Synthetic And Photophysical Study Of Novel Perylene/Pyrene-Derived Heavy Atom-Free Triplet Photo Sensitizers

Generation of long-lived triplet excited state of a chromophore is crucial for the triplet photosensitizers(PSs).Regarding this,heavy atom-free triplet PSs have attracted much attention due to their low cost,less toxicity and sufficiently long triplet state lifetime.However,the molecular design strategy is not mature,and it is still a major challenge for chemists to design heavy atom-free triplet PSs with simple molecular structures and long triplet state lifetimes.This thesis focuses on the design of heavy atom-free triplet PSs by utilizing the concept of spin-orbit charge transfer intersystem crossing(SOCT-ISC)and radical enhanced intersystem crossing mechanisms.Intersystem crossing(ISC)involving charge separation(CS)and charge recombination(CR)processes is one of the most important research areas of modern photochemistry.Therefore,a series of compact donor-acceptor(DA)organic molecules based on SOCT-ISC have been prepared.The ISC efficiency and properties of charge transfer(CT)state were studied by several factors such as electronic coupling(the magnitude of matrix element,VDA),chromophore orientation,the solvent polarity effect and distance between donor and acceptor,etc.Moreover,radical enhanced ISC mechanism was employed to explore the electron spin dynamics and prolong the triplet state lifetime.The study will be helpful in the design of novel heavy atom-free triplet PSs.The research outcome is as follows:Perylene chromophore as an electron acceptor was attached with phenothiazine(PTZ)via C-N/C-C linkage to prepare compact D-A dyads.Pery-N-PTZ adapts orthogonal geometry(φ=91.5°)while Pery-C-PTZ has relatively coplanar geometry(φ=60.6°),both showed efficient ISC ability and the singlet oxygen quantum yields(φΔ)were 60%and 50%,respectively.It showed that orthogonal conformation favored SOCT-ISC but not a necessary prerequisite,electronic coupling between the 1CT state and triplet state(Tn)is also important.In line with orthogonal geometry,perylene-phenoxazine(PXZ)/PTZ triads were designed using the concept of symmetry-breaking CT.Compared with corresponding analogue dyads,faster CT kinetics were observed in the triads and Pery-2PTZ triad had longer triplet state lifetime(τt＝359 μs)and higher ISC ability(ΦΔ=70%).The dyads and triads were applied as PSs to induce delayed fluorescence using perylenebisimide(PBI)and Solvent Green-5(SG-5)as acceptors and long luminescence lifetimes were observed(τDF=71～94 μs).In bodipy-derived SOCT-ISC dyads,bodipy can only act as electron acceptor,while the dyads with bodipy as electron donor don’t show SOCT-ISC.Herein,we utilize the versatility of perylene chromophore,a dyad(Pery-NI)with perylene as electron donor and naphthalimide(NI)as electron acceptor was designed.Pery-NI showed high ΦΔ(80%)although the geometry was not exactly orthogonal(ψ=72.5°).Excitation wavelengthdependent photophysical properties were observed.Upon selective excitation into NI unit,there was a competition between Forster resonance energy transfer(FRET)and CS processes,then perylene triplet state was populated.While both perylene triplet state and perylene radical cation were observed when excited into perylene moiety.Pery-NI was used as triplet PSs,long-lived delayed fluorescence of PBI was observed(delayed fluorescence lifetime τDF=57.3 μs).The electron-donating ability of the electron donor or mutual orientation of the chromophores can change the energy level of the CT state in compact D-A systems.Herein,perylene was replaced with pyrene and attached to NI at 4-C and 3-C positions to prepare compact D-A dyads.Steady-state UV-vis absorption spectra showed a broad CT absorption band for dyads.This demonstrated that the strong electronic coupling between the electron donor and acceptor could be obtained by controlling the molecular geometry or rotation around the linker.Furthermore,the ISC ability was moderate to high in different polarity solvents and maximal in polar solvents(ΦΔ=52%),which proved the fluctuation of CT state energy levels with solvent polarity and close-lying 1CT and triplet states produced efficient ISC.Radical enhanced intersystem crossing mechanism was employed to enhance the ISC of perylene,long triplet excited state(τT=9.5 μs)was achieved in contrast to the short triplet state(τT=0.1 μs)for the same chromophore reported in the literature.The spin-spin exchange interaction between radical and chromophore was responsible for triplet generation.The fs-TA study revealed the ultrafast direct ISC(Dn→ D1 transition)within 0.5 ps and interconversion of excited doublet and quartet states in 80 ps.Time-resolved electron paramagnetic resonance(TREPR)spectroscopy unravel the simultaneous formation of quartet and triplet states for Pery-Ver-1.That could be due to different molecular geometries at excited state,which is rare in compact radical-chromophore systems.