Studies On Ultrafast Spectroscopy Of Donor–Acceptor Organic Optoelectronic Molecules In Mono–disperse And Aggregation Systems

At this stage,due to the increasing demand for clean energy and its technological development,organic optoelectronic materials have become an emerging research field.Discovering and designing sustainable materials in the field can provide energy power for future generations without causing serious damage to the natural environment.The organic molecules have a simple preparation process and can be applied as solution–processed films on large flexible substrates.Compared to more expensive inorganic compounds that may contain rare earth metals,many organic materials can be synthesized or grown from inexpensive materials to reduce operating costs for large–scale production.In addition,most organic molecules are non–toxic,and the resulting waste can be decomposed without causing environmental pollution.With the rapid development of interdisciplinary technologies,researchers are constantly striving to discover and improve materials for next–generation devices.Through a bottom–up approach,a fundamental understanding of the photophysical or photochemical processes of materials can be obtained,which can effectively facilitate substantial progress in material design.With a more comprehensive understanding of the inner workings of organic materials,the "microscopic" molecular structures can be rationally designed to improve the "macroscopic" functional properties of materials.Light–matter interaction is an efficient way to characterize the electronic ground stateand excited state of these molecules,especially for organic optoelectronic molecules that inherently rely on this light–induced process for their functionality.With the increasing commercial availability of ultrashort pulse lasers,its emergence has subverted scientific research in various fields such as chemistry,physics,biology,etc.,and finally provides possible feasibility for interdisciplinary scientific research.Ultrafast spectroscopy techniques resolve the complexity of photoinduced processes in a unique way,helping to analyze processes such as exciton delocalization,charge transfer,and energy transfer.The study of organic molecules usually begins in solution to observe intramolecular transfer processes in monodispersed system.Systematic studies from solution to thin films can be used to describe the working mechanisms of these organic materials in device applications.Pump–probe techniques using femtosecond lasers and nonlinear optics can provide sufficient temporal resolution to track the radiative or nonradiative decay of excited state dynamics.Combining steady–state and ultrafast spectroscopy techniques provide insight into light–induced processes in organic molecules to elucidate intramolecular interactions in solution in monodispersed system and intermolecular interactions in thin films in aggregated system.These findings help bridge the gap between spectroscopic and materials science research,while fostering interdisciplinary collaboration and exchange.In this paper,steady–state absorption and photoluminescence spectroscopy combined with time–correlated single-photon counting technology,aperture Z–scan technology,and femtosecond time–resolved pump–probe spectroscopy is used.The photophysical properties of donor–acceptor organic optoelectronic molecules are studied from monodispersed solutions and aggregated films.Two kinds of organic conjugated molecules with the same triphenylamine electron–donor unit and different electron–acceptor units are investigated in detail,respectively.The main contents of this article are as follows:In Chapter 3,the intramolecular charge transfer process in BTTM molecule with triphenylamine as electron donor unit,benzothiazole as electron acceptor unit and vinyl group as spacer in monodisperse system has been studied.From the solvent polarity–dependent steady–state absorption and photoluminescence spectra,it can be determined that the organic molecule has intramolecular charge transfer properties,and the dipole–dipole interaction increases with the solvent polarity.The photoluminescence dynamics measured by the time–correlated single photon counting technique show that in the low–polarity solvent exhibited a single e–index relaxation behavior,which may be caused by the excited state in the low–polarity solvent being hybridized state of localized excited(LE)state and charge transfer(CT)state(HLCT state).Combined with the global fitting of singular value decomposition to analyze the femtosecond pump–probe spectra,it is considered that the HLCT state dominated by the LE state is the lowest excited state in the low–polarity solvent,and the energy level of the CT state decreases gradually with the increase of solvent polarity.The results of pump–dump/push–probe experiments with secondary perturbation pulses confirm the existence of conformationally stable HLCT state during the relaxation of excited states in medium and high polarity solvents due to the existence of solute–solvent interactions.In Chapter 4,the intermolecular interactions and exciton dynamics of aggregated BTTM films are analyzed through the tests of temperature–dependent steady–state spectroscopy,time-correlated single-photon counting techniques,and temperature–dependent femtosecond pump–probe experiments.When the temperature decreases from 360 K to 77 K,the average lifetime and fluorescence quantum yield of the aggregated films increases,while the radiative lifetime decreases.Femtosecond pump–probe spectroscopy data indicate that strong intermolecular interactions in aggregated system replace intramolecular charge transfer in monodispersed system,and the exciton–exciton annihilation phenomenon occurs during the exciton relaxation process.In Chapter 5,the linear and nonlinear optical properties of the TOND molecule with benzothiazole and triphenylamine units in monodispersed system are investigated detail by using steady–state and transient spectroscopy techniques combining Z–scan,two–photon fluorescence,and femtosecond pump–probe spectroscopy.Steady–state spectra show that in the monodispersed system,the generation of absorption band is attributed to the transition of the LE state,and the the red-shift of the fluorescence peak with the increase of solvent polarity is due to the transition of the electronic structure from the LE state to the CT state.Due to the strong dipole coupling associated with the intramolecular charge transfer properties,TOND molecules exhibit obvious two–photon fluorescence properties,and the two–photon absorption cross–section at 800 nm increases with increasing solvent polarity.The photo–excited relaxation behaviors of TOND molecules in monodispersed system are investigated by femtosecond pump–probe spectroscopy and pump–dump–probe techniques,the results confirme that only LE state–dominated HLCT exists in low–polarity solvent,and the solute–solvent interaction increases with the increase of the solvent polarity.After the equilibrium is broken by the dump pulse,the process of restoring the equilibrium between the solute and solvent molecules can be analyzed.In Chapter 6,the photoluminescence mechanism and radiation properties of TOND thin film in aggregated state are comprehensively analyzed by using temperature–dependent steady–state absorption spectra,photoluminescence spectra combined with time–dependent single–photon counting techniques.From the temperature–dependent steady–state and transient spectra of aggregated TOND films,it can be seen that when the temperature decreases from 295 K to 77 K,the photoluminescence intensity is significantly enhanced,the fluorescence quantum yield increases nearly tenfold,and the average lifetime increases by about 2.5 times.Since most of the nonradiative processes are inactive at low temperature,the probability of thermal excitation increases with increasing temperature,but the the effect of nonradiative processes is more obvious.The color coordinates of the organic green light–emitting device based on TOND molecules are(0.47,0.51),which means that it may have the potential to combine with blue light–emitting chips to make white light–emitting diodes.These research results can help people understand the energy transfer and charge transfer process of organic optoelectronic molecules,which is beneficial to the design and realization of high–performance organic optoelectronic devices.