Redox-Gated\nOptical Modulation of Coumarin-Triphenyliminophosphorane\nFluorophores

Novel coumarin-triphenyliminophosphorane (TPIPP) fluorophores,\nsynthesized via a nonhydrolytic Staudinger reaction, exhibit remarkable\nredox-responsive optical properties. Upon chemical and electrochemical\noxidation, these compounds display a hypsochromic shift in absorption\nfrom 430 to 350 nm, accompanied by up to 11-fold fluorescence enhancement\nunder 405 nm excitation. The fluorescence switching occurs at an electrochemical\noxidation potential of approximately +2.0 V. This enhanced one-photon\nexcited fluorescence is attributed to an emissive radical effect,\nstemming from in situ generated radical cations at the polarizable\niminophosphorane (P=N) bond. The radical formation was confirmed by\ntrapping experiments using tetracyanoquinodimethane, which produced\ncharacteristic absorption of radical anions around 850 nm, and by\nelectron spin resonance studies using 5,5-dimethyl-1-pyrroline <i>N</i>-oxide as a spin trap. Conversely, two-photon excited fluorescence\nunder 800 nm pulsed laser excitation decreases upon oxidation, likely\ndue to reduced two-photon absorption resulting from altered π-conjugation.\nThis work demonstrates that external redox modulation can induce significant\nchanges in absorption profiles and enable switching between enhanced\none-photon and diminished two-photon excited fluorescence, highlighting\nthe potential of leveraging the controllable radical character of\nthe P=N bond in designing redox-responsive fluorophores.