Ultrafast Spectroscopy And Optical Anisotropy Of Two Fluorescent Molecules Modified By Styrene Skeleton

Human development is inseparable from the innovation and change of materials.The development of new materials and the improvement of the properties of existing materials are of great importance to social progress.However,the process of innovation is a huge project,and the discovery of new materials often has to go through a large number of experiments,which has high capital and time cost and low efficiency.Through the analysis and understanding of the properties of the material itself,in turn,it can guide some possible material design schemes,and regulate the macro properties from micro changes,which can effectively improve the development progress of materials.A wide variety of semiconductor materials,widely used,can be divided into inorganic semiconductor materials and organic semiconductor materials,especially organic semiconductor materials,the molecular structure is more flexible,can be adjusted through chemical synthesis and other methods,so as to be used in flexible electronics,display technology,biosensing and other fields.Luminescent materials play a very important role in organic semiconductors.According to their luminescence state,they are generally divided into three molecular forms:ACQ,AIE and DSE.ACQ molecules emit strong fluorescence in dilute solution due to their rigid planar structure.However,fluorescence quenching occurs when the molecules are aggregated.In the monodisperse state,AIE molecules will consume excitation energy due to intramolecular rotation or vibration,resulting in fluorescence quenching.However,in the aggregation state,the free rotation of the group is inhibited,the decay of non-radiative energy is hindered,and the emission is enhanced.DSE molecules can be conjugated to induce rigid structure,introduce into receptor（D-A）structure or introduce large steric groups,and other molecular design strategies to achieve solid-liquid bistate efficient luminescence.A typical aggregation state of organic luminescent molecules is organic crystals,which are generally composed of highly regularly arranged molecules.This special property makes them have good applications in optics,electricity,magnetism and other fields.The high-quality single crystal formed by orderly assembly of small organic molecules has the advantages of few defects,easy molecular structure modification,high thermal stability,and its highly ordered molecular arrangement often brings optical anisotropy characteristics,which is suitable for the production of polarization-related devices.The study of organic luminescent molecules,from solutions to crystals,provides insight into how they work in device applications.In this paper,femtosecond transient absorption technology is combined with steady-state spectroscopy to detect the intermolecular interactions of organic molecules with different luminescence mechanisms in different states,so as to obtain the photophysical processes of organic molecules after photoexcitation.The optical anisotropy of organic molecular monocrystals is studied by using the polarization-related test system built by the laboratory,which provides a new choice for the display equipment in the wide band polarization-sensitive lighting and the whole visible wavelength range.The main contents of this paper are as follows:In Chapter 3,the photoluminescence mechanism of SO₂-Bri-DSB in dichloromethane solution and crystal state was studied.The absolute fluorescence quantum yields of the molecules in solution and crystal can reach 55%and 89%,which shows obvious bistate luminescence characteristics.SO₂-Bri-DSB solution is excited by a femtosecond laser at 400 nm and initially undergoes a rapid vibrational energy dissipation（VED）process,from the high vibrational energy level of excited state to the low vibrational energy level,and then undergoes a relatively long molecular structure change process of single double bond length averaging（BLA）,and the energy is transferred to the intermediate BLA state.Finally,the excited species relaxes back to the ground state and fluoresces.Combined with global fitting and singular value decomposition,the differential spectra and time evolution curves of the three transient components are proved analytically,and the lifetimes of the three components are fitted.When SO₂-Bri-DSB molecules aggregate into a crystal state,their transient absorption spectra are simple,with only one excited state absorption（ESA）signal relaxation process with time,and the VED and BLA processes disappear due to the enhanced intermolecular interaction in the crystal.In Chapter 4,the optical anisotropy of SO₂-Bri-DSB crystal is studied,and the evolution law of the intrinsic luminescence anisotropy and anisotropy of SO₂-BRI-DSB crystal is analyzed in detail from the aspects of steady state,time and space by using the polarization system built by the laboratory.The polarization absorption spectra show that the electron transition anisotropy is caused by the special orientation of the transition dipole moment of the crystal.The steady-state emission spectrum of the crystal is period-dependent on both the polarization direction of the excitation light and the polarization direction of the fluorescence.The amplitude and relaxation rate of the polarization time-resolved fluorescence dynamic curve are significantly different under the mutually perpendicular excitation polarization direction.The variation curve of the anisotropy value with the delay time consists of two parts:The step part caused by the anisotropy of the electron transition and the slow rise process caused by the difference in exciton relaxation rate,the photoexciton recombination and radiation rate are slower at 0°excitation polarization direction.Due to the different refraction and scattering effects of the crystal on fluorescence in different vibration directions,the crystal has the characteristics of optical waveguide anisotropy,and the effective propagation length is15.15 mm.In Chapter 5,a 3-（furan-2-group）-2-（4-{[（2-hydroxy-5-methylphenyl）methylene]amino-}phenyl）propylene-2-enonitrile（LAXYUI）molecule with AIE properties is studied.The femtosecond transient absorption spectra show that,compared with LAXYUI solution,due to the enhanced molecular interaction in LAXYUI crystals,Laxyui solution is more effective than Laxyui solution.As a result,the lifetime of the excited state is prolonged,the non-radiating channel is inhibited,and the luminescence is enhanced.The ordered arrangement of molecules brings anisotropy characteristics to the crystal,and the polarization direction-dependent emission spectra show that the crystal has the characteristics of absorption and polychromism.Due to the special orientation of transition dipole moment,the photogenerated carrier is easier to diffuse when the vibration direction is the same as the transition dipole moment orientation,while the carrier may dissipate or transfer energy to the surrounding area when it is different from the transition dipole moment orientation,resulting in a shorter lifetime.With the decrease of temperature,the anisotropy of the crystal is enhanced.The birefringence effect of the crystal makes the fluorescence refractive index of 0°excitation direction larger when the fluorescence propagates along the long axis of the crystal,which leads to the propagation anisotropy of the fluorescence in the crystal.These results contribute to the understanding and application of the optical anisotropy of organic luminescent crystals,and lay a foundation for improving the performance of polarization display devices.