INVESTIGATIVE PHOTOCHEMISTRY: ELUCIDATING THE MECHANISM OF THE PHOTO-FAVORSKII REARRANGEMENT

Abstract

The p-hydroxyphenacyl chromophore (pHP) is a versatile phototrigger for several biological nucleofuges, such as gamma-aminobutyric acid (GABA). The photochemistry is trenchant; irradiation discharges the nucleofuge and induces rearrangement of pHP, analogous to the ground state Favorskii rearrangement. Structure-reactivity relationships were explored with pHP GABA's. Electron donating groups lowered the release efficiency, Φdis, relative to the unsubstituted analog. Electron withdrawing carbonyl groups attenuated Φdis. Fluoro moieties modulated the pKa of pHP; protonated pHP manifested higher Φdis than their conjugate base counterparts. The reactive excited states were determined as triplets by Stern-Volmer quenching and laser flash photolysis studies. The triplet lifetimes were ~ 10-9 s with release rate constants of 107 to 108 s-1. Auxochrome substitutions lengthened the absorption wavelength of pHP but lowered Φdis. Added perchlorate salts markedly improved Φdis for pHP GABA's, imputing intermediate ion-radical pairs. Comprehensive photo-Favorskii mechanisms were posited for protonated and conjugate base forms of pHP GABA.