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=-⁺PR₃) and donor (D=-NPh₂) 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=-(C₆H₄)_n- and -(C₁₀H₆C₆H₄)- exhibit an unusual dual emission (F₁ and F₂ bands) in weakly polar or nonpolar solvents. Time-resolved fluorescence studies show a successive temporal evolution from the F₁ to F₂ emission, which can be rationalized by an ICT-driven counterion migration. Upon optically induced ICT, the counterions move from -⁺PR₃ to -NPh₂ 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.