Research On Red-emissive Through-space Charge Transfer Complexes Based On Aggregation-induced Emission Molecules

Red/near-infrared(NIR)luminescent materials have caused tremendous interests in bioimaging for their strong tissue penetrability,few interference of background noise andnegligible damage to biological tissues.However,these molecules not only possess strong donor-acceptor(D-A)structure but also have large π-conjugation plane.Therefore,they often emit strongly in solution,while the emission is quenched by theπ–π stacking effect by the aggregation,which is called as aggregation-caused quenching(ACQ)phenomenon.Aggregation-induced emission(AIE)is a novel photo-physical concept which was termed by Tang,et al in 2001.AIE revealed that some fluorescent molecules with twisted conformation emit weakly or give no emission in the solution,while the emission gets intense by the aggregation.However,the preparation of AIE-active red/NIR needs throughtful structural design and trivial organic synthesis,therefore increasing the complexity of material development and preparation cost.Thus,it is of great significance to develop a much easier strategy to get the red/NIR materials.In this paper,we choose pyrazine-based AIE molecules as electron acceptors and phenylamines as electron donors.The red emissions can be obtained readily based on the through-space charge transfer between the acceptors and donors as well as their ―twist plus twist‖ interaction.Our work is given as following:Firstly,the pyrazine-based AIE molecules of A1 and A2 with twisted conformation prepared by the one-step reaction act as electron acceptors,while the commercial phenylamines(e.g.triphenylamine)were selected as electron donors.The through-space charge transfer between them as well as "twist plus twist" interaction,induce a remarkable red emission.The structure of the pyrazine-based AIE compounds was confirmed by the NMR and HRMS.The properties of acceptor/donor molecules as well as their complex films were studied by the absorption/excitation/fluorescence spectra,fluorescent quantum efficiency,luminescent lifetime and two-photon fluorescence,etc..The results show that the A2/D1 and A2/D2 films with a molar ratio of 1:1 have a good luminescent property and the obvious two-photon fluorescence can be collected.Then,A2/D1 nanoparticles(NPs)were used for the two-photon fluorescence imaging of mouse brain blood vessels.Under the excitation of 800 nm fs laser,the imaging depth can reach 400 μm.A 3D reconstruction of images of the mouse brain blood vessels from 0-400 μm give clear information of major blood vessels and some small capillaries.At a depth of 100 μm,the full width at half maximum(defined as the diameter)of capillaries calculated by Gaussian fitting is 4.3 μm,and the signal-to-noise ratio is 13.4,further confirming an excellent imaging contrast,which paves a simple strategy to construct fluorescent dyes for the deep-tissue imaging.Finally,to extend the application of red complex with through-space charge transfer effect and "twist plus twist" strategy,pyrazine-based AIE-active chiral molecules of AR and AS with twisted conformations were synthesized.The NMR,absorption/fluorescence spectra,fluorescent quantum yield,circular dichroism spectroscopy and scanning electron microscopy were utilized to confirm the molecular structure,photophysical properties,chirality and self-assembly.Using the chiral molecule and phenylamines as the electron acceptors and donors,respectively,the red through-space charge transfer complex with tunable emssions can be obtained readily.