Capture of the steric trap state in azobenzene MOF thin films using femtosecond transient absorption spectroscopy

Azobenzene-based metal-organic frameworks (MOFs) are generally used as photoswitches for gas separation, drug transportation, and other functions. The photoswitching function of these materials is mainly determined by the photoisomerization of the azobenzene ligands. A detailed understanding of the excited-state dynamics of azobenzene in MOFs is significant for the synthesis of more efficient photoswitching materials. Here, the excited-state photoisomerization dynamics of AzoBPDC in THF solution and MOF thin films were investigated using femtosecond transient absorption spectroscopy. Except for the ultrafast photoisomerization dynamics of AzoBPDC (∼5 ps), an additional long-lived component (∼58 ps) was revealed solely in the MOF thin film. Further experiments conducted on side-chain MOF thin films of varying sizes indicate that the observed time component can be modulated by varying the substituent groups on the azobenzene linkers. The long-lived component was therefore attributed to a steric trap state, which manifests the confinement effect imposed by the MOF framework on the ligand molecules. These findings provide clear guidance for the synthesis of functional MOF materials.