Luminescence performance and photoisomerization control of sensitized energy transfer in a series of Eu(acac)(3)De 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 Eu-III 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 D-5(1) and D-5(0) of the Eu(III )ion, which facilitates forward energy transfer from De to the Eu-III ion. The closed-ring isomer, for the extended conjugated system formed in cyclization, has a much lower triplet energy level than (5)D(0)( )of the Eu-III 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 in different channels are also calculated for the series of complexes. A statistics diagram is obtained to exhibit the change trend of energy-transfer rates in the forward and backward directions as a function of the triplet energy level, which indicates the contribution of different channels to energy transfer in each level region and figures out that the optimal triplet energy level should be in the range of 21740-19532 cm(-1).