Emissive Covalent organic Frameworks: Hydrogen bond switch-driven metal ion sensing and Cellular imaging

Covalent organic frameworks (COFs) with highly conjugated it skeletons hold great promise for developing emissive materials. However, designing tunable and highly emissive COFs remains a significant challenge. In this study, we introduce hydrazone-linked COFs (HL-COF and HL-COF-OH) that exhibit exceptional and tunable emission properties through hydrogen bond interactions in their frameworks. Notably, the emission wavelengths of HL-COF and HL-COF-OH can be tuned from 473 nm (blue) to 536 nm (yellow) in water. HL-COF-OH demonstrates superior sensitivity and selectivity for metal ion sensing compared to HL-COF, owing to the presence of open hydrogen bonds. The detection limit for Cu2+ was reduced to 208 nmol, positioning it among the bestperforming COFs reported to date. Additionally, these emissive COFs successfully detected Cu2+ in cells without causing cytotoxicity, marking a significant milestone in the field. This strategy opens new avenue for designing color-tunable emissive materials for biological applications.