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

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.