The S1 ← S0 electronic transitions of 4- and 5-phenyl imidazole in the gas phase

The S1 ← S0 electronic spectrum of 4-phenyl imidazole (4PI), previously studied in the gas phase by Hockridge et al. [Chem. Phys. Lett. 302 (1999) 538], has been re-examined at both low and high resolution using fluorescence excitation and isotopic substitution techniques. The data establish a planar structure in both states; a pure a-type origin band indicates that the S1 state is 1La in character. Analysis of the torsional progressions in the S1 ← S0 electronic spectrum of the 4PI tautomer, 5-phenyl imidazole (5PI), indicates a planar excited state but a non-planar ground state, with a twist angle between the two ring systems of ∼27° and a barrier to planarity of ∼90 cm−1.