Highly Luminescent Metal-Organic Frameworks Through Quantum Dot Doping

This permits the incorporation of the QDs within the evolving framework during the reaction. The resulting QD@MOF-5 composites are characterized using small-angle X-ray scattering/diffraction, which proofed the undistorted incorporation of the ODs into the MOF structure. Such structures showed the synergistic combination of luminescent QDs and the controlled porosity of MOF-5 in the QD@MOF-5 composites demonstrated within a prototype molecular sensor that can discriminate on the basis of molecular size.
To date, the synthesis of host–guest luminescent MOFs has only been achieved by post impregnation with organic dyes or through a two-step procedure functionalizing a ceramic microparticlewith QDs and then using it as a seed to grow MOF-5.A one-step approach is considered better suited to industrial processes when compared with multistep procedures.The emission properties of QD@MOF composites have critical advantages over organic counterparts, in terms of long-term stability, quantum yield, and precise control of the emission wavelength.Maintaining the optical quality of QDs through processing into QD@MOF-5 composites is a critical challenge that must be overcome in order to utilize such MOF materials in the still unexplored fields of photocatalysis, photovoltaics, and optoelectronics.
The one-pot methodology of growing QD@MOF-5 composites reported in this work has the potential to result in significant inhomogeneity of the QD distribution within the framework, as well as to cause distortion of the original MOF-5 framework crystal lattice. These features could result in undesired deterioration of both the QD optical properties and the framework microstructure. A campaign was therefore launched to screen the experimental variables that influence both the retention of optical properties and the framework crystal structure of the composites. By careful optimization of these variables the desired result has been obtained.

The quality of the red QD@MOF-5 samples was assessed by comparing synchrotron small angle X-ray scattering (SAXS) patterns taken at the Austrian SAXS beamline for the composite with a control sample of un-doped MOF-5 powder. The diffraction results confirm the crystalline order of the framework matrix surrounding the QDs. The intense and sharp diffraction peaks at 2 theta = 6.9°({200}plane, d = 12.8 Å) and 9.7°({220}plane, d = 9.1 Å) indicate long-range modular arrangement of large pores, indicative of a typical MOF-5 cubic lattice; the diffraction of the QD@MOF-5 sample is consistent with the diffraction pattern of the control powder. In addition, the relative intensity of the low-angle reflections and the low intensity of the 13.8° peak ({400}plane, d = 6.4 Å) indicate that the crystals are of high crystallinity and present very limited framework interpenetration, respectively. The analysis demonstrates that although the QD diameter (8.3 nm) is ten times bigger than the MOF-5 cavities (0.8 nm), the overall microstructural arrangement of the framework matrix has not been significantly distorted. It is important to note here that no diffraction from the embedded QDs has been detected, given their low volume concentration within the crystals. In synthesis, the structural identity of the MOF matrix has been preserved in the QD@MOF-5 sample, thus indirectly indicating that the QDs sit within the MOF-5 crystals with very little distortion of the framework microstructure.