Astruc D. - Modern arene chemistry (2002)(en)

7.6 Self-Assembly of PPEs on Surfaces: From Jammed Gel Phases to Nanocables and Nanowires 243

(solvent, evaporation rate, support), either nanowires (Figure 16b) or nanoribbons (Figure 16c) could be formed. The Mu¨llen–Rabe wires are 3–5 microns long and 30–40 nm wide. They correspond to the features observed by Perahia et al. [60, 61]. Using more concentrated solutions, Mu¨llen and Rabe observed the formation of these structures on a smaller scale, and dense nanoribbons were observed by a combination of transmission electron microscopy and AFM. These features are approximately 1 nm wide and several hundred nm long, and represent single polymer lamellae in which the ‘‘wing span’’ of 12a dictates the width of the lamellae. In simple terms, each of the structures in Figure 16c would represent one or several polymer chains stacked on top of one another.

While the formation of the nanoribbons can be explained in terms of a crystallization process that leads to these structures on solid surfaces, the formation of the nanocables and nanowires is not so straightforward. The origin of the cable-like structures thus became a topic of interest. They could either arise by a crystallization process when a homogeneous solution of 12 is dried on a surface, or these superstructures could be preformed in solution and act as seeds for the subsequent growth of cables.

It was observed early on that solutions of PPEs in toluene are colorless at temperatures above 35 C, while they turn yellow at room temperature. Almost all PPE samples prepared for 13C NMR spectroscopy show gel-like behavior at concentrations exceeding 5 % weight. Perahia et al. [61] investigated the behavior of the yellow/colorless toluene solutions of PPEs. At elevated temperatures, colorless PPEs form molecular solutions in which the rigid PPE chains are almost fully extended. On cooling these solutions below 30 C, a slightly exothermic phase transition (0.02 kcal mol 1 (repeating unit) 1) occurs, accompanied by a color change to yellow. In these yellow solutions, neutron scattering is enhanced by a factor of 40– 50, indicative of the presence of large superstructures. Calculational fitting of the scattering data suggests the formation of a jammed gel phase in these yellow solutions (Figure 17). This gel phase has not been described, and consists of aggregates that are approximately 7 nm in diameter and several mm long (Figure 17). Due to their size, the flat aggregates cannot flow freely, restrict the movement of the solvent, and form a gel. Upon casting onto surfaces, these nanocylinders seed the growth of the nanocables observed in thin solid films. The nanostructures observed in thin films must be preformed in solution. Several of these cylinders (from their dimensions, approximately 10–15) aggregate to form nanocables (Figure 16a). Schnablegger et al. [62] reported similar behavior for a di erent PPE derivative.

However, their polymer was a highly carboxylated polyelectrolyte, in which strong intermolecular interactions are expected due to the preeminent electrostatic forces between charged particles. The dialkyl-PPEs 12 are devoid of any strong intermolecular forces, and so only van der Waals interactions and p–p stacking interactions need to be considered. The principles

Fig. 17. Formation of gel phases upon cooling solutions of dialkyl-PPEs 12 in toluene.

2447 The ADIMET Reaction: Synthesis and Properties of Poly(dialkylparaphenyleneethynylene)s

that guide the size and the shape of these fascinating intermolecular complexes must be subtle and are under investigation by Perahia et al.

7.7

PPE-Based Organic Light-Emitting Diodes (OLEDs)

Conjugated polymers are superb candidates for applications as active emissive layers in light-emitting diodes [63]. Flexibility in synthesis and processing allows the fabrication of large-area devices by spin-coating and doctor-blade techniques, while their electronic and mechanical properties can be easily modified by selection of the appropriate substituent pattern and polymer backbone. Until recently, the conjugated polymer that has found the most widespread application in OLEDs is PPV, either as the unsubstituted parent or as di- alkoxy-PPV. PPVs possess well-balanced properties for applications in OLEDs. PPV multilayer LEDs with hole-blocking and/or electron-injecting layers combine long lifetimes with high brightness of more than 2000 Cd m 2 at reasonable turn-on voltages [64].

Despite the overall excellent profile of the PPVs, they have delicate processing requirements and are not particularly stable under ambient conditions. This leads to the formation of photo-oxidized products and device degradation. In addition, the band gap of PPVs is relatively small so that only green, yellow, orange, and red emitters can be realized; for organic blue emitters other backbone structures have to be employed [64]. PPEs are easily prepared in large quantities, are stable, and show emission in the solid state that can range from bluish-green to yellow depending on the side chain and the solid-state ordering of the sample.

First attempts by Barton and Shinar [65] to utilize PPE types as active emitter layers were somewhat disappointing, and the fabricated devices performed poorly. Subsequently, Weder [66, 67] and Neher and Bunz [68] reinvestigated PPEs as active emitter layers in OLEDs. According to both groups, there are big di erences between PPEs and PPVs, and one reason for the poorer performance of the PPEs is the unsatisfactory hole injection. The presence of alkyne groups instead of vinyl groups in the backbone leads to an increased band gap with a considerably lowered HOMO and LUMO. The lowered HOMO stifles hole injection, but the lowered LUMO has the advantage that electron injection from the cathode is less di cult than in the case of PPVs. As a positive consequence of the decreased LUMO, aluminum works considerably better as a cathode material than calcium does in PPE-based light-emitting devices. This gives testimony to the facilitated electron injection into the lower-lying LUMO of the PPEs. To increase the performance of PPEs, Neher et al. [67] fabricated multilayer diodes. The known problems with hole injection were overcome by adding a hole-injecting layer, Baytron P (a water-soluble mixture of polystyrene sulfonate and oxidized poly(ethylenedioxythiophene); PEDOT), onto the indium tin oxide (ITO) substrate. To increase the emissive intensity of the PPE-types in the solid state, as well as the band gap, a copolymer 53 was utilized in which 25–75 % of the phenyl rings were replaced by naphthalene units (53a 25 %; b 33 %; c 50 %; d 75 % naphthyl units). Increasing naphthalene content increased the solid-state quantum yield and led to a hypsochromic shift of the solid-state emission. Figure 18 shows the solid-state emission spectra of 53a–d, and Figure 19 displays a typical OLED architecture that was utilized for 53. The organic emitter layer is capped with an evaporated layer of LiF, upon which Al is sublimed in a high vacuum metal evaporator.

7.8 Conclusions and Outlook 245

Fig. 18. Solid-state emission of copolymers 53a–d.

On charging, the sandwich emits blue or blue-green light depending upon the naphthalene content in 53. The higher the naphthalene content the more blue-shifted the emission. The brightest diodes (see Figure 20), up to 100 Cd m 2 at 10 V, were obtained for a PPE copolymer that contained 33 % naphthyl units. The electroluminescence spectrum of this diode is shown in Figure 20. The pure blue emitter 53b shows peak brightness values of 20 Cd m 2 at a driving bias of 18 V.

Recently, Weder [67] demonstrated that PPEs can show a device brightness of up to 300 Cd m 2 in OLEDs if Alq3 is utilized as a hole-injecting and emissive layer. At the moment, it is not clear as to what limits the e ciency and brightness of PPEs in OLED applications because charge transport does not seem to be a problem. The exploration of di erent hole-injecting layers combined with other cathode materials, interspersed inorganic nanoparticles, or carbon nanotubes would be directions worth exploring when optimizing PPEbased device architectures.

7.8

Conclusions and Outlook

Alkyne metathesis is an exciting method that furnishes small molecules, oligomers, and cycles in excellent yields, and at the same time ADIMET makes conjugated alkyne-bridged polymers of high molecular weight available. A series of di erent catalysts for alkyne metathesis and ADIMET is available; these range from the sophisticated but sensitive defined tungsten and molybdenum carbynes of the Schrock, Cummins, and Fu¨rstner types to sim-

Fig. 19. OLED architecture used for 53b,c.

246 7 The ADIMET Reaction: Synthesis and Properties of Poly(dialkylparaphenyleneethynylene)s

Fig. 20. (a) Normalized electroluminescence (EL) spectra of devices with 53b and 53c as emissive layers. (b) Spectrally integrated EL intensity vs. voltage characteristics for multilayer devices made from

53b and 53c.

Scheme 21

References 247

ple but robust ‘‘shake-and-bake’’ catalysts formed in situ. The in situ catalysts give excellent results in the formation of polymers and oligomers, and most of the described targets have been made utilizing this reliable ‘‘low-tech’’ approach.

The main thrust of this contribution has been to present results obtained for PPEs. While processible PPEs have been known for more than a decade, many of their fundamental properties, including liquid crystallinity, aggregation and emission behavior, as well as thermoand solvatochromicity, have been explored only recently. Although the PPEs represent the dehydrogenated congeners of the PPVs, their physical and spectroscopic properties are more di erent than one might expect from the small changes in chemical structure. The facile access to large amounts of PPEs of high molecular weight and purity by ADIMET has been the key to the discovery of most of the interesting properties of these unusual polymers. The simplicity and ease of scale-up make the ADIMET process with mixtures of Mo(CO)6 and phenols valuable for industrial applications. At the same time, this process is useful in an academic environment as a tool for the discovery and synthesis of novel alkynebridged polymers due to its simplicity, reliability, and low cost. Many future developments are anticipated, and so far we have only scratched the surface of the possibilities o ered by this powerful and exciting reaction and these fascinating structures.

Acknowledgements

This work has been supported by the National Science Foundation (CHE-9814118, CHE 0138-659). The Petroleum Research Funds, the Camille and Henry Dreyfus Foundation (UB is Camille Dreyfus Teacher Scholar 2000–2004), the Research Corporation, the Commission of Higher Education of the State of South Carolina, and the University of South Carolina are acknowledged for generous funding of this project. I am very much indebted to my coworkers Dr. Lioba Kloppenburg, Dr. Neil G. Pschirer, Dr. Winfried Ste en, Dr. Ping-Hua Ge, Dr. Paul M. Windscheif, Glen Brizius, Carlito G. Bangcuyo, James Wilson, Dschun Song, Alan R. Marshall, Steve Kroth, Liam Palmer, and Rhonda Roberts. My thanks as well to the invaluable collaborators (and friends), Prof. Dr. Dieter Neher; Prof. Dr. Ulli Scherf; Prof. Mark A. Berg, Ph.D.; Prof. Michael L. Myrick, Ph.D.; Prof. Cathy J. Murphy, Ph.D.; Prof. Dvora Perahia, Ph.D.; and Prof. Miguel Garcia-Garibay, Ph.D., who have all helped to develop the physical understanding of the PPEs.

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250

8

The Chromium-Templated Carbene Benzannulation Approach to Densely Functionalized Arenes (Do¨tz Reaction)

Karl Heinz Do¨tz and Joachim Stendel jr.

Abstract

The benzannulation of readily accessible arylor vinylcarbene chromium complexes by alkynes provides a straightforward synthesis of densely functionalized benzenoid and fused arenes selectively labeled with a Cr(CO)3 fragment. The reaction occurs under mild conditions, is regioselective, and tolerates a variety of functional groups both on the alkyne and on the carbene ligand. The chromium fragment undergoes a haptotropic metal migration controlled by the substitution pattern of the arene; moreover, it activates the coordinated benzene ring towards selective nucleophilic addition and aromatic substitution. Final decomplexation occurs upon ligand exchange or oxidation reactions. Chiral information imposed on either the carbene ligand or the alkyne allows for a diastereoselective benzannulation providing optically active arene-Cr(CO)3 complexes.

8.1

Introduction

The regiospecific preparation of polyfunctionalized aromatic compounds still represents a major challenge. Aiming at the realization of complex substitution patterns, conventional synthetic methodologies starting from a given aromatic starting material, such as electrophilic or nucleophilic substitutions, coupling reactions, and metalation–functionalization reactions, are often inconvenient. This inconvenience is caused by the (linear) multistep reaction sequence needed, the di erent activating or deactivating and orientating e ects of the substituents, and the di culties associated with the regioselective introduction of a specific substitution pattern (especially in the case of polycyclic compounds).

As a consequence, new methodologies have been designed to overcome these bottlenecks. In this context, strategies that simultaneously construct the aromatic skeleton and the target substitution pattern are both elegant and powerful solutions to the problem because only a few steps are required and the formation of regioisomeric mixtures can be avoided in most cases. Reppe’s research [1] with nickel catalysts in the 1940s initiated the development of the alkyne cyclotrimerization with organometallic compounds leading to benzene derivatives. An elegant result of this development has been the work by the Vollhardt group [2] in

Modern Arene Chemistry. Edited by Didier Astruc

Copyright 8 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30489-4

8.1 Introduction 251

applying cobalt templates to the synthesis of natural products such as steroids [2a] and strained polycyclic compounds such as oligophenylenes [2b].

Another attractive route to densely substituted arenes is based on transition metal carbene chemistry. In Fischer-type carbene complexes, the carbene carbon atom is linked by a formal double bond to a metal carbonyl fragment incorporating a low-valent metal of Groups VI to VIII [3]. The electron-deficient carbene carbon typically bears at least one stabilizing heteroatom p-donor substituent. The second carbene substituent may be either a saturated or an unsaturated hydrocarbon chain. The metal center is coordinated by p-acceptors such as carbon monoxide, phosphines, or cyclopentadienyl ligands. Reactions of carbene complexes [4] may either proceed at the metal center or occur within the carbene ligand. The metal carbene fragment represents a potent electron-acceptor moiety, which activates adjacent CbC and CcC bonds towards the addition of nucleophiles or cycloaddition reactions; moreover, it considerably enhances the CaH acidity of a-hydrogen atoms, which allows for CaC coupling reactions via metal carbene anions.

On the other hand, as demonstrated by early examples, carbene complexes may serve as stable carbene equivalents and undergo [2þ1] cycloaddition with alkenes to give cyclopropanes [5]. However, no indication of a free carbene intermediate has yet been obtained; in contrast, the principal reaction mechanism involves a carbene transfer within the coordination sphere of the metal, which acts as a ‘‘template’’. In an early extension of this idea, when pentacarbonyl(a-methoxybenzylidene)chromium was reacted with tolane, our aim was the in situ generation of an alkyne–carbene–carbonyl complex in which the carbonyl chromium ‘‘template’’ keeps these three di erent ligands in a facial configuration favorable for subsequent interligand coupling. The experimental result was quite exciting: We did not observe any type of two-ligand (alkyne/carbene, alkyne/CO, or carbene/CO) coupling to give cyclopropene or ketene derivatives; instead, a three-ligand coupling of the alkyne, phenylcarbene, and carbonyl ligand to a ord a naphthohydroquinone skeleton had occurred at the Cr(CO)3 template, which remained h6-coordinated to the benzannulation product (Scheme 1) [6].

Scheme 1. Reaction of pentacarbonyl(a-methoxybenzylidene)chromium with diphenylethyne: the first example of benzannulation.

The benzannulation product may be viewed as a formal [3þ2þ1] cycloaddition product, in which the alkyne (C2 synthon) is connected to the carbon monoxide ligand (C1 unit) and the carbene carbon atom of the unsaturated carbene ligand (C3 building block), the b-carbon

2528 The Chromium-Templated Carbene Benzannulation Approach to Densely Functionalized Arenes

atom of which, in turn, is further coupled to the carbon monoxide ligand. However, a closer insight into the mechanism of this reaction, as discussed in Section 8.2, indicates a stepwise carbon–carbon bond formation starting from the alkyne–carbene–carbonyl complex B as the key intermediate, formed in a decarbonylation equilibrium from the pentacarbonyl carbene complex A and the alkyne (Scheme 2). The Cr(CO)3 template assists the interligand coupling and remains coordinated to the newly formed arene ring C.

Scheme 2. Connectivity in the chromium-templated benzannulation reaction.

Chromium(0) in an octahedral configuration is the metal of choice for this type of reaction. There have been a few examples in which metals other than chromium, such as tungsten and molybdenum [7a], manganese [7b, 7c], and iron [7d] have been reported as alternative templates, but all of them turned out to be far less e cient and general.

The benzannulation of a,b-unsaturated chromium carbene complexes with alkynes, sometimes referred to as the ‘‘Do¨tz reaction’’ [8], has stimulated organic synthesis along the borderline of organometallic and synthetic organic chemistry. Since the end of the 1970s, the reaction has been applied to the synthesis of biologically active compounds by Do¨tz, Semmelhack, and Wul and their groups, in order to define its scope. In the early 1980s, Do¨tz and co-workers started to elucidate the mechanism of the reaction. At the end of the 1980s, the research temporarily shifted to amino chromium carbene complexes in order to determine the factors that control the competitive formation of benzannulation and pentannulation products. In the 1990s, related additional and useful benzannulation procedures employing chromium carbene complexes, such as the photochemical benzannulation methodology of Merlic, were developed. More recently, part of the focus has been shifted to strained arenes and to the chiral plane present in the benzannulation products, and e orts