ABSTRACT
Aiming to combine the advantages of both prompt fluorescence and thermally activated delayed fluorescence in single emitter, molecular design of emitters with hybridized locally excited and charge transfer states were investigated by computational approaches and optical spectroscopy. Taking into account the results of the theoretical screening, the most promising derivatives of 9,9-dimethyl-9,10-dihydroacridine and 10-phenyl-10H-phenothiazine-5,5-dioxide based with the different linking topology (meta- and para-isomers) were selected for the synthesis and experimental investigations. Both the compounds exhibit ultraviolet LE emission peaking at ca. 360 nm, green ICT peaking at ca. 510 nm, and deep-blue HLCT emission peaking at ca. 430 nm when they are molecularly dispersed in the solid media of the different polarity. The developed emitters allow to obtain deep-blue electroluminescence for the host-containing OLEDs and green electroluminescence of host-free devices with the efficiency of exciton production of 42 and 73%, respectively. Efficient exciton production is due to the spin-flip switching via thermally activated processes which is much more efficient than prompt fluorescence. Showing the impact of the linking topology, the para-isomer demonstrated more efficient triplet harvesting in OLEDs than meta-isomer. A detailed discussion on the structure-property relationships and on some discrepancies between the results of the results of theoretical calculations and spectral analysis allows to obtain important insights on the photophysical properties of these compounds.
ABSTRACT
Six new hole transporting materials based on the commercially available m-MTDATA core were synthesized by introducing different electron donating groups, such as methoxy, ethoxy and dimethylamino groups. To achieve the target compounds simple two-step synthesis was used by employing the Buchwald-Hartwig reaction. New derivatives were investigated experimentally and their thermal, optical, photoelectrical, and charge transporting properties were compared with the respective results of their commercial counterpart (m-MTDATA). In solid-state and DCM solutions, target compounds and commercial m-MTDATA showed comparable ionization potential values. The compound with methoxy groups at the meta position showed a hole mobility of 1.06 × 10-3 cm2 V-1 s-1 at an electric field of 4.62 × 105 V cm-1. These values exceed those reported for m-MTDATA. The synthesized derivatives were employed for the preparation of hole transporting layers in multi-resonant TADF OLEDs. Excellent colour purity and high maximum external quantum efficiency reaching 13.8% were achieved for OLEDs employing the synthesized hole-transporting material and the deep-blue MR-TADF emitter t-DABNA.
