ACS Physical Chemistry Au最新文献

The electronic structure of the S₁ state of photosystem II (PSII) was investigated using selective hole burning of Q-band pulsed electron paramagnetic resonance. The free induction decay and spin-echo signals of the tyrosine radical Y_D ^• in the plant PSII oscillated because of the magnetic dipole-dipole interaction with the S₁ state Mn cluster. The initial period was 410 ns (2.44 MHz) and was assigned to the S = 1 spin state. Based on the oscillation analysis, both Mn1 and Mn4 and both Mn2 and Mn3 were assigned as Mn-(III) and Mn-(IV), respectively, which is consistent with the quantum chemical calculations. The 410 ns period was accounted for in the simplified model using the isotropic spin density distribution ratio [1.6:-1.1:-1.1:1.6] for Mn1-4 ions. This oscillation was identical with that observed in the presence of methanol. The oscillation decreased in PsbP/Q- and PsbO/P/Q-depleted PSII. In Thermosynechococcus vulcanus, two periods, 390 ns (2.56 MHz) and 630 ns (1.59 MHz), were detected, indicating that the cyanobacterial S₁ state includes two isomers, S = 1 and S ≥ 2 spins. The S ≥ 2 spin was not detected in PsbO/U/V-depleted PSII without polyethylene glycol. The S ≥ 2 state was consistent with the reported quantum chemical calculation using S = 3. A simplified model accounted for the S = 1 state as the spin density distribution [1.8:-1.3:-1.3:1.8] and for the S ≥ 2 state as the isotropic spin density distribution [-0.5:0.5:0.5:0.5] for Mn1-4 ions. In combination with quantum chemical calculations, the most probable protonated structure is W1 = H₂O, W2 = H₂O, O4 = O^2-, and O5 = O^2- for the S₁ state. These results demonstrate that the selective hole burning method is a powerful tool to complement X-ray studies to determine the valence and protonation structure of manganese clusters, not only in the S₁ state but also in higher S-states and general metal clusters, which would provide important insights into the water oxidation mechanism.

Excited state intramolecular proton transfer (ESIPT) in organic dyes is one of the most studied photophysical processes by experimental and theoretical chemists alike. Of the various subclasses of ESIPT, the azo-hydrazo tautomerism occurs in ortho azo-dyes derived from phenols, mainly β-naphthol. In the current study, we present a steady-state and time-resolved spectroscopic investigation on the azo-hydrazo proton transfer process in two azo dyes derived from β-naphthol, 1-phenylazo-2-naphthol (PDNO) and 1-naphthylazo-2-naphthol (NDNO), which differ by a phenyl ring in solvents of differing polarity and proticity. Steady-state investigations implicate the presence of the ground-state azo-hydrazo tautomerism. The emission lifetime measurements reveal that the azo and hydrazo tautomers have different radiative lifetimes, with the hydrazo tautomer decaying faster than the azo tautomer. The femtosecond-picosecond transient absorption measurements provide critical information on the nonradiative decay processes associated with the dyes and demarcate the temporal behavior of the nonradiative relaxation between the two dyes. NDNO, having an extra phenyl ring, reveals slower relaxation times compared to PDNO, irrespective of the solvent. The relaxation time is longest in chloroform and shortest for nonpolar hexane, irrespective of dye. Considering both the steady-state and time-resolved measurements, we propose how the spectrodynamics in azo dyes derived from β-naphthol could be manipulated by tuning the annulation as well as the surrounding solvent system, which aids the fundamental understanding of excited-state photoprocesses like the azo-hydrazo tautomerism.

In scanning tunneling microscopy of molecules, an insulating buffer layer is often introduced to reduce interactions between adsorbed molecules and the substrate. We demonstrate that the buffer itself strongly influences the wave function of the tunneling electron at the molecule for tunneling through the molecule's electronic transport gap. We use a theory, which provides a very good agreement with prior experiments on platinum phthalocyanine on a NaCl buffer, to study effects of varying the buffer's composition and thickness and show the importance of the buffer's lattice parameter. Expanding the tunneling electron's wave function using molecular orbitals (MOs) additionally shows how to control the relative weights at the highest occupied MO (HOMO), lowest unoccupied MO (LUMO), and energetically low-lying MOs with few nodal surfaces. Those with significant weight are key for manipulating molecules with tunneling electrons. When used for up-conversion molecular luminescence, in which emitted photons exceed the input tunneling bias, we find that an intricate competition occurs between tunneling through the HOMO or LUMO versus low-lying MOs. The buffer choice provides a substantive handle for controlling such processes. We predict that it can influence the up-conversion efficiency by an order of magnitude.

Ionomers critically impact catalyst layer performance in polymer electrolyte fuel cells (PEFCs). While sulfonated aromatic polyether-ketone (SPEEK) and Perfluorinated sulfonic acid-Nafion ionomers are widely used, their structure-property relationships under operating conditions remain insufficiently understood. In this study, we systematically analyzed Pt-supported SPEEK and Nafion films (13 nm) using in situ ellipsometry, grazing incidence wide-angle X-ray scattering (GIWAXS), and electrochemical impedance spectroscopy (EIS) to investigate their morphology, water uptake, and proton transport properties. Compared to Nafion, the ultrathin SPEEK films exhibit dense, well-defined crystalline domains at the buried interface, attributed to a relatively stronger affinity for the Pt substrate. Analysis revealed that at 90% RH, the buried interface layer of SPEEK maintained a stable thickness of ∼13.8 Å, while Nafion exhibited a more pronounced swelling, increasing to ∼18.7 Å. Consequently, SPEEK thin films show reduced water uptake rates and conductivity, particularly under dry conditions. The proton conductivity measurements further highlighted a significant gap between in-plane and out-of-plane directions for SPEEK, with out-of-plane conductivity being 1-2 orders of magnitude lower, whereas Nafion displayed a smaller anisotropy. We propose that the intramolecular forces between ether groups, along with the interactions between Pt and oxygen atoms in the repeating units of SPEEK, contribute to the differences observed in the buried interface structure. This study enhances our understanding of ionomer-containing films in relation to molecular structure, confinement effects, and substrate interactions, thereby facilitating further fundamental investigations of the catalyst-ionomer interface.

Contributions of protein and coupled solvent configurational fluctuations to the molecular mechanism of enzyme catalysis are addressed in the adenosylcobalamin (coenzyme B₁₂)-dependent ethanolamine ammonia-lyase (EAL) enzyme from Salmonella enterica serovar Typhimurium. Full-spectrum, time-resolved electron paramagnetic resonance (EPR) spectroscopy is used to measure the temperature dependence of the first-order kinetics of the substrate radical reaction in EAL surrounded by successive hydration and aqueous-cosolvent (aminoethanol; added dimethyl sulfoxide, glycerol, and sucrose) layers in frozen solution. At different temperature (T) values in each system, the piecewise-continuous Arrhenius relation displays the characteristic bifurcation, from high-T monoexponential dependence (reaction from substrate radical macrostate, S ^• ) to the low-T biexponential dependence (reaction from sequential substates, S ₁ ^• and S ₂ ^• ). Parallel measurements of the solvent dynamics around EAL by using EPR spin probe mobility or electric permittivity detect a dynamical transition from collective cluster to individual fluctuations of coupled protein surface groups and hydration water, with decreasing T, that precisely coincides with the kinetic bifurcation T in each solvent system. When shifted along their common monotonic high-T relation, the Arrhenius dependences collapse onto a single, universal pattern. The results indicate that specific, or select, collective fluctuations in the EAL protein hydration layer are coupled to active-site configurational fluctuations, providing low-barrier portals through the configuration space. The direct mechanistic link between solvent dynamics and turnover expands the understanding of radical-mediated reactions in EAL and supports the model that select collective configurational dynamics are a fundamental feature of enzyme catalysis.

Supercapacitors are key to sustainable energy storage due to their high power density and long lifespan, though their energy density remains limited. This study explores natural deep eutectic solvents (NADES) as alternative electrolytes for graphene-based supercapacitors via molecular dynamics simulations. Three NADEScomposed of betaine chloride and the amino acids arginine, histidine, or lysineare assessed for their biocompatibility, cost-effectiveness, and hydrogen-bonding capabilities. Simulations at 300 and 600 K reveal distinct physicochemical behaviors: histidine-based NADES shows the highest density and cohesive energy, attributed to imidazole-mediated interactions, while lysine-based NADES offers the greatest ionic mobility. In supercapacitor models, asymmetric electric double layers (EDLs) form, with amino acids dominating the positive EDL and betaine the negative. Interaction energy analyses underscore the stabilizing role of amino acids in the EDL structure. Capacitance values range from 2.2 to 2.8 μF/cm², aligning with those of conventional electrolytes. These results highlight the promise of NADES as sustainable and tunable electrolytes, offering a viable route to enhance the performance of next-generation supercapacitors.

A molecular orbital or electronically excited state may change its character, from Rydberg or mixed valence-Rydberg to valence, as dissociation progresses. This geometrical dependency of the electronically excited states is known as Rydbergization. Recently, we proposed a new approach to characterizing the nature of electronically excited states based on the stabilization method [Randi P. A S, et al. J. Phys. Chem. A, 2025, 129, 5820-5828.]. Here, we demonstrate that the stabilization method can effectively describe the Rydbergization phenomenon in the low-lying excited states of water. To this end, we analyze both the symmetric and asymmetric dissociation pathways, comparing our findings to previously reported results whenever possible. In addition to reproducing established data, we present new insights into the symmetric dissociation of states with B ₂ symmetry, as well as previously unexplored behavior along the asymmetric dissociation pathway. We conclude also that Rydbergization is pathway-dependent and that conclusions drawn from one geometric distortion cannot be uncritically generalized to others.

We present a transferable force field for water in proton-exchanged, alkali (Li, Na, K, Rb, and Cs) metal-exchanged, and alkaline-earth (Mg, Ca, Sr, and Ba) metal-exchanged zeolites. The fitting methodology is based on adsorbate-adsorbent interaction energies obtained from periodic density functional theory calculations and corrected using the coupled-cluster method applied to small model clusters. To ensure an accurate prediction of both adsorption and diffusion properties of water, sets of configurations that sample both adsorption sites and intracrystalline hopping transition states were used in the fitting. The quality of the force field is assessed for a wide range of zeolites with different topologies and chemical compositions, demonstrating good agreement between theoretical predictions and experimental measurements of water adsorption and diffusion.

X-ray and XUV diffraction experiments have visualized both the outer shape and quantum vortices inside individual superfluid helium droplets. Both features are effective on the helium induced signature observed as the spectral shape and position of the electronic transitions of molecules doped into helium droplets. In this article the helium induced signature at the electronic band origin of phthalocyanine is re-examined systematically comprising previous analytical results as well as newly reported experimental investigations. Helium-induced effects such as a nonmonotonous evolution of the solvent shift and the emergence of an optical anisotropy, both observed for rather large helium droplets, are the spectroscopic response on the analytical results reported from diffraction experiments. All helium induced spectroscopic features can be explained as an expression of London dispersion interaction under the varying structural conditions of helium droplets.

Jet-cooled excitation spectra of the phenalenyl radical are obtained using resonance enhanced multiphoton ionization. The excitation spectra reveal previously unobserved transitions, up to 17,000 cm^-1 above the D₁ origin, including transitions to electronically forbidden A₂ ^″ electronic states. A quasi-diabatic approach is applied to construct a vibronic Hamiltonian, including both Jahn-Teller and pseudo-Jahn-Teller interactions, between seven excited electronic surfaces. This is employed to calculate the electronic excitation spectrum of the phenalenyl radical in its entirety, providing vibronic assignments and spectral parameters to help decode the spectroscopy of this key radical.