Yin Jun, Liu Shenghua group, central China Normal University: stimulated response benzothiadiazole near infrared fluorescent dyes

In recent years, near-infrared fluorescent dyes have been widely concerned, and have been widely used in biological imaging, intelligent sensing and other fields In the aspect of biological imaging, NIR fluorescent dye has the advantages of small background interference, strong penetrability and low phototoxicity In addition, as sensor materials, they can also be used in stimulus response, defect detection, information storage and other aspects Therefore, it is of great significance to design and develop new near-infrared fluorescent dyes for biological imaging and fluorescence sensing The molecules with benzothiadiazole / benzoselenidazole as the basic framework have been widely used in photoelectric materials Based on this, Yin Jun and Liu Shenghua of central China Normal University selected benzothiadiazole / benzoselenidazole as electron acceptors and constructed a series of D - π - A - π - D compounds By changing the strength of electron donor or electron acceptor, the relationship between molecular structure and near-infrared emission, color change behavior of multi stimulus response was explored (as shown in Figure 1) Figure 1: schematic diagram of molecular structure, molecular design and mechanical discoloration First, the author selects 2,1,3-benzothiadiazole and benzoselenidazole as electron absorption centers, and vinyl as conjugated bridges, which are respectively connected to electronic groups: pyridyl, p-methoxyphenyl and n, Six compounds, btd-565, btd-580, btd-675, btd-685, btd-695 and btd-se-740, were designed and synthesized The calculation of solvent effect density functional theory shows that there is obvious intramolecular charge transfer in these six D - π - A - π - D compounds In the solution, except btd-565, the charge transfer process of the other five compounds is easily affected by the polarity of the solvent and shows a significant solvent effect The AIE effect is worth mentioning that the propeller like tetrastyrene structure introduced in btd-se-740 has successfully realized the near-infrared region aggregation induced fluorescence (AIE) of the compounds in solution state Molecular structure and solid-state fluorescence emission through the study of molecular structure and solid-state fluorescence emission, the author found that the red shift of solid-state fluorescence emission of compounds occurred with the increase of the electron delivery ability of terminal group or the electron absorption ability of central group Specifically, benzothiadiazole was used as the electron acceptor center to introduce the donor with enhanced electron donating ability in turn: pyridyl, methoxyphenyl, N, n-dimethylphenyl The solid emission of the compound was red shifted from 565 nm to 685 nm After introducing fluorine atom or replacing sulfur atom with selenium atom on the basis of benzothiadiazole, the electron absorption ability of the central group can be further enhanced, so that the compound solid fluorescence can be further red shifted to near infrared 740 nm (as shown in Figure 1) All in all, for D - π - A - π - D compounds, more fluorescent dyes with near-infrared emission can be obtained by simply adjusting the electron donor or electron absorption center The molecular mechanochemical properties explore its solid-phase properties The author found that these compounds have reversible mechanochemical properties and can achieve reversible transformation under the condition of mechanical grinding / solvent fumigation (or heating) (as shown in Figure 2) Except for the mechanical blue shift of btd-565 (565-550 nm) modified by bipyridyl group (which can be used as a strong electron absorption group), the other five molecules show the phenomenon of mechanical red shift Among them, the mechanical discoloration of btd-675 (675-715 nm), btd-685 (685-725 nm) and btd-695 (695-745 nm) moved from visible region to near infrared region And for the first time, the author realized the visualization of fluorescence changes in the near infrared region through the living imaging system (as shown in Figure 3) In addition, btd-se-740 (740-800 nm) has fully realized the mechanical discoloration in the near infrared region Fig 2: response behavior and crystal accumulation of molecular multistimulation (source: SCI Chin Chem 2019, 62, 440-450) Fig 3: Xenogen IVIS spectral images of btd-695 before grinding (a, b), after grinding (C, d) and after fumigation with dichloromethane (E, f); (a, C, e: excitation wavelength: 470 nm, collection fluorescence region: 640 nm); (B, D, f: Excitation wavelength: 470 nm, collection fluorescence region: 745 nm) (source: chem EUR J 2018, 24, 3671-3676) mechanism of molecular mechanical discoloration the author found that there are many weak interactions in the aggregation state of these compounds through the analysis of single crystal structure and powder diffraction Under the action of mechanical force, the orderly accumulation of molecules is destroyed and transformed into an amorphous state, thus showing the mechanical discoloration property As we all know, near-infrared fluorescent dye has been widely used in biological imaging, intelligent sensing and other fields due to its advantages of small background interference and strong penetration ability Through reasonable molecular design and optimization, a series of near-infrared fluorescent molecules were obtained Among them, compound btd-695 can transfer and erase the information on the film surface by mechanical force and solvent fumigation (as shown in Figure 4) Btd-se-740 has been successfully applied to intracellular lysosome tracing and zebrafish in vivo imaging due to its far-reaching near-infrared emission and AIE characteristics (as shown in Figure 5) Figure 4: simple application on the film surface: write "NIR" on the filter paper attached with btd-695 compound with a knife, and then eliminate it by solvent fumigation The above: photos taken under 365nm ultraviolet lamp, the following: photos taken under fluorescent lamp (source: chem EUR J 2018, 24, 3671-3676) Figure 5: (a) confocal microscopy of lysosomal fluorescence imaging of MCF-7 cells; (b) zebrafish fluorescence imaging (source: org Lett 2019, 21, 7213-7217) This study provides a new idea for the design of more multifunctional near-infrared fluorescent materials by exploring the relationship between molecular structure and function At the same time, these compounds have potential application value in mechanical sensing, biological imaging and other fields The results were recently published in organic letters (org Lett 2019, 21, 7213-7217) Master student Ye Fengying is the first author, Professor Yin Jun and Professor Zhao Wenbo are the corresponding authors.