Theoretical Study of Carbazole Dimers: Does Carbazole Form an Excimer
That Undermines the Performance of Organic Light Emitting Diodes?
Abstract
Carbazole (Cz) dimers in various cofacial conformations, including
staggered (Stg), anti, and syn, were explored by
means of ab initio calculations at SOS-MP2, SOS-CIS(D0),
and additional coupled cluster calculation levels. As in other
π-conjugated molecules, strong Cz excimers form in the syn
conformation in both the S1 and T1
states, leading to significantly reduced optical excitation energies,
whereas the dimers in the Stg and anti conformations, upon
excitation, remain as simple excited dimers, showing similar optical
energy gaps to that of the monomer. Being far more stable in the ground
state, however, the Stg dimer turned out to be nearly
isoenergetic to the syn dimer in the S1 state,
and even more stable in the T1 state. In addition, a
considerable potential energy barrier between the syn and
Stg dimers was found in the calculated S1-state
potential energy surface. Given that the ground-state intermolecular
interactions are expected to govern the dimer conformations of Cz-based
materials in the solid-state films of organic electronics, these results
strongly demonstrate that the electronic excitation of Cz dimers do not
necessarily lead to the strong excimer formation, unless Cz molecules
were forced to be arranged in the syn conformation.