Читать книгу Handbook of Aggregation-Induced Emission, Volume 2 - Группа авторов - Страница 41

A Schiff base (named after Hugo Schiff [1]) is a compound with the general structure R₂C = NR′ (R′ ≠ H). It is a subclass of imines, being either secondary ketimines or aldimines formed by the condensation of active carbonyl groups of ketones or aldehydes with primary amines, respectively [2]. In salicylaldehyde Schiff base (SSB) derivatives, including salicylaldehyde azine and salicylidene aniline, imine and ortho‐hydroxyl groups can form stable six‐membered ring structures through intramolecular hydrogen bonding, which allows the entire molecule to rotate freely around nitrogen–nitrogen or carbon–nitrogen single bonds (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).