The three compounds, labeled 5a, 5b and 5c, follow the classic donor–π–acceptor (D–π–A) blueprint that underlies many modern organic dyes. Diphenylamine serves as the electron-rich donor, a thiazole heterocycle acts as the rigid π-conjugated bridge, and a terminal benzoyl acrylonitrile group functions as the electron acceptor. The only variable across the series is the para-substituent on the terminal benzoyl ring: nitro in 5a, cyano in 5b and carboxylic acid in 5c. The molecules were assembled through a Knoevenagel condensation between a formylated diphenylamino-thiazole precursor and three phenacyl cyanide derivatives, with yields ranging from 68.8 to 78.4 percent, and were confirmed by infrared, nuclear magnetic resonance and mass spectrometry.

Quantum-chemical modelling formed the theoretical backbone of the study. The team optimized each structure using the B3LYP hybrid functional with a 6-311G(++) basis set and simulated absorption and emission spectra with time-dependent DFT, employing a polarizable continuum solvation model to mimic DMSO. The calculations revealed near-planar geometries and a clear division of electronic labor: the highest occupied molecular orbitals sat on the diphenylamino-thiazole donor, while the lowest unoccupied orbitals extended toward the electron-withdrawing termini. The nitro derivative 5a showed the smallest frontier orbital gap in the ground state, 4.97 electron-volts, compared with 5.11 eV for the cyano compound and 5.13 eV for the carboxylic acid, and its gap contracted most dramatically upon excitation, falling to 3.68 eV—a hallmark of strong intramolecular charge transfer.

The experimental photophysics tracked the computations closely. In solution, all three dyes displayed an intense low-energy absorption band assigned to charge-transfer excitation, with maxima that shifted to longer wavelengths as solvent polarity increased. Compound 5a was the most solvent-sensitive, its absorption maximum moving 34 nanometers between tetrahydrofuran and DMSO, while 5b and 5c shifted only 13 and 14 nanometers respectively. Emission followed the same logic: 5a glowed furthest into the red, peaking at 613 nanometers in DMSO, whereas the cyano and carboxylic analogues emitted at shorter wavelengths. The carboxylic acid derivative 5c posted the largest Stokes shifts of the series, roughly 7,100 to 7,200 per centimeter, which the authors attribute to hydrogen bonding and extensive excited-state relaxation.

Fluorescence efficiency told a subtler story. Despite its strong charge-transfer character, the nitro compound 5a showed a slightly reduced quantum yield of 0.678, likely because nitro groups open nonradiative decay channels. The cyano derivative 5b achieved the highest quantum yield of the series at 0.748, suggesting that moderate electron withdrawal sustains efficient charge transfer while suppressing competing relaxation pathways. Time-dependent DFT reproduced these trends, attributing the dominant visible emission in all three dyes to HOMO-to-LUMO charge-transfer transitions, with the calculated emission maximum of 5a red-shifted to 646 nanometers, consistent with the ordering of acceptor strength from nitro to cyano to carboxyl.

The biological results are where the study takes on its most striking dimension. Using the MTT assay, the team tested the dyes against HepG2 liver cancer, MCF-7 breast cancer and HT-29 colon cancer cells, alongside normal WI38 fibroblasts as a selectivity control. All three compounds proved more toxic to cancer cells than to normal fibroblasts, with selectivity indices generally between two and three. The cyano compound 5b emerged as the standout against HT-29 colon cancer cells, halting growth at a concentration of 21.05 micromolar while leaving normal cells largely unharmed up to 77.44 micromolar—a therapeutic window the authors note compares favorably with the reference drug doxorubicin in terms of selectivity. Compound 5c was most active against MCF-7 breast cancer cells at 28.46 micromolar, while 5a showed more uniform but weaker activity across the panel.

The mechanism behind this cytotoxicity remains hypothetical, and the authors are careful to frame it that way. They propose that the heteroaromatic scaffolds may trigger apoptosis through mitochondrial membrane depolarization, cytochrome c release and caspase activation, or alternatively arrest the cell cycle by inhibiting cyclin-dependent kinases. The strong charge-transfer absorption of the dyes also raises the possibility of electron-transfer reactions that elevate intracellular oxidative stress, a vulnerability that cancer cells, already living near their redox limits, are poorly equipped to tolerate. None of these mechanisms was directly tested; the authors recommend follow-up flow cytometry, caspase assays and reactive oxygen species measurements to confirm the pathways.

More concrete evidence came from the kinase experiments. Vascular endothelial growth factor receptor 2, or VEGFR-2, is a validated anticancer target because blocking it starves tumors of their blood supply. In vitro assays showed that all three chromophores inhibit the enzyme at sub-micromolar concentrations, with the nitro compound 5a the most potent at 0.44 micromolar, followed by 5c at 0.59 micromolar and 5b at 0.81 micromolar—placing the entire series in the same potency range as the clinical drug sorafenib. Molecular docking against the VEGFR-2 crystal structure (PDB entry 1YWN) lent mechanistic texture to these numbers. Compound 5a achieved the best binding score, minus 6.59 kilocalories per mole, forming hydrogen bonds with the hinge residues Cys917 and Cys1043 through its nitro and olefinic groups. Compound 5c, though slightly weaker in score, showed the most stable docked pose with three distinct bonding interactions, while 5b relied on two acceptor-type contacts with Arg831 and Lys856. Sorafenib itself docked at minus 6.25 kilocalories per mole with a higher conformational instability than the new ligands.

To verify that the docked poses were not computational artifacts, the team ran a 100-nanosecond molecular dynamics simulation on the VEGFR-2–5a complex. The system equilibrated within the first 10 nanoseconds and remained stable thereafter, with the binding-site residues Cys917 and Cys1043 showing minimal fluctuation and two to three hydrogen bonds persisting between ligand and protein throughout the trajectory. The radius of gyration stayed constant, indicating the protein remained compact, and MM-PBSA free-energy calculations returned favorable negative values driven by van der Waals and electrostatic terms—together confirming that the complex is both dynamically and thermodynamically stable.

The authors caution that docking scores are qualitative guides rather than quantitative predictions of biological activity, and that factors such as solubility and membrane permeability also shape cellular outcomes. They likewise note that structure–activity conclusions drawn from only three analogues are necessarily preliminary. Even so, the study delivers a coherent structure–property framework: acceptor strength orders the frontier orbital gaps, the solvatochromic response, the emission wavelengths and, intriguingly, the biological profile. The nitro group maximizes charge transfer and kinase affinity, the cyano group maximizes fluorescence yield and colon-cancer selectivity, and the carboxylic acid group maximizes Stokes shift and breast-cancer activity. For researchers hunting molecules that can image a tumor and attack it at the same time, this diphenylamino-thiazole platform offers a rare and tunable starting point.

Subject of Research: Donor–π–acceptor diphenylamino-thiazole fluorescent chromophores with tunable intramolecular charge transfer, cytotoxic activity and VEGFR-2 kinase inhibition

Article Title: Molecular modelling and cytotoxic activity of donor-π-acceptor fluorescent diphenylamino-thiazole chromophores

Article References: Albalawi, S., & Abualnaja, M. M. (2026). Molecular modelling and cytotoxic activity of donor-π-acceptor fluorescent diphenylamino-thiazole chromophores. Journal of Saudi Chemical Society, 30(3), Article 28. https://doi.org/10.1007/s44442-026-00063-z

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00063-z

Keywords: fluorescent chromophores, donor-π-acceptor, diphenylamine, thiazole, intramolecular charge transfer, TD-DFT, solvatochromism, cytotoxicity, HT-29 colon cancer, VEGFR-2 kinase, molecular docking, sorafenib

The rationale behind the work rests on molecular hybridization, a drug-design strategy in which two pharmacophores are fused into a single molecular entity to achieve synergistic effects. Pyridazine-based compounds have long attracted attention in medicinal chemistry because they can modulate key oncogenic targets; clinically successful kinase inhibitors such as Vatalanib, Talazoparib and Simmiparib all contain the pyridazine heterocycle. Adding a thiophene ring to form the thienopyridazine scaffold increases electron density and enhances the pharmacological profile of the pyridazine system. Thiazole rings, meanwhile, appear in established anticancer drugs including Dasatinib and Tiazofurin, and are known to improve pi-pi stacking, hydrogen bonding and lipophilicity. The researchers reasoned that merging the two frameworks could amplify target binding, cellular uptake and pharmacokinetic behaviour simultaneously.

The synthesis began from 3-chloro-4-cyano-5,6-dimethylpyridazine, which was converted through an addition-elimination reaction with thiourea into a mercapto intermediate and then cyclized to the key building block, 6-acetyl-5-amino-3,4-dimethylthieno[2,3-c]pyridazine. This bifunctional compound served as the branching point for two synthetic routes. In the first, condensation with thiosemicarbazide produced a thiosemicarbazone derivative in 74.4 percent yield, which was then reacted with alpha-halo-carbonyl reagents in Hantzsch-type cyclizations to yield thiazole-bearing hydrazones, or with chloroacetic acid to form a thiazolin-4-one ring that could subsequently undergo Knoevenagel condensations with substituted benzaldehydes. The second route converted the amino group of the building block into a chloroacetamide, which reacted with ammonium thiocyanate to close a thiazolidin-4-one ring, again followed by benzylidene elaboration. All products were verified by infrared, nuclear magnetic resonance and mass spectrometry, together with elemental analysis.

Quantum-chemical calculations at the DFT/B3LYP level with a 6-311++G(d,p) basis set revealed how structural modifications reshape the electronic landscape of the molecules. The optimized geometries were largely non-planar, with torsion angles showing that phenyl substituents introduce significant twisting while thiazolidinone ring closure produces a more rigid, near-planar architecture. Frontier molecular orbital analysis showed that in the parent aminothienopyridazine the highest occupied and lowest unoccupied molecular orbitals both sit on the fused ring system, corresponding to pi-pi* transitions with limited charge separation. In the nitro-substituted benzylidene analogues, by contrast, the LUMO localizes strongly on the benzylidene phenyl group, creating a pronounced donor-acceptor separation. HOMO-LUMO gaps ranged from 2.81 to 3.80 electron volts, with the nitro compounds showing the smallest gaps, the highest softness and the greatest electrophilicity, all hallmarks of strong intramolecular charge-transfer capability. Computed dipole moments, polarizabilities and first hyperpolarizabilities exceeded those of urea by factors of up to 5.25 and 22.80 respectively, underlining the strong charge separation in these conjugated systems.

The biological screening used the MTT assay against three cancer cell lines, HepG2 liver carcinoma, HT-29 colorectal adenocarcinoma and MCF-7 breast cancer, alongside normal WI-38 fibroblasts, with doxorubicin as the reference drug. The results traced a clear structure-activity relationship. The parent compounds, lacking extended heterocyclic conjugation, were weakly active. Introducing a methyl-substituted thiazole boosted activity markedly against HT-29 and MCF-7, while the phenyl-substituted regioisomer acted selectively on HepG2, showing that subtle changes in heteroatom orientation can redirect cellular sensitivity. Benzylidene substitution on the thiazolidinone rings further enhanced potency, particularly with electron-donating methoxy or halogen substituents, whereas the strongly electron-withdrawing nitro group diminished activity across the series.

The standout compound was the chloroacetamide analogue 8, which achieved IC50 values of 12.57 micromolar against HT-29 cells and 17.66 micromolar against MCF-7 cells, close to or better than doxorubicin at the same cell lines, while showing the highest tolerance in normal WI-38 fibroblasts at 88.21 micromolar. The authors attribute this performance to the synergistic effect of the electrophilic chloroacetamide linkage with an optimal electronic distribution, adequate molecular size and enhanced hydrogen-bonding capability. A related thiazolidinone imine analogue showed the strongest inhibition of HepG2 cells at 19.40 micromolar, suggesting that scaffold flexibility can confer organ-specific sensitivity. Most analogues displayed high selectivity indices against the normal cell line, a critical safety signal for any prospective anticancer lead.

To rationalize these findings computationally, the team docked the molecules into the N-terminal ATP-binding domain of Hsp90alpha, a molecular chaperone that stabilizes numerous oncogenic proteins and is an established cancer drug target. Binding scores ranged from -4.9488 to -7.2492 kilocalories per mole, with the lead chloroacetamide compound achieving the strongest score, driven by a hydrogen-acceptor bond between its chlorine atom and Lys58. Derivatives carrying thiazole and thiazolidinone rings consistently bound better than their simpler precursors because those rings increase hydrophobic and pi-stacking interactions within the binding cavity. Docking of doxorubicin itself yielded a score of -6.6103 kilocalories per mole, confirming the protocol and demonstrating that several novel derivatives match or exceed the reference drug’s computed affinity.

A multiple linear regression analysis connecting quantum-chemical descriptors to measured IC50 values produced determination coefficients between 0.9567 and 0.9823 across the four cell lines, indicating that frontier orbital energies largely account for the biological variation. Lower LUMO energies favoured cytotoxicity, consistent with the importance of electron-accepting ability in stabilizing interactions with biological targets, while the positive contribution of the HOMO-LUMO gap suggested that optimal activity requires balanced reactivity rather than excessive molecular softness. Electronegativity enhanced activity in all models, whereas hardness and softness effects varied by cell line, reflecting differences in how each tumour type responds to electronic flexibility.

In silico pharmacokinetic profiling with SwissADME added a further layer of assessment. The lead compound 8, together with analogues 9 and 10a, showed high gastrointestinal absorption, good aqueous solubility and full Lipinski compliance, although several were predicted to be P-glycoprotein substrates, which could modestly limit oral bioavailability. Importantly, none of the synthesized analogues were predicted to cross the blood-brain barrier, which the authors interpret as evidence that the series is best suited to peripheral, non-CNS diseases rather than central nervous system indications.

The authors caution that the cytotoxic data represent a preliminary screening; mechanistic studies involving apoptosis markers, cell cycle analysis and Western blotting on the most active analogues are the logical next step. Nevertheless, the convergence of potent and selective antiproliferative activity, docking affinities rivalling a clinical drug, and acceptable predicted drug-likeness positions the thienopyridazine-thiazole hybrids, and the chloroacetamide lead in particular, as promising starting points for further optimization in the search for safer and more effective anticancer agents.

Subject of Research: Synthesis, computational modelling and cytotoxic evaluation of thienopyridazine-thiazole hybrid compounds as potential anticancer agents

Article Title: Synthesis, molecular modelling, and cytotoxic activity of new thienopyridazine analogues clubbed thiazole ring systems

Article References: Mazi, W., & Alenazi, N. A. (2026). Synthesis, molecular modelling, and cytotoxic activity of new thienopyridazine analogues clubbed thiazole ring systems. Journal of Saudi Chemical Society, 30(4), Article 46. https://doi.org/10.1007/s44442-026-00076-8

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00076-8

Keywords: thienopyridazine, thiazole, anticancer agents, cytotoxicity, DFT, molecular docking, Hsp90alpha, drug-likeness, molecular hybridization, HT-29, MCF-7, structure-activity relationship