Crystalline solids dominate the field of metal-organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of 3D MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs. The glasses formed upon vitrification are chemically and structurally distinct from the three other existing categories of melt-quenched glasses (inorganic nonmetallic, organic, and metallic), and retain the basic metal-ligand connectivity of crystalline MOFs, which connects their mechanical properties to their starting chemical composition. The transfer of functionality from crystal to glass points toward new routes to tunable, functional hybrid glasses.
Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.
Linear poly(p‐phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H2 evolution rate. The most active dibenzo[b,d]thiophene sulfone co‐polymer has a UV photocatalytic activity that rivals TiO2, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co‐polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride.
The
energy-efficient separation of alkylaromatic compounds is a
major industrial sustainability challenge. The use of selectively
porous extended frameworks, such as zeolites or metal–organic
frameworks, is one solution to this problem. Here, we studied a flexible
molecular material, perethylated pillar[n]arene crystals
(n = 5, 6), which can be used to separate C8 alkylaromatic
compounds. Pillar[6]arene is shown to separate para-xylene from its structural isomers, meta-xylene
and ortho-xylene, with 90% specificity in the solid
state. Selectivity is an intrinsic property of the pillar[6]arene
host, with the flexible pillar[6]arene cavities adapting during adsorption
thus enabling preferential adsorption of para-xylene
in the solid state. The flexibility of pillar[6]arene as a solid sorbent
is rationalized using molecular conformer searches and crystal structure
prediction (CSP) combined with comprehensive characterization by X-ray
diffraction and 13C solid-state NMR spectroscopy. The CSP
study, which takes into account the structural variability of pillar[6]arene,
breaks new ground in its own right and showcases the feasibility of
applying CSP methods to understand and ultimately to predict the behavior
of soft, adaptive molecular crystals.
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