Synthesis And Photophysical Study Of The Naphthalimide-Derived Triplet Photosensitizers

Organic triplet-state photosensitizers have shown unique advantages in the fields of triplet-triplet annihilation up-conversion（TTA-UC）,photodynamic therapy（PDT）and photocatalysis.It has been an important topic to design photosensitizer molecules with long triplet state lifetime,strong absorption in the visible region,and structures that can be easily modified and tuned.The heavy-atom-free triplet-state photosensitizer molecular system based on the spin-orbit charge transfer intersystem crossing（SOCT-ISC）mechanism,because of its simple and easily modifiable molecular structure,has become the focus of recent investigations.Therefore,in this paper,based on SOCT-ISC,the compact dyads were synthesized in the orthogonal configuration with naphthalimide（NI）chromophores as the electron acceptors.The photophysical properties of the molecules were systematically investigated by steady-state and transient spectroscopy,expecting to obtain triple excited states（³LE and ³CT）with high energy levels and long lifetimes.Firstly,compact dyads were prepared by linking two NI units by C-C bond at the3-position（NI-Dimer-2）or 4-position（NI-Dimer-1）.In this case,the significant charge separation（CS）process occurs only for NI-Dimer-2,with structurally different electron donor（D）and electron acceptor（A）,where D（NI-NH）is NI substituted with an amino group at the4-C position and A is unsubstituted NI.Femtosecond/nanosecond transient absorption（fs/ns-TA）spectra show that the CS process occurs within 11.6 ps,followed by a slow charge recombination（CR,（29）1.5 ns）,which results in a long-lived triplet state（intrinsic triplet lifetime of 206.4μs）,populated on the NI-NH unit.Fs/ns-TA spectroscopy did not give direct evidence of CR-induced ISC.Time-resolved electron paramagnetic resonance（TREPR）spectroscopy,which gives zero-field splitting parameters（|D|=1894 MHz,|E|=111 MHz）and electron spin polarization patterns of（e,e,e,a,a,a）for NI-Dimer-2,which exclude the RP-ISC mechanism and infers SOCT-ISC mechanism.In contrast,NI-Dimer-1,which is structurally identical to D and A（both unsubstituted NI）,does not undergo CS（symmetry breaking charge transfer,SBCT）upon photoexcitation.Electrochemical results show that its higher oxidation potential and lower reduction potential lead to a thermodynamically difficult CS process.Interestingly,the singlet and triplet excited state of the compound is delocalization over the whole molecular due to the introduction of substituents on native NI.These photophysical studies play an important role for the design of triplet photosensitizers with high energy and long lifetime.Secondly,a compact electron donor/acceptor dyad（NI-PTZ）with orthogonal configuration was synthesized by linking the naphthalimide（NI）and phenothiazine（PTZ）moieties,and by sulfidation for NI-PTZ,the redox potential was altered and subsequently the energy of CT state decreases,expecting the delocalization T₁ state of PTZ-NI-2S.Fluorescence spectra and fs-TA spectra show that the compound NI-PTZ undergoes a fast CS process（0.6 ps）in acetonitrile,followed by the ISC within 14.6 ps,and eventually yielding the ³CT state.In contrast,in a non-polar solvent（n-hexane）,the compound undergoes vibrational relaxation,CS and ISC processes upon photoexcitation,eventually producing a mixed ³LE/³CT state.Similarly,the compound PTZ-NI-2S shows the solvent polarity dependence.DFT calculations and ns-TA spectra indicate that PTZ-NI-2S undergoes ISC,with the triplet state populated on the[NI-2S]moiety in non-polar solvents,while possibly delocalized on the whole molecule in a polar solvent.The studies provide important references for the subsequent design and synthesis of organic photosensitizer molecules with high-energy,long-lived CT states based on electron spin-control methods.