Large Magnetoresistance in Tetracyanoquinodimethane (TCNQ) Charge–Transfer Complex of a Stable S = 1/2 Tetrazolinyl Radical

We report charge-transfer complexes derived from TCNQ and thermally ultrarobust S = 1/2 tetrazolinyl radical, of which we obtain an extraordinary polymorph with large magnetoresistance (MR) at low temperature. Mixing solutions of tetrazolinyl radical and TCNQ in acetonitrile provides 1:1 donor–acceptor (D–A) complexes that crystallize into two distinct polymorphs 1 and 2 . X-ray crystallography of 1 and 2 reveals tetrameric units with D–A–A–D packing motifs, including two molecules of acetonitrile, with distinct π-stacking within A–A dimers. Both types of A–A dimers exhibit strong π-bonding, indicated by plane-to-plane distances of 3.1 Å and singlet–triplet energy gaps, Δ E ST ≈ −3 kcal mol –1 . Experimental values of both plane-to-plane distances and Δ E ST are well-reproduced at the UB3LYP-D3BJ/6-31G(d,p)/IEF-PCM-UFF+ZPVE (in acetonitrile) level of theory. Single crystal devices are constructed to investigate electrical conductivity (σ) and magnetoresistance (MR) of randomly selected crystals of 1 and 2 . Polymorph 1 possesses relatively high room temperature σ (σ RT ) of up to 100 S cm –1 and low MR < 10%, while 2 is a modest conductor with 1 order of magnitude lower σ RT and large MR ≈ +380 ± 50%. These large MRs and decreased conductivities at low temperatures for 2 are consistent with a Mott variable range hopping model. Characterization of 1 and 2 in solution, by 1 H NMR, EPR, UV–vis–NIR absorption and fluorescence spectroscopies, indicates complete spin/charge transfer. Magnetic and spectroscopic characterization of polycrystalline 1 and 2 is carried out by EPR, SQUID, vis-NIR absorption and fluorescence spectroscopy, with the partial spin/charge transfer determined by Raman spectroscopy.