Synthesis and photophysical studies of chiral fluorescent compounds

OLEDs appear as one of the most promising technologies for screen displays. They are based on the use of emissive organic compounds embedded in a matrix that emit light after electrical excitation. Their main advantages are a higher contrast ratio, a faster response time and a minimal thickness. However, the main issue encountered with the use of fluorescent organic materials in OLEDs resides in the hole-electron recombination statistic. Due to quantum mechanics, 25% of the electrical energy is converted into singlet state excitons and 75% into triplet state ones. Thus, for normal fluorescence emitters only up to 25% of the energy input can be converted into light. In this context, the use of thermally activated delayed fluorescence (TADF) materials has emerged as an alternative to construct efficient OLED. TADF occurs when the energy difference between singlet and triplet states (ΔEST) of the molecule is small enough, so that a reverse intercrossing system (rISC) allows triplet states to be converted into singlet ones. The ΔEST is proportional to the HOMO–LUMO integral overlap, and can be tuned through molecular design. In parallel to this approach, the design of efficient circularly polarized luminescent (CPL) emitters emerged as an important gateway to improve efficiency while decreasing the power consumption of portable OLED displays. Indeed those devices need antiglare filters to avoid reflection of external light sources off their emitting surface. Unfortunately, such an architecture reduces the intensity of non-polarized emitted light by at least 50%. A solution to this significant energy loss could lies in mastering the design of efficient CPL materials.
Recently, we developed a synthetic approach that allows to combine both TADF and CPL properties within a single emitter. The concept that relies on the introduction of a chiral pertubator on a TADF active fluorophore was validated and extended to gain a deeper understanding of the structure – activity relationship. Based on these results we wish to explore new ways of combining and maximizing both properties within a single molecular frame. A promising approach is to induce a stable chiral arrangement of the Donor-Acceptor TADF moiety thanks to the chiral perturbator / inducer. The PhD work would thus combine organic synthetic work to prepare libraries of active compounds, full photophysical and chiroptical characterizations (absorption, fluorescence, transient absorption, electronic circular dichroism, CPL) and quantum calculation to support the interpretation of the experimental results and establish structure-properties relationship. The most promising compounds would then be tested in OLED devices.

For more information, contact Dr G. Clavier

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