Rational Design of Water-Soluble Dipyrroethene-Based Red-Light Emissive Chromophores for Rapid and Selective...

Design and synthesis of water-soluble π-conjugated chromophores are one of the significant challenges due to the intrinsic hydrophobicity of the conjugated frameworks. Consequently, rational molecular design to access novel water-soluble biologically relevant organic chromophores with red-to-NIR emission, large Stokes' shifts, and good fluorescent quantum yields always attracts significant attention. Here, we establish a straightforward synthetic strategy to access a novel class of water-soluble red-light-emissive (up to 645 nm) cationic dipyridyl dipyrroethene derivatives, achieved through strategic engineering of the (E)-dipyrroethene (DPE) core. The X-ray structures of cationic species 8 revealed a very interesting supramolecular nanoarchitectonics. These sets of molecules are showcasing a fluorescence in both the solid state and solution phase across a wide range of solvents, including water, with a decent fluorescent quantum yield and a large Stokes' shift. Detailed spectroscopic and computational studies support that the balanced intramolecular charge transfer (ICT) leads to a redshift in its absorption/emission. We further expanded the applicability of the water-soluble chromophore 10 as a rapid, selective, and effective fluorescent-turn-on sensor for recognition of serum albumin proteins with a strong binding constant of 2.7 × 10⁶ M^-1. This work provides a fundamental platform for designing robust, bio-responsive water-soluble π-conjugated systems with tenable structural and electronic properties.