Predicting the Operational Stability of Phosphorescent OLED Host Molecules from First Principles: A Case Study

Abstract

Low operational stability is the main limiting factor for commercialization of the blue phosphorescent organic light emitting diodes (PhOLEDs). The high energy and long lifetime of triplet excitons in blue PhOLEDs makes them more prone to degradation. Degradation of the host molecules in the emitting layer of PhOLEDs is one of the possible mechanisms leading to the luminosity loss in the course of device operation. Although possible degradation mechanisms are proposed in the literature, predicting the degradation kinetics is not straightforward because the evolution of excited states should be accurately described. We propose a computational scheme to assess the operational stability of PhOLED host materials. Our protocol relies on the usage of the multireference CASSCF/XMCQDPT2 method. In the present work we consider the degradation of four prototypical blue PhOLED host molecules in the charged and excited states as well as the degradation induced by exciton–polaron and exciton–exciton annihilation processes with the focus on breaking of exocyclic C–C or C–N bonds and triazine ring fission. By analyzing the calculated activation energies for different mechanisms we found the least stable states and the most probable dissociation pathways. On the basis of our computations, we derived a stability series for the studied molecules and determine the structural features that provide higher stability with respect to the unimolecular dissociation.