Reversible Color Change of Chromophores in Zeolites by Direct Interaction with Alkali Metal Cations

Reversible color change of a neutral chromophore in a cationic zeolite was achieved by controlling the direct interaction between alkali-metal cations and chromophores. As a typical combination of the cationic host and the neutral chromophore, Na + -type mordenite accommodating N,N -dimethyl- p -nitroaniline (DMpNA) was prepared. The UV−vis spectra showed an extraordinarily large shift of the absorption band of DMpNA by 85 nm after the dehydration treatment of the hydrated sample. Furthermore, the absorption band was shifted reversibly by the dehydration and hydration treatments. Solid-state 23 Na MAS NMR with a cross-polarization technique revealed that DMpNA molecules were present in the neighborhood of the dehydrated Na + in the dehydrated form while the molecules were distant from 23 Na in the hydrated form. These results indicated that adsorbed water molecules controlled the strength of the interaction between Na + and DMpNA molecules.