Origin of Ferromagnetic Exchange Coupling in Donor-Acceptor Biradical Analogues of Charge-Separated Excited States.

A new donor-acceptor biradical complex, TpZn(SQ-VD) (TpZn = zinc(II) hydro-tris(3-cumenyl-5-methylpyrazolyl)borate complex cation; SQ = orthosemiquinone; VD = oxoverdazyl), which is a ground-state analogue of a charge-separated excited state, has been synthesized and structurally characterized. The magnetic exchange interaction between the = 1/2 SQ and the = 1/2 VD within the SQ-VD biradical ligand is observed to be ferromagnetic, with = +77 cm ( = -2̂·̂) determined from an analysis of the variable-temperature magnetic susceptibility data. The pairwise biradical exchange interaction in TpZn(SQ-VD) can be compared with that of the related donor-acceptor biradical complex TpZn(SQ-NN) (NN = nitronyl nitroxide, = 1/2), where ≅ +550 cm. This represents a dramatic reduction in the biradical exchange by a factor of ∼7, despite the isolobal nature of the VD and NN acceptor radical SOMOs. Computations assessing the magnitude of the exchange were performed using a broken-symmetry density functional theory (DFT) approach. These computations are in good agreement with those computed at the CASSCF NEVPT2 level, which also reveals an = 1 triplet ground state as observed in the magnetic susceptibility measurements. A combination of electronic absorption spectroscopy and CASSCF computations has been used to elucidate the electronic origin of the large difference in the magnitude of the biradical exchange coupling between TpZn(SQ-VD) and TpZn(SQ-NN). A Valence Bond Configuration Interaction (VBCI) model was previously employed to highlight the importance of mixing an SQ → NN charge transfer configuration into the electronic ground state to facilitate the stabilization of the high-spin triplet ( = 1) ground state in TpZn(SQ-NN). Here, CASSCF computations confirm the importance of mixing the pendant radical (e.g., VD, NN) LUMO (VD and NN) with the SOMO of the SQ radical (SQ) for stabilizing the triplet, in addition to spin polarization and charge transfer contributions to the exchange. An important electronic structure difference between TpZn(SQ-VD) and TpZn(SQ-NN), which leads to their different exchange couplings, is the reduced admixture of excited states that promote ferromagnetic exchange into the TpZn(SQ-VD) ground state, and the intrinsically weaker mixing between the VD and the SQ compared to that observed for TpZn(SQ-NN), where this orbital mixing is significant. The results of this comparative study contribute to a greater understanding of biradical exchange interactions, which are important to our understanding of excited-state singlet-triplet energy gaps, electron delocalization, and the generation of electron spin polarization in both the ground and excited states of (bpy)Pt(CAT-radical) complexes.