Linear Oligothiophenes Research Articles

Solid-state mechanochemical cross-coupling of insoluble substrates into insoluble products by removable solubilizing silyl groups: uniform synthesis of nonsubstituted linear oligothiophenes.

Conventional solution-based organic reactions that involve insoluble substrates are challenging and inefficient. Furthermore, even if the reaction is successful, the corresponding products are insoluble in most cases, making their isolation and subsequent transformations difficult. Hence, the conversion of insoluble compounds into insoluble products remains a challenge in practical synthetic chemistry. In this study, we showcase a potential solution to address these solubility issues by combining a mechanochemical cross-coupling approach with removable solubilizing silyl groups. Our strategy involves solid-state Suzuki-Miyaura cross-coupling reactions between organoboron nucleophiles bearing a silyl group with long alkyl chains and insoluble polyaromatic halides. The silyl group on the nucleophile can act as a solubilizing group that enables product isolation via silica gel column chromatography and can be easily removed by the addition of fluoride anions to form the desired insoluble coupling products with sufficient purity. Furthermore, we demonstrate that after aromatic electrophilic bromination of the desilylated products, sequential solid-state cross-coupling of the obtained insoluble brominated substrates, followed by desilylation, afforded further π-extended functional molecules. Using this conceptually new protocol, we achieved the first uniform synthesis of the longest nonsubstituted linear insoluble 9-mer oligothiophene.

Self-assembled nanostructures of a series of linear oligothiophene derivatives adsorbed on surfaces

Many previous studies have shown that the molecular structures of oligothiophene derivatives including molecular skeleton and alkyl chains have a significant effect on their self-assemblies on the surface. In this work, a series of linear oligothiophene derivatives (DCV-nT-Hex, n = 3∼11) modified with terminal dicyanovinyls and alkyl chains were adopted to further investigate the different assembly behaviors at liquid-solid interface by scanning tunneling microscopy (STM). Interestingly, via the hydrogen bonding and van der Waals interactions, DCV-3T-Hex formed zigzag and flower structures while DCV-nT-Hex (n = 4∼11) formed lamellar structures. Density functional theory (DFT) calculations show that for the most energetically favorable configurations of DCV-nT-Hex, the different distribution of alkyl chains affected intermolecular interactions, and ultimately led to the different assembled structures. The zigzag and flower structures of DCV-3T-Hex had preferential thermodynamic stability compared to other structures of DCV-nT-Hex (n = 4∼11). In addition, self-assembled nanostructures of DCV-nT-Hex molecules with even numbers (n = 4, 6, 8, 10) were overall more stable than those with odd numbers (n = 5, 7, 9, 11), and the stability of the self-assembled structure was weakened with the extension of the molecular backbone, individually. The orientation of molecular alkyl chains was found to greatly affect the intermolecular interactions and thus leading to various self-assembly structures of DCV-nT-Hex (n = 3∼11). The self-assembly behaviors of linear oligothiophene derivatives DCV-nT-Hex (n = 3∼11) driven by hydrogen bond and van der Waals force were investigated by STM and DFT calculations, and the orientation of molecular alkyl chains was found to greatly affect the intermolecular interactions.

Approaching the Integer-Charge Transfer Regime in Molecularly Doped Oligothiophenes by Efficient Decarboxylative Cross-Coupling.

A library of symmetrical linear oligothiophene was prepared employing decarboxylative cross-coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross-coupling partners without the need of co-catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.

Approaching the Integer‐Charge Transfer Regime in Molecularly Doped Oligothiophenes by Efficient Decarboxylative Cross‐Coupling

AbstractA library of symmetrical linear oligothiophene was prepared employing decarboxylative cross‐coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross‐coupling partners without the need of co‐catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.

Chain-Length-Dependent Exciton Dynamics in Linear Oligothiophenes Probed Using Ensemble and Single-Molecule Spectroscopy

Exciton dynamics in π-conjugated molecular systems is highly susceptible to conformational disorder. Using time-resolved and single-molecule spectroscopic techniques, the effect of chain length on the exciton dynamics in a series of linear oligothiophenes, for which the conformational disorder increased with increasing chain length, was investigated. As a result, extraordinary features of the exciton dynamics in longer-chain oligothiophene were revealed. Ultrafast fluorescence depolarization processes were observed due to exciton self-trapping in longer and bent chains. Increase in exciton delocalization during dynamic planarization processes was also observed in the linear oligothiophenes via time-resolved fluorescence spectra but was restricted in L-10T because of its considerable conformational disorder. Exciton delocalization was also unexpectedly observed in a bent chain using single-molecule fluorescence spectroscopy. Such delocalization modulates the fluorescence spectral shape by attenuating the 0-0 peak intensity. Collectively, these results provide significant insights into the exciton dynamics in conjugated polymers.

Oligothiophene Semiconductors: Synthesis, Characterization, and Applications for Organic Devices

Oligothiophenes provide a highly controlled and adaptable platform to explore various synthetic, morphologic, and electronic relationships in organic semiconductor systems. These short-chain systems serve as models for establishing valuable structure-property relationships to their polymer analogs. In contrast to their polymer counterparts, oligothiophenes afford high-purity and well-defined materials that can be easily modified with a variety of functional groups. Recent work by a number of research groups has revealed functionalized oligothiophenes to be the up-and-coming generation of advanced materials for organic electronic devices. In this review, we discuss the synthesis and characterization of linear oligothiophenes with a focus on applications in organic field effect transistors and organic photovoltaics. We will highlight key structural parameters, such as crystal packing, intermolecular interactions, polymorphism, and energy levels, which in turn define the device performance.

Concise Synthesis of Well‐Defined Linear and Branched Oligothiophenes with Nickel‐Catalyzed Regiocontrolled Cross‐Coupling of 3‐Substituted Thiophenes by Catalytically Generated Magnesium Amide

The synthesis of linear and branched oligothiophenes of well-defined structures is performed with regioselective deprotonation of 3-substituted thiophenes and nickel-catalyzed cross-coupling of the thus formed metalated species with a bromothiophene. The reaction of 3-hexylthiophene with EtMgCl and 2,2,6,6-tetramethylpiperidine (TMP-H, 10 mol%) induces the metalation selectively at the 5-position by use of the catalytically generated hindered magnesium amide (TMPMgCl) and the subsequent reaction of a 2-halo-3-hexylthiophene (bromide or chloride) in the presence of a nickel catalyst affords a head-to-tail (HT)-type dimer. By repeating the same sequence, the linear oligothiophene up to a 4-mer is synthesized in good yield. The reaction of 3-hexylthiophene with 2,3-dibromothiophene also takes place to afford a branched oligothiophene 3-mer in quantitative yield. The obtained 3-mer is also metalated at the sterically less-hindered position in a regioselective manner furnishing a 7-mer in >99% yield after a further coupling reaction with 2,3-dibromothiophene. These dendrimers react with several multifunctionalized organic electrophiles, leading to a variety of branched oligothiophenes.

Theory and simulation of organic solar cell model compounds: How packing and morphology determine the electronic conductivity

We approach the electronic conductivity of simple models of organic solar cells containing linear and branched αα'-oligothiophenes and buckminsterfullerene. Close-packed model geometries are generated using a Monte Carlo method, this procedure is verified making use of an analogue model. The electronic structure is described by an extended Su-Schrieffer-Heeger Hamiltonian, the resulting potential energy surfaces relevant to charge transfer can be analyzed using Marcus' theory, leading to local and--via Kirchhoff's rule--global conductivities for uniform oligothiophene and fullerene systems and their mixtures. Dense fullerene systems or subsystems always exhibit a conductivity in excess of 100 S/cm. In contrast, oligothiophenes show a comparable conductivity only for uniform, well-ordered arrangements of layers. Branched oligomers show only a slight improvement over linear oligothiophenes. Our results support the bulk heterojunction approach as a design principle of organic solar cells from a theoretical perspective.

Properties of Oligothiophene Dendrimers as a Function of Molecular Architecture and Generation Number

Density functional and time-dependent density functional calculations using the B3LYP method combined with the 6-31G(d) and 6-311++G(d,p) basis sets are performed on symmetric and unsymmetric all-thiophene dendrimers containing up to 45 thiophene rings. Calculations consider both the neutral and the oxidized states of each dendrimer. The results are used to examine the molecular geometry, the ionization potential, the lowest π-π* transition energy, and the shape of the frontier orbitals. The molecular and electronic properties of these systems depend not only on the number of thiophene rings, as typically occurs for linear oligothiophenes, but also on their symmetric/unsymmetric molecular architecture. Two mathematical models developed to predict the lowest π-π* transition energy of all-thiophene dendrimers that are inaccessible to quantum mechanical calculations are tested on a dendrimer with 90 thiophene rings.

Excited-State Processes in First-Generation Phenyl-Cored Thiophene Dendrimers

First generation dendrimers with three oligothiophene arms (meta-arranged, 3G1-nS) and four arms (ortho- and para-arranged, 4G1-nS) connected to a central phenyl core were investigated spectroscopically in solution. In all dendrimers, on an ultrafast time scale (<10 ps), two "cooling" processes convert the initially generated, "hot" exciton into the geometrically relaxed, "cold" exciton. A decrease in the triplet yield, particularly evident for the 4-arm dendrimers; intersystem crossing rate; and nonradiative triplet decay time with increasing number of bridging thiophene units n all meet with expectations from prior studies on linear oligothiophenes. A relatively fast internal conversion process (>0.6 ns(-1)) is observed in both dendrimer series, possibly due to increased twisting about the phenyl core that reduces the triplet yields considerably with respect to oligothiophenes. An anomalous shifting of the triplet-triplet absorption spectra characterizes the 4G1-nS dendrimers as unique from the 3G1-nS series in terms of the hindrance of torsional motion and confinement of excited states enforced by the arrangement of dendrons.

Molecular and electronic structure of cyclo[10]thiophene in various oxidation states: polaron pair vs. bipolaron

The molecular structure of a cyclic oligothiophene, C10T, has been determined by single-crystal X-ray structure analysis. The exclusive syn-conformation of all thiophene units as confirmed in the solid state and the ring strain in this macrocycle result in its unusual and optoelectronic properties. This does not only apply to neutral C10T but also to its oxidized states, as demonstrated by absorption and ESR spectroscopy, supporting the formation of a polaron-pair structure upon oxidation of C10T to C10T2(·+) as has been discussed for linear oligothiophenes. To the best of our knowledge, C10T2(·+) represents an unambiguous example comprising a two-polaron structure (polaron-pair) of a thiophene-based conjugated macrocycle.

An atomistic simulation of the liquid-crystalline phases of sexithiophene

We have investigated, using atomistic molecular dynamics simulations, the high temperature molecular organization of the linear oligothiophene α-sexithienyl (T6), well known for its organic electronics applications. We have found a smectic and a nematic liquid crystalline phase in the same temperature range where they had been experimentally reported but not fully characterized. We have microscopically characterized the phases and connected the change of mesophase and of orientational and positional order to variations in the T6 conformation and effective shape. T6 phases obtained by rapid cooling from the ordered melts have also been simulated.

Synthesis of Monochlorosilyl Derivatives of Dialkyloligothiophenes for Self-Assembling Monolayer Field-Effect Transistors

Unsymmetrical dimethylchlorosilyl-substituted α, α′- dialkylquater-, quinque-, and sexithiophenes were designed and successfully synthesized by a combination of Kumada and Suzuki cross-coupling reactions followed by hydrosilylation. Optimization possibilities of the hydrosilylation of low-soluble linear oligothiophenes by dimethylchlorosilane as well as the nonreactive byproducts formed are described. The molecular structures of the obtained dimethylchlorosilyl-functionalized oligothiophenes were proven by NMR and DCI MS techniques. These compounds were found to be stable and reactive enough, even in the presence of the nonreactive byproducts, to form semiconducting monolayers on dielectric hydroxylated SiO2 surfaces by self-assembly from solution. The semiconducting properties of these oligothiophene SAMs were as good as those of bulk oligothiophenes. This allowed the production of stable, even under ambient conditions, SAMFETs with a mobility of up to 0.04 cm2/(V s) and an on/off ratio up to 1 × 10 8. © 2010 American Chemical Society.

Spiro Oligothiophenes

Linear oligothiophenes, containing annelated dithieno[2,3-b:3',2'-d]thiophene units, undergo unexpected spirocyclizations upon treatment with trifluoroacetic acid. The resultant spiro oligothiophenes possess novel structures in which the spiro or dispiro skeleton is within an all-thiophene/dihydrothiophene framework. Racemic dispiro octathiophene is obtained with high diastereoselectivity and in high yield.

Complexes of C60 with Cyclic Oligothiophenes: A Theoretical Study

Complexes of C60 and cyclic and linear oligothiophenes containing 8 and 12 repeating units have been modeled at the M05-2X/6-311G**//M05-2X/6-31G* level of theory. BSSE-corrected binding energies of neutral donor-acceptor complexes vary from 5 to 12 kcal/mol depending on the complex and donor type. Inclusion complexes formed by C60 and cyclooligothiophenes containing 8 repeating units were found to be the most stable ones. Only weak charge transfer from oligothiophene to C60 fragment (<0.1 electron) is observed in the ground state, whereas complete electron transfer from oligothiophene fragment to C60 has been found in the excited state. One electron oxidation or reduction increases binding energies of "tight" complexes and decreases donor-acceptor interaction for "loose" complexes. In the case of cation radicals, positive charge is totally concentrated at the oligothiophene fragment, whereas in anion radicals, negative charge is located at the C60 moiety. Calculations demonstrated that plane oligothiophene conformation and weak binding in a donor-acceptor complex favor the photoinduced charge-carrier formation.

Tetrahedral Oligothiophenes; Synthesis, X‐ray Analysis, and Optoelectronic Properties of Highly Symmetrical, 3D‐Branched Oligothiophenes

Tetrakis(bithienyl)methane and tetrakis(terthienyl)methane have been synthesized from tetrakis(2-thienyl)methane by use of Suzuki-Miyaura coupling as a key reaction. Their trimethylsilyl (TMS) derivatives are also synthesized. X-ray analysis reveals that each oligothiophene moiety tends to adopt anti-conformations and show relatively small torsion angles between the adjacent thiophene rings. While the longest absorption maxima of these tetrakis(oligothienyl)methanes exhibit only a slight bathochromic shift compared to the corresponding linear oligothiophene derivative, tetrakis(bithienyl)methane and its TMS derivative exhibit an appreciable red-shift in their fluorescence spectra. The intramolecular interaction between thienyl groups of tetrakis(2-thienyl)methane is supported by DFT calculation.

Novel Highly Fluorescent Triphenylamine-Based Oligothiophenes†

A homologous series of novel amorphous oligothienyl-triphenylamines, comprised of head-to-tail coupled oligo(3-hexylthiophene)s covalently linked to a triphenylamine core, are disclosed. These hybrid systems, which differ by the length of the oligothiophene units from a monothiophene up to a quaterthiophene, were created via palladium-catalyzed cross-coupling reactions in good yields. The optical and electrochemical properties of these compounds were thoroughly investigated and are compared to the corresponding parent linear non-functionalized oligothiophenes, clearly demonstrating that these newly synthesized hybrid systems are characterized by outstanding absorption and emission properties. These molecules exhibit fluorescence quantum yields as high as 65%, which is unexpectedly high for thiophene-based materials. Based on this available data set, structure−property relationships were established, providing a deeper insight into the electronic nature of these materials and in this respect elucidating electronic differences between these star-shaped structures and their corresponding parent linear oligothiophenes.

V-Shaped Thiophene-Based Oligomers with Improved Electroluminescence Properties

The synthesis via the Stille coupling of a new family of oligomers derived from benzo[b]thiophene is reported. Owing to their branched molecular structure lacking any symmetry element, these compounds display a low tendency to crystallization and better film-forming properties than their linear counterparts. Spin-coated films show photoluminescence efficiencies up to 50 %. Light-emitting diodes with spin-coated films as the active layers display markedly improved performance with respect to similar devices based on linear oligothiophenes, with luminance values up to more than 10000 cd m - 2 . Semiempirical PM3 and ZINDO/S calculations provide insight into the molecular geometries and electron distribution of the frontier orbitals of the new compounds. Cyclic voltammetry data indicates that the transformation of the thienyl sulfur of benzo[b]thiophene to the corresponding thienyl-S,S-dioxide leads an increase in electron affinity by 0.5-0.7 V, analogous to that of the corresponding linear oligomers.

Theoretical description of the Raman spectrum of a vinylene-bridged quaterthiophene oligomer

The Raman spectrum of a quaterthiophene oligomer incorporating a central vinylene spacer has been investigated using density functional theory B3LYP/6-31G** calculations. The spectrum has been fully assigned with the aid of the calculations and in comparison with unsubstituted quaterthiophene. The spectrum preserves most of the vibrational features of linear oligothiophenes. The vibrations of the vinylene spacer are clearly differentiated from those of the rest of the chain. The vinylene spacer increases the conjugation length of the molecule and induces a frequency downshift of ≈20 cm −1 for the normal modes associated with the ν as(CC) and ν s(CC) vibrations of the thiophene rings. The molecule is predicted to have a structure in which both the thiophene rings and the vinylene spacer are in an all- anti conformation.