Confinement-Driven Aggregate Formation of Photoacids within Porous Metal-Organic Frameworks

Structurally driven properties of hybrid materials are a fascinating feature of metal-organic framework (MOF) materials that can serve as hosts for various responsive dye molecules. In particular, the formation of aggregates and the related shift of the emission of fluorophors can be tuned as a function of pore confinement. In this work, the fluorosolvatochromic methylated photoacid tris(2,2,2-trifluoroethyl) 8-methoxypyrene-1,3,6-trisulfonate (MePhos) and the free photoacid tris(2,2,2-trifluoroethyl) 8-hydroxypyrene-1,3,6-trisulfonate (Phos) were inserted into various MOF scaffolds, and the resulting emission properties were found to be far beyond the observed red shifts for polar solvents such as methanol or ethanol. Instead of the modulation of the band gap by the local environment given by the physicochemical properties of the MOF pores, there is aggregation of the MePhos molecules depending on the MOF structure, leading either to H- or t o J-like character.