A geographical analysis of cropland productivity in Bangladesh.

Originally published in Optics Express on 23 March 2015 (oe-23-6-8073

Media 1: Ultrafast imaging of terahertz Cherenkov waves and transition-like radiation in LiNbO₃

Originally published in Optics Express on 23 March 2015 (oe-23-6-8073

Media 3: Ultrafast imaging of terahertz Cherenkov waves and transition-like radiation in LiNbO₃

Originally published in Optics Express on 23 March 2015 (oe-23-6-8073

Synthesis and Electronic Properties of Conjugated Pentacene Dimers

Conjugated pentacene dimers 1−3 were synthesized in two steps from readily available precursors. Noteworthy is the initial step, which assembles five independent fragments to form the carbon-rich molecular framework. Solution-cast films of these materials are air stable. Photocurrent measurements for solution-deposited thin films show that dimer 3 exhibits photoconductive gain >10

Pentacene-Based Dendrimers: Synthesis and Thin Film Photoconductivity Measurements of Branched Pentacene Oligomers

The synthesis of pentacene-based dendrimers has been achieved via esterification of 1,3,5-benzenetricarboxylic acid and unsymmetrical pentacene 4 possessing a hydroxy group. Dendrimers 1 (C183H204O9Si9, 2800 g mol−1) and 2 (C540H570O30Si24, 8214 g mol−1) are characterized by 1H and 13C NMR, IR, UV−vis, and fluorescence spectroscopy, as well as mass spectrometry. These branched oligomeric materials are benchtop stable and soluble in common organic solvents, allowing for solution cast formation of thin films. Photocurrent and photocurrent yield measurements of these films reveal improved efficiency in photogenerated conduction for dendrimers in comparison to linearly connected pentacene-based polymers

Generation of Terahertz Radiation by Optical Excitation of Aligned Carbon Nanotubes

We have generated coherent pulses of terahertz radiation from macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs) excited by femtosecond optical pulses without externally applied bias. The generated terahertz radiation is polarized along the SWCNT alignment direction. We propose that top-bottom asymmetry in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized terahertz radiation by a transient photocurrent surge directed along the nanotube axis

Size <i>vs</i> Surface: Tuning the Photoluminescence of Freestanding Silicon Nanocrystals Across the Visible Spectrum <i>via</i> Surface Groups

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3–4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies

Synthesis, Structure, and Nonlinear Optical Properties of Diarylpolyynes

A series of α,ω-diarylpolyynes has been synthesized. In addition to the synthesis of three hexaynes (3a−c), a notably improved synthesis of 1,16-diphenylhexadecaoctayne (5) is described. The third-order nonlinear optical characteristics for these molecules have been studied and show a substantial increase in molecular hyperpolarizability (γ) as a function of increasing length. The unusual solid-state structures of compounds 3a and 3b are reported

Polyynes as a Model for Carbyne: Synthesis, Physical Properties, and Nonlinear Optical Response

With the Fritsch−Buttenberg−Wiechell rearrangement as a primary synthetic route, a series of conjugated, triisopropylsilyl end-capped polyynes containing 2−10 acetylene units has been assembled. In a few steps, significant quantities of the polyynes are made available, which allow for a thorough analysis of their structural, physical, and optical properties. Molecules in the series have been characterized in detail using 13C NMR spectroscopy, differential scanning calorimetry, mass spectrometry, and, for four derivatives including octayne 6, X-ray crystallography. UV−vis spectroscopy of the polyynes 1−7 shows a consistent lowering of the HOMO−LUMO gap (Eg) as a function of the number of acetylene units (n), fitting a power-law relationship of Eg ∼ n-0.379±0.002. The third-order nonlinear optical (NLO) properties of the polyyne series have been examined, and the nonresonant molecular second hyperpolarizabilities (γ) increase as a function of length according to the power-law γ ∼ n4.28±0.13. This result exhibits an exponent that is larger than theoretically predicted for polyynes and higher than is observed for polyenes and polyenynes. The combined linear and nonlinear optical results confirm recent theoretical studies that suggest polyynes as model 1-D conjugated systems. On the basis of UV−vis spectroscopic analysis, the effective conjugation length for this series of polyynes is estimated to be ca. n = 32, providing insight into characteristics of carbyne

Understanding the Origin of Phosphorescence in Bismoles: A Synthetic and Computational Study

A series of bismuth heterocycles, termed bismoles, were synthesized via the efficient metallacycle transfer (Bi/Zr exchange) involving readily accessible zirconacycles. The luminescence properties of three structurally distinct bismoles were explored in detail via time-integrated and time-resolved photoluminescence spectroscopy using ultrafast laser excitation. Moreover, time-dependent density functional theory computations were used to interpret the nature of fluorescence versus phosphorescence in these bismuth-containing heterocycles and to guide the future preparation of luminescent materials containing heavy inorganic elements. Specifically, orbital character at bismuth within excited states is an important factor for achieving enhanced spin–orbit coupling and to promote phosphorescence. The low aromaticity of the bismole rings was demonstrated by formation of a CuCl π-complex, and the nature of the alkene-CuCl interaction was probed by real-space bonding indicators derived from Atoms-In-Molecules, the Electron Localizability Indicator, and the Non-Covalent Interaction index; such tools are of great value in interpreting nonstandard bonding environments within inorganic compounds