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

Rotationally resolved fluorescence excitation spectra of several vibronic b ands in the S-1 <-- S-0 electronic transitions of styrene (STY) and phenyla cetylene (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 correspo nding band of STY is an a-type band, showing that the S-1 state of PA is L- 1(b) in character (like that of most other monosubstituted benzenes! but th at the corresponding state of STY is L-1(a). The observed changes in the ro tational 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'' conj ugating - CH=CH2 group, a suggestion that is supported by the observed pola rizations of higher vibronic bands in both molecules. (C) 1999 American Ins titute of Physics. [S0021-9606(99)00240-8].