K. Sato

Electronic-spin and columnar crystal structures of stable 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl radical

Abstract 1,3-Diazaphenalenyl is a typical example of the isoelectronic mode of heteroatomic modification for phenalenyl. Recently, we have designed and isolated 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl (6) as the first example of stable azaphenalenyl. For further elucidation of the electronic and solid-state structures of 6, 15N atoms incorporated 1,3-diazaphenalenyl 7 was designed. New nitration reaction by K15NO3 of 2,7-di-tert-butylnaphthalene has enabled us to accomplish effective introduction of 15N atoms and synthesis of 7. The spin structure of 1,3-(15N2)diazaphenalenyl 7 was unequivocally determined by the ratio of the hfcc for 14N and 15N (A14N/A15N=0.292/0.409=0.714), which was equal to the ratio of each gyromagnetic ratio (γ14N/γ15N=0.713). Under an air atmosphere, the radical 7 decomposes slowly, but most of it remains unchanged for weeks. By comparing the spin densities of 7 with those of parent phenalenyl radical 2, it appears that the spin densities of the 1- and 3-positions decrease appreciably, while those of the 4-, 6-, 7-, and 9-positions increase. The radical 7 forms the syn-dimer with gable structure in the crystal, and the dimer stacks in a columnar structure motif.

Synthesis and spin structure of 8-(p-halophenyl)-6-oxophenalenoxyl derivative

2,5-Di-tert-butyl-8-(p-iodophenyl)-6-oxophenalenoxyl has been synthesized as the first 6-oxophenalenoxyl derivative with halophenyl group, and its π-spin structure was unequivocally determined in terms of ESR/ENDOR/IRIPLE spectroscopies and the DFT calculation.

Redox-based spin diversity: a reversible topological spin switching in oxophenalenoxyl systems

A 6-oxophenalenoxyl system is found to be the first example of the molecular system with redox-based spin diversity nature. To generalize this nature, we focus on the 3-oxophenalenoxyl system as a topological isomer of 6-oxophenalenoxyl system. The radical dianion derivative 3 which is the redox state in the 3-oxophenalenoxyl system was successfully generated by the reduction of the corresponding hydroxyphenalenone 5 with potassium mirror. Our ESR/ENDOR/TRIPLE measurements and DFT calculation of 3 demonstrated that the 3-oxophenalenoxyl system has also redox-based spin diversity nature. In addition, for the elucidation of the interplay between spin and charge fluctuation in 3, we discuss a VB approach derived from MO calculations.

Generation and properties of 2,5,8-tri-tert-butyl-6-oxo-7,9-diazaphenalenoxyl

Novel neutral radical, 2,5,8-tri-tert-butyl-6-oxo-7,9-diazaphenalenoxyl, was designed on the basis of 6-oxophenalenoxyl and generated by the chemical oxidation of the corresponding tetraethylammonium salt. The spin density distribution was determined by ESR, ENDOR/TRIPLE measurements and the DFT calculations, showing that then-spin structure is similar to that of the 6-oxophenalenoxyl.

Hyperfine structure of ESR spectra as a probe for heisenberg exchange couplings in nitroxide triradicals serving as building blocks for molecule-based ferrimagnets

A study on electron spin resonance (ESR) spectroscopic determination of exchange interactions in organic oligoradicals is given. When the intramolecular exchange couplingJ between, unpaired electron spins in nitroxide-based oligoradicals falls within the order of 10 Oe (1 mK or 10−3 cm−1 forg=2), which is on the same order as the hyperfine couplingA of magnetic nuclei such as nitrogen atoms of nitroxide radicals, the magnitude ofJ can be determined from the hyperfine splitting pattern of ESR spectra in solutions. This range of the exchange couplingJ is not detectable in conventional magnetic susceptibility measurements. We demonstrate an application of hyperfine ESR spectroscopy as a probe for the exchange coupling to a series of organic oligoradicals, which the authors have recently developed as building blocks for molecule-based magnetic materials.

A novel organic neutral radical system: topological effects in oxophenalenoxyls

Abstract An idea of molecular topology for spin alignment in π-electron networks, which has been widely accepted in high-spin chemistry, can be expanded to monoradical π-systems. In this paper, we describe the importance of the topology of π-electron network for elucidating spin structures in the monoradical systems. An oxophenalenoxyl is a phenalenyl-based neutral monoradical and has many topological isomers depending on the positions of introduced two oxygen atoms. We have focused on a series of α, α-type of oxophenalenoxyls, 3-, 4-, 6- and 9-oxophenalenoxyl systems with two oxygen substituents at α-positions on the phenalenyl skeleton. Our experimental studies have demonstrated that the spin-delocalized nature of the oxophenalenoxyl systems extremely depends on the topological symmetry of the introduced two oxygen atoms. The π-spin density distributions have been calculated by density functional theory, supporting the topology-dependent spin diversity nature inherent in a series of oxophenalenoxyl systems.

Syntheses and spin structures of 1,6-dithiapyrene derivatives having imino nitroxide or oxoverdazyl moiety

Abstract 1,6-Dithiapyrene (DTPY) derivatives containing imino nitroxide or 1,5-dimethyl-6-oxoverdazyl moiety 1 and 2 have been designed as new neutral radicals, and their syntheses were carried out from DTPY in three steps. The isotropic hyperfine coupling constants of the protons and nitrogen atoms for 1 and 2 were determined by ESR/ENDOR/TRIPLE measurements and ESR simulations. Their assignments were made with the help of the density functional theory by using gaussian -98 with the UBLYP/6-31G**//UBLYP/6-31G** method. The syntheses of 1 and 2 contribute to the expansion of structural diversity in the field of the magnetic and conductive materials.

A synthetic study of a homospin diradical based on 6-oxophenalenoxyl

A novel homospin diradical, 8,8-bis(2,5-di-tert-butyl-6-oxophenalenoxyl) was designed based on 6-oxophenalenoxyl. A diradical precursor, an 8,8-bis(6-hydroxyphenalenone) derivative was synthesized by the coupling reaction of the lithium enolate of phenalanone derivative as a key step. The monoradical of the precursor was generated and characterized in terms of ESR/ 1 H-ENDOR/TRIPLE measurements and the DFT calculations.

Mechanism for spin alignments in organic molecule-based ferrimagnetics

A mechanism for spin alignments in molecule-based ferrimagnets is proposed in terms of a Heisenberg spin Hamiltonian and molecular orbital calculations for a supramolecule composed of an S = 1 biradical and an S = 1/2 radical. Spin polarization in the supramolecule with an effective S = 1/2 spin is found to give ferromagnetic interactions between the supramolecules, leading to the ferrimagnetic ground state.

A cw/pulsed ESR study of a calix[4]arene biradical with two nitroxides

A calix[4]arene based biradical with two tert-butyl nitroxide radicals has been studied by cw and pulsed ESR spectroscopy. Two nitroxide radicals antiferromagnetically interact with each other, generating a thermally accessible triplet state. The spin multiplicity of the biradical was identified by a pulsed ESR-based 2D-electron spin transient nutation (ESTN) spectroscopy. Based on spin Hamiltonian parameters of the biradical determined from a spectrum simulation, a salient electronic local structure of the radical sites with π-orbitals on the nitrogen atoms facing each other was determined.