Organic, Organometallic, and Metallic Luminogens
by Ben Zhong Tang
August 15, 2016
Organic materials and metallic substances have distinct characteristics. For example, organic materials are readily processible, but metals are more durable and last longer. Materials based on organometallic compounds have some properties of each and can combine the advantages of the two.
Scientists are continuously exploring new luminescent materials, partially because of their potential value in high-tech innovations. Recent reports disclose the development of advanced organic, organometallic, and metallic luminogens. They luminesce efficiently in the solid state, and their emission is easily tuned by applying conventional methods.
A common way to tune the properties of an organic luminogen is to change its molecular structure. Doing so, however, can result in rather complex structures. Can properties be tuned while keeping structures simple? Keiji Morokuma, Gen-ichi Konishi, and colleagues at the Tokyo Institute of Technology, Kyoto University, Kyushu University (Japan), and the Japan Science and Technology Agency (Tokyo) answered in the affirmative by devising an elegant strategy.
The researchers modulated the emission behavior of 9,10-bis(N,N-dialkylamino)anthracenes (1 in Figure 1) via molecular “engineering” without compromising structural simplicity. By placing dialkylamine substituents at the para positions, tuning the twist angles of the substituents, and increasing the flexibility of the alkyl chains, they prepared unique luminogens with such desirable photophysical properties as high susceptibility to their steric environment, large Stokes shifts, and efficient solid-state light emission. (J. Am. Chem. Soc. DOI: 10.1021/jacs.6b03749)
Metal carbonyl (MC) structural units are often found in organometallic complexes. Numerous MC complexes are known; but their luminescence processes have seldom been investigated because they are normally nonluminescent, and they usually function as efficient emission quenchers. Xiaosong Wang and coauthors at the University of Waterloo (ON) and National Chiao Tung University (Hsinchu, Taiwan) cleverly created an MC that contains an organometallic luminogen (2) that can self-assemble into blue-light–emitting MC nanovesicles (MCsomes).
Although 2 is hydrophobic, its molecules can self-assemble in water into MCsomes because H2O–CO interactions stabilize its colloids. In the bilayer membrane structure, the bithiophene groups in 2 are interdigitated within the inner wall, whereas the Fe–CO units are exposed to water. Separating the conjugated bithiophene domain from the Fe–CO units with butanoyl spacers prevents quenching by the metallic iron and makes the MCsomes emissive. (J. Mater. Chem. C DOI: 10.1039/C6TC01222A)
- A Stokes shift is the difference between the wavelengths of the band maxima of the adsorption and emission spectra of a molecule.
- A ditopic ligand is a molecule with multiple binding sites that can coordinate different metals.
- A bathochromic shift is a change in emission to a longer wavelength (lower frequency). Its opposite is called a hypsochromic shift.
- In the WLED context, down-conversion is a process in which white light is generated by using single or multiple phosphors that are excited by a blue or near-UV LED chip.
- CIE is the Commission Internationale de l’Eclairage (International Commission on Illumination), a Vienna-based organization that “advanc[es] knowledge and provid[es] standardization to improve the lighted environment”, according to its Web site.
Small-molecule MCsomes are not the only ones that can be highly luminescent in the aggregated state. Large metal coordination polymers (MCPs) are useful as well. Alessandra Forni, Chiara Botta, Nicolas Mercier, and coauthors at the University of Angers (France), the Institute for the Study of Macromolecules (Milan), the University of Milan, and the University of Milan-Bicocca prepared MCPs that are based on bismuth(III) and the simple ditopic ligand 4,4’-bipyridine-N-oxide (3). Whereas the MCPs are nonemissive in solution, their powders luminesce in quantum yields as high as 85% with long lifetimes (up to 18 ms). When the powders are ground further, their phosphorescence is bathochromically shifted. Heating or exposure to water vapor recovers the initial light emission. (Angew. Chem., Int. Ed. DOI: 10.1002/anie.201602602)
Metallic phosphors that are based on rare-earth elements (REEs) are the key components in down-conversion, white-light–emitting diodes (WLEDs). Recovering REEs from the Earth is often environmentally destructive, and growing demand is creating a serious supply shortage... These circumstances have triggered an ongoing search for REE-free, inexpensive phosphors for WLEDs. Copper is an Earth-abundant element, but very little has been done to develop WLEDs based on copper nanoclusters (CuNCs).
Andrey L. Rogach and colleagues at the City University of Hong Kong and the Beijing Institute of Technology fashioned an WLED that uses only CuNCs as phosphors. They synthesized orange- and blue-light–emitting CuNCs by simple procedures at low cost.
The researchers prepared orange CuNCs by using glutathione as the Cu2+ reductant and stabilizer, followed by ethanol-induced aggregation. The blue NCs were made by reducing poly(vinylpyrrolidone)-supported Cu2+ with ascorbic acid, followed by surface treatment with sodium citrate. Combining the orange and blue CuNCs on a commercial gallium nitride LED chip resulted in a CuNC-based WLED that emits white light with a CIE color coordinate of (0.36, 0.31). (Adv. Sci. DOI: 10.1002/advs.201600182)
In the search for new luminogens with desirable properties, the final products often have complex structures and cost more than simpler materials. The examples given here run in the opposite direction: The luminogens are simple organic or organometallic compounds or metallic clusters with Earth-abundant, inexpensive elements. This is the right way to go—and researchers in the field hope that even simpler, less costly luminogens with better performance will be developed through intelligent molecular engineering.