Ligand-to-Ligand Charge Transfer within Metal–Organic Frameworks Based on Manganese Coordination Polymers with Tetrathiafulvalene-Bicarboxylate and Bipyridine Ligands

A systematic study
on ligand-to-ligand charge-transfer (LLCT) properties of three closely
related metal–organic frameworks (MOFs) is presented. These
compounds are formulated as [MnL­(4,4′-bpy)­(H2O)]n·nCH3CN (1), [MnL­(bpe)0.5­(DMF)]n·2nH2O (2), and [MnL­(bpa)­(H2O)]n·2nH2O (3) (L = dimethylthio-tetrathia­fulvalene-bicarboxylate,
4,4′-bpy = 4,4′-bipyridine, bpe = 1,2-bis­(4-pyridyl)­ethene,
bpa = 1,2-bis­(4-pyridyl)­ethane). The X-ray single-crystal diffractions
show that complexes 1–3 are all two-dimensional
(2-D) coordination polymers with different frameworks in crystal lattices.
Charge-transfer (CT) interactions within these MOFs are visually apparent
in colors and vary according to the conjugated states of the bipyridine
ligands (4,4′-bpy, bpe, and bpa). Theoretical calculations
show that the charge transfer occurs from ligand L to bipyridine.
The intensity of the LLCT is in the order of 2 > 1 > 3 investigated by theoretical calculations
and ESR, which indicates that the intensity of CT is related to the
bipyridyl conjugated state. Photocurrent responses of these compounds
are consequently studied, and the results are in agreement with the
intensity of charge transfer and linearly related to the LLCT energy.