Formation efficiency of radical cations of stilbene and methoxy-substituted stilbenes during resonant two-photon ionization with a XeCl excimer laser

Formation efficiency of radical cations of trans-stilbene and the methoxy-substituted stilbenes (S=1–6) during the resonant two-photon ionization (RTPI) in acetonitrile has been studied by laser flash photolysis with a XeCl excimer laser (308 nm, 25 ns). The transient absorption spectra of S radical cations (S+) were observed with a peak around 450–540 nm. The formation quantum yield of S+ (Φion) was 0.06–2.1%. No relation between Φion and Eox of S was observed, although Φion increased with the increasing lifetime of S in the lowest excited singlet state (S1). These results are coincident with a two-step two-photon excitation from the ground state (S0) to S1 and from S1 to higher excited singlet state (Sn), and ionization from Sn. Considerably large Φion was obtained for the RTPI of 3,5-dimethoxystilbene (5) with relatively high oxidation potential and a long S1 lifetime among S. This is explained by the intermediacy of the internal charge-transfer excited state for the RTPI of 5 based on the fluorescence lifetime measurement, solvatochromic measurement, and the dipole moment calculation. The concentration of S+ increased with the increasing laser fluence (F). The log–log plot of [S+] versus F gave a linear line with a slope of approximately 2 for the RTPI of 1–4, while approximately 1 for the RTPI of 5 and 6. The slope of 2 indicates clearly two-photon excitation during the RTPI. The slope of 1 is explained by the photostationary state among S0, S1, and Sn states attained during the laser pulse, particularly for the RTPI of S with a long S1 lifetime.