Time-resolved resonance Raman spectrum of chrysene in the S1 and T1 states

Abstract

The time-resolved Sn–S1 and Tn–T1 resonance Raman spectra of highly purified chrysene in tetrahydrofuran were observed at room temperature using the third harmonic of a Q-switched Nd:YAG laser as an electronic excitation pulse and a dye laser pumped by the second harmonic of another Q-switched Nd:YAG laser as a Raman probe pulse. We observed the Raman lines attributed to the S1 state at 1549, 1369, 1344, 1110, and 990 cm−1, and to the T1 state at 1528, 1494, 1353, 1199, 1110, 990, 678, 554, 472, and 286 cm−1. The decay time of 1369 cm−1 (S1) line, the formation time of 1528 cm−1 (T1) line, that of 1199 cm−1 (T1) line, and the fluorescence lifetime were obtained to be 35+6−5, 51+19−12, 41+10−7, and 41+− 1 ns, respectively. The observed Sn–S1 and Tn–T1 resonance Raman spectra were compared with the previously reported fluorescence and phosphorescence spectra in Shpolskii matrices, Sn–S1 and Tn–T1 resonance CARS spectra, and Tn–T1 spontaneous resonance Raman spectra. Discussion was made on the geometrical changes in the excited states on the assumption that the changes were not very large for the rigid molecular structure. We came to the following conclusions: (1) The resonance-coupled excited singlet state is different from the ground state in geometry mainly along the normal coordinates of the vibrational modes which have the Sn–S1 resonance Raman peaks at 1549 and 1344 cm−1. (2) The geometry of the resonance-coupled excited triplet state differs from that of the ground state mainly along five coordinates. Four of them correspond to the vibrational bands observed in the Tn–T1 resonance Raman spectrum at 1528, 1353, 1110, and 990 cm−1. The other corresponds to the vibrational bands observed in the Shpolskii spectrum of phosphorescence at 1385 cm−1 in literature. (3) The S1 and T1 states differ along the normal coordinates of the modes which have the Tn–T1 resonance Raman bands at 678, 554, 472, and 286 cm−1.