Effects and Influence of External Electric Fields on the Equilibrium Properties of Tautomeric Molecules

Abstract

In this review, we have attempted to briefly summarize the influence of an external electric field on an assembly of tautomeric molecules and to what experimentally observable effects this interaction can lead to. We have focused more extensively on the influence of an oriented external electric field (OEEF) on excited-state intramolecular proton transfer (ESIPT) from the studies available to date. The possibilities provided by OEEF for regulating several processes and studying physicochemical processes in tautomers have turned this direction into an attractive area of research due to its numerous applications.

Keywords: excited state; oriented electrical field; proton transfer; tautomerism; transient spectroscopy.

Figure 1

Scheme of the shape of the Stark spectrum line of an isotropic, stationary sample with a dominant contribution of $\Delta \overrightarrow{\mathit{\mu }}$. In red, blue and green are shown vectors corresponding to ∆µ parallel, antiparallel and perpendicular to the applied field ${\overrightarrow{\mathit{F}}}_{\mathit{e}\mathit{x}\mathit{t}}$. The energy shift, in the absence of an EEF, is shown by the following equation: $\Delta \mathit{E}=-\Delta \overrightarrow{\mathit{\mu }}·{\overrightarrow{\mathit{F}}}_{\mathit{ext}}$. The consequence for the spectrum is shown on the right: some orientational subpopulations are shifted to lower energy, some to higher energy and some remain about the same. The broadened spectrum is the result of the difference between the field-applied spectrum and the fieldless spectrum, which has the second derivative line shape. Reprinted with permission from [30]: Copyright 2009 American Chemical Society.

Figure 2

Scheme of the energy level diagrams showing three types of the so-called nonclassical Stark effects in which the absorption or emission line shape is affected. All levels in blue correspond to absence of the applied field, while in green the shift in level for orientation of the field is given. (A). Application of the field, for population effects, shifts one of the states to lower energy and so the populations are supposed to respond as the system returns to equilibrium. (B) A similar effect is expected to occur for excited-state electron transfer. The rate of electron transfer is changed as the charge-separated-state energy is shifted by the field change. Because of the mixing of the locally excited and charge-separated state, the ground-state is also affected by the electric fields, which leads to “resonance Stark effects”. (C) The direct transfer of an electron from one part of a molecule to another corresponds to intervalence charge transfer bands. Theoretical investigations, such as [33,34], can be used as a description for the intensity, position, and line width of these transitions. It is expected that an application of a field will shift the relative energy of the states involved in the transition and will lead to intervalence band Stark effect line shapes. Reprinted with permission from [30]: Copyright 2009 American Chemical Society.

Figure 3

Some factors that affect the strength of electrostatic effects on the stability of a species R–X, including the nature of the interaction (a), the orientation of the charge respect to the bond dipole (b), the distance of the charge from the bond (c) and the polarity of the reaction medium (d) as quantified by its dielectric constant. Used with permission of Royal Society of Chemistry, from [60]: permission conveyed through Copyright Clearance Center, Inc.

Figure 4

Sketch of a tautomer process (a) with contribution of ground and excited states (S₀, S₁, T₀ and T₁ represent ground and excited states of enol- and keto- forms, respectively); a closed four-level-cycle process of proton transfer (b) of a tautomeric compound. Reprinted from [66]: Copyright (2020), with permission from Elsevier.

Figure 5

Snapshots from molecular dynamics (MD) simulations of a CH₃CN solvent box at different positive values of electrical field F_X values (in V/Å), noted in red below (or next to) each individual snapshot. The directions of the global dipole moment of the solvent ensemble in the box (in Debye units) are shown by the red arrows; the convention for the dipole moment vector assigns the head of the arrow as the positive pole. Reproduced from [77]: Copyright (2020), with permission from American Chemical Society with CC-BY license.

Figure 6

The four reactor types for investigations of EEF influence on processes: (a) scanning tunnelling microscope (STM) probe “nanoreactors”; (b) probe−bed−probe (PBP) reactors, in which a catalyst bed is placed in the gap between two probes; (c) continuous circuit (CC) reactors, in which the catalyst bed is integrated into an electric circuit (the surface charge gradient is represented with a color gradient in the wire) and (d) capacitor−type reactors. Red arrows indicate the general field structure; R and R′ denote adsorbed reactants. Reprinted with permission from [59]: Copyright 2018 American Chemical Society.