A pentacene intermediate via formal intramolecular photoredox of a 6,13-pentacenequinone in aqueous solution

Abstract

The formal intramolecular photoredox reaction (or tandem phototautomerizations) of aromatic ketones in aqueous solution discovered in our laboratory has been extended to a number of acenequinones. In particular, we were interested in whether the photoredox reaction could be applied to 2-(hydroxymethyl)-6,13-pentacenequinone (4), which would result in 2-formyl-6,13-dihydroxypentacene (10) and hence offer a photochemical method for synthesizing a pentacene derivative. Whereas a number of acenequinones displayed a range of photoredox reactivity, photolysis of 4 in acidic aqueous solution (pH < 3) resulted in a clean intramolecular photoredox reaction, via an enol intermediate, to give 10 (green compound; Φ ~ 0.2 at pH 1), which was too reactive for isolation or trapping by standard ArOH trapping agents such as acetic anhydride. These reactions may be viewed as a one-way photochemical intramolecular “redox switch” from quinone to hydroquinone with concurrent oxidation of an attached hydroxymethyl (alcohol) moiety. Without the attached alcohol moiety, these acenequinones are photostable in aqueous solution. The trend in observed relative reactivity may be partially rationalized by examining changes in molecular orbital coefficients observed in the calculated HOMOs and LUMOs (at the AM1 level).Key words: pentacene, acenequinones, photoredox, enol, acid catalysis.