Detection of chemical explosives with zinc (salicylaldimine) complexes: Mechanism and applications
The detection of chemical explosives is an important problem that poses unique challenges for chemists. Common explosives such as nitro-organics and peroxides lack chromophores, and differ greatly in chemical structures and properties. A family of Zn(salicylaldimine) (ZnL) complexes were prepared, and investigated for fluorescence sensory applications. ZnLs are strong fluorophores, Φ = 0.3, with sub-nanosecond lifetimes and highly reducing excited states. The fluorescence of ZnL is effectively quenched by both nitroaromatics and nitroalkanes. Quenching occurs via photoinduced electron transfer from the phenolate ring of ZnL, creating a phenoxyl radical species that was observed with EPR.
ZnL was synthetically varied with electron withdrawing and donating substituents, and Stern-Volmer quenching experiments revealed mixed quenching pathways, depending upon the steric bulk of the substituent. The energetic contributions from the electron withdrawing or donating substituents changed the driving force for electron transfer, and high steric bulk favored dynamic quenching over the static pathway. The kinetics of dynamic quenching were treated with Marcus theory, predicting a modest reorganization energy, λ = 24 kcal/mol, governed by solvent effects. A sensor array was formulated for the discrimination of structurally similar nitro-organic compounds. Fingerprint patterns were generated for each quencher based on the unique interactions with each ZnL. Using statistical analysis, 100% of unknown samples were accurately identified.
Solid state structural investigations reveal a penta-coordinate Zn with solvent bound axially. The axial ligand, EtOH, THF and pyr, influenced the degree of π-stacking in the unit cell, and shifted the solid state λ em. Preliminary investigations into solid state sensors involved formation and characterization of polycrystalline structures, ZnL doped thin films, and ZnL polymers.
Fluorescence turn-on methods are highly sensitive compared to fluorescence quenching. A fluorescence turn-on sensor was developed for peroxide based explosives using the oxidative deboronation of a masked prochelator to form H2L, which chelated Zn2+ and produced fluorescent ZnL. The rate of fluorophore formation was limited both by peroxide concentration and structure of the diamine backbone of the prochelator. Limits of detection for H2O2, benzoyl peroxide and triacetone triperoxide, a highly energetic peroxide-based explosive, were below 10nM in solution.
0490: Organic chemistry