Responsive Mechanism And Molecular Design Of Two-photon Fluorescence Probes For Zinc Ions

Two-photon absorption is a third-order nonlinear optical phenomenon.It has great potential applications in biology,chemistry,material science,and so on.The organic molecular materials with strong two-photon absorption properties are widely used in the study of optical limiting,3-dimensional optical information storage and two-photon fluorescence microscope?TPEM?.TPEM makes the cell imaging in vivo come true,and then the two-photon fluorescent probe attracts much attentions to the researchers.Various metal ions such as Ca²⁺,Zn²⁺,Mg²⁺,Pb²⁺and so on,present in living organisms and play important roles in cell growth,functional regulation and the transformation of active enzymes in the body.Then,accurate detection of their content and distribution is of vital importance.At present,many two-photon fluorescent probes for metal ions have been synthesized and their applications in ion analysis have been explored.However,the related theoretical studies are still very limited.Theoretical study on the responsive mechanism of two-photon fluorescent probe,the relationship between molecular structure and properties and the molecular design of two-photon fluorescent probe could provide important guidelines for experimental synthesis.In this paper,the one-photon absorption,emission properties and two-photon absorption of a series of zinc ion probes are calculated by means of density functional theory and response theory method.The main content is divided into two parts.The first part explores the response mechanism of a kind of two-photon zinc ion probe.Based on the model sensor,we have designed a series of two-photon fluorescent probes for zinc ions with better performance.In the second part,the effects of isomerism and coordination mode on the optical properties of two kinds of zinc ion sensors are discussed at length.1.Responsive mechanism and molecular design of two-photon fluorescent probes for zinc ionsThe one-photon absorption?OPA?,emission and two-photon absorption?TPA?properties of a kind of naphthalene-based zinc ion sensor have been investigated employing the density functional theory and response theory method.The responsive mechanism is analyzed.The result are quite consistent with experimental data.Because the intra-molecular charge transfer?ICT?is increased upon zinc ion binding,the TPA intensity is enhanced dramatically.On the basis of the model sensor,we have designed a series of zinc ion probes which differ by conjugation center,acceptor and donor moieties and the OPA,emission and TPA properties of the designed molecules have been calculated.Results demonstrate that the OPA and emission wavelengths of the designed probes have large red-shifts after metal ion binding.In comparison with the model sensor,the TPA intensities of the designed probes are enhanced significantly at longer wavelength range.Furthermore,the TPA intensity can be improved greatly upon zinc ion binding based on the increased ICT mechanism.2.Isomerism and coordination mode effects on optical properties of two-photon fluorescent probes for zinc ions?i?The one-and two-photon absorption,as well as the emission properties of a naphthalene-based zinc ion probe have been theoretically investigated.Various coordination geometries have been considered.Special emphasis is placed on the effects of coordination mode on the optical properties.Upon combining with zinc ion,ICT is increased and OPA,emission and TPA wavelengths show considerable red shift and their intensities are enhanced to some extent.It is also shown that the red shifts are quite different for various coordination geometries.When the zinc ion is connected with the electron acceptor of the fluorophore,the OPA,emission and TPA wavelengths have larger red shifts induced by the lower energy gaps between the related molecular orbitals.?ii?The OPA and TPA properties of two tris?picolyl?amine-based zinc ion sensors have been theoretically investigated.Special emphasis is placed on the isomerism and coordination mode effects on the optical properties.It is shown that the isomerism have some effects on TPA properties of free ligands and can not be neglected.Both the TPA wavelength and the cross section are highly dependent on the coordination mode.When the zinc ion connects with the picolyl unit in the middle of ligand,the zinc complex has the large TPA intensity at long wavelength region due to the increased ICT mechanism.It is also found that the coordination mode effects are enhanced when the donor strength increases.