Synthesis of dipolar molecular rotors as linkers for metal-organic frameworks.

Sebastian Hamer,Xianghui Zhang,Rainer Herges,Ian A Howard,Fynn Röhricht,Marius Jakoby,Christian Näther

doi:10.3762/bjoc.15.132

Sebastian Hamer, Xianghui Zhang + Show 5 more

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https://doi.org/10.3762/bjoc.15.132

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Abstract

We report the synthesis of five dicarboxylic acid-substituted dipolar molecular rotors for the use as linker molecules in metal-organic frameworks (MOFs). The rotor molecules exhibit very low rotational barriers and decent to very high permanent, charge free dipole moments, as shown by density functional theory calculations on the isolated molecules. Four rotors are fluorescent in the visible region. The linker designs are based on push–pull-substituted phenylene cores with ethynyl spacers as rotational axes, functionalized with carboxylic acid groups for implementation in MOFs. The substituents at the phenylene core are chosen to be small to leave rotational freedom in solids with confined free volumes. The dipole moments are generated by electron-donating substituents (benzo-1,3-dioxole, benzo-1,4-dioxane, or benzo-2,1,3-thiadiazole annelation) and withdrawing substituents (difluoro, or dicyano substitution) at the opposite positions of the central phenylene core. A combination of 1,4-dioxane annelation and dicyano substitution generates a theoretically predicted, very high dipole moment of 10.1 Debye. Moreover, the molecules are sufficiently small to fit into cavities of 10 Å3. Hence, the dipolar rotors should be ideally suited as linkers in MOFs with potential applications as ferroelectric materials and for optical signal processing.

Highlights

Rotors are among the fundamental functional units in engineering in our macroscopic world, as well as at the molecular level
Aiming at high dipole moments our design was inspired by recent reports of Müllen et al who reported on very high dipole moments of 1,2-dicyano-4,5-diamino-substituted phenyl derivatives [35]
The predicted dipole moment of 1 with 2.6 D is small, but the molecule should be suitable for metalorganic frameworks (MOFs) preparation because the structurally similar parent compound 1,4-benzenedipropynoic acid has been successfully used for MOF synthesis [43]

Summary

Introduction

Rotors are among the fundamental functional units in engineering in our macroscopic world, as well as at the molecular level. We report on the synthesis of five different dipolar rotors (Figure 2) that are designed to meet the criteria 1–4 listed above, for the use as building blocks in the construction of functional MOFs. Molecular rotors with permanent dipole moments can be oriented by an external electric field as shown by Michl [26,27] and Price [28,29], or undergo spontaneous ordering by intermolecular dipole–dipole interactions. MOFs [31,32] und particular SURMOFs [33,34] are ideally suited to achieve an ordered 3D arrangement and to maximize intermolecular interactions, because the dipolar rotors are used as functional units as well as building blocks for construction of the lattice (linker).

Results

Conclusion