Time-Resolved Electron Paramagnetic Resonance Study of the First Excited Triplet States of Dibenzocycloheptadienylidene and Diphenylmethylene Derivatives

Abstract

Transient triplet electron paramagnetic resonance (EPR) spectra of dibenzocycloheptadienylidene (DBC) and substituted diphenylmethylenes were observed in a rigid glassy matrix at low temperatures. On excitation with the wavelength of the T−T transition of DBC, polarized EPR spectra due to the both ground (T0) and excited (T1) states were observed at a short delay time (180 ns) after the laser pulse. The T1 components in the spectrum decayed faster than the other and diminished at 1 μs after the laser pulse irradiation. The remaining signals after the decay of the T1 state were unambiguously assigned to the T0 state, taking into account the steady-state (SS) EPR results. From analysis of the spectrum of the short-lived triplet species, the zero-field splitting (ZFS) parameters, |D| = 0.201 and |E| = 0.008 cm-1, were determined for the T1 state directly. The parameters obtained were comparable to those of the T1 state of diphenylmethylene (DPM). The polarization patterns of the transient spectra match well with interpretation as anisotropic intersystem crossing induced by the spin−orbit couplings between the Sn and the triplet states. On the basis of the results of magnetophotoselection experiments, we also confirmed that the principal axes in the T0 and T1 states did not differ significantly from each other. The behavior of the methyl-substituted DPM resembles that of DPM and DBC, indicating that the methyl group does not affect the photophysical and photochemical properties of the T0 and T1 states. Although the SS EPR spectra were observed for the T0 state of substituted DPM derivatives, the chemical properties of the excited states were affected by substitution of electron-donating (OCH3) or -withdrawing (CN) groups.