TADF type high-stability near-infrared laser material as well as preparation method and application thereof

Abstract

The invention discloses a TADF type high-stability near-infrared laser material and a preparation method and application thereof, the structural formula of the TADF type high-stability near-infrared laser material is # imgabs 0 # imgabs 1 #, and R is selected from hydrogen, benzene, methylbenzene, alkyl, alkoxy, alkylthio or dialkylamino; x is oxygen, sulfur, nitrogen, one or more toluene or one or more methyl. The TADF type high-stability near-infrared laser material provided by the invention has ESIPT activity on the basis of having TADF characteristics, successfully realizes near-infrared TADF laser with low threshold and high stability, and solves the problem of poor stability of near-infrared organic laser in the prior art.

Description

Technical Field

[0001] The present invention relates to the technical field of near-infrared laser materials, and particularly to a thermally activated delayed fluorescence (TADF)-type highly stable near-infrared laser material, a preparation method thereof, and an application thereof. Background Art

[0002] Since the laser technology was first reported in 1960, this field has achieved rapid development. Among them, near-infrared (NIR) lasers have shown great application potential in the fields of sensing, display, and photonic integrated circuits due to their unique advantages. Among various implementation methods of NIR lasers, organic solid-state lasers (OSSLs) using organic materials as gain media are particularly remarkable. Such lasers have become a research hotspot due to their adjustable chemical structures, relatively simple preparation processes, and large stimulated emission cross-sections.

[0003] However, despite the many advantages of OSSLs, they still face two major challenges in practical applications: reducing the threshold and improving stability. These two problems mainly stem from the inherent characteristics of organic materials, namely rapid non-radiative decay and the accumulation of triplet excitons caused by intersystem crossing. The accumulation of triplet excitons will further trigger the quenching of singlet excitons and the transition to higher-energy triplet excitons, thus seriously affecting the performance of the laser.

[0004] To solve the above problems, researchers have proposed various strategies. Among them, using thermally activated delayed fluorescence (TADF) materials as gain materials is considered an effective approach. TADF materials can transfer triplet excitons to singlet excitons through the reverse intersystem crossing (RISC) process, thereby effectively managing triplet excitons and improving the performance of the laser. In addition, an effective four-level system is crucial for achieving stimulated emission. However, in NIR materials, due to the energy level degeneracy caused by molecular vibration, it is difficult to ensure a quasi-four-level system. Therefore, constructing an effective four-level system has become the key to realizing NIR TADF lasers. The excited-state intramolecular proton transfer (ESIPT) process provides the possibility of forming a true four-level system. However, no TADF laser material with ESIPT activity has been reported so far. Summary of the Invention

[0005] To solve the problems of high laser threshold and poor laser stability of near-infrared organic solid-state lasers, the present invention provides a thermally activated delayed fluorescence (TADF)-type highly stable near-infrared laser material, a preparation method thereof, and an application thereof. The near-infrared laser material is triggered based on the excited-state intramolecular proton transfer (ESIPT) process and has both ESIPT activity and TADF characteristics.

[0006] The present invention is achieved through the following technical solutions:

[0007] In the first aspect of the present invention, a TADF-type highly stable near-infrared laser material is provided. The structural formula of the TADF-type highly stable near-infrared laser material is shown as Formula (I), Formula (II), or Formula (III):

[0008]

[0009] Wherein, R is selected from hydrogen, benzene, toluene, alkyl, alkoxy, alkylthio, or dialkylamino; X is oxygen, sulfur, nitrogen, one or more toluenes, or one or more methyl groups.

[0010] The TADF-type highly stable near-infrared laser material provided by the present invention has ESIPT activity. ESIPT-active molecules can undergo proton transfer in the excited state, thereby changing the energy level structure of the molecules, making it possible to construct a four-level system. Given the important role of ESIPT in realizing NIR organic lasers, introducing ESIPT into TADF molecules and combining the advantages of both is an effective strategy for designing TADF gain materials with high gain characteristics.

[0011] Further, R is selected from hydrogen, benzene, toluene, C1-C20 alkyl, C1-C20 alkoxy, C1-C20 alkylthio, or C1-C20 dialkylamino.

[0012] Preferably, R is selected from methyl, tert-butyl, methoxy, methylthio, dimethylamino, or diethylamino.

[0013] Further, the TADF-type highly stable near-infrared laser material is selected from the compounds shown in the following structures:

[0014]

[0015] In the second aspect of the present invention, a preparation method of the TADF-type highly stable near-infrared laser material described in the first aspect is provided, including the following steps:

[0016] S1. The compound shown in Formula (IV) is subjected to hydroxymethylation to obtain the compound shown in Formula (V);

[0017] S2. The compound shown in Formula (V) described in S1 is connected with substituents through a Suzuki coupling reaction and then deprotected to obtain the TADF-type highly stable near-infrared laser material shown in Formula (I)-(III);

[0018] The structural formulas of the above Formula (I), Formula (II), Formula (III), Formula (IV), and Formula (V) are shown as follows:

[0019]

[0020] Among them, R is selected from hydrogen, benzene, toluene, alkyl, alkoxy, alkylthio or dialkylamino; X is oxygen, sulfur, nitrogen, one or more toluenes or one or more methyl groups; Ar is selected from halogens; Z is selected from C1-C20 alkyl.

[0021] Further, the R is selected from hydrogen, benzene, toluene, C1-C20 alkyl, C1-C20 alkoxy, C1-C20 alkylthio or C1-C20 dialkylamino.

[0022] Preferably, the R is selected from methyl, tert-butyl, methoxy, methylthio, dimethylamino or diethylamino.

[0023] The third aspect of the present invention provides an application of the TADF-type highly stable near-infrared laser material described in the first aspect in a near-infrared organic solid laser.

[0024] The present invention utilizes the two characteristics of TADF and ESIPT of the TADF-type highly stable near-infrared laser material, and can effectively achieve near-infrared laser emission with low threshold and high stability.

[0025] The fourth aspect of the present invention provides a polystyrene microsphere, and the polystyrene microsphere includes the TADF-type highly stable near-infrared laser material described in the first aspect.

[0026] Further, the doping concentration of the TADF-type highly stable near-infrared laser material in the polystyrene microsphere is greater than 0.1 wt%.

[0027] Preferably, the doping concentration of the TADF-type highly stable near-infrared laser material in the polystyrene microsphere is 0.1-3 wt%.

[0028] The fifth aspect of the present invention provides an application of the polystyrene microsphere described in the fourth aspect in an organic solid laser.

[0029] Further, the laser wavelength generated by the polystyrene microsphere is 750-2500 nm, and it has a low threshold and excellent laser stability.

[0030] Advantages of the present invention:

[0031] 1. The present invention provides a laser active material with low threshold and high laser stability, its preparation method and application. A molecule with ESIPT activity is used as an acceptor to construct a TADF material to obtain a TADF-type highly stable near-infrared laser material. This near-infrared laser material has both ESIPT activity and TADF properties. While constructing an effective four-level system, it effectively manages triplet excitons through TADF, so that it has laser properties of low threshold and high stability, and the laser wavelength generated by it is in the near-infrared light region (750 nm - 2500 nm).

[0032] 2. The TADF-type highly stable near-infrared laser material provided by the present invention provides a feasible solution to solve the problem of poor stability of existing NIR organic lasers, which will greatly promote the development of future NIR organic lasers and thus push forward the research process of electrically pumped organic near-infrared lasers. Description of the Drawings

[0033] Figure 1 Schematic energy level diagram of proton transfer of Compound 1 prepared in Example 1; wherein, a is a schematic diagram of the ESIPT process of Compound 1, and b is a diagram of the relative energies of Compound 1 in the ground state (S 0 ) and the first singlet state (S 1 ) in toluene calculated at the TD-DFT / B3LYP / 6-31+G(d,p) level.

[0034] Figure 2 Electron-hole distribution diagrams of the first singlet state and the first triplet state of Compound 1 prepared in Example 1 in the normal and tautomeric forms; wherein, a is a schematic diagram of the ESIPT process of Compound 1, and b are electron-hole distribution diagrams of the first singlet state (S 1 ) and the first triplet state (T 1 ) of Compound 1 in the normal form and the tautomeric form.

[0035] Figure 3 Absorption and emission spectra of Compound 1 prepared in Example 1 in different solvents; wherein, a are absorption and emission spectra of Compound 1 in cyclohexane, toluene and dichloromethane, and b are absorption and emission spectra of Compound 1 in acetone, acetonitrile and methanol.

[0036] Figure 4 Fluorescence and phosphorescence spectra of Compound 1 prepared in Example 1 in a 77K toluene solution.

[0037] Figure 5 Transient fluorescence spectra of polystyrene films doped with Compound 1 prepared in Example 1; wherein, a is the transient fluorescence spectrum of a polystyrene film doped with Compound 1 under vacuum conditions, and b are transient fluorescence spectra of polystyrene films doped with Compound 1 at different temperatures.

[0038] Figure 6Transient absorption spectra of polystyrene films doped with Compound 1 prepared in Example 1; where a is the femtosecond transient absorption spectrum of the polystyrene film doped with Compound 1, b is the relaxation kinetic data of the femtosecond transient absorption spectrum monitored at 600 nm, c is the nanosecond transient absorption spectrum of the polystyrene film doped with Compound 1, and d is the nanosecond transient spectral curve of the polystyrene film doped with Compound 1.

[0039] Figure 7 SEM image of polystyrene microspheres prepared in Example 2.

[0040] Figure 8 Laser spectra of polystyrene microspheres prepared in Example 2; where a is the laser spectrum of the polystyrene microspheres (pump laser: 532 nm, the inset is a fluorescence microscope image), and b is the plot of laser intensity and full width at half maximum as a function of energy density.

[0041] Figure 9 Laser stability diagram of polystyrene microspheres prepared in Example 2; where a is the plot of laser intensity of the polystyrene microspheres as a function of time, and b is the laser spectra of the polystyrene microspheres at 0 min and 580 min. Detailed implementation manners

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this invention belongs. The terms used in the description of this invention herein are only for the purpose of describing specific embodiments and are not intended to limit the invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0043] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the embodiments given are not intended to limit the present invention.

[0044] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods, and the materials, reagents, etc. used, unless otherwise specified, can all be obtained from commercial sources. The substrate used in the examples and comparative examples is a silica substrate.

[0045] Example 1

[0046] A preparation method of a TADF-type highly stable near-infrared laser material, comprising the following steps:

[0047] S1. Add intermediate 1 (2.46 g), potassium carbonate (7.92 g), acetone (400 mL), and dimethyl sulfate (4.8 mL) into a 1000 mL three-necked flask in sequence. After the addition, heat under reflux and stir at 80 °C for 24 hours. After the reaction is completed, cool to room temperature, perform suction filtration, separate the solid and liquid, and wash the obtained solid with a large amount of dichloromethane. Then, combine the dichloromethane washing solutions, remove the solvent under reduced pressure, and perform column chromatography at atmospheric pressure (petroleum ether:dichloromethane = 3:2) to obtain a yellow solid (intermediate 2).

[0048] S2. Add intermediate 2 (1.04 g), intermediate 3 (1.64 g), potassium carbonate (1.10 g), and tetrakis(triphenylphosphine)palladium(0) (0.12 g) into a 100 mL Schlenk flask in sequence. Evacuate the air, change the gas three times, and protect with argon. Then, add N,N-dimethylformamide (20 mL) and water (4 mL) thereto, heat and stir the reaction at 90 °C for 24 hours, and then cool to room temperature naturally. Then, carefully add 15 mL of 1 mol / L hydrochloric acid solution thereto, perform suction filtration, and perform column chromatography at atmospheric pressure (petroleum ether:dichloromethane = 1:1) to obtain a red solid (intermediate 4).

[0049] Add intermediate 4 (0.23 g) into a 50 mL Schlenk flask, evacuate the air, and change the gas with argon three times. Under an argon atmosphere, add dichloromethane (10 mL) thereto and place it at -78 °C. Then, dropwise add a dichloromethane solution of boron tribromide (1.0 M, 18.7 mL) thereto. After the addition is completed, react at -78 °C for 10 min and then continue to react at room temperature for 1 hour. After the reaction is completed, add water (15 mL) to quench the reaction. Separate the layers, extract the aqueous phase with dichloromethane (3 × 10 mL). Combine the organic phases, wash with water (3 × 20 mL), wash with saturated brine (40 mL), dry over anhydrous sodium sulfate, and perform column chromatography at atmospheric pressure (petroleum ether:dichloromethane = 2:1) to obtain a dark black solid (compound 1).

[0050] The reaction equation for preparing the TADF-type highly stable near-infrared laser material in Example 1 is as follows:

[0051]

[0052] The NMR data of compound 1 are as follows: 1 H NMR(400MHz,CDCl 3 )δ13.64(s,2H),7.96(d,J=7.9Hz,2H),7.79(d,J=7.9Hz,2H),7.62(d,J=8.8Hz,4H),7.36 - 7.29(m,8H),7.25 - 7.14(m,12H),7.10(t,J=7.3Hz,4H). 1313C NMR (101 MHz, CDCl 3 ) δ 187.06, 159.50, 147.26, 146.30, 136.26, 135.27, 130.55, 129.16, 128.38, 128.23, 124.02, 122.49, 122.49, 121.26, 118.66, 115.31.

[0053] Example 2

[0054] A method for preparing polystyrene microspheres, comprising the following steps:

[0055] (1) Preparation of the mother liquor: Accurately weigh compound 1 (1.0 mg) prepared in Example 1 and polystyrene powder (M W : 260,000, 100 mg), and prepare a 2 mL dichloromethane solution. Oscillate with ultrasonic waves until completely dissolved, and use the obtained solution as the mother liquor for standby.

[0056] (2) Preparation of cetyltrimethylammonium bromide (CTAB) solution: Accurately weigh CTAB (14.6 mg), dissolve it in 20 mL of deionized water to prepare a 2 mM CTAB aqueous solution, and use ultrasonic oscillation until completely dissolved and then set aside.

[0057] (3) Preparation of polystyrene microspheres: Add CTAB aqueous solution (2 mL) to a clean sample bottle, add the mother liquor (0.2 mL) under vigorous stirring conditions, continue to stir vigorously for 30 min and then stir slowly for 12 h. Filter, wash the solid with deionized water, and the obtained solid is the polystyrene microsphere. Among them, the doping concentration of compound 1 in the polystyrene microsphere is 1.0 wt%.

[0058] Test Example 1

[0059] Density functional theory (DFT) calculations were carried out through the Gaussian 16 program. The ground state (S 0 ), the first singlet state (S 1 ), and the first triplet state (T 1 ) structures and energies of compound 1 prepared in Example 1 in the normal form and tautomer form were calculated at the DFT and TD-DFT / B3LYP / 6-31+G(d,p) levels.

[0060] Figure 1 It is the energy level schematic diagram of the proton transfer of compound 1 prepared in Example 1; among them, a is the schematic diagram of the ESIPT process of compound 1, and b is the ground state (S 0 ) and the first singlet state (S 1Relative energy schematic diagram on. From Figure 1 It can be seen that the small energy barrier between the normal - form excited state (N*) and the transition state (TS*) indicates that the transfer process of protons from the normal form (N*) to the tautomer form (TA*) can proceed smoothly. However, the huge energy barrier between TA* and the second - transition state (TS'*) hinders the second proton transfer from TA* to the second tautomer (TB*), thus indicating that compound 1 can only undergo one proton transfer.

[0061] Figure 2 Figure showing the electron - hole distribution diagrams of the first singlet state and the first triplet state of compound 1 prepared in Example 1 in the normal and tautomer forms. From Figure 2 It can be seen that in the normal form, the energy levels of the first singlet state (S 1 ) and the first triplet state (T 1 ) of compound 1 are 1.64 eV and 1.48 eV respectively. The energy difference (ΔE ST ) between S 1 and T 1 is 0.16 eV. In the tautomer form, the S 1 and T 1 energy levels of compound 1 are 1.40 eV and 0.89 eV respectively, and ΔE ST is 0.51 eV. Therefore, compound 1 has TADF activity in the normal form and can utilize triplet excitons through the reverse intersystem crossing process.

[0062] Test Example 2

[0063] Optical performance tests were carried out on compound 1 prepared in Example 1.

[0064] (1) Absorption and emission spectra in different solvents: Prepare cyclohexane, toluene, dichloromethane, acetone, acetonitrile, and methanol solutions of compound 1 with a concentration of approximately 1×10 -5 mol / L and a volume of 5 mL. Use a UV - visible spectrophotometer to test the UV - visible absorption spectra of the compound 1 solutions. Use a fluorescence spectrometer to test the fluorescence emission spectra of the compound 1 solutions. Among them, the excitation light sources for cyclohexane, toluene, and dichloromethane solutions are xenon lamps with an excitation wavelength of 500 nm, and the excitation light sources for acetone, acetonitrile, and methanol solutions are xenon lamps with an excitation wavelength of 400 nm.

[0065] The test results are as Figure 3 shown, Figure 3 Figure showing the absorption and emission spectra of compound 1 prepared in Example 1 in different solvents. From Figure 3It can be seen that compared with those in medium- and low-polarity solvents (cyclohexane, toluene, dichloromethane), the absorption and emission spectra of Compound 1 show an obvious blue shift in high-polarity solvents (acetone, acetonitrile, methanol). This is because the carbonyl, carboxyl, and nitrile groups in acetone, methanol, and acetonitrile have strong polarity, which stabilizes the normal form of Compound 1 through intermolecular hydrogen bonding, thus inhibiting the ESIPT process in Compound 1 and only showing the emission of the normal form, thereby confirming the existence of the ESIPT process in Compound 1.

[0066] (2) Fluorescence and phosphorescence spectra in toluene solution at 77 K: Prepare a toluene solution of Compound 1 with a concentration of about 1×10 -5 mol / L and a volume of 1 mL. Cool the prepared toluene solution of Compound 1 to 77 K using liquid nitrogen, and use a fluorescence spectrometer to measure the fluorescence and phosphorescence spectra of the toluene solution of Compound 1 at 77 K. The excitation light is a xenon lamp source with a wavelength of 500 nm.

[0067] The test results are as Figure 4 shown, Figure 4 which is the fluorescence and phosphorescence spectra of Compound 1 prepared in Example 1 in toluene solution at 77 K. It can be seen from Figure 4 the fluorescence spectrum and phosphorescence spectrum that the values of the first singlet state and the first triplet state of Compound 1 are calculated to be 1.78 eV and 1.74 eV respectively, and the corresponding ΔE ST value is 0.04 eV. The smaller ΔE ST further confirms the TADF characteristics of Compound 1.

[0068] (3) Transient fluorescence spectrum and transient absorption spectrum of polystyrene film doped with Compound 1: Add 1 mg of Compound 1 and 100 mg of polystyrene to 2 mL of dichloromethane, and ultrasonicate for 15 min to fully dissolve Compound 1 and polystyrene. Drop the mixed solution onto a quartz wafer, and after the dichloromethane has completely evaporated, obtain a polystyrene film doped with Compound 1. Use a transient fluorescence spectrometer to measure the transient fluorescence spectrum of the polystyrene film doped with Compound 1, and use a transient absorption spectrum detector to measure the transient absorption spectrum of the polystyrene film doped with Compound 1.

[0069] The test results are as Figure 5 and Figure 6 shown, Figure 5 which is the transient fluorescence spectrum of the polystyrene film doped with Compound 1 prepared in Example 1. It can be seen from Figure 5 the figure that prompt and delayed fluorescence lifetimes can be observed in the polystyrene film doped with Compound 1, indicating that Compound 1 has TADF characteristics. In addition, as the temperature increases, the polystyrene film doped with Compound 1 shows a more obvious delayed lifetime, further verifying the TADF characteristics of Compound 1.

[0070] Figure 6 Transient absorption spectrum of polystyrene film doped with Compound 1 prepared in Example 1. From Figure 6 It can be seen that in the normal form, after the triplet excitons of Compound 1 are converted into singlet states, they can be effectively utilized through the ESIPT process, thereby reducing the adverse effects on the threshold and stability of the laser caused by the accumulation of triplet excitons.

[0071] Test Example 3

[0072] Characterize the polystyrene microspheres prepared in Example 2 and test their laser performance.

[0073] (1) Characterization of polystyrene microspheres: Disperse the obtained polystyrene microspheres into absolute ethanol, take a drop of the dispersion and drop it on a clean glass slide, and obtain a sample for characterization and laser performance testing after the solvent has completely evaporated.

[0074] The scanning electron microscope (SEM) image of the polystyrene microspheres prepared in Example 2 is as Figure 7 shown. It can be seen from the figure that the polystyrene microspheres have a regular morphology and a smooth surface. Such polystyrene microspheres with a smooth surface can achieve light reflection and can therefore be used as a resonant cavity.

[0075] (2) Laser performance testing: Using a microarea spectroscopy system, under the excitation of a pulsed laser at 532 nm (pulse width: 10 ns, frequency: 10 Hz), the laser spectrum of the polystyrene microspheres can be obtained.

[0076] The test results are as Figure 8 and Figure 9 shown. Figure 8 is the laser spectrum of the polystyrene microspheres prepared in Example 2, Figure 9 is the laser stability diagram of the polystyrene microspheres prepared in Example 2. From Figure 8 it can be seen that the polystyrene microspheres can achieve laser emission with a central wavelength of 820 nm and a threshold of 6.3 μJ / cm 2 . From Figure 9 it can be seen that after continuous excitation for 580 min, the laser intensity of Compound 1 at a wavelength of 815 nm still remains at about 80% of the initial laser intensity, indicating that Compound 1 has good laser stability.

[0077] In summary, the present invention designs and synthesizes a near-infrared TADF material with ESIPT activity. By utilizing its two characteristics of TADF and ESIPT, near-infrared laser emission with low threshold and high stability can be effectively achieved.

[0078] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. Those skilled in the art should understand that other different forms of changes or variations can be made on the basis of the above description. It is not necessary and impossible to enumerate all the implementation manners here. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims

1. A TADF type highly stable near-infrared laser material, characterized in that: The structural formula of the TADF type highly stable near-infrared laser material is shown in formula (I), formula (II) or formula (III): Wherein, R is selected from hydrogen, benzene, toluene, alkyl, alkoxy, alkylthio or dialkylamino; X is oxygen, sulfur, nitrogen, one or more toluene or one or more methyl.

2. The TADF type highly stable near-infrared laser material according to claim 1, characterized in that: The R is selected from hydrogen, benzene, toluene, C1-C20 alkyl, C1-C20 alkoxy, C1-C20 alkylthio or C1-C20 dialkylamino.

3. The TADF type highly stable near-infrared laser material according to claim 1, characterized in that: The R is selected from methyl, tert-butyl, methoxy, methylthio, dimethylamino or diethylamino.

4. The TADF type highly stable near-infrared laser material according to claim 1, characterized in that: The TADF type highly stable near-infrared laser material is selected from the compounds shown in the following structure:

5. A method for preparing the TADF type highly stable near-infrared laser material according to any one of claims 1 to 4, characterized in that: The steps include: S1. The compound represented by formula (IV) is subjected to hydroxymethylation to obtain the compound represented by formula (V); S2. The compound represented by formula (V) described in S1 is connected to the substituent by Suzuki coupling reaction and then deprotected to obtain a TADF type highly stable near-infrared laser material represented by formula (I)-(III); The structural formulas of the above formula (I), formula (II), formula (III), formula (IV) and formula (V) are shown below: Wherein, R is selected from hydrogen, benzene, toluene, alkyl, alkoxy, alkylthio or dialkylamino; X is oxygen, sulfur, nitrogen, one or more toluenes or one or more methyl groups; Ar is selected from halogen; and Z is selected from C1-C20 alkyl.

6. Use of the TADF type highly stable near-infrared laser material according to any one of claims 1 to 4 in a near-infrared organic solid laser.

7. A polystyrene microsphere, characterized in that: The polystyrene microspheres include the TADF type highly stable near-infrared laser material according to any one of claims 1 to 4.

8. The polystyrene microspheres according to claim 7, characterized in that: The doping concentration of the TADF type highly stable near-infrared laser material in the polystyrene microspheres is greater than 0.1 wt %.

9. Use of the polystyrene microspheres according to claim 7 or 8 in organic solid-state lasers.

10. The use according to claim 9, characterized in that: The laser wavelength generated by the polystyrene microspheres is 750-2500nm.

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