A hybrid ab initio/free electron computational model for conjugated dye molecules: Simple cyanines and oxonols

Justifications developed for the application the free electron model to the pi-orbitals of conjugated molecules suggest that the optical properties of these molecules would be well described by a one-dimensional free electron model with a potential chosen to reproduce the energy level spacing of the ground state occupied pi-orbitals. Such a hybrid ab initio/free electron m odeling approach, where the free electron potential parameters are optimize d on a molecule-by-molecule basis, is developed, and applied to a series of simple cyanine and oxonol dyes. The ensuing predictions for lambda(max), o scillator strengths, and redox properties compare well to available experim ental information. Two important strengths of this approach are that no exp licit calculations of the excited electronic state are required, and that t he ab initio determination of the occupied pi-orbital level spacing conside rs all the electrons (pi and sigma) of the entire molecule in a specified g eometry, environment, etc. This second characteristic gives the ability to efficiently model modifications of the optical properties of conjugated mol ecules resulting from chemical and/or physical modifications occuring withi n and remote to the conjugated region of the molecule. (C) 2000 John Wiley & Sons, Inc.