CHARGE-TRANSFER IN TRIARYL PYRYLIUM CATIONS - THEORETICAL AND EXPERIMENTAL-STUDY

The spectroscopic properties of 2,4,6-triaryl pyrylium cations bearing electron-donating groups (dialkylamino) are investigated. Calculations based on the CS-INDO-MRCI (conformations spectra - intermediate neglect of differential overlap - multi reference configuration interaction) method reveal that the two lower-lying singlet excited states have very different dipolar contributions. The corresponding electronic transitions are close in energy, polarized in the plane of the pyrylium core and orthogonal to one another. Their existence is confirmed by fluorescence anisotropy measurements leading to resolution of the visible absorption spectrum. Calculations show the formation of a non-fluorescent TICT (twisted intramolecular charge transfer) state involving rotation of the dialkylaminophenyl group without energy barrier; this formation is corroborated by the influence of the rotor size and the solvent viscosity on fluorescence properties. Photo-induced charge transfer is studied by second harmonic generation in solution using time-resolved non-degenerate six-wave mixing. It is shown that the charge transfer direction is parallel to the corresponding transition. The photo-induced variation of the dipole moment experimentally measured (18 D) is in good agreement with the calculated value (15.8 D). Comparison of the fluorescence decay and the second harmonic decay proves the formation of a non-fluorescent transient charge transfer state.