Covalent
organic frameworks (COFs), consisting of covalently connected organic building
units, combine attractive features such as crystallinity, open porosity and
widely tunable physical properties. For optoelectronic applications, the
incorporation of heteroatoms into a 2D COF has the potential to yield desired
photophysical properties such as lower band gaps, but can also cause lateral
offsets of adjacent layers. Here, we introduce dibenzo[g,p]chrysene (DBC) as a
novel building block for the synthesis of highly crystalline and porous 2D
dual-pore COFs showing interesting properties for optoelectronic applications.
The newly synthesized terephthalaldehyde (TA), biphenyl (Biph), and
thienothiophene (TT) DBC-COFs combine conjugation in the a,b-plane with a tight
packing of adjacent layers guided through the molecular DBC node serving a
specific docking site for successive layers. The resulting DBC-COFs exhibit a
hexagonal dual-pore kagome geometry, which is comparable to COFs containing
another molecular docking site, namely
4,4′,4″,4‴-(ethylene-1,1,2,2-tetrayl)-tetraaniline (ETTA). In this context, the
respective interlayer distances decrease from about 4.60 Å in ETTA-COFs to
about 3.6 Å in DBC-COFs, leading to well-defined hexagonally faceted single
crystals sized about 50-100 nm. The TT DBC-COFs feature broad light absorption
covering large parts of the visible spectrum, while Biph DBC-COF shows extraordinary
excited state lifetimes exceeding 10 ns. In combination with the large number
of recently developed linear conjugated building blocks, the new DBC
tetra-connected node is expected to enable the synthesis of a large family of
strongly p-stacked, highly ordered 2D COFs with promising
optoelectronic properties.
Pseudo-resonance and energy band gaps in plasmonic crystals
