Seed-assembly-mediated fabrication and application of highly branched gold nanoshells having hollow and porous morphologies.

Highly branched gold nanoshells (BAuNSs) having hollow and porous morphologies have been fabricated via a seed-assembly-mediated strategy. Gold seed assemblies can be prepared by removal of SiO2nanotemplates with help of polyvinylpyrrolidone (PVP) molecules, which weakly link gold nanoparticles together even after SiO2etching. L-3,4-dihydroxy phenylalanine (L-DOPA) and AgNO3are employed as shape-directing agents to induce the anisotropic growth of gold. BAuNSs exhibit 7.4 and 4.4 times stronger activities than SiO2@Au nanoparticles in catalysis and surface-enhanced Raman scattering (SERS) applications, respectively, due to their large surface areas and numerous hot spots. It is necessary to find the optimal amount of gold deposition in fabrication to effectively utilize the hollow and porous morpologies of BAuNSs for catalysis and SERS applications. Overgrown nanobranches can fill the nanopores and nanogaps of BAuNSs, resulting in decrease of activities in applications. Overall, the seed-assembly-mediated fabrciation can be employed to produce plasmonic nanostructures having unique morphologies and high application activities.

Boosting Visible-Light Photocatalytic Redox Reaction by Charge Separation in SnO2 /ZnSe(N2 H4 )0.5 Heterojunction Nanocatalysts.

In this work, environmentally friendly photocatalysts with attractive catalytic properties are reported that have been prepared by introducing SnO2 quantum dots (QDs) directly onto ZnSe(N2 H4 )0.5 substrates to induce advantageous charge separation. The SnO2 /ZnSe(N2 H4 )0.5 nanocomposites could be easily synthesized through a one-pot hydrothermal process. Owing to the absence of capping ligands, the attached SnO2 QDs displayed superior photocatalytic properties, generating many exposed reactive surfaces. Moreover, the addition of a specified amount of SnO2 boosted the visible-light photocatalytic activity; however, the presence of excess SnO2 QDs in the substrate resulted in aggregation and deteriorated the performance. The spectroscopic data revealed that the SnO2 QDs act as a photocatalytic mediator and enhance the charge separation within the type II band alignment system of the SnO2 /ZnSe(N2 H4 )0.5 heterojunction photocatalysts. The separated charges in the heterojunction nanocomposites promote radical generation and react with pollutants, resulting in enhanced photocatalytic performance.

Photothermal structural modification of porous gold nanoshells via pulsed-laser irradiation: effects of laser wavelengths and surface conditions.

Pulsed lasers are promising candidates for fabricating plasmonic nanoparticles having unique structures, and there is a strong need for studies on the detailed effects of various experimental conditions on laser-induced fabrications. In this work, we demonstrate the effects of laser wavelengths and nanoparticle surface conditions, as well as laser fluences, in the structural modification of porous gold nanoshells induced by picosecond pulse irradiation. Laser wavelengths play a critical role in the modification because irradiating laser pulses excite not only porous gold nanoshells but also gold nanospheres, which have been produced via the melting of irradiated porous gold nanoshells. Significantly different localized surface plasmon resonances of gold nanospheres and porous gold nanoshells make the effect of laser wavelengths noticeable. The polyvinylpyrrolidone (PVP) concentration of colloid containing porous gold nanoshells also modifies the deformation of the structures. Gold nanostructures become positively charged by the irradiation, strengthening gold-PVP attraction. The stronger binding affinity of PVP is considered to reduce the deformation of irradiated porous gold nanoshells.

Formation and decay of charge carriers in aggregate nanofibers consisting of poly(3-hexylthiophene)-coated gold nanoparticles.

Thin nanofibers (NFs) of J-dominant aggregates with a thickness of 15 nm and thick NFs of H-dominant aggregates with a thickness of 25 nm were fabricated by the self-assembly of poly(3-hexylthiophene)-coated gold nanoparticles. The formation and decay dynamics of the charge carriers, which are dependent on the aggregate types of NFs, was investigated by time-resolved emission and transient-absorption spectroscopy. With increasing excitation energy, the fraction of the fast emission decay component decreased, suggesting that the fast formation of polaron pairs (PP), localized (LP), and delocalized polarons (DP) results from higher singlet exciton states produced by the singlet fusion. The faster decay dynamics of DP and LP in the thick NFs than in thin NFs is due to the increased delocalization of DP and LP. As the interchain aggregation is weaker than intrachain aggregation, PP decays faster in thin NFs than in thick NFs. In both thin and thick NFs, although triplet (T1) excitons were barely observed with excitation at 532 nm on a nanosecond time scale, they were observed with excitation at 355 nm, showing that T1 excitons within NFs are generated mainly through the singlet fission from a higher singlet exciton state rather than through intersystem crossing.

Electrophoretic separation of gold nanoparticles according to bifunctional molecules-induced charge and size.

Gold nanospheres modified with bifunctional molecules have been separated and characterized by using agarose gel electrophoresis as well as optical spectroscopy and electron microscopy. The electrophoretic mobility of a gold nanosphere capped with 11-mercaptoundecanoic acid (MUA) has been found to depend on the number of MUA molecules per gold nanosphere, indicating that it increases with the surface charge of the nanoparticle. The extinction spectrum of gold nanospheres capped with MUA at an MUA molecules per gold nanosphere value of 1000 and connected via 1,6-hexanedithiol (HDT) decreases by 33% in magnitude and shifts to the red as largely as 22 nm with the increase of the molar ratio of HDT to MUA (R(HM)). Gold nanospheres capped with MUA and connected via HDT have been separated successfully using gel electrophoresis and characterized by measuring reflectance spectra of discrete electrophoretic bands directly in the gel and by monitoring transmission electron microscope images of gold nanoparticles collected from the discrete bands. Electrophoretic mobility has been found to decrease substantially with the increment of HDT to MUA, indicating that the size of aggregated gold nanoparticles increases with the concentration of HDT.

Laser-induced silver nanojoining of gold nanoparticles.

Gold nanoparticles have been silver-joined to fabricate nanowires by irradiating gold nanospheres of 25 nm in diameter and silver nanospheres of 8 nm in diameter held together on a carbon-coated copper grid with a 30 ps laser pulse of 532 nm for 20 min at a fluence of 3.0 mJ/cm2. Laser-induced nanojoining of silver nanoparticles as well as that of gold nanoparticles has also been carried out by varying the wavelength and fluence of irradiation laser pulses. Irradiation at an optimum condition of laser fluence is essential for the proper silver nanojoining of gold nanospheres to produce gold@silver core-shell composite nanowires. The excitation of the surface plasmon resonances of the base-metallic gold nanospheres rather than the filler-metallic silver nanospheres paves the way for the silver nanojoining of gold nanoparticles.

Excited-state hydrogen relay along a blended-alcohol chain as a model system of a proton wire: deuterium effect on the reaction dynamics.

The excited-state deuteron transfer (ESDT) of deuterated 7-hydroxyquinoline (7DQ) along a heterogeneous hydrogen (H)-bonded chain composed of two deuterated alcohol (ROD) molecules having different acidities, as a model system of a proton wire consisting of diverse amino acids, has been investigated. To understand dynamic differences between deuteron transfer and proton transfer, solvent-inventory experiments have been performed with variation of the combination as well as the composition of alcohols in a H-bonded mixed-alcohol chain. Deuteron transfer from the adjacent ROD molecule to the basic imino group of 7DQ via tunneling, which is the rate-determining step, initiates ESDT, and subsequent barrierless deuteron relay from the acidic enolic group of 7DQ to the alkoxide moiety along the H-bonded chain completes ESDT. Whereas the acceleration of the reaction has been observed in excited-state proton transfer because of the accumulated proton-donating abilities of two alcohol molecules in a H-bonded chain by a push-ahead effect, such acceleration is not observed in ESDT. Because the energy barrier of deuteron relay is much higher than that of proton relay due to the low zero-point energy of 7DQ·(ROD)(2) and a deuteron is twice as heavy as a proton, it is hard for a deuteron to pass through the barrier via tunneling. Moreover, both the H-bonding ability and the acidity of ROD molecules are so weak that their deuteron-donating abilities cannot be accumulated at the rate-determining step of ESDT. Consequently, the rate constant of ESDT is determined mostly by the acidity of the ROD molecule H-bonded directly to the imino group of 7DQ.

Excited-state proton-relay dynamics of 7-hydroxyquinoline controlled by solvent reorganization in room temperature ionic liquids.

The excited-state triple proton relay of 7-hydroxyquinoline (7HQ) along a hydrogen-bonded methanol chain in room temperature ionic liquids (RTILs) has been investigated using picosecond time-resolved fluorescence spectroscopy. The rate constant of the proton relay in a methanol-added RTIL is found to be slower by an order of magnitude than that in bulk methanol and to have unity in its kinetic isotope effect. These suggest that the excited-state tautomerization dynamics of 7HQ in methanol-added RTILs is mainly controlled by the solvent reorganization dynamics to form a cyclically hydrogen-bonded complex of 7HQ·(CH(3)OH)(2) upon absorption of a photon due to high viscosity values of RTILs. Because the cyclic complex of 7HQ·(CH(3)OH)(2) at the ground state is unstable in RTILs, the collision-induced slow formation of the cyclic complex should take place upon excitation prior to undergoing subsequent intrinsic proton transfer rapidly.

Ground-state proton-transfer dynamics governed by configurational optimization.

The ground-state proton transfer (GSPT) of 7-hydroxyquinoline along a hydrogen-bonded alcohol chain has been investigated in n-alkanes using time-resolved transient-absorption spectroscopy with variation of alcohols, media, isotopes, and temperatures. As a 7-hydroxyquinoline molecule associates with two alcohol molecules via hydrogen bonding to form a cyclic complex in a nonpolar aprotic medium, the intrinsic GSPT dynamics of the cyclic complex in a n-alkane has been observed directly without being interfered with by solvent association to form the cyclic complex. GSPT occurs concertedly without accumulating any reaction intermediate and yet asymmetrically with a rate-determining tunneling process. Both the rate constant and the kinetic isotope effect of GSPT increase rapidly with the proton-donating ability of the alcohol but decrease greatly with the molecular size of the alcohol. The reorganization of the hydrogen-bond bridge to form an optimal precursor configuration for efficient proton tunneling takes place prior to intrinsic GSPT, and configurational optimization becomes more important as the molecular size of the alcohol increases. Consequently, the larger contribution of configurational optimization to GSPT leads to the weaker asymmetric character of GSPT.

Solvent effect on the excited-state proton transfer of 7-hydroxyquinoline along a hydrogen-bonded ethanol dimer.

We have studied the solvent effect on structures and potential energy surfaces along proton transfer in the ground and the excited states of 7-hydroxyquinoline interacting with an ethanol dimer using ab initio calculations. The proton transfer is forbidden in the ground state not only in vacuum but also in solvents of n-heptane, ethanol, and dimethyl sulfoxide. In the excited state, although the proton transfer is forbidden in vacuum, it is possible in solvent due to its greatly reduced barrier (∼10 kcal mol(-1)) and highly stabilized product. It has also been found from the calculations that the proton-transfer barrier in the excited state decreases as the dielectric constant of a solvent increases. Our calculations are consistent with experimental results that the proton transfer does not take place in the ground state and that the excited-state proton-transfer rate increases as the solvent polarity increases. Our calculated absorption and emission properties are in excellent agreement with experimental results. Projection factors (reflecting geometrical change from the ground state to the excited state) and reorganization energies for several low frequency vibrations in connection with the excited-state proton transfer are discussed as well.

Accumulated proton-donating ability of solvent molecules in proton transfer.

Solvent-inventory experiments on the excited-state proton transfer of a 7-hydroxyquinoline molecule complexed cyclically with two alcohol molecules in the host medium of n-alkane have been carried out with various combinations of alcohols having different proton-donating abilities. Alcohol molecules participating in the hydrogen-bonded chain of the cyclic complex accelerate proton transfer in a concerted fashion by accumulating their proton-donating abilities. The rate-determining deprotonation of the alcohol molecule hydrogen-bonded directly to the imino group of 7-hydroxyquinoline is stimulated by the push-ahead effect of the next alcohol molecule in the hydrogen-bonded chain. Our results provide a clue at the level of molecules on the fundamental mechanistic elucidation of proton transport occurring through a proton wire consisting of diverse amino acids.

Excited-state prototropic equilibrium dynamics of 6-hydroxyquinoline encapsulated in microporous catalytic faujasite zeolites.

The excited-state proton transfer and geminate recombination of 6-hydroxyquinoline (6HQ) encaged in catalytic Na(+)-exchanged faujasite zeolites X (NaX) and Y (NaY) have been explored by measuring steady-state and picosecond time-resolved spectra. The pathways and rate constants of proton transfer of excited 6HQ are determined by the microscopic environment of zeolitic hosts surrounding the guest molecules. The excited-state proton transfer of a 6HQ molecule encapsulated in a zeolitic nanocavity is initiated by deprotonation of the enolic group to form an anionic intermediate and completed by subsequent protonation of the imino group to form a zwitterionic tautomer. Geminate recombination occurs to compete with proton transfer at each tautomerization step of excited-state 6HQ because of the confined environment of dehydrated zeolitic supercages. Consequently, excited-state equilibria among three prototropic species of 6HQ are established in microporous catalytic faujasite zeolites. Kinetic differences in NaX and NaY are attributed to dissimilarities in acidity/basicity.

Excited-state double proton transfer dynamics of model DNA base pairs: 7-hydroxyquinoline dimers.

The excited-state double proton transfer of model DNA base pairs, 7-hydroxyquinoline dimers, in benzene has been investigated using picosecond time-resolved fluorescence spectroscopy. Upon excitation, whereas singly hydrogen-bonded noncyclic dimers do not go through tautomerization within the relaxation time of 1400 ps, doubly hydrogen-bonded cyclic dimers undergo excited-state double proton transfer on the time scale of 25 ps to form tautomeric dimers, which subsequently undergo a conformational change in 180 ps to produce singly hydrogen-bonded tautomers. The rate constant of the double proton transfer reaction is temperature-independent, showing a large kinetic isotope effect of 5.2, suggesting that the rate is governed mostly by tunneling.

A white-light-emitting molecule: frustrated energy transfer between constituent emitting centers.

White-light-emitting single molecules are promising materials for use in a new generation of displays and light sources because they offer the possibility of simple fabrication with perfect color reproducibility and stability. To realize white-light emission at the molecular scale, thereby eliminating the detrimental concentration- or environment-dependent energy transfer problem in conventional fluorescent or phosphorescent systems, energy transfer between a larger band-gap donor and a smaller band-gap acceptor must be fundamentally blocked. Here, we present the first example of a concentration-independent ultimate white-light-emitting molecule based on excited-state intramolecular proton transfer materials. Our molecule is composed of covalently linked blue- and orange-light-emitting moieties between which energy transfer is entirely frustrated, leading to the production of reproducible, stable white photo- and electroluminescence.

Proton transport of water in acid-base reactions of 7-hydroxyquinoline.

The sequential, Grotthuss-type, proton-transport mechanism of water has been experimentally observed in the excited-state tautomerization of a 7-hydroxyquinoline molecule complexed cyclically with two water molecules in diethyl ether.

Triple proton transfer of excited 7-hydroxyquinoline along a hydrogen-bonded water chain in ethers: secondary solvent effect on the reaction rate.

A large secondary solvent effect on the reaction rate has been experimentally observed in the excited-state tautomerization of a 7-hydroxyquinoline (7HQ) molecule complexed cyclically with two water molecules in ethers. The proton acceptance of a water molecule from the enolic group of 7HQ is the rate-determining step while the proton donation of a water molecule to the imino group of 7HQ is followed rapidly to complete the triple proton transfer of the 7HQ.(H2O)2 complex in both diethyl ether and di-n-propyl ether. The rate constant of the tautomerization is larger in diethyl ether than in di-n-propyl ether due to the more polar environment around the complex in diethyl ether. Although the activation energies of the proton transfer are similar in both ethers, the kinetic isotope effect of the rate constant is larger in di-n-propyl ether than in diethyl ether. We attribute these kinetic differences to dissimilarity in the polarities of the two secondary solvents.

Triplet-state acid-base reactions of 1-methyl-7-oxyquinolinium in water.

Upon absorption of a photon, the 1-methyl-7-oxyquinolinium zwitterion in neutral water undergoes a proton-transfer cycle involving protonation at the lowest excited triplet state and redeprotonation at the ground state. On one hand, the proton transfer of a water molecule to the triplet-state zwitterion takes place on the time scale of 28 micros with an activation energy of 12.6 kJ mol(-1). On the other hand, the ground-state redeprotonation occurs with a rate constant of (13 micros)(-1), although its reverse reaction of protonation competes with a rate constant of (37 micros)(-1).

Laser-induced shape transformation and electrophoretic analysis of triangular silver nanoplates.

The shape transformation of silver triangular nanoplates induced by nanosecond laser pulses has been monitored using agarose gel electrophoresis as well as optical spectroscopy and electron microscopy. Laser irradiation truncates the vertices of the triangular nanoplates, having an average edge size of 110 nm and an average thickness of 14 nm, to form nearly regular-hexagonal and spherical silver nanoplates gradually. The surface-enhanced Raman scattering of 4-nitrobenzenethiol absorbed on silver nanoparticles has been found to decrease enormously with laser irradiation, showing its strong dependence on the shape transformation of nanostructures. Electrophoretic mobility values have been found to decrease substantially with the increase of irradiation time, indicating that the surface charge density of silver nanoplates has been modified largely by laser pulses.

One-step fabrication of well-defined hollow CdS nanoboxes.

Hollow CdS nanoboxes, having paper-thin walls of well-defined facets, were synthesized at 170 degrees C via a simple reaction using Na(2)SeO(3) for interior quasitemplates and ethylenediamine for exterior molecular templates.

Shape transformation and relaxation dynamics of photoexcited TiO2/Ag nanocomposites.

The laser-induced sintering of TiO2 nanoparticles into larger nanospheres is accelerated by adsorbed silver particles. For the same weight fraction of silver, silver nanoparticles of 5 nm in diameter modify TiO2 nanoparticles more effectively than those of 1.5 nm do, suggesting that the photocatalysis of TiO2 nanoparticles as well as their stability is highly dependent on the sizes, the shapes, and the distribution of adsorbed metal nanoparticles. The photoexcited electrons of TiO2 nanoparticles are quenched at trap sites and surface states by transfer to the conduction band of silver, implying that the presence of adsorbed silver nanoparticles enhances the photocatalytic effect of TiO2.