Design, Synthesis, And Properties Of Λ-Shaped Pyridinium Salts Optoelectronic Functional Materials

Organic optoelectronic materials have been studied extensively for their important applications in organic light-emiting diodes(OLEDs),organic laser diode,organic solar cells,chemical sensors,and so on.However,most organic chromophores are highly emissive in solution but become weakly luminescent in the solid state,which was called aggregation-caused quenching(ACQ).It is mainly attributed to the formation of theÏ€-Ï€close stacking,excimer,and H-aggregation.Aggregation quenching has been the thomiest problem in the development of optoelectronic devices because the luminescent materials are predominately used as thin solid films.In this sense,studying and searching of excellent optical materials,especially those strongly emissive in the solid state,would be of great importance in both theory and practice.Rencently,an unusual optical phenomenon,namely aggregation-induced emission (ALE),has attracted much attention.Opposite to traditional ACQ materials,they are nonemissive in solution but highly luminescent in the solid state.The discovery of AIE-active materials resolves primarily the problem of fluorescence quenching resulted from the aggregation.And they have shown potential applications in optoelectronic fields such as OLEDs.A-shaped geometry configuration of Tr(o|¨)ger's base is theoretically disadvantageous to formÏ€-Ï€close stacking,which commonly results in fluorescence quenching in the solid state.Organic pyridinium salts are historically of special interest in photoelectric field due to their good chemical and thermal properties. Considering these two aspects,we developed a number of A-shaped pyridinium salts based on Tr(o|¨)ger's Base which exhibit a typical AIE behavior.In this paper,we studied their photophysical properties and the relationship between structure and properties in detail.Meanwhile,we also explored their application in bioprobes field.The synthetic routes and purification of these TB analogues are simple,mainly according to the Knoevenagel E and ion-exchange reaction and without column chromatography.These compounds were characterized and confirmed by ¹H NMR,¹³C NMR,and elemental analysis methods.Crystal structure is very important for the study of the relationship between the structure and properties.Five new TB analogues(TB1, TB2,DMDPS,DMDPN,and DMDP-Hg) crystals and one planar pyridinium salt DPPS crystal were obtained by evaporation and diffusion methods,and these crystal structures were investigated by X-ray diffraction.The molecular conformation, molecule stacking and intermolecular interactions in the aggregation state were investigated in detail.It is found that all of these five TB analogues doesn't formÏ€-Ï€close stacking,which proved our molecular design theory.For organic-inorganic hybrids DMDP-Hg,it comprises a three-dimensional supramolecular network constructed from one-dimensional Hg(SCN)⁴ anionic chains,which only constructed by the building block Hg(SCN)^4- linked via S…S interaction and two-dimensional cationic layers.To explore the reason of AIE phenomenon of DMDPS and study the relationship between structure and properties,a planar pyridinium salt DPPS was designed and synthesized as a contrast.Different from DMDPS,DPPS has a planar conformation and shows normal optical properties.It exhibits efficient fluorescence in solution and weak emission in the solid state.Compared the optical properties and structures of DMDPS and DPPS,we speculate that the enhanced emission of DMDPS may mainly be attributed to the twisted geometry configuration which sterically disturbs close packing by increasing intermolecular distances.The distances between two neighboring molecules of DMDPS and DPPS are found to be approximately 3.8(?) and 3.3(?), respectively.The enantiomerization and/or intramolecular vibrational motion which induced the nonradiative deactivation process caused fluorescence quenching in solution while loose stacking resulted from twisted molecular geometry configuration possibly reduced the distance-dependent intermolecular quenching effect,restrict of dynamic processes,and effective intramolecular charge-transporting to produce intense fluorescence in the aggregation state.Fluorescence(FL) bioprobes especially the turn-on bioprobes for protein detection and quantification have received great attention due to their high sensitivity,low background noises,and accordingly,they show potential applications in chemistry, materials science,biology and medicine.Water-soluble A-shaped pyridinium salts DMDPS,DMDPI,and DMDPN,which possess AIE phenomenon and excellent photophysical properties,such as the absorption in the near-UV region(λ_max≈400 nm) and the large Stokes' shift(△λ≈147 nm),can be used as fluorescence turn-on bioprobes for protein detection.Further more,the plots of photoluminescence intensity versus BSA concentration(0-70μg/mL) display a good linear relationship,indicating quantitative detection can be achieved.Moreover,the AIE nature allows the use of large fluorophore/protein ratios,enabling the detection of trace amounts of proteins.In summary,in the process of the exploration of highly emissive organic solids,we presented a new design strategy and established a new system of AIE materials. Meanwhile,we gave reasonable explanation of the AIE phenomenon of the A-shaped pyridinium salts by analyzing the crystal structures and through comparative experiments.In addition,the water-soluble A-shaped pyridinium salts were used in bioprobes field and the results are satisfactory.In view of their special structural feature and excellent optical properties,A-shaped pyfidinium salts are good candidates for theoretical research and with potential applications.