Photoinduced charge transfer in bichromophoric molecules in the gas phase

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Abstract
We have studied a series of bichromophores of the form A-(CH2)n-D where 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 redshifted anthracene spectrum. No redshifted emission was observed in these cases. For the n=2 molecules and a second conformer of the n=4 molecules we observed a redshifted emission in the gas phase. This emission is assigned as coming from a charge transfer (CT) state or exciplex. On the basis of the excitation and emission spectra we observed two different pathways to reach the CT state. For the molecules with donor N-methyl-N-alkylaniline, excitation took place to the locally excited (LE) state followed by relaxation to the CT state. For the molecules with donor N-methyl-N-alkyl-p-methoxyaniline, the CT state was low enough to interact with the ground state forming an intramolecular electron–donor–acceptor (EDA) complex. From this EDA ground state direct excitation to the CT state occurred. In the molecule with n=2 and donor 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 molecules exhibits the full range of spectroscopic behavior expected in bichromophores.