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 Å 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.