A series of novel functionalised dumbbell-shaped bifullerenes in which two [5.0] pentakis-adducts of C₆₀ are covalently connected by cyclic bismalonates were synthesised. These dimeric compounds, carrying various combinations of hydrophilic and hydrophobic addends, self-assemble in aqueous solution towards supramolecular architectures of different structural complexity as observed by cryogenic transmission electron microscopy (cryo-TEM). The detailed analysis of the image data revealed an unprecedented hierarchical aggregation behaviour. Whereas completely hydrophilic substituted bifullerenes formed profoundly monodisperse populations of small oligomeric elementary micelles consisting of only three or four bifullerene molecules in a supposedly bent conformation, their amphiphilic equivalents underwent a hierarchical two-step assembly process towards larger spherical and even rod-like structures. The data suggest that the hierarchical assembly process is driven by hydrophobic interactions of preformed tetrameric elementary micelles.

We report on the very facile access of a new family of amphiphilic and bola-amphiphilic fullerene [5:1]hexakisadducts 9 a–f and 11 a–e. The key point for this successful approach is the use of C_2v-symmetrical fullerene pentakisadduct precursors 2 b–f allowing for the completely regioselective addition of a sixth malonate addend to complete the octahedral [5:1] addition pattern. For the synthesis of the new amphiphiles we first developed a new second-generation dendrimer containing 9 tert-butoxycarbonyl (Boc)-protected amino functions at the periphery and two new malonates containing 6 or 18 Boc-protected amino termini, respectively. The hexakisadducts contain up to 18 positive or negative charges at the dendritic moiety and either no or ten positive or negative charges at the unbranched malonate positions after deprotection with trifluoroacetic acid (TFA). The charge state at the termini is pH-dependent. Complete structural characterization of the new compounds was carried out by ESI mass spectrometry and by UV/Vis, FTIR, ¹H NMR and ¹³C NMR spectroscopy. We were also able to obtain the first X-ray crystal structure of pentakisadduct (2 a) with a C_2v-symmetrical addition pattern. The new amphiphilic hexakisadducts show interesting solubility properties in water. Initial investigations on the aggregation properties of the amphiphilic hexakisadduct 12 c by using dynamic light scattering (DLS) and conductivity measurements, show aggregates with a radius up to 200 nm and a critical micelle concentration (CMC) of approximately 8 mg L⁻¹.

We report here on the facile synthetic access of a new family of bis-, tetra-, hexa-, and heptafullerenes (prototypes I–IV), which can be easily converted into very water soluble polyelectrolytes with up to 60 charges located on their periphery. Their very regioselective formation is based on the use of C_2v-symmetrical pentakisadducts 3 and hexakisadducts 2 as key intermediates. All fullerene moieties incorporated in these macromolecular structures involve a complete or partial octahedral addition pattern. Tripod-shaped tetrafullerenes 9 a,b (type II), which can accumulate up to thirty positive or negative charges, are very soluble in acidic or basic water, respectively. Hexafullerenes 13 a,b (type III) were synthesized via isoxazolinofullerenes 10 followed by photolytic cleavage of the isoxazoline group. The giant heptafullerene 1 b (type IV) representing the anionic counterpart of the previously synthesized polyelectrolyte 1 a can store up to 60 negative charges on its periphery within a defined three-dimensional structure. We also discovered a new cyclopropanation reaction of C₆₀ involving dibromomalonates and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). This reaction allows even for the highly regioselective formation of hexakisadducts with an octahedral addition pattern without requiring activation with reversibly binding addends such as 9,10-dimethylanthracene (DMA).

The synthesis and structural characterization of a first family of dendronized polycationic perylenetetracarboxylic acid diimides (PDIs) 7, 8, and 10 is reported. They were obtained by amide coupling reactions of pyridinium salt head groups with terminal carboxylic groups of Newkome-dendron-type functionalized PDIs. Resulting pyridinium-terminated PDIs 7, 8, and 10 are highly water-soluble, independent of the pH value. This is due to 6, 18, and 9 permanent positive charges, respectively. PDIs 7 containing the smaller first-generation dendrons exhibit a pronounced aggregation behavior in water. This was studied by absorption and fluorescence spectroscopy, by pulsed-gradient spin-echo (PGSE, DOSY) NMR measurements, and by cryo-transmission electron microscopy (cryo-TEM). The counterparts containing the bulkier second-generation dendrons hamper aggregation, and as a consequence intermolecular π–π stacking interactions between the perylene cores is suppressed.

A new family of dipolar ionic dendrimers that consist of a cationic pyridinium core surrounded by a negatively charged shell of carboxylic acids was successfully synthesized and characterized. Both spherical architectures and examples of amphiphiles with a dipolar head group were synthesized. Studies of the aggregation behaviour in aqueous solution by determining the cmc values were performed. This new type of dipolar core–shell dendrons can be considered as molecular architectures that contain a positively charged nucleus covalently incorporated within a negatively charged shell.

We report here on the selective synthesis of fullerene pentakisadducts 3 with an incomplete octahedral addition pattern by means of mixed [5:1]hexakisadducts 1 that involve an isoxazoline moiety as a protection group. The isoxazoline addend can be cleanly cleaved by irradiation with light. By using this protection–deprotection strategy, a variety of fullerene pentakisadducts 3 were synthesized in 29–44 % overall yield without the need of HPLC purification. This novel photolytic deprotection of 1 can be explained by an initial electron transfer that leads to a biradical, which can easily eliminate the isoxazoline added. The very efficient and straightforward syntheses of the bisfullerene 4 and the globular hexakisadduct 7, each of which involves mixed octahedral addition patterns, clearly demonstrate the advantage of fullerene pentakisadducts 3 as suitable precursors for the construction of highly functional and complex [5:1]hexakisadduct architectures. Complete structural characterization of all new compounds was carried out by MALDI mass spectrometry, UV/Vis, FTIR, ¹H NMR and ¹³C NMR spectroscopy, as well as X-ray diffraction.

A series of truly water-soluble C₆₀/porphyrin electron donor–acceptor conjugates has been synthesized to serve as powerful mimics of photosynthetic reaction centers. To this end, the overall water-solubility of the conjugates was achieved by adding hydrophilic dendrimers of different generations to the porphyrin moiety. An important variable is the metal center of the porphyrin; we examined zinc(II), copper(II), cobalt(II), nickel(II), iron(III), and manganese(III). The first insights into electronic communication between the electron donors and the electron acceptors came from electrochemical assays, which clearly indicate that the redox processes centered either on C₆₀ or the porphyrins are mutually affected. Absorption measurements, however, revealed that the electronic communication in terms of, for example, charge-transfer features, remains spectroscopically invisible. The polar environment that water provides is likely to be a cause of the lack of detection. Despite this, transient absorption measurements confirm that intramolecular charge separation processes in the excited state lead to rapid deactivation of the excited states and, in turn, afford the formation of radical ion pair states in all of the investigated cases. Most importantly, the lifetimes of the radical ion pairs were found to depend strongly on several aspects. The nature of the coordinated metal center and the type of dendrimer have a profound impact on the lifetime. It has been revealed that the nature/electronic configuration of the metal centers is decisive in powering a charge recombination that either reinstates the ground state or any given multiplet excited state. Conversely, the equilibrium of two opposing forces in the dendrimers, that is, the interactions between their hydrophilic regions and the solvent and the electronic communication between their hydrophobic regions and the porphyrin and/or fullerene, is the key to tuning the lifetimes.

The physicochemical characterization, that is, ground and excited state, of a new series of dendronized porphyrin/fullerene electron donor–acceptor conjugates in nonaqueous and aqueous environments is reported. In contrast to previous work, we detail the charge-separation and charge-recombination dynamics in zinc and copper metalloporphyrins as a function of first- and second-generation dendrons as well as a function of ortho, meta, and para substitution. Both have an appreciable impact on the microenvironments of the redox-active constituents, namely the porphyrins and the fullerenes. As a matter of fact, the resulting charge-transfer dynamics were considerably impacted by the interplay between the associated forces that reach from dendron-induced shielding to dipole–charge interactions.

Dedicated to Professor François Diederich on the occasion of his 60th birthday

Photophysical investigations on a series of (2,4,6)-tris-substituted metalloporphyrin-fullerene conjugates revealed the effects of an electron-rich microenvironment surrounding the electron-donating porphyrin as a function of the metal center. On one hand, for all conjugates—water-soluble and non-water-soluble—ultrafast charge separation was observed upon photoexcitation. On the other hand, when examining the charge recombination dynamics for the non-water-soluble conjugates it becomes obvious that the (2,4,6)-tris-substitution stabilizes the radical-ion-pair state relative to the mono-substitution in the ortho-, meta-, and para-position. The more efficient protection of the electron-donating porphyrin from solvation is thought to be the major cause for this impact. Nevertheless, the situation is slightly different for the water-soluble conjugates. At first glance, the radical-ion-pair state lifetimes are, also in the case of the (2,4,6)-tris-substitution, longer than for the mono-substituted ortho-, meta- and para-conjugates. Upon closer inspection, they fail, however, to exhibit any metal dependence. Competing with the protection from solvation of the dendrons, dipole-charge interactions impact the stabilization in the polar aqueous environment and, in turn, become the dominant force governing the electron-transfer dynamics.

Carbon nanotubes and graphene are outstanding materials of the 21^st century with a broad spectrum of applications. However, major challenges are faced such as the intrinsically low solubility of both sp² carbon allotropes. To overcome this hurdle the potential of noncovalent functionalization is summarized with a special focus on the establishment of the perylene bisimide unit as aromatic anchor to the graphitic surface. Rational surfactant design is unmasked as the key to solubilization of the carbon allotropes, while at the same time tailoring their surface properties, or even electronic properties in a fully reversible fashion.

We report here on the two-step synthesis of fullerene dimer 3 from monomer 2. The structures were confirmed by MALDI mass spectrometry, UV/Vis, FTIR, ¹H NMR, and ¹³C NMR spectroscopy. The magnetic properties of the analogous species of dimer 3, N@C₁₄₀H₂₄O₁₂, were studied by continuous-wave electron-spin resonance (CW-EPR), indicating that the functionalization introduces a permanent zero-field splitting (ZFS) D = 8.6 MHz and E = 0.45 MHz. The same synthetic procedure could be used to produce a ¹⁴N–¹⁵N dimer enabling spin resonance experiments that could demonstrate entanglement between electron spins.

A new series of dendronized metalloporphyrin–fullerene conjugates as photosynthetic reaction center mimics was developed in a highly regioselective fashion through tether-controlled synthesis. The microenvironment around the porphyrin core is dependent on the spatial substitution pattern and the nature and generation number of the dendrons, which was proven by cyclic voltammetry.

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.

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.

A modular construction kit with two orthogonal noncovalent binding sites for self-assembly of supramolecular architectures is presented. The heteroditopic building blocks contain a terpyridine (tpy) unit for coordination of metal ions and a Hamilton receptor for multiple H-bonding of cyanuric acid derivatives. The association constants of ligand binding of M^II complexes (M=Ru, Zn, Fe, and Pt) with a dendritic end cap were determined to be in the range of 10² and 10⁴ L mol⁻¹ in chloroform. The capabilities for binding of metal ions were investigated by ¹H NMR and UV/Vis spectroscopy. The Fe complexes are most appropriate for the generation of discrete and high-ordered architectures due to their strong tendency to form FeL₂ complexes. Superstructures are readily formed in a one-pot procedure at room temperature. No mutual interactions between the orthogonal binding motifs were observed, and this demonstrates the highly specific nature of each binding process. Decomplexation experiments were carried out to examine the reversibility of Fe–tpy coordination. Substitution of the terminal end cap with a homoditopic bis-cyanurate linkage leads to formation of an iron-containing supramolecular strand. Formation of coordination polymers was confirmed by viscosity measurements. The supramolecular polymer strands can be reversibly cleaved by addition of a terminating cyanuric acid building block, and this proves the dynamic nature of this noncovalent polymerization process.

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.

Two novel supramolecular building blocks, namely cis- and trans-symmetric zinc–porphyrins bearing Hamilton receptors as their focal points (10 and 15), were self-assembled with (i) first-, second- and third-generation depsipeptide cyanurates (21–23); (ii) second-generation dendrofullerene (24), and (iii) cyanuric acid substituted fullerene (20), and probed in a series of photophysical investigations. Importantly, an overall 1:2 stoichiometry was confirmed with binding constants K_n, and Hill coefficients n_H that indicate remarkable cooperativities. As a general feature, stronger binding evolves from the trans-symmetric porphyrin isomer, although the increased steric demand of the depsipeptide ligandsmeans that the binding constants decrease with each generation. The self-assembly of 10 or 15 with 20 affords novel electron donor–acceptor hybrids that reveal, upon excitation,intra-hybrid charge-transfer events.

Not only the self-aggregation of dendritic polycarboxylates into structurally persistent micelles, but also that of the micelles themselves into superlattices is controlled by alkali-metal counterions and shows a pronounced sodium effect. Our combined experimental and computational work has revealed the formation of superlattices for the first time. The behavior of a variety of amphiphilic carboxylates and the different effects of the alkali cations Li⁺, Na⁺, and K⁺ have been investigated by conductivity measurements, cryogenic transmission electron microscopy (cryo-TEM), and molecular-dynamics (MD) simulations. Together, these show that sodium salts of the amphiphiles give the most stable micelles, followed by lithium and potassium. Our results suggest that ion multiplets in bridging positions, rather than contact ion pairs, are responsible for the enhanced stability and the formation of hexagonally ordered superlattices with sodium counterions. Potassium ions do not form such ion multiplets and cannot therefore induce aggregation of the micelles. This sodium effect has far-reaching consequences for a large number of biological and technical systems and sheds new light on the origin of specific-ion effects.

We have realized for the first time a series of truly water-soluble and tightly coupled porphyrin/C₆₀ electron-donor–acceptor conjugates in which the charge separation and charge recombination dynamics are controlled by modifying the nature of the dendrimer and/or the choice of the central metal atom.

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.

A novel dendritic C₆₀-H₂P-(ZnP)₃ (P=porphyrin) conjugate gives rise to the successful mimicry of the primary events in photosynthesis, that is, light harvesting, unidirectional energy transfer, charge transfer, and charge-shift reactions. Owing, however, to the flexibility of the linkers that connect the C₆₀, H₂P, and ZnP units, the outcome depends strongly on the rigidity/viscosity of the environment. In an agar matrix or Triton X-100, time-resolved transient absorption spectroscopic analysis and fluorescence-lifetime measurements confirm the following sequence. Initially, light harvesting is seen by the peripheral C₆₀-H₂P- *(ZnP)₃ conjugate. Once photoexcited, a unidirectional energy transfer funnels the singlet excited-state energy to H₂P to form C₆₀-*(H₂P)-(ZnP)₃, which powers an intramolecular charge transfer that oxidizes the photoexcited H₂P and reduces the adjacent C₆₀ species. In the correspondingly formed (C₆₀)^.−-(H₂P)^.+-(ZnP)₃ conjugate, an intramolecular charge-shift reaction generates (C₆₀)^.−-H₂P-(ZnP)₃^.+, in which the radical cation resides on one of the three ZnP moieties, and for which lifetimes of up to 460 ns are found. On the other hand, investigations in organic media (i.e., toluene, THF, and benzonitrile) reveal a short cut, that is, the peripheral ZnP unit reacts directly with C₆₀ to form (C₆₀)^.−-H₂P-(ZnP)₃^.+. Substantial configurational rearrangements— placing ZnP and C₆₀ in proximity to each other—are, however, necessary to ensure the required through space interactions (i.e., close approach). Consequently, the lifetime of (C₆₀)^.−-H₂P-(ZnP)₃^.+ is as short as 100 ps in benzonitrile.

New [3:3]hexakisadducts 3, 6, and 7 have been synthesized by a stepwise addition of two tripodal malonate tethers to pristine [60]fullerene. For the first time, [3:3]hexakisadducts of two sets of asymmetrically substituted malonate addends could be prepared as single in/out isomers by following this synthetic protocol. Thereby, four spherically defined addend zones I–IV were created with various similar degrees of latitude on the fullerene sphere. Four amphiphilic [3:3]hexakisadducts 12, 14, 18, and 19 were prepared with different relative arrangements of both the hydrophilic and lipophilic moieties in the addend zones I/II and III/IV, respectively.

e,e,e-Trisadducts 13 and 15 have been prepared by a highly regioselective threefold cyclopropanation of tripodal malonates 10 and 12 with C₆₀. The yield and regioselectivity depend on the length and structure of the tethers that connect the malonate units to the focal benzene core of 13–15. As a consequence of the template-directed synthesis, all e,e,e-trisadducts were formed as in/out isomers exclusively and contain two spherically well-defined addend zones with equatorial and polar orientation, respectively. By variation of the outer malonate termini of the tethers, selective functionalization of the equatorial addend zone could be achieved, thus leading to fine-tuning of intermolecular interactions, such as solubility or aggregation phenomena. After removal of the focal benzene moiety in 14 and 15, selective functionalization of the polar addend zone could be achieved. Strong intramolecular hydrogen-bonding networks of the polar substituents in the polar addend zone could be observed by ¹H NMR spectroscopic analysis. By orthogonal functionalization of both addend zones, fullerene derivatives 44–48 could be synthesized as one single in/out isomer, thus greatly enhancing the potential of e,e,e-trisadducts as building blocks in supramolecular architectures.

We report here on the synthesis of three new prototypes (types I–III) of very large fullerene-based polyelectrolytes which can carry up to 60 charges on their periphery. All fullerene moieties incorporated in these macromolecular structures have an octahedral hexakisaddition pattern. Dumbbell-shaped icosacarboxylate 5 (type I), which can accumulate up to twenty negative charges, is very soluble in methanol as well as in neutral and basic water. On the other hand, Janus dumbbell 13 (type II) contains both positively and negatively chargeable fullerene building blocks and is very soluble in acidic and basic media. However, in the region of the isoelectric point at pH 6.0–6.5 it precipitates as a pale orange solid due to pronounced intermolecular Coulomb interactions. Giant heptafullerene 15 (type III) can store up to 60 positive charges in its periphery and is the largest molecular polyelectrolyte with defined three-dimensional structure.

A new approach towards the synthesis of the elusive heterofullerene C₅₈N₂ is presented. The synthetic strategy is based on the intramolecular 1,3-dipolar cycloaddition of a spacer-linked azide to a monoazaheterofullerene (C₅₉N) core. Since this cycloaddition can theoretically result in 16 different isomeric products, the design of the spacer moiety was based on extensive molecular modelling. Two fundamental synthetic routes towards suitably functionalized C₅₉N derivatives were designed and carried out. Subsequent one-pot experiments, each making use of the same conditions as had previously been employed for the introduction of the first nitrogen atom into the fullerene framework, were performed. Immediate analysis by FAB mass spectrometry indicated that up to 20 % of the submitted material indeed gave C₅₈N₂ species as fragmentation products. This result not only represents the first strategic synthesis of a C₅₈N₂ derivative, but also holds great promise for further efforts directed towards the preparative isolation of a member of the diaza[60]fullerene family. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The cover picture shows the synthetic pathway for the substitution of two carbon atoms of the fullerene C₆₀ framework with two nitrogen atoms, bringing the new heterofullerene football C₅₈N₂ into play. The easily accessible (C₅₉N)₂ is used as a starting material for a tether-based functionalization sequence leading to a direct C₅₈N₂ precursor. The chemical transformation of this precursor molecule under the same conditions used for the introduction of one nitrogen atom leads to a reaction product that, according to FAB mass spectroscopic analysis, contains up to 20 % of a C₅₈N₂ species. This represents the first strategic chemical synthesis of the C₅₈N₂ cage. Details are discussed in the article by A. Hirsch et al. on p. 4109 ff. The authors acknowledge the support of this work by Dipl. Chem. Benjamin Gebhardt, who designed the cover page.