Influence of Molecular Separation on Through-Space Intervalence Transient Charge Transfer in Metal-Organic Frameworks with Cofacially arranged Redox Pairs

We demonstrate direct evidence of photoinduced through-space intervalence charge transfer (IVCT) between two cofacially arranged redox-active pairs in metal-organic frameworks and their dynamic variation with their molecular separation. Two homologous MOFs [Co₂ (NDC)₂ (DPTTZ)₂ ]. DPTTZ. DMF, 1 and [Co₂ (BDC)₂ (DPTTZ)₂ ]. DMF, 2 (where NDC=naphthalene dicarboxylate, BDC=benzene dicarboxylate, DPTTZ=N, N'-di(4-pyridyl)thiazolo-[5,4-d]thiazole, DMF=N, N'-dimethyl formamide) are considered for this, whose intra-dimer distance of redox-active DPTTZ ligands differs ca. 1 Å from one system to another. Spectroelectrochemical study detects the formation of IVCT band at the NIR region between cofacially oriented DPTTZ molecules in both MOFs. Transient spectroscopy shows faster charge separation as well as charge recombination when intra-dimer distance is lesser (in MOF 2) due to stronger electronic coupling. We quantify the extent of IVCT by charge transfer integral calculation; and also by optical pump terahertz probe spectroscopy, where MOF 2 shows three times higher carrier mobility due to lesser inter-DPTTZ distance than MOF 1. These findings reveal a more localized aspect of through-space IVCT between cofacially organized redox-active pair in a three-dimensional framework.