The nature of the initially excited state of the primary electron donor or special pair has been investigated by Stark effect spectroscopy for reaction centers from the photosynthetic bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides at 77 K. The data provide values for the magnitude of the difference in permanent dipole moment between the ground and excited state, [unk]Δμ[unk], and the angle [unk] between Δμ and the transition dipole moment for the electronic transition. [unk]Δμ[unk] and [unk] for the lowest-energy singlet electronic transition associated with the special pair primary electron donor were found to be very similar for the two species. [unk]Δμ[unk] for this transition is substantially larger than for the Qy transitions of the monomeric pigments in the reaction center or for pure monomeric bacteriochlorophylls, for which Stark data are also reported. We conclude that the excited state of the special pair has substantial charge-transfer character, and we suggest that charge separation in bacterial photosynthesis is initiated immediately upon photoexcitation of the special pair. Data for Rhodobacter sphaeroides between 340 and 1340 nm are presented and discussed in the context of the detection of charge-transfer states by Stark effect spectroscopy
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