Influence of a nearby substrate on the reorganization energy of hole exchange between dye molecules

A numerical method is presented to estimate the influence of a nearby substrate on the polarization energy and outer sphere reorganization energy (lambda(o)) for intermolecular hole transfer for a series of dye molecules. The calculation considers the net charge distribution of the oxidised molecule (determined from quantum chemical calculation of the highest occupied molecular orbital of the neutral molecule within the frozen orbital approximation) encapsulated within a conformal cavity, by the molecules total electron density. An analytical point charge approximation was used at longer range. The molecular cavity was either surrounded by a single polarizable continuum, or, to simulate a nearby substrate, embedded at different positions relative to the interface between two semi-infinite slabs with different dielectric constants. The calculated lo values in the single dielectric medium were linearly related to the outer-sphere reorganisation energy calculated from DFT with a polarizable continuum model, validating the approach. In the two phase system, variations in lo was sensitive to the position of the substrate relative to the molecule and differences in the Pekar factor (1/epsilon(o) - 1/epsilon(r)) for the media. For dye molecules in ACN positioned touching a TiO2 substrate lambda(o) was typically about 20% lower than in pure ACN depending on the molecular configuration. Our approach can be adapted to systems of more than two media.