Effect of Axially Projected Oligothiophene Pendants and Nitro-Functionalized Diimine Ligands on the Lowest Excited State in Cationic Ir(III) bis-Cyclometalates

Schwartz, K. R.; Chitta, R.; Bohnsack, J. N.; Ceckanowicz, D. J.; Miró, P.; Cramer, C. J.; Mann, K. R.
Inorg. Chem. 2012, 51, 5082 (doi:10.1021/ic202573y).

The novel terthiophene (3T) oligomer 6 and a series of cationic Ir(III) bis-cyclometalates [Ir(C^N)₂(N^N)]PF₆ 9 - 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 ppy and the mesityl group. The diimine ligands (N^N) are 2,2'-bipyridine (bpy) or 4-NO₂-bipyridine (4-NO₂-bpy). Hybrid metal-organic complexes 11 and 12 bear 3T-pendants ligated through the ppy cap, 10 and 12 contain NO₂ functionalized diimines, whereas 9 contains neither. Structural characterization of 10 by single crystal X-ray diffraction confirms the presence of the NO₂ substituent and pseudo-ocathedral coordination geometry about the Ir(III) ion. Cyclic voltammetry highlights the large electron withdrawing effect of the NO₂ substituent, providing an 850 mV shift towards 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 NO₂ and 3T functionalized complexes 10 - 12 display near total quenching of luminescence. Computations of the ground and excited states with DFT and TD-DFT indicate that both the NO₂ and 3T substituents play an important role in excited state deactivation of complexes 10 - 12. A substantial electronic contribution of the NO₂ substituent results in stabilization of the diimine based MO and offers an efficient non-radiative decay pathway for the excited state. Spin-orbit coupling effects of the Ir(III) ion lead to efficient population of the low lying, non-luminescent, triplet states centered on the 3T-pendants.