Novel molecular staircase self-assembly from the cations of 1-(4′-nitrobenzyl)pyridinium: Synthesis, crystal structure, magnetic property and spin dimer analyses

Two salts consisting of 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical and 1-(4′-nitrobenzyl)pyridinium ([NO₂BzPy]⁺) with molar ratio of 1:1 (2) and 2:1 (3) have been prepared and structurally characterized. The change of molar ratio between radical of TCNQ and [NO ₂BzPy]⁺leads two salts to exhibit different packing structures. A strong dimer of [(TCNQ)₂]^2- is formed, and the adjacent dimers slide to each other along the direction of the shorter molecular axis of TCNQ^- in 2. The weak H-bonding interactions between O-atom of nitro groups and H-atom of benzene rings are observed, which construct a cationic chain as a supramolecular staircase. The radical anions, [(TCNQ)₂]^-, stack into a tetrad column, in which two types of TCNQ entries, TCNQ^-1 and TCNQ⁰, are found in 3, and this kind of charge-ordered state is further supported from the results of both IR spectra and molecular orbital calculation. Two neighboring cations of [NO₂BzPy]⁺overlap into a dimer via π⋯π stacking between benzene rings. The EPR spectra of polycrystalline samples at room temperature indicated the feature of isotropic signals with g-factor of 2.0056 for 2 and 2.0068 for 3. The temperature dependences of magnetic susceptibility (2-350 K) show spin gap of singlet-triplet feature for two salts. The best simulations to susceptibility data in the range 2-350 K yielded the parameters Δ/k_B=1813 K and zJ′/k_B=-6 K for 2; Δ/k_B=1882 K and zJ′/k_B=-11 K for 3.