Fluorescent Vapochromism in Synthetic
Polymers
Pierpaolo Mineia and Andrea Puccia,b
aDipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Moruzzi 13, 56124 Pisa, Italy
bCNR-Istituto per i Processi Chimico Fisici, UOS Pisa, Via Moruzzi 1, 56124 Pisa, Italy
Corresponding author:
Prof. Andrea Pucci, Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Moruzzi 13, 56124 Pisa, Italy; E-mail: andrea.pucci@unipi.it
Abstract
The detection of volatile organic compounds (VOCs) by means polymeric materials through fluorescence variations has attracted considerable interest over the past ten years. The detection of VOCs is of great concern because of their undesirable impact on both environment and human health. Moreover, the sensing of vapours of organic compounds such as explosives is one pressing issue in anti-terrorism and homeland security. In this mini- review, the current state of knowledge concerning the preparation of fluorescent polymers with vapochromic features is summarized. Two types of procedures are illustrated and thoroughly discussed along with their respective structure– property relationships: the first resides on the covalent insertion of chromophoric units into the macromolecule backbone or side chains; the second involves the physical dispersion of the fluorophore in a preformed polymer matrix. Herein we review the simplicity of the preparative routes available, and their influence over the properties of the resulting fluorophore–polymer systems, by focusing on the most illustrative examples described in the literature. Finally, conclusive remarks and future perspectives are presented and discussed.
1. Introduction
Fluorescence was used for the first time as an analytical tool for the determination of various and diverse organic and/or inorganic species.1, 2 It was found that sensing based on fluorescence is one of the most effective methods for the detection of analytes with high sensitivity and selectivity thanks to the specially designed emitting probes.3-6 Materials which respond to external solicitations
(physical or chemical) through variations in their fluorescence are called fluorogenic.7 Nature
illustrates several ways on how colour response towards external stimuli may be obtained. For example, chameleons, cephalopods and lizards are well known for their awesome abilities to change the skin colour in response to mood, temperature and light intensity.8, 9 These and other inspiring examples, together with the growing knowledge on the photophysics of chromophores, will stimulate the design of new effective tools for the development of sensors, probes and information displays. Nowadays, the detection of volatile organic compounds (VOCs) is an important aspect considering that VOCs are continuously released to the environment by different sources like industrial processes, transportation, agriculture, etc.10-13 Some of them have adverse effects on
human health.14, 15 Also, current pressing concerns in global security strongly stimulated the development new fluorescent materials with various sensing mechanisms for detecting explosives in the vapour phase in order to achieve super-sensitivity, ultra-selectivity, as well as fast response time.16-18 Various detection methods have been proposed in recent years.19-21 Leading examples are
based on changes in electrochemical, conducting, and chromic properties of the corresponding sensor matrices.16-18, 22-24 Due to the non-fluorescent 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 vapour phase. Notably, any phenomenon that results in a change of fluorescence intensity (quenching or enhancement), wavelength, anisotropy, or lifetime as a function of vapours exposure has the potential to be used to sense VOCs. In this context,
colorimetric sensor systems are of particular interest thanks to their effectiveness and simplicity and have attracted exponential interests among academic and technological scientists (Figure 1).20, 21, 25
40 30 20 10
C
ita
tio
n
s
2015 2010 2005 2000 1995 1990publications in each year
Figure 1. Annual numbers of publications and citations on “vapochromism” (data searched from Scifinder® on January 2016)
Emerging strategies for the colorimetric and specific detection of VOCs, are based on fluorophores embedded in plastic materials or in fluorescent conjugated polymers.24, 26-30 Notable is the fact that the optical behaviour of these polymer-based VOCs sensors is tuneable by specific interactions between the polymer matrix and the fluorophore units. These synergic interactions of the fluorophore with the polymer structure also provide innovative material features and responsive character upon external stimuli. Since their chemical and physical properties may be tailored over a wide range of characteristics, the use of such vapochromic polymers is finding a permanent place in sophisticated measuring devices such as sensors.
Within such a fascinating frame, the present mini-review focuses on examples demonstrating the use of synthetic polymers as vapochromic probes for the detection of volatile organic compounds. In the examples discussed, the fluorophore can be either covalently bound to or simply dispersed in the synthetic polymer. These bi-component tools (macromolecule + fluorophore), which act as
responsive materials upon a specific vapour analyte stimulus, can work as active units for sensing, tracking and identification purposes, as well as for information storage.
The examples here reported come from the work performed in the authors’ laboratories, with complementary additional examples selected from the recent relevant literature. Eventually, the objective of this mini-review is to relate molecular parameters to final optical response through the understanding of the effective interactions among the different components. Therefore, the following sections are organized on the basis of a molecular descriptive approach by reporting selected examples and their vapochromic behaviour in two separate sections. The first is devoted to homogeneous systems, which are typically represented by covalently bound macromolecule and fluorophore, while the second resides on polyphase systems that are normally observed when the fluorophore is molecularly dispersed in the polymer matrix.
2. Fluorescent vapochromic materials from covalently linked fluorophore–polymer systems
In the case of fluorophores covalently linked to polymers, the chromophoric unit should be appropriately positioned within a polymer chain. The introduction of chromophoric moieties into polymers through covalent bonding is generally achieved by different pathways (Scheme 1), depending on the type of polymers to be obtained.31, 32
A
B
C
D
Scheme 1. Polymerization strategies towards fluorescent vapochromic polymers
The fluorescent vapochromic unit can be incorporated in a macromolecule through its direct linking (Scheme 1A) or copolymerization with other non fluorescent monomers (Scheme 1B). Moreover, the fluorescent vapochromic unit can also be attached on the polymer as side chains. In scheme 1C and D, a polymerizable monomer is labelled with the vapochromic moiety, which then undergoes homopolymerization or copolymerization to afford linear polymers. Notably, the amount of fluorophore covalently linked to the polymer can be modulated by adjusting the content of the labelled co-monomer in the feed.
2.1 Conjugated polymers
Fluorescent conjugated polymers are an intriguing class of materials for fluorescent vapochromism, since their highly efficient exciton migration results in high sensitivity for fluorescence quenching sensing. Notably, the electron-poor vapours might adsorb onto the electron-rich polymer backbone, thus favouring the photoinduced electron transfer from the conjugated macromolecule (donor) to the adsorbed analyte (acceptor) via a non radiative pathway (Scheme 2).1, 33
Scheme 2. Energy diagram of the photoinduced electron transfer resulting in fluorescence quenching
This process requires a wavefunction overlap between the donor and acceptor, which means it can only occur at short distances such as those of vapours adsorption on the macromolecular backbone. Binding of a single molecule can quench the fluorescence from hundreds of polymer repeating units, resulting in an amplification of the quenching response. This ability, called also “superquenching”, has led to them being referred to as amplifying fluorescent polymers (AFPs). Leading examples are represented by poly(phenyleneethynylene) (PPEs) and poly(phenylenevinylene) (PPVs), which were extensively reported for the trace detection of explosives such as nitroaromatic compounds. These fluorescent conjugated polymers contain electron-rich aryl rings, which form favourable interactions to bind the electron-poor nitroaromatic derivatives. Yang and Swager demonstrated that efficient sensing requires the incorporation of bulky groups within the polymer in order to prevent self-quenching between
polymer chains in the solid state.34 They efficiently designed a PPE possessing pentiptycene
moieties integrated in series in the polymer backbone (Figure 2a), which was obtained with a Mn of 56000 by palladium-catalyzed cross-coupling of the corresponding disubstituted diiodobenzene and pentiptycene diacetylene.35 n RO OR R= C14H29 n R= 2-ethylhexyl OC16H33 C16H33O OR OR RO RO
a
b
c
d
n O OFigure 2. Pentiptycene-derived PPE (a), poly(triphenylene ethynylene) (b), chiral pentiptycenyl PPE (c) polymers and mechanism of analyte binding (d). Adapted with permission from @@@
The material demonstrated to be highly sensitive to TNT vapours (fluorescence quenching of 50±5 % by 30 s of TNT vapours exposure) as well as other common contaminants of land mines. The device, which is commercially available with the name of “Fido” allows for the real-time monitoring of fluorescence intensity as a function of the exposure against nitro-aromatics vapours. Femtogram sensitivity to TNT was demonstrated to be comparable to those of trained canines.36 The
same research group has reported several effective modifications of the PPE polymers aimed at enhancing the sensitivity towards TNT vapours. For example, a series of PPEs that incorporated
triphenylene moieties into the backbone of a conjugated polymer (Figure 2b) showed longer excited-state lifetimes, and a more sensitive response to TNT were achieved.37 Moreover, chiral
aggregates of PPE (Figure 2c) were demonstrated with a 4-fold increase in sensitivity towards TNT vapour compared with PPE in figure 2a. This increased sensitivity was addressed to the enhanced exciton transport caused by more effective three dimensional electronic interactions among polymer aggregates able to retain the majority of its fluorescence intensity (quantum yield = 0.61).38
Recent advances on vapochromic PPE derivatives for the detection of explosives were proposed by Mishra et al.39 They prepared a series of poly(arylene ethynylene)s (PAEs) containing t-butyl
pyrene in the main chain via Sonogashira cross coupling reaction.
Figure 3. tButyl pyrene containing poly(arylene ethynylene) and time dependent fluorescence intensity of the derived drop-cast film upon exposure to TNT vapors. Adapted with permission from @@@
Sensing studies of drop-cast film exposed to TNT vapours revealed that polymers having tbutyl substituted pyrene in the main chain quenched 20% of its fluorescence after an exposure time of 10 s (Figure 3), as similarly displayed by Swager et al. with PPE vapochromic polymers.34, 35 Notably, for an exposure time of 20 min, the vapochromic polymer lost almost its emission. The efficient quenching of TNT to the fluorescence emission of the investigated polymer was addressed to the high interaction between the TNT and the pyrene moieties in the copolymer, which strongly favoured the photo induced electron transfer.
In addition to the research on PPE polymers and copolymers, other groups have reported that fluorescent conjugated polymers based on Si-based materials have shown excellent gas permeability. On the basis of this effective combination of properties which sensitivity, Si-based conjugated polymers could be exceptional materials for fabrication of a thin-film-based fluorescent sensor for vapours of neutral analytes. For example, Schanze et al. reported fluorescence from thin films of poly[1-phenyl-2-(4-trimethylsilylphenyl)ethyne] that is strongly quenched by the vapours of a variety of nitroaromatic compounds present at levels ranging from ppm to ppb in air (Figure 4).40 The polyacetylene was obtained in high yields and with a Mn of 293000 (PDI = 1.6) by
polymerization of 1-phenyl-2-(4-trimethylsilylphenyl)ethyne using a TaCl5/n-Bu4Sn catalyst.
Figure 4. Poly[1-phenyl-2-(4-trimethylsilylphenyl)ethyne] and fluorescence from the derived 7 nm thick film at various times following introduction of solid 2,6-DNT to the fluorescence cuvette. The inset shows the fluorescence intensity as a function of exposure time. Adapted with permission from @@@
Also in this case, the mechanism for the quenching process was addressed to the amplified fluorescence quenching caused by the formation of adducts between the nitroaromatic acceptors and the electron-rich polymer chain. The fluorescence dropped quickly within the first 2 min after addition of 2,6-DNT and then it decreased more slowly until reaching a plateau with about 10% of the remaining intensity. The rate of the vapochromic response was reported to be determined by the sublimation rate of the analyte and its permeation within the polymer film.
The amplified fluorescence quenching mechanism was recently proposed by Chen et al. for a series of conjugated polymers based on benzo[5]helicene, elegantly obtained via catalysed cross-coupling reactions.30 Helicenes are a kind of polycyclic aromatic compounds with unique helical structures
and characterized by a strong emission not only solution but also solid states.41 These electron-rich
conjugated polymers were reported to display a vapochromic response to electron-deficient nitroaromatics with similar structures.
Figure 5. Benzo[5]helicene-based polymer and fluorescence behaviour of the derived spin-coated film upon exposure to saturated vapour of DNT at different time interval. Adapted with permission from @@@
Notably, the polymer films revealed a reduction of 87% of their fluorescence upon exposure to DNT vapour (Figure 5), and of 70% to TNT vapour in only 10 s. Such difference was addressed to the higher vapour pressure of DNT with respect to TNT. The pronounced vapochromic response was attributed to the nature of helicene units, whose twisted structure and extended π‐conjugation effectively increase the interaction between the macromolecules and the nitroaromatics, thus amplifying the quenching effect.
Besides the experimental explosive sensing studies, other interesting examples were reported in literature on the use of inorganic conjugated fluorescent polymers such as poly(silole)s for the detection of nitroaromatics by amplified photoinduced electron-transfer quenching.42 Poly(silole)s
conjugated Si–Si backbone, along which of electrons are delocalized to provide an effective exciton migration pathway. Polytetraphenylsiloles feature a helical structure and fluoresce efficiently in the solid state. The fluorescence of the derived thin film experienced quenching due to electron-transfer mechanism (Scheme 2) on exposure to TNT vapour in oxygenated air. Noteworthy, a decrease in emission intensity of 8.2% occurred over a 10 min exposure to 4 ppb TNT.43
Fluorescent conjugated polymers have been also investigated for their possible vapochromic response towards volatile amines. In this context, Yashima et al. reported the preparation of novel optically active polymer consisting of riboflavin units as the main chain by base-mediated polycondensation of 8α-bromoriboflavin (Figure 6).44
N N N N O O OAc OAc AcO AcO n NH2 HClO4
Figure 6. Optically active riboflavin-containing polymer and vapochromic behaviour of a films drop-cast on thin glass plates exposed to 2-propylamine vapors at room temperature for 5 seconds. The original colour was recovered by the treatment with diethyl ether containing HClO4
The riboflavin residues were then converted to the corresponding 5-ethylriboflavinium cations, which demonstrated to self-assemble in a supramolecularly twisted, helical structure through intermolecular face-to-face π-stacking of the riboflavinium units. Being riboflavinium cations reactive towards nucleophiles, the polymer films showed a rapid and reversible vapochromic response to primary and secondary amine vapours, leading to the direct visual detection of the
amines. The absorption spectral pattern of the film was indeed changed upon treatment with the amine vapour (Figure 4), and the characteristic absorption peaks due to the conjugated flavinium cation units of the polymer almost completely disappeared only after 5 s of exposure. This system effectively contributes to the recent examples on fluorescent vapochromic polymers, even if sensitivity tests should be carried out before final validation.
2.2 Aggregation induced emission (AIE) polymers
Another interesting report recently emerged involved a phenomenon called “Aggregation Induced Emission” (AIE).45, 46 Materials showing the AIE phenomenon, i.e. more efficient emitters while in the aggregated state than in the dissolved form, were reported for the first time in 2001.47 The AIE
phenomenon is based on the fact that luminogen aggregation played a constructive, instead of a destructive, role in the light-emitting process: a series of molecules were found to be non-luminescent in the solution state but emissive in the aggregated state. For example, tetraphenylethylene (TPE) is non-emissive in dilute solutions but becomes highly luminescent when their molecules result aggregated in concentrated solutions or cast into solid films.48, 49 In detail, in a dilute solution, TPE undergoes dynamic intramolecular rotations against its double bond and renders its molecule non-luminescent.50, 51 Conversely, in the aggregate state, the intramolecular rotations of its aryl rotors are greatly restricted, thus opening up the radiative channel. 52 AIE was
first realized in propeller-like small molecules and only very recently extended to polymeric systems which exhibit an array of functional properties which suggest potential applications as explosive chemosensors, light-emitting materials, including bioprobes for in vitro and in vivo imaging applications.53-62 Notably, VOCs can be easily detected taking advantages of AIE features.
For example, conjugated poly(fluorene) polymers based on o-carborane moieties were reported as vapochromic materials thanks to the C2B10 cage’s unique bonding scheme as well as its bent geometry and its bent geometry, which is able promote specific interactions with analyte
molecules.63 The polymers were obtained following the widely used Ni(0) dehalogenative
Yamamoto-type polymerization64 of the o-carborane fluorene monomer.65
a
b
AIE
Figure 7. (a) Optical images under UV light of the poly(fluorine-o-carborane) coated over squares of laboratory wipes to common laboratory solvent vapours and (b) photoluminescence emission of the same polymer during exposure to amine vapours. Adapted with permission from @@@
In detail, exposure of polymer coated over squares of laboratory wipes to common laboratory solvent vapours led to a fluorescent vapochromism under UV light due to a combination of effects (Figure 7): the first resides on the strong decreasing of the green AIE emission of the polymer and the second is founded on solvatochromic effects due to the adsorbed vapours. This phenomenon produced a clear shifting of the observed emission colour toward the red. Moreover, exposure to certain amines quenched emission completely by both disrupting the AIE green emission and interacting with the Lewis acidic boron cages. This is thought to occur via the amine attacking the boron adjacent to the C-C bond, which is the most electron deficient.
Another interesting example based on AIE vapochromism was reported by Tang et al. that prepared polyacrylates with glycogen-like structures via radical polymerization of
tetraphenylethene-containing di- and tetra-acrylates.66 The process furnished high molecular weight (Mw up to 87100)
and closed-loops in nearly quantitative yields (isolation yield up to 99%).
+DCM
-DCM
O O
O O
Figure 8. TPE-containing polyacrylates and optical images under UV light of the poly(fluorine-o-carborane) coated over squares of laboratory wipes to common laboratory solvent vapours
The polymer was then deposited over a TLC plate and the spots generated after solvent evaporation demonstrated strong vapochromism (Figure 8). The polymer emission was accordingly turned “off” and “on” continuously and reversibly by wetting and de-wetting processes by VOCs. Similar vapochromic behaviour was also reported by Zhu et al. for analogous AIE polymers based on amorphous fluorene-based fluorophores that have been end-capped with tetraphenylethene (TPE).67
Preliminarily results showed potential application of the system in solid state vapour sensing using dichloromethane that shows “on” and “off” fluorescence behaviour.
3. Fluorescent vapochromic materials from fluorophore-polymer blends
The mixing together of functional molecules into polymers has attracted large interest because of the advantage to attain new properties without synthesizing a novel polymer structure.68-73 The
macromolecules will remain chemically and structurally unaltered, and the system will be generally biphasic, unless the dyes result fully soluble (compatible) in the original polymer. Indeed, a fundamental issue of these multicomponent materials is the control of the additive phase dispersion within the polymer matrix, such condition being governed mainly by the affinity between the components’ mixture. Component affinity, which strongly affects the miscibility of dye-polymer
blends, is promoted by intermolecular interactions such as hydrogen bonding, dipole–dipole and Van der Waals forces.74
Fluorophore/polymer dispersions can be realised either in solution or in the molten polymer mass by using compounding apparatuses depending on the physico-chemical characteristics of the mixture’s components.68-72, 75 In the case of polymer matrices composed of repeating functional units that can be highly compatible with the chemical structure of fluorophores, homogeneous fluorophore/polymer solid mixtures can be obtained by film casting. Film casting involves dissolving polymer granules or powder, along with the desirable amount of dye in a suitable solvent. The solution is then poured into a mould and the film is obtained after solvent evaporation at room temperature or by heating. An efficient alternative aimed at controlling the coating process of fluorophore/polymer blends is represented by the dip-coating technique. Dip-coating is precision controlled immersion and withdrawal of any substrate into a reservoir of solution for the purpose of depositing a layer of dye-doped material.
Problems of can, however, arise when the polymer matrix is completely apolar and non-interacting with the dye, thus strongly limiting miscibility and compatibility. This is the case of fluorophore/polyolefin mixtures that can show phase-separation during casting and drying.76, 77 In this case, fluorophore/polymer blends can be realised by means of continuous mechanical mixers, which are able to disperse the dye thoroughly the polymer bulk. In a melt-blending process, the phase morphology is developed as a result of the shearing forces overcoming the interfacial tension, which tends to resist the deformation and break-up of fluorophore agglomerates. When the blending process is stopped, the high viscosity of the polymer matrix mostly prevents the coalescence of dye particles into larger phase-separated aggregates.
The design of fluorescent vapochromic materials from fluorophore-polymer blends is based on fluorophore-analyte interactions that are stronger than those that cause simple physical adsorption. The chemically responsive fluorophores generally fall into four classes: a) fluorophores with large
permanent dipoles that respond to local polarity (e.g., solvatochromic fluorophores), b) pH indicators that respond to Brønsted acidity/basicity, c) dyes containing metal ions that respond to Lewis basicity (that is, electron-pair donation, metal-ion bonding) and d) fluorophores with segmental mobility that respond to local viscosity (e.g., AIE molecules and fluorescent molecular rotors).
For example, the solvatochromic dimethylaminostyryl terpyridine fluorophore (Figure 9), was investigated by Schmehl et al.71 for use in vapochromic sensing. The polarity-sensitive fluorophore
was incorporated into several polymer films such as poly(methyl methacrylate) (PMMA) and polystyrene (PS) and exposed to a variety of solvents.
Figure 9. Dimethylaminostyryl terpyridine fluorophore
Thin films (420±10 nm thick) were prepared by dip-coating from chloroform solutions containing a fluorophore/polymer ratio of 1:25 by weight. Rapid (<2 s) and reversible changes in the emission of the fluorophore/polymer films were observed for the series of vapours comprising a range of polarities. The polymers absorbed and concentrated the analyte vapours, thereby influencing the environment surrounding the fluorophore embedded in the polymer. Notably, PMMA showed larger vapochromic red-shifts (>100 nm) with more polar solvents than PS being the more hydrophilic nature of the former polymer matrix. Nevertheless, polar solvents that have very low vapour pressures at room temperature (DMF and DMSO) did not affect the emission behaviour of the fluorophore in the polymer. Overall, this system might find use as a sensory material, even if sensitivity tests should be carried out before final validation.
Fluorescent vapochromism was also extended to organometallic fluorophores dispersed via solution casting in a series of PMMA films as in the case of square-planar platinum(II) complex of the 4-dodecyloxy-2,6-bis(N-methylbenzimidazol-20-yl) pyridine ligand (Figure 10).
+CH3CN
Figure 10. Platinum(II) complex and images showing the luminescence from a 10 wt% of complex in PMMA thin film under UV irradiation (365 nm) before and after exposure to acetonitrile vapours. The emission spectra were recorded during exposure to acetonitrile vapours (exc = 377 nm) at 30 s intervals over a 900 s period. Adapted with permission from @@@
The exposure of the fluorophore-doped PMMA films to acetonitrile vapour results in a red/orange emission with the maximum shifting to 600 nm, and the intensity noteworthy raising, reaching 3–5 times that of the intensity of the pristine film. Vapochromism occurred within approximately 15 min and that only 30% of the total emission change occurred within the first 3 minutes. Notably, the phenomenon was reversible since heating the films to drive off the solvent restored the original film emission. The mechanism behind the vapochromism was shown to be related to a structural rearrangement of the Pt(II) complex promoted by acetonitrile vapours, which results in an increased number of shorter intermolecular Pt–Pt interactions. No details on VOCs selectivity and system sensitivity were provided by authors.
Another contribution of organometallic fluorophores was recently demonstrated by Platonova et al. that reported the vapochromic behaviour of PMMA films doped with small amounts of a Zn adduct of the bisthienylethynylbipyridine ligand.30 The Zn(II) complex exhibited polarity-dependent
changes in the fluorescence spectral properties when dissolved in solvents with different polarity index, indicating that its fluorescence states are of intramolecular charge transfer character (Figure 11a). When dissolved in PMMA films (<1 wt.%) the material showed a significant variation of its emission with a 40 nm red-shift when exposed to a saturated atmosphere of dichloromethane due to the effective solvation of the fluorophore by the absorbed solvent (Figure 11b).
6 0 0 x 1 03 5 0 0 4 0 0 3 0 0 2 0 0 1 0 0 E m is si o n ( a. u .) 6 5 0 6 0 0 5 5 0 5 0 0 4 5 0 4 0 0 w a v e l e n g t h ( n m ) 4 7 5 4 7 0 4 6 5 4 6 0 4 5 5 4 5 0 4 4 5 4 4 0 4 3 5 w av el en gt h (n m ) 5 0 0 4 0 0 3 0 0 2 0 0 1 0 0 0 t i m e ( s )
a
b
Figure 11. (a) Images (λexc = 366 nm) of 10-5 M Zn(II) complex dissolved in solvents at different polarity index (toluene = 2.4, dichloromethane (DCM) = 3.1 and ethyl acetate (AcOEt) = 4.4)68 and
(b) progressive changes in the fluorescence of the 0.5 wt.% Zn(II) complex/PMMA film as a function of the exposition to dichloromethane vapours. The spectra were collected in succession over a range of 500 s with 30 s data interval. Inset: wavelength shift reported with exposition time and mono-exponential growth fitting function. Adapted with permission from @@@
This solvent dependence was addressed to the ICT character of the Zn(II) complex emission and resides in the reorganization energy of the transition with increasing polarity.1 The Zn(II) complex
solvation besides decreasing the energy gap between the electronic states of fluorophore, promotes the electron transfer rate thus leading to a faster and higher amplitude relaxation to a permanently quenched state.72 This phenomenon was reported to be possibly responsible of the decreasing of the
emission intensity of the fluorophore along with its shift to higher wavelengths. Conversely, the films appeared much less responsive when methanol, a solvent barely interacting with PMMA, was
used. Notably, the rate constant associated with the vapochromic behaviour was found to be affected by solvent composition and easily determined by luminescent experiments, thus opening future perspectives to the use of such system for the realization of vapochromic polymers.
Recently, fluorescent vapochromism was proposed by utilizing a pyrene-containing polystyrene (Py-PS) polymer film with self-assembled three-dimensionally ordered nanopores (Figure 12a).70 In
detail, self-assembled fluorescent nanostructured film expressing regular breath-figure nanopores was generated by dip-coating of a mixture of PS and the fluorophore Py onto a glass slide. The morphology of the three-dimensional nanoporous array of holes was effectively controlled by variation of the dip-coating parameters and the ratio of PS to Py. The proposed system revealed a rapid and selective vapochromic behaviour when exposed to nitroaromatic vapours. Vapochromism relies on the fluorescence quenching mechanism due to the photo-induced electron transfer from the excited Py excimers within the nanoporous film to the electron-deficient nitroaromatics.
Figure 12. (a) 3D-AFM image of the nanoporous array film collected in tapping mode for a 5x5 m section and (b) fluorescence quenching profiles of the 3D nanoporous Py–PS fluorescent film (0.1 M Py and 4 wt% PS). Adapted with permission from @@@
Upon exposure of the best sensing film to saturated DNT vapours, the fluorescence quenching of the excimer peak at 470 nm was observed to be 60% within 6 min and to reach a plateau of 90% after 30 min (Figure 12b). The geometry of the PS scaffold possibly enabled effective co-facial stacking, allowing pyrene to be arranged in and between the phenyl moieties of PS, thus
contributing to the formation of extended conjugation of electrons. Such structure resembles those of conjugated polymers in which long-range energy migration resulted in the amplification of the quenching response.34, 35 Excellent selectivity of the self-assembled Py-PS films against common interferences was also reported.
Among the different classes of fluorophores utilized as vapochromic probes in polymer blends, those that show viscosity- or aggregation-dependent fluorescence have been recently reported as one of the most effective for the detection of VOCs. For example, fluorescent molecular rotors (FMRs) are flexible chromophores with a fluorescence response that depends on the local viscosity of the environment.69, 71, 74, 75, 77, 78 FMRs have become rather popular in the last 10 years thanks to their easy applicability as non-mechanical viscosity sensors, tools for protein characterization, and local microviscosity imaging. Remarkably, their sensitivity towards viscosity and viscosity changes has reached a precision comparable to that of commercial mechanical rheometers with shorter measurement time.71 p-N,N-Dimethylaminobenzonitrile (DMABN),77 9-(dicyanovinyl)julolidine
(DCVJ), 9-(2-carboxy-2-cyanovinyl)julolidine(CCVJ)77, 79, 80 and tetraphenylethylene (TPE)46, 81-83 are among the most known examples of FMRs and AIE fluorophores. A number of mechanistic pathways have been hypothesized aimed at unveiling the different working mechanisms, including conformational planarization, J-aggregate formation, E/Z isomerization, twisted intramolecular charge transfer (TICT), and excited-state intramolecular proton Transfer (ESIPT). An excellent review that dissected all the proposed mechanisms has been recently proposed by Tang et al.82 and a
complete coverage of the topic is out of the scope of this paper.
Martini et al. explored for the first time the use of julolidine derivatives blended in PS for the detection of several VOCs.28 Julolidine-based FMRs are fluorescent molecules composed by an
electron donor unit in conjugation with an electron acceptor moiety and are reported to undergo radiative relaxation from their locally excited (LE) state. When molecular internal rotation is hindered, e.g. due to an increase in viscosity or sterical constraints, the radiative decay of LE state is
favoured, and an increase in quantum yield is obtained. Conversely, solvent relaxation causes rapid internal torsional motion, thus resulting in a non emissive TICT excited state (Scheme 3).1
!"# $% # L E s t a t e p l a n a r ! "# $% # T I C T s t a t e t w i s t e d e xc ita tio n e m is si on f ro m L E non-radiative
disexcitation from TICT
reaction coordinate Scheme 3. Potential energy diagram for FMRs
In case of FMRs, the excited-state energy is lower in the TICT state, whereas the ground-state energy is higher in the TICT state than in the LE state. Therefore, the S1 – S0 energy gap is lower in the TICT state with correspondingly lower relaxation energy. Notably, in DCVJ or CCVJ the TICT energy gap is much smaller than the LE energy gap, so that the disexcitation from the TICT state occurs without photon emission.74, 77, 79
Martini et al. showed that julolidine-based FMRs exhibited viscosity-dependent emission properties when dissolved in solvents with different viscosity or 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 (e.g., chloroform and toluene) caused a significant drop of FMR fluorescence due to their favoured relaxation from the non-emissive TICT excited state. This behaviour can be interpreted as follows: the interactions between a polymer and vapours of a suitable solvent are able to induce a relaxation of macromolecular chains which is followed by a greater mobility with an increase in the free volume and a consequent decrease in the local microviscosity It is therefore expected that the
fluorescence emission of an FMR embedded in a polymer matrix could be affected by vapour exposure (Figure 13).
Figure 13. 9-Dicyanovinyljulolidine (DCVJ, left), 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ, middle) and 9-(2-(1H,1H,2H,2H -perfluorodecyloxycarbonyl)-2-cyanovinyl)julolidine (F8CVJ, right). (a) Progressive changes in the fluorescence emission of 0.05 wt.% DCVJ/PS film as a function of the exposure to chloroform vapours and (b) Variation of the fluorescence maximum intensity with exposure time for all 0.05 wt.% FMR/PS films with inset images of the 0.05 wt.% F8CVJ/PS film before (t = 0 s) and after (t = 420 s) chloroform exposure
It is worth noticing that vapochromism was much more evident and rapid when a perfluorodecyl chain was linked to the julolidine core (F8CVJ). This behaviour was addressed to the preferential molecule concentration in a ~10 nm outer layer of the film surface, due to the selective segregation of the low surface energy fluorinated fluorophore at the polymer air interface. Therefore, this segregation enabled F8CVJ to interact more readily with chloroform. Conversely, poor or absent vapochromism was detected for PS films exposed to methanol and hexane vapours, being class of solvents barely interacting with PS. This characteristic response made plastic films able to discern vapours of different composition, providing also a sensitive and reproducible fluorescence response in successive cycles of solvent exposure.
A similar approach was reported by Minei et al. that proposed an interesting vapochromic system suitable for sensing VOCs based on thermoplastic 90–120 m thick PMMA and polycarbonate (PC)
films doped with 0.05–0.1 wt% of 4-(diphenylamino)phthalonitrile (DPAP), a fluorescent molecular rotor sensitive to both solvent polarity and viscosity.29 DPAP was reported to have a
contrasting deactivation pattern of the intramolecular charge transfer state: in low and medium polar solvents DPAP shows a strong emission, whereas in high polar and protic solvents DPAP is not emissive since the rotor's ICT state is stabilized. Moreover, likewise common FMRs, DPAP showed a viscosity-dependent emission properties.84 Minei reported that DPAP/PMMA films show a good
and reversible vapochromism when exposed to VOCs with high polarity index and favourable interaction with polymer such as CHCl3, and acetonitrile. The vapochromic response occurred within 4 min of exposure and consisted in a significant decrease and a red-shift of the main emission of about 50 nm (Figure 14a). The authors attributed the origin of this effect to solvent induced changes in the local polarity and viscosity of the medium, this last analogous to julolidine derivatives in PS as reported earlier by Martini et al.28 The vapochromic response was also found to
be reproducible even after several successive cycles of vapour exposure. By contrast, low polar and barely interacting VOCs such as n-hexane, toluene and THF did not affect film fluorescence.
N CN
CN
a b
DPAP
Figure 14. DPAP and (a) progressive changes in the emission 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 5 min (red box) of vapours exposure. The spectra were collected for 5 min with a time interval of 1 min; (b) progressive changes in the emission of 0.05 wt% DPAP/PC films as a function of exposure to CHCl3 vapours. The spectra were collected for 38 min with a time
with permission from @@@
Even more interesting was the behaviour reported for DPAP/PC, since PC matrix experienced solvent-induced crystallization during VOCs exposure (Figure 14b). In this case, an initial vapochromic response analogous to that showed by PMMA films was recorded. Noteworthy, at longer VOCs exposure (>8 min), fluorescence experienced a further red-shift and a steadily increase of its intensity. This phenomenon was rationalized by a polymer crystallization process which provoked a general increase in the viscosity of the matrix, thus enhancing DPAP radiative decay processes. Once more, this phenomenon occurred only for vapours of highly interacting solvents with the polymer matrix.
4. Conclusive Remarks and Future Perspectives
Polymeric materials which respond to chemical solicitations such as vapours of volatile organic compounds through variations of their optical features are called vapochromic. Fluorescent vapochromic polymers was reported for the first time in 1998 by the pioneering work of Swager et al. with the preparation of PPE derivatives for the trace detection of high explosives such as TNT. This approach has been repeatedly demonstrated in wide range of conjugated polymers by using different classes of chromophoric repeating units. This method involves the electron-poor vapours adsorption onto the electron-rich polymer backbone, thus favouring the photoinduced electron transfer from the conjugated macromolecule (donor) to the adsorbed analyte (acceptor) via a non radiative pathway. Evidences demonstrated that the highest sensitivity was due to more effective three dimensional electronic interactions among polymer backbones which then caused the enhanced exciton transport. Binding of a single molecule can quench the fluorescence from hundreds of polymer repeating units, resulting in an amplification of the quenching response. This method has the advantage of obtaining a material with homogeneously distributed dye molecules
whose diffusion and segregation is prevented. Other inspiring examples are founded on solvatochromic or pH sensitive probes.
The synthetic possibility is however limited by the need of appropriate functional groups on both macromolecules and fluorophores. Dye dispersion into polymers is certainly a sustainable procedure and largely applied to proprietary polymers, even if efforts must be taken into account to prevent dye segregation or to modulate the dispersion degree as a function of the ultimate properties of the material. This approach allowed the realization of vapochromic polymers starting from different categories of fluorescent probes, e.g. solvatochromic fluorophores, pH indicators and fluorescent organometallic materials. In this case, the design of fluorescent vapochromic materials should be based on fluorophore-analyte interactions that are stronger than those that cause simple physical adsorption.
Even if a lot of progress has been made during the last decades in the field of chromogenic polymers, various new and intriguing challenges remain to be resolved. For example, the design of polymers able to respond with increasingly larger colour change at modest analyte concentration is a desirable objective. The inspiring examples reported in this mini review, together with the growing knowledge on the photophysics of fluorophores, will stimulate the design of new effective plastic tools for the development of sensors, probes and information displays.
In both formulations (i.e., covalently linked fluorophore–polymer systems and fluorophore-polymer blends) recent advances have been focused on vapochromic materials based on fluorophores that show viscosity- or aggregation-dependent fluorescence. AIE and FMR fluorophores were reported for the first time by Tang47 and Theodorakis79 in 2001 and have become rather popular in the last 10
years thanks to their easy applicability as explosive chemosensors, light-emitting materials, non-mechanical viscosity sensors, tools for protein characterization, and local microviscosity imaging. Remarkably, their sensitivity towards viscosity and viscosity changes has reached a precision
comparable to that of commercial mechanical rheometers with shorter measurement time. The vapochromic polymers based on the turn-on/turn-off nature of the AIE and FMRs probes appear very promising for the detection of analytes at small concentration and during the first moment of vapour exposure. Trace amounts of the well interacting analyte with the polymer matrix rapidly disrupt the AIE features and cause a significant drop of FMR emission, thus switching off the polymer fluorescence.
Overall, the enormous number of available fluorophores and synthetic and bio-based polymers, opens unlimited possibilities for the production of smart materials with vapochromic features. We expect that these concepts assisted by sustainable routes may steer innovative intelligent materials to be used in the everyday life.
Acknowledgements
The financial support by MIUR-PRIN (2010XLLNM3) is kindly acknowledged.
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Nowadays, fluorescent vapochromic polymers is finding a permanent place in sophisticated measuring devices such as sensors thanks to their sensitivity, reversibility and rapid response
