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

Abstract

The authors have previously proposed a theoretical model for exotic spin alignment in organic molecular assemblages: The alternating chain of organic biradicals in a singlet (Sb=0) ground state and monoradicals with S=1/2 has a ferrimagnetic ground state for the whole chain, which has been termed generalized ferrimagnetism. An important feature of the generalized ferrimagnetic spin alignment has been found in the deviation of the expectation value Sb2 of the biradical spin from zero. Even a triplet-like spin state Sb2=2 (Sb=1) has been predicted in the theoretical calculations. In this study, we have found experimental evidence for the pseudo-triplet state appearing in the ground-state singlet biradical of a real open-shell compound. At first, we have demonstrated from theoretical calculations that the singlet biradical has Sb2=2 (Sb=1) in a molecular pair with an S=1 metal ion as well as with the S=1/2 monoradical. The pseudo-triplet state of the biradical affords a singlet state of the whole system of the biradical-metal ion pair, which is readily detectable in experiments for verifying the theoretical prediction. As a model compound for the biradical-metal ion pair, a transition metal complex, [(bnn)(Ni(hfac)2)1.5(H2O)] (1), has been synthesized from a nitronyl nitroxide-based ground-state singlet biradical bnn and Ni(hfac)2. From X-ray crystallographic analyses, the compound contains a molecular pair of bnn and Ni(hfac)2, which serves as a model system under the above theoretical studies. It has been found from the analysis of the temperature dependence of magnetic susceptibility that the bnn-Ni(hfac)2 pair has the singlet (S=0) ground state. The singlet ground state of the pair results from an antiparallel coupling of the pseudo-triplet of the biradical and the S=1 spin on the Ni ion. The pseudo-triplet state in the ground-state singlet biradical has thus been verified experimentally, which is crucially important to realize the generalized ferrimagnetic spin alignment.