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