Intramolecular charge transfer in photoexcited hydroxyterphenyls: Evidence for formation of terphenyl quinone methides in aqueous solution

The photochemistry of a number of p-terphenyls substituted with hydroxy and hydroxymethyl substituents at the terminal benzene rings has been studied in aqueous solution. Previous work has shown that a simple hydroxyl group can strongly activate appropriately substituted benzenes or biphenyls towards dehydroxylation, via its highly electron-donating nature upon deprotonation in S1. The intermediates formed are the corresponding quinone methides (QMs) or biphenyl quinone methides (BQM). We show in this work that the methodology also works for appropriately substituted p-terphenyls. Thus, a number of p-terphenyls with the appropriate substituents at the terminal benzene rings were synthesized via consecutive Suzuki-type coupling reactions from commonly available starting materials. Although these terphenyls are much less soluble than their simpler biphenyl analogs, they were sufficiently soluble in aqueous CH3CN or CH3OH for their photosolvolytic reactivity to be studied by product studies and fluorescence measurements which are consistent with formation of terphenyl quinone methides (TQMs) as intermediates. Their quantum yields of formation are attenuated compared to the corresponding biphenyl and benzene analogs and one possible reason is the highly fluorescent nature of both phenol and phenolate forms.