Energetically More Stable Singlet Cyclopentane-1,3-diyl Diradical with π-Single Bonding Character than the Corresponding σ-Single Bonded Compound.

Carbon-carbon σ-single bonds are crucial for constructing molecules like ethane derivatives (R3C-CR3), which are composed of tetrahedral four-coordinate carbons. Molecular functions, such as light absorption or emission, originate from the π-bonds existing in ethylene derivatives (R2C═CR2). In this study, a relatively stable cyclopentane-1,3-diyl species with π-single bonding system (C-π-C) with planar four-coordinate carbons is constructed. This diradicaloid is energetically more stable than the corresponding σ-single bonding system. The π-electron single bonding system provides deeper insights into the chemical bonding and the physical properties derived from the small energy gaps between the bonding and antibonding molecular orbitals.

Photoinduced α-Aminoalkylation of Sulfonylarenes with Alkylamines.

α-Aminoalkylation of sulfonylarenes with alkylamines was found to be induced by photoirradiation. Here various types of alkylamines, such as trialkylamines, dialkylamines, N,N-dialkylanilines and N-alkylanilines as well as sulfonylarenes containing an azole, azine, heterole or benzene ring are available. The reaction proceeds through a homolytic aromatic substitution (HAS) process consisting of addition of an α-aminoalkyl radical to a sulfonylarene and elimination of the sulfonyl radical to give the α-arylalkylamine, where photoirradiation is considered to induce homolysis of sulfonylarenes leading to the generation of α-aminoalkyl radicals that make a radical chain operative.

Generation and Characterization of a Tetraradical Embedded in a Curved Cyclic Paraphenylene Unit.

Unique spin-spin (magnetic) interactions, ring-size effects on ground-state spin multiplicity, and in-plane aromaticity has been found in localized 1,3-diradicals embedded in curved benzene structures such as cycloparaphenylene (CPP). In this study, we characterized the magnetic interactions in a tetraradical consisting of two localized 1,3-diradical units connected by p-quaterphenyl within a curved CPP skeleton by electron paramagnetic resonance (EPR) spectroscopy and quantum chemical calculations. Persistent triplet species with zero-field splitting parameters similar to those of a triplet 1,3-diphenylcyclopentane-1,3-diyl diradical were observed by continuous wave (CW) or pulsed X-band EPR measurements. The quintet state derived from the ferromagnetic interaction between the two triplet diradical moieties was not detected at 20 K under glassy matrix conditions. At the B3LYP/6-31G(d) level of theory, the singlet state was lower in energy than the triplet and quintet states. These findings will aid in the development of open-shell species for material science application.

Thermal Reaction Behavior of Triphenylimidazolyl Radical with a Bulky Substituent.

The triphenylimidazolyl radical (TPIR) is generated by the irradiation of the photochromic molecule hexaarylbiimidazole (1,2'-HABI). Usually, the unsubstituted TPIRs form 1,2'-HABI thermally at room temperature. In this study, we report the thermal reaction behavior of TPIR with a tBu group (tBu-TPIR) under N2 atmosphere and the novel reactivities of TPIRs. Under N2 atmosphere at room temperature, tBu-TPIRs form SpI with a spiro carbon and a novel HABI isomer tBu-1,4'-HABI, whose bonding pattern is different from that of the original unsubstituted HABI (1,2'-HABI). The results of 1H NMR spectroscopy, EPR measurements, and DFT calculations revealed that SpI is generated via three steps: (1) intramolecular hydrogen transfer from the tBu group to the nitrogen atom of the imidazole ring, (2) intramolecular cyclization of alkyl radicals, and (3) intermolecular hydrogen transfer with another tBu-TPIR. Furthermore, we found that the thermal reaction of tBu-TPIR at a low temperature affords the diastereomers of other isomers (tBu-4,4'-HABI_RS and tBu-4,4'-HABI_RR); in other words, the thermal reaction of tBu-TPIR depends on temperature.

A stable silylborane with diminished boron Lewis acidity.

A new dimethyl(phenyl)silylborane having a naphthalene-1,8-diaminato (dan) substituent on the boron center, PhMe2Si-B(dan), was synthesized. Owing to the diminished boron Lewis acidity, it is highly stable toward air. Synthetic application of the silylborane to catalytic silylboration and silylation of alkynes is also described.

1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity.

Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1,3-diradicals (DRs) embedded in curved cycloparaphenylene (CPPs) structures, DR-(n+3)CPPs (n = 0-5), was investigated to understand the effects of the curvature and system size on the spin-spin interactions and singlet versus triplet state, as well as their unique characteristics such as in-plane aromaticity. A triplet ground state was predicted for the larger 1,3-diradicals, such as the seven- and eight-paraphenylene-unit-containing diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5), by quantum chemical calculations. The smaller-sized diradicals DR-(n+3)CPPs (n = 0-3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of DR-6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-DR-4CPP (n = 1), as proven by nucleus-independent chemical shift calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that homoconjugation between the 1,3-diradical moiety arises because of the curved and distorted bonding system.

Impact of the macrocyclic structure and dynamic solvent effect on the reactivity of a localised singlet diradicaloid with π-single bonding character.

Localised singlet diradicals are key intermediates in bond homolysis processes. Generally, these highly reactive species undergo radical-radical coupling reaction immediately after their generation. Therefore, their short-lived character hampers experimental investigations of their nature. In this study, we implemented the new concept of "stretch effect" to access a kinetically stabilised singlet diradicaloid. To this end, a macrocyclic structure was computationally designed to enable the experimental examination of a singlet diradicaloid with π-single bonding character. The kinetically stabilised diradicaloid exhibited a low carbon-carbon coupling reaction rate of 6.4 × 103 s-1 (155.9 µs), approximately 11 and 1000 times slower than those of the first generation of macrocyclic system (7.0 × 104 s-1, 14.2 µs) and the parent system lacking the macrocycle (5 × 106 s-1, 200 ns) at 293 K in benzene, respectively. In addition, a significant dynamic solvent effect was observed for the first time in intramolecular radical-radical coupling reactions in viscous solvents such as glycerin triacetate. This theoretical and experimental study demonstrates that the stretch effect and solvent viscosity play important roles in retarding the σ-bond formation process, thus enabling a thorough examination of the nature of the singlet diradicaloid and paving the way toward a deeper understanding of reactive intermediates.

Self-Propelled Motion of a Camphor Disk on a Photosensitive Amphiphilic Molecular Layer.

We have studied the self-propelled motion of a camphor disk on a 2,2'-bis(2-chlorophenyl)-4,4',5,5'-tetraphenyl-1,2'-biimidazole ( o-Cl-HABI) molecular layer, which was developed on water, as a photomechanical sensing system. The o-Cl-HABI dimer changed to its monomeric form upon UV light irradiation, and as a result, the surface pressure of the o-Cl-HABI molecular layer decreased. The reciprocating motion of a camphor disk in the absence of UV light irradiation was observed at A = 45 Å2 molecule-1, of which the surface pressure was ∼10 mN m-1. Random motion was observed under UV light irradiation at A = 45 Å2 molecule-1, of which the surface pressure was ∼5 mN m-1. Therefore, the nature of motion of a camphor disk changes depending on the photosensitivity of the o-Cl-HABI molecular layer. We have discussed the mechanism of the change in the motion of the camphor disk in relation to the photoreaction of the o-Cl-HABI molecular layer with the surface pressure acting as the driving force.

Generation of "Sumanenylidene": A Ground-State Triplet Carbene on a Curved π-Conjugated Periphery.

We have observed the generation of sumanenylidene (2), a divalent, neutral-carbon species at the benzylic position of sumanene (1). We also clarified both experimentally and theoretically that the ground state of compound 2 was a triplet state and that its singlet-triplet energy gap (ΔEST ) was similar to that in fluorenylidene. The curved structure of compound 2 led to slightly better spin delocalization over the two adjacent aromatic rings than in planar systems, because of the unpaired spins on the σ and π orbitals. Synthetic application of the carbene precursor, diazosumanene (5), with a variety of thiocarbonyl compounds revealed its utility for the preparation of tetrasubstituted alkene compounds (e.g., that contain a strongly electron-donating unit) that are directly conjugated to the sumanene (1) moiety.

Anthranilamide (aam)-substituted diboron: palladium-catalyzed selective B(aam) transfer.

An unsymmetrical diboron bearing an anthranilamide (aam) substituent was synthesized and was found to be coupled with aryl halides through the palladium-catalyzed Miyaura-Ishiyama-type reaction. The selective transfer of aam-substituted boryl moiety led to exclusive formation of diverse Ar-B(aam).

Extremely Long Lived Localized Singlet Diradicals in a Macrocyclic Structure: A Case Study on the Stretch Effect.

Localized singlet diradicals have attracted much attention, not only in the field of bond-homolysis chemistry, but also in nonlinear optical materials. In this study, an extremely long lived localized singlet diradical was obtained by using a new molecular design strategy in which it is kinetically stabilized by means of a macrocycle that increases the molecular strain of the corresponding σ-bonded compound. Notably, the lifetime of this diradical (14 µs) is two orders of magnitude longer than that of a standard singlet diradical without a macrocyclic structure (≈0.2 µs) at 293 K. The species is persistent below a temperature of 100 K. In addition to the kinetic stabilization of the singlet diradical, the spontaneous oxidation of its corresponding ring-closed compound at 298 K produced oxygenated products under atmospheric conditions. Apparently, the "stretch effect" induced by the macrocyclic structure plays a crucial role in extending the lifetime of localized singlet diradicals and increasing the reactivity of their corresponding σ-bonded compounds.

B(MIDA)-Containing Diborons.

Unsymmetrical B(MIDA)-containing diborons of potential synthetic utility were found to be synthesized from the readily available (neop)B-B(neop) and (HO)2B-B(OH)2, and the procedure was extended to the generation of symmetrical (MIDA)B-B(MIDA). NMR and X-ray crystal structure studies revealed that B(MIDA) units of all of the diborons obtained in this study were in rigid tetrahedral environment.

Control of the Termination Mechanism in Radical Polymerization by Viscosity: Selective Disproportionation in Viscous Media.

The termination mechanism of radical polymerization, that is, disproportionation (Disp) versus combination (Comb), determines the chain length and end-group structure of the resulting polymer as well as polymer properties, and yet factors governing the mechanism are still unclear. Furthermore, no attempts have been made to control the mechanism. Here, the effects of temperature and viscosity on the termination of methyl methacrylate (MMA) and styrene (St) polymerization were elucidated by using small molecular model-radicals and the corresponding polymer radicals in various solvents. The results showed that Disp was preferred over Comb if the temperature was decreased and the viscosity of the media was increased for all the radicals examined. Although the temperature effect on the Disp/Comb selectivity is counterintuitive because Disp should be favored entropically over Comb considering the decrease in the number of polymer chains in Comb, the results clearly showed that the observed inverse temperature effect was a result of the viscosity effect. Disp was favored over Comb at lower temperatures and in more viscous solvents because the transition state leading to Disp is more flexible than that for Comb. Because of the significant viscosity effect, Disp selectively occurred in highly viscous solvents; the Disp/Comb selectivity was 97/3 in both MMA and St termination. For the first time, the termination mechanism was intentionally controlled and such a high Disp selectivity was observed. In particular, the termination mechanism in St is described as Comb in textbooks, but nearly complete inversion of the selectivity from Comb to Disp is realized by simply changing the viscosity of the media.

A Puckered Singlet Cyclopentane-1,3-diyl: Detection of the Third Isomer in Homolysis.

In the photochemical denitrogenation of 1,4-diaryl-2,3-diazabicyclo[2.2.1]heptane (AZ6) bearing sterically hindered substituents, a curious new absorption band at about 450 nm was observed under low-temperature matrix conditions, together with the previously well-characterized planar singlet diradical pl-(1) DR6 with λmax =≈580 nm. The 450 nm species was electron paramagnetic resonance (EPR)-silent. Instead of generating the planar diradical pl-(1) DR6 and the precursor azoalkane AZ6 upon warming, the ring-closed bicyclo[2.1.0]pentane derivative SB6, that is, the AZ6 denitrogenation product was identified. Based on product analysis, low-temperature spectroscopic observations, high-level quantum-mechanical computations, viscosity effect, and laser-flash photolysis, the puckered singlet diradicaloid puc-(1) DR6 was assigned to the new 450 nm absorption. The latter was detected experimentally at the same time as the planar singlet diradical pl-(1) DR6. Sterically demanding substituents as well as viscosity impediments were essential for the detection of the experimentally hitherto unknown puckered singlet cyclopentane-1,3-diyl diradicaloid puc-(1) DR6, that is, the third isomer in homolysis. The present findings should stimulate future work on the mechanistically fascinating stereoselectivity documented in the formation of bicyclo[2.1.0]pentanes during the 2,3-diazabicyclo[2.2.1]heptane denitrogenation.

Entropy-controlled biradical-quinoid isomerization of a π-conjugated delocalized biradical.

Biradicaloid species have been extensively studied for their characteristic features in electric conductivity, magnetism, and optical nonlinearity. Theoretical investigations of rigid biradicaloid species have been suggesting that they are represented as a resonance hybrid of open-shell biradical and closed-shell quinoid structures. However, much is still unknown about flexible biradicaloid species whether the activation free energy barrier between these states exists or not. Herein, we investigated the thermal isomerization from the photogenerated unstable biradical to the stable quinoid species observed for the photochromic dimer of a bisimidazolyl radical and found that the large negative activation entropy for the valence isomerization causes the activation free energy barrier between these two states.

Dual association modes of the 2,5,8-tris(pentafluorophenyl)phenalenyl radical.

The 2,5,8-tris(pentafluorophenyl)phenalenyl radical was obtained by a straightforward synthesis in 11 steps from 2,7-dibromonaphthalene. This radical crystallized as a σ dimer from a solution in MeCN and as a π-stack from a melted liquid. The π stack was not confined to dimerization, but extended into a uniform 1D stack with an interplanar distance of 3.503 Å. This unique duality in association mode arose from the thermodynamic stability of the phenalenyl moiety.

Multistate photochromism of 1-phenylnaphthalene-bridged imidazole dimer that has three colorless isomers and two colored isomers.

A new type of the bridged imidazole dimer with a 1-phenylnaphthalene moiety that bridges two diphenylimidazole units at the 2- and 2'-positions was synthesized and the photochemical and thermochemical properties were investigated. This molecule shows unique multistate photochromism, in which the stable colorless 1,2'-isomers A and B photochemically isomerize to the colorless 2,2'-isomer through the short-lived biradical with a half-life of 180 ns at 25 °C. The 2,2'-isomer thermally returns to the 1,2'-isomers A and B through the colored isomer at elevated temperatures. The 1,2'-isomers A and B, the 2,2'-isomer, and the colored isomer were isolated, and their molecular structures were determined by X-ray crystallographic analysis. These isomers are stable at room temperature and can be almost fully converted to the 2,2'-isomer by light irradiation. This study serves the useful strategy for the molecular design of a new type of negative photochromic molecules applicable to switch molecular properties by visible light irradiation.

Insights into the recombination of radical pairs in hexaarylbiimidazoles.

The (13)C NMR characterization of the 2,2'- and the 4,4'-hexaarylbiimidazoles is reported for the first time as resulting from the recombination of the radical pairs produced upon irradiation of the 1,2'-dimer, without any isolation processes, directly in a 500 MHz NMR spectrometer coupled with in situ laser irradiation.

Unusual negative photochromism via a short-lived imidazolyl radical of 1,1'-binaphthyl-bridged imidazole dimer.

We have synthesized a new photochromic compound that exhibits unusual negative photochromism, in which the stable colored species photochemically converts into the metastable colorless species via a short-lived radical. This compound has a 1,1'-binaphthyl moiety bridging the two diphenylimidazole units. Its photochemical properties were investigated by nanosecond laser flash photolysis. The colored species isomerizes to the colorless species upon exposure to visible light and thermally returns to the original colored species within 20 min at room temperature. Moreover, the photodecoloration reaction proceeds via a short-lived radical with a half-life of 9.4 µs in benzene at room temperature. Both the colored and colorless species show the photoinduced homolytic bond cleavage reaction of the C-N bond between the nitrogen atom of the imidazole ring and the carbon atom of the 1-position of the 1,1'-binaphthyl moiety and that of the C-C bond between each of the carbon atoms of the 2-position of the imidazole ring, respectively, followed by their formation by rapid radical coupling.

A peroxide-bridged imidazole dimer formed from a photochromic naphthalene-bridged imidazole dimer.

2,4,5-Triphenylimidazole (lophine) is known as the first chemiluminescence substrate, and its oxidized derivative, the 2,4,5-triphenylimidazolyl radical, corresponds to the coloured species in the photochromic reaction of hexaarylbiimidazole (HABI). We report the first direct observation of the O(2) adduct of the imidazolyl radical that forms the end-on peroxide-bridged imidazole dimer. The ring-opening reaction of the peroxide-bridged imidazole dimer leading to the formation of an N-benzoylbenzamidine derivative supports the presence of the 4,5-epidioxide of lophine as a reaction intermediate of its chemiluminescence.