Tuning the Ground State and Excited State Properties of Monocationic Iridium(III) Complexes by Varying the Site of Benzannulation on Diimine Ligand

Abstract

Extending π-conjugation of the diimine ligand (N^N ligand) via benzannulation is a common way to tune the absorption and emission energies of cationic iridium(III) complexes. However, it can cause either a red- or blue-shift of the absorption and emission bands depending on the site of benzannulation. To understand the mechanism of changes in optical transitions upon benzannulation on the diimine ligand, a series of new cationic iridium(III) complexes [Ir(dppi)(N^N)]PF (1-6) (where dppi =1,2-diphenylpyreno[4,5-d]imidazole; N^N = 2-(pyridin-2-yl)quinoline (1), 2-(pyridin-2-yl)[7,8]benzoquinoline (2), 2,2'-bisquinoline (3), 2-(quinolin-2-yl)[7,8]benzoquinoline (4), 2-(pyridin-2-yl)[6,7]benzoquinoline (5), 2-(quinolin-2-yl)[6,7]benzoquinoline (6)) containing diimine ligand with varied degrees of π-conjugation via benzannulation at different sites of the 2-(pyridin-2-yl)quinoline ligand were synthesized. Experimental results and density functional theory (DFT) calculations revealed that benzannulation at the 6,7-position of quinoline and/or the 5',6'-position of pyridine (3, 5, and 6) induced red-shifts in their absorption and emission bands with respect to the parent complex 1; while benzannulation at the 7,8-position of quinoline resulted in blue-shifts (2 vs 1 and 4 vs 3). This phenomenon was rationalized by the symmetry of the frontier molecular orbitals at the site of benzannulation, which stabilized or destabilized the lowest unoccupied molecular orbital (LUMO) upon interactions with 1,3-butadiene, while the energy of the highest occupied molecular orbital (HOMO) remained nearly the same. This discovery would enable a rational design of organic or organometallic compounds that have predetermined absorption and emission energies.

Monocationic Iridium(III) Complexes with Far‐Red Charge‐Transfer Absorption and Near‐IR Emission: Synthesis, Photophysics, and Reverse Saturable Absorption

“…The similar spectral feature reflected the similar configuration of their T 1 states. The positive absorption bands are similar to those observed in other Ir(III) complexes bearing the dppi ligands , . Taking into account the fact that the T 1 state contains the 3 LLCT/ 3 LMCT characters, the observed positive absorption bands could be ascribed to the absorption from the dppi cation radical.…”

“…All chemicals and solvents were purchased from commercial sources and used as received without further purification. The ligands dpp ( L1 ), 2,3–di(pyridin–2‐yl)quinoxaline ( L2 ), 2,3–di(pyridin–2‐yl)benzo[ g ]quinoxaline ( L3 ), dppi, and the Ir(III) dimer [Ir(dppi) 2 Cl] 2 , were prepared following the reported procedures. Column chromatography was carried out using silica gel (60 Å, 230–400 mesh) or Al 2 O 3 gel (activated, neutral).…”

“…The emission energies of Ir1–Ir3 were obtained using the ΔSCF approach based on TDDFT. With this method, we computed the phosphorescence energy using the triplet equilibrium geometry for calculating the optical transition between the singlet ground state (S 0 ) and the lowest triplet excited state (T 1 ) . The nature of the singlet absorption (S 0 → S n ) and the triplet emission (T 1 → S 0 ) was demonstrated by computing the natural transition orbitals (NTOs) for relevant optical transitions.…”

Site-Selective Benzannulation of N-Heterocycles in Bidentate Ligands Leads to Blue-Shifted Emission from [(P^N)Cu]2(μ-X)2 Dimers

“…Whilst the results reveal more pronounced lower energy absorption bands upon extending the -system, a counter-intuitive blue shift in emission maximum is observed. This contrary behavior was not observed for Pt 5,7 or Ir, 6 where the direction of the shift was the same for both absorption and emission. A strong dependence of the luminescence efficiency on the identity of the halide bridge has also been discovered.…”

“…15 In some cases, meanwhile, a small S1-T1 energy gap can open up thermally activated delayed fluorescence (TADF) from T1 to S1 as another mechanism by which otherwise non-emissive triplet states can be harvested. 16 While the use of site-selective benzannulation in tuning emission has been reported for Pt 5,7 and Ir 6 emitters, such strategies have not, to our knowledge, been explored in Cu(I) 5 coordination complexes. Whilst the results reveal more pronounced lower energy absorption bands upon extending the -system, a counter-intuitive blue shift in emission maximum is observed.…”