Norbert Jux

Towards Tunable Graphene/Phthalocyanine–PPV Hybrid Systems†

The sheer explosion of interest in graphene has undoubtedly shown that it is the rising star in the emerging field of nanotechnology. Its extraordinary properties render it an outstanding material for electronics, material sciences, and photoconversion systems. As a zero-gap semiconductor for example, a flat monolayer of graphene is almost transparent and exhibits the lowest known electrical resistivity for any material at room temperature. The remarkably high electron mobility of graphene gives rise to its implementation in transparent conducting electrodes as a viable alternative to indium tin oxide (ITO). 4] Recent results demonstrate, however, that doping is a necessity to harvest the full potential of graphene. 6] Therefore, the aim herein is the tuning/altering of the features of photochemically transparent graphene by integrating a versatile electron donor system in solution. High-quality graphene flakes have been formed by means of solution processing, which involves exfoliating and dispersing them directly from graphite. 8] Such mild strategies stand in strong contrast to high throughput exfoliation of graphite with the assistance of strong oxidants. Moreover, reduction of graphene oxide to graphene is by no means quantitative and results in irreversible coagulation and permanent lattice defects. 13] To date, samples that exhibit high charge mobilities are large-area graphene samples obtained by micromechanical cleavage of pyrrolitic graphite. Other notable breakthroughs in this area rely on sheets grown onto solid substrates. The investigation of novel electron donor–acceptor hybrids involving low-dimensional allotropes of carbon is far more challenging than the exploitation of carbon nanotubes in the same context. The reason is primarily the lack of photospectroscopic signatures/markers, which makes the study of graphene more challenging. In fact, we have selected a spectator molecule to circumvent this impediment and to assist in identifying and visualizing electron donor–acceptor interactions. The unique absorption with high extinction coefficients in the red and near infrared regions, fluorescence, and the strong electron-donating character of zinc phthalocyanines (ZnPc) make a ZnPc-based PPV oligomer (1) PPV = poly(p-phenylene vinylene) the molecule of choice (Supporting Information, Scheme S1). Apart from supporting several ZnPc units, the oligomeric backbone is expected to be a great asset with regard to graphite exfoliation and stabilization of novel nanographene (NG) hybrids bearing ZnPc oligomer 1 that are thus formed (Figure 1).

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.

Improving Photocurrent Generation: Supramolecularly and Covalently Functionalized Single-Wall Carbon Nanotubes-Polymer/Porphyrin Donor- Acceptor Nanohybrids

Novel nanohybrids based on covalently and noncovalently functionalized single-wall carbon nanotubes (SWNTs) have been prepared and assembled for the construction of photoactive electrodes. Polymer-grafted SWNTs were synthesized by free-radical polymerization of (vinylbenzyl)trimethylammonium chloride. Poly[(vinylbenzyl)trimethylammonium chloride] (PVBTAn+) was also noncovalently wrapped around SWNTs to form stable, positively charged SWNT/PVBTAn+ suspensions in water. Versatile donor-acceptor nanohybrids were prepared by using the electrostatic/van der Waals interactions between covalent SWNT-PVBTAn+ and/or noncovalent SWNT/PVBTAn+ and porphyrins (H2P8- and/or ZnP8-). Several spectroscopic, microscopic, transient, and photoelectrochemical measurements were taken to characterize the resulting supramolecular complexes. Photoexcitation of the nanohybrids afforded long-lived radical ion pairs with lifetimes as long as 2.2 micros. In the final part, photoactive electrodes were constructed by using a layer-by-layer technique on an indium tin oxide covered glass support. Photocurrent measurements gave remarkable internal photon-to-current efficiencies of 3.81 and 9.90 % for the covalent ZnP8-/SWNT-PVBTAn+ and noncovalent ZnP8-/SWNT/PVBTAn+ complex, respectively, when a potential of 0.5 V was applied.

Multiwalled carbon nanotubes in donor–acceptor nanohybrids—towards long-lived electron transfer products

Novel multiwalled carbon nanotube/metalloporphyrin nanohybrids are devised and probed as versatile donor-acceptor hybrids.

Toward combining graphene and QDs: assembling CdTe QDs to exfoliated graphite and nanographene in water.

Herein, we report for the first time on a full-fledged investigation of water-soluble CdTe quantum dots (QD) that are immobilized onto exfoliated graphite (EG) and/or nanographene (NG). Particular emphasis was placed on a top-down preparation of stable aqueous dispersions-starting from natural graphite rather than graphene oxide-while preserving the intrinsic properties of graphene. To this end, we circumvented the harsh conditions commonly employed for the pre-exfoliation (i.e., Hummers method). First, a hydrophobic-hydrophobic/π-π stacking motif was tested between EG and pyrene, to which QDs are covalently attached (QD-pyrene). Second, we employed the combination of hydrophobic-hydrophobic/π-π stacking and electrostatic interactions to build up hierarchical structures composed of NG, positively charged pyrene (pyrene(+)), and negatively charged QDs. The novel nanohybrids-QD-pyrene/EG and QD/pyrene(+)/NG-were characterized with specific emphasis on electron-transfer chemistry. In fact, both assays provide kinetic and spectroscopic evidence that support electron transfer dynamics that vary, however, between EG and NG as a reflection of the different degree of graphite exfoliation.

The Porphyrin Twist: Hückel and Möbius Aromaticity

The M bius strip is the classic example of a single-sided nonorientable surface. It can be constructed by taking a strip of paper, twisting it by 1808, and joining its ends to form a single strip. An excellent visualization of a M bius strip is the famous ant walk in “M bius Strip II” by the Dutch artist M. C. Escher. The intriguing concept of M bius aromaticity was proposed more than 40 years ago by Heilbronner, who predicted that M bius strip-like twisted annulenes with 4n p electrons should be closed-shell species and aromatic (Scheme 1). Despite the high number of papers in which the properties of such compounds were calculated, the synthesis of a M bius aromatic annulene was reported only recently by Herges and co-workers. The clever conceptual combination of a flexible polyene chain containing oneE double bond with a rigid bianthraquinodimethane moiety resulted in M biusstabilized [16]annulene 3 (Scheme 2). Its synthesis was

Iron Catalysis for In Situ Regeneration of Oxidized Cofactors by Activation and Reduction of Molecular Oxygen: A Synthetic Metalloporphyrin as a Biomimetic NAD(P)H Oxidase

A major challenge in biomimetic catalysis is the development of synthetic low-molecular-weight compounds that are able to mimic the catalytic function of enzymes. Thus, biomimetic redox enzymes should, on the one hand, be able to function as a catalyst in water and on the other hand accept cofactors, in particular NADH and NADPH (NAD(P) = nicotinamide adenine dinucleotide (phosphate)) and their oxidized forms NAD and NADP, respectively, as co-substrates. The in situ recycling of the expensive cofactors, a process carried out mostly by means of biotransformations, is considered a key technique for conducting enzymatic redox reactions in an attractive fashion. For the reduction mode of the cofactor regeneration (to regenerate the reduced forms NADH and NADPH) Steckhan et al. developed a “biomimetic formate dehydrogenase” for the regeneration of NAD(P)H by oxidation of formic acid into carbon dioxide by using a suitable rhodium complex. For the oxidation mode of the cofactor regeneration, NAD(P)H oxidases as natural catalysts have been applied as well as chemoenzymatic, electrochemical, and biomimetic catalyst systems. However, the biomimetic catalysts developed so far produce undesired hydrogen peroxide as a by-product instead of (preferably) water. To the best of our knowledge no biomimetic catalyst for the regeneration of NAD(P) from NAD(P)H by activation and reduction of molecular oxygen into water, similar to the mode of action of a NAD(P)H oxidase, is known. The mode of action of such a water-producing NAD(P)H oxidase is depicted in Scheme 1. In addition only a few synthetically suitable NAD(P)H oxidases, which serve as key tools for the “oxidative cofactor regeneration”, are known. However, in part these enzymes show a lack of stability under process conditions, different preferences for the two cofactors NADH and NADPH, and the production of unwanted hydrogen peroxide (instead of water) as a byproduct. Additionally, from this perspective the availability of an “artificial” biomimetic, water-producing NAD(P)H oxidase would be desirable and a valuable alternative to NAD(P)H-oxidase-type enzymes in preparative syntheses. Herein we report the application of a synthetic, watersoluble iron(III) porphyrin as an artificial, biomimetic waterproducing NAD(P)H oxidase. In analogy to enzymes, the metalloporphyrin is suitable for the in situ regeneration of both cofactors NAD and NADP by activation and reduction of molecular oxygen, and is also compatible with different preparative enzymatic oxidative reactions. Furthermore, to the best of our knowledge, this represents the first application of a synthetic metalloporphyrin as a catalyst for the activation and reduction of molecular oxygen into water by means of a natural cofactor in aqueous solution. Additionally a novel alternative is presented for carrying out enzymatic oxidation reactions under in situ regeneration of the oxidized cofactor NAD(P) by means of a non-enzymatic, synthetic catalyst serving as an “artificial enzyme mimic”. At the beginning of our work we searched for a lowmolecular-weight and water-soluble metal complex that accepts the natural cofactors NAD(P)H as a hydride donor for the activation of molecular oxygen, and would thereby be able to reduce oxygen into water while simultaneously being recycled as a catalyst. As the Fe porphyrin subunit, located in the active site of monooxygenases, exhibits comparable characteristics in the initial steps of monohydroxylation (though here a one-electron transfer involving a further cofactor takes place), we focused our preliminary screening on low-molecular-weight Fe complexes having a waterScheme 1. Concept of the NAD(P)H-oxidase-catalyzed or biomimetic in situ cofactor regeneration of NAD(P).

On the Energetics of Conformational Switching of Molecules at and Close to Room Temperature

We observe and induce conformational switching of individual molecules via scanning tunneling microscopy (STM) at and close to room temperature. 2H-5,10,15,20-Tetrakis-(3,5-di-tert-butyl)-phenylporphyrin adsorbed on Cu(111) forms a peculiar supramolecular ordered phase in which the molecules arrange in alternating rows, with two distinct appearances in STM which are assigned to concave and convex intramolecular conformations. Around room temperature, frequent bidirectional conformational switching of individual molecules from concave to convex and vice versa is observed. From the temperature dependence, detailed insights into the energy barriers and entropic contributions of the switching processes are deduced. At 200 K, controlled STM tip-induced unidirectional switching is possible, yielding an information storage density of 4.9 × 10(13) bit/inch(2). With this contribution we demonstrate that controlled switching of individual molecules at comparably high temperatures is possible and that entropic effects can be a decisive factor in potential molecular devices at these temperatures.

Integrating Water‐Soluble Graphene into Porphyrin Nanohybrids

Scheme 1. Schematic representation of a graphene sheet to which positively charged pyrene 1 is immobilized and the electrostatic interaction of NG/1 with negatively charged porphyrin 2. An overwhelming interest rests on graphene, the rising star in the field of nanotechnology.[1] Its extraordinary properties[2] render it a promising material for electronics,[3] material sciences,[4] and photoconversion systems.[5] The high electron mobility of graphene at room temperature calls for its implementation into transparent conducting electrodes.[6] In accordance with recent results doping is, however, crucial to harvest the full potential of graphene.[7] Solution processed graphene layers essentially have to be stabilized by means of surface active molecules prior to their transfer to solid substrates.[8] Usually graphene oxide[9] is used, which, however, requires chemical reduction or annihilation under high temperature[10] or UV-light assisted[11] conditions to afford either reduced graphene oxide also known as chemically converted graphene.[12] Notably, the overall reaction conditions (i.e., oxidation and reduction) are rather harsh and oxygen residues remain within the reduced graphene oxide structure.[13] In other words, irreversible damages of the conjugated π-system of graphene stay behind, which exert appreciable changes to the electronic properties such as conductivity.[14] A viable alternative to the aforementioned comprises the covalent[15] and/ or the non-covalent[16] functionalization of just graphite using reaction protocols similar to those developed in the context of the chemistry of fullerenes and/or of carbon nanotubes.[17] As a matter of fact, non-covalent functionalization of graphene as conducted by ultrasonicating graphite in presence of building blocks assists in the exfoliation to graphene,[18] on one hand, and ensures the stability of the resulting dispersions, on the other hand. Recently, we reported on the exfoliation of graphite and the concomitant non-covalent functionalization of graphene by a chromophoric electron donor. Beneficial was its use as a spectroscopic marker for probing electron transfer processes in solution.[19] A major concern related with this approach is the stability of the chromophores during the ultrasound treatment. Therefore, we report here on a novel non-invasive functionalization approach, namely tuning/altering the intrinsic features of photochemically “transparent” graphene, by integrating water-soluble porphyrins. The integration by means of electrostatic interactions necessitates an anchoring group, which is provided by a positively charged water-soluble pyrene

Donor-acceptor nanoensembles of soluble carbon nanotubes{

Donor-acceptor nanoensembles, prepared via electrostatic interactions of single wall carbon nanotubes and porphyrin salts, give rise to photoinduced intra-complex charge separation that lasts tens of microseconds.