Handbook of Aggregation-Induced Emission, Volume 2. Группа авторов
href="#ulink_ecb53b24-4af9-550c-a61e-e800a079999b">Figure 3.1). In good solvent, the free rotation of the molecule around the single bond can dissipate the energy of the excited molecule, and the molecule appears to be weakly fluorescent; in poor solvent, the aggregated molecules are emissive due to the restriction of the free single bond rotation as the excited electrons return to their ground state. Such an emission mechanism follows the restricted intramolecular rotation (RIR) process of typical AIE molecules.
Distinct from most AIEgens, SSBs are widely followed and studied because the unique molecular structure renders the AIE process often accompanied by excited‐state intramolecular proton transfer (ESIPT) procedure. ESIPT refers to a phototautomerization process by which organic molecules undergo a proton transfer via intramolecular hydrogen bonding between adjacent proton donors and acceptors in the excited state after light irradiation [3]. Such a procedure always proceeds extremely fast at a subpicosecond time scale. Because molecules with ESIPT properties always have large Stokes shifts, they can effectively avoid the self‐absorption or the internal filtering effects of fluorescence and therefore have wide applications in designing or constructing molecular probes and luminescent materials [4]. ESIPT process is easily affected by the environment (temperature, pressure, polarity, viscosity, and acidity, etc.); its application in the field of fluorescent sensors has thus attracted widespread attention.
SSB is a representative class of ESIPT compounds. The research on the ESIPT of SSB first started in the 1960s. Cohen et al. found a large Stokes shift in the spectrum of salicylanilide and summarized this to the role of proton transfer heterogeneity [5]. Further research found that keto tautomer also exists in two different configurations, cis‐keto and trans‐keto. Subsequently, the strongly emissive yellow‐green luminescence of 3‐hydroxyflavonoids was observed in aprotic solvents, which was also attributed to the luminescence of the proton transfer isomer. This was also the first time for the proposal of the concept of ESIPT. As Figure 3.2 shows, the adjacent position of hydroxyl and imine groups in SSB molecules enables tautomerization of enol to keto form at the excited state and is thus accompanied by ESIPT. The energy band gap between the excited keto (N*) state and ground state (N) is much narrower than that of the non‐ESIPT state. As a result, the ESIPT process (enol–keto tautomerism) generates a red‐shifted emission with a large Stokes shift. Given the beneficial impact of the ESIPT process on the fundamental principles of photophysics, a great deal of researches have been conducted to not only understand but also utilize this process.
Figure 3.1 Schematic illustration of intramolecular rotation and excited‐state intramolecular proton transfer (ESIPT) of two typical salicylaldehyde Schiff base (SSB) derivatives.
Figure 3.2 Schematic illustration of the ESIPT process of SSB derivatives.
Source: Reprinted from Ref. [6] (Copyright 2015 American Chemical Society).
3.1.2 Universal Design of SSB‐based AIEgens
The current generic SSB AIEgens are designed in two ways. As mentioned above, the key factor for SSB derivatives to generate aggregation‐induced fluorescence is the intramolecular hydrogen bonding that helps the entire molecule to rotate around the nitrogen–nitrogen or carbon–nitrogen single bond and ensure the AIE and ESIPT processes. Therefore, the protection and deprotection of hydroxyl groups enable the design and synthesis of most SSB‐based AIE probes and stimuli‐responsive materials. Substitution of protons on the hydroxyl group into specific recognition groups by chemical modifications, the probe can achieve fluorescence “off–on” switch after interaction with an analyte.
Another characteristic property of SSB is the coordination ability with metal ions. The nitrogen atom on the imine structure and the oxygen atom on the hydroxyl group are affluent in lone‐pair electrons, and the spatial conformation is close in size to that of metal ions such as copper(II) and zinc(II). Coordination with metal ions results in quenching or enhancement of fluorescence, depending on the nature of metal ions. This is also one common design approach for the SSB fluorescent probes. For metal ions or other analytes that can interact with metal ions, highly sensitive detection based on changes in the fluorescence intensity of SSB or SSB–metal complexes can be performed.
Since it was first reported in 2009 [7], the AIE properties of SSB molecules have been widely applied in designing fluorescent probes and fluorescent functional materials in chemistry, biology, and environmental science. This chapter summarizes the design and application of SSB as AIEgens of fluorescent probes and materials, for detection and imaging of metal ions, for biologically and environmentally related molecules, and as stimuli‐responsive materials and nanoparticles (NPs).
3.2 Fluorescent Probes
3.2.1 Metal Ion Detection and Imaging
Metal elements exist widely in nature and have applications in various fields of human daily life. Many metal ions support normal life processes and play an irreplaceable role in the organism. For example, as the second messenger in cells, calcium ions are of great importance in the process of signal transmission. Another example is iron ions, which are converted to each other in the form of ferrous and iron in human body. Inadequate intake of iron ions can cause diseases such as anemia and dysplasia, while excessive intake of iron ions can cause oxidization to damage the body, thereby endangering the human heart and circulatory system. In contrast, some metal ions, even when present in trace amounts in the environment, can cause great harm to living organisms. For example, even traces of chromium(VI) ions enter the human body; it will cause serious damage to human skin, respiratory system, kidneys and other tissues, and even cancer. Therefore, the development of simple and practical metal ion detection methods has always attracted great interests of researchers.
Schiff base compounds have lone‐pair electrons on the nitrogen atom in the structure, so that they can complex with metal ions, thereby changing the overall structure and properties of the compound. Due to the simple synthesis of Schiff base compounds, high yields, rich sources of raw materials, and wide choices, especially their good coordination properties, they have been widely used in the preparation of metal ion complexes. SSB fluorophores are therefore with unique advantages in AIE probes for metal ion detection. Additionally, nitrogen in the imine bond and hydroxyl oxygen atoms contained in SSB have excellent metal coordination ability, and the formed metal ion complex has a large stability constant, which is helpful to reduce the detection limit [8]. Because the coordination ability is influenced dramatically by the hydroxyl oxygen atom, which will experience deprotonation at high pH conditions, by adjusting the pH value, the probe can be recycled, and “logic gate” systems for the detection of both metal ions and pH can be designed accordingly [9–11]. In this section, SSB‐based metal ion probes are classified according to different response mechanisms, and three different types of turn‐off, turn‐on, and ratiometric are included.
Turn‐off metal ion probes have strong fluorescence in their aggregate state and fluorescence quenching after binding with target metal ions. Strong fluorescence comes from the AIE property of the probe, and after binding with metal ions, charge transfer between the metal ion and the probe molecule, i.e. metal‐to‐ligand charge transfer (MLCT), occurs, thus resulting in chelated fluorescence quenching. Most reported typical turn‐off metal ion probes are for Cu2+, which is due to the special electronic structure of divalent copper ions. The d9 valence electron layer configuration, which has a single electron, is prone to fluorescence