Distyrylbenzene-based segmented conjugated polymers: Synthesis, thin film morphology and chemosensing of hydrophobic and hydrophilic nitroaromatics in aqueous media

Article abstract of DOI:10.1016/j.polymer.2017.02.069

Two new segmented conjugated polymers bearing distyrylbenzene chromophoric units and their model compounds were synthesized. The tendency of the model compounds to form H- and J-type aggregates in the amorphous matrix was greatly diminished by the twisted polymeric architecture. Fluorescence anisotropy measurements indicated good exciton mobilities in condensed phase. Fluorescence quenching by nitroaromatic aqueous solutions was fast, complete, selective and reversible pointing to a rapid diffusion of analytes into the films. The quenching response to nitrophenols was superior to that against nitrotoluenes. The increase of the electron-donating capabilities by diethoxy-substitution was detrimental to the amorphous morphology and it did not increase sensitivity to NACs. Quenching efficiencies of polymers were not modified when MeOH was used instead of water. The solubility parameter distances, R_a. indicate that the sensing materials show higher responses when their affinity with the analytes is lower. This observation could help in the designing of fluorescent sensors.

Full text of DOI:10.1016/j.polymer.2017.02.069

Accepted Manuscript
Distyrylbenzene-based segmented conjugated polymers: Synthesis, thin film
morphology and chemosensing of hydrophobic and hydrophilic nitroaromatics in
aqueous media
Marcela F. Almassio, Maria J. Romagnoli, Pablo G. Del Rosso, Ana Belén Schwal,
Raúl O. Garay
PII:
S0032-3861(17)30210-0
10.1016/j.polymer.2017.02.069
JPOL 19470
DOI:
Reference:
To appear in: Polymer
Received Date: 30 November 2016
Revised Date: 18 February 2017
Accepted Date: 22 February 2017
Please cite this article as: Almassio MF, Romagnoli MJ, Del Rosso PG, Schwal AB, Garay RO,
Distyrylbenzene-based segmented conjugated polymers: Synthesis, thin film morphology and
chemosensing of hydrophobic and hydrophilic nitroaromatics in aqueous media, Polymer (2017), doi:
10.1016/j.polymer.2017.02.069.
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ACCEPTED MANUSCRIPT
Distyrylbenzene-based segmented conjugated polymers: Synthesis, thin film morphology and
chemosensing of hydrophobic and hydrophilic nitroaromatics in aqueous media
Marcela F. Almassio,ǂ Maria J. Romagnoli,ǂ Pablo G. Del Rosso, Ana Belén Schwal and Raúl O.
Garay*
INQUISUR, Departamento de Química, CONICET, Universidad Nacional del Sur, Alem 1253, CP
8000 Bahía Blanca, Argentina
*Corresponding author: Prof. Raúl O. Garay. Tel.: +54 291 4595101; fax: +54 291 4595187. E-
mail address: rgaray@criba.edu.ar
ǂBoth authors contributed equally to this work
Abstract
Two new segmented conjugated polymers bearing distyrylbenzene chromophoric units and their
model compounds were synthesized. The tendency of the model compounds to form H- and J-type
aggregates in the amorphous matrix was greatly diminished by the twisted polymeric architecture.
Fluorescence anisotropy measurements indicated good exciton mobilities in condensed phase.
Fluorescence quenching by nitroaromatic aqueous solutions was fast, complete, selective and
reversible pointing to a rapid diffusion of analytes into the films. The quenching response to
nitrophenols was superior to that against nitrotoluenes. The increase of the electron-donating
capabilities by diethoxy-substitution was detrimental to the amorphous morphology and it did not
increase sensitivity to NACs. Quenching efficiencies of polymers were not modified when MeOH
was used instead of water. The solubility parameter distances, Ra. indicate that the sensing materials
show higher responses when their affinity with the analytes is lower. This observation could help in
the designing of fluorescent sensors.
Keywords
Segmented conjugated polymer
Fluorescence quenching
Film sensor
Nitroaromatics
Aqueous phase
1

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1. Introduction
The remarkable development of molecular and polymer electronics relies in a large extent upon the
potential of organic material architectures to be tuned for improvement of color, band gap, morphology and
other properties required for their optimal performance in specific applications such as chemical and
biological sensing [1,2,3], emission [4,5] or storage of light [6]. Conjugated polymers are the paradigm of
electronic organic materials and the subject of intense research endeavoring to tailor their luminescence and
expand their fields of application [7]. In this context, an efficacious concept of macromolecular design aiming
to customize the physical and electronic properties of conjugated polymers consists in the regular insertion in
the polymer backbone of atomic or molecular motifs which tailor the π-conjugation length. Thus, a variety of
regularly segmented conjugated polymers (SCPs) have been synthesized by linking aromatic segments of
discrete size with spacers such as biphenylene [8], binaphthylene [9], paracyclophane [10], linear alkylene
[11,12] and oxyethylene chains [13], oxygen atoms [14], silicon atoms [15,16] and sp3-carbon atoms of the
norbornylene [17], fluorenylidene [18] and isopropylidene groups [19-21] all of which break conjugation in a
controlled manner.
In fact, the well-defined electro-optical properties of the aromatic segments are usually retained by the
SCPs only in diluted solutions. In solid state, the aromatic units of SCPs often form inter-chain species that led
to various morphological organizations at the molecular or nanoscopic level [12,13,22-24]. Thus, molecular
anisometries, specific attractive forces and microsegregation often generates aggregates and excimers which
have quite different electro-optical responses from those of the isolated unit in solution. The modification, and
even degradation [23], of the photophysical properties of the chromophoric repeating units is an especially
acute problem in SCPs with flexible spacers, whose conformational flexibility and amphilicity promote
ordering of the anisometric rigid units [12,23]. But when the size of the spacer dwindles down to a single sp3-
carbon atom the gem-chromophores are forced into angular dispositions bringing forth twisted polymer
microstructures which introduce a great deal of disorder into polymer morphologies. Therefore, in our studies
we focused on the small isopropylidene spacer as the key element in designing regularly SCPs with
amorphous morphologies which would ideally present optical properties insensible to the aggregation state.
Thus, we were successful in obtaining polymeric amorphous assemblies of biphenylene [20], terphenylene
[21], quaterphenylene [24] and diphenylfluorenylene [25] groups with good film-forming properties that are
solution processable and each show similar optical responses either in solution or in solid state. Besides,
despite the absence of side chains in the polymer architecture which is usually a condition to obtain soluble
aromatic polymeric materials, we found that polymer solubilities are notably enhanced when the aromatic
segments are tethered by the meta rather than para positions [25,26]; the same finding has been reported for
quaterphenylenes linked with oligo(ethylene glycol) spacers [12]. Finally, the frustrated packing of semi-rigid
segments causes void spaces that ease analyte exchange, thus making films of these nonaggregating
amorphous luminescent materials good candidates for chemosensing by fluorescence quenching [1,3,25-28].
In this contribution, we describe the synthesis and report on the optical and chemosensing properties of two
SCPs, Pa and Pb, in which the backbone consists of distyrylbenzene-based (DSB) moieties and isopropylene
spacers (Scheme 1). The DSB family of chromophores displays excellent photophysical properties and has
been the subject of intense research both as molecular materials [29-31] and as SCPs with flexible spacers
[12,32-34]. However, the DSB groups tend to aggregate making their morphology at the molecular level
highly dependent on subtle modifications of their structures. Thus, closely related members could arrange in
different ground state aggregates, namely, J- and H-aggregates, the latter with π-stacking in either face-to-face
and or in herringbone edge-to-face dispositions, all of them presenting their own optical signatures [29]; in
addition, their excited states could form excited dimers. So, we decided to focus on the DSB chromophores
partly because of its remarkable photophysical properties, and partly to test the effectiveness of the
isopropylene spacer to yield polymeric amorphous assemblies of DSB chromophores, which could become a
2

ACCEPTED MANUSCRIPT
good chemical sensing platform in the solid state. The related model compounds Ma and Mb were also
synthetized (Scheme 1) and their optical and chemosensing properties were correlated with those of their
parent polymers. The central benzene ring of the chosen DSB fluorophores for this study was either plain, Ma
and Pa, or dimethoxy substituted, Mb and Pb, in order to find whether the increase in the electron-donor
capacity of the electroactive aromatic segment was beneficial regarding their quenching performance with
various hydrophilic and hydrophobic nitroaromatics. Because our main interest resides in environmental
issues concerning detection of pollutants in soil and water bodies of city and industrial areas, the luminescence
quenching response of thin films to NACs was evaluated mainly in aqueous solutions [35,36].
Scheme 1 (
PO(OEt)2
X
Cl
t-Bu
t-Bu
OH
X
P(OEt)3
1) Na2CO3/CH3I
2) POCl3/DMF
reflux, 24 h
CHO
OCH3
X
X
(EtO)2OP
2a, X = H
Cl
1a, X = H
1b, X = OEt
3
2b, X = OEt
H3CO
H3CO
CH2Cl
H3CO
CHO
ZnCl2/HCl/(HCOH)n
CH3
CH3
CH3
CH3
EtONa/i-PrNO2
DMSO
CH3
CH3
dioxane/70 ºC
H3CO
H3CO
CH2Cl
H3CO
CHO
4
5
H3C CH3
t-Bu
X
X
H3CO
H3CO
NaH/THF:DMF(1:1)
NaH/THF
2a,b
3
5
OCH3
OCH3
X
X
Pa, X = H
t-Bu
n
Ma, X = H
Mb, X = OEt
Pb, X = OEt
2. Experimental part
2.1 Materials
Reagents were purchased from Sigma and used without further purification unless otherwise specified. The
solvents used for polymerization were purified by standard methods, thus THF was purified by distillation
from Na/benzophenone while DMF was dried over molecular sieves and stored under Ar atmosphere.
Chloroform used for spectroscopic measurements was of spectroscopic or equivalent grade.
2.2 Measurements and calculations
Melting points reported were not corrected. 1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were
recorded on a Bruker AVANCE III spectrometer at 25 °C. Chemical shifts are provided in ppm relative to
TMS (δ = 0 ppm) for 1H and 13C NMR. Elemental analyses (C, H) were performed in an EXETER CE-440
instrument at UMYMFOR (Argentina). FT-IR spectra were recorded on a FT-IR Nicolet spectrometer in KBr.
Gel permeation chromatography analyses were carried out on THF solutions at room temperature using a
Waters model 600 equipped with a Waters 2487 UV detector set at 254 nm. Calibration of the instrument was
done using polystyrene standards. Thermal analysis was carried out on a TAQ20 instrument under nitrogen
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flow. The scan rate was 5 °C/min. The thermal behavior was observed on an optical polarizing microscope
(Leitz, Model Ortolux) equipped with a hot stage (Mettler).
UV/vis spectra were obtained from a UV-Visible GBC Cindra 20 spectrometer. The absorption
measurements were done either on dilute samples (less than 0.01 g/ml) or on thin films cast on quartz plates,
which were placed at a 30º angle with respect to the incident beam. Steady-state fluorescence studies were
conducted using an SML AMINCO 4800 spectrofluorimeter at 25 ºC. The emission measurements were
carried out on dilute samples (less than 0.02 mg/ml) using a quartz cuvette with a path length of 1 cm and
keeping the optical densities below 0.1 to minimize aggregation and reduce artifacts introduced by self-
absorption in fluorescence. Thin film spectra were recorded by front-face (30º) detection. Film specimens
were drop-cast from a CHCl3 solution on quartz substrates and dried at room temperature. Fluorescence
anisotropy was measured using a couple of film polarizers on the excitation and emission beams in the
spectrofluorimeter operating in an L-format. The fluorescence anisotropy is calculated according to ‹r› = Ivv –
GIvh/(Ivv + G2Ivh) where Iexc,em is the intensity of the emission, v and h are the vertical or horizontal alignment
of the excitation and emission polarizers, and G = Ihv/Ihh is the instrumental correction factor which accounts
for the difference in sensitivities for the detection and emission in the perpendicular and parallel polarized
configurations.
2.3. Molecular Modeling
All calculations were performed with the ORCA program package (3.03 ed.) [37]. Pre- and post-processing
operations were performed by using the graphical interface Gabedit 2.4.8 [38]. Structures were initially built
graphically and optimized with the semi-empirical AM1 method. Molecular modeling of the model
compounds and polymers was carried out using the HF-3c method [39]. This HF hybrid program use a small
basis set MINIX, coupled with three empirical corrections, the atom-pairwise dispersion correction with the
Becke-Johnson damping scheme (D3BJ) [40,41], the geometrical counterpoise correction gCP [42], and a
short-ranged basis incompleteness correction, SRB. It was noted that the HF-3c yields rather flattened stilbene
and DSB geometries quite similar to those obtained with DFT methods such as B3LYP/6-31G*. It was also
found that for stilbene the B3LYP/def-SVP calculations coupled with the dispersion correction D3 and the
basis set superposition error correction gCP gives a less planar conformation with dihedral angles of around 5-
7° but still very far from the experimental value of 27°. Only a post-Harthree Fock method such as MP2 gives
near experimental dihedral angles values. However, the computational cost of MP2 geometries is too high for
our current purposes.
2.4 Fluorescence quenching studies
The films were cast onto carefully leveled quartz or glass substrates (2.4x0.8 cm) by spreading over the
whole area 0.1 ml of a chloroform solution of the compound. The film was allowed to evaporate slowly in a
nitrogen filled chamber and finally was kept under vacuum for 12 hours at room temperature. The thickness of
cast films was measured in at least five different regions using a UV-visible interferometer (Model F20;
Filmetrics, Inc.) operated in reflectance mode with a spot size of ~0.5 mm. Stock solutions (10-3 M) of each
NAC were prepared by dissolving the adequate amount of the desired NAC in 3 mL of MeOH and by
completing the volume up to 10 mL with water. Solid phase fluorescence quenching was investigated for the
NACs at different concentrations on films whose thickness was ca. 200 nm for the polymers and ca. 50 nm for
the model compounds. Quenching experiments were performed by inserting diagonally the films down to two-
thirds of the height of the fluorescence cell. The 1 cm quartz cell was then filled with the solvent (2.4 mL) and
spectra were acquired by front-face (30º) detection at room temperature after the addition of microliter
aliquots of the quencher solution. Each fluorescence spectrum was recorded immediately after fluorescence
intensity stabilization.
2.5 Synthesis
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Bisphenol A (BPA) was recrystallised from ethanol prior to use. Bis(chloromethyl)benzene (1a) is
commercially available. 1,4-Bis(chloromethyl)-2,5-diethoxybenzene (1b) was prepared from diethoxybenzene
by chloromethylation according to the procedure of Wood and Gibson [43]. 2,2-bis(4-methoxyphenyl)propane
(4) was prepared according to the literature [21].
The synthetic procedures are outlined in Scheme 1. The synthesis of the monomers started with the
preparation of the bisphosphonates 2a and 2b which were obtained from the bischloromethylenes 1a and 1b
using the Arbuzov reaction. Then, the monoaldehyde 3 was synthetized by formylation of the aromatic ring
using the Vilsmeier-Haack reaction while the dialdehyde 5 was prepared in a sequence of two reactions from
4. Thus, a mixture of 4, formalin, ZnCl2 and HCl afforded 2,2-bis(3-(chloromethyl)-4-methoxyphenyl)propane
which after purification was treated with EtONa and nitro propane in DMSO to yield the dialdehyde 5.
Finally, the Wittig-Horner coupling of the bisphosphonates 2a and 2b with the dialdehyde 5 yielded the
polymers Pa and Pb. Similar procedure, but using instead the monoaldehyde 4, gave the model compounds
Ma and Mb.
2.5.1 Synthesis of bisphosphonates 2a y 2b
Bisphosphonates were synthesized by a literature method [44] heating under reflux for 24 h a solution of
the aromatic dichloride (24.0 mmol) in and triethylphosphite (50.4 mmol) under nitrogen atmosphere.
1,4-Bis(diethoxyphosphinylmethyl)benzene (2a) was recrystallized in cyclohexane to yield white plates.
Yield: 73%; mp: 63-65 °C (lit. 75-76 °C) [45]. 1H NMR (CDCl3) δ: 7.25 (s, 4H), 4.00 (m, 8H), 3.12 (d, 4H,
JH-P = 20.4 Hz), 1.23 (t, 12H, J= 7.1 Hz). 13C NMR (CDCl3) δ: 130.3, 129.9, 62.1, 33.5 (JC-P = 141.6 Hz),
16.3.
1,4-Bis(diethoxyphosphinylmethyl)-2,5-diethoxybenzene (2b) was recrystallized in cyclohexane to yield pale
yellow needles. Yield: 65%; mp: 95-97 °C. 1H NMR (CDCl3) δ: 6.92 (d, 2H), 4.01 (m, 8H), 3.64 (c, 4H, J =
7.0 Hz), 3.25 (d, 4H, JH-P = 20.2 Hz), 1.38 (t, 6H, J = 7.0 Hz), 1.24 (t, 12H, J = 7.1 Hz). 13C NMR (CDCl3) δ:
150.4, 119.7, 115.2, 64.6, 61.8, 26.4 (JC-P = 140.3 Hz), 16.3, 14.9.
2.5.2 Synthesis of 2-methoxy-5-tert-butylbenzaldehyde (3)
DMF (33.2 g, 0.455 mol) was cooled at 0 °C and then POCl3 (58.0 g, 0.378 mol) was added drop wise with
agitation. The mixture was warmed at 35 °C and a solution of tert-butyl-4-methoxybenzene (3.0 g, 18.4
mmol) in 1,2-dichloroethane (30 mL) was added slowly. Afterwards, the mixture was stirred for 6 days at 35
°C. The reaction was quenched by addition of sodium acetate (50 mL) and water (100 mL) cooling at 0 °C,
then extracted with CH2Cl2 (3x100 mL), dried with Na2SO4 and evaporated under reduced pressure. 1H NMR
(CDCl3) δ: 10.5 (s, 1H), 7.85 (d, 1H, Jm =2.67 Hz), 7.58 (dd, 1H, Jm =2.67 Hz, Jo =8.77 Hz), 6.93 (d, 1H,
Jo=8.77 Hz), 3.91 (s, 3H), 1.31 (s, 9H). 13C NMR (CDCl3) δ: 190.4, 160.3, 143.9, 133.4, 125.4, 124.7, 111.8,
56.1, 34.6, 31.8.
2.5.3 Synthesis of 2,2-bis(3-formyl-4-methoxyphenyl)propane (5)
First, to a solution of 2,2-bis(4-methoxyphenyl) propane (4) (9.56 g, 37.3 mmol) in dioxane (47 mL) was
added powdered ZnCl2 (5.0 g, 37.3 mmol). The stirred suspension was cooled to 5 °C and 35% HCl (5.03 g,
138 mmol) was added. The mixture was heated at 70 °C and 37% formalin (3.0 mL) was added. Additional
37% formalin was added (3 mL) when temperature reached 85 °C and then the mixture was then stirred for 8
h. After cooling at r.t., the organic layer was washed with NaHCO3 (50 mL), water (3x50 mL), dried with
Na2SO4 and the solvent was evaporated to yield 2,2-bis(3-(chloromethyl)-4-methoxyphenyl)propane as a
white solid which was used as such in the next synthetic step. Yield: 13.5 g, 98 %. 1H NMR (CDCl3) δ: 7.21
(d, 1H ,J = 2.4 Hz), 7.12 (dd, 1H, J = 8.6, 2.4 Hz), 6.78 (d, 1H, J = 8.6 Hz), 4,61 (s, 2H), 3.85 (s, 3H), 1.64
(s, 3H). 13C NMR (CDCl3) δ: 155.3, 142.7, 128.8, 128.2, 125.0, 110.5, 66.9, 55.5, 41.8, 30.9.
5

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To a stirred solution of Na (0.321 g, 13.6 mmol) in ethanol (19 mL) was added 2-nitropropane (1.92 g,
21.5 mmol) drop wise, then the emulsion was added to a solution of 2,2-bis(3-(chloromethyl)-4-
methoxyphenyl)propane (2.00 g, 5.7 mmol) in DMSO (20 mL) under nitrogen atmosphere over a period of 10
min. The resulting mixture was stirred overnight and then quenched by addition of cold water (60 mL). The
white solid was filtered off and dried in vacuo. Recrystallization from cyclohexane afforded 5. Yield: 1.37 g,
77%; mp: 120-125 °C. 1H NMR (CDCl3) δ: 10.45 (s, 1H), 7.76 (d, 1H, J = 2.6 Hz,), 7.32 (dd, 1H, J = 8.8, 2.6
Hz,), 6.88 (d, 1H, J = 8.8 Hz,), 3.90 (s, 3H), 1.67 (s, 3H). 13C NMR (CDCl3) δ: 189.9, 160.2, 142.6, 134.8,
125.9, 124.2, 111.7, 55.7, 40.9, 30.6.
2.5.4 Synthesis of 1,4-bis(5-tert-butyl-2-methoxystyryl)benzene (Ma)
NaH 60% (0.125 g, 5.3 mmol) was washed 5 times with dry hexane under Argon atmosphere and
suspended in dry DMF (2 mL). Then a solution of 1a (0.295 g, 0.78 mmol) in dry DMF (2 mL) was added
drop wise under Ar. The solution turned to greenish color. This mixture was stirred for 3 h and then a solution
of 3 (0.30 g, 1.56 mmol) in dry DMF (3 mL) was added under Ar atmosphere. The solution was stirred at
room temperature overnight, and quenched by the addition of water (3 mL) and HCl 5% (1.0 mL), extracted
with CHCl3 (3x10 mL), dried with Na2SO4 and evaporated in vacuo. The resulting solid was purified by
column chromatography using CHCl3 as eluent. Yield: 110 mg, 32%. 1H NMR (CDCl3) δ: 7.60 (d, 2H, J =
2.4 Hz), 7.52 (s, 4H), 7.48 (d, 2H, J = 16.5 Hz), 7.26 (dd, 2H, J = 8.5 Hz, J = 2.4 Hz), 7.12 (d, 2H, J = 16.5
Hz), 6.84 (d, 2H, J = 8.5 Hz), 3.88 (s, 6H), 1.35 (s, 18H). 13C NMR (CDCl3) δ: 155.0, 143.4, 137.2, 128.6,
126.8, 125.8, 125.5, 123.9, 123.6, 110.7, 55.7, 34.2, 31.6. FT-IR (KBr, cm-1): 3050, 2963, 2853, 1251, 1980,
965, 807.
2.5.5 Synthesis of 1,4-bis(5-tert-butyl-2-methoxystyryl)-2,5-diethoxybenzene (Mb)
The procedure used for the model compound Ma (see above) was repeated for Mb with NaH (0.10 g, 2.48
mmol) in DMF (3 mL) and 1b (0.269 g, 0.62 mmol) in DMF (3 mL). In this case the solution turned to
reddish color. Then, 3 (0.25 g, 1.30 mmol) in DMF (3 mL) was added. Yield: 150 mg, 45%. mp: 145-146 °C.
1H NMR (CDCl3) δ: 7.55 (d, 2H, J = 2.3 Hz), 7.40 (s, 4H), 7.17 (dd, 2H, J = 8.6 Hz, J = 2.3 Hz), 7.09 (s,
2H), 6.76 (d, 2H, J = 8.6 Hz), 4.05 (c, 4H, J = 6.9 Hz), 3.79 (s, 6H), 1.40 (t, 6H,J = 6.6 Hz), 1.27 (s, 18H).
13C NMR (CDCl3) δ: 154.7, 151.0, 143.1, 127.5, 126.3, 125.1, 124.8, 123.7, 123.5, 111.3, 110.5, 65.3, 55.5,
34.0, 31.3, 14.9. FT-IR (KBr, cm-1): 3060, 2962, 2866, 1248, 1053, 1032, 970, 810.
2.4.6 Synthesis of poly(2,2´´-dimethoxy-p-distyrylbenzene-5,5´´-ylene)propylene (Pa)
To a solution of 1a (0.500 g, 1.32 mmol) in a 1:1 mixture of THF and DMF (6 mL) was added 60%
sodium hydride oil dispersion (95 mg, 3.96 mmol) under nitrogen atmosphere. After cooling the mixture to 0
°C, a solution of 5 (0.413 g, 1.32 mmol) in 1:1 THF-DMF (6 mL) was added drop wise, then the reaction
mixture was allowed to reach room temperature and stirred for 12 h at the same temperature. Afterward, a
solution of the end-capping agent diethyl phenylphosphonate (15 mg, 0.066 mmol) in 1:1 THF-DMF (1.0 mL)
was added and the mixture was stirred for additional 3 h. Benzaldehyde (14 mg, 0.132 mmol) in dry 1:1THF-
DMF (1.0 mL) was then added, the mixture was stirred overnight. and then quenched with water (20 mL) and.
After neutralization with HCl 35%, a yellow solid was obtained. The obtained polymer was purified by
fractional precipitation from CHCl3-methanol. Yield: 150 mg, 32%. 1H NMR (CDCl3) δ: 7.51 (d, 2H, J = 1.7
Hz), 7.48 (s, 2H), 7.44 (d, 2H, J = 16.5 Hz), 7.07 (dd, 2H J = 8.1 Hz, J = 1.7 Hz), 7.07 (d, 2H, J = 16.5 Hz),
6.79 (d, 2H; J = 8.5 Hz), 3.85 (s, 3H), 1.72 (s, 3H). 13C NMR (CDCl3, 5% DMF-d7) δ: 154.9, 142.8, 137.1,
128.6, 127.2, 126.6, 125.5, 124.7, 123.6, 110.5, 53.3, 41.8, 31.0. FT-IR (KBr, cm-1): 2990, 2950, 2830, 1255,
1120, 1036, 966, 806.
2.5.7 Synthesis of Poly(2,2´´-dimethoxy-2´,2´-diethoxy-p-distyrylbenzene-5,5´´-ylene)propylene (Pb).
The procedure used for polymer Pa (see above) was repeated for Pb. 1b (0.500 g, 0.92 mmol) in dry THF-
DMF 1:1 (7 mL), sodium hydride (0.066 g, 2.76 mmol), 5 (0.287 g, 0.92 mmol) in 6 mL THF-DMF (1:1).
6

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Diethyl phenylphosphonate (0.010 mg, 0.046 mmol) in dry THF-DMF 1:1 (1.0 mL), benzaldehyde (0.010 g,
0.092 mmol) in dry THF-DMF 1:1 (1.0 mL). Yield: 184 mg, 43%. 1H NMR (CDCl3) δ: 7.54 (d, 2H, J = 1.3
Hz), 7.12 (s, 2H), 7.42 (s, 4H), 7.07 (dd, 2H J = 8.7 Hz, J = 1.3 Hz), 6.79 (d, 2H, J = 8.7 Hz), 4.07 (c, 4H, J =
6.8 Hz), 3.85 (s, 3H), 1.72 (s, 3H), 1.42 (t, 6H, J = 6.8 Hz). 13C NMR (CDCl3, 5% DMF-d7) δ: 154.9, 151.0,
142.9, 127.4, 126.9, 126.1, 124.9, 123.9, 123.8, 111.2, 110.4, 65.3, 55.4, 41.8, 30.9, 14.9.FT-IR (KBr, cm-1):
3050, 2961, 2831, 1246, 1186, 1028, 973, 819.
3. Results and discussion
3.1 Synthesis
The synthetic routes used to obtain model compounds M and polymers P are shown in Scheme 1. First, the
bisphosphonates 2a,b were prepared in almost quantitative yields from the corresponding bischloromethylene
compounds 1a,b. Then, the synthesis of the monoaldehyde 3 was attempted using standard Vilsmeier-Haack
reaction conditions with 2 equivalents of POCl3 in DMF, but to obtain only traces of 3. However, the use of a
large excess of the reagent (1:20) and very long reaction times afforded 3 in 67% yield after chromatographic
purification. Finally, the model compounds Ma and Mb were synthesized by Wittig-Horner coupling between
3 and bisphosphonates 2a and 2b. A trans configuration was assigned to these model compounds by 1H NMR
and FTIR. Thus, Ma presents coupling constants of vinylic protons (Jtrans) of ca. 16 Hz. In addition, both Ma
and Mb show intense bands in the 960 cm-1 region which corresponds to the out-of-plane deformation band in
trans carbon-carbon double bonds. Due to the poor performance of the Vilsmeier route in the direct
diformylation of 4, the key dialdehyde monomer 5 for copolymerization with the bisphosphonates was instead
obtained by chloromethylation followed by oxidation with an overall yield of 75%.
.....
a)
b)
Figure 1. a) 1H(CDCl3) and 13C NMR(CDCl3 with 5% DMF) spectra of polymer Pa . b) 1H(CDCl3) and 13C
NMR(CDCl3 with 5% DMF-d7) spectra of polymer Pb. * 13C signal from either DMF or DMF-d7.
The new segmented conjugated polymers Pa and Pb with meta-linked bis(styrylarylene) units were
synthesized using the Wittig-Horner reaction with yields of 30-60% after repeated reprecipitations. We
7

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observed that the polymers precipitated in the reaction media when THF was used as solvent. In contrast,
polymers obtained in THF:DMF (1:1) reaction mixtures remained soluble throughout the entire reaction
without precipitation or gel formation and consistently gave higher degrees of polymerization (DPn).
Polymerizations were terminated by addition of diethyl benzylphosphonate and benzaldehyde as end-capping
agents and the as-made polymers were purified by fractional reprecipitation from CHCl3-methanol. Pa and Pb
were soluble in a variety of organic solvents like CHCl3, CH2Cl2, DMF, toluene or THF despite the absence in
their backbone of solubilizing long chains. Gel permeation chromatography (GPC) indicated that relatively
medium to high number average molecular weights were achieved (Pa, Mn = 30.0 kDa, Mn/Mw = 2.1, DPn =
78; Mb, Mn = 11.4 kDa, Mn/Mw = 2.1, DPn = 23). The 1H and 13C NMR spectra indicated that both polymers P
have high structural homogeneity with an all-trans stereochemistry (Fig. 1). Polymers Pa and Pb showed
coupling constants of ca. 16 Hz and an intense IR band at ≈ 960 cm-1, that is, similar to those observed for
their corresponding model compounds
3.2. Morphology. Thermal and optical studies.
The new distyrylbenzene model compounds are highly soluble light yellow powders. The DSC trace on
Fig. 2 of Ma showed a crystallization exotherm at 42 °C, a weak glass transition at 66 °C followed by a broad
melting that occurs in the 140-180 °C range, that is, approximately 100 °C lower than that of its unsubstituted
parent 1,4-bis[(E)-2-phenylvinyl]benzene (264-266 °C) [46]. Ma shows strong supercooling of about 130 °C
before the broad transition around 46 °C sets in. The model compound with ethoxy substituents, Mb, presents
a supercooling of 90 °C and a more defined crystallization exotherm than that of Ma (Table 1). Hence, Mb is
less prone to form disordered assemblies than Ma. Thus, the overall thermal behavior of model compounds
can be traced to the presence of bulky t-butyl groups which are known to hinder crystallization in
bis(styryl)arylenes [29]. Although bulk samples of Ma and Mb do not give kinetically stable amorphous
phases, transparent homogeneous thin films, with thickness ranging from 20 to 200 nm, casted from
chloroform solutions were suitable for optical measurements.
Ma
Mb
Pa
Pb
0
50
100
150
200
250
300
Temperature (°C)
Figure 2. DSC traces of model compound Ma and Mb and polymers Pa and Pb.
Furthermore, thicker films remained transparent with no birefringence detected by POM observations even
after several weeks. According to evidence from DSC and POM experiments polymers Pa and Pb are
8

ACCEPTED MANUSCRIPT
amorphous; their DSC traces registered at a temperature range between 50 and 300 °C showed only distinct
glass transitions and no melting transitions were found upon heating beyond the glass transition temperature
(Table 1 and Fig. 2). Moreover, birefringent textures were not observed either for Pa or Pb during POM
experiments carried out in the same temperature range. Both polymers easily gave smooth thin films casted on
quartz plates. These findings are consistent with our current thinking that the forced angular disposition of the
gem-chromophores linked by a single sp3-carbon atom brings about highly distorted main chains that hamper
ordering processes [21]. The twisted nature of Pa is highlighted by the HF-3c molecular model of the tetramer
shown in Fig. 3a.
The macroscopic information from DSC and POM analysis was coupled with the results acquired using
spectroscopic methods which can provide information about local ordering of the chromophores. The optical
absorption is a highly localized process which samples the entire diversity of absorbing species within the
film, and as such it indicates disorder much more than emission will. On the contrary, excitations usually
move across the chromophoric assembly by resonance energy transfer processes to finally emit from sites for
which the energy of the excited state is smallest; therefore emission usually occurs from a single species. This
delocalization is also relevant for our sensing studies because exciton migration boost its interaction with
quenchers thus improving detection sensitivity. Therefore, bearing in mind both film characterization and
sensing applications, energy migration in the thin films was evaluated by fluorescence depolarization
measurements which yielded low residual values of the steady-state anisotropy in the range of <r> ~ 0.05 -
0.09 (Table 1), thus indicating good exciton mobility in model compounds M as well as in polymers P.
a)
b)
c)
Figure 3. a) HF-3c relaxed geometry of a tetramer of Pa, the isopropylidene linking group and
the two end-capping t-butyl groups are shown in black. b) A representation of a herringbone H-
arrangement of Ma is shown in a side view (left) and a front view (right). b) A representation
of a J-arrangement of Mb is shown in a side view (left) and front view (right).
9

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Table 1. Thermal and photophysical properties of model compound Ma,b and polymers Pa,b.
DSCa
Abs.
Fluo.
c
d
e
Tg
66
Tm, Tc
mediab
λ max
fwhm
λ max
fwhmf
SSg
‹r›h
A2 , A1
E1 , E2
416, 439
178, 46(broad)
CHCl3
film
310, 369 4810
313, 383, 414 8540
312, 364 7580
3120
2650
3200
4410
2500
2650
2430
2780
3060
1270
3780
2930
3500
1870
3550
5920
-
0.064
-
Ma
Pa
437, 462, 492
422, 444, 468
427, 452
453, 481
513
175
-
-
CHCl3
film
317, 378 7550
0.069
-
189, 99(broad)
-
CHCl3
film
319, 391 4470
Mb
Pb
368, 422, 468 10930
318, 394 4860
0.055
-
155
CHCl3
film
458, 483, 526
524
328, 400 6630
0.087
a Determined by DSC at scan rates of 5 ºC/min, in ºC. Tg = Glass transition temperature and Tm = melting temperature
from second heating cycle, Tc = crystallization temperature from first cooling cycle.
bMeasured from dilute CHCl3 solutions and pristine thin films.
cAbsorption maxima measured in dilute CHCl3 solutions and on films, bold data indicate the major peaks.
dFull width at half-maximum of the absorption bands (in cm-1).
eEmission maxima measured in dilute CHCl3 solutions and on films. Bold data indicate the major peaks.
fFull width at half-maximum of the fluorescence bands (in cm-1).
gStokes shifts in dilute CHCl3 solutions and on films (λE1 - λ A1 in cm-1).
hAverage anisotropy measured in thin films in an emission range of 60 nm around the λmax,em
.
Optical studies in distyrylbenzenes have been most useful in characterizing their polymorphism in solid
state [29]. In particular, comparison of the solution and solid state optical data often provide insight into the
multiple environments that could be present in the solid state. Shown in Fig. 4 are the normalized UV-vis and
fluorescence spectra of Ma, Mb, Pa and Pb in solutions and thin films. The absorption and emission data is
listed in Table 1. Fig. 4a compares the absorption and emission spectra of the model compound Ma and
polymer Pa in dilute chloroform solution. Same data is presented for Mb and Pb in Fig. 4b. The maxima,
vibronic structure with a prominent A1 absorption transition peak and two distinct emission bands, E1 and E2,
full width at half-maximum (FWHM) of the absorption and fluorescence bands and stokes shifts of Ma and
Pa on one side and Mb and Pb on the other are quite similar in solution (see also Table 1), thereby arguing for
the effective electronic isolation of gem-chromophores by isopropyl groups in the polymer main chain [21,25].
10

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Mb
Pb
Ma
Pa
a
b
300 350
400
450 500 550
600
300
350
400 450
500
550
600
Wavelength (nm)
Wavelength (nm)
A
B
A
B
c
d
Ma
Mb
- - - - C
300
350
400
450
500
550
Pa
600
300 350
400 450 500
Wavelength (nm)
550
600
Wavelength (nm)
A
B
A
B
e
f
Pb
- - - - C
300
350
400
450
500
550
600
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 4. a) Absorption (left) and emission (right) spectra. CHCl3 solutions of a) Ma and Pa and b)
Mb and Pb. Pristine (A), annealed (B) and aged (C) thin films of b) Ma, d) Mb, e) Pa and f) Pb.
Spectra in the solid state were recorded periodically from the pristine films, then after mild annealing at 50
°C for 3 h under vacuum, and finally after annealing for 2 h under nitrogen at 100 °C in the case of the model
compounds and at 180 °C for the polymers. Spectra were also recorded from the as-made films after an
extended storage period of 10 months at room temperature in the dark. In another experiment, the pristine
films were immersed in water while the spectra of the hydrated films were measured for up to 24 hours. Fig.
4c and 4e illustrate the optical spectra of Ma and Pa from pristine films (A) and after mild annealing at 50 °C
(B). No significant spectral variations were observed between spectra recorded from films annealed initially at
50 °C and then at higher temperatures. The absorption and fluorescent spectra of as-made films of Ma and Pa
show no distinctive differences against those in solution, in fact, they are only slightly red-shifted. These
bathochromic shifts appear to be due mainly to the moderate isotropic polarizability of a disordered assembly
11

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of chromophores similar to those observed in other amorphous tert-butyl substituted DSBs [29,47]. The
spectral similarities indicate first the absence of strong interactions between neighboring chromophores in the
solid state and second that similar disordered assemblies are bring about either by the bulky tert-butyl groups
present in Ma or the twisted main chain of Pa. Other common features shared by Ma and Pa films are that
annealing as well as water solvation caused only slight spectral variations. Although hydration smoothed the
vibronic pattern of emission the λmax change very little, in fact, λmax(solution) = 439 nm ≈ λmax(pristine) = 462
nm ≈ λmax(hydrated) = 439 nm for Ma and λmax(solution) = 444 nm ≈ λmax(pristine) = 449 nm ≈ λmax(hydrated)
= 451 nm for Pa. However, a blue-shift of the absorption and a broadening in the emission were observed in
the aged films after a ten month storage period (see trace C in Fig. 4c and 4e) which indicate that
modifications in molecular segmental arrangements sluggishly occurred at room temperature leading to better
ordered domains within the overall amorphous films. It is worth mentioning that, in DSB-type compounds,
distinct blue-shifts of the main absorption band and moderate red-shifts in the emission band in going from
solution to condensed phase are optical signatures of H-type aggregation with edge-to-face arrangement of
parallel adjacent molecules [47]. This herringbone organization prevents the strong intermolecular vibronic
coupling between chromophores that in π-stacked H-aggregates leads to large red-shifted excimer-like
emission spectra and to very low luminescence intensities. The spatial arrangements of two HF-3c geometries
of Ma depicted in Fig. 3b shows that the herringbone alignment could be accomplished for at least some of
the rotamers of DSB chromophores with two t-butyl groups in meta positions forming weak H-aggregates,
while it has been observed that this organization is very difficult to achieve when there are four t-butyl groups
[29,48].
The EtO substitution in the central aromatic ring produces in solution a modest ~ 30 nm red shift in both
absorption and emission spectra of Mb and Pb against those of Ma and Pa. However, this substitution brings
more pronounced modifications of the optical behavior in solid state. Thus, the absorption spectrum of a
pristine film of Mb is red-shifted against that in solution and shows an increase in the A1/A2 ratio while the
emission spectrum presents an almost unstructured band with a λmax shifted about 80 nm from the value
observed in solution and a decrease in the E1/E2 ratio (Table 1, Fig. 4d). Such variations in the A1/A2 and the
E1/E2 ratios are usually attributed to an increase in molecular order [49]. The large broadening of the
absorption band points to the formation of aggregates in the pristine films while the unstructured and
moderately red shifted emission band suggest that emission comes mainly from weak H-aggregates similar to
those observed in Ma. The shape of the emission band of Mb is highly dependent on the casting conditions,
type of substrate and temperature. Moreover, it is noticeable in Fig. 4d the small Stoke´s shift due to the
increased A1/A2 ratio and red-shift of the absorption and the blue-shifted emission observed after annealing,
which produced a vibronically structured emission with λmax at 481 nm and shoulders at 454 and 509 nm.
Therefore, after mild annealing the films modified their spectral features which then resemble more closely
the ones observed in alkoxy co-substituted DSBs for aggregates organized in a J-type disposition which
consists in the slip-stacked parallel alignment of neighboring chromophores [29,48]. Such spatial organization
is illustrated in Fig. 3c for a chromophore J-aggregate formed from two HF-3c geometries of Mb. Indeed, the
same blue shift of the emission band can also be induced by immersing the pristine film in water. Although
the diffusion of water molecules into a film usually increases molecular reorientations as well as the polarity
of the excited state environment, more probably this blue shift is originated from a morphological
modification that change the population ratio of the emitters from H- to J-aggregates due to increased
rotational and conformational mobility since the same optical shift was brought about by the thermal
treatment. On the other hand, and in contrast with Ma, the long term storage do not seems to increase local
order already present in Mb films, as the absorption and emission spectra registered after 10 months are
similar to those obtained from pristine films (not shown). On the contrary, absorption spectra of pristine films
of the amorphous polymer Pb (Fig. 4f) closely follow the ones recorded in dilute solution but they are
markedly different from those of Mb recorded in solid state. Interestingly, no changes in the polymer
12

ACCEPTED MANUSCRIPT
absorption are produced by thermal treatments or hydration. However, the unstructured red-shifted emission
band of Pb is similar to that observed in Mb. So, although the absorption features suggest that the local order
in Pb is much reduced in comparison with Mb, the nature of the low energy emission species in both
compounds seems to be quite similar. Likewise, the thermal treatment of Mb and Pb films yield in both cases
a new blue-shifted structured emission band at ca. 480 nm with the coincident disappearance of the original
band (see Fig. 4d and 4f). But, though the same blue-shift occurred on hydration in Mb films, the hydrated
films of Pb retained the ~ 530 nm original emission band. In summary, thin films of Ma and Pa showed
closely related spectral features. But the EtO-substitution enhances the interchromophoric electronic
interactions in Mb that can only be overcome in Pb by the contorted polymeric main chain. In addition, the
EtO-substitution generates blue-shifted emitting species whose fate depends on the structural capacity of the
material to retain the original arrangements in the pristine film. The tendency of DSB moieties to form
aggregates in the model compound and the fluctuating appearance of the emission band on the casting
conditions is greatly diminished by this polymeric architecture. The isotropic disposition of chromophores in
polymer films is stabilized to such an extent that thermal annealing has no effect in increasing the overall
order. Indeed, solutions and pristine, annealed or hydrated thin films of Pa showed similar absorption and
emission spectra. The same behavior is seen for solutions and pristine or hydrated films of Pb.
3.3. Fluorescence quenching studies with NACs in hydroxylic media
Next, the sensing properties of these DSB-materials were evaluated since the morphological and optical
properties indicate that they could present good performances as fluorescent sensing platforms.The steady-
state quenching experiments were performed in the solid state on highly fluorescent thin films immersed in a
hydroxylic solvent. This sensing configuration involves interchange between the film and its bathing of
mobile species (solvent and analytes) and their subsequent interactions of electronic nature with excitons and
of cohesive character with the organic environment in the film interior. The films were casted onto
dodecyltrichlorosilane, DTS, coated glass substrates. We have found previously that the insertion of a
monolayer of nonpolar aliphatic chains between the organic sensing layer and the glass substrate, which
largely reduce the solvent affinity for the surface, improve adhesion and the physico-mechanical stability of
the organic film [28]. Moreover, the presence of the nonpolar layer was decisive in producing transparent
films of Mb and, to a lesser degree, of Ma which in general showed lesser tendency to form opaque films.
Fluorescence control experiments on films of the model compounds indicated that there was no leakage from
the film towards water media after 24 hours. However, increasing levels of fluorescence were observed in the
solvating media when the films were immersed in H2O/MeOH (1:1) or pure MeOH. No leakage was detected
for the polymer films in H2O, H2O/MeOH (1:1) mixture or plain MeOH. Pristine films were immersed in the
hydroxylic solvent and allowed to solvate prior to record their steady-state fluorescence response to
increments in the concentration of various hydrophilic (i.e.: 4-nitrophenol, NP; 2,4-dinitrophenol, DNP; 2,4,6-
trinitrophenol, TNP) and hydrophobic (i.e.: 4-nitrotoluene, NT, 2,4-dinitrotoluene, DNT; 2,4,6-trinitrotoluene,
TNT) nitroaromatics.
Fluorescence quenching was fast, complete and reversible showing that analytes diffuse easily into the
DSB-based films. The reversibility of the sensing response was studied by quenching-recovery experiments.
Thus, a solvated film of Pa was first exposed to a large concentration of the quencher for about one minute
which was then removed with care and the film washed twice with water. This procedure was repeated several
times. The fluorescence responses in the presence and absence of the quencher plotted in Fig. 5a show that the
quenching of the fluorescence was a reversible process. Thus, even after six cycles the film showed near
complete quenching and fluorescence recovery. Same experiment was repeated for Pb though in this case
fluorescence was measured twice, first after one and then after five minutes (Fig. 5b). No further decrease in
the fluorescence was observed at the longer time thus corroborating that quenching was quick.
13

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Film washed with water
Film washed with water
Pa
Pb
400
300
200
100
0
500
400
300
200
100
0
~1'
~1' ~5'
Film exposed to [TNP] = 140 µM
Film exposed to [TNP] = 320 µM
1
2
3
4
5
6
1
2
3
4
5
Quenching Cycles
Quenching Cycles
a)
b)
Figure 5. a) Six continuous cycles of quenching-recovery test of a film of Pa. The quenching was
measured after exposing the film to [TNF] = 1.4 10-4 M for ~ 1 min (λex/λem = 379/451 nm). b) Five
continuous cycles of quenching-recovery test of a film of Pb. The quenching was measured after
exposing the film to [TNF] = 3.2 10-4 M for ~ 1 min and ~ 5 min (λex/λem = 430/532 nm).
Fig. 6 displays the quenching of the emission spectrum of thin films of Pa and Pb in water upon addition
of microliter aliquots of NACs. The plots of the Stern-Volmer (S-V) equation, (Io/I) -1 = KSV [Q], for
fluorescence quenching are shown in Fig. 5a (inset) and Fig. 5b (inset). The S-V constant (KSV) and the values
of half of the maximum quench, Q50%, that is, the quencher concentration needed to reach (Io/I) - 1 = 1 of
polymer and model compounds are shown in Table 2.
5
a
1,0
b
4
3
0,5
2
R=0.996
R=0.994
1
0,0
0
0
20
40
[Q] /
60
80
20
40
60
80
100
µM
[Q] / µM
500
550
600
650
700
400
500
600
λ
(nm)
λ
(nm)
Figure 6. a) Fluorescence spectra change (λex = 379 nm, λem = 451 nm) of a Pa film as a function of
added TNP in water; [TNP] = 3.7 10-6 – 1.1 10-4 M (top to bottom). b) Fluorescence spectra change
(λex = 430 nm, λem = 532 nm) of a Pb film as a function of added TNP in water; [TNP] = 2.5 10-6 –
2.6 10-4 M (top to bottom). The Stern-Volmer plots for the linear region are shown in the insets.
Linear S-V relationships (R2 > 0.99) were observed model compound and Pa for most the NACs tested
even though only in the range ~20-30 to ~65-70% of fluorescence quenching. Pb showed linear relationships
in narrower ranges with less satisfactory correlation coefficients (R2 ≥ 0.98). A non-linear S-V relationship
with upward curvature usually occurred at the higher end of quencher concentrations for the polymers (Fig.
6a) while the model compounds showed a more linear behavior. The non-linear response observed as the
14

ACCEPTED MANUSCRIPT
analyte concentration increases is most probably because of increasingly efficient quenching of migrants
excitons [2,28]
Table 2. Data analysis of Stern-Volmer plots and quenching efficiencies
a)
a)
Quencher
(Q)
Q50%
µM
,
Q50%
µM
,
Solvent
KSV, M-1
KSV, M-1
Ma
Mb
H2O
H2O
97900
17400
9.9
60
80200
18100
20
TNP
TNT
63
Pa
Pb
H2O
H2O
5920
170
47
-
-
-
NP
41900
60900
23200
-
DNP
TNP
TNP
H2O
34
12500
11100
90
110
MeOH
37
H2O
H2O
8500
120
200
570
970
-
-
-
NT
3040
b)
-
b)
DNT
TNT
TNT
H2O
1100
640
MeOH
940
3540
a [Q] for (Io/I) - 1 = 1. b) Non-linear S-V relationships.
Pa, TNP
6
Ma, TNP
Mb, TNP
Ma, TNT
Mb, TNT
6
5
4
3
2
1
0
Pb, TNP
Pa, TNT
Pb, TNT
5
4
3
2
1
0
0
50µ
100µ
150µ
200µ
0
50µ
100µ
150µ
200µ
[Q] / M
[Q] / M
Figure 7. Stern-Volmer plots of polymers and model compounds in response to TNP and TNT.
Since the development of small molecule fluorescent sensors is an intense field of research [50], we also
tested the model compounds in the quenching studies and their quenching efficiencies were correlated with
those of their parent polymers. All compounds tested in this study are much more sensitive to TNP than to
TNT in aqueous media (see Fig. 7). It also was found that Pa and Ma showed higher overall quenching
efficiencies than Pb and Mb despite the fact that Pb and Mb have better electron-donating capabilities
because of the diethoxy substitution. In fact, the efficiency ratio for TNP is Q50%,Pa/Q50%,Pb ≈ 3 while for TNT
is Q50%,Pa/Q50%,Pb ≈ 2. This outcome cannot be rationalized in terms of electronic considerations based on the
accepted mechanism of fluorescence sensing of NACs that consists of the photo-induced electron transfer
from an electron-rich chromophore to an electron-deficient NAC. Moreover, the results with a series of
15

ACCEPTED MANUSCRIPT
quenchers with increased electron-deficiency are contradictory, while quenching efficiencies for Pa increase
with the number of nitro group for the hydrophilic nitrophenols (TNP > DNP > NP), the opposite was
observed with the hydrophobic nitrotoluenes (NT > DNT > TNT). However, the presence of as least one
strongly electron accepting nitro group in the aromatic ring of the analyte was required to achieve acceptable
quenching efficiencies of the emission of the electron-rich polymer Pa. Thus, either the parent compounds
lacking nitro groups, i.e.: phenol and toluene, or the diphenolic compounds such as hydroquinone, HQ, or 1,5-
dihydroxynaphtalene, DHN, which have two electron donating groups produced only very slight negative
(toluene,phenol and HQ) or positive (DHN) responses which were from two to three orders of magnitude
smaller than the one observed for TNP (see Fig. 8). Additionally, we tested the response of Pa films in
aqueous media against other chemicals which are normally found in industrial wastewaters [51,52]. These
chemicals could act as interferents and alter the detection of NACs. It is therefore important to assess the
selectivity of these polymers toward NACs. No significant change in Pa fluorescence was detected by
exposition to micromolar quantities of several possible interferents (Fig. 8). Moreover, the same lack of
response was observed towards milimolar quantities of benzene, toluene, DMF, NaCl, HCl and NaOH.
2
1
0
µ
M
Figure 8. Fluorescence quenching efficiencies of TNF and several commonly found interferents
measured on Pa thin films immersed in water.
Clearly, other factors affecting the fluorescence quenching of the sensing layer exceed electronic
phenomena like the electron-donor and electron-acceptor abilities of the fluorophores/quencher pairs,
probably its morphology, the electrostatic interaction with the analyte or the mobility of the excited states
within the polymeric matrix. Thus, the quenching responses of the model compounds against TNP and TNT
were higher than those observed in the corresponding polymers, Q50%,P/Q50%,M ~ 3-4 (Fig. 7b). Noteworthy,
Ma and Mb show good response and comparable sensitivity towards TNT, indeed, their response were highly
linear and of similar magnitude; and the quenching concentrations needed to reach Q50% were one order of
magnitude lower than those of the polymers.
On the other hand, the general trend in Q50% was not modified when MeOH was used instead of water in
the quenching experiments of Pa and Pb with TNP and TNT. Actually, upon solvent change to MeOH
quenching efficiencies for Pa and Pb against TNP were high and quite similar to those recorded in water
while their already poor efficiencies against TNT decrease ~70% for Pa and increased ~70% for Pb. These
observations suggest that the solvent does not play a substantial role in modifying the quenching ability of
polymers Pa and Pb and that the dynamic interaction between three partners, i. e., solvent, analyte and sensing
material, could in this case be reduced to binary analyte-polymer interactions. The nature and extent of
molecular interactions between two compounds can be evaluated using the semi-empirical Hansen solubility
16

ACCEPTED MANUSCRIPT
parameters (HSPs) [53]. Thus, the solubility parameter distance, Ra, indicates the closeness in cohesive energy
densities, ced, between sensing material and NACs, Ra = [4(δd,sm - δd,NAC)2 + (δp,sm – δp,NAC)2 + (δhb,sm
–
δhb,NAC)2]1/2, where the cohesive energy densities are expressed as the square of the total solubility parameter,
2
2
2
ced = δt2 = δd + δp + δhd , which is the sum of squares of the partial HSP for the atomic dispersive
interactions, δd, permanent dipole molecular interactions, δp, and hydrogen-bonding interactions, δhb.
Therefore, small Ra indicates high compatibility, that is, the closer the HSP of two interacting species are, the
bigger the interaction. Hence, the HSPs of the model compounds and polymers were first calculated using the
Stefanis–Panayiotou group-contribution method [54,55] and then the analyte-sensing material distances Ra
were evaluated for TNP and TNT.
Table 3. Hansen solubility parameters, HSPs, and solubility arameter distances, Ra.
Raa)
vs TNP
Raa)
vs TNT
HSPs
δd
δp
δhb
5.9
5.0
3.0
3.0
δt
Ma
Mb
Pa
21.4
21.5
22.9
22.9
2.4
5.3
7.3
9.9
22.3
22.7
24.2
25.1
19.4
17.0
16.4
14.4
16.9
14.0
12.3
9.9
Pb
a) Calculated using the HSPs for TNP y TNT from ref. 28.
Interestingly, Ra values shown in Table 3 indicate that affinity of the sensing materials with hydrophilic
TNP increase in the order Ma < Mb < Pa < Pb, however, the quenching efficiency of TNP, evaluated by their
Q50% values, increase in the opposite sense, Ma > Mb > Pa > Pb. The same trend was observed with the Ra
and Q50% in the case of the hydrophobic TNT. We have reported similar observations regarding two segmented
conjugated polymers, one bearing non-polar aliphatic side chains and another with polar oxyethylene side
chains [28]. It is relevant at this point to stress again the fast, complete (> 90%) and reversible nature of the
quenching process (see Fig. 5). These observations indicate that equilibrium between the concentrations in
solution and inside the sensing layer is reached rapidly between successive additions of the quencher. It was
also inferred that the mobility of the excited species was comparable in these DSB compounds (Table 1). So,
it seems that the kinetic effects due to analyte diffusion inside the film or excited states toward the quencher
cannot to be held responsible for the observed trend in quenching efficiencies. The interplay of molecular
affinity and morphology could be determining the quenching efficiencies. Although it is not yet clear which
are indeed the factors determining that the sensing material turns out to be more sensitive when the affinity
with the analytes is lower, this observation could be turned into a useful criteria that could help in the
designing of fluorescent sensors along with others like electron donor capacity, amorphous morphology or
microporosity.
4. Conclusions
The Wittig-Horner reaction was employed to synthesize new segmented conjugated polymers showing good
molecular weights, structural regularity and solution processability despite the absence of solubilizing side
chains. The same synthetic route was used to obtain their corresponding model compounds. The
morphological studies based on DSC and POM analysis of bulk samples of the model compound and
polymers were complemented with the results acquired using spectroscopic methods on thin films. They
showed that the amorphous matrix in the thin films contained varying amounts of H- and J-type aggregates
whose appearance depend on the thermal history, aging and solvation. Thus, pristine thin films of Ma and Pa
showed closely related spectral features. But the EtO-substitution enhanced the interchromophoric electronic
interactions in Mb that could only be overcome in Pb by the contorted polymeric main chain. The tendency of
DSB moieties to form aggregates in the model compounds and the fluctuating behavior of the emission band
in Mb on the casting conditions, thermal history and solvation is greatly diminished by the polymeric
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architecture. The isotropic disposition of chromophores in polymer films is stabilized to such an extent that
thermal annealing has no effect in increasing the overall order. Pristine, annealed or hydrated thin films of Pa
showed similar optical features and the same behavior was seen for and pristine or hydrated films of Pb.
Fluorescence quenching by nitroaromatic analytes was fast and reversible pointing to a rapid and easy
diffusion of analytes into the DSB-based polymer films. The quenching response to nitrophenols was superior
to that against nitrotoluenes. Pa and Ma showed higher overall quenching efficiencies than Pb and Mb
despite the fact that Pb and Mb have better electron-donating capabilities. Therefore, the EtO substitution was
detrimental to the amorphous morphology and it did not increase sensitivity to NACs. Though it was not
anticipated, the sensing abilities of model compounds were higher than those displayed by their parent
polymers, however, their thin films are mechanically stable only in plain water, while the casting of
transparent thin films and control of the amount of aggregates was much more difficult than in the case of the
corresponding polymers. Quenching efficiencies of Pa and Pb were not modified when MeOH was used
instead of water. Thus, in this case the solvent does not play a substantial role in modifying the quenching
ability. Moreover, phenol and diphenols as well as commonly found interferents showed negligible effects on
the fluorescence emission of Pa. Finally, the molecular interactions between analytes and sensing materials
were evaluated by their solubility parameter distances, Ra. Interestingly, Ra values indicate that the sensing
materials show higher responses when their affinity with the analytes is lower. This observation could be used
in the designing of fluorescent sensors along with others more like electron donor capacity, amorphous
morphology or material microporosity.
Acknowledgments
Financial support from SGCyT-UNS, CIC-PBA and CONICET is acknowledged. MJR and ABS thank
CONICET for a fellowship. MFA is member of the research staff of CIC-PBA. PGDR and ROG are members
of the research staff of CONICET.
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Film washed with water
t = 0'
400
300
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100
0
t = 1'
Film exposed to TNP
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Quenching Cycles

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Research Highlights
Distyrylbenzene-based segmented conjugated polymers and model compounds were
synthesized
Aggregate formation in the amorphous matrix was impeded by the twisted polymeric chain
Fluorescence quenching by nitroaromatic aqueous solutions was fast, complete, reversible
and selective
Thin film quenching response to nitrophenols was superior to that against nitrotoluenes
Higher quenching responses were produced by lower analyte-sensing materials cohesive
interactions