Jenny Malig

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.

Low dimensional nanocarbons – chemistry and energy/electron transfer reactions

This Minireview sheds light onto the electronic communication between, on one hand, low dimensional nanocarbons – single and multiwalled 1D carbon nanotubes and 2D graphene – and, on the other hand, a variety of electroactive species en-route to novel electron donor–acceptor conjugates and hybrids in relation to their covalent and non-covalent chemistry, respectively. A common denominator to any of the highlighted conjugates/hybrids is charge transport across different scales, that is, from individual molecular conjugates/hybrids to morphologically controlled devices.

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.

Direct exfoliation of graphite with a porphyrin – creating functionalizable nanographene hybrids

Exfoliation of graphite was achieved using a free-base porphyrin 1 resulting in an efficient fabrication of single-layer nanographene (NG)- hybrid platelets that can be further functionalized with other nanomaterials. The novel nanographene-porphyrin hybrids reveal efficient charge transfer in the excited state.

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

Electron Accepting Porphycenes on Graphene

The versatility of nanographene (NG) as an electron donor is demonstrated when integrated together with an electron accepting porphycene into a novel electron NG hybrid. This feature is further exploited in dye-sensitized solar cells (DSSCs), in which photoanodes (ZnO) reveal a cascade of electron flow as modus operandi.

Poly‐Ortho‐Functionalizable Tetraarylporphycene Platform–Synthesis of Octacationic Derivatives Towards the Layer‐by‐Layer Design of Versatile Graphene Oxide Photoelectrodes

Porphycene, the fi rst isolated and most stable constitutional isomer of porphyrin, was synthesized for the fi rst time in 1986 by Vogel and co-workers. [ 1 ] To this end, 2,7,12,17-tetran -propylporphycene is by far the most commonly studied porphycene derivative owing to its ease of synthesis and its remarkable solubility. [ 2 ] In the early years, porphycenes were investigated as therapeutic agents in the fi eld of photodynamic therapy. [ 3 ]

Interfacing Nanocarbons with Organic and Inorganic Semiconductors: From Nanocrystals/Quantum Dots to Extended Tetrathiafulvalenes

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 low-dimensional 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.

Synthesis and physico-chemical properties of porphycenes

A new and improved synthesis for porphycenes is presented. In particular, treating pyrroles with iodine monochloride results in quantitative yields and easier work up. This key step was employed for the synthesis of different 2,7,12,17-substituted porphycenes (TPrPc, TPPc, TtBPPc). The synthetic part of this project is followed by a global analysis of the physico-chemical properties of three exemplary porphycenes, including absorption, emission, and electrochemistry. In-situ UV-vis/nIR spectroelectrochemistry was used to determine the spectral signatures of the radical anion and cation species. Temperature-dependent fluorescence lifetimes were determined under air as well as under inert atmosphere.