Name: Mr. Amitav Sahu

Title: Fluorescence-detected Multidimensional Electronic Spectroscopy (fMES)with a Visible White-light Continuum: New Experimental Approaches and Vibronic Exciton Models

Date &Time : Friday, 26th April 2024 at 4.00 p.m.

Venue: Rajarshi Bhattacharya Memorial Lecture Hall, Chemical Sciences Building

Multidimensional Electronic Spectroscopy (MDES)[1]represents the state-of-the-art optical spectroscopy method within the umbrella of time-resolved four-wavemixing techniques. MDES works by resolving the femtosecond quantum dynamics within the overlapping vibrational-electronic (vibronic) manifolds in the condensed phase initiated upon photoexcitation,as 2D contour map snapshots along the excitation and detection frequency axes for each pump-probe waiting time T. Thesensitivity of MDES to both amplitude and phase of non-linear signals[1]has revolutionized our understanding of ultrafast phenomenathat determinefunctionality in proteins[2], photovoltaic polymers[3], layered materials[4], etc. At the same time, this has also led to improved theoretical models which aim to describe such complex phenomena. However, conventional MDES implementations[5]have onlylimited applicability[6]when applied to scatter-prone samples such as photosynthetic cells, where probing ultrafast energy transfer across light-harvesting protein networks within a cell has remained an outstanding challenge in the field.In this work, which lies at the interface of experiment and theory, we address this challenge.

We demonstrate MDES experiments withsensitivity~100x superior to the current state-of-the-art[7], apply this tool to probe exciton diffusion within the intact light-harvesting apparatus of photosynthetic cells, and elucidate how superior signal-to-noise ratio (SNR) measurements of vibrational wavepackets eventually pavethe way for improved vibronic exciton description[8] of ultrafast energy/charge transfer. We apply these vibronic exciton models to resolve[9] conflictingspectroscopic observations[10,11,12]and propose MDES experiments[13] that can uniquely identify the ‘reaction coordinates’ for excited state photophysics.

In this work, I describe in detail[7]the development of a home-built fMES spectrometerthat takes a visible white-light continuum (WLC, 550-700 nm) as input and generates fully collinear phase-locked pulse pairs,whicharerouted into a microscope objective for sample excitation. Passive phase stabilization across the visible WLC bandwidth is achieved through the acousto-optic phase modulation approach[14]. Unlike the conventional MDES approaches that detect hetero- or homodyned-detected electric fields in a background-free direction, our spectrometer replaces directional filtering with phase-sensitive lock-in frequency filtering across the visible WLC bandwidth and detects an incoherent non-linear population signal such as photocurrent or fluorescence (in our case).Unlike ~80 MHz Ti:Sapp oscillator repetition rates which may be easier to implement[6] for fMES, repetition ratesof ~1MHz are optimal for microscopy applications[15] but not facile to implement due to ~100x lower repetition rates as well as WLC pulse energies.Circumventing these hurdles, this is the first such demonstration of a visible WLC-based fMES spectrometer[7].

The light-harvesting apparatus of photosynthetic cyanobacterial cells relies on energetically downhill shuttling of excitations across nanometre lengthscales to a photo-susceptible reaction center protein (RC)within 10-100 ps, where a stabilized charge-separated state is created with nearunity quantum efficiency, and utilized for further metabolic reactions. Annihilation of excess excitations (exciton-exciton annihilation, EEA) is an essential photoprotection mechanism in this design, but it remains unclear whether EEA occurs equally efficiently across all proteins in the downhill energy funnel, or whether it occurs preferentially in only low-energy sites before reaching the RC.Because of overwhelming scatter from cells, all studies so far have focussed only on detergent-isolated protein supercomplexes, without essential components such as the RC.Detection of fluorescence, in our case, enablesfacile scatter suppression through optical filtering, enabling measurements of exciton diffusion and annihilationwithin the intactlight harvesting apparatus, with a broadband WLC excitation capable of probing the entire visible photosynthetic spectrum.We apply our fMES spectrometer on cyanobacterial intact cells and demonstrate howthe peak locations and amplitudes in the 2D spectra allow us to trace exciton-exciton annihilation pathways within the protein network.Our measurements show a surprising aspect of EEA where downhill energy transfer is preferred over EEA, and the excess excitations are dominantly annihilated in the lowest site energy protein.

Quantum beats along the pump-probe waiting time Thold important mechanistic clues regarding couplings between fast electronic and slow nuclear motions that can efficiently drive excited state photophysics such as exciton delocalization and dissociation[16]. I will discuss our efforts to push the sensitivity of our fMES spectrometer ~100x better than the current state-of-art to enable high SNR detection of quantum beats. By introducing biased sampling and rapid stage scan along T that is synchronized with the reference signal, reduction in 1/f experimental noise allows us to detect vibrational quantum beats on the ground and excited electronic states of a molecule for sample OD as low as 1mOD[7], a 300x reduction in sample concentrations typically required for all conventional MDES implementations so far.Pushing this even further withreduction in experimental time and faster averaging,we have achieved fMES signal detection sensitivity down to a few nanomolar concentrations.

High SNR detection of 2D quantum beat maps, that is, amplitude distribution of quantum beats on the 2D contour map,eventually led us to questionthe resulting 2D lineshapes,for instance, the nearly zero amplitude of vibrational oscillation at the 2D diagonal (same excitation and detection frequency)[7]. Curiously, similar spectroscopic signatures were reported in both monomers[10]as well as multichromophoric [11,12]photosynthetic proteins. Our analysis of 2D lineshapes in the quantum beat maps resolves these conflicting observations by connecting the macroscopic observables to underlying microscopic quantum dynamics.

We start by first questioning the validity of approximations in the current vibronic exciton models.For extended systems like multichromophoric photosynthetic proteins, constructing an exact vibronic exciton model that explicitly includes the vibrational coordinate in the system Hamiltonian is computationally expensive and often relies on approximations, such as the one-particle approximation, which does not account for vibrational excitations on electronically unexcited sites. Considering a simple donor-acceptor dimer model, we show that essential features such as vibronic exciton delocalization, population transfer rates, and vibrational distortion field around the site of excitation, which dictate excited state wavepacket motions, are severely underestimated by the one-particle approximation[8]. We argue thatthe use of exact basis sets, or alternatively an effective mode approach[17]is necessary to adequately capture the non-adiabatic dynamics.We then apply these ideas in a refined donor-acceptor vibronic exciton Hamiltonian that accounts for population and coherence transfer during T. We show that the destructive interference between picosecond long ground and excited-state vibrational wavepackets causes narrowing on the 2D diagonal in case of monomers[9]. In contrast, in presence of donor-to-acceptor energy transfer, we show that the signal cancellation at the 2D diagonal is attributed to the transfer of the excited-state vibrational wavepackets from donor to the acceptor state along ‘spectator’ vibrational modes that play no role in the electronic relaxation process[9].

Based on the above developed ideas, we argue that vibrational coherences or the emergence ofnew vibrational frequencies in the photoproduct may notbe sufficient to identify the‘reaction coordinates’for electronic relaxation and can lead to anambiguous mechanistic interpretations. In this context,we extend our vibronic exciton Hamiltonian to propose readily implementable polarization-based MDES experiments to uniquely identify the ‘promoter’vibrational modes in electronic relaxation that facilitate vibronic couplings. We also extend the applicability of these proposed experiments to a crystalline environment for elucidating vibronic couplings in the context of singlet exciton fission[13].

Overall, this thesis presents powerful spectroscopic strategies, experimental and theoretical, for probing ultrafast processes across diverse photophysical systems. The ideas developed in this work holdsignificant implications for the spectroscopic communityseeking to elucidate the quantum mechanical details of electronic relaxation based on high SNR detection of quantum beats.


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[6] V. Tiwari, “Multidimensional Electronic Spectroscopy in High-Definition – Combining Spectral, Temporal and Spatial Resolutions,” J. Chem. Phys., vol. 154, no. 23, p. 230901, 2021, doi: 10.1063/5.0052234.

[7] A. Sahu, V. N. Bhat, S. Patra, and V. Tiwari, “High-sensitivity fluorescence-detected multidimensional electronic spectroscopy through continuous pump–probe delay scan,” J. Chem. Phys., vol. 158, no. 2, p. 24201, 2023, doi: 10.1063/5.0130887.

[8] A. Sahu, J. S. Kurian, and V. Tiwari, “Vibronic resonance is inadequately described by one-particle basis sets,” J. Chem. Phys., vol. 153, no. 22, 2020, doi: 10.1063/5.0029027.Feature Article (top 3% selected by Editors’)

[9] A. Sahu and V. Tiwari, “Vibrations That Do Not Promote Vibronic Coupling Can Dominate Observed Lineshapes in Two-Dimensional Electronic Spectroscopy,” J. Phys. Chem. Lett., vol.14, no. 19, pp. 4617–4624, 2023, doi: 10.1021/acs.jpclett.3c00753.

[10] V. R. Policht, A. Niedringhaus, and J. P. Ogilvie, “Characterization of Vibrational Coherence in Monomeric Bacteriochlorophyll a by Two-Dimensional Electronic Spectroscopy,” J. Phys. Chem. Lett., vol. 9, no. 22, pp. 6631–6637, 2018, doi: 10.1021/acs.jpclett.8b02691.

[11] V. R. Policht et al., “Hidden vibronic and excitonic structure and vibronic coherence transfer in the bacterial reaction center,” Sci. Adv., vol. 8, no. 1, p. eabk0953, 2022, doi: 10.1126/sciadv.abk0953.

[12] D. Paleček, P. Edlund, S. Westenhoff, and D. Zigmantas, “Quantum coherence as a witness of vibronically hot energy transfer in bacterial reaction center,” Sci. Adv., vol. 3, no. 9, p. e1603141, 2017, doi: 10.1126/sciadv.1603141.

[13] A. Bhattacharyya, A. Sahu, S. Patra, and V. Tiwari, “Low- and high-frequency vibrations synergistically enhance singlet exciton fission through robust vibronic resonances,” Proc. Natl. Acad. Sci., vol. 120, no. 49, p. e2310124120, 2023, doi: 10.1073/pnas.2310124120.

[14] P. F. Tekavec, G. A. Lott, and A. H. Marcus, “Fluorescence-detected two-dimensional electronic coherence spectroscopy by acousto-optic phase modulation,” J. Chem. Phys., vol. 127, no. 21, pp. 214307–214327, 2007, doi: 10.1063/1.2800560.

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[16] C. Andrea Rozzi et al., “Quantum coherence controls the charge separation in a prototypical artificial light-harvesting system,” Nat.Commun., vol. 4, no. 1, p. 1602, 2013, doi: 10.1038/ncomms2603.

[17] S. Patra, A. Sahu, and V. Tiwari, “Effective normal modes identify vibrational motions which maximally promote vibronic mixing in excitonically coupled aggregates,”J. Chem. Phys., vol. 154, no. 11, p. 111106, 2021, doi: 10.1063/5.0037759.