Crown ethers with spiropyran units incorporated in the ring frameworks for pH-triggered ion recognition at the air-water interface

The stimuli-responsive macrocycles are smart materials of high interest. Crown ethers with the ring frameworks enlarged with one and two spiropyran units were synthesized in this work, whose ring conformation, ring sizes and charge states could be responsively altered. The cyclic spiropyran derivatives changed from neutral SP form to zwitterionic ME form at pH < 7. The cyclic spiropyran derivatives in either SP or ME form formed stable Langmuir monolayers on the water surface. The monolayer of cyclic spiropyran derivatives in the SP form showed recognition of specific metal ions according to analysis of the surface pressure-molecular area isotherms, whereas the cyclic spiropyran derivatives in ME form lost the ability of specific recognition based on size-fitting complexation. The stimuli-responsive macrocycles hold promising potential to act as responsive molecular gates and catalysts, to mimic ion channels of bio-membranes, etc.