Aggregation Induced Emission: new emerging
fluorophores for environmental sensing
Nicola Giudugli,1 Riccardo Mori,1 Fabio Bellina,1 Ben Zhong Tang,2 Andrea Pucci11Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Moruzzi 13, 56124 Pisa, Italy
2Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China
*Corresponding author:
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Moruzzi 13, 56124 Pisa, Italy Email: andrea.pucci@unipi.it,
Abstract
PMMA films containing tetraphenylethylene (TPE) derivatives as red-emissive AIE fluorophores (AIEgen) were demonstrated to be sensitive to both volatile organic compounds (VOCs) and thermal stress. Notably, a novel AIE fluorophore (TPE_RED) was synthesized and used as the initiator to prepare red-emitting poly(methyl methacrylate) polymers (PMMA_TPE_RED1.5) via atom transfer radical polymerization (ATRP). The sensing performances of the spin-coated films (thickness of 2 µm) demonstrated significant vapochromism when exposed to VOCs characterized by high vapour pressure and favourable interaction with the polymer matrix. It was worth noting that PMMA_TPE_RED1.5 displayed substantial vapochromism already at concentration of CHCl3 vapours of 40 ppm, that is about 4 times smaller than those ever registered in our laboratory with the same apparatus. This threshold was even more decreased in the case of PMMA films containing the TPE_RED physically mixed or another TPE-based derivative already tested in literature, i.e. the barbituric acid-functionalized tetraphenylethene derivative (TPE-HPh-Bar). PMMA films containing TPE_RED AIEgen resulted also sensitive to temperature variations showing an evident thermochromic response close to the glass transition temperature of the polymer matrix.
1. Introduction
Over the last few decades, chromogenic compounds are being effectively used for the study and development of optically-active materials that are sensitive to different external stimuli. The fluorescence sensing based on organic sensory materials has attracted lot of attention due to its desirable features such as non-invasiveness, excellent sensitivity, simplicity, and high
signal-to-noise ratio.1-8 Notably, a ground-breaking class of fluorophores featuring fluorescence development
with aggregation has triggered great attention in sensor applications since their discovery by Ben
Zhong Tang in 20019-12 The effect, called aggregation induced emission (AIE), arises from the
restriction of fluorophore intramolecular motions (RIM) that is typical of those molecules whose
structure consists of two or more units that can dynamically rotate against each-other.9-11
Noteworthy, by allowing light emission in the aggregate and solid state, AIE fluorophores (AIEgens) demonstrate to show a striking impact on energy, optoelectronics, life science and
environment.9,13-18 Notably, for AIE systems with donor-acceptor moieties, the emission quenching
is often addressed to the formation of a non-emissive twisted intramolecular charge transfer (TICT) state that occurs in solution, while in aggregates or in viscous media, transition from locally excited
(LE) state to TICT is inhibited.10 Those molecules have become popular in the last decade due to
their facile applicability as viscosity sensors and local micro-viscosity imaging.19-22 AIEgens in solution has been explored since years, and examples of their use in combination with structured
and polymeric materials have been already reported for environment change due to temperature
variations23 or the presence of volatile organic compounds (VOCs).18,22,24-26 The detection of VOCs is an important concern since that they are delivered into the environment by human and natural processes and owing to their toxic nature, regulations setting a limit to VOCs emission are emerging.27,28 Moreover, current pressing issues in global security are still encouraging the design of novel AIEgens aimed at detecting environment changes with even more sensitivity and reproducibility of the optical response. For example, thermoplastic indicators containing fluorophore sensitive to viscosity variations have been successfully designed and utilized for the
detection of VOCs.29 Notably, plastic films containing moderate amounts of the selected AIEgen (<0.1 wt.%) experience a fluorescence drop with an ON-OFF mechanism upon the exposure to well-interacting VOCs due to an induced plasticization of the supporting polymer matrix.24-26,29-31 Moreover, being polymer viscosity also regulated by temperature, the incorporation of AIEgens into
polymer matrices appears also effective for the visualization of temperature variations.32,33
Notwithstanding the effectiveness of the chromogenic response towards environmental variations, challenges are still open to drive future improvements in the direction of even more rapid and sensitive optical responsiveness. Examples to modulate the optical output are based on the use of different strategies for the incorporation of AIEgens, or in general the chromogenic probes, within
the polymer matrices.3,34-36 It is expected that covalent binding of AIEgens to the macromolecular
chains could ensure a more homogeneous distribution of the active AIE units in the polymer matrix, i.e. without promoting the formation of stacked supramolecular structures potentially useless to
provide the fastest response to minimal external solicitations.3,24 On the other hand, physically
blended AIEgens with polymers could be, for example, more promptly solvated by VOC molecules than the covalently linked counterparts, thus providing a faster response.
The present dilemma is taken into consideration in this work. Notably, a novel AIE fluorophore (TPE_RED, Figure 1a) was synthesized and used as the initiator to prepare red-emitting poly(methyl methacrylate) polymers (PMMA_TPE_RED) via atom transfer radical polymerization (ATRP).37 ATRP was selected to control the introduction of the fluorophore on the macromolecular backbone to assure the homogeneous distribution of the active AIE units. The AIE-doped polymers
were used in the form of thin films obtained by spin-coating on glass plate surfaces and exposed to
different concentrations of VOCs of different nature or to temperature solicitations. The obtained results were then compared to those collected from PMMA films containing the TPE_RED AIEgen physically mixed or another TPE-based derivative already tested in literature, i.e. the barbituric
acid-functionalized tetraphenylethene derivative (TPE-HPh-Bar, Figure 1b).38
Experimental part
Materials
Unless otherwise stated, all reactions were performed under argon by standard syringe, cannula and septa techniques. CuBr (Sigma-Aldrich) was purified by washing with glacial acetic acid followed by diethyl ether, dried under vacuum and stored under nitrogen. Anisole (Sigma-Aldrich) was refluxed over sodium for three hours, distilled and stored under nitrogen. DCM was refluxed over CaH2 for one hour, distilled and stored under Argon. THF was refluxed over LiAlH4 for 3 hours, distilled and stored under Ar. Zinc dust Aldrich, < 10 micron), DMF anhydrous (Sigma-Aldrich) and ethane1,2-diol anhydrous (Sigma-(Sigma-Aldrich) were used as received. Sigma-Aldrich precoated silica gel PET foils were used for TLC analyses. TPE_RED and TPE-HPh-Bar were prepared following the synthetic procedure reported in the literature.38,39 Poly(methyl methacrylate) (PMMA, Sigma-Aldrich, Mw = 350,000 g/mol, acid number <1 mg KOH/g) was used as received. Synthesis of PMMA_TPE_RED polymers
As one example, CuBr (10 mg, 0.07 mmol), TPMA (20 mg, 0.07 mmol), PMDETA (0.106 g, 0.61 mmol), MMA (2 g, 19.9 mmol) and 4 mL of anhydrous anisole were introduced in a schlenk tube and degassed with three freeze-pump-thaw cycles. Then TPE_RED (10 mg, 0.014 mmol) was added and after three freeze-pump-thaw cycles the polymerization was left to proceed for 24 h at 90 °C under nitrogen atmosphere. The polymer was purified by repeated precipitations from chloroform into hexane and then dried at reduced pressure. 0.980 g of a red solid were recovered (yield 49%).
Preparation of PMMA_TPE_RED films
PMMA_TPE_RED films were obtained by spin-coating on glass substrate. A 2.4 × 2.4 cm2 glass cover slip was cleaned and then placed on the vacuum chuck hold-down of a WS-400B-6NPP-LITE (Laurell Technologies Corp., North Wales, PA, USA) spin-coater. A viscous solution of the copolymer (5 mg) in CHCl3 (40 µL) was placed in the centre of the glass, and the coating was performed at a 750 rpm for 22 s, with an acceleration index of 004 (~448 rpm s−1). The obtained
films were allowed to slowly dry at rt for 24 h before any measurement. Film thickness was measured with a CM1S dial indicator (Borletti, Milan, Italy) with ruby movement bearing.
Characterizations
Gel permeation chromatography (GPC) was used to determine molecular weights and molecular weight dispersion (Mw/Mn) of polymer samples with respect to polystyrene standards. GPC measurements were performed in CHCl3 as solvent on a four-channel pump PU-2089 Plus chromatograph (Jasco, Easton, MD, USA) equipped with a Jasco RI 2031 Plus refractometer and a multichannel Jasco UV-2077 Plus UV-Vis detector set at 252 and 360 nm. The flow rate was 1 mLmin−1 at a temperature of 30 °C held through a Jasco CO 2063 Plus Column Thermostat. A series composed by two PLgel™ MIXED D columns and a PLgel™ precolumn (Polymer Laboratories, Church Stretton, UK) packed with polystyrene-divinylbenzene was used to perform the analysis (linearity range 100 Da–400 kDa).
The thermal behaviour was evaluated by differential scanning calorimetry (DSC) under nitrogen atmosphere by using a Mettler Toledo StarE System, equipped with a DSC822c module. Films were heated from 25 to 150 °C at 10 °C min-1 (1st heating), cooled to 0 °C at the same scan rate (1st cooling), then heated again to 150 °C at 10 °C min-1 (2nd heating) after 5 min of annealing.
Spectrophotometric measurements were performed using a Perkin-Elmer Lambda 650 spectrometer with temperature control to within ±0.1 °C. The chemical composition of polymers was evaluated by UV–Vis spectroscopy by means of a calibration curve obtained from 5 × 10−7 – 1 × 10−5 CHCl3 solutions of TPE_RED. The fluorescence measurements in solution (with temperature control to within ±0.1°C) and in the solid state were performed using a Horiba Jobin-Yvon Fluorolog®-3 spectrofluorometer equipped with a 450 W xenon arc lamp and double-grating both excitation and emission monochromators. The emission quantum yields of the solid samples were obtained by means of a 152 mm diameter "Quanta-phi" integrating sphere, coated with Spectralon® and mounted in the optical path of the spectrofluorometer, using as excitation source the 450 W Xenon lamp coupled with a double-grating monochromator for selecting wavelengths.
Emission spectra of polymer films were recorded on the same spectrofluorometer in the dark by using a F-3000 Fibre Optic Mount apparatus (Horiba Jobin-Yvon) coupled with optical fibre bundles. Light generated from the excitation spectrometer is directly focused on the sample using optical fibre bundles. Emission from the sample is then directed back through the bundle into the collection port of the sample compartment. The emission response of the films towards vapours of volatile organic compounds (VOCs) was tested by exposing the sample held by a steel tripod in a 50 mL beaker closed by a pierced aluminium foil lid, to different concentrations of various organic solvents of different vapour pressure and PMMA-solvent Flory–Huggins interaction parameter χ (Table 1), at 25 °C and atmospheric pressure.26,40 Annealing experiments were performed by placing the films on a temperature controlled hot stage in the temperature range 25–130 °C.41
Results and discussion
Optical properties of TPE_RED and TPE-HPh-Bar solutions
The preparation of the ATRP initiator TPE_RED was performed according to a synthetic pathway which involved the assembly of the AIE core of the molecule by a McMurry reaction between the bis-(dimethylamino) benzophenone and the bromobenzophenone.39 The resulting tetraphenylene derivative was then decorated with the ATRP arm by a reaction sequence involving at first the formylation with DMF of the aryllithium derivative, followed by a Knoevenagel-type condensation of the cyanoester 6 obtained from 2-bromo-2-methylpropanoyl bromide and cyanoacetic acid. TPE-HPh-Bar was synthesized according to a Sonagashira cross-coupling between 2-(4-ethynylphenyl)-1,1,2-triphenylethene and 4-bromo-2,5-bis(hexyloxy)benzaldehyde. The resulting derivative was then reacted with barbituric acid by a Knoevenagel-type condensation.38
TPE_RED shows an absorption maximum at 498 nm (Figure 2a) with an extinction coefficient of 7.54・103 M-1cm-1 and a poor fluorescence when dissolved in DMSO. At water fractions larger than 80%, TPE_RED experiences the typical AIE behavior with fluorescence maximum at 695 nm (Figure 2b).
<Figure 2 near here>
TPE-HPh-Bar shows in THF absorption maxima located at 388 nm and 447 nm with molar absorptivities of 2.63・105 M-1cm-1 and 2.99・105 M-1cm-1, respectively. Dilute THF solutions (10 mM) of TPE-HPh-Bar show a yellow emission at 545 nm upon excitation. When water was added up to 60 vol.%, the emission from the solution was weakened progressively due to the typical TICT effect. Conversely, at water content higher than 70-80 vol.% fluorescence intensity starts to rise due to the dominant AIE effect, with a maximum located at 630 nm that is 100 nm red-shifted from that in pure THF solution.38
TPE_RED polymerization and polymer main features
The red-emitting PMMA_TPE_RED polymer was obtained by atom transfer radical polymerization (ATRP) of methylmethacrylate (MMA) using TPE_RED as initiator. The feeding ratio of TPE_RED to MMA was 1.5 wt.% and the polymerization was conducted under the catalyst system CuBr/TPMA in solution of anhydrous anisole at 90 °C for 24 h. The resultant PMMA_TPE_RED1.5 polymer was purified by repeated precipitations from chloroform into hexane and characterized by standard methods. 1H NMR spectroscopy showed (Figure S8) the signals attributed to PMMA only being the aromatic signals of the TPE below the signal to noise ratio. TPE_RED quantitative determination was then accomplished by UV–Vis spectroscopy by means of a calibration curve obtained from 5 × 10−7–1 × 10−5 M CHCl3 solutions of TPE_RED. Notably, the variation of the TPE_RED molar extinction coefficient was retained negligible after polymerization.31 The TPE_RED amount in the polymer was 3 wt.%, owing to the 50% of MMA conversion. PMMA_TPE_RED1.5 has a Mn of 36400 with a polydispersity index (PDI) of 1.29, in agreement with that of typical ATRP systems.42 The glass transition temperature (Tg) was calculated from DSC analysis and resulted of 68.3 °C.
PMMA_TPE_RED1.5 displayed an absorption maximum peaked at 495 nm and a faint fluorescence in 2 g/L CHCl3 solution (Figure 3). Conversely, at hexane (non-solvent)
contents higher than 80%, PMMA_TPE_RED1.5 experienced aggregation, thus activating the AIE at 618 nm. It was worth noting that at 95% hexane fraction, the fluorescence intensity was 40–fold higher than that in 100% CHCl3. Moreover, the emission ranged from 618 to 607 nm with dielectric constant passing from 2.56 (80% hexane) to 2.14 (95% hexane).
<Figure 3 near here> Vapochromism of PMMA_TPE_RED1.5 polymer films
PMMA_TPE_RED1.5 films of 2 µm of thickness were obtained by spin-coating chloroform
solutions on 2.4 × 2.4 cm cleaned glass cover slides. Absorption features appeared similar to those
in solution (maximum peak at 481 nm) and an apparent fluorescence at 623 nm was gathered from the solid film with a quantum yield of 12%, due to the highly viscous and glassy polymer matrix (Tg = 64.3 °C) that activates the AIE process. The emission behaviour of PMMA_TPE_RED1.5 films was then studied by exposing them to several volatile organic solvents with different vapour pressure and Flory–Huggins interaction parameter χ (Table 1), as analogously performed earlier for polymer films doped with fluorescent molecular rotor entities.22,25,26,29,31
<Table 1 near here>
Notably, the parameter χ is introduced to provide an useful measure of the interaction between the polymer and the solvent: for example, χ values are associated to well-interacting solvent-polymer pairs and viceversa.43
An example of the fluorescence emission variation on exposure time to chloroform vapours is reported in Figure 4.
<Figure 4 near here>
It is worth noting that PMMA_TPE_RED1.5 films experienced a complete suppression of the fluorescence just after 200 s of chloroform exposure. This extremely sensitive response agrees well with the characteristics of the functionalized TPE moiety. As a matter of fact, chloroform is a good solvent for PMMA (χ = 0.44, Table 1), whose empty channels and holes of molecular dimensions are progressively filled by vapours of solvent, which in turn swell both the polymer and the
fluorophore, hence causing a rapid microviscosity decreasing. This phenomenon promptly triggers the AIE deactivation since phenyl rings of TPE are more allowed to rotate around the stator, thus inducing a progressive and eventually complete fluorescence drop at the end of the exposure time. The vapochromic response was then monitored for other class of VOCs and the fluorescence peak intensity variation was plotted as a function of VOCs exposure time. The fluorescence variation of PMMA_TPE_RED1.5 films resulted almost similar when highly interacting solvents were tested as probing VOCs (Figure 4). More specifically, acetone ( = 0.48), Et2O ( = 0.56), CHCl3 ( = 0.44) and toluene ( = 0.45) displayed a very fast decreasing rate, i.e. showing 50% of emission decreasing within the first 5-40 s of exposure. Notably, for toluene and Et2O, the fluorescence of PMMA_TPE_RED1.5 films was not completely suppressed within the exposure time investigated. In the case of toluene, this phenomenon could be possibly due to the adverse combination of (0.45) and vapour pressure (2.9 kPa), this last limiting the complete swelling of the polymer film in the time interval of the analysis. In the case of Et2O, the fluorescence drop was delayed and appeared uncomplete as analogously gathered earlier from vapochromic polystyrene films.26,31 In detail, this phenomenon was addressed to the nature of the Et2O molecule that caused a rapid adsorption, diffusion and macromolecules rearrangement into a less swellable matrix that behaves as a negative feedback for the vapochromic behaviour. Conversely, PMMA_TPE_RED1.5 films appeared scarcely responsive to DMSO, MeOH ( = 1.00), hexane ( = 2.08) and dioxane ( = 0.70) vapours due to their restricted affinity with the polymer matrix and low vapour pressures (Table 1) that hamper solvent adsorption by the film during the first periods of exposure.
We also investigated the sensibility threshold of the PMMA_TPE_RED1.5 films towards CHCl3 vapours, i.e. the solvent with the lowest Flory–Huggins interaction parameter χ (0.44). Notably, different amounts of CHCl3 (from 10 to 50 ppm) were introduced within the compartment box and the results of the vapochromic response reported in Figure 5.
It was worth noting that PMMA_TPE_RED1.5 films displayed an evident vapochromism already at concentration of CHCl3 vapours of 40 ppm, that is about 4 times smaller than those ever registered in our laboratory with the same apparatus. At 50 ppm of CHCl3, the fluorescence quenching was almost complete at the end of the analysis (200 s). This noteworthy result is addressed to the prompt fluorescence quenching caused by the AIE deactivation induced by the effective chloroform solvation of TPE moieties.
Vapochromism of TPE_RED/ and TPE-HPh-Bar/PMMA films
Aimed at confirming the significant sensibility threshold provided by the AIE features, the earlier vapochromic experiments were repeated on PMMA films with the same thickness (2 µm) but prepared by spin-coating from a commercially available polymer matrix and TPE_RED or TPE-HPh-Bar physically mixed at the same dye content (3 wt.%). Both molecules are red-emitting AIE fluorophores, even if the latter characterized by a more complex and bulkier structure. As far as the vapochromism of the films is concerned, no substantial differences with the behavior shown by PMMA_TPE_RED1.5 films towards the different investigated VOCs were evidenced. As expected, TPE_RED/PMMA and TPE-HPh-Bar/PMMA films were extremely sensitive to vapours of well interacting solvents with the PMMA matrix, whereas no fluorescence variation occurred with those featured with high χ and low vapour pressures. We therefore focused on the sensibility threshold of TPE_RED/PMMA and TPE-HPh-Bar/PMMA films towards CHCl3 vapours (Figure 6) as analogously accomplished for PMMA_TPE_RED1.5 samples.
<Figure 6 near here>
Surprisingly, at very low CHCl3 vapours concentration, both blend films showed a positive fluorescence variation, more evident in the case of TPE_RED/PMMA films. This phenomenon was tentatively addressed to the film plasticization induced by the first molecules of chloroform that are adsorbed by the PMMA matrix. This caused an increased mobility of the TPE derivatives that are allowed to complete their aggregation (and their maximum of fluorescence), possibly hampered by the fast solvent evaporation during film formation by spin-coating. This phenomenon was
particularly evidenced for films prepared from a commercially available PMMA being characterized by a molecular weight (Mw = 350,000 g/mol), about ten times higher than that of PMMA_TPE_RED1.5 samples. It was worth noting that negative fluorescence variation evidently occurred for just 15 ppm and 25 ppm of CHCl3 for TPE_RED/PMMA and TPE-HPh-Bar/PMMA films, respectively, i.e. about less than half of that required for PMMA_TPE_RED1.5. Therefore, it is possible to claim that the presence of a chemical bond between the AIE fluorophore and the polymer matrix induced a substantial delay in the vapochromic response that needs more amount of chloroform vapours to be effective and detectable. In fact, the fluorescence intensity of TPE_RED/PMMA films dropped by 50% just after 80 s of exposure to 20 ppm of CHCl3, whereas the same emission reduction was detected for PMMA_TPE_RED1.5 films only after 120 s of exposure to 50 ppm of vapours. The effect of the molecular complexity of the AIE fluorophore optical responsiveness was confirmed by the behavior of TPE-HPh-Bar/PMMA films. Their sensitivity threshold resulted higher than that shown by TPE_RED/PMMA films possibly due to the more complex and bulkier structure of TPE-HPh-Bar compared to TPE_RED.
Thermochromic behavior of TPE_RED containing PMMA films
PMMA_TPE_RED1.5 and TPE_RED/PMMA films were thermally stressed at temperatures ranging from 25 °C to 130 °C, by placing them in contact with a thermostatically controlled (±0.1 °C) metal surface. The effect provided by temperature changes was evaluated by means of fluorescence spectroscopy by collecting emission spectra about 15 s after the temperature increase to ensure film relaxation to a new thermal equilibrium. Upon heating, both PMMA_TPE_RED1.5 and TPE_RED/PMMA films experienced fluorescence variations with emission decreasing and blue shift of the maxima peaks of about 10 nm (Figure 7).
<Figure 7 near here>
Blue shift of the fluorescence maximum could be addressed to minimal variations of the dielectric constant of the PMMA matrix with temperature.44 Notably, the largest variations in wavelength occurred near the glass transition temperature of the polymer matrix, that is close to 70 °C for
PMMA_TPE_RED1.5 and 110 °C for the commercially available PMMA sample in TPE_RED/PMMA films. The same trend was also observed looking at the intensities variation versus temperature as plotted in figure 8.
<Figure 8 near here>
The reported fluorescence quenching was caused by the alteration of molecular packing modes among AIE molecules, i.e. triggered by the decreased viscosity of the polymer matrix due to the thermal stress. It was actually noteworthy that PMMA_TPE_RED1.5 and TPE_RED/PMMA films started lessening their emission in large extent only in proximity of their Tg. At temperatures higher that Tg, viscosity reduction resulted substantial and well enough to promote fluorescence deactivation due to more allowed rotation of phenyl rings of the TPE nucleus. However, the thermochromic phenomenon was less evident than the vapochromic one since fluorescence variation was less than 33% and 60% for PMMA_TPE_RED1.5 and TPE_RED/PMMA films, respectively. More specifically, the higher Tg of TPE_RED/PMMA films allowed superior annealing temperatures thus ensuring a more complete fluorescence deactivation process. Nevertheless, the experimental setup did not affect film size and aspect after within the temperature range investigated. This allowed the complete recovery of the film emission after less than one minute suggesting the complete reversibility (and reuse) of the designed thermochromic system.
Conclusions
We have demonstrated that PMMA containing covalently linked or physically mixed red-emitting AIEgens with D-A features can be suitable for the preparation of thin films with significant vapochromic and thermochromic features. All films disclosed viscosity-dependent fluorescence once exposed to volatile and well interacting VOCs/polymer pairs. The pronounced drop in their fluorescence was addressed to solvent-induced changes in the local viscosity of the polymer matrix, that in turn deactivated the AIE phenomenon due to the more allowed rotation of the phenyl rings of the TPE nuclei. It was worth noting that physically dispersed TPE-based AIEgens showed the
lowest sensitivity threshold towards CHCl3 vapours, that is about ten times lower than that previously gathered for the most efficient vapochromic system developed in our laboratory. This superior performance of the physically dispersed AIEgens was addressed to the absence of a chemical bond between the AIE fluorophore and the polymer matrix that induced a substantial delay in the vapochromic response that also needed more vapours to be effective and detectable. The fluorescence of PMMA films containing TPE_RED AIEgen resulted also sensitive to thermal stress being polymer viscosity also regulated by temperature. Notably, evident thermochromic response close to the glass transition temperature of the polymer matrix. Overall, all evidences support the application of AIEgens containing PMMA thin films as innovative vapochromic and thermochromic plastic sensors.
References
1. A. Seeboth, D. Loetzsch, R. Ruhmann and O. Muehling, Chemical Reviews, 2014, 114, 3037-3068.
2. G. Prampolini, F. Bellina, M. Biczysko, C. Cappelli, L. Carta, M. Lessi, A. Pucci, G. Ruggeri and V. Barone, Chemistry--A European Journal, 2013, 19, 1996-2004.
3. F. Ciardelli, G. Ruggeri and A. Pucci, Chemical Society Reviews, 2013, 42, 857-870.
4. S. Achelle, L. Bodiou, J. Charrier and F. R.-l. Guen, Comptes Rendus Chimie, 2016, 19, 279-285.
5. O. Korostynska, K. Arshak, E. Gill and A. Arshak, Sensors, 2007, 7, 3027-3042.
6. D. T. McQuade, A. E. Pullen and T. M. Swager, Chemical Reviews, 2000, 100, 2537-2574. 7. B. Adhikari and S. Majumdar, Progress in Polymer Science, 2004, 29, 699-766.
8. A. Pucci, T. Biver, G. Ruggeri, L. Itzel Meza and Y. Pang, Polymer, 2005, 46, 11198-11205. 9. J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam and B. Z. Tang, Chemical Reviews, 2015,
115, 11718-11940.
10. J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang and B. Z. Tang, Adv. Mater., 2014, 26, 5429-5479. 11. R. Hu, N. L. C. Leung and B. Z. Tang, Chem. Soc. Rev., 2014, 43, 4494-4562.
12. J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chemical Communications (Cambridge, United Kingdom), 2001, 1740-1741.
13. R. T. K. Kwok, C. W. T. Leung, J. W. Y. Lam and B. Z. Tang, Chemical Society Reviews, 2015, 44, 4228-4238.
14. S. Chen, H. Wang, Y. Hong and B. Z. Tang, Mater. Horiz., 2016, 3, 283-293. 15. S. Lim, B. Z. Tang and Y. Hong, ACS Symp. Ser., 2016, 1227, 199-216.
16. H. Nie, J. Huang, Z. Zhao and B. Z. Tang, ACS Symp. Ser., 2016, 1227, 173-198.
17. G. Liang, F. Ren, H. Gao, F. Zhu, Q. Wu and B. Z. Tang, Journal of Materials Chemistry A, 2017, 5, 2115-2122.
18. G. Liang, F. Ren, H. Gao, Q. Wu, F. Zhu and B. Z. Tang, ACS Sensors, 2016, 1, 1272-1278. 19. E. Y. Choi, S. Y. Nam, C. E. Song, K.-j. Kong, C. Lee, I. H. Jung and S. C. Yoon, RSC Advances,
2015, 5, 107540-107546.
20. M. A. Haidekker, T. P. Brady, D. Lichlyter and E. A. Theodorakis, Bioorganic Chemistry, 2005,
33, 415-425.
21. M. A. Haidekker, T. Ling, M. Anglo, H. Y. Stevens, J. A. Frangos and E. A. Theodorakis, Chemistry & Biology, 2001, 8, 123-131.
22. P. Minei, M. Koenig, A. Battisti, M. Ahmad, V. Barone, T. Torres, D. M. Guldi, G. Brancato, G. Bottari and A. Pucci, Journal of Materials Chemistry C: Materials for Optical and Electronic Devices, 2014, 2, 9224-9232.
23. H. Zhou, F. Liu, X. Wang, H. Yan, J. Song, Q. Ye, B. Z. Tang and J. Xu, Journal of Materials Chemistry C, 2015, 3, 5490-5498.
24. P. Minei and A. Pucci, Polymer International, 2016, 65, 609-620.
25. P. Minei, M. Ahmad, V. Barone, G. Brancato, E. Passaglia, G. Bottari and A. Pucci, Polymers for Advanced Technologies, 2016, 27, 429-435.
26. G. Iasilli, F. Martini, P. Minei, G. Ruggeri and A. Pucci, Faraday Discussions, 2017, 196, 113-129.
27. M. E. Jenkin, S. M. Saunders and M. J. Pilling, Atmospheric Environment, 1997, 31, 81-104. 28. Y. M. Kim, S. Harrad and R. M. Harrison, Environ. Sci. Technol., 2001, 35, 997-1004.
29. G. Martini, E. Martinelli, G. Ruggeri, G. Galli and A. Pucci, Dyes and Pigments, 2015, 113, 47-54.
30. P. Minei, M. Koenig, A. Battisti, M. Ahmad, V. Barone, T. Torres, D. M. Guldi, G. Brancato, G. Bottari and A. Pucci, Journal of Materials Chemistry C, 2014, 2, 9224-9232.
31. M. Borelli, G. Iasilli, P. Minei and A. Pucci, Molecules, 2017, 22. 32. M. Gao and B. Z. Tang, ACS Sensors, 2017, 2, 1382-1399.
33. Q. Zhu, W. Yang, S. Zheng, H. H. Y. Sung, I. D. Williams, S. Liu and B. Z. Tang, Journal of Materials Chemistry C, 2016, 4, 7383-7386.
34. F. Ciardelli, M. Bertoldo, S. Bronco, A. Pucci, G. Ruggeri and F. Signori, Polym. Int., 2013, 62, 22-32.
35. F. Ciardelli, S. Bronco, O. Pieroni and A. Pucci, in Molecular Switches (2nd, Completely Revis. Enlarged Ed.), eds. B. L. Feringa and W. R. Browne, Wiley-VCH Verlag GmbH & Co, Weinheim, Germany Editon edn., 2011, vol. 1, pp. 321-360.
36. A. Pucci, G. Ruggeri, S. Bronco, M. Bertoldo, C. Cappelli and F. Ciardelli, Progr. Org. Coat., 2007, 58, 105-116.
37. K. Matyjaszewski and J. Xia, Chemical Reviews, 2001, 101, 2921-2990.
38. E. Wang, J. W. Y. Lam, R. Hu, C. Zhang, Y. S. Zhao and B. Z. Tang, Journal of Materials Chemistry C, 2014, 2, 1801-1807.
39. R. Mori, G. Iasilli, M. Lessi, A. B. Munoz-Garcia, M. Pavone, F. Bellina and A. Pucci, Polymer Chemistry, 2017.
40. M. Borelli, G. Iasilli, P. Minei and A. Pucci, Molecules, 2017, 22.
41. M. Carlotti, G. Gullo, A. Battisti, F. Martini, S. Borsacchi, M. Geppi, G. Ruggeri and A. Pucci, Polymer Chemistry, 2015, 6, 4003-4012.
42. T. Grimaud and K. Matyjaszewski, Macromolecules, 1997, 30, 2216-2218.
43. T. Tadros, in Encyclopedia of Colloid and Interface Science, ed. T. Tadros, Springer Berlin Heidelberg, Berlin, Heidelberg, Editon edn., 2013, pp. 523-524.
44. B. Shekar, M. Na, J. Lee and S.-W. Rhee, Molecular Crystals and Liquid Crystals, 2004, 424, 43-53.
45. D. R. Lide, CRC Handbook of Chemistry and Physics, 86th Edition, CRC Press LLC, Boca Raton, FL, USA, 2004.
46. J. E. Mark, Physical properties of polymers handbook, 2nd edn., Springer Science + Business Media, LLC, New York, 2007.
Table 1. Vapour pressure of different solvents at 25 °C,45 PMMA-solvent Flory-Huggins interaction parameter χ,46 for the investigated volatile organic compounds (VOCs).
Solvent Vapour Pressure (kPa) χ
hexane 17 2.08 toluene 2.9 0.45 dioxane 4.1 0.70 CHCl3 21.3 0.44 Et2O 71.7 0.56 acetone 30.8 0.48 MeOH 16.9 1.00 DMSO 0.79
25x103 20 15 10 5
E
m
is
si
on
(
a.
u
.)
840 800 760 720 680 640 600wavelength (nm)
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 95%(a)
20 15 10 5 0I/I
0 90 80 70 60 50 40 30 20 10 0% vol water
0% 95%(b)
Figure 2. a) Fluorescence spectra of TPE_RED in DMSO/water mixture as a function of the water
content (vol.%) and b) emission intensity variations plotted as a function of the water content. In the inset, pictures of the same solutions taken under illumination with a near-UV lamp at 366 nm
50x103 45 40 35 30 25 20 15 10 5
E
m
is
si
on
(
a.
u.
)
800 760 720 680 640 600 560wavelength (nm)
618 nm 608 nm 607 nm 95% 90% 80% 70% 60% 50% 0%Figure 3. Emission (λexc = 490 nm) spectra of 2 g/L PMMA_TPE_RED1.5 in CHCl3/hexane mixtures with different hexane fractions (vol.%). Inset: photographs of the same solutions taken under 365 nm UV light irradiation.
-0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0
F
lu
or
e
sc
en
ce
v
ar
ia
tio
n
200 160 120 80 40 0exposure time (s)
CHCl3 acetone hexane dioxane Et2O toluene DMSO MeOHFigure 4. Fluorescence variation of the maximum intensity (λexc = 490 nm) with exposure time to
VOCs for PMMA_TPE_RED1.5 films. Fluorescence was collected for a total time of 200 s in each experiment
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0
F
lu
or
e
sc
en
ce
v
ar
ia
tio
n
200 160 120 80 40 0exposure time (s)
10 ppm 25 ppm 40 ppm 50 ppm (a) -1.0 -0.8 -0.6 -0.4 -0.2 0.0F
lu
or
e
sc
en
ce
v
a
ria
tio
n
10 25 40 50ppm CHCl
3 (b)Figure 5. (a) Fluorescence variation of the maximum intensity (λexc = 490 nm) with exposure time
to different concentration of CHCl3 vapours. (b) Maximum fluorescence variation plotted as a function of CHCl3 vapours concentration after 200 s of exposure
-0.50 -0.40 -0.30 -0.20 -0.10 0.00
F
lu
or
e
sc
e
nc
e
va
ria
tio
n
5 7.5 10 15 20ppm CHCl
3 TPE_RED/PMMA (a) -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0F
lu
or
es
ce
nc
e
va
ria
tio
n
10 25 40 50 100ppm CHCl
3 TPE-HPh-Bar/PMMA (b)Figure 6. Maximum fluorescence variation plotted as a function of CHCl3 vapours concentration
after 200 s of exposure for a) TPE_RED/PMMA (λexc = 490 nm) and b) TPE-HPh-Bar/PMMA films (λexc = 447 nm).
350x103 300 250 200 150 100 50
E
m
is
si
o
n
(a
.u
.)
800 760 720 680 640 600 560wavelength (nm)
25 °C 35 °C 45 °C 55 °C 65 °C 75 °C 85 °C 95 °C 105 °C 115 °C (a) 350x103 300 250 200 150 100 50E
m
is
si
on
(
a
.u
.)
720 680 640 600 560wavelength (nm)
40 °C 50 °C 60 °C 70 °C 80 °C 90 °C 100 °C 110 °C 120 °C 130 °C (b)Figure 7. Fluorescence spectra (λexc = 490 nm) of a) PMMA_TPE_RED1.5 and b)
-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0
F
lu
or
es
ce
nc
e
va
ria
tio
n
120 100 80 60 40 20temperature (°C)
PMMA_TPE_RED1.5 TPE_RED/PMMAFigure 8. Fluorescence maximum variations (λexc = 490 nm) of PMMA_TPE_RED1.5 and TPE_RED/PMMA films as a function of the annealing temperature
