Alternating copolymers containing fluorene and oxadiazole derivatives for fluorescent chemosensors

Synthetic Metals 191 (2014) 12–18

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Alternating copolymers containing fluorene and oxadiazole derivatives for fluorescent chemosensors Chin-Yang Yu ∗ , Tsung-Yu Shih Department of Materials Science and Engineering, National Taiwan University of Science and Technology, 43, Section 4, Keelung Road, Taipei 10607, Taiwan

a r t i c l e

i n f o

Article history: Received 1 November 2013 Received in revised form 13 February 2014 Accepted 16 February 2014 Available online 12 March 2014 Keywords: Oxadiazole Bathochromic effect Fluorescence quenching Fluorescent chemosensor Metal ion

a b s t r a c t The novel alternating copolymers containing 9,9-dioctylfluorene and meta-phenylene linked 2-pyridyl1,3,4-oxadiazole or 2-pyrimidyl-1,3,4-oxadiazole were synthesized through a palladium catalyzed cross coupling reaction. The thermal and optical properties of the polymers were substantially affected by the heteroaryl substituted oxadiazole units. The absorption spectra of the copolymers in the solid state exhibited a bathochromic shift when compared to those in solution. Fluorescence quenching as examined using a fluorescence spectrophotometer was found to be directly related to concentration of the metal ions. Due to the photoinduced electron transfer mechanism, both polymer solutions act as a turn-off fluorescent chemosensor. The sensing behavior to various metal ions such as Ni2+ , Cu2+ , Zn2+ and Hg2+ ions revel that the polymers were highly sensitive to both Ni2+ and Cu2+ ions. The results clearly demonstrate that the copolymers can act as a potential sensory material for the detection of metal ions. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

1. Introduction Several heavy metals are hazardous to health or to the environment, therefore the development of sensors displaying both sensitivity and selectivity has been dramatically raised up in recent years [1]. The facile detection and sensitivity of portable sensors are inherently more desirable than performing numerous complex analytical methods such as flame photometry, atomic absorption spectrometry, ion sensitive electrodes and electron microprobe analysis that require sophisticated instruments and are time-consuming. Conjugated polymers have emerged as one of the most promising classes of sensor materials in recent years due to their high sensitivity and selectivity, which enables for detection of a variety of analytes ranging from ions to explosives [2]. Compared to small organic compounds, polymer based chemosensors have numerous advantages such as their simplicity of use, signal amplification, easy device fabrication and a combination of different outputs [3]. Swager [4] demonstrated that conjugated polymer sensors with the receptors connected in conjugation with each other show several advantages over small molecules for sensing applications. Under photon irradiation, the sensor molecule is promoted to an excited state and the resulting exciton can rapidly migrate along the conjugated polymer backbone to a low energy acceptor site, the emission intensity

∗ Corresponding author. Tel.: +886 2 27376525; fax: +886 2 27376544. E-mail address: [email protected] (C.-Y. Yu). http://dx.doi.org/10.1016/j.synthmet.2014.02.012 0379-6779/Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

of conjugated polymers dramatically decreases as a turn-off sensor. The other type of conjugated polymer sensor is based on the fluorescence changes in response to the conformation of the polymer backbone which was reported by Wasielewski and Wang [5]. A bipyridyl group with a dihedral angle of 20◦ between the two pyridine planes in the 2,2-bipyridine system was introduced into the polymer backbone. It was forced into a planar configuration after chelation to metal ions. This resulted in an increase of the conjugation length of the polymer which can be monitored by the fluorescence and absorption spectroscopy. Another approach is utilizing a conjugated polymer backbone with pendants and the concept of a polyreceptor which was demonstrated by Swager’s group [6]. They attached a cyclophane-based receptor onto a poly(para-phenylene-enthynylene) backbone. Upon addition of viologen, a decrease in the fluorescence was observed as well as a 65-fold increase in the selectivity when compared to a small molecule receptor. It should be noted that the receptors are placed at a para position relative to the phenylene ring of the polymer chain which allows an electronic resonance interaction between the receptor and conjugated polymer backbone. Recently, fluorene derived polymers furnished with bipyridine units have been shown to exhibit fluorescence quenching of polymer solutions by the addition of transition metal ions [7]. The influence of the polymer main chain rigidity in metal cation sensing has also been reported. Fluorene and 2,2-bipyridyl containing polymers displayed fluorescence quenching and variations in the UV–vis absorption spectra upon interaction with a wide set of transition metal ions [8]. The higher backbone flexibility of the

C.-Y. Yu, T.-Y. Shih / Synthetic Metals 191 (2014) 12–18

polymer provided the weakest resistance to coplanarity of the two pyridine rings during chelation with metal ions, giving rise to the higher selectivity. 1,3,4-Oxadiazoles have attracted attention due to their unique optical properties and are currently exploited as signaling components in sensory systems [9]. Photoemissive 2,5diaryl-1,3,4-oxadiazoles have found wide-spread applications as electronic and photonic materials [10] and their application as the signaling component in molecular sensory systems have recently been described. There are a variety of examples of analyte binding from oxadiazole-based fluorescent chemosensors such as fluoride sensing [11], explosive nitroaromatics sensing [12] and metal ion sensing systems [13]. This structural framework allows a bidentate metal-chelating environment to be established when a metal binding site is incorporated into the 5-aryl ring. As the O and N atoms in the 1,3,4-oxadiazole moiety can be combined with this additional binding site, two options of bidentate chelation may be created. Herein, we report the preparation of alternating copolymers containing fluorene and the ortho-pyridyl or ortho-pyrimidyl groups at the C-5 position of the oxadiazole ring. The oxadiazole derivative is placed at the meta position along the polymer main chain, therefore the resonance interaction from the oxadiazole center is only transmitted up to the meta-phenylene bridge. The molecular design, therefore, limits the resonance interaction between the oxadiazole and polymer backbone. This electronic connection could be exploited to attenuate the signal transfer between the oxadiazole sensing site and fluorescent polymer backbone [14]. This chelating ability and chelating mode should allow individual metal ions or a combination of them as input with fluorescence output in terms of both intensity and wavelength [15]. This was expected to couple with the 1,3,4-oxadiazole ring to form (O,N) or (N,N) bidentate chelation for metal ions. 2. Experimental 2.1. Instrumentation 1 H and 13 C NMR spectra were recorded in deuterated chloroform on a Bruker AVIII 500 MHz spectrometer. (Abbreviations used: s, singlet; d, doublet; t, triplet; m, multiplet; br, broad.) Electron Impact (EI) mass spectra were determined on Shimadzu LCMSIT-TOF Mass Spectrometer. Elemental analysis was performed on a Heraeus varioIII-NCSH elemental analyzer. Gel permeation chromatography (GPC) was carried in THF solution using low polydispersity polystyrene standards for calibration with Water 2414 RI detector. UV–vis measurements were obtained on a Jasco V-670 UV-Vis-NIR spectrophotometer. Photoluminescence measurements were carried out on a Jasco FP6300 fluorescence spectrophotometer. Polymer thin films were spin-coated on a glass plate from tetrahydrofuran (THF) solution at 1000 rpm for 60 s. Thermal properties were studied by thermogravimetric analysis on a TA TGA Q500 at a heating rate of 10 ◦ C/min and differential scanning calorimetry on a Perkin-Elmer DSC4000 thermal analyzer at a heating or cooling rate of 10 ◦ C/min. The melting point was recorded using MEL-TEMP 1001D melting point apparatus.

2.2. Materials Unless otherwise noted, all reagents were used as received from Alfa or Aldrich without further purification. THF was distilled under a nitrogen atmosphere over sodium/benzophenone. The monomer 1, 2,2 -(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5tetramethyl-1,3,2-dioxaborolane) was prepared by lithiation of the 2nd and 7th position of the dibromodioctyl substituted fluorene followed by addition with 2-isopropoxy-4,4,5,5-tetramethyl[1,3,2]dioxaborolane. The reaction mixture was then warmed to

13

room temperature and stirred for 18 h. The crude compound was purified by recrystallization in methanol and column chromatography. The 2-(5-(3,5-dibromophenyl)-1,3,4-oxzdiazol-2-yl)pyridine, 2, and 2-(5-(3,5-dibromophenyl)-1,3,4-oxzdiazol-2-yl)pyrimidine, 3, can be synthesized by a modification of established procedures [16]. 2.2.1. Synthesis of 2,2 -(9,9-dioctyl-9H-fluorene-2,7diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (1) To a solution of 2,7-dibromo-9,9-dioctylfluorene (4.0 g, 7.3 mmol) in dry THF (80 mL) was added n-butyllithium (12 mL, 19.2 mmol, 1.6 M) dropwise over 10 min and the mixture was stirred at −78 ◦ C for 2 h. 2-Isopropoxy-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (4.6 mL, 19.8 mmol) was then added and the resulting mixture was stirred for a further 1.5 h at −78 ◦ C, allowed to warm to room temperature and stirred for a further 18 h. The reaction mixture was poured into water (200 mL), extracted with diethyl ether (100 mL), washed with water (2× 50 mL) and dried with anhydrous MgSO4 . The solvent was evaporated and the crude product was purified by recrystallization from a solvent system of DCM and methanol and then by column chromatography using 5% of ethyl acetate and 95% of hexane as eluent to give white crystals in a yield of 67%. 1 H NMR (500 MHz, CDCl3 , ı): 7.81 (d, J = 7.5 Hz, 2H), 7.75 (s, 2H), 7.72 (d, J = 7.5 Hz, 2H), 2.0 (m, 4H), 1.39 (s, 24H), 1.23–0.97 (m, 20H), 0.81 (t, J = 7.2 Hz, 6H), 0.56 (m, 4H); 13 C NMR (167 MHz, CDCl3 , ı): 150.30, 143.74, 133.21, 128.48, 118.92, 83.27, 54.73, 39.64, 31.33, 29.48, 28.74, 28.70, 24.49, 23.15, 22.14, 13.61. Mass spectrum (EI+, m/z) 643 (M+ ). m.p. 128 ◦ C. 2.2.2. Synthesis of 2-(5-(3,5-dibromophenyl)-1,3,4-oxadiazol-2-yl)pyridine (2) To a mixture of picolic acid (0.62 g, 5 mmol), 3,5-dibromobenzohydrazide (1.47 g, 5 mmol) and diisopropylethylamine (1.94 g, 15 mmol) in acetonitrile (80 mL) at room temperature was added O-(benzotriazol-1-yl)-N,N,N ,N tetramethyluronium tetrafluoroborate (1.77 g, 5.5 mmol) and the resulting mixture was stirred for 14 h. N,N-Diisopropylethylamine (1.3 g, 10 mmol) was successively added, followed by 4-methyl benzenesulfonyl chloride (2.86 g, 15 mmol) and the reaction mixture was stirred for 12 h before pouring into a 14% NH3 aqueous solution. The crude mixture was stirred at room temperature for 30 min and then extracted with DCM and water. The organic layers were combined, washed with 2 M NaOH aqueous solution, dried over MgSO4 , filtered and evaporated. The residue was chromatographed using DCM as eluent to give a white solid in a yield of 58%. 1 H NMR (500 MHz, CDCl3 , ı): 8.84 (dd, J = 4.8, 1.0 Hz, 1H), 8.34 (d, J = 7.8 Hz, 1H), 8.32(d, J = 1.7 Hz, 2H), 7.94 (td, J = 7.8, 1.0 Hz, 1H), 7.86 (t, J = 1.7 Hz, 1H), 7.52 (dd, J = 7.8, 4.8 Hz, 1H); 13 C NMR (167 MHz, CDCl3 , ı): 164.33, 163.11, 150.39, 143.18, 137.34, 137.32, 128.69, 126.63, 126.14, 123.70, 123.50. Mass spectrum (EI+, m/z) 381 (M+ ). m.p. 206 ◦ C. 2.2.3. Synthesis of 2-(5-(3,5-dibromophenyl)-1,3,4-oxadiazol-2-yl)pyrimidine (3) To a mixture of pyrimidine-2-carboxylic acid (0.62 g, 5 mmol), 3,5-dibromo-benzohydrazide (1.47 g, 5 mmol) and diisopropylethylamine (1.94 g, 15 mmol) in acetonitrile (80 mL) at room temperature was added O-(benzotriazol-1-yl)-N,N,N ,N tetramethyluronium tetrafluoroborate (1.77 g, 5.5 mmol) and the resulting mixture was stirred for 5 h. N,N-Diisopropylethylamine (1.3 g, 10 mmol) was successively added, followed by 4-methyl benzenesulfonyl chloride (2.86 g, 15 mmol) and the reaction mixture was stirred for 5 h before pouring into a 14% NH3 aqueous solution. The crude mixture was stirred at room temperature for 30 min and then extracted with DCM and water. The organic

14

C.-Y. Yu, T.-Y. Shih / Synthetic Metals 191 (2014) 12–18

Scheme 1. Synthetic routes to polymers 4 and 5.

layers were combined, washed with 2 M NaOH aqueous solution, dried over MgSO4 , filtered and evaporated. The residue was chromatographed using DCM as the eluent to give a white solid in a yield of 49%. 1 H NMR (500 MHz, CDCl3 , ı): 9.01 (d, J = 4.9 Hz, 2H), 8.34 (d, J = 1.7 Hz, 2H), 7.88 (t, J = 1.7 Hz, 1H), 7.52 (t, J = 4.9 Hz); 13 C NMR (167 MHz, CDCl3 , ı): 163.65, 163.08, 158.15, 153.11, 127.67, 128.87, 126.38, 123.79, 122.49; Mass spectrum (EI+, m/z) 382 (M+ ). m.p. 270 ◦ C

Fig. 1.

1

2.2.4. Synthesis of poly(2,7-(9,9-dioctyl)fluorene-alt-2,5–4-[(2 pyridyl)-1,3,4 -oxadiazol-5 -yl]phenylene) (4) Under an argon atmosphere, 2,2 -(9,9-dioctyl-9H-fluorene(0.482 g, 2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) 0.75 mmol), 2-(5-(3,5-dibromophenyl)-1,3,4-oxazdiazol-2yl)pyridine (0.286 g, 0.75 mmol), Pd(PPh3 )4 (86 mg, 0.075 mmol), 1,4-dioxane (20 mL), and 2 M K2 CO3 (3 mL) were placed in a two

H NMR spectra of monomers and polymers. (a) Monomer 2, (b) monomer 3, (c) polymer 4, (d) polymer 5 in CDCl3 . The alkyl chain region was omitted for clarity.

C.-Y. Yu, T.-Y. Shih / Synthetic Metals 191 (2014) 12–18

2.2.5. Synthesis of poly(2,7-(9,9-dioctyl)fluorene-alt-2,5–4-[(2 pyrimidyl)-1,3,4 -oxadiazol-5 -yl]phenylene) (5) Under an argon atmosphere, 2,2 -(9,9-dioctyl-9H-fluorene2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.482 g, 2-(5-(3,5-dibromophenyl)-1,3,4-oxadiazol0.75 mmol), 2-yl)pyrimidine (0.287 g, 0.75 mmol), Pd(PPh3 )4 (86 mg, 0.075 mmol), 1,4-dioxane (25 mL), and 2 M K2 CO3 (3 mL) were placed in a two necked flask and refluxed for 72 h. After cooling, the mixture was extracted with CHCl3 , washed with water and dried with anhydrous MgSO4 . The solvent was evaporated and then the resulting crude powder was washed with methanol and further purified by Soxhlet extraction with methanol and acetone for 24 h before solubilizing in hot chloroform. The obtained precipitate was collected to a give yellow solid in a yield of 66%. 1 H NMR (500 MHz, CDCl3 , ı): 9.01–9.06 (d, J = 4.8 Hz, 2H), 8.51 –8.56 (br. s, 2H), 8.11–8.17 (m, 1H), 7.86–7.93 (m, 2H), 7.70–7.83 (m, 4H), 7.49–7.54 (t, J = 4.8 Hz, 1H), 2.08–2.22 (br. s, 4H), 1.06–1.16 (br. s, 1H), 0.70–0.79 (m, 10H). Anal. calcd for C41 H46 N4 O: C 80.62, H 7.59, N 9.17; found: C 78.65, H 7.49, N 8.56. 3. Results and discussion 3.1. Polymerization Poly{2,7-(9,9-dioctyl)fluorene-alt-2,5-4-[(2 pyridyl-1,3,4 -oxadiazol-5 -yl]phenylene} 4 and poly{2,7-(9,9-dioctyl)fluorene-alt-2,5-4-[(2 -pyrimidyl)-1,3,4 oxadiazol-5 -yl]phenylene} 5 were synthesized by the palladium catalyzed cross coupling reaction of 1 with 2 or 3 (Scheme 1). The high purity of monomers 1 and 2 or 1 and 3 with 10 mol% Pd(PPh3 )4 were dissolved in a solvent mixture of 1,4-dioxane and aqueous 2 M K2 CO3 under an argon atmosphere. The solution was then refluxed with vigorous stirring for 72 h. After extraction and removal of the solvent, the crude powder was further purified by Soxhlet extraction in methanol and acetone for 24 h to remove the oligomers and catalyst residue before washing out from hot chloroform. The resulting polymers 4 and 5 were soluble in common organic solvents such as THF, dichloromethane and chloroform. Polymers 4 and 5 were obtained in a yield of 76 and 66%, respectively. 3.2. NMR spectroscopy and molecular weight of polymers The 1 H NMR spectrum of 2 is shown in Fig. 1a. Signals appear at 8.32 and 7.86 ppm with J = 1.7 Hz associated with hydrogens of the dibromo substituted aromatic ring. The signal of the ortho-pyridyl hydrogen generate a doublet of doublets at 8.84 ppm with J = 4.8, 1.0 Hz. The signals for the meta-pyridyl hydrogens are observed as a doublet at 8.34 ppm with J = 7.8 Hz and a doublet of doublet at 7.52 ppm with J = 7.8, 4.8 Hz. The para-pyridyl hydrogen appears at 7.94 ppm with J = 7.8, 1.0 Hz. The 1 H NMR spectrum of 3 is shown in Fig. 1b. Signals appear at 8.34 and 7.88 ppm with a J = 1.7 Hz

4

0.8

5

Absorbance

0.6

0.4

0.2

0.0 300

400

500

600

Wavelength (nm)

4 0.2

Absorbance

necked flask and refluxed for 72 h. After cooling, the mixture was extracted with CHCl3 , washed with water and dried with anhydrous MgSO4 . The solvent was evaporated and then the resulting crude powder was washed with methanol and further purified by Soxhlet extraction using methanol and acetone for 24 h before solubilizing in hot chloroform. The obtained precipitate was collected to give a yellow solid in a yield of 76%. 1 H NMR (500 MHz, CDCl3 , ı): 8.85–8.86 (d, J = 4.5 Hz, 1H), 8.47–8.54 (br. s, 2H), 8.35–8.44 (d, J = 7.6 Hz, 1H), 8.09–8.16 (m, 1H), 7.84–7.98 (m, 3H), 7.64–7.82 (m, 4H), 7.47–7.54 (m, 1H), 1.98–2.20 (br. s, 4H), 1.08–1.22 (br. s, 20H), 0.63– 0.82 (m, 10H). Anal. calcd for C42 H47 N3 O: C 82.72, H 7.77, N 6.89; found: C 80.00, H 7.35, N 6.30.

15

5

0.1

0.0 400

600

800

1000

Wavelength (nm) Fig. 2. (a) The solution UV–vis spectra. (b) The solid state UV–vis spectra of polymers 4 and 5.

associated with the hydrogens of the dibromo substituted aromatic ring. The signals generated by the ortho-pyrimidyl hydrogens are observed as a doublet at 9.01 ppm with a J = 4.9 Hz. A triplet signal can be observed for the meta-pyrimidyl hydrogens at 7.52 ppm with a J = 4.9 Hz. The structures of the monomers have been fully characterized by 1 H–1 H COSY and mass spectrometry (supplementary information). The 1 H NMR spectra of and polymers 4 and 5 are shown in Fig. 1c and d. Signals of the aromatic hydrogens in the 2,5-diaryl oxadiazole rings of the polymers 4 and 5 were observed and compared to those in monomers 2 and 3. It should be noted that the chemical shifts from the hydrogens in the phenyl ring of the oxadizole unit are shifted downfield after polymerization. The molecular weights of polymers 4 and 5 were determined by gel permeation chromatography (GPC) in tetrahydrofuran solution, calibrated against narrow polydispersity index polystyrene standards, using refractive index (RI) detection. The polymer 4 had a Mn of 11,900 and a polydispersity index of 1.64. The polymers 5 had the Mn of 10,900 and the polydispersity index of 1.88. 3.3. Optical properties of polymers The absorption spectra of polymers 4 and 5 were recorded in dilute THF solution. Fig. 2a showed that the polymers exhibited three absorption peaks. As expected, the shorter absorption between 250 and 300 nm originates from the absorption of the oxadiazole segments which is confirmed by the absorption maxima of monomers. For both polymers, the longer absorption wavelength

C.-Y. Yu, T.-Y. Shih / Synthetic Metals 191 (2014) 12–18

100

0.00 eq 0.02 eq 0.05 eq 0.10 eq 0.20 eq 0.30 eq 0.60 eq 1.20 eq

PL Intensity

80 60 40

100

0.00 eq 0.02 eq 0.05 eq 0.10 eq 0.20 eq 0.30 eq 0.60 eq 1.20 eq

80

PL Intensity

16

20

60 40 20

0

0 350

400

450

500

550

600

350

400

Wavelength (nm)

500

550

600

100

100

60 40

0.00 eq 0.02 eq 0.05 eq 0.10 eq 0.20 eq 0.30 eq 0.60 eq 1.20 eq

80

PL Intensity

0.00 eq 0.02 eq 0.05 eq 0.10 eq 0.20 eq 0.30 eq 0.60 eq 1.20 eq

80

PL Intensity

450

Wavelength (nm)

60 40 20

20

0

0 350

400

450

500

550

350

600

400

450

500

550

600

Wavelength (nm)

Wavelength (nm)

Fig. 3. Fluorescence spectra of 4 (2 × 10−7 M) in the presence of increasing amounts of (a) Ni2+ , (b) Zn2+ , (c) Cu2+ and (d) Hg2+ in THF solution.

0.00 eq 0.02 eq 0.05 eq 0.10 eq 0.20 eq 0.30 eq 0.60 eq 1.20 eq

PL Intensity

60

40

80 0.00 eq 0.02 eq 0.05 eq 0.10 eq 0.20 eq 0.30 eq 0.60 eq 1.20 eq

60

PL Intensity

80

20

40

20

0

0 350

400

450

500

550

600

350

400

Wavelength (nm)

0.00 eq 0.02 eq 0.05 eq 0.10 eq 0.20 eq 0.30 eq 0.60 eq 1.20 eq

PL Intensity

60

40

500

550

20

80

600

0.00 eq 0.02 eq 0.05 eq 0.10 eq 0.20 eq 0.30 eq 0.60 eq 1.20 eq

60

PL Intensity

80

450

Wavelength (nm)

40

20

0

0 350

400

450

500

Wavelength (nm)

550

600

350

400

450

500

550

600

Wavelength (nm)

Fig. 4. Fluorescence spectra of 5 (2 × 10−7 M) in the presence of increasing amounts of (a) Ni2+ , (b) Zn2+ , (c) Cu2+ and (d) Hg2+ in THF solution.

C.-Y. Yu, T.-Y. Shih / Synthetic Metals 191 (2014) 12–18

26

1.0

(I0-I)/I0

0.6

2+ Ni 2+ Zn 2+ Cu 2+ Hg

0.4

0.2

4 5

24

Heat Flow Endo Up (mV)

0.8

17

0.0

22

20

18

16 50

100

150

200

250

300

o

Temperature ( C)

1.0

(I0-I)/I0

0.6

0.4

0.2

0.0 Fig. 5. (I0 − I)/I0 (a) polymer 4 and (b) polymer 5 in the presence of 1.20 equivalents of a given metal ion.

is attributed to the ␲–␲* transition along the polymer backbone. Polymers 4 and 5 exhibited an absorption maximum at 333 nm and 335 nm, respectively. The absorption maxima of polymers 4 and 5 are blue shifted compared to the polyfluorene prepared in previous literature. This indicates an effective conjugation interruption at meta-phenylene linkages [17] between the fluorene and oxidiazole units. The photoluminescence (PL) quantum yields for polymer 4 and 5 were determined in THF solution relative to a quinine sulfate standard. The PL quantum yields of polymer 4 and 5 were 0.17 and 0.13, respectively. The low quantum yields of polymers 4 and 5 compared to polyfluorene are possibly due to the heteroatom effect that favors the non-radiative pathway. The polymer films were prepared by spin-casting of a solution (1 mg/mL) onto glass slides. The solid state UV–vis absorption spectra of 4 and 5 are shown in Fig. 2b. Polymer 4 exhibits an absorption maximum at 350 nm and polymer 5 exhibits an absorption maximum at 372 nm. The wavelength of the absorption maximum for these two copolymers are clearly red-shifted by approximately 20–40 nm from those carried out in solution due to enhanced interchain interactions in solid state. It should be noted that the solid state UV–vis absorption spectra of 4 and 5 exhibited very long tails extending through the whole visible region. This is due to the scattering effect as the quality of films of 4 and 5 are poor due to the relatively low molecular weights of polymers. 3.4. Fluorescence quenching behavior of polymers The effects of the molecular recognition sites of the oxadiazole for polymers 4 and 5 on metal sensing have been investigated as

100 4 5

90 80

Weight (%)

0.8

2+ Ni 2+ Zn 2+ Cu 2+ Hg

70 60 50 40 0

200

400 600 o Temperature ( C)

800

Fig. 6. (a) DSC and (b) TGA analysis of polymers 4 and 5 under a nitrogen atmosphere at heating or cooling rate of 10 ◦ C/min.

depicted in the fluorescence emission spectra of polymers with increasing amounts of various metal ions. The concentrations of the metal free polymers were at 2 × 10−7 M. Figs. 3 and 4 show the fluorescence spectra of polymers 4 and 5 upon addition of Ni2+ , Zn2+ , Cu2+ and Hg2+ ions. It should be noted that after addition of the metal ions, in all cases the fluorescence was quenched. The addition of Ni2+ and Cu2+ ions in polymer solutions result in significant fluorescence quenching of polymer 4 compared to the addition of Zn2+ and Hg2+ ions. Partial fluorescence quenching was observed by Zn2+ and Hg2+ ions when the ratio of polymer and metal ions was 1.2. A possible explanation is that the meta-positioned oxidazole group does not have a resonance interaction with the chromophore along the polymer backbone. This structural feature weakens the electronic connection between the chromophore in the polymer backbone and the metal-chelation site, thereby attenuating the fluorescence quenching by metal ions to allow the observed selectivity. A similar fluorescence quenching behavior but to a smaller extent was also observed for polymer 5 upon addition of Ni2+ , Cu2+ and Hg2+ ions. In addition, a small change in the emission maxima wavelength was observed for both polymers in response to a series of metal ions which indicates little formation of intramolecular charge transfer complexes [18]. For both polymers, the fluorescence quenching toward the metal ions can be attributed to photoinduced electron-transfer quenching when the ions coordinate with the bidentate O,N or N,N site of the 5-pyridyland 5-pyrimidyl oxadiazole moieties. The fluorescence quenching

18

C.-Y. Yu, T.-Y. Shih / Synthetic Metals 191 (2014) 12–18

behavior can be attributed to many factors, including quenching mechanisms, nature of the metal ions, and the metal–ligand binding strength. The stronger quenching effects from Cu2+ and Ni2+ ions could be due to a combination of their strong binding with the ligand and their paramagnetic property. The fluorescence quenching factor, (I0 − I)/I0 , of polymers 4 and 5 in the presence of 1.2 equivalents of a given metal ion are shown in Fig. 5. Screening of different metal ions showed that both polymers were the most sensitive to Ni2+ ions (Fig. 5) with the sensitivity in the order of Ni2+ > Cu2+ > Hg2+ > Zn2+ for polymer 4 and Ni2+ > Cu2+ > Zn2+ > Hg2+ for polymer 5. 3.5. Thermal properties of polymers The thermal properties of polymers 4 and 5 can be analyzed by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating or cooling rate of 10 ◦ C/min (Fig. 6). The result of the DSC shows that polymer 5 exhibited a higher glass transition temperature (Tg ) at 165 ◦ C than that of polymer 4 (Tg = 121 ◦ C), whereas their melting or recrystallization temperatures were not observed on heating or cooling. This indicates that the polymers are amorphous. The decomposition temperatures (5% weight loss) of the polymers 4 and 5 are 393 and 407 ◦ C, respectively. Both polymers exhibited a one-step degradation in the loss of weight due to the decomposition of the side chains and the polymer backbone and remain around 45% solid residue even heating to 800 ◦ C. 4. Conclusions Two soluble alternating copolymers incorporating fluorene and oxadiazole pendants have been successfully prepared by utilizing the palladium catalyzed cross coupling reaction. The photophysical properties of the copolymers were significantly affected by the oxadiazole pendants. The polymers exhibited good thermal stabilities up to 350 ◦ C. Additionally, the conjugated polymers performed their intended role as fluorescent chemosensors for metal ions. The results indicate that polymer 4 shows a higher sensitive in sensing of Ni2+ , Cu2+ and Hg2+ ions than polymer 5. Such polymers

may be provided a crucial template for the synthesis and design of enhanced and highly sensitive chemosensory materials. Acknowledgement Financial support from the National Science Council and the National Taiwan University of Science and Technology is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2014.02.012. References [1] L.J. Fan, Y. Zhang, C.B. Murphy, S.E. Angell, M.F.L. Parker, B.R. Flynn, W.E. Jones, Coord. Chem. Rev. 253 (2009) 410–422. ˜ Polym. Rev. 51 (2011) 341–390. [2] J.M. García, F.C. García, F. Serna, J.L. de la Pena, [3] H.N. Kim, Z. Guo, W. Zhu, J. Yoon, H. Tian, Chem. Soc. Rev. 40 (2011) 79–93. [4] T.M. Swager, Acc. Chem. Res. 31 (1998) 201–207. [5] B. Wang, M.R. Wasielewski, J. Am. Chem. Soc. 119 (1997) 12–21. [6] D.T. McQuade, A.E. Pullen, T.M. Swager, Chem. Rev. 100 (2000) 2537–2574. [7] Y. Chen, K.Y. Pu, Q.L. Fan, X.Y. Qi, Y.Q. Huang, X.M. Lu, W. Huang, J. Polym. Sci. A: Polym. Chem. 47 (2009) 5057–5067. [8] B. Liu, W.L. Yu, J. Pei, S.Y. Liu, Y.H. Lai, W. Huang, Macromolecules 34 (2001) 7932–7940. [9] J.A. Zhou, X.L. Tang, J. Cheng, Z.H. Ju, L.Z. Yang, W.S. Liu, C.Y. Chen, D.C. Bai, Dalton Trans. 41 (2012) 10626–10632. [10] U. Mitschke, P. Bauerle, J. Mater. Chem. 10 (2000) 1471–1507. [11] G. Zhou, Y. Cheng, L. Wang, X. Jing, F. Wang, Macromolecules 38 (2005) 2148–2153. [12] T.H. Kim, H.J. Kim, C.G. Kwak, W.H. Park, T.S. Lee, J. Polym. Sci. A: Polym. Chem. 44 (2006) 2059–2068. [13] Y. Liu, L. Zong, L. Zheng, L. Wu, Y. Cheng, Polymer 48 (2007) 6799–6807. [14] V. Banjoko, Y. Xu, E. Mintz, Y. Pang, Polymer 50 (2009) 2001–2009. [15] A.F. Li, Y.B. Ruan, Q.Q. Jiang, W.B. He, Y.B. Jiang, Chem. Eur. J. 16 (2010) 5794–5802. [16] P. Stabile, A. Lamonica, A. Ribecai, D. Castoldi, G. Guercio, O. Curcuruto, Tetrahedron Lett. 51 (2010) 4801–4805. [17] C.Y. Yu, M. Horie, A.M. Spring, K. Tremel, M.L. Turner, Macromolecules 43 (2010) 222–232. [18] S.H. Mashraqui, S. Sundaram, A.C. Bhasikuttan, Tetrahedron 63 (2007) 1680–1688.