Excited state intramolecular proton transfer (ESIPT) in dihydroxyphenyl anthracenes

Abstract

The photochemistry of three 9-(dihydroxyphenyl)anthracenes 6–8 was studied in neat CH₃CN and selected organic solvents, to investigate excited state intramolecular proton transfer (ESIPT) from the phenol to the anthracene moiety. In D₂O–CH₃CN mixtures, the observed deuterium exchange of 6–8 is consistent with water-mediated (formal) ESIPT process from the orthophenolic OH to the 10′-position of the anthracene ring, giving rise to quinone methide (QM) intermediates 12–14. There is no ESIPT for the corresponding methoxy-substituted compounds. Introduction of an extra hydroxyl group onto the phenol ring at different positions led to a range of deuterium exchange quantum yields (Φ = 0.03 to 0.15). In addition to the anticipated ESIPT process to the 10′-position, in neat CH₃CN and other organic solvents, 6 (but not 7 or 8) undergoes a clean photocyclization to give bridged product 19 in quantitative yield. The mechanism of this unique photocyclization may involve a direct ESIPT or a 1,4-hydrogen transfer from the orthophenolic OH to the 9′-position of the anthracene ring, generating a zwitterion (20) or diradical (21) intermediate, respectively, followed by ring closure. Fluorescence studies of 6 in various solvents show the existence of both local excited and intramolecular charge transfer states whereas only the former was present for 7 and 8, offering a possible rationalization for the photocyclization pathway.