Yoshifumi Maegawa

Transparent and visible-light harvesting acridone-bridged mesostructured organosilica film

Transparent and visible light-harvesting acridone-bridged periodic mesoporous organosilica (PMO) films were prepared by acidic sol–gel polycondensation of non-methylated and N-methylated acridone-bridged bis-triethoxysilane precursors in the presence of a template surfactant via evaporation-induced self-assembly (EISA). A muddy film containing small aggregates was obtained from the non-methylated precursor. The aggregate was formed by strong intermolecular hydrogen bonds between N–H and CO of the acridone groups during EISA. However, a transparent PMO film was successfully formed from the N-methylated precursor. Capping of the amine group hindered the intermolecular hydrogen bonds and effectively suppressed aggregate formation. The obtained acridone-bridged PMO film showed a visible light absorption band with an edge at 430 nm and fluorescence emission centered at 500 nm. Furthermore, doping of a fluorescent dye into the mesochannels of the acridone–PMO promoted efficient energy funneling from the framework acridone groups into the dye, resulting in a strong fluorescence emission centered at 600 nm from the dye.

Heterogeneous Catalysis for Water Oxidation by an Iridium Complex Immobilized on Bipyridine‐Periodic Mesoporous Organosilica

Heterogenization of metal-complex catalysts for water oxidation without loss of their catalytic activity is important for the development of devices simulating photosynthesis. In this study, efficient heterogeneous iridium complexes for water oxidation were prepared using bipyridine-bridged periodic mesoporous organosilica (BPy-PMO) as a solid chelating ligand. The BPy-PMO-based iridium catalysts (Ir-BPy-PMO) were prepared by postsynthetic metalation of BPy-PMO and characterized through physicochemical analyses. The Ir-BPy-PMOs showed high catalytic activity for water oxidation. The turnover frequency (TOF) values for Ir-BPy-PMOs were one order of magnitude higher than those of conventional heterogeneous iridium catalysts. The reusability and stability of Ir-BPy-PMO were also examined, and detailed characterization was conducted using powder X-ray diffraction, nitrogen adsorption, (13) C DD MAS NMR spectroscopy, TEM, and XAFS methods.

Synthesis of visible-light-absorptive and hole-transporting periodic mesoporous organosilica thin films for organic solar cells

Periodic mesoporous organosilica (PMO) thin films that possess both visible-light absorption and hole-transporting properties were synthesized from a newly designed 4,7-dithienyl-2,1,3-benzothiadiazole (DTBT)-bridged organosilane precursor using polystyrene-block-poly(ethylene oxide) (PS-b-PEO) as a template. DTBT-based PMO films were successfully obtained from the 100% organosilane precursor without the addition of other alkoxysilanes such as tetraethyoxysilane and by control of the tetrahydrofuran–ethanol solvent ratio and the PS/PEO group ratio in the block copolymer. The PMO films possess connected cage-like mesopores with diameters of ca. 15–20 nm, which could be derived from templating with spherical micelles. The PMO films exhibited absorption in the visible range between 400 and 650 nm and hole-transport mobility in the order of 10−5 cm2 V−1 s−1. We demonstrate that the PMO thin film functions as a p-type layer for organic solar cells by filling an n-type [6,6]-phenyl C61 butyric acid methyl ester into the mesopores.

Synthesis of 9,9′-spirobifluorene-based conjugated microporous polymers by FeCl3-mediated polymerization

An easy, safe and low-cost synthesis of conjugated microporous polymers and related carbonized microporous materials is highly desirable for practical use. Here, we report the synthesis of 9,9′-spirobifluorene-based conjugated microporous organic polymers (COPs) starting from unsubstituted spirobifluorene using an inexpensive FeCl3 mediator. Three kinds of COPs, synthesized by FeCl3-mediated oxidative polymerization, Friedel–Crafts polymerization, and competitive oxidative/Friedel–Crafts polymerization of the spirobifluorene precursor, showed large surface areas (940–1980 m2 g−1) and high micropore volumes (0.5–0.9 cm3 g−1). The COPs synthesized by competitive oxidative/Friedel–Crafts polymerization were found to show a high gas uptake ability which is almost comparable to that of the previously reported spirobifluorene-based microporous organic polymers prepared by Yamamoto polymerization using an expensive Ni catalyst. The COPs were easily transformed into microporous carbons by direct carbonization without the addition of any activating agents. The carbonization process enhanced the gas uptake ability of the COPs at low pressure (1 atm), although the surface area and micropore volume were almost unchanged or slightly decreased. The FeCl3-mediated competitive oxidative/Friedel–Crafts polymerization of non-functionalized aromatics would be a useful synthetic approach for an easy, safe and low-cost synthesis of conjugated microporous polymers.

Mesoporous organosilica nanotubes containing a chelating ligand in their walls

We report the synthesis of organosilica nanotubes containing 2,2′-bipyridine chelating ligands within their walls, employing a single-micelle-templating method. These nanotubes have an average pore diameter of 7.8 nm and lengths of several hundred nanometers. UV-vis absorption spectra and scanning transmission electron microscopy observations of immobilized nanotubes with an iridium complex on the bipyridine ligands showed that the 2,2′-bipyridine groups were homogeneously distributed in the benzene-silica walls. The iridium complex, thus, immobilized on the nanotubes exhibited efficient catalytic activity for water oxidation using Ce4+, due to the ready access of reactants to the active sites in the nanotubes.

Ab initio molecular orbital study on the excited states of [2.2]-, [3.3]-, and siloxane-bridged paracyclophanes.

Paracyclophanes are simple idealized model molecules for the study of interacting π-stacking systems. In this study, the excited states of [2.2]paracyclophane ([2.2]PCP), [3.3]paracyclophane ([3.3]PCP), and siloxane-bridged paracyclophane (SiPCP) are systematically investigated using the multiconfiguration quasi-degenerated perturbation theory (MCQDPT) method. The excited states of the alkyl- and silyl-substituted benzene monomers and benzene dimer, which can be regarded as the building blocks of paracyclophanes, are also examined at the same level of theory for more detailed understanding. The accuracy of the time-dependent density functional theory (TD-DFT) method required for excited state geometry optimization of the paracyclophanes is confirmed from calculations of the benzene dimer. The equilibrium distances between the benzene rings of the paracyclophanes in the first excited states are shorter than those in the ground state, and the benzene rings at the excited state optimized geometries are in an almost eclipsed parallel configuration, which indicates excimer formation. The calculated transition energies and oscillator strengths are generally in good agreement with the corresponding experimental results. A clear correlation between the excited state properties and the molecular structures is systematically demonstrated based on the calculation results for the substituted benzene monomers and benzene dimer. The transition energies of SiPCP are close to the corresponding absorption and fluorescence energies of the experimentally studied phenylene-silica hybrids, which indicates that the electronic properties of organic-silica hybrids, which is a new class of material with potential in photofunctional applications, can be approximated by simple siloxane-bridged cyclophane derivatives.

Enhanced sol–gel polymerization of organoallylsilanes by solvent effect

We investigated solvent effects on the acid-catalyzed deallylation of organoallylsilane precursors to identify mild sol–gel polymerization conditions. Organoallylsilanes are expected to be alternative precursors for preparation of functionalized organosilica hybrids but they undergo sol–gel polymerization with difficulty due to their low reactivity towards hydrolysis. Sol–gel polymerization of model organoallylsilane precursors was conducted in various organic solvents and deallylation was monitored by 1H NMR spectroscopy. The nature of the solvent was found to strongly influence the deallylation rate and a significant correlation was observed between reaction rate and solvent basicity, which suggests that proton activity is a key factor in enhancing the reaction rate. In particular, acetonitrile was found to most effectively enhance the rate, and it accelerated the formation of a spirobifluorene-bridged organosilica hybrid film from its allylsilane precursor under a mild acidic condition. This key finding can be generally utilized for the preparation of organoallylsilane-derived highly functionalized organosilica hybrids.

Re(bpy)(CO)3Cl Immobilized on Bipyridine-Periodic Mesoporous Organosilica for Photocatalytic CO2 Reduction

This paper describes the physicochemical properties of a rhenium (Re) complex [Re(bpy)(CO)3 Cl] immobilized on a bipyridine-periodic mesoporous organosilica (BPy-PMO) acting as a solid support. The immobilized Re complex generated a metal-to-ligand charge transfer absorption band at 400 nm. This wavelength is longer than that exhibited by Re(bpy)(CO)3 Cl in the polar solvent acetonitrile (371 nm) and is almost equal to that in nonpolar toluene (403 nm). The photocatalytic activity of this heterogeneous Re complex was lower than that of a homogeneous Re complex due to the reduced phosphorescence lifetime resulting from immobilization. However, the catalytic activity was enhanced by the co-immobilization of the ruthenium (Ru) photosensitizer [Ru(bpy)3 ]2+ on the PMO pore surfaces. Quantum chemical calculations suggest that electron transfer between the Ru and Re complexes occurs through interactions between the molecular orbitals in the pore walls. These results should have applications to the design of efficient heterogeneous CO2 reduction photocatalysis systems.

Transfer hydrogenation of nitrogen heterocycles using a recyclable rhodium catalyst immobilized on bipyridine-periodic mesoporous organosilica

Transfer hydrogenation of unsaturated nitrogen heterocycles using a rhodium catalyst immobilized on bipyridine-periodic mesoporous organosilica (BPy-PMO) is described. The immobilized catalyst was prepared by mixing [Cp*RhCl2]2 (Cp* = η5-C5Me5) with BPy-PMO powder in DMF at 60 °C and characterized by nitrogen adsorption measurements, solid-state NMR spectroscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. In the presence of the catalyst, a wide variety of unsaturated nitrogen heterocycles underwent transfer hydrogenation to afford the corresponding products in good yields. The immobilized catalyst could be readily recovered by centrifugation and reused several times in the transfer hydrogenation process.