Time-Dependent DFT Study of Emission Mechanism of 8-Hydroxyquinoline Derivatives as Fluorescent Chemosensors for Metal Ions

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8-Hydroxyquinoline derivatives (BQOH and BQClOH) were synthesized as fluorescent chemosensors for metal ions. BQOH and BQClOH hardly show fluorescence themselves, but they showed strong fluorescence with the addition of zinc or cadmium ions. The order of the maximum values of fluorescence intensity (I_f,max) was BQClOH-Zn²⁺ > BQClOH-Cd²⁺ >> BQOH-Cd²⁺ > BQOH-Zn²⁺. Therefore, the ab initio calculations (by Gaussian 98) using the time-dependent density functional method with 6-31+G(d) basis set were carried out on the Zn²⁺ complexes of quinoline chromophore, BQOH and BQClOH, in order to investigate the cause of the difference in the fluorescent intensity (BQClOH-M²⁺ >> BQOH-M²⁺, M²⁺ = Zn²⁺ or Cd²⁺). The results of the calculations showed that the T₂ state of the [ZnBQO]⁺ lay just below its S₁ state, while that was not the case for [ZnBQClO]⁺.