Magnet-Free Time-Resolved Magnetic Circular Dichroism with Pulsed Vector Beams

Diverse Transient Chiral Dynamics in Evolutionary Distinct Photosynthetic Reaction Centers

The evolution of photosynthetic reaction centers (RCs) from anoxygenic bacteria to higher-order oxygenic cynobacteria and plants highlights a remarkable journey of structural and functional diversification as an adaptation to environmental conditions. The role of chirality in these centers is important, influencing the arrangement and function of key molecules involved in photosynthesis. Investigating the role of chirality may provide a deeper understanding of photosynthesis and the evolutionary history of life on Earth. In this study, we explore chirality-related energy transfer in two evolutionarily distinct RCs: one from the anoxygenic purple sulfur bacterium Thermochromatium tepidum (BRC) and the other from the oxygenic cyanobacterium Thermosynechococcus vulcanus (PSII RC), utilizing two-dimensional electronic spectroscopy (2DES). By employing circularly polarized laser pulses, we can extract transient chiral dynamics within these RCs, offering a detailed view of their chiral contribution to energy transfer processes. We also compute traditional 2DES and compare these results with spectra related to circular dichroism. Our findings indicate that two-dimensional circular dichroism spectroscopy effectively reveals chiral dynamics, emphasizing the structural symmetries of pigments and their interactions with associated proteins. Despite having similar pigment-protein architectures, the BRC and PSII RC exhibit significantly different chiral dynamics on an ultrafast time scale. In the BRC, the complex contributions of pigments such as BChM, BPhL, BCh, and PM to key excitonic states lead to more pronounced chiral features and dynamic behavior. In contrast, the PSII RC, although significantly influenced by ChlD1 and ChlD2, shows less complex chiral effects and more subdued chiral dynamics. Notably, the PSII RC demonstrates a faster decay of coherence to localized excitonic populations compared to the BRC, which may represent an adaptive mechanism to minimize oxidative stress in oxygenic photosystems. By examining and comparing the chiral excitonic interactions and dynamics of BRC and PSII RC, this study offers valuable insights into the mechanisms of photosynthetic complexes. These findings could contribute to understanding how the functional optimization of photosynthetic proteins in ultrafast time scales is linked to biological evolution.

Experimental and theoretical aspects of magnetic circular dichroism and magnetic circularly polarized luminescence in the UV, visible and IR ranges: A review

A historical sketch of the MCD (magnetic circular dichroism) spectroscopy is reported in its experimental and theoretical aspects. MCPL (magnetic circularly polarized luminescence) is also considered. The main studies are presented encompassing porphyrinoid systems, aggregates and materials, as well as simple organic molecules useful for the advancement of the interpretation. The MCD of chiral systems is discussed with special attention to new studies of natural products with potential pharmaceutical valence, including Amaryllidaceae alkaloids and related isocarbostyrils. Finally, the vibrational form of MCD, called MVCD, which is recorded in the IR part of the spectrum is also discussed. A final brief note on perspectives is given.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Efficient Protocol for Computing MCD Spectra in a Broad Frequency Range Combining Resonant and Damped CC2 Quadratic Response Theory

Coupled cluster response theory offers a path to high-accuracy calculations of spectroscopic properties, such as magnetic circular dichroism (MCD). However, divergence or slow convergence issues are often encountered for electronic transitions in high-energy regions with a high density of states. This is here addressed for MCD by an implementation of damped quadratic response theory for resolution-of-identity coupled cluster singles-and-approximate-doubles (RI-CC2), along with an implementation of the MCD A term from resonant response theory. Combined, damped and resonant response theory calculations provide an efficient strategy to obtain MCD spectra over a broad frequency range and for systems that include highly symmetric molecules with degenerate excited states. The protocol is illustrated by application to zinc tetrabenzoporphyrin in the energy region of 2-8 eV and comparison to experimental data. Timings are reported for the resonant and damped approaches, showing that a greater part of the calculation time is consumed by the construction of the building blocks for the final MCD ellipticity. A recommendation on how to use the procedure is outlined.