Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F25: Chemical Physics of Multichromophores IIIFocus
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Sponsoring Units: DCP Chair: David Reichman, Columbia University Room: 288 |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F25.00001: Dynamics of photosynthetic light harvesting: from conformational changes to excitation energy transfer Invited Speaker: Gabriela Schlau-Cohen In photosynthetic light harvesting, absorbed energy migrates through a protein network to reach a dedicated location for conversion to chemical energy. This energy flow is efficient, directional, and, in oxygenic photosynthesis, regulated. The regulatory response, known as non-photochemical quenching (NPQ), safely dissipates excess energy to protect the system against deleterious photoproducts. In recent research, a protein within this network, light-harvesting complex stress related (LHCSR), has been implicated in dissipation in green algae and moss, yet the mechanisms of photoprotection remain poorly understood. We explore these mechanisms with single-molecule spectroscopy of LHCSR. By characterizing the conformational dynamics, we identify the extent of energy dissipation in single LHCSR proteins and how this changes under conditions that mimic high and low light. While this approach reveals the conformational dynamics of solubilized photosynthetic proteins, we also present experiments measuring ultrafast energy transfer dynamics within proteins held in their native membrane environment. Together, these experiments explore how the protein network produces an efficient and adaptable energy flow. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:03PM |
F25.00002: Ultrafast decoherence dynamics in polymer solar cells Eric Bittner, Hao Li I will briefly discuss our joint theoretical and experimental efforts concerning the role of excitonic dephasing as governing the photocarrier efficiencies in organic polymer heterojunction solar cells. [Preview Abstract] |
Tuesday, March 14, 2017 12:03PM - 12:15PM |
F25.00003: Molecular aggregates in cavities: from Bose-Einstein condensation to chemistry and back Joel Yuen-Zhou In this talk, I will discuss recent theoretical work discussing how thermodynamics and kinetics of molecular processes can be nontrivially altered when organic dye aggregates strongly interact with confined electromagnetic fields. I will discuss some generalizations of transition state theory that are suitable to polariton ensembles. Finally, I will argue that room-temperature nonequilibrium polariton condensates can be harnessed to drive a photochemical reactions to near 100{\%} selectivity even when the bare photochemical reaction has democratic branching ratios. [Preview Abstract] |
Tuesday, March 14, 2017 12:15PM - 12:51PM |
F25.00004: Light-induced nonadiabatic dynamics in molecular assemblies and nanostructures Invited Speaker: Roland Mitric The combination of mixed quantum-classical dynamics with efficient electronic structure methods was developed in order to simulate the light-induced processes in complex molecules, multichromophoric aggregates and metallic nanostructures. We will demonstrate how the combination of nonadiabatic dynamics with experimental pump-probe techniques such as time-resolved photoelectron imaging (TRPEI) allows to fully resolve the mechanism of excited state relaxation through conical intersections in several prototype organic- and biomolecules. Specifically, the role of the solvent in the excited state relaxation in microsolvated and fully solvated systems will be addressed. Currently there is growing evidence that nonadiabatic relaxation processes also play a fundamental role in determining the efficiency of excitonic transfer or charge injection in multichromophoric assemblies. Since such systems are currently out of the reach of the state-of-the-art quantum chemistry a development of even more efficient quantum chemical approaches is necessary in order to describe the excited state dynamics in such assemblies. For this purpose we have recently developed long-range corrected time-dependent density functional tight binding (LC-TDDFTB) nonadiabatic dynamics and combined it with the QM/MM approach in order to simulate exciton relaxation in complex systems. The applications of the method to the investigation of the optical properties and dynamics in multichromophoric assemblies including stacked pi-conjugated organic chromophores, model molecular crystals as well as self-organized dye aggregates will be presented. Finally, we will address exciton transport dynamics coupled with the light propagation in hybrid exciton-plasmon nanostructures, which represent promising materials fort the development of novel light-harvesting systems. [Preview Abstract] |
Tuesday, March 14, 2017 12:51PM - 1:03PM |
F25.00005: The coherence lifetime-borrowing effect in vibronically coupled molecular aggregates under non-perturbative system-environment interactions. Shu-Hao Yeh, Gregory S. Engel, Sabre Kais Recently it has been suggested that the long-lived coherences in some photosynthetic pigment-protein systems, such as the Fenna-Matthews-Olson complex, could be attributed to the mixing of the pigments' electronic and vibrational degrees of freedom. In order to verify whether this is the case and to understand its underlying mechanism, a theoretical model capable of including both the electronic excitations and intramolecular vibrational modes of the pigments is necessary. Our model simultaneously considers the electronic and vibrational degrees of freedom, treating the system-environment interactions non-perturbatively by implementing the hierarchical equations of motion approach. Here we report the simulated two-dimensional electronic spectra of vibronically coupled molecular dimers to demonstrate how the electronic coherence lifetimes can be extended by borrowing the lifetime from the vibrational coherences. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:15PM |
F25.00006: Torsional Dynamics, Intramolecular Charge Transfer, and Solvent Friction in the S$_{\mathrm{2}}$ (1$^{\mathrm{1}}$B$_{\mathrm{u}}^{\mathrm{+}})$ Excited State of Peridinin: A Mechanism for Enhanced Mid-Visible Light Harvesting in the Peridinin--Chlorophyll $a$ Protein Warren Beck, Jerome Roscioli, Soumen Ghosh, Michael Bishop, Amy LaFountain, Harry Frank The structural mechanism that allows peridinin to provide one of the highest quantum efficiencies for excitation energy transfer to chlorophyll (Chl) $a$ acceptors in the peridinin--chlorophyll $a$ protein (PCP) from dinoflagellates involves an order-of-magnitude slowing of the S$_{\mathrm{2}}$ (1$^{\mathrm{1}}$B$_{\mathrm{u}}^{\mathrm{+}}) \quad \to $ S$_{\mathrm{1}}$ (2$^{\mathrm{1}}$A$_{\mathrm{g}}^{\mathrm{-}})$ nonradiative decay pathway compared to carotenoids lacking carbonyl substitution. Using femtosecond transient grating spectroscopy with heterodyne detection, we have determined for the first time that the decay of an intermediate state termed S$_{\mathrm{x}}$, which we assign to a twisted form of the S$_{\mathrm{2}}$ state, is substantially slowed by solvent friction in peridinin due to its intramolecular charge transfer (ICT) character. The S$_{\mathrm{x}}$ intermediate exhibits a long enough lifetime to serve as an efficient excitation energy transfer donor to Chl $a$ in PCP. The possibility that the Franck--Condon S$_{\mathrm{2}}$ state also transfers excitation via quantum coherent mechanisms is being considered currently using broadband two-dimensional electronic spectroscopy. [Preview Abstract] |
Tuesday, March 14, 2017 1:15PM - 1:27PM |
F25.00007: Multiscale Electrodynamics/Time-Dependent Density Functional Theory Modeling of Coupled Plasmon/Molecule Excitations Kenneth Lopata, Holden Smith The coupled dynamics of molecular chromophores and plasmons at surface of metal nanostructures are important for a range of processes such as molecular sensing, light harvesting, and near-field photochemistry. Modeling these dynamics from first principles, however, is challenging, as the large system sizes precludes a purely quantum mechanical treatment. In this talk I will present an approach based on propagating the plasmonic currents and fields using electrodynamics (finite-difference time-domain) with each chromophore described using an isolated quantum sub-region embedded in the overall classical background. This approach can be readily parallelized over these quantum regions, which enables large multiscale simulations of tens or hundreds of dyes, each of which is described individually by real-time time-dependent density functional theory. Application to gold nanoparticles coated with malachite green and rhodamine 6G monolayers shows good agreement with experimentally measured coupling spectra, including the polariton peaks, as well as the plasmon and molecular depletions. [Preview Abstract] |
Tuesday, March 14, 2017 1:27PM - 1:39PM |
F25.00008: Modeling electronic trap state distributions in nanocrystalline anatase Nam Le, Igor Schweigert The charge transport properties of nanocrystalline TiO$_2$ films, and thus the catalytic performance of devices that incorporate them, are affected strongly by the spatial and energetic distribution of localized electronic trap states. Such traps may arise from a variety of defects: Ti interstitials, O vacancies, step edges at surfaces, and grain boundaries. We have developed a procedure for applying density functional theory (DFT) and density functional tight binding (DFTB) calculations to characterize distributions of localized states arising from multiple types of defects. We have applied the procedure to investigate how the morphologies of interfaces between pairs of attached anatase nanoparticles determine the energies of trap states therein. Our results complement recent experimental findings [1] that subtle changes in the morphology of highly porous TiO$_2$ aerogel networks can have a dramatic effect on catalytic performance, which was attributed to changes in the distribution of trap states. [1] P. A. DeSario et al. J.~Phys.~Chem.~C 119, 17529 (2015). [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 1:51PM |
F25.00009: Ultrafast Dynamics of Vibration-Cavity Polariton Modes Jeff Owrutsky, Adam Dunkelberger, Kenan Fears, Blake Simpkins, Bryan Spann Vibrational modes of polymers, liquids, and solvated compounds can couple to Fabry-Perot optical cavity modes, creating vibration-cavity polariton modes whose energy tunes with the cavity length and incidence angle. Here we report the pump-probe infrared spectroscopy of vibration-cavity polaritons in cavity-coupled W(CO)$_{\mathrm{6}}$. At very early times, we observe quantum beating between the two polariton states find an anomalously low degree of excitation. After the quantum beating, we directly observe spectroscopic signatures of excited-state absorption from both polariton modes and uncoupled reservoir modes. An analytical expression for cavity transmission reproduces these signatures. The upper polariton mode relaxes ten times more quickly than the uncoupled vibrational mode and the polariton lifetime depends on the angle of incidence of the infrared pulses. Coupling to an optical cavity gives a means of control of the lifetime of vibration-cavity polaritons and could have important implications for chemical reactivity in vibrationally excited molecules. [Preview Abstract] |
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