Chapter 16: Luminescent Metal Phosphonate Materials Check Access

This chapter surveys the synthesis and characterization of metal phosphonates that display luminescent properties. The first part of the chapter concentrates on giving a quick overview of the photophysical processes which give rise to luminescence and continues to consider materials based upon lanthanide metals. Whilst the luminescent properties of the lanthanides are unique among the elements, they are at the same time almost entirely predictable. The penetration of the 4f electrons means that they play no part in bonding, and hence they behave in core-like manner. As such the electronic energy levels of the lanthanide (Ln³⁺) ions are largely independent of the nature of the co-ordination environment and the transitions which give rise to luminescent processes occur at well-defined wavelengths. It is, however, possible to alter the colour of the light emitted by changing the local symmetry of the Ln³⁺ ion, and changing the response of these materials becomes more a question of crystal engineering rather than a simple consideration of chemistry alone.

The second part of the chapter is devoted to examples of phosphonates of divalent and monovalent transition metals, which are perhaps of more general interest. Whilst the emissions observed for these ions are not as narrow and well defined as those of the lanthanides, there is an enormous variety in the colours emitted. Zinc phosphonates, for example, can be made to give a yellow photoluminescent (PL) response rather than a green one, by changing the nature of the phosphonate anion.

In addition to the changes in response that can be derived from changing the phosphonate anion, there exists the possibility of doping PL active materials with different metal ions. The effects of doping can range from changing the colour of the PL response to increasing (or decreasing) the brightness of the response. Effective doping, however, depends upon robust synthetic procedures which result in the formation of single-phase products with a homogenous dispersion of the dopant ion rather than phase separation.