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.
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