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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 (λex = 400 nm) of the series affords strong luminescence from the 3T oligomer 6 and the unsubstituted 9, with φem = 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.