Local Excitation Dominance Governs the Photophysics of Hemi-Cyanine Dyes: Structural Modulation and Fluorescence Sensing of Butyrylcholinesterase

Balancing red-shifted emission and high luminescence efficiency remains a central challenge in developing near-infrared (NIR) fluorophores. Conventional charge-transfer (CT) designs can effectively red-shift emission; however, excessive spatial separation of electron density often leads to fluorescence quenching and low brightness. A synergistic approach involving donor modification is reported, π-bridge engineering, and ionic modulation to construct three series of D–π–A hemi-cyanine dyes with Local Excitation (LE)-dominated excited states. The optimized dyes exhibit NIR emissions up to 802 nm (ε_max × Φ = 1716 L mol⁻¹ cm⁻¹) and ultrahigh brightness reaching 10 799 L mol⁻¹ cm⁻¹ at 735 nm. Both theoretical calculations and photophysical analyses consistently indicate dominant local excitation (LE) character, evidenced by limited spatial separation between hole and electron distributions (Sr, D, t index) and negligible dependence of emission maxima on solvent polarity (E_T(30)). This LE-preserving excited-state design enables red-shift extension without compromising fluorescence output. Moreover, the retained reactive hydroxyl site allows straightforward functionalization, leading to two “turn-on” probes for selective detection of butyrylcholinesterase. This work establishes a structure–excited-state–function design paradigm that achieves both spectral tunability and bio-responsive functionality, providing a new direction for constructing bright, LE-based NIR fluorophores for biological applications.