Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y2: Focus Session: Charge & Energy Transfer V |
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Sponsoring Units: DCP Chair: Tim Lian, Emory University Room: 102 |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y2.00001: Bipolar Charge Transport Properties of Poly(imide-thienyl(thienylenevinylene)) Evan Lafalce, Xiaomei Jiang, Cheng Zhang The charge transport properties of $\pi $--conjugated polymers are of interest from a fundamental perspective and also because they are a limiting factor for many optoelectronic device applications. In this work, we study the charge carrier mobility and recombination in Poly(imide-thienyl(thienylenevinylene)) (imide-PTV), a novel PTV derivative with imide side group. The electron and hole mobility are determined separately through the Space Charge Limited Current (SCLC) analysis of single carrier diodes. These devices are fabricated using interfacial layers that provide carrier selective contacts. A mobility asymmetry factor of approximately 20 that favors hole transport is observed, with the hole mobility of the order of 10$^{-5}$ [cm$^2$/V*s]. Similar results are obtained from the analysis of the intensity dependence of photoconductivity. Complimentary analysis of the ambipolar carrier mobility through carrier extraction under linearly increasing voltage (CELIV) and double injection transient techniques are also presented. The effects of carrier recombination and trapping are discussed. We conclude that the hole transport is not the limiting factor for power conversion efficiency of photovoltaic device based on imide-PTV and PCBM. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y2.00002: A State Representation Approach for Atomistic Time-Dependent Transport Calculations in Molecular Junctions Tamar Zelovich, Leeor Kronik, Oded Hod A new method for simulating electron dynamics in open quantum systems out of equilibrium, motivated by the intuitive and practical nature of the damped Liouville von-Neumann equation approach of S\'anchez et al. [J. Chem Phys, 124, 214708 (2006)], is presented. The new approach is based on a transformation of the Hamiltonian matrix from an atomistic to a state representation of the molecular junction. This allows us to define the bias voltage across the system uniquely while maintaining a proper thermal distribution within the lead models. Furthermore, it allows us to investigate time-dependent effects in non-linear and multi-lead configurations. We investigate the degree of conservation of exact conditions such as the N-representability of the density matrix and suggest ways to remedy the violation of Pauli's exclusion principle. We believe that the new approach offers a practical and physically sound route for performing atomistic time-dependent transport calculations in realistic models of molecular electronics junctions. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y2.00003: Femtosecond Nonlinear Optical Studies of Radiationless Decay in Carotenoids and in the Peridinin--Chlorophyll $a$ Protein Soumen Ghosh, Michael Bishop, Jenny Jo Mueller, Nolan Shepherd, Warren Beck, Harry Frank Femtosecond transient-grating spectroscopy with optical heterodyne detection was employed to observe the time evolution of the absorption and dispersion components of the third-order nonlinear optical signal following resonant excitation of the S$_{2}$ (1B$_{u}^{+})$ states of $\beta ${\-}carotene in benzonitrile and peridinin in methanol. The absorption and dispersion components exhibit distinct time profiles owing to the population of dark intermediate states. An initial intermediate is populated on an ultrashort (\textless 30~fs) time scale in both carotenoids owing to the onset of torsional distortions on the S$_{2}$-state potential surface. The time-resolved transient-grating spectra obtained for peridinin in the peridinin--chlorophyll $a$ protein from \textit{Amphidinium carterae} indicate that the intermediate is formed even more rapidly than in solution. This finding suggests that the twisted conformation of the peridinin chromophore is controlled in the binding site so as to optimize energy transfer to chlorophyll $a$ by enhancing the formation of an intramolecular charge-transfer character. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y2.00004: Energy and Charge Transfer in Dinuclear Ru-based Complexes Invited Speaker: Valeria Kleiman In this work, the excited state dynamics of a series of dinuclear compounds combining Ru based cromophores with M$=$Ru(II), Fe(II), Fe(III), Cr(III) are explored. Ru-$\mu $-NC-M dimers are good candidates to investigate the competition between electron and energy transfer in arrays of chromophores. The presence of a $\mu $-NC bridge affords a strong coupling between the moieties without providing acceptor states that might act as electron traps. Polypyridyl Ru based compounds play an important role on light-harvesting antennas for energy conversion. With proper knowledge of the excited state dynamics, multinuclear arrays of chromophores can be developed. Our studies focus on (i) energy/electron transfer from the Ru(II) to a 2$^{\mathrm{nd}}$ M center through the cyanide bridge, and (ii) geometry changes due to the exchange of one of the Ru(II) polypiridyl ligands . Broadband ultrafast spectroscopy shows excited state dynamics in the psec time regime. These dynamics depend strongly on the nature of the acceptor and the orientation of the ligand involved in the photoinduced transition. Hence, the competition between energy and electron transfer across the bridge is modulated by the selective choice of the secondary M center. We conclude that transition metals from the 3$^{\mathrm{rd}}$ row are good candidates for longer arrays since their lack of low-lying MC states precludes thermal deactivation.\\[4pt] Work done in collaboration with Shiori Yamazaki and Jaired Tate, University of Florida; Alejandro Cadranel and Luis Baraldo, Universidad de Buenos Aires. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y2.00005: Optical Excitations in Cycloparaphenylene Molecules of Various Sizes Lyudmyla Adamska, Iffat Nayyar, Anna Swan, Steven Doorn, Sergei Tretiak Cycloparaphenylene ([$n$]CPP) molecule can be imagined as $n$ benzene molecules connected in a periodic chain. [$n$]CPPs with even number of links have alternating dihedral angles of $+$/- 34 degrees, whereas odd-numbered [$n$]CPPs cannot adopt such a high symmetry configuration, so they have a ``defect'': one of the rings is connected to its neighbors by dihedral angles of about 20 degrees. This ``defect'' plays a role of a localization site for an exciton. In this work we show that in [$n$]CPPs with $n$\textgreater 8 the exciton is localized on 5-6 rings, which strongly reduce their dihedral angles, while preserving the ground state geometry on the rest of the rings. This occurs both in odd-numbered and, surprisingly, in even-numbered [$n$]CPPs. We use electronic structure theory to address the spatial extent/properties of electronic wavefunctions and resulting electronic functionalities in [$n$]CPP molecular chromophores. Localization of excitonic states due to electron-phonon coupling in cycloparaphenylenes invalidates Condon approximation and breaks optical selection rules, making these materials to be efficient emitters. The effect of solvent is also discussed. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y2.00006: Signature of Nonadiabatic Coupling in Excited-State Vibrational Modes Tammie Nelson, Miguel Soler, Adrian Roitberg, Sergei Tretiak, Sebastian Fernandez-Alberti Using analytical excited-state gradients, vibrational normal modes are obtained at the minimum of the electronic excited-state potential energy surfaces for a set of extended conjugated molecules. In regions of strong coupling, the contribution to the forces in the direction of the corresponding non-adiabatic (NA) coupling vector (i.e., the Pechuckas force) is the dominant driving force for nuclear motion and should be reflected in the specific adiabatic excited-state equilibrium normal modes (ES-ENMs) responsible for the coupling. Specifically, the projection of the NA coupling vector on the basis on ES-ENMs with a significant agreement with a single ES-ENM indicates an effective decoupled direction for NA energy transfer. The influence of the nonadiabatic coupling on the excited-state equilibrium normal modes is revealed as a unique highest frequency adiabatic vibrational mode that overlaps with the coupling vector. Comparison with vibrational modes computed in a locally diabatic representation demonstrates that the effect of nonadiabaticity is confined to only a few modes. Such an approach is encouraging as it suggests that the nonadiabatic character of a system may be detected spectroscopically by identifying these unique high frequency modes. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y2.00007: Non-Adiabatic, Multi-State Ring-Polymer Molecular Dynamics Franziska Bell, Artur Menzeleev, Thomas Miller III Ring-polymer molecular dynamics (RPMD) has been shown to be a promising method for studying mechanisms and rates in large systems which require the inclusion of quantum effects, such as zero-point energies and tunneling. Examples involve electron and/or proton transfer reactions in enzymes and artificial catalysts. However, the traditional formulation of RPMD has several shortcomings: (i) it is restricted to migrations of only one distinguishable electron, (ii) it cannot describe photophysical processes, and (iii) it cannot be used in conjunction with potential energy surfaces obtained from electronic structure methods. Here I present a parameter-free extension of the RPMD method that addresses these issues and allows for the direct simulation of non-adiabatic processes involving many-electron wavefunctions without prior assumptions of the reaction mechanism. The new approach is demonstrated to provide a quantitative description of electron-transfer reaction rates and mechanisms throughout (i) the normal and inverted regimes and (ii) the weak- and strong-coupling regimes. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y2.00008: Exploring the Vibrational Stark Effect in Fullerene and Derivatives from First Principles Peter Doak, Yajing Li, Douglas Natelson, Leeor Kronik, Jeffrey Neaton Fullerene (C60) and its derivatives have played a central role in molecular and organic electronics, where its electron affinity and high symmetry result in key functionality. Understanding the impact of local fields on C60 properties in situ is of considerable interest, and here we determine how electric fields alter vibrational modes via the vibrational Stark effect. Using density functional theory-based finite-difference approach, we calculate the shifts in mode energy and symmetry in electric fields in gas-phase C60, PCBM, and other derivatives of fullerene. We examine the effect of high and low symmetry electronic field orientations, symmetry breaking functionalization, and doping on the the magnitude and mode-dependence of the vibrational Stark effect. The implications for fullerene-based materials under device conditions is discussed. This work is supported by DOE and computational resources were provided by NERSC. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y2.00009: Excitation Energy Transfer in Peridinin-Chlorophyll $a$-Protein Complex Using F\"{o}rster Resonance Energy Transfer William Bricker, Cynthia Lo Peridinin-Chlorophyll $a$-Protein (PCP) is a trimeric light-harvesting protein complex containing peridinin and chlorophyll $a$ pigment molecules, and the excitation energy transfer (EET) efficiency within this system is estimated at close to 90{\%}. To study the EET in PCP, we use the F\"{o}rster Resonance Energy Transfer (FRET) model, assuming that energy transfer occurs incoherently. We calculate the FRET coulombic coupling and spectral overlap terms using multi-reference configuration interaction (MR-CI) with semi-empirical basis sets in MOPAC. The two dominant EET pathways within PCP are from the S$_{\mathrm{2}}$ state of peridinin to the Q$_{\mathrm{x}}$ band of chlorophyll $a$, and from the S$_{\mathrm{1}}$ state of peridinin to the Q$_{\mathrm{y}}$ band of chlorophyll $a$. In peridinin, absorption in the visible spectrum is due to the strongly allowed S$_{\mathrm{0}}$ to S$_{\mathrm{2}}$ transition, while the S$_{\mathrm{0}}$ to S$_{\mathrm{1}}$ transition is optically forbidden and has double excitation character. To more accurately describe the FRET coulombic coupling term, we are working on an extension to the MOPAC source code, which will provide transition densities for all transitions. Our EET study of PCP using FRET reveals the interplay of pigment geometry and environment on the EET rates and efficiencies within the complex. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y2.00010: Electric field manipulation of magnetoresistance in a single molecular spin-valve device Kamal Dhungana, Ranjit Pati Manipulation of spin transport in a molecular spin-valve device using external electric field is a challenging as well as an exciting task from both fundamental and technological points of view. The weak spin-orbit and hyperfine interactions in organic molecules make them potential candidates for spin conserved tunneling. Tunable spin transport properties in single molecular junctions have recently been demonstrated using spin polarized scanning tunneling microscope. Here, we model a molecular spin-valve device by attaching an organic molecule between two ferromagnetic electrodes. A single-particle many-body Green's function approach together with unrestricted density functional theory is employed to ca [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y2.00011: Electron transfer in a two-level system within a Cole-Davidson vitreous bath Mark Ratner, Mehdi Zarea, Michael Wasielewski We study electron transfer (ET) in a two level quantum system coupled to a glassy viscous bath. The bath is modeled by the Cole-Davidson (CD) spectral density. The ET in this model is compared to the ET in a normal Drude-Debye (DD) model. It is shown that at low temperatures and when the coupling to the bath is weak, the viscous bath preserves the quantum coherence for a longer time. However in the strong coupling regime, the tunneling rate is higher in the CD. In the classical high temperature limit the difference between the CD and DD models is negligible. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y2.00012: Exact factorization of the electron-nuclear wave function: General theory and applications Federica Agostini, Ali Abedi, Seung Kyu Min, Yasumitsu Suzuki, E.K.U. Gross The exact factorization of the molecular wave function [1, 2] to the product of an electronic factor, parametrically depending on nuclear positions, and a nuclear wave function is presented. This starting point is used to decomposed the time dependent Schr\"odinger equation into two equations, that generate the evolution of the electronic and nuclear components. In this formulation, time dependent scalar and vector potentials mediate the coupling between the two sets of degrees of freedom, in a formally exact way. They represent what is usually referred to as electronic ``back-reaction'' on the nuclei. In a model system for non-adiabatic electron transfer, we investigate the properties of the potentials [3] and we analyze the classical approximation [4] of nuclear dynamics, in comparison to exact dynamics. This last point will lead to the development of a practical scheme to deal with non-adiabatic dynamics in the mixed quantum-classical approximation. \\[4pt] [1] A. Abedi, N.T. Maitra and E.K.U. Gross, Phys. Rev. Lett. 105 (2010)\\[0pt] [2] A. Abedi, N.T. Maitra and E.K.U. Gross, J. Chem. Phys. 137 (2012)\\[0pt] [3] A. Abedi, F. Agostini, Y. Suzuki and E.K.U. Gross, Phys. Rev. Lett. 110 (2013)\\[0pt] [4] F. Agostini, A. Abedi, Y. Suzuki and E.K.U. Gross, accepted in Mol. Phys. DOI:10.1080/00268976.2013.84373 [Preview Abstract] |
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