George L. Gaines

Spin-Polarized Radical Ion Pair Formation Resulting from Two-step Electron Transfer from the Lowest Excited Singlet State of a Fixed-Distance Photosynthetic Model System at 5 K

Abstract A photosynthetic reaction center model consisting of a zinc porphyrin primary electron donor, ZP, positioned between a naphthoquinone electron acceptor, NQ, and an N,N,N,N–tetraalkyl–p–phenylenediamine secondary electron donor, TAPD, was synthesized. The resulting rigid structure places the TAPD donor at a fixed 23 A center–to–center distance from the NQ acceptor. Excitation of ZP with 540 nm light in butyronitrile glass at 5 K results in two–step sequential electron transfer: TAPD–1*ZP–NQ > TAPD–ZP+−NQ− −> TAPD+−ZP–NQ−. The final TAPD+−ZP–NQ− radical pair lives for 4 ms and exhibits a spin–polarized EPR spectrum characteristic of a spin–correlated radical pair. The EPR spectrum of this long–lived, spin–polarized radical ion pair closely mimics the bacteriochlorophyll cation – quinone anion radical pair produced in photosynthetic reaction centers.

Low Temperature Ultrafast Charge Separation: Rate Versus Free Energy

The rate of electron transfer from the lowest excited singlet state of a porphyrin to an acceptor was measured in 2-MTHF glass at 77 K for a series of molecules. Analysis of the rate data as a function of free energy of reaction reveals that the ion-pair state of the oxidized donor and reduced acceptor is destabilized in solid solution by about 0.7 eV relative to its energy in polar liquids.

Multi-Step Electron Transfer in Rigid Photo-synthetic Models at Low Temperature: Requirements for Charge Separation and Spin-Polarized Radical Ion Pair Formation

Photoinduced, multi-step charge separation in bacterial photosynthetic reaction centers proceeds from the lowest excited singlet state of the dimeric bacteriochlorophyll electron donor in two steps to yield a weakly interacting dimer cation — quinone anion radical pair, P+-Q−, separated by 28 A.1 The chromophores within the reaction center are positioned at precise distances and orientations to insure that the electronic coupling between P+ and Q− is sufficiently weak to allow P+-Q− to live for about 100 ms.2 At long distances the electron-electron exchange interaction, J, between radicals within a charge separated ion pair is sufficiently weak that differences in local magnetic fields surrounding each radical result in S-T0 mixing of the radical pair spin sublevels.3 This mixing produces a non-Boltzmann population of the spin sublevels of the radical pair and may result in the appearance of spin-polarized EPR spectra. Such spectra have been reported extensively for both bacterial and green plant reaction centers,4–6 but have not been observed previously in rigid model systems.