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Heptacene
Heptacene is an organic compound and a polycyclic aromatic hydrocarbon and the seventh member of the acene or polyacene family of linear fused benzene rings. This compound has long been pursued by chemists 〔Clar, E. (1942), ''Heptacen ein einfacher, „ultragrüner” Kohlenwasserstoff (Aromatische Kohlenwasserstoffe, XXXV. Mitteil.).'' Ber. dtsch. Chem. Ges. A/B, 75: 1330–1338. 〕〔''Cyclic Dienes. XI. New Syntheses of Hexacene and Heptacene'' William J. Bailey and Chien-Wei Liao Journal of the American Chemical Society 1955 77 (4), 992-993 〕〔''519. Four higher annellated pyrenes with acene character'' B. Boggiano and E. Clar J. Chem. Soc., 1957, 2681-2689 〕 because of its potential interest in electronic applications and was first synthesized but not isolated in 2006.〔''Photogeneration of Heptacene in a Polymer Matrix'' Rajib Mondal, Bipin K. Shah, and Douglas C. Neckers J. Am. Chem. Soc.; 2006; 128(30) pp 9612 - 9613; (Communication) 〕〔a| Diels-Alder reaction of dibromonaphthalene forming a naphthyne and ''bicyclo()oct-2,3,5,6,7-pentaene'' with ''n''-butyllithium in toluene 3 hours at -50 to -60°C 53% chemical yield b| organic oxidation with P-chloranil in toluene 2 hours reflux and 81% yield c] Bishydroxylation with N-Methylmorpholine N-oxide and osmium tetroxide in acetone and t-butanol at room temperature for 48 hours, 83% yield d] Swern oxidation with trifluoroacetic acid in dimethyl sulfoxide and dichloromethane at -78°C, 51% yield e] photochemical decarbonylization in a PMMA matrix at 395 nm〕
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The final step is a photochemical decarbonylization with a 1,2-dione bridge extruded as carbon monoxide. In solution heptacene is not formed because it is very unstable being a reactive DA diene and quickly reacts with oxygen or forms dimers. When on the other hand the dione precursor is dissolved in a PMMA matrix first, heptacene can be studied by spectroscopy. Heptacene has been studied spectroscopically at cryogenic temperatures in a matrix.〔''Synthesis, Stability, and Photochemistry of Pentacene, Hexacene, and Heptacene: A Matrix Isolation Study'' Rajib Mondal, Christina Tönshoff, Dmitriy Khon, Douglas C. Neckers, and Holger F. Bettinger Journal of the American Chemical Society 2009 131 (40), 14281-14289 〕 When dissolved in sulfuric acid the heptacene dication is reported to be stable at room-temperature for more than a year in absence of oxygen.
== Derivatives ==
7,16-Bis(tris(trimethylsilyl)silylethynyl)heptacene was synthesised in 2005. This compound is stable in the solid state for a week but decomposes in contact with air. Its synthesis started from anthraquinone and naphthalene-2,3-dicarboxaldehyde. More stable substituted heptacenes have been reported: with stabilizing ''p''-(''t''-butyl)thiophenyl substituents and with phenyl and triisopropylsilylethynyl groups.
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