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

Multidimensional Electronic Spectroscopy (MDES) presents the state-of-the-art spectroscopy method within the umbrella of time-resolved four-wavemixing techniques. MDES works by resolving the femtosecond quantum dynamics within the overlapping vibrational-electronic 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 [1]. The sensitivity of MDES to both amplitude and phase of non-linear signals has revolutionized our understanding of ultrafast phenomena that determine functionality in proteins, photovoltaic polymers, layered materials, etc. At the same time, this has also led to improved theoretical models which aim to describe such complex phenomena. However, conventional MDES implementations have only limited applicability[2] 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 these challenges.

We demonstrate MDES experiments with sensitivity at least ~300x superior to the current state-of-the-art [3], apply this tool to probe exciton diffusion within the intact light-harvesting apparatus of photosynthetic cells, and elucidate how superior signal-to-noise ratio measurements of vibrational wavepackets eventually pave the way for improved vibronic exciton description[4] of ultrafast energy and charge transfer. We apply these vibronic exciton models to resolve conflicting spectroscopic observations [5] and propose MDES experiments [6] that can uniquely identify the ‘reaction coordinates’ for excited state photophysics.

Overall, this thesis presents powerful spectroscopic strategies, experimental and theoretical, for probing ultrafast processes across diverse photophysical systems. The ideas developed in this work hold significant implications for the spectroscopic community seeking to elucidate the quantum mechanical details of electronic relaxation based on high signal-to-noise ratio detection of quantum beats.

[1] D. M. Jonas, “Two-Dimensional Femtosecond Spectroscopy,” Annu. Rev. Phys. Chem., vol. 54, no. 1, pp. 425–463, 2003, doi: 10.1146/annurev.physchem.54.011002.103907.

[2] 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.

[3] 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.

[4] 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.

[5] 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.

[6] 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.