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Final Activity Report Summary - POLYPHONIC (Poly-nuclear luminescent complexes for photonics applications)

Luminescence (emission of light) from lanthanide (Ln) ions is an area of immense technological significance because of possible applications in lighting and display devices, fibre-optic based telecommunications systems, and medical diagnosis and imaging. Lanthanide-based luminescence is generally long-lived and line-like (at a specific narrow wavelength) and therefore easy to detect and measure.

Stimulating this luminescence however is more difficult, because it requires that the Ln ion is an energetic excited-state to begin with, such that it has excess energy available to emit as light. Often such excited states are generation by absorption of light, and there are many compounds and materials that absorb light at one wavelength, store the energy in the form of an electronically excited state, and then re-emit light at a longer wavelength when the excited state collapses. However Ln ions are very poor at absorbing light (they are essentially colourless) which makes it harder to get them into their excited states to start with.

There are many ways around this, such as the use of electric fields (in 'OLED' display devices). The aim of this project was to make materials in which lanthanide ions are combined with transition metal ions that absorb light strongly and consequently enter relatively long-lived excited states themselves. If these transition metal units are chosen correctly, they will be able to transfer their excited-state energy to nearby lanthanide ions rather than use it themselves to emit light or heat. This 'energy-transfer' allows the transition metal fragments (often intensely coloured) to absorb light, then pass the resultant energy on to the Ln ions which can use it to generate luminescence.

We have approached this problem by combining transition metal species such as [M(diimine)(CN)4]2- (M = Ruthenium, Osmium; diimine = an organic molecule bound to the metal centre) with Ln ions (which have a charge of +3). The two components combine partly because of their opposite charges, and partly because the cyanide groups of the [M(diimine)(CN)4]2- units have externally-directed N atoms which bind well to the lanthanide cations. In addition, these transition-metal species are known to absorb light strongly and to have long-lived excited states, which makes it easy for them to pass their energy on to other species.

Combination of the [M(diimine)(CN)4]2- negative ions and Ln(+3) ions results in formation cyanide-bridged solid-state crystalline materials containing cross-linked structures based on M-CN-Ln bridges. The structures can be complicated, because each [M(diimine)(CN)4]2- unit has four cyanide groups radiating ouwards, and each Ln cation can bind several such cyanides from different [M(diimine)(CN)4]2- fragments. Many examples of such network materials have been structurally characterised by X-ray crystallography.

The function of the cyanide bridges is twofold. Firstly, they propagate the structure by acting as bridges linking the two types of metal centre. Secondly, they facilitate the essential energy-transfer process, by providing a short connection through which, following absorption of light by the transition metal centre, the energy can be transferred to the lanthanide centre. This allows the all-important Ln-based excited state to form, followed by the desired luminescence.

Photophysical analysis of these compounds showed that in nearly every case laser irradiation with visible light (which is absorbed selectively by the transition metal fragment) results in transfer of the energy to, and subsequent luminescence from, the Ln centres (especially Yb and Nd, which have characteristic long-wavelength luminescence; 980 nm for Yb, 880 and 1060 nm for Nd). Thus the compounds, in addition to having unusual types of new structure, also show the desired property of allowing a transition metal ion to 'sensitise' luminescence from a nearby Ln ion.

Reported by

University of Sheffield
Western Bank
S10 2TN Sheffield
United Kingdom
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