Nature of partial sigma bond.

This study investigates the formation of partial sigma (σ) covalent bonds in experimentally synthesizable biradicals formed from hydrogenated and fluorinated C8, C20, and C60 cage structures, by assessing their stability, geometry, and bonding character in singlet and triplet states using restricted B3LYP-D3/6-31+G(d,p) theory, natural bond orbital (NBO) analysis, and complete active space self-consistent field (CASSCF) method. The results show that these partial σCC bonds have Wiberg bond orders of 0.38 to 0.48 and bond lengths ranging from 2.62 Å to 5.93 Å. Cage size influences the characteristics of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), with electrons favoring more antibonding orbitals in smaller cages where electrons reside more on the exterior of the cage and favoring bonding orbitals in larger ones where electrons are more in the interior. Fluorination enhances electron density on bonding orbitals. The analysis further clarified that the differentiation between antibonding and bonding features of HOMOs and LUMOs extends beyond merely electron transfer from s- to p-atomic orbitals, also noting possible interactions of the same symmetry repel. The study also introduces hyperconjugation from α-position CH bonds as a factor in stabilizing partial σ-bond formation. The results also caution against the use of broken symmetry methodology in unrestricted SCF wavefunctions for biradicals, such as those in this study as it may cause large spin contamination and thus errors in the calculated electronic properties results.

Rational Design of a Phosphorus-Centered Disbiradical.

Phosphorus-centered disbiradicals, in which the radical sites exist as individual spin doublets with weak spin-spin interaction have not been known so far. Starting from monoradicals of the type [⋅P(µ-NTer)2 P-R], we have now succeeded in linking two such monoradical phosphorus centers by appropriate choice of a linker. To this end, biradical [⋅P(µ-NTer)2 P⋅] (1) was treated with 1,6-dibromohexane, affording the brominated species {Br[P(µ-NTer)]2 }2 C6 H12 (3). Subsequent reduction with KC8 led to the formation of the disbiradical {⋅[P(µ-NTer)]2 }2 C6 H12 (4) featuring a large distance between the radical phosphorus sites in the solid state and formally the highest biradical character observed in a P-centered biradical so far, approaching 100 %. EPR spectroscopy revealed a three-line signal in solution with a considerably larger exchange interaction than would be expected from the molecular structure of the single crystal. Quantum chemical calculations revealed a highly dynamic conformational space; thus, the two radical sites can approach each other with a much smaller distance in solution. Further reduction of 4 resulted in the formation of a potassium salt featuring the first structurally characterized P-centered distonic radical anion (5- ). Moreover, 4 could be used in small molecule activation.

Visible-Light-Mediated Activation of Remote C(sp3)-H Bonds by Carbon-Centered Biradical via Intramolecular 1,5- or 1,6-Hydrogen Atom Transfer.

In this study, we introduce a novel intramolecular hydrogen atom transfer (HAT) reaction that efficiently yields azetidine, oxetane, and indoline derivatives through a mechanism resembling the carbon analogue of the Norrish-Yang reaction. This process is facilitated by excited triplet-state carbon-centered biradicals, enabling the 1,5-HAT reaction by suppressing the critical 1,4-biradical intermediates from undergoing the Norrish Type II cleavage reaction, and pioneering unprecedented 1,6-HAT reactions initiated by excited triplet-state alkenes. We demonstrate the synthetic utility and compatibility of this method across various functional groups, validated through scope evaluation, large-scale synthesis, and derivatization. Our findings are supported by control experiments, deuterium labeling, kinetic studies, cyclic voltammetry, Stern-Volmer experiments, and density functional theory (DFT) calculations.

Photoisomerization of "Partially Embedded Dihydropyridazine" with a Helical Structure.

Herein, we report the synthesis of two "partially embedded fused-dihydropyridazine N-aryl aza[5]helicene derivatives" (PDHs) and the demonstration of their intrinsic photo-triggered multi-functional properties based on a Kekulé biradical structure. Introducing bulky electron-withdrawing trifluoromethyl or pentafluoroethyl groups into the aza[5]helicene framework (PDH-CF3 and -C2 F5 ) gives PDH axial chirality based on the helicity of the P and M forms, even at room temperature. Upon photo-irradiation of PDH-CF3 in a frozen solution, an ESR signal from the triplet biradical with zero-field splitting values, generated by N-N bond dissociation, was observed. However, when the irradiation was turned off, the ESR signal became silent, thus indicating the existence of two equilibria: between the biradical and quinoidal forms based on the Kekulé structure, and between N-N bond cleavage and recombination. The observed photo- and thermally induced behaviors indicate that T-type photochromic molecules are involved in the photoisomerization mechanism involving the two equilibria. Inspired by the photoisomerization, chirality control of PDH by photoracemization was achieved. Multiple functionalities, such as T-type photochromism, photo-excitation-mediated triplet biradical formation, and photoracemization, which are attributed to the "partially embedded dihydropyridazine" structure, are demonstrated.

Enhancing the Magnetization Blocking Energy of Biradical-Metal System by Merging Discrete Complexes into One-Dimensional Chains.

The reaction of nitronyl nitroxide biradical NITPhMeImbis [5-(2-methylimidazole)-1,3-bis(1-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)-benzene] with Ln(hfac)3 ⋅ 2H2 O and Cu(hfac)2 (hfac=hexafluoroacetylacetonate), led to two series of 2p-3d-4f complexes, namely, nona-spin clusters, [Ln2 Cu3 (hfac)12 (NITPhMeImbis)2 ] (Ln=Gd 1, Dy 2), or one-dimensional chains [LnCu2 (hfac)7 (NITPhMeImbis)] (Ln=Y 3, Dy 4, Tb 5) depending on the temperature of the reaction. All five complexes contain a biradical-Ln unit in which the biradical chelates the LnIII ion by the means of one aminoxyl (i. e. NO) group of each NIT unit. For the discrete complexes, a Cu(hfac)2 links two biradical-Ln units via one of the remaining NO groups, while for the chain compounds, the two remaining NO groups of the biradical-Ln moiety are each coordinated to a Cu(hfac)2 unit to form a 1D coordination polymer. Moreover, a terminal Cu(hfac)2 unit is coordinated to the imidazole-N atom of the NITPhMeImbis ligand. Spin dynamics investigations evidenced the onset of slow relaxation of the magnetization for 2, whereas 4 and 5 exhibit a typical single-chain magnet behavior. This highlights the vital role of the 1D spin correlation in the blocking of the magnetization. These results illustrate that from the same basic building blocks, magnetic relaxation can be carefully modulated by structural adjustments.

Inorganic Bergman Cyclization: An Appeal From Theory.

The scope of Bergman cyclization is expanded computationally by exploring the cyclization in inorganic B, N substituted derivative. This substitution has introduced polarity into the transition state, which resulted in dramatic lowering of the activation barrier. Natural charge distribution throughout the reaction profile has ascertained this hypothesis. Single B and N atom substitution at 1 and 6 terminal positions lowers the activation barrier by almost half of the original value which becomes even lower in the complete B, N analogue. The parent Bergman and single B, N substituted products were characterized by significant biradical character while the complete B, N substituted analogue was characterized by significant zwitterionic character. Reduction in electron delocalization is also observed in the complete B, N substituted analogue.

Photoinduced Processes in Lysine-Tryptophan-Lysine Tripeptide with L and D Tryptophan.

Optical isomers of short peptide Lysine-Tryptophan-Lysine (Lys-{L/D-Trp}-Lys) and Lys-Trp-Lys with an acetate counter-ion were used to study photoinduced intramolecular and intermolecular processes of interest in photobiology. A comparison of L- and D-amino acid reactivity is also the focus of scientists' attention in various specialties because today, the presence of amyloid proteins with D-amino acids in the human brain is considered one of the leading causes of Alzheimer's disease. Since aggregated amyloids, mainly Aß42, are highly disordered peptides that cannot be studied with traditional NMR and X-ray techniques, it is trending to explore the reasons for differences between L- and D-amino acids using short peptides, as in our article. Using NMR, chemically induced dynamic nuclear polarization (CIDNP) and fluorescence techniques allowed us to detect the influence of tryptophan (Trp) optical configuration on the peptides fluorescence quantum yields, bimolecular quenching rates of Trp excited state, and the photocleavage products formation. Thus, compared with the D-analog, the L-isomer shows a greater Trp excited state quenching efficiency with the electron transfer (ET) mechanism. There are experimental confirmations of the hypothesis about photoinduced ET between Trp and the CONH peptide bond, as well as between Trp and another amide group.

Cyclic [Cu-biRadical]2 Secondary Building Unit in 2p-3d and 2p-3d-4f Complexes: Crystal Structure and Magnetic Properties.

Employing the new nitronyl nitroxide biradical ligand biNIT-3Py-5-Ph (2-(5-phenyl-3-pyridyl)-bis(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide)), a 16-spin Cu-radical complex, [Cu8(biNIT-3Py-5-Ph)4(hfac)16] 1, and three 2p-3d-4f chain complexes, {[Ln(hfac)3][Cu(hfac)2]2(biNIT-3Py-5-Ph)2}n (LnⅢ= Gd 2, Tb 3, Dy 4; hfac = hexafluoroacetylacetonate), have been prepared and characterized. X-ray crystallographic analysis revealed in all derivatives a common cyclic [Cu-biNIT]2 secondary building unit in which two bi-NIT-3Py-5-Ph biradical ligands and two CuII ions are associated via the pyridine N atoms and NO units. For complex 1, two such units assemble with four additional CuII ions to form a discrete complex involving 16 S = 1/2 spin centers. For complexes 2-4, the [Cu-biNIT]2 units are linked by LnIII ions via NO groups in a 1D coordination polymer. Magnetic studies show that the coordination of the aminoxyl groups with Cu or Ln ions results in behaviors combining ferromagnetic and antiferromagnetic interactions. No slow magnetic relaxation behavior was observed for Tb and Dy derivatives.

A Triplet/Singlet Ground-State Switch via the Steric Inhibition of Conjugation in 4,6-Bis(trifluoromethyl)-1,3-phenylene Bisnitroxide.

Ground triplet 4,6-bis(trifluoromethyl)-1,3-phenylene bis(tert-butyl nitroxide) (TF2PBN) reacted with [Y(hfac)3(H2O)2] (hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dionate), affording a doubly hydrogen-bonded adduct [Y(hfac)3(H2O)2(TF2PBN)]. The biradical was recovered from the adduct through recrystallization. Crystallographic analysis indicates that the torsion angles (|θ| ≤ 90°) between the benzene ring and nitroxide groups were 74.9 and 84.8° in the adduct, which are larger than those of the starting material TF2PBN. Steric congestion due to o-trifluoromethyl groups gives rise to the reduction of π-conjugation. Two hydrogen bonds enhance this deformation. Susceptometry of the adduct indicates a ground singlet with 2J/kB = -128(2) K, where 2J corresponds to the singlet-triplet gap. The observed magneto-structure relation is qualitatively consistent with Rajca's pioneering work. A density functional theory calculation at the UB3LYP/6-311+G(2d,p) level using the atomic coordinates determined provided a result of 2J/kB = -162.3 K for the adduct, whilst the corresponding calculation on intact TF2PBN provided +87.2 K. After a comparison among a few known compounds, the 2J vs. |θ| plot shows a negative slope with a critical torsion of 65(3)°. The ferro- and antiferromagnetic coupling contributions are balanced in TF2PBN, being responsible for ground-state interconversion by means of small structural perturbation like hydrogen bonds.

Influence of the Coordinated Transition Metal Ion on Magnetic Relaxation of Lanthanide Based Complexes with Imino Nitroxide Biradical Ligands.

In spite of achievement of a lot of Ln-radical SMMs, how to improve magnetic behavior of Ln-radical system remains challenging. Here, two series of Ln-radical complexes have successfully been built using an imino nitroxide biradical, namely, [Ln2 (hfac)6 (ImPhPyobis)2 ] (LnIII =Gd 1, Tb 2, Dy 3) and [Ln2 Cu2 (hfac)10 (ImPhPyobis)2 ] (LnIII =Gd 4, Dy 5; hfac=hexafluoroacetylacetonate and ImPhPyobis=5-(4-oxypyridinium-1-yl)-1,3-bis(1'-oxyl-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). For these biradical-metal complexes, two imino nitroxide biradicals bind two Ln(III) ions via their oxygen atoms coming from 4-oxypyridinium units to produce a binuclear {Ln2 O2 } unit. Those imino nitroxide groups are free for complexes 1-3, however one of imino nitroxide groups of the biradical is ligated to the copper(II) ion for complexes 4 and 5. The distinct magnetic relaxation behaviors are observed for two Dy derivatives, as revealed by ac magnetic studies: complex 3 presents one magnetic process with the effective energy barrier(Ueff ) of 74.0 K while complex 5 exhibits dual relaxation processes with Ueff values for the fast- and slow-relaxation being 20.2 K and 30.9 K, respectively, which implies that the second coordination sphere of Dy ion plays a critical role for magnetic relaxation.

A Mesoionic Diselenolene Anion and the Corresponding Radical Dianion.

Dichalcogenolenes are archetypal redox non-innocent ligands with numerous applications. Herein, a diselenolene ligand with fundamentally different electronic properties is described. A mesoionic diselenolene was prepared by selenation of a C2-protected imidazolium salt. This ligand is diamagnetic, which is in contrast to the paramagnetic nature of standard dichalcogenolene monoanions. The new ligand is also redox-active, as demonstrated by isolation of a stable diselenolene radical dianion. The unique electronic properties of the new ligand give rise to unusual coordination chemistry. Thus, preparation of a hexacoordinate aluminum tris(diselenolene) complex and a Lewis acidic aluminate complex with two ligand-centered unpaired electrons was achieved.

Excitation wavelength- and intensity-dependent stepwise two-photon-induced photochromic reaction.

The photochromic molecules showing wavelength-selective or light intensity-dependent photoresponse are receiving increased attention in recent years. Although a photoswitch with a single chromophore can control the ON and OFF states of a function, that consisting of multi-chromophores would be useful for the specific control in complex systems. Herein, we designed stepwise two-photon induced photochromic molecules (PABI-PIC and PABI-PIC2) consisting of two different photochromic units (PABI and PIC). One-photon absorption reaction in the UV light region of PABI-PIC generates the short-lived transient biradical (BR) that absorbs an additional photon in the visible and UV light region in a stepwise manner to produce the two-photon photochemical product, the quinoidal species (Quinoid). The photochromic properties of these transient species are completely different in color and fading speed. In addition, PABI-PIC also shows the excitation wavelength-dependent photochromism because the excited states of the PABI and PIC units are electronically orthogonal. Therefore, the stepwise photochromic properties of PABI-PIC are easily controlled depending on the excitation light intensity and wavelength. These molecular designs are important for the development of advanced photoresponsive materials.

Synthesis, Crystal Structure and Magnetic Properties of a Trinuclear Copper(II) Complex Based on P-Cresol-Substituted Bis(α-Nitronyl Nitroxide) Biradical.

Trinuclear copper(II) complex [CuII3(NIT2PhO)2Cl4] was synthesized with p-cresol-substituted bis(α-nitronyl nitroxide) biradical: 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (NIT2PhOH). The crystal structure of this heterospin complex was determined using single-crystal X-ray diffraction analysis and exhibits four unusual seven-membered metallocycles formed from the coordination of oxygen atoms of the N-O groups and of bridging phenoxo (µ-PhO-) moieties with copper(II) ions. The crystal structure analysis reveals an incipient agostic interaction between a square planar copper center and a hydrogen-carbon bond from one methyl group carried on the coordinated nitronyl-nitroxide radical. The intramolecular Cu∙∙∙H-C interaction involves a six-membered metallocycle and may stabilize the copper center in square planar coordination mode. From the magnetic susceptibility measurements, the complex, which totals seven S = 1/2 spin carriers, has almost a ground state spin S = 1/2 at room temperature ascribed to strong antiferromagnetic interaction between the nitronyl nitroxide moieties and the copper(II) centers and in between the copper(II) centers through the bridging phenoxo oxygen atom.

Effects of Spiro-Cyclohexane Substitution of Nitroxyl Biradicals on Dynamic Nuclear Polarization.

Spiro-substituted nitroxyl biradicals are widely used as reagents for dynamic nuclear polarization (DNP), which is especially important for biopolymer research. The main criterion for their applicability as polarizing agents is the value of the spin-spin exchange interaction parameter (J), which can vary considerably when different couplers are employed that link the radical moieties. This paper describes a study on biradicals, with a ferrocene-1,1'-diyl-substituted 1,3-diazetidine-2,4-diimine coupler, that have never been used before as DNP agents. We observed a substantial difference in the temperature dependence between Electron Paramagnetic Resonance (EPR) spectra of biradicals carrying either methyl or spirocyclohexane substituents and explain the difference using Density Functional Theory (DFT) calculation results. It was shown that the replacement of methyl groups by spirocycles near the N-O group leads to an increase in the contribution of conformers having J ≈ 0. The DNP gain observed for the biradicals with methyl substituents is three times higher than that for the spiro-substituted nitroxyl biradicals and is inversely proportional to the contribution of biradicals manifesting the negligible exchange interaction. The effects of nucleophiles and substituents in the nitroxide biradicals on the ring-opening reaction of 1,3-diazetidine and the influence of the ring opening on the exchange interaction were also investigated. It was found that in contrast to the methyl-substituted nitroxide biradical (where we observed the ring-opening reaction upon the addition of amines), the ring opening does not occur in the spiro-substituted biradical owing to a steric barrier created by the bulky cyclohexyl substituents.

Triggering near-infrared luminescence of vanadyl phthalocyanine by charging.

Probing electrofluorochromism (EFC) at the molecular level remains challenging. Here we study the strongly charge state-dependent photoluminescence of vanadyl phthalocyanine. We report vibrationally resolved absorption and laser-induced fluorescence (LIF) spectra of samples comprising both the mass-selected neutral molecule (VOPc⋅, a stable radical) and its cation produced upon electron ionization (EI) isolated in 5 K neon matrices. Ionization of the essentially non-emissive VOPc⋅ forms a high-spin diradical cation (VOPc+.. ) which shows profound photoluminescence (PL) in the NIR range. This unique phenomenon is potentially of interest for NIR-emitting electro-optic devices.

Singlet Biradical Versus Triplet Biradical/Zwitterion Characteristics in Isomers of C6 -C5 -C6 -C7 -C6 -Fused Pentacyclic Aromatic Hydrocarbons Revealed through Reactivity Patterns.

Among various polycyclic aromatic hydrocarbons, C6 -C5 -C6 -C7 -C6 fused pentacyclic aromatic hydrocarbons have the unique potential to adopt quinonoid, zwitterion, singlet, or triplet biradical electronic configurations. Two such hybrid structures between pentacene and azulene were synthesized and their ground state electronic configurations were deduced from the reactivity patterns they exhibit respectively. Compound 6, where the radicaloid carbons are linked through a para-phenylene, forms a head-to-head dimer like a singlet biradical. In contrast, isomer 7, where the para-linkage was switched to meta, reacts readily with oxygen which resembles the reactivity of a triplet state. The oxidized intermediate(s) then undergoes rearrangement to furnish the C6 -C5 -C6 -C6 -C6 ring contraction product 13. Cation 14, the protonated form of 7, was synthesized, which implies 7 also reacts like a zwitterion. It was revealed the oxidative rearrangement takes place even with mesityl dibenzotropylium cation despite its perceived aromaticity. DFT calculations confirm the most stable forms of 6 and 7 are singlet and triplet diradical, which is consistent with the observed reactivity of respective molecules.

Reduction and Rearrangement of a Boron(I) Carbonyl Complex.

The one-electron reduction of a cyclic (alkyl)(amino)carbene (CAAC)-stabilized arylborylene carbonyl complex yields a dimeric borylketyl radical anion, resulting from an intramolecular aryl migration to the CO carbon atom. Computational analyses support the existence of a [(CAAC)B(CO)Ar].- radical anion intermediate. Further reduction leads to a highly nucleophilic dianionic (boraneylidene)methanolate.

Dicyclopentaannelated Hexa-peri-hexabenzocoronenes with a Singlet Biradical Ground State.

Synthesis of two dicyclopentaannelated hexa-peri-hexabenzocoronene (PHBC) regioisomers was carried out, using nonplanar oligoaryl precursors with fluorenyl groups: mPHBC 8 with two pentagons in the "meta"-configuration was obtained as a stable molecule, while its structural isomer with the "para"-configuration, pPHBC 16, could be generated and characterized only in situ due to its high chemical reactivity. Both PHBCs exhibit low energy gaps, as reflected by UV-vis-NIR absorption and electrochemical measurements. They also show open-shell singlet ground states according to electron paramagnetic resonance (EPR) measurements and density functional theory (DFT) calculations. The use of fully benzenoid HBC as a bridging moiety leads to significant singlet biradical characters (y0 ) of 0.72 and 0.96 for mPHBC 8 and pPHBC 16, respectively, due to the strong rearomatization tendency of the HBC π-system; these values are among the highest for planar carbon-centered biradical molecules. The incorporation of fully unsaturated pentagons strongly perturbs the aromaticity of the parent HBC and makes the constituted benzene rings less aromatic or antiaromatic. These results illustrate the high impact of cyclopentaannelation on the electronic structures of fully benzenoid polycyclic aromatic hydrocarbons (PAHs) and open up a new avenue towards open-shell PAHs with prominent singlet biradical characters.

Highly Oxidized States of Phthalocyaninato Terbium(III) Multiple-Decker Complexes Showing Structural Deformations, Biradical Properties and Decreases in Magnetic Anisotropy.

Presented here is a comprehensive study of highly oxidized multiple-decker complexes composed of TbIII and CdII ions and two to five phthalocyaninato ligands, which are stabilized by electron-donating n-butoxy groups. From X-ray structural analyses, all the complexes become axially compressed upon ligand oxidation, resulting in bowl-shaped distortions of the ligands. In addition, unusual coexistence of square antiprism and square prism geometries around metal ions was observed in +4e charged species. From paramagnetic 1 H NMR studies on the resulting series of triple, quadruple and quintuple-decker complexes, ligand oxidation leads to a decrease in the magnetic anisotropy, as predicted from theoretical calculations. Unusual paramagnetic shifts were observed in the spectra of the +2e charged quadruple and quintuple-decker complexes, indicating that those two species are actually unexpected triplet biradicals. Magnetic measurements revealed that the series of complexes show single-molecule magnet properties, which are controlled by the multi-step redox induced structural changes.

Highly Conjugated π-Systems Arising from Cannibalistic Hexadehydro-Diels-Alder Couplings: Cleavage of C-C Single and Triple Bonds.

We have investigated the cannibalistic self-trapping reaction of an ortho-benzyne derivative generated from 1,11-bis(p-tolyl)undeca-1,3,8,10-tetrayne in an HDDA reaction. Without adding any specific trapping agent, the highly reactive benzyne is trapped by another bisdiyne molecule in at least three different modes. We have isolated and characterized the resulting products and performed high-level calculations concerning the reaction mechanism. During the cannibalistic self-trapping process, either a C≡C triple bond or an sp-sp3 C-C single bond is cleaved. Up to seven rings and nine C-C bonds are formed starting from two 1,11-bis(p-tolyl)undeca-1,3,8,10-tetrayne molecules. Our experiments and calculations provide considerable insight into the variety of reaction pathways which the ortho-benzyne derivative, generated from a bisdiyne, can take when reacting with another bisdiyne molecule.