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

摘要

Rotationally resolved fluorescence excitation spectra of several vibronic bands in the S_1 implied by S_0 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 S_1 state of PA is ~1L_b in character (like that of most other monosubstituted benzenes) but that the corresponding state of STY is ~1L_a. 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 S_1 state in STY is attributed to the presence of the "off-axis" conjugating -CH = CH_2 group, a suggestion that is supported by the observed polarizations of higher vibronic bands in both molecules.