Electroabsorption measurements and ab initio calculations of the dipolar properties of 2-(2′-hydroxyphenyl)-benzothiazole and -benzoxazole: two photostabilizers that undergo excited-state proton transfer

Electroabsorption is used to determine the difference dipole moment, ∣Δμ∣, and the difference polarizability, Δα, of HBT and HBO, two molecules that undergo excited-state intramolecular proton transfer (ESIPT). These measurements are compared to ab initio (6-31G∗) and semi-empirical (INDO1/s) calculations. Good agreement is obtained between ab initio and experimental ∣Δμ∣ values. However, the negative Δα values obtained experimentally for both molecules are in disagreement with the calculated Δα values which are positive. The absorption maxima, measured as a function of solvent polarity, support the electroabsorption results. Agreement between the observed and calulated shift requires that Δα for both molecules be negative.