Dimers of triarylpyrylium salts: Geometry and electronic transitions

The present paper deals with the geometry and the electronic transitions of ion-pair dimers formed by 2,6-diphenyl-4-(4'-(dialkylamino)phenyl)pyrylium tetrafluoroborates. The geometry is determined theoretically by minimizing the potential interaction energy calculated at a molecular level. The most stable configuration corresponds to a noncentrosymmetric oyster-like arrangement where the anions are sandwiched between the pyrylium cores forming an angle of ca. 30 degrees. Such an arrangement is corroborated by H-1-NMR and nonlinear optical measurements. The properties of the electronic transitions of the dimers are determined in the framework of the exciton theory, taking into account the calculated geometry, and compared to those obtained experimentally. It is shown that fluorescence originates from an excited state localized on one of the chromophores. Localization of the excitation is accompanied by a moving of the counterions by ca. 1.5 Angstrom, due to the variation of the atomic charge distribution. The calculated decrease (3350 cm(-1)) in the transition energy upon relaxation is practically the same as the experimentally observed Stokes shift (3000 +/- 400 cm(-1)).