Phosphorescent Ir(III) complexes attract enormous attention because they allow highly efficient electrophosphorescence. In pursuing the development of Ir(III) complexes during the last decade, significant progress has been made in terms of the colour-tunability, thermal- and photo-stability, phase homogeneity, and phosphorescence efficiency. By far, extensive synthetic efforts have been focused on the molecular design of ligands to achieve a wide range of phosphorescence colour that is compatible with organic light-emitting device (OLED) applications. In contrast, less has been known about a collective structure–property relationship for phosphorescence quantum efficiency. In fact, a few rule-of-thumbs for high phosphorescence quantum efficiency have been occasionally reported, but a collective rationale is yet to be investigated. In this article, we provide a comprehensive review of 8 different methods reported so far to achieve high phosphorescence quantum efficiency from Ir(III) complexes. The methods included herein are limited to the cases of intramolecular controls, and thus are discussed in terms of variations in ligand structures: (1) geometric isomer control, (2) rigid structure and restricted intramolecular motion, (3) larger mixing of ¹MLCT and ³LC states, (4) de-stabilizing a thermally accessible non-emissive state, (5) introducing dendrimer structures, (6) control in substituents of ligands, (7) confining the phosphorescent region of a mixed ligand Ir(III) complex and (8) sensitized phosphorescence by using attached energy donors. Each method is closely related to intramolecular excited state interactions, which strongly affect radiative or non-radiative transitions. A comprehensive understanding of these methods leads us to conclude that the modulation in ligand structures has a profound effect on both the phosphorescence colour and phosphorescence quantum efficiency. Thus, the judicious selection of ligand structures and their chelate disposition should be considered before synthesis. We expect that the guidelines for attaining a high phosphorescence efficiency, summarized in this Perspective, would be helpful in developing highly phosphorescent Ir(III) complexes.

The last few years have seen a steady increase in small molecule based conjugated materials, which promise innovative (opto)electronic applications. This requires however a systematic understanding of structure–property relationships, which can only be achieved via libraries of structurally well-defined single crystalline materials based on systematically designed molecular structures. In this feature article, we are presenting structure–property relationships of functionalized distyrylbenzene (DSB), which is one of the most extensively investigated π-conjugated molecular materials. This will provide a general insight into the specific implications of intermolecular arrangements on their solid state optoelectronic properties, discussing H- vs. J-aggregation, herringbone vs. π-stacks, the occurrence of excimers, size effects, and polycrystallinity. The systematic insight into DSB functionalization will then suggest pathways towards targeted molecular design strategies, with special focus on the cyano-vinylene motif (DCS materials) which allows for highly fluorescence solid state samples due to synergetic packing effects promoted by its twist elasticity and secondary bonding interaction. Finally, recent advances in the application of DSB-/DCS-based materials are shortly reviewed.