Virtual Winter School on Computational Chemistry
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Department of Chemistry, Lomonosov Moscow State University, Russia
Light of various wavelengths and intensity can be used as a probe of molecular structure and dynamics. Absorption and photoelectron spectroscopies provide invaluable information not only about the electronic structure of molecules, but also about the photoinduced dynamics of vibrational wave packets in their excited states. Knowledge of vibrational modes, which become active upon electronic transitions, can greatly enhance our understanding of mechanisms, specificity, and speed of primary photochemical reactions, such as photoisomerization in human vision and photoinduced electron transfer in photoactive proteins. Vibronic couplings are essential for transitions otherwise electronically forbidden by symmetry; however, even for allowed transitions, vibronically induced contributions can be significant and define, for example, the origin of markedly different spectral shapes in one- and two-photon absorption. We will discuss mechanisms of radiative and non-radiative transitions and approaches for modeling vibronic band shapes of polyatomic molecules using a linear coupling scheme, which accounts for both the Franck-Condon and Herzberg-Teller contributions. We will further analyze the differences in one-photon and two-photon absorption profiles for the lowest-lying transitions using a simple two-level model, which allows one to describe two-photon cross-sections and vibronic contributions using readily available physical properties of the two states involved in the transition. We will then apply the basic linear model for simulating and analyzing one-photon and two-photon spectral profiles of retinal-containing visual and microbial rhodopsins and the green fluorescent protein (GFP). We will show that the photoresponse of the retinal chromophore is highly mode-specific inside the proteins, resulting in excitation of those vibrational modes that facilitate photoisomerization of a particular double bond. Furthermore, by calculating instantaneous population of vibrational modes, which mediate barrier-controlled photochemical reactions in excited states, such as nuclear-driven photoinduced electron transfer in GFP, we can also estimate their non-thermal rate constants as a function of excitation wavelength in the linear and non-linear regime. Finally, we will extend our discussion to simulations of non-resonant and resonant photoelectron spectra of isolated biologically relevant chromophore anions to identify direct and indirect electron emission processes and to reveal molecular resonances and weakly bound electronically excited states as gateways for electron transfer processes. Non-adiabatic coupling matrix elements in the nuclear configuration space will be used to redefine Franck-Condon integrals for treating vibrational autodetachment of photoelectrons.
P.A. Kusochek, A.V. Scherbinin, A.V. Bochenkova. Insights into the Early-Time Excited-State Dynamics of Structurally Inhomogeneous Rhodopsin KR2. J. Phys. Chem. Lett. 2021, 12, 8664–8671, DOI: 10.1021/acs.jpclett.1c02312.
C.S. Anstoter, G. Mensa-Bonsu, P. Nag, M. Rankovic, K. Ragesh T. P., A.N. Boichenko, A.V. Bochenkova, J. Fedor, J.R.R. Verlet. Mode-Specific Vibrational Autodetachment Following Excitation of Electronic Resonances by Electrons and Photons. Phys. Rev. Lett. 2020, 124, 203401. DOI: 10.1103/PhysRevLett.124.203401.
A.V. Bochenkova, C.R.S. Mooney, M.A. Parkes, J.L. Woodhouse, L. Zhang, R. Lewin, J.M. Ward, H.C. Hailes, L.H. Andersen, H.H. Fielding. Mechanism of resonant electron emission from the deprotonated GFP chromophore and its biomimetics. Chemical Science 2017, 8, 3154-3163. DOI: 10.1039/C6SC05529J.
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