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Theoretical Chemistry Day

Room: 
MoSE G011
Date: 
Tuesday, October 15, 2013 - 8:30am to 5:00pm
  Schedule

8:30-9:15  -  Registration 9:15-9:30  -  Welcome/Opening Remarks, Paul Goldbart 9:30-10:20  -  Roald Hoffmann 10:20-11:10  -  Peter Rossky 11:10-11:30  -  Break 11:30-12:20  -  George Schatz 12:20-12:45  -  Jean-Luc Bredas CCMST Overview 1:00-2:15  -  Poster Session and Buffet Lunch 2:30-3:20  -  Josef Michl 3:20-4:10  -  Mark Ratner 4:10-4:20  -  Closing remarks, Andrew Lyon 4:30-5:30  -  Reception

 

Roald Hoffmann, Cornell University

The Chemical Imagination at Work in Very Tight Places

Diamond anvil cells now permit the study of matter under multimegabar (i.e. several hundred GPa) pressures; properties in this pressure regime differ drastically from those known at 1 atm. Just how different chemistry and physics is at high pressure and the role that a chemical intuition for bonding and structure can have in understanding matter at high pressures will be explored in this lecture. I will discuss in detail an overlapping hierarchy of structural and electronic responses to increased density, consisting of (a) squeezing out van der Waals space; (b) increasing coordination; (c) decreasing the bond length of covalent bonds and the size of anions; and (d) an extreme regime of electrons moving off atoms and new modes of correlation. Examples of the startling chemistry and physics that emerge under such extreme conditions will alternate in this account with qualitative chemical ideas about the bonding involved.

 

Josef Michl, University of Colorado, Boulder

Singlet Fission for Enhancing Solar Cell Efficiency

Singlet fission is a process in which a singlet excited chromophore shares some of its energy with a ground state neighbor and both end up in their respective triplet states. The phenomenon converts an absorbed photon of sufficiently high energy into two excited states and possibly, two electron-hole pairs, instead of the usual one. The theoretical efficiency limit for a solar cell composed of a layer of singlet-fission-capable high band-gap chromophores that absorb the short-wavelength part of solar radiation and an layer of ordinary low band-gap chromophores that absorb the long-wavelength part, without any current matching requirements, has a theoretical maximum efficiency limit close to 1/2, well above the usual Shockley-Queisser limit of 1/3.

Before single fission finds practical use, it is necessary to identify chromophores that perform singlet fission efficiently and meet the myriad other requirements for use in solar cells. The talk will address the use of fundamental molecular quantum theory for finding efficient singlet fission chromophores.

Mark Ratner, Northwestern University

Molecular Mesoscopics: Transport in Molecular JunctionsMolecular Mesoscopics: Transport in Molecular Junctions

The two phenomena of electron transfer in molecules and electron transport through molecules are closely related to one another.  Some of the phenomena exhibited in one of these areas can be mirrored in the other, but there are also differences.  In this talk, we discuss the transport situation and different mechanisms for transport that occur under different temperature conditions and with different molecular structures.  In particular, we will examine transport through more complex organic molecules than usual, and the interference phenomena that can result from cross-coupling, from meta linkages, and from simultaneous transport through more than one molecule.  Emphasis will be conceptual (no complicated equations, no harping on methodology), and some concepts of physical organic chemistry, and their relationship to transport, will be addressed.

Peter J. Rossky, The University of Texas at Austin

Exciton Migration and Dissociation in Conjugated Molecular  Materials

In order to develop a working chemical intuition about electronically active organic materials, and particularly with the goal of developing design principles for organic photovoltaic materials, it is imperative to understand the relationship between molecular-level structure and the electronic excited state phenomena of exciton migration and charge separation dynamics both within conjugated polymers and at organic donor/acceptor interfaces. In this presentation, recent progress in simulating these processes, using a mixed quantum/classical molecular dynamics approach that employs an all-atom description of the intermolecular interactions coupled with semi-empirical electronic structure will be described. The results of exploring several systems at ambient temperature will be discussed, including phenylene-vinylene and thiophene oligomers, as well as cyanine and fullerene components.  The roles of molecular structural fluctuations and intermolecular electronic couplings, as well as the roles of donor and acceptor excited state energy alignments will be discussed in the context of exciton transport and dissociation.  The development of a molecular-level interpretation of experimental ultrafast time-resolved spectroscopic probes of these processes in a layered phtalocyanine-C60 system will illustrate the mechanistic insight accessible via such simulations.

The results reported here are based on work supported as part of the Energy Frontier Research Center “Understanding Charge Separation and Transfer at Interfaces in Energy Materials” (EFRC:CST), funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001091.

 

George Schatz, Northwestern University

Modeling active plasmonic response

This talk describes recent projects where we have developed theory to describe the interaction of plasmons with excited states of molecules.  In one project that was done with Emily Weiss, we considered the influence of ligands on the time-resolved absorption spectrum of few nm gold nanoparticles.  Here we used electronic structure calculations to show that ligands such as thiolates contribute significantly to the plasmonic density of states near the Fermi level, and also influence the electron-phonon coupling strength, but other ligands such as amines do not have this affect.  As a result it is possible for thiolates to alter the rate of energy flow between electrons and phonons, and also the flow of energy to the surrounding solvent. In the second project, we have collaborated with Teri Odom’s group to study the time evolution of exciton populations in dye molecules that are near to plasmonic particle array structures.  Here we modeled the dye excited states using a 4-level model that incorporates plasmon-enhanced fields that are obtained from FDTD calculations.  We find that femtosecond pumping leads to inverted populations in the dye, and laser emission above an intensity threshold that is strongly coupled to lattice plasmon modes.

 

 

Contact Information: 

Prof. Jean-Luc Bredas (404-385-4985)

Map of Georgia Tech

School of Chemistry & Biochemistry

901 Atlantic Drive Atlanta, GA 30332-0400

(404) 894-4002 (phone) | (404) 894-7452 (fax)