Effect of axially projected oligothiophene pendants and nitro-functionalized diimine ligands on the lowest excited state in cationic Ir(III) bis-cyclometalates

The novel terthiophene (3T) oligomer 6 and a series of cationic Ir(III) bis-cyclometalates [Ir(C ^{^}N) ₂(N ^{^}N)]PF ₆9-12 were prepared. The synthesis, characterization, electrochemical, and photophysical properties are reported. The cyclometalating ligands (C ^{^}N) are 2-phenylpyridinato (ppy) or the 3T oligomer (3T-ppy), asymmetrically capped in the 5 and 5″ positions with the ppy and mesityl groups. The diimine ligands (N ^{^}N) are 2,2′-bipyridine (bpy) or 4-NO ₂-bipyridine (4-NO ₂-bpy). Hybrid metal-organic complexes 11 and 12 bear 3T-pendants ligated through the ppy cap, 10 and 12 contain NO ₂ functionalized diimines, whereas 9 contains neither. Structural characterization of 10 by single crystal X-ray diffraction confirms the presence of the NO ₂ substituent and pseudo-octahedral coordination geometry about the Ir(III) ion. Cyclic voltammetry highlights the large electron withdrawing effect of the NO ₂ substituent, providing an 850 mV shift toward lower potentials for the first diimine centered reduction of 10 and 12. Strong overlap of the intense π → π* absorptions of the 3T-pendants with Ir(III) charge transfer bands is evident in complexes of 11 and 12, precluding the possibility for selective excitation of either chromophore. Photoexcitation (λ _ex = 400 nm) of the series affords strong luminescence from the 3T oligomer 6 and the unsubstituted 9, with φ _em = 0.11. In stark contrast the NO ₂ and 3T functionalized complexes 10-12 display near total quenching of luminescence. Computations of the ground and excited state electronic structure using density functional theory (DFT) and time-dependent DFT (TD-DFT) indicate that both the NO ₂ and 3T substituents play an important role in excited state deactivation of complexes 10-12. A substantial electronic contribution of the NO ₂ substituent results in stabilization of the diimine based molecular orbital (MO) and offers an efficient nonradiative decay pathway for the excited state. Spin-orbit coupling effects of the Ir(III) ion lead to efficient population of the low lying, nonluminescent, triplet states centered on the 3T-pendants.