Asit Patra

Poly(3,4-ethylenedioxyselenophene).

The first highly conductive polyselenophene, namely, poly(3,4-ethylenedioxyselenophene) (PEDOS), was synthesized by taking advantage of a novel method for efficiently contracting the selenophene ring. PEDOS shows a relatively low band gap (1.4 eV), very high stability in the oxidized state, and a well-defined spectroelectrochemistry.

Rubrenes: Planar and Twisted

Surprisingly, despite its very high mobility in a single crystal, rubrene shows very low mobility in vacuum-sublimed or solution-processed organic thin-film transistors. We synthesized several rubrene analogues with electron-withdrawing and electron-donating substituents and found that most of the substituted rubrenes are not planar in the solid state. Moreover, we conclude (based on experimental and calculated data) that even parent rubrene is not planar in solution and in thin films. This discovery explains why high mobility is reported in rubrene single crystals, but rubrene shows very low field-effect mobility in thin films. The substituted rubrenes obtained in this work have significantly better solubility than parent rubrene and some even form films and not crystals after evaporation of the solvent. Thus, substituted rubrenes are promising materials for organic light-emitting diode (OLED) applications.

Controlling Rigidity and Planarity in Conjugated Polymers: Poly(3,4-ethylenedithioselenophene)**

Conjugated oligomers and polymers 2] attract considerable interest owing to their application in photovoltaic cells, organic light-emitting diodes (OLEDs), 6] organic fieldeffect transistors (OFETs), and electrochromic devices. Generally, planarity and good conjugation are required so that organic materials can achieve band gaps in the semiconductor region, high conductivity, high mobility, and an electrooptical response. Polythiophenes are among the most promising and best-studied conducting polymers. 2] However, even parent bithiophene is not planar in the gas phase (according to both experiment and theory), and crystal packing forces are responsible for the planarity of oligothiophenes in the solid state. Various small substituents (such as two adjacent alkyl chains on the same or neighboring rings: 3,4 or 3,3’-substitution) cause oligothiophene to become nonplanar, and the availability of oligoand polythiophenes with substituents that do not disturb planarity is very limited (for example, poly(3-hexylthiophene) is planar). 11] Although twisting of the oligothiophene backbone requires very little energy, it results in a significant increase in the HOMO–LUMO gap. The fact that small conformational changes to conjugated polymers may produce large band-gap effects has been utilized in the development of polythiophene-based sensors. Poly(3,4-ethylenedioxythiophene) (PEDOT) has many advantages over other conducting polymers in organic electronics applications. However, it cannot be applied as a light-absorbing donor in organic solar cells, for example, owing to its very low oxidation potential and, consequently, very low work function. PEDOT is believed to be planar; however, its analogue, poly(3,4-ethylenedithiothiophene) (PEDTT), in which oxygen atoms are replaced by sulfur atoms, is assumed to be twisted, as manifested by its significantly wider band gap (2.2 eV for PEDTT vs. 1.6 eV for PEDOT). 21] Indeed, the dimer of 3,4-ethylenedithiothiophene (bis-EDTT) has an inter-ring twist angle of 458, whereas bis-EDOT has a planar structure in the solid state. 18,20, 22, 23] Recently, we obtained the first conductive polyselenophene, poly(3,4-ethylenedioxyselenophene) (PEDOS), which has a relatively narrow band gap and excellent electrochromic properties. 25] Synthesis of stable and conductive PEDOS enables the development of applications of polyselenophenes as organic electronic materials. Designing such materials demands the identification of more rigid conjugated systems capable of bearing various substituents on their backbone whilst retaining their planarity. Herein, we report that the range of substituents that polyselenophenes can bear whilst still maintaining their planarity is wider than that of polythiophenes, and is mostly due to the more rigid backbone of the polyselenophenes. Poly(3,4-ethylenedithioselenophene) (PEDTS) has a significantly narrower optical band gap (0.6–0.8 eV) than PEDTT, which can be attributed to its planarity. Moreover, PEDTS is a conducting polymer that is not as electron-rich as PEDOS and PEDOT. The top of the valence band of PEDTS is about 0.7 eV (0.64 eV experimental, 0.81 eV calculated) lower than that of PEDOT, which makes PEDTS a very attractive material for organic solar cell applications. The energy required to twist around inter-ring bonds in decaselenophene is small; however, it is notably greater (by a factor of 1.2–1.8; Supporting Information, Figure S7) than in decathiophene. Twisting to a 608 inter-ring dihedral angle requires only 2.6 kcalmol 1 per inter-ring bond for decaselenophene (2.1 kcalmol 1 for decathiophene) and twisting to a [*] Y. H. Wijsboom, Dr. A. Patra, Dr. S. S. Zade, Dr. Y. Sheynin, Dr. M. Li, Dr. M. Bendikov Department of Organic Chemistry Weizmann Institute of Science, Rehovot 76100 (Israel) Fax: (+ 972)8934-4142 E-mail: michael.bendikov@weizmann.ac.il Homepage: http://www.weizmann.ac.il/oc/bendikov/

Synthesis, Structure, and Electropolymerization of 3,4-Dimethoxytellurophene: Comparison with Selenium Analogue

3,4-Dimethoxytellurophene (5) was synthesized via a new ring construction reaction. The crystal structure of 5 is characterized by unusually short Te...Te distances. The electropolymerization of 5 probably produces some amount of poly-5. Since the product was unstable under experimental conditions, a definitive assignment could not be made. However, the UV-vis spectrum recoded during electropolymerization of 5 shows an absorption peak at 679 nm with an onset at 820 nm (1.51 eV), closely matching the calculated band gap of poly-5.

Tuning of electronic properties and rigidity in PEDOT analogs

The electronic properties, rigidity, and planarity of conjugated polymers of the PEDOT type were tuned by changing the conjugated backbone from polythiophene to the more rigid polyselenophene and by replacing one or both oxygen atoms in the ethylenedioxy bridge (peripheral ring) with sulfur. While the band gaps of the obtained polyselenophenes are ∼1.4 eV, the orbital energy levels shift significantly because of changes in the electronic nature of the peripheral ring and the peak-width of the absorbance spectrum varies because of changes to backbone rigidity.

Flat conjugated polymers combining a relatively low HOMO energy level and band gap: polyselenophenes versus polythiophenes

In this work, we prepared a series of new conjugated polyselenophenes that, in the 3,4-positions of the selenophene ring, have oxygen or sulfur substituents bridged by a phenylene moiety. Such substitution of a conjugated backbone produces a skeleton that has only planar units, does not have stereo centers, and offers the potential to structurally modify the polymer without impairing its conjugation. The reported polyselenophenes exhibit significantly different properties as a function of the heteroatom. The selenophene backbone combined with a phenylene periphery creates the rare combination of a low-band gap, low HOMO energy level, and a flat skeleton, which is desired for many optoelectronic applications. The properties of the phenylene-bridged polyselenophenes were compared with those of their polythiophene analogs. The polyselenophenes obtained in this work have a lower band gap and higher planarity than polythiophenes and their monomers electropolymerize more easily. Theoretical studies support the experimental findings about rigidity and band gap changes.

Unusual Doping of Donor−Acceptor-Type Conjugated Polymers Using Lewis Acids

Conjugated polymers that can undergo unusual nonoxidative doping were designed. A series of polymers based on donor-acceptor-donor (DAD) moieties 2,1,3-benzoselenadiazole, 2,1,3-benzothiadiazole, 2,1,3-benzoxadiazolebenzo[2,1,5]oxodiazole, and 2-hexylbenzotriazole as acceptor fragments and 3,4-ethylenedioxyselenophene (EDOS) and 3,4-ethylenedioxythiophene (EDOT) as donor fragments was prepared. When the studied polymers were reacted with Lewis acids and bases, notable optical switching and conductivity changes were observed, evidencing the exceptional case of efficient nonoxidative doping/dedoping. Remarkably, in previously reported works, coordination of Lewis acids causes band gap shift but not doping of the conductive polymer, while in the present study, coordination of Lewis acid to highly donating EDOT and EDOS moieties led to polymer doping. The polymers show remarkable stability after numerous switching cycles from neutral to doped states and vice versa and can be switched both electrochemically and chemically. The reactivity of the prepared polymers with Lewis acids and bases of different strengths was studied. Calculation studies of the Lewis acid coordination mode, its effect on polymer energies and band gap, support the unusual doping. The reported doping approach opens up the possibility to control the conjugation, color change, and switching of states of conjugated polymers without oxidation.

Solvent Retention, Thermodynamics, Rheology and Small Angle X-ray Scattering Studies on Thermoreversible Poly(vinylidene fluoride) Gels

Solvent retention power of poly(vinylidene fluoride) (PVDF) gels has been studied for various homologues of phthalate (aromatic diesters). The thermal stability has been examined for gels of varying morphology. Solvent evaporation, gelation, gel melting, and polymer degradation temperatures have increased with increasing aliphatic chain length of phthalates. The thermodynamics and polymer-solvent compound formations in the PVDF-phthalate gels have been explored. The weight fraction of polymer in compound has decreased with increasing aliphatic chain length. SAXS studies have confirmed the lamellar organization inside the fibrils, and interlamellar distance increases with aliphatic chain length of diesters. The scattering patterns follow the power law behavior (I(q) approximately q(-alpha)), and polymer gels consist of high-density mass (fibril), voids, and interlamellar region. Dynamic mechanical properties indicate the splintering and reformation of network structure in gels whose percolation frequency has reduced for higher aliphatic chain length phthalate. Morphology-dependent moduli have been observed, and greater mechanical strength has been verified for thicker fibrillar gels both for steady and dynamic measurements.

Thermoreversible gelation of poly(vinylidene fluoride-co-hexafluoro propylene) in phthalates.

The thermoreversible gelation of poly(vinylidene fluoride-co-hexafluoro propylene) copolymer have been studied in a series of phthalates, Ph-(COO C(n)H(2n+1))(2) with n = 1-8. The gelation rate increases with increasing aliphatic chain length up to n = 6, and the gelation phenomena does not occur for higher n > 6. The fibrillar morphology is evident for dried gels whose dimension (both lateral and thickness) becomes shorter and thinner with increasing n. The structures of the gels formed in various phthalates have been investigated by small-angle neutron scattering and small-angle X-ray scattering techniques, suggesting sheet-like structure, where the interplanner distance increases with increasing aliphatic chain length. The scattering intensity I(q) decreases with q according to the Ornstein-Zernike model, where q = (4pi/lambda) sin theta (2theta and lambda are scattering angle and wavelength of neutron) and the correlation length, xi, assigned to the average distance between the neighboring crystallites, also increases with increasing aliphatic chain length of diesters. The detailed thermal analyses and phase diagrams of the copolymer gels have been studied in a wide range of phthalates. Further, polymer-solvent complexes leading to the formation of two distinct compounds have been reported. A systematic change of compound composition has also been observed in the whole range of phthalates studied here. On the basis of electronic structure calculation, a model has been proposed to elucidate the conformation of copolymer chain in presence of various phthalates and their complexes, which offer the cause of higher gelation rate for longer aliphatic chain length up to n = 6, no gelation phenomena occurs for n > 6, and formation of two copolymer-solvent compounds. The mechanical properties (storage modulus and viscosity) decrease with increasing aliphatic chain length of phthalates and realignment of fibrils occurs at particular frequency depending on the strength of fibrillar gels.

Thermally activated and field dependent hole transport in poly(3-hexylthiophene)

Here, we investigate the hole transport mechanism in poly(3-hexylthiophene) (P3HT). First, ohmic contact has been established at indium tin oxide (ITO)/P3HT interface by molybdenum oxide (MoOx) hole injection layer. Thickness of MoOx layer is observed to play a crucial role with ohmic contact being observed even for 1 nm layer. However, device with less than 5 nm layer are found to be extremely unstable. A device with a 5 nm layer of MoOx is found to be stable and ohmic injection at ITO/P3HT layer enabled to observe ohmic conduction at low voltages ( 3 V. At higher voltages, effect of field on charge carrier mobility is also observed. Observation of SCLC enabled us to directly evaluate the hole mobility in P3HT which is calculated to be 5.4 × 10 -5 cm 2 /Vs. Conductivity is calculated from the low voltage region and found to be 6.85 × 10 -8 S/cm. Temperature dependent mobility is used to study the charge transport behavior and it has been observed that mobility is thermally activated with an extremely low activation energy of 39 meV. Copyright