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This article can be cited before page numbers have been issued, to do this please use: C. Wu, Y. Lin and Y. Chen, Org. Biomol. Chem., 2013, DOI: 10.1039/C3OB42054J.

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Organic conjugated materials have attracted a great deal of attention because of their potential

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applications light-emitting diodes (LEDs),1,2 chemical sensors,3,4 organic solar cells,5,6 and field-effect transistors.7,8 Recently, there has been a growing need for developing highly sensitive and selective probes for the detection of metal ions in biological and environmental samples. Analytical techniques based on fluorescence detection are very popular because fluorescence measurements are usually very sensitive, easy to perform, and inexpensive.9,10 Therefore, a large number of papers involving fluorescent chemical sensors have been reoprted.11 Some heavy metal ions, such as Fe3+, Zn2+, Cu2+, Co2+, Mn2+, and Mo4+, are essential for the maintenance of human metabolism. However, high concentrations of these ions can lead to many adverse health effects.12 Therefore, the development of selective and sensitive methods for the determination of heavy metal ions is currently receiving considerable attention.9 On the other hand, to achieve highly electroluminescent efficiency it is necessary that both electrons and holes are efficiently injected/transported and balanced.13,14 However, for conventional electroluminescent

MEH-PPV

{poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene]},

holes are more readily injected and transported than electrons, leading to imbalanced carriers currents in PLEDs.15-19 Moreover, for development of the PLEDs industrialization, the cathode must be stable to environment, the environmentally stable high-work-function metals such as Al, Cu, Ag, and Au as the cathode has recently attracted extensive attention. Use of a water- or alcohol-soluble electron injection layer (EIL) based on a conjugated polymer grafted with amino, ammonium salt, or diethanolamino groups has been demonstrated to allow the use of a high-work-function metal as the cathode.20-23 This is because interfacial dipole or space charge is formed between the emitting layer (EML) and the cathode that reduces the electron injection barrier. Among the typical fluorescent materials, fluorene-based derivatives show good thermal and chemical stabilities, exhibit wide energy band gap, and high photoluminescence (PL) efficiency (60~80%) in solution and solid states. 24-26 Moreover, relatively high reactivity of the protons at C-9 position of fluorene unit renders the chemical modification of polyfluorene easy without perturbing its electrochemical properties. Accordingly, the fluorene-based derivatives are widely used in

2

Organic & Biomolecular Chemistry Accepted Manuscript

Introduction

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electroluminescent (EL) devices, solar cells and chemical sensory applications. In additional, crown ethers are designed as molecular hosts able to complex alkali and alkaline earth cations, transition

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dependent on the number of ether donor atoms. Of particular importance is the size and shape of the cavity relative to the cation size.28 The crown ether-metal ion complex forms in a 1:1 ratio when the ion is able to fit within the cavity. However, the ion is larger than that of the crown cavity, there is a tendency for the crown ether moieties to ‘‘sandwich’’ the metal ions between adjacent crown units.29 An azacrown ether is a crown ether that has nitrogen donor atoms as well as oxygen donor atoms to coordinate to the metal iron. The nitrogen atoms in azacrown ether should play important role in coordination with metal cations. To our best knowledge, a few researches focus on small molecule which could be not only applied as chemosensor for cations but also as electron injection layer to enhance electroluminescence. In particular, small-molecular-weight materials have received interest due to their easy purification by re-crystallization or column chromatographic techniques, and mono-disperse with well-defined chemical structures, and synthetically well-reproducibility. In this study, we report the synthesis and characterization of a novel and small-molecule fluorene-based material containing triple azacrown ether groups FTC. The azacrown ether groups attached on FTC are expected not only to coordinate with certain metal ions to result in fluorescence quenching but also to be applied as electron injection layer to enhance significantly the electroluminescence of MEH-PPV. Moreover, vinyl groups also have been introduced between fluorene-cored and triple azacrown ethers to extend the conjugation of mono-fluorene part. This is not only to make the PL emission of FTC locate visible region but also achieve more suitable LUMO and HOMO energy levels. The PL quenching is caused by the energy transfer from fluorene moiety to the adjacent coordinated metal ions. The electroluminescent enhancement is attributable to reduce the electron injection barrier to obtain more balance of charges in MEH-PPV. The results indicate that the FTC is potentially not only applicable in chemical sensors if high selectivity and sensitivity are realized but also in enhancing device performance of PPV derivatives. Experimental Section All new synthesized compounds were identified by 1H NMR and elemental analysis (EA). The 1H NMR spectra were recorded on a Bruker AMX-400 MHz FT-NMR, and the chemical shifts 3

Organic & Biomolecular Chemistry Accepted Manuscript

metal cations and ammonium cations.27 The stability of the crown ether-metal ion complex is

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are reported in ppm using tetramethylsilane (TMS) as an internal standard. The elemental analysis was carried out on a Heraus CHN-Rapid elemental analyzer. Absorption and photoluminescence

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spectrofluorometer, respectively. Cyclic voltammograms were measured with a voltammetric apparatus (model CV-50W from BAS) equipped with a three-electrode cell. The cell was made up of an oligomer-coated glassy carbon as the working electrode, an Ag/AgCl electrode as the reference electrode, and a platinum wire as the auxiliary electrode. The electrodes were immersed in acetonitrile containing 0.1 M (n-Bu)4NClO4. The energy levels were calculated using the ferrocene (FOC) value of -4.8 eV with respect to vacuum level, which is defined as zero.30 Synthesis and Characterization N,N-Bis(acetoxyethyl)phenylamine (1) A mixture of N-phenyldiethanolamine (8.0 g, 44 mmol), triethylamine (13.3 g, 132 mmol) and THF (40 mL) was added to a solution of acetyl chloride (9.3 mL, 132 mmol) in THF (40 mL). The mixture was stirred at 35 oC for 20 h. Then, 40 mL water was added dropwise to the mixture slowly. After removal of THF by rotary evaporation, the mixture was extracted with chloroform and the organic layer was dried (MgSO4) and concentrated under reduced pressure. The residue was distillated in vacuo (210 oC, 7 torr ) to obtain 1 (9.7 g , 82.9 %). 1H NMR (400 MHz, CDCl3, TMS, 25 oC): δ 7.20-7.24 (t, 2H, J = 16 Hz, Ar-H), 6.70-6.76 (m, 3H, Ar-H), 4.21-4.24 (t, 4H, J = 12.5 Hz , -CH2-), 3.59-3.63 (t, 4H, J = 12.6 Hz, -CH2-), 2.04 (s, 6H, -CH3).Anal. Calcd. for C14H19NO4 (%): C, 63.38; H, 7.22; N, 5.28. Found: C, 62.98 7.18; N, 5.22. FT-IR (KBr, cm-1): ν 1735 (C=O stretch), 2963 (-CH stretch). 4-(Bis(2-acetoxylethyl)amino)benzaldehyde (2) Phosphoryl trichloride (6.9 g, 45 mmol) was added slowly to N,N-dimethylformamide (13.8 g, 188 mmol) under ice bath and stirred at room temperature for 1 h, and then reacted at 75 oC for additional 12 hr after adding compound 1. Chloroform and sodium acetate solution (24.7 g, 0.3 mol) was added slowly under ice bath and stirred at room temperature overnight. The mixture was extracted with chloroform and the organic layer was dried (MgSO4) and concentrated under reduced pressure to give 2 (12.2 g, 92.8 %). 1H NMR (400 MHz, CDCl3, TMS, 25 oC): δ 9.74 (s, H, -CHO) 6.81-6.83.(d, 2H, J = 8.6 Hz, Ar-H), 7.73-7.75 (d, 2H, J = 8.7 Hz, Ar-H), 4.26-4.29 (t, 4H, J = 12.2 Hz, -CH2-), 3.71-3.74 (t, 4H, J = 12.3 Hz, -CH2-), 2.04 (s, 6H, -CH3). Anal. Calcd. for C15H19NO5 4

Organic & Biomolecular Chemistry Accepted Manuscript

(PL) spectra were measured on a Jasco V-550 spectrophotometer and a Hitachi F-4500

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(%): C, 61.42; H, 6.53; N, 4.78. Found: C, 61.09; H, 6.56; N, 4.65. FT-IR (KBr, cm-1): ν 1676

4-Bis(2-hydroxyethyl)amino)benzaldehyde (3) Published on 23 December 2013. Downloaded by St. Petersburg State University on 23/01/2014 12:07:26.

A mixture of 2 (10 g, 34 mmol), methanol (150 mL) and sodium carbonate solution (10.9 g, 102 mmol) was stirred at room temperature overnight. Few drops of 1N HCl were added to the mixture; and then the methanol was removed by rotary evaporation. The mixture was extracted with chloroform and the organic layer was dried (MgSO4) and concentrated under reduced pressure. The residue was purified by column chromatography eluted with ethyl acetate to give 3 (4.9 g, 69.5 %). 1

H NMR (300 MHz, CDCl3, TMS, 25 oC): δ 9.62 (s, H, -CHO) 7.63-7.66 (d, 2H, J = 9.0 Hz, Ar-H),

6.68-6.71 (d, 2H, J = 9.0 Hz, Ar-H), 3.86-3.89 (t, 4H, J = 9.8 Hz, -CH2-), 3.65-3.68 (t, 4H, J = 9.7 Hz, -CH2-), 3.40-3.45 (d, 2H, J = 16.4 Hz, -OH). Anal. Calcd. for C11H15NO3 (%): C, 63.14; H, 7.23; N, 6.69. Found: C, 63.07; H, 7.28; N, 6.59. FT-IR (KBr, cm-1): ν 1667 (-CHO stretch), 2753(-CH aldehyde), 3352 (-OH stretch). Triethyleneglycol Ditosylate (4) Mixture of p-toluenesulfonyl chloride (5.75 g, 0.03 mol), triethylene glycol (1.5 g, 0.01 mol) and THF (15 mL) was mechanically stirred at 0 oC. Aqueous solution of KOH (16 M, 4 mL) was added portion wise to the mixture within 1 h and then stirred at room temperature for 7 h. The mixture was poured into ice water, the appearing white precipitate was collected by filtration and re-crystallized from methanol/water to afford white crystal of 4 (3.214 g, 70 %, m.p. 81-83 oC). 1H NMR (400 MHz, CDCl3, TMS, 25 oC): 7.25-7.79 (m, 8H, aromatic region), 4.1 (m, 4H), 3.6 (m, 4H), 3.5 (s, 4H), 2.4 (s, 6H). FT-IR (KBr, cm-1): ν 1600 (aromatic ring), 1129 (C-O-C), 1175 (sulfonate). Anal. Calc. for C20H26O8S2: C, 52.39; H, 5.72. Found: C, 52.37; H, 5.72. 4-(Monoaza-15-crown-5)benzaldehyde (5) A mixture of compound 3 (0.4 g, 2.09 mmol) and NaH (0.8 g, 60 % in mineral oil, 20 mmol) in dried THF (300 mL) was refluxed under nitrogen atmosphere for 30 min. The mixture was added dropwise with a solution of compound 4 (1.0 g, 2.1 mmol) in THF (200 mL) and then stirred at 70 o

C for 2 days. After adding an aqueous solution of H2SO4 (2 M, 50 mL), the THF was removed by

rotary evaporation. The mixture was then extracted with chloroform and the organic layer was dried (MgSO4) and concentrated under reduced pressure. The crude product was purified by column

5

Organic & Biomolecular Chemistry Accepted Manuscript

(-CHO stretch), 1739 (C=O stretch), 2815 (-CH aldehyde), 2963 (-CH stretch).

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chromatography (eluent: acetone/n-hexane = 3/2) to give 5 (0.3 g, 44.49 %). 1H NMR (400 MHz, = 8.7 Hz), 3.52 (m, 4H, crown ether), 3.77-3.80 (m, 16H, crown ether). Anal. Calcd. for C17H25NO5

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(%): C, 63.14; H, 7.79; N, 4.33. Found: C, 62.68; H, 7.77; N, 4.14. FT-IR (KBr, cm-1): ν 2798 (-CH aldehyde), 1667 (-CHO stretch), 1124 (C-O stretch). 2,4,7-Tri(bromomethyl)-9,9-dihexylfluorene (6) Into the mixture of 9,9-dihexylfluorene (3.25 g, 9.5 mmol) and paraformaldehyde (9.3g, 0.31 mole) under ice bath was added drop-wise with HBr aqueous solution (33 wt%). The mixture was stirred at 70 oC for 2 day under nitrogen atmosphere, poured into ice water and extracted with CH2Cl2. After removal of CH2Cl2 by a rotary evaporator, the residues were purified by column chromatography using n-hexane/toluene (10/1, v/v) as eluent to afford colorless liquid of 6. (4.19 g, 72%) 1H NMR (400 MHz, CDCl3, TMS, 25 oC): δ 7.87-7.89 (d, 1H, J = 8 Hz), 7.3-7.4 (m, 2H ,ArH), 7.30-7.35 (m, 2H, ArH), 4.8 (s, 2H, -CH2-), 4.5 (s, 2H, -CH2-), 4.6 (s, 2H, -CH2-), 1.93-1.97 (m, 4H, -CH2-), 0.56-1.25 (m, 22H, -CH2-). Anal. Calc. for C28H37Br3: C, 54.83; H, 6.08. Found: C, 54.41; H, 6.01. 2,4,7-Tri[methylene(triphenylphosphonium bromide)]-9,9-dihexylfluorene (7) To a mixture of compound 6 (4.3 g, 7.5 mmol) and triphenylphosphine (15.7 g, 59.8 mmol) and DMF (60 mL) was stirred at 100 oC for 2 days under nitrogen atmosphere. The mixture was poured into excess of benzene and the appearing solid was collected by filtration, which was dried in vacuo at 100 oC to get the desired product 7 (8.7 g, 82.5 %). 1H NMR (400 MHz, CDCl3, TMS, 25 oC): δ 7.84~7.52 (m, 45H, Ar-H), 7.12 (s, 1H, Ar-H), 6.96 (s, 2H, Ar-H), 6.87 (s, 1H, Ar-H), 6.71 (s, 1H, Ar-H), 5.56~5.52 (d, 2H, J = 16Hz, -CH2Br), 5.35~5.31 (d, 2H, J = 16 Hz, -CH2Br), 5.16~5.12 (d, 2H, J = 16Hz, -CH2Br), 1.43~1.33 (m, 4H, -CH2-), 1.15~1.1 (m, 4H, -CH2-), 0.95~0.79 (m, 14H, -CH3- and -CH2-), 0.046 (m, 4H, -CH2-). Anal. Calc. forC82H82Br3P3: C, 70.3; H, 5.9. Found: C, 70.1; H, 5.99. 2,4,7-Tri[4-(monoaza-15-crown-5)styryl]-9,9-dihexylfluorene (FTC) A mixture of compound 5 (3.36 g, 10.4 mmol) and compound 7 (1.6 g, 1.1 mmol) in dried ethanol (20 mL) was added dropwise with sodium ethoxide (3.4 g, 10.4 mmol) and stirred at room temperature overnight under nitrogen atmosphere. The mixture was extracted with chloroform and

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Organic & Biomolecular Chemistry Accepted Manuscript

CDCl3, TMS, 25 oC): δ 9.73 (s, H, -CHO) 7.70-7.73 (d, 2H, J = 8.8 Hz, Ar-H), 6.71-6.74 (d, 2H, J

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the organic layer was dried (MgSO4) and concentrated under reduced pressure. The crude product NMR (400 MHz, CD2Cl2, TMS, 25 oC): δ 7.88~7.86 (d, 1H, Ar-H), 7.70~7.66 (d, 1H, Ar-H), 7.6 (s,

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1H, Ar-H),7.54~7.42 (m, 8H, Ar-H), 7.38 (s, 1H, Ar-H), 7.20~6.98 (m, 5H, Ar-H), 6.76~6.68 (m, 6H, Ar-H), 3.80~3.53 (m, 60H, crown ether), 2.06~2.02 (t, 4H, -CH2-), 1.16~1.03 (m, 12H, -CH2-), 0.79~0.75 (m, 6H, -CH3-), 0.67~0.65 (m, 4H, -CH2-). Anal. Calc. for C79H109N3O12: C, 73.40; H, 8.50; N, 3.25. Found: C, 73.42; H, 8.72; N, 3.11. Results and Discussion The

FTC

was

synthesized

from

monoaza-15-crown-5

derivative

(5)

and

2,4,7-tri[methylene(triphenylphosphonium-bromide)]-9,9-dihexylfluorene (7) by the Wittig reaction (Scheme 1). The chemical structure of FTC was characterized by 1H NMR, COSY and NOESY spectra and elemental analysis. The optical properties of FTC were investigated using absorption spectroscopy (UV/Vis) and photoluminescence spectroscopy (PL). Cyclic voltammetry (CV) was used to measure onset reduction and oxidation potentials of FTC film, which in turn were used to estimate the LUMO and HOMO levels respectively.

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Organic & Biomolecular Chemistry Accepted Manuscript

was purified by column chromatography (eluent: acetone/n-hexane = 1/1, EA) (0.51 g, 34.5 %).1H

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H3C

N

OAc

Cl N

TEA

OH

OAc

1

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methanol

O

N

DMF

OAc

2

O O S O 4

O H

O

OAc

C

H

O O S O

sodium carbonate

O

POCl3

OH

C

CH3

N

CH3

OH

O

O H

NaH, THF

3

Br

paraformaldehyde

t-BuOK, DMF

Br

HBr

6

Br

O

O P+Ph3Br-

H

-

DMF

BrPh3+P -

BrPh3+P

O

7

O

O

O

Sodium ethoxide, Ethanol

O O

O

N

C 5

PPh3

O

5

1-bromohexane

N

O

N

O

O O

O

FTC

Scheme 1. Synthetic procedures of FTC.

8

O O

O

N

O

N

C

O

Organic & Biomolecular Chemistry Accepted Manuscript

OH

O C

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Characterization of Fluorene-Based Material FTC

FTC. Assignments of each proton were assisted by the two-dimensional NMR spectra (Figure temperatures (Td) (at 5 wt% loss) and glass transition temperatures (Tg) of of FTC were evaluated with thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The Td at 5% weight loss, glass transition temperature (Tg) and melting point (Tm) of FTC was observed at 394 oC, 49 °C and 64 °C, respectively [Figure S4 (SI)], but no obvious Tc was detected between 30°C and 150°C. The asymmetric structure of FTC and the twist of 4-azacrown ether moiety relative to the fluorene plane effectively prevent close packing between the molecules.

FTC in film state FTC in CHCl3

1.2

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 300

400

500

600

Normalized PL Intensity (a.u.)

1.2

Normalized Abs. Intensity (a.u.)

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S2-S3), which agree well with the proposed molecular structure of FTC. Thermal decomposition

0.0

Wavelength (nm)

Figure 1. Absorption spectra and photoluminescence spectra of FTC in the film state and CHCl3 solution. Optical Properties Figure 1 illustrates the absorption and photoluminescence (PL) spectra of FTC in solution (CHCl3) and as film spin-coated from CHCl3 solution. The absorption spectra of FTC in solution and film state are peaked at ca. 416 nm and 424 nm, respectively. The absorption could be attributed to the n-π* and π-π* electronic transitions of crown ether and conjugated fluorenyl vinyl phenyl moiety. The PL spectra of FTC in solution and solid state locate at ca. 470 nm and ca. 535 nm, respectively. Solid state PL maximum of FTC is greatly red-shifted (65 nm) relative to solution

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Organic & Biomolecular Chemistry Accepted Manuscript

Figure S1-S3 (Supporting Information) shows the 1H NMR, COSY and NOESY spectra of

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state, probably due to excimer formation via inter-molecular interactions. The interactions are

Acetonitrile Chloroform Methanol Toluene Ethanol DMF Acetone THF Ethyl acetate

Normalized Abs. Intensity (a.u.)

0.8

0.6

0.4

0.2

0.0 350

400

450

500

550

Wavelength (nm)

Figure 2. Absorption spectra of FTC in various solvent. The concentration of FTC was fixed at 10-5 M.

Acetonitrile Chloroform Methanol Toluene Ethanol DMF Acetone THF Ethyl acetate

1.0

Normailzed PL Intensity (a.u.)

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1.0

0.8

0.6

0.4

0.2

0.0 450

500

550

600

650

700

Wavelength (nm)

Figure 3. Photoluminescence spectra of FTC in various solvent. The concentration of FTC was fixed at 10-5 M.

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Organic & Biomolecular Chemistry Accepted Manuscript

attributed to π-π stacking in solid state.

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Figure 2 and Figure 3 show the absorption and photoluminescence (PL) spectra of FTC in various solvents with the characteristic optical data summarized in Table 1. In general, the

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solvent dipoles can reorient or relax around μE, which lowers the energy of the excited state via excitation. This means that the absorption and PL emission are red-shifted with increasing the solvent polarities. Therefore, FTC showed red-shifts in absorption and PL emission with gradually increasing the polarity of solvent.31 However, absorption maximum are slightly red-shifted (ca. 10 nm) relative to PL emission (ca. 40 nm) due to the fluorescence lifetimes (1–10 ns) are usually much longer than the time required for solvent relaxation (10–100 ps). Table 1. Absorption and photoluminescence maximum wavelength of FTC in various solvents. Wavelength

Solvent λabs,max (nm)

λfluo,max (nm)

Toluene

396s, 418, 442s

468, 489s

THF

389s, 417, 440s

488

Chloroform

391s, 416, 440s

473, 491s

Ethyl acetate

390s, 414, 438s

475

Methanol

389s, 411, 433s

497

Acetone

391s, 415, 440s

501

Ethanol

388s, 412, 434s

496

Acetonitrile

392s, 414, 438s

504

DMF

395s, 421, 445s

508

Chemosensory Properties of FTC To investigate the applications of FTC as chemical sensor for cations, the chemosensory characteristics of FTC were investigated in two aqueous mixture solvent systems (THF/H2O = 9/1 and methanol/H2O = 9/1). The chloride salts of Li+, Na+, K+, Ca2+, Ba2+, Co2+, Ni2+, Pb2+, Zn2+, Co2+, Fe2+ and Fe3+ were dissolved in a mixture solvent of THF/H2O (9/1, v/v), methanol/H2O (9/1, v/v), or ethanol/H2O (9/1, v/v) to prepare their corresponding stock solutions (10-4 M). In the 11

Organic & Biomolecular Chemistry Accepted Manuscript

fluorophore has a larger dipole moment in the excited state (μE) than in the ground state (μG). The

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presence of Fe2+, the absorption and PL spectra slightly diminished intensities. However, the absorption spectral intensity and wavelength remain almost unchanged in the presence of other

the presence of Fe3+.

1.2 FTC Li+ Na+ K+ Ca2+ Ba2+ Co2+ Ni2+ Pb2+ Zn2+ Cd2+ Fe2+ Fe3+

Abs. Intensity (a.u.)

1.0

0.8

0.6

0.4

0.2

0.0 300

350

400

450

500

550

Wavelength (nm)

Figure 4. Absorption spectra of FTC with various metal ions in solution (THF/H2O = 9/1, v/v).The concentration of FTC and metal ions were fixed at 10-5 M and 10-4 M, respectively. 140 FTC Li+ Na+ K+ Ca2+ Ba2+ Co2+ Ni2+ Pb2+ Zn2+ Cd2+ Fe2+ Fe3+

120

100

PL Intensity (a.u.)

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obviously blue-shift from 419 nm to 376 nm as well as the PL intensity significantly decreased in

80

60

40

20

0 450

500

550

Wavelength (nm)

12

600

650

Organic & Biomolecular Chemistry Accepted Manuscript

cations tested as shown in Figure 4 and Figure 5. However, the absorption spectra demonstrated

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Similarly, the same tendency also could be found in methanol/H2O = 9/1 system (Figure S5 and absorption spectrum with decreasing PL intensity concomitantly. 0.1

0.0

-0.1

(I-I0)/I0

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Figure S6 in SI). The addition of Fe3+ caused obvious blue-shift from 412 nm to 367 nm in

-0.2

-0.3

-0.4

-0.5

-0.6

Metal ions Fe3+ Li+ Ni2+ Co2+ Zn2+ Fe2+ Na+ Cd2+ Ca2+ Pb2+ Ba2+ K+ Diameter (Å) 1.28 1.36 1.44 1.48 1.48 1.52 1.90 1.94 1.98 2.66 2.98 3.04

Figure 6. Fluorescence emission response profile of FTC upon addition of various metal ions in solution (THF/H2O = 9/1, v/v). The concentration of FTC and metal ions were fixed at 10-5 M and 10-4 M, respectively. Excitation wavelength was 418 nm. Figure 6 and Figure S7 (in SI) shows the fluorescence responses [(I-I0)/I0] of FTC toward various metal ions in two systems. The quenching ability of FTC showed great sensitivity towards Fe3+, suggesting it is a potential chemosensor to detect Fe3+. The quenching response towards Fe3+ is mainly governed by the interaction between metal ion and the lone pair electrons of nitrogen atom on the azacrown ether. The interaction is mainly depends on : (1) charge density of metal ions, (2) size of metal ions relative to that of crown ether, and (3) solvent in the system.32 Moreover, quenched fluorescence intensities also depends partially on the charge density of the complexed metal ion.33 The obvious blue-shift of absorption spectra and the quenching effect of fluorescence spectra towards Fe3+ have been further discussed. For instance, the photo-excited energy may relax via

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Organic & Biomolecular Chemistry Accepted Manuscript

Figure 5. Photoluminescence spectra of FTC with various metal ions in solution (THF/H2O = 9/1, v/v).The concentration of FTC and metal ions were fixed at 10-5 M and 10-4 M, respectively. Excitation wavelength was 418 nm.

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three possible ways after exciting the mixed solution of FTC with Fe3+. Three types of molecules probably coexisted in solution, which are Q, M, and [MQ]. Those symbols Q, M, and [MQ] mean

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The first way is the excitation of M resulted in the immediately energy transfer to Q. It is well known that Fe3+ is d block metal. The d block metals are per se inclined towards easy and reversible electron-transfer processes and have access to different and consecutive oxidation states. Such an access can be typically controlled by varying the coordinative environment around the metal. Therefore, quenching can be induced from the transfer of an electron from the excited state of the fluorophore directly to the metal center.34 Also the quenching efficiency depends on the degree of overlap between the absorption spectrum of metal ion (Fe3+) and fluorescence spectrum of the fluorophore.35,36 As shown in Figure S8 and Figure S9 (SI), the overlap leads to the possibility of energy transfer in both two systems. The second is the excited M* which did not collide and energy transfer with Q, so light emission would be detected. The final possibility is the formation of complex [MQ] which leads to the photoinduced charge transfer (PCT) phenomenon. When the cation interacts with the donor group (azacrown ether), the excited state is more strongly destabilized by the cation than the ground state, results in a blue-shift of the absorption and PL emission spectra. Therefore, the blue-shifted absorption and PL emission spectra were induced by PCT. However, the blue-shift of PL emission is much smaller than absorption. The electron density of the nitrogen atom is reduced when the cation interacts with the fluorophore. This nitrogen atom becomes a non-coordinating atom due to the positive polarization of nitrogen atom. Therefore, excitation induces a photo-disruption of the interaction between the cation and the nitrogen atom of the azacrown. Only slight shift is found in the PL emission spectra, because most of the fluorescence is emitted from the fluorophore which does not interact with cation anymore or the interaction is much weaker. Also the formation of non-fluorescent complex is the characteristic of static quenching, resulting in the reduction of fluorescence.37 Therefore, in our case those three possible ways contributed to the blue-shift of absorption spectra and the quenching effect of fluorescence spectra towards Fe3+. And the slight changes in spectra of FTC towards Fe2+ were found which was attributed to the partially oxidation of Fe2+ in the presence of H2O. These results indicate the selectivity of FTC towards Fe3+.

14

Organic & Biomolecular Chemistry Accepted Manuscript

the free Fe3+ metal ion, vacant FTC, the complex of FTC with Fe3+, respectively.

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DOI: 10.1039/C3OB42054J

Response Sensitivity towards Fe3+

fluorescence spectra of FTC with various concentration of Fe3+ were investigated in two mixture changed to ethanol/H2O (9/1) when the solubility was poor due to high concentration. As a quantitative measure of fluorescence quenching, the Stern-Volmer equation (Equation 1) is a useful method, where [Q] is the concentration of the quencher, Ksv is the Stern-Volmer constant which is proportional to sensitivity, F0 and F are the fluorescence intensity in the absence and presence of the quencher, respectively.38 F0/F = 1 + Ksv [Q]

(Eq. 1)

FTC 1.0x10-6 M 5.0x10-6 M 1.0x10-5 M 1.5x10-5 M 2.0x10-5 M 2.5x10-5 M 3.0x10-5 M 3.5x10-5 M 4.0x10-5 M 5.0x10-5 M 7.5x10-5 M 1.0x10-4 M 2.5x10-4 M 5.0x10-4 M 7.5x10-4 M 1.0x10-3 M 1.0x10-2 M

140

120

100

PL Intensity (a.u.)

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solvent systems (THF/H2O = 9/1 and ethanol/H2O = 9/1). In addition, the solvent composition was

80

60

40

20

0 450

500

550

600

650

700

Wavelength (nm)

Figure 7. Photoluminescence spectra of FTC with varying concentration of Fe3+in solution (THF/H2O = 9/1, v/v). The concentration of FTC was fixed at 10-5 M. Excitation wavelength was 418 nm. As shown in Figure 7 and Figure S10 (in SI), the PL emission intensity of FTC decreased gradually with increasing concentration of Fe3+ from 1.0×10-6 M to 1.0×10-2 M in two solvent systems. Figure S11 and Figure S12 (in SI) demonstrated the Stern-Volmer plots of FTC. From the Stern-Volmer plot, it was an upward curvature and concaved towards y-axis. The slopes increased gradually at the beginning and rose dramatically as the concentration of Fe3+ was more than 15

Organic & Biomolecular Chemistry Accepted Manuscript

In order to further investigate the sensitivity of FTC with Fe3+, the absorption and

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1.0×10-4 M. It is well known that the upward curvature is the characteristic of combination of were 2.90×103 M-1 and 1.54×104 M-1 in ethanol/H2O (v/v = 9/1) and 1.15×104 M-1 and 1.59×105

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M-1 in THF/H2O (v/v = 9/1) for the static and dynamic quenching regions, respectively. As the concentration of quencher (cation) increases, the possibility of collision raises as well as the distance between quencher and fluorophore is shortened which leads to the immediately energy transfer from fluorophore to quencher. Table 2. Stern-Volmer constants of ethanol and THF systems. Ksv(1)

Ksv(2)

ethanol/H2O = 9/1

2.90 ×103

1.54×104

THF/H2O = 9/1

1.15×104

1.59×105

Further comparison of the Ksv values between two mixture solvent systems (THF/H2O = 9/1 and ethanol/H2O = 9/1). Table 2 reveals that the Ksv values of THF system were almost one order magnitude larger than those of the ethanol system. The Ksv value can be estimated by equation (2), where kq is bimolecular quenching constant, τ0 was the lifetime of fluorophore in the absence of quencher, fQ is the quenching efficiency, k0 is diffusion controlled bimolecular rate constant, R is the collision radius which is generally assumed to be the sum of the molecular radii of the fluorophore (Rf) and quencher (Rq), and D is the sum of the diffusion coefficients of the fluorophore (Df) and quencher (Dq), and N was Avogadro's number. And equation 3 is used to calculate D (diffusion coefficient), where κ is Boltzmann's constant, T is temperature, η is the solvent viscosity, and R is the molecular radius.38

K sv  kq 0  ( f Q k0 ) 0  f Q 0 ( 4RDN / 1000)

(Eq. 2)

D  T / 6R

(Eq. 3)

Accordingly, the Ksv value is inversely proportional to viscosity. Moreover, the viscosity of THF (0.48 cps) is much lower than that of ethanol (1.07 cp), which leads to the larger Ksv value in THF/H2O mixture. Furthermore, it has long been known that conjugated polymers-based fluorescent sensors possess high sensitivity, which is due to the signal is transformed and amplified 16

Organic & Biomolecular Chemistry Accepted Manuscript

dynamic and static quenching. 38 Therefore, the Stern-Volmer constants (Ksv) of FTC toward Fe3+

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by the highly conjugated backbone.39,40 Comparing to small molecules, conjugated polymers is azacrown ether as the recognition moieties; the Ksv are comparable with our previously conjugated THF/H2O and ethanol/H2O are detectable by naked eye (Figure 8). These results suggest that FTC is a promising candidate for sensing Fe3+. (b)

(a)

Figure 8. Color variations of FTC added with various concentrations of Fe3+ in solution. (a) ethanol/H2O = 9/1 and (b) THF/H2O = 9/1. (bottom images: excited with 365 nm wavelength)

140 FTC FTC + metal ions FTC + metal ions +Fe3+

120

100

PL Intensity (a.u.)

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polymer sensors PFC (Ksv =1.1 ×103 M-1).41 In addition, color variations of the solutions in both

80

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40

20

0 450

500

550

600

650

Wavelength (nm)

Figure 9. Fluorescence spectral quenching of FTC by Fe3+ interfered by various metal ions (Li+ + Na+ + K+ + Ca2+ + Ba2+ + Co2+ + Ni2+ + Pb2+ + Zn2+ + Co2+) (THF/H2O = 9/1, v/v). The concentration of FTC and metal ions were fixed at 10-5 M and 10-3 M, respectively. Excitation wavelength was 419 nm. Selectivity of FTC towards Fe3+

17

Organic & Biomolecular Chemistry Accepted Manuscript

more sensitive towards analytes. However, FTC which is a small molecular, possesses triple

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In the real applications, various metal ions often coexist in the contaminated solution. Therefore, it is important to investigate the selectivity of FTC towards Fe3+ in the stock solution.

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ions (Figure 9). However, further addition of Fe3+ quenches the fluorescence significantly. Therefore, FTC shows very high selectivity to Fe3+ against the interfering cations. Influence of pH on Absorption and Fluorescence Spectra of FTC The azacrown ether (monoaza-15-crown-5) in FTC is directly linked to the conjugated fluorenyl core via nitrogen atom. There is one lone pair within the nitrogen atom which might be protonated by the addition of acid.42 The absorption and PL spectra of FTC with hydrochloric acid in mixture solvent system (THF/H2O = 9/1) had been investigated (Figure S13 and Figure S14 in SI). As the concentration of hydrochloric acid reached 1.0×10-2 M, the absorption and fluorescence spectra of FTC started to blue-shift and decrease emission intensity. This is probably attributed to THF was also protonated by hydrochloric acid, the start point of spectra change was 1.0×10-2 M which was larger than the concentration of FTC (1.0×10-5 M). The remarkable blue-shift was attributed to the protonation of nitrogen atom, which resulted in PCT phenomenon. The proton (H+) readily interacted with the lone pair electrons. The excited state was more strongly destabilized by the H+ than the ground state, resulting in a blue-shift of the absorption and emission spectra. Therefore, FTC was not only applicable as a Fe3+ sensor but also as a pH sensor. Electrochemical properties Cyclic voltammetry (CV) was employed to investigate the electrochemical properties of FTC. The cyclic voltammograms are shown in Figure S15 in (SI), with the representative electrochemical data summarized in Table 3. HOMO and LUMO energy levels were estimated by the equations: EHOMO (eV) = − (Eox,FOC + 4.8) and ELUMO (eV) = − (Ered,FOC + 4.8), where Eox,FOC and Ered,FOC are the

onset

oxidation

and

onset

reduction

potentials,

respectively,

relative

to

the

ferrocene/ferrocenium couple whose energy level is already known (-4.8 eV). The HOMO and LUMO energy levels of FTC were estimated to be -5.88 eV and -2.88 eV, respectively as well as its electrochemical band-gap (Egel) was 3.0 eV. On the other hand, the optical band gap (Egopt.) was estimated to 2.55 eV from the onset absorption wavelength 486 nm obtained from the absorption spectrum of FTC in the film state

18

Organic & Biomolecular Chemistry Accepted Manuscript

The fluorescence spectral intensity of FTC diminishes only slightly in the presence of ten metal

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(Figure 1). The energy band gaps obtained by optical method and cyclic voltammetry were 2.55 eV LUMO − HOMO, is slightly different from optical band-gap (Egopt) obtained from onset

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absorption. The electrochemical band-gap (Egel) is the energy difference between the LUMO and HOMO levels, which in turn are estimated from the onset oxidation and reduction potentials, respectively. However, oxidation and reduction may start from different parts of a molecule which is composed of electron-donating and electron-withdrawing groups. Therefore, the discrepancy between the band-gaps determined electrochemically (Egel) and optically (Egopt) is probably due to their different estimating bases. Due to FTC is comprised of conjugated fluorene core and three crown ethers, oxidation may start from the electron-donating crown ether parts, and reduction may start from the conjugated core of FTC. Table 3. Electrochemical properties of FTC Eonset(ox)

Eonset(red)

EHOMO

ELUMO

Egel

(eV)b

(eV)b

(eV)c

-5.88

-2.88

3.00

Compound vs. FOC(V)a vs. FOC(V)a FTC

1.08

-1.92

a

EFOC= 0.49 V vs. Ag/AgCl. EHOMO= -(Eonset(ox), FOC +4.8) eV; ELUMO = -(Eonset(red), FOC + 4.8 ) eV. c Eg = |LUMO – HOMO| b

Electroluminescent Enhancement of PLEDs by inserting FTC According to the energy band diagrams depicted in Figure 10, the energy barrier between aluminum (A1) and MEH-PPV (1.60 eV) is larger than that between PEDOT:PSS and MEH-PPV (0.02 eV). Moreover, holes are usually more readily transported than electrons in conjugated materials, leading to reduced recombination ratio of holes and electrons. However, the charge injection/transport imbalance in MEH-PPV can be alleviated by inserting the electron injection and hole-blocking layer FTC in which both LUMO and HOMO energy levels are lower than those of MEH-PPV. The lowered LUMO and HOMO levels give rise to enhanced electron injection/transport and hole-blocking, respectively, in MEH-PPV based devices, resulting in more balanced charges recombination.

19

Organic & Biomolecular Chemistry Accepted Manuscript

and 3 eV, respectively. Electrochemically-estimated band-gap (Egel), calculated using Egel =

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DOI: 10.1039/C3OB42054J

-2.88

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MEH-PPV Al FTC

ITO

-4.3

PEDOT

-4.8

-5.0

-5.02

-5.88

Figure 10. The energy band diagrams of MEH-PPV, with those of FTC, PEDOT:PSS, and electrodes. Multi-layer

polymer

light-emitting

diodes

(PLEDs)

with

a

configuration

of

ITO/PEDOT:PSS/MEH-PPV(90~100 nm)/EIL/Al(80 nm) were fabricated to investigate their electroluminescent characteristics. In this work, we propose the use of a water/alcohol-soluble FTC blended with M2CO3 (M = Na, K, Cs) as electron injection layer (EIL) in a PLED with MEH-PPV as the emitting layer (EML), the optimal condition for processing the EIL materials in PLEDs was to use water and alcohol (v/v = 1/5) as solvent for spin-coating. The ratio of FTC/M+ was fixed at 1:9

(azacrown/M+

=

1:3)

and

the

concentration

was

1

mg

FTC/ml.

Current

density–luminance–voltage (J–L–V) and luminous efficiency–current density (LE–J) characteristics of the devices are shown in Figure 11 and Figure 12, with the corresponding optoelectronic data summarized in Table 4. Electroluminescent performances are significantly enhanced by inserting the electron-injection layer (FTC or doped FTC) between emitting layer (MEH-PPV) and Al cathode. The turn-on voltages are slightly decreased from 4.8 V to 4.6 V by inserting the electron-injection layer (EIL: neat FTC) between emitting layer (MEH-PPV) and Al cathode (Table 4). This is mainly attributed to effectively promoted electron injection/transport with FTC as EIL.

20

Organic & Biomolecular Chemistry Accepted Manuscript

-2.70

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Non EIL FTC FTC + Na2CO3 FTC + K2CO3 FTC + Cs2CO3

103

102

101 0

2

4

6

8

10

12

8

10

12

Voltage (V) 1000

2

Current density (mA/cm )

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2

Luminance (cd/m )

104

Non EIL FTC FTC + Na2CO3

800

FTC + K2CO3 FTC + Cs2CO3

600

400

200

0 0

2

4

6

Voltage (V)

Figure 11. Luminance versus voltage and current density versus voltage characteristics of PLEDs. Device structure: ITO/PEDOT:PSS/MEH-PPV(90~100 nm)/EIL/Al(80 nm). Moreover, maximum luminance and current efficiency of devices were greatly increased from 230 cd/m2 and 0.03 cd/A to 1990 cd/m2 and 0.65 cd/A, respectively, by inserting the electron-injection layer (EIL: neat FTC). The performance enhancement is attributable to the hole-blocking and electron injection effect of FTC that raises charges recombination ratio. The lower HOMO level of 21

Organic & Biomolecular Chemistry Accepted Manuscript

DOI: 10.1039/C3OB42054J

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FTC (-5.88 eV) than emitting layer MEH-PPV (-5.02 eV) results in the accumulation of holes in the interface between emitting and FTC layers. Moreover, the lower LUMO level of FTC (-2.88

recombination ratio.

1.6 1.4

Luminous efficiency (cd/A)

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holes will readily combine with electrons passing through the interface to increase the

1.2 1.0 0.8 Non EIL FTC FTC + Na2CO3

0.6 0.4

FTC + K2CO3

0.2

FTC + Cs2CO3

0.0 0

200

400

600

800

Current density (mA/cm2)

Figure 12. Luminous efficiency versus current density characteristics of PLEDs. Device structure: ITO/PEDOT:PSS/MEH-PPV(90~100 nm)/EIL/Al(80 nm). The device performances using MEH-PPV as emitting layer were further significantly enhanced in the presence of metal carbonates M2CO3 (M = Na, K, Cs) (Table 4). The turn-on voltages with doped-FTC are greatly decreased from 4.8 V to 3.1 V due to abundant azacrown-chelated M+. The chelated metal cation (M+) can further reduce the electron-injection barrier through the formation of an interfacial dipole and the establishment of an intermediate step for electron injection.43 The devices using K2CO3-doped FTC as EIL reveal the best performance, i.e., the maximum luninance and current efficiencies of the device based on MEH-PPV were further promoted to 11630 cd/m2 and 1.47 cd/A, respectively (Table 4). Moreover, these performances are superior to those obtained with conventional Ca/Al cathode (3230 cd/m2 and 0.29 cd/A) (Table 4). In addition, The 1931 CIE coordinates (x, y) of the EL emission shift slightly from (0.51, 0.49) to 22

Organic & Biomolecular Chemistry Accepted Manuscript

eV) than emitting layer MEH-PPV (-2.70 eV) promotes electron injection (Figure 10). Therefore,

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(0.57, 0.43) with non-FTC, FTC and doped-FTC, which originated from MEH-PPV emission (Figure 13). Consequently, the potassium carbonate-doped FTC as EIL effectively promoted

Organic & Biomolecular Chemistry Accepted Manuscript

electron injection/transport to significantly enhance the device performance.

Von

L max

CE max

CIE

(V)b

(cd/m2)c

(cd/A)d

(x, y)f

None

4.8

230

0.03

(0.51, 0.49)

Ca26e

3.3

3230

0.29

(0.54, 0.46)

FTC

4.6

1990

0.65

(0.57, 0.43)

FTC + Na2CO3

3.6

5190

1.13

(0.56, 0.43)

FTC + K2CO3

3.1

11630

1.47

(0.54, 0.46)

FTC + Cs2CO3

3.6

7240

1.42

(0.55, 0.45)

EIL

a

[ITO/PEDOT:PSS/MEH-PPV (90~100 nm)/EIL/Al (80nm)]. Turn-on voltage at 10 cd/m2. c Maximum luminance. d Maximum current efficiency. e Maximum luminous power efficiency. f The 1931 CIE coordinate at Maximum luminance. b

1.2

Normalized EL Intensity (a.u.)

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Table 4. Optoelectronic Properties of the Light-emitting Diodes Based on MEH-PPVa

non-EIL FTC FTC + Na2CO3

1.0

FTC + K2CO3

0.8

FTC + Cs2CO3

0.6 0.4 0.2 0.0 250

300

350

400

450

500

550

600

650

700

750

800

Wavelength (nm)

Figure 13. Emission spectra of PLEDs. ITO/PEDOT:PSS/MEH-PPV(90~100 nm)/EIL/Al(80 nm). 23

Device

configuration:

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Conclusions

monoaza-15-crown-5 ether and fluorene-based moiety. The FTC was satisfactorily characterized by

Published on 23 December 2013. Downloaded by St. Petersburg State University on 23/01/2014 12:07:26.

1

H NMR, COSY, NOESY, FT-IR and elemental analysis. The absorption and fluorescence spectra

of FTC red shifted with gradually increasing the polarity of solvents. Fluorescence of FTC solution was selectively quenched by Fe3+ cation, with static (dynamic) Stern-Volmer constants (Ksv) being 1.15×104 M-1 (1.59×105 M-1) in THF/H2O and 2.90×103 M-1 (1.54×104 M-1) in ethanol/H2O mixtures. The selective recognition of FTC toward Fe3+ was still maintained in the presence of ten interfering metal cations. The absorption and fluorescence spectra of FTC were blue-shifted and quenched in the presence of hydrochloric acid, respectively. The HOMO and LUMO energy levels of FTC were estimated to be -5.88 eV and -2.88 eV, respectively. The FTC was a highly efficient EIL, especially after addition with metal carbonates M2CO3 (M = Na, K, Cs). The maximum luminance and current efficiency of devices were greatly increased from 230 cd/m2 and 0.03 cd/A to 1990 cd/m2 and 0.65 cd/A, respectively, by inserting the electron-injection layer (EIL: neat FTC). Furthermore, the device performance based on MEH-PPV with K2CO3-doped FTC as EIL were further promoted to 11630 cd/m2 and 1.47 cd/A, respectively which are superior to those obtained with conventional Ca/Al cathode (3230 cd/m2 and 0.29 cd/A). Current results demonstrate that FTC is not only a promising chemical sensor for the detection of Fe3+ but also applied as electron injection layer effectively promoted electron injection/transport to significantly enhance the device performance of MEH-PPV. References (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Bruns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (2) Akcelrud, L. Prog. Polym. Sci. 2003, 28, 875. (3) (a) Satrijo, A.; Swager, T. M. J. Am. Chem. Soc. 2007, 129, 16020. (b) Kim, J.; McQuade, D. T.; McHugh, S. K.; Swager, T. M. Angew. Chem. Int. Ed. 2000, 39, 3868. (4) Pu, K. Y.; Pan, S. Y. H.; Liu, B. J. Phys. Chem. B 2008, 112, 9295. (5) Colladet, K.; Fourier, S.; Cleij, T. J.; Lutsen, L.; Gelan, J.; Vanderzande, D.; Nguyen, L. H.; Neugebauer, H.; Sariciftci, S.; Aguirre, A.; Janssen, G.; Goovaerts, E. Macromolecules 2007, 40,

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Organic & Biomolecular Chemistry Accepted Manuscript

We have successfully synthesized the multi-functional FTC comprising of tiple

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DOI: 10.1039/C3OB42054J

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Organic & Biomolecular Chemistry Accepted Manuscript

(6) Liu, C. L.; Tsai, J. H.; Lee, W. Y.; Chen, W. C.; Jenekhe, S. A. Macromolecules 2008, 41, 6952.

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(28) Greenwood, N. N., Earnshaw, A. , Chemistry of the Elements. Pergamon Press Inc.: Tarrytown,

A fluorene-based material containing triple azacrown ether groups: synthesis, characterization and application in chemosensors and electroluminescent devices.

We design a novel multifunctional fluorene-based material containing triple azacrown ether (FTC) not only for application in aqueous solution as a che...
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