Synthesis and spectroscopic characterization of a fluorescent pyrrole derivative containing electron acceptor and donor groups

Article abstract of DOI:10.1016/j.saa.2014.03.012

The synthesis and fluorescence characterization of a new pyrrole derivative (PyPDG) containing the electron donor-acceptor dansyl substituent is reported. The effects of temperature and solvent polarity on the steady-state fluorescence of this compound are investigated. Our results show that PyPDG exhibits desirable fluorescent properties which makes it a promising candidate to be used as the photoactive material in optical thermometry and thermography applications. Further, the electrochemical and emission properties of polymeric films obtained from the oxidation polymerization of PyPDG are also analyzed.

Full text of DOI:10.1016/j.saa.2014.03.012

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 812–818
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and spectroscopic characterization of a fluorescent pyrrole
derivative containing electron acceptor and donor groups
A.K.A. Almeida a, M.P. Monteiro b, J.M.M. Dias c, L. Omena b, A.J.C. da Silva a, J. Tonholo a, R.J. Mortimer d,
M. Navarro c, C. Jacinto b, A.S. Ribeiro a,d, I.N. de Oliveira b,
⇑
a Instituto de Química e Biotecnologia, Universidade Federal de Alagoas, 57072-970 Maceió-AL, Brazil
b Instituto de Física, Universidade Federal de Alagoas, 57072-970 Maceió-AL, Brazil
c Departamento de Química Fundamental, CCEN, Universidade Federal de Pernambuco, 50670-901 Recife-PE, Brazil
d Department of Chemistry, Loughborough University, LE11 3TU Leicestershire, United Kingdom
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
The synthesis of a new pyrrole
derivative containing the dansyl
group is reported.
The emission of PyPDG is high
sensitive to the temperature and
polarity environment.
The emission intensity of PyPDG
enhances as the solvent temperature
is increased.
The fluorescence of PyPDG is
analyzed in light of a thermally
activated mechanism.
We characterize the electrochemical
and emission properties of the
polymerized material.
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and fluorescence characterization of a new pyrrole derivative (PyPDG) containing the elec-
tron donor–acceptor dansyl substituent is reported. The effects of temperature and solvent polarity on
the steady-state fluorescence of this compound are investigated. Our results show that PyPDG exhibits
desirable fluorescent properties which makes it a promising candidate to be used as the photoactive
material in optical thermometry and thermography applications. Further, the electrochemical and emission
properties of polymeric films obtained from the oxidation polymerization of PyPDG are also analyzed.
Ó 2014 Elsevier B.V. All rights reserved.
Received 26 August 2013
Received in revised form 24 February 2014
Accepted 8 March 2014
Available online 20 March 2014
Keywords:
Dansyl
Pyrrole
Twisted Intramolecular Charge Transfer (TICT)
Fluorescence
Thermal effects
Introduction
possibility of exploring such materials in the design of new elec-
tro-optical devices [1–3]. In fact, highly conjugated compounds
The synthesis of new organic
attracted remarkable interest over the past decade due to the
p
-conjugated compounds has
have been reported as an excellent alternative to sensing and
imaging applications which require an active matrix with lumines-
cent properties that are sensitive to the external conditions, such
as the medium polarity [4,5] and the environment temperature
[6,7]. In this context, the class of organic molecules containing
⇑
Corresponding author. Tel.: +55 8232141778.
E-mail address: italo@fis.ufal.br (I.N. de Oliveira).
http://dx.doi.org/10.1016/j.saa.2014.03.012
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

A.K.A. Almeida et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 812–818
813
electron donor and acceptor groups has played an important role
once they present a rich phenomenology related to the mechanism
of intramolecular charge transfer (ICT) [8]. In particular, dimethyl-
amino-substituted aromatic compounds may exhibit a dual fluo-
rescence upon a single excitation which is directly associated
with a large charge separation and the emergence of highly polar
excited states from the process of the Twisted Intramolecular
Charge Transfer (TICT) [8]. Indeed, the TICT state corresponds to
a rotational isomer stabilized by a polar medium in which a
complete charge transfer has occurred [8,9]. In addition to the
short wavelength emission arising from the locally excited (LE)
state [10], the deexcitation from the TICT state takes place at a
lower energy level, thus leading to a red-shifted second emission
which is sensitive to the temperature, the polarity, and the
viscosity of the solvent environment [8,10,11]. As a consequence,
compounds presenting the TICT mechanism have been successfully
used in distinct timely applications, such as in real-time microvis-
cosity probes [12], molecular recognition [13], as well as in the
imaging of living cells [14].
temperature on the steady-state fluorescence of PyPDG are
investigated. We show that the resultant compound preserves
the fluorescent properties of the dansylglycine and presents an
electrochemical response inherent to polypyrrole.
Experimental
Materials
Unless otherwise stated, all chemical reagents were purchased
from Sigma–Aldrich, Vetec, or Acros and used as received. The sol-
vents of analytical grade were dried by conventional procedures
and distilled prior to use. The compounds were characterized by
1H NMR spectroscopy, FTIR and elemental analysis.
Instrumentation
The 1H NMR spectra were recorded using a Bruker spectrometer
operating at a frequency of 400 MHz. The FTIR spectra were
acquired with a Bruker IFS66 spectrophotometer using KBr pellets.
The elemental analysis determinations were performed using a
Carlo Erba equipment. Melting points were determined on a Micro
Química MQAPF 301 melting point apparatus and are uncorrected.
Number- and weight-average molecular weights (Mn, Mw) were
measured by size exclusion chromatography (SEC) against polysty-
rene (PS) standards, using a Polymer Laboratory PLGel 10 mm
Mixed-C column, a Shodex RI-71RI detector, and a Shimadzu
LC-10 AD pump, in THF, at a 1.0 mL minꢁ1 flow rate. Cyclic voltam-
mograms were recorded on an Autolab PGSTAT30 galvanostat/
potentiostat by using a three-electrode cell.
Since the prominent work reporting the assembly of a light-
emitting diode based on a polymeric material [15], several studies
have been devoted to the design of new
p-conjugated polymers
containing fluorescent groups in their architectures [16–18]. In
particular, such systems exhibit optical and electronic properties
similar to inorganic semiconductor materials, giving rise to a
new generation of organic semiconductors with useful characteris-
tics, such as mechanical flexibility, thermal stability, and low cost
of production. In fact, a large variety of optoelectronic devices
based on conjugated polymers has been demonstrated from the
electronic transport and/or the light emission in these systems,
including organic light emitting diodes (OLED) [19], organic field-
effect transistors (OFET) [20,21], and photovoltaic devices
[22,23]. Although many conjugated polymers have been explored
in the development of optoelectronic devices, polypyrrole and its
derivatives constitute an important class of polymeric compounds
due to their low oxidation potential, high stability, and the good
capability of being chemically modified without the loss of the
conductive properties [24,25]. As a result, properly functionalized
pyrrole derivatives have been identified as ideal photoactive mate-
rials for a large variety of chemosensing applications, such as DNA
recognition [26], diagnosis of hepatitis C virus [27], electronic
tongue [28], and gases detection [29].
An attractive feature of polymeric materials is the possibility to
alter their electronic and spectral properties through the attach-
ment of different functional groups in their polymeric backbone
[30–32]. Such a desirable characteristic has been widely used in
the fluorescent label technique, which explores the side attach-
ment of fluorophore groups in the polymeric chain [33]. Among
the large variety of fluorophore groups, dansyl probes have played
an important role due to their high sensitivity to the environmen-
tal conditions which is associated with the formation of TICT states
[18,4,34,35]. As a consequence, several practical applications have
been realized from the covalent linkage of dansyl groups in a poly-
mer backbone [18,36,37]. By using blends of electroluminescent
polymers and polysilanes functionalized with dansyl moieties, it
was demonstrated that blend-based OLEDs present an improved
performance as compared with OLEDs based on single electrolumi-
nescent polymer [37]. Recently, the synthesis was reported of a
fluorescent polythiophene derivative bearing a donor–acceptor
dansyl substituent that exhibits fluorescent and electrochromic
properties [17].
Steady-state fluorescence and absorption
In order to investigate the solvation and thermal effects, the
steady-state fluorescence of PyPDG was probed by exciting the
sample with the Innova 90 CW Argon laser (Coherent) tuned at
457 nm. The excitation power was fixed at P0 ¼ 5:2 mW in all
measurements to allow a comparative analysis of the emission
intensity. The emission was collected by an optical fiber connected
to a monochromator Sciencetech, model 9057. The signal was
detected by a photomultiplier tube model S-20 and it was ampli-
fied with a lock-in amplifier SR530 (Stanford Research Systems).
By using a homemade electric oven with a precision of 0.1 K, the
sample temperature was varied from 295 K up to 316 K, in steps
of 3 K. The solutions of PyPDG presented a concentration of
1.0 mg/mL and all emission measurements were carried out at
the same experimental conditions. Absorption spectra were
obtained using a spectrophotometer Perkin Elmer LAMBDA 1050.
Synthesis
1-(3-Bromopropyl)pyrrole and 1-(3-iodopropyl)pyrrole were
synthesized according to the procedure described in previous liter-
ature [38] and were obtained in 64% and 57% yield, respectively.
The data of 1H NMR, FTIR and elemental analysis are in agreement
with those previous reported for both pyrrole derivatives [38].
3-(N-pyrrolyl)propyl dansylglycinate (PyPDG)
1-(3-Iodopropyl)pyrrole (0.66 g, 2.82 mmol) and 1,8-bis(dimethyl-
amino)naphthalene (proton-spongeÒ, 0.44 g, 2.09 mmol) were
added to a solution of dansylglycine (0.63 g, 2.05 mmol) in 15 mL
dry CH3CN. The reaction mixture was stirred at 50 °C for 1.5 h,
and the white precipitate was removed by filtration. CH3CN
(15 mL) was added to the crude product, which was followed by
stirring, and the precipitate was removed again by filtration. The
In the present work, we report the synthesis and spectroscopic
characterization of a new pyrrole derivative containing the elec-
tron donor–acceptor dansyl substituent. 3-(N-pyrrolyl)propyl
dansylglycinate (PyPDG) was prepared by simple route using
dansylglycine as a precursor [38]. The effects of solvation and

814
A.K.A. Almeida et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 812–818
filtration step was repeated until no more precipitate was formed.
The crude product was chromatographed on silica using CH2Cl2 as
eluent to give 0.50 g (59% yield) of the compound as a pale yellow
solid. M.p. 84.9–85.5 °C; 1H NMR (400 MHz, methanol-d4, d): 8.56
(d, J = 8.6 Hz, 1H), 8.37 (d, J = 8.6 Hz, 1H), 8.18 (dd, J = 7.3 and
1.2 Hz, 1H), 7.62–7.54 (m, 2H), 7.27 (dd, J = 7.5 and 1.2 Hz, 1H),
6.57 (m, 2H), 5.99 (m, 2H), 3.80–3.70 (m, 6H), 2.87 (s, 6H),
materials. 1-(3-Iodopropyl)-pyrrole was prepared by nucleophilic
substitution of the brominated precursor. The esterification of
the 1-(3-Iodopropyl)-pyrrole with dansylglycine using proton-
spongeÒ as a selective proton abstractor affords PyPDG with good
yield. Finally, the oxidative polymerization of PyPDG with FeCl3 in
CHCl3 generates a black polymer (PPyPDG).
1.92–1.78 (m, 2H); FTIR (KBr): 3287 (s,
m (NAH)), 3098 (w, m
Fluorescence of PyPDG
(CAH ) pyrrole)), 2928 (m, mas (CAH)), 2776 (m, mas (CAH)), 1752
a
(s,
m (C@O)), 1576 (w, mas (C@C)), 1325 (w, d (NAH)), 1227 (m, d
In Fig. 2, we exhibit the absorption and emission spectra of
PyPDG in toluene. In both cases, the sample temperature was kept
at 295 K. From the absorption spectrum, we observe that the
PyPDG presents two absorption bands at ka ¼ 260:5 nm and
(CAH, naphthalene)), 1160 (m,
m (CAO)), 789 (m, dout-of-plane (CAH,
naphthalene)), 721 (s, dout-of-plane (CAH pyrrole)) cmꢁ1. Anal. calcd
a
for C21H25N3O4S: C 60.70, H 6.06, N 10.11, O 15.42, S 7.72; found:
C 63.05, H 6.27, N 9.09, O 14.05, S 7.54.
kb ¼ 338:4 nm, corresponding to the lowest
p !
pꢃ transitions
which are typical of the aminonaphthalene derivatives [39]. In par-
Poly[3-(N-pyrrolyl)propyl dansylglycinate] (PPyPDG)
ticular, the absorption band centered at ka ¼ 260:5 nm corresponds
to the
p !
pꢃ transition from the fundamental state, S0, to the
PyPDG (0.40 g; 1.01 mmol) dissolved in 100 mL dry CHCl3 was
added by dropwise to a FeCl3 suspension (0.80 g; 5.04 mmol) in
20 mL dry CHCl3, under N2. The mixture was stirred for 48 h at
room temperature. The polymer was precipitated by addition of
CH3OH, filtered, and purified by Soxhlet extraction with CH3OH.
The purified polymer was dried under vacuum at 50 °C for 6 h. A
black solid (0.16 g) was obtained. 1H NMR (400 MHz, dimethyl
sulfoxide-d6, d): 8.53, 8.35, 8.22, 8.12–7.97, 7.52, 7.37, 7.29, 2.83,
short-axis polarized state, 1La, while the absorption band centered
at ka ¼ 338:4 nm is associated with the transition from S0 to the
long-axis polarized state 1Lb. It is important to stress that the low-
est absorption bands of the aminonaphthalene derivatives are
associated with the formation of locally excited states (LE), which
tend to be insensitive to the solvent polarity. In fact, we have not
observed any effect associated with the solvent polarity on the
absorption spectrum of PyPDG. Concerning with the fluorescence,
we observe that PyPDG presents a broad emission spectrum rang-
ing from 420 nm to 650 nm, with a maximum intensity at
kf ¼ 501:9 nm. The fluorescence spectrum of PyPDG is quite simi-
lar to the emission spectrum of dansylglycine (not shown), indicat-
ing a minor contribution of the pyrrole group to the emission state
of PyPDG. The broad emission of PyPDG can be directly attributed
to the radiative relaxation from the LE and TICT states, with the
later being very sensitive to the polarity of the solvent. Indeed, it
was previously reported that dansyl moieties exhibit a dual fluo-
rescence which is characterized by a double exponential decay of
the transient fluorescence intensity [40,41]. More specifically,
the mixing of 1La and 1Lb polarized states of the aminonaphthalene
group constitute the emitting state of PyPDG, with the emergence
of a charge-transfer state from the promotion of a lone-pair
electron of the amino group into a pꢃ-antibonding orbitals of the
naphthalene ring [39,41]. As consequence, we observe a noticeable
red shift in the emission of PyPDG as the polarity of the solvent is
increased. Although the red shift in the fluorescence spectrum is
3.80–3.65, 1.78–1.90. FTIR (KBr): 3460 (s,
(CAH)), 2779 (m, mas (CAH)), 1752 (s,
m
m
(NAH)), 2928 (m, mas
(C@O)), 1560 (w, mas
(C@C)), 1322 (s, d (NAH)), 1203 (m, d (CAH, naphthalene)), 1143
(m,
m .
(CAO)), 791 (m, dout-of-plane (CAH, naphthalene)) cmꢁ1
Mw ¼ 2:67 ꢂ 104; Mn ¼ 1:93 ꢂ 104 and polydispersity index
(Mw=Mn) of 1.38.
Film deposition
The polymer films were prepared by dissolving 1.0 mg PyPDG
in CH2Cl2 (1.0 mL), followed by their coating onto ITO/glass
(Delta Technologies, 8–12
via casting of the polymer solution onto the electrode
(100
L cmꢁ2). The films were then dried at room temperature.
X
, coated area = 1.0 cm2) electrodes
l
The polymer films had been rinsed with CH3CN prior to the elec-
trochemical analysis.
Electrochemistry
typical of
p !
pꢃ electronic transition, several works have reported
The polymer films deposited onto ITO/glass were characterized
by cyclic voltammetry in 0.1 mol Lꢁ1 LiClO4/CH3CN solution as
supporting electrolyte, using a Pt wire as the counter electrode
and an Ag/Ag+ (0.1 mol Lꢁ1, CH3CN) electrode as reference, at a
scan rate of 20 mV sꢁ1. Cyclic voltammograms were acquired with-
in the potential scan range of ꢁ1:8 6 E 6 0:85 V vs. Ag/Ag+
(0.1 mol Lꢁ1, LiClO4/CH3CN).
a red shift on the fluorescence phenomenon involving a TICT state
[8]. The data of absorption and fluorescence from PyPDG are
summarized in Table 1.
The Stokes shift ma
ꢁ
mf can be used to estimate the variation in
the dipole moment of the PyPDG molecule upon excitation [6]. In
particular, we use the empirical polarity parameter ENT introduced
by Reichardt and Welton [5,42], which reduces the effects associ-
ated with the error estimative of the Onsager cavity radius of the
molecule of interest. Further, the ENT parameter provides a better
description of the microscopic environment of molecular dipoles
in solution rather than the other bulk polarity functions based on
the permittivities and the refractive indices of the solvents, by
including the formation of hydrogen bonding and the distinct
mechanisms of the intramolecular charge transfer [42,43]. More
specifically, the variation of molecular dipole moment can be ob-
tained from:
Results and discussion
Synthesis
The synthetic route to obtain PyPDG and its polymer was
divided into three steps: the first one involved preparation of
pyrrole derivatives, the second was the esterification step and
the third was the polymerization of the monomer using FeCl3
(Fig. 1). Pyrrole derivative was prepared by condensation of the
primary amines with 2,5-dimethoxytetrahydrofuran, in glacial
acetic acid, to give in one step N-substituted pyrrole. This method
is applicable to a large variety of substituted aliphatic and
aromatic amines and it has the advantages of simplicity, mild
conditions and good yields from readily available starting
"
#
ꢀ
ꢁ
2
ꢂ
ꢃ
3
d
l
aB
a
ma
ꢁ
mf ¼ 11307:6
ENT þ constant:
ð1Þ
d
lB
Here, ma and mf are respectively the absorption and fluorescence
maximum wavenumbers in cmꢁ1. dlB ¼ 9 D and aB ¼ 6:2 Å are

A.K.A. Almeida et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 812–818
815
Fig. 1. Synthetic route for the preparation of 3-(N-pyrrolyl)propyl dansylglycinate (PyPDG) and polymerization of PyPDG, to obtain PPyPDG.
respectively the dipole moment change upon excitation and the
Onsager cavity radius of the betaine dye [5]. d and a corresponds
2.5
2.0
1.5
1.0
0.5
0.0
2.5
2.0
1.5
1.0
0.5
0.0
l
to the variation of the dipole moment and the Onsager cavity
radius of the molecule of interest. In Fig. 3, we present the Stokes
shift of PyPDG as a function of the solvent polarity parameter ENT .
By determining the slope m from linear regression of the data
(dashed line), we can evaluate the dipole moment variation for
PyPDG by using the following relation:
LE
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
d
l2B ꢂ m ꢂ a3
dl
¼
:
ð2Þ
11307:6 ꢂ aB
TICT
500
From the molar mass and the density of PyPDG,
MP ¼ 415:50 g=mol and qP ¼ 1:21 g=cm3, we estimate the Onsager
cavity radius a ¼ 5:14 Å and the variation of the dipole moment
200
300
400
600
700
Wavelength (nm)
d
l ¼ 3:48 D for PyPDG molecule. This value is in good agreement
with previous results reported for other dansyl derivatives
[4,39,49]. However, the small variation of dipole moment indicates
that the radiative decay from locally excited state corresponds to
the main mechanism for the fluorescence spectrum of the dansyl
Fig. 2. Absorption (solid black line) and fluorescence (solid red line) of PyPDG in
toluene. The dashed lines correspond to the Gaussian deconvolution of the
fluorescence spectrum of PyPDG. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
Table 1
Solvatochromic data and solvents parameters: ka (ma) and kf (mf ) are the absorption and fluorescence peak wavelengths (wavenumbers), respectively. (ma ꢁ mf ) is the Stokes shift.
ꢀ is the solvent dielectric constant and ENT is the Reichardt solvent parameter [5].
ETN
Solvent
ꢀ
ka (nm)
kf (nm)
ðma
ꢁ
mf Þ (cmꢁ1
)
Toluene
2.36
4.81
8.93
21.01
36.64
338.4
338.9
338.5
338.1
338.4
501.9
512.0
515.0
525.6
528.2
9626.5
9975.9
10124.6
10551.1
10618.6
0.099
0.259
0.309
0.355
0.460
Chloroform
Dichloromethane
Acetone
Acetonitrile

816
A.K.A. Almeida et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 812–818
The maximum of the emission intensity ImaxðTÞ can be directly
related with fluorescence quantum yield UF by [12]:
11.0
10.5
10.0
9.5
ImaxðTÞ ¼
c
ꢄ c ꢄ Iexc
ꢄ
UFðTÞ;
ð3Þ
where represents the gain of the detection apparatus, c is the fluo-
c
rophore concentration, and Iexc is excitation intensity. If a reference
temperature T0 is defined, we can compute
ImaxðTÞ
UFðTÞ
:
¼
ð4Þ
ImaxðT0Þ UF ðT0Þ
By considering that the fluorescence of PyPDG is governed by a
thermally activated mechanism, we can write the effective
emission rate as [8]
0,0
0.1
0.2
0.3
0.4
0.5
e
B
a
ENT
kf ¼ k0 þ k1eꢁ
;
ð5Þ
k
T
Fig. 3. Stokes shift as a function of the solvent polarity parameter ENT . The dashed
line represents the linear regression of the data.
where ea is the activation energy. k0 is the residual emission rate at
low temperature, while k1 is the amplitude of the thermally
activated contribution. Disregarding the thermal effects on the
non-radiative decay from the excited state, it is straightforward to
show that the quantum yield satisfies an Arrhenius relation which
is given by:
moiety. In this case, the deactivation of the TICT state is expected
to occur through a non-radiative decay which tend to be more
pronounced as the solvent temperature is enhanced.
In order to investigate the thermal effects on the fluorescence
properties of the system, we show in Fig. 4 the emission spectra
of the PyPDG dissolved in acetonitrile for different temperatures.
For all measurements, the excitation laser power was fixed at
P0 ¼ 5:2 mW and it was rigorously monitored in order to allow a
comparative analysis of the measured spectra. As one can note,
the emission intensity of PyPDG increases as the solvent tempera-
ture is raised, indicating that the reduction of the solvent viscosity
does not play a major role in the fluorescence of this compound.
Such a result contrasts with the typical behavior reported for
molecular rotors based on the TICT phenomenon. Indeed, the emis-
sion intensity of molecular rotors decreases as the solvent viscosity
diminishes due to the enhancement of the molecular torsional
relaxation that induces the radiationless decay [12,44,45]. This is
not the case of PyPDG, although it presents the pyrrolyl propyl as
a very flexible group. In fact, the enhancement of emission inten-
sity with the solvent temperature shows that the fluorescence of
PyPDG takes place through a thermally activated mechanism.
Further, we observe that the fluorescence peak wavelength is not
shifted as the solvent temperature is raised and thus the
thermochromic effect on this system can be disregarded.
ꢄ
ꢅ
ꢀ
ꢁ
U
ðTÞ
ea
kB T0
1
1
T
ln
¼
ꢁ
:
ð6Þ
UðT0Þ
In Fig. 5 we exhibit the Arrhenius plot for the quantum yields of
the PyPDG as a function of temperature, in solvents with distinct
polarities. As one can observe, the quantum yield increases in both
solvents as the sample temperature is raised. By using Eq. (6), we
can estimate the activation energy from the angular or linear
coefficients of the linear regression (dashed lines). Here, we notice
that the activation energy is very sensitive to the solvent polarity,
presenting a higher value in acetonitrile (ea ¼ 0:13 eV) than that
obtained in toluene (ea ¼ 0:08 eV). Such a result shows that the
radiative decay process of PyPDG is favored in a non-polar environ-
ment, in agreement with previous solvatochromic analysis that
revealed that the fluorescence phenomenon in PyPDG is governed
by the radiative decay from the locally excited state. The enhance-
ment of the fluorescence intensity of PyPDG as the environment
temperature is increased opens the possibility of using such
compound as the photoactive material in optical thermometry
and thermography applications.
0.4
8
Toluene
T = 295 K
T = 301 K
T = 307 K
T = 313 K
Acetonitrile
0.3
0.2
6
0.1
4
0.0
2
0
-0.1
0.92
0.94
0.96
0.98
1.00
1.02
400
450
500
550
600
650
700
T0/T
Wavelength (nm)
Fig. 5. Thermal dependence of the quantum yields of PyPDG in solvents with
distinct polarities: toluene (circle) and acetonitrile (square). Dashed lines represent
the linear regression by using Eq. (6).
Fig. 4. Emission spectra of PyPDG in acetonitrile for different temperatures. Notice
the increase of the emission intensity as the solvent temperature is raised.

A.K.A. Almeida et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 812–818
817
Electrochemistry and fluorescence of PPyPDG
PyPDG. The corresponding band gap energy was determined from
konset of PPyPDG as being Eogpt ¼ 2:72 eV. The electrochemical band
gap, Eegc of the polymer is clearly lower than the optical gap. This
discrepancy shows that the process of absorption and emission
associated to the band at 338 nm must occur in strictly related
electronic transitions by molecular orbitals HOMO/LUMO of the
dansyl chromophore. The pyrrol group (PyPDG) or polymer chain
(PPyPDG) is not associated with the fluorescence process. The
Stokes shift observed between monomer and polymer emissions
occurs due to the change in the molecular dipole moment of these
molecules.
As we have presented above, PyPDG presents a broad fluores-
cence spectrum which is sensitive to the polarity and temperature
of the environment. The presence of pyrrole moiety on its molecu-
lar structure provides the possibility of producing polymeric films
with interesting conductive and fluorescent properties. In order to
characterize the conductive properties of PPyPDG, we present in
Fig. 6 the cyclic voltammogram of the PPyPDG films deposited by
casting onto ITO/glass electrodes. It was observed that PPyPDG
films display an irreversible anodic wave with anodic peak poten-
tial (Epa) at 0:52 V vs. Ag/Ag+ and a poorly defined redox pair in the
cathodic region. Such a result is associated with n-doping of the
polymer, a process in which the cation enters in the polymeric
structure and neutralizes the negative charge formed during the
reduction process.
The cyclic voltammetry can be used to estimate the relative
position of HOMO and LUMO energy levels of the conjugated
polymer, EHOMO and ELUMO, respectively. According to empirical
relationship proposed in previous works [46,47], HOMO and LUMO
energy levels can be obtained from the onset oxidation potential,
Eooxnset, and the onset reduction potential, Eorendset, as follows:
In Fig. 7a, we present the emission spectra of the PPyPDG and
PyPDG dissolved in dichloromethane, at
a temperature of
T = 23 °C. As one can note, PPyPDG presents a broad emission band
that almost covers the entire range of the visible spectrum, with a
maximum intensity taking place at kf ¼ 562 nm. Further, a large
red shift is observed as compared to the fluorescence of PyPDG,
which presents a maximum intensity at 512 nm. Such a red shift
of the polymer emission is associated with the enhancement of
conjugation length in the polymeric backbone. Due to the broad
emission band of PPyPDG, it is interesting to characterize the chro-
matic sensation of human eyes to this specific fluorescence spec-
trum from the chromaticity diagram. In what follows, we use the
CIE 1931 representation which is based on the tristimulus values
of a color, representing the intensity combination of primary colors
basis [50]. The chromaticity is represented by a point coordinate
(x,y), which is obtained from the color match functions [51]. In
EHOMO ¼ ꢁðEonset þ 4:40ÞeV
ð7Þ
ð8Þ
ð9Þ
ox
ELUMO ¼ ꢁðEonset þ 4:40ÞeV
red
Eegc ¼ ðELUMO ꢁ EHOMOÞeV;
where Eegc represents the electrochemical band gap energy. Here, the
energy value of ꢁ4:40 eV corresponds to the HOMO/LUMO energy
levels of the reference ferrocene/ferricenium redox pair [48]. Thus,
the HOMO and LUMO energy levels as well as the electrochemical
band gap energy of the PPyPDG films are respectively
EHOMO ¼ ꢁ4:72 eV, ELUMO ¼ ꢁ3:84 eV, and Eegc ¼ 0:92 eV. The data
obtained from the PPyPDG cyclic voltammetry show that the
electrochemical oxidation process involves the electron withdraw-
ing from the HOMO of the polypyrrole moiety, while the reduction
process involves the electron addition to the LUMO of the dansyl
moiety. Therefore, a possible electronic transition between polypyr-
role and dansyl groups (Eegc ¼ 0:92 eV) could be expected for an
absorption band at konset ¼ 1347 nm. Although, this electronic
transition was not observed.
1.2
(a)
PyPDG
PPyPDG
1.0
0.8
0.6
0.4
0.2
0.0
Optical band gap energy Eogpt of PPyPDG was empirically
calculated from the respective onset absorption wavelength konset
,
400
500
600
700
800
determined from UV–Vis spectrum in CH2Cl2. PPyPDG shows an
absorption band at kmax ¼ 338 nm (not shown), similar to the
Wavelength (nm)
0.9
(b)
G
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
PyPDG
PPyPDG
R
B
0.0
0.1 0.2
0.3
0.4
0.5
0.6
0.7
0.8
x
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
E (V vs. Ag/Ag+)
Fig. 7. (a) Emission spectra of PyPDG (black line) and PPyPDG (red line) in CH2Cl2,
under a 457 nm laser excitation. (b) CIE 1931 chromaticity diagram for the emission
of PyPDG and PPyPDG in CH2Cl2. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
Fig. 6. Cyclic voltammogram of the PPyPDG film deposited onto ITO by casting,
recorded in 0.1 mol Lꢁ1 LiClO4/CH3CN, with
m
¼ 0:02 V sꢁ1
.

818
A.K.A. Almeida et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 812–818
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Summary and conclusion
In summary, we have synthesized a new fluorescent pyrrole
derivative (PyPDG) containing the electron donor–acceptor dansyl
substituent from a simple route. The thermal and solvatochromic
effects on the fluorescence spectra of PyPDG were investigated.
We showed that the emission intensity of the PyPDG presents a
pronounced enhancement as the solvent temperature is increased,
indicating that the fluorescence of this compound consists of a
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The dependence of emission spectra on the temperature and sol-
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the fluorescence phenomenon of this compound. Our results show
that PyPDG is a promising candidate to be used as a photoactive
material in sensor applications, due to the high sensitivity of its
fluorescent properties on the temperature and polarity of the envi-
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the emission band of PPyPDG was observed when compared to the
original emission band of PyPDG. The resulting polymer is there-
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Acknowledgments
The authors wish to thank the granting authorities CNPq,
CAPES, FINEP (FUNTEL and CTENERG Programs) and FAPEAL for
financial support. The authors are particularly thankful to Instituto
Nacional de Ciência e Tecnologia de Fluidos Complexos (INCT-FCx),
Instituto Nacional de Ciência e Tecnologia de Nanotecnologia para
Marcadores Integrados (INCT-INAMI), and INCT de NanoBioE-
struturas e Simulação NanoBioMolecular (INCT – NANO(BIO)SIMES).
We also wish to thank Dr. Mark Edgar and Mr. Mark D. Weller
(Loughborough University) for NMR and GPC analysis.
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