Electronic properties of small model compounds that undergo excited-state intramolecular proton transfer as measured by electroabsorption spectroscopy

Electroabsorption (Stark spectroscopy) is used to measure the electronic properties of o-hydroxybenzaldehyde (oHBA) and o-hydroxyacetophenone (oHAP), two molecules that undergo excited-state intramolecular proton transfer (ESIPT) in a non-interacting organic glass matrix. We report the change in dipole moment, |Δμ→|, and the average change in polarizability, Δα, for both oHBA and oHAP as well as for a molecular analog of oHAP, o-methoxyacetophenone (oMAP), in which proton transfer is blocked by methyl substitution. The experimental results for the two ESIPT molecules are found to compare well to the results of ab initio calculations. In contrast, the measured values of |Δμ→| and Δα for these systems compare poorly to the INDO/SCI results. Preparation for ESIPT in the Franck–Condon region of the potential energy surface in oHAP and oHBA is apparent from the calculated atomic charge distributions as an increase of the intramolecular hydrogen bond strength and a decrease in the aromaticity of the phenyl ring.