New Electron Donor Acceptor Carbazole-Based Fluorescent Chromophores: Synthesis, Molecular Modelling and Antimicrobial Activity.

The synthetic methodology of the desired chromophores, benzothiazolyl-carbazole-cyanoacetanilide 6a-c, was finally involved a condensation reaction of 6-benzothiazolyl-9-hexyl-9H-carbazole-3-carbaldehyde 4 with three distinct substituted cyanoacetanilide compounds 5a-c. In DMSO, the synthesized chromophores' absorption and emission spectra presented valuable Stokes' shift ( $\Delta \overline{\nu }$ = 4782-3680 cm^-1). The DFT calculations of DMSO solvated ground and excite states (S_o and S₁) of derivatives 6a-c showed that the benzothiazole and N-phenyl cyanoacrylamide served as electron releasing and receiving portions, respectively, and HOMO-LUMO charge-transfer (CT) could be denoted as donor-π-acceptor transition. Moreover, the conjugates 6a-c absorption (S_o → S₁) and emission (S₁ → S_o) spectra have been simulated in DMSO using the TD-DFT/B3LYP and compared with the experimental spectra. Moreover, the chromophores' antimicrobial effectiveness was investigated through the inhibition zone (IZ) and minimum inhibitory concentration (MIC) techniques, in comparison to the reference drugs. The results showed that the chromophore 6b demonstrated the lowest MIC value (6.25 μg/mL) towards S. aureus, B. subtilis, and E. coli, suggesting its probable as a wide-spectrum antibacterial agent. Moreover, molecular docking study with the target protein PDB: 1BDD showed that conjugate 6b established the strongest bindings affinity, however conjugate 6a disclosed a moderate binding.