Post-synthetic modification (PSM) is an effective and simple approach for tailored functionalization of metal-organic architectures, but its generalizability remains challenging. Anthracene is a useful molecule that has the reactivity to undergo both photochemical and thermal cycloadditions in a mild condition across 9 and 10 positions with a variety of suitable substrates. However, there are not many examples of constructing metal-organic cages (MOCs) by using anthracene as a reactive group for PSM, especially the PSM by introducing anthracene groups into PdnL2n-type MOCs for directed functionalization has not been reported yet. We report here a general PSM strategy to functionalize the PdnL2n-type MOCs (MOCs, n = 2, 12) via a rapid and efficient Diels−Alder cycloaddition between the anthracene substituents and the functional motifs bearing a maleimide group. The MOC Pd12L24 containing 24 anthracene groups was firstly synthesized, which can be efficiently grafted with functional groups (e.g., cyclohexyl, phenethyl, BINOL moiety, tetraphenylethylene group, and pyrene substituent) by using covalent PSM process (Figure 1). As results, the solubility of cyclohexyl-modified MOCs has been greatly improved, BINOL-modified MOCs are endowed with chirality and concentration-dependent circular dichroism, and tetraphenylene-modified MOCs exhibited aggregation-induced emission property. More interestingly, a step-by-step PSM method is introduced to decorate two different types of functional groups on a MOC (e.g., chiral BINOL and pyrene substituents), which can make achiral pyrene chromophores produce circularly polarized luminescence (CPL) properties through chiral induction by chiral BINOL groups. Moreover, this general method can be easily extended to a similar MOC Pd2L4 system, and functionalized products can be obtained efficiently and approximately quantitatively. This work provides a general strategy and new platform to design and synthesize advanced materials based on MOCs.

Tailor-made PdnL2n metal-organic cages with customized functions via covalent post-synthetic modification

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