Greg Engel Associate Professor
Born: West Chester, PA, 1977
Princeton University, A.B., 1999
Harvard University, A.M., 2001
Harvard University, Ph.D., 2004
Harvard University, Postdoctoral Scholar, 2004-2005
University of California, Berkeley, Postdoctoral Fellow, 2005-2007
University of Chicago, Associate Professor, 2007-
2012 Llewellyn John and Harriet Manchester Quantrell Award for Undergraduate Teaching
2012 Dreyfus Teacher-Scholar Award
2012 Sloan Research Fellowship
2012 Coblentz Award
2010 DTRA Young Investigator Award
2010 DARPA Young Faculty Award
2009 PECASE Recipient
2009 Searle Scholar
2008 AFOSR Young Investigator Program
2007 Scientific American Top 50 Leaders in Research
2007 Camille and Henry Dreyfus New Faculty Award
2005-2007 Miller Research Fellow
2003-2004 EPA STAR Fellow
1999-2002 NSF Graduate Research Fellow
OFFICE: 929 E. 57th St., GCIS E 119, Chicago, IL 60637
Group Webpage: http://engelgroup.uchicago.edu/
Observing and Controlling Excited State Dynamics
Research in the Engel Group focuses on excited state reactivity including excitonic transport, non-radiative relaxation to photochemical products, and new methods to image excited state dynamics. Excited states in the condensed phase have an extremely high chemical potential thereby making them highly reactive and difficult to control. Our control strategy involves exploiting coherent response of the environment to the excitation event. In particular, we develop methodologies to manipulate two fundamental components of excited state dynamics: exciton migration and non-radiative relaxation.
Our approach is inspired by biological systems optimized by evolution to exploit manifestly quantum mechanical phenomena to drive coherent energy transfer, to steer trajectories through conical intersections and to protect long-lived quantum coherence. Currently, we are focusing on four key scientific efforts: (1) new techniques to image excited state dynamics, (2) understanding mechanisms of quantum transport in photosynthesis, (3) dynamics of conical intersections in the condensed phase, and (4) engineering quantum dynamics in new classes of synthetic materials.
New Spectroscopy for Imaging Excited State Dynamics
Gradient Assisted Photon Echo Spectroscopy (GRAPES) uses tilted wavefronts to create an array of delay times spatially within the sample and then images the signal with a CCD. An entire 2D spectrum can be acquired in a single laser shot.
Excited state dynamics prove extremely sensitive to couplings to the environment and to other electronic states. Such subtle couplings in complex excitonic systems require new spectroscopic approaches to collect data and new strategies to interpret spectra. We have created the first optical analog to MRI by recasting the electronic spectroscopy problem into an imaging problem. Our GRAPE approach can capture the entire 2D spectrum in a single laser shot. We plan to use this technique to study photodamage, oxidative aging, and photoprotection to understand how these processes affect electronic structure.
Understanding Mechanisms of Quantum Transport in Photosynthetic Systems
The discovery of long-lived quantum coherence and coherence transport in photosynthetic systems has spawned interest in “Quantum Biology” – aspects of biology where evolution has taken advantage of non-classical phenomena. The inclusion of a wavelike mechanism of energy transfer provides an opportunity for robust and efficient operation in disordered environments and represents a new paradigm for transport in materials. The Engel group endeavors to understand design principles evident in photosynthetic light harvesting and then to incorporate these elements into synthetic systems. We maintain an active research program in biophysics that allows us to probe the mechanism and to develop new characterization methods.
We have isolated a new signature in our 2D data that provides direct evidence of quantum transport. The first observation of its type, we discovered population oscillations due to the protein bath coupling population (diagonal) elements of the density matrix to oscillatory coherence (off-diagonal) elements of the matrix. This observation suggests an opportunity to engineer chemical consequences of quantum coherence, such as oscillatory reaction rates, into synthetic systems.
The quantum beat signals extracted from the FMO 2D data set agree well in both frequency and phase as temperature increases. The approximately two cycles of oscillation observed match theoretical predictions for optimally efficient energy transfer.
Direct evidence of quantum transport, these population oscillations also demonstrate entanglement with the bath because energy within the excitonic system is not conserved.
Engineering Quantum Coherence
The Engel Group is developing small-molecule test cases for long-lived coherence. We strive to maintain control over the relative dipole orientations, the solvation environment (solution, polymer, glass, etc), and the energetics of the coupling. To this end, we have synthesized functionalized fluorescein molecules and linked them together to form dimeric pairs. Our synthetic scheme is specifically designed to tune properties related to coherent energy transfer such as distance, coupling, dipolar orientation, and stiffness. The synthetic strategy outlined here can also be easily modified to give similar compounds with rotated transition dipoles, different transition energy gaps, different distances between chromophores and different numbers of chromophores (dimers, trimers, etc).
Synthetic scheme to create fluorescein dimers. We have carried out the synthetic scheme described at left on a small scale and successfully isolated compound 8 (confirmed by NMR). We intend to explore different length linkers, trimers, tetramers, other coupling geometries, and other halogenations schemes to test models of excited state dynamics by effectively changing the detuning and coupling parameters. Different solvent conditions will also be tested including glassy and polymeric environments.
Theoretical Efforts: Excitonic Transport in Photosynthesis and Beyond
Phenomenological environmentally-assisted quantum transport (EnAQT) models explain the benefits of coherent transport. However, these models lack the detail required to enable new excitonic materials that operate with this mechanism. We are exploring new models of coherent energy transport to isolate design principles that will enable new quantum materials.
Our efforts include advances in both theory and analysis. For example, different pairs of excitons show radically different dephasing times and these dephasing times do not directly correlate to energy, spatial overlap or distance. To extract this data, we adapted the Z-transform and applied it to two-dimensional spectra for the first time. This new analysis tool permits simultaneous extraction of decay constants and beating frequencies.
Also, we found that experiments were probing the ensemble dephasing of the photosynthetic complexes, but that the efficiency of transport depends not on the dephasing time, but on the decoherence time. That is, inhomogeneity across the ensemble leads to dephasing, but transport efficiency depends only on the system-bath interactions within each complex. These observations led us to develop new approaches to separate experimentally dephasing from decoherence using the rephasing nature of 2D electronic spectroscopy.
An ensemble of 2500 FMO complexes were simulated with 15 cm-1 of energetic disorder among the chlorophyll site energies. The entire ensemble was excited, and then the individual coherences (pink) were averaged to create the observable ensemble average. The ensemble dephases with a lifetime of 310 fs, which would be measured in an experiment. However, the decoherence time of a typical trajectory (red) is much longer. This decoherence time rather than the dephasing time determines transport efficiency.
Capturing Reactivity at Conical Intersections
A main goal of our laboratory remains probing and controlling reactivity near conical intersections in the condensed phase. Photochemistry has proven very difficult to understand and to intuit in solution because trajectories through conical intersections typically depend on solvent coordinates in the condensed phase (22,23). Within photoenzymes, motion along these coordinates can be precisely controlled, but in solution, it is random. Ultimately, we aim to manipulate macromolecular scaffolds to affect excited state reactivity near conical intersections. The critical control parameters for such a strategy are the non-Born-Oppenheimer coupling elements that mix the two electronic states and that determine the trajectories through the conical intersection. Yet, no spectroscopy exists that can dissect these coupling elements. We are working to create a new spectroscopic technique to directly image these couplings.
Novel 3D Nonlinear Spectroscopies
Two-dimensional electronic spectroscopy often fails to resolve the underlying Hamiltonian fully or to provide a complete map of the excited state dynamics. We strive to develop novel and tractable higher order spectroscopic methods to better isolate specific signals. Extending the capabilities of our 2D electronic spectroscopy into three (or more) dimensions reveals additional information and exploits the improved phase stability and acquisition speed of our GRAPE spectrometer.
Expanding on the 3D idea, we realized that we can apply some of the same analysis strategy to our third order spectra. By eliminating exponential population dynamics and subsequently examining long-lived coherences, we have resolved 19 individual cross-peaks to enable extraction of the Hamiltonian (Figure 9). In contrast, the 2D electronic spectrum (Figure 4, inset) shows only a single resolvable cross-peak, which the 3D experiment shows to be actually composed of two cross-peaks. This work provides a new paradigm for dissecting excitonic Hamiltonians in highly congested spectra.
A peak from IR144 lies approximately on the main diagonal of the spectrum. The unexpected off-diagonal peak can only be reproduced in our simulations by including coupled vibrational motion. No such cross-peak appears in third order spectra of IR144.
Nineteen distinct cross peaks can be isolated in the FMO complex using 3D FT electronic spectroscopy. This approach has allowed us to experimentally reconstruct the excitonic Hamiltonian for the first time.
Selected References (Click here for full list)
E. Harel and G.S. Engel, "Quantum Coherence Spectroscopy Reveals Complex Dynamics in Bacterial Light Harvesting Complex 2 (LH2)" PNAS 109(3) 706-711 2012.
K.M. Pelzer, G.R. Griffin, S.K. Gray, and G.S. Engel, "Inhomogeneous Dephasing Masks Coherence Lifetimes In Ensemble Measurements," JCP 136, 164508 2012.
G. Panitchayangkoon, D.V. Voronine, D. Abramavicius, J.R. Caram, N.H.C. Lewis, S. Mukamel, and G.S. Engel, “Direct Evidence of Quantum Transport in Photosynthetic Light-harvesting Complexes." PNAS, 108(52) 20908-20912 2011.
E. Harel, A. Fidler, and G.S. Engel, "Real-time Mapping of Electronic Structure with Single-shot Two-dimensional Electronic Spectroscopy."PNAS, 107:16444-16447 2010.
G. Panitchayangkoon, D. Hayes, K.A. Fransted, J.R. Caram, E. Harel, J. Wen, R.E. Blankenship, and G.S. Engel. “Long-lived quantum coherence in photosynthetic complexes at physiological temperature.” PNAS, 107:12766-12770 2010.