Manipulation of Phosphorescence Efficiency of Cyclometalated Iridium Complexes by Substituted o‐Carboranes

Abstract

A series of [(C^N)2 Ir(acac)] complexes [{5-(2-R-CB)ppy}2 Ir(acac)] (3 a-3 g; acac=acetylacetonate, CB=o-carboran-1-yl, ppy=2-phenylpyridine; R=H (3 a), Me (3 b), iPr (3 c), iBu (3 d), Ph (3 e), CF3 C6 H4 (3 f), C6 F5 (3 g)) with various 2-R-substituted o-carboranes at the 5-position in the phenyl ring of the ppy ligand were prepared. X-ray diffraction studies revealed that the carboranyl CC bond length increases with increasing steric and electron-withdrawing effects from the 2-R substituents. Although the absorption and emission wavelengths of the complexes are almost invariant to the change of 2-R group, the phosphorescence quantum efficiency varies from highly emissive (ΦPL ≈0.80 for R=H, alkyl) to poorly emissive (R=aryl) depending on the 2-R group and the polarity of the medium. Theoretical studies suggest that 1) the almost nonemissive nature of the 2-aryl-substituted complexes is mainly attributable to the large contribution to the LUMO in the S1 excited state from an o-carborane unit and 2) the variation in the CC bond length between the S0 and T1 state structures increases with increasing steric (2-alkyl) and electronic effects (2-aryl) of the 2-R substituent and the polarity of the solvent. The solution-processed electroluminescence (EL) devices that incorporated 3 b and 3 d as emitters displayed higher performance than the device based on the parent [(ppy)2 Ir(acac)] complex. Along with the high phosphorescence efficiency, the bulkiness of the 2-R-o-carborane unit is shown to play an important role in improving device performance.

Solid‐State Thermochromic Luminescence through Twisted Intramolecular Charge Transfer and Excimer Formation of a Carborane−Pyrene Dyad with an Ethynyl Spacer

“…Boron cluster compounds have recently attracted significant attention as a key component in the development of stimuli‐responsive luminescent solid films, which hold great promise as versatile sensors and probes . To obtain clear responses, such materials need to show dramatic luminochromism that is triggered by the target stimuli.…”

A Convenient Approach To Synthesize o‐Carborane‐Functionalized Phosphorescent Iridium(III) Complexes for Endocellular Hypoxia Imaging

“…According to cyclic voltammetry studies, energy gaps in all the o ‐carborane‐incorporated complexes have been increased, in accordance with the calculated results. This suggests that modification with o ‐carborane can increase the oxidation potentials and lower the HOMO energy levels of iridium(III) complexes, owing to stabilization of the HOMO levels by the inductive electron‐withdrawing effect ,…”

Nido‐Carboranes: Donors for Thermally Activated Delayed Fluorescence

“…Icosahedral carboranes (dicarba‐ closo ‐dodecaboranes, closo ‐carboranes) are well‐known boron clusters that possess unique properties such as steric bulkiness, three‐dimensional σ‐aromaticity, and high thermal and chemical stability . Owing to these properties, closo ‐carboranes have been widely explored as steric and electronic auxiliaries to organic and organometallic luminophores, which can serve as efficient emitting materials for optoelectronic applications such as organic light‐emitting diodes (OLEDs) ,,,,. In contrast, nido ‐carboranes (dicarba‐ nido ‐undecaboranes), which are the anionic, open‐cage analogues of closo ‐carboranes, have attracted less attention in the field of luminescent materials although they are important members of the carborane family.…”