Intramolecular Donor−Acceptor Interactions in Jet-Cooled Bichromophoric Molecules: Comparison between SiMe₂ and CMe₂ Spacers

Abstract

The laser-induced excitation and dispersed fluorescence spectra of a series of bichromophores consisting of para-substituted benzene moieties linked by a CMe2 or a SiMe2 spacer have been investigated under solvent-free conditions in a supersonic expansion. The spectroscopic properties of three different systems involving the benzonitrile−benzonitrile, the anisole−benzonitrile, and the N,N-dimethylaniline−benzonitrile pairs have been examined and compared to the monomeric units. For the isolated bichromophores, the lowest fluorescent excited state is localized on the aromatic moiety with the lowest excited state and does not reveal strong perturbation due to the presence of the second aromatic subunit. In the anisole−benzonitrile system where the localized excitations are expected to be close in energy, the lowest excited state is localized on the anisole moiety for the CMe2 spacer and on the benzonitrile moiety in the case of the SiMe2 spacer. In this latter example, a second intense transition has been assigned to the excitation of the anisole chromophore. The emission following excitation of this upper level is characteristic of the benzonitrile chromophore, indicating that irreversible electronic energy transfer takes place in this system. In the dimethylaniline−benzonitrile bichromophores, the excitation and dispersed emission show that the lowest excited state is localized on the dimethylaniline subunit and that no electron-transfer process takes place under isolated conditions. Complexation of carbon- and silicon-bridged donor−acceptor compounds with acetonitrile results in the appearance of an intense continuous absorption extending over a large energy range and a Stokes shifted emission that can be assigned to a charge-transfer state.