Conjugated Oligothiophene Derivatives Based on Bithiophene with Unsaturated Bonds as Building Blocks for Solution‐Processed Bulk Heterojunction Organic Solar Cells

A new building block ATVTA that uses stiff carbon–carbon triple bonds (A) on 1,2‐di(2‐thienyl)‐ethene (TVT) has been developed. Oligothiophene derivatives S‐01 with a TVT unit, S‐02 with a 5,5′‐diethynyl‐2,2′‐dithienyl (AT2) unit and S‐03 with ATVTA were synthesized to compare their effects in a systematic study. Due to the better π‐conjugation extension of the TVT unit, S‐01 exhibits the most red‐shifted absorption profile among them, whereas S‐02 possesses the deepest HOMO level. While the HOMO level of S‐03 is down‐shifted by 0.02 eV relative to that of S‐01 , the alkyne linkages can effectively down‐shift the HOMO level. By replacing the terminal units of S‐03 with stronger electron acceptors, S‐04 and S‐05 exhibited broader absorption profiles and lower HOMO levels than those of S‐03 . Organic solar cells based on these molecules were fabricated and an S‐03 :PC₆₀BM (1:1, w/w) based device afforded the highest V_oc value of 0.96 V and a power conversion efficiency (PCE) of 2.19 %.     

Light-Responsive Oligothiophenes Incorporating Photochromic Torsional Switches

We present a quaterthiophene and sexithiophene that can reversibly change their effective π-conjugation length through photoexcitation. The reported compounds make use of light-responsive molecular actuators consisting of an azobenzene attached to a bithiophene unit by both direct and linker-assisted bonding. Upon exposure to 350 nm light, the azobenzene undergoes trans-to-cis isomerization, thus mechanically inducing the oligothiophene to assume a planar conformation (extended π-conjugation). Exposure to 254 nm wavelength promotes azobenzene cis-to-trans isomerization, forcing the thiophenic backbones to twist out of planarity (confined π-conjugation). Twisted conformations are also reached by cis-to-trans thermal relaxation at a rate that increases proportionally with the conjugation length of the oligothiophene moiety. The molecular conformations of quaterthiophene and sexithiophene were characterized by using steady-state UV-vis spectroscopy, X-ray crystallography and quantum-chemical modeling. Finally, we tested the proposed light-responsive oligothiophenes in field-effect transistors to probe the photo-induced tuning of their electronic properties.     

Synthesis and photovoltaic properties of functional dendritic oligothiophenes

Novel p‐type and low bandgap functional dendritic oligothiophenes bearing hole‐transporting carbazole as peripheral substituents and an electron‐withdrawing dicyanovinyl core group, namely, DCT(n)‐DCN, where n = 1 or 2 for solution‐processable photovoltaic (PV) applications have been synthesized. With electron‐donating carbazole surface‐functionalized moieties conjugated with dicyanovinyl core group, the optical bandgap of these functional dendritic oligothiophene thin‐films greatly reduces to 1.74 eV with a strong spectral broadening and a high ionization potential at ～5.5 eV as determined by UV photoelectron spectroscopy. The bulk heterojunction PV cells fabricated from these dendrimers blended with PC₇₁BM as an acceptor showed a power conversion efficiency up to 1.64% with an open circuit voltage of (V_oc) = 0.93 V in the annealed device. We have demonstrated that the desirable molecular and PV properties of dendritic oligothiophenes can be obtained/tuned by the incorporation of functional group(s) onto peripheral of the dendron and into the core. In addition, these functional dendritic oligothiophenes show superior functional properties even at low dendritic generation as compared to the unsubstituted higher generation dendritic oligothiophenes as a p‐type, low‐bandgap semiconductor for solution‐processable bulk heterojunction PV cells. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis and characterization of cross‐shaped p–n diblock oligomers

The cross‐shaped p–n diblock oligomers based on oligothiophenes (OTs) and 1,3,4‐oxadiazole (OXD) were synthesized and investigated regarding optical properties, electrochemistry, quantum chemical calculations, and intramolecular energy transfer. Since only one emission peak is observed in PL spectra of the oligomers, it is evidenced that effective energy transfer from the OXD to OTs branch. The electrochemical experiments show that almost complete spatial separation of HOMO and LUMO with the thiophene number increasing. The theoretical calculations were carried out regarding which conformer is the lowest in energy, the torsion angle between thiophene/oxadiazole and the central benzene ring, and electron densities distributions of the oligomers. Based on all data, a model for intramolecular energy‐transfer process has been put forward to explain the optical properties. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1066–1073, 2007     

Palladium-catalyzed coupling reactions towards the synthesis of well-defined thiophene-oligomers

Heck- and Ullmann-type palladium-catalyzed reactions were used in order to perform the synthesis of 3′,4″-dioctyl-2,2′,5′2″,5″,2′″-tetrathienyl. The desired oligothiophene was synthesized in five steps with a total yield of 38% by a Heck-type reaction of bromo-derivatives and a yield of 45% by an Ullmann-type reaction of iodo-precursors.     

π‐Conjugated Oligothiophene‐Based Polycatenar Liquid Crystals: Self‐Organization and Photoconductive,Luminescent, and Redox Properties

A series of liquid‐crystalline (LC) π‐ ‐conjugated oligothiophenes bearing three or two alkoxy chains at their extremities has been designed and synthesized. These polycatenar oligothiophenes form various LC nanostructures including smectic, columnar, and micellar cubic phases. These properties depend on the number and length of the terminal alkoxy chains. The hole mobilities for the oligothiophenes have been measured. The layered smectic and columnar structures are capable of transporting holes, leading to mobilities of up to 0.01 cm² V^?1 s^?1. The columnar LC assemblies have also been explored to produce linearly polarized light‐emission. Fine red polarized fluorescence is observed from a uniaxially aligned film of the oligothiophenes. The redox properties of the oligothiophenes both in solutions and in films have been examined. The oligothiophenes exhibit electrochromism upon applying an oxidative potential. The present design strategy is useful for fabricating a variety of functional electro‐active molecular assemblies.     

Solution‐Processed Bulk‐Heterojunction Solar Cells Based on Monodisperse Dendritic Oligothiophenes

A novel family of soluble conjugated dendritic oligothiophenes (DOTs) as monodisperse 3D macromolecular architectures was characterized with respect to optical and redox properties in solution and in solid films. Band gaps of 2.5–2.2 eV, typical for organic semiconductors, were determined as well as HOMO/LUMO energy levels ideal for efficient electron transfer to acceptors such as [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) identifying them as suitable materials for solar cell applications. Solution‐processed bulk‐heterojunction solar cells using DOTs as electron donor and PCBM as acceptor were prepared and investigated. High open‐circuit voltages V_OC of 1.0 V and power‐conversion efficiencies up to 1.72% were obtained for the DOT‐based devices. The higher generations DOTs provide the highest efficiencies. Based on the monodispersity of the DOTs, an analysis of the molar ratio between donor and acceptor in the blended film was possible leading to an optimal value of five to six thiophene units per PCBM.     

Naphthyl End‐Capped Terthiophene‐Based Chemiresistive Sensors for Biogenic Amine Detection and Meat Spoilage Monitoring

A reliable and sensitive detection of biogenic amines (BAs) is essential to ensure food safety and maintain public health. In this study, two naphthyl end‐capped terthiophene derivatives, namely, 5‐(naphthalen‐1‐yl)‐2,2′:5′,2′′‐terthiophene ( NA‐3T ) and 5,5′′‐di(naphthalen‐1‐yl)‐2,2′:5′,2′′‐terthiophene ( NA‐3T‐NA ), were employed to develop chemiresistive sensors for detecting gaseous BAs. In contrast to NA‐3T , the NA‐3T‐NA ‐based sensor showed a higher sensitivity for trimethylamine (TMA) with an experimental detection limit lower than 22 ppm, and for aromatic BAs, including dopamine, histamine, tryptamine, and tyramine. Additionally, the recovery time for TMA was found to be shorter than 23 s. In addition, both sensors were successfully used for an in situ evaluation of meat freshness by monitoring the concentration of relevant volatile BAs. The difference in the sensing performances of the two chemiresistive sensors was tentatively ascribed to different packing structures of the derivatives and the adlayer structures of the films developed with the compounds.     

General synthesis of extended fused oligothiophenes consisting of an even number of thiophene rings

The intramolecular double cyclization of bis(3-bromo-2-thienyl)acetylenes though a lithium-halogen exchange reaction with tBuLi followed by treatment with elemental sulfur produces two thiophene-fused thieno[3,2-c](1,2-dithiin)s. The subsequent dechalcogenation from the 1,2-dithiins with copper nanopowder affords tetrathienoacenes. On the basis of this two-step procedure, a series of trialkylsilyl-terminated, fused oligothiophenes, including hexathienoacene (a six thiophene-fused system) and octathienoacene (an eight thiophene-fused system), were synthesized. In the UV/Vis absorption and fluorescence spectra of the fused oligothiophenes, the absorption and emission maxima shift to longer wavelengths, as the pi-conjugation length increases. Their maximum wavenumbers have linear relationships with the reciprocal number of thiophene rings consisting of the pi-conjugated frameworks. In the cyclic voltammograms, all the compounds show reversible oxidation waves, the first oxidation potential of which shifts to less positive as the conjugation length increases. Among them, the octathienoacene also shows a reversible second oxidation process. Indeed, its chemical oxidation with an excess amount of NO(+)SbF(6) (-) produces the dication as a golden crystal. The crystal structures of the neutral octathienoacene and its dication were determined by X-ray crystallography. While in the neutral state, the octathienoacene has a benzenoid structure with a large bond alternation of about 0.04 A, its dication has a quinoid structure in which two cationic charges are mainly localized on the terminal rings.     

Dicyanovinyl–Substituted Oligothiophenes: Structure‐Property Relationships and Application in Vacuum‐Processed Small Molecule Organic Solar Cells

Efficient synthesis of a series of terminally dicyanovinyl (DCV)‐substituted oligothiophenes, DCVnT 1–6, without solubilizing side chains synthesized via a novel convergent approach and their application as electron donors in vacuum‐processed m‐i‐p‐type planar and p‐i‐n‐type bulk heterojunction organic solar cells is described. Purification of the products via gradient sublimation yields thermally highly stable organic semiconducting materials in single crystalline quality which allows for X‐ray structure analysis. Important insights into the packing features and intermolecular interactions of these promising solar cell materials are provided. Optical absorption spectra and electrochemical properties of the oligomers are investigated and valuable structure–property relationships deduced. Photovoltaic devices incorporating DCVnTs 4–6 showed power conversion efficiencies up to 2.8% for planar and 5.2% for bulk heterojunction organic solar cells under full sun illumination (mismatch corrected simulated AM 1.5G sunlight). The 5.2% efficiency shown here represents one of the highest values ever reported for organic vacuum‐deposited single heterojunction solar cells.