Relativistic effects in a phosphorescent Ir(III) complex

We compare high field magnetic circular dichroism, absorption, and photoluminescence spectra for Ir(ptz)${}_3$ with time-dependent density functional theory. By gradually turning on the relativistic effects we identify several distinct relativistic effects in the spectra of this complex. We show that relativistic effects must be included to accurately predict the low-temperature spectra. This leads to new insights into the low-lying excitations responsible for the observed phosphorescence, and suggests new avenues to improve the performance of organic light emitting diodes.

Figure 1

Fac-tris (1-methyl-5-phenyl-3-$n$-propyl-[$1,2,4$]triazolyl) iridium(III), Ir(ptz)${}_3$, (left) and a schematic MO energy level diagram for the NR and SR DFT calculations.

Figure 2

Absorption (solid line) and MCD (dashed line) spectra of Ir(ptz)${}_3$ collected at 10 K under an applied field of 5 T compared to the “stick” absorption spectra calculated by (a) NR-TDDFT, (b) SR-TDDFT, and (c) perturbative SOC correction to the SR calculation. Degenerate (E) states are denoted with * (** marks two nearby E states). In the NR and SR calculations the (formally forbidden) triplet excitations are given small arbitrary oscillator strengths for clarity.

Figure 3

Electron densities in the frontier orbitals of Ir(ptz)${}_3$ calculated from SR-DFT.

Figure 4

Scaled absorption and PL spectra of Ir(ptz)${}_3$ at room temperature (RT) and LT. A clear mirror image rule is observed in the LT data. The Stokes shift is significantly reduced at LT due to the freezing of the solvent.