Mechanistic Origin of Charge Separation and Enhanced Photocatalytic Activity in D-$π$-A-Functionalized UiO-66-NH$_2$ MOFs

Donor-$π$-acceptor (D-$π$-A) functionalization of MOF linkers can enhance visible-light photocatalytic activity, yet the mechanisms responsible for these effects remain unclear. Here we combine EPR spectroscopy, transient photoluminescence, and first-principles calculations to examine how diazo-coupled anisole, diphenylamine (DPA), and N,N-dimethylaniline (NNDMA) groups modify the photophysics of UiO-66-NH$_2$. All donor units introduce new occupied states near the valence-band edge, enabling charge separation through dye-to-framework electron transfer. Among them, the anisole-modified material stands out for facilitating efficient intersystem crossing into a triplet charge-transfer configuration that suppresses fast recombination and yields long-lived charge carriers detectable by photo-EPR. Meanwhile, bulkier donors such as DPA and NNDMA - despite their stronger electron-donating character - also tend to introduce defect-associated trap states. These results underscore the interplay between donor-induced electronic-structure changes, triplet pathways, and defect-mediated recombination, offering a mechanistic basis for tuning photocatalytic response in D-$π$-A-modified MOFs.

💡 Research Summary

The authors investigate how post‑synthetic donor‑π‑acceptor (D‑π‑A) functionalization of the Zr‑based metal‑organic framework UiO‑66‑NH₂ influences its photophysical properties and visible‑light photocatalytic activity. Three electron‑donating aryl groups—anisole (–OCH₃), N,N‑dimethylaniline (–N(CH₃)₂), and diphenylamine (–NHPh)—were covalently attached to the amino‑terephthalate linkers via diazonium coupling, forming stable –N=N– azo bridges. UV‑Vis spectroscopy shows that all three modifications extend absorption well into the visible region (up to ~520 nm for anisole, ~650 nm for NNDMA and ~700 nm for DPA), confirming successful dye incorporation and band‑gap narrowing.

Density‑functional theory (DFT) calculations reveal that each donor introduces occupied states just above the valence‑band maximum (VBM) of the pristine UiO‑66‑NH₂, thereby reducing the band gap by ~0.1 eV (anisole) to ~0.7 eV (NNDMA, DPA). Projected density of states indicates that the highest occupied orbital is localized on the donor‑azo fragment, while the conduction‑band minimum remains on the original linker/Zr‑node framework. This spatial separation of frontier orbitals embodies the D‑π‑A concept and should promote intraframework charge transfer (CT) upon photoexcitation: an electron moves from the donor to the framework, leaving a hole on the donor moiety.

Photoluminescence (PL) measurements support this picture. The pristine MOF exhibits a single emission at 451 nm, whereas the functionalized samples show dual emission: a residual band from unmodified linkers and a new, red‑shifted band (600–715 nm) attributable to the dye‑modified linkers. Time‑resolved PL indicates that anisole‑functionalized UiO‑66‑NH₂ possesses the longest excited‑state lifetime (hundreds of nanoseconds), suggesting efficient intersystem crossing (ISC) to a triplet CT state. In contrast, the bulkier NNDMA and DPA introduce broader, less defined emission, consistent with faster non‑radiative recombination.

Electron paramagnetic resonance (EPR) spectroscopy provides direct evidence of charge separation. Dark‑state EPR detects a weak NH• radical common to all samples and two additional organic radicals (Species 1 and 2) with g‑values close to the free‑electron value (g≈2.004). The concentration of these radicals is lowest for the anisole‑modified material, implying minimal structural disturbance. Upon broadband UV illumination, a pronounced photo‑induced EPR signal appears, especially for the anisole sample, indicating the formation of long‑lived spin‑polarized charge carriers (triplet CT radicals). The larger donors generate slightly higher radical concentrations, reflecting greater steric strain, possible missing‑linker defects, and the creation of trap states that accelerate recombination.

Photocatalytic tests using methylene‑blue (MB) degradation under simulated solar light reveal that the anisole‑functionalized MOF achieves the highest degradation efficiency, despite its modest red‑shift compared with NNDMA and DPA. This decoupling of absorption breadth from catalytic performance underscores that efficient charge separation, long‑lived triplet states, and low defect density are more decisive than mere band‑gap narrowing.

In summary, the study demonstrates that D‑π‑A functionalization can endow UiO‑66‑NH₂ with visible‑light activity, but the ultimate photocatalytic outcome depends on a delicate balance: (i) the donor’s electron‑donating strength must be sufficient to raise occupied states, (ii) the donor’s size should not introduce excessive steric strain or defects, (iii) the system must facilitate rapid ISC to a triplet CT configuration that suppresses fast electron‑hole recombination, and (iv) defect‑related trap states must be minimized. These insights provide a mechanistic framework for rational design of MOF‑based photocatalysts, suggesting that small, well‑aligned π‑conjugated donors (or post‑synthetic treatments that heal defects) are optimal for achieving high visible‑light photocatalytic efficiencies.