Acenes electronic effects

Electronic Effects in the Formation of Diradicals. Syntheses of Naphthalenes and Acenes... 

The role played by the electron-donating or electron-withdrawing effects of the various ligands in rendering the oxidation process easier for [VO(salen)] compared to [VO(saloph)] or [VO(acen)] is evident. The influence of the solvent on the redox potentials is also clear. 

Functionalization of pentacene with the specific aim of improving performance in devices is a recent endeavor - the first use of a functionalized pentacene in a field-effect transistor was reported only recently (2003) [26], Functionalization of pentacene has led to the ability to engineer the solid-state arrangement, electronic, and solubility properties of this important semiconductor and to improve its stability and film-forming ability. Recent functionalized pentacene materials have yielded devices with properties comparable with those of the parent acene, have enabled the formation of devices from solution-deposited films, and have even changed the semiconductor behavior of this organic molecule from p-type to n-type. As functionalization strategies are refined, materials with all of the properties necessary for commercial device applications should soon be developed. 

Oxidation of anthracene or higher acenes also occurs in the absence of a dye, since the same substrates act as sensitizers in singlet oxygenation [9c,42a]. These compounds react mainly at the mexo-positions of the aromatic nucleus [49a], but electron-donating groups have a strong directing effect when placed at the 1- and 4-positions (Sch. 33) [49b]. 

The linear acenes illustrate the effect of extended conjugation. As the number of rings is extended in both the hydrocarbons and perfluorinated hycrocarbons, the Ea increases. However, for a nonlinear extension the Ea may or may not increase. For example, the Ea of phenanthrene is 0.30 0.02 eV, while that for anthracene is 0.68 0.02 eV. Likewise, the Ea of benzanthracene is less than that of tetracene. The curves for the linear acenes are an upper limit for the electron affinities of polycyclic aromatic hydrocarbons with the same number of six membered rings. The inclusion of five or seven membered rings will increase the electron affinity above those with solely six rings. The simplest example is the Ea of azulene, 0.8 eV, versus that of naphthalene, 0.16 eV. Interestingly, the calculated electron affinity of the perfluorinated azulene is 2.6 eV or about 0.2 eV per fluorine atom. 

Fig. 9 Effective number of unpaired electrons for the Ag and B3 states of the n-acenes with and without the HONO and LUNO included using the 7t-MR-AQCC/RAS/CAS(4,4)/AlIXy6-31G method...

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