Structural Control of Band Gap and Polaron Delocalization in 2D Azatriangulene Covalent Organic Frameworks

New two-dimensional (2D) covalent organic frameworks
(COFs) with
a tri(oxa)azatriangulene (TANG) node and different polarity of the
imine linkers (CN vs NC) and internode distance have
been synthesized using a transimination polymerization. The orientation
of the imine linker controls the donor–acceptor interactions
in these π-conjugated COFs, resulting in a large modulation
of the valence band maximum (−4.2 to −5.4 eV) and the
band gap (Eg ≈ 1.2–1.6 eV). p-Doping of COFs in iodine vapor results in the formation
of highly delocalized polarons with an absorption band extending into
the far-infrared (THz band) region. Reducing the TANG–TANG
internodal distance increases the 2D bandwidth dispersion and leads
to the highest (in the series) electrical conductivity (10–1 S/cm), the lowest conductance activation energy (50 meV near room
temperature), and the most red-shifted polaron absorption (λedge ∼ 160 μm, 8 meV) in the series.