FREE-ENERGY AND STRUCTURE DEPENDENCE OF INTRAMOLECULAR TRIPLET ENERGY-TRANSFER IN ORGANIC-MODEL COMPOUNDS

A series of compounds, each containing a triplet energy donor and an acceptor separated by a rigid spacer, has been designed and synthesized. The 1,4-cyclohexanediyl moiety is employed as the spacer for the series. The rates of intramolecular triplet energy transfer (TT) have been measured for the series. The rate of TT shows an inverted parabolic, i.e., Marcus, dependence on the thermodynamic driving force for a selected subset of the compounds wherein the donor is maintained constant throughout and the acceptors are "rigid", having no low-frequency internal degrees of freedom. The internal low-frequency torsional mode of a biphenylyl acceptor can be accounted for quite satisfactorily as an additional contribution to the solvent reorganization energy, lambda-s. The driving force dependence of the rate of TT is not modeled well by the conventional Marcus-Jortner equation for weakly coupled nonadiabatic electron transfer. Generalization of the Marcus-Jortner equation to include coupling to a high-frequency harmonic mode which is both displaced and distorted along the reaction coordinate provides a somewhat better fit to the experimental data with fewer adjustable parameters.