Two-photon absorption material

Abstract

To provide a material with large two-photon absorption cross-sectional area.SOLUTION: A two-photon absorption material is provided, containing a molecule having a thiolate ligand and a metal thiolate complex bound to a surface of a small atomic cluster nucleus. An element constituting the small atomic cluster nucleus and a metal element constituting the metal thiolate complex are Au, and the small atomic cluster nucleus of the molecule of the metal thiolate complex has an atomic number in a range of 10-56.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an organic-inorganic hybrid two-photon absorption material having a large two-photon absorption cross section. [Background technology]

[0002] Two-photon absorption is an electronic excitation process in which a molecule or cluster simultaneously absorbs two photons and transitions to an excited state. Because the probability of two-photon absorption is proportional to the square of the irradiated light intensity (nonlinear optics), it is possible to induce photoexcitation more spatially selectively than single-photon absorption, which occurs proportional to the first power of the irradiated light intensity. In other words, two-photon absorption can selectively induce photoexcitation only in a small space with high light intensity. Furthermore, the transition to an excited state via two-photon absorption can occur with photon energy half the transition energy between the initial and final states. Therefore, while single-photon absorption requires photon energy in the ultraviolet or visible range, two-photon absorption can induce such transitions using near-infrared light (700–1300 nm), which is highly biotransparent. Because of this characteristic behavior, materials that undergo two-photon absorption (two-photon absorbing materials) are being considered for application in various technical fields, such as luminescence imaging (multiphoton excitation laser microscopes), photodynamic therapy, high-density optical storage, and three-dimensional microfabrication (see, for example, Non-Patent Documents 1 and 2). On the other hand, many inorganic two-photon absorption materials have been discovered to date, but inorganic materials have less freedom in molecular design than organic compounds, making it difficult to optimize their two-photon absorption properties and other properties depending on the application. Therefore, development of organic compounds that exhibit two-photon absorption properties is underway, and for example, Non-Patent Document 3 reports that two-photon absorption has been confirmed in the molecules of the following organic compound (Hex represents a hexyl group).

[0003] [ka] [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] K. Kamata, Near-infrared two-photon absorbing materials and their applications. Two-photon absorption of organic molecules, ITE Technical Report 40,25 (2016). [Non-patent document 2] F. Terenziani et al., Adv. Mater. 20, 4641 (2008). [Non-patent document 3] M. Pawlicki et al., Angew. Chem. Int. Ed. 48, 3244 (2009). Summary of the Invention [Problem to be solved by the invention]

[0005] The strength of two-photon absorption per molecule of a two-photon absorbing material is expressed as the two-photon absorption cross section. The two-photon absorption cross section is named after Goppert-Mayer, the discoverer of two-photon absorption, and is expressed as GM (1GM = 10 -50 ·cm 4 ·s·photon -1 molecule -1 ) and, for the same molecular size, the larger this value, the higher the two-photon absorption efficiency. However, the two-photon absorption cross-sections of organic compounds proposed to date are not sufficiently high and cannot be used practically for the applications mentioned above. Furthermore, guidelines for molecular design are limited to expanding the π-conjugated system of the organic compound itself, improving planarity, and promoting intramolecular charge transfer (introduction of donor or acceptor groups), making it difficult to significantly improve the two-photon absorption cross-section. Therefore, in order to solve such problems of the conventional technology, the present inventors have carried out extensive research with the aim of providing a material with a large two-photon absorption cross section. [Means for solving the problem]

[0006] As a result of intensive research to solve the above problems, the present inventors have found that organic-inorganic hybrid molecules in which thiolate ligands or metal thiolate complexes are bound to the surface of a small-atom cluster nucleus, in which the constituent elements of the small-atom cluster nucleus and the metal element of the metal thiolate complex are Au, and the small-atom cluster nucleus has an atomic number of 10 to 56, have a large two-photon absorption cross section and are useful as two-photon absorption materials. The present invention has been proposed based on these findings and specifically has the following configuration.

[0007] [1] A two-photon absorption material comprising a molecule in which a thiolate ligand is bound to the surface of a minority-atom cluster nucleus, wherein the only element constituting the minority-atom cluster nucleus is Au, and the number of atoms in the minority-atom cluster nucleus of the molecule is 10 to 56. [2] The two-photon absorption material according to [1], wherein the number of atoms in the small-atom cluster core of the molecule is 10, 16, 20, 35, 40, 52 or 56. [3] The two-photon absorption material according to [1] or [2], wherein the thiolate ligand has a structure in which a coordinated sulfur atom is bonded to a carbon atom constituting a ring skeleton of a hydrocarbon ring or a heterocycle. [4] The two-photon absorption material according to [3], wherein the hydrocarbon ring and the heterocycle have a π-conjugated system. [5] The two-photon absorption material according to [4], wherein the hydrocarbon ring is an aromatic ring and the heterocycle is an aromatic heterocycle. [6] The two-photon absorption material according to [5], wherein the aromatic ring and the aromatic heterocycle have a fused ring structure consisting of 2 to 4 constituent rings. [7] The two-photon absorption material according to any one of [1] to [6], wherein the thiolate ligand has a structure in which a coordinating sulfur atom is bonded to a naphthalene ring. [8] The two-photon absorption material according to [7], wherein the thiolate ligand is a substituted or unsubstituted 2-naphthalene thiolate. [9] The two-photon absorption material according to [3], wherein the thiolate ligand is cyclopentanethiol.

[10] The two-photon absorption material according to any one of [1] to [9], wherein in the molecule, the thiolate ligand is coordinated to Au that does not constitute the minority atom cluster nucleus to form a metal thiolate complex.

[11] The two-photon absorption material according to

[10] , wherein the metal thiolate complex has a structure represented by the following general formula (1a): General formula (1a) *-SR-(Au-SR) n1 -* [In general formula (1a), SR represents a thiolate ligand, S represents a coordinating sulfur atom, and R represents an atomic group. n1 represents an integer of 1 or more. * represents the bonding position to the minority atom cluster nucleus. The thiolate ligand in general formula (1a) bonds to Au via the coordinating sulfur atom. Multiple SRs may be the same or different.]

[12] The two-photon absorption material according to any one of [1] to

[11] , wherein the molecule undergoes two-photon absorption in a wavelength range of 700 to 1300 nm. [Effects of the Invention]

[0008] The molecules used in the present invention have a large two-photon absorption cross section. Furthermore, the molecular size, two-photon absorption characteristics, and physical properties of the molecules can be effectively controlled by molecular design of the organic thiolate ligand. Therefore, the molecules used in the present invention are highly useful as two-photon absorption materials. [Brief explanation of the drawings]

[0009] [Figure 1] This figure shows the structure of a molecule [Au20(NP3Au2)8] in which a complex of Au and 2-naphthalenethiolate NP is bound to a gold cluster core Au20. [Figure 2] This figure shows the structure of a molecule [Au20(Cy3Au2)8] in which a complex of Au and cyclopentanethiolate Cy is bound to a gold cluster core Au20. [Figure 3] 1 is a graph showing the incident light wavelength dependence of the two-photon absorption cross section of Au20(NP3Au2)8 and Au20(Cy3Au2)8. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in detail below. The following description of the constituent elements may be based on representative embodiments or specific examples, but the present invention is not limited to such embodiments. In this specification, a numerical range expressed using "to" means a range including the numerical values ​​before and after "to" as the lower and upper limits. In addition, the isotopes of hydrogen atoms present in the molecules of the compounds used in the present invention are not particularly limited, and for example, when all hydrogen atoms in the molecule are 1 H, or part or all of 2 It may also be H (deuterium D).

[0011] <Two-photon absorption materials> The two-photon absorption material of the present invention is a two-photon absorption material comprising a molecule in which a thiolate ligand is bound to the surface of a minority-atom cluster nucleus, the element constituting the minority-atom cluster nucleus is Au, and the number of atoms in the minority-atom cluster nucleus of the molecule is 10 to 56. In a preferred embodiment of the present invention, the molecule used in the present invention comprises a thiolate complex in which a thiolate ligand bound to a minority atom cluster nucleus is coordinated to a second metal atom (hereinafter referred to as a "second metal atom") other than the minority atom cluster nucleus. In other words, the molecule used in the present invention is preferably a molecule in which a metal thiolate complex in which a thiolate ligand is coordinated to a second metal atom is bound to the surface of the minority atom cluster nucleus. Here, the metal thiolate complex may have further thiolate ligands in addition to the thiolate ligand bound to the minority atom cluster nucleus, and these thiolate ligands may be coordinated to the second metal atom. Specifically, the "two-photon absorption material" of the present invention is a material in which molecules contained in the material simultaneously absorb two low-energy photons in an initial state (two-photon absorption), thereby transitioning to a final state corresponding to the sum of the energies of the two photons and becoming excited. The two-photon absorption material of the present invention preferably has a maximum two-photon absorption cross-section in the near-infrared range of 1500 GM or more, more preferably 2000 GM or more, and may be, for example, 5000 GM or more. The wavelength of light that undergoes two-photon absorption by the two-photon absorption material of the present invention (two-photon absorption wavelength) is not particularly limited, but two-photon absorption preferably occurs in the near-infrared range of 700 to 1300 nm, more preferably in the range of 800 to 1000 nm, and even more preferably in the range of 800 to 900 nm. Because near-infrared light is permeable through living organisms, two-photon absorption materials with two-photon absorption wavelengths in the near-infrared range can be excited by two-photon absorption of near-infrared light from outside the body when placed in the body. Therefore, such two-photon absorption materials can be effectively used in applications in which they are placed in a living body, such as photosensitizers used in photodynamic therapy. Furthermore, the two-photon absorption material of the present invention is preferably one in which molecules excited by two-photon absorption subsequently emit light and are deactivated. The wavelength of the emitted light is shorter than the two-photon absorption wavelength. For example, when the two-photon absorption wavelength is in the near-infrared region, the wavelength is usually 750 to 1000 nm, and may be 1000 to 2500 nm. The molecular small-atom cluster nucleus, thiolate ligand, and metal thiolate complex used in the present invention will be described below.

[0012] (small number of atom cluster nuclei) The "small number of atom cluster nucleus" of a molecule used in the present invention means an assembly consisting of a small number of atoms. An assembly of atoms is formed by the accumulation of multiple atoms to form a three-dimensional shape as a whole. In the following explanation, the three-dimensional shape formed by an assembly of atoms, specifically the three-dimensional shape formed by connecting the centers of atoms, may be referred to as a "cluster shape." Furthermore, in this specification, the small number of atom cluster nucleus is referred to as the cluster shape of its atoms M a Using the number m of a m For example, M a20 is 20 atoms M a It means that it is a small number of atomic cluster nuclei consisting of Au 20 means that the gold cluster nucleus is composed of 20 gold atoms.

[0013] The element constituting the minority-atom cluster nucleus is Au. Because Au has high biocompatibility, it can be suitably used as a constituent element of the minority-atom cluster nucleus in two-photon absorbing materials, particularly for applications applied to living organisms (e.g., photosensitizers for photodynamic therapy).

[0014] The number of atoms constituting the minority-atom cluster nucleus is 10 to 56, preferably 10, 16, 20, 35, 40, 52, or 56. A minority-atom cluster nucleus having 10, 20, 35, or 56 atoms forms a regular tetrahedron cluster shape, while a minority-atom cluster nucleus having 16, 40, or 52 atoms forms a truncated tetrahedron cluster shape. When Au takes on a regular tetrahedron or truncated tetrahedron cluster shape, it is believed to have a stable electron configuration, resulting in a stable molecule that is difficult to decompose. Furthermore, by limiting the number of atoms in the minority-atom cluster nucleus to 10 to 56, the molecular size can be kept small, making it possible to realize a two-photon absorption material with a large two-photon absorption cross-section and strong two-photon absorption intensity. The minority-atom cluster nucleus may be a regular tetrahedron or a truncated tetrahedron, or may have a structure in which some atoms have been removed from an atomic assembly in the shape of a regular tetrahedron or a truncated tetrahedron, a structure in which at least one atom has been bonded to an atomic assembly in the shape of a regular tetrahedron or a truncated tetrahedron, or a structure in which some atoms have been removed from an atomic assembly in the shape of a regular tetrahedron or a truncated tetrahedron, and at least one further atom has been bonded to the structure.

[0015] (thiolate ligand) The "thiolate ligand" molecule used in the present invention refers to an organic ligand with a sulfur atom as the coordinating atom. In other words, a thiolate ligand is an organic ligand that has a terminal sulfur atom and that bonds to an atom constituting the surface of a small-atom cluster nucleus or a second metal atom of a metal thiolate complex via the terminal sulfur atom. In this specification, the sulfur atom that serves as the coordinating atom is sometimes referred to as a "coordinating sulfur atom." The thiolate ligand forms a strong bond with Au constituting the small-atom cluster nucleus, effectively protecting and stabilizing the small-atom cluster nucleus and transferring energy to and from the small-atom cluster nucleus, thereby absorbing and emitting light.

[0016] The number of coordinating sulfur atoms in the thiolate ligand may be 1 or 2 or more. The number of coordinating sulfur atoms in the thiolate ligand is preferably 1 to 8, more preferably 1 to 5, and even more preferably 1 to 2. The portion of the thiolate ligand other than the coordinating sulfur atoms is an atomic group, and may be composed only of atoms selected from the group consisting of carbon atoms, hydrogen atoms, nitrogen atoms, oxygen atoms, sulfur atoms, boron atoms, halogen atoms, and phosphorus atoms, for example.

[0017] The thiolate ligand of the molecule used in the present invention preferably has a structure in which a coordinating sulfur atom is bonded to a carbon atom constituting the ring skeleton of a hydrocarbon ring or heterocycle. The bond between the carbon atom constituting the ring skeleton and the coordinating sulfur atom may be a single bond or a bond via a linking group. Examples of the linking group include substituted or unsubstituted alkylene groups. The number of carbon atoms in the alkylene group can be selected, for example, from the range of 1 to 6. The substituent of the alkylene group can be selected, for example, from the following Substituent Group A.

[0018] The hydrocarbon ring constituting the thiolate ligand may be an aromatic ring or an alicyclic ring, and may be a monocyclic ring or have a fused ring structure. Alicyclic rings and aromatic rings having a π-conjugated system are preferred, aromatic rings are more preferred, and aromatic rings having a fused ring structure are even more preferred. Molecules containing a π-conjugated system in the ring structure of the thiolate ligand tend to emit light after two-photon absorption. In particular, molecules containing an aromatic ring in the thiolate ligand can effectively stabilize the minority atom cluster nucleus due to the resonance effect of the aromatic ring, and double resonance may occur, dramatically increasing the two-photon absorption cross-section. The number of carbon atoms in the hydrocarbon ring is, for example, 6 to 30, and may be 6 to 22, 6 to 16, 6 to 14, or 6 to 10. Furthermore, when the hydrocarbon ring has a fused ring structure, the number of fused rings is, for example, 2 to 6, preferably 2 to 4, and more preferably 2 or 3. In particular, when the hydrocarbon ring is an aromatic ring having a fused ring structure, the number of constituent rings is preferably within the range of 2 to 4. This prevents excessive light absorption due to the expansion of the π-conjugated system. Specific examples of hydrocarbon rings include monocyclic hydrocarbon rings consisting of a benzene ring, a cyclobutane ring, a cyclobutene ring, a cyclobutadiene ring, a cyclopentane ring, a cyclopentene ring, a cyclopentadiene ring, a cyclohexane ring, a cyclohexene ring, a cyclohexadiene ring, a cycloheptane ring, a cycloheptene ring, a cycloheptadiene ring, or a cycloheptatriene ring, as well as fused rings formed by condensing two or more of these hydrocarbon rings. A preferred example of a fused ring is a fused ring formed by condensing two to four benzene rings. Specific examples of fused rings include a naphthalene ring, an azulene ring, a fluorene ring, a phenalene ring, a phenanthrene ring, an anthracene ring, a triphenylene ring, a pyrene ring, and a tetracene ring, with a naphthalene ring being particularly preferred. Examples of thiolates having a naphthalene ring include substituted or unsubstituted 1-naphthalene thiolate and substituted or unsubstituted 2-naphthalene thiolate, with substituted or unsubstituted 2-naphthalene thiolate being preferred.

[0019] The heterocycle constituting the thiolate ligand may be an aromatic heterocycle or an alicyclic heterocycle, and may be a monocycle or a fused ring structure. Examples of heteroatoms in the heterocycle include nitrogen, oxygen, sulfur, and phosphorus atoms. The heterocycle may contain one or more heteroatoms. Alicyclic heterocycles and aromatic heterocycles having a π-conjugated system are preferred as heterocycles, with aromatic heterocycles being more preferred. Molecules containing a π-conjugated system in the ring structure of the thiolate ligand tend to emit light after two-photon absorption. In particular, molecules containing an aromatic heterocycle in the thiolate ligand can effectively stabilize the minority atom cluster nucleus due to the resonance effect of the aromatic heterocycle, and double resonance may occur, dramatically increasing the two-photon absorption cross-section. Here, the electrons forming the π-conjugated system of the heterocycle may be π electrons forming a π bond or lone electron pairs of heteroatoms. The number of atoms constituting the ring skeleton of the heterocycle is, for example, 4 to 30, and may be, for example, 5 to 20 or 5 to 14, or may be, for example, 5 to 10. Furthermore, when the heterocycle has a fused ring structure, the number of fused rings is, for example, 2 to 6, preferably 2 to 4, and more preferably 2 or 3. In particular, when the heterocycle is an aromatic heterocycle having a fused ring structure, the number of rings is preferably within the range of 2 to 4. This can prevent excessive light absorption due to the expansion of the π-conjugated system. Specific examples of the heterocycle include a monocyclic heterocycle consisting of a pyridine ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, a triazine ring, a pyran ring, a pyrrole ring, a furan ring, a thiophene ring, or a phosphole ring; a fused ring formed by condensing two or more of these heterocycles; and a fused ring formed by condensing these heterocycles with a hydrocarbon ring. Specific examples of the fused ring include a carbazole ring, a dibenzofuran ring, a dibenzothiophene ring, a benzo[b]phosphindole, a xanthene ring, an acridine ring, and a thienothiophene ring.

[0020] The hydrogen atoms of the hydrocarbon ring and heterocyclic ring constituting the thiolate ligand may be substituted with a substituent, which can be selected from, for example, the following Substituent Group A.

[0021] The thiolate ligand may have only one ring structure selected from the hydrocarbon ring and heterocyclic ring as described above, or may have two or more ring structures. The number of ring structures contained in the thiolate ligand is, for example, 1 to 3, e.g., 1 or 2. When the ring structure is a fused ring structure, the entire fused ring structure is counted as one ring structure when counting the "number of ring structures" above. When the thiolate ligand has two or more ring structures, the two or more ring structures may be bonded by a single bond or may be bonded via a linking group. Examples of the linking group include substituted or unsubstituted alkylene groups. The number of carbon atoms in the alkylene group can be selected, for example, from the range of 1 to 6. The substituent of the alkylene group can be selected, for example, from the following substituent group A. When the thiolate ligand has two or more ring structures, a coordinated sulfur atom is bonded to a carbon atom constituting the ring skeleton of at least one of the ring structures. The coordinating sulfur atom may be bonded to only one ring structure or to two or more ring structures, and the number of coordinating sulfur atoms bonded to each ring structure may be one or more per ring structure.

[0022] In this specification, "substituent group A" means one atom or group, or a combination of two or more selected from the group consisting of alkyl groups having 1 to 20 carbon atoms, alkoxy groups having 1 to 20 carbon atoms, aryl groups having 6 to 40 carbon atoms, alkenyl groups having 2 to 10 carbon atoms, alkynyl groups having 2 to 10 carbon atoms, heteroaryl groups having 3 to 40 carbon atoms, amino groups, halogen atoms, hydroxy groups, aldehyde groups, carboxy groups, cyano groups, nitro groups, silyl groups, and sulfo groups or salts thereof.

[0023] The thiolate ligand molecules used in the present invention can be obtained, for example, by using a thiol compound as a precursor and dissociating the hydrogen atom from the thiol group. Specific examples of thiol compounds that can be used as precursors of thiolate ligands are listed below, and specific examples of thiolate ligands are those obtained by removing a hydrogen atom from the thiol group of the specific thiol compounds listed below. However, the thiolate ligands that can be used in the present invention should not be construed as being limited by these specific examples. First, preferred specific examples of the thiolate ligand include those obtained by removing a hydrogen atom from the thiol group of the following compound examples.

[0024] [ka] [ka]

[0025] Specific examples of the thiolate ligand include compounds in which a hydrogen atom has been removed from the thiol group of the following compound examples. [ka] [ka]

[0026] The molecule used in the present invention has a thiolate ligand bonded to the surface of a small-atom cluster nucleus. Specifically, the thiolate ligand is bonded to an atom constituting the surface of the small-atom cluster nucleus through its coordinated sulfur atom. In molecules in which thiolate ligands are bonded to the surface of a small-atom cluster nucleus, the number of thiolate ligands bonded to one small-atom cluster nucleus is preferably three or more, more preferably four or more. The upper limit of the number of thiolate ligands bonded to one small-atom cluster nucleus is three times or less than the number of atoms constituting the surface of the small-atom cluster nucleus, and may be one or more or two or more less than this upper limit. For example, in a regular tetrahedral cluster shape, when the total number of atoms is 10, 20, 35, or 56, the number of surface atoms is 10, 20, 34, or 52, respectively. Therefore, for example, when the number of atoms in the small-atom cluster nucleus is 20, the number of thiolate ligands bonded to the small-atom cluster nucleus is 60 or less, preferably 12 to 40, and more preferably 20 to 28. When two or more thiolate ligands are bonded to one small-atom cluster nucleus, the multiple thiolate ligands may be the same or different.

[0027] The molecule used in the present invention may be a molecule in which the thiolate ligand constituting the metal thiolate complex is bonded to the surface of a small number of atom cluster nucleus. The metal thiolate complex will be described below.

[0028] (Metal thiolate complexes) The metal thiolate complex that can be used in the present invention is a complex that contains Au as a second metal atom and a thiolate ligand coordinated to the Au as the second metal atom. The second metal atom is Au, which is different from the minority atom cluster nucleus, and is coordinated with a thiolate ligand. The number of second metal atoms contained in the metal-thiolate complex may be one or more. For an explanation of the thiolate ligand, please refer to the description in the above section (Thiolate Ligand). The metal thiolate complex contains at least one thiolate ligand, preferably two, that is bonded to the second metal atom through its coordinated sulfur atom and also to the minority atom cluster nucleus. The metal thiolate complex may also contain one or more thiolate ligands that are bonded to two second metal atoms through their coordinated sulfur atoms but are not bonded to the minority atom cluster nucleus. The chemical structures of the multiple thiolate ligands contained in the metal thiolate complex may be the same or different. In a metal thiolate complex, the second metal atom and the thiolate ligand are arranged alternately via bonds between the second metal atom and the coordinating sulfur atom, for example, so that both ends are thiolate ligands, and the coordinating sulfur atoms of the thiolate ligands on both ends are bonded to atoms constituting the surface of the minority-atom cluster nucleus.

[0029] An example of the metal thiolate complex is a complex represented by the following general formula (1a). General formula (1a) *-SR-(Au-SR) n1 -* In general formula (1a), the metal atom SR represents a thiolate ligand, S represents a coordinating sulfur atom, and R represents an atomic group. n1 represents an integer of 1 or more, preferably 1 to 5, and more preferably 1 to 3. * represents the bonding position to the minority atom cluster nucleus. The thiolate ligands at both ends of general formula (1a) are bonded to the second metal atom and atoms constituting the minority atom cluster nucleus via coordinating sulfur atoms, and the other thiolate ligands are bonded to the adjacent second metal atom via coordinating sulfur atoms. Multiple SRs may be the same or different, but are preferably the same. For an explanation of the thiolate ligands, their preferred ranges, and specific examples, please refer to the description in the above section (Thiolate Ligands). This metal thiolate complex has a staple-like shape and binds to the small-atom cluster nucleus via the sulfur atoms of the thiolate ligands at both ends, effectively protecting and stabilizing the small-atom cluster nucleus.

[0030] In molecules in which metal thiolate complexes are bound to the surface of a small-atom cluster nucleus, the number of metal thiolate complexes bound to one small-atom cluster nucleus is preferably three or more, more preferably four or more. The upper limit of the number of metal thiolate complexes bound to one small-atom cluster nucleus is 1.5 times or less than the number of atoms constituting the surface of the small-atom cluster nucleus, and may be one or more or two or more less than this upper limit. For the number of atoms constituting the surface of the small-atom cluster nucleus, see the description in the above section (Thiolate Ligand). For example, if the small-atom cluster nucleus has 20 atoms, the number of metal thiolate complexes bound to the small-atom cluster nucleus is 30 or less, preferably 4 to 20, more preferably 4 to 12, and particularly preferably 4 to 8. When two or more metal thiolate complexes are bound to one small-atom cluster nucleus, the multiple metal thiolate complexes may be the same or different.

[0031] (Molecular composition) In this specification, the composition of the molecule used in the present invention is represented by the following general formula (1). General formula (1) [(M a m )(SR r M b n2 ) x ] β In general formula (1), M a m represents a few-atom cluster nucleus, and M a represents Au. m represents the atom M that constitutes the minority atom cluster nucleus. a It represents the number of , and is an integer between 10 and 56. (SR r M b n2 ) represents a metal thiolate complex, SR represents a thiolate ligand, and M b represents the second metal atom, which is Au; r represents the number of thiolate ligands SR contained in the metal thiolate complex and is an integer of 1 or more; n2 represents the number of second metal atoms, M, contained in the metal thiolate complex;b x represents the number of metal thiolate complexes (SR r M b n2 ) and is an integer of 1 or more. β represents the valence of the ion. However, when n2 is 0, r is 1, and (SR r M b n2 ) represents a thiolate ligand, and x represents the number of thiolate ligands in the molecule. For example, (Au 20 )(NP3Au2)8 (where NP stands for naphthalene thiolate) represents a molecule in which eight metal thiolate complexes, each consisting of three naphthalene thiolates and two Au atoms, are bound to the surface of a gold cluster core consisting of 20 gold atoms. Also, when r = n2 + 1 (where n is 1 or more), (SR r M b n2 The metal thiolate complex represented by formula (1a) preferably has a structure represented by formula (1a) above. In the general formula (1), when r is 2 or more, the plurality of SRs may all be the same thiolate ligand, or may contain two or more types of thiolate ligands. When x is 2 or more, the plurality of (SRs r M b n2 ) may be the same or different. a m , thiolate ligand SR and metal thiolate complexes (SR r M b n2 For an explanation of (Small number of atom cluster nuclei), (Thiolate ligands), and (Metal thiolate complexes), please refer to the descriptions in the above sections. m is 10 to 56, and preferably 10, 16, 20, 35, 40, 52, or 56. r is preferably 1 or greater, more preferably 1 to 10, and even more preferably 1 to 5. n2 is preferably 0 or greater, more preferably 0 to 5, and even more preferably 0 to 3. x is preferably 3 or greater, more preferably 4 to 16, and even more preferably 4 to 8. β is preferably -15 to 15, more preferably -10 to 10, and even more preferably -5 to 5.

[0032] As described above, the molecules used in the two-photon absorption material of the present invention are molecules in which a thiolate ligand or a metal thiolate complex is bound to the surface of a small number of atom cluster nucleus. The molecules bound to the surface of the small number of atom cluster nucleus may be only thiolate ligands (thiolate ligands that are not coordinated to elements other than the constituent elements of the small number of atom cluster nucleus), only metal thiolate complexes, or both thiolate ligands and metal thiolate complexes. Furthermore, the small number of atom cluster nucleus may further be bound to an organic ligand other than a thiolate ligand or a complex other than a metal thiolate complex. As described above, the number of atoms in the minority-atom cluster nucleus of the molecule used in the present invention is 10 to 56. Here, the "number of atoms in the minority-atom cluster nucleus" means the number of atoms contained in the minority-atom cluster nucleus in a molecule in which a metal thiolate complex or other complex is bound to the surface of the minority-atom cluster nucleus, and does not include the number of atoms coordinated with organic ligands in the complex.

[0033] A preferred example of the molecule used in the present invention is a molecule in which a metal thiolate complex is bound to the surface of a small-atom cluster nucleus containing Au, and the thiolate ligand constituting the metal thiolate complex contains an aromatic ring having a fused ring structure. A more preferred example of the molecule used in the present invention is a molecule in which a metal thiolate complex is bound to the surface of a small-atom cluster nucleus containing Au, and the second metal atom constituting the metal thiolate complex is Au, and the thiolate ligand contains an aromatic ring having a fused ring structure. In these examples, the aromatic ring is preferably a hydrocarbon ring, and more preferably a naphthalene ring. Furthermore, it is particularly preferred that the thiolate ligand is 2-naphthalene thiolate.

[0034] The two-photon absorption material of the present invention contains one or more types of molecules in which a thiolate ligand or a metal thiolate complex is bound to the surface of a small number of atom cluster nucleus. The two-photon absorption material of the present invention may contain only one type of molecule in which a thiolate ligand or a metal thiolate complex is bound to the surface of a small number of atom cluster nucleus, or may contain two or more types. Furthermore, the two-photon absorption material may contain components (other components) other than the molecules in which a thiolate ligand or a metal thiolate complex is bound to the surface of a small number of atom cluster nucleus.

[0035] (Method for synthesizing molecules in which thiolate ligands / metal thiolate complexes are bound to the surface of a small number of atom cluster cores) The molecules used in the present invention can be synthesized, for example, by a ligand exchange reaction using a precursor, a ligand-bonded few-atom cluster nucleus that has a common few-atom cluster nucleus with the target molecule but a different ligand or metal complex structure. Examples of the ligand in the precursor include triphenylphosphine, and examples of the metal complex include a gold-triphenylphosphine complex in which triphenylphosphine is coordinated to Au. Other known compounds can be appropriately selected and used depending on the target molecule. For specific conditions and procedures for the synthesis method, please refer to the description in the Examples section.

[0036] (Usefulness of two-photon absorption materials) The molecules used in the two-photon absorption material of the present invention have a large two-photon absorption cross-section. Furthermore, the molecular size, two-photon absorption characteristics (two-photon absorption cross-section and two-photon absorption wavelength), and molecular properties (e.g., water solubility) of the molecules used in the present invention can be controlled by molecular design of the thiolate ligand, which is an organic ligand. For example, the two-photon absorption cross-section in the near-infrared region, which allows for biological permeability, can be dramatically increased. Therefore, the two-photon absorption material of the present invention can be effectively applied to luminescence imaging (multiphoton excitation laser microscopes), photodynamic therapy, high-density optical storage, three-dimensional microfabrication, and the like. [Example]

[0037] The features of the present invention will be explained in more detail below with reference to synthesis examples and working examples. The materials, amounts used, ratios, treatment details, treatment procedures, etc. shown in the following working examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below. The geometry of the ground state S0 of the molecule was optimized by density functional theory (DFT) using the calculation package Gaussian 16 Rev. B 01, Rev. C 01. The functional was B3LYP, and the basis set was LANL2DZ for Au and S, and 3-21G for C and H. The two-photon absorption cross section was measured using the open aperture Z-scan method. Specifically, the sample solution in dimethylformamide was placed in a quartz cell with an optical path length of 2 mm, and the transmittance was measured. The two-photon absorption cross section δ (unit: GM) was calculated using the following formula: δ=hvα (2) / N In the formula, hv is the energy of the incident photon (J photon -1 ), α (2) is the two-photon absorption coefficient (cm s J) calculated from the measured transmittance using a theoretical formula. -1 ), N is the number density of molecules in the sample (molecule cm -3 ) are shown respectively. The incident light used for the transmittance measurements was light (wavelength 803-953 nm, pulse width 75-85 fs, repetition rate 1 kHz) from a femtosecond optical parametric amplifier (TOPAS-Prime manufactured by Spectra-Physics). The average incident power during the measurements was 0.80 mW, the Rayleigh length (zR) of the optical system was 5.5-6.2 mm, and the peak light intensity was 340-420 GW cm. -2 It was. The theoretical equation for the light intensity calculation and transmittance was based on a model with a spatial and temporal Gaussian light intensity distribution. The number density N of molecules is cN A / 1000(N A is the Avogadro constant, and c is the molar concentration. The two-photon absorption cross section δ was corrected using a standard sample (1,4-bis(2,5-dimethyl-4-{2-[4-(N-methyl)pyridinium]ethenyl}phenyl)butadiene triflate (MPPBT) in dimethyl sulfoxide). For details of the open aperture Z-scan method, see M. Sheik-Bahae, AA Said, TH Wei, DJ Hagan, and EW Van Stryland, IEEE J. Quantum Electron., 1990, 26, 760.

[0038] Example 1: A gold thiolate complex with 2-naphthalene thiolate (NP) as a ligand was synthesized. 20 Synthesis and structural analysis of molecules bound to Gold cluster nucleus Au 11 A molecule with eight triphenylphosphine atoms attached to [Au 11 An excess of 2-naphthalenethiol was added to (triphenylphosphine)8 and stirred at 40-50°C to obtain a yellow reaction solution. The reaction solution was separated by gel permeation chromatography (GPC) using an open column (gel used: SX-1 manufactured by BIO-RAD) to recover the reaction product. The reaction product was analyzed by single crystal X-ray structure analysis, and the ground state S0 structure was optimized. The optimized structure is shown in Figure 1. As shown in Figure 1, the resulting reaction product is a gold cluster core Au with eight gold thiolate complexes (NP3Au2). 20 The molecule [Au 20 (NP3Au2)8], where Au is the gold atom. 20 It was confirmed that the gold cluster core has a regular tetrahedral shape, and the remaining gold atoms are bound to the coordinated sulfur atoms of the 2-naphthalenethiolate ligand at both ends to form a complex (-NP-Au-NP-Au-NP-). This structure is the same as that described in Q. Shi et al., Nanoscale, 12, 4982 (2020), but the document does not suggest its usefulness as a two-photon absorption material.

[0039] Example 2: A gold thiolate complex with cyclopentanethiolate (Cy) as a ligand is Au 20 Synthesis and structural analysis of molecules bound to Au 20 An excess amount of cyclopentanethiol was added to a toluene solution of (NP3Au2)8, and the mixture was heated overnight at 80°C to react. The reaction solution was separated by gel permeation chromatography (GPC) using toluene as a developing solvent and an open column (gel used: SX-1 manufactured by BIO-RAD) to recover the reaction product. The reaction product was analyzed by single-crystal X-ray structure analysis, and the ground state S0 structure was optimized. The optimized structure is shown in Figure 2. As shown in Figure 2, the resulting reaction product was composed of eight gold thiolate complexes (Cy3Au2) attached to the gold cluster core Au 20 The molecule [Au 20 (Cy3Au2)8], where Au is the gold atom. 20 It was confirmed that the gold cluster core is a tetrahedron, and the remaining gold atoms are bound to cyclopentathiolate-coordinated sulfur atoms at both ends to form a complex (-Cy-Au-Cy-Au-Cy-). This structure is the same as that described in Q. Shi et al., Nanoscale, 12, 4982 (2020), but the document does not suggest its usefulness as a two-photon absorption material.

[0040] [Evaluation of two-photon absorption cross section] Au synthesized in each example 20 (NP3Au2)8 and Au 20 Figure 3 shows the two-photon absorption cross section spectrum of (Cy3Au2)8 measured with incident light in the near-infrared region. As shown in Figure 3, Au 20 (NP3Au2)8 and Au 20 (Cy3Au2)8 exhibited large two-photon absorption cross sections of over 2000 GM in the near-infrared region. 20 (NP3Au2)8 showed an extremely large two-photon absorption cross section of 5200 GM for incident light of 850 nm, which has high biological permeability. This is presumably due to the contribution of the absorption band of the naphthalene ring of the thiolate ligand to double resonance. 20 (NP3Au2)8 and Au 20 (Cy3Au2)8 did not decompose during this measurement, confirming its high stability. Also, Au 20 The two-photon absorption cross section per molecular volume was calculated for (NP3Au2)8 and the following comparative compound 1 (Hex represents a hexyl group). 20 (NP3Au2)8 at 0.143GM bohr -3 (Incident light wavelength: 850 nm), 0.129 GM bohr for comparative compound 1 -3 The two-photon absorption cross section of Comparative Compound 1 used in this calculation is the actual measured value (1000 GM) at 730 nm described in M. Pawlicki et al., Angew. Chem. Int. Ed. 48, 3244 (2009) (Non-Patent Document 3).

[0041] [ka] [Industrial Applicability]

[0042] According to the present invention, it is possible to provide a two-photon absorption material having a large two-photon absorption cross section. Such a two-photon absorption material can be effectively applied to luminescence imaging (multiphoton excitation laser microscope), photodynamic therapy, high-density optical storage, three-dimensional microfabrication, etc. Therefore, the two-photon absorption material of the present invention has high industrial applicability.

Claims

1. A two-photon absorbing material comprising a molecule having a thiolate ligand attached to a surface of a small number of atom cluster core, the element constituting the minority atom cluster nucleus is Au only, The two-photon absorbing material has a number of atoms in the minority atom cluster core of the molecule of 10 to 56.

2. 2. The two-photon absorption material of claim 1, wherein the number of atoms in the minority atom cluster core of the molecule is 10, 16, 20, 35, 40, 52, or 56.

3. The two-photon absorption material according to claim 1 , wherein the thiolate ligand has a structure in which a coordinated sulfur atom is bonded to a carbon atom constituting a ring skeleton of a hydrocarbon ring or a heterocycle.

4. The two-photon absorption material of claim 3 , wherein the hydrocarbon ring and the heterocyclic ring have a π-conjugated system.

5. The two-photon absorption material according to claim 4 , wherein the hydrocarbon ring is an aromatic ring, and the heterocycle is an aromatic heterocycle.

6. 6. The two-photon absorption material according to claim 5, wherein the aromatic ring and the aromatic heterocycle have a fused ring structure consisting of 2 to 4 constituent rings.

7. The two-photon absorption material according to claim 1 , wherein the thiolate ligand has a structure in which a coordinated sulfur atom is bonded to a naphthalene ring.

8. 8. The two-photon absorption material according to claim 7, wherein the thiolate ligand is a substituted or unsubstituted 2-naphthalene thiolate.

9. 2. The two-photon absorbing material of claim 1, wherein the thiolate ligand is cyclopentanethiol.

10. The two-photon absorption material according to any one of claims 1 to 9, wherein in the molecule, the thiolate ligand is coordinated to Au that does not constitute the minority atom cluster nucleus to form a metal thiolate complex.

11. The two-photon absorption material according to claim 10 , wherein the metal thiolate complex has a structure represented by the following general formula (1a): General formula (1a) *-SR-(Au-SR) ｎ１ -* [In general formula (1a), SR represents a thiolate ligand, S represents a coordinating sulfur atom, and R represents an atomic group. n1 represents an integer of 1 or more. * represents the bonding position to the minority atom cluster nucleus. The thiolate ligand in general formula (1a) bonds to Au via the coordinating sulfur atom. The multiple SRs may be the same or different.]

12. The two-photon absorption material according to any one of claims 1 to 9, wherein the molecule has a two-photon absorption wavelength within a wavelength range of 700 to 1300 nm.

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