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

The resonant two-photon ionization (TPI) of trans-stilbene and substituted trans-stilbenes (S) in acetonitrile was studied by laser flash photolysis using a Nd3+:YAG laser (266- or 355-nm). The transient absorption spectra of S radical cations (S+) with a peak around 450–540 nm were observed. Formation of S+ can be explained by two-step two-photon excitation from the ground state (S0) to the lowest singlet excited state (S1) and from the S1 to the higher singlet excited state (Sn), from which ionization occurs. The formation quantum yield of S+ (ϕion) was 0.005–0.11. Little or no relation between ϕion and Eox 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, ϕion increased with the increase of the fluorescence lifetime (τf) for all S. Interestingly, relatively large ϕion was observed for 17–21 even though they have relatively short τf among those of S. It is suggested that the CT electronic character of the S1 state and, therefore, the large molar absorption coefficient are responsible for the efficient TPI of 17–21.