Spiro Compounds for Organic Light-Emitting Diodes

The development of cutting-edge optoelectronic devices is rapidly changing science and technology. The organic light-emitting diodes (OLEDs), as one of the most applicable techniques of organic electronics, are experiencing the process of materials' upgrade. Among all the organic units to construct OLED materials, the Spiro structure and its derived compounds have been widely studied due to the good thermal stability and unique orthogonally rigid structure. It had shown great potential in first generation fluorescent OLEDs (FOLEDs) and second generation phosphorescent OLEDs (PhOLEDs). Recently, it has achieved outstanding performance in third generation thermally activated delayed fluorescence (TADF) OLEDs, which is attributed to its orthogonal structure caused by the Spiro atom; thus the spiro compound can innately separate the hole and electron which is beneficial for minimizing the singlet-triplet splitting energy (Delta E-ST). The most classic aromatic Spiro unit is 9,9'-spriobifluorene (SBF), which was first reported in 1930. The study of aromatic spiro compounds is closely related to the development of organic semiconductors; now, the structure-property relationship of spiro compounds is getting clearer and attracting wide attention from the material community. In this Account, we briefly summarize the efforts on the functionalization and application of spiro compounds in OLEDs. The functionalization of spiro compounds in our group mainly focuses on the development of electron-rich Spiro backbone and the adjustment of different substitution positions. In this regard, we have systematically studied the electron-rich Spiro backbones which could greatly facilitate the carrier injection/transport and the new derivation positions (C1, C3, and C4 at fluorene moiety) can markedly affect the energy levels of triplet excitons, which is crucial for host materials in OLEDs, especially for the blue ones. The two main methods, i.e., backbone modification and position engineering, can cooperate to construct a rigid scaffold to realize spatial donor/acceptor intramolecular interaction, which confines the donor/acceptor distance. Afterward, the application of spiro compounds in OLEDs is elucidated from FOLEDs to PhOLEDs, then to TADF OLEDs. The spiro[acridine-9,9'-fluorene] (SAF) unit is our first example to enhance the performance of blue and violet fluorescent emitters, which have reached the theoretical upper limit of FOLEDs. For PhOLEDs, the Spiro-type compounds are mainly used as host materials. The substitution positions of the SBF unit are thus thoroughly investigated to afford host materials with high triplet energy. And the SAF unit is modified to form ambipolar host materials to balance hole/electron injection for better white PhOLEDs. During this research, we understood the importance of the orthogonal arrangement in the spiro structure for ambipolar material design. Finally, TADF materials based on the spiro compounds are also presented. The spiro materials used in TADF are usually performed as an individual donor or acceptor before our contribution. Our tendency is to utilize the through-space charge transfer, i.e., charge transfer occurring between the spatial-close donor and acceptor, to construct efficient TADF materials, which paves a new avenue for the future study of spiro compounds.