In natural photosynthesis the reaction time imposed by thermodynamics is exploited for kinetic control and efficient photochemical conversion of solar energy. In bacterial photosynthetic reaction centers, excitation of the special pair (P) couples to specific low frequency intradimer and protein modes to provide an efficient channel for vibrationally assisted mixing of charge transfer states into excitons. This process has been described with a modified Redfield Hamiltonian that includes four energy levels and two collective modes. Very recently we have resolved the reaction coordinate for a 3D minimal model of the special pair and its protein environment, guided by solid state NMR spectroscopy of the electronic ground state that reveals how the special pair is tuned for electron transfer by packing effects.1 The axial histidines produce PLd-PMe- charge transfer character. This configuration is unstable and oscillates between the two halves of P due to intradimer vibrations, which can be detected with solid state NMR spin diffusion methods.2, 3 Every time the pair is in a favourable charge transfer configuration, charge separation can proceed efficiently, and electrons can tunnel from the donor into the redox chain of the active branch of the RC. The 3-D specific motions involve classically coherent charge transfer intermediate formation between the two halves of P, HisM202 rotation, proton displacement coupled to this motion involving water and proton displacement upon oxidation of P involving HisL168 and the 31-acetyl of PL.1 Overall the reaction is slowed down by the oscillatory behaviour in P, but it can proceed with little energy loss because of temporal lowering of the transition state by dynamic structural changes that lower the energy levels and can confine charge for dissipative, proton-assisted tunnelling. This is different from the Marcus transfer mechanism that requires a match between the free energy difference and the reorganization energy in an incoherent process, and it is also different from the depletion region or �band bending� that drives charge separation in majority carrier solar cell and PEC systems. The time-dependent alternation between quantum delocalization and classical confinement makes effective use of non-stationary states that evolve in time, and get �dressed� with selected vibrations on a very short time scale, a few tens of femtoseconds. The protein can control the vibration dressing, and by adapting the structure the selected modes that can contribute to a smooth reaction coordinate will be the modes that are enhanced. This will result in semi-classical evolution along the reaction coordinate with an effective particle mass of ~ 100 electron masses, which can appear as an effective driver of natural and artificial photosynthesis.

Responsive matrices for solar to fuel conversion: Trading time for efficiency

247th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.