Solvent-dependent excited state intramolecular proton transfer (ESIPT) pathways from phenol to carbon in 2,5-dihydroxyphenyl arenes.

The ESIPT of three 2,5-dihydroxyphenyl-substituted arenes 9-11 was studied in various solvent systems, to investigate the direction of the proton transfer from the phenol to the respective carbons of naphthyl, phenanthrenyl and anthryl aromatic rings. In neat CH3CN, 9-11 undergo direct ESIPT from the phenolic OH to the ipso-position of the corresponding aromatic carbon acceptors, via an intramolecular charge transfer state (S(1,ct)), giving rise to observable zwitterions, ZIs 35, 25, 27, respectively. Surprisingly, the generated ZI in 9 proceeds via a 1,2-phenyl migration followed by re-aromatization to afford 16 (a structural isomer of 9) in quantitative yield. In 10 and 11, the corresponding ZIs proceed via electrocyclic ring closure to furnish 20 and 28, respectively. In the case of 10, another intrinsic ESIPT pathway takes place to the 10-position of a phenanthrenyl ring, giving QM 26 in high quantum efficiency (Φex = 0.72). In aqueous solution, 9 undergoes formal ESIPT to the more distal 2'- and 7'-positions of the naphthalene ring, delivering QMs 18 and 38, which either revert back to the starting material or proceed via electrocyclic ring closure, respectively. In 11 in aqueous solution, formal ESIPT to the 10-position of the anthracene ring takes place delivering QM 29, which readily aromatizes to regenerate starting material.