Synthesis, optical studies and DFT analysis of benzo[b][1,4]oxazepines and thiazepines: A promising fluorescent probe for sensing of picric acid

Halogen atoms are commonly present in organic molecules and halogen functionalization is of immense significance in organic synthesis to achieve the desired material with promising biological and fluorescent properties. Herein, we report synthesis of halogenated derivatives of benzo[b] [1,4] oxazepines 1-4 and benzo[b] [1,4] thiazepines 5-8 through a simple and facile synthetic pathway.The synthesized molecules exhibit promising optical properties and excellent sensing ability with picric acid. In terms of absorption and emission studies, the halogenated derivatives of both benzo[b] [1,4] oxazepines and benzo[b] [1,4] thiazepines display red-shift for heavier halogens. The molecules synthesized have been characterized through various spectroscopic techniques such as FT-IR, 1H and 13C NMR, DR-UV and PL. One of the derivatives such as Iodine-based benzo[b] [1,4] oxazepine 4 has been evaluated for the sensing of toxic and explosive substance like picric acid through fluorescence spectroscopy in which the quenching of fluorescence takes place on incremental addition of picric acid and the LOD value of the system comes out to be 1.416 x 10-6 M-1. Further, DFT studies were carried out to calculate the HOMO-LUMO band gaps of the synthesized molecules which comes in agreement with the experimental results. Moreover, the molecular electrostatic potential maps of the synthesized molecules were evaluated using Gaussian 09 set of codes which throws light on the donor-acceptor tendency of the synthesized derivatives.