Photochemical Properties of Host–Guest Supramolecular Systems with Structurally Confined Metal–Organic Capsules

Abstract

Inspired by natural photosynthesis, researchers have designed symmetric metal–organic hosts with large inner pockets that are spontaneously generated through preorganized ligands and functionalized metallocorners to construct dye-containing host–guest systems. The abundant noncovalent interaction sites in the pockets of the hosts facilitated substrate–catalyst interactions for possible enrichment, fixation, and activation of substrates/reagents, providing special electron transfer pathways for regio- or stereoselectively photocatalytic chemical transformations. In this Account, we focus our attention on metal–organic hosts that contain photoactive or redox-active units to evaluate electron transfer and charge separation between host and guest units in these supramolecular systems and elucidate the related photoinduced chemical reactions controlled by these electron transfer processes within the structurally confined pockets of these interesting metal–organic hosts. We have been engaged in developing methods to isolate a series of chromophores for charge separation in supramolecular systems, incorporating organic dyes as photosensitizers in metal–organic hosts with electron acceptor/donor guests is a promising way to enable typical enzyme-like photocatalytic transformations within a confined microenvironment. Related to these inter- and intramolecular photoinduced electron transfer (PET) processes, the formation of host–guest supramolecular systems to fix and isolate the donor–acceptor pair with a short through-space distance provided a new PET pathway to stabilize the charge-separated ion pair. Highly efficient photosynthetic systems can be obtained when charge transfer to electron donors/acceptors occurs faster than the charge recombination. This Account starts with a brief summary of the potential approaches for constructing photoactive metal–organic hosts through the incorporation of dye molecules within ligand backbones or as a part of the metal nodes of the architecture. Following the methodological summary is a discussion on the mechanisms governing the photoinduced charge separation and electron transfer pathways within the dye-incorporated metal–organic hosts. We also searched for strategies for constructing photoactive supramolecular systems through encapsulating dye molecules within the inner space of redox-active hosts. The photochemistry of these systems demonstrated the following advantages due to the structural confinement: avoiding excited state quenching caused by other chemical species, including aggregated dyes, stabilizing the radical intermediate and tuning the absorption or emission of the guest through electron/energy transfer pathways. The photoinduced dye to redox-active host electron transfer is a new and efficient pathway that is meaningful for chemists to realize and understand many important enzymatic processes and to reveal the secrets of a substance and energy metabolism in biological systems. The confined interactions between the host and the guest have shown fascinating effects of ...

Catalysis of an Aldol Condensation Using a Coordination Cage

“…The use of self-assembled molecular containers such as coordination cages as catalysts for reactions that occur in the central cavity has provided some remarkable examples of synthetic hosts providing enzyme-like levels of the rate acceleration of reactions. The range of reactions that has been shown to be catalysed is now extensive [1][2][3][4][5][6][7][8][9][10].…”

The Redox Coupling Effect in a Photocatalytic RuII‐PdII Cage with TTF Guest as Electron Relay Mediator for Visible‐Light Hydrogen‐Evolving Promotion

“…Along this line,synthetic strategies have been reported to engineer functional coordination innerspace of nanocages via incorporation of either redox-active or photoactive centers, [31][32][33][34] thus generating MOC-based photocatalytic hostguest systems to imitate natural photosynthesis, [9,[35][36][37][38][39] where the spatially separated electron and/or energy transfer processes can facilitate photocatalytic water/H 2 Ss plitting, as well as other chemical transformations.T oa chieve efficient conversion from solar energy to chemical energy in such hostguest systems,one of the fundamental factors is to control the photo-induced redox events in the confined chemical nanospace. [39,40] So far, many research efforts have been devoted to the exploration of electron and energy transfer processes between host and guest molecules,e ither depending on construction of photoactive hosts or resorting to encapsulation of dye guests,r espectively. [40,41] Nevertheless,t he potential synergistic coupling interactions between photoredox host and redox-active guest remains rarely explored, which might provide an alternative communication protocol to enzyme-mimicking photochemical molecular device (PMD) at as upramolecular level.…”

“…[39,40] So far, many research efforts have been devoted to the exploration of electron and energy transfer processes between host and guest molecules,e ither depending on construction of photoactive hosts or resorting to encapsulation of dye guests,r espectively. [40,41] Nevertheless,t he potential synergistic coupling interactions between photoredox host and redox-active guest remains rarely explored, which might provide an alternative communication protocol to enzyme-mimicking photochemical molecular device (PMD) at as upramolecular level.…”

The Redox Coupling Effect in a Photocatalytic RuII‐PdII Cage with TTF Guest as Electron Relay Mediator for Visible‐Light Hydrogen‐Evolving Promotion

“…Along this line, synthetic strategies have been reported to engineer functional coordination innerspace of nanocages via incorporation of either redox‐active or photoactive centers, thus generating MOC‐based photocatalytic host–guest systems to imitate natural photosynthesis, where the spatially separated electron and/or energy transfer processes can facilitate photocatalytic water/H 2 S splitting, as well as other chemical transformations. To achieve efficient conversion from solar energy to chemical energy in such host–guest systems, one of the fundamental factors is to control the photo‐induced redox events in the confined chemical nanospace . So far, many research efforts have been devoted to the exploration of electron and energy transfer processes between host and guest molecules, either depending on construction of photoactive hosts or resorting to encapsulation of dye guests, respectively .…”

“…To achieve efficient conversion from solar energy to chemical energy in such host–guest systems, one of the fundamental factors is to control the photo‐induced redox events in the confined chemical nanospace . So far, many research efforts have been devoted to the exploration of electron and energy transfer processes between host and guest molecules, either depending on construction of photoactive hosts or resorting to encapsulation of dye guests, respectively . Nevertheless, the potential synergistic coupling interactions between photoredox host and redox‐active guest remains rarely explored, which might provide an alternative communication protocol to enzyme‐mimicking photochemical molecular device (PMD) at a supramolecular level.…”