Electron Spin Transient Nutation Spectroscopy Research Articles

Organic polyanionic high-spin molecular clusters: topological-symmetry controlled models for organic ferromagnetic metals

Trianionic spin-quartet and tetraanionic spin-quintet molecular clusters derived from m-dibenzoylbenzene in solution were identified by CW-ESR/pulse-ESR based two-dimensional electron spin transient nutation spectroscopy, and their spin and clustering structures in the ground state were determined in terms of a D-tensor based phenomenological approach and DFT calculations. The molecular structures obtained semiempirically are supported by DFT-based quantum chemical calculations. The DFT calculations have been tested for a sodium ion bridged fluorenone-based cluster, [fluorenone(-)˙ {Na(+)(dme)(2)}](2), whose crystal structure was reported in the literature [H. Bock, H.-F. Herrmann, D. Fenske and H. Goesmann, Angew. Chem., Int. Ed. Engl., 1988, 27, 1067], reproducing the experimentally determined moelcular structure of the dimer cluster. It is suggested that both the quartet and quintet clusters in the 2-MTHF glass and solution form the cross-typed structures with the two m-dibenzoylbenzene moieties in cis-configuration. A dianionic spin-triplet m-dibenzoylbenzene derivative was detected for the first time and its charge and spin densities were studied by the quantum chemical calculations. The high-spin states of the open-shell entities under study were confirmed by X-band pulse-ESR based electron spin nutation spectroscopy in organic frozen glasses. The D values and other spin Hamiltonian parameters of all the polyanionic high-spin species were determined by the hybrid eigenfield spectral simulation for fine-structure ESR spectra. m-Dibenzoylbenzene provides pseudo-degenerate π-LUMOs arising from its topological symmetry of the π-electron network and its dianion in the triplet ground state is a prototypical model for topologically-controlled genuinely organic ferromagnetic metals.

Electronic and molecular structures of C 60-based polyanionic high-spin molecular clusters: Direct spin identification and electron spin transient nutation spectroscopy for high-spin chemistry

C 60-based polyanionic high-spin clusters ( S = 1–3) in their ground state have been prepared by successive chemical reductions of pristine C 60 fullerene with potassium in the presence of dicyclohexano-18-crown-6-ether in solution. Intermolecular spin-triplet, quintet, and septet states arising from the C 60-based polyanionic molecular clusters have been generated at ambient temperature and identified by CW-ESR and pulse-ESR-based two-dimensional (2D) electron spin transient nutation (2D-ESTN) spectroscopy in organic rigid glasses, for the first time. Intermolecular exchange interactions between mono- and polyanionic C 60 fullerenes are ferromagnetic via bridging potassium metal cations. The molecular structures of the polyanionic high-spin C 60 clusters in solution have been proposed by a well-established phenomenological spin Hamiltonian approach in terms of the D -tensor-based calculations for the high-spin clusters. The findings of the C 60-based high-spin molecular clusters evidence the occurrence of an intramolecular triplet C 60 dianion in the ground state. Unequivocal spin identification for molecular high-spin entities by 2D-ESTN spectroscopy and its powerfulness have been illustrated, emphasizing that the 2D-ESTN spectroscopy is useful for mixtures of molecular species with different spin multiplicities, characterized by a small g-anisotropy, for which the powerfulness of advanced high-frequency ESR spectroscopy is hampered.

Spin-multiplicity of a moderately coupled triplet–doublet spin pair in a biphenylene-linked porphyrin dimer

Electron spin transient nutation spectroscopy has been applied to a moderately coupled excited triplet and doublet spin-pair. In a biphenylene-linked copper(II)-free base porphyrin dimer, laser irradiation of either the free base half or the copper half, leads to excitation to the excited states and relaxation to the lowest excited state of the dimer. In this lowest excited state, the free base moiety in the lowest triplet state is coupled to the copper porphyrin ground doublet state, and thus forming a triplet–doublet spin pair. Time-resolved electron paramagnetic resonance (TREPR) spectra of the dimer in frozen solution at 40 K show much wider spectral width and a prominent band in the centre of the spectra than either of those for the free base monomer triplet or that of the copper(II) monomer porphyrin. These features are quite different from the strongly-coupled and weakly-coupled triplet–doublet spin pairs. Transient nutation method by pulsed EPR is used to examine the spin-multiplicity of the excited state in the dimer at three magnetic field positions. It was found that spin-multiplicity depends largely on the magnetic field. This is rationalized in terms of an intermediate size of coupling. In such intermediate coupling cases, the degree of mixing of the triplet and doublet basis wavefunctions depends greatly on the relative orientation between magnetic field and molecular axis because the ratio of the coupling size to an energy difference between the unperturbed species, triplet and doublet, varies with magnetic field direction. A splitting of TREPR spectra in partially oriented samples suggests that exchange coupling |J| is around 10 mT, which reasonably matches the results of the pulsed experiments.

A Bindschedler's Green-Based Arylamine: Its Polycations with High-Spin Multiplicity

Intramolecular high-spin correlation in a series of the successively generated polycationic species of Bindschedler's green-based arylamine, N,N,N‘,N‘,N‘‘,N‘‘-hexakis[4-(dimethylamino)phenyl]-1,3,5-benzenetriamine (1), has been investigated by continuous wave (CW) and pulsed EPR spectroscopy. Cyclic voltammetry shows multiredox behavior of 1 that can be reversibly oxidized from monocation to hexacation. Depending on the quantity of the added oxidant, the characteristic EPR spectra are observed for polycations of 1 in frozen solution. Unequivocal determination of the spin state at each oxidation stage of 1 is given by a pulsed EPR technique, that is, electron spin transient nutation spectroscopy.

Electronic and Molecular Structures of Dendrimeric High-spin Polycarbenes as Studied by CW and Pulsed ESR-based Electron Spin Transient Nutation Spectroscopy

Highly symmetric high-spin polycarbenes of π-conjugation, which are the first and second generations of the spin-mediated dendrimers based on phenylacetylenes, have been studied by cw and pulsed ESR spectroscopy. We measured pulsed ESR-based two-dimensional electron-spin transient nutation (2D-ESTN) spectra of the stabilized polycarbene systems connecting the phenylacetylene moieties at the meta-positions of benzene rings, and we determined their spin multiplicity with the help of a transition moment analysis. A septet ground state of the triscarbene system, which is the first generation of the phenylacetylene dendrimers, was unequivocally discriminated. It is indicated that the stable diphenylcarbene serves as a useful building block and the phenylacetylene dendrimer as a ferromagnetic coupling unit with robust spin polarization for super high-spin polymeric systems and super paramagnetic materials.

Electronic and Molecular Structures of Dendrimeric High-spin Polycarbenes as Studied by CW and Pulsed ESR-based Electron Spin Transient Nutation Spectroscopy

Highly symmetric high-spin polycarbenes of π-conjugation, which are the first and second generations of the spin-mediated dendrimers based on phenylacetylenes, have been studied by cw and pulsed ESR spectroscopy. We measured pulsed ESR-based two-dimensional electron-spin transient nutation (2D-ESTN) spectra of the stabilized polycarbene systems connecting the phenylacetylene moieties at the meta-positions of benzene rings, and we determined their spin multiplicity with the help of a transition moment analysis. A septet ground state of the triscarbene system, which is the first generation of the phenylacetylene dendrimers, was unequivocally discriminated. It is indicated that the stable diphenylcarbene serves as a useful building block and the phenylacetylene dendrimer as a ferromagnetic coupling unit with robust spin polarization for super high-spin polymeric systems and super paramagnetic materials.

Extremely π-Delocalized Nature of Dianthrylcarbene-Based High Spin Systems as Studied by CW and Pulsed 2D Electron Spin Transient Nutation Spectroscopy

Electronic and molecular structures of bis(9,9′-dianthrylcarbene) and tris(9,9′-dianthrylcarbene) coupled by 1,4-diethynylbenzene and 1,3,5-triethynylbenzene, respectively, have been investigated by cw and pulsed ESR spectroscopy. Their spin multiplicities were identified by two-dimensional electron spin transient nutation (2D-ESTN) method based on the pulsed ESR technique. The fine-structure ESR spectra observed after UV-irradiation of the corresponding diazo precursor were analyzed with the help of a hybrid eigenfield method. The biscarbene is spin-singlet in the ground state and a thermally accessible triplet state with an axial symmetry was detected, showing that the biscarbene undergoes a linear and perpendicular conformation of the two 9,9′-dianthrylcarbene moieties. Such a conformation gives rise to an extremely large amount of π-spin delocalization in ethynylbenzene-based 9,9′-dianthrylcarbene derivatives. The electronic and molecular structures of tris(9,9′-dianthrylcarbene) in the septet ground state are also discussed in this context. The oligocarbenes under study are a novel series of Skell-type high-spin carbenes characterized by extremely π-delocalized nature.

ChemInform Abstract: Synthesis of 1,3‐Bis(diarylamino)benzenes (I) as Model Precursors for One‐Dimensional Organic Ferromagnetic Metals; Characterization of the Dications by Cyclic Voltammetry and Electron Spin Transient Nutation Spectroscopy.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis of 1,3-bis(diarylamino)benzenes as model precursors for one-dimensional organic ferromagnetic metals; characterization of the dications by cyclic voltammetry and electron spin transient nutation spectroscopy

A series of 1,3-bis(diarylamino)benzenes as model precursors for 1D organic ferromagnetic metals were synthesized. Cyclic voltammetric measurements showed that the corresponding monoand dication radicals with protecting groups were stable enough even at ambient temperature. A triplet state for the dication was unequivocally identified by a novel electron spin transient nutation spectroscopy.

An FT-Pulsed ESR/Electron Spin Transient Nutation Study of Hyperbranched π-Aryl Stable Triplet and Quartet Molecules

Abstract An electron spin transient nutation (ESTN) method based on FT-pulsed ESR spectroscopy was for the first time carried out for identifying the ground-state high spin multiplicities of stable π-aryl based molecules in organic glasses. The transient nutation frequencies (ωn) observed from the molecules in frozen toluene were interpreted using an equation, ωn=ω1 × [S(S+1)-MS(MS-l)]1/2, expected for an IS, MS-1 > ↔ IS, MS > single transition in the weak microwave irradiation limit. ω1 is a frequency that depends on the strength of an irradiated microwave pulse field. The observed nutation frequencies clearly depended on the S and MS, identifying directly that the ground states of the molecules were triplet and quartet from the viewpoint of quantum spin nutation phenomena. The usefulness of the ESTN spectroscopy for the identification of spin multiplicity was illustrated to be a complement of cw-ESR spectroscopy.

FT Pulsed ESR/Electron Spin Transient Nutation Spectroscopy in the Study of Molecular Based Magnetism: Applications to High-Spin Polymers and Ferromagnetic Materials

Abstract Recently, organic high-spin polymers and clusters have been emerging. With the increasing effective molecular spin quantum number S and molecular weight of the polymers, however, cw ESR spectroscopy manifests its inherent disadvantages in discriminating high spins from S=½ and in determining the S's for the complex mixture of various spin assemblages. An electron spin transient nutation method based on pulsed ESR spectroscopy has been for the first time applied to a quasi 1D high-spin polymer as one of the most complex amorphous spin assemblages, identifying that the polymer is comprised of high-spin assemblages with the S's greater than two. It can be concluded that electron spin transient nutation spectroscopy is a facile and useful method for the exclusive identification of S and ESR transitions even for the cases of apparently vanishing fine-structure splittings and for spin systems with residual fine-structure terms in the spin Hamiltonian. Fundamental bases for the transient nutation method...

Excitation and detection schemes in pulsed EPR

Abstract