Heptacenes

The linear acenes, benzene to pentacene, are used as examples of the CURES-EC procedure. The results obtained utilizing MINDO/3 and AMI are compared. In addition to calculating the Ea by subtracting the energies of the optimized form, the LUMO of the neutral is compared with the experimental Ea. The electron affinity of hexacene has been estimated from the electronegativity and experimental ionization potential. As a further example of the use of CURES-EC, both the ionization potential and electron affinity of heptacene are estimated. The Ea of octacene and nova-cene are calculated for comparison to values obtained by using Koopman s theorem and a semi-empirical method based on a variable-parameter modification of the Pariser Parr Pople (PPP) approximation to the Hartree Fock equation [10]. 

The charge densities for the anions can be displayed in HYPERCHEM as illustrated in Figure 7.5, where the charges on C and H are shown for the naphthalene anion. The Values of the linear acenes vary from —0.06 to 0.26q for benzene to —0.05 to 0.15q for naphthalene, to 0.04 to 0.18q for anthracene and tetracene. For pentacene the range is lowered to 0.03 to 0.14q. Thus, the solution energy differences should be smaller for the higher acenes and fullerenes. In predicting the reduction potential value for heptacene, this variation should be included. The reduction potential based on a value of 0.6 V versus Hg pool for hexacene is predicted to be —0.5 V versus the Hg pool. The value, if we assume a constant... 

While the longest commercially available oligoacene, pentacene, also exhibits the highest OTFT performance, very few longer acenes or derivatives thereof have been reported. Recently silylethynylated hexacene and heptacene derivatives were synthesized (Figure 3.1.3), but their transistor characteristics were not reported.[78]... 

Bond numbers and bond lengths in tetrabenzo de, no, st, cj, dj) heptacene and other condensed aromatic hydrocarbons a valence bond treatment. Acta Cryst. B36 (1980) 1898-1901. 

By considering the mild conditions required for the generation of the bisbenzyne species in such cycloaddition reactions, Wudl employed compound 146 as the bisbenzyne precursor in the synthesis of 6,8,15,17-tetraphenyl-1.18,4.5,9.10,13.14-tetrabenzoheptacene (147) (Equation 12.40) [80]. In comparison to Pascal s protocol, Wudl s method afforded the cycloaddition product, heptacene 147, in a much higher (22%) yield. 

French chemist Charles Marschalk also synthesized hexacene in the late 1930s and reported properties similar to those described above [13]. An interesting correspondence between Clar and Marschalk is described in Clar s 1939 paper in a September 1, 1938 letter by Marschalk to Clar, Marschalk communicates that he is on the way to the synthesis of heptacene. Clar described having finished and had already communicated the results of his work oti hexacene to the chemical industry on April 2, 1938. Clar replied to Marschalk on September 11, 1938 that he had... 

In 1942 Clar reported the synthesis of heptacene [41]. As with hexacene, the key to the synthesis is the dehydrogenation of dihydroheptacenes (such as the 6,17-dihydro... 

Fig. 12 Catalytic dehydrogenation of hexacosahydroheptacene 31 was claimed to yield heptacene [15]...

Another synthesis of a heptacene was reported by Bailey and Liao in 1955 [15]. These authors obtained heptacene in 76% yield by dehydrogenation of a hexacosahydroheptacene 31 with palladium-on-carbon at 340-375°C (Fig. 12). Bailey and Liao give a satisfactory elemental analysis and describe their product as greenish-black material that decomposes at approximately 400°C [15]. This work received criticism by Boggiano and Clar [42]. No further attempts at heptacene synthesis were described in the twentieth century, with the exception of aPh.D. thesis [47]. 

Further matrix isolation experiments were reported by Bettinger et al. using solid noble gas matrices (Ar and Xe) under cryogenic conditions [22,52]. Heptacene was generated photochemically as reported by the Neckers group [48]. These experiments allowed measurement of the UV/vis absorption spectrum of heptacene between 200 and 800 nm (Fig. 14) and its IR spectrum (Fig. 15). 

Fig. 13 Photogeneration of heptacene 7 in a PMMA matrix from a-diketone 37 [48]...

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