Systematic Investigation into the Photoswitching and Thermal Properties of Arylazopyrazole-based MOF Host–Guest Complexes

Griffiths, Kieran and Greenfield, Jake L. and Halcovitch, Nathan R. and Fuchter, Matthew J. and Griffin, John M. (2023) Systematic Investigation into the Photoswitching and Thermal Properties of Arylazopyrazole-based MOF Host–Guest Complexes. Crystal Growth and Design, 23 (10). pp. 7044-7052. ISSN 1528-7483

Abstract

A series of arylazopyrazole-loaded metal–organic frameworks were synthesized with the general formula Zn2(BDC)2(DABCO)(AAP)x (BDC = 1,4-benzenedicarboxylate; DABCO = 1,4-diazabicyclo-[2.2.2]octane; AAP = arylazopyrazole guest). The empty framework adopts a large pore tetragonal structure. Upon occlusion of the E-AAP guests, the frameworks contract to form narrow pore tetragonal structures. The extent of framework contraction is dependent on guest shapes and pendant groups and ranges between 1.5 and 5.8%. When irradiated with 365 nm light, the framework expands due to the photoisomerization of E-AAP to Z-AAP. The proportion of Z-isomer at the photostationary state varies between 19 and 57% for the AAP guests studied and appears to be limited by the framework which inhibits further isomerization once fully expanded. Interestingly, confinement within the framework significantly extends the thermal half-life of the Z-AAP isomers to a maximum of approximately 56 years. This finding provides scope for the design of photoresponsive host–guest complexes with high stability of the metastable isomer for long-duration information or energy storage applications.