Effect of Intra-molecular Disorder and Inter-molecular Electronic Interactions on the Electronic Structure of Poly-p-Phenylene Vinylene (PPV)

We investigate the role of intra-molecular conformational disorder and inter-molecular electronic interactions on the electronic structure of disorder clusters of poly-p-phenylene vinylene (PPV) oligomers. Classical molecular dynamics is used to determine probable molecular geometries, and first-principle density functional theory (DFT) calculations are used to determine electronic structure. Intra-molecular and inter-molecular effects are disentangled by contrasting results for densely packed oligomer clusters with those for ensembles of isolated oligomers with the same intra-molecular geometries. We find that electron trap states are induced primarily by intra-molecular configuration disorder, while the hole trap states are generated primarily from inter-molecular electronic interactions.Comment: 4 pages, 4 figures. Compile with pdflate

Sample-Averaged Biexciton Quantum Yield Measured by Solution-Phase Photon Correlation

The brightness of nanoscale optical materials such as semiconductor nanocrystals is currently limited in high excitation flux applications by inefficient multiexciton fluorescence. We have devised a solution-phase photon correlation measurement that can conveniently and reliably measure the average biexciton-to-exciton quantum yield ratio of an entire sample without user selection bias. This technique can be used to investigate the multiexciton recombination dynamics of a broad scope of synthetically underdeveloped materials, including those with low exciton quantum yields and poor fluorescence stability. Here, we have applied this method to measure weak biexciton fluorescence in samples of visible-emitting InP/ZnS and InAs/ZnS core/shell nanocrystals, and to demonstrate that a rapid CdS shell growth procedure can markedly increase the biexciton fluorescence of CdSe nanocrystals.United States. Dept. of Energy. Office of Basic Energy Sciences (DE-FG02-07ER46454)United States. Dept. of Energy. Office of Basic Energy Sciences (DE-SC0001088)National Institutes of Health (U.S.) (9P41EB015871-26A1

From computational discovery to experimental characterization of a high hole mobility organic crystal

For organic semiconductors to find ubiquitous electronics applications, the development of new materials with high mobility and air stability is critical. Despite the versatility of carbon, exploratory chemical synthesis in the vast chemical space can be hindered by synthetic and characterization difficulties. Here we show that in silico screening of novel derivatives of the dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene semiconductor with high hole mobility and air stability can lead to the discovery of a new high-performance semiconductor. On the basis of estimates from the Marcus theory of charge transfer rates, we identified a novel compound expected to demonstrate a theoretic twofold improvement in mobility over the parent molecule. Synthetic and electrical characterization of the compound is reported with single-crystal field-effect transistors, showing a remarkable saturation and linear mobility of 12.3 and 16 cm2 V−1 s−1, respectively. This is one of the very few organic semiconductors with mobility greater than 10 cm2 V−1 s−1 reported to date

Exciton/Charge-transfer Electronic Couplings in Organic Semiconductors

Charge transfer (CT) states and excitons are important in energy conversion processes that occur in organic light emitting devices (OLEDS) and organic solar cells. An ab initio density functional theory (DFT) method for obtaining CT−exciton electronic couplings between CT states and excitons is presented. This method is applied to two organic heterodimers to obtain their CT−exciton coupling and adiabatic energy surfaces near their CT−exciton diabatic surface crossings. The results show that the new method provides a new window into the role of CT states in exciton−exciton transitions within organic semiconductors.United States. Dept. of Energy (DEFG02- 07ER46474)David & Lucile Packard Foundation (Fellowship

From Discrete Molecules To One-Dimensional Coordination Polymers Containing Mn(Ii), Zn(Ii) Or Cd(Ii) Pyridine-2-Aldoxime Building Unit

The syntheses, spectroscopic characterization and crystal structures for eight coordination compounds [Cd(HCO2)2(pya)2] (1), {[M(bdc)(pya)].1.5DMF}n (M = Mn(II), Zn(II), Cd(II) (2-4), [Zn(SO4)(pya)(H2O)3] (H2O) (5), [(ZnSO4)2(pya)4] DMF.2H2O (6), [Zn(SO4)(pya)(H2O)2]n (7), and [Cd(SO4)(pya)(H2O)]n (8) [pya = pyridine-2-aldoxime, bdc = 1,4-benzenedicarboxylate, DMF = N,N′-dimethylformamide] are reported. Compounds 1-4 and 6 contain two, while 5, 7, and 8 contain one neutral pya ligand coordinated in a chelate mode to the metal center through the N pyridyl and Noxime atoms. The dicarboxylic bdc anions provide an access to 1D polymeric arrays in 2-4. The hybrid sulfates including discrete mononuclear (5), binuclear (6), and 1D polymeric (7, 8) coordination arrays were obtained due to different binding modes of coordinated sulfate anion. While solid state fluorescence of new polymeric compounds 2-4 is similar to that of the H2bdc ligand, a rare dual emission is observed for compounds 7 and 8. According to TD-DFT calculations, emission at 400 and 650 nm originates from nπ* and ππ* states of the pya ligand respectively. © 2013 Elsevier Ltd. All rights reserved

Polymeric Luminescent Zn(Ii) And Cd(Ii) Dicarboxylates Decorated By Oxime Ligands: Tuning The Dimensionality And Adsorption Capacity

Ten Zn(II) and Cd(II) metal-organic materials were synthesized and studied by the X-ray method. Among these 10 structures, two represent binuclear clusters, and two are one-dimensional (1D) coordination polymers, while five are laminar two-dimensional (2D) solids and one is the three-dimensional (3D) framework. The investigation has been aimed at rational design of coordination polymers decorated by oxime ligands to increase the accessible adsorption area in these newly synthesized solids. The ligands used include three aliphatic dicarboxylic acids, HOOC-(CH2)n-COOH [n = 1, 2, 4 corresponding to malonic (H2mal), succinic (H2suc), and adipic (H2adi) acids], and three neutral oxime ligands [pyridine-2-aldoxime (2-pyao), pyridine-4-aldoxime (4-pyao), and 1,2-cyclohexanedionedioxime (Niox)]. These novel hybrid solids with the compositions [Zn2(suc)2(2-pyao)4] ·2H2O 1, [Cd2(suc)(2-pyao)4(H 2O)2][BF4]2 2, [Cd(suc)(2-pyao) 2]n 3, [Zn(mal)(4-pyao)(H2O)]n 4, [Cd(mal)(4-pyao)(H2O)]n 5, [Zn(suc)(4-pyao)]n 6, [Zn(adi)(4-pyao)2]n 7, {[Cd(adi)(4-pyao) 2]·dmf}n 8, [Zn(adi)(Niox)]n 9, and [Cd(adi)(Niox)]n 10 [dmf - N,N\u272-dimethylformamide] demonstrate a variable class of coordination supramolecular architectures dictated by the distinctions in the metals\u27 and oxime ligands\u27 coordination capacities and preferences, and length and flexibility of the dicarboxylic linkers. The discrete aggregates 1 and 2 differ by the components\u27 ratio and conformation of the bridging succinate anion; compounds 3 and 7 are 1D arrays, and compounds 4, 5, 6, 8, and 9 represent 2D layers of different topologies. Compound 10 is a 3D grid afforded by the concerted contribution of the longest in this series adipate anion, and the bigger atomic radius Cd(II) vs. Zn(II). The adsorptive properties of 7 and 9 are reported. For the laminar solid 9, the quantum chemical simulations of the adsorption capacity are in line with the experimental results. All new materials reveal dual green-blue wavelength emission in the solid state. © 2014 American Chemical Society

Mechanism Of Nonlinear Optical Enhancement And Supramolecular Isomerism In 1D Polymeric Zn(Ii) And Cd(Ii) Sulfates With Pyridine-4-Aldoxime Ligands

Interaction of zinc(II) and cadmium(II) sulfates with pyridine-4-aldoxime (4-pyao) and pyridine-4-amidoxime (4-pyamo) ligands resulted in four 1D metal-organic materials (MOMs) with identical composition, [M(SO 4)A2(H2O)2]n, where M = Zn(II), A = 4-pyao for 1, M = Cd(II), A = 4-pyao for 2, M = Zn(II), A = 4-pyamo for 3, M = Cd(II), A = 4-pyamo for 4, and mononuclear [Zn(SO4)(4- pyamo)2(H2O)3] 5. New coordination polymers represent the mixed-ligand supramolecular isomers different by the twisting of two pyridine-4-oxime ligands in the metal coordination environments, and crystallizing in the different space groups. Conformational preferences and nonlinear optical properties of the 4-pyao and 4-pyamo complexes were investigated using density functional theory. Spectral properties of 1-3 have been also evaluated. The solid-state emission of 1D polymers 1-3 appears to be ligand-based, as the positions of the emission maxima remain practically unchanged from free ligand to complexes. The enhancement of luminescence and two-photon absorption in polymers in comparison with the pure ligands is attributed to the chelation of the ligand to the metal center. The detailed mechanism of this enhancement upon complex formation is analyzed and can be used in future design of metal-organic nonlinear optical materials. © 2014 American Chemical Society