Mapping electronic coupling within photogenerated radical ion pairs at fixed distances using magnetic field effects: implications for artificial photosynthesis

Abstract

We will report on a series of electron donor-acceptor (D-A) dyads and triads that undergo singlet-initiated charge separation to produce spin-coupled radical ion pairs, which subsequently undergo charge recombination to produce a locally excited triplet state analogous to what is observed in photosynthetic reaction centers from both bacteria and green plants. This mechanism is rarely observed within covalently linked donor-acceptor molecules, yet is critical to developing an understanding of the analogous process within photosynthetic reaction centers. The molecules consist of either 4-(N-piperidinyl)naphthalene-1,8-imide or 4-(N-pyrrolidinyl)naphthalene-1,8-imide chromo-phoric donors, a 1,8:4,5-naphthalenediimide, acceptor, and in the case of the triads, a series of para-substituted secondary donors. A phenyl spacer separates the chromophores and the acceptor. Time-resolved optical absorption spectroscopy of both the radical ion pair state and the triplet state in the presence of a magnetic field is used to characterize the spin-spin exchange interaction, 2J, between the radicals directly by observing resonances in the reaction yield of the triplet state vs. magnetic field. The magnitude of 2J is directly related to that of V2, the electronic coupling matrix element for the charge recombination reaction. By varying the structures of these molecules, the dependence of V2 on structure can be determined directly. Understanding this fundamental property of the dyad or triad structure provides insights into what structural features of the electron transport cofactors are important for efficient, unidirectional charge separation within the reaction center protein.