Y.F.Wang

The Diels–Alder reactivity of different bowl-shaped polycyclic aromatic hydrocarbons (namely, corannulene, cyclopentacorannulene, diindenochrysene, hemifullerene, and circumtrindene) has been explored computationally within the DFT framework. To this end, both the increase in reactivity with the size of the buckybowl and complete [6,6]-regioselectivity in the process have been analyzed in detail by using the activation strain model of reactivity in combination with the energy decomposition analysis method. These results have been compared with the parent C₆₀ fullerene, which also produces the corresponding [6,6]-cycloadduct exclusively. The behavior of the buckybowls considered herein resembles, in general, that of C₆₀. Whereas the interaction energy between the deformed reactants along the reaction coordinate mainly controls the regioselectivity of the process, it is the interplay between the activation strain energy and the transition-state interaction that governs the reactivity of the system.

Bigger bowls, better reactivity: Starting from corannulene, there is a smooth convergence to the C₆₀ energy barrier and reaction energy for the Diels–Alder reaction with cyclopentadiene when the size of the buckybowl is increased (see figure). Through density functional calculations, the origins of both this trend of reactivity and the observed exclusive [6,6]-regioselectivity are analyzed in detail.

Three new unsymmetrical phenothiazine-based platinum(II) bis(acetylide) complexes PT1–PT3 with different electron-donating arylacetylide ligands were synthesized and characterized. Their photophysical, electrochemical, and photovoltaic properties have been fully investigated and the density functional theory (DFT) calculations have been carried out. Under AM 1.5 irradiation (100 mW cm⁻²), the PT1-based dye-sensitized solar cell (DSSC) device exhibited an attractive power conversion efficiency (η) up to 5.78 %, with a short-circuit photocurrent density (J_sc) of 10.98 mA cm⁻², an open-circuit photovoltage (V_oc) of 0.738 V, and a fill factor (ff) of 0.713. These findings provide strong evidence that platinum–acetylide complexes have great potential as promising photosensitizers in DSSC applications.

An attractive power conversion efficiency (η) of up to 5.78 %, a short-circuit photocurrent density of 10.98 mA cm⁻², an open-circuit photovoltage of 0.738 V, and a fill factor of 0.713 are displayed by the newly synthesized PT1-based dye-sensitized solar cell (DSSC) device. These findings provide strong evidence that platinum–acetylide complexes have great potential as promising photosensitizers in DSSC applications.

A general strategy based on the nanoscale Kirkendall effect is developed to grow hollow transition metal (Fe, Co or Ni) oxide nanoparticles on graphene sheets. When applied as lithium-ion battery anodes, these hollow transition metal oxide-based composites exhibit excellent electrochemical performance with high reversible capacities and long-term stabilities at a high current density, superior to most transition metal oxides reported to date. More information can be found in the Full Paper by Y. Chen, C. Sun, P. Gao et al. (DOI: 10.1002/chem.201503897).

The unprecedented dependence of final charge separation efficiency as a function of donor–acceptor interaction in covalently-linked molecules with a rectilinear rigid oligo-p-xylene bridge has been observed. Optimization of the donor–acceptor electronic coupling remarkably inhibits the undesirable rapid decay of the singlet charge-separated state to the ground state, yielding the final long-lived, triplet charge-separated state with circa 100 % efficiency. This finding is extremely useful for the rational design of artificial photosynthesis and organic photovoltaic cells toward efficient solar energy conversion.

Optimization of donor–acceptor electronic coupling remarkably inhibits the undesirable rapid decay of the singlet charge-separated state to the ground state, yielding the final long-lived, triplet charge-separated state with circa 100 % efficiency. This finding is relevant to the rational design of artificial photosynthesis and organic photovoltaic cells.

Energy conversion schemes have attracted considerable attention in recent years. A large amount of research effort has focused on fullerenes, particularly C₆₀ and its derivatives, as suitable electron acceptors, owing to their outstanding properties. In this context, C₅₉N-based donor–acceptor systems have lately attracted attention, owing to their exceptional energy-and electron-transfer properties. As a result, chemical derivatization of C₅₉N plays an important role in the realization of the aforementioned systems. The current Minireview aims to familiarize researchers with the main aspects of azafullerene synthesis, chemistry, and photophysical properties, while it mainly focuses on the synthetic methodologies employed for as well as on energy conversion schemes of azafullerene-based donor–acceptor systems.

Energy conversion schemes: Donor–acceptor systems based on the azafullerene C₅₉N are reviewed. While focusing on the synthetic strategies, the properties of systems employing this azafullerene as electron or energy acceptor are also presented, unfolding both the synthetic access as well as the unique electronic properties of these systems. Both in dyads in solution as well as in solar cell devices, C₅₉N is revealed as a promising acceptor for energy conversion schemes.

A coupled light-harvesting antenna–charge-separation system, consisting of self-assembled zinc chlorophyll derivatives that incorporate an electron-accepting unit, is reported. The cyclic tetramers that incorporated an electron acceptor were constructed by the co-assembly of a pyridine-appended zinc chlorophyll derivative, ZnPy, and a zinc chlorophyll derivative further decorated with a fullerene unit, ZnPyC₆₀. Comprehensive steady-state and time-resolved spectroscopic studies were conducted for the individual tetramers of ZnPy and ZnPyC₆₀ as well as their co-tetramers. Intra-assembly singlet energy transfer was confirmed by singlet–singlet annihilation in the ZnPy tetramer. Electron transfer from the singlet chlorin unit to the fullerene unit was clearly demonstrated by the transient absorption of the fullerene radical anion in the ZnPyC₆₀ tetramer. Finally, with the co-tetramer, a coupled light-harvesting and charge-separation system with practically 100 % quantum efficiency was demonstrated.

Dual functionality: A coupled light-harvesting and charge-separation system is constructed by coordination-directed assembly of specifically functionalized chlorophyll molecules, in which nearly 100 % charge separation is achieved once a photon is absorbed (see figure; EnT=energy transfer, ET=electron transfer).

D_5h-symmetric fullerene C₇₀ (D_5h-C₇₀) is one of the most abundant members of the fullerene family. One longstanding mystery in the field of fullerene chemistry is whether D_5h-C₇₀ is capable of accommodating a rare-earth metal atom to form an endohedral metallofullerene M@D_5h-C₇₀, which would be expected to show novel electronic properties. The molecular structure of La@C₇₀ remains unresolved since its discovery three decades ago because of its extremely high instability under ambient conditions and insolubility in organic solvents. Herein, we report the single-crystal X-ray structure of La@C₇₀(CF₃)₃, which was obtained through in situ exohedral functionalization by means of trifluoromethylation. The X-ray crystallographic study reveals that La@C₇₀(CF₃)₃ is the first example of an endohedral rare-earth fullerene based on D_5h-C₇₀. The dramatically enhanced stability of La@C₇₀(CF₃)₃ compared to La@C₇₀ can be ascribed to trifluoromethylation-induced bandgap enlargement.

The missing link: The molecular structure of a missing metallofullerene La@C₇₀ was determined for the first time through in situ trifluoromethylation of the fullerene cage, followed by single-crystal X-ray diffraction. La@C₇₀ is greatly stabilized as a consequence of trifluoromethylation-induced bandgap enlargement.

The assembly of a discrete hydrogen-bonded molecular tube from eight small identical monomers is reported. Tube assembly was accomplished by means of selective heterodimerization between isocytosine and ureidopyrimidinone hydrogen-bonding motifs embedded in an enantiopure bicyclic building block, leading to the selective formation of an octameric supramolecular tube. Upon introduction of a fullerene guest molecule, the octameric tube rearranges into a tetrameric inclusion complex and the hydrogen-bonding mode is switched. The dynamic behavior of the system is further explored in solvent- and guest-responsive self-sorting experiments.

Be my guest: An octameric supramolecular tube was assembled from identical small bicyclic building blocks by means of selective heterodimerization between isocytosine and ureidopyrimidinone hydrogen-bonding units. Upon treatment with C₆₀, the hydrogen-bonding mode is switched, leading to rearrangement of the tube into a tetrameric inclusion complex.

Exohedral derivatization of Lu₃N@I_h-C₈₀ was successfully conducted by the photoreactions with disilirane and digermirane. The single-crystal X-ray structural analyses firmly established the 1,4-bis-silylated and bis-germylated structures, which represent rare examples of the crystallographic studies of functionalized Lu₃N@I_h-C₈₀. The electrochemical and theoretical studies revealed that the redox properties of the bis-silylated and bis-germylated derivatives were remarkably altered compared to those of the parent Lu₃N@I_h-C₈₀. For more details see the Full Paper on page 16411 ff. by M. Kako, M. M. Olmstead, A. L. Balch, S. Nagase, T. Akasaka et al.

A series of wide-band capturing, self-assembled supramolecular donor–acceptor conjugates comprised of bis(donor styryl)BODIPY (donor=triphenylamine or phenothiazine; BODIPY=4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) and C₆₀ have been newly assembled. These conjugates are shown to undergo ultrafast photoinduced charge separation leading to radical ion pairs by femtosecond transient spectroscopy. For more details see the Full Paper by F. D’Souza and co-workers on page 16005 ff.

Herein, we report a newly developed C₆₀ fullerene-bonded silica monolith in a capillary with unique retention behavior due to the structure of C₆₀ fullerene. N-Hydroxysuccinimide (NHS)-conjugated C₆₀ fullerene was successfully synthesized by a thermal coupling agent, perfluorophenyl azide (PFPA), and assigned by spectroscopic analyses. Then, NHS-PFPA-C₆₀ fullerene was attached onto the surface of a silica monolith in a capillary. The capillary provided specific separation ability for polycyclic aromatic hydrocarbons in liquid chromatography by an effective π–π interaction. Furthermore, corannulene, which has a hemispherical structure, was selectively retained in the capillary based on the specific structural recognition due to the spherical C₆₀ fullerene. This is the first report revealing the spherical recognition ability by C₆₀ fullerene in liquid chromatographic separation.

Spherical recognition: A C₆₀ fullerene-bonded silica monolithic medium was prepared by using a thermal coupling agent conjugated with C₆₀ fullerene. The C₆₀ fullerene-bonded monolith prepared in a capillary showed a very strong π–π interaction in liquid chromatography. Also, the hemispherical molecule corannulene was dramatically retained in the C₆₀ fullerene-bonded monolith by the unique molecular recognition based on a buckyball (see figure).

An open-cage C₆₀ tetraketone with a large opening was able to encapsulate N₂ and CO₂ molecules after its exposure to high pressures of N₂ and CO₂ gas. A subsequent selective reduction of one of the four carbonyl groups on the rim of the opening induced a contraction of the opening (2) and trapped the guest molecules inside 2. The thus-obtained host–guest complexes N₂@2 and CO₂@2 could be isolated by recycling HPLC, and were found to be stable at room temperature. The molecular structures of N₂@2 and CO₂@2 were determined by single-crystal X-ray diffraction analyses, and revealed a short NN triple bond for the encapsulated N₂, as well as an unsymmetric molecular structure for the encapsulated molecule of CO₂. The IR spectrum of CO₂@2 suggested that the rotation of the encapsulated molecule of CO₂ is partially restricted, which was supported by DFT calculations.

Single-molecule trap: After insertion of N₂ and CO₂ molecules into open-cage C₆₀ tetraketone, a subsequent selective reduction of one of the four carbonyl groups induced a contraction of the opening and trapped the guest molecules. The isolation of the thus obtained molecular complexes was accomplished by recycling HPLC. These structures were unambiguously determined by single-crystal X-ray analyses.

A two-step solution processing approach has been established to grow void-free perovskite films for low-cost high-performance planar heterojunction photovoltaic devices. A high-temperature thermal annealing treatment was applied to drive the diffusion of CH₃NH₃I precursor molecules into a compact PbI₂ layer to form perovskite films. However, thermal annealing for extended periods led to degraded device performance owing to the defects generated by decomposition of perovskite into PbI₂. A controllable layer-by-layer spin-coating method was used to grow “bilayer” CH₃NH₃I/PbI₂ films, and then drive the interdiffusion between PbI₂ and CH₃NH₃I layers by a simple air exposure at room temperature for making well-oriented, highly crystalline perovskite films without thermal annealing. This high degree of crystallinity resulted in a carrier diffusion length of ca. 800 nm and a high device efficiency of 15.6 %, which is comparable to values reported for thermally annealed perovskite films.

A breath of fresh air: Simple room-temperature air exposure can drive the interdiffusion between perovskite precursor layers and crystallize the perovskite thin films. The obtained perovskite films show high crystallinity and well-aligned orientation. The devices with and without a TiO₂ electron transporting layer yielded high efficiencies of 15.6 % and 13.8 %, respectively.