Metal-Carbodithioate-Based 3D Semiconducting Metal-Organic Framework: Porous Optoelectronic Material for Energy Conversion

Solar energy conversion requires the working compositionsto generatephotoinduced charges with high potential and the ability to delivercharges to the catalytic sites and/or external electrode. These twoproperties are typically at odds with each other and call for newmolecular materials with sufficient conjugation to improve chargeconductivity but not as much conjugation as to overly compromise theoptical band gap. In this work, we developed a semiconducting metal-organicframework (MOF) prepared explicitly through metal-carbodithioate(-CS2) (n) Mlinkage chemistry, entailing augmented metal-linker electroniccommunication. The stronger ligand field and higher covalent characterof metal-carbodithioate linkages-when combined withspirofluorene-derived organic struts and nickel-(II) ion-based nodes-provideda stable, semiconducting 3D-porous MOF, Spiro-CS2Ni. ThisMOF lacks long-range ordering and is defined by a flexible structurewith non-aggregated building units, as suggested by reverse MonteCarlo simulations of the pair distribution function obtained fromtotal scattering experiments. The solvent-removed closed porematerial recorded a Brunauer-Emmett-Teller area of similar to 400m(2)/g, where the open pore form possesses90 wt % solvent-accessible porosity. Electrochemical measurementssuggest that Spiro-CS2Ni possesses a band gap of 1.57 eV(sigma = 10(-7) S/cm at -1.3 V bias potential),which can be further improved by manipulating the d-electron configurationthrough an axial coordination (ligand/substrate), the latter of whichindicates usefulness as an electrocatalyst and/or a photoelectrocatalyst(upon substrate binding). Transient-absorption spectroscopy revealsa long-lived photo-generated charge-transfer state (tau(CR) = 6.5 mu s) capable of chemical transformation under a biasedvoltage. Spiro-CS2Ni can endure a compelling range of pH(1-12 for weeks) and hours of electrochemical and photoelectrochemicalconditions in the presence of water and organic acids. We believethis work provides crucial design principles for low-density, porous,light-energy-conversion materials.

Automated summary

Solar energy conversion requires materials with high potential and charge delivery ability. In this study, a semiconducting metal-organic framework (MOF) was synthesized with improved charge conductivity and optical band gap. The MOF showed stable 3D-porous structure and had a band gap of 1.57 eV. Transient absorption spectroscopy revealed long-lived photo-generated charge-transfer state.