Heteroleptic cationic iridium(iii) complexes bearing phenanthroline derivatives with extended π-conjugation as potential broadband reverse saturable absorbers

Three cationic heteroleptic iridium(iii) complexes [Ir(piq)(N^N)] (where piq refers to 1-phenylisoquinoline and N^N represents the fluorenyl-substituted phenanthroline (phen, 1), dipyrido-[3,2-a:2′,3′-c]phenazine (dppz, 2) and benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine (dppn, 3)) were synthesized and their singlet and triplet excited-state characteristics were investigated via spectroscopic methods and theoretical calculations. The electronic absorption spectra of 1-3 in toluene all featured strongly absorbing ligand-center π,π∗ transitions below 380 nm, broad and intense diimine ligand-localized ILCT (intraligand charge transfer)/π,π∗ transitions at 380-450 nm, weaker LLCT (ligand-to-ligand charge transfer)/MLCT (metal-to-ligand charge transfer) transitions at 450-520 nm, and very weak spin-forbidden LLCT/MLCT/π,π∗ transitions at 550-680 nm. Variations of the core diimine ligand from phen to dppz caused a slight red-shift of the absorption bands at <450 nm for 2 compared to that of 1; while changing the core diimine ligand to dppn in 3 induced a more pronounced red-shift of the bands at 380-680 nm compared to those of 1 and 2. All three complexes exhibited structured room-temperature phosphorescence maximized at 590 nm and broad triplet excited-state absorption at 450-800 nm in toluene. However, the emission quantum yield of 3 was more than two orders of magnitude lower than those of 1 and 2 in toluene because the emitting state of 3 was the 2nd triplet excited state (T). The nature of the lowest triplet excited state (T) in toluene varied from the piq ligand localized π,π∗ mixed with some MLCT/LMCT (ligand-to-metal charge transfer) characters in 1 and 2 to dppn ligand-based π,π∗/ILCT configuration in 3. The different nature of the T state in 3 resulted in a much stronger transient absorption (TA) at ca. 540 nm and 600-800 nm with a much longer TA lifetime (∼23 μs). In addition, the T state varied from pig ligand-localized π,π∗/MLCT/LMCT in toluene to dppz localized π,π∗/ILCT state in CHCN for 2, which resulted in a shorter triplet lifetime, reduced emission quantum yield, but stronger TA signals in the NIR regions for 2 in CHCN. Because of the strong triplet excited-state absorption of 1-3 at 532 nm, all three complexes manifested strong reverse saturable absorption (RSA) at 532 nm, with the RSA trend following 3 > 2 > 1. Fluorenyl substitution not only enhanced the RSA at 532 nm for 2 and 3, but also made them potential broadband reverse saturable absorbers.