Light-Induced Structural Change in Iridium Complexes Studied by Electron Spin Resonance

Abstract

Iridium-based compounds are materials of great interest in the production of highly efficient organic light emitting diodes and several other applications. However, these organometallic compounds present relative low stability due to photodegradation processes still not well understood. In this work we investigated paramagnetic states induced by UV photoexcitation on iridium(III) bis[(4,6-fluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) and iridium(III)-tris(2-phenylpyridine) (Ir(ppy) 3 ) complexes dispersed in different polymeric matrices by electron spin resonance (ESR). Photogenerated charged states with relatively strong hyperfine interactions were observed and attributed to matrix/complex charge-transfer processes. Measurements of the signal amplitude decay after photoexcitation interruption were performed as a function of temperature. The photoinduced centers are thermally activated with energy barrier between 0.3 and 0.6 eV. Electronic structure calculations suggest that the signals observed by ESR are associated with metastable negatively charged Ir complexes distorted structures.

Degradation Mechanisms and Reactions in Organic Light-Emitting Devices

“…209 Using EPR, it is possible to study structural changes in emitting molecules (like FIrpic or Ir(ppy) 3 ) during UV excitation. 213 …”

Luminescent Iridium Complexes Used in Light-Emitting Electrochemical Cells (LEECs)

“…Entry 1 effectively demonstrates this principle with the device based on 1 exhibiting a remarkably long lifetime of 668 h. The authors attribute this stability to the relatively large calculated 3 MC-T 1 energy gap for 1 [ 23 ]. Theory and experimental observations have implicated 3 MC states in an elongation of the N pyridyl -Ir bond of the C^N ligands of [Ir(C^N) 2 (N^N)] + complexes, which accounts for the efficient non-radiative quenching resulting from these states [ 40 ]. In addition, work on ruthenium(II) complexes has suggested that this bond lengthening/breaking process within the 3 MC state introduces a free coordination site that allows small molecules such as water to coordinate to the metal, quenching the emission and leading to degradation products within the device [ 13 , 41 ].…”

FIrpic: archetypal blue phosphorescent emitter for electroluminescence

“…In addition to problems common to all OLEDs, blue phosphors present additional challenges, 100 especially thermally accessible non-radiative metal centred states. 101,102 FIrpic is further disadvantaged because of chemical complication arising from its chemical structure.…”