Anything glows

Cool light © PhotoDisc

What do the following have in common - paper, clothes, budgie feathers, banknotes, biological cells, grass, petrol, computer screens, and a gin and tonic? A clue - have you ever noticed your clothes glowing blue under bright disco lights, or the bluish-yellow colour of petrol as it is poured into a car's petrol tank? What you are seeing is luminescence, in particular fluorescence. And all the materials listed, and myriad more, contain luminescent compounds. So what is luminescence and how useful is it?   

Getting excited? Just relax .   

Essentially luminescence is cold light because, unlike an incandescent light bulb, no heat is involved. When ultraviolet-visible light falls on certain molecules, their 'electronic configuration' changes, ie the orientation of the electrons holding the molecules together changes. The electrons, with this input of energy, are said to be 'excited' and unstable. The natural thing for them to do is to relax and lose this excess energy. If they are relatively rigid molecules, they can't wriggle or vibrate and loose this energy as heat to the surroundings. Instead they create a photon - a small amount (quantum) of light, and they glow. Depending on how long the excited state takes to relax, defines two types of luminescence: if it happens very quickly, typically within 10ns (10-8s), this is fluorescence; if it is slower, over a period of seconds, this is phosphorescence.   

Molecules that luminesce are chromophores - the 'colour bringers'. They tend to have alternating double and single bonds, ie they are conjugated, the electrons are 'delocalised' and thus able to move freely. Examples include anthracene (1) and chlorophyll, the green pigment in grass.    

Here's looking at luminescence   

To study luminescence requires a light source to excite the sample and a detector; in between sits a coloured glass filter, which allows the chemist to select the appropriate wavelength of light from the source. Together these elements form the basis of a spectrometer (though these contain a prism in place of the glass filter). The detector is attached to a computer which collects the emission data and allows chemists to produce various spectra - graphs of intensity versus wavelength, intensity versus time etc.

Luminescence imaging at Durham University

Photochemist, Dr Andrew Beeby, at the University of Durham, studies luminescent materials. Beeby and his group use one of three light sources:   

• a laser - these are expensive, but they produce very short (10-12-10-9s) pulses of intense light; 
• a xenon flash gun - similar to those found in a camera flash gun, these are cheap and emit light of all wavelengths;   
• light emitting diodes (LEDs) - these cost just a couple of pence each, the most recent ones produce light in the uv-visible region of the electromagnetic spectrum.      

Light is detected using either a photon multiplier tube, a photodiode or a CCD (charge coupled device) camera. The latter comprises a small grid or an array of photosensitive elements (pixels) on a silicon chip. Each pixel measures the intensity of light falling on it.   

Shining examples   

As well as providing information on electronic and molecular structure and motion, luminescent materials are used in a variety of applications. 

Luminescence spectroscopy catches out slick operators © PhotDisc

• Identifying compounds. Different chromophores absorb light differently, and a record of their emission spectra provides chemists with a characteristic fingerprint of such molecules and thus a way to identify them. This is exploited in security labels on banknotes. Most banknotes now contain a rich signature of luminescent dyes in specific places, as can be revealed by shining a uv light on them. Luminescence spectroscopy is used to verify the authenticity of such notes. In another example, chemists have obtained 'fingerprints' of crude oil from ships that empty their tanks at sea, with the owners denying all knowledge of the oil slick left behind. The oil contains polycyclic aromatic hydrocarbons, eg anthracene (1), which fluoresce. By matching the spectra from the oils from a ship and the slick, chemists have played their part in catching the offenders.   
• In molecular probes. Some groups, notably Professor David Parker's at Durham University, are making phosphorescent probes, based on lathanide complexes, to investigate cellular processes. When bound to ions such as HCO3-, the luminescent spectra of these molecules changes, allowing chemists to monitor these ions and gain an insight into their metabolism inside the cell. Other groups use fluorescent dyes to monitor metal ions such as Ca2+ and Zn2+, and DNA to develop our understanding of cellular processes. 
• In electroluminescent displays. The next generation light emitting displays could be based on electroluminescent materials, replacing cathode ray tubes and even liquid crystal displays (LCDs). In these devices, an electric current is passed through a very thin layer of material, eg the organometallic polymer Ir(ppy)3 (2). The electricity excites the bonding electrons in the metal and as they relax, the material glows. In comparison with LCDs, these displays are brighter, cheaper, have a better viewing angle, and importantly, don't have to be mounted on a glass substrate. Electroluminecent displays could, therefore, be put onto plastic, which would give a new dimension to display technology. It might be possible, for example, to print a moving display on a disposable carrier bag. Professor Richard Friend and his team at Cambridge University are developing this technology while Beeby's team are synthesising new electroluminescent compounds.   
• In sunscreens and medicine. Recent research by Beeby, working in collaboration with Brian Diffey, a leading expert in skin protection from uv light, has pointed to fluorescent sunscreens as a way of investigating how efficiently and evenly people put suncream on. This is particularly important for people suffering from solar urticaria, a rare allergic reaction to sunlight, who have to wear sunscreen on exposed parts of their body when they go out. Other research groups, notably Professor David Philips at Imperial College, London have developed fluorescent dyes for use in photodynamic therapy (PDT) for cancer treatment. The dye is taken up selectively by the cancer, and red light is shone on the site. The excited dye molecule gives up its excess energy to nearby oxygen molecules, which become energised. This form of oxygen (known as singlet oxygen) is toxic to nearby cells. However its lifetime is short so that PDT only occurs where the light shines, making it a precise technique.   
• In molecular wires and switches. Beeby's group is also making luminescent compounds that could be used as potential molecular wires and switches. In particular they are making oligomers (small parts of a polymer) of arylethynylenes (3) - ie alternating phenyl rings and ethynes.        

Other chemists have attached sulphur groups to each end, which stick to gold. In principle this highly conjugated molecule can bridge the gap between two gold electrodes, resulting in an organic conductor. By changing the conformation of the oligomer, eg by attaching bulky CH3 or C2H5 groups to neighbouring aryl groups, which get in the way of each other, Beeby's group has been able to control the conformation. This should allow the oligomer to be used as a molecular switch or transistor - the smallest electronic component you can get. Kathryn Roberts