Student Hosted Colloquia: Professor Lin X. Chen, Northwestern University

"Excited State Trajectories in Photoactive Transition Metal Complexes Probed by Ultrafast Laser and X-ray Spectroscopies and Scattering"

Host: Augustin Braun

About the Seminar

Many photochemical events start from initial light-matter interactions that cause atomic and electronic displacements in the excited states away from the energy minima of their potential energy surfaces. When certain photochemical events, such as bond breaking, intersystem crossing and electron/energy transfer taking place within the periods of key vibrational modes, the excited state energies are determined by the trajectories defined by the actual nuclear movements that may lead to different reaction pathways and outcome. Examples will be given in the work of tracking excited state pathways for transition metal complexes on the time scales from femtosecond to a few picoseconds, such as excited state Cu(I) and Pt(II) dimer complexes. Using fs broadband transient spectroscopy and fs X-ray solution scattering, coherent vibrational wavepacket motions can be examined.

From the time evolution of key vibrational modes in the Pt dimer complexes, particularly the Pt-Pt stretching mode, we mapped out excited state trajectories on potential energy surfaces of Pt-dimer complexes for light conversion, including coherent nuclear motions. These studies were carried out in a series of model platinum dimer complexes in solution and their electron donor acceptor complexes, featuring rich photochemistry and a set of intricate excited state potential energy surfaces on time scale previously unattainable. 

For the time evolution of the structural dynamics of Cu(I) complex, the experimental results revealed the Jahn-Teller distortion that transforms the symmetry of the molecule from the D2d in the ground state to D2 in the triplet metal-to-ligand-charge-transfer (MLCT) state along the two key cooperative coordinates, the Cu-N distances and the angle between the two phenanthroline ligand planes. The detailed structural trajectories have been reproduced by quantum mechanical calculations to map out the actual excited state nuclear motions from the Frank-Condon structure to an intermediate triplet state and finally the triplet MLCT state.

The combined molecular design, experimental and theoretical approaches enabled quantitative evaluation of the excited state trajectories in terms of actual nuclear motions as well as the energetic variations. These results will guide future molecular design and synthesis to steer photochemical reactions to optimal directions.

About the Speaker

Lin X. Chen is a Professor of Chemistry in Northwestern University and a Distinguished Fellow in Argonne National Laboratory.  She received her Ph.D. from the University of Chicago. After her postdoctoral research at UC Berkeley, she joined Argonne as a staff scientist.  In 2007, she joined Northwestern University where her research are focused on fundamental light-matter interactions of different solar energy conversion platforms, including excited state molecular structural dynamics in photocatalytic processes and photovoltaic materials; understanding roles of ultrafast and coherent electronic and atomic motions in in photochemical reactions, and functional structural dynamics of biomacromolecules on multiple spatial and temporal scales. Her main tools for research are ultrafast laser and X-ray spectroscopy/scattering and other property/structural methods in collaborations with theorists and chemists making molecules and materials. She was awarded one of the highly cited scientists in 2019 by the Web of Science, with >270 publications, and >210 invited lectures.  She has been members of the Research Council for the Chemical, Biological and Geological Sciences Division, and Basic Energy Science Advisory Committee, Basic Energy Science, US Department of Energy, the Advisory Editorial Board of Journal of Physical Chemistry and Chemical Physics Letters, Journal of Materials Chemistry C, Aggregates, Senior Editor of ACS Energy Letters, Associate Editor of Chemical Science (RSC) and the International Science Advisory Committee for π-Functional Materials.  She is an AAAS, RCS and ACA Fellow and has won Experimental Physical Chemistry Award from ACS Physical Chemistry Division in 2020 was awarded the distinguished performance award at Argonne.   Her group website is at http://chemgroups.northwestern.edu/chen_group/.

Image Credit: Northwestern University