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. Calculation s based on the CS-INDO-MRCI (conformations spectra - intermediate negl ect of differential overlap - multi reference configuration interactio n) method reveal that the two lower-lying singlet excited states have very different dipolar contributions. The corresponding electronic tra nsitions are close in energy, polarized in the plane of the pyrylium c ore and orthogonal to one another. Their existence is confirmed by flu orescence anisotropy measurements leading to resolution of the visible absorption spectrum. Calculations show the formation of a non-fluores cent TICT (twisted intramolecular charge transfer) state involving rot ation of the dialkylaminophenyl group without energy barrier; this for mation is corroborated by the influence of the rotor size and the solv ent viscosity on fluorescence properties. Photo-induced charge transfe r is studied by second harmonic generation in solution using time-reso lved non-degenerate six-wave mixing. It is shown that the charge trans fer direction is parallel to the corresponding transition. The photo-i nduced variation of the dipole moment experimentally measured (18 D) i s in good agreement with the calculated value (15.8 D). Comparison of the fluorescence decay and the second harmonic decay proves the format ion of a non-fluorescent transient charge transfer state.