Solvent Polarity Steers Double Proton Transfer in a Promising OLED Fluorophore

Some molecules do something remarkable when they absorb light: they shuffle protons internally, transforming into a different chemical form before they ever get the chance to fluoresce. This phenomenon, known as excited-state intramolecular proton transfer, or ESIPT, produces molecules that emit at two distinct wavelengths and exhibit enormous Stokes shifts, the gap between the light absorbed and the light given off. Those properties make ESIPT compounds attractive for molecular logic gates, luminescent materials, and biological probes. Now, a computational study published in the Journal of Saudi Chemical Society has dissected how the surrounding solvent can tip the scales of this delicate proton-shuttling dance in a fluorophore with real promise for organic light-emitting diode technology.

The research, carried out by Jiahe Chen and Jinfeng Zhao of Shenyang Normal University in China, focuses on a derivative of 2,2′-bipyridyl-3,3′-diol, a compound abbreviated H2BP-(OH)2DC-NH2. This molecule was originally designed and reported by Trannoy and co-workers, and it has already attracted attention because it emits efficiently not only in solution but also in the crystalline state, and it shows excellent electroluminescence performance, making it a candidate for OLED devices. What makes it particularly interesting to theorists is that it carries two internal hydrogen bonds, O1-H2···N3 and O4-H5···N6, each capable of shuttling a proton when the molecule is excited by light. That opens the possibility of excited-state double proton transfer, or ESDPT, in which two protons move rather than one.

Double proton transfer matters far beyond the chemistry of a single fluorophore. The vast majority of reactions in biological systems involve the transfer of multiple protons, so a single-proton model is simply too crude to capture the intricacies of processes such as those that stabilize DNA or drive enzyme catalysis. The history of the field dates back to 1969, when Taylor, El-Bayoumi, and Kasha provided the first experimental evidence of excited-state two-proton tautomerism in doubly hydrogen-bonded 7-azaindole dimers, observing a broad green fluorescence quite distinct from the violet glow of the monomer. Since then, researchers have worked to establish whether such double transfers happen simultaneously or step by step, a question the new study addresses directly for H2BP-(OH)2DC-NH2.

To probe the mechanism, the team turned to density functional theory and time-dependent density functional theory, the workhorse quantum chemical methods for studying molecules in their ground and excited states. All calculations were performed with the Gaussian 16 program using the B3LYP functional with a triple-zeta valence basis set including polarization functions, augmented with Grimme’s D3 dispersion correction to describe the weak hydrogen bond interactions accurately. The solvents themselves were modeled with the polarizable continuum model, allowing the researchers to simulate three environments of increasing polarity: cyclohexane, chloroform, and acetonitrile. Geometries were optimized without constraints in both the ground state and the first excited singlet state, and vibrational analyses confirmed that every optimized structure was a true minimum with no imaginary frequencies.

The first line of evidence came from geometry. When the molecule is promoted from the ground state to the first excited state, the hydrogen bonds in both O1-H2···N3 and O4-H5···N6 shorten, regardless of solvent. In cyclohexane, for example, the H2···N3 distance contracts from 1.6862 angstroms to 1.5966 angstroms, while the covalent O1-H2 and O4-H5 bonds stretch from 1.0024 angstroms to 1.0241 angstroms. The bond angles at both hydrogen-bonding sites widen from 149.26 degrees to 151.29 degrees. Shorter hydrogen bonds and larger bond angles are classic signatures of hydrogen bond strengthening, and a strengthened hydrogen bond is precisely what lowers the barrier for a proton to hop across it. The same trends, to varying degrees, appeared in chloroform and acetonitrile.

Spectroscopic and topological analyses reinforced the geometric picture. Simulated infrared spectra showed that the stretching frequencies of the two O-H bonds redshift markedly upon excitation, dropping from 3047.66 to 2624.12 wavenumbers in cyclohexane, from 3016.61 to 2596.64 in chloroform, and from 2989.32 to 2569.76 in acetonitrile. A redshift in an O-H stretch means the covalent bond has weakened, which happens when the hydrogen bond pulling on that proton grows stronger. The researchers also computed the electron localization function and derived from it the core-valence bifurcation index, a topological metric that correlates linearly with hydrogen bond strength. More negative CVB values in the excited state confirmed stronger hydrogen bonds, with acetonitrile showing the strongest and chloroform the weakest, hinting already that higher polarity favors proton transfer. Predicted hydrogen bond energies and electron densities at the bond critical points rose on excitation as well, and did so more dramatically in the more polar solvents.

Charge redistribution provided the electronic explanation for why excitation primes the molecule for proton motion. Analyzing the frontier molecular orbitals, the team found that the dominant S0 to S1 transition accounts for roughly 96.7 to 96.9 percent of the excited-state character in all three solvents, justifying their focus on this single excitation under Kasha’s rule. Charge density difference maps and electron-hole analyses revealed that upon photoexcitation, electron density shifts away from the oxygen atoms O1 and O4 and accumulates on the nitrogen atoms N3 and N6. In plain terms, the oxygens become better at letting go of their protons while the nitrogens become more eager to accept them. This photoinduced charge redistribution is a significant motivating factor for the ESIPT reaction, effectively priming both hydrogen bonds for proton transfer the moment the molecule absorbs a photon.

The decisive test came from mapping the potential energy surfaces. The team scanned both hydrogen bond coordinates from 0.9 to 2.1 angstroms in steps of 0.1 angstrom in both electronic states and all three solvents. In the ground state, the energy barrier climbs steadily as the O-H bonds stretch, meaning proton transfer is essentially shut down before excitation. In the excited state, the landscape transforms completely. Comparing the direct pathway in which both protons move at once against the stepwise route in which one proton transfers first and the second follows, the stepwise path consistently presents smaller barriers. In acetonitrile, the barriers for the two stepwise steps are 0.1286 and 2.6619 kilocalories per mole, while the concerted route costs 2.8564 kilocalories per mole. In cyclohexane, the corresponding values are 0.3589, 6.1076, and 3.5831 kilocalories per mole. The verdict is clear: ESDPT in H2BP-(OH)2DC-NH2 proceeds step by step, and every barrier shrinks as solvent polarity rises.

The implications reach beyond one molecule. Because the luminescence of ESIPT fluorophores depends intimately on whether and how fast protons transfer, the demonstration that solvent polarity systematically tunes the barriers offers a practical lever for controlling emission color and efficiency. The authors note that their conclusions provide a theoretical basis for regulating the luminescent properties of organic molecules simply by changing the polarity of the surrounding medium. For device engineers, that suggests the emissive behavior of H2BP-(OH)2DC-NH2 and related bipyridyl-diol derivatives could be engineered through matrix selection in OLED architectures, where the local dielectric environment plays the role of the solvent. For chemists studying biological proton transfer, the stepwise mechanism confirmed here offers a computationally tractable template for the multi-proton reactions that underpin so much of biochemistry.

The study also showcases how modern computational tools can dissect phenomena once accessible only through spectroscopy. By combining geometric parameters, infrared vibrational shifts, interaction region indicator visualizations, core-valence bifurcation indices, hydrogen bond energy predictions, frontier orbital analysis, and full potential energy surface scans, the researchers assembled a mutually consistent case in which every independent probe pointed to the same conclusion: excitation strengthens both hydrogen bonds, charge redistribution drives the protons toward their acceptors, and the double transfer unfolds one proton at a time, faster in more polar surroundings. Cross-checks with an alternative functional, Cam-B3LYP, reproduced the same reaction tendencies, bolstering confidence in the results. As ESIPT and ESDPT fluorophores continue to find roles in sensing, imaging, and display technologies, work of this kind turns an empirical art into a predictive science, showing exactly which molecular levers, hydrogen bond geometry, charge flow, and environmental polarity, determine whether a photon in becomes a shifted photon out.

Subject of Research: Solvent-polarity effects on excited-state double proton transfer in the H2BP-(OH)2DC-NH2 fluorophore investigated by DFT and TDDFT calculations

Article Title: A computational decode of photoinduced dual hydrogen bonding interactions and ESDPT behaviors for H2BP-(OH)2DC-NH2 fluorophore

Article References: Chen, J., & Zhao, J. (2026). A computational decode of photoinduced dual hydrogen bonding interactions and ESDPT behaviors for H2BP-(OH)2DC-NH2 fluorophore. Journal of Saudi Chemical Society, 30(2), Article 18. https://doi.org/10.1007/s44442-026-00067-9

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00067-9

Keywords: ESIPT, ESDPT, hydrogen bonding, solvent polarity, DFT, TDDFT, fluorophore, OLED, potential energy surface, charge redistribution, photochemistry, bipyridyl-diol