2.B.150.  The Light-driven Anion-translocating Molecular Machine (LAMM) Family

New designs of light-driven anion-translocating molecular machines that do not involve conformational changes have been described (Qu et al. 2025). In the designed structures, the dramatic redistribution of positive charge from the electron acceptor to the donor moiety in the dipolar cation dye is driven by excited-state intramolecular charge transfer (ESICT). This shifts the anion binding site to the opposite side of the molecule, facilitating a fast and directional ion motion. The continuous reversible cycle arises from the fact that the forward motion occurs during the excited-state lifetime on the high-energy potential energy surface, whereas the reverse reaction proceeds on the ground-state potential energy surface. Thus, the light quanta not only provide the energy source but also serve as the factor that drives the ion in the specified direction.The unexpected observation about the anomalous dual-emission behavior of various phosphonium and pyridinium salts in nonpolar solvents has prompted the proposal of such a photoinduced counterion migration mechanism.

Unlike the ultrafast ESICT process, which occurs on a subpicosecond time scale, the appearance of a strongly Stokes-shifted emission band ─ attributed to anion translocation ─ is observed over tens to hundreds of picoseconds. Furthermore, it was shown that the increase in ion radius results in the retardation of anion motion, which can be adequately explained by the proposed mechanism. The interpretation of ion motion as a relaxation process toward electrostatic equilibrium is supported by the observed monoexponential decay of the spectral response function C(t) that is commonly used to describe the dynamics of solvent relaxations. Based on C(t) analysis, the dependence of the motion rate on the temperature and solvent viscosity demonstrated the absence of significant energy barriers during the process. Through structural modification of functional groups, the appended photoinduced intramolecular proton-transfer group anchored on the donor side enhances the efficiency of ion translocation.

Qu et al. 2025 summarized recent reports on photoinduced counterion migration and highlight its potential for enabling transmembrane ion transport. Although challenges in future practical applications still need to be addressed, the core principle of modulating the directionality of anion migration along the dipolar cationic backbone via ESICT offers a promising opportunity for a concise and general design strategy for molecular machines that simulate the active translocation of ions in biological systems.

Compared with natural membrane proteins, artificial ion transport systems across membranes have advantages in terms of structural simplicity, stability and cost-effectiveness. The review by Liu et al. 2025 outlines recent advances in artificial molecular machines to mimic the ion transport function of natural membrane proteins and achieve efficient and selective transmembrane transport. Different types of molecular machine-based transmembrane ion transport systems, such as unimolecular transmembrane ion transport systems, polymer transmembrane ion transport systems and supramolecular transmembrane ion transport systems, are analyzed to explore their potential applications in lipid bilayers. Subsequently, the challenges and future directions of molecular machines in regulating transmembrane ion transport are discussed (Liu et al. 2025).