The kinetics of formation of twisted intramolecular charge transfer (TICT) states in p-substituted dialkylanilines: consequences of conical intersections along the reaction coordinate

Abstract

The dual fluorescence spectra of a series of dialkylaminobenzonitriles and dialkylaminobenzoesters measured at room and low temperature in polar and non-polar solvents are compared. The esters always show a higher ratio of long-wavelength to short-wavelength fluorescence. This ratio strongly depends on the type of dialkylamino substituent. In non-polar solvents it is highest for the compound with the lowest donor ionization potential (D ≡ 2,5-dimethylpyrrolidino). The fluorescence spectra show clear evidence of dual fluorescence (twisted intramolecular charge transfer (TICT) emission) even for the corresponding nitrile in alkane solvents. This is rationalized by invoking thermodynamic control of TICT state formation. In the case of polar solvents the order of the rate constants for TICT state formation is changed such that for rotating parts of similar size it correlates with the initial Franck-Condon twist angle, indicating kinetic control. Low temperature quantum yield measurements and direct laser kinetic data show that in n-butyl chloride at low temperatures the rate of formation of the TICT states of the esters is a factor of 2 – 13 greater than that of the nitriles. This is ascribed to the presence of a conical intersection along the reaction coordinate in the case of the nitriles. Microviscosity theory is used to discuss some effets which depend on the size of the dialkylamino substituents and on the nature of the solvent in terms of the available free volume for rotation.