Spin transport in higher n-acene molecules

We investigate the spin transport properties of molecules belonging to the acenes series by using density functional theory combined with the non-equilibrium Green's function approach to electronic transport. While short acenes are found to be non-magnetic, molecules comprising more than nine acene rings have a spin-polarized ground state. In their gas phase these have a singlet total spin configuration, since the two unpaired electrons occupying the doubly degenerate highest molecular orbital are antiferromagnetically coupled to each other. Such an orbital degeneracy is however lifted once the molecule is attached asymmetrically to Au electrodes via thiol linkers, leading to a fractional magnetic moment. In this situation the system Au/n-acene/Au can act as an efficient spin-filter with interesting applications in the emerging field of organic spintronics.Comment: 13 pages,9 figure

Charge Delocalization in a Homologous Series of a,a’-Bis(dianisylamino)-Substituted Thiophene Monocations

A homologous series of three molecules containing thiophene, bithiophene, and terthiophene bridges between two redox-active tertiary amino groups was synthesized and explored. Charge delocalization in the one-electron-oxidized forms of these molecules was investigated by a combination of cyclic voltammetry, near-infrared optical absorption spectroscopy, and EPR spectroscopy. All three cation radicals can be described as organic mixed-valence species, and for all of them the experimental data are consistent with strong delocalization of the unpaired electron. Depending on what model is used for analysis of the optical absorption data, estimates for the electronic coupling matrix element (HAB) range from ∼5000 to ∼7000 cm–1 for the shortest member of the homologous series. According to optical absorption and EPR spectroscopy, even the terthiophene radical appears to belong either to Robin–Day class III or to a category of radicals commonly denominated as borderline class II/class III systems. The finding of such a large extent of charge delocalization over up to three adjacent thiophene units is remarkable

Electrochemical Route to Solution-Processable Polymers of Thiophene/Selenophene Capped Didodecyloxybenzo[1,2‑<i>b</i>:4,3‑<i>b</i>′]dithiophene and Their Optoelectronic Properties

Two new solution-processable polymers <b>P1</b> and <b>P2</b> are being reported here, which were prepared by electrochemical polymerization of thiophene and selenophene capped 7,8-didodecyloxybenzo­[1,2-<i>b</i>:4,3-<i>b</i>′]­dithiophene (BdT-Dod), respectively and characterized by gel permeation chromatography (GPC) and ¹H NMR. The selenophene containing polymer possesses lower band gap than the thiophene analogue. Density functional theory (DFT) calculation showed the highly curved structure of the polymers and reproduced the trend in their optical band gaps. <b>P2</b> showed larger bathochromic shift in the absorption spectrum from solution to film state compared to that of <b>P1</b>, which indicates better π-stacking interaction in the solid state for <b>P2</b>. In spite of having highly curved chains, the polymers successfully exhibited electrochromic switching. The exchange of the end-caps from thiophene to selenophene have manifested with higher electrochromic switching ability and better polaronic and bipolaronic features in spectroelectrochemical measurement of <b>P2</b> than that of <b>P1</b>. Kinetic study on the polymer films using chronoamperometry revealed that the selenophene containing polymer <b>P2</b> afforded Δ%<i>T</i> of ∼60 in the visible region with a coloration efficiency of 100 cm² C^–1. Electrochemical polymerization of BdT-Dod using different solvent/electrolyte systems was unsuccessful

The isolation and crystal structure of a cyclic selenenate ester derived from bis(2,6-diformyl-4-tert-butylphenyl)diselenide and its glutathione peroxidase-like activity

Enzyme mimicry: Halogenation of diselenide 1 affords the cyclic selenenate ester 2 (see scheme) through a highly unstable selenenic acid intermediate. Compound 2 exhibits a strong intramolecular nonbonding Se···O interaction. Both 1 and 2 show excellent glutathione peroxidase-like catalytic activity

Benzoselenadiazole Containing Donor–Acceptor–Donor Small Molecules: Nonbonding Interactions, Packing Patterns, and Optoelectronic Properties

Herein, we describe the fine-tuning of intermolecular Se···N interaction in benzoselenadiazole (BDS) derivatives to form head-to-head dimers in the solid state. The structures and photophysical properties of phenyl-, thiophene-, and selenophene-capped BDS (<b>1–3</b>, respectively) are studied here. Because of the presence of the strong intramolecular Se···N interaction, selenophene-capped BDS <b>3</b> showed <i>syn</i> arrangement of two capped selenophene rings, whereas two thiophene rings in <b>2</b> showed an <i>anti</i> orientation. Compounds <b>1</b> and <b>2</b> showed the tendency to form head-to-head dimers in the solid state through the intermolecular Se···N interactions. In contrast to compounds <b>1</b> and <b>2</b>, compound <b>3</b> does not form a dimer in the solid state and, instead, shows strong intramolecular Se···N interactions. The tendency to form dimers largely depends on the nonbonding interactions and the steric effect of capped rings

Synthesis of diaryl selenides using the in situ reagent SeCl<SUB>2</SUB>

Reactions of in situ prepared SeCl<SUB>2</SUB> with Grignard reagents (prepared from bromobenzene, o-tolyl bromide, 2,6-dimethyl-4-tert-butyl-1-bromobenzene, and 1-bromo-2-methylnaphthalene) and dilithiated benzamides (prepared from N-phenyl, N-cyclohexyl, and N-isopropyl benzamide) are described

Formation of Acene-Based Polymers: Mechanistic Computational Study

Understanding the mechanism of linear acene decomposition and its reactivity is a prerequisite for controlling the stability of acenes and their future applications. Previously, we suggested that long acenes may undergo polymerization since the polymerization product is thermodynamically more stable than the dimerization product. However, due to kinetic considerations, the most thermodynamically stable product, the polymer, might not necessarily be formed. To elucidate the situation, we investigated the mechanisms of acene polymerization computationally, using pentacene, hexacene, and heptacene as representative examples. Similarly to dimerization, acene polymerization follows a stepwise biradical pathway. Structural and steric hindrance of the polymer backbone forces acene polymerization to proceed via the less reactive noncentral benzene rings. Consequently, dimerization is always kinetically more favorable than polymerization, irrespective of acene length. Although, for long acenes starting from hexacene, both polymerization and dimerization are barrierless pathways relative to the reactants, polymerization is thermodynamically preferred for hexacene and heptacene and even more so for longer acenes (since polymerization forms four new C–C bonds while dimerization forms only two). Indeed, reinvestigation of available experimental data suggests that acene-based polymers were probably obtained experimentally previously

Cyclopenta[<i>c</i>]thiophene-Based D–A Conjugated Copolymers: Effect of Heteroatoms (S, Se, and N) of Benzazole Acceptors on the Properties of Polymers

Three new donor–acceptor (D–A) type copolymers <b>P1</b>, <b>P2</b>, and <b>P3</b> have been synthesized by Stille condensation between the distannyl derivative of thiophene-capped cyclopenta­[<i>c</i>]­thiophene (CPT) with 4,7-dibromo­[2,1,3]­benzothiadiazole, 4,7-dibromo­[2,1,3]­benzoselenadiazole, and 4,7-dibromo­[2,1,3]­benzotriazole, respectively. These new CPT-based D–A copolymers showed an interesting trend of visible color (red, green, and blue) in solution as the acceptor was varied keeping the donor constant. The optical band gaps of the polymers, which were estimated by measuring the absorption onset in the UV–vis spectra of the film, were found to be 1.57, 1.44, and 1.86 eV for <b>P1</b>, <b>P2</b>, and <b>P3</b>, respectively. DFT calculations correlated the strength of the acceptors with the interesting trend in the colors of these (D)_nonvariant–(A)_variant copolymers. Compared with the solution, the film state absorption of <b>P2</b> and <b>P3</b> was significantly red-shifted compared to that of <b>P1</b>, indicating the presence of strong interchain interactions due to efficient self-π-stacking in the solid state

Cyclopentac]thiophene oligomers based solution processable D-A copolymers and their application as FET materials

Two new solution processable, low band gap donor-acceptor (D-A) copolymers (P1 and P2) comprising a cyclopentac] thiophene (CPT) based oligomers as donors and benzoc]1,2,5] selenadiazole (BDS) and 2-dodecyl1,2,3]-benzotriazole (BTAz) as acceptors were synthesized and characterized and their field effect transistor properties were studied. The internal charge transfer interaction between the electron-donating CPT based oligothiophene and the electron-accepting BDS or BTAz unit effectively reduces the band gap in polymers to 1.3 and 1.66 eV with low lying highest occupied molecular orbital (HOMO). The absorption spectrum of P1 was found to be more red shifted than that of P2 because of incorporation of the more electron-withdrawing BDS unit. The color of neutral P1 was found to be green in both solution and film states with two major bands in the absorption spectra; however, neutral P2 revealed one dominant absorption exhibiting red color in both solution and film state which could be attributed to the less electron-withdrawing effect of the BTAz unit. The polymers were further characterized by GPC, TGA, DSC and cyclic voltammetry. P1 and P2 exhibited charge carrier mobilities as high as 9 x 10(-3) cm(2) V-1 s(-1) and 2.56 x 10(-3) cm 2 V-1 s(-1), respectively with the current on/off ratio (I-on/I-off) in the order of 10(2)