David B. Lingerfelt

Time-dependent non-equilibrium dielectric response in QM/continuum approaches

The Polarizable Continuum Models (PCMs) are some of the most inexpensive yet successful methods for including the effects of solvation in quantum-mechanical calculations of molecular systems. However, when applied to the electronic excitation process, these methods are restricted to dichotomously assuming either that the solvent has completely equilibrated with the excited solute charge density (infinite-time limit), or that it retains the configuration that was in equilibrium with the solute prior to excitation (zero-time limit). This renders the traditional PCMs inappropriate for resolving time-dependent solvent effects on non-equilibrium solute electron dynamics like those implicated in the instants following photoexcitation of a solvated molecular species. To extend the existing methods to this non-equilibrium regime, we herein derive and apply a new formalism for a general time-dependent continuum embedding method designed to be propagated alongside the solutes electronic degrees of freedom in the time domain. Given the frequency-dependent dielectric constant of the solvent, an equation of motion for the dielectric polarization is derived within the PCM framework and numerically integrated simultaneously with the time-dependent Hartree fock/density functional theory equations. Results for small molecular systems show the anticipated dipole quenching and electronic state dephasing/relaxation resulting from out-of-phase charge fluctuations in the dielectric and embedded quantum system.

“Watching” Polaron Pair Formation from First-Principles Electron–Nuclear Dynamics

The formation of polaron pairs is one of the important photophysical processes that take place after the excitation in semiconducting organic polymers. First-principles Ehrenfest excited-state dynamics is a unique tool to investigate ultrafast photoinduced charge carrier dynamics and related nonequilibrium processes involving correlated electron-nuclear dynamics. In this work the formation of polaron pairs and their dynamical evolution in an oligomer of seven thiophene units is investigated with a combined approach of first-principles exciton-nuclear dynamics and wavelet analysis. The real-time formation of a polaron pair can be observed in the dipole evolution during the excited-state dynamics. The possible driving force of the polaron pair formation is investigated through qualitative correlation between the structural dynamics and the dipole evolution. The time-dependent characteristics and spectroscopic consequences of the polaron pair formation are probed using the wavelet analysis.

Ab initio two-component Ehrenfest dynamics

We present an ab initio two-component Ehrenfest-based mixed quantum/classical molecular dynamics method to describe the effect of nuclear motion on the electron spin dynamics (and vice versa) in molecular systems. The two-component time-dependent non-collinear density functional theory is used for the propagation of spin-polarized electrons while the nuclei are treated classically. We use a three-time-step algorithm for the numerical integration of the coupled equations of motion, namely, the velocity Verlet for nuclear motion, the nuclear-position-dependent midpoint Fock update, and the modified midpoint and unitary transformation method for electronic propagation. As a test case, the method is applied to the dissociation of H2 and O2. In contrast to conventional Ehrenfest dynamics, this two-component approach provides a first principles description of the dynamics of non-collinear (e.g., spin-frustrated) magnetic materials, as well as the proper description of spin-state crossover, spin-rotation, and spin-flip dynamics by relaxing the constraint on spin configuration. This method also holds potential for applications to spin transport in molecular or even nanoscale magnetic devices.

Direct ab Initio (Meta-)Surface-Hopping Dynamics

Tractable methods for studying the molecular dynamics of chemical processes driven by electronic nonadiabaticity are highly sought after to provide insight into, for example, photochemical reaction mechanisms, molecular collisions, and thermalized electronic band structures. Starting from the time-dependent Schrödinger equation for a many-body system, a direct ab initio trajectory surface-hopping (TSH) method relying on an analytical treatment of nonadiabatic couplings between electronic states is developed in this work. An approach that combines time-dependent perturbation theory and explicit time evolution via TSH to expedite calculation of nonadiabatic transition rates, namely, meta-surface-hopping dynamics, is presented, and an extrapolatory approach using time-dependent perturbation theory for recovering unbiased transition rates is assessed. The meta-surface-hopping method is applied to the problem of estimating nonradiative relaxation rates of a photoexcited iminium ion, CH₂NH₂⁺, and evidence for internal consistency of the combined dynamics/perturbation theory approach is presented.

Coupling Real-Time Time-Dependent Density Functional Theory with Polarizable Force Field

Real-time time-dependent density functional theory (RT-TDDFT) is a powerful tool for obtaining spectroscopic observables and understanding complex, time-dependent properties. Currently, performing RT-TDDFT calculations on large, fully quantum mechanical systems is not computationally feasible. Previously, polarizable mixed quantum mechanical and molecular mechanical (QM/MMPol) models have been successful in providing accurate, yet efficient, approximations to a fully quantum mechanical system. Here we develop a coupling scheme between induced dipole based QM/MMPol and RT-TDDFT. Our approach is validated by comparing calculated spectra with both real-time and linear-response TDDFT calculations. The model developed within provides an accurate method for performing RT-TDDFT calculations on extended systems while accounting for mutual polarization between the quantum mechanical and molecular mechanical regions.

Ab Initio Excited-State Transient Raman Analysis

Time-resolved Raman spectroscopy has proven useful for studying the formation of polarons in conjugated polymers, verifying the presence of reactive intermediates in photochemical reactions, investigating nonradiative transitions in the short lifetime of the photoexcited species, and resolving electron-phonon coupling strengths and exciton dissociation in crystalline materials. In this paper, we present an excited state transient Raman analysis protocol combining ab initio direct molecular dynamics, transient excited state Hessian, and excited state nonresonant Raman activities evaluations. Prototypical molecules are used as test cases, showing the evolution of the transient Raman signatures that follow electronic excitation. This protocol provides a direct route to assigning the vibrations implicated in the (photo)dynamics of several (photoactive) systems, complementary to the transient infrared analysis.

Can Excited State Electronic Coherence Be Tuned via Molecular Structural Modification? A First-Principles Quantum Electronic Dynamics Study of Pyrazolate-Bridged Pt(II) Dimers

Materials and molecular systems exhibiting long-lived electronic coherence can facilitate coherent transport, opening the door to efficient charge and energy transport beyond traditional methods. Recently, signatures of a possible coherent, recurrent electronic motion were identified in femtosecond pump-probe spectroscopy experiments on a binuclear platinum complex, where a persistent periodic beating in the transient absorption signals anisotropy was observed. In this study, we investigate the excitonic dynamics that underlie the suspected electronic coherence for a series of binuclear platinum complexes exhibiting a range of interplatinum distances. Results suggest that the long-lived coherence can only result when competitive electronic couplings are in balance. At longer Pt-Pt distances, the electronic couplings between the two halves of the binuclear system weaken, and exciton localization and recombination is favored on short time scales. For short Pt-Pt distances, electronic couplings between the states in the coherent superposition are stronger than the coupling with other excitonic states, leading to long-lived coherence.

Role of Vibrational Dynamics on Excited-State Electronic Coherence in a Binuclear Platinum Complex

Long-lived quantum coherence between excited electronic states can enable highly efficient energy and charge transport processes in chemical systems. Recent pump-probe experiments on binuclear platinum complexes identified persistent, periodic beating of transient absorption anisotropy signals, indicating long excited-state coherence lifetimes. Our previous simulations of the electronic dynamics of these complexes indicate that coherence lifetimes are sensitive to the balance between competitive electronic couplings. The complexes with shorter Pt-Pt distances underwent no appreciable dephasing in the limit of static nuclei, motivating the inclusion of nuclear motion into our simulations. The tert-butyl-substituted complex is studied in this work using the Ehrenfest method for mixed quantum-classical dynamics to investigate the role of vibrational dynamics on a complex shown to support long coherence lifetimes. Results indicate that the inclusion of excited-state vibrations drives a rapid collapse of the two-state coherence prior to the experimentally determined intersystem crossing. This further suggests singlet excited-state coherences may not be prerequisites for long-lived triplet coherences.