Through-space Coupling Research Articles

Site-selective dynamics of ligand-free and ligand-bound azidolysozyme.

Azido-modified alanine residues (AlaN3) are environment-sensitive, minimally invasive infrared probes for the site-specific investigation of protein structure and dynamics. Here, the capability of the label is investigated to query whether or not a ligand is bound to the active site of lysozyme and how the spectroscopy and dynamics change upon ligand binding. The results demonstrate specific differences for center frequencies of the asymmetric azide stretch vibration, the longtime decay, and the static offset of the frequency fluctuation correlation function (FFCF)-all of which are experimental observables-between the ligand-free and the ligand-bound N3-labeled protein. The center-frequency shifts range from 1 to 8 cm-1, which is detectable from state-of-the art experiments. Similarly, the nonvanishing static component Δ0 of the FFCF between ligand-free and ligand-bound protein can differ by up to a factor of 2.5. This makes the azide label a versatile and structurally sensitive probe to report on the dynamics of proteins in a variety of environments and for a range of different applications. Ligand-induced differences in the dynamics are also mapped onto changes in the local and through-space coupling between residues by virtue of dynamical cross correlation maps. This demonstrates that the position where the label is placed also influences the local and global protein motions.

Interrupted intramolecular donor-acceptor interaction compensated by strong through-space electronic coupling for highly efficient near-infrared TADF with emission over 800 nm

Near-infrared (NIR) organic emitters with emission wavelength from 700 to 900 nm show promising application in phototherapy, bioimaging and night vision products. The traditional fluorescent compounds composed of strong donor (D) and acceptor (A) units can readily achieve NIR emission via the extensive conjugation over the molecular skeleton. However, the development of NIR thermally activated delayed fluorescence (TADF) is still an unaddressed issue mainly due to the limited conjugation between D and A within TADF molecules. Here, the strategy is to introduce strong D/A fragments into the molecular skeleton and to incorporate the strong through-space electronic coupling between TADF molecules simultaneously. With this strategy, we demonstrate an unprecedented NIR TADF material (AQTC-DTPA) with emission over 800 nm. TADF OLEDs based on AQTC-DTPA show the record-high maximum External quantum efficiencies (EQE) of 0.51%/0.41%/0.30%/0.23% with Electroluminescence (EL) peak wavelength at 810/828/852/894 nm, respectively.

Purely Organic Room-Temperature Phosphorescence Endowing Fast Intersystem Crossing from Through-Space Spin-Orbit Coupling.

Purely organic room-temperature phosphorescence endowing very fast intersystem crossing from through-space systems has not been well investigated. Here we report three space-confined bridged phosphors, where phenothiazine is linked with dibenzothiophene, dibenzofuran, and carbazole by a 9,9-dimethylxanthene bridge. Nearly pure phosphorescence is observed in the crystals at room temperature. Interestingly, phosphorescence comes solely from the phenothiazine segment. Experimental results indicate that bridged counterparts of dibenzothiophene, dibenzofuran, and carbazole contribute as close-lying triplet states with locally excited (LE) character. The through-space spin–orbit coupling principle is proposed in these bridged systems, as their 1LE and 3LE states have intrinsic spatial overlap, degenerate energy levels, and tilting face-to-face alignment. The resulting effective through-space spin–orbit coupling leads to efficient intersystem crossing a with rate constant as high as 109 s–1 and an overwhelming triplet decay channel of the singlet excited state.

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

Interplay between Intermolecular and Intramolecular Singlet Fission in Thin Films of a Covalently Linked Terrylenediimide Dimer

Covalent chromophore dimers having the required energetics can undergo intramolecular singlet fission (SF) in solution; however, in the solid state, intra- and intermolecular SF can compete. Here, the structure and excited-state dynamics of a linear terrylene-3,4:11,12-bis(dicarboximide) (TDI) dimer, TDI-Ph-TDI, in which the two TDI molecules are linked via one of their imide nitrogen atoms to a 2,5-di-t-butylphenyl spacer at its 1,4-positions are studied in solution and in thin films to understand the interplay between these two SF mechanisms. TDI-Ph-TDI undergoes slow intramolecular SF in toluene due to weak through-bond interactions and a lack of through-space electronic coupling. The TDI-Ph-TDI dimers in the films are π-stacked, allowing for through-space interactions between neighboring TDI moieties. As a result, TDI-Ph-TDI undergoes intermolecular SF two orders of magnitude faster than intramolecular SF. Using film-processing techniques, the SF dynamics can be modified. The (T1T1) states in the unannealed and thermally annealed films dissociate to form free triplet excitons, whereas a long-lived (T1T1) state with mixed charge-transfer character is observed in a chlorobenzene solvent vapor annealed film. Although intramolecular SF in TDI-Ph-TDI cannot compete with intermolecular SF, the structure of TDI-Ph-TDI has a strong influence on the possible film morphologies and the role of the charge-transfer state in SF.

A Self-Assembled Homochiral Radical Cage with Paramagnetic Behaviors.

Condensation of an inherently C3 -symmetric polychlorotriphenylmethyl (PTM) radical trisaldehyde with tris(2-aminoethyl)amine (TREN) yields a [4+4] tetrahedral radical cage as a racemic pair of homochiral enantiomers in 75 % isolated yield. The structure was characterized by X-ray crystallography, confirming the homochirality of each cage framework. The homochirality results from intramolecular [CH⋅⋅⋅π] and hydrogen-bonding interactions within the cage framework. The four PTM radicals in a cage undergo weak through-space coupling. Magnetic measurements demonstrated that each cage bears 3.58 spins.

Conformational Preference of 2'-Fluoro-Substituted Acetophenone Derivatives Revealed by Through-Space 1H-19F and 13C-19F Spin-Spin Couplings.

The conformational properties of 2′-fluoro-substituted acetophenone derivatives were elucidated based on Hα–F and Cα–F through-space spin–spin couplings (TS-couplings), which occur between two atoms constrained at a distance smaller than the sum of their van der Waals radii. This study revealed that 2′-fluoro-substituted acetophenone derivatives in solutions form exclusively s-trans conformers by analyzing their NMR spectra focused on the TS-couplings. The magnitudes of the coupling constants 5J (Hα, F) and 4J (Cα, F) correlate linearly with the value of the dielectric constant of the solvents. Furthermore, s-trans conformations of the two derivatives were confirmed by X-ray crystallographic analysis. These conformational preferences were consistent with the DFT calculations. The s-cis conformer, in which fluorine and oxygen atoms lie in a syn-periplanar mode, may be subject to strong repulsion between the two polar atoms and become unstable. The s-trans preference of the 2′-fluoro-substituted acetophenone derivatives may be utilized in drug design.

Magnetic Coupling in a Tris-hydroxo-Bridged Chromium Dimer Occurs through Ligand Mediated Superexchange in Conjunction with Through-Space Coupling.

Quantum chemistry can explain and predict many properties of molecules and materials. However, there are some systems, among which are magnetic systems, that remain a challenge for all electronic structure methods. The tris-hydroxo-bridged chromium dimer, known as Kremer's dimer, contains two antiferromagnetically coupled chromium(III) metal ions and provides a classic example of a magnetic molecular system that poses a challenge for computational quantum chemistry. In this study we show that combining multiconfiguration pair-density functional theory with large-active-space density matrix renormalization group wave functions can predict the correct spin-state ordering, energy spacing, and magnetic coupling constant. We also use the unpaired electron density to analyze the superexchange contribution. This methodology offers promise for revolutionary rational design of molecular magnets.

Phosphorus-Bismuth Peri-Substituted Acenaphthenes: A Synthetic, Structural, and Computational Study.

A series of acenaphthene species with a diisopropylphosphino group and a variety of bismuth functionalities in the peri positions were synthesized and fully characterized, including single-crystal X-ray diffraction. The majority of the reported species feature a relatively rare interpnictogen P-Bi bond. The series includes the phosphine-bismuthine Acenap(PiPr2)(BiPh2) (2; Acenap = acenaphthene-5,6-diyl), which was subjected to a fluorodearylation reaction to produce Acenap(PiPr2)(BiPhX) (5-8 and 10; X = BF4-, Cl, Br, I, SPh), displaying varying degrees of ionicity. The geminally bis(acenaphthyl)-substituted [Acenap(PiPr2)]2BiPh (3) shows a large through-space coupling of 17.8 Hz, formally 8TSJPP. Coupling deformation density calculations confirm the double through-space coupling pathway, in which the P and Bi lone pairs mediate communication between the two 31P nuclei. Several synthetic routes toward the phosphine-diiodobismuthine Acenap(PiPr2)(BiI2) (9) have been investigated; however, the purity of this, surprisingly thermally stable potential synthon, remains poor.

Analysis of Vibrational Circular Dichroism Spectra of Peptides: A Generalized Coupled Oscillator Approach of a Small Peptide Model Using VCDtools.

Vibrational circular dichroism (VCD) is one of the major spectroscopic tools to study peptides. Nevertheless, a full understanding of what determines the signs and intensities of VCD bands of these compounds in the amide I and amide II spectral regions is still far from complete. In the present work, we study the origin of these VCD signals using the general coupled oscillator (GCO) analysis, a novel approach that has recently been developed. We apply this approach to the ForValNHMe model peptide in both α-helix and β-sheet configurations. We show that the intense VCD signals observed in the amide I and amide II spectral regions essentially have the same underlying mechanism, namely, the through-space coupling of electric dipoles. The crucial role played by intramolecular hydrogen bonds in determining VCD intensities is also illustrated. Moreover, we find that the contributions to the rotational strengths, considered to be insignificant in standard VCD models, may have sizable magnitudes and can thus not always be neglected. In addition, the VCD robustness of the amide I and II modes has been investigated by monitoring the variation of the rotational strength and its contributing terms during linear transit scans and by performing calculations with different computational parameters. From these studies—and in particular, the decomposition of the rotational strength made possible by the GCO analysis—it becomes clear that one should be cautious when employing measures of robustness as proposed previously.

Synthesis and properties of chiral amide-bonded porphyrin dimers with various functional bridging blocks

Eight porphyrin dimers with various functional bridging blocks and chiral amide-bonds were synthesized and characterized. An analysis of the spectroscopy and electrochemistry has been carried out to demonstrate that the chiral properties can be modified by changing the interchromophoric through-space coupling distance between the two porphyrin chromophores by introducing various bonding and bridging blocks.

Photoisomerization of Enediynyl Linker Leads to Slipped Cofacial Hydroporphyrin Dyads with Strong Through-Bond and Through-Space Electronic Interactions

Photoisomerization of 3,4-di(methoxycarbonyl)-enediyne linker in hydroporphyrin (chlorin or bacteriochlorin) dyads leads to thermally stable cis isomers, where macrocycles adopt a slipped cofacial mutual geometry with an edge-to-edge distance of ∼3.6 Å (determined by density functional theory (DFT) calculations). Absorption spectra exhibit a significant splitting of the long-wavelength Qy band, which indicates a strong electronic coupling with a strength of V = ∼477 cm-1 that increases to 725 cm-1 upon metalation of hydroporphyrins. Each dyad features a broad, structureless emission band, with large Stokes shift, which is indicative of excimer formation. DFT calculations for dyads show both strong through-bond electronic coupling and through-space electronic interactions, due to the overlap of π-orbitals. Overall, geometry, electronic structure, strength of electronic interactions, and optical properties of reported dyads closely resemble those observed for photosynthetic special pairs. Dyads reported here represent a novel type of photoactive arrays with various modes of electronic interactions between chromophores. Combining through-bond and through-space coupling appears to be a viable strategy to engineer novel optical and photochemical properties in organic conjugated materials.

Weak Pnictogen Bond with Bismuth: Experimental Evidence Based on Bi-P Through-Space Coupling.

To study pnictogen bonding involving bismuth, flexible accordion‐like molecular complexes of the composition [P(C6H4‐o‐CH2SCH3)3BiX3], (X=Cl, Br, I) have been synthesised and characterised. The strength of the weak and mainly electrostatic interaction between the Bi and P centres strongly depends on the character of the halogen substituent on bismuth, which is confirmed by single‐crystal X‐ray diffraction analyses, DFT and ab initio computations. Significantly, 209Bi–31P through‐space coupling (J=2560 Hz) is observed in solid‐state 31P NMR spectra, which is so far unprecedented in the literature, delivering direct information on the magnitude of this pnictogen interaction.

Electronic Coupling and Electron Transfer between Two Mo2 Units through meta- and para-Phenylene Bridges.

A series of three Mo2 dimers bridged by a meta-phenylene group has been studied in terms of electronic coupling (EC) and electron transfer (ET) in comparison with the para isomers. Optical analyses on the mixed-valence complexes indicate that by replacing a para-phenylene bridge with a meta one, the EC between the two Mo2 centers is dramatically weakened; consequently, the ET rates (ket ) are lowered by two to three orders of magnitude. In the para series, the EC parameters (Hab ) and ET rates (ket ) are greatly affected by O/S atomic alternation of the bridging ligand. However, for the meta analogues, similar EC and ET parameters are obtained, that is, Hab =300-400 cm-1 and ket ≈109 s-1 . These results suggest that through-σ-bond and/or through-space coupling channels become operative as the π conjugation is disabled. DFT calculations reveal that destructive quantum interference features seen for the meta series arise from the cancellation of two π-conjugated coupling pathways.

Through-space spin–spin coupling constants involving fluorine: benchmarking DFT functionals

ABSTRACTThrough-space spin–spin coupling constants (SSCCs) involving fluorine are computed applying Density Functional Theory and compared with experimental data to benchmark the performance of various functionals. In addition to the most often analysed J(FF) constants, we consider examples of J(FN), J(FP), J(FC) and J(FSe) constants. Basis sets optimised for the study of SSCCs are applied and thus we find the choice of the functional to be more important than the choice of the basis set. Different performance of DFT functionals is observed for different SSCCs, with the hybrid DFT functionals generally superior for the through-space couplings. When all the SSCCs are considered, PBE0 appears to be the most robust functional.

Probing through-space and through-bond magnetic exchange couplings in a new benzotriazinyl radical and its metal complexes.

The synthesis and characterization of a new chelating benzotriazinyl radical (Rad2) are described. Crystallographic studies coupled with SQUID magnetometry on Rad2 reveal the presence of discrete radical pairs which are antiferromagnetically coupled. The reaction of Rad2 with the 3d transition metal complexes M(hfac)2·xH2O (hfac- = hexafluoroacetylacetonate) led to mononuclear metal complexes of general formula M(hfac)2(Rad2) [M = Zn(ii) (1); Ni(ii) (2) and Co(ii) (3)] whose structures have been determined by single crystal X-ray diffraction. Compounds 1-3 are isostructural and crystallize in the monoclinic space group P21/n with two molecules in the asymmetric unit. In the case of the Zn(ii) complex (1) through-space intermolecular radicalradical antiferromagnetic exchange interactions viaπ*π* contacts are observed, whereas strong intramolecular through-bond metal-radical ferromagnetic interactions [J = +59.3(9) cm-1] are observed for the Ni(ii) complex (2). For the Co(ii) complex (3), computational and magnetic studies reveal substantial zero field splitting and ferromagnetic metal-radical interactions.

Substituent Effects on the Reactivity of the 2,4,6-Tridehydropyridinium Cation, an Aromatic σ,σ,σ-Triradical.

2,4,6-Tridehydropyridinium cation (7) undergoes three consecutive atom or atom group abstractions from reagent molecules in the gas phase. By placing a π-electron-donating hydroxyl group between two radical sites, their reactivity can be quenched by enhancing their through-space coupling via a favorable resonance structure. Indeed, 3-hydroxy-2,4,6-tridehydropyridinium cation (8) abstracts only one atom or group of atoms from reagents. On the other hand, an electron-withdrawing cyano group between two of the radical sites (9) destabilizes the analogous resonance structure and diminishes through-space coupling between the radical sites, resulting in abstraction of three atoms, just like 7. However, the cyano-substituent also increases acidity to the point that 9 reacts pre-dominantly via proton transfer instead of undergoing radical reactions. Therefore, acidic triradicals may undergo nonradical, barrierless proton transfer reactions faster than radical reactions, which are usually accompanied by barriers. Examination of the analogous cyano-substituted mono-and biradicals revealed behavior similar to that of the corresponding unsubstituted species, with the exception of substantially greater reactivities due to their greater (calculated) vertical electron affinities. Finally, the 3-cyano-2,6-didehydropyridinium cation with a singlet ground state (S-T splitting: -11.9 kcal mol-1) was found to react exclusively from the lowest-energy triplet state by fast proton transfer reactions.

Enantiopure Phospha[1]ferrocenophanes: Textbook Examples of Through-Space Nuclear Spin-Spin Coupling.

Three enantiopure phospha[1]ferrocenophanes (2R ) equipped with either a phenyl, an isopropyl, or a tert-butyl group at the bridging phosphorus atom were synthesized by a salt-metathesis approach in isolated yields between 52 and 63 %. The chirality in these strained sandwich compounds stems from the planar-chiral ferrocene moiety, which is symmetrically equipped with two iPr groups adjacent to phosphorus. Surprisingly, all three phospha[1]ferrocenophanes show an uncommon through-space nuclear 1 H-31 P coupling. As a result of the embedded symmetry, these new compounds are ideal examples to differentiate between through-space and through-bond coupling mechanisms in NMR spectroscopy.

Dihydrogen contacts observed by through-space indirect NMR coupling.

"Through-space" indirect spin-spin couplings between hydrogen atoms formally separated by up to 18 covalent bonds have been detected by NMR experiments in model helical molecules. It is demonstrated that this coupling can provide crucial structural information on the molecular conformation in solution. The coupling pathways have been visualised and analysed by computational methods. The conformational dependence of the coupling is explained in terms of orbital interactions.

Lower trifluoromethyl[70]fullerene derivatives: novel structural data and an survey of electronic properties

We revisit a series of trifluoromethylated C70(CF3)n compounds with n=2–12 to provide a comprehensive comparative characterization thereof by means of a combination of physico-chemical methods. X-ray diffraction studies, including first structural determinations of the major isomers of C70(CF3)2, C70(CF3)4, and the third most abundant isomer of C70(CF3)8 as co-crystals with octaethylporphyrin Ni(II), produce accurate CF3 group rotation angles and intergroup F⋯F distances that enable more reliable 19F NMR signal assignment based on the through-space JFF spin-spin coupling relation to the F⋯F distances. Cyclic voltammetry measurements were performed in a potential range that covers oxidation on the one end and reduction to tri- or tetraanionic states on the other. It was demonstrated that the CF3 addition patterns have marked effects on the redox potentials. A consistent set of the experimental HOMO energy values of the C70(CF3)n molecules is reported for the first time in complement to the LUMO data, and the both sets of results were found to demonstrate good correlation with quantum-chemical DFT predictions. Most of the compounds exhibit electrochemically reversible one-electron reductions up to trianionic (sometimes even tetraanionic) state, a notable exception being C70(CF3)10 with irreversibility of its very first reduction. Bulk electrolysis of C70(CF3)12 at its 4-th reduction potential identifies CF3 detachment as the most likely cause of the observed irreversible processes yielding C70(CF3)11−anions as was identified by means of MALDI MS.