2.8 Molecular Design
95
HOMO
LUMO
Fig. 2.93 The HOMO and the LUMO patterns of DPA-TRZ molecule based on the TD-M06-2Xcc-pVDZ calculation. Reprinted with permission from Shizu et al. (2015a). Copyright 2015 by the
American Physical Society
twisted from that of the DPA in the left (indicated by red) to suppress the ST
value while remaining the value of |m 01 | as much as possible by inspection of the
detailed contents of the expressions of ST and |m 01 |. This DPA-TRZ molecule
has been designed so that intramolecular charge transfer may easily take place from
the DPA to the TRZ motif. This is understood by that the HOMO is localized at
the left side and the LUMO at the right side as shown in Fig. 2.93. Note that these
two motifs are rather twisted in order to obtain a small ST value (0.11 eV).
The external quantum efficiency of fabricated OLED using this molecule reaches
13.8%. Furthermore, another improved molecule, 9-(4-(4,6-diphenyl-1,3,5-triazin2-yl)phenyl)-9
-phenyl-9H,9
H-3,3
-bicarbazole (BCzT) shown in Fig. 2.94 has
achieved the external quantum efficiency of 21.7% (Shizu et al. 2015b).