Synthesis and electrochemical properties of novel, donor-acceptor pyrrole derivatives with 1,8-naphthalimide units and their polymers

Article abstract of DOI:10.1016/j.electacta.2013.10.163

A new class of bipolar monomers with pyrrole or thiophene-pyrrole-thiophene as electron donor and 1,8-naphthalimide as electron acceptor unit is reported. Donor-acceptor conjugated polymers were generated electrochemically. The synthesis of monomers, opti

Full text of DOI:10.1016/j.electacta.2013.10.163

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Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical properties of novel, donor–acceptor
pyrrole derivatives with 1,8-naphthalimide units and their polymers
Przemyslaw Ledwona, Alina Brzeczekb, Sandra Pluczyka, Tomasz Jarosza,
Wojciech Kuznika, Krzysztof Walczakb, Mieczyslaw Lapkowskia,c,∗,1
a Department of Physical Chemistry and Technology of Polymers, Faculty of Chemistry, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland
b Department of Organic Chemistry, Bioorganic Chemistry and Biotechnology, Faculty of Chemistry, Silesian University of Technology, Krzywoustego 4,
44-100 Gliwice, Poland
c Centre of Polymer and Carbon Materials, Polish Academy of Sciences, Curie-Sklodowskiej 34, 41-819 Zabrze, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2013
Received in revised form 15 October 2013
Accepted 23 October 2013
Available online xxx
A new class of bipolar monomers with pyrrole or thiophene–pyrrole–thiophene as electron donor and
1,8-naphthalimide as electron acceptor unit is reported. Donor–acceptor conjugated polymers were
generated electrochemically. The synthesis of monomers, optical, electrochemical and spectroelectro-
chemical properties supported by theoretical calculations are presented. 1,8-naphthalimide units were
attached directly to pyrrole in compounds 1a and 2a or by different bridges in the case of 1b and 2b. Intra-
molecular donor–acceptor interactions of the monomers and its polymers were investigated using cyclic
voltammetry, in-situ UV–Vis-NIR, electron spin resonance (ESR) spectroelectrochemistry and fluores-
cence spectroscopy. Studied compounds present large discrepancy (up to 1.31 eV for 2a) between energy
gap values determined through electrochemical and optical measurements. The Time-dependent density
functional theory (TDDFT) calculations help to explain this discrepancy. This is caused by weak HOMO to
LUMO transition, 2000 times weaker than HOMO−2 to LUMO or HOMO to LUMO+1 transition. Altering the
structure of monomers yields different stability and properties of obtained polymers. The p- and n-doping
processes are separated. Anions are localized mainly on 1,8-naphthalimide units. Cations are localized
mainly on pyrrole or thiophene–pyrrole–thiophene moiety and their polymer chains. Attachment of the
additional thiophene units decreases the oxidation potential of the monomer and reduces the influence
of the steric hindrance between 1,8-naphthalimide moiety and polymer/oligomers chain. This new class
of model compounds is promising for use as a material with enhanced charge separation for wide range
of optoelectronic, electrochromic and photovoltaic applications.
Keywords:
Donor–acceptor
Bipolar
Polypyrrole
Naphthalimide
Spectroelectrochemistry
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
further this goal, discussing and systematizing existing develop-
ments in the field [1–4].
Organic electronics is a dynamically growing branch of materials
science, attracting the attention of numerous researchers world-
wide. Based on conductive molecules and macromolecules, such
as conjugated polymers, the field introduces novel concepts, such
as printable and flexible electronics. Furthermore, the operating
parameters of such materials and devices can be tuned through
structural modification of the electroactive layers, leading to a mul-
titude of potential systems. Combining versatility with outstanding
mechanical properties, organic materials are well equipped to com-
pete with inorganic semiconductors in a bid for supremacy in
commercial applications. A number of excellent reviews strive to
Recently, modification of conducting polymers’ physicochem-
ical properties, through the functionalization with a variety of
chromophore groups has garnered considerable interest [5–7]. The
reported, ␲-conjugated, semiconducting systems can be divided
into p- or n-type, depending on the nature of the charge carriers
occurring in each material. Initially, most works were concerned
with p-type organic semiconductors. Lately, however, the study
of n-type systems has achieved significant progress as reflected in
most recent reports. The importance of improvements in design
and synthesis of such semiconductors is not to be underestimated,
as they are essential for the construction of an array of electronic
devices, including organic photovoltaic cells (OPVs) [8], organic
light-emitting diodes (OLEDs) [9] and organic field-effect transis-
tors (OFETs) [3,10].
Among known, n-type organic materials, aromatic imides are
acclaimed as one of the most stable, demonstrating relatively high
electron affinity and electron mobility [10]. They are a versatile
∗
Corresponding author. Tel.: +48 322371509, fax: +48 322371509.
E-mail address: mieczyslaw.lapkowski@polsl.pl (M. Lapkowski).
The ISE member.
1
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.10.163
Please cite this article in press as: P. Ledwon, et al., Synthesis and electrochemical properties of novel, donor–acceptor pyrrole derivatives with
1,8-naphthalimide units and their polymers, Electrochim. Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.10.163

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material building block, useful for supplementing materials with
electron-withdrawing properties, due to their planar, conjugated
structure, substituted with one or two ␲-accepting imide groups
[11–13]. 1H-benzo[de]isoquinoline-1,3(2H)-dione, imide of 1,8-
naphthalenedicarboxylic acid is a relatively simple representative
of aromatic imides. The compound has been successfully used in
the construction of OPVs [14], white polymer light emitting diodes
(WPLEDs) [15], fluorescent probes for analytical sensing and opti-
cal imaging [16], DNA intercalators [17] as well as colorimetric and
fluorescent anion sensors [18].
After completion of the reaction (progress was monitored by means
of TLC: MeOH:CHCl3 10:90 v/v), the mixture was cooled to room
temperature, the obtained precipitate was filtered off and purified
by column chromatography to give yellow solid. Yield: 87% (0.92 g).
Mp 263–264 ◦C; 1H NMR (600 MHz, DMSO-d6, ı):8.50 (dd, J = 7.4,
0.7 Hz, 2H, Ar–H), 8.45 (dd, J = 8.2, 0.7 Hz, 2H, Ar–H), 7.87 (dd, J = 8.2,
7.4 Hz, 2H, Ar–H), 5.80 (s, 2H, NH2); 13C NMR (151 MHz, DMSO-d6,
ı): 160.422, 134.42, 131.21, 130.71, 127.21, 125.95, 121.63. ESI–MS
(m/z (%)): 213.2 (100) [M + H]+, 235.2 (40) [M + Na]+, 229.2 (100)
[M + OH]−.
p-Type conjugated polymer semiconductors, on the other hand,
are well-represented by polypyrrole and its substituted deriva-
tives. The nature of the substituent, in such systems, determines
the properties of the polymer, predestining it for application in a
given type of devices, such as electrochromics [19], sensors [20]
and solar cells [21,22]. For N-substituted pyrroles, large groups may
stabilize the radical cation generated during electrochemical poly-
merization, possibly inhibiting the process. Steric interactions in
resultant polymers are particularly important, as they may induce
a loss of conjugation across the polymer backbone, leading to lower
conductivity of the material [23].
Building upon the highlights of both p- and n-type polymer
semiconductors, the donor–acceptor (D–A) approach, introduced
by Havinga et al. [24] creates a new quality. The importance of
structural modification is retained and supplemented as HOMO
and LUMO energy levels of the material may be individually tuned.
Two distinct strategies for D–A semiconductors have been reported,
composed of either a D–A copolymeric chain [25] or a homopoly-
mer donor chain substituted with acceptor pendant groups [26,27].
The first architecture allows for better band gap control, while the
second exhibits improved charge separation, both features well
sought after [8].
2.2.2.
2-(2-aminoethyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione(2)
To the solution of 1,2-diaminoethane (0.6 g, 10 mmol) in EtOH
(100 mL) catalytic amount of DMAP 4-dimethylaminopyridine was
added (0.005 g). The mixture was stirred under reflux for 1 h while
the naphthalic 1,8-anhydride (0.99 g, 5 mmol) was added in por-
tions. The mixture was refluxed for additional 4 h. The course of
the reaction was monitored by means of TLC (MeOH:CHCl3:NH3,
20:80:1 v/v/v). After completion of the reaction the mixture was
cooled and the precipitate was filtered off. The filtrate was con-
centrated under reduced pressure and the residual was purified
by column chromatography to give pale yellow solid. Yield: 59%
(0.71 g). Mp 140–141 ◦C; 1H NMR (600 MHz, CDCl3, ␦): 8.61 (dd,
J = 7.3, 1.0 Hz, 2H, Ar–H), 8.22 (dd, J = 8.2, 1.0 Hz, 2H, Ar–H), 7.76
(dd, J = 8.2, 7.3 Hz, 2H, Ar–H), 4.29 (t, J = 6.7 Hz, 2H, CH2N(CO)2), 3.08
(t, J = 6.7 Hz, 2H, CH2NH2), 1.30 (bs, 2H, NH2); 13C NMR (151 MHz,
CDCl3, ␦): 164.48, 133.99, 131.60, 131.33, 128.23, 126.94, 122.60,
43.18, 40.55. ESI–MS (m/z (%)): 241.4 (70) [M + H] +, 481.4 (100)
[2M + H]+, 721.8 (70) [3M + H] +
.
2.2.3.
Adhering to the above approach, monomers composed of an
n-type, aromatic imide acceptor core and a p-type, pyrrole-based
moiety have been envisioned. For the exploration of this new class
of D–A systems, a simple 1,8-naphthalimide was chosen as the
acceptor core. D–A monomers were prepared either through a
direct N–N linkage between donor and acceptor moiety or through
the use of a linking group (Fig. 1), similar to previously reported
systems, where 1,8-naphthalimide was used as a pendant group of
a conducting polymer [15,28].
2-(1H-pyrrol-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione(1a)
N-Amino-1,8-naphthalimide (0.106 g, 0.5 mmol) and 2,5-
dimethoxytetrahydrofuran (0.06 mL 0.5 mmol) were refluxed
in acetic acid (6 mL) for 3 h. Acetic acid was evaporated under
reduced pressure. The crude residue was purified by column
chromatography (MeOH:CHCl35:95, v/v) to give pale brown solid.
Yield: 50% (65 mg). Mp 304–306 ◦C; 1H NMR (600 MHz, CDCl3, ␦):
8.69 (dd, J = 7.3, 1.0 Hz, 2H, Ar–H), 8.31 (dd, J = 8.3, 1.0 Hz, 2H, Ar–H),
7.82 (dd, J = 8.3, 7.3 Hz, 2H, Ar–H), 6.77–6.77 (m, 2H, Ar–H), 6.40 (t,
J = 2.3 Hz, 2H, Ar–H); 13C NMR (151 MHz, CDCl3, ı): 162.45, 135.08,
132.39, 131.94, 128.13, 127.21, 122.14, 121.29, 108.58; IR: ꢀ = 3078
(C–H Ar), 1716 (C=O), 1688 (C=O), 1585 (C–C Ar), 1359 (C–N–C),
1071 (C–H Het) cm−1; ESI–MS (m/z (%)): 263.2 (45) [M + H]+, 284.8
(100) [M + Na]+.
2. Experimental
2.1. Materials
All used reagents were purchased from Aldrich or Alfa Aesar.
Solvents were used without further purification. Ethyl alcohol
and toluene were dried with molecular sieves of 4 A prior to
2.2.4. 2-(2-(1H-pyrrol-1-yl)ethyl)-1H-benzo[de]isoquinoline-
1,3(2H)-dione(1b)
˚
Synthesis was performed according to the same procedure as
1a. Yield 81% (117 mg), pale yellow solid. Mp 160–162 ◦C; 1H NMR
(600 MHz, CDCl3, ␦): 8.59 (dd, J = 7.3, 1.0 Hz, 2H, Ar-H), 8.23 (dd,
J = 8.2, 1.0 Hz, 2H, Ar–H), 7.76 (dd, J = 8.2, 7.3 Hz, 2H, Ar–H), 6.74 (t,
J = 2.1 Hz, 2H, Ar–H), 6.12 (t, J = 2.1 Hz, 2H, Ar–H), 4.56–4.52 (m, 2H,
CH2N(CO)2), 4.26–4.22 (m, 2H, CH2N(CH)2); 13C NMR (151 MHz,
CDCl3, ␦): 163.97,134.21, 131.64, 131.42, 128.21, 127.00, 122.32,
121.01, 108.51, 46.78, 41.05; IR: ꢀ=3072 (C–H Ar), 1692 (C=O),
1655 (C=O), 1589 (C–C Ar), 1440 (C–H Pyr), 1384 (C–N–C), 1074
use. Column chromatography was performed using silica gel
60 (0.040–0.063 mm) purchased from Merck. NMR spectra were
recorded at 600 MHz for 1H NMR and 151 MHz for 13C NMR on a
Varian Mercury Plus 600 MHz spectrometer; ı values are in parts
per million (ppm) relative to tetramethylsilane (TMS) as an internal
standard. Mass spectra were recorded on ESI Mass Spectrometer
ABSciex System 4000 QTRAP® at positive and negative modes of
ionization. Melting points were measured with a Boetius apparatus
(Franz Kustner Nachf. KG Dresden HMK) and are uncorrected.
(C–H Pyr), 1048 (C–H Pyr), 1033 (C–H Pyr), 943 (C–H Pyr) cm−1
;
ESI–MS (m/z (%)): 291.4 (80) [M + H]+, 313.4 (80) [M + Na]+, 531.6
2.2. Preparation of monomers
(35) [2 M + H3O + MeOH]+,603.6 (100) [2M + Na]+.
2.2.1. 2-amino-1H-benzo[de]isoquinoline-1,3(2H)-dione(1)
To the suspension of 1,8-naphthalic anhydride (0.99 g, 5 mmol)
in chloroform (40 mL), hydrazine monohydrate (0.75 mL, 10 mmol)
was added. The reaction mixture was heated under reflux for 4 h.
2.2.5. 2-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-yl-1H-
benzo[de]isoquinoline-1,3(2H)-dione(2a)
Synthesis of the monomer was carried out in two steps. The
first one was acylation of thiophene with succinyl dichloride
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to give 1,4-di(thiophene-2-yl)butane-1,4-dione, according to
literature [29], followed by condensation of dione with 2-
Graphical User Interface [32] was used to generate input files and
to produce graphs from output files, including UV–vis spectra.
amino-1H-benzo[de]isoquinoline-1,3(2H)-dione.
A solution of
1,4-di(thiophen-2-yl)butane-1,4-dione(63 mg, 0.25 mmol), N-
amino-1,8-naphthalimide (53 mg, 0.25 mmol) and p-TSA (5 mg,
0.25 mmol) in dry toluene (6 mL) was heated under reflux for 20 h
2.5. X-ray single crystal measurement
The crystal chosen for X-ray analysis was a deep red block with
the approximate dimensions of 1 × 0.2 × 0.2 mm. C24H14N2O2S2,
(426.49 g mol−1) crystallizes in the monoclinic system, space
˚
under nitrogen atmosphere in the presence of molecular sieves 4 A.
After completion of the reaction molecular sieves were filtered off
and the solvent was removed under reduced pressure. The crude
product was purified by column chromatography eluting with
˚
˚
˚
group P21/c, a = 12.4258 (25) A, b = 9.3899 (19) A, c = 16.7899 (34) A,
◦
3
−1
˚
ˇ=95.672 (30) , V = 1949.40 (7) A , Z = 4, ꢁ(Mo K␣) = 0.277 mm ,
CH2Cl2 to give of red solid. Yield: 50% (50 mg). Mp 265–267 ◦C; 1
H
and Dcalcd = 1.453 cm−3. A total of 3807 reflections were collected
to 2ꢂmax = 31.02◦ (h: −13 → 13, k: 0 → 10, l: 0 → 18), of which
2703 were unique. In refinements, weights were used according
to the scheme w = 1/[ꢃ2(Fo2) + (0.1453P)2], where P = (Fo2 + 2Fc2)/3.
The refinement of 322 parameters (data-to-parameter ratio being
8.4) converged to the final agreement factors R = 0.0677 for 1904
reflections with Fo > 4ꢃ(Fo) and Rw = 0.2201, and S = 1.208 for all
observed reflections. The electron density of the largest difference
NMR (600 MHz, CDCl3, ı): 8.66 (dd, J = 7.3, 1.0 Hz, 2H, Ar–H), 8.30
(dd, J = 8.3, 1.0 Hz, 2H, Ar–H), 7.80 (dd, J = 8.3, 7.3 Hz, 2H, Ar–H),
7.06 (dd, J = 3.7, 1.2 Hz, 2H, Ar–H), 7.02 (dd, J = 5.1, 1.2 Hz, 2H,
Ar-H), 6.85 (dd, J = 5.1, 3.7 Hz, 2H, Ar–H), 6.66 (s, 2H, Ar–H); 13C
NMR (151 MHz, CDCl3, ␦): 162.63, 135.20, 132.71, 132,08, 132.04,
129.00, 128.27, 127.28, 127.17, 124.81, 124.66, 121.98, 109.22; IR:
ꢀ = 3104 (C–H Ar), 2919 (C–H Ar), 1722 (C=O), 1698 (C=O), 1581
(C–C Ar), 1504 (C–H Het), 1373 (C–N–C), 1072 (C–H Het) cm−1
;
peak was found to be 0.51 e A−3, while that of the largest differ-
˚
ESI–MS (m/z (%)): 426.4 (100) [M]+, 449.4 (60)[M + Na]+.
ence hole was −0.53 e A−3.Measurements of diffraction intensities
˚
were performed on a KUMA KM4 four-circle diffractometer, Mo K␣
radiation, ω/2ꢂ scan mode, ꢂ range 7.8–31.02◦; temperature of
the measured crystals: 293 K. Detailed crystallographic data for the
structure of 2a has been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication CCDC 910970.
2.2.6. 2-(4-(4-(2,5-di(thiophen-2-yl)-1H-pyrrol-1-
yl)phenoxy)phenyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione(2b)
The monomer was obtained in two steps. (4-(2,5-di(thiophen-2-
yl)-1H-pyrrol-1-yl)-4ꢀ-aminodiphenyl) ether was obtained accord-
ing to the literature [30], and next it was condensed with naphthalic
1,8-anhydride according to the same procedure as compound 2
(solvent: AcOH). The product was purified by column chromatog-
raphy eluting with CH2Cl2.to give pale yellow solid. Yield: 25%. Mp
245–247 ◦C; 1H NMR (400 MHz, CDCl3, ı): 8.66 (dd, J = 7.3, 1.0 Hz,
2H, Ar–H), 8.29 (dd, J = 8.3, 1.0 Hz, 2H, Ar–H), 7.81 (dd, J = 8.3, 7.3 Hz,
2H, Ar–H), 7.32 (dd, J = 8.8, 2.0 Hz, 2H, Ar–H), 7.24–7.19 (m, 2H,
Ar–H), 7.18–7.13 (m, 2H, Ar–H), 7.09 (dd, J = 5.1, 1.1 Hz, 2H, Ar–H),
6.86 (dd, J = 5.1, 3.6 Hz, 2H, Ar–H), 6.64 (dd, J = 3.6, 1.1 Hz, 2H, Ar–H),
6.54 (s, 2H, Ar–H); 13C NMR (101 MHz, CDCl3, ␦): 164.43, 157.16,
144.31, 142,47, 134.35, 134.07, 131.70, 131.57, 130.58, 130.19,
127.75, 127.07, 126.89, 124.39, 124.16, 120.87, 119.89, 119.39,
114.18, 109.77; IR: ꢀ = 3072 (C–H Ar), 2920, (C–H Ar), 1705 (C=O),
1663 (C=O), 1588 (C–C Ar), 1498 (C–H Het), 1374 (C–N–C), 1234
3. Results and discussion
3.1. Synthesis of monomers
The monomers were synthesized via Paal Knorr condensa-
tion reaction. Monomers: 1a, 1b were obtained directly from
2,5-dimethoxytetrahydrofurane, while monomer 2a and 2b were
prepared from 1,4-di(thiophene-2-yl)butane-1,4-dione. The latter
was synthesized by Friedel Crafts acylation of thiophene with 1,4-
dichlorobutanedione as shown in (Scheme 1). The structures of the
compounds were confirmed by 1H, 13C NMR and ESI-MS analyses.
3.2. Single crystal structure analysis
(C–O–C) cm−1
.
Single crystal of 2a suitable for structural analysis was obtained
after recrystallization from the dichloromethane/hexane mixture.
The crystal belongs to the monoclinic space group P21/c with 4
molecules in the unit cell. The unit cell parameters are: a =◦12.4258
2.3. Spectroelectrochemical measurements
Electrosynthesis and studies on polymer films were per-
formed on CH Instrument Electrochemical Analyzer model 620,
in dichloromethane (Sigma Aldrich ≥99.8%) or acetonitrile (POCH
99.9%) containing 0.1 M tetrabutylammonium hexafluorophospate
(Sigma Aldrich) as supporting electrolyte. Polymer films were syn-
thesized at a platinum wire or ITO electrodes, with a scan rate
of 0.1 V s−1. An Ag pseudo-reference electrode was used and its
potential was standardized versus ferrocene. Platinum electrode
served as a counter electrode. Spectral and fluorescence mea-
surements were carried out using respectively an UV–vis Hewlett
Packard 8453 spectrophotometer and a Hitachi f-2500 fluorescence
spectrophotometer. Electron spin resonance investigation utilised
a JEOL JES FA 200, X-band CW-EPR spectrometer, operating at
100 kHz field modulation.
˚
(25), b = 9.3899 (19), c = 16.7899 (34) A, ˇ = 95.672 (30) . Fig. 2
shows the molecular structure of 2a. The whole molecule is not
coplanar. It can be divided into two nearly orthogonal planes, one
of which contains the pyrrole with adjacent thiophene rings, and
the other one the naphthalimide ring. Torsion angle between adja-
cent thiophene and pyrrole rings C(10)–C(9)–C(8)–C(7) is equal to
10.69◦. The naphthalimide unit is twisted out of the pyrrole plane
with the torsion angle along C(5)–N(1)–N(2)–C(24) equal to 87.80◦.
One of the thiophene rings contains a disorder. Atoms: S2, C3ꢀꢀ and
corresponding S2ꢀ, C3ꢀꢀꢀ sustain the planarity of the thiophene ring
with intact position of the C4 atom. Atom C3ꢀ was found in order
to achieve the best refinement, however, it causes the nonaromatic
share of the disorder by adapting non-planar positions with respect
to the other atoms.
2.4. Calculations
3.3. Fluorescence
Density Functional Theory (DFT) calculations were performed
using GAUSSIAN09 [31] package. Geometry optimization per-
formed at 6-31G(d,p) DFT B3LYP level of theory, excited states and
UV–vis spectra were obtained from a TDDFT run. GABEDIT 2.3.6
Fluorescence spectra of the studied monomers registered in
CH2Cl2 are presented in Fig. 3, whereas important information is
extricated into Table 1. Strong fluorescence was found for com-
pounds 1a, 1b and 2a, exhibiting slightly differing emission bands.
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Fig. 1. Structure of monomers.
Scheme 1. Synthesis of monomers. (i) EtOH, DMAP, reflux, 4 h. (ii) CH3COOH, reflux, 3 h., (iii) p-TSA, toluene, reflux, 20 h (iv) CH3COOH, reflux, 8 h.
In case of 2b, strong quenching of emission is observed. The anal-
ysis of the fluorescence spectra reveals their complex character.
The excitation and emission spectra are not exact mirror images.
In the long wavelength region the excitation spectra of the stud-
ied compounds present the S0 → S1 transition and well resolved
Table 1
Fluorescence properties where ꢄex is fluorescence excitation; ꢄex is fluorescence
emmision.
vibronic bands. At least three maxima, at approximately 315 nm,
330 nm and 340 nm, are observed. In the shorter wavelength range
S0 → S2 and higher energy transitions are observed. The emission
spectra related to the S1 → S0 transition present partial symme-
try to the S0 → S1 transition. No less than three peaks, located at
approximately 360 nm, 380 nm and 398 nm, are more separated
and present different intensity ratio than excitation peaks. This
indicates some structure differences between the ground state
S0 and the excited state S1. The excitation and emission fluo-
rescence peaks are in the range characteristic for N-substituted
1,8-naphthalimides [33]. However, some differences, such as small
shift of fluorescence peaks and change of their shape indicate
Compound
ꢄex [nm]/[eV]
ꢄem [nm]/[eV]
1a
318/3.90
327/3.80
338/3.67
322/3.85
332/3.73
346/3.58
318/3.90
330/3.76
341/3.64
328/3.78
357/3.47
373/3.32
391/3.17
362/3.42
379/3.32
397/3.17
360/3.44
380/3.26
397/3.12
415/2.99
1b
2a
2b
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1a
1b
2a
2b
15
10
μ
5
0
-5
-10
-15
-2.0 -1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
potential (vs. Fc/Fc+) / V
Fig. 4. Cyclic voltammetric curves of 2 mM monomer solutions; scan rate 0.1 V s–1
;
0.1 M, Bu4NBF4/CH3CN.
is known about the behaviour of N-substituted pyrrole systems
[23]. The addition of two thiophene moieties to the donor frag-
ment leads to increased conjugation, as evidenced by the lowering
of the oxidation potential in respect to the bare pyrrole derivative.
For every monomer, repeated oxidation results in the deposi-
tion of an electroactive layer on the electrode surface (Fig. 5). The
resultant film, in each case, undergoes oxidation at lower poten-
tials than the corresponding monomer. In the case of oxidation
of 1a in CH3CN, the deposition process is induced by a potential
just beyond the onset of the monomer oxidation peak, yielding an
electroactive polymer. The application of more positive potentials,
however, results in a loss of electroactivity of the system.
Fig. 2. Molecular structure of 1-(N-naphthalimidyl)-2,5-di(thiophen-2-yl)-1H-
pyrrole (2a), showing 50% probability displacement ellipsoids and atomic
numbering scheme.
slight affect of pyrrole moieties on 1,8-naphthalimide fluores-
cence.
3.4. Cyclic voltammetry
Poly1a shows only average electrochemical properties, as
expected due to the imide substituent at the pyrrole nitrogen.
The ethyl substituted 1b, on the other hand, shows improved
electropolymerization ability, as indicated by the generation of a
redox system at less positive potentials and its rapid development
in subsequent potential cycles. When attempting electropolymer-
ization at potentials beyond the monomer’s oxidation peak, no
such behaviour is observed. The lack of an electroactive deposit
is attributed to overoxidation of the polymer upon its formation
and deposition on the electrode surface.
Electrochemical properties of monomers and their polymers
were estimated by cyclic voltammetry. Each of the investigated
monomers (Fig. 4) exhibits a reversible, cathodic redox pair and
an irreversible anodic oxidation signal. Cathodic behaviour of
the monomers is congruent, as is the acceptor fragment of the
molecules. The redox pair shifts from −1.57 V to −1.79 V, possi-
bly due to electron donating effects from the linker and donor
fragments. The redox potentials correspond to those reported in
literature for 1,8-naphthalimide [34]. Anodic properties vary with
the donor fragment.
The oxidation of 2a results in coating the working electrode with
a yellow film. In the second potential cycle, a new oxidation peak
forms at 0.4 V and develops in subsequent cycles. The oxidation of
2b in CH3CN leads to the formation of soluble products. The use of
CH2Cl2/MeOH 1:5 for the oxidation of 2b results in the formation
of an electroactive film on the electrode surface.
The oxidation potential of 1a is 1.29 V, whereas 1b, featuring
an ethylene separator between the pyrrole and imide nitrogens,
exhibits an oxidation potential of +0.91 V. This effect is replicated
for 2a and 2b (0.52 V and 0.44 V, respectively), consistent with what
1200
Cyclic voltammetry curves of polymer films were recorded in
monomer-free electrolyte. Weak oxidation and reduction peaks
can be distinguished in the voltammograms of poly1a, confirming
the average electrochemical properties of electrodeposited films.
Poly1b shows an onset of oxidation at approximately −0.3 V, with
the peak maximum at 0.37 V. Voltamograms in anodic range over-
lap during multiple doping and dedoping cycles, indicating good
electrochemical stability. Potential cycling in cathodic potential
ranges reveals the presence of a redox pair, corresponding to n-
doping and dedoping of the polymer films. Upon repeated cycling,
the pair undergoes rearrangement, implying restructuring of the
polymer film. During potential scanning in a wide range, so as to
incorporate both p- and n-doping of the polymer, poly2a exhibits
reversible, stable behaviour. Reduction is observed at −1.77 V and
oxidation at 0.46 V. Poly2b, on the other hand shows progressive
flow of the reduction peak to more negative potentials, starting
from −1.83 V in the first cycle. Eventually, this leads to a loss of
electrochemical activity.
1a
1b
2a
1000
800
600
400
200
0
250
300
350
400
450
500
Wavelength / nm
Fig. 3. Fluorescence excitation and emission spectra of 10 ␮M solutions of 1a, 1b
and 2a, in CH2Cl2.
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1,8-naphthalimide units and their polymers, Electrochim. Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.10.163

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8
1a
1b
15
poly1a
poly1b
backgrond
background
6
10
μ
4
μ
5
0
2
0
-2
-4
-5
-2.0 -1.5 -1.0 -0.5 0.0
0.5
1.0
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
potential (vs. Fc/Fc+) / V
potential (vs. Fc/Fc+) / V
4
2
2b
2a
15
10
5
poly2b
background
poly2a
background
μ
0
μ
0
-2
-4
-5
-10
-15
-2.0
-1.5
-1.0
-0.5
0.0
0.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
potential (vs. Fc/Fc+) / V
potential (vs. Fc/Fc+) / V
Fig. 5. Cyclic voltammetric curves during polymerization of 2 mM monomer solutions of, 1a, 1b, 2a in Bu4NBF4/CH3CN and 2b in CH2Cl2/MeOH 1:5 (solid lines). Cyclic,
voltammetric curves of polymer film in free monomer solution (dashed lines) in Bu4NBF4/, CH3CN. Scan rate 0.1 V s−1
.
3.5. HOMO LUMO levels and TDDFT calculations
approximately 330 nm. This is due to lack of spatial overlap
between the HOMO and LUMO orbitals of the molecules, as seen
in Figs. 6 and 7. For each of the monomers, HOMO is localized
mainly on the pyrrole moiety, whereas HOMO−2 and LUMO incor-
porate the 1,8-naphthalimide system. Such a localization of the
HOMO and LUMO orbitals essentially marks the HOMO–LUMO
transition as forbidden. Thus, in this case, the onset energy of
the electro-neutral monomer’s absorption peak corresponds to
the energy of the symmetry-allowed HOMO−2–LUMO transition,
rather than to the band gap. This is demonstrated graphically by the
TDDFT-derived UV–vis spectra enclosed within the supplementary
materials to the article.
Cyclic voltammetry was used to evaluate the HOMO and LUMO
energy (EHOMO and ELUMO) levels of the monomers and, therefore,
the electrochemical band gap. Based on UV–vis spectra, the optical
energy gap values were determined. Discrepancy between energy
gap from experimental electrochemical and optical measurements
is observed (up to 1.31 eV for 2a), implying the presence of an
additional effect. TDDFT calculations were employed to investi-
gate the source of this difference. Experimental values of EHOMO
ELUMO, and Eg are presented in Table 2 alongside values calculated
by TDDFT.
,
Interestingly, the HOMO to LUMO spectral transition has been
found to exhibit a very low oscillator strength, as much as 2000
times weaker than the first significant transition (HOMO–LUMO+1
in the case of 2b and HOMO−2–LUMO in the case of other
studied compounds), which manifests as absorption signals at
Polymers’ EHOMO and ELUMO are collected in Table 3. ELUMO is in
the range from −3.08 eV to −3.25 eV for all polymers. Larger dif-
ferences are observed in the case of EHOMO, between the pyrrole
(poly1a) and thiophene–pyrrole–thiophene derivatives (poly2a
and poly2b).
Table 2
Properties of monomers where EHOMO and ELUMO are HOMO and LUMO energy levels Eg is energy gap estimated: (a) electrochemically from equations EHOMOel = −4.8 −-
Eoxonset, ELUMOel = −4.8 −- Eoxonset; Egel = EHOMO −- ELUMOel; (b) optically Egopt = 1240/ꢄonset where ꢄonset is absorption onset; (c) from (TD)DFT calculations Egc; EHOMO
;
el
c
ELUMOc = EHOMOc(DFT)- Egc(TDDFT); (d) for compounds 1a, 1b, 2a oscillator strength of HOMO−2 to LUMO transition; for compound 2b oscillator strength of HOMO to
LUMO+1 transition.
Egel [eV]
Egopt [eV]
EHOMO [eV]
ELUMO [eV]
Egc [eV]
Oscillator
strength HOMO
to LUMO
Oscillator
strength d
el
el
c
c
Compound
EHOMO [eV]
ELUMO [eV]
1a
1b
2a
2b
−5.95
−5.59
−5.18
−5.12
−3.19
−3.19
−3.36
−3.10
2.76
2.54
1.82
2.02
3.39
3.41
3.13
3.21
−5.32
−5.49
−5
−2.94
−3.16
−3.07
−2.62
2.38
2.33
1.93
2.16
0.0008
0.0011
0.0001
0
0.205
0.1853
0.1991
0.4714
−4.88
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Fig. 6. HOMO−2, HOMO and LUMO orbitals of 1a, 1b, 2a calculated at 6-31G(d,p) DFT B3LYP level of theory.
Fig. 7. The HOMO, LUMO and LUMO+1 orbitals of 2b calculated at 6-31G(d,p) DFT B3LYP level of theory.
3.6. In situ UV–vis and ESR spectroelectrochemistry of monomers
favour of the developing absorption band, centred around 415 nm,
characteristic of the absorbance of radical anion of N-substituted
1,8-naphthalimide derivatives [37]. The absorption spectrum of the
reduced 2a molecule comprises four well-developed systems, at
416 nm, 489 nm, 754 nm and 845 nm. Similar absorbance charac-
teristics are observed for 1a and 1b. In the case of 2b, the primary
change, in respect to the other systems, is the lack of peaks at
longer wavelength, implying stronger interaction between 1,8-
naphthalimide moiety and the rest of the molecule. Reduction
of each monomer takes place on the 1,8-naphthalimide moiety
and, therefore, was expected to undergo in an uniform fashion.
Registered UV–vis absorption spectra for each of the investigated
monomers confirm this assumption, exhibiting a significant degree
of similarity. The observed batochromic shift of reduced molecules
indicates an increase in conjugation, related to the improved pla-
narity of the negatively charged molecule. The emergence potential
of the new absorption band is consistent with cyclic voltammetry
data, as it corresponds to the onset potential of the reduction
peak. This is the case for each monomer, further confirming the
localization of the negative charge on the 1,8-naphthalimide moi-
ety. Charge carriers produced during reduction are initially stable,
The UV–vis spectra of studied compounds in their neutral
state consist of absorption of all their chromophore systems. The
absorbance of 2a and 2b exhibits overlapping of predominant
ꢅ–ꢅ* transition of 1,8-naphthalimide system (330 nm, 340 nm)
[35] and ꢅ–ꢅ* transition of thiophene–pyrrole–thiophene system
(345 nm) [36]. In the case of reduction of every monomer (Fig. 8
and supplementary materials), the absorption signal, correspond-
ing to electro-neutral molecules, gradually decreases in intensity in
Table 3
el
el
Properties of polymers where EHOMO and ELUMO are HOMO and LUMO estimated
electrochemically from equations EHOMOel = −4.8 − Eoxonset, ELUMOel = − 4.8 −Eoxonset
;
;
el
el
Eg is energy gap estimated electrochemically from equation Egel = EHOMO − ELUMO
*not determined.
el
el
Compound
EHOMO [eV]
ELUMO [eV]
Egel [eV]
Poly1a
Poly1b
Poly2a
Poly2b
*
*
*
−4.53
−4.97
−4.98
−3.25
−3.24
−3.08
−1.28
−1.73
−1.9
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0,7
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2a
1a
0,6
0,5
0,4
0,3
0,2
0,1
0,0
300
400
500
600
700
800
900
300
400
500
600
700
800
900
Wavelength / nm
Wavelength / nm
Fig. 8. UV–vis spectra recorded during reduction of 1a and 2a; in 0.1 M Bu4NBF4/CH3CN.
process, indicating the proceeding generation of anion radicals. The
radical anions of derivatives of naphthalimide are fairly stable.
Oxidation of the monomers was found to induce elec-
tropolymerization and, thus, registered UV–vis absorption spectra
comprise a record of the polymerization process, as presented in
Fig. 10. Generally, the monomer absorption signal decreases as a
broad absorption band, positioned towards the low-energy side of
the spectrum is generated and progressively increases in magni-
tude. An important feature of the spectra is the presence of an
absorption signal at 800–900 nm, standing out in intensity from
the broad band polymer signal. This is attributed to soluble species,
possibly oxidised oligomers, as hinted at in literature [38,39].
100
75
50
25
0
-25
-50
-75
-100
3.7. In situ spectroelectrochemistry of polymer films
334.0 334.5 335.0 335.5 336.0 336.5 337.0 337.5
Magnetic field / mT
In order to investigate the changes in polymer film in the
course of the redox process, UV–vis spectroelectrochemistry was
employed. Spectra, registered during polymer film oxidation, are
presented in Fig. 11 and as supplementary materials. In contrast to
the spectral study of the monomer oxidation process, no individual
signal at 800–900 nm is observed, supporting its correlation to sol-
uble polymerization products. Poly2a thin film, in its neutral state,
shows two absorption peaks, at 345 nm and at 415 nm. This shift
in relation to monomer absorption signals is a result of increased
length of the ␲-conjugated bond system. Upon polymer oxidation,
the intensity of the signal at 415 nm declines, as a broad absorp-
tion band arises. In the oxidized state, the polymer’s 345 nm peak
remains virtually unchanged. This similarity to the monomer spec-
Fig. 9. ESR spectrum of 1a radical anion recorded in Bu4NBF4/CH2Cl2.
however, after several minutes a consecutive reaction takes place,
leading to products with broken conjugation. The nature of the
charge carriers in question was further studied through electron
spin resonance spectroelectrochemical measurements.
Registered ESR spectra of the anion radicals generated through
reduction of each monomer have revealed an extremely rich hyper-
fine structure, as shown for the spectrum of 1a in Fig. 9. This
structure arises as a result of interaction of the radical with both
naphthalic hydrogens and the pyrrole system for 1a and 2a, or the
linker group’s protons in case of 1b and 2b. ESR signals appeared
at a potential, corresponding to the onset of monomer reduction
*
trum supports the attribution of this signal to the ␲–␲ transition
of the 1,8-naphthalimide system. The new broad bands, located
at longer wavelengths, are attributed to the formation of charge
carriers (polarons and bipolarons) in the polymer backbone. These
1.4
0.25
2b
2a
1.2
0.20
0.15
0.10
0.05
1.0
0.8
0.6
0.4
0.2
0.0
300 400 500 600 700 800 900 1000 1100
300 400 500 600 700 800 900 1000 1100
Wavelength / nm
Wavelength / nm
Fig. 10. UV–vis spectra recorded during oxidation of 2 mM solutions of 2a 0.1 M Bu4NBF4/CH3CN and 2b in Bu4NBF4/CH2Cl2.
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0.09
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
poly2a
poly1b
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
400 500 600 700 800 900 1000 1100
300 400 500 600 700 800 900 1000
Wavelength / nm
Wavelength / nm
Fig. 11. UV–vis spectra recorded during oxidation of polymer films in 0.1 M Bu4NBF4/CH3CN.
optoelectronic applications such as bipolar OFETS and OLEDs with
exciplex emission.
0.09
poly2a
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
Utilising different linking groups and donor moieties, the ability
to tune the band gap of the investigated systems is demonstrated.
Due to orbital symmetry issues, the spectral method of band gap
determination was found unfeasible for this class of compounds, as
the HOMO–LUMO transition is negligible compared to the HOMO−2
to LUMO or HOMO to LUMO+1 transition.
Acknowledgements
This study was partially supported by the PCSS in Poznan com-
putational grant No. 147. Przemyslaw Ledwon, Alina Brzeczek,
Sandra Pluczyk, Wojciech Kuznik are scholars in SWIFT project
POKL.08.02.01-24-005/10 which is co-financed by European Union
within European Social Fund. Tomasz Jarosz is a scholar supported
by the “Doktoris—scholarship program for an innovative Silesia”,
co-financed by European Union within European Social Fund.
300 400 500 600 700 800 900 1000 1100
Wavelength / nm
Fig. 12. UV–vis spectra recorded during reduction of poly2a in 0.1 M
Bu4NBF4/CH3CN.
Appendix A. Supplementary data
absorption bands are specific for conjugated polymers in the oxi-
dized state, for which polarons and bipolarons absorb lower energy
light. The oxidation of poly2b and poly1b proceeds in a similar fash-
ion, this behaviour further supporting the aforementioned relation.
During reduction of poly2a, sharp peak located at 345 nm,
decreases and a new peak appears at approximately 440 nm
(Fig. 12). The comparison of spectra recorded during reduction and
oxidation of poly2a indicate that these processes occur at different
moieties. Oxidized species are located mainly at the polymer main
chain, whereas reduced species at 1,8-naphthalimide side groups.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.electacta.
2013.10.163.
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Please cite this article in press as: P. Ledwon, et al., Synthesis and electrochemical properties of novel, donor–acceptor pyrrole derivatives with
1,8-naphthalimide units and their polymers, Electrochim. Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.10.163