Nonadiabatic excited-state intramolecular proton transfer in 3-hydroxyflavone: S2 state involvement via multi-mode effect

Abstract

The excited-state intramolecular proton transfer process in 3-hydroxyflavone is investigated based on the computed structural parameters and energetics of stationary points of vibronically coupled S1-S2 potential energy surfaces. A conical intersection close to the Franck-Condon point on S1 is identified. The minimum energy of the conical intersection is found to be near-degenerate with the equilibrium minimum of S2. Quantum nuclear wavepacket simulations revealed a small population transfer from the “bright” S1 to “dark” S2 on a time scale shorter than the O—H stretching vibrational period. Such a nonadiabatic transition opens up the possibility of new photophysical and photochemical pathways, including the proton transfer via S2.

Nature of Fast Relaxation Processes and Spectroscopy of a Membrane-Active Peptide Modified with Fluorescent Amino Acid Exhibiting Excited State Intramolecular Proton Transfer and Efficient Stimulated Emission

“…The emission from the excited state normal form (N*) of F was not observed, presumably because of relatively fast (∼ ps timescale) ESIPT processes that were frequently observed for similar 3HF derivatives. 51 , 55 − 57 According to the 3D emission maps of 1 ( Figure 3 ), the shape of the obtained fluorescence spectra is independent of the excitation wavelength. The fundamental anisotropy spectrum of 1 , r 0 (λ), was obtained in viscous glycerol solution ( Figure 2 , curve 3) and exhibited relatively high (≥ 0.36) and nearly constant values in the main long-wavelength absorption band.…”

“…The steady-state linear absorption spectra of 1 (Figure , curve 1) exhibited structureless long-wavelength absorption bands at ∼399–412 nm with relatively weak intensity (maximum extinction coefficient, ε max ≈ (24–25)·10 3 M –1 cm –1 ) and mild dependence on solvent polarity (see Table ). Taking into account the spectral and electronic properties of similar 3HF derivatives, , it is reasonable to assume that the observed long-wavelength bands can be assigned to the S 0 → S 1 transition with π → π* character , (S 0 and S 1 are the ground and first excited electronic state, respectively). The absorption spectra belong to the normal (N) form of the fluorophore part ( F ) in 1 (see the molecular structure in Figure ), which can exhibit ESIPT processes under photoexcitation. ,,, The steady-state fluorescence spectra of 1 revealed only one emission band with relatively large Stokes shifts (∼6000 cm –1 ) that can be assigned to the excited state tautomer form (T*) fluorescence of F .…”

Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview

“…Both investigations led to similar conclusions on a critical role of a conical intersection between bright S and dark S states, of respective * and n * character, which was interpreted as the reason for observation of two ESIPT rate constants for these molecules in the experiment. Anand et al applied the same methodology to study ESIPT also in similar 3-hydroxyflavone [ 150 ] and 3-hydroxypyran-4-one [ 151 ] systems, confirming the important role of the S state in their photorelaxation. Finally, recent thorough work by Cao et al provided theoretical insights on ESIPT-driven mechanism and quantum dynamics of thermally activated delayed fluorescence in triquinolonobenzene [ 152 ], in which singlet-state ultrafast proton transfer occurs within a dense manifold of low-lying triplet states.…”

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

“…It should be noted though at this point that other alternatives to the adiabatic S 1 process involving triplet states and higher singlet states (S 2 , S 3 ) have also been invoked. Recently, a detailed analysis of the effect of the S 2 /S 1 conical intersection in 3-HF using several quantum chemical methods and a combined quantum dynamical study demonstrated that non-negligible S 2 populations occurred immediately after the Franck–Condon excitation . The calculated S 2 populations depended significantly on the method used for the calculation of the energy surface.…”

“…Recently, a detailed analysis of the effect of the S 2 /S 1 conical intersection in 3-HF using several quantum chemical methods and a combined quantum dynamical study demonstrated that non-negligible S 2 populations occurred immediately after the Franck−Condon excitation. 28 The calculated S 2 populations depended significantly on the method used for the calculation of the energy surface. Time-dependent density functional theory (TDDFT) using the Becke 3-parameter Lee, Yang, and Parr (B3LYP) and the Coulomb-attenuating method B3LYP showed average S 2 populations below 5%, whereas the secondorder approximate coupled-cluster (CC2) method gave much larger S 2 populations since the minimum energy conical intersection was located below the Franck−Condon energy.…”