Thermally-Driven Conformational Twist in Organic Azobenzene Linker Activates Molecular Doping Effect in Thin Films of Lanthanide MOFs

Abstract

Azobenzene-based photo-switchable molecules have shown significant potential in stimuli-responsive systems, especially when incorporated into metal-organic frameworks (MOFs). This study reports thin films of lanthanide-based metal-organic frameworks (Ln-MOFs) with 4,4′-azobenzene dicarboxylic acid (H2ADA) as the organic linker – Tb-ADA, Eu-ADA, and Gd-ADA – using an electrodeposition method. Upon heating to 400 K, a reversible structural transition was observed via variable temperature grazing-incidence X-ray diffraction (GIXRD) and Raman spectroscopy, not due to trans-cis isomerization but rather a thermally-induced conformational twist of the ADA linker. Density functional theory (DFT) combined with molecular dynamics (MD) simulations supports this interpretation, revealing high-energy atropisomeric states stabilized by MOF confinement. Molecular doping of these films with 7,7,8,8-tetracyanoquinodimethane (TCNQ) significantly enhanced their electrical conductivity, increasing by two orders of magnitude at 400 K. This enhancement is attributed to improved π-π stacking and charge-transfer interactions facilitated by the conformational twist. Temperature-dependent X-ray photoelectron spectroscopy (XPS) confirmed redox activity in TCNQ@Tb-ADA films, showing reversible conversion between Tb(III) and Tb(IV), with back electron transfer at 400 K restoring Tb(III). These findings introduce a new mechanism of thermally-driven conformational switching in MOFs and open avenues for developing responsive electronic materials based on azobenzene linkers.