Styrene and phenylacetylene: Electronic effects of conjugating substituents “off” and “on” the axis of a benzene ring

Abstract

Rotationally resolved fluorescence excitation spectra of several vibronic bands in the S1←S0 electronic transitions of styrene (STY) and phenylacetylene (PA) have been obtained. Confirming earlier low resolution results, we find that the origin band of PA is a b-type band but that the corresponding band of STY is an a-type band, showing that the S1 state of PA is Lb1 in character (like that of most other monosubstituted benzenes) but that the corresponding state of STY is La1. The observed changes in the rotational constants of PA and STY that occur when the photon is absorbed are consistent with these assignments. Reversal in the electronic character of the S1 state in STY is attributed to the presence of the “off-axis” conjugating –CH=CH2 group, a suggestion that is supported by the observed polarizations of higher vibronic bands in both molecules.