Exchange Interaction in Covalently Bonded Biradical−Monoradical Composite Molecules

Abstract

As a novel molecular designing for genuinely organic molecule-based ferrimagnets, we have proposed a strategy of "single-component ferrimagnetics". When a pi-biradical with an S = 1 ground state and a pi-monoradical with S = (1)/(2) are united by sigma-bonds, the pi-conjugation between the biradical and the monoradical moieties should be truncated in the resultant triradical. This gives magnetic degrees of freedom for both S = 1 and (1)/(2) in the single molecule, serving as a building block for organic molecule-based ferrimagnets under favorable conditions (single-component ferrimagnetics). We have designed and synthesized a triradical, 3-(1'-oxyl-3'-oxido-4',4',5',5'-tetramethylimidazolin-2-yl)benzoic acid 2,4-bis(1' '-oxyl-3' '-oxido-4' ',4' ',5' ',5' '-tetramethylimidazolin-2-yl)phenyl ester (4), as a model compound for the novel approach to genuinely organic ferrimagnets. In the triradical 4, a m-phenylene-bis(nitronyl nitroxide) biradical with a triplet (S = 1) ground state is united with a phenyl nitronyl nitroxide monoradical (S = (1)/(2)) by an ester coupler. Solution-phase ESR spectra from 4 exhibited a complex hyperfine splitting due to (14)N and (1)H nuclei. The analysis of the hyperfine structure based on perturbation calculations has revealed that the exchange interaction within the biradical moiety is much larger than those between the biradical and the monoradical moieties and the magnetic degrees of freedom for both S = 1 and (1)/(2) are retained in 4. An X-ray crystal structure analysis showed that the triradical molecules are arranged in a one-dimensional molecular chain in the crystal. The magnetic susceptibility in a crystalline solid state is consistent with the crystal structure.

EPR spectroscopy of thermally induced and light-induced spin transitions in heterospin exchange clusters of compounds Cu(hfac)2LR

“…1- 23 In addition to the static magnetic susceptibility technique and X ray diffraction analysis, in many cases EPR spectroscopy be comes a key method of investigations of the structural and magnetic properties of such exchange coupled systems. [24][25][26][27][28][29][30][31] This review is devoted to a new class of molecular magnet ics, viz., "breathing crystals" and summarizes for the first time the results of systematic EPR investigations of ther mally induced and light induced spin transitions in these compounds. 32-38 R = Me, Et, Pr n , Bu n The family of heterospin "breathing crystals" Cu(hfac) 2 L R based on copper(II) hexafluoroacetylacetonate and pyr azolyl substituted nitronyl nitroxides was discovered quite recently.…”

Dynamic mixing processes in spin triads of “breathing crystals” Cu(hfac)2LR: a multifrequency EPR study at 34, 122 and 244 GHz

“…Electron paramagnetic resonance (EPR) spectroscopy is widely used in the studies of exchange-coupled systems. [18][19][20][21][22][23][24] We have shown previously that the EPR of strongly-coupled spin triads is specific and informative. [13][14][15][16] First, the ground multiplet state of a triad (Fig.…”

Experimental Evidence for the Triplet-Like Spin State Appearing in Ground-State Singlet Biradicals as a Key Feature for Generalized Ferrimagnetic Spin Alignment

“…1 Ferrimagnetism is one of the categories in magnetism, the physical picture of which has been initiated by Ne ´el: 2 An antiparallel coupling of different spin quantum numbers, e.g., S ) 1 and S ) 1 / 2 , gives an ordered state with bulk and net magnetization due to incomplete cancellation of magnetic moments. Our previous theoretical papers 3,4 have dealt with the possibility of ferrimagnetic-like spin alignment in an alternating chain consisting of a biradical in a singlet (S ) 0) ground state and a monoradical with S ) 1 / 2 . The spin Hamiltonian describing the system is written as [3][4][5][6] where S i,b1 , S i,b2 , and S i,m denote the spin-1 / 2 operators and N stands for the number of unit cells.…”

“…Our previous theoretical papers 3,4 have dealt with the possibility of ferrimagnetic-like spin alignment in an alternating chain consisting of a biradical in a singlet (S ) 0) ground state and a monoradical with S ) 1 / 2 . The spin Hamiltonian describing the system is written as [3][4][5][6] where S i,b1 , S i,b2 , and S i,m denote the spin-1 / 2 operators and N stands for the number of unit cells. The Hamiltonian (eq 1) is schematically shown in Figure 1a.…”