Isomeric State Research Articles

Hydroxysilylene (HSi-OH) in the gas phase.

The hydroxysilylene (HSiOH) molecule has been spectroscopically identified in the gas phase for the first time. This highly reactive species was produced in a twin electric discharge jet using separate precursor streams of 16O2/18O2 and Si2H6/Si2D6, both diluted in high pressure argon. The strongest and most stable laser induced fluorescence (LIF) signals were obtained by applying an electric discharge to each of the precursor streams and then merging the discharge products just prior to expansion into vacuum. Bands of the Ã1A-X~1A' electronic transition of HSiOH were found in the 455-420nm region, and single vibronic level emission spectra showed only transitions attributable to the trans-hydroxysilylene ground state isomer. High resolution, rotationally resolved spectra were obtained for the 0-0 bands of HSi16OH and HSi18OH. The rotational constants were used to obtain ground and excited state molecular structures of HSiOH, with some necessary constraints. The derived ground state structure is trans-HSiOH, with geometric parameters similar to theoretical predictions from the literature. In the excited state, a skew-HSiOH structure was obtained with a dihedral angle of 102°. Our own CASSCF/aug-cc-pVTZ calculations predict a similar excited state skew geometry. The lack of odd quantum number changes in the torsional mode in emission and our difficulties in obtaining DSiOD spectra, despite considerable effort, all suggest that further experimental and theoretical efforts will be necessary to thoroughly understand the electronic spectrum of hydroxysilylene.

Radiative decay of the Th229m nuclear clock isomer in different host materials

A comparative vacuum ultraviolet spectroscopy study conducted at ISOLDE-CERN of the radiative decay of the Th229m nuclear clock isomer embedded in different host materials is reported. The ratio of the number of radiative decay photons and the number of Th229m embedded are determined for single crystalline CaF2, MgF2, LiSrAlF6, AlN, and amorphous SiO2. For the latter two materials, no radiative decay signal was observed and an upper limit of the ratio is reported. The radiative decay wavelength was determined in LiSrAlF6 and CaF2, reducing its uncertainty by a factor of 2.5 relative to our previous measurement. This value is in agreement with the recently reported improved values from laser excitation. Published by the American Physical Society 2025

Simultaneous Ring-Opening and Dehydrogenation of Diarylethene Induced by Tunneling Electrons.

Understanding the reversible transformation between two isomeric states of organic molecules under external stimulation is essential for advancing single-molecule device development. Photochromic diarylethene (DAE) derivatives are promising candidates for single molecular switching elements. This study investigates the single-molecule reactions of the closed-form isomer of a DAE derivative on Cu(111) using scanning tunneling microscopy (STM). A novel ring-opening pathway, distinct from the well-known photochromic isomerization, was discovered. Electron injection into the lowest unoccupied molecular orbitals induces the sequential anchoring of molecules to the substrate through the dehydrogenation of a methyl group at the 2-position of the thiophene ring. This mechanism was revealed by density functional theory calculations and STM simulations. The adsorption configurations for singly and doubly dehydrogenated DAEs were identified. Based on these findings, a new reaction mechanism extending beyond reversible isomeric reactions is proposed for DAE on Cu(111). The present work establishes a novel framework for future studies on single-molecule switching phenomena on metal substrates.

Single GAF Domain Phytochrome Exhibits a pH-Dependent Shunt on the Millisecond Timescale.

The light-sensing activity of phytochromes is based on the reversible light-induced switching between two isomerization states of the bilin chromophore. These photo-transformations may not necessarily be only unidirectional, but could potentially branch back to the initial ground state in a thermally driven process termed shunt. Such shunts have been rarely reported, and thus our understanding of this process and its governing factors are limited. Here, we aim to close this gap by providing coherent experimental evidence of a shunt process using UV/Vis laser flash photolysis. We studied the Pfr to Pr dynamics of the single GAF domain (g1) construct of the knotless phytochrome All2699 from cyanobacterium Nostoc punctiforme. We identified a shunt that can be switched on and off by ambient buffer conditions. In combination with H/D exchange and kinetic modeling, we propose a keto-enol tautomerism to allow for the thermal isomerization of the chromophore and act as the driver of the shunt transition.

Distinguishing Charged Lepton Flavor Violation Scenarios with Inelastic μ→e Conversion.

The Mu2e and COMET experiments are expected to improve existing limits on charged lepton flavor violation (CLFV) by roughly 4 orders of magnitude. μ→e conversion experiments are typically optimized for electrons produced without nuclear excitation, as this maximizes the electron energy and minimizes backgrounds from the free decay of the muon. Here we argue that Mu2e and COMET will be able to extract additional constraints on CLFV from inelastic μ→e conversion, given the ^{27}Al target they have chosen and backgrounds they anticipate. We describe CLFV scenarios in which inelastic CLFV can induce measurable distortions in the near-endpoint spectrum of conversion electrons, including cases where certain contributing operators cannot be probed in elastic μ→e conversion. We extend the nonrelativistic EFT treatment of elastic μ→e conversion to include the new nuclear operators needed for the inelastic process, evaluate the associated nuclear response functions, and describe several new-physics scenarios where the inelastic process can provide additional information on CLFV.

Hemiindigoids as Prominent Photoswitch Scaffolds

AbstractHemiindigo photoswitches, a class of chromophores derived from the well‐known indigo dye, have attracted considerable interest over the past two decades. These compounds are unique in their ability to absorb light in the visible region of the spectrum in both isomeric states, making them ideal for applications where high‐energy UV light is not tolerated. A particularly attractive feature of hemiindigoids is that, despite their rigid structures, they undergo significant and controllable changes during the photoisomerisation process. Moreover, they exhibit a combination of high thermal bistability, pronounced photochromism, and fast, efficient photoisomerisation, positioning hemiindigoid photoswitches as versatile chromophores. This review summarises the current developments and advances in hemiindigoid photoswitches, including their synthesis, photoswitching performance, and applications, with a strong focus on their practical use as reversible triggers in fields ranging from photopharmacology to advanced materials.

TWO-PHOTON RESONANCE MECHANISM OF OPTICAL PUMPING OF THE 8.3-eV ISOMER 229mTh IN NEUTRAL ATOMS

The possibility of refining the energy of the nuclear isomer 229mTh with the energy of 8.36 eV, the most likely candidate for the role of a nuclear frequency standard, using resonant optical pumping is discussed. Attention is focused on the broadening of theresonance in order toreduce scanning time. The proposed twophoton method uses radical broadening of the isomer line due to mixing with an electronic transition. This method is not burdened by cross-section reduction, in contrast with internal-conversion-based resonance broadening or intended extra-broadening of the spectral line of a scanning laser. In the case under consideration, it turns out to be two orders of magnitude more effective. It applies to both ionized and neutral thorium atoms. The realization of the method supposes excitation of both the nucleus and the electron shell in the final state.

γ-Ray spectroscopy above the 9+ isomeric state in the N = Z nucleus 66As

Excited states above the 9+ isomer in the odd-odd N = Z nucleus 66As were studied by employing the 40Ca(28Si, pn) fusion-evaporation reaction at the Accelerator Laboratory of the University of Jyväskylä, Finland. A key method in this study was the use of conversion electrons emitted in the de-excitation of the isomeric state in 66As as a tag for prompt γ rays. Several new states have been added to the 66As level scheme, which was extended up to a tentative spin of 23ħ. The previously reported even-spin yrast band has been reassigned to have odd spin values. The odd-spin states above the 9+ isomer are compared with shell-model calculations using the jj44b and JUN45 interactions. Additionally, the recoil-β tagging efficiency of the recently developed scintillator detector named Tuike has been determined experimentally for the first time.

When can flexible weak polyelectrolytes be treated as effective rigid objects?

Conformational and ionization equilibria of flexible weak polyelectrolytes (PEs) are, in general, strongly coupled. In this article, we analyze the effect of averaging over (or "contracting") the conformational degrees of freedom so that the original flexible molecule is replaced by an effective rigid object with the same ionization properties. As a result, one obtains the so-called Site Binding (SB) model, much easier to treat both theoretically and computationally, and extensively used to characterize the ionization properties of PE. The conformational averages can be performed in a systematic way by means of the Conformational Contraction Equations (CCEs), which relate the SB parameters to the underlying conformational equilibrium. The conditions for the convergence of the CCE are evaluated in the presence of both Short Range (SR) and Long Range (LR) electrostatic interactions. Two analytically solvable models based on the Freely Jointed Chain (FJC), involving only SR interactions, are analyzed at first. Despite the large chain flexibility, the resulting SB model reproduces the ionization properties with high accuracy. In the case of independent bonds, a very flexible chain can be exactly replaced by an effective rigid object with neighboring pairwise interactions. In general, however, triplet and higher order interactions emerge at the SB level. When LR electrostatic interactions are introduced and combined with the FJC large chain flexibility, the convergence of the CCE for long chains becomes problematic since the SB free energy must be truncated. Similar conclusions are reached for the freely rotating chain and rotational isomeric state models.

Photophore-Anchored Molecular Switch for High-Performance Nonvolatile Organic Memory Transistor

Over the last decade, there has been a marked increase in interest in memory devices embedded with molecular switches, especially field-effect transistors (FETs). These molecular switches are integrated to regulate the resistance or current levels within FETs. Despite significant efforts, achieving a large memory window with extended retention time remains challenging. This difficulty arises because the isomeric state of a molecular switch cannot function as a stable deep trap state in the semiconductor layer. This study addresses this issue by introducing chemical bonding between the molecular switch and the conjugated polymeric semiconductor. This bonding enables the closed isomer of diarylethene (DAE) to act as a morphologically stable deep trap state. Azide- and diazirine-anchored DAEs are synthesized to form chemical bonds with the polymer through photocrosslinking, thus creating permanent and controllable trapping states near the conjugated backbone of the polymer semiconductor. As a result, when diazirine-anchored DAE is blended with F8T2 and subjected to photocrosslinking, the resulting organic FETs demonstrate impressive memory performance, including a memory window of 22 V with a retention time exceeding 106 seconds, a high photoprogrammable on/off ratio greater than 103, and high operational stability over more than 100 photocycles. Additionally, photophore-anchored DAEs allow for precise patterning, which enables meticulous control over the semiconductor layer structure. Figure 1

(Invited) Atomically Thin Electronic Channels with Programmed Molecular and Ionic Interactions

The electronic properties of two-dimensional materials are highly susceptible to the types of nearby chemical substances, including molecules and ions. Furthermore, when electrons transport through materials with widths narrowed down to the nanoscale, their motions can be significantly modulated by interactions with chemicals similar in size to the channel width. Here, I present two such material platforms, including graphene nanoribbon grids and grain boundaries of monolayer MoS2. The atomic structures of ultra-narrow electronic channels are designed during growth by chemical vapor deposition to host programmed interactions with targeted chemicals. I show examples of how these platforms can be used to electrically identify the isomer states of nearby molecules and the types of ions.

Second Generation Zwitterionic Aza‐Diarylethene: Photoreversible CN Bond Formation, Three‐State Photoswitching, Thermal Energy Release, and Facile Photoinitiation of Polymerization

AbstractDiarylethenes are a well‐studied and optimized class of photoswitches with a wide range of applications, including data storage, smart materials, or photocontrolled catalysis and biological processes. Most recently, aza‐diarylethenes have been developed in which carbon‐carbon bond connections are replaced by carbon‐nitrogen connections. This structural elaboration opens up an entire new structure and property space expanding the versatility and applicability of diarylethenes. In this work, we present the second generation of zwitterionic aza‐diarylethenes, which finally allows for fully reversible photoswitching and precise control over all three switching states. High‐yielding photoswitching between the neutral open form and a zwitterionic Z isomer is achieved with two different wavelengths of light. The third zwitterionic E isomeric state can be reached in up to 87 % upon irradiation with a third wavelength. Its high energy content of >10 kcal/mol can be released thermally by deliberate solvent change as trigger mechanism, rendering aza‐diarylethenes into interesting candidates for molecular solar thermal energy storage (MOST) applications. The third state also serves as locking state, allowing to toggle light‐responsiveness reversibly between thermally labile and thermally stable switching. Further, irradiation of the zwitterionic states leads to highly efficient photopolymerization of methyl acrylate (MA), directly harnessing the unleashed chemical reactivity of our aza‐diarylethene in a materials application.

Multiscale molecular modeling of bulk amorphous structure of poly(propylene oxide)

An efficient method of multiscale molecular modeling to generate the bulk amorphous structures of coarse-grained (CG) and fully atomistic poly(propylene oxide), PPO, models was developed. The intramolecular short-range interaction of PPO chains was described by the Rotational Isomeric State (RIS) model and the non-bonded intermolecular interaction was represented by the discretized version of Lennard-Jones (LJ) potential energy. Monte Carlo (MC) simulation was implemented to effectively generate and equilibrate the CG models of amorphous structures at the bulk density of PPO, CH3(OCH(CH3)CH2)29OCH3 with the molecular weight of 1728 g/mol, on the second nearest diamond (2nnd) lattice. The reverse mapping of CG models was done to generate the fully atomistic PPO model followed by energy minimization to obtain the relaxed structures. To validate this atomistic PPO model, some molecular and material properties were compared to experimental data with reasonable agreement including chain dimension, conformational statistics, solubility parameter, and X-ray/neutron scatterings.

Laser-based approach to measure small nuclear cross sections in plasma

We demonstrate an approach successfully measuring very small nuclear isomeric excitation cross sections (on the order of 10 to 100 picobarns) via laser-cluster interaction. The interaction between an intense laser pulse and Kr atomic clusters generates a high-temperature and high-density plasma ball in which nuclear excitations are facilitated by inelastic electron scattering. The electron temperature reaches several hundred keV (corresponding to 10[Formula: see text] K), similar to a stellar environment. The very small nuclear excitation cross sections are extremely challenging to measure otherwise, for example using traditional accelerator-based approaches. Our method opens a broad avenue of measuring small nuclear cross sections and holds promise for advancing understanding of nuclear transition mechanisms and exploring pathways in the evolutionary dynamics of elements within stellar environments.

Laser-based approach to verify nuclear excitation by electron capture

Laser-based approach to verify nuclear excitation by electron capture

A Rationally Designed Azobenzene Photoswitch for DNA G-Quadruplex Regulation in Live Cells.

G-quadruplex (G4) DNA structures are increasingly acknowledged as promising targets in cancer research, and the development of G4-specific stabilizing compounds may lay a fundamental foundation in precision medicine for cancer treatment. Here, we propose a light-responsive G4-binder for precise modulation of drug activation, providing dynamic and spatiotemporal control over G4-associated biological processes contributing to cancer cell death. We developed a specialized fluorinated azobenzene (AB) switch equipped with a quinoline unit and a positively charged carboxamide side chain, Q-Azo4F-C, designed for targeted binding to G4 structures within cells. Biophysical studies, combined with molecular dynamics simulations, provide insights into the unique coordination modes of the photoswitchable ligand in its trans and cis configurations when interacting with G4s. The observed variations in complexation processes between the two isomeric states in different cancer cell lines manifest in more than 25-fold reversible cytotoxic activity. Immunostaining conducted with the structure-specific G4 antibody (BG4), establishes a direct correlation between cytotoxicity and the varying extent of G4 induction regulated by the two isoforms. Finally, we demonstrate the photo-driven reversible regulation of G4 structures in lung cancer cells by Q-Azo4F-C. Our findings highlight the potential of light-responsive G4-binders in advancing precision cancer therapy through dynamic control of G4-mediated pathways.

A Rationally Designed Azobenzene Photoswitch for DNA G‐Quadruplex Regulation in Live Cells

AbstractG‐quadruplex (G4) DNA structures are increasingly acknowledged as promising targets in cancer research, and the development of G4‐specific stabilizing compounds may lay a fundamental foundation in precision medicine for cancer treatment. Here, we propose a light‐responsive G4‐binder for precise modulation of drug activation, providing dynamic and spatiotemporal control over G4‐associated biological processes contributing to cancer cell death. We developed a specialized fluorinated azobenzene (AB) switch equipped with a quinoline unit and a positively charged carboxamide side chain, Q‐Azo4F‐C, designed for targeted binding to G4 structures within cells. Biophysical studies, combined with molecular dynamics simulations, provide insights into the unique coordination modes of the photoswitchable ligand in its trans and cis configurations when interacting with G4s. The observed variations in complexation processes between the two isomeric states in different cancer cell lines manifest in more than 25‐fold reversible cytotoxic activity. Immunostaining conducted with the structure‐specific G4 antibody (BG4), establishes a direct correlation between cytotoxicity and the varying extent of G4 induction regulated by the two isoforms. Finally, we demonstrate the photo‐driven reversible regulation of G4 structures in lung cancer cells by Q‐Azo4F‐C. Our findings highlight the potential of light‐responsive G4‐binders in advancing precision cancer therapy through dynamic control of G4‐mediated pathways.

Spectral Flux Enhancement of X Rays for Addressing Ultranarrow Nuclear Transitions.

Recently, the 1.4 feV ultranarrow nuclear transition at 12.4keV energy in ^{45}Sc was resonantly excited for the first time using radiation from the self-seeded EuXFEL laser [Y. Shvyd'ko et al., Resonant x-ray excitation of the nuclear clock isomer ^{45}Sc, Nature (London) 622, 471 (2023)NATUAS0028-083610.1038/s41586-023-06491-w], establishing ^{45}Sc as a promising candidate for a future Mössbauer nuclear clock. While this experiment demonstrated a high potential of x-ray free-electron laser sources for resonantly exciting nuclear isomers in the hard x-ray range, it also highlighted a severe limitation in the achievable excitation level caused by their extremely large spectral bandwidth ∼1 eV. In this Letter, we propose a method to enhance the spectral flux of x-ray free-electron laser radiation using a resonant absorber with a longitudinal gradient in the nuclear transition frequency. A portion of the incident pulse can be absorbed into a nuclear collective excitation, and converted back into x-ray radiation by reversing the sign of the frequency gradient. Spectral narrowing and flux enhancement of this re-emitted x-ray field is achieved by using a reversed frequency gradient with a smaller magnitude than the initial one. About a hundredfold of such spectral flux enhancement is feasible in ^{45}Sc_{2}O_{3} single crystal, rendering a more efficient source for nuclear excitation and facilitating the experimental observation of resonant fluorescence and coherent forward scattering at the 12.4keV transition, both of which are essential for realizing a nuclear clock.

Effective M3Y-P2 interaction of study nuclear energy levels for 48Ti in fp shell model

In the fp-shell area of the 48Ti isotope (Z=22, N=26), examine the nuclear excitation energies using the nuclear shell model. The energy levels, total angular momentum, and parity of the nucleons outside the locked core 40Ca are calculated using the OXBASH algorithm for GX2, GX1A, KB3, and effective M3Y-P2 interactions. mixing together two orbital configurations. There was a good agreement between the model space vectors and energy levels and experimental data, but the agreement decreased when the realistic M3YP-2 interaction was used. The core polarisability effect has been used to accommodate the rejected space: core and higher organization via the L-S shell with an effective M3Y-P2 interaction. A comparison of the theoretical and experimental results has been conducted.

On the low-energy electromagnetic dipole modes in 151,153,155Sm Nuclei

The recently observed low-energy magnetic dipole (M1) and electric dipole (E1) excitations in deformed 151,153,155Sm are theoretically analysed. Rotational Invariant (RI-) and Translational-Galilean Invariant (TGI-) Quasiparticle Nuclear Model (QPNM) are used in the calculation of M1 and E1 properties, respectively. Both theories consider monopole pairing between nucleons, and the deformed Woods-Saxon potential is used as the mean-field potential. Pyatov's symmetry restoration procedure is applied in these models to eliminate the spurious modes from the intrinsic nuclear excitations. Model calculations show that although E1 transitions dominate the low-energy dipole spectra of 151,153,155Sm, many low-lying M1 transitions exist in these nuclei. It is shown that the most significant contribution to E1 and M1 excitation comes from ΔK=±1 transitions. The theory satisfactorily reproduces the gross features of low-lying dipole modes determined from the Oslo Method analysis of the experimental spectra.