Multifunctional molecular carbon materials--from fullerenes to carbon nanotubes

Shape- and size-controlled nanomaterials for artificial photosynthesis

Nanomaterials with various shapes and sizes have been developed to mimic functions of photosynthesis in which solar energy conversion is achieved by using nanosized proteins with controlled shapes and sizes. Artificial photosynthesis consists of light-harvesting and charge-separation processes together with catalytic units of water oxidation and reduction. Nanosized mesoporous silica-alumina was utilized to encapsulate organic charge-separation molecules inside the nanospace to elongate the lifetimes of the charge-separated states, as observed in the photosynthetic reaction centers. Metal nanoparticles with controlled shapes and sizes have also been utilized as efficient catalysts for photocatalytic hydrogen evolution from water with reductants by using electron donor-acceptor organic molecules as photocatalysts. The control of the shape and size of metal nanoparticles plays a very important role in achieving high catalytic performance in catalytic hydrogen evolution in water reduction and also in catalytic oxygen evolution in water oxidation.

Carbon nanomaterials for electronics, optoelectronics, photovoltaics, and sensing

In the last three decades, zero-dimensional, one-dimensional, and two-dimensional carbon nanomaterials (i.e., fullerenes, carbon nanotubes, and graphene, respectively) have attracted significant attention from the scientific community due to their unique electronic, optical, thermal, mechanical, and chemical properties. While early work showed that these properties could enable high performance in selected applications, issues surrounding structural inhomogeneity and imprecise assembly have impeded robust and reliable implementation of carbon nanomaterials in widespread technologies. However, with recent advances in synthesis, sorting, and assembly techniques, carbon nanomaterials are experiencing renewed interest as the basis of numerous scalable technologies. Here, we present an extensive review of carbon nanomaterials in electronic, optoelectronic, photovoltaic, and sensing devices with a particular focus on the latest examples based on the highest purity samples. Specific attention is devoted to each class of carbon nanomaterial, thereby allowing comparative analysis of the suitability of fullerenes, carbon nanotubes, and graphene for each application area. In this manner, this article will provide guidance to future application developers and also articulate the remaining research challenges confronting this field.

Quenching and sensitizing fullerene photoreactions by natural organic matter

Effects of natural organic matter (NOM) on the photoreaction kinetics of fullerenes (i.e., C60 and fullerenol) were investigated using simulated sunlight and monochromatic radiation (365 nm). NOM from several sources quenched (slowed) the photoreaction of C60 aggregates in water (aqu/nC60), but sensitized (accelerated) photoreaction of fullerenol. Kinetic studies indicated that the quenching occurred through a static mechanism involving NOM molecules adsorbed on the aqu/nC60 surface. Quenching constants for the photoreaction of aqu/nC60 correlated approximately with optical parameters related to the aromaticity and molecular size of the NOM. Association of aqu/nC60 particles with NOM was investigated indirectly via the study of the aggregation kinetics of colloidal C60 in the presence and absence of NOM as a function of NaCl strength at pH 7. In contrast to aqu/nC60, the photoreaction efficiencies of the hydrophilic fullerene, fullerenol, increased linearly with increasing NOM concentrations and kinetic parameters for the sensitized photoreactions increased as the spectral slope coefficients and ratio of absorption coefficients at 254 to 365 nm (E2:E3) of the NOM increased. The results indicate that triplet excited states of the NOM are key intermediates in the photosensitized reactions.

Nanocarbon Hybrids: The Paradigm of Nanoscale Self-Ordering/Self-Assembling by Means of Charge Transfer/Doping Interactions

The scope of this Perspective is to highlight the potential of nanoscale self-ordering/self-assembling nanocarbons-fullerenes, single-wall carbon nanotubes, and graphene-en route toward novel charge transfer hybrids that unify several functions such as light harvesting, charge separation, and, eventually, catalysis. All of the given examples are fully characterized by a broad range of spectroscopic as well as microscopic techniques and, as such, document the success in tuning the electronic structure of functional nanocarbons by means of charge transfer/doping interactions.

Excitation-wavelength-dependent, ultrafast photoinduced electron transfer in bisferrocene/BF2-chelated-azadipyrromethene/fullerene tetrads

Donor-acceptor distance, orientation, and photoexcitation wavelength are key factors in governing the efficiency and mechanism of electron-transfer reactions both in natural and synthetic systems. Although distance and orientation effects have been successfully demonstrated in simple donor-acceptor dyads, revealing excitation-wavelength-dependent photochemical properties demands multimodular, photosynthetic-reaction-center model compounds. Here, we successfully demonstrate donor- acceptor excitation-wavelength-dependent, ultrafast charge separation and charge recombination in newly synthesized, novel tetrads featuring bisferrocene, BF2 -chelated azadipyrromethene, and fullerene entities. The tetrads synthesized using multistep synthetic procedure revealed characteristic optical, redox, and photo reactivities of the individual components and featured "closely" and "distantly" positioned donor-acceptor systems. The near-IR-emitting BF2-chelated azadipyrromethene acted as a photosensitizing electron acceptor along with fullerene, while the ferrocene entities acted as electron donors. Both tetrads revealed excitation-wavelength-dependent, photoinduced, electron-transfer events as probed by femtosecond transient absorption spectroscopy. That is, formation of the Fc(+)-ADP-C60(.-) charge-separated state upon C60 excitation, and Fc(+)-ADP(.-)-C60 formation upon ADP excitation is demonstrated.

Toward multifunctional wet chemically functionalized graphene-integration of oligomeric, molecular, and particulate building blocks that reveal photoactivity and redox activity

Many technological applications indispensable in our daily lives rely on carbon. By altering the periodic binding motifs in networks of sp(3), sp(2), and sp-hybridized carbon atoms, researchers have produced a wide palette of carbon allotropes. Over the past two decades, the physicochemical properties of low-dimensional nanocarbons, including fullerenes (0D), carbon nanotubes (1D), and, most recently, graphene (2D), have been explored systematically. An entire area of research has focused on the chemistry of 1D nanocarbons, particularly single-wall carbon nanotubes. These structures exhibit unique electronic, mechanical, and optical properties. These properties are, however, only discernible for single-wall carbon nanotubes that are debundled, individualized, and stabilized, often in solution. Most prominently, they are small band gap, p-type semiconductors or metals with conductances that reach ballistic dimensions. These structures can have poor solubility in many media, and large bundles can originate from attractive interactions such as π-π stacking and London dispersion forces. Therefore, both covalent and noncovalent modifications of single-wall carbon nanotubes have emerged as powerful approaches to overcome some of these problems. Noncovalent functionalization is especially useful in improving the solubility without altering the electronic structure. We expect that many of the strategies that have recently been exploited and established in the context of 1D nanocarbons can be applied to the chemistry of 2D nanocarbons, especially graphene. Two-dimensional nanocarbons are currently attracting extensive attention due to their striking mechanical, optical, and electrical features. Nanocarbons that are a single atom thick are gapless semiconductors and exhibit electron mobilities reaching values of up to 15000 cm(2) V(-1) s(-1) at room temperature. Researchers have made rapid progress in the covalent and/or noncovalent functionalization of graphene with photoactive and or redox active building blocks. In this Account, we summarize our work on the integration of photoactive and/or redox active building blocks, including oligomers, molecules, and particulates, onto graphenoid materials to yield multifunctional electron donor-acceptor conjugates and hybrids. Intriguingly, we produce graphene in the form of single-layer, bilayer, and multilayer graphene through the exfoliation of graphite by surface active agents. The exfoliation occurs through π-π, hydrophobic, van der Waals, electrostatic, and charge transfer interactions, and the surface active agents also serve as versatile anchor groups. We studied the electronic interactions in terms of photoactivity and/or redox activity in depth by steady-state and time-resolved spectroscopy. Finally, we present examples of proof-of-principle solar energy conversion devices.

Non-destructive inhibition of metallofullerenol Gd@C(82)(OH)(22) on WW domain: implication on signal transduction pathway

Endohedral metallofullerenol Gd@C₈₂(OH)₂₂ has recently been shown to effectively inhibit tumor growth; however, its potential adverse bioeffects remain to be understood before its wider applications. Here, we present our study on the interaction between Gd@C₈₂(OH)₂₂ and WW domain, a representative protein domain involved in signaling and regulatory pathway, using all-atom explicit solvent molecular dynamics simulations. We find that Gd@C₈₂(OH)₂₂ has an intrinsic binding preference to the binding groove, particularly the key signature residues Y28 and W39. In its binding competition with the native ligand PRM, Gd@C₈₂(OH)₂₂ is shown to easily win the competition over PRM in occupying the active site, implying that Gd@C₈₂(OH)₂₂ can impose a potential inhibitory effect on the WW domain. Further analyses with binding free energy landscapes reveal that Gd@C₈₂(OH)₂₂ can not only directly block the binding site of the WW domain, but also effectively distract the PRM from its native binding pocket.

Endohedral and exohedral hybrids involving fullerenes and carbon nanotubes

Since fullerenes and carbon nanotubes (CNTs) were discovered, these materials have attracted a great deal of attention in the scientific community due to their unique structures and properties. The properties of both carbon allotropes can be modulated by chemical functionalization, and merging fullerenes and CNTs combines the electronic and optical properties of CNTs with the excellent electron acceptor characteristic of fullerenes; moreover, a synergistic effect of these hybrids can be found, as the properties of both the nanotube and the fullerene are affected by the presence of the other. In these hybrids, the fullerene can be located inside (endohedral) or outside (exohedral) the CNT and both types of hybrid have specific features. CNT-fullerene hybrids have been studied for various applications, including photovoltaics, optical limiting and flame retardancy amongst others. This review outlines the progress in research on CNT-fullerene hybrids, including endohedral and exohedral combinations, their properties, functionalization, applications and outlook.

Graphene as a quencher of electronic excited states of photochemical probes

Graphene sheets quench the singlet and triplet excited states of a series of six photochemical probes including pyrene, acridine orange, tris(2,2́-bipyridyl)ruthenium(II) dichloride, methylene blue, meso-tetrakis(phenylsulphonate)porphyrin, and 5,10,15,20-tetraphenyl-21H,28H-porphine zinc. It was found that Stern-Volmer fluorescence quenching can fit to one or two different quenching regimes depending on the probe. In addition, the quenching can be either static or dynamic depending on the fluorophore. The occurrence of several quenching regimes has been interpreted considering that quenching arises from the crowding of the fluorophore on both graphene faces, or site isolation on the graphene sheets. Laser flash photolysis has shown that the triplet lifetime of the probes generally decreases due to graphene quenching and that no new transients appear except in the case of methylene blue, where a new absorption spectrum characterized by a continuous absorption band is observed and attributed to graphene radical ion. This spectroscopic evidence suggests that the most general quenching mechanism is energy transfer from the singlet or triplet excited state of the dye to graphene. This raises the issue of determining the energy of the electronic excited states of graphene.

Supramolecular hetero-porphyrin SWNT complexes

The complexation of single walled carbon nanotubes (SWNTs) with neutral, anionic and cationic porphyrins has been investigated under identical complex forming conditions. The determination of the porphyrin loading reveals large differences depending on the nature of the porphyrin used. Combinations of different porphyrins to form mixed hetero-porphyrin complexes shows that the mixture of a cationic and anionic porphyrin results in loading which is an order of magnitude larger than in all other complexes. This complex also exhibits high adsorption and emission intensities and can be regarded as an extended co-operative binary ionic (CBI) solid. The complexes were further studied using Raman spectroscopy, elemental analysis, AFM and cyclic voltammetry.

Photoelectrochemical properties of donor-acceptor nanocomposite films composed of porphyrin-functionalized cup-shaped nanocarbon materials

Porphyrin-functionalized cup-shaped nanocarbons (CNC-H2P) have been assembled onto nanostructured SnO2 films using an electrophoretic deposition method to examine the photoelectrochemical properties. The obtained CNC-H2P nanohybrid films were examined by a series of steady-state and time-resolved spectroscopic measurements and photoelectrochemical measurements. The resulting nanohybrid film afforded drastic enhancement in the photoelectrochemical performance as well as broader photoresponse in the visible region as compared with the reference CNC system without porphyrins. The enhancement of photocurrent generation may be caused by the efficient electron injection from the long-lived charge-separated state of CNC-H2P upon photoexcitation. This feature makes cup-shaped nanocarbon materials a useful candidate for developing efficient photoelectrochemical and photovoltaic cells.

A charge-transfer challenge: combining fullerenes and metalloporphyrins in aqueous environments

A series of truly water-soluble C(60)/porphyrin electron donor-acceptor conjugates has been synthesized to serve as powerful mimics of photosynthetic reaction centers. To this end, the overall water-solubility of the conjugates was achieved by adding hydrophilic dendrimers of different generations to the porphyrin moiety. An important variable is the metal center of the porphyrin; we examined zinc(II), copper(II), cobalt(II), nickel(II), iron(III), and manganese(III). The first insights into electronic communication between the electron donors and the electron acceptors came from electrochemical assays, which clearly indicate that the redox processes centered either on C(60) or the porphyrins are mutually affected. Absorption measurements, however, revealed that the electronic communication in terms of, for example, charge-transfer features, remains spectroscopically invisible. The polar environment that water provides is likely to be a cause of the lack of detection. Despite this, transient absorption measurements confirm that intramolecular charge separation processes in the excited state lead to rapid deactivation of the excited states and, in turn, afford the formation of radical ion pair states in all of the investigated cases. Most importantly, the lifetimes of the radical ion pairs were found to depend strongly on several aspects. The nature of the coordinated metal center and the type of dendrimer have a profound impact on the lifetime. It has been revealed that the nature/electronic configuration of the metal centers is decisive in powering a charge recombination that either reinstates the ground state or any given multiplet excited state. Conversely, the equilibrium of two opposing forces in the dendrimers, that is, the interactions between their hydrophilic regions and the solvent and the electronic communication between their hydrophobic regions and the porphyrin and/or fullerene, is the key to tuning the lifetimes.

Syntheses, electrochemistry, and photodynamics of ferrocene-azadipyrromethane donor--acceptor dyads and triads

A near-IR-emitting sensitizer, boron-chelated tetraarylazadipyrromethane, has been utilized as an electron acceptor to synthesize a series of dyads and triads linked with a well-known electron donor, ferrocene. The structural integrity of the newly synthesized dyads and triads was established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a 'molecular clip'-type structure for the triads wherein the donor and acceptor entities were separated by about 14 Å. Differential pulse voltammetry combined with spectroelectrochemical studies have revealed the redox states and estimated the energies of the charge-separated states. Free-energy calculations revealed the charge separation from the covalently linked ferrocene to the singlet excited ADP to yield Fc(+)-ADP(•-) to be energetically favorable. Consequently, the steady-state emission studies revealed quantitative quenching of the ADP fluorescence in all of the investigated dyads and triads. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of photoinduced electron transfer in these donor-acceptor systems by providing spectral proof for formation of ADP radical anion (ADP(•-)) which exhibits a diagnostic absorption band in the near-IR region. The kinetics of charge separation and charge recombination measured by monitoring the rise and decay of the ADP(•-) band revealed ultrafast charge separation in these molecular systems. The charge-separation performance of the triads with two ferrocenes and a fluorophenyl-modified ADP macrocycle was found to be superior. Nanosecond transient absorption studies revealed the charge-recombination process to populate the triplet ADP as well as the ground state.