Acylhydrazone-based reversibly photoswitchable ion pair transporter with OFF–ON cotransport activity

Abstract

The cellular membrane transport of physiologically important cations and anions is omnipresent and regulates different physiological functions. Whereas a notable number of cation–anion transporters are being developed to transport salts across the membrane, developing an artificial cation–anion symporter with stimulus-responsive activities is an immense obstacle. Herein, for the first time, we report reversibly photoswitchable acylhydrazone-based transporter 2 that has distinctive OFF–ON cation–anion co-transport abilities. The substituent was modified in 1a–1c and 2, to change the to-and-fro movement of the transporter to enhance the ion transport efficiency. Ion transport experiments across the lipid bilayer membrane demonstrate that 1a has the highest transport activity among the series with irreversible photoisomerization properties, whereas 2 has a unique reversible photoisomerization property. A detailed transport study indicated that the E-conformer of compound 2 facilitates Na⁺/Cl⁻ transport via the symport process by following the carrier mode of ion transport. ²³Na NMR and chloride selective electrode assays confirmed the OFF and ON state of ion transport of compound 2 with photoirradiation. An assembly of [(2_E)₂ + NaCl] was subjected to geometry optimization to understand the responsible ion binding motif. Geometry optimization followed by the natural bond orbital analysis of 1a_Z and 2_Z demonstrated that 1a_Z forms comparatively stronger intramolecular H-bonding than 2_Z, making it accessible for reversible photoisomerization.