Excited state acidity of bifunctional compounds. 6. A novel, high fluorescence quantum yield, excited state intramolecular proton transfer compound: 2-hydroxyphenyl-lapazole in non-protic solvents

The synthesis of 2-hydroxyphenyl-lapazole (HPL) is reported for the first time. HPL in the first excited singlet state undergoes excited state intramolecular proton transfer (ESIPT) in cyclohexane, with an overall fluorescence quantum yield of 0.15. This is an order of magnitude greater than that observed for 2-hydroxyphenyl-benzoxazole (HBO), which has been extensively studied and has a similar structure. At room temperature in non-protic solvents, ESIPT in HPL attains equilibrium, whose constant (between normal and tautomer species) is approximately 1.4. These observations suggest that HPL (and its derivatives) is well worth investigating because it is easier to study (and may work more efficiently as a laser dye) than HBO. (C) 1999 Elsevier Science S.A. All rights reserved.