Sanjio S. Zade

Organoselenium chemistry: role of intramolecular interactions.

In 1836 the first organoselenium compound, diethyl selenide, was prepared by Löwig,1 and it was isolated in the pure form in 1869.2 Early selenium chemistry involved the synthesis of simple aliphatic compounds such as selenols (RSeH), selenides (RSeR), and diselenides (RSeSeR); however, because of their malodorous nature, these compounds were difficult to handle. This, combined with the instability of certain derivatives and difficulties in purification, meant that selenium chemistry was slow to develop. By the 1950s, the number of known selenium compounds had increased significantly, but it was not until the 1970s, when several new reactions leading to novel compounds with unusual properties were discovered, that selenium chemistry began to attract more general interest.3-9 Aryl-substituted compounds were synthesized that were found to be less volatile and more pleasant to handle than the earlier aliphatic compounds. Compounds containing selenium in high oxidation states are relatively easy to manipulate using modern techniques.4c Organoselenium chemistry has now become a well-established field of research, and recent advances have been brought about by the potential technical applications of selenium compounds. Today selenium compounds find application in many areas including organic synthesis,4 biochemistry,5 xerography,6 the synthesis of conducting materials7 and semiconductors,8 and ligand chemistry.4c,9 Many of these aspects of selenium chemistry are wellcovered elsewhere in the literature; however, the subject of hypervalency has not attracted much attention and is the focus of this review.10

From Short Conjugated Oligomers to Conjugated Polymers. Lessons from Studies on Long Conjugated Oligomers

Given their utility in a variety of electronic devices, conjugated oligomers and polymers have attracted considerable research interest in recent years. Because polymeric materials consist of very large molecules with a range of molecular weights (that is, they are polydisperse), predicting their electronic properties is a complicated task. Accordingly, their properties are typically estimated by extrapolation of oligomeric properties to infinite chain lengths. In this Account, we discuss the convergence behavior of various electronic properties of conjugated oligomers, often using thiophene oligomers as a representative example. We have observed some general trends in our studies, which we briefly summarize below for five properties. Most of the calculated values are method dependent: the absolute values can be strongly dependent on the computational level used. Band Gap. The generally accepted approximation used to estimate polymer band gap, whereby a plot of HOMO-LUMO gap versus 1/n (where n is the number of monomer units) is extrapolated to infinite n, fails for long oligomers, because convergence behavior is observed for band gaps. At the B3LYP/6-31G(d) level, it is possible to extrapolate oligomer HOMO-LUMO gaps with a second-order polynomial equation. Alternatively, PBC/B3LYP/6-31G(d) is a very good method to reliably predict the band gap of conjugated polymers. Reorganization Energy. Values of the internal reorganization energy (λ) do not scale linearly with 1/n, instead exhibiting an inverse correlation with the square-root of the number of monomer units for n = 2-12. For larger n (10-50), a linear relationship is observed between reorganization energy and the reciprocal chain length, and the extrapolation approaches λ ≈ 0 for infinite numbers of oligomer rings. Ionization Potential. The relationship between the first adiabatic ionization potential IP(1a) of oligothiophenes and oligoselenophenes and chain length linearly correlates with an empirically obtained value of 1/(n(0.75)). The first vertical ionization potential (IP(1v)) linearly correlates with a similarly empirically obtained value of 1/(n(0.70)). Polaron-Bipolaron Balance. The contribution of a polaron pair to the electronic structure of the short oligothiophene dication is small; for medium-length oligothiophene chains, the contribution from the polaron pair state begins to become significant. For longer (above 20-mer) oligothiophenes, the polaron pair state dominates. A similar picture was observed for multications as well as doped oligomers and polymers. The qualitative polaron-bipolaron picture does not change when a dopant is introduced; however, quantitatively, the bipolaron-polaron pair equilibrium shifts toward the bipolaron state. Disproportionation Energy. The stability of a single oligothiophene dication versus two cation radical oligothiophene molecules increases with increasing chain length, and there is an excellent correlation between the relative disproportionation energy and the inverse of chain length. A similar trend is observed in the disproportionation energies of oligothiophene polycations as well as doped oligomer and polymers. We also examine doped oligothiophenes (with explicitly included counterions) and polymers with a repeating polar unit. From our experience, it is clear that different properties converge in different ways, and long oligomers (having about 50 double bonds in the backbone) must often be used to correctly extrapolate polymer properties.

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.

Heptacene and Beyond: The Longest Characterized Acenes

Acenes (1) are polycyclic aromatic hydrocarbons consisting of linearly fused benzene rings. The smallest acenes, benzene, naphthalene, and anthracene, are among the most studied organic molecules, while pentacene has received much attention as an active semiconducting material in organic field-effect transistors (OFETs) owing to its high charge-carrier mobility. Since increased conjugation length is expected to be beneficial for some applications in organic electronics, interest in the synthesis of acenes larger than pentacene has been increased in the last decade, and significant efforts have been devoted to the development of appropriate synthetic methods. However, the synthesis of larger stable acenes is a difficult and challenging task because of their very low solubility, poor light and oxygen stability, and tendency to dimerize, as well as the difficult multistep synthetic approaches required. Consequently, successful experimental studies on larger acenes are very limited. An excellent review by Anthony covered the literature on larger acenes up to 2007. In recent years and, particularly, in the last one and a half years, significant progress has been made in the synthesis of larger acenes, and stable and fully characterized heptacene derivatives were obtained. Larger acenes can be considered to be the building blocks of carbon nanotubes and graphene, and studies on larger acenes may increase understanding of their properties. For example, the chirality of carbon nanotubes can be described as arising from different arrangements of the acene chains that are responsible for its electronic properties (metallic or semiconducting). Although the electronic properties of larger acenes have been examined extensively using computational techniques, their electronic structure, aromaticity, and HOMO–LUMO gaps are still not completely understood. A singlet disjoint biradical character in the ground states of larger acenes is predicted based on UB3LYP/6-31G(d) calculations. Using spin-polarized DFT, Jiang and Dai predict antiferromagnetic ground states for larger acenes (n> 7) and polyacenes. As the number of rings increases, acenes become increasingly reactive, with the central ring being the most reactive. Photooxidation with molecular oxygen and dimerization of the longer acenes are the major degradation pathways. Although the synthesis of heptacene (1, n = 7) was claimed in 1942, later reports in 1943 and 1955 questioned this synthesis and it was withdrawn in 1957. No significant progress in this area was made until 1986, when the synthesis of larger acenes was reported in the PhD dissertation of Fang which was written under the supervision of Chapman. Thermolysis of the heptacene dimer was reported to produce heptacene. However, pure heptacene was not obtained, since it was always contaminated with heptacene dimer and dihydroheptacene. Heptacene formation was confirmed by accurate mass measurement (using mass spectrometry) and by the lmax value for the highest wavelength absorption band in the sublimed film (968 nm) and in 1-methylnaphthalene solution (at 220 8C, 752 nm). Twenty years later, Neckers and co-workers obtained unsubstituted (parent) heptacene in a poly(methyl methacrylate) (PMMA) matrix by photodecarbonylation of a dione precursor at 395 nm (Scheme 1). The lmax value (roughly 760 nm, for the central vibronic peak) recorded in the PMMA matrix concurred with Fang s report on unsubstituted

Oligo- and Polyselenophenes: A Theoretical Study

Recently, a family of conducting polyselenophenes was synthesized, and they were shown to have a number of interesting properties. Here we have studied oligoselenophenes, their cation radicals and dications up to the 50-mer (50 Se), as well as polyselenophene at the B3LYP/6-31G(d) level of theory, and compared them with the corresponding oligothiophenes. Although the calculations reveal many similarities between oligo- and polyselenophenes and their thiophene-based counterparts, they also show the important differences between those two types of conjugated systems. Oligo- and polyselenophenes have a more quinoid character, lower band gap, and importantly, they are more difficult to twist. The theoretical results suggest that the HOMO-LUMO gap (band gap), bond-length alternation (BLA), and charge distribution in oligo- and polyselenophenes are strongly dependent on inter-ring twisting, yet twisting costs little energy. The inter-ring distances in oligo- and polyselenophenes are shorter than the related distances in oligothiophenes, whereas the bond lengths within the selenophene rings are comparable to those of the corresponding oligothiophenes.

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/

Products and mechanism of acene dimerization. A computational study.

The high reactivity of acenes can reduce their potential applications in the field of molecular electronics. Although pentacene is an important material for use in organic field-effect transistors because of its high charge mobility, its reactivity is a major disadvantage hindering the development of pentacene applications. In this study, several reaction pathways for the thermal dimerization of acenes were considered computationally. The formation of acene dimers via a central benzene ring and the formation of acene-based polymers were found to be the preferred pathways, depending on the length of the monomer. Interestingly, starting from hexacene, acene dimers are thermodynamically disfavored products, and the reaction pathway is predicted to proceed instead via a double cycloaddition reaction (polymerization) to yield acene-based polymers. A concerted asynchronous reaction mechanism was found for benzene and naphthalene dimerization, while a stepwise biradical mechanism was predicted for the dimerization of anthracene, pentacene, and heptacene. The biradical mechanism for dimerization of anthracene and pentacene proceeds via syn or anti transition states and biradical minima through stepwise biradical pathways, while dimerization of heptacene proceeds via asynchronous ring closure of the complex formed by two heptacene molecules. The activation barriers for thermal dimerization decrease rapidly with increasing acene chain length and are calculated (at M06-2X/6-31G(d)+ZPVE) to be 77.9, 57.1, 33.3, -0.3, and -12.1 kcal/mol vs two isolated acene molecules for benzene, naphthalene, anthracene, pentacene, and heptacene, respectively. If activation energy is calculated vs the initially formed complex of two acene molecules, then the calculated barriers are 80.5, 63.2, 43.7, 16.7, and 12.3 kcal/mol. Dimerization is exothermic from anthracene onward, but it is endothermic at the terminal rings, even for heptacene. Phenyl substitution at the most reactive meso-carbon atoms of the central ring of acene blocks the reactivity of this ring but does not efficiently prevent dimerization through other rings.

Study of Hopping Transport in Long Oligothiophenes and Oligoselenophenes: Dependence of Reorganization Energy on Chain Length

Internal reorganization energies for self-exchange hole-transfer process were calculated at the B3LYP/6-31G(d) level of theory for a series of oligothiophenes and oligoselenophenes up to the 50-mers. This is the first study of reorganization energy in very long pi-conjugated systems. We observed a linear correlation between reorganization energy and the reciprocal chain length for these long pi-conjugated heterocyclic oligomers, which can be explained by the changes in bond length that occur between the neutral and cation radical species and by the charge distribution in the cation radicals. In contrast to the saturation behavior observed for the HOMO-LUMO gaps of long pi-conjugated heterocyclic oligomers, the reorganization energy does not show saturation behavior for any length of the oligomers in this study, even up to the 50-mers. Interestingly, the reorganization energy approaches zero for infinite numbers of oligomer units (at the B3LYP/6-31G(d) level of theory), that is, for polythiophene and polyselenophene. The absolute values of the reorganization energies of oligoselenophenes, and the changes that occur in those energies with chain length, are similar to those found for oligothiophenes.

Poly(cyclopenta[c]selenophene): a new polyselenophene

Cyclopenta[c]selenophene-(CH(2)OMe)(2) has been synthesized by a new synthetic approach, successfully electrochemically polymerized and studied by spectroelectrochemistry.

Dicyanovinyl terthiophene as a reaction based colorimetric and ratiometric fluorescence probe for cyanide anions

3′-Dicyanovinyl terthiophene 3 was synthesized and the changes in absorption and emission properties of 3 in the presence of various anions were evaluated. Compound 3 acts as a selective colorimetric as well as a ratiometric fluorescence probe for cyanide anions in aqueous THF solution even in the presence of other anions such as AcO−, F−, Cl−, Br−, NO2−, N3−, HS− and ClO4−. Obstruction in an intramolecular charge transfer (ICT) by the nucleophilic addition of a cyanide anion to the dicyanovinyl group induces remarkable changes in the absorption and emission spectra of 3. The signal transduction mechanism was investigated by absorption and emission spectroscopy, 1H NMR titration and DFT calculations.