Study On Recognition Mechanism Of Novel Two-photon Molecular Fluorescent Probes

The emergence of laser makes various nonlinear optical phenomena that are difficult to be detected under the traditional light source develop rapidly.Under this background,organic molecular two-photon absorption materials show a good application prospect in the fields of medicine,biology and so on.Two-photon fluorescence microscopy is a powerful tool for the detection of anions and small molecules in organisms and the environment.At present,a large number of molecular fluorescent probes have been used to detect organic small molecules in experiments,but the relevant theoretical research work is relatively s carce,especially the explanation of the microscopic mechanism of fluorescence probes.In this paper,at the level of quantum chemical calculation,the combination of density functional theory（DFT）and quadratic response theory is used to study the optical properties of a series of novel molecular fluorescent probes synthesized in the laboratory and characterize the recognition mechanism,revealing the nature of the optical response of the fluorescent probes.It provides a theoretical basis for further design and synthesis of novel,efficient and excellent molecular fluorescence probes.The main research contents of this paper are as follows:1.Optical properties and recognition mechanism of N₂H₄fluorescent probe based on SpirodifluoreneThe one-photon absorption（OPA）and fluorescence emission properties of two molecular fluorescent probes based on Spirodifluorene were calculated on DFT level,and the two-photon absorption（TPA）properties of the probes were calculated using the quadratic response theory method.The orbital transition distribution and the change of electric dipole moment of transition in molecular system were theoretically characterized.The reason for the change of optical properties of fluorescent probe was elucidated,and the proce ss of intramolecular charge transfer（ICT）and photoinduced electron transfer（PET）was revealed.In addition,we combine the two-photon transition matrix elements in the reaction between the two probes and N₂H₄ to reveal the response mechanism of the TPA phenomenon.Finally,the effect of charge transfer on the OPA properties of the probe is characterized theoretically by combining the charge transfer process from the ground state to the maximum one-photon absorption state.2.Study on the optical properties of fluorescent probes for targetingβ-amyloid protein and its application in the diagnosis of Alzheimer’s diseaseThe effects ofβ-amyloid on the optical properties of a series of PET fluorescent probes with different donor-acceptor structures were studied by molecular docking simulation and quantum mechanics/molecular mechanics（QM/MM）calculation.Theoretical calculation results showed that the fluorescence effect of the probe was obviously enhanced after the reaction withβ-amyloid protein.In addition,we calculated the TPA properties of the probes before and after their reaction withβ-amyloid.The results showed that the two-photon absorption cross section ofβ-amyloid protein in the environment was much larger than that in vacuum,which is due to the greater degree of electron delocalization and increased electric dipole moment in the fiber-like environment.The above calculation results can reasonably explain the experimental measurement phenomenon,and also demonstrate that this series of probes can be used as a good fluorescence microscopic imaging tool for detectingβ-amyloid protein.In addition,the probes with naphthalene ring as donor or more double bonds in the spacer chain will have better detection ability.3.Optical properties of two-photon molecular fluorescent hypochlorous acid probes targeting endoplasmic reticulumThe OPA properties and fluorescence emission properties of three PET type fluorescent probes（Probe 1,Probe 2 and Probe 3）with naphthalimide as fluorophore and different recognition groups were studied at the DFT level.At the same time,the TPA properties of Probe 1,Probe 2 and Probe 3 reactimg with hypochlorous acid（HCLO）were calculated using the quadratic response theory.Theoretically,the possibility of Probe 1 and Probe 2 as fluorescent"off-on"probes and Probe 3 as fluorescent"on-off"probes was characterized,and the reason for the change of optical properties of fluorescence probe was elucidated in combination with the distribution of frontier molecular orbitals.Finally,the effect of charge transfer on the OPA properties of the probe is characterized theoretically by combining the charge transfer process from the ground state to the maximum one-photon absorption state.It is important that our calculated results are qualitatively consistent with experimental measurements.The results show that the presence of HCLO changes the TPA cross section of the fluorescent probe,and different recognition groups also have a certain influence on its optical properties.