The synthesis, physicochemical properties, and electrochemical polymerization of fluorene-based derivatives as precursors for conjugated polymers

Tetrahedron Letters 56 (2015) 2574–2578

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The synthesis, physicochemical properties, and electrochemical polymerization of fluorene-based derivatives as precursors for conjugated polymers Ammar Khelifa Baghdouche ⇑, Mounia Guergouri, Salima Mosbah, Lotfi Benmekhbi, Leila Bencharif ⇑ Laboratoire de Chimie des Matériaux Constantine, Université Constantine 1, Constantine 25000, Algeria

a r t i c l e

i n f o

Article history: Received 3 October 2014 Revised 14 March 2015 Accepted 31 March 2015 Available online 4 April 2015 Keywords: 2,7-Bis[(thien-2-yl)cyanovinyl]-9,9dipentylfluorene 2,7-Bis[(2,3-dihydrothieno[3,4b][1,4]dioxin-5-yl)cyanovinyl]-9,9dipentylfluorene Electrochemical polymerization Cyclic voltammetry Band gap

a b s t r a c t Two novel monomers, 2,7-bis[(thien-2-yl)cyanovinyl]-9,9-dipentylfluorene (FPT) and 2,7-bis-[(2,3dihydrothieno[3,4-b][1,4]dioxin-5-yl)cyanovinyl]-9,9-dipentylfluorene (FPE) are synthesized and their electrochemical polymerization is achieved via potentiostatic methods. The corresponding polymers, poly(2,7-bis[(thien-2-yl)cyanovinyl]-9,9-dipentylfluorene) (PFPT) and poly(2,7-bis[(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)cyanovinyl]-9,9-dipentylfluorene) (PFPE), are characterized by cyclic voltammetry, FT-IR, and UV–vis spectroscopy. The band gap values (Eg) of the polymers are found to be 2.21 and 1.92 eV for PFPT and PFPE, respectively. Ó 2015 Published by Elsevier Ltd.

Conjugated polymers have attracted significant attention due to the possibility of their applications in photovoltaic cells, electroluminescent displays, field effect transistors, plastic lasers, and optical sensors.1,2 Among these applications, polymer-based light-emitting diodes have attracted the particular interest of researchers.3,4 The first demonstration of efficient polymer lightemitting diodes (PLEDs) in 19905 resulted in great interest in display applications for conjugated polymers.6 Compared with conventional LEDs, PLEDs offer a wide variety of advantages, such as easy fabrication by spin coating and low cost.7,8 Among the large number of conjugated polymers with different emissive colors, polyfluorenes (PFs) are of significant importance as blue light emitting emissive layers, not only due to their high thermal and chemical stability, but also for their high photoluminescence efficiency and good photostability.9–11 These properties make polyfluorene a material of interest and a large number of PF derivatives have been reported. Moreover, PFs can be substituted at the C-9 position; this facile process provides the opportunity to improve both the solubility and functionality of the resulting polymers. Conducting polymers are prepared by chemical or electrochemical ⇑ Corresponding authors. Tel.: +213 554 013 909 (A.K.B.), +213 773 947 384 (L.B.). E-mail addresses: [email protected] (A. Khelifa Baghdouche), [email protected] (L. Bencharif). http://dx.doi.org/10.1016/j.tetlet.2015.03.130 0040-4039/Ó 2015 Published by Elsevier Ltd.

oxidation of monomeric compounds.12–15 Electrochemical methods of preparation include potentiostatic, potentiodynamic, and galvanostatic.13 The potentiodynamic method is reported to produce films with superior adhesion, smoothness, and optical properties.16–18 PF films obtained by constant potential electrolysis are brittle and hydrogen-rich with electrical conductivity of 104 S/cm. Rault-Berthelot et al. have reported several articles on anodic polymerization and spectroelectrochemical studies of PFs in acetonitrile.19–21 In this letter, we describe the electrochemical behavior and the anodic electropolymerization of new fluorene derivatives consisting of a central alkylfluorene unit substituted at the 2,7-positions with two cyanovinylene-thiophenes or two cyanoviny-lene-(3,4ethylene-dioxythiophenes). Scheme 1 shows the synthetic route to the monomers. The syntheses of 2,7-bis[(thien-2-yl)cyanovinyl]-9,9-dipentylfluorene (FPT) and 2,7-bis[(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)cyanovinyl]-9,9-dipentylfluorene (FPE) started with the alkylation of fluorene followed by chloromethylation to give 2,7-bis(chloromethyl)-9,9-dipentylfluorene (2). Compound 3 was obtained by cyanation of compound 2. Finally, condensation reactions of compound 3 with thiophene-2-carboxaldehyde and 3,4-ethylenedioxythiophene-2-carboxaldehyde gave the desired monomers, FPT and FPE.

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2

HCHO/HCl

Br KOH/DMSO

Cl

Cl

Ac2O

74%

50%

1

2 KCN DMF

O

2 S

NC

H

CN

S

S

CN

NC

t-BuOK/EtOH

65%

O

FPT

O

60%

2

O

O S

S

3

O

O

O

S H t-BuOK/EtOH

CN

NC

50%

FPE Scheme 1. Synthesis of the monomers FPT and FPE.

3.0 S

S

O

O

O

O S

2.0

CN

NC

2.5 CN

NC

S

FPT 1

I/mA

Next, the electrochemical behavior of these fluorene-based monomers was investigated by cyclic voltammetry. During the first positive scan, FPT and FPE exhibited irreversible peaks (EOx m) at 1.40 V and 1.20 V versus SCE, respectively. (Fig. 1), which correspond to the transfer of an electron from the HOMO level of the monomer to the working electrode of the electrochemical system. After the determination of the redox behavior of FPT and FPE, repetitive anodic scans were performed to obtain their corresponding polymers, PFPT and PFPE. For FPE, The electrochemical polymerization was performed with a platinum disk electrode between 0.36 V and 1.26 V. Due to the limited solubility of FPE in MeCN, a mixture of CH2Cl2 and MeCN (1:10 by volume) was used as the solvent. On the other hand, repetitive cycling between 0.36 V and 1.46 V was used for the polymerization of FPT in Bu4NBF4 (0.1 M)/MeCN electrolyte solution. As shown in Figure 2, new redox couples intensified during each successive scan indicating the formation of electroactive polymer films on the surface of the working electrode, with increasing polymer film thickness.22 The electrochemical behavior of the polymeric films previously

1.5

FPE

1.0 0.5 0 0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

E/V vs. SCE Figure 1. Cyclic voltammograms of FPE (2.0  103 M), FPT (2.0  103 M) and 9,9dipentylfluorene (1) (2.0  103 M) in Bu4NBF4 (0.1 M)/MeCN solution at a scan rate of 100 mV/s.

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S

4

CN

NC

S

S

NC

CN

FPT

O

O

O

6

Electrochemical Polymerization

O

O

S

S

n

CN

NC

Electrochemical Polymerization

O

O

O S

S

NC

CN

S n

4

PFPT

PFPE

FPE

2

I/mA

I/mA

2

0

0

(b)

-2

(a) -2

-4

0.4

0.6

0.8

1.0

1.2

0.4

1.4

0.6

0.8

1.0

1.2

E/V vs. SCE

E/V vs. SCE

Figure 2. Electropolymerization of 2.0  103 M (a) FPT in Bu4NBF4 (0.1 M)/MeCN and (b) FPE in Bu4NBF4 (0.1 M)/CH2Cl2/MeCN (1:10) at a scan rate of 100 mV/s.

(a)

(b) 200 mV/s S

NC

CN

8

8

S n

6 4

I/mA

I/mA

PFPT

4

Ia.c 2

20 mV/s

Ic.c

0

0

-2

-4 0.4

0.6

0.8

1.0

1.2

1.4

0

1.6

20

40

60

O

O

O NC

CN

200 mV/s

S

8

n

8

6 4

20 mV/s

0

I/mA

PFPE

4

I/mA

10

O S

100 120 140 160 180 200

Scan rate/mV s-1

E/V vs. SCE 12

80

Ia.c

2 0

Ic.c

-2 -4

-4

-6 -8

-8 0.4

0.6

0.8

1.0

1.2

E/V vs. SCE

0

20

40

60

80 100 120 140 160 180 200 220

Scan rate/mV s-1

Figure 3. (a) Scan rate dependence of polymer films on a Pt disk electrode in Bu4NBF4 (0.1 M)/MeCN and (b) the relationship of anodic (Ia,c) and cathodic (Ic,c) current peaks as a function of scan rate between doped and dedoped states.

deposited on the working electrode was examined in another cell containing only the MeCN/Bu4NBF4 (0.1 M) electrolytic solution without monomer. These films were cycled repeatedly between doped and dedoped states without significant decomposition of the material. The corresponding cyclic voltammograms are presented in Figure 3. As can be seen from Figure 3, PFPT and PFPE exhibited single reversible redox couples at 1.30 V and 1.05 V, respectively. The reversible redox system obtained during the successive scans corresponds to a positive doping polymer derived

from oxidation and from the accumulation of charges due to BF4 anions (known as polarons or bipolarons) in the polymer matrix; these charges are at the root of the conductivity of the material.23 The scan rate dependence of the anodic and cathodic peak current densities was studied in a monomer-free electrolyte solution (Fig. 3b). A linear increase in the peak currents as a function of the scan rate confirmed well adhered electroactive polymer films on the electrode surface as well as non-diffusional redox processes.24

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(a)

(b) PFPE

PFPT

FPE

FPT

1000

2000

1000

3000

2000

3000 -1

-1

Wavenumber (cm )

Wavenumber (cm )

Figure 4. IR spectra of (a) PFPT and FPT, (b) PFPE and FPE.

In addition, IR spectroscopic analysis of the monomers and the polymers gave valuable information on the polymer structure. Figure 4 shows the IR spectra of the PFPT and PFPE polymers and their corresponding monomers. According to the spectra of the monomers, FPT and FPE exhibit characteristic peaks at 3108–3047 cm1 (aromatic C–H stretching), at 2857–2952 cm1 (C–H stretching of the pentyl chains), and at 1170 and 1266 cm1 (C–O stretching). The peaks at 680–718 cm1 are due to the 1,2-disubstituted benzene rings of the fluorene precursor.25 When the monomers are formed after the coupling reaction, the peak around 680–718 cm1 lost intensity and a new peak appeared at 819 cm1, which is characteristic of 1,2,4-trisubstituted benzene rings. The peak observed at 1677 cm1 corresponds to the vibration of the carbon-carbon bond between two successive monomer units and appears only in the polymer spectrum.26 The peak at 2210 cm1 indicates the presence of the nitrile group in the polymer chain. The new peak that appears at 1068 cm1 is attributed to the presence of a BF4 counterion in the polymer chains.27 Finally, the presence of signals recorded at 709 cm1 and at 1409 cm1 in the IR spectrum of PFPT, which are attributed to the bending of the Ca–H bond, prove that this position is not the only one which participates in the polymerization process. In fact, the b-position also participates in the polymerization, but to a

(a)

1.0

291 nm

0.8

0.8

0.6

0.6

0.4 opt

Band gap : 2.12 eV 585 nm

opt

Band gap : 2.87 eV

0.2

(b) 340 nm

FPT PFPT

498 nm

Absorbance

Absorbance

1.0

lesser degree. The presence of multi Ca–H bonds in the polymer was confirmed by the presence of an oxidative peak at 1.30 V in the electrochemical response of the polymer (Fig. 3a) accompanying the redox peak of the doped/dedoped polymer. In the case of PFPE, there is only one redox peak recorded in the electrochemical behavior of the polymer, due to the regioselectivity of the polymerization. Figure 5 depicts the UV–vis spectra recorded in DMSO solution of each monomer and polymer in the oxidized state. The absorption maximum (kmax) of the FPT and FPE monomers is centered at 291 and 340 nm, respectively. On the other hand, the kmax of PFPT was observed at 498 nm, while that of PFPE occurred at 567 nm. The difference between the kmax corresponding to the monomer and the corresponding polymer is linked to the shorter effective conjugation length of PFPT than that of PFPE. Using the optical spectra of the monomers and polymers in the oxidized state, and their voltammograms including p and n-doping cycles, it is possible to estimate the energy gap for the conducting polymers. The optical gap is calculated according to Eopt g = 1240/konset, and konset is obtained from the intersection between the baseline and the tangent of the UV–vis band-end. Cyclic voltammetry (CV) was employed to investigate the redox behavior of the conjugated polymers and to estimate their highest occupied molecular

0.4

opt

Band gap : 1.88 eV 658 nm opt

Band gap : 2.72 eV 456 nm

0.2

432 nm

0

FPE PFPE

567 nm

0 300

400

500

600

700

Wavelength (nm)

800

900

300

400

500

600

700

Wavelength (nm)

Figure 5. UV–vis absorption spectra of (a) FPT and PFPT, (b) FPE and PFPE in DMSO.

800

900

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8

el

8

Band gap : 2.21 eV HOMO: -5.59 eV ox E onset: 1.19 V

HOMO: -5.19 eV ox E onset: 0.79 V

4

I/mA

I/mA

4

el

Band gap : 1.92 eV

0

0 -4

red

E

: -1.02 V

onset

PFPT

LUMO: -3.38 eV

-4

red

E onset: -1.13 V LUMO: -3.27 eV

-1

0

-8

PFPE -2

1

-1

0

1

E/V vs. SCE

E/V vs. SCE

Figure 6. Cyclic voltammograms of PFPT and PFPE films in Bu4NBF4/MeCN at a scan rate of 100 mV s1.

Supplementary data

Table 1 Electrochemical and optical band gaps

FPT PFPT FPE PFPE

Eox onset (V)a

Ered onset (V)b

EHOMO (eV)c

ELUMO (eV)d

Eel g (eV)e

kmax (nm)

konset (nm)

Eopt g (eV)f

1.21 1.19 1.05 0.79

1.48 1.02 1.46 1.13

5.61 5.59 5.45 5.19

2.92 3.38 2.94 3.27

2.69 2.21 2.51 1.92

291 498 340 567

432 585 456 658

2.87 2.12 2.72 1.88

a First oxidation potentials of the monomers and polymers from CV measurements. b First reduction potentials of the monomers and polymers from CV measurements. c Energy of the highest occupied molecular orbital calculated from EHOMO = (Eox onset + 4.4). d Energy of the lowest unoccupied molecular orbital calculated from ELUMO = (Ered onset + 4.4). e Energy of the band gap calculated from the difference between the energy of the HOMO and the LUMO. f Energy of the band gap calculated from UV–vis spectroscopy, Eopt g = 1240/konset.

orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels from the onset of oxidation and onset of reduction potentials according to the equations:28

EHOMO ðeVÞ ¼ ½Eox onset ðvs SCEÞ þ 4:4 ELUMO ðeVÞ ¼ ½Ered onset ðvs SCEÞ þ 4:4 red where Eox onset and Eonset are the onset potentials of oxidation and reduction respectively; the onset potentials are determined from the intersection of the two tangents drawn at the rising current and baseline charging current of the CV traces. The obtained results are presented in Figures 5 and 6, and in Table 1. In summary, we have successfully synthesized two novel monomers, FPT and FPE, and their corresponding polymers. The PFPT and PFPE polymers were obtained via potential cycling in appropriate solvent–electrolyte mixtures and they were characterized by electrochemical and spectroscopic methods. The electrochemical gap of PFPE (1.92 eV) is lower than that of PFPT (2.21 eV).

Acknowledgments The authors are grateful to the Department of Chemistry, Constantine 1 University, for the financial support.

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