Alkyl Chain-Tuned Fluorescent Sensors: Phenanthroimidazole Derivatives for Selective Detection of Nitroaromatic...

In this paper, a series of phenanthroimidazole derivatives 2-(1 H-indol-3-yl)-3a,11b-dihydro-1 H-phenanthro[9,10-d]imidazole(NZ1), 1-hexyl-2-(1 H-indol-3-yl)-3a,11b-dihydro-1 H-phnanthro[9,10-d] imidazole (NZ2) and 1-hexyl-2-(1-hexyl-1 H-indol-3-yl)-1 H-phenanthro[9,10-d] imidazole (NZ3) were synthesized for selective sensing of nitroaromatic compounds used in explosives. The structure was confirmed through nuclear magnetic resonance (NMR) spectroscopy, infrared and single crystal x-ray diffraction data. To ensure the molecules suitability for sensing application, several photophysical characterization were performed. Though these molecules differ structurally with different substitution of alkyl chains, the absorption spectra remain approximately same which signifies no significant influence of alkyl chain addition. The molecules (NZ1-NZ3) are fluorescent in blue region with peak emission wavelengths at 412 nm, 397 nm, and 395 nm respectively, the variation in emission wavelength suggest the contribution of charge transfer state, due to the fact that indole here work as a donor and phenanthrene imidazole as an acceptor moiety. The Crystal structure of NZ1-picric acid complex was used as a base model to understanding the mechanism of fluorescence quenching. Time-resolved fluorescence measurements reveal that the quenching is dynamic. The decrease in LOD from NZ1 to NZ3 with respect to alkyl substitution suggest that the alkyl chain enhance the sensing ability of the compound probably by increasing solubility and interaction with solvent and nitroaromatic and making NZ3 as an efficient sensor.