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Phosphole-Containing Π-Systems |2 VU Research Portal Phosphole and Phosphine Acetylene Building Blocks for n-Conjugated Systems Weymiens, W. 2012 document version Publisher's PDF, also known as Version of record Link to publication in VU Research Portal citation for published version (APA) Weymiens, W. (2012). Phosphole and Phosphine Acetylene Building Blocks for n-Conjugated Systems. 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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,
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