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

The solvatochromism of the 10-hydroxybenzo[h]quinoline molecule (HBQ) has b een investigated for the S-0 --> S-1 absorption of the normal tautomer, and for the S-1(') --> S-0(') fluorescence emission of the proton-transfer (PT ) tautomer. The wavelength shifts of the first absorption band depend princ ipally on polarity of the solvent used with a minor contribution of acidity , contrary to the fluorescence band wavelength shifts, which exhibit a grea ter dependence on acidity than on polarity. These observations preclude the use of HBQ for probing polarity in cavities of biopolymers, Theoretical ca lculations show that the local minimum of electrostatic potential (V-min) w hich is found at the carbonyl oxygen center in the S-1(') state of the PT t automer is much deeper than that V-min, located at the hydroxy oxygen cente r in the SI state of the normal tautomer. This observation is interpreted a s a significant increase in basicity of the carbonyl oxygen center, this be ing approximately twice more basic than the hydroxy oxygen center for the f irst singlet excited electronic state. Therefore, the oxygen center is the most subjected to be perturbed by solvent acidity. The steps followed in th is paper for analyzing the response of HBQ to solvent effects can be applie d to any molecular system to be used for probing polarity of the environmen t (e.g., in biopolymers). (C) 2001 Elsevier Science B.V. All rights reserve d.