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).

Femtosecond Pump – Probe Spectroscopy Reveals the Photo-excited State and Charge Transfer of a Photocatalytic Metal-Organic Framework

“…In those regions, the spectral features were, apart from solvatochromic shifts, also the same for both solvents. As mentioned, Ti 4+ is reduced to Ti 3+ after photo-excitation, and NH2-MIL-125 containing Ti3+ shows a broad increase in absorbance in the visible range [5]. Our transient absorbance measurements confirm this, and the found life times of 10 ps, 1 ns and a significantly longer component, correspond well with the transient mid-infrared spectra (figure 2 left).…”

“…It has been shown before that the photoexcited electron reside on Ti 4+ leading to Ti 3+ [5]. The vibrations of the bonds with the N-atom (C-N stretch, and N-H stretch vibrations) of the amino group lead to the largest changes in the transient absorption spectra.…”

Effects of ligand functionalization on the photocatalytic properties of titanium-based MOF: A density functional theory study

“…10 However, because of the large optical band gap (3.6 eV), 10 MIL-125 is only active under UV irradiation. Therefore, it is necessary to modify the organic ligands or 5,16 After introducing the amine group, the band gap of MIL-125 is obviously reduced to about 2.6 eV, 12,18,19 and similar decreasing of the band gap by amine-functionalization is also observed for zirconium-based MOF (NH 2 -Uio-66). 11 A recent investigation indicates that the number of amine group also has obviously influence on the photocatalytic performances of NH 2 -MIL-125.…”

Metal–Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production

“…[31][32][33][34] Thev ersatility in the composition of the organic linkers and metal nodes and the possibility of post-synthetic modification, together with the large surface area and porosity allowing incorporation of guests means that in many respects MOFs ( Figure 4) could be similar to zeolites and probably also semiconducting metal oxides. [36][37][38] Moreover,a romatic compounds have intense absorption bands above 250 nm and depending on the substituents can easily move to 300 nm or even into the visible region (l > 400 nm) and, therefore,M OFs can be designed in principle to exhibit visible-light photoresponses. In MOFs,t he most common ligands are aromatic polycarboxylates which because of their excess of electron density can, upon photon absorption, transfer an electron to the positive metal ions bound to them.…”