Sensitivity of the photophysical properties of organometallic complexes to small chemical changes

Abstract

We investigate an effective model Hamiltonian for organometallic complexes that are widely used in optoelectronic devices. The two most important parameters in the model are J, the effective exchange interaction between the π and π∗ orbitals of the ligands, and ε∗, the renormalized energy gap between the highest occupied orbitals on the metal and on the ligand. We find that the degree of metal-to-ligand charge transfer character of the lowest triplet state is strongly dependent on the ratio ε∗/J. ε∗ is purely a property of the complex and can be changed significantly by even small variations in the complex’s chemistry, such as replacing substituents on the ligands. We find that small changes in ε∗/J can cause large changes in the properties of the complex, including the lifetime of the triplet state and the probability of injected charges (electrons and holes) forming triplet excitations. These results give some insight into the observed large changes in the photophysical properties of organometallic complexes caused by small changes in the ligands.

Spin–Orbit Coupling in Phosphorescent Iridium(III) Complexes

“…We study the excited states of two iridium(III) complexes with potential applications in organic light-emitting diodes: factris(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).…”

“…Phosphorescent materials are particularly desirable over fluorescent emitters due to their potential to harvest both singlet and triplet excitations generated in a device. [1,2] However, the development of efficient deep-blue phosphorescent emitters, which are required for full-colour OLED displays, [3][4][5][6] has remained a persistent problem since the field was established with the discovery of the green phosphorescent complex factris(2-phenylpyridyl)iridium(III) [Ir(ppy) 3 ; Figure 1 and Table 1]. [7] Fac-tris(1-methyl-5-phenyl-3-n-propyl- [1,2,4]…”

Properties of materials for organic light emitting diodes

“…However, Jacko et al 40,173,205 have recently found that an additional dependence on the energy gap between T 1 and S 1 , which is also inverse square to leading order, arises from the details of the hybridisation between ligand centred and MLCT excitations required to form T 1 . Thus Jacko et al's 205 work predicts that the radiative rate of T 1 varies as the fourth power of the inverse of the energy gap between T 1 and S 1 .…”

“…Phosphorescent materials are particularly desirable over fluorescent emitters due to their potential to harvest both singlet and triplet excitations generated in a device. 37,173 However, the development of efficient deep-blue phosphorescent emitters, which are required for full colour OLED displays, 19,51,73,191 has remained a persistent problem since the field was established with the discovery of the green phosphorescent complex fac-tris(2-phenylpyridine)iridium(III) [Ir(ppy) 3 ;…”

“…55 However, while some general design principles are now known 51 less is understood about why small variations can cause large changes in, for example, the photoluminescence quantum yield (PLQY). 19,40,72,172,173 Understanding the photophysical and electronic properties of these complexes remains a major challenge and the major impediment to the rational design of new, highly emissive materials and complexes used in photovoltaic devices.…”