The dependence of photoinduced adiabatic transformations and fluorescence in 2,4,6-triarylsubstituted pyridinium cations on environment

Investigation of the intermolecular interaction effect on the fluorescent p roperties of the 2,4,6-triarylsubstituted pyridinium cations has been carri ed out by steady-state absorption and fluorescent spectroscopy using the st ructural modelling in solvents with different polarities and viscosities at room temperature as well as when the viscosity changes with variations of the temperature from 180 to 340 K. The unspecific Coulomb and polarization cation-solvent interactions result in the small absorption and fluorescence band spectral shifts (Delta nu(a)(max) approximate to 800 cm(-1) and Delta nu(a)(max) = 300 cm(-1)) and therefore cannot be responsible for the fluor escence band anomalous Stokes shift (ASS) (Delta nu(a-f) approximate to 10 000 cm(-1)) The ASS is conditioned by the barrierless process of the adiaba tic structural relaxation (ASR) associated with rotation of aryls in the 2n d and 6th positions. The ASR is described satisfactorily by the continual r elaxation model and proceeds through the several main stages differing in t he ASS dependence on viscosity. One of the stages is the linear ASS depende nce on the logarithm of the solvent viscosity. The ASS is considered as a f unction of the interaction between the cation and its microenvironment in t he concrete solvent ('microviscosity') and therefore, its dependence on vis cosity may be considered as the microviscosity-macroviscosity correlation. The qualitative interpretation of dependence of the specific fluorescence p arameters (quantum yield, band half-width, lifetime) on temperature (viscos ity) is also discussed. (C) 1999 Elsevier Science S.A. All rights reserved.