Ultrafast bimolecular radical reaction between S₁p‐terphenyl and carbon tetrachloride: mode‐specific acceleration of vibrational dephasing in reactant molecule

Ultrafast bimolecular radical reaction proceeds between photoexcited p ‐terphenyl and carbon tetrachloride. The lifetime of the first excited singlet (S 1 ) state of p ‐terphenyl in carbon tetrachloride measured with picosecond time‐resolved fluorescence spectroscopy is 5.6 ps, shorter than the typical lifetime in ordinary solvents, 0.95–2.8 ns, by a factor of 170 or more. Time‐resolved infrared spectroscopy reveals the generation of the trichloromethyl (CCl 3 ) radical as a reaction intermediate. The decay kinetics of the CCl 3 radical indicates that the CCl 3 radical and the p ‐terphenyl–Cl radical adduct, both of which are produced simultaneously by a radical reaction between S 1 p ‐terphenyl and carbon tetrachloride, recombine to form the product. The picosecond time‐resolved Raman spectrum of the reactant S 1 p ‐terphenyl shows that the dephasing process for the four vibrational modes at 1640, 1497, 1180, and 1017 cm −1 is selectively accelerated in carbon tetrachloride. The four vibrational modes probably have a large contribution from the motion of a specific atom or atoms where the intermolecular interaction that induces the bimolecular reaction is present.