Judicious Design of Cationic, Cyclometalated Ir(III) Complexes for Photochemical Energy Conversion and Optoelectronics

Abstract

The exponential growth in published studies on phosphorescent metal complexes has been triggered by their utilization in optoelectronics, solar energy conversion, and biological labeling applications. Very recent breakthroughs in organic photoredox transformations have further increased the research efforts dedicated to discerning the inner workings and structure-property relationships of these chromophores. Initially, the principal focus was on the Ru(II)-tris-diimine complex family. However, the limited photostability and lack of luminescence tunability discovered in these complexes prompted a broadening of the research to include 5d transition metal ions. The resulting increase in ligand field splitting prevents the population of antibonding e* orbitals and widens the energy range available for color tuning. Particular attention was given to Ir(III), and its cyclometalated, cationic complexes have now replaced Ru(II) in the vast majority of applications. At the start, this Account documents the initial efforts dedicated to the color tuning of these complexes for their application in light emitting electrochemical cells, an easy to fabricate single-layer organic light emitting device (OLED). Systematic modifications of the ligand sphere of [Ir(ppy)bpy] (ppy: 2-phenylpyridine, bpy: 2,2'-bipyridine) with electron withdrawing and donating substituents allowed access to complexes with luminescence emission maxima throughout the visible spectrum exhibiting room temperature excited state lifetimes ranging from nanoseconds to dozens of microseconds and quantum yields up to 15 times that of [Ru(bpy)]. The diverse photophysical properties were also beneficial when using these Ir(III) complexes for driving solar fuel-producing reactions. For instance, photocatalytic water-reduction systems were explored to gain access to efficient water splitting systems. For this purpose, a variety of water reduction catalysts were paired with libraries of Ir(III) photosensitizers in high-throughput photoreactors. This parallelized approach allowed exploration of the interplay between the diverse photophysical properties of the Ir compounds and the electron-accepting catalysts. Further work enhanced and simplified the critical electron transfer processes between these two species through the use of bridging functional groups installed on the photosensitizer. Later, a novel approach summarized in this Account explores the possibility of using Zn metal as a solar fuel. Structure-activity relationships of the light-driven reduction of Zn to Zn metal are described. DFT calculations along with cyclic voltammetry were utilized to gain clear insights into the complexes' electronic structures responsible for the effective photochemical properties observed in these dyes. While [Ir(ppy)bpy] and its derivatives were found to be much more photostable than the Ru(II)-tris-diimine complex family, mass spectrometry indicated that the bpy ligand still photodissociated under extensive illumination. An interesting new approach involved the substitution o...

Successive modification of polydentate complexes gives access to planar carbon- and nitrogen-based ligands

“…Previous studies have found that the incorporation of nitrogen and carbon atoms into a polydentate ligand framework significantly influences the optical and electronic properties of the resulting complex, highlighting the potential applications of these species in the fields of biomedical and optoelectronic materials 7,10,11,13 . We therefore embarked on photophysical studies on four structurally well-defined polydentate chelates, 2 , 3 , 4 , and 5 .…”

“…As shown in Fig. 1a, complexes with combinations of N and C coordination sites, such as bidentate NC cores 13,14 , tridentate NCN cores 15,16 , and CNC/CCN cores 17,18 , exhibit multiple functions and have had a substantial impact on emerging areas in chemistry. For complexes with higher planar coordination configurations, examples are limited, and the majority of coordinating sites are occupied by nitrogens.…”

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Electron‐donor substituents on the dppz‐based ligands to control luminescence from dark to bright emissive state in Ir(III) complexes

“…Typical cationic cyclometalated Ir(III) complexes have [Ir(C ^ N) 2 (N ^ N)] + as general formula, where C ^ N is the cyclometalating ligand and N ^ N the ancillary ligand . These complexes display a wide range of colors according to the ligand structure and/or the incorporation of electron‐donor or acceptor substituents onto the ancillary or cyclometalating ligands . In this context, the outstanding emission properties of Ir‐iTMCs, have been barely overcome with other metal‐complexes, therefore, the exploration of new Ir‐iTMC follow a challenge to discover new luminescent materials.…”