Mechanism of Two-photon Absorption Enhancement for a Piperazine-based Zinc Ion Probe

Zinc ion is an essential component in enzymes and proteins and involves in many biological processes. Although many two-photon zinc ion probes have been synthesized and applied for detecting the zinc ion, the related theoretical study is still in the primary stage. To explore the mechanism of coordination induced two-photon absorption (TPA) enhancement for a piperazine-based zinc ion probe, molecular dynamics simulations in combination with quantum chemical calculations are employed to calculate the TPA properties of the probe and its coordination complexes. The configuration evolution of the probe and the coordination process in water are investigated by molecular dynamics simulations. It is found that there are two coordination modes, which include mono-coordinated mode and bi-coordinated mode, to form zinc complexes. In comparison with the probe, the coordination complexes have more planar backbones and more stable structures. On the basis of the simulated configurations, a series of geometries for the probe and the zinc complexes with two coordination modes are optimized by quantum chemical methods and their TPA properties are calculated by the response theory and two-state model. The results show that the coordination mode and the structure of piperazine unit have important effects on TPA. The twisted piperazine ring in the mono-coordinated complex is very beneficial for improving the strength of TPA without changing the wavelength. The bi-coordination always decreases TPA and produces a blue-shifted wavelength. Through configurational sampling, the averaged TPA spectra for the probe and the complexes are calculated and the influence of the structural flexibility is analyzed. Because the probe has a more flexible structure, its averaged TPA intensity decreases, and then the great increase of TPA can be observed in the zinc complexes with mono-coordinated mode. Therefore, it is the special complex with mono-coordinated mode and the flexibility of the probe that produce the TPA enhancement after coordination in experiment. The present research would be helpful for understanding and predicting of the photophysical properties for two-photon zinc ion probes.