Theoretical study on effect of thiophene substitution on the structure and phosphorescence quantum yields of red-emitting iridium(III) emitters in OLEDs

• The absorption and phosphorescent properties of thiophene substituted Ir(III) complexes are investigated.
• The nature of high efficiency has been revealed by the analyses of kr and knr.
• The designed material has good potential for applications in red emitters for OLEDs.

The electronic structures and photophysical properties of Ir(tpy)2(acac) complex (1) and the derivative (2, 3) with thiophene at different positions of tpy ligands have been investigated with density functional theory (DFT) and time-dependent density functional theory (TD-DFT), where tpy = 2,2-thienylpyridine, acac = acetylacetonate. 3a and 3b are studied to get insight into the influence of different positions of fluorine atoms on the photophysical properties of 3. The calculated results reveal that the introduction of thiophene are beneficial to enhance absorption intensity, decrease LUMO energy levels, and reduce the energy barrier for electron injection compared with 1. However, the decreased amount of metal orbitals involved in the transitions, small participation of metal to ligand charge transfer contribution in both the absorption (MLCT%) and emission spectra (3MLCT%), large energy difference between the S1 and Tm (ΔES1−Tm) for 2 (m = 1–2) and 3 (m = 1–3) compared with those of 1 account for their relatively low phosphorescence quantum yields (Φp) observed experimentally. Moreover, the incorporation of F atoms into complex 3 (3a, 3b) can be efficient approach of tuning the electron injection ability, balance of charge transfer process, and emitting color. Especially, the high quantum yield of 3b compared with 3a is explained based on the detailed analysis of the triplet energy (ET1), transition dipole moment (μS1) upon the S0 → S1, SOC matrix element between the Tm and Sn states, ΔES1−Tm, 3MLCT% in the phosphorescent spectra, energy difference between 3MLCT and electronic configurations of the triplet metal-centered (3MC) d–d excited states. These structure-property relationship is expected to provide useful information for synthesis highly efficiency phosphorescence emitters.