Improving triplet excited-state absorption and lifetime of cationic iridium (III) complexes by extending a-conjugation of the 2-(2-quinolinyl) quinoxaline ligand

The synthesis and photophysical properties (UV-vis absorption, emission, and transient absorption) of four cationic Ir(III) complexes (C<^>N)(2)Ir(R-quqo)(+) (HC<^>N = 1-phenylisoquinoline (piq) and 1,2-diphenylpyreno [4,5-d] imidazole (dppi), quqo = 2-(2-quinolinyl)quinoxaline, R = H or fluorenyl) are reported. The UV-vis absorption and emission were simulated by time-dependent density functional theory (TDDFT). Influences of extending pi-conjugation of the C<^>N ligand and the diimine ligand on the singlet and triplet excited-state absorption and lifetimes of these complexes were explored. All complexes exhibited intense ligand-localized (1)pi,pi transitions, broad and structureless metal-to-ligand charge transfer ((MLCT)-M-1)/ligand-to-ligand charge transfer ((LLCT)-L-1) transitions, and very weak spin-forbidden (MLCT)-M-3/(LLCT)-L-3/(3)pi,pi transitions in their UV-vis absorption spectra. The two complexes that bear fluorenyl-substituted quqo ligands (Ir-3 and Ir-4) also possessed a broad intraligand charge transfer ((ILCT)-I-1)/(1)pi,pi band at 430-550 nm. The predominant (ILCT)-I-3/(3)pi,pi characters of the triplet excited states of Ir-3 and Ir-4 improved their phosphorescent emission quantum yields and prolonged their triplet lifetimes compared to the weaker and short-lived emission of Ir-1 and Ir-2. In contrast to the very weak nanosecond transient absorption (TA) of Ir-1 and Ir-2, Ir-3 and Ir-4 possessed much stronger TA signals at 520 - 800 nm upon nanosecond laser excitation. These complexes exhibited moderate to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses, with the RSA trend following Ir-1 > Ir-2 Ir-3 > Ir-4. Considering the long triplet excited-state lifetimes and broadband TA, complexes Ir-3 and Ir-4 could be potential broadband RSA materials.