Synthesis and Characterization of Oxygen-Embedded Quinoidal Pentacene and Nonacene

Extension of oxygen-embedded polycyclic aromatic hydrocarbons (PAHs), in particular with a defined topology, is synthetically challenging primarily because of limited regio-specific methods and poor solubility of PAHs. We reported herein an efficient way to construct quinoidal pentacenes and nonacenes with regular zigzag O-inserted edges. These O-embedded backbones composed of benzene, pyranyl, and cyclohexa-1,4-diene moieties provided access to a new class of longitudinally conjugated acenes with superior stability. Their structures, confirmed by single-crystal XRD analysis, indicated that they possessed rich hydrogen/halogen-bonding interactions, which likely contribute to the strengthened aggregation. In contrast to many other O-annulated PAHs generally displaying short-absorption wavelengths due to partially interrupted electron communication, the O-embedded quinoidal acene exhibited highly red-shifted absorptions (up to ∼699 nm) and narrowed energy gaps (down to ∼1.5 eV). As with more O-containing rings and quinoidal subunits in the backbone, the conjugation size was enlarged, and the molar absorption coefficients (ε) of the λ_max significantly increased further, in particular, a noticeable lower-energy peak at ∼790 nm for O-doping nonacenes N₁-OH/OMe. By the thin-film-based organic field-effect transistor measurements, the relatively ordered O-doping pentacene P₁-OMe possessed a hole transporting efficiency (μ_h) of 0.00406 cm^-2 V^-1 s^-1 in in-air fabricated devices, while O-pentacene P₁-PFB with two perfluorobutoxyl substituents witnessed an improved μ_h up to 0.0152 cm^-2 V^-1 s^-1. In addition, one- or two-electron oxidation of O-pentacene/nonacene generated the corresponding radical cations or dications, in which electronic properties were dependent on the number of O-containing six-membered rings and quinoidal subunits. The study provided insights into the relationships between molecule structures and optoelectronic properties for the unique class of O-embedded PAHs.