β‐Ketonitrile‐Modified Prodan Derivatives as Large‐Stokes‐Shift Solvatochromic Fluorophores With Enhanced Brightness and Ratiometric Thermometric Response

We report a systematic investigation of β‐ketonitrile–modified Prodan‐type solvatochromic fluorophores as an alternative to conventional π‐extension. A series of donor–π–acceptor dyes incorporating a β‐ketonitrile acceptor and a unified piperidyl donor and compared with the corresponding acetyl analogues. The β‐ketonitrile substitution produced pronounced bathochromic shifts in both absorption and emission, together with substantial enhancement of fluorescence quantum yields ( Φ fl ) in nonpolar solvents, effectively overcoming the low‐ Φ fl limitation of Prodan. These improvements are attributed to reduced intersystem crossing arising from destabilization of the n–π* triplet manifold. The dyes also exhibit markedly increased molar absorption coefficients at 405 nm, facilitating efficient excitation at 405 nm. In polar media, polarity‐dependent formation of TICT states or promotion of internal conversion modulates the emission response, with the π‐extended dye FπAc‐CN displaying exceptionally strong positive temperature sensitivity from −10 to 75°C, functioning as a ratiometric fluorescent thermometer. In membrane‐mimetic lipid bilayers, the naphthalene and fluorene derivatives ( NAc‐CN , FRAc‐CN ) showed significantly larger phase‐dependent λ fl shifts than Prodan or Laurdan, exhibiting enhanced sensitivity to membrane‐mimetic polarity environments. These results establish β‐ketonitrile substitution as a powerful design principle that achieves large‐Stokes‐shift emission, enhanced brightness, temperature responsiveness, and phase‐dependent fluorescence response in model membranes.