Topics in multidimensional optical spectroscopy I. Theoretical studies of finite pulse effects II. Applications to plant light-harvesting complex and nanomaterials

• Main Author: Do, Thanh Nhut
• Other Authors: Tan Howe Siang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/136996
• Institution: Nanyang Technological University

In Nature, there are numerous important processes happening at the picosecond, femtosecond and attosecond timescales. With the development of the ultrafast laser and techniques such as ultrafast multidimensional optical spectroscopy, scientists are able to study many of such ultrafast dynamic processes. In this thesis, we present both theoretical and experimental studies on multidimensional optical spectroscopy (MDOS) and the applications of ultrafast two-dimensional electronic spectroscopy (2DES) to the study of photosynthetic pigment-protein complexes and nanomaterials. MDOS spectra are affected by the use of finite bandwidth excitation pulses due to the convolution properties of the multi-pulse interactions. Disentangling the effects of finite bandwidth is not trivial. In Chapters 2 and 3 of this thesis, we present theoretical studies on how the finite bandwidth and the duration of excitation pulses alter the MDOS spectra. We derive an approximate and simplified expression that can be used to calculate the MDOS spectra that includes finite bandwidth effects. We found that for third-order two-dimensional optical spectra, in certain conditions, the obtained expressions are exact and contain only simple multiplications of the pulse spectra and the system response functions. We also obtained expressions for fifth-order optical spectra, such as two-quantum two-dimensional and three dimensional optical spectra. In these cases, due to the higher complexity caused by a higher number of pulse interactions, the obtained formulae are only approximate. However, by comparing the simulations using the exact formalism and our approximated expressions, the approximated formula can reproduce most of the main spectral features very well. These results will be beneficial to the MDOS community by providing simpler and more resource-efficient ways of incorporating the finite bandwidth effects of excitation pulses into the simulation of MDOS spectra. It will also facilitate the extraction from experimental MDOS spectra, of relevant information contained in the systems’ response functions. Chapters 4 and 5 of this thesis present the application of 2DES on studying the ultrafast excitation energy transfer (EET) processes in Light-harvesting complex II (LHCII) the most abundant light-harvesting antenna on Earth. LHCII plays an important role in photosynthesis due to its main functions of harvesting sunlight energy and funnelling it to the reaction center to regulate photosynthesis. As the first pigment protein complex in eukaryotic plants having its crystal structure solved, LHCII has been intensively studied during the past decades. Many experimental and theoretical studies have been proposed to try to solve the EET scheme. However, due to the highly congested spectral features and ultrafast dynamics in the femtosecond timescale, there is no consensus on the model for LHCII EET dynamics. In our studies, we implement 2DES with the capability of correlating the excitation and emission frequencies to track the ultrafast EET dynamics of LHCII. 2DES measurements of LHCII at various temperatures elucidate the temperature-dependent excitation energy transfer dynamics of the Chl a manifold of LHCII. Global analysis results reveal three main timescales for the energy equilibration dynamics of LHCII. A clear indication of the temperature dependence of uphill/downhill energy transfer processes was also discerned, which follows the detailed-balance condition. In Chapter 5, we take a closer look at the cryogenic 77 K 2DES measurement. We construct a phenomenological model to fit our 2DES data and extract the spectral information of the excitonic states, as well as the kinetic parameters. Due to the restriction of the model and complexity of the system, the EET network cannot be constructed uniquely. However, based on the agreement of our fitting results with the previously proposed structure-based calculations, we have proposed two EET schemes with tentative pigment assignments. In addition, the existence of a ‘bottleneck’ state in the intermediate wavelength region (660-670 nm) is supported by our results. Our model also determines three terminal excitons having the lowest energy levels, and the excitation can always be equilibrated among these three states without any significant EET directly connecting them. These equilibration processes happen via the uphill energy flow to the higher excitonic levels bridging the three lowest termini and this equilibration mechanism accelerates the EET at higher temperatures and results in the temperature dependence of EET processes in LHCII. Chapter 6 presents the application of 2DES on studying the photophysics of nanomaterials. Due to intrinsic limitations in synthesis procedure, the nanomaterials, like quantum dots (QDs) and nanoplatelets (NPLs) always exhibit heterogeneity in size, shape, and spectroscopic properties. We perform 2DES measurements on zero-dimensional CdSe QDs and two-dimensional CdSe NPLs. Employing the nodal line slope and central line slope analysis, we can extract information about the systems’ homogeneity, inhomogeneity and spectral diffusion dynamics from our measured 2DES data. The results show that no spectral diffusion dynamics occur for the CdSe QDs. On the other hand, spectral diffusion was observed in the 5 mono-layers CdSe NPLs’ heavyhole transition. The normalized frequency fluctuation correlation function of the CdSe NPLs’ heavy-hole transition was measured to have a major fast decay component at 80 fs, followed by a slower diffusion component of 2.5 ps.