Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B38: Focus Session: The Chemical Physics of Biological and Biologically-inspired Solar Energy Harvesting II |
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Sponsoring Units: DCP Chair: Bern Kohler, Ohio State University Room: 410 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B38.00001: Coherence and Decoherence in the Excited States of Light Harvesting Complexes Invited Speaker: |
Monday, March 16, 2009 11:51AM - 12:27PM |
B38.00002: Electronic coherence in electronic energy transfer despite fast dephasing Invited Speaker: F\"{o}rster resonance energy transfer (FRET) is a common and fundamental photophysical process in life and materials sciences. FRET is an interchromophore relaxation process that transmits the electronic excitation from an initially excited donor to a ground state acceptor chromophore (light-absorbing moleule). FRET is used, for example, to harvest light in photosynthesis, measure distances in proteins, and it accelerates the photodegradation of polymers. In recent years attention has turned to the study of FRET in complex assemblies of molecules. While F\"{o}rster theory has enabled the efficiency of FRET to be predicted and analyzed in numerous and diverse areas of study, recent work has aimed to discover ways beyond the F\"{o}ster mechanism by which electronic energy can be transferred. The talk will compare and contrast theoretical and experimental studies of excitation relaxation in photosynthetic antenna systems with the conjugated polymer poly[2-methoxy,5-(2'-ethyl-hexoxy)-1,4-phenylenevinylene] (MEH-PPV). I will report new work where we have used a new anisotropy experiment to examine coherent energy transfer and a complementary technique using two-dimensional electronic spectroscopy expose the role of coherence transfer in the fastest time dynamics. We find that coherent energy transfer occurs for many tens of femtoseconds, even at room temperature. That leads us to examine the nature and implications of the so-called intermediate coupling regime for EET. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B38.00003: Photosynthetic nanoparticle complexes Alexander Govorov We investigate structures composed of a photosynthetic molecule and a semiconductor (metal) nanoparticle [1]. The rate of optical generation of electron--hole pairs inside a photosynthetic system can be greatly increased through conjugation with nanoparticles. In the case of a semiconductor nanoparticle, the enhancement effect comes from the essentially larger optical absorption cross-section of a semiconductor nanoparticle compared to a photosynthetic system. In this hybrid complex, excitons are transferred via the Forster mechanism to the photosynthetic system, where charge separation takes place. For metal nanoparticles conjugated with a photosynthetic system, we predicted a strong enhancement effect due to the plasmon resonance. Such an enhancement effect was recently observed at Munich U. [2]. In summary, we have shown that one can use crystalline nanoparticles to create a 10-fold enhancement of the initial stage of photosynthesis, i.e. the absorption process. Potential applications of nanocrystal complexes are in light-harvesting. [1] A. O. Govorov and I. Carmeli, Nano Lett. \textbf{7}, 620 (2007); A. O. Govorov, Adv. Materils, online, DOI: 10.1002/adma.200702999. [2] S.Mackowski, S. W\"{o}rmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Br\"{a}uchle, Nano Lett.~\textbf{8}, 558 (2008). [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B38.00004: Quantum coherence, decoherence and entanglement in light harvesting complexes Martin Plenio, Filippo Caruso, Alex Chin, Animesh Datta, Susana Huelga Transport phenomena in networks allow for information and energy to be exchanged between individual constituents of communication systems, networks or light-harvesting complexes. Environmental noise is generally expected to hinder transport. Here we show that transport of excitations across dissipative quantum networks can be enhanced by dephasing noise. We identify two key processes that underly this phenomenon and provide instructive examples of quantum networks for each. We argue that Nature may be routinely exploiting this effect by showing that exciton transport in light harvesting complexes and other networks benefits from noise and is remarkably robust against static disorder. These results point towards the possibility for designing optimized structures for transport, for example in artificial nano-structures, assisted by noise. Furthermore, we demonstrate that quantum entanglement may be present for short times in light-harvesting complexes. We describe how the presence of such entanglement may be verified without the need for full state tomography and with minimal model assumptions. This work is based on M.B. Plenio \& S.F. Huelga, New J. Phys. 10, 113019 (2008) and F. Caruso, A. Chin, A. Datta, S.F. Huelga \& M.B. Plenio, in preparation [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B38.00005: Non-radiative decay processes in InAs nanocrystals Marco Califano The mechanisms governing excited state relaxation in semiconductor nanocrystals (NCs) are still not well understood. The validity of the Auger electron cooling and multiexciton recombination hypotheses, which would explain much of the experimental data available to date, has recently been questioned. Moreover the recent observation of sub-picosecond electron relaxation times and biexciton recombination rates of the order of 0.1-1 ps$^{-1}$ in InAs, although qualitatively ascribed to Auger processes, still awaits a quantitative theoretical interpretation. Multiexciton recombination is particularly important as its signatures are used to detect and quantify carrier multiplication efficiency in NCs. Furthermore efficient non-radiative (multi-) exciton decay represents a major obstacle for application of NCs in lasing and photovoltaics. A quantitative theoretical understanding of these processes is therefore critical for any technological implementation of quantum-dot-based devices. The results of a detailed investigation using the pseudopotential method provide an explanation of the observed lifetimes in terms of Auger-like decay mechanisms, supporting the Auger interpretation of excited state relaxation in NCs. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B38.00006: Exciton Transport Simulations in Phenyl Cored Thiophene Dendrimers Kwiseon Kim, Muhammet Erkan Kose, Peter Graf, Nikos Kopidakis, Garry Rumbles, Sean E. Shaheen Phenyl cored 3-arm and 4-arm thiophene dendrimers are promising materials for use in photovoltaic devices. It is important to understand the energy transfer mechanisms in these molecules to guide the synthesis of novel dendrimers with improved efficiency. A method is developed to estimate the exciton diffusion lengths for the dendrimers and similar chromophores in amorphous films. The approach exploits Fermi's Golden Rule to estimate the energy transfer rates for an ensemble of bimolecular complexes in random orientations. Using Poisson's equation to evaluate Coulomb integrals led to efficient calculation of excitonic couplings between the transition densities. Monte-Carlo simulations revealed the dynamics of energy transport in the dendrimers. Experimental exciton diffusion lengths of the dendrimers range 10 $\sim $ 20 nm, increasing with the size of the dendrimer. Simulated diffusion lengths correlate well with experiments. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors that determine the exciton diffusion length in amorphous films. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B38.00007: Efficient and Long-lived Charge Separation in a Heteroleptic Ruthenium(II) Polypyridyl Complex Joseph Henrich, Haoyu Zhang, Jeremy White, Prabir Dutta, Bern Kohler The excited-state dynamics of a tris-bidentate mononuclear ruthenium(II) complex, [(bpy)$_{2}$RuL$_{DQ}$]$^{4}$ (where bpy = bipyridine, L$_{DQ}$ = 1-[4-(4'-methyl)-2,2'-bipyridyl)]-2-[4-(4'-N,N'-tetramethylene-2,2'-bipyridinium]) was investigated by femtosecond transient absorption spectroscopy in bulk solution and tethered to a zeolite nanocrystal. [(bpy)$_{2}$RuL$_{DQ}$]$^{4}$ is a promising photosensitizer molecule for artificial photosynthesis. Broadband transient absorption experiments in bulk acetonitrile solution reveal that excitation of the MLCT absorption band transfers an electron within one picosecond from the metal center to the bipyridinium (DQ) ligand. Back electron transfer then takes place with a time constant of 1.45 ns. Highly efficient charge separation is attributed to the conjugated nature of the bipyridinium-terminated ligand. When [(bpy)$_{2}$RuL$_{DQ}$]$^{4}$ is tethered to a zeolite Y particle, charge can be transferred to a methyl viologen molecule encapsulated in the zeolite. Zeolites are promising materials for solar energy conversion because of their ability to slow rates of charge recombination. The effects of the zeolite on the photoprocesses of the ruthenium polypyridyl complex will be presented. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B38.00008: First-Principles Studies of Single-Molecule Photovoltaics Peter Doak, R. A. Segalman, T. D. Tilley, J. B. Neaton Organic photovoltaics consist of electron donor and acceptor polymers or molecules blended together, and are promising inexpensive, lightweight alternatives to conventional silicon solar cells. However, many of the physical processes responsible for their poor efficiencies are not well understood. Here, using first-principles calculations based on density functional theory, including self-energy corrections within the GW approximation and a discussion of excitonic effects, we examine the relationship between molecular structure and electronic level alignment at a covalent donor-acceptor interface. We consider small asymmetric molecules subdivided into discrete covalently linked moieties based on thiophene, tetrafluorobenzene, pyridine, and durene. Excited states of each of these moieties, as well as their covalently-linked combinations, are computed and discussed in the context of their ability to absorb photons and separate charge. Work supported in part by the DOE Helios SERC. Computational resources provided by NERSC. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B38.00009: Intramolecular Interactions in Novel Macrocyclic Materials Theodore Goodson In this presentation I will report a strongly interacting new dendrimer system with an extended spectroscopic unit (coherent domain) beyond the trimer configuration. Strong cooperative enhancement of two-photon absorption cross-section was observed when going from the trimer arrangement to the next generation. Combination of a variety of femtosecond spectroscopy methods such as femtosecond time-resolved fluorescence upconversion, transient absorption, transient grating, three pulse photon echo peak shift experiments complemented with those of steady state spectroscopy allowed us to compare the properties of absorption states with those of fluorescence states, to estimate the reorganization energies, and the extent of inhomogeneous broadening. Our measurements indicated that spectroscopic unit (domain) is different for the trimer system and for the dendrimers of higher generation numbers. This coherent domain extends over the trimer geometry and its size is comparable with the size of the dendrimer G1 comprising nine linear segments. We have also investigated the novel applications of a two-dimensional carbon network structure's building blocks. The material shows very interesting two-photon absorption properties as well as strongly coupled optical excitations. They have also been suggested as good building blocks for molecular electronics applications. [Preview Abstract] |
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