Theoretical Investigation Of Photophysical Properties In A Series Of Organic Small-molecule Two-photon Excited Fluorescent Probes

With the development of laser technology,non-linear optics and two photon absorption(TPA)phenomenon have attracted more and more attention.The materials with excellent TPA properties are widely used in a variety of fields,especially their potential applications in biology society field have drawn much attention,such as two-photon fluorescence imaging,biological labeling,photodynamic therapy and so on.Until now,the ideal and practical TPA fluorescent probes are extremely scarce.For the development of fluorescence imaging,it is urgent to study and design the organic small-molecule two-photon excited fluorescent(TPEF)probes,which have large TPA cross section and high fluorescence quantum yield.Therefore,we thoroughly study the influence of molecular structures on photophysical properties in a series of organic small-molecule TPEF probes from four aspects.We successfully explain the fluorescent mechanism and provide useful theoretical guidelines for designing and synthesizing new TPA fluorescence probes with exceptional properties.The main research contents are given as follows:1.Analyzing the Effect of Substituents on the Photophysical Properties of Carbazole-Based Two-Photon Fluorescent Probes for Hypochlorite in MitochondriaThe development and utilization of two-photon fluorescent probes with large two-photon absorption(TPA)cross sections(?TPA)and strong TP-excited fluorescence are highly needed for monitoring the level of hypochlorite in mitochondria.In this contribution,we successfully explain the fluorescence “off-on” effect observed in experiment,and clarify the fluorescent quenching mechanism of the probe containing hydroxyl oxime from the aspect of excited-state dynamics for the first time,while explore the influence of alkyl chain length on TPA cross section and fluorescent quantum yield.It is proposed that the alkyl chain length affects significantly the fluorescent efficiency by the vibronic coupling.The proper alkyl chain length should also be one of the smart strategies for designing highly efficiency fluorescent two-photon probes applied in biological system.We demonstrate that the designed compound HCB-CNO has exceptional optical properties,such as the larger TPA cross section(467 GM)and higher fluorescence quantum yield(0.485),compared to the experimental molecule.Moreover,the TPA tensor elements are increased remarkably by introducing strong electron-donors at R1 position,which promote to intramolecular charge transfer,and improve significantly two photon absorption cross sections.2.Theoretical Investigations on the Photophysical Properties for a Series of Symmetrical and Asymmetrical Carbazole-based Cationic Two-Photon Fluorescent Probes: The Magic of Methyl GroupsThe biological fluorescence imagining for nucleic acid has attracted attention due to the essential role of nucleic acid in living system.Two-photon(TP)fluorescent probes are important molecular tools for biological imaging due to their high resolution and low photo-damage to tissues.However,the practically applicable TP fluorescent probes are still limited,because of indistinct fluorescent mechanism and ambiguous relationship between molecular structures and two-photon excited fluorescent properties.In this contribution,we researched the photophysical properties for a series of symmetrical and asymmetrical carbazole-based cationic TP fluorescent probes for nucleic acid,and explained the fluorescent mechanism of the nucleic acid probes from the aspect of excited-state dynamics.It is firstly proposed that the fluorescence quenching for the cationic probes with terminal methyl chains is derived from methyl rotating motion in low-frequency regime rather than vinyl rotating motion in water solvent.We illuminated the origin of better TPA properties for cationic compounds compared with corresponding neutral compounds.The TPA efficiency is much higher for symmetrical compound than that for asymmetrical compound,and the symmetrical compound is more stable due to restricted molecular bending vibrations.Besides,the electron-donating/withdrawing ability of substituent groups and the position of methyl at acceptor also have important influence on TPA properties of cationic compounds.3.An Excellent Benzocoumarin-based Ratiometric Two-Photon Fluorescent Probe for H₂O₂The level of hydrogen peroxide(H₂O₂)plays an essential role in regulating biological process.Detecting H₂O₂ in vivo or vitro deep tissue by utilizing two-photon(TP)fluorescence probes can significantly alleviate the detection damage of living organisms to obtain high-resolution imaging compared with one-photon(OP)fluorescence probes.But few of TP fluorescence probes possess both high fluorescence efficiency and easily distinguishable spectra for measuring H₂O₂.Therefore,in-depth understanding of the relationship between electronic structure and two-photon fluorescence properties and finding the probes with excellent performance are still challenging.Aiming to such a target,we designed a series of benzocoumarin-based ratiometric TP fluorescent probes and corresponding product molecules for H₂O₂.Then,we theoretically evaluated the two-photon recognition performance of these compounds and studied the relationship between their molecular structure and TP performance by time-dependent density functional theory and quadratic response theory.Besides,we also simulated the spectral properties and fluorescence efficiency.Fortunately,in this contribution,an excellent TP probe BC-3 and corresponding product molecule DCCA-3 are put forward,which manifest large TPA cross sections in the NIR region(3420 GM/988 nm,316 GM/939 nm),large Stokes shifts(116 nm,60 nm)and emission shift(225 nm),thus the probe enables simultaneous NIR and two-photon imaging of H₂O₂ which is unique and has never been previously reported.What's more,we thoroughly illuminated the effect of benzene-fused positon in the coumarin backbone on transition dipole moment and non-radiative decay channel,explaining the fluorescent near-quenching mechanism of benzo[f]coumarin derivative DCCA-4 for the first time.4.Two Novel Ratiometric Two-Photon Excited Fluorescent and Room-Temperature Phosphorescent Probes for HOCl —— Design Strategies and Theoretical InvestigationsHypochlorous acid(HOCl)plays a critical role in natural defense system,but excessive production of HOCl eads to many diseases.It's very essential to monitoring the distribution and level of endogenous HOCl.However,the applicable two-photon excited fluorescent(TPEF)and phosphorescent(TPEP)probes for biological imaging and quantitative detection is rare due to low efficiency and vague mechanism.Therefore,we thoroughly researched the electron structures and luminous mechanisms of turn-on and ratiometric TPEF probe compounds,and provide design strategies for high efficient ratiometric TPEF probes.The designed ratiometric TPEF probe benza1-COCL and product benza1-CO have quite large TPA cross sections(143 GM/898 nm,168 GM/988 nm),high fluorescence efficiencies(0.15,0.28)and spectral resolution,contributing to real-time monitoring level of HOCl in vivo.Besides,a long-lived two-photon excited room-temperature phosphorescent probe benza2-COCL is provided firstly,which possesses large TPA cross section(144 GM/838 nm)and time-resolved biological imaging(1.09 ms)...