Structure-Function Correlation in Switchable DTE@MOF Hybrids: Tracking Local Dynamics and Fatigue Pathways.

The origin of optical properties of non-covalently inserted photoswitchable molecules inside porous metal-organic frameworks (MOFs) remains a challenge due to the high mobility and spatial freedom of the embedded dye. Both before and after switching, dye-MOF interaction motifs are subject to changes, which might significantly impact the resulting photochromic response with respect to fatigue resistance. Within this work, the light-induced isomerization processes of a P-type chromophore (dithienylethene) inside the robust UiO-67 are studied with respect to structural rearrangements. For the first time, solid-state NMR (ssNMR) data were utilized next to X-ray diffraction experiments to characterize the embedment and physical state of the photoswitchable dye by tracing changes in both the MOF and guest signature. Moreover, diffuse reflectance spectroscopy measurements revealed slight fatigue originating from a unique nanoconfinement effect within the MOF pore. The open photoinactive form is preferably present in an MOF side pore, causing fatigue of the photoswitching. Molecular dynamics (MD) simulations at the semiempirical quantum chemical level of theory provided the atomic perspective to understand the underlying host-guest interaction, involving the occupation of different pores of UiO-67 by the open and closed form of the embedded chromophore and the tremendous stabilization of the compact photoinactive conformer.