Cordula D. Schmidt

Manipulating single-wall carbon nanotubes by chemical doping and charge transfer with perylene dyes

Single-wall carbon nanotubes (SWNTs) are emerging as materials with much potential in several disciplines, in particular in electronics and photovoltaics. The combination of SWNTs with electron donors or acceptors generates active materials, which can produce electrical energy when irradiated. However, SWNTs are very elusive species when characterization of their metastable states is required. This problem mainly arises because of the polydispersive nature of SWNT samples and the inevitable presence of SWNTs in bundles of different sizes. Here, we report the complete and thorough characterization of an SWNT radical ion-pair state induced by complexation with a perylene dye, which combines excellent electron-accepting and -conducting features with a five-fused ring π-system. At the same time, the perylene dye enables the dispersion of SWNTs by means of π–π interactions, which gives individual SWNTs in solution. This work clears a path towards electronic and optoelectronic devices in which regulated electrical transport properties are important. Using carbon nanotubes in electronic or photovoltaic devices generates active metastable states. These elusive species are hard to characterize because of the polydisperse and aggregate nature of nanotube bundles. A complete characterization of the radical–ion pair state has now been achieved using a range of techniques.

High Population of Individualized SWCNTs through the Adsorption of Water-Soluble Perylenes

The aqueous dispersion of SWCNTs in the presence of the water-soluble perylene derivatives 1-3 is reported. Significantly, even very low concentrations of the perylenes such as 0.01 wt% of the amphiphilic derivative 3, cause an efficient dissolution of the SWCNTs in water accompanied by a very pronounced individualization. The individualization of SWCNTs in water after ultrasonication in the presence of water-soluble aromatic perylenes was investigated in detail by absorption, emission, and Raman spectroscopy as well as by AFM and cryo-TEM. These studies also revealed that the individualization of the SWCNTs caused by the adsorption of 3 is much more effective than that induced by SDBS, which is the most frequently used surfactant for SWCNT dispersion in water. The pi-pi-stacking interaction and the electronic interaction between the perylene unit and the nanotube surface is reflected, for example, by the distinct absorption and emission features in the UV/vis/nIR, which differ significantly from those observed for SWNTs dispersed in the presence of SDBS and by the quenching of the perylene fluorescence of 3 when being in contact with the tubes.

Nanotube Surfactant Design: The Versatility of Water‐Soluble Perylene Bisimides

The synthesis of perylene-based single-walled carbon nanotube (SWCNT) surfactants and the dispersion and exfoliation of SWCNTs in water by a variety of designed surfactants is investigated. The quality of the nanotube dispersions is evaluated by optical absorption and emission spectroscopy, zeta-potential measurements and statistical atomic force microscopy (AFM). Significantly the dispersion efficiency can be increased at higher pH, as water solubility of the surfactants is ensured by peripheral derivatization with carboxyl-functionalized first- and second-order Newkome dendrimers. Even at very low perylene concentrations of 0.1 g L(-1) and a nanotube-to-surfactant ratio of 1:1, the nanotube supernatant after centrifugation contains up to 73% of the pristine material with exfoliation degrees (the number of fractions of individualized nanotubes N(I)/N(T)) of up to 76%. The adsorption of the perylene core to the nanotube scaffold is indicated by red-shifted perylene-absorption and SWCNT-emission features except for the smallest perylene amphiphile, where solubilization is presumably based on a micellar arrangement. The nanotube fluorescence is significantly altered and reduced in intensity compared to nanotubes dispersed in sodium dodecylbenzene sulfonate (SDBS) being strongly dependent on the structure of the perylene surfactant. We attribute this observation to the homogeneity of the surfactant coverage, e.g., the supramolecular arrangement onto the nanotube backbone. This study represents a step forward in understanding the structure-property relationship of nanotube surfactants. Furthermore high-quality nanotube dispersions with increased degrees of exfoliation are highly desirable, as the efficiency of nanotube separation techniques relies on highly individualized samples.

Non‐Covalent Chemistry of Graphene: Electronic Communication with Dendronized Perylene Bisimides

Graphene is the youngest representative of synthetic carbon allotropes. Since its discovery in 2004, [ 1 ] a series of outstanding physical properties has been revealed. As a consequence, this single-layer graphite is considered to be one of the most promising materials for high-performance applications, [ 2 ] for example in the fi eld of molecular electronics. Although chemistry on graphene and highly dispersed graphite offers unprecedented opportunities, wet chemical functionalization of intact graphene [ 3 ] remains almost completely unexplored. Wet chemistry of graphene is highly attractive because: a) its unique properties can be combined with those of other compound classes, b) solubility and processability can be increased, c) fi ne tuning of the electronic characteristics (doping) can be achieved, d) synthetic routes to novel macromolecular architectures, for instance, graphanes as 2D-polymers, can be provided, and e) the inherent principles of graphene reactivity can be revealed. Herein, we report for the fi rst time on the electronic communication between graphene with the perylene bisimide (PBI) 1 [ 4 ] ( Figure 1 ) when both are deposited on a surface or dispersed in homogeneous solution. This interaction is provided by the non-covalent binding of their conjugated π -systems. Recently we have shown that amphiphilic PBIs, which are related to 1 but contain deprotected carboxylic acid groups, are very effi cient for the exfoliation of single-walled carbon nanotubes (SWNTs) [ 4 , 5 ] and graphite [ 6 ] in water. Moreover, we have demonstrated that the π – π -stacking interaction of electrondefi cient PBIs with SWNTs in water is accompanied by a p -doping of the tubes. [ 7 ] So far, evidence for graphene-dye interactions has been obtained for the solid state, exclusively, for instance, after gas-phase deposition of the dye in ultrahigh vacuum on epitaxial graphene, [ 8 ] after soaking mechanically exfoliated Kish graphite in a dye solution, [ 9 ] on gold or silver colloids, [ 10 ] and on H-passivated substrates. [ 11 ]

Interfacing Strong Electron Acceptors with Single Wall Carbon Nanotubes

The complementary use of steady-state and time-resolved spectroscopy in combination with electrochemistry and microscopy are indicative of mutual interactions between semiconducting SWNTs and a water-soluble strong electron acceptor, i.e., perylenediimide. Significant is the stability and the strong electronic coupling of the perylenediimide/SWNT electron donor-acceptor hybrids. Several spectroscopic and spectroelectrochemical techniques, i.e., Raman, absorption, and fluorescence, confirmed that distinct ground- and excited-state interactions occur and that kinetically and spectroscopically well characterized radical ion pair states form within a few picoseconds.

Fractioning HiPco and CoMoCAT SWCNTs via density gradient ultracentrifugation by the aid of a novel perylene bisimide derivative surfactant.

HiPco and CoMoCAT single-walled carbon nanotubes (SWCNT) were fractionated with the aid of a novel perylene bisimide surfactant by combined co-surfactant and replacement density gradient ultracentrifugation (DGU).

A Facile Route to Water-Soluble Coronenes and Benzo[ghi]perylenes

Click reactions at the bay-position of perylenes and a new route to benzo[ghi]perylenes and coronenes are presented. Irradiation with light leads to an electrocyclic reaction of the newly formed triazole ring(s) with the neighbouring bay-positions of the perylene core and after oxidation by air, the benzo[ghi]perylenes and coronenes are obtained. By using Newkome dendrimers as substituents for perylene diimides (PDIs), water solubility can be achieved after removal of the tert-butyl protecting groups. The aggregation and optical properties of the bay-functionalised PDIs, benzo[ghi]perylenes and coronenes are investigated by absorption and fluorescence spectroscopy.

Perylene-Based Nanotweezers: Enrichment of Larger-Diameter Single-Walled Carbon Nanotubes

The solubilization of single-walled carbon nanotubes (SWCNTs) by a novel tweezer-shaped molecule with perylene bisimide moieties that act as aromatic anchoring groups is presented. Encouraging results of the tweezer-dispersion concept is combined with the outstanding exfoliation and dispersion efficiencies of designed perylene bisimide derivatives, which have previously turned out as most-powerful SWCNT dispersants. Based on the preferred interaction between the nanotweezer and SWCNTs with diameters larger than 0.8 nm, the supernatant was depleted after mild centrifugation in SWCNT species of smaller diameters. Characterization was carried out by a combination of UV/Vis and nIR absorption spectroscopy as well as emission spectroscopy of the SWCNTs and perylene. This study presents the foundation for a further improvement of selective SWCNT dispersion and sorting by designed molecules.

Enhanced Adsorption Affinity of Anionic Perylene-Based Surfactants towards Smaller-Diameter SWCNTs

We present evidence from multiple characterization methods, such as emission spectroscopy, zeta potential, and analytical ultracentrifugation, to shed light on the adsorption behavior of synthesized perylene surfactants on single-walled carbon nanotubes (SWCNTs). On comparing dispersions of smaller-diameter SWCNTs prepared by using cobalt-molybdenum catalysis (CoMoCAT) with the larger-diameter SWCNTs prepared by high-pressure carbon monoxide decomposition (HiPco), we find that the CoMoCAT-perylene surfactant dispersions are characterized by more negative zeta potentials, and higher anhydrous specific volumes (the latter determined from the sedimentation coefficients by analytical ultracentrifugation), which indicates an increased packing density of the perylene surfactants on nanotubes of smaller diameter. This conclusion is further supported by the subsequent replacement of the perylene derivatives from the nanotube sidewall by sodium dodecyl benzene sulfonate (SDBS), which first occurs on the larger-diameter nanotubes. The enhanced adsorption affinity of the perylene surfactants towards smaller-diameter SWCNTs can be understood in terms of a change in the supramolecular arrangement of the perylene derivatives on the scaffold of the SWCNTs. These findings represent a significant step forward in understanding the noncovalent interaction of π-surfactants with carbon nanotubes, which will enable the design of novel surfactants with enhanced selectivity for certain nanotube species.

Nickel oxide nanostructured electrodes towards perylenediimide-based dye-sensitized solar cells

In this work, we have realized nickel oxide (NiO) electrodes that serve as photocathodes in p-type dye-sensitized solar cells (p-DSSCs) sensitized by dendronized perylenediimides (PDIs). To this end, two different approaches in terms of preparing NiO nanoparticle pastes were pursued to fabricate mesoporous electrodes on conductive fluorine doped tin oxide (FTO) glass substrates. Firstly, commercially available NiO nanoparticles were dispersed in a mixture of ethanol and terpineol. Here, in order to obtain a mesoporous network two types of ethylcelluloses (EC), that is, EC 5–15 and 30–50 mPa s, were added in 1 : 1 weight ratios. Following the evaporation of ethanol, the resulting pastes were spread on FTOs by doctor blading and calcinated at different temperatures. Importantly, the calcination temperature evolved as a crucial aspect in developing efficient electrodes. Nevertheless, the visual appearance of these NiO electrodes prompts a fairly heterogeneous coverage. To circumvent the aforementioned problem, a second approach en route to homogenous electrodes was investigated. In that particular case, commercial NiO nanoparticles were mixed with a mixture of EC 5–15 and 30–50 mPa s at a 1 : 1 weight ratio, with triacetin as a plasticizer in ethanol. In doing so, pastes containing 7 wt% EC, 3 wt% triacetin, and 3 to 20 wt% NiO nanoparticles were prepared. Most importantly, scanning electron microscopy (SEM) images corroborated the fact that the resulting electrodes revealed a dense coverage on FTOs. In addition, further characterizations ranging from UV/Vis transmission spectroscopy and conductivity measurements to Barrett–Joyner–Halenda (BJH) pore size and volume analysis were carried out. In the final step, the applicability of the new NiO photoelectrodes for p-DSSCs was successfully demonstrated by utilizing two types of PDIs, namely a symmetric 1 and a non-symmetric dendronized 2. Key aspects such as time dependence of dye uptake, hole lifetime and resistance features of the electrodes under operation conditions were investigated.