Understanding the solvatochromism of 10-hydroxybenzo[h]quinoline. An appraisal of a polarity calibrator Original Research Article

The solvatochromism of the 10-hydroxybenzo[h]quinoline molecule (HBQ) has been investigated for the S0→S1 absorption of the normal tautomer, and for the S1′→S0′ fluorescence emission of the proton-transfer (PT) tautomer. The wavelength shifts of the first absorption band depend principally on polarity of the solvent used with a minor contribution of acidity, contrary to the fluorescence band wavelength shifts, which exhibit a greater dependence on acidity than on polarity. These observations preclude the use of HBQ for probing polarity in cavities of biopolymers. Theoretical calculations show that the local minimum of electrostatic potential (Vmin) which is found at the carbonyl oxygen center in the S1′ state of the PT tautomer is much deeper than that Vmin located at the hydroxy oxygen center in the S1 state of the normal tautomer. This observation is interpreted as a significant increase in basicity of the carbonyl oxygen center, this being approximately twice more basic than the hydroxy oxygen center for the first singlet excited electronic state. Therefore, the oxygen center is the most subjected to be perturbed by solvent acidity. The steps followed in this paper for analyzing the response of HBQ to solvent effects can be applied to any molecular system to be used for probing polarity of the environment (e.g., in biopolymers).