29th Annual Meeting of the American Society of Photobiology

Downtown Marriot

Chicago, Il.

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 that subsequently undergo charge recombination to produce a triplet state. Selective photoexcitation of D within D-A produces the radical ion pair ¹[D^.+-A^.-] quantitatively. This is followed by radical pair intersystem crossing to yield ³[D^.+-A^.-] via the hyperfine mechanism. Charge recombination within the triplet radical pair yields [D-^3*A] or [^3*D-A] 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 reactions centers. The molecules consist of either 4-(N-piperidinyl)naphthalene-1,8-imide or 4-(N-pyrrolidinyl)naphthalene-1,8-imide chromophoric 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 electronic 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 V, the electronic coupling matrix element for the charge recombination reaction. By varying the structures of these molecules the dependence of V on structure can be determined directly. Understanding this fundamental property of the dyad or triad structure provides insights into the design of molecules that can better mimic the key charge separation and storage chemistry within photosynthetic organisms.(This work was supported by the Div. of Chemical Sciences, Office of Basic Energy Sciences, US DOE under grant no. DE-FG02-99ER14999.)

Keywords: photosynthetic reaction center, radical ion pair, magnetic field effect, time-resolved spectroscopy