Time-resolved resonance Raman study of 4-phenylpyridine

Abstract

The triplet-triplet absorption spectrum of 4-phenylpyridine has been measured and shown to have a maximum at 365 nm with a lifetime of 1.5 μs. TR 3 data for this molecule in the triplet state indicates increasing planarity, relative to its 40° ground state geometry, reflected in the shift of the aromatic CC vibrational frequency from 1612 cm −1 in the ground state to 1578 cm −1 in the triplet state. Potential energy surface calculations confirm the quasi-planar nature of the triplet state and the shortening of the inter-ring bond distance in going from the ground state to the lowest triplet. Normal mode calculations identify the specific A-mode vibration, which accounts for the observed enhanced resonance Raman band of the triplet. A single exponential fluorescence decay process is observed only in aqueous solutions, indicating a strongly hydrogen bonded system in the ground state, which facilitates the photoinduced proton transfer upon excitation into the excited singlet state.