Achievement of Highly Efficient and Strong Emission of Eu(III) Complex in a Host-Guest Film by Triplet Sensitization

Trivalent lanthanide (Ln(III)) materials are ideal for luminescence applications owing to their narrow-band emission and superior color purity. However, they suffer from the inherent drawback of the tiny absorption coefficient. To overcome their tiny absorption coefficient due to the forbidden f-f transitions, tremendous effort has been devoted to developing photoluminescent Ln(III) complexes using molecular technology focused on realizing efficient energy transfer from antenna ligands to Ln(III) as well as strong light absorption of the antenna ligands. However, the development of antenna ligands that can be used for sensitization is limited due to difficulties in controlling the coordination structures of lanthanides.<br/>In this study, we overcame the challenge by a simple strategy using solution-processed host-guest films: host-molecule-based sensitization of trivalent europium (Eu(III)) via triplet exciton. When compared to conventional luminescent Eu(III) complexes, a system composed of mT2T (2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine) host molecules and Eu(hfa)₃(TPPO)₂ (hfa: hexafluoroacetylacetonato, TPPO: triphenylphosphine oxide) significantly increased overall photoluminescence intensity (<i>I</i>_PL). We achieved a 500-fold enhancement of the <i>I</i>_PL of an Eu(III) complex with a common ligand. Moreover, the combination realized sensitization from more than 40 host molecules per Eu(III).<br/>To investigate the source of the relatively high <i>I</i>_PL, we used time-resolved photoluminescence spectroscopy (TR-PL) and femtosecond transient absorption spectroscopy (fs-TAS) in a multiscale temporal range from sub-picoseconds to hundreds of microseconds and investigated the emission mechanisms of the host-guest film. Beginning with the initial excitation of the host molecules and ending with the emission of the Eu(III) complex, we elucidated the mechanisms of all processes in the film: (1) the intersystem crossing (ISC) in the host molecule, (2) the inter-molecular energy transfer process from the host molecules to the ligands of the guest Eu(III) complex, (3) the intra-molecular energy transfer process from the ligands to Eu(III), and (4) the emission processes of f-f transitions in Eu(III). Furthermore, we discovered that the yields of all energy transfer processes, (1)-(3), are nearly unity and that the yield of the Eu(III) emission process (4) determines the overall quantum yield of the film. This highly efficient photoluminescence is attributed to the ideal triplet sensitization processes: rapid and efficient ISC in mT2T results in efficient triplet-triplet inter-molecular energy transfers with no loss. Finally, we propose clear design strategies for efficient light harvesting of Eu(III): the host-guest system with host molecules (1) which shows efficient ISC, and (2) whose triplet state energy matches that of the ligand of the Eu(III) complex.<br/>Our discovery paves the way for efficient light harvesting of Eu(III) complexes with simple fabrication using a solution process.