Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E25: Chemical Physics of Multichromophores IIFocus
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Sponsoring Units: DCP Chair: Tim Berkelbach, University of Chicago Room: 288 |
Tuesday, March 14, 2017 8:00AM - 8:36AM |
E25.00001: Conditional energy transfer: Towards molecular excitonic Gates. Invited Speaker: Alan Aspuru-Guzik In this talk, I will describe an approach that we developed to carry out conditional energy transfer. In particular, we propose a multi-chromophore ultrafast approach that based on a defined pulse sequence, allows for directing energy transfer on selected pathways. If experimentally realized, this proposal opens the doors to applications in a variety of fields, although we concentrate on biomolecular labeling as one of the most promising ones. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E25.00002: Quantum-classical master equation approach to energy transfer dynamics in multichromophoric systems Aaron Kelly, William Pfalzgraff, Andres Montoya-Castillo, Thomas Markland Quantum-classical and semiclassical dynamics methods offer a hierarchy of rigorous approaches to treat non-equilibirum condensed phase relaxation processes, such as electronic excitation energy transfer in multi-chromophore systems. Each tier of this hierarchy offers a different balance between accuracy and computational cost. However, for problems containing large numbers of degrees of freedom, or that involve many quantum states, or where an ab initio treatment of the electronic states is required, only the lowest tiers of this hierarchy are likely to be practical due to computational limitations. In this talk I will discuss our recent work related to combining these methods with the generalized quantum master equation (GQME) framework. In many cases these techniques can be made both more accurate and more efficient, allowing large systems to be simulated with good accuracy. I will demonstrate the abilities and benefits of this approach in describing electronic energy transfer processes in multi-chromophoric light harvesting systems, such as LHC-II, and in developing new theoretical tools for modeling nonlinear optical spectra. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E25.00003: Interference of Interchromophoric Energy Transfer Pathways in π-Conjugated Macrocycles Tammie Nelson, Laura Alfonso-Hernandez, Maxim Gelin, John Lupton, Sergei Tretiak, Sebastian Fernandez-Alberti The interchromophoric energy transfer pathways between weakly coupled chromophore units in a $\pi$-conjugated phenylene-ethynylene macrocycle and its half ring analogue have been investigated using the nonadiabatic excited state molecular dynamics (NA-ESMD) approach. To track the flow of electronic transition density between macrocycle units, we formulate a transition density flux analysis adapted from the statistical minimum flow (SMF) method previously developed to investigate vibrational energy flow. Following photoexcitation, transition density is primarily delocalized on two chromophore units and the system undergoes ultrafast energy transfer creating a localized excited state on a single unit. In the macrocycle, distinct chromophore units donate transition density to a single acceptor unit but do not interchange transition density among each other. We find that energy transfer in the macrocycle is slower than in the corresponding half ring due to the presence of multiple interfering energy transfer pathways. Simulation results are validated by modeling the fluorescence anisotropy decay. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:36AM |
E25.00004: Interfacial disorder drives charge separation in molecular semiconductors Invited Speaker: Adam Willard One of the fundamental microscopic processes in photocurrent generation is the dissociation of neutral photo-excitations (i.e., Frenkel excitons) into free charge carriers (i.e., electrons and holes). This process requires the physical separation of oppositely charged electrons and holes, which are held to together by an attractive electrostatic binding energy. In traditional inorganic-based photovoltaic (PV) materials, this binding energy is generally small and easily overcome, however, in organic-based PVs (OPVs) the exciton binding energy can significantly exceed thermal energies. The inability of bound charges to overcome this large binding energy has been implicated as a primary source of efficiency loss in OPVs. Here I present results from our recent efforts to explore the role of static molecular disorder in mediating this process. Using a simple lattice model of exciton dynamics we demonstrate that random spatial variations in the energetic landscape can mitigate the attractive Coulomb interaction between electrons and holes. We show that this effect manifests as a reduction in the free energy barrier for exciton dissociation that grows more pronounced with increasing disorder. By considering the competition between this thermodynamic effect and the disorder-induced slowing of dissociation kinetics we demonstrate that exciton dissociation yields are expected to depend non-monotonically on the degree of static disorder. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E25.00005: A coarse-gained model of exciton dynamics on long-chain conjugated polymer system Elizabeth Lee, William Tisdale, Adam Willard A comprehensive understanding of exciton dynamics in conjugated polymers is challenging, given the effects of electron-electron interactions , electron-nuclear coupling, and disorder at the molecular level up to the device scale has on electronic and optical properties. We present a new phenomenological model for simulating the dynamics of excitons in long-chain organic conjugated molecules. In our model, the polymer is described as a time-dependent array of ring-ring torsion angles mapped on a three-dimensional coarse-grained beads. Exciton dynamics arise in direct response to the evolution of this torsional landscape along its excited state potential energy surface, which includes exciton-induced forces (such as those that lead to self-trapping). The framework for generating an accurate description of these heterogeneous excited state forces was developed based on the analysis of QCFF/PI, a type of mixed QM/MM simulations. We show that this model can reproduce transient pump-probe experiments; we remark on the importance on the excited state force field when describing these systems. Then we go on to present molecular-level physical insights into exciton dynamics in these polymer materials, which have been previously speculative, to help better engineer organic solar cells. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E25.00006: First-principles simulation of transient x-ray absorption spectroscopy of photo-excited molecular systems Das Pemmaraju, Kristina Closser, David Prendergast We explore the utility of beyond-ground-state density functional methods such as constrained density functional theory (c-DFT) and time-dependent density functional theory (TDDFT) in the interpretation of time-resolved X-ray absorption spectroscopies investigating chemical dynamics in photo-excited molecular systems. Recent results based on a methodology that combines a state-by-state self-consistent field description such as c-DFT with a frequency domain linear-response TDDFT approach to model the core-level spectroscopy of photo-excited ring-opening dynamics in small molecular systems is discussed. Illustrative case studies involving the strong-field ionization of Selenophene and resonant UV-vis excitation of 1,3-cyclohexadiene are presented. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:36AM |
E25.00007: Atomistic absorption spectra and non-adiabatic dynamics of the LH2 complex with a GPU-accelerated \textit{ab initio} exciton model Invited Speaker: David Glowacki Recently, we outlined an efficient multi-tiered parallel excitonic framework that utilizes time dependent density functional theory (TDDFT) to calculate ground/excited state energies and gradients of large supramolecular complexes in atomistic detail. In this paper, we apply our \textit{ab initio }exciton framework to the 27 coupled bacteriocholorophyll-a chromophores which make up the LH2 complex, using it to compute linear absorption spectra and short-time, on-the-fly nonadiabatic surface-hopping (SH) dynamics of electronically excited LH2. Our \textit{ab initio} exciton model includes two key parameters whose values are determined by fitting to experiment: d, which is added to the diagonal elements, corrects for the error in TDDFT vertical excitation energies on a single chromophore; and e, which occurs on the off-diagonal matrix elements, describes the average dielectric screening of the inter-chromophore transition-dipole coupling. Using snapshots obtained from equilibrium molecular dynamics simulations (MD) of LH2, best-fit values of both d and e were obtained by fitting to the thermally broadened experimental absorption spectrum within the Frank-Condon approximation, providing a linear absorption spectrum that agrees reasonably well with the experimental observations. We follow the nonadiabatic dynamics using surface hopping to construct time-resolved visualizations of the EET dynamics in the sub-picosecond regime following photoexcitation. This provides some qualitative insight into the excitonic energy transfer (EET) that results from atomically resolved vibrational fluctuations of the chromophores. The dynamical picture that emerges is one of rapidly fluctuating eigenstates that are delocalized over multiple chromophores and undergo frequent crossing on a femtosecond timescale as a result of the underlying chromophore vibrational dynamics. The eigenstate fluctuations arise from disorder in both the diagonal chromophore site energies and the off-diagonal inter-chromophore couplings. The scalability of our excitonic computational framework across massively parallel architectures opens up the possibility of addressing a wide range of questions, including how specific dynamical motions impact both the pathways and efficiency of electronic energy-transfer within large supramolecular systems. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E25.00008: On-the-fly ab initio semiclassical dynamics for computing vibrationally resolved electronic spectra Jiri Vanicek, Marius Wehrle, Miroslav Sulc, Solene Oberli We combine the thawed Gaussian approximation (TGA) with an on-the-fly ab initio (OTF-AI) scheme to calculate the vibrationally resolved emission spectra of oligothiophenes with up to five rings as well as absorption and photoelectron spectra of ammonia. The efficiency of the OTF-AI-TGA permits treating all vibrational degrees of freedom on an equal footing even in pentathiophene with 105 vibrational degrees of freedom, thus obviating the need for the global harmonic approximation, popular for large systems. Besides reproducing almost perfectly the experimental emission spectra, in order to provide a deeper insight into the associated physical and chemical processes, we also develop a novel systematic approach to assess the importance and coupling between individual vibrational degrees of freedom during the dynamics. This allows us to explain how the vibrational line shapes of the oligothiophenes change with increasing number of rings. [1]~M. Wehrle, M. \v{S}ulc, and J. Van\'{\i}\v{c}ek, J. Chem. Phys. \textbf{140}, 244114 (2014). \newline [2] M. Wehrle, S. Oberli, and J. Van\'{\i}\v{c}ek, J. Phys. Chem. A \textbf{119}, 5685 (2015). [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E25.00009: Anharmonic Densities of States -- A General Dynamics-Based Solution. Darya Aleinikava, Julius Jellinek Density of states (DOS) is a fundamental property that allows for construction of all the statistical mechanical characteristics of systems. It also plays a central role in chemical kinetics providing for reaction rate constants. Regarding the vibrational DOS, the almost ubiquitous current practice is to use the framework of the harmonic approximation, within which an exact solution for the DOS is available. A considerable effort over the last eight decades to go beyond the harmonic approximation produced a number of solutions, all of which, however, are approximate and/or suffer from other limitations. Here we present an exact solution to the general problem of anharmonic DOS. The solution is based on following the dynamical evolution of a system of interest on the relevant time-scale. As a consequence, the resulting anharmonic DOSs are dynamically informed and reflect the actual dynamical evolution of a system. In general, they may depend on initial conditions and/or time, and can be used to characterize both equilibrium and noneqilibrium processes. As such, they lay the foundation for formulation of new statistical mechanical frameworks that incorporate time and are, by construction, ergodic with respect to actual dynamical behavior of systems. We illustrate our methodology through applications to highly anharmonic atomic clusters. [Preview Abstract] |
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