Showing papers by "Vivek Tiwari published in 2023"

Coherence Transfer and Destructive Interference in Two-Dimensional Coherence Maps

Abstract: Coherence maps (CMs) in multidimensional spectroscopy report total interference of all quantum coherent pathways. Detailed understanding of how this interference manifests spectroscopically is vital for deciphering mechanistic origins of impulsively generated wavepackets, but currently lacking. Here we explain the origin of recently reported diagonal node-like features in CMs of bacteriochlorophyll monomers and photosynthetic reaction centers (RCs), where the apparent resemblance in the two disparate systems was reportedly perplexing. We show that both spectroscopic signatures have distinct physical origins. Node-like lineshapes in monomers arise from unique phase twists caused by destructive interference between ground and excited state vibrational coherences. In contrast, nodal lines in RCs are explained by coherence transfer of vibrational wavepackets which do not participate in the ultrafast energy transfer and their destructive interference with ground state pathways. Our results resolve recent spectroscopic observations and illustrate new mechanistic insights gained from understanding interference effects in multidimensional spectroscopy.

Vibrations That Do Not Promote Vibronic Coupling Can Dominate Observed Lineshapes in Two-Dimensional Electronic Spectroscopy.

Abstract: Impulsively generated wavepackets can report on vibronic couplings underlying ultrafast electronic relaxation. Coherence maps (CMs) in multidimensional spectroscopy resolve beating amplitudes of such wavepackets as two-dimensional contour maps. A precise understanding of how these wavepackets manifest spectroscopically is vital for unambiguously deciphering their mechanistic significance and establishing CMs as a powerful tool for guiding the synthetic design of functional vibronic couplings. Here we take a step in this direction. We establish physically distinct origins of recently reported apparent similarities between diagonal node-like CM lineshapes in bacteriochlorophyll monomers and multichromophoric photosynthetic reaction centers (RCs). The former arise when vibrational wavepackets survive on both ground and excited electronic states, while nodal lines in RCs arise when vibrational wavepackets that do not participate in energy transfer are coherently transferred to the acceptor. We resolve recent spectroscopic observations and illustrate new mechanistic insights gained by connecting microscopic interference between electronic relaxation pathways and observed CMs.

Low and High Frequency Vibrations Synergistically Enhance Singlet Exciton Fission Through Robust Vibronic Resonances

Abstract: Singlet exciton fission (SEF) is initiated by ultrafast internal conversion of a singlet exciton into a correlated triplet pair (TT)1. The `reaction coordinates' for ultrafast SEF even in archetypal systems such as pentacene thin film remain unclear with synthetic design principles broadly relying on tailoring electronic couplings to achieve new templates for efficient SEF materials. Spectroscopic detection of vibrational coherences in the (TT)1 photoproduct has motivated theoretical investigations into a possible role of vibronic resonance in driving SEF, akin to that reported in several photosynthetic proteins. However, a precise understanding of how prominent low-frequency vibrations and their modulation of intermolecular orbital overlaps, equally prominent high-frequency vibrations, and order of magnitude larger Huang-Rhys factors in SEF chromophores compared to photosynthetic pigments, collectively influence the mechanistic details of SEF remains starkly lacking. Here we address this gap and identify previously unrecognized effects which are quite contrasting from those known in photosynthesis excitons, and vitally enhance non-adiabatic internal conversion in SEF. Our findings have direct implications for the broad experimental interest in synthetically tailoring molecules to promote vibronically enhanced internal conversion.