Search

EPR line shapes in fluid solution and electron spin relaxation times and lineshapes in frozen solution were measured at X-band for two dinitroxides. For dinitroxide 1, which has two proxyl nitroxides connected by an amide linkage, the fluid solution spectra are in the strong exchange limit (J >> nitrogen hyperfine coupling) and the interspin distance in glassy water:glycerol is about 8.8 A. For dinitroxide 2, which has two proxyl nitroxides connected by a bridging ethylenediamine and two amide linkages, the fluid solution spectra are characteristic of dynamic exchange between conformations with strong exchange (J > 850 MHz) and conformations with weaker exchange interaction. For 2, the interspin distance in glassy water:glycerol is about 9.2 A and J > 600 MHz. T1 and Tm for both dinitroxides in 1:1 water:glycerol were measured at 10 to 160 K. Relaxation times are within experimental uncertainty of values for a pyrrolinoxyl monoradical and very similar to values for a nitroxide with a piperidinyl ring. These data demonstrate that even for these relatively flexible linkages dipolar interaction with an interspin distance of about 9 A does not provide an effective spin–lattice relaxation mechanism for dinitroxides in glassy water:glycerol.

Singlet diradicaloids hold great potential as semiconductors for organic field-effect transistors (OFETs). However, their relative low material and device stabilities impede the practical applications. Here, to achieve balanced stability and performance, two isomeric dibenzoheptazethrene derivatives with singlet diradical character were synthesized in a concise manner. Benefitting from the aromatic stabilization, both compounds display a small diradical character and large singlet-triplet gap, as corroborated by variable-temperature electron paramagnetic resonance spectra, single-crystal analysis, and theoretical calculations. OFET devices based on single crystals showed a high hole mobility of 0.15 cm2 V-1 s-1 , which is the highest for zethrene-based semiconductors. Both isomers exhibited remarkable material stability in air-saturated solutions as well as excellent bias-stress and storage stability in device under ambient air.

The synthesis and characterization of two 1,8-naphthalene-bridged cyclic oligo-para-phenylenes with four or eight phenylene units are reported. Both molecules exhibit a highly strained structure with two nearly face-to-face arranged p,p'-biphenylylene/p,p'-quaterphenylylene chains. Their dications were obtained by chemical oxidation with NO·SbF6, and both showed open-shell singlet diradical character with a small singlet-triplet energy gap.

The synthesis of cages with a polyradical framework remains a challenging task. Herein is reported an enantiomeric pair of quinoid-bridged polyradical tetrahedral palladium(II) cages that are stabilized by an unusual dianionic diradical form (dhbq..2- ). These cages have been characterized by electron paramagnetic resonance and UV-visible spectroscopy, squid magnetometry and mass spectrometry. Single-crystal-derived X-ray investigations of the iso-structural cages built on fluoranilate linkers confirm the tetrahedral structure of the obtained radical cages. Theoretical calculations showed that the diradical state of the dhbq anions is more stable than the usual monoradical state. A weak ferromagnetic exchange between adjacent radical centers was observed in DFT studies.

The σ-dimer of a kinetically protected olympicenyl radical, which evaded the experimental detection, was revealed by conversion into biolympicenylidene with E-configuration in a regioselective manner. The complicated stereochemistry and energetics of the σ-dimers derived from C2v symmetry and uneven spin distribution of the olympicenyl radical were revealed by the theoretical calculations, and the energetic preference of π-dimer over σ-dimer by a minute gap was disclosed. The E-biolympicenylidene, a polycyclic ene structure previously considered as reactive intermediate in the phenalenyl radical system, exhibited exceptional stability, which allowed for a detailed investigation on its singlet diradical character and physical properties by means of X-ray crystallography, UV-vis-NIR absorption/emission spectroscopy and cyclic voltammetry, and assisted by theoretical calculations. The E-biolympicenylidene showed high resistance towards both thermal and photochemical ring-cyclization reactions, which was attributed to high activation energies for the rate-determining electrocyclization operated on both disrotatory and conrotatory mode, as well as a small spin density at the bonding sites for the radical-radical coupling process.

A novel and convenient method has been developed for the facile synthesis of functionalized succinimide derivatives via intramolecular Alder-ene reaction of 1,6-enynes. This reaction features mild and metal-free reaction conditions, which offers a green and efficient entry to synthetically important succinimide scaffolds. Preliminary mechanistic studies suggest that a diradical intermediate might be involved in this transformation.

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.

The synthesis of robust high-spin carbon radicals is an important topic in organic chemistry. Toward this end, several porphyrin-stabilized radicals have been systematically explored. A singly naphthalene-fused porphyrin radical was synthesized by a reaction sequence consisting of a Suzuki-Miyaura coupling of β-borylated porphyrin with 2-bromobenzaldehyde, addition of mesityl Grignard reagent, intramolecular Friedel-Crafts alkylation, and final oxidation with DDQ or tBuOK/O2 . This strategy was also used to synthesize doubly naphthalene-fused porphyrins and syn- and anti-fused-anthracene-bridged porphyrin dimers. While singly naphthalene-fused porphyrin radical has been shown to be a stable monoradical, doubly naphthalene-fused porphyrins and anti-fused-anthracene-bridged porphyrin dimers have been shown to be closed-shell molecules. Finally, the syn-dimer was characterized as a surprisingly stable radical (t1/2 =28 days under ambient air and at 80 °C) that is storable for more than several months, despite its high-spin triplet ground-state carbon diradical.

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...

The first SnI diradical [(ADCPh)Sn]2 (4) based on an anionic dicarbene (ADCPh={CN(Dipp)}2CPh; Dipp=2,6‐iPr2C6H3) scaffold has been isolated as a green crystalline solid by KC8 reduction of the corresponding bis‐chlorostannylene [(ADCPh)SnCl]2 (3). The six‐membered C4Sn2‐ring of 4 containing six π‐electrons shows a diatropic ring current, thus 4 may also be regarded as the first 1,4‐distannabenzene derivative. DFT calculations suggest an open‐shell singlet (OS) ground state of 4 with a remarkably small singlet–triplet energy gap (ΔEOS–T=4.4 kcal mol−1), which is consistent with CASSCF (ΔES–T=6.6 kcal mol−1 and diradical character y=37 %) calculations. The diradical 4 splits H2 at room temperature to yield the bis‐hydridostannylene [(ADCPh)SnH]2 (5). Further reactivity of 4 has been studied with PhSeSePh and MeOTf., A 1,4‐distannabenzene derivative 4 with two‐coordinated SnI atoms has been isolated as a green crystalline solid. The ground state of 4 is an open‐shell singlet (OS) with the singlet–triplet energy gap (ΔE OS–T) of 4.4 kcal mol−1 (according to CASSCF, ΔE S–T=6.6 kcal mol−1). Consequently, 4 exhibits a half‐field EPR signal at 100 K and undergoes H2 splitting at room temperature to quantitatively yield the SnII hydride 5 as an orange solid.

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.

The reaction of (LSi:)2 (1; L = PhC(NtBu)2) with 2 equiv of Me3SiC2C2SiMe3 resulted in the formation of (Me3SiC2)2(Me3Si)2C4Si2(L)2 (2). 2 exhibited a one-electron transfer when treated with 1 equiv of [Ph3C]+[B(C6F5)4]- to yield [(Me3SiC2)2(Me3Si)2C4Si2(L)2]·+[B(C6F5)4]- (3) and Ph3CCPh3, respectively. When compound 2 was treated with 2 equiv of AgOSO2CF3 a transfer of two electrons occurred to produce [(Me3SiC2)2(Me3Si)2C4Si2(L)2]2+·2[OSO2CF3]- (4) and elemental silver. The 1,4-disilabenzene 2 is disclosed of an open-shell singlet diradical character, and 3 and 4 are, respectively, the elusive stable radical cation and dication species of the 1,4-disilabenzene (2). Furthermore, 2 reacted with group 16 elements of O, S, and Se by oxidative addition to form (Me3SiC2)2(Me3Si)2C4Si2(L)2(μ-O2) (5) and (Me3SiC2)2(Me3Si)2C4Si2(L)2(μ-E) (E = S (6) and Se (7)), respectively.

High-spin conjugated radicals have great potential in magnetic materials and organic spintronics. However, to obtain high-spin conjugated radicals is still quite challenging due to their poor stability. We report the successful synthesis and isolation of a stable triplet conjugated diradical, 10,12-diaryldiindeno[1,2-b:2',1'-e]pyrazine (m-DIP). With the m-xylylene analogue skeleton containing electron-deficient sp2 -nitrogen atoms, m-DIP displays significant aromatic character within its pyrazine ring and its spin density mainly delocalizes on the meta-pyrazine unit, making it a triplet ground state conjugated diradical. Our work provides an effective "spin density tuning" strategy for stable high-spin conjugated radicals.

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.

The aromaticity of cyclic 4n pi-electron molecules in their first pi pi* triplet state (T-1), labeled Baird aromaticity, has gained growing attention in the past decade. Here we explore computationally the limitations of T-1 state Baird aromaticity in macrocyclic compounds, [n]CM's, which are cyclic oligomers of four different monocycles (M = p-phenylene (PP), 2,5-linked furan (FU), 1,4-linked cyclohexa-1,3-diene (CHD), and 1,4-linked cyclopentadiene (CPD)). We strive for conclusions that are general for various DFT functionals, although for macrocycles with up to 20 pi-electrons in their main conjugation paths we find that for their T-1 states single-point energies at both canonical UCCSD(T) and approximative DLPNO-UCCSD(T) levels are lowest when based on UB3LYP over UM06-2X and UCAM-B3LYP geometries. This finding is in contrast to what has earlier been observed for the electronic ground state of expanded porphyrins. Yet, irrespective of functional, macrocycles with 2,5-linked furans ([n]CFU's) retain Baird aromaticity until larger n than those composed of the other three monocycles. Also, when based on geometric, electronic and energetic aspects of aromaticity, a (3)[n]CFU with a specific n is more strongly Baird-aromatic than the analogous (3)[n]CPP while the magnetic indices tell the opposite. To construct large T-1 state Baird-aromatic [n]CM's, the design should be such that the T-1 state Baird aromaticity of the macrocyclic perimeter dominates over a situation with local closed-shell Hiickel aromaticity of one or a few monocycles and semilocalized triplet diradical character. Monomers with lower Hiickel aromaticity in S-0 than benzene (e.g., furan) that do not impose steric congestion are preferred. Structural confinement imposed by, e.g., methylene bridges is also an approach to larger Baird-aromatic macrocycles. Finally, by using the Zilberg-Haas description of T-1 state aromaticity, we reveal the analogy to the Bickel aromaticity of the corresponding closed-shell dications yet observe stronger Hiickel aromaticity in the macrocyclic dications than Baird aromaticity in the T-1 states of the neutral macrocycles.

While quinoidal moieties are considered as emerging platforms showing efficient charge transport and interesting open-shell diradical characteristics, whether these properties could be changed by extension to the conjugated polymer structure remains as a fundamental question. Here, we developed and characterized two conjugated polymers incorporating quinoids with different lengths, which have a stable close- and open-shell diradical character, respectively, namely, poly(quinoidal thiophene-thienylene vinylene) (PQuT-TV) and poly(quinoidal bithiophene-thienylene vinylene) (PQuBT-TV). A longer length of a quinoidal core led to enhanced diradical characteristics. Therefore, the longer core length of QuBT was favorable for the formation of an open-shell diradical structure in its monomer and in the quinoidal polymer. PQuBT-TV exhibited high spin characteristics observed by the strong ESR signal, a low band gap, and improved electrochemical stability. On the other hand, as QuT maintained a closed-shell quinoid structure, PQuT-TV exhibited high backbone coplanarity and strong intermolecular interaction, which was beneficial for charge transport and led to high hole mobility (up to 2.40 cm2 V-1 s-1) in organic field-effect transistors. This work successfully demonstrated how the control of the closed/open-shell character of quinoidal building blocks changes charge transport and spin properties of quinoidal conjugated polymers via quinoid-aromatic interconversion.

Cycloalkanes and alkenes are important components of real fuels but there is little kinetic and mechanistic data on the dissociation of most large cyclic and olefinic molecules at elevated temperatures. We present here the first experimental and theoretical investigation of dissociation of cycloheptane and the initial product from ring opening, 1-heptene. Experiments were performed in a diaphragmless shock tube using laser schlieren densitometry. Pyrolysis of cycloheptane (0.5–4% in Kr) was studied over 1450–2000 K and 30–120 Torr. Experiments with 1-heptene (1–4% in Kr) covered 1200–1650 K and 30–120 Torr. A newly developed chemical kinetic mechanism for pyrolysis of cycloheptane and 1-heptene is presented herein. Simulations are in very good agreement with the experimental measurements. Rate coefficients for the initial ring-opening process in cycloheptane, k1, and dissociation of 1-heptene, k2, were determined from the experiments. Both k1 and k2 are in falloff, and the pressure and temperature dependencies were well reproduced by theoretical calculations allowing extrapolation to conditions beyond the scope of this work. These calculations yielded the following expressions for k1 and k2 with the uncertainties estimated as ±40% and ±50% respectively: k 1 , ∞ = 5.94 × 10 17 exp ( − 44 , 521 T ) s − 1 and k 2 , ∞ = 8.86 × 10 16 exp ( − 35 , 887 T ) s − 1 . The results of this study indicate that cycloheptane dissociates similarly to cyclopentane and cyclohexane, i.e. ring-opening via C C scission to a diradical that rapidly isomerizes to a conjugate 1-alkene. The secondary chemistry is dominated by the dissociation products of the 1-alkenes i.e. allyl and n-alkyl radicals. Furthermore, rates of dissociation of the cycloalkanes are size dependent and kcyclopentane

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.

A highly regioselective and diastereoselective [2 + 2] cross-photocycloaddition between electron-poor and electron-rich/electron-neutral alkenes under visible-light irradiation was developed. In the absence of an external photocatalyst, additive and solvent, the substrates 1 and 4 reacted with rigid cyclic alkoxyethenes to generate cis-anti-head-to-head heterocoupled [2 + 2] products in high yields with regiospecificity and good diastereoselectivity. Meanwhile, the reactions of 1 and 4 with (Z)- and (E)-1,2-diphenylethenes yielded the desired products in good yields with a high dr. It is important to note that no geometric isomerization of olefins was observed during the reaction. Mechanistic studies and DFT calculations suggested that 1 and 4 having ortho-ethynyl and cyano groups as a self-photocatalyst play a very important role in the reaction and a five-membered ring diradical intermediate (Int1) was formed via an intramolecular radical addition. The obtained regioselectivity and diastereoselectivity were confirmed by X-ray structural analysis of the representative products.

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.

Two-electron oxidation of five aniline-based compounds 4,4′′-p/m-terphenyldiamines afforded a class of terphenyldiamine dications 12+–52+. Their electronic structures were systematically investigated by theoretical calculations and various experiments, including single-crystal X-ray diffraction, UV/Vis absorption spectroscopy, EPR spectroscopy and SQUID measurements. p-Terphenyldiamine dications 12+–32+ are open-shell singlets in the ground state with large singlet–triplet energy gaps. The inherent instability of dication 42+ precludes the determination of its ground state, whereas the observed linear Curie plot for m-terphenyldiamine dication 52+ indicates that its ground state is triplet. The calculated singlet–triplet energy gap (ΔEOS–T) value is 1.27 kcal mol−1 for 42+ and 1.10 kcal mol−1 for 52+. This work provides the first examples of isolable aniline-based diradical dications.

A series of viologen derivatives containing aggregation-induced emission (AIE)-active 9,10-azaboraphenanthrene (BNPV2+) is reported. The AIE properties of BNPV2+ were attributed to the strong electron transfer between an electron donating azabora-phenanthrene (BNP) and electron-accepting viologen (MV2+) moieties via a twisted intramolecular charge transfer (TICT) mechanism in the aggregated and solid states. BNPV2+ with good redox and AIE properties exhibited electro-chromic (EC) behavior in solution, and simultaneous EC and electrofluorochromic (EFC) behaviors in the aggregated and film states, where the AIE phenomenon can be turned “on/off” by redox processes. The formation of three redox states (dication, radical cation and diradical) was rationalized and supported by the experimental data and DFT calculations. More importantly, the EC and EFC properties of BNPV2+ were successfully used to realize a reversible data encryption system, contributing towards the continuous efforts in the development of new information security technologies.

Thermal decomposition of 1-hexene at temperatures 295-1410 K was conducted using a flash pyrolysis micro-reactor coupled to laser-based vacuum ultraviolet photoionization time-of-flight mass spectrometer (VUV-PI-TOFMS). The decomposition mechanism of 1-hexene was developed with the help of theoretical calculation performed at the MRCI/cc-pvtz//CASSCF/6–31+G(d,p) level. The γ-scission and diradical retro-ene reactions were determined as the main initial decomposition reactions in the temperature range 990-1240 K. Two diradical retro-ene reaction channels, 1,5-diradical and 1,6-diradical reactions, were proposed in order to interpret the appearance of the C4H8 species. The 1,5-diradical retro-ene reaction involved a 1,5-diradical intermediate that subsequently decomposed via C C β-scissions to the C2, C3 and C4 products. The 1,6-diradical retro-ene reaction proceeded via a 1,6-diradical intermediate and C C β-scissions to produce the C2 and C4 species. The proposed diradical retro-ene mechanism was evidenced indirectly by the early product distribution of 1-hexene pyrolysis in a flow reactor at 1173 K determined by synchrotron radiation VUV-PI-TOFMS. It was verified in the flash pyrolysis of 1-heptene as well.

The state-specific Brillouin-Wigner multireference perturbation theory [which employs Jeziorski-Monkhorst parametrization of the wave function] using improved virtual orbitals, denoted as IVO-BWMRPT, is applied to calculate excitation energies (EEs) for methylene, ethylene, trimethylenemethane, and benzyne systems exhibiting various degrees of diradical character. In IVO-BWMRPT, all of the parameters appearing in the wave function ansatz are optimized for a specific electronic state. For these systems, the IVO-BWMRPT method provides EEs that are in close agreement with the benchmark results and experiments, where available, indicating that the method does not introduce imbalance in the target-specific treatment of closed- and open-shell states involved. The good performance of the present methodology is primarily related to structural compactness of the formalism. Overall, present findings are encouraging for both further development of the approach and chemical applications on the energy differences of strongly correlated systems.

We explored the mechanism and the origin of chemoselectivity of the manganese porphyrin-catalyzed aliphatic C–H isocyanation reaction with density functional theory (DFT) calculations. In this reaction, a diradical intermediate complex of a hydroxomanganese porphyrin and a substrate-derived radical is formed by the H-abstraction process. Then the axial ligand exchange of OH with NCO leads to a di-isocyanate manganese porphyrin intermediate, which provides the possibility for the NCO-rebound process to form the alkyl isocyanate product. The competition of different axial ligands rebounding to substrate-derived radicals is the origin of different products. The computational results suggest that the NCO-rebound pathway is more favorable than the OH-rebound pathway by 2.7 kcal mol−1, which is consistent with the experimental results that the major product is an alkyl isocyanate instead of the oxygenation product. The NCO-rebound pathway through the di-isocyanate manganese porphyrin intermediate is more favorable than that through the hydroxomanganese porphyrin by 5.9 kcal mol−1 due to the stronger trans effect of the axial ligand NCO compared to OH. We also found that the electron-donating aryl ligand can promote the C–H bond isocyanation. The mechanism and the controlling factors are also applicable to the manganese porphyrin-catalyzed C–H bond azidation reaction.

A novel state-averaged version of ab initio nonorthogonal valence bond method is described, for the sake of accurate theoretical studies of excited states in the valence bond framework. With respect to standard calculations in the molecular orbital framework, the state-averaged breathing-orbital valence bond (BOVB) method has the advantage to be free from the penalizing constraint for the ground and excited state(s) to share the same unique set of orbitals. The ability of the BOVB method to faithfully describe excited states and to compute accurate transition energies from the ground state is tested on the five lowest-lying singlet electronic states of ozone and sulfur dioxide, among which 11B2 and 21A1 are the challenging ones. As the 11A2, 11B1, and 11B2 states are of different symmetries than the ground state, they can be calculated at the state-specific BOVB level. On the other hand, the 21A1 states and the 11A1 ground states, which are of like symmetry, are calculated with the state-averaged BOVB technique. In all cases, the calculated vertical energies are close to the experimental values when available, and at par with the most sophisticated calculations in the molecular framework, despite the extreme compactness of the BOVB wave functions, made of no more than 5-9 valence bond structures in all cases. The features that allow the combination of compactness and accuracy in challenging cases are analyzed. For the "ionic" 11B2 states, which are the site of important charge fluctuations, it is because of the built-in dynamic correlation inherent to the BOVB method. For the 21A1 ones, this is the fact that these states have the degree of freedom of having different orbitals than the ground states, even though they are of like symmetry and calculated simultaneously using the newly implemented state-average BOVB algorithm. Finally, the description of the excited states in terms of Lewis structures is insightful, rationalizing the fast ring closure for the 21A1 state of ozone and predicting some diradical character in the so-called "ionic" 11B2 states.

Diradical character and excited-state aromaticity serve as guidelines to identify molecules that show nonlinear optical properties. Cumulenes are known to have small singlet-triplet gaps resulting in significant diradical character. Herein, we report a computational investigation on the electronic structure and excited-state properties of cumulenes of different lengths and with various terminal carbene groups. Intriguingly, cumulenes with an even number of cumulative double bonds, which barely have been studied experimentally, are predicted to be thermodynamically more stable than their odd counterparts. We propose that this is due to the stabilizing effect of electron delocalization in the helical Möbius-type frontier orbitals. Accordingly, we delineate how to control the energies of the excited states by the choice of carbene and length of the cumulene. We find that π-acceptor carbenes decrease the diradical character, whereas donors as well as captodative substitution or potentially a biscationic charge leads to an open-shell ground state. We also predict that bent allenes are better in stabilizing organic radicals than carbenes. Eventually, we identify suitable candidates for experimental endeavors toward new singlet fission molecules.

The preparation of a series of dibenzo- and tetrabenzo-fused fluoreno[3,2-b]fluorenes is disclosed, and the diradicaloid properties of these molecules are compared with those of a similar, previously reported series of anthracene-based diradicaloids. Insights on the diradical mode of delocalization tuning by constitutional isomerism of the external naphthalenes has been explored by means of the physical approach (dissection of the electronic properties in terms of electronic repulsion and transfer integral) of diradicals. This study has also been extended to the redox species of the two series of compounds and found that the radical cations have the same stabilization mode by delocalization that the neutral diradicals while the radical anions, contrarily, are stabilized by aromatization of the central core. The synthesis of the fluorenofluorene series and their characterization by electronic absorption and vibrational Raman spectroscopies, X-ray diffraction, SQUID measurements, electrochemistry, in situ UV-vis-NIR absorption spectroelectrochemistry, and theoretical calculations are presented. This work attempts to unify the properties of different series of diradicaloids in a common argument as well as the properties of the carbocations and carbanions derived from them.

Helicenes and extended helical π-conjugated compounds have been widely studied, but most of the systems contain only aromatic benzene or heterocyclic rings, showing local aromatic character. Herein, new S-shaped double [6]helicene 1, which has two embedded para-quinodimethane (p-QDM) units, is reported. Due to the existence of a proaromatic quinoidal substructure, it has open-shell diradical character. Its model compound, C-shaped single [6]helicene 2 containing one p-QDM unit, was also synthesized and compared. Their ground-state structures and electronic properties were systematically studied by a combination of various experimental methods assisted by theoretical calculations. Compound 1 has a double-helical structure in the crystal, with the two terminal [6]helicene units bent in opposite directions (i.e., anti form). However, an anti/syn isomerization process with a moderate interconversion energy barrier was observed on the NMR timescale. Compound 1 shows amphoteric redox behavior. It also exhibits open-shell diradical character (y0 =12.1 %) and a small singlet-triplet gap. On the other hand, compound 2 has a typical closed-shell nature. The dication and dianion of 1 also show open-shell diradical character. The dianion of 2 and the tetraanion of 1 exhibit similar electronic structures to their respective isoelectronic structures, that is, [6]helicene and a double [6]helicene. This work provides some insights into the design and synthesis of stable helical π systems with open-shell diradical character and magnetic activity.

The investigation of π-conjugated diradical compounds, featuring radical centers in the ground state, is key to understanding the nature of chemical bonds.[1] Occasionally, these systems can form long σ C-C bonds between two unpaired electrons resulting in macrocyclic or staircase oligomers or polymers by self-assembly processes. Furthermore, these new C-C bonds are longer than an ordinary bond between two sp3 carbon resulting in reversible dissociation/formation behavior between isolated radical species and cyclophane structures. Therefore, these materials are potential building blocks for dynamic covalent chemistry (DCC).[2] Hereinto, we present an experimental and theoretical study of carbazole and indolocarbazole-based diradicals (Figure 1) with dicyanomethylene (DCM) groups in different positions (para-DCM or meta-DCM) to identify new building blocks to obtain multi-responsive materials.[3-5] To this end, we investigated the dynamic interconversion between the isolated diradical and the cyclophane structures under external stimuli such as temperature, pressure and so on. Specifically, our main aim is to study how the DCM substitution and the elongation of the conjugated core affect the diradical character and to understand the connection between this parameter and the cyclophanes stability. In addition, we want to investigate if this transformation is reversible or not. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech.

A new tetrathiafulvalene (TTF) derivative is synthesized, which is substituted with two phenoxy radicals on one 1,3-dithiole ring, and may have either open-shell diradical or closed-shell extended-quinoidal ground states. X-ray single crystal analysis and NMR measurements prove that this molecule has a closed-shell extended quinoidal structure both in the solid state and in solution. DFT calculations show the donor-acceptor electronic properties of this molecule with a well-separated HOMO-LUMO distribution and a small HOMO-LUMO energy gap. Because of this donor-acceptor character, this molecule gives both the dication and the dianion species by electrochemical oxidation and reduction. Furthermore, during the redox process between the neutral and dication states, this molecule exhibits unique changes in the cyclic voltammogram upon repeating the cycles or varying the scan rate. The observed electrochemical behavior is explained by the conformational changes in the electrochemically generated species, thus indicating that this molecule is classified as a dynamic redox system.

Visible-light-induced dearomatization reaction via energy-transfer mechanism is an emerging strategy for the synthesis of highly strained polycyclic molecules. Transient, high-energy diradical spec...

This is a longish, theoretical paper dealing with the molecular conductance of H2 and π-stacked ethylene. At different levels of theory ranging from single-determinant to multi-reference, from open...

Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt2−) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt2−. UV-vis-Near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt2− π-manifold from the Fe(II)-centered spin-crossover. The TTFtt2− centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character.

EPR spectroscopic evidence for intramolecular electron transfer in anionic N-substituted naphthalimides to yield persistent diradical anions and intermolecular electron transfer from a variety of carbanions to 6-bromo-N-phenyl-naphthalimide to yield persistent radical-radical anion pairs was recently claimed in two papers by Zhang et al. In this comment, it is shown that the EPR spectra published in both papers do not agree with the proposed triplet-state species. Rather, the spectra are due to various doublet-state radicals, deriving from minor side reactions. The misinterpretations invalidate the general conclusions of the papers.

Metallonitrenes (M–N) are complexes with a subvalent atomic nitrogen ligand that have been proposed as key reactive intermediates in nitrogen atom transfer reactions. However, in contrast to the common classes of nitride complexes (M≡N) and organic nitrenes (R–N), structurally and spectroscopically well defined ‘authentic’ metallonitrenes with a monovalent atomic nitrogen ligand remain elusive. Here we report that the photolysis of a platinum(ii) pincer azide complex enabled the crystallographic, spectroscopic, magnetic and computational characterization of a metallonitrene that is best described as a singly bonded atomic nitrogen diradical ligand bound to platinum(ii). The photoproduct exhibits selective C–H, B–H and B–C nitrogen atom insertion reactivity. Despite the subvalent metallonitrene character, mechanistic analysis for aldehyde C–H amidation shows nucleophilic reactivity of the N-diradical ligand. Ambiphilic reactivity of the metallonitrene is indicated by reactions with CO and PMe3 to form isocyanate and phosphoraneiminato platinum(ii) complexes, respectively. Transient metallonitrenes (M–N) have been proposed as key intermediates in nitrogen atom transfer reactions, but well-defined examples have remained elusive. Now, a platinum complex with an atomic nitrogen ligand, best described as a subvalent nitrogen diradical singly bonded to a platinum(ii) ion (Pt–N), has been isolated and shows ambiphilic reactivity.