Structural control of photoconductivity in a flexible titanium-organic framework

Abstract

The soft nature of Metal-Organic Frameworks (MOFs) sets them apart from other non-synthetic porous materials. Their flexibility allows the framework components to rearrange in response to environmental changes, leading to different states and properties. Our work extends this concept to titanium frameworks, demonstrating control over charge transport in porous molecular crystals. MUV-35 is a two-fold catenated framework composed of heterometallic TiMn 2 trimers and electron donor 4,4',4''-(benzo[1,2-b:3,4-b':5,6-b'']trithiophene-2,5,8triyl)tribenzoic acid (H3 BTTTB) linkers, forming a rare sit-c net topology that can fold to reduce its volume by about 40% through a single-crystal transformation controlled by linker conformation in open, intermediate, and closed states. This process, driven by a free energy difference of nearly 300 kJ·mol-1 , originates from the formation of a continuous network of non-covalent interactions that force the spontaneous loss of the solvent in the pores of the framework to establish charge transport pathways that afford photocurrents of 2.5 × 10 -3 S·m-1 under visible light for an ON/OFF ratio (∆R) of four orders of magnitude. This photoconductivity rivals the best conductivity values described for though-transport conductive MOFs while maintaining a porosity of nearly 1.000 m2 ·g-1 .