A Facile Synthesis of Fluorescent Conjugated Polyelectrolytes using

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A Facile Synthesis of Fluorescent Conjugated Polyelectrolytes using Polydentate Sulfonate as Highly Selective and Sensitive Copper (II) Sensors Wei Wu, Anting Chen, Linyue Tong, Ziqi Qing, Kevin P. Langone, William Bernier, and Wayne E Jones ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00400 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 12, 2017

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A Facile Synthesis of Fluorescent Conjugated Polyelectrolytes using Polydentate Sulfonate as Highly Selective and Sensitive Copper (II) Sensors Wei Wu, Anting Chen, Linyue Tong, Ziqi Qing, Kevin P. Langone, William E. Bernier, Wayne E. Jones Jr.* Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902-6016, USA KEYWORDS: Fluorescence; conjugated polyelectrolyte; chemical sensor; copper; multi-dentate; computational modeling

ABSTRACT: Fluorescent conjugated polyelectrolytes represent an exciting area of research into new chemosensors. By virtue of their rapid electron and energy transfer paths, these highly correlated, one-dimensional systems have been depicted as “molecular wires” and show “million-fold” sensitivity compared to monomolecular sensor analogs. In this paper, a novel polyelectrolyte sensor, the ttp-PPESO3, has been designed by incorporating terpyridine and sulfonate functional groups into the polyelectrolyte. This specifically tailored sensor has displayed remarkable quenching response towards copper (II) with a detection limit of 14.7 nM (0.93 ppb). It is capable of selectively screening copper without interference from 12 common cations. Molecular modeling suggests the binding occurs through a coordination interaction of the terpyridine and sulfonate. The additional multidentate nature from the sulfonate offers extraordinary chelating ability to the analyte. We anticipate this unique binding mode will provide insight for the design of future more sensitive and selective systems.

Fluorescent conjugated polymers (FCPs) have drawn growing attention for a variety of modern applications including fabrication of light-emitting diodes, photovoltaic devices, optical probes and bio-/chemo-sensors.1-4 However, the inherent poor solubility of the conjugated backbones brings a great challenge for their practical use. Introducing side-groups onto the backbone, such as long alkyl chains or ionic functional groups, have been demonstrated to increase their solubility in organic solvents or water.5-6 Fluorescent conjugated polyelectrolytes (FCPEs) bearing positively or negatively charged groups have been developed as chemosensors to detect a variety of chemical species in both environmental and biological systems.7-9 FCPEs have good water solubility, which is more effective for aqueous sensory applications. The intrinsic energy or exciton migration along the backbones offers FCPEs with “million-fold” signal amplification. In addition, the charged nature of FCPE has more affinity to bind ionic targets compared to their neutral sensory counterparts. For instance, many FCPEs-based sensors employ the electrostatic interaction between the charged portions and the ionic targets of interest, such as metal cations, phosphate anions, proteins, polyelectrolytes, and polynucleic acids.8, 10-11 Among the important trace elements in the living creatures, copper plays an essential role in versatile biological processes.12-14 At low concentrations, it is requisite to maintain the fundamental metabolic and physiological functions. However, exposure to a high level of copper even for a short time period may subject to gastrointestinal disturbance; while long-term intake could lead to chronic disease, neurological disorder, and liver or kidney

damage.15-18 In drinking water, restrict regulations have been set regarding the allowable copper concentrations. For example, the World Health Organization (WHO) has recommended 2 mg/L or 2 ppm of copper as the maximum acceptable concentration.19 Also, the U.S. Environmental Protection Agency (EPA) has regulated an action level of copper to be 1.3 ppm (~ 20 µM).20 In the human body, the average concentration of the total copper level is 63.7140.12 μg/dL or 10-22 µM.21 Up to now, a variety of instrumental techniques have been used to quantify the concentrations of copper, including atomic absorption spectrometry22, inductively coupled plasma mass spectroscopy23, and electrochemical methods (cathodic or anodic stripping voltammetry)24-25, to name a few. Although the equipment has been demonstrated with good accuracy, it is quite costly and often requires sophisticated pretreatment as well as trained expertise. Therefore, a simple, rapid, yet sensitive and selective method is of particular importance. Fluorescent chemosensors have been widely explored due to the merits of easy signal read-out, lowcost, facile structural adjustment and the fact that fluorescence is extremely sensitive. In this connection, substantial efforts have been made to synthesize fluorescent conjugated polyelectrolyte chemosensors that are selective, sensitive, and well adapted for on-site use.10, 26 Many copper-selective FCPE sensors have been reported and applied with some success in environmental systems.27-28 However, some of them have drawbacks for practical use such as crosssensitivities towards other metal cations and inadequate detection limits.29-30 Despite the fact that the fluorescence

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signal transduction of many polyelectrolyte sensors has been recorded on a photoinduced electron transfer basis, the studies of direct binding sites of target species remain scarce. To develop more efficient chemosensors, a detailed binding location regarding the involvement of atoms, ligands, and functional groups needs to be understood. The inclusion of polydentate feature from one or more ligands has been applied in diverse fields such as chemical recognition,31 material separation,32 and anticancer medicine.33 The multidentate chelating often involves with a coordination of two or more atoms, including nitrogen, oxygen, and sulfur. For instance, ethylenediaminetetraacetic acid or EDTA is one of the most common chelating agents to treat a certain heavy metal poisoning. It is a hexadentate ligand, which has a maximum of 6 places (2 nitrogen atoms and 4 oxygens) to bind metal cations in solution. The nitrogens and oxygens synergize with each other to offer extraordinary binding abilities. Inspired by this synergistic interaction in targeting analytes, a tridentate terpyridine receptor has been rationally assembled with a sulfate functional group to achieve higher denticity in our newly constructed copper sensor. Herein, the design and synthesis of the next generation of FCPE chemosensory material, poly[(thienylene terpyridine)-alt- (phenylene ethynylene) sulfonate (ttp-PPESO3, Scheme 1) is presented demonstrating enhanced selectivity and sensitivity for the detection of copper. Computational tools are employed to provide additional support for a potential binding mechanism and the impact on chemosensor behavior.

EXPERIMENTAL SECTION Chemicals and Reagents. Tetrakis(triphenylphosphine) palladium ((PPh3)4Pd, copper (I) iodide (CuI), N,Ndimethylformamide (DMF), diisopropylamine (i-Pr2NH), and all other chemicals and solvents were purchased from Sigma-Aldrich or Fisher Scientific Co. and used as received without further purification unless otherwise noted. For air sensitive synthetic steps, solvents were freshly distilled and stored under a nitrogen atmosphere prior to use. General Methods. 1H-NMR spectra were recorded on a Bruker Advanced III-600 MHz spectrometer. The FT-IR spectra were collected on a Shimadzu IR Affinity-1s spectrophotometer with potassium bromide (KBr) pellets. The UV-visible spectra were obtained on a Hewlett-Packard 8452A diode array spectrophotometer with 0.5 seconds integration time and 1 nm intervals. The photoluminescence (PL) emission spectra were measured on a Panorama Fluorat 02 spectrofluorometer with 4 nm slit width and high PMT sensitivity. Gel permeation chromatography (GPC) measurements were performed on a Waters Alliance 2695 system with dimethyl sulfoxide/water mixture (DMSO/H2O 50:50) as the mobile phase. The columns are calibrated using polystyrene standards ranging from 1606,980,000 using EasiCal PS-1&2 kits. The instrument parameters include using a flow rate at 1.0 ml/min with a column temp at 40 oC. The injection volume is 100 ul using a 100 uL sample loop with a run time of 35 minutes at isocratic conditions. The concentration of polyelectrolyte aqueous solutions was held at 5 µM with respect to the repeating unit of the conjugated backbone. All the cationic

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stock solutions (0.01 M) were freshly prepared by dissolving their chloride salts in distilled water. The titration of the cations into the fluorescent polyelectrolyte solution was performed by pipetting small aliquots of the cation stock solutions into a 3 mL quartz cuvette, followed by thoroughly mixing in order to achieve homogeneous solutions at the desired cation concentration. All the titrations were repeated at least three times, and the data was very reproducible with negligible variance. The fluorescence quantum yields of the polyelectrolytes in solution were calculated relative to the standard quinine sulfate in 0.5 M sulfuric acid solutions with a quantum yield of 0.546, excited at 365 nm.34 Computational studies were performed using Spartan ‘14v1.1.0 (Wavefunction, Inc., Irvine, CA) software for molecular dynamics on a private computer. Molecular models of different binding sites between the polyelectrolyte repeating unit and the metal ion were built, and subject to molecular mechanical optimization via the energy minimization function in Spartan using the molecular mechanics force field (MMFF) basis set.35-36 Synthesis of Monomers and the Polymer. 1,4-diiodo2,5-di(propyloxysulfonate)benzene (M1), 1,4-diethynyl benzene (M2) were synthesized according to published procedures.37-38 The synthesis of 4’-(4-(2-(2,5dibromothiophen-3-yl)-vinyl)-phenyl)-2,2’:6’,2” terpyridine (M3) was previously reported.39 The synthetic routes for the three monomers were provided in Scheme S1 in the supporting information. The polyelectrolyte ttp-PPESO3 was prepared by a step growth polymerization employing the Sonogashira reaction40: a palladium catalyzed cross-coupling of aromatic halides on M1 and M3, together with terminal alkynes on M2 (Scheme 1). Scheme 1. The synthesis of ttp-PPESO3 using Sonogashira coupling protocol.

M1 (white powder, yield 69%). 1H-NMR (600 MHz, DMSO-d6, δppm): 7.29 (2H), 4.05-4.03 (4H), 2.63-2.60 (4H), 2.00-1.98 (4H). FT- IR (νmax, cm-1, KBr): 3092, 2940, 2867, 2359, 2330, 1619, 1495, 1349, 1219, 1050, 847, 621, 553. M2 (white crystal, yield 92%). 1H-NMR (600 MHz, DMSO-d6, δppm): 7.48 (4H), 4.35 (2H). FT- IR (νmax, cm-1, KBr): 3264, 2102, 1922, 1487, 1402, 1255, 1107, 836, 634, 549.


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M3 (light yellow solid, yield 80%). 1H-NMR (600 MHz, DMSO-d6, δppm): 8.78-8.68 (6H), 8.06-8.03 (2H), 8.007.98 (2H), 7.80-7.78 (3H), 7.55-7.53 (2H), 7.42-7.40 (1H), 7.10-7.08 (1H). FT- IR (νmax, cm-1, KBr): 3048, 3008, 2355, 2326, 1684, 1558, 1466, 1386, 1266, 1191, 1008, 951, 785, 733, 664, 618, 469. Poly[(thienyleneterpyridine)-alt-(phenylene ethynylene) sulfonate] (ttp-PPESO3). 0.6502 g (1 mmol) of M1, 0.2523 g (2 mmol) of M2 and, 0.5754 g (1 mmol) of M3 were placed into a Schleck flask and purged with a gentle flow of nitrogen. Next, 0.1156 g (0.1 mmol) of Pd(PPh3)4 and 0.0190 g (0.1 mmol) of CuI were added to the flask. The whole reaction apparatus was purged with nitrogen for half an hour. Then, 36 mL pre-deoxygenated solvent mixtures DMF/water/i-Pr2NH (3:2:1 in volume ratio) was carefully transferred into the reaction flask. The final mixture was stirred at 80 oC for 24 hours. Upon cooling to room temperature, the resulting mixture was slowly added to 100 mL of a methanol/acetone/ether (10:40:50) solution. The crude product was precipitated and collected via centrifugation. It was then re-dissolved in 20 mL of water/methanol (7:3) mixed solvents, followed by treatment with 0.01 g of sodium sulfide. The mixture was filtered, and the filtrate was titrated into a large amount of methanol/acetone/water (10:40:50) solution. The upper liquid was decanted carefully, and the rest mixture was centrifuged to obtain the ttp-PPESO3 product as a brown powder (0.752 g, yield 71%). GPC (in DMSO/H2O, with polystyrene standard): = 1.47×104 g/mol; polydispersity index (PDI) = 3.5. 1H-NMR (600 MHz, DMSO-d6, δppm): 8.908.35 (6H), 8.16-7.30 (15H), 7.28-7.10 (2H), 4.31-4.06 (4H), 2.77-2.63 (4H), 2.18-1.90 (4H). FT-IR (νmax, cm-1, KBr): 2932, 2869, 2360, 2329, 2192, 2108, 1600, 1437, 1136, 1004, 878, 828, 790, 677, 614, 539. The formation of internal ethynyl link was confirmed by the presence of the 2192 cm-1 infrared band stretch.

of the 2192 cm-1 stretch can be assigned to the internal ethynyl link. This suggests the polymerization was successful (also see Figure S1 in the supporting information). The ttp-PPESO3 polyelectrolyte structure with the incorporation of terpyridyl functionality, sulfonate ionic groups, and bisalkyne linkers has not been previously reported in the literature. The poly[p-(phenyleneethynylene)-alt(thienyleneethynylene)] backbone was selected based on the intense fluorescence in the visible light spectrum.41 The terpyridyl group was chosen for the tridentate nitrogen atoms which have strong chelating ability to attract metal cations. The sulfonate ionic characteristics introduce both increased water solubility and enhanced fluorescence thanks to the better separation from the electrostatic repulsion among the negatively charged moieties. It has been reported that the sulfonate groups could also contribute to bind certain heavy metals.42-43

RESULTS AND DISCUSSION Synthesis and Characterization. The use of palladiumcatalyzed cross coupling reactions to synthesize conjugated polymers is well established in the literature.40 In this case, we have used known protocols to prepare monomers for the ABAB polymerization as described previously.39 The polymerization reaction was carried out under the palladium-catalyzed Sonogashira coupling protocol as shown in Scheme 1. The monomer components including two dihaloarenes (M1 and M3) and one bisalkyne (M2) were synthesized according to literature descriptions (scheme S1 in the supporting information). All the monomers and the polyelectrolyte ttp-PPESO3 were then characterized by routine techniques such as 1H NMR and FT-IR with the results captured above in the experimental section. The NMR spectra are shown in Figure 1. All the peaks corresponding to the protons in ttp-PPESO3 have been clearly identified. It is worth mentioning that the terminal C-H triplet peak of M2, which occurs at 4.35 ppm in the 1H NMR, does not show up in the final polyelectrolyte product. Also, the disappearance of the terminal C-H groups, together with the broadening of the other peaks are consistent with the formation of the polyelectrolyte. The NMR finding is also consistent with FT-IR in which the presence

Figure 1. 1H-NMR characterization of the monomers and the ttp-PPESO3 (600 MHz, in DMSO-d6, * denotes the solvent residue peak)

The polyelectrolyte ttp-PPESO3 was soluble in water and several organic solvents, such as methanol and DMSO. Water is an economical solvent of choice for green chemistry, especially when solubility in organic media is limited. Thus, ttp-PPESO3 is more flexible and “greener” than most conjugated polymer analogs which require the use of nonaqueous solvents. The molecular weight of ttp-PPESO3 was determined by GPC analysis with polystyrene as the standard (Figure S2). The weight average molecular weight 4 ( ) is 1.47×10 g/mol (polydispersity index = 3.5) corresponding to a weight average degree of polymerization Xw ≈ 28. The relatively high polydispersity might result from the interactions between the columns and the polyelectro-


lyte. The three amino pendent groups on the conjugated polyelectrolyte may prolong the eluent time through the column and cause overestimation of the polydispersity.7 The ttp-PPESO3 has a degree of polymerization around 28. This moderate conjugation will not only provide it with excellent processability as a promising sensory material, but also maintain superb “molecular wire” effect for signal amplification. Photophysical Properties. The normalized UV-visible absorption and photoluminescence emission spectra of ttp-PPESO3 in dilute water solutions are shown in Figure 2. The polyelectrolyte concentration is 5 µM with respect to the backbone repeating unit. It has an absorption between 350 nm and 500 nm with a maximum absorbance at 442 nm. This result is consistent with the expected π-π* transition of the conjugated polymer backbone. The photoluminescence emission glows broadly from 450 nm to 700 nm with a peak emission at 530 nm. The polyelectrolyte ttpPPESO3 aqueous solution displays light yellowish color under daylight, and shows a strong green fluorescence when excited at 365 nm under the illumination of a commercial UV lamp.

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Figure 3. Normalized absorbance spectra (left) and PL intensity spectra (right) of the ttp-PPESO3 in different solvents.

Detailed photophysical data of ttp-PPESO3 has been summarized in Table 1. There is a large Stock shift of 3757 cm-1 in water, indicating a visible color change under UV illumination. The quantum yield is calculated according to the following equation:

where Ar, As are the absorbance of the reference and sample solutions at the excitation wavelength; Fs，Fr are the corresponding emission integration areas; ns，nr are refractive indices of the solvents used in sample and reference solutions; and φs，φr are quantum efficiencies of the sample and reference compound. In our experiments, quinine sulfate was used as a standard reference, which has a quantum yield of 0.546 in 0.5 M sulfuric acid aqueous solution, when excited at 365 nm.34 Table 1. Photophysical data of ttp-PPESO3 (5 µM with respect to repeating unit) in different solvents.

Figure 2. The plot of normalized absorbance spectra, PL intensity of the ttp-PPESO3 in water (left), under routine laboratory lighting, and its photoluminescence image under UV light (right).

To further investigate the photophysical properties of ttp-PPESO3 in various solvents, DMSO and methanol were used in comparison with water. As shown in Figure 3, both the absorption and emission spectra were blue-shifted in DMSO and methanol than those in water. It is noteworthy to mention that the emission spectrum of ttp-PPESO3 in DMSO and methanol is sharper, with the emergence of a shoulder peak on the right, compared to that in water. This would result from the solubility of the polyelectrolyte in different solvent environments. The thienylene terpyridine (ttp) moiety tends to dissolve in organic solvents, while the sulfonate group dissolves readily in water. Due to the bulk effect of ttp adduct, the resulting polyelectrolyte slightly prefers the DMSO and methanol, which produce narrow emission spectra. The ttp-PPESO3 dissolves in water in an aggregated form. This can be seen from the broadening emission spectrum in the aqueous media.

The quantum yield of ttp-PPESO3 in DMSO and methanol is around 0.20, while the value of 0.05 is lower in water. The quantum yield decrease, plus the large Stokes shift in water suggest that the electronic transition involved might be different. Also, the ground and excited dipole moment, as well as the aggregation state, tends to vary in a high polar solvent like water. Nevertheless, the 5% quantum efficiency in aqueous solution is a typical yield for linear poly(phenylene ethynylene) polyelectrolytes and consistent with that reported in the literature.44 It is sufficient to be used as a chemosensory material, especially, having the advantage of being dissolved in water. Since water is the green solvent of choice, all cation screening experiments are conducted in aqueous media. Cation Screening. Twelve representative metal ions (Ca2+, Cd2+, Cu2+, Fe2+, Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Zn2+) plus H+ cation were selected as representative analytes in this study. The cation titrations were carried out the same way as those reported in previous studies.7 All the measurements were performed three times, and negligible variations were found. The average values of the flu-


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orescence intensity (I) and the intensity change (I0/I) at the emission maximum upon titrating different cations were collected as shown in Figure 4. Both the ttp-PPESO3 polyelectrolyte and analyte concentrations were held at 5 µM. It is noteworthy that the fluorescence of ttp-PPESO3 was quenched dramatically by Cu2+, only about 17% of the initial intensity remained; while the other cations slightly reduced the fluorescence (Figure 4a). The quenching effect of different cations could be vividly seen through the photoluminescence intensity change I0/I (Figure 4b). Cu2+ resulted in 5.61-fold fluorescence change; whereas the second responsive cation Ni2+ only caused 1.14-fold intensity quench. For a real sensory application, it is important to consider the interference of a variety of metal ions other than the one of interest. As shown in Figure S3 in the supporting information, control experiments were conducted in the absence and presence of Cu2+. Cuvette (a) contains the ttp-PPESO3 aqueous solution. Cuvette (b) contains ttpPPESO3 in the presence of all thirteen selected cations mentioned above, and cuvette (c) contains ttp-PPESO3 in the presence of twelve cations but without Cu2+. The cuvette (b) and (c) show a large difference in fluorescence quenching response, demonstrating that copper (II) ions are specifically distinguished. no cations 2+ Ca 2+ Cd 2+ Cu 2+ Fe + H 2+ Hg + K + Li 2+ Mg 2+ Mn + Na 2+ Ni 2+ Zn

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To further quantify the quenching efficiency of the ttpPPESO3, the fluorescence intensities of the polyelectrolyte and the concentrations of copper were fitted into the Stern-Volmer equation listed below: 1 + [ ] where I0 and I are the PL intensity of the polyelectrolyte in the absence and presence of a quencher, copper ion in this case; and Ksv is the Stern-Volmer quenching constant. The fitting is shown in the inset graph in Figure 5. The linear regression equation is I0/I = 1.1021+ 1.6318×10-6 [Cu2+] (mol/L), with a correlation coefficient R2 = 0.9932. Thus, the Ksv was obtained to be 1.63×106 L/mol. The intercept does not start from 1 on the y-axis in our regression equation, which might result from certain form of superquenching mechanism in operation.30 The limit of detection (LoD) for copper ions was determined to be 1.47×10-8 M, or 0.93 parts-per-billion (ppb). It is defined by the equation LoD = 3 σ/s, where σ is the standard deviation of the blank signals; and s is the slope of the calibration curve. The high sensitivity of ttp-PPESO3 is capable of screening trace copper under the EPA standard (1.3 ppm). 100%

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Sensitivity. To evaluate the efficiency of ttp-PPESO3 as a Cu2+ sensor probe, the fluorescence titration spectra were recorded, as shown in Figure 5 and Figure S4. The emission intensities of ttp-PPESO3 were decreasing constantly as a function of the Cu concentrations. The fluorescence maxima intensities at 530 nm were analyzed as there was no obvious shift in PL wavelength. At low Cu2+ concentrations (0 – 1.2 ×10-6 M), the fluorescence quenching of ttpPPESO3 increased linearly. Above this range, the quenching percentage increased steeply until 20 µM Cu2+ was added, where the fluorescence was quenched by 90%. Then the quenching trend slowed down due to partial saturation of copper ions around the ttp-PPESO3.

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Figure 5. Fluorescence quenching of the ttp-PPESO3 to a wide concentration range of Cu2+. The quenching percentages were calculated according to the ratio of the remainder fluorescence to the initial fluorescence [(I0-I)/I0] ×100%. The inset graph is the Stern-Volmer plot at low Cu2+ concentrations.


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Binding site investigation. The ttp-PPESO3 polyelectrolyte has seen outstanding selectivity and sensitivity towards copper ions. The question is where exactly the polyelectrolyte binds to the Cu2+. The ttp-PPESO3 has three potential binding sites that could interact with the analytes: 1) the ttp receptor; 2) the SO32- charged moiety; and 3) the S atom on the thiophene ring. The ttp receptor alone does not provide a good selectivity, as it is known in the literature that the polymer PPETE with the ttp functional group displayed response to several transition-metal ions, including Ni2+, Cu2+, and Mn2+.39 Due to the varying chelating ability of the terpyridine receptor with different transition metals, ttp itself might not correspond to the high selectivity to Cu2+. A second example, the polymer PPESO3 with no other functional groups has been demonstrated to show quenching response to Cu2+, because of the strong electrostatic interaction as well as the electron transfer between the polymer and copper ions.30, 45 However, the limit of detection for Cu2+ with that PPESO3 sensor30 was reported to be 3.0×10-7 M, which is higher than that of ttpPPESO3 (1.47×10-8 M). The ultra-sensitivity (low LoD) of our polyelectrolyte indicates that the binding may involve, but not limit to the sulfonate groups. Thus, we suspect that the increased selectivity and enhanced sensitivity of ttpPPESO3 could result from the coordination interaction of the ttp receptor and the SO32- charged moieties. Terpyridine coordinating with sulfonate, this multi-dentate feature of the ttp-PPESO3 creates a tailored “cave” for the analyte. The analyte of interest is expected to fit in with the right size and structure. In this case, ttp-PPESO3 is selected for copper (II) ions. The detection mechanism results from that Cu2+ being a d9 system behaves distorted octahedral geometry on account of Jahn-Teller effect. The resulting increase in non-radiative decay upon binding and the resulting quenching are illustrated in Scheme 2.46

Scheme 2. Illustration of energy transfer migration induced fluorescence quenching through a conjugated polymeric sensory system. Molecular modeling was employed to confirm our hypothesis. The structures of the complexes formed between the ttp-PPESO3 repeating unit and the Cu2+ were established through molecular mechanical optimization in Spartan. During the simulation, the water molecules have been omitted for clarity, although they might slightly lower the molecular energy due to the stabilization of solvation process. When copper coordinates with both the ttp receptor and the sulfonate group (Figure 6), the minimized energy

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(Emin) of this complex is approximately 625.80 kJ/mol. There are four other binding models together with their optimized structures listed in Figure S5 in the Supporting Information. The minimized energy conformations increase in the following order: ttp and SO32- binding (625.80 kJ/mol) < ttp binding (655.28 kJ/mol) < S atom binding (747.23 kJ/mol) < SO32- binding (833.82 kJ/mol) < S atom and SO32- binding (838.48 kJ/mol). As can be clearly seen that the ttp and SO32- binding has the lowest energy among these models. It is the most stable and dominant form when the polyelectrolyte traps a copper cation. Therefore, it is highly likely that the modeled binding event takes place with three nitrogen atoms and one oxygen. Due to the amorphous nature of polyelectrolyte, direct evidence from spectroscopic studies hasn’t been achieved. However, such binding behavior has been reported in small crystalline complexes.47-48 The crystal structures, in both copperterpyridine complex47 and copper-phytate-terpyridine complex,48 show that the copper ion coordinates to 3 nitrogen atoms of the terpyridine unit and 1 oxygen atom of the sulfate or the phosphate groups. These theoretical simulations and crystal structures from molecular analogs shed important insight into which parts of the ttp-PPESO3 function to detect the analytes. The coordination of SO32- and the tridentate ttp group increases the denticity from 3 to 4. The additional dentate provides exceptional chelating ability to bind metals. This unique binding mode may find use in future molecular design for polymer chemosensory materials to meet with specific sensing requirements.

Figure 6. Computation modeling of the proposed binding event involving the ttp and sulfonate moieties (circled in green) of the ttp-PPESO3 and Cu ions. Atoms coding as follows: Carbon: grey; Copper: green; Oxygen: red; Nitrogen: blue-grey; Sulfur: yellow; Sodium: yellow (small); Hydrogen: white.

CONCLUSION The new FCPE, ttp-PPESO3, with the PPETE conjugated backbone, terpyridine tridentate receptor and negatively charged sulfonate groups was synthesized and characterized. It has intense green emission in aqueous media. It is highly selective to copper and is capable of differentiating Cu2+ in the presence of 12 other cations. The detection lim-


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it of ttp-PPESO3 to Cu2+ is 0.93 ppb, far below the EPA action level (1.3 ppm). Computation studies suggest that the binding occurs via a coordination interaction of the terpyridine and sulfonate. The additional dentate from the sulfonate offers remarkable chelating ability to copper ions. This unique binding mode can be expanded and will provide insight to design future more sensitive and selective chemosensory systems.

ASSOCIATED CONTENT Supporting Information The monomer synthesis, Infrared characterization, interference studies, and molecular modeling. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to thank Susan Sonner from Corning Inc. for the GPC measurements. The research was supported by the Army Research Office (ARO) W911NF1310235. Additional funding was provided by the Joint Science and Technology Office for Chemical Biological Defense (JSTO-CBD) under contract BA13PHM210. The Bruker Advanced III-600 is funded by grant CHE-0922815.

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for TOC only


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A Facile Synthesis of Fluorescent Conjugated Polyelectrolytes using

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