Synthesis, photochemical isomerization and photophysical properties of hydrazide-hydrazone derivatives

Hydrazide-hydrazone derivatives have been utilized in molecular switches, sensing, metallo assemblies, drugs and other applications. The hydrazide-hydrazone functional group contains the -CO-NH-N = CH- moiety that yields the hydrazide-hydrazone group its physical, chemical and biochemical properties; however, the application in security protection has not been explored before. Herein, the 2-(ortho-), 3-(meta-), 4-(para-) substituted pyridine conjugated to 2,6-dihydroxybenzoic acid, which containing hydrazide-hydrazone as photoswitch linker were designed and synthesized (3a-3c). Interestingly, when irradiating the ortho-substituted pyridine of molecule 3a with a light of 365 nm, obvious emission intensity enhancements were detected. However, 3a-3c before irradiation and 3b-3c after irradiation show no fluorescence. UV-vis, fluorescence, H-1 NMR and Fourier transform infrared spectra and density functional theory calculations were carried out, which revealed that the hydrazide-hydrazone derivative of 3a could undergo a characteristic E to Z photoisomerization after light irradiation for 365 nm. The Z isomer of 3a may have additional intramolecular hydrogen bonds to restrict the motions of the molecule, thus increasing the fluorescence intensity of 3a. The experimental and molecular calculation results agreed and thus demonstrated the first example of invisible ink based on the hydrazide-hydrazone motif.