Position‐Dependent Vibronic Coupling Control in Hydrazine‐Hydrazone BODIPY Fluorophores: A Computational Framework for Rational Chemosensor Design

4,4‐Difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY)‐based “turn‐on” fluorescent probes are paramount for chemosensing, yet the mechanisms governing their photophysical switching remain incompletely understood. This study investigates hydrazine and hydrazone‐functionalized BODIPY derivatives to elucidate the molecular origins of fluorescence enhancement upon reaction with formaldehyde. Using spin‐flip time‐dependent density functional theory, we analyzed derivatives substituted at the 1‐, 2‐, 3‐, and 8‐positions. Our findings overturn the prevailing twisted intramolecular charge transfer (TICT) hypothesis. Unlike the TICT model, which attributes quenching to large‐scale geometric rotation, we establish that fluorescence switching is predominantly controlled by vibronic coupling modulation in the Franck–Condon region. Specifically, hydrazone formation suppresses direct S 1 → S 0 internal conversion by decreasing reorganization energies and increasing activation barriers, independent of the twisted geometries typically associated with TICT. Crucially, we reveal profound position‐dependent effects: substitution at the 2‐position yields the highest fluorescence enhancement (600‐fold) via exceptional intramolecular charge transfer character, while the 3‐position affords the highest quantum yield (Φ = 0.95) suitable for high‐brightness applications. These results provide definitive mechanistic insight and establish a robust computational framework for the rational design of next‐generation chemosensors with optimized sensitivity and brightness.