Impact of Benzannulation Site at the Diimine (N^N) Ligand on the Excited-State Properties and Reverse Saturable Absorption of Biscyclometalated Iridium(III) Complexes

Ten biscyclometalated monocationic Ir­(III) complexes were synthesized and studied to elucidate the effects of extending π-conjugation of the diimine ligand (N^N = 2,2′-bipyridine in Ir1, 2-(pyridin-2-yl)­quinoline in Ir2, 2-(pyridin-2-yl)­[6,7]­benzoquinoline in Ir3, 2-(pyridin-2-yl)-[7,8]­benzoquinoline in Ir4, phenanthroline in Ir5, benzo­[f]­[1,10]­phenanthroline in Ir6, naphtho­[2,3-f]­[1,10]­phenanthroline in Ir7, 2,2′-bisquinoline in Ir8, 3,3′-biisoquinoline in Ir9, and 1,1′-biisoquinoline in Ir10) via benzannulation at 2,2′-bipyridine on the excited-state properties and reverse saturable absorption (RSA) of these complexes. Either a bathochromic or a hypsochromic shift of the charge-transfer absorption band and emission spectrum was observed depending on the benzannulation site at the 2,2′-bipyridine ligand. Benzannulation at the 3,4-/3′,4′-position or 5,6-/5′,6′-position of 2,2′-bipyridine ligand or at the 6,7-position of the quinoline ring on the N^N ligand caused red-shifted charge-transfer absorption band and emission band for complexes Ir2, Ir8, Ir10 vs Ir1 and Ir3 vs Ir2, while benzannulation at the 4,5-/4′,5′-position of 2,2′-bipyridine ligand or at the 7,8-position of the quinoline ring on the N^N ligand induced a blue shift of the charge-transfer absorption and emission bands for complex Ir9 vs Ir1 and Ir4 vs Ir2. However, benzannulation at the 2,2′,3,3′-position of 2,2′-bipyridine or 5,6-position of phenanthroline ligand had no impact on the energy of the charge-transfer absorption band and emission band of complexes Ir5–Ir7 compared with those of Ir1. The observed phenomenon was explained by the frontier molecular orbital (FMO) symmetry analysis. Site-dependent benzannulation also impacted the spectral feature and intensity of the triplet transient absorption spectra and lifetimes drastically. Consequently, the RSA strength of these complexes varied with a trend of Ir7 > Ir5 ≈ Ir6 ≈ Ir1 > Ir3 > Ir2 > Ir10 > Ir4 > Ir8 > Ir9 at 532 nm for 4.1 ns laser pulses.