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  • Title: Role of ligand-to-metal charge transfer state in nontriplet photosensitization of luminescent europium complex.
    Author: Fu LM, Ai XC, Li MY, Wen XF, Hao R, Wu YS, Wang Y, Zhang JP.
    Journal: J Phys Chem A; 2010 Apr 08; 114(13):4494-500. PubMed ID: 20210293.
    Abstract:
    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.
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