ELECTRONIC-ENERGY TRANSFER - DENSITY-OF-STATES

Intramolecular electronic energy transfer has been observed in one iso mer of the bichromophoric D-Sp-A where the donor (D) is naphthalene, t he spacer (Sp) is cyclohexane, and the acceptor (A) is dimethylaniline (DMA). Because the emission spectra of both the donor and acceptor ov erlap, the wavelength could not be used as a diagnostic for energy tra nsfer. However, the donor and acceptor have very different fluorescenc e lifetimes, and large changes in the fluorescence lifetimes of the vi bronic states of naphthalene due to the presence of DMA could be used as a measure of the mixing and energy transfer between the two chromop hores. The methyl groups of DMA cause the density of states to rise ve ry rapidly at energies close to the zero-point level, and in the cis i somer this leads to energy transfer from naphthalene vibronic levels t hat are only slightly above the zero-point level of DMA. The energy-tr ansfer mte was measured as a function of the excited vibronic-state en ergy. The onset of energy transfer in cis isomers occurs at a lower en ergy than in the trans isomer, corresponding to a lower density of sta tes.