Lowest triplet excited states of a novel heteroleptic iridium(III) complex and their role in the emission colour

The [Ir(ppy-F2)2Me4phen]+1 complex, where ppy-F2 is 2-(2′,4′-fluorophenyl)pyridine and Me4phen is 3,4,7,8-tetramethyl-1,10-phenanthroline, has been theoretically investigated by means of DFT calculations. The molecular and electronic properties calculated for [Ir(ppy-F2)2Me4phen]+1 are compared with those obtained for the simpler [Ir(ppy)(bpy)]+1 complex. The introduction of fluorine substituents in the ppy ligands and the use of phenanthroline instead of 2,2′-bipyridine as the diimine ligand increase the HOMO-LUMO energy gap and blue-shift the emission colour. The phenanthroline ligand causes the appearance of two nearly-degenerate LUMO orbitals of different symmetry in [Ir(ppy-F2)2Me4phen]+1 and determines that two almost isoenergetic doublet states are obtained for the reduced complex. Calculations predict three lowest-energy triplet excited states of different nature in an energy difference of only 0.10 eV. Emission from the 3LC π-π* state takes place at higher energies and accounts for the structured blue emission observed in solution. Emission from the lowest-energy 3MLCT state occurs in the green region and explains the structureless broad band observed in solid films.