| 1. |
- Ahn, Chi Woo, et al.
(författare)
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Direct Observation of a Transiently Formed Isomer During lodoform Photolysis in Solution by Time-Resolved X-ray Liquidography
- 2018
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 9:3, s. 647-653
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Tidskriftsartikel (refereegranskat)abstract
- Photolysis of iodoform (CHI3) in solution has been extensively studied, but its reaction mechanism remains elusive. In particular, iso-iodoform (iso-CHI2-I) is formed as a product of the photolysis reaction, but its detailed structure is not known, and whether it is a major intermediate species has been controversial. Here, by using time-resolved X-ray liquidography, we determined the reaction mechanism of CHI3 photodissociation in cyclohexane as well as the structure of iso-CHI2-I. Both iso-CHI2-I and CHI2 radical were found to be formed within 100 ps with a branching ratio of 40:60. Iodine radicals (I), formed during the course of CHI3 photolysis, recombine nongeminately with either CHI2 or I. Based on our structural analysis, the I-I distance and the C-I-I angle of iso-CHI2-I were determined to be 2.922 +/- 0.004 angstrom and 133.9 +/- 0.8 degrees, respectively.
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| 2. |
- Huijser, Annemarie, et al.
(författare)
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Shedding Light on the Nature of Photoinduced States Formed in a Hydrogen-Generating Supramolecular RuPt Photocatalyst by Ultrafast Spectroscopy
- 2018
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 122:31, s. 6396-6406
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Tidskriftsartikel (refereegranskat)abstract
- Photoinduced electronic and structural changes of a hydrogen-generating supramolecular RuPt photocatalyst are studied by a combination of time-resolved photoluminescence, optical transient absorption, and X-ray absorption spectroscopy. This work uses the element specificity of X-ray techniques to focus on the interplay between the photophysical and -chemical processes and the associated time scales at the catalytic Pt moiety. We observe very fast (<30 ps) photoreduction of the Pt catalytic site, followed by an ∼600 ps step into a strongly oxidized Pt center. The latter process is likely induced by oxidative addition of reactive iodine species. The oxidized Pt species is long-lived and fully recovers to the original ground state complex on a >10 μs time scale. However, the photosensitizing Ru moiety is fully restored on a much shorter ∼300 ns time scale. This reaction scheme implies that we may withdraw two electrons from a catalyst that is activated by a single photon.
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| 3. |
- Liu, Cunming, et al.
(författare)
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Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites
- 2019
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 141:33, s. 13074-13080
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Tidskriftsartikel (refereegranskat)abstract
- Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of Vk center (Br2 - dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of ∼59 μs, which is ∼3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient "background" under continuous light illumination, influencing the charge carrier transport along the lattice framework. ©
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| 4. |
- Naumova, Maria A., et al.
(författare)
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Nonadiabatic Charge Transfer within Photoexcited Nickel Porphyrins
- 2024
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Ingår i: Journal of Physical Chemistry Letters. - 1948-7185. ; 15:13, s. 3627-3638
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Tidskriftsartikel (refereegranskat)abstract
- Metalloporphyrins with open d-shell ions can drive biochemical energy cycles. However, their utilization in photoconversion is hampered by rapid deactivation. Mapping the relaxation pathways is essential for elaborating strategies that can favorably alter the charge dynamics through chemical design and photoexcitation conditions. Here, we combine transient optical absorption spectroscopy and transient X-ray emission spectroscopy with femtosecond resolution to probe directly the coupled electronic and spin dynamics within a photoexcited nickel porphyrin in solution. Measurements and calculations reveal that a state with charge-transfer character mediates the formation of the thermalized excited state, thereby advancing the description of the photocycle for this important representative molecule. More generally, establishing that intramolecular charge-transfer steps play a role in the photoinduced dynamics of metalloporphyrins with open d-shell sets a conceptual ground for their development as building blocks capable of boosting nonadiabatic photoconversion in functional architectures through “hot” charge transfer down to the attosecond time scale.
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| 5. |
- Vincent, Jonathan, et al.
(författare)
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Solvent dependent structural perturbations of chemical reaction intermediates visualized by time-resolved x-ray diffraction
- 2009
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 130:15, s. 154502-
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Tidskriftsartikel (refereegranskat)abstract
- Ultrafast time-resolved wide angle x-ray scattering from chemical reactions in solution has recently emerged as a powerful technique for determining the structural dynamics of transient photochemical species. Here we examine the structural evolution of photoexcited CH2I2 in the nonpolar solvent cyclohexane and draw comparisons with a similar study in the polar solvent methanol. As with earlier spectroscopic studies, our data confirm a common initial reaction pathway in both solvents. After photoexcitation, CH2I2 dissociates to form CH2I center dot+I center dot. Iodine radicals remaining within the solvent cage recombine with a nascent CH2I center dot radical to form the transient isomer CH2I-I, whereas those which escape the solvent cage ultimately combine to form I-2 in cyclohexane. Moreover, the transient isomer has a lifetime approximately 30 times longer in the nonpolar solvent. Of greater chemical significance is the property of time-resolved wide angle x-ray diffraction to accurately determine the structure of the of CH2I-I reaction intermediate. Thus we observe that the transient iodine-iodine bond is 0.07 A +/- 0.04 A shorter in cyclohexane than in methanol. A longer iodine-iodine bond length for the intermediate arises in methanol due to favorable H-bond interaction with the polar solvent. These findings establish that time-resolved x-ray diffraction has sufficient sensitivity to enable solvent dependent structural perturbations of transient chemical species to be accurately resolved.
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| 6. |
- Zhang, Xiaoyi, et al.
(författare)
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Highly Accurate Excited-State Structure of [Os(bpy)(2)dcbpy](2+) Determined by X-ray Transient Absorption Spectroscopy
- 2014
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 136:24, s. 8804-8809
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Tidskriftsartikel (refereegranskat)abstract
- Determining the electronic and geometric structures of photoexcited transient species with high accuracy is crucial for understanding their fundamental photochemistry and controlling their photoreactivity. We have applied X-ray transient absorption spectroscopy to measure the XANES and EXAFS spectra of a dilute (submillimolar) solution of the osmium(II) polypyridyl complex [Os(bpy)(2)dcbpy](PF6)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) (OsL2L') in methanol at the Os L-III edge. We have obtained spectra of superb quality for both the ground state and the photoinduced (MLCT)-M-3 excited state that have allowed us not only to extract detailed information about the Os 5d orbitals but also to resolve very small differences of 0.010 +/- 0.008 angstrom in the average Os-N bond lengths of the ground and excited states. Theoretical calculations using a recently developed DFT-based approach support the measured electronic structures and further identify the nature of the molecular orbitals that contribute to the main absorption bands in the XANES spectra.
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| 7. |
- Zheng, Kaibo, et al.
(författare)
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Direct Experimental Evidence for Photoinduced Strong-Coupling Polarons in Organolead Halide Perovskite Nanoparticles
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:22, s. 4535-4539
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Tidskriftsartikel (refereegranskat)abstract
- Echoing the roaring success of their bulk counterparts, nano-objects built from organolead halide perovskites (OLHP) present bright prospects for surpassing the performances of their conventional organic and inorganic analogues in photodriven technologies. Unraveling the photoinduced charge dynamics is essential for optimizing the optoelectronic functionalities. However, mapping the carrier-lattice interactions remains challenging, owing to their manifestations on multiple length scales and time scales. By correlating ultrafast time-resolved optical and X-ray absorption measurements, this work reveals the photoinduced formation of strong-coupling polarons in CH3NH3PbBr3 nanoparticles. Such polarons originate from the self-trapping of electrons in the Coulombic field caused by the displaced inorganic nuclei and the oriented organic cations. The transient structural change detected at the Pb L3 X-ray absorption edge is well-captured by a distortion with average bond elongation in the [PbBr6]2- motif. General implications for designing novel OLHP nanomaterials targeting the active utilization of these quasi-particles are outlined.
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