Experimental manifestation of redox-conductivity in metal-organic frameworks and its implication for semiconductor/insulator switching

Abstract

Electric conductivity in metal-organic frameworks (MOFs) follows either a band-like or a redox-hopping charge transport mechanism. While conductivity by the band-like mechanism is theoretically and experimentally well established, the field has struggled to experimentally demonstrate redox conductivity that is promoted by the electron hopping mechanism. Such redox conductivity is predicted to maximize at the mid-point potential of the redox-active units in the MOF, and decline rapidly when deviating from this situation. Herein, we present direct experimental evidence for redox conductivity in fluorine-doped tin oxide surface-grown thin films of Zn(pyrazol-NDI) (pyrazol-NDI = 1,4-bis[(3,5-dimethyl)-pyrazol-4-yl]naphthalenediimide). Following Nernstian behavior, the proportion of reduced and oxidized NDI linkers can be adjusted by the applied potential. Through a series of conductivity measurements, it is demonstrated that the MOF exhibits minimal electric resistance at the mid-point potentials of the NDI linker, and conductivity is enhanced by more than 10000-fold compared to that of either the neutral or completely reduced films. The generality of redox conductivity is demonstrated in MOFs with different linkers and secondary building units, and its implication for applications that require switching between insulating and semiconducting regimes is discussed.

Fig. 1. Schematic diagrams of various metal organic framework (MOF) conductors and representative examples.

a, b Ohmic-conductor, where a fixed level of conductivity or resistance is associated so that a linear relationship is presented between the applied voltage and the current output (Ohms law, a); representative metallic conducting FeTHT (THT = 2,3,6,7,10,11-triphenylenehexathiolate) MOF with extensive electron delocalization throughout two-dimensional sheets (b, adapted with permission from ref. Copyright 2019 American Chemical Society). c, d Semiconductor, where the magnitude of conductivity is specified by the Fermi-Dirac statistics. Here, the level of conductivity is presented at room temperature (c); representative semiconducting Fe₂(DSBDC) (DSBDC = 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid) MOF with band gap of 1.92 eV (d, picture is reproduced using the Cif file in ref. ). e, f Redox conductor, where the probability of redox-hopping is determined by the occupancy of neighboring sites. It maximizes when the population of oxidized and reduced sites is equal (at the standard potential according to the Nernst equation, e); representative redox-conductive Zn-NDI (NDI is pyrazolate-functionalized redox-active naphthalene diimide-based linkers) MOF with linker-to-linker localized redox-hopping (f, reconstructed based on simulated model from ref. ).

Fig. 2. Structural characterization, cyclic voltammetry (CV) and spectroelectrochemical measurements of Zn(pyrazol-NDI) thin-films.

a The crystal structure of Zn(pyrazol-NDI) with the projection of the pyrazolate-bridged Zn²⁺ chain (top) and viewing along the c direction (bottom); hydrogen atoms are omitted for clarity; C, N, O, and Zn atoms or ions are presented with gray, blue, red, and brown spheres, respectively (the crystal structure was reconstructed based on the reported structure in ref. ). b Experimental thin-film X-ray diffraction (XRD) data on fluorine-doped tin oxide surface (FTO) surfaces and simulated powder XRD data. c Scanning electron microscopy (SEM) cross-section image of the Zn(pyrazol-NDI) thin-film on FTO. d A representative CV with a scan rate of 5 mV s⁻¹: points i-v indicate various potentials, the applications of which result in different redox states of the film as described by the mole fraction of electrons (x, ranging from 0 to 2 with a step of 0.5) with respect to the NDI linkers. e, f UV–vis spectroelectrochemical measurements of Zn(pyrazol-NDI) thin-film at controlled potentials to access different mole fraction of electrons within the film, x-Zn(pyrazol-NDI), x = 0.0, 0.5, 1.0 (e) and x = 1.0, 1.5, 2.0 (f), calculated spectra are also included for x = 0.5, 1.5 using linear combination of x = 0.0 and 1.0, x = 1.0 and 2.0, respectively. All electrochemistry data were collected in Ar-saturated DMF with KPF₆ as the supporting electrolyte (0.1 M).

Fig. 3. Redox-conductivity characteristics of Zn(pyrazol-NDI) thin-films, together with activation barrier and conductivity switching measurements.

a Bode plots of Zn(pyrazol-NDI) thin-film at different applied potentials (the impedance data point at the frequency of 0.1 Hz was magnified for each measurements to highlight the effect of redox state); before each measurement, a stabilization time of 120 s was applied at respective potentials to achieve steady redox states. b Evolution of the steady-state thin-film conductivity as a function of applied electrochemical potential, which determines the mole fraction of electron reduction, x-Zn(pyrazol-NDI), 0.0 ≤ x ≤ 2.0, raw data is available in the source data file. Gaussian fit was performed for NDI/NDI^•− based (red line) and NDI^•−/NDI²⁻ (blue line) bell-shaped redox conductivity. c Steady-state conductivity of 0.0-Zn(pyrazol-NDI) (black sphere) and 0.5-Zn(pyrazol-NDI) (red sphere) thin-films as the function of temperature, activation energy is derived based on the Arrhenius equation. d Switchability of the redox conductivity between 0.0-Zn(pyrazol-NDI) (bottom sphere) and 0.5-Zn(pyrazol-NDI) (top sphere) over 100 cycles (~24 h operation).

Fig. 4. Crystal structures, redox-active linkers, cyclic voltammetry (CV) and redox-conductivity of Zr(dcphOH-NDI) and UU-100(Co) thin-films.

a Crystal structure of Zr(dcphOH-NDI) viewing along a axis and displaying two interpenetrated frameworks having a 12-c fcu net (the crystal structure was reconstructed based on the reported structure in ref. ). b Structure of the redox-active dcphOH-NDI linker. c Representative CVs (100 mV s⁻¹) and redox-conductivity characterizations of Zr(dcphOH-NDI) thin-films. Gaussian fit was performed for NDI/NDI^•− based (blue line) and NDI^•−/NDI²⁻ (brown line) bell-shaped redox conductivity. d Crystal structure of UU-100(Co) viewing along the c axis and displaying a tetragonal unit cell (a = b = 27.3 Å, and c = 19.6 Å) with P4/mbm space group (the crystal structure was reconstructed based on the reported structure in ref. ). e Structure of the cobaloxime-based redox-active linker. f Representative CVs (100 mV s⁻¹) and redox-conductivity characterizations of UU-100 thin-films. Gaussian fit was performed for Co²⁺/Co¹⁺ based (blue line) bell-shaped redox conductivity. All measurements were conducted in Ar-saturated DMF with KPF₆ as the supporting electrolyte (0.1 M). Hydrogen atoms are omitted for clarity; C, N, O, Co, Cl, and Zr atoms are presented with brown, light blue, red, yellow and green spheres, respectively. Redox conductivity data is available in the source data file.

Fig. 5. Electrochemical and steady-state redox conductivity measurements with different counter cations.

Cyclic voltammetry (CV) scan (5 mV s⁻¹) and steady-state redox conductivity measurement of Zn(pyrazol-NDI) thin-films in Ar-saturated DMF with KPF₆ (a), LiClO₄ (b), and TBAPF₆ (c) as the supporting electrolyte (0.1 M). Gaussian fit was performed for NDI/NDI^•− based (blue line) and NDI^•−/NDI²⁻ (brown line) bell-shaped redox conductivity for all three different counter cations. Redox conductivity data is available in the source data file.