Acid-Responsive Inks via Shuttling in a Pseudorotaxane Complex

Pseudorotaxanes are host-guest complexes where a guest molecule threads through a ring-shaped host via noncovalent interactions. If the guest has two recognition sites, the host can dynamically shuttle between them. These complexes enable stimulus-responsive molecular shuttles, where a stimulus, such as pH change, can control the position of the host, affecting properties like color and solubility. In this study, a guest molecule (Gc) with two recognition sites-1,4-dialkoxyphenylene and pH-sensitive 1,5-diaminonaphthalene-is synthesized. These sites interact strongly with cyclobis(paraquat-p-phenylene) (CBPQT^{4 +}), a cationic host. CBPQT^{4 +} binds to the neutral diaminonaphthalene group to produce a green solution. Upon protonation of the diaminonaphthalene group, the host shifts to the dialkoxyphenylene site, turning the solution light orange. This color-switching behavior remains stable over multiple protonation-deprotonation cycles. The pseudorotaxane complex can also be disassembled via slow solvent diffusion, allowing recovery of the Gc and CBPQT^{4 +} components. Additionally, cellulose nanocrystal films incorporating the Gc⊂CBPQT^{4 +} complex show similar green-to-orange color shifts, demonstrating their potential for information encryption applications.