Figure 5. Schematic energy diagram for the excimer (left) and excited
dimer (right), in general; BEgs and BEesdenote the biding energies of dimer in the ground and excited states,
respectively; REgs corresponds to the repulsive energy
in the ground state; EoptM and
EoptD are the optical energy gaps of
the monomer and dimer, respectively.
The results thus far combine to explain why many Cz-based host materials
have successfully been employed for blue phosphorescent
OLEDs,[3-5,
16-19,
21-24] although Cz dimers in thesyn conformation can undermine their device performances upon
forming excimers.[9]In order to prohibit excimer formation of π-conjugated molecules, in
general, bulky substituents are introduced. Without such bulky
substituents, however, Cz molecules can avoid an excimer formation that
is detrimental to the device performance of OLEDs, owing to their much
stronger ground-state intermolecular interactions in the Stgconformation than the syn conformation that is prone to the
excimer formation. Given that the Cz dimer conformations in the solid
film are governed by the ground-state intermolecular interactions, theStg conformation would prevail in the emissive layers of OLEDs.
In addition, the considerable energy barrier between the syn andStg dimers in the S1 state (Figure 2) also
inhibits the formation of the singlet Cz excimer. Therefore, the Cz
excimer would hardly form, unless the Cz dimer is forced to be arranged
in the synconformation.[7-9]
- Conclusions
In the present study, carbazole (Cz) dimers in various conformations
were investigated theoretically. As expected, the Cz dimer in thesyn conformation forms a strongly bound excimer; however, it is
not the most stable in the excited state. On the contrary, theStg dimer is at least as stable as the syn dimer in the
S1 state, and even more stable in the T1state. Such a remarkable stability of Stg dimer originates from
the strong intermolecular interactions in the ground state. Provided
that the film morphology is governed by the ground-state intermolecular
interactions, Cz-based materials are more likely in the Stgconformation, and stay in the same conformation even in the excited
states. Our results are also consistent with those of many successful
Cz-based host materials for the blue phosphorescent OLEDs reported in
the literature. Although our present study is limited to the Cz dimer
itself, we believe that the results discussed here help better
understand not only the morphology of various Cz-based π-conjugated
molecules in solid-state films, but also the performance of organic
electronics employing such materials.