Stark effect spectroscopy of exciton states in the dimer of acridine orange

Low temperature absorption and Stark effect (electroabsorption) spectra of the cationic dye acridine orange (AO) were recorded in the wide spectral range comprising the lowest energy electronic transition at 20,150 cm(-1) (496 nm) and the intense UV band at 36,800 cm(-1)(272 nm). By changing the water, glycerol and ethanol contents in the solvent, different proportions of monomers, dimers and higher oligomers can be obtained in frozen glasses, enabling the decomposition of spectra and determination of dipole moment and polarizability changes for electronic transitions in AO monomer and dimer. These data are analysed on grounds of exciton theory for a dimer in an electric field. While the split U-V transition fits the simplified theory well, the lower- and higher-energy dimer transitions in the VIS region exhibit significantly increased polarizability changes of opposite sign. Comparison with literature data for similar molecular systems points to orbital overlap as a source of the effects observed. (C) 2003 Elsevier Science B.V. All rights reserved.