Spin–Orbit Coupling in Phosphorescent Iridium(III) Complexes

Abstract

We study the excited states of two iridium(III) complexes with potential applications in organic light-emitting diodes: fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)(3)] and fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) [Ir(ptz)(3)]. Herein we report calculations of the excited states of these complexes from time-dependent density functional theory (TDDFT) with the zeroth-order regular approximation (ZORA). We show that results from the one-component formulation of ZORA, with spin-orbit coupling included perturbatively, accurately reproduce both the results of the two-component calculations and previously published experimental absorption spectra of the complexes. We are able to trace the effects of both scalar relativistic correction and spin-orbit coupling on the low-energy excitations and radiative lifetimes of these complexes. In particular, we show that there is an indirect relativistic stabilisation of the metal-to-ligand charge transfer (MLCT) states. This is important because it means that indirect relativistic effects increase the degree to which SOC can hybridise singlet and triplet states and hence plays an important role in determining the optical properties of these complexes. We find that these two compounds are remarkably similar in these respects, despite Ir(ppy)(3) and Ir(ptz)(3) emitting green and blue light respectively. However, we predict that these two complexes will show marked differences in their magnetic circular dichroism (MCD) spectra.

Properties of materials for organic light emitting diodes

“…86 Treating the core electrons on the iridium atom within the frozen core approximation has little effect on the calculated excitations. Including SOC as a perturbation to the scalar ZORA TDDFT gives essentially the same results as those obtained from more expensive two-component methods, and is more easily related to the underlying spin restricted excitations.…”

“…It is interesting to compare this result with Hofbeck and Yersin's spectroscopic studies of Ir(ppy) 3 , which shares many similarities with complexes 1-4. 86 On the basis of these measurements Hofbeck and Yersin argued that in Ir(ppy) 3 the lowest triplet excitation, T 1 , is split into three substates. This led them to argue that the symmetry of Ir(ppy) 3 is lowered from C 3 by distortions induced from a host material, whether a solvent or a solid matrix.…”

“…19,85,86 The LUMO on the other hand is predominantly found on the pyridine moieties of the ligands. Therefore modification of the phenyl ring will adjust the energy of the HOMO while changing the pyridine moiety affects the energy of the LUMO.…”

“…11,13,15,86,113,[194][195][196]220 Most approaches taken have included SOC perturbatively. In Chapter 3 it was shown that this approximation accurately reproduces the results of calculations in the two-component formalism, which includes SOC to all orders, in these complexes.…”

“…In Chapter 3 it was shown that this approximation accurately reproduces the results of calculations in the two-component formalism, which includes SOC to all orders, in these complexes. 86 Furthermore, it was seen in Chapter 2 that scalar relativistic effects are also sizeable in these iridium(III) complexes, and play a key role in determining the degree of metal-to-ligand charge transfer (MLCT) character and hence their optical properties. 196 Both Matsushita et al 11 and Jansson et al 113 studied Ir(ppy) 3 with SOC included via the semi-empirical effective nuclear charge method, which includes SOC on top of non-relativistic calculations.…”