Mechano-fluorochromic behavior of AEE polyurethane films and their high sensitivity to halogen acid gas

Three polyurethanes with different contents of tetraaryl-buta-1,3-diene derivatives in the soft segment (STMPU-25/STMPU-50/STMPU-75) have been synthesized and found to present aggregation-enhanced emission features. The fluorescence intensity of polymer films was greatly enhanced with increasing tensile stress. Also, polyurethanes with higher aggregation-induced emission fluorogen content had stronger mechano-fluorochromic behavior in the same tension state. Moreover, the resulting polyurethane films possessed high sensitivity for halogen acid gas, suggesting their potential applications in environmental monitoring fields.

However, AIE/AEE polymers possessing mechano-uorochromic feature were rarely studied. [32][33][34] Kokado et al. found that the AIE elastomer based on PDMS and TPE exhibited stimuli-sensitive uorescence against organic solvents and temperature. 35 Tang et al. reported a TPE-containing memory chromic polyurethane, which showed that the emission intensity of resulting polyurethane gave negative correlation with shape xity, temperature, and existence of solvent. 36 In our previous work, an AEE polyurethane containing 4,4 0 -((1Z,3Z)-1,4-diphenylbuta-1,3-diene-1,4-diyl) dibenzaldehyde (TABDAA2) was prepared and it was found that the uorescence of polyurethane lms was enhanced in the tension state. 37,38 So far, the effect of tension on the uorescent intensity of polymer lms was poorly understood.
Meanwhile, the detection of halide is important for monitoring excessive halide levels in the environment (i.e. pollution in air and water). There are many ways to determine the concentration of the halide in the solutions, such as amperometry and chemistry titration, [39][40][41][42] but few researches are about detecting the presence of halide gas in air.
Considering that the content of AIE molecules TABDAA2 might affect the uorescence behavior of polyurethane lms, three polyurethanes with different mass fraction (0.25%, 0.5% and 0.75%) of TABDAA2 in the so segments (STMPU-25/ STMPU-50/STMPU-75) have been synthesized in this work. And their mechano-uorochromic behavior were investigated. Meanwhile, the polyurethane lm was sensitive to halogen acid gas like hydrogen iodide (HI), proving that it could be good solid probe to halogen acid gas.
Firstly, the uorescent emission spectra of STMPU-50 in the solution and aggregate states was investigated as shown in Fig. 1(a) and (b). The uorescent intensity of STMPU-50 increased continuously until water fraction raised up to 80%. When the water fraction reached beyond 80%, the uorescent emission started to decrease. These results demonstrated that resulting polyurethane indeed had an AEE feature. The uorescent emission spectra of STMPU-25 and STMPU-75 in the solution and aggregate states were also studied ( Fig. S1 †). It can be found that STMPU-75 had the highest uorescent intensity. And all three polyurethanes possessed AEE property. Dong et al. reported that tetraaryl-buta-1,3-diene (TABD) small molecules had typical AIE effect. 43,44 When the moiety TABDAA2 was introduced into polyurethane systems, these polymers usually exhibited AEE feature rather than the typical AIE effect. Since the AIE small molecules were chemically located within the polymer matrix, which limited their intramolecular rotations to some extent. So, it could block the nonradioactive pathway and populate the radiative excitons, leading to the luminescence of the polyurethane in the solution state.
To further investigate the mechano-uorochromic properties of AEE polyurethanes, three polyurethanes with different mass fraction of TABDAA2 (0.25%, 0.50% and 0.75%) were synthesized and their FL-true strain spectra were depicted in Fig. 2(a)-(c). The uorescent intensity of all STMPUs rstly increased and then decreased with the increased true strain. And their true stress-strain curves were conducted by tension experiment at room temperature ( Fig. 2(d)), which exhibited typical elastomeric feature. The true strain (3 T )-dependent uorescence spectra of three STMPUs were also summarized in Fig. 2(d). In the low strain scope (3 T # 0.3), the uorescence intensity of the lm was increased, while the stress was also increased with increased strain in the linear correlation coefficients (3 T # 0.3) of 0.9180, 0.8729 and 0.8608 for STMPU-25, STMPU-50 and STMPU-75, respectively, proving that polyurethanes with higher concentration of TABDAA2 had more positive correlation with the tensile strain when 3 T # 0.3.
XRD proles of the STMPU-50 lm before and aer stretching were measured (Fig. S2 †). Aer stretching, peaks at about 21.5 and 29 became sharper a little than the original state, suggesting that stretching can induce the more regular structure in the polyurethane lm. Thus, we could assume that the tension stress could bring two opposite effects on the FL emission intensity of the STMPU lms. In the initial stage of stretching, the structure of the polyurethane lms became more regular and the aggregation state of AIE moiety in the so segments of the polyurethanes was enhanced, thus leading to a rising uorescence intensity. At the same time, the concentration of the TABDAA2 in the lms dropped due to the enlarged volume of the lm accompanied with the stretching, causing the reduction of the uorescence intensity when the strain was further increased. 45 As a result, polyurethanes with higher concentration of TABDAA2 might prevent some negative effect of stretching on uorescence intensity, implying that high mass fraction of AIE moiety in the polyurethanes would lead to the more positive mechano-uorochromic feature in a larger scope of strain. This novel positive mechano-uorochromic feature has the potential to be applied widely in strain or stress sensors.
Meanwhile, it was observed that the uorescent intensity of polyurethane lms was very sensitive to hydrogen iodide. 1 mL halogen acid and 1 mL ammonia water (25-28% wt) was put into 5 mL beakers to prepare halogen acid gas and ammonia atmosphere. As seen in Fig. 3(a), when the STMPU-50 lm was fumed with hydroiodic acid (HI, 45% wt) for 5 seconds, the uorescence of the lm was almost quenched. And aer fumigating the lm with ammonia, the FL intensity increased and surpassed the original state of the lm. The bright and dark states could be inter-converted to each other 3 times in the uorescent emission intensity (Fig. 3(b)), and these chemical stimuli were non-destructive.
When hydrobromic acid (HBr, 47% wt) was used to fume the polyurethane lm, a new small shoulder peak was shown in Fig. 3(c). While aer fuming the lm with ammonia, the shoulder peak was disappeared, and the FL intensity was enhanced comparing to the original state of the lm.
Further, Fig. 3(d) showed that the STMPU-50 lm was fumigated with hydrochloric acid (HCl, 37% wt), an obvious new emission peak at around 610 nm appeared, but the original peak at 480 nm disappeared mostly. When the lm was smoked with ammonia, the original uorescent emission peak was  enhanced, but the new emission peak in 610 nm became shoulder peak. Therefore, it could be assumed that the polyurethane lm had different and obvious response to hydrogen iodide and hydrochloric acid. Simultaneously, this phenomenon could be observed visually in Fig. 4(a) and (b). The STMPU-50 lm was deep yellow aer smoked with hydroiodic acid, then turned to pale yellow when fumed with NH 3 $H 2 O. Under 365 nm ultraviolet lamp, the original lm exhibited blue uorescence, then got dim greenish blue aer fumigated with hydroiodic acid. The uorescence of lm became bright blue when fumigated with NH 3 $H 2 O. In contrast, the lm was painted reddish brown aer smoked with hydrochloric acid, and then turned to pale yellow when fumed with NH 3 $H 2 O. Under 365 nm ultraviolet lamp, the original lm exhibited blue uorescence, and yellow aer smoked with hydrochloric acid. The uorescence of lm became green when fumigated with NH 3 $H 2 O.
According to the uorescent color and brightness changes of the STMPU-50 lm, the proposed sensing mechanism was illustrated in Fig. 5. Most of the Schiff base moiety of STMPU-50 with Cl À formed a highly rigid uorophore so that the conjugation extent of the molecular chain was enlarged, leading to a red shi peak at 610 nm. And for HBr, part of TABDAA2 molecules could form a rigid ring with the Schiff base moiety so that the new peak in 610 nm was weak as a shoulder peak. However, I À ion was too large to insert into the space between C]N and the benzene ring of the TABD. 46,47 Thus, I À ion reacted with TABD uorogen by electrostatic interactions and offered more diffusion channels for the excitons to migrate, allowing them to be more quickly annihilated by the hydrogen iodide. 48 Next, when these lms were fumed with ammonia gas, halogen ion could react with NH 4 + due to the acid-base neutralization.
Thus, the emission peak at around 470 nm was enhanced again. In summary, three aggregation-enhanced emission polyurethanes with 0.25%, 0.5% and 0.75% of tetraaryl-buta-1,3diene derivatives in the so segments, that is, STMPU-25/ STMPU-50/STMPU-75, have been synthesized. The    uorescence intensity of the polymer lms was greatly increased with the increase of tensile stress. Also, polyurethanes with higher content of AIE uorogen presented better positive correlation between uorescent intensity and tensile strain when 3 T # 0.3. Moreover, the resulting polyurethane lms had high sensitivity for halogen acid gas, suggesting their potential applications in environment monitoring elds.

Conflicts of interest
There are no conicts to declare.