Excitation Energy-Transfer Processes in the Sensitization Luminescence of Europium in a Highly Luminescent Complex.

Yan‐Jie Huang,Ying‐Chao Ma,Can Ke,Shu‐Feng Wang,Yu Li,Yuan Wang,Li‐Min Fu,Jan‐Ping Zhang

doi:10.1002/open.201900012

Abstract

The excitation energy transfer (EET) pathways in the sensitization luminescence of EuIII and the excitation energy migration between the different ligands in [Eu(fod)3dpbt] [where fod=6,6,7,7,8,8,8‐heptafluoro‐2,2‐dimethyl‐3,5‐octanedione and dpbt=2‐(N,N‐diethylanilin‐4‐yl)‐4,6‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,3,5‐triazine], exhibiting well‐separated fluorescence excitation and phosphorescence bands of the different ligands, were investigated by using time‐resolved luminescence spectroscopy for the first time. The data clearly revealed that upon the excitation of dpbt, the sensitization luminescence of EuIII in [Eu(fod)3dpbt] was dominated by the singlet EET pathway, whereas the triplet EET pathway involving T1(dpbt) was inefficient. The energy migration from T1(dpbt) to T1(fod) in [Eu(fod)3dpbt] was not observed. Moreover, upon the excitation of fod, a singlet EET pathway for the sensitization of EuIII luminescence, including the energy migration from S1(fod) to S1(dpbt) was revealed, in addition to the triplet EET pathway involving T1(fod). Under the excitation of dpbt at 410 nm, [Eu(fod)3dpbt] exhibited an absolute quantum yield for EuIII luminescence of 0.59 at 298 K. This work provides a solid and elegant example for the concept that singlet EET pathway could dominate the sensitization luminescence of EuIII in some complexes.

Highlights

In most investigated lanthanide complexes, the triplet states of antenna ligands dominate the excitation energy transfer in the sensitization luminescence of lanthanide ions.[9,10,11,12,13,14] while the direct experimental evidence for the singlet EET pathway in the lanthanide sensitization luminescence, which does not involve the triplet states of antenna ligands, had been very limited, it was proposed by Kleinerman[15] in 1966 and supported by some indirect evidences.[16,17]
A further investigation on the energy migration processes among the excitation states of different ligands in a dpbt-sensitized EuIII luminescence complex is helpful to demonstrate the dominant role of the singlet EET pathway.[8]
The energy migration processes between the different ligands and the EET pathways for EuIII sensitization luminescence in a highly luminescent complex [Eu(fod)3dpbt]

Summary

Luminescence of Europium in a Highly Luminescent

Yan-Jie Huang+,[a] Can Ke+,[a] Li-Min Fu,[b] Yu Li,[c] Shu-Feng Wang,[c] Ying-Chao Ma,[b] JanPing Zhang,*[b] and Yuan Wang*[a]. The excitation energy transfer (EET) pathways in the sensitization luminescence of EuIII and the excitation energy migration between the different ligands in [Eu(fod)3dpbt] [where fod =. S1(dpbt) exhibits an obvious feature of charge transfer.[18,19] After that more europium luminescent complexes capable of being sensitized by singlet intra-ligand charge transfer states were reported.[20,21,22] Due to the avoidance of energetic constraint from the dpbt T1 state, the singlet pathway endows [Eu (tta)3dpbt] and its analog [Eu(tta)3bpt] (bpt, 2-(N,N-diethylanilin4-yl)-4,6-bis(pyrazol-1-yl)-1,3,5-triazine) with efficient EuIII luminescence under one-photo excitation in visible region or twophoton excitation in near-infrared (NIR) region,[19,23,24] which has been applied in studying the tumor-targeting dynamics of nanocarriers in living animals using a home-built two-photon excitation time-resolved imaging system.[3].

Renmin University of China

Results and Discussion

Conclusions

Experimental Section

The authors express their thanks for the support from the Chinese