Yang Jian-Gong, Song Xiu-Fang, Cheng Gang, Wu Siping, Feng Xingyu, Cui Ganglong, To Wai-Pong, Chang Xiaoyong, Chen Yong, Che Chi-Ming, Yang Chuluo, Li Kai
College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, People's Republic of China.
College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, People's Republic of China.
ACS Appl Mater Interfaces. 2022 Mar 23;14(11):13539-13549. doi: 10.1021/acsami.2c01776. Epub 2022 Mar 14.
Carbene-Au-amide (CMA) type complexes, in which the amide and carbene ligands act as an electron donor (D) and acceptor (A), respectively, can exhibit strong delayed fluorescence (DF) from a ligand to ligand charge transfer (LLCT) excited state. Although the coplanar donor-acceptor (D-A) conformation has been suggested to be a crucial factor favoring radiative decay of the charge-transfer excited state, the geometric structural factor underpinning the excited-state mechanism of CMA complexes remains an open question. We herein develop a new class of carbene-Au-carbazolate complexes by introducing large aromatic substituents onto the carbazolate ligand, the presence of which are conceived to restrict the rotation of the Au-N bond and thus confine a twisted D-A conformation in both ground and excited states. A highly twisted D-A orientation is found for the complexes in their crystal structures. Photophysical studies reveal that the twisted conformation induces a decrease in the gap (Δ) between the lowest singlet excited state (S) and the triplet manifold (T) and thus a faster reverse intersystem crossing (RISC) from T to S at the expense of oscillator strength for an S radiative transition. In comparison with the coplanar analogue, the twisted complexes exhibit comparable or improved DF with quantum yields of up to 94% and short emission lifetimes down to sub-microseconds. The tuning of excited-state dynamics has been well interpreted by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, which unveil much faster RISC rates for twisted complexes. Solution-processed organic light-emitting diodes (OLEDs) based on the new CMA complexes show promising performances with almost negligible efficiency rolloff at a brightness of 1000 cd m. This work implies that neither a coplanar ground-state D-A conformation nor a dynamic rotation of the M-N bond is the key to the realization of efficient DF for CMA complexes.
在卡宾-金-酰胺（CMA）型配合物中，酰胺和卡宾配体分别作为电子供体（D）和受体（A），能够展现出从配体到配体电荷转移（LLCT）激发态的强烈延迟荧光（DF）。尽管共平面供体-受体（D-A）构象被认为是有利于电荷转移激发态辐射衰变的关键因素，但支撑CMA配合物激发态机制的几何结构因素仍是一个悬而未决的问题。我们在此通过在咔唑配体上引入大的芳基取代基，开发了一类新型的卡宾-金-咔唑配合物，设想其存在会限制金-氮键的旋转，从而在基态和激发态均限制扭曲的D-A构象。在其晶体结构中发现这些配合物具有高度扭曲的D-A取向。光物理研究表明，扭曲构象导致最低单重激发态（S）和三重态（T）之间的能隙（Δ）减小，从而使从T到S的反向系间窜越（RISC）更快，代价是S辐射跃迁的振子强度降低。与共平面类似物相比，扭曲的配合物表现出相当或更高的DF，量子产率高达94%，发射寿命短至亚微秒。激发态动力学的调控已通过密度泛函理论（DFT）和含时密度泛函理论（TDDFT）计算得到很好的解释，这些计算揭示了扭曲配合物的RISC速率要快得多。基于新型CMA配合物的溶液加工有机发光二极管（OLED）表现出有前景的性能，在1000 cd m的亮度下效率滚降几乎可以忽略不计。这项工作表明，对于CMA配合物，实现高效DF的关键既不是共平面的基态D-A构象，也不是M-N键的动态旋转。
