Structural, Magnetic and DFT studies on a Charge-Transfer Salt of a Tetrathiafulvalenepyridyl-(1,5-diisopropyl) verdazyl Diradical Cation.

A new tetrathiafulvalene (TTF) donor covalently appended with a 1,5-diisopropylverdazyl radical through a cross-conjugated pyridyl linker (3) has been prepared and characterised. Reaction of 3 with tetracyanoquinonedimethane (TCNQ) afforded the 2:1 charge-transfer complex (3) ₂ ⋅TCNQ (4), in which the IR and structural data are consistent with 0.25 e ^- charge transfer from the TTF donor (D) to the TCNQ acceptor (A). The TTF and TCNQ molecules adopt a mixed-stack D⋅⋅⋅D⋅⋅⋅A arrangement that does not facilitate conduction. A solution EPR spectrum of 4 comprises a broad featureless singlet, which is consistent with the presence of a TCNQ radical anion. Theoretical studies were performed to probe the exchange interactions within selected fragments of 4 with and without charge transfer. In the absence of charge transfer, DFT calculations reveal weak antiferromagnetic exchange between verdazyl radicals within the (3) ₂ monoradical unit. However, partial oxidation of the dimer (3) ₂ to the diradical cation leads to an S= 1 / 2 ground state, in which the verdazyl radical spins are now aligned co-parallel as a consequence of antiferromagnetic exchange to the additional delocalised TTF-based spin containing unit. The magnetic properties of 4 are consistent with a net S= 1 / 2 spin state per formula unit with dominant antiferromagnetic interactions between spin-bearing building blocks.
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