Potential Energy Surfaces and Dynamical Behavior of Two Rigidly Linked Bichromophoric Molecules Studied by CASSCF Computations and Ab Initio Classical Trajectory Simulations

Potential energy surface analysis and classical trajectory simulations have been used to study intramolecular electronic energy transfer (IEET) in two aliphatic diketones that each contains two rigidly linked carbonyl chromophores. This article provides evidence that two geometric coordinates (the carbonyl bond lengths) and two singlet diabatic states (characterized by specific excitation of either one carbonyl chromophore or the other) are needed to understand the mechanism of IEET in these systems. Although computations show that a nonadiabatic transition step is involved in the process, IEET is mainly an adiabatic mechanism of intramolecular vibrational redistribution (IVR) on the lowest excited state into the vibrations of the chromophore that is not initially excited. Dynamics calculations show that this redistribution is slower in the molecule characterized by the more extended σ -bond bridge. It is demonstrated that the σ -bond bridge not only enhances interchromophoric electronic coupling via through-bond interactions compared to through-space interactions, it also controls the IEET process by promoting IVR from one side of the molecule into the other.