Pillar[5]arene-Based Ion-Pair Recognition for Encapsulation of a Stilbazolium-Type Dye with Enhanced Photophysical...

Constructing organic composite materials through molecular recognition has emerged as an important theme in materials science. Here we report an ion-pair recognition system involving the use of a propoxylated pillar[5]arene (PrP5) to modulate the solid-state photophysical properties of dye trans-4'-(dimethylamino)-N-methyl-4-stilbazolium hexafluorophosphate (DMASP). Single crystal X-ray diffraction analysis reveals that the dye guest DMASP is encapsulated by PrP5 to form a 2 : 1 host-guest complex 2PrP5⸧DMASP in the crystalline state. The macrocyclic skeleton of PrP5 imposes restrictions on the intramolecular motions of the dye guest, leading to a significant enhancement of its fluorescence emission. Additionally, within the 2PrP5⸧DMASP complex crystal structure, DMASP molecules are found to display two possible opposite orientations in the one-dimensional channels formed by PrP5 molecules. This arrangement is believed to alter the overall solid-state packing structure of DMASP, thereby activating its nonlinear optical activity. This work not only reports a novel ion-pair molecular recognition system based on pillararenes but also provides valuable insights into the modulation of the crystalline state photophysical properties of organic dyes via cocrystal engineering.