Singlet ground states in an organic S = 1/2 spin ladder and a novel double spin chain of ferromagnetic dimers formed by an organic tetraradical

We have synthesized the novel organic polyradicals BIP-BNO and BIP-TENO, which crystallize to form spin systems with a double spin chain, where BIP-BNO and BIP-TENO denote 3,5′-bis(N-tert-butylaminoxyl)-3′,5-dibromobiphenyl and 3,3′,5,5′-tetrakis(N-teri-butylaminoxyl)biphenyl, respectively. The crystals of BIP-BNO form a two-leg ladder of S = 1/2 with antiferromagnetic legs and rungs. The ground state of this material is singlet. The BIP-TENO molecule includes S = 1/2 spin pairs (dimers) in which the two spins are coupled ferromagnetically. The BIP-TENO crystals form a double spin chain consisting of dimers. All dimers interact antiferromagnetically with each other. This system can be regarded as an antiferromagnetic two-leg ladder when the dimer is taken as a unit. The ground state of this material is singlet and the existence of excitation gaps is suggested by the temperature dependence of the static magnetic susceptibility.