Cascade Dynamics of Multiple Molecular Rotors in a MOF: Benchmark Mobility at a Few Kelvins and Dynamics Control by CO2.

Jacopo Perego,Luciano Marchiò,Silvia Bracco,Giacomo Prando,Mattia Negroni,Piero Sozzani,Charl X Bezuidenhout,Angiolina Comotti,Pietro Carretta

doi:10.1021/jacs.1c03801

Jacopo Perego, Luciano Marchiò + Show 7 more

Open Access

https://doi.org/10.1021/jacs.1c03801

Copy DOI

Abstract

Achieving sophisticated juxtaposition of geared molecular rotors with negligible energy-requirements in solids enables fast yet controllable and correlated rotary motion to construct switches and motors. Our endeavor was to realize multiple rotors operating in a MOF architecture capable of supporting fast motional regimes, even at extremely cold temperatures. Two distinct ligands, 4,4′-bipyridine (bipy) and bicyclo[1.1.1]pentanedicarboxylate (BCP), coordinated to Zn clusters fabricated a pillar-and-layer 3D array of orthogonal rotors. Variable temperature XRD, 2H solid-echo, and 1H T1 relaxation NMR, collected down to a temperature of 2 K revealed the hyperfast mobility of BCP and an unprecedented cascade mechanism modulated by distinct energy barriers starting from values as low as 100 J mol–1 (24 cal mol–1), a real benchmark for complex arrays of rotors. These rotors explored multiple configurations of conrotary and disrotary relationships, switched on and off by thermal energy, a scenario supported by DFT modeling. Furthermore, the collective bipy-ring rotation was concerted with the framework, which underwent controllable swinging between two arrangements in a dynamical structure. A second way to manipulate rotors by external stimuli was the use of CO2, which diffused through the open pores, dramatically changing the global rotation mechanism. Collectively, the intriguing gymnastics of multiple rotors, devised cooperatively and integrated into the same framework, gave the opportunity to engineer hypermobile rotors (107 Hz at 4 K) in machine-like double ligand MOF crystals.

Highlights

The open porosity of samples was proven by CO2 adsorption isotherms at 195 K yielding a maximum adsorbed amount of 1.7 mmol/g, which corresponded to the filling of the accessible free volume estimated by the crystal structure (Figure S15)
Activation energy for all rotational phenomena previously observed at lower activation energies in the empty compound (SI). These results clearly demonstrate the ease of manipulating the dynamics of two distinct rotors by an external stimulus due to the porosity of the MOF, which enables diffused-in molecules to act directly on the rotators
The rotors operate in spatiotemporal succession, covering a temperature range from 390 K down to 2 K, with a multiple pyrotechnic motional evolution, while exploring systematically ultrafast dynamics

Summary

■ INTRODUCTION

The mechanics of motion in solids has been attracting increasing interest from the perspective of designing organized molecular rotors, motors, and machines, the goal being to control their functions and properties, such as the commutation of light into mechanical work, dielectric and optical properties, and ferroelectricity.[1−6] Different strategies have been addressed for the targeted construction of dynamic materials, including the use of self-assembly principles, host− guest compounds, and hybrid materials.[7−15]. Within the realm of porous materials, MOFs are outstanding for their synthetic versatility and the opportunity they provide to design modular structures, while preserving crystalline order and periodicity.[25,26] MOFs have been shown to support extensive dynamics without disrupting the primary architecture For this reason, MOFs were successfully employed to insert rotors in the frameworks as ligands bridging the metal ions or cluster nodes.[27−30] The project to assemble multiple fast rotors of increasing complexity within a single ordered framework is further challenging, since the Received: April 10, 2021 Published: August 13, 2021. We engineered bicomponent MOFs built from two distinct ultrafast and interacting molecular rotors of diverse chemical nature and symmetry (saturated vs unsaturated moieties and 2-fold vs 3-fold symmetry) They are organized as 2D layers comprising bicyclopentane dicarboxylate (BCP) units and bipyridine pillars forming a multidynamical architecture, wherein the rotors experience sequential motional behavior activated at distinct temperatures. The inclusion of CO2 greatly affected the mechanism, speed, and activation energy of the rotators

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES