Michael Bendikov

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.

“Donor–Two-Acceptor” Dye Design: A Distinct Gateway to NIR Fluorescence

The detection of chemical or biological analytes upon molecular reactions relies increasingly on fluorescence methods, and there is a demand for more sensitive, more specific, and more versatile fluorescent molecules. We have designed long wavelength fluorogenic probes with a turn-ON mechanism based on a donor-two-acceptor π-electron system that can undergo an internal charge transfer to form new fluorochromes with longer π-electron systems. Several latent donors and multiple acceptor molecules were incorporated into the probe modular structure to generate versatile dye compounds. This new library of dyes had fluorescence emission in the near-infrared (NIR) region. Computational studies reproduced the observed experimental trends well and suggest factors responsible for high fluorescence of the donor-two-acceptor active form and the low fluorescence observed from the latent form. Confocal images of HeLa cells indicate a lysosomal penetration pathway of a selected dye. The ability of these dyes to emit NIR fluorescence through a turn-ON activation mechanism makes them promising candidate probes for in vivo imaging applications.

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.

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/

α‐Oligofurans: An Emerging Class of Conjugated Oligomers for Organic Electronics

While the field of organic electronics has developed extensively in recent years, it remains limited by number of materials available. Further expansion requires the innovation of new types of π-conjugated backbones, but suitable candidates are discovered only very rarely. The recent introduction of a new class of conjugated materials, long α-oligofurans, was therefore greeted with considerable interest. α-Oligofurans possess many of the properties required to excel in applications as organic electronic materials, can be manufactured from renewable resources, and are expected to be biodegradable. This Minireview provides an account of long oligofurans from the perspectives of their synthesis, molecular properties, chemical reactivity, and use in electronic devices.

Poly(3,4-ethylenedioxyselenophene) and Its Derivatives: Novel Organic Electronic Materials

Since the discovery of high conductivity in iodine-doped polyacetylene, many interesting conducting polymers have been developed. Of these, polythiophenes have been most studied as electronic materials, with poly(3,4-ethylenedioxythiophene) (PEDOT) and the water-soluble PEDOT-PSS being the most successful commercially used conducting polymers. The polyselenophene family together with poly(3,4-ethylenedioxyselenophene) (PEDOS) and its derivatives have been shown to have slightly different properties compared to these of polythiophene and PEDOT because of their different electron donating characters, aromaticities (selenophene vs thiophene), oxidation potentials, electronegativities, and polarizabilities (Se vs S). As a result, the polyselenophenes, especially PEDOS and its derivatives, show a lower band gap and higher-lying highest occupied molecular orbital (HOMO) levels compared with those of thiophene and the PEDOT family. In an organic materials context, the PEDOS family offers some advantages over PEDOT derivatives. This Account draws on computational studies, synthetic methods, electrochemical polymerizations, chemical polymerizations, and the materials properties of PEDOS and its derivatives to demonstrate the importance of these novel materials, which lie at the frontier of conducting polymer research. In particular, we show that (i) PEDOS derivatives have a lower band gap (about 0.2 eV) than the corresponding PEDOT derivatives. Consequently, PEDOS derivatives can absorb the solar spectrum more efficiently compared to PEDOT derivatives and the properties of optoelectronic devices based on neutral and doped PEDOS should be somewhat different from these of PEDOT. (ii) EDOS derivatives have a greater tendency to undergo electrochemical polymerization compared to EDOT derivatives and offer stable and smooth polymer films. (iii) The PEDOS backbone is more rigid than the PEDOT backbone. (iv) PEDOS derivatives are excellent electrochromic materials with high transparency, and have higher contrast ratio and coloration efficiency. (v) The PEDOS/C electrode offers better control over the formation and size of nanoparticles through Se···Pt interactions compared with the PEDOT/C electrode. In addition to this, we summarize the synthesis, electrochemical polymerization, materials properties, and computational studies of fused polyselenophene analogues, namely, poly(cyclopenta[c]selenophene), and a series of low band gap thieno- or selenolo-fused polyselenophenes and selenolo-fused polythiophene. Additionally, we discuss oxidative and solid state polymerization to obtain conducting PEDOS, and its derivatives, and made throughout comparison with S-analogue where applicable. We found that EDOS-based derivatives have a greater tendency toward solid state polymerization and working at a temperature about 20 °C lower than that required for EDOT-based compounds. Our results demonstrate the utility of EDOS unit for generating promising materials PEDOS and its derivatives for electronic devices. Consequently, EDOS structure is the basis for many functionalized polymers and copolymers with tunable optoelectronic and redox properties. These interesting properties, which include high conductivity, lower band gap, rigidity, multicolor electrochromism, and rapid redox switching, allow them to be used in a variety of electronic applications.

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.