Synthesis and characterization of fluorescent nitrobenzoyl polythiophenes

Article abstract of DOI:10.1016/j.electacta.2015.08.006

Two thiophene derivatives named 4"-nitrobenzoyl (thiophene-3′-yl)-1-ethylate (NTh) and 3",5"-dinitrobenzoyl (thiophene-3′-yl)-1-ethylate (DNTh) were synthesized and characterized. Both monomers were electrochemically polymerized onto platinum or glassy ca

Full text of DOI:10.1016/j.electacta.2015.08.006

Electrochimica Acta 178 (2015) 134–143
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and characterization of fluorescent nitrobenzoyl
polythiophenes
Elaine C.S. Coelhoa, Valberes B. Nascimentob, Adriana S. Ribeiroc, Marcelo Navarroa, ,1
*
a
Universidade Federal de Pernambuco, CCEN, Departamento de Química Fundamental, 50670-901, Recife, PE, Brazil
Universidade Federal Rural de Pernambuco, Departamento de Química, 52171-030, Recife, PE, Brazil
b
c
Universidade Federal de Alagoas, Instituto de Química e Biotecnologia, 57072-970, Maceió, AL, Brazil
A R T I C L E I N F O
A B S T R A C T
Two thiophene derivatives named 4”-nitrobenzoyl (thiophene-30-yl)-1-ethylate (NTh) and 3”,5”-
dinitrobenzoyl (thiophene-30-yl)-1-ethylate (DNTh) were synthesized and characterized. Both mono-
mers were electrochemically polymerized onto platinum or glassy carbon electrodes by potentiody-
namic method in 0.1 mol Lꢀ1 TBABF4/CH2Cl + boron trifluoride diethyl etherate (BFEE) (1:1, v/v). These
monomers were also successfully polymerized by chemical oxidation using FeCl3/CHCl3. Spectroscopic
and electrochemical properties of the monomers and polymers were investigated. Films of both
polymers showed well-defined reversible redox system at the anodic branch (1.0 V vs. Ag/AgCl, KCl (sat.)),
attributed to thiophene doping/dedoping process; and another redox process at the cathodic branch
(PNTh) (or two redox processes for PDNTh) was attributed to the reduction of the 4-nitrobenzoyl or 3,5-
dinitrobenzoyl substituent groups. The THF solutions of monomers and polymers are fluorescent, with
emission bands at lmax = 488 nm (NTh), 511 nm (DNTh), 440 nm (PNTh) and 546 nm (PDNTh). The
monomers are also fluorescent in the solid state.
Article history:
Received 29 June 2015
Received in revised form 28 July 2015
Accepted 2 August 2015
Available online 4 August 2015
Keywords:
electropolymerization
nitrobenzoyl thiophene
fluorescent polymer
BFEE
ã 2015 Elsevier Ltd. All rights reserved.
1. Introduction
modification of the monomer structure by attaching different
functional groups, such as, electron-donating and/or electron-
The synthesis and characterization of novel
p
-conjugated
withdrawing, n-dopable, or fluorescent substituents [10–13]. For
instance, the appendage of a nitro group on the monomer structure
would make the resulting polymers amenable for use in sensors or
electrochemical capacitors [14,15].
The presence of nitro groups in conjugated polymers may
provide interesting applications: i) during the redox process of
corresponding conjugated polymers, the nitro group can be
reduced, ii) the NO2 group can be reduced to NH2 at less negative
potential, and iii) the introduction of the nitro groups in the
conjugated polymer structure can provide a post-functionalization
approach, e.g., reduction of NO2 into NH2 [16]. In addition,
polymers bearing nitro groups have not been extensively
investigated.
The presence of the electron-withdrawing nitro group in the
monomer structure usually leads to a higher oxidation potential,
culminating in poor polymer film quality or no electropolymeri-
zation [16–18]. The nitro group can stabilize the radical cation
intermediates, allowing its diffusion away from the electrode that
prevents the polymerization on the anode surface [19,20].
Therefore, its preparation still remains as a challenge.
polymers have attracted a great deal of interest, mainly because of
their properties: electrical conductivity, electrochromism, electro-
luminescence, nonlinear optical characteristics and chemical
sensing [1], which are useful for a wide range of applications,
including electrochromic devices [2,3], organic light emitting
diodes (OLEDs) [4], organic solar cells (OSCs) [5], organic field
effect transistors (OFETs) [6], energy storage [7] and biosensors [8].
All these applications usually require the rational design and
modification of the monomer structure. Therefore, the creative
design and strategies to synthesize new derivatives of conjugated
polymers can lead to materials with enhanced performance.
Some of the conjugated polymer properties are assigned to
their ability on changing the electronic and spectral features, upon
the modification of their chemical structure, i.e., the tailoring of the
band gap (Eg) of these polymers allows the shift of the absorption
and emission wavelengths [9], or even in the electrochemical
response [10]. Therefore, the properties can be adjusted by
Thiophene and pyrrole electropolymerizations has been largely
studied, and follow the same reaction mechanism. The pyrrole
* Corresponding author. Tel.:+ 55 81 21267460; fax: +55 81 21268442.
E-mail address: navarro@ufpe.br (M. Navarro).
1
ISE member.
http://dx.doi.org/10.1016/j.electacta.2015.08.006
0013-4686/ã 2015 Elsevier Ltd. All rights reserved.

E.C.S. Coelho et al. / Electrochimica Acta 178 (2015) 134–143
135
oxidation potential (ꢁ1.2 V vs. Ag/AgCl) is lower than the thiophene
2. Experimental
oxidation potential (ꢁ1.6 V) that can reach 2.4 V, depending on the
substituent group; thus, thiophene electropolymerization is very
difficult to be carried out in aqueous medium. Polypyrrole is more
susceptible to oxidation than polythiophene, leading to the
carbonyl group formation at the position 3 and breakup of the
conjugation. Polythiophene presents a better thermal stability than
polypyrrole,andtheconductivityofbothpolymerscanvary(10ꢀ3 to
103 S cmꢀ1) according to the substituent, dopant and experimental
conditions of electropolymerization [21,22]. Therefore, numerous
applications can be associated to intrinsic characteristics and
properties of these polymers.
A series of polypyrrole and polythiophene derivatives bearing
nitro- and dinitro- substituents have been synthesized and
characterized by our research group aiming their application in
electrochromic devices (ECDs) [3,23–25], fluorescent layers [26] or
as material for energy storage [14]. In an earlier study, Welzel et al.
[19] synthesized two thiophene derivatives substituted with nitro-
and dinitro- groups and tried to electropolymerize these deriv-
atives under standard conditions (such as TBAP/CH3CN), but they
were not successful. According to their results, the high potential
(up to 2.3 V vs. Ag/AgCl) necessary for the monomers oxidation
caused an overoxidation process destroying the aromatic structure
of the thiophene units.
The replacement of the usual organic media (CH3CN) by a Lewis
acid, such as boron trifluoride diethyl etherate (BFEE) is a good
strategy to prepare good-quality conjugated polymer films bearing
electron-withdrawing substituents [17,23]. BFEE solution has been
widely used as catalyst to electropolymerize aromatic monomers,
such as thiophene and its derivatives, selenophene and pyrene
[27–29]. In the presence of BFEE, the monomer oxidation occurs at
a lower potential, rather than in CH3CN, by virtue of the interaction
between the Lewis acid and the aromatic monomer [30]. It occurs
because the strong electrophilic nature of BFEE catalyzes the
deprotonation of aromatic compounds, decreasing the oxidation
potential of such compounds [31,32]. In the specific case of
2.1. Chemicals and equipments
Most chemicals and solvents were purchased and used without
further purification. The solvents CH3CN, CHCl3 and CH2Cl2 were
dried and then distilled before the use, the first two under P2O5 and
the last one under CaH2 [34]. The starting materials 3-(20-
Bromoethyl) thiophene and 3-(20-Iodoethyl) thiophene were
synthesized according to the previous literature [35]. The
monomers NTh and DNTh were obtained as described below.
BFEE was distilled and freshly used. Tetrabutylammonium tetra-
fluoroborate (TBABF4) was dried under high vacuum at 100 ꢂC for
3 h before use.
NMR spectra (1H, 13C and Cosy NMR) were obtained in a Varian
spectrometer (Unity Plus, 300 and 400 MHz) using CDCl3 as
solvent. Infrared spectra were registered using a Bruker FTIR
(IFS66, 1% sample KBr pellets) and ATR Varian FTIR (640-IR, film
electropolymerized on Pt foil) spectrometers. Melting points were
determined on Electrothermal equipment (Mel-Temp, analogic
mode). Elemental analyses were performed in a Carlo Erba (EA
1110) and a Perkin-Elmer (CHN 2400) equipment. Gas chromato-
grams were carried out in a Varian (CP-3380) (30 m ꢃ 0.25 mm
ꢃ 0.25
m
m DB-5 capillary column) using a 60-200 ꢂC temperature
range (10 ꢂC minꢀ1). Gas chromatograms/mass spectra were
carried out in a Shimadzu (GC/MS-QP5050A), using the same
column and temperature ramp. UV-Vis spectra of the monomers
and polymers, both solubilized in THF solvent, were carried out in a
Perkin Elmer spectrophotometer (Lambda 650, double beam).
Excitation and emission fluorescence spectra were recorded in a
Shimadzu spectrofluorophotometer (RF-5301PC). All electrochem-
ical experiments were carried out using an Autolab potentiostat/
galvanostat PGSTAT30 (NOVA software v.1.10).
2.2. Synthesis of the monomers
thiophene derivatives, the formation of
p
-complexes with BFEE
2.2.1. 4”-Nitrobenzoyl (thiophene-30-yl)-1-ethylate - NTh
suppresses the resonance stability of the thiophene ring, thus
facilitating the loss of electrons from the ring [33]. So the catalytic
effect of BFEE enables the formation of high quality polymer films.
In the present work, we report a new synthetic route for the
preparation of two thiophene derivatives bearing nitro- and
dinitro- groups: 4”-nitrobenzoyl (thiophene-30-yl)-1-ethylate
(NTh) and 3”,5”-dinitrobenzoyl (thiophene-30-yl)-1-ethylate
(DNTh), improving the previous route proposed by Welzel et al.
[19]. Polymer films of PNTh and PDNTh were successfully
electropolymerized by direct anodic oxidation of the monomers
onto platinum or glassy carbon electrodes by using a mixed
electrolyte containing TBABF4/CH2Cl2 and BFEE (1:1, v/v). The
electrochemical and spectroscopic properties of the monomers
and polymers were investigated in details.
3-(20-Iodoethyl) thiophene (0.712 g, 3.0 mmol), 4-nitrobenzoic
acid (0.500 g, 3.0 mmol) and proton-sponge1 (1.280 g, 6.0 mmol)
were stirred for 5 hours in dry CH3CN (10 mL) at 75 ꢂC. In the first
hour of reaction, a white solid precipitated (proton-sponge iodine).
This solid was removed by filtration and rigorously washed with
diethyl ether. The reaction solution was rotaevaporated for diethyl
ether extraction, and the reaction was continued until its
completeness to be confirmed by GC. The remaining proton-
sponge iodine precipitate was filtrated and washed with ethyl
ether. The resulting solution (orange color) was rotaevaporated
and the crude product was extracted with 0.01 mol Lꢀ1 HNO3/
CHCl3 (1:1) biphasic solution (2 ꢃ 50 mL, each extraction). The
organic phase was dried with anhydrous Na2SO4 and rotaevapo-
rated. The crude product was solubilized in 10 mL of ethyl acetate/
Scheme 1. NTh and DNTh monomer preparations.

136
E.C.S. Coelho et al. / Electrochimica Acta 178 (2015) 134–143
hexane mixture (3:7), and precipitated at -10 ꢂC. The isolated
product (Scheme 1) was recrystallized using the same procedure
above, and dried under high vacuum for 3 hours.
2.3. Chemical polymerization
A typical poly-condensation reaction was used to obtain the
polymers PNTh and PDNTh (Scheme 2). Dry FeCl3 (ꢁ100 mg,
0.6 mmol), used as oxidant, was suspended in dry CHCl3 (2 mL) and
stirred for 20 min under N2. To the dark green solution formed, it
was slowly dropwise added (during 30 min) the monomer solution
of NTh (50 mg, 0.18 mmol) or DNTh (50 mg, 0.16 mmol), both
dissolved in dry CHCl3 (2 mL) and kept previously under N2 for
20 min. The mixture was stirred and purged overnight with N2 at
room temperature. Each insoluble oxidized polymer was then
filtered off, washed with water, and reduced in a mixture of
aqueous hydrazine (25%)/ethyl acetate (1:9), under stirring for 3 h
[26]. The neutral polymers were filtered off and washed with water
and ethyl acetate. Then, the purified polymers were dried under
high vacuum for 24 h. The polymers presented low solubility in
common organic solvents, such as acetonitrile, chloroform, DMF,
DMSO and acetone.
Pale yellow solid, 60% yield (500 mg). M.p. 78-80 ꢂC; 1H NMR
(300 MHz, CDCl3) d= 8.29 (d, j = 9.0 Hz, 2H, -ArH), 8.18 (d, j = 9.0 Hz,
2H, -ArH), 7.31 (dd, J = 4.9, 2.9 Hz, 1H, -ThH), 7.09 (ddd, J = 3.0, 1.4,
0.8 Hz, 1H, -ThH), 7.02 (dd, J = 4.9, 1.3 Hz, 1H, -ThH), 4.59 (t,
j = 4.0 Hz, 2H, -CH2), 3.14 (t, j = 6.0 Hz, 2H, -CH2); 13C NMR (75 MHz,
CDCl3)
d= 164.6, 150.5, 137.5, 135.5, 130.7, 128.1, 125.9, 123.6, 121.8,
65.7, 29.5; IR (KBr) cmꢀ1: 3105, 3081, 2925, 2861, 1718, 1606, 1527,
1455, 1350, 1270, 1100, 1010, 951, 870, 834, 783, 717, 690, 630,
582 and 505; Anal. calcd. for C13H11NO4S (%) (277.3005 g molꢀ1): C,
56.31; H, 4.00; N, 5.05; S 11.56. Found: C, 56.19; H, 3.92; N, 4.97; S,
10.98. MS, m/z (relative intensity): 150 (14), 111 (15), 110 (100), 104
(18), 97 (26), 76 (23), 53 (14), 50 (25), 45 (35).
2.2.2. 3”,5”-Dinitrobenzoyl (thiophene-30-yl)-1-ethylate - DNTh
The same NTh synthetic procedure was carried out for the
DNTh monomer (Scheme 1), employing the following amounts: 3-
(20-iodoethyl) thiophene (0.579 g, 2.4 mmol), 3,5-dinitrobenzoic
acid (0.508 g, 2.4 mmol) and proton-sponge1 (1.01 g, 6.8 mmol).
The mixture, initially orange, turned purple in few minutes. After
extracted from the organic layer (2 ꢃ 50 mL of 0.01 mol Lꢀ1 HNO3/
CHCl3 (1:1)), the crude product was eluted with CHCl3 in a silica gel
column (70-230 Mesh) to eliminate the purple dye. After this
procedure, the precipitation, recrystallization and drying steps
were performed according to the NTh synthetic procedure.
Pale green solid, 45% yield (348 mg). M.p. 107-109 ꢂC; 1H NMR
2.3.1. NTh polymer - P NTh
A reddish brown solid was obtained in 35 mg (70%) for PNTh. IR
(KBr) cmꢀ1: 3105, 2957, 2919, 2854, 1723, 1603, 1524, 1456, 1346,
1269, 1169, 1106, 1010, 973, 964, 829, 778, 714 and 500; Anal. calcd.
for C13H9NO4S (%) (275.2847 g molꢀ1): C, 56.72; H, 3.30; N, 5.09; S,
11.65. Found: C, 55.95; H, 3.29; N, 4.77; S 11.62.
2.3.2. DNTh polymer - PDNTh
A reddish brown solid was obtained, 38 mg (74%) for PDNTh. IR
(KBr) cmꢀ1: 3095, 1730, 1626, 1543, 1456, 1340, 1269, 1157, 1072,
978, 916, 825, 770, 717, 656, 507 and 457; Anal. calcd. for
(300 MHz, CDCl3)
d= 9.21 (t, j = 3.0 Hz, 1H, -ArH), 9.12 (d, j = 3.0 Hz,
2H, -ArH), 7.32 (dd, j = 4.9, 2.9 Hz, 1H, -ThH), 7.11 (dt, j = 2.9, 1.0 Hz,
1H, -ThH), 7.03 (dd, j = 4.9, 1.3 Hz, 1H, -ThH), 4.67 (t, j = 9.0 Hz, 2H,
-CH2), 3.19 (t, j = 6.0 Hz, 2H, -CH2); 13C NMR (75 MHz, CDCl3)
C
13H10N2O6S (%) (320.2826 g molꢀ1): C, 48.75; H, 2.52; N, 8.75; S,
10.01. Found: C, 48.47; H, 2.94; N, 8.58; S, 10.60.
d
= 162.6, 149.0, 137.3, 134.2, 129.6, 128.2, 126.4, 122.6, 122.2, 66.8,
29.8; IR (KBr) cmꢀ1: 3095, 1731, 1630, 1540, 1457, 1440, 1342, 1276,
1174, 1074, 976, 924, 833, 804, 775, 725, 698, 593, 511 and 425;
Anal. calcd. for C13H10N2O6S (%) (322.2984 g molꢀ1): C, 48.45; H,
3.13; N, 8.69; S, 9.95. Found: C, 48.64; H, 3.31; N, 8.61; S, 9.13. MS,
m/z (relative intensity): 149 (10), 111 (11), 110 (100), 97 (41), 75
(23), 53 (13), 45 (30).
2.4. Electrochemical measurements
Electropolymerizations (NTh and DNTh monomers) and redox
behavior of monomers and polymers were carried out in one-
compartment cell containing a glassy carbon (GC) or platinum (Pt)
disk electrode (both with
w= 3 mm), platinum wire and Ag/AgCl,
KCl (sat.) as the working, counter and reference electrodes,
respectively. Before every assay, the working electrode was
Scheme 2. Chemical and electrochemical polymerization of NTh and DNTh.

E.C.S. Coelho et al. / Electrochimica Acta 178 (2015) 134–143
137
carefully polished in a felt-pad soaked with aqueous alumina
slurry (0.3 m) and rinsed with plenty of distilled water, followed
(a)
m
by acetone and, then, dried in a soft paper. Compartments with
sintered glass walls were used to keep the reference and counter
electrodes separated from the monomer solutions. The electrolytic
medium for electropolymerizations was a mixture (1:1, v/v) of
0.1 mol Lꢀ1 TBABF4/CH2Cl2 and freshly distilled BFEE. The polymers
were electrosynthesized from that medium, containing 10 mmol
Lꢀ1 monomer, via cyclic voltammetry in a potential range from
0.00 V to +1.90 V vs. Ag/AgCl, KCl (sat.), at a scan rate of 20 mV sꢀ1
.
Immediately after obtaining the films, they were rinsed thoroughly
with CH2Cl2 and transferred to another cell containing 0.1 mol Lꢀ1
TBABF4/CH2Cl2 supporting electrolyte. The redox behavior of the
polymers was monitored during 10 cyclic voltammetric scans at
20 mV sꢀ1, between the neutral and doped states, which also
served as a pre-treatment step. The same electrolytic medium was
employed to study the cathodic behavior of the monomers. All
experiments were carried out after degassing for 10 min, main-
taining the system at 20 ꢂC - 22 ꢂC under argon atmosphere.
(b)
3. Results and Discussion
3.1. Chemical syntheses of monomers and polymers
The NTh and DNTh monomers were synthesized through a
simple two-step route (Scheme 1), giving an overall yield of 60%
and 45%, respectively, and gave satisfactory elemental, mass
spectral and spectroscopic analyses. Firstly, the thiophene
halogenated precursor 3-(20-iodoethyl) thiophene was obtained,
followed by esterification reaction with 4-nitrobenzoic or 3,5-
dinitrobenzoic acid in presence of proton-sponge1 (1,8-bis
(dimethylamino) naphthalene) (Scheme 1). The sterically hindered
base, proton-sponge1, was alternatively employed in the place of
triethylamine to avoid the by-product formation (3-vinylthio-
phene), formed by HI elimination, [23]. The structures of both
compounds were fully characterized by routine analysis techni-
ques (melting point, 1H and 13C NMR, FTIR, GC/MS, and elemental
analysis).
The polymers were synthesized by using FeCl3 as oxidant agent
in dry CHCl3 medium (Scheme 2), employing a Fe3+/monomer
4:1 molar ratio, 16 hours reaction time, room temperature and
argon atmosphere. Pure neutral polymers were obtained by
exhaustive washes with methanol and water, followed by
reduction with hydrazine, exhaustive washes with water and
ethyl acetate, and dried under high vacuum for 24 hours. The
products PNTh (70%) and PDNTh (74%) were obtained in good
yields. The dedoping process was investigated by elemental
analysis of the polymers, showing CHNS data very close to
calculated values.
Fig. 1. Cyclic voltammograms of 3.0 mmol Lꢀ1 NTh (a) and DNTh (b) monomers on
GC electrode vs. Ag/AgCl, KCl (sat.),
n
= 20 mV sꢀ1. Sweeps were carried out in
0.1 mol Lꢀ1 TBABF4/CH2Cl2. Inset: anodic sweeps were carried out in 0.1 mol Lꢀ1
TBABF4/CH2Cl2 + BFEE (1:1, v/v).
described [26], the presence of the pendant nitrobenzoyl moieties
does not interfere in the oxidation process. In addition, BFEE is a
volatile compound, and the variation on its concentration can
result on slight shifts of the thiophene Epa. Commonly, the Epa
decreases with the BFEE ratio increasing [27,35].
It is important to highlight that in the presence or absence of
BFEE the onset oxidation potential remains the same (Table 1),
while the oxidation peak potential is shifted to lower potentials in
the presence of BFEE, confirming the catalytic effect of this reagent
[27,35].
At the cathodic branch (Fig. 1), the sweep was carried out in the
absence of BFEE (0.1 mol Lꢀ1 TBABF4/CH2Cl2) due to the reactivity
of the reduced species generated. It was observed a one electron
reduction process, concerning the nitro group reduction (aprotic
medium) for both monomers, identified by the two pairs of peaks
Ic/Ia (4-nitrobenzoyl moiety) and IIc/IIa (3,5-dinitrobenzoyl moie-
ty), Eqs. (1) and (2), respectively, Table 1.
3.2. Redox behavior of monomers
Fig. 1 shows cyclic voltammograms of 3.0 mmol Lꢀ1 NTh and
DNTh monomer solutions (0.1 mol Lꢀ1 TBABF4/CH2Cl2) on GC disk
electrode, in two potential windows: 0.0 V to -1.5 V and 0.0 V to
2.5 V vs. Ag/AgCl, KCl (sat.). One irreversible peak was observed in
the anodic branch, related to oxidation (one electron) of the
thiophene group for both compounds (NTh:EpIa' = 2.25 V, Fig 1(a);
DNTh EpIIa' = 2.30 V, Fig. 1(b)). The high oxidation potential is in
agreement to data described in literature [19], and no polymeri-
zation is observed. Thus, the anodic sweep was also carried out in
the presence of BFEE (0.1 mol Lꢀ1 TBABF4/CH2Cl2 + BFEE (1:1, v/v)),
where it can be observed a small shift on the oxidation peak
potentials (NTh: EpIa' = 1.96 V, inset of Fig.1(a); DNTh: EpIIa' = 2.07 V,
inset of Fig. 1(b)), allowing the electropolymerization process at
higher (10.0 mmol Lꢀ1) monomer concentration (Fig. 2). As already
ꢄ
(COOR)-Ar-(NO2) + 1 eꢀ Ð (COOR)-Ar-(NO2)ꢀ
(1)
(COOR)-Ar-(NO2)2 + 1 eꢀ Ð (COOR)-Ar-(NO2)2
(2)
ꢀ
ꢄ
The electron transfer process of soluble nitrocompounds is
quasi-reversible [36,37], as observed here from the anodic to the
cathodic peak potential separations (NTh: EpIa - EpIc = 130 mV;
DNTh: EpIIa - EpIIc = 110 mV). Besides, it can be observed a second
electron transfer process in DNTh molecule (EpIIc” = -1.18 V/EpIIa” = -
1.07 V), normally assigned to the nitro radical anion reduction,
giving the respective dianion intermediate (Eq. (3)) [36]. However,
a
recent work involving ESR-spectroelectrochemistry and

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E.C.S. Coelho et al. / Electrochimica Acta 178 (2015) 134–143
(a)
(b)
Fig. 3. Cyclic voltammograms of PNTh (red) and PDNTh (black and dashed dot line)
films onto Pt disk electrode vs. Ag/AgCl, KCl (sat.), in 0.1 mol Lꢀ1 TBABF4/CH2Cl2,
= 20 mV sꢀ1. PNTh and PDNTh films were obtained according to Fig. 2. (For
n
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
while unsubstituted thiophene polymerizes at 1.3 V [27]. The
higher oxidation potential of substituted thiophene monomers
may be associated to the steric hindrance and/or electronic density
decrease of the ring. The electropolymerization of thiophenes
substituted with strong electron withdrawing groups, directly
attached to
b positions has already been reported and assigned to
the high reactivity of the corresponding radical cation [34].
Electropolymerization of both monomers (NTh and DNTh) can
be accomplished by sweeping the potential between 0.6 and 1.9 V
in the presence of BFEE. On the first scan, the anodic current
increases at around 1.8 V and a typical current loop can be clearly
seen between 1.8 and 1.9 V when the scan is reversed (Fig. 2). The
presence of this loop is indicative of a nucleation process, as
reported in the literature [38]. Upon subsequent scans, a reversible
peak, typical of adsorbed polythiophene films [34] can be observed
between 0.7 V and 1.0 V with a continuous growth of the peak
current intensity, indicating the formation and growth of a
polymeric film. The redox couples, EpIc”' = 0.84 V/EpIa”' = 0.87 V for
PNTh and EpIIc”' = 0.90 V/EpIIa”' = 0.94 V for PDNTh,can be attributed
to the p-doping/dedoping processes of polythiophene films [35].
The redox behavior of PNTh and PDNTh was also investigated.
Fig. 3 shows cyclic voltammograms of the polymer films (obtained
after 10 cycles on Pt disk electrode) in a monomer and BFEE free
solution (0.1 mol Lꢀ1 TBABF4/CH2Cl2). At the anodic region, it can
Fig. 2. Cyclic voltammograms of the potentiodynamic electrodeposition of (a) NTh
and (b) DNTh,both 10.0 mmol Lꢀ1, in 0.1 mol Lꢀ1 TBABF4/CH2Cl2 + BFEE (1:1, v/v), on
Pt electrode vs. Ag/AgCl, KCl (sat.),
n
= 20 mV sꢀ1, 10 scans.
quantum chemical studies about the methyl 3,5-dinitrobenzoate
reduction have indicated to a rare biradical dianion formation
(Eq. (4)) [37]. These intermediate species are strong bases that can
react in the presence of water (or electrophilic species), generating
the respective nitro reduction product, hydroxylamine [36,37].
(COOR)-Ar-(NO2)2ꢀ. + 1 eꢀ Ð (COOR)-Ar-(NO2)2
(3)
2ꢀ
(COOR)-Ar-(NO2)2ꢀ. + 1 eꢀ Ð (COOR) ꢀ.-Ar-(NO2)2
ꢄ
(4)
ꢀ
be observed a reversible peak centered at around 1.00 V (EpIc”'
=
3.3. Electrochemical polymerization of NTh and DNTh
0.99 V/EpIa”' = 1.02 V for PNTh and EpIIc”' = 0.97 V/EpIIa”' = 1.01 V for
PDNTh), which corresponds to the oxidation/reduction of the
polythiophene backbone. At the cathodic region it appears a quasi-
reversible peak at EpIc = -0.97 V/EpIa = -0.74 V for PNTh,and EpIIc = -
0.72 V/EpIIa = -0.54 V for PDNTh, which are assigned to the
reduction of 4-nitrobenzoyl and 3,5-dinitrobenzoyl moieties,
respectively [37]. When the sweep is carried out to more negative
potential (-1.5 V) a small irreversible cathodic peak is observed
(EpIIc” = -0.97 V), and the film becomes electro inactive at the
Unlike similar nitrobenzoyl pyrrole derivatives synthesized and
recently described [26], the electropolymerization of NTh and
DNTh required the use of BFEE as catalyst, 1:1 (v/v) in 0.1 mol Lꢀ1
TBABF4/CH2Cl2 solution. Fig. 2 shows successive cyclic voltammo-
grams carried out for the electropolymerization of both 10.0 mmol
Lꢀ1 monomer solutions, on Pt disk electrode. It can be seen that the
onset oxidation potential of NTh and DNTh are 1.78 V and 1.86 V vs.
Ag/AgCl, KCl (sat.), respectively, in the presence of BFEE (Table 1),
Table 1
Electrochemical data, EHOMO,ELUMO levels and Eg for NTh and DNTh monomers, PNTh and PDNTh polymers in 0.1 mol Lꢀ1 TBABF4/CH2Cl2.
ec
onset
onset
LUMO
ec
entry
comp.
Epc
(V)
Epa
(V)
Epa'
(V)
Eox
(V)
EThHOMO(eV)
Ered
(V)
E
(eV)
Eg
’
-NO2
(eV)
1
2
3
4
NTh
-0.83
-0.79
-0.97
-0.72
-0.70
-0.68
-0.74
-0.54
2.25/1.96a
2.30/2.07a
1.01
1.78/1.78a
1.85/1.86a
0.90
-6.18/-6.18a
-6.25/-6.26a
-5.30
-0.64
-0.62
-0.67
-0.50
-3.76
-3.78
-3.73
-3.90
2.42
2.47
1.57
1.43
DNTh
PNTh
PDNTh
1.02
0.93
-5.33
a
0.1 mol Lꢀ1 TBABF4/CH2Cl2 + BFEE (1:1, v/v).

E.C.S. Coelho et al. / Electrochimica Acta 178 (2015) 134–143
139
cathodic region. Therefore, the second electron transfer enables
the generation of very reactive species (dianion or diradical
dianion, Eqs. (3) and (4), respectively), which can react with
organic functional groups (or other species) present in the film.
In the PNTh film, the 4-nitrobenzoyl moiety electron transfers
occur at more negative potentials (-0.97 V/-0.74 V, Eꢂ = -0.86 V),
compared to the respective NTh monomer (-0.83 V/-0.70 V, Eꢂ = -
0.76 V), meaning that the redox process becomes more difficult to
occur on the film/electrode interface than for monomer/electrode
surface. When analyzing the corresponding data for the PDNTh
film (-0.72 V/-0.54 V, Eꢂ = -0.63 V) and DNTh monomer (-0.79 V/-
0.68 V, Eꢂ = -0.73 V), it can be observed the opposite effect, i.e., the
electron exchange occurs at less negative potential in the PDNTh
film/electrode than for the DNTh monomer/electrode surface.
These potential shifts can be attributed to the structural differ-
ences between 4-nitrobenzoyl and 3,5-dinitrobenzoyl moieties, as
well, the electron transfer mechanisms involved into the film. It is
known that the electrochemical potentials for these reactions are
strongly dependent on the nitro group structural position in the
aromatic system [36,39], and also on the structural changes
occurring during the electron transfer processes [40,41]. Therefore,
in a general way, the electron-withdrawing effect of the additional
nitro group in the 3,5-dinitrobenzoyl moiety can make the PDNTh
film electrochemically more accessible than DNTh monomer,
during the first electron transfer process.
Welzel et al. [19] described the preparation of eight thiophene
derivatives, including the NTh and DNTh monomers described in
this work, but did not observe the electrochemical homopolyme-
rization of these monomers in acetonitrile medium. Thus,
electrochemical copolymerizations of NTh and DNTh were carried
out in the presence of 3-methylthiophene, furnishing stable
copolymers with characteristic redox properties of polythiophene.
The cathodic behavior of these copolymers was not described. In
turn, Li et al. [42] described the preparation of two similar
monomers (3-thiophene acetic acid-4-nitrobenzyl ester and 3-
thiophene acetic acid-4-(4-nitrophenyl) butyl ester), which were
electropolymerized in acetonitrile and high monomer concentra-
tion (0.2 mol Lꢀ1). The polymer films presented a well-defined
redox behavior for the polythiophene backbone (anodic region), as
well, a distinct redox behavior for the nitrophenyl moiety (cathodic
region).
Fig. 4. Successive four cyclic voltammograms of the polymer PNTh in 0.1 mol Lꢀ1
TBABF4/CH2Cl2,
10 mmol Lꢀ1 monomer solution in 0.1 mol Lꢀ1 TBABF4/CH2Cl2 + BFEE (1:1, v/v) on Pt
electrode vs. Ag/AgCl, KCl (sat.),
= 20 mV sꢀ1
n
= 20 mV sꢀ1. Film obtained from 10 scans (0.0 to 1.85 V), from
n
.
reduced/oxidated without affecting other functional groups [43].
As described above, the electroactivity of the nitrobenzoyl moiety
in PNTh and PDNTh disapears after 5 scans, but does not affect the
conjugated
p-electron system of the polymeric chain (Fig. 4),
which remains electroactive. The study of similar nitrobenzoyl
substituted polypyrroles (poly-4”-nitrobenzoyl(pyrrol-10-yl)-1-
propylate and poly-3”,5”-dinitrobenzoyl(pyrrol-10-yl)-1-propy-
late) [26], showed an irreversible reduction system for the nitro
moieties, which become inactive after the first scan, decreasing the
polypyrrole backbone electroactivity. Therefore, the polythio-
phene is more stable than polypyrrole films in the presence of
the nitrobenzoyl reduced species.
To investigate the electroactivity and stability of the polymers,
NTh (Fig. S1a) and DNTh (Fig. S1b) were electropolymerized by
3
cycles on Pt disk electrode. Cyclic voltammograms were
monitored between neutral and oxidized states at scan rates from
20 mV sꢀ1 to 200 mV sꢀ1 in a monomer and BFEE free electrolyte.
The scan rate (n) dependence with the anodic (Ipa) and cathodic
(Ipc) peak currents is linear, evidencing that the redox process is
non-diffusional in the range studied and the electroactive films are
well adhered on Pt electrode surface.
The electropolymerization of the NTh and DNTh monomers
employing BFEE can be carried out at small concentrations
(10.0 mmol Lꢀ1) of monomers and improve the quality of the
resulting polymer films (PNTh and PDNTh), showing a well-
3.4. Spectroscopic properties of monomers and polymers
3.4.1. FTIR measurements
.
defined redox couple, R-NO2/R-NO2ꢀ , for both 4-nitrobenzoyl and
The FTIR spectra of both monomers (NTh and DNTh) and
respective polymers (PNTh and PDNTh, obtained via chemical
polymerization) were carried out and revealed characteristic
bands, in agreement to the proposed structures (experimental
section). Fig. 5 shows the FTIR spectra of the NTh monomer
(Fig. 5a) and respective polymers, chemically (PNTh powder,
Fig. 5b) and electrochemically (PNTh film, Fig. 5c) synthesized.
3,5-dinitrobenzoyl moieties into the polymeric films. Thus, the
electroactivity of pendant groups in conducting polymers should
depend not only on the monomer structure but also on the
experimental conditions employed during the electropolymeriza-
tion process [18,26].
Fig. 4 shows the behavior and stability of the nitro group redox
process in the PNTh film (prepared after 10 scans, on Pt disk
electrode), using 0.1 mol Lꢀ1 TBABF4/CH2Cl2 as electrolyte solution.
After consecutive scans, it can be observed a drop on the oxidation/
reduction currents of the 4-nitrobenzoyl moiety of the film, until
they are no longer observed. At the anodic branch, in turn, the
redox activity of the polythiophene backbone was maitained,
showing a positive shift of the anodic peak associated to a small
increase of the current. The cathodic peak of the polythiophene
backbone remains unaltered. These results contrast with the data
described by Li et al. [18], which showed a complete lost of the
charge/discharge capability of the polymer after two scans in such
wide potential range.
These spectra presents similar major bands, such as carbonyl C O
¼
stretching at 1718 cmꢀ1; asymmetric and symmetric Ar–NO2
stretching at 1527 cmꢀ1 and 1350 cmꢀ1, respectively; aromatic
C–H in-plane vibration at 1270 cmꢀ1; ester C–O stretching at
1100 cmꢀ1; symmetric aromatic C–H vibration at 717 cmꢀ1. The
bands around 3104–2861 cmꢀ1, related to thiophene and benzene
aromatic rings symmetric/asymmetric stretching are not observed
in ATR mode (Fig. 5c), probably due to the low penetration depth of
photons in these region.
3.5. UV-Vis measurements
As shown in Scheme 2, the NTh and DNTh monomers present
thiophenyl, 4-nitrobenzoyl and 3,5-dinitrobenzoyl chromophores
The cyclovoltammograms of NTh and DNTh monomers (Fig. 1)
show a characteristic propertie of nitro groups, which can be easily

140
E.C.S. Coelho et al. / Electrochimica Acta 178 (2015) 134–143
0,5
0,4
0,3
0,2
0,1
0,0
0,5
0,4
0,3
0,2
0,1
0,0
(a)
2
(b)
(c)
300
350
400
450
500
550
300
350
400
450
500
550
Wavelength/nm
1
0
Wavelength/nm
PNTh
NTh
PDNTh
DNTh
200
300
400
500
600
700
Wavelength / nm
Fig. 6. UV-Vis absorbance spectra of NTh and DNTh monomers and PNTh and
PDNTh dedoped polymers in THF. The polymers were synthesized from oxidation
with FeCl3. Inset: PNTh and PDNTh backbone bands.
3200 2800 2400 2000 1600 1200 800
Wavenumber / cm-1
Fig. 5. FTIR spectra of (a) NTh, (b) PNTh prepared from oxidation with FeCl3, and (c)
PNTh prepared by electropolymerization on Pt electrode (ATR mode) in 0.1 mol Lꢀ1
TBABF4/CH2Cl2 + BFEE (1:1, v/v).
where, the -4.40 eV energy correspond to the HOMO/LUMO energy
levels of the reference ferrocene/ferricenium redox pair (vs. Ag/
AgCl, KCl (sat.)), as described in the literature and used as external
standard [47].
(Table 2). In THF, these monomers present absorption bands at
lmax = 232 nmthatcorrespondto p-p*primaryelectronictransition
of the thiophene ring (Fig. 6). An overlapping band was observed for
the DNTh monomer at 238 nm, related to the 3,5-dinitrobenzoyl
chromophore. The absorption band at lmax = 278 nm correspond to
the 4-nitrobenzoyl chromophore of the NTh monomer.
Normally, the calculated Egec (Eq. (7)) corresponds to the optical
band gap energy (Egopt) of the polymeric chain. For PNTh and
PDNTh, it does not occur because the nitrobenzoyl moiety
reduction potential is higher than the reduction potential of the
conjugated polymeric chain (polythiophene). That is, both oxida-
tion and reduction occur in distinct moieties of the monomer or
polymer, and cannot be associated to the optical band gap. The
electrochemical behavior of the studied substances (NTh, DNTh,
PNTh and PDNTh) should take into account the electron transfer
processes as separated phenomena, as specified in the Scheme 3.
Therefore, the Egec corresponds to energy difference between two
distinct moieties (thiophene/polythiophene HOMO and nitro-
benzoyl LUMO) separated by a non-conjugated bridge (ethyl
group).
The same absorption bands
(lmax = 232 nm and 278 nm,
respectively) were observed for the chemically synthesized
polymer, PNTh, with an additional broad band at lmax = 410 nm
related to the polythiophene band (
UV-Vis absorption spectrum presented a different behavior, the
* electronic transition (lmax = 239 nm) of the 3,5-dinitroben-
p-p* transition). The PDNTh
p-p
zoyl chromophore was improved, and additional bands at 277 nm
and 427 nm can be observed, which also corresponds to the
polythiophene backbone band.
At the same time, the UV-Vis optical absorption spectroscopy is
a well-known technique to evaluate the band gap of materials,
which can be associated to HOMO/LUMO electronic transitions
caused by absorption of electromagnetic energy. These electronic
3.6. Electro-optical properties of monomers and polymers
As described in a previous work [26], the cyclic voltammetry
was used to estimate the relative position of HOMO (EHOMO) and
LUMO (ELUMO) energy levels of conjugated polymers [9,44]. These
transitions (n-
in the molecule, which Eg
p*, p-p*) normally occur in chromophores present
opt
can be estimated from the onset
levels, as well as the electrochemical band gap energy (Egec), were
absorption wavelength (lonset) of the chromophore electronic
transition (Scheme 3) [9,44–47].
onset
calculated from the onset oxidation potential (Eox
) and
reduction potential (Eredonset), respectively, for the monomers
(NTh and DNTh) and polymers (PNTh and PDNTh), according to
relationships proposed by Bredas et al. (Eqs. (5) to (7)) [45,46]:
3.6.1. Electrochemical measurements
The EHOMO and ELUMO (Eqs. (5) and (6)) were determined by
cyclic voltammetry in 0.1 mol Lꢀ1 TBABF4/CH2Cl2,
n
= 20 mV sꢀ1
E
HOMO = - (Eoxonset + 4.40) eV
(5)
(6)
(7)
(Table 1). For the monomers NTh and DNTh, the EHOMO calculated
onset
from Eox
of the thiophene moieties are EThHOMO = -6.18 eV and
E
LUMO = - (Eredonset + 4.40) eV
-6.25 eV, respectively. With addition of BFEE the onset reduction
potential of the thiophene moieties remains the same
Eoxonset = 1.78 and 1.85 V, respectively; and the estimated molecular
orbital energy values are EThHOMO = -6.18 eV and -6.26 eV. The ELUMO
Egec = (ELUMO – EHOMO) eV
Table 2
Absorbance and Egopt data for thyophene, nitrobenzoyl and dinitrobenzoyl chromophores present in NTh and DNTh monomers, PNTh and PDNTh polymers (THF).
Absorbance
w-NO2 chromophore
Absorbance thyophene chromophore
opt
opt'
entry
comp.
lmax (nm)
lonset (nm)
Eg (eV)
lmax (nm)
lonset (nm)
Eg
(eV)
1
2
3
4
NTh
278
238
278
239
307
284
307
304
4.03
4.36
4.03
4.07
232
232
232, 410
277, 427
243
243*
498
510
5.10
5.10*
2.49
2.43
DNTh
PNTh
PDNTh
*
Assuming the same value for NTh. E (eV) = 1239 (eV nm)/l (nm).

E.C.S. Coelho et al. / Electrochimica Acta 178 (2015) 134–143
141
for DNTh/PDNTh, both in THF medium. When excited at 421 nm
(NTh, Fig. 7a) and 430 nm (DNTh, Fig. 7b), both monomers show
emission bands at lmax = 488 nm and lmax = 511 nm, respectively,
with Stokes shifts of 67 nm for NTh and 81 nm for DNTh.
The PNTh solution was excited at three different wavelengths
(262 nm, 314 nm or 371 nm, Fig. 7a), in which an emission band
was observed only at lmax = 440 nm, with Stokes shift of 69 nm for
the maximum intensity of emission. PDNTh also was excited at
different emission wavelengths (Fig. 7b): at 355 nm, it was
observed an emission band at lmax = 530 nm and Stokes shift of
175 nm. When exciting PDNTh at 519 nm, it was observed an
elevated intensity emission at lmax = 546 nm and a corresponding
Stokes shift of 27 nm.
For the NTh/PNTh pair, the red shift of the monomer emission
band, if compared to the respective polymer emission band
(Fig. 7a), shows an opposite behavior than typical fluorescent
conductive polymers, in which the monomer absorbs and emits
light at lower wavelengths than the polymer, i.e., higher energy is
necessary to excite the monomer than polymer [45,46]. The
different behavior of PNTh can be associated to electronic
distribution, structure or fluorescence mechanism of the polymer.
On the other hand, the PDNTh polymer can be excited at shorter
and longer wavelengths than its corresponding DNTh monomer.
The fluorescence mechanism of the related compounds must be
associated to a photoinduced electron transfer process [48]. The
monomer molecules are free in solution and can rotate via -CH2-
bond, hindering the formation of the excimer that allows the
electron transfer between nitrobenzoyl and thiophene moieties,
thus, the excitation process occurs at shorter wavelengths. The
Scheme 3. HOMO/LUMO energy levels and related electrochemical and optical
electron transfers for NTh or PNTh.
onset
calculated from Ered
of the 4-nitrobenzoyl (E
LUMO = -
-NO2
’
3.76 eV) and 3,5-dinitrobenzoyl (E
presented a small variance. The Ered
LUMO = -3.78 eV) moieties
’
-NO2
onset
of DNTh is less cathodic
due the second nitro substituent on the benzoyl group. Thus, the
ec
Eg values were calculated according to Eq. (7) for NTh (2.42 eV)
and DNTh (2.47 eV) monomers.
The EHOMO and ELUMO calculated for the respective polymers
followed the same procedure described above, allowing the
ec
determination of Eg for PNTh (1.57 eV) and PDNTh (1.43 eV),
which show a decrease on the electrochemical gap energy values
when compared to monomers data (Table 1). This behavior is
onset
caused by decreasing on the Eox
of the polythiophene moiety
due to the formation of the conjugated polymeric chain, which
correspond to an increasing of the HOMO energy (PNTh, ETh
HOMO = - 5.30 eV and PDNTh, ETh HOMO = - 5.33 eV) for both
polymers. On the other hand, at the cathodic branch it can be
observed almost the same values for LUMO energies (PNTh, E
’
-
LUMO = - 3.73 eV and PDNTh, E LUMO = - 3.90 eV).
’-NO2
NO2
3.6.2. Optical measurements
The corresponding band gap energies of the electronic
transitions for thiophene (Egopt') and nitrobenzoyl (Egopt) chromo-
phores were determined from lonset (E (eV) = 1239 (eV nm)/
l
(nm)) of the respective absorption bands as already explained in
Scheme 3, either to monomers as to polymers, and are described in
Table 2.
opt'
According to the Eg
calculated for thiophene (5.10 eV, NTh,
and 5.10 eV, DNTh) and polythiophene chromophores (2.49 eV,
PNTh, and 2.43 eV, PDNTh), it was observed a drop on the band gap
energy after the polymerization process, as expected. Normally,
the polymeric chain gives rise to
p-p* electronic transitions
opt
occurring at lower HOMO/LUMO energy gap [42]. The Eg
calculated values for the nitrobenzoyl chromophore of monomers
and polymers studied are between 4.03 eV and 4.36 eV (Table 2).
3.6.3. Fluorescence measurements
Fig. 7a shows the excitation and emission spectra for the
monomer/polymer pair NTh/PNTh, while Fig. 7b shows the spectra
Fig. 7. Excitation and emission spectra in THF for: (a) NTh and PNTh; (b) DNTh and
PDNTh. Inset: Color of polymers (left) and monomers (right) under ambient light
(up) and irradiated with UV light at 256 nm (down).

142
E.C.S. Coelho et al. / Electrochimica Acta 178 (2015) 134–143
same photoinduced electron transfer should occur in the
polymeric chain; however, at lower energy since thiophene and
nitrobenzoyl groups are arranged in a rigid chain. Therefore, the
fluorescence mechanism of the monomers and polymers should be
deeply studied in due course, and will be reported in a future paper.
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4. Conclusion
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Two nitrobenzoyl thiophene compounds (NTh and DNTh) and
respective polymers (PNTh and PDNTh) were synthesized,
characterized, and their electro-optical properties were investi-
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for the monomers and polymers are distinct, corroborating the
proposed model in Scheme 3, in which optoelectronic transitions
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ec
value Eg is not specifically associated to the
p-p* electronic
transition occurring on the polymeric chain. An interesting
fluorescence effect was observed for all monomers and polymers
studied and should be more deeply studied.
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properties, and characterization, CRC, Press, New York, 2007.
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Acknowledgements
The authors wish to thank granting authorities INCT-INAMI/
CNPq, CAPES, and FACEPE for financial support and fellowship of
Elaine C. S. Coelho.
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Appendix A. Supplementary data
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polypyrrole derivatives, Electrochim. Acta 123 (2014) 441.
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
electacta.2015.08.006.
[27] W. Chen, G. Xue, Low potential electrochemical syntheses of heteroaromatic
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