Fullerene electron acceptor properties

The UV/Vis spectra (Figure 3.2) of the chestnut brown solutions of the monoadducts CjqHR, particularly the intensive bands at = 213, 257 and 326 nm, are close to those of Cjq, demonstrating their electronic similarity [4]. The biggest changes in the spectra compared with Cjq appear in the visible region. The typical features of Cjq between X = 400 and 700 nm are lost, and a new and very characteristic band at X = 435 nm appears, which is independent of the nature of R. Also, the electrochemical properties of CjqHR are comparable with those of Cjq [5, 19]. The first three reversible reduction waves shift about 100 mV to more negative potentials. Therefore, the fullerene core in these monoadducts still exhibits remarkable electron-acceptor properties, which is one reason for almost the identical chemical reactivity compared with CgQ. 

Linking the fullerene core to a tetracyanoanthraquinodimethane (TCAQ) ( 1/2 = -0.38 V relative to the SCE) (12) or a 2,3-dibromonaphthoquinone acceptor ( 1/2 = —0.21 V relative to the SCE) [315] (13) afforded two rare examples of dyads in which an organic addend is covalently attached to the fullerene core that exhibits better electron-acceptor properties than the parent fullerene core [329]. 

Davydov et al. [46] used IGC to determine several adsorption thermodynamic properties (equilibrium constants and adsorption heats) for the adsorption of organic compounds on C q crystals, and compared them with those obtained for graphitized carbon black. The adsorption potential of the surface of fiillerene crystals was much lower than that of a carbon black surface. The dispersive interaction of organic molecules with C q is much weaker than with carbon black. The adsorption equilibrium constant for alkanes and aromatic compounds is therefore lower in the case of fullerenes. Aliphatic and aromatic alcohols as well as electron-donor compounds such as ketones, nitriles and amines were adsorbed more efficiently on the surface of fiillerene crystals. This was taken as proof that fiillerene molecules have electron-donor and electron-acceptor properties, which is in agreement with the results of Abraham et al. [44]... 

Langa, who has reported several C o derivatives with intramolecular PET processes, recently described the synthesis under microwave irradiation conditions of two pyrazolino [60]fullerene (58a,b) bearing electron-withdrawing substituents. The resulting fullerene derivatives were highly soluble in several polar and nonpolar solvents, and electrochemical studies revealed improved electron-acceptor properties compared with unmodified Ceo [63]. 

Very recently, several new classes of solution processible small molecule donors with favorable optical and electronic properties have been reported for use in OPV devices. When coupled with fullerene electron acceptors, such as [6,6]-phenyl-C7i-butyric acid methyl ester (PCy BM), solution processible small molecule based solar cells have achieved record PCEs over 8% for single layers devices and 10% for tandem cells. This chapter provides an overview of several important classes of solution processible small molecules used as donor materials in high performance OPV devices. For greater details on materials and devices, the reader is referred to several excellent reviews by Meerholz, Nguyen, Bauerle, Zhan, and Roncali. ... 

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). 

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]. 

A variation of the above strategy has been used to produce the related [2]-rotax-ane 16 in 15% yield. In this case, two potentially redox-active fullerenes act as the stoppers. CgQ was chosen for inclusion in this product because of its interesting electrochemical and electronic properties and, in particular, because it is a strong electron acceptor. Although the substituted fullerene stoppers appear to have a significant influence on the redox properties of the metal centre (an anodic shift occurs), the converse is not true and the metal does not significantly affect the redox properties of the fullerene groups. 

Thus, the higher fullerene is expected to exhibit organic electron donor properties, and acceptor-doped materials should be stable. This exciting possibility is currently being actively pursued. 

Thus the thermodynamic characteristics of adsorption at small coverage of different classes organic compounds determined by gas chromatography show that surface of ful-lerene molecular crystals and surface of graphitized carbon black have essentially different adsorption properties. On adsorption on fullerene crystals the electron-acceptor and electron-donor properties of fullerene molecules are manifested. Adsorption data on fullerenes Ceo nd C70 show that properties of fullerene Ceo a-nd C70 molecules arranged in surface layer of crystals are different. 

Abstract. Copper phthalocyanine (CuPc)-fullerene (C60) photovoltaic cells are produced by organic vapour phase deposition reaching efficiencies of 3%. The electronic transport properties of the devices are investigated as a function of the CuPc C60 absorber blend layer composition and its preparation temperature. The analysis of the transport properties of the devices employs the one-diode model. It is shown that the dominant recombination process takes place at the donor-acceptor interfaces of the CuPc and C60 absorber domains. The activation energy of recombination is related to the effective band gap of the blend layer. 

The electron affinity of fullerenes is noteworthy. They easily accept one or even several electrons. Obviously they are strong electrophiles, which markedly influences their reactivity. However, Cgo can also act as an electron donor, yet this feature is far less pronounced than the acceptor properties. 

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