Ultrafast dynamics in rubrene and its spectroscopic manifestation

Abstract

A multimode Brownian oscillator model is employed to investigate absorption line shapes of rubrene in solution and thin film. Excellent agreement has been obtained between simulated and measured absorption spectra. Furthermore, using parameters obtained from fitting absorption spectra of rubrene, dynamics of singlet fission is explored by the Dirac-Frenkel time-dependent variation with multiple Davydov trial states. By comparing the absorption spectra between a conical intersection model and the multimode Brownian oscillator model, the optimal system-bath coupling parameter of η = 0.2 is determined, and ensuing population dynamics of the S1 state is found to concur with a previously proposed coherent singlet fission mechanism. We further simulate the two-dimensional spectra to reveal the wavepacket dynamics on both excited state and ground state potential energy surfaces. It is demonstrated that the wavepacket motion of tuning modes play a significant role in the evolution of two-dimensional spectra.

Efficient simulation of time- and frequency-resolved four-wave-mixing signals with a multiconfigurational Ehrenfest approach

“…Since an arbitrary nuclear wave function can be expanded as a linear combination of time-dependent coherent states, the Gaussian basis methods can in principle achieve a formally exact description of quantum dynamics once the Gaussian basis is sufficiently expanded toward completeness. Typical approaches using time-dependent Gaussian basis functions include the multiple spawning (MS) method [21,22], the coupled coherent states (CCS) [23,24], the multiconfigurational Ehrenfest (MCE) method [25,26], the methods of variational Multiconfigurational Gaussians (vMCG) [27], the hierarchy of the Davydov ansätze (DA) [28][29][30][31][32][33], and the Gaussian-based multiconfiguration time-dependent Hartree (G-MCTDH) method [34]. It has been shown that the Gaussian basis method with the center of each Gaussian moving along the classical trajectory already yields accurate spectra of time-resolved stimulated emission in molecular systems if sufficient Gaussians are included [35].…”

Exciton Transfer in Organic Photovoltaic Cells: A Role of Local and Nonlocal Electron-Phonon Interactions in a Donor Domain

“…The off-diagonal elements representing the interaction between the ground and excited state are ∆ 12 = ∆ 21 = 0.32ω. It should be noted that, while similar single crystals, which include rubrene 52,53 and pentacene 54 have been modeled and investigated, the parameter values of the offdiagonal element of tetracene has not been well explored. Here we employ the value of pentacene as a representative case, while the interactions between the excited states are chosen to be ∆ 23 = ∆ 32 = 1.23ω for the pair 1 and ∆ 23 = ∆ 32 = 0.12ω for the pair 2, respectively.…”

The hierarchy of Davydov's Ansätze and its applications

“…51,52 Based on many-body wave functions, the multiple Davydov Ansatz method is nonperturbative and numerically exact if provided sufficiently large multiplicities of the trial states, capable to compute nonlinear spectroscopic responses for close comparisons with experiment as will be elaborated in the next section. 52,127…”

“…Without the contribution of excited-state absorption, the PE third polarization P 3 ð Þ t ð Þ can be decomposed into four nonlinear response functions R 1À4 , which form the theoretical bases for the simulation of various 2D spectra. 4,6 The third-order response is directly given by the response functions in the impulsive limit, which can be expressed through the multi-D 2 parameters in Equation ( 18) as follows 27,52,59,104 :…”

“…To extend wave function-based methods such as the HDA approach to finite-temperature dynamics of many-body systems, several useful techniques can be borrowed, including the method of Monte Carlo importance sampling to initialize vibrational modes in order to properly account for temperature effects, 39,40 the method of thermo-field dynamics (TFD) that maps the Liouville-von Neumann equation for the density matrix to the TFD Schrödinger equation with twice as many degrees of freedom, [41][42][43][44][45][46][47][48][49] and the method of displaced number states exploiting the fact that initial excitation of the vibrational manifold can be conveniently described by displaced number states of the bath degrees of freedom. 50,52 The focus of this review is on the HDA method, that is, the formalism of employing Davydov's Ansätze 53,54 and their multistate extensions (i.e., linear superpositions of the single Davydov Ansätze) 26,27,29,55 to reveal accurate dynamics of quantum many-body systems and corresponding spectroscopic manifestations. By using the time-dependent variational principle, those Davydov trial states have been applied, with great success, to describe the ground-state as well as the dynamic properties of the paradigmatic spin-boson model (SBM) as well as the Holstein molecular crystal model.…”