Takao Tani

Light harvesting by a periodic mesoporous organosilica chromophore.

Light aqueduct: Periodic mesoporous organosilica exhibits strong light absorption due to densely packed organic chromophores within the pore walls. Light energy absorbed by 125 biphenyl groups in the pore walls is funneled into a single coumarin 1 molecule in the mesochannels with almost 100% quantum efficiency, and results in significant enhancement of emission from the coumarin 1 dye.

Isothermally Reversible Fluorescence Switching of a Mechanochromic Perylene Bisimide Dye

Isothermally rewritable fluorescence mechanochromism has been realized for a perylene bisimide dye with bulky and flexible substituents. Fluorescent patterns drawn by mechanical stimuli can be erased by thermal stimuli, treatment with solvent vapors, or spontaneous structural transition from orange-fluorescent to green-fluorescent states. The isothermal fluorescence switching of solid dye films is applicable to displays and sensory materials.

Luminescent periodic mesoporous organosilicas

Progress in the development of periodic mesoporous organosilicas (PMOs) bearing luminescent organic groups is reviewed and directions for future research are highlighted. The inclusion of luminescent organic groups in the PMO framework allows for the preparation of materials with dissimilar luminescent groups in two spatially separated regions; the framework and the mesopore channels. Mesoporous silicas, in contrast, only bear luminescent dyes in the channels of the material structure. In multi-dye systems, the transfer of excitation energy from the framework donors to acceptors in the mesochannels is observed. Certain PMOs, such as biphenyl PMO, exhibit efficient light absorption due to the dense packing of chromophores in the framework, and high luminescence quantum yields, demonstrating the potential of luminescent PMOs as a lighting technology. Fluorescence studies have revealed that the interaction among bridging organic groups in PMOs differs between crystal-like and amorphous frameworks. These recent developments highlight the potential of luminescent PMOs as a new technology, which should be supported by further investigation of the optical properties and functionalization of PMOs.

Hole-transporting periodic mesostructured organosilica.

Hole-transporting framework is formed by surfactant-templated sol-gel polycondensation of an electroactive phenylenevinylene-based organosilane precursor. Molecular geometry of the three-armed precursor contributes to both formation of periodic mesostructures and introduction of hole conductivity in the organosilica hybrids. Electroactive organosilicas with mesopores and large surface areas have great potentials for novel photovoltaic and photocatalytic systems.

Efficient Visible‐Light Emission from Dye‐Doped Mesostructured Organosilica

2009 WILEY-VCH Verlag Gmb Much attention has been paid to luminescent materials for use in various devices, such as fluorescent lamps, light-emitting diodes (LEDs), and plasma displays. A large variety of phosphors with various excitation and emission wavelengths have been developed for these specific applications. Most commercial phosphors are transition metals or rare-earth metal-ion-doped metal-oxide powders because of their high chemical and physical stability and easy handling. However, scientific focus has also been placed on fluorescent polymers and organic–inorganic hybrid materials because of their unique features such as transparency (no light-scattering loss), flexibility in shape, and easy processing, which can broaden the use of these luminescent materials. Dye-doped fluorescent polymers have been widely investigated for electroluminescent layers of organic LEDs. The use of fluorescent polymers is suited for the fabrication of large-area devices because they can be deposited by solution processes. The optical properties of p-conjugated polymer materials can be fine-tuned by manipulating the chemical structures and incorporating various organic and organometallic dyes. The development of polymer-based phosphors has been accompanied by an increased understanding of the emission processes such as charge transportation in polymer films and energy transfer from emissive host polymers to guest dye molecules. On the other hand, periodic mesostructured and mesoporous organosilica hybrids, which are synthesized by surfactanttemplated polycondensation of bridged organosilane precursors ((RO)3Si R Si(OR)3), have great potential for various functional materials including luminescent applications. For mesostructured organosilicas, various organic bridging groups ( R ) can be densely and covalently embedded within the mesochannel walls based on robust siloxane networks, suggesting that the organosilica frameworks themselves as well as the inner parts of the mesochannels are available for functionalization. Mesostructured organosilica materials are also advantageous for optical applications because they can be obtained as transparent films by acidic sol–gel polycondensation. Recently, we examined the optical properties of mesostructured organosilica films consisting of aromatic–silica hybrid frameworks and found that biphenyl–silica mesostructured films showed exceptionally strong absorption and high photoluminescence quantum efficiency. Expansion of the aromatic and p-conjugated organic bridges has led to a red shift of the absorption bands and resulted in efficient fluorescence emission in the visible-light region. These recent developments can broaden the potential applications of periodic mesostructured organosilicas as visible-light-responsive photofunctional materials. Well-defined mesostructures are also appropriate for the confinement of dyes, suggesting the versatility of mesostructured materials for luminescence and efficient energy transfer. We recently reported efficient fluorescence resonance energy transfer (FRET) from an organosilica framework to a fluorescent dye incorporated into the mesopores, which occurred in coumarin-doped biphenyl–silica mesoporous materials. In periodic mesostructures, the excitation energy of the biphenyl groups is completely transferred to the coumarin dye in the mesopores with almost no loss even at low dye levels (only 0.80 mol%), that is, mesoporous biphenyl–silica hybrids function as hierarchically structured light-harvesting antenna. In addition, ordered mesochannels are advantageous for a high dispersion of fluorescent dyes and stabilization of their optical properties. Such unique features of mesostructured organosilicas can lead the way to multicolor fluorescence with high quantum yields. Here we report on color-tunable visible-light emission with high quantum yields that can be achieved in fluorescent dye-doped organosilica mesostructured films. Doping of fluorescent dyes into the mesopores of fluorescent organosilicas should induce efficient FRET from the organosilica framework to the guest dye molecules, which can thus be utilized to tune the emission colors. We can also expect an enhancement of the fluorescence quantum yield by the FRET process when the quantum yield of the doped dye (energy acceptor) is higher than that of the fluorescent host film (energy donor). In order to realize this, we selected molecular components of mesostructured organosilica films (Fig. 1) and designed an energy transfer scheme as shown in Figure 2. A fluorescent host mesostructured film (mOPV-F) was prepared using a hexyloxy-substituted oligo(phenylenevinylene) (OPV) precursor (OPV-C6), tetraethyl orthosilicate (TEOS), and a template-surfactant P123. This film shows strong blue-light emission and definitely possesses a mesochannel array with a periodicity of 10–14 nm as shown in Figure 2a and reported in the literature. The surfactant P123 was chosen for the present light-emitting system because it tends to direct the organization of silica species not into lamellae but into mesochannel arrays. Guest molecules are likely to be well dispersed in these

Fluorescence emission from 2,6-naphthylene-bridged mesoporous organosilicas with an amorphous or crystal-like framework.

We report that 2,6-naphthylene-bridged periodic mesoporous organosilicas exhibit unique fluorescence behavior that reflects molecular-scale periodicities in the framework. Periodic mesoporous organosilicas consisting of naphthalene-silica hybrid frameworks were synthesized by hydrolysis and condensation of a naphthalene-derived organosilane precursor in the presence of a template surfactant. The morphologies and meso- and molecular-scale periodicities of the organosilica materials strongly depend on the synthetic conditions. The naphthalene moieties embedded within the molecularly ordered framework exhibited a monomer-band emission, whereas those embedded within the amorphous framework showed a broad emission attributed to an excimer band. These results suggest that the naphthalene moieties fixed within the crystal-like framework are isolated in spite of their densely packed structure, different from conventional organosilica frameworks in which only excimer emission was observed for both the crystal-like and amorphous frameworks at room temperature. This key finding suggests a potential to control interactions between organic groups and thus the optical properties of inorganic/organic hybrids.

Ab initio studies of aromatic excimers using multiconfiguration quasi-degenerate perturbation theory.

The aromatic excimers of benzene, naphthalene, anthracene, pyrene, and perylene are systematically investigated using the multiconfiguration quasi-degenerate perturbation theory (MCQDPT) method, which is one of high-level ab initio quantum chemical methods. The reference configuration space for MCQDPT is carefully designed for an appropriate description of the target electronic state with a tractable computational cost. The dimers with eclipsed parallel arrangement are investigated. The basis set dependence of the selected spectroscopic parameters is examined for the benzene and naphthalene dimers, and that of the excimer binding energy is found to be significant. In contrast, the equilibrium intermolecular distance and excimer fluorescence energy are less sensitive to the size of the basis sets used, and they agree with the corresponding experimental values, even with a nonextensive basis set size. The calculated spectroscopic parameters for anthracene, pyrene, and perylene dimers are also in good agreement with the experimental results. The electronic properties of the excimers are discussed in relation to those of the corresponding monomers. The wave functions of the excimers are analyzed in detail to clarify the origin of the attractive nature between the two monomers.

A periodic mesoporous organosilica-based donor-acceptor system for photocatalytic hydrogen evolution.

A new solid-sate donor-acceptor system based on periodic mesoporous organosilica (PMO) has been constructed. Viologen (Vio) was covalently attached to the framework of a biphenyl (Bp)-bridged PMO. The diffuse reflectance spectrum showed the formation of charge-transfer (CT) complexes of Bp in the framework with Vio in the mesochannels. The transient absorption spectra upon excitation of the CT complexes displayed two absorption bands due to radical cations of Bp and Vio species, which indicated electron transfer from Bp to Vio. The absorption bands slowly decayed with a half-decay period of approximately 10 mus but maintained the spectral shape, thereby suggesting persistent charge separation followed by recombination. To utilize the charge separation for photocatalysis, Vio-Bp-PMO was loaded with platinum and its photocatalytic performance was tested. The catalyst successfully evolved hydrogen with excitation of the CT complexes in the presence of a sacrificial agent. In contrast, reference catalysts without either Bp-PMO or Vio gave no or little hydrogen generation, respectively. In addition, a homogeneous solution system of Bp molecules, methylviologen, and colloidal platinum also evolved no hydrogen, possibly due to a weaker electron-donating feature of molecular Bp than that of densely packed Bp in Bp-PMO. These results indicated that densely packed Bp and Vio are essential for hydrogen evolution in this system and demonstrated the potential of PMO as the basis for donor-acceptor systems suitable for photocatalysis.

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.

Crystal-like periodic mesoporous organosilica bearing pyridine units within the framework

Periodic mesoporous organosilica with densely packed pyridine units within the framework and crystal-like molecular-scale periodicity was synthesized. The framework pyridines were chemically active and fully accessible for protonation and Cu(2+) adsorption.