Ionicity and paramagnetism of strong organic charge-transfer complexes

Abstract

Ionic charge-transfer (CT) complexes were prepared by mixing neutral solutions of the potent electron acceptor TCNQF4 (7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane) with the strong donors M2P(5,10-dihydro-5,10-diemethylphenazine) and TMPD(N,N,N′,N′-tetramethyl-p-phenylenediamine). M2P–TCNQF4 [complex (1)] and TMPD–TCNQF4 [complex (2)] were obtained as deep blue solids, the latter only in polycrystalline form. Complex (1) crystallizes in the triclinic space group P1̄ with a = 7.002 (2), b = 8.787(2), c = 9.551(3), α = 108.53(2), β = 99.68(2), γ = 91.88(2), and Z = 1. The planar M2P+⋅ and TCNQF−4⋅ radical ions form mixed regular stacks. The structure refinement gave R = 0.039 and Rw = 0.033 for 1044 counter data. The ionicity γ and structure of mixed ⋅⋅⋅D+γA−γD+γA−γ⋅⋅⋅ stacks are closely related to the paramagnetic susceptibility χ(T). The two TCNQF4 complexes are shown to be the first highly ionic (γ≳0.9) systems. χ(T) for complex (1) follows a Heisenberg antiferromagnetic chain with J = 175 cm−1 for T≳122 K, below which χ(T) decreases steeply as expected for a dimerization transition. χ(T) for complex (2) can be fit to a power law T−α, with α = 0.75±0.10, and also indicates γ≳0.9. Both complexes (1) and (2) are on the paramagnetic side of the paramagnetic–diamagnetic interface, while the finite activation energy for paramagnetism in the less ionic M2P–TCNQ and TMPD–TCNQ complexes puts them on the diamagnetic side. The elementary excitations of these related CT complexes with different ionicities are modeled.