Inclusion Complexes of Cyclodextrins with 1-(4-Carboxybenzyl)-4-[2-(4-pyridyl)vinyl]pyridinium Chloride: Photochromism, Erasable Inkless Printing, and Color Tuning

The formation of 1:1 and 2:1 inclusion complexes between the hosts cyclodextrins (α-, β-, and γ-CD) and the guest 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)vinyl]pyridinium chloride (guest CPP+·Cl–) was demonstrated by NMR spectroscopy. The inclusion complexes CPP+@α-CD, CPP+@β-CD and CPP+@γ-CD2 display photochromic properties from pale yellow to red in the solid state. The electron-spin resonance (ESR) spectroscopic data of the CPP+@γ-CD2 suggest that the photochromic behavior arises from the generation of the free radicals. The CPP+@γ-CD2 can be easily coated on the filter paper and used in erasable inkless printing and anticounterfeiting. Most importantly, the mixture of CPP+@γ-CD2 and 1+@β-CD (an inclusion complex previously reported, whose color after irradiation was blue) also can be coated on the filter paper, and the color of printing on the filter paper is tunable by controlling the mass proportions of CPP+@γ-CD2 and 1+@β-CD.