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

Solar energy conversion requires the working compositions
to generate
photoinduced charges with high potential and the ability to deliver
charges to the catalytic sites and/or external electrode. These two
properties are typically at odds with each other and call for new
molecular materials with sufficient conjugation to improve charge
conductivity but not as much conjugation as to overly compromise the
optical band gap. In this work, we developed a semiconducting metal–organic
framework (MOF) prepared explicitly through metal–carbodithioate
“(−CS2)nM”
linkage chemistry, entailing augmented metal–linker electronic
communication. The stronger ligand field and higher covalent character
of metal–carbodithioate linkageswhen combined with
spirofluorene-derived organic struts and nickel(II) ion-based nodesprovided
a stable, semiconducting 3D-porous MOF, Spiro-CS2Ni. This
MOF lacks long-range ordering and is defined by a flexible structure
with non-aggregated building units, as suggested by reverse Monte
Carlo simulations of the pair distribution function obtained from
total scattering experiments. The solvent-removed “closed pore”
material recorded a Brunauer–Emmett–Teller area of ∼400
m2/g, where the “open pore” form possesses
90 wt % solvent-accessible porosity. Electrochemical measurements
suggest that Spiro-CS2Ni possesses a band gap of 1.57 eV
(σ = 10–7 S/cm at −1.3 V bias potential),
which can be further improved by manipulating the d-electron configuration
through an axial coordination (ligand/substrate), the latter of which
indicates usefulness as an electrocatalyst and/or a photoelectrocatalyst
(upon substrate binding). Transient-absorption spectroscopy reveals
a long-lived photo-generated charge-transfer state (τCR = 6.5 μs) capable of chemical transformation under a biased
voltage. Spiro-CS2Ni can endure a compelling range of pH
(1–12 for weeks) and hours of electrochemical and photoelectrochemical
conditions in the presence of water and organic acids. We believe
this work provides crucial design principles for low-density, porous,
light-energy-conversion materials.

太阳能转换要求功能组分能够生成具有高电势的光生电荷，并可将电荷输送至催化位点与/或外电极。这两种特性通常相互矛盾，因此亟需开发新型分子材料：这类材料需具备足够的共轭结构以提升电荷传导能力，但共轭程度又不能过高，以免过度牺牲光学带隙。本研究通过二硫代甲酸金属“(−CS2)nM”键合化学，精准合成了一种半导体金属有机框架（metal–organic framework, MOF），该框架可强化金属-配体间的电子通信。二硫代甲酸金属键合具有更强的配体场与更高的共价特征，当与螺芴衍生有机支柱以及镍(II)离子节点结合时，可得到稳定的半导体三维多孔MOF——Spiro-CS2Ni。该MOF不具备长程有序结构，其结构具有柔性特征且结构基元未发生聚集，这一结论由全散射实验得到的径向分布函数（pair distribution function, PDF）的反向蒙特卡洛（reverse Monte Carlo, RMC）模拟所证实。脱溶剂后的“闭孔”材料的布鲁诺尔-埃米特-特勒（Brunauer–Emmett–Teller, BET）比表面积约为400 m²/g，而“开孔”形式则拥有90 wt%的溶剂可及孔隙率。电化学测试结果表明，Spiro-CS2Ni的带隙为1.57 eV（在−1.3 V偏置电位下，电导率σ=10–7 S/cm），且可通过轴向配位（配体/底物）调控d电子构型进一步优化其性能，这表明该材料可用作电催化剂与/或光电催化剂（结合底物时）。瞬态吸收光谱研究显示，该材料存在寿命长达6.5 μs的长寿命光生电荷转移态（τCR=6.5 μs），可在偏置电压下发生化学转化。Spiro-CS2Ni可耐受宽泛的pH区间（1~12，持续数周），以及在水与有机酸存在下数小时的电化学与光电化学工况。我们认为，本研究为低密度、多孔型光能转换材料提供了关键的设计原则。