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 λmax 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 (μh) 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 μh 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.