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

Abstract

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 +) 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 (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 + ( ϕ ion) was 0.005–0.11. Little or no relation between ϕ 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, ϕ ion increased with the increase of the fluorescence lifetime ( τ f) for all S. Interestingly, relatively large ϕ ion was observed for 17– 21 even though they have relatively short τ 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.