Important factors for the formation of radical cation of stilbene and substituted stilbenes during resonant two-photon ionization with a 266-or 355-nm laser

摘要

The resonant two-photon ionization(TPI)of trans-stilbene and substituted trans-stilbenes(S)in acetonitrile was studied by laser flash photolysis using a Nd~(3+):YAG laser(266-or 355-nm).The transient absorption spectra of S radical cations(S~(centre dot +))with a peak around 450-540 nm were observed.Formation of S~(centre dot +)can be explained by two-step two-photon excitation from the ground state(S_0)to the lowest singlet excited state(S_1)and from the S_1 to the higher singlet excited state(S_n),from which ionization occurs.The formation quantum yield of S~(centre dot +)(phi_(ion))was 0.005-0.11.Little or no relation between phi_(ion)and E_(OX)was observed for non-substituted(1),mono-p-substituted(2-7)and di-p-substituted(8-11),methoxy-substituted(12-16),and donor-acceptor-p-substituted trans-stilbenes(17-21).On the other hand,phi_(ion)increased with the increase of the fluorescence lifetime(tau_f)for all S.Interestingly,relatively large phi_(ion)was observed for 17-21 even though they have relatively short tau_f among those of S.It is suggested that the CT electronic character of the S_1 state and,therefore,the large molar absorption coefficient are responsible for the efficient TPI of 17-21.