PHOTOPHYSICS OF 10-HYDROXYBENZO[H]QUINOLINE IN AQUEOUS-SOLUTION

Various 7,8-benzoquinoline (BQ) derivatives have been synthesized and studied in order to determine the fluorescence species and dynamics of excited-state proton transfer (ESPT) of 10-hydroxybenzo[h]quinoline ( HBQ) in water. The acidity of the hydroxyl proton and protonated benzo quinolinic nitrogen for various BQ derivatives in both ground and exci ted states has been measured by pH-dependent absorption and emission s pectroscopies. The results, in combination with fluorescence decay dyn amics, conclude that for HBQ the keto-tautomer formation in the excite d state is highly exergonic and a dominant process in a broad pH range , regardless of whether HBQ is in a neutral, protonated, or deprotonat ed form. This process is only limited dynamically by the proton-donati ng or -accepting rate associated with free water molecules. The result s also indicate that the excited-state resonance charge transfer betwe en the hydroxyl oxygen and benzoquinolinic nitrogen acts as a driving force for the ESPT reaction.