Dispersing carbon nanotubes by star-like water soluble C60 derivatives.

The properties of three water soluble [3:3] hexakisadducts of C60 1-3 are investigated both in bulk aqueous solutions and at air/water interface. 1-3 have a star-like molecular structure with six hydrophilic quarternary ammonium groups located terminally. 2 and 3 also have increasing number of ethylene oxide groups inserted into their peripheral hydrophilic arms. This molecular structure makes 1-3 highly water soluble, thus difficult to form aggregates in bulk aqueous solutions as revealed by transmission electron microscope observations. But surface tension measurements shows 1-3 still own some amphiphilic properties and can lower the surface tension of water. These unique C60 derivatives are also subjected to disperse carbon nanotubes (CNTs) in water for the first time. It was found the ability of each water soluble C60 derivative to disperse CNTs was mainly dominated by their amphiphilic feature as well as molecular weight.

Cooperativity and tunable excited state deactivation: modular self-assembly of depsipeptide dendrons on a Hamilton receptor modified porphyrin platform.

A series of novel supramolecular architectures were built around a tin tetraphenyl porphyrin platform 6--functionalized by a 2-fold 1-ethyl-3-3-(3-dimethylaminopropyl)carbodiimide (EDC) promoted condensation reaction--and chiral depsipeptide dendrons of different generations 1-4. Here, implementation of a Hamilton receptor provided the necessary means to keep the constituents together via strong hydrogen bonding. Characterization of all architectures has been performed, including 4 which is the fourth generation, on the basis of NMR and photophysical methods. In particular, several titration experiments were conducted suggesting positive cooperativity, an assessment that is based on association constants that tend to be higher for the second binding step than for the first step. Importantly, molecular modeling calculations reveal a significant deaggregation of the intermolecular network of 6 during the course of the first binding step. As a consequence, an improved accessibility of the second Hamilton receptor unit in 6 emerges and, in turn, facilitates the higher association constants. The features of the equilibrium, that is, the dynamic exchange of depsipeptide dendrons 1-4 with fullerene 5, was tested in photophysical reference experiments. These steady-state and time-resolved measurements showed the tunable excited-state deactivations of these complexes upon photoexcitation.

Testing electron transfer within molecular associates built around anionic C60 and C70 dendrofullerenes and a cationic zinc porphyrin.

Mono- and bis-functionalized C(60) and C(70) fullerene derivatives (DF, 1-10) that carry one or two oligoanionic dendritic termini in their malonate addends and an oligocationic octapyridinium zinc porphyrin salt (ZnP) were found to self-assemble in buffered aqueous solution to yield a novel series of 1:1 and/or 1:2 electron transfer hybrid associates. Remarkably high association constants-typically on the order of 10(8) M(-1)-were derived that corroborate stable complex formations. A combination of electrostatic and charge-transfer interactions that are operative between the electron-accepting DF and the electron-donating ZnP is considered to contribute to the uniquely high complex stability. First insight into intracomplex excited state interactions came from steady-state and time-resolved fluorescence quenching experiments that were performed with the molecular ZnP/DF hybrid associates. Excited state quenching processes are, for example, evident in form of a bi-exponential fluorescence decay of ZnP-corresponding to a distribution of associated and non-associated ZnP. Unambiguous evidence for an intracomplex electron transfer quenching, namely, formation of ZnP(.+)/C(60) (.-) and ZnP(.+)/C(70) (.-) radical ion pairs, was gathered in time-resolved transient absorption measurements. Lifetimes of these radical ion-pairs range from nanoseconds to a few microseconds.

Implementation of a Hamiliton-receptor-based hydrogen-bonding motif toward a new electron donor-acceptor prototype: electron versus energy transfer.

A new modular concept for the self-assembly of electron donor-acceptor complexes is presented that ensures (i) fine-tuning the strength of the complexation, (ii) controlling the electronic coupling to impact electron and energy transfer processes, and (iii) high solubility of the corresponding hybrid architectures. This task has been realized through developing a series of porphyrin-fullerene donor-acceptor systems held together by a Hamilton-receptor-based hydrogen-bonding motif. In this context, novel libraries of C60 monoadducts (1) containing cyanuric acid side chains and of tetraphenylporphyrin derivatives (2) involving the complementary Hamilton-receptor unit were synthesized. The association constants of the corresponding 1:1 complexes (1.2) connected by six hydrogen bonds were determined complementary by NMR and fluorescence assays. Their strength, which was found to be in the range between 3.7 x 10(3) and 7.9 x 10(5) M-1, depends on the nature of the spacers, namely, hexylene versus propylene chains. Finally, transient absorption studies revealed photoinduced electron transfer from ZnP to C60 in the corresponding 1.2 complexes, which generate radical ion pair states that are persistent well beyond the ns time scale. In the case of the analogous SnP complexes, energy instead of electron transfer was observed. This is due to the shift of oxidation potential caused by presence of Sn in the oxidation state of +4.

Water solubility, antioxidant activity and cytochrome C binding of four families of exohedral adducts of C60 and C70.

Over the past decade, surface-modified, water soluble fullerenes have been shown by many different investigators to exhibit strong antioxidant activity against reactive oxygen species (ROS) in vitro and to protect cells and tissues from oxidative injury and cell death in vivo. Nevertheless, progress in developing fullerenes as bona fide drug candidates has been hampered by three development issues: 1) lack of methods for scalable synthesis; 2) inability to produce highly purified, single-species regioisomers compatible with pharmaceutical applications; and 3) inadequate understanding of structure-function relationships with respect to various surface modifications (e.g., anionic versus cationic versus charge-neutral polarity). To address these challenges, we have designed and synthesized more than a dozen novel water soluble fullerenes that can be purified as single isomers and which we believe can be manufactured to scale at reasonable cost. These compounds differ in addition pattern, lipophilicity and number and type of charge and were examined for their water solubility, antioxidant activity against superoxide anions and binding of cytochrome C. Our results indicate that dendritic water soluble fullerene[60] monoadducts exhibit the highest degree of antioxidant activity against superoxide anions in vitro as compared with trismalonate-derived anionic fullerenes as well as cationic fullerenes of similar overall structure. Among the higher adducts, anionic derivatives have a higher antioxidant activity than comparable cationic compounds. To achieve sufficient water solubility without the aid of a surfactant or co-solvent at least three charges on the addends are required. Significantly, anionic in contrast to cationic fullerene adducts bind with high affinity to cytochrome C.

A chiral dumbbell shaped bis(fullerene) oligoelectrolyte.

The first icosacationic bis(fullerene) whose water solubility is provided by twenty peripheral ammonium groups has a rigid, almost cylindrical shape and was synthesized by connecting two fullerene cores with a chiral cyclo-bis(malonate), followed by the regioselective addition of ten amino-terminated malonates into the octahedral positions of the fullerene moieties and subsequent cleavage of the Boc-protecting groups.

Electrophoresis of electrostatically assembled fullerene-porphyrin conjugates.

The formation of electrostatically coupled assemblies of a series of anionic dendritic fullerene derivatives and cationic porphyrins in buffered aqueous media was studied with gel electrophoresis. Of central interest in these investigations was the variation of the amount of charge carried by the molecules, their size, shape and self-aggregation. Ferric cytochrome c 1 and the more rigid zinc porphyrin served as octacationic species. The two new anionic fullerene derivates 3 and 6 were synthesized. The formation of electrostatic complexes of the fullerene polyelectrolytes 3 and 4 with 1and 2 was clearly evident in the gel electrophoresis experiment. Compounds and showed a similar behaviour towards 2. The electrophoresis experiments confirmed previous results obtained with other techniques on a qualitative level and gave new insights into aggregation phenomena.

Functional single-wall carbon nanotube nanohybrids--associating SWNTs with water-soluble enzyme model systems.

We succeeded in integrating single-wall carbon nanotubes (SWNTs), several water-soluble pyrene derivatives (pyrene(-)), which bear negatively charged ionic headgroups, and a series of water-soluble metalloporphyrins (MP(8+)) into functional nanohybrids through a combination of associative van der Waals and electrostatic interactions. The resulting SWNT/pyrene(-) and SWNT/pyrene(-)/MP(8+) were characterized by spectroscopic and microscopic means and were found to form stable nanohybrid structures in aqueous media. A crucial feature of our SWNT/pyrene(-) and SWNT/pyrene(-)/MP(8)(+) is that an efficient exfoliation of the initial bundles brings about isolated nanohybrid structures. When the nanohybrid systems are photoexcited with visible light, a rapid intrahybrid charge separation causes the reduction of the electron-accepting SWNT and, simultaneously, the oxidation of the electron-donating MP(8)(+). Transient absorption measurements confirm that the radical ion pairs are long-lived, with lifetimes in the microsecond range. Particularly beneficial are charge recombination dynamics that are located deep in the Marcus-inverted region. We include, for the first time, work devoted to exploring and testing FeP(8)(+) and CoP(8)(+) in donor-acceptor nanohybrids.