Homoconjugation in triptycenes: an inquiry through photochromism

Abstract

The spatial disposition of three benzene rings in triptycene permits through-space overlap of the orbitals of fused benzene sp² carbons, leading to ‘homoconjugation’; the latter is a contentious issue in the literature. To inquire into its existence, we have designed and synthesized a diagnostic photochromic molecular system, i.e., triptycene annulated with diphenylpyran (Trip-chrom). Persistence of the photogenerated colored o-quinonoid intermediate, generated by photoirradiation of Trip-chrom for a few min at r.t. lends credence to the operation of homoconjugation.