Design, Synthesis And Properties Of AIE Organic Materials Based On Triphenylamine

The emission wavelength and intensity of aggregation-induced emission (AIE)-active compounds in the solid or aggregation state can be modulated by changing the composition of molecules, twist conformation and accumulation patterns, which endows these materials with promising application in organic light emitting diode (OLED), fluorescent probes, etc. We have designed and synthesized two series of compounds based on triphenylamine and studied their properties systematacially. Preliminary explorations of mechanism and application of piezallochromy are also described in this article.Main details as followings:1. Based on related literatures, we obtained some useful information about the AIE materials and put forward the design philosophy of the whole article.2. The synthesis of materials involve reaction of formylation, condensation between aldehyde and amine. Six benzimidazole-based Schiff-bases with new structures are characterized through1H-NMR、13C-NMR、MOLDI-TOF-MS and IR. These compounds are also demonstrated to have the typical intramolecular-charge transfer (ICT) according to the DFT (Density Functional Theory). The photophysical properties in different states are studied in details, which confirms the presence of AIE. On the one hand, all the compounds have similar maximum absorption wavelength and maximum emission wavelength, which is indicative of the similar dipole moment. On the other hand, no red-shift of emissions can be observed as the polarity increased, which contributes to the enhancement of emission caused by the restriction of ICT. The SEM, TEM and ED of aggregations are also studied to further explore the mechanism. Different rates and degree of crystallization, which leads to the AIE behavior, could be also responsible for the notably different intensities of fluorescence enhancement. In other words, these derivatives both possess aggregation-induced and crystallization enhanced emission (AIE and CEE) property. The following DLS (dynamic lighting scattering) measurement further demonstrate our inference above. 3. During synthesizing new AIE fluorophors based on triphenylamine, we accidentally found that the reactant4-Diphenylamino-benzaldehyde (1) is not only AIE-active but also PIE (piezofluorochromism-induced emission)-active. We made some simple modifications to afford three compounds:2-(4-Diphenylamino-benzylid ene)-malononitrile (2),4-[Bis-(4-ethoxy-phenyl)-amino]-benzaldehyde (3),2-{4-[Bis-(4-ethoxy-phenyl)-amino]-benzylidene}-malononitrile (4). Compounds1-3can be smartly switched in the fluorescence emission by various external stimuli such as grinding, annealing and solvent-fuming. According to the WXRD and SEM, the change can be explained by the transformation from the crystalline state to the amorphous state and vice versa. The determination of crystals structure provides the further confirmation that the twisted conformation and weak interactions between different atoms or groups are the prerequisite of morphological change.