Nitroaromatic compounds sensing. Synthesis, photophysical characterization and fluorescence quenching of a new amorphous segmented conjugated polymer with diphenylfluorene chromophores

Sensors and Actuators B 160 (2011) 524–532

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Nitroaromatic compounds sensing. Synthesis, photophysical characterization and fluorescence quenching of a new amorphous segmented conjugated polymer with diphenylfluorene chromophores Pablo G. Del Rosso, Marcela F. Almassio, Gerardo R. Palomar, Raúl O. Garay ∗ INQUISUR, Departamento de Química, Universidad Nacional del Sur, CP 8000 Bahía Blanca, Argentina

a r t i c l e

i n f o

Article history: Received 11 April 2011 Received in revised form 14 July 2011 Accepted 10 August 2011 Available online 19 August 2011 Keywords: Segmented conjugated polymer Fluorescence quenching Nitroaromatics Film sensor

a b s t r a c t A new regularly segmented conjugated polymer bearing 2,7-diphenylfluorene chromophores tethered by isopropylidene connectors was synthesized by a relatively short synthetic route starting from easily available monomer synthons. Its photophysical properties were investigated using UV–vis absorption, steady state and time-resolved emission spectroscopies. The bent microstructure produces a highly soluble amorphous polymer and fluorescence depolarization showed that exciton mobility within the polymer film is not hindered. These properties are of practical significance in view of the high sensitivity and fast response of its fluorescence quenching by nitro aromatics. Half of the maximum quench (Q50% ) of a polymer film occurred with dinitrobenzene in methanol solution at the micromolar range in less than 1 min in a reversible manner. We demonstrate that amorphous segmented conjugated polymers bearing relatively short chromophores can be used as sensing materials with performances comparable to those presented by conjugated polymers with more elaborate structural design. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Fluorescent polymers are functional materials with vast applications. Conjugated polymers have become one of the most extensively studied classes of luminescent materials for use in organic optoelectronic devices such as light-emitting diodes (LEDs) [1], solar cells [2], electrochromic displays and smart windows [3], and fluorescent chemical sensors [4]. The electrooptical properties of conjugated polymers are intrinsic and often differ substantially from the ones shown by its independent repeating units [5]. In that manner, the ability of conjugated polymer chains to self-amplify their fluorescence quenching response by energy migration compared to that observed for isolated repeating units has led to an intense evaluation of conjugated polymers for sensing applications [4]. The use of conjugated polymers [4,6], conjugated polyelectrolytes [4,6,7] and nanomaterials containing conjugated polymers [8] have been explored for chemo- and biodetection of a wide variety of analytes. The amplified fluorescence quenching of conjugated polymers has been used for the detection of nitroaromatic compounds [6,9] whose quantification is important for environmental pollution control, industrial applications, munitions remediation sites and explosives detection. A rapid and sensitive response of a conjugated

∗ Corresponding author. Tel.: +54 291 4595101; fax: +54 291 4595187. E-mail address: [email protected] (R.O. Garay). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.08.021

polymer to nitroaromatics depends mainly on polymer–analyte interactions and film thickness and porosity [10,11]. While film thickness can be controlled by processing techniques [10] or by formation of monolayer assemblies on glass plates [12] and nanoparticules [13], the tailoring of film porosity is a rather difficult task, which usually implies careful material design and elaborate syntheses. For example, the sensory responses have been considerably improved by the incorporation of rigid and bulky ipticene moieties of different sizes and shapes into the backbones of poly(phenyleneethynylene)s [10,14] or [2,2,2] bicyclic ring moieties in the main chain of poly(phenylenevinylene)s [15], these structural modifications led to open morphologies where the steric hindrance for the diffusion of the analytes within the film is greatly reduced. We have been interested in the synthesis and characterization of a subclass of conjugated polymers, namely, regularly segmented conjugated polymers [16] whose structure is composed by non interacting [17] or weakly interacting active units tethered along the main chain [18–20] and isolated by saturated spacers. We observed that if the spacer is shrunk down to a single saturated carbon atom the polymer main chain is forced to adopt bent conformations that lead to the formation of amorphous glasses with decreased intermolecular interactions [21]. We have also extended this approach to meta-linked p-terphenylene segmented conjugated polymers whose contorted polymer chains form a very stable amorphous morphology, substantially hinder interactions between chromophores and promote high solubility [22]. On the basis of

P.G. Del Rosso et al. / Sensors and Actuators B 160 (2011) 524–532

H3C CH3 C

H3C CH3 C

H3C CH3 C H3 C

C

Q H3C CH3 C

CH3

C H3C CH3

CH3 H3C C

C H3C CH3

Q

H3C C CH3

H3 C C CH3 C CH3 CH3

Scheme 1. Schematic illustration of how the isopropilene spacer induced porosity in the film.

the above considerations, we hypothesise that such amorphous structure could foster analyte exchange (see Scheme 1) and testing its quenching performance against nitroaromatics would be worthwhile. However, it was less clear whether the array of short chromophoric units could retain in the solid state the amplified quenching effect of conjugated polymers. We report herein the synthesis, characterization and fluorescence quenching response of a segmented conjugated polymer composed of diphenylfluorenylene units tethered by their meta positions along the polymer main chain and separated by isopropylene spacers. Its photophysical properties were investigated using UV–vis absorption and both steady state and time-resolved photoluminescence emission (PL) spectroscopies. The polymer was synthesized from easily available starting materials such as biphenyl A and fluorine derivatives and did not require an elaborate synthetic route. The 2,7-diphenylfluorene chromospheres was chosen to test our concept because of its superior absorption coefficient and quantum yield. In addition, we reasoned that the presence of MeO groups would increase the electron donor nature of the diphenylfluorene moiety increasing its sensitivity to the electron deficient nitroaromatics.

2. Experimental 2.1. Materials and characterization Bisphenol A (BPA), p-tert-Butylphenol, 9,9-dihexyl-2,7fluorenylenediboronic acid and 2,7-dibromofluorene were purchased from Aldrich and used without further purification unless otherwise specified. THF was purified by distillation from Na/benzophenone. BPA was recrystallized from ethanol prior to use. Pd2 (dba)3 was prepared according to the literature [23]. Tetrakis(triphenylphosphine)palladium(0) was prepared as described in the literature and then was washed with ethyl ether and EtOH [24,25]. Melting points reported are not corrected. 1 H NMR and 13 C NMR spectra were recorded on a Bruker ARX300 spectrometer on samples dissolved in CDCl3 ; all signals assigned with the help of polarization transfer DEPT135 13 C and 2D(1 H–13 C) correlation NMR experiments. Gel permeation chromatography analyzes 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 PerkinElmer DSC7 under nitrogen flow. The scan rate was 10 ◦ C/min. The thermal behavior was observed on an optical polarizing microscope (Leitz, Model Ortolux) equipped with a hot stage (Mettler). The film

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thickness was recorded by an atomic force microscope (Agilent model 5500) on slits made in the polymer thin films. 2.1.1. Photophysical methods UV/vis spectra were obtained from a UV–vis GBC Cindra 20 spectrometer. The absorption measurements were done either on dilute samples (less than 10−5 M) or on thin films cast on quartz plates, which were placed at a 30◦ angle with respect to the incident beam. The molar extinction coefficients (ε) were obtained from the slope of the plot of the absorption with six solutions of different concentrations vs. the concentration (correlation values ≥0.99). Steady-state fluorescence studies were conducted using a SML AMINCO 4800 spectrofluorimeter at 25 ◦ C. The time-resolved measurements were obtained using a TimeMaster PT1 spectrofluorimeter with band pass-filters (excitation, UG1; emission, WG375) at 25 ◦ C. The emission measurements were carried out on dilute samples (less than 10−6 M) 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 selfabsorption in fluorescence. Thin film spectra were recorded by front-face (30◦ ) detection. Film specimens were drop-cast from a CHCl3 solution on quartz substrates and dried at room temperature. Fluorescence quantum yields of compound in CHCl3 solution were determined relative to equiabsorbing solutions of quinine sulphate (˚F = 0.546 in 0.5 M sulphuric acid) [26]. 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 [27]. Reported values are an average of the anisotropy measurements in a range of ca. 60 nm. 2.1.2. Fluorescence quenching studies Solid phase fluorescence quenching was investigated at different concentrations of nitrotoluene (NT), 2,4-dinitrotoluene (DNT), 2,4-dinitrofluorbenzene (DNF), 4-nitroaniline (NA) and nitropropane (NP) on drop-cast films whose thickness was estimated to be ca. 0.6 ␮m. Thinner films were obtained by spin casting from a CHCl3 solution on quartz substrates and dried at room temperature (d = 35 ± 2 nm, 5 mg/mL, 500 rpm and d = 9 ± 3 nm, 1 mg/ml, 500 rpm). Quenching experiments were carried out by diagonally inserting the film specimens casted on quartz substrates down to two-thirds of the height of a quartz fluorescence cell to allow analyte equilibration. The 1 cm quartz cell was then filled with methanol (2 mL) and spectra were repeatedly acquired by front-face (45◦ ) detection at room temperature after the addition of microliter aliquots of a quencher solution. Each fluorescence spectrum was recorded immediately after fluorescence intensity stabilization (ca. 30–50 s) at an excitation wavelength of 327 nm. 2.2. Molecular modeling Molecular modeling of the trimer was carried out at the semiempirical level using the PM3 MO program implemented in the suite of programs Hyperchem (Hypercube Inc., release 8.0.4 for windows, serial # 12-800-1501800054, 2007). Modeling was assumed to be carried out in the gas phase at 0 K. Minimization operations were performed using the conjugate gradient method and halted by setting the gradient option at 0.01 kcal/mol. The lowest potential energy conformations were found by minimization of the energy function in conjunction with a conformational search around the

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single bond that links the aromatic ring to the isopropylene spacer using the MM+ program. 2.3. Monomer synthesis The synthesis of the monomer 2,2-bis-(2-bromo-4methoxyphenyl)propane (1) has been already reported [22]. 2.3.1. 2,2-Bis[3-(1,3,2-dioxaborinan-2-yl)-4-methoxyphenyl] propane (2) A solution of 2,2-bis(4-methoxy-2,5-dibromophenyl)propane (1) (14.80 g, 35.6 mmol) in dry THF (125.0 mL) was added slowly with stirring to a mixture of magnesium turnings (1.72 g, 64.0 mmol) and dry THF (85.0 mL) under Argon. The resulting mixture was refluxed for 24 h. The Grignard reagent solution was cooled to −78 ◦ C. A solution of trimethyl borate (17.2 g, 165.0 mmol, 19.5 mL), in dry THF (83.0 mL) was added dropwise over 1 h. The temperature was then allowed to rise gradually up to room temperature and the mixture was stirred for 5 days. The resulting suspension was cooled to −78 ◦ C followed by the addition of 6% hydrochloric acid (110 mL). The cooling bath was removed and the mixture was stirred for 1 h at room temperature. The solution was extracted with ether (3 × 50 mL), dried with SO4 Mg and the solvent was evaporated. The product was purified by recrystallization from acetonitrile–water 50:20 to give pure isopropyl-2,2-enebis(2-methoxyphenyl-5-ene)boronic acid as white crystals. (5.0 g, 60%) mp 210 ◦ C. 1 H NMR (DMSO-d6 ): ı = 7.69 (s, 1H, OH), 7.53 (d, 1H, Jm = 2.5 Hz), 7.27 (dd, 1H, Jm = 2.5 Hz, Jo = 8.8 Hz), 6.95 (d, 1H, Jo = 8.8 Hz), 3.86 (s, 3H), 1.66 (s, 3H). 13 C NMR (DMSO-d6 ): ı = 160.3, 141.0, 132.3, 128.5, 108.7, 54.2, 40.0, 29.74. A mixture of isopropyl-2,2-ene-bis(2-methoxyphenyl-5ene)boronic acid (5.7 g, 16.4 mmol), 1,3-dihidroxypropane (2.96 g, 38.6 mmol, 2.82 mL) and toluene (175.0 mL) was refluxed for 2.5 h. Water that was formed during the reaction was removed by azeotropic distillation and collected in a Dean-Stark trap. After this period the toluene was removed under reduced pressure. The resulting semisolid was dissolved in CHCl3 (50 mL), dried with SO4 Mg and the solvent was evaporated. The obtained product was purified by column chromatography on silica gel using hexane/CH2 Cl2 as eluent and purified by recrystallization from toluene/hexane 5:6 to give white crystals. (3.5 g, 52%) mp 149 ◦ C. 1 H NMR (CDCl3 ): ı = 7.56 (d, 1H, Jm = 2.7 Hz), 7.11 (dd, 1H, Jm = 2.7 Hz, Jo = 8.6 Hz), 6.72 (d, 1H, Jo = 8.6 Hz), 4.16 (t, 2H, J = 5.5 Hz), 3.79 (s, 3H), 2.04 (q, 4H, J = 5.5 Hz), 1.64 (s, 3H). 13 C NMR (CDCl3 ): ı = 162.0, 141.0, 133.9, 130.9, 110.5, 62.5, 56.2, 40.0, 31.57, 27.7. 2.3.2. 2,7-Bis(1,3,2-dioxaborinan-2-yl)-9,9-dihexylfluorene (3) It was synthesized by the same procedure used for the boronic ester 2 except that 9,9-dihexyl-2,7-fluorenediborónic acid (0.950 g, 2.25 mmol) and 1,3-dihidroxypropane (0.405 g, 5.29 mmol, 0.38 mL) were used. White crystals (0.901 g, 80%) mp 119 ◦ C. 1 H NMR (CDCl3 ): ı = 7.75 (dd, 1H, Jo = 7.5 Hz, Jp = 1.0 Hz), 7.67 (dd, 1H, Jo = 7.5 Hz Jm = 0.7 Hz), 7.72 (s, 1H), 4.20 (t, 2H, J = 5.5 Hz), 2.08 (q, 4H, J = 5.5 Hz), 1.98 (m, 2H), 1.04 (m, 8H), 0.74 (t, 3H, J = 7.1 Hz). 13 C NMR (CDCl3 ): ı = 150.7, 143.9, 132.7, 128.3, 119.5, 62.4, 55.3, 40.7, 31.9, 30.2, 27.8, 24.1, 22.9, 14.3. 2.4. Model compound synthesis 2.4.1. Synthesis of 2-bromo-4-tert-butylanisol (4) This compound was obtained in two steps. First, a mixture of p-tert-butylphenol (14.3 g, 0.10 mmol), K2 CO3 (16.5 g, 0.120 mol) and acetone (115 mL) was heated under reflux and stirred for 1 h. The reaction mixture was cooled to room temperature and

CH3 I (19.0 g, 0.134 mol) was added slowly. Then, stirring was continued for 3 days at 30–35 ◦ C. The reaction was quenched with water (100 mL), the organic layer was separated, and the aqueous layer was extracted with CHCl3 ·(3 × 80 mL). The combined organic layers were washed with water (100 mL), 5% aqueous NaOH solution (2 × 100 mL), 5% aqueous HCl solution (2 × 100 mL) and water (2 × 100 mL), dried with Na2 SO4 and the solvent was removed under reduced pressure to yield crude p-tert-butylanisol (15.5 g, 75%) as an oil which was used without further purification. 1 H RMN (CDCl3 ): ı = 1.30 (s, 9H), 3.78 (s, 3H), 6.84 (d, 2H, Jo = 8.97 Hz), 7.31 (d, 2H, Jo = 8.97 Hz). 13 C RMN: ı = 31.5, 34.0, 55.2, 113.4, 126.2,143.4, 157.4. Then, Br2 (2.92 g, 18.3 mmol) was added dropwise over a period of 80 min to a solution of p-tert-butylanisol (3.00 g, 18.3 mmol) in CH2 Cl2 (14 mL) placed in a water-ice bath. The mixture was stirred for 3 h and quenched with 10% aqueous NaOH solution (35 mL). The mixture was then extracted with CH2 Cl2 (3 × 15 mL), the organic layer was dried with Na2 SO4 and the solvent was removed under reduced pressure. Compound 5 was obtained as a very dense liquid. (4.14 g, 93%). 1 H NMR (CDCl3 ): ı = 1.29 (s, 9H), 3.87 (s, 3H), 6.83 (d, 1H, Jo = 8.58 Hz), 7.27 (dd, 1H, Jo = 8.58 Hz, Jm = 2.47 Hz), 7.54 (d, 1H, Jm = 2.47 Hz). 13 C RMN (CDCl3 ): ı = 31.4, 34.1, 56.3, 111.4, 111.7, 125.2, 130.5, 145.1, 153.7. 2.4.2. Synthesis of 9,9-dihexyl-2,7-bis(2-methoxy-5-tertbutylphenyl)fluorene (5) A Schlenk tube was charged with 2 (0.095 g, 0.39 mmol), 2,7-bis(1,3,2-dioxaborinan-2-yl)-9,9-dihexylfluorene (3) (0.09 g, 0.18 mmol,), Pd2 (dba)3 (0.006 g, 0.007 mmol), P(o-tolil)3 (0.013 g, 0.043 mmol) and Na2 CO3 (0.250 g, 2.4 mmol,) and the mixture was kept under Ar atmosphere. THF (3.3 mL) and water (3.3 mL) were added via a syringe. The mixture was heated at 80 ◦ C for 48 h. The reaction mixture was quenched with methanol (15 mL), extracted with CHCl3 (3 × 15 mL), dried with Na2 SO4 and the solvent was removed under reduced pressure. The solid was collected by filtration and purified by column chromatography using hexane-CH2 Cl2 as eluent. (0.086 g, 58%) mp = 163 ◦ C. 1 H NMR (CDCl3 ): ı = 7.73 (d, 1H, Jo = 7.82 Hz), 7.54 (s, 1H), 7.48 (dd, 1H, Jo = 7.82, Jm = 1.52 Hz), 7.42 (d, Jm = 2.48 Hz 1H), 7.33 (dd, 1H, Jo = 8.58 Hz, Jm = 2.48 Hz), 6.95 (d, 1H, Jo = 8.58 Hz), 3.79 (s, 3H), 1.98 (m, 2H), 1.37 (s, 6H), 1.11 (m, 6H), 0.88 (m, 2H), 0.77 (t, 3H, J = 7.05 Hz). 13 C NMR (CDCl3 ): ı = 154.5, 150.8, 143.7, 139.7, 137.5, 130.9, 128.1, 128.1, 125.1, 124.6, 119.1, 111.4, 55.8, 54.9, 40.2, 34.2, 31.6, 29.9, 29.7, 24.0, 22.6, 13.9. C47 H62 O2 (659.0): Calcd. C, 85.66; H, 9.48; Found: C, 85.57; H, 9.40. 2.5. Polymer synthesis 2.5.1. Poly[9,9-dihexyl-2,7-bis(2-methoxyphenyl-1,5-ene) fluorenylenisopropilene] (6a) A 50 mL Schlenk tube was charged with Pd(PPh3 )4 (0.44 g, 0.38 mmol), 3 (0.805 g, 1.6 mmol), Na2 CO3 (2.82 g, 26.6 mmol) and comonomer 1 (0.665 g, 1.6 mmol) and the mixture was kept under Ar atmosphere. Dry THF (13.3 mL) and water (13.3 mL) were added via a syringe. The mixture was heated at 80 ◦ C for 7 days. The reaction mixture was poured into methanol (30 mL). The precipitate was collected by filtration and dissolved in CHCl3 (4.0 mL). The solution was filtered and poured into methanol (30.0 mL). The precipitate was collected by filtration and dried under vacuum to yield the polymer as a whitish powder (0.86 g; 90%). 1 H NMR (CDCl3 ): ı = 7.63 (d, 1H, Jo = 7.8 Hz), 7.49 (s, 1H), 7.37 (d, 1H, Jo = 7.8 Hz), 7.32 (s, 1H), 7.12 (d, 1H, Jo = 8.7 Hz), 6.84 (d, 1H, Jo = 8.7 Hz), 3.72 (s, 3H), 1.90 (m, 2H), 1.70 (s, 3H), 1.02 (m, 6H), 0.78 (m, 2H), 0.67 (t, 3H, J = 6.5 Hz). 13 C NMR (CDCl3 ): ı = 153.6, 149.6, 142.3, 138.6, 136.3, 129.6, 128.2, 127.1, 125.9, 123.6, 118.1, 110.2, 54.7, 53.9, 40.9, 39.2,

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30.5, 30.2, 28.8, 23.0, 21.6, 12.9. (C25 H26 O4 )n (390.5)n : Calcd. C, 76.90; H, 6.71; Found: C, 76.48; H, 6.52.

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birefringence was detected for these three compounds by POM observations carried out in the same temperature range. 3.2. Photophysical properties

2.5.2. Poly[2,7-bis(2-methoxyphenyl-1,5-ene) fluorenylenisopropylene)] (6b) It was synthesized by the same procedure used for 4a except that 2,7-dibromofluorene (0.648 g, 2.00 mmol) and diboronic ester 2 (0.824 g, 2.00 mmol) were used. The compound was obtained as a mixture of oligomers (0.43 g; 52%). 1 H NMR (CDCl3 ): ı = 7.95–6.74 (m, 6H), 3.98 (s, 1H), 3.81 (m, 3H), (2.05–1.41) (m, 3H).

3. Results and discussion 3.1. Synthesis and thermal properties A summary of the synthesis of monomers, the model compound and polymers is shown in Scheme 2. We have already reported the synthesis of the dibromide 1 from bisphenol A [22]. Now, a new diboronic acid was prepared starting from 1 by treatment of its dilithium salt with isopropyl borate followed by acidic hydrolysis. The diboronate ester 2 was then obtained from the acid by azeotropic esterification with 1,3-propanediol. Same esterification technique was used to prepare monomer 3. Then, Suzuki cross-coupling reactions between either the dibromide 1 and the diboronic ester 3 or 2,7-dibromofluorene and the diboronic ester 2 were carried out to obtain polymer 6a (X = n-octyl) and oligomeric 6b (X = H). We also synthesized the corresponding model compound 5 to facilitate the elucidation of the structure of polymer 6a as well as the analysis of its thermal and optical properties. The monobromide 4 needed for the Suzuki coupling was prepared in good yields by methylation followed by slow bromination at low temperature to avoid polybromination. Polymer 6a was obtained in high yield after reprecipitation and best results were observed with freshly prepared (PPh3 )4 Pd(0) [24] that was washed with ethyl ether and EtOH [28]. Gel permeation chromatography (GPC) showed that 6a had a monomodal distribution and that a fair molecular weight and a regular polydispersity index (Mw = 13000, DPI = 1.9) had been achieved. The polymer 6a is highly soluble in CHCl3 (50 wt.%) and soluble in other common organic solvents. Its high solubility is probably related to its non-linear structure rather than to the pending alkyl chains. Films drop-cast on quartz plates and dried under vacuum gave smooth films suitable for optical measurements. When concentrated solutions were used, the films could also be pulled out from the glass substrate to form homogeneous transparent free-standing films. On the contrary, the GPC analysis showed that 6b was an oligomeric material with a normal polydispersity after reprecipitation from MeOH (Mw = 1400, DPI = 1.8). As expected, the 1 H and 13 C NMR spectra showed that 6b has a much higher structural heterogeneity than 6a due to the higher relative weight of chain ends in oligomers. However, despite their lower molecular weights both oligomeric 6b and model compound 5 also formed homogeneous transparent films on quartz plates. The thermal properties were investigated using DSC and polarized optical microscopy (POM). DSC was performed at a temperature range between −50 and 250 ◦ C (Table 1). Compound 5 was a semisolid mass whose DSC showed only a glass transition at −8 ◦ C; likely the bulky tert-butyl substituents efficiently hinder crystallization. 6a and 6b are amorphous in nature; their DSC traces showed only distinct glass transitions at 95 ◦ C and 155 ◦ C and no melting transitions were found upon heating beyond the glass transition temperature. The rather high Tg temperature showed by 6b, regardless of its low degree of polymerization, reflects the absence of conformationally rich aliphatic side chains. Besides, no

The optical properties of 5, 6a and 6b were investigated using UV–vis absorption and steady state and time-resolved photoluminescence emission. The absorption and PL spectra of 5, 6a and 6b in dilute chloroform solutions and in the solid state are shown in Fig. 1. The corresponding photophysical data are reported in Table 1. The absorption and emission spectra in chloroform solution of the model compound 5 and polymer 6a are very similar (cf. Fig. 1a and b). The absence of mirror symmetry between absorption and fluorescence in solution shown by the 2,7-diphenylfluorene chromophore in both 5 and 6a can be attributed to planarization of the first excited state structure as compared to the non-planar ground state. In both cases, such planarization results in moderate Stokes shifts, SS, of 40 nm and ca. 55 nm for the chromophore 0-0 and 0-1 emission vibronic bands. We have observed such insensitiveness of optical properties to the molecular mass in structurally related segmented conjugated polymers bearing oligophenylene chromophores where the isopropylidene spacer effectively isolates the gem-chromophore electronic systems [21,22]. Moreover, both model compound 5 and polymer 6a have strongly allowed –* transitions which give rise to equally large molar (per repeating unit) absorption coefficients. Both materials have also high fluorescence quantum yields of comparable magnitude (see Table 1). Thus, the observed similarity between absorption and emission spectra as well as the photophysical properties argues for a regular microstructure of 6a composed of well defined non-interacting chromophores. In contrast, the thin film absorption and emission spectra of 5 and 6a showed discrepancies. First, the absorption spectrum of 5 presents a broader full width at half-maximum, FWHM, and a larger red tail in the solid film as compared to that in dilute solution. The similarity of the emission spectra (not shown here) obtained by exciting at different energies from 340 to 380 nm on the red edge of the absorption spectrum of 5 indicates that fluorescence is emitted by the same species, i.e.: the isolated excited chromophores, thus ruling out the presence of ground-state aggregates. Therefore, the band broadening of the absorption spectrum of 5 in the condensed phase is probably due to an increasing population of more planar conformers in the distribution of ground state chromophores with different ring-torsion angles. Interestingly, such broadening was not observed for polymer 6a. We observed that 6a shows no considerable differences between its solution and film absorption spectra indicating that the interchain ␲-stacking of the chromophores is substantially reduced in the amorphous state by bent conformations imposed by the polymer microstructure (see Fig. 2). Likewise, the thin film emission spectrum of 6a as compared to that of 5 showed a smaller SS and reduced fwhm arguing again for the beneficial effects of the contorted microstructure of vicinal polymer chains not only in reducing intrachain interactions but also bulk interactions between the linear chromophores. The overall absorption and emission behavior of 5 and 6a in dilute chloroform solutions is closely followed by 6b with few deviations (cf. Fig. 1a–c), e.g., the very small hypsochromic shift of the absorption spectrum is most probably related to electronic effects due to the absence of the pendant alkyl chains while the comparatively larger molar absorption coefficient and the reduced fluorescence quantum yield could be related to the presence of fluorenone moieties (vide infra) which usually have a higher extinction coefficient than the parent fluorene chromophore. More remarkably, while both 5 and 6a are blue-emitting materials in the solid state, 6b turns out to be a green emitter with a broad emission band centered at ca. 532 nm with very weak vibronic coupling.

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H3C

CH3

HO

H3C

a) K2CO3/CH3I OH

CH3

Br

b) Br2/CH2Cl2, 0º

H3CO

(HO)2B

1

Br

a) BuLi/THF b) B(OiPr)3

OCH3

c) H2O/H2SO4 d) HO(CH2)3OH/toluene

O B O

HO(CH2)3OH

B(OH)2

toluene, reflux H13C6

C6H13

a) K2CO3/CH3I

O

OCH3

2

O

3

C6H13

H3CO

3 Pd2(dba)3/P(o-tolyl)3/ aq CO3Na2/THF

t-Bu

OCH3

t-Bu

H13C6 C6H13

5 H3C

or

O B

CH3

O

4

1 + 3

H3CO

t-Bu

b) Br2/CH2Cl2, 0º t-Bu

H3C

B H13C6

OCH3 Br

OH

O

O B

CH3

H3CO

Pd(PPh3)4

2 + Br

Br aq Na2CO3/THF

OCH3

X

n

X

6a; X = n-C6H13 6b; X = H Scheme 2. Synthetic routes of model compound 5, polymer 6a and oligomer 6b.

This green band, or “g-band” for short, has been reported by many groups to appear with varying relative strength in a range of polyfluorene homo- and copolymer films and taint the desired blue emission [29–32]. It has been proposed that the g-band is attributable to emission from fluorenone-based excimers [30]. Indeed, a noticeable signal appeared at 1718 cm−1 in the IR spectrum of 6b which indicates that some of the methylene bridges of the fluorene moieties were oxidized during the synthetic procedure thereby opening the possibility for excimer formation. The luminescence decay of the g-band observed in 6b was monoexponential with a lifetime range of 3.5 ± 0.1, a value close to that observed for fluorenone in toluene solutions ( = 3.0 ns) but much lower than the one ( = 13.0 ns) observed in fluorenone crystals [30]. We noticed that although the cofacial interchromophoric interaction is apparently not prevented by the bent microstructure of the oligomer, it has a liquid-like nature in the amorphous state. In contrast, monoexponential decay curves of the

luminescence were observed in films of 5 and 6a which show comparable lifetimes of the single-chromophore excited states, that is,  = ca. 0.44 ns (see Table 1), values surprisingly close to those measured in pentafluorene ( = 343 ps) [33] and polyfluorene films ( = 430 ps) [34] despite the obvious differences in chromophore lengths. Moreover, after having characterized the undesired fluorenone-based defects we were pleased that their spectral features were absent in 6a films. Equally significant, a photoluminescence spectrum completely dominated by excimer emission implies both mobility of the initially formed excitons in 6b films as well as very efficient excitation transfer from the excited 2,7-diphenylfluorene units to the fluorenone-based moieties. Energy migration in 6b films was confirmed by fluorescence depolarization measurements, which are especially pertinent in rigid media where the reorientation of chromophores is halted and exciton migration becomes the only possible source of emission depolarization. In segmented

Table 1 Thermal and photophysical properties of model compound 5 and polymers 6. Tg a

Mediab

Abs. max ( εmax)

5

−8

6a

95

6b

155

a b c d e f g h i j

CHCl3 Film CHCl3 Film CHCl3 Film

Fluo. c

324 (38000) 326 327 (36600) 327 321 (50700) 322

FWHM

max

6220 9010 5580 6110 7900 9860

364, 377 395, 413 367, 383 391, 407 363, 379 498, 532

d

e

SSg

˚F h

3390 6460 4470 5010 4770 12260

0.74

fi

rj

0.43 0.45

0.130 0.071 0.046

3.50

0.073

f

FWHM 3000 3540 3760 3140 4000 3660

0.73 0.29

Glass transition temperature (◦ C) determined by DSC at scan rates of 5 ◦ C/min. Measured from dilute CHCl3 solutions and thin films. Absorption maxima measured in dilute CHCl3 solutions and on films (Extinction coefficients, in M−1 cm−1 ; based upon the molar repeating units for 6a and b). Full witdth at half-maximum of the absorption bands (cm−1 ). Emision maxima measured in dilute CHCl3 solutions and on films (ex = 330 nm). Bold data indicate the major peaks. Full witdth at half-maximum of the fluorescence bands (cm−1 ). Stokes shifts in dilute CHCl3 solutions and on films (max,em − max,abs , in cm−1 ). Fluorescence quantum yields measured in CHCl3 (ex = 325 nm). Observed lifetime at max ,em on thin films (ns). Average anisotropy measured in thin films (ex = 330 nm) in an emission range of 60 nm around the max ,em .

P.G. Del Rosso et al. / Sensors and Actuators B 160 (2011) 524–532

529

Absorption/Emission (a.u.)

5

Fig. 2. AM1 molecular model of a trimer of 6a.

300

350

400

450

500

550

500

550

Wavelength (nm)

Absorption/Emission (a.u.)

6a

is operative and contributes to emission depolarization in these polymer films. But we cannot assess if conformational relaxation is also taking place and to what extent. However, the glass transition of polymer 6a that occurs at 90 ◦ C suggests that conformational relaxation in 6a films would play a minor role in its depolarization at room temperature. The residual value of the steady-state anisotropy, r, of 6b falls within the range found for polyfluorene films [35,36] (see Table 1). An equally low anisotropy value was also measured for the blue emission of 6a. These results bear particular significance considering that mobility of excited states towards quenching sites increases polymer sensitivity to analytes. Finally, identical film fluorescence spectra of 6a were recorded on the same sample before and after thermal annealing up to 6 h in a nitrogen atmosphere at 10 ◦ C above their Tg values, without showing any band broadening or decrease in fluorescence intensity. Therefore, the amorphous morphology of 6a proved to be very stable under thermal stress. 3.3. Photoluminiscence quenching of polymer 6a

300

350

400

450

Wavelength (nm)

6b

379 nm

Absorption/Emission (a.u.)

532 nm

300

350

400

450

500

550

Wavelength (nm) Fig. 1. Normalized absorption and emission spectra of CHCl3 solutions (dashed curves) and films (solid curves) of 5 (a), 6a (b) and 6b (c).

conjugated polymers exciton mobility is blocked along the polymer chain by the saturated connecting groups; therefore energy migration in the solid state rests on homo resonance energy transfer (RET) while depolarization could be due to RET and conformational relaxation. The emission behavior of polymer 6b indicated that RET

Having established the high structural regularity, the stable amorphous morphology and fine photophysical properties of polymer 6a, we finally analyzed its fluorescence quenching behavior in the solid state. The steady-state fluorescence response of polymer 6a films to increasing concentrations of various analytes was observed for methanol solutions of nitrobenzene (NB), 1,3-dinitrobenzene (DNB), 2,4-dinitrofluorobenzene (DNFB), 4nitroaniline (NA) and nitropropane (NP). Considering that the structural stability of polymer films immersed in a solvent has been a concern [12], we performed control experiments which indicated that there was no polymer leakage towards the liquid media as well as that fluorescence intensity of pristine films was not reduced by immersing them into methanol for several hours; on the contrary, we observe that wet films showed a 15–20% increase in fluorescence intensity. Fig. 3 shows the fluorescence intensity of 6a as a function of added DNB. Each fluorescence spectrum was recorded subsequently to the addition of microliter aliquots of the quencher solution and immediately after reaching fluorescence intensity stabilization. The film response was rapid, i.e., steady fluorescence intensity was always reached in ca. 30–50 s. This data implies fast diffusion of the analyte within the film so the solid/liquid distribution ratio is reached in the mentioned period of time. Such findings can be put into perspective considering that the penetration depth of light into strongly absorbing conjugated materials is in the range of 100 nm [37], meaning that the active sensing layer where equilibrium has to be established could extend over a 100 nm depth within the film. The reversibility of the sensing was examined with NB as an example nitro aromatic compound. Fig. 4 shows ten continuous cycles of fluorescence quenching–recovery. The fluorescence intensity was recorded first in neat methanol and

P.G. Del Rosso et al. / Sensors and Actuators B 160 (2011) 524–532

14

14

Fluorescence intensity (a.u.)

12

(I /I) -1)

10

DNB 10

NA

12

(I0/I)-1

530

8

5

10

6

NB

d = 9 nm d = 35 nm

2 0 0.0

1.0µ

2.0µ 3.0µ [Q] / M)

4.0µ

(I0/I)-1

4

0

8

50µ

100µ

150µ

[Q] / M

6 4

DNFB

2 NP

0 0.0

200.0µ

400.0µ

600.0µ

800.0µ

[Q] / M

400

450

500

550

Wavelength (nm) Fig. 3. Fluorescence spectra change (ex = 327 nm) of a 6a film as a function of added DNB in MeOH; [DNB] = 4.5 × 10−8 –4.4 10−6 M (top to bottom). The corresponding Stern–Volmer plot is shown in the inset.

then after the quencher solution was added. Next the quencher solution was carefully withdrawn, the film was washed twice with methanol and without further delay measurements in absence and in presence of the analyte were repeated several times. The results of the quenching–recovery test indicate that quenching is intrinsically reversible. Thus, both fast equilibration of the fluorescence response and reversible quenching with nearly complete fluorescence recovery suggest that the amorphous morphology of the polymer 6a is an important factor that determines the efficacy of the material in sequestering the quencher molecules from the liquid media in a reversible manner. The Stern–Volmer (S–V) plot of fluorescence quenching shown in Fig. 3(inset) points out that 6a has sensitivity towards DNB at the submicromolar range, i.e., a concentration as low as 0.05 ppm decreases the fluorescence intensity to 60% of its initial value. In that manner, half of the maximum quench (Q50% ) of 6a occurred with DNB at the micromolar range (0.9 × 10−6 mol/l) while almost complete quenching (Q10% ) was achieved with a 3.4 ␮M solution.

Fluorescence intensity (a. u.)

400

NB

300

0M

200

100

3,22 μ M 0 0

2

4

6

8

10

Quenching cycles Fig. 4. Ten continuous cycles of quenching–recovery test of a drop-cast 6a film. The quenching was measured after exposing the film to [NB] = 3.22 10−6 M for less than 1 min (ex = 327 nm, em = 390 nm).

Fig. 5. Stern–Volmer plots of 6a in response to DNB, NA, NB, DNFB and NP. Stern–Volmer plots of thinner 6a films with two different thicknesses (d = 9 and 35 nm) in response to nitroaniline are shown in the inset.

To our knowledge, such sensitivity has been observed only for nitrobenzene in aqueous media [38]. In addition, the S–V relationship for DNB exhibits a non-linear dependence of exponential nature and analogous data was obtained for the other nitro aromatic compounds tested. The Stern–Volmer plots of fluorescence quenching response in 6a films to different analytes tested are pictured in Fig. 5 and the corresponding results of data analysis is shown in Table 2. Numerous studies indicate that quenching of conjugated polymers in the solution state, where mass transport has no bearing, usually affords linear S–V plots [39–42] although non-linear with upward curvature S–V plots are also occasionally observed [14,43]. However, in addition to the exergonicity of electron transfer between nitro compounds and conjugated materials, the fluorescence quenching of chromophores in films immersed into quencher solutions is also influenced by the diffusion ability of analytes through films and the solid/liquid distribution ratio [44]. The last variable is related to the binding strength between analyte and sensing material. This multiplicity of factors originates S–V relationships of diverse nature as it can be seen in the fewer reports dealing with this sensing configuration. A linear dependence has been observed for oligosilane monolayers grafted on glass slides [38] while benzofluorantene dispersed in poly(vinyl alcohol) films [45], nanosized aggregates of polytriazoles [43] and conjugated polymer-grafted silica nanoparticules [13] presented non-linear curves with upwards curvature. Finally, polythiophenegrafted silica nanoparticules [46] showed a non-linear S–V plot with downward curvature. All nitro aromatics tested in this study showed non-linear S–V plots with upward curvature whose data could be fitted into single-exponential equations with correlation coefficients above 0.99 for all fittings (see Table 2). Moreover, similar non-linear dependence of fluorescent quenching was observed on thinner polymer films suggesting that the diffusion to the interior of the film is not the fundamental reason for the non-linear Stern–Volmer plots (see Fig. 5, inset). Therefore, it can be reasonably considered that this common feature depends on the polymer film intrinsic characteristics. Whether this exponential dependence involves non-specific solute-polymer interactions, i.e.: non-linear solid/liquid distribution ratio, which derives from nonlinear solubility of analytes in the polymer, or an increasingly efficient quenching of migrant excitons as the analyte concentration augments within the sensing film is not clear. On the other hand, the electronic and structural effects of the nitro compounds on its quenching efficiency can be easily

P.G. Del Rosso et al. / Sensors and Actuators B 160 (2011) 524–532

531

Table 2 Data analysis of Stern–Volmer plots and quenching efficiencies. Quencher (Q)

Regression equation (R2 )a [Q ]/1.8×10−6

Rangeb

1,3-DNB

I0 /I = 1.6e

− 0.5 (0.999)

0.005–0.67

4-NA

I0 /I = 1.3e[Q ]/4.3×10

− 0.2 (0.999)

0.7–14

NB

I0 /I = 5.0e[Q ]/3.2×10

− 0.2 (0.994)

0.025–250

2,4-FDNB NP

I0 /I = 4.1e[Q ]/1.7×10 − 3.1 (0.999) I0 /I = (− 0.001 + 77[Q])0.37 (0.992)

a b c

−5

−4 −3

7–370 1.8–1780

Q50% c 0.15 4.0 4.4 71 1340

Q10% c 0.57 11 44 372 –

Nonlinear regression equation and its correlation coefficients. Quencher concentration interval in which fluorescence intensity of 6a was sampled, in ␮g/ml. Quencher concentration at which either half (Q50% ) or ninety percent (Q90% ) of the maximum quench was observed, in ␮g/ml.

evaluated from the values of half of the maximum quench (Q50% ) listed in Table 2. It can be observed that the fluorescence quenching follows the order of DNB > NA > NB  DNFB  NP. The redox potentials (vs. SCE) of DNB, NA and NB are −0.9 V [10], −0.92 [47] and −1.1 [10] respectively. Clearly, the response of the film to these three quenchers can be attributed to its reduction potential. We note also that the fluorescence maximum and band shape of 6a were unchanged in the presence of either DNB (see Fig. 3) or the other nitro compounds tested, which indicates that the interaction between the electron-poor nitro compounds and the electron-rich 6a does not afford emissive (exciplet) states. Likewise, the absorption spectra of 6a in chloroform solution were not modified even up to relative DNB concentrations of 1:2.5, ruling out the formation of non-emissive ground state complexes. The observed trend of fluorescence attenuation indicates that the mechanism of photoluminescence quenching is attributable to electron-transfer from the excited polymer to the analyte. The relatively lower quenching efficiency of DNFB compared to that of DNB deserves a comment. Unfortunately, the reduction potential of DNFB is not available in the literature, however, judging from the electron attractor properties of fluorine (Hammett sigma value: 0.06) [48] its redox value should not be lower than that of DNB, and consequently should show a comparable Q50% . Moreover, the close resemblance ˚ of hydrogen and fluorine sizes (van der Waals radii: 1.2 vs. 1.35 A) [49] precludes that significant steric impediment for DNFB diffusion into the film will arise upon substitution of hydrogen by fluorine in DNB. In our view, the reduced DNFB efficiency arises instead from the decrease of the solid/liquid distribution ratio of DNFB owing to hydrogen bonding between the fluorine substituent and methanol. Finally, the very low quenching efficiency of the nitro aliphatic NP points up the fundamental role of the aromatic moiety in the quenching process by enhancing the electron acceptor properties of the analyte.

3.4. Conclusions A new regularly segmented conjugated polymer bearing 2,7diphenylfluorene chromophores was synthesized through Suzuki cross-coupling by a relatively short synthetic route and starting from easily available monomer synthons. The small size of the isopropylidene connector lead to a high density of short but welldefined chromophores tethered into a bent microstructure which produces a highly soluble amorphous polymer and precludes the formation of aggregated species commonly observed in conjugated polymers. Thus, the polymer formed homogeneous transparent films with stable optical properties, which are insensible to thermal stress. The fine optical properties of the fluorene chromophore are retained by the polymer in solution and enhanced in the condensed phase. Thus, fluorescence depolarization measurements showed that exciton mobility within the amorphous polymer film is not hindered. The morphological and optical properties shown by polymer 6a are of practical significance in view of the high sensitivity and fast response of its fluorescence quenching by nitro aromatics.

It was observed that half of the maximum quench (Q50% ) of a 6a film occurred with DNB at the micromolar range in less than 1 min in a reversible manner showing that our approach to tailor film porosity was effective. Thus, this study demonstrates that amorphous segmented conjugated polymers bearing relatively short chromophores can be used as sensing material with performances comparable to those presented by conjugated polymers with more elaborate structural design.

Acknowledgments Financial support from SGCyT-UNS and SPU-MCyT is acknowledged. PGDR and MFA are members of the research staff of CIC-PBA. ROG is member of the research staff of CONICET.

References [1] A. Kraft, A.C. Grimsdale, A.B. Holmes, Electroluminescent conjugated polymers – seeing polymers in a new light, Angew. Chem. Int. Ed. 37 (1998) 402–428. [2] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Plastic Solar Cells, Adv. Funct. Mater. 11 (2001) 15–26. [3] A.A. Argun, P.H. Aubert, B.C. Thompson, I. Schwendeman, C.L. Gaupp, J. Hwang, Multicolored electrochromism in polymers: structures and devices, Chem. Mater. 16 (2004) 4401–4412. [4] D.T. McQuade, A.E. Pullen, T.M. Swager, Conjugated polymer-based chemical sensors, Chem. Rev. 100 (2000) 2537–2574. [5] A. Mathy, K. Ueberhofen, H. Gregorius, R.O. Garay, K. Müllen, C. Bubeck, Third-harmonic generation spectroscopy of poly(p-phenylenevinylene): a comparison with oligomers and scaling laws for conjugated polymers, Phys. Rev. B 53 (1996) 4367–4376. [6] S.W. Thomas, G.D. Joly, T.M. Swager, Chemical sensors based on amplifying fluorescent conjugated polymers, Chem. Rev. 107 (2007) 1339–1386. [7] Y. Liu, K. Ogawa, K.S. Schanze, Conjugated polyelectrolytes as fluorescent sensors, J. Photochem. Photobiol. C: Photochem. Rev. 10 (2009) 173–190. [8] T. Rajesh, D. Ahuja, Kumar, Recent progress in the development of nanostructured conducting polymers/nanocomposites for sensor applications, Sens. Actuators B: Chem. 136 (2009) 275–286. [9] S.J. Toal, W.C. Trogler, Polymer sensors for nitroaromatic explosives detection, J. Mater. Chem. 16 (2006) 2871–2883. [10] J.-S. Yang, T.M. Swager, Fluorescent porous polymer films as TNT chemosensors: electronic and structural effects, J. Am. Chem. Soc. 120 (1998) 11864–11873. [11] Y. Liu, R.C. Mills, J.M. Boncella, K.S. Schanze, Fluorescent polyacetylene thin film sensor for nitroaromatics, Langmuir 17 (2001) 7452–7455. [12] G. He, G. Zhang, F. Lu, Y. Fang, Fluorescent film sensor for vapor-phase nitroaromatic explosives via monolayer assembly of oligo(diphenylsilane) on glass plate surfaces, Chem. Mater. 21 (2009) 1494–1499. [13] J. Feng, Y. Li, M. Yang, Conjugated polymer-grafted silica nanoparticles for the sensitive detection of TNT, Sen. Actuators B: Chem. 145 (2010) 438–443. [14] D. Zhao, T.M. Swager, Sensory responses in solution vs solid state: a fluorescence quenching study of poly(iptycenebutadiynylene)s, Macromolecules 38 (2005) 9377–9384. [15] Y. Kim, Z. Zhu, T.M. Swager, Hyperconjugative and inductive perturbations in poly(p-phenylene vinylenes), J. Am. Chem. Soc. 126 (2004) 452–453. [16] A. Greiner, Design and synthesis of polymers for light-emitting diodes, Polym. Adv. Technol. 9 (1998) 371–389. [17] M.A. Hargadon, E.A. Davey, T.B. McIntyre, D. Gnanamgari, C.M. Wynne, R.C. Swift, Alternating block copolymers consisting of oligo(phenylene) and oligo(ethylene glycol) units of defined length: synthesis. Thermal characterization, and light-emitting properties, Macromolecules 41 (2008) 741–750. [18] C.A. Sierra, P.M. Lahti, A photoluminescent, segmented oligopolyphenylenevinylene copolymer with hydrogen-bonding pendant chains, Chem. Mater. 16 (2004) 55–61.

532

P.G. Del Rosso et al. / Sensors and Actuators B 160 (2011) 524–532

[19] Y. Morisaki, Y. Chujo, Novel [2.2]paracyclophane-fluorene-based conjugated copolymers: synthesis, optical, and electrochemical properties, Macromolecules 37 (2004) 4099–4103. [20] J. Osio Barcina, M.R. Colorado Heras, M. Mba, R. Gomez Aspe, N. Herrero-Garcia, Efficient electron delocalization mediated by aromatic homoconjugation in 7,7-diphenylnorbornane derivatives, J. Org. Chem. 74 (2009) 7148–7156. [21] P.G. Del Rosso, M.F. Almassio, S.S. Antollini, R.O. Garay, Optical characterization of amorphous oligo(4,4(-biphenylene-1,1-substituted methylene)s, Opt. Mater. 30 (2007) 478–485. [22] P.G. Del Rosso, M.F. Almassio, P. Aramendia, S.S. Antollini, R.O. Garay, Poly[(2,2,5,2-tetramethoxy-p-terphenyl-5,5-ylene)propylene].;1; synthesis and physical properties of a novel amorphous regularly segmented conjugated polymer, Eur. Polym. J. 43 (2007) 2584–2593. [23] U. Toshinao, H. Kawazura, Y. Ishii, Chemistry of dibenzylideneacetone-palladium(0) complexes: I. Novel tris(dibenzylideneacetone)dipalladium (solvent) complexes and their reactions with quinones, J. Organomet. Chem. 65 (1974) 253–266. [24] M. Ranger, D. Rondeau, M. Leclerc, New well-defined poly(2,7-fluorene) derivatives: photoluminescence and base doping, Macromolecules 30 (1997) 7686–7691. [25] M. Sunagawa, H. Matsumara, Y. Kitamara, Crystalline palladium tetrakis(triphenyl-phosphine) and process for preparing the same, European Patent 0493032A2, Sumimoto Pharmaceuticals Company (1991). [26] M. Montalti, A. Credi, L. Prodi, M.T. Gandolfi, Handbook of Photochemistry, third ed., Taylor & Francis Group, Boca Raton, 2006, p. 572. [27] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Academic Press, New York, 1999. [28] M. Sunagawa, H. Matsumara, Y. Kitamara, Crystalline palladium tetrakis(triphenyl-phosphine) and process for preparing the same, European Patent 0493032A2. Sumimoto Pharmaceuticals Company, 1991. [29] A.C. Grimsdale, K. Müllen, Oligomers, Polymers based on bridged phenylenes as electronic materials, Macromol. Rapid Commun. 28 (2007) 1676–1702. [30] M. Sims, D.D.C. Bradley, M. Ariu, M. Koeberg, A. Asimakis, M. Grell, D.G. Lidzey, Understanding the origin of the 535 nm emission band in oxidized poly(9,9dioctylfluorene): the essential role of inter-chain/inter-segment interactions, Adv. Funct. Mater. 14 (2004) 765–781. [31] W. Zhao, T. Cao, J.M. White, On the origin of green emission in polyfluorene polymers: the roles of thermal oxidation degradation and crosslinking, Adv. Funct. Mater. 14 (2004) 783–790. [32] D. Neher, Polyfluorene homopolymers conjugated liquid-crystalline polymers for bright blue emission and polarized electroluminescence, Macromol. Rapid Commun. 22 (2001) 1365–1385. [33] C. Chi, C. Im, G. Wegner, Lifetime determination of fluorescence and phosphorescence of a series of oligofluorenes, J. Chem. Phys. 124 (024907) (2006) 1–8. [34] M. Ariu, D.G. Lidzey, M. Sims, A.J. Cadby, P.A. Lane, D.D.C. Bradley, The effect of morphology on the temperature-dependent photoluminescence quantum efficiency of the conjugated polymer poly(9, 9-dioctylfluorene), J. Phys. Condens. Matter 14 (2002) 9975–9986. [35] R.F. Cossiello, M.D. Susman, P.F. Aramendía, T.D.Z. Atvars, Study of solventconjugated polymer interactions by polarized spectroscopy: MEH–PPV and Poly(9,9-dioctylfluorene-2,7-diyl), J. Lumin. 130 (2010) 415–423. [36] U. Lemmer, S. Heun, R.F. Mahrt, U. Scherf, M. Hopmeier, U. Siegner, E.O. Gobel, K. Mullen, H. Bassler, Aggregate fluorescence in conjugated polymers, Chem. Phys. Lett. 240 (1995) 373–378. [37] L. Stolz Roman, O. Inganäs, T. Granlund, T. Nyberg, M. Svensson, M.R. Andersson, J.C. Hummelen, Trapping light in polymer photodiodes with soft embossed gratings, Adv. Mater. 12 (3) (2000) 189–195.

[38] Y. Zhang, G. He, T. Liu, M. Yang, L. Ding, Y. Fang, Sensing performances of oligosilane functionalized fluorescent film to nitrobenzene in aqueous solution, Sens. Lett. 7 (2009) 1141–1146. [39] L. Chen, D.W. McBranch, H-L. Wang, R. Helgeson, F. Wudl, D.G. Whitten, Highly sensitive biological and chemical sensors based on reversible fluorescence quenching in a conjugated polymer, PNAS 96 (1999) 12287–12292. [40] H. Sohn, M.J. Sailor, D. Magde, W.C. Trogler, Detection of nitroaromatic explosives based on photoluminescent polymers containing metalloles, J. Am. Chem. Soc. 125 (2003) 3821–3830. [41] J.H. Wosnick, T.M. Swager, Enhanced fluorescence quenching in receptorcontaining conjugated polymers: a calix[4] arene-containing poly(phenylene ethynylene), Chem. Commun. 274 (2004) 2744–2745. [42] A. Kumara, M.K. Pandeya, R. Anandakathira, R. Mosurkala, V.S. Parmard, A.C. Wattersona, J. Kumar, Sensory response of pegylated and siloxanated 4,8dimethylcoumarins: a fluorescence quenching study by nitro aromatics, Sen. Actuators B: Chem. 147 (2010) 105–110. [43] A. Qin, J.W.Y. Lam, L. Tang, C.K.W. Jim, H. Zhao, J. Sun, B.Z. Tang, Polytriazoles with aggregation-induced emission characteristics: synthesis by click polymerization and application as explosive chemosensors, Macromolecules 42 (2009) 1421–1424. [44] H.K. Karapanagioti, I. Klontza, Testing phenanthrene distribution properties of virgin plastic pellets and plastic eroded pellets found on Lesvos island beaches (Greece), Mar. Environ. Res. 65 (2008) 283–290. [45] D. Patra, A.K. Mishra, Fluorescence quenching of benzo[k]fluorantene in poly(vinyl alcohol) film: a possible optical sensor for nitro aromatic compounds, Sen. Actuators B: Chem. 80 (2001) 278–282. [46] A.L. Holt, J.P. Bearinger, C.L. Evans, S.A. Carter, Chemically robust conjugated polymer platform for thin-film sensors, Sen. Actuators B: Chem. 143 (2010) 600–605. [47] A.A. Jbarah, R. Holze, A comparative spectroelectrochemical study of the redox electrochemistry of nitroanilines, J. Solid State Electrochem. 10 (2006) 360–372. [48] R.W. Taft, A survey of Hammett substituent constants and resonance and field parameters, Chem. Rev. 91 (1991) 165–195. [49] A. Bondi, Van der Waals, Volumes and radii, Phys. Chem. 68 (3) (1964) 441–451.

Biographies Pablo G. Del Rosso received his PhD in chemistry from the Universidad Nacional del Sur (UNS) in 2006. He did postdoctoral work at the Max-Planck Institut für Polymerforschung from 2007 to 2008. Currently he holds a permanent position as Instructor in the Departamento de Química at UNS (2006–Present). His research interests are mainly focused in the synthesis and applications of organic materials. Marcela F. Almassio received his PhD in chemistry from the Universidad Nacional del Sur (UNS) in 2007. He did postdoctoral work at the University of Coimbra from 2010 to 2011. Currently he holds a permanent position as Instructor in the Departamento de Química at UNS (2006–Present). Her research interests are mainly focused in the synthesis and electrooptical characterizations of organic and bioorganic materials. Raúl O. Garay received his PhD in chemistry from the Universidad Nacional del Sur (UNS) in 1985. He was an assistant professor of chemistry at UNS during 1987. After postdoctoral work in the Department of Polymer Science and Engineering at UMASS, at the Max-Planck Institut für Polymerforschung and BASF SE from 1988 to 1993, he was a lecturer of chemistry at UMASS from 1993 to 1994. Currently he is a Professor of Chemistry in the Departamento de Química at UNS (1995–Present). His research interests are in the areas of luminescent polymers and liquid crystals.