Steric and Electronic Substituent Effects Influencing Regioselectivity of Tetracene Endoperoxidation

Abstract

This paper describes the influence of steric and electronic factors in the regioselectivity of endoperoxide formation of tetracene derivatives using (1)O2. A combination of kinetics experiments and product distributions resulting from these photosensitized oxidations demonstrates that, while the steric effect of o-alkyl groups on aryl substituents is highly localized to the substituted ring, the resistance to oxidation based on phenylethynyl substituents is more evenly distributed between the two reactive rings. These results are important for the rational design of highly persistent acenes.

Benzo- and Thieno-Annulated Tetracenes: A One-Pot Synthesis via Cross-Dehydrogenative Annulation

“…For example, the representative tetracene derivative rubrene shows an exceptionally high charge-carrier mobility but simultaneously shows a high propensity to engage in rapid photooxidative degradation . Whereas a number of oligoacene derivatives have been intensively studied, the development of novel synthetic strategies for the versatile functionalization of acene derivatives and the improvement of their photooxidative stability remain of paramount importance for the continued progress of acene-based materials chemistry.…”

“…Even though the photooxidation of acenes and their substituted derivatives has been extensively studied, the underlying mechanism remains a matter of debate. , A lowered LUMO level plays an important role for the stabilization of TIPS-pentacene by suppressing the generation of the radical anion O 2 •– via an electron transfer . Cyclic voltammograms of 2a and 2e in CH 2 Cl 2 each show one reversible reduction wave ( E 1/2 = −1.58 V and −1.59 V vs Fc/Fc + , respectively), indicating that the LUMO levels of 2a and 2e are comparable.…”

Short Excited-State Lifetimes Enable Photo-Oxidatively Stable Rubrene Derivatives

“…Tetracene is another model compound with high reactivity to oxygen that can be used to investigate chemical stability. Several derivatives of substituted tetracenes have been reported, including diethynyltetracenes, tetraalkyltetracenes, dialkoxytetracenes, and rubrene derivatives. ,− Notably, rubrene has been extensively studied for many years because of its extremely high carrier mobility. − Indeed, it is well-known that rubrene undergoes a fast oxidation process if exposed to air. This oxidation process is strongly enhanced in the presence of light and may be thermally reversed to form the parent compound .…”

“…For example, as oligoacenes increase in length, their absorption red shifts, their intermolecular electronic couplings become stronger, and their larger conjugated cores result in reduced reorganization energies. − Unfortunately increasing oligoacene size also results in deleterious changes to some properties including reduced solubility and rapid degradation, such as photo-oxidation and dimerization. − We have previously reported the adoption of an unconventional “angular” mode of annulation to afford more stable oligoacenes. − This type of motif increases both the aromaticity and required distortion energy to form the oxidized endo -peroxide. However, one of the most common strategies to increase solubility and stability in this class of small molecules is through the addition of bulky substituents, such as phenyl, arylthio, or ethynyl moieties, at peri-positions. − The enhanced stability of oligoacenes via addition of certain substituents has been attributed to the combination of steric resistance and electronic effects. − Inclusion of bulky substituents at the peri-positions has proven to be a successful approach to reducing dimerization that has also served to improve solubility. However, even substituted oligoacenes still suffer from photo-oxidation in both solution and the solid state …”

Spectroscopy and Reactivity of Dialkoxy Acenes

“…Therefore, when the central rings of tetracene derivatives are not identically substituted, the distribution of observed endoperoxide regioisomers gives direct comparisons of how substituents influence cycloaddition reactivity. Our previous work used this approach to elucidate steric and electronic substituent effects in endoperoxide formation . Here, exposing 5,12‐dioctyloxytetracene 4 to 1 O 2 through irradiation of methylene blue in CDCl 3 yielded the expected two endoperoxides, with cycloaddition across the alkoxy‐substituted ring as the major product in a 9:1 molar ratio (Figure and Figure S16); the minor regioisomer resulted from endoperoxidation across the unsubstituted central ring.…”