Yozo Miura

Excited high spin states of novel π conjugated verdazyl radicals: photoinduced spin alignment utilizing the excited molecular field

This paper reports the excited quartet (S = 3/2) and quintet (S = 2) states arising from the intramolecular radical-triplet pair in the purely organic π conjugated spin systems. A previous paper reported the excited quartet and quintet states of 9-anthracene-(4-phenyliminonitroxide) and 9,10-anthracene-bis(4-phenyliminonitroxide), respectively, in which iminonitroxide radicals are linked to the phenyl- or diphenylanthracene moiety (a spin-coupler) through the π conjugation. The similar excited quartet and quintet states were observed for the 9-anthra-cene-(4-phenylverdazyl) radical (1) and 9,10-anthracene-bis(4-phenylverdazyl) diradical (2) by time resolved electron spin resonance (TRESR). The TRESR spectrum was analysed by the ordinary spin Hamiltonian with the Zeeman and fine structure terms. For the quartet state of 1, the g value, fine structure splitting, and relative population of the Ms sublevels have been determined to be g = 2.0035, D = 0.0230 cm−1, E = 0.0, P 1/2′ = P −1/2′ = 0.5 and P 3/2′ = P −3/2′ = 0.0, respectively, by spectral simulation. The spin Hamiltonian parameters of the quintet state of 2 were determined to be g = 2.0035, D = 0.0128 cm−1, E = 0.0, P 2′ = P −2′ = 0.0, P 1′ = P −1′ = 0.37 and P 0′ = 0.26, respectively. Direct observation of the excited high spin state showed that photoinduced intramolecular spin alignment is realized between the excited triplet state (S = 1) of the phenyl- or diphenylanthracene moiety and the doublet spin (S = 1/2) of the dangling verdazyl radicals. Ab initio MO calculations (DFT) were carried out in order to clarify the mechanism of the photoinduced spin alignment.

Radical polymerization of butyl acrylate and random copolymerization of styrene and butyl acrylate and styrene and methyl methacrylate mediated by monospiro- and dispiropiperidinyl-N-oxyl radicals

Abstract Radical polymerization of butyl acrylate (BA) and random copolymerizations of styrene (St) and BA and St and methyl methacrylate (MMA) in the presence of 7-aza-15-hydroxydispiro[5.1.5.3]hexadecane-7-yloxyl (1) and 1-aza-2,2-dimethyl-4-hydroxyspiro[5.6]dodecane-1-yloxyl (2) were carried out. Radical polymerization of BA at 120 °C in the presence of 1 gave poly(BA) with Mn=20200 and Mw/Mn=1.30 at 23% conversion. The termination of polymerization observed around ∼20% conversion was solved to a certain extent by an addition of small amounts of dicumyl peroxide, and poly(BA) with Mn=37400 and Mw/Mn=1.33 was obtained in 46% yield. Random copolymerizations of St and BA and St and MMA in the presence of 1 and 2 at 80 °C gave the corresponding random copolymers with narrow polydispersities of 1.12–1.38 at the molar fraction above 0.30 of St in feed. The kinetic study for the NO–C bond homolysis of the corresponding alkoxyamines prepared from 1 and 2 were carried out, and evaluation of the preexponential factors (Aact) and the activation parameters (Eact) showed that the steric factors of the nitroxides are reflected mainly on Eact.

Design, synthesis, and uniquely electron-spin-polarized quartet photo-excited state of a π-conjugated spin system generated via the ion-pair state

A functionalized verdazyl radical, 1, was synthesized, which consists of a bodipy acceptor (A), a phenyl-anthracene donor (D), and a stable verdazyl radical (R). Optical measurements, electron spin resonance (ESR), time-resolved ESR (TRESR), and laser-excitation pulsed ESR experiments were carried out. The efficient intra-molecular energy transfer (EnT) from the anthracene moiety to the bodipy functional component was observed by time-resolved fluorescence spectroscopy and TRESR measurements. A unique dynamic electron-spin polarization (DEP) was detected for the quartet photo-excited state of 1 by TRESR and pulsed ESR. Such a unique DEP was not detected for either the parent verdazyl radical2 without the bodipy acceptor or new compound 3 with an anthraquinone moiety instead of the bodipy component. The spectral simulation of the quartet spectrum of 1 revealed that the unique DEP was generated by competition between a mechanism involving the intra-molecular ion pair *A−–D+–R and spin–orbit intersystem crossing. The dynamic electron-spin polarization via the ion-pair state and the mechanism were discussed based on the experimental data and theoretical calculations. The electronic structure and the spin delocalization in the photo-excited quartet state of 1 were discussed by comparison with the results of 3.

Enzymatic reduction-resistant nitroxyl spin probes with spirocyclohexyl rings

To suppress enzymatic reduction of nitroxyl group of spin probes, this study designed two new nitroxyl probes, 4-hydroxy and 4-oxopiperidine-N-oxyls having 4′-hydroxyspirocyclohexyl groups at the 2- and 6-positions of the piperidine ring (hydroxy-DICPO and oxo-DICPO, respectively). The decay of the EPR signal of these probes in mouse liver homogenates was significantly suppressed compared with that of 4-hydroxy- and 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (hydroxy-TEMPO and oxo-TEMPO, respectively), although hydroxy-DICPO and oxo-DICPO showed no difference in the reactivities with ascorbic acid. While both hydroxy- and oxo-DICPO reacted with hydroxyl radicals, only hydoxy-DICPO lost its EPR signal by the reaction with superoxide anion radical in the presence of cysteine. This feature is similar to that observed for hydroxy- and oxo-TEMPO. These results suggest that the introduction of spirocyclohexyl groups to nitroxyl spin probes is effective for protecting the nitroxyl group against enzymatic reduction without changing the characteristics of the reaction with oxygen radicals.

Syntheses of Functional Alkoxyamines and Application to Syntheses of Well‐Defined Star Polymers

Three kinds of 2,2,6,6-tetramethylpiperidine-N-oxyl (EMPO)-based alkoxyamines, 1-(4-iodophenyl)- (3), 1-(4-ethynylphenyl)- (4), and 1-[4-(1, 3, 2-dioxaborinan-2-yl)phenyl]-(2,2,6,6-tetramethyl-1-piperidinyloxyl)ethanes (5) were prepared. The Pd-catalyzed cross-coupling reaction of 3 with 1,3,5-triethynylbenzene or 1,3,5-tribromobenzene with 4 gave the corresponding 1,3,5-tris(alkoxyaminophyenylethynyl)benzene 11, and the Pd-catalyzed cross-coupling reaction of 5 with 1,3,5-tribromobenzenegave the corresponding 1,3,5-tris(alkoxyaminophenyl)benzene 12. Bulk polymerization of styrene (St) at 120°C initiated with 11 and 12 were investigated. The first-order plots, linear relationships between and conversion, and low M w /M n s of the formed poly(St) showed that the polymerization proceeded in the living fashion, leading to formation of well-defined three-arm star polymers with M w /M n of 1.20-1.40.

SPIN ALIGNMENT BETWEEN THE TRIPLET EXCITED STATE OF PHENYLANTHRACENE AND THE DANGLING VERDAZYL RADICAL AS STUDIED BY TIME-RESOLVED ELECTRON SPIN RESONANCE

The photo-excited quartet (S=3/2) high spin state of a radical-excited triplet pair of a novel compound, phenylanthracene-verdazyl radical, was detected by time-resolved electron spin resonance (TRESR) technique. The TRESR spectrum was well analyzed by the ordinary spin Hamiltonian with the Zeeman and fine-structure terms. The g value, fine-structure splitting, and relative population of the Ms sublevels have been determined to be g=2.0043, D=0.0230 cm-1, E=0.0 cm-1 and P1/2′=P-12′=0.5 and P3/2′=P-3/2′=0.0, respectively, by the spectral simulation. The direct observation of the excited quartet state shows tha the photo-induced intramolecular spin alignment is realized between the excited triplet state (S=1) of the phenylanthracene moiety and the doublet spin (S=1/2) of the dangling verdazyl radical.

Synthesis and magnetic behaviour of polyradical: poly(1,3-phenyleneethynylene) with π-toporegulated pendant stable aminoxyl and imine N-oxide–aminoxyl radicals

The palladium-catalysed polycondensation of N-tert-butyl-N-(3,5-diethynylphenyl)aminoxyl with 2-(3,5-diiodophenyl)-4,4,5,5-tetramethyl-3-oxido-2-imidazolin-3-ium-1-yloxyl in pyridine–triethylamine at room temperature afforded poly(1,3-phenyleneethynylene) with pendant aminoxyl and imine N-oxide–aminoxyl radicals as a light blue powder in 78–83% yields. The number-average molecular weights (n) of the polyradical determined by GPC were 2670–3030 and the spin concentrations determined by EPR were 1.91–2.02 × 1021 spins g−1. Both solution (CH2Cl2) and powder EPR measurements of the polyradicals gave a hyperfine-smeared single line spectrum, suggesting that spin-exchange narrowing took place due to the high spin concentrations of the polyradicals. The magnetic susceptibility (χ) measurements of the polyradicals were carried out on a superconducting quantum interference device (SQUID) magnetometer in the temperature range of 1.8–300 K. The χ Tvs. T plots showed that the polyradicals were a paramegnetic species yielding a weak antiferromagnetic coupling (θ = −1.5 K) below 30 K. The absence of the through-bond ferromagnetic interaction is accounted for by masking of the interaction by the stronger through-space antiferromagnetic interactions.

Syntheses of well-defined poly(siloxane)-b-poly(styrene) and poly(norbornene)-b-poly(styrene) block copolymers using functional alkoxyamines

Abstract Functional alkoxyamines, 1-[4-(4-lithiobutoxy)phenyl]-1-(2,2,6,6-tetramethylpiperidinyl-N-oxyl)ethane (2) and 1-[4-(2-vinyloxyethoxy)phenyl]-1-(2,2,6,6-tetramethylpiperidinyl-N-oxyl)ethane (3) were prepared, and well-defined poly(hexamethylcyclotrisiloxane)-b-poly(styrene)[poly(D3)-b-poly(St)] and poly(norbornene)-b-poly(St) [poly(NBE)-b-poly(St)] were prepared using the alkoxyamines. The first step was preparation of poly(D3) and poly(NBE) macroinitiators, which were obtained by the ring-opening anionic polymerization of D3 using 2 as an initiator and the ring-opening metathesis polymerization of NBE using 3 as a chain transfer. The radical polymerization of St by the poly(D3) and poly(NBE) macroinitiators proceeded in the ‘living’ fashion to give well-defined poly(D3)-b-poly(St) and poly(NBE)-b-poly(St) block copolymers.

Syntheses and magnetic properties of Cu(II)(hfac)2 and Mn(II)(hfac)2 complexes of 4-pyridyl-substituted thioaminyl radicals

Two types of Cu(II)(hfac)2 and Mn(II)(hfac)2 complexes of N-(4-pyridylthio)-4-ethoxycarbonyl-2,6-bis(4-chlorophenyl)phenylaminyl (1) and N-(4-pyridylthio)-2,4,6-tris(4-chlorophenyl)phenylaminyl (2) were prepared and their X-ray crystallographic and magnetic studies were performed. Mixtures of Cu(II)(hfac)2 and 1 and Mn(II)(hfac)2 and 2 in anhydrous heptane-benzene solution gave 1 : 2 complexes of M(II)(hfac)2 (M = Cu, Mn) and 1 or 2 in 73-75% yields. For Cu(II)(hfac)2(1)2 and Mn(II)(hfac)2(2)2 X-ray crystallographic analyses were successfully performed. The magnetic behaviors for the two metal complexes were investigated with a SQUID magnetometer. The analyses for the chimolTvs. T plots of Cu(II)(hfac)2(1)2 were carried out by the numerical diagonalization of the Heisenberg Hamiltonian matrix (4096 x 4096 matrix) for the four repeating units of the complex (12-spin system). The exchange interaction between the copper(II) ion and the thioaminyl radicals is ferromagnetic (J1/kB = +28 K) and the interactions between the complexes is antiferromagnetic (J2/kB = -13 K). The magnetic behavior of Mn(II)(hfac)2(2)2 complexes is well analyzed with the theoretical equation of a 1/2-5/2-1/2 three-spin system taking the intermolecular interaction (theta) into account. The exchange interaction between the Mn(II) ion and the thioaminyl radicals is antiferromagnetic (J/kB = -4.2 K) and theta = -1.0 K. These magnetic behaviors could be well explained in terms of their crystal structures.

Synthesis and magnetic behaviour of poly(1,3-phenylene)-based polyradical carrying N-tert-butylaminoxyl radicals

The synthesis and magnetic behaviour of poly[6-(N-oxyl-N-tert-butylamino)biphenyl-3,3′-ylene] 2 are described. Polyradical 2 was prepared by the Pd-catalyzed polycondensation of N-tert-butyl-2,4-dibromoaniline and 1,3-phenylenebis(trimethylene boronate), followed by oxidation with 3-chloroperoxybenzoic acid. The degree of polymerisation determined by SEC (size exclusion chromatography) was ∼18. Polyradical 2 was stable in both solution and the solid state. The spin concentrations of 2 determined by EPR and SQUID were 0.75 and 0.82 spin per repeating unit, respectively. Magnetic susceptibility (χ) measurements on 2 were carried out on a SQUID magnetometer and these showed that the magnetic interaction between the unpaired electron spins was antiferromagnetic (θ=–0.98 K). The fact that no ferromagnetic interaction was observed was ascribed to a twisting of the nitroxide moieties from the π-conjugated main chain, and this was confirmed by X-ray crystallographic analysis of the corresponding model monoradical, N-tert-butyl-N-(2,4-diphenylphenyl)aminoxyl.