Reverse saturable absorbing cationic iridium(III) complexes bearing the 2-(2-quinolinyl)quinoxaline ligand: effects of different cyclometalating ligands on linear and nonlinear absorption

To develop broadband reverse saturable absorbing materials, four Ir(III) complexes containing the 2-(2-quinolinyl)quinoxaline (quqo) ligand and different cyclometalating ligands ((quqo) Ir(C<^>N)(2)PF6, where C<^>N = 2-[4-(7-benzothiazolylfluoren-2-yl)phenyl]pyridine (1), 2-(7-benzothiazolylfluoren-2-yl)pyridine (2), 2-[3-(7-benzothiazolylfluoren-2-yl)phenyl]pyridine (3), and 2-[3-(7-naphthalimidylfluoren-2-yl)phenyl]pyridine (4)), were synthesized. Their linear and nonlinear absorption and emission characteristics were systematically investigated via spectroscopic techniques (i.e. UV-vis absorption, emission, and fs and ns transient absorption), nonlinear optical characterization techniques (i.e. Z-scan and the time-resolved pump-probe technique with a phase object (PO pump-probe)), and theoretical calculations. The effect of quqo as a strong electron-withdrawing and extensively pi-conjugated ligand, and the effect of the substitution position (meta versus para) at the 2-phenylpyridine ligand on the lowest-energy charge transfer absorption band and the triplet excited-state lifetimes of these complexes were explored. It was found that all these complexes possess a very weak but broad charge-transfer ground-state absorption band from 450 to 750 nm, where their excited states absorb strongly. Therefore, they exhibit the broadest optical window in the visible to the near-IR region among reverse saturable absorbers reported to date. The strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses was demonstrated for these complexes. In order to maintain the long-lived triplet excited state while red-shifting the charge-transfer ground-state absorption band, attaching the 7-R-fluoren-2-yl substituent at the 4-position of the cyclometalating phenyl ring instead of the 3-position of the cyclometalating phenyl ring is the solution.