Excited state intramolecular proton transfer (ESIPT) from phenol OH (OD) to adjacent “aromatic” carbons in simple biphenyls

This work reports results of further studies on a new class of excited state intramolecular proton transfer (ESIPT), from phenol OH to adjacent aromatic carbon atoms of suitably designed biphenyl systems. For this purpose, a number of 2-phenylphenols 3–6 with methyl and methoxy substituents on the adjacent proton accepting phenyl ring were synthesized. In particular, we were also interested in studying the effect of an acetyl (ketone) substituent on the proton accepting ring (biphenyl 7) and the effect on the photochemistry when the ketone is reduced to alcohol (biphenyl 8). All compounds except for 7 were found to undergo deuterium exchange (Ф_ex = 0.019–0.079) primarily at the 2′-position on photolysis in 1:3 D₂O–CH₃CN. This is consistent with a reaction mechanism involving initial ESIPT from the phenol OH to the 2′-position of the adjacent phenyl ring, to generate a biphenyl quinone methide intermediate which rapidly tautomerizes back to starting material. Biphenyl 8 also undergoes a competing photosolvolysis reaction (overall loss of water). Both photosolvolysis and ESIPT reactions react via isomeric quinone methide intermediates and are best interpreted as arising from an excited singlet state that possesses a large degree of charge transfer character, from the phenol ring to the attached phenyl ring. The failure of 7 to react may be due to two possible reasons: (i) high intersystem crossing rate to a non-polarized triplet excited state and/or (ii) a polarized singlet state that is now much more basic at the carbonyl oxygen. The results are consistent with qualitative examination of calculated HOMOs and LUMOs (AM1).