PHOTOINDUCED CHARGE-TRANSFER IN BICHROMOPHORIC MOLECULES IN THE GAS-PHASE

We have studied a series of bichromophores of the form A-(CH2)(n)-D wh ere A is a 9-anthryl group, D is either a N-methylanilino group or a N -methyl-p-methoxyanilino group, and n = 1,2,3, or 4. The fluorescence excitation and emission spectra of these molecules have been observed in a supersonic jet and in a variety of solvents. In the gas phase, in the n = 1 and n = 3 molecules, and in an extended conformer of the n = 4 molecules interactions are weak resulting only in a slightly redsh ifted anthracene spectrum. No redshifted emission was observed in thes e cases. For the n = 2 molecules and a second conformer of the n = 4 m olecules we observed a redshifted emission in the gas phase. This emis sion is assigned as coming from a charge transfer (CT) state or excipl ex. On the basis of the excitation and emission spectra we observed tw o different pathways to reach the CT state. For the molecules with don or N-methyl-N-alkylaniline, excitation took place to the locally excit ed (LE) state followed by relaxation to the CT state. For the molecule s with donor N-methyl-N-alkyl-p-methoxyaniline, the CT state was low e nough to interact with the ground state forming an intramolecular elec tron-donor-acceptor (EDA) complex. From this EDA ground state direct e xcitation to the CT state occurred. In the molecule with n = 2 and don or N-methyl-p-methoxyaniline the EDA interaction is very weak allowing excitation to the LE state as well as to the CT state. This set of mo lecules exhibits the full range of spectroscopic behavior expected in bichromophores.