Evidence of Photoinduced Charge Separation in the Metal-Organic Framework MIL-125(Ti)-NH2

Herein, we describe the photochemical behavior of the porous metalorganic framework MIL-125(Ti)-NH2, built up from cyclic Ti8O8(OH)(4) oxoclusters and 2-aminoterephthalate ligands. While MIL-125(Ti)-NH2 does not emit upon excitation at 420 nm, laser flash photolyses of dry samples (diffuse reflectance) or aqueous suspensions (transmission) of the solid have allowed detecting a transient characterized by a continuous absorption from 390 to 820 nm decaying in the sub-millisecond timescale, which is quenched by oxygen. This transient has been attributed to the charge-separation state. Firm evidence for this assignment was obtained by lamp irradiation of aqueous suspensions of MIL-125(Ti)-NH2 in the presence of electron-donor (N,N,N'N'-tetramethyl-p-phenylenediamine) or electron-acceptor (methylviologen) probe molecules, which has allowed the visual detection of the corresponding radical ions, in agreement with the occurrence of photoinduced charge separation in MIL-125(Ti)-NH2.