Towards a Better Understanding of Magnetic Interactions within m‐Phenylene α‐Nitronyl Nitroxide and Imino Nitroxide Based Radicals, Part III: Magnetic Exchange in a Series of Triradicals and Tetraradicals Based on the Phenyl Acetylene and Biphenyl Coupling Units

Abstract

The present work completes and extends our previous reports on the determination of the magnetic ground state and on the strength of the through bond exchange coupling within series of biradicals. This knowledge was subsequently exploited for the analysis of the magnetic interactions in their crystals. We report here the studies of series of triradicals incorporating alpha-nitronyl nitroxides (NN) or alpha-imino nitroxides (IN) as terminal radical fragments connected through a m-phenylene coupling unit in one case and a phenyl acetylene unit in other case. Tetraradical derivatives have also been studied. The studies of isolated molecules (EPR in solution and DFT calculations) allow the assessment of the magnetic interactions through the magnetic coupling unit. All triradical derivatives are found to exhibit a quartet ground state, whereas a singlet ground state is determined for the tetraradical. This last result reinforces previous findings that the singlet ground state is favoured in related biradicals involving similar m-phenylene couplers. Moreover, the through bond magnetic exchange coupling for the ortho-meta connectivity could be demonstrated as being ferromagnetic, thus ascertaining our previous hypotheses. The magnetic properties of the triradicals and tetraradicals in their solid state have been rationalized by using a previously proposed methodology, allowing to identify the most relevant magnetic pathways.

Intramolecular Spin Alignment in Photomagnetic Molecular Devices: A Theoretical Study

“…[32,66,67] It is worth noting that a spin polarization pattern is present on the phenyl bridging subunits and also on the diamagnetic anthracene core, as revealed by the spin-density maps computed for Dd 1 and Dd 2 (Figure 2). Nevertheless, if the spin polarization mechanism yields larger spin-density patterns on the phenyl (with an overall negative spin density of 0.1 e À on this subunit), the much smaller fraction of spin density computed on each atom of the anthracene unit as well as the negligible overall spin density localized on it, comply with the very weak coupling found between the two dangling radicals of Td 1 and Td 2.…”

Isolation and Characterization of the Triradical 1,3,5‐Trimethylenebenzene

“…Thus, 1,3‐dimethylenebenzene ( m ‐xylylene) 1 was shown to have a robust triplet ground state 5. 6 The spectroscopic properties, structure, and photochemistry of 1 was recently investigated using matrix isolation spectroscopy 7. Prerequisite for these studies was the observation that high yields of 1 are obtained by flash vacuum pyrolysis (FVP) of 1,3‐bis(iodomethyl)benzene 2 at 450 °C and subsequent trapping of the products in argon at 10 K (Scheme ).…”

Isolation and Characterization of the Triradical 1,3,5‐Trimethylenebenzene

“…Thus, 1,3-dimethylenebenzene (m-xylylene) 1 was shown to have a robust triplet ground state. [5,6] The spectroscopic properties, structure, and photochemistry of 1 was recently investigated using matrix isolation spectroscopy. [7] Prerequisite for these studies was the observation that high yields of 1 are obtained by flash vacuum pyrolysis (FVP) of 1,3bis(iodomethyl)benzene 2 at 450 8C and subsequent trapping of the products in argon at 10 K (Scheme 1).…”