Bulky Substituent Effect on Reactivity of Localized Singlet Cyclopentane‐1,3‐diyls with π‐Single Bonding (C‐π‐C) Character

Thorough investigation of key intermediates, such as long‐lived singlet diradicals, is essential to understand the homolytic bond cleavage reactions. In this study, we evaluate the effect of bulky substituents at the meta‐position of the phenyl ring on the bond formation process in singlet 2,2‐diethoxy‐1,3‐diarylcyclopentane‐1,3‐diyls. The bulky groups have significant influence on the diradical lifetime, as such, when the triisopropylphenyl group was used, the lifetime was 45 times longer than that of the parent diradical in benzene at 293 K.     

Systematic approach to design organic magnetic molecules: strongly coupled diradicals with ethylene coupler

The intramolecular magnetic coupling constant (J) values of diradical systems linked with two monoradicals through a coupler (para-substituted phenyl acetylene (Model I), meta-substituted phenyl acetylene (Model II), ethylene (Model III)) were investigated by unrestricted density functional theory calculations. We divided eight monoradicals into α-group and β-group according to Mulliken spin density values of the connected atoms. The overall trends in the strength of magnetic interactions of diradicals were found to be identical in three different model systems. The diradicals with para-substituted phenyl acetylene coupler resulted in almost twice stronger intramolecular magnetic coupling interactions of the corresponding diradicals as compared to the meta-substituted one with opposite magnetism. NN-Ethylene-PO (nitronyl nitroxide radical coupled to phenoxyl radical via ethylene coupler) was calculated to have the strongest magnetic coupling constant with ferromagnetism, and to be even stronger (more than twice) than NN-ethylene-NN (nitronyl nitroxide diradical with ethylene coupler), which was reported to have strong antiferromagnetic interactions in a previous experiment. It was found that the spin density values of the connected atoms are closely related to the determination of magnetic interactions and J values. The spin states of the ground state in diradical systems were explained by means of the spin alternation rule.     

1,3-Diborata-2,4-diphosphoniocyclobutane-1,3-diyls communicate through a para-phenylene linker

The presence of a second 1,3-diborata-2,4-diphosphoniocyclobutane-1,3-diyl in the para-position of a phenylene linker favors the diradical form over the 1,3-diborata-2,4-diphosphoniobicyclo[1.1.0]butane bond stretch isomer, as long as the two planar diradical units are coplanar with the linker. This demonstrates that two BPBP diradicals communicate through the phenyl ring.     

A high-spin diradical dianion and its bridged chemically switchable single-molecule magnet

Triplet diradicals have attracted tremendous attention due to their promising application in organic spintronics, organic magnets and spin filters. However, very few examples of triplet diradicals with singlet–triplet energy gaps (ΔE_ST) over 0.59 kcal mol⁻¹ (298 K) have been reported to date. In this work, we first proved that the dianion of 2,7-di-tert-butyl-pyrene-4,5,9,10-tetraone (2,7-tBu₂-PTO) was a triplet ground state diradical in the magnesium complex 1 with a singlet–triplet energy gap ΔE_ST = 0.94 kcal mol⁻¹ (473 K). This is a rare example of stable diradicals with singlet–triplet energy gaps exceeding the thermal energy at room temperature (298 K). Moreover, the iron analog 2 containing the 2,7-tBu₂-PTO diradical dianion was isolated, which was the first single-molecule magnet bridged by a diradical dianion. When 2 was doubly reduced to the dianion salt 2K2, single-molecule magnetism was switched off, highlighting the importance of diradicals in single-molecule magnetism.

We report a triplet diradical dianion in magnesium complex with ΔE_ST = 0.94 kcal mol⁻¹ (473 K). Its iron analog is the first single-molecule magnet bridged by a diradical dianion, and the SMM property is switched off through two-electron reduction.     

Direct Detection of a Chemical Equilibrium between a Localized Singlet Diradical and Its σ‐Bonded Species by Time‐Resolved UV/Vis and IR Spectroscopy

Localized singlet diradicals are key intermediates in bond homolyses. The singlet diradicals are energetically much less stable than the σ‐bonded species. In general, only one‐way reactions from diradicals to σ‐bonded species are observed. In this study, a thermal equilibrium between a singlet 1,2‐diazacyclopentane‐3,5‐diyl diradical and the corresponding σ‐bonded species was directly observed. The singlet diradical was more stable than the σ‐bonded species. The solvent effect clarified key features, such as the zwitterionic character of the singlet diradical. The effect of the nitrogen atoms is discussed in detail.     

The chemistry of localized singlet 1,3-diradicals (biradicals): from putative intermediates to persistent species and unusual molecules with a π-single bonded character

Localized singlet diradicals (biradicals) are key intermediates in chemical reactions involving homolytic bond-cleavage and formation processes. The molecular structure and electronic structure had been historically elusive due to the short-lived character of the reactive intermediates. In the last 15 years, a significant development of singlet diradical chemistry was achieved after the pioneering findings of long-lived singlet diradicals. In this tutorial review, the recent development of localized singlet diradical chemistry is summarized and discussed. The following subjects are included (a) the mechanism by which the ground state spin-multiplicity of localized 1,3-diradicals is controlled; (b) the substituent and heteroatom effect on the most stable electronic configuration of the singlet 1,3-diradicals, type-1 versus type-2; (c) the molecular design for the long-lived singlet ground state diradicals; (d) the generation and characterization of the singlet diradicals; and (e) the future prospects.     

On the electronic character of localized singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals: substituent effects on the lifetime

Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe > OMe, CN > CN, CN > OMe, H > Cl, Cl approximately CN, H approximately Me, Me > H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures.     

Theoretical Study on the Relationship between Diradical Character and Second Hyperpolarizabilities of Four‐Membered‐Ring Diradicals Involving Heavy Main‐Group Elements

By using spin‐unrestricted density functional theory methods, the relationship between the diradical character y and the second hyperpolarizability γ (the third‐order nonlinear optical (NLO) properties at the molecular scale) for four‐membered‐ring diradical compounds, that is, cyclobutane‐1,3‐diyl, Niecke‐type diradicals, and Bertrand‐type diradicals, were investigated by focusing on the substitution effects of heavy main‐group elements as well as of donor/acceptor groups on the y and γ values. It has been found that i) γ is enhanced in the intermediate y region for these four‐membered‐ring diradicals, ii) Niecke‐type diradicals with intermediate y values, which are realized by tuning the combination of the main‐group elements involved, exhibit larger γ values than Bertrand‐type diradicals, and iii) the y value and thus γ value can be controlled by modifying the both‐end donor/acceptor substituents attached to carbon atoms in Nicke‐type C₂P₂ diradicals. These results demonstrate that four‐membered‐ring diradicals involving heavy main‐group elements exhibit high controllability of the y and γ, which indicates the potential applications of four‐membered‐ring diradicals as a building block of highly efficient open‐shell NLO materials.     

Solvent effects on mechanism of tetramethylene polymerization

The polymerization mechanism of tetramethylenes was reinvestigated under inclusion of solvent effects. The approach of a methanol molecule to a borderline diradical, a typical diradical, and a typical zwitterion was studied by a valence, charge, and dipole moment analysis of SINDO 1 calculations. Whereas the solvent molecule has no effect on the character of the zwitterion, the borderline diradical was found to switch to a zwitterion at the approach of the methanol molecule if the distance between the donor carbon and the methanol oxygen is below 2 Å. A similar switch of character was observed for the typical diradical at CO distances below 1.5 Å. From energy considerations it is concluded that borderline diradicals can follow a zwitterionic polymerization mechanism in polar solvents, whereas typical diradicals are much less likely to do so.     

Disjoint molecular orbitals in nonalternant conjugated diradical hydrocarbons

The concept of disjoint NBMOs in diradical alternant hydrocarbons (AHs) is reviewed and extended to diradical nonalternant hydrocarbons (nonAHs). A valence-bond (VB) method for recognizing disjoint versus nondisjoint NBMOs (nonbonding molecular orbitals) in diradicals is presented. When circumscribed with hexagonal rings, disjoint diradicals produce nonradical polycyclic successors and nondisjoint diradicals produce diradical polycyclic successors. The interconnection between the topological VB terms of cross-conjugated, disjoint NBMOs, and essentially disconnected polycyclic hydrocarbons is delineated.     

Reversible Self-Assembling of Boryl Radical Anions to Their Diradicals with Tunable Singlet Ground States

Two novel boron-centered diradicals based on dimesitylpyridine borane ( 1 ) were synthesized by the self-assembling of the corresponding radical sodium and potassium salts, respectively. The sodium diradical was obtained by re-dissolving the crystals of the radical salt 1Na in toluene, while the potassium diradical was directly obtained by the reduction of 1 with potassium in THF. The diradicals could be converted back to their radical anions in THF solution, forming a reversible process. EPR spectroscopy and SQUID measurements, together with theoretical calculations, show that the diradicals have singlet ground states with excited triplet states. Their singlet–triplet energy gaps are tunable with metals.     

Spin-orbit coupling in oxygen containing diradicals

Intermediate diradicals which occur in the Paterno-Büchi photocycloaddition and in the Norrish type I photoreactions have been calculated taking into account the spin-orbit coupling (SOC) between the singlet (S) and triplet (T) states. Reaction paths for the photocycloaddition of formaldehyde to ethene and the diradical products of the -cleavage of cyclohexanone have been optimized by the MNDO CI method for a number of different singlet and triplet states. SOC integrals are calculated by an effective one-electron approximation. Intermediate diradicals in the Paterno-Büchi reaction and the SOC effects are also studied ab initio with CAS SCF geometry optimization in a TZV basis set. Both methods predict a large SOC matrix element between the S and T states in the course of the C-C attack, while the SOC integral is two orders of magnitude smaller for the diradical produced in the C-O attack. In the Norrish type I photoreaction the oxygen atom also produces some nonzero contribution to the SOC integral which governs intersystem crossing in a ·C-C· diradical. For the diradicals produced by the -cleavage of cyclohexanone a vibronic interaction is responsible for the SOC mixing between the lowest S and T states. The importance of one-center versus two-center SOC contributions in diradicals is briefly discussed.     

An acyclic enediyne anticancer compound attributed to a Bergman cyclization at physiological temperature

Enediyne cytotoxic drugs have attracted much attention because of their unique structure and potent anticancer activity. However, acyclic enediynes are long considered as inactive at physiological temperature due to their long C1-C6 distance. By adjusting the steric bulkiness of the functional groups at the alkynyl termini and the electron-withdrawing effect at the ene moiety, herein, a simple acyclic enediyne was designed and synthesized to achieve the onset of thermal Bergman cyclization at physiological temperature in polar solvents. The spontaneous generation of diradical intermediates was confirmed through EPR analysis and further supported by spin trapping experiment, radical indicator experiment, and high resolution MS analysis. The reactive diradicals generated in aqueous media induced single and double stranded cleavage of DNA, and showed high cytotoxicity to Hela cells. The IC₅₀ value of the enediyne compound is comparable to many clinical used anti-tumor agents.     

Competing Pathways in the Photogeneration of Didehydrotoluenes from (Trimethylsilylmethyl)aryl Sulfonates and Phosphates

The scope of the photochemical generation of α,n‐didehydrotoluene diradicals from aryl sulfonates and phosphates and their chemistry are explored. The thermally inaccessible α,2‐ and α,4‐ intermediates are efficiently obtained by irradiation of ortho‐ and para‐(trimethylsilylmethyl)phenyl triflates through heterolytic splitting of the ester anion from the substrate in the triplet state. Triplet phenyl cations are formed and the loss of trimethylsilyl cation from them affords the desired diradicals (³Me₃SiCH₂C₆H₄‐OZ→³Me₃SiCH₂C₆H₄⁺→ ^? CH₂C₆H₄ ^? ). Triplet sensitization is required, for which acetone is used throughout. Direct irradiation leads, on the contrary, to photo‐Fries fragmentation (¹Me₃SiCH₂C₆H₄O‐Z→Me₃SiCH₂C₆H₄O ^? +Z ^? ). With mesylates, where ester cleavage is less convenient, a further competition from the triplet is direct desilylation. Didehydrotoluenes are also obtained from the corresponding phosphates, although with poor efficiency.     

Polyacene spacers in intramolecular magnetic coupling

We predict the intramolecular magnetic exchange coupling constant (J) for eleven nitronyl nitroxide diradicals (NN) with different linear and angular polyacene couplers from broken-symmetry density functional treatment. For the linear acene couplers, J initially decreases with increase in the number of fused rings. But from anthracene coupler onward, the J value increases with the number of benzenoid rings due to an increasing diradical character of the coupler moiety. The J value for the diradical with a fused bent coupler is always found to be smaller than that for a diradical with a linear coupler of the same size. The nuclear independent chemical shift (NICS) is calculated, and it is observed that the average of the NICS values per benzenoid ring in the diradical is less than that in the normal polyacene molecule. An empirical formula for the magnetic exchange coupling constant of a NN diradical with an aromatic spacer is obtained by combining the Wiberg bond order (BO), the angle of twist (phi) of the monoradical (NN) plane from the plane of the coupler, and the NICS values. A comparison of the formula with the computed values reveals that, from tetracene onward, the diradical nature of the linear acene couplers becomes prominent thereby leading to an increase in the ferromagnetic coupling constant. Isotropic hyperfine coupling constants are calculated by using a polarized continuum model for the diradicals in different solvents and in vacuum.     

Decomposition of 1,1‐dimethyl‐1‐silacyclobutane on a tungsten filament—evidence of both ring C?C and ring Si?C bond cleavages

The decomposition of 1,1‐dimethyl‐1‐silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum ultraviolet laser single photon ionization time‐of‐flight mass spectrometry. It is found that the decomposition of DMSCB on the W filament to form ethene and 1,1‐dimethylsilene is a catalytic process. In addition, two other decomposition channels exist to produce methyl radicals via the Si? CH₃ bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage of a ring C? C bond and a ring Si? C bond, respectively, to form diradical intermediates, followed by the breaking of the remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C? C bond is dominant over that of propene via an initial cleavage of a ring Si? C bond. When the collision‐free condition is voided, secondary reactions in the gas‐phase produce various methyl‐substituted 1,3‐disilacyclobutane molecules. The dominant of all is found to be 1,1,3,3‐tetramethyl‐1,3‐disilacyclobutane originated from the dimerization of 1,1‐dimethylsilene. Copyright © 2010 John Wiley & Sons, Ltd.     

Spin–orbit coupling in oxygen containing diradicals

Intermediate diradicals which occur in the Paterno–Büchi photocycloaddition and in the Norrish type I photoreactions have been calculated taking into account the spin–orbit coupling (SOC) between the singlet (S) and triplet (T) states. Reaction paths for the photocycloaddition of formaldehyde to ethene and the diradical products of the α-cleavage of cyclohexanone have been optimized by the MNDO CI method for a number of different singlet and triplet states. SOC integrals are calculated by an effective one-electron approximation. Intermediate diradicals in the Paterno–Büchi reaction and the SOC effects are also studied ab initio with CAS SCF geometry optimization in a TZV basis set. Both methods predict a large SOC matrix element between the S and T states in the course of the C–C attack, while the SOC integral is two orders of magnitude smaller for the diradical produced in the C–O attack. In the Norrish type I photoreaction the oxygen atom also produces some nonzero contribution to the SOC integral which governs intersystem crossing in a ·C–C· diradical. For the diradicals produced by the α-cleavage of cyclohexanone a vibronic interaction is responsible for the SOC mixing between the lowest S and T states. The importance of one-center versus two-center SOC contributions in diradicals is briefly discussed.     

DFT study on singlet diradical character of zethrenes

The diradical character of zethrenes was investigated using a symmetry-broken UB3LYP/6-311G(d,p) method. The number of hexagons in the investigated molecules ranges from 6 to 12. It was found that all zethrenes are singlet diradicals, whose diradical character increases with the increasing size of the molecules. A singlet diradical structure provides a possibility for an electron pair to occupy different parts of space, and allows for achieving aromatic stabilization. It can be predicted, on the basis of the singlet-triplet values, that even higher zethrenes will be singlet, but not triplet molecules.     

Crystalline Diradical Dianions of Pyrene-Fused Azaacenes

Although diradicals and azaacenes have been greatly attractive in fundamental chemistry and functional materials, the isolable diradical dianions of azaacenes are still unknown. Herein, we describe the first isolation of pyrene-fused azaacene diradical dianion salts [(18-c-6)K(THF)₂]⁺[(18-c-6)K]⁺⋅ 1 ^2−.. and [(18-c-6)K(THF)]²⁺⋅ 2 ^2−.. by reduction of the neutral pyrene-fused azaacene derivatives 1 and 2 with excess potassium graphite in THF in the presence of 18-crown-6. Their electronic structures were investigated by various experiments, in conjunction with theoretical calculations. It was found that both dianions are open-shell singlets in the ground state and their triplet states are thermally readily accessible owing to the small singlet–triplet energy gap. This work provides the first examples of crystalline diradical dianions of azaacenes with considerable diradical character.     

DFT calculations on the effects of para substituents on the energy differences between singlet and triplet states of 2,2-difluoro-1,3-diphenylcyclopentane-1,3-diyls

UB3LYP/6-31g* calculations have been performed on a series of para-substituted 2,2-difluoro-1,3-diphenylcyclopentane-1,3-diyls (4). The singlet is computed to be the ground state for each of the diradicals, regardless of the nature of the para substituents, which range from strongly pi-electron-donating (amino) to strongly pi-electron-withdrawing (nitro). In the symmetrically para-disubstituted diradicals, the size of the singlet-triplet energy gap (Delta E(ST)) increases with the pi-electron-donating ability of the substituents, but in the unsymmetrically substituted diradicals, large values of Delta E(ST) are calculated even when one of the substituents is a pi electron acceptor. The origins of the competitive and cooperative substituent effects, predicted for diradical 4, are discussed in light of the calculated effects of the same substituents on the singlet and triplet states of diradical 6, which lacks the geminal fluorines at C-2 that are present in 4.