Study On Intersystem Crossing Of Electron Donor-Acceptor Dyads With Long-lived Excited State

Electron donor-acceptor dyads showing spin-orbit charge transfer intersystem crossing(SOCT ISC)mechanism are novel heavy-atom free triplet photosensitizers,with good properties of simple synthesis,low biotoxicity and long-lived triplet state,which can be potentially used in photon upconversion,photocatalysis,artificial photosynthesis,photodynamic therapy fields,and so on.However,there are only few studies on the photophysical properties of this kind of photosensitizer.Thus,it is still a major challenge to design new electron donor-acceptor triplet photosensitizers showing efficient SOCT ISC.Here,we designed a series of new electron donor-acceptor dyads,and studied the excited state properties with steady-state/time-resolved transient spectroscopy,time-resolved electron paramagnetic resonance(TREPR)spectroscopy,electrochemical study,density functional theory calculation,and so on.We also explored the potential applications in triplet-triplet annihilation upconversion(TTA UC)and photodynamic therapy(PDT).First,a series of compact dyads were directly connected with the tw o units in different orientation,or by different substitution sites,showing absorption in UV light and green light range.After the rapid charge separation(on picosecond scale),SOCT ISC occurred during the slow charge recombination(on nanosecond scale).The TREPR spectra indicated that the electron spin polarization patterns were not only due to the conformation,but also the structures of the electron donor.Besides,both the orientations and geometry restrictions between the two units could affect the SOCT ISC efficiency.Based on the above studies,we designed D-A dyads with red light range absorption,by derivatization or conjugation expansion on the 3,5 positions of Bodipy chromophore,which showed efficient SOCT ISC and long-lived triplet states(the lifetime was extended by 168 times compared to the reference heavy-atom triplet photosensitizer);we also used electron spin control method to introduce electron donor/acceptor with strong electron donating/accepting capacities in compact dyads,to attain long-lived triplet charge transfer states(?T was 8.0 ?s).Theses triplet photosensitizers could be used in TTA UC with a high upconversion quantum yield of 15.8%;for in vitro PDT,these photosensitizers could kill tumor cells with high photo toxicity(IC50 is 0.1 ?M)and low dark toxicity(IC50>70 ?M);in hypoxia condition.singlet oxygen production was still efficient(?? is 19%,while ?? is almost 0 for the reference heavy-atom triplet photosensitizer),since the triplet state lifetime was prolonged.