When a molecule absorbs light, this often leads to a movement of the electrons in the molecule to form what is known as an excited state. Ligands are also very important in determining how a metal complex will interact with light. For example, for the metal complexes used as drugs for cancer treatment, if the ligands are too large and bulky, the metal complex will not be able to pass through various channels in the body to reach the site where treatment is required. The properties and behaviour of a metal complex are not just dictated by the type of metal at the centre, but also very much by the type of ligands attached. Ligands are like tree branches stemming from the central metal core and there are a multitude of types of ligands, differing in their size, shape and chemical composition.
IRIDIUM METALLIC COLOR SERIES
The original uploader was Tomihahndorf at German Wikipedia./CC BY-SAĪ metal complex is a molecule that contains one or more metal atoms, surrounded by a series of ligands attached to the metal core. Various luminescent molecules are based on iridium. Prof Aoki and his group have been designing and synthesising new Ir-based compounds for a variety of application, including as chemical pH sensors and anticancer agents. By tuning the chemical structure of such Ir-containing metal complexes, it is also possible to tune their emission colours and properties, something which Prof Shin Aoki at the Tokyo University of Science is an expert in. Ir-containing molecules have proved very popular for this application for their high luminescence yields, long emission lifetimes and stability with respect to chemical and light-induced degradation. The properties and behaviour of a metal complex are not just dictated by the type of metal the centre, but also very much by the type of ligands attached. This allows for better contrast and generally higher image qualities. OLED devices have rapidly become a popular alternative to liquid crystal display (LCD) screens as they do not require backlighting and pixels in the screen can be switched completely off to give a true black, rather than just a region where the backlight is dimmed. This has largely been driven by the development of more efficient organic light emitting diode (OLED) devices for use in electronic displays. While humans have found many ways to make use of luminescence, from using glow sticks as emergency lighting to security ink in bank notes, there has been a recent surge of interest in luminescent molecules, particularly those based on iridium (Ir). Glowing jellyfish: deep in the ocean, luminescence is generated as a result of a chemical reaction. Chemiluminescence is the name for light emitted as a result of a chemical reaction, and is what fireflies use to generate the characteristic lights seen on their tails. Not all luminescence has to be triggered by absorption of light either.
The absorbed and emitted light do not have to be of the same wavelength, or colour, so it is possible to design substances that will absorb visible light coming from the sun, but emit a specific colour such as red. Usually luminescence occurs when a substance has absorbed a small packet of light energy, known as a photon, and later reemits that energy in the form of light. Luminescence occurs when a substance emits light without having been heated beforehand.
The power of luminescence is also harnessed in many of our electronic devices, from high-quality television displays to ‘flexible’ electronic displays that may be the basis of wearable electronic clothes and electronic paper in the future.
It is responsible for the distinctive glow of fireflies and the eery glow and fantastic colours of many deep-sea creatures. Luminescence is a phenomenon found everywhere, from the deepest depths of the ocean to outer space. Professor Shin Aoki at the Tokyo University of Science has been designing new Ir-based complexes for not just the aforementioned applications, but cancer treatments as well.
This has made them widely used as molecules for bioimaging probes and organic light emitting diodes (OLEDs) that can now be found in most display devices, from phones to computer screens. Such complexes have unusual and interesting interactions with light, with many complexes exhibiting strong luminescence on long timescales. A rare metal found in the Earth’s crust, Ir can bond with a variety of other elements to make different metal complexes.
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