Realizing luminescent single crystals of covalent organic polymers with color-tunable emission through isomer engineering.

Preparing luminescent single crystals of B-N-containing covalent organic polymers (COPs) is very important but highly challenging, since most conditions only produce luminescence-silent crystals. To address this issue, we employ an isomer engineering strategy to successfully prepare luminescent (CityU-50) and luminescence-silent (CityU-60) single crystals of B-N linked COPs based on two distinct chromophore isomers, 3,3',6,6'-tetra(pyridin-4-yl)-9,9'-bicarbazole (3,6-TPy-BCz) and 2,2',7,7'-tetra (pyridin-4-yl)-9,9'-bicarbazole (2,7-TPy-BCz). Single-crystal X-ray diffraction analysis reveals that 3,6-TPy-BCz can lead to a "loose" stacking model of helical polymer chains in CityU-50, effectively suppressing interchain aggregation and endowing CityU-50 with unprecedented visible-light-excitable luminescence, while 2,7-TPy-BCz results in a "close" packing model of helical polymer chains in CityU-60, leading to a non-emissive structure. This factor validates our design principle. We also find that CityU-50 can display excitation-dependent and solvent-responsive luminescence due to the different aggregation states within the crystal and solvent-triggered packing changes in the polymer chains of CityU-50. Based on these responses, we demonstrate the application of CityU-50 in advanced information encryption. This work not only delivers a new class of smart crystalline materials but also establishes a powerful strategy for engineering crystalline polymers with precisely tailored structures and properties.