Electronic interactions in photosynthetic light-harvesting complexes: The role of carotenoids

The origin and distance dependence of the electronic interactions which promote energy transfer within photosynthetic light-harvesting complexes is investigated. A model based on localized molecular orbitals is related to canonical molecular orbital calculations, therefore demonstrating its practical utility and allowing us to interpret the results of CAS-SCF calculations of the coupling between donor-acceptor pairs. We then focus on the mechanism of energy transfer involving the carotenoid 2¹A_g (S₁) electronic state: [carotenoid (2¹A_g) (Car) to carotenoid (2¹A_g)] and [carotenoid (2¹A_g) to bacteriochlorophyll (Q_y) (Bchl)] interactions. The Car-Car coupling is found to involve reasonably long-range interaction terms, with a primary contribution from dispersion-type interactions, which have an R⁶ distance dependence. The primary contributor to the Car-Bchl S₁ → S₁ energy transfer mechanism is suggested to be proportional to the product of dipole-dipole and polarization interactions. In neither case does the electronic interaction resemble the Dexter exchange integral in origin or distance dependence. Some model CAS-SCF calculations of electronic interactions in 2,4,6-octatriene dimers are presented which support the predictions of the theory: the calculated interaction is found to be (i) small in comparison to the overlap-dependent triplet-triplet interaction at close separations; (ii) small in comparison to a dipole-dipole (S₂) interaction at all separations; and (iii) quite weakly distance dependent at larger separations. The implications for the role of carotenoids in photosynthetic light-harvesting complexes are discussed.