Polypyridyl Ru(II) or cyclometalated Ir(III) functionalized architectures for photocatalysis

Abstract

The chemistry of polypyridyl Ru(II) and cyclometalated Ir(III) derivatives provides long-lasting interest to researchers due to the inherent advantage of their triplet states in a variety of photoactivities. The introduction of Ru(N^N)₃ and Ir(C^N)₂(X^N) modules into well-defined architectures extends the research areas of both photoactive metal complexes and network chemistry, generating a lot of new opportunities with interesting structural aesthetics and profound functional possibilities. The rapid development of research in integrating Ru(II) or Ir(III) metallotecons into the architectures has been apparent in recent years which makes this a fascinating subject for reviewing. This review focuses on the design and syntheses of Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized architectures of metal–organic frameworks (MOFs), covalent–organic frameworks (COFs), metallasupramolecules, organic supramolecules and supramolecular organic frameworks (SOFs). Furthermore, the photocatalytic applications including the hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), photocatalytic oxidation and photoredox catalysis of organic transformation are also presented.

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Yan-Lin Li

Yan-Lin Li received her BS degree (2015) and MS degree (2018) from the China University of Petroleum. She is currently a PhD candidate under the supervision of Prof. Sheng-Li Huang at the Beijing Institute of Technology. Her current research interests are the preparation of polypyridyl Ru(ii) or cyclometalated Ir(iii) functionalized COFs for photocatalysis.

Ai-Juan Li

Ai-Juan Li is currently a PhD candidate under the supervision of Prof. Sheng-Li Huang at the Beijing Institute of Technology. Her current research interests are the preparation of polyoxometalate functionalized architectures, and polypyridyl Ru(ii) or cyclometalated Ir(iii) based supramolecular architectures for photocatalysis.

Sheng-Li Huang

Sheng-Li Huang received his PhD in chemistry in 2013 from Fudan University. Then he worked as a research scientist at the Institute of Materials Research & Engineering of A*STAR in Singapore. At the beginning of 2018, he moved to the Beijing Institute of Technology. His research interests contain well-organised architectures, functional polyoxometalates, Ru (Ir) chemistry and photosynthesis.

Jagadese J. Vittal

Jagadese J. Vittal obtained his PhD from the Indian Institute of Science, Bangalore (1982). He was a postdoctoral research associate and service crystallographer at the University of Western Ontario, Canada, before moving to NUS as a faculty member in 1997. He has been an Emeritus Professor since 2021. JJ is interested in solid-state science and structural chemistry including new materials, chemical reactivities, and structure–property relationships.

Guo-Yu Yang

Guo-Yu Yang received his MS and PhD degrees from Jilin University in 1991 and 1998, respectively. During the period 1998–2001, he worked as a Postdoctoral Fellow at the University of Notre Dame and Stockholm University. He became a full professor at FJIRSM-CAS in 2001. He was awarded the Cheung Kong Professorship in 2009. In 2014, he moved to the Beijing Institute of Technology. His research interests include oxo cluster chemistry and function development of oxo clusters.

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1. Introduction

The octahedral d⁶-metal complexes, namely, polypyridyl–Ru(II) and cyclometalated–Ir(III) complexes have been exploited in photonic and optoelectronic applications due to their interesting electronic transitions and furthermore, charge-transfer of their triplet states is tunable over a wide range.^1,2 Specifically, Ru(N^N)₃ and Ir(C^N)₂(X^N) (X = C or N), where (N^N) and (C^N) respectively represent the diimine ligand and cyclometalated ligand, have been mostly utilized for this purpose since the presence of Ru(II) and Ir(III) enhances the spin–orbit coupling and promotes the intersystem crossing of electrons from the excited singlet state to the triplet state, and their excited states are simultaneously influenced by the metal-to-ligand charge transfer (MLCT) and intraligand charge transfer (LLCT).³ The bonding of d⁶-metal ions and coordinating units provide enormous possibilities for assembling intriguing photoactive metal complexes.^4,5 The highest occupied molecular orbital (HOMO) –lowest unoccupied molecular orbital (LUMO) gap, light absorption region, triplet lifetime, excitation potential, emission quantum yields (Φ_PL) and other optoelectronic characteristics are readily modulated via substituent modification of the (C^N) and (N^N) units. The strong visible light absorption capacity and stable long-lived photoexcited states of Ru(N^N)₃ and Ir(C^N)₂(X^N) derivatives enabled their wide application in photophysics and photochemistry fields.^6,7 For example, photoexcited Ir(ppy)₃ (H-ppy = 2-phenylpyridine) exhibits special reducing features in photosynthesis owing to the strong electron-donating power of three ppy anions.⁸ Ru(N^N)₃ and Ir(C^N)₂(X^N) units have reliable synthesis and high chemical stability, and hence, their combinations with elaborate synthons yield desirable architectures with great significance in both skeletons and photo-functions (Fig. 1).

Fig. 1 Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized architectures and their photocatalytic applications.

In the past two decades, well-defined architectures of MOFs,^9–12 COFs,^13–15 metallasupramolecules,^16–18 organic supramolecules^19–21 and SOFs^22–25 have received greater attention due to their rich and designable structures, modifiable pore space and tunable functionality.²⁶ These architectures provide the best models for the structure–property relationship study, and the well-organized skeletons commonly possess special functions for olefin separation in hierarchical porous MOFs^27,28 for enzyme separation in mesoporous COFs,^29,30 for unusual reactions in metallasupramolecules which act as molecular flasks,³¹ for photodynamic therapy efficacy in organic supramolecules³² and for accessible film production in SOFs.³³ Like building skyscrapers, the molecular architectures can be precisely constructed by the judicious choice of divergent building blocks.³⁴ The combination of the octahedral core of d⁶-metal ions and peripheral coordination components produces various three-dimensional (3D) building blocks,^35,36 that may not be possible with pure organic components alone (Table 1). Furthermore, incorporation of Ru(N^N)₃ and Ir(C^N)₂(X^N) into well-defined architectures will not only yield unexpected and unusual properties of these materials but is also expected to have synergistic effects. For example, the structural confinement of the Ru(N^N)₃ or Ir(C^N)₂(X^N) moieties in MOFs, produced new photofunctional entities due to the weak conjugation of crystalline coordination frameworks.⁷² In photocatalysis, photogenerated charges are easily separated and transferred to Lewis acid sites for redox reactions.

Table 1 Structural diversity of Ru(N^N)₃ and Ir(C^N)₂(X^N) cores (AS = Active Site)

No. of AS	Structure diagram	Specific structure	Ref.
1			37–42
2			43–46
			47–52 53–58
3			59–61
			62
			63–65
4			66
			67
6			35 and 68–70
			68 and 71

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Efficient utilization of solar energy is an important way to solve the current global energy shortage and environmental crisis, and furthermore, photocatalysis can accomplish the transformation of the abundant solar energy into chemical energy.⁷³ Molecular Ru(N^N)₃ and Ir(C^N)₂(X^N) complexes have been widely employed in various photosynthesis reactions, such as the HER,^74,75 CO₂RR,^76–78 water oxidation,^79,80 and particularly in organic transformations including electron transfer reactions, H atom transfer reactions and energy transfer reactions.^81–83 Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized architectures exhibit numerous applications from luminescence,^7,84,85 sensing,⁸⁶ solar fuels^87,88 to bioimaging^89–91 and furthermore, photocatalytic activity is always the most intriguing performance.^92–98 They have great advantages in the photocatalysis system due to the following: (1) their light adsorption ranges are tunable from visible light to the near-infrared region.^6,99 The integration of Ru(II)/Ir(III)–metallotecons within the architectures can broaden the spectral region, providing a platform for the efficient utilization of solar energy. (2) In these architectures, the varied potentials of building units linked to Ru(II)/Ir(III)-metallotecons lead to the efficient separation of photogenerated electrons and holes.^100–102 (3) The conjugation of architectures is beneficial for highly efficient charge transfer, and the porous frameworks permitting more substrate to access the surface of the architectures and increase the utilization of charges.¹⁰³ (4) Such architectures provide the required nano-space to govern the host–guest optoelectronic interactions, which accelerates photocatalysis arising from the photophysical interaction between bound guests and the emitting hosts. The nano-space can also accommodate other nanoparticles, producing interesting catalytic performance via the synergistic effect.^70,104,105 (5) Most of these architectures are insoluble in common solvents, hence these noble metal catalysts can be easily recycled and reused.¹⁰⁶ (6) These designed architectures are expected to provide a meaningful structure–function relationship which will help to design better catalysts and to improve our understanding of the catalytic process.⁵⁴ Although some types of classical photocatalysts have been well explored, they have their disadvantages and limitations. Organic dyes have a tunable visible light absorption area, but they are readily decomposed during use.^107,108 The cheap inorganic semiconductors of TiO₂, Bi₂WO₆, α-Fe₂O₃, CdS and ZnO have better photostability, however the unavailable synthesis of nanoparticles without surfactant, narrow absorption spectrum of sunlight, high reduction potential, inefficient photogenerated charge separation and toxicity, in some cases restrict their wide application. The construction of heterojunctions and Schottky junctions led by doping of precious metals could improve some photochemical characteristics,^109–111 but suffer from aggregation and deactivation of precious metals.¹¹² In sharp contrast to other emerging photocatalysts, such as organic polymers, inorganic–organic hybrid materials, MOFs or COFs assembled from net organic building modules, the necessary conditions for photocatalysis including light absorption capacity, charge separation and utilization efficiency, interaction between substrate and catalyst of Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized architectures could be greatly improved.

This review is focused on the design and synthesis of Ru(N^N)₃ and Ir(C^N)₂(X^N) containing architectures, which will be presented and discussed in separate sections based on MOFs, COFs, metallasupramolecules, organic supramolecules and SOFs. The Ru(N^N)₃ and Ir(C^N)₂(X^N) units can be obtained via reliable synthetic routes, the incorporation of these units into well-defined architectures commonly involves two methodologies (Fig. 2),¹¹³viz. deployment of the metalloligands^70,71,114 and post-synthetic modification (PSM).^115–118 High chemical stability of the Ru(N^N)₃ and Ir(C^N)₂(X^N) units makes the synthesis viable by the first method, even under harsh reaction conditions such as strong base or acid environments and high temperature. The post-synthetic incorporation of Ru(N^N)₃ and Ir(C^N)₂(X^N) units are achieved by the coordination linkage of (N^N) groups from the skeleton with Ru(N^N)₂- or Ir(C^N)₂- precursor. These architectures have many photocatalytic applications. The HER, CO₂RR, photocatalytic oxidation and photoredox catalysis of organic transformation are concluded.

Fig. 2 Two synthetic strategies for metalloarchitectures: (a) metalloligands; (b) PSM.

2. Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized MOFs

MOF, also known as porous coordination polymer, is one of the fastest growing research fields in both chemistry and materials science in the last two decades.^34,119–123 The structural diversity, ultrahigh surface area, crystalline skeleton and tunable functionality allow MOFs to be widely utilized in various disciplines.^124–131 As featuring periodic networks, MOFs are formed by the self-assembly of organic linkers with metal ions or metal clusters (secondary building units (SBUs)).³⁴ A large number of MOFs have been reported arising from different types of organic linkers, variable coordination numbers and flexible coordination modes of metal ions, as well as the in situ formed “OH⁻” and “O²⁻” species leading to the formation of wide-ranging MOFs.^132,133 A judicious combination of organic linkers and SBUs control the topology of the resulting MOFs, whose photofunctions are commonly inherited from the photo-responsive ligands or post-introduced photosensitizers.^34,134 In general, Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized MOFs are synthesized by two routes: (1) direct reaction of metalloligands with metal ions.^{35,49,135–140} (2) PSM: the bis-chelating sites of ligand skeleton in the MOFs, such as (N^N) groups, could be bonded to the Ru(II) or Ir(III) precursors, thus producing MOFs containing Ru(N^N)₃ or Ir(C^N)₂(X^N) units.^50,141,142 In addition, the metalloligand could also be anchored at the unsaturated coordination sites of the MOFs via PSM.^37,118 In the syntheses of Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized MOFs, the nature of metal ions has great influence on the SBUs, topology and function of the resulting MOFs. In the following section, the photoactive MOFs will be divided into three categories according to the metal ion species in the SBUs, viz. 3d metal ions, d⁰ metal ions and 4f metal ions.

2.1 Photoactive MOFs constructed from the 3d metal ions

Transition metals of Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, have been commonly used in the construction of various MOFs with the following advantages: (1) the variable coordination numbers and flexible coordination modes of these transition metal ions provide different SBUs for the construction of diverse MOFs; (2) the reaction conditions of solvents, acid or base, and temperature have great influence on the formation of metal nodes, providing infinite possibilities to synthesize different SBUs; (3) weak metal–ligand interactions facilitate the crystallization of the MOFs. But most of MOFs incorporating transition metal ions have low chemical stability, which has adverse effects on the catalytic applications. However, these reactions produce photoactive MOFs with diverse structures and interesting functions.

A combination of Ru(II) and phenba (phenba = 4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzoic acid) formed a C₃ symmetric tridentate metalloligand [Ru(phenba)₃]²⁺. The reaction of [Ru(phenba)₃]²⁺ with Co(NO₃)₂ produced [Ru(phenba)₃+Co]-MOF with two-dimensional (2D) hexagonal layers and large open channels. Mesoporous [Ru(phenba)₃+Co]-MOF exhibited a selective gas sorption property, and its CO₂ sorption capacities were far better than N₂ (Fig. 3).⁶¹[Ru(ip)₃+Co]-MOF (Hip = 1H-imidazo[4,5-f][1,10]phenanthroline) revealed a high symmetry ctn network consisting of 3-connected [Ru(ip)₃]²⁺ and 4-connected Co²⁺ units. Under visible light, the overall conversion of CO₂ reduction and H₂O oxidation was achieved simultaneously by this multifunctional photocatalyst with a high conversion rate and CO selectivity.⁵⁹ Ir(C^N)₃-carboxy metalloligand was synthesized in a stepwise manner from the cyclometallation of (C^N)-ester derivatives and Ir(III) precursor, and then ester hydrolysis. Ir(C^N)₃-cores have two stereochemical forms, namely, mer and fac. The fac isomer has better stability and a longer emission lifetime than mer, and in fac-isomer all nitrogen atoms are trans to carbon atoms. The introduction of a carboxy group into the para-position of the coordinated C or N atoms, resulting in the bowl shaped metalloligand, whose carboxy groups are nearly perpendicular to each other. Using the Ir(ppy-COOH)₃-metalloligand, (H-ppy-COOH = 3-(pyridin-2-yl)benzoic acid) [Ir(ppy-COO)₃+Co]-MOF and [Ir(ppy-COO)₃+Zn₄]-MOF were synthesized. [Ir(ppy-COO)₃+Co]-MOF was applied as an efficient O₂-evolution–reaction electrocatalyst.⁶² The highly phosphorescent [Ir(ppy-COO)₃+Zn₄]-MOF was used in O₂ sensing⁶³ as well as photocatalytic pollutant degradation.¹⁴³ The position of the carboxyl groups at Ru(N^N)₃^38,66,144 and Ir((C^N))₂(N^N)-metalloligands can influence the topology of MOFs.^57,145

Fig. 3 [Ru(phenba)₃+Co]-MOF.⁶¹

In (C^N)-derivatives, the introduction of the carboxy groups into both the para positions of the coordinated C and N generated the hexadentate ligands Ir(H₂dcppy)₃ and [Ir(H₂dcppy)₂(H₂dcbpy)]⁺ (H-H₂dcppy = 2-(3-carboxyphenyl)pyridine-4-carboxylic acid, H₂dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid), and the carboxy groups of these two octahedral metalloligands linked with linear mono-node [Ni(cyclam)]²⁺ and formed the two isostructural complexes [Ir(dcppy)₃+Ni]-MOF and [Ir(dcppy)₂(dcbpy)+Ni]-MOF with pcu topology. They were 2-interpenetrating networks and exhibited a good CO₂/N₂ separation property (Fig. 4).³⁵

Fig. 4 [Ir(dcppy)₃+Ni]-MOF.³⁵

2.2 Photoactive MOFs constructed from d⁰ metal ions

The d⁰ metal ions of Ti⁴⁺, Zr⁴⁺ and Hf⁴⁺ have high charge density and strong oxophilicity, and they have a particularly strong bonding ability with carboxylate groups. These d⁰ metal salts can easily form metal hydroxides or oxides when encountering water, further increasing the chemical stability of metal-carboxylate clusters. The MOFs synthesized from Ti⁴⁺, Zr⁴⁺ or Hf⁴⁺ have strong chemical stability and their skeletons are maintained even under a strong acid environment. They are commonly used as heterogeneous catalysts based on their stable and porous frameworks. The strong metal-carboxylate interactions result in a lower control over the crystallization process, and difficulty in growing the single crystals suitable for X-ray diffraction was encountered.¹⁴⁶ Many d⁰ metal-based MOFs exhibited versatile catalytic reactions. The bonding energy between Zr⁴⁺ and carboxylate groups was weaker than Ti⁴⁺ but stronger than Hf⁴⁺, hence Zr₆-MOFs have medium levels of chemical stability and crystallization ability, promoting the rapid development of functional Zr₆-MOFs.^146–148 The reaction of Zr⁴⁺ and COO⁻ groups unexpectedly afford a Zr₆(μ₃-O)₄(μ₃-OH)₄(CO₂)₁₂ cluster.

Zhou and co-workers reported the synthesis of mesoporous MOFs, termed (TPTB+Zr₆)-MOF (H₄TPTB = 5′,5′′′-bis(4-carboxylatophenyl)-4′′′,6′-dimethoxy-[1,1′:3′,1′′:4′′,1′′′:3′′′,1′′′′-quinquephenyl]-4,4′′′′-dicarboxylate), via assembling the Zr₆ clusters with the tetratopic carboxylate linkers H₄TPTB. The carboxylate group of [Ru(bpy)₂(H₂bpydc)]²⁺-metalloligand (H₂bpydc = 2,2′-bipyridine-5,5′-dicarboxylic acid) linked to the uncoordinated sites of the Zr₆ cluster, leading to photoactive Ru(bpy)₂(bpydc)@(TPTB+Zr₆)-MOF with the expected porous network. It showed high catalytic efficiency in the aza-Henry reaction (Fig. 5).¹¹⁸

Fig. 5 Ru(bpy)₂(H₂bpydc)-metalloligand-functionalized (TPTB+Zr₆)-MOF.¹¹⁸

In the construction of Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized MOFs, the most popular synthetic method was the direct reaction of metalloligand and metal source. In this way, the structural advantages of Ru(N^N)₃ or Ir(C^N)₂(X^N) units could be passed to the target frameworks and the photoactive units could be well organized in the pore. Using metalloligand [Ru(bpy)₂(H₂bpydc)] and 4,4′-biphenyldicarboxylic acid (H₂bpdc), Lin and co-workers synthesized several heteroleptic MOFs that were isostructural with UIO-67 and exhibited high photocatalytic activity in aza-Henry reactions.⁴³ The introduction of Cu(H_xPO₄)_y into Cu@[Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF produced the dual-functional catalyst Cu@[Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF, and it promoted the conversion of CO₂ to C₂H₅OH with light irradiation and CH₃OH in the absence of light. TEM analysis showed that the catalysts formed Cu⁰ nanoparticles in the dark, which were the active species for CO₂ transformation to CH₃OH. With light illumination, {[Ru(bpy)₂(bpydc)]²⁺}* transferred e⁻ to Cu²⁺ and generated Cu⁺, simultaneously, {[Ru(bpy)₂(bpydc)]²⁺}* was quenched by electron injection from Cu⁰ and generated Cu⁺, which were responsible for C₂H₅OH production.¹³⁸ The encapsulation of [Ni(bpet)(H₂O)₂]²⁺ (bpet = 1,2-bis[(pyridin-2-ylmethyl)thio]ethane) into [Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF produced Ni⊂[Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF, and the photogenerated electron reduced the Ni center with enhanced CO₂ affinity and formed a Ni–CO₂ adduct, achieving highly efficient CO₂ transformation with good CO selectivity.¹³⁹ Zhang and co-workers synthesized Co@[Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF by the insertion of Co²⁺ as a single-site catalyst in the open (N^N)-sites of [Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF, which supplied a molecular platform capable of rapid injection of multiple electrons from the photosensitizer to Co-catalyst, resulting in a MOF-based composite photocatalyst for efficient syngas production. The photogenerated electrons transferred from the photosensitizer to Co-catalyst, then reduced CO₂ and H₂O to syngas. In the photocatalytic process, the H₂/CO ratios could be well controlled by adjusting the water content or the ratio of PS/Cat in the MOFs.¹⁴⁹

In the construction of Ru(N^N)₃-(UIO-67), the Ru(N^N)₃-metalloligand was simultaneously used with other linear dicarboxylate linkers because the small cavity in UIO-67 could not accommodate Ru(N^N)₃-units due to its small pore size. This problem was overcome by increasing the length between two terminal carboxyl groups of the metalloligands, producing more space for the accommodation of Ru(N^N)₃ or Ir(C^N)₂(X^N) units inside the framework. The linkage of Ru(bpy)₂(H₂bpydc)-metalloligands and [Zr₆(μ₃-O)₄(μ₃-OH)₄]-SBUs produced Ru(bpy)₂(dbpydc)-Zr₆-MOF (H₂dbpydc, 4,4′-([2,2′-bipyridine]-5,5′-diyl)dibenzoic acid). In the synthetic procedure, the addition of Wells–Dawson {P₂W₁₈} polyoxometalates (POMs) resulted in the formation of {P₂W₁₈}⊂[Ru(bpy)₂(dbpydc)+Zr₆]-MOF (Fig. 6).¹¹⁴{Ni₄P₂}⊂[Ru(bpy)₂(dbpydc)+Zr₆]-MOF, and {Ni₄P₂}⊂[Ir(ppy)₂(dbpydc)+Zr₆]-MOF were obtained via similar synthetic methods. Both [Ru(bpy)₂(dbpydc)+Zr₆]-MOF and [Ir(ppy)₂(dbpydc)+Zr₆]-MOF had the same topological network with UIO-67. The incorporation of the photosensitizer and POMs {Ni₄P₂} into a single framework shortened their distance, promoting the photo-generated electron transfer from photosensitizer to POMs, and the accumulation of electrons at POMs enhanced its reduction ability, thus achieving photocatalytic hydrogen production. However, {Ni₄P₂}⊂[Ru(bpy)₂(dbpydc)+Zr₆]-MOF could not photocatalyze hydrogen production because the Ru(N^N)₃ photosensitizer does not have a low enough reduction potential to permit electron accumulation at {Ni₄P₂}.¹⁵⁰ In POMs⊂Ru(Ir)-MOF, the enhanced multi-electron transfer from Ru(N^N)₃ or Ir(C^N)₂(X^N) photosensitizer to POMs catalyst was responsible for the photocatalytic hydrogen production.^114,150 Ye and co-workers synthesized a series of Ir(pqc)₂(N^N)-metalloligands (H-pqc = H-2-phenylquinoline-4-carboxylic acid), in which the (C^N)-units were located around the Ir(III) center with controllable trans arrangement and the carboxyl group was located at the para position of the N atom, resulting in linear [Ir(pqc)₂(N^N)]⁺ metalloligands. The reaction of these metalloligands with ZrCl₄ yielded [Ir(pqc)₂(N^N)+Zr₆]-MOF (N^N derivatives = bpy, 4,4′-dimethoxy-2,2′-bipyridine, or 1,10-phenanthroline (phen)) which showed excellent photocatalytic activity in the selective sulfide oxidation under a O₂ atmosphere (Fig. 7).⁴⁸ The attachment of the (N^N^N)-unit with the carboxylate group produced similar metalloligands, such as [Ru(cptpy)₂]²⁺ (cptpy = 4′-(4-carboxyphenyl)-terpyridine), and its combination with simple Zr-node produced 1D framework [Ru(cptpy)₂+Zr]-MOF, which had high stability and selectively photocatalyzed CO₂ to formate (HCOOH) under visible light irradiation.¹⁵¹

Fig. 6 {P₂W₁₈}⊂[Ru(bpy)₂(dbpydc)+Zr₆]-MOF.¹¹⁴

Fig. 7 [Ir(pqc)₂(N^N)+Zr₆]-MOF.⁴⁸

2D materials have unique photoelectric properties because the delamination has a great influence on their functions. Metal–organic layers (MOLs) are a new type of 2D organic–inorganic material.^152,153 They are highly dispersible and have enough exposed active sites.^154,155 A series of MOLs were constructed by linking Ir(C^N)₂(N^N)-^{49,50,135,136,156,157} and Ru(N^N)₃-^44,158 metalloligands with SBUs of Hf₁₂ or Hf₆, exhibiting numerous applications. Ir(C^N)₂(N^N) or Ru(N^N)₃ derivatives are two efficient photosensitizers for the generation of ¹O₂.^159,160 Hf atoms in the SBUs efficiently absorb X-rays and transfer its energy to Ir(C^N)₂(N^N) and Ru(N^N)₃ moieties, thus generating reactive oxygen species (ROSs) for photodynamic therapy. Hf₆-MOLs were synthesized from the reaction of HfCl₄ and H₃BPY (H₃BPY, 4′,6′-dibenzoato-[2,2′-bipyridine]-4-carboxylic acid), and the open (N^N)-coordination site of Hf₆-MOL was occupied by the reaction with [Ir(ppy)₂Cl]₂ or Ru(bpy)₂Cl₂. An X-ray excited Hf₆ cluster transferred energy to photosensitizer moieties, generating ¹O₂.¹⁵⁶ The incorporation of POMs enhanced the generation of various ROSs. Under X-ray irradiation, different functional modules of {P₂W₁₈}⊂[Ir(ppy)₂(dbpydc)+Hf₁₂]-MOL synergistically produced the corresponding ROSs (˙OH from Hf₁₂ cluster, ¹O₂ from Ir-photosensitizer and O²⁻ from {P₂W₁₈}).⁴⁹ Similarly, [Ir(dFCFppy)₂(dbpydc)+Hf₁₂]-MOL (H-dFCFppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine) and Ir(dFCFppy)₂(N^N)@(BPY+Hf₆)-MOL could simultaneously generate ¹O₂ from the Ir(C^N)₂(N^N) unit and ˙OH from Hf₆ or Hf₁₂ SBUs.⁵⁰ The weakly coordinated trifluoroacetic acid of [Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL was replaced by 2-(5′-methyl-[2,2′-bipyridin]-5-yl)acetic acid, which was further coordinated with M(CO)₃X (M = Mn or Re, X = Cl or Br), leading to Re@[Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL or Mn@[Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL with efficient photocatalytic CO₂RR activities. The photogenerated electrons transferred from the Ru(N^N)₃ photosensitizer to Mn or Re catalyst, and the coordination interaction between CO₂ and catalyst enhanced its electron grabbing capacity from the catalyst, enhancing the selective CO₂ transformation to CO.¹⁵⁵ Furthermore, OTf@[Ir(dFCFppy)₂(dbpydc)+Hf₁₂]-MOL (OTf = triflate) could effectively catalyse dehydrogenative cross-couplings. In this system, dehydrogenation occurred on [Ir(dFCFppy)₂(H₂dbpydc)]⁺ bridging ligands, and the OTf on the Hf₁₂ SBUs working as Lewis acid.¹³⁶ When [Ir(dFCFppy)₂(dbpydc)+Hf₁₂]-MOL was decorated with Ni(2-(4′-methyl-[2,2′-bipyridin]-4-yl)acetate)Cl₂ at Hf₁₂ centers (Fig. 8), the corresponding Ni@[Ir(dFCFppy)₂(dbpydc)+Hf₁₂]-MOL showed high photoactivities in C–S, C–O, and C–C cross-coupling reactions.¹³⁵OTf/Co@[Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL hierarchically integrated three active sites into one complex, OTf as the strong Lewis acid, [Ru(bpy)₂(H₂dbpydc)]²⁺ ligand as the photosensitizer, and [Co(dimethylglyoxime)₂(4-pyridinepropionate)]Cl as a hydrogen-transfer catalyst. They worked synergistically for tandem catalysis.¹⁵⁸

Fig. 8 Ni@[Ir(dFCFppy)₂(dbpydc)+Hf₁₂]-MOL.¹³⁵

In contrast to numerous Zr-MOFs and Hf-MOFs, only a few Ti-MOFs were reported because Ti⁴⁺ ions are easily hydrolyzed and strong Ti–carboxylate bonds make the crystallization of Ti-MOFs difficult.¹⁴⁶ The reaction of [Ti₆O₆(isopropoxy)₆(4-aminobenzoate)₆] and Ru(Ir)-metalloligand or 4,4′-biphenyldicarboxylic acid yielded [Ru(bpy)₂(bpydc)/bpdc+Ti₃]-MOF or [Ir(ppy)₂(bpydc)/bpdc+Ti₃]-MOF, and the combination of photosensitizer and Ti₃ cluster facilitated the photocatalytic HER under visible light.¹⁴⁰

2.3 Photoactive MOFs constructed from 4f metal ions

Rare-earth elements possess rich optical, electrical, and magnetic properties due to their large atomic magnetic moment, unique 4f sublayer electronic structure, and strong spin–orbit coupling. Lanthanide (Ln) ions have flexible coordination modes, high coordination numbers, strong Lewis acid¹⁴² and strong oxophilicity based on the Hard–Soft–Acid–Base theory.

A few Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized Ln-MOFs have been reported with interesting functions.^99,161–164 The reaction of planar triangular-shaped metalloligand [Ru(phenba)₃]²⁺ and Co(NO₃)₂ generated a 2D network, however its reaction with Eu(NO₃)₃ produced [Ru(phenba)₃+Eu₂]-MOF with a 3D porous network, which efficiently promoted the selective transformation of CO₂ to HCOOH.¹³⁷ Under visible-light irradiation, (Eu³⁺)₂-SBU obtained two electrons from the excited {[Ru(phenba)₃]²⁺}* units and was reduced to (Eu²⁺)₂, enabling the selective reduction of CO₂ to HCOOH via a two-electron process. In [Ru(bpy)₂(dcbpy)+Ln]-MOFs and [Ir(ppy)₂(dcbpy)+Ln]-MOFs (Ln = Yb, Er, Nd, Gd, Yb, Dy),^54,99,164 the photosensitizer groups acted as a good light-harvesting antenna to effectively sensitize Ln ions by d → f energy transfer, and achieving near-infrared luminescence. In addition, [Ru(dcbpy)₃+La_1.75]-MOF was constructed by the reaction of [Ru(H₂dcbpy)₃]²⁺ with LaCl₃ in a molar ratio of 4 : 7. Water adsorption/desorption of [Ru(dcbpy)₃+La_1.75]-MOF triggered a reversible structural transformation, accompanying great influence on its ³MLCT emission energy and ion conductivity. Vapochromic luminescence occurred via water vapor adsorption–desorption, in addition, the increased ion conductivity rate was proportional to humidity.¹⁶³ In isostructural complexes of [Ru(dcbpy)₃+Ce_1.75]-MOF and [Ru(dcbpy)₃+Nd_1.75]-MOF, different Ln ions have a great influence on their properties. MOFs synthesized with larger Ln ions could recover the ion conduction activation energy and the original porous structure at lower relative humidities.¹⁶² Similarly, in [RuH_5.5(dpbpy)₃+La_1.5]-MOF (H₄dpbpy = 2,2′-bipyridine-4,4′-bis(phosphonic acid)) and [RuH_5.5(dpbpy)₃+Pr_1.5]-MOF, the ³MLCT emission exhibited a blue-shift following the decreased humidity and red-shift with increased humidity, because water-adsorption triggered proton release and reconstruction of the porous structure. The proton conductivity of [RuH_5.5(dpbpy)₃+La_1.5]-MOF was much higher than [Ru(dcbpy)₃+La_1.75]-MOF due to the higher acidity of the phosphonic acid groups.¹⁶¹

Ln(III) ions have a high coordination number, and the combination of multifunctional carboxylate linker and Ln(III) ions can also form Ln₆-MOFs. Like [Ru(Ir)+Zr₆]-MOFs, the construction of [Ru(Ir)+Ln₆]-MOFs should also simultaneously use both metalloligands and its isometric dicarboxylate ligand when the metalloligands have no sufficient length. Cu@[Ru(bpy)₂(bpydc)/bpydc+Eu₆]-MOF was constructed by mixed use of [Ru(bpy)₂(H₂bpydc)]²⁺, H₂bpydc, and [Cu(H₂bpydc)]Cl₂, which photocatalyzed the selective transformation of CO₂ to HCOOH with a high conversion rate (Fig. 9).¹⁶⁵

Fig. 9 Cu@[Ru(bpy)₂(bpydc)/bpydc+Eu₆]-MOF.¹⁶⁵

PSM is another effective method to fabricate multinuclear Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized Ln-MOFs. Ce₆-MOLs. The combination of H₃BTB (H₃BTB = 1,3,5-benzenetribenzoate) and the photosensitive [Ir(ppy)₂(HMBA)]⁺ (HMBA = 2-[5′-methyl-(2,2′-bipyridin)-5-yl]acetate) or [Ru(bpy)₂(HMBA)]²⁺ group was introduced into the framework via the coordination with the Ce₆ cluster. Ir(ppy)₂(MBA)@(BTB+Ce₆)-MOL and Ru(bpy)₂(MBA)@(BTB+Ce₆)-MOL showed good photocatalytic HER performance due to the synergistic effect of photosensitizer and Ce₆ center (Fig. 10).³⁷Ru(bpy)₂(bpydc)@(bpydc+Ln₆)-MOFs (Ln = Tb, Gd, Eu) were synthesized via the coordination linkage of Ru(N^N)₂ and (N^N) groups of frameworks, exhibiting photocatalytic thioether oxidation with high sulfoxide selectivity under mild conditions.¹⁴²

Fig. 10 Ru(bpy)₂(MBA)@(BTB+Ce₆)-MOL.³⁷

The reaction of Ru(bpy)₂Cl₂ or Re(CO)₃Cl with (N^N)-sites of Al-MOF produced [Re/Ru(bpy)₂(bpydc)]@(bpydc+Al)-MOF, which showed high activity in photocatalytic CO₂ reduction under visible light.¹⁶⁶ The Al-OTf Lewis acid sites and Ir(ppy)₂(bpydc) photocatalytic sites were installed on Al-MOF. The obtained [OTf/Ir(ppy)₂(bpydc)]@(bpydc+Al)-MOF effectively catalysed the reductive cross-coupling to afford new aza arene derivatives. The close proximity of substrate binding centers and catalytic centers in the mesopore gave this heterogeneous catalyst better performance than the homogeneous counterpart. Furthermore, this MOF facilitated coupling reaction between the activated vinyl- or alkynyl-azaarenes and alkyl radicals.¹⁶⁷

3. Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized COFs

A wide range of covalent organic frameworks (COFs) have been developed in the past two decades, because of their unique properties of good chemical and thermal stability, high specific surface area, and low skeletal density.¹⁶⁸ They have been widely employed in photocatalytic systems due to their chromophore and long-range π–π conjugated structure providing a platform for light-harvesting and efficient charge transfer.¹⁰⁸ Herein both crystalline COFs and amorphous COFs are described,²⁶ which are classified by the type of covalent bonds, topology, porosity and functions.²⁶

3.1 Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized crystalline COFs

Similar to MOFs, COFs are also crystalline materials which have well-defined skeletons formed by the orderly and infinitely extended molecular tecons.¹⁶⁹ Different from MOFs, the covalent linkages give the whole framework with high conjugation and associated electronic properties.^170,171 Crystalline COFs are also more advantageous than amorphous species because crystalline networks possess well-ordered building units and better framework conjugation that is beneficial for the charge transport.¹⁷² They exhibited numerous applications from drug delivery,^105,173,174 gas adsorption^175,176 and separation^177,178 to optoelectronics.¹⁷⁹ The introduction of the chelating coordination sites including N, O, and S elements into COFs makes them excellent metal catalyst supporters. The chelating sites could anchor the metal ions, permitting the uniform dispersion of metal catalysts into the supporters and preventing catalyst detachment. COFs can be easily functionalized to help the improvement of product selectivity. The high porosity and well-ordered pore channels of COFs accelerate the diffusion of substrates, and increase the approaching possibility between the substrates and catalytic center. These metal functionalized COFs have adjustable catalytic performances concerning high activity, product selectivity and reuse.

The construction of metal functionalized COFs also includes two main methodologies of metalloligand and PSM (Fig. 2).¹¹³ The use of metalloligands is the most direct approach to fabricate metal-containing COFs with diverse topologies. For example, several interesting COFs were synthesized from metal–porphyrin^180–183 and glyoximate¹⁸⁴ precursors. PSM is another efficient approach to incorporate a metal catalyst into the frameworks via metal–ligand interactions, thus providing a reliable way to produce metal functionalized COFs for specific applications.¹⁸⁵ Although porous, the crystallinity and structure of COFs are maintained^186–190 by the PSM route, and an even distribution and precise location of the metal catalyst is difficult to achieve.

The condensation reaction of [Ru(bpy-OEt)₃]²⁺ (bpy-OEt = 5,5′-bis(diethoxymethyl)-2,2′-bipyridine) metalloligands and multi-amine precursor including ETTA (ETTA = 4,4′,4′′,4′′′-(ethene-1,1,2,2-tetrayl)tetraaniline), TPB (TPB = 1,2,4,5-tetrakis-(4-aminolphenyl)benzene) or ETTBA (ETTBA = 4′,4′′′,4′′′′′,4′′′′′′′-(ethene-1,1,2,2-tetrayl)tetrakis([1,1′-biphenyl]-4-amine)) produced three 3D stp topological networks [Ru(bpy-OEt)₃+ETTA]-COF, [Ru(bpy-OEt)₃+TPB]-COF and [Ru(bpy-OEt)₃+ETTBA]-COF (Fig. 11). Based on the photosensitive Ru(N^N)₃ unit, they possessed a strong light-harvesting ability and showed high photocatalytic activity in the HER process.⁷¹ Most of the Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized COFs were assembled via PSM, and the (N^N)-unit of COFs was a commonly used bis-chelating group to grab metal catalysts.^187–189 For instance, Re(CO)₃Cl(N^N)-functionalized COFs efficiently reduced CO₂ to form CO under visible light illumination with high selectivity.¹⁸⁶ The reaction of BPDCA (BPDCA = 2,2′-bipyridine-5,5′-dicarbaldehyde) and TAB (TAB = 1,3,5-tris-(4-aminophenyl) benzene) afforded an imine-linked 2D framework (BPDCA+TAB)-COF with an open (N^N)-site (Fig. 12), and its metalation with Ru(bpy)₂Cl₂ resulting in the formation of Ru(N^N)₃@(BPDCA+TAB)-COF. The site-specific functionalization of sp³ C–H bonds of the C atom adjacent to a N atom is one less explored organic transformation. Ru(N^Ru(N^N)₃@(BPDCA+TAB)-COF photocatalyzed cross-dehydrogenative coupling between secondary amines and indoles with excellent yield.¹¹⁶ Under visible light irradiation, the Ru(N^N)₃ moiety was excited into [Ru(N^N)₃]*, and it received one electron from the secondary amine offering a radical intermediate. The radical intermediate underwent deprotonation and formed a radical imine cation intermediate, and its quenching by indoles afforded the desired indolylglycine product. Another (N^N)-site-based 2D (phendda+TAB)-COF (phendda = 4,4′-(1,10-phenanthroline-3,8-diyl)dibenzaldehyde) was synthesized from TAB and phenanthroline-containing linear dialdehyde. The respective immobilization of [Ir(dFCFppy)₂Cl]₂ and Ni²⁺ into the porous framework led to [Ni/Ir(dFCFppy)₂(N^N)]@(phendda+TAB)-COF, enabling heterogeneous photocatalysis of C–C cross-couplings.¹¹⁵ A similar complex, [Ni/Ir(C^N)₂(N^N)]@(Tp+abpy)-COF (Tp = 1,3,5-triformylphloroglucinol; abpy = 5,5′-diamino-2,2′-bipyridine) exhibited good performance in photocatalyzing C–N cross-coupling reactions with broad substrate diversity and high recyclability (Fig. 13).¹¹⁷ The reaction of TAPM (TAPM = tetra(p-aminophenyl)methane) and BPDCA produced a 3D diamondoid network (BPDCA+TAPM)-COF with 9-fold interpenetration, and its coordination with Ru(bpy)₂Cl₂ enabled visible light mediated cross-dehydrogenative coupling of tertiary amines and indoles under mild conditions (Fig. 14).¹⁹¹

Fig. 11 3D [Ru(bpy-OEt)₃+ETTA]-COF.⁷¹

Fig. 12 2D Ru(N^N)₃@(BPDCA+TAB)-COF.¹¹⁶

Fig. 13 2D [Ni/Ir(C^N)₂(N^N)]@(Tp+abpy)-COF.¹¹⁷

Fig. 14 3D Ru(N^N)₃@(BPDCA+TAPM)-COF.¹⁹¹

Via host–guest interactions, the Ru(N^N)₃ or Ir(C^N)₂(X^N) derivatives could be incorporated into the pores of COFs,¹⁰⁵ yielding PS⊂COFs with diverse photocatalytic properties. The combined use of hydrazone-based COF photosensitizer and [Ir(terpyridine)(ppy)Cl]⁺ catalyst exhibited efficient and durable photocatalytic CO₂RR. The reactivity and durability were highly improved compared with the bare [Ir(terpyridine)(ppy)Cl]⁺, because the pore confinement effect from the COF facilitated the dispersion and weakened the aggregation of the Ir-catalyst.¹⁰⁸

3.2 Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized amorphous COFs

The covalent bond strength determines the reversibility of the bond and crystallization of the COFs, thus imine-based COFs are readily formed with good crystallinity, while C–C bond-based COFs are extremely difficult to produce with crystalline networks. Amorphous COFs have disordered arrangement of building units and less porosity, but they commonly have strong chemical stability and are emerging as versatile platforms for exploring new functional materials.^192–194 These cross-linked amorphous COFs are usually formed with inert covalent bonds through Suzuki reactions, alkyne trimerization reactions and oxidative coupling reactions, thus have high chemical stability. Amorphous COFs are not readily decomposed even under harsh reaction conditions,^195–198 but with less skeleton conjugation and a low surface area.

Heterogeneous photocatalysts can be formed by the incorporation of a photoredox catalyst into stable COF with excellent catalytic efficiency and easy separation from the reaction system.¹⁹⁹ A series of amorphous [Ru(N^N)₃+TPMB]-COF (TPMB = tetrakisphenylmethane borate) were synthesized from Pd-catalysed coupling reactions between one TPMB and two Br-Ru(N^N)₃-derivatives, and they showed visible-light driven photocatalytic activity in enantioselective alkylation of aldehydes.¹⁹⁹ The Suzuki coupling reaction of linear ditopic Ir(C^N)₂(tbubpy)-linker (tbubpy = 4,4′-di(tert-butyl)-2,2′-bipyridine) and TPMB afforded diamond type [Ir(C^N)₂(tbubpy)+TPMB]-COF, exhibiting good photocatalytic performance in a wide range of organic reactions, including aerobic oxidations of sulfides and oxidative hydroxylation of arylboronic acids, desulfurative conjugate addition to Michael acceptors, and Smiles-Truce rearrangement of alkyliodides (Fig. 15).⁵² Their photocatalytic efficiency is better than the homogeneous prototype Ir(III) complexes owing to the porous structure and conjugated backbone of COFs. The cross-coupling of [Ru(Ir)+TEPM]-COF (TEPM = tetra(4-ethynylphenyl)methane) was obtained via Co₂(CO)₈-catalysed alkyne trimerization reactions of TEPM and Ru(N^N)₃ or Ir(C^N)₂(N^N)-based linear alkyne-linker. [Ru(N^N)₃+TEPM]-COF and [Ir(C^N)₂(N^N) + TEPM]-COF were highly effective in a variety of important organic transformations, such as aza-Henry reactions, α-arylation of bromomalonate, and oxyamination of an aldehyde.²⁰⁰ FeCl₃-promoted oxidative coupling reaction of carbazole derivatives was an efficient methodology for the construction of organic polymers, and the self-polymerization of Ir(C^N)₃-carbazole derivatives produced [Ir(C^N)₃+carbazole]-COF, showing intense phosphorescence and high activity in photocatalytic aza-Henry reactions.¹⁶⁸ The Ir(C^N)₂(N^N) analogue [Ir(C^N)₂(N^N)+carbazole]-COF also showed good performance in aerobic photooxidation, such as sulfide oxidation, oxidative hydroxylation of arylboronic acids and cross-dehydrogenative coupling reactions.²⁰¹ The self-polymerization of 4,4′-di(4-vinylphenyl)-2,2′-bipyridine led to 1D polymer with open (N^N)-sites, and its bondage with Ru(bpy)₂Cl₂ or [Ir(ppy)₂Cl]₂ produced Ru(N^N)₃- or Ir(C^N)₂(N^N)@polyethylene-COF with an electrochemiluminescence response to tri-n-propylamine.²⁰²

Fig. 15 Diamond type [Ir(C^N)₂(tbubpy)+TPMB]-COF.⁵²

4. Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized metallasupramolecules

Different from the polymeric networks of MOFs, COFs¹⁶⁷ and SOFs, metallasupramolecules are discrete architectures with sufficient solubility in solvents, that exhibit unique properties in solution, such as host–guest and liquid luminescence.^203,204 Like the synthesis of MOFs, the construction of metallasupramolecules is also based on the coordination-driven assembly of metal ions and ligands. However, the structures of target architectures are commonly precisely foreseen via the controlled combination of metal ions and organic linkers.^203,205,206 The fruitful function and abundant structural configurations of these metallasupramolecules could be achieved by rational selection of well-designed ligands and metal precursors. The use of metalloligands with predesigned symmetry and configuration could contribute to the precise control of the architecture's geometry and their functionality could also be transferred to the metallasupramolecules.²⁰⁷

4.1 Ir(C^N)₂(N^N)-based metallasupramolecules

Similar to Pd(ethylenediamine) and cis-Pt(triethylphosphine)₂^208,209 groups, [Ir(ppy)₂Cl]₂ could also be utilized as a metal-corner with two available coordination sites and the ppy ligands are arranged in C,C-cis-N,N-trans orientation, which would sufficiently facilitate the self-assembly of Ir(C^N)₂(N^N)-based metallasupramolecules.²¹⁰ The combination of [Ir(ppy)₂Cl]₂-precursor and tritopic 1,3,5-tricyanobenzene produced octahedral metallasupramolecule [Ir₆(1,3,5-tricyanobenzene)₄], and compared to the mononuclear complex, the incorporation of the Ir(C^N)₂(N^N) unit into the multimetallic array led to a significant luminescence enhancement (Fig. 16).²¹¹ Two homochiral [Ir₃(tpmc)₂] metallo-cryptophane cages (tpmc = tris(4-pyridyl-methyl)-cyclotriguaiacylene) were synthesized from the assembly of cyclotriveratrylene-based chiral tripodal ligand and [Ir(ppy)₂Cl]₂-precursor. Interestingly, homochiral self-sorting of both the metal-corner and ligand occurred simultaneously, which was accelerated by the addition of a chiral guest (Fig. 17).⁵⁶ A similar [Ir₃(ttpadtc)₂] complex (ttpadtc = 2,7,12-trimethoxy-3,8,13-tris(4,4′-pyridyl-azophenylcarboxy)-10,15-dihydro-5H-tribenzo[a,d,g] cyclononane) was synthesized from the azobenzene-functionalized tripodal cyclotriguaiacylene derivative, showing reversible photo-isomerisation under blue light irradiation.²¹²

Fig. 16 Ir(C^N)₂(N^N)-based octahedral cage [Ir₆(1,3,5-tricyanobenzene)₄].²¹¹

Fig. 17 Homochiral Ir(C^N)₂(N^N)-based cryptophane [Ir₃(tpmc)₂].⁵⁶

The reaction of [Ir(ppy)₂Cl]₂-precursor and ligands with both bis-chelating (N^N)-unit and mono-pyridyl units produced [Ir(mesppy)₂(qpy)]⁺ (H-mesppy = 2-phenyl-4-mesitylpyridinato; qpy = 4,4′:2′,2′′,4′′,4′′′-quaterpyridine) and [Ir(dFmesppy)₂(qpy)]⁺ (H-dFmesppy = 2-(4,6-difluorophenyl)-4-mesitylpyridinato) metalloligands, and the assembly of them with Pd(MeCN)₄(BF₄)₂ (MeCN = acetonitrile) afforded homochiral hetero-metallacages [Ir₈Pd₄]. The blue-emitting [Ir(dFppy)₂(MeCN)₂]⁺ ((H-dFppy = 2-(2,4-difluorophenyl)-4-phenyl)pyridine) guest was encapsulated into the cavity of [Ir₈Pd₄], and energy transfer between the red-emitting Δ-[Ir₈Pd₄] cage and the guest molecules was observed (Fig. 18).²¹³ Another isostructural [Ir₈Pd₄] metallacage was constructed by [Ir(ppy)₂(qpy)]⁺ and [Pd(MeCN)₄](BF₄)₂. Under visible light irradiation, the cubic barrel-shaped [Ir₈Pd₄] showed large ¹O₂ quantum yields, which exhibited great potential in the photodynamic therapy and organelles-targeted cell imaging. The [Ir₈Pd₄] showed higher mitochondria-targeting efficiency and less dark toxicity compared with the [Ir(ppy)₂(qpy)]⁺ metalloligand. The correlation coefficient of mitochondrial affected by the [Ir(ppy)₂(qpy)]⁺ metalloligand was smaller than [Ir₈Pd₄], which might be attributed to the higher positive charge of the metallacage.⁵⁸

Fig. 18 Ir(ppy)₂(qpy)-based polyhedral cage [Ir₈Pd₄].²¹³

4.2 Ir(C^N)₃-based metallasupramolecules

Ir(C^N)₃-derivatives are widely used in light-emitting diodes because of their strong Φ_PL and large Stokes shift. The optical properties of Ir(C^N)₃-based materials involving phosphorescence lifetime and emission colour can be regulated via modification of conjugation, heteroatom type, and push–pull electron groups of ligands.^214,215 A triple helical organic cage was pre-organised with three (C–H^N)-units, and its cyclometallation with one Ir(III) ion formed a fac-Ir(C^N)₃ metallacage with a wide range of light emission and suffering from photoluminescence quenching in the presence of O₂ (Fig. 19).^216,217

Fig. 19 Fac-Ir(C^N)₃-based triple helical architecture.²¹⁷

Like organic linkers, the metallotecons containing organic active groups, such as amino and aldehyde groups could further react with organic precursors. The reaction of metallotecon fac-tris-Ir(appy)₃ (H-appy = 4-(2-pyridinyl)phenylamine), 2-formylpyridine and Zn(BF₄)₂ afforded [Ir₂Zn₃] with the properties of photoluminescence, CO₂ fixation and nitrite separation (Fig. 20).³⁶ The isostructural complex [Ir₂Co₃] contained both a photosensitizer and Lewis catalytic center. Co²⁺ active sites were all in an open environment for favouring substrate bonding. [Ir₂Co₃] worked as a typical dual catalyst that synergistically combined photo-redox catalysis and metal coordination activation for a trichloromethylation of 2-acylpyridines with BrCCl₃.⁶⁵ In [Ir₂Zn₃] and [Ir₂Co₃] cages, the fac conformation of the Ir(C^N)₃ moiety constrained the geometry of Zn²⁺ and Co²⁺ with four N donors positioned in a controllable fashion.

Fig. 20 Ir(appy)₃-based heterometallic polyhedron [Ir₂Zn₃].³⁶

4.3 Ru(N^N)₃-based metallasupramolecules

Ru(N^N)₃ is one type of renowned photosensitizer²¹⁸ with intriguing photochemical applications in light-harvesting and photocatalysis.⁴⁶ Ru(N^N)₃ derivatives have a moderate redox state,⁶ and they have been widely employed in various types of photocatalytic reactions.^219–222 As stable and functional building blocks, Ru(N^N)₃ units are also used in the construction of interesting metallasupramolecules.^223–227

[Pd₆Ru₈] metallacages attached with a Ru(N^N)₃-unit were synthesized from the reaction of bulky triangular [Ru(piphen)₃]²⁺-metalloligand (piphen, 2-(pyridin-3-yl)-1H-imidazo[4,5-f][1,10]phenanthroline) and Pd(MeCN)₄(BF₄)₂, and these metallacages featured a quite high Φ_PL.³⁹ The [Pd₆Ru₈] cage (Fig. 21) has a rhombododecahedral shape with a 5350 ų cavity and 12 open windows. The large hydrophobic size-selective cavity facilitated effective trapping of a nonpolar and water-immiscible aromatics guest in a water-containing hydrophilic solvent. The ¹H NMR study of guest inclusion confirmed the formation of noticeable interactions between hosts and guests due to the encapsulation of aromatic guests, which was expected to form facile π–π interactions with the phenanthroline moieties, thus [Pd₆Ru₈] had potential applications in drug delivery. In addition, inclusion tests on photosensitive guest molecules against UV light radiation confirmed that the cage offered better photoprotection than the pure [Ru(piphen)₃]²⁺-metalloligand, suggesting that the [Pd₆Ru₈] metallacage could well shield the guests to prevent undesired photolysis.²²⁸ Furthermore, the [Pd₆Ru₈] cage was incorporated with multiple photo- and Lewis acid centers,²²⁹ in which the Ru(N^N)₃-unit acted as a photosensitizer and Pd²⁺ as the catalyst. Electron transfer from the [Ru(piphen)₃]²⁺ photoactive center to the Pd²⁺ center resulted in intramolecular charge separation, and they worked synergistically for the efficient photocatalyzed HER. In the host–guest system, TTF (TTF = tetrathiafulvalene) guests acted as an electron relay mediator to improve the overall electron transfer efficiency by virtue of redox-guest modulation of the photo-induced electron transfer process. In this process, TTF strongly quenched [Pd₆Ru₈] emission due to the formation of a host–guest adduct. In contrast, without host–guest interactions, TTF-derivatives slightly influenced the emission quenching of [Pd₆Ru₈] and H₂ evolution. This host–guest interaction between redox-active metallacages and a guest provided a model to understand and optimize redox events, such as photocatalytic activities in a confined nanosapce.²³⁰ The homochiral Δ/Λ-[Ru(piphen)₃]²⁺-metalloligand led to the formation of homochiral [Pd₆Ru₈] (Δ-[Pd₆Ru₈] and Λ-[Pd₆Ru₈]) featured with large D₄-symmetric chiral space, which was imposed by the predetermined [Ru(piphen)₃]²⁺-octahedral stereo configuration. They had an enantio-separation ability for atropisomeric compounds with C₂ symmetry, thus recognizing R- and S-BINOL (BINOL = 1,1′-bis(2-naphthol)) enantiomeric guests in solution.⁶⁰ The homochiral metallasupramolecules possessed dual functionality of photoredox reactivity and stereoselectivity. Naphthol guests were encapsulated into the racemic or enantiopure cages, then underwent a regiospecific 1,4-coupling rather than the normal 1,1-coupling and formed 4-(2-hydroxy-1-naphthyl)-1,2-napthoquinones. This photoinduced regio- and enantioselective coupling was achieved in the confined chiral space of homochiral [Pd₆Ru₈]. Under aerobic conditions, the photo-excited [Ru(piphen)₃]²⁺ centers were quenched by O₂, affording hydrogen peroxide (H₂O₂) and hydroxyl radical (˙OH). The naphthol substrates were oxidized by [Pd₆Ru₈]⁺via single-electron-transfer to give radical species, and its further reaction with ˙OH produced intermediate naphthalene-1,2-dione. Through regioselective 1,4-coupling, 4-(2-hydroxy-1-naphthyl)-1,2-napthoquinone was exclusively obtained.²³¹ By virtue of the molecular cage confinement effect and multi-functions coupling synergism, racemic [Pd₆Ru₈] accelerated [2+2] photodimerization of symmetric acenaphthylene with seteroselectivity and formed anti-products. Homochiral Δ- or Λ-[Pd₆Ru₈] photocatalyzed dimerization of 1-Br-acenaphthylene offered the corresponding (6bS,6cS,12bR,12cR)-6b,6c-dibromo-6b,6c,12b,12c-tetrahydrocyclobuta [1,2-a:3,4-a′]diacenaphthylene or (6bR,6cR,12bS,12cS)-6b,6c-dibromo-6b,6c,12b,12c-tetrahydrocyclobuta [1,2-a:3,4-a′]diacenaphthylene, respectively.²³² Synergistic actions arising from imidazole-N coordination, imidazole-N protonation-deprotonation of [Pd₆Ru₈], cage hydrophobicity, and host–guest electrostatic interactions facilitated carbanionic intermediate stabilization of terminal alkynes and C–H activation to achieve Glaser-coupling and unusual H/D-exchange. The immiscible mixture of alkynes and [Pd₆Ru₈] in aqueous solution turned into a homogeneous phase rapidly, clearly owing to the inclusion of alkynes by the hydrophobicity of the cage via a phase transfer process. This provided a useful catalytic method combining advantages from heterogeneous, homogeneous, and phase transfer to enzymatic catalysis.²³³

Fig. 21 Ru(piphen)₃-based [Pd₆Ru₈].²²⁸

A Ru(bpy)₂Cl₂ fragment was used as a 90° acceptor tecton, and the Cl⁻ sites of cis-Ru(bpy)₂Cl₂ could be occupied by pyridine of the pyridyl linker, producing a [Ru₄(4,4′-bipyridine)₄] molecular square or [Ru₆(TPT)₄] (TPT = 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine) truncated tetrahedral cage respectively. The [Ru₆(TPT)₄] cage possessed emergent properties attributed to its unique electronic structure, resulting in increased visible-light absorption and biexponential decay of an emission band.⁵³ A heteronuclear metallacage [Pd₄Ru₈] was assembled by [Ru(tbubpy)₂(qpy)]²⁺-metalloligand and Pd(MeCN)₄(BF₄)₂ with high Φ_PL.²³⁴

5. Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized organic supramolecules

Supramolecular architectures could be classified into metallasupramolecules and organic supramolecules. Metallasupramolecules are constructed by the metal–ligand interactions, and organic supramolecules are assembled via covalent linkages.¹⁹ The Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized organic supramolecules are described here. Luminescent tetrahedral molecular cages [Ru(phen)₂(N^N)+tris(2-aminoethyl)amine] were formed via the reaction of linear precursor {Ru(phen)₂[(3,3′:6′,2′′:5′′,3′′′-quaterpyridine)-6,6′′′-dicarbaldehyde]}²⁺ with tris(2-aminoethyl)amine and Zn{bis(trifluoromethanesulfonyl)imide}₂. The stereoisomeric chirality of the Ru(N^N)₃-center was passed to the synthesized cage (Fig. 22).⁴⁶

Fig. 22 Luminescent tetrahedral cage [Ru(phen)₂(N^N)+tris(2-aminoethyl)amine].⁴⁶

Mitochondria DNA-targeted photodynamic therapy was used for cancer treatment with a series of [fac-Ir(ppy-CHO)₃+diamine] metallohelices (H-ppy-CHO = 4-(2-Pyridyl)benzaldehyde). Aldehyde groups of fac-Ir(ppy-CHO)₃ were helically arranged around the Ir(III) center with a facial propeller-like configuration. Connecting the preorganized Δ/Λ-fac-Ir(ppy-CHO)₃ modules with different diamines by dynamic imine-coupling chemistry allowed for the formation of [fac-Ir(ppy-CHO)₃+diamine] metallohelices with stereochemical information and suitable size. The odd-even character and length of the diamine alkyl linkers determined the conformations of two Ir(III) centers of one metallohelices with the same chirality or heterochirality (Fig. 23). The high degree of narcissistic chiral self-sorting occurred during the condensation, and a pair of enantiomers were obtained even when diamines with an even number of C atoms were introduced. For an odd number C-based diamine, only one achiral cage was obtained with two heterochiral Ir(III) centers. To improve their stabilities for biological applications, these imine-linked [fac-Ir(ppy-CHO)₃+diamine] metallahelices were further reduced into amine-inked metallohelices by NaBH₄. Notably, in sharp contrast to the corresponding imine complex, the Φ_PL and ¹O₂ quantum yields of these amine-linked metallohelices were significantly enhanced. DNA-binding affinities influenced the photodynamic therapy treatment. 1,5-Pentanediamine had an appropriatelength between diamine spacers, thus showed stronger DNA-binding affinities in a minor groove manner with high photodynamic therapy efficacy.³² Employing Δ/Λ-fac-Ir(ppy-CHO)₃ modules with chiral RR/SS-trans-1,2-diaminecyclohexane spacers, homochiral dinuclear ΔR-[fac-Ir(ppy-CHO)₃+cyclohexylamine] and ΛS-[fac-Ir(ppy-CHO)₃+cyclohexylamine] were obtained. Their chiral porous crystals could be used for the effective enantioseparation of atropisomers, yielding >99% ee for BINOL in a single separation cycle. These molecular crystals maintained consistent ee values even after 10 cycles of enantioseparation. Changing the helicands as achiral ethylenediamine, the resulting crystalline homochiral helicates Δ-[fac-Ir(ppy-CHO)₃+cyclohexylamine] and Λ-[fac-Ir(ppy-CHO)₃+cyclohexylamine] showed no enantioselectivity despite displaying a similar adsorption capacity. The strict chirality matching between the extrinsic 3D chiral channels and the enantiopure guests may dominate the enantioseparation process.⁶⁴

Fig. 23 [fac-Ir(ppy-CHO)₃+diamine]-helicates.³²

A chiral Ir(C^N)₂(N^N)-based building block was regionally coordinated to a bipyridyl-based strand, which subsequently extended into a chiral building block with six vacant pyridine-based chelating sites, and the building block was dimerized through coordination with four Zn(OTf)₂ and formed a Ir(C^N)₂(N^N)-based closed-loop helicate (Fig. 24). Helicate crossing was captured covalently by ring closing metathesis of pendant alkenes and a topological chiral star of David catenane was formed, where the Zn²⁺ could be removed with the maintenance of an interlocked structure. The kinetic stability of the coordinated Ir(III) center ensured that the circular helicate was formed as a single enantiomer. In the star of David [2] catenane, both complexes with and without coordinated Zn²⁺ ions retained the photophysical characteristic of the Ir(C^N)₂(N^N) unit.⁶⁷

Fig. 24 Ir(C^N)₂(N^N)-based triply interlocked star.⁶⁷

6. Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized SOFs

SOFs are crystalline, ordered supramolecular polymers with defined structures. Many reaction conditions including solvents, temperature, pH and reaction time could have a great influence on the synthesis of MOFs and COFs, however the construction of SOFs depends on the mild formation of intermolecular interactions,²³⁵ including hydrogen bonds, electrostatic, and π–π interactions, etc.²³⁵

Using photosensitized building blocks, the homogeneity permitted SOFs with maximum visible light radiation.¹⁰⁴Via host–guest interaction, the combination of six-armed Ru(N^N)₃-based precursors with cucurbit[8]uril generated a {Ru(N^N)₃+cucurbit[8]}-SOF at room temperature, which maintains its solid-state structure in solution, achieving the encapsulation of {P₂W₁₈} in a one-cage-one-guest manner. This material realized homogeneous catalytic H₂ production under visible light irradiation with high efficiency (Fig. 25).⁷⁰ Another similar cubic {Ru(N^N)₃+cucurbit[8]}-SOF was obtained from the reaction of [Ru(bpy)₃]²⁺-based acyl hydrazine with aldehyde in the presence of cucurbit[8]uril. The remarkable size could adsorb discrete anionic {P₂W₁₈} through electrostatic interaction, which was further stabilized in the cationic framework. The corresponding SOF remarkably facilitated visible light-induced electron transfer from Ru(N^N)₃ units to POM guests, enabling highly efficient photocatalytic H₂ production.⁶⁹

Fig. 25 {P₂W₁₈}⊂{Ru(N^N)₃+cucurbit[8]}-SOF.⁷⁰

In addition, Ru(bpy)₂(dcbpy) and [Ru(dcbpy)₃]⁴⁻ photosensitizers, {P₂W₁₈} and {PW₁₂} POMs could also be encapsulated in the pores of SOFs as guests through host–guest interactions. The nanospace provided by SOFs simultaneously encapsulated Ru(N^N)₃-photosensitizer and POMs. The improvement of electron transfer between the Ru(N^N)₃-photosensitizer and POMs catalyst greatly improved the photocatalytic H₂ production.¹⁰⁴

7. Photocatalysis investigation of Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized architectures

Ru(N^N)₃ and Ir(C^N)₂(X^N) derivatives are commonly employed as photocatalysts owing to their strong visible light absorption capability, long-lived photoexcited states, dually effective excited state oxidant and reductant. The light absorption range and redox potentials of three types of Ru(N^N)₃, Ir(C^N)₂(C^N) and Ir(C^N)₃ samples are quite different. As shown in Table 2,⁸¹ photoexcited Ir(ppy)₃ has a stronger reduction capability than [Ru(bpy)₃]²⁺ and [Ir(ppy)₂(bpy)]⁺ owing to its lower redox potentials. For the same type of PS, the redox potential of [Ir(ppy)₂(bpy)]⁺ is higher than [Ir(ppy)₂(tbubpy)]⁺, but lower than [Ir(dFCFppy)₂(tbubpy)]⁺, indicating the influence of the electron donating or withdrawing group of (N^N) on redox potential. Their photophysical properties could be finely regulated via the judicious choice of (N^N) or (X^N) ligands, providing a designable and flexible photocatalyst platform for specific reactions.

Table 2 Redox potentials E_1/2 (V vs. SCE) and selected photophysical properties of commonly utilized Ru(N^N)₃ and Ir(C^N)₂(X^N) photocatalysts^a 81

Entry	Complex	Excitation λmax/(nm)	Emission λmax/(nm)	Excited-state lifetime t/(ns)	E 1/2(M/M^−)	E 1/2(M^+/M*)	E 1/2(M*/M^−)	E 1/2(M^+/M)
SCE = standard calomel electrode. M represents the Ru(N^N)3 or Ir(C^N)2(X^N) complex.a Measurements were performed in MeCN at room temperature unless otherwise noted.b Determined in 1:1 CH5OH/CH3OH at 77 K.
1		452	615	1100	−1.33	−0.81	+0.77	+1.29
2		375	494^b	1900	−2.19	−1.73	+0.31	+0.77
3		420	585	307	−1.42	−0.87	+0.70	+1.25
4		410	581	557	−1.51	−0.96	+0.66	+1.21
5		380	470	2300	−1.37	−0.89	+1.21	+1.69

---

Ru(N^N)₃ and Ir(C^N)₂(X^N) derivatives are good photosensitizers (PSs), and the combined use of the PS and Lewis acid catalyst (Cat) generated novel photocatalytic activities. For example, Ru(N^N)₃ derivatives are good PS for water splitting because of their appropriate redox ability. According to thermodynamic criteria, Ru(N^N)₃ catalysts can drive the photolysis of H₂O, but this is rarely observed due to the high reaction barriers. Only when Ru(N^N)₃ PS and other Cat are employed together, can the photocatalytic H₂O splitting be achieved. Ru(N^N)₃ or Ir(C^N)₂(X^N) based PS and Cat photocatalytic systems commonly follow the three-component system of “PS-Cat-D/A” (D = electron donor, A = electron acceptor). As shown in Fig. 26, PS* acts as a mediator and transfers an electron from the electron donor D to Cat, which involves two mechanisms of oxidative quenching and reductive quenching.²³⁶ The oxidative quenching process involves single-electron-transfer from PS* to Cat, accompanied with the formation of high-oxidation-state PS⁺ and reduced Cat⁻. The PS⁺ gets one electron from D to achieve recovery. The reduction quenching process involves PS* grabbing the electrons from D associated with the formation of reduced PS⁻ and single-electron-transfer occurs from PS⁻ to Cat. In a word, reductive refers to the reduction of PS*, whereas the external D is oxidized in the same process. Oxidative means oxidation of the PS* concomitant with the reduction of Cat.²³⁷ The incorporation of both PS and catalytic sites into a confined nanospace not only increases the apparent concentration of both PS and Cat¹⁰⁴ but also facilitates multielectron transfer to drive high photocatalytic performance, as well as stabilizing the PS and Cat via site isolation.^{47,114,238–240} Many types of photocatalytic reactions have been achieved using the Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized architectures.

Fig. 26 Mechanisms of oxidative quenching and reductive quenching.²³⁷

7.1 Photocatalytic hydrogen evolution reaction (HER)

Photocatalytic HER is an important approach to achieve solar-to-chemical energy conversion. The HER involves both oxidative quenching and reductive quenching mechanisms.^140,150 Reductive quenching-based photoreduction can take place when both requirements of E(PS/PS⁻) < E(Cat/Cat⁻) < E(H⁺/H₂) and E(PS*/PS⁻) > E(D⁺/D) are met. Oxidative quenching-based photoreduction takes place when both requirements of E(PS⁺/PS*) < E(Cat/Cat⁻) < E(H⁺/H₂) and E(PS⁺/PS) > E(D⁺/D) are satisfied. Different Ru(N^N)₃ and Ir(C^N)₂(X^N) PSs should be matched with the corresponding Cat to achieve the HER. For instance, when [Ir(ppy)₂(N^N)]⁺ and [Ru(bpy)₂(N^N)]²⁺ were used as photosensitizers and {Ni₄P₂}-based POMs as catalysts. [Ir(ppy)₂(N^N)]⁺ drove a highly efficient HER process, but [Ru(bpy)₂(N^N)]²⁺ was proved useless, because −0.70 eV of E{[Ir(ppy)₂(N^N)]²⁺/([Ir(ppy)₂(N^N)]⁺)*} was negative enough to permit electron accumulation at {Ni₄P₂} POMs, enabling {Ni₄P₂} to have enough reduction activity for H₂ production. In contrast, −0.62 eV of E{[Ru(bpy)₂(N^N)]³⁺/([Ru(bpy)₂(N^N)]²⁺)*} was higher than the −0.65 eV redox potential of {Ni₄P₂}, thus cannot give {Ni₄P₂} with a more negative reduction potential for driving H₂ production (Fig. 27).¹⁵⁰

Fig. 27 HER process catalysed by {Ni₄P₂}⊂[Ir(ppy)₂(dbpydc)+Zr₆]-MOF.¹⁵⁰

The incorporation of POMs into Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized architectures produced interesting photocatalytic activities arising from the synergistic effect of the photosensitizer, POMs and well-defined nanospace. In {P₂W₁₈}⊂[Ru(bpy)₂(dbpydc)+Zr₆]-MOF¹¹⁴ and {Ni₄P₂}⊂[Ru(bpy)₂(dbpydc)+Zr₆]-MOF,¹⁵⁰ the POMs were encapsulated into the porous cavity of the photosensitizer-functionalized frameworks via electrostatic force and host–guest interaction, and the proximity of POMs to the photosensitizer allowed a facile multi-electron transfer and enabled an efficient visible-light-driven HER with high turnover numbers. Photophysical and electrochemical studies proved that the oxidative quenching of PS* by POMs was the initiating step of the HER. The simultaneous encapsulation of Ru(N^N)₃ derivatives and POMs into a crystalline framework, producing an excellent photocatalyst for the HER. In these networks, the electron transition distance was shortened because the photosensitizer unit and POM catalyst were confined together via electrostatic force and host–guest interactions, leading to sharply increased photocatalytic activity in the HER process.

Apart from POMs, the metal cluster in SBUs can also act as a catalyst. In [Ru(bpy)₂(bpydc)/bpdc+Ti₃]-MOF and [Ir(ppy)₂(bpydc)/bpdc+Ti₃]-MOF, photophysical and electrochemical studies proved that the photocatalytic HER proceeded reductive quenching of PS* and then an electron was transferred from the reduced PS to Ti₃ SBUs. Density functional theory calculations revealed that key steps of the HER process included the Ti³⁺-OH was protonated to generate the Ti³⁺ species with a vacant coordination site, and proton-coupled electron transfer to provide the key Ti⁴⁺-H intermediate.¹⁴⁰ In Ir(ppy)₂(MBA)@(BTB+Ce₆)-MOL and Ru(bpy)₂(MBA)@(BTB+Ce₆)-MOL, the proximity of photosensitizing ligands and Ce₆-SBUs facilitated electron transfer to drive the photocatalytic HER under visible light. The PS* in the MOL was reductively quenched and then transferred electrons to Ce₆-SBUs to generate reduced Ce³⁺ centers, and they were photoexcited to Ce³⁺* species for further HERs.³⁷

In addition, the metal center anchored at the ligands can also be employed as a catalyst, and they were combined with a Ru(N^N)₃ or Ir(C^N)₂(X^N) photosensitizer for bifunctional catalysis. The incorporation of both photosensitizer and metal catalyst into one network, enabled the fast electron–hole separation and transfer to metal catalyst for excellent photocatalytic performance. The network of [Ir(ppy)₂(bpydc)/bpydc+Zr₆]-MOF had open (N^N)-sites that can capture K[PtCl₃(C₂H₄)]. The catalyst-incorporated framework not only promoted photoelectron transfer from the Ir(C^N)₂(X^N) photosensitizer to the Pt catalyst, but also increased the stability of the backbone.⁴⁷ The stable Co@[Ru(bpy)₂(bpydc)/bpydc+Zr₆]-MOF exhibited extra high photocatalytic activity for H₂ production with recycle and reuse performances. The rate-limiting step was the formation of Co⁺ intermediate species reduced by a photoelectron.²⁴¹

7.2 Photocatalytic CO₂ reduction reactions (CO₂RR)

The ever-increasing atmospheric CO₂ from fossil fuel consumption raises growing concerns about global warming.^242,243 Photocatalytic CO₂ transformation to other valued chemicals attracted great interest. However, selective CO₂ transformation remains a challenge as the existing catalyst usually has low catalytic efficiency and poor product selectivity and often suffers from the competing HER.^165,244

In recent years, several Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized architectures have been employed in the CO₂RR process, in which Ru(N^N)₃ and Ir(C^N)₂(X^N) units are used as the photosensitizer, and metal clusters^137,151 or metal complexes are used as coordination catalytic centers, such as Cu,^138,165 Re, Mn,¹⁵⁵ Ni,¹³⁹ and Co.¹⁴⁹ The CO₂RR also obeys the three-component form of “PS-Cat-D” involving two mechanisms of oxidative quenching and reductive quenching. The electrons are transferred from PS* to Cat, and the photogenerated holes in PS* abstract the electrons from D.

CO₂ can be reduced to various products of CO, HCOOH, HCHO, CH₃OH, and CH₄ by gaining different numbers of electrons and protons.²⁴⁴ During the CO₂RR, the light-excitation attributes, band structure, and separation efficiency of photogenerated charge carriers are three important factors to influence photocatalytic CO₂ transformation. Moreover, CO₂ adsorption/activation, catalytic active sites, and intermediate adsorption/desorption are also critical in regulating the product selectivity.²⁴⁴ Redox potentials of photocatalytic CO₂ reduction to different products (Fig. 28) can theoretically control the product selectivity of CO₂ transformations. In addition, the C, H, and O affinity of the photocatalyst has a great influence on the adsorption and desorption of reactants/intermediates, the sequence of hydrogenation and deoxygenation reactions, reaction pathways and product selectivity.

Fig. 28 Reduction potentials of various products for the CO₂RR (SHE = standard hydrogen electrode, CB = conduction band, VB = valence band).

Herein, we elaborate on the materials based on the types of products formed. HCOOH is one potential compound for fuel cells and hydrogen storage.^245,246[Ru(phenba)₃+Eu₂]-MOF has Eu₂ clusters as connecting nodes and Ru(phenba)₃-metalloligand as linkers, which selectively reduced CO₂ to HCOOH in a two-electron process with an excellent rate. The efficient electron transfer from [Ru(phenba)₃]²⁺-units to Eu₂ clusters allowed high HCOOH production rates. The (Eu–H₂O–Eu)-active sites not only accepted electrons but also captured CO₂, thus could achieve highly efficient transformation of CO₂ to HCOOH (Fig. 29).¹³⁷[Ru(cptpy)₂+Zr]-MOF was composed of [Ru(cptpy)₂]²⁺-metalloligand and ZrO₈ cluster and had both high chemical stability and photostability, and it efficiently photocatalyzed the reduction of CO₂ to HCOOH under visible light irradiation.¹⁵¹ In addition, bimetallic catalysis can reduce CO₂ to HCOOH. Highly selective photoreduction of CO₂ to HCOOH was achieved with a high yield by hierarchical integration of [Ru(bpy)₂(H₂bpydc)]²⁺ photosensitizer and monometallic Cu(bpydc)Cl₂ catalyst into a stable Cu@[Ru(bpy)₂(bpydc)/bpydc+Eu₆]-MOF.¹⁶⁵ The multinuclear complexes of [Ru(N^N)₃]_x{Ru(N^N)₂(CO)₂}_y combined PS and Cat together via covalent linkage, exhibited high photocatalytic efficiency and HCOOH selectivity in CO₂ transformation. The ratio between PS and Cat strongly affected the photocatalytic activities, and the higher PS ratio gave a better yield of HCOOH. Weak conjugation of PS and Cat could be also beneficial for better photocatalytic performance.²⁴⁷

Fig. 29 Pathways of electron transfer from Ru(phenba)₃ to the catalytic Eu₂ oxo cluster in [Ru(phenba)₃+Eu₂]-MOF.¹³⁷

Among all the possible technological routes, visible-light-driven two-electron reduction of CO₂ to CO is a kinetically favourable option because it has a lower reaction barrier compared to one-electron and multi-electron reactions.^248,249 Bimetallic synergistic catalysts are commonly explored for CO synthesis with enhanced performance over their monometallic counterparts because of multielectron accumulation at the catalytic centers. The carboxylate exchange at the Hf₁₂ cluster of [Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL with metal complexes yielded bimetallic catalysts M@[Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL (M = Mn or Re, X = Cl or Br), that possessed both Ru(N^N)₃ photosensitizer and M(CO)₃X catalyst for efficient photocatalytic CO₂ reduction (Fig. 30).¹⁵⁵ The molecular catalyst [Ni(bpet)(H₂O)₂]²⁺ was encapsulated into a photo-responsive [Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF to fabricate a composite Ni⊂[Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF for the photocatalytic transformation of CO₂ to CO.¹³⁹ Artificial photocatalytic CO₂RR is a big challenge as the efficient cooperation of multiple functional units is difficult. Without use of an electron donor, the multifunctional photocatalyst [Ru(ip)₃+Co]-MOF could convert CO₂ to CO with a high CO production rate and nearly 100% selectivity accompanied by water oxidation to O₂. Suitable photocatalytic redox potentials, efficient electron-hole separation and CO₂ adsorption ability all contributed to this artificial photosynthesis.⁵⁹

Fig. 30 Photocatalytic CO₂RR to CO occurred in M@[Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL.¹⁵⁵

Syngas (a mixture of H₂ and CO) is a versatile fuel precursor for producing chemical and synthetic liquid fuels. Photocatalytic CO₂RR will open an avenue for consecutive production of syngas.²⁵⁰ Zhang and collaborators reported a Co@[Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF composite photocatalyst for efficient syngas production. Interestingly, the H₂/CO ratios could be well regulated by precisely adjusting the water content of the reaction solution and the molar ratio of PS/Cat in the network.¹⁴⁹

Photoirradiation CO₂ hydrogenation to ethanol is of practical importance yet poses a significant challenge due to the formation of a C–C bond with an intact C–O bond. Lin and co-workers used low-intensity light to activate a Cu@[Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF catalyst for the selective hydrogenation of CO₂ to C₂H₅OH. Under light illumination, [Ru(N^N)₃]* underwent single-electron-transfer to Cu²⁺ to generate Cu⁺ which was active for C₂H₅OH production. In contrast, Cu⁰ nanoparticles were formed as the active species for CH₃OH production under darkness (Fig. 31).¹³⁸

Fig. 31 Selective photocatalytic CO₂RR with or without light irradiation catalysed by Cu@[Ru(bpy)₂(bpydc)/bpdc+Zr₆]-MOF.¹³⁸

7.3 Aerobic photooxidation

Aerobic photooxidation is very important in organic synthesis because photocatalysis provides a high-efficiency, safe and economic O₂ utilization for developing diverse aerobic organic transformations and O₂ could be transformed to other ROSs, such as H₂O₂, ¹O₂, ˙O₂⁻, and ˙OH.²⁵¹ ROSs of ˙OH, H₂O₂, ˙O₂⁻ and ¹O₂ could be generated in different ways. A stepwise reduction of O₂ respectively generates ˙O²⁻, H₂O₂, and ˙OH.²⁵¹ The quantum yield of ˙O₂⁻ is significantly higher, and its lifetime is only lower than H₂O₂. In the absence of reactants, ˙O₂⁻ is converted to the stable H₂O₂via a disproportionation process. The ˙OH radical has the highest oxidation activity, thus its oxidation performance is almost limited by the substrate diffusion rate. In addition, ˙OH is commonly pursued in photocatalytic organic decomposition. Although ¹O₂ has a lifetime of ten milliseconds in air, it was shortened to only 3 μs in H₂O and thus it might be decayed by energy transfer with no chance of participating in any reaction.²⁵¹

By combining three intrinsically nontoxic components, namely, photosensitizer, light, and tissue O₂ to generate cytotoxic ROS, photodynamic therapy provides an effective phototherapy against cancer.²⁵² Ultrathin MOLs facilitate the diffusion of ROS to cell milieu to exert cytotoxic effects.^50,156 Cationic [Ir(ppy)₂(dbpydc)+Hf₁₂]-MOL was built from Hf₁₂-SBUs and [Ir(ppy)₂(H₂dbpydc)]⁺-metalloligand and then was loaded with POMs {P₂W₁₈} to afford {P₂W₁₈}⊂[Ir(ppy)₂(dbpydc)+Hf₁₂]-MOL. Upon X-ray irradiation, {P₂W₁₈}⊂[Ir(ppy)₂(dbpydc)+Hf₁₂]-MOL significantly enhanced ˙OH generation from Hf₁₂-SBUs, ¹O₂ generation from [Ir(ppy)₂(H₂dbpydc)]⁺-metalloligand, and ˙O₂⁻ generation from {P₂W₁₈}, achieving synergistic cell killing by these ROSs (Fig. 32).⁴⁹

Fig. 32 Three distinct ROS generated from {P₂W₁₈}⊂[Ir(ppy)₂(dbpydc)+Hf₁₂]-MOL upon X-ray irradiation.⁴⁹

Aerobic photocatalytic sulfide oxidation to sulphoxide is a green pathway for sulphoxide production using O₂ as an oxidant. Highly stable [Ir(pqc)₂(phen)+Zr₆]-MOF was constructed by [Ir(pqc)₂(phen)]⁺-metalloigand linkers and Zr₆-SBUs. [Ir(pqc)₂(phen)+Zr₆]-MOF exhibited high catalytic activity upon aerobic photooxidation of sulfide into sulfoxide in water at room temperature.^48,142 Under visible light irradiation, the [Ir(pqc)₂(phen)]⁺ photosensitizer was excited to a high-energy excited singlet state ¹{[Ir(pqc)₂(phen)]⁺}*, which quickly underwent intersystem crossing to an excited triplet due to the highly efficient spin–orbit coupling. The ³{[Ir(pqc)₂(phen)]⁺}* transferred an electron to O₂ to form ˙O₂⁻ and generated [Ir(pqc)₂(phen)]²⁺. Sulfide was oxidized by [Ir(pqc)₂(phen)]²⁺ to generate a sulfide radical cation and achieve the regeneration of neutral [Ir(pqc)₂(phen)]⁺ photosensitizer in the photocatalytic cycle. The sulfide radical cation reacted with the ˙O₂⁻ to generate a persulfoxide intermediate, which further reacted with sulfide to afford the corresponding sulfoxides. In this system H₂O not only accelerated the conversion of persulfoxide into sulfoxide but also prevented the over oxidation of sulfoxide into sulfone.⁴⁸ Moreover, ˙O₂⁻ was the main ROS for efficient photocatalytic sulfide oxidation. Interestingly, by changing the auxiliary ligand of the Ir(pqc)₂(N^N)-metalloligand, the excitation lifetime of the triplet state, the generation of ROS quantum yields, and the sulfide photooxidation efficiency could be tuned.²⁵³Ru(bpy)₂(bpydc)@(bpydc+Ln₆)-MOFs (Ln = Tb, Gd, Eu) showed excellent photocatalytic activity in sulfide oxidation with high conversion and sulfoxide selectivity under an O₂ atmosphere, ˙O²⁻ was the main ROS and h⁺ also took effect in oxidation. Ru(bpy)₂(bpydc)@(bpydc+Ln₆)-MOFs were excited by visible light and generated electrons and holes. Here, O₂ accepted an electron and produced ˙O₂⁻, which further attacked sulfide, with the assistance of h⁺, the active intermediate persulphoxide formed, and its reaction with another sulfide molecule led to the formation of sulfoxide products.¹⁴²[Ru(cptpy)₂+Ce]-MOF was synthesized from [Ru(cptpy)₂]²⁺-metalloligands and Ce(NO₃)₃. [Ru(cptpy)₂+Ce]-MOF exhibited high photocatalytic activities in sulfide oxidation, which is better than the isomorphic [Ru(cptpy)₂+Zr]-MOF, revealing the important role of Ln(III) ions in the photocatalytic process. The doping of Zr(IV) enhanced the chemical stability of [Ru(cptpy)₂+Ce/Zr]-MOF, and [Ru(cptpy)₂+Ce/Zr]-MOF possessed both high chemical stability and catalytic efficiency in photocatalytic sulfide oxidation.²⁵⁴

Overall photocatalytic water splitting is a great challenge. Thermodynamically, the conduction band/valence band position of the semiconductor must match the reduction/oxidation potential of water, so that the excited electron–hole pair has a sufficient ability to carry out overall water splitting. Dynamically, the slow desorption rate of H₂ and O₂ generated by photocatalytic water splitting on the surface of the material also becomes the kinetic limiting factor, and the desorption rate is slower than the recombination of photo-generated electrons and holes.^255–258HER-MOF and WOR-MOF nanosheets were integrated into liposomal structures for separation of the photogenerated charges. For example, the HER-MOF were constructed from hydrophobically modified Hf₆ clusters and Zn–porphyrin or Pt–porphyrin linkers, while WOR-MOF were fabricated by Zr₁₂ clusters and Ru(N^N)₃ or Ir(C^N)₂(N^N) metalloligands. The lipid membrane separated the oxidative and reductive components to prevent charges recombination, and this system was used in overall photocatalytic water splitting through the ‘Z-scheme’ electron-transfer.²⁵⁹

[Ru(bpy)₂(bpydc)/BTB+Zr₆]-MOF was assembled via the linkage of BTB-Zr₆ layers and [Ru(bpy)₂(H₂bpydc)]²⁺-metalloligand, and it served as a highly efficient catalyst in the photooxidation reactions, including photooxidation of sulfide, coupling of amines and oxidative hydroxylation of arylboronic acids (Fig. 33). The main ROS of three types of reaction was ˙O²⁻, and the generated ROS was determined by the photocatalyst itself.²⁶⁰ Mesoporous Ru(bpy)₂(bpydc)@(TPTB+Zr₆)-MOF was composed of Zr₆ clusters and Ru(bpy)₂(H₂bpydc)-metalloligand, and it exhibited exceptional high photocatalytic activity in the oxidation of dihydroartemisinic acid to artemisinin.¹¹⁸ The 3D diamondoid (BPDCA+TAPM)-COF had open (N^N)-sites, and the incorporation of the Ru(bpy)₂Cl₂ unit led to Ru(N^N)₃@(BPDCA+TAPM)-COF, which performed as a highly efficient, visible-light-mediated photocatalyst for the oxidative cyanation reaction of tertiary amines with excellent yields.¹⁹¹

Fig. 33 Aerobic photooxidation catalysed by [Ru(bpy)₂(bpydc)/BTB+Zr₆]-MOF.²⁶⁰

7.4 Photocatalytic organic transformation

Organic transformations play a key role in the synthesis of pharmaceutical drugs and functional molecules. Photocatalytic reactions have appeared as a new tool to solve some critical problems met in the traditional reaction methods because photocatalysis could directly activate the substrates and bypass the reaction barrier. The last decade has witnessed the rapid development of photoredox catalysis.^6,219,261 In particular, Ru(N^N)₃ or Ir(C^N)₂(X^N) derivatives have been widely used as the photocatalyst.^262–264 A large number of Ru(N^N)₃ or Ir(C^N)₂(X^N) functionalized architectures have been explored for the photocatalytic organic transformation, including single-site catalysts,^116,118 dual active sites^{115,135,136,141,265} and ternary active sites¹⁵⁸ for synergistic catalysis.

Ru(N^N)₃ or Ir(C^N)₂(X^N) derivatives acted as both photosensitizer and catalyst in the well-defined architectures. The Ru(N^N)₃@(BPDCA+TAB)-COF displayed unprecedented photocatalytic activity of visible light and achieved cross-dehydrogenation coupling of secondary amine and indole with an excellent rate (Scheme 1).¹¹⁶ Mesoporous Ru(bpy)₂(bpydc)@(TPTB+Zr₆)-MOF photocatalyzed the aza-Henry reaction with high conversion yields (Scheme 2), showing a similar catalytic performance with [Ru(bpy)₃]²⁺.¹¹⁸ Stereoselectivity in photoredox reactions was very difficult to control. One strategy to achieve enantioselectivity in photoredox reactions is to combine a photoredox catalyst and a stereochemically controlled cocatalyst into one catalytic system.^266–268Δ/Λ-[Pd₆Ru₈] metallasupramolecules had dual functionality of photoredox reactivity and stereoselectivity. The photoinduced regio- and enantioselective coupling of naphthols and its derivatives thereof was achieved in the confined chiral space of [Pd₆Ru₈]. The enantiomer cages encased naphthol guests, and then underwent a regionally specific 1,4-coupling, instead of the normal 1,1-coupling, forming 4-(2-hydroxy-1-naphthyl)-1,2-napthoquinones.²³¹Δ/Λ-[Pd₆Ru₈] was also an effective supramolecular reactor to achieve the enantioselective cycloaddition of 1-subsituted acenaphthylene derivatives.²³²

Scheme 1 Photocatalytic cross-dehydrogenative coupling of secondary amines with indoles catalysed by Ru(N^N)₃@(BPDCA+TAB)-COF.¹¹⁶

Scheme 2 Aza-Henry reactions photocatalyzed by Ru(bpy)₂(bpydc)@(TPTB+Zr₆)-MOF.¹¹⁸

In the well-defined architectures, Ru(N^N)₃ and Ir(C^N)₂(X^N) units are synergistic with another catalyst in the catalytic reactions. The proximity and cooperativity between different catalytic sites are key issues in the design of dual-functional catalytic systems.²⁶⁹ The precise location of dual catalytic sites at porous architectures should be a feasible strategy.^{115,135,136,141,265} The proximity of two catalytic components in the cavities of the frameworks greatly facilitates charge transfer from photosensitizing centers to catalytic centers, reaching a higher turnover number than the free counterpart.^135,136,265 Incorporation of Ni(N^N)Cl₂ Cat into [Ir(dFCFppy)₂(dbpydc)/dbpydc+Zr₁₂]-MOL,²⁶⁵[Ir(dFCFppy)₂(dbpydc)+Hf₁₂]-MOL,¹³⁵Ir(dFCFppy)₂(N^N)@(phendda+TAB)-COF and Ir(ppy)₂((N^N))@(Tp+abpy)-COF^115,117 efficiently catalysed four important cross-coupling reactions with broad substrate scopes, including C–S cross-coupling between thiols and aryl iodides,^135,265 C–O cross-coupling between aryl bromides and alcohols,¹³⁵ and C–C cross-coupling between aryl bromides and potassium benzyltrifluoroborates (Scheme 3)^115,135 as well as C–N cross-coupling between iodides and amines.¹¹⁷ For example, upon light irradiation, [Ir(dFCFppy)₂(H₂dbpydc)]⁺ was excited to {[Ir(dFCFppy)₂(H₂dbpydc)]⁺}* and it was quenched by thiol and NiCl₂ to generate the thiol radical and Ni⁺–Cl. The binding of thiol and Ni⁺–Cl formed Ni²⁺–Cl–sulphide species (Fig. 34). The concomitantly obtained Ir(dFCFppy)₂(H₂dbpydc) returned to [Ir(dFCFppy)₂(H₂dbpydc)]⁺ by transferring one electron to Ni²⁺–Cl–sulphide thus formed Ni⁺–sulphide. Oxidative addition of an aryl iodide to Ni⁺–sulphide delivered a Ni³⁺-complex, which underwent a facile reductive elimination process, leading to a C–S cross-coupled product and regeneration of Ni²⁺–Cl. The distance between the Ir(III) photosensitizer and Ni(II) catalytic center was shortened to only 0.6 nm, which facilitated both electron and thiophenol radical transfers and significantly enhanced its photocatalytic activity.²⁶⁵ Multifunctional OTf@[Ir(dFCFppy)₂(dbpydc)+Hf₁₂]-MOL had a synergistic catalytic performance in dehydrogenative cross-couplings of heteroarenes with unactivated alkanes, amines, and ethers. Furthermore, it could also catalyse late-stage functionalization of bioactive and drug molecules such as caffeine, Fasudil, and Metyrapone.¹³⁶ A mesoporous [OTf/Ir(ppy)₂(bpydc)]@(bpydc+Al)-MOF had both strong Lewis acidic Al-OTf sites and photoredox sites, which effectively catalysed reductive cross-coupling to afford new azaarene derivatives.¹⁴¹ The employment of a homogeneous Au-catalyst typically required high catalyst loadings due to the relatively low reactivity of Au complexes and rapid catalyst deactivation. Deactivation of Au catalysts via a ligand redistribution formed [Au(phosphine)]⁺ and unstable Au⁺ complexes, then Au⁺ complexes disproportionated into catalytically inactive Au³⁺ species and Au⁰ nanoparticles.²⁷⁰ MOLs could hierarchically integrate photosensitizers and Au catalysts together, not only preventing Au catalyst deactivation but also enhancing photoredox catalytic activities with proximately placed photosensitizers and {Au[4-(diphenylphosphino)phenylacetate]}Cl. [Au/Ru(N^N)₃]@(BPY+Hf₆)-MOL was built from Ru(bpy)₂Cl₂, [Au(phosphine)]Cl, H₃BPY ligands and Hf₆ SBUs. Ru(bpy)₂(N^N) photosensitizer and Au⁺ catalysts worked synergistically for the cross-coupling of allenoates, alkenes, or alkynes.²⁷¹

Scheme 3 C–S, C–O, and C–C cross-coupling reactions photocatalyzed by Ni@[Ir(dFCFppy)₂(dbpydc)+Hf₁₂]-MOL.¹³⁵

Fig. 34 Proposed mechanism for the C–S cross-coupling reaction catalysed by Ni@[Ir(dFCFppy)₂(dbpydc)/dbpydc+Zr₁₂]-MOL.²⁶⁵

The design of functional architectures with three or more catalytic centers remains a challenge.²⁷² A multifunctional catalyst allows the combination of multiple transformations in a one-pot synthetic route, rather than the traditional stop-and-go approach.²⁷³ With three synergistic active sites, OTf/Co@[Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL contained a Ru(bpy)₂(N^N) photosensitizer, strong Lewis acid OTf-Hf₁₂-SBUs and hydrogen transfer catalyst [Co(dimethylglyoxime)₂(4-pyridinepropionate)]Cl, which efficiently catalysed dehydrogenative tandem transformations (Scheme 4). The hierarchical integration of three different catalytic centers into the MOL could prevent mutual inactivation and promote charge transfer to enhance the synergistic tandem catalytic activity. Under light irradiation, the excited {[Ru(bpy)₂(N^N)]²⁺}* transferred one electron to the Co³⁺ unit and generated a Co²⁺ unit and [Ru(bpy)₂(N^N)]³⁺, then indoline was oxidated to the radical cation intermediate by [Ru(bpy)₂(N^N)]³⁺. Deprotonation of the radical cation intermediate formed the α-carbon radical, and its radical addition reaction with Co²⁺ formed Co³⁺–C bonded intermediate. Subsequently, the β-H elimination from the Co³⁺–C intermediate produced the indole and Co³⁺–H intermediate, and Co³⁺–H was protonated and enabled the H₂ production and regeneration of Co³⁺ catalysts. Simultaneously, upon coordinating to the strongly Lewis acidic Hf₁₂ SBUs, vinyl ketone became electron deficient and attacked the electron rich β-position of indole, and the resulting intermediate further reacted with vinyl ketone to form the corresponding product.¹⁵⁸

Scheme 4 Dehydrogenative tandem transformations of indolines photocatalyzed by OTf/Co@[Ru(bpy)₂(dbpydc)+Hf₁₂]-MOL.¹⁵⁸

8. Summary and future prospects

Here we reviewed the application of Ru(N^N)₃ and Ir(C^N)₂(X^N) based architectures for in-light-driven catalysis, which provide an interesting platform for photosynthesis by organizing photoactive components in well-organised architectures. The incorporation of Ru(N^N)₃ and Ir(C^N)₂(X^N) building blocks into well-defined skeletons can create interesting architectures with novel structures and the required photo-function, because the structural aesthetics and functional advantages of Ru(N^N)₃ or Ir(C^N)₂(X^N) are readily inherited in the target architectures, and the combination of functional tectons, nano-cavity, crystalline network and Lewis acid site produced new photofunctional entities. The absorption ability of light, absorption band, and the diversity of structures could be enhanced and broadened by the integration of Ru(N^N)₃ or Ir(C^N)₂(X^N) into distinct architectures. These Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized architectures exhibited novel photocatalytic performances with a synergistic catalytic effect, high catalytic activity, product selectivity and recyclability. As highlighted in POMs⊂Ru(Ir)-MOF, the components of the photosensitizer and POMs were organised in close proximity, to promote the photogenerated electron transfer from the photosensitizer to POMs, and the accumulation of electrons at POMs achieved photocatalytic hydrogen production. The integration of Ir(C^N)₂(N^N) and Ni²⁺ into the porous network of 2D (N^N)-COF enabled highly efficient photocatalysis of C–C or C–N cross-couplings. In the [Pd₆Ru₈] cage, the supramolecular interactions between host and guest can regulate the redox potentials of the photosensitizer and catalyst, enhance charger transfer, and stabilize the catalytic intermediates, resulting in special photocatalytic activity.

The design and synthesis of Ru(N^N)₃ and Ir(C^N)₂(X^N) based architectures is ongoing research, while the use of these novel complexes for photosynthesis still remains underexplored. There are still some serious problems needed to be solved compared with plant photosynthesis. (1) Understanding of photo-generated charge and energy transfer processes, efficient charge separation, and stabilization of unstable intermediates should provide more insights into the fundamental mechanistic aspects for the development of efficient photocatalysts. (2) The simultaneous consumption of both electrons and holes is difficult, and the addition of sacrificial agents results in resource waste and pollution. (3) Ru(N^N)₃ and Ir(C^N)₂(X^N) functionalized architectures still face limited broadband light absorption and low Φ_PL due to inefficient separation of photogenerated electrons and holes. (4) The design and construction of Ru(N^N)₃ and Ir(C^N)₂(X^N) photo-functionalized architectures remain sustainable developments because the architecture type, architecture porosity, linkage type and alignment mode of the photosensitizer and catalyst, and the introduction of a guest have a great influence on the functionality. These architectures showing interesting catalytic properties are also expected to show other photophysical properties like multi-photon absorption cross-sections, and second and third harmonic generations. Furthermore, the architectures may be able to include both the Ru(N^N)₃ and Ir(C^N)₂(X^N) moieties into the same architectures. Although this has not been accomplished successfully, the synergistic effect of the presence of these photofunctional metal complexes in the same materials will be of immense academic interest and may shed more light on understanding the design principles of new functional materials. The incorporation of Ru(N^N)₃ and Ir(C^N)₂(X^N) moieties into other well-defined architectures like a molecular sieve could produce stable mesoporous photocatalysts. Immobilizing these Ru(N^N)₃ or Ir(C^N)₂(X^N) based complexes on a conductive supporter will allow their implementation on photo-electrochemical devices. These are expected to promote their closer commercial applications.

Abbreviations

MLCT	Metal-to-ligand charge transfer
ILCT	Intraligand charge transfer
MOFs	Metal–organic frameworks
COFs	Covalent organic frameworks
SOFs	Supramolecular organic framework
MOL	Metal–organic layer
2D	Two-dimensional
3D	Three-dimensional
Φ PL	Emission quantum yields
bpy	2,2′-Bipyridine
H-ppy	2-Phenylpyridine
SBUs	Secondary building units
PSM	Post-synthetic modification
H2bpydc	2,2′-Bipyridine-5,5′-dicarboxylic acid
H4TPTB	5′,5′′′-Bis(4-carboxylatophenyl)-4′′′,6′-dimethoxy-[1,1′:3′,1′′:4′′,1′′′:3′′′′,1′′′′-quinquephenyl]-4,4′′′′-dicarboxylate
H2bpdc	4,4′-Biphenyldicarboxylic acid
H-pqc	H-2-Phenylquinoline-4-carboxylic acid
H3BPY	4′,6′-Dibenzoato-[2,2′-bipyridine]-4-carboxylic acid
Hip	1H-Imidazo[4,5-f][1,10]phenanthroline
H-H2dcppy	2-(3-Carboxyphenyl)pyridine-4-carboxylic acid
H2dcbpy	2,2′-Bipyridine-4,4′-dicarboxylic acid
H2dbpydc	4,4′-([2,2′-Bipyridine]-5,5′-diyl)dibenzoic acid
H4dpbpy	2,2′-Bipyridine-4,4′-bis(phosphonic acid)
phenba	4-(1H-Imidazo[4,5-f][1,10]phenanthrolin-2-yl)benzoic acid
phen	1,10-Phenanthroline
H-ppy-COOH	3-(Pyridin-2-yl)benzoic acid
cptpy	4′-(4-Carboxyphenyl)-terpyridine
Ln	Lanthanides
H3BTB	1,3,5-Benzenetribenzoate
HMBA	2-[5′-Methyl-(2,2′-bipyridin)-5-yl]acetate
bpy-OEt	5,5′-Bis(diethoxymethyl)-2,2′-bipyridine
ETTA	4,4′,4′′,4′′′-(Ethene-1,1,2,2-tetrayl)tetraaniline
TPB	1,2,4,5-Tetrakis-(4-aminolphenyl)benzene
BPDCA	2,2′-Bipyridine-5,5′-dicarbaldehyde
TAB	1,3,5-Tris-(4-aminophenyl) benzene
ETTBA	4′,4′′′,4′′′′′,4′′′′′′′-(Ethene-1,1,2,2-tetrayl)tetrakis([1,1′-biphenyl]-4-amine)
phendda	4,4′-(1,10-Phenanthroline-3,8-diyl)dibenzaldehyde
abpy	5,5′-Diamino-2,2′-bipyridine
Tp	1,3,5-Triformylphloroglucinol
TAPM	Tetra(p-aminophenyl)methane
TPMB	Tetrakisphenylmethane borate
MeCN	Acetonitrile
TEPM	Tetra(4-ethynylphenyl)methane
piphen	2-(Pyridin-3-yl)-1H-imidazo[4,5-f][1,10]phenanthroline
TTF	Tetrathiafulvalene
BINOL	1,1′-Bis(2-naphthol)
H-dFCFppy	2-(2,4-Difluorophenyl)-5-(trifluoromethyl)pyridine
H-ppy-CHO	4-(2-Pyridyl)benzaldehyde
tbubpy	4,4′-Di(tert-butyl)-2,2′-bipyridine
H-mesppy	2-Phenyl-4-mesitylpyridinato
qpy	4,4′:2′,2′′,4′′,4′′′-Quaterpyridine
H-dFmesppy	2-(4,6-Difluorophenyl)-4-mesitylpyridinato
H-dFppy	(2-(2,4-Difluorophenyl)-4-phenyl)pyridine
H-appy	4-(2-Pyridinyl)phenylamine
tpmc	Tris(4-pyridyl-methyl)-cyclotriguaiacylene
ttpadtc	2,7,12-Trimethoxy-3,8,13-tris(4,4′-pyridyl-azophenylcarboxy)-10,15-dihydro-5H-tribenzo[a,d,g] cyclononene
bpet	1,2-Bis[(pyridin-2-ylmethyl)thio]ethane
OTf	Triflate
TPT	2,4,6-Tri(pyridin-4-yl)-1,3,5-triazine
HOMO	Highest occupied molecular orbital
LUMO	Lowest unoccupied molecular orbital
PS	Photosensitizer
Cat	Catalyst
D	Electron donor
A	Electron acceptor
POMs	Polyoxometalates
{P2W18}	[P2W18O62]^6−
{PW12}	[PW12O40]^3−
{Ni4P2}	[Ni4(H2O)2(PW9O34)2]^10−
CO2RR	Carbon dioxide reduction reactions
CO2	Carbon dioxide
HCOOH	Formate
CO	Carbon monoxide
HCHO	Formaldehyde
CH3OH	Methanol
CH4	Methane
O2	Oxygen
ROS	Reactive oxygen species
H2O2	Hydrogen peroxide
^1O2	Singlet oxygen
˙O2^−	Superoxide radical
˙OH	Hydroxyl radical
HER	Hydrogen evolution reaction.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Foundation of China (21971011 and 21831001).

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