Phosphorescence vs fluorescence in cyclometalated platinum(II) and iridium(III) complexes of (oligo)thienylpyridines

Two newly prepared oligothienylpyridines, 5-(2-pyridyl)-5'-dodecyl-2,2'-bithiophene, HL(2), and 5-(2-pyridyl)-5'-dodecyl-2,2':5',2'-ter-thiophene, HL(3), bind to platinum(II) and iridium(III) as N∧C-coordinating ligands, cyclometallating at position C(4) in the thiophene ring adjacent to the pyridine, leaving a chain of either one or two pendent thiophenes. The synthesis of complexes of the form PtL(n)(acac)] and Ir(L(n))(2)(acac)] (n = 2 or 3) is described. The absorption and luminescence properties of these four new complexes are compared with the behavior of the known complexes PtL(1)(acac)] and Ir(L(1))(2)(acac)] {HL(1) = 2-(2-thienyl)pyridine}, and the profound differences in behavior are interpreted with the aid of time-dependent density functional theory (TD-DFT) calculations. Whereas PtL(1)(acac)] displays solely intense phosphorescence from a triplet state of mixed ππ*/MLCT character, the phosphorescence of PtL(2)(acac)] and PtL(3)(acac)] is weak, strongly red shifted, and accompanied by higher-energy fluorescence. TD-DFT reveals that this difference is probably due to the metal character in the lowest-energy excited states being strongly attenuated upon introduction of the additional thienyl rings, such that the spin-orbit coupling effect of the metal in promoting intersystem crossing is reduced. A similar pattern of behavior is observed for the iridium complexes, except that the changeover to dual emission is delayed to the terthiophene complex Ir(L(3))(2)(acac)], reflecting the higher degree of metal character in the frontier orbitals of the iridium complexes than their platinum counterparts.