MOF-74(M) Films Obtained through Vapor-Assisted Conversion—Impact on Crystal Orientation and Optical Properties

In recent years, metal–organic frameworks (MOFs) with the structure MOF-74 have attracted much interest owing to their tunable pore aperture, high surface area, and electrical conductivity. The synthesis of well-defined, highly crystalline thin films of MOF-74 is of paramount importance for their implementation into device-based applications such as in chemical sensing, optoelectronics, gas storage, and separations. Here, we present the synthesis of highly crystalline MOF-74 (M = Zn2+, Mg2+, Ni2+, and Co2+) films by vapor-assisted conversion. MOF-74(M) thin films were grown on bare glass, quartz, gold, and silicon surfaces, showing high crystallinity, crystal orientation, and average thicknesses of 500 nm. By including a benzoic acid modulator, oriented MOF-74(Zn) films, with the crystallographic c-axis of the MOF crystallites oriented horizontally to the surface, were obtained on all substrates. In addition, highly crystalline MOF-74(Mg) was grown on glass and gold substrates with the crystallographic c-axis aligned orthogonally to the surface. Moreover, randomly oriented highly crystalline MOF-74(Co) and MOF-74(Ni) films were synthesized on glass, quartz, gold, and silicon. The pore accessibility of the obtained films was examined by means of krypton sorption measurements, revealing permanent and accessible porosity, reaching a BET surface area of 975 cm2/cm2 for MOF-74(Mg). Steady-state and time-resolved photoluminescence studies show emission in the blue spectral region of MOF-74(Zn and Mg) on quartz with a biexponential decay. In addition, confocal photoluminescence mapping confirmed a homogeneous MOF film surface with a similar emission profile over the whole examined area of 70 μm × 70 μm.