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

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
toward 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.