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Phosphole and Phosphine Acetylene Building Blocks for n-Conjugated Systems Weymiens, W.

2012

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Download date: 28. Sep. 2021 Phosphole-Containing π-Systems |2

An overview is given of the most important developments and the current status of phosphole-containing π-conjugated materials. The emergence of organic electronics as well as phospholes is discussed, as such placing the field of phosphole-based π-systems in a historic perspective. The electronic structure of the phosphole ring combines a low degree of aromaticity, hyperconjugation and the presence of a reactive phosphorus center, as such creating a handle to fine-tune the photophysical properties of phosphole-containing π-systems. Influencing the extent of conjugation over the diene moiety enabled the construction of a great variety of luminescent π-systems, which may be grouped in two distinct categories; 2,5-bisarylphospholes and phosphole- containing ‘ladder-type’ ring systems. 10 Chapter 2: Phosphole-containing π-systems

2| Phosphole-Containing π-Systems

In this chapter, a literature overview is given on the role of phospholes

(phosphacyclopentadiene, C4H5P, 1) in π-conjugated materials. Phosphole- containing materials have attracted huge interest lately, for their potential incorporation into organic electronics. Therefore, this chapter begins with a short introduction of organic electronics, before the subject of phospholes is explored in depth.

2.1. Organic electronics In the quest for making electronic devices more versatile, cheaper and energy efficient, organic electronics play an increasingly important role. In this respect, displays made from organic molecules, the so-called organic light emitting diode (OLED) displays,[1] have a multitude of benefits over conventional displays (LCD, plasma), such as the possibility for complete flexibility and transparency, increased brightness, wider viewing angles and shorter response times, less energy consumption and easy and potentially cheap manufacture.[1] In general, an OLED is a stack of fluorescent layers, each consisting of (an) organic compound(s). These layers are sandwiched between a conducting anode and cathode. When sufficient voltage is applied to the device, a current flows from the cathode to the anode. Electrons are injected into the ‘electron-transporting layer’ (ETL) from the cathodic side of the device and holes into the ‘hole-transporting layer’ (HTL) from the anodic side. Recombination of holes and electrons in the emissive layer (EL) results in electroluminescence.[1]

In OLEDs display, the emissive layers are formed by organic semi-conductor materials, which may either be polymeric or ‘small molecule’. The color of the OLED is determined by the fluorescence wavelength of the emissive layer, i.e. the energy of the emitted photon. Tuning this emission to the desired wavelength may be accomplished by the addition of dopants to the emissive layer, or by fine-tuning of the photophysical properties of the organic material of the emissive layer itself. Grouping OLEDs together, containing different EL materials and thus having different colors of emission, enables the manufacture of multi-colored displays. Compounds suitable as emissive layer should at least exhibit efficient visible light fluorescence. Semi-conducting organic compounds form an important class of candidates, as their relatively narrow band gap often gives rise to emission of visible light. Organic compounds having such a narrow band gap can be obtained by extensive π-conjugation. Increasing the degree of conjugation lowers the energy gap between Chapter 2: Phosphole-containing π-systems 11 the π- and π*-orbitals, thus decreasing the HOMO-LUMO gap or bad gap. This reduction in band gap reaches an asymptotic value above about ten monomeric units.[2]

The first conducting polymer was observed as early as 1862.[3] Letheby oxidized analine electrochemically and chemically, and obtained deep blue polyanaline, but did not recognize its true nature. In the 1950s and 1960s, more publications on conductive polymers appeared, such as perylene charge transfer complexes [4] and iodine doped polypyrrole.[5] Melanin is a naturally occurring class of pigments, present in most organisms, which consists of polymers and co-polymers of aniline, pyrrole and acetylene. It was suggested in 1972 that melanin might be suitable for incorporation in electronic devices, having conductivities resembling those of chalcogenide glasses.[6] Two years later, the actual production of such a device enabled the generation of a flash of light from the melamine polymers upon switching on an electric circuit (electro- luminescence).[7] As such, the first organic electronic device had been developed. These discoveries were largely overlooked, when the Nobel committee awarded Heeger, MacDiarmid and Shirakawa the Nobel Prize in Chemistry 2000,[8] “for the discovery and development of electrically conductive polymers”, work that was performed in 1977.[9] They found that the oxidation of polyacetylene by chlorine, bromine or iodine vapor led to spectacular increase in conductivity.

In 1963, the first electroluminescence from a ‘small’ organic molecule was observed. Pope reported blue emission from 20 μm crystals of anthracene (2, Figure 2.1), when a voltage of 780 V was applied thereon.[10] The amount of voltage needed to achieve electroluminescence was decreased significantly, when it was applied across vacuum- deposited 0.6 μm anthracene films, enabling naked eye visualization of the blue emission in a dark room at about 12 V.[11] The appearance of small organic molecules as potential candidates in electronics was [12] triggered by the intriguing properties of tetrathiafulvalene (TTF, (H2C2S2C)2). The chlorine salt hereof exhibits semi-conducting behavior,[13] while the tetracyanoquino- dimethane salt [TTF]⊕[TCNQ]⊖ (3, Figure 2.1) has a narrow band gap, showing metal- like conductivity below 66K, and exists as alternating positively and negatively charged stacks in the solid state.[14]

The first working OLED was developed at the Kodak Eastman laboratories in 1980, which contained only one organic (emissive) layer, primarily containing phthalocyanine (4) derivatives (Figure 2.1).[15] Employing voltages below 10 V, green emission with a 2 brightness of over 1000 cd/m could be obtained. A few years later, the first dual layer

12 Chapter 2: Phosphole-containing π-systems

Figure 2.1. Early electroluminescent compounds.

Figure 2.2. Organic compounds employed as emissive layer in OLEDs.

OLEDs, based on tris(8-hydroxyquinolinato)aluminium (Al(q)3, 5) were developed at the same laboratory (Figure 2.1).[16]

Over time, a great variety of organic molecules have been employed as emissive layer [15] [17] or as dopant therein (Figure 2.2), such as Al(q)3 (5), amino-oxadiazole-fluorine 6, [18] [19] DPVBi (7), Ru(bipy)3 (8).

2.2. History of phospholes It was not until 1953 that the first phosphole derivative, phenyl-dibenzophosphole (9) was prepared by Wittig and Geisler (Figure 2.3),[20] which is rather late compared to the first appearance of related compounds, such as pyrrole in 1834,[21] furan in 1780[22] and thiophene in 1883.[23] During the first ten years of phosphole chemistry, only a few novel derivatives were reported. In this respect, pentaphenylphosphole (10)[24] and 1,2,5-triphenylphosphole (11)[25] are representative early examples (Figure 2.3). It [26] lasted until 1983 until the parent phosphole ring (C4H5P, 1) was prepared.

Figure 2.3. Early examples of phosphole compounds. Chapter 2: Phosphole-containing π-systems 13

2.2.1. Synthesis Over the years, several synthetic approaches towards the phosphole ring were developed (Scheme 2.1). Here a short overview is given of the most important synthetic approaches.

The first synthesis of a phosphole ring system employed the treatment of

2,2’-dilithiated biphenyl (12) with PhPCl2, as such forming phenyl-dibenzophosphole (9, Scheme 2.1(i)).[20] Other early syntheses of monocyclic phosphole rings initiated from diphenylethyne (13). Treatment thereof with metallic lithium gave dilithiated tetraphenylbutadiene (14), which upon reaction with PhPCl2 or BnPCl2 gave the corresponding pentaphenyl-phosphole (10)[24b,27] or its 1-benzyl derivative 15[28] respectively (Scheme 2.1(ii)). 10 was also prepared by treatment of diphenylethyne with Fe(CO)5, via an ironcyclopentadiene-iron (16) intermediate, which subsequently [24a] reacted with PhPCl2 (Scheme 2.1(iii)). In 1967, Märkl reported the synthesis of phospholes from disubstituted diynes (17) and primary phosphines in the presence of a strong base (e.g. PhLi), thus enabling the introduction of three different ring-substituents for the first time (Scheme 2.1(iv)).[29] In this respect, the synthesis of 2,5-(2-bromophenyl)-1-phenyl-phosphole (18)[29] via this route is worth mentioning, as the bromine moieties can potentially be used for further functionalization. No yield was reported for this compound. An alternative of this method for the preparation of phosphole-containing fused ring systems is developed in 1993, also by the group of Märkl, in which a benzene ring, ortho-substituted with a secondary phosphine and an alkynyl moiety (19) ring-closes internally to phosphindole 20 (Scheme 2.1(v)).[30] A different approach is presented by the McCormack reaction, which was developed by McCormack at DuPont Company.[31] This reaction initially affords halo- phospholenenium halides (21) by the addition of RPCl2 to 1,3-dienes via a pericyclic [1+4] cycloaddition, which may be hydrolyzed to phospholenes (dihydrophospholes). When no water is added, intermediates 21 may dehydrohalogenate to form phospholes, which is however hampered by relative low yields (50 to maximally 70 % [25,32] in excess PhPCl2). With the recognition that the addition of a base results in smooth dehydrohalogenation (Scheme 2.1(vi)),[33] the first versatile synthesis of phosphole analogues, bearing a wide variety of different substituents, was born (22).

In analogy to the above-described reaction between diphenylethyne and Fe(CO)5, symmetrically substituted acetylenes react with zirconocene, activated by treatment with n-BuLi, to zirconocyclopentadiene intermediates (22), which, in analogy to the [34] iron-cycle, forms a phosphole ring when reacted with PhPCl2 (Scheme 2.1(vii)).

Exemplary is the treatment of 2-butyne with activated “ZrCp2” and subsequent [35] reaction with PhPCl2 gave tetramethyl-phenyl phosphole 23 in 81% yield.

14 Chapter 2: Phosphole-containing π-systems

Scheme 2.1. Overview of synthetic approaches towards phosphole rings. Chapter 2: Phosphole-containing π-systems 15

In these reactions, the regiochemistry is not easily controlled to its full extent, and the use of asymmetrically substituted acetylenes results mostly in isomeric mixtures. Only when the difference in steric bulk of both substituents is large enough, the larger substituent ends up at the 2- and 5-positions. The low selectivity is dramatically improved by linking two acetylenes via a C3- or C4-spacer, which then ends up at the 3,4-positions (Scheme 2.1(viii)). Using this approach, a wide variety of substituents could be regiospecifically introduced at the 2,5-positions, while an inert ring substituent is introduced at the 3,4-positions. In this respect, substituted 1,6-heptadiyne (24a, n = 1) and 1,7-octadiyne (24b, n = 2) are found to be valuable building blocks. This synthetic procedure towards 2,5-disubstituted phospholes is coined the Fagan-Nugent reaction, after its developers. Due to the versatility and elegance of this method, the synthesis of 2,5-bisaryl phospholes took a big flight (see section 2.3.1).

2.2.2. Aromaticity Ever since the phosphole ring was first prepared, the scientific community debated the extent of its aromaticity. It was not until the mid 1960s that phosphole rings became accessible through effective synthetic approaches, which enabled a detailed study of its electronic structure. Somewhere during the 1970s, consensus on the aromaticity of phosphole rings had been reached;[36] namely, the phosphole ring is hardly aromatic. In this respect, the first X-ray structure of a phosphole-containing compound, 1-benzyl- phosphole (27) in 1970 (Figure 2.4), proved to be a benchmark publication, in that therein the pyramidal nature of phosphorus centre within a phosphole ring was unambiguously confirmed for the first time.[37] The bond length alteration (BLA) analysis of the crystal structures of phospholes, in comparison with related heterocycles, showed that phosphole rings contained localized C–C single bonds and C=C double bonds with considerable difference in length,[38] whereas highly conjugated rings show small differences in C–C and C=C bond lengths. In order of decreasing aromaticity, the BLA values are 0.035 Å for pyrrole, 0.053 Å for thiophene, 0.07 Å for furan, 0.095 Å for phosphole and 0.127 Å for cyclopentadiene. A fine-tuning of this method by Bird, the Bird Index (BI) of aromaticity,[39] gave a slightly different order in aromaticity,[40] i.e. thiophene (BI = 66) > pyrrole (BI = 59) ≈ selenophene (BI = 59) > furan (BI = 43) > phosphole (BI = 35.5) > cyclopentadiene (BI = 29),[41] with the same general conclusion; phosphole is hardly aromatic. More recently, the availability of computational chemistry led to the development of the “nucleus independent chemical shift” (NICS),[42] which is a measure of the NMR chemical shift of a dummy atom in the centre of a (conjugated) ring. The NICS value of phosphole has been determined to be –5.3, which places phosphole near cyclopenta- diene (NICS = –3.2) and far removed from pyrrole (NICS = –15.1).[42] 16 Chapter 2: Phosphole-containing π-systems

Figure 2.4. Early examples of phosphole compounds.

It is generally recognized that the increased barrier of inversion of phosphines [43] [44] (31.5 kcal/mol for PH3) versus amines (5.7 kcal/mol), causes the great difference in aromaticity between phosphole and pyrrole. In pyrrole, the energy cost of flattening the nitrogen centre, which corresponds to its the barrier of inversion, is more than compensated by the energy gain of aromatic electron delocalization (20.8 kcal/mol).[45] Accordingly, such an energy gain by electron delocalization is too small to overcome the barrier of inversion of the phosphole phosphorus atom. Hence, the phosphorus atom retains its pyramidal nature, and the phosphorus lone electron pair resides in an essentially sp3-type orbital and has little to no overlap with the carbon-based π-orbitals. In fact, the low but not negligible amount of conjugation within the phosphole ring mainly originates from hyperconjugation between the diene π-system and the exocyclic P–C σ-bond.[46] Due to its aromatic planar transition state, the phosphorus centre of phospholes has a significantly lowered barrier of inversion of ~16 kcal/mol.[47] It has been determined that the electron donor capacity of planar phosphorus rivals that of nitrogen.[48] In view thereof, phosphole would display an even higher degree of aromaticity than pyrrole and thiophene, when the phosphorus atom would be forced into a planar configuration.[49] The planarity of phosphorus is efficiently measured by the summation of the three angles around phosphorus (Σ P). In this respect, a summation to 360° reflects a completely planar phosphorus centre, while a regular pyramidal phosphine exhibits [50] [51] values for Σ P ranging from 280.5° for PH3 to 309.0° for PPh3. Quite similarly, 1-benzylphosphole (27) exhibits a Σ P of 302.7°.[38]

The extent of conjugation within the phosphole ring is directly related to the pyramidality of the phosphorus centre;[52] the higher the planarity of the phosphorus tripod, the higher the aromatic character of the phosphole ring. The extent of aromaticity can thus be enhanced by forcing phosphorus into more or less planar configuration, but also by reducing the diene-type conjugation. The latter effect is associated with the presence of conjugated moieties at the 2- and 5-positions of the phosphole ring, which stimulate the establishment of a conjugated path over the four carbon atoms of the phosphole ring, and thus omitting the phosphorus atom Chapter 2: Phosphole-containing π-systems 17 completely from conjugation. Hence, introducing such π-conjugated substituents on the 2- and 5-positions results increased phosphorus pyramidality and even further decreased phosphole intraring conjugation. The first phosphole derivatives in which such an increased phosphorus pyramidality was observed, were 2,5-bis(2-phenylethynyl)phosphole 28[53] and 2,5-bis(2-pyridyl) phosphole 29,[54] exhibiting a significantly lowered Σ P of 292.4° and 299.3° respectively, when compared to 1-benzylphosphole 27 (Figure 2.4). In these molecular frameworks, an effective conjugated path is established, in which the π-systems of the 2,5-substituents interact with the π-orbitals of the phosphole diene. In this respect, the Bird Index of 1,2,5-triphenylphosphole 11 is worth noting. Its BI of 20[55] is even lower than the generally encountered BI of 35.5 in 2,5-unsubstituted phospholes, indicative of a further decreased extent of aromaticity.

The reason for the above-described deviation from regular bond angles around the phosphorus centre is electronic in nature. On the other hand, such a deviation can also be accomplished by steric effects. To analyze such a steric effect in more detail, the series of 1-arylphospholes 30 is of interest (Figure 2.5). The phosphorus tripod of isytyl-substituted 30c and supermesityl-substituted 30e are significantly flattened with respect to less bulky substituted phospholes, i.e. Σ P(30c) = 314.4°;[56] Σ P(30e) = 331.7°).[57] Such an increased planarization enforces an increased parallel alignment of the phosphorus lone pair with respect to the carbon-based π-orbitals, thus enabling enhanced conjugation within the phosphole ring. In accordance with the increased aromaticity of phospholes bearing bulky phosphorus substituents is the low lone pair ionization energies of 30d (7.55 eV)[58] and 30e (7.5 eV)[59], which however hardly effected the reactivity of 30d.[60] Removal of an electron from a delocalized π-system is easier than from a localized orbital, as the resulting negative charge is stabilized by distribution over more atoms in delocalized systems. A Bird Index of 56.5 for 30e also supports its increased aromaticity.[57]

Interesting to note is that the incorporation of one or two further phosphorus atoms within the phosphole ring enables nearly full or even completely full planarization. In this respect 1,3-diphosphole 31 (Σ P = 352°),[61] and 1,2,4-triphosphole 32

R1 R2 R3 Ar 30a H H H phenyl (Ph) 30b Me Me Me mesityl (Mes) 30c i-Pr i-Pr i-Pr isityl (Is) 30d Me t-Bu t-Bu 30e t-Bu t-Bu t-Bu Supermesityl (Mes*)

Figure 2.5. Bulky phosphorus substituents increase conjugation in the phosphole ring. 18 Chapter 2: Phosphole-containing π-systems

Figure 2.6. Phosphole ring wherein the phosphorus atom approaches planarity.

(Σ P = 358°)[62] are noteworthy (Figure 2.6). The latter constitutes the first planar phosphole, having a Bird Index as high as 84 and a NICS value of –12.3. Moreover, 1-supermesityl-1,2,4-triphosphole (33) is predicted to be fully planar and aromatic, having a NICS value of –14.7, which is remarkably close to that of pyrrole (–15.1).[63]

2.2.3. Reactivity at phosphorus Because of its low aromaticity, the pyramidal phosphorus centre of phosphole rings is sensitive towards oxidation and other chemical modification, such as alkylation, metal complexation and Staudinger ligation. The possible approaches towards modification of the phosphorus centre will be briefly discussed here below.

Thus, exposing phospholes to air or common oxidizing agents gradually converts them into the corresponding phosphole oxides.[28,64] It has been found that oxidation of the pyramidal phosphorus centre to its P(V) oxidation state, by for example oxygen or sulfur, changes the electronic status of the ring from slightly aromatic to slightly anti-aromatic.[65] Because of this slight anti- aromaticity, the diene system of phosphole oxides is prone to Diels-Alder reactions, and consequently phosphole oxides generally undergo rapid dimerization (Scheme 2.2). Exemplary is the oxidation of 3,4-dimethyl-1-phenyl-phosphole (34) with t-BuOOH to the corresponding oxide 35, which subsequently dimerizes to 7-phosphanorbornene 36, displaying two distinct signals in its 31P NMR spectrum.[66] The presence of sterically demanding substituents at the 2- and 5-positions or at the 3- and 4-positions hampers this reaction, giving rise to stable phosphole oxides.[28,64,67] Dienophiles have been used as trapping agent, reacting with in situ formed phosphole oxides. In this respect, maleimide[66,68] is commonly used as dienophile, but reactions with oxyallyl cations[69] and acetylenes[64,70] are also known. However, in some cases in the latter reaction has been found to result in loss of the phosphorus bridge from the 7-phosphanorbornadiene system.[64,70a] Also P(III) phospholes are able to undergo Diels-Alder reactions with strong dienophiles, which was first recognized in 1981, when 3,4-dimethyl-1-phenyl- phosphole (34) was reacted with N-phenylmaleimide.[71] Several years later, this [66] reaction was repeated with the corresponding phosphole oxide. Interestingly, the

Chapter 2: Phosphole-containing π-systems 19

Scheme 2.2. Dimerization of phosphole oxides via a Diels-Alder reaction.

regioselectivity of the products of these two reactions was exactly opposite. In the phosphole product, the P–phenyl group was oriented syn to the maleimide moiety, while it was oriented anti to this moiety in the phosphole oxide product. Of interest in this respect is the intramolecular Diels-Alder, which occurs in 1-(3-butenyl)- 3,4-dimethyl phosphole.[72] Another consequence of the anti-aromaticity of phosphole oxides is that P(V) phospholes prepared from substituted 1,6-heptadiynes and 1,7-octadiynes cannot be treated with strong bases, such as n-BuLi, as these bases deprotonate the α-hydrogen of the ‘backbone’ ring at the 3- or 4-position, thereby triggering a rearrangement in [65] which the C2–C3 double bond shifts to the C3–Cα position. The P(III) centers are also reactive towards other chalcogens, such as sulfur[28,33,73] and selenium.[64,74] Surprisingly, the resulting phosphole sulfides and selenophides are not prone to dimerization via the Diels-Alder reaction. However, the Diels-Alder reaction of phosphole sulfides with dienphiles does occur, and is observed for N-methyl- 1,2,4-triazoline-3,5-dione,[75] maleic anhydride[76] and tert-butylphosphaalkyne.[77]

The phosphorus lone pair is also susceptible to alkylation. Thus, reaction of 1,2,5-trimethylphosphole with methyl iodide results in the formation of the iodide salt of the tetramethylphospholium cation.[78] Likewise, protonation of the phosphorus atom of the phosphole ring is possible.[79] However, the resulting 1H-phospholium cations are unstable and rapidly dimerize or rearrange via 1,5-sigmatropic hydrogen shifts.

Iminophospholes may be prepared via the Staudinger reaction,[80] a well-known reaction to convert azides to amines upon treatment with triphenylphosphine, via a phosphazene intermediate.[81] When this reaction is conducted with phospholes instead of phosphines, the outcome is a P(V) phosphole, bearing a double bonded nitrogen. Exemplary is the formation of iminophosphole 37, which is prepared by the reaction of 1,2,5-triphenylphosphole with phenylazide (Scheme 2.3).[82] The phosphorus can also act as a Lewis base by donating electron density to a Lewis acid via its lone pair. In this respect, BH3 is commonly used as Lewis base, such as for the complexation to 3,4-dimethylphosphole.[82] 20 Chapter 2: Phosphole-containing π-systems

Scheme 2.3. Staudinger ligation between phosphole 11 and phenylazide.

Phospholes, like regular phosphines, are versatile ligands for (transition) metal complexes.[84] Contrary to phosphines, a multitude of coordination modes is available for a phosphole as ligand (Figure 2.7). Most regularly encountered is coordination via the phosphorus atom (η1-phosphole A), but also coordination via the carbon-based π-system is possible (η4-phosphole B), or a combination thereof (C).[85] The options increase when the exocyclic phosphorus substituent is removed by a strong base. The phospholyl ligand is encountered in many coordination modes. Most studies are done on η5-phospholyl complexes (D), in which the phosphorus lone pair is available for further coordination (E). Coordination may also occur on phosphorus alone (η1-phospholyl F). In the absence of an exocyclic substituent, the phosphorus atom may also act as a bridging ligand (μ2-phospholyl G), in which the conjugation in the phospholyl ring is broken. In analogy to the non-conugated phosphole ring, coordination to the carbon-based π-system has been observed, giving trimetallic complex (H).[86] Intriguing in this respect is heptametallic complex 38, in which four “type H” phosphole ligands bridge three palladium and four manganese centers (Figure 2.8). The number of possibilities increases even further when additional donor sites are available in other parts of the phosphole-containing ligand. This is discussed for the 2-pyridylphosphole moiety in section 2.3.1.

Figure 2.7. Coordination modes of the phosphole ring (A – C) and the phospholyl anion (D – H).

Figure 2.8. Heptametallic tetra-phospholyl anion complex 38. Chapter 2: Phosphole-containing π-systems 21

For the interested reader, reference is made to the numerous reviews available on phosphole chemistry throughout the years.[87]

2.3. Phospholes in π-conjugated molecular frameworks Phosphorus is being introduced in π-conjugated molecular frameworks with great success for approximately one decade.[88] This sudden interest in phosphorus incorporation mainly originates from the recognition that phospholes are extremely suitable in this respect. It was not until recently that the scientific community realized that the electronic properties of the phosphole ring, as described in section 2.2.2, makes the phosphole an ideal building block for the construction of π-conjugated molecular frameworks. Its low but not negligible degree of aromaticity, the participation of the exocyclic σ*(P–R) orbital in hyperconjugation with the diene π-system and the presence of a reactive phosphorus atom, enables efficient conjugation over the diene moiety and offers a handle to fine-tune the photophysical properties of the π-system by reaction at the phosphorus centre. Furthermore, as discussed before, interring conjugation is enhanced when intraring conjugation (i.e. aromaticity) is rather low.[89] Thus, it can be concluded that low aromatic phospholes are especially suited for the construction of extended π-conjugated systems. The majority of phosphole-containing π-conjugated molecular frameworks investigated to date can be grouped in two distinct classes, the 2,5-bisarylphospholes (section 2.3.1) and systems of fused aromatic rings containing at least one phosphole ring, the so-called ladder-type ring systems (section 2.3.2). Here below, both classes will be discussed separately.

2.3.1. 2,5-Bisarylphospholes Introducing π-orbital containing substituents, such as aromatic rings, on the 2- and 5-positions of the phosphole ring effectively lengthens the conjugated path over the diene π-system. In this respect, a great variety of substituents have been attached to the phosphole ring, from commonly encountered aromatic substituents as phenyl, pyridyl and thienyl, to more extended and complex systems. The resulting oligomers can be classified according to their monomeric units. In this respect, phosphole [P] is often flanked by phenyl [Ph], pyridine [Py] and thiophene [T], but trans-stilbene [St], 1,2,3-triazole [Tr], pyrrole [Pr] and 9,9-dimethyl-9H-fluorene [Fl] may also be encountered. These abbreviations will be used throughout this section, in which the individual rings or ring-systems are joined via C–C bonds.

Early phosphole-containing oligomers have trimeric [P][P][P][P][90] and hexameric [T][T][P][P][T[[T][91] arrangements. However, interring conjugation was hampered by rotational disorder between the individual rings.[92] Pentameric [Py][P][T][P][Py] was 22 Chapter 2: Phosphole-containing π-systems found to be a double P,N-chelate towards Ru(II) centers.[93] The intriguing coordinative properties of these systems will be discussed in more detail further below. After these initial attempts, a multitude of conjugated phosphole-containing compounds has been prepared over the years employing the Fagan-Nugent reaction.[94] In 2001, the first attempts led to the [T][P][T] (39), [Ph][P][Ph] (40) and [Py][P][Py] (29) trimers and the [T][P][T][T][P][T] hexamer.[92] Further extension of the π-conjugated backbone resulted in tetrameric [Ph][P][Ph][Ph], [T][P][Ph][Ph], [Ph][P][St], [T][P][St].[95] Such an extension of the π-conjugated path led to emission of red light (λem ([T][P][St]) = 647 nm), indicative of the dramatically narrowed HOMO- LUMO gap. The unexpectedly large Stokes shift of 228 nm was attributed to a significant planarization of the phosphorus tripod in the excited state.[96]

Generally, modification of the π-conjugated backbone is an efficient approach towards fine-tuning of the photophysical properties. In this respect, DFT calculations suggested that the introduction of electron-withdrawing substituents in the oligophosphole backbone has been found to enhance the n-type semi-conductor abilities of the oligomer.[97] Thus, increasing the number of monomeric units in these chains of aromatic rings has been found to decrease the HOMO-LUMO gap, but also alternating thiophene and phosphole units within such a chain increases the conjugation. In this respect, heptamer [T][P][T][P][T][P][T] has been shown to exhibit an absorption [98] maximum at λabs = 550 nm. The increased conjugation in phosphole-thiophene alternating oligomers has been confirmed by Raman spectroscopy, comparing [T][P][T] and [T][T][T].[99]

Further fine-tuning of the photophysical properties of phosphole-containing π-systems is accomplished by reaction at the phosphorus center, as discussed above. These systems contain a reactive phosphorus atom, since its lone pair does not contribute to the conjugated framework. On the other hand, modifications at phosphorus do have an influence on the electronic properties of the π-system, because of the hyperconjugation between the π-system and the exocyclic σ(P–R) orbital. This provides a handle to influence the electronic properties of the phosphole-containing π-systems through chemical modifications at the phosphorus center. The possibilities in this respect are outlined in Scheme 2.4. In view hereof, several approaches towards the modification of phosphorus have been explored; (1) oxidation by chalcogens (Scheme 2.4(i – iii)), (2) alkylation (Scheme 2.4(iv)) and (3) metal complexation (Scheme 2.4(v – vi)). All reactions have been performed for [T][P][T] 39. The [T][P][T] trimer is by far the most extensively studied 2,5-bisaryl-phosphole. Chapter 2: Phosphole-containing π-systems 23

Scheme 2.4. Chemical modification of the phosphorus center of [T][P][T] (39).

Generally, the electronic properties of a π-system are analyzed by fluorescence spectroscopy. Lower energy of emitted photons indicates smaller HOMO-LUMO gaps

(ΔEHL) and thus enhanced conjugation throughout the π-system. The relevant data of the modified π-systems are given in Scheme 2.4, from which it is clear that functionalization of phosphorus in general decreases ΔEHL, when compared to the parent π-system. Another important characteristic of such a fluorophore is its quantum yield (ΦF), i.e. the efficiency of fluorescence, in which ΦF = 1 means that every photon absorbed by the compound results in the emission of a photon. As is apparent from Scheme 2.4, the quantum yields generally decrease on functionalization of the phosphorus centre. For the [T][P][T] system 39, the phosphole gold chloride complex [T][P(AuCl)][T] is the only exception.

These 2,5-bisarylphospholes exhibit rigidochromic behaviour, as their fluorescence quantum yield is dramatically enhanced in the solid state. This is exemplified for pentaphenylphosphole oxide, which exhibits a quantum yield rise from ΦF = 0.00056 in [100] solution to ΦF = 0.49 in the crystal phase. This 880-fold increase is attributed to restricted interring rotations in the solid state. Solvatochromic behavior, i.e. solvent polarity dependent fluorescence, was encountered in the [P][T][T] trimer and the [P][T][T][T] tetramer, in which the [101] phosphorus lone pair acted as donor for a BH3 Lewis acid. For these oligomers, transition state dipole moment (Δμ(S0-S1)) of 3.8 and 4.4 debye respectively were measured.

The hyperconjugation between the diene-based π-orbitals and the exocyclic σ(P–R) bond is employed as a handle to further fine-tune the photophysical properties of the 24 Chapter 2: Phosphole-containing π-systems phosphole containing π-systems. In this respect, dimers wherein two phosphole rings are joined via a P–P bond are prepared,[102] thus creating an electronic connection between two π-systems via a σ-bond (Figure 2.9). Two [Ph][P][Ph] or [T][P][T] π-conjugated frameworks, linked via a P–P bond between both phosphole phosphorus centers, exhibited a lowering of the HOMO-LUMO gap of the resulting σ-π-system when compared to the standalone bisarylphosphole systems.[103] A series of thienylphosholes, such as 2,3,4,5-tetrathienylphosphole, and their P–P bridged dimers have been prepared and their electronic properties analyzed.[104] Especially the increase in LUMO energy of the 3-thienyl with respect to the 2-thienyl derivatives is noteworthy, and proves that the conjugations over the phosphole diene unit is less efficient at the 3-position.

Figure 2.9. The interaction of two phosphole-containing π-systems via a σ-bridge.

Originally thought to be inert, the C4-backbone, which originates from the Fagan- Nugent reaction to prepare phospholes (see section 2.2.1 and Scheme 2.1(viii)) and bridges the 3,4-positions in these 2,5-bisarylphospholes, has a distinct effect on the electronic properties of the phosphole-based π-systems. This effect is steric in nature, as the presence of this bridge enforces a slight deviation from coplanarity between the different aromatic rings in these oligomers, as such weakening the interring [105] conjugation. More recently, this C4-bridge has been used to as a handle to further fine-tune the photophysical properties of these conjugated frameworks. Incorporating sulphur atoms at the α-positions in such a four atom bridge was found to lower the [106] HOMO-LUMO gap. The C4-bridge has also been used to introduce chirality into the system, by incorporation of enantiomerically pure substituents at the β-positions of this bridge. [107]

As mentioned before, the combination of phospholes and thiophene building blocks has been especially successful. It has been found that a strong orbital interaction exists between the respective LUMOs of the thiophene and phosphole rings.[105] The success of the [T][P][T] framework (39) as electronic material is exemplified by the preparation of the first working OLED in this respect in 2001, using [T][P(AuCl)][T] as emissive Chapter 2: Phosphole-containing π-systems 25 layer.[95] Doping of this emissive layer with red-emitting dopant DCJTB enabled [108] emission of the OLED at λem = 623 nm. Introduction of 9,9-dimethyl-9H-fluorene [Fl] substituents at the 2,5-positions of the phosphole ring gave [P(AuCl)][Fl] and [Fl][P(AuCl)][Fl] complexes, which were unsuitable for OLED preparation, because of their rapid decomposition. The corresponding thioxophospholes performed much better as emissive layer in OLED displays.[109] The suitability of these phosphole-based π-conjugated systems in the manufacture of displays has triggered the University of Rennes,[110] Fuji[111] and Konica-Minolta[112] to patent various aspects of this technology.

The quest for bright fluorescing WOLEDs – a combination of OLEDs emitting at different wavelengths, which together make up a white light emitting device – led to the functionalization of the [T][P][T] trimer with electron donating methyl- and methoxy-groups at the 5-positions of both thiophene rings.[113] OLEDs prepared therefrom emit orange light at λem = 575 nm and 610 nm respectively. Square planar palladium complexes, trans-substituted with asymmetrically substituted 2,5-bisaryl- phospholes ([Py][P][T]–OMe, [Py][P][Ph]–NBu2), as such creating bis-dipoles, have been found to exhibit non-linear optical activity.[114]

Over the last five years, the scope of aromatic substituents on the 2- and 5-positions of phospholes has greatly expanded. Aza[4]helicenes[115] and aza[6]helicenes[116] have been incorporated as a substituent on the 2-position of 5-phenylphospholes, to induce chirality in these systems as P,N-ligands towards Cu and Pd. Their photophysical properties have also been investigated, which showed that π-conjugation extended throughout the entire molecule.[117] 2,5-bisaza[4]helicene-phosphole displayed extended π-conjugation and supramolecular coordination chemistry upon copper complexation. Phosphole-cored dendrimers based on the [Ph][P][Ph] trimers have been prepared. The phenyl-substituents thereof are functionalized with poly(benzylether) at their meta-positions. Dendrimers have been prepared up to the third generation, and the intensity of the blue fluorescence increased with every additional generation.[118] However, intramolecular rotations of the dendritic moieties hampered emission in solution, and fluorescence was only observed in aggregated or solid state.[119] These phosphole-cored dendrimers eventually led to the development of sensors for nitro- compounds (see below). Recently, an efficient synthesis towards 2,5-diethynylphospholes (42), analogues of 28, has been established, wherein the inner acetylene moieties of substituted 1,3,8,10-undecatetraenes 41 are regioselectively subjected to the Fagan-Nugent [120] reaction with activated “ZrCp2” (Scheme 2.5). The pendant acetylenes of the silyl- deprotected analogues 42b enabled easy extension of the conjugated π-system, by 26 Chapter 2: Phosphole-containing π-systems introducing (i) 1,2,3-triazole substituents via the Cu-catalyzed 1,3-dipolar Huisgen addition with arylazides (coined click chemistry), or (ii) ethynylphenyl moieties via the Pd-catalyzed Sonogashira coupling with aryliodides. In this respect, the [Ph][Tr][P][Tr][Ph] pentamer 43 was easily prepared via a click reaction with azidobenzene,[121] and the conjugated [Ph]–C≡C–[P]–C≡C–[Ph] 44 via a Sonogashira coupling with iodobenzene.[120] The electronic effects of the para-substituents

(H, OMe, CF3) on the outer phenyl substituents was only small, but Pd chelation to the phosphole phosphorus and one of the triazole nitrogen atoms perturbed the electronic properties more rigorously. The [Ph][P][Tr][Fl][Tr][P][Ph] heptamer was prepared in similar fashion.[121] Pentamer [Ph][P][P][P][Ph] was obtained by the phosphole synthesis as developed by Märkl (see Scheme 2.1(iv)) by diyne bridged [Ph][P] dimer ([Ph][P]–C≡C–C≡C–[P][Ph]).[120] Subjecting 1,6-heptadiynes, which are capped with pyrrole rings, to a Fagan-Nugent reaction led to the formation of [Pr][P][Pr] trimers and [Ph][Pr][P][Pr][Ph] pentamers. The incorporation of pyrrole rings in the pentamer led to a decreased HOMO-LUMO gap, when compared to [Ph][P][P][P][Ph], in which phosphole rings are present instead.[122]

Recently, 2,5-bisarylphospholes bearing two azido moieties have been prepared, from which extension of the π-system was elegantly accomplished via click chemistry, resulting in [X][Tr][Ph][P][Ph][Tr][X], wherein [X] = [Ph], [Py], [T].[123] This is the subject- matter of chapter 4.

Very similar, but not falling directly in the scope of the 2,5-bisarylphospholes, are those compounds in which a conjugated linker is introduced between phosphole ring and one or both of the aromatic groups at the 2- and/or 5-positions. The 2,5-diethynylphospholes, discussed above, are an elegant example hereof. Exemplary is MeO–[Ph]–C≡C–[P]–C≡C–[Ph]–OMe, which exhibits emission at λem = 518 nm, with [120] reasonable quantum yield (ΦF = 0.10). Also, a series of 1-aryl-5-styryl-phospholes has been synthesized, such as “push-pull” system MeO–[T][P]–C=C–CO2Me having an electron donating methoxy moiety at one end of the π-system, and an electron withdrawing ester moiety at the other. This compound displayed yellow-orange [124] emission (λem = 622 nm), but with low quantum yield (ΦF = 0.0013).

One or more of the four pyrrole rings in porphyrins have been replaced by phospholes or other heterocyclopentadienes (Figure 2.10), as such constructing phosphole- containing conjugated macrocycles. The 18 π-electron conjugated ring system [P][Pr][T][Pr] (45) exhibits a high degree of aromaticity.[125] The palladium complex of 45, wherein all four heteroatoms coordinate to the metal center, has also been reported.[126] Chapter 2: Phosphole-containing π-systems 27

Scheme 2.5. Phosphole-acetylene building blocks; synthesis and extension of the π-system.

18 π-electrons 22 π-electrons yellow emission red absorption Figure 2.10. Extension of the π-system upon oxidation of phosphole-containing porphyrin rings.

A remarkable example of fine-tuning the photophysical properties of a π-system by phosphorus modification is observed in the related [P][Pr][Pr][Pr] macrocycle 46.[127] The P(III) phosphole-containing 46a contains a conjugated 18 π-electron ring system, analogous to 45, exhibiting yellow emission. Intriguingly, upon oxidation of the phosphorus atom to P(V), the electronic structure of the ring was changed to a 22 π-electron system (46b), showing a significantly enhanced degree of conjugation (Figure 2.10). The lowest energy absorption band of 46b displays a broad band extending as far as the near-IR (λabs = 730 – 1100 nm), indicative of a very small HOMO- LUMO gap.

Conjugated polymeric phosphole-containing materials have also been prepared (Figure

2.11). The first occurrence of such a polymer regarded a ([Ph][Ph][P])n structure (47).[128] The zirconocene mediated polymerization of 4,4’-bis(1-hexynyl)biphenyl resulted in the formation of 80% 2,4-disubstitued cross-conjugated phospholes in the chain, and only 20% 2,5-disubstitution. This majority of cross-conjugation disabled extensive conjugation through the polymer, and thus a rather wide HOMO-LUMO gap and blue fluorescence (λem = 470 nm) was observed. 28 Chapter 2: Phosphole-containing π-systems

Br–[Ph][P][Ph]–Br (48) was prepared via the Fagan-Nugent procedure from 1,8-bis(4-bromophenyl)-octa-1,7-diyne, and polymerized with para-bisethylbenzene derivatives via a Sonogashira/Heck reaction (Figure 2.11).[129] The resulting polymers 49 had a degree of polymerization of 13 – 15 and exhibited blueish-green fluorescence

(λem = 435 – 490 nm). Introduction of only one fluorine substituent on the bisethynyl- [130] benzene monomer, decreased the band gap and gave emission at λem = 502 nm.

Oligomers with a phosphole core and terminal thiophene substituents can be electrooxidatively polymerized.[131] In this process the terminal thiophene units undergo oxidation to the corresponding radical cations, which subsequently undergo homocoupling. Electronic cycling in a cyclic voltammetry set-up results in thiophene polymers. In this respect, [T][P][T], and [T][Py][P][Py][T] have been subjected to electropolymerization, affording ([T][P][T])n 50a and ([T][Py][P][Py][T])n 51 (Figure 2.11).[132] The gold chloride complex of the [T][P][T] trimer has also been polymerized by anodic electropolymerisation at the 2,5-positions thiophene rings, resulting in [133] ([T][P(AuCl)][T])n polymers 50b with strong chalcogenide sensing abilities.

Analogous ([T][P][T])n polymers 50c, wherein the 3,4-positions of the thiophene rings are functionalized with ethylenedioxy bridges, emitted at λem = 715 nm, indicative of extensive conjugation and a dramatically decreased HOMO-LUMO gap.[134]

Figure 2.11. Phosphole-containing π-conjugated polymers. Chapter 2: Phosphole-containing π-systems 29

Polyphospholes with general structure [P]n (52a) have recently been prepared by Stille- type coupling reactions between 2,5-diiodophospholes and 2,5-distannyl- [135] phospholes. Especially the resulting polyphosphole oxides [P(O)]n 52b (Figure 2.11) exhibited dramatically reduced band gaps (λabs = 655 nm), but were non-fluorescent in solution.

Apart for their interesting photophysical properties, novel applications for these 2,5-bisarylphospholes emerge rapidly. The most intensely studied application in this respect is the coordination chemistry and catalytic activity of such P-ligands.

Numerous catalytic activities have been reported when 2,5-bisarylphospholes are employed as ligands to (transition) metals. In this respect, the [Ph][P][Ph] trimer (40) was found effective in the Au-catalyzed cycloisomerization of enynes,[136] the Rh-catalyzed hydroformylation of mono- and disubstituted olefins,[137] the Pd-catalyzed deallylation of allyl ethers[138] and the Pd-catalyzed carboxylation of benzyl- bromides.[139] Recently, a xantphos (53) analogue emerged, in which the phosphines are replaced by [Ph][P][Ph] trimers (54, Figure 2.12). This conjugated trimer proved to be a strong π-acceptor and thus a suitable ligand in the Pd-catalyzed allylation of [140] amines. Replacement of the PPh2 moieties of xantphos with [Ph][P][Ph] trimers in 54 greatly enhanced the efficiency in this reaction, and a broad scope of amines were converted.[141] This ligand also gave unprecedented activity and selectivity in the Rh- catalyzed hydroformylation of 2-octene to nonanal[142] and was recently found to be active in the Au-catalyzed dehydrogenative silylation of alcohols.[143] Protonation of the free ligand resulted in the formation of an unprecedented cyclic phospholium dihydrophospholene (55).[144] The general concept of the use of the 2,2’-diphosphole-biphenyl ligand in asymmetric catalysis is patented by Takasago International Company.[145]

P,N-ligands were obtained by introducing –CH2Py or –CH2(R-phenyloxazoline) substituents as exocyclic phosphorus substituent on the [Ph][P][Ph] trimer.[146] The

PdCl2 complex of latter chiral ligand was successfully employed in asymmetric Miyaura and Heck coupling reactions, albeit with low enantiomeric excess. The [Ph][P][Ph]

Figure 2.12. Xantphos, ‘xantphosphole’ and protonated xantphosphole. 30 Chapter 2: Phosphole-containing π-systems

trimer, bearing –CH2Py as exocyclic phosphorus substituent, has been used as P,N-ligand in the Ru-catalyzed transfer hydrogenation of ketones with high TON and TOF values.[147] The Pd-complex of a chiral P,P-ligand has also been reported, based on the [P][P] dimer in which the both phosphorus atoms are joined via a chiral C3-bridge, [148] i.e. –CH(Me)–CH2–CH(Me)–. The [Ph][P][Ph] trimer, covalently attached to the chiral ligand IndolPhos, acted as P,P-chelate to Pd, which enabled asymmetric allylic alkylation with high activity and enantioselectivity, but low regioselectivity.[149] A chiral environment around phosphorus has also been created by introduction of two binaphthyl moieties at the 2- and 5-positions of a phosphole ring. Such a ligand has been tested with reasonable enantioselectivity in Pd-catalyzed asymmetric hydrosilylation.[150] The [Py][P][Py] trimer has also been employed as a P,N-ligand,[151] first as chelating ligand to Pd[152] and later in the Pd-catalyzed telomerisation of isoprene with good selectivity.[153] Palladium complexes of related [Ph][P][Py] and [T][P][Py] P,N-ligands[154] have been used in the olefin/CO copolymerization with high catalytic activity,[155] and in allylic substitution reactions.[156] The corresponding nickel complexes have been used in the dimerization of ethylene with high selectivity for 1-butene,[157] The P,N-chelating ability of [Py][P][Py] towards ruthenium centers has also been investigated.[158] In none of these cases, [Py][P][Py] acted as a tridentate N,P,N-chelate, thus leaving one pyridine ring uncomplexed. The [Ph][P][Ph] ligand has been combined with the P,N-chelating abilities of the [P][Py] unit in the Pd- and Ni-complexes of pyridine, disubstituted at the 2- and 6-positions with [Ph][P][Ph], via their phosphorus atom.[159] The Pd-complex catalyzes the coupling between pinacolborane and haloarenes with good TON. Tetrameric [Ph][P][P][Ph] has been complexed to platinum and palladium, and the latter complex has been used in Pd-catalyzed asymmetric allylic substitution, with high activity but moderate enantioselectivity.[160]

Because of its various donor sites for metal complexation, the [Py][P][Py] trimer has been used to bring two or more metal centers together, in which the phosphorus center bridges two metal centers.[161] In this respect, a Pd–Pd moiety has been bridged by two [Py][P][Py] ligands, wherein both ligands are complexed as two distinctly different P,N-chelates to each Pd centre.[162] Similar complexes are known, wherein one to three [Py][P][Py] ligands bridge a heterobimetallic Pt–Pd moiety[163] and up to four copper centers.[164] The versatility in copper coordination modes has been employed in the construction of extended supramolecular frameworks, the structure of which is governed by π-π interactions.[165] The coordination chemistry of [Py][P][Py] towards Ag and Au has also been extensively studied, giving rise to numerous interesting coordination modes,[166] and even thallium complexes have been prepared with the [Py][P][Py] trimer as ligand.[167] Chapter 2: Phosphole-containing π-systems 31

Recently, an attempt has been made to extent the scope of coordination compounds based on 2,5-bisarylphospholes as ligand, by incorporating the [Ph][P][Ph] moiety in the side-chains of naturally occurring amino acids tyrosine and phenylalanine.[168] As such, a first step towards artificial metalloproteins has been made. The η5-complexation mode of phospholes is encountered in phosphametallocenes, prepared by complexing [Ph][P] dimers and [Ph][P][Ph] trimers to Ru,[169] Fe,[150,170] Mn,[171] Ca, Sr and Ba.[172]

Substituted [Ph][P][Ph] trimers have been used as sensors for various substrates. In general, complexation of the substrate to the appropriately functionalized trimer led to alteration of the fluorescence intensity, which enabled detection of this substrate. The fluorescent properties of [Ph][P][Ph], functionalized with sugar moieties, have been employed to detect sugar binding enzyme (lectins). In this respect, the fluorescence intensity increased when such a protein bound to the [Ph][P][Ph] fluorophore.[173] Analogously, benzyloxy functionalized [Ph][P][Ph] is able to detect nitro compounds,[174] which enabled the detection of explosives, such as TNT.[175]

2,5-bisarylphospholes, the AuCl complex of [Py][P][Py] in particular, has even found application in the human body, as inhibitor of glutathione[176], disulfide[177] and thioredoxin[178] reductases. This therapeutic use of [T][P][Py] and [Py][P][Py] as P,N-ligands to gold and platinum has been patented.[179]

Before leaving the subject of 2,5-bisarylphosphole, the interested reader is referred to some general reviews.[180]

2.3.2. Phosphole-containing ‘ladder-type’ ring systems Fused ring-systems containing phospholes have been known since the very first emergence of phospholes, as dibenzophosphole [Ph]:[P]:[Ph] (9, wherein the colon “:” designates ring fusion) was the first phosphole to be prepared.[20] The next ladder-type phosphole ring system, trimer [T]:[P]:[T] (56), was prepared in 1974 (Figure 2.13).[181]

–OH, –NH2 and –Cl are encountered as exocyclic phosphorus substituent of the corresponding phosphole oxide [T]:[P(O)]:[T], which have been used in preparation of [182] cyclotriphosphazenes (P3N3-rings).

However, the intriguing electronic properties of phosphole-containing fused ring systems were not recognized until 2004, when Baumgartner reported the modification of the phosphorus centre of the [T]:[P]:[T] trimer.[183] These π-systems exhibit efficient conjugation and high quantum yields, as the conjugated path is not disrupted by twisting between adjacent rings, i.e. co-planarity is enforced on the system. In addition, the possibilities of non-radiative decay pathways, e.g. by rotation or twisting, 32 Chapter 2: Phosphole-containing π-systems

Figure 2.13. [T]:[P]:[T] (56) and [Ph]:[T]:[P]:[T]:[Ph] (57) ladder-type π-systems.

are limited in these systems. For these reasons, band-gap tuning is efficiently accomplished in conjugated systems of annulated rings.[184] Moreover, the favorable interaction between the LUMOs of the phosphole and the thiophene rings (discussed in section 2.3.1)[105] led to an exponential increase in amount of literature on the fused [T]:[P]:[T] trimer and analogues thereof over the last few years.[180d,185]

Indeed, band gap tuning is possible by modification of the phosphorus center, which is accomplished by the same chemical transformations as those depicted in Scheme 2.4 for 2,5-bisarylphospholes. Importantly, these modifications did not lead to a significant decrease in quantum yield, quite opposite to what was observed in modification of 2,5-bisarylphospholes. Thus, modification of the phosphorus center of parent [T]:[P]:[T] led to variation of the emission wavelength of λem = 415 nm and/or of the quantum yield ΦF = 0.779. In this respect, complexation by a Lewis acid (BH3), giving [T]:[P(BH3)]:[T], was found to slightly decrease energy of emission and the quantum yield (λem = 424 nm; [183] ΦF = 0.690). Oxidation of the phosphorus centre in the [T]:[P]:[T] system has been accomplished upon treatment with H2O2, affording [T]:[P(O)]:[T] (λem = 453 nm; [183] [186] ΦF = 0.565), and with S8, affording [T]:[P(S)]:[T] (λem = 460 nm; ΦF = 0.556). The electron donating phosphorus atom is also reactive towards alkyl cations. In this respect, [T]:[P]:[T] reacts with MeOTf to [T]:[P⊕(Me)]:[T] + ⊖OTf, which exhibited [187] emission at λem = 467 nm. Furthermore, a series of metal centers have been complexed to the phosphorus atom of such [T]:[P]:[T] ring system, as is exemplified by the preparation the AuCl,

Rh(COD)Cl, W(CO)5, Fe(CO)4 and cis- and trans-Pd(Cl)2([T]:[P]:[T])2 complexes of

5,5’-bis(SiMe2t-Bu)-[T]:[P]:[T]. Such an interaction with a metal center led to increased electron accepting capacity of the phosphorus atom and lowered the LUMO energy of the π-system.[188] The resulting narrowed band gap gave a lowering of the emission [186,188] energy (λem = 445 – 470 nm). The same phosphorus modification reactions have been shown to have similar impact on the photophysical properties of pentamer [Ph]:[T]:[P]:[T]:[Ph] (57, Figure 2.13).[189] The parent [Ph]:[T]:[P]:[T]:[Ph], prepared from a bis-benzothiophene precursor, as well as the corresponding P-modified analogues, exhibited a slight red-shift in emission Chapter 2: Phosphole-containing π-systems 33 with respect to the trimeric [T]:[P]:[T] derivatives (e.g. λem (57) = 440 nm), which is indicative of extended conjugation.

Another useful functionality of the thiophene ring is the high susceptibility of the carbon atoms at the 2- and 5-positions to electrophiles, which enables easy functionalization of the carbon atoms adjacent to sulfur. This phenomenon is not lost upon fusion with a phosphole ring, which enabled easy extension of the π-system. Thus, the π-conjugated path is extended with a plethora of functional groups, by reaction at the 5,5’-positions of the [T]:[P]:[T] trimer. Exemplary is the introduction of pinacolboryl substituents.[190] The first [T]:[P]:[T] trimers contained various trialkylsilyl-groups at the 5,5’-positions, which mainly altered the absorption properties, leaving the fluorescence properties largely unaffected. 5,5’-bis(SiMe3)-[T]:[P]:[T] and 5,5’-bis(SiMe2t-Bu)-[T]:[P]:[T] both exhibited emission at λem = 422 nm, a red-shift of only 7 nm with respect to the parent [T]:[P]:[T]. On the other hand, the lowest energy absorption maximum shifted from

λabs = 344 nm for the –SiMe3 analogue to λabs = 364 nm for the –SiMe2t-Bu [186] analogue. Also, –SiMe2H functionalized [T]:[P]:[T] trimers have been prepared, in which the reactivity of the Si–H moieties towards acetylenes enables further functionalization via Pt-catalyzed hydrosilation.[191] As such, π-conjugated moieties have been incorporated, such as cis-stilbene upon treatment with tolane. Reaction with 1,7-octadiyne afforded polymeric materials. However, the presence of an sp3 silicon atom within the π-conjugated path disrupted the conjugation, and no further reduction of the HOMO-LUMO gap was observed in these polymers. A lowering of the HOMO-LUMO gap was observed when aromatic moieties were introduced on the 5,5’-positions of the [T]:[P]:[T] unit. Reaction of [T]:[P(O)]:[T] with

N-bromosuccinimide (NBS) resulted in 5,5’-Br2-[T]:[P(O)]:[T] (58), which was subjected to various coupling reactions to extent the conjugated path (Scheme 2.6). Especially the 5,5’-[T]2 (59; λem = 545 nm) and the 5,5’-(4-PhNPh2)2 (60; λem = 566 nm) derivatives showed a significant red-shift in emission wavelength.[192] Compound 60 exhibits bright orange fluorescence, which is switched to green upon protonation of the NMe2 [193] moieties. A mixture of 60 and the corresponding 5,5’-bis(SiMe2t-Bu) derivative led to intense white fluorescence. Incorporation of perfluorinated phenyl rings, both at the 5,5’-positions and as exocyclic phosphorus substituent, resulted in significant stabilization of the LUMO levels [194] [195] (λem = 526 – 539 nm). The AuCl complexes thereof have also been reported. Introduction of formyl-groups at the 5,5’-positions of [T]:[P]:[T] enabled Wittig-Horner reactions with phenylvinylene moieties. As such, both symmetrically[196] and asymmetrically[197] substituted conjugated dendrimer-like structures have been incorporated at the 5,5’-positions. The latter encompassed donor-acceptor systems(push-pull), which exhibited favorable nonlinear optical features. 34 Chapter 2: Phosphole-containing π-systems

Scheme 2.6. Extension of the π-system in [T]:[P]:[T] analogues.

The introduction of only one bromine atom, at the 5-position, enabled monofunctionalization of the [T]:[P]:[T] trimer. In this respect, two [T]:[P]:[T] scaffolds were bridged with π-conjugated spacers, such as 1,4-phenyl, 4,4’-biphenyl and 2,7-fluorene, to effectively extend the conjugated path.[198] Also, regioselective monoacylation was possible in a Friedel-Crafts acylation procedure with methyl- and thienylacyl chloride.[199] In these systems, the fluorescent properties could be further tuned by Lewis acid coordination to the carbonyl oxygen atom. The functionalization of the 5-position of [T]:[P]:[T] with terpyridinyl-acetylene[200] or 2,2’-bipyridyl-acetylene[201] enabled Zn(II), Ru(II) and Pt(II) complexation, in which the extended frontier orbitals were responsible for the intense fluorescence.

More recently, the [Ph]:[P]:[P]:[Ph] (61) tetramer has gained popularity. This P,P-bridged trans-stilbene has been prepared via an intramolecular cascade cyclization reaction from 2,2’-PPh(NEt2)-tolane, affording both the cis- and the trans-isomer (Scheme 2.7(i)).[202] Theoretical calculations suggest that the cascade cyclization is initiated by the nucleophilic attack of the phosphorus lone pair on the acetylene carbon.[203] The trigonal phospholes as well as the phosphole oxides (62a) and sulfoxides (62b) have been prepared. Especially the oxo-derivatives exhibit intense blue fluorescence (λem = 480 nm; ΦF = 0.98 – 0.99) and efficient electron accepting ability. These intriguing properties prompted the inventors to patent their finding.[204] Remarkably, the corresponding thioxo-derivatives exhibited no fluorescence at all.[205] The same synthetic approach has been applied to asymmetrically substituted tolanes, with –PR2 (R = cHex, t-Bu) on one side and –BMes2 on the other. In this respect, [Ph]:[P]:[B]:[Ph] (63a, [B] = borole) and [T]:[P]:[B]:[T] (63b) tetramers have been prepared, as well as the hexamers [Ph]:[P]:[B]:[Ph]:[B]:[P]:[Ph] (63c), [Ph]:[B]:[P]:[Ph]:[P]:[B]:[Ph] (63d)[206] and [T]:[B]:[P]:[Ph]:[P]:[B]:[T] (63e)[207] (Scheme 2.7(ii)). These ladder-type π-conjugated systems are zwitterionic, bearing a positively

charged phosphorus atom and a negatively charged boron atom. The P,B-bridged

Chapter 2: Phosphole-containing π-systems 35

Scheme 2.7. P,P-bridged, P,B-bridged trans-stilbenes and analogous thereof.

trans-stilbene 63a exhibits the longest emission wavelength ever observed for bridged trans-stilbenes (λem = 517 nm), which is even longer for [T]:[P]:[B]:[T] (λem = 623 nm), having thiophene end-caps. Also the hexamers show long emission wavelengths of

λem = 578 nm for 63c and λem = 614 nm for 63d, which are all indicative of a large degree of conjugation. In 2009, the corresponding P,C-bridged trans-stilbene has been [208] reported, having P(O)Ph and CPh2 bridges. Analogous ladder-type structures up to ten fused rings have been reported. The octamer [Ph]:[P]:[Ph]:[P]:[P]:[Ph]:[P]:[Ph] has been used in a display as light-emitting layer,[209] and decamer [Ph]:[Ph]:[Ph]:[P]:[T]:[T]:[P]:[Ph]:[Ph]:[Ph] is found to be effective as oxidation-resistant semiconductor.[210]

A [T]:[P]:[T]-containing polymer was prepared by co-polymerizing [T]:[P]:[T], in which the exocyclic P–phenyl substituent is functionalized with a para-vinylene moiety, with a 30-fold excess of styrene.[183] In this reaction, the π-conjugated path was not extended, but instead a multitude of trimeric π-systems [T]:[P]:[T] were brought in close proximity. This hardly affected the emission maximum and quantum yield

(λem = 424 nm; ΦF = 0.743). Extending the conjugated path of the [T]:[P]:[T] unit itself has been accomplished by polymerizing 5,5’-SnBu4–[T]:[P]:[T] with para-diiodobenzene derivatives via Stille-coupling procedures. The resulting polymer ([T]:[P]:[T]–[Ph])n 36 Chapter 2: Phosphole-containing π-systems

exhibited a significantly higher emission wavelength (λem = 555 nm), indicative of a decreased HOMO-LUMO gap by an increased extent of conjugation.[186]

Cationic [T]:[P (Me)]:[T] reacts with N-bromosuccinimide to 5,5’-Br2-[T]:[P (Me)]:[T], which can subsequently be polymerized with fluorenebis(boronic) acid to cationic [187] ([T]:[P (Me)]:[T]–[Fl]]n polymers. Similar polymers have been prepared from the [T]:[P(O)]:[T] analogue as well.[211] These polymers exhibited narrow band gaps (1.7 eV) suitable for solar cell applications. The 5,5’-bisboronic acid derivative of [T]:[P]:[T] has been subjected to Suzuki co-polymerization with dibromo-spirobisfluorene, which resulted in twisted polymers, in which extended conjugation was not possible.[212] Co-polymers have also been prepared from [Ph]:[P]:[P]:[Ph] as one of the monomers, together with monomeric thiophene or tetrafluorophenyl, which are used as emissive layer.[213]

Recently, the scope of phosphole-containing ladder-type π-systems has been broadened with P,S-bridged trans-stilbenes,[214] which is the subject of chapter 5.

2.4. Concluding remarks Both the fields of small molecule electronics and phosphole-containing π-systems has expanded greatly over the last decade. Especially since the recognition of the intriguing electronic properties of the phosphole ring, π-systems containing such a ring are more extensively studied than ever before. The presence of a reactive phosphorus atom, which is in electronic contact with the neighboring π-system offers great possibilities of fine-tuning its photophysical properties. Additional strategies to alter the photophysical properties are also investigated, such as modifying the π-conjugated backbone and enforcing hybridization changes of the phosphorus center. As such, novel compounds are being prepared at high rate, each one having equally intriguing properties. To conclude, it is believed that the full potential of this class of compounds has not yet been reached, and the near future will provide is with more and surprising possibilities. The greatest challenge still resides in the synthetic accessibility and stability of phosphole-containing π-systems. However, also in this respect, major steps have been made recently, providing us with elegant syntheses towards stable, but still interesting, π-systems.

2.5. References 1. (a) Organic Light Emitting Devices: A Survey (ed.: J. Shinar), 2004, Springer-Verlag, New York, USA. (b) Organic Light Emitting Diodes: Principles, Characteristics and Processes (ed.: J. Kalowski), 2005, Marcel Dekker, New York, USA. (c) Organic Light Emitting Devices: Chapter 2: Phosphole-containing π-systems 37

Synthesis, Properties, and Applications (eds.: K. Müllen, U. Scherf), 2006, Wiley-VCH, Weinheim, Germany. (d) Organic Electronics: Materials, Manufacturing and Applications (ed.: H. Klauk), 2006, Wiley-VCH, Weinheim, Germany. (e) Organic Light Emitting Materials and Devices (eds.: Z.R. Li, H. Meng), 2007, CRC Press, Boca Raton, USA. (f) Highly Efficient OLEDs with Phosphorescent Materials (ed.: H. Yersin), 2008, Wiley-VCH, Weinheim, Germany. (g) Organic Electronics: Materials, Processing, Devices and Applications (ed.: F. So), 2009, CRC Press, Boca Raton, USA. (h) Device Architecture and Materials for Organic Light Emitting Devices (ed.: S. Schols), 2011, Springer Science+Business Media, Dordrecht, Netherlands. 2. A.J. Heeger, S. Kivelson, J.R. Schrieffer, W.P. Su, Rev. Mod. Phys. 1988, 60, 781 – 850. 3. H. Letheby, J. Chem. Soc. 1862, 15, 161 – 163. 4. H. Akamatsu, H. Inokuchi, Y. Matsunaga, Nature 1954, 173, 168 – 170. 5. (a) R. McNeill, R. Siudak, J.H. Wardlaw, D.E. Weiss, Austr. J. Chem. 1963, 16, 1056 – 1075. (b) B.A. Bolto, D.E. Weiss, Austr. J. Chem. 1963, 16, 1076 – 1089. (c) B.A. Bolto, R. McNeill, D.E. Weiss, Austr. J. Chem. 1963, 16, 1090 – 1103. 6. J. McGinness, Science 1972, 177, 896 – 897. 7. J. McGinness, P. Corry, P. Proctor, Science 1974, 183, 853 – 855. 8. The Nobel Prize in Chemistry 2000: Conductive polymers (www.nobelprize.org/nobel_prizes/chemistry/laureates/2000/chemadv.pdf). 9. H. Shirakawa, E.J. Louis, A.G. MacDiarmid, C.K. Chiang, A.J. Heeger, J. Chem. Soc. Chem. Comm. 1977, 578 – 580. 10. (a) M. Pope, H.P. Kallmann, P. Magnante, J. Chem. Phys. 1963, 38, 2042 – 2043. (b) M. Sano, M. Pope, H.P. Kallmann, J. Chem. Phys. 1965, 43, 2920 – 2921. 11. P.S. Vincett, W.A. Barlow, R.A. Hann, G.G. Roberts, Thin Solid Films 1982, 94, 171 – 183. 12. M. Bendikov, F. Wudl, D.F. Perepichka, Chem. Rev. 2004, 104, 4891 – 4945. 13. F. Wudl, D. Wobschall, E.J. Hufnagel, J. Am. Chem. Soc. 1972, 94, 670 – 672. 14. J. Ferraris, D.O. Cowan, V.V. Walatka Jr., J.H. Perlstein, J. Am. Chem. Soc. 1973, 95, 948 – 949. 15. C.W. Tang, US 4,356,429 (filing: 7 July 1980, publication: 26 October 1982). 16. C.W. Tang, S.A. VanSlyke, Appl. Phys. Lett. 1987, 51, 913 – 915. 17. H. Tang, F. Li, J. Shinar, Appl. Phys. Lett. 1997, 71, 2560 – 2562. 18. (a) C. Hosokawa, H. Tokailin, H. Higashi, T. Kusumoto, Appl. Phys. Lett. 1993, 63, 1322 – 1324. (b) C. Hosokawa, H. Tokailin, H. Higashi, T. Kusumoto, J. Appl. Phys. 1995, 78, 5831 – 5833. (c) H. Tokailin, H. Higashi, C. Hosokawa, US 5,130,603 (filing: 8 March 1990, publication: 14 July 1992). 19. F.G. Gao, A.J. Bard, J. Am. Chem. Soc. 2000, 122, 7426 – 7427. 20. G. Wittig, G. Geissler, Liebigs Ann. Chem. 1953, 580, 44 – 57. 21. F.F. Runge, Ann. Physik. 1834, 107, 65 – 78. 22. C.W. Scheele, Mem. Acad. Roy. Sci. Stockholm 1780. 23. V. Meyer, Ber. Dtsch. Chem. Ges. 1883, 16, 1465 – 1478. 24. (a) E.H. Braye, W. Hübel, Chem. Ind. (London) 1959, 1250. b) F.C. Leavitt, T.A. Manuel, F. Johnson, J. Am. Chem. Soc. 1959, 81, 3163 – 3164. 25. I.G.M. Campbell, R.C. Cookson, M.B. Hocking, Chem. Ind. (London) 1962, 359. 26. C. Charrier, H. Bonnard, G. de Lauzon, F. Mathey, J. Am. Chem. Soc 1983, 105, 6871 – 6877. 27. F.C. Leavitt, T.A. Manuel, F. Johnson, L.U. Matternas, D.S. Lehman, J. Am. Chem. Soc. 1960, 82, 5099 – 5102. 28. E.H. Braye, W. Hübel, I. Caplier, J. Am. Chem. Soc. 1961, 83, 4406 – 4413. 29. G. Märkl, R. Potthast, Angew. Chem. Int. Ed. Engl. 1967, 6, 86. 30. G. Märkl, G.Y. Jin, K.P. Berr, Tetrahedron Lett. 1993, 34, 3103 – 3106. 31. (a) W.B. McCormack, US 2,663,737 (filing: 7 August 1951, publication: 22 December 1953). (b) W.B. McCormack, Org. Synth. Coll. Vol. 1973, 5, 787. 38 Chapter 2: Phosphole-containing π-systems

32. B. Lukas, R.M.G. Roberts, J. Silver, A.S. Wells, J. Organomet. Chem. 1983, 256, 103 – 110. 33. F. Mathey, C. R. Hebd. Seances Acad. Sci., Ser. C 1969, 269, 1066. 34. (a) P.J. Fagan, W.A. Nugent, J. Am. Chem. Soc. 1988, 110, 2310 – 2312. (b) P.J. Fagan, W.A. Nugent, Org. Synth. 1992, 70, 272. 35. P.J. Fagan, W.A. Nugent, J.C. Calabrese, J. Am. Chem. Soc. 1994, 116, 1880 – 1889. 36. For reviews, see: (a) Multiple Bonding and Low Coordination in Phosphorus Chemistry (eds.: M. Regitz, O. Scherer), 1990, Thieme, Stuttgart, Germany. (b) L. Nyulászi, Chem. Rev. 2001, 101, 1229 – 1246. (c) A.T. Balaban, D.C. Oniciu, A.R. Katritzky, Chem. Rev. 2004, 104, 2777 – 2812. (d) L. Nyulászi, Z. Benkö, Top. Heterocycl. Chem. 2009, 19, 27 – 82. 37. P. Coggon, J.F. Engel, A.T. McPhail, L.D. Quin, J. Am. Chem. Soc. 1970, 92, 5779 – 5780. 38. P. Coggon, A.T. McPhail, J. Chem. Soc., Dalton Trans. 1973, 1888 – 1891. 39. C.W. Bird, Tetrahedron 1992, 48, 335 – 340. 40. C.W. Bird, Tetrahedron 1985, 41, 1409 – 1414. 41. L. Nyulászi, P.v.R. Schleyer, J. Am. Chem. Soc. 1999, 121, 6872 – 6875. 42. P.v.R. Schleyer, C. Märker, A. Dransfeld, H. Jiao, N.J.R. van Eikema-Hommes, J. Am. Chem. Soc. 1996, 118, 6317 – 6318. 43. (a) C. Kölmel, C. Ochsenfeld, R. Ahlrichs, Theor. Chim. Acta 1991, 82, 271 – 284. (b) P. Schwerdtfeger, L. Laakkonen, P. Pyykkõ, J. Chem. Phys. 1992, 96, 6807 – 6819. 44. J.D. Swalen, J.A. Ibers, J. Chem. Phys. 1962, 36, 1914 – 1918. 45. E. Vessally, J. Struct. Chem. 2008, 49, 979 – 985. 46. W. Schäfer, A. Schweig, F. Mathey, J. Am. Chem. Soc. 1976, 98, 407 – 414. 47. (a) W. Egan, R. Tang, G. Zon, K. Mislow, J. Am. Chem. Soc. 1970, 92, 1442 – 1444. (b) W. Egan, R. Tang, G. Zon, K. Mislow, J. Am. Chem. Soc. 1971, 93, 6205 – 6216. 48. (a) L. Nyulászi, A. Belghazi, S. Kis Szétsi, T. Veszprémi, J. Heinicke, J. Mol. Struct. (Theochem) 1994, 313, 73 – 81. (b) J. Kapp, C. Schade, A.M. El-Nahasa, P.v.R. Schleyer, Angew. Chem. Int. Ed. Engl. 1996, 35, 2236 – 2238. 49. (a) D.B. Chesnut, L. Quin, J. Am. Chem. Soc. 1994, 116, 9638 – 9643. (b) A. Dransfeld, L. Nyulászi, P.v.R. Schleyer, Inorg. Chem. 1998, 37, 4413 – 4420. (c) L. Nuylászi, J. Phys. Chem. 1995, 99, 586 – 591. (d) L. Nuylászi, J. Phys. Chem. 1996, 100, 6194 – 6198. 50. D.P. Stevenson, J. Chem. Phys. 1940, 8, 285 – 287. 51. J.J. Daly, J. Chem. Soc. 1964, 3799 – 3810. 52. D. Delaere, A. Dransfeld, M.T. Nguyen, L.G. Vanquickenborne, J. Org. Chem. 2000, 65, 2631 – 2636. 53. S. Holland, F. Gandolfo, L. Ricard, F. Mathey, Bull. Soc. Chim. Fr. 1996, 133, 33 – 37. 54. C. Hay, D. Le Vilain, V. Deborde, L. Toupet, R. Réau, Chem.Commun. 1999, 345 – 346. 55. C.W. Bird, Tetrahedron 1990, 46, 5697 – 5702. 56. G. Keglevich, L.D. Quin, Z. Böcskei, G.M. Keserü, R. Kalgutkar, P.M. Lathi, J. Organomet. Chem. 1997, 532, 109 – 116. 57. G. Keglevich, Z. Böcskei, G.M. Keserü, K. Ujszászy, L.D. Quin, J. Am. Chem. Soc. 1997, 119, 5095 – 5099. 58. L. Nyulászi, G. Keglevich, L.D. Quin, J. Org. Chem. 1996, 61, 7808 – 7812. 59. L. Nyulászi, L. Soos, G. Keglevich J. Organomet. Chem. 1998, 566, 29 – 35. 60. L.D. Quin, G. Keglevich, A.S. Ionkin, R. Kalgutkar, G. Szalontai, J. Org. Chem. 1996, 61, 7801 – 7807. 61. A.S. ionkin, W.J. Marshall, B.M. Fish, M.F. Schiffhauer, F. Davidson, C.N. McEwen, D.E. Keys, Organometallics 2007, 26, 5050 – 5058. 62. F.G.N. Cloke, P.B. Hitchcock, P. Hunnable, J.F. Nixon, L. Nyulászi, E. Niecke, V. Thelen, Angew. Chem. Int. Ed. 1998, 37, 1083 – 1086. 63. L. Nyulászi, J.F. Nixon, J. Organomet. Chem. 1999, 588, 28 – 31. 64. (a) I.G.M. Campbell, R.C. Cookson, M.B. Hocking, A.N. Hughes, J. Chem. Soc. 1965, 2184 – 2193. (b) E. Deschamp, L. Ricard, F. Mathey, Heteroatom Chem. 1991, 2, 377 – 383. Chapter 2: Phosphole-containing π-systems 39

65. L. Nyulászi, O. Holloczki, C. Lescop, M. Hissler, R. Réau, Org. Biomol. Chem. 2006, 4, 996 – 998. 66. L.D. Quin, X.P. Wu, Heteroatom Chem. 1991, 2, 359 – 367. 67. (a) F.B. Clarke III, F.H. Westheimer, J. Am. Chem. Soc. 1971, 93, 4541 – 4545. (b) L.D. Freedman, B.R. Ezzell, R.N. Jenkins, R.M. Harris, Phosphorus 1974, 4, 199. (c) K.S. Fongers, H. Hogeveen, R.F. Kingma, Tetrahedron Lett. 1983, 24, 1423 – 1426. 68. L.D. Quin, B.G. Marsi, J. Am. Chem. Soc. 1985, 107, 3389 – 3390. 69. Y. kashman, O. Awerbouch, Tetrahedron 1975, 31, 53 – 62. 70. (a) J.K. Stille, J.L. Eichelberger, J. Higgins, M.E. Freeburger, J. Am. Chem. Soc. 1972, 94, 4761 – 4763. (b) K. Matsumoto, S. Hashimoto, T. Uchida, Heterocycles 1990, 30, 201 – 203. 71. F. Mathey, F. Mercier, Tetrahedron Lett. 1981, 22, 319 – 322. 72. E. Mattmann, F. Mercier, L. Ricard, F. Mathey, J. Org. Chem. 2002, 76, 5422 – 5425. 73. F. Mathey, R. Mankowski, Bull. Soc. Chim. Fr. 1970, 4433. 74. F. Mathey, Tetrahedron 1972, 13, 4171 – 4181. 75. R. Hussong, H. Heydt, M. Regitz, Phosphorus, Sulfur Relat. Elem. 1985, 25, 201 – 212. 76. Y. Kashman, I. Wagenstein, A. Rudi, Tetrahedron 1976, 32, 2427 – 2431. 77. W. Rösch, M. Regitz, Z. Naturforsch. B 1986, 41, 931 – 933. 78. A.N. Hughes, C. Srivanavit, Can. J. Chem. 1971, 49, 879 – 884. 79. L.D. Quin, S.E. Belmont, F. Mathey, C. Charrier, J. Chem. Soc., Perkin Trans. 2 1986, 629 – 633. 80. (a) G. Wittig, E. Kochendörfer, Chem. Ber. 1964, 97, 741 – 746. (b) J.I.G. Cadogan, R.J. Scott, R. Gee, J. Chem. Soc., Perkin Trans. 1 1974, 1694 – 1702. 81. H. Staudinger, J. Meyer, H. Chim. Acta 1919, 2, 635 – 646. 82. J.I.G. Cadogan, R.D. Gee, R.J. Scott, J. Chem. Soc., Chem. Commun. 1972, 1242a. 83. A. Brèque, F. Mathey, P. Savignac, Synthesis 1981, 983 – 985. 84. F. Mathey, J. Fisher, J.H. Nelson, Struct. Bonding 1984, 55, 153 – 201. 85. A.P. Sadimenko, A.D. Garnosakii, N. Retta, Coord. Chem. Rev. 1993, 126, 237 – 318. 86. F. Mathey, Coord. Chem. Rev. 1994, 137, 1 – 52. 87. (a) A.N. Hughes, C. Srivanavit, J. Heterocycl. Chem. 1970, 7, 1 – 24. (b) F.G. Mann, in The Heterocyclic Chemistry of Phosphorus, Arsenic, Antimony and Bismuth (eds.: A. Weissberger, E.C. Taylor), 1970, 2nd edition, Wiley-Interscience, New York, USA, 13 – 30. (c) A.N. Hughes, D. Kleemona, J. Heterocycl. Chem. 1976, 13, 1 – 12. (d) A.N. Hughes, in New trends in heterocyclic chemistry (eds.: R.B. Mitra, B.D. Tilak), 1979, Elsevier, Amsterdam, Netherlands, 216. (e) F. Mathey, Top. Phosphorus Chem. 1980, 10, 1 – 128. (f) L.D. Quin, in The Heterocyclic Chemistry of Phosphorus: Systems Based on the Phosphorus-Carbon Bond, 1981, Wiley-Interscience, New York, USA, 30 – 104. (g) F.Mathey Chem. Rev. 1988, 88, 429 – 453. (h) L.D. Quin, A.N. Hughes, in The Chemistry of Organophosphorus Compounds (ed.: F.R. Hartley), 1990, Wiley-Interscience, Chichester, UK, 295 – 384. (i) A.N. Hughes, in Handbook of Organophosphorus chemistry (ed.: R. Engel), 1992, Marcel Dekker, New York, 483 – 558. (j) F. Mathey, Phosphorus, Sulfur, Silicon Relat. Elem. 1994, 87, 139 – 148. (k) L.D. Quin, in Comp. Heterocycl. Chem. II (eds.: A.R. Katritzky, C.W. Rees, E.F.V. Scriven), 1996, Elsevier, Oxford, UK, 757 – 856. (l) K.B. Dillon, F. Mathey, J.F. Nixon, Phosphorus: The carbon copy, (eds.: K.B. Dillon, F. Mathey, J.F. Nixon), 1998, Wiley-Interscience, Chichester, UK, 181 – 256. (m) L.D. Quin, G.S. Quin, in Phosphorus Carbon Heterocyclic Chemistry: The rise of a new domain (ed.: F. Mathey), 2001, Elsevier, Oxford, UK, 219 – 362. (n) F. Mathey, Science of Synthesis, 2002, 9, 553 – 600. (o) F. Mathey, Angew. Chem. Int. Ed. 2003, 42, 1578 – 1604. (p) L.D. Quin, Curr. Org. Chem. 2006, 10, 43 – 78. (q) D.W. Allen, Organophosphorus Chem. 2010, 39, 1 – 48. 88. (a) T. Baumgartner, R. Réau, Chem. Rev. 2006, 106, 4681 – 4727. (b) M. Hissler, P. W. Dyer, R. Réau, Top. Cur. Chem. 2005, 250, 127 – 163. 40 Chapter 2: Phosphole-containing π-systems

89. (a) D. Delaere, M.T. Nguyen, L.G. Vanquickenborne, J. Organomet. Chem. 2002, 643 – 644, 194 – 201. (b) D. Delaere, M.T. Nguyen, L.G. Vanquickenborne, Phys. Chem. Chem. Phys. 2002, 4, 1522 – 1530. 90. E. Deschamps, L. Ricard, F. Mathey, Angew. Chem. Int. Ed. Engl. 1994, 33, 1158 – 1161. 91. (a) M.O. Bevière, F. Mercier, L. Ricard, F. Mathey, Angew. Chem. Int. Ed. Engl. 1990, 29, 655 – 657. (b) M.O. Bevière, F. Mercier, F. Mathey, A. Jutand, C. Amatore, N. J. Chem. 1991, 15, 545 – 550. 92. C. Hay, M. Hissler, C. Fischmeister, J. Rault-Berthelot, L. Toupet, L. Nyulászi, R. Réau, Chem. Eur. J. 2001, 7, 4222 – 4236. 93. C. Hay, M. Sauthier, V. Deborde, M. Hissler, L. Toupet, R. Réau, J. Organomet. Chem. 2002, 494 – 497. 94. (a) M. Hissler, C. Lescop, R. Réau, J. Organomet. Chem. 2005, 690, 2482 – 2487. (b) M. Hissler, C. Lescop, R. Réau, C. R. Chim. 2005, 8, 1186 – 1193. (c) M. Hissler, C. Lescop, R. Réau, C. R. Chim. 2008, 11, 628 – 640. (d) J. Crassous, R. Réau, Dalton Trans. 2008, 6865 – 6876. 95. H.C. Su, O. Fadhel, C.J. Yang, T.Y. Cho, C. Fave, M. Hissler, C.C. Wu, R. Réau, J. Am. Chem. Soc. 2006, 128, 983 – 995. 96. Y.L. Liu, J.K. Feng, A.M. Ren, J. Comp. Chem. 2007, 28, 2500 – 2509. 97. P.T. Nguyen-Nguyen, M.T. Nguyen, C. R. Chim. 2010, 13, 912 – 922. 98. C. Hay, C. Fave, M. Hissler, J. Rault-Berthelot, R. Réau, Org. Lett. 2003, 5, 3467 – 3470. 99. J. Casado, R. Réau, V. Hernández, J.T. López-Navarrete, Synth. Mater. 2005, 153, 249 – 252. 100. A. Fukazawa, Y. Ichihashi, S. Yamaguchi, New J. Chem. 2010, 34, 1537 – 1540. 101. N.H. Tran Huy, B. Donnadieu, F. Mathey, A. Muller, K. Colby, C.J. Bardeen, Organometallics 2008, 27, 5521 – 5524. 102. J.K. Vohs, P. Wei, J. Su, B.C. Beck, S.D. Goodwin, G.H. Robinson, Chem. Commun. 2000, 1037 – 1038. 103. (a) C. Fave, M. Hissler, T. Kárpáti, J. Rault-Berthelot, V. Deborde, L. Toupet, L. Nyulászi, R. Réau, J. Am. Chem. Soc. 2004, 126, 6058 – 6063. (b) J. Casado, R. Réau, J.T. López- Navarrete, Chem. Eur. J. 2006, 12, 3759 – 3767. 104. O. Fadhel, D. Szieberth, V. Deborde, C. Lescop, L. Nyulászi, M. Hissler, R. Réau, Chem. Eur. J. 2009, 15, 4914 – 4924. 105. D. Delaere, M.T. Nguyen, L.G. Vanquickenborne, J. Phys. Chem. A 2003, 107, 838 – 846. 106. O. Fadhel, Z. Benko, M. Gras, V. Deborde, D. Joly, C. Lescop, L. Nyulászi, M. Hissler, R. Réau, Chem. Eur. J. 2010, 16, 11340 – 11356. 107. A.I. Aranda-Perez, T. Biet, S. Graule, T. Agou, C. Lescop, N.R. Branda, J. Crassous, R. Réau, Chem. Eur. J. 2011, 17, 1337 – 1351. 108. C. Fave, T.Y. Cho, M. Hissler, C.W. Chen, T.Y. Luh, C.C. Wu, R. Réau, J. Am. Chem. Soc. 2003, 125, 9254 – 9255. 109. D. Joly, D. Tondelier, V. Deborde, B. Geffroy, M. Hissler, R. Réau, New J. Chem. 2010, 34, 1603 – 1611. 110. N. Lemaitre, B. Geffroy, O. Fadhel, M. Hissler, R. Réau, EP 2047534 B1 (filing: 24 July 2007, publication: 16 March 2011). 111. T. Igarashi, JP 4211920 B2 (filing: 25 April 2003, publication: 21 January 2009). 112. M. Matsuura, T. Yamada, H. Kita, JP 4103442 B2 (filing: 25 April 2002, publication: 18 June 2008). 113. O. Fadhel, M. Gras, N. Lemaitre, V. Deborde, M. Hissler, B. Geffroy, R. Réau, Adv. Mater. 2009, 21, 1261 – 1265. 114. C. Fave, M. Hissler, K. Senechal, I. Katell, J. Zyss, R. Réau, Chem. Commun. 2002, 1674 – 1675. 115. W. Shen, S. Graule, J. Crassous, C. Lescop, H. Gornitzka, R. Réau, Chem. Commun. 2008, 850 – 852. Chapter 2: Phosphole-containing π-systems 41

116. S. Graule, M. Rudolph, N. Vanthuyne, J. Autschbach, C. Roussel, J. Crassous, R. Réau, J. Am. Chem. Soc. 2009, 131, 3183 – 3185. 117. S. Graule, M. Rudolph, W. Shen, J.A.G. Williams, C. Lescop, J. Autschbach, J. Crassous, R. Réau, Chem. Eur. J. 2010, 16, 5976 – 6005. 118. T. Sanji, K. Shiraisha, M. Tanaka, Org. Lett. 2007, 9, 3611 – 3614. 119. K. Shiraishi, T. Kashiwabara, T. Sanji, M. Tanaka, New J. Chem. 2009, 33, 1680 – 1684. 120. Y. Matano, M. Nakashima, H. Imahori, Angew. Chem. Int. Ed. 2009, 48, 4002 – 4005. 121. Y. Matano, M. Nakashima, H. Imahori, Org. Lett. 2009, 11, 3338 – 3341. 122. Y. Matano, M. Fujita, A. Saito, H. Imahori, C. R. Chim. 2010, 13, 1035 – 1047. 123. W. Weymiens, F. Hartl, J.C. Slootweg, A. Ehlers, J.R. Mulder, K. Lammertsma, Eur. J. Org. Chem. 2012, accepted, DOI: 10.1002/ejoc.201201148. 124. Y. Matano, T. Miyajima, H. Imahori, Y. Kimura, J. Org. Chem. 2007, 72, 6200 – 6205. 125. Y. Matano, T. Nakabuchi, T. Miyajima, H. Imahori, H. Nakano, Org. Lett. 2006, 8, 5713 – 5716. 126. Y. Matano, T. Miyajima, T. Nakabuchi, H. Imahori, N. Ochi, S. Sakaki, J. Am. Chem. Soc. 2006, 128, 11760 – 11761. 127. Y. Matano, M. Nakashima, T. Nakabuchi, H. Imahori, S. Fujishige, H. Nakano, Org. Lett. 2008, 10, 553 – 556. 128. S.S.H. Mao, T. Don Tilley, Macromolecules 1997, 30, 5566 – 5569. 129. (a) Y. Morisaki, Y. Aiki, Y. Chujo, Macromolecules 2003, 36, 2594 – 2597. (b) H.S. Na, Y. Morisaki, Y. Aiki, Y. Chujo, Polym. Bull. 2007, 58, 645 – 652. (c) H.S. Na, Y. Morisaki, Y. Aiki, Y. Chujo, J. Polym. Sci. A: Polym. Chem. 2007, 45, 2867 – 2875. 130. Y. Morisaki, H.S. Na, Y. Aiki, Y. Chujo, Polym. Bull. 2007, 58, 777 – 784. 131. J. Roncali, Chem. Rev. 1992, 92, 711 – 738. 132. C. Hay, C. Fischmeister, M. Hissler, L. Toupet, R. Réau, Angew. Chem. Int. Ed. 2000, 39, 1812 – 1815. 133. (a) M. Sebastian, M. Hissler, C. Fave, J. Rault-Berthelot, C. Odin, R. Réau, Angew. Chem. Int. Ed. 2006, 45, 6152 – 6155. (b) R. Réau, M. Hissler, C. Odin, J. Rault-Berthelot, P. Bäuerle, WO 2006/122909 A1 (filing: 12 May 2006, publication: 23 November 2006). 134. (a) V. Lernau de Talance, M. Hissler, L.Z. Zhang, T. Kárpáti, L. Nyulászi, D. Caras-Quintero, P. Bäuerle, R. Réau, Chem. Commun. 2008, 2200 – 2202. (b) P.M. Carrasco, C. Pozo- Gonzalo, H. Grande, J.A. Pomposo, M. Cortazar, V. Deborde, M. Hissler, R. Réau, Polym. Bull. 2008, 61, 713 – 724. 135. A. Saito, Y. Matano, H. Imahori, Org. Lett. 2010, 12, 2675 – 2677. 136. N. Mezailles, L. Ricard, F. Gagosz, Org. Lett. 2005, 7, 4133 – 4136. 137. C. Bergounhou, D. Neilbecker, R. Mathieu, J. Mol. Cat. A: Chem. 2004, 220, 167 – 182. (b) G. Mora, S. van Zutphen, C. Thoumazet, X.F. le Goff, L. Ricard, H. Grutzmacher, P. le Floch, Organometallics 2006, 25, 5528 – 5532. 138. G. Mora, O. Piechaczyk, X.F. le Goff, P. le Floch, Organometallics 2008, 27, 2565 – 2569. 139. U.R. Kalkote; M.K. Gurjar; S.V. Joshi; S.M. Kadam; S.J. Naik, US 6,818,786 (filing: 31 March 2003, publication: 16 November 2004). 140. C. Thoumazet, H. Grutzmacher, B. Deschamp, L. Ricard, P. le Floch, Eur. J. Inorg. Chem. 2006, 19, 3911 – 3922. 141. (a) G. Mora, O. Piechaczyk, R. Houdard, N. Mezailles, X.F. le Goff, P. le Floch, Chem. Eur. J. 2008, 14, 10047 – 10057. (b) G. Mora, B. Deschamp, S. van Zutphen, X.F. le Goff, L. Ricard, P. le Floch, Organometallics 2007, 26, 1846 – 1855. 142. L. van der Veen, P.C.J. Kamer, P.W.N.M. van Leeuwen, Organometallics 1999, 18, 4765 – 4777. 143. (a) A. Escalle, G. Mora, F. Gagosz, N. Mezailles, X. le Goff, Y. Jean, P. le Floch, Inorg. Chem. 2009, 48, 8415 – 8422. (b) S. Labouille, A. Escalle-Lewis, Y. Jean, N. Mezailles, P. le Floch, Chem. Eur. J. 2011, 17, 2256 – 2265. 42 Chapter 2: Phosphole-containing π-systems

144. A. Escalle, X.F. le Goff, P. le Floch, J. Org. Chem. 2009, 74, 7540 – 7543. 145. H. Shimizu, S. Takao, I. Hagasaki, EP 1419815 B1 (filing: 10 November 2003, publication: 9 January 2008). 146. C. Thoumazet, M. Mohand, L. Ricard, P. le Floch, C. R. Chim. 2004, 7, 823 – 832. 147. C. Thoumazet, M. Mohand, L. Ricard, P. le Floch, Organometallics 2003, 22, 1580 – 1581. 148. C. Ortega, M. Gouygou, J.C. Daran, Chem. Commun. 2003, 1154 – 1155. 149. J. Wassenaar, S. van Zutphen, G. Mora, P. le Floch, M.A. Siegler, A.L. Spek, J.N.H. Reek, Organometallics 2009, 28, 2724 – 2734. 150. M. Ogasawara, A. Ito, K. Yoshida, T. Hayashi, Organometallics 2006, 25, 2715 – 2718. 151. C. Lescop, M. Hissler, in Tomorrow’s Chemistry Today: Concepts in Nanoscience and Environmental Chemistry (ed.: B. Pignataro) 2009, Wiley-VCH, Weinheim, Germany, 295 – 319. 152. F. Leca, M. Sauthier, B. Le Guennic, C. Lescop, L. Toupet, J.F. Halet, R. Réau, Chem. Commun. 2003, 1774 – 1775. 153. F. Leca, R. Réau, J. Cat. 2006, 238, 425 – 429. 154. (a) M. Sauthier, B. Le Guennic, V. Deborde, L. Toupet, J.F. Halet, R. Réau, Angew. Chem. Int. Ed. 2001, 40, 228 – 231. (b) F. Leca, C. Lescop, L. Toupet, R. Réau, Organometallics 2004, 23, 6191 – 6201. 155. M. Sauthier, F. Leca, L. Toupet, R. Réau, Organometallics 2002, 21, 1591 – 1602. 156. F. Leca, F. Fernandez, G. Muller, C. Lescop, R. Réau, M. Gomez, Eur. J. Inorg. Chem. 2009, 5583 – 5591. 157. R.F. de Souza, K. Bernardo-Gusmao, G.A. Cunha, C. Loup, F. Leca, R. Réau, J. Cat. 2004, 226, 235 – 239. 158. A. Caballero, F.A. Jalon, B.R. Manzano, M. Sauthier, L. Toupet, R. Réau, J. Organomet. Chem. 2002, 663, 118 – 126. 159. M. Melaimi, C. Thoumazet, L. Ricard, P. le Floch, J. Organomet. Chem. 2004, 689, 2988 – 2994. 160. E. Robe, C. Ortega, M. Mikina, M. Mikolajczyk, J.C. Daran, M. Gouygou, Organometallics 2005, 24, 5549 – 5559. 161. for a review, see: C. Lescop, Act. Chim. 2006, 297, 30 – 33. 162. F. Leca, M. Sauthier, V. Deborde, L, Toupet, R. Réau, Chem. Eur. J. 2003, 9, 3785 – 3795. 163. F. Leca, C. Lescop, E. Rodriguez-Sanz, K. Costuas, J.F. Halet, R. Réau, Angew. Chem. Int. Ed. 2005, 44, 4362 – 4365. 164. (a) B. Nohra, S. Graule, C. Lescop, R. Réau, J. Am. Chem. Soc. 2006, 128, 3520 – 3521. (b) B. Nohra, E. Rodriguez-Sanz, C. Lescop, R. Réau, Chem. Eur. J. 2008, 14, 3391 – 3403. (c) B. Nohra, C. Lescop, M. Hissler, R. Réau, Phosphorus, Sulfur, Silicon Relat. Elem. 2008, 183, 253 – 257. 165. T. Agou, M. Sebastian, C. Lescop, R. Réau, Inorg. Chem. 2011, 50, 3183 – 3185. 166. (a) S. Welsch, C. Lescop, M. Scheer, R. Réau, Inorg. Chem. 2008, 47, 8592 – 8594. (b) S. Welsch, B. Nohra, E.V. Peresypkina, C. Lescop, M. Scheer, R. Réau, Chem. Eur. J. 2009, 15, 4685 – 4703. 167. S. Welsch, C. Lescop, R. Réau, M. Scheer, Dalton Trans. 2009, 2683 – 2686. 168. F. Bisaro, P. le Floch, Synlett 2010, 3081 – 3085. 169. D. Carmichael, L. Ricard, N. Seeboth, Organometallics 2007, 26, 2964 – 2970. 170. (a) D. Carmichael, L. Ricard, N. Seeboth, J.M. Brown, T.D.W. Claridge, B. Odell, Dalton Trans. 2005, 2173 – 2181. (b) R. Shintani, G.C. Fu, Angew. Chem. Int. Ed. 2003, 42, 4082 – 4085. (c) R. Shintani, G.C. Fu, Org. Lett. 2002, 21, 3699 – 3702. (d) L. Zhang, M. Hissler, H.B. Bu, P. Bäuerle, C. Lescop, R. Réau, Organometallics 2005, 24, 5369 – 5376. (e) W. Ahlers, T. Mackewitz, R. Paciello, F. Mathey, P. le Floch, X. Sava, WO 2002/005955 A1 (filing: 13 July 2001, publication: 24 January 2002). Chapter 2: Phosphole-containing π-systems 43

171. (a) V.V. Bashilov, A.G. Ginzburg, A.F. Smol’yakov, F.M. Dolguschin, P.V. Petrovskii, V.I. Sokolov, J. Organomet. Chem. 2009, 694, 4121 – 4123. (b) V.V. Bashilov, A.G. Ginzburg, A.F. Smol’yakov, F.M. Dolguschin, P.V. Petrovskii, V.I. Sokolov, Russ. Chem. Bull. 2010, 59, 486 – 487. (c) A.G. Ginzburg, V.V. Bashilov, F.M. Dolgushin, A.F. Smol’yakov, A.S. Peregudov, V.I. Sokolov, J. Organomet. Chem. 2009, 694, 72 – 76. 172. K. Wimmer, C. Berg, R. Kretschmer, T.M.A. Al-Shboul, H. Görls, S. Krieck, M. Westerhausen, Z. Naturforsch. 2009, 64, 1360 – 1368. 173. (a) T. Sanji, K. Shiraishi, M. Tanaka, ACS Appl. Mater. & Interfaces 2009, 2, 270 – 273. (b) M. Tanaka, T. Sanji, K. Shiraishi, WO 2010/027108 A1 (filing: 7 September 2009, publication: 11 March 2010). 174. K. Shiraishi, T. Sanji, M. Tanaka, JP 2010/156662 A ((filing: 5 January 2009, publication: 15 July 2010). 175. K. Shiraishi, S. Kentaro, M. Tanaka, ACS Appl. Mater. & Interfaces 2009, 1, 1379 – 1382. 176. M. Deponte, S. Urig, L.D. Arscott, K. Fritz-Wolf, R. Réau, C. Herold-Mende, S. Koncarevic, M. Meyer, E. Davioud-Charvet, D.P. Ballou, C.H. Williams Jr., K. Becker, J. Biol. Chem. 2005, 280, 20628 – 20637. 177. S. Urig, K. Fritz-Wolf, R. Réau, C. Herold-Mende, K. Toth, E. Davioud-Charvet, K. Becker, Angew. Chem. Int. Ed. 2006, 45, 1881 – 1886. 178. E. Viry, E. Battaglia, V. Deborde, T. Muller, R. Réau, E. Davioud-Charvet, D. Bargek, ChemMedChem 2008, 3, 1667 – 1670. 179. E. Davioud-Charvet, K. Becker-Brandenburg, V. Deborde, R. Réau, R.H. Schirmer, EP 1771181 B1 (filing: 29 July 2005, publication: 10 September 2008). 180. (a) M. Hissler, P.W. Dyer, R. Réau, Coord. Chem. Rev. 2003, 244, 1 – 44. (b) M. Hissler, C. Lescop, R. Réau, Pure Appl. Chem. 2005, 77, 2099 – 2104. (c) M. Hissler, C. Lescop, R. Réau, Pure Appl. Chem. 2007, 79, 201 – 212. (d) M.G. Hobbs, T. Baumgartner, Eur. J. Inorg. Chem. 2007, 3611 – 3628. (e) R. Réau, P.W. Dyer, in Comprehensive Heterocyclic Chemistry III (eds.: A.R. Katrintzky, C.A. Ramsden, E.F.V. Scriven, R.J.K. Taylor), 2008, Elsevier, Amsterdam, Netherlands, 1029 – 1147. (f) Y. Matano, H. Imahori, Org. Biomol. Chem. 2009, 7, 1258 – 1271. 181. J.P. Lampin, F. Mathey, J. Organomet. Chem. 1974, 71, 239 – 255. 182. (a) M. Taillefer, F. Plenat, C. Combes-Chamalet, V. Vicente, H.J. Cristau, Phosphorus Res. Bull. 1999, 10, 696 – 701. (b) V. Vicente, A. Fruchier, H.J. Cristau, Magn. Reson. Chem. 2003, 41, 183 – 192. (c) V. Vicente, A. Fruchier, M. Taillefer, C. Combes-Chamalet, I.J. Scowen, F. plenat, H.J. Cristau, New J. Chem. 2004, 28, 418 – 424. 183. T. Baumgartner, T. Neumann, B. Wirges, Angew. Chem. Int. Ed. 2004, 43, 6197 – 6201. 184. (a) J. Roncali, Chem. Rev. 1997, 97, 173 – 205. (b) M. Pomerantz, Handbook of Conducting Polymers (eds.: T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds), 1998, Marcel Dekker, New York, USA, 277 – 309. (c) X. Zhang, A.J. Matzger, J. Org. Chem. 2003, 68, 9813. 185. T. Baumgartner, J. Inorg. Organomet. Polym. & Mater. 2005, 15, 389 – 409. 186. T. Baumgartner, W. Bergmans, T. Kárpáti, T. Neumann, M. Nieger, L. Nyulászi, Chem. Eur. J. 2005, 11, 4687 – 4699. 187. S. Durben, Y. Dienes, T. Baumgartner, Org. Lett. 2006, 8, 5893 – 5896. 188. Y. Dienes, M. Eggenstein, T. Neumann, U. Englert, T. Baumgartner, Dalton Trans. 2006, 1424 – 1433. 189. Y. Dienes, M. Eggenstein, T. Kárpáti, T. Sutherland, L. Nyulászi, T. Baumgartner, Chem. Eur. J. 2008, 14, 9878 – 9889. 190. T. Neumann, Y. Dienes, T. Baumgartner, Org. Lett. 2006, 8, 495 – 497. 191. T. Baumgartner, W. Wilk, Org. Lett. 2006, 8, 503 – 506. 192. Y. Dienes, S. Durben, T. Kárpáti, U. Englert, L. Nyulászi, T. Baumgartner, Chem. Eur. J. 2007, 13, 7487 – 7500. 44 Chapter 2: Phosphole-containing π-systems

193. C. Romero-Nieto, S. Durben, I.M. Kormos, T. Baumgartner, Adv. Funct. Mater. 2009, 19, 3625 – 3631. 194. Y. Ren, Y. Dienes, S. Hettel, M. Parvez, B. Hoge, T. Baumgartner, Organometallics 2009, 28, 734 – 740. 195. Y. Dienes, U. Englert, T. Baumgartner, Z. Anorg. Allg. Chem. 2009, 635, 238 – 244. 196. C. Romero-Nieto, S. Merino, J. Rodriguez-Lopez, T. Baumgartner, Chem. Eur. J. 2009, 15, 4135 – 4145. 197. C. Romero-Nieto, K. Kamada, D.T. Cramb, S. Merino, J. Rodriguez-Lopez, T. Baumgartner, Eur. J. Org. Chem. 2010, 5225 – 5231. 198. S. Durben, T. Linder, T. Baumgartner, New J. Chem. 2010, 34, 1585 – 1592. 199. T.J. Gordon, L.D. Szabo, T. Linder, C.P. Berlinguette, T. Baumgartner, C.R. Chim. 2010, 13, 971 – 979. 200. D.R. Bai, C. Romero-Nieto, T. Baumgartner, Dalton Trans. 2010, 39, 1250 – 1260. 201. D.R. Bai, T. Baumgartner, Organometallics 2010, 29, 3289 – 3297. 202. A. Fukazawa, M. Hara, T. Okamoto, E.C. Son, C. Xu, K. Tamao, S. Yamaguchi, Org. Lett. 2008, 10, 913 – 916. 203. A. Fukazawa, E. Yamaguchi, E. Ito, H. Yamada, J. Wang, S. Irle, S. Yamaguchi, Organometallics 2011, 30, 3870 – 3879. 204. S. Yamaguchi, A. Fukazawa, M. Hara, T. Okamoto, JP 4231929 B2 (filing: 1 September 2006, publication: 4 March 2009). 205. A. Fukazawa, T. Murai, L. Li, Y. Chen, S. Yamaguchi, C.R. Chim. 2010, 13, 1082 – 1090. 206. A. Fukazawa, H. Yamada, S. Yamaguchi, Angew. Chem. Int. Ed. 2008, 47, 5582 – 5585. 207. A. Fukazawa, H. Yamada, Y. Sasaki, S. Akiyama, S. Yamaguchi, Chem. Asian J. 2010, 5, 466 – 469. 208. A. Fukazawa, Y. Ichihashi, Y. Kosaka, S. Yamaguchi, Chem. Asian J. 2009, 4, 1729 – 1740. 209. T. Tanaka, H. Kita, JP 2010/225984 A (filing: 25 March 2009, publication: 7 October 2010). 210. M. Watanabe, T. Ohashi, JP 2009/143835 A (filing: 13 December 2007, publication: 2 July 2009). 211. S. Durben, D. Nickel, R.A. Kruger, T.C. Sutherland, T. Baumgartner, J. Polym. Sci. A: Polym. Chem. 2008, 46, 8179 – 8190. 212. Z. Zhang, J. Li, B. Huang, J. Qin, Chem. Lett. 2006, 35, 958 – 959. 213. N. Yasukawa, Y. Okubo, T. Hattori, JP 2011/049204 A (filing: 25 August 2009, publication: 10 March 2011). 214. (a) Y. Ren, T. Baumgartner, J. Am. Chem. Soc. 2011, 133, 1328 – 1340. (b) W. Weymiens, M. Zaal, J.C. Slootweg, A. Ehlers, K. Lammertsma, Inorg. Chem. 2011, 50, 8516 – 8523.