Speaker : Dr Benjamin Mourot

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Speaker : Prof Pachaiyappan RAJAMALLI

Abstract : Thermally activated delayed fluorescence (TADF) emitters have garnered much attention due to 100% exciton utilization and toxic metal-free design. However, most of the TADF emitters experience a concentration-quenching effect due to which emitting layers are dispersed into the host matrix. There is an urgent need to develop emitters that give the same performance and emission wavelength irrespective of the concentration of emitters. Herein, two TADF emitters (2BPy-pTC and 2BPy-oTC) are designed and synthesized. For both emitters, the nature and energetics of the lowest excited singlet and triplet together with the extent of through-bond exciton transfer (TBET) and through-space exciton transfer (TSET) are unveiled using reliable quantum-chemical calculations. While 2BPy-pTC exhibits pre-dominantly TBET, a greater extent of TSET is found in 2BPy-oTC. 2BPy-pTC displays blue color with emission maxima at 469 nm while 2BPy-oTC exhibits green color with emission maxima at 509 nm in toluene. Both emitters show a low singlet-triplet energy gap (ΔE_ST) of 0.20 eV for 2BPy-pTC and 0.01 eV for 2BPy-oTC and a delayed lifetime of 147.4 μs for 2BPy-pTC and 7.4 μs for 2BPy-oTC. 2BPy-pTC shows EQE_max of 12% with an Electroluminescence (EL) peak at 467 nm while 2BPy-oTC shows EQE_max of 24% with EL maxima of 500 nm. In the case of 2BPy-pTC, upon increasing the concentration of the dopant from 5 wt% to 100 wt%, the EL peak experiences a bathochromic shift from 467 to 495 nm and EQE_max drops from 12% to 5.5%. On the other hand, 2BPy-oTC maintains EQE_max of ~24% and EL maxima of 500 nm while increasing the concentration of dopant from 5 wt% to 100 wt%. Hence, 2BPy-oTC acts as a universal dopant for both doped and non-doped OLEDs through which the tedious co-deposition process can be avoided.

References

[1] Chem. Commun., 2024,60, 9234.

[2] J. Mater. Chem. C, 2023, 11, 16368.

[4] ACS Appl. Electron. Mater, 2023, 5, 4959. [5] Adv. Optical Mater.2024, 2402820.

Olivier Maury (ENS, https://perso.ens-lyon.fr/olivier.maury/)

Old polymethine dyes, discovered in the middle of the XIX^th century for photography application, continue to hold a real fascination in the scientific community owing to their unique spectroscopic properties. In the last decade, these dyes found a renewal of interest for near-infrared (NIR) applications in biological imaging or for the design of advanced photonic materials (laser dyes, nonlinear optics…). Generally speaking, polymethine dyes are charged compounds where the positive (resp. negative) charge is delocalized between two electron-donating (resp. withdrawing) groups via an odd number of sp² carbon atoms. In spite of their wide range of use, the complete rationalization of their very particular photophysical properties remains a matter of debate from both experimental and theoretical points of view.