Boosting the brightness of aggregation-caused quenching chromophore-based covalent organic frameworks via energy level matching strategy

Most chromophores exhibit an aggregation-caused quenching (ACQ) effect, resulting in minimal or weak emission when incorporated into covalent organic frameworks (COFs). Consequently, the development of luminescent COFs, particularly those utilizing ACQ chromophores, presents both a compelling and formidable challenge. Herein, we report the construction of highly emissive ACQ chromophore-based COFs by an energy level matching strategy. This approach effectively modulates the electronic structures within the COFs, thereby suppressing intralayer charge transfer to ACQ chromophores, such as the imine linkage and dibenzo[g,p]chrysene unit, while simultaneously inhibiting interlayer charge transfer and non-radiative decay pathways. As a result, the undesired ACQ effect is mitigated, enabling the realization of highly emissive COFs. Notably, COF-BT-PhDBC demonstrates high brightness with a solid-state photoluminescence quantum yield of 14.7%, standing among the best ACQ chromophore-based COFs. Furthermore, COF-BT-PhDBC has been successfully utilized for in vivo three-photon fluorescence imaging of murine brain vasculature, achieving an impressive imaging depth of approximately 800 µm.