A Three-State Nonadiabatic Model for Intramolecular Electronic Energy Transfer (IEET) in 9-Anthryl-1‘-naphthylalkanes Studied by Molecular Mechanics/Valence Bond Dynamics

Classical trajectory simulations with the inclusion of nonadiabatic transitions have been used to study intramolecular electronic energy transfer (IEET) in 9-anthryl-1'-naphthylalkanes. We provide evidence that a model with two geometric coordinates (the naphthalene transannular bond and the anthracene transannular bonds) and involving three covalent (dot-dot) singlet diabatic states (N*-A, N*-A* and N-A*) is needed to describe the mechanistic aspects of IEET in these systems. Although the computations show that the initial photoexcitation is to the third covalent state, which corresponds to the N*-A state in the Franck-Condon region, the process of IEET occurs on the lowest energy covalent state. Dynamics results show that intramolecular vibrational redistribution (IVR) on the lowest energy covalent state into the anthracene transannular vibrations on the N*-A diabatic potential surface is faster for the naphthalene-(CH2)3-anthracene system, where interchromphoric exchange interactions are possible due to a 'sandwich' type conformation, than for the naphthalene-(CH2)1-anthracene system where a more rigid 'spacer' (one CH2 group) only allows a 'T' shape conformation. A similar conclusion can be drawn for the transition from the N*-A* diabatic surface to the N-A*diabatic surface.