A Brownian Oscillator Approach to the Kennard−Stepanov Relation

Homogeneous Dephasing in Photosynthetic Bacterial Reaction Centers: Time Correlation Function Approach

A new tractable linear electronic transition dipole moment time correlation function (ETDMTCF) that accurately accounts for electronic dephasing, asymmetry, and width of 1-phonon profile, which the zero-phonon line (ZPL) contributes to it, in Rhodopseudomonas viridis bacterial reaction center is derived. This time correlation function proves to be superior to other frequency-domain expressions in case of strong electron-phonon coupling (which is often the case in bacterial RCs and pigment-protein complexes), many vibrational modes involved, and high temperature, whereby more vibronic and electronic (sequence) transitions would arise. The Fourier transform of this ETDMTCF leads to asymmetric multiphonon profiles composed of Lorentzian distribution and Gaussian distribution on the high- and low-energy sides, respectively, whereby the overtone widths fold themselves with that of the one-phonon profile. This ETDMTCF also features expedient computation in large systems using asymmetric phonon profiles to account correctly for dephasing and pigment-protein interaction (electron-phonon coupling). The derived ETDMTCF allows computing all nonlinear optical signals in both time and frequency domains, through the nonlinear dipole moment time correlation functions (as guided by nonlinear optical response theory) in line with the eight Liouville space pathways. The linear transition dipole moment time correlation function is of a central value as the nonlinear transition dipole moment time correlation function is expressed in terms of the linear transition dipole moment time correlation function, derived herein. One of the great advantages of presenting this ETDMTCF is its applicability to nonlinear transition dipole moment time correlation functions in line with the eight Liouville space pathways needed in computing nonlinear signals. As such, there is more to the utility and applicability of the presented ETDMTCF besides computational expediency and efficiency. Results show good agreement with the reported literature. The intimate connection between a one-phonon profile and the corresponding bath spectral density in photosynthetic complexes is discussed.

Exciton Dynamics and Time-Resolved Fluorescence in Nanocavity-Integrated Monolayers of Transition-Metal Dichalcogenides

We have developed an ab initio-based, fully quantum, numerically accurate methodology for the simulation of the exciton dynamics and time- and frequency-resolved fluorescence spectra of the cavity-controlled two-dimensional materials at finite temperatures and applied this methodology to the single-layer WSe₂ system. Specifically, the multiple Davydov D₂ Ansatz has been employed in combination with the method of thermofield dynamics for the finite-temperature extension of accurate time-dependent variation. This allowed us to establish dynamical and spectroscopic signatures of the polaronic and polaritonic effects as well as uncover their characteristic time scales in the relevant range of temperatures. Our study reveals the pivotal role of multidimensional conical intersections in controlling the many-body dynamics of highly intertwined excitonic, phononic, and photonic modes.

Simulation of Emission Spectra of Transition Metal Dichalcogenide Monolayers with the Multimode Brownian Oscillator Model

The multimode Brownian oscillator model is employed to simulate the emission spectra of transition metal dichalcogenide (TMD) monolayers. Good agreement is obtained between measured and simulated photoluminescence spectra of WSe₂, WS₂, MoSe₂, and MoS₂ at various temperatures. The Huang-Rhys factor extracted from the model can be associated with that from the modified semiempirical Varshni equation at high temperatures. Individual mechanisms leading to the unique temperature-dependent emission spectra of those TMDs are validated by the multimode Brownian oscillator (MBO) fitting, while it is, in turn, confirmed that the MBO analysis is an effective method for studying the optical properties of TMD monolayers. Parameters extracted from the MBO fitting can be used to explore exciton-photon-phonon dynamics of TMDs in a more comprehensive model.

Singlet fission dynamics and optical spectra of pentacene and its derivatives

A multimode Brownian oscillator model is employed to investigate the absorption spectra of pentacene and its derivatives in solution and thin films. Excellent agreement has been obtained between simulated and measured absorption spectra. Furthermore, using parameters obtained from fitting the absorption spectra of these pentacene derivatives, the singlet fission dynamics and two-dimensional electronic spectra of an ab initio Hamiltonian are investigated by Dirac-Frenkel time-dependent variation with multiple Davydov trial states. It is found that the periodic wave packet motion induced in the displaced excited state, and the accompanying vibrational relaxation, can be visualized by two-dimensional electronic spectra at short times.

Elucidating the enhancement in optical properties of low band gap polymers by tuning the structure of alkyl side chains

We carry out a computational study of optical properties of two novel 5,6-difluorobenzo[c][1,2,5]-thiadiazole-based polymers, PFBT-T20TT and PFBT-T12TT, to elucidate the surprisingly superior performance of polymer solar cells based on the former, when it differs from the latter only in the alkyl side chains. Density Functional Theory (DFT) based geometry optimization at the B3LYP/6-31G(d) level reveals differences in internal coordinates, which are important in tuning the electronic and optical properties. We further calculate the electronic structure at room temperature by employing molecular dynamics (MD) simulations in combination with DFT techniques. The energies of the highest occupied molecular orbital (HOMO) are found to be in reasonable agreement with the available experimental data and the HOMO-lowest unoccupied MO energy gap is found to be similar for both the molecules. The electronic density of the HOMO in PFBT-T20TT is, however, found to be significantly more delocalized along the backbone, which is proposed to be conducive for the formation of charge separated states leading to an improved device performance. Furthermore, via fitting the absorption spectra calculated with the multi-mode Brownian Oscillator model, we have also extracted a weaker exciton-phonon coupling parameter in PFBT-T20TT, consistent with the trends revealed via the DFT-MD results.

Vibronic bandshape of the absorption spectra of dibenzoylmethanatoboron difluoride derivatives: analysis based on ab initio calculations

The nature of absorption bandshapes of dibenzoylmethanatoboron difluoride (DBMBF₂) derivatives is investigated using TDDFT and time-dependent linear vibronic coupling model.

Simulating the peptide folding kinetic related spectra based on the Markov State Model

Optical spectroscopic tools are used to monitor protein folding/unfolding dynamics after a fast triggering such as the laser induced temperature jump. These techniques provide new opportunities for comparison between theory and simulations and atom-level understanding protein folding mechanism. However, the direct comparison still face two main challenges: a gap between folding relevant timescales (microseconds or above) and length of molecular dynamics simulations (typically tens to hundreds of nanoseconds), and difficulty in directly calculating spectroscopic observables from simulation configurations. Markov State Model (MSM) approach is one of the most powerful means which can increase simulations timescale up to microsecond or even millisecond. We address progress on modeling infrared and fluorescence spectroscopic signals of temperature jump induced unfolding dynamics for a few small proteins. The harmoniousness between experiment and theoretical can both improve our understanding of protein folding mechanisms and provide direct validation of those theoretical models.

Investigating the structural origin of trpzip2 temperature dependent unfolding fluorescence line shape based on a Markov state model simulation

Vibrationally resolved fluorescence spectra of the β-hairpin trpzip2 peptide at two temperatures as well as during a T-jump unfolding process are simulated on the basis of a combination of Markov state models and quantum chemistry schemes. The broad asymmetric spectral line shape feature is reproduced by considering the exciton-phonon couplings. The temperature dependent red shift observed in the experiment has been attributed to the state population changes of specific chromophores. Through further theoretical study, it is found that both the environment's electric field and the chromophores' geometry distortions are responsible for tryptophan fluorescence shift.

Green Luminescence Band in ZnO: Fine Structures, Electron−Phonon Coupling, and Temperature Effect

The green emission band of ZnO has been investigated by both experimental and theoretical means. Two sets of equally separated fine structures with the same periodicity (close to the longitudinal optical (LO) phonon energy of ZnO) are well resolved in the low-temperature broad green emission spectra. As the temperature increases, the fine structures gradually fade out and the whole green emission band becomes smooth at room temperature. An attempt to quantitatively reproduce the variable-temperature green emission spectra using the underdamped multimode Brownian oscillator model taking into account the quantum dissipation effect of the phonon bath is done. Results show that the two electronic transitions strongly coupled to lattice vibrations of ZnO lead to the observed broad emission band with fine structures. Excellent agreement between theory and experiment for the entire temperature range enables us to determine the dimensionless Huang-Rhys factor characterizing the strength of electron-LO phonon coupling and the coupling coefficient of the LO and bath modes.