Direct and water-mediated excited state intramolecular proton transfer (ESIPT) from phenol OH to carbon atoms of extended ortho-substituted biaryl systems

Abstract

In our continuing studies of the generality and mechanistic details of a new type of excited state intramolecular proton transfer (ESIPT), from phenol to adjacent carbon atoms of suitably designed biaryl systems, the photochemical deuterium incorporation at adjacent (proximal) ortho and more distal positions of 2-hydroxy-p-terphenyl (3), 2-hydroxy-o-terphenyl (4), and 2-(hydroxyphenyl)-9H-fluorene (5) has been studied in D₂O–CH₃CN. The observed deuterium exchange in these molecules is entirely consistent with direct and water-mediated (formal) ESIPT processes from the phenol OH to the ring carbons, formally migrating over a range of 2.18–8.12 Å (4–10 bonds). These are examples of the longest solvent-mediated ESIPTs reported. Multiple sites of protonation (sites of formal ESIPT) observed requires the intermediacy of a wide-variety of unique quinone methide intermediates. The overall higher deuterium exchange efficiency and greater number of proton transfer sites observed for 5 may be attributed to the enhanced structural rigidity of the proton-accepting ring (a rigid fluorene system).