Ion Translocation Driven by Electric Field Generated in Excited-State Reactions

Abstract

The fundamental mechanism of ion translocation against the concentration gradient in biological systems has become a central focus of research. The variation of the electric field in response to external stimuli can be an essential trigger in this process. The introduction of molecular machines has enriched this field by providing a direct approach to converting energy into mechanical work. However, existing models mainly rely on photoisomerization dynamics that alter the location of ion-carrying molecular segments to achieve transportation. In a recent series of works, we present a new design of light-driven anion-translocating molecular machines that do not involve any conformational changes. 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 mechanism we proposed. 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.

In this Account, we briefly summarize 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.