Merocyanine-Linked Zwitterionic Covalent Organic Frameworks

Merocyanines are classic π-conjugated systems with significant zwitterionic character and distinct optoelectronic properties. Here, we report the synthesis of the first merocyanine-based C = C-linked COFs through the condensation of N-alkylcollidinium salts with hydroxytrimesic aldehyde. These zwitterionic COFs are chemically stable, with a pronounced charge-transfer absorption band to ∼850 nm. Strong dipolar interactions within the COFs result in pore contraction/expansion in response to the medium polarity, manifested in a macroscopic expansion and an increased surface area in a high dielectric constant medium. The COFs can reversibly transition between a black neutral and a yellow cationic form upon protonation. Density functional theory calculations suggest an "omnidirectional" π-conjugation with large band dispersions. Thin films of EtMER-COF prepared by heterogeneous nucleation on glass/ITO substrates reveal a vertical orientation of its 2D sheets. Time-resolved spectroscopy of the thin film suggests the presence of stimulated emission as well as a long-lived excited state.