Heptacenes substituted

LDI-TOF measurements [33]. The energy of dimerization of the substituted compounds is likely smaller than that of the parent acenes. It is not clear whether the dimer mass of heptacene observed is indeed due to the presence of (a structurally not characterized) heptacene dimer or to laser induced breaking of heptacene oligomers and polymers, but it is noted that the monomer mass was not observed [22]. A recent computational study by the Bendikov group shows that the dimerization of heptacene to the [4 + 4] dimer, a thermally forbidden reaction, is strongly exothermic [53]. 

The first tmambiguous report of a substituted heptacene was provided by Payne et al. in 2005 [37]. The authors synthesized silylethynyl substituted heptacenes similar to hexacenes nucleophilic addition of lithium(silylacetylide) to heptacenequinone 39 followed by reduction of the diol by SnCl2 (Fig. 19) [37]. 

Qu and Chi added four electron-withdrawing p-trifluoromethylphenyl groups in addition to two TIPS-ethynyl groups to the heptacene core [57]. The synthesis of the required substituted heptacenequinone 49 was achieved by a Diels-Alder reaction of the transient isonaphthofuran with benzoquinone followed by dehydration. Addition of the TIPS-acetylide and subsequent oxidation with SnCl2/H20 furnished the desired substituted heptacene 50 (Fig. 24) [57]. 

The 2008 investigation by Chun et al. [56] nicely demonstrates that the introduction of bulky groups such as phenyl that may protect the acene from dimerization are not sufficient for stabilizing the heptacene core towards air oxidation in the presence of laboratory light. Two more aryl substituted heptacenes (53 and 54) were prepared and investigated by the Miller group in 2009 (Fig. 26) [58]. 

As expected, the 7,16-di(p-ferf-butylphenyl)heptacene 53 decomposed rapidly in solution in an ambient atmosphere in the presence of light, while the four additional o,o-dimethylphenyl substituents in 54 enhance the stability somewhat [58]. The half-life in the presence of light was not given, but appears to lie in the region of 10 min for this derivative from the displayed time dependence of acene absorption. The solids have significant lifetimes" [58] and could be studied by LDI-TOF-MS. In addition to the molecular ion M", the less substituted derivative 53 also showed the presence of the oxygen adduct (M" + 32) and its fragment (M-" + 16) [58]. 

The Miller group also reported the synthesis of nonacene derivatives 75 and 76 using their thioaryl approach discussed above in the context of substituted heptacenes [62]. The synthesis is reminiscent of that of heptacene derivatives [58], with the key step being the Diels-Alder reaction between anthracenequinone 72 and the bis-ortho-quinodimethane precursor 73 (Fig. 35). The persistence of the less substituted derivative 76 is insufficient for characterization by solution phase methods. Derivative 75, however, could be investigated by NMR, UV/vis-NIR, and fluorescence spectroscopies as well as by LDI-TOF-MS [62]. 

Fig. 34 Left. UV/vis-NIR spectrum of nonacene 70c measured in toluene at RT. Right, the long-wavelength portion of the absorption spectra for silylethyne-substituted acenes from anthracene to heptacene and nonacene 70c. Reprinted with permission from [61], Copyright 2011 John Wiley and Sons...

This method has gained importance in recent time in view of the importance of linear acenes in molecular electronics applications. The following synthesis of substituted heptacene demonstrates the utility of this methodology (Scheme 16.28) [31]. 

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