Triggered single-photon emission and cross-correlation properties in InAlAs quantum dot

Triggered single-photon generation from InAlAs quantum dot (QD) was demonstrated for the first time. Emitted photon energy coincides with high detection efficiency range of Si single-photon detectors, which is highly suitable for free-space communication. Single-QD spectroscopy and crossed photon correlation measurements unambiguously revealed that several emitting lines observed in a single mesa structure originated from the identical QD, and two temporary competing decay processes associated with neutral states and charged states were identified. Presence of the competing process is also inferred from an analysis of steady-state photoluminescence intensities. Formation process of charged exciton in QD is also discussed.     

Boundary Harnack Principle for <Emphasis Type="Italic">p</Emphasis>-harmonic Functions in Smooth Euclidean Domains

We establish a scale-invariant version of the boundary Harnack principle for p-harmonic functions in Euclidean C ^1,1-domains and obtain estimates for the decay rates of positive p-harmonic functions vanishing on a segment of the boundary in terms of the distance to the boundary. We use these estimates to study the behavior of conformal Martin kernel functions and positive p-superharmonic functions near the boundary of the domain. H. A. was partially supported by Grant-in-Aid for (B) (2) (No. 15340046) Japan Society for the Promotion of Science. N. S. was partially supported by NSF grant DMS-0355027. X. Z. was partially supported by the Taft foundation.     

Hydrogen Production from a Methanol–Water Solution Catalyzed by an Anionic Iridium Complex Bearing a Functional Bipyridonate Ligand under Weakly Basic Conditions

An efficient catalytic system for the production of hydrogen from a methanol–water solution has been developed using a new anionic iridium complex bearing a functional bipyridonate ligand as a catalyst. This system can be operated under mild conditions [weakly basic solution (0.046 mol L^?1 NaOH) below 100 °C] without the use of an additional organic solvent. Long‐term continuous hydrogen production from a methanol–water solution catalyzed by the anionic iridium complex was also achieved.     

Coronenetetraimide‐Centered Cruciform Pentamers Containing Multiporphyrin Units: Synthesis and Sequential Photoinduced Energy‐ and Electron‐Transfer Dynamics

A series of coronenetetraimide (CorTIm)‐centered cruciform pentamers containing multiporphyrin units, in which four porphyrin units are covalently linked to a CorTIm core through benzyl linkages, were designed and synthesized to investigate their structural, spectroscopic, and electrochemical properties as well as photoinduced electron‐ and energy‐transfer dynamics. These systems afforded the first synthetic case of coroneneimide derivatives covalently linked with dye molecules. The steady‐state absorption and electrochemical results indicate that a CorTIm and four porphyrin units were successfully characterized by the corresponding reference monomers. In contrast, the steady‐state fluorescence measurements demonstrated that strong fluorescence quenching relative to the corresponding monomer units was observed in these pentamers. Nanosecond laser flash photolysis measurements revealed the occurrence of intermolecular electron transfer from triplet excited state of zinc porphyrins to CorTIm. Femtosecond laser‐induced transient absorption measurements for excitation of the CorTIm unit clearly demonstrate the sequential photoinduced energy and electron transfer between CorTIm and porphyrins, that is, occurrence of the initial energy transfer from CorTIm (energy donor) to porphyrins (energy acceptor) and subsequent electron transfer from porphyrins (electron donor) to CorTIm (electron acceptor) in these pentamers, whereas only the electron‐transfer process from porphyrins to CorTIm was observed when we mainly excite porphyrin units. Finally, construction of high‐order supramolecular patterning of these pentamers was performed by utilizing self‐assembly and physical dewetting during the evaporation of solvent.     

Tuning the Redox Properties of a Nonheme Iron(III)–Peroxo Complex Binding Redox‐Inactive Zinc Ions by Water Molecules

Redox‐inactive metal ions play important roles in tuning chemical properties of metal–oxygen intermediates. Herein we report the effect of water molecules on the redox properties of a nonheme iron(III)–peroxo complex binding redox‐inactive metal ions. The coordination of two water molecules to a Zn²⁺ ion in (TMC)Fe^III‐(O₂)‐Zn(CF₃SO₃)₂ ( 1 ‐Zn²⁺) decreases the Lewis acidity of the Zn²⁺ ion, resulting in the decrease of the one‐electron oxidation and reduction potentials of 1 ‐Zn²⁺. This further changes the reactivities of 1 ‐Zn²⁺ in oxidation and reduction reactions; no reaction occurred upon addition of an oxidant (e.g., cerium(IV) ammonium nitrate (CAN)) to 1 ‐Zn²⁺, whereas 1 ‐Zn²⁺ coordinating two water molecules, (TMC)Fe^III‐(O₂)‐Zn(CF₃SO₃)₂‐(OH₂)₂ [ 1 ‐Zn²⁺‐(OH₂)₂], releases the O₂ unit in the oxidation reaction. In the reduction reactions, 1 ‐Zn²⁺ was converted to its corresponding iron(IV)–oxo species upon addition of a reductant (e.g., a ferrocene derivative), whereas such a reaction occurred at a much slower rate in the case of 1 ‐Zn²⁺‐(OH₂)₂. The present results provide the first biomimetic example showing that water molecules at the active sites of metalloenzymes may participate in tuning the redox properties of metal–oxygen intermediates.     

Lewis Acid Coupled Electron Transfer of Metal–Oxygen Intermediates

Redox‐inactive metal ions and Brønsted acids that function as Lewis acids play pivotal roles in modulating the redox reactivity of metal–oxygen intermediates, such as metal–oxo and metal–peroxo complexes. The mechanisms of the oxidative C?H bond cleavage of toluene derivatives, sulfoxidation of thioanisole derivatives, and epoxidation of styrene derivatives by mononuclear nonheme iron(IV)–oxo complexes in the presence of triflic acid (HOTf) and Sc(OTf)₃ have been unified as rate‐determining electron transfer coupled with binding of Lewis acids (HOTf and Sc(OTf)₃) by iron(III)–oxo complexes. All logarithms of the observed second‐order rate constants of Lewis acid‐promoted oxidative C?H bond cleavage, sulfoxidation, and epoxidation reactions of iron(IV)–oxo complexes exhibit remarkably unified correlations with the driving forces of proton‐coupled electron transfer (PCET) and metal ion‐coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes were taken into account. The binding of HOTf and Sc(OTf)₃ to the metal–oxo moiety has been confirmed for Mn^IV–oxo complexes. The enhancement of the electron‐transfer reactivity of metal–oxo complexes by binding of Lewis acids increases with increasing the Lewis acidity of redox‐inactive metal ions. Metal ions can also bind to mononuclear nonheme iron(III)–peroxo complexes, resulting in acceleration of the electron‐transfer reduction but deceleration of the electron‐transfer oxidation. Such a control on the reactivity of metal–oxygen intermediates by binding of Lewis acids provides valuable insight into the role of Ca²⁺ in the oxidation of water to dioxygen by the oxygen‐evolving complex in photosystem II.     

Phase structure change and ER effect in liquid crystalline polymer/dimethylsiloxane blends

The mechanism of the electrorheological (ER) effect in two types of liquid crystalline polymer (LCP)/dimethylsiloxane (DMS) blends was investigated by rheological measurements and by structure observation under electric field and shear flow. The results show that the phase structures of these immiscible blends can be categorized into slipping (low viscosity) and non-slipping (high viscosity) states. In the non-slipping state, higher viscosity LCP domains connect the electrodes. In the slipping state, on the other hand, LCP domains do not connect the electrodes and the shear is mainly confined in the lower viscosity DMS domains. The ER effect (electrically induced viscosity increase) originates from the electrically induced slipping to non-slipping transition. In one of the blends, the ER effect occurs only at high shear rate, since this blend is in non-slipping state even under no field if the shear rate is low. Received: 29 April 1997 Accepted: 3 November 1997     

Formation of double helix self-assembled monolayers of ethynylhelicene oligomer disulfides on gold surfaces

Optically active ethynylhelicene pentamers and hexamers linked by disulfide bonds were synthesized. They formed self-assembled monolayers (SAMs) with double helix structure on gold surfaces, which were analyzed by infrared reflection-absorption spectroscopy (IR-RAS), quartz crystal microbalance (QCM), surface plasmon resonance (SPR), and circular dichroism (CD). Double helix SAMs could be formed on gold surfaces either from double helices or random coils in solution. The double helices on the surface were more stable than in solution. This result suggested the presence of strong intercomplex interactions between double helix complexes on the surface.     

Synthesis and photophysics of silicon phthalocyanine-perylenebisimide triads connected through rigid and flexible bridges

Three new bisperylenebisimide-silicon phthalocyanine triads [(PBI)(2)-SiPcs 1, 2, and 3] connected with either rigid or flexible bridges were synthesized and characterized. A new synthetic approach to connect SiPc and PBI moieties through click chemistry produced triad 3 with an 80% yield. In (PBI)(2)-SiPc 1, PBI and SiPc are orthogonal and were connected with a rigid connector; triads 2 and 3 bear flexible aliphatic bridges, resulting in a tilted (2) or nearly parallel arrangement (3) of PBI and SiPc. Photoinduced intramolecular processes in these (PBI)(2)-SiPcs were studied and the results are compared with those of the reference compounds SiPc-ref and PBI-ref. The occurrence of electron-transfer processes between the SiPc and PBI units was confirmed by time-resolved emission and transient absorption techniques. Charge-separated (CS) states with lifetimes of 0.91, 1.3 and 2.0 ns for triads 1, 2, and 3, respectively, were detected using femtosecond laser flash photolysis. Upon the addition of Mg(ClO(4))(2), an increase in the lifetime of the CS states to 59, 110 and 200 μs was observed for triads (PBI)(2)-SiPcs 1, 2, and 3, respectively. The energy of the CS state (SiPc(·+)-PDI(·-)/Mg(2+)) is lower than the energy of both silicon phthalocyanine ((3)SiPc*-PDI) and perylenebisimide (SiPc-(3)PDI*) triplet excited states, which decelerates the metal ion-decoupled electron-transfer process for charge recombination to the ground state, thus increasing the lifetime of the CS state. The photophysics of the three triads demonstrate the importance of the rigidity of the spacer and the orientation between donor and acceptor units.     

Photochemical activation of ruthenium(II)-pyridylamine complexes having a pyridine-N-oxide pendant toward oxygenation of organic substrates

Ruthenium(II)-acetonitrile complexes having η(3)-tris(2-pyridylmethyl)amine (TPA) with an uncoordinated pyridine ring and diimine such as 2,2'-bipyridine (bpy) and 2,2'-bipyrimidine (bpm), [Ru(II)(η(3)-TPA)(diimine)(CH(3)CN)](2+), reacted with m-chloroperbenzoic acid to afford corresponding Ru(II)-acetonitrile complexes having an uncoordinated pyridine-N-oxide arm, [Ru(II)(η(3)-TPA-O)(diimine)(CH(3)CN)](2+), with retention of the coordination environment. Photoirradiation of the acetonitrile complexes having diimine and the η(3)-TPA with the uncoordinated pyridine-N-oxide arm afforded a mixture of [Ru(II)(TPA)(diimine)](2+), intermediate-spin (S = 1) Ru(IV)-oxo complex with uncoordinated pyridine arm, and intermediate-spin Ru(IV)-oxo complex with uncoordinated pyridine-N-oxide arm. A Ru(II) complex bearing an oxygen-bound pyridine-N-oxide as a ligand and bpm as a diimine ligand was also obtained, and its crystal structure was determined by X-ray crystallography. Femtosecond laser flash photolysis of the isolated O-coordinated Ru(II)-pyridine-N-oxide complex has been investigated to reveal the photodynamics. The Ru(IV)-oxo complex with an uncoordinated pyridine moiety was alternatively prepared by reaction of the corresponding acetonitrile complex with 2,6-dichloropyridine-N-oxide (Cl(2)py-O) to identify the Ru(IV)-oxo species. The formation of Ru(IV)-oxo complexes was concluded to proceed via intermolecular oxygen atom transfer from the uncoordinated pyridine-N-oxide to a Ru(II) center on the basis of the results of the reaction with Cl(2)py-O and the concentration dependence of the consumption of the starting Ru(II) complexes having the uncoordinated pyridine-N-oxide moiety. Oxygenation reactions of organic substrates by [Ru(II)(η(3)-TPA-O)(diimine)(CH(3)CN)](2+) were examined under irradiation (at 420 ± 5 nm) and showed selective allylic oxygenation of cyclohexene to give cyclohexen-1-ol and cyclohexen-1-one and cumene oxygenation to afford cumyl alcohol and acetophenone.