Intramolecular donor-acceptor interactions in jet-cooled bichromophoric molecules: Comparison between SiMe2 and CMe2 spacers

The laser-induced excitation and dispersed fluorescence spectra of a series of bichromophores consisting of para-substituted benzene moieties linked b y a CMe2 or a SiMe2 spacer have been investigated under solvent-free condit ions in a supersonic expansion. The spectroscopic properties of three diffe rent systems involving the benzonitrile-benzonitrile, the anisole-benzonitr ile, and the N,N-dimethylaniline-benzonitrile pairs have been examined and compared to the monomeric units. For the isolated bichromophores, the lowes t fluorescent excited state is localized on the aromatic moiety with the lo west excited state and does not reveal strong no perturbation due to the pr esence of the second aromatic subunit. In the anisole-benzonitrile system w here the localized excitations are expected to be close in energy, the lowe st 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 o f the anisole chromophore. The emission following excitation of this upper level is characteristic of the benzonitrile chromophore, indicating that ir reversible electronic energy transfer takes place in this system. In the di methylaniline-benzonitrile bichromophores, the excitation and dispersed emi ssion show that the lowest excited state is localized on the dimethylanilin e subunit and that no electron-transfer process takes place under isolated conditions. Complexation of carbon- and silicon-bridged donor-acceptor comp ounds with acetonitrile results in the appearance of an intense continuous absorption extending over a large energy range and a Stokes shifted emissio n that can be assigned to a charge-transfer state.