The precursor itself is prepared in three steps from two molecules of α,α,α',α'-tetrabromo-o-xylene with a 7-tert-butoxybicyclo[2.2.1]hepta-2,5-diene by first heating with sodium iodide in dimethylformamide to undergo a series of elimination and Diels–Alder reactions to form the ring system, then hydrolysing the tert-butoxy group to an alcohol and followed by its oxidation to the ketone.[13]
The product is reported to have some solubility in chloroform and is therefore amenable to spin coating. Pentacene is soluble in hot chlorinated benzenes, such as 1,2,4-trichlorobenzene, from which it can be recrystallized to form platelets.
Pentacene derivatives
Monomeric pentacene derivatives
6,13-Substituted pentacenes are accessible through pentacenequinone by reaction with an aryl or alkynyl nucleophile (for example Grignard or organolithium reagents) followed by reductive aromatization.[14][15][16] Another method is based on homologization of diynes by transition metals (through zirconacyclopentadienes) [17][18][19][20][21] Functionalization of pentacene has allowed for control of the solid-state packing of this chromophore.[22][23] The choice of the substituents (both size and location of substitution on the pentacene) influences the solid-state packing and can be used to control whether the compound adopts 1-dimensional or 2-dimensional cofacial pi-stacking in the solid-state, as opposed to the herringbone packing observed for pentacene.
Although pentacene's structure resembles that of other aromatic compounds like anthracene, its aromatic properties are poorly defined; as such, pentacene and its derivatives are the subject of much research.
This equilibrium is entirely in favor of the methylene compound. Only by heating a solution of the compound to 200 °C does a small amount of the pentacene develop, as evidenced by the emergence of a red-violet color. According to one study[24] the reaction mechanism for this equilibrium is not based on an intramolecular1,5-hydride shift, but on a bimolecularfree radical hydrogen migration. In contrast, isotoluenes with the same central chemical motif easily aromatize.
In the crystal phase the molecules display aromatic stacking interactions, whereby the distance between some sulfur atoms on neighboring molecules can become less (337 pm) than the sum of two Van der Waals radii (180 pm)
has an end-to end twist of 144° and is sterically stabilized by the six phenyl groups. The compound can be resolved into its two enantiomers with an unusually high reported optical rotation of 7400° although racemization takes place with a chemical half-life of 9 hours.
Oligomers and polymers based on pentacene have been explored both synthetically as well as in device application settings.[27][28] Polymer light emitting diodes (PLEDs) have been constructed using conjugated copolymers (1a–b) containing fluorene and pentacene.[29] A few other conjugated pentacene polymers (2a–b and 3) have been realized based on Sonogashira and Suzuki coupling reactions of a dibromopentacene monomer.[30][31] Non-conjugated pentacene-based polymers have been synthesized via esterification of a pentacene diol monomer with bis-acid chlorides to form polymers 4a–b.[32][33]
OLIGOMERS
Various synthetic strategies have been employed to form conjugated oligomers of pentacene 5a–c including a one-pot-four-bond forming procedure which provided a solution-processable conjugated pentacene dimer (5c) which exhibited photoconductive gain >10,[34] placing its performance within the same order of magnitude as thermally evaporated films of non-functionalized pentacene which exhibited photoconductive gain >16 using analogous measurement techniques.[35] A modular synthetic method to conjugated pentacene di-, tri- and tetramers (6–8) has been reported which is based on homo- and cross-coupling reactions of robust dehydropentacene intermediates.[36] Non-conjugated oligomers 9–10 based on pentacene have been synthesized,[32][33] including dendrimers 9–10 with up to 9 pentacene moieties per molecule with molar absorptivity for the most intense absorption > 2,000,000 M−1•cm−1. Dendrimers 11–12 were shown to have improved performance in devices compared to analogous pentacene-based polymers 4a–b in the context of photodetectors.[37]
Combined with buckminsterfullerene, pentacene is used in the development of organic photovoltaic prototypes.[40][41] Organic photovoltaic cells are cheaper and more flexible than traditional inorganic cells, which could potentially open doors to solar cells in new markets.[42]
Pentacene is a popular choice for research on organic thin-film transistors and OFETs, being one of the most thoroughly investigated conjugated organic molecules with a high application potential due to a hole mobility in OFETs of up to 5.5 cm2/(V·s), which exceeds that of amorphous silicon.[43][44][45]
Pentacene, as well as other organic conductors, is subject to rapid oxidation in air, which precludes commercialization. If the pentacene is preoxidized, the pentacene-quinone is a potential gate insulator, then the mobility can approach that of rubrene – the highest-mobility organic semiconductor – namely, 40 cm2/(V·s). This pentacene oxidation technique is akin to the silicon oxidation used in the silicon electronics.[44]
↑in the synthesis of this compound, the starting material is treated with 1,4-naphthoquinone and DPT. DTP converts the oxo-norbornadiene to an intermediary furan. The second step is oxidation by PPTS
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