Mixed-Ligand Metal-Organic Framework for Two-Photon Responsive Photocatalytic C-N and C-C Coupling Reactions

A multiple-photon photochemical process breaks the existing energy limitation of visible-light photo catalysis. Through careful incorporation of both bis(3,5-dicarboxyphenyl)pyridine and bis (3,5-dicarboxyphenyl)-methylpyridinium ligands into a single metal-organic framework (MOF), we report herein photocatalytic C-N and C-C oxidative coupling reactions that evidenced the direct two photon response process. Time-dependent luminescence decay studies demonstrated that the framework reached the same excited state by a two-photon absorption process as that reached via a single-photon absorption process, and the excited state could activate substrates even under NIR-light irradiation. Under 660 nm light-emitting diode (LED) irradiation, both the photooxidative C-N coupling reaction of benzylamine to form benzylidene-1-phenylmethanamine and the C-C coupling reaction between nitromethane and N-phenyltetrahydroisoquinoline were accelerated directly in a heterogeneous manner. Control experiments suggested that minimal byproducts were formed under the NIR-light irradiation compared to those of the UV light catalytic reactions. Importantly, photon intensity-dependent studies exhibited a nonlinear correlation between catalytic efficiency of multivariate MOFs (MTV-MOFs) and photon power intensity, verifying the two photon responsive nature, which indicated that metal-organic framework with high density and ordered photoactive motifs could enhance the two-photon absorption ability, thereby improving photocatalytic efficiency even under LED irradiation.