Cationic and neutral copper(I) iodide cluster MOFs derived from tridentate N-donor functionalized P(V) ligands: synthesis, structure and photophysical properties

Abstract

Formation of cationic and neutral CuI cluster MOFs have been reported starting from tridentate phosphoramide ligands, [(NHR)3P=E] (L1: R=3-aminoquinolinyl (AQ), E=S; L2: R=3-pyridyl (PY), E=S; L3: R=3-aminoquinolinyl (AQ), E=O). By utilizing L1, a cationic 2D-MOF {[(L1)2(Cu6I5)](OH)⋅3DMF⋅4MeOH}n, 1 containing a rugby ball shaped discrete Cu6I5 cluster has been reported earlier. Formation of a new 3D-MOF {[(L2)2(Cu6I4)](OH)2⋅2DMF}n containing a Zintl type [(Cu6I4)2+]n cluster chains is reported in this paper. A neutral cluster MOFs 3 with formula unit of {[Cu4I4L3(CH3CN)]⋅2DMF⋅3H2O}n has been prepared from the ligand L3. Formation of the smaller Cu4I4 clusters in the MOF 3 is due to the presence of a MeCN ligation at one of the Cu(I) atoms which not only precludes the extension of the assembly in three dimension but also reduces the size of the obtained cluster. Unlike 1 which showed a ligand-assisted thermochromism, photophysical studies on the 3D-MOF 2 exhibited green phosphorescence at both 298 K and 77 K. The occurrence of the phosphorescence at 77 K in 2 is due to triplet cluster centered (3CC) excited state of the cluster as there is no ligand-centered transition observed at 298 K. The 2D-MOF 3 does not show any characteristic luminescence behavior as the presence of the acetonitrile coordination at one of the Cu(I) ion is believed to quench the emission by non-radiative pathways. Further, luminescence quenching experiments on 1 and 2 with aromatic nitro-analytes showed a very high sensing selectivity for picric acid (TNP) over other aromatic nitro-analytes.