Handbook of Aggregation-Induced Emission, Volume 2. Группа авторов
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range from 0 to 250 μM. However, in the concentration range of 0–75 and 150–225 μM, the linear responses can be observed, thus providing a platform to detect H2S at low concentration (Figure 1.1). Besides, the sensor shows a good selectivity against other anions (e.g. AcO−, F−, ClO−, IO4−, N3−, NO2−, and OH−, etc.), except for CO32−, which exerts subtle effect on the detection. The biothiols of cysteine, homocysteine, and glutathione also pose some effect because of the presence of thiol in the structures. However, such an influence can be neglected in comparison to the strong signal in the presence of H2S.
Study on the influence of molecular structure on the sensing property is particularly valuable to provide the clues for designing advanced functional materials. With this regard, Tang prepared three conjugated isomers of TPP‐p‐TPE, TPP‐m‐TPE, and TPP‐o‐TPE by connecting TPP and triphenylethene at the para‐, meta‐, and ortho‐positions, respectively (Figure 1.2a) [55]. The isomers show the AIE effect due to the melding of the typical AIEgens of TPP and triphenylethene in the structures. Their conformations evolve from an extended to a folded one due to the different linkages between two units, which makes the emissions blue‐shifted and luminescence efficiency to decrease gradually. It is reasonable because the molecular conjugation gets worse as the structure changes.
Interestingly, TPP‐o‐TPE is an easy‐to‐form organic porous crystal thanks to its fold conformation locked by the strong intramolecular C–H···N hydrogen bond. It is an easy‐to‐form DCM‐captured porous crystal (Figure 1.2b). In the crystal, every four molecules coordinately generate a cylinder‐like pore with a volume of around 0.27 nm3, which is enough for accommodating two DCM molecules. The pores are distributed uniformly in the crystal and connected in a straight line as long‐ranged nanochannels. The porous crystal produces a fixed framework structure regardless of the guest molecules. For example, each pore can also capture two THF molecules, with the structure of crystal less changed (Figure 1.2c). However, the size of the pore is tunable to fit different volumes of guest molecules.
Considering the difference in the influence of molecular conjugation and porosity caused by the isomerization effect, it is desirable to investigate their sensing behavior. 2,4,6‐Trinitrophenol (picric acid, PA) is chosen as the first analyte because it is a well‐known model explosive and the accurate detection of explosive meets current needs in antiterrorist and protection of country safety. The detection is basically carried out with the nanoaggregates in solution because of the strong emissions of AIE isomers in the aggregate state. Upon addition of PA, the isomers show similar quenching behavior as the concentration of PA increases. The lifetime of the three sensors has a negligible change before and after analyte addition, demonstrating that a static quenching model dominates the sensing mechanism. Since the process normally takes place from the ground state of isomers to the excited state of PA, no excited‐state behaviors of sensors such as fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET) will occur, while the interaction between the sensor and the analyte plays a crucial role. Overall, the quenching effect of TPP‐p‐TPE is somewhat higher than those of the others at high PA concentration. It is due to the stronger Lewis acid–base interactions preferred to take place between PA and AIE isomers with better conjugation. On the other hand, although TPP‐o‐TPE possesses the worst molecular conjugation, its quenching effect is a bit larger than TPP‐m‐TPE at a PA concentration of 400–500 μM, which is because of its best porosity that increases the binding capacity between TPP‐o‐TPE and PA. Therefore, both factors collectively determine the detection of PA by nanoprobes (Figure 1.2d).
The study is then focused on the sensing of another analyte of Ru3+ by the nanoaggregates because it is important in catalysis but harmful to the environment and toxic to the human beings (Figure 1.2e) [55]. Different from the above research, the isomers show a partial quenching behavior upon addition of Ru3+. The quenching keeps linear at a low‐concentration range of Ru3+ and reaches plateaus at high concentrations. Besides, the quenching constant of TPP‐p‐TPE, TPP‐m‐TPE, and TPP‐o‐TPE increases accordingly. The lifetime study indicates that an obvious decrease of lifetime of sensors is observed after addition of Ru3+. Thus, the quenching mechanism is mainly ascribed to a dynamic (collisional) quenching model, which requires a close contact of the excited molecules and the analyte. The Dexter energy transfer is dominated because a short distance of the donor and the acceptor may help the electron exchange between them. On the other hand, the possible influence of FRET on the sensing is ruled out because all the emissions should be quenched if it exists. It seems that the change of the quenching constant of the sensors is related to the variation in the PL quantum efficiency of isomers. It is more likely that the interaction of the isomers and Ru3+ plays a major role and a much strong interaction will be produced between Ru3+ and isomers with better conjugation.
Figure 1.2 Fluorescent detection of PA and Ru3+ by AIE isomers. (a) Molecular structures of AIE isomers. (b) DCM‐ and (c) THF‐containing organic porous crystals of TPP‐o‐TPE. (d) Plots of I0/I‐1 versus PA concentration. Inset: molecular structure of PA. (e) Plots of I0/I‐1 versus Ru3+ concentration. Inset: molecular structure of PA. In (d) and (e), I0 and I are the peak intensity of probes before and after the addition of analytes, respectively, and the excitation wavelengths of TPP‐p‐TPE, TPP‐m‐TPE, and TPP‐o‐TPE are 350, 336, and 332 nm, respectively.
Although the quenching efficiency of TPP‐m‐TPE is larger than that of TPP‐o‐TPE at low concentrations, it displays the same quenching extent at high concentrations. As the TPP‐o‐TPE is an easy‐to‐form pore in the aggregates, the sensor will encapsulate more Ru3+ into the nanoaggregates, which will enhance their interactions. On the other hand, the presence of 3D voids in the probes may help Ru3+ to diffuse and contact more fluorogens to annihilate the emission. Both factors enable the TPP‐o‐TPE to possess similar quenching extent with TPP‐m‐TPE, while the plateaus of the former will be reached at a higher Ru3+ concentration. However, the isomerization effect has less influence on the selectivity of the sensors.
For examples, by addition of other metal ions like Ag+, Fe3+, and Cu2+, etc., no fluorescent response can be observed.
1.3.3 Chiral Cage for Self‐assembly to Achieve White‐light Emission
Organic cage compounds have drawn considerable attention due to their wide functionalities in self‐assembly, gas adsorption, and catalysis, etc [56]. Currently, the organic cages are basically designed based on the classical organic reactions of specific building units. The conjugated units are promising in organic cage design, and the study of optical properties of organic cage is attractive by introduction of these units. However, most of the conjugated units possess ACQ effect. There are few studies on AIE unit‐based organic cages because of lack of AIE prototype structures with good symmetry for design.
As we know, TPP is an AIEgen with four peripheral phenyl rings symmetrically attached to the central pyrazine. Thus, utilization of TPP to develop a luminescent cage is very promising. Tang reported the synthesis of an amphiphilic organic cage with tetrahydroxyl‐substituted TPP