Effect of extending conjugation via thiophene-based oligomers on the excited state electron transfer rates to ZnO nanocrystals.

Oligothiophene dyes with two to five thiophene units were anchored to oleate-capped, quantum-confined zinc oxide nanocrystals (ZnO NCs) through a cyanoacrylate functional group. While the fluorescence of the bithiophene derivative was too weak for meaningful quenching studies, the fluorescence of the dyes with three, four and five thiophene rings was quenched upon binding to the NCs. Ultrafast pump-probe spectroscopy was used to observe the singlet excited states of the free dyes dissolved in dichloromethane as well as attached to a ZnO NC dispersed in the same solvent. When the dyes were bound to ZnO NCs, ultrafast spectroscopic measurements revealed rapid disappearance of the singlet excited state and appearance of a new transient absorption at higher energy that was assigned to the oxidized dye based on the similar absorption observed upon oxidation of the dye using nitrosonium ion. The appearance lifetimes of the oxidized dyes were assigned to the excited state electron transfer and were 36 ± 2, 22.3 ± 3.9, 26.5 ± 1.5 and 19.4 ± 0.8 ps for bi-, ter-, quarter- and quinquethiophene dyes respectively. Two factors contributed to the similarity in the electron transfer lifetime. First the excited state energies of the dyes were similar, and second, the free energy for electron transfer reaction was sufficiently large to move the event into the energy-independent regime.

Computational Thermochemistry of Mono- and Dinuclear Tin Alkyls Used in Vapor Deposition Processes.

Hexamethylditin has been reported to be a more effective precursor compared to monotin analogs in hybrid molecular beam epitaxy depositions of perovskite oxides. To understand the differences, a library of 68 monotin- and ditin-containing molecules bearing hydrido and/or carbon-based ligands was generated, and their structures were optimized using density functional theory. On the basis of a modified W1-F12 composite thermochemical method, thermochemical data (enthalpy of formation, entropy, and heat capacity) were calculated for each structure over a range of temperatures (298-5000 K). The application of the modified W1-F12 method to heavy element compounds was benchmarked against existing experimental and computational studies of mononuclear hydrido, alkyl, and mixed hydridoalkyl complexes of silicon, germanium, and tin. The library of thermodynamic data was used in partial equilibrium calculations from 300 to 1500 K to determine gas phase compositions resulting from the pyrolysis of tetramethyltin and hexamethylditin at 10-6 and 760 Torr.

Impact of dihydrogen bonding on lattice energies and sublimation enthalpies of crystalline [H2GaNH2]3, [H2BNH2]3 and [H2GeCH2]3.

The lattice energies of [H2GaNH2]3, [H2BNH2]3 and [H2GeCH2]3 in their experimentally determined space groups, P21/m, Pmn21 and Pbcm, respectively, were calculated using density functional methods for periodic structures with the ab initio periodic code CRYSTAL17. Using the basis set pob-TZVP for all calculations, B3LYP including Grimme's D3 dispersion correction was found to reproduce experimental bond distances and angles most accurately. CRYSTAL17 was also used to optimize geometries and calculate energies of the molecular structures in the gas phase. While the chair conformation of the six-membered rings is found in all of the crystals, only [H2GeCH2]3 retains this as the preferred conformation in the gas phase. By contrast, a twist-boat conformation is preferred for both [H2GaNH2]3 and [H2BNH2]3 in the gas phase, and thus a correction for this change in conformation must be included in corresponding sublimation enthalpy calculations. In addition to the D3 dispersion correction, all lattice energies included a correction for basis set superposition error. The lattice energies for [H2GaNH2]3, [H2BNH2]3 and [H2GeCH2]3 were 153.5, 120.8 and 84.9 kJ mol-1, respectively. These values were used to calculate the sublimation enthalpies, which exhibited good agreement for the single case where an experimental measurement is available, namely [H2BNH2]3 (exp ΔH sub(298), 119 ± 12 kJ mol-1; calcd, 119.4 kJ mol-1). The energetic impact of the crystal structure was assessed by minimizing the structures of each molecule in each of the three space groups spanned by them experimentally and calculating their respective lattice energies. In every case, the experimentally observed space group was the one computed to be the most stable.

Effects of a phosphonate anchoring group on the excited state electron transfer rates from a terthiophene chromophore to a ZnO nanocrystal.

Terthiophene dyes were synthesized having a carboxylate or a phosphonate moiety at the 2-position which serves as an anchoring group to zinc oxide nanocrystals (ZnO NCs). Electronic absorption and fluorescence measurements, combined with reduction potentials, provided estimates of -1.81 and -1.86 V vs. NHE for the excited state reduction potential of the carboxylate and phosphonate, respectively. Static quenching was observed when the dyes were bound to the surface of acetate-capped ZnO NCs having a diameter of 2.8 nm. Stern-Volmer studies conducted at several dye concentrations established that a minor fraction of the adsorbed dye remained unquenched even at 1 : 1 dye to NC ratios. Adsorption isotherm measurements established that the phosphonate binds more strongly than the carboxylate and that saturation coverage was ∼1.2 dyes per nm2 for both dyes. Ultrafast transient absorption spectroscopic experiments were used to probe excited state dynamics. In the presence of ZnO NCs, disappearance of the singlet excited state of the dye corresponded to appearance of the spectroscopic signature of the oxidized dye with a time constant of 1.5 ± 0.1 and 6.1 ± 0.2 ps, respectively, for the carboxylate and phosphonate dye. The difference in the electron transfer rates was attributed to a larger electronic coupling for the dye having the carboxylate anchoring group.

Poly(cyclohexylethylene)-block-Poly(lactide) Oligomers for Ultrasmall Nanopatterning Using Atomic Layer Deposition.

Poly(cyclohexylethylene)-block-poly(lactide) (PCHE-PLA) block polymers were synthesized through a combination of anionic polymerization, heterogeneous catalytic hydrogenation and controlled ring-opening polymerization. Ordered thin films of PCHE-PLA with ultrasmall hexagonally packed cylinders oriented perpendicularly to the substrate surface were prepared by spin-coating and subsequent solvent vapor annealing for use in two distinct templating strategies. In one approach, selective hydrolytic degradation of the PLA domains generated nanoporous PCHE templates with an average pore diameter of 5 ± 1 nm corroborated by atomic force microscopy and grazing incidence small-angle X-ray scattering. Alternatively, sequential infiltration synthesis (SIS) was employed to deposit Al2O3 selectively into the PLA domains of PCHE-PLA thin films. A combination of argon ion milling and O2 reactive ion etching (RIE) enabled the replication of the Al2O3 nanoarray from the PCHE-PLA template on diverse substrates including silicon and gold with feature diameters less than 10 nm.

Zinc oxide nanocrystal quenching of emission from electron-rich ruthenium-bipyridine complexes.

A series of heteroleptic bipyridine ruthenium complexes were prepared using known synthetic methods. Each compound incorporated one electron withdrawing 4,4'-dicarboxylic acid-2,2'-bipyridine and two bipyridines each of which had electron donating dialkylamine substituents in the 4 and 4' positions. The electronic absorption spectra exhibited absorptions that moved to lower energy as the donor ability of the amine substituent increased. Density functional calculations established that the HOMO was delocalized over the metal and two pyridine groups located trans to the pyridines of the dicarboxylic acid bipyridine. The LUMO was delocalized over the dicarboxylic acid bipyridine. Cyclic voltammetry of the deprotonated compounds exhibit one quasi-reversible oxidation and three reductions. Coupled with the emission data, the excited state reduction potentials were estimated to range from -0.93 to -1.03 V vs. NHE. Monodispersed 3.2 nm diameter nanocrystals (NCs) of zinc oxide were found to quench partially the excited state of the dyes via a static quenching electron transfer process involving the formation of a dyad of the complex and the NC. The magnitude of the partial quenching of complexed dyes was correlated to the distribution of band gaps for the NCs, which is an inverse function of diameter. Dyes attached to the NCs on the small end of the particle size distribution had electron transfer rates that were uncompetitive with radiative and nonradiative decay mechanisms.

Size-tuned ZnO nanocrucible arrays for magnetic nanodot synthesis via atomic layer deposition-assisted block polymer lithography.

Low-temperature atomic layer deposition of conformal ZnO on a self-assembled block polymer lithographic template comprising well-ordered, vertically aligned cylindrical pores within a poly(styrene) (PS) matrix was used to produce nanocrucible templates with pore diameters tunable via ZnO thickness. Starting from a PS template with a hexagonal array of 30 nm diameter pores on a 45 nm pitch, the ZnO thickness was progressively increased to narrow the pore diameter to as low as 14 nm. Upon removal of the PS by heat treatment in air at 500 °C to form an array of size-tunable ZnO nanocrucibles, permalloy (Ni80Fe20) was evaporated at normal incidence, filling the pores and creating an overlayer. Argon ion beam milling was then used to etch back the overlayer (a Damascene-type process), leaving a well-ordered array of isolated ZnO nanocrucibles filled with permalloy nanodots. Microscopy and temperature-dependent magnetometry verified the diameter reduction with increasing ZnO thickness. The largest diameter (30 nm) dots exhibit a ferromagnetic multidomain/vortex state at 300 K, with relatively weakly temperature-dependent coercivity. Reducing the diameter leads to a crossover to a single-domain state and eventually superparamagnetism at sufficiently high temperature, in quantitative agreement with expectations. We argue that this approach could render this form of block polymer lithography compatible with high-temperature processing (as required for technologically important high perpendicular anisotropy ordered alloys, for instance), in addition to enabling separation-dependent studies to probe interdot magnetostatic interactions.

Combined plasma gas-phase synthesis and colloidal processing of InP/ZnS core/shell nanocrystals.

Indium phosphide nanocrystals (InP NCs) with diameters ranging from 2 to 5 nm were synthesized with a scalable, flow-through, nonthermal plasma process at a rate ranging from 10 to 40 mg/h. The NC size is controlled through the plasma operating parameters, with the residence time of the gas in the plasma region strongly influencing the NC size. The NC size distribution is narrow with the standard deviation being less than 20% of the mean NC size. Zinc sulfide (ZnS) shells were grown around the plasma-synthesized InP NCs in a liquid phase reaction. Photoluminescence with quantum yields as high as 15% were observed for the InP/ZnS core-shell NCs.

Tuning electron transfer rates via systematic shifts in the acceptor state density using size-selected ZnO colloids.

We report direct measurements of the influence of the available density of acceptor states on the rate of near-barrierless electron transfer between a dye sensitizer and an oxide semiconductor. The electron donor was the excited state of a zinc porphyrin, and the acceptors were a series of size-selected ZnO nanocrystals. The available density of states was tuned by controlling the relative position of the ZnO band edge using quantum confinement. The resulting change in the rate was consistent with a simple model of the state density as a function of energy above the ZnO band edge.

Thermally degradable ligands for nanocrystals.

We exchanged the oleate ligands on as-prepared PbSe/CdSe core/shell nanocrystals with octyldithiocarbamate to enable the removal of insulating ligands by gentle heating. The octyldithiocarbamate ligand could readily be stripped from the surface by heating briefly to temperatures from 140 to 205 degrees C, which is substantially lower than the temperature (330 degrees C) required to remove oleate from the nanocrystal surface. X-ray diffraction and transmission electron microscopy reveal that the nanocrystals sinter around 250 degrees C, resulting in a loss of quantum confinement. Heating for 1 min to 205 degrees C removed 92% of the organics from the surface. We could therefore prepare densely packed films of quantum-confined nanocrystals via dithiocarbamate treatment. Conductivity increased by up to 4 orders of magnitude after annealing. In addition to PbSe/CdSe core/shell nanocrystals, we also examined the applicability of our ligand removal procedure to CdSe nanocrystals.

Zinc oxide nanocrystals stabilized by alkylammonium alkylcarbamates.

Nearly monodispersed, spherical ZnO nanocrystals were synthesized from the reaction of an amide precursor, [Zn(N(i)Bu(2))(2)](2), with hexylamine followed by reactions of the as-formed solution in a moist air flow. Extensive experiments were conducted to optimize the synthesis and to characterize the nanocrystals. The room temperature reactions led to 3.3-5.3 nm nanocrystals with the sizes increasing in direct proportion to the relative humidity. Purification afforded high yields of free-flowing nanocrystals that were dispersible in nonpolar solvents. The overall synthesis requires several days, but it results in multigram quantities of stable, redispersible nanocrystals. The nanocrystals were characterized using elemental analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), solution and solid-state NMR, IR, UV-vis absorption, and photoluminescence spectroscopies. In addition to providing H(2)O to serve as the source of oxygen in the ZnO, the air flow adds CO(2) that converts the alkylamine into an alkylammonium alkylcarbamate, which serves as the surfactant. Elemental analysis, TGA, and XPS results established that the total number of N-hexyl fragments on a 3.7 nm nanocrystal was 200, where they exist as an equal number of anionic carbamates and cationic ammonium ions. The addition of pure hexylammonium hexylcarbamate to ZnO nanocrystals prepared by literature methods resulted in the formation of a product that was similar to the ZnO formed using [Zn(N(i)Bu(2))(2)](2). Larger nanocrystals up to 7.3 nm were also obtained by heating smaller nanocrystals in a mixture of hexylamine and toluene at 119 degrees C.

Hydrido and chloro gallium and aluminium complexes with the tridentate bis(2-dimethylaminoethyl)amide ligand.

Five-coordinate gallium and aluminium dihydrides, H2Ga[N(CH2CH2NMe2)2] () and H2Al[N(CH2CH2NMe2)2] (), were synthesized and found to be volatile and thermally stable. and reacted with H3Ga(NMe3) and H3Al(NMe3), respectively, to form H2Ga[N(CH2CH2NMe2)2]GaH3 () and H2Al[N(CH2CH2NMe2)2]AlH3 (), in which the amido nitrogen bridged between the MH2 and MH3 groups (M=Ga or Al). A mixed metal complex, H2Al[N(CH2CH2NMe2)2]GaH3 () was obtained from the reaction of with H3Al(NMe3) or with H3Ga(NMe3), and a crystal consisting of a mixture of and was structurally characterized. The five-coordinate chloro derivative, Cl2Ga[N(CH2CH2NMe2)2] (), was synthesized and characterized.

Pseudo-two-dimensional structures (HXYH)3n2H6n (XY = GaN, SiC, GeC, SiSi, or GeGe; n = 1-3): density functional characterization of structures and energetics.

Hybrid density functional calculations with effective core potential basis sets are performed for monomeric group 13/15 and group 14/14 analogues of cyclohexane, as well as for three different pseudo-two-dimensional structures that can be formed from expanding one and two concentric rings around the central one (trans-fused chairs, a rolling combination of trans- and cis-fused chairs, and cis-fused boats). Varying contributions from torsional strain, angle strain, electrostatics, and nontraditional H-H hydrogen bonding lead to different orderings and magnitudes of motif energies in the various systems: Homoatomic SiSi and GeGe systems prefer the trans-fused chair alternative and heteroatomic systems GaN, SiC, and GeC prefer the rolling chair. Decomposition of structure energies into characteristic fragment contributions indicates that pseudo-one-dimensional rods of poly(imidogallane) are thermodynamically more stable than any of the pseudo-two-dimensional structures.

Mono- and digallane complexes of a tridentate amido-diamine ligand.

Bis(2-dimethylaminoethyl)amido gallane, H2GaN(CH2CH2NMe2)2, that melts at 27 degrees C and remains stable upon heating at 55 degrees C for two days, was synthesized either from the reaction of the quinuclidine adduct of monochlorogallane with the lithium salt of the corresponding amine, or from the reaction of trimethylamine gallane and the amine; the latter affords an unusual co-product with both GaH2 and GaH3 bonded to the same amido nitrogen.

Oligomeric rods of alkyl- and hydridogallium imides.

Reaction of [RGa(NMe(2))(2)](2), where R = Me, Et, Bu, and Hx, with ammonia at 150 degrees C in an autoclave produced insoluble white powders formulated as oligomers of [RGaNH](n). The analogous reaction between NH(3) and MeGa[N(SiMe(3))(2)](2) at low temperature (<25 degrees C) formed an isolable intermediate, [MeGa(mu-NH(2))N(SiMe(3))(2)](2), that was characterized using single-crystal X-ray diffraction. Infrared spectroscopy and X-ray diffraction of the oligomers were consistent with a rodlike structure comprised of six-membered, [RGaNH](3) rings stacked perpendicular to the long axis of the rod. The method of synthesis, formula, and diffraction results suggested a structural similarity between the alkyl, [RGaNH](n)(), and the previously reported hydride, [HGaNH](n). The structural and electronic properties of rods having the general formula H(3)[(HXYH)(3)](n)H(3) (XY = GaN, GeC; n = 1-9) were investigated using density functional theory. Atomic electronegativity differences between the group 13/15 and 14/14 systems were found to play important roles in the geometrical structures of the two rods and also caused significant differences in the electronic structures. Energetically, it was found to be increasingly favorable to add additional cyclotrigallazane rings to the GaN rods, while for the GeC rods, there was a roughly constant energy cost associated with each additional ring. The electric dipole moments of the GaN rods increased substantially with length; in the GeC rods, charge separation occurred to a much smaller extent and had a polarization opposite to that found in GaN. In addition, increased dipole moments correlated with smaller electronic excitation energies, as predicted by time-dependent density functional theory. All of the powders exhibited luminescence in the visible spectrum at room temperature. Structure observed in the photoluminescence spectra of [HGaNH](n) and [MeGaNH](n) was interpreted as arising from rods of different length.

Gallium and indium hydrazides. Molecular and electronic structure of In[N(SiMe3)NMe2]3 and related compounds.

Gallium and indium hydrazides, Ga[N(SiMe(3))NMe(2)](3) (1) and In[N(SiMe(3))NMe(2)](3) (2), were synthesized from the reactions of metal chlorides and Li[N(SiMe(3))NMe(2)]. Single crystal X-ray crystallographic analysis revealed that compound 2 was monomeric with trigonal planar geometries on the indium and the indium-bonded nitrogen atoms. The average In[bond]N distance of 2.078(3) A and the N[bond]In[bond]N[bond]N dihedral angles did not provide clear structural evidence of In[bond]N pi-bonding. The electronic absorption spectra of the indium hydrazido complex revealed transitions at significantly lower energies compared to those observed in the tris(amido) compounds, In[N(SiMe(3))(2)](3) (3) and In[N((t)Bu)(SiMe(3))](3) (4). The absorptions of the indium and gallium compounds were attributed to ligand-metal charge transfer transitions. Trends in the electronic transitions for compounds 2 and 3 calculated at the time-dependent density functional and configuration interaction including single excitations levels, both using a minimal basis set, were consistent with the experimental data, and Mulliken charge analyses support the assignment to ligand-to-metal charge transfer transitions. These calculations also demonstrated the presence of pi-bonding between the In and N p-orbitals, and an analogy is drawn to the frontier molecular orbitals of trimethylenemethane. The low-lying spectroscopic transition in 2, and thus its yellow color, results from mixing of the lone pair electrons on the beta-nitrogens of the hydrazido ligands with the HOMO of the InN(3) core.

Synthesis and structures of gallium amido imido phenyl clusters (PhGa)(4)(NH(i)Bu)(4)(N(i)Bu)(2) and (PhGa)(7)(NHMe)(4)(NMe)(5).

The phenylgallium-containing clusters constructed with bridging imido and amido ligands, (PhGa)(4)(NH(i)Bu)(4)(N(i)Bu)(2) (1) (51% yield) and (PhGa)(7)(NHMe)(4)(NMe)(5) (2) (31% yield), were synthesized from the room-temperature reactions of bis(dimethylamido)phenylgallium, [PhGa(NMe(2))(2)](2), with isobutylamine and methylamine, respectively. The reaction of [PhGa(NMe(2))(2)](2) in refluxing isobutylamine (85 degrees C) afforded (Ph(2)GaNH(i)Bu)(2) as one of the products, while the reaction of [PhGa(NMe(2))(2)](2) with methylamine at 150 degrees C afforded compound 2 in only 9% yield. Compound 1 possessed an admantane-like Ga(4)N(6) core, whereas compound 2 had a novel Ga(7)N(9) core constructed with both chair- and boat-shaped Ga(3)N(3) rings. The presence of several isomers of compounds 1 and 2 in solution is discussed along the structural similarities with other known gallium-nitrogen clusters and with gallium nitride.

Synthesis and thermolysis of alkyl- and phenylamido diphenylgallium, [Ph(2)GaN(H)R](2). isolation and structural characterization of (PhGaNMe)(7) and (PhGaNPh)(4).

Alkyl- and phenylamido diphenylgallium compounds, [Ph(2)GaN(H)R](2) (R = Me, 1; Et, 2; (n)Pr, 3; (i)Bu, 4; Ph, 5), were prepared from the reactions of Ph(3)Ga with the corresponding primary amines and aniline at elevated temperatures and were characterized by elemental analysis, mass spectroscopy, and (1)H NMR and IR spectroscopy. These dimeric compounds contained bridging amido groups and exhibited both trans and cis isomers in solution. Thermolysis of compounds 1 and 5 was carried out either without solvent or in dodecane solutions, and two clusters, (PhGaNMe)(7) 6 and (PhGaNPh)(4) 7, were isolated in 24% and 55% yields and characterized. The structure of 6 consisted of a heptameric Ga(7)N(7) core constructed with Ga(2)N(2) and Ga(3)N(3) rings, and the structure of 7 possessed a Ga(4)N(4) cubane core.