Primary charge-recombination in an artificial photosynthetic reaction center

Abstract

Photoinduced primary charge-separation and charge-recombination are characterized by a combination of time-resolved optical and EPR measurements of a fullerene-porphyrin-linked triad that undergoes fast, stepwise charge-separation processes. The electronic coupling for the energy-wasting charge recombination is evaluated from the singlet-triplet electronic energy gap in the short-lived, primary charge-separated state. The electronic coupling is found to be smaller by Ϸ40% than that for the primary charge-separation. This inhibition of the electronic interaction for the charge-recombination to excited triplet state largely results from a symmetry-broken electronic structure modulated by configuration interaction between 3 (b1u,b3g) and 3 (au, b3g) electronic states of the free-base porphyrin.electronic coupling ͉ long-range electron transfer ͉ molecular orbital symmetry T o construct nanoscale devices or molecular wires that efficiently convert photon energies to chemical potentials, extensive studies have produced photoinduced, long-distance charge separation (CS) states by using donor (D)-acceptor (A)-linked multiarray systems that mimic natural photosynthesis (1-11). In each intramolecular, sequential electron transfer (ET) step, an electron or a hole tunnels between D and A over substantial distances (Ͼ10 Å) as in the natural photosynthetic reaction centers (12). From Marcus theory (13)(14)(15)(16)(17)(18)(19)(20), the ET rate constant (k ET ) is described (in the weak coupling regime) aswhere -h is Planck's constant divided by 2 and V, the electronic coupling matrix element, represents the electronic tunneling interaction between D and A. FCWDS denotes Franck-Condon weighted density of states parameterized by the reorganization energy ( ) and the driving force (Ϫ⌬G) for the ET reaction. The FCWDS term is given by (4where k B is the Boltzmann constant and T is the temperature. Although for several long-range ET systems, V has been interpreted in terms of a superexchange coupling model (21) that bridges the redox sites (22-28), much remains to be resolved regarding the tunneling mechanism (29-34). In addition, characterizations of V will be useful for designing molecular electronic devices (35). Key to efficient photoinduced CS is prevention of the energywasting charge recombination (CR) characterized in part by V. Although V and have been investigated for various ET steps by experimentally measuring k ET s in systems such as D 2 -D 1 -A triad (7, 9, 36) and protein complexes (24,25,34), primary CR,-A has escaped elucidation, especially in the efficient stepwise CS systems because the CR kinetics is hidden by subsequent CS processes,In the photosynthetic reaction center protein of Rhodobacter sphaeroides, which contains as cofactors a special pair (P), two bacteriopheopheophytins (H A and H B ), and two quinones (Q A and Q B ), the CR kinetics of the primary CS state of P ؉• H A Ϫ• Q A cannot easily be observed, because H A Ϫ• 3 Q A forward ET (Ϸ200 ps) is much faster than the primary CR (10-20 ns, which has b...

Hole‐Transfer Dyads and Triads Based on Perylene Monoimide, Quaterthiophene, and Extended Tetrathiafulvalene

“…The rational molecular design of photomolecular assemblies for efficient light-induced charge separation is a topic of considerable importance owing to its fundamental and practical implications for solar energy conversion (artificial photosynthesis [1][2][3][4][5] and photovoltaics [6][7][8] ). Towards this goal, the general approach, which is essentially inspired by the strategy adopted by Nature in photosynthesis, involves creating a multistep electron-transfer chain along a molecular assembly that exhibits a defined redox gradient that allows the vectorial spatial separation of the photogenerated charges.…”

Magnetic field effects on electro-photoluminescence of photoinduced electron transfer systems in a polymer film

“…A large number of molecular systems composed of donor (D) and acceptor (A) with a rigid spacer or a bridging element or composed of D and A as the separate constituents, which mimic the PIET in photosynthetic reaction center, have been designed and synthesized with the aim of the construction of an artificial PIET system equivalent to biological system. For the proper exploitation of such D-A systems, however, the elucidation of the kinetics and mechanisms of forward electron transfer as well as back electron transfer in the excited states is prerequisite [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”

Creation of Fullerene-Based Artificial Photosynthetic Systems

“…65 A similar diagram can be drawn for the H 2 P-C 60 dyad 10 in polar solvents. 63,69,95 Photoinduced CS from the freebase porphyrin singlet excited state ( 1 H 2 P Ã (1.89 eV)) to C 60 in benzonitrile occurs to yield H 2 P þ -C 60 À (1.59 eV) with k ET(CS1) of 6:7 Â 10 8 s À1 (È CS1 ( 1 H 2 P Ã ) = 87%). In contrast to the porphyrinatozinc-C 60 dyad 9, the resulting charge-separated state decays with a rate constant of 5:0 Â 10 7 s À1 , generating the porphyrin ( 3 H 2 P Ã ) and fullerene ( 3 C 60 Ã ) triplet states rather than the ground state, because the former processes are much faster than the latter process owing to the small of C 60 .…”