Fig. 3.

Concentration-dependent optical properties of encapsulated spiropyran. a UV/Vis absorption spectra of a concentrated (yellow; 5.0 mM) and diluted (blue; 0.31 mM) solution of 5⊂4 in water. b Concentration dependence of the absorbance of 5⊂4 at 345 nm (where all isomers of 5 absorb equally) and at 592 nm (where only the MC isomer absorbs). c Dilution-induced deprotonation of the MCH⁺ isomer of 5 encapsulated within 4. d Writing in a 5⊂4-soaked paper with a pen filled with pure water. Local delivery of water induces the deprotonation of 4-encapsulated MCH⁺ and consequently the appearance of a deep-blue color. The created message gradually disappears as water evaporates