Recording medium, information recording method, and information reading method

Abstract

A recording medium according to the present disclosure comprises a recording layer containing an organic compound having nonlinear light absorption properties, and the organic compound has a molar absorption coefficient of at least 90 mol-1·L·cm-1 for light having a wavelength of 400-405 nm. In the transient absorption spectrum of the organic compound, the absorbance change ΔAbs at a wavelength of 400-405 nm is a positive value.

Description

Recording medium, information recording method, and information reading method

[0001] The present disclosure relates to a recording medium, a method for recording information, and a method for reading information.

[0002] Among optical materials such as light-absorbing materials, materials that exhibit nonlinear optical effects are called nonlinear optical materials. The nonlinear optical effect refers to the occurrence of optical phenomena in a material that are proportional to the square or higher-than-squared electric field of the irradiated light when the material is irradiated with intense light such as laser light. Examples of optical phenomena include absorption, reflection, scattering, and light emission. Examples of second-order nonlinear optical effects proportional to the square of the electric field of the irradiated light include second-harmonic generation (SHG), the Pockels effect, and the parametric effect. Examples of third-order nonlinear optical effects proportional to the cube of the electric field of the irradiated light include two-photon absorption, multi-photon absorption, third-harmonic generation (THG), and the Kerr effect. In this specification, multi-photon absorption such as two-photon absorption is sometimes referred to as nonlinear optical absorption. Materials capable of nonlinear optical absorption are sometimes referred to as nonlinear optical absorption materials. In particular, materials capable of two-photon absorption are sometimes referred to as two-photon absorption materials. Note that nonlinear optical absorption is also sometimes referred to as nonlinear absorption.

[0003] Much research has been conducted on nonlinear optical materials. In particular, inorganic materials that can be easily prepared as single crystals have been developed as nonlinear optical materials. In recent years, the development of nonlinear optical materials made from organic materials has been anticipated. Compared to inorganic materials, organic materials not only have a high degree of design freedom but also have large nonlinear optical constants. Furthermore, organic materials exhibit high-speed nonlinear response. In this specification, nonlinear optical materials containing organic materials may be referred to as organic nonlinear optical materials.

[0004] Patent No. 6448042 Patent No. 5738554

[0005] Harry L. Anderson et al, "Two-Photon Absorption and the Design of Two-Photon Dyes", Angew. Chem. Int. Ed. 2009, Vol. 48, p. 3244-3266.

[0006] There is a demand for new recording media using nonlinear optical materials.

[0007] A recording medium according to one aspect of the present disclosure includes a recording layer containing an organic compound having nonlinear optical absorption properties, and the organic compound has a molar absorption coefficient of 90 mol / L for light having a wavelength of 400 nm or more and 405 nm or less. -1 ・L・cm -1 or more, and in the transient absorption spectrum of the organic compound, the absorbance change ΔAbs at a wavelength of 400 nm or more and 405 nm or less is a positive value.

[0008] The present disclosure provides a new recording medium using a nonlinear optical material.

[0009] 1A is a flowchart illustrating a method for recording information using a recording medium including a nonlinear optical absorbing material according to an embodiment of the present disclosure. FIG. 1B is a flowchart illustrating a method for reading information using a recording medium including a nonlinear optical absorbing material according to an embodiment of the present disclosure. FIG. 2 is a flowchart illustrating a compound represented by formula (2). 13A is a graph showing the recording and reproduction characteristics of the resin thin film of Example 1. FIG. 3B is a graph showing the recording and reproduction characteristics of the resin thin film of Example 2. FIG. 3C is a graph showing the recording and reproduction characteristics of the resin thin film of Comparative Example 1. FIG. 3D is a graph showing the recording and reproduction characteristics of the resin thin film of Comparative Example 2. FIG. 3E is a graph showing the recording and reproduction characteristics of the resin thin film of Comparative Example 3. FIG. 3F is a graph showing the recording and reproduction characteristics of the resin thin film of Comparative Example 4. FIG. 3G is a graph showing the recording and reproduction characteristics of the resin thin film of Comparative Example 5. FIG. 4A is a transient absorption spectrum of a solution containing the compound of Example 1. FIG. 4B is a transient absorption spectrum of a solution containing the compound of Example 1 when the vertical axis is converted from absorbance change to excited state molar absorption coefficient. FIG. 5A is a transient absorption spectrum of a thin film containing the compound of Example 1. FIG. 5B is a transient absorption spectrum of a thin film containing the compound of Example 1 when the vertical axis is converted from absorbance change to excited state molar absorption coefficient. 6A and 6B show transient absorption spectra of a solution containing the compound of Comparative Example 1 and the compound of Comparative Example 2, respectively.

[0010] (Findings that form the basis of the present disclosure) Among organic nonlinear optical materials, two-photon absorption materials have attracted particular attention. Two-photon absorption refers to a phenomenon in which a compound absorbs two photons almost simultaneously and transitions to an excited state. Known two-photon absorption types include simultaneous two-photon absorption and stepwise two-photon absorption. Simultaneous two-photon absorption is also called non-resonant two-photon absorption. Simultaneous two-photon absorption refers to two-photon absorption in a wavelength range where no single-photon absorption band exists. Stepwise two-photon absorption is also called resonant two-photon absorption. In stepwise two-photon absorption, a compound absorbs a first photon and then further absorbs a second photon, thereby transitioning to a higher excited state. In stepwise two-photon absorption, a compound sequentially absorbs two photons.

[0011] In simultaneous two-photon absorption, the amount of light absorbed by a compound is usually proportional to the square of the intensity of the irradiated light and exhibits nonlinearity. The amount of light absorbed by a compound can be used as an indicator of the efficiency of two-photon absorption. When the amount of light absorbed by a compound exhibits nonlinearity, for example, light absorption by the compound can occur only near the focus of a laser beam with a high electric field strength. That is, in a sample containing a two-photon absorbing material, the compound can be excited only at the desired position. Compounds that exhibit simultaneous two-photon absorption thus provide extremely high spatial resolution, and therefore are being considered for application in applications such as recording layers for three-dimensional optical memories and photocurable resin compositions for stereolithography. If the two-photon absorbing material also has fluorescent properties, it can also be used as a fluorescent dye material used in two-photon fluorescence microscopes, etc. If this two-photon absorbing material is used in a three-dimensional optical memory, it may be possible to adopt a method of reading the ON / OFF state of the recording layer based on changes in fluorescence from the two-photon absorbing material. Current optical memories employ a method for reading the ON / OFF state of a recording layer based on changes in the light reflectance and light absorbance of a two-photon absorbing material. However, when this method is applied to a three-dimensional optical memory, crosstalk may occur due to the fact that the two-photon absorption efficiency of conventional two-photon absorbing materials is lower than the one-photon absorption efficiency.

[0012] In two-photon absorption materials, the two-photon absorption cross section (GM value) is used as an index of two-photon absorption efficiency. The unit of the two-photon absorption cross section is GM (10 -50 cm 4 ・s・molecule -1 ・photon -1 ) Up to now, many organic two-photon absorption materials with large two-photon absorption cross sections have been proposed. For example, many compounds with large two-photon absorption cross sections exceeding 500 GM have been reported (e.g., Non-Patent Document 1). However, in most reports, the two-photon absorption cross section is measured using laser light with a wavelength longer than 600 nm. In particular, near-infrared light with a wavelength longer than 750 nm is sometimes used as the laser light.

[0013] However, in order to apply two-photon absorption materials to industrial applications, materials that exhibit two-photon absorption properties when irradiated with laser light having a shorter wavelength are required. For example, in the field of three-dimensional optical memory, laser light having a short wavelength can realize a finer focused spot, thereby improving the recording density of the three-dimensional optical memory. In the field of stereolithography, laser light having a short wavelength can also realize modeling with higher resolution. Furthermore, the Blu-ray (registered trademark) disc standard uses laser light having a central wavelength of 405 nm. Thus, the development of a compound that has excellent two-photon absorption properties for light in the same wavelength range as short-wavelength laser light could greatly contribute to industrial development.

[0014] Furthermore, light-emitting devices that emit ultrashort pulsed lasers with high optical intensity tend to be large and unstable in operation. Therefore, such light-emitting devices are difficult to adopt for industrial applications from the viewpoints of versatility and reliability. In consideration of this, in order to apply two-photon absorption materials to industrial applications, materials that exhibit two-photon absorption properties even when irradiated with low-intensity laser light are required.

[0015] Examples of light sources that can be used include femtosecond lasers such as titanium sapphire lasers, or pulsed lasers with picosecond to nanosecond pulse widths, such as semiconductor lasers. Semiconductor lasers are suitable for industrial applications due to their compact size, versatility, and stable operation. When an organic nonlinear optical material is irradiated with light having a pulse width of picoseconds to nanoseconds or longer, and the light is focused with a lens to increase the photon density, electrons undergo one-photon or two-photon excitation on the order of femtoseconds and then relax to the lowest excited state within several hundred femtoseconds to picoseconds. Even when electrons relax to the lowest excited state, they are still in the middle of pulse irradiation. Therefore, excitation from the lowest excited state to a higher excited state can occur. This phenomenon is called excited state absorption (ESA). Subsequently, excited state absorption and relaxation to the lowest excited state are repeated as long as pulse irradiation continues. Except for special cases such as azulene, this relaxation is an extremely fast process that is completed within picoseconds at the latest and is nonradiatively deactivated. In other words, deactivation occurs not by emitting light such as fluorescence or phosphorescence, but by generating heat. Thus, when a nonlinear optically absorbing material is locally excited using a laser with a pulse width of picoseconds to nanoseconds or longer, if the nonlinear optically absorbing material further undergoes excited-state absorption, it is possible to locally generate heat. This makes it possible, for example, to use the nonlinear optically absorbing material as a heat source for locally altering a recording medium, thereby enabling three-dimensional recording.

[0016] The excited state absorption proceeds, for example, as follows.

[0017] (1-1) By one-photon absorption or two-photon absorption, electrons transition from the ground state (S0) to an excited state and quickly relax to the first excited state (singlet, S1). (1-2) By further one-photon absorption from the S1 state, electrons transition to a higher singlet excited state (S n) or (2-1) By one-photon absorption or two-photon absorption, the electron transitions from the ground state (S0) to an excited state and quickly relaxes to the first excited state (singlet, S1). (2-2) Intersystem crossing (ISC) occurs from the S1 state to the triplet excited state (T1). (2-3) By another one-photon absorption from the T1 state, the electron transitions to a higher triplet excited state (T n ) is excited.

[0018] In the absence of excited state absorption, for example, the electronic transition proceeds as follows:

[0019] (3-1) One-photon or two-photon absorption causes electrons to transition from the ground state (S0) to an excited state and quickly relax to the first excited state (singlet, S1). (3-2) Electrons relax from the S1 state to the S0 state and are deactivated. Or, (4-1) One-photon or two-photon absorption causes electrons to transition from the ground state (S0) to an excited state and quickly relax to the first excited state (singlet, S1). (4-2) Intersystem crossing occurs from the S1 state to the triplet excited state (T1). (4-3) Electrons relax from the T1 state to the S0 state and are deactivated.

[0020] As described above, excited-state absorption is a sequential multiphoton absorption and a type of nonlinear optical absorption. Like two-photon absorption, excited-state absorption occurs only when a sample is irradiated with high-intensity light. The probability of excited-state absorption occurring when a sample is irradiated with low-intensity light is negligibly small.

[0021] In a compound having nonlinear optical absorption properties, the relationship between light intensity and absorption properties when irradiated with laser light having a pulse width of picoseconds to nanoseconds or longer is expressed by the following formulas (i) and (ii). In this specification, a compound having nonlinear optical absorption properties may be referred to as a nonlinear optical absorbing compound. Formulas (i) and (ii) are formulas for calculating the decrease in light intensity -dI when a sample containing a nonlinear optical absorbing compound and having a minute thickness dz is irradiated with light of intensity I.

[0022] In formula (ii), α is the one-photon absorption coefficient (cm -1) β is the simultaneous two-photon absorption coefficient (cm / W) γ is the simultaneous three-photon absorption coefficient (cm 3 / W 2 ) σ ESA is the excited state absorption cross section (cm 2 ) where τ is the lifetime of the excited state (s). h-(Hbar) is the Dirac constant (J·s). ω is the angular frequency of the incident light (rad / s).

[0023] Furthermore, α and β can be expressed by the following formulas (iii) and (iv), respectively. In formulas (iii) and (iv), ε is the molar absorption coefficient (mol -1 ・L・cm -1 ) N is the number of molecules of the compound per unit volume of the sample (mol cm -3 ) N A is the Avogadro constant. σ is the two-photon absorption cross section (GM).

[0024] Absorption coefficient (cm -1 The molar extinction coefficient (L mol ) is the rate at which light is absorbed per unit length as it travels through a substance. -1 ・cm -1 ) refers to the rate at which photons are absorbed per mole of molecules as light travels through a substance. 2 ) indicates the ratio of photons absorbed per particle (molecule) when light travels through a substance. The absorption coefficient can be converted to the absorption cross section by dividing it by the number of molecules per unit volume of the sample (number density of molecules). The absorption cross section is calculated using Avogadro's constant (6.02 x 10 23 mol -1 ) to convert the unit, it can be converted to the molar extinction coefficient.

[0025] When the intensity of the irradiated light is low, the contribution of the third-order nonlinear optical absorption is small. For example, when irradiating light from a small, versatile semiconductor laser, the third-order term in equation (i) is considered to be negligibly small. For simplicity, we use the CI 3 The nonlinearity of optical absorption will be explained below using the following equation (v) which ignores

[0026] From equation (v), the linear absorption (first-order term: AI) and nonlinear absorption (second-order term: BI) are determined for the sample. 2 It can be seen that the intensity I of the incident light when A / B is equal to A / B is expressed as A / B. In other words, when the intensity I of the incident light is smaller than A / B, linear optical absorption occurs preferentially in the sample. When the intensity I of the incident light is larger than A / B, nonlinear optical absorption occurs preferentially in the sample. Therefore, the smaller the value of A / B in the sample, the more likely it is that nonlinear optical absorption can be preferentially exhibited by laser light with a low optical intensity. Here, A / B is expressed by the following formula (vi).

[0027] For materials where excited state absorption does not occur, σ ESA = 0, the following formula (vii) holds: Therefore, when the incident light intensity is smaller than α / β, linear light absorption occurs in preference to nonlinear light absorption.

[0028] On the other hand, in the case of a material that exhibits excited-state absorption, the term on the left side of formula (vii) has a value, and formula (vii) does not hold. Therefore, the threshold of the incident light intensity I for preferentially causing nonlinear optical absorption over linear optical absorption can be lowered. If the material has high excited-state absorption, it becomes possible to cause nonlinear optical absorption to predominate even at very low incident light intensities.

[0029] Patent Document 1 discloses that an optical information recording material containing a nonlinear absorbing dye and having a multilayer diffraction grating formed thereon is irradiated with a laser having a central wavelength of 401 nm and a pulse width of 8 nanoseconds to locally destroy the diffraction grating, thereby forming a recording mark. Examples of nonlinear absorbing dyes disclosed include 1,1,4,4-tetraphenyl-1,3-butadiene, 1,3,6,8-tetraphenylpyrene, pyrene-ethylene glycol-pyrene, 1,4-bis(phenylethynyl)benzene, 1,2,4,5-tetrakis(phenylethynyl)benzene, 9,10-diphenylanthracene, 5,6,11,12-tetraphenylnaphthacene, fluorene, 2,7-dibromofluorene, 1-bromopyrene, 4-bromopyrene, and pyrene.

[0030] Patent Document 2 discloses a holographic recording medium containing a nonlinear sensitizer that transitions to a higher triplet excited state upon irradiation with a laser having a wavelength of 405 nm and a pulse width of 5 nanoseconds. The nonlinear sensitizer disclosed is a platinum ethynyl complex.

[0031] However, if the absorption coefficient of the nonlinear absorbing dye in the ground state is too small, the value of -dI / dz in formula (v) will be small, resulting in insufficient recording sensitivity. Furthermore, if a nonlinear absorbing dye absorbs light and becomes excited, then passes through a triplet excited state before returning to the ground state, there is a concern that lightfastness may be insufficient. This is because atmospheric oxygen molecules exist in a triplet state in the ground state and undergo an energy transfer reaction with the dye in the triplet excited state, generating singlet oxygen. The triplet excited state has a long lifetime because it involves spin inversion when returning to the ground state, with some having an excitation lifetime on the order of several hundred milliseconds. The more dyes that pass through the triplet excited state after excitation, i.e., the higher the quantum yield of the dye's intersystem crossing, the higher the probability of reaction with oxygen molecules. The longer the lifetime of the triplet excited state, the higher the probability of reaction with oxygen molecules. Singlet oxygen is electron-deficient and highly active, reacting with and altering surrounding dyes or polymer compounds. When a dye reacts with singlet oxygen, its optical properties change, such as fading.

[0032] As a result of extensive research, the present inventors have newly discovered that the following characteristics are required for a recording medium using an organic compound, which is a nonlinear light absorbing material: (a) for light having a wavelength in the short wavelength region, the one-photon absorption coefficient α is in an appropriate range, and (b) for light having a wavelength in the short wavelength region, the excited state absorption cross section σ is in an appropriate range. ESAIt is required that the value of -dI / dz described above be sufficiently large. In this case, in addition to the value of -dI / dz being sufficiently large, the value of the ratio A / B (formula (vi)) of the magnitude B of linear optical absorption to the magnitude A of nonlinear optical absorption is small, and the nonlinearity of optical absorption tends to be high. Furthermore, when the excited state absorption occurring in the short wavelength region is from a singlet excited state, the lifetime of the excited state is not too long, and reaction with oxygen in the atmosphere is unlikely to occur. Therefore, deterioration due to the generation of singlet oxygen is unlikely to occur. In this specification, the short wavelength region means a wavelength region including 405 nm, for example, a wavelength region of 390 nm to 420 nm.

[0033] (Summary of one aspect of the present disclosure) A recording medium according to a first aspect of the present disclosure includes a recording layer containing an organic compound having nonlinear optical absorption properties, and the organic compound has a molar absorption coefficient of 90 mol / L for light having a wavelength of 400 nm or more and 405 nm or less. -1 ・L・cm -1 or more, and in the transient absorption spectrum of the organic compound, the absorbance change ΔAbs at a wavelength of 400 nm or more and 405 nm or less is a positive value.

[0034] According to the first aspect, a new recording medium using a nonlinear optical material can be provided.

[0035] In a second aspect of the present disclosure, for example, in the recording medium according to the first aspect, the molar absorption coefficient of the organic compound is 1000 mol -1 ・L・cm -1 It may be the following:

[0036] In a third aspect of the present disclosure, for example, in the recording medium according to the first or second aspect, the molar absorption coefficient of the organic compound is 100 mol -1 ・L・cm -1 Over 500 mol -1 ・L・cm -1 It may be the following:

[0037] In a fourth aspect of the present disclosure, for example, in the recording medium according to any one of the first to third aspects, the organic compound may have an excited state with a lifetime of 1 millisecond or less.

[0038] Organic compounds having the properties described in the second to fourth aspects are suitable for the recording medium of the present disclosure.

[0039] In a fifth aspect of the present disclosure, for example, in the recording media according to the first to fourth aspects, the organic compound may be a compound capable of undergoing a change in its three-dimensional structure in an excited state. By having such a property, the excited state absorption cross section σ of the organic compound in the short wavelength region can be reduced. ESA may show high values.

[0040] An information recording method according to a sixth aspect of the present disclosure includes: preparing a light source that emits light having a wavelength of 390 nm or more and 420 nm or less; and concentrating the light from the light source and irradiating the light onto a recording layer in a recording medium according to any one of the first to fifth aspects.

[0041] According to the sixth aspect, the recording medium has improved nonlinear optical absorption characteristics for light having a wavelength in the short wavelength range. According to the information recording method using the recording medium containing the organic compound having nonlinear optical absorption characteristics, information can be recorded at a high recording density.

[0042] An information reading method according to a seventh aspect of the present disclosure is, for example, a method for reading information recorded by the recording method according to the fifth aspect, the reading method including measuring optical characteristics of the recording layer by irradiating light onto the recording layer in the recording medium, and reading information from the recording layer.

[0043] In an eighth aspect of the present disclosure, for example, in the information reading method according to the seventh aspect, the optical property may be the intensity of light reflected by the recording layer.

[0044] According to the seventh or eighth aspect, when reading out information, it is possible to suppress the occurrence of crosstalk due to other recording layers.

[0045] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0046] The recording medium of this embodiment has a recording layer containing an organic compound having nonlinear optical absorption characteristics. The organic compound has a molar absorption coefficient of 90 mol / L for light having a wavelength of 400 nm or more and 405 nm or less. -1 ・L・cm -1 In the transient absorption spectrum of the organic compound, the absorbance change ΔAbs at wavelengths of 400 nm or more and 405 nm or less is a positive value.

[0047] The absorbance change ΔAbs is the absorbance Abs in the excited state ex From the ground state absorbance Abs noex The absorbance Abs is calculated by subtracting the absorbance change ΔAbs from the absorbance change ΔAbs. The absorbance Abs is calculated by the Beer-Lambert law. A positive absorbance change ΔAbs indicates that excited state absorption (ESA) occurs predominantly in the wavelength range. In other words, excited state absorption can be caused by light with a wavelength of 400 nm or more and 405 nm or less, and the heat generated during relaxation can be utilized. On the other hand, irradiation with low-intensity light of the same wavelength generates almost no heat. These properties are suitable for forming recording marks on recording media and reading information.

[0048] Transient absorption refers to the transient absorption of light. Transient absorption spectroscopy allows for the tracking of instantaneous excited-state absorption caused by irradiation with a pulsed laser with high time resolution. For example, a single-wavelength pump light is irradiated onto a sample to transition it to an excited state, followed by a white probe light after an appropriate delay. A transient absorption spectrum is obtained by detecting the difference in absorbance between the excited and ground states, ΔAbs. By focusing on the increase in absorbance compared to the ground state, the wavelength range in which excited-state absorption occurs can be determined. In the ground-state absorption wavelength range, a decrease in absorbance due to a decrease in the number of ground-state molecules due to light absorption can sometimes be observed, resulting in a bleaching phenomenon in which the absorbance change takes a negative value.

[0049] Transient absorption spectroscopy can be used on both solution and thin-film samples. To prevent deactivation of the excited state by atmospheric oxygen, it is desirable to remove dissolved oxygen. For solutions, deoxidation can be achieved by bubbling argon or nitrogen for a certain period of time. For thin-film samples, deoxidation can be achieved by replacing the gas in the cell with argon or nitrogen. The lifetime of the excited state can also be determined from the time evolution of the transient absorption spectrum. When the lifetime of the excited state observed by tracking the transient absorption spectrum is on the order of picoseconds to hundreds of nanoseconds, it is generally considered to be a transition from a singlet excited state to a higher singlet excited state. On the other hand, when the lifetime of the excited state is longer than a microsecond, it is generally considered to be a transition from a triplet excited state to a higher triplet excited state. Furthermore, the triplet excited state is significantly less stable to oxygen than the singlet excited state. Therefore, if the transient absorption intensity remains almost constant with or without oxygen, it can be concluded that the observed state originates from a singlet excited state. If the transient absorption intensity changes significantly depending on the presence or absence of oxygen, it can be concluded that the observed state originates from the triplet excited state.

[0050] Excited state absorption is a sequential light absorption that occurs following one-photon absorption or two-photon absorption. To efficiently generate excited state absorption, it is desirable that an excited state is formed by one-photon absorption of an organic compound. A desirable range of the molar extinction coefficient of an organic compound when one-photon absorption occurs is, for example, 90 mol -1 ・L・cm -1 When the one-photon absorption and excited-state absorption satisfy the desired conditions, the desired nonlinear optical absorption characteristics can be achieved.

[0051] The molar absorption coefficient of the above organic compound is 1000 mol -1 ・L・cm -1 It may be 100 mol or less, -1 ・L・cm -1 Over 500 mol -1 ・L・cm -1If the molar extinction coefficient is not too small, a sufficient amount of excited species can be formed. If the molar extinction coefficient is not too large, nonlinear absorption is likely to predominate over linear absorption.

[0052] The excited state of the organic compound has a lifetime of, for example, 1 millisecond or less. The lifetime of the excited state of 1 millisecond or less means that the excited state is a singlet excited state. The singlet excited state is more stable against oxygen in the air than the long-lived triplet excited state. This is advantageous for improving the light resistance of the recording medium.

[0053] An example of an organic compound having the above-described properties is a compound a represented by the following formula (1).

[0054] In formula (1), R 1 From R 12 each independently represents a group containing at least one atom selected from the group consisting of H, B, C, N, O, F, Si, P, S, Cl, I, and Br.

[0055] Compound a has a sufficiently high absorption of light having wavelengths in the short wavelength range. The breakdown of absorption tends to be a small ratio (A / B) of the magnitude of linear optical absorption (A) to the magnitude of nonlinear optical absorption (B), i.e., a tendency for the nonlinearity of optical absorption to be high. Compound a has excellent light resistance because it does not pass through a triplet excited state during the relaxation process. Thus, Compound a has been improved in terms of both nonlinear optical absorption characteristics and light resistance for light having wavelengths in the short wavelength range. Furthermore, recording sensitivity has been improved. Compound a is excited by irradiation with laser light having a wavelength in the short wavelength range, relaxes to the lowest singlet excited state while undergoing structural changes, and then transitions from the lowest singlet excited state to a higher singlet excited state by absorbing further laser light. The structural change after excitation occurs due to twisting of the double bond connecting the two six-membered rings. In the adiabatic state, the π-electron conjugated system is shortened. This causes the excited-state optical absorption band to blue-shift to the short wavelength range, and the σ in the short wavelength range ESA shows high values.

[0056] In formula (1), R 1 From R 12may be, independently of one another, a hydrogen atom, a halogen atom, a hydrocarbon group, a halogenated hydrocarbon group, a group containing an oxygen atom, a group containing a nitrogen atom, a group containing a sulfur atom, a group containing a silicon atom, a group containing a phosphorus atom, or a group containing a boron atom.

[0057] Examples of halogen atoms include F, Cl, Br, and I. In this specification, halogen atoms may be referred to as halogen groups.

[0058] The hydrocarbon group is an alkyl group or an unsaturated hydrocarbon group.

[0059] The number of carbon atoms in the alkyl group is not particularly limited and is, for example, 1 or more and 20 or less. From the viewpoint of facilitating the synthesis of compound a, the number of carbon atoms in the alkyl group may be 1 or more and 10 or 1 or more and 5 or less. By adjusting the number of carbon atoms in the alkyl group, the solubility of compound a in a solvent or resin composition can be adjusted. The alkyl group may be linear, branched, or cyclic. At least one hydrogen atom contained in the alkyl group may be substituted with a group containing at least one atom selected from the group consisting of N, O, P, and S. Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, a 2-methylbutyl group, a pentyl group, a hexyl group, a 2,3-dimethylhexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an eicosyl group, a 2-methoxybutyl group, and a 6-methoxyhexyl group.

[0060] The unsaturated hydrocarbon group contains an unsaturated bond such as a carbon-carbon double bond or a carbon-carbon triple bond. The number of unsaturated bonds contained in the unsaturated hydrocarbon group is, for example, 1 to 5. The number of carbon atoms in the unsaturated hydrocarbon group is not particularly limited and is, for example, 2 to 20, or alternatively 2 to 10, or alternatively 2 to 5. The unsaturated hydrocarbon group may be linear, branched, or cyclic. At least one hydrogen atom contained in the unsaturated hydrocarbon group may be substituted with a group containing at least one atom selected from the group consisting of N, O, P, and S. Examples of unsaturated hydrocarbon groups include a vinyl group, an ethynyl group, and an aryl group.

[0061] A halogenated hydrocarbon group refers to a group in which at least one hydrogen atom contained in a hydrocarbon group has been substituted with a halogen atom. The halogenated hydrocarbon group may be a group in which all hydrogen atoms contained in the hydrocarbon group have been substituted with halogen atoms. Examples of the halogenated hydrocarbon group include a halogenated alkyl group and a halogenated alkenyl group.

[0062] Examples of halogenated alkyl groups include -CF3, -CH2F, -CH2Br, -CH2Cl, -CH2I, -CH2CF3, etc. Examples of halogenated alkenyl groups include -CH=CHCF3, etc.

[0063] The group containing an oxygen atom is, for example, a substituent having at least one group selected from the group consisting of a hydroxyl group, a carboxyl group, an aldehyde group, an ether group, an acyl group, and an ester group.

[0064] Examples of the substituent having a hydroxyl group include a hydroxyl group itself and a hydrocarbon group having a hydroxyl group. In this substituent, the hydroxyl group can be deprotonated to form -O - Examples of hydrocarbon groups having a hydroxyl group include -CHOH, -CH(OH)CH, -CHCH(OH)CH, and -CHC(OH)(CH).

[0065] Examples of substituents having a carboxyl group include the carboxyl group itself and hydrocarbon groups having a carboxyl group. In this substituent, the carboxyl group is deprotonated to form -CO2 - The hydrocarbon group having a carboxyl group may be in the form of -CH2CH2COOH, -C(COOH)(CH3)2, -CH2CO2 - Examples include:

[0066] Examples of the substituent having an aldehyde group include the aldehyde group itself and hydrocarbon groups having an aldehyde group, such as —CH═CHCHO.

[0067] Examples of the substituent having an ether group include an alkoxy group, a halogenated alkoxy group, an alkenyloxy group, an oxiranyl group, and a hydrocarbon group having at least one of these functional groups. At least one hydrogen atom contained in the alkoxy group may be substituted with a group containing at least one atom selected from the group consisting of N, O, P, and S. Examples of the alkoxy group include a methoxy group, an ethoxy group, a 2-methoxyethoxy group, a butoxy group, a 2-methylbutoxy group, a 2-methoxybutoxy group, a 4-ethylthiobutoxy group, a pentyloxy group, a hexyloxy group, a heptyloxy group, an octyloxy group, a nonyloxy group, a decyloxy group, an undecyloxy group, a dodecyloxy group, a tridecyloxy group, a tetradecyloxy group, a pentadecyloxy group, a hexadecyloxy group, a heptadecyloxy group, an octadecyloxy group, a nonadecyloxy group, an eicosyloxy group, and -OCHO. - , —OCH2CH2O - , —O(CH2)3O - Examples of halogenated alkoxy groups include -OCHF2, -OCH2F, and -OCH2Cl. Examples of alkenyloxy groups include -OCH=CH2. Examples of hydrocarbon groups having a functional group such as an alkoxy group include -CHOCH3, -C(OCH3)3, a 2-methoxybutyl group, and a 6-methoxyhexyl group.

[0068] Examples of substituents having an acyl group include the acyl group itself and hydrocarbon groups having an acyl group. Examples of acyl groups include -COCH3. Examples of hydrocarbon groups having an acyl group include -CH=CHCOCH3.

[0069] Examples of substituents having an ester group include alkoxycarbonyl groups, acyloxy groups, and hydrocarbon groups having at least one of these functional groups. Examples of alkoxycarbonyl groups include -COOCH3, -COO(CH2)3CH3, and -COO(CH2)7CH3. Examples of acyloxy groups include -OCOCH3. Examples of hydrocarbon groups having a functional group such as an acyloxy group include -CH2OCOCH3.

[0070] The group containing a nitrogen atom is, for example, a substituent having at least one selected from the group consisting of an amino group, an imino group, a cyano group, an azido group, an amide group, a carbamate group, a nitro group, a cyanamide group, an isocyanate group, and an oxime group.

[0071] Examples of substituents having an amino group include primary amino groups, secondary amino groups, tertiary amino groups, quaternary amino groups, and hydrocarbon groups having at least one of these functional groups. In these substituents, the amino group may be protonated. Examples of tertiary amino groups include -N(CH3)2. Examples of hydrocarbon groups having a functional group such as a primary amino group include -CH2NH2, -CH2N(CH3)2, -(CH2)4N(CH3)2, and -CH2CH2NH3. + , -CH2CH2NH(CH3)2 + , -CH2CH2N(CH3)3 + Examples include:

[0072] Examples of the substituent having an imino group include the imino group itself and hydrocarbon groups having an imino group. Examples of the imino group include -N=CCl2.

[0073] Examples of substituents having a cyano group include the cyano group itself and hydrocarbon groups having a cyano group, such as -CHCN and -CH=CHCN.

[0074] Examples of the substituent having an azido group include the azido group itself and a hydrocarbon group having an azido group.

[0075] Examples of substituents having an amide group include the amide group itself and hydrocarbon groups having an amide group. Examples of amide groups include -CONH2, -NHCHO, -NHCOCH3, -NHCOCF3, -NHCOCH2Cl, and -NHCOCH(CH3)2. Examples of hydrocarbon groups having an amide group include -CH2CONH2 and -CH2NHCOCH3.

[0076] Examples of substituents having a carbamate group include the carbamate group itself and hydrocarbon groups having a carbamate group, such as -NHCOOCH3, -NHCOOCH2CH3, and -NHCO2(CH2)3CH3.

[0077] Examples of substituents having a nitro group include the nitro group itself and hydrocarbon groups having a nitro group, such as —C(NO)(CH).

[0078] Examples of the substituent having a cyanamide group include the cyanamide group itself and a hydrocarbon group having a cyanamide group. The cyanamide group is represented by —NHCN.

[0079] Examples of the substituent having an isocyanate group include an isocyanate group itself and a hydrocarbon group having an isocyanate group. The isocyanate group is represented by —N═C═O.

[0080] Examples of the substituent having an oxime group include the oxime group itself and a hydrocarbon group having an oxime group. The oxime group is represented by —CH═NOH.

[0081] The group containing a sulfur atom is, for example, a substituent having at least one selected from the group consisting of a thiol group, a sulfide group, a sulfinyl group, a sulfonyl group, a sulfino group, a sulfonic acid group, an acylthio group, a sulfenamide group, a sulfonamide group, a thioamide group, a thiocarbamide group, and a thiocyano group.

[0082] Examples of the substituent having a thiol group include a thiol group itself and a hydrocarbon group having a thiol group. A thiol group is represented by —SH.

[0083] Examples of substituents having a sulfide group include alkylthio groups, alkyldithio groups, alkenylthio groups, alkynylthio groups, thiacyclopropyl groups, and hydrocarbon groups having at least one of these functional groups. At least one hydrogen atom contained in the alkylthio group may be substituted with a halogen group. Examples of alkylthio groups include -SCH3, -S(CH2)F, -SCH(CH3)2, and -SCH2CH3. Examples of alkyldithio groups include -SSCH3. Examples of alkenylthio groups include -SCH=CH2 and -SCH2CH=CH2. Examples of alkynylthio groups include -SC≡CH. Examples of hydrocarbon groups having a functional group such as an alkylthio group include -CH2SCF3.

[0084] Examples of the substituent having a sulfinyl group include the sulfinyl group itself and hydrocarbon groups having a sulfinyl group. Examples of the sulfinyl group include —SOCH3.

[0085] Examples of substituents having a sulfonyl group include the sulfonyl group itself and hydrocarbon groups having a sulfonyl group. Examples of sulfonyl groups include -SO2CH3. Examples of hydrocarbon groups having a sulfonyl group include -CH2SO2CH3 and -CH2SO2CH2CH3.

[0086] Examples of the substituent having a sulfino group include the sulfino group itself and a hydrocarbon group having a sulfino group. In this substituent, the sulfino group can be deprotonated to form -SO2 - It may be in the state of

[0087] Examples of the substituent having a sulfonic acid group include the sulfonic acid group itself and a hydrocarbon group having a sulfonic acid group. In this substituent, the sulfonic acid group is deprotonated to form -SO3 - It may be in the state of

[0088] Examples of substituents having an acylthio group include the acylthio group itself and hydrocarbon groups having an acylthio group. Examples of the acylthio group include -SCOCH3.

[0089] Examples of the substituent having a sulfenamide group include the sulfenamide group itself and hydrocarbon groups having a sulfenamide group. Examples of the sulfenamide group include —SN(CH3)2.

[0090] Examples of the substituent having a sulfonamide group include the sulfonamide group itself and hydrocarbon groups having a sulfonamide group. Examples of the sulfonamide group include -SO2NH2 and -NHSO2CH3.

[0091] Examples of substituents having a thioamide group include the thioamide group itself and hydrocarbon groups having a thioamide group. Examples of thioamide groups include -NHCSCH3 and -CH2SC(NH2)2. + Examples include:

[0092] Examples of substituents having a thiocarbamide group include the thiocarbamide group itself and hydrocarbon groups having a thiocarbamide group. Examples of the thiocarbamide group include -NHCSNHCH2CH3.

[0093] Examples of substituents having a thiocyano group include the thiocyano group itself and hydrocarbon groups having a thiocyano group, such as —CH2SCN.

[0094] The group containing a silicon atom is, for example, a substituent having at least one group selected from the group consisting of a silyl group and a siloxy group.

[0095] Substituents having a silyl group include the silyl group itself and hydrocarbon groups having a silyl group. Examples of silyl groups include -Si(CH3)3, -SiH(CH3)2, -Si(OCH3)3, -Si(OCH2CH3)3, -SiCH3(OCH3)2, -Si(CH3)2OCH3, -Si(N(CH3)2)3, -SiF(CH3)2, -Si(OSi(CH3)3)3, and -Si(CH3)2OSi(CH3)3. Examples of hydrocarbon groups having a silyl group include -(CH2)2Si(CH3)3.

[0096] Examples of the substituent having a siloxy group include the siloxy group itself and hydrocarbon groups having a siloxy group, such as —CH2OSi(CH3)3.

[0097] The group containing a phosphorus atom is, for example, a substituent having at least one group selected from the group consisting of a phosphino group and a phosphoryl group.

[0098] Examples of the substituent having a phosphino group include the phosphino group itself and hydrocarbon groups having a phosphino group, such as -PH2, -P(CH3)2, -P(CH2CH3)2, -P(C(CH3)3)2, and -P(CH(CH3)2)2.

[0099] Examples of substituents having a phosphoryl group include the phosphoryl group itself and hydrocarbon groups having a phosphoryl group, such as —CH2PO(OCH2CH3)2.

[0100] The group containing a boron atom is, for example, a substituent having a boronic acid group. Examples of the substituent having a boronic acid group include the boronic acid group itself and a hydrocarbon group having a boronic acid group.

[0101] In formula (1), R 5 From R 12 may each be a hydrogen atom. In this case, the aromatic ring in compound a represented by formula (1) has no substituent. Therefore, it is possible to suppress an increase in the energy of the highest occupied molecular orbital (HOMO: Highest Occupied Molecular Orbital) of the compound and a decrease in the energy of the lowest unoccupied molecular orbital (LUMO: Lowest Unoccupied Molecular Orbital) due to the electron-withdrawing or electron-donating properties of the substituent. In other words, it is possible to suppress a decrease in the energy gap between the HOMO and the LUMO. This prevents a peak resulting from one-photon absorption from shifting to longer wavelengths, and suppresses an increase in the value of the ratio A / B (formula (vi)) of linear optical absorption to the magnitude of nonlinear optical absorption.

[0102] In formula (1), R 1 and R 2 may be the same group. 1 and R 3 may be the same group. According to this configuration, the compound represented by formula (1) can be easily synthesized.

[0103] In formula (1), R 1 From R 4 may be the same group. According to this configuration, the compound represented by formula (1) can be easily synthesized.

[0104] In formula (1), R 1 From R 4 may each be a hydrocarbon group or a halogenated hydrocarbon group having 5 or less carbon atoms. 1 From R 4 may each be a methyl group or a CF3 group.

[0105] In formula (1), R 1 From R 12may be a group that does not contain an aromatic ring.

[0106] In particular, the compounds of the present disclosure may be represented by the following formula (2):

[0107] The compound represented by formula (2) exists as a cis isomer and a trans isomer. 1 From R 4 Compared to compounds in which all stilbenes are hydrogen atoms, the stability of the cis isomer is low due to steric hindrance. Even when isomerized by light irradiation, the compound represented by formula (2) quickly reverts to the trans isomer at room temperature. Due to this property, the trans:cis ratio obtained during synthesis is 100:0 (Michael Oelgemoller et al., "Synthesis, structural characterization and photoisomerization of cyclic stilbenes", Tetrahedron, 2012, 68, 4048-4056.). Therefore, materials or devices containing the compound represented by formula (2) can stably exhibit their inherent properties without necessarily needing to be stored in a light-blocking environment.

[0108] The synthesis method of compound a is not particularly limited, and for example, a McMurry coupling reaction can be used. Compound a represented by formula (1) can be synthesized, for example, by the following method. First, compound b represented by formula (3) below and compound c represented by formula (4) below are prepared.

[0109] Next, compound b and compound c are subjected to a coupling reaction, thereby synthesizing compound a. The conditions for the coupling reaction can be appropriately adjusted depending on, for example, the types of substituents contained in compound b and compound c.

[0110] The compound b represented by formula (3) can be synthesized, for example, by the following method. First, a compound d, which is a tetralone derivative represented by formula (5) below, and a compound R 1 -X and R 2A halide represented by -X is prepared. X is a halogen atom. Examples of halogen atoms include Br and I.

[0111] Next, compound d and R 1 -X. As a result, compound e represented by the following formula (6) can be synthesized. The coupling reaction conditions are, for example, 1 This can be appropriately adjusted depending on the type of substituent contained in each of the -X's.

[0112] Next, compound e and R 2 -X. This allows the synthesis of compound b represented by formula (3). The coupling reaction conditions are, for example, 2 This can be appropriately adjusted depending on the type of substituent contained in each of the -X's.

[0113] Another example of an organic compound suitable for the recording medium of the present disclosure is a compound a′ represented by the following formula (7).

[0114] In formula (7), R 21 From R 30 R each independently represent a group containing at least one atom selected from the group consisting of H, B, C, N, O, F, Si, P, S, Cl, I, and Br. 21 From R 30 Examples of R in compound a represented by formula (1) include 1 From R 12 The following explanation about compound a can also be applied to compound a'.

[0115] Both the compound a represented by formula (1) and the compound a' represented by formula (7) are compounds that can undergo a change in their conformation in an excited state. By having such a property, the excited state absorption cross section σ of these compounds in the short wavelength region is ESA The "conformational change" typically involves a twisting around the carbon-carbon double bond.

[0116] Compound a represented by formula (1) has excellent nonlinear optical absorption properties for light having a wavelength in the short wavelength region. The second-order nonlinear absorption coefficient is expressed as the sum of the product of the one-photon absorption coefficient, the excited-state absorption cross section, and the excited-state lifetime and the two-photon absorption coefficient.

[0117] The two-photon absorption cross-section of compound a for light having a wavelength of 405 nm may be greater than 1 GM, 10 GM or more, 20 GM or more, 100 GM or more, 400 GM or more, or 600 GM or more. The upper limit of the two-photon absorption cross-section of compound a is not particularly limited, and may be, for example, 10,000 GM or 1,000 GM. The two-photon absorption cross-section can be measured, for example, by the Z-scan method described in J. Opt. Soc. Am. B, 2003, Vol. 20, p. 529. The Z-scan method is widely used as a method for measuring nonlinear optical constants. In the Z-scan method, a measurement sample is moved along the irradiation direction of a laser beam near the focal point where the beam is focused. At this time, changes in the amount of light transmitted through the measurement sample are recorded. In the Z-scan method, the power density of the incident light changes depending on the position of the measurement sample. Therefore, when a measurement sample exhibits nonlinear optical absorption, the amount of transmitted light is attenuated when the measurement sample is positioned near the focus of the laser beam. The two-photon absorption cross section can be calculated by fitting the change in the amount of transmitted light to a theoretical curve predicted from the intensity of the incident light, the thickness of the measurement sample, the concentration of compound a in the measurement sample, etc.

[0118] The molar absorption coefficient of compound a for light having a wavelength of 405 nm is, for example, 4000 mol -1 ・L・cm -1 less than 2000 mol -1 ・L・cm -1 It may be 1000 mol or less, -1 ・L・cm -1 It may be 500 mol or less, -1 ・L・cm -1 The lower limit of the molar extinction coefficient of the compound a is not particularly limited, and may be, for example, 90 mol-1 ・L・cm -1 The molar extinction coefficient can be measured, for example, by a method conforming to the provisions of Japanese Industrial Standards (JIS) K0115:2004. In measuring the molar extinction coefficient, a light source is used that irradiates light with a photon density that causes almost no two-photon absorption by compound a. Furthermore, in measuring the molar extinction coefficient, the concentration of compound a is adjusted to, for example, 1 mmol / L. The molar extinction coefficient can be used as an index of one-photon absorption.

[0119] When compound a undergoes two-photon absorption, compound a absorbs approximately twice the energy of light irradiated onto compound a. The wavelength of light having approximately twice the energy of light having a wavelength of 405 nm is, for example, 200 nm. When compound a is irradiated with light having a wavelength around 200 nm, one-photon absorption may occur in compound a. Furthermore, compound a may undergo one-photon absorption with light having a wavelength close to the wavelength range in which two-photon absorption occurs.

[0120] Compound a represented by formula (1) can be used, for example, as a component of a light-absorbing material. That is, in another aspect, the present disclosure provides a light-absorbing material containing compound a represented by formula (1). The light-absorbing material contains, for example, compound a as a main component. "Main component" means the component contained in the light-absorbing material in the largest amount by weight. The light-absorbing material, for example, consists essentially of compound a. "Consisting essentially of..." means excluding other components that alter the essential characteristics of the referenced material. However, the light-absorbing material may contain impurities in addition to compound a.

[0121] Compound a is used, for example, in devices that utilize light having a wavelength in the short wavelength range. As an example, compound a is used in devices that utilize light having a wavelength of 390 nm or more and 420 nm or less. Examples of such devices include recording media, modeling machines, and fluorescence microscopes. Examples of recording media include three-dimensional optical memories. A specific example of three-dimensional optical memories is a three-dimensional optical disk. Examples of modeling machines include optical modeling machines such as 3D printers. Examples of fluorescence microscopes include two-photon fluorescence microscopes. Light used in these devices has a high photon density, for example, near its focus. The power density near the focus of light used in the device is, for example, 0.1 W / cm. 2 Above 1.0 x 10 20 W / cm 2 The power density near the focal point of this light is 1.0 W / cm 2 or more, and may be 1.0 × 10 2 W / cm 2 or more, and may be 1.0 × 10 5 W / cm 2 As the light source of the device, for example, a femtosecond laser such as a titanium sapphire laser, or a pulse laser having a pulse width of picoseconds to nanoseconds such as a semiconductor laser can be used.

[0122] The recording medium includes, for example, a thin film called a recording layer. In the recording medium, information is recorded in the recording layer. As an example, the thin film serving as the recording layer contains compound a. That is, from another aspect thereof, the present disclosure provides a recording medium containing the above-mentioned compound a.

[0123] The recording layer may further contain a polymer compound that functions as a binder in addition to compound a. The recording medium may include a dielectric layer in addition to the recording layer. The recording medium may include, for example, a plurality of recording layers and a plurality of dielectric layers. In the recording medium, a plurality of recording layers and a plurality of dielectric layers may be alternately laminated.

[0124] Next, a method for recording information using the above-mentioned recording medium will be described. FIG. 1A is a flowchart illustrating the method for recording information using the above-mentioned recording medium. First, in step S11, a light source emitting light having a wavelength of 390 nm or more and 420 nm or less is prepared. As the light source, for example, a femtosecond laser such as a titanium sapphire laser, or a pulsed laser having a pulse width of picoseconds to nanoseconds such as a semiconductor laser can be used. Next, in step S12, the light from the light source is focused using a lens or the like and irradiated onto the recording layer of the recording medium. More specifically, the light from the light source is focused using a lens or the like and irradiated onto the recording region of the recording medium. The NA (numerical aperture) of the lens used for focusing is not particularly limited. As an example, a lens with an NA in the range of 0.8 to 0.9 can be used. The power density near the focus of this light is, for example, 0.1 W / cm. 2 Above 1.0 x 10 20 W / cm 2 The power density near the focal point of this light is 1.0 W / cm 2 or more, and may be 1.0 × 10 2 W / cm 2 or more, and may be 1.0 × 10 5 W / cm 2 In this specification, the recording area means a spot that exists in the recording layer and that can record information when irradiated with light.

[0125] In the recording area irradiated with the light, a physical or chemical change occurs. For example, heat is generated when compound a that has absorbed light returns from a transition state to a ground state. This heat alters the binder present in the recording area. This changes the optical properties of the recording area. For example, the intensity of light reflected in the recording area, the reflectance of light in the recording area, the absorbance of light in the recording area, and the refractive index of light in the recording area change. In the recording area irradiated with light, the intensity or wavelength of the fluorescent light emitted from the recording area may also change. This allows information to be recorded in the recording layer, specifically in the recording area (step S13).

[0126] Next, a method for reading information using the above-described recording medium will be described. FIG. 1B is a flowchart illustrating a method for reading information using the above-described recording medium. First, in step S21, light is irradiated onto the recording layer of the recording medium. More specifically, light is irradiated onto the recording area of ​​the recording medium. The light used in step S21 may be the same as or different from the light used to record information on the recording medium. Next, in step S22, the optical characteristics of the recording layer are measured. More specifically, the optical characteristics of the recording area are measured. In step S22, for example, the intensity of light reflected from the recording area may be measured as the optical characteristics of the recording area. In step S22, the optical characteristics of the recording area may include the reflectance of light in the recording area, the absorptance of light in the recording area, the refractive index of light in the recording area, the intensity of fluorescent light emitted from the recording area, and the wavelength of the fluorescent light. Next, in step S23, information is read from the recording layer, more specifically, the recording area.

[0127] In the information reading method, the recording area where information is recorded can be found by the following method. First, light is irradiated onto a specific area of ​​the recording medium. This light may be the same as or different from the light used to record information on the recording medium. Next, the optical characteristics of the area irradiated with light are measured. Examples of optical characteristics include the intensity of light reflected from the area, the reflectance of light in the area, the absorbance of light in the area, the refractive index of light in the area, the intensity of fluorescent light emitted from the area, and the wavelength of fluorescent light emitted from the area. Whether or not the area irradiated with light is a recording area is determined based on the measured optical characteristics. For example, if the intensity of light reflected from the area is equal to or less than a specific value, the area is determined to be a recording area. On the other hand, if the intensity of light reflected from the area is greater than a specific value, the area is determined to be not a recording area. Note that the method for determining whether or not the area irradiated with light is a recording area is not limited to the above method. For example, if the intensity of light reflected from the area is greater than a specific value, the area may be determined to be a recording area. Alternatively, if the intensity of light reflected from the area is equal to or less than a specific value, the area may be determined to be a non-recorded area. If it is determined to be a non-recorded area, the same operation is performed on other areas of the recording medium. This allows the search for a recorded area.

[0128] The information recording and reading methods using the above-mentioned recording medium can be performed, for example, by a known recording device that includes, for example, a light source that irradiates a recording area of ​​the recording medium with light, a measuring device that measures the optical characteristics of the recording area, and a controller that controls the light source and the measuring device.

[0129] The present disclosure will be described in more detail below with reference to examples. Note that the following examples are merely illustrative and the present disclosure is not limited to the following examples.

[0130] Example 1 Compound (2) represented by formula (2) was synthesized by the following procedure.

[0131] First, 50 g (0.342 mol) of α-tetralone (Tokyo Chemical Industry Co., Ltd.) and 500 mL of anhydrous tetrahydrofuran (Fujifilm Wako Pure Chemical Industries, Ltd.) were placed in a 2 L reactor under an argon atmosphere. After cooling the resulting solution to -20°C, 376 mL (0.376 mol) of sodium bis(trimethylsilyl)amide (1.0 mol / L tetrahydrofuran solution) (Fujifilm Wako Pure Chemical Industries, Ltd.) was added dropwise. After stirring at -20°C for 15 minutes, 23.4 mL (0.376 mol) of iodomethane (Fujifilm Wako Pure Chemical Industries, Ltd.) was slowly added dropwise. The mixture was allowed to warm to room temperature over 3 hours. The resulting suspension was added to 1.5 L of hydrochloric acid (0.5 mol / L) and separated into two phases. The aqueous phase was extracted with toluene. The organic phase was washed successively with sodium bicarbonate water, tap water, and 1 L of saturated saline solution, and then dried over anhydrous magnesium sulfate. The extract was then concentrated to give a light brown liquid, which was then purified by distillation to give a colorless liquid precursor of compound (2).

[0132] Next, 480 mL of anhydrous tetrahydrofuran (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was placed in a 1 L reactor under an argon atmosphere and cooled to -15 °C. Then, 11.0 mL (100 mmol) of titanium(IV) chloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was slowly added dropwise. After stirring at -15 °C for 30 minutes, 19.7 g (301 mmol) of zinc powder (manufactured by Aldrich) was added all at once and stirred at -15 °C for 30 minutes. To the resulting bluish-brown suspension, 12.0 g (66.9 mmol) of a precursor of compound (2) diluted with 120 mL of anhydrous tetrahydrofuran was added dropwise over 10 minutes. The cooling bath was removed, the bath temperature was raised to 75 °C, and the mixture was stirred at 75 °C for 30 minutes. The resulting dark brown suspension was added dropwise to 1 L of a 2.0 mol / L aqueous solution of potassium carbonate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and stirred at room temperature for 1 hour. The precipitated solid was filtered through Celite, and the filter bed was washed with 500 mL of ethyl acetate (Fujifilm Wako Pure Chemical Industries, Ltd.). Of the filtrate, the aqueous phase was extracted with 500 mL of a 1 / 1 mixture of heptane (Fujifilm Wako Pure Chemical Industries, Ltd.) and ethyl acetate. The mixture of the extract and organic phase was washed successively with tap water and saturated saline, and dried using anhydrous magnesium sulfate. The liquid obtained by the drying process was concentrated using a rotary evaporator. The resulting light brown liquid was purified by silica gel column chromatography to obtain a light yellow liquid. 12 mL of ethanol (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to the light yellow liquid and ultrasonically irradiated. The precipitated solid was filtered, and the filter bed was washed with methanol (Fujifilm Wako Pure Chemical Industries, Ltd.) and dried to synthesize compound (2), a colorless powder. Figure 2 shows the structure of compound (2). 1 1 is a graph showing the H-NMR spectrum of compound (2). 1 The H-NMR spectrum was as follows: 1H-NMR (400 MHz, CHLOROFORM-D) δ 7.22-7.07 (m, 8H), 2.85-2.79 (m, 4H), 1.77-1.74 (m, 4H), 1.06 (s, 6H), 0.64 (s, 6H).

[0133] [Example 2 and Comparative Examples 1 to 5] Compounds of Example 2 and Comparative Examples 1 to 5 were prepared. The compound of Example 2 is represented by the following formula (8). The compounds of Comparative Examples 1 to 5 are represented by the following formulas (9) to (13), respectively. Compounds of formulas (8), (10), (11), and (12) were obtained from Aldrich Chemical Company. Compounds of formulas (9) and (13) were obtained from Tokyo Chemical Industry Co., Ltd.

[0134] <Measurement of Two-Photon Absorption Cross Section> The two-photon absorption cross section of the compounds of Examples 1 and 2 and Comparative Examples 1 to 5 was measured for light having a wavelength of 405 nm. The two-photon absorption cross section was measured using the Z-scan method described in J. Opt. Soc. Am. B, 2003, Vol. 20, p. 529. A titanium sapphire pulsed laser was used as the light source for measuring the two-photon absorption cross section. Specifically, the second harmonic of the titanium sapphire pulsed laser was irradiated onto the sample. The laser pulse width was 80 fs. The laser repetition rate was 1 kHz. The average power of the laser was varied in the range of 0.01 mW to 0.08 mW. The light from the laser had a wavelength of 405 nm. Specifically, the light from the laser had a central wavelength of 403 nm to 405 nm. The full width at half maximum of the light from the laser was 4 nm.

[0135] <Measurement of Molar Absorption Coefficient> The molar absorption coefficients of the compounds of Examples 1 and 2 and Comparative Examples 1 to 5 were measured by a method conforming to the provisions of JIS K0115:2004. Specifically, a measurement sample was first prepared in which the compound concentration was adjusted to 500 mmol / L. The absorption spectrum of the measurement sample was measured. From the obtained spectrum, the absorbance at a wavelength of 405 nm was read. The molar absorption coefficient was calculated based on the concentration of the compound in the measurement sample and the optical path length of the cell used for the measurement.

[0136] The two-photon absorption cross section σ (GM) and molar absorption coefficient ε (mol -1 ・L・cm -1) are shown in Table 1. In Table 1, "SA" means that saturated absorption occurred during two-photon absorption measurement by the Z-scan method, and the cross section value could not be obtained.

[0137]

[0138] The molar absorption coefficients of the compounds of Examples 1 and 2 for light at 405 nm are 160 mol -1 ・L・cm -1 and 435 mol -1 ・L・cm -1 and 90 mol -1 ・L・cm -1 Over 1000 mol -1 ・L・cm -1 The molar absorption coefficients were within the following ranges: In contrast, the molar absorption coefficients of the compounds of Comparative Examples 1 and 2 were large, and the molar absorption coefficients of the compounds of Comparative Examples 4 and 5 were very small.

[0139] <Recording / Reproducing Characteristics> [Preparation of Thin Film Containing Dye] First, the following materials were mixed by stirring to obtain a uniformly mixed coating solution. The weight ratio of the resin to the solvent was fixed at 9 wt %. Although the solubility of the dye in the solvent varies depending on the dye, each dye was dissolved to its upper limit of solubility. Poly(9-vinylcarbazole) (manufactured by Aldrich Chemical Co.) was used as the resin. Chlorobenzene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the solvent. The composition ratios of the coating solutions for preparing thin films containing the compounds of Example 1, Example 2, and Comparative Examples 1 to 5 as dyes are shown in Table 2.

[0140]

[0141] Next, a glass substrate was prepared. The dimensions of the glass substrate were 26 mm in length, 38 mm in width, and 0.9 mm in thickness. The glass substrate was placed on a spin coater, and 400 μL of the coating solution prepared by the above method was dropped onto the glass substrate and rotated at 3000 rpm for 30 seconds. The glass substrate was then dried on a hot plate at 80° C. for 30 minutes to obtain a resin thin film containing any of the compounds of Example 1, Example 2, and Comparative Examples 1 to 5. Hereinafter, these resin thin films will be referred to as the thin film of Example 1, the thin film of Example 2, and the thin films of Comparative Examples 1 to 5.

[0142] [Reproducing operation before recording] Pulsed light with a central wavelength of 405 nm, a peak power of 3 mW, a pulse width of 200 nanoseconds, and a repetition rate of 100 Hz was irradiated through a lens with an NA of 0.85 while being focused on a thin resin film on a glass substrate. The reflected light signal intensity at this time was obtained as the initial reflected light signal intensity. If the reflected light signal intensity after the recording operation changed from the initial reflected light signal intensity, it was determined that a recording mark had been formed.

[0143] [Recording Operation] Recording was performed by irradiating one pulse of recording light having a central wavelength of 405 nm and a peak power of 100 mW through a lens with an NA of 0.85. The pulse width was adjusted between 10 nanoseconds and 5 milliseconds.

[0144] [Reproducing Operation] Light with a central wavelength of 405 nm, a peak power of 3 mW, and a pulse width of 200 nanoseconds was irradiated onto the recording portion of the resin thin film at a repetition rate of 100 Hz through a lens with an NA of 0.85, and the reflected light signal intensity was measured. The rate of change in the reflected light signal intensity after the recording operation relative to the reflected light signal intensity before the recording operation was calculated.

[0145] [Evaluation of recording and reproducing characteristics] Since the molar absorption coefficients and solubilities of the dyes are different from one another, the prepared resin thin films have different light absorption characteristics. In order to make an equal comparison of the recording and reproducing characteristics, the incident light intensity during recording was converted into the energy (J / cm) of light with a wavelength of 405 nm that is absorbed by a resin thin film with a thickness of 1 cm. The light energy (J / cm) absorbed by a resin thin film with a thickness of 1 cm is calculated by multiplying the intensity (W) of the irradiated light by the recording time (seconds) and the absorption coefficient (cm) of the resin thin film. -1The absorption coefficient of the resin thin film was calculated by multiplying the dye concentration (mol / L) in the thin film by the molar absorption coefficient of the dye (mol -1 ・L・cm -1 3A to 3G are graphs plotting the rate of change in reflected light signal intensity versus the light energy absorbed by a 1 cm thick resin thin film.

[0146] 3A to 3G are graphs showing the recording and reproduction characteristics of the resin thin films of Example 1, Example 2, and Comparative Examples 1 to 5, respectively. In FIGS. 3A to 3G, the horizontal axis represents the absorbed light energy changed depending on the irradiation time (pulse width) of the laser light. The vertical axis represents the rate of change in the reflected light signal intensity after the recording operation relative to the reflected light signal intensity before the recording operation. In FIGS. 3A to 3G, a small change in the reflected light signal intensity means that the resin film was hardly altered by the irradiation of the laser light. A large change in the reflected light signal intensity means that the resin thin film was altered by the irradiation of the laser light, resulting in the formation of a recording mark.

[0147] 3A and 3B, in Examples 1 and 2, when the laser light irradiation time (pulse width) was increased to increase the absorbed light energy, the rate of change in the reflected light signal intensity increased sharply at around 3 mJ / cm. The thin films of Examples 1 and 2 had threshold characteristics in which the reflected light signal intensity hardly changed when irradiated with light of low intensity, but the reflected light signal intensity changed significantly when irradiated with light of high intensity. In other words, the thin films of Examples 1 and 2 hardly changed even after repeated reproduction operations, and had high durability and reliability. The thin films of Examples 1 and 2 were not easily altered by the weak light used during reproduction, so it was possible to avoid the formation of recording marks even when no recording operations were performed.

[0148] Furthermore, in Examples 1 and 2, the rate of change in the reflected light signal intensity was less likely to saturate and increased to 50% or more. The higher the rate of change in the reflected light signal intensity, the greater the difference between the reflected light signal intensity of the recording mark and the reflected light signal intensity around the recording mark. In other words, the S / N ratio, which is the ratio of signal to noise, is improved, making it easier to read the recording mark.

[0149] The threshold characteristics of the thin films of Examples 1 and 2 show that nonlinear optical absorption, specifically excited state absorption, occurs significantly in the range where the absorbed optical energy exceeds 3 mJ / cm.

[0150] Compound (2) of Example 1 has a structure in which tetralin rings are connected by a carbon-carbon double bond. It is presumed that this structure contributed to the favorable recording and reproducing characteristics. Compound (8) of Example 2 has a structure in which two benzene rings are connected to a hexatriene skeleton. Both compounds (2) and (8) are compounds capable of changing their conformation in an excited state. It is presumed that this property contributed to the favorable recording and reproducing characteristics. Compound (2) can also be considered a compound having a structure in which the benzene ring and double-bond carbon of trans-stilbene are linked by an alkyl chain. It is presumed that this structure improves the isomerization rate of the compound, thereby improving the recording and reproducing characteristics.

[0151] On the other hand, as can be seen from Figures 3C and 3G, in Comparative Examples 1 to 5, the rate of change of the reflected light signal intensity increased linearly with the absorbed light energy and saturated at a low rate of change. In addition, in Comparative Examples 1 to 3, even when the absorbed light energy was 1 mJ / cm or less, changes in the reflected light signal intensity of about 8% to 30% occurred. This means that the resin thin film is easily altered by repeated reproduction operations. In other words, the alteration of the thin film by weak light during reproduction means that recording marks are formed even when no recording operation is performed.

[0152] In Comparative Example 4, the rate of change of the reflected light signal intensity changed linearly with the absorbed light energy, and the rate of change of the reflected light signal intensity with respect to the absorbed light energy was small, which means that the S / N ratio was small and it was difficult to read the presence or absence of a recording mark.

[0153] In the case of the thin film of Comparative Example 5, no change occurred in the reflected light signal intensity within the range of light intensity used in the experiment.

[0154] <Measurement of Transient Absorption Spectrum> The transient absorption spectrum of the dye (compound) of Example 1 was measured. A sample was dissolved in chloroform to prepare a solution, and the solution was placed in a quartz cell with an optical path length of 1 cm. The concentration of the solution was adjusted so that the absorbance at a wavelength of 355 nm and an optical path length of 1 cm was in the range of 0.1 to 0.2. The quartz cell was sealed and degassed by argon bubbling for 30 minutes. A UNISOKU TSP-2000 system was used to measure the transient absorption spectrum. A Surelite II excitation light was used, and light with a wavelength of 355 nm, a repetition rate of 10 Hz, an average power of 8 mW, and a beam diameter of 6 mm was irradiated onto the sample with an optical path length of 1 cm. The results are shown in Figure 4A.

[0155] FIG. 4A shows the transient absorption spectrum of a solution containing the compound of Example 1. The vertical axis represents the absorbance difference ΔAbs between the excited state and the ground state. The horizontal axis represents the wavelength of light. The "0 nanosecond" graph shows the absorbance change when the delay time between irradiation of the excitation light (pump light) and irradiation of the probe light is 0 nanoseconds, and is ΔAbs measured almost simultaneously with irradiation of the excitation light. ΔAbs at 0 nanoseconds is the value when the delay time is closest to zero in the device used and can be considered to be the value closest to ΔAbs at the moment of excitation at that wavelength. ΔAbs varies depending on the pulse width of the excitation light and the time resolution of the device. As can be seen from FIG. 4A, the transient absorption spectrum of the compound of Example 1 had a large peak at wavelengths from 400 nm to 405 nm, showing large excited-state absorption in the short-wavelength region. The absorbance change ΔAbs at wavelengths from 400 nm to 405 nm was a positive value. Furthermore, the graph in Figure 4A indicates that the lifetime of the observed excited species is approximately 10 nanoseconds. After measuring the transient absorption spectrum, the quartz cell was opened, the sample solution was exposed to air, and the measurement was repeated, but a similar transient absorption spectrum was obtained. This indicates that the observed transient absorption was not affected by oxygen in the air, and was therefore light absorption from the singlet excited state.

[0156] 4B shows the transient absorption spectrum of the solution of the compound of Example 1, where the vertical axis is converted from the absorbance change to the excited-state molar absorption coefficient. The excited-state molar absorption coefficient is calculated as follows.

[0157] As shown in the following formula (viii), the molar extinction coefficient ε1 (mol -1 ・L・cm -1 ) is obtained by dividing the absorbance change ΔAbs obtained by transient absorption spectroscopy by the product of the concentration c (mol / L) of molecules that have transitioned to the excited state and the optical path length z (cm).

[0158] As shown in the following formula (ix), the concentration c of molecules that have transitioned to an excited state is expressed as the number density N (number / cm) of molecules that have transitioned to an excited state by absorbing pump light. 3 ) is the Avogadro constant N A (mol -1 ) and convert the units.

[0159] As shown in the following formula (x), the number density N1 of molecules that have transitioned to an excited state is the number of photons absorbed by the sample per unit time n (photon / (s cm 2 )) and the laser pulse width τ (s) divided by the optical path length z (cm).

[0160] The number of photons, n, is the incident photon flux Φ (photon / (s cm 2 The incident photon flux Φ is expressed by the following equation (xi) using the laser power W (W) and the irradiation area S (cm 2 ) and the optical power density (W / cm 2 ) by the photon energy of the irradiated light (J / photon).

[0161] As can be seen from FIG. 4B, the molar absorption coefficient of the compound of Example 1 in the excited state (singlet excited state) at wavelengths of 400 nm or more and 405 nm or less is 2000 to 3000 mol -1 ・L・cm -1 was in the range of

[0162] Transient absorption spectrum measurement was carried out on a thin film prepared by the method described in [Preparation of a thin film containing a dye] for the compound of Example 1. The thin film was placed upright in a quartz cell with an optical path length of 1 cm, the inside of the cell was replaced with argon, and the cell was sealed. Measurement was carried out in the same manner as for the measurement of the solution.

[0163] Figure 5A shows the transient absorption spectrum of a thin film containing the compound of Example 1. Figure 5B shows the transient absorption spectrum of a thin film containing the compound of Example 1 when the vertical axis is converted from the absorbance change to the excited-state molar absorption coefficient. The vertical and horizontal axes are the same as those in Figures 4A and 4B. As can be seen from Figure 5A, the compound of Example 1 exhibited a large transient absorption peak near 400 nm and large excited-state absorption in the short-wavelength region, even in a polymer thin film with different polarity and dispersion state than in a chloroform solution. The lifetime of the observed excited species was several nanoseconds to approximately 10 nanoseconds, which was no different from that in a chloroform solution.

[0164] Next, solutions of the compounds of Comparative Examples 1 and 2 were prepared in the same manner as in Example 1, and transient absorption spectra were measured. The results are shown in Figures 6A and 6B.

[0165] Figure 6A shows the transient absorption spectrum of a solution containing the compound of Comparative Example 1. The vertical and horizontal axes are the same as those in Figure 4A. As shown in Figure 6A, the compound of Comparative Example 1 bleached in the wavelength region below 420 nm, making it impossible to calculate the molar extinction coefficient of the excited state. Bleaching is a phenomenon in which the number of dyes in the ground state decreases significantly when one-photon absorption at the observed wavelength is large, resulting in a negative absorbance change. Bleaching is also sometimes called bleaching. However, in the wavelength region from 430 nm to 500 nm, a tail of the transient absorption peak increasing toward shorter wavelengths was observed. This suggests that the compound of Comparative Example 1 exhibits excited-state absorption in the short-wavelength region around 400 nm. However, the excited-state lifetime was very long, approximately 1 millisecond. After the measurement, the quartz cell was opened, the sample solution was exposed to air, and the measurement was repeated; the lifetime was shortened. Based on these facts, the transient absorption observed in the graph of Figure 6A is likely due to the influence of oxygen in the atmosphere and is therefore due to light absorption from the triplet excited state. In this way, by confirming that the lifetime and absorption intensity differ depending on the presence or absence of oxygen in the surrounding environment, it is possible to determine whether the light absorption is from a singlet excited state or a triplet excited state.

[0166] Figure 6B shows the transient absorption spectrum of a solution containing the compound of Comparative Example 2. The vertical and horizontal axes are the same as those in Figure 4A. As shown in Figure 6B, like the compound of Comparative Example 1, the compound of Comparative Example 2 also experienced bleaching in the wavelength region of 430 nm or shorter, making it impossible to calculate the molar extinction coefficient of the excited state. On the other hand, unlike the compound of Comparative Example 1, the compound of Comparative Example 2 did not exhibit a tail of the transient absorption peak in the wavelength region longer than 430 nm, where the bleaching effect is minimal. This suggests that the dye of Comparative Example 2 exhibits almost no excited-state absorption in the short-wavelength region around 400 nm. The lifetime of the barely detectable excited state was also very long, on the order of several milliseconds. After the measurement, the quartz cell was opened, the sample solution was exposed to air, and the measurement was repeated; the lifetime was shortened. From these facts, it is believed that the compound of Comparative Example 2 rapidly transitions to the triplet excited state after excitation.

[0167] The transient absorption spectrum of the compound of Example 2 is described in Nicole Marie Dickson, "Exploration of the Excited States of Organic Molecules and Metal Complexes Using Ultrafast Laser Spectroscopy," Dissertation, Ohio State University (2011). The transient absorption spectrum of the compound of Example 2 has a peak at approximately 440 nm. From the transient absorption spectrum described in the literature, it can be seen that significant excited state absorption also occurs near 400 nm, i.e., the absorbance change ΔAbs is positive.

[0168] The transient absorption spectrum of trans,trans-1,4-distyrylbenzene (unsubstituted), a derivative of the compound of Comparative Example 3, is described in Gabriella Ginocchietti et al., "Photobehavior of thio-analogues of stilbene and 1,4-distyrylbenzene studied by time-resolved absorption techniques," Chem. Phys. 2008, 352, pp. 28-34. From the transient absorption spectrum described in the document, it can be seen that the light absorption from the singlet excited state has a peak at approximately 750 nm, that no excited state absorption occurs around 400 nm, and that the absorbance change ΔAbs is negative.

[0169] The transient absorption spectrum of the compound of Comparative Example 5 is described in Paolo Foggi et al., "Transient absorption and vibrational relaxation dynamics of the lowest excited singlet state of pyrene in solution", J. Phys. Chem. 1995, 99, pp. 7439-7445. The transient absorption spectrum described in the literature has a peak at about 460 nm. From the transient absorption spectrum described in the literature, it can be seen that slight excited state absorption occurs around 400 nm.

[0170] From the above, it has become clear that the light absorption characteristics required for an organic compound as a dye are such that it has sufficient recording sensitivity and nonlinearity (threshold characteristics).

[0171] The compounds of the present disclosure can be used as nonlinear optical absorption materials in applications such as recording layers for three-dimensional optical memories and photocurable resin compositions for stereolithography. The compounds of the present disclosure have optical absorption properties that exhibit high nonlinearity for light having wavelengths in the short wavelength range. Therefore, the compounds of the present disclosure can achieve extremely high spatial resolution in applications such as three-dimensional optical memories and modeling machines. Compared to conventional compounds, the compounds of the present disclosure can exhibit nonlinear optical absorption more predominantly than single-photon absorption, even when irradiated with laser light of low optical intensity.

Claims

1. A recording layer including an organic compound having a nonlinear optical absorption characteristic, The molar absorption coefficient of the organic compound for light having a wavelength of 400 nm or more and 405 nm or less is 90 mol -1 ・L・cm -1 That's all. In the transient absorption spectrum of the organic compound, an absorbance change ΔAbs at a wavelength of 400 nm or more and 405 nm or less is a positive value, A recording medium in which the organic compound does not undergo an absorption saturation phenomenon when irradiated with light having a wavelength of 405 nm at an average power of 0.01 mW or more and 0.08 mW or less.

2. The molar absorption coefficient of the organic compound is 1000 mol -1 ・L・cm -1 2. The recording medium according to claim 1, wherein:

3. The molar absorption coefficient of the organic compound is 100 mol -1 ・L・cm -1 More than 500 mol -1 ・L・cm -1 2. The recording medium according to claim 1, wherein:

4. 2. The recording medium according to claim 1, wherein the organic compound has an excited state lifetime of 1 millisecond or less.

5. 2. The recording medium according to claim 1, wherein the organic compound is a compound capable of undergoing a change in its three-dimensional structure in an excited state.

6. providing a light source that emits light having a wavelength of 390 nm or more and 420 nm or less; The light from the light source is condensed and irradiated onto a recording layer of the recording medium according to any one of claims 1 to 5. (a) how information is recorded;

7. A method for reading information recorded by the recording method according to claim 6, comprising the steps of: The reading method includes: measuring optical characteristics of the recording layer by irradiating the recording layer of the recording medium with light; Reading information from the recording layer; A method of reading information, comprising:

8. The method of claim 7 , wherein the optical characteristic is the intensity of light reflected by the recording layer.

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