Steps To Demarcate the Effects of Chromophore Aggregation and Planarization in Poly(phenyleneethynylene)s. 1. Rotationally Interrupted Conjugation in the Excited States of 1,4-Bis(phenylethynyl)benzene

A series of photophysical measurements and semiempirical calculations were carried out with 1,4-bis(phenylethynyl)benzene in search of evidence on the effects of phenyl group rotation and chromophore aggregation of oligo- and poly(phenyleneethynylene)s. It is suggested that planarization gives rise to relatively modest shifts of ca. 20-30 nm, which preserve the vibronic structure of the monomer and retain a high emission quantum yield. In contrast, it is proposed that aggregation gives rise to larger shifts and loss of vibronic structure.