Ken-ichi Yamanaka

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.

Efficient Light-Harvesting Antenna with a Multi-Porphyrin Cascade

A light-harvesting antenna 1 comprising three varieties of porphyrins, each having a different number of ethynyl groups at its meso positions, was designed and synthesized. Antenna 1 exhibits intense absorption throughout the visible region up to 700 nm. Steady-state and time-resolved fluorescence studies showed that singlet-excited-state energy transfer occurs from the peripheral porphyrins to the central porphyrin with >90% efficiency and rate constants on the order of 10(10) s(-1).

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.

Photoinduced Charge-Separation and Charge-Recombination Processes of Fullerene[60] Dyads Covalently Connected with Phenothiazine and Its Trimer

Photoinduced charge-separation and charge-recombination processes of fullerene[60] dyads covalently connected with phenothiazine and its trimer (PTZ n -C 60, n = 1 and 3) with a short amide linkage were investigated. A time-resolved fluorescence study provided evidence of charge separation via the excited singlet state of a C 60 moiety ( (1)C 60*), which displayed high efficiencies in various solvents; Phi (S) CS (quantum yield of charge separation via (1)C 60*) = 0.59 (toluene) to 0.87 (DMF) for PTZ 1-C 60 and 0.78 (toluene) to 0.91 (DMF) for PTZ 3-C 60. The transient absorption measurement with a 6 ns time resolution in the visible and near-IR regions showed evidence of the generation of radical ion pairs in relatively polar solvents for both dyads. In nonpolar toluene, only PTZ 1- (3)C 60* was observed for PTZ 1-C 60, whereas PTZ 3- (3)C 60* as well as the radical ion pair state in equilibrium were observed for PTZ 3-C 60. The radical ion pairs had relatively long lifetimes: 60 (DMF) to 910 ns ( o-dichlorobenzene) for (PTZ) 1 (*+)-C 60 (*-) and 230 (PhCN) to 380 ns ( o-dichlorobenzene) for (PTZ) 3 (*+)-C 60 (*-). The small reorganization energy (lambda) and the electronic coupling element (| V|) were estimated by the temperature dependence of the charge-recombination rates, i.e., lambda = 0.53 eV and | V| = 1.6 cm (-1) for (PTZ) 3 (*+)-C 60 (*-).

Energy and electron transfer from fluorescent mesostructured organosilica framework to guest dyes.

Energy and electron transfer from frameworks of nanoporous or mesostructured materials to guest species in the nanochannels have been attracting much attention because of their increasing availability for the design and construction of solid photofunctional systems, such as luminescent materials, photovoltaic devices, and photocatalysts. In the present study, energy and electron-transfer behavior of dye-doped periodic mesostructured organosilica films with different host-guest arrangements were systematically examined. Fluorescent tetraphenylpyrene (TPPy)-silica mesostructured films were used as a host donor. The location of guest perylene bisimide (PBI) dye molecules, acting as an acceptor, could be controlled on the basis of the molecular design of the PBI substituent groups. PBI dyes with bulky substituents and polar anchoring groups were located at the pore surface with low self-aggregation, which induced efficient energy or electron transfer because of the close host-guest arrangement. However, PBI dye with bulky and hydrophobic substituents was located in the center of template surfactant micelles; the fluorescence emission from the host TPPy groups was hardly quenched when the host-guest distance was longer than the critical Förster radius (ca. 4.5 nm). The relationship between the energy or electron-transfer efficiency and the location of guest species in the channels of mesostructured organosilica was first revealed by molecular design of the PBI substituents.

Over 100 ns intrinsic radiative recombination lifetime in type II InAs/GaAs1−xSbx quantum dots

We report very long intrinsic radiative recombination lifetime τrad in type II InAs quantum dots embedded in GaAs1−xSbx. The dependence of photoluminescence (PL) decay time τPL on both the Sb composition (x = 0–0.18) and excitation intensity (38–460 mW/cm2) was systematically investigated by time-resolved PL measurements with a time-correlated single-photon counting method. All PL decay curves exhibited non-exponential profiles, and τPL was strongly dependent on the excitation intensity. These properties were well explained by solving rate equations of carrier density with neglecting nonradiative process, in which τrad is inversely proportional to carrier density. The 18% Sb sample exhibited a τPL of over 100 ns under weak excitation, which is longer than twice the previously reported values. We evaluated the value of τrad in InAs/GaAs1−xSbx QDs relative to that in type I InAs/GaAs QDs based on an effective mass approximation and found that the observed extremely long τPL corresponds to τrad.

Intramolecular photoinduced electron-transfer processes in buta-1,3-diynyl-benzene-linked porphyrin-fullerene dyad

Intramolecular electron-transfer process of porphyrin-fullerene dyad linked by phenyl buta-1,3-diynyl-phenyl unit (ZnP-sp-C60) was studied by laser flash photolysis. Picosecond fluorescence lifetime and transient absorption measurements revealed that photoinduced charge-separation takes place via the excited singlet state (1ZnP*) with the rate constant of (1-2) × 1010s−1. For the charge recombination, about a half of the radical-ion pair decayed quickly with 2.9 × 109s−1 as evaluated from picosecond transient absorption measurements, whereas the remaining half was long-lived with slow decay (1.6 × 106s−1) as estimated from nanosecond transient absorption measurements. The lifetime of the radical-ion pair of ZnP-sp-C60 was longer than those of directly connected dyads with a buta-1,3-diynyl bridge and buta-1,3-diynyl-phenyl bridge by the insertion of an extra phenyl group in addition to the pyrrodino ring.

Energy transfer between Ni2+ sensitizers and Er3+ emitters in broadband-sensitive upconverters La(Ga,Sc,In)O3:Er,Ni,Nb

We have analyzed broadband-sensitive upconversion from 1.1–1.6 μm to 0.98 μm in La(Ga,Sc,In)O3 doped with Er, Ni, and Nb, which could significantly boost the conversion efficiency of crystalline silicon solar cells, in particular, energy transfer from the Ni2+ sensitizers to the Er3+ emitters and back transfer from the Er3+ to the Ni2+. We have compared these processes and the resultant upconversion emission intensities depending on the host material compositions. With increasing the bond length between the Ni2+ and surrounding oxygen ions, the Ni2+ emission band located at around 1.2–1.6 μm red-shifts and hence overlaps more significantly with the Er3+ absorption band ranging from 1.45 μm to 1.6 μm, resulting in more rapid energy transfer from the Ni2+ to the Er3+. However, back energy transfer from the Er3+ to the Ni2+ deteriorates the performance more considerably, because of more significant overlap between the Er3+ emission band and Ni2+ absorption band. This trade-off relationship strongly affects t...

Ionic Valence Change of Metal Ions in Solution by Femtosecond Laser Excitation Accompanied by White-Light Laser

Three lanthanide ions (Ln^(3+)), Ln = Eu, Sm, and Yb, and two transition metals, Fe^(3+) and Ag^+, were found to be reduced to the corresponding Ln^(2+), Fe^(2+), and Agn in methanol or aqueous solution upon irradiation with intense femtosecond laser pulses. The major excitation wavelength was 800 nm and single-photon-non-resonant with the electronic transitions of metal ion solutions. Laser pulses with wavelengths of 970, 1190, and 1930 nm were used for particular cases. Whenever the white-light laser was generated, the reductions were observed. The reduction mechanisms would be explained in terms of self-focusing, solvated electron formation followed by trapping the electron. The electron ejection under focused beam conditions in solution has been known to be accompanied by white-light laser. In the exceptional case of Fe^(3+) at 800 nm, two-photon excitation of the charge transfer state followed by the reduction would be operative. Fe^(2+) was detected even with an intensity lower than the threshold of the white-light laser generation.

Synthesis and Photoinduced Energy‐ and Electron‐Transfer Processes of C60–Oligothienylenevinylene–C70 Dumbbell Compounds

Unsymmetric dumbbell molecules based on N-methylpyrrolidine[60]fullerene, oligothienylenevinylenes (nTV; n=2, 4), and N-methylpyrrolidine[70]fullerene, namely, C(60) -nTV-C(70) were synthesized and their photophysical properties were studied. In nonpolar solvents, photoinduced energy-transfer process predominantly takes place from the singlet excited state of nTV to C(60) and C(70) , as was confirmed by time-resolved emission and transient absorption spectroscopy. In polar solvent, charge-separation processes take place instead of energy transfer. The generated charge-separated radical-ion pairs decay to the neutral molecules by a fast charge-recombination process; for n=4, a rate constant of 2×10(7) s(-1) and lifetime of 50 ns were evaluated.