Spontaneous fluctuations of transition dipole moment orientation in OLED triplet emitters

TL;DR

This paper investigates the spontaneous fluctuations in the orientation of transition dipole moments in triplet emitters used in OLEDs, revealing non-deterministic behavior that impacts emission efficiency and molecular orientation analysis.

Contribution

It uncovers spontaneous symmetry breaking in triplet emitters leading to polarization fluctuations, which influence OLED light out-coupling and orientation measurements.

Findings

Triplet emitter tris(2-phenylisoquinoline)iridium(III) does not behave as a linear dipole.

Spontaneous symmetry breaking causes random ligand selection in excited states.

Polarization scrambling may enhance out-coupling efficiency.

Abstract

The efficiency of an organic light-emitting diode (OLED) depends on the microscopic orientation of transition dipole moments of the molecular emitters. The most effective materials used for light generation have threefold symmetry, which prohibit a priori determination of dipole orientation due to the degeneracy of the fundamental transition. Single-molecule spectroscopy reveals that the model triplet emitter tris(2-phenylisoquinoline)iridium(III) (Ir(piq)3) does not behave as a linear dipole, radiating with lower polarization anisotropy than expected. Spontaneous symmetry breaking occurs in the excited state, leading to a random selection of one of the three ligands to form a charge transfer state with the metal. This non-deterministic localization is revealed in switching of the degree of linear polarization of phosphorescence. Polarization scrambling likely raises out-coupling efficiency and should be taken into account when deriving molecular orientation of the guest emitter within the OLED host from ensemble angular emission profiles.