ELECTRONIC-ENERGY TRANSFER - DENSITY-OF-STATES

Intramolecular electronic energy transfer has been observed in one isomer of the bichromophoric D-Sp-A where the donor (D) is naphthalene, the spacer (Sp) is cyclohexane, and the acceptor (A) is dimethylaniline (DMA). Because the emission spectra of both the donor and acceptor overlap, the wavelength could not be used as a diagnostic for energy transfer. However, the donor and acceptor have very different fluorescence lifetimes, and large changes in the fluorescence lifetimes of the vibronic states of naphthalene due to the presence of DMA could be used as a measure of the mixing and energy transfer between the two chromophores. The methyl groups of DMA cause the density of states to rise very rapidly at energies close to the zero-point level, and in the cis isomer this leads to energy transfer from naphthalene vibronic levels that are only slightly above the zero-point level of DMA. The energy-transfer mte was measured as a function of the excited vibronic-state energy. The onset of energy transfer in cis isomers occurs at a lower energy than in the trans isomer, corresponding to a lower density of states.