Heterogenization of Photochemical Molecular Devices: Embedding a Metal-Organic Cage into a ZIF-8-Derived Matrix To Promote Proton and Electron Transfer.

ABSTRACT

Application of a molecular catalyst in artificial photosynthesis is confronted with challenges such as rapid deactivation due to photodegradation or detrimental aggregation in harsh conditions. In this work, a metal-organic cage [Pd6(RuL3)8]28+ (MOC-16), characteristic of a photochemical molecular device (PMD) concurrently integrating eight Ru2+ light-harvesting centers and six Pd2+ catalytic centers for efficient homogeneous H2 production, is successfully heterogenized through incorporation into a metal-organic framework (MOF) of ZIF-8 and then transformed into a carbonate matrix of Znx(MeIm)x(CO3)x (CZIF), leading to hybridized MOC-16@CZIF. This MOC@MOF integrated photocatalyst inherits a highly efficient and directional electron transfer in the picosecond domain of MOC-16 and possesses one order increased microsecond magnitude of the triplet excited-state electron in comparison to that of the primitive MOC-16. The carbonate CZIF matrix endows MOC-16@CZIF with water wettability, serving as a proton relay to facilitate proton delivery by virtue of H2O as proton carriers. Electron transfer during the photocatalytic process is also enhanced by infiltration of a sacrificial agent of BIH into the CZIF matrix to promote conductivity, owing to its strong reducing ability to induce free charge carriers. These synergistic effects contribute to the extra high activity for H2 generation, making the turnover frequency of this heterogeneous MOC-16@CZIF photocatalyst maintain a level of ∼0.4 H2·s-1, increased by 50-fold over that of a homogeneous PMD. Meanwhile, it is robust enough to tolerate harsh reaction conditions, presenting an unprecedented heterogenization example of homogeneous PMD with a MOF-derived matrix to mimic catalytic features of a natural photosystem, which may shed light on the design of multifunctional PMD@MOF materials to expand the number of molecular catalysts for practical application in artificial photosynthesis.