Pyridine-Functionalized AIE-Materials: Design, Preparation And Application

The concept of aggregation-induced emission,or AIE,provides new strategies for the development of organic optoelectronic materials.Up till now,a great many of AIE-active small molecules,polymers and supramolecular complexes have been prepared and widely used in technical fields such as chemo-/biodetection,optoelectronic devices and stimuli-responsive materials.Among these materials,pyridine-functionalized ones count for a large part.Varied design strategies,like linking pyridine moieties to classical AIE-gens or fabricating novel AIE-gens following the RIM principal based on pyridine units,ensure the structural diversity of this category of materials.The ability of pyridyl groups,sometimes as pyridinium precursors,to form hydrogen bonds,coordinate bonds or electrostatic interactions can be used for the fabrication of different supramolecular systems.Moreover,pyridine moieties endow AIE-active materials with nucleophilicity,basicity,potential net charges or water solubility,which makes them intensively studied both in fundamental science and in applied technology.Based on the aspects above,we did the following work on the topic of pyridine-functionalized AIE-active materials.(I)AIE-active molecules are usually used for the preparation of stimuli-responsive materials and many AIE-active molecules have been reported to repond to mechanical force,heat,electricity,environmental polarity,etc,but few of their structure-property relationships are investigated in detail.We synthesized pyridine-functionalized tetraphenylethene(TPE)derivatives through triple bond bridging and separated a pair of stereoisomers.They both display typical AIE-activity and multi-stimuli-responsive behaviours.By correlating to their single crystal structures,we discussed the difference in their emission wavelength and fluorescence quantum efficiency.Unlike common fluorescent molecules,they show negative solvatofluorochromism.Distinct from the cis-isomer,the fluorescence of the trans-isomer in the ground powder can go back spontaneously in the air,and this was explained by humidity-triggered slow change in molecular conformation.Moreover,this pair of stereoisomers show quick response to acidic/basic vapors.Among the over 1100 publications on TPE derivatives,our work is the 9th of these which clearly separated the stereoisomers and elustrates the structure-property relationship in detail unprecidentally.(II)The systhesis of fluorescence turn-on bioprobes of novel structure,good biocompatibility and high contrast has always been an important research subject.We got an ionic AIE-active probe of good performance through the reaction between the trans-isomer mentioned above and benzyl bromide.It also shows solvato-and mechanofluorochromism.By mechanical grinding and solvent fuming,it can swich between the yellow-greenish-emissive crystalline state and the orange-red-emissive amorphous powder.Due to the good water solubility plus positive net charges the pyridium units endow the AIE-gen with,this compound can bind to polyphosphorous backbones via electrostatic interaction,and can be used to detect DNA in buffer solution.Moreover,it also manifests good membrane permeability and cell viability,and can image mitochondria in living cells.(III)Resctriction of intramolecular motions(RIM),which has been recognized as the mechanism of AIE phenomenon based on previous research,can be the principle for the design of novel pyridine-functionalized AIE-gens of various topology.By substituting the TPE core of the product mentioned above in(?)with a phenyl ring,we got a fluoregen of greater simplicity.And with this building block we synthesized cyclic and linear fluorescent molecules.On the one hand,their photopysical behaviors feature AIE characteristics,while on the other hand,their AIE behavior is“atypical”:their fluorescence intensities soar like an exponential curve with no significant inflection point.For this,we put forward a water-cluster triggered RIM mechanism.(?)Based on the aforementioned“atypical AIE-gens",we considered investigating its relationship with“typical”ones.Starting from the linear compound above,we synthesied two catagories of fluorophores following two routes(adding rotors or adding substituents)and thoroughly investigated their photophysical behaviours.In a word,we prepared a series of novel pyridine-functionalized AIE-active materials and studied their applications in stimuli-repsonsive materials and fluorescent bioprobes.We also put forward new understanding of AIE mechanism based on them.