Linkages Make a Difference in the Photoluminescence of Covalent Organic Frameworks

Covalent organic frameworks (COFs) with structural designability and tunability of photophysical properties enable them to be a promising class of organic luminescent materials by incorporating well-designed fluorescent units directly into the periodic skeletons. The photophysical properties of COFs are mainly affected by the structural features, which determine the conjugation degree, charge delocalization ability, and exciton dynamics of COFs. To understand the relationship between COF structures and their photophysical properties, two COFs with the same pyrene chromophore units but different linkages (imine or vinylene) were designed and synthesized. Interestingly, different linkages endow COFs with huge differences in solid-state photoluminescence quantum yield (PLQY) for imine- and vinylene-linked pyrene-based COFs, which possess PLQY values of 0.34 % and 15.43 %, respectively. The femtosecond-transient absorption spectra and time-dependent density functional theory reveal the different charge-transfer pathways in imine- and vinylene-linked COFs, which influence the exciton relaxation way and fluorescence intensity. In addition, an effective white-light device was obtained by coating the vinylene-linked COF on a light-emitting diode strip. Different linkages endow COFs with huge differences in solid-state photoluminescence quantum yield for imine-linked and vinylene-linked pyrene-based COFs. Different charge-transfer pathways in imine-linked and vinylene-linked COFs influence the exciton relaxation way and quenching of fluorescence.+image

Automated summary

Covalent organic frameworks (COFs) show promise as organic luminescent materials due to their structural designability and tunability of photophysical properties. The photophysical properties of COFs are influenced by their structural features, which dictate the conjugation degree, charge delocalization ability, and exciton dynamics. By comparing two COFs with the same pyrene chromophore units but different linkages (imine or vinylene), significantly different solid-state photoluminescence quantum yields (PLQYs) were observed: 0.34% for imine-linked COF and 15.43% for vinylene-linked COF. The differences in charge transfer pathways between the two linkages influence exciton relaxation and fluorescence intensity. Additionally, a white-light device was successfully obtained by coating the vinylene-linked COF on a light-emitting diode strip.