Thermally-activated delayed fluorescence (TADF) emitters are typically used in OLEDs by doping them into a host matrix to prevent emission losses caused by aggregation, concentration quenching, and unwanted excimer emission.TADF molecules that are constructed of donor and acceptor moieties separated by a single chemical bond or a flexible bridge can easily provide soft twisted states, leading to a wide range of shapes and hence CT energies even at room temperature [1, 2, 3]. While small molecules are usually chosen for the host matrix design, polymers could offer better solution processing and lower production costs for light-emitting structures. In this study, we demonstrate that ethyl cellulose (EC), an environmentally friendly polymer, can serve as a host matrix for specific TADF molecules, enabling the preparation of high-quality films through solution processing. We found that the photoluminescence (PL) quantum yield of composite samples using the EC matrix is enhanced by about ten times compared to neat solid-state films of the same dyes. Our PL emission spectroscopy and fluorescence microscopy studies on two pairs of donor-acceptor-donor molecules—each pair having the same donor units but different sulfonyl or carbonyl acceptor groups—revealed distinct morphologies and complex emission bands. The emission color was determined by the interplay of emission components in the multicomponent charge-transfer band. Neat dye films showed aggregated species with red-shifted charge-transfer emission, while guest-host systems exhibited smooth morphology with brighter, more even, and blue-shifted emission. The enhanced PL emission in composite films is attributed to a model where emitter molecules are frozen in the polymer matrix, adopting a restricted twisting disorder. In contrast, the PL emission in neat dye films is weakened due to a more free molecular twisting. We suggest that the specific interface between EC and emitter molecules facilitates specific intermolecular packing and suppresses intramolecular twisting upon photoexcitation, leading to a dominant locally excited state and a resulting blue shift in the emission spectrum. This work was supported by the Research Council of Lithuania (Grant No. S-LU-24-7) and bilateral Ukraine-Lithuania project (М/54-2024).
ETHYL CELLULOSE AS A HOST MATERIAL FOR THERMALLY-ACTIVATED DELAYED FLUORESCENCE EMITTERS
Gintare Krucaite1, Daiva Tavgeniene1, Saulius Grigalevičius1, Alla Bogoslovska2, Amjad Ali3, Glib Baryshnikov3, Petro Smertenko2, Mats Fahlman3, Andrei Smertenko4, Oleg Dimitriev2, 3
1 Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenuplentas 19, Kaunas, LT50254, Lithuania
2 V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, Pr. Nauki 41, Kyiv03028
3 Laboratory of Organic Electronics and Wallenberg Wood Science Center, Linköping University, Norrköping 60174, Sweden
4 Institute of Biological Chemistry, Washington State University, Pullman, WA 99164-7411, USA
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