Quantum chemical calculations of electron affinities of alkaline earth metal atoms (Ca, Sr, Ba, and Ra).

We performed high-level ab initio quantum chemical calculations, incorporating higher-order excitations, spin-orbit coupling (SOC), and the Gaunt interaction, to calculate the electron affinities (EAs) of alkaline earth (AE) metal atoms (Ca, Sr, Ba, and Ra), which are notably small. The coupled-cluster singles and doubles with perturbative triples [CCSD(T)] method is insufficient to accurately calculate the EAs of AE metal atoms. Higher-order excitations proved crucial, with the coupled-cluster singles, doubles, and triples with perturbative quadruples [CCSDT(2)Q] method effectively capturing dynamic electron correlation effects. The contributions of SOC (ΔESOs) to the EAs calculated using the multireference configuration interaction method with the Davidson correction, including SOC, positively enhance the EAs; however, these contributions are overestimated. The Dirac-Hartree-Fock (DHF)-CCSD(T) method addresses this overestimation and provides reasonable values for ΔESO (ΔESO-D). Employing additional sets of diffuse and core-valence correlation basis sets is critical for accurately calculating the EAs of AE metal atoms. The contributions of the Gaunt interaction (ΔEGaunt) to the EAs of AE metal atoms are negligible. Notably, the CCSDT(2)Q with the complete basis set limit + ΔESO-D + ΔEGaunt produced EA values for Ca, Sr, and Ba that closely aligned with experimental data and achieved accuracy exceeding the chemical accuracy. Based on our findings, the accurately proposed EA for Ra is 9.88 kJ/mol.

Phosphorus-Ligand Redox Cooperative Catalysis: Unraveling Four-Electron Dioxygen Reduction Pathways and Reactive Intermediates.

The reduction of dioxygen to water is crucial in biology and energy technologies, but it is challenging due to the inertness of triplet oxygen and complex mechanisms. Nature leverages high-spin transition metal complexes for this, whereas main-group compounds with their singlet state and limited redox capabilities exhibit subdued reactivity. We present a novel phosphorus complex capable of four-electron dioxygen reduction, facilitated by unique phosphorus-ligand redox cooperativity. Spectroscopic and computational investigations attribute this cooperative reactivity to the unique electronic structure arising from the geometry of the phosphorus complex bestowed by the ligand. Mechanistic study via spectroscopic and kinetic experiments revealed the involvement of elusive phosphorus intermediates resembling those in metalloenzymes. Our result highlights the multielectron reactivity of phosphorus compound emerging from a carefully designed ligand platform with redox cooperativity. We anticipate that the work described expands the strategies in developing main-group catalytic reactions, especially in small molecule fixations demanding multielectron redox processes.

Increasing Chemical Diversity of B2 N2 Anthracene Derivatives by Introducing Continuous Multiple Boron-Nitrogen Units.

Increasing the chemical diversity of organic semiconductors is essential to develop efficient electronic devices. In particular, the replacement of carbon-carbon (C-C) bonds with isoelectronic boron-nitrogen (B-N) bonds allows precise modulation of the electronic properties of semiconductors without significant structural changes. Although some researchers have reported the preparation of B2 N2 anthracene derivatives with two B-N bonds, no compounds with continuous multiple BN units have been prepared yet. Herein, we report the synthesis and characterization of a B2 N2 anthracene derivative with a BNBN unit formed by converting the BOBN unit at the zigzag edge. Compared to the all-carbon analogue 2-phenylanthracene, BNBN anthracene exhibits significant variations in the C-C bond length and a larger highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap. The experimentally determined bond lengths and electronic properties of BNBN anthracene are confirmed through theoretical calculations. The BOBN anthracene organic light-emitting diode, used as a blue host, exhibits a low driving voltage. The findings of this study may facilitate the development of larger acenes with multiple BN units and potential applications in organic electronics.

Do Planar Tetracoordinate Fluorine Atoms Exist? Revisiting a Theoretical Prediction.

We re-examined the existence of planar tetracoordinate F (ptF) atoms, which was proposed recently by using high-level ab initio methods such as coupled-cluster singles and doubles with perturbative triples (CCSD(T)) with large basis sets. Our calculations indicate that the planar structures of FIn4+ (D4h), FTl4+ (D4h), FGaIn3+ (C2V), FIn2Tl2+ (D2h), FIn3Tl+ (C2V), and FInTl3+ (C2V) are not the minimum energy states; by contrast, they are transition states. Density functional theory calculations overestimate the size of the cavity formed by the four peripheral atoms, leading to erroneous conclusions regarding the existence of ptF atoms. Our analysis suggests that the preference for non-planar structures in the six cations studied is not due to the pseudo Jahn-Teller effect. Additionally, spin-orbit coupling does not alter the main conclusion that the ptF atom does not exist. If sufficiently large cavity formation by group 13 elements to accommodate the central F- ion is guaranteed, then the existence of ptF atoms is plausible.

Ultrafast photoisomerization mechanism of azaborine revealed by nonadiabatic molecular dynamics simulations.

1,2-Dihydro-1,2-azaborine is an isoelectronic analog of benzene with a B-N substitution, and its unique photoisomerization behavior, which is distinct from that of benzene, has drawn significant attention. To understand the detailed mechanism of azaborine photochemistry considering the dynamical effect and gain a comprehensive understanding of photochemical reactions, we investigated the photoisomerization dynamics of azaborine using nonadiabatic molecular dynamics simulations with Tully's surface hopping algorithm. Herein, the structural and energetic analyses of the trajectories revealed three different paths: direct relaxation (path 1), relaxation via a prefulvene-like intermediate (path 2), and formation of the Dewar isomer as a photoproduct (path 3). Our results confirmed that the photoisomerization of azaborine follows the energetically favored pathway predicted by the previous minimum energy path (MEP) calculations, exclusively forming the Dewar isomer, which is consistent with the experimental observations. Additionally, despite the low quantum yield found in our simulations, the high-level excitation energy calculations support the complete conversion observed in the experiments.

Photochemical Reaction Mechanism of Intramolecular H Transfer Reaction of H2 C3 O+ ⋠Radical Cation.

The photochemical reaction mechanism underlying the intramolecular H-transfer of the H2 C3 O+ ⋅ radical cation to the H2 CCCO+ ⋅ methylene ketene cation was elucidated using time-dependent density functional theory and high-level ab initio methods. Once the D1 state of H2 C3 O+ ⋅ is populated, the reaction proceeds to form an intermediate (IM) in the D1 state (IM4D1 ). The molecular structure of the conical intersection (CI) was optimized using a multiconfigurational ab initio method. The CI is readily accessible because it lies slightly above the IM4D1 in energy. In addition, the gradient difference vector of the CI is almost parallel to the intramolecular H-transfer reaction coordinate. Once the vibration mode of IM4D1 which is parallel to the reaction coordinate is populated, the degeneracy of the CI is readily lifted and H2 CCCO+ ⋅ was formed via a relaxation pathway in the D0 state. Our calculated results clearly describe the photochemical intramolecular H transfer reaction reported in a recent study.

High level ab initio and density functional study of TeF6+ and TeCl6+: Attainability of +7 oxidation state for tellurium.

Discovery of a new oxidation state for an element expands its chemistry. A high oxidation state, such as +7, is rare for sp-block elements except for halogens. In this study, we determined that Te can attain a +7 oxidation state through the existence of a distorted octahedron (DOH) structure of TeCl6+ based on coupled cluster singles and doubles with perturbative triples calculations. We propose a new type of isomerization that resembles pseudorotation. The octahedron structure of TeF6+ bearing one elongated axial bond isomerizes to a DOH via an associated pseudorotation.

Extracting Kinetics and Thermodynamics of Molecules without Heavy Atoms via Time-Resolved Solvent Scattering Signals.

Time-resolved X-ray liquidography (TRXL) has emerged as a powerful technique for studying the structural dynamics of small molecules and macromolecules in liquid solutions. However, TRXL has limited sensitivity for small molecules containing light atoms only, whose signal has lower contrast compared with the signal from solvent molecules. Here, we present an alternative approach to bypass this limitation by detecting the change in solvent temperature resulting from a photoinduced reaction. Specifically, we analyzed the heat dynamics of TRXL data obtained from p-hydroxyphenacyl diethyl phosphate (HPDP). This analysis enabled us to experimentally determine the number of intermediates and their respective enthalpy changes, which can be compared to theoretical enthalpies to identify the intermediates. This work demonstrates that TRXL can be used to uncover the kinetics and reaction pathways for small molecules without heavy atoms even if the scattering signal from the solute molecules is buried under the strong solvent scattering signal.

Organic-Inorganic Hybrid Device with a Novel Deep-Blue Emitter of a Donor-Acceptor Type, with ZnO Nanoparticles for Solution-Processed OLEDs.

Two new deep-blue emitters with bipolar properties based on an organoboron acceptor and carbazole donor were newly synthesized: 2-(9H-carbazol-9-yl)-5-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho [3,2,1-de]anthracen-7-yl)-5H-benzo[b]carbazole (TDBA-BCZ) and 5-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho [3,2,1-de]anthracen-7-yl)-8-phenyl-5,8-dihydroindolo[2,3-c]carbazole (TDBA-PCZ). The two emitters showed deep-blue and real-blue photoluminescence emission in their solution and film states, respectively. The doped spin-coated films were prepared using synthesized materials and showed a root-mean-square roughness of less than 0.52 nm, indicating excellent surface morphology. The doped devices, fabricated via a solution process using TDBA-BCZ and TDBA-PCZ as the dopants, showed electroluminescence peaks at 428 and 461 nm, corresponding to the Commission Internationale de L'éclairage (CIE) coordinates of (0.161, 0.046) and (0.151, 0.155), respectively. The external quantum efficiency (EQE)/current efficiency (CE) of the solution-processed forward devices, with TDBA-BCZ and TDBA-PCZ as dopants, were 7.73%/8.67 cd/A and 10.58%/14.24 cd/A, respectively. An inverted OLED device fabricated using rod-shaped ZnO nanoparticles as an electron injection layer showed a CE of 1.09 cd/A and an EQE of 0.30%.

Photoactivation of triosmium dodecacarbonyl at 400 nm probed with time-resolved X-ray liquidography.

The photoactivation mechanism of Os3(CO)12 at 400 nm is examined with time-resolved X-ray liquidography. The data reveal two pathways: the vibrational relaxation following an internal conversion to the electronic ground state and the ligand dissociation to form Os3(CO)11 with a ligand vacancy at the axial position.

PN-Doped tetraphenylnaphthalene: a straightforward synthetic strategy analogous to BN-annulation.

Compared to BN heterocycles, few studies on PN heterocycles have been reported to date. Herein, we developed an efficient synthetic strategy analogous to BN-annulation to simultaneously incorporate a PN bond and a halogen group into the naphthalene core. Subsequently, we prepared PN-containing tetraphenylnaphthalene using this method, followed by palladium-catalyzed cross-coupling and reduction reactions. The prepared molecule was characterized via X-ray crystallography, NMR spectroscopy, UV-vis spectroscopy, and cyclic voltammetry.

Optical Kerr Effect of Liquid Acetonitrile Probed by Femtosecond Time-Resolved X-ray Liquidography.

Optical Kerr effect (OKE) spectroscopy is a method that measures the time-dependent change of the birefringence induced by an optical laser pulse using another optical laser pulse and has been used often to study the ultrafast dynamics of molecular liquids. Here we demonstrate an alternative method, femtosecond time-resolved X-ray liquidography (fs-TRXL), where the microscopic structural motions related to the OKE response can be monitored using a different type of probe, i.e., X-ray solution scattering. By applying fs-TRXL to acetonitrile and a dye solution in acetonitrile, we demonstrate that different types of molecular motions around photoaligned molecules can be resolved selectively, even without any theoretical modeling, based on the anisotropy of two-dimensional scattering patterns and extra structural information contained in the q-space scattering data. Specifically, the dynamics of reorientational (libration and orientational diffusion) and translational (interaction-induced motion) motions are captured separately by anisotropic and isotropic scattering signals, respectively. Furthermore, the two different types of reorientational motions are distinguished from each other by their own characteristic scattering patterns and time scales. The measured time-resolved scattering signals are in excellent agreement with the simulated scattering signals based on a molecular dynamics simulation for plausible molecular configurations, providing the detailed structural description of the OKE response in liquid acetonitrile.

Filming ultrafast roaming-mediated isomerization of bismuth triiodide in solution.

Roaming reaction, defined as a reaction yielding products via reorientational motion in the long-range region (3 - 8 Å) of the potential, is a relatively recently proposed reaction pathway and is now regarded as a universal mechanism that can explain the unimolecular dissociation and isomerization of various molecules. The structural movements of the partially dissociated fragments originating from the frustrated bond fission at the onset of roaming, however, have been explored mostly via theoretical simulations and rarely observed experimentally. Here, we report an investigation of the structural dynamics during a roaming-mediated isomerization reaction of bismuth triiodide (BiI3) in acetonitrile solution using femtosecond time-resolved x-ray liquidography. Structural analysis of the data visualizes the atomic movements during the roaming-mediated isomerization process including the opening of the Bi-Ib-Ic angle and the closing of Ia-Bi-Ib-Ic dihedral angle, each by ~40°, as well as the shortening of the Ib···Ic distance, following the frustrated bond fission.

Mapping the emergence of molecular vibrations mediating bond formation.

Fundamental studies of chemical reactions often consider the molecular dynamics along a reaction coordinate using a calculated or suggested potential energy surface1-5. But fully mapping such dynamics experimentally, by following all nuclear motions in a time-resolved manner-that is, the motions of wavepackets-is challenging and has not yet been realized even for the simple stereotypical bimolecular reaction6-8: A-B + C â†’ A + B-C. Here we track the trajectories of these vibrational wavepackets during photoinduced bond formation of the gold trimer complex [Au(CN)2-]3 in an aqueous monomer solution, using femtosecond X-ray liquidography9-12 with X-ray free-electron lasers13,14. In the complex, which forms when three monomers A, B and C cluster together through non-covalent interactions15,16, the distance between A and B is shorter than that between B and C. Tracking the wavepacket in three-dimensional nuclear coordinates reveals that within the first 60 femtoseconds after photoexcitation, a covalent bond forms between A and B to give A-B + C. The second covalent bond, between B and C, subsequently forms within 360 femtoseconds to give a linear and covalently bonded trimer complex A-B-C. The trimer exhibits harmonic vibrations that we map and unambiguously assign to specific normal modes using only the experimental data. In principle, more intense X-rays could visualize the motion not only of highly scattering atoms such as gold but also of lighter atoms such as carbon and nitrogen, which will open the door to the direct tracking of the atomic motions involved in many chemical reactions.

Ultrafast structural dynamics of in-cage isomerization of diiodomethane in solution.

Despite extensive studies on the isomer species formed by photodissociation of haloalkanes in solution, the molecular structure of the precursor of the isomer, which is often assumed to be a vibrationally hot isomer formed from the radical pair, and its in-cage isomerization mechanism remain elusive. Here, the structural dynamics of CH2I2 upon 267 nm photoexcitation in methanol were probed with femtosecond X-ray solution scattering at an X-ray free-electron laser. The determined molecular structure of the transiently formed species that converts to the CH2I-I isomer has the I-I distance of 4.17 Å, which is longer than that of the isomer (3.15 Å) by more than 1.0 Å and the mean-squared displacement of 0.45 Å2, which is about 100 times larger than those of typical regular chemical bonds. These unusual structural characteristics are consistent with either a vibrationally hot form of the CH2I-I isomer or the loosely-bound radical pair (CH2I˙⋯I˙).

Theoretical Investigation of Excited-State Intramolecular Proton Transfer and Photoisomerization of 2-(Iminomethyl)phenol.

Time-dependent density functional theory and high-level ab initio calculations were performed to investigate the excited-state intramolecular proton transfer (ESIPT) and subsequent isomerization of 2-(iminomethyl)phenol (IMP). According to the results of the correlated theoretical methods, ESIPT is a barrierless process; subsequently, the isomerization (rotation of the torsion angle) of IMP also readily occurs. Fictitious intermediates are found due to an insufficient theoretical level. The molecular structure of the conical intersection (CI) during the isomerization process is optimized, and its branching plane is characterized. Both the gradient difference vector and the derivative coupling vector are significantly correlated to the C═O and H2N-C antiparallel stretching coordinates, and the dynamic electron correlation effect is crucial to optimize the molecular structure of the real CI of IMP. The relaxation pathway from the CI in the S0 state was examined; the dominant pathway proceeds to the trans-keto form of IMP. However, if the C═O and H2N-C antiparallel stretching mode is sufficiently populated, then the reaction proceeds to the cis-keto form of IMP and can eventually recover to the cis-enol form.

Electronic and Structural Dynamics of Dicyanoaurate Trimer in Excited State.

A trimer of dicyanoaurate has been studied as a model system of the covalent chemical bond formation. Here, we report the dynamics of dicyanoaurate trimer in water upon photoexcitation by femtosecond time-resolved luminescence (TL) and luminescence spectra at cyrogenic temperature. Temperature was varied as a means to control the medium flexibility as well as the population of isomers. A unique parallelism between the luminescence spectrum vs. time and vs. temperature was observed, which enables unambiguous luminescence band assignments and facilitates investigation of the dynamics. Upon photoexcitation to S1, intersystem crossing proceeds in an ultrafast manner within 20 fs due to the large spin-orbit coupling followed by a structural change from a loose bent to a tight linear form in 1.5 ps. Higher oligomerization occurs above the melting temperature. TL reveals a strong coherent excitation of the symmetric Au-Au stretching vibration at 74 cm-1 through the non-Condon effect.

Fate of transient isomer of CH2I2: Mechanism and origin of ionic photoproducts formation unveiled by time-resolved x-ray liquidography.

Diiodomethane, CH2I2, in a polar solvent undergoes a unique photoinduced reaction whereby I2 - and I3 - are produced from its photodissociation, unlike for other iodine-containing haloalkanes. While previous studies proposed that homolysis, heterolysis, or solvolysis of iso-CH2I-I, which is a major intermediate of the photodissociation, can account for the formation of I2 - and I3 -, there has been no consensus on its mechanism and no clue for the reason why those negative ionic species are not observed in the photodissociation of other iodine-containing chemicals in the same polar solvent, for example, CHI3, C2H4I2, C2F4I2, I3 -, and I2. Here, using time-resolved X-ray liquidography, we revisit the photodissociation mechanism of CH2I2 in methanol and determine the structures of all transient species and photoproducts involved in its photodissociation and reveal that I2 - and I3 - are formed via heterolysis of iso-CH2I-I in the photodissociation of CH2I2 in methanol. In addition, we demonstrate that the high polarity of iso-CH2I-I is responsible for the unique photochemistry of CH2I2.