Molecules that exhibit emission in the solid state, especially those known as aggregation-induced emission (AIE) chromophores, have found applications in areas as varied as light-emitting diodes and biological sensors. Despite numerous studies, the mechanism of fluorescence quenching in AIE chromophores is still not completely understood. To this end, much interest has focused on understanding the low frequency vibrational dynamics of prototypical systems such as tetraphenylethylene (TPE), in the hope that such studies would provide more general principles towards the design of new sensors and electronic materials. We hereby show that a perdeuterated TPE-based metal-organic framework (MOF) serves as an excellent platform for studying the low energy vibrational modes of AIE-type chromophores. In particular, we use solid-state 2H and 13C NMR experiments to investigate the phenyl ring dynamics of TPE cores that are coordinatively trapped inside a MOF and find a phenyl ring flipping energy barrier of 43(6) kJ/mol. DFT calculations are then used to deconvolute the electronic and steric contributions to this flipping barrier. Finally, we couple the NMR and DFT studies with variable temperature X-ray diffraction experiments to propose that both the ethylenic C=C bond twist and the torsion of the phenyl rings are important for quenching emission in TPE, but that the former may gate the latter. To conclude, we use these findings to propose a set of design criteria for the development of tunable turn-on porous sensors constructed from AIE-type molecules, particularly as applied to the design of new multifunctional MOFs.
We show that fluorescent molecules incorporated as ligands in rigid, porous metal-organic frameworks (MOFs) maintain their fluorescence response to a much higher temperature than in molecular crystals. The remarkable high-temperature ligand-based fluorescence, demonstrated here with tetraphenylethylene- and dihydroxyterephthalate-based linkers, is essential for enabling selective and rapid detection of analytes in the gas phase. Both Zn2(TCPE) (TCPE = tetrakis(4-carboxyphenyl)ethylene) and Mg(H2DHBDC) (H2DHBDC(2-) = 2,5-dihydroxybenzene-1,4-dicarboxylate) function as selective sensors for ammonia at 100 °C, although neither shows NH3 selectivity at room temperature. Variable-temperature diffuse-reflectance infrared spectroscopy, fluorescence spectroscopy, and X-ray crystallography are coupled with density-functional calculations to interrogate the temperature-dependent guest-framework interactions and the preferential analyte binding in each material. These results describe a heretofore unrecognized, yet potentially general property of many rigid, fluorescent MOFs and portend new applications for these materials in selective sensors, with selectivity profiles that can be tuned as a function of temperature.
Organochalcogen-nitrogen heterocycles such as the 1,2,5-chalcogenadiazoles have a distinct tendency to establish intermolecular links in the solid state through secondary bonding interactions E...N (E = S, Se, Te). The association of these molecules was examined in detail using relativistic density functional theory. Although there is an important electrostatic component, the interaction between these molecules is dominated by contributions arising from orbital mixing, which can be interpreted as the donation of a nitrogen lone pair into the chalcogen-centered antibonding orbitals. Because of its more polar character and lower-lying antibonding orbitals, the tellurium derivatives possess the strongest association energies; these are so large that the binding strength is comparable to that of some hydrogen bonds. In the absence of steric constraints, telluradiazoles associate in a coplanar fashion forming ribbon polymers. However, bulky susbstituents could be used to direct the formation of either helical chains or discrete dimers. In addition to its strength, the coplanar dimer is characterized by being rigid, yet no activation barrier is expected for the association/dissociation process. These attributes strongly indicate that tellurium-nitrogen heterocycles have great potential as building blocks in supramolecular architecture.
DFT calculations were used to compare the magnitude of steric repulsion to the strength of secondary bonding
interactions (SBIs) in the (Te−N)2 supramolecular synthon to explain or predict the supramolecular structures assembled by two
derivatives of the 1,2,5-telluradiazole ring: benzo-2,1,3-telluradiazole (4c) and 3,6-dibromobenzo-2,1,3-telluradiazole (5). The
crystallographically determined structures were consistent with the computational predictions. In sharp contrast with the previously
known structures of its sulfur and selenium analogues, 4c forms infinite ribbon chains in the solid state with 2.682(7)−2.720(7) Å
Te−N SBIs. Steric hindrance in 5 restricted the supramolecular association to form discrete dimers with 2.697(8) Å Te−N SBIs.
In addition to discrete dimers, the dibromo derivative crystallizes as solvated dimers in 5·DMSO with 2.834(5) Å Te−O SBIs.
Other weaker SBIs were identified in the crystal lattices and were assessed by the computational method.
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