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 benzoquinolinic nitrogen for various BQ derivatives in both ground and excited states has been measured by pH-dependent absorption and emission spectroscopies. The results, in combination with fluorescence decay dynamics, conclude that for HBQ the keto-tautomer formation in the excited state is highly exergonic and a dominant process in a broad pH range, regardless of whether HBQ is in a neutral, protonated, or deprotonated form. This process is only limited dynamically by the proton-donating or -accepting rate associated with free water molecules. The results also indicate that the excited-state resonance charge transfer between the hydroxyl oxygen and benzoquinolinic nitrogen acts as a driving force for the ESPT reaction.