The title compound undergoes efficient photoaddition of a molecule of a hydroxylic solvent (H2O, MeOH, (Me)(2)CHOH) across the 9- and 10-positions of the anthracene moiety to give isolable triphenyl-methanol or triphenylmethyl ether type products. The reaction is believed to proceed via a mechanism involving water-mediated formal excited state intramolecular proton transfer (ESIPT) from the phenolic OH to the 10-position of the anthracene ring, generating an o-quinone methide intermediate that is observable by nanosecond laser flash photolysis, and is trappable with nucleophiles. A "water-relay" mechanism for proton transfer seems plausible but cannot be proven directly with the data available. Irradiation in deuterated solvents led to incorporation of one deuterium atom at the methylene position in the photoaddition product, and partial deuterium exchange of the 10-position of recovered starting material, consistent with the proposed formal excited state proton transfer mechanism. The deuterium exchange and photoaddition reach maximum quantum efficiency at similar to5 M water (in CH3CN or CH3OH), with no reaction observed in the absence of a hydroxylic solvent, demonstrating the sensitivity of this type of ESIPT to solvent composition.