Uncovering Two Principles of Multivariate Hierarchical Metal-Organic Framework Synthesis via Retrosynthetic Design

Abstract

Multivariate (MTV) hierarchical metal-organic frameworks (MOFs), which contain multiple regions arranged in ordered structures, show promise for applications such as gas separation, size-selective catalysis, and controlled drug delivery. However, the complexity of these hierarchical MOFs is limited by a lack of control during framework assembly. Herein, we report the controlled generation of hierarchical MOF-on-MOF structural formation under the guidance of two design principles, surface functionalization and retrosynthetic techniques for stability control. Accordingly, the tunability of spatial distributions, compositions, and crystal sizes has been achieved in these hierarchical systems. The resulting MOF-on-MOF hierarchical structures represent a unique crystalline porous material which contains a controllable distribution of functional groups and metal clusters that are associated together within a framework composite. This general synthetic approach not only expands the scope and tunability of the traditional MTV strategy to multicomponent materials, but also offers a facile route to introduce variants and sequences to sophisticated three-dimensional hierarchical and cooperative systems. As a proof of concept, the photothermal effects of a porphyrinic core-MOF are exploited to trigger the controlled guest release from a shell-MOF with high guest capacity, highlighting the integrated cooperative behaviors in multivariate hierarchical systems.

Scheme 1. Preparation of Multivariate Hierarchical Metal–Organic Frameworks through Retrosynthetic Design

(a) Direct reaction between PCN-222 and MOF-5 precursor leads to the homogeneous nucleation and growth of MOF-5 as a separate phase; (b) overcoming the energy barrier of heterogeneous nucleation of MOF-5 by surface functionalization; (c) retrosynthetic analysis of bimetallic MOFs starting from monometallic MOFs exhibiting surface defects. This strategy can be utilized to guide the stepwise synthesis of hierarchical PCN-222@MOF-5 composites. Note that the ball-and-stick model of MOF-5 refers to the whole crystal lattice. The scale of PCN-222 is narrowed to better present the apportionment of PCN-222 inside MOF-5.

Figure 1

Tunable compositions in hierarchical PCN-222@MOF-5. (a) Models and optical images showing controlled incorporation of PCN-222 inside MOF-5, from 0% to 14%. (b–c) SEM images of the surface (b) and interior (c) of PCN-222@MOF-5-(8%). The internal areas of PCN-222@MOF-5-(8%) were accessed by physically crushing samples. (d–e) N₂ sorption isotherms and PXRD patterns of PCN-222@MOF-5-(R%, R = 1, 8, and 14).

Figure 2

Versatility of kinetically guided retrosynthesis of hierarchical MOFs. (a) Sequence-defined combinations of clusters and linkers into hierarchical MOFs with controllable compositions and distributions. More stable MOFs with high valence metals (M⁴⁺, M³⁺) should be designed as core MOFs, while less stable MOFs with low valence metals (M⁺, M²⁺) would be assigned as shell MOFs. (b–d, f–j) Optical images of hierarchical MOFs with sequence-defined combinations guided by retrosynthetic stability consideration. Scale bar is 100 μm in all optical images. (e) Unlimited combination of multivariate hierarchical MOFs that can be synthesized under the guidance of the two principles.

Figure 3

Tuning spatial distributions in hierarchical PCN-222@MOF-5. (a–b) Evolution of well-mixed and center-concentrated PCN-222@MOF-5 hierarchical MOFs. (c–d) Preparation of an asymmetric dispersion of PCN-222 in MOF-5 shell (similar to Janus particles). Scale bar is 100 μm in inset images.

Figure 4

(a) Tunability of the pore environment and guest penetration behavior in hierarchical MOFs for tailored applications. (b) Relative activity of PCN-222(Fe) and hierarchical PCN-222(Fe)@ZIF-8 for the oxidation of o-phenylenediamine (o-PDA) and ABTS.

Figure 5

Phototriggered guest release by multivariate hierarchical MOFs. (a) Shell MOFs without photoresponsive units cannot release guests upon light exposure; (b) core MOFs with photothermal effects lack the controlled release of guests and have overall guest capacity; (c) physical mixtures of (a, b) result in low photoresponsive efficiency due to the heat loss to surrounding solvents; (d) multivariate hierarchical MOFs with well designed apportionments can achieve efficient phototriggered release with high guest capacity and controllable release.

Figure 6

Phototriggered release of 4-nitrophenol as a function of time with and without lamp irradiation determined by UV–vis spectra using the kinetic mode. Hierarchical PCN-223@ZIF-8 and PCN-223@MOF-177 can achieve more efficient phototriggered release than its single component or physical mixtures.