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Metallic charge transport and porosity appear almost mutually exclusive. Whereas metals demand large numbers of free carriers and must have minimal impurities and lattice vibrations to avoid charge scattering, the voids in porous materials limit the carrier concentration, provide ample space for impurities, and create more charge-scattering vibrations due to the size and flexibility of the lattice. No microporous material has been conclusively shown to behave as a metal. Here, we demonstrate that single crystals of the porous metal–organic framework Ln 1.5 (2,3,6,7,10,11-hexaoxytriphenylene) (Ln = La, Nd) are metallic. The materials display the highest room-temperature conductivities of all porous materials, reaching values above 1,000 S/cm. Single crystals of the compounds additionally show clear temperature-deactivated charge transport, a hallmark of a metallic material. Lastly, a structural transition consistent with charge density wave ordering, present only in metals and rare in any materials, provides additional conclusive proof of the metallic nature of the materials. Our results provide an example of a metal with porosity intrinsic to its structure. We anticipate that the combination of porosity and chemical tunability that these materials possess will provide a unique handle toward controlling the unconventional states that lie within them, such as charge density waves that we observed, or perhaps superconductivity. 

Solar thermal fuels (STFs) offer a unique way of harnessing energy from the sun by absorbing photons and storing the energy in a metastable photoisomerized state. The reverse isomerization process can then be catalytically or thermally triggered to release the stored energy and return the fuel to its stable configuration. Functionalization of these compounds is necessary to reach practical energy storage densities, but substitutions that increase the energy storage density may adversely impact performance at other steps along the fuel cycle. Recent computational screening efforts to identify high-performance STF candidates have focused on properties that can be estimated from ground-state electronic structure methods. Here we argue that computational screening of STF candidates across the full fuel cycle benefits from a multifactor approach with excited-state properties like excitation energies and photoisomerization quantum yields addressed alongside key ground-state properties like energy storage densities and reverse isomerization barriers. As a critical step toward multifactor high-throughput screening and optimization of STFs, in this work we first systematically simulate the specific storage energy and excitation energy of substituted azobenzene- and norbornadiene-based STFs through electronic structure calculations. Density-functional tight-binding (DFTB) predictions are benchmarked against density functional theory (DFT) and experimental measurements where available. To encompass the complete solar thermal fuel cycle in these compounds, we then apply DFT methods to analyze the reverse isomerization process and its relationship to the photoisomerization quantum yield. We find that DFTB provides a useful balance between accuracy and computational efficiency for virtual screening of STF photoabsorption and energy storage, while isomerization barrier and quantum yield predictions require more sophisticated approaches.