The Origin of Dual Emission in Antiparallel-Stacked Two-Dimensional Covalent Organic Frameworks

Most
previous reports on covalent organic frameworks (COFs) are
centered on eclipsed (or presumably eclipsed) structures, while the
properties peculiar to the antiparallel stacked structure are little
known. Recently, we discover that two-dimensional (2D) acylhydrazone
COFs with alkyoxy side chains adopt an antiparallel stacked structure.
This provides us with the opportunity to re-examine the origin of
dual emission in such COFs in the context of an antiparallel stacked
structure, in lieu of an earlier proposal based on incorrectly assigned
eclipsed stacked structure. We discover that the intralayer and interlayer
hydrogen bonding in the antiparallel stacked structure work synergistically
to restrict intermolecular bond rotation, thus turning on fluorescence.
The interlayer hydrogen bonding between the acyl oxygen and the side-chain
hydrogen is stronger in the dual emissive COF, compared to those in
the single emissive ones, underscoring the roles of competing interhydrogen
and intrahydrogen bonds in the excited-state conformers.