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&z.dbd;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, 1 H 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. The structure-property relationships for the hydrazide-hydrazone derivatives were investigated to provide new insights into the design of photo-responsive materials.