Electronic transitions and intramolecular hydrogen bonding in anthralin. UV-VIS linear dichroism spectroscopy and quantum chemical calculations

The electronic and molecular structure of the antipsoriatic drug anthralin (1,8-dihydroxy-9(10H)-anthracenone) is investigated by UV-VIS linear dichroism (LD) spectroscopy in stretched polyethylene and by quantum chemical model calculations. Seven individual electronic transitions are resolved below 47?000 cm⁻¹ and assigned to calculated π–π* transitions. The low-energy region is characterized by a relatively broad band around 28?000 cm⁻¹ that can be assigned to two overlapping, differently polarized electronic transitions involving a considerable degree of intramolecular charge transfer from the phenolic moieties to the carbonyl group. Computed wavenumbers for these transitions depend significantly on the assumed geometrical parameters for the intramolecular H-bonds in anthralin; best agreement with observed data is obtained with a geometry corresponding to strong H-bonding. The calculations also indicate that excited state intramolecular proton transfer (ESIPT) is likely to occur, leading to the prediction of a very large Stoke's shift.