OPTICAL-PROPERTIES OF TRIARYLPYRYLIUM DIMERS

The present paper deals with ion-pair dimers (two chromophores and the ir counterions) formed by 2,6-diphenyl-4-(4'-dialkylaminophenyl)pyryli um tetrafluoroborates in solution. They are studied at room temperatur e by electronic absorption and fluorescence spectroscopy and using a t ime-resolved nondegenerate six-wave mixing technique. Spectral analysi s gives evidence that the excited states of the dimers are delocalized on both cationic chromophores. The properties (energy, oscillator str ength, relative polarization) of the Franck-Condon transitions are det ermined. It is found that the Franck-Condon transition that is lowest in energy is orthogonal to the transition corresponding to fluorescenc e, proving that relaxation results in a drastic change in the wave fun ction of the lowest excited state. It is shown, for the first time, th at dimers formed upon aggregation are capable of generating second har monic in solution. This observation leads to the conclusion that the d imer geometry in the ground state is noncentrosymmetric and that excit ation induces an important variation of the atomic charge distribution .