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]
  1. 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.