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).