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December 7th, 2021 Webinar Speakers

Our next webinar will take place via the internet on Tuesday December 7th at 8 PM EST/ 1 AM GMT. Sign up on our mailing list to receive the Zoom link!

We hope to see/hear from you all at one of our sessions or as one of the next speakers. If you are an early career scientist and would like to present your research, don't hesitate to submit an abstract today! For now, please learn more about our current speakers and their research below. We also thank the generous support from Cell Reports Physical Science, Merck, and the Royal Society of Chemistry.

Our featured speakers this week are Dr Vinícius Wilian D. Cruzeiro (postdoctoral researcher, Stanford University, USA), and Dr Michele Myong (postdoctoral researcher, Brookhaven National Laboratory, USA).


DR VINICIUS WILIAN D. CRUZEIRO (on Twitter @vwcruzeiro)

Biography: Dr. Cruzeiro explores computational/theoretical chemistry aiming at accurately describing the behavior of proteins, biomolecules, and related systems using molecular simulations, quantum mechanics, and machine learning representations. His research digs into fundamental aspects of nature at the intersection of physics, chemistry, and biology.

Dr. Cruzeiro is part of the Amber developer’s team. He has participated in developing different methodologies, including molecular simulations with coupled electrochemical and pH effects, enhanced sampling techniques, and quantum mechanics/molecular mechanics approaches. Amber is a popular software package for molecular simulations used by several research groups worldwide.

Title of Talk: Towards an accurate description of the X-ray emission spectrum of liquid water

Abstract: Liquid water displays some remarkable anomalous properties that are still not fully understood. Among them, there is a splitting observed in the 1b1 band of the X-ray emission (XE) spectrum of liquid water. Previous publications have presented different hypotheses to explain this splitting, such as associating it to the local hydrogen-bonding environment; However, even though several theoretical studies have attempted to describe the 1b1 splitting using a variety of methods, no theoretical approach has successfully reproduced it before.

In a recent publication, we introduced a novel theoretical/computational QM/MM approach. This approach builds upon the demonstrated accuracy of the successful MB-pol many-body potential energy function. It combines path-integral molecular dynamics (PIMD) simulations, which contain quantum nuclei effects, with time-dependent density functional theory (TD-DFT) calculations to model the X-ray emission (XE) spectrum of water and ice Ih. To the best of our knowledge, this study reports the first calculations that predict the split of the 1b1 peak in the XE spectrum of liquid water. A systematic analysis in terms of the underlying local structure of liquid water at ambient conditions indicates that several different hydrogen-bonding motifs contribute to the overall XE lineshape in the energy range corresponding to emissions from the 1b1 orbitals, which suggests that it is not possible to unambiguously attribute the split of the 1b1 peak to only two specific structural arrangements of the underlying hydrogen-bonding network; this assessment conflicts with the mainstream hypotheses to explain the origin of the splitting.

Our theoretical lineshapes achieve nearly quantitative agreement between theory and experiment for ice Ih. In the case of liquid water, the theoretical lineshape correctly captures the overall linewidth, although the experimentally observed 1b_1 splitting is significantly underestimated.

While these findings represent a further step towards understanding the behavior of liquid water at the molecular level and allow for reconciling apparent inconsistencies between experimental measurements and computer simulations, a quantitative description of the XE spectrum is still missing. The next step in this study is to go beyond the accuracy of densify-functional theory by employing rigorous electronic structure methods, namely equation-of-motion coupled-cluster theory, to describe the XE spectrum. Since the computational cost of this methodology is commonly prohibitively expensive, we will take advantage of the GPU-accelerated TeraChem software. We will investigate whether a more accurate treatment of the electronic structure calculations could contribute to a better description of the XE spectrum of liquid water, which is a problem that has remained unsolved for more than one decade.

DR MICHELE MYONG (on Twitter @michele_myong)

Biography: Michele earned her B.A. in chemistry from Columbia University in 2016 and graduated with her PhD from Northwestern University in August 2021. As an NSF Graduate Research Fellow, she worked with Prof. Michael Wasielewski to study the photophysics of self-assembled organic chromophore systems relevant to solar energy harvesting. She recently began a postdoc with Dr. Matthew Bird in the Electron- and Photo-Induced Processes Group at Brookhaven National Laboratory where she will use pulse radiolysis to study energy converting molecular systems.

Title of Talk: Excimer Diffusivity in 9,10-Bis(phenylethynyl)anthracene Assemblies on Anodic Aluminum Oxide Membranes

Abstract: To meet the world’s energy demands, solar energy must be captured, transferred, and stored efficiently. The efficiency of natural photosynthetic light harvesting systems depends on the electronic energy transfer between molecules in chromophore assemblies that surround a reaction center. Because the electronic coupling between the chromophores determines the degree of energy transfer and can be tuned by the modifying the distance and orientation between molecules, research efforts have focused on creating self-assembled supramolecular chromophore structures. Excimers usually serve as low energy trap sites in supramolecular chromophore assemblies; however, if the trap is not too deep, excimers may diffuse throughout the structure, making it possible to delivery excitation energy to distant sites. To investigate this phenomenon, a supramolecular assembly of 9,10-bis(phenylethynyl)anthracene (BPEA) chromophores was prepared by covalently linking a BPEA molecule to the walls of nanoporous anodic aluminum oxide (AAO) membranes. Using transient absorption spectroscopy, we calculate diffusion coefficients to compare the excimer mobility in BPEA assemblies to that of other chromophore aggregates. The BPEA molecules self-associate and form excimers upon photoexcitation. The excimer diffusivity in the BPEA on the AAO membranes is higher than that of other reported excimers, approaching that of singlet excitons in efficient organic photovoltaic systems. The AAO membrane geometry confines the mobility of the excimer to one dimension, highlighting the ability of the nanostructure to serve as a platform that forces many chromophores into specific geometric relationships. BPEA molecules self-assembled on AAO membranes also represent the mesoscopic regime between the solution phase and solid state in which we can study both how bimolecular diffusion in solution and close packing in the AAO membrane influence the mechanism of excimer formation and diffusion.

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