Identifying efficient blue-phosphorescent polymer light-emitting diode host materials based on carbazole derivatives...

Two series of carbazole derivatives were designed and studied that could potentially be used in polymer light-emitting diodes (PLEDs) as host molecules. These carbazole-based host molecules incorporated substituents with C or Si at their centers at the 3- and 6-positions on the carbazole moiety. Density functional theory calculations were performed to investigate the influence of the substituent on energy and charge transfer, and to predict whether each molecule could act as an effective host material in PLEDs. The results show that, for the series in which the carbazole moiety is linked to the substituent via the central C/Si atom ("cbz-sub series"), triplet-state electron transitions and triplet excitons arise from the carbazole moiety. Members of the cbz-sub series also exhibited higher triplet energies (ET) than the series in which the carbazole moiety is linked to the C/Si-centered substituent via an extra phenyl group ("cbz-ph-sub series"). Moreover, members of the cbz-sub series presented strong molecular orbital interactions and suitable singlet and triplet energy differences (ΔEST). Further investigations showed that, in each series, the presence of an Si atom was more likely to inhibit charge and exciton delocalization, and inserting a methyl or tert-butyl group at the 2- and 7-position, respectively, of the carbazole moiety in the cbz-ph-sub molecules improved their ET values and led to clear intramolecular charge-transfer character. A comparison of the energies of host and guest molecules showed that all of the molecules designed in this work are suitable for use with blue-light guest materials.