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

Abstract

Investigation of the intermolecular interaction effect on the fluorescent properties of the 2,4,6-triarylsubstituted pyridinium cations has been carried out by steady-state absorption and fluorescent spectroscopy using the structural 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 (Δ ν max a≈800 cm −1 and Δ ν max a≈400 cm −1) and therefore cannot be responsible for the fluorescence band anomalous Stokes shift (ASS) (Δ ν a-f ≈ 10 000 cm −1). The ASS is conditioned by the barrierless process of the adiabatic structural relaxation (ASR) associated with rotation of aryls in the 2nd and 6th positions. The ASR is described satisfactorily by the continual relaxation model and proceeds through the several main stages differing in the ASS dependence on viscosity. One of the stages is the linear ASS dependence on the logarithm of the solvent viscosity. The ASS is considered as a function of the interaction between the cation and its microenvironment in the concrete solvent (`microviscosity') and therefore, its dependence on viscosity may be considered as the microviscosity–macroviscosity correlation. The qualitative interpretation of dependence of the specific fluorescence parameters (quantum yield, band half-width, lifetime) on temperature (viscosity) is also discussed.