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.