CHCl3-Dependent Emission Color and Jumping Behavior of Cyclic Chalcone Single Crystals: The Halogen Bond Network Effect

Abstract

As a conventional strategy to modulate the structure and properties of inorganic single crystals, hydration/solvation is rarely found to function in pure organic single crystal. Herein, we report chalcone single crystals with CHCl3-dependent emission color and jumping behavior. Two crystals: a pure crystal phase (1) with green-yellow emission and a CHCl3-containing co-crystal phase (1•2CHCl3) with orange-red emission were constructed by fine-controlling the crystallization conditions. The special halogen bond network in the crystal packing structure effectively narrows the bandgap and thereby redshift the emission of 1•2CHCl3. 1•2CHCl3 would revert to 1 together with emission color change when losing CHCl3. These findings are similar to the effect of H2O in hydrated inorganic crystals. Notably, owing to a special pre-organized “molecular pair” structure for [2 + 2] cycloaddition, the pure crystal phase 1 exhibits violently photo-induced jumping phenomenon, indicating large potentials in intelligent devices. This work would overturn the previous perception that the structurally simple solvent molecules without conjugation cannot greatly affect the structure and properties of pure organic single crystals and provide a new strategy to construct multi-colored organic fluorescent crystals.

Recent advances in photoresponsive fluorescent materials based on [2+2] photocycloaddition reactions

“…19b). 109 A pure crystalline phase (1) with greenish-yellow emission and a CHCl 3 -containing eutectic phase (1–2CHCl 3 ) with orange-red emission were designed by incorporating the D–A units. Notably, the [2+2] photocycloaddition reaction of the pure crystalline phase ( 1 ) exhibited a strong light-induced jumping behaviour due to the molecular stacking effect.…”

Halogen effect in photomechanical molecular crystals

“…4e). 97 In this crystal, chlorine atoms may interact with the lone pair electrons of nitrogen in the triphenylamine moiety (C-ClÁ Á ÁN, 3.993 Å). Meanwhile, the intermolecular hydrogen bonds (C-HÁ Á ÁO, C-HÁ Á ÁCl), halogen bonds (C-ClÁ Á ÁN, 3.993 Å) and p-p interactions are responsible for the similar molecular packing in the crystal of 7 to that of 6, and the distance of the ''olefin pair'' was 3.622 Å.…”

“…As can be seen from the crystalline structure, hydrogen bonds involving fluorine atoms are abundant. It was suggested that the contraction along the long axis at the beginning of UV irradiation led to the bending toward the light source and the reaction gradient with a prolonged 97 irradiation time caused the crystal to swing (Fig. 5e).…”

“…96 (e) Microscopic images of thin flakes of the crystal of 7 showing photo-induced jumping and splitting. Reproduced from with permission from MDPI 2021 97.…”