Time-resolved infrared spectra and structures of the excited singlet and triplet states of fluorenone

Nanosecond time-resolved infrared spectroscopy has been used to study the excited electronic states of fluorenone in polar and nonpolar solvents. By using the singular-value-decomposition analysis of the observed time-resolved spectra, infrared spectra of fluorenone in the lowest excited singlet (S1) and the lowest excited triplet (T1) states have been obtained for a polar solvent, as well as that in the T1 state in a nonpolar solvent. The assignments of the CO stretching modes for the S0, S1 and T1 states have been made on the basis of isotopic frequency shifts. The CO stretch frequency of fluorenone in the S1 state in a polar solvent and those in the T1 states in polar and nonpolar solvents are much lower than that of the ground state, but are still located in the double-bond frequency region. These findings indicate that these excited states are the same in character (ππ∗), but differ in the degree of electron delocalization between the benzene rings and the CO bond.