Stilbenes, like resveratrol, are natural small molecules with several applications in the treatment of chronic illnesses, diabetes, and neurological and cardiovascular conditions. Considering the molecular structure of stilbenes and the biological activities of resveratrol, we investigated the synthesis and biological activity of a novel styryl quinolinium (SQ) derivative. The SQ was synthesized using the alkylation of lepidine and methyl 4-methylbenzenesulfonate and further Knoevenagel condensation with 2-hydroxy naphthaldehyde. Its structure was determined using NMR, IR, Raman, UV-Vis, and MS. The current trend of research has shifted toward the synthesis of novel SQ as resveratrol’s analog. Therefore, its anti-inflammatory and antioxidant activities were evaluated. The compound exerted very good anti-inflammatory effects in preventing albumin denaturation, which were confirmed by ex vivo immunohistochemical studies. Density functional theory (DFT) analyses were conducted on the SQ structure to gain detailed insights into its active sites, energy gap, quantum properties, and electronic behavior. The biological evaluation of the compound was completed, assessing its antioxidant potential using DPPH radical scavenging assay. In addition, molecular docking simulations were utilized to assess the SQ compound’s potential inhibitory effect on human serum albumin (HSA).

It was found that the irradiation of aqueous solution of isocytosine with UVC light provokes an oxo-hydroxy phototautomerism of the compound with a rate constant of 5.29× 10-3 min-1. It was observed a backward reaction, after removing the UV light source, with a rate constant of 0.12×10-3 min-1. Two mechanisms of the phototautomerism were investigated at the B3LYP/aug-cc-pVDZ theoretical level in water surroundings (PCM). The first one showed a consecutive dissociation and association of a proton through conical intersections S0/S1 whose structures were located at the same theoretical level in the gas phase. It occurs along the 1πσ* excited-state reaction pathway. The more probable mechanism includes an excited-state H-transfer supported by a water molecule as a catalyst. It occurs along the 1ππ* excited-state reaction pathway which we found over the IRC ground-state energy curve. The water molecule drastically reduces the energy barrier in the ground state as well in the excited state.

Two previously synthesized styrylquinolinium dyes, namely (E)-1-butyl-4-(4-(dimethylamino)styryl)quinolinium iodide (D36) and (E)-1-butyl-4-(4-hydroxystyryl)quinolinium iodide (D34), were compared in terms of their properties by single-crystal X-ray diffraction (XRD), Hirshfeld surface analysis, Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR), fluorescence, and ultraviolet–visible (UV–Vis) spectroscopy, and ¹H- and ¹³C-NMR methods. Both dyes D36 and D34 crystallized in the triclinic and monoclinic systems in the centrosymmetric space groups P-1 and P2₁/n, respectively. The unit cell of D36 contains two molecules of the dye, participating in weak intermolecular interactions, whereas that of D34 contains four formula units. The phenolic hydroxy group of D34 participates in the formation of a hydrogen bond with the iodide anion. The 4-styrylquinolinium moieties of the cationic dye molecules are nearly planar. The dihedral angle between the mean planes through the ten-membered quinolinium system and the benzene ring is 7.5° in D36 and 5.9(1)° in D34. The structural parameters planarity and bond length alternation (BLA) are discussed, which are important for the evaluation of the first hyperpolarizability β at the molecular level, even in a centrosymmetric crystal. The UV–visible spectra of the dyes in 14 solvents of different polarities were investigated. The reversible solvatochromic behavior of the dyes is demonstrated experimentally and compared with known “binuclear dyes” by evaluating the Rezende model. Dye D36 does not fluoresce, and D34 has a very low emission in the solvents tested.

The mechanism of excited state proton transfer between guanine and cytosine in the Watson-Crick base pair was studied with experimental and theoretical methods. When irradiated with UV light the water solution of the equimolecular mixture of guanine and cytosine indicates a photoreaction with a rate constant 48.07×10-4 min-1. The backward dark reaction occurs with a rate constant 12.03×10-4 min-1. One of the identified processes is an excited-state proton transfer (ESPT). The mechanism of the photoreaction was followed at the B3LYP/aug-cc-pVDZ level of theory in water surroundings (according to PCM). We tested two mechanisms: intrinsic reaction coordinate (IRC) and Linear interpolation of internal coordinates (LIIC) between a base pair and a conical intersection S0/S1. They both showed that the ESPT occurs along 1pp* excited-state reaction paths.

A new styryl quinolinium dye (E)-4-(2-(2-hydroxynaphthalene-1-yl)ethenyl-1-octyl quinolinium iodide (3) was obtained by Knoevenagel condensation. Single crystal X-ray diffraction, spectral methods IR, 1H and 13C-NMR spectroscopy and TGA-DTA analysis were used to characterize the compound. The title compound crystallized in the triclinic space group P-1 and exhibited thermal stability up to 212 °C. The vibrational spectra were completely determined through the quantum chemical calculations.

The use of styrylium dyes as organic nonlinear optical materials in many photonics domains has been the subject of research for decades. It has been noted that over time, research has also looked into the biological activity of styrylium dyes, namely their antibacterial effects, as well as attempts to establish links between structure and property by choosing particular structural pieces. These investigations’ scope is still very limited. Therefore, our main goal was to synthesize a styrylium compound with antimicrobial potential. A novel styrylquinolinium compound (D) was synthesized using Knoevenagel condensation. Spectroscopic techniques, including IR, 1D and 2D NMR (COSY, HSQC, and HMBC), HRMS spectra, and X-ray analysis, were used to confirm its structure. The antimicrobial and anti-inflammatory activity of the compound was assessed. The compound was found to have very good antimicrobial activity against five Gram-positive strains, three Gram-negative strains, and fungi. The most pronounced effect of the compound was against Escherichia coli and Pseudomonas aeruginosa. The compound’s anti-inflammatory activity was evaluated through its ex vivo immunohistochemistry. DFT calculations, such as geometry optimization, Molecular Electrostatic Potential (MEP), HOMO–LUMO, reactivity parameters and molecular docking simulation were applied to investigate the electronic features of the compound and confirm the biological activity. The compound (D) demonstrated a promising antibacterial and immunomodulatory profile. Its ability to induce IL-1β and at the same time moderately reduce NOS3 can be considered as a controlled adaptation of the immune response, especially in cases requiring local immune activation. Docking simulation revealed that (D) binds effectively to the active site of the bacterial protein, supporting the experimental findings of the compound’s antibacterial activity.