Multi-state Photoswitching in Thienoquinoid-Based Fluorescent Trithiophenes

Achieving multi-site photoisomerization within a single π-conjugated scaffold while retaining luminescent functionality remains a formidable challenge, as electronic coupling between isomerizable units typically promotes intramolecular energy transfer that suppresses sequential switching, and the additional nonradiative pathways inherent to multiple isomerizable bonds further impede emissive behavior. Herein, we report a methine-bridged trithiophene bearing 2,6-dichlorophenyl substituents, in which two C═C double bonds embedded in a thienoquinoid framework undergo photoisomerization to afford three interconvertible geometrical isomers (ZZ, EZ, and EE). The steric constraint imposed by the ortho-chloro substituents suppresses nonradiative torsional relaxation, yielding a fluorescence quantum yield of 0.40 for the ZZ-isomer in cyclohexane, which decreases upon photoisomerization to the EZ- and EE-isomers-a significant enhancement over the corresponding derivative lacking ortho-substituents on the aryl groups, accompanied by a markedly reduced Stokes shift. The photostationary state composition varies systematically with the irradiation wavelength, primarily reflecting differences in the molar absorption coefficients of the three isomers, thereby enabling wavelength-selective multi-state control over isomer distributions inaccessible under thermodynamic equilibrium. Fluorescence lifetime measurements revealed isomer-dependent excited-state lifetimes, confirming that molecular geometry governs the photophysical properties within this conjugated framework. This study establishes a molecular design strategy for luminescent multi-state photoswitches applicable to multi-level optical information processing.