Ab Initio Studies of Triplet-State Properties for Organic Semiconductor Molecules

Triplet–triplet annihilation (TTA) leads to a
reduced efficiency
of organic light-emitting diodes (OLEDs) at high current densities.
Spacial confinement of the triplet excitons, which is mainly dependent
on triplet energy differences, can reduce the TTA rate. Therefore,
a deliberate choice of the organic semiconductor materials with particular
attention to their triplet energies can help to considerably increase
the device efficiency. Organic solid-state lasers are, on the other
hand, efficiently quenched by singlet–triplet annihilation
(STA), which is closely related to the triplet–triplet absorption
of the organic semiconductors. To establish a useful set of parameters
related to the processes in organic semiconducting devices, we provide
theoretical estimates for the triplet energy of 31 organic semiconductor
molecules using state-of-the art ab initio quantum chemical methods.
For a subset of 22 molecules, the triplet–triplet absorption
spectra were calculated as well. We also discuss related features
like localizations of excitations to molecular fragments, driven by
the structural changes of the molecules in the excited triplet state.
The calculated excited-state properties can assist experimentalists
and serve as input parameters in simulations of organic electronics.