The Synthesis And Application Of Thiol Fluorescent Probes Based On Diketopyrrolopyrroles(DPPs)

The biological thiols play a vital role. These compounds participate in several important reactions in the human body, and the abnormality of their level will cause kinds of diseases. The most important thiols are cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), and the detection of thiols is of great importance. Fluorescent probes are widely used in the detection of thiols due to their simple operation, rapid response, low cost and good biocompatibility etc. Although lots of thiol probes has been reported, much room is left for development of new fluorescent probes for selective detection of thiols. First of all, almost all of the thiol probes use traditional coumarin, rhodamine, BODIPY as the fluorophore, other type of fluorophores were rarely reported. Second, most thiol probes are fluorescent OFF-ON type, very few are ratiometric, but simultaneously colorimetric and ratiometric thiol probes are more rare. Finally, the recognition mechanism of the probes is commonly traditional single mechanism, such as the cleavage of sulfonamide and sulfonate ester and Michael addition. Novel and mixed recognition mechanism is very rare. On the other hand, diketopyrrolopyrroles (DPP) are a new class of fluorescent dyes developed in recent years. Because they have a brilliant color, high fluorescence quantum yields, good photo stability and are easy to be derivatized, as a result this chromophore is widely used in polymer solar cells, organic semiconductors, two-photon absorption materials, organic light emitting diodes, etc. However, fluorescent molecular probes that based on DPP are rarely reported.Based on the above considerations, herein we designed and synthesized thiol probe P-1and P-2with DPP as the chromophore, and they use aldehydes and benzal diethyl malonatc as the binding site for detection of thiols, respectively. Upon the addition of thiols such as cysteine, the color of the two probes change from orange to green, and the fluorescence also change from orange to green. UV-vis absorption and fluorescence emission spectrum show a blue shift about15nm and50nm respectively. Therefore the two probes can be described as colorimetric and ratiometric with high selectivity. Mass spectrum results show the classic aldehyde cyclization and Michael addition recognition mechanism, DFT/TDDFT theoretical calculation verify the mechanism of the reaction.On the basis of the first two probes, colorimetric and ratiometric thiol probe P-3with DPP as the chromophore were prepared, and malononitrile as the recognition moiety. Probe P-3shows longer absorption (523nm) and emission (666nm) wavelength than the previous two probes, the color of the probe solution is purple, and the fluorescence is deep red. Upon addition of cysteine, the color gradually turned into light yellow, and the fluorescence turned into yellow. The absorption and emission spectrum has blue shifted to479nm and540nm respectively, the shift of wavelength was up to44nm and126nm, respectively, and the intensity at540nm increased by140-fold. Mass spectral analysis demonstrated that besides the traditional Michael addition of thiols, the CN groups of the malonitrile moieties also react with thiols. We also verified this mechanism through the theoretical calculation. Probe P-3was successfully used for fluorescence imaging of intracellular thiols in MDA-231cells.