Spectroscopy in Chlorophyll Dimer Embedded in Mixed Quantum-Classical Environment

27 February 2025

03:45 PM - 04:30 PM

Abstract

The electronic transition dipole moment 4-point time correlation function for a dimeric photosynthetic complex, from which nonlinear optical time-domain signals may be obtained. This 4-point time correlation function draws on an experimentally fit spectral density of the surrounding phonons of the photosynthetic protein. This spectral density of the photosynthetic phonons renders a phonon-side band characterized by its asymmetry, caused by the unequal contribution from the photosynthetic phonons (bath) to the low- and high-energy sides of the optical signals. This spectral density manifests its asymmetry explicitly in the 1-phonon profile, due to the intimate spectral connection between them, which will in turn reflect in the entire phononic part of the absorption spectrum. This asymmetry turns out to play an important role in characterizing the exciton-phonon coupling strength and the phonon relaxation mechanism. To this end, the obtained nonlinear optical electronic transition dipole moment time correlation functions (Liouville space pathways) whereby both excitonic and exciton-phonon couplings are accounted for are deemed convenient, more tractable, and computationally expedient, a unique advantageous feature in the case of a multimode system, which is often the case in photosynthetic complexes. Linear spectra and photon echo signals to probe pigment-protein complexes, in which pure electronic dephasing, vibrational relaxation effects, 1-phonon profile asymmetry, exciton-exciton coupling, and exciton-phonon coupling in bacterial reaction centers and photosynthetic complexes are provided.

Professor Dr. Mohamad Toutounji

Ph.D. at Iowa State University. Research Associate at Northwestern University in Chicago and University of Toronto

Bio

Dr. Mohammed Toutounji is an accomplished researcher in the fields of theoretical chemical physics and quantum systems. He received his BSc in Mathematics and Physical Science from the University of Central Oklahoma in 1990, followed by graduate work in Mathematics and Quantum Physics at the University of Oklahoma in 1991. Dr. Toutounji earned his Ph.D. in Theoretical Chemical Physics from Iowa State University in 1998, where he focused on the interplay between quantum mechanics and chemical systems.

His postdoctoral research took him to prestigious institutions such as Northwestern University and the University of Toronto, where he studied Quantum Dissipative Systems in 1999 and 2000, respectively. Dr. Toutounji’s research spans a variety of topics, including electronic dephasing, vibronic coupling, and quantum-classical molecular systems. His work has notably advanced our understanding of the dynamics of photosynthetic bacterial reaction centers and the spectroscopic properties of polyatomic molecules in condensed phases.

Among his key contributions is the development of linear and nonlinear electronic transition dipole moment time correlation function for photosynthetic systems, which accounts for electronic dephasing, asymmetry, and phonon interactions. He has also explored the spin-boson model for electronic dephasing in mixed quantum-classical systems. His work has been published in leading journals, including the Journal of Chemical Physics, Journal of Chemical Theory and Computation, and Chemical Physics, Annals of Physics, and Biophysical Reports.

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