ELECTRONIC INTERACTIONS IN PHOTOSYNTHETIC LIGHT-HARVESTING COMPLEXES - THE ROLE OF CAROTENOIDS

The origin and distance dependence of the electronic interactions whic h promote energy transfer within photosynthetic Light-harvesting compl exes is investigated. A model based on localized molecular orbitals is related to canonical molecular orbital calculations, therefore demons trating 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 carot enoid 2(1)A(g) (S-1) electronic state: [carotenoid (2(1)A(g)) (Car) to carotenoid (2(1)A(g))] and [carotenoid (2(1)A(g)) to bacteriochloroph yll (Q(y)) (Bchl)] interactions. The Car-Car coupling is found to invo lve reasonably long-range interaction terms, with a primary contributi on from dispersion-type interactions, which have an R-6 distance depen dence. The primary contributor to the Car-Bchl S-1 --> S-1 energy tran sfer mechanism is suggested to be proportional to the product of dipol e-dipole and polarization interactions. In neither case does the elect ronic interaction resemble the Dexter exchange integral in origin or d istance dependence. Some model CAS-SCF calculations of electronic inte ractions in 2,4,6-octatriene dimers are presented which support the pr edictions of the theory: the calculated interaction is found to be (i) small in comparison to the overlap-dependent triplet-triplet interact ion at close separations; (ii) small in comparison to a dipole-dipole (S-2) interaction at all separations; and (iii) quite weakly distance dependent at larger separations. The implications for the role of caro tenoids in photosynthetic light-harvesting complexes are discussed.