The Electronic Spectra of Aromatic Molecules V: The Interaction of Two Conjugated Systems

The electronic spectrum of a composite alternant hydrocarbon R-S is discussed theoretically in relation to the excited electronic states of the separate fragments RH and SH. The lowest excited states of RS are interpreted as arising from two types of excited configuration. In the first type an electron is raised from an occupied orbital of one fragment to an unoccupied orbital of the same fragment, and in the second, an electron is transferred from one fragment to the other. The self-consistent molecular orbital theory is used to evaluate the interactions between the various configurations, and the molecules butadiene, biphenyl and styrene are examined quantitatively. The following results are worthy of mention: (i) In butadiene the near ultra-violet transition arises primarily from an in-phase combination of local excitations in the two double bonds. (ii) In biphenyl a similar situation arises, there being strong configurational interaction in the perpendicular as well as in the planar conformation.