Molecular ENgINeering of the triplet state energy level of BipoLar organic semi-condUctors: Towards nEw molecular designs. – Men in blue

The research topic we wish to develop herein deals with novel molecular designs of host materials for high-efficiency/stability blue Phosphorescent Organic Light Emitting Diodes (PhOLEDs). Indeed, in order to reduce the energy consummation, new efficient sources of low-energy white light are required. OLEDs offer a promising alternative to usual lighting but are still not efficient and stable enough to drastically change the future. One cause lies in the lack of highly efficient and stable blue PhOLEDs compared to red and green ones. Thus, from a fundamental point of view, controlling exciton migration in pi-systems is a fundamental concept in these emerging technologies of Organic Electronics (OE). For the last decade, numerous families of Organic Semi-Conductors (OSCs) have been investigated with the aims of optimizing p-conjugation, the energies of singlet (ES) and triplet (ET) excited states, energy transfer processes…In addition, bipolar molecules, which can accept and transport both holes and electrons, are highly attractive candidates for OE as they gather the properties of both hole and electron transporting materials. If these bipolar OSCs further possess a very high ET (>2.75 eV), they become ideal host materials for blue PhOLEDs. Indeed, in a prototypical PhOLED, the emitting layer is constituted of a phosphorescent emitter dispersed into an organic host to avoid triplet-triplet annihilation. However, a major drawback of bipolar materials as hosts for guest emitters is the compression of the HOMO/LUMO gap due to intramolecular charge transfer (between the electron-donating and the electron-accepting moieties) and pi-conjugation extension. This results in lowering of ES and ET, which, in turn, leads to back-transfer of energy from the guest to the host, thereby dramatically reducing the device efficiency. This challenge of combining bipolarity and high ET appears to be even more difficult as the ideal host material for PhOLEDs should also possess high thermal and morphological stability, essential to obtain stable devices. Thus, combining all these properties in a single molecule is far to be an easy task and requires precise molecular designs, which can be only obtained through a perfect knowledge and control of the intra and intermolecular interactions (physical and electronic) occurring in the material. There is even clearly a trade-off between increasing the HOMO-LUMO gap of a material to increase ES and ET and decreasing the length of the pi-system, which may affect the charge transport properties and the morphological stability. Thus, we wish to address this apparent antinomy by synthesizing, through a unique molecular design, new bipolar OSCs possessing high ET, coupled to high thermal/morphological stability as host for blue PhOLEDs.The proposal deals hence with the synthesis of 12 OSCs (OSC1-12) based on 4 platforms (PF1-4), the study of their electrochemical, optical and physical properties and their applications as host in green and blue PhOLEDs.
The design of OSC1-12 is built on:
-A high ET fluorene unit substituted in ortho position with a hole or electron transporting unit. The substitution in ortho position will partially interrupt the pi-conjugation along the fluorene core, avoiding a dramatic decrease of ES/ET, while adjusting the HOMO/LUMO energy levels
-A spiroconjugated Diazafluorene, Thioxanthene or Dioxothioxanthene unit to achieve excellent electron/hole transporting properties directly incorporated within the structure
-A spiro bridge in order to (i) interrupt the electronic coupling between the pi-systems avoiding the consequent decrease of ES/ET through the separation of the hole and electron transporting units (ii) achieve a high thermal/morphological stability
There is hence a 'double' conjugation breaking: the ortho linkage and the spiro bridge. This design may pave the way to the development of these OSCs as hosts for highly efficient blue PhOLEDs, key feature for the future of low energy lighting