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.