Excited charge transfer states in donor-acceptor indole derivatives

Solvent-dependent electronic structure of the selected donor (D)-acceptor (A) derivatives of 5-methylindole and 5-methoxyindole containing benzonitrile as an electron acceptor in the fluorescent charge transfer (CT) states has been investigated. The mechanism of the radiative charge recombination (CT)-C-1 --> S-0 is discussed in terms of the Mulliken-Murrell model of the CT complexes and the Marcus theory of photoinduced electron transfer (ET). Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra point to a considerable CT character of the emitting singlet states of all the compounds studied. The determination of the electronic transition dipole moments corresponding to the (CT)-C-1 --> S-0 fluorescence and the band-shape analysis of the corresponding spectra leads to the quantities relevant for the charge recombination in the Marcus inverted region. It is shown that some of the photophysical properties of the compounds can be predicted in terms of a simple model from the properties of individual chromophores. (C) 1999 Elsevier Science B.V. All rights reserved.