Y.F.Wang

Chiral fullerene–metal hybrids with complete control over the four stereogenic centers, including the absolute configuration of the metal atom, have been synthesized for the first time. The stereochemistry of the four chiral centers formed during [60]fullerene functionalization is the result of both the chiral catalysts employed and the diastereoselective addition of the metal complexes used (iridium, rhodium, or ruthenium). DFT calculations underpin the observed configurational stability at the metal center, which does not undergo an epimerization process.

Chirality at the metal: Fullerene hybrids with iridium-, rhodium-, or ruthenium-centered chirality were prepared by precise control of four newly formed stereogenic centers. The resulting complexes are configurationally stable and do not undergo an epimerization process.

Selective addition to the C₇₀ cage divides its π-conjugated system into various smaller π-conjugated systems with enhanced fluorescent properties. Key reactions include chlorination, methoxylation, ozonation, and Bingel or Bingel–Hirsch reactions. The maximum emission wavelength of the C₇₀ multiadducts ranges from 450 to 655 nm. Among the C₇₀ multiadducts, C₇₀(OMe)₈(C(COOEt)₂)₃ showed the highest quantum yield (Φ_F=0.18) and largest Stokes shift (402 nm), with maximum emission at 655 nm.

Amazing Technicolor Dream-cage: Selective addition to the C₇₀ cage divides its π-conjugated system into various smaller π-conjugated systems with enhanced fluorescence properties. Key reactions include chlorination, methoxylation, ozonation, and Bingel or Bingel–Hirsch reactions. C₇₀(OMe)₈(C(COOEt)₂)₃ (10) showed a quantum yield of Φ_F=0.18 and a Stokes shift of 402 nm, with maximum emission at 655 nm.

Methanol (CH₃OH) and formaldehyde (H₂CO) molecules were inserted into an open-cage C₆₀ derivative with a large opening, under high-pressure and high-temperature conditions in solution. Isolation of their molecular complexes in pure form was achieved by the use of recycling HPLC with Buckyprep columns. ¹H NMR spectroscopy, single-crystal X-ray diffraction studies, and DFT calculations revealed the orientation of the encapsulated CH₃OH and H₂CO, both in solution and in the solid state, and the results show that the CH₃ group of the CH₃OH and the carbonyl group of the H₂CO point to the bottom of the cages. Furthermore, the dynamic behavior of the CH₃OH and H₂CO were studied at the molecular level.

One and only: Molecular complexes encapsulating CH₃OH and H₂CO inside open-cage C₆₀ derivatives were synthesized. The orientations of the encapsulated species in solution and in the solid state were revealed by NMR analysis, single crystal X-ray analysis, and DFT calculations. Each species shows only one orientation: the CH₃ group of the CH₃OH sits on the curve of the carbon cage, whereas the CH₂ group of the H₂CO is directed toward the opening.

Stereoselective electrosynthesis of the first individual (^f,tA)- and (^f,tC)-1,4-fullerene derivatives with a non-inherently chiral functionalization pattern is described, as well as the first example of an optically pure protected primary amino acid directly linked to the fullerene through only the chiral α-amino-acid carbon atom. An application of an auxiliary chiral nickel-Schiff base moiety as derivatizing agent allowed separation of (^f,tA)- and (^f,tC)-1,4-fullerene derivatives using an achiral stationary phase, a separation which has never been done before.

Separation of the unseparable: The first separation of (^f,tA)- and (^f,tC)-1,4-fullerene derivatives using an achiral stationary phase is reported. The compounds are the first example of an optically pure, protected primary amino acid directly linked to fullerene exclusively through the chiral α-amino-acid carbon atom.

A multichromophoric perylenediimide–silicon phthalocyanine–C60 system has been synthetized. Successful occurrence of sequential energy transfer followed by electron transfer in a newly synthesized covalently linked triad composed of perylenediimide, silicon phthalocyanine and C60 as building blocks, in both polar and nonpolar solvents, has been demonstrated. The cover represents the beauty of the molecule as a mermaid where the C60 is the head, the body the phthalocyanine and the perylenediimide the tail. More information can be found in the Full Paper by F. D′Souza, A. Sastre-Santos et al. on page ▪▪ ff. (DOI: 10.1002/chem.201603741).

Organic–inorganic halide perovskite solar cells have rapidly come to prominence in the photovoltaic field. In this context, CH₃NH₃PbI₃, as the most widely adopted active layer, has been attracting great attention. Generally, in a CH₃NH₃PbI₃ layer, unreacted PbI₂ inevitably coexists with the perovskite crystals, especially following a two-step fabrication process. There appears to be a consensus that an appropriate amount of unreacted PbI₂ is beneficial to the overall photovoltaic performance of a device, the only disadvantageous aspect of excess residual PbI₂ being viewed as its insulating nature. However, the further development of such perovskite-based devices requires a deeper understanding of the role of residual PbI₂. In this work, PbI₂-enriched and PbI₂-controlled perovskite films, as two extreme cases, have been prepared by modulating the crystallinity of a pre-deposited PbI₂ film. The effects of excess residual PbI₂ have been elucidated on the basis of spectroscopic and optoelectronic studies. The initial charge separation, the trap-state density, and the trap-state distribution have all been found to be adversely affected in PbI₂-enriched devices, to the detriment of photovoltaic performance. This leads to a biphasic recombination process and accelerates the charge carrier recombination dynamics.

Too much of a good thing? Excess residual PbI₂ in perovskite solar cells has been found to affect their charge-separation/trap-state properties (see figure), which impairs their photovoltaic performance.

Additions of adamantylidene (Ad) to M₃N@I_h-C₈₀ (M=Sc, Lu) and Sc₃N@D_5h-C₈₀ have been accomplished by photochemical reactions with 2-adamantyl-2,3′-[3H]-diazirine (1). In M₃N@I_h-C₈₀, the addition led to rupture of the [6,6]- or [5,6]-bonds of the I_h-C₈₀ cage, forming the [6,6]-open fulleroid as the major isomer and the [5,6]-open fulleroid as the minor isomer. In Sc₃N@D_5h-C₈₀, the addition also proceeded regioselectively to yield three major isomeric Ad mono-adducts, despite the fact that there are nine types of C−C bonds in the D_5h-C₈₀ cage. The molecular structures of the seven Ad mono-adducts, including the positions of the encaged trimetallic nitride clusters, have been unambiguously determined through single-crystal XRD analyses. Furthermore, results have shown that stepwise addition of Ad to Lu₃N@I_h-C₈₀ affords several Ad bis-adducts, two of which have been isolated and characterized. The X-ray structure of one bis-adduct clearly revealed that the second Ad addition took place at a [6,6]-bond close to an endohedral metal atom. Theoretical calculations have also been performed to rationalize the regioselectivity.

Metallofullerene derivatization: Mono- and bis-adamantylidene adducts of metallofullerenes M₃N@I_h-C₈₀ (M=Sc, Lu) and Sc₃N@D_5h-C₈₀ have been prepared by photochemical reactions with 2-adamantyl-2,3′-[3H]-diazirine (see graphic). The addition sites have been determined by single-crystal X-ray diffraction analysis.

An alternative approach to replacing the isothiocyantate ligands of the N3 photosensitizer with light-harvesting bidentate ligands is investigated for application in dye-sensitized solar cells (DSSCs). An in-depth theoretical analysis has been applied to investigate the optical and redox properties of four non-innocent ligand platforms, which is then corroborated with experiment. Taking advantage of the 5- and 7-positions of 8-oxyquinolate, or the carboxyaryl ring system of the N-arylcarboxy-8-amidoquinolate ligand, fluorinated aryl substituents are demonstrated as an effective means of tuning complex redox potentials and light-harvesting properties. The non-innocent character, resulting from mixing of both the central metal-dπ and ligand-π manifolds, generates hybrid metal–ligand frontier orbitals. These play a major role by contributing to the redox properties and visible electronic transitions, and promoting an improved power conversion efficiency in a Ru DSSC device featuring non-innocent ligands.

Oxyquinolate and carboxyamidoquinolate ligands were investigated experimentally and theoretically for their influence on the optical and redox properties of ruthenium dyes for application in dye-sensitized solar cells. Fluorination of these non-innocent ligands has a positive influence on the frontier orbital mixing with the central ruthenium metal, thereby contributing to (metal-ligand)-to-ligand charge-transfer electronic transitions (see figure).

Fused-pentagons results in an increase of local steric strain according to the isolated pentagon rule (IPR), and for all reported non-IPR clusterfullerenes multiple (two or three) metals are required to stabilize the strained fused-pentagons, making it difficult to access the single-atom properties. Herein, we report the syntheses and isolations of novel non-IPR mononuclear clusterfullerenes MNC@C₇₆ (M=Tb, Y), in which one pair of strained fused-pentagon is stabilized by a mononuclear cluster. The molecular structures of MNC@C₇₆ (M=Tb, Y) were determined unambiguously by single-crystal X-ray diffraction, featuring a non-IPR C_2v(19138)-C₇₆ cage entrapping a nearly linear MNC cluster, which is remarkably different from the triangular MNC cluster within the reported analogous clusterfullerenes based on IPR-obeying C₈₂ cages. The TbNC@C₇₆ molecule is found to be a field-induced single-molecule magnet (SMM).

Fused box: Mononuclear clusterfullerenes MNC@C₇₆ (M=Tb, Y) which do not obey the isolated-pentagon rule (IPR) contain one pair of fused-pentagons stabilized by a nearly linear mononuclear cyanide cluster. This situation contrasts with the triangular MNC clusters within the analogous clusterfullerenes of IPR-obeying C₈₂ cages. TbNC@C₇₆ is identified to be a field-induced single-molecule magnet (SMM).

Hydrophobically capped nanocrystals of formamidinium lead bromide (FAPbBr₃) perovskite (PNC) show bright and stable fluorescence in solution and thin-film states. When compared with isolated PNCs in a solution, close-packed PNCs in a thin film show extended fluorescence lifetime (ca. 4.2 μs), which is due to hopping or migration of photogenerated excitons among PNCs. Both fluorescence quantum efficiency and lifetime decrease in a PNC thin film doped with fullerene (C₆₀), which is attributed to channeling of exciton migration into electron transfer to C₆₀. On the other hand, quenching of fluorescence intensity of a PNC solution is not accompanied by any change in fluorescence lifetime, indicating static electron transfer to C₆₀ adsorbed onto the hydrophobic surface of individual PNCs. Exciton migration among close-packed PNCs and electron transfer to C₆₀ places C₆₀-doped PNC thin films among cost-effective antenna systems for solar cells.

Exciton migration among closely packed formamidinium lead bromide perovskite nanocrystals in thin films extends their fluorescence lifetime. By doping the film with C₆₀ molecules, migrating excitons can be efficiently trapped, which quantitatively quenches the fluorescence of the nanocrystals.

Hybrid covalent/supramolecular porphyrin–fullerene structures were synthesized as highly efficient molecular wires with a remarkably low attenuation factor (β=0.07±0.01 Å⁻¹). Hydrogen-bonding interactions and p-phenylene oligomers of different lengths are responsible for efficient electron transfer in the molecular wires.

Whizzing along the wire: A series of electron-donor–acceptor hybrids of a zinc porphyrin and C₆₀ connected through molecular wires based on a combination of covalent and noncovalent interactions (see picture) exhibited efficient charge transport. Hydrogen-bonding interactions and p-phenylene oligomers of different lengths were responsible for electron transfer in these molecular wires.

We report that Ce@C_2v(9)-C₈₂ forms a centrosymmetric dimer when co-crystallized with Ni(OEP) (OEP = octaethylporphyrin dianion). The crystal structure of {Ce@C_2v(9)-C₈₂}₂⋅2[Ni(OEP)]⋅4 C₆H₆ shows that a new C−C bond with a bond length of 1.605(5) Å connects the two cages. The high spin density of the singly occupied molecular orbital (SOMO) on the cage and the pyramidalization of the cage are factors that favor dimerization. In contrast, the treatment of Ni(OEP) with M@C_2v(9)-C₈₂ (M = La, Sc, and Y) results in crystallization of monomeric endohedral fullerenes. A systematic comparison of the X-ray structures of M@C_2v(9)-C₈₂ (M = Sc, Y, La, Ce, Gd, Yb, and Sm) reveals that the major metal site in each case is located at an off-center position adjacent to a hexagonal ring along the C₂ axis of the C_2v(9)-C₈₂ cage. DFT calculations at the M06-2X level revealed that the positions of the metal centers in these metallofullerenes M@C_2v(9)-C₈₂ (M = Sc, Y, and Ce), as determined by single-crystal X-ray structure studies, correspond to an energy minimum for each compound.

An unusual pairing: Ce@C_2v(9)-C₈₂ forms a centrosymmetric dimer when co-crystallized with Ni(OEP) (OEP = dianion of octaethylporphyrin; see picture). In marked contrast, the treatment of Ni(OEP) with the analogous metallofullerenes M@C_2v(9)-C₈₂ (M = Sc, Y, La, Gd, Yb, and Sm) results in crystallization of monomeric endohedral fullerenes.

Macrocyclic compounds have been extensively studied because of their widespread applications. Most classical macrocyclic compounds are planar molecules. In their Communication on page 14590 ff., L. B. Gan and co-workers report a N,O-heterocycle created on the C₆₀ cage skeleton through a multistep procedure, which includes repeated PCl₅-induced hydroxylamino N−O bond cleavage, and also piperidine-induced peroxo O−O bond cleavage. The fullerene-based macrocycle showed unique reactivity towards fluoride ions and copper salts.

The oxygen reduction reaction (ORR) is a key step in H₂–O₂ fuel cells, which, however, suffers from slow kinetics even for state-of-the-art catalysts. In this work, by making use of photocatalysis, the ORR was significantly accelerated with a polymer semiconductor (polyterthiophene). The onset potential underwent a positive shift from 0.66 to 1.34 V, and the current was enhanced by a factor of 44 at 0.6 V. The improvement was further confirmed in a proof-of-concept light-driven H₂–O₂ fuel cell, in which the open circuit voltage (V_oc) increased from 0.64 to 1.18 V, and the short circuit current (J_sc) was doubled. This novel tandem structure combining a polymer solar cell and a fuel cell enables the simultaneous utilization of photo- and electrochemical energy, showing promising potential for applications in energy conversion and storage.

Light up: The oxygen reduction reaction (ORR) can be significantly enhanced by using polymer semiconductor photoelectrodes, thus combining electro- and photocatalysis. Upon illumination of such a H₂–O₂ fuel cell, the onset potential of the ORR was shifted from 0.66 to 1.34 V, and the V_oc increased from 0.64 to 1.18 V.

Understanding photoinduced charge separation in fullerene-based dye-sensitized solar cells is crucial for the development of photovoltaic devices. We investigate here how the driving force of the charge separation process in conjugates of M@C₈₀ (M=Sc₃N, Sc₃CH, Sc₃NC, Sc₄O₂, and Sc₄O₃) with triphenylamine (TPA) depends on the nature of the metal cluster. Both singlet and triplet excited-state electron-transfer reactions are considered. These results based on TD-DFT calculations demonstrate that the driving force of charge separation in TPA-M@C₈₀ can be tuned well by varying the structure of the metal cluster encapsulated inside the fullerene cage.

The driving force of the photoinduced charge separation process in conjugates of M@C₈₀ (M=Sc₃N, Sc₃CH, Sc₃NC, Sc₄O₂, and Sc₄O₃) with triphenylamine (TPA) depends on the nature of the metal cluster. These results based on TD-DFT calculations demonstrate that charge separation in TPA-M@C₈₀ can be tuned by varying the structure of the metal cluster encapsulated inside the fullerene cage.

Size-complementary cyclotriveratrylene (CTV)-based hosts can incarcerate C₇₆, C₇₈, and C₈₄, thus allowing the selective isolation of these higher-order fullerenes from a commercially available mixture of fullerenes. The hemicarceplexes, formed after the encapsulation of the size-complementary fullerenes within the hosts, are isolated by column chromatography and released at elevated temperature, thereby leading to the isolation of C₇₆/C₇₈ and C₈₄ in good purities (up to 95 and 88 %, respectively).

A successful release: Two cyclotriveratrylene (CTV)-based hosts can form hemicarceplexes with higher-order fullerenes, which allows the selective isolation of C₇₆/C₇₈ and C₈₀ in high purities (80–95 and 81–88 %, respectively) from a commercially available mixture of higher-order fullerenes (see picture).