Fluorescent Detection of Nitroaromatics and 2,3-Dimethyl- 2,3-dinitrobutane (DMNB) by a Zinc Complex: (salophen)Zn

Fluorescent sensors for the detection of chemical explosives are in great demand. It is shown herein that the
fluorescence of ZnL* (H2L = N,N‘-phenylene-bis-(3,5-di-tert-butylsalicylideneimine)) is quenched in solution by
nitroaromatics and 2,3-dimethyl-2,3-dinitrobutane (DMNB), chemical signatures of explosives. The relationship between
the structure and fluorescence of ZnL is explored, and crystal structures of three forms of ZnL(base), (base =
ethanol, tetrahydrofuran, pyridine) are reported, with the base = ethanol structure exhibiting a four-centered hydrogen
bonding array. Solution structures are monitored by 1H NMR and molecular weight determination, revealing a
dimeric structure in poor donor solvents which converts to a monomeric structure in the presence of good donor
solvents or added Lewis bases to form five-coordinate ZnL(base). Fluorescence wavelengths and quantum yields
in solution are nearly insensitive to monomer−dimer interconversion, as well as to the identity of the Lewis base;
in contrast, the emission wavelength in the solid state varies for different ZnL(base) due to π-stacking. Nitroaromatics
and DMNB are moderately efficient quenchers of ZnL*, with Stern−Volmer constants KSV = 2−49 M-1 in acetonitrile
solution.