Phosphorescence versus Thermally Activated Delayed Fluorescence. Controlling Singlet鈥揟riplet Splitting in Brightly Emitting and Sublimable Cu(I) Compounds

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• 作者：Markus J. Leitl ; Valentina A. Krylova ; Peter I. Djurovich ; Mark E. Thompson ; Hartmut Yersin
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：November 12, 2014
• 年：2014
• 卷：136
• 期：45
• 页码：16032-16038
• 全文大小：457K
• ISSN：1520-5126

文摘

Photophysical properties of two highly emissive three-coordinate Cu(I) complexes, (IPr)Cu(py₂-BMe₂) (1) and (Bzl-3,5Me)Cu(py₂-BMe₂) (2), with two different N-heterocyclic (NHC) ligands were investigated in detail (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; Bzl-3,5Me = 1,3-bis(3,5-dimethylphenyl)-1H-benzo[d]imidazol-2-ylidene; py₂-BMe₂ = di(2-pyridyl)dimethylborate). The compounds exhibit remarkably high emission quantum yields of more than 70% in the powder phase. Despite similar chemical structures of both complexes, only compound 1 exhibits thermally activated delayed blue fluorescence (TADF), whereas compound 2 shows a pure, yellow phosphorescence. This behavior is related to the torsion angles between the two ligands. Changing this angle has a huge impact on the energy splitting between the first excited singlet state S₁ and triplet state T₁ and therefore on the TADF properties. In addition, it was found that, in both compounds, spin鈥搊rbit coupling (SOC) is particularly effective compared to other Cu(I) complexes. This is reflected in short emission decay times of the triplet states of only 34 渭s (1) and 21 渭s (2), respectively, as well as in the zero-field splittings of the triplet states amounting to 4 cm^鈥? (0.5 meV) for 1 and 5 cm^鈥? (0.6 meV) for 2. Accordingly, at ambient temperature, compound 1 exhibits two radiative decay paths which are thermally equilibrated: one via the S₁ state as TADF path (62%) and one via the T₁ state as phosphorescence path (38%). Thus, if this material is applied in an organic light-emitting diode, the generated excitons are harvested mainly in the singlet state, but to a significant portion also in the triplet state. This novel mechanism based on two separate radiative decay paths reduces the overall emission decay time distinctly.