Unraveling Competitive Electron and Energy-Transfer Events at the Interfaces of a 2D MOF and Nile Red Composites: Effect of the Length and Structure of the Linker.

The distribution and interactions of organic molecules adsorbed on the surface of materials play important roles in many catalytic and photonic processes. Here, we show that the length and chemical structure of the linker in new Al-ITQ metal-organic frameworks (MOFs) are fundamental for the dynamics of the dye Nile Red (NR) adsorbed on its surface. For the studied composites using Al-ITQ-4-ethylbenzoic acid (EB), Al-ITQ-4-aminobenzoic acid (AB), and Al-ITQ-EB exposed to the aniline (AN) or N, N-dimethylaniline (DMA) atmospheres, we observed a very fast (∼1.2 ps) intramolecular charge-transfer reaction in adsorbed NR molecules. For NR@Al-ITQ-EB, where the linker has a shorter aliphatic chain (two carbons), the dye molecules present a homoenergy-transfer (ET) process, which is faster (∼90 ps) than in the previously reported NR@Al-ITQ-4-heptylbenzoic acid composite with longer aliphatic chain (seven carbons, ∼220 ps). The more polar environment created by the Al-oxide nodes in Al-ITQ-EB surface around the NR populations strongly favors the ET event. When the linker structure contains phenyl amine moieties, the resulting NR@Al-ITQ-AB composites show different and rich photodynamics, in which a fast electron transfer reaction from the MOF aniline moiety to the adsorbed NR occurs in ∼17 ps, inhibiting the ET process between the dye molecules near the MOF surface. This process also was confirmed in Al-ITQ-EB MOF exposed to AN and DMA gas atmospheres, as well as NR in pure aniline. The obtained results demonstrate how modifications in the length and structure of the organic linker in this MOF change the interface interactions and outcome of the photoinduced processes in the composites. Our findings on dye-MOF interface photobehavior are relevant to the design of new materials in which the interface plays a key role in their performance in the fields of catalysis and photonics.