A theoretical study of carbazole dimers: Does carbazole form an excimer that undermines the performance of organic light emitting diodes?

Carbazole (Cz) dimers in various cofacial conformations, including staggered (Stg), anti, and syn, were explored by means of ab initio calculations at scaled opposite-spin (SOS)-MP2, SOS-CIS(D₀), and additional coupled cluster calculation levels. Similar to other π-conjugated molecules, strong Cz excimers form in the syn conformation in both S₁ and T₁ states, leading to significantly reduced optical excitation energies. Upon excitation, the dimers in the Stg and anti-conformations remain simple excited dimers, exhibiting similar optical energy gaps to those of the monomer. Being far more stable in the ground state, however, the Stg dimer is nearly isoenergetic to the syn dimer in the S₁ state and even more stable in the T₁ state. Given that the intermolecular interactions in the ground state are expected to govern the dimer conformations of Cz-based materials in the solid-state films of organic electronics, our results strongly demonstrate that the electronic excitation of Cz dimers do not necessarily lead to the strong excimer formation unless Cz molecules are forced to be arranged in the syn conformation.