DIMERS OF TRIARYLPYRYLIUM SALTS - GEOMETRY AND ELECTRONIC-TRANSITIONS

The present paper deals with the geometry and the electronic transitio ns of ion-pair dimers formed by 2,6-diphenyl-4-(4'-(dialkylamino)pheny l)pyrylium tetrafluoroborates. The geometry is determined theoreticall y by minimizing the potential interaction energy calculated at a molec ular level. The most stable configuration corresponds to a noncentrosy mmetric oyster-like arrangement where the anions are sandwiched betwee n the pyrylium cores forming an angle of ca. 30 degrees. Such an arran gement is corroborated by H-1-NMR and nonlinear optical measurements. The properties of the electronic transitions of the dimers are determi ned in the framework of the exciton theory, taking into account the ca lculated geometry, and compared to those obtained experimentally. It i s shown that fluorescence originates from an excited state localized o n one of the chromophores. Localization of the excitation is accompani ed by a moving of the counterions by ca. 1.5 Angstrom, due to the vari ation of the atomic charge distribution. The calculated decrease (3350 cm(-1)) in the transition energy upon relaxation is practically the s ame as the experimentally observed Stokes shift (3000 +/- 400 cm(-1)).