Photoinduced electron transfer fullerenes

Imahori H and Sakata Y 1997 Donor-linked fullerenes photoinduced electron transfer and its potential application Adv. Mater. 9 537-46... 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

Kuciauskas D, Lin S, Seely G R, Moore A L, Moore T A, Gust D, Drovetskaya T, Reed C A and Boyd P D W 1996 Energy and photoinduced electron transfer in porphyrin-fullerene dyads J. Phys. Chem. 100 15 926-32... 

Recently, photochemical and photoelectrochemical properties of fullerene (Cto) have been widely studied [60]. Photoinduced electron-transfer reactions of donor-Qo linked molecules have also been reported [61-63]. In a series of donor-Cfio linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin-Qo linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. 

Fullerenes have shown particular promise as acceptors in molecular electronics, and numerous interesting TTF/Cgo ensembles have been reported.42 For example, Orduna and co-workers75,76 prepared the TTF/C60 dyad 13 and observed photoinduced electron-transfer from the TTF to the fullerene. Martin et al 1 observed two separate one-electron transfer events in their conjugated dyads 14 (where n = 2). The TTF-porphyrin-fullerene triad 15, prepared by Carbonera et al.7 showed long-lived photoinduced charge separation. 

Braun M, Atalick S, Guldi Dirk M, Lanig H, Brettreich M, Burghardt S, Hatzimarinaki M, Ravanelli E, Prato M, Van Eldik R, Hirsch A (2003) Electrostatic Complexation and Photoinduced Electron Transfer between Zn-Cytochrome c and Polyanionic Fullerene Dendrimers. Chem. Eur. J. 9 3867-3875. 

The photochemical addition of some cyclic oligosilanes to Ceo has also been found interesting. Scheme 8.8 shows some examples of such a transformation. Irradiation (X > 300 nm) of a toluene solution of disilirane 36 with Ceo afforded the fullerene derivative 37 in a 82% yield [37]. The reaction mechanism is still unknown. When toluene is replaced by benzonitrile the bis-silylated product of the solvent was obtained in good yields. In these experiments a photoinduced electron transfer between 36 and Ceo is demonstrated, indicating the role of Ceo as sensitizer [38]. The photoinduced reactions of disilirane 36 with higher fullerenes such as C70, Cv8(C2v)and CuiDi) have also been reported... 

Some cation-radicals can appear as hydrogen acceptors. Thus, fullerene Cgg is oxidized to the cation-radical at a preparative scale by means of photoinduced electron transfer. As in the case of anion-radical, the fullerene Cgo cation-radical bears the highly delocalized positive charge and shows low electrophilicity. This cation-radical reacts with various donors of atomic hydrogen (alcohols, aldehydes, and ethers) yielding the fullerene 1,2-dihydroderivatives (Siedschlag et al. 2000). 

The photoinduced electron-transfer dynamics has also been examined for a series of porphyrin-fullerene-linked molecules with the same spacer employed for Fc-ZnP-H2P-C6o ZnP-Ceo (edge-to-edge distance Ree = 11-9 A), Fc-ZnP-Ceo (Ree = 30.3 A) and ZnP-H2P-Ceo (Ree = 30.3 A), shown in Chart 1 [53]. The driving force dependence of the electron-transfer rate constants ( et) of these dyad, triads, and tetrad molecules is shown in Fig. 3, where log et is plotted against the driving force (-AGet) [47]. 

The Sc -promoted photoinduced electron transfer can be generally applied for formation of the radical cations of a variety of fullerene derivatives, which would otherwise be difficult to oxidize [135]. It has been shown that the electron-transfer oxidation reactivities of the triplet excited states of fullerenes are largely determined by the HOMO (highest occupied molecular orbital) energies of the fullerenes, whereas the triplet energies remain virtually the same among the fullerenes [135]. 

Photoinduced electron transfer [22] from reductants such as l-benzyl-l,4-di-hydronicontinamide [27], the Hantzsch-ester [22] (diethyl-2,6-dimethyl-l,4-di-hydropyridine-3,5-dicarboxylate) or 10-methyl-9,10-dihydroacridine [27, 28] to the fullerene and successive proton transfer leads selectively to l,2-dihydro[60]fullerene. These reductions usually proceed under mild conditions. 

Figure 14.19 Schematic representation of a photoinduced electron transfer in a fullerene-based donor-acceptor dyad.

Ito O, Yamanaka K (2009) Roles of molecular wires between fullerenes and electron donors in photoinduced electron transfer. Bull Qiem Soc Jpn 82 316—332... 

Takai A, Chkounda M, Eggenspiller A et al (2010) Efficient photoinduced electron transfer in a porphyrin tripod-fullerene supramolecular complex via pi-pi interactions in nonpolar media. J Am Chem Soc 132 4477-4489... 

A few other interesting molecular architectures exhibiting uncharacteristic electrochemical behavior have been constructed on the basis of the methanofullerene building block. These include two methanofullerene-substituted bipyridine ligands complexed to rhenium and ruthenium ((35) and (36) in Fig. 17), which were prepared as possible candidates for photoinduced electron transfer processes [138]. In addition, three fullerene crown ether conjugates ((38), (39), and (40) in Fig. 18) have... 

TTF-based D-A systems have been extensively used in recent years to play around photoinduced electron transfer processes. Typically, when an electron acceptor moiety that emits fluorescence intrinsically is linked to TTF (D), the fluorescence due to the A moiety may be quenched because of a photoinduced electron transfer process (Scheme 15.1). Accordingly, these molecular systems are potentially interesting for photovoltaic studies. For instance, efficient photoinduced electron transfer and charge separation were reported for TTF-fullerene dyads.6,7 An important added value provided by TTF relies on the redox behavior of this unit that can be reversibly oxidized according to two successive redox steps. Therefore, such TTF-A assemblies allow an efficient entry to redox fluorescence switches, for which the fluorescent state of the fluorophore A can be reversibly switched on upon oxidation of the TTF unit. 

Preparation of mono-adducts of fullerene - for studies on electrostatic interactions - was undertaken by cyclopropanation of fullerene with appropriately functionalised malonic esters 1 (Bingel reaction) to form 2. Coupling with the tert-butyl protected oligoamide-amino-dendron 3 and subsequent hydrolysis lead to the water-soluble fullerene dendron 5, which can carry up to nine negative charges after depro to nation. After association with the zinc complex of cytochrome C, photoinduced electron transfer (PET) from the redox protein to the fullerene can be accomplished, which was studied by fluorescence spectroscopy. 

Investigations of the photoinduced electron transfer between fullerenes and porphyrines show that electron transfer occur from the porphyrin to 3C o/3C o-Additionally, due to the absorption of the porphyrines at the excitation wavelength, electron transfer from triplet-excited porphyrine to fullerene may occur. Both pathways lead to the radical ion pair, the ratio depends on the ratio of con-... 

The first really successful experiments to generate the fullerene radical cation by photoinduced electron transfer were carried out by Foote and coworkers (Fig. 19) [167], They used singlet excited /V-methylacridinium hexafluorophosphate (MA+) as an electron acceptor which has a reduction potential of +2.31 V vs SCE, enough to oxidize C6o [Eq. (6)] [19]. 

The radical anion Cw, can also be easily obtained by photoinduced electron transfer from various strong electron donors such as tertiary amines, fer-rocenes, tetrathiafulvalenes, thiophenes, etc. In homogeneous systems back-electron transfer to the reactant pair plays a dominant role resulting in a extremely short lifetime of Qo. In these cases no net formation of Qo is observed. These problems were circumvented by Fukuzumi et al. by using NADH analogues as electron donors [154,155], In these cases selective one-electron reduction of C6o to Qo takes place by the irradiation of C6o with a Xe lamp (X > 540 nm) in a deaerated benzonitrile solution upon the addition of 1-benzyl-1,4-dihydronicoti-namide (BNAH) or the corresponding dimer [(BNA)2] (Scheme 15) [154], The formation of C60 is confirmed by the observation of the absorption band at 1080 nm in the near infrared (NIR) spectrum assigned to the fullerene radical cation. 

Photoinduced electron transfer from the amine to C6o to yield a radical ion pair is suggested to be the initial step for the formation of 54a-b. This is followed by deprotonation of the amine cation by the fullerene anion to give an a-aminoalkyl and HC6o radical pain [134], Subsequent combination of the radical pair leads to the final product. Formation of 55 is likely to be initiated by PET from 54b to C6o. This is then followed by successive intermolecular proton transfer, hydrogen abstraction, and ring closure to give l,2-H2C6o and 55 (Scheme 21). 

Sun et al. further investigated the photoinduced electron transfer reactions of C6o and triethylamine, both in deoxygenated solution and air saturated solution [79], Three types of cycloadducts of fullerenes 33 and 92a-b were obtained, whereas the formation of the monoalkylated l,2-dihydro[60]fullerene 29 as described by Liou et al. [230] in the reaction of trimethylamine and /V,/V-dimcthy-laniline with C6o, was not observed (see Fig. 32). 

Photoinduced electron transfer from the amine to the fullerene core leading to a radical ion pair is suggested to be the initial step in the reaction mechanism (Scheme 39). Formation of the bis-[6,6] closed adduct proceeds via [3 + 2] cycloaddition of the tertiary amine followed by a [2 + 2] cycloaddition of the vinyl group and the C6o double bond adjacent to the previously formed ring connection leading to a structure analogous to 1,2,3,4-C6oH4-... 

Novel hybrid materials have been realized in which fullerenes participate in composite films with 7r-conjugated-polymer electron donors such as oligothio-phenes. Established studies have already shown that the photoinduced electron transfer is rather enhanced between 7r-conjugated polymers and fullerenes, while back electron transfer is considerably slower [145,149,171,172].Electrosynthe-sized polythiophene with pendant fullerene substituents was recently obtained from the corresponding biothiophene-fulleropyrrolidine dyad [173]. The novel material described has the potential of a double-cable polymer, heavily loaded with fullerene electron-conducting moieties. 

In 2002, a triad molecule including dithienylethene (DTE), porphyrin (P), and fullerene (DTE-P-C60) was first designed and synthesized as a photoinduced electron transfer (PET) switch by D. Gust et al. [33], as shown in Scheme 5. This molecule was synthesized by modified procedures reported literature Porphyrin chromophore was covalently linked to substituted dithienylethene by Sonogashira coupling reaction. Fullerene was introduced by the cycloaddition reaction of dyad aldehyde (dithienylethene-porphyrin aldehyde) with the fullerene and the N-methyl-glycine. 

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