Handbook of Aggregation-Induced Emission, Volume 3. Группа авторов

This emission was found to progressively decrease upon heating from −10 to 60 °C with a 35‐fold variation of the overall fluorescence intensity. Since at temperature higher than 30 °C PCL‐crystalline phase melts, TPE moieties increase their mobility, thus allowing de‐excitation processes via nonradiative pathways more probable. The reversible thermochromic behavior suggests a new avenue in the development of optical temperature sensors based on AIE‐doped polymer films.

      3.3.3 Vapochromic AIE‐doped Polymer Films

      The detection of VOCs is an important concern since they are delivered into the environment by human and natural processes and owing to their toxic nature, regulations setting a limit to VOC emission are emerging [72, 73]. Leading examples are based on changes in electrochemical, conducting, and chromic properties of the corresponding sensor matrices [74–79]. Due to the nonfluorescent properties of VOCs, the fluorescence‐based detection is an indirect method to utilize fluorophore species that undergo fluorescence changes upon interactions with analytes in the vapor phase. Therefore, current pressing issues in global security are encouraging in the design of novel AIEgens with RIR and/or TICT features aimed at detecting VOC exposure with even more sensitivity and reproducibility of the optical response. Thermoplastic or thermoset indicators containing fluorophore sensitive to viscosity variations have been successfully designed and utilized for the detection of VOCs [49]. Notably, Tang et al. prepared polyacrylates with glycogen‐like structures via radical polymerization of TPE‐containing di‐ and tetra‐acrylates [80]. The polymer films were then deposited over a thin‐layer chromatography (TLC) plate and the spots were generated after solvent evaporation evidenced strong vapochromism. This behavior was addressed to the solvating activity of the absorbed VOC molecules that strongly reduced the aggregation among the TPE molecules, thus favoring the collapse of the emission band. Such (secondary) interaction being reversible, as soon as the VOCs are removed, the polymer would restore its original fluorescence. As a matter of fact, the polymer emission continuously and reversibly experienced an ON–OFF–ON behavior by wetting and dewetting processes by VOCs. A similar vapochromic behavior was also reported by Zhu et al. [81] for end‐capped TPE‐doped polymers based on amorphous fluorene‐based fluorophores. The system demonstrated vapochromism by typical ON–OFF fluorescence response upon exposure to dichloromethane vapors.

Martini et al. [49] proposed the use of julolidine‐based AIEgens as FMR, i.e. showing the typical viscosity‐dependent emission properties when dispersed at low loadings (<0.1 wt.%) in PS plastic films. The exposure of FMR/PS films to a saturated atmosphere of well‐interacting VOCs such as chloroform and toluene caused a significant drop of system fluorescence due to their favored relaxation from the nonemissive TICT excited state. Conversely, no chromogenic response was detected when vapors of poorly interacting solvents were utilized such as hexane or methanol. This behavior suggested that the typical ON–OFF vapochromic characteristic is governed by the ability of the polymer to interact with the target vapor molecule. The authors also demonstrated the reproducibility of the optical response toward successive cycles of VOC exposure and they were able to increase the rapidity of such a response by embedding AIEgen functionalized by a perfluorinated alkyl chain. This molecule accumulated close to the film–air contact surface during film formation, thus increasing the ability of the plastic system to detect the approaching VOC molecules.

A similar approach was reported by the same research group that utilized 4‐(diphenylamino)phthalonitrile (DPAP) as the FMR probe embedded in thermoplastic poly(methyl methacrylate) (PMMA) and polycarbonate (PC) thin films (90–120‐μm thick) at a concentration of 0.05–0.1 wt.% [82, 83]. DPAP was reported to have a fluorescence deactivation pattern due to the formation of the TICT state. Notably, in low and medium polar solvents, DPAP shows a strong emission, whereas in high polar and protic solvents, DPAP is not emissive due to the stabilization of the TICT state. The authors reported that DPAP/PMMA films experience a reversible vapochromism when exposed to CHCl₃ and acetonitrile, i.e. VOCs with high polarity index and favorable interaction with the polymer matrix. The optical variation occurred within four minutes of VOC exposure, with the emission red‐shifted by about 50 nm in addition to the intensity drop. Further expanding the behavior recorded in PMMA, DPAP/PC films were reported to show not only vapochromic features but also fluorescence variation when the polymer matrix PC crystallizes due to the VOC exposure. Notably, the emission of the films progressively shifted at longer wavelengths flanked by an increased intensity due to the progressive enhancement of the DPAP constraints as the crystalline content of the polymer grows (Figure 3.9).

Figure 3.9 DPAP and (a) emission band variations of 0.05 wt.% DPAP/PMMA films as a function of exposure to CHCl₃, and (inset) pictures of the same film taken under illumination at 366 nm at 0 (blue box) and five minutes (red box) of vapor exposure. The spectra were collected for five minutes with a time interval of one minute; (b) progressive changes in the emission of 0.05 wt.% DPAP/PC films as a function of exposure to CHCl₃ vapors. The spectra were collected for 38 minutes with a time interval of 1 minute, whereas the images were collected at 366 nm after 0 and 20 minutes.

      Source: Adapted from Ref. [82] with permission of the Royal Society of Chemistry.

      Attempts were addressed in the literature to increase the sensitivity and the rate of the response of the AIE‐doped polymer films toward different VOC molecules. For this purpose, Iasilli [84] and Borelli [85] et al. proposed to covalently link julolidine‐based FMR with styrene macromolecules. Following this approach, as soon as the VOC molecules get in contact with the polymer film surface, the plasticization of the matrix caused by VOC absorption occurs rapidly, and the information promptly transfers to the FMR probe being linked to the macromolecular system by means of a primary bond. With this strategy, the decrease in the fluorescence intensity was seven times higher than the first experiments proposed by Martini et al. [49] that was conversely based on physically dispersed FMR probes in the same polymer matrix and with the same wt.% content.

      In connection with these findings, Sorgi et al. [67] lately proposed two strategies in one FMR: covalent‐linking the fluorophore and the polymer matrix and using perfluorinated moieties to selectively distribute the FMR near the film surface. On this basis, new fluorinated styrene‐based block copolymers were prepared and labeled by a small amount (i.e. 0.11–0.13 mol.%) of the julolidine‐based FMR probe. Spray‐coating of the AIE‐doped fluorinated copolymers over the surface of a PS film yielded two‐layer films, in which the segregation of the perfluorinated tails bearing the AIE probe occurred at the polymer–air interface. Physical proximity of the probe FMR to the air–polymer surface enables the film to show fast and highly sensitive response toward VOC exposure. Notably, the vapochromic response of these sensing films was at least six times more rapid than the previous outcomes and with sensitivity toward chloroform vapors of about 25 ppm.

Following the general idea of boosting vapochromic features of AIE‐doped films by the covalent incorporation of the fluorescent probes, Guidugli et al. proposed to design a new TPE molecule with donor–acceptor features and a 2‐bromo‐2‐methylpropanoate unit used as the initiator to prepare red‐emitting poly(methyl methacrylate) polymers (PMMA_TPE_RED_1.5) via atom transfer radical polymerization (ATRP) [86]. The sensing performances of the spin‐coated films (thickness of 2 μm) demonstrated significant vapochromism when exposed to VOCs characterized by high vapor pressure and favorable interaction with the polymer matrix such as chloroform, acetone, and toluene. According to the authors’ explanation, the empty channels and holes of the polymer are progressively filled by vapors of solvents, thus causing a rapid decrease in microviscosity. This phenomenon caused the prompt deactivation of the TPE probe, its phenyl rings being allowed to rotate around the stator, thus provoking a strong drop at the emission during VOC exposure. Noteworthily, PMMA_TPE_RED_1.5 films displayed substantial vapochromism already in the presence of 10 ppm of CHCl₃ vapors, which is the