Showing papers by "Stephen Barlow published in 2010"

Design of Polymethine Dyes with Large Third-Order Optical Nonlinearities and Loss Figures of Merit

Abstract: All-optical switching applications require materials with large third-order nonlinearities and low nonlinear optical losses. We present a design approach that involves enhancing the real part of the third-order polarizability (γ) of cyanine-like molecules through incorporation of polarizable chalcogen atoms into terminal groups, while controlling the molecular length to obtain favorable one- and two-photon absorption resonances that lead to suitably low optical loss and appreciable dispersion enhancement of the real part of γ. We implemented this strategy in a soluble bis(selenopyrylium) heptamethine dye that exhibits a real part of γ that is exceptionally large throughout the wavelength range used for telecommunications, and an imaginary part of γ, a measure of nonlinear loss, that is smaller by two orders of magnitude. This combination is critical in enabling low-power, high-contrast optical switching.

High‐Strain Shape‐Memory Polymers

Abstract: Shape-memory polymers (SMPs) are self-adjusting, smart materials in which shape changes can be accurately controlled at specific, tailored temperatures. In this study, the glass transition temperature (Tg) is adjusted between 28 and 55 °C through synthesis of copolymers of methyl acrylate (MA), methyl methacrylate (MMA), and isobornyl acrylate (IBoA). Acrylate compositions with both crosslinker densities and photoinitiator concentrations optimized at fractions of a mole percent demonstrate fully recoverable strains at 807% for a Tg of 28 °C, at 663% for a Tg of 37 °C, and at 553% for a Tg of 55 °C. A new compound, 4,4'-di(acryloyloxy)benzil (referred to hereafter as Xini) in which both polymerizable and initiating functionalities are incorporated in the same molecule, was synthesized and polymerized into acrylate shape-memory polymers, which were thermomechanically characterized yielding fully recoverable strains above 500%. The materials synthesized in this work were compared to an industry standard thermoplastic SMP, Mitsubishi's MM5510, which showed failure strains of similar magnitude, but without full shape recovery: residual strain after a single shape-memory cycle caused large-scale disfiguration. The materials in this study are intended to enable future applications where both recoverable high-strain capacity and the ability to accurately and independently position Tg are required.

Effects of electronegative substitution on the optical and electronic properties of acenes and diazaacenes

Abstract: Large acene molecules are common components of organic electronics. Appleton et al. show that embedding pyrazine units in acenes results in unexpected red-shifted optical transitions upon electronegative substitution, which may aid the design of acene-type materials for organic electronics applications.

Dithienopyrrole-based donor–acceptor copolymers: low band-gap materials for charge transport, photovoltaics and electrochromism

Abstract: A series of highly soluble donor–acceptor (D–A) copolymers containing N-(3,4,5-tri-n-decyloxyphenyl)-dithieno[3,2-b:2′,3′-d]pyrrole (DTP) or N-(2-decyltetradecyl)-dithieno[3,2-b:2′,3′-d]pyrrole (DTP′) as donor and three different acceptors, 4,7-dithien-2-yl-[2,1,3]-benzothiadiazole, 4,9-dithien-2-yl-6,7-di-n-hexyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline and 4,8-dithien-2-yl-2λ4δ2-benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BThX, X = BTD, TQHx2, BBT, respectively) were synthesized by Stille coupling polymerizations. The optical and electrochemical properties of these copolymers were investigated, along with their use in field-effect transistors and photovoltaic devices. The band gaps (eV) estimated from UV-vis-NIR spectra and electrochemical measurements of the copolymers varied from ca. 1.5–0.5 eV, and were consistent with quantum-chemical estimates extrapolated using density functional theory. Oxidative and reductive spectroelectrochemistry of the copolymers indicated they can be both p-doped and n-doped, and three to four differently colored redox states of the polymers can be accessed through electrochemical oxidation or reduction. The DTP-BThBTD and DTP-BThTQHx2 copolymers exhibited average field-effect hole mobilities of 1.2 × 10−4 and 2.2 × 10−3 cm2/(Vs), respectively. DTP-BThBBT exhibited ambipolar field-effect characteristics and showed hole and electron mobilities of 1.2 × 10−3 and 5.8 × 10−4 cm2/(Vs), respectively. Bulk heterojunction photovoltaic devices made from blends of the copolymers with 3′-phenyl-3′H-cyclopropa[1,9](C60-Ih)[5,6]fullerene-3′-butanoic acid methyl ester (PCBM) (1:3 weight ratio) exhibited average power conversion efficiencies as high as 1.3% under simulated irradiance of 75 mW/cm2.

Acceptor energy level control of charge photogeneration in organic donor/acceptor blends

Abstract: In this paper we focus upon the role of interfacial energetics in influencing the separation of charge transfer states into dissociated charge carriers in organic donor/acceptor blend films. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) electron acceptors. For this film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron acceptor 3′-phenyl-3′H-cyclopropa[1,9](C60-Ih)[5,6]fullerene-3′-butanoic acid methyl ester (PCBM).

A Molybdenum Dithiolene Complex as p-Dopant for Hole-Transport Materials: A Multitechnique Experimental and Theoretical Investigation

Abstract: Molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3) is investigated as a p-dopant for organic semiconductors. With an electron affinity of 5.6 eV, Mo(tfd)3 is a strong oxidizing agent suitable for the oxidation of several hole transport materials (HTMs). Ultraviolet photoemission spectroscopy confirms p-doping of the standard HTM N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl-4,4′-diamine (α-NPD). Strong enhancement of hole injection at α-NPD/Au interfaces is achieved via doping-induced formation of a narrow depletion region in the organic semiconductor. Variable-temperature current−voltage measurements on α-NPD: Mo(tfd)3 (0−3.8 mol %) yield an activation energy for polaron transport that decreases with increasing doping concentration, which is consistent with the effect of the doping-induced filling of traps on hopping transport. Good stability of Mo(tfd)3 versus diffusion in the α-NPD host matrix is demonstrated by Rutherford backscattering for temperatures up to 110 °C. Densi...

Electronic and optical properties of 4H-cyclopenta[2,1-b:3,4-b']bithiophene derivatives and their 4-heteroatom-substituted analogues: a joint theoretical and experimental comparison.

Abstract: The electronic and optical properties of 2,6-dialkyl and 2,6-bis(5-alkyl-2-thienyl) derivatives of the fused-ring systems 4H-cyclopenta[2,1-b:3,4-b′]bithiophene, 4,4-di-n-hexyl-4H-cyclopenta[2,1-b:3,4-b′]bithiophene, 4H-cyclopenta[2,1-b:3,4-b′]bithiophene-4-one, 4-alkyl and 4-aryldithieno[3,2-b:2′,3′-d]pyrrole, 4-phenyldithieno[3,2-b:2′,3′-d]phosphole, 4-phenyldithieno[3,2-b:2′,3′-d]phosphole 4-oxide, dithieno[3,2-b:2′,3′-d]thiophene, dithieno[3,2-b:2′,3′-d]thiophene 4-oxide, and dithieno[3,2-b:2′,3′-d]thiophene 4,4-dioxide have been compared to those of the analogous unbridged 5,5′-substituted 2,2′-bithiophene derivatives using electrochemistry, UV−visible absorption and emission spectroscopy, and DFT and TD-DFT calculations. The planarization in the fused-ring compounds means that the methylene-bridged cyclopentabithiophenes are more readily oxidized than their unbridged bithiophene analogues. In each case, the bridging group (X) lies on a nodal plane of the HOMO; accordingly, within each series of fuse...

Remote doping of a pentacene transistor: Control of charge transfer by molecular-level engineering

Abstract: We demonstrate that holes from a p-doped N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) layer transfer to an adjacent pentacene film. The spatial separation of carriers from dopants, or remote doping, is demonstrated with a combination of photoemission spectroscopy and current-voltage measurements for a p-doped α-NPD/pentacene heterojunction. Increased conductivity of the pentacene film is observed in both nongated temperature-dependent conductivity and gated thin-film transistor measurements.

Using end groups to tune the linear and nonlinear optical properties of bis(dioxaborine)-terminated polymethine dyes.

Abstract: Six anionic pentamethine dyes with different 2,2-difluoro-4-aryl-1,3,2(2 H)-dioxaborin-6-yl termini were synthesized and isolated as tetra-n-octylammonium salts with a variety of aryl groups appended to increase conjugation beyond the dioxaborine termini. The increased conjugation was expected to decrease the energy of the lowest-lying excited state, and increase the transition dipole moment linking this state to the ground state, which would be anticipated to result in an increase in the real part of the third-order polarizability, Re(gamma). UV/Vis-NIR absorption spectroscopy indicates that the absorption maxima in DMSO vary from 691 to 761 nm, with the longest wavelength transitions observed for a derivative where the aryl group is 4-nitrophenyl. Closed-aperture Z-scan measurements at 1.3 microm in DMSO indicate that Re(gamma) varies from -2.9x10(-33) to -5.4x10(-33) esu in these systems. The largest magnitude of Re(gamma) was observed for a dye with E-4-styrylphenyl aryl groups. This result can be rationalized using a two-state expression which relates Re(gamma) to the energy and transition dipole moment of the transition from the ground state to the lowest-lying excited state. A nonamethine analogue of this compound was also synthesized and exhibits a slightly larger Re(gamma) with respect to a previously reported bis(dioxaborine)-terminated nonamethine. The extension of conjugation beyond the dioxaborine termini seems to result in an overall increase in Re(gamma). However, the effects are smaller than those found by increasing conjugation in the polymethine bridge due to reduced participation of terminal groups in the HOMO.

Electronic properties of the 2,6-diiododithieno[3,2-b:2',3'-d]thiophene molecule and crystal: a joint experimental and theoretical study.

Abstract: The electronic properties of the 2,6-diiododithieno[3,2-b:2′,3′-d] thiophene molecule and crystal are investigated by means of UV−vis spectroscopy, cyclic voltammetry, X-ray crystallography, and density functional theory. The experimental and calculated properties of the compound are compared to those exhibited by the parent molecule, dithieno[3,2-b:2′,3′-d]thiophene. Quantum-chemical studies of the 2,6-diiododithieno[3,2-b:2′,3′-d]thiophene crystal suggest uniaxial hole-transport character with an effective mass of about 2m0, comparable to that in the pentacene single crystal.

Organic Materials for All-Optical Signal Processing and Optical Limiting

Abstract: Third-order nonlinearities and optical switching and limiting figures of merit are reported for several conjugated organic materials. Polymethines with large real to imaginary hyperpolarizability ratios and conjugated polymers with strong nonlinear absorption will be discussed.

Polymeric Ambipolar Hosts for Large-Area Phosphorescent Light-Emitting Diodes

Abstract: We report on a polymer bearing pendant ambipolar carbazole / oxadiazole moieties that can be used as an efficient host material for green-emitting iridium-based phosphorescent guests and discuss large-area diodes without indium-tin oxide electrodes.

Cyanine dyes with exceptional third-order nonlinear optical figures-of-merit for all-optical switching

Abstract: A molecular design strategy that involves favorable control of one- and two-photon absorption resonances to produce exceptional nonlinear optical figures-of-merit for all-optical switching has been realized in a series of cyanines dyes.