Scientists have explored the application of coordination complexes in different materials. One of the most important classes of materials is optoelectronic materials. Optoelectronic materials are subordinate to molecular electronics and photonics. The well-documented inherent limitations of current semiconductor-based electronic devices in terms of both speed and scale of operation are the principal motivation behind much molecular electronics and photonics research. Among many different types of optoelectronic compounds (e.g., organic compounds, quantum dots, semiconductor nanomaterials), coordination complexes are very promising candidates for development of molecular optoelectronic materials and devices due to their diverse structural, electronic, and optical properties, such as a narrow-band luminescence with a high quantum yield and structurally variable ligands.
- Directing molecular coordination
The great versatility and directionality offered by metal-ligand coordinative bonding allow for the precise control over the shape and stereochemistry of the assemblies. With multidentate ligands, coordination complexes with high porosity and chemical stability can be realized. This is essential for reasonable control of the optoelectronic properties of the resulting coordination complexes.
- Enhancing radiative transition
For efficient optoelectronic devices, molecules with high radiative rates are generally selected as the emitters. The complexation of metal ions by conjugated organic ligands can improve structural rigidity and sustain many excitation-emission cycles. Therefore, these coordination complexes can be applied in optoelectronic fields by enhancing the radiative conversion.
- Improving charge transport characteristics
Charge-transfer properties are the key attributes that optoelectronic materials should possess. Interestingly, the metal ion centers of the coordination complexes have delocalized valence electrons and different oxidation states, which are favorable for the injection and transport of charge carriers.
Coordination complexes, combining easy synthesis and tunable optical properties, have attracted a lot of attention over the past decades owing to their great potential for applications in optoelectronic materials. Figure 1 shows important milestones in the development of coordination complexes for optoelectronic applications. Discovery of the first organic semiconductor copper phthalocyanine (CuPc) can be dated back to 1948 . In the late 1980s, Tang and Van Slyke at Kodak fabricated the first thin film light-emitting diode using tris(8-quinolinolato) aluminum (III) (Alq3) as the emitting layer . Research into organic light-emitting diodes culminated in 1998 with the work of Forrest and co-workers, who reported the discovery of organic light-emitting devices (OLEDs) employing phosphorescent coordination complexes . These pioneering studies have laid the groundwork for our understanding of the relationship between the chemical structure of coordination complexes and their optical properties. In addition, these studies have driven the rapid development of optoelectronic devices.
Figure 1. Milestones in the development of optoelectronic complexes
Alfa Chemistry discusses the application of coordination complexes as optoelectronic materials from the following five aspects:
- Eley, D. D. Phthalocyanines as Semiconductors. Nature. 1948, 162: 819.
- Tang, C. W.; et al. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51: 913-915.
- Forrest, S. R.; et al. Highly efficient phosphorescent emission from organic electroluminescent devices. Nature. 1998, 395: 151-154.