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A judicious change in the selected transition used for circular polarization excitation will overcome the low oscillator strength limitation of the currently allowed magnetic-dipole 5D1 ← 7F2 (Eu(III)) transition chosen for circularly polarized luminescence (CPL) measurement. The proposed allowed magnetic-dipole 5D1 ← 7F0 (Eu(III)) transition will facilitate the detection of CPL from the Eu(III) systems of interest. CPL on the acetonitrile solution of the chiral tris complex of Eu(III) with (R,R)-N,N′-bis(1-phenylethyl)-2,6-pyridinedicarboxamide ([Eu((R,R)-1)3]3+), recently suggested as an effective and reliable CPL calibrating agent, confirms the feasibility of the proposed experimental procedure. A comparable CPL activity exhibited by the acetonitrile solution of [Eu((R,R)-1)3]3+ following direct excitation in the spectral range of the 5D1 ← 7F0 transition and upon indirect excitation through the ligand absorption bands (λexc = 308 nm) was observed. This confirms that the recommended magnetic-dipole allowed absorption transition, 5D1 ← 7F0, is the transition to be considered in the measurement of CPL. This work provides critical direction for the continued instrumental improvements that can be done for developing CPL into a biomolecular structural probe.


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