| Abstract
| - It has been known for 30 years that the oxidized special pair radical cation P+ is as efficient as the neutralground-state species P in quenching excitation from the neighboring accessory bacteriochlorophylls BL andBM, but the mechanism for this process has remained elusive. Indeed, simple treatments based on applicationof standard Förster theory to the most likely acceptor candidate fails by 5 orders of magnitude in the predictionof the energy transfer rates to P+. We present a qualitative description of the electronic energy transfer (EET)dynamics that involves mixing of the strongly allowed transitions in P+ with a manifold of exotic lower-energy transitions to facilitate EET on the observed time scale of 150 fs. This description is obtained usinga three-step procedure. First, multireference configuration-interaction (MRCI) calculations are performed usingthe semiempirical INDO/S Hamiltonian to depict the excited states of P+. However, these calculations arequalitatively indicative but of insufficient quantitative accuracy to allow for a fully a priori simulation of theEET and so, second, the INDO results are used to establish a variety of scenarios, empirical parameters thatare then fitted to describe a range of observed absorption and circular dichroism data. Third, EET accordingto these scenarios is predicted using a generalized Förster theory that uses donor and acceptor transitiondensities, which together account for the large size of the chromophores in relation to the interchromophorespacings. The spectroscopic transitions of P+ that facilitate the fast EET are thus unambiguously identified.
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