Discrimination of Rotational Isomers of Jet-Cooled Acetophenone Derivatives as Studied by Sensitized Phosphorescence Excitation Spectroscopy

Abstract

The S₁(n,π^*)←S₀ and T(n,π^*)←S₀ sensitized phosphorescence excitation spectra of jet-cooled o- and m-substituted (fluoro- and methyl-) acetophenones have been measured. The two rotational isomers (O-cis and O-trans isomers) of each molecule arising from the orientation of the acetyl group with respect to o- or m-substituent have been discriminated. The energy difference in the 0–0 transition between the two isomers is less than 100 cm⁻¹ for m-substituted acetophenone but more than 300 cm⁻¹ for o-substituted acetophenone. The characteristic energy difference can be explained from the fact that the electronic excitation is localized on the C=O group.