Bioinspired Electron-Transfer Systems and Applications

Abstract

Bioinspired electron-transfer systems including artificial photosynthesis and respiration are presented herein together with some of their applications. First, multi-step electron-transfer systems composed of electron donor-acceptor ensembles have been developed, mimicking functions of the photosynthetic reaction center. However, a significant amount of energy is lost during the multi-step electron-transfer processes. Then, as an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer within redox cascades, simple donor-acceptor dyads have been developed to attain a long-lived and high energy charge-separated state without significant loss of excitation energy, by fine control of the redox potentials and of the geometry of donor-acceptor dyads that have small reorganization energies of electron transfer. Such simple molecular dyads, capable of fast charge separation but extremely slow charge recombination, have significant advantages with regard to synthetic feasibility, providing a variety of applications including construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion. An efficient four-electron reduction of dioxygen to water by oneelectron reductants such as ferrocene derivatives as well as by an NADH analog has also been achieved as a respiration model by using a cofacial dicobalt porphyrin that can form the -peroxo Co(III)-O 2 -Co(III) complex. The catalytic mechanism of the four-electron reduction of dioxygen has been clarified based on the detailed kinetic study and the detection of the intermediate.There has been considerable interest in the photosynthetic reaction center of purple bacteria ever since the three-dimensional X-ray crystal structures of reaction centers of Rhodobacter (Rb.) sphaeroides 1 and other purple bacteria including Rhodopseudomonas (Rh.) viridis 2 have been disclosed. The photoinduced electron-transfer processes occur in a membrane-bound protein, which contains a number of cofactors, including four bacteriochlorophylls (BChl). Of these, the central part in Fig. 1 is referred to as the special pair [(BChl) 2 ], while the other two bacteriochlorophylls (BChl) are referred to as ''accessory'' bacteriochlorophylls. There are also two bacteriopheophytins (BPhe), two ubiquinones (Q A and Q B ), and a nonheme iron atom (not shown in Fig. 1), which together with the special pair are organized in pseudo-C 2 symmetry forming two branches (A and B). Light-initiated charge separation occurs between the special pair [(BChl) 2 ] and the neighboring pigments, leading to a radical cation [(BChl) 2 þ ]. Despite the quasi-symmetrical arrangement of the cofactors, the electrons are transported unidirectionally along the A-branch of the reaction center, 4-6 which suggests that the symmetry-breaking specific interactions with the protein are fundamentally important. The electron-transfer process is found to occur very rapidly from the special pair toward the quinones to pro...

Face‐to‐Face Pacman‐Type Porphyrin–Fullerene Dyads: Design, Synthesis, Charge‐Transfer Interactions, and Photophysical Studies

“…[4][5][6][7][8][9][10][11][12] As a consequence, C 60 promotes photoinduced charge separation (CS), but retards the charge-recombination (CR) process, which results in the formation of much desired long-lived charge-separated states. [4][5][6][7][8][9][10][11][12] Several comprehensive reviews of the literature of donor-fullerene dyads, which includes larger hybrids (triads, tetrads, pentads, etc. ), that incorporate a wide variety of electron donors and linkers have been published.…”

Synthetic Multifunctional Nanoarchitecture in Lipid Bilayers: Ion Channels, Sensors, and Photosystems

“…44,45 This may include variation in mode of detection (conductance), [117][118][119] 45 enzymes for inhibitor screening, [73][74][75][76] enzymes for signal generation, [43][44][45][73][74][75][76] reactive amplifiers, analytes, matrix, and so on. 44,45 The mainly academic nature of artificial photosynthesis in lipid bilayers, finally, invites to apply the lessons learned to advanced rigid-rod nanoarchitecture on conducting surfaces such as gold or ITO 102,103 with the hope to end up with photovoltaic devices.…”

“…[101][102][103][104][105] Upon excitation with light, intramolecular transfer of an electron to the acceptor and, eventually, coinciding electron transfer from the donor to the photooxidized chromophore causes a transient separation of charges. Donor and acceptor are selected to convert as much photonic energy as possible into chemical energy.…”

“…In synthetic photosystems, porphyrins are most frequent chromophores, fullerenes have replaced quinones as most common acceptors. Studied with regard to photovoltaics in several groups including Fukuzumi 102 and Kobuke, 103 photosynthesis in lipid bilayer membranes with covalent donor-chromophore-acceptor triads has been pioneered by Gust, Moore, and Moore. 101 Support for the functional significance of advanced nanoarchitecture for molecular (opto)electronics came from studies on core-substituted perylenediimides (PDIs) in the Wasielewski group.…”

Synthesis of Covalently Linked Porphyrin Heterodimers with Potential Use in the Solar Energy Conversion Devices

“…[1][2][3][4][5][6][7][8][9] Porphyrin-based assemblies have important applications in the development of artificial devices for solar energy conversion into chemical potential or electrical energy in common with that effected by natural photosynthetic systems. [7,[10][11][12][13][14] Studies performed with many donor-acceptor model systems [1][2][3][4][5][15][16][17][18][19][20] revealed the effects of structural factors, such as distance and orientation, as well as electronic nature of the spacer, driving force, temperature and solvent on the rates and quantum yields of the energy transfer and charge separation processes. Porphyrin dimers are good photosynthetic models in which the geometry, distance and angle between the porphyrin moieties strongly influence on the efficiency of the photophysical processes.…”