Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals: Candidates for Quantum Magnets

Stable neutral organic biradicals are of special interest because they offer the possibility to tune the magnetic interactions through the appropriate design of a spacer.(1) Thus, by considering the topological rules of physics and at the same time employing the advanced methods of synthetic organic chemistry, molecules with predictable magnetic properties can be synthesized.(2) Among the biradical family, antiferromagnetically (AF) coupled species can be considered as a source of interacting bosons.(3) Consequently, such biradicals can serve as molecular models of a gas of magnetic excitations which can be used for quantum computing application.(4) Notably, the vanishing triplet state population at very low temperature (2–4 K) in weakly AF coupled biradicals can be switched into a larger triplet population upon application of an external magnetic field. Therefore, such biradical systems are promising molecular models for studying the phenomena of magnetic-field-induced Bose–Einstein condensation in the solid state.(5) To observe such phenomena, it is very important to control intra- as well as intermolecular magnetic exchange interactions. The intramolecular magnetic exchange interactions can be tuned by either changing the length of a π-spacer molecule carrying the radical moieties or changing the radical moiety while maintaining the same π-spacer.(6) The intermolecular magnetic exchange interactions, which usually operate through hydrogen bond or other short intermolecular contacts, highly depend on the crystal packing and are quite difficult to predict or control.(7) To some extent, the intermolecular magnetic exchange interactions can be regulated by employing a crystal engineering approach.(8) For instance, introduction of hydrogen bond donor or acceptor (or a combination of both) functional groups in the π-spacer fragment can result in an additional hydrogen bonding, which in turn is advantageous for smooth transmission of magnetic interactions through the lattice. As a result, multidimensional spin-networks can be constructed