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"MoCl3(dme)" (dme = 1,2-dimethoxyethane) is an important precursor for midvalent molybdenum chemistry, particularly for triply Mo-Mo bonded compounds of the type Mo2X6 (X = bulky anionic ligand). However, its exact structural identity has been obscure for more than 50 years. In search of a convenient, large-scale synthesis, we have found that trans-MoCl4(Et2O)2 dissolved in dme can be cleanly reduced with dimethylphenylsilane, Me2PhSiH, to provide khaki Mo2Cl6(dme)2 in ∼90% yield. If the reduction is performed on a small scale, single crystals suitable for X-ray crystallography can be obtained. Two different crystal morphologies were identified, each belonging to the P21/n space group, but with slightly different unit cell constants. The refined structure of each form is an edge-shared bioctahedron with overall Ci symmetry and metal-metal separations on the order of 2.8 A. The bulk material is diamagnetic as determined by both the Gouy method and SQUID magnetometry. Density functional theory calculations suggest a σ2π2δ*2 ground state for the dimer with the diamagnetism arising from a singlet diradical "broken symmetry" electronic configuration. In addition to a definitive structural assignment for "MoCl3(dme)", this work highlights the utility of organosilanes as easy to handle, alternative reductants for inorganic synthesis.

This work describes the synthesis and properties of a dicyanomethylene‐substituted indolo[3,2‐b]carbazole diradical ICz‐CN. This quinoidal system dimerises almost completely to (ICz‐CN)2, which contains two long C(sp3)−C(sp3) σ‐bonds between the dicyanomethylene units. The minor open‐shell ICz‐CN component in the solid‐state mixture was identified by EPR spectroscopy. Cyclic voltammetry and UV–visible spectroelectrochemical data, as well as comparison with reference monomer ICz‐Br reveal that the nature of the one‐electron oxidation of (ICz‐CN)2 at ambient temperature and ICz‐CN at elevated temperature is very similar in all these compounds due to the prevailing localization of their HOMO on the ICz backbone. The peculiar cathodic behaviour reflects the co‐existence of (ICz‐CN)2 and ICz‐CN. The involvement of the dicyanomethylene groups stabilizes the close‐lying LUMO and LUMO+1 of (ICz‐CN)2 and especially ICz‐CN compared to ICz‐Br, resulting in a distinctive cathodic response at low overpotentials. Differently from neutral ICz‐CN, its radical anion and dianion are remarkably stable under ambient conditions. The UV/Vis(–NIR) electronic transitions in parent (ICz‐CN)2 and ICz‐CN and their different redox forms have been assigned convincingly with the aid of TD‐DFT calculations. The σ‐bond in neutral (ICz‐CN)2 is cleaved in solution and in the solid‐state upon soft external stimuli (temperature, pressure), showing a strong chromism from light yellow to blue–green. Notably, in the solid state, the monomeric diradical species is predominantly formed under high hydrostatic pressure (>1 GPa). The work at the University of Málaga was funded by the MICINN (PID2019-110305GB-I00, PID2019-104293GB-I00 and PCI2019-111825-2), Instituto de Salud Carlos III (ISCIII) (RETIC ARADYAL, RD16/0006/0012) and Junta de Andalucía (UMAFEDERJA-080, UMA-FEDERJA-007 and P09FQM-4708) projects. S.R.G. also thanks the Ayuda para la incorporación de doctores of the University of Malaga. The work at the University of Alicante was supported by the MICINN (PID2019-106114GB-I00) and Generalitat Valenciana (AICO/2018/175) projects. The work at SIOC was supported by National Natural Science Foundation of China (grants no. 21875279 and 21790362). The authors thankfully acknowledge the computer resources, technical expertise, and assistance provided by the SCBI (Supercomputing and Bioinformatics) center of the University of Malaga. The spectro-electrochemical studies (R.N.) were fully funded by Spectroelectrochemistry Reading (a spinout company of the University of Reading). M.P.-A. would like to acknowledge the support of the European Research Council (ERC) Grant “Hecate”, reference no. 695527, held by Prof. Graeme Ackland. This work has been also supported by MCIU/MINECO through the projects CTQ2013-48252-P, CTQ2015-67755-C02-01-R and PGC2018-094814-B-C21.

Density functional theory calculations with plane-wave basis sets are often used for theoretical investigations of solid materials; nevertheless, analysis techniques for open shell structures are i...

Guanine is the most susceptible base to oxidation damage induced by reactive oxygen species including singlet oxygen (1O2, 1Δg). We clarify whether the first step of guanine oxidation in B−DNA proceeds via either a zwitterionic or a diradical intermediate. The free energy profiles are calculated by means of a combined quantum mechanical and molecular mechanical (QM/MM) method coupled with the adaptive biasing force (ABF) method. To describe the open‐shell electronic structure of 1O2 correctly, the broken‐symmetry spin‐unrestricted density functional theory (BS−UDFT) with an approximate spin projection (AP) correction is applied to the QM region. We find that the effect of spin contamination on the activation and reaction free energies is up to ∼8 kcal mol−1, which is too large to be neglected. The QM(AP−ULC−BLYP)/MM‐based free energy calculations also reveal that the reaction proceeds through a diradical transition state, followed by a conversion to a zwitterionic intermediate. Our computed activation energy of 5.2 kcal mol−1 matches experimentally observed range (0∼6 kcal mol−1)., Realistic (free energy) profile: We demonstrate how singlet oxygen reacts with guanine in B−DNA by means of the spin‐projected QM(AP−ULC−BLYP)/MM free energy simulations. The present approach gives a realistic free energy profile, which agrees well with experimental findings. The rate‐limiting syn‐addition of 1O2 is found to proceed via a diradical transition state, followed by the smooth conversion from the open‐shell surface to the closed‐shell one.

New kinds of diradical rare-earth metal complexes supported by diazabutadiene (DAD) ligands, [(DAD)2LnN(TMS)2] (1; Ln = Dy, Lu; TMS = SiMe3), were synthesized and studied. They showed a new [radical-Ln-radical] alignment with distorted square-pyramidal geometry. Structural and density functional theory analysis illustrated the radical anionic nature of the ligands. Magnetic studies revealed antiferromagnetic coupling of the two radicals in 1-Lu. 1-Dy showed typical single-molecule-magnet (SMM) behavior with an effective energy barrier of 231 K, which is much higher than those of similar radical-containing SMMs. Magnetostructural analysis suggests that the anionic [N(TMS)2]- group plays a vital role in the SMM property. This study provides a new platform for further improving the performance of radical-Ln SMMs.

Sufficiently strong molecular magnets are used in small modern electronic and spintronic devices. Diradical organic magnetic molecules (OMMs) are promising options due to their lightness, flexibility, and the low energy required for their synthesis. In this field, the coupler is a dominating component, though acetylene, the shortest coupler, has not been studied for this use. In this study, we conceptualize several model systems using an acetylene link to couple different monoradicals. We simulated their magnetic strength using the broken symmetry method via density functional theory calculations. Our results show that the acetylene (–CC–) coupler gives much larger J values than the ethylene (–CC–) coupler. Acetylene coupling a phenoxyl (PO) and a nitronyl nitroxide (NN) diradical is the strongest ferromagnetic diradical OMM, and acetylene coupling a PO bisradical is the strongest anti-ferromagnetic diradical OMM. Moreover, we found that the magnetic strength is dependent on the coupler length and torsion, but is independent of the bending. Since only a very small energy barrier is needed, it is feasible to use torsion to tune a diradical OMM's magnetic strength. Our results are also beneficial for the rational design and synthesis of diradical OMMs.

Stable organic semiconductors (OSCs) with a high-spin ground-state can profoundly impact emerging technologies such as organic magnetism, spintronics, and medical imaging. Over the last decade, there has been a significant effort to design π-conjugated materials with unpaired spin centers. Here, we report new donor–acceptor (D–A) conjugated polymers comprising cyclopentadithiophene and cyclopentadiselenophene donors with benzobisthiadiazole (BBT) and iso-BBT acceptors. Density functional theory calculations show that the BBT-based polymers display a decreasing singlet–triplet energy gap with increasing oligomer chain length, with degenerate singlet and triplet states for a N = 8 repeat unit. Furthermore, a considerable distance between the unpaired electrons with a pure diradical character disrupts the π-bond covalency and localizes the unpaired spins at the polymer ends. However, replacing the BBT acceptor with iso-BBT leads to a closed-shell configuration with a low-spin ground-state and a localized spin density on the polymer cores. This study shows the significance of the judicious choice of π-conjugated scaffolds in generating low- (S = 0) and high-spin (S = 1) ground-states in the neutral form, by modulation of spin topology in extended π-conjugated D–A polymers for emergent optoelectronic applications.

An interaction of M(hfac)2 (M = Mn or Ni) with N-(bis(4,4,5,5-tetramethyl-3-oxido-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)methylene)-2-methyl-propan-2-amine oxide (a nitronyl nitroxide diradical with theC[double bond, length as m-dash]N(O)-tert-Bu coupler) was investigated under various conditions. It was found that prolongation of reaction time caused transformation of the initial diradical into new diradicals with the uniqueC[double bond, length as m-dash]N-OH coupling unit and formation of binuclear Mn(ii) and Ni(ii) complexes, which were characterized by X-ray diffraction analysis. The resulting binuclear heterospin complexes have a complicated magnetic structure with six paramagnetic centers and a number of exchange interaction channels between them, as well as between neighboring complexes. To adequately describe the magnetic properties of these complexes, high-level ab initio calculations of their electronic structure and parameters of the spin-Hamiltonian were carried out. The accuracy of the conventional broken-symmetry density functional theory approach in the calculation of the exchange interaction parameters was also verified.

Enyne-allene compounds undergo Myers-Saito cyclization at physiological temperature to generate diradical intermediates that are capable of inducing DNA damage and cell death. The high reactivity of enyne-allene however limits their promising prospect as anticancer agents due to the spontaneous cyclization during storage and delivery. Regulating the cyclization process by taking advantage of the characteristics of a tumor cellular microenvironment, such as employing a low pH value to activate the cyclization process, is thus of essential importance. In this work, a novel enediyne (EDY) system with locked carbonyl groups was specifically designed and synthesized. Unlocking the protected carbonyl groups in the presence of acid would facilitate the rearrangement of propargyl moieties into an allene group, enabling the formation of an enyne-allene structure and occurrence of Myers-Saito cyclization. The pH-dependent diradical generation and DNA-cleavage ability of the designed EDY system were confirmed by electron paramagnetic resonance analysis and DNA gel electrophoresis. A promising cytotoxicity against HeLa cells with half inhibition concentrations (IC50) as low as 1.40 μM was obtained, which was comparable to those of many commercially applied anticancer drugs. Further in vitro experiments revealed that this EDY system induced intracellular DNA damage and subsequently resulted in S-phase arrest and cytotoxicity through programmed apoptosis.

Of particular interest in radiation-induced charge transfer processes in DNA is the extent of hole localization immediately after ionization and subsequent relaxation. To address this, we considered double stranded oligomers containing guanine (G) and 8-oxoguanine (8OG), i.e., ds(5'-GGG-3') and ds(5'-G8OGG-3') in B-DNA conformation. Using DFT, we calculated a variety of properties, viz., vertical and adiabatic ionization potentials, spin density distributions in oxidized stacks, solvent and solute reorganization energies and one-electron oxidation potential (E0) in the aqueous phase. Calculations for the vertical state of the -GGG- cation radical showed that the spin was found mainly (67%) on the middle G. However, upon relaxation to the adiabatic -GGG- cation radical, the spin localized (96%) on the 5'-G, as observed in experiments. Hole localizations on the middle G and 3'-G were higher in energy by 0.5 kcal mol-1 and 0.4 kcal mol-1, respectively, than that of 5'-G. In the -G8OGG- cation radical, the spin localized only on the 8OG in both vertical and adiabatic states. The calculated vertical ionization potentials of -GGG- and -G8OGG- stacks were found to be lower than that of the vertical ionization potential of a single G in DNA. The calculated E0 values of -GGG- and -G8OGG- stacks are 1.15 and 0.90 V, respectively, which owing to stacking effects are substantially lower than the corresponding experimental E0 values of their monomers (1.49 and 1.18 V, respectively). SOMO to HOMO level switching is observed in these oxidized stacks. Consequently, our calculations predict that local double oxidations in DNA will form triplet diradical states, which are especially significant for high LET radiations.

Thermodynamically stable La@C2v(24 107)-C78, La@D3h(24 109)-C78, La@C1(22 595)-C78, Ce@D3h(24 109)-C78, Sm@C2v(24 107)-C78, and Sm@D3h(24 109)-C78 based on density functional theory and statistical thermodynamic analysis are studied in theory. C1(22 595)-C78, violating the isolated pentagon rule, is a second novel isomer stabilized by encaging a La atom. In addition, three- and two-electron transfers occur in M@C78 (M = La and Ce) and Sm@C78, respectively. Although there are two electrons transferred from Sm to C78, these electrons transferred to the surface of C2v(24 107)-C78 are unpaired. Thus, the surface of the endohedral metallofullerene, Sm@C2v(24 107)-C78, firstly displays diradical characteristics. Notably, the spin states of the two electrons transferred from Sm to D3h(24 109)-C78 are different from those on C2v(24 107)-C78, leading to different spin ground states. Furthermore, the natural bond orders and bond critical point analyses on thermodynamically stable M@C78 (M = La, Ce, and Sm) reveal that the distance between the metal and carbon atom plays an important role in the covalent interaction between the inner metal atoms and C78. Because of the strong ionic interaction, the studies on the magnetic character of M@C78 (M = La, Ce, and Sm) show that Sm@C78 is a promising candidate for single-molecule magnets with high isotropic susceptibility. The infrared spectra were simulated to facilitate further experimental study on the stable M@C78 (M = La, Ce, and Sm). We believe that this work will provide good guidance and assistance for the further study of mono-metallofullerenes and coordination compounds in both experiment and theory.

An N-heterocyclic-carbene-ligated 3-benzoborepin with a bridged structure has been synthesized by double radical trans-hydroboration of benzo[3,4]cycloundec-3-ene-1,5-diyne with an N-heterocyclic carbene borane. The thermal reaction of the NHC-ligated borepin at 150 °C gives an isolable NHC-boranorcaradiene. Experiments and density functional theory calculations support a mechanism whereby the borepin initially rearranges to a boranorcaradiene by a thermal 6π-electrocyclic reaction. This is followed by 1,5-boron shift to give a rearranged boranorcaradiene. This shift occurs with stereoinversion at boron through a transition state with open-shell diradical character. This is the first example of the isolation of a boranorcaradiene from a thermal reaction of a borepin.

We report an air-stable diradical dication of chiral D(2)-symmetric conjoined bis[5]diazahelicene with unprecedented high spin (triplet) ground state, singlet triplet energy gap, ΔE(ST) = 0.3 kcal mol(−1). The diradical dication possesses closed-shell (Kekulé) resonance forms with 16 π-electron perimeters. The diradical dication is monomeric in dibutylphthalate (DBP) matrix at low temperatures, and it has a half-life of more than two weeks at ambient conditions in the presence of excess oxidant. A barrier of ~35 kcal mol(−1) has been experimentally determined for inversion of configuration in the neutral conjoined bis[5]diazahelicene, while the inversion barriers in its radical cation and diradical dication were predicted by the DFT computations to be within a few kcal mol(−1) of that in the neutral species. Chiral HPLC resolution provides the chiral D(2)-symmetric conjoined bis[5]diazahelicene, enriched in (P,P)- or (M,M)-enantiomers. The enantiomerically enriched triplet diradical dication is configurationally stable for 48 h at room temperature, thus providing the lower limit for inversion barrier of configuration of 27 kcal mol(−1). The enantiomers of conjoined bis[5]diazahelicene and its diradical dication show strong chirooptical properties that are comparable to [6]helicene or carbon-sulfur [7]helicene, as determined by the anisotropy factors, |g| = |Δε|/ε = 0.007 at 348 nm (neutral) and |g| = 0.005 at 385 nm (diradical dication). DFT computations of the radical cation suggest that SOMO and HOMO energy levels are near-degenerate.

Triangulene is a prototypical non-Kekulé diradical known as Clar’s hydrocarbon. It is the smallest non-Kekulé graphene fragment comprised of six benzenoid rings fused in a triangular shape. In accordance with Hund’s rule, a triplet ground state is predicted for this hydrocarbon. However, because of its high reactivity, synthesis of a stable or persistent derivative of neutral triangulene to validate this prediction has been an elusive goal for almost 70 years. Herein, we report the synthesis and characterization of the first persistent derivative of triangulene, kinetically stabilized by three sterically demanding substituents installed at the most reactive positions in the centers of the zigzag edges. The triplet ground state of this hydrocarbon is unequivocally confirmed by pulse electron paramagnetic resonance spectroscopy and supported by density functional theory. The realization of a long-sought-after persistent triangulene derivative will motivate the synthesis of analogous open-shell graphene fragments as prospective building blocks of high-spin materials for the newly emerging field of organic spintronics.

Sudden polarization (SP) is one of the manifestations of electron transfer in the electronically excited states of molecules. Proposed initially to explain the unusual reactivity of photoexcited olefins, SP often occurs in the excited states of molecules possessing strongly correlated diradical ground state. Theoretical description of SP involves mixing between the singly excited and the doubly excited zwitterionic states, which makes it inaccessible with the use of the popular linear-response time-dependent density functional theory methods. In this work, an extended variant of the state-interaction state-averaged spin-restricted ensemble-referenced Kohn–Sham (SI-SA-REKS, or SSR) method is applied to study SP in a number of organic diradical systems. To this end, the analytical derivative formalism is derived and implemented for the SSR(3,2) method (see the main text for explanation of the acronym), which enables the automatic geometry optimization and obtains the relaxed density matrices as well as the electron binding energies and respective Dyson’s orbitals. Application of the new method to SP in the lowest singlet excited state of ethylene agrees with the results obtained previously with the use of multireference methods of wavefunction theory. A number of interesting manifestations of SP are observed, such as the charge transfer in photoexcited tetramethyleneethene (TME) diradical mediated by the vibrational motion and conductivity switching in the excited state of a donor–acceptor dyad placed in an external electric field.

The larger stability of phenacenes compared to their acene isomers in their ground states is attributed to the larger aromaticity of the former. To our knowledge the relative stability of acenes and phenacenes in their lowest-lying triplet states (T1) has not been discussed yet. Using unrestricted density functional theory calculations, our results show that for the smallests members of the series, acenes in their T1 states are more stable than the corresponding phenacenes. However, when the number of the rings (n) involved increases, the energy difference is reduced and for n > 12, phenacenes become more stable than acenes in their T1 states. To rationalize this trend, we analyze the aromaticity of acenes and phenacenes using a set of aromaticity descriptors. We find that in the T1 states of both acenes and phenacenes, the outer rings form aromatic Clar π-sextets. In acenes, delocalization of spin density in the central rings leads to the preferred formation of the largest antiaromatic diradical. Resonant structures in the form of antiaromatic diradical Baird π-octadectets and π-tetradectets are the major contributors, while the smaller ones, such as π-doublets and π-sextets, contribute the least. In phenacenes, structures with diradical antiaromatic Baird π-sextets in some of the central rings contribute the most. These results are relevant to understand the (anti)aromaticity of larger polycyclic aromatic hydrocarbons in their triplet states R.B-G. is grateful for the financial support of FONDECYT Postdoctorado 3210037. Powered@NLHPC. This research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02). M.S. acknowledges the financial support from the Ministerio de Economía y Competitividad of Spain (project CTQ2017-85341-P) and the Generalitat de Catalunya (project 2017SGR39)

The course of the Barton-Kellogg (BK) reaction for alkyl- and aryl-substituted substrates has been investigated at the DLPNO-CCSD(T)/def2-TZVPP//ωB97X-D/def2-TZVPP level of theory, with results compared to available experimental kinetic data. Through comparison with the unsubstituted parent system, the preference for the formation of 1,3,4-dihydrothiadiazole over the isomeric 1,2,3-dihydrothiadiazole was observed to result from reduced steric repulsion in the relevant transition-state structure. Nitrogen extrusion [retro-(3 + 2)-cycloaddition] from the intermediate dihydrothiadiazole was found to be the rate-determining step. The barrier for this process was, however, significantly lower for aromatic substrates, which is consistent with the difficulty in isolating aryl-substituted dihydrothiadiazoles. The electronic structure of the transient thiocarbonyl ylide was also investigated, highlighting the contradictory results from wave-function theory- and density functional theory-based methods. Correlation of unrestricted natural orbital eigenvalues with previous experimental models suggested that the dipole intermediates possess low diradical character and are therefore considered to be closed-shell species. Exergonic conrotatory electrocyclization of the dipole led to sterically congested thiirane products, even for very bulky systems (di-t-butyl). These results complement the recent work of Mloston et al. Finally, DLPNO-CCSD(T)//ωB97X-D was found to be a reliable method for estimating the feasibility of the BK reaction, which should assist experimentalists in the selection of viable substrates.

Understanding the properties of the coupler influencing spin-spin interactions in organic diradicals are crucial for designing of molecular spintronic systems with long-range effects. In this quest, using density functional theory (DFT) based broken symmetry method, magnetic exchange coupling (2J) between localized radical centers at a given length connected via spacers with different radicaloid character (y) is investigated. The nitroxy radical sites are coupled through prototypical decacene coupler keeping them ~ 27 Å apart. Couplers with different y values, modeled by addition of benzenoid rings in the spin confining region of decacene are used as different spacers between nitroxide diradicals. The strength of magnetic coupling between the localized spin centers is found to be correlated with the radicaloid character of polyaromatic hydrocarbon (PAH) spacers. The stronger exchange interaction is observed for the diradical with higher radicaloid character. It is further observed that the fractional occupied molecular orbitals (MOs) responsible for the open-shell nature of the coupler predominantly constitute the Frontier MOs (FMOs), while in diradicals SOMOs constitute the FMOs.

The development of stimuli responsive systems that can switch between two distinct spin states under the application of an external stimuli has always remained an illusory challenge. Here, we report a stimuli-based spin filter by utilizing photo-responsive endoperoxide (EPO) based single molecule device. The photo-irradiation on EPO triggers the homolytic cleavage of the peroxide O-O bond generating diradical intermediate centered on two O-atoms which facilitates high spin filtering efficiency when placed between gold electrodes. The broken conjugated scenario due to peroxide bridge of EPO hinders the propagation of de-Broglie waves across the molecular skeleton. While the diradical intermediate of EPO yields high conductance for one of the spin configuration. The transmission characteristics of various photoproducts along the photochemical reaction pathway of EPO are also investigated using density functional theory in combination with non-equilibrium Green’s function (NEGF-DFT) technique. We demonstrate the key role played by Quantum Interference (QI) effects in dramatic modulation of conductance arising due to different degree of conjugation along the reaction pathway of EPO.

Mechanisms and regioselectivities in 1,3-dipolar cycloadditions of diazomethane with alkenes have been investigated with density functional theory (DFT) and DLPNO-CCSD(T) computations. The concerted but asynchronous pathway is shown to be favored by at least 6 kcal/mol over the stepwise diradical pathway in all diazomethane-alkene cycloadditions. Frontier molecular orbital calculations indicate a dominant HOMOdipole → LUMOdipolarophile interaction in diazomethane cycloadditions involving electron-deficient alkenes, whereas for electron-rich alkenes, both frontier interactions contribute significantly, and the regioselectivity is governed by frontier coefficients.

Two bis(triarylamine) diradical dication salts 12+ 2[Al(ORF )4 ]- and 22+ 2[Al(ORF )4 ]- bridged with an orthogonal oriented bisanthrancene motif were synthesized. Their solid state and electronic structures were investigated by various experimental approaches in conjunction with theoretical calculations. Superconducting quantum interference device measurements and density functional theory calculations reveal that both of 12+ and 22+ feature open-shell singlet ground states with rather small singlet-triplet energy gaps; thus, they can be regarded as nearly pure diradicals (biradicals). Their diradical characters are enhanced by the orthogonal configuration of the bisanthrancene moiety which demonstrates the importance of the geometry of bridging units in turning the electronic structures of bis(triarylamine) dications.

Using the spin-unrestricted density functional theory method, we have investigated a novel class of open-shell singlet fused-ring molecules with helical scaffolds toward the development of highly efficient third-order nonlinear optical (NLO) systems. It turns out that the helical molecules possess pancake bonding with intermediate diradical character and thereby exhibit large enhancement of orientationally-averaged second hyperpolarizability γs values (third-order NLO properties at the molecular scale) as compared to their closed-shell analogues. It is also found that the asymmetric charge distribution between the cofacial radical sites located on both the terminal rings of the helical scaffold leads to further enhancement of the γ value in the intermediate open-shell character region as compared to the open-shell analogues without asymmetric charge distributions. These results indicate that the helical systems with intermediate diradical character are potential candidates for the building block of highly...

A reliable first-principles description of singlet diradical character is essential for predicting nonlinear optical and magnetic properties of molecules. As diradical and closed-shell electronic structures differ in their distribution of single, double, triple, and aromatic bonds, modeling electronic diradical character requires accurate bond-length patterns, in addition to accurate absolute bond lengths. We therefore introduce structural diradical character, which we suggest as an additional measure for comparing first-principles calculations with experimental data. We employ this measure to identify suitable exchange-correlation functionals for predicting the bond length patterns and electronic diradical character of a biscobaltocene with the potential for photoswitchable nonlinear optical activity. Out of four popular approximate exchange-correlation functionals with different exact-exchange admixtures (BP86, TPSS, B3LYP, TPSSh), the two hybrid functionals TPSSh and B3LYP perform best for diradical bond length patterns, with TPSSh being best for the organometallic validation systems and B3LYP for the organic ones (for which the D3 dispersion correction was included). Still, none of the functionals is suitable for correctly describing relative bond lengths across the range of molecules studied, so that none can be recommended for predictive studies of (potential) diradicals without reservation. © 2018 Wiley Periodicals, Inc.

This work explores possible reaction paths for the inversion of a series of trigonal pyramidal phosphorus trihalides, PF3, PCl3, PBr3, and PI3, and it especially addresses the question of whether and when the bonding of the lowest-energy species along the inversion paths should be described as a hyper-open-shell diradical. The various paths for inversion are calculated using a single-reference method within the framework of Kohn-Sham density functional theory and also with multireference wave function methods. Our calculated results using both kinds of methods show that, for all the halogens studied (F, Cl, Br, and I), the lowest-energy singlet path for the inversion occurs by the formation of a C2 v transition structure rather than a D3 h transition structure. This geometrical preference agrees with what has been inferred previously based on closed-shell singlet calculations. But in the present study, we examined not only closed-shell singlet transition states but also open-shell singlet states and triplet states for calculating stationary points and inversion paths, and for some of the phosphorus trihalides, we found that paths involving open-shell configurations are lower in energy than those restricted to closed-shell configurations. We analyzed the changes along the paths in terms of hybridization and orientation of the frontier orbitals and in terms of locally avoided crossings, and the extent of the diradical character was quantified by calculating the effective number of unpaired electrons. Even for the singlet inversion path that goes via a D3 h structure, the barrier for PF3, PCl3, and PBr3 is higher for a closed-shell singlet spin state than for the open-shell singlet configuration. Furthermore, the energy of the triplet D3 h structure is below even that of the open-shell D3 h singlet for PCl3, PBr3, and PI3. This necessitates rethinking the role of open-shell states in nominally closed-shell processes.

Some of the most promising materials for application in molecular electronics and spintronics are based on diradical chains. Herein, the proposed relation between increasing conductance with length and diradical character is revisited using ab initio methods that account for the static electron correlation effects. Electron transmission was previously obtained from restricted single determinant wavefuntions or tight-binding approximations, which are unable to account for static correlation. Broken Symmetry Unrestricted Kohn–Sham Density Functional Theory (BS-UKS-DFT) in combination with electron transport analysis based on electron deformation orbitals (EDOs) reflects an exponential decay of the electrical conductance with length. Also, other important effects such as quantum interference are correctly accounted for, leading to a decrease of the conductance as the diradical character increases. As a proof-of-concept, the electrical conductance obtained from BS-UKS-DFT and CASSCF(2,2) wavefunctions were compared in diradical graphene strips in the frame of the pseudo-π approach, obtaining very similar results.

Due to their multiconfigurational nature featuring strong electron correlation, accurate description of diradicals and diradicaloids is a challenge for quantum chemical methods. The recently developed mixed-reference spin-flip (MRSF)-TDDFT method is capable of describing the multiconfigurational electronic states of these systems while avoiding the spin-contamination pitfalls of SF-TDDFT. Here, we apply MRSF-TDDFT to study the adiabatic singlet-triplet (ST) gaps in a series of well-known diradicals and diradicaloids. On average, MRSF displays a very high prediction accuracy of the adiabatic ST gaps with the mean absolute error (MAE) amounting to 0.14 eV. In addition, MRSF is capable of accurately describing the effect of the Jahn-Teller distortion occurring in the trimethylenemethane diradical, the violation of the Hund rule in a series of the didehydrotoluene diradicals, and the potential energy surfaces of the didehydrobenzene (benzyne) diradicals. A convenient criterion for distinguishing diradicals and diradicaloids is suggested on the basis of the easily obtainable quantities. In all of these cases, which are difficult for the conventional methods of density functional theory (DFT), MRSF shows results consistent with the experiment and the high-level ab initio computations. Hence, the present study documents the reliability and accuracy of MRSF and lays out the guidelines for its application to strongly correlated molecular systems.

Polyradical character and global (anti)aromaticity are fundamental concepts that govern rational design of cyclic conjugated macromolecules for optoelectronic applications. Here, we have designed donor-acceptor (D-A) macromolecules with ring topology along with their pi-spacer derivatives, and analyzed their unique electronic properties with density functional theory (DFT). The macromolecules with n = 8 and 16 repeat D-A units display similar singlet-triplet energy gap and have closed-shell electronic configuration with no diradical character (y0 = 0). These macromolecules also display global nonaromatic character, which is supported by NICS(0), AICD, and 2D-ICSS analysis. However, the derivatives with pi-spacer (n = 8-pi) develop near pure open-shell character (y0=1) with increased singlet-triplet gap. Furthermore, these derivatives display global antiaromaticity in the singlet ground-state according to Huckel's rule, and global aromaticity in the lowest triplet state following Baird's rule, and have a very high polyradical character, which is not reported for D-A type macromolecules. The calculated absorption spectra with time-dependent DFT (TDDFT) indicates intensive light absorption in the near-infrared (NIR) region broadening to 2500 nm. These macrocycles also display a size-dependent absorption maxima when the number of repeat units increased from n = 8 to 16. Furthermore, due to the development of open-shell character in the pi-spacer derivatives, a significant red-shift in the absorbance wavelength is observed.

In the preceding paper, we examined the addition of nucleophiles to homosymmetric diradicals and showed that the reaction occurs with no symmetry restrictions or other electronic impediments. In this work, we examine the addition of nucleophiles to heterosymmetric diradicals, by using the addition of chloride to m-dehydrotoluene as a case study. Using CASPT2 and density functional theory calculations, we show that the addition of chloride to m-dehydrotoluene is predicted to be barrierless at the asymptotic limit if Cs symmetry is broken, and the reaction is allowed to proceed through a nonplanar geometry. A nonplanar cyclic allene acts as the transitioning structure between open-shell and closed-shell species for the addition of chloride, with a continuous and smooth changing of the wave function by the evolution of orbital and configuration interaction coefficients, such that there is no abrupt switch from diradical to closed-shell species along the reaction coordinate. The overall conclusion from our analysis is that both homosymmetric and heterosymmetric diradicals can undergo reaction with closed-shell reagents without a barrier, and one cannot rule out the direct addition of nucleophiles to diradicals when considering the reaction mechanism.

In the current work, the nitroxide radical groups are utilized to functionalize the nucleobases, obtaining the nucleobase diradical building blocks for magnetic DNA with significant ferromagnetic or antiferromagnetic coupling characteristics. The nitroxide functionalization strategies include introduction of nitroxide radical group to the carbon site and oxidization of the amino group in nucleobases, and the diradical-functionalized nucleobases are denoted by 2NOX, where X = A, G, T, and C bases. The density functional theory calculations reveal that these nitroxide diradicalized nucleobases are stable and have large magnetic spin coupling magnitudes. Almost all of them possess antiferromagnetic-like spin coupling characteristics with considerably large spin coupling constants [J = −671.7 (2NOA1), −463.3 (2NOA3), −370.5 (2NOG), −494.9 (2NOC1), −3265.5 (2NOT), and −2445.5 cm–1 (2NOC3)] expect for 2NOC2 and 2NOA2 which have the ferromagnetic-like spin coupling characteristics (J = 149.1 and 440.7 cm–1), res...

Deoxypodophyllotoxin is a secondary metabolite lignan possessing potent anticancer activity with potential as a precursor for known anticancer drugs, but its use is limited by scarcity from natural sources. We here report the total synthesis of racemic deoxypodophyllotoxin in seven steps using an intramolecular styryl Diels-Alder reaction strategy uniquely suited to assemble the deoxypodophyllotoxin core. Density functional theory was used to analyze concerted, polar, and singlet-open-shell diradical reaction pathways, which identified a low-energy concerted [4 + 2] Diels-Alder pathway followed by a faster di-t-butylhydroxytoluene assisted [1,3]-formal hydrogen shift.

Tunability of the open-shell character, charge asymmetry, and third-order nonlinear optical (NLO) properties of covalently linked (hetero)phenalenyl dimers are investigated by using the density functional theory method. By changing the molecular species X and substitution position (i, j) for the linker part, a variety of intermonomer distances R and relative alignments between the phenalenyl dimers can be realized from the geometry optimizations, resulting in a wide-range tuning of diradical character y and charge asymmetry. It is found that the static second hyperpolarizabilities along the stacking direction, γyyyy , are one-order enhanced for phenalenyl dimer systems exhibiting intermediate y, a feature that is in good agreement with the "y-γ correlation". By replacing the central carbon atoms of the phenalenyl rings with a boron or a nitrogen, we have also designed covalently linked heterophenalenyl dimers. The introduction of such a charge asymmetry to the open-shell systems, which leads to closed-shell ionic ground states, is found to further enhance the γyyyy values of the systems having longer intermonomer distance R with intermediate ionic character, that is, charge asymmetry. The present results demonstrate a promising potential of covalently linked NLO dimers with intermediate open-shell/ionic characters as a new building block of highly efficient NLO systems.

International audience; We propose a family of photonic materials based on nickel bis(dithiolene) [NiBDT] and benzene (B). Linear and planar oligomers of 1D and 2D polymers, together with several rich π-electron linkers (e.g. C14H8, C24H8, and C4O2H4), have been employed in order to demonstrate the superior performance of the proposed materials. A key property for the design of efficient photonic materials is the second hyperpolarizability. We have, thus, computed it together with several other properties, such as the dipole moment, polarizability, first excitation energy, etc. We have also used two indices to estimate the degree of the diradical character of the considered derivatives. The calculations have been performed by employing density functional theory. Considerable care has been taken to validate the computational procedure we have used. The main findings of the present work are as follows: (a) the 1D and 2D coordination polymers based on nickel bis(dithiolene) and benzene and in particular the linear ones, (B-NiBDT)n, are very likely to be efficient photonic materials, since they have gigantic second hyperpolarizability. (b) The second hyperpolarizability greatly depends on the organic linker with which NiBDT is fused and on the structure; for example, the linear oligomers have much higher second hyperpolarizability than the planar ones. The present work together with a recent article (A. Avramopoulos et al., J. Phys. Chem. C, 2016, 120, 9419-9435) demonstrates a mechanism for designing efficient photonic materials.

Using the spin-unrestricted density functional theory method, we investigate the effect of external static electric field application on the first and second hyperpolarizabilities (β and γ) – nonlinear optical (NLO) properties at the molecular scale – of a 1,2,3,5-dithiadiazolyl π-radical dimer. It is found that these properties drastically vary depending on the external electric field intensity, and that such drastic variation occurs with gigantically enhanced peaks around the transition region from the diradical to charge transfer state. These results qualitatively accord with the previous theoretical prediction based on the asymmetric two-site diradical model. It also turns out that the field-induced enhancement ratio of γ amplitude reaches 9 × 104 in the maximum positive peak and −3 × 105 in the maximum negative peak for the dimer with an intermolecular distance d = 4.8 A. This result demonstrates the promising potential of open-shell molecular aggregates as a novel class of highly-active external-field-driven NLO switches.

We report a thorough comparison between the spin-unrestricted semi-empirical molecular orbital (SE-UMO) and density functional theory (UDFT) calculations for a variety of π-conjugated diradical molecules which exhibit through-bond magnetic exchange interactions. The spin-unrestricted neglect of diatomic differential overlap (NDDO)-based methods (UPM6 and UrPM6) and self-consistent charge density functional tight-biding (UDFTB) are examined. The UCAM-B3LYP method is used as a representative of UDFT. We have found that NDDO-based and UDFTB methods give different performances with respect to the results of UCAM-B3LYP. The degree of diradical character in the target systems decreases in the order of UPM6 > UrPM6 ≈ UCAM-B3LYP > UDFTB. The conventional UPM6 and UDFTB calculations tend to overestimate and underestimate the diradical character, respectively, whereas UrPM6 is able to provide much the same results as UCAM-B3LYP. This is convincing evidence that UrPM6 will be useful for in silico screening of materials science at much lower computational cost.

The diradical characters (y) and third‐order nonlinear optical (NLO) properties of open‐shell quinoidal oligothiophene derivatives with phenoxyl groups, and the corresponding reduced (hydrogenated)‐state oligomers, are investigated by using the broken‐symmetry density functional theory method. The oxidized (dehydrogenated) states are predicted to have an open‐shell singlet ground state and their y values increase with the number of units. Static second hyperpolarizabilities (γ) of the open‐shell oligomers with intermediate y are shown to be enhanced significantly compared with those of the closed‐shell analogues. Furthermore, owing to the effective diradical distances, the γ values of open‐shell oligomers are found to exceed that of s‐indaceno[1,2,3‐cd;5,6,7‐c′d′]diphenalene, which is known as an organic molecule with the largest two‐photon absorption cross‐section in this size of the pure hydrocarbons. This feature extends the range of efficient open‐shell third‐order NLO materials to a novel class of one‐dimensional conjugated oligomers with redox‐based high tunability of third‐order NLO properties.

A dinuclear, Cu(I)-catalyzed reductive CO2 coupling reaction was recently developed to selectively yield a metal–oxalate product through electrochemical means, instead of the usual formation of carbonate and CO (Science 2010, 327, 313). To shed light on the mechanism of this important and unusual reductive coupling reaction, extensive and systematic density functional theory (DFT) calculations on several possible pathways and spin states were performed in which a realistic system up to 164 atoms was adopted. Our calculations support the observation that oxalate formation is energetically more favorable than the formation of carbonate and CO products in this cationic Cu(I) complex. Spatial confinement of the realistic catalyst (a long metal–metal distance) was found to further destabilize the carbonate formation, whereas it slightly promotes oxalate formation. Our study does not support the proposed diradical coupling mechanism. Instead, our calculations suggest a new mechanism in which one CO2 molecule is...

Using the spin-unrestricted density functional theory method, we investigate the interplay between the diradical character y and second hyperpolarizabilities γ (the third-order nonlinear optical (NLO) properties at the molecular scale) of corannulene derivatives with two phenoxyl radicals. This molecule in the singlet state exhibits intermediate y and thus displays a significantly larger γ value than the triplet state and the closed-shell bis-phenol analogue. We also examine the planar molecules involving a coronene moiety in place of the curved corannulene. The intermediate y and large γ values of the corannulene systems are found to originate not from their curved skeleton but from the equilibrium between benzenoid/quinoid resonance forms due to delocalization of the radical electrons of the terminal phenoxyl rings. The longitudinal γ value of the singlet state is found to be comparable to that of s-indaceno[1,2,3-cd;5,6,7-c'd']diphenalene, which is known to be one of the organic molecules with the largest two-photon absorption cross section in this size of pure hydrocarbons. The present system is thus expected to be a promising candidate for highly efficient open-shell NLO molecules.

Quasi-one-dimensional open-shell ladder oligomers composed of fully conjugated five-and six-membered rings are investigated to describe their open-shell singlet character and their third-order nonlinear optical (NLO) responses using density functional theory calculations. The nature of the repeating structures is shown to modify the multi-radical nature described by multiple diradical characters, the bond-length alternation, and the local aromaticity. The calculation of the third-order NLO properties (γ, the second hyperpolarizability) reveals that the open-shell ladder oligomers with small multiple diradical characters exhibit the largest γ values. At the infinite chain length limit, the static γ value per fused-ring is estimated to reach about 3.6 times that of polyacene, and about half that of all-trans-polyacetylene. The present study gives the first detailed clarification of the multi-radical character and the local aromaticity in real multi-radical ladder oligomers, and the first demonstration that these oligomers are efficient NLO candidates with both large γ and high rigidity.

Carbon–carbon double bonds (C═C) are ubiquitous in natural and synthetic polymers. In bulk studies, due to limited ways to control applied force, they are thought to be mechanically inert and not to contribute to the extensibility of polymers. Here, we report a single molecule force spectroscopy study on a polymer containing C═C bonds using atomic force microscope. Surprisingly, we found that it is possible to directly observe the cis-to-trans isomerization of C═C bonds at the time scale of ∼1 ms at room temperature by applying a tensile force ∼1.7 nN. The reaction proceeds through a diradical intermediate state, as confirmed by both a free radical quenching experiment and quantum chemical modeling. The force-free activation length to convert the cis C═C bonds to the transition state is ∼0.5 A, indicating that the reaction rate is accelerated by ∼109 times at the transition force. On the basis of the density functional theory optimized structure, we propose that because the pulling direction is not parall...

The geometry, aromaticity, and electronic properties of benzo[3,4]cyclobutathiophenes (BCTs) and their homologues have been examined theoretically using density functional theory calculations. The harmonic oscillator measure of aromaticity and nucleus-independent chemical shift analyses revealed the aromaticity characteristics of the two regioisomers benzo[3,4]cyclobuta[1,2-b]thiophene and benzo[3,4]cyclobuta[1,2-c]thiophene. When the aromaticity of one of the six-π-electron rings increases, it concomitantly decreases in the other ring. The anti-aromaticity of the four-membered ring varies depending on the π-electron density of the shared bond with the thiophene ring. This leads to a large difference of the highest occupied molecular orbital–lowest unoccupied molecular orbital gap between the isomers. Linear BCT homologues show medium diradical characters and the smallest EGap values. In the angular and branched homologues, the π-electrons of central benzene rings are localized avoiding the shared bonds, ...

The ability to synthesize targeted molecules hinges on detailed mechanistic insight of the reaction. The 1,3-dipolar cycloaddition reaction between diazomethane derivatives and 7-isopropylidenebenzonorbornadiene have been extensively studied using density functional theory (DFT) at the M06–2X/6-311G(d,p) level of theory in order to delineate the peri-, regio-, and stereo-selectivities of the reaction. The diazomethane is shown to periselectively add across the endocyclic olefinic bond of the 7-isopropylidenebenzonorbornadiene and stereoselectively in the exo fashion, yielding the exo-cycloadduct as the major product, with a rate constant of 3.83 × 104 s−1. The endo approach of this periselective path is the closest competing pathway with a rate constant of 8.78 × 101 s−1. Neither electron-donating groups (R = methyl, ethyl, amine, cyclopropyl) nor electron-withdrawing groups (R = cyano, nitro, carbonyl) on the diazomethane alters the peri- and stereo-selectivity of the reaction. However, the substituents do have an effect on whether the addition follow normal or inverse electron demand mechanisms. EDGs favor a normal electron demand mechanism while EWGs favor an inverse electron demand 1,3-dipolar cycloaddition reaction. While EDGs-substituted diazomethane derivatives behave as nucleophiles in reactions with 7-isopropylidenebenzonorbornadiene, EWGs-substituted diazomethane derivatives behave as electrophiles. The 1,3-dipole adds across the dipolarophile via a concerted asynchronous mechanism, but a stepwise diradical mechanism has been ruled out. The selectivities observed in the title reaction are kinetically controlled. Analysis of the nucleophilic Parr function ( P K − ) at the different reaction sites in the dipolarophile indicates that the diazomethane adds across the atomic centers with highest NBO and Mulliken atomic spin densities.

MnV═O functionalities are important in synthetic and bioinorganic chemistry, being relevant to both C–H activation and the O–O bond formation steps in enzymatic water oxidation, for example. The triplet and quintet spin states are believed to be active in these reactions, but they have only been sparingly characterized experimentally. Density functional theory (DFT) gives varying results, depending on the exchange-correlation functional employed, leading to ambiguity about whether the triplet MnV═O is better represented as MnIV–O•. While recent CASPT2 studies confirmed that the MnIV–O• character is exaggerated by hybrid functionals, questions still remain about the nature of this bonding. Using high-level wave function methods, we investigated the fundamental relationship between the spin polarization, diradical character, and the physical oxidation state assignments. We conclude that, in terms of formal oxidation assignment, these species are best described as being between the MnV═O and MnIV–O• extremes...

Organic radicals display unique physical structures and could become next generation functional materials. However, design and synthesis of stable neutral radicals with a significant polyradical character has been an enormous challenge for chemists. In this work, we synthesized a series of stable 3,6-linked, kinetically blocked fluorenyl radical oligomers up to hexamer (FR-n, n = 1-6). Their ground-state geometric and electronic structures were systematically studied by various experimental methods including X-ray crystallographic analysis, variable temperature nuclear magnetic resonance, electron spin resonance, and superconducting quantum interference device measurements, supported by density functional theory and ab initio calculations. Moderate antiferromagnetic coupling between the fluorenyl radicals was observed, and moderate to large diradical and polyradical characters were calculated from dimer onward. Furthermore, their photophysical properties were estimated by steady-state, transient absorption, and two-photon absorption measurements, and their electrochemical properties were investigated by cyclic voltammetry/differential pulse voltammetry and spectro-electrochemical measurements. A clear chain length dependence of their optical, electrochemical, and magnetic properties was found for the oligomers with an odd or even number of spin centers, respectively.