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 π-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 π-orbitals. Such a hybrid ab initio/free electron modeling approach, where the free electron potential parameters are optimized on a molecule-by-molecule basis, is developed, and applied to a series of simple cyanine and oxonol dyes. The ensuing predictions for λ_max, oscillator strengths, and redox properties compare well to available experimental information. Two important strengths of this approach are that no explicit calculations of the excited electronic state are required, and that the ab initio determination of the occupied π-orbital level spacing considers all the electrons (π and σ) of the entire molecule in a specified geometry, environment, etc. This second characteristic gives the ability to efficiently model modifications of the optical properties of conjugated molecules resulting from chemical and/or physical modifications occuring within and remote to the conjugated region of the molecule.