Herein, we have designed and synthesized an effective fluorogenic chemosensor, BTCP for the sensing of DCP, a very low toxic simulant of sarin both in the vapour and solution phases. The poor emission property (λ_max = 470 nm, quantum yield (φ) = 0.051) of BTCP is significantly increased in the presence of DCP with a red shift of the emission wavelength (λ_max = 542 nm, quantum yield (φ) = 0.241) in CH₃CN/H₂O (1/1, v/v, pH = 7.2) medium. The limit of detection is found to be as low as 15.8 nM for the detection of DCP. Moreover, the fluorescent probe (BTCP) is very useful for the visualization of DCP within human lung cancer cell lines (A549). Sensing of the probe is based on the formation of a new adduct, BTCP-DCP and inhibition of photo-induced electron transfer (PET) process present in the free BTCP. The structure of the probe (BTCP) is confirmed by single crystal X-ray crystallography. Density functional theory (DFT) and time dependent density functional theory (TDDFT) computational techniques are used to interpret the electronic structure and Uv–vis spectra of both BTCP and BTCP-DCP adduct.