Fluorescent polystyrene films for the detection of volatile organic compounds using the twisted intramolecular charge transfer mechanism ?

Article abstract of DOI:10.3390/molecules22081306

Thin films of styrene copolymers containing fluorescent molecular rotors were demonstrated to be strongly sensitive to volatile organic compounds (VOCs). Styrene copolymers of 2-[4-vinyl(1,1-biphenyl)-4-yl]-cyanovinyljulolidine (JCBF) were prepared with different P(STY-co-JCBF)(m) compositions (m% = 0.10–1.00) and molecular weights of about 12,000 g/mol. Methanol solutions of JCBF were not emissive due to the formation of the typical twisted intramolecular charge transfer (TICT) state at low viscosity regime, which formation was effectively hampered by adding progressive amounts of glycerol. The sensing performances of the spin-coated copolymer films (thickness of about 4 μm) demonstrated significant vapochromism when exposed to VOCs characterized by high vapour pressure and favourable interaction with the polymer matrix such as tetrahydrofurane (THF), CHCl₃ and CH₂Cl₂. The vapochromic response was also reversible and reproducible after successive exposure cycles, whereas the fluorescence variation scaled linearly with VOC concentration, thus suggesting future applications as VOC optical sensors.

Full text of DOI:10.3390/molecules22081306

molecules
Article
Fluorescent Polystyrene Films for the Detection of
Volatile Organic Compounds Using the Twisted
Intramolecular Charge Transfer Mechanism †
Mirko Borelli 1, Giuseppe Iasilli 1, Pierpaolo Minei 1 and Andrea Pucci 1,2,
*
ID
1
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via G. Moruzzi 13, 56124 Pisa, Italy;
mirkobore@live.it (M.B.); giuseppe.iasilli@gmail.com (G.I.); Pierminei@gmail.com (P.M.)
INSTM, UdR Pisa, Via G. Moruzzi 13, 56124 Pisa, Italy
2
*
Correspondence: andrea.pucci@unipi.it
†
Dedicated to Professor Ben Zhong Tang on the occasion of his 60th birthday.
Received: 8 June 2017; Accepted: 3 August 2017; Published: 6 August 2017
Abstract: Thin films of styrene copolymers containing fluorescent molecular rotors were demonstrated
to be strongly sensitive to volatile organic compounds (VOCs). Styrene copolymers of 2-[4-vinyl(1,10-
biphenyl)-40-yl]-cyanovinyljulolidine (JCBF) were prepared with different P(STY-co-JCBF)(m)
compositions (m% = 0.10–1.00) and molecular weights of about 12,000 g/mol. Methanol solutions
of JCBF were not emissive due to the formation of the typical twisted intramolecular charge transfer
(TICT) state at low viscosity regime, which formation was effectively hampered by adding progressive
amounts of glycerol. The sensing performances of the spin-coated copolymer films (thickness of about
4 µm) demonstrated significant vapochromism when exposed to VOCs characterized by high vapour
pressure and favourable interaction with the polymer matrix such as tetrahydrofurane (THF), CHCl3
and CH2Cl2. The vapochromic response was also reversible and reproducible after successive exposure
cycles, whereas the fluorescence variation scaled linearly with VOC concentration, thus suggesting
future applications as VOC optical sensors.
Keywords: fluorescent molecular rotors; polymer films; VOC; vapochromism
1. Introduction
Nowadays, chromogenic fluorescent (fluorogenic) materials are being effectively used for the
preparation of devices showing optical features sensitive to different external stimuli [1–8]. The
fluorescent properties generally depend on different molecular parameters, such as structural flexibility,
electron donor or acceptor moieties in addition to an extended conjugation. Nevertheless, self-assembly
of fluorophore molecules usually suffer of emission quenching phenomena (aggregation-caused
quenching (ACQ)), which strongly restricts the number of accessible fluorophores for practical
applications. In this context, a revolutionary class of luminophores demonstrating the fluorescence
development with aggregation has drawn great attention since their discovery sixteen years ago
by Ben Zhong Tang [
restriction of fluorophore intramolecular motions (RIM), which promotes bright luminescence in
the aggregate and solid state [ ]. By enabling light emission in the practically useful solid state,
AIE fluorophores demonstrate to show a striking impact on energy, optoelectronics, life science
and environment [ 13 18]. Notably, for AIE systems with donor-acceptor structure, the emission
9–12]. The effect, called aggregation induced emission (AIE), arises from the
5,7
9, –
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, called also fluorescent molecular
rotors (FMRs), have become rather popular in the last 5–10 years thanks to their easy applicability
Molecules 2017, 22, 1306; doi:10.3390/molecules22081306
www.mdpi.com/journal/molecules

Molecules 2017, 22, 1306
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as non-mechanical viscosity sensors, tools for protein characterization, and local microviscosity
imaging [19 22]. Viscosity measurements by means of FMRs require shorter measurements times and
–
smaller amounts of samples. Moreover, since the fluorescence viscometry does not apply shear to
the sample, it is more practical for biofluids, which have apparent non-Newtonian properties. FMRs
in solution has been explored since years, and examples of their use in combination with polymers
have been efficiently reported for the detection of volatile organic compounds (VOCs) [22–25]. The
detection of VOCs is an important issue considering that they are continuously released into the
environment by different sources such as industrial processes, transportation, agriculture, as well
as indoor applications, and some of them have adverse effects on human health [26,27]. VOCs
are usually characterized by low boiling point and high vapour pressure at standard conditions,
thus they may rapidly fill an enclosed environment. Because of their toxic nature, in many states
there are regulations setting a limit to VOCs emission. Furthermore, current pressing concerns in
global security have stimulated the development of new fluorescent materials, with various sensing
mechanism, aimed at detecting chemicals in the vapour phase. For example, the vapochromism of
julolidine-based FMRs were explored in polystyrene (PS) films [28]. The exposure of FMR/PS films
to well-interacting VOCs induced plasticization of the supporting polymer matrix, thus favouring
a striking drop of FMR fluorescence due to the favoured relaxation from the non-emissive TICT
state. Nevertheless, efforts must be still pursued to confer the fastest response under VOCs exposure.
For example, it is expected that covalent binding of FMR moieties to the macromolecular chains
should provide the fastest response to minimal amounts of VOCs. Notably, the amount of FMR
covalently linked to the polymer can be easily modulated by adjusting the content of the labelled
co-monomer in the feed [3,23]. This procedure would also ensure a more homogeneous distribution of
the active FMR units within the macromolecular chains, without promoting the formation of stacked
supramolecular structures potentially useless for the vapochromic response. In the present work,
0
0
styrene copolymers of 2-[4-vinyl(1,1 -biphenyl)-4 -yl]-cyanovinyljulolidine (JCBF) were prepared with
different P(STY-co-JCBF)(m) compositions with m% = 0.10–1.00. Copolymer thin films were obtained
by spin-coating on glass plate surfaces, and their sensing performances towards different VOCs were
investigated at room temperature.
2. Results and Discussion
2.1. Synthesis of JCBF
The synthesis of the 2-[4-vinyl(1,10-biphenyl)-40-yl]-cyanovinyljulolidine (JCBF, Scheme 1) and
its inclusion into styrene polymers by conventional radical polymerization have never been reported
before. JCBF should act as a viscosity-sensitive FMR unit in glassy PS environment owing to the
presence of the cyanovinyljulolidine moiety. Notably, the FMR unit is linked to the vinylbiphenyl
moiety that is more chemically related to the styrene repeating units than the methacrylate functions
used earlier [25].
Julolidine was formylated with phosphorus oxychloride and dimethylformamide to
afford 9-formyljulolidine (FJUL). The 2-(40-vinyl-[1,10-biphenyl]-4-yl)acetonitrile was obtained via
cross-coupling between 2-(4-bromophenyl)acetonitrile and 4-vinylphenylboronic acid as reported in
literature [29]. The product was then reacted with FJUL, thus yielding after purification the desired
target monomer JCBF with a yield of 50%. The appropriate amount of JCBF was then copolymerized
with styrene through radical polymerization with Azobisisobutyronitrile (AIBN) for one week to afford
P(STY-co-JCBF)(m) copolymers, with m% = 0.10–1.00 (Scheme 2).

Molecules 2017, 22, 1306
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O
H
1) DMF, POCl3, 4.5h, 90°C
2) H2O, 0°C
3) CH3COONa, Overnight, rt
FJUL
71% yield
N
N
Br
1) Toluene, Aliquat 366, Na2CO3
Pd[P(Ph)3]4, 24h, 90°C
2) H2O, EtOAc
+
CBF
65% yield
B
HO
OH
N
N
O
H
THF, tBuOK
2h, rt
+
JCBF
50% yield
N
N
N
N
0
0
Scheme 1. Synthesis of the 2-[4-vinyl(1,1 -biphenyl)-4 -yl]-cyanovinyljulolidine (JCBF) monomer. DMF:
dimethylformamide; rt: room temperature; THF: tetrahydrofurane; FJUL: 9-formyljulolidine; CBF: of
0
0
2-[4-vinyl(1,1 -biphenyl)-4 -yl]-acetonitrile.
Scheme 2. Synthesis of the P(STY-co-JCBF) copolymers. AIBN: Azobisisobutyronitrile. dd: days.
Copolymer compositions, molecular weights and thermal behaviour are reported in Table 1.
Table 1. Characteristics of the P(STY-co-JCBF) copolymers.
JCBF
(alimentation wt %)
◦ ◦
Mw/Mn Tg ( C) Td ( C)
Copolymer
m (mol. %)
Mn (g/mol)
P(STY-co-JCBF)(0.10)
P(STY-co-JCBF)(0.34)
P(STY-co-JCBF)(0.90)
P(STY-co-JCBF)(1.04)
0.39
0.98
3.41
3.90
0.10
0.34
0.90
1.04
13,800
11,000
10,900
12,200
1.6
1.6
1.5
1.5
103
102
105
108
415
420
430
430
Mw: weight average molecular weight; Mn: number average molecular weight; Tg: glass transition temperature;
Td: degradation temperature

Molecules 2017, 22, 1306
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A long polymerization time (7 days) was required to maximize JCBF conversion. The JCBF
monomer was barely inclined to propagate, possibly due to an effective stabilization of the radical
moiety by its chromophoric unit.
Four distinct copolymers were prepared and containing progressive amounts of JCBF, i.e., from
0.10 to 1.04 mol %, as determined by 1H-NMR and UV-Vis experiments, that is supposing negligible
variation of the molar extinction coefficient of JCBF after polymerization. No evidence of JCBF content
on the copolymers molecular weight and thermal properties is present. Notably, P(STY-co-JCBF)
◦
copolymers display a glass transition temperature in between 100 and 110 C, and a degradation onset
above 415 ◦C.
2.2. Optical Properties of JCBF Chromophore in Solution
JCBF chloroform solution showed absorption maximum around 407 nm (
with emission maximum at about 500 nm with a negligible quantum yield (
to the favored formation of the typical radiationless TICT excited state (Figure S1) [30,31]. Similar
ε
= 21,500 M−1 cm−1),
Φ
f) of 2.5·× 10−3 due
absorption (Amax = 407 nm) and emission (Emmax = 500 nm) were recorded in methanol solutions as
well as for tetrahydrofurane (THF) (Amax = 407 nm, Emmax = 505 nm) and toluene (Amax = 403 nm,
Emmax = 494 nm). Surprisingly, a strongly different behavior was observed when glycerol was added
to methanol solutions (Figure 1 and Table 2).
−5
Figure 1. UV-Vis absorption spectra of 1
glycerol volume contents.
×
10 M JCBF in methanol/glycerol solutions with different
−6
Table 2. Optical features of 5 × 10 M JCBF solutions.
ε (F/m) a
Φf
d
η (mPas) b
Solvent
Amax (nm)
Emmax (nm)
−3
−3
−3
−3
−3
−3
−3
−2
−2
−2
methanol
407
411
415
426
427
428
428
435
440
465
500
505
506
508
510
512
515
519
521
525
33
0.6 (0.6) c
1.8
1.26 × 10
1.56 × 10
1.79 × 10
2.36 × 10
3.15 × 10
4.18 × 10
6.26 × 10
1.06 × 10
1.89 × 10
3.73 × 10
methanol/glycerol 90:10 v/v
methanol/glycerol 80:20 v/v
methanol/glycerol 70:30 v/v
methanol/glycerol 60:40 v/v
methanol/glycerol 50:50 v/v
methanol/glycerol 40:60 v/v
methanol/glycerol 30:70 v/v
methanol/glycerol 20:80 v/v
methanol/glycerol 10:90 v/v
34.4
35.6
36.7
37.6
38.6
39.3
40.1
40.8
41.4
4.8
7.7
13
28 (30) c
58 (55) c
100 (120) c
250 (250) c
630 (620) c
Dielectric constants [32], b viscosity at 20 ◦C [33] and at c 22 ◦C [34]; d Fluorescence (λexc. = 410 nm) quantum yield
a
(Φf) determined at room temperature relative to perylene (Φsf = 0.92 in EtOH) [35].

Molecules 2017, 22, 1306
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JCBF exhibited an apparent solvatochromism (55 nm in absorption and 20 nm in emission) when
glycerol was added to methanol solutions (Figures 1 and 2, Table 2). The red-shift in the absorption
spectrum with increasing polarity is not typical in FMRs, which usually exploit a solvent-independent
absorption features [24,36]. To the best of our knowledge, Karpenko et al. have reported one of the
few examples of FMR with evident solvatochromism also in absorption, even if at lower extent than
JCBF. [37] This feature was addressed to the presence of a significant dipole moment in the ground
state, which further increases on electronic excitation to the Frank-Condon state. Nevertheless, all
these findings do not completely explain the effect of glycerol on JCBF methanol solution, and more
accurate investigations are required. Although meaningful, we deferred the study to a future work.
As instead expected, in the excited state the molecule undergoes Intramolecular Charge Transfer
(ICT) as it is favored in fluorophores bearing electron donor and acceptor moieties, thus showing
fluorescent solvatochromism. More than that, the significant viscosity-dependent fluorescence of the
JCBF monomer was noteworthy (Figure 2 and Table 2).
Figure 2. Fluorescence (
λ
= 410 nm) of 1
×
10−5 M JCBF in methanol/glycerol solutions with
exc.
different glycerol volume contents. In the inset picture, JCBF solutions were excited with a Dark Reader
46B transilluminator (~450 nm, Clare Chemical Research, Dolores, CO, USA).
Notably, JCBF underwent a strong increase in quantum yield (about 30 times higher) when
dissolved in viscous environments like glycerol-containing solutions (viscosity
η
= 630 mPas at 20 ◦C
for methanol/glycerol 10:90 v/v mixture, as compared to 0.6 mPas for pure methanol). According to
the FMR behaviour the molecular internal rotation of the molecule is hampered in viscous media, thus
promoting the emission from the LE and, in turn, the increase in quantum yield. As expected, JCBF
solutions ensued the typical Förster-Hoffmann behaviour [38] (Equation (1) and Figure S2), which
relates the (double logarithmic) quantum yield with viscosity:
logΦf = C + x·logη
(1)
where C and x are constants.
Equation (2) [33]:
η was predicted by the Grunberg-Nissan simplified additive rule in
ηmix = φ1η1 + φ2η2
(2)
where the subscripts 1 and 2 are referred to solvent 1 and 2, respectively, φi are the volume fractions,
and ηi are the viscosities of pure substances [33]. The x parameter, defined as the viscosity sensitivity

Molecules 2017, 22, 1306
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of the FMR, was found to be 0.56, which is comparable with the highest values reported for similar
systems, thus making JCBF suitable as microviscosity probe of environments.
The JCBF chromophore did not experience any significant variation of its optical features
when incorporated in the PS backbone. Notably, P(STY-co-JCBF) dissolved in chloroform displayed
−3
absorption peaked at 410 nm with emission pointed at about 500 nm with a negligible Φf of 2.5
×
10
(Figure S3). Moreover, the progressive increase of the JCBF content in the PS backbone did not alter the
position of the absorption and emission maxima peaks (Figure S4). Being P(STY-co-JCBF) polymers
insoluble in methanol, no comparison with JCBF methanol solution was reported.
2.3. Vapochromism of P(STY-co-JCBF)(m) Thin Films
Well homogeneous P(STY-co-JCBF)(m) thin polymer films (4
µm) were obtained by spin-coating
chloroform solutions on 2.4 2.4 cm cleaned glass cover slides. Absorption features appeared very
×
similar to those gathered in solution and with absorbance maxima in agreement with the JCBF content
(Figure S5a). As far as the emission is concerned, strong fluorescence was gathered from the solid films
being the highly viscous and glassy (Tg of about 100–110 ◦C, Table 1) polymer matrix favour light
emission from the Locally Excited (LE) states of the covalently bonded JCBF moieties. The red-shift
of the emission band with JCBF content (Figure S5b) could be possibly addressed to auto-absorption
phenomena (inner-filter effect) [39].
The emission behaviour of P(STY-co-JCBF)(m) thin films was then studied by exposing them
to different kinds of volatile organic solvents. An example of the fluorescence emission variation
on exposure time to chloroform vapours is reported in Figure 3. It is worth noting that the
P(STY-co-JCBF)(0.34) films experienced a significant variation in emission intensity just during the first
minutes of VOC exposure (Figure 3).
Figure 3. Progressive changes in the fluorescence emission (
λ
= 410 nm) of P(STY-co-JCBF)(0.34)
exc.
film as a function of the exposure to chloroform vapours. The spectra were collected for a total time of
5–6 min.
The emission intensity dropped by about 80% after 8 s of exposure. This extremely sensitive
solvent dependence, never achieved previously by our research group [22,24,25,28,40], agrees well
with the characteristics of JCBF. Notably, the progressive VOC adsorption and successive absorption by
the polymer matrix triggers the reorganization energy of the JCBF excited transition state from the LE to
the non-emissive TICT. As already known, chloroform is a good solvent for PS (Table 3), whose empty
channels and holes of molecular dimensions are filled by solvent vapours. Their progressive diffusion
and the resulting swelling of the polymer causes rapid local microviscosity decrease, thus enabling
fluorescence quenching. Notably, after a few minutes, the swelled polymer reached an equilibrium
and its emission did not change any longer for prolonged exposure times.

Molecules 2017, 22, 1306
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The monitoring of the fluorescence peak intensity variation as a function of VOCs exposure time
(Figure 4) furnishes a better perception of the vapochromic response. All P(STY-co-JCBF)(m) thin films
were monitored and exhibited a similar behaviour upon exposure to chloroform vapours. Fluorescence
decreasing resulted faster for polymer films with the lowest JCBF content (i.e., P(STY-co-JCBF)(0.10)
and P(STY-co-JCBF)(0.34)) and reached a plateau after less than 60 s (Figure 4). This phenomenon was
addressed to the fact that low amounts of emitting species are expected to be readily solvated by the
absorbed solvent and, therefore, to provide the greatest variation in emission during the early stages
of VOCs exposure. Conversely, at higher fluorophore content, possible cluster formation due to π-π
staking interactions might occur and retard the complete solvation of the single chromophoric units,
which, in turn causes a substantial delay of the fluorescent quenching.
(a)
(b)
Figure 4.
to chloroform vapours for all P(STY-co-JCBF)(m) films. Fluorescence was collected for a total time
of 10 min in each experiment. ( ) Comparison between P(STY-co-JCBF)(0.34) and the most sensitive
(
a) Variation of the fluorescence maximum intensity (
λ
= 410 nm) with exposure time
exc.
b
julolidine-based Fluorescent Molecular Rotors (FMR) dispersed in polystyrene (PS) [28] towards
chloroform vapours.
More specifically, the P(STY-co-JCBF)(0.10) film lost 50% of its pristine emission upon less than
10 s of exposure to chloroform vapours. By contrast, the most efficient julolidine-based FMRs dispersed
in PS films displayed a pretty much slower reduction in fluorescence emission, i.e., showing 50%
decreasing only after 180 s of exposure without levelling off before 4 minutes [28]. This outstanding

Molecules 2017, 22, 1306
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result was addressed to the prompt and simultaneous response of all the thickness of the films
provided by the well isolated and homogeneously distributed covalently linked FMR moieties. In the
covalent approach, the external contamination experienced by the polymer matrix is readily perceived
by the julolidine-based FMRs, which enable a fast fluorescence response. Conversely, physically
dispersed FMR molecules may form clustered supramolecular assemblies, which might adversely
affect the fluorescence quenching rate. In fact, blend PS films containing dispersed julolidine-based
FMR with a thickness lower than 20–30
poor film homogeneity.
µm were not found suitable for sensing experiments due to
P(STY-co-JCBF)(0.34) films were then exposed to hexane vapours, i.e., a solvent with different
vapour pressures and Flory–Huggins interaction parameter (Table 3). The parameter is defined to
give an effective measure of the interaction between the polymer and the solvent. Notably, small
χ
χ
χ
values are characteristic of well-interacting solvent-polymer pairs [41] and, therefore, associated to
stronger fluorescence decreasing.
◦
Table 3. Vapour pressure of different solvents at 25 C, [42] PS-solvent Flory-Huggins interaction
parameter χ, [43] for the investigated volatile organic compounds (VOCs).
Solvent
Vapour Pressure (kPa)
χ
hexane
toluene
THF
CHCl3
CH2Cl2
Et2O
13
2.9
1.49–1.14
0.42–0.31
0.16–0.70
0.52–0.17
0.78–0.71
0.17–0.05
0.81–0.94
2.44
21.6
21.3
58.2
71.7
30.8
5.7
acetone
EtOH
As a matter of fact, the exposure of P(STY-co-JCBF)(0.34) film to hexane vapours did not cause
any significant variation of their emission intensity (Figure 5).
Figure 5. Progressive changes in the fluorescence emission (
λ
= 410 nm) of P(STY-co-JCBF)(0.34)
exc.
film as a function of the exposure to hexane vapours. The spectra were collected for a total time of
5–6 min.

Molecules 2017, 22, 1306
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Significant fluorescence variation of P(STY-co-JCAEM)(0.34) films actually occurred when
highly interacting solvents were utilized as VOCs (Figure 6). Notably, Et2O, THF, chloroform and
dichloromethane produced the fastest decreasing rate (Figure 6) thanks to the favourable combination
of
χ and vapour pressure. In the case of Et2O, the fluorescence drop was not completed since the
intensity of the emission started recovering after a certain exposure time. This phenomenon was
tentatively addressed to the very high vapour pressure and reduced size of the Et2O molecule. These
factors caused a rapid adsorption, diffusion and macromolecules rearrangement into a less swellable
system that acts as a negative feedback for the vapochromic behaviour. This phenomenon possibly
occurs for all the VOCs investigated, most probably shifted at longer exposure times.
Figure 6. Variation of the fluorescence maximum intensity (
λ
= 410 nm) of P(STY-co-JCBF)(0.34)
exc.
films with exposure time to all VOCs investigated.
The vapochromic response differ significantly for acetone (
= 0.42–0.31). While for acetone the slower vapochromism was addressed to its higher
(i.e., = 0.81–0.94 for acetone against 0.17–0.05 for Et2O), for toluene the fluorescence variation was
mostly delayed due to its lower vapour pressure (i.e., 2.9 kPa for toluene against 71.7 kPa for Et2O,
at 25 ◦C). Moreover, the fluorescence intensity resulted barely affected by hexane (
= 1.49–1.14) and
χ
= 0.81–0.94) and toluene
(
χ
χ
value
χ
χ
ethanol (χ = 2.44) vapours, i.e., for VOCs with limited affinity with the polymer matrix.
It is worth noting that the experimental setup did not affect P(STY-co-JCBF) film size and aspect
after the time interval investigated, i.e., within 20 min of VOCs exposure. Actually, solvent can be
effectively desorbed at 20 ◦C and 1 Atm, allowing complete recovery of the film emission after one
day. This allowed also the complete reversibility (and reuse) of the designed vapochromic system
as evidenced by the good reproducibility of the vapochromic response during the second cycle of
chloroform vapours exposure (Figure S6).
Lastly, we investigated the sensibility threshold of the P(STY-co-JCBF) films, by placing progressive
amounts of chloroform within the measurement box (Figure S7 and Figure 7). The results reported in
Figure 7 show that the system started to be optically responsive for concentration of chloroform less
than 150 ppm, a threshold that is fairly far from that required in sensors applications.
Nevertheless, the linear relationship between the fluorescence maximum variation and chloroform
concentration renders the designed vapochromic polymer films as one of the most promising in the
development of modern optical sensors to VOCs.

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Figure 7. Fluorescence maximum variation as a function of chloroform concentration (ppm, expressed
as vol ratio of vaporized chloroform in the measuring chamber).
3. Experimental Section
3.1. Materials and Methods
All the chemical agents and solvents were obtained from commercial sources and used as
received. Phosphorous oxychloride (Sigma-Aldrich, Milan, Italy), was purified by distillation at
reduced pressure.AIBN was recrystallized from acetone. N,N-dimethylformamide (DMF), THF and
toluene were refluxed over CaH2 for 2 h and distilled under nitrogen. Styrene was distilled at
reduced pressure in presence of butylated hydroxytoluene (BHT, Aldrich) as polymerization inhibitor.
Spectroscopy grade solvents (Carlo Erba , Milan, Italy or Sigma-Aldrich, Milan, Italy) were utilized
without further purification.
Fourier transform infrared (FT-IR) spectra were recorded on a Spectrum GX FT-IR (Perkin-Elmer,
USA) at room temperature, on pellets made from grinding mixtures of anhydrous KBr with ~1% w/w
of the solid product, and pressing the mixture. Electron ionization mass (EI-MS) spectra were recorded
at 70 eV by gas-liquid chromatography and mass spectrometry (GLC-MS), performed on an Agilent
6890N gas-chromatograph interfaced with Agilent 5973N mass detector (Agilent, Santa Clara, CA,
USA). NMR spectra (at 400 MHz (1H) and 100 MHz (13C)) were recorded with a Bruker Advance DRX
400 spectrometer (Bruker, Billerica, MA, USA) at room temperature and were referenced to the residual
protons of deuterated solvents.
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 Pl◦us UV-Vis detector set at 252 and 360 nm. The flow rate was
1 mL
·
min−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).
Thermogravimetric (TG) analyses were carried out by means of a TGA/SDTA 851 apparatus
(Mettler Toledo, Columbus, OH, USA). Samples were heated from 25 to 700 ◦C at 10 ◦C min−1 under
·
a nitrogen flow. Differential Scanning Calorimetry (DSC) thermograms were recorded under nitrogen
atmosphere by a Mettler Toledo DSC 922e Module Stare apparatus equipped with a liquid nitrogen
cooling system. Samples were heated from 25 to 180 ◦C at 10 ◦C min−1, then cooled to 0 ◦C at the
·
same speed. The heating was repeated in the same conditions after 3 min of annealing at 0 ◦C.

Molecules 2017, 22, 1306
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Absorption spectra were recorded at room temperature on a Perkin-Elmer Lambda 650 spectrometer.
Fluorescence spectra were measured at room temperature on a Fluorolog®-3 spectrofluorometer (Horiba
Jobin-Yvon, Palaiseau, France) equipped with a 450 W xenon arc lamp, double-grating excitation and
single-grating emission monochromators.
The fluorescence quantum yield (
Φ) in methanol/glycerol mixtures was determined at room
temperature relative to perylene (Φsf = 0.92 in EtOH) [35] according to the relation:
R
ꢀ
ꢁ
∞ I(ν)dν
1 − 10−A
n2
n2ST
ST
0
R
Φ = ΦST
(3)
∞ IST(ν)dν
(1 − 10−A
)
0
R
R
where the subscripts ST are referred to standard, the integrals ∞ I(ν)dν and ∞ IST(ν)dν are the areas
under the emission curves of the investigated compound and standard, A and AST are the absorbances
0
0
of the investigated compound and standard at the excitation wavelength (410 nm),
n and nST are the
refractive index of the solvents, i.e., 1.332 for methanol, 1.474 for glycerol. The refractive index of
MeOH/Glycerol mixtures was predicted by the Arago-Biot additive rule [44]:
nmix = φ1n1 + φ2n2
(4)
The chemical composition of copolymers was evaluated by UV–Vis spectroscopy by means of
a calibration curve obtained form 5 × 10−7–1 × 10−5 CHCl3 solutions of JCBF.
Emission spectra (λexc 410 nm) of polymer films were recorded on the same spectrofluorometer
in the dark by using a F-3000 Fibre Optic Mount apparatus (Horiba Jobin-Yvon, Palaiseau, France)
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 was
tested by exposing the sample held by a steel tripod in a 50-mL beaker closed by a pierced aluminium
foil lid (Scheme 1), to 20 mL of various organic solvents of different vapour pressure and PS-solvent
Flory–Huggins interaction parameter χ (Table 3), at 25 ◦C and atmospheric pressure.
3.2. Synthesis of 9-Formyljulolidine (FJUL)
The synthesis of 9-formyljulolidine was carried out modifying a reported procedure [45,46]. In brief,
phosphorous oxychlori◦de (1.1 mL, 11.55 mmol) was added dropwise to N,N-dimethyl-formamide
(2 mL, 25.85 mmol) at 0 C. A solution of julolidine (2.0015 g, 11.55 mmol) in DMF (3.5 mL, 45.24 mmol)
◦
was then added and the mixture was stirred at 90 C for 4.5 h. The solution was allowed to cool
at room temperature (rt) and neutralized to pH 6–8 by the addition of a saturated sodium acetate
solution (~30 mL). After stirring overnight at rt, a greenish-yellow solid precipitate was recovered
via filtration, washed with water (30 mL) and dried under high vacuum. The crude product was
purified through column chromatography on silica gel using ethyl acetate/CHCl3 (70/30 v/v) as
eluent mixture. 1.65 g of FJUL were recovered (71% yield). FT-IR (KBr, cm−1): 2758, 1651, 1594,
1527, 1321. 1H-NMR (CDCl3):
4H, NCH2), 2.7 (t, J = 6.3 Hz, 4H, NCH2CH2CH2), 1.9 (m, 4H, NCH2CH2). 13C-NMR (CDCl3):
(ppm) = 190.1 (- HO), 147.9 (-N- (-C-)=C-), 129.5 (-C(=C)- H=C(-C)-CH=), 124.0 (-CH-(CH=) -CHO),
120.33 (-CH2- (=C-)-CH(=C)), 50.0 (-N(- H2)-), 27.7 (-N(-CH2-CH2- H2-)-), 21.3 (-N(-CH2- H2-CH2-)-).
EI-MS m/z (%): 201 (100, M+).
δ (ppm) = 9.6 (s, 1H, CHO), 7.3 (s, 2H, aromatic), 3.3 (t, J = 5.8 Hz,
δ
C
C
C
C
C
C
C
C
3.3. Synthesis of 2-[4-Vinyl(1,10-biphenyl)-40-yl]-acetonitrile (CBF)
2-(4-Bromophenyl)acetonitrile (0.5562 g, 2.84 mmol), 4-vinylphenylboronic acid (0.4469 g,
3.02 mmol) and palladium-tetrakis(triphenylphosphine) (0.045 g, 0.0389 mmol) were dissolved
in a mixture of toluene (20 mL), Aliquat 336 (0.16 g, 0.5 mmol) and 2 M aqueous potassium
carbonate solution (10 mL). The mixture was stirred at room temperature for 30 min under N2

Molecules 2017, 22, 1306
12 of 15
gas and then heated to 90 ◦C for 24 h. After that the mixture was poured into water and extracted
three times with ethyl acetate. The organic layer was dried over anhydrous magnesium sulfate.
The crude product was purified through column chromatography on silica gel using petroleum
ether/chloroform (3:7) as eluent to a solid white powder. 0.404 g of CBF was recovered (65%
yield). FT-IR (KBr, cm−1): 2908, 2862, 2250, 1600, 1498, 900, 800. 1H-NMR (CDCl3):
7.61 (d, J = 7.5 Hz, 2H), 7.57 (d, J = 7.5 Hz, 2H), 7.44 (d, J = 6.9 Hz 2H), 6.78 (dd, J = 7.9, 2.1 Hz,
1H), 5.85 (d, J = 16 Hz, 1H), 5.33 (d, J = 10 Hz, 1H), 3.82 (s, 2H). 13C-NMR (CDCl3):
(ppm) =
140.6 ((-CH-(CH=) -C(=CH)-CH)), 139.6 ((-CH-(CH=)C- (=CH)-CH)),), 137.1 ((-CH-(CH=) -CH),
136.3 ((-CH-(CH=)-C- H=CH2), 128.9 (-CH2- -(=CH)-CH)), 128.4 (-CH2-C-(= H)-CH)), 126.8
(NC-CH2-C-(= H)-CH)), 117.8 (N -CH2-C-(=CH)-CH)), 114.2 (((-CH-(CH=)-C-CH= H2)), 23.34
NC-CH2-C-(=CH)-CH)).
δ (ppm)
δ
C
C
C
C
C
C
C
C
C
3.4. Synthesis of 2-[4-Vinyl(1,10-biphenyl)-40-yl]-cyanovinyljulolidine (JCBF)
tBuOK (190 mg, 1.70 mmol) was added to a solution of FJUL (0.2538 g, 1.2572 mmol) and
CBF (0.3452 mL, 1.574 mmol) in anhydrous THF (10 mL). The mixture was kept under stirring at
rt for 2 h. The solvent was then removed under reduced pressure. The crude product was purified
through column chromatography on silica gel using chloroform as eluent to a solid white powder.
0.250 g of JCBF was recovered (50% yield). FT-IR (KBr, cm−1): 2928, 2854, 2210, 1593, 1523, 1311,
1136. 1H-NMR (CDCl3):
-C =CH2), 5.83 (d, J = 16 Hz, 1H, -CH=CH2-), 5.31 (d, J = 10 Hz, 1H, -CH=CH2-), 3.31 (t, J = 5.8 Hz,
4H, -N-CH2-), 2.82 (t, J = 6.3 Hz, 4H, -N-CH2-CH2-CH2), 2.03 (m, 4H, -N-CH2-CH2). 13C-NMR
(CDCl3): (ppm) = 145.1 (CH-(CH=)C- H-C), 142.3 (-N- (-C)=C-), 139.8 ((-CH-(CH=) -C(=CH)-CH)),
139.72 ((-CH-(CH=)C- (=CH)-CH))), 136.84 ((-CH-(CH=) -CH), 136.4 ((-CH-(CH=)-C- H=CH2),
129.1 (-CH2-C-(= H)-CH)), 125.7 (NC-CH2-C-(= (=C-)CH(=C)), 119.7
(N H2)-),
δ (ppm) = 7.75–7.48 (m, 10H, aromatics), 6.79 (dd, J = 7.9, 2.1 Hz, 1H,
H
δ
C
C
C
C
C
C
C
CH)-CH)), 120.8–120.6 (-CH2-C
C
-C-C-(=CH)-CH)), 114.1 (((-CH-(CH=)-C-CH=
C
H2)), 102.28 (NC-
C
-C-(=CH)-CH)), 49.9 (-N(-
C
27.7 (-N(-CH2-CH2-CH2-)-), 21.5 (-N(-CH2-CH2-CH2-)-). EI-MS m/z (%): 402 (100, M+).
3.5. Synthesis of Random Copolymers P(STY-co-JCBF)(m)
We describe in detail the typical preparation for the P(STY-co-JCAEM)(0.34). A solution of JCBF
(0.0104 g, 0.025 mmol), styrene (0.55 mL, 4.800 mmol) and AIBN (0.0060 g, 0.037 mmol) in anhydrous
toluene (5.2 mL) was introduced into a dry reaction tube with a Rotaflow Polytetrafluoroethylene
(PTFE) tap under nitrogen. After three freeze-pump-thaw cycles, the tube was sealed under vacuum
◦
through the PTFE screw at the top and the polymerization was let to proceed at 60 C for 7 days.
After cooling to rt, the polymer was then recovered by precipitation into methanol. The polymer
was centrifuged and the supernatants collected and then dried at reduced pressure. The polymer
was finally purified by repeated precipitations from dichloromethane solutions into methanol. Yield
40%; Mn = 11,000 g mol−1 (by GPC). FT-IR (KBr, cm−1): 3100–3000, 2925, 2852, 2204, 1942–1745,
1
1600, 1493, 900, 800. H-NMR (CDCl3):
δ (ppm) = 7.20 (aromatic), 6.60 (aromatic), 1.86 (broad, 1H,
-CH2-CH(-C6H5)-), 1.45 (broad, 2H, -CH2-CH(-Ph)-) typical of polystyrene and 3.20 (4H, -N-CH2-),
2.82 (4H, -N-CH2-CH2-CH2), characteristic of JCBF.
3.6. Preparation of P(STY-co-JCBF)(m) Films
Thin polymer films (4
A 2.4 2.4 cm 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 L) 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
−1). The obtained
µm) of P(STY-co-JCBF)(m) were obtained by spin-coating on glass substrate.
×
µ
·
s
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.

Molecules 2017, 22, 1306
13 of 15
4. Conclusions
We have demonstrated that 2-[4-vinyl(1,10-biphenyl)-40-yl]-cyanovinyljulolidine (JCBF), a new
julolidine-based FMR, once copolymerized with styrene, endows the resulting P(STY-co-JCBF)(m),
with m% = 0.10–1.00 thin films with significant vapochromic features. Thin films of styrene
copolymers 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, and appeared about 18 times faster than that of the
most efficient julolidine-based FMR dispersed in PS films. Moreover, the films’ size and morphology
resulted unaffected by VOCs exposure and the vapochromic response appeared also reproducible.
By contrast, the films fluorescence appeared unaltered when scarcely interacting VOCs like ethanol
and hexane were utilized. The sensitivity threshold was found to be less than 150 ppm, but the
linear relationship between the fluorescence maximum variation and VOC concentration supports the
application of julolidine-enriched styrene copolymers as modern vapochromic plastic sensors.
Supplementary Materials: The following are available online: Figure S1: UV-Vis absorption and emission
(
λ
exc. = 410 nm) of 1x10-5 M JCBF in chloroform, Figure S2: Förster-Hoffmann relationship of 1x10-5 M JCBF
soelxuctions in methanol/glycerol mixtures with different glycerol volume contents, Figure S3: UV-Vis absorption and
emission (
(b) emission (
emission (
λ = 410 nm) of 0.5 mg/mL P(STY-co-JCBF)(0.34) in chloroform, Figure S4: (a) UV-Vis absorption and
exc.
λexc. = 410 nm) of 0.5 mg/mL P(STY-co-JCBF) in chloroform, Figure S5: (a) UV-Vis absorption and (b)
λexc. = 410 nm) of P(STY-co-JCBF) thin films, Figure S6: Variation of the fluorescence maximum intensity
of P(STY-co-JCBF)(0.34) film as a function of successive cycles of chloroform exposure, Figure S7: Fluorescence
variation for all the P(STY-co-JCBF)(m) films as a function of progressive concentration of chloroform (ppm).
See Figure 6 for the exact concentration.
Acknowledgments: The authors are grateful to Marco Lessi (marco.lessi@unipi.it) of the Dipartimento di
Chimica e Chimica Industriale, Università di Pisa, Via G. Moruzzi 13, 56124 Pisa, Italy, for assistance with
EI-MS measurements.
Author Contributions: Mirko Borelli, Giuseppe Iasilli and Pierpaolo Minei synthesized and characterized the
samples. Andrea Pucci leaded the research and wrote the manuscript
Conflicts of Interest: The authors declare no conflicts of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
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