Photophysics of Azobenzene Constrained in a UiO Metal–Organic Framework: Effects of Pressure, Solvation and Dynamic Disorder

• Published in: Chemistry : a European journal 2021-10, Vol.27 (60), p.14871-14875
• Main Authors: Sussardi, Alif, Marshall, Ross J., Moggach, Stephen A., Jones, Anita C., Forgan, Ross S.
• Format: Article
• Language: English
• Subjects: Azo compounds; azobenzene; Chemistry; Chromophores; Conformation; Crystal structure; Crystallography; External stimuli; Fluorescence; high-pressure chemistry; Isomerization; Metal-organic frameworks; Optical properties; Pressure; Pressure effects; Solvation; Topology; X-ray diffraction
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202101879

Photophysical studies of chromophoric linkers in metal–organic frameworks (MOFs) are undertaken commonly in the context of sensing applications, in search of readily observable changes of optical properties in response to external stimuli. The advantages of the MOF construct as a platform for investigating fundamental photophysical behaviour have been somewhat overlooked. The linker framework offers a unique environment in which the chromophore is geometrically constrained and its structure can be determined crystallographically, but it exists in spatial isolation, unperturbed by inter‐chromophore interactions. Furthermore, high‐pressure studies enable the photophysical consequences of controlled, incremental changes in local environment or conformation to be observed and correlated with structural data. This approach is demonstrated in the present study of the trans‐azobenzene chromophore, constrained in the form of the 4,4’‐azobenzenedicarboxylate (abdc) linker, in a UiO topology framework. Previously unobserved effects of pressure‐induced solvation and conformational distortion on the lowest energy, nπ* transition are reported, and interpreted the light of crystallographic data. It was found that trans‐azobenzene remains non‐fluorescent (with a quantum yield less than 10−4) despite the prevention of trans‐cis isomerization by the constraining MOF structure. We propose that efficient non‐radiative decay is mediated by the local, pedal‐like twisting of the azo group that is evident as dynamic disorder in the crystal structure. Putting the pressure on azobenzene photophysics: When locked in a UiO metal–organic framework, the azobenzene chromophore is geometrically constrained but exists in spatial isolation, unperturbed by inter‐chromophore interactions. This is a unique and advantageous environment in which to explore fundamental photophysical properties. Optical spectroscopy in a diamond anvil cell enables the photophysical consequences of pressure‐induced changes in solvation and conformation of the chromophore to be observed and correlated with crystallographic data.