Molecular Dynamics Simulation of the Excited-State Proton Transfer Mechanism in 3-Hydroxyflavone Using Explicit Hydration Models

Abstract

3-Hydroxyflavon (3-HF) represents an interesting paradigmatic compound to study excited-state intramolecular proton transfer (ESIPT) and intermolecular (ESInterPT) processes to explain the experimentally observed dual fluorescence in solvents containing protic contamination (water) as opposed to single fluorescence in highly purified nonpolar solvents. In this work, adiabatic on-the-fly molecular dynamics simulations have been performed for isolated 3-HF in an aqueous solution using a polarizable continuum model and including explicit water molecules to represent adequately hydrogen bonding. For the calculation of the excited state, time-dependent density functional theory and the Becke-3-Lee-Yang-Parr (B3LYP) functional have been used. For the isolated 3-HF, ultrafast ESIPT from the enol group to the neighboring keto group has been observed. The calculated PT time of 48 fs agrees well with the experimental value of 39 fs. Addition of one water molecule quenches this ESIPT process but shows an intermolecular concerted or stepwise tautomerization process via the bridging water molecule. Adding a second or more water molecules inhibits this ESInterPT process to a large degree. Most of the trajectories do not show any PT, preserving the initial excited-state enol structure, which is the origin of the violet-blue fluorescence appearing in the solvents contaminated with protic components.

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“…However, for the most part, a quantum mechanics/molecular mechanics (QM/MM) level is considered to account for bulk solvation on the proton shuttle mechanism in the chromophore, , thus providing a reliable description of the solvent effects, but it requires an expensive computation. So, we implemented distinct quantum mechanical (QM) microsolvation augmented with a continuum , approach, which enhances the description of solute–solvent interactions that are cost-effective and represent a good approximation of the solvent effects. , Hence, a combined microsolvation-continuum model was employed to systematically investigate and improve the description of specific solute–solvent interactions coupled to the bulk effects.…”

“…Thereafter, S 1 dynamics simulations were carried out in the NEWTON-X program in the solvent phase with a microcanonical ensemble using Born–Oppenheimer energies and gradients provided by the TDDFT/CAM-B3LYP/6-31G­(d,p) theoretical level with 0.5 fs integration time step and a maximum time of 300 fs. This approach has previously been employed and was found to be suitable in dynamics simulation studies. − The nuclear motions of all the atoms at each step of the dynamics simulations were computed using numerical integration of Newton’s equation by the Velocity–Verlet algorithm . The trajectories for each complex were analyzed and classified into two categories based on the results obtained from the on-the-fly dynamics simulations: (a) ESIPT, where the intermolecular proton transfer occurs in the excited-state during given simulation time via solvent molecules and (b) NO PT, where there occurs no PT within the simulation time.…”

“…The trajectories for each complex were analyzed and classified into two categories based on the results obtained from the on-the-fly dynamics simulations: (a) ESIPT, where the intermolecular proton transfer occurs in the excited-state during given simulation time via solvent molecules and (b) NO PT, where there occurs no PT within the simulation time. The PT time is defined as the time at which the breaking bonds and forming bonds intersect, implemented in previous investigations. ,, Moreover, the details of the PT time, PT probability, and PT mechanism along the H-bonded chain and relative energy difference were obtained by the statistical analysis of all the active PT trajectories.…”

“…The PT time is defined as the time at which the breaking bonds and forming bonds intersect, implemented in previous investigations. 63,65,66 Moreover, the details of the PT time, PT probability, and PT mechanism along the H-bonded chain and relative energy difference were obtained by the statistical analysis of all the active PT trajectories.…”

“…So, we implemented distinct quantum mechanical (QM) microsolvation augmented with a continuum 76,77 approach, which enhances the description of solute−solvent interactions that are cost-effective and represent a good approximation of the solvent effects. 65,66 Hence, a combined microsolvationcontinuum model was employed to systematically investigate and improve the description of specific solute−solvent interactions coupled to the bulk effects.…”