A new arXiv preprint reports that, in computer simulations of excited fluorophore molecules, selected molecular vibrations were associated with different proton-transfer outcomes: symmetric-mode trajectories ended in double transfer, while antisymmetric-mode trajectories ended in single transfer. The process is ESIPT — excited-state intramolecular proton transfer, a proton shift within a molecule after excitation.
The authors combined TD-DFT electronic-structure calculations with molecular dynamics on the first excited state. Baseline trajectories were followed for up to 1 ps unless transfer occurred earlier.
Transfer was rare in the baseline runs
The baseline comparison used p-DAPA-CF3 and p-DAPA-CN. At 300 K, the simulations recorded zero transfers among 10 p-DAPA-CF3 trajectories and one transfer among 10 p-DAPA-CN trajectories.
At 1000 K, 13 of 14 p-DAPA-CF3 trajectories and all 14 p-DAPA-CN trajectories transferred a proton. Exponential fits gave transfer times of 345 femtoseconds for p-DAPA-CF3 and 165 femtoseconds for p-DAPA-CN. A femtosecond is one millionth of a billionth of a second.
Among reactive trajectories at 1000 K, the reported mean transfer times were 280 ± 145 femtoseconds for p-DAPA-CF3 and 159 ± 66 femtoseconds for p-DAPA-CN, with the table presenting these averages within 95% confidence.
The energy calculations also indicated a lower barrier to the transferred state for p-DAPA-CN than for p-DAPA-CF3, with the transferred state calculated to be lower in energy relative to the N* state.
The selected mode tracked the product
The researchers then energized selected symmetric or antisymmetric normal modes — coordinated patterns of atomic movement — and compared the resulting trajectories. In these control simulations, symmetric modes ended in double transfer, while antisymmetric modes ended in single transfer.
At 0 K and 2.48 eV of vibrational energy, p-DAPA-CF3 transferred in 34 femtoseconds with the symmetric mode, where double transfer occurred, and in 214 femtoseconds with the antisymmetric mode. For p-DAPA-CN, the corresponding times were 32 femtoseconds with double transfer and 13 femtoseconds with the antisymmetric mode.
At 300 K with 1.22 eV activation, the reacting fractions were 4/4 for p-DAPA-CF3 symmetric runs, 3/4 for p-DAPA-CF3 antisymmetric runs, 4/4 for p-DAPA-CN symmetric runs and 4/4 for p-DAPA-CN antisymmetric runs. The p-DAPA-CN symmetric condition included one double-transfer event.
In the simulations, the authors also report ultrafast transfer under a one-photon, targeted-frequency condition at 300 K.
A result still confined to a model
On the authors’ interpretation, the room-temperature baseline points to ESIPT being essentially thermally activated, while targeted vibrational activation was associated with different single- and double-transfer outcomes.
The conclusion is bounded by the calculation. Excited-state nuclear dynamics were treated classically, omitting vibrational zero-point energy and proton tunneling, so the reported rates and product patterns do not settle how quantum nuclear effects would alter them.
Nor is this an experimental result. The paper is a computational study of molecular models, and p-DAPA-CN is presented as a proposed rather than synthesized design. The reported times and transfer counts therefore describe calculated trajectories, not measured fluorescence lifetimes or quantum yields.
The baseline sample was modest: 10 trajectories per system at 300 K and 14 at 1000 K, while the thermal control comparison used four trajectories per condition. The authors’ picture will need testing in synthesized molecules and with calculations that include quantum nuclear motion.
Paper data and sources
Original title: Vibrational Activation Triggers Ultrafast Excited State Intramolecular Proton Transfer in Single-Benzene Fluorophores
Authors: Brieuc Le Dé, Simon Huppert, Riccardo Spezia, Alex W. Chin
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-20
DOI: Not available
Original paper · Full text