Theoretical Study On Two-photon Absorption Properties Of Transition Metal Organic Compounds And Two-photon Fluorescent Probe For Transition Metal Ions In Life

In recent years, organic two-photon materials are expected to have potential applications in many fields; especially their potential applications in biology society are of great importance, such as three-dimensional photolysis release, two-photon biological labeling, two-photon fluorescence imaging（TPFM）. TPFM is a new technology combinating laser scanning confocal microscopy and two-photon excitation technique. Compared with the traditional one-photon fluorescence imaging（OPFM）, TPFM has outstanding advantages: accurate positioning, strong penetration, high resolution, avoiding photodamage and photobleaching. In addition, TPFM can achieve dark field imaging and lower SNR. These features of two-photon fluorescence imaging technology make it important to the development of bio-imaging applications. The combination of probe and TPFM has become an indispensable tool for imaging in biomedical research. Howere, so far, the two-photon probe molecules is very limited that can be effectively used in the practical application, which leading the severely limits of the TPFM applications in biology and medical research field. Therefore, it is very important to improve the theoretical basis and design principles of two-photon fluorescence probes and design and synthesis of a variety of effective two-photon probe molecules for the wider range of applications of two-photon fluorescence microscopy. The research of life transition metal ion two-photon fluorescent probe is needed to be improved, and it is the area with a strong development potential and broad prospects.The thesis studies the TPA properties of the transition metal compounds, as well as the two-photon fluorescent probe for life transition metal ions(Zn²⁺, Cu²⁺) by utilizing quantum chemical computational methods at the ab-initio level. Purpose is to reveal the internal relations between probe molecular structure and the two-photon absorption properties, to provide guidance and important information for exploring new probes owing larger TPA cross sections（δmax） and higher up-conversion fluorescence intensity, and to provide the excellent performance of alternative two-photon fluorescent probe molecules. The main contents are summarized as follows:1. The two-photon absorption properties of a series of D-π￠-A-π-[M]-π-A-π￠-D type and A￠-π￠-A-π-[M]-π-A-π￠-A￠ nickel（II）, palladium（II）, platinum（II） acetylides have been explored in detail by using DFT, ZINDO programs and SOS equation. Calculated results show that Pt center is a well electron donor; these molecules have OPA peaks in range of 330-570 nm and have obvious TPA peaks in range of 610-760 nm. Through the analysis of factors influenced TPA cross-section, it is concluded that:（1） δmax values are decreased in the order of Pt, Pd and Ni compounds, while the metal palladium has the outstanding contributions on one- and two-photon absorption spectrum blue shift.（2） Charge transfer between center metal and p-conjugated organic fragment dominates in TPA transitions, in which center metal increases conjugation length in the direction of long ligand, results in large TPA cross section.（3） The maxima δmax of D-π￠-A-π-[M]-π-A-π￠-D type molecules are smaller than A￠-π￠-A-π-[M]-π-A-π￠-A￠ type molecules.（4） For D-π￠-A-π-[M]-π-A-π￠-D type molecules, using triphenylamine end group, 2,1,3-benzothiadiazole bridge and trimethylphosphine ligand can get a large δmax.（5） For A￠-π￠-A-π-[M]-π-A-π￠-A￠ type molecules, using 2,2￠-bithiophene end group and 2,1,3-benzothiadiazole bridge can receive a larger TPA cross-section, 4-methylpyridine and trimethylphosphine are good ligands. This contribution laid the groundwork for finding excellent two-photon metal ions probes.2. The geometrical structure, electronic structure, OPA, TPA, and fluorescence properties of a series of intracellular free zinc ion two-photon fluorescent probes have been studied by theoretical computations. The binding energies（ΔE）, binding enthalpies（ΔH）, Gibbs free energies（ΔG）, and stabilization interaction（E2） analysis show that the designed probe molecules have good sensitivity for the zinc ion. The OPA property studies indicate that OPA is almost unchanged after binding to Zn²⁺. The fluorescence properties and the mechanism of the PET probe studies suggest that, except for AZn5 and AZn6, the fluorescence of other molecules is quenching and can be turned on after coordination with Zn²⁺. These results reveal that this series of compounds can become good OFF-ON-type fluorescent bioimaging reagents except for AZn5 and AZn6. During the TPA process, in most of the molecules the TPA peaks are located in the NIR region, 720-860 nm. Also, the increase in δmax is about 3-7-fold when the probes coordinate with the zinc ion. Therefore, reasonably changing and modifying the fluorophores can produce ideal TP fluorescent sensors for imaging the intracellular zinc ion. The series of compounds with DCS possesses great potential to be excellent TP fluorescent bioimaging reagents in the application of Zn²⁺ rapid detection in vivo.3. On the basis of 5,5’-bis（（E）-2-（9-methyl-9H-carbazol-3-yl）vinyl）-2,2’-bi-pyridine（GBC）, a series of new bipyridine-based probe molecules were designed by means of the modification on different active point and the substitution with the different end group. The geometry optimization, NBO analysis, stability of product molecules, one?photon absorption（OPA） spectra and electronic structure, TPA properties as well as fluorescence properties of these probes in acetonitrile solvent were studied by the density functional theory（DFT） and time dependent-DFT（TD-DFT）. The results indicate when the cis-bipyridine as the center, the GBC has the most stable structure. The spectra results show that, these probe molecules have the obvious OPA in the range of 300-390 nm and the TPA in the range of 600-672 nm. After combining with Zn²⁺, the OPA and TPA spectra red-shift and the TPA cross-section（d） increase for most molecules. Moreover, probes have the fluorescence emission in the range of 368-530 nm, and the fluorescence spectra can red-shift in the level of 37-118 nm upon coordinated with Zn²⁺, except for molecule T-8 and T-9. The analysis of transition nature indicates that, the Zn²⁺ is almost not involved in the optical processes; it only increases the electron-accepting ability of molecular center part. What’s more, the substituent on 5, 5’ site can get the maximum d value and the near infrared fluorescence wavelength, which are beneficial to the accuracy of Zn²⁺ recognition. Moreover, the increase of electron-donating ability of end group at 5,5’-site is beneficial to the red-shift of spectra and the increase of d value; the substitution with electron-accepting group can lead to the blue-shift of spectra and the decrease of d value. These substitutions and modifications do not affect the ability of probes to chelate Zn²⁺. It is worth noting that, molecule T-1 and T-3 have the larger TPA response than GBC; the molecule T-6 have the same maximum TPA wavelength with the product T-6-Zn²⁺, which is beneficial to recognize for Zn²⁺.4. A new two-photon ratiometric fluorescent probe 2a for the detection of copper ion(Cu²⁺) is computational designed and studied. Consequently, we confirmed that 2a is dominated by F?rster resonance energy transfer（FRET） mechanism for fluorescence imaging, and analysed FRET efficiency. The new probe 2a has OPA peak at 450 nm and fluorescence emission at 500 nm. With addition of Cu²⁺ to probe 2a, 2a is catalytically hydrolyzed by Cu²⁺, the hydrazide ring is opened to form 2b molecule, and its emission spectrum is shifted from 500 nm to 630 nm, thus the obvious changes in the fluorescence signal is conductive to the recognition of Cu²⁺. The analysis of the FRET efficiency illustrates that 2a has a higher energy transfer efficiency. Therefore, the probe 2a is a more effective FRET-based ratiometric fluorescent probe for detecting Cu²⁺. Significantly, we predicted the novel probe molecule 2a has a large TPA peak in the near-infrared light region. Therefore, it is deduced that the probe 2a is a two-photon excited FRET-based ratiometric fluorescent probe for Cu²⁺. We hope the detailed investigations can provide a theoretical basis, benefiting to synthesize copper-ion-responsive two-photon FRET ratiometric fluorescent bioimaging reagents.