Figure 4. Natural transition orbital pairs for the S1and T1 states of the carbazole dimer in thestaggered -cofacial (Stg ) conformation. The most dominant pair of hole and electron wave functions for a given excited state are placed below and above the weight of each pair, λ , respectively.
The optical excitation energy reductions of dimers with respect to that of monomer, ΔEopt., in Table 3 are consistent with these NTO pictures. Compared to the monomer, for example, the syn dimer shows considerable reductions in the singlet and triplet excitation energies (by ca. 0.62 and 0.42 eV, respectively), indicating significant red-shifts in emission spectra. This can give rise to the energy back transfer from the light-emitting guest to the Cz-based host in blue OLEDs, and thus, possibly undermine device performance as pointed out in the literature.[9]On the other hand, the singlet excitation energy of the Stg dimer only marginally reduces (by ca. 0.12 eV). In particular, the Stgdimer exhibits the triplet energy no smaller (by ca. 20 meV) than that of the monomer, which prohibits blue emitters from yielding their exciton energy to the Cz-based hosts in which the triplet exciton is held by Cz unit. In the case of the anti dimer, moderate decreases in both singlet and triplet energies are calculated (ca. 0.16 and 0.15 eV, respectively), suggesting that this conformation should not lead to the significant exciton leakage from the light emitting molecules in blue OLEDs.
Table 3. Excited-state binding energies (BEes), adiabatic excitation energies (EoptD), excitation (optical) energy reduction (ΔEopt.) of carbazole dimer (in eV), and the interplanar distances (Rπ-π: in Å) between carbazoles in various conformationsa