The motivation behind the study lies in one of the central dilemmas of modern renewable energy. Silicon photovoltaics dominate the market, but their production is expensive, energy-intensive and dependent on complex industrial processes. Organic photovoltaic cells, first developed in the 1990s, offer an attractive alternative: they absorb light efficiently, are cheap to manufacture, and can be made into flexible, lightweight panels. Their Achilles heel has always been efficiency, which still trails crystalline silicon by a considerable margin. Closing that gap requires better molecular materials, and triazine-based compounds, with their highly conjugated electronic systems and dual ability to trap and transport electrons, have emerged as promising building blocks for the next generation of organic semiconductors.

The synthetic route devised by the team is elegantly simple. A diazonium ion was generated from 3-amino-5,6-dimethyl-1,2,4-triazine using sodium nitrite and concentrated sulfuric acid at temperatures between minus five and zero degrees Celsius in dimethyl sulfoxide. This reactive intermediate was then coupled with three 2-aminobenzothiazole derivatives bearing different substituents, an ethoxy group, a nitro group and a methoxy group, yielding three azo compounds designated 4a, 4b and 4c. In a further reaction, the diazonium ion was coupled with the parent triazine itself to produce a symmetrical bis-triazine derivative, compound 5, in an impressive 95.7 percent yield. Establishing the exact structures of these products was no trivial matter, because azo couplings can proceed through several different pathways. The researchers resolved the ambiguity using a combination of infrared spectroscopy, one- and two-dimensional nuclear magnetic resonance, high-resolution mass spectrometry and elemental analysis, showing for example that compound 4a forms by electrophilic substitution on the benzothiazole ring while compound 4b arises from addition at the ring nitrogen.

With the molecules in hand, the team turned to quantum chemistry. All four compounds were modeled using the B3LYP hybrid functional with the 6-311+G(d,p) basis set, a combination chosen for its proven reliability on conjugated organic systems. Calculations were performed in the gas phase and in two polar solvents, methanol and dimethyl sulfoxide, using the polarizable continuum model to mimic the real environments in which such materials would be used. The authors are candid about the limitations of their approach: global hybrid functionals like B3LYP tend to underestimate the excitation energies of charge-transfer transitions and to overestimate hyperpolarizabilities compared with long-range-corrected alternatives, but they retained it for internal consistency across the full set of properties and because it correctly reproduces the qualitative trends observed experimentally for this series.

The electronic structure calculations revealed a striking split within the family. Compounds 4a and 4c, which carry electron-donating alkoxy groups, behave almost identically despite differing only by a methyl group in their side chains. Their HOMO-LUMO energy gaps shrink from roughly 3.3 electron volts in the gas phase to about 2.95 electron volts in polar solvents, a solvent-induced narrowing that favors visible-light absorption. Orbital analysis showed that in these two molecules the HOMO sits predominantly on the donor benzothiazole-alkoxy fragment while the LUMO resides on the acceptor triazine unit, a textbook donor-acceptor architecture that drives intramolecular charge transfer across the conjugated azo bridge. Compound 4b, bearing the strongly electron-withdrawing nitro group, proved far less sensitive to solvation and emerged as the most electronically stable member of the series, with the highest electrophilicity index and ionization potential, marking it out as a natural electron acceptor.

Charge transport, the lifeblood of any semiconductor, was assessed through reorganization energies calculated within the framework of Marcus theory. These values measure how much a molecule must geometrically distort when it gains or loses an electron, and low values translate into faster, more efficient charge hopping. Here again the alkoxy compounds shone: 4a and 4c posted hole and electron reorganization energies of just 0.61 to 0.67 electron volts, with electron values only slightly exceeding hole values, suggesting a nearly balanced ambipolar character that is highly prized in organic electronics. Compound 5 showed moderate values, while the nitro-substituted 4b was a dramatic outlier, with a hole reorganization energy of 4.09 electron volts, roughly 6.7 times that of 4c, indicating that its geometry changes too profoundly upon oxidation to serve as an efficient charge carrier. The authors benchmarked their results against pentacene, the reference p-type organic semiconductor, whose hole reorganization energy of 0.08 to 0.12 electron volts remains far lower; their dyes are competitive but not yet optimal.

Perhaps the most eye-catching results concern nonlinear optics, the branch of photonics concerned with materials whose optical response changes with light intensity. Such materials enable frequency doubling, optical switching and signal processing. The first-order hyperpolarizability of compounds 4a and 4c surged from around 45 to 46 times ten to the minus thirty esu in the gas phase to roughly 171 times ten to the minus thirty esu in polar solvents, a solvent-driven amplification of more than a factor of three. Set against para-nitroaniline, the classical benchmark donor-pi-acceptor chromophore, whose hyperpolarizability reaches only about 20 to 25 times ten to the minus thirty esu even in water, the two alkoxy dyes outperform the reference by a factor of seven to eight. Intriguingly, compound 4b, despite its strong acceptor credentials, showed a hyperpolarizability less than half that of its alkoxy siblings, demonstrating that a large dipole moment alone does not guarantee a strong nonlinear response and that the spatial delocalization of charge separation matters more.

The optoelectronic and optical calculations rounded out the picture. Compounds 4a and 4c exhibited high dielectric constants and refractive indices above 2.4 in methanol, values typical of high-performance pi-conjugated organic materials and attractive for light confinement in integrated photonic waveguides. Time-dependent DFT placed the lowest-energy absorption of 4a and 4c in the blue-green region of the visible spectrum, near 480 and 490 nanometers respectively, with mixed multi-orbital transitions characteristic of intramolecular charge transfer. The high-energy pi to pi-star bands were reproduced with excellent accuracy against the experimental ultraviolet-visible spectra, with errors below three percent, while the lowest charge-transfer band showed the larger deviations expected from the known weaknesses of B3LYP. Thermodynamic calculations confirmed that all four compounds are stabilized in polar media, with the nitro compound 4b showing the lowest Gibbs free energy and hence the greatest thermodynamic stability.

The authors are careful to frame their conclusions as computationally motivated hypotheses rather than proven device performance. No solar cell or light-emitting device was fabricated in this study, and experimental validation at the material and device level will be needed before any of these dyes reaches a photovoltaic panel or a photonic modulator. Nevertheless, the work illustrates a powerful and increasingly standard paradigm in materials discovery: synthesize a chemically diverse family of candidate chromophores, pin down their structures rigorously, and use solvent-explicit quantum chemical screening to decide which members deserve the expense of device fabrication. On that basis, compounds 4a and 4c, with their narrow gaps, balanced charge transport and record-beating nonlinear optical response, have earned their place at the front of the queue, while the stable but transport-limited 4b may find its calling as an electron acceptor in multilayer organic architectures.

Subject of Research: Synthesis and DFT characterization of triazine-benzothiazole hybrid azo dyes as pi-conjugated materials for organic photovoltaic and nonlinear optical applications

Article Title: Synthesis photophysical characterization and DFT Study of triazine benzothiazole hybrid azo dyes as π conjugated materials for organic photovoltaic and nonlinear optical applications

Article References: Tsemeugne, J., Ebode, R. D. P. N., Fomekong, L. T., Eckhardt, P., Kamsi, R. A. Y., Mvot, C. A., Ottou, M. T. A., Ngoupo, A. T., Sielinou, V. T., Ejuh, G. W., Mkounga, P., Opatz, T., Ndjaka, J.-M. B., Sopbué, E. F., & Nkengfack, A. E. (2026). Synthesis photophysical characterization and DFT Study of triazine benzothiazole hybrid azo dyes as π conjugated materials for organic photovoltaic and nonlinear optical applications. Discover Chemistry, 3(1), Article 554. https://doi.org/10.1007/s44371-026-00999-6

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00999-6

Keywords: azo dyes, triazine, benzothiazole, DFT, organic photovoltaics, nonlinear optics, pi-conjugated materials, charge transport, reorganization energy, hyperpolarizability, TD-DFT, organic semiconductors

The fascination with organic molecules as NLO materials is not new. Photonic technologies, including second-harmonic generation, optical switching, frequency conversion, and electro-optical modulation, all depend on substances whose electrons respond nonlinearly to intense light. Inorganic crystals have long dominated the field, but organic systems offer synthetic versatility, easily tunable structures, and extended pi-electron delocalization. When electron density sloshes between donor and acceptor regions inside a conjugated molecule, the result is a large dipole moment, high polarizability, and, most importantly, a sizable first-order hyperpolarizability, the parameter that governs second-order NLO behavior. What makes the new study distinctive is its proposal that drug molecules, whose structures often already contain aromatic rings, heteroatoms, and donor-acceptor subunits, could serve double duty as optoelectronic components.

Hydroxyzine, chemically designated 2-[2-[4-[(4-chlorophenyl)(phenyl)methyl]piperazin-1-yl]ethoxy]ethanol, turns out to be structurally well suited for such scrutiny. Its architecture stacks two phenyl rings, a piperazine heterocyclic core, ether linkages, and a terminal hydroxyl group into a framework rich in electron donors and acceptors. The researchers optimized the molecule’s geometry at the B3LYP/6-311G(d,p) level of theory, a workhorse combination of hybrid functional and triple-zeta basis set widely trusted for organic pharmaceuticals. The computed structural parameters told a coherent story: aromatic carbon-carbon bond lengths between 1.389 and 1.399 angstroms, midway between single and double bonds, confirmed substantial pi-electron delocalization, while the carbon-oxygen bonds in the ether linkages, at 1.411 to 1.419 angstroms, were shorter than standard single bonds, indicating partial double-bond character as oxygen lone pairs delocalize into the adjacent carbon framework.

To make sure the quantum picture was not a computational mirage, the team validated it against experiment. The simulated FT-IR spectrum matched the measured one across a broad range, reproducing the telltale broad hydroxyl stretch between 3400 and 3200 inverse centimeters, the split aromatic and aliphatic C-H stretching regions, and the ether C-O-C fingerprint bands between 1250 and 1050 inverse centimeters. The ultraviolet-visible spectrum, measured experimentally with peaks near 260 and 300 nanometers, was reproduced by time-dependent DFT calculations across seven solvent environments spanning gas phase to water, with computed absorptions falling between 259 and 273 nanometers. The dominant electronic transitions, assigned as HOMO-1 to LUMO, HOMO to LUMO, and HOMO-1 to LUMO+1 with near-complete contributions, confirmed that excitation is dominated by the delocalized aromatic pi-system. Curiously, the solvatochromic shifts were tiny, less than 3 nanometers across every solvent tested, suggesting the electronic excitations are localized well within the aromatic core and only weakly coupled to their surroundings.

The frontier molecular orbital analysis supplied the electronic heart of the story. The highest occupied molecular orbital sat at minus 6.24 electron volts, the lowest unoccupied one at minus 1.08 electron volts, yielding a gap of 5.16 electron volts. That figure places hydroxyzine in the same electronic stability neighborhood as other biologically active pharmaceuticals, such as the cyclic dipeptide cyclo(L-phenylalanyl-L-proline) at 5.19 electron volts, and implies a molecule that is neither too chemically inert nor too fragile, a balance quantified by derived descriptors like a chemical hardness of 2.58 electron volts. Mulliken population analysis then mapped the charge landscape: the two oxygen atoms and two piperazine nitrogens carried the deepest negative charges, between minus 0.37 and minus 0.40 elementary units, marking them as the principal nucleophilic centers, while positively polarized hydrogens completed an effective donor-pi-acceptor architecture that channels intramolecular charge transfer through the conjugated scaffold.

That charge asymmetry is precisely what drives the headline result. The calculated first-order hyperpolarizability came out at 1.79 times ten to the minus thirty electrostatic units, roughly 4.81 times larger than the benchmark value for urea, the traditional reference compound in NLO studies. The dominant tensor components, beta-zxx at 172.69 atomic units and beta-xxx at minus 75.92 atomic units, pointed to charge redistribution along specific molecular directions as the engine of the response. The researchers were careful to keep expectations grounded: the absolute magnitude places hydroxyzine among moderate NLO materials, well below star chromophores like para-nitroaniline derivatives and push-pull stilbenes, whose hyperpolarizabilities can exceed urea’s by one to two orders of magnitude. Yet the conceptual payoff is considerable. A clinically approved molecule already produced at industrial scale exhibits optical nonlinearity, implying that the structural features responsible for biological activity, aromatic rings, heteroatoms, and charge-transfer pathways, can be recruited for photonic purposes without designing a new scaffold from scratch.

The study went further, connecting molecular topology to electronic behavior through graph-theoretical descriptors. Representing the molecule as a graph of 28 vertices and 29 edges, the authors computed a battery of degree-based topological indices, including a first Zagreb index of 116, a second Zagreb index of 128, a hyper Zagreb index of 528, and a forgotten index of 272. These values, noticeably higher than those of simple aromatic molecules, quantify the structural complexity introduced by the piperazine bridge, the two chemically distinct phenyl rings, and the ether-linked side chain. In the authors’ framework, this topological complexity underwrites the extended electronic delocalization and donor-acceptor charge transfer responsible for the optical response, establishing a quantitative bridge between the shape of a molecular graph and its electro-optical performance.

The pharmaceutical side of the analysis held its own surprises. SwissADME predictions placed hydroxyzine squarely in the yellow yolk of the Boiled-Egg model, indicating high passive gastrointestinal absorption and probable blood-brain barrier penetration, findings consistent with its known sedative and anxiolytic actions. The molecule passed Lipinski’s rule of five with zero violations and was predicted not to be a P-glycoprotein substrate, easing concerns about efflux at the barrier. Molecular docking against histamine N-methyltransferase, a key enzyme of the histaminergic pathway, using the protein with PDB identifier 3Q01, yielded a binding energy of minus 6.01 kilocalories per mole, stabilized by hydrogen bonds to asparagine 200 and glycine 199 at 2.1 and 2.6 angstroms, plus a cluster of hydrophobic contacts. Laboratory agar well diffusion assays added a microbial footnote: compounds identified during characterization showed moderate antibacterial activity, generally stronger against Staphylococcus aureus than against Escherichia coli, a selectivity the authors attribute to the impermeable outer membrane of Gram-negative bacteria.

Not every piece of the puzzle snapped cleanly into place, and the authors were candid about it. Several computed proton NMR shifts, particularly for the flexible ethoxy side chain, diverged substantially from experimental values, with one proton predicted near 2.6 parts per million measured at 8.72. The team attributes these discrepancies to solvent-induced deshielding, concentration-dependent aggregation, possible assignment issues, and the limitations of relying on a single lowest-energy conformer rather than the full conformational ensemble present in solution, flagging conformational sampling and explicit solvent models as priorities for future work. Still, the overall verdict is that a familiar allergy pill, viewed through the combined lenses of quantum chemistry, spectroscopy, topology, and biology, reveals itself as a genuine structure-property laboratory. The findings suggest that piperazine-based drug scaffolds, already synthetically accessible and pharmacologically validated, could seed the next generation of sustainable, multifunctional organic materials, blurring the boundary between the pharmacy shelf and the photonics bench.

Subject of Research: Density functional theory investigation of the charge transfer and nonlinear optical properties of the antihistamine hydroxyzine

Article Title: Computational insight into charge transfer and nonlinear optical response of hydroxyzine: a DFT-guided approach towards sustainable optoelectronic and pharmaceutical applications

Article References: Sasikala, L., Kumar, K. B., Vimal, S., Prabhaharan, M., & Amirthakumar, C. (2026). Computational insight into charge transfer and nonlinear optical response of hydroxyzine: a DFT-guided approach towards sustainable optoelectronic and pharmaceutical applications. Results in Chemistry, 30, Article 103890. https://doi.org/10.1016/j.rechem.2026.103890

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103890

Keywords: hydroxyzine, density functional theory, nonlinear optics, hyperpolarizability, intramolecular charge transfer, piperazine, TD-DFT, topological indices, molecular docking, drug-likeness, antimicrobial activity, optoelectronics