Thermochromic and Mechanochromic Luminescence Umpolung in Isostructural Metal–Organic Frameworks Based on Cu6I6 Clusters

Abstract

Two isostructural metal–organic framework (MOF) materials, namely, {[MeSi(3Py)3]6(Cu6I6)}n (1) and {[ MeSi(3Qy)3]6(Cu6I6)}n (2), featuring Cu6I6 clusters were synthesized from tridentate arylsilane ligands of the type MeSi(3Py)3 (3Py = 3-pyridyl) and MeSi(3Qy)3 (3Qy = 3-quinolyl), respectively. While the MOF 1 displays the usual thermochromism associated with traditional Cu4I4Py4 clusters, the MOF 2 shows 3XLCT/3MLCT emission due to the Cu6I6 cluster core at both 298 and 77 K, albeit with some marginal variations in its emission wavelengths. Interestingly, an unusual reversal in the mechanochromic luminescent behavior was observed for these isostructural MOFs at 298 K wherein a pronounced blue-shifted high energy emission for 1 (from orange to yellowish-orange) and a red-shifted low-energy emission for 2 (from green to orange) were obtained upon grinding these samples. This is primarily due to the variations in their cuprophilic interactions as 1 displays shorter Cu···Cu distances (2.745(1) Å) in comparison with those present in 2 (3.148(0) Å). As a result, the ground sample of 2 exhibits a prominent red shift in luminescence owing to the reduction of its Cu···Cu distances to an unknown value closer to the sum of van der Waals radii between two Cu(I) atoms (2.80 Å). However, the blue-shifted emission in 1 is presumably attributed to the rise in its lowest unoccupied molecular orbital energy levels caused by changes in the secondary packing forces. Furthermore, the absorption and emission characteristics of 1 and 2 were substantiated by time-dependent density functional theory calculations on their discrete-model compounds. In addition, the syntheses, reactivity studies, and photophysical properties of two one-dimensional MOFs, namely, {[MeSi(3Qy)3]2(Cu2I2)}n (3) and {[MeSi(3Qy)3](CuI)}n (4), having dimeric Cu2I2 and monomeric CuI moieties, respectively, were examined.