Intramolecular charge transfer properties in the excited state of para-disubstituted biaryls

Abstract

The photophysical properties of four donor—acceptor biaryl derivatives substituted in the para positions by cyano and N,N′-dimethylamino groups were investigated in cyclohexane and acetonitrile solutions. The absorption and fluorescence spectra in non-polar cyclohexane were analysed in terms of geometrical restrictions to planarity resulting from the steric hindrance between the two aromatic subunits. The intramolecular charge transfer interactions lead to a large Stokes shift in polar solvents and to a large dipole moment in the fluorescent state. The transition dipole moment of the relaxed emitting state in acetonitrile, deduced from quantum yield and lifetime measurements, decreases with an increase in the initial inter-ring torsion angle. These observations have been interpreted to result from the solvent-induced increase in the charge transfer character of the fluorescent state. 4-Dimethylamino-4′-cyano-1,1′-binaphthyl exhibits dual fluorescence when excited in the second excited singlet state. The emission spectrum, intensity and decay times of the blue component of this fluorescence are not sensitive to solvent polarity, whereas the red component exhibits the same dependence on solvent polarity as the fluorescence of other biaryls. An asymmetric double-minimum S1 potential curve corresponding to cisoid and transoid conformers has been suggested to be responsible for this specific behaviour.