The synthesis of 10 novel P-substituted dithienophosphole oxide compounds applying phenylcarbazole and indolocarbazole donors is presented. Based on photo-physical and theoretical investigations, the study reveals that the pyramidal geometry of the phosphorus allows for the synthesis of charge transfer materials by introducing strong exocyclic donor groups but suppresses intramolecular charge transfer below a certain donor strength threshold, which is an appealing structural feature for the design of donor–acceptor materials. The triplet energies of the phenylcarbazole based compounds are in the range of 2.49–2.65 eV, sufficiently high for potential applications as host materials in PhOLEDs. By contrast, the introduction of indolocarbazole, the weakest employed donor, yields materials exhibiting a significantly higher triplet energy of up to 2.87 eV and a remarkably low singlet–triplet splitting (0.18 eV). In addition an interesting example of an intramolecular electronic through-space interaction has been observed for the ortho-linked phenylcarbazole derivative.

A series of 6 novel triarylamine-containing oxadiazole compounds (o-PCzPOXD, o-ICzPOXD, o-TPATOXD, o-PCzTOXD, o-ICzTOXD, o-CzTOXD) have been designed, synthesized and characterized concerning applications as host materials in PHOLED devices. To further improve the ortho-linkage concept, the impact of incorporating planarized electron-donating triarylamine (TAA) structures on intramolecular charge transfer was examined. The effect was evaluated for two series of electron-accepting oxadiazole scaffolds, realizing ortho-linkage on the benzene (POXD) and the thiophene (TOXD) core. Thermal analysis shows increased glass-transition temperatures for planarized structures indicating an improved morphological stability. A higher degree of planarization also results in significantly increased singlet and triplet energy values, revealing the impact on the intramolecular charge transfer. Employing the developed materials, red (o-TPATOXD: CE_max: 28.8 cd A^-1, EQE_max: 16.9%), green (o-PCzPOXD: CE_max: 62.9 cd A^-1, EQE_max: 17.1%) and blue (o-PCzPOXD: CE_max: 29.8 cd A^-1, EQE_max: 13.4%) devices were achieved showing remarkably low efficiency roll-off for planarized donors. Hence, this is the first report of efficient blue devices for this specific class of host materials. It is proposed that the results correlate with an increasing ortho-linkage effect and decreasing donor strength of the TAA moiety by planarization and, thus, tackling one of the major challenges in PHOLED research: improving both triplet energy and compound stability.