Study Of The Photophysical Properties Of Aryl-imide Triplet Photosensitizers

Heavy atom free photosensitizers have the advantages of high singlet oxygen quantum yield,long triplet excited state lifetime,low cost and low toxicity,which are widely used in photoredox catalytic synthetic organic reactions,photovoltaics,photodynamic therapy,and triplet-triplet annihilation upconversion.The molecular structures of compounds showing spin orbit charge transfer intersystem crossing（SOCT-ISC）are simple,and they are easy to be synthesized,which is the focus of current research.However,there are still some challenges for the investigation of SOCT-ISC,for instance,the chromophores used for SOCT-ISC are limited,and the relationship between molecular structure and the efficiency of SOCT-ISC is not fully studied.In this thesis,we designed and synthesized a new compact electron donor-acceptor dyad perylenemonoimide-phenoxazine（PMI-PXZ）,with perylenemonoimide（PMI）as the electron acceptor and phenoxazine（PXZ）as the electron donor,and then studied the photophysical properties with steady and transient spectra.The dyad shows a weak charge transfer absorption band,efficient ISC（singlet oxygen quantum yield:75.7%in n-hexane）and a long-lived triplet state（lifetime:182μs in n-hexane）.Ultrafast charge separation（ca.0.14 ps）and slow charge recombination（1.4 ns）were observed with femtosecond transient absorption spectra.With the time-resolved electron paramagnetic resonance（TREPR）spectroscopy,we confirmed the ISC mechanism as SOCT-ISC.The electron spin polarization（ESP）phase pattern of the PMI-PXZ triplet state TREPR spectrum is（e,e,a,e,a,a）,which is drastically different from the previously reported analogues dyad perylenemonoimide-phenothiazine（PMI-PTZ,a,e,a,e,a,e）,indicating that ESP pattern of the triplet state of a specific chromophore in the electron donor-acceptor dyads is not only dependent on the mutual orientation of the donor and acceptor,but also dependent on the structure of the electron donor.The ESP can be influenced by even one atom variation in the donor structure.We designed three electron donor-acceptor-acceptor triads（NI-NI-PTZ,NO2-NI-NI-PTZ and NDI-NI-PTZ）with phenothiazine（PTZ）as electron donor,naphthalimide（NI）as the bridge acceptor,and NI,4-nitro-naphthalimide（NO2-NI）or naphthalenediimide（NDI）as the terminal acceptor,inspired by the previously reported NI-PTZ dyad,which has a long-lived charge transfer（CT）state and thermally-activated delayed fluorescence（TADF）.For these triads,red TADF were observed in non-polar solvent,with a long lifetime（0.56-1.27μs）.In nanosecond transient absorption spectra,the charge transfer states were observed for NI-NI-PTZ,with a long lifetime of 0.71μs for NI-NI^-·-PTZ^+·in cyclohexane.The types of charge transfer states formed in the triads were changed by regulating the polarity of solvents.For NO2-NI-NI-PTZ,CT state NO₂-NI-NI^-·-PTZ^+·with a lifetime of 0.57μs was observed in cyclohexane,but the CT state[NO₂-NI]^-·-NI-PTZ^+·with a lifetime of 1.08μs was observed in acetonitrile.For NDI-NI-PTZ,both NDI-NI^-·-PTZ^+·and NDI^-·-NI-PTZ^+·were observed in cyclohexane with a long lifetime of 1.95μs,but only NDI^-·-NI-PTZ^+·with a lifetime of 1.39μs was observed in acetonitrile.Through comparing the energies of ¹CT,³CT and ³LE state of these compounds,we propose the ³CT and ³LE states are both involved in the TADF processes.Long-lived charge transfer states of electron donor/acceptor triads were obtained successfully in solution,which are useful for developing organic compounds with simple structure showing efficient intersystem crossing,and control of charge transfer state with polarity of solvents,as well as designing novel TADF materials.