EVIDENCE FOR STRONG MIXING BETWEEN THE LC AND MLCT EXCITED-STATES IN BIS(2-PHENYLPYRIDINATO-C2,N') (2,2'-BIPYRIDINE)IRIDIUM(III)

The well-resolved absorption, excitation, and luminescence spectra of [Ir(ppy)2bpy]+ (ppyH = 2-phenylpyridine, bpy = 2,2'-bipyridine) in different media at cryogenic temperatures are presented. In solutions and glasses at ambient temperature the lowest energy excited state corresponds to an Ir --> bpy charge-transfer excitation whereas in the crystalline host lattice [Rh(ppy)2bpy]PF6 the lowest excited state at 21 450 cm-1 is assigned to a 3pi-pi* excitation localized on the cyclometalating ppy- ligands. The next higher excited Ir --> bpy charge-transfer state has shifted to 21 820 cm-1, only 300 cm-1 above the 3LC excited state. The close proximity of the 3LC and 3MLCT excited states and the large spin-orbit coupling constant of Ir3+ induce a strong mixing of charge-transfer character into the 3LC lowest excited states, resulting in increased oscillator strengths, reduced lifetimes, short axis polarized transitions, and a large zero-field splitting of 10-15 cm-1.