The Electronic Spectrum of Diphenylhexatriyne. The Interaction of Mutually Perpendicular π and π′ Systems

Abstract
The electronic absorption spectrum of diphenylhexatriyne has been divided into two components polarized along the long and the short molecular axes by means of dichroism analysis using the stretched-polymer-film technique; i.e., a reduced polarization spectrum has been obtained. This reduced polarization spectrum can be explained by a modified PPP calculation including the interaction between mutually perpendicular π and π′ systems in the conjugated system. It has been clarified that the relative intensities of the first and second bands of diphenylhexatriyne reflect the magnitude of interaction between mutually perpendicular π and π′ systems, which is quite great. The magnitude of the electron-repulsion integral between π and π′ electrons is estimated to be about 20–30 per cent of that between ordinary π (or π′) electrons.
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