A non-covalent strategy to prepare electron donor-acceptor rotaxanes.

A straightforward non-covalent synthetic strategy for producing donor-acceptor rotaxanes is reported. Femtosecond absorption spectroscopy illustrates that the use of this strategy gives rise to supramolecular chromophores with different electron transfer behavior.

A three-level luminescent response in a pyrene/ferrocene rotaxane.

A solvent switchable rotaxane equipped with a pyrene stopper and with two ferrocenyl units on the macrocycle is reported, in which three different states, two nondegenerate and one degenerate, can be obtained in different solvents at room temperature. This is accompanied by high contrast changes in fluorescence intensity of the pyrene stopper by the presence of the ferrocenyl moieties on the macrocycle, which quench the emission of pyrene more efficiently with proximity.

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.

Tetrathiafulvalene-based nanotweezers-noncovalent binding of carbon nanotubes in aqueous media with charge transfer implications.

Electron donor-acceptor hybrids based on single wall carbon nanotubes (SWCNT) are one of the most promising functional structures that are currently developed in the emerging areas of energy conversion schemes and molecular electronics. As a suitable electron donor, π-extended tetrathiafulvalene (exTTF) stands out owing to its recognition of SWCNT through π-π stacking and electron donor-acceptor interactions. Herein, we explore the shape and electronic complementarity between different types of carbon nanotubes (CNT) and a tweezers-shaped molecule endowed with two exTTFs in water. The efficient electronic communication between semiconducting SWCNT/multiwall carbon nanotubes (MWCNT), on one hand, and the water-soluble exTTF nanotweezers 8, on the other hand, has been demonstrated in the ground and excited state by using steady-state as well as time-resolved spectroscopies, which were further complemented by microscopy. Importantly, appreciable electronic communication results in the electronic ground state having a shift of electron density, that is, from exTTFs to CNT, and in the electronic excited state having a full separation of electron density, that is oxidized exTTF and reduced CNT. Lifetimes in the range of several hundred picoseconds, which were observed for the corresponding electron transfer products upon light irradiation, tend to be appreciably longer in MWCNT/8 than in SWCNT/8.

Charge transfer events in semiconducting single-wall carbon nanotubes.

Electron-donating ferrocene units have been attached to SWNTs, with different degrees of functionalization. By means of a complementary series of novel spectroscopic techniques (i.e., steady-state and time-resolved), we have documented that mutual interactions between semiconducting SWNT and the covalently attached electron donor (i.e., ferrocene) lead, in the event of photoexcitation, to the formation of radical ion pairs. In the accordingly formed radical ion pairs, oxidation of ferrocene and reduction of SWNT were confirmed by spectroelectrochemistry. It is, however, shown that only a few semiconducting SWNTs [i.e., (9,4), (8,6), (8,7), and (9,7)] are susceptible to photoinduced electron transfer processes. These results are of relevant importance for the development of SWNT-based photovoltaics.

Immobilizing water-soluble dendritic electron donors and electron acceptors-phthalocyanines and perylenediimides-onto single wall carbon nanotubes.

The complementary use of spectroscopy and microscopy sheds light onto mutual interactions between semiconducting single wall carbon nanotubes (SWNT) and either a strong dendritic electron acceptor-perylenediimide-or a strong dendritic electron donor-phthalocyanine. Importantly, the stability of the perylenediimide/SWNT electron donor-acceptor hybrids decreases with increasing dendrimer generation. Two effects are thought to be responsible for this trend. With increasing dendrimer generation we enhance (i) the hydrophilicity and (ii) the bulkiness of the resulting perylenediimides. Both effects are synergetic and, in turn, lower the immobilization strength onto SWNT. Owing to the larger size of the phthalocyanines, phthalocyanine/SWNT electron donor-acceptor hybrids, on the other hand, did not reveal such a marked dependence on the dendrimer generation. Several spectroscopies confirmed that distinct ground- and excited-state interactions prevail and that kinetically and spectroscopically well-characterized radical ion pair states are formed within a few picoseconds.

Efficient functionalization of carbon nanotubes with porphyrin dendrons via click chemistry.

The attempt to decorate carbon nanotubes with organic molecules as a powerful means to form new functional materials has attracted broad attention in the scientific community. Here, we report the functionalization of single-walled carbon nanotubes (SWNTs) with zinc porphyrins (ZnP) using very mild conditions to afford a series of SWNTs-ZnP (1 and 2) electron donor-acceptor conjugates. Owing to the presence of either one or two ZnP, introduced via "click chemistry", different absorption cross sections were realized. Important in this context is that the covalent linkages between SWNT and ZnP were corroborated by monitoring the diagnostic signature of the nitrogen atoms as part of the formed triazole ring by X-ray photoelectron spectroscopy (XPS). The resulting SWNTs-ZnP 1 and 2 were fully characterized. This characterization was complemented by a full-fledged investigation of their electrochemical and photophysical properties. In particular, appreciably strong electronic coupling between the photo- and electroactive constituents (i.e., SWNT and ZnP) led to rapid excited-state deactivation of ZnP via charge transfer to the nanotubes. Here, the different absorption cross sections throughout the visible part of the solar spectrum turned out to be valuable in enhancing the overall light-harvesting features. Upon photoexcitation, for both SWNTs-ZnP 1 and 2, radical ion pair states (i.e., reduced SWNT and oxidized ZnP) are formed. The charge-separated states decay to regenerate the singlet ground state with lifetimes of 820 and 200 ps for 1 and 2, respectively.

Synthesis, photophysical and electrochemical characterization of phthalocyanine-based poly(p-phenylenevinylene) oligomers.

The synthesis of two poly(p-phenylenevinylene) oligomers (oPPV) laterally substituted by phthalocyanines (Pc) is described. The preparation of Pc-based oligomers and was accomplished by means of Knoevenagel and Wadsworth-Horner-Emmons reactions, respectively. Diformylphthalocyanine was employed as a monomer in these reactions, thus providing functionalised conjugated oligomers upon reaction with the corresponding co-monomers, and . Photophysical and electrochemical experiments have been carried out with both oligomeric derivatives, revealing excited state interactions such as transduction of singlet excited state energy.

A carbon nano-onion-ferrocene donor-acceptor system: synthesis, characterization and properties.

We describe the synthesis and characterization of a novel ferrocene-carbon onion derivative, where ferrocene acts as an electron-donating moiety, while the carbon nano-onion (CNO) serves as the electron acceptor. CNOs were functionalized by 1,3-dipolar cycloaddition and the resulting products were characterized by transmission electron microscopy, thermogravimetric analysis, Raman and energy dispersive spectroscopies. The electronic properties of the Fc-CNO derivative were investigated by electrochemical and photophysical techniques, complemented by quantum chemical calculations. On average, the CNOs have a spherical appearance with six shells. Functionalization saturates one carbon atom in 36 carbon atoms on the outer cage of the CNO. Through-space interactions between the Fc moiety and the CNO core were detected electrochemically. Fluorescence was observed at low and high energies with an intrinsic decay that is faster at lower energies. Based on theory and experiment, we conclude that, after absorption of a photon at low energy, there is emission from CNOs characterized by larger external shells and a lower degree of functionalization. At high energy, emission comes from CNOs with smaller external shells and a higher degree of functionalization.

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.

Charge transfer reactions along a supramolecular redox gradient.

A rotaxane scaffold is used to align three photo/electroactive units along a supramolecular redox gradient leading to a cascade of through-space charge transfer reactions.

Facile decoration of functionalized single-wall carbon nanotubes with phthalocyanines via "click chemistry".

We describe the functionalization of single-wall carbon nanotubes (SWNTs) with 4-(2-trimethylsilyl)ethynylaniline and the subsequent attachment of a zinc-phthalocyanine (ZnPc) derivative using the reliable Huisgen 1,3-dipolar cycloaddition. The motivation of this study was the preparation of a nanotube-based platform which allows the facile fabrication of more complex functional nanometer-scale structures, such as a SWNT-ZnPc hybrid. The nanotube derivatives described here were fully characterized by a combination of analytical techniques such as Raman, absorption and emission spectroscopy, atomic force and scanning electron microscopy (AFM and SEM), and thermogravimetric analysis (TGA). The SWNT-ZnPc nanoconjugate was also investigated with a series of steady-state and time-resolved spectroscopy experiments, and a photoinduced communication between the two photoactive components (i.e., SWNT and ZnPc) was identified. Such beneficial features lead to monochromatic internal photoconversion efficiencies of 17.3% when the SWNT-ZnPc hybrid material was tested as photoactive material in an ITO photoanode.

Spectroscopic characterization of photolytically generated radical ion pairs in single-wall carbon nanotubes bearing surface-immobilized tetrathiafulvalenes.

We succeeded in establishing for the first time a conclusive spectroscopic signature for reduced single-wall carbon nanotubes (SWNT), which evolves from electron donor-acceptor interactions between SWNT and electron-donating pi-extended tetrathiafulvalene (exTTF). In particular, pi-pi interactions were employed to anchor the electron donor to the surface of SWNT. New conduction band electrons, injected from photoexcited exTTF, shift the transitions that are associated with the van Hove singularities to lower energies.

Dispersion of single-walled carbon nanotubes with an extended diazapentacene derivative.

The dispersion of SWCNTs by using a novel diazapentacene derivative is reported. The proposed pi-pi interactions between the diazapentacene derivative and SWCNTs leave their inherent properties virtually intact, as observed by several photophysical measurements. This approach is very attractive for manipulation of SWCNTs for electronic applications.

An electrochemically driven molecular shuttle controlled and monitored by C60.

Herein we describe a fullerene rotaxane, in which shuttling between two well-defined and distant co-conformations is both induced and monitored by the C60 stopper.

Synthesis, characterization and photophysical properties of a SWNT-phthalocyanine hybrid.

We report the synthesis, characterization and photophysical features of a new nanometer scale carbon nanostructure, that is, a single-wall carbon nanotube bearing phthalocyanine chromophores.

Single-wall carbon nanotubes bearing covalently linked phthalocyanines--photoinduced electron transfer.

HiPco single-walled carbon nanotubes (SWNTs) have been sidewall-functionalized with phthalocyanine addends following two different approaches: a straightforward Prato reaction with N-octylglycine and a formyl-containing phthalocyanine, and a stepwise approach that involves a former Prato cycloaddition to the double bonds of SWNTs using p-formyl benzoic acid followed by esterification of the derivatized nanotubes with an appropriate phthalocyanine molecule. The two materials obtained by these routes comprise different carbon/Pc-addenda ratios, as evidenced by Raman, TGA, and photophysical studies. The occurrence of electron transfer from photoexcited phthalocyanines to the nanotube framework in these ZnPc-SWNT ensembles is observed in transient absorption experiments, which confirm the absorption of the one-electron oxidized ZnPc cation and the concomitant bleaching of the van Hove singularities typical from SWNTs. Charge-separation (i.e., 2.0 x 1010 s(-1)) and charge-recombination (i.e., 1.5 x 106 s(-1)) dynamics reveal a notable stabilization of the radical ion pair product in dimethylformamide.

Synthesis, characterization, and photoinduced electron transfer in functionalized single wall carbon nanohorns.

Single-wall carbon nanohorns (SWNHs) are a new class of material that is closely related to single-wall carbon nanotubes. Here, we describe the synthesis and characterization of a series of SWNHs functionalized with ethylene glycol chains and porphyrins. Functionalization of carbon nanohorns has been achieved using two different synthetic protocols: (1) direct attack of a free amino group on the nanohorn sidewalls (nucleophilic addition) and (2) amidation reaction of the carboxylic functions in oxidized nanohorns. The nanohorn derivatives have been characterized by a combination of several techniques, and the electronic properties of the porphyrin/nanohorn assemblies (SWNH/H2P) have been investigated by electrochemistry, spectroelectrochemistry, and a series of steady-state and time-resolved spectroscopy. The cyclic voltammetry curve of nanohorn/porphyrin conjugate 6 showed a continuum of faradic and pseudocapacitive behavior, which is associated with multiple-electron transfers to and from the SWNHs. Superimposed on such a pseudocapacitive current, the curve also displays three discrete reduction peaks at -2.26, -2.57, and -2.84 V and an oxidation peak at 1.12 V (all attributed to the porphyrin moiety). Steady-state and time-resolved fluorescence demonstrated a quenching of the fluorescence of the porphyrin in SWNH/H2P conjugates 5 and 6 compared to the reference free base porphyrin. Transient absorption spectra permitted the electron-transfer process between the porphyrins and the carbon nanostructures to be highlighted.

Photophysical and electrochemical properties of a fullerene-stoppered rotaxane.

The photophysical and electrochemical properties of a fumaramide rotaxane stoppered with C(60) are reported. The results evidenced the strong binding interactions between the template and the macrocycle, which are also supported by molecular modelling.