4-Spirobifluorene derivatives - A new family of host materials for PhOLEDs

Since 1970, the lighting market has seen two major developments: fluorescent bulbs and light-emitting diodes (LEDs). As early as 1980, organic compounds were integrated as an active layer in OLEDs (Organic Light-Emitting Diodes). OLED performance is limited to 25% conversion of electrical energy into light energy, which is insufficient for high-performance lighting. One way of improving OLED performance has been to use phosphorescent molecules as dopants in an organic layer, leading to phosphorescent organic diodes (PhOLEDs). PhOLED performance depends on the choice of organic matrix/phosphorescent dopant pair. Good energy transfer from the matrix to the dopant requires the energy levels of the singlet and triplet states of the matrix to be matched to those of the dopant, since the color emitted by the PhOLED is directly linked to the triplet level of the dopant. While host materials adapted to red or green dopants are relatively common today and lead to PhOLEDs with good performance, host materials adapted to blue dopants are much rarer, as they must possess a wide HOMO-LUMO energy gap and a triplet state above 2.8 eV. The search for low-conjugation matrices is therefore still a major research topic in the PhOLED field. The strategies used to restrict the conjugation of an organic semiconductor material are varied, and conjugation breaks by "steric hindrance" have been described. The latter strategy is used with spirobifluorene (SBF) derivatives substituted in the ortho position (on the C4 carbon), and the literature is beginning to present interesting PhOLED device performances using 4-SBFs as the host matrix for blue PhOLEDs. This is the background to the work presented in this article.