CONFORMATION-DEPENDENT INTRAMOLECULAR ELECTRONIC-ENERGY TRANSFER IN A MOLECULAR-BEAM

The electronic spectrum of a molecule consisting of two chromophores separated by a spacer was observed in a supersonic molecular beam. The chromophores were anisole and dimethylaniline (DMA), the spacer was cyclohexane, and the chromophores were attached to the 1- and 4-positions of the spacer. Each chromophore could be attached axial (a) or equatorial (e) with respect to the cyclohexane and the conformers ae, ea, and ee were observed. The aa conformer is not energetically allowed and was not observed. Following excitation of the S1 state of the anisole moiety, emission was observed from both the anisole and DMA parts of the molecule. Since there was no direct absorption by the DMA, the emission from the DMA was produced by intramolecular electronic energy transfer. Measurement of the relative intensities of the anisole-like and DMA-like emissions provided a measure of the relative electronic energy transfer rates of the various conformers. It was found that energy transfer was considerably slower in the two trans (ee) conformers than in the four cis (ae and ea) conformers. This observation is not consistent with either simple Forster energy transfer theory, which predicts that the electronic energy transfer rate of all six conformers should be nearly the same, or the Dexter formalism, predicting that the trans isomers should be faster.