Substituent effects on the electronic structures and fluorescent properties of salicylaldehyde acylhydrazone probes:...

Salicylaldehyde acylhydrazone fluorescent probes exhibit excellent optical properties, making them valuable for fluorescence analysis and environmental monitoring. In this work, we investigate four representative probes (SBH, SSH, SNH, SNH-OH) to elucidate the substituent effects on their electronic structures and photophysical behavior. Our calculations reveal that all free probes are non-fluorescent due to an efficient non-radiative decay pathway mediated by rotation around the hydrazone bond, which leads to a conical intersection. Crucially, the introduction of an ortho-hydroxyl group enhances molecular planarity, while extending π-conjugation with a naphthalene ring significantly red shifts the absorption and emission. Upon binding with Al³⁺, the hydrazone bond rotation is restricted, effectively blocking the nonradiative pathway and triggering "turn-on" fluorescence response. This work provides theoretical insights into how subtle structural modifications can precisely modulate the excited-state properties and thereby control the fluorescence of acylhydrazone-based sensors.