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One commercial application of emissive lanthanide complexes is in fluoroimmunoassays, which combine FRET with time-resolved detection of emission to glean information on biological interactions. Herein the development of highly emissive europium complexes to act as FRET donors in this process is described. Each complex reported in this thesis incorporates a coordinating ligand, which chelates to the metal ion centre, and a sensitising group, capable of populating the europium excited state. To maximise the molar extinction coefficient and emission quantum yield of the complexes, variation of the ligand and sensitising group, was addressed. An initial study into the use of 3-azaxanthone as a sensitising group was undertaken. A constitutional isomer of this chromophore is known to sensitise europium emission. The new system was developed to allow inclusion of four sensitising groups per complex, with a view to maximising the overall molar extinction coefficient. The resulting systems showed little improvement upon previously reported complexes and a new chromophore was devised, which possesses a much higher molar extinction coefficient and is able to sensitise europium emission efficiently. A series of complexes was synthesised in which the chelating ligand was varied to investigate the effect upon the photophysical properties. The complex with the properties best suited to the FRET application combines phenylphosphinate donating groups with a high degree of symmetry about the metal ion centre. The ability of this complex to withstand quenching of emission by a number of competitive anions and cations was explored and it was found that the complex possesses high kinetic stability. By bringing together the favourable photophysical properties imparted by the phenylphosphinate ligand and the high molar extinction coefficient of the new sensitising group, a final europium complex was synthesised with an extraordinarily high brightness, defined as the product of the emission quantum yield and molar extinction coefficient. This final complex met the specification for a FRET donor set out at the beginning of the project.