Yana Sheynin

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.

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/

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.

α-Oligofurans show a sizeable extent of π-conjugation as probed by Raman spectroscopy

A Raman spectroscopic analysis revealed that π-conjugation does not reach saturation at least up to the octamer in long α-oligofurans and spreads over 14-15 furan units in the polyfuran. Comparing DFT calculations with experimental results suggests that a considerable amount of HF exchange is required to reproduce computationally the observed conjugation.

Controlling Pt nanoparticle formation through Se···Pt interactions on the electrode surface.

We show that interactions between the electrode surface and the transition metal during the initial step of metal nanoparticle formation can be utilized to control the formation and size of metal nanoparticles deposited on a conducting surface. Pt nanoparticles formed on the PEDOS surface are of smaller size compared to the PEDOT surface.