2-Styryl-quinoline-4-carboxylic acids: Synthesis, characterization, and investigation of their optical properties and antimicrobial activity

Styrylquinoline-based fluorophores 1–10 were synthesized via Knoevenagel and Pfitzinger reactions. The molecules were fully characterized and studied for the impact of their substitution patterns (A-D) on their optical and bioactivity properties. With exception of nitro substituted compounds 9-D and 10-D, all compounds were fluorescent in solution with a significant solvatochromic behavior measured in toluene, dioxane, tetrahydrofurane, ethyl acetate, dichloromethane, dimethylformamide, dimethylsulfoxide, acetonitrile, isopropanol, and methanol. The maximum wavelength of their emission ranged from 408 to 539 nm. Additionally, there was a significant variation in the photoluminescence (PL) quantum yield concerning different solvent polarities. The PL quantum yields ranged from small values, such as 1.7 % in methanol for compound 1-A, to good PL quantum yields, such as 44.4 % in dioxane for 7-C. For comparison, the quantum yield of 7-C in methanol was 3.3 %, a total range variation of 41.1 %. 4-B and 5-B with O-acetyl groups at ortho and at para position to the CC double bond, respectively, presented an interesting acidochromism with a redshift of about 102 nm in their emission bands along with a visible emitted color change from blue to green. In addition, the antimicrobial activity for all compounds and the theoretical mechanism of action of the more significant bioactive compounds were investigated. In inverse virtual screening studies, the enzymes Dehydrosqualene synthase and Squalene synthase were indicated as putative biological targets for 3-B and 4-B. Molecular docking indicated the amino acid residues critical for interaction in bacteria and fungi.