Transition moment directions in substituted benzenes and the effect of unsymmetrical substitution on hybrid character

Abstract

Transition moments in symmetrically and unsymmetrically substituted benzenes are determined by the band contour method. This procedure allows us to ascertain the symmetry and nodal properties of the lowest lying π∗ ← π state. Band contour analyses of the 0-0 bands of meta- and ortho-difluorobenzene and o-chlorofluorobenzene are performed. This results in a determination of the changes in rotational constants on excitation and the direction of the transition moment. The difluorobenzenes are found to be A-type bands while the o-chlorofluorobenzene band is a 65% A- 35% B-type hybrid. This allows the o-difluorobenzene transition to be assigned as a 1A1 ← 1A1 and the m-difluorobenzene transition as a 1B2 ← 1A1. It is pointed out that the symmetry designation of the excited states are different but the nodal properties are in fact the same.Styrene is reinvestigated in light of the unusual nature of a 1A1 ← 1A1 electronic transition (when classified according to C2v symmetry). We substantiate this result and compare styrene to other monosubstituted benzenes which all have 1B2 excited states.