Synthesis and DFT Study of New Pyrano[3,2-c]Quinoline Derivatives for Antioxidant, Forensic, and Security Ink Applications

A series of heterocyclic derivatives incorporating the pyrano[3,2-c]quinoline-2,5-dione framework were successfully synthesized via nucleophilic substitution reactions of 6-ethyl-4-chloropyrano[3,2-c]quinoline-3-carbaldehyde 2 with several sulfur nucleophiles. Furthermore, aldehyde 2 underwent condensation reactions with various nucleophilic reagents, such as primary amines, and selected cyclic and acyclic active methylene compounds. The optimized geometrical structure of the starting aldehyde 2 was obtained through density functional theory (DFT) calculations performed at the B3LYP/6–311++G(d,p) level of theory. These computations were employed to evaluate several molecular parameters, including the molecular electrostatic potential (MEP), Fukui functions, frontier molecular orbital (FMO) characteristics, and nuclear magnetic resonance (NMR) properties. The DPPH radical scavenging activity of all synthesized derivatives was evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. The results revealed that compounds 4, 7, and 9 exhibited the highest radical scavenging activity among the synthesized derivatives, although their activity remained lower than that of ascorbic acid. PL emission spectra and CIE 1931 chromaticity coordinates confirmed tunable fluorescence across the synthesized compounds, governed by molecular structure and physical state. Compounds 5 (yellow–green) and 6 (gold–yellow) proved highly effective for latent fingerprint visualization, yielding sharp ridge details on porous, nonporous, and multicolored substrates. Additionally, compounds 11 (violet), 8 (blue), and 3 (green–yellow) were formulated into stable, humidity-resistant luminescent inks, highlighting strong potential for anticounterfeiting and secure data encoding.