Multielectrochromic amide-based poly(2,5-dithienylpyrrole) bearing a fluorene derivative: Synthesis, characterization, and optoelectronic properties

Article abstract of DOI:10.1016/j.electacta.2021.138173

A novel 2,5-dithienylpyrrole derivative bearing a fluorene substituent (SNSFCA) was synthesized and successfully electropolymerized on ITO electrodes in acetonitrile (CH₃CN) containing tetrabutylammonium tetrafluoroborate ((C₄H₉)₄NBF₄). The fluorescence properties of SNSFCA and its polymer (PSNSFCA) were investigated upon laser excitation at 337 nm, however, the polymer was not fluorescent, which may be explained by DFT methods. PSNSFCA films present multielectrochromism in a narrow range of applied potential (0.0 ≤ E ≤ 0.4 V vs. Ag/Ag⁺), as shown by the track of the CIE 1931 xy chromaticity coordinates, besides high absorption in the near infrared (NIR) region. The electrochromic properties of PSNSFCA films, such as good chromatic contrast (Δ%T), coloration efficiency (η) in the range of 110–350 cm² C^?1, and stability to redox cycling aroused the possibility of its application as an electrochromic material in optoelectronic devices.

Full text of DOI:10.1016/j.electacta.2021.138173

Electrochimica Acta 379 (2021) 138173
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Multielectrochromic amide-based poly(2,5-dithienylpyrrole) bearing a
fluorene derivative: Synthesis, characterization, and optoelectronic
properties
a
a
a
b
c
Jorge L. Neto , Luis P.A. da Silva , Joel B. da Silva , Raul L. Ferreira , Ana Júlia C. da Silva ,
a
b
a
a,1,∗
Júlio C.S. da Silva , Ítalo N. de Oliveira , Dimas J.P. Lima , Adriana S. Ribeiro
a
Chemistry and Biotechnology Institute, Federal University of Alagoas, Maceió, AL 57072-970, Brazil
b
Physics Institute, Federal University of Alagoas, Maceió, AL 57072-970, Brazil
c
Federal Institute of Alagoas, Campus Viçosa, Viçosa, AL 57700-000, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel 2,5-dithienylpyrrole derivative bearing a fluorene substituent (SNSFCA) was synthesized and suc-
cessfully electropolymerized on ITO electrodes in acetonitrile (CH3CN) containing tetrabutylammonium
tetrafluoroborate ((C4H9)4NBF4). The fluorescence properties of SNSFCA and its polymer (PSNSFCA) were
investigated upon laser excitation at 337 nm, however, the polymer was not fluorescent, which may be
explained by DFT methods. PSNSFCA films present multielectrochromism in a narrow range of applied
Received 23 January 2021
Revised 11 March 2021
Accepted 13 March 2021
Available online 17 March 2021
+
Keywords:
potential (0.0 ≤ E ≤ 0.4 V vs. Ag/Ag ), as shown by the track of the CIE 1931 xy chromaticity coordi-
Synthesis
nates, besides high absorption in the near infrared (NIR) region. The electrochromic properties of PSNS-
Conjugated polymer
Fluorescence
FCA films, such as good chromatic contrast (ꢀ%T), coloration efficiency (η) in the range of 110–350 cm2
−1
C
, and stability to redox cycling aroused the possibility of its application as an electrochromic material
Spectroelectrochemistry
in optoelectronic devices.
©
2021 Elsevier Ltd. All rights reserved.
1
. Introduction
ate pyrrole and thiophene coupling at their α-positions [10]. The
high reactivity of this π-extended system creates the possibil-
ity of introducing a series of functional groups through the ni-
trogen atom in the pyrrole ring of the SNS unit, such as alkyl
and phenyl derivatives [11–14], ferrocene [15], BODIPY [16], lu-
minol [17], dansyl [7], including a series of dyes [2,18] and fluo-
rophores [19–21]. In addition, the relatively low oxidation poten-
The design and synthesis of novel π-conjugated polymers has
attracted considerable interest due to their electrical and optical
properties, leading to the development of multifunctional mate-
rials for technological applications in the field of electronics and
photonics, sensors, and devices [1]. Polythiophene, polypyrrole and
their derivatives have been highlighted as active layers in elec-
trochromic device applications owing to their low band gap, good
conductivity, high optical contrast, and multicolor electrochromism
+
tial (about 0.70 V vs. Ag/Ag ) [20], multielectrochromic proper-
ties, high chemical and electrochemical stability, and effortless syn-
thesis methods, make PSNS derivatives one of the most promis-
ing conjugated polymers aimed at applications in optoelectronics
[20,22].
[
2–4]. Furthermore, various strategies have been proposed in re-
cent years to fine-tune the optical properties of these multifunc-
tional materials, including the incorporation of fluorescent sub-
stituents [5,6], preparation of copolymers [7,8], and synthesis of
fused-aromatic rings or extended π-conjugated systems [9].
Improved electrochromic properties than those of individual
pyrrole and thiophene ones, may be achieved by using trimeric
thiophene-pyrrole-thiophene derivatives, namely N-functionalized
According to Cihaner and Algi [21,23], the synthesis of extended
π-conjugated systems based on the introduction of a fluorene ap-
pendage in the SNS main chain can render multifunctional mate-
rials that exhibit both electrochromic and fluorescent properties.
Fluorene and polyfluorene derivatives have been widely employed
in solar cells [24], sensors [25,26] and optical devices [27,28], due
to their rigid planar structure, excellent hole-transporting prop-
erties, good solubility, exceptional chemical stability and photo-
luminescence efficiencies. Particularly, fluorene-9-carboxylic acid
2
,5-dithienylpyrrole (SNS), which are synthezised from appropri-
∗
Corresponding author.
(
FCAc), which contains an electron-withdrawing substituent, have
E-mail address: aribeiro@qui.ufal.br (A.S. Ribeiro).
rarely been studied [29], because its anodic oxidation is hampered
1
ISE member
https://doi.org/10.1016/j.electacta.2021.138173
013-4686/© 2021 Elsevier Ltd. All rights reserved.
0

J.L. Neto, L.P.A. da Silva, J.B. da Silva et al.
Electrochimica Acta 379 (2021) 138173
in common organic solvents, such as acetonitrile. However, the
presence of a carboxyl group on the conjugated polymer backbone
has several advantages owing to its electron-deficient functionality.
Therefore, the synthetic versatility of the SNS main chain along
the aforementioned properties of fluorene-9-carboxylic acid moi-
ety, has attracted a considerable interest in the synthesis of a
novel SNS-fluorene derivative, namely N-(2-(2,5-di(thiophen-2-yl)-
N–H amide), 1440 (v C=C aromatic ring), 1210 (vC–N amide), 1075
β/β’
(vC=C thiophene ring), 840 (δC–H
thiophene ring), 730 (δC–H
β
pyrrole ring) and 678 (δC–Hα thiophene ring) (see Supplementary
Material, Fig. S3).
2.3. Electropolymerization of SNSFCA
1
H-pyrrol-1-yl)ethyl)-9H-fluorene-9-carboxamide (SNSFCA), for ap-
Films of PSNSFCA were electrodeposited on ITO electrodes
2
plication as electrochromic material in optoelectronic devices.
(Delta Technologies, specific resistivity (Rs) = 8-12 ꢁ cm, 1.0 cm )
+
in a single compartment cell. A home-built non-aqueous Ag/Ag
−
+
2. Experimental
(0.10 mol L 1 AgNO /CH CN), calibrated to the Fc/Fc redox sys-
3 3
tem [30], was used as reference electrode and a Pt foil was em-
−
3
2
.1. Materials and Instrumentation
ployed as counter electrode. A solution of SNSFCA (5.0 × 10 mol
−1
−1
L
) in 0.1 mol L
(C H ) NBF / CH CN was used for the elec-
4
9
4
4
3
All chemicals were purchased from Sigma-Aldrich or Acros as
trodeposition. The polymer was electrodeposited by cyclic voltam-
−
1
analytical grade. Previously to the synthetic procedures, the sol-
vents CH Cl2 and toluene were treated with P O , being subse-
metry at scan rate (ν) = 20 mV s
in a potential range of 0.00
+
≤ E ≤ 0.65 V vs. Ag/Ag . After electrodeposition, the films were
2
2
5
quently distilled. For the electrochemical experiments, anhydrous
washed several times with CH CN to remove unreacted monomers
3
acetonitrile 99.8% (CH CN < 0.001% water) and tetrabutylammo-
and the excess of electrolyte.
3
nium tetrafluoroborate ((C H ) NBF ) were used as received.
4
9
4
4
NMR spectra were recorded on a Bruker Ascend 600 spectrom-
eter at 600 MHz for 1H NMR and 150 MHz for 13C NMR, using
CDCl3 as solvent. Chemical shifts (δ) were given relative to tetram-
ethylsilane (TMS) as the internal standard. The compounds were
analyzed by using attenuated total reflection Fourier transformed
infrared spectroscopy (ATR-FTIR) on a Shimadzu IR Prestige - 21
2.4. Spectroelectrochemistry
The PSNSFCA films deposited on ITO were characterized
by cyclic spectrovoltammetry and double potential step spec-
trochronoamperometry in 0.1 mol L 1 (C H ) NBF / CH CN so-
−
4
9
4
4
3
lution as supporting electrolyte, using a Pt wire as the counter
−
1
−1
+
spectrophotometer, operating between 4000 cm
and 400 cm
,
electrode and an Ag/Ag (CH CN) electrode as reference. The cyclic
3
with a spectral resolution of 4 cm 1. Scanning Electron Microscopy
−
voltammograms were registered in a potential range of -0.20 ≤ E
+
−1
and chronoam-
(
SEM) images of the polymer film were obtained in a Jeol JSM –
≤ 0.60 V vs. Ag/Ag (CH CN) at ν = 20 mV s
3
6
8
610 (Thermo Scientific NSS Spectral Image). A Hewlett-Packard
453A diode array spectrophotometer was used for spectroelectro-
perograms were acquired by applying pulses of E1 = 0.00 V and
E2 = 0.40 V for 40 s. In situ spectroelectrochemistry was performed
by recording the UV-vis-NIR spectra simultaneously with the elec-
trochemical experiments in kinetic mode at intervals of 2.5 s.
The CIE (Commission Internationale de l’Eclairage) 1931 xy color
coordinates [31] were calculated by using a Microsoft® Excel®
spreadsheet developed by Mortimer and Varley [32,33] using a
D55 standard illuminant. The CIE 1931 xy chromaticity coordinates
in the CIE chromaticity diagram were displayed by the Spectra Lux
Software v.2.0 Beta [34].
chemistry and kinetic studies.
2
.2. Synthesis of N-(2-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)ethyl)-
H-fluorene-9-carboxamide
9
(SNSFCA)
The starting materials 1,4-di(thiophen-2-yl)butane-1,4-dione
and 2-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)ethanamine were syn-
thesized according to the procedure previously decribed [7].
2
-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl)ethanamine (55 mg,
2.5. Fluorescence Spectroscopy
0
.20 mmol), fluorene 9-carboxylic acid (FCAc) (84 mg, 0.40 mmol)
and 4-dimethylaminopyridine (DMAP) (6.0 mg, 0.05 mmol) were
The photoluminescence emission spectra of SNSFCA solubilized
in CHCl3 (0.5 mg mL 1) and its polymer film deposited on ITO and
in N-methylpyrrolidone (NMP) solution were recorded in an UV-
Vis spectromer (USB2000, Ocean Optics). The samples were excited
by using a pulsed Nitrogen laser (MNL-103 PD LTB Lasertechnik
Berlin) at 337 nm and pulse width of 3.0 ns. The repetition rate
was set in 30 Hz.
−
added to CH Cl2 (5.0 mL) under stirring and argon atmosphere.
2
The mixture was cooled with ice-salt bath (−2 to 0 °C). Then a so-
lution of N, N’-Dicyclohexylcarbodiimide (DCC) (41 mg, 0.20 mmol)
in CH Cl2 (10 mL) was added dropwise. After this, the ice bath
2
was removed, and the reaction mixture stirred for 20 h at room
temperature. Dicyclohexylurea precipitated and was removed by
filtration. The filtrate was extracted with CH Cl (5 × 15 mL)
2
2
and H O, the organic solution was dried with anhydrous Na SO
4
2.6. Computational methods
2
2
and the solvent was removed under reduced pressure. The crude
product was purified by chromatography on silica gel using hex-
ane/ethyl acetate 8:2 as eluent, to afford SNSFCA as grayish solid
The quantum calculations were performed using the C.01 ver-
sion of the Gaussian 09 program [35]. Ground state structures for
SNSFCA and PSNSFCA were evaluated using the density functional
theory (DFT) level of the three-parameter compound functional of
Becke (B3LYP), including the D3 dispersion correction proposed by
Grimme and co-workers [36]. All atoms of the monomer and its
polymer were described using cc-pVDZ basis set [37].
(
65 mg, 73% yield), Scheme 1. m.p.: 191 ± 1 °C; 1H NMR (600
MHz, CDCl , δ (ppm)): 7.8 (d, J = 7.56 Hz, 2 H), 7.5 (dd, J = 0.72,
3
7
.56 Hz, 2 H), 7.4 (t, J = 7.47 Hz, 2 H), 7.3 (d, J = 1.08, 7.47 Hz,
2
6
H), 7.2 (d, J = 1.08 Hz, 2 H), 7.0 (dd, J = 3.54, 5.16 Hz, 2 H),
.9 (dd, J = 1.08, 3.54 Hz, 2 H), 6.3 (s, 2 H), 5.1 (s, 1 H), 4.7
(
s, 1 H), 4.3 (t, J = 6.51 Hz, 2 H), 3.2 (q, J = 6.34 Hz, 2 H) (see
Supplementary Material, Fig. S1); 13C NMR (150 MHz, CDCl , δ
3. Results and discussion
3
(ppm)): 172.0, 141.3, 141.1, 134.2, 128.6, 128.2, 127.7, 127.5, 126.2,
1
25.5, 125.4, 120.1, 111.4, 55.6, 43.8, 39.5 (see Supplementary Ma-
3.1. Synthesis and characterization of SNSFCA
terial, Fig. S2); FTIR (ATR) (cm 1): 3270 and 3100 (vN–H amide),
060 (vC–H aromatic ring), 2960-2850 (vsC–H and vas C–H), 1652
vC=O amide), 1608 (v C=C aromatic ring), 1540 (vC–N and δ
−
3
The synthetic route to obtain SNSFCA was accomplished into
three steps, as shown in Scheme 1. The first step involved the
(
2

J.L. Neto, L.P.A. da Silva, J.B. da Silva et al.
Electrochimica Acta 379 (2021) 138173
Scheme 1. The synthetic route of SNSFCA.
with the repetitive cyclic voltammetric scans, which implies that
the deposition of a redox-active layer of a conducting material is
occuring on the electrode.
3
.3. Morphological characterization
The SEM image of the PSNSFCA film deposited on ITO displays
a smooth, homogenous and globular morphology. The potentiody-
namic method (cyclic voltammetry) used for the electrodeposition
of the polymer on ITO usually results in multi-nucleation lead-
ing to small, but well-dispersed, grains [45], as can be seen in
Fig. 2. Such structures are similar to those found for other poly(2,5-
dithienylpyrrole) derivatives reported in the literature [46].
3
.4. Spectroelectrochemical characterization
Fig. 1. Cyclic voltammograms of SNSFCA in 0.1 mol L 1 (C4H9)4NBF4/CH3CN.
−
The cyclic voltammogram of the PSNSFCA film deposited on ITO
(
Fig. 3a) displayed an anodic wave with anodic peak potential (Epa)
preparation of the diketone involving a Friedel-Crafts acylation
with 82% yield [12]. Then, a Paal-Knorr condensation between
the diketone and ethylenediamine gave the SNS intermediate with
at 0.33 V and a cathodic wave with cathodic peak potential (Epc)
+
at 0.22 V vs. Ag/Ag . The difference (ꢀEp) of 0.11 V between the
Epa and Epc is within the range of commonly observed values for
conjugated polymers and have been associated to the kinetic lim-
itations, such as slow heterogeneous electron transfer, effects of
structural reorganization processes within the polymer film, and
electronic charging of a sum of two interfacial exchanges, namely
the electrode/polymer and the polymer solution interfaces [47,48].
The polymer film was cycled between reduced, neutral and
oxidized states at various scan rates, in order to investigate the
scan rate dependence of anodic (Ipa) and cathodic (Ipc) peak cur-
rents. The peak currents were linearly proportional to the scan
rate indicating a non-diffusional redox process and a well-adhered
electroactive polymer films to the working electrode surface [49],
Fig. 3b.
7
5% yield [38], and lastly, the condensation reaction between
the FCAc and the amine present in the SNS intermediate by us-
ing DCC/DMAP to afford SNSFCA as a greyish solid in 73% yield,
Scheme 1. Such method has the simplicity, the use of mild condi-
tions, and good yields from readily available starting materials as
advantages.
3
.2. Electropolymerization
The cyclic voltammograms recorded during the electropolymer-
ization of SNSFCA show that the onset oxidation potential (Eonset)
+
of the monomer is ca. 0.60 V vs. Ag/Ag (CH CN), which is lower
3
than the oxidation potential reported for pyrrole (~ 0.90 V vs.
The changes in the absorbance spectra of the PSNSFCA film ac-
cording to the applied potential are shown in Fig. 4. In this case,
a potential range of 0.00 V to 0.40 V was sufficient to cause a
perceptible optical contrast in a reversible way. At 0.00 V (neu-
tral state), the absorption spectrum of the polymer film exhibited
a band with maximum wavelength (λmax) at 360 nm, assigned to
+
Ag/Ag [39]), thiophene [40], and fluorenes [29,41,42]. When com-
pared with most of SNS derivatives reported in the literature [21–
2
3,42,43], the introduction of an electron donating fluorene amide-
based substituent into the SNS main chain displaces the Eonset to-
wards less anodic potentials [44], facilitating the electropolymer-
ization process. Furthermore, from cyclic voltammograms shown
in Fig. 1, a new redox pair in the range of 0.10 ≤ E ≤ 0.40 V vs.
∗
π-π transition. The band gap energy (Eg) of 2.40 eV was calcu-
∗
lated from the onset of the π-π transition (λonset = 517 nm) in
+
Ag/Ag arised. It was observed that the current density increases
the absorption spectrum of the film at neutral state. According to
3

J.L. Neto, L.P.A. da Silva, J.B. da Silva et al.
Electrochimica Acta 379 (2021) 138173
Fig. 2. SEM image of PSNSFCA film electrodeposited on ITO.
−
1
−1
Fig. 3. Cyclic voltammograms of the PSNSFCA film deposited on ITO in 0.1 mol L (C4H9)4NBF4/CH3CN, (a) at ν = 0.02 V s and (b) at different scan rates between 0.025
and 0.300 V s 1. The inset shows a plot of the dependence of the peak current density on the potential sweep rate.
−
4

J.L. Neto, L.P.A. da Silva, J.B. da Silva et al.
Electrochimica Acta 379 (2021) 138173
Table 1
Optical properties of PFCAc and PSNS-fluorene derivatives.
Polymer
λ
max (nm)
Eg (eV)
Colors
Ref.
3
74
3.10
transparent (0.00 V), brownish-orange (1.25 V)a
[42]
4
45
2.18
yellow (0.00 V), green/blue (intermediate), violet (0.80 V)b
[21,23]
3
23
2.59
yellow (0.00 V), blue (1.00 V)c
[52]
3
60
2.40
yellow (0.00 V), green (intermediate), blue (0.40 V)c
This work
a
E (V) vs. Ag wire,
b
E (V) vs. Ag/AgCl,
c
E (V) vs. Ag/Ag+. The conversion factors for the reference electrodes are + 0.10 V relative to pseudo reference Ag wire [53] and
-
0.20 V relative to Ag/AgCl [54,55].
served at 470 nm indicating that polymer film is undergoing inter-
conversion between its neutral and oxidized states. At higher oxi-
dation levels, two broad bands at ca. 620 and 1030 nm appeared,
owing to the formation of polaronic and bipolaronic states, respec-
tively [56,57].
It is well known that the absorption spectrum of an elec-
trochromic film provides an objective measure for color absorption
in visible region. However, such maesurent is limited in terms of
how the color is perceived by the human eye. To overcome this
drawback, the field of colorimetry has been developed for descrip-
tion of color in an objective way [32]. Hence, in-situ colorimetric
analysis were performed for quantitative examination of PSNSFCA
spectral properties, thus providing a direct correlation between the
changes in the spectral absorption bands as the colors seen by the
human eye according to the potential applied to the system. The
−1
Fig. 4. Spectroelectrochemistry of PSNSFCA film deposited on ITO in 0.1 mol L
CIE 1931 xy chromaticity coordinates were calculated from the in
situ spectra of the polymer film at potentials varying from 0.00
to 0.40 V and its trajectory is shown in Fig. 5, together with the
images of the film in different oxidation states. The colors of the
film change from yellow (x = 0.388, y = 0.425) in the neutral state
(0.00 V), green (x = 0.336, y = 0.382) in the intermediate state, to
blue (x = 0.317, y = 0.354) in the oxidized state.
(
C4H9)4NBF4 / CH3CN. Spectra were registered at each 50 mV from 0.00 to 0.40 V
vs. Ag/Ag+ (CH3CN).
the literature, Eg values for conjugated polymers are in the range
of 1.5–3.0 eV [9,50,51]. Therefore, the Eg value found for PSNSFCA
films is close to the values reported for similar π-conjugated sys-
tems, including other PSNS-fluorene derivatives [21,23,52], and it is
lower than that exhibited by PFCAc [42], as can be seen in Table 1.
With increasing potential, the peak intensity of the band at
It is noteworth the PSNSFCA film presents multielec-
trochromism where a gradual change in the chromatic coordinates
takes place as the potential is varied in a narrow range of 0.40 V.
Such a high electrochromic response of PSNSFCA film contrasts
3
60 nm slightly decreased, and an isosbestic point may be ob-
5

J.L. Neto, L.P.A. da Silva, J.B. da Silva et al.
Electrochimica Acta 379 (2021) 138173
Fig. 5. (a) Calculated colour trajectory in the CIE 1931 xy color space and (b) images of the PSNSFCA film deposited on ITO, registered during potential scan from 0.00 to
+
0
.40 V vs. Ag/Ag (CH3CN) (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.).
tively, indicating that the polymer films present pronounced chro-
matic contrast in the NIR region. This characteristic is an advantage
due to the possibility of application of this polymer in smart win-
dows that controls the temperature of the ambient by absorbing
NIR radiation.
The coloration efficiency (η) was calculated from the amount of
the charge injected in the polymer as a function of the change in
the optical density during switching. The η values calculated for
the PSNSFCA film at 620 and 1030 nm were 110 and 350 cm2
C
−1
,
respectively. These values are higher than those cited in the litera-
ture for PSNS-fluorene derivatives (78 – 107 cm2
C
−1
) [21,23,52].
Therefore, concerning energy economy, PSNSFCA is a promising
material for application in electrochromic devices, because it needs
only a small amount of charge injected per area to show a percep-
tible change of its color.
The PSNSFCA film is stable with respect to the switch between
the oxidized and reduced states by over than 100 cycles, show-
ing no significant loss of its electrical and optical responses, Fig. 6.
Coulombic efficiency was calculated as being 91% in the initial cy-
cles, reaching 94% in subsequent cycles, such behavior may be at-
tributed to the conformational changes in the structure of the film
that occurs during the redox process [59].
Fig. 6. Transmittance variation at (a) λ = 620 nm and (b) 1030 nm, and (c) current
density (j) variation for the PSNSFCA film during double potential step chronoam-
perometry with E1 = 0.0 V, E2 = 0.4 V and tstep = 40 s. Full line (black) corresponds
to the first cycle and dashed line (red) corresponds to the 100th cycle (For interpre-
tation of the references to color in this figure legend, the reader is referred to the
web version of this article.).
3
.6. Fluorescence properties
The fluorescence properties of polyfluorene and its deriva-
tives are well-known and broadly reported in the literature as
having a strong blue emission about between 400 and 500 nm
[
21,28,29,60]. Therefore, it would expected that both monomer and
with the typical behavior of similar SNS-based polymers, in which
the potential range for a significant electrochromic response may
reaches up to 3.0 V [58]. Thus, these features make it suitable for
application as active layer in displays and electrochromic devices.
polymer derivatized with a fluorene moiety were fluorescent. In-
deed, the monomer presents a blue emission, which is charac-
terized by a dual fluorescence with bands centered at 424 and
5
26 nm, indicating that distinct radiative mechanisms in SNSFCA
photoluminescence occur. For organic compounds, this behavior
is usually attributed to the excited states associated with distinct
molecular conformations [61], excimer formation [62,63], and pho-
todimerization [62,64].
3
.5. Electrochromic properties
The electrochromic performance of the PSNSFCA film with re-
spect to chromatic contrast (ꢀ%T), coloration efficiency (η) and
stability to redox cycling was investigated by double step spec-
trochronoamperometry. The variation in the transmittance (%) of
the polymer film recorded simultaneously at 620 and 1030 nm in
function of the number of redox cycles is depicted in Fig. 6 and the
Interestigly, it was observed a pronounced color change in the
SNSFCA solution from colorless to brownish after the laser excita-
tion. In order to investigate such behavior, the modification in the
emission spectrum of SNSFCA in different time intervals after the
initial UV excitation was analyzed, Fig. 7a. After the initial UV exci-
tation, a modification in the SNSFCA fluorescence takes place with
ꢀ
%T calculated at such wavelengths were 11.8% and 41.3%, respec-
6

J.L. Neto, L.P.A. da Silva, J.B. da Silva et al.
Electrochimica Acta 379 (2021) 138173
Fig. 7. (a) Normalized fluorescence spectra of SNSFCA according to the time after UV laser exposure from t = 0, 5 min, 25 min and 45 min. Inset: images of SNSFCA before
t = 0) and after UV laser exposure (45 min.) under visible and UV light. (b) Calculated colour trajectory (emission) in the CIE 1931 xy color space before (t = 0) and after
UV laser exposure at 5, 25 and 45 min.
(
Fig. 8. The HOMO and LUMO Kohn-Sham molecular orbitals of the SNSFCA and PSNSFCA.
a strong suppression of the fluorescence band centered at 424 nm.
In this case, the exposure of SNSFCA solution to UV laser for a long
period of time leads to an irreversible change of its optical proper-
ties due to photodimerization [62,64].
3.7. Computational Results
According to Fig. 8, the HOMO orbital for SNSFCA is distributed
along the conjugated rings of thiophene and pyrrole of the SNS
system, while the LUMO orbitals are strictly located in the fluorene
group. However, the polymerization drastically affects the LUMO
orbital of the molecular structure of the compound, in which for
the PSNSFCA, both HOMO and LUMO orbitals are symmetrically
distributed in the conjugated region of the SNS aromatic rings. This
result is consistent with the fluorescence behavior of the material,
Although the fluorescence emission of SNSFCA solution is main-
tained after the formation of stable dimers, the photoluminescence
of PSNSFCA film and solution was not observed. In such a case, the
SNS polymer chain seems to suppress the radiative decaying of ex-
cited electrons, as discussed by the analysis of HOMO and LUMO
theoretical calculations of the monomer and polymer structures.
7

J.L. Neto, L.P.A. da Silva, J.B. da Silva et al.
Electrochimica Acta 379 (2021) 138173
since it is possible to observe that the monomer is a blue/green
light emitter, whilst the polymer is not fluorescent.
[3] K.H. Chang, H.P. Wang, T.Y. Wu, J.W. Sun, Optical and electrochromic charac-
terizations of four 2,5-dithienylpyrrole-based conducting polymer films, Elec-
trochim. Acta 119 (2014) 225–235.
Moreover, the band gap energy of the SNSFCA and PSNS-
FCA were calculated using the HOMO–LUMO difference as being
[
4] T.Y. Wu, J.W. Liao, C.Y. Chen, Electrochemical synthesis, characteriza-
tion and electrochromic properties of indan and 1,3-benzodioxole-based
poly(2,5-dithienylpyrrole) derivatives, Electrochim. Acta 150 (2014) 245–262.
5] W.M. Pazin, A.K.A. Almeida, V. Manzoni, J.M.M. Dias, A.C.F. de Abreu,
M. Navarro, A.S. Ito, A.S. Ribeiro, I.N. de Oliveira, Thermal and solvatochromic
effects on the emission properties of dansyl-based thiophene derivative, RSC
Adv. 10 (2020) 28484.
3
.85 eV and 2.71 eV, respectively. Such result is in good agreement
[
with the PSNSFCA Egop of 2.40 eV.
4. Conclusions
[
[
6] R. Ayranci, M. Ak, A fluorescence and electroactive surface design electropoly-
merization of dansyl fluorophore functionalized PEDOT, J. Electrochem. Soc.
164 (2017) H925–H930.
The monomer SNSFCA was prepared by using a simple syn-
7] J.L. Neto, L.P.A. da Silva, J.B. da Silva, A.J.C. da Silva, D.J.P. Lima, A.S. Ribeiro,
thetic route with good yield (73%) and its electropolymerization
on ITO was successfully achieved. PSNSF films exhibited multielec-
trochromic behavior, presenting distinct colors from yellow in the
neutral state, green in the intermediate state, to blue in the ox-
idized state. It would expected that both monomer and polymer
derivatized with a fluorene moiety were fluorescent, however, the
monomer presents a blue emission, whilst the photoluminescence
of the polymer was not observed. Such behavior was interpreted
theoretical calculations in good agreement with the results.
The electrochromic properties showed by the PSNSFCA films,
such as good chromatic contrast (ꢀ%T), coloration efficiency (η)
and stability to switching, including high absorption in the NIR re-
gion (ꢀ%T at 1030 nm = 41.3%), aroused the interest in the appli-
cation of this polymer as active layer in electrochromic devices and
displays.
A
rainbow multielectrochromic copolymer based on 2,5-di(thienyl)pyrrole
derivative bearing a dansyl substituent and 3,4-ethylenedioxythiophene, Synth.
Met. 269 (2020) 116545.
8] A.J.C. Silva, V.C. Nogueira, T.E.A. Santos, C.J.T. Buck, D.R. Worrall, J. Ton-
[
holo, R.J. Mortimer, A.S. Ribeiro, Copolymerisation as
a way to enhance
the electrochromic properties of an alkylthiophene oligomer and a pyrrole
derivative: copolymer of 3,3”ꢀꢀ dihexyl-2,2’:5’,2ꢀꢀ:5ꢀꢀ,2”ꢀꢀ-quaterthiophene with
(R)-(-)-3-(1-pyrrolyl)propyl-N-(3,5-dinitrobenzoyl)-α-phenylglycinate, Sol. En-
ergy Mater. Sol. Cells 134 (2015) 122–132.
[
9] A.S. Ribeiro, R.J. Mortimer, Conjugated conducting polymers with elec-
trochromic and fluorescent properties, in: SPR Electrochemistry, v. 13, 1st ed.,
Cambridge, UK, 2016, pp. 21–49.
10] E. Rende, C.E. Kilic, Y.A. Udum, D. Toffoli, L. Toppare, Electrochromic properties
of multicolored novel polymer synthesized via combination of benzotriazole
and N-functionalized 2,5-di(2-thienyl0-1H-pyrrole units, Electrochim. Acta 138
(2014) 454–463.
[
[
11] P. Camurlu, C. Gültekin, Z. Bicil, Fast switching, high contrast multichromic
polymers from alkyl-derivatized dithienylpyrrole and 3,4-ethylenedioxythio-
phene, Electrochim. Acta 61 (2012) 50–56.
[
12] S. Tarkuc, M. Ak, E. Onurhan, L. Toppare, Electrochromic properties of
trimeric”thiophene-pyrrole-thiophene derivative grown from electrodeposited
-(2,5-di-thiophen-2-yl-1H-pyrrolyl-1-yl)hexan-1-amine and its copolymer, J.
Macromol Sci. Part A Pure Appl. Chem. 45 (2008) 164–171.
“
6
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
[13] N.A. Lengkeek, J.M. Harrowfield, G.A. Koutsantonis, Synthesis and electropoly-
merization of N-(4”-carboxyphenyl)-2,5-di(2”-thienyl)pyrrole, Synth. Met. 160
(
2010) 72–75.
[
[
[
14] E. Yildiz, P. Camurlu, C. Tanyeli, I. Akhmedov, L. Toppare, A soluble conducting
polymer of 4-(2,5-di(thiophene-2-yl)-1H-pyrrol-1-yl)benzamine and its multi-
chromic copolymer with EDOT, J. Electroanal. Chem. 612 (2008) 247–256.
15] P. Camurlu, Z. Bicil, C. Gültekin, N. Karagoren, Novel ferrocene derivatized
poly(2,5-dithienylpyrrole)s: optoelectronic properties, electrochemical copoly-
merization, Electrochim. Acta 63 (2012) 245–250.
Credit authorship contribution statement
Jorge L. Neto: Methodology, Formal analysis, Investigation, Val-
idation, Writing – original draft. Luis P.A. da Silva: Investigation,
Validation. Joel B. da Silva: Investigation, Validation. Raul L. Fer-
reira: Investigation, Validation. Ana Júlia C. da Silva: Methodology,
Formal analysis, Investigation, Supervision. Júlio C.S. da Silva: For-
mal analysis, Writing – original draft. Ítalo N. de Oliveira: Formal
analysis, Investigation, Writing – original draft. Dimas J.P. Lima:
Conceptualization, Formal analysis, Resources, Writing – original
draft, Supervision, Funding acquisition. Adriana S. Ribeiro: Con-
ceptualization, Formal analysis, Resources, Writing – review & edit-
ing, Project administration, Funding acquisition.
16] A. Cihaner, F. Algi,
A
new conducting polymer bearing 4,4-difluo-
ro-4-bora-3a,4a-diaza-s-indacene (BODIPY) subunit: synthesis and charac-
terization, Electrochim. Acta 54 (2008) 786–792.
[17] D. Asil, A. Cihaner, F. Algi, A.M. Önal, A diverse-stimuli responsive chemilu-
minescent probe with luminol scaffold and its electropolymerization, Electro-
analysis 22 (2010) 2254–2260.
[18] A. Cihaner, F. Algi, Electrochemical and optical properties of new soluble
dithienylpyrroles based on azo dyes, Electrochim. Acta 54 (2009) 1702–1709.
[19] B. Hu, C. Li, Z. Liu, X. Zhang, W. Luo, L. Jin, Synthesis and multi-electrochromic
properties of asymmetric structure polymers based on carbazole-EDOT and 2,
5
–dithienylpyrrole derivatives, Electrochim. Acta 305 (2019) 1–10.
[20] S. Koyuncu, C. Zafer, E. Sefer, F.B. Koyuncu, S. Demic, I. Kaya, E. Ozdemir, S. Icli,
A new conducting polymer of 2,5-bis(2-thienyl)-1H-(pyrrole) (SNS) contain-
ing carbazole subunit: electrochemical, optical and electrochromic properties,
Synth. Met. 159 (2009) 2013–2021.
[
21] A. Cihaner, F. Algi, Processable electrochromic and fluorescent polymers based
Acknowledgments
on N-substituted thienylpyrrole, Electrochim. Acta 54 (2008) 665–670.
[
[
[
[
22] W.H. Wang, J.C. Chang, T.Y. Wu, 4-(Furan-2-yl)phenyl-containing poly-
dithienylpyrroles as promising electrodes for high contrast and coloration effi-
ciency electrochromic devices, Org. Electron. 74 (2019) 23–32.
The authors wish to thank the granting authorities CNPq (proc.
4
3
42320/2014-0), CAPES and FAPEAL (proc. 1033/2016 and 60030
93/2017) for financial support. Also wish thank to Prof. J.D. de
23] A. Cihaner, F. Algi, A processable rainbow mimic fluorescent polymer and its
unprecedent coloration efficiency in elecetrochromic device, Electrochim. Acta
Freitas at Federal Institute of Alagoas for the SEM image.
53 (2008) 2574–2578.
24] J. Hu, J. You, C. Peng, S. Qiu, W. He, C. Li, X. Liu, Y. Mai, F. Guo, Polyflu-
orene copolymers as high-performance hole-transport materials for inverted
Perovskite solar cells, RRL Sol. 4 (2020) 1900384.
25] Q. Hu, W. Zhang, Q. Yin, Y. Wang, H. Wang, A conjugated fluorescent poly-
mer sensor with amidoxime and polyfluorene entities for effective detection
of uranyl ion in real samples, Spectrochim. Acta Part A Mol. Biomol. Spectrosc.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.electacta.2021.138173.
244 (2021) 118864.
[
26] S. Roy, A. Gunukula, B. Ghosh, C. Chakraborty, A folic acid-sensitive polyfluo-
rene based “turn-off” fluorescence nanoprobe for folate receptor overexpressed
cancer cell imaging, Sens. Actuators B Chem. 291 (2019) 337–344.
References
[
27] F.A.R. Nogueira, A.J.C. Silva, J.D. Freitas, A.S. Tintino, A.P.L.A. Santos, I.N. Oliveira,
A.S. Ribeiro, Transmissive to dark electrochromic and fluorescente device based
on poly(fluorene-bisthiophene) derivative, J Braz. Chem. Soc. Spec. Issue Braz.
Woman Sci. 30 (12) (2019) 2702–2711.
[
[
1] M.P. Algi, A. Cihaner, F. Algi, Design, synthesis, photochromism and electro-
chemistry of a novel material with pendant photochromic units, Tetrahedron
7
0 (2014) 5064–5072.
2] D. Yi g˘ it, S¸ .O. Hacıo g˘ lu, M. Güllü, L. Toppare, Novel poly(2,5-dithienylpyrrole)
PSNS) derivatives functionalized with azobenzene, coumarin and fluorescein
[
28] S.P. Mucur, C. Kök, H. Bilgili, B. Canımkurbey, S. Koyuncu, Conventional and in-
verted organic light emitting diodes based on bright green emmisive polyflu-
orene derivatives, Polymer 151 (2018) 101–107.
(
chromophore units: Spectroelectrochemical properties and electrochromic de-
vice applications, New J. Chem. 39 (2015) 3371–3379.
8

J.L. Neto, L.P.A. da Silva, J.B. da Silva et al.
Electrochimica Acta 379 (2021) 138173
[
29] G. Nie, T. Cai, J. Xu, S. Zhang, Low-potential facile electrosyntheses of
high-quality free-standing poly(fluorene-9-carboxylic acid) films, Electrochem.
Commun. 10 (2008) 186–189.
[46] T. Soganci, S. Soyleyici, H.C. Soyleyici, M. Ak, Optoelectronic characterization
and smart windows application of bi-functional amid sibstituted thienyl pyr-
role derivative, Polymer 118 (2017) 40–48.
[
30] G. Gritzner, J. Kuta, Recommendations on reporting electrode potentials in
nonaqueous solvents, Pure Appl. Chem. 56 (1984) 461–466.
31] G. Wyszecki, W.S. Stiles, Color Science: Concepts and Methods, Quantitative
Data and Formulae, 2nd ed., John Wiley Ans Dons, New York, USA, 1982.
[47] G. Nie, L. Qu, J. Xu, S. Zhang, Electrosyntheses and characterizations of a new
soluble conducting copolymer of 5-cyanoindole and 3,4-ethylenedioxythio-
phene, Electrochim. Acta 53 (2008) 8351–8358.
[
[48] G. Inzelt, M. Pineri, J.W. Schutze, M.A. Vorotyntsev, Electron and proton con-
ducting polymers: recent developments and prospects, Electrochim. Acta 45
(2000) 2403–2421.
[
32] R.J. Mortimer, T.S. Varley, Quantification of colour stimuli through the calcula-
tion of CIE chromaticity coordinates and luminance data for application to in
situ colorimetry studies of electrochromic materials, Displays 32 (2011) 35–44.
33] R.J. Mortimer, T.S. Varley, In situ spectroelectrochemistry and colour measure-
ment of a complementary electrochromic device based on surface-confined
prussian blue and aqueous solution-phase methylviologen, Sol. Energy Mater.
Sol. Cells 99 (2012) 213–220.
[49] J.F. Rusling, S.L. Suib, Characterizing materials with cyclic voltammetry, Adv.
Mater. 6 (1994) 922–930.
[
[50] P.M. Beaujuge, J.R. Reynolds, Color control in π-conjugated organic polymers
for use in electrochromic devices, Chem. Rev. 110 (2010) 268–320.
[51] P. Camurlu, Polypyrrole derivatives for electrochromic applications, RSC Adv. 4
(2014) 55832–55845.
[34] P.A. Santa-Cruz, F.S. Teles, Spectra Lux Software v. 2.0 Beta, Ponto Quântico
Nanodispositivos/RENAMI, Brasil, 2003.
[52] N. Guven, P. Camurlu, Optoelectronic properties of poly(2,5-dithienylpyrrole)s
with fluorophore groups, J. Electrochem. Soc. 162 (2015) H867–H876.
[
35] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-
man, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji,
M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida,
T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery
Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin,
V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Bu-
rant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox,
J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann,
O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Mo-
rokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich,
A.D. Daniels, Ö. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, GAUS-
SIAN 09, Revision A.1, Gaussian, Inc., Wallingford, CT, 2009.
[53] T. Soganci, H.C. Soyleyici, M. Ak,
A soluble and fluorescent new type
thienylpyrrole based conjugated polymer: optical, electrical and electrochemi-
cal properties, Phys. Chem. Chem. Phys. 18 (2016) 14401–14407.
[54] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applica-
tion, John Wiley & Sons, New York, 1980.
[55] L.B. Groenendaal, G. Zotti, P.H. Aubert, S.M. Waybright, J.R. Reynolds, Electro-
chemistry of poly(3,4-alkylenedioxythiophene) derivatives, Adv. Mater. 15 (11)
(2003) 855–879.
[56] M. Sato, S. Tanaka, K. Kaeriyama, Electrochemical preparation of conducting
poly(3-methylthiophene): comparison with polythiophene and poly (3-ethylth-
iophene), Synth. Met. 14 (1986) 279–288.
[57] T.C. Chung, J.H. Kaufman, A.J. Heeger, F. Wudl, Charge storage in doped
poly(thiophene): Optical and electrochemical studies, Phys. Rev. B 30 (1984)
702–710.
[58] T. Soganci, S. Soyleyici, H.C. Soyleyici, M. Ak, High contrast elec-
trochromic polymer and copolymer materials based on amide-substituted
poly(dithienylpyrrole), J. Electrochem. Soc. 164 (2017) H11–H20.
[59] L.M.O. Ribeiro, J.Z. Auad, J.G. Silva Júnior, M. Navarro, A. Mirapalheta, C. Fon-
seca, S. Neves, J. Tonholo, A.S. Ribeiro, The effect of the conditions of electrode-
position on the capacitive properties of dinitrobenzoyl-derivative polypyrrole
films, J. Power Sources 177 (2008) 669–675.
[60] S.P. Mucur, R. Kacar, C. Meric, S. Koyuncu, Thermal annealing effect on light
emmision profile of polyfluorenes containing double bond subunit, Org. Elec-
tron. 50 (2017) 55–62.
[
36] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, A consistent and accurate ab initio
parametrization of density functional dispersion correction (DFT-D) for the 94
elements H – Pu., J. Chem. Phys. 132 (2010) 154104.
[
37] T.H. Dunning Jr., Gaussian basis sets for use in correlation molecular calcu-
lations. I. The atomas boron through neon and hydrogen, J. Chem. Phys. 90
(1989) 1007.
[38] G. Cooke, J.F. Garety, B. Jordan, N. Kryvokhyzha, A. Parkin, G. Rabani,
V.M. Rotello, Favin-based [2]rotaxanes, Org. Lett. 8 (2006) 2297–2300.
[39] S. Cosnier, A. Karyakin, Electropolymerization: Concepts, Materials and Appli-
cations, Wiley-VCH Verlag, Weinhein, Germany, 2010.
[
40] N. Atilgan, A. Cihaner, A.M. Onal, Electrochromic performance and ion sensi-
tivity of a terthienyl based fluorescent polymer, React. Funct. Polym. 70 (2010)
2
44–250.
[61] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Structural changes accompanying in-
tramolecular electron transfer: focus on twisted intramolecular charge-transfer
states and structures, Chem. Rev. 103 (2003) 3899–4032.
[
41] J. Xu, Z. Wei, Y. Du, W. Zhou, S. Pu, Low-potential electrochemical polymeriza-
tion of fluorene and its alkyl-polymer precursor, Electrochim. Acta 51 (2006)
4
771–4779.
[63] T. Hayashi, N. Malaga, Y. Sakata, S. Misumi, M. Morita, J. Tanaka, Ex-
cimer fluorescence and photodimerization of anthracenophanes and 1,2-di-
anthrylethanes, J. Am. Chem. So. 98 (1976) 5910–5913.
[
42] B. Bezgin, A. Cihaner, A.M. Önal, Electrochemical polymerization of 9-fluo-
renecarboxylic acid and its electrochromic device application, Thin Solid Films
5
16 (2008) 7329–7334.
[62] W.T. Yip, D.H. Levy, Excimer/exciplex formation in van der Waals dimers of
aromatic molecules, J. Phys. Chem. 100 (1996) 11539–11545.
[64] H. Bouas-Laurent, A. Castellan, J.P. Desvergne, R. Lapouyade, Photodimerization
of anthracenes in fluid solutions: (part 2) mechanistic aspects of the photo-
cycloaddition and of the photochemical and thermal cleavage, Chem. Soc. Rev.
30 (2001) 248–263.
[
43] E. Sefer, H. Bilgili, B. Gultekin, M. Tonga, S. Koyuncu, A narrow range multi-
electrochromism from 2,5-di-(2-thienyl)-1H-pyrrole polymer bearing pendant
perylenediamine moiety, Dyes Pigm. 113 (2015) 121–128.
44] J. Roncali, Conjugated poly(thiophenes): synthesis, functionalization, and ap-
plications, Chem. Rev. 92 (1992) 711–738.
[
[
45] A.S. Ribeiro, L.M.O. Ribeiro, J.G. Silva Jr., M. Navarro, J. Tonholo, Characteriza-
tion by atomic force microscopy of electrodeposited films of polypyrrole-dini-
trobenzoyl derivative, Microsc. Microanal. 11 (2005) 146–149.
9