Open Access Article

This Open Access Article is licensed under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence

Chao-Ping
Hsu

128 Academia Road Section 2, Institute of Chemistry, Academia Sinica, Nankang, Taipei, 11529, Taiwan. E-mail: cherri@sinica.edu.tw; Fax: +886 2 27831237; Tel: +886 2 55728658

Received
3rd June 2020
, Accepted 13th August 2020

First published on 17th August 2020

In describing the dynamics of electron transfer or charge transport, the reorganization energy and the spectral density function describe the influence of nuclei motion to the transporting electron. The spectral density can be obtained using various theoretical approaches: from a model of dielectric response, or calculated with various computational means. With the vast advancement of modern computational techniques, many details in vibronic coupling can be obtained, including those described in the early literature. In this work, we provide a comprehensive understanding of the nature of vibronic coupling in light of some early literature. The theoretical connection among different quantities for vibronic coupling is discussed, followed by a brief review of the spectral density function. Various approaches and some of the results for the spectral density function are also reviewed. The importance of low-frequency bands in nonpolar systems that can be overlooked is also discussed, for both Holstein and Peierls types of electron–phonon couplings.

To develop systems with good charge transport, non-polar (or less dipolar) molecules with low band gaps have been commonly considered, for the benefit of having low reorganization energy (often denoted as λ). One of the key insights of Marcus theory is that the activation energy of the reaction arises essentially from the reorganization of polarization from the environment. The less polar molecular device offers an environment with low dielectric response, which is quite different from systems discussed in the traditional electron transfer (ET) literature, where electrolytes are mainly in a polar or even aqueous solution. Most of the insights derived previously with polar environments in mind might still be valid in the nonpolar systems. Therefore, one of the goals of the present work is to lay out aspects of the reorganization energy with a stress on our current understanding of a nonpolar system.

With the vast advancement of computational hardware and software, it is now routinely possible to calculate and dissect the reorganization energy in various forms. Reorganization energy, electron–phonon coupling, and spectral density function are frequently discussed in the ET literature or related topics. Modern works simulating the charge transport dynamics have been developed using various schemes.^{12–18} However, because of the complex nature of the problem, there are inevitable discrepancies among different theoretical models and simulation approaches. For example, to date, most charge transport simulation works rarely considered the feedback of the polaron to the classical nuclear motions, whereas recent improvements have been developed^{19–21} to include such feedbacks. Some early literature, mainly in the field of non-equilibrium statistical mechanics, has offered rigorous and insightful bases, such that modern computational work can be relied on, helping us derive further understanding for the system.^{22–26} Many of these works are well-written, with great insights; some do not have apparent relevance, whereas others might appear to be difficult to understand especially in the theoretical derivations. For facilitating the development of modern simulation, a summary and overview on the theoretical grounds is helpful, which is what we aim to provide in the present work. Therefore, another goal of the present work was to introduce such works and connect their results to the modern computational chemistry. Therefore, this work is far from any comprehensive review for modern computational chemistry work on charge-transport materials. Instead, we aim to provide an integrated and yet modest set of relevant information from the traditional statistical mechanics field, for deriving more insights from computational results.

In this work, we first review various similar quantities for the reorganization energy and their background theoretical models, followed by a section on the dielectric continuum models and their theoretical predictions. We then outline some of the computational chemistry results and conclude with speculation on future perspectives.

(1) |

Fig. 1 Marcus' parabolic free energy curves (a), broken down into harmonic oscillators, showing one of them as x_{i} as shown in (b), with various quantities labelled. |

We note that, for a classical harmonic oscillator, a standard theoretical model does not need any parameter, because the units for displacement and the energy can be chosen to fit any harmonic oscillator in application. For ET problems, the minimal number of parameters describing the set of two parabola with the same curvature is 2, with one being the free-energy difference ΔG^{0}, measuring the vertical shift of the two curves, and another can be defined by various means: the curvature, the displacement (horizontal shift), or the coefficient for the difference of the two states (which is a linear function), depending on the setting of the theoretical models.

The reorganization energy is further divided into two different contributions: one arising from the surrounding solvent, the outer sphere component, (λ_{out}) and the other from internal degrees of freedom (or the first solvation shell, λ_{in}).

For λ_{out}, because the process of ET involves a change in charge distribution of the donor–acceptor pair, the most important effect from the environment is its polarization, or the dielectric response. An expression considering such changes, in a simplified spherical cavity model, was given by Marcus:^{28,29}

(2) |

Another component for reorganization is the internal contribution, which is derived from the internal degrees of freedom for a molecule, or the inner solvation shell for an ion in a polar solution. In this case it is common to express the reorganization energy in terms of the structural difference (Δ) in the reactant and product states and project such a structural difference to the normal modes (in mass-weighted coordinates {Q_{i}}) of the reactant (or the product).

(3) |

(4) |

λ_{i} = S_{i}ħω_{i}, | (5) |

(6) |

(7) |

(8) |

(9) |

(10) |

(11) |

(12) |

(13) |

(14) |

(15) |

(16) |

It is also possible to further generalize the electron–phonon coupling to the transfer integral, leading to the Peierl electron–phonon coupling, which is also studied in the literature^{36–38}

(17) |

In both electron–phonon coupling models, g_{i} is a dimensionless parameter for electron–phonon coupling. It can be shown that

(18) |

(19) |

(20) |

However, we note that, for an array of molecules, periodic or not, assigning a single-site location for a vibrational mode in general can be difficult. As eigenvectors of the Hessian matrix, the motion of nuclei can be distributed all over the system. Expressions such as eqn (15) and (20) were written when the coupling among local vibrations is not large, or, in an amorphous condensed phase where vibrational modes are somewhat localized to certain sites. Therefore, the frequently used expression in eqn (16) is more general and could better describe the long-range interaction between a general vibrational mode q and site n.

A great proportion of interesting problems in chemistry are not in the gas phase, nor do they take place in a periodic environment. The classical redox systems mainly involve electrolytes in solutions, and the modern charge transporting devices are mostly in amorphous solid-state phase, not to mention that many interesting ET problems are in biomolecules such as proteins or DNAs, mainly in aqueous solution. A convenient treatment for the model is to keep electrons in a desirable basis,‡ and to use vibrational modes for the system as a whole. Depending on the computational setting, usually it is easy to separate the inner-shell contribution, the vibrational modes of the donor and acceptor molecules (or fragments) from those arising from the surrounding molecules. Here we note that the expression in eqn (17) can be generalized further, with an index for all vibrational modes of the whole system, collective or not.^{38,45} The Holstein–Peierls (HP) coupling term can be expressed as,

(21) |

(22) |

(23) |

X(t) = e^{itH/ħ}Xe^{−itH/ħ}, | (24) |

C(t) ≡ 〈δX(t)δX(0)〉 | (25) |

(26) |

With C_{i}x_{i}, the displacement of each mode is C_{i}/m_{i}ω_{i}^{2}, and the contribution to the reorganization energy is C_{i}^{2}/2m_{i}ω_{i}^{2}. Therefore, the reorganization can be related to J(ω) as well:

(27) |

Just as λ is divided into the inner and outer-shell contribution, the spectral density can also be divided in the same way. The inner-shell component is much easier to define. As discussed in Section 2.1, the individual contribution λ_{i} can be defined and calculated routinely (eqn (3)). Therefore the “inner” spectral density function can be

(28) |

In theoretical studies, one commonly uses analytical functions such as

J(ω) = Aω^{s}e^{−ω/ωc}, | (29) |

We note that in Marcus theory, all the influence of the nuclei motion is integrated into λ. The internal vibrational modes of the donor and acceptor contribute to λ_{in}, which comes with a specific set of frequencies. The polarization of the environment constitutes λ_{out}, which usually forms a “band” of continuous modes. However, in the SBH and HP models, all the nuclei motions are treated with the harmonic oscillator models, with an infinite number of harmonic oscillators as the model onto which all influences can be projected.^{25} Such theoretical models are flexible for many different problems, but model setting for this infinite number of coupling coefficients becomes a problem. The dielectric continuum models introduced in the following section are practical solutions.

The hypothetical 1-dimensional reaction coordinate is also a central concept in Marcus theory. The harmonic oscillator model for the bath naturally leads to a many (or infinite) dimensional picture, with each dimension similar to that depicted in Fig. 1(b). However, the dynamics of the system can be described using a 1-dimensional model, and the simplest definition for this coordinate is the energy difference between the reactant and product states, ΔE(≡X). As will be shown in the following section and the references cited therein, ΔE offers a means to model, calculate and understand the dynamics of the system. The fluctuation–dissipation theorem and its corresponding (linear) response function can be built with ΔE. Moreover, a parabolic free energy curve along the reaction coordinate implies a Gaussian distribution in the ensemble.^{27} However, the linear assumption could be challenged, and it can be corrected with theoretical and numerical efforts.^{50}

ΔE_{int}(t) can be generally written as

(30) |

α(τ) = 〈[δX(τ),δX(0)]〉 = 2ImC(τ)/ħ, | (31) |

J(ω) = −Im(ω). | (32) |

(33) |

ΔẼ_{int}(ω) = −(ω)(ω), | (34) |

1. An early version with the general formulation mainly for a uniform electric (and displacement) field developed in ref. 22, leading to,§

(35) |

(36) |

ε(ω) = ε′(ω) + iε′′(ω). | (37) |

2. For the Marcus' sufficiently separated donor and acceptor model, the dielectric image effect can be neglected, such that

(38) |

(39) |

3. Using the Onsager model, the system can also be modeled as a closely spaced donor and acceptor in a spherical cavity of radius a and change in the dipole moment Δμ. In this case we can obtain,¶

(40) |

(41) |

• movement of the molecules (mainly charged particles),

• changes in the orientation of the polar molecule,

• changes in the bond angle or other internal degrees of freedom of the solvent molecules,

• stretching or suppression of polar bonds, and the polarization of electronic distribution,

• polarization of electrons.

For ET, the donor–acceptor system has a change in its charge distribution. The first term, the movement of charge particles does not really occur in the regular solvents composed of neutral molecules. The second item, the orientational effects in polarization, gives rise to Debye dielectric solvation. For ET, the electronic polarization is faster than the charge transfer process; thus an optical dielectric constant ε_{op} is used to represent the “infinite frequency” limit in the theories.

3.2.1 Low frequency: Debye dielectric relaxation.
For polar solvents, the Debye dielectric relaxation model can be used to describe the dielectric dispersion ε(ω), where the response of polarization was assumed to be an exponential decay function, with lifetime τ. τ can be considered as the orientational lifetime for a fluctuating solvent. This exponential response yields to a dielectric function as,

where τ is the characteristic relaxation time. We note that in most of the literature the dielectric response at the high-frequency limit is often denoted as ε_{∞}, but here we use ε_{op} for better coherence with expressions shown above. With this model, it gives

and

Therefore, there should be a diffuse peak at 1/τ for the spectral function J(ω) for a polar solvent.

(42) |

(43) |

(44) |

3.2.2 Higher frequency: resonances.
When a dielectric material is placed under the oscillatory electric field, the oscillations at various levels, as listed above, affect the polarization of the material. If the frequency is low, the motion of the oscillation follows, leading to a cancellation of the electric field applied, and thus dielectric shielding is obtained (in the real part of ε(ω)). When the frequency of the applied field increases, to a point at which the frequency matches the intrinsic frequency of the oscillation, a resonance is observed and there is an absorption of electric energy. An absorption peak (dielectric loss) appears in the imaginary part of ε(ω). For frequencies above the resonance, the oscillation can no longer catch up with the external field, and thus this mode no longer contributes to the cancelation of the electric field, which leads to a lower value in the real part of ε(ω). Therefore, the real part, ε′(ω), steps down to a lower value with each resonance, whereas the imaginary part, ε′′(ω), has successive absorption peaks.

For systems composed of polar molecules, the lowest frequency motion that leads to a dielectric effect is perhaps the reorientation. Therefore, the system starts with a (large) static dielectric constant in the real part, with Debye-like relaxation accounting for the rotational relaxation, followed perhaps by low and high-frequency vibrations that can contribute to the polarity, finally reaching the optical frequency such that it is no longer relevant to the ET problem.

The mathematical description for the above process, (i.e., a resonance in the imaginary part together with a step down in the real part of the response function) can be described by the Kramer–Kronig relationship, which arises from the general causality of the response function,^{57,58} but will not be recapitulated here.

With experimental results, we include the spectral density function J(ω) for 5 different solvents as shown in Fig. 2. The experimental dielectric data were obtained from ref. 60–70. From Fig. 2 dielectric response spectra for the polar solvents have a large component for the low frequency region (≤1000 cm^{−1} for water, ≤2–300 cm^{−1} for methanol and acetonitrile). The low-frequency response consists of intermolecular modes such as constrained translational and rotational modes. For polar molecules, an external field may induce an orientational motion, leading to a dielectric response. These are known to contribute significantly to the dynamic Stokes shift in time-dependent fluorescence.^{51,52}

Fig. 2 Im[1/ε(ω)] as a model for J(ω) with experimentally measured ε(ω) (black lines, data derived from ref. 52, 60, 63, 64, 66–68 and 70). At low frequency, a red curve for the Debye or multiple Debye model is included, with parameters from ref. 62 and 65 for water and acetonitrile, and interpolation and adjustment to 25 °C according to ref. 69. Inset: log–log scaled plots between 1 and 1000 cm^{−1} and Im[1/ε(ω)] between 0.0001 and 1. |

The intramolecular bending and OH stretching bands are also seen for both water and methanol, which are broader than the vibrational bands in the nonpolar system. All of these bands contribute to λ_{out}.

For nonpolar or weakly polar systems, sharp, narrow vibrational bands occur in the high-frequency area, and the low-frequency responses are very weak. For benezene and toluene, the largest contribution is from a band near 700 cm^{−1}, which has been assigned as the out-of-plane CH bending. Indeed the C–H bond is weakly polar, and its bending can create a polarization in response to a change in the solute. With these results, one might conclude that the high-frequency intramolecular vibration is important for the nonpolar solvation, whereas the influence is not so great for the low-frequency, orientational, or acoustic phonon-like region. However, the dielectric response model has its own assumption and limitations. For periodic or MD simulations, some response is still observed in the low-frequency region, as outlined below.

Holstein type | Peierls type | ||
---|---|---|---|

Isolated molecules | Obtained | • Breaking down λ_{in} to all intramolecular vibrational modes. |
• Scanning few intermolecular degrees of freedom for coupling; directly evaluating the nuclear derivative of electronic coupling is also seen. |

• Mainly high-frequency, intramolecular contribution. | • Most works reported intermolecular contributions with scanning. | ||

Limitation | • Difficult to obtain the low-frequency, intermolecular contribution. | • With scanning: need to specify the mode and scan. Difficult to obtain all the modes. | |

• Difficult to obtain the corresponding frequencies and their dispersion in a condensed state. | |||

Periodic models | Obtained | • General approaches can account for contributions from all phonons, including both optical (intramolecular) and acoustic (intermolecular) phonons | • Can fully account for all modes for the off-diagonal matrix elements. |

Limitation | • Can be difficult to derive “chemical insights” in real space. | • Involves 3 sets of indexes for the reciprocal vectors and corresponding electron and phonon bands. Even more complicated to gain chemical insights. | |

With MD simulation | Obtained | • Depending on the setting, can obtain purely outer-sphere component, or both inter and intra-molecular contribution | • The off-diagonal coupling can be obtained from MD snap shots. |

• Mainly arises from low-frequency, intermolecular motions | |||

Limitation | • Quality depends on the force field used. | • Large cost of quantum chemistry calculation, and therefore, poor ensemble averaging quality. |

The commonly seen charge transporting materials often involve the aromatic π moiety that has alternate C–C single and double bonds. Ionization or excitation in these molecules generally involves a change in this alternacy: shorter single bonds and longer double bonds. Therefore, it is common to see a large contribution from this alternating C–C stretch mode in the projection λ_{in}, leading to peaks in the region of 1300–1500 cm^{−1}, whereas many other modes could have some contribution as well.^{73,74}

For a mode with large vibration frequency, the population will mainly reside at the zero-point energy level, thus resulting in weak temperature dependence. Such a behavior was believed to contribute to the weak or even inverse temperature dependence reported in charge mobility in molecular crystals.^{72} However, such a model ignores the low-frequency modes which mainly arise from intermolecular libration or reorientation in response to a change in the charge position, an effect that exists predominantly in any condensed phase as further discussed below.

The electronic coupling calculation^{75} is becoming routinely available in many quantum chemistry packages, but the off-diagonal Peierls type of vibronic coupling has been also reported.^{73,74,76} In most of these works, the electronic coupling factor was scanned via a few degrees of freedom between two organic molecules. The scan is typically performed with planar π-conjugated organic molecules with one or two degrees of freedom in their relative position of π–π stacking, and oscillatory coupling is observed owing to the nature of wavefunctions in the HOMO or LUMO, thus showing the importance of the intermolecular modes in a condensed array of molecules. Another well-known property is the exponential intermolecular distance dependence of the electronic coupling.^{77} These observed behaviors contribute to important Peierls coupling for the intermolecular acoustic-like modes. A direct computation for the nuclear derivative for the electronic coupling is available nowadays,^{78,79} which allows for direct computation for the Peierls coupling with any given vibrational mode, which would facilitate such characterization without having to scan for the nuclear degrees of freedom.

Computational schemes for electron–phonon coupling for periodic systems have also been developed.^{89–91} In general, the electron–phonon coupling is a function of the band and the reciprocal vector for the electron and the band and the reciprocal vector for the phonon. A lot of such works have been reported for covalent systems such as inorganic semiconductors or graphene,^{92,93} whereby the rigid covalent bonds are likely to offer a steep electron phonon coupling. Such approaches can be used for organic molecular crystals, but a careful treatment of the density functionals for a good intermolecular dispersion interaction would be necessary.

A direct evaluation for the electron–phonon coupling in a periodic system can also be achieved by using derivative coupling^{94} or directly accounting for the matrix elements.^{95} Both diagonal and off diagonal electron–phonon couplings were reported in these works. The diagonal Holstein type of electron–phonon coupling is affected by both the low- and high-frequency modes, whereas the coupling for off-diagonal elements, the Peierls type of coupling, is mostly in the low-frequency acoustic phonons.^{94,95}

For example, the spectral densities have been reported for water and methanol.^{59,99} In these early works, the general features are remarkably similar to those depicted in Fig. 2: an intramolecular contribution as well as low-frequency intermolecular bands. Nevertheless, such results imply that both inter- and intra-molecular bands contribute to the dielectric response, or to the reorganizational energy for an ET process.

In Fig. 3, we have included the spectral density derived from a Fourier transformation of the autocorrelation function C(t), for ethylene, a nonpolar molecule in its liquid state,|| as a test case for the nonpolar system. In this case, sharp high-frequency peaks are seen, similar to those with the dielectric response for benzene and toluene. The 1200 cm^{−1} peak for the CH_{2} rocking mode, together with weaker peaks near 1700 cm^{−1} for possibly the CC stretching, and 2900 cm^{−1} for C–H stretching modes, is observed.

Fig. 3 Spectral density derived from a MD simulation for a periodic box of small nonpolar molecules of ethylene. Shown is the Fourier transformation of the autocorrelation function C(t), scaled by coth(βħω/2) (eqn (26)), in arbitrary units. |

An interesting feature in Fig. 3 is the diffuse signal at the low-frequency region, at about 100 cm^{−1} and below. In contrast to the spectral density derived from the dielectric response for the nonpolar benzene that has nearly zero contribution in this region, with a similar nonpolar system, and similar to the direct calculation for electron–phonon coupling for nonpolar molecular crystals^{94,95} a low-frequency response is seen with MD. The current dielectric response model assumes that the surrounding medium is a homogeneous dielectric, and the solute charge is assumed to be in a spherical cavity. In MD, ΔE collected includes all the electrostatic interaction between a cationic ethylene and the surrounding neural molecules. Near the cation, a change in the orientation of a neutral ethylene leads to a different electrostatic interaction. It is also possible that higher order polarity effects could contribute (e.g. the quadruple moment could interact with a spatial change in the electric field). Therefore, low-frequency orientational or translational motion can contribute. Omitting such low-frequency contribution, even for nonpolar systems, might lead to an incorrect description for the system.

The off-diagonal electron–phonon coupling has also been studied with MD simulation and INDO/S semiempirical Hamiltonian calculations, or DFT.^{36,41,104} Again, the importance of low-frequency modes close or under 100 cm^{−1} is observed, similar to those observed with periodic first-principle calculations.^{94,95} With the off-diagonal element of the Hamiltionian, the electronic coupling is sensitive to the relative position and orientation of the two molecules where ET is being considered, and thus, the low-frequency modes are important in this case. A future possibility is to use machine learning for a better ensemble averaging and higher resolution in the spectral density function.^{105}

The quality of MD simulation results highly depends on the force field used.^{96} A polarizable force field should offer a better solution. Meanwhile, the development for combined quantum mechanical/molecular mechanical (QM/MM) calculations^{106} is worth noting. QM/MM allows for treating one or several molecules using a high-level method while accounting for the effects of the rest, which will offer a much better description for the reorganization λ and its break-down, electron–phonon coupling factors g. By combining QM/MM with MD, a spectral density function can be obtained. This approach has been shown useful to simulate the spectra for pigment-binding proteins, and it can also be useful for describing ET systems.^{107}

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## Footnotes |

† This article is dedicated to the memory of Prof. Ching-Liang Lin (1931–2019), for her warm love to students and her firm support to women physicists. |

‡ The electrons can be described with adiabatic^{13,14,16,21} or diabatic basis,^{44} but the discussion is out of the scope of the present work. |

§ α(ω) has a dimension of energy. Here, for the clarity of theories, an expression similar to eqn (12) of ref. 22 was loosely given without a rigorous ground. |

¶ The expression in eqn (40) was slightly different from that described in ref. 25, by a constant factor (ε_{c} + 2), and it is closer to those in ref. 51 and 52. |

|| The simulation was performed using the Berendsen NPT ensemble, with temperature set at 130 K (for a liquid state) and pressure set at 1 bar, under optimized potentials for the liquid simulations force field^{100} using GROMACS (v 2016.4).^{101} ΔE was simulated with the Coulomb interaction of a cationic ethylene (modeled by Mulliken population analysis via DFT at the LC-BLYP/DZ* level^{102,103}) and the equilibrated surrounding molecules. |

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