Influence of Ligand Orientation on the Absorption Spectra in MOF

Structural disorder is not usually related to crystalline solids. Still, a considerable percentage of the deposited structures are reported with some disorder, especially in Metal-Organic Frameworks (MOF), formed by metal atoms and organic molecules, in which the organic part could have rotatable groups that can oriented distinctively of each other during synthesis but are indistinguishable once the structure is resolve. MIRO-101 is an MOF composed of zinc nodes and 3-pyridyl-tetrazole ligands that exhibit exceptional nonlinear optical properties and unusual optical absorption behavior. Previous studies reported that millimeter-sized single crystals of MIRO-101 exhibit absorption near 350 nm. At the same time, polycrystalline samples are entirely transparent in a similar range. According to computational simulations based on density functional theory (DFT), different spatial orientations of pyridyl groups on the crystal structure can generate significant variations in the absorption spectra with two main variants studied. Recently a pink-orange hue was observed in larger MIRO-101 single crystals with no clear indicative of critical structural differences concerning polycrystalline or previous XRD patterns. To explore this phenomenon, we examined the landscape of pyridyl orientations inside the unit cell of MIRO-101, generating several distinct structures and calculating absorbance with DFT. Our results show the contribution of each spatial distribution to the absorption spectra.