Deciphering the influence of short-range electronic couplings on optical properties of molecular dimers: application to "special pairs" in photosynthesis.

The excited states of chromophore dimers are, in general, delocalized, and the transition energies and transition dipoles are different from those of the monomers. The intermolecular interaction that is responsible for these effects has two contributions: Forster-type Coulomb coupling and a short-range coupling, which depends on the intermolecular overlap of electronic wave functions. The latter contains the Dexter-type exchange coupling and the coupling of excited states to intermolecular charge-transfer (CT) states. Recently, we developed a method (TrEsp) for an accurate and numerically efficient calculation of the Forster-type Coulomb part (Madjet et al. J. Phys. Chem. B 2006, 110, 17268). Here, we combine the latter with quantum chemical calculations to evaluate the short-range contribution, extending a method developed earlier by Scholes et al. (J. Phys. Chem. B 1999, 103, 2543). An effective two-state model is used, which relates the transition energies and transition dipole moments obtained by quantum chemical calculations of the monomers to those calculated for the dimer. From this relation, the short-range excitonic coupling and effective shifts of the local transition energies due to the coupling to intermolecular CT states can be inferred including a consistency check to evaluate quantum chemical methods that differ in the treatment of electron correlation. The method is applied to the special pairs of the reaction centers of purple bacteria (bRC) and photosystem I (PSI). We find that the short-range coupling represents the dominant contribution to the total excitonic coupling in both special pairs (80% in PSI and 70% in the bRC) and exhibits a monoexponential dependence on the distance between the pi-planes of the pigments with an attenuation factor of 2.8 A(-1). We obtain significant red-shifts of the local transition energies, which show a biexponential distance dependence with one attenuation factor being 2.8 A(-1) and another factor being in the range 0.3-0.7 A(-1) for PSI and 0.8-0.9 A(-1) for bRC. Both effects of the short-range coupling determine the excitation energy sink in the reaction centers at the special pairs.