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

TL;DR

This study elucidates how D-$\\pi$-A functionalization modifies the electronic structure of UiO-66-NH$_2$ MOFs, enhancing charge separation and photocatalytic activity through specific donor groups and triplet charge-transfer states.

Contribution

It provides a mechanistic understanding of how donor groups influence charge dynamics and photocatalytic efficiency in D-$\pi$-A-functionalized MOFs using combined spectroscopic and computational methods.

Findings

Anisole-modified MOFs facilitate efficient intersystem crossing to triplet states.

Donor groups introduce new occupied states near the valence band.

Bulkier donors can create defect-related trap states.

Abstract

Donor-$\pi$-acceptor (D-$\pi$-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-$\pi$-A-modified MOFs.