Redox-Gated Optical Modulation of Coumarin-Triphenyliminophosphorane Fluorophores

Novel coumarin-triphenyliminophosphorane (TPIPP) fluorophores, synthesized via a nonhydrolytic Staudinger reaction, exhibit remarkable redox-responsive optical properties. Upon chemical and electrochemical oxidation, these compounds display a hypsochromic shift in absorption from 430 to 350 nm, accompanied by up to 11-fold fluorescence enhancement under 405 nm excitation. The fluorescence switching occurs at an electrochemical oxidation potential of approximately +2.0 V. This enhanced one-photon excited fluorescence is attributed to an emissive radical effect, stemming from in situ generated radical cations at the polarizable iminophosphorane (P=N) bond. The radical formation was confirmed by trapping experiments using tetracyanoquinodimethane, which produced characteristic absorption of radical anions around 850 nm, and by electron spin resonance studies using 5,5-dimethyl-1-pyrroline <i>N</i>-oxide as a spin trap. Conversely, two-photon excited fluorescence under 800 nm pulsed laser excitation decreases upon oxidation, likely due to reduced two-photon absorption resulting from altered π-conjugation. This work demonstrates that external redox modulation can induce significant changes in absorption profiles and enable switching between enhanced one-photon and diminished two-photon excited fluorescence, highlighting the potential of leveraging the controllable radical character of the P=N bond in designing redox-responsive fluorophores.