Resolving the Role of Zeaxanthin in qE with Annihilation-Free Transient Absorption Spectroscopy
Zeaxanthin (Zea) plays a key role in the energy-dependent non-photochemical quenching processes for protecting photosynthetic organisms from excessive light exposure. Previous transient absorption studies suggested that Zea can participate in direct quenching via chlorophyll to Zea energy transfer. However, the kinetic profiles of Zea S1 are typically distorted by the intrinsic exciton-exciton annihilation dynamics, leading to ambiguity in assigning transient absorption signals. In this talk, I will present the annihilation-free transient absorption study on N. benthamiana thylakoid membrane, including wild type and three NPQ mutants (npq1, npq4, and lut2), without contamination from multi-exciton dynamics. The results show a strong correlation between Chl Qy to Zea S1 energy transfer and the xanthophyll cycle during NPQ activation. Additionally, the analysis of fifth-order signals using diffusion models shows a reduction in exciton diffusion length under high light illumination, consistent with that reducing range of exciton migration is a key aspect of plants’ response to excess light.
Tsung-Yen Lee is a 5 th year physical chemistry PhD student in Graham Fleming’s group. He grew up in Taiwan and received his B.S. and M.S. in chemistry from National Tsing Hua University. His current research focuses on the mechanism of non-photochemical quenching in photosynthetic organisms.
Self-assembly of Electrostatically Stabilized Semiconductor Nanocrystals
Self-assembly of nanocrystals (NCs) into ordered arrays, or superlattices (SLs), is an appealing approach to generate hierarchically organized materials with new functionalities. Typically, SLs are formed directly from NCs colloidally suspended in solution; however, by using electrostatics to tune the interactions between NCs, self-assembly was recently shown to result in the formation of a metastable liquid phase that forms via fluid-fluid phase separation. We systematically investigate the phase behavior of electrostatically-stabilized PbS NCs in situ and in real time using small angle X-ray scattering (SAXS). By quantitatively fitting SAXS patterns to colloid, liquid, and SL models, we obtain the time evolution of each phase. The extracted phase diagram of this system is consistent with predictions for particles with short-ranged attractive interactions. We can reliably direct the self-assembly to proceed either directly from the colloidal phase or with a liquid intermediate. The liquid increases the SL formation kinetics without sacrificing SL order, providing a design principle for SL self-assembly. We are exploring additional design principles by using photoexcitation to alter the solvation shell of ions around the NC, which would tune the NC interactions. Preliminary results show that above-bandgap photoexcitation enhances the solvation shell of PbS NCs and that of CdSe NCs, which do not form ordered SLs in equilibrium. Future work will explore the time evolution of photoinduced solvation shell enhancement to develop non-equilibrium optical illumination protocols for ordered assembly of all semiconductor NCs.
Vivian Wall is a 3rd-year Chemistry Ph.D. student in Naomi Ginsberg’s group studying nanocrystal superlattice self-assembly with a variety of X-ray scattering techniques. She obtained her B.S. in Chemistry from UCLA in 2020, where she worked for Sarah Tolbert studying thermally insulating mesoporous silica materials. In her free time, she enjoys theater, soccer, baking, and hiking.