Читать книгу Handbook of Aggregation-Induced Emission, Volume 3 - Группа авторов - Страница 41

Mechanochromic AIEgens show sensitive photophysical response to external stimulation, manifested in their emission color, intensity, as well as the lifetime. The underlying reason is the change in their molecular packing, conformation, and intermolecular interaction induced by the external forces. AIEgens are enabled to act as fluorescent probes to visualize the structural and morphological variations occurred in materials due to any possible contamination event that possibly changes the extent of their aggregation. On this account, AIEgens have been effectively utilized as mechanochromic fluorescent additives in commodity polymers [54] and first attempts for the detection of mechanical stimuli via an ON–OFF optical output are summarized below.

Tetraphenylethylene (TPE) is reported as one of the classic building cores of most of the investigated AIEgens [55]. It was therefore proposed as a lead moiety for the design of mechanochromic polymer films obtained by either physically mixing or the covalent linkage approaches (Figure 3.1). TPE was firstly utilized in polymer films based on styrene‐containing polymers such as the poly(b‐styrene‐b‐butadiene‐b‐styrene) (SBS) thermoplastic elastomer as a reversible mechanochromic optical sensor [56]. SBS films containing the 0.01 wt.% of TPE obtained by solution casting from chloroform blends were found to be particularly responsive toward uniaxial mechanical deformation. As soon as uniaxial deformation occurred at a draw ratio >1, the typical emission of TPE aggregates embedded into the polymer at about 460‐nm collapses flanked by a clearly visible ON–OFF behavior as detected by the irradiation with a long‐range ultraviolet (UV) lamp at 366 nm (Figure 3.4).

Figure 3.4 Fluorescence spectra ( λ _exc = 360 nm) of a 0.01 wt.% of the TPE‐doped SBS film before (draw ratio = 1) and after (draw ratio = 3) uniaxial deformation (drawn) at room temperature and pictures of the same film under the irradiation at 366 nm. A draw ratio of 1 means the pristine undeformed and relaxed film.

This phenomenon was attributed to a combination of effects: the first resides on the film thickness modification upon drawing that, in turn, diminished the concentration of the AIEgen in the longitudinal axis direction of the excited area; the second involves the deaggregation of TPE fluorescent molecules that triggered the reduction of the barrier energy associated with the phenyl ring rotation, thus leading to the drop in the emission intensity. More than that, profiting of the elastomeric features of the SBS matrix, the responsive character was also found to be reversible. The film subjected to several drawing cycles restored its original fluorescence as soon as the mechanical solicitation was stopped. This ON–OFF–ON behavior is visible by the naked eye under the excitation of a UV lamp at 366 nm, thus rendering the TPE‐doped SBS films useful as mechanochromic sensors.

However, the optical output is characterized by an overall poor contrast since the orientation of the amorphous phase during uniaxial deformation proved to be inadequate for the break‐up of TPE aggregates. In order to overcome this issue, Kokado et al. proposed a new AIE elastomer based on covalently linked TPE molecules to poly(dimethylsiloxane) (PDMS) macromolecules [57]. Notably, tetravinyl AIE luminogen based on TPE was actually synthesized and then reacted at different contents (up to 7.8 wt.%) with H‐terminated PDMS via hydrosilylation to obtain AIE‐doped elastomeric polymer films. Those films showed the typical elastomeric behavior as revealed by the tensile test, while the mechanical properties varied in agreement with the chain length of PDMS and crosslinking degree. Covalently bonded polymer–AIE resulted in a better distribution control of the TPE content and increased the phase stability of the AIEgen toward different drawing cycles.

The same approach was also adopted by Tang et al. [58] who covalently incorporated TPE derivatives into polyurethane elastomers for the development of mechanochromic fluorescent materials also characterized by shape memory features. The luminescent polymers were realized by reacting PCL diol (M_n = 4000) with 1,6‐hexamethylene diisocyanate (HDI) and 1,4‐butanediol (BDO) as diisocyanate and chain extender to form a hard segment (about 25%). The TPE–diol was synthesized and directly connected to the polymer backbone at different contents, i.e. from 0.02 to 0.1 wt.%. Polymer films with a thickness of 0.1 mm were then obtained by casting from dimethylformamide (DMF) polymer solutions and solvent evaporation at 70 °C for 12 hours. As analogously reported in TPE/SBS blend films, the emission green band around 479 nm experienced a strong decrease in intensity with the progression of the uniaxial film deformation up to 213%. The fluorescence drop was also accompanied by a slight original blue‐shift from 479 to 468 nm after stretching. All these phenomena were addressed to morphological changes of TPE aggregates; AIEgen supramolecular arrangements were basically caused to deaggregate to some degree during the stretching process. The emission intensity of polymer films is notably sensitive to mechanical stretching until a shape fixity of 100%, and the phenomenon was also clearly visible by the naked eye under the excitation with a long‐range UV light.

A more intriguing and effective solution for the preparation of high‐contrast mechanochromic fluorescent sensors based on the OFF–ON mechanism was then proposed by Moore et al. who took advantages of the core–shell microcapsule technology [59]. This strategy relies on a one‐component design, whose mechanochromic fluorescent response is not provided by the perturbation of the intermolecular interactions but based on the encapsulation chemistry. This approach was reported as highly accessible for a variety of systems due to the facile incorporation of AIEgen‐containing microcapsules into polymeric materials. Core–shell microcapsules were prepared according to a single‐batch in situ emulsification condensation polymerization method [60–63]. Specifically, a nonfluorescent 1 wt.% solution of TPE solution in hexyl acetate was encapsulated into double‐walled polyurethane/poly(urea‐formaldehyde) microcapsules with a diameter of 112 ± 10 μm and characterized by a thermal stability up to 220 °C. Transparent epoxy coatings incorporating 10 wt.% of TPE microcapsules were prepared by conventional chemical methods, and no emission (OFF state) emerged from the pristine material under a long‐range UV lamp. After the mechanical damage of the surface of the doped epoxy resin by means of a razor blade, the rupture of the microcapsules occurs, thus releasing the encapsulated solution around the failure. The aggregation of TPE molecules occurs after solvent evaporation and is flanked by the rapid emersion of the typical fluorescence at about 450 nm, whose maximum was reached after five minutes. The phenomenon was clearly visible under exposure to UV light and was addressed by stereomicroscopy and control experiments to the fluorescence response of individually broken microcapsules (Figure 3.5). Since the area outside of the solicitation maintains its OFF state, excellent resolution and image contrast were obtained and with a fluorescence intensity that accordingly depends on the TPE concentration and incorporation of more microcapsules. This feature also allowed for a quantitative assessment of the extent of the mechanical solicitation.

Figure 3.5 Evaluation of damage detection in encapsulated AIE polymer coatings: (a) drawings of the working mechanism of an epoxy resin containing 10 wt.% TPE‐containing microcapsules under illumination with white light and UV light after being scratched with a razor blade; (b) time‐dependent fluorescence microscopy illustrating the emersion of fluorescence after damage; (c) the same behavior on passing from thermoset epoxy resins to thermoplastic polymers.

Source: Reprinted with permission from Ref. [59]. Copyright (2016) American Chemical Society.

The authors also demonstrated that the proposed methodology could be accessible for not only thermoset epoxy resins but also thermoplastic polymers such as PS, acrylic polymers, and PDMS. Following this suggestion, Young II Park et al. proposed associating the mechanochromic fluorescence provided by the encapsulated TPE AIEgens with the extrinsic self‐healing features of epoxy resins [61]. A blend composed by methacryloxypropyl‐terminated polydimethylsiloxane, styrene, benzoin isobutyl ether, and TPE represented the healing core that was encapsulated in the urea–formaldehyde shell. When the mechanical stress is applied to the epoxy matrix through a razor blade, the selected UV lamp served not only to detect the damaged regions but also to activate the photoinitiator (i.e. the benzoin isobutyl ether) to initiate the copolymerization between the styrene‐reactive monomers and the methacrylic‐reactive terminal units of the siloxane crosslinker.

Very recently, Tang and his group have proposed the 1,1,2,2‐tetrakis(4‐nitrophenyl)ethene (TPE‐4N; Figure 3.6) as a versatile AIEgen with a very sensitive emission to mechanical stress [64]. In this case, the reported ON–OFF fluorescence behavior was addressed to the intersystem crossing (ISC) process from the excited singlet state to the highly sensitive triplet state that is characterized by a nonemissive character. Experimental investigations reported that the Ph‐NO₂ moieties of TPE‐4N in the crystalline phase activate the nonradiative ISC pathway and hence the fluorescence quenching. Conversely, in the amorphous state, the TPE‐4N luminogen recovered its emission with a bright green fluorescence at 520 nm, thus suggesting that the ISC relaxation process is hindered/caused by AIE morphology modification.

Figure 3.6 Chemical structure of TPE‐4N and fluorescence intensity changes of the AIEgen‐coated steel specimen before and after drawing at fracture under the illumination with a long‐range UV lamp.

Profiting from these chromogenic characteristics and the capability of TPE‐4N to form homogeneous films over different materials, the authors proposed the use of this AIEgen as the mechanochromic coating for the detection of stress–strain solicitations of metal surfaces [65]. Notwithstanding the use of TPE‐4N without the presence of a supporting polymer matrix, the coating procedure was identical to those proprietary of commodity plastics and therefore possibly extended to AIE‐doped plastic films. A uniform 1‐μm thick TPE‐4N film was realized over a steel specimen by dip‐coating from a chloroform solution. After drying, the TPE‐4N film displayed the typical strong green fluorescence at 520 nm, owing to the amorphous character of the coated AIEgen. The emission was then totally quenched upon annealing the specimen for 20 minutes at 150 °C due to the promotion of the nonradiative ISC process that was activated by the fluorophore crystallization. The annealed specimen was then utilized for the mechanochromic tests by means of an in situ testing machine. Notably, the fluorescence variation was detected with a charge‐coupled device (CCD) camera under the irradiation with a coaxial UV light. When a uniaxial force was applied, green fluorescence emerged again from the specimen and progressively increased as a function of the external strain (Figure 3.6). External strain cracks and transforms the nonemissive crystalline TPE‐4N into amorphous TPE‐4N, which shows a green emission.

Owing to the OFF–ON mechanism provided by the TPE‐4N probe, the mechanochromic response was detected even at a very moderate strain of 0.13% and with a progressive enhancement up to deformations of 20%. It was reported that the green emission intensity at drawings of 65% resulted seven times higher than that of the pristine undeformed metal.