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 anthr alin (1,8-dihydroxy-9(10H)-anthracenone) is investigated by W-VIS line ar dichroism (LD) spectroscopy in stretched polyethylene and by quantu m chemical model calculations. Seven individual electronic transitions are resolved below 47 000 cm(-1) and assigned to calculated pi-pi tr ansitions. The low-energy region is characterized by a relatively broa d band around 28 000 cm(-1) that can be assigned to two overlapping, d ifferently polarized electronic transitions involving a considerable d egree 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 intramolec ular H-bonds in anthralin; best agreement with observed data is obtain ed with a geometry corresponding to strong H-bonding. The calculations also indicate that excited state intramolecular proton transfer (ESIP T) is likely to occur, leading to the prediction of a very large Stoke 's shift. (C) 1997 Elsevier Science B.V.