Novel poly(2,5-dithienylpyrrole) (PSNS) derivatives functionalized with azobenzene, coumarin and fluorescein chromophore units: Spectroelectrochemical properties and electrochromic device applications

Article abstract of DOI:10.1039/c4nj02098g

Three novel 2,5-dithienylpyrrole (SNS) derivatives containing strong chromophore units such as azobenzene, coumarin and fluorescein were synthesized to investigate the effects of chromophore substituents on the electrochemical and spectroelectrochemical p

Full text of DOI:10.1039/c4nj02098g

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ARTICLE TYPE
Novel poly(2,5ꢀdithienylpyrrole) (PSNS) derivatives functionalized with
azobenzene, coumarine and fluorescein chromphore units:
Spectroelectrochemical properties and electrochromic device
applications
5
Deniz Yiğit,a ꢁerife O. Hacıoğlu,b Mustafa Güllü*a and Levent Toppareb,c,d,e
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Three novel 2,5ꢀdithienylpyrrole (SNS) derivatives containing strong chromophore units such as
azobenzene, coumarine and fluorescein were synthesized to investigate the effects of chromophore
10 substituents on the electrochemical and spectroelectrochemical properties of resulting polymers.
Electrochemical and optoelectronic characteristics of the homopolymers, poly(1ꢀ(2ꢀ(4ꢀ
(phenyldiazenyl)phenoxy)ethyl)ꢀ2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrole) (PTPTAz), poly(4ꢀ(2ꢀ(2,5ꢀ
di(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethoxy)ꢀ2Hꢀchromenꢀ2ꢀone) (PTPTCo) and poly(methyl 2ꢀ(6ꢀ(2ꢀ(2,5ꢀ
di(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethoxy)ꢀ3ꢀoxoꢀ3Hꢀxanthenꢀ9ꢀyl)benzoate) (PTPTFlo), were
15 investigated in detail. The combination of a strong chromophore pendant group with poly(2,5ꢀ
dithienylpyrrole) (PSNS) backbone significantly influences electronic and optoelectronics behaviors of
conducting polymers. PTPTAz and PTPTCo exhibited different colors in their neutral and oxidized states
while PTPTFlo showed yellow color in all states. The optical band gap (Eg) values of PTPTAz, PTPTCo
and PTPTFlo films were calculated as 2.81 eV, 2.44 eV and 2.31 eV respectively. Moreover, a
20 PTPTFlo/PEDOT based dual type solid state electrochromic device (ECD) was constructed. The ECD
exhibited quite well longꢀterm stability with reasonable optical memory performance under ambient
conditions.
charge carriers (polarons and bipolarons) and incorporation of
Introduction
45 counter ions into polymer backbones (doping process). The
structural modification on polymer chain results in a second
colored or transmissive state.25,26 The color and other
spectroelectrochemical behaviors of a conducting polymer mostly
depend on the band gap (Eg). Hence, in the context of the band
50 gap tuning, the design and synthesis of monomer is a crucial tool
to control electrochromic properties of a conducting polymer and
obtain unique electrochromic materials. Till now, various
directions have been developed for structural modification of
conducting polymers however, polymer backbone alteration and
55 diverse pendant group or substituent approaches can be
considered as the most effective ones among different
concepts.27ꢀ32
Due to such advantages of low cost synthesis, easy derivatization
25 and impeccable integration into many electronic devices, πꢀ
conjugated conducting polymers (CPs) have attracted great
attention over the last decade. CPs have been extremely used in a
wide field of industrial applications including organic light
emitting diodes (OLEDs),1,2 field effect transistors (FETs),3,4
30 electrochromic devices,5ꢀ7 sensors,8,9 organic photovoltaics
(OPVs),10ꢀ12 and supercapacitors (SCs).13ꢀ16 Particularly, the use
of CPs as electrochromic materials has been widely investigated
since πꢀconjugated conducting polymers exhibit reversible and
steady color changes with low potentials, high optical contrast in
35 visible and near infraꢀred (NIR) region and have high coloriation
efficiency and fast switching times.17ꢀ22
Polythiophene, polypyrrole and their derivatives have been most
60 commonly utilized as active layers in electrochromic device
applications owing to their low band gap, good conductivity, low
oxidation potential, high optical contrast, high stability in their
oxidized form and multicolor electrochromism.33ꢀ39 In addition to
polythiophene and polypyrrole derivatives, recently, poly(2,5ꢀ
65 dithienylpyrrole) derivatives (PSNS) have become promising πꢀ
conjugated conducting polymers for electrochromic applications
The electrochromism phenomenon in conducting polymers is
related to dopingꢀundoping processes of electroactive polymers.
The great majority of electrochromic polymers are colored in
40 their neutral states (πꢀπ* transition) since their band gap (Eg) lies
within the visible region of electromagnetic spectrum.23,24 Upon
oxidation or reduction, new lowꢀenergy transitions emerge in the
band gap (Eg) of conducting polymer on account of formation of
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due to their relatively lower oxidation potential, multichromic
toluene (anhydrous, 99.8%), pꢀtoluenesulfonyl chloride
( ≥
properties, high chemical and electrochemical stability, effortless 60 99.0%), triethylamine (Et3N,
≥
99%), lithium perchlorate
synthesis methods. 2,5ꢀDithienylpyrrole type monomers (SNS)
can be also easily modified with multifarious electronꢀdonating
and electronꢀwithdrawing functional groups through the nitrogen
atom in pyrrole ring of SNS unit.45ꢀ48 Even though many
(LiClO4, 99.999%), sodium perchlorate (NaClO4, ≥ 98%), cesium
carbonate (Cs2CO3, 99%), acetonitrile (ACN, HPLC grade), N,Nꢀ
dimethylformamide (DMF, anhydrous, 99.8%), dichloromethane
(DCM, anhydrous, ≥ 99.8% ), 4ꢀhydroxyazobenzene, 4ꢀhydroxyꢀ
5
researchers reported on the
synthesis and electrochromic 65 2Hꢀchromenꢀ2ꢀone,
3,4ꢀethylenedioxythiophene
(EDOT),
properties of PSNS derivatives in the literature, the majority of
these PSNS derivatives have been functionalized using similar
platinum wire (Pt, 0.25 mm diameter, 99.999%) and silver wire
(Ag, 0.05 mm diameter, 99.9%) were purchased from Sigma
Aldrich and used without any further purification. Methyl 2ꢀ(6ꢀ
hydroxyꢀ3ꢀoxoꢀ3Hꢀxanthenꢀ9ꢀyl)benzoate,51 1,4ꢀdi(thiophenꢀ2ꢀ
10 substituted aryl and alkyl amine compounds.45ꢀ48 Hence, design
and synthesis of novel SNS derivatives containing different
pendant substituents is quite necessary in terms of obtaining 70 yl)butaneꢀ1,4ꢀdione (1)41 and 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ
excellent precursor compounds for producing SNS type πꢀ
conjugated conducting polymers with better electrochemical and
15 spectroelectrochemical properties.
1ꢀyl)ethanol (2)41 were synthesized according to previously
described methods. Microwaveꢀassisted nucleophilic substitution
reactions were performed using a CEM Discover SꢀClass singleꢀ
mode microwave synthesis instrument. Electrochemical studies
In the context, we report herein the synthesis and electrochromic 75 were carried out with a Radiometer VoltaLab PST 050
properties of three novel 2,5ꢀdithienylpyrrole (SNS) type πꢀ
conjugated conducting polymers, namely poly(1ꢀ(2ꢀ(4ꢀ
20 (phenyldiazenyl)phenoxy)ethyl)ꢀ2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀ
pyrrole) (PTPTAz), poly(4ꢀ(2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ
potentiostat/galvanostat in a threeꢀelectrode cell configuration
consisting an indium tin oxide doped glass slide (ITO, 12 ꢁ sqꢀ1),
a platinum wire and a silver wire as the working, counter and
pseudoꢀreference (+0.3 V vs Fc/Fc+) electrodes, respectively. The
1ꢀyl)ethoxy)ꢀ2Hꢀchromenꢀ2ꢀone) (PTPTCo) and poly(methyl 2ꢀ 80 homopolymers were electrochemically deposited with cyclic
(6ꢀ(2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethoxy)ꢀ3ꢀoxoꢀ3Hꢀ
xanthenꢀ9ꢀyl)benzoate) (PTPTFlo), functionalized with strong
25 chromophore units such as azobenzene, coumarine and
fluorescein which are highly optically potent in the UVꢀVis
voltammetry technique (CV) in the presence of 0.05 M monomer
in 0.1 M solution of NaClO4ꢀLiClO4/ACN/DCM (95:5, v/v)
supporting electrolyte under nitrogen atmosphere at a scan rate of
100 mV sꢀ1 for 10 cycles. The spectroelectrochemical studies of
region. The new approach based on the combination of 85 the homopolymers films on ITO surfaces were done via a Varian
chromophore groups with monomer chains was reported
previously by our group and interesting electrochemical and
30 spectroelectrochemical behaviors were observed for terthienyl
derivatives containing the same chromophore units.49,50 In the
Carry 5000 spectrophotometer at a scan rate of 2000 nm minꢀ1.
The potentials were controlled by using a Solartron 1285
potentiostat/galvanostat. Konica Minalto CSꢀ100 colorimeter in a
Pantone Colorviewing Light PJC/CV L3 Light Box 3 was used to
light of our preliminary results, it is expected that similar 90 perform color analysis of the polymers. FTIR spectra were
electrochromic behaviors can be obtained through the
incorporation of the strong chromophore groups and 2,5ꢀ
35 dithienylpyrrole (SNS) monomer chain. For this purpose, the
electroactive SNS type monomers bearing an azobenzene, a
recorded on a PerkinꢀElmer Spectrum 100 spectrometer. 1H NMR
and 13C NMR spectra were recorded in CDCl3 at room
temperature on
a VarianꢀMercury 400 MHz (FT)ꢀNMR
spectrometer. All chemical shifts were given in ppm downfield
coumarine or a fluorescein unit attached to 2,5ꢀdithienylpyrrole 95 from tetramethylsilane (TMS). Mass spectra were recorded using
monomer chain via an ethylene bridge, respectively TPTAz,
TPTCo and TPTFlo, were firstly synthesized and their
40 homopolymers, PTPTAz, PTPTCo and PTPTFlo, were
electrochemically prepared on ITO glass slide electrodes. The
electronic and optical properties of homopolymers were 100
investigated by cyclic voltammetry and spectroelectrochemical
techniques. The effects of type of chromophore units as a pendant
45 substituent on electrochemical and spectroelectrochemical
behaviors of SNS type πꢀconjugated conducting polymers were
also evaluated and compared in detail. Besides, a dual type
a DIꢀ2010 direct inlet probe on a Shimadzu GCꢀMS QP2010
mass spectrometer operating at ionization potential (EI) of 70 eV.
Elemental analyses were conducted by Eurovector CHNS
Elemental Analyzer.
Syntheses
The SNS type novel monomers (TPTAz, TPTCo and TPTFlo)
containing strong chromophore groups were synthesized in three
105 steps as explained below.
electrochromic
device
(ECD)
based
on
poly(3,4ꢀ
Synthesis of 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethyl 4ꢀ
methylbenzenesulfonate (3)
ethylenedioxythiophene) (PEDOT) / PTPTFlo was constructed
50 and its electrochromic performance was characterized.
2ꢀ(2,5ꢀDi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethanol (2) was firstly
110 prepared according to an earlier reported work.41 In a 100 mL
twoꢀnecked roundꢀbottomed flask, 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀ
pyrrolꢀ1ꢀyl)ethanol (2) (10 mmol) and pꢀtoluenesulfonyl chloride
(10.54 mmol) were dissolved in THF (30 mL). Aqueous solution
of NaOH (30 mmol) /10 mL H2O was added dropwise to the
115 solution at 0˚C and then, the suspension was stirred at room
temperature overnight. The solvent was removed on a rotary
Experimental section
Materials and instrumentation
55
All chemicals, including thiophene (≥ 99.0%), succinyl dichloride
(95%), aluminium (III) chloride (AlCl3, anhydrous, 99.999%),
ethanolamine (≥ 99%), pꢀtoluenesulfonic acid (PTSA, ≥ 99.5%),
2
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evaporator under reduced pressure. The oily residue was
extracted with chloroform (3 x 25 mL), the combined organic
methylbenzene sulfonate (3) and 4ꢀhydroxyꢀ2Hꢀchromenꢀ2ꢀone.
The crude product was subjected to column chromatography
phase was washed with water and brine, then dried over 60 using hexane/CH2Cl2 (5:1, v/v) as the eluent (Rf, 0.35). TPTCo
anhydrous Na2SO4. After removal of the chloroform by
evaporation, the crude product was purified by column
chromatography using dichloromethane as eluent (Rf, 0.6).
2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethyl 4ꢀmethylbenzene
was obtained as a dark yellow solid (0.36 g, 0.87 mmol, 87%, mp
148ꢀ151˚C). FTIR νmax/cmꢀ1 3109, 3083, 2980, 2959, 1710, 1653,
1620, 1610, 1500, 1491, 1455, 1351, 1317, 1236, 1181, 1109,
1039. 1H NMR δH (400 MHz, CDCl3, Me4Si) 4.08 (t, J = 6 Hz,
5
sulfonate (3) was obtained as a light brown solid (3.98 g, 9.3 65 2H, CH2), 4.77 (t, J = 6 Hz, 2H, CH2), 5.36 (s, 1H, ꢀC=CHꢀ), 6.38
mmol, 93%, mp 68ꢀ70˚C). FTIR νmax/cmꢀ1 3090, 2930, 1729,
(s, 2H, Ar H), 7.08ꢀ7.18 (m, 4H, Ar H), 7.34ꢀ7.35 (dd, J = 6.5 Hz
and J = 1.2 Hz, 2H, Ar H), 7.4 (dd, J = 5.0 Hz and J = 1.2 Hz,
1H, Ar H), 7.47ꢀ7.51 (m, 1H, Ar H), 7.64 (dd, J = 5.2 Hz and J =
1.2 Hz, 1H, Ar H), 7.81 (dd, J = 5.2 Hz and J = 1.2 Hz, 1H, Ar
1
10 1593, 1492, 1415, 1385, 1365, 1295, 1187, 1169. H NMR δH
(400 MHz, CDCl3, Me4Si) 2.45 (s, 3H, CH3), 3.92 (t, J= 6 Hz,
2H, CH2ꢀ), 4.39 (t, J= 6 Hz, 2H, CH2ꢀ), 6.2 (s, 2H, ArH), 6.96ꢀ
7.06 (m, 4H, ArH), 7.25ꢀ7.31 (m, 4H, ArH), 7.53 (d, J= 8.4 Hz, 70 H); 13C NMR δC (400 MHz, CDCl3) 43.3, 67.7, 90.6, 111.8,
2H, ArH). MS (EI): m/z 429.1 (M+, 100%), 339 (2), 274.1 (10),
15 258 (12), 244.1 (20), 230.1 (30), 199.1 (15), 155.1 (10), 121.1
(10), 91.1 (35).
115.3, 116.5, 123.3, 123.7, 125.9, 126.5, 127.6, 128.8, 132.1,
132.3, 133.6, 134.1, 153.1 (ꢀCꢀOꢀ), 162.5 (ꢀC=O), 165.9 (Oꢀ
C=CHꢀ). MS (EI): m/z 419 (M+, 100%), 385 (2), 335 (3), 310 (2),
258 (12), 244 (20), 230 (18), 199 (10), 155 (43), 139 (10), 111
75 (80), 91 (50). Anal. Calcd. for C23H17NO2S2: C, 65.85%; H,
4.08%; N, 3.34%. Found: C, 65.63%; H, 3.87%; N, 3.11%.
General procedure for microwaveꢀassisted nucleophilic
substitution reactions
20 A mixture of 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethyl 4ꢀ
methylbenzene sulfonate (3) (1.0 mmol), chromophore compound
(1.0 mmol) and Cs2CO3 (2 mmol) were placed in a microwave
Methyl 2ꢀ(6ꢀ(2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethoxy)ꢀ
3ꢀoxoꢀ3Hꢀxanthenꢀ9ꢀyl) benzoate (TPTFlo)
reaction vessel (30 mL) and DMF (5 mL) was added. The slurry 80 This monomer was synthesized via nucleophilic substitution
reaction mixture was heated at 85˚C under microwave irradiation
25 (110 W singleꢀmode power) for 8 min. Then, the mixture was
cooled to room temperature, poured into iceꢀwater (100 mL) and
vigorously stirred for 1 h. The solid crude product was filtered
reaction between 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethyl 4ꢀ
methylbenzene sulfonate (3) and methyl 2ꢀ(6ꢀhydroxyꢀ3ꢀoxoꢀ3Hꢀ
xanthenꢀ9ꢀyl)benzoate. The crude product was subjected to
column chromatography using hexane/ethyl acetate (1:3, v/v) as
under reduced pressure, dried and purified by column 85 the eluent (Rf, 0.40). TPTFlo was obtained as a dark red solid
chromatography to afford pure target SNS type monomer. The
(0.47 g, 0.78 mmol, 78%, mp 167ꢀ171˚C). FTIR νmax/cmꢀ1 3094,
3047, 2949, 2935, 1729, 1642, 1591, 1509, 1455, 1439, 1414,
1276, 1257, 1213, 1151, 1105, 1081. 1H NMR δH (400 MHz,
CDCl3, Me4Si) 3.62 (s, 3H, CH3), 4.04 (t, J = 6 Hz, 2H, CH2),
90 4.62 (t, J = 6 Hz, 2H, CH2), 6.43ꢀ6.62 (m, 4H, CH=CHꢀ, and Ar
H), 7.09ꢀ7.13 (m, 4H, Ar H), 7.25ꢀ7.27 (m, 1H, Ar H), 7.35ꢀ7.36
(dd, J = 6 Hz and J = 1 Hz, 2H, Ar H), 7.63ꢀ7.73 (m, 2H, Ar H),
8.21ꢀ8.23 (d, J = 6.8 Hz, 1H, Ar H). 13C NMR δC (400 MHz,
CDCl3) 43.4, 52.4, 67.2, 100.6, 105.7, 111.7, 113.5, 115, 117.6,
95 125.9, 126.7, 128.5, 128.7, 129.6, 129.9, 130.1, 130.5, 131.1,
132.6, 134.2, 134.5, 150, 154, 158.8, 162.4, 165.5 (ester C=O),
185.6 (quinoid C=O). MS (EI): m/z 603 (M+, 30%), 529 (3), 454
(2), 440 (3), 389 (12), 364 (45), 288 (15), 258 (50), 202 (20), 150
(5), 111 (100). Anal. Calcd. for C35H25NO5S2: C, 69.63%; H,
100 4.17%; N, 2.32%. Found: C, 70.08%; H, 3.92%; N, 2.11%.
1
30 monomer was characterized by FTIR, H NMR, 13C NMR, mass
spectrometer techniques and elemental analysis.
1ꢀ(2ꢀ(4ꢀ(Phenyldiazenyl)phenoxy)ethyl)ꢀ2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ
1Hꢀpyrrole (TPTAz)
35 This monomer was synthesized via nucleophilic substitution
reaction between 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethyl 4ꢀ
methylbenzene sulfonate (3) and 4ꢀhydroxyazobenzene. The
crude product was subjected to column chromatography using
hexane/CH2Cl2 (10:1, v/v) as the eluent (Rf, 0.55). TPTAz was
40 obtained as a bright orange solid (0.42 g, 0.92 mmol, 92%, mp
132ꢀ140˚C). FTIR νmax/cmꢀ1 3094, 3064, 2953, 2919, 2873,
1603, 1501, 1456, 1406, 1330, 1308, 1239, 1142, 1076. 1H NMR
δH (400 MHz, CDCl3, Me4Si) 4.06 (t, J = 6 Hz, 2H, CH2), 4.61 (t,
J = 6 Hz, 2H, CH2), 6.39 (s, 2H, Ar H), 6.75 (d, J = 8.8 Hz, 2H,
45 Ar H), 7.11ꢀ7.17 (m, 4H, Ar H), 7.36ꢀ7.38 (dd, J = 6.4 Hz and J =
1.2 Hz, 2H, Ar H), 7.44ꢀ7.52 (m, 3H, Ar H), 7.81ꢀ7.88 (m, 4H,
Ar H). 13C NMR δC (400 MHz, CDCl3) 43.7, 66.8, 111.5, 114.6,
122.5, 124.6, 125.8, 126.7, 127.4, 128.5, 129, 130.4, 134.3,
147.1, 152.7, 160.5. MS (EI): m/z 455 (M+, 100%) 429 (4), 350
50 (4), 317 (20), 258 (30), 225 (10), 186 (8), 147 (10), 97 (5), 77
(28). Anal. Calcd. for (C26H21N3OS2): C, 68.54%; H, 4.65%; N,
9.22%. Found: C, 68.77%; H, 4.18%; N, 8.89%.
The construction of electrochromic device (ECD)
Both polymers (PTPTFlo and PEDOT) were electrochemically
105 coated onto the ITOꢀcoated glass and ECD was constructed by
arranging two electrochromic polymers (with two different states,
doped and neutral) facing each other separated by gel
electrolyte.45,48 The gel electrolyte was prepared according to
literature.52 Schematic representation of the electrochromic
110 device based on PTPTFlo/PEDOT is shown in Fig. 1.
4ꢀ(2ꢀ(2,5ꢀDi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethoxy)ꢀ2Hꢀ
55 chromenꢀ2ꢀone (TPTCo)
This monomer was synthesized via nucleophilic substitution
reaction between 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethyl 4ꢀ
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20 multicolored characteristics, high optical contrasts and fast
switching times for terthienylꢀbased πꢀconjugated conducting
polymers. In the present study, the new kind of 2,5ꢀ
dithienylpyrrole (SNS) type monomers containing an
azobenzene, a coumarine and a fluorescein were synthesized for
25 the first time according to a threeꢀstep synthetic methodology, as
briefly illustrated in Scheme 1. Initially, synthesis of 2ꢀ(2,5ꢀ
di(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl) ethanol (2) was performed with
a PaalꢀKnorr condensation reaction between 1,4ꢀdi(2ꢀthiophenyl)ꢀ
1,4ꢀbutanedione (1) and ethanolamine in the presence of
30 catalytical amount pꢀtoluenesulfonic acid. The second step
involves treatment of 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀ
yl)ethanol (2) with pꢀtoluenesunfonyl chloride to obtain 2ꢀ(2,5ꢀ
di(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl)ethyl 4ꢀmethylbenzenesulfonate
(3). Microwaveꢀassisted nucleophilic substitution reaction of
35 tolueneꢀ4ꢀsulfonic acid 2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀ
yl)ethyl ester with the chromophore compound (4ꢀ
hydroxybenzene, 4ꢀhydroxyꢀ2Hꢀchromenꢀ2ꢀone and methyl 2ꢀ(6ꢀ
hydroxyꢀ3ꢀoxoꢀ3Hꢀxanthenꢀ9ꢀyl)benzoate) was finally carried out
in order to connect a SNS unit and a chromophore dye through a
40 ethylene bridge. The chemical structures of SNS type
Fig.2 Schematic representation of the PTPTFlo/PEDOT ECD
Results and discussions
5
Synthesis
The design and synthesis of novel precursor molecules are one of
the most effective tools in order to control electrochemical and
optical properties of πꢀconjugated conducting polymers. In this
10 context, the novel conducting polymer derivatives bearing
azobenzene, coumarine and fluorescein dyes have been
synthesized in our previous study and their electrochemical and
spectroelectrochemical properties have been investigated in
detail.49,50 The new approach based on the incorporation of
15 chromophore units with thiophene polymer backbones
significantly enhanced electronic and optoelectronic behaviors of
conducting polymers.
monomers,1ꢀ(2ꢀ(4ꢀ(phenyldiazenyl)phenoxy)
ethyl)ꢀ2,5ꢀ
di(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrole (TPTAz), 4ꢀ(2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀ
yl)ꢀ1Hꢀpyrrolꢀ1ꢀyl) ethoxy)ꢀ2Hꢀchromenꢀ2ꢀone (TPTCo) and
methyl 2ꢀ(6ꢀ(2ꢀ(2,5ꢀdi(thiophenꢀ2ꢀyl)ꢀ1Hꢀpyrrolꢀ1ꢀyl) ethoxy)ꢀ3ꢀ
45 oxoꢀ3Hꢀxanthenꢀ9ꢀyl)benzoate (TPTFlo), were confirmed by
FTIR, 1H NMR, 13C NMR, mass spectroscopy and elemental
analysis techniques.
This favorable effect of chromophore groups on electrochemical
and spectroelectrochemical properties resulted in low band gaps,
50
Scheme 1 Synthetic route to SNS type monomers TPTFlo, TPTCo and TPTAz
Cyclic voltammetry studies
properties of polymers. In this study after polymerization via
electrochemically, electroactivity and HOMO and LUMO energy
levels of resulting polymers (PTPTCo, PTPTFlo and PTPTAz)
were determined by cyclic voltammetry (CV) to get a deeper
Cyclic voltammetry is a useful technique that can be used for
55 both electrochemical polymerization and to investigate redox
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ꢀ4.93 eV and ꢀ5.07 eV, respectively. Since all polymers have only
pꢀtype doping character, LUMO energies were calculated from
HOMO energy levels and optical band gaps (Egop) were
35 calculated as ꢀ2.62 eV (PTPTFlo), ꢀ2.64 eV (PTPTCo) and ꢀ2.26
eV (PTPTAz). For these calculations, the reference electrode was
calibrated with respect to Fc/Fc+ (+0.30 V) and the HOMOꢀ
LUMO energies were calculated relative to the vacuum level
taking the SHE value as ꢀ4.75 eV vs. vacuum.
insight on redox behaviors. TPTFlo, TPTCo and TPTAz were
polymerized electrochemically on ITO coated glass slides in a
mixture of dichloromethane (DCM) and acetonitrile (ACN) (5:95,
v:v) in the presence of 0.1 M supporting electrolyte formed of
equimolar amounts of sodium perchlorate (NaClO4) and lithium
perchlorate (LiClO4) mixture with repeated scan intervals
between 0 V and +1.2 V for TPTFlo, 0 V and +1.5 V for TPTCo
and 0 V and +1.5 V for TPTAz. Fig. 2 represents the cyclic
5
10 voltammograms (CV) of all monomers at 100 mV sꢀ1 scan rate.
During electropolymerization, irreversible monomer oxidation
peaks at +0.82 V (TPTFlo), +0.96 V (TPTCo) and +1.05 V
(TPTAz) were observed in the first cycle of voltammograms (Fig.
2). After observing monomer oxidations in the first cycle,
15 oligomers and polymers were electrodeposited on the ITO
surfaces which can be proved with increasing current intensity of
each successive cycle.
40
When TPTFlo, TPTCo and PTPAz were compared in terms of
their monomer oxidations and polymer oxidations of
corresponding polymers, it can be easily seen that TPTFlo has
the lowest value. Low oxidation potential TPTFlo can be
45 attributed to the electronic nature of chromophore groups.
Ethylene bridges between SNS monomer chains and
chromophore units hinder the steric and mesomeric effects of
these strong chromophore groups on electronic nature of
50 electroactive polymers which make inductive effect dominant
over the others. The carbonyl groups in the structure of
fluorescein group decrease the electron withdrawing capability of
chromophore unit which increase the electron density on the main
polymer chain and makes it more ready for redox reactions with
55 lower oxidation potential.
Resulting polymers (PTPTFlo, PTPTCo and PTPTAz) were
20 subjected to CV in a monomer free 0.1 M NaClO4ꢀLiClO4/CAN
solution in order to explore the doping property of resulting
electroactive polymers as illustrated in Fig. 3. As depicted in Fig.
3, all polymers have pꢀtype doping property with a reversible
redox couple centered at +0.74 V/ +0.63 V for PTPTFlo, at
25 +1.01 V/ +0.63 V for PTPTCo and at +0.87 V/ +0.62 V for
PTPTAz. HOMOꢀLUMO energy levels of conducting polymers
are very important for the application fields and could be tailored
via changing the donor and acceptor units in the donorꢀacceptorꢀ
donor systems. Oxidation and reduction onset values were used to
30 calculate the HOMO energy levels for all polymers as ꢀ5.08 eV,
60
Fig.2 Repeated potential scan polymerization of (a) TPTFlo (b) TPTCo and (c) TPTAz in 0.1 M LiClO4/ NaOCl4/DCM/ACN (5:95,v:v) solution at 100
mV sꢀ1 scan rate
65
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5
Fig.3 Single scan cyclic voltammograms for (a) PTPTFlo (b) PTPTCo and (c) PTPTAz in a monomer free, 0.1 M NaClO4/LiClO4/ACN solution
40
Spectroelectrochemistry
nm. Although formation of new bands (polaron and bipolaron) at
610 nm/1120 nm were observed it has a yellow color at both
states (neutral and oxidized), since there was no decrease in the
neutral absorption during stepwise oxidation. This may be due to
45 the fact that the tail of the polaron band intersects with that of the
neutral band absorption peak. The optical band gaps (Egop) of
PTPTFlo, PTPTCo and PTPTAz were also calculated from the
onsets of the lowest energy πꢀπ* transitions on the neutral state
absorption spectra (Fig. 4). They were calculated as 2.31 eV, 2.44
50 eV and 2.81 eV respectively. Summary of all electrochemical and
spectroelectrochemical data were depicted in Table 1.
10 Spectroelectrochemical studies were performed in order to
investigate the optical and electrochromic properties of
conjugated polymers upon doping process. PTPTFlo, PTPTCo
and PTPTAz were electrochemically coated on ITO surface and
spectral
changes
were
recorded
by
UVꢀvisꢀNIR
15 spectrophotometer. Similar to the single scan cyclic
voltammograms, a monomerꢀfree 0.1 M NaClO4ꢀLiClO4/ACN
solution was used for recording the electronic absorption spectra.
During the spectral studies, recording neutral state absorption is
very crucial for comparison with other states. For this purpose,
20 polymer films were subjected to 0 V to reduce and to remove any
trapped charge and dopant ion arising from electrochemical
polymerization. Then stepwise oxidation was performed via
incrementally increasing applied potential between +0.3 V and
+1.4 V for PTPTFlo, +0.5 V and +1.1 V for PTPTCo and 0.0 V
25 and +1.0 V for PTPTAz. The normalized electronic absorption
spectra of all polymers were depicted in Fig. 4.
The structures and colors of PTPTFlo, PTPTCo and PTPTAz at
neutral and oxidized states were illustrated in Fig. 4.
55 Spectroelectrochemical studies were also consistent with
recorded colors. While all polymers have yellow color in the
neutral state, PTPTCo reveals gray and PTPTAz has blue color
in the fully oxidized states. On the other hand, PTPTFlo shows
all states yellow character and as a result of this unique property
60 we constructed a PTPTFlo/PEDOT based electrochromic device
(ECD). Characterization of PTPTFlo/PEDOT ECD will be
discussed in the next section with some important properties such
as; stability, open circuit memory and spectral response.
Maximum wavelength (λmax) can be defined as the wavelength at
which a polymer shows πꢀπ*transition. In this study, they are at
30 459 nm/490 nm for PTPTFlo, 370 nm for PTPTCo and 347 nm
for PTPTAz. Via oxidation, while the absorption in the visible
region decreasing for PTPTCo and PTPTAz, new absorption
bands called as polaron bands (radical cations), bipolaron bands
(bications) appeared at 620 nm/920 nm for PTPTCo and 610 nm/
35 970 nm for PTPTAz. As illustrated in Fig. 4a PTPTFlo has two
absorption maxima in the visible region centered at 459 nm/ 490
65 Electrochromic
device
(ECD)
application
and
characterization
As mentioned before because of the all state yellow character of
PTPTFlo, we constructed a prototype electrochromic device
70 based on PTPTFlo and PEDOT in order to investigate the
electrochromic performance of ECD in ITO/PTPTFlo/ITO
configuration.
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Fig.4 Electronic absorption spectra of (a) PTPTFlo (b) PTPTCo (c) PTPTAz in 0.1 M NaClO4ꢀLiClO4/ACN electrolyte solution between +0.3 V and
+1.4 V for PTPTFlo +0.5 V and +1.1 V for PTPTCo and 0.0 V and +1.0 V for PTPTAz
,
5
Table 1 Summary of electrochemical and optical properties of PTPTFlo, PTPTCo and PTPTAz
ox
op
Polymer
Film
Emon
(V)
Epꢀdoping
(V)
Epꢀdedoping
(V)
HOMO *LUMO
λmax
(nm)
Eg
Polaron/Bipolaron
(nm)
(eV)
ꢀ4.93
ꢀ5.08
ꢀ5.07
(eV)
ꢀ2.62
ꢀ2.64
ꢀ2.26
(eV)
2.31
2.44
2.81
0.82
0.96
1.05
0.74
1.01
0.87
0.63
0.63
0.62
459/ 490
370
610/ 1120
620/ 920
610/ 970
PTPTFlo
PTPTCo
PTPTAz
347
After construction of the ECD, working range was determined
The optoelectronic properties of the PTPTFlo/PEDOT ECD
from cyclic voltammogram of PTPTFlo/PEDOT device at 100 25 were investigated via UVꢀvis spectrophotometer while
10 mV sꢀ1. As illustrated in Fig. 5a, single scan cyclic
voltammogram bears characteristic redox couples for both
PTPTFlo and PEDOT.
incrementally increasing the applied potential between ꢀ1.8 V and
+1.8 V. The spectroelectrochemistry study of the device was
shown in Fig. 6. At ꢀ1.8 V PTPTFlo/PEDOT ECD has two
absorption maxima centered at 458 nm and 490 nm as a result of
The longꢀterm switching stability of the PTPTFlo/PEDOT ECD 30 the neutral state yellow color of the PTPTFlo, at that time
15 were investigated by chronoamperometry technique while
switching the device between two extreme states (ꢀ1.8 V and +2.0
V) at every 5 seconds. In Fig. 5b the stability of the device
initially and after 1500 seconds (300th cycles) were reported and
PEDOT is expected to have transmissive color due to oxidized
state. When the applied potential was changed to +1.8 V, the
yellow color of the device changed to blue with an absorption
maximum at 580 nm. The color change was caused by the
it can be clearly stated that after 300 cycles PTPTFlo/PEDOT 35 oxidation of PTPTFlo layer whereas PEDOT layer started to get
20 ECD retain its electrochromic performance without any
significant electroactivity loss.
neutralized (Fig. 6).
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20 Fig.6 Electronic absorption spectra of PTPTFlo/PEDOT ECD at various
applied potentials (between ꢀ1.8 V and +1.8 V)
5
Fig.5 (a) Cyclic voltammogram (b) stability after 1st and 300th
cycles of PTPTFlo/PEDOT ECD
The open circuit memory (optical memory) is another parameter
for electrochromic device characterization. This can be defined as
Fig.7 Open circuit memory of PTPTFlo/PEDOT ECD recorded at 580 nm,
25 +2.0 V and ꢀ1.8 V pulse were applied for 1 s every 200 s
10 the time which the device maintains its color under open circuit
conditions.45,48 For openꢀcircuit stability experiment, 1 s pulse of
+2.0 V or 1.8 V was applied to PTPTFlo/PEDOT ECD at 580 nm
and then for the next 200 s the device was kept under openꢀcircuit
condition while simultaneously probing the percent transmittance
15 for 20 min. As summarized in Fig. 7, the optical properties of the
device remained almost the same after applying 1 s pulse during
the period.
Conclusions
To conclude, three novel 2,5ꢀdithienylpyrrole (SNS) derivatives
containing strong chromophore substituents such as azobenzene,
30 coumarine and fluorescein were designed, synthesized and
characterized successfully. The corresponding conducting
polymer films of SNS type monomers, PTPTAz, PTPTCo and
PTPTFlo, were electrochemically synthesized on ITO coated
glass slide electrodes and then, their electrochemical and
35 spectroelectrochemical properties were investigated in detail.
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8
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for PTPTFlo, PTPTCo and PTPTAz respectively. PTPTAz and
PTPTCo exhibited different colors in their neutral and fully
oxidized states whereas PTPTFlo showed all states yellow
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