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In recent years, the ability of carbon to exist in different allotropic forms has provided, besides C₆₀, new varieties of nanoscale size shapes with fascinating properties, such as higher fullerenes, endohedral fullerenes or single and multi-wall carbon nanotubes within others.¹

In this context, chemical functionalization is an especially attractive goal as it can improve solubility and processability of carbon nanostructures and allows the unique properties of these carbonaceous species to be coupled to those of other materials.

With the long term objective of constructing versatile and functional nanosized ensembles for photoinduced electron transfer, we lately embarked on the preparation of donor-acceptor nanohybrids considering the combination of these new carbon nanostructures with electron-donor moieties following different, covalent and non-covalent methodologies.

In a first approach, we have prepared single-walled carbon nanotubes (SWNT) endowed with covalently linked tetrathiafulvalene (TTF) derivatives by using simple esterification reactions (Figure 1). Results from near-infrarred and transient absorption measurements showed that the charge recombination dynamics is a function of the spacer linking the TTF moiety to the nanotube and the donor ability of the different TTF derivatives investigated.²

Figure 1. SWNT-exTTF covalent nanostructures.

Although the covalent approach is very versatile, an alternative strategy to control the organization between donor and acceptor units, while preserving the p system of the graphene sheets and, therefore, the electronic structure of CNTs, consists in the supramolecular funtionalization of SWNTs by means of hydrophobic, p-stacking, or van der Waals interactions with the sidewalls of SWNTs. For this purpose, we have prepared pyrene-exTTF³ or pyrene-TTF⁴ (Figure 2) derivatives and investigated their electron donor–acceptor interactions with different types of CNTs.

Figure 2. Non-covalent functionalization of SWNTs with pyrene-TTF.

In the context of photoinduced electron transfer important differences are observed for the CNT-TTF series: charge injection into the conduction band of CNTs afforded stable radical ion pair states only for MWNTs, while the lifetimes observed for SWNTs are much shorter, as the rate constant decay for the radical ion pair state indicates ( >3 x 10¹¹ s^-1).⁴

Endohedral metallofullerenes possess larger absorptive coefficients than C₆₀ in the visible region of the electromagnetic spectrum and a low HOMO-LUMO energy gap, while preserving a remarkable electron accepting ability, similar to that of C₆₀. Considering these electronic features, Sc₃N@C₈₀, Y₃N@C₈₀,La₂@C₈₀ and Ce₂@C₈₀ have shown to be applicable as part of electron-donor/acceptor systems in combination with powerful donors such as ferrocene^5,6 metalloporphyrins⁷ or p-extended tetrathiafulvalene derivatives (exTTFs)^6,8(Figure 3).The photophysical investigations revealed a significant stabilization of the radical ion pair state of Sc₃N@C₈₀-ferrocene when compared to an analogous C₆₀-ferrocene dyad.⁵ This results might indicate a promising prospect of endohedral fullerenes in solar-energy conversion applications.

Figure 3. exTTF derivative of [6,6]-I_h-Y₃N@C₈₀

References

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With the aim to explore more deeply the chemistry of fullerenes and to obtain more sophisticated structures, pyrrolidino[60]fullerenes derivatives endowed with different nucleofilic groups have been chosen as starting building blocks to test a series of reactions. For this way, Fullerenines ( R1 is an alkyne group) displayed a very interesting reactivity giving rise to thermal ([2+2] or “ene” ) or metal mediated (Pauson-Khand) cycloadditions. Similarly, alcohols, phenols and thioalcohols linked to the pyrrolidino fullerenes underwent nucleofilic addition to the cage. All of these cycloadditions occur regioselectively to the cis-1 double bond of the pyrrolidinofullerenes giving rise to a wide range of different bisadducts with two or more fused ring built on the fullerene cage. (Figure 1).

The employ of transition metals in fullerene chemistry has also been successfully proven in the removal of functionality from the carbon cage. Particularly differently substituted pyrrolidinofullerenes, considered chemically stable species, undergo a highly efficient retro-cycloaddition which quantitatively afford the parent unsubstituted fullerene. This protocol hasemerged as an useful protection-deprotection protocol for fullerenes (figure 1).

The preparation of chiral fullerenes in a controlled way has been recognized as an important topic. The lack of a general method for the preparation of chiral derivatives has conditioned the possible application of fullerene compounds. In this regard, often, racemic compounds have been used, even in fields such as medicinal chemistry (HIV protease inhibition) where the stereochemical configuration is of primary importance.

The main obstacle for a general method for the synthesis of chiral fullerene derivatives lies, in the unavailability of most of the known catalytic systems for the selective addition to a non coordinating double bond of the fullerene sphere.

Figure 2

Recently, we have reported the first synthesis of chiral pyrrolidinofullerenes (the most widely studied fullerene derivatives) prepared “á la carte” by metal catalyzed enantioselective 1,3-dipolar cycloaddition of N-metalated azomethine ylides onto the non ligand fullerene C₆₀ sphere used as dipolarophile. In contrast to previous classical thermal conditions used for the racemates, the reaction proceeds smoothly under very mild conditions at low temperature and in higher yields. A most outstanding finding is the complete control on the stereochemical outcome of the reaction by modifying the metal catalyst [Ag(I) or Cu(II)] in combination with a variety of chiral ligands. This methodology has proven to be quite general affording ee > 90%. Furthermore, well-defined chiral carbon atoms linked to the fullerene sphere are able to perturb the inherent symmetryof the fullerene p-system as revealed by the CD measurements.

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The convex surface of fullerenes is particularly well-suited for its molecular recognition by concave conjugated fragments.^1,2 We noticed that the shape complementarity between the concave aromatic face of p-extended derivatives of tetrathiafulvalene (exTTF) and the convex exterior of fullerenes should lead to large and positive non-covalent interactions (see Figure). In the last five years, we have developed a series of electroactive receptors for fullerenes based on exTTF. Our hosts were initially based on simple tweezers-like designs³ and showed binding constants in the order of Log K_a = 3.5 in chlorobenzene at room temperature.^4-6 Building on these simple tweezers, we have been able to construct self-assembled donor-acceptor nanostructures, like linear⁷ and hyperbranched⁸ supramolecular polymers, and large covalent dendrimers —up to 4^th generation— capable of associating several units of C₆₀.⁹ More recently, by substituting the exTTF units with the geometrically equivalent tetracyanoanthraquinodimethane (TACQ), we have also constructed all-acceptor linear and hyperbranched supramolecular oligomers¹⁰.

Based on the same recognition motif, we have very recently developed some of the best receptors for fullerenes reported in the literature, either decorated with two exTTF units as part of macrocyclic architectures, with Log K_a = 6.5 towards C₆₀ in PhCI and 7,5 in PhCN at room temperature ^11,12 or featuring three units of exTTF covalently connected to a cyclotriveratrilene (CTV) unit as preorganizing fragment, with Log K_a = 5.3 towards C₆₀ and Log K_a = 6.3 towards C₇₀ (all binding constants measured in PhCl, at 298 K).¹³. We have also investigated the cooperativity between p-p and n-p interactions, by decorating a single exTTF unit with two crown ether moieties, to obtain log K_a = 7.0 in PhCN at room temperature¹⁴

Some exTTF-based receptors for fullerene, showing their binding constants in PhCl at room temperature. Also included, a linear head-to-tail supramolecular polymer, as an example of self-assembled architecture based on the exTTF-fullerene interaction.

Based on the same concave-convex complementarity principle, and with the aim to construct an electron-donor fragment that would: 1) show good electron-donor properties, 2) absorb light efficiently, preferably in the visible region, and 3) self-assemble with them in a controlled fashion, we designed and synthesized a new family of exTTF derivatives, truxene-TTFs.

Truxene-TTFs feature three dithiole units connected to a truxene core. To accommodate the dithioles, the truxene moiety breaks down its planar structure and adopts an all-cis sphere-like geometry with the three dithiole rings protruding outside, as shown in its X-ray crystal structure. The concave bowl-shape configuration adopted by the truxene core perfectly mirrors the convex surface of fullerenes, suggesting that van der Waals and concave-convex p-p interactions between them should be maximized. Indeed, the association of trux-TTF and fullerenes in solution was investigated by ¹H NMR titrations with C₆₀ and C₇₀ as guests affording binding constants of Log K_a = 3.1 and 3.9 for C₆₀ and C₇₀, respectively.¹²

X-ray crystal structure of truxene-TTF and energy-minimized (MPWB1K/6-31G** level) structure of its complex with C₇₀.

Pioneering work carried out by Sessler¹³ and Therien¹⁴ demonstrated that the electronic communication through hydrogen-bonding interfaces is more efficient than in comparable s- or p-bonding networks.

Following this seminal work, we have investigated a set of non-covalently associated C₆₀-porphyrin ensembles interfaced by a two-point amidinium∙carboxylate pair that facilitates an efficient charge separation process to afford microsecond-lived P∙⁺-C₆₀∙- radical pairs. In solvents that do not interfere significantly with either the electrostatic or the hydrogen bonding interactions binding constants as high as Log K_a = 7.3 were observed.¹⁵ Exceptionally strong electronic couplings stem from this self-assembly, which in turn facilitate faster, more efficient and longer-lived formation of radical ion pair states (i.e., ~10 ms in THF) – when compared to similar covalent C₆₀ conjugates (i.e., ~1 ms in THF). Most importantly, such remarkable radical ion pair lifetimes outperform previously reported ensembles based on i) non amidinium-carboxylate binding motif or ii) non fullerene electron acceptors by several orders of magnitude.

We have also utilized a combination of hydrogen bonds between and exTTF-fullerene interaction to construct extremely stable (Log Ka = 6.2) exTTF-C₆₀ dyads. To do so, we synthesized an exTTF derivative bearing a crown ether moiety and a Bingel-type C₆₀ adduct featuring an ammonium salt. The cooperative effect of the ammonium-crown ether (typically Log Ka ~ 3) and the exTTF-C₆₀ interactions give rise to the extraordinary stability of the complex in solution. Furthermore, we have demonstrated that upon light irradiation, this supramolecular species yield a charge separated state with a lifetime of 9.3 ps.¹⁶

Supramolecular exTTF-C₆₀ dyad, held together by a combination of hydrogen bonds and concave-convex interactions.

References:

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Our group has recently initiated a research line to explore the potential biological and pharmacological properties of fullerenes. The first objective is to obtain suitably functionalized fullerenes compatible with life systems. Owing to their rigid spherical shape, fullerene hexakis-adducts with a Th-symmetrical octahedral addition pattern appear to be a very attractive core molecule for the synthesis of unique globular multivalent derivatives. Employing an approach based on click chemistry, we have efficiently prepared hexakis-adducts decorated with sugar residues¹.

Synthesis of hexakis-adducts of fullerenes by click chemistry.

A carbohydrate–protein interaction is a key step in many biological processes. This interaction is highly selective, calcium dependent in some cases, and multivalent. The glycans are the ligands responsible to interact, in a multivalent manner, with cell surface receptors (lectins). We have shown that the mannoses in this spherical presentation are able to be recognised by lectins in a multivalent manner. In this study, we have used isothermal titration calorimetry (ITC) and concanavalin A (Con A) as a lectin that recognizes mannoses².

Titration of Con A with the fullerene derivative with 24 mannoses in 0.1 M sodium acetate buffer (pH 5.3) with 100 mM NaCl, 5 mM CaCl2 and 5 mM MnCl2 at 25 ºC.

These calorimetric studies allow us to demonstrate that fullerene is a unique and rather efficient platform for the multivalent presentation of ligands, leading to a remarkable increase of affinity with a simple multivalent model.

References

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The implementation of photovoltaics as a clean and reliable energy source requires the availability of efficient, cheap, and flexible devices which, mimicking natural processes such as photosynthesis, are able to transform sunlight into power. The foremost feature of organic photovoltaics is its flexibility (Figure 1) which represents their major advantage over the inorganic devices currently available in the market.¹

Figure 1. Flexible organic solar cells (image taken from http://www.afrl.af.mil/accomprpt/sep03/accompsep03.asp.

As stated before, and inspired by Nature, photovoltaic devices utilize photoinduced electron transfer events to convert sunlight into energy. Nonetheless, photoinduced electron transfer is not the only important process involved in this complex transformation. In this regard, the three basic steps to achieve power from sunlight are:

The sandwich-like architecture —showing all its components - of solar cells is depicted in Figure 2:

Figure 2. Typical sandwich-like architecture of organic solar cells.

Typically, an organic solar cell is constituted by a transparent substrate of either glass or plastic. On top of it are deposited subsequent layers of: a) indium tin oxide (ITO), acting as the anode, b) the polymer PEDOT-PSS, c) the active material, d) an interface salt, usually LiF, and, finally, the cathode, usually aluminium. The different work function of both metallic electrodes provides enough energy to break down the initial exciton (electron-hole pair intimately bonded).

Regarding the photoactive layer, the most successful strategy utilized to obtain high PV performances is the bulk-heterojunction (BHJ).² In those organic solar cells, the active layer is formed by an interpenetrating network of a p-type material —usually a semiconducting polymer based in polyphenylene vinylene or polythiophene) and a soluble fullerene derivative—the most used one is PCBM— acting as n-type material (Figure 3). This strategy has already reached power conversion efficiencies higher than 5%.³

Figure 3. Compounds usually utilized as p- and n-type materials in the construction of organic solar cells.

We have recently reported the synthesis and the photovoltaic properties of a new family of soluble C₆₀-based derivatives, called DPMs, which show outstanding power efficiency conversion values of around 2.3% in blends with P3HT as electron donor (Figure 4).⁴Stimulated by an idea to combine good electron accepting and light harvesting properties, we have introduced recently a new class of fullerene-based materials, constituted by two covalently connected fullerene units (C₆₀ and/or C₇₀).⁵ The resulting dimers exhibit increased light-harvesting properties, due to the presence of two units of identical or different fullerenes linked through a 2-pyrazoline-pyrrolidine rings (Figure 4). Photovoltaic devices prepared with blends of these materials and P3HT exhibited promising external quantum and power conversion efficiencies.

Figure 4. Chemical structure of DPM and I/V curves for different mixtures DPM/P3HT (top). [60]- and [70]fullerene-based dimers (down).

We have recently reported the preparation of a new cyanine-cyanine organic salt displaying exceptional light harvesting properties in the NIR spectral range. Preliminary results of molecular bulk heterojunction solar cells based on this compound and [60]PCBM as the only active layer, revealed this new dye as a promising light-harvesting material for photovoltaics.⁶

A particular active strategy towards new PV designs is the molecular heterojunction. ⁷ In this case, C₆₀-based conjugates are integrated as the only component of the photoactive layer of the device. We are currently studying this class of devices by using different hybrids in which a variety of electron-donor fragments, such as p-conjugated oligomers or p-extended tetrathiafulvalene derivatives, are covalently linked to one or more fullerene units (Figure 5).⁸

Figure 5. Chemical structures of different C60-based conjugates utilized in the preparation of molecular heterojunction solar cells..

The preparation of low-cost photovoltaic devices based on organic or hybrid materials, and in particular the development of metal-free organic sensitizers for dye-sensitized solar cells (DSSCs) is a challenging research field for the scientific community.⁹ Recently, we have reported the use of the π-extended tetrathiafulvalene (exTTF) fragment for the development of new sensitizers for DSSCs.¹⁰ Indeed, the butterfly shape of exTTF (Figure 7a), that may prevent aggregation, together with its exceptional electron-donor character, are making it an excellent candidate for the molecular engineering of sensitizers. Due to their redox properties, these compounds showed surprising kinetic behavior: indeed, even though the measured driving force for regeneration of the sensitizer by the redox mediator (3I-/I3-) was small (150 mV), an efficient regeneration could be observed. Photovoltaic devices with a power conversion efficiency of 3.8% (AM 1.5G) were fabricated (Figure 7c). This proof-of-concept showed that sensitizers with a small driving force can efficiently operate in DSSCs.

Figure 7: a) exTTF; b) Electron transfer between an exTTF-based dye and TiO2; c) corresponding photocurrent-voltage curve.

References

 

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The rational design of molecular-sized materials for electronic and photonic applications is currently a topic of great interest. In particular, the preparation and integration of multifunctional molecules into architectures of higher order -the so called bottom-up approach- is the basis for the realization of molecular-scale electronic.

A molecular wire consists of a molecular unit connected to two continuum reservoirs of electrons where the wire provides a pathway for the transport of electrons from one reservoir to another. Hereby, p-conjugation emerges as a particular crucial factor to realize i) small attenuation factors (i.e., electron conducting over large distances), ii) good contacts with the termini (i.e., electron donors and electron acceptors) and iii) good orbital mixing (i. e., with donor and acceptor states).¹

We have demonstrated wire-like behaviour in a series of oligo-p-phenylenevinylenes (oPPVs) connected to a C₆₀ electron acceptor and an extended tetrathiafulvalene (exTTF)² or a zinc tetraphenylporphyrin (ZnTPP)³ as electron donors.

Figure 1. Electron transfer through "Molecular-scale wires" (up) and minimum energy calculated structure for exTTF-oPPV7-C60 (bottom).

Exceptionally small attenuation factors (b) and strong electronic donor-acceptor coupling (V) are responsible for the nanowire behaviour, assisting charge-transfer processes that exhibit a rather weak dependence on distance.

A simple exchange of C-C double bonds (oPPV) with C-C triple bonds (oligo-p-phenyleneethynylene, oPPE) alters considerably the long-range electron transfer in electron donor-acceptor conjugates.⁴ Thus, the differences in the electronic structure between oPPE and oPPV systems (Figure 2) have strong influence in the charge-separation process and justify the weak wire-like behaviour found in the C₆₀-oPPE-exTTF systems.

C₆₀-(oPPV)3-exTTF -C₆₀-(oPPE)3-exTTF

Figure 2. Electron Affinity Maps of C₆₀-oPPV-exTTF and C₆₀-oPPE-exTTF.

When we connect the donor (exTTF) and acceptor (C₆₀) moieties by means of m-phenyleneethynylene spacers, electron transfer events depend on the concentration ⁵. Thus, at low concentrations (10^-6 M) quite ineffective "through space" intermolecular electron transfer takes place and lead to excited-state deactivations. In contrast, at high concentrations (10^-4 M) excited-state deactivations are dominated by intracomplex electron transfer events, namely, between exTTF of one molecule and C₆₀ of another molecule that are self-organized into wormlike structures. This self-organization assists in stabilizing the resulting radical ion pair state with lifetimes reaching 4.0 μs.

In order to obtain a reliable comparison between the different p-conjugated oligomers, it is important to refer to the same donor and acceptor moieties. In this sense, we have studied the exTTF-oligofluorene-C₆₀ systems represented in Figure 3.⁶ The calculated electron affinity map shows a channel of high electron affinity through the bifluorene bridge with a maximum at C₆₀, proving the charge-transfer features of these systems. A b value of 0.09 Å^-1 was obtained for these systems, indicating that the ability of oligofluorenes to conduct charges lies between that of oPPVs and that of oPPEs

Figure 3. Chemical structure of exTTF-oFln-C60 (up) and electron affinity map calculated with Parasurf A07 for exTTF-oFl2-C₆₀down); from blue to red low to high.

The understanding of electronic-transport properties through single molecules sandwiched between metal contacts has recently become possible by several strategies, including mechanically controllable break junctions (MCBJ) or scanning tunneling microscopy (STM) techniques. For this purpose, we have prepared TTF-based molecular wires endowed with thioacetyl end groups for their attachment to gold surfaces⁷.

Figure 4. Tetrathiafulvalene-based molecular nanowire. (A) Conductance traces for an Au–Au junction (trace a) and two molecular junctions comprising one or a few molecules (traces b and c). (B) Conductance histogram constructed from the conductance traces that show evidence of trapped molecules.

Charge transport measurements carried out by the break junction technique show encouraging resistance values which emphasize the potential of TTF derivatives as molecular wires with an electrostatically adjustable conductance.

References

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The aim of our group is to produce new molecules of interest in different areas of Nanoscience. In particular, we are developing photo- and electro-active molecules (mostly TTF derivatives and fullerenes) for the coating of gold surfaces in the search of a control on the final morphology of the material since the precise nanometer-scale organization of complementary electroactive materials is key prerequisite to improve the performance of optoelectronic devices.

Ultrahigh Vacuum (UHV) STM experiments of mixtures of exTTF and the fullerene derivative PCBM¹performed on Au(111) substrates provided evidence supporting a high degree of organization in these systems2. These mixtures rearrange into a lateral organization of interdigitated stripes with characteristic widths of 10-20 nm. a morphology that has been predicted to optimize the efficiency of organic solar cells³.

Figure1 . STM images on Au(111) of the organization of exTTF and PCBM at the nanometer-scale.

In addition, progress in organic electronics requires a detailed understanding of all the relevant chemical and physical processes that occur at organic-film/metal interfaces.Our recent work in collaboration with the group of Prof Miranda (IMDEA-Nanociencia) illustrates that electrom transfer at the interface between a Cu(100) metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. The adsorption of TCNQ molecules, atracted to the surface by van der Waals forces, causes a lateral displacement of the electronic charge, and effectively pushes back the electron density of the underlying metal substrate⁴.

Figure 2. Self-assembled islands of TCNQ on Cu(100).

References

Prof. Nazario Martín
Organic Chemistry Department
Chemistry Faculty
Universidad Complutense de Madrid
Email: nazmar@quim.ucm.es
Phone: (+34) 91.394.42.27
Fax: (+34) 91.394.41.03

Feb 08, 2011

Interview Prof. Nazario
Martín León. President
of Spanish Royal Society
of Chemistry, "Somos todo
moleculas" El Pais.
Read more...

Jan 25, 2011

Interview Prof. Nazario
Martín León. President
of Spanish Royal Society
of Chemistry, ."El ser
humano es el ejemplo
más representativo de
lo que la química
puede llegar a hacer"
Tribuna Complutense.
Read more..

Organic Chemistry Department
Chemistry Faculty
Universidad Complutense de Madrid
E-28040 Madrid. Spain.
Tel: +34 91 3944227
Fax: +34 91 3944103
e-mail: nazmar@quim.ucm.es