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Graphical Abstract
Abstract
Research into phosphorescent Ir(III) complexes has grown immensely since their first report. Talented chemists have successfully synthesized complexes capable of emitting from the ultraviolet to the near-infrared regions of the electromagnetic spectrum. Tuning the emission energy in a selective manner requires knowledge of how ligand substitution affects not only energy levels such as the highest occupied molecular orbital and lowest unoccupied molecular orbital, but also the emissive triplet energy level. This review describes fundamental principles involved in energy-level engineering and substituent selection as well as fundamental methods of device characterization.
FUNDING
This work was supported by the European Commission H2020-ICT-2014-1, SOLEDLIGHT project, grant agreement N°: 643791, and the Swiss State Secretariat for Education, Research, and Innovation (SERI).
NOMENCLATURE
| Alq3 | = | tris(8-hydroxyquinolinato)aluminum |
| Acac | = | acetylacetonate |
| Bpy | = | 2,2’-bipyridine |
| BQ | = | benzoquinone |
| CIE | = | commission internationale de l’éclairage |
| CV | = | cyclic voltammetry |
| DCM | = | dichloromethane |
| Dfppy | = | 2-(2’,4’-difluorophenyl)pyridine |
| DMF | = | N,N-dimethylformamide |
| DMSO | = | dimethylsulfoxide |
| e− | = | electron |
| E(S+/S) | = | ground-state energy level |
| E(S/S-) | = | reduced-state energy level |
| EQE | = | external quantum efficiency |
| fac | = | facial isomer |
| FIrPic | = | bis(2-(2’,6’-difluorophenyl)pyridine)iridium (III) picolinate |
| FMO | = | frontier molecular orbital |
| h+ | = | hole |
| IR | = | infrared |
| ISC | = | intersystem crossing |
| J | = | current |
| kr | = | radiative rate |
| knr | = | non-radiative rate |
| K | = | kelvin |
| λmax | = | wavelength of maximum emission |
| L | = | luminance |
| LC | = | ligand-centered |
| LFER | = | linear free energy relationship |
| μs | = | microsecond |
| ms | = | millisecond |
| MeCN | = | acetonitrile |
| MLCT | = | metal-to-ligand-charge-transfer |
| MO | = | molecular orbital |
| ns | = | nanosecond |
| NHC | = | N-heterocyclic carbene |
| NIR | = | near infrared |
| NUV | = | near ultraviolet |
| OEP | = | octaethylporphine |
| OLED | = | organic light emitting diodes |
| PE | = | power efficiency |
| PhIm | = | 1-methyl-3-phenyl-imidazol-2-ylidene |
| Pic | = | 2-picolinate |
| PL | = | photoluminescence |
| PLQY | = | photoluminescence quantum yield |
| ΦPL | = | photoluminescence quantum yield |
| PMMA | = | poly(methylmethacrylate) |
| ppy | = | phenylpyridine |
| PyrIm | = | 1-methyl-3-(pyridin-2-yl)-imidazol-2-ylidene |
| Qquin | = | 2-quinoxalylquinoline |
| rt | = | room temperature |
| SFR | = | structure function relationship |
| SOC | = | spin orbit coupling |
| T | = | temperature |
| THF | = | tetrahydrofuran |
| TM | = | transition metal |
| Tol | = | toluene |
| TPA | = | triplet-polaron annihilation |
| TTA | = | triplet-triplet annihilation |
| UV | = | ultraviolet |
| V | = | voltage |
| WOLED | = | white organic light emitting diodes |
| ZFS | = | zero field splitting |