Interacting networks of purely organic spin–1/2 dimers

Abstract

In the present study we report the synthesis of some novel nitronyl nitroxide biradical systems 1-4c with various p-bridges between the radical centres. UV-Vis, IR, EPR and X-ray diffraction studies, along with MS and NMR data where appropriate, are described. Magnetic measurements revealed that the biradicals 1c, 3c and 4c exhibit a moderately strong antiferromagnetic intra-molecular exchange, whereas nitroxide 2c shows a significantly higher exchange coupling, which can only be explained by the presence of strong inter-molecular interactions. From DFT calculations performed on the basis of the Xray crystal structure of compound 4c, a theoretical value of the intra-dimer coupling constant J intra ¼ À8.6 K is obtained. Direct proof also for inter-molecular arrangement with J inter $ À2 K was provided by the low temperature AC studies of biradical 4c. According to the magnetic characterization, the nitronyl biradical 4c is a promising candidate for a purely organic-based low-dimensional quantum magnet. † Electronic supplementary information (ESI) available: Fig. S1-12 UV-Vis spectra of the nitronyl nitroxides 1-4c, IR spectra of dialdehyde 4a, imidazolidine 4b, biradicals 2c, 3c and 4c, EPR spectra of the biradical 3c, magnetic susceptibility data of compounds 1-3c, TOC graphic of 4c, respectively. CCDC 822066, 816633, 816635. For ESI and crystallographic data in CIF or other electronic format see Scheme 3 Synthetic route towards dialdehydes 3a and 4a. Scheme 4 Synthesis of the imidazolidine 4b and biradical 4c. Scheme 2 Synthetic route towards dialdehyde 2a. 6620 | J. Mater. Chem. C, 2014, 2, 6618-6629 This journal is

Tuning the Magnetic Exchange Interactions in Organic Biradical Networks

“…These radical entities were chosen since they can be prepared without impurities after column chromatography and handled under air and ambient conditions. For scaling the intramolecular antiferromagnetic exchange interactions, several para‐phenylenyl‐bridged bis(nitronyl nitroxides) 1 – 4 were prepared and investigated by theoretical analysis and experimental magnetization experiments . Thus the influence of the bridging length on the intramolecular exchange interaction was elucidated (Figure ).…”

“…Thus the influence of the bridging length on the intramolecular exchange interaction was elucidated (Figure ). Here the distance of the central imidazolyl positions C2 to C2′ varied from 1.01 nm for the biphenyl spacer to 1.51 nm for the diphenyldiacetylene spacer …”

“Tschitschibabin type biradicals”: benzenoid or quinoid?

“…Serendipitously during the course of our study on organic biradical systems for the Bose-Einstein condensation we found the molecule BPNO (N,N'-([1,1'-biphenyl]-4,4'-diyl)bis(N-(tertbutylaminoxyl)) which exists in semi-quinoid form ( Figure 3). [27][28][29] At room temperature BPNO showed doublet EPR spectrum. Even after several purifications by column followed by repetitive crystallization no improvement in EPR spectrum was observed.…”

A Weakly Antiferromagnetically Coupled Biradical Combining Verdazyl with Nitronylnitroxide Units

“…Depending on the solvent, the prepared biradical crystallized in two different forms. [5] Earlier, we tested two nitronylnitroxides (NN) depicted as A [6,7] in Figure 1 and two iminonitroxides (IN) shown as B [8] with a tolane bridge (1,2-bis(phenyl) acetylene) for achieving 2D and 3D ordering of triplon excitations of these spin dimers in the crystal lattice.. Further, DFT calculations explained this phenomenon and indicated the advantage of Form I for further in-depth investigations.…”

“…[3] Therefore a control of J intra as well as J inter is essential and we have shown earlier that this is possible in organic biradicals, where the intramolecular interactions can be fine-tuned through the applied spacer between the radical units. [5] Earlier, we tested two nitronylnitroxides (NN) depicted as A [6,7] in Figure 1 and two iminonitroxides (IN) shown as B [8] with a tolane bridge (1,2-bis(phenyl) acetylene) for achieving 2D and 3D ordering of triplon excitations of these spin dimers in the crystal lattice.. [5] Earlier, we tested two nitronylnitroxides (NN) depicted as A [6,7] in Figure 1 and two iminonitroxides (IN) shown as B [8] with a tolane bridge (1,2-bis(phenyl) acetylene) for achieving 2D and 3D ordering of triplon excitations of these spin dimers in the crystal lattice..…”