Heteroleptic cationic iridium(iii) complexes bearing phenanthroline derivatives with extended π-conjugation as potential broadband reverse saturable absorbers. (12th December 2019)

• Record Type: Journal Article
• Authors: Wang, Li; Cui, Peng; Lystrom, Levi; Lu, Jiapeng; Kilina, Svetlana; Sun, Wenfang
• Is Part Of: New journal of chemistry. Volume 44:Number 2(2020)
• Page Start: 456
• Page End: 465
• Publication Date: 2019-12-12
• Subjects: Chemistry -- Periodicals; Chimie -- Périodiques; 540
• DOI: 10.1039/c9nj03877a
• Languages: English
• ISSNs: 1144-0546

Abstract

Abstract : Fluorenyl substitution at the diimine ligand broadened the excited-state absorption to near-IR, and enhanced reverse saturable absorption at 532 nm for the cationic Ir(iii ) complexes. Abstract : Three cationic heteroleptic iridium(iii ) complexes [Ir(piq)2 (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 1 π, π* transitions below 380 nm, broad and intense diimine ligand-localized 1 ILCT (intraligand charge transfer)/ 1 π, π* transitions at 380–450 nm, weaker 1 LLCT (ligand-to-ligand charge transfer)/ 1 MLCT (metal-to-ligand charge transfer) transitions at 450–520 nm, and very weak spin-forbidden 3 LLCT/ 3 MLCT/ 3 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 (T2 ). The nature of the lowest triplet excited state (T1 ) in toluene varied from the piq ligand localized 3 π, π* mixed with some 3 MLCT/ 3 LMCT (ligand-to-metal charge transfer) characters in 1 and 2 to dppn ligand-based 3 π, π*/ 3 ILCT configuration in 3 . The different nature of the T1 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 T1 state varied from pig ligand-localized 3 π, π*/ 3 MLCT/ 3 LMCT in toluene to dppz localized 3 π, π*/ 3 ILCT state in CH3 CN for 2, which resulted in a shorter triplet lifetime, reduced emission quantum yield, but stronger TA signals in the NIR regions for 2 in CH3 CN. 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.