Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation.

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

The Dichotomy of Mn-H Bond Cleavage and Kinetic Hydricity of Tricarbonyl Manganese Hydride Complexes.

Acid-base characteristics (acidity, pKa, and hydricity, ΔG°H- or kH-) of metal hydride complexes could be a helpful value for forecasting their activity in various catalytic reactions. Polarity of the M-H bond may change radically at the stage of formation of a non-covalent adduct with an acidic/basic partner. This stage is responsible for subsequent hydrogen ion (hydride or proton) transfer. Here, the reaction of tricarbonyl manganese hydrides mer,trans-[L2Mn(CO)3H] (1; L = P(OPh)3, 2; L = PPh3) and fac-[(L-L')Mn(CO)3H] (3, L-L' = Ph2PCH2PPh2 (dppm); 4, L-L' = Ph2PCH2-NHC) with organic bases and Lewis acid (B(C6F5)3) was explored by spectroscopic (IR, NMR) methods to find the conditions for the Mn-H bond repolarization. Complex 1, bearing phosphite ligands, features acidic properties (pKa 21.3) but can serve also as a hydride donor (ΔG≠298K = 19.8 kcal/mol). Complex 3 with pronounced hydride character can be deprotonated with KHMDS at the CH2-bridge position in THF and at the Mn-H position in MeCN. The kinetic hydricity of manganese complexes 1-4 increases in the order mer,trans-[(P(OPh)3)2Mn(CO)3H] (1) < mer,trans-[(PPh3)2Mn(CO)3H] (2) ≈ fac-[(dppm)Mn(CO)3H] (3) < fac-[(Ph2PCH2NHC)Mn(CO)3H] (4), corresponding to the gain of the phosphorus ligand electron-donor properties.

Fac-to-mer isomerization triggers hydride transfer from Mn(I) complex fac-[(dppm)Mn(CO)3H].

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex fac-[(dppm)Mn(CO)3H] and Lewis acids (LA = Ph3C+, B(C6F5)3) involving the formation of so-far elusive meridional hydride species mer-[(dppm)Mn(CO)3H⋯LA] and unusual dearomatization of the Ph3C+ cation upon hydride transfer.

Substitution pattern dependent behavior of the singlet excited states in symmetrically fluorinated biphenyls.

Biphenyls are important basic chromophore systems that offer a possibility to study the effects of chemical substitution on the lower-lying excited states without complications from photoisomerization or other side processes. For several symmetric biphenyls, pristine biphenyl (bP0), 4,4'-difluorobiphenyl (bP2), 2,3,5,6,2',3',5',6'-octafluorobiphenyl (bP8), and perfluorobiphenyl (bP10), we report stationary and ultrafast solution-phase spectra rationalized with the aid of computations by means of the XMCQDPT2 multi-configuration perturbation theory and TDDFT. Polyfluorination tends to broaden the gap between the nearly degenerate S1 + S2 pair of states and the S3 state in bP8 and bP10, yet relaxation from any sheet of the S1-S3 manifold leads through a system of state crossings to the same stationary points in S1. Unlike bP0 and bP2 where the relaxed excited state is planar and non-polar, excited bP8 and bP10 exhibit sudden polarization to give a symmetry-lowered excited state via pseudo-Jahn-Teller interactions involving S1 and S2. Of particular interest is excited bP10 which reveals both sudden polarization and loss of planarity of one phenyl ring. We also demonstrate the unsatisfactory performance of the TDDFT methodology as applied to the biphenyls.

Type-I Energy Level Alignment at the PTCDA-Monolayer MoS2 Interface Promotes Resonance Energy Transfer and Luminescence Enhancement.

Van der Waals heterostructures consisting of 2D semiconductors and conjugated molecules are of increasing interest because of the prospect of a synergistic enhancement of (opto)electronic properties. In particular, perylenetetracarboxylic dianhydride (PTCDA) on monolayer (ML)-MoS2 has been identified as promising candidate and a staggered type-II energy level alignment and excited state interfacial charge transfer have been proposed. In contrast, it is here found with inverse and direct angle resolved photoelectron spectroscopy that PTCDA/ML-MoS2 supported by insulating sapphire exhibits a straddling type-I level alignment, with PTCDA having the wider energy gap. Photoluminescence (PL) and sub-picosecond transient absorption measurements reveal that resonance energy transfer, i.e., electron-hole pair (exciton) transfer, from PTCDA to ML-MoS2 occurs on a sub-picosecond time scale. This gives rise to an enhanced PL yield from ML-MoS2 in the heterostructure and an according overall modulation of the photoresponse. These results underpin the importance of a precise knowledge of the interfacial electronic structure in order to understand excited state dynamics and to devise reliable design strategies for optimized optoelectronic functionality in van der Waals heterostructures.

Insights into Charge Transfer at an Atomically Precise Nanocluster/Semiconductor Interface.

The deposition of an atomically precise nanocluster, for example, Ag44 (SR)30 , onto a large-band-gap semiconductor such as TiO2 allows a clear interface to be obtained to study charge transfer at the interface. Changing the light source from visible light to simulated sunlight led to a three orders of magnitude enhancement in the photocatalytic H2 generation, with the H2 production rate reaching 7.4 mmol h-1 gcatalyst -1 . This is five times higher than that of TiO2 modified with Ag nanoparticles and even comparable to that of TiO2 modified with Pt nanoparticles under similar conditions. Energy band alignment and transient absorption spectroscopy reveal that the role of the metal clusters is different from that of both organometallic complexes and plasmonic nanoparticles: A type II heterojunction charge-transfer route is achieved under UV/Vis irradiation, with the cluster serving as a small-band-gap semiconductor. This results in the clusters acting as co-catalysts rather than merely photosensitizers.

Evaluating the Solvent Stark Effect from Temperature-Dependent Solvatochromic Shifts of Anthracene.

The solvent Stark effect on the spectral shifts of anthracene is studied with temperature-dependent solvatochromic measurements. The Stark contribution ΔvStark to the absorption shift Δvp in polar solvents is measured to be ΔvStark =(53±35) cm-1 , in reasonable agreement with dielectric continuum theory estimate of 28 cm-1 , whereas the major shift Δvp ∼300 cm-1 presumably originates from the solute quadrupole. We pay attention to the accurate correction of Δvp for the nonpolar contribution that is crucial when the shifts are modest in magnitude.

A Simple and Convenient Method for the Synthesis of 1-Methyl-7-arylfuro[3,2-g]pteridine-2,4(1H,3H)-diones and Their Substituted Derivatives.

A simple and effective method for the synthesis of unknown 1-methyl-7-arylfuro[3,2-g]pteridine-2,4(1H,3H)-diones by dehydration of the corresponding 1-methyl-6-phenacyl-pteridine-2,4,7(1H,3H,8H)-triones is reported in the article. It was shown that their alkylation by butyl chloroacetate in basic medium proceeded by the N3-atom of the heterocycle. The structure and purity of the synthesized compounds were confirmed by IR, 1H, 13C NMR spectroscopy, gas chromatography-mass spectrometry, mass spectrometry, as well as X-ray diffraction analysis. The proposed mechanism of the dehydration reaction was discussed.

Time-Resolved Photochemistry of Stiffened Stilbenes.

Broadband transient absorption spectroscopy is used to study the photoisomerization of stiffened stilbenes in solution, specifically E/ Z mixtures of bis(benzocyclobutylidene) (t4, c4) and ( E)-1-(2,2-dimethyltetralinylidene)-2-2-dimethyltetraline (t6). Upon excitation to S1, all evolve to perpendicular molecular conformation P, followed by decay to S0, while the spectra and the kinetic behavior crucially depend on the size of the stiffening ring. In 4, contrary to all previously studied stilbenes, the trans and cis absorption and excited-state spectra are nearly indistinguishable, while the corresponding isomerization times are comparable: τi = 166 ps for t4 and τi = 64 ps for c4 in n-hexane, as opposed to 114 and 45 ps in acetonitrile, respectively. Faster isomerization in polar solvents agrees with the zwitterionic character of the P state. In t6, torsion to P is effectively barrier-less and completes within 0.3 ps, the S1 → P evolution being directly traceable through the transient spectra of stimulated emission and that of excited-state absorption. In n-hexane, the P state is remarkably long-lived, τP = 1840 ps, but the lifetime drops down to 35 ps in acetonitrile. The trans-to-cis photoisomerization yield for t6 is measured to be 20%, while for t4, it remains uncertain. We discuss the effects of stiffening and substitution on the formation and lifetime of the intermediate states through which the stilbene molecules evolve on the S1 energy surface.

Mechanism-Dependent Modulation of Ultrafast Interfacial Water Dynamics in Intrinsically Disordered Protein Complexes.

The recognition of intrinsically disordered proteins (IDPs) is highly dependent on dynamics owing to the lack of structure. Here we studied the interplay between dynamics and molecular recognition in IDPs with a combination of time-resolving tools on timescales ranging from femtoseconds to nanoseconds. We interrogated conformational dynamics and surface water dynamics and its attenuation upon partner binding using two IDPs, IBB and Nup153FG, both of central relevance to the nucleocytoplasmic transport machinery. These proteins bind the same nuclear transport receptor (Importinß) with drastically different binding mechanisms, coupled folding-binding and fuzzy complex formation, respectively. Solvent fluctuations in the dynamic interface of the Nup153FG-Importinß fuzzy complex were largely unperturbed and slightly accelerated relative to the unbound state. In the IBB-Importinß complex, on the other hand, substantial relative slowdown of water dynamics was seen in a more rigid interface. These results show a correlation between interfacial water dynamics and the plasticity of IDP complexes, implicating functional relevance for such differential modulation in cellular processes, including nuclear transport.

Atomically Precise Bimetallic Nanoclusters as Photosensitizers in Photoelectrochemical Cells.

The atomically precise bimetallic nanocluster (NC), Au24 Ag20 (PhCC)20 (SPy)4 Cl2 (1) (Py=pyridine), was employed for the first time as a stable photosensitizer for photoelectrochemical applications. The sensitization of TiO2 nanotube arrays (TNA) with 1 greatly enhances the light-harvesting ability of the composite because 1 shows a high molar extinction coefficient (ϵ) in the UV/Vis region. Compared to a more standard Au25 (SG)18 -TNA (2-TNA; SG=glutathione) composite, 1-TNA shows a much better stability under illumination in both neutral and basic conditions. The precise composition of the photosensitizers enables a direct comparison of the sensitization ability between 1 and 2. With the same cluster loading, the photocurrent produced by 1-TNA is 15 times larger than that of 2-TNA. The superior performance of 1-TNA over 2-TNA is attributed not only to the higher light absorption ability of 1 but also to the higher charge-separation efficiency. Besides, a ligand effect on the stability of the photoelectrode and charge-transfer between the NCs and the semiconductor is revealed. This work paves the way to study the role of metal nanoclusters as photosensitizers at the atomic level, which is essential for the design of better material for light energy conversion.

Modulation of the expression of genes related to the system of amyloid-beta metabolism in the brain as a novel mechanism of ceftriaxone neuroprotective properties.

BACKGROUND: The dominant hypothesis about the pathogenesis of Alzheimer's disease (AD) is the "amyloid cascade" concept and modulating the expression of proteins involved in the metabolism of amyloid-beta (Aß) is proposed as an effective strategy for the prevention and therapy of AD. Recently, we found that an antibiotic ceftriaxone (CEF), which possesses neuroprotective activity, reduced cognitive deficits and neurodegenerative changes in OXYS rats, a model of sporadic AD. The molecular mechanisms of this effect are not completely clear, we suggested that the drug might serve as the regulator of the expression of the genes involved in the metabolism of Aß and the pathogenesis of AD. The study was aimed to determine the effects of CEF on mRNA levels of Bace1 (encoding ß-secretase BACE1 involved in Aß production), Mme, Ide, Ece1, Ace2 (encoding enzymes involved in Aß degradation), Epo (encoding erythropoietin related to endothelial function and clearance of Aß across the blood brain barrier) in the frontal cortex, hippocampus, striatum, hypothalamus, and amygdala of OXYS and Wistar (control strain) male rats. Starting from the age of 14 weeks, animals received CEF (100 mg/kg/day, i.p., 36 days) or saline. mRNA levels were evaluated with RT-qPCR method. Biochemical parameters of plasma were measured for control of system effects of the treatment. RESULTS: To better understand strain variations studied here, we compared the gene expression between untreated OXYS and Wistar rats. This comparison showed a significant decrease in mRNA levels of Ace2 in the frontal cortex and hypothalamus, and of Actb in the amygdala of untreated OXYS rats. Analysis of potential effects of CEF revealed its novel targets. In the compound-treated OXYS cohort, CEF diminished mRNA levels of Bace1 and Ace2 in the hypothalamus, and Aktb in the frontal cortex. Furthermore, CEF augmented Mme, Ide, and Epo mRNA levels in the amygdala as well as the levels of Ece1 and Aktb in the striatum. Finally, CEF also attenuated the activity of ALT and AST in plasma of OXYS rats. CONCLUSION: Those findings disclosed novel targets for CEF action that might be involved into neuroprotective mechanisms at early, pre-plaque stages of AD-like pathology development.

Targeted sequencing reveals complex, phenotype-correlated genotypes in cystic fibrosis.

BACKGROUND: Cystic fibrosis (CF) is one of the most common life-threatening genetic disorders. Around 2000 variants in the CFTR gene have been identified, with some proportion known to be pathogenic and 300 disease-causing mutations have been characterized in detail by CFTR2 database, which complicates its analysis with conventional methods. METHODS: We conducted next-generation sequencing (NGS) in a cohort of 89 adult patients negative for p.Phe508del homozygosity. Complete clinical and demographic information were available for 84 patients. RESULTS: By combining MLPA with NGS, we identified disease-causing alleles in all the CF patients. Importantly, in 10% of cases, standard bioinformatics pipelines were inefficient in identifying causative mutations. Class IV-V mutations were observed in 38 (45%) cases, predominantly ones with pancreatic sufficient CF disease; rest of the patients had Class I-III mutations. Diabetes was seen only in patients homozygous for class I-III mutations. We found that 12% of the patients were heterozygous for more than two pathogenic CFTR mutations. Two patients were observed with p.[Arg1070Gln, Ser466*] complex allele which was associated with milder pulmonary obstructions (FVC 107 and 109% versus 67%, CI 95%: 63-72%; FEV 90 and 111% versus 47%, CI 95%: 37-48%). For the first time p.[Phe508del, Leu467Phe] complex allele was reported, observed in four patients (5%). CONCLUSION: NGS can be a more information-gaining technology compared to standard methods. Combined with its equivalent diagnostic performance, it can therefore be implemented in the clinical practice, although careful validation is still required.

Tuning Stilbene Photochemistry by Fluorination: State Reordering Leads to Sudden Polarization near the Franck-Condon Region.

Spontaneous polarization of a nonpolar molecule upon photoexcitation (the sudden polarization effect) earlier discussed for 90°-twisted alkenes is observed and calculated for planar ring-fluorinated stilbenes, trans-2,3,5,6,2',3',5',6'-octofluorostilbene (tF2356) and trans-2,3,4,5,6,2',3',4',5',6'-decafluorostilbene (tF23456). Due to the fluorination, Franck-Condon states S1FC and S2FC are dominated by the quasi-degenerate HOMO-1 → LUMO and HOMO-2 → LUMO excitations, while their interaction gives rise to a symmetry-broken zwitterionic S1 state. After optical excitation of tF2356, one observes an ultrafast (∼0.06 ps) evolution that reflects relaxation from initial nonpolar S3FC to long-lived (1.3 ns in n-hexane and 3.4 ns in acetonitrile) polar S1. The polarity of S1 is evidenced by a solvatochromic shift of its fluorescence band. The experimental results provide a sensitive test for quantum-chemical calculations. In particular, our calculations agree with the experiment, and raise concerns about the applicability of the common TDDFT approach to relatively simple stilbenic systems.

Effect of a Tertiary Butyl Group on Polar Solvation Dynamics in Aqueous Solution: Femtosecond Fluorescence Spectroscopy.

We monitor the time-dependent Stokes shift (TDSS) of fluorescence from the zwitterionic probe N-methyl-6-oxyquinolinium betaine in water. A spectral relaxation time τsolv = 0.57 ps (at 20.5 °C) is attributed to a solvation process involving water in the hydration layer. In this article we show that a tertiary butyl group, when attached to the chromophore, slows the dynamics to τsolv = 0.76 ps and increases the corresponding activation energy by 5 kJ/mol. In a companion paper (10.1021/acs.jpcb.7b05039), simulations suggest that the observed slow-down indicates coupling of solute vibrations to hydration water. Thus, a new angle on a thoroughly researched topic, solvation dynamics, has been opened.

A facility to search for hidden particles at the CERN SPS: the SHiP physics case.

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, [Formula: see text] and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

Perpendicular State of an Electronically Excited Stilbene: Observation by Femtosecond-Stimulated Raman Spectroscopy.

In the photoisomerization path of stilbene, a perpendicular state P on the S1 potential energy surface is expected just before internal conversion through a conical intersection S1/S0. For decades the observation of P was thwarted by a short lifetime τP in combination with slow population flow over a barrier. But these limitations can be overcome by ethylenic substitution. Following optical excitation of trans-1,1'-dicyanostilbene, P is populated significantly (τP = 27 ps in n-hexane) and monitored by an exited-state absorption band at 370 nm. Here we report stimulated Raman lines of P. The strongest, at 1558 cm-1, is attributed to stretching vibrations of the phenyl rings. Transient electronic states, resonance conditions, and corresponding Raman signals are discussed.

Sensitized Two-NIR-Photon Z→E Isomerization of a Visible-Light-Addressable Bistable Azobenzene Derivative.

Two-NIR-photon-triggered Z→E isomerization of an azobenzene was accomplished by covalently linking a two-photon-harvesting triarylamine antenna to a thermally stable ortho-fluorinated azobenzene derivative. The obtained photoswitch is fully addressable with visible and NIR light by using one-photon and two-photon excitation, respectively, with the latter offering enhanced penetration depth and improved spatial resolution.

Two-State Intramolecular Charge Transfer (ICT) with 3,5-Dimethyl-4-(dimethylamino)benzonitrile (MMD) and Its Meta-Isomer mMMD. Ground State Amino Twist Not Essential for ICT.

From X-ray structure analysis, amino twist angles of 90.0° for 2,4-dimethyl-3-(dimethylamino)benzonitrile (mMMD), 82.7° for 4-(di-tert-butylamino)benzonitrile (DTABN), and 88.7° for 6-cyanobenzoquinuclidine (CBQ) are determined, all considerably larger than the 57.4° of 3,5-dimethyl-4-(dimethylamino)benzonitrile (MMD). This large twist leads to lengthening of the amino-phenyl bond, 143.5 pm (mMMD), 144.1 pm (DTABN), 144.6 pm (CBQ), and 141.4 pm (MMD), as compared with 136.5 pm for the planar 4-(dimethylamino)benzonitrile (DMABN). As a consequence, the electronic coupling between the amino and phenyl subgroups in mMMD, DTABN, CBQ, and MMD is much weaker than in DMABN, as seen from the strongly reduced molar absorption coefficients. The fluorescence spectrum of MMD in n-hexane at 25 °C consists of two emissions, from a locally excited (LE) and an intramolecular charge transfer (ICT) state, with a fluorescence quantum yield ratio Φ'(ICT)/Φ(LE) of 12.8. In MeCN, a single ICT emission is found. With mMMD in n-hexane, in contrast, only LE fluorescence is observed, whereas the spectrum in MeCN originates from the ICT state. These differences are also seen from the half-widths of the overall fluorescence bands, which in n-hexane are larger for MMD than for mMMD, decreasing with solvent polarity for MMD and increasing for mMMD, reflecting the disappearance of LE and the onset of ICT in the overall spectra, respectively. From solvatochromic measurements the dipole moments µe(ICT) of MMD (16 D) and mMMD (15 D) are obtained. Femtosecond excited state absorption (ESA) spectra at 22 °C, together with the dual (LE + ICT) fluorescence, reveal that MMD in n-hexane undergoes a reversible LE ⇄ ICT reaction, with LE as the precursor, with a forward rate constant ka = 5.6 × 10(12) s(-1) and a back-reaction kd ∼ 0.05 × 10(12) s(-1). With MMD in the strongly polar solvent MeCN, ICT is faster: ka = 10 × 10(12) s(-1). In the case of mMMD in n-hexane, the ESA spectra show that ICT does not take place, contrary to MeCN, in which ka = 2.5 × 10(12) s(-1). The ICT reactions with MMD and mMMD are much faster than that of the parent compound DMABN in MeCN, with ka = 0.24 × 10(12) s(-1). Because of the very short ICT reaction times of 180 fs (MMD, n-hexane), 100 fs (MMD, MeCN), and 400 fs (mMMD, MeCN), it is clear that the picosecond fluorescence decays of these systems appear to be single exponential, due to the insufficient time resolution of 3 ps. It is concluded that the faster LE → ICT reaction of MMD as compared with DMABN (ka = 0.24 × 10(12) s(-1) in MeCN) is caused by a smaller energy gap ΔE(S1,S2) between the lowest singlet excited states and not by the large amino twist angle. Similarly, the larger ΔE(S1,S2) of mMMD as compared with MMD is held responsible for its smaller ICT efficiency (no reaction in n-hexane).

ß-Carotene Revisited by Transient Absorption and Stimulated Raman Spectroscopy.

ß-Carotene in n-hexane was examined by femtosecond transient absorption and stimulated Raman spectroscopy. Electronic change is separated from vibrational relaxation with the help of band integrals. Overlaid on the decay of S1 excited-state absorption, a picosecond process is found that is absent when the C9 -methyl group is replaced by ethyl or isopropyl. It is attributed to reorganization on the S1 potential energy surface, involving dihedral angles between C6 and C9 . In Raman studies, electronic states S2 or S1 were selected through resonance conditions. We observe a broad vibrational band at 1770 cm(-1) in S2 already. With 200 fs it decays and transforms into the well-known S1 Raman line for an asymmetric C=C stretching mode. Low-frequency activity (<800 cm(-1) ) in S2 and S1 is also seen. A dependence of solvent lines on solute dynamics implies intermolecular coupling between ß-carotene and nearby n-hexane molecules.