Smart, chiral, and nonconjugated cyclohexane-based bis-salicylaldehyde hydrazides: multi-stimuli-responsive, turn-on, ratiometric, and thermochromic fluorescence, single-crystal structures via DFT calculations

A unique series of smart, chiral, and multi-stimuli-responsive cyclohexane-based bis-salicylaldehyde hydrazides have been designed and prepared. Their molecular structures, alignments, and fluorescence properties are highly dependent on the environmental conditions. They show unusual polar-solvent-, cation-, anion-, and amino-acid-induced fluorescence enhancements with emission quantum yield of up to 0.40. Based on single-crystal structures and multi-molecule density functional theory calculations, it was shown that polar-solvent-induced fluorescence can be caused by the intermolecular hydrogen bonds that would affect the molecular structure, charge distribution, and transition process. Cation-and anion/amino-acid-induced fluorescence are caused by coordination and deprotonation, respectively. Through cation/anion exchange reactions, ratiometric fluorescence along with extending the detection range for Mg2+ has been achieved for the first time. Thermochromic fluorescence from white into blue would be observed when the temperature is increased from 77 K to room temperature. Moreover, these nonconjugated materials with lower cytotoxicity and better biocompatibility have potential applications in cell imaging and chiral recognition.