Y.F.Wang

The pressure to move towards renewable energy has inspired researchers to look for ideas in photovoltaics that may lead to a major breakthrough. Recently the use of perovskites as a light harvester has lead to stunning progress. The power conversion efficiency of perovskite solar cells is now approaching parity (>22 %) with that of the established technology which took decades to reach this level of performance. The use of a hole transport material (HTM) remains indispensable in perovskite solar cells. Perovskites can conduct holes, but they are present at low levels, and for efficient charge extraction a HTM layer is a prerequisite. Herein we provide an overview of the diverse types of HTM available, from organic to inorganic, in the hope of encouraging further research and the optimization of these materials.

Hole for whole: Semiconductor hole-transport materials (HTMs) are an essential component for perovskite solar cells. The three classes of materials available, inorganic, polymeric, and small molecule HTMs are reviewed, particularly the optoelectrical properties of molecular HTMs, which seem to be the most effective materials.

Despite being widely used as electron acceptor in polymer solar cells, commercially available PC₇₁BM (phenyl-C₇₁-butyric acid methyl ester) usually has a “random” composition of mixed regioisomers or stereoisomers. Here PC₇₁BM has been isolated into three typical isomers, α-, β₁- and β₂-PC₇₁BM, to establish the isomer-dependent photovoltaic performance on changing the ternary composition of α-, β₁- and β₂-PC₇₁BM. Mixing the isomers in a ratio of α/β₁/β₂=8:1:1 resulted in the best power conversion efficiency (PCE) of 7.67 % for the polymer solar cells with PTB7:PC₇₁BM as photoactive layer (PTB7=poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]). The three typical PC₇₁BM isomers, even though sharing similar LUMO energy levels and light absorption, render starkly different photovoltaic performances with average-performing PCE of 1.28–7.44 % due to diverse self-aggregation of individual or mixed PC₇₁BM isomers in the otherwise same polymer solar cells.

Special blend: PC₇₁BM has been isolated into three typical isomers, α-, β₁- and β₂-PC₇₁BM, to optimize the best electron acceptor of ternary blends of isomeric PC₇₁BM. A α-β₁-β₂ triangle “PCE phase diagram” (see figure) for polymer solar cells with the structure ITO/PEDOT:PSS/PTB7:PC₇₁BM/Ca/Al has also been plotted for the first time.

Uranyl polyoxometalate clusters are both fundamentally fascinating and potentially relevant to nuclear energy applications. With only ten years of development, there is still much to be discovered about heterometal derivatives and aqueous speciation and behavior. Herein, we show that it is possible to encapsulate the polyoxocations [Bi₆O₈]²⁺ and [Pb₈O₆]⁴⁺ in [(UO₂)(O₂)(OH)]₂₄²⁴⁻ (denoted Bi@U₂₄ and Pb@U₂₄) in pure form and high yields despite the fact that under aqueous conditions, these compounds are stable on opposite ends of the pH scale. Moreover, [Pb₈O₆]⁴⁺ is a formerly unknown Pb^II polynuclear species, both in solution and in the solid state. Raman spectroscopic and mass spectrometric analysis of the reaction solutions revealed the very rapid assembly of the nested clusters, driven by bismuth- or lead-promoted decomposition of excess peroxide, which inhibits U₂₄ formation. Experimental and simulated small-angle X-ray scattering data of Bi@U₂₄ and Pb@U₂₄ solutions revealed that this technique is very sensitive not only to the size and shape of the clusters, but also to the encapsulated species.

Heavy and heavier: Polyoxocations of the heavy metals bismuth and lead were nested inside uranyl polyoxoanion capsules, creating very distinctive X-ray scattering profiles. Matching symmetry and electrostatic attraction between the core and shell clusters enabled the formation of endohedral Pb@U₂₄ and Bi@U₂₄ in high yield and purity.

Improving our comprehension of diverse CO₂ activation pathways is of vital importance for the widespread future utilization of this abundant greenhouse gas. CO₂ activation by uranium(III) complexes is now relatively well understood, with oxo/carbonate formation predominating as CO₂ is readily reduced to CO, but isolated thorium(III) CO₂ activation is unprecedented. We show that the thorium(III) complex, [Th(Cp′′)₃] (1, Cp′′={C₅H₃(SiMe₃)₂-1,3}), reacts with CO₂ to give the mixed oxalate-carboxylate thorium(IV) complex [{Th(Cp′′)₂[κ²-O₂C{C₅H₃-3,3′-(SiMe₃)₂}]}₂(μ-κ²:κ²-C₂O₄)] (3). The concomitant formation of oxalate and carboxylate is unique for CO₂ activation, as in previous examples either reduction or insertion is favored to yield a single product. Therefore, thorium(III) CO₂ activation can differ from better understood uranium(III) chemistry.

A winning combination: The thorium(III) complex [Th{C₅H₃(SiMe₃)₂-1,3}₃] reacts with CO₂ to give a mixed carboxylate–oxalate product (see figure). This reactivity contrasts with better understood uranium(III) CO₂ activation, which typically gives oxo and carbonate products via the elimination of CO.

Buckybowls are fascinating components of supramolecular assemblies owing to their unique bowl-shaped π-surfaces. Herein we present a protocol for the functionalization of a sulfur-doped buckybowl, trithiasumanene, via a brominated intermediate, from which thiolated trithiasumanenes were derived. The curved surface and electron-donating properties of thiolated trithiasumanenes promote their ready assembly with fullerenes to form concave–convex complexes. The supramolecular assembly behavior in solution was investigated by NMR analysis. The structures of supramolecular complexes were unambiguously characterized by crystallography. The crystals of the concave–convex complexes showed high thermal stability and photoconductivity.

Like a hand in a glove: When decorated with electron-donating thiol groups, trithiasumanenes assembled with fullerenes both in solution and in the solid state. The supramolecular complexes were unambiguously characterized by X-ray diffraction, which revealed their concave–convex supramolecular recognition (see picture). The crystals of the supramolecular complexes showed remarkable thermal stability and high photoconductivity.

The synthesis of a donor–acceptor silicon phthalocyanine (SiPc)-azafullerene (C₅₉N) dyad 1 and of the first acceptor–donor–acceptor C₅₉N-SiPc-C₅₉N dumbbell triad 2 was accomplished. The two C₅₉N-based materials were comprehensively characterized with the aid of NMR spectroscopy, MALDI-MS as well as DFT calculations and their redox and photophysical properties were evaluated with CV and steady-state and time-resolved absorption and photoluminescence spectroscopy measurements. Notably, femtosecond transient absorption spectroscopy assays revealed that both dyad 1 and triad 2 undergo, after selective photoexcitation of the SiPc moiety, photoinduced electron transfer from the singlet excited state of the SiPc moiety to the azafullerene counterpart to produce the charge-separated state, with lifetimes of 660 ps, in the case of dyad 1, and 810 ps, in the case of triad 2. The current results are expected to have significant implications en route to the design of advanced C₅₉N-based donor–acceptor systems targeting energy conversion applications.

Union is strength: The synthesis and photophysical properties of SiPc-C₅₉N dyad 1 and the first azafullerene-based acceptor–donor–acceptor C₅₉N-SiPc-C₅₉N dumbbell triad 2 (see figure) are reported. Femtosecond laser-induced transient absorption spectral measurements reveal that both of these undergo photoinduced electron transfer from the singlet excited state of the SiPc moiety to the azafullerene counterpart to produce the charge-separated state.

Labile bis-triazoline adducts of C₆₀ are supposed to be the precursors of bis-azafulleroids, but the formation mechanism is still unclear because of the incomplete isolation of the thermolized products and the lack of X-ray structures. A rigid-tethered reagent 1,2-bis(azidomethyl)benzene (1) was used to regioselectively synthesize the labile 1,2,3,4-bis(triazolino)[60]-fullerene (2), the structure of which was determined by single-crystal X-ray crystallography. Further thermolysis of 2 produces four products (3 a–3 d), which were all characterized by X-ray crystallography. Although 3 a and 3 b have traditional bis-azafulleroid structures, as proposed previously, 3 c and 3 d show unprecedented structures with either the coexistence of [5,6]-open and [6,6]-closed patterns or an oxidized structure with an 11-membered ring on the cage. A thermolysis mechanism is proposed to clarify long-term confusion about the transformation process from bis-triazoline adducts to bis-azafulleroids of C₆₀.

A rigid-tether strategy is developed to regioselectively synthesize the labile 1,2,3,4-bis(triazolino)[60]fullerene. Subsequent thermolysis treatment gives rise to several unprecedented structures (see picture), which were all characterized with single-crystal X-ray crystallography, providing concrete evidence for the formation mechanism of [60]bis-azafulleroids to clarify the long-term confusion.

A new low-band gap dyad DPP-Ful, which consists of covalently linked dithiafulvalene-functionalized diketopyrrolopyrrole as donor and fullerene (C₆₀) as the acceptor, has been designed and synthesized. Organic solar cells were successfully constructed using the DPP-Ful dyad as an active layer. This system has a record power-conversion efficiency (PCE) of 2.2 %, which is the highest value when compared to reported single-component organic solar cells.

Climbing alone: The combination of dithiafulvalene-functionalized diketopyrrolopyrrole (DPP) as donor with fullerene (Ful) as acceptor has been successfully explored. Its utilization in single-component organic solar cells (SC-OSCs) was investigated, and it was shown to have a record power-conversion efficiency. ITO=indium tin oxide.

Supramolecular Chemistry. The use of noncovalent interactions between untethered residues to amplify the regio-, stereo-, and atropselective formation of a C₆₀ fullerene bisadduct racemate is described by G. Bottari, D. M. Guldi, T. Torres et al. in their Communication on page 11020 ff.

A 14-membered heterocycle is created on the C₆₀ cage skeleton through a multistep procedure. Key steps involve repeated PCl₅-induced hydroxylamino N−O bond cleavage leading to insertion of nitrogen atoms, and also piperidine-induced peroxo O−O bond cleavage leading to insertion of oxygen atoms. The hetero atoms form one pyrrole, two pyran, and one diazepine rings in conjunction with the C₆₀ skeleton carbon atoms. The fullerene-based macrocycle showed unique reactivities towards fluoride ion and copper salts.

N,O your product: A N,O-heterocycle is created on the C₆₀ cage skeleton through a multistep procedure, which includes repeated PCl₅-induced hydroxylamino N−O bond cleavage leading to insertion of nitrogen atoms, and also piperidine-induced peroxo O−O bond cleavage leading to insertion of oxygen atoms. The fullerene based-macrocycle showed unique reactivity towards fluoride ion and copper salts.

Cyclobuteno[3,4:1,2][60]fullerenes have been prepared in a straightforward manner by a simple reaction between [60]fullerene and readily available allenoates or alkynoates as organic reagents under basic and mild conditions. The chemical structure of the new modified fullerenes has been determined by standard spectroscopic techniques and confirmed by X-ray diffraction analysis. Some of these new fullerene derivatives exhibit a remarkable intrinsic electron mobility (determined by using flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements), which surpasses that of the well-known phenyl-C61-butyric acid methyl ester, thus behaving as promising n-type organic semiconductors.

Cyclobutene[60]fullerenes are now available! A new synthetic approach based on alkynoates and/or allenoates leads to a new family of fullerene derivatives exhibiting remarkable semiconducting behavior.

We directly observed charge separation and a space-charge region in an organic single-crystal p–n heterojunction nanowire, by means of scanning photocurrent microscopy. The axial p–n heterojunction nanowire had a well-defined planar junction, consisted of P3HT (p-type) and C₆₀ (n-type) single crystals and was fabricated by means of the recently developed inkjet-assisted nanotransfer printing technique. The depletion region formed at the p–n junction was directly observed by exploring the spatial distribution of photogenerated carriers along the heterojunction nanowire under various applied bias voltages. Our study provides a facile approach toward the precise characterization of charge transport in organic heterojunction systems as well as the design of efficient nanoscale organic optoelectronic devices.

Junction box: Single-crystal organic axial P3HT/C₆₀ p–n heterojunction nanowires each having a well-defined junction region were fabricated by means of inkjet-assisted nanotransfer printing and were exploited for direct observation, by means of scanning photocurrent microscopy, of charge separation and depletion width in the junction region under external electric fields.

An integrated cell for the solar-driven splitting of water consists of multiple functional components and couples various photoelectrochemical (PEC) processes at different length and time scales. The overall solar-to-hydrogen (STH) conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as on the component integration, overall device architecture, and system operating conditions. This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components. The underlying physics and interactions at the cell level is are reviewed and discussed, followed by an overview of the use of the cell model to provide target properties of materials and guide the design of a range of traditional and unique device architectures.

Catching the sun: Significant advances have been made in recent years on the modeling- and simulation-guided development of integrated solar-driven water-splitting devices. Multidimensional multiphysics models have provided design guidelines for semiconductors, electrocatalysts, as well as liquid and membrane electrolytes. This Review discusses the guiding principles and key findings of these activities.

A high potential donor–acceptor dyad composed of zinc porphyrin bearing three meso-pentafluorophenyl substituents covalently linked to C₆₀, as a novel dyad capable of generating charge-separated states of high energy (potential) has been developed. The calculated energy of the charge-separated state was found to be 1.70 eV, the highest reported for a covalently linked porphyrin–fullerene dyad. Intramolecular photoinduced electron transfer leading to charge-separated states of appreciable lifetimes in polar and nonpolar solvents has been established from studies involving femto- to nanosecond transient absorption techniques. The high energy stored in the form of charge-separated states along with its persistence of about 50–60 ns makes this dyad a potential electron-transporting catalyst to carry out energy-demanding photochemical reactions. This type of high-energy harvesting dyad is expected to open new research in the areas of artificial photosynthesis especially producing energy (potential) demanding light-to-fuel products.

Keep 'em separated: A donor–acceptor dyad composed of a high oxidation potential zinc porphyrin covalently linked to C₆₀ has been synthesized and shown to generate a high-energy charge-separated state on the order of 1.70 eV and a lifetime in the range of 50–60 ns during photoinduced electron transfer, which is sufficient to carry out many energy (potential) demanding photocatalytic reactions.

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed-metal Sc–Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH₄ and Ti/CH₄ systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH₄ system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH₄ system are Sc₄C₂@C₈₀ (the most abundant EMF from this synthesis), Sc₃C₂@C₈₀, isomers of Sc₂C₂@C₈₂, and the family Sc₂C_2 n (2 n=74, 76, 82, 86, 90, etc.), as well as Sc₃CH@C₈₀. The Sc–Ti/CH₄ system produces the mixed-metal Sc₂TiC@C_2 n (2 n=68, 78, 80) and Sc₂TiC₂@C_2 n (2 n=80) clusterfullerene families. The molecular structures of the new, transition-metal-containing endohedral fullerenes, Sc₂TiC@I_h-C₈₀, Sc₂TiC@D_5h-C₈₀, and Sc₂TiC₂@I_h-C₈₀, were characterized by NMR spectroscopy. The structure of Sc₂TiC@I_h-C₈₀ was also determined by single-crystal X-ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc₂TiC- and Sc₂TiC₂-containing clusterfullerenes have Ti-localized LUMOs. Encapsulation of the redox-active Ti ion inside the fullerene cage enables analysis of the cluster–cage strain in the endohedral fullerenes through electrochemical measurements.

Captured in a cage: The redox-active Ti ion in endohedral metallofullerenes enables the estimation of the inner strain through variation of the redox potential with the size of the cluster (see figure).

The formation of endohedral metallofullerenes in an electric arc is studied for the mixed-metal Sc–Ti system utilizing methane as a reactive gas. The main metallofullerenes produced in such conditions are Sc₂TiC@C₈₀ and Sc₂TiC₂@C₈₀ as well as similar clusterfullerenes with other cages. Comparison of these results with those from the Sc/CH₄ and Ti/CH₄ systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. More information can be found in the Full Paper by M. M. Olmstead, A. L. Balch, A. A. Popov et al. (DOI: 10.1002/chem.201601655).

Subphthalocyanine (SubPc), a unique ring-reduced member of the common phthalocyanines family, although known for its higher absorptivity, reveals narrow absorption with peak maxima around 570 nm thus limiting its utility in light-energy-harvesting applications. In the present study, by peripheral thio–aryl substitution of SubPc macrocycle, the spectral properties have been modulated to extend the absorption and emission well into the visible/near-IR region. Additionally, for α-ring-substituted derivatives, facile oxidation of SubPc was witnessed, thus making these derivatives better electron donors. Next, the preparation of donor–acceptor dyads containing the well-known electron acceptor C₆₀ connected to the central boron atom of SubPc was accomplished by making use of the 1,3-dipolar cycloaddition reaction. Control experiments and free-energy calculations using the redox and spectral data suggested that the observed fluorescence quenching of SubPc in these dyads is due to electron transfer. Accordingly, transient spectral studies performed both in polar and nonpolar solvents conclusively proved electron transfer to be the quenching mechanism in these dyads. The measured rate constants by fitting kinetic data revealed efficient charge separation and charge recombination processes, suggesting that these dyads could be useful materials for the construction of light-to-electricity or light-to-fuel production devices.

Supramolecular electron transfer: A new series of dyads comprised of structurally modified (to extend the absorption and singlet emission in to near-IR region) subphthalocyanine covalently linked to fullerene, as light energy harvesting capable functional materials is synthesized and occurrence of photoinduced electron transfer is demonstrated (see figure).

The synthesis of unprecedented multimeric Kdo glycoclusters based on fullerene and calix[4]arene central scaffolds is reported. The compounds were used to study the mechanism and scope of multivalent glycosyltransferase inhibition. Multimeric mannosides based on porphyrin and pillar[5]arenes were also generated in a controlled manner. Twelve glycoclusters and their monomeric ligands were thus assayed against heptosyltransferase WaaC, which is an important bacterial glycosyltransferase that is involved in lipopolysaccharide biosynthesis. It was first found that all the multimers interact solely with the acceptor binding site of the enzyme even when the multimeric ligands mimic the heptose donor. Second, the novel Kdo glycofullerenes displayed very potent inhibition (K_i=0.14 μm for the best inhibitor); an inhibition level rarely observed with glycosyltransferases. Although the observed “multivalent effects” (i.e., the enhancement of affinity of a ligand when presented in a multimeric fashion) were in general modest, a dramatic effect of the central scaffold on the inhibition level was evidenced: the fullerene and the porphyrin scaffolds being by far superior to the calix- and pillar-arenes. We could also show, by dynamic light scattering analysis, that the best inhibitor had the propensity to form aggregates with the heptosyltransferase. This aggregative property may contribute to the global multivalent enzyme inhibition, but probably do not constitute the main origin of inhibition.

Better together? The synthesis of unprecedented multimeric Kdo glycoclusters based on fullerene and calixarene central scaffolds is reported (see figure). The compounds were used to study the mechanism and the scope of multivalent heptosyltransferase inhibition, which is an important bacterial enzyme. The novel Kdo glycofullerenes displayed very potent inhibition (K_i=0.14 μm), at a level that is rarely observed with glycosyltransferases.

Azafullerenes are as yet the only synthetically available heterofullerenes. Herein, we present plausible reaction pathways towards pentaaryl azafullerenes, focusing on the reactivity of hydro-azafullerene intermediates and their regiochemistry. The X-ray structure of a β′-hydro-tetraaryl adduct is presented for the first time. The reactivity of dihydro-azafullerene adducts is demonstrated here through H-abstraction in mass spectrometric experiments. Moreover, hydride abstraction and subsequent hydroxylation is possible, as well as deprotonation followed by alkylation.

Regioselectivity in hydro-azafullerene reactions: The reactivity of hydro-azafullerene intermediates en route to pentaaryl derivatives has been explored. Experimental results have shown that both hydride and proton abstraction occur preferentially at the α′-positions of dihydro-azafullerenes.