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 [Co-2(NDC)(2)(DPTTZ)(2)]. DPTTZ. DMF, 1 and [Co-2(BDC)(2)(DPTTZ)(2)]. DMF, 2 (where NDC=naphthalene dicarboxylate, BDC=benzene dicarboxylate, DPTTZ=N, N & PRIME;-di(4-pyridyl)thiazolo-[5,4-d]thiazole, DMF=N, N & PRIME;-dimethyl formamide) are considered for this, whose intra-dimer distance of redox-active DPTTZ ligands differs ca. 1 & ANGS; 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.

Automated summary

We provide evidence of photoinduced through-space intervalence charge transfer (IVCT) between two redox-active pairs in metal-organic frameworks and show that the extent of charge transfer varies with their molecular separation. Two similar MOFs [Co-2(NDC)(2)(DPTTZ)(2)]. DPTTZ. DMF, 1 and [Co-2(BDC)(2)(DPTTZ)(2)]. DMF, 2 were studied, with a difference of approximately 1 Å in the intra-dimer distance of redox-active DPTTZ ligands. Spectroelectrochemical study detected the formation of IVCT band at the near-infrared region in both MOFs. Transient spectroscopy revealed faster charge separation and recombination when the intra-dimer distance was shorter (in MOF 2) due to stronger electronic coupling. IVCT extent was quantified by charge transfer integral calculation and optical pump terahertz probe spectroscopy, with MOF 2 showing three times higher carrier mobility due to a shorter inter-DPTTZ distance compared to MOF 1. These findings reveal a more localized aspect of through-space IVCT between cofacially arranged redox-active pairs in a three-dimensional framework.