A Facile Molecular Machine: Optically Triggered Counterion Migration by Charge Transfer of Linear Donor‐π‐Acceptor Phosphonium Fluorophores

The D‐π‐A type phosphonium salts in which electron acceptor (A=‐ + PR 3 ) and donor (D=‐NPh 2 ) groups are linked by polarizable π‐conjugated spacers show intense fluorescence that is classically ascribed to excited‐state intramolecular charge transfer (ICT). Unexpectedly, salts with π=‐(C 6 H 4 ) n ‐ and ‐(C 10 H 6 C 6 H 4 )‐ exhibit an unusual dual emission (F 1 and F 2 bands) in weakly polar or nonpolar solvents. Time‐resolved fluorescence studies show a successive temporal evolution from the F 1 to F 2 emission, which can be rationalized by an ICT‐driven counterion migration. Upon optically induced ICT, the counterions move from ‐ + PR 3 to ‐NPh 2 and back in the ground state, thus achieving an ion‐transfer cycle. Increasing the solvent polarity makes the solvent stabilization dominant, and virtually stops the ion migration. Providing that either D or A has ionic character (by static ion‐pair stabilization), the ICT‐induced counterion migration should not be uncommon in weakly polar to nonpolar media, thereby providing a facile avenue for mimicking a photoinduced molecular machine‐like motion.