General

RESCHEDULED: Inorganic Chemistry Seminar: Dr. Markus Ribbe, UCI

Tue, 2018-09-18 12:00 - 13:00
Speaker: 

Dr. Markus Ribble

Seminar Rescheduled 5/7/2019

 

"Nitrogenase M-Cluster Assembly:"


"Tracing the ‘9th Sulfur’ of the Nitrogenase Cofactor via a Semi-Synthetic Approach"

"The  M-cluster  is  the  active  site  of  nitrogenase  that  contains  an  8Fe-core  assembled  via coupling and rearrangement  of  two [Fe4S4]  clusters  concomitant  with  the  insertion  of  an interstitial carbon and a ‘9th  sulfur’. Combining synthetic [Fe4S4] clusters with an assembly protein template, we show that sulfite gives rise to the ‘9th sulfur’ that is incorporated in the catalytically important belt region of the cofactor after the radical SAM-dependent carbide insertion and the concurrent 8Fe-core rearrangement have already taken place. This work provides a semi-synthetic tool for strategically labeling the cofactor—including its ‘9th  S’ in the belt region—for mechanistic investigations of nitrogenase while suggesting an interesting"
"link between nitrogen fixation and sulfite detoxification in diazotrophic organisms."

Location: 

Carol Lynch Lecture Hall

Chemistry Complex

Attached Document: 

Host: Dr. Tomson

inquiries rvargas@sas.upenn.edu

Inorganic Chemistry Seminar: Dr. Smaranda Marinescu, John Hopkins

Thu, 2018-09-13 12:00 - 13:00
Speaker: 
Dr. Smaranda Marinescu
Bio-Inspired Coordination Complexes and Polymers for Energy Applications

Research in the Marinescu group focuses on fundamental research to understand, design, and synthesize novel catalytic systems essential to the development of efficient solar-to-fuel technologies. Inspired by biological systems, we innovate molecular catalysts that involve hydrogen bonding networks capable of small molecule activation, and multiple proton and electron transfers. We have shown that cobalt complexes with pendant proton relays (NH groups) act as highly efficient catalysts for the reduction of CO2 to CO, and that the presence of the pendant NH moiety is crucial for catalysis.

 

 

 

We also explore the immobilization of metal complexes to electrodes as a method to combine homogeneous and heterogeneous catalysis. Metal-organic frameworks (MOFs) have emerged as a promising class of materials; however, the insulating nature of MOFs has limited their application as electrocatalysts. We have shown that metal dithiolene units can be successfully integrated into one- and two-dimensional (1D/2D) frameworks. The generated coordination polymers display unique electronic properties – they catalyze with remarkable activity the electrocatalytic conversion of water into hydrogen, and their electrical conductivity is among that of the best coordination polymers. We expect the design principles discovered in these studies to have a profound impact towards the development of advanced materials and sustainable technologies.

 

Location: 

Carol Lynch Lectrue Hall

Chemistry Complex

 Host: Dr. Schelter

inquiries rvargas@sas.upenn.edu

Physical Chemistry Seminar: Dr. Sean Roberts, University of Texas-Austin

Thu, 2018-12-13 13:00 - 14:00
Speaker: 

Dr. Sean Roberts

 

Manipulating Energy and Spin for Photon Up- and Down-conversion The negligible spin-orbit coupling in many organic molecules creates opportunities to alter the energy of excited electrons by manipulating their spin. In particular, molecules with a large exchange splitting have garnered interest due to their potential to undergo singlet fission (SF), a process where a molecule in a high-energy spin-singlet state shares its energy with a neighbor, placing both in a low-energy spin-triplet state. When incorporated into photovoltaic and photocatalytic systems, SF can offset losses from carrier thermalization, which account for ~50% of the energy dissipated by these technologies. Likewise, compounds that undergo SF’s inverse, triplet fusion (TF), can be paired with infrared absorbers to create structures that upconvert infrared into visible light. In this presentation, I will review our group’s efforts to create organic:inorganic structures that use SF and TF for improved light harvesting and photon upconversion.

Location: 

Carol Lynch Lectrue Hall

Chemistry Complex

Host: Dr. Anna

inquiries rvargas@sas.upenn.edu

Physical Chemistry Seminar: Dr. Wei Xiong, UC San Diego

Thu, 2018-11-01 13:00 - 14:00
Speaker: 

Dr. Wei Xiong

 

Ultrafast Nonlinear IR Spectroscopy for Exotic Molecular Materials


In this seminar, I will discuss two developments in ultrafast nonlinear IR spectroscopy for exotic molecular materials: (1) 2D IR spectroscopy for molecular vibrational polaritons and (2) transient electric field induced VSFG spectroscopy for probing interfacial charge transfer. Both show the advantages of ultrafast nonlinear IR spectroscopic technique: to decipher hidden physics of exotic molecular materials. 

2D IR of Molecular Polaritons.1 Molecular vibrational polaritons, half-light, half-matter hybrid quasiparticles, are studied using ultrafast, coherent 2D IR spectroscopy. Molecular vibrational-polaritons are anticipated to produce new opportunities in the photonic and molecular phenomena. Many of these developments hinge on fundamental understanding of physical properties of molecular vibrational polaritons. Using 2D IR spectroscopy to study vibrational-polaritons, we obtained results that challenge and advance both polariton and spectroscopy fields. These results invoke new developments in theory for the spectroscopy, discover observation of new nonlinear optical effects and unexpected responses from hidden dark states. We expect these results to have significant implications in novel infrared photonic devices, lasing, molecular quantum simulation, as well as new chemistry by tailoring potential energy landscapes. 

Transient E-field induced VSFG for Direct Interfacial Charge Transfer.2 We describe direct electron-transfer at buried interfaces between an organic polymer semiconductor film and a gold substrate, by observing the transient electric-field-induced vibrational sum frequency generation (VSFG).  We observe dynamic responses (<150 fs) where electrons are directly transferred from the Fermi level of gold to the LUMO of organic semiconductor. Transient spectra further reveal that, although the interfaces are prepared without deliberate alignment control, a sub-ensemble of surface molecules can adopt conformations for direct electron transfer, supported by DFT calculations. This result will have implications for implementing novel direct electron transfer in energy materials.

References.

1.        Xiang, B. et al. Two-dimensional infrared spectroscopy of vibrational polaritons. Proc. Natl. Acad. Sci. 115, 4845–4850 (2018).

2.        Xiang, B., Li, Y., Pham, C. H., Paesani, F. & Xiong, W. Ultrafast Direct Electron Transfer at Organic Semiconductor and Metal Interfaces. Sci. Adv. 3, e1701508 (2017). 

 

 

Location: 

Carol Lynch Lecture Hall 

Chemistry Complex

Host: Dr. Saven

inquiries rvargas@sas.upenn.edu

Physical Chemistry Seminar: Dr. Antoine Kahn, Princeton University

Thu, 2018-11-08 13:00 - 14:00
Speaker: 
Dr. Antoine Kahn
" Electron spectroscopy and the study of metal halide perovskite surfaces and interfaces"

 

 

  

 The formidable promises of the “re-discovered” class of organic/inorganic metal halide perovskites (MHP) such as methylamonium lead tri-iodide (MAPbI3), and the rapid and steady rise in device performance achieved with these materials over the past seven years, have triggered a flurry of research all over the world. This talk reviews our efforts to understand key surfaces and interfaces of these materials. We first report a combined ultra-violet/inverse photoemission spectroscopy (UPS/IPES) - DFT study of the surface electronic structure of several 3D-MHPs, e.g., MAPbI3, MAPbBr3, and CsPbBr3, which yield valence and conduction band edge positions (VBM, CBM), ionization energy and electron affinity (IE, EA), energy gap. An unusually low density of states is found at the VBM of these materials, with potential consequences on device physics. We then look at recent measurements of the electronic structure of MHP interfaces with hole and electron transport layers used for carrier extraction/blocking in photovoltaic devices. Specifically, the role of p-doping is investigated in the case of interfaces between the HTL poly(triarylamine) (PTAA) and CsPbBr3.  We then turn to two-dimensional metal halide perovskites (2D-MHP) and present electronic structure measurements on several 2D butylammonium methylammonium lead iodide and bromide compounds, BA2MAn-1PbnI3n+1, n=1 - 4. XRD, AFM, UV-vis absorption, and UPS/IPES spectroscopies are used to investigate these compounds.  Their single-particle gap is obtained from UPS/IPES results, and compared with optical absorption measurements to deduce an exciton binding energy (EB). In agreement with previous results, EB is found to be large for n=1 and 2 (390 and 110 meV, respectively), but drops rapidly for n=3 and above. Finally, a simple model is presented to justify the electron and hole levels and the single particle gap in these quantum wells structures.  

 

Location: 

Carol Lynch Lecture Hall

Chemistry Complex

Host: Dr. Rappe

inquiries rvargas@sas.upenn.edu

Physical Chemistry Seminar: Dr. Tom Miller, Caltech

Thu, 2018-10-18 13:00 - 14:00
Speaker: 

Dr. Tom Miller

 

 

Getting Something For Nothing:

Classical and Machine-Learning Methods for Quantum Simulation

 


 

A focus of my research is to the develop simulation methods that reveal the mechanistic details of quantum mechanical reactions that are central to biological, molecular, and heterogenous catalysis. The nature of this effort is three-fold: we work from the foundation of quantum statistical mechanics and semiclassical dynamics to develop methods that significantly expand the scope and reliability of condensed-phase quantum dynamics simulation; we develop quantum embedding and machine learning methods that improve the description of molecular interactions and electronic properties; and we apply these methods to understand complex chemical systems.

The talk will focus on recent developments [1] and applications [2] of Feynman path integral methods for the description of non-adiabatic chemical dynamics, including proton-coupled electron-transfer and long-ranged electron transfer in protein systems.  Additionally, we will describe a machine-learning approach [3] to predicting the electronic structure results on the basis of simple molecular orbitals properties, yielding striking accuracy and transferability across chemical systems at low computational cost.

 

[1] "Path-integral isomorphic Hamiltonian for including nuclear quantum effects in non-adiabatic dynamics." X. Tao, P. Shushkov, and T. F. Miller III, J. Chem. Phys., 148, 102327 (2018).

 

[2] "Fluctuating hydrogen-bond networks govern anomalous electron transfer kinetics in a blue copper protein." J. S. Kretchmer, N. Boekelheide, J. J. Warren, J. R. Winkler, H. B. Gray, and T. F. Miller III, Proc. Natl. Acad. Sci. USA, 115, 6129 (2018).

 

[3] "Transferability in machine learning for electronic structure via the molecular orbital basis." M. Welborn, L. Cheng, and T. F. Miller III, J. Chem. Theory Comput., in press, DOI: 10.1021/acs.jctc.8b00636.
Location: 

Carol Lynch Lecture Hall

Chemistry Complex

Host: Dr. Subotnik

inquiries rvargas@sas.upenn.edu

Physical Chemistry Seminar: Dr. Jeanne Stachowiak, University of Texas- Austin

Thu, 2018-10-04 13:00 - 14:00
Speaker: 

Dr. Jeanne Stachowiak

 

" Stochastic Mechanisms in Membrane Traffic"

-Membrane traffic, an essential cellular process that plays a role in many human diseases, requires key biophysical steps including formation of membrane buds, loading of these buds with specific molecular cargo, separation from the parent membrane, and fusion with the target membrane. The prevailing view has been that structured protein motifs such as wedge-like amphipathic helices, crescent-shaped BAR domains, curved coats and constricting dynamin rings drive these processes. However, many proteins that contain these structural motifs also contain large intrinsically disordered protein (IDP) domains of 300-1500 amino acids, including many clathrin and COPII coat components. While these IDP domains have been regarded primarily as flexible biochemical scaffolds, we have recently discovered that IDPs are highly efficient physical drivers of membrane budding. Further, our work demonstrates that IDP domains serve as strong drivers of membrane fission. How can molecules without a defined structure drive membrane budding and fission? Our results support the idea that disordered domains generate entropic pressure at membrane surfaces, which is critical to overcoming key biophysical barriers to membrane traffic. IDPs are particularly efficient generators of entropic pressure owing to their very large hydrodynamic radii, potential for electrostatic repulsion owing to high net charge, and the substantial entropic cost of extending them. More broadly our findings suggest that any protein,regardless of structure, can contribute to membrane remodeling by increasing entropic pressure, and paradoxically, that proteins that lack a defined secondary structure, IDPs, may be among the most potent drivers of membrane traffic. Our ongoing work focuses on understanding how entropic pressure influences membrane traffic, and designing biophysical tools for manipulating receptor recycling and signaling.
Location: 

Carol Lynch Lectrue Hall

Chemistry Complex

Host: Dr. Baumgart

 inquiries rvargas@sas.upenn.edu

Physical Chemistry Seminar: Dr. Paul Cremer, Pennsylvania State University

Thu, 2018-09-20 13:00 - 14:00
Speaker: 

Dr. Paul Cremer

" Probing the Interactions of Ions and Small Molecules with Phospholipid Membranes"

The interactions of ions and small molecules with lipid head groups depends intimately on the structure of the analytes and the chemical composition of the membrane. In this talk, it will be shown that these interactions can be facilely probed by fluorescence microscopy when using a pH sensitive probe. The ability of small organic species to insert into the membrane depends on the charge on the membrane, the charge on the small molecule, the presence of cholesterol, as well as the presence of other specific membrane lipids (e.g. phosphatidylethanolamine or phosphatidylglycerol). Further information on the insertion process can be obtained from sum frequency generation (SFG), Langmuir isotherm, and fluorescence recovery after photobleaching (FRAP) measurements. Intriguingly, it is revealed that the perturbation of the interfacial water structure can be correlated with the mechanisms of small molecule-membrane interactions under certain conditions. These types of interactions are significant because soluble drug molecules typically are meant to interact with specific protein receptors and membrane-drug interactions present an additional and often unintended influence.

 

 

Location: 

Carol Lynch Lecture Hall

Chemistry Complex

Attached Document: 

Host: Dr. Gai

 

inquiries rvargas@sas.upenn.edu

Physical Chemistry Seminar: Dr. Akbar Salam, Wake Forest

Thu, 2018-09-06 13:00 - 14:00
Speaker: 

Dr. Akbar Salam

 Molecular QED Theory of Resonance Energy Transfer 

 

 Abstract: The fundamental theory of electron-photon interactions is quantum electrodynamics (QED). Its characteristic feature is the quantisation of the electromagnetic field. One of the successes of the molecular version of QED [1,2] is the unified description it provides of resonance energy transfer (RET) [1]. This process is viewed as arising from the exchange of a single virtual photon between an initially excited donor species and a ground state acceptor moiety. Asymptotic limits of the Fermi golden rule transition rate reveal a radiationless Förster type of exchange mechanism in the near-zone that has inverse sixth power separation distance dependence, and a radiative inverse square law behaviour at large displacements.

 

 

 

In this seminar an overview of the theory of molecular QED and its application to pair RET will be given first, before recent results are presented concerning the role that one and two additional passive polarisable molecules have in modifying the transfer rate [3,4]. Insight is gained into migration of energy in an environment of bath molecules by comparing these microscopic models of RET occurring between individual particles with a polariton based approach [5] in which direct excitation energy transfer is facilitated by medium-dressed photons.

 

 

 

[1] A. Salam, Molecular Quantum Electrodynamics, John Wiley & Sons, Inc., Hoboken, 2010.

 

[2] D. L. Andrews, G. A. Jones, A. Salam and R. G. Woolley, J. Chem. Phys. 148, 040901 (2018).

 

[3] A. Salam, J. Chem. Phys. 136, 014509 (2012).

 

[4] D. L. Andrews and J. S. Ford, J. Chem. Phys. 139, 014107 (2013).

 

[5] G. Juzeliunas and D. L. Andrews, Adv. Chem. Phys. 112, 357 (2000). 

 

 

Location: 

Carol Lynch Lecture Hall

Chemistry Complex

Attached Document: 

Host: Dr. Nitzan and Subotnik

inquiries rvargas@sas.upenn.edu

Inorganic Chemistry Seminar: Dr. Yunho Lee, KAIST

Tue, 2018-08-14 12:00 - 13:00
Speaker: 

Dr. Yunho Lee

Coordination Chemistry of 1st-row Transition Metal Pincer Complexes

 Transition metal adduct formations with small molecules such as N2, H2, CO and CO2 are drawing much attention due to their importance in developing synthetic catalysts for various industrial processes. In our laboratory, a series of such species with low-valent 1st row transition metals are currently under investigation. This effort is to show their respective roles in small-molecule transformations that include the COx and NOx (x = 1 – 3) conversions for modeling ACS/CODH active site chemistry and biological denitrification processes, respectively. In this presentation, a particular study with pincer complexes (PEP)M-L (E = N, P or Si and M = Co, Ni, Cu), where the L site is occupied by various ligands such as NHR2, N2, COx and COOR will be discussed. Regarding the geometry and reactivity relationship, a (PPP)M scaffold reveals the interconversion between square planar and tetrahedral geometry in which reversible group transfer occurs between a phosphide moiety and a nickel ion via unanticipated metal-ligand cooperation. This unusual group transfer reaction is tightly coupled with metal’s local geometry and its 0/I/II redox couples. In contrast, a (PNP)M scaffold shows a selective reaction pattern occurring at the structurally rigidified nickel center. With a structurally rigidified acriPNP ligand, a T-shaped nickel(I) metalloradical species was successfully stabilized. Having a sterically exposed half-filled dx2-y2 orbital, this nickel(I) species reveals unique open-shell reactivity. Such modification is successful in selective conversion of CO2. The reduction of {(acriPNP)Ni(CO)}{BF4} also succeeded in generating mono- and zero-valent nickel carbonyl complexes. In fact, the Ni(0)-CO species reveals the selective addition of CO2 to give a nickel(II)-carboxylate species with the expulsion of CO. The closed synthetic cycle for CO2 reduction to CO was finally established with a (acriPNP)Ni system.

 


·       “Selective Transformation of CO2 to CO at a Single Nickel Center” Acc. Chem. Res., 2018, 51, 1144-1152.

·       “Direct CO2 addition to a Ni(0)-CO species allowing the selective generation of a nickel(II) carboxylate with expulsion of CO” J. Am. Chem. Soc., 2018, 140, 2179-2185.

·       “A T-Shaped Ni(I) Metalloradical Species” Angew. Chem., Int. Ed. 2017, 56, 9502.

·       “Carbon Dioxide Binding at a Ni/Fe Center; Synthesis and Characterization of Ni(η1-CO2-κC) and Ni-μ-CO2-κC:κ2O,O’-Fe” Chem. Sci. 2017, 8, 600.

·       “Phosphinite-Ni0 Mediated Formation of a Phosphide-NiII-OCOOMe Species via Uncommon Metal-Ligand Cooperation.” J. Am. Chem. Soc. 2015, 137, 4280.

Location: 

Carol Lynch Lecture Hall

Chemistry Complex

Host: Dr. Mindiola

inquiries rvargas@sas.upenn.edu

Department of Chemistry

231 S. 34 Street, Philadelphia, PA 19104-6323

215.898.8317 voice | 215.573.2112 fax | web@chem.upenn.edu

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