Covalent organic frameworks: a platform for the experimental establishment of the influence of intermolecular distance on phosphorescence

We report herein a two-dimensional (2D) COF (BZL-COF) based on an representative crystallization-induced phosphorescence (CIP) organic phosphor, benzil, for phosphorescence study. In BZL-COF, the CIP building blocks are arranged in an eclipsed fashion similar to the face-to-face mode in H-aggregates. The phosphorescence behavior significantly changed from on to off at cryogenic temperature upon altering the interlayer distance from 3.7 angstrom to 3.4 angstrom by removing the solvent molecules trapped between the 2D polymeric layers. In comparison, a single crystal of the model compound that adopts an H-type aggregation with an intermolecular distance of 4.1 angstrom can exhibit strong phosphorescence at room temperature. Our study shows that, in addition to the restriction of intramolecular rotation and H-aggregated packing, suitable intermolecular distance also plays a vital role in enhancing the phosphorescence.