The project consisted of four work packages (WPs). In WP1, we modeled four proposed target deep-red (DR) boron diiminate (BDI) emitters containing two strong donor 9,9-dimethyl-9,10-dihydroacridine (DMAC) using DFT/TD-DFT calculations at the PBE0/6-31G(d,p) level of theory in the gas phase. These calculations indicated that these emitters have the potential to be TADF emitters. WP2 involved the synthesis of these TADF BDI emitters and their chemical characterization using modern analytical techniques such as NMR spectroscopy, ESI-HRMS, XRD, and HPLC. In WP3, we investigated the detailed photophysical and electrochemical properties of the BDI TADF emitters to assess their suitability for OLED fabrication. To ensure high purity, the BDI emitters were sublimed and the purity evaluated using melting point measurements, elemental analysis, and HPLC analysis. Photophysical measurements were conducted both in a solution state and as films doped in 10 wt% PMMA, as well as neat films. The optical gap of the BDI emitters was determined from UV-Vis absorption spectra measured in toluene, and singlet and triplet state energies were estimated from the steady-state PL measurements in air and nitrogen at low temperatures. The aggregation-induced emission (AIE) properties of these emitters were demonstrated in THF/water or CHCl3/hexane solvent mixtures, showing a significant increase in emission from 4- to 32-fold in the aggregate state compared to the solution state. Oxidation and reduction potentials were measured by cyclic voltammetry. Unfortunately, the photoluminescence quantum yields (PLQYs) of the emitters were relatively low, measuring below 39% in the 10 wt% doped films in PMMA and even lower as neat films (below 11%). As a result, these compounds were not suitable for OLEDs, and OLED devices were not fabricated due to cost-benefit considerations, as initially planned in WP4. However, it is worth noting that these emitters displayed mechanochromism and enhanced emission induced by crystallization, offering interesting properties beyond their TADF and AIE properties.
Results of this Marie Skłodowska Curie Action (MSCA) project will be published in forthcoming papers on the synthesis of boron diiminate (BDI) emitters with aggregation-induced emission and thermally activated delayed fluorescence characteristics.
