Conventional responsive fluorescent probes often suffer from high background signals, laborious multi-step synthesis, and limited photophysical tunability, which constrain their practical applications in biosensing. Developing a generalizable platform that integrates facile synthesis, efficient quenching, and modular target responsiveness is urgently needed. Herein, we address this challenge by exploring N-substituent electronic modulation on tricyclic boronic ester hemicyanine (TBEH) dyes as a unified strategy for constructing high-performance turn-on NIR probes. Through N-substituent engineering, electron-withdrawing groups (EWGs) effectively quench fluorescence via a photoinduced electron transfer (PET) mechanism, while analyte-triggered conversion to electron-donating species or elimination restores emission. Three probes were developed for hypoxia (2d), glutathione (2f), and bioorthogonal (2g) detection, exhibiting 130-, 54-, and 13-fold fluorescence turn-on with detection limits of 16, 90, and 33 nM, respectively. All probes showed rapid response (20-40 min), excellent selectivity, and good biocompatibility in live-cell imaging. DFT calculations confirmed that EWGs lower acceptor LUMO energies to enable PET quenching, and target interactions reverse this process to recover fluorescence. This work establishes a universal TBEH-based fluorescence analysis platform that combines synthetic simplicity, electronically tunable photophysical properties, and high sensitivity. The N-substituent modulation strategy offers a straightforward blueprint for developing diverse responsive NIR probes for real-time biosensing and theranostic applications.