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

Sahu, Amitav; Tiwari, Vivek

doi:10.1021/acs.jpclett.3c00753

Cited by 3 publications

(1 citation statement)

References 45 publications

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“…When an impulsive photoexcitation creates a superposition of such vibronic eigenvectors, an amplitude enhancement of quantum beats on the excited and ground electronic state is expected ( 18 ). Quantum beats, enhanced or not, can serve as a sensitive probe ( 49 ) of the underlying mechanism of energy/charge transfer. For example, dephasing rates of vibrational quantum beats can identify ( 28 ) reaction coordinates for electron transfer.…”

Section: Resultsmentioning

confidence: 99%

Low- and high-frequency vibrations synergistically enhance singlet exciton fission through robust vibronic resonances

Bhattacharyya,

Sahu,

Patra

et al. 2023

Proc. Natl. Acad. Sci. U.S.A.

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Singlet exciton fission (SEF) is initiated by ultrafast internal conversion of a singlet exciton into a correlated triplet pair ( T T ) 1 . The “reaction coordinates” for ultrafast SEF even in archetypal systems such as pentacene thin film remain unclear. Couplings between fast electrons and slow nuclei are ubiquitous across a range of phenomena in chemistry. Accordingly, spectroscopic detection of vibrational coherences in the ( T T ) 1 photoproduct motivated investigations into a possible role of vibronic coupling, akin to that reported in several photosynthetic proteins. However, acenes are very different from chlorophylls with 10× larger vibrational displacements upon photoexcitation and low-frequency vibrations modulating intermolecular orbital overlaps. Whether (and if so how) these unique features carry any mechanistic significance for SEF remains a poorly understood question. Accordingly, synthetic design of new molecules aiming to mimic this process across the solar spectrum has broadly relied on tuning electronic couplings. We address this gap and identify previously unrecognized synergistic interplay of vibrations, which in striking contrast to photosynthesis, vitally enhances SEF across a broad, nonselective and, therefore, unavoidable range of vibrational frequencies. We argue that attaching mechanistic significance to spectroscopically observed prominent quantum beats is misleading. Instead, we show that vibronic mixing leads to anisotropic quantum beats and propose readily implementable polarization-based two-dimensional electronic spectroscopy experiments which uniquely distinguish vibrations which drive vibronic mixing and promote SEF, against spectator vibrations simply accompanying ultrafast internal conversion. Our findings introduce crucial ingredients in synthetic design of SEF materials and spectroscopy experiments aiming to decipher mechanistic details from quantum beats.

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Section: Resultsmentioning

confidence: 99%

Low- and high-frequency vibrations synergistically enhance singlet exciton fission through robust vibronic resonances

Bhattacharyya,

Sahu,

Patra

et al. 2023

Proc. Natl. Acad. Sci. U.S.A.

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Coherence in Chemistry: Foundations and Frontiers

Schultz,

Yuly,

Arsenault

et al. 2024

Chem. Rev.

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Coherence refers to correlations in waves. Because matter has a wave-particle nature, it is unsurprising that coherence has deep connections with the most contemporary issues in chemistry research (e.g., energy harvesting, femtosecond spectroscopy, molecular qubits and more). But what does the word "coherence" really mean in the context of molecules and other quantum systems? We provide a review of key concepts, definitions, and methodologies, surrounding coherence phenomena in chemistry, and we describe how the terms "coherence" and "quantum coherence" refer to many different phenomena in chemistry. Moreover, we show how these notions are related to the concept of an interference pattern. Coherence phenomena are indeed complex, and ambiguous definitions may spawn confusion. By describing the many definitions and contexts for coherence in the molecular sciences, we aim to enhance understanding and communication in this broad and active area of chemistry.

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Incoherent ultrafast energy transfer in phycocyanin 620 from Thermosynechococcus vulcanus revealed by polarization-controlled two dimensional electronic spectroscopy

Wang,

Zhu,

Zou

et al. 2024

The Journal of Chemical Physics

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Phycocyanin 620 (PC620) is the outermost light-harvesting complex in phycobilisome of cyanobacteria, engaged in light collection and energy transfer to the core antenna, allophycocyanin. Recently, long-lived exciton–vibrational coherences have been observed in allophycocyanin, accounting for the coherent energy transfer [Zhu et al., Nat. Commun. 15, 3171 (2024)]. PC620 has a nearly identical spatial location of three α84–β84 phycocyanobilin pigment pairs to those in allophycocyanin, inferring an existence of possible coherent energy transfer pathways. However, whether PC620 undergoes coherent or incoherent energy transfer remains debated. Furthermore, accurate determination of energy transfer rates in PC620 is still necessary owing to the spectral overlap and broadening in conventional time-resolved spectroscopic measurements. In this work, the energy transfer process within PC620 was directly resolved by polarization-controlled two dimensional electronic spectroscopy (2DES) and global analysis. The results show that the energy transfer from α84 to the adjacent β84 has a lifetime constant of 400 fs, from β155 to β84 of 6–8 ps, and from β155 to α84 of 66 ps, fully conforming to the Förster resonance energy transfer mechanism. The circular dichroism spectrum also reveals that the α84–β84 pigment pair does not form excitonic dimer, and the observed oscillatory signals are confirmed to be vibrational coherence, excluding the exciton–vibrational coupling. Nodal line slope analysis of 2DES further reveals that all the vibrational modes participate in the energy dissipation of the excited states. Our results consolidate that the ultrafast energy transfer process in PC620 is incoherent, where the twisted conformation of α84 is suggested as the main cause for preventing the formation of α84–β84 excitonic dimer in contrast to allophycocyanin.

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

Cited by 3 publications

References 45 publications

Low- and high-frequency vibrations synergistically enhance singlet exciton fission through robust vibronic resonances

Low- and high-frequency vibrations synergistically enhance singlet exciton fission through robust vibronic resonances

Coherence in Chemistry: Foundations and Frontiers

Incoherent ultrafast energy transfer in phycocyanin 620 from Thermosynechococcus vulcanus revealed by polarization-controlled two dimensional electronic spectroscopy

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