Optical properties of triarylpyrylium dimers

The present paper deals with ion-pair dimers (two chromophores and their counterions) formed by 2,6-diphenyl-4-(4'-dialkylaminophenyl)pyrylium tetrafluoroborates in solution. They are studied at room temperature by electronic absorption and fluorescence spectroscopy and using a time-resolved nondegenerate six-wave mixing technique. Spectral analysis gives evidence that the excited states of the dimers are delocalized on both cationic chromophores. The properties (energy, oscillator strength, relative polarization) of the Franck-Condon transitions are determined. It is found that the Franck-Condon transition that is lowest in energy is orthogonal to the transition corresponding to fluorescence, proving that relaxation results in a drastic change in the wave function of the lowest excited state. It is shown, for the first time, that dimers formed upon aggregation are capable of generating second harmonic in solution. This observation leads to the conclusion that the dimer geometry in the ground state is noncentrosymmetric and that excitation induces an important variation of the atomic charge distribution.