Fullerenes: From Synthesis to Optoelectronic Properties

Although extensively studied in the last decade, the fullerenes continue to be a most exciting and promising challenge in current chemical research. After a first phase characterized by theoretical, physical and physicochemical studies, a well established “organic chemistry of the fullerenes” developed in a very rapid progression to the point that, currently, a wide variety of functionalized fullerenes are available through simple and accessible synthetic routes [1,2]. Among the many reactions that were successfully developed, the 1,3-dipolar cycloaddition of azomethine ylides provides a valuable synthetic procedure for the preparation of functionalized fullerenes [3–5]. The product of the reaction, named fulleropyrrolidine because a pyrrolidine ring is fused to a ring-junction of a fullerene, retains the basic properties of the parent fullerene and can be used as a starting material for further functionalization. Whereas monofunctionalization of the highly symmetrical [60]fullerene affords a single fulleropyrrolidine, the same addition to [70]fullerene [6, 7] gives rise to mixtures of regioisomers. For this reason, current fulleropyrrolidine syntheses are predominantly carried out using [60]fullerene.

Following the discovery of the macroscopic-scale C₆₀ synthesis by Krätschmer, Huffman and co-workers, the physical properties of this fascinating carbon cage have been intensively investigated [1–8]. Among the most spectacular findings, C₆₀ was found to behave like an electronegative molecule able to reversibly accept up to six electrons, to become a supraconductor in M₃C₆₀ species (M = alkali metals), or to be an interesting material with non-linear optical properties [1–8]. However, this new molecular material aggregates very easily and is insoluble or only sparingly soluble in most solvents. Therefore, pristine C₆₀ is difficult to handle. This serious obstacle for practical applications can be, at least in part, overcome with the help of the organic modification of C₆₀. Effectively, the recent developments in the functionalization of fullerenes allow the preparation of highly soluble C₆₀ derivatives easier to handle, and the electronic properties such as facile multiple reducibility, optical non-linearity or efficient photosensitization that are characteristic of the parent fullerene are maintained for most of the C₆₀ derivatives [1–8].

During the last recent years fullerenes and their derivatives have shown a wide variety of interesting properties and applications. Most of these modified fullerenes have been prepared by different chemical approaches developed during the last decade. The most important chemical methodologies designed for the synthesis of novel organofullerenes will be presented in the first chapters of this book for a better understanding of the importance that synthetic aspects have in the search of novel properties and applications of functionalized fullerenes.

Aromaticity has played an important role in modern organic chemistry, and is one of the most frequently used and widely accepted concept for understanding, systematizing and predicting peculiarities of structure, stability, reactivity and magnetic properties of organic compounds. Aromaticity is a concept of time-dependent phenomenon and its historical development has been well reviewed recently [1–4].

The remarkable electron accepting character of fullerenes make them attractive molecules in several fields of chemistry [1, 2], For instance, this property has been fruitfully exploited in photochemistry [3], electrochemistry [4], as well as in synthetic [5] and materials chemistry [6]. As far as photochemistry is concerned, it has been widely demonstrated that in chemical systems containing fullerenes and suitable electron donors (either in solution or in the solid state), photoinduced electron transfer may occur.

A considerable amount of work concerning systems in which C₆₀ is an electron acceptor has been published in the past three years, representing a substantial advance in knowledge over that summarized in the reviews published by Guldi and Kamat in 2000 [1] and by Martin et al. in 1998 [2]. Other reviews covering specific topics in this area have also appeared in the interim [3–5]. Accordingly, the present chapter will concentrate on new developments in this field, with only occasional reference to work published before 1999. The fundamental principles behind fullerene donor-acceptor systems are revisited and, for the first time, a section summarizing the experimental methods available for the study of these systems is presented. Other chapters in this volume deal with subjects that are very closely interwoven with the present discussion, specifically “Energy Transfer in Functionalized Fullerenes” (Armaroli), “Reorganization Energy in Functionalized Fullerenes” (Guldi), and “Photovoltaic Applications” (Hummelen). Where these subjects arise, as they will repeatedly, the reader will be referred to these chapters for more extensive discussions.

EPR spectroscopy of modified fullerenes is considered. In particular are described fullerene derivatives in the first excited triplet state, radical species produced by photoexcitation and the peculiar shape of their EPR spectra, and high spin species and processes involving the interaction of photoexcited fullerenes with free radicals. A brief introduction is included on EPR spectroscopy of excited state molecules and on time resolved EPR techniques, used for studying them. A section deals with the spectra of triplet excited mono- and poly-adducts of C₆₀, and a section is devoted to the peculiar spin polarisation characteristics of the spectra of radical pairs produced by photo electron transfer from an electron donor to fullerene. Spin polarisation effects, which are always present in the spectra of photoexcited systems, are described in relation to the process of generation of the excited triplet, and in the interaction of excited triplets with free radicals. One of the last sections deals with the EPR spectra of fullerenes covalently linked to one or two stable free radicals and in particular with the effect of pulsed light irradiation on their spectra.

In the context of optimizing charge-separation processes in artificial model systems, meaningful incentives are lent from bacterial photosynthetic reaction centers [1]. Whereas in green or purple bacteria only one photosynthetic unit — PS II — is carrying out the light-to-chemical product conversion, green plants are using two systems — PS I and PS II [2]. Essential to all these systems is a relay of short-range energy/electron transfer reactions, evolving among chlorophyll- and quinone-moieties embedded in a transmembrane protein matrix. Ultimately the product of these cascades is transformation of light into usable chemical energy. The latter governs water cleavage to O₂ and reduction of NADP to NADPH, which is used to produce in its final instant sugars from CO₂.

More than ten years after the initial isolation of gram-quantities of fullerene compounds in 1990 [1], the field has matured considerably but continues to evolve in new and interesting directions. The unique properties of fullerenes and their derivatives continue to find applications in potentially useful areas, some of which are reviewed in other chapters in this book. Some of the properties that caught a lot of the initial attention in fullerene research, such as superconductivity [2], have found renewed interest in view of very recent observations of increased critical temperature or T_C values in hole-doped C₆₀ [3]. At the time of writing of this article a new report has appeared describing the use of bromoform to expand the crystal lattice of C₆₀ and the consequent observation of superconductivity at 117 K for hole-doped single crystals of this material [4]. This is the highest transition temperature ever observed for a non-cuprate superconductor and qualifies the fullerenes as the second class of materials that have broken the 77 K T_c barrier. Due to the greater ease of processability compared to the corresponding cuprate ceramics, it is possible that these C₆₀-based superconductors will catch up and maybe surpass the ceramic materials in technological applications. Perhaps more interestingly, the work reported suggests that further expansion of the lattice parameter from the observed 14.45 Å for the bromoform-based material to 14.7 Å without breaking the crystal could result in superconductivity around room temperature.

Most promising applications of C₆₀ fullerene are in the field of optoelectronics and nonlinear optics (NLO). Photovoltaic cells and infrared detectors are among actively pursued optoelectronic applications while, in the field of NLO, after some initial emphasis on third-order responses, such as intensity dependent refractive index, the focus is now mainly on optical limiting based on the mechanisms of reverse saturable absorption (RSA) and possibly other multiphoton processes.

There are several kinds of materials and devices requiring a peculiar and specific organization of active moieties, which is accomplished through distinctive techniques. A well-known example is represented by the Langmuir-Blodgett (LB) technique that nowadays is described as a frontier method for the deposition of ultra-thin and homogeneous films with a pre-determined architecture, composition, thickness and usually with a resulting elevated level of anisotropy. Another impressive aspect is that the LB technique is among the few methods allowing the realization of different electrical conjunctions between the two ends of a single molecule.

And then there was light. Since the beginning of time man has harvested the energy of the sun in a secondary manner, for instance by growing crops or by burning wood or fossil fuels. More recently, more direct ways of attaining solar energy conversion have evolved. Direct conversion of solar irradiative power into electrical power using the photovoltaic effect constitutes a clean and sustainable source of energy. Already in 1839, Bequerel discovered the photovoltaic effect while irradiating one of two platinum or gold electrodes, immersed in silver halide solutions [1]. The first usable solar cell, based on monocrystalline silicon, was reported in 1954 [2]. While at present silicon-based solar cells are still most important, commercially, a plethora of photovoltaic (PV) devices is being investigated (and some of them developed for commercial application). Apart from silicon, several inorganic semiconductor materials (especially GaAs, CdTe, Copper-Indium-diselenide (CIS)) are being applied as PV active materials. Concurrently, organic molecules and materials are being investigated as potential PV active layer constituents. A driving force is the notion that organic thin-film PV devices offer intriguing new possibilities, in combination with the existence of cheap technology, developed for other kinds of ‘plastic’ thin-film applications.