Photoinduced Spin Centers in Photocatalytic Metal-Organic Framework UiO-66

TL;DR

This study investigates the photoinduced spin centers in UiO-66 MOF photocatalyst using advanced spectroscopic techniques and DFT calculations, revealing fundamental charge transfer processes and identifying three distinct radical species under UV light.

Contribution

It provides the first detailed analysis of photoinduced spin centers in UiO-66, enhancing understanding of its photocatalytic mechanisms through combined experimental and theoretical approaches.

Findings

Identification of three distinct photoinduced radical species

Observation of delayed fluorescence from triplet excitons

Insights into charge transfer processes in UiO-66

Abstract

Metal-Organic frameworks (MOFs) are promising candidates for advanced photocatalytically active materials. These porous crystalline compounds have large active surface areas and structural tunability and are thus highly competitive with oxides, the well-established material class for photocatalysis. However, due to their complex organic and coordination chemistry composition, photophysical mechanisms involved in the photocatalytic processes in MOFs are still not well understood. Employing electron paramagnetic resonance (EPR) spectroscopy and time-resolved photoluminescence spectroscopy (trPL), the fundamental processes of electron and hole generation are investigated, as well as capture events that lead to the formation of various radical species in UiO-66, an archetypical MOF photocatalyst. A manifold of photoinduced electron spin centers is detected, which is subsequently analyzed and identified with the help of density-functional theory (DFT) calculations. Under UV illumination, the symmetry, g-tensors and lifetimes of three distinct contributions are revealed: a surface O2-radical, a light-induced electron-hole pair, and a triplet exciton. Notably, the latter was found to emit (delayed) fluorescence. Our findings provide new insights into the photoinduced charge transfer processes, which are the basis of photocatalytic activity in UiO-66. This sets the stage for further studies on photogenerated spin centers in this and similar MOF materials.