Evidence of photoinduced charge separation in the metal-organic framework MIL-125(Ti)-NH2

Abstract

Herein, we describe the photochemical behavior of the porous metal-organic framework MIL-125(Ti)-NH(2), built up from cyclic Ti(8)O(8)(OH)(4) oxoclusters and 2-aminoterephthalate ligands. While MIL-125(Ti)-NH(2) 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)-NH(2) 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)-NH(2).