Unveiling the Relationship between Energy Transfer and the Triplet Energy Level by Tuning Diarylethene within Europium(III) Complexes

Luminescence performance and photoisomerization control of sensitized energy transfer in a series of Eu(acac)3De complexes that contain photochromic diarylethene (De) as the ligand are studied by theoretical methods. Both the open-ring and closed-ring isomers exhibit a consistent coordination mode between the EuIII ion and De. An unneglected weak interaction originating from electrostatic attraction is found in the region of the coordinate bond Eu–N. The open-ring isomer has higher triplet energy levels than 5D1 and 5D0 of the EuIII ion, which facilitates forward energy transfer from De to the EuIII ion. The closed-ring isomer, for the extended conjugated system formed in cyclization, has a much lower triplet energy level than 5D0 of the EuIII ion. The energy-gap deficit makes energy transfer unavailable. By utilization of this phenomenon, regulation of energy transfer and reversible on/off luminescence switching of the europium(III) complex can be achieved. The forward and backward energy-transfer rates ...