Role of ligand-to-metal charge transfer state in nontriplet photosensitization of luminescent europium complex.
We have investigated, by means of steady-state and time-resolved optical spectroscopies, the excited-state dynamics of the luminescent europium complex Eu(III)(tta)(3)dpbt (tta = henoyltrifluoroacetonate; dbpt = 2-(N,N-diethylanilin-4-yl)-4,6-bis(3,5-dimethylpyrazol-1-yl)-1,3,5-triazine) with Gd(III)(tta)(3)dpbt and Tb(III)(tta)(3)dpbt as the reference complexes that cannot be photosensitized. In the Eu(III)(tta)(3)dpbt complex, the ligand dpbt exhibited biphasic fluorescence decay kinetics; the faster component (decay time constant, 8.5 ps) is ascribed to the rapid conversion of the lowest-lying singlet excited state of dpbt (S(1) or (1)dpbt*) to a ligand-to-metal charge transfer singlet state of the complex ((1)LMCT*), whereas the slower one (1.8 ns) is shown by temperature-dependent luminescence spectroscopy to be delayed fluorescence due to the LMCT-to-dpbt backward energy transfer and represents the time scale of efficient excitation energy flow from the (1)LMCT* state to the (5)D(1) state of Eu(III). On the basis of the spectroscopic results of the Ln(III)(tta)(3)dpbt complexes (Ln = Eu, Gd, and Tb), the crucial role of the (1)LMCT* state in photosensitization of the Eu(III)(tta)(3)dpbt complex is established, and a LMCT-mediated nontriplet sensitization mechanism is proposed, which is advantageous in high efficiency and low excitation photon energy as well as in low susceptibility against oxygen quenching.
Fu
Li-Min
LM
Department of Chemistry, Renmin University of China, Beijing 100872, PR China. lmfu@chem.ruc.edu.cn
Ai
Xi-Cheng
XC
Li
Man-Yu
MY
Wen
Xiao-Fan
XF
Hao
Rui
R
Wu
Yi-Shi
YS
Wang
Yuan
Y
Zhang
Jian-Ping
JP
D004735:Energy Transfer; D005063:Europium / Q000737:chemistry*; D005682:Gadolinium / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D049449:Luminescence; D049408:Luminescent Agents / Q000737:chemistry*; D008163:Luminescent Measurements / Q000379:methods; D009942:Organometallic Compounds / Q000737:chemistry*; D017785:Photons; D017319:Photosensitizing Agents / Q000737:chemistry*; D013725:Terbium / Q000737:chemistry; D014227:Triazines / Q000737:chemistry*
D008024:Ligands; D049408:Luminescent Agents; D009942:Organometallic Compounds; D017319:Photosensitizing Agents; D014227:Triazines; D013725:Terbium; D005063:Europium; D005682:Gadolinium