Hydration-dependent exciton-phonon coupling in a metal-organic framework photocatalyst

Metal-organic frameworks (MOFs) have potential for light-harvesting applications owing to their tunable optoelectronic properties via building block selection. The presence of bound solvent in MOFs is known to impact gas adsorption properties, but little is known about their effect on optical properties. Here, we investigate how the presence of bound water can modulate the optical properties of a reported MOF photocatalyst, SU-101. Using the GW approximation and the ab initio Bethe-Salpeter equation (BSE) and treating exciton-phonon coupling with a finite difference method, we show that exciton-phonon interactions in SU-101 are strong and highly sensitive to the presence of bound water molecules. Specifically, we find that the presence of bound water is associated with a stronger localization of excitons due to phonons. Guided by our calculations, we synthesize SU-101 and study this MOF under different water loadings. We measure photoluminescence (PL) emission and UV-Vis absorption spectra of the MOFs, the latter in good agreement with our computational predictions. By considering the interplay of water loading, phonons, and excitons, our calculations rationalize measured UV-Vis absorption and PL spectra. Our work also highlights the pivotal role of exciton-phonon interactions and their sensitivity to hydration on the optoelectronic properties of MOFs with large pores and flexible organic linkers.