Quantum Chemistry Insight into the Multifaceted Structural Properties of Two-Dimensional Covalent Organic Frameworks

2D covalent organic frameworks (COFs) possess a wide
range of desirable
mechanical and optoelectronic properties, rendering them a versatile
compound class that is widely investigated in many areas of chemistry.
However, experimental structure determination remains challenging
as mostly inseparable isomers are measured while computational models
mostly account for a very limited number of model structures, making
the establishment of comprehensive structure–property relationships
difficult. Thus, here, we go computationally beyond the conventional
but insufficient description of eclipsed stacked COF building blocks.
We emphasize the presence of a variety of stacking and structural
motifs and investigate their impact on the electronic properties.
Given the very large size of the investigated systems, we employ the
fast and accurate tight-binding based semiempirical quantum mechanical
(SQM) GFN-xTB method. Taking an imine-linked COF as a representative
system, we discuss our structural, energetic, electronic, and spectroscopic
results calculated at the periodic GFN1-xTB quantum chemistry level
in comparison to experimental findings.