Achievement of highly efficient and strong photoluminescence of Eu(III) complex in a host-guest film by triplet sensitization Shiori Miyazaki 1 , Kiyoshi Miyata 1 , Kenichi Goushi 2 , Yuichi Kitagawa 3,4 , Yasuchika Hasegawa 3,4 , Chihaya Adachi 2 , Ken Onda 1 1 Department of Chemistry, Kyushu University, Japan, 2 OPERA, Kyushu University, Japan, 3 Faculty of Engineering, Hokkaido University, Japan, 4 WPI-ICReDD, Hokkaido University, Japan Trivalent lanthanide (Ln(III)) complexes are expected to be used as light-emitting materials, such as organic light emitting diodes (OLEDs), because of their high color purity.[1] Nonetheless, they are hindered by the inherent limitation of their small absorption coefficient. To address this challenge arising from the forbidden f-f transitions, extensive efforts have been dedicated to creating efficient photoluminescent Ln(III) complexes.[2] These efforts center around molecular technology, aiming to achieve effective energy transfer from antenna ligands to Ln(III) ions while ensuring robust light absorption by the antenna ligands. However, the advancement of antenna ligands capable of sensitization is hampered by difficulties in precisely controlling the coordination structures of lanthanides. In this study, we have overcome this challenge through a straightforward approach employing solution- processed host-guest films. Specifically, we achieved sensitization of trivalent europium (Eu(III)) via triplet exciton through host-molecule-based mechanisms. In comparison to conventional luminescent Eu(III) complexes, our system comprising host molecules of mT2T (2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine) and Eu(hfa) 3 (TPPO) 2 (hfa: hexafluoroacetylacetonato, TPPO: triphenylphosphine oxide) as guest components exhibited a remarkable enhancement in overall photoluminescence intensity ( I PL ). Notably, we achieved a 500-fold increase in I PL compared to a typical Eu(III) complex with a standard ligand. To delve into the source of this relatively high I PL , we employed time-resolved photoluminescence spectroscopy (TR-PL) and femtosecond transient absorption spectroscopy (fs-TAS) across a wide temporal range spanning from sub-picoseconds to hundreds of microseconds. This comprehensive investigation shed light on the emission mechanisms within the host-guest film. Starting from the initial excitation of host molecules and culminating in the emission of the Eu(III) complex, we elucidated the stepwise processes within the film: (1) intersystem crossing (ISC) within the host molecule, (2) inter-molecular energy transfer from host molecules to ligands of the Eu(III) complex, (3) intra-molecular energy transfer from ligands to Eu(III) ions, and (4) emission processes involving f-f transitions in Eu(III). Notably, we discovered that the efficiencies of all energy transfer processes (1)-(3) are nearly perfect, and the efficiency of the Eu(III) emission process (4) dictates the overall quantum yield of the film. This remarkable photoluminescence efficiency is attributed to ideal triplet sensitization processes, whereby the swift and effective ISC in mT2T results in efficient triplet–triplet inter-molecular energy transfers with minimal losses.[3] Ultimately, our findings propose well-defined design strategies for efficient light harvesting of Eu(III) complexes. This entails a host-guest system featuring host molecules that (1) exhibit efficient ISC and (2) possess triplet state energies aligned with those of the ligands in the Eu(III) complex. Our discovery opens the pathway for achieving efficient light harvesting in Eu(III) complexes through a straightforward fabrication process utilizing solution
processing. References 1. L. Wang, et al ., Adv. Optical Mater. 2019 , 7 , 1801256. 2. J.-C. G. Bünzli, Chem. Rev. 2010 , 110 , 5, 2729. 3. S. Miyazaki et al. , Chem. Sci. 2023 , 14 , 6867.
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