Impact of Transition-State Aromaticity on Selective Radical-Radical Coupling of Triarylimidazolyl Radicals.

Radical coupling reactions are generally known to have a low selectivity due to the high reactivity of radicals. In this study, high regio and substrate selectivity was discovered in the dimerization of triarylimidazolyl radicals (), a versatile photochromic reaction. When two different radicals, 2-(4-cyanophenyl)-4,5-diphenyl-1-imidazolyl radical () and 2-(4-methoxyphenyl)-4,5-diphenyl-1-imidazolyl radical (), were simultaneously generated in situ, a hexaarylbiimidazole, formed by selective coupling at the nitrogen atom at position 1 of and the carbon atom at position 2 of , was isolated with high selectivity as the main product among 24 possible radical dimer hexaarylbiimidazole derivatives.

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 (RC-CR), 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 (RC═CR). 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.

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.

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 Bu group (Bu-TPIR) under N atmosphere and the novel reactivities of TPIRs.

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, PhMeSi-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.

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.

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 (s) embedded in curved cycloparaphenylene (s) structures, -(+3)s ( = 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 - ( = 4) and - ( = 5), by quantum chemical calculations.

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 Å molecule, of which the surface pressure was ∼10 mN m.

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 (ΔE ) 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.

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.

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)B-B(OH), 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.

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.

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.

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.

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.

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.

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.

Unprecedented radical-radical reaction of a [2.2]paracyclophane derivative containing an imidazolyl radical moiety.

Pseudogem-DPIM-DPI[2.2]PC dimer (3), with a C-N bond between the [2.2]paracyclophane ([2.

Photochromism of a radical diffusion-inhibited hexaarylbiimidazole derivative with intense coloration and fast decoloration performance.

We report the synthesis and the photochromic behavior of a newly designed, photochromic, radical diffusion-inhibited hexaarylbiimidazole (HABI) derivative with markedly improved photochromic performance in coloration and decoloration rates as well as greater optical density in the colored state. The thermal bleaching rate (tau1/2 = 260 ms at 295 K) is the fastest among the reported ones for HABI derivatives.

Activation parameters for the recombination reaction of intramolecular radical pairs generated from the radical diffusion-inhibited HABI derivative.

Activation parameters were determined for the recombination of radical pairs arising from newly designed, photochromic, radical diffusion-restricted hexaarylbiimidazole (HABI) derivative. We have developed a new type of radical diffusion-inhibited HABI derivative, which contains two equivalent HABI units and yields a tetraradical with four equivalent 2,4,5-triphenylimidazolyl radical (TPIR) units by photoirradiation. This radical dimerization proceeds by a successive first-order reaction from the tetraradical to the parent molecule via a diradical.