Controlled self-assembly of a trinitrofluorenone-appended gemini-shaped amphiphilic hexabenzocoronene selectively formed nanotubes or microfibers with different photochemical properties. In these nanotubes, which are 16 nanometers in diameter and several micrometers long, a molecular layer of electron-accepting trinitrofluorenone laminates an electron-donating graphitic layer of pi-stacked hexabenzocoronene. The coaxial nanotubular structure allows photochemical generation of spatially separated charge carriers and a quick photoconductive response with a large on/off ratio greater than 10(4). In sharp contrast, the microfibers consist of a charge-transfer complex between the hexabenzocoronene and trinitrofluorenone parts and exhibit almost no photocurrent generation.

Highly efficient hydrogen generation from dehydrogenation of formic acid is achieved by using bioinspired iridium complexes that have hydroxyl groups at the ortho positions of the bipyridine or bipyrimidine ligand (i.e., OH in the second coordination sphere of the metal center). In particular, [Ir(Cp*)(TH4BPM)(H2 O)]SO4 (TH4BPM: 2,2',6,6'-tetrahydroxyl-4,4'-bipyrimidine; Cp*: pentamethylcyclopentadienyl) has a high turnover frequency of 39 500 h(-1) at 80 °C in a 1 M aqueous solution of HCO2 H/HCO2 Na and produces hydrogen and carbon dioxide without carbon monoxide contamination. The deuterium kinetic isotope effect study clearly indicates a different rate-determining step for complexes with hydroxyl groups at different positions of the ligands. The rate-limiting step is β-hydrogen elimination from the iridium-formate intermediate for complexes with hydroxyl groups at ortho positions, owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of hydrogen generation. In contrast, the reaction of iridium hydride with a proton to liberate hydrogen is demonstrated to be the rate-determining step for complexes that do not have hydroxyl groups at the ortho positions.

Hydrogen generation from formic acid (FA), one of the most promising hydrogen storage materials, has attracted much attention due to the demand for the development of renewable energy carriers. Catalytic dehydrogenation of FA in an efficient and green manner remains challenging. Here, we report a series of bio-inspired Ir complexes for highly robust and selective hydrogen production from FA in aqueous solutions without organic solvents or additives. One of these complexes bearing an imidazoline moiety (complex 6) achieved a turnover frequency (TOF) of 322,000 h −1 at 100 ºC, which is higher than ever reported. The novel catalysts are very stable and applicable in highly concentrated FA. For instance, complex 3(1 µmol) affords an unprecedented turnover number (TON) of 2,050,000 at 60 ºC. Deuterium kinetic isotope effect experiments and density functional theory (DFT) calculations employing a "speciation" approach demonstrated a change in the rate determining step with increasing solution pH. This study provides not only more insight into the mechanism of dehydrogenation of FA, but also offers a new principle for the design of effective homogeneous organometallic catalysts for H 2 generation from FA.

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Proton-responsive half-sandwich Cp*Ir(III) complexes possessing a bipyridine ligand with two hydroxy groups at the 3,3′-, 4,4′-, 5,5′-, or 6,6′-positions (3DHBP, 4DHBP, 5DHBP, or 6DHBP) were systematically investigated. UV–vis titration data provided average pK a values of the hydroxy groups on the ligands. Both hydroxy groups were found to deprotonate in the pH 4.6–5.6 range for the 4–6DHBP complexes. One of the hydroxy groups of the 3DHBP complex exhibited a low pK a value of <0.4 because the deprotonation is facilitated by the strong intramolecular hydrogen bond formed between the generated oxyanion and the remaining hydroxy group, which in turn leads to an elevated pK a value of ∼13.6 for the second deprotonation step. The crystal structures of the 4- and 6DHBP complexes obtained from basic aqueous solutions revealed their deprotonated forms. The intramolecular hydrogen bond in the 3DHBP complex was also observed in the crystal structures. The catalytic activities of these complexes in aqueous phase reactions, at appropriate pH, for hydrogenation of carbon dioxide (pH 8.5), dehydrogenation of formic acid (pH 1.8), and transfer hydrogenation reactions using formic acid/formate as a hydrogen source (pH 2.6 and 7.2) were investigated to compare the positional effects of the hydroxy groups. The 4- and 6DHBP complexes exhibited remarkably enhanced catalytic activities under basic conditions because of the resonance effect of the strong electron-donating oxyanions, whereas the 5DHBP complex exhibited negligible activity despite the presence of electron-donating groups. The 3DHBP complex exhibited relatively high catalytic activity at low pH owing to the one strong electron-donating oxyanion group stabilized by the intramolecular hydrogen bond. DFT calculations were employed to study the mechanism of CO2 hydrogenation by the 4DHBP and 6DHBP complexes, and comparison of the activation free energies of the H2 heterolysis and CO2 insertion steps indicated that H2 heterolysis is the rate-determining step for both complexes. The presence of a pendent base in the 6DHBP complex was found to facilitate the rate-determining step and renders 6DHBP a more effective catalyst for formate production.

The catalytic cycle for the production of formic acid by CO2 hydrogenation and the reverse reaction have received renewed attention because they are viewed as offering a viable scheme for hydrogen storage and release. In this Forum Article, CO2 hydrogenation catalyzed by iridium complexes bearing sophisticated N^N-bidentate ligands is reported. We describe how a ligand containing hydroxy groups as proton-responsive substituents enhances the catalytic performance by an electronic effect of the oxyanions and a pendent-base effect through secondary coordination sphere interactions. In particular, [(Cp*IrCl)2(TH2BPM)]Cl2 (Cp* = pentamethylcyclopentadienyl; TH2BPM = 4,4',6,6'-tetrahydroxy-2,2'-bipyrimidine) enormously promotes the catalytic hydrogenation of CO2 in basic water by these synergistic effects under atmospheric pressure and at room temperature. Additionally, newly designed complexes with azole-type ligands were applied to CO2 hydrogenation. The catalytic efficiencies of the azole-type complexes were much higher than that of the unsubstituted bipyridine complex [Cp*Ir(bpy)(OH2)]SO4. Furthermore, the introduction of one or more hydroxy groups into ligands such as 2-pyrazolyl-6-hydroxypyridine, 2-pyrazolyl-4,6-dihydroxypyrimidine, and 4-pyrazolyl-2,6-dihydroxypyrimidine enhanced the catalytic activity. It is clear that the incorporation of additional electron-donating functionalities into proton-responsive azole-type ligands is effective for promoting further enhanced hydrogenation of CO2.

Zinc porphyrin-appended dendrimers, 12PZn, 18PZn, 24PZn, and 36PZn, containing 12, 18, 24, and 36 zinc porphyrin units, respectively, were synthesized using zinc porphyrin dyad (2PZn) and triad (3PZn) as precursors. Although these dye-functionalized dendrimers all serve as chiroptical sensors for an asymmetric bipyridine (RR- and SS-Py2), the sensing capability is highly dependent on the structure of the dendritic scaffold. 2PZn, which is chiroptically silent toward Py2, turns cooperative and displays a large ICD (induced circular dichroism) response in the visible region when incorporated into 12PZn. Judging from the extents of contribution of each zinc porphyrin unit to the CD amplitudes ([Deltaepsilonmax]), the cooperativity in 24PZn (112 M-1 cm-1) is lower than that in 12PZn (196 M-1 cm-1) and much lower in dendron 4PZn (59 M-1 cm-1). In contrast, 3PZn, which is ICD-active toward Py2, hardly shows such an enhanced cooperativity when incorporated into 18PZn and 36PZn and dendron 6PZn, as well. Absorption spectroscopy suggests some unique conformational characteristics of the zinc porphyrin units in highly cooperative 12PZn.

Organic field-effect transistors with hydrogen-bonded diketopyrrolopyrrole-thiophene co-oligomers were fabricated by a solution-process method with annealing at 200 °C, showing ambipolar charge-carrier transfer with field-effect mobilities up to μ(h) = 6.7 × 10(-3) cm(2) V(-1)s(-1) and μ(e) = 5.6 × 10(-3) cm(2) V(-1) s(-1).