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Posted: Mar 29, 2016

Querying excited electrons

(Nanowerk News) Electrons are often nudged or jolted from their low-energy state to trigger change in a plethora of phenomena, from plant photosynthesis to biofuel production. Yet traditional theoretical approaches have relied on studying molecules and materials where the electrons were in their low-energy state, or at equilibrium. Achieving a fundamental understanding of the underlying processes, which can lead to better atomic-scale control, also requires studying the nature of chemical bonding and structure away from equilibrium.
To answer this need, scientists at DOE's Pacific Northwest National Laboratory and three universities have devised theoretical approaches to study the excited state responses of molecules and materials. Their approaches were developed within NWChem, which takes advantage of large-scale computing systems to answer challenging questions in chemistry, biology, and materials science.
In the ground state, the molecular complex, zinc phthalocyanine, absorbs in the yellow, orange, and red parts of the spectrum
In the ground state, the molecular complex, zinc phthalocyanine, absorbs in the yellow, orange, and red parts of the spectrum. As the intensity of the light increases, more molecules are transferred to the excited state with a corresponding increase in the absorption of yellow, green and blue light. These results show that this complex has the ability to be a limiting material for light in the yellow to violet range, depending on the intensity of light. Correctly predicting how excited materials respond to light would help scientists create materials of interest for solar power and other energy technologies. (Artwork by Niri Govind, PNNL)
"When things change, you want to understand what's happening at the fundamental length and time scales," said Dr. Niri Govind, a chemical physicist at PNNL who led the research and is a member of the NWChem development team. "Our approach helps fill in the gaps and gives a fuller picture."
The team members were from PNNL, the University of Washington, University of California, Irvine, and the University of Minnesota.
Why It Matters
The new approaches let scientists theoretically interrogate complex molecules and materials in the excited state and complement state-of-the-art transient and ultrafast spectroscopies. The result is new insights into the chemical and materials transformations that lead to new and more efficient materials and processes.
Methods
The team performed complex calculations, based on real-time and linear-response time-dependent density functional theory, to study the excited state response of optical chromophores and transition metal complexes in the visible and X-ray wavelengths. The team used the supercomputing facilities at PNNL and two DOE Office of Science scientific user facilities: EMSL located at PNNL in Washington State, and the National Energy Research Scientific Computing Center located at the Lawrence Berkeley National Laboratory in California.
emission spectrum from an excited molecular complex
The teamís theoretical approach which simulates the emission spectrum from an excited molecular complex (top) sheds light on the chemical bonding between a metal atom and the ligands bonded to it (bottom). The results of this method allow scientists to determine the structure of catalysts and other materials important for clean energy technologies. (Artwork by Niri Govind, PNNL)
What's Next?
This work can be used to provide new insights into the excited state and transformations that happen on very fast time scales. Experiments to probe these regimes are currently possible with the new generation of DOE light sources. In some ways, a light source, which uses X-rays to probe fast dynamical events, is a camera with an extremely fast shutter speed that can capture short-lived transformations on the excited state. The team is applying their theoretical approach to complement and interpret experiments on more complex molecular systems using the Linac Coherent Light Source located at the SLAC National Accelerator Laboratory in California.
References
Fischer SA, CJ Cramer, and N Govind. 2016. "Excited State Absorption from Real-Time Time-Dependent Density Functional Theory: Optical Limiting in Zinc Phthalocyanine." Journal of Physical Chemistry Letters. DOI: 10.1021/acs.jpclett.6b00282
Fischer SA, CJ Cramer, and N Govind. 2015. "Excited State Absorption from Real-Time Time-Dependent Density Functional Theory." Journal of Chemical Theory and Computation 11(9):4294-4303. DOI: 10.1021/acs.jctc.5b00473
Zhang Y, S Mukamel, M Khalil, and N Govind. 2015. "Simulating Valence-to-Core X-ray Emission Spectroscopy of Transition Metal Complexes with Time-Dependent Density Functional Theory." Journal of Chemical Theory and Computation 11(12):5804-5809. DOI: 10.1021/acs.jctc.5b00763
Source: Pacific Northwest National Laboratory
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