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1. Vibronic coherence evolution in multidimensional ultrafast photochemical processes

   https://doi.org/10.1038/s41467-019-13503-9  

   Gaynor, James D.; Sandwisch, Jason; Khalil, Munira (December 2019, Nature Communications)

   Abstract The complex choreography of electronic, vibrational, and vibronic couplings used by photoexcited molecules to transfer energy efficiently is remarkable, but an unambiguous description of the temporally evolving vibronic states governing these processes has proven experimentally elusive. We use multidimensional electronic-vibrational spectroscopy to identify specific time-dependent excited state vibronic couplings involving multiple electronic states, high-frequency vibrations, and low-frequency vibrations which participate in ultrafast intersystem crossing and subsequent relaxation of a photoexcited transition metal complex. We discover an excited state vibronic mechanism driving long-lived charge separation consisting of an initial electronically-localized vibrational wavepacket which triggers delocalization onto two charge transfer states after propagating for ~600 femtoseconds. Electronic delocalization consequently occurs through nonadiabatic internal conversion driven by a 50 cm⁻¹coupling resulting in vibronic coherence transfer lasting for ~1 picosecond. This study showcases the power of multidimensional electronic-vibrational spectroscopy to elucidate complex, non-equilibrium energy and charge transfer mechanisms involving multiple molecular coordinates. 

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2. Optical spectra of complex aggregates and crystals: Vibronic band structure and Davydov splitting

   https://doi.org/10.1063/5.0263317  

   Giavazzi, Davide; Schwarzl, Robert; Painelli, Anna; Spano, Frank C (May 2025, The Journal of Chemical Physics)

   A thorough understanding of the nature and pattern of intermolecular interactions in molecular aggregates and crystals is a prerequisite for the design of the next generation of functional materials. In systems with multiple, symmetrically equivalent molecules per unit cell, each excited state of the isolated molecule splits into several Davydov components that appear in the absorption spectra in up to three orthogonally polarized transitions. In this work, a Frenkel–Holstein Hamiltonian is adopted to simulate the vibronic structure of the Davydov components in aggregates and crystals with up to four molecules per unit cell, where electrostatic intermolecular interactions define either 1D or 2D structures. Analysis shows that vibronic signatures report directly on the electronic couplings that contribute to the Davydov splitting and the exciton band shapes. Specifically, the vibronic signature of a given Davydov component is solely determined by its free excitonic shift. For crystals with two molecules per unit cell, the lower and upper Davydov components can each exhibit J-like or H-like behavior, resulting in JJ, JH, and HH components in order of increasing energy, all with unique vibronic signatures. Under certain conditions, null points can exist in either band, leading to a monomer-like absorption spectrum for the corresponding Davydov component. In crystals with four symmetrically equivalent molecules per unit cell, the J- or H-nature of the three orthogonally polarized Davydov components results in four possible combinations, JJJ, JJH, JHH, and HHH in order of increasing energy, all readily identified through vibronic signatures. 

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3. Implementation of Broadband near-UV Pump Pulses for Ultrafast 2D Electronic-Vibrational Spectroscopy

   https://doi.org/10.1364/UP.2020.Tu2A.2  

   Sandwisch, Jason W.; Gaynor, James D.; Khalil, Munira (January 2020, Optica Publishing Group)

   We present the generation of efficient, tunable sub-20 fs broadband near UV pulses. The tunable pulses are used in two-dimensional electronic-vibrational spectroscopy and reveal additional vibronic states in an excited state intramolecular proton transfer process. 

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4. Hidden vibronic and excitonic structure and vibronic coherence transfer in the bacterial reaction center

   https://doi.org/10.1126/sciadv.abk0953  

   Policht, Veronica R.; Niedringhaus, Andrew; Willow, Rhiannon; Laible, Philip D.; Bocian, David F.; Kirmaier, Christine; Holten, Dewey; Mančal, Tomáš; Ogilvie, Jennifer P. (January 2022, Science Advances)

   We report two-dimensional electronic spectroscopy (2DES) experiments on the bacterial reaction center (BRC) from purple bacteria, revealing hidden vibronic and excitonic structure. Through analysis of the coherent dynamics of the BRC, we identify multiple quasi-resonances between pigment vibrations and excitonic energy gaps, and vibronic coherence transfer processes that are typically neglected in standard models of photosynthetic energy transfer and charge separation. We support our assignment with control experiments on bacteriochlorophyll and simulations of the coherent dynamics using a reduced excitonic model of the BRC. We find that specific vibronic coherence processes can readily reveal weak exciton transitions. While the functional relevance of such processes is unclear, they provide a spectroscopic tool that uses vibrations as a window for observing excited state structure and dynamics elsewhere in the BRC via vibronic coupling. Vibronic coherence transfer reveals the upper exciton of the “special pair” that was weakly visible in previous 2DES experiments. 

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5. Variety, the spice of life and essential for robustness in excitation energy transfer in light-harvesting complexes

   https://doi.org/10.1039/c9fd00081j  

   Oh, Sue Ann; Coker, David F.; Hutchinson, David A. W. (January 2020, Faraday Discussions)

   For over a decade there has been some significant excitement and speculation that quantum effects may be important in the excitation energy transport process in the light harvesting complexes of certain bacteria and algae, in particular via the Fenna–Matthews–Olsen (FMO) complex. Whilst the excitement may have waned somewhat with the realisation that the observed long-lived oscillations in two-dimensional electronic spectra of FMO are probably due to vibronic coherences, it remains a question whether these coherences may play any important role. We review our recent work showing how important the site-to-site variation in coupling between chloroplasts in FMO and their protein scaffold environment is for energy transport in FMO and investigate the role of vibronic modes in this transport. Whilst the effects of vibronic excitations seem modest for FMO, we show that for bilin-based pigment–protein complexes of marine algae, in particular PC645, the site-dependent vibronic excitations seem essential for robust excitation energy transport, which may again open the door for important quantum effects to be important in these photosynthetic complexes. 

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