Enhanced Energy Transfer from a Metal-Organic Framework to a Highly Confined Organic Phosphorescent Dye.

Three-Dimensional Metal-Organic Frameworks (MOFs) are crystalline, porous hybrid materials with well-defined pore structures, offering isotropic adsorption of molecules in their pore system. We designed and synthesized an organic, phosphorescent di-tert-butyl-carbazole dibenzophenazine chromophore, 11-DTCz-BP, with an appropriate size for a tight hand-to-glove fit into the 1D pore channels of MIL-68(In). MOF particles are synthesized, and homogeneous films are prepared on Au-substrates through electrochemical deposition. The steric demand of 11-DTz-BP yields a crystallographic ordered confinement inside the pores of MIL-68(In), proven by XRD and DFT calculations. Through this approach, it was possible to obtain controlled packing of the chromophore and lower emission quenching factors. Detailed spectroscopic analysis was performed using (cryo)fluorescence spectroscopy on powders and thin films. We consider Förster Resonance Energy Transfer (FRET) as a process of energy transfer between the pore walls of MIL-68 and 11-DTCz-BP. The aggregation control leads to a quantum yield enhancement, while FRET enables long lifetimes of phosphorescence at room-temperature.