Study Of Intersystem Crossing In Orthogonal Naphthalimide Electron Donor-Acceptor Dyad

Heavy atom-free triplet photosensitizers based on charge recombination-induced intersystem crossing(ISC)have attracted much attention in the field of photochemistry and photophysics in recent years because of its long triplet lifetime,easy synthesis,low biological toxicity,and low cost of prearation.However,the factors affecting the ISC efficiency of such triplet photosensitizers are still unclear,the further research is needed.Therefore,in this thesis designed the naphthalimide derivatives(NI/NDI)with different electron donor and acceptor(D-A)dipole moment orientations and different D-A distances were prepared to study the influence on the ISC process of the heavy-atom free triplet photosensitizers.The photophysical properties and the characteristic of electron spin polarization(ESP)of naphthalimide derivatives were studied.First,this paper prepared orthogonal 4-aminonaphthalimide-perylene(NI-Py)dyads in which perylene(Py)as an electron donor and 4-aminonaphthimide(NI)as an electron acceptor.Due to the steric hindrance imposed by the 4-amino substituent on the 4-aminonaphthimide moiety,the molecule has an orthogonal geometric conformation.The photophysical properties of NI-Py were studied by steady-state spectroscopy,femtosecond/nanosecond transient absorption spectroscopy and density functional theory calculations.The results show that the ground state interaction between NI and Py moieties is negligible,but fluorescence quenching of NI-Py in polar solvents indicates that charge separation process can occur(fluorescence quantum yield(Φ_F)in toluene is 61.9%,andΦ_F in methanol is 0.2%).The femtosecond transient absorption spectra in dichloromethane show that the time constant for charge separation is 1.7ps and the time constant for charge recombination is 6.9 ns.At the same time,the generation of Py triplet states was observed,which proves that the charge recombination is very fast,and ISC mechanism is the spin-orbit charge transfer intersystem crossing(SOCT-ISC).The nanosecond transient absorption spectra show that triplet state is localized on the perylene for NI-Py,and the triplet state lifetime(175μs)is much longer than the triplet state produced by the heavy atom effect in 3-bromoperylene(16μs).In dichloromethane,the singlet oxygen quantum yield(ΦΔ)of NI-Py is 10%,which is much lower compared with the orthogonal naphthimide-perylene without amino substituent dyad.The perpendicular conformation is not enough to achieve effecient SOCT-ISC in a compact electron donor-acceptor compound,and the efficiency of ISC may be related to the orientation of the dipole moment of the D-A moieties.Second,based on tuning the distance between D and A by introducing benzene ring as a bridging moiety and changing the orientation of the D-A by using 2 or 9-substituted anthracene,a series of NDI-An derivatives were prepared to further study the effect of molecular structure on ISC processes.In this thesis,steady-state spectroscopy,nanosecond transient absorption spectroscopy,time-resolved electron paramagnetic resonance(TREPR)spectroscopy and theoretical calculations were studied to investigate photophysical process.The nanosecond transient absorption results show that the triplet states of these derivatives are located on anthracene moiety,and the longest lifetime of the triplet state is 321?s.Time-resolved electron paramagnetic resonance spectroscopy shows that the ISC mechanism is SOCT-ISC.These results show that different molecular structures can adjust the energy level of the charge separate state and affect the ISC process.