π‐Conjugated 2‐Arylindoles with High Quantum Yields and Tunable Spectroscopic Features

Indole‐based fluorophores represent a promising class of fluorescent probes due to their tunable optical properties, structural versatility, and compatibility with biological systems. In this work, the design, synthesis, and photophysical characterization of a new series of 2‐arylindole derivatives functionalized with (hetero)aryleneethynylene units and a pendant ester group are reported. The compounds are synthesized via Fischer indole synthesis followed by Sonogashira coupling, enabling also the introduction of electron‐donating (–NMe 2 ) and electron‐withdrawing (–CN) substituents. Furthermore, they are decorated with an N‐hydroxysuccinimidyl (NHS) moiety for facile bioconjugation. Spectroscopic studies conducted in solvents of varying polarity reveal strong solvatochromic effects, high fluorescence quantum yields (up to 94%), and emission wavelengths spanning the blue to green region. Selected derivatives are successfully conjugated to bovine serum albumin, thanks to the NHS reactive group. Photophysical measurements of the resulting protein conjugates reveal that the fluorophores remain solvent‐exposed, maintaining their emission profiles in aqueous media—an important feature for potential bioimaging and sensing applications. These findings highlight the utility of indole‐based scaffolds for the development of environmentally responsive fluorescent probes.