| 1. |
- Cotter, Laura F., et al.
(författare)
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Solvent and Temperature Effects on Photoinduced Proton-Coupled Electron Transfer in the Marcus Inverted Region
- 2021
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 125:35, s. 7670-7684
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Tidskriftsartikel (refereegranskat)abstract
- Concerted proton-coupled electron transfer (PCET) in the Marcus inverted region was recently demonstrated (Science 2019, 364, 471-475). Understanding the requirements for such reactivity is fundamentally important and holds promise as a design principle for solar energy conversion systems. Herein, we investigate the solvent polarity and temperature dependence of photoinduced proton-coupled charge separation (CS) and charge recombination (CR) in anthracene-phenol-pyridine triads: 1 (10-(4-hydroxy-3-(4-methylpyridin-2-yl)benzyl)anthracene-9-carbonitrile) and 2 (10-(4-hydroxy-3-(4-methoxypyridin-2-yl)benzyl)anthracene-9-carbonitrile). Both the CS and CR rate constants increased with increasing polarity in acetonitrile:n-butyronitrile mixtures. The kinetics were semi-quantitatively analyzed where changes in dielectric and refractive index, and thus consequently changes in driving force (-.G degrees) and reorganization energy (lambda), were accounted for. The results were further validated by fitting the temperature dependence, from 180 to 298 K, in n-butyronitrile. The analyses support previous computational work where transitions to proton vibrational excited states dominate the CR reaction with a distinct activation free energy (Delta G(CR)* similar to 140 meV). However, the solvent continuum model fails to accurately describe the changes in Delta G degrees and lambda with temperature via changes in dielectric constant and refractive index. Satisfactory modeling was obtained using the results of a molecular solvent model [J. Phys. Chem. B 1999, 103, 9130-9140], which predicts that lambda decreases with temperature, opposite to that of the continuum model. To further assess the solvent polarity control in the inverted region, the reactions were studied in toluene. Nonpolar solvents decrease both Delta G(CR)degrees and lambda, slowing CR into the nanosecond time regime for 2 in toluene at 298 K. This demonstrates how PCET in the inverted region may be controlled to potentially use proton-coupled CS states for efficient solar fuel production and photoredox catalysis.
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| 2. |
- D'Amario, Luca, et al.
(författare)
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Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells
- 2015
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 6:5, s. 779-783
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Tidskriftsartikel (refereegranskat)abstract
- Mesoporous nickel oxide has been used as electrode material for p-type dye-sensitized solar cells (DSCs) for many years but no high efficiency cells have yet been obtained. One of the main issues that lowers the efficiency is the poor fill factor, for which a clear reason is still missing. In this paper we present the first evidence for a relation between applied potential and the charge recombination rate of the NiO electrode. In particular, we find biphasic recombination kinetics: a fast (15 ns) pathway attributed to the reaction with the holes in the valence band and a slow (1 ms) pathway assigned to the holes in the trap states. The fast component is the most relevant at positive potentials, while the slow component becomes more important at negative potentials. This means that at the working condition of the cell, the fast recombination is the most important. This could explain the low fill factor of NiO-based DSCs.
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| 3. |
- Eliasson, Nora, et al.
(författare)
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Ultrafast Dynamics in Cu-Deficient CuInS2 Quantum Dots : SubBandgap Transitions and Self-Assembled Molecular Catalysts
- 2021
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Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:27, s. 14751-14764
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Tidskriftsartikel (refereegranskat)abstract
- The photophysical properties of Cu-deficient Cu01.2In1Sx quantum dots synthesized through a facile aqueous-based procedure have been investigated. Transient absorption experiments were carried out probing in the UV-vis, near-IR, and mid-IR regions, with the aim to (i) study the photophysical properties of the quantum dots and (ii) monitor kinetics of electron transfer to a molecular catalyst. When pumping subbandgap transitions, negative (bleach) signals were observed that were spectrally and kinetically distinct from those observed with bandgap pump wavelengths. Herein, these distinct contributions are suggested to result from the overlapping bleaching of state filling electrons and trapped holes. Such an interpretation highlights the importance of considering the hole-contributions to the bleach for the proper determination of carrier kinetics in similar systems. A model complex of the [Fe-2]-hydrogenase active site was introduced to explore the potential of the quantum dots as photosensitizers for molecular catalysts. The quantum dot photoluminescence was quenched upon catalyst addition, and direct evidence of the singly reduced catalyst was found by transient absorption in the UV-vis and mid-IR. The catalyst accepted reducing equivalents on a subpicosecond time scale upon photoexcitation of the quantum dots, despite no covalent linking chemistry being applied. This implies that charge transfer is not limited by diffusion rates, thus confirming the presence of spontaneous quantum dot and catalyst self-assembly.
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| 4. |
- Materna, Kelly L., et al.
(författare)
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Using Surface Amide Couplings to Assemble Photocathodes for Solar Fuel Production Applications
- 2020
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Ingår i: ACS Applied Materials and Interfaces. - : AMER CHEMICAL SOC. - 1944-8244 .- 1944-8252. ; 12:4, s. 4501-4509
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Tidskriftsartikel (refereegranskat)abstract
- A facile surface amide-coupling method was examined to attach dye and catalyst molecules to silatrane-decorated NiO electrodes. Using this method, electrodes with a push-pull dye were assembled and characterized by photoelectrochemistry and transient absorption spectroscopy. The dye-sensitized electrodes exhibited hole injection into NiO and good photoelectrochemical stability in water, highlighting the stability of the silatrane anchoring group and the amide linkage. The amide-coupling protocol was further applied to electrodes that contain a molecular proton reduction catalyst for use in photocathode architectures. Evidence for catalyst reduction was observed during photoelectrochemical measurements and via photocathodes.
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| 5. |
- Parada, Giovanny A., et al.
(författare)
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Concerted proton-electron transfer reactions in the Marcus inverted region
- 2019
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 364:6439, s. 471-475
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Tidskriftsartikel (refereegranskat)abstract
- Electron transfer reactions slow down when they become very thermodynamically favorable, a counterintuitive interplay of kinetics and thermodynamics termed the inverted region in Marcus theory. Here we report inverted region behavior for proton-coupled electron transfer (PCET). Photochemical studies of anthracene-phenol-pyridine triads give rate constants for PCET charge recombination that are slower for the more thermodynamically favorable reactions. Photoexcitation forms an anthracene excited state that undergoes PCET to create a charge-separated state. The rate constants for return charge recombination show an inverted dependence on the driving force upon changing pyridine substituents and the solvent. Calculations using vibronically nonadiabatic PCET theory yield rate constants for simultaneous tunneling of the electron and proton that account for the results.
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| 6. |
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| 7. |
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| 8. |
- Pettersson-Rimgard, Belinda, et al.
(författare)
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Proton-coupled energy transfer in molecular triads
- 2022
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 377:6607, s. 742-746
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Tidskriftsartikel (refereegranskat)abstract
- We experimentally discovered and theoretically analyzed a photochemical mechanism, which we term proton-coupled energy transfer (PCEnT). A series of anthracene-phenol-pyridine triads formed a local excited anthracene state after light excitation at a wavelength of similar to 400 nanometers (nm), which led to fluorescence around 550 nm from the phenol-pyridine unit. Direct excitation of phenolpyridine would have required similar to 330-nm light, but the coupled proton transfer within the phenolpyridine unit lowered its excited-state energy so that it could accept excitation energy from anthracene. Singlet-singlet energy transfer thus occurred despite the lack of spectral overlap between the anthracene fluorescence and the phenol-pyridine absorption. Moreover, theoretical calculations indicated negligible charge transfer between the anthracene and phenol-pyridine units. We construe PCEnT as an elementary reaction of possible relevance to biological systems and future photonic devices.
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| 9. |
- Pettersson Rimgard, Belinda, 1992-
(författare)
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Tailing Charges on New Paths : Ultrafast intramolecular charge transfer in chromophores
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- By tailing charges on their paths within a molecule, one can gain fundamental knowledge of their inherent reactivity. The charges, originally residing in their lowest energy configuration, can upon light absorption move across the system, or transfer in between them. The focus of this thesis has been to study how electrons and protons transfer between different fragments of a molecule, following an absorption of laser light. The relevance of this work is not only attributed to its mechanistic insights, but also that it might provide a foundation for future designs of renewable energy systems, such as solar cells and fuel cells. This thesis entails the investigation of a derivative of the famous N3 dye, developed for dye sensitized solar cells. This dye is a ruthenium-complex, with two bipyridyl and two isothiocyanate coordinated ligands. There was no real consensus in the literature whether the initial excitation of this complex, would cause the localization of an electron on one bipyridyl or, if it would delocalize over both of them. Moreover, if the charge did initially localize, would it, at some point in time, transfer in between the ligands, hence perform interligand electron transfer? The results confirm the existence of an initial localization onto one ligand, and that interligand electron transfer occurs on a sub-picosecond time scale. This diminishes the risk of e.g. slow electron injection into a semiconductor surface such as in a solar cell, due to a charge localization onto a surface unbound ligand. In photocatalysis, a concerted proton and electron transfer is sought after, as the total charge neutralizes thus avoids the formation of high energy intermediates. In a set of anthracene-phenol-pyridine triads a concerted mechanism was investigated, where the charges were shown to separate upon excitation, but later slowly recombine. The slow recombination followed a Marcus inverted region behavior, with a counter-intuitive decrease in rate with an increase in driving force. A concerted mechanism in the inverted region had previously never been observed. As the driving forces were altered, with the help of solvent and temperature, alternative reaction paths in the triads became visible. The anthracene-localized excitation was observed to transfer across the molecule, to the phenol-pyridine, where a concomitant proton transfer occurred. This proton-coupled energy transfer mechanism, is a new phenomenon that further adds to the knowledge of charge transfer mechanisms.
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| 10. |
- Pettersson-Rimgard, Belinda, et al.
(författare)
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Ultrafast Interligand Electron Transfer in cis-[Ru(4,4’-dicarboxylate-2,2’-bipyridine)2(NCS)2]4- and Implications for Electron Injection Limitations in Dye Sensitized Solar Cells
- 2018
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Ingår i: Chemical Science. - : RSC Publishing. - 2041-6520 .- 2041-6539. ; 9:41, s. 7958-7967
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Tidskriftsartikel (refereegranskat)abstract
- Interligand electron transfer (ILET) of the lowest metal-to-ligand charge transfer (MLCT) state of N712 (cis-[Ru(dcb)2(NCS)2]4−, where dcb = 4,4′-dicarboxylate-2,2′-bipyridine) in a deuterated acetonitrile solution has been studied by means of femtosecond transient absorption anisotropy in the mid-IR. Time-independent B3LYP density functional calculations were performed to assign vibrational bands and determine their respective transition dipole moments. The transient absorption spectral band at 1327 cm−1, assigned to a symmetric carboxylate stretch, showed significant anisotropy. A rapid anisotropy increase (τ1 ≈ 2 ps) was tentatively assigned to vibrational and solvent relaxation, considering the excess energy available after the excited singlet–triplet conversion. Thereafter, the anisotropy decayed to zero with a time constant τ2 ≈ 240 ps, which was assigned to the rotational correlation time of the complex in deuterated acetonitrile. No other distinctive changes to the anisotropy were observed and the amplitude of the slow component at time zero agrees well with that predicted for a random mixture of MLCT localization on either of the two dcb ligands. The results therefore suggest that MLCT randomization over the two dcb ligands occurs on the sub-ps time scale. This is much faster than proposed by previous reports on the related N3 complex [Benkö et al., J. Phys. Chem. B, 2004, 108, 2862, and Waterland et al., J. Phys. Chem. A, 2001, 105, 4019], but in agreement with that found by Wallin and co-workers [J. Phys. Chem. A, 2005, 109, 4697] for the [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) complex. This suggests that electron injection from the excited dye into TiO2 in dye-sensitized solar cells is not limited by ILET.
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