It is strongly believed that further progress in organic light emitting technologies depends on if we can develop heavy-metal-free materials with fast reverse intersystem crossing (rISC) and fluorescence rates in subnanosecond domain. Nowadays, the most promising uprising all-organic OLED technologies including those using thermally activated delayed fluorescence (TADF) phenomenon and TADF combined with Förster resonance energy transfer (FRET), a so-called “hyperfluorescence”, rely on the donor-acceptor TADF materials with the fastest rISC. However, understanding of mechanism of basic phostophysical processes in such materials still remains poorly investigated. Obviously, general theory of TADF is also highly required.
This presentation will focus on the features of popular and most demanded blue and red TADF emitters, which deviate from our understanding within the classic photophysical model. An alternative TADF model will be described [1, 2], which explains these deviations suggesting that spin-flip transitions between the singlet (1CT) and triplet (3CT) states of the charge-transfer character are actually not as “forbidden” as stated by selection rules. The presented model emphasizes the importance of the 3CT-1CT transition, which molecular vibrations/rotations are crucial for rISC and which aspects of molecular design can improve TADF materials.
Bibliography
[1]DOI: 10.1039/d2tc00476c;
[2]DOI: 10.1021/acs.jpcb.0c10605
Financial support: LIDER XI grant (LIDER/47/0190/L-11/19/NCBR/2020) and CHEMFIZ program (WND-POWR.03.02.00-00-I059/1) of National Centre for Research and Development. Sonata 16 project (UMO-2020/39/D/ST5/03094) of National Science Centre, Poland. DFT calculations were performed on the computers of the Wroclaw Centre for Networking and Supercomputing (WCSS), Poland.
Currently, the major problem of most thermally activated delayed fluorescence (TADF) emitters is low rate of reverse intersystem crossing (rISC), a crucial process, responsible for conversion of dark triplet excitons into emissive singlet ones. One of the solutions to accelerate rISC is to increase spin-orbit coupling (SOC) between triplet and singlet states. It can be achieved by heavy-atom effect (HAE). Described here research is aimed to verify HAE concept for TADF materials through detailed experimental and DFT investigations conducted on blue and red/NIR derivatives of TADF emitters with high potential for application [1,2].
Thermally activated delayed fluorescent (TADF) emitters are promising organic materials for application in OLED. The most demanded OLED emitters are the blue ones due to low stability of devices of this color. The main problem is the rate of spin flip transition from the lowest excited triplet to singlet state called reverse intersystem crossing (rISC). The rISC efficiency has the direct influence on the external quantum efficiency (EQE) of OLED. In organic blue emitters the spin-orbit coupling (SOC) is low which causes low rISC and low EQE. In our work, we are looking for the way to increase SOC and rISC in blue TDAF emitters. Previously we found that interactions of triplet and singlet states of charge transfer (CT) nature are playing the key role in rISC [1,2]. The influence of different substituents and their position on 3CT-1CT transition in blue TADF emitter DMAC-DPS was studied. We introduced different substituents at donor, acceptor and linker fragment of DMAC-DPS molecule. The substituent effects on the geometry (torsion angle over the σ-bound between donor and acceptor fragments), energy of states, SOC and 3CT-1CT gaps were analyzed. The impact of triplet states localized on donor and acceptor fragments was also analyzed. Our conclusions are helpful in further understanding of different rISC transitions. References: 1) DOI: 10.1039/d2tc00476c; 2) DOI: 10.1021/acs.jpcb.0c10605. Financial support: V.I is grateful to the National Science Centre, Poland within the Sonata 16 project No. UMO-2020/39/D/ST5/03094. Quantum chemical calculations were performed on the computers of the Wroclaw Centre for Networking and Supercomputing (WCSS), Poland.
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