Читать книгу Handbook of Aggregation-Induced Emission, Volume 2 - Группа авторов - Страница 30

Acidichromism refers to the phenomenon that the absorption and fluorescence spectra of organic dye molecules change significantly with the change of pH. Organic luminescence materials with acidichromism properties are often used in acid sensor, biological imaging, etc. It is worth noting that the molecules containing nitrogen organic functional groups such as pyridines are prone to protonation to form hydrogen bonds, so the molecules under protonation–deprotonation stimuli will show obvious acidichromism properties in both solution and solid state.

Figure 2.3 (a) Reversible structural isomerization between 1‐11 (DSA‐2SP) and DSA‐2MC under different stimuli; (b) solid‐state photoinduced luminescence switch of 1‐11 for advanced anti‐counterfeiting and super‐resolution imaging applications; (c) photoluminescence spectra of 1‐11 (DSA‐2SP) powders before and after UV irradiation, ex: 410 and 560 nm; (d) overlap of MC absorption and DSA emission spectra, ex: 410 nm [44].

Source: Reprinted (adapted) with permission from Ref. [44]. Copyright © 2017 American Chemical Society.

The fluorescence of DSA derivative 1‐13 powders changed from green to orange after being fumed with hydrochloride (HCl) vapor and returned to its initial state after being fumed with triethylamine [46]. The investigation results of X‐ray structural analysis and theoretical calculation confirmed that the change of fluorescence is the result of the enhanced excitonic coupling and electron delocalization.

Different from compound 1‐13, the protonation degree of DSA derivative 1‐2 (i.e. BP4VA in Figure 2.4) crystals can be regulated by adding different acids and supramolecular interactions between molecules [47]. The neutral single crystal can emit bright green fluorescence. However, the symmetrical protonation state crystal obtained by adding H₂SO₄ shows a red‐shifted orange fluorescence (see BP4VA‐2H in Figure 2.4a), and the fluorescence of the asymmetrical protonation state crystal obtained from adding HCl is further red‐shifted to red (see BP4VA‐1H in Figure 2.4a). Figure 2.4b and c shows the photophysical properties of these 1‐2 molecular crystals. The different protonation states of crystals lead to various supramolecular interactions, different aggregation states, and even tunable optical properties. It provides a mechanism for understanding the role of protonation in organic conjugated molecules and a method for expanding the range of organic functional materials.

Figure 2.4 (a) The symmetrical and asymmetrical protonation states of compound 1‐2 (i.e. BP4VA in the figure) and fluorescence images of BP4VA, BP4VA‐1H, and BP4VA‐2H crystals; normalized (b) photoluminescence spectra and (c) absorption spectra of BP4VA, BP4VA‐1H, and BP4VA‐2H crystals [47].

Source: Reprinted (adapted) with permission from Ref. [47]. Copyright © 2017 American Chemical Society.