Phenyl Ring Dynamics in a Tetraphenylethylene-Bridged Metal–Organic Framework: Implications for the Mechanism of Aggregation-Induced Emission

Abstract

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.

Synthesis, Crystal Structure, and Solid-State NMR Investigations of Heteronuclear Zn/Co Coordination Networks — A Comparative Study

“…Therefore, non-crystalline materials can be characterized with solid-state NMR which is increasingly used for the characterization of structure and dynamics in these materials. [25][26][27] Furthermore, using this method it is easy to determine the presence of small mobile molecules, while X-ray analysis is more complicated or impossible. 28 In this paper, we report on the synthesis and detailed structural elucidation of two Zn MOFs together with their corresponding protonated ligands by means of X-ray structure analysis, 1 32 which has already been proven to be a reliable tool in characterizing the NMR properties of crystalline solids.…”

Ultra‐Fast Molecular Rotors within Porous Organic Cages

“…The larger E a value obtained for [D 12 ]TCC2‐ R versus [D 12 ]TCC3‐ R is again consistent with stronger steric interactions in the terphenylene cage structure. The k 0 values obtained are on the low side of the ≈10 12  Hz22 value often associated with para ‐phenylene rotation, although these values vary significantly with the systems studied and k 0 in the 10 8 –10 41  Hz range are known 19, 23, 24, 27, 28, 29, 32, 36, 37, 40, 41. The associated change in entropy (Δ S ) is negative and is tentatively assigned to correlated rotational motion (Table S2) 41, 42…”

Complex from ionic β-cyclodextrin polyrotaxane and sodium tetraphenylthiophenesulfonate: restricted molecular rotation and aggregation-enhanced emission

“…With the beneficial emission properties in the aggregated state, lots of AIE-active luminogens containing tetraphenylethylene, 3,4 phenothiazine, 5,6 anthracene, 7,8 triphenylethylene, 9.10 distyrylanthracene, 11,12 carbazolyl 13,14 groups and AIE-active metal complexes 15,16 had been prepared and characterized and for the practical aspects, AIE-active materials in biomedical applications [17][18][19] were also explored extensively.…”