Synthesis And Photophysical Properties Investigation Of Bodipy-based Polyiodinated And Dimeric Derivative Photosensitizer

In recent years,due to the excellent characteristics of green,heighta-djustable,and high safety of light energy,photochemistry has attracted extensive attention from researchers in academia and industry in the context of the energy transition and energy revolution.The research on organic triplet photosensitizers,as an important development direction of modern molecular organic photochemistry,has made a lot of progress and achievements in various frontier fields such as material science,life science,synthetic science,environmental science,energy science,and communication science.In particular,for BODIPY,because of its strong visible light absorption ability and easy modification,occupies an important position among organic dyes and becomes an excellent candidate building block and one of the most interesting research objects.The variety of BODIPY-based photosensitizers has been greatly enriched by the increasing demand and high quality of practical applications.However,studies targeting their photophysical processes,especially the mechanistic studies related to the triplet excited states,still need to be further supplemented.Here,we have used iodine atoms and benzenetetetrol,respectively,for the modification of BODIPY,and studied them by various steady-state spectroscopy,nanosecond and femtosecond ultrafast spectroscopy,and electron paramagnetic resonance spectroscopy,in order to providing more theoretical basis for the rational design of iodine-substituted and dimeric BODIPY derivatives.The main research contents of this thesis are:（1）Iodine substitution is a classical and reliable method in enhancing the efficiency of intersystem crossing.However,there is still space for research and improvement,especially for BODIPY,and this work has made new discoveries in multi-iodine substitution.A series of BODIPY derivatives with different degrees of iodination BDP,I₁-BDP,I₂-BDP and I₄-BDP（and their corresponding reference mBDP,I₁-mBDP and I₂-mBDP）were prepared.Characterized by steady-state spectroscopic,the fluorescence quantum yields of BDP,I₁-BDP,I₂-BDP and I₄-BDP are 3.43%,0.43%,0.43%and 3.23%respectively,and singlet oxygen yields are 0%,81%,89%and 94%respectively.Both are enhanced on I₄-BDP,which is a non-negligible contradiction,since the accelerated rate of intersystem crossing mediated by heavy atomic effects often leads to further quenching of fluorescence.This leads us to speculate that a decrease in the intersystem crossing rate and an increase in the singlet oxygen yield occur for another reason.Femtosecond transient absorption spectroscopy provides direct evidence for the slowing down of the I₄-BDP intersystem crossing rate(τ_ISC=350 ps).In contrast,calculations based on the intrinsic triplet state lifetimes from nanosecond transient absorption measurement and the first single excited state internal conversion quantum yields demonstrate that the internal conversion rates of the lowest excited states are both limited after tetraiodine substitution.In short,the intersystem crossing rate slows down while it is still competitive for the more dramatic decrease in the internal conversion rate,leading to a continued increase in the singlet state oxygen yield.Finally,theoretical calculations are used to extend the understanding of the intersystem crossing process by iodine atoms,and it is found that tetraiodine substitution enhances the spin-orbit coupling but reduces the effective intersystem crossing channel,which eventually leads to the slowdown of the intersystem crossing rate.（2）Heavy atom-free triplet photosensitizer is more conducive to the acquisition of properties such as low dark toxicity,high photostability and long triplet lifetime.At present,one of the key points of the development of pure organic triplet photosensitizers is the molecular design based on charge recombination induced intersystem crossing.Dimers also belong to this kind of idea,but recently received attention,there is still huge development space left.The dimer BODIPY derivatives mBDP₂-Ph,mBDP₂-An（and their corresponding parent mBDP-Ph,mBDP-An）based on 1,2,4,5-phenyltetraphenol bridging were prepared.The molecular structure and configuration of mBDP₂-Ph were determined by single crystal analysis.Characterized by steady-state spectroscopic,mBDP₂-Ph and mBDP₂-An have negligible fluorescence quantum yields and singlet state oxygen yields.Nanosecond transient absorption spectra also have difficulty in capturing good signals（τ_T<25μs）.We speculate that the overly compact and rigid orthorhombic configuration leads to an overly strong charge transfer effect,making the triple excited state energy and its efficiency decreased.However,after adjusting the excitation wavelength from around 500 nm to around 350 nm,we found a gradual recovery of the dimeric BODIPY derivative fluorescence（from 0%to 2%with UV irradiation for five minutes）with the appearance of an ultra-long radical signal（τ_T>3 ms）.And combined with the electron paramagnetic resonance spectra further confirmed the radical(BDP^·+)generation.Finally,the influence of electronic structure on photophysical properties is studied based on DFT and TD-DFT.