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1. Electronic-vibrational density evolution in a perylene bisimide dimer: mechanistic insights into excitation energy transfer

   https://doi.org/10.1039/D1CP02135D  

   Kundu, Sohang ; Makri, Nancy ( July 2021 , Physical Chemistry Chemical Physics)

   null (Ed.)

   The process of excitation energy transfer (EET) in molecular aggregates is etched with the signatures of a multitude of electronic and vibrational time scales that often are extremely difficult to resolve. The effect of the motion associated with one molecular vibration on that of another is fundamental to the dynamics of EET. In this paper we present simple theoretical ideas along with fully quantum mechanical calculations to develop a comprehensive mechanistic picture of EET in terms of the time evolution of electronic-vibrational densities (EVD) in a perylene bisimide (PBI) dimer, where 28 intramolecular normal modes couple to the ground and excited electronic states of each molecule. The EVD motion exhibits a plethora of dynamical features, which impart physical justification for the composite effects observed in the EET dynamics. Weakly coupled vibrations lead to classical-like motion of the EVD center on each electronic state, while highly nontrivial EVD characteristics develop under moderate or strong exciton–vibration interaction, leading to the formation of split or crescent-shaped densities, as well as density retention that slows down energy transfer and creates new peaks in the electronic populations. Pronounced correlation effects are observed in two-mode projections of the EVD, as a consequence of indirect vibrational coupling between uncoupled normal modes induced by the electronic coupling. Such indirect coupling depends on the strength of exciton–vibration interactions as well as the frequency mismatch between the two modes and leaves nontrivial signatures in the electronic population dynamics. The collective effects of many vibrational modes cause a partial smearing of these features through dephasing. 

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2. Energy transfer and spatial scrambling of an exciton in a conjugated dendrimer

   https://doi.org/10.1039/c8cp05852k  

   Ondarse-Alvarez, D. ; Oldani, N. ; Roitberg, A. E. ; Kleiman, V. ; Tretiak, S. ; Fernandez-Alberti, S. ( December 2018 , Physical Chemistry Chemical Physics)

   Photoexcitation of multichromophoric light harvesting molecules induces a number of intramolecular electronic energy relaxation and redistribution pathways that can ultimately lead to ultrafast exciton self-trapping on a single chromophore unit. We investigate the photoinduced processes that take place on a phenylene-ethynylene dendrimer, consisting of nine equivalent linear chromophore units or branches. meta -Substituted links between branches break the conjugation giving rise to weak couplings between them and to localized excitations. Our nonadiabatic excited-state molecular dynamics simulations reveal that the ultrafast internal conversion process to the lowest excited state is accompanied by an inner → outer inter-branch migration of the exciton due to the entropic bias associated with energetically equivalent conjugated segments. The electronic energy redistribution among chromophore units occurs through several possible pathways in which through-bond transport and through-space exciton hopping mechanisms can be distinguished. Besides, triple bond excitations coincide with the localization of the electronic transition densities, suggesting that the intramolecular energy redistribution is a concerted electronic and vibrational energy transfer process. 

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3. Stacking‐Order‐Driven Optical Properties and Carrier Dynamics in ReS 2

   https://doi.org/10.1002/adma.201908311  

   Zhou, Yongjian ; Maity, Nikhilesh ; Rai, Amritesh ; Juneja, Rinkle ; Meng, Xianghai ; Roy, Anupam ; Zhang, Yanyao ; Xu, Xiaochuan ; Lin, Jung‐Fu ; Banerjee, Sanjay K. ; et al ( April 2020 , Advanced Materials)

   Two distinct stacking orders in ReS₂are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one‐unit cell displacement along theaaxis. First‐principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm⁻¹for AA stacking, and 20 cm⁻¹for AB stacking, making a simple tool for determining the stacking orders in ReS₂. Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump–probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular tobaxis. The findings underscore the stacking‐order driven optical properties and carrier dynamics of ReS₂, mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

    

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4. Topological aspects of system-bath Hamiltonians and a vector model for multisite systems coupled to local, correlated, or common baths

   https://doi.org/10.1063/5.0147135  

   Makri, Nancy ( April 2023 , The Journal of Chemical Physics)

   Some topological features of multisite Hamiltonians consisting of harmonic potential surfaces with constant site-to-site couplings are discussed. Even in the absence of Duschinsky rotation, such a Hamiltonian assumes the system-bath form only if severe constraints exist. The simplest case of a common bath that couples to all sites is realized when the potential minima are collinear. The bath reorganization energy increases quadratically with site distance in this case. Another frequently encountered situation involves exciton-vibration coupling in molecular aggregates, where the intramolecular normal modes of the monomers give rise to local harmonic potentials. In this case, the reorganization energy accompanying excitation transfer is independent of site-to-site separation, thus this situation cannot be described by the usual system-bath Hamiltonian. A vector system-bath representation is introduced, which brings the exciton-vibration Hamiltonian in system-bath form. In this, the system vectors specify the locations of the potential minima, which in the case of identical monomers lie on the vertices of a regular polyhedron. By properly choosing the system vectors, it is possible to couple each bath to one or more sites and to specify the desired initial density. With a collinear choice of system vectors, the coupling reverts to the simple form of a common bath. The compact form of the vector system-bath coupling generalizes the dissipative tight-binding model to account for local, correlated, and common baths. The influence functional for the vector system-bath Hamiltonian is obtained in a compact and simple form.

    

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5. B800-to-B850 relaxation of excitation energy in bacterial light harvesting: All-state, all-mode path integral simulations

   https://doi.org/10.1063/5.0093828  

   Kundu, Sohang ; Dani, Reshmi ; Makri, Nancy ( July 2022 , The Journal of Chemical Physics)

   We report fully quantum mechanical simulations of excitation energy transfer within the peripheral light harvesting complex (LH2) of Rhodopseudomonas molischianum at room temperature. The exciton–vibration Hamiltonian comprises the 16 singly excited bacteriochlorophyll states of the B850 (inner) ring and the 8 states of the B800 (outer) ring with all available electronic couplings. The electronic states of each chromophore couple to 50 intramolecular vibrational modes with spectroscopically determined Huang–Rhys factors and to a weakly dissipative bath that models the biomolecular environment. Simulations of the excitation energy transfer following photoexcitation of various electronic eigenstates are performed using the numerically exact small matrix decomposition of the quasiadiabatic propagator path integral. We find that the energy relaxation process in the 24-state system is highly nontrivial. When the photoexcited state comprises primarily B800 pigments, a rapid intra-band redistribution of the energy sharply transitions to a significantly slower relaxation component that transfers 90% of the excitation energy to the B850 ring. The mixed character B850* state lacks the slow component and equilibrates very rapidly, providing an alternative energy transfer channel. This (and also another partially mixed) state has an anomalously large equilibrium population, suggesting a shift to lower energy by virtue of exciton–vibration coupling. The spread of the vibrationally dressed states is smaller than that of the eigenstates of the bare electronic Hamiltonian. The total population of the B800 band is found to decay exponentially with a 1/ e time of 0.5 ps, which is in good agreement with experimental results.

    

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