Fullerene Electron-acceptor Systems

Once the synthetic methodologies were developed, the research efforts were focused on the preparation of photoactive systems where fullerenes had already shown potential applicability, such as organic photovoltaic materials. In these molecular-scale engineered systems, a fullerene electron acceptor contained in one submolecular fragment is coupled with an electron donor contained in the opposite component. Zinc porphyrins were thus coupled to in many different architectures such as, for instance, in that shown in Scheme 9.11, where the ZnP was appended to... 

The bis-dienes 13-15 form ladder-type bis-fullerene adducts. Cycloadditions proceed in good yields also with electron-deficient dienes such as 21-23. Porphyrin donor systems such as 25 or 30 or quinoidal acceptor systems such as 19 and 20 were introduced to study the properties of the charge transfer in Cjq donor-acceptor systems. 

The simplest covalently linked systems consist of porphyrin linked to electron acceptor or donor moiety with appropriate redox properties as outlined in Figure 1. Most of these studies have employed free base, zinc and magnesium tetrapyrroles because the first excited singlet state is relatively long-lived (typically 1-10 ns), so that electron transfer can compete with other decay pathways. Additionally, these pigments have relatively high fluorescence quantum yields. These tetrapyrroles are typically linked to electron acceptors such as quinones, perylenes , fullerenes , acetylenic fragments (14, 15) and aromatic spacers and other tetrapyrroles (e.g. boxes and arrays). 

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. 

Ferrocene is composed of a pair of 6-7r-electron carbon arrays and a 6-d-electron iron(II) atom. Ferrocene-fullerene donor-acceptor dyads carry all the requisites for electron-transfer phenomena. However, data for the formation of ferrocene-fullerene hybrids are not abundant. Some such dyads have already been synthesized following the methodology of 1,3-dipolar cycloaddition of the appropriate azome-thine ylides to C60, with either variable-spacing building blocks or a rigid-bridge all-cj-bonded framework, in order to tune the redox properties of the system [40,234, 248-251]. Another novel dyad that contained two covalently bound ferrocene units was recently synthesized via cyclopropanation of the fullerene core [252]. 

Electron acceptors, such as 11,11,12,12-tetracyanoanthraquinodimethane (TCAQ) and its 9,10-anthraquinone (AQ) precursor have been also attached to fulleropyrrolidine materials, since they are good candidates for obtaining fullerene-based electroactive hybrid systems [245,258,259]. 

Similarly, the pyridyl fullerenes 30 and 31 also axially bind to ZnNcCBu)4 (28) to form supramolecular triads in which the zinc(II) naphthalocyanine acts as an electron donor, the pyridyl fullerenes act as primary electron acceptors, and either the ferrocene (Fc) or iV,iV-dimethylaminophenyl (DMA) moiety serves as a second electron donor [43], Fig. 2 shows the optimized structures of the resulting triads calculated by density functional theory (DFT), showing that the three components are arranged in a linear fashion. The binding constants of these systems (7.4 x 104M 1 for [ZnNc( Bu)4] 30 and 10.2 x 104M 1 for [ZnNc( Bu)4] 31 in o-dichlorobenzene) determined by UV-Vis spectroscopy are comparable with... 

The significant effect of metal ions to stabilize the CS state described above can also make it possible to use fullerene as an electron donor rather than an electron acceptor (see below). Fullerene normally was used only as an electron acceptor in electron donor-acceptor linked systems, because the ET... 

There continues to be an enormous amount of activity in the area of PET, much of it directed towards the development of systems capable of delivering artificial photosynthesis. Many of these systems involve porphyrin units as electron-donors and thus it is appropriate to consider them in this section of the review. A number of new fullerene-porphyrin dyads have been reported. A pyrazolinofullerene (155) has been constructed which facilitates efficient PET when strong donors such as iV,Ar-diethylaniline or ferrocene are linked to the pyrazoline ring. A photosynthetic multi-step ET model (156) based on a triad consisting of a meso,meso- inked porphyrin dimer connected to ferrocene and Ceo as electron-donor and electron-acceptor, respectively, has been synthesized and its ET dynamics (Scheme 38) have been investigated using time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. ... 

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