Interfacing Nanocarbons with Organic and Inorganic Semiconductors

Invited Feature Article pubs.acs.org/Langmuir

Interfacing Nanocarbons with Organic and Inorganic Semiconductors: From Nanocrystals/Quantum Dots to Extended Tetrathiafulvalenes Georgios Katsukis, Carlos Romero-Nieto, Jenny Malig, Christian Ehli, and Dirk M. Guldi* Department of Chemistry and Pharmacy & Interdisciplinary Center of Molecular Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany. ABSTRACT: There is no doubt that the outstanding optical and electronic properties that low-dimensional carbon-based nanomaterials exhibit call for their implementation into optoelectronic devices. However, to harvest the enormous potential of these nanocarbons it is essential to probe them in multifunctional electron donor−acceptor systems, placing particular attention on the interactions between electron donors/electron acceptors and nanocarbons. This feature article outlines challenges and recent breakthroughs in the area of interfacing organic and inorganic semiconductors with lowdimensional nanocarbons that range from fullerenes (0D) and carbon nanotubes (1D) to graphene (2D). In the context of organic semiconductors, we focus on aromatic macrocycles and extended tetrathiafulvalenes, and CdTe nanocrystals/quantum dots represent the inorganic semiconductors. Particular emphasis is placed on designing and probing solar energy conversion nanohybrids.

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INTRODUCTION Of all the elements in the periodic table, only carbon provides the basis for life on earth. Carbon is the key to many technological applications from drugs to synthetic materials that have become indispensable in daily life and have influenced the world’s civilization for centuries. Importantly, the structural diversity of organic compounds and molecules results in an endless number of chemical and physical properties. Altering the periodic binding motifs in networks of sp3-, sp2-, and sphybridized C-atoms represents the conceptual starting point for constructing a wide palette of carbon allotropes. To this end, the past two decades have served as a test bed for measuring the physicochemical properties of nanocarbon-based materials starting with the advent of fullerenes (0D), followed in chronological order by carbon nanotubes (1D) and, most recently, by graphene (2D).1−3 These species are now poised for use in wide-ranging applications because of their unique mechanical, thermal, optical, and electronic properties.4,5 The development of nanocarbon-based nanohybrids has been fueled up to generate potential materials for highperformance transistors, sensors, drug delivery, and solar cells.6,7 A wide variety of electron donors and electron acceptors as well as nanoparticles have successfully been introduced, including tetrathiafulvalenes (TTF), tetracyanoethylenes (TCNE), and tetracyanoquinodimethane (TCNQ), resulting in major alterations of the electronic properties of carbon nanotubes and graphene (Figure 1).8−14 As a matter of fact, it has been shown that the G mode of graphene shifts to higher frequencies when electron-accepting TCNEs or electron-donating TTFs are adsorbed.8,15 This goes hand in © XXXX American Chemical Society

hand with chemical doping with, for example, boron or nitrogen,16 where the G band tends to shift to higher frequencies for electron as well as hole doping. In this feature article, we set our focus on a selected set of molecular and particulate building blocks, that are, on one hand, representative of this contemporary area of research and, on the other hand, exhibit an exceptional electron-donating ability. One emerging area is the field of CdTe quantum dots (QDs), whose superior tunability in terms of chemical, optical, and electronic properties is a key asset as unique light harvesters in solar cells and as remarkable electron donors.17 To this end, CdTe QDs in conjunction with nanocarbons may provide advantages over conventional QD-based photoelectronic materials. Another emerging area is the field of 9,10bis(1,3-dithiol-2-ylidene)-9,10-dihidroanthracenes/extended tetrathiafulvalenes (exTTF) that feature a proaromatic nature and butterfly-like geometry. exTTF derivatives are soluble in commonly used organic solvents and represent an interesting class of electron donors.18,19 Additionally, the field of aromatic macrocycles should be considered because it has advanced our understanding of nanocarbons in the ground and excited states.20

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NANOCARBONS Fullerenes. The first successful preparation of fullerenes in macroscopic quantities by the evaporation and recondensation Received: March 19, 2012 Revised: June 4, 2012

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dx.doi.org/10.1021/la301152s | Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 1. Electronic band gap structure for semiconducting SWNTs (left) and for graphene (right).

of graphite was reported in 1990.21 Fullerenes are nowadays readily available and exhibit exciting characteristics. For example, the delocalization of charges within the giant, spherical carbon framework together with the rigid and confined structure of the aromatic π-sphere offers unique opportunities for stabilizing charged entities.4 It is due to the small reorganizational energies in electron-transfer reactions that fullerenes have led to a notable breakthrough in synthetic electron donor−acceptor systems by providing accelerated charge separation and decelerated charge recombination processes.6 Carbon Nanotubes (CNTs). Conceptually, 1D singlewalled carbon nanotubes (SWNTs) are considered to be small strips of graphene sheets that have been rolled up to form perfectly seamless single-walled nanocylinders. The graphene sheets can be wrapped in a variety of ways that are denoted by a pair of indices (n, m) that define both the diameter and chirality of SWNTs. SWNTs, where the difference between n and m is a multiple of 3, are metallic, which is always true for armchair SWNTs. This condition is truly fulfilled only by large-diameter zigzag SWNTs; the rest are semiconducting (Figure 2). The structure and, in particular, the chirality/helicity determines the electronic and optical properties of SWNTs, namely, the

conductance, the lattice structure, and other characteristics. The variety of diameters and the large aspect ratios of SWNTs render them ideal 1D quantum wires, which moreover can be concentrically organized to form multiwalled carbon nanotubes (MWNTs). Importantly, the movement of electrons is dependent on the type of carbon nanotube (CNT). The electrical transport in metallic CNTs is ballistic;22 that is, electrons are not subjected to any scattering events over a length scale of several micrometers or to any electromigration, even at room temperature. As a consequence, CNTs can carry current densities approximately 1000 times that of a typical copper wire.23 Electron transport is also ballistic for semiconducting SWNTs but only over distances of a few hundred nanometers.24 The spectroscopic characterization of SWNTs and their metastable states is made difficult by the polydisperse nature of SWNT samples and the inevitable presence of SWNTs in bundles of different sizes. In addition, SWNTs are completely insoluble in organic and aqueous solvents. The latter calls for suitable methodologies to disperse them. Toward this aim, two general approaches, namely, covalent functionalization and noncovalent interactions, are commonly pursued.18,25−37 The two approaches may eventually lead to similar results but differ substantially in the change in hybridization of SWNT carbon atoms, that is, from sp2 to sp3, which occurs only during the course of covalent functionalization. Heavy modification of this latter kind can therefore lead to an extensive loss of conjugation, with dramatic effects on the electron acceptor and/or electron-transport properties. This process can be monitored in the visible to the near-infrared region of the electronic absorption spectrum, where photoabsorption takes place between characteristic maxima in the electronic density of states (i.e., E11, E22, etc.). Upon covalent functionalization, a loss of these features is observed because of increased exciton recombinations at defect sites.30−37 Alternatively, several hybrids have been prepared by the noncovalent adsorption of electron donors to SWNTs, utilizing supramolecular forces such as π−π stacking and/or electrostatic interactions while keeping the sp2 character of the SWNT network intact.18,25−29,38 Graphene. The youngest representative of nanocarbons is 2D graphene. Single graphene layers were successfully prepared in 2004 by the simple mechanical exfoliation of graphite using Scotch Tape.39 Other fabrication methodologies, including epitaxial growth and solubilization from bulk graphite, have

Figure 2. Schematic representation of rolling a sheet of graphene into SWNTs with armchair (left), zigzag (center), and chiral (right) configurations. B


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Figure 3. Example of noncovalently functionalized C60/QD.

stabilization of the resulting graphene layers.53 A common denominator of these methods is a suitable amphiphilic intercalator and/or solvent that guarantees efficient exfoliation. The intercalator should be multifunctional, and desirable features include, among many others, photoactivity and redoxactivity.

been established, which nowadays pave the way for fundamental experiments and technological applications.40−43 Transport measurements show that graphene has a remarkably high electron mobility at room temperature, with values exceeding 15 000 cm2/Vs.39 Additionally, conductance measurements suggests not only high mobilities for electrons but also high mobilities for holes.5 An ideal monolayer of graphene has an optical transmittance of 97.7 %, which together with the aforementioned features makes such a carbon-based nanomaterial a cost-effective and abundant source for transparent conductive electrode applications. So far, major breakthroughs, including the fabrication of single-layer graphene, have come from substrate-related preparation procedures.44−46 An alternative approach is the formation of graphene oxide from graphite under highly oxidizing conditions, which has also become a viable method of producing chemically modified graphene, especially in the context of realizing the mass production of graphene flakes.47−50 In light of the altered sp2 networks, graphene oxide lacks the electronic quality of pristine graphene. Therefore, restoring the sp2 network by reduction becomes a crucial step. Notably, the reduction processes lead irreversibly to partial amorphous carbon.51 As a matter of fact, the resulting reduced graphene oxide exhibits features that are similar but not identical to that of pristine graphene. More recently, milder wet chemical approaches toward high-quality graphene flakes encompass the use of graphite as a starting material, which is mechanically individualized into atomic planes by means of ultrasonic forces in the liquid phase.52 Notably, such a wet chemical approach cannot be performed without chemical modification/functionalization or without the subsequent

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NANOCARBON/QD NANOHYBRIDS QDs. Quantum dots (QDs) are crystalline nanoparticles that exhibit more versatile electronic properties than those found in bulk inorganic semiconductors.54 Exciton confinement in QDs along all three spatial dimensions leads to size-dependent electronic features. In this context, smaller crystal sizes result in larger band gaps. Water-processed CdTe QDs exhibit, for example, outstanding absorption characteristics throughout the visible and near-infrared regions of the solar spectrum.17,54 The latter go hand in hand with highly efficient band gap luminescence. In addition, control over the size, shape, and surface in the synthetic stage allows us to obtain QDs with tunable optical properties.17 This renders QDs particularly interesting as light-harvesters and electron donors for optoelectronic applications, such as solar energy conversion. Major scientific and technological breakthroughs will depend, however, on key achievements such as electronical coupling of these crystalline QDs to functional materials such as nanocarbons. Fullerenes and QDs. In the context of interfacing QDs with fullerenes, pioneering experiments were conducted with three different size-quantized QDs, on one hand, that were synthesized with either L-cysteine or thioglycolic acid as surface stabilizers and two regioisometric and positively charged C


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Figure 4. Example of noncovalently functionalized SWNT/pyrene+/QD.

fullerene bis-adducts, on the other hand.55 In particular, green (2.4 nm), yellow (3.4 nm), and red (5.0 nm) QDs as photoexcited-state electron donors and trans-2 and trans-3 isomers of fullerene as electron acceptors were used to fine tune QD/fullerene nanohybrid interactions (Figure 3). There are several advantages of the electrostatic approach with association constants on the order of 105 M−1. They include control over the nanohybrid composition in solution and favorable electrontransfer behavior. When analyzing the QD/fullerene nanohybrids by TEM, a nanometer-sized pattern of larger objects is seen. In the context of electron-transfer, quite stable radical ion pair states were noticed on a time scale of up to several hundred microseconds with a remarkable lifetime of 1.5 ± 0.5 ms. Either the facile corrosion of the QDs or the reducing features of the L-cysteine/thioglycolic acid stabilizer are responsible for the overall stability. The interesting electron density features seen for the fullerene/QD nanohybrids in TEM open exciting new possibilities for further transferring and engineering the photoredox processes at electrode surfaces.56 This was accomplished by following the protocol of the layer-by-layer technique, in which electrostatic forces between oppositely charged species are at work. Two different steps were taken. First, sandwich layers consisting of negatively charged QDs and the positively charged poly(diallyl-dimethylammonium) polyelectrolyte (PDDA) were constructed. Second, the PDDA layers were substituted with those of the fullerene derivative, which led to fullerene/QD sandwich stacks. From the monochromatic light, an IPCE response of 1.7% was determined for single sandwich coverage. Further amplifications of the IPCE values were realized for sandwich stacks, yielding the highest IPCE value of 5.4% for five fullerene/QD sandwich stacks.56 These numbers are lower than for record-setting solidstate devices, although they exceed many of them. The fullerene/QD nanohybrids set the example for the molecular design of an efficient, nearly stable charge-separation process as corroborated in condensed media, forming the foundation of photovoltaic cells.

One of the areas of future research will be the acceleration of the charge transport in films by means of rationally designing multilayer stack of QDs with energy and redox gradients. Here, recent results on a cascade Förster resonant energy transfer on the performance of QD-based photoelectrochemical cells provide important incentives.57 SWNTs and QDs. SWNTs and MWNTs suspended by positively charged 1-(trimethylammonium acetyl) pyrene were linked to thioglycolic acid-stabilized QDs through electrostatic interactions (Figure 4).58 The novel SWNT/QD nanohybrids were characterized both in the ground and excited states with a specific accent on electron-transfer chemistry. Both assays provide kinetic and spectroscopic evidences that support strong electronic interactions that are responsible for favorable electron-transfer characteristics. In fact, the rapid formation of microsecond-lived radical ion pair states is seen to develop. Subsequently, electrostatic and van der Waals interactions between individual components have been employed for the sequential integration of photoactive SWNT/QD sandwich stacks into novel hybrid cells. In response to visible light irradiation, appreciable photoelectrochemical device performances were registered. It is important that, depending on the sequence of deposition, the photocurrent mechanism is altered with the highest monochromatic IPCEs of up to 2.3% for hybrid cells consisting of single SWNT/QD stacks. SWNTs and QD-Pyrene. In complementary work, we have documented that by virtue of covalent (i.e., peptidecondensation) and noncovalent (i.e., π−π forces) interactions, complex formation is obtained from water-soluble QDs and SWNTs en route to versatile SWNT/QD-pyrene nanohybrids (Figure 5).59 The use of spectroscopic and microscopic techniques confirmed the hierarchical integration of the electronically coupled constituents, namely, QD, 1-pyrenemethylamine, and SWNT. A full-fledged photophysical investigation shed light onto the formation of hot excited states, vibrational relaxation, the formation of strongly correlated excitonic electron hole pairs, and trapping as well as radiative recombination of excitonic electron hole pairs. In the SWNT/ D


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a short circuit current of 0.22 mA, an open circuit voltage of 0.1 V, a fill factor of 20.4%, and an efficiency of 0.0044% were measured for a monolayered SWNT/QD-pyrene configuration. SWNTs and QDs. Another strategy of coupling QDs to SWNTs was wrapping shells of polyelectrolytes, that is, poly(diallyldimethylammonium chloride) (PDDA) and poly((vinylbenzyl)trimethylammonium chloride) (PVBTA) around SWNTs (Figure 6).60 Here, simple and effective π−π stacking, hydrophobic interactions, and van der Waals attractive forces guarantee the water solubility of the resulting SWNT/PDDA and SWNT/PVBTA. The subsequent association of QDs is achieved through electrostatic interactions with the surface of SWNT/PDDA. With respect to the association, TEM corroborated that linearly ordered structures are present, which implies aligned SWNT/PDDA/QDs. Nevertheless, in a few rare cases it is noted that some SWNT/PDDA areas are incompletely decorated with QDs. It is likely that the homogeneous PDDA coating of SWNTs is limited by surface defects. This hypothesis finds further support from Raman measurements in which ultrasonicating SWNTs with PDDA results in an unreasonably intense D band, especially when compared to SWNT/SDBS and SWNT/PVBTA that were prepared under the same conditions (i.e., solvent, time, temperature, etc.). However, efficient electronic communication between QDs and SWNT/PDDA is established by photophysical techniques. For example, when comparing the emission of QDs and their lifetimes in the absence and presence of SWNT/PDDA, significant differences emerge. Taking the aforementioned results into account, electronic communication in the form of charge transfer between SWNT/ PDDA and QDs was confirmed. This is additionally underlined by femtosecond pump probe experiments. A 600 ps component correlates with fast emission decay kinetics and reflects the charge-transfer reaction evolving from the QD excited states.

Figure 5. Example of noncovalently functionalized SWNT/QDpyrene.

QD-pyrene nanohybrids, a competitive pathway of charge transfer (i.e., 62 ps) transforms the excitonic state of QD into a charge-separated state that lives for several nanoseconds. To explore their potential application as an integrative component for the construction of a photovoltaic device, SWNT/QDpyrene nanohybrids were probed as a photoactive layer. To this end, we combined a sprayed thin-film of SWNT and a selfassembled QD-pyrene monolayer onto ITO and a Pt-coated FTO in a photoelectrochemical cell. Under AM 1.5 conditions,

Figure 6. Examples of noncovalently functionalized SWNT/QD: SWNT/PDDA/PSS/PDDA (left), SWNT/PDDA/QD (center) and SWNT/ PVBTA/QD (right). E


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Figure 7. Examples of noncovalently functionalized graphene/QD: graphene/QD-pyrene (left) and graphene/pyrene+/QD (right).

Graphene and QDs. New insights have been obtained into the communication between organic and inorganic building blocks by extending our concepts to graphene/QD related systems. Particular emphasis was placed on the preparation of stable dispersions in water by a wet chemical approach. Starting from graphite, exfoliation was performed in water with the help of positively charged 1-(trimethylammonium acetyl) pyrene, from which π−π and hydrophobic interactions originate (Figure 7). 61 Raman investigations verify the efficient exfoliation of graphite into single and few-layer graphene flakes (i.e., nanographene (NG)). In particular, the D band intensifies during exfoliation as a reflection of diminished flake sizes, and the 2D band transforms into a highly symmetric peak. A deconvolution of the latter into one Lorentzian fit points to decoupled few-layer graphene flakes. NGs were then electrostatically linked to water-soluble QDs, which are stabilized with thioglycolic acid. As a matter of fact, hierarchical structures were realized that are composed of exfoliated graphite, positively charged pyrene, and negatively charged QDs. Complementary microscopic assays reveal that the QDs are quantitatively immobilized onto NG. Furthermore, detailed photophysical characterizations support the notion that from the excitonic states of QDs a fast energy and/or electrontransfer evolves to single and few-layer graphene on the picosecond time scale next to the filling of deep and shallow traps. Graphene and QD-Pyrene. In an alternative approach, we pursued the integration of QDs, to which pyrene is covalently attached to the basal plane of graphene via π−π interactions (Figure 7).61 Spectroscopic and microscopic assays indicate

that exfoliation is, however, far from being quantitative. In comparison to pristine graphite, the presence of multilayer graphene (i.e., exfoliated graphite (EG)) rather than NG was observed. The coexistence of few-layer and multilayer graphene is, for example, discernible in HRTEM and TEM with flake sizes from 0.27 up to 12.7 mm2, on which the QDs are immobilized. Nevertheless, the photophysical investigation highlights that the rather poor degree of exfoliation exerts a negative impact, particularly on the efficiency of the electronic communication between QDs and EG. In time-resolved transient absorption measurements, lifetime reductions of the processes are noted (i.e., filling of shallow and deep traps) after the excitation of QDs by approximately 40% because of interactions with the graphenoid material.

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NANOCARBON/AROMATIC MACROCYCLE NANOHYBRIDS

Aromatic Macrocycles. Aromatic macrocycles offer immense potential as integrative components for the design of novel electron donor−acceptor systems. Owing to their broad absorption cross sections throughout the visible and near-infrared part of the spectrum and their good electrondonating ability, they have emerged as interesting building blocks in dye-sensitized solar cells. As a matter of fact, the past decade has witnessed a doubling of the overall efficiency from around 2−4% to a record value of 7%.62 Importantly, a systematic tailoring of the molecular structure is crucial to achieving radical ion pair state lifetimes that are of interest in the areas of photocatalysis and/or electric circuits.63,64 F


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Figure 8. Example of a noncovalently functionalized SWNT/zinc porphyrin-pyrene.

Fullerene-Aromatic Macrocycles. The field of porphryin and/or phthalocyanine containing electron donor−acceptor systems is probably best described as one of most intensively studied fields of nanocarbons and is subject to numerous excellent reviews.65−67 In brief, the synthesis and the study of either covalently linked or noncovalent electron donor− acceptor conjugates and hybrids, respectively, utilizing porphyrins and/or phthalocyanines have opened a wealth of information on the principles of electron-transfer (i.e., reorganization energies, electronic coupling, Marcus theory, etc.).19,63−65 In this regard, the exploration of noncovalent means such as π−π interactions, ion pairing, cation−dipole, metal coordination, and hydrogen bonding had a significant impact on recent advances in the field of fullerene chemistry.65,67 A particularly remarkable example of aromatic macrocycles is azulenocyanines, which consist of four azulene units fused to a tetraazaporphyrin macrocycle.68 What is so striking about azulenocyanines is that they absorb in the UV, visible, and nearinfrared regions all the way out to 1300 nm, and as such, bear great potential for efficient light harvesting. To this end, a noncovalent azulenocyanine/fullerene hybrid has been reported with panchromatic absorptions.69 Despite the extremely shortlived nature of the azulenocyanine-centered excited state of approximately 10 ps, it was possible to observe a nearly nanosecond lived (i.e., 0.87 ns) radical ion pair state. SWNT-Aromatic Macrocycles and SWNTs and Aromatic Macrocycles. SWNT/aromatic macrocycle-based electron donor−acceptor systems and electron-transfer reactions therein have been intensively investigated.65,67 At this point, we refer to the original literature and limit our discussion to a few cutting-edge aspects. One of the most intriguing contributions is based on SWNTspecific electron-transfer properties. In fact, the first reports are appearing, in which enriched SWNT samples reveal SWNT-

type-dependent electron-transfer rates and efficiencies.70 For example, in that context 5,10,15,20-tetra(phenyl-4pyrenylbutanoyl)porphyrinatozinc(II) derivatives were synthesized − Figure 8. Because of the presence of four different pyrenes, efficient immobilization onto SWNT surfaces yields extremely stable SWNT/porphyrin suspensions. Comparing (7,6)- and (6,5)-enriched SWNT/porphyrin hybrids, slightly longer lived radical ion pair states were noted for (6,5) SWNTs than for (7,6) SWNTs with lifetimes of 48 and 37 ns, respectively. In line with this finding is the fact that (7,6) SWNTs feature higher efficiencies in complementary solar cell device performance tests. A different direction of contemporary SWNT research is to optimize the noncovalent forces between SWNTs, which are intrinsically p-doped, and either p-type and n-type poly(pphenylenevinylene) (PPV) oligomers, to which zinc phthalocyaninesare are linked (Figure 9).25,71 Interestingly, only the ntype PPVs bearing electron-withdrawing CN substituents interact tightly with SWNT affording stable SWNT/n-type PPV zinc phthalocyanine suspensions. P-type PPVs, however, fail to disperse SWNTs with respect to p-type SWNTs because of an overall weakening of the π−π interactions. Notable, upon photoexciting SWNT/n-type PPV zinc phthalocyanine a fast singlet excited state deactivation occurs leading to the transient spectrum of the electron-oxidized phthalocyanine radical cation with characteristic transitions at 425−550 and 840 nm. The radical ion pair state is stable for approximately 530 ps, during which it recovers quantitatively to the ground state. Finally, panchromatic absorption should be considered because it has evolved into a hot topic in SWNT chemistry. In preliminary work, the noncovalent interactions between SWNTs and the aforementioned azulenocyanines (i.e., a symmetrical Zn(II) octa-tert-butylazulenocyanine or a Zn(II) azulenocyanine-phthalocyanine bearing a pyrene unit) were tested.72 In transient absorption measurements with such G


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Figure 9. Example of a noncovalently functionalized SWNT/PPV zinc phthalocyanine.

Figure 10. Example of a noncovalently functionalized graphene/porphyrin.

SWNT/azulenocyanine hybrids, new transients species evolved with distinct minima at 550, 950, and 1050 nm as well as maxima at 630 and 1140 nm. Spectroelectrochemical measure-

ments revealed that these attributes resemble those of the oneelectron-oxidized radical cations of azulenocyanines. As a matter of fact, it was concluded that after excitation the singlet H


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Figure 11. Examples of covalently functionalized C60-exTTF.

minima decay within 4 ps with the concomitant formation of new characteristics. In particular, a maximum at 840 nm, which corresponds to the one-electron oxidized form of the phthalocyanine, and a minimum at 1290 nm, which resembles the reduction of SWNTs and, as such, suggests the presence of reduced graphene, are seen. The lifetime of the radical ion pair state is given as 360 ps.

excited state deactivation within 20 ps leads to an electron- or energy-transfer product that features lifetimes of around 130 ps. Graphene-Aromatic Macrocyles. A fairly straightforward approach to integrating aromatic macrocycles infers chemically converted graphene (CCG). To CCG, porphyrins have been linked by means of a metal-catalyzed coupling reaction between iodophenyl-functionalized CCG and 5,10,15-tris(3,5-di-tertbutylphenyl)-20-(4,4,5,5-tetramethyl[1,3,2]dioxaborolan-2-yl)porphyrinatozinc(II).73 In the resulting CCG-porphyrin, only energy transfer from the porphyrins to CCG (i.e., 38 ps) followed by a fast relaxation of the CCG-centered excited state (i.e., 0.3 ps) was observed in the 600 to 700 nm region. A likely rationale for this photoreactivity is the distance and the orientation between CCG and the porphyrins. Graphene and Aromatic Macrocycles. Usually, graphene oxide (GO) or reduced graphene oxide (rGO) are used en route toward exfoliated graphite in the form of single layers as well as multilayers, which are integrated into electron donor− acceptor systems.47,73−78 Overall, the dispersibility of GO and rGO is further increased upon interactions with positively charged aromatic macrocyles because of strong π−π stacking, van der Waals, and, most importantly, electrostatic forces. A leading example is the noncovalent association of rGO with cationic 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (Figure 10).74 In this particular case, a significant decrease in the singlet excited-state lifetime, namely, from 4.9 ns to less than 250 ps, goes hand in hand with the detection of a long-lived transient absorbing below 550 nm. Novel pathways en route to the direct exfoliation of graphite are currently developed, in which aromatic macrocycles serve as powerful exfoliation agents. Here, the use of the aforementioned n-type PPVs and p-type PPVs is particularly promising, especially in the context of immobilization onto the basal plane of graphene.79 Importantly, interfacing p-type PPVs raises the Fermi level into the conduction band, whereas n-type PPVs reverse the trend. None of the PPVs resulted exclusively in single-layer graphene regardless of the doping type. A detailed Raman analysis revealed the coexistence of single-layer and fewlayer graphene. Most notable, the asymmetric 2D band is displaced by a strongly symmetric 2D band. Turning to transient absorption measurements, graphene-related transitions are verified by minima at 615 and 1200 nm. These

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NANOCARBON/exTTF NANOHYBRIDS ExTTF. 2-[9-(1,3-Dithiol-2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithioles (exTTF) have emerged as a promising electron-donating counterpart to nanocarbons.19,80,81 Unlike many known electron donors such as porphyrins, aniline derivatives, ferrocenes, phthalocyanines, and π-conjugated oligomers, exTTFs undergo aromatization upon oxidation.19,81 In particular, upon losing a single electron exTTFs turn into a stable aromatic radical cation intermediate, while still retaining to some extent their p-quinonoid character. Overall, the aromatization affords thermodynamically stable radical cationic and dicationic species at relatively low oxidation potentials. Notably, the formation of such aromatic intermediate species is further accompanied by geometrical changes. In fact, the latter leads to an appreciable stabilization of either the radical cationic and/or the dicationic species. C60-exTTF. Following the aromatization principle undergone by exTTFs upon oxidation, we pursued the synthesis of C60-exTTF conjugates.82 Fluorescence and transient absorption spectroscopy confirmed that in C60-exTTF the fullerene singlet excited state deactivates to the corresponding C60•−-exTTF•+ radical ion pair state. Lifetimes of around 200 ns (i.e., benzonitrile) are 2 orders of magnitude longer than those found in C60-TTF conjugates. This stabilization is rationalized in terms of aromaticity and planarity that are gained during the transformation of the ground state to the radical cation species. In the aforementioned example, C60 and exTTF are directly linked to each other, that is, without the implementation of a spacer. However, the role played by the spacers is far from just structural because its chemical nature governs the electronic communication between terminal units. Another important feature of the spacer is its modular composition, which allows the alteration of the separation without affecting the electronic nature of the connection. To address these central issues, I


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Figure 12. Example of a covalently functionalized SWNT-exTTF.

charge transfer is deactivated. In this off state, unquenched fullerene fluorescence emerges. SWNT-exTTF. SWNTs endowed with covalently linked exTTF derivatives were prepared by using simple esterification reactions (Figure 12) leading to an intensification of the D-/Gband ratio.32 Time-resolved spectroscopy helped to identify reduced SWNTs and oxidized exTTFs as metastable states in a series of novel donor−acceptor nanoconjugates. Following nanosecond excitation of fine dispersions of SWNT-exTTF in THF the well-known signatures of the exTTF radical cations were observed at 660 nm. In the near-infrared region of the spectrum, the radical cation absorptions of the donor fragments are negligible and the absorption features of SWNTs dominate. Overall, remarkable lifetimes in the range of hundreds of nanoseconds are noted. Most important is that we succeeded for the first time in the control over the rate of electron-transfer (i.e., charge separation and charge recombination) by either systematically altering the relative electron donor−acceptor separations or integrating different electron donors. SWNTs and exTTF-Pyrene. Whereas covalent functionalization causes dramatic changes in the electronic properties of SWNTs, π−π interactions leave the graphitic structure intact. In fact, the use of π−π interactions to anchor an electrondonating exTTF to the surface of the SWNT was successfully demonstrated by using a pyrene-tethered moiety that strongly adsorbs on the surface of an SWNT (Figure 13).18 A complete and concise characterization of the radical ion pair state was achieved, especially in light of injecting electrons into the conduction band of SWNTs. Furthermore, π−π interactions between the concave hydrocarbon skeleton of exTTF and the convex surface of SWNT add further strength and stability to the SWNT/pyrene-exTTF nanohybrid. Because of the close proximity of the exTTF to the electron-acceptor SWNT, very rapid intrahybrid electron-transfer affords a photogenerated

different connectivities and different separation lengths between the 1,3-dithiole moiety and C60 were considered (Figure 11).83 In particular, a Diels−Alder cycloadduct and a fulleropyrrolidine-based C60-exTTF conjugate were first comparatively subjected to study. Transient absorption measurements revealed radical ion pair state lifetimes in the range of 200 ns for the Diels−Alder cycloadduct as well as for the fulleropyrrolidine. In conclusion, the data proved that connecting exTTF at either its anthracene or its 1,3-dithiole functionality to C60 exerts no appreciable impact on the radical ion pair state lifetime. As a complement to this study, the impact of the distance between the different constituents onto the charge separation was investigated in C60-exTTF conjugates separated by two single bonds (i.e., 200 ns): one (i.e., 725 ns) and two vinylene (i.e., 1465 ns) units as spacers. In addition, the radical cation that resides on the oxidized donor was found to be delocalized over the two 1,3-dithiole rings rather than localized on just one of them. Finally, it is important to emphasize that the aromatization, which is applicable to the one-electron oxidation of exTTF, indeed plays a crucial role in the stabilization of the corresponding radical ion pair states. The Diels−Alder cycloadduct also represents an interesting example of a thermal on−off electron-transfer switch.84 Photophysical assays corroborated the reaction pattern. In particular, the fullerene singlet excited state is subjected to a rapid charge-transfer deactivation that yields a 200 ns-lived C60•‑-exTTF•+ radical ion pair state. Much more interesting are temperature effects. The Diels−Alder cycloadduct acts as an efficient, thermally reversible material, whose fluorescence switches. In the on state, an activated charge transfer is the inception for strong fullerene fluorescence quenching, when compared with a fullerene reference. Upon heating, the Diels− Alder cycloadduct reverts to the starting materials and the J


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supra and bears strong promise for the preparation of systems for photoinduced energy conversion based on electroactive tweezers vide inf ra, in which exTTF moieties act as an efficient template for the supramolecular organization of SWNT-based electron donor−acceptor complexes. SWNTs and exTTF-Nanotweezers. The interactions between exTTFs and SWNTs/MWNTs are further strengthened when employing tweezers-like architectures. A careful synthetic design allows for linking two exTTFs to a central and rigid unit yielding tweezers-like geometry (Figure 14).85 Chemical modification of the central unit has furthermore been employed to incorporate organic fragments that promote solubility in organic/aqueous media. The resulting ensemble confers on both exTTF fragments the suitable disposition to interact in a complementary way with SWNTs via π−π and charge-transfer interactions. It is important to note that the flexible nature of the linkages renders, moreover, size-adaptable interactions with CNTs of different diameters including multiwalled carbon nanotubes (MWNT) feasible. As a result, exTTF nanotweezers are capable, on one hand, of strongly interacting with the concave surfaces of various CNTs to yield finely dispersed suspensions and, on the other hand, of manipulating the electronic features of SWNTs. In fact, the optical transitions are altered as denoted by red-shifted absorption features. Likewise, the SWNT emission is bathochromically shifted and strongly quenched. The irradiation of

Figure 13. Example of a noncovalently functionalized SWNT/exTTFpyrene.

radical ion pair state whose lifetime is only a few nanoseconds. The present method for the preparation of SWNT/exTTF nanohybrids nicely complements the covalent approach vide

Figure 14. Example of a noncovalently functionalized SWNT/exTTF-nanotweezers. K


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(2) Bettencourt, L. M. A.; Kaiser, D. I.; Kaur, J.; Castillo-Chavez, C.; Wojick, D. E. Population modeling of the emergence and development of scientific fields. Scientometrics 2008, 75, 495−518. (3) Barth, A.; Marx, W. Graphene - A rising star in view of scientometrics. arXiv:0808.3320v3, 2008. (4) Hirsch, A.; Brettreich, M. Fullerenes: Chemistry and Reactions; Wiley-VCH: Weinheim, Germany, 2005. (5) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183−191. (6) Fullerenes: Principles and Applications; Langa, F., Nierengarten, J.F., Eds.; Royal Society of Chemistry: Cambridge, U.K., 2007. (7) Geim, A. K. Graphene: status and prospects. Science 2009, 324, 1530−1534. (8) Rao, C. N. R.; Voggu, R. Charge-transfer with graphene and nanotubes. Mater. Today 2010, 13, 34−40. (9) Voggu, R.; Rao, K. V.; George, S. J.; Rao, C. N. R. A simple method of separating metallic and semiconducting single-walled carbon nanotubes based on molecular charge transfer. J. Am. Chem. Soc. 2010, 132, 5560−5561. (10) Kamat, P. V. Graphene-based nanoarchitectures. anchoring semiconductor and metal nanoparticles on a two-dimensional carbon support. J. Phys. Chem. Lett. 2010, 1, 520−527. (11) Kaminska, I.; Das, M. R.; Coffinier, Y.; Niedziolka-Jonsson, J.; Woisel, P.; Opallo, M.; Szunerits, S.; Boukherroub, R. Preparation of graphene/tetrathiafulvalene nanocomposite switchable surfaces. Chem. Commun. 2012, 48, 1221−1223. (12) Lu, Y. H.; Chen, W.; Feng, Y. P.; He, P. M. Tuning the electronic structure of graphene by an organic molecule. J. Phys. Chem. B 2009, 113, 2−5. (13) Shiraishi, M.; Swaraj, S.; Takenobu, T.; Iwasa, Y.; Ata, M.; Unger, W. E. S. Spectroscopic characterization of single-walled carbon nanotubes carrier-doped by encapsulation of TCNQ. Phys. Rev. B 2005, 71. (14) Wildgoose, G. G.; Banks, C. E.; Compton, R. G. Metal nanopartictes and related materials supported on carbon nanotubes: methods and applications. Small 2006, 2, 182−193. (15) Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S. K.; Waghmare, U. V.; Novoselov, K. S.; Krishnamurthy, H. R.; Geim, A. K.; Ferrari, A. C.; Sood, A. K. Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor. Nat. Nanotechnol. 2008, 3, 210−215. (16) Panchakarla, L. S.; Subrahmanyam, K. S.; Saha, S. K.; Govindaraj, A.; Krishnamurthy, H. R.; Waghmare, U. V.; Rao, C. N. R. Synthesis, structure, and properties of boron- and nitrogen-doped graphene. Adv. Mater. 2009, 21, 4726−4730. (17) Semiconductor Nanocrystal Quantum Dots: Synthesis, Assembly, Spectroscopy, and Applications; Rogach, A. L., Eds.; Springer-Verlag: Vienna, 2008. (18) Herranz, M. A.; Ehli, C.; Campidelli, S.; Gutierrez, M.; Hug, G. L.; Ohkubo, K.; Fukuzumi, S.; Prato, M.; Martín, N.; Guldi, D. M. Spectroscopic characterization of photolytically generated radical ion pairs in single-wall carbon nanotubes bearing surface-immobilized tetrathiafulvalenes. J. Am. Chem. Soc. 2008, 130, 66−73. (19) Martín, N.; Sanchez, L.; Herranz, M. A.; Illescas, B.; Guldi, D. M. Electronic communication in tetrathiafulvalene (TTF)/C60 systems: toward molecular solar energy conversion materials? Acc. Chem. Res. 2007, 40, 1015−1024. (20) Imahori, H.; Umeyama, T.; Kurotobi, K.; Takano, Y. Selfassembling porphyrins and phthalocyanines for photoinduced charge separation and charge transport. Chem. Commun. 2012, 48, 4032− 4045. (21) Krätschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Solid C60 - a new form of carbon. Nature 1990, 347, 354−358. (22) Frank, S.; Poncharal, P.; Wang, Z. L.; de Heer, W. A. Carbon nanotube quantum resistors. Science 1998, 280, 1744−1746. (23) Hong, S.; Myung, S. Nanotube electronics - A flexible approach to mobility. Nat. Nanotechnol. 2007, 2, 207−208. (24) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. J. Ballistic carbon nanotube field-effect transistors. Nature 2003, 424, 654−657.

SWNT/exTTF nanotweezers and MWNT/exTTF nanotweezers with visible light is the inception of photoinduced electrontransfer. At first glance, the corresponding electron-transfer products seemed nearly identical with lifetimes of several hundred picoseconds. A closer look reveals that the presence of several concentric nanotubes in, for example, MWNT is beneficial in prolonging the lifetime of the radical ion pair state, that is, 160 ps for SWNT/exTTF nanotweezers versus 380 ps for MWNT/exTTF nanotweezers. Importantly, the synthetic straightforward design and the versatility of tweezerslike compounds make the use of exTTF nanotweezers, as such, an elegant strategy for promoting the disaggregation of SWNTs, yielding soluble and electroactive nanohybrids.

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CONCLUSIONS AND OUTLOOK In summary, a variety of sophisticated approaches have been established to obtain stable dispersions of individual SWNTs and single-layer/few-layer graphene. Differing between topdown and bottom-up approaches, few preparations are known for the latter, whereas for the former starting from bundles of SWNTs and graphite myriad techniques are at hand. However, to overcome the π−π stacking energy between individual SWNTs and graphene layers in graphite crystals, an activation step needs to be implemented. This is where the immobilization of functional groups, either by covalent or noncovalent means, has led to sizable breakthroughs in the realization of interfacing organic (i.e., extended tetrathiafulvalenes and aromatic macrocycles) and inorganic (i.e., CdTe nanocrystals) semiconductors with low-dimensional nanocarbons. Promising results in terms of solar energy conversion applications, that is, photoelectrochemical cells that were processed by layer-bylayer means from aqueous solution onto ITO, are particularly relevant. Scientists pledge nanocarbons and their integration into state-of-the-art nanotechnology. Nevertheless, vigorous efforts that are at the interface of chemistry, physics, and materials science are needed to outperform today’s technology that is almost exclusively driven by silicon. Importantly, nanocarbons and their hybrids/conjugates reveal numerous unique features. However, it is now time to seize such uniqueness to take full advantage of the potential.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge our long-standing collaboration with Professor Nazario Martin and Professor Tomas Torres on the cutting edge science of nanocarbons/extended tetrathiafulvalenes and nanocarbons/aromatic macrocycles, respectively. Financial support from the DFG is greatly appreciated. The ZMP (Zentralinstitut für Materialien ud Prozesstechnik) is acknowledged for intellectual support.

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REFERENCES

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Interfacing Nanocarbons with Organic and Inorganic Semiconductors

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