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