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An organic solar cell fabrication kit has been developed for demonstration in both undergraduate teaching classes and high school laboratories to promote the growing field of renewable energy and t...

Monodispered ZnO nanocrystals (NCs) were found to quench the fluorescence of two donor−π system–acceptor (D−π–A) dyes; (E)-2-cyano-3-[5-[4-(diethylamino)phenyl]thiophen-2-yl]-2-propenoic acid, 1, a...

X-ray structural determinations and computational studies were used to investigate halogen interactions in two halogenated oxindoles. Comparative analyses of the interaction energy and the interaction properties were carried out for Br···Br, C-H···Br, C-H···O and N-H···O interactions. Employing Møller–Plesset second-order perturbation theory (MP2) and density functional theory (DFT), the basis set superposition error (BSSE) corrected interaction energy (Eint(BSSE)) was determined using a supramolecular approach. The Eint(BSSE) results were compared with interaction energies obtained by Quantum Theory of Atoms in Molecules (QTAIM)-based methods. Reduced Density Gradient (RDG), QTAIM and Natural bond orbital (NBO) calculations provided insight into possible pathways for the intermolecular interactions examined. Comparative analysis employing the electron density at the bond critical points (BCP) and molecular electrostatic potential (MEP) showed that the interaction energies and the relative orientations of the monomers in the dimers may in part be understood in light of charge redistribution in these two compounds.

Direct (hetero)arylation polymerization (DHAP) of a monobrominated benzo[1,2-b:4,5-b′]dithiophene monomer using the Herrmann–Beller catalyst with a tertiary phosphine provided benzodithiophene homopolymers in good yields. Employing both P(o-OMePh)3 and P(o-NMe2Ph)3 as the phosphine ligands gave well-defined polymers—with the later phosphine providing a higher molecular weight polymer. The preparation of a benzodithiophene (BDT) trimer was used to assist in the assignment of the 1H NMR spectra of the synthesized polymers which show largely defect-free couplings. The optical spectra of polymers formed via DHAP and those prepared using traditional Stille couplings are essentially identical, which further confirms the presence of well-defined BDT–BDT couplings along the conjugated polymer chain. These results confirm that carboxylic acid additives are not always necessary to suppress defects in DHAP polymerizations and that DHAP is a viable alternative to traditional Stille coupling for the preparation of benzodithiophene homopolymers.

The crystal structure of 2-(5-bromo­thio­phen-2-yl)aceto­nitrile, previously reported as a liquid, has short centrosymmetric Type I Br⋯Br halogen inter­actions., The title compound, C6H4BrNS, crystallizes in the space group P21/n with one complete mol­ecule in the asymmetric unit. The non-H atoms are nearly planar (r.m.s for non-H atoms = 0.071 Å), with the nitrile group oriented anti­periplanar with respect to the thio­phene S atom. Inter­molecular Type I centrosymmetric Br⋯Br halogen inter­actions are present at a distance of 3.582 (1) Å and with a C—Br⋯Br angle of 140.7 (1)°. Additional weaker C—H⋯N, C—H⋯S, and S⋯π inter­actions are also present. A Hirshfeld analysis indicates Br⋯Br inter­actions comprise only 1.9% of all the inter­atomic contacts.

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.

Reactions between 3,4-propylenedioxythiophenes (ProDOTs) and N-alkyl isatins under ambient conditions result in isomerically pure indophenine materials as confirmed by TLC and 1H NMR analysis. The resulting low band gap materials exhibit favorable inter- and intramolecular interactions, high thermal stabilities, low energy electronic transitions, and amphoteric redox behavior.

Addition of tert-butoxycarbonyl (Boc) groups to 6,6′-dibromoisoindigo (3a) provides a highly soluble material that displays π-stacking in the solid state as evidenced by a single crystal X-ray study. This Boc-substituted molecule (3b) was compared to an alkyl-substituted isoindigo using techniques such as TGA, UV–Vis and cyclic voltammetry. TGA analysis reveals the thermolabile Boc groups of 3b can be removed at elevated temperatures to afford the parent isoindigo. Electrochemical analyses, in conjunction with density functional theory (DFT) calculations, reveals enhanced electron affinities of 3b relative to alkyl-substituted isoindigos.

New stilbenoid and thiophenic compounds terminally functionalized with donor-donor, acceptor-acceptor, or donor-acceptor moieties and possessing a central [2.2]paracyclophane unit have been prepared, and their properties interpreted in terms of through-bond and through space π-electron delocalization (i.e., π-conjugations). Based on photophysical data, their excited-state properties have been described with a focus on the participation of the central [2.2]paracyclophane in competition with through-bond conjugation in the side arms. To this end, two-photon and one-photon absorption and emission spectroscopy, as a function of temperature, solvent polarity, and pressure in the solid state have been recorded. Furthermore, charge delocalization through the [2.2]paracyclophane in the neutral state and in the oxidized species (radical cations, dications and radical trications) has been investigated, allowing the elucidation of the vibrational Raman fingerprint of through-space charge delocalization. Thus, a complementary approach to both "intermolecular" excitation and charge delocalizations in [2.2]paracyclophane molecules is shown which can serve as models of charge and exciton migration in organic semiconductors.

Quinoidal-vs.-aromatic synergy: Cyclopentadithiophene-vinylene oligomers with different sizes, in different oxidation states and as pro-aromatic quinoidals are studied considering balanced contributions of aromatic and quinoidal forms., A new series of π-conjugated oligomers based on the 4,4 dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene vinylene repeating unit has been prepared and characterized by X-ray, electrochemical, spectroscopic (UV-Vis absorption, emission and Raman) and density functional theory methods. The oligomers in their neutral, oxidized and reduced forms have been investigated. The neutral compounds show a longer mean conjugation length than oligothiophenes and oligothiophene-vinylenes and display very rich redox chemistry with the stabilization of polycationic states of which the radical cations and dications are strong NIR absorbers, the latter displaying singlet diradicaloid character. An interesting complementarity between the sequence of aromatic-quinoidal structural segments in the radical cations and dications has been described and interpreted. Two derivatives with the 4,4 dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene vinylene unit, disubstituted either with electron donor, bis(triaryl amino) groups, or acceptors bis(dicyano-methylene) caps enforcing a quinoidal structure in the dithiophene-vinylene bridge, have been also synthesized and characterized. The radical cation of the triarylamine compound and the radical anion of the tetracyano compound similarly display hole and electron charge localization, or confinement, in the nitrogen and dicyano surrounding parts, or class II mixed valence systems, while their dication and dianion species, conversely, are open-shell diradical (i.e., polaron pair) and closed-shell (i.e., bipolaron), respectively. The preparation of these new π-conjugated oligomers gives way to the realization of compounds with new electronic properties and unique structures potentially exploitable in organic electronics.

Benzo[1,2-b:4,5-b']dithiophenes were oxidized under mild conditions with m-CPBA to yield the corresponding bis-sulfones (or tetraoxides). These sulfones possess red-shifted absorption and emission spectra relative to the parent molecules. Electrochemical analyses reveal that the benzodithiophene molecules are transformed from electron donors to electron acceptors.

A series of experiments for undergraduate courses (e.g., organic, physical) have been developed in the area of small molecule organic materials. These experiments focus on understanding the electronic and redox properties of a donor–acceptor molecule that is prepared in a convenient one-step microscale reaction. The resulting intensely colored material exhibits an electronic transition in the visible portion of the electromagnetic spectrum and is indicative of intramolecular charge transfer (ICT). The ICT and redox properties are investigated with conventional analytical techniques and are understood with the use of density functional theory. These experiments illustrate the importance of organic materials in areas such as solar energy conversion. As a result of these experiments, students are exposed to a number of important chemical concepts as well as useful physical and theoretical methods used by experts in the field.

We have synthesized several new push-pull oligothiophenes based on the BODIPY moiety as the electron acceptor and the well known oligothiophenes substituted with N,N-dialkylamino groups to enhance the electron donor ability. A complete characterization of the electronic properties has been done consisting of their photophysical, electrochemical and vibrational properties. The compounds have been studied after chemical treatment with acids and after oxidation. In this regard, they can be termed as NIR dyes and amphoteric redox electroactive molecules. We have described the presence of dual fluorescence in these molecules and fluorescence quenching either by energy transfer or, in the push-pull molecules, by the electron exchange. The combination of electrochemical and proton reversibility combined with the interesting optical properties of the new species offer an interesting platform for sensor and material applications., DGES, MEC (Spain) and Fundação para a Ciência e a Tecnologia (FCT)

New direct syntheses of [Pt(trpy)(NCCH3)](CF3SO3)2 2 (where trpy = 2,2′:6′,2′′-terpyridine) and [Pt(tBu3-trpy)(NCCH3)](CF3SO3)2 3 (where tBu3-trpy = 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine) via the displacement of acetonitrile from [Pt(NCCH3)4](CF3SO3)2 have been developed. The synthetic utility of 2 was investigated in reactions with triphenylphosphine (PPh3), 2,6-dimethylphenyl isocyanide (CN-Xyl), 2,5-dimethyl-2,5-diisocyanohexane (TM4), and tert-butyl isocyanide (CN-tBu). Whereas the expected substitution products were observed for reactions with PPh3, CN-Xyl, and CN-tBu, dealkylation of TM4 occurred to afford [Pt(trpy)(CN)](CF3SO3) 6. The structures of [Pt(trpy)L]2+ dications show little intermolecular interactions in the solid state, with the exception of the tBu3-trpy complex 3 which exists as head-to-tail dimers with a Pt–Pt distance of 3.29 A. The cyano product 6 was found to stack in infinite chains of cations with a Pt–Pt distance of 3.45 A.

A series of experiments for undergraduate laboratory courses (e.g., organic, polymer, inorganic) have been developed. These experiments focus on understanding the regiochemistry of the conducting polymer poly(3-hexylthiophene) (P3HT). The substitution patterns in P3HTs control their conformational features, which, in turn, dictates the π conjugation and subsequent electronic properties of the materials. The regiochemistry in P3HTs can be controlled synthetically, and this experiment includes the synthesis of regioregular P3HT via Grignard metathesis chemistry. The regiochemistry is established unambiguously using techniques such as 1H NMR and UV−vis spectroscopy; comparisons to the regiorandom isomer are then made. Density functional theory calculations are also used to shed light on the electronic and conformational features of the polymers by analyzing suitable oligomers. These experiments illustrate the importance of regiochemistry in organic materials and expose students to a number of important chemi...

Photophysical data for a series of end substitued 3',4'-dibutyl-2,2':5',2''-terthiophenes are reported. Static absorption and fluorescence, quantum yields, time-resolved fluorescence, and time- and frequency-resolved pump-probe spectra are applied to investigate excited state relaxation in bromo, nitro, and tricyanovinyl substituted species in a variety of solvents. The effect of solvent polarizability and end-group substitution is discussed in the context of charge transfer in the excited state and its impact on nonradiative decay rates. In solution at room temperature, both symmetric and asymmetric addition of electron withdrawing end groups generate an excited state with substantial charge transfer character. Solvent polarizability has a significant influence on the excited state dynamics in the charge transfer compounds. Examples include a 20-fold reduction in the intersystem crossing rate going from hexane to toluene and an order of magnitude increase in the internal conversion rate between toluene and acetone. The results demonstrate that the impact of the substituents on intramolecular charge transfer, and the resulting amplification of the interactions between the excited state(s) and the local molecular environment, can dramatically change the excited state relaxation dynamics in substituted terthiophenes.

In this project, an alternative approach to synthesize ammonia electrochemically from wind energy, nitrogen gas, and ethanol is reported. Initial electrolyses have been attempted using room temperature ionic liquids (RTILs) as the electrolyte solvent in the presence of lithium ions. Bis(trifluoromethanesulfonyl)imide salts of 1-Ethyl-3-methylimidizolium (EMI-TFSI) and 1-Butyl-1-methylpyrrolidinium (PYR14-TFSI) were tested in this investigation. These results are compared to the use of tetrahydrofuran as the solvent. The data suggests EMI-TFSI is not an appropriate ionic liquid for the lithium mediated reduction of N2, whereas the increased electrochemical stability of PYR14-TFSI offers greater promise. Current efficiencies (3.8%) for NH3 formation in THF are consistently higher than PYR14-TFSI at atmospheric temperatures and pressures. The amount of ammonia synthesized was quantified by performing the Berthelot reaction.

Oligothiophene sulfones of up to six rings can be conveniently prepared by the direct oxidation of butyl-substituted thiophene oligomers with m-CPBA in dichloromethane. Reverse selectivity of oxidized rings is observed relative to previously reported systems without beta-substitution. The selectivity in the trimer and hexamer is confirmed with single-crystal X-ray structure data. The sulfones possess red-shifted absorptions and increased electron affinities relative to the parent oligomers.

We report the structural and electrical characterization of thin films of organic semiconductor molecules consisting of an oligothiophene core capped with electron-withdrawing tricyanovinyl (TCV) groups. X-ray diffraction and atomic force microscopy of evaporated films of three different TCV-capped oligothiophenes showed that the films were highly crystalline. Electrical transport was measured in thin film transistors employing silver source and drain contacts and channel probes to correct for contact resistance. Three compounds exhibited n-channel (electron) conduction consistent with cyclic voltametry data that indicated they undergo facile reduction. Maximum electron mobilities were 0.02 cm2/V.s with an on/off current ratio of 10(6). A fourth end-capped molecule, TCV-6T-TCV, which had six thiophene rings, exhibited both p- and n-channel transport. Overall, these results confirm that substitution of oligothiophene cores with electron-withdrawing groups is a useful strategy to achieve electron-transporting materials.

A series of new quinodimethane-substituted terthiophene and quaterthiophene oligomers has been investigated for comparison with a previously studied quinoid oligothiophene that has demonstrated high mobilities and ambipolar transport behavior in thin-film transistor devices. Each new quinoidal thiophene derivative shows a reversible one-electron oxidation between 0.85 and 1.32 V, a quasi-reversible one-electron second oxidation between 1.37 and 1.96 V, and a reversible two-electron reduction between -0.05 and -0.23 V. The solution UV-vis-NIR spectrum of each compound is dominated by an intense (epsilon congruent with 100 000 M(-1) cm(-1)) low energy pi-pi transition that has a lambda(max) ranging between 648 and 790 nm. All X-ray crystal structures exhibit very planar quinoidal backbones and short intermolecular pi-stacking distances (3.335-3.492 A). Structures exhibit a single pi-stacking distance with parallel cofacial stacking (sulfur atoms of equivalent rings pointed in the same direction) or with alternating distances and antiparallel cofacial stacking (sulfur atoms of equivalent rings pointed in the opposite direction). Examples of the layered and herringbone-packing motifs are observed for both the parallel and the antiparallel cofacial stacking. Analysis of the X-ray structures and molecular orbital calculations indicates that all of these compounds have one-dimensional electronic band structures as a result of the pi-stacking. For structures with a unique pi-stacking distance, a simple geometric overlap parameter calculated from the shape of the molecule and the slip from perfect registry in the pi-stack correlates well with the transfer integrals (t) calculated using molecular orbital theory. The calculated valence (633 meV) and conduction (834 meV) bandwidths for a quinoid quaterthiophene structure are similar to those calculated for the benchmark pentacene and indicate that both hole and electron mobilities could be significant.

A series of radial thiophene-based structures consisting of a central benzene or thiophene ring surrounded by acetylene-bridged terthienyl arms has been investigated by physical and theoretical methods. Fourier transform Raman spectroscopy of the neutral solids shows that the terthiophene arms are weakly coupled across the core (benzene plus acetylene groups) likely due to cross-conjugation or meta-conjugation effects that may prevent full delocalization. By increasing the number of arms around the central ring, the electronic structure of the molecules seems to be affected only at the core, whereas the outer terthiophene arms remain almost unaltered. Raman spectroelectrochemistry and quantum chemical calculations provide further insight into the charge delocalization of the oxidized species. There is no evidence to suggest that these oxidized forms, obtained upon electrochemical doping of the molecules, show charge delocalization across the core.

A series of quaternized poly(2-vinylpyridine) (PVP) polymers has been synthesized with the general formula [(PVP) R ]I, where R =H, Me, Bu. Addition of AgI to the polymers results in new silver ion conducting solid polymer materials. Conductivities as high as 6×10 −3 S/cm have been observed for the 3:1 AgI/[(PVP)Me]I composition. Analogous materials with a 3:1 composition for the protonated and butyl-substituted polymers display lower conductivities near 2×10 −4 S/cm. Conductivity studies, as a function of temperature, show these materials to be thermodynamically stable with a low activation energy for ion conduction. Differential scanning calorimetry experiments suggest that a new phase with the general formula (PVP) R Ag 3 I 4 forms with silver ions in a new amorphous state.

A quinoid-type oligothiophene, 3 0 ,4 0 -dibutyl-5,5 00 -bis(dicyanomethylene)-5,5 00 -dihidro-2,2 0 :5 0 ,2 00 -terthiophene, which can be viewed as an analog of TCNQ, has been investigated by spectroelectrochemistry and density functional theory calculations, in its neutral and dianionic states. Electrochemical data show that the molecule can be both reduced and oxidized at relatively low potentials. Upon reduction, both experiments and theory agree well with the generation of a dianionic charged species. The model shows that the electronic structure of the dianion is consistent with two anionic dicyanomethylene groups attached to a central terthienyl spine having an aromatic structure. The two negatively charged dicyanomethylene groups conjugate with the terthiophene allowing for the extension of the p-delocalized system over the whole molecule. The stability of the dianionic species, due to its aromatic character, gives support to the low value for the electrochemical two-electron reduction process. Infrared spectra of the neutral and of the dianion species have been assigned and correlated. q 2003 Elsevier Science B.V. All rights reserved.

[structure: see text] A series of thiophene-based homologues with an aromatic core surrounded by terthiophene "arms" with acetylene linkages has been synthesized by using Sonogashira coupling methods. The homologues were investigated spectroscopically in solution and in the solid state. They display extended pi-conjugation through the aromatic core that affects the strong emission and redox properties.

Two new oligothiophenes, the dinitro compound 3',4'-dibutyl-5,5' '-dinitro-2,2':5',2' '-terthiophene (1) and the quinodimethane 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (2), have been synthesized and studied with electrochemistry, UV-vis-NIR-IR spectroscopy, ESR, and X-ray crystallography. These compounds, designed to be both electron and hole carriers, show redox properties that are unusual for oligothiophenes. Cyclic voltammetry and spectroelectrochemistry demonstrated that each compound could be oxidized to a cation radical and reduced to an anion radical and dianion. The spectra of 2 and its three redox partners were analyzed in terms of a limiting structure in which the neutral 2 has orbitals corresponding to those of a substituted-terthiophene dication. Compound 1 crystallizes with the thiophene rings held in an unusual nonplanar, cisoid configuration in face-to-face pi-stacks, with a spacing between molecules of 3.65 A. The C-C bond lengths of the outer nitro-substituted rings have quinoid character. Compound 2 crystallizes with the thiophene rings in a planar, transoid configuration. The molecules are held in pi-stacks formed from pi-dimers with a spacing between molecules of 3.47 and 3.63 A. The C-C bond distances of the thiophene rings of 1 and 2 and other oligomers were analyzed by a principal component analysis. The analysis found that 93% of the structural variance resided in one principal component related to the quinoid structure of the oligothiophene moiety. The analysis reliably demonstrated a quinoid contribution to the structure of 1. This method should be applicable to understanding the structure of other conjugated molecules in which quinoid structures contribute.

The electronic and the Fourier transform Raman (FT-Raman) spectra of a novel well−barrier−well vinylene-bridged sexithiophene in neutral and in doped states have been recorded. The samples were oxidized chemically, with trifluoroacetic acid or ferric chloride, and electrochemically in dichloromethane solutions. The evolution of the Raman spectral pattern upon oxidation at different anodic potentials has been analyzed for a thin solid film of the material. Experimental observations are explained by the initial generation of a radical cation species followed by the generation of a dication species. A comprehensive assignment of the electronic and Raman spectroscopic features observed in both neutral and oxidized states has been performed on the basis of density functional theory (DFT) calculations. Time-dependent DFT calculations have been performed to calculate the electronic excitation energies. The experimental data and theoretical results give a clear picture of the structure and electronic properties o...

The infrared absorption and Raman scattering spectra recorded during the electrochemical oxidation of two sexithiophenes are reported. Using in situ spectroelectrochemical methods five oxidation states, neutral, radical cation, dication, radical trication, and tetracation of an α,α‘-diphenyl sexithiophene, were studied. The experimental spectra are in agreement with the predictions of the effective conjugation coordinate theory and are in good accordance with theoretical density functional theory calculations.

A simple two-step procedure is elaborated for the preparation of dialkylated benzodithiophenes (III) and (V) from dione (I).

A series of eight conjugated oligomers consisting of central dithieno[3,2-b:2',3'-d]pyrroles (DTPs) end-capped with either thienyl or phenyl groups have been prepared from N-alkyl-, N-aryl-, and N-acyl-dithieno[3,2-b:2',3'-d]pyrroles via Stille and Suzuki cross-coupling. The DTP-based quaterthiophene, N-phenyl-2,6-bis(2-thienyl)dithieno-[3,2-b:2',3'-d]pyrrole was characterized via X-ray crystallography and was found to crystallize in the orthorhombic space group Pna2(1) with a = 10.8666(3) Å, b = 22.8858(6) Å, c = 7.4246(2) Å, and Z = 4. The full oligomeric series was thoroughly investigated via photophysical, electrochemical, and DFT calculations in order to correlate the cumulative effects of both aryl end-groups and N-functionalization on the resulting optical and electronic properties. Through such molecular tuning, it was found to be possible to modulate the HOMO energy by as much as 0.32 V and to generate highly fluorescent oligomers with solution fluorescence efficiencies as high as 92%.

13 páginas, 13 figuras, 3 tablas, 2 esquemas.-- et al., In this paper we have prepared a new series of oligothiophenes capped with hexyl groups and a variety of strong acceptors, mainly cyanovinyl moieties. An exhaustive analysis of the absorption, photophysical, electrochemical, solid state, nonlinear optical and vibrational properties has been presented guided by theoretical calculations. The investigation is centered on the efficiency of the intramolecular charge transfer (i.e., chain length and acceptor dependence) and its impact on all the relevant electronic, structural, optical, and vibrational properties. The most significant features imparted by the acceptors through the π–conjugated oligothiophene path are (i) intense visible electronic absorptions, (ii) tuned fluorescence wavelength emissions, (iii) solid state π–stacking, (iv) ambipolar redox behavior, (v) S1 -- S0 internal conversion as being the major route for the deactivation of the excited state, and (vi) large electronic and vibrational contributions to their nonlinear optical response (hyperpolarizability). The analysis establishes connections between the different properties of the materials and structure–function relationships useful in organic electronics., We are indebted to the Ministerio de Ciencia e Innovaci on through the project references CTQ2009-10098, CTQ2008- 02942, and MAT2008-06522-C02-02. S.R.G. and R.A. acknowledge the Junta de Andalucía and the BES 2006-12104 grant, respectively. The Gobierno de Arag on through the project reference E39 and the Junta de Andalucía by means of the research project PO9-4708 are also acknowledged.

Oligothiophene-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2þ 2] cycloaddition reactions between tetracyanoethylene and oligothiophene alkynes. The TCBD moiety is compared to other electron acceptors attached to dibutylterthiophene including dicyanovinyl (DCV) and tricyanovinyl (TCV).These donor-acceptor molecules (TCBD-3T, DCV-3T, andTCV-3T) show red-shifted absorption spectra relative to the unsubstituted oligothiophene as a result of intramolecular charge-transfer (ICT). Monosubstituted terthiophenes bearing the electron acceptors show both oxidation and reduction processes as characterized by cyclic voltammetry. Density functional theory (DFT) calculations are used to explain the electronic and redox properties of the materials. Electrochemical oxidation of a bis(terthienyl)-substitutedTCBDmolecule (3T-TCBD-3T) yields a conducting polymer exhibiting balanced ambipolar redox conduction with similar values for the oxidized and reduced states of the polymer (1 _ 10-3 S cm-1). Raman spectra of the asymmetric donor-acceptor materials are characterized by two intense bands characteristic of the aromatic and quinoidal regions in the conjugated À-system of the oligothiophene.

We have prepared a new series of mixed thiophene-pyrrole oligomers to investigate the electronic benefits arising from the combination of these two heterocycles. The oligomers are functionalized with several hexyl and aryl groups to improve both processability and chemical robustness. An analysis of their spectroscopic (absorption and emission), photophysical, electrochemical, solid state, and vibrational properties is performed in combination with quantum-chemical calculations. This analysis provides relevant information regarding the use of these materials as organic semiconductors. The balance between the high aromatic character of pyrrole and the moderate aromaticity of thiophene allows us to address the impact of the coupling of these heterocycles in conjugated systems. The data are interpreted on the basis of the aromaticity, molecular conformations, ground and excited electronic state structures, frontier orbital topologies and energies, oxidative states, and quinoidal versus aromatic competition.

Direct addition of tetracyanoethylene to N-(p-hexylphenyl)dithieno[3,2-b:2',3'-d]pyrrole yields not only the aromatic mono- and bis-tricyanovinyl-substituted products but also a quinoidal product with dicyanomethylene groups. The analogous reaction with dithieno[3,2-b:2',3'-d]thiophene yields exclusively the aromatic mono-tricyanovinyl product. The aromatic and quinoidal products possess red-shifted absorptions, increased electron affinities, and favorable pi-stacking motifs in comparison to the unsubstituted oligomers.

Oligothiophene sulfones of up to six rings can be conveniently prepared by the direct oxidation of butyl-substituted thiophene oligomers with m-CPBA in dichloromethane. Reverse selectivity of oxidi...

We analyze the electronic and molecular structures for the ground and excited electronic states of aromatic terthiophene (3T), the quinodimethane 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (3Q), and isologues with the middle ring S-oxidized (3TO2, 3QO2). These represent extremes of electron rich and deficient ground states, often exhibiting complementary properties. Oxidizing the central sulfur atom affects the molecular structure, electron affinity, and photophysical properties of both pi systems. The consequences for 3T include de-aromatization of the central thiophene, red-shifting of the electronic absorption spectrum, and lowering of the reduction potential. The electron deficient quinoid 3QO2 shows an enhancement of electron affinity from reducing the electron-donor ability of sulfur, and a blue-shifting of its electronic absorption spectrum was seen. Fluorescence emission is quenched in the sulfonated terthiophene, and the contrary effect again would be expected upon sulfonation of a quinoid emitter. Raman vibrational spectroscopy, electrochemistry, and UV-vis and fluorescence spectroscopies are analyzed in conjunction with theoretical calculations.

13 pages, 13 figures, 3 tables., A series of tricyanovinyl (TCV)-substituted oligothiophenes was synthesized and investigated with a number of physical methods including UV/Vis, IR, and Raman spectroscopy, nonlinear optical (NLO) measurements, X-ray diffraction, and cyclic voltammetry. Mono- or disubstituted oligomers were prepared by the reaction of tetracyanoethylene with mono- or dilithiated oligomers. The comparative effects of the symmetric and asymmetric substitutions in the electronic and molecular properties have been addressed. These oligomers display dramatic reductions in both their optical and electrochemical band gaps in comparison with unsubstituted molecules. The analysis of the electronic properties of the molecules was assisted by density functional theory calculations, which are in excellent agreement with the experimental data. TCV substitution influences the energies of the frontier orbitals, especially with respect to the stabilization of LUMO orbitals. X-ray structural characterization of a monosubstituted oligomer exhibits -stacking with favorable intermolecular interactions. NLO results agree with the role of the intramolecular charge-transfer feature in the asymmetric samples. These results furthermore exalt the role of conformational flexibility in the disubstituted compounds and reveal an unexpected nonlinear optical activity for symmetric molecules. Regarding the electronic structure, the interpretation of the vibrational data reflects the balanced interplay between aromatic and quinoid forms, finely tuned by the chain length and substitution pattern. The electronic and structural properties are consistent with the semiconducting properties exhibited by these materials in thin film transistors (TFTs)., J.C. is grateful to the Ministerio de Ciencia y Tecnología (MCyT) of Spain for a Ramón y Cajal research position of Chemistry at the University of Málaga. The present work was supported in part by the Dirección General de Enseñanza Superior (DGES, MEC, Spain) through the research projects BQU2003–03194 and BQU2002-00219. We are also indebted to Junta de Andalucía (Spain) (FQM-0159) and Gobierno de Aragón (E39) funding for our research groups. M.C.R.D. thanks the MEC of Spain for a personal grant. T.M.P. acknowledges a Grant-in-Aid of Research, Artistry and Scholarship from the Office of the Dean of the Graduate School of the University of Minnesota.

The UV/Vis, infrared absorption, and Raman scattering spectra of 3',4'-dibutyl-5,5"-bis(dicyanomethylene)-5,5"-dihydro-2,2':5',2"-terthiophene have been analyzed with the aid of density functional theory calculations. The compound exhibits a quinoid structure in its ground electronic state and presents an intramolecular charge transfer from the terthiophene moiety to the C(CN)2 groups. The molecular system therefore consists of an electron-deficient terthiophene backbone end-capped with electron-rich C(CN)2 groups. The molecule is characterized by a strong absorption in the red, due to the HOMO-->LUMO pi-pi* electronic transition of the terthiophene backbone that shifts hypsochromically on passing from the solid state to solution and with the polarity of the solvent. The analysis of the vibrational spectra confirms the structural conclusions and supports the existence of an intramolecular charge transfer. Vibrational spectra in several solvents and as a function of temperature have also been studied. Significant frequency upshifts of the vibrations involved in the pi-electron-conjugated pathway have been noticed upon solution in polar solvents and with the lowering of the temperature. Finally, we propose a quinoid molecule as a reliable structural and electronic model for dication species in doped oligothiophenes or for bipolaron charged defects in doped polythiophene.

Tricyanovinyl-capped oligothiophenes of up to six rings have been synthesized and characterized. The substituted oligomers display dramatic reductions in both their optical and electrochemical band gaps in comparison to unsubstituted oligomers. The solvatochromic behavior of the terthiophene-substituted molecule was investigated in a variety of solvents. Stable oxidations and reductions were exhibited by the sexithiophene-substituted molecule on the CV time scale. [structure: see text]

A novel series of terthiophenes bearing electron-donor and electron-acceptor groups at the end alpha-positions has been prepared. The analysis of the UV-vis, infrared, and Raman spectra, performed with the aid of density functional theory calculations, shows that the asymmetrically substituted nitro compounds PhT(3)NO(2) and BrT(3)NO(2) behave as push-pull systems and present an intense photoinduced charge transfer in the visible spectrum. The symmetrically substituted dinitro compound NO(2)T(3)NO(2) displays a highly delocalized structure with a low single-double bond length alternation and also displays a low-energy absorption band in the visible region. The novel nitroterthiophenes possess attractive electrochemical properties since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the oligothiophene backbone and leads to the formation of stable cations even for NO(2)T(3)NO(2). Reduction is mainly nitro-centered but also affects the conjugated structure. Radical anions and dianions are formed for PhT(3)NO(2) and BrT(3)NO(2). Dianions, not radical anions, and trianions are obtained for NO(2)T(3)NO(2). Nitro-functionalized terthiophenes are shown to be very promising as electroactive molecular materials since they behave as push-pull systems, present a very intense photoinduced charge transfer in the visible region, and could act as both n- and p-channel conductors in organic electronic transistors.

Two quinonoid bis(dicyanomethylene) oligothiophenes, terthiophene and quaterthiophene analogues of TCNQ, have been investigated by spectroelectrochemical experiments and density functional theory calculations. Electrochemical data show that the molecules can be both reduced and oxidized at relatively low potentials, and that the quaterthiophene derivative forms four stable redox species, the dianion, neutral, cation radical, and dication. The neutral oligomers are characterized by a strong electronic absorption in the red or near-infrared region and can be viewed as structural and electronic analogues of aromatic oligothiophenes in the dication or bipolaron state. Upon reduction, dianions, not anion radicals, are formed which absorb in the visible region. The theoretical calculations show that the dianions have aromatic oligothiophene moieties with two anionic dicyanomethylene groups. The transition from a quinonoid to an aromatic structure is fully supported by UV-vis-near-IR and vibrational spectroscopic data. Oxidation, generating cation radicals and dications, occurs at rather low potentials similar to those reported for oligothiophenes. The electronic spectra of these cations are understood from the calculations, which suggest that the oxidized species are stabilized by the partial aromatization of the oligothiophene backbone. IR spectra of the species, especially the CN stretching frequencies, confirm the structural conclusions and allow comparison with TCNQ and the TCNQ dianion.

A terthiophene-based quinodimethane, 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (1) was synthesized and crystallized. Compound 1 has a planar quinoid geometry that is stabilized by dicyanomethylene groups at each end of the molecule. In the crystal each molecule is part of a dimerized face-to-face pi-stack, with intermolecular spacings of 3.47 and 3.63 A, respectively. Cyclic voltammetry showed that 1 could be reversibly reduced and oxidized in methylene chloride solution. Thin film transistors (TFTs) were prepared by vacuum evaporation of 1 onto SiO2(300 nm)/Si substrates, followed by evaporation of Ag source and drain contacts. The doped Si substrate served as the gate electrode. X-ray diffraction and atomic force microscopy indicate the films are polycrystalline, with the long axes of the molecules approximately perpendicular to the substrate. The TFT measurements revealed n-channel conduction in films of 1, with room-temperature electron field effect mobilities as high as 0.005 cm2/Vs. The butyl side chains give 1 appreciable solubility in a range of common solvents, and preliminary TFT results on films cast from chlorobenzene show electron mobility as high as 0.002 cm2/Vs. These results indicate that pi-stacked quinoidal thiophene oligomers are a promising new class of soluble n-channel organic semiconductors.

A new series of bipyridine-capped oligothiophene ligands and their binuclear Ru(II) complexes of the general formula [(bpy)(2)Ru-bpy(th)(x)bpy-Ru(bpy)(2)](4+) (where bpy = 2,2'-bipyridyl, th = 2,5-thienyl, and x = 1, 3, or 6) has been synthesized. Comparison of the bipyridine-capped oligothiophenes to the corresponding uncapped oligomers shows that there is coupling of the bipyridine to the oligothiophene-conjugated pi-system. Each of the bipyridine-capped ligands reduces at a bipyridine cap at potentials less negative than those of free bipyridine. The lambda(max) values of the lowest pi-pi* transition increase and the E(pa) values for the first oxidation decrease with an increase in the number of thiophene rings in the bridge. The increase in the number of thiophene rings also leads to more accessible oxidation states, up to four for the x = 6 compound. Upon complexation of the ligands with the (bpy)(2)Ru(2+) moiety, the electronic spectra of the complexes show a significant redshift in comparison to the spectra of Ru(bpy)(3)(2+), the oligothiophene-based oxidation(s) shifts to more positive potentials, and new oxidation and reduction processes are also observed. A single concurrent Ru(II/III) oxidation process is observed in all cases at about the same potential (E(ave)(0) = 1.32 V (acetonitrile solution)) as is observed in the parent complex Ru(bpy)(3)(2+), suggesting that the couples are not strongly coupled. The series of complexes shows a unique range of ligand-based oxidation processes with respect to the position of the Ru(II/III) redox process. In the case of x= 1, no ligand-based oxidation process is observed; for x = 3, the first oligothiophene oxidation occurs at potentials less positive than those of the Ru(II/III) redox process, and for x = 6, two oligothiophene oxidations are less positive than the Ru(II/III) redox process. A series of bpy reduction processes that are similar to those observed for Ru(bpy)(3)(2+) also occurs. The x = 1 compound shows two, one-electron reductions and then two, two-electron reductions; the x = 3 and x = 6 compounds show three, two-electron reduction processes. The first reductions occur at the capping bipyridines of the bridging ligand in all three complexes, and subsequent reductions occur at the ancillary bipyridine ligands in a stepwise fashion.

A simple two-step procedure is elaborated for the preparation of dialkylated benzodithiophenes (III) and (V) from dione (I).