RESUMO
The high reactivity of acenes can reduce their potential applications in the field of molecular electronics. Although pentacene is an important material for use in organic field-effect transistors because of its high charge mobility, its reactivity is a major disadvantage hindering the development of pentacene applications. In this study, several reaction pathways for the thermal dimerization of acenes were considered computationally. The formation of acene dimers via a central benzene ring and the formation of acene-based polymers were found to be the preferred pathways, depending on the length of the monomer. Interestingly, starting from hexacene, acene dimers are thermodynamically disfavored products, and the reaction pathway is predicted to proceed instead via a double cycloaddition reaction (polymerization) to yield acene-based polymers. A concerted asynchronous reaction mechanism was found for benzene and naphthalene dimerization, while a stepwise biradical mechanism was predicted for the dimerization of anthracene, pentacene, and heptacene. The biradical mechanism for dimerization of anthracene and pentacene proceeds via syn or anti transition states and biradical minima through stepwise biradical pathways, while dimerization of heptacene proceeds via asynchronous ring closure of the complex formed by two heptacene molecules. The activation barriers for thermal dimerization decrease rapidly with increasing acene chain length and are calculated (at M06-2X/6-31G(d)+ZPVE) to be 77.9, 57.1, 33.3, -0.3, and -12.1 kcal/mol vs two isolated acene molecules for benzene, naphthalene, anthracene, pentacene, and heptacene, respectively. If activation energy is calculated vs the initially formed complex of two acene molecules, then the calculated barriers are 80.5, 63.2, 43.7, 16.7, and 12.3 kcal/mol. Dimerization is exothermic from anthracene onward, but it is endothermic at the terminal rings, even for heptacene. Phenyl substitution at the most reactive meso-carbon atoms of the central ring of acene blocks the reactivity of this ring but does not efficiently prevent dimerization through other rings.
RESUMO
Surprisingly, despite its very high mobility in a single crystal, rubrene shows very low mobility in vacuum-sublimed or solution-processed organic thin-film transistors. We synthesized several rubrene analogues with electron-withdrawing and electron-donating substituents and found that most of the substituted rubrenes are not planar in the solid state. Moreover, we conclude (based on experimental and calculated data) that even parent rubrene is not planar in solution and in thin films. This discovery explains why high mobility is reported in rubrene single crystals, but rubrene shows very low field-effect mobility in thin films. The substituted rubrenes obtained in this work have significantly better solubility than parent rubrene and some even form films and not crystals after evaporation of the solvent. Thus, substituted rubrenes are promising materials for organic light-emitting diode (OLED) applications.
RESUMO
To understand the reactivity of acenes, particularly pentacene, the addition of HCl and water to acenes was studied for the benzene-nonacene series at the B3LYP/6-31G(d) level of theory. Surprisingly, the reactivity of the acenes increases along the series up to hexacene and remains constant from hexacene and above due to the biradical character of the ground state of higher acenes. While the exothermicity of HCl and water additions are very similar, the activation barriers for HCl and water additions differ by a constant factor of ca. 27 kcal/mol. The barrier for the addition of HCl varies from 44 kcal/mol for benzene to 16-18 kcal/mol for pentacene-nonacene, whereas the barrier for the addition of water varies from 71 kcal/mol for benzene to 43-46 kcal/mol for pentacene-nonacene. The transition states (TSs) for the addition of water to acenes are relatively "late" on the reaction coordinate, compared to the "earlier" TSs for the addition of HCl. There is a substantial substituent effect on the energy barriers for these reactions. HCl behaves as an electrophile, with rhoHCl (vs rho p) = -4.48 and -3.39 for anthracenes and pentacenes, respectively, while water behaves as a nucleophile, with rhoHCl (vs rho p) = 2.35 and 1.39 for anthracenes and pentacenes, respectively.
RESUMO
An interesting change in mechanism (from concerted to biradical) is described for the reaction of acenes (benzene through pentacene) with molecular oxygen (either singlet oxygen, 1Deltag-O2, or triplet oxygen, 3Sigmag-O2).
