About the electronic and photophysical properties of iridium(iii)-pyrazino[2,3-f][1,10]-phenanthroline based complexes for use in electroluminescent devices

Abstract

A family of cyclometalated Ir(III) complexes was studied through quantum chemistry calculations to get insights into their applicability in light electrochemical cells (LECs). The complexes are described as [Ir(R-C^N)2(ppl)](+), where ppl is the pyrazino[2,3-f][1,10]-phenanthroline ancillary ligand. The modification of the HOMO energy in all the complexes was achieved by means of different R-C^N cyclometalating ligands, with R-ppy (phenylpyridine), R-pyz (1-phenylpyrazole) or R-pypy (2,3'-bipyridine); in addition, inductive effects were taken into account by substitution with the R groups (R = H, F or CF3). Then, compounds with HOMO-LUMO energy gaps from 2.76 to 3.54 eV were obtained, in addition to emission energies in the range of 438 to 597 nm. The emission deactivation pathways confirm the presence of metal-to-ligand transitions in all the complexes, which allow the strong spin-orbit coupling effects, and then improving the luminescence performance. However, the coupling with ligand and metal centered excited states was observed for the blue-shifted emitters, which could result in a decrease of the luminescence efficiencies. Furthermore, ionization potentials, electron affinities and reorganization energies (for holes and electrons) were obtained to account for the injection and transport properties of all the complexes in electroluminescent devices.

Insights into the luminescent properties of anionic cyclometalated iridium(III) complexes with ligands derived from natural products

“…Hence, the orbital stabilization of C2 is highly influenced by inductive effects exerted by the CF 3 group, which are high enough to exchange the order of the HOMO and HOMO − 1 orbitals with respect to complex C1 (Figure ), explaining the different HOMO topologies in these compounds. However, the Δ HL values of systems C4 ‐ C6 increase in up to 0.09 eV with respect to systems C1 ‐ C3 due to the inductive effects exerted by the fluorine atoms in the F 2 ppy ligands, which also compensate the inductive effects on the ancillary ligand.…”

“…Consequently, the accessibility to the 3 MC states is related to the k nr in cyclometalated Ir(III) complexes, and a high splitting energy between the 3 MC and T 1 states is required to avoid the radiationless processes. To get insights into the accessibility to the non‐radiative processes, the 3 MC states of all the complexes were optimized at the unrestricted PBE0 level of theory, starting from a distorted T 1 geometry by elongation of the metal‐ligand distances as reported in related studies . The 3 MC states involves the electron promotion between the t 2g and e g metal orbitals; due to the antibonding character of the e g orbital, the elongation of the IrC^N and/or IrO^O bonds are the main structural change in the 3 MC states .…”

“…To get insights into the accessibility to the non‐radiative processes, the 3 MC states of all the complexes were optimized at the unrestricted PBE0 level of theory, starting from a distorted T 1 geometry by elongation of the metal‐ligand distances as reported in related studies . The 3 MC states involves the electron promotion between the t 2g and e g metal orbitals; due to the antibonding character of the e g orbital, the elongation of the IrC^N and/or IrO^O bonds are the main structural change in the 3 MC states . Indeed, the IrN(C^N) distances of the 3 MC states of complexes C1 ‐ C10 are increased in up to ∼0.5 Å compared to the ground states (see Table S2 in the Supporting Information).…”

Electron‐donor substituents on the dppz‐based ligands to control luminescence from dark to bright emissive state in Ir(III) complexes

“…In general, the energy difference between λ hole and λ electron (Δ λ ) are smaller in all systems suggesting a charge transport balance between hole and electron injections. Apparently, the Δ λ of Ir‐2 is slightly out of the trend, however, all differences in the reorganization energies can be classified as properly to improve the hole (electron) transfer properties in the electroluminescent devices based on cationic Ir(III) complexes, according to comparison with the performance of an analogous complex showing a maximum luminance of 177 cd/m 2 with a Δ λ = 0.20 …”

“…Five different functionals (B3LYP, CAM‐B3LYP, M06‐2x, wB97XD, and BHandHLYP) were tested to reproduce the emission energy of a reference Ir(III) complex ( Ir‐ref: [Ir(ppy) 2 (dppz)] + , where ppy is 2‐phenylpyridine), and the B3LYP functional showed the best agreement with the experimental data (see section S1 in Supporting Information [SI]). In addition, this functional has been extensively utilized in theoretical studies of analogous cyclometalated Ir(III) complexes, obtaining accurate results related to molecular structures and photophysical parameters …”