To achieve the goals of the project, we first focused on the modeling of the isolated emitter molecules in so-called multi-resonant TADF (MR-TADF) emitters, an emerging class of TADF emitters. Unlike the original design of TADF emitters, MR-TADF emitters rely on the mixing of resonance structures induced by electroactive groups and essentially look like doped nanographenes. In order to describe the energetics of triplet and singlet states in those architectures, we revised the commonly used computational strategy by utilizing the second-order perturbation theory (namely, SCS-CC2) instead of the time-dependent density functional theory. Unlike TDDFT, this approach turned out to yield very accurate values of the singlet-triplet gaps (delEST), and became a predictive tool for computational screening for perspective MR-TADF emitters. Most importantly, for the first time ever, we demonstrated that small delEST for the alike lowest singlet and triplet states may coexist with bright and narrow emission (which was considered as one of the major challenges in the field) because MR-TADF molecules sustain highly delocalized excitations with short-range charge-transfer character. We first applied our methodology to boron and nitrogen doped triangulene molecules, developed by Prof. Hatakeyama. In collaboration with Prof Zysman-Colman at the University of St Andrews, we designed a blue-emitting nitrogen and diketone substituted triangulene derivative which once in a OLED device exhibits an EQE up to 21 % with limited roll-off. In a collaboration with Prof. Wang at Queen University in Canada, we also designed DABNA-1 derivatives where boron and nitrogen have exchanged their respective positions. These compounds show the typical difference density patterns of MR-TADF characteristic of excited states displaying short-range, extended, charge transfer. Interestingly, recently Prof. Hatakeyama exploited this strategy and was able to synthesize a larger boron and nitrogen-substituted MR-TADF showing a reduced delEST together with a higher oscillator strength. This ultrapure blue emitter (469-nm emission wavelength) MR-TADF yields an unprecedented OLED record EQE of 34%, which is the most efficient blue device up to date based on MR-TADF.
Furthermore, we focused on the impact of the environment on the performance of OLED devices. In collaboration with the experimental group of Prof. R. Friend at the University of Cambridge, we developed a detailed understanding of the time evolution of singlet and triplet states in a film and solution for TXO-TPA emitter. Here, with the help of an explicit QM/MM model, we outlined the role of solvent polarization, which activates certain structural reorganizations (absent in a film) with a large impact on the efficiency of the intersystem-crossing processes. We also investigated further effects of the medium that interfere with the intersystem-crossing process, such as exciton-spin interaction and triplet-triplet annihilation, TTA (as well as the down-conversion as a counterpart of TTA). Regarding the former, we proposed a mechanism that explains the optical activation of triplet states in organic molecules by Yb-doped lanthanide nanoparticles, showing that the process involves a double spin-exchange process with the quantum dot. These findings triggered various experimental efforts and the efficient transfer of photogenerated triplets between the molecules and the inorganic shell or even up-conversion was demonstrated experimentally. In the case of TADF-based OLED, the coupling of the emitter to a spin-containing nanoparticle resulted in the increase of ISC rate by three orders of magnitude. Note that although a similar working principle is utilized in e.g. phthalocyanine copper complexes, the effect is usually attributed to a large induced spin-orbit coupling. Here, we have shown that it rather originates from the presence of the non-zero spin, converting the molecular singlet and triplet states into the same (doublet) spin-manifold, in contrast to a common believe.
To further understand the triplet-triplet annihilation processes in organic materials, in collaboration with Dr. A. Rao group, we developed several new emitters with the preferred energy alignment of singlet and triplet states. In particularly, we elaborated on the role of entropy in the down-conversion processes that turned out to be responsible for overcoming a large energy barrier of ~0.3 eV. Moreover, we demonstrated and rationalized the up-conversion of TIPS-anthracene films with a huge anti-Stokes shift of > 1 eV from near-infrared to blue light, that is among the largest reported to date. Note that the latter finding is particularly useful for both blue OLEDs and bio-medical applications.
The results of MILORD were disseminated in a form of scientific publications (7 papers in total including two in high impact journals) and were presented during two scientific events.
