A Water-Soluble Organometallic Conjugated ... - ACS Publications

Dec 31, 2010 - †Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University,. Waterloo Road, Kowloon Tong, ...
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Macromolecules 2011, 44, 483–489

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DOI: 10.1021/ma102373y

A Water-Soluble Organometallic Conjugated Polyelectrolyte for the Direct Colorimetric Detection of Silver Ion in Aqueous Media with High Selectivity and Sensitivity Chuanjiang Qin,† Wai-Yeung Wong,*,† and Lixiang Wang*,‡ †

Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China, and ‡State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China Received October 18, 2010; Revised Manuscript Received December 10, 2010

ABSTRACT: A water-soluble organometallic conjugated polyelectrolyte P1 and its corresponding model complex M1 based on an aspartic acid-substituted fluorene spacer are reported, which possess good water solubility as well as intriguing fluorescent and phosphorescent dual-emissive properties in a completely organic-free aqueous medium at room temperature. A new colorimetric silver ion sensor based on P1 is developed, which shows high selectivity and sensitivity for Agþ ions in buffered water solution because of the Agþ-induced intersystem crossing from the singlet to triplet states. The obvious color change from colorless to yellow upon exposure to Agþ ion is visible to the naked eyes and can be quantified colorimetrically by the visible absorption spectroscopic method. On the basis of the fluorescence intensity of P1 obtained in the fluorescence titration curves, a linear relationship is observed in the Stern-Volmer plot at low concentrations (1-5 μM), and the corresponding Stern-Volmer quenching constant (KSV) of 1.9  105 M-1 for P1 is comparable to that obtained from the fluorescence titration studies. As determined by the Benesi-Hildebrand plot obtained from the absorption spectra, a 1:1 complex formation is anticipated between the Pt compound and Agþ ion. The limit of detection is low at 0.5 μM, i.e., at concentrations in the ppb range. The present study represents an original approach using a water-soluble organometallic conjugated polyelectrolyte for the accurate and rapid detection of trace amounts of Agþ ion in pure water. It also establishes a new system featuring dual-emissive properties of platinum(II) acetylide-based conjugated polymers for chemosensing application.

*Corresponding authors. E-mail: [email protected] (W.-Y.W.); [email protected] (L.W.).

certainly important for practical applications in environmental and biological assays in aqueous media.6 Conjugated polyelectrolytes (CPEs) have found wide applications in chemo- and biosensing technology because they combine the amplified quenching of fluorescent conjugated polymers with the good water solubility and processability and ionic nature of the polyelectrolytes.7 However, most of the works in this field have proliferated largely in conjugated materials consisting of organic building blocks, and only a few examples of metal ion sensors (excluding those for Agþ ion, however) using watersoluble organometallic conjugated polyelectrolytes can be found in the literature.8 In fact, platinum(II) acetylide-derived conjugated polymers have attracted much attention due to their rich room-temperature phosphorescence properties with potential applications in various research domains.9 Although previous studies have demonstrated that triplet excitons are much less delocalized in the conjugated backbone as compared to singlet excitons, their long exciton lifetimes and inter- or intrachain triplet exciton migration can lead to the development of promising chemosensing materials with high sensitivity.8b We can also observe fluorescent and phosphorescent dual emissions simultaneously in a single conjugated polymer through the copolymerization of organic chromophores with platinum acetylide conjugated segments that would facilitate efficient intersystem crossing (ISC).10 Interestingly, no published report involving the dualemissive properties of platinum(II) acetylide-based conjugated polymers for chemosensing application is yet available, even though a dual-emissive-materials design concept has recently

r 2010 American Chemical Society

Published on Web 12/31/2010

1. Introduction Silver ion is an environmental pollutant of great public concern and has adverse biological effects on human health.1 Thus, there is an upsurge of interest in the development of rapid and selective detection method of Agþ ions in water or food resource within the scope of health science. Various methods, such as atomic absorption spectroscopy, inductively coupled plasma-mass spectroscopy, and electrochemical methods based on ion-selective electrodes, have been developed in such research endeavor.2 However, most of these procedures are relatively time-consuming and not cost-effective in practice. On the other hand, fluorimetric methods are widely used for bioimaging and sensing applications owing to their high sensitivity, rapid response, and facile detection by purely optical means.3 To date, there are a handful of literature reports on using molecular probes as fluorescent chemosensors for Agþ ion that are only applicable in organic or mixed aqueous-organic solutions,4 and to our knowledge, only one example is presently known in exploiting conjugated organic polymers for Agþ ion sensing in nonaqueous medium.4k Recently, one successful example was reported by using a dye-labeled silverspecific oligonucleotide and graphene oxide in a buffer solution and river water.5 Even that, there is no precedented report of using water-soluble conjugated polymers (either organic or organometallic in nature) for the accurate and rapid detection of trace amounts of Agþ ion in pure water, and this research frontier is

pubs.acs.org/Macromolecules

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Macromolecules, Vol. 44, No. 3, 2011 Scheme 1. Chemical Structures of P1 and M1

been put forward by Fraser et al.11 In fact, silver nitrate was shown to be a good candidate to function as a heavy-atominduced room-temperature phosphorescent reagent.12 This raises up a simple question here: Can we make use of the properties of both worlds (i.e., platinum acetylide-based conjugated polymer and Agþ ion) together to develop a new probe for Agþ ion sensing? In order to establish a selective and sensitive Agþ analyte sensor in an aqueous medium, the integration of water-soluble fluorene unit and platinum acetylide moiety appears to be a wise choice. It is also well-known that polyfluorene CPEs are promising class of materials.13 Here, we report for the first time a novel water-soluble platinum acetylide-based CPE P1 (Scheme 1) based on an aspartic acid-substituted fluorene spacer possessing high electron density and dendritic structure as the water solubilizer which can serve multiple functions: good water solubility coupled with strong fluorescence and phosphorescence as well as selective and colorimetric response to Agþ ion in water. The corresponding dinuclear model complex M1 was also prepared for comparison studies. 2. Results and Discussion 2.1. Synthesis and Characterization. The neutral diester precursor polymer of P1 was obtained from the Sonogashira coupling reaction between [Pt(PMe3)2Cl2] and the diethynyl ligand (Scheme 2).8-10 This precursor polymer is soluble in organic solvents such as CHCl3 and THF, and its molecular weight was characterized by gel permeation chromatography (GPC) using polystyrene standards (Mn = 26 340 g mol-1 and Mw = 62 360 g mol-1 with polydispersity index PDI = 2.36). The anionic water-soluble CPE P1 was obtained by hydrolysis of the ester using 2 M KOH solution as a base, and the desired polymer was purified by dialysis against Milli-Q water using an 8 kDa molecular weight cutoff dialysis membrane. The final polyelectrolyte P1 containing ionic side groups was characterized via 1H NMR, 31P NMR, and IR spectroscopies. 2.2. Photophysical Characterization. As shown in Figure 1, P1 exhibits an absorption peak at 390 nm due to the longaxis polarized π,π* transition of platinum acetylide backbone in water. The emission from P1 features two bands with λmax at 405 and 540 nm in degassed water (Φem = 0.23%). The high-energy band belongs to the fluorescence from 1π,π* state (τF = 0.8 ns), whereas the low-energy one is due to the

Qin et al.

phosphorescence from 3π,π* manifold (τP = 11.3 μs). Both the absorption and emission spectra of P1 are similar to those found for common organic-soluble fluorene-based platinum acetylide polymers in CH2Cl2.9c The observation of intense fluorescent and phosphorescent dual emissions (peak intensity ratio ∼1:1) for P1 at room temperature in water is a manifestation of the fact that it has a good water solubility, and the H2O molecule could increase the ISC efficiency from the singlet to the triplet states.14 Previous studies demonstrated that the absorption and PL spectra of CPEs are pH-dependent because the pH of water solution can influence the charge amount of these anionic CPEs.7e,15 A series of absorption and PL spectra of our dendronized polymer were recorded in 5 mM phosphate buffer solutions at pH values ranging from 3 to 11 (at the pH interval of 2). As shown in Figure S1a of the Supporting Information, the absorption peak of P1 is slightly red-shifted from 390 nm at pH = 11 to 393 nm at pH = 3. Simultaneously, its peak intensity is reduced by almost half. This is due to the protonation of the carboxylate groups which results in an aggregation of conjugated polymer chains.15c From the pH dependence of PL spectra of P1 as shown in Figure S1b of the Supporting Information, when the pH value is 7, its fluorescent emission maximum can be kept quite stable, but the intensity of phosphorescent emission decreased to about 31% of its original intensity. When the pH value is 3, the fluorescent emission maximum slightly decreased with a red shift of 12 nm. However, the phosphorescent emission intensity was only about 4% of the original one when the pH value decreased from 11 to 3, and the peak position is only redshifted by 3 nm. Such result would be expected since aggregation of the conjugated polymer chains typically exerts a stronger effect on the fluorescent peak position than that for the phosphorescence, but for the emission intensity, the observation is just the opposite. This phenomenon was also demonstrated in poly(p-phenyleneethynylene)-type Pt-acetylide-based CPE.8a 2.3. Silver Ion Sensing Studies. Our initial investigations were focused on the response of P1 to different metal ions based on the absorption spectra. As illustrated in Figure 2, upon addition of an excess amount of each metal ion (10 μM), i.e., Ca2þ, Cd2þ, Cu2þ, Co2þ, Mg2þ, Ni2þ, Zn2þ, Pb2þ, Fe3þ, Liþ, and Csþ, only a very slight decrease in intensity of the absorption peak (at 390 nm) was noted. When 10 μM Hg2þ was added into the solution of P1, the absorption intensity decreased dramatically with a hypsochromic shift of about 12 nm in absorption wavelength. In a sharp contrast, the addition of 10 μM Agþ resulted in a distinct intensity change and a large bathochromic shift of 25 nm. At the same time, we also examined the PL response of P1 to different metal ions in degassed water (Figure S2 of the Supporting Information). For most cations, they quenched both fluorescence and phosphorescence of P1 to some extent. While the addition of 10 μM Hg2þ induced a 6-fold fluorescence quenching, the intensity of phosphorescence increased slightly. All of the above metal ions mentioned could not induce a change of the emission peak position of P1, no matter for the fluorescence or the phosphorescence. However, akin to the absorption spectral changes of P1, both the fluorescent and phosphorescent peak positions experienced obvious red shifts (40 and 30 nm, respectively) on adding 10 μM Agþ into the polymer solution. The relative intensity of the two kinds of emission (IF: IP) changed from 1.4 to 0.5. Especially, when [Agþ] is 1 μM, IF decreased while IP increased with a red shift in wavelength of 5 nm (Figure S3 of the Supporting Information), and τP changed from 11.3 μs in the absence of Agþ to 12.7 μs at [Agþ] = 1 μM. Such an

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Scheme 2. Synthetic Routes of P1 and M1

increase in τP upon Agþ binding was also observed previously in the phosphorescence signal from azurin.16 These results indicate that Agþ could promote the ISC from the singlet to triplet states, and the ISC efficiency is high even at such a low concentration, presumably due to the coordinative interactions of Agþ ion with the conjugated backbone at a molecular level.17 Meanwhile, τP decreased to 6.1 μs at [Agþ] > 5 μM. Thus, the effect of heavy Agþ ion on the

lifetime of phosphorescence becomes rather complicated which depends on the solution concentration and viscosity and temperature.16 Indeed, this represents the first literature study involving the possible utilization of dual-emissive properties of water-compatible platinum(II) acetylide-based conjugated polymers for chemosensing application. This can be further confirmed from the spectral and NMR titration studies for the model compound M1 (Figure S4 of the

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Figure 1. Absorption and PL spectra of P1 and M1 in degassed water (concentration = 5  10-6 M). Figure 4. Partial 1H NMR (400 MHz) spectral changes of M1 (a) in D2O and upon addition of (b) 0.5 equiv of AgNO3 and (c) 1.0 equiv of AgNO3 at 298 K.

Figure 2. Absorption spectra of P1 in the presence of various metal ions in degassed water. [P1] = 5  10-6 M. [Metal ion] = 1  10-5 M.

Figure 3. Benesi-Hildebrand plot of M1 and Agþ in degassed water.

Supporting Information). Notably, the absorption maximum of M1 gradually diminished in intensity and red-shifted in wavelength upon the addition of Agþ ion. To provide a possible explanation of why Agþ is the only metal that induces a change in color, the model complex M1 was used for the study of its absorption spectral changes upon binding with Agþ. Most likely, M1 and Agþ formed a 1:1 complex as determined by the Benesi-Hildebrand plot obtained from the absorption spectra (see Figure 3). Such apparent absorption spectral changes indicate that the heavy Agþ ion binds with the electron-rich amino acid pendant of M1 of each fluorene unit.18 The idea of using amino acids stems from the fact that they are readily susceptible to chemical substances involving metal ions such as Agþ ion.18e It is known that

silver-amine complexes form readily in aqueous solution because of their large formation constants and the preference of Agþ for linear coordination.18a-c The interaction between CdO group and Agþ ion is also possible.18e 1H NMR spectra of M1 in the presence of different concentrations of Agþ ion demonstrated that the signal at 1.9 ppm due to protons of the amino acid pendants was gradually splitted and broadened upon increasing the amount of Agþ ion (Figure 4). This further suggests the coordination of M1 with Agþ and, hence, provides a good evidence for the interactions between P1 and Agþ. We found that the selectivity of P1 for Agþ over other metal ions is high for both P1 and M1 systems. Remarkably, we can detect Agþ ion via a unique absorption spectral change which would not be interfered by the severe oxygen quenching problem commonly encountered using the phosphorimetric analytical methods and also makes the detection procedures simpler and quicker (e.g., no need for careful degassing of the solution before measurements). While M1 shows a similar chemosensing function as P1, P1 has a much broader detection range (0.5 μM-2 mM) than M1 (1-35 μM). Simultaneously, P1 shows a clear visual color change upon adding Agþ; however, the color of M1 solution displayed no virtual change under such condition. So, polymer P1 appears to possess a better silver sensory property than model compound M1. Next, cross-contamination experiments were performed to further gauge the selectivity for Agþ ion over other metal ions. By monitoring the wavelength change of absorption maximum (Δλ), Agþ can be distinguished apparently from other metal ions. From Figure 5a, other competitive metal ions such as Hg2þ, Pb2þ, and Cu2þ have very little impact on the Δλ value of P1 and Agþ solution mixture. These results clearly illustrate that P1 shows an excellent selectivity for Agþ ion in the presence of other interfering metal ions. More importantly, the color change of P1 solution from colorless to yellow upon exposure to Agþ was readily visible to the naked eyes. From Figure 5b, the color of aqueous solution containing 5 μM P1 did not show any change upon addition of an excess of each metal cation (10 μM), i.e., Ca2þ, Cd2þ, Cu2þ, Co2þ, Mg2þ, Ni2þ, Zn2þ, Pb2þ, Fe3þ, Hg2þ, Liþ, and Csþ. However, the color of solution changed promptly to yellow as Agþ was added to it. Meanwhile, the yellow color gradually deepened with increasing [Agþ]. Actually, the color of P1 solution had significantly changed at [Agþ] g 3 μM. We also investigated the possible interference owing to the counteranion(s) used, and it was shown that different anions have almost no interaction with P1 in water. These

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of Agþ ion (Figure 6a). The response range to [Agþ] is very broad from 1 μM to 4 mM. Since the absorption maximum showed a gradual red shift upon adding Agþ ion at the concentration range of 1 μM to 4 mM, we can also study the relationship between [Agþ] and Δλ to quantify the response of P1 to a given amount of Agþ ion. P1 exhibited a linear relationship within the [Agþ] range employed (1 μM to 2 mM, see Figure 6b). The limit of detection (LOD) is about 0.5 μM, i.e., at concentrations in the ppb range. These results reveal that there exists a wider linear response range to [Agþ] via a change of the absorption wavelength in the present system. Besides the colorimetric detection, P1 allows for fluorescence detection of Agþ. We also can quantify the response of P1 to a given amount of Agþ by the Stern-Volmer (SV) equation19 because the fluorescence signal also gradually decreased in intensity after adding a low concentration of Agþ, and so Agþ ion could serve as a quencher. The SternVolmer equation becomes Figure 5. (a) Graph showing the wavelength change of the absorption maximum (Δλ) of P1. Gray bars represent the wavelength change in the presence of metal ions. Black bars represent the change in the presence of the indicated metal ions, followed by 1 equiv of Agþ. (b) Color change of P1 in the presence of various representative metal ions: 0, blank; 1, Agþ; 2, Hg2þ; 3, Pb2þ; 4, Mg2þ; 5, Csþ; 6, Liþ; 7, Cd2þ; 8, Ca2þ; 9, Fe3þ; 10, Ni2þ; 11, Cu2þ; 12, Zn2þ; 13, Co2þ. [P1] = 5  10-6 M. [Metal ion] = 1  10-5 M. Solvent: H2O.

I0 =I ¼ 1 þ KSV ½Q

ð1Þ

where I0 and I are fluorescence intensities observed in the absence and presence of the quencher, respectively. [Q] is the quencher concentration, and KSV is the SV quenching constant. On the basis of the fluorescence intensity of P1 obtained in the fluorescent titration curves (Figure S3a), the SV plot could be obtained which displayed a linear relationship at low concentrations (1-5 μM) (Figure S3b). The corresponding KSV value of P1 was determined to be 1.9  105 M-1 (R2 = 0.999 78). Therefore, both absorption and fluorescence detection limits of P1 toward Agþ are as low as ppb range. 3. Conclusions In summary, we describe a water-soluble fluorene-based platinum(II) acetylide conjugated polyelectrolyte, which shows attractive water solubility as well as fluorescent and phosphorescent dual-emissive properties in a completely organic-free aqueous medium at room temperature. This new system demonstrated remarkably high selectivity and sensitivity for Agþ ion because of the associated Agþ-induced ISC from the singlet to triplet states. Saliently, a selective color modulation was observed in the presence of Agþ and the Agþ-responsive coloration change from colorless to yellow is readily visible to the naked eyes and can be quantified using visible absorption spectroscopy. The present work definitely opens a new door to develop a sensitive and quick absorption spectrometric method for Agþ ion sensing in pure water, which is in complete contrast with the commonly used method relying on emission spectral changes. In addition, the detection of Agþ ion via such a unique absorption spectral change would be simpler and quicker than the typical phosphorimetric analytical methods because it would not be restricted by the undesirable oxygen quenching experienced for the phosphorescence emission. Work is still underway to better understand the detailed silver binding mechanism involved for the chemosensing phenomenon.

Figure 6. (a) Absorption spectral response of P1 upon addition of Agþ ions in water and (b) the change of absorption maximum as a function of Agþ concentration. [P1] = 5  10-6 M. Solvent: H2O.

results render P1 a highly selective and sensitive chemosensory material for Agþ ion in a purely aqueous medium. 2.4. Titration Experiments. An absorption titration study of Agþ ion was conducted using a 5 μM solution of P1 in water. As expected, the absorption maximum of P1 decreased in intensity and was red-shifted upon the addition

4. Experimental Section General Information. All reactions were performed under nitrogen. Solvents were carefully dried and distilled from appropriate drying agents prior to use. The water used in all experiments was prepared in a Millipore Milli-Q plus purification system and displayed a resistivity of g18.2 MΩ cm-1. Commercially available reagents were used without further purification unless otherwise stated. All reactions were monitored by thinlayer chromatography (TLC) with Merck precoated aluminum

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plates. Flash column chromatography was carried out using silica gel obtained from Merck (230-400 mesh). Fast atom bombardment (FAB) mass spectra were recorded on a Finnigan MAT SSQ710 system. Proton NMR spectra were measured in CDCl3 or D2O on a Varian Inova 400 MHz FT-NMR spectrometer; chemical shifts were quoted relative to tetramethylsilane. Physical Measurements. UV-vis spectra were obtained on a Cary-300 spectrophotometer. The photoluminescent properties and lifetimes of the compounds were probed on the Photon Technology International (PTI) Fluorescence Master Series QM1 system. Solutions of M1 and P1 were deoxygenated by bubbling with argon for 30 min prior to all measurements. The photoluminescent quantum yields were determined in degassed water at 293 K against quinine sulfate in 1 N sulfuric acid as a reference standard (Φem = 0.546).20 Fluorescence Titration Procedure. A 10  10 mm quartz curet was used for recording the solution spectra, and the emission spectrum was collected at 90° relative to the excitation beam. The solutions of polymer P1 and model compound M1 were successively diluted to a concentration of 5  10-6 M in water (the concentration of polymer is based on the number of moles of the repeating unit). Solutions of perchlorate or nitrate salts of different metal ions with the concentration of typically 5  10-3 M were prepared by successive dilution in water. Fluorescence quenching experiment was carried out by sequentially adding small aliquots of stock solutions of metal salts to 3.00 mL of the polymer solutions by using a calibrated microliter pipet. The solutions were vibrated and bubbled with argon for 30 min prior to obtaining the fluorescence spectra. The pH-dependent experiment was carried out by adding 30 μL of P1 or M1 (5  10-4 M) to a 3 mL solution at different pH, and then the solutions were vibrated and bubbled with argon for 30 min prior to obtaining the fluorescence spectra. Material Synthesis. All chemicals and reagents were used as received from commercial sources without further purification. Solvents for chemical synthesis were purified according to the standard procedures. All chemical reactions were carried out under an inert atmosphere. Compounds 115c and [Pt(PMe3)2Cl2]21 were synthesized as previously described. Synthesis of Compound 2. To a degassed ice-cooled mixture of compound 1 (0.726 g, 1.0 mmol) in dry triethylamine (10 mL) and CH2Cl2 (10 mL) was added CuI (20 mg), triphenylphosphine (20 mg), and Pd(OAc)2 (60 mg). After the solution was stirred for 30 min, trimethylsilylacetylene (0.57 mL, 4.0 mmol) was then added, and the suspension was stirred for 30 min in an ice bath before being warmed to room temperature. After reacting for 30 min at room temperature, the mixture was heated to 60 °C for 24 h under a nitrogen atmosphere. The solution was then allowed to cool to room temperature, and the solvent mixture was evaporated in vacuo. The crude product was purified by column chromatography on silica gel with a solvent combination of hexane/ethyl acetate (2:1, v/v) as eluent to provide 2 as a white solid (0.65 g, 85%). 1H NMR (400 MHz, CDCl3, δppm): 7.61 (d, J = 8.0 Hz, 2H, Ar), 7.51 (s, 2H, Ar), 7.48 (dd, J = 1.3 and 8.0 Hz, 2H, Ar), 7.37 (d, J = 8.4 Hz, 2H, NH), 4.95 (m, 2H, CH), 3.75 (s, 6H, CH3), 3.66 (s, 6H, CH3), 3.26 (d, J = 13.6 Hz, 2H, CCH2), 3.02 (dd, J = 4.4 and 16.8 Hz, 2H, CHCH2), 2.81 (dd, J = 4.4 and 16.8 Hz, 2H, CHCH2), 2.43 (d, J = 13.6 Hz, 2H, CCH2), 0.25 (s, 18H, SiMe3). 13C NMR (100 MHz, CDCl3, δppm): 171.38, 171.08, 170.34 (CdO), 148.85, 138.85, 132.20, 127.89, 122.41, 120.33 (Ar), 105.30, 94.97 (CtC), 52.89, 52.12 (alkyl), 50.46 (C9-fluorene), 48.39, 42.61, 36.07 (alkyl), 0.05 (SiMe3). FAB-MS: m/z 761.3 (Mþ). IR (KBr): ν(CtC) 2135 cm-1. Synthesis of Compound 3. A solution of 2 (0.38 g, 0.5 mmol) in THF (10 mL) was stirred, and then 1.5 mL of 1 M n-Bu4NF in THF was added at room temperature under nitrogen for 30 min. Solvent was removed under reduced pressure to leave a gray white residue. The residue was purified by column chromatography on silica gel using hexane/ethyl acetate (1:1, v/v) as eluent

Qin et al. to afford a white solid of 3 (0.28 g, 90%). 1H NMR (400 MHz, CDCl3, δppm): 7.66 (d, J = 8.0 Hz, 2H, Ar), 7.57 (s, 2H, Ar), 7.51 (dd, J = 1.3 and 8.0 Hz, 2H, Ar), 7.33 (d, J = 8.4 Hz, 2H, NH), 4.95 (m, 2H, CH), 3.75 (s, 6H, CH3), 3.67 (s, 6H, CH3), 3.22 (d, J = 13.6 Hz, 2H, CCH2), 3.11 (s, 2H, CtCH), 3.00 (dd, J = 4.8 and 17.2 Hz, 2H, CHCH2), 2.82 (dd, J = 4.8 and 17.2 Hz, 2H, CHCH2), 2.50 (d, J = 13.6 Hz, 2H, CCH2). 13C NMR (100 MHz, CDCl3, δppm): 171.27, 171.02, 170.24 (CdO), 148.96, 138.98, 132.27, 128.16, 121.37, 120.37 (Ar), 83.83, 77.91 (CtC) 52.83, 52.07 (alkyl), 50.43 (C9-fluorene), 48.42, 42.48, 36.05 (alkyl). FAB-MS: m/z 617.1 (Mþ). IR (KBr): ν(CtC) 2124 cm-1. Synthesis of P1 Precursor. A solution of 3 (123.3 mg, 0.2 mmol), [Pt(PMe3)2Cl2] (83.6 mg, 0.2 mmol), and CuI (2 mg) in a mixture of CH2Cl2/Et3N (10 mL, 1:1, v:v) was degassed with nitrogen for 10 min. The reaction mixture was stirred at room temperature for 24 h. The solvent was evaporated under vacuum, and the product was purified by silica gel chromatography using CH2Cl2 as the eluent. The product was obtained in 70% yield (136 mg). 1H NMR (400 MHz, CDCl3, δppm): 8.43 (br, 2H, Ar), 7.60 (br, 2H, Ar), 7.18 (br, 2H, Ar), 4.62 (br, 2H, CH), 3.63 (d, J = 8.4 Hz, 12H, CH3), 2.73-2.50 (br, 8H, alkyl), 1.72 (br, 18H, PCH3). 31P NMR (75 MHz, CDCl3, δppm): -20.24 (JPt-P = 2277 Hz). GPC (THF, polystyrene standards): Mn = 26 340, Mw = 62 360, PDI = 2.36. Synthesis of P1. A solution of P1 precursor (98 mg, 0.1 mmol), 1,4-dioxane (10 mL), and 2 M aqueous KOH (5 mL) was degassed for 30 min and stirred at 100 °C for 12 h. After cooling to room temperature, the solvent was evaporated under vacuum. The residue was redissolved in 20 mL of water and filtered through a 0.22 μm cellulose membrane. The filtrate was dialyzed against deionized water using a 8000 molecular weight cutoff cellulose for 3 days, and water was changed every 6 h. The polymer was isolated by freeze-drying of the water as a yellow solid in 65% yield (69 mg). 1H NMR (400 MHz, D2O, δppm): 7.58 (br, 2H, Ar), 7.42 (br, 2H, Ar), 7.34 (br, 2H, Ar), 3.87 (br, 2H, CH), 3.54 (br, 1H, CHCH2), 2.96 (br, 3H, CHCH2), 2.00 (br, 4H, CCH2), 1.73 (br, 18H, PCH3). 31P NMR (75 MHz, D2O, δppm): -20.37 (JPt-P = 2135 Hz). Synthesis of Compound 4. A solution of [Pt(PMe3)2Cl2] (500 mg, 1.1 mmol) and 100 μL of phenylacetylene (1.1 mmol) in a mixture of CH2Cl2/Et3N (12 mL, 3:1, v:v) was degassed with nitrogen for 10 min. The reaction mixture was stirred at room temperature for 3 h. The solvent was evaporated under vacuum, and the product was purified by silica gel chromatography using toluene/ethyl acetate mixture as an eluent. The product was obtained in 38% yield (180 mg). 1H NMR (400 MHz, CDCl3, δppm): 7.31-7.15 (m, 5H, Ar), 1.70-1.61 (m, 18H, CH3). Synthesis of M1 Precursor. A solution of 3 (61.7 mg, 0.1 mmol), 4 (107 mg, 0.22 mmol), and CuI (5 mg) in a mixture of CH2Cl2/ Et3N (20 mL, 1:1, v:v) was degassed with nitrogen for 10 min. The reaction mixture was stirred at room temperature for 12 h. The solvent was evaporated under vacuum, and the product was purified by silica gel chromatography using toluene/ethyl acetate (10:1, v/v) as the eluent. The product was obtained in 70% yield (81 mg). 1H NMR (400 MHz, CDCl3, δppm): 7.49-7.11 (m, 18H, Ar and NH), 4.89 (m, 2H, CH), 3.73 (s, 6H, CH3), 3.65 (s, 6H, CH3), 3.12 (d, J = 13.6 Hz, 2H, CCH2), 2.98 (dd, J = 4.4 and 16.8 Hz, 2H, CHCH2), 2.74 (dd, J = 4.4 and 16.8 Hz, 2H, CHCH2), 2.55 (d, J = 13.6 Hz, 2H, CCH2), 1.78 (m, 36H, PCH3). 31P NMR (75 MHz, CDCl3, δppm): -20.40 (JPt-P = 2298 Hz). FAB-MS: m/z 1511.7 (Mþ). Synthesis of M1. A solution of M1 precursor (151 mg, 0.1 mmol), 1,4-dioxane (30 mL), and 2 M aqueous KOH (5 mL) was degassed for 30 min and stirred at 100 °C for 12 h. After cooling to room temperature, the solvent was evaporated under vacuum. The residue was redissolved in 20 mL of water and filtered through a 0.22 μm cellulose membrane. The filtrate was dialyzed against deionized water using a 500 molecular weight cutoff cellulose for 3 days, and water was changed every 6 h. The polymer was isolated by freeze-drying of the

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water as a yellow solid in 80% yield (126 mg). 1H NMR (400 MHz, D 2O, δppm): 7.52-7.16 (m, 16H, Ar) 3.82 (t, J = 5.6 Hz, 2H, CH), 2.90 (m, 4H, CCH2), 1.95 (m, 4H, CHCH 2), 1.64 (m, 36H, PCH3). 31P NMR (75 MHz, CDCl3, δppm ): -20.41 (JPt-P = 2136 Hz).

Acknowledgment. This work was supported by the Hong Kong Baptist University (FRG2/08-09/111), the Hong Kong Research Grants Council (HKBU202709), and State Administration of Foreign Experts Affairs International Partnership Program for Creative Research Teams, Chinese Academy of Sciences. W.-Y. W. also thanks the Croucher Foundation for the Croucher Senior Research Fellowship.

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Supporting Information Available: Photophysical properties and fluorescence quenching studies of P1 and M1. This material is available free of charge via the Internet at http:// pubs.acs.org. (10)

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