Electrochemically-Induced TCNQ/Mn[TCNQ](2)(H2O)(2) (TCNQ=7,7,8,8-Tetracyanoquinodimethane) Solid-Solid Interconversion: Two Voltammetrically Distinct Processes That Allow Selective Generation of Nanofiber or Nanorod Network Morphologies

摘要

Unlike the case with other divalent transition metal M[TCNQ](2)(H2O)(2) (M=Fe, Co, Ni) analogues, the electrochemically induced solid-solid phase interconversion of TCNQ microcrystals (TCNQ=7,7,8,8-tetracyanoquinodimethane) to Mn[TCNQ](2)(H2O)(2) occurs via two voltammetrically distinct, time dependent processes that generate the coordination polymer in nanofiber or rod-like morphologies. Careful manipulation of the voltammetric scan rate, electrolysis time, Mn-(aq)(2+) concentration, and the method of electrode modification with solid TCNQ allows selective generation of either morphology. Detailed ex situ spectroscopic (IR, Raman), scanning electron microscopy (SEM), and X-ray powder diffraction (XRD) characterization clearly establish that differences in the electrochemically synthesized Mn-TCNQ material are confined to morphology. Generation of the nanofiber form is proposed to take place rapidly via formation and reduction of a Mn-stabilized anionic dimer intermediate, [(Mn2+) (TCNQ-TCNQ)(2)(center dot-)], formed as a result of radical-substrate coupling between TCNQ(center dot-) and neutral TCNQ, accompanied by ingress of Mn2+ ions from the aqueous solution at the triple phase TCNQ/electrode/electrolyte boundary. In contrast, formation of the nanorod form is much slower and is postulated to arise from disproportionation of the [(Mn2+)(TCNQ-TCNQ)(center dot-)(2)] intermediate. Thus, identification of the time dependent pathways via the solid-solid state electrochemical approach allows the crystal size of the Mn[TCNQ](2)(H2O)(2) material to be tuned and provides new mechanistic insights into the formation of different morphologies.