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1. From antiferromagnetic and hidden order to Pauli paramagnetism in U M ₂ Si ₂ compounds with 5 f electron duality

   https://doi.org/10.1073/pnas.2005701117  

   Amorese, Andrea; Sundermann, Martin; Leedahl, Brett; Marino, Andrea; Takegami, Daisuke; Gretarsson, Hlynur; Gloskovskii, Andrei; Schlueter, Christoph; Haverkort, Maurits W.; Huang, Yingkai; et al (December 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U 5 f electrons in U M 2 S i 2 (M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U 5 f 2 configuration with the Γ 1 ( 1 ) and Γ 2 quasi-doublet symmetry. The amount of the U 5 f 3 configuration, however, varies considerably across the U M 2 S i 2 series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments in U P d 2 S i 2 and U N i 2 S i 2 , the availability of orbital degrees of freedom needed for the hidden order in U R u 2 S i 2 to occur, as well as the appearance of Pauli paramagnetism in U F e 2 S i 2 . A unified and systematic picture of the U M 2 S i 2 compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centered T h C r 2 S i 2 structure. 

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2. Pushing the heterometal doping limit while preserving long-lived charge separation in a Ti-based MOF photocatalyst

   https://doi.org/10.1063/5.0174664  

   Long, Conor L; Zhang, Xiaoyi; Lockard, Jenny V (November 2023, The Journal of Chemical Physics)

   This study explores the nature, dynamics, and reactivity of the photo-induced charge separated excited state in a Fe3+-doped titanium-based metal organic framework (MOF), xFeMIL125-NH2, as a function of iron concentration. The MOF is synthesized with doping levels x = 0.5, 1 and 2 Fe node sites per octameric Ti-oxo cluster and characterized by powder x-ray diffraction, UV-vis diffuse reflectance, atomic absorption, and steady state Fe K-edge X-ray absorption spectroscopy. For each doping level, time-resolved X-ray transient absorption spectroscopy studies confirm the electron trap site role of the Fe sites in the excited state. Time scan data reveal multiexponential decay kinetics for the charge recombination processes which extend into the microsecond range for all three concentrations. A series of dye photodegradation studies, based on the oxidative decomposition of Rhodamine B, demonstrates the reactivity of the charge separated excited state and the photocatalytic capacity of these MOF materials compared to traditional heterometal-doped semiconductor photocatalysts. 

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3. Two-dimensional electronic–vibrational sum frequency spectroscopy for interactions of electronic and nuclear motions at interfaces

   https://doi.org/10.1073/pnas.2100608118  

   Deng, Gang-Hua; Qian, Yuqin; Zhang, Tong; Han, Jian; Chen, Hanning; Rao, Yi (August 2021, Proceedings of the National Academy of Sciences)

   Interactions of electronic and vibrational degrees of freedom are essential for understanding excited-states relaxation pathways of molecular systems at interfaces and surfaces. Here, we present the development of interface-specific two-dimensional electronic–vibrational sum frequency generation (2D-EVSFG) spectroscopy for electronic–vibrational couplings for excited states at interfaces and surfaces. We demonstrate this 2D-EVSFG technique by investigating photoexcited interface-active ( E )-4-((4-(dihexylamino) phenyl)diazinyl)-1-methylpyridin-1- lum (AP3) molecules at the air–water interface as an example. Our 2D-EVSFG experiments show strong vibronic couplings of interfacial AP3 molecules upon photoexcitation and subsequent relaxation of a locally excited (LE) state. Time-dependent 2D-EVSFG experiments indicate that the relaxation of the LE state, S 2 , is strongly coupled with two high-frequency modes of 1,529.1 and 1,568.1 cm −1 . Quantum chemistry calculations further verify that the strong vibronic couplings of the two vibrations promote the transition from the S 2 state to the lower excited state S 1 . We believe that this development of 2D-EVSFG opens up an avenue of understanding excited-state dynamics related to interfaces and surfaces. 

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4. On the Origin of the Photostability of DNA and RNA Monomers: Excited State Relaxation Mechanism of the Pyrimidine Chromophore

   https://doi.org/10.1021/acs.jpclett.0c00935  

   Arpa, E. M.; Brister, M. M.; Hoehn, S. J.; Crespo-Hernández, C. E.; Corral, I. (June 2020, The Journal of Physical Chemistry Letters)

   Today’s genetic composition is the result of continual refinement processes on primordial heterocycles present in prebiotic Earth and at least partially regulated by ultraviolet radiation. Femtosecond transient absorption spectroscopy and state-of-the-art ab initio calculations are combined to unravel the electronic relaxation mechanism of pyrimidine―the common chromophore of the nucleobases. Excitation of pyrimidine at 268 nm populates the S1(nπ*) state directly. A fraction of the population intersystem crosses to the triplet manifold within 7.8 ps, partially decaying within 1.5 ns, while another fraction recovers the ground state in >3 ns. The pyrimidine chromophore is not responsible for the photostability of the nucleobases. Instead, C2 and C4 amino and/or carbonyl functionalization is essential for shaping the topography of pyrimidine’s potential energy surfaces, which present accessible conical intersections between the initially populated electronic excited state and the ground state. 

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5. Energy- and conformer-dependent excited-state relaxation of an E / Z photoswitchable thienyl-ethene

   https://doi.org/10.1039/c9cp01226e  

   Young, Jamie D.; Honick, Chana R.; Zhou, Jiawang; Pitts, Cody R.; Ghorbani, Fereshte; Peters, Garvin M.; Lectka, Thomas; Tovar, John D.; Bragg, Arthur E. (July 2019, Physical Chemistry Chemical Physics)

   Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E / Z photoisomerization following absorption of visible light. cis -to- trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565–420 nm) transient absorption (TA) studies to probe the photophysics of cis -to- trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (P A and P B ) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the P B conformer is found to contribute to triplet population and the switching of cis -4TCE: high-energy excitation (<470 nm) of P B involves direct excitation to S 2 , relaxation from which prepares an ISC-active S 1 geometry (ISC QY 0.4–0.67, k ISC ∼ 1.6–2.6 × 10 −9 s −1 ) that is the gateway to triplet population and isomerization. We ascribe low cis -to- trans isomerization yield to excitation of the nonreactive P A conformer (75–85% loss) as well as loses along the P B S 2 → S 1 → T 1 cascade (10–20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive “antiparallel” ring conformation. 

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