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The aim of this study was to investigate the transition from non-covalent reversible over covalent reversible to covalent irreversible inhibition of cysteine proteases by making delicate structural changes to the warhead scaffold. To this end, dipeptidic rhodesain inhibitors with different N-terminal electrophilic arenes as warheads relying on the SNAr mechanism were synthesized and investigated. Strong structure–activity relationships of the inhibition potency, the degree of covalency, and the reversibility of binding on the arene substitution pattern were found. The studies were complemented and substantiated by molecular docking and quantum-mechanical calculations of model systems. Furthermore, the improvement in the membrane permeability of peptide esters in comparison to their corresponding carboxylic acids was exemplified. [ABSTRACT FROM AUTHOR]

Radicals of the lightest group 13 element, boron, are well established and observed in numerous forms. In contrast to boron, radical chemistry involving the heavier group 13 elements (aluminum, gallium, indium, and thallium) remains largely underexplored, primarily attributed to the formidable synthetic challenges associated with these elements. Herein, we report the synthesis and isolation of planar and twisted conformers of a doubly CAAC (cyclic alkyl(amino)carbene)‐radical‐substituted dialane. Extensive characterization through spectroscopic analyses and X‐ray crystallography confirms their identity, while quantum chemical calculations support their open‐shell nature and provide further insights into their electronic structures. The dialane‐connected diradicals exhibit high susceptibility to oxidation, as evidenced by electrochemical measurements and reactions with o‐chloranil and a variety of organic azides. This study opens a previously uncharted class of dialuminum systems to study, broadening the scope of diradical chemistry and its potential applications. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We investigate the quantum and classical wave packet dynamics in an harmonic oscillator that is perturbed by a disorder potential. This perturbation causes the dispersion of a Gaussian wave packet, which is reflected in the coordinate-space and the momentum-space Shannon entropies, the latter being a measure for the amount of information available on a system. Regarding the sum of the two quantities, one arrives at an entropy that is related to the coordinate–momentum uncertainty. Whereas in the harmonic case, this entropy is strictly periodic and can be evaluated analytically, this behavior is lost if disorder is added. There, at selected times, the quantum mechanical probability density resembles that of a classical oscillator distribution function, and the entropy assumes larger values. However, at later times and dependent on the degree of disorder and the chosen initial conditions, quantum mechanical revivals occur. Then, the observed effects are reversed, and the entropy may decrease close to its initial value. This effect cannot be found classically. [ABSTRACT FROM AUTHOR]

A novel trimeric perylenediimide (PDI) macrocycle was synthesized, and its intramolecular electronic couplings were investigated by bulk and single-molecule optical spectroscopy and by various theoretical approaches. In polarization-resolved excitation spectroscopy at 1.2 K in a PMMA matrix, the appearance and disappearance of the three zero-phonon lines (ZPLs) of an individual trimer by changing the polarization in steps of 60° nicely reflect an approximate triangular geometry of the macrocycle and indicate localized excitations that are transferred by incoherent hopping processes at time scales of around 1 ps as inferred from the ZPL linewidths. The electronic coupling strength deduced from the low temperature data is found to be in good agreement with theoretical estimates. Bulk spectroscopy in toluene at room temperature indicates that the excitations are also localized under these conditions. Theory reveals that the reasons for the localized nature of the excitations at room and low temperatures are different. For a rigid macrocycle, the excitations are predicted to be delocalized, but molecular dynamics simulations point to considerable structural flexibility at ambient temperatures, which counteracts excitation delocalization. At 1.2 K in a PMMA matrix, this effect is too small to lead to localization. Yet, supported by simple model calculations, the disorder in the PMMA host induces sufficient differences between the PDI chromophores, which again result in localized excitations. By addressing crucial aspects of excitation energy transfer, our combined approach provides a detailed and quantitative account of the interchromophore communication in a trimeric macrocycle. [ABSTRACT FROM AUTHOR]

The efficient population of the triplet excited states in heavy metal-free organic chromophores has been one of the long-standing research problems to molecular photochemists. The negligible spin–orbit coupling matrix elements in the purely organic chromophores and the large singlet–triplet energy gap (ΔES–T) pose a hurdle for ultrafast intersystem crossing (ISC). Herein we report the unprecedented population of triplet manifold in a series of nitrogen-annulated perylene bisimide chromophores (NPBI and Br-NPBI). NPBI is found to have a moderate fluorescence quantum yield (Φf = 68 ± 5%), whereas Br-NPBI showcased a low fluorescence quantum yield (Φf = 2.0 ± 0.6%) in toluene. The femtosecond transient absorption measurements of Br-NPBI revealed ultrafast ISC (kISC = 1.97 × 1010 s−1) from the initially populated singlet excited state to the long-lived triplet excited states. The triplet quantum yields (ΦT = 95.2 ± 4.6% for Br-NPBI, ΦT = 18.7 ± 2.3% for NPBI) calculated from nanosecond transient absorption spectroscopy measurements showed the enhancement in triplet population upon bromine substitution. The quantum chemical calculations revealed the explicit role of nitrogen annulation in tuning the excited state energy levels to favor the ISC. The near degeneracy between the singlet and triplet excited states observed in NPBI and Br-NPBI (ΔES–T = −0.01 eV for NPBI, ΔES–T = 0.03 eV for Br-NPBI) facilitates the spin flipping in the molecules. Nitrogen annulation emerges as a design strategy to open up the ISC pathway and the rate of which can be further enhanced by the substitution of a heavier element. [ABSTRACT FROM AUTHOR]

Perylene‐based organic semiconductors are widely used in organic electronic devices. Here, we studied the ultrafast excited state dynamics after optical excitation at interfaces between the electron donor (D) diindenoperylene (DIP) and the electron acceptor (A) dicyano‐perylene‐bis(dicarboximide) (PDIR‐CN2) using femtosecond time‐resolved second harmonic generation (SHG) in combination with large scale quantum chemical calculations. Thereby, we varied in bilayer structures of DIP and PDIR‐CN2 the interfacial molecular geometry. For an interfacial configuration which contains a edge‐on geometry but also additional face‐on domains an optically induced charge transfer (CT) is observed, which leads to a pronounced increase of the SHG signal intensity due to electric field induced second harmonic generation. The interfacial CT state decays within 7.5±0.7 ps, while the creation of hot CT states leads to a faster decay (5.3±0.2 ps). For the bilayer structures with mainly edge‐on geometries interfacial CT formation is suppressed since π‐π overlap perpendicular to the interface is missing. Our combined experimental and theoretical study provides important insights into D/A charge transfer properties, which is needed for the understanding of the interfacial photophysics of these molecules. [ABSTRACT FROM AUTHOR]

We report a study on the fragmentation of core-ionized and core-excited isocyanic acid, HNCO, using Auger-electron/photoion coincidence spectroscopy. Site-selectivity is observed both for normal and resonant Auger electron decay. Oxygen 1s ionization leads to the CO+ + NH+ ion pairs, while nitrogen 1s ionization results in three-body dissociation and an efficient fragmentation of the H–N bond in the dication. Upon 1s → 10a′ resonant excitation, clear differences between O and N sites are discernible as well. In both cases, the correlation between the dissociation channel and the binding energy of the normal Auger electrons indicates that the fragmentation pattern is governed by the excess energy available in the final ionic state. High-level multireference calculations suggest pathways to the formation of the fragment ions NO+ and HCO+, which are observed although the parent compound contains neither N–O nor H–C bonds. This work contributes to the goal to achieve and understand site-selective fragmentation upon ionization and excitation of molecules with soft x-ray radiation. [ABSTRACT FROM AUTHOR]

Covalent peptidomimetic protease inhibitors have gained a lot of attention in drug development in recent years. They are designed to covalently bind the catalytically active amino acids through electrophilic groups called warheads. Covalent inhibition has an advantage in terms of pharmacodynamic properties but can also bear toxicity risks due to non-selective off-target protein binding. Therefore, the right combination of a reactive warhead with a well-suited peptidomimetic sequence is of great importance. Herein, the selectivities of well-known warheads combined with peptidomimetic sequences suited for five different proteases were investigated, highlighting the impact of both structure parts (warhead and peptidomimetic sequence) for affinity and selectivity. Molecular docking gave insights into the predicted binding modes of the inhibitors inside the binding pockets of the different enzymes. Moreover, the warheads were investigated by NMR and LC-MS reactivity assays against serine/threonine and cysteine nucleophile models, as well as by quantum mechanics simulations. [ABSTRACT FROM AUTHOR]

The development of ligands capable of effectively stabilizing highly reactive main‐group species has led to the experimental realization of a variety of systems with fascinating properties. In this work, we computationally investigate the electronic, structural, energetic, and bonding features of proximity‐enforced group 13–15 homodimers stabilized by a rigid expanded pincer ligand based on the 1,8‐naphthyridine (napy) core. We show that the redox‐active naphthyridine diimine (NDI) ligand enables a wide variety of structural motifs and element‐element interaction modes, the latter ranging from isolated, element‐centered lone pairs (e.g., E = Si, Ge) to cases where through‐space π bonds (E = Pb), element‐element multiple bonds (E = P, As) and biradical ground states (E = N) are observed. Our results hint at the feasibility of NDI‐E2 species as viable synthetic targets, highlighting the versatility and potential applications of napy‐based ligands in main‐group chemistry. [ABSTRACT FROM AUTHOR]

A naphthyridine diimine (NDI) supported bis(germylene) NDI-Ge2 containing two dicoordinate, coplanar Ge(II) atoms has been synthesised. Computational investigations on NDI-Ge2 indicated the two Ge(II) atoms are nearly independent. The EDA-NOCV analysis of the [NDI-Ge2][Fe2(CO)6] complex revealed the six-electron donor behavior of NDI-Ge2, the first example for group-14-element-based bidentate ligands. [ABSTRACT FROM AUTHOR]

The reaction of a cyclic alkyl(amino)carbene (CAAC)‐stabilized thiazaborolo[5,4‐d]thiazaborole (TzbTzb) with strong Brønsted acids, such as HCl, HOTf (Tf=O2SCF3) and [H(OEt2)2][BArF4] (ArF=3,5‐(CF3)2C6H3), results in the protonation of both TzbTzb nitrogen atoms. In each case X‐ray crystallographic data show coordination of the counteranions (Cl−, OTf−, BArF4−) or solvent molecules (OEt2) to the doubly protonated fused heterocycle via hydrogen‐bonding interactions, the strength of which strongly influences the 1H NMR shift of the NH protons, enabling tuning of both the visible (yellow to red) and fluorescence (green to red) colors of these salts. DFT calculations reveal that the hydrogen bonding of the counteranion or solvent to the protonated nitrogen centers affects the intramolecular TzbTzb‐to‐CAAC charge transfer character involved in the S0→S1 transition, ultimately enabling fine‐tuning of their absorption and emission spectral features. [ABSTRACT FROM AUTHOR]

The stability of N-alkoxypyridine-2(1H)thione as precursors of photochemical alkoxy radicals is enhanced by the blue shift of the broad absorption band at 360 nm. This is achieved by the substitution at the pyridinethione heterocycle.

Femtosecond time-resolved photoionization and photoelectron spectroscopy are used to investigate the excited-state dynamics of the tert-butyl radical, t-[C.sub.4][H.sub.9]. Ab initio calculations of the electronic ground state and a number of electronically excited states are performed in order to understand the shapes and characters of the excited-state potential energy surfaces.

A synthesis for the unsubstituted bicyclo[1.1.0]but-2-ylmethanols from 1,3-butadiene is developed. Solvolyses of their sulfonates took entirely different courses, as the endo compound gave rise exclusively to rearranged products such as cyclopent-3-en-1-ol, while the exo compound underwent only the substitution of the tosylate group with complete retention of the exco-bicyclo[1.1.0]but-2-ylmethyl skeleton.

The accuracy of theoretical approaches to describe electronic absorption spectra of N-(hydroxy)- and N-(methoxy)- derivatives of pyridine-2(1H)-thione and thiazole-2(3H)-thione are examined. Its aim is to identify methods that are applicable for a rational design of new photochemically active oxyl radical precursors.

Accurate association constants of xanthosine and inosine dimers are determined by concentration dependent H NMR experiments, and a structural characterization of individual complexes formed in solution is performed through measurements under slow exchange conditions at very low temperatures. The influence of polar medium on the bonding properties is also estimated and the importance of the various effects is discussed.

An intermolecular hydrogen bonding network establishes the X-ray structure analysis of the aziridinyl peptide EtO-Azi-Gly-OBn indicating the linear conformation adopted in the crystalline state. However, it is concluded that the intramolecular interactions in the crystal are stronger than the intermolecular ones, which for a single molecule would lead to a bent structure.

The Moore (C(super 2)-C(super 7)) cyclization and its regio alternative C(super 2)-C(super 6) cyclization of enyne-ketenes are studied. Results of this experiment are compared to biradical cyclization of enyne-allenes to study similarities.

The six-membered cyclic allenes, cyclohexa-1,2,4-triene, 1-oxacyclohexa-2,3,5-triene (3delta(super 2)pyran) are studied theoretically by means of an MR-Cl approach. In the complete active space SCF computations, an eight electron in eight orbital active space ([8,8]-CAS) was used for all species, and for planar structures which possess C(sub s) symmetry, it consists of [4,4] space for A' and a [4,4] space for A'' in case of monocycles and [2,2] space for A' and a [6,6] space for A'' in case of benzannulated systems.

The isolation of simple, fundamentally important, and highly reactive organometallic compounds remains among the most challenging tasks in synthetic chemistry. The detailed characterization of such compounds is key to the discovery of novel bonding scenarios and reactivity. The dimethylbismuth cation, [BiMe2(SbF6)] (1), has been isolated and characterized. Its reaction with BiMe3 gives access to an unprecedented dative bond, a Bi→Bi donor–acceptor interaction. The exchange of methyl groups (arguably the simplest hydrocarbon moiety) between different metal atoms is among the most principal types of reactions in organometallic chemistry. The reaction of 1 with BiMe3 enables an SE2(back)‐type methyl exchange, which is, for the first time, investigated in detail for isolable, (pseudo‐)homoleptic main‐group compounds. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We report the unique heterobimetallic dodecanuclear oxamate-based {Co II6 Cu II6 } nanowheel obtained using an environmentally friendly synthetic protocol. The effective Hamiltonian methodology employed herein allows the rationalisation of magnetic isotropic or anisotropic metal clusters, being a significant advance for future studies of exciting properties only observed at low and ultralow temperatures. [ABSTRACT FROM AUTHOR]

The development of complexes featuring low‐valent, multiply bonded metal centers is an exciting field with several potential applications. In this work, we describe the design principles and extensive computational investigation of new organometallic platforms featuring the elusive manganese‐manganese bond stabilized by experimentally realized N‐heterocyclic carbenes (NHCs). By using DFT computations benchmarked against multireference calculations, as well as MO‐ and VB‐based bonding analyses, we could disentangle the various electronic and structural effects contributing to the thermodynamic and kinetic stability, as well as the experimental feasibility, of the systems. In particular, we explored the nature of the metal‐carbene interaction and the role of the ancillary η6 coordination to the generation of Mn2 systems featuring ultrashort metal‐metal bonds, closed‐shell singlet multiplicities, and positive adiabatic singlet‐triplet gaps. Our analysis identifies two distinct classes of viable synthetic targets, whose electrostructural properties are thoroughly investigated. [ABSTRACT FROM AUTHOR]

We present a general method for analyzing the character of singly excited states in terms of charge transfer (CT) and locally excited (LE) configurations. The analysis is formulated for configuration interaction singles (CIS) singly excited wave functions of aggregate systems. It also approximately works for the second-order approximate coupled cluster singles and doubles and the second-order algebraic-diagrammatic construction methods [CC2 and ADC(2)]. The analysis method not only generates a weight of each character for an excited state, but also allows to define the related quasi-diabatic states and corresponding coupling matrix elements. In the character analysis approach, we divide the target system into domains and use a modified Pipek-Mezey algorithm to localize the canonical MOs on each domain, respectively. The CIS wavefunction is then transformed into the localized basis, which allows us to partition the wavefunction into LE configurations within domains and CT configuration between pairs of different domains. Quasi-diabatic states are then obtained by mixing excited states subject to the condition of maximizing the weight of one single LE or CT configuration (localization in configuration space). Different aims of such a procedure are discussed, either the construction of pure LE and CT states for analysis purposes (by including a large number of excited states) or the construction of effective models for dynamics calculations (by including a restricted number of excited states). Applications are given to LE/CT mixing in π-stacked systems, charge-recombination matrix elements in a hetero-dimer, and excitonic couplings in multi-chromophoric systems. [ABSTRACT FROM AUTHOR]

The NHC‐stabilised diboryne (B2(SIDep)2; SIDep=1,3‐bis(2,6‐diethylphenyl)imidazolin‐2‐ylidene) undergoes a high‐yielding P−P bond activation with tetraethyldiphosphine at room temperature to form a B2P2 heterocycle via a diphosphoryldiborene by 1,2‐diphosphination. The heterocycle can be oxidised to a radical cation and a dication, respectively, depending on the oxidant used and its counterion. Starting from the planar, neutral 1,3‐bis(alkylidene)‐1,3‐diborata‐2,4‐diphosphoniocyclobutane, each oxidation step leads to decreased B−B distances and loss of planarity by cationisation. X‐ray analyses in conjunction with DFT and CASSCF/NEVPT2 calculations reveal closed‐shell singlet, butterfly‐shaped structures for the NHC‐stabilised dicationic B2P2 rings, with their diradicaloid, planar‐ring isomers lying close in energy. [ABSTRACT FROM AUTHOR]

Copyright of Angewandte Chemie is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)