Vapor-phase fluorescence spectra from the second excited singlet state of pyrene and its derivatives

Abstract

Vapor-phase fluorescence spectra have been measured for pyrene and its simple derivatives, i.e., pyrene- d 10, 1-methylpyrene, and 4-methylpyrene. Each of these derivatives shows a weak fluorescence emission which is similar to the fluorescence from the second excited singlet state (S 2) of pyrene and is to be assigned to the S 2-fluorescence. The methyl substitution causes frequency shifts (Δ ν) in both absorption and fluorescence, and the Δ ν values for absorption and fluorescence transitions that are associated with the same excited state, i.e., the first excited singlet state (S 1) or the second excited singlet state (S 2), are approximately equal to each other. When the excitation energy increases, the S 2-fluorescence shifts gradually to the red in almost the same way as the S 1-fluorescence. The S 2-fluorescence spectrum has a sort of mirror-image relation to the S 2-absorption. A comparison of S 2-emissions of pyrene and pyrene- d 10 suggests that the ratio between the quantum yields of S 2- and S 1-fluorescence may be related to a ratio ϱ 2 ϱ 1 , where ϱ 1 and ϱ 2 are the densities of vibrational states in S 1 and S 2 at the energy of excitation.