Comparison of Reorganization Energies for Intra- and Intermolecular Electron Transfer

Imahori, Hiroshi; Yamada, Hiroko; Guldi, Dirk M.; Endo, Yuichi; Shimomura, Akihisa; Kundu, Santi; Yamada, Koji; Okada, Tadashi; Sakata, Yoshiteru; Fukuzumi, Shunichi

doi:10.1002/1521-3773(20020703)41:13<2344::aid-anie2344>3.0.co;2-7

Cited by 196 publications

(121 citation statements)

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“…30 This observation may imply a rise of the HOMO energy level and lowering of the LUMO energy level of B compared with the corresponding MO levels of the amide bond of compound 2. These changes in the LUMO of B decrease the energy barriers of the electron transfer from ZnP* to B in the CS (CS-1).…”

Section: ¹1mentioning

confidence: 98%

Roles of Molecular Wires between Fullerenes and Electron Donors in Photoinduced Electron Transfer

Ito

Yamanaka

2009

Bulletin of the Chemical Society of Japan

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In the photoinduced intramolecular electron transfer events of donorbridgeacceptor molecular systems, the bridges play important roles. In the first part of this account, we review charge separation and charge recombination of porphyrinbridgefullerene systems, in which the porphyrin acts as a photosensitizing electron donor, whereas the ground state of the C 60 moiety acts as an electron acceptor. In such systems, the charge separation usually takes place through the LUMO of the bridges via a super-exchange mechanism and/or a hopping mechanism, depending on their relative LUMO energy levels. On the other hand, the charge recombination of the radical ion pairs usually takes place through the HOMO of the bridges. In the second part, research of charge separation via the excited state of fullerene through the HOMO of the bridges is reviewed. So far, very small damping factors have been reported for ³-conjugated bridges such as oligophenyleneacetylenes, oligothiophenes, and oligothiophenevinylenes. On summarizing these results, it is revealed that switching from the super-exchange mechanism in short bridges to hopping mechanism in longer bridges is important to achieve long distance electron transfer through the bridges.Photoinduced electron transfer (ET) of donoracceptor systems is the most elementary of all photochemical reactions, and plays a crucial role in many essential photoactive devices.14 The functions of conjugated nano-scale donor acceptor molecules are useful for molecular electronic devices 5 and photovoltaic cells. 6,7 Photo-induced electron-transfer systems exhibiting charge-separation (CS) and charge-recombination (CR) processes of the covalently connected donorspacer acceptor systems, in which the spacers are flexible methylene chains, have been extensively studied by the Okada, Mataga, Sakata, and Misumi groups. 811 Recently, fullerene derivatives have been widely applied to construct photovoltaic cells. 12,13 For further advances of these chemically modified fullerenes as molecular electronic devices, it is very important to reveal the general roles of the bridges in the covalently connected donor bridgeacceptor (DBA) systems, in which the donor and acceptor can be photo-excited as illustrated in Figure 1. 14 When the D moiety in a DBA system is photo-excited, the excited state of D (D*) acts as a photosensitizing electron donor, thus CS takes place from D* to the A moiety through the LUMO of B, giving finally the radical ion pair D•+ BA•¹ as illustrated in Figure 2. In this photosensitizing CS process via D*, two cases can be considered depending on the energy level of the LUMO of the B unit relative to the LUMO level of D. One is super-exchange, in which the LUMO energy level of B is higher than the LUMO levels of D and A and another is electron hopping, in which the LUMO level of B is lower than the LUMO level of D. 15,16 Thus, in the superexchange mechanism, the first-step CS (CS-1) after initial photo-excitation (CS-0) is endothermic and the second-step CS (CS-2) is exothermic as shown in F...

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Section: ¹1mentioning

confidence: 98%

Roles of Molecular Wires between Fullerenes and Electron Donors in Photoinduced Electron Transfer

Ito

Yamanaka

2009

Bulletin of the Chemical Society of Japan

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“…As chromophores, they have extended absorption throughout the visible spectrum. Importantly, they have been found to have small internal and solvent reorganization energies, and low sensitivity to solvent stabilization of their anions [9][10][11]. Those features lead to desirable ET properties such as rapid photoinduced ET and slow charge recombination.…”

Section: Introductionmentioning

confidence: 99%

EPR-detected photoinduced electron transfer in three structurally related molecular triads

et al. 2006

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A comparative electron paramagnetic resonance (EPR) study has been performed on a series of structurally related molecular triads which undergo photoinduced electron transfer and differ one from the other in terms of the acceptor or donor moieties. The molecular triads, C-P-C6o , TTF-P-C6o and C-P-PF, share the same free-base tetraarylporphyrin (P) as the primary electron donor, which after light excitation initiates the electron transfer process, but differ either in terms of the electron acceptor (fullerene derivative, C6o , versus fluorinated free-base porphyrin, PF), or in terms of the final electron donor (carotenoid polyene, C, versus tetrathiafulvalene, TTF). AII these molecular triads can be considered artificial photosynthetic reaction centers in their ability to mimic several key properties of the reaction center primary photochemistry. Photoinduced charge separation and recombination have been followed by time-resolved EPR in a glass of 2-methyltetrahydrofuran and in the nematic phase of the uniaxial liquid crystal E-7. All the triads undergo photoinduced electron transfer, with the generation of charge-separated states in both the low-dielectric environment of the 2-methyltetrahydrofuran glass and in anisotropic E-7 medium. Different photochemical pathways have been recognized depending on the specific donor and acceptor moieties constituting the molecular triads. In the presence of the tetrathiafulvalene electron donor singlet-and triplet-initiated electron transfer routes are concurrently active. Recombination to the low-lying carotenoid triplet state occurs in the carotene-based triads, while singlet recombination is the only active route for the TTF-P-C6o triad, where a low-lying triplet state is lacking. Long-lived charge separation has been observed in the case of TTF-P-C6o: about 8 p.s for the singlet-born radical pair in the glassy isotropic matrix and about 7 ~ts for the triplet-born radical pair in the nematic phase of E-7. For all the molecular triads, a weak exchange interaction (J ~ I G) between the electrons in the final spin-correlated radical pair has been evaluated by simulation of the EPR spectra, providing evidence for superexchange electronic interactions mediated by the tetraarylporphyrin bridge.

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“…In particular, fullerenes have small reorganization energies of electron transfer, which results in remarkable acceleration of photoinduced charge separation and of charge shift as well as deceleration of charge recombination [29][30][31][32][33][34]. Thus, they have been frequently employed as an electron acceptor in donor-fullerene composites to yield a long-lived charge-separated state with a high quantum yield [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28].…”

Section: Generalmentioning

confidence: 99%

Novel Electron Donor Acceptor Nanocomposites

Imahori

Guldi

Fukuzumi

2010

Chemistry of Nanocarbons

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Comparison of Reorganization Energies for Intra- and Intermolecular Electron Transfer

Cited by 196 publications

References 28 publications

Roles of Molecular Wires between Fullerenes and Electron Donors in Photoinduced Electron Transfer

Roles of Molecular Wires between Fullerenes and Electron Donors in Photoinduced Electron Transfer

EPR-detected photoinduced electron transfer in three structurally related molecular triads

Novel Electron Donor Acceptor Nanocomposites

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