2.5 Organic Light-Emitting Diodes (OLEDs)
Generally, nonconventional luminescent materials possess relatively low QY due to the absence of large conjugations. Therefore, they are considered inapplicable in OLEDs, which requires high PL performance. In recent years, nonconventional luminophores come into sight in OLEDs. Cheng et al. fabricated a three-layered OLED device with improved fluorescence purity and QY from 3,6-dipyrenylcarbazole-POSS hybrids (POSS-DPCz).[75] The POSS nanoparticles serve as a compatibilizer to facilitate the formation of 3D structures, and suppressed the aggregation and enhanced the color stability of the chromophore. Specifically, the three-layered OLED device exhibits a luminous efficiency of 1.4 cd/A with the maximum brightness of 8900 cd/m2 at 450 nm. Zhang et al. designed a white LED basing on a linear siloxane containing poly(hydroxyurethane). The flexible Si-O-Si segment benefits the aggregation of hydroxyurethane chromophores, leading to a high luminance of 8222 cd/m2.[1] The above OLED devices facilitates the realization of polysiloxane lighting devices with enhanced luminescence efficiency.
Conclusion
In summary, this review summarizes recent progress in nonconventional luminescent polysiloxanes without classic conjugated structures. The polysiloxanes generally consisted of Si-O-Si or Si-O-C chain segments with other electron rich auxochromic groups, such as -OH, -NH2, C=O, ether, etc. The unexpected luminescence behavior is related to the aggregation caused by the flexible Si-O bonds and the electron delocalization generated by electron-rich atoms. With the decreased molecular distance in the polysiloxane aggregates, the electron cloud of electron-rich groups overlaps with each other and generated electron delocalization, lowing the excitation energy and rigidifying the molecular conformation, thus favoring the PL process. The empty 3d orbitals of Si atoms can generate coordination bonds like N → Si and O → Si in polysiloxanes, altering the electron delocalization of the material and resulting in external emissions. For HBPSis that contains Si-O-C chain segment, their unique bond angle renders HBPSis good flexibility and rigidity simultaneously, leading to enhanced luminescence, such as enhanced QY and multicolour fluorescence with high purity.
The outstanding photophysical properties of polysiloxanes have attracted considerable attention in recent years, and remarkable progress have been made in variety enrichment, mechanism exploration, and application extension. However, nonconventional polysiloxanes with AIE behaviors still have several drawbacks, such as low QY, short fluorescent lifetime and indistinct PL mechanism. Thereby, the further developments of polysiloxanes may include several aspects. 1) Developing polysiloxanes with good water-solubility, high QY and infrared luminescence plays an important role in expand their applications. 2) Reveal the emission mechanism of polysiloxanes, especially polysiloxanes with Si-O-C chain segment, are critical to the design of multicolor luminescent nonconventional materials. So far, the emission mechanism of polysiloxanes is still under debate. Some believes that their luminescence comes from the electron rearrangement in the split 3d orbitals of Si atoms by N → Si and O → Si coordination bonds. While others suggest that the TSC generated from Si-O bonds and other auxochromic groups leads to the PL emissions. Emphasizing the role of Si atoms in CTE mechanism may provide a new way to understand their emission behaviors. 3) To control the topologies of polysiloxanes precisely, exploring new synthesis methods is of central importance. Besides the frequently-used methods, such as hydrosilylation, hydrolysis polycondensation and transesterification polycondensation reaction, atom transfer radical polymerization (ATRP) and proton transfer polymerization (PTP) can also help to obtain nonconventional polysiloxanes. We believe the understanding of polysiloxanes would benefit in unveiling the emission mechanism of nonconventional luminophores, and expanding the application of nonconventional luminescent materials.
Acknowledgements
This work was financially supported by the Guangdong Basic and Applied Basic Research Foundation (Program No. 2020A1515110540), the Key Research and Development Program of Shaanxi (Program No. 2022SF-599), and the National Natural Science Foundation of China (Program No. 22175143).