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, HL2, and 5-(2-pyridyl)-5 ''-dodecyl-2,2':5',2 ''-ter-thiophene, HL3, bind to platinum(II) and iridium(III) as N boolean AND 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 [PtLn(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 [PtL1(acac)] and [Ir(L-1)(2)(acac)] {HL1 = 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 [PtL1(acac)] displays solely intense phosphorescence from a triplet state of mixed pi pi*/MLCT character, the phosphorescence of [PtL2(acac)] and [PtL3(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.