Synthesis Of Tetraphenylethylene Macrocycles With Aggregation-Induced Emission Effect And Their Applications In Fluorescent Probes

Since aggregation-induced emission (AIE) phenomenon was discovered by Tang and coworkers in 2001, it has attracted tremendous interests. During past several years, a mountain of AIE compounds have been synthesized by researchers. Among the most common AIE fluorophores, relatively simple tetraphenylethylene (TPE) has proven to be the best popular building block for construction of AlE-based molecular motifs. The reason that the TPE derivatives overwhelmingly prevail should be ascribed to the following features:1. While TPE unit is easily synthetically available, a wide variety of functional groups can be also facilely introduced into the four phenyl rings of TPE.2. TPE core has a very stable AIE effect even after it is connected by a large number of functional groups.3. With a simple C2 symmetry, tetratopic reaction positions and their combination, TPE unit is an ideal building block for preparation of ligands, monomers, linkers, receptors, and so on.4. TPE unit is stable under general conditions except some extreme conditions such as very strong ultraviolet light and high temperature more than 500 ℃, which will result in cis-trans isomerization of TPE.1. A TPE-cyclohexanediamine Schiff base macrocycle was synthesized, which could aggregate into nanospheres and emit yellow fluorescence in aqueous media. Thanks to its AIE effect, the macrocycle showed a sensitive and selective response to 2,4,6-trinitrophenol (TNP) and 2,4-dinitrophenol (DNP) among a number of nitroaromatic compounds, which could be used to detect TNP and DNP at nanomolar level. Moreover, it exhibited a superamplified quenching effect by DNP rather than by TNP, which could be possible to discern DNP from TNP. By comparison with open chain TPE-cyclohexylamine Schiff base, the cave of the macrocycle played a key role in exhibiting the selectivity for DNP over TNP. In addition, both quantitative analysis of DNP and TNP in real water samples and qualitative detection of these two analytes in solid state by the macrocycle were tested. The reliability of the quantitative analysis was confirmed by HPLC method. This demonstrated that the TPE Schiff base macrocycle 30 had a great potential as an excellent sensor of DNP and TNP.2. A series of TPE-based macrocycles with an AIE effect for the 0.2-4 ppb level detection of TNT among a number of nitro-aromatic compounds through fluorescence quenching in natural water sources were developed, whereas the contact mode approach using portable paper sensors exhibited a high sensitivity for the detection of TNT at 1.0 x 10-13M level. The reliability of the quantitative analysis has been confirmed by HPLC Our findings demonstrate that the TPE-based macrocycles have great potential as excellent sensors for TNT. Moreover, it was found for the first time that the macrocycles could selectively recognize nitroaromatics explosives bearing methyl group through a CH3-∏ interactions, and even exhibit a sole selectivity for TNT among the very difficultly differentiating nitroaromatics including trinitrophenol and trinitrobenzene.3. A TPE-o-phenylenediamine Schiff base macrocycle displaying aggregation-induced emission (AIE) effect was synthesized by condensation reaction of the TPE dialdehyde and 1,2-benzenediamine. The macrocycle could aggregate into nanofibers in an aqueous solution to give a stable and fluorescence suspension. The fluorescence nanofibers showed a highly selective response to copper ion in aqueous solution with detection sensitivity up to nanomolar level. Meanwhile, the emission of the nanofibers displayed a very large Stokes shift up to 260 nm because of the AIE effect, which brought the emission into the red area and avoided the background interference. Therefore, the macrocycle probe showed a great potential for the detection of Cu (II) in real water samples including pork juice-containing water. In addition, the macrocycle only reacted with Cu(II) to give a color change and could detect copper ion at 10 μM level by naked eyes.4. Three new TPE-BODIPY conjugates, composed of a tetraphenylethylene (TPE) core bearing BODIPY units, were synthesized. The TPE-BODIPY conjugate bearing two neighboring BODIPY units could emit strong fluorescence up to 100%quantum yield and multicolor light from green, yellow, to red (GYR) both in solutions and in poly(methyl methacrylate) films with aggregation of it. Moreover, the GYR multicolor emissions in solution could interconvert by adding and removing THF. The crystal structure of the compound disclosed that two BODIPY units were completely perpendicular to the phenyl ring of TPE core but parallel with each other. This extraordinary conformation resulted in intra/intermolecular slipped π-π stackings and even the synergism of these two slipped π-π stackings, rendering the compound the multicolor fluorescence. In contrast, other two TPE-BODIPY derivatives had no intramolecular π-π stacking and only displayed green and yellow emission.5. Several tetraphenylethylene-pyridine derivatives with AIE effect were prepared and their solid-state emission can be repeatedly tuned from blue or green to yellow by grinding-heating processes due to the transformation from the crystalline to the amorphous state and vice versa. The investigation of the co-crystal with distinctly different fluorescent emissions(yellow to orange) indicated that the process for the acid-stimuli-response underwent a two-step transformation, i.e. protonation of the pyridine moiety, and then planarization of the resultant complex. To further demonstrate the unique photophysical property, density functional theory (DFT) analysis was carried out for these compounds. Such multicolored switching features of TPE-pyridine derivatives may enable this novel AIE dyes to promise an array of potential applications in the fields of chemosensors, optical displays and rewritable optical media.