Photophysical And Photochemical Reaction Kinetics Of Thermally Activated Delayed Fluorescent Molecules

Thermally activated delayed fluorescence（TADF）emitters are a new type of organic electroluminescence materials,which can capture triplet excitons for luminescence without using precious metals.Since the high-performance TADF organic light emitting diode（OLED）device was reported in 2012,a widespread concern over organic light emitting materials with TADF properties in academia has increased the exposure of various methods in designing and synthesizing TADF molecules,which then were used in high-efficiency OLED devices.In addition,TADF materials have shown potential for initial applications in triplet-triplet annihilation（TTA）upconversion and photocatalysis.In fact,the most critical factor for organic molecules to achieve the TADF is the very small energy gap between the lowest single state（S1）and the lowest triplet state（T---₁）（AES-T）.And this value is hard to be measured accurately via experiments.Besides,it is generally acknowledged that solvents have a great influence on the energy level,photophysical properties and decay rate of excited state of solute molecules.As a result,solvents may affect the whole photochemical reaction process and even the final product.Therefore,studying the solvent effect of TADF molecules is of great importance for an in-depth understanding of the relevant kinetic processes.At present,there are only few studies on the solvent effects of TADF molecules,and inadequate researches on the kinetics of TTA upconversion using TADF molecules as triplet photosensitizer.To solve the above problems,we established the physical model of TADF molecule based on Jablonski diagram,and proposed a new,simple and efficient method for measuring ΔE_S-T.The solvent effects of TADF molecules 1,3-bis（4-（9,9-dimethylacridin-10（9H）-yl）-phenyl）-3-hydroxyprop-2-en-1-one （DMACPDO）and 1,3-bis（4-（1 OH-phenoxazin-10-yl）phenyl）-3-hydroxyprop-2-en-l-one（PXZPDO）were studied by transient fluorescence spectroscopy and kinetic measurements,combined with steady-state UV-Vis absorption spectroscopy,fluorescence emission spectroscopy and nanosecond transient absorption spectroscopy,and other experimental means and theoretical calculations.Finally,DMACPDO was applied to TTA upconversion as an organic triplet photosensitizer,and the kinetics of TTA upconversion was studied.The details are as follows:（1）Building a physical model of thermally activated delayed fluorescence moleculesIn this chapter,a physical model is established based on the Jablonski diagram of the TADF molecule to describe the corresponding singlet-triplet coupling system,and a new,simple and efficient method for measuring ΔE_S-T is proposed accordingly.By fitting the observed fluorescence kinetics of TADF molecules,the intersystem crossing（ISC）and reverse intersystem crossing（RISC）rates,which are difficult to measured in experiments,can be extracted from it,and then the value of ΔE_S-T can be calculated.The validity of the method was verified in the classical TADF molecule 4CZIPN,as well as in the DMACPDO and PXZPDO mentioned above.（2）Solvent effects on photophysical processes of DMACPDO and PXZPDOThe effects of solvents on the geometric structures and photophysical properties of DMACPDO and PXZPDO molecules are studied by using theoretical calculations,steady-state spectra and transient spectra.It was found that the change of solvent polarity had a slight effect on the S0,S1 and T---₁ molecular geometric configuration and absorption spectra of DMACPDO and PXZPDO molecules,and a significant effect on the fluorescence spectra.Using our self-established transient fluorescence spectroscopy method,we obtained the ΔE_S-T values in different solvents and the decay rate of each channel in the excited state.The results show that the change of solvent polarity has a great influence on the decay rates of excited states of both molecules.When the solvent polarity increase,The ΔE_S-T of DMACPDO will decreases.Instead,the rate of RISC will increases gradually,which leads to the shortening of the lifetime and the increase of the intensity of the delayed fluorescence（DF）component.However,the lifetime of DF component in PXZPDO decreases with the increase of solvent polarity,while the intensity also decreases.After the study,we found that this anomaly stems from the difference in T---₁ state decay channels.In PXZPDO molecules,the non-radiative decay rate of T---₁ state is much higher than that of RISC rate,and the non-radiative decay rate is positively correlated with the solvent polarity.In contrast,in DMACPDO molecules,the non-radiative decay rate of T---₁ state is comparable to the RISC rate,and the RISC rate is positively correlated with the solvent polarity.Therefore,it is important to study the solvent effect of DMACPDO and PXZPDO molecules for their application in solution phase.（3）DMACPDO as a triplet photosensitizer for TTA upconversionTADF molecules have great potential for practical applications as triplet photosensitizers,owing to their long triplet lifetimes,almost negligible energy loss in intersystem crossing,and excellent light-harvesting ability.Recently,several TTA upconversion systems using TADF photosensitizer have been reported.However,there are only few studies on the kinetics of TTA upconversion process using TADF molecules as triplet photosensitizer.We used the recently reported TADF molecule DMACPDO as a triplet photosensitizer and then selected 9,10-diphenylanthracene（DPA）as an annihilator.The upconversion quantum yield of the new TTA upconversion system was determined to be 22.3%in toluene,21.9%in benzene,and 8.1%in chlorobenzene,while no upconversion fluorescence was observed in tetrachloromethane.To uncover the root cause of this solvent effect,we investigated every fundamental photophysical and photochemical process involved in this system using experimental methods such as transient absorption and fluorescence spectroscopy.Based on the rates derived from these kinetics and the second part of the work,the quantum yields of ISC,triplet-triplet energy transfer from photosensitizer to annihilator,and fluorescence emission of DPA were calculated and compared.The fluorescence quantum yield of DPA itself,as well as solvent polarity and viscosity,are confirmed to have significant influences in the title system.This complex effect suggests that we need to consider all factors together when applying TADF molecules as triplet photosensitizer in TTA upconversion.