ReviewSupramolecular iridium(III) assemblies
Graphical abstract
Introduction
The term molecular self-assembly applies to pathways that produce a final product directly and spontaneously when the correct components are mixed under appropriate conditions [1]. Self-assembly plays an integral role in the structure and function of biological systems [2], [3], [4] and it is implicated in a variety of functional materials [5]. In Nature self-assembly is generally based on numerous hydrogen bounding, electrostatic, van der Waals, and other weak inter- and intra-molecular interaction working synergistically to assemble, for example, secondary and tertiary structures of proteins, which then provide well-defined local environments to mediate biochemical transformations [6]. Similarly, in natural photosynthesis organisms optimize solar energy conversion through the self-organized assembly of photofunctional chromophores [7], [8]. Over the last two decades, molecular self-assembly has played a key role in the construction of a variety of elegant and intricate synthetic nanostructures, including molecular crystals and liquid crystals [9], [10], colloids [11] and micelles [12], gels [13], [14], polymers [15], [16] and nanoscale structures of high symmetry, such as 3D-frameworks [17], metal-organic polygons and polyhedra [18], [19]. As the properties of these materials highly depend both on the nature of their components and the interactions between them, the explicit manipulation of the building blocks and the non-covalent forces that hold the constituents together has promoted the evolution of functional properties, which have been exploited in numerous advanced technologies. For example, liquid crystals have found application as anisotropic organic semiconductors in organic field effect transistors (OFETs), Organic Light-Emitting Diodes (OLEDs) and Organic Photovoltaic devices (OPVs) [9], [10], [20]. Due to their large surface area and biocompatibility, colloids and micelles are very important for water purification, cleansing action of soap and food formulation [21], [22]. Hydrogels and polymers are key components in materials for medicine, food science and cosmetics [23], [24], [25]. Nanostructured materials such as molecular crystals, frameworks or 3D-polyhedral structures exhibit interesting optical, magnetic and catalytic properties, which have been rapidly exploited in diverse applications such as in catalysis, magnetic devices and gas purification [26], [27], [28].
In recent years, there has been an increasing interest in the construction of photoactive supramolecular assemblies through the incorporation of luminescent building blocks [29], [30], [31], [32]. This immediately generates possibilities for assembling in very close proximity a high concentration of chromophoric units through non-covalent interactions, thereby achieving photophysical properties that are difficult to obtain in conventional molecular materials. Besides modulating the optoelectronic properties of the emissive compounds as a function of the assembly, their organization into ordered structures can also radically change the physical properties of the bulk materials [33]. As a result, nanomaterials that exhibit both fascinating physical and photoactive properties have been one of the main areas of interest in supramolecular chemistry in recent years [29], [34], [35].
Cyclometalated iridium(III) complexes exhibit efficient phosphorescence, unprecedented facile emission color tunability across the visible spectrum, and high chemical and thermal stability [36], [37], [38], [39], [40]. They have thus found use in myriad applications such as in sensing [41], [42], bio-imaging [43], photoredox catalysis [44], solar fuels [45], [46] and in electroluminescent devices [47], [48]. The use of iridium complexes as luminescent components for self-assembly has also become increasingly popular. This review article provides an exhaustive summary on the development of photoactive self-assembled materials based on iridium(III) complexes, giving special emphasis to their photophysical properties, and highlighting their applications. Depending on the nature of the ligands, iridium(III) complexes can be cationic, neutral and anionic. The vast majority of cationic iridium(III) complexes possess the general motif [Ir(C^N)2(N^N)]+, where C^N is a cyclometalating ligand such as 2-phenylpyridinato (ppy) and N^N is a neutral ancillary ligand such as 2,2′-bipyridine (bpy). Homoleptic neutral iridium complexes possess the general formula [Ir(C^N)3] and are frequently studied as their facial geometric isomer. Heteroleptic neutral complexes generally possess the structural motif [Ir(C^N)2(X)], where X is an anionic bidentate ligand such as acetylacetonates, oxazolines or thiazolines whereas, negatively charged cyclometalated iridium(III) complexes typically possess the composition [Ir(C^N)2(Y)2]−, where Y is typically an anionic monodentate ligand such as CN−, NCS− and NCO−.
We begin by describing iridium-based soft materials such as ion-paired iridium complexes commonly known as soft-salts, liquid crystals, supramolecular gels, colloidal structures and assemblies developed through H-bonding and π-π-stacking interactions. Next, we turn our attention to describing luminescent iridium-based coordination polymers, metal-organic frameworks (MOFs) and discrete structures, followed by an overview of luminescent Ir-based macrocycles, capsules and cages. The last chapter of this review highlights the emerging field of dynamic encapsulation of mononuclear iridium complexes into the cavities of 3D MOFs and cages. In this chapter, emphasis will be placed in describing the changes of the luminescent properties of the guest iridium complexes due to their physical and optoelectronic interactions with the host materials. It is worth noting that a number of studies involving luminescent Ir(III) complexes covalently assembled within multinuclear dyads, triads and arrays [49], [50], [51], or covalently linked onto polymeric structures [52], [53], [54] have been reported. As these systems have been extensively described elsewhere, they are not included within the present review.
Section snippets
Soft materials
Molecular aggregation induced by non-covalent interactions between Ir(III) chromophores can have tremendous impact on the properties of materials. Desirable photophysical properties such as emission tuning, enhanced photoluminescence quantum yield, longer excited state lifetimes, and energy and electron transfer processes can be achieved by controlling the aggregation and organization of Ir(III) emitters in soft materials.
Coordination-driven self-assembly
Coordination-driven self-assembly, which is based on the formation of metal-ligand bonds, has proven to be a powerful method to prepare supramolecular well-defined nanostructures of varying shapes, sizes and functional properties, featuring considerable synthetic advantages such as facile and rapid construction of the final products and high yields [123], [124], [125]. In this section, we highlight the use of Ir(III) complexes as luminescent scaffolds in coordination-driven self-assembly by
Encapsulation of Ir(III) chromophores
Several studies have demonstrated that the photophysical properties of luminescent Ir(III) metal complexes emitting from CT states strongly depend on the local environment [101], [107], [111], [167]. In this context, the encapsulation of iridium complexes into the cavities of photoactive cages, capsules or MOFs has been demonstrated to be an efficient approach to tuning of the emission properties of the assembly as a function of host-guest energy transfer.
Umakoshi and co-workers [168]
Conclusions
The self-assembly of Ir(III) luminophores into supramolecular materials clearly offers possibilities for tuning the physical and optoelectronic properties of Ir(III) complexes and opens up opportunities for exploiting these materials in many applications, ranging from catalysis to electroluminescent devices. Iridium-based soft materials generally exhibit highly organized structures with enhanced emission when compared to their mononuclear counterparts. Three-dimensional iridium-based polymers
Acknowledgements
EZ-C acknowledges the University of St Andrews for financial support and the Engineering and Physical Sciences Research Council for financial support (EP/M02105X/1).
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