Transient Absorption and Two-Step Laser-Induced Fluorescence Studies of Excited States of Substituted Diphenylcarbenes

Article abstract of DOI:10.1021/j100339a039

Several para-substituted diphenylcarbenes were generated from the corresponding diphenyldiazomethanes at room temperature and 77 K.The transient absorption (T_n <- T₁ and T_n <- T₀) and fluorescence (T₁ -> T₀) spectra of the triplet states of a series of para-substituted diphenylcarbenes were obtained by laser flash photolysis and two-step laser-induced fluorescence (TS-LIF) spectroscopy at room temperature.A significant fluorescence spectral shift of these excited triplet states diphenylcarbenes between TS-LIF at room temperature and fluorescence at 77 K was observed.No spectral difference was detected in a structurally rigid carbene, 10,11-dihydrodibenzocycloheptenylidene.The facts are discussed in terms of a temperature dependence of the rotation of phenyl rings around a C-C<*> bond and/or of a bending motion of a C-C<*>-C bond in the fluorescent state of the diphenylcarbenes.Very small heavy-atom and deuterium isotope effects on the fluorescence (T₁ -> T₀) lifetimes of these diphenylcarbenes were observed.The data suggests that there is no S_n state of diphenylcarbene involved in the T₁ relaxation process.