Ab initio QM/MM study of excited state electron transfer between pyrene and 4,4′‐bis(dimethylamino)‐diphenylmethane with different solvent systems: Role of hydrogen bonding within solvent molecules

The exciplex is a charge transfer species formed in the process of electron transfer between an electron donor and an electron acceptor and hence is very sensitive to solvent polarity. In order to understand the role of solvent in exciplex formation between pyrene (PY) and 4,4′‐bis(dimethylamino)diphenylmethane (DMDPM), we used two types of solvent approximations: an implicit solvent model and an explicit solvent model. The difference in energies between the excited and the meta‐stable Frank–Condon state (ΔE) of the structures were assumed to correspond to the emission maximum of the exciplex in different solvents. The ΔE values show the trend of stabilization of the exciplex with an increase in solvent polarity. This trend in stabilization is substantially more prominent in the explicit solvent model than that with the implicit solvent model. The ΔE value obtained in methanol reflects equal stabilization compared to that in a more polar solvent, N,N‐dimethylformamide. This extra stabilization of the exciplex may be explained on the basis of the H‐bonding capability of the protic solvent, methanol. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005