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Heptacene
Published in Klaus D. Sattler, 21st Century Nanoscience – A Handbook, 2020
Holger F. Bettinger, Peter Grüninger
Heptacene is a member of the class of polycyclic aromatic hydrocarbons (PAH), called acenes, that is comprised of rectilinearly fused benzene rings (Figure 4.1). The smallest acene is naphthalene, followed by anthracene, tetracene (the name recommended by the International Union of Pure and Applied Chemistry is naphthacene, but the compound is often called tetracene in the literature and also in this chapter), pentacene, and hexacene. Naphthalene and anthracene have been isolated from coal tar in the first part of the 1820s and are nowadays very well-investigated fundamental organic compounds with multiple applications. The next higher homologs of the acenes, tetracene, pentacene, and hexacene were first synthesized in the 1930s (Fieser, 1931; Clar and John, 1929; Clar, 1939; Marschalk, 1939). These acenes, in particular tetracene and pentacene, are well characterized, and it is well understood how the properties such as absorption and fluorescence spectra, ionization potentials, and chemical reactivity change by successive elongation of the π systems. One particularly noteworthy property of acenes is their very rapid increase in chemical reactivity with increasing the size. For example, the bimolecular rate constants for Diels-Alder reaction with maleic anhydride, measured at 91.5 ± 0.2 °C, increase by more than three orders of magnitude when going from anthracene to hexacene (Biermann and Schmidt, 1980). Likewise, the reactivity towards molecular oxygen, particularly in the presence of light, increases within the series and requires researchers to take appropriate caution when storing and handling the compounds, in particular the larger ones.
Relative energies of increasingly large [n]helicenes by means of high-level quantum chemical methods
Published in Molecular Physics, 2023
The largest [n]helicene that has been synthesised includes 16 ortho-fused benzene rings ([16]helicene) [13,14]. Expanded helicenes (i.e. helicenes composed of alternating linear and ortho-fused rings) [15] of up to [23]helicene have been synthesised [16]. For comparison, linear acenes with only up to 7 benzene rings (heptacene) have been isolated in bulk [17], and the largest metastable linear acene that has been investigated experimentally includes 12 benzene rings (dodecacene) [18–20]. Thus, despite both helicenes and linear acenes being catacondensed PAHs, large helicenes are synthetically more accessible. Indeed, [n]acenes become more reactive with an increasing number of rings [21].