Supramolecular Inclusion Complexes Based on Cucurbit[7]uril and Triazine-Bridged Viologens Displaying Near-Infrared Photochromism and Photomodulable Fluorescence

The design and construction of near-infrared (NIR) photochromic materials that show absorption and responsiveness in the NIR region remain a challenge. In this work, we prepared two triazine-bridged viologens, one being 2,4,6-tri(4-pyridinium)-1,3,5-triazine trichloride (TPA·Cl3) and the other one being its tribenzyl-substituted derivative (TPB·Br3). 1H NMR spectroscopy reveals that the cucurbit[7]uril (Q[7]) can bind with the two viologens to form quaternary inclusion complexes TPA3+@Q[7]3 and TPB3+@Q[7]3. Both inclusion complexes demonstrate NIR photochromic behavior, and the color changed from white to bluish green. Electron-spin resonance (ESR) spectra indicate that their photochromism is due to the formation of free radicals TPA•2+/TPB•2+. In addition, the inclusion complexes show photomodulable fluorescence emission. The excellent photochromic behavior and photomodulable fluorescence endow the inclusion complexes with practical utility in erasable inkless printing and anticounterfeiting.