Synthesis, characterization and semiconducting behavior of N,2,5-trisubstituted pyrroles

Article abstract of DOI:10.1016/j.molstruc.2018.05.086

In this work, N,2,5-trisubstituted pyrroles were synthesized and characterized by IR, UV–Vis and NMR spectroscopies. These materials' electronic and optical properties were then evaluated. Theoretical calculations were carried out by means of the Gaussian

Full text of DOI:10.1016/j.molstruc.2018.05.086

Accepted Manuscript
Synthesis, characterization and semiconducting behavior of N,2,5-trisubstituted
pyrroles
Olivia Monroy, Lioudmila Fomina, María Elena Sánchez-Vergara, Rubén Gaviño,
Alonso Acosta, José Ramón Álvarez Bada, Roberto Salcedo
PII:
S0022-2860(18)30665-3
10.1016/j.molstruc.2018.05.086
MOLSTR 25259
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Accepted Date:
06 February 2018
28 May 2018
Please cite this article as: Olivia Monroy, Lioudmila Fomina, María Elena Sánchez-Vergara, Rubén
Gaviño, Alonso Acosta, José Ramón Álvarez Bada, Roberto Salcedo, Synthesis, characterization
and semiconducting behavior of N,2,5-trisubstituted pyrroles, Journal of Molecular Structure (2018),
doi: 10.1016/j.molstruc.2018.05.086
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Graphical Abstract

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Synthesis, characterization and semiconducting behavior of N,2,5-trisubstituted
pyrroles
Olivia Monroya, Lioudmila Fominaa, María Elena Sánchez-Vergarab, Rubén Gaviñoc, Alonso
Acostaa, José Ramón Álvarez Badab & Roberto Salcedoa
a Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior
s/n, Ciudad Universitaria, Coyoacán 04510, Ciudad de México.
b Universidad Anáhuac México, campus Norte, Avenida Universidad Anáhuac 46, Col. Lomas Anáhuac,
Huixquilucan 52786, Estado de México, México.
c Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad
Universitaria, Coyoacán 04510, Ciudad de México.
ABSTRACT
In this work, N,2,5-trisubstituted pyrroles were synthesized and characterized by IR, UV-Vis and
NMR spectroscopies. These materials’ electronic and optical properties were then evaluated.
Theoretical calculations were carried out by means of the Gaussian09 software and all the involved
species were geometrically optimized in order to obtain the theoretic HOMO, LUMO and band
gaps. From the HOMO and LUMO calculations, it turns out that monomer 1-(3,5-dinitrophenyl)-
2,5-diphenyl-1H-pyrrole could be used as a p-type semiconductor or n-type semiconductor,
depending on the substituent. The experimental optical band gaps were obtained by the Tauc and
Cody methods and compared to the ones calculated through DFT. The results show that the
semiconducting behavior is found in all the monomers and depends on the functional group of the
structure, its crystallinity degree, as well as its HOMO and its LUMO energies.
KEYWORDS: Diphenyl-pyrrole derivatives; semiconductor monomers; DFT calculations; optical
band gap
INTRODUCTION
In the last decades, the study of organic semiconductors has been of great importance due to their
potential applications in electronic devices, such as organic light-emitting diodes (OLEDs) [1],

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field-effect transistors [2], sensors [3] and solar cells [4]. The distinctive feature of organic
semiconductors, in contrast to inorganic semiconductors, is their solid-state structure, which is
based on weak interactions, mainly Van der Waals and dipole-dipole interactions among nearest
neighbors, resulting in a wide range of conductive behaviors, from insulators to conductors [5]. This
type of materials also presents other special properties, such as flexibility for chemical synthesis,
ease to modify their structures and simple processing, that make them attractive for several
applications. This functionality depends mainly on the interaction between HOMO (High Occupied
Molecular Orbital) and LUMO (Low Unoccupied Molecular Orbital) energy levels [6], the
energetic difference between LUMO and HOMO being known as band gap [7]. A band gap
characteristic of semiconductor materials permits low-energy excitations, meaning that the material
has the capability to easily donate an electron (from the HOMO), or receive it (in the LUMO),
which is the elementary process occurring in every organic electronic device. In commercial
devices, the desirable band gaps for optimal efficiency are in the 1.59-3.18 eV range [8] within the
wider semiconductor range of 1-4 eV [9]. There are different methods to determine the band gap in
organic materials. Ab initio density functional (DFT) calculations can be used to estimate the
HOMO and LUMO energy levels for the band gap determination [10], while the Tauc and Cody
methods can be used to determine the optical band gap [11,12]. The Tauc method [11,13] is
frequently used to derive the band gap in terms of the incident energy. The Tauc band gap
associated with semiconductor materials is determined through an extrapolation of the linear trend
observed in the spectral dependence of (αhν)n over a limited range of photon energies hν [11,14].
The Cody model assumes that this behavior is due to a fundamental curvature in the spectral
dependence of (αhν)n = f(hν), which is held responsible for increases in the Tauc optical gap related
to decreasing thickness [12,14]. In accordance to this model, as defined by the Cody method, the
band gap associated with semiconductors should be determined by extrapolating the linear trend
observed in the spectral dependence of (α/hν)n versus hν.
Among the semiconductor materials under study, those that contain substituted heterocycles in their
structure, such as pyrrole, stand out [15]. Substituted pyrrole is well known for its different
applications in drug chemistry [16], organic electronics [17] and photoluminescence materials [18].
Trisubstituted pyrroles have also been subject of interest [19-23]. Materials of this type can be
monomers or polymers with electron-withdrawing groups, but all the materials under consideration
in this work are monomers. These monomers have the structure shown in Diagram 1, where the
substituents are –O-Me, –OH, –CN, –COOH and –NO2 and are located in the para position of the
aromatic ring joined to the nitrogen atom from pyrrole. In all cases, the capability of the substituents
for withdrawing electron density is well-known, resulting in two main effects: (i) promoting an

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inductive effect and (ii) tuning HOMO and LUMO energy levels. Therefore, these materials show a
decrease in the band gap energy, obtaining smaller values than those for the unsubstituted monomer
(4.451 eV). However, in the case of –NO2 substituents, these effects are stronger than the ones
shown by other compounds of this family. This feature leads to a dipolar behavior and an electronic
current through a route directed towards the electron-withdrawing substituent. The narrowest band
gap has a value of 3.02 eV; consequently, this material has a semiconducting behavior. The
electronic structure and properties of this type of materials depend on the degree of conjugation
along the molecular skeleton. Interest in monomers has been growing due to their versatility, since
their properties can be modified by changing from amorphous to crystalline structure. High hole
mobility (of up to 0.1 cm2/Vs) occurs in ordered monomer structures, unlike polymers, in which
polydispersity linked to the synthesis occurs. The mixture of compounds that make up the polymer
does not usually show the same composition and, thus, the reproducibility of its optoelectronic
properties cannot be fully guaranteed. Moreover, the high probability of finding defects in the
conjugated polymer chains and their low crystallinity make charge mobility in the polymers lower
than that in the monomers.
The purpose of this work is to report the synthesis and characterization of monomers prepared from
1,4-diphenyl-1,3-butadiyne and aromatic amines with one or two different electron-withdrawing
substituents in order to study the source of the semiconducting behavior of the materials and
propose new materials with interesting electronic behavior. The theoretical band gap of these
monomers was estimated from DFT calculations and compared with the values obtained from Cody
and Tauc semi-empirical methods, based on spectroscopic measurements. Band gap determination
also provides fundamental information regarding the electronic properties of semiconductors. For
instance, the band gap controls the nature of the electroluminescent signals in OLEDs and, in
photovoltaic cells, it determines light absorption efficiency.
Diagram 1. Pyrrole derivative with electron-withdrawing groups

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EXPERIMENTAL
All chemicals were provided by Aldrich Chemical Company and used as received. The FT-IR
spectra data were collected with a Thermo Scientific, Nicolet 6700 spectrophotometer with an ATR
1
Smart Orbit attachment. H and 13C NMR spectra for all compounds were registered with Bruker,
Avance 300 MHz and Bruker, Avance III HD 400 MHz spectrometers. The chemical shifts are
reported in ppm, scaled relative to TMS. All melting points were registered on a Fischer-Jones
melting-point apparatus and are uncorrected.
Synthesis of the precursor compound. 1, 4-diphenylbuta-1,3-diyne [24-26]. To a solution of phenyl
acetylene (2.042 g, 20 mmol) in iso-propanol was added CuCl (I) (0.4 g, 4 mmol) and N, N, N´, N´-
tetramethylethylendiamine (TMEDA), 0.1 ml (0.077 g, 0.6 mmol). The mixture was stirred for 3
hours under a stream of oxygen, and the subsequent solution added to acidified water. The product
was separated by filtration, dried in vacuum and purified by recrystallization from hexane. Yield:
1
95%, white solid, m.p. 86 °C. FT-IR: ν(C≡C) = 2148 cm-1, ν(C-H) = 3050 cm-1. H NMR (300
13
MHz, CDCl3): δ 7.34 (m, 3H), 7.53 (m, 2H). C NMR (75 MHz, CDCl3): δ 73.92, 81.55, 121.81,
128.43, 129.19, 132.49.
Synthesis of monomers (1-8). The monomers were synthesized using the procedure reported
previously by some authors [19,21,23]. A mixture of precursor (0.50 g, 2.47 mmol), CuCl (I) (0.04
g, 0.49 mmol) and the appropriate amine (4.94 mmol), in DMF (10 ml), was refluxed under a
nitrogen atmosphere for 48 hours at 153 °C in an oil bath, then allowed to cool at room temperature.
The solvent was removed in vacuum and the product was purified by column chromatography using
hexane or a hexane-acetone mixture as eluent.
(1) 1-(3,5-dinitrophenyl)-2,5-diphenyl-1H-pyrrole. Yield: 50%, orange solid, m.p.: 216 °C. FT-IR:
ν(C-N) = 1382 cm-1, ν(C=C) = 1448, 1463, 1483, 1599 cm-1, νas(NO2) = 1537 cm-1, νs(NO2) = 1335
cm-1, δ(NO2) = 847 cm-1. 1H NMR (400 MHz, CDCl3): δ 6.55 (s, 2H, H-pyrrole), 7.09 (m, 4H), 7.27
(m, 6H), 8.08 (d, 2H), 8.86 (t, 1H).13C NMR (100 MHz, CDCl3): δ 110.98 (C-pyrrole), 111.78,
116.59, 127.68, 128.44, 128.62, 129.33, 131.74, 135.88, 147.98.
(2) 1-(4-nitro-3-(trifluoromethyl)phenyl)-2,5-diphenyl-1H-pyrrole. Yield: 80%, yellow solid, m.p.:
185 °C. FT-IR: ν(C-N) = 1365 cm-1, ν(C=C) = 1417, 1449, 1488, 1598 cm-1, νas(NO2) = 1535 cm-1,
νs(NO2) = 1301 cm-1, δ(NO2) = 852 cm-1, ν(CF3) = 1245 cm-1. 1H NMR (400 MHz, CDCl3): δ 6.51
(s, 2H, H-pyrrole), 7.04 (m, 6H), 7.13 (m, 1H), 7.26 (m, 5H), 7.75 (d, 1H). 13C NMR (100 MHz,
CDCl3): δ 111.70 (C-pyrrole), 124.09, 125.75, 127.39, 127.85, 128.47, 128.65, 129.18, 130.65,
132.11, 135.61, 142.56, 145.40.

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(3) 1-(2-nitro-4-(trifluoromethyl)phenyl)-2,5-diphenyl-1H-pyrrole. Yield: 50%, white solid, m.p.:
79°C. FT-IR: ν(C-N) = 1360 cm-1, ν(C=C) = 1461, 1603 cm-1, νas(NO2) = 1510 cm-1, νs(NO2) =
1376 cm-1, δ(NO2) = 842 cm-1, ν(CF3) = 1243 cm-1. 1H NMR (400 MHz, CDCl3): δ 6.78 (s, 2H, H-
pyrrole), 7.32 (t, 3H), 7.45 (t, 5H), 7.80 (d, 5H). 13C NMR (100 MHz, CDCl3): δ 107.25 (C-
pyrrole), 123.74, 127.36, 128.73, 130.80, 153.37.
(4) 1-(2,4-dinitrophenyl)-2,5-diphenyl-1H-pyrrole. Yield: 40%, white solid, m.p.: 56°C. FT-IR:,
ν(C-N) = 1379 cm-1, ν(C=C) = 1441, 1489, 1570, 1596 cm-1, νas(NO2) = 1539 cm-1, νs(NO2) = 1382
cm-1, δ(NO2) = 848 cm-1. 1H NMR (300 MHz, CDCl3): δ 6.73 (s, 2H, H-pyrrole), 7.27 (m, 3H), 7.39
13
(m, 5H), 7.75 (m, 5H). C NMR (75 MHz, CDCl3): δ 107.20 (C-pyrrole), 123.71, 127.33, 128.69,
128.80, 130.77, 153.35.
(5) 1-(4-nitro-2-(trifluoromethyl)phenyl)-2,5-diphenyl-1H-pyrrole. Yield: 50%, white solid, m.p.:
80°C. FT-IR: ν(C-N) = 1363 cm-1, ν(C=C) = 1439, 1484, 1568, 1592 cm-1, νas(NO2) = 1529 cm-1,
νs(NO2) = 1385 cm-1, δ(NO2) = 849 cm-1, ν(CF3) = 1285 cm-1. 1H NMR (400 MHz, CDCl3): δ 6.78
(s, 2H, H-pyrrole), 7.31 (m, 3H), 7.45 (t, 5H), 7.79 (m, 5H). 13C NMR (100 MHz, CDCl3): δ 107.24
(C-pyrrole), 123.73, 127.36, 128.72, 130.80, 153.37.
(6) 1-(4-fluoro-3-(trifluoromethyl)phenyl)-2,5-diphenyl-1H-pyrrole. Yield: 80%, white solid, m.p.:
211°C. FT-IR: ν(C-N) = 1319 cm-1, ν(C=C) = 1433, 1482, 1504, 1600 cm-1, ν(CF3) = 1321 cm-1,
ν(C-F) = 1120 cm-1. 1H NMR (400 MHz, CDCl3): δ 6.48 (s, 2H, H-pyrrole), 7.04 (m, 5H), 7.16 (m,
1H), 7.21 (m, 7H). 13C NMR (100 MHz, CDCl3): δ 110.48 (C-pyrrole), 117.18, 117.39, 126.83,
127.41, 128.14, 128.94, 132.57, 133.85, 133.94, 135.11, 135.87.
(7) 2,5-diphenyl-1-(4-(trifluoromethyl)phenyl)-1H-pyrrole. Yield: 90%, white solid, m.p.: 209°C.
FT-IR: ν(C=C) = 1463, 1485, 1520, 1600 cm-1, ν(CF3) = 1332 cm-1. 1H NMR (300 MHz, CDCl3): δ
6.49 (s, 1H, H-pyrrole), 7.03 (m, 2H), 7.10 (d, 1H), 7.19 (m, 3H), 7.48 (d, 1H, H-9). 13C NMR (75
MHz, CDCl3): δ 110.68 (C-pyrrole), 122.03, 125.75, 125.79, 125.84, 125.89, 126.64, 128.10,
128.88, 129.06, 132.81, 135.77, 142.01.
(8) 1-(4-fluorophenyl)-2,5-diphenyl-1H-pyrrole. Yield: 70%, white solid, m.p.: 256°C. FT-IR: ν(C-
N) = 1287 cm-1, ν(C=C) = 1398 cm-1, ν(C-F) = 1229 cm-1. 1H NMR (400 MHz, CDCl3): δ 6.50 (s,
1H, H-pyrrole), 6.96 (m, 1H), 7.02 (m, 1H), 7.09 (m, 2H), 7.21 (m, 3H). 13C NMR (100 MHz,
CDCl3): δ 109.99 (C-pyrrole), 115.61, 115.83, 126.39, 127.98, 128.78, 130.33, 130.41, 133.05,
135.02, 135.89.
X-ray diffraction analysis was performed with the θ-2θ technique using a Bragg-Brentano geometry
with a Bruker, D8 Avance diffractometer and working with CuK-α (λ=0.15405 nm) radiation. For
the optical properties, the UV-vis spectroscopy of the monomers was measured on a Unicam
spectrophotometer, model UV300, in the wavelength range of 200-1100 nm. The Tauc and Cody

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band gap values of monomers were calculated from the absorption coefficient and photon energy.
Additionally, the types of electronic transitions in the N,2,5-trisubstituted pyrroles were obtained.
THEORETICAL
All calculations were carried out by means of the Gaussian09 code [27]. For the structures of all the
derivatives, Becke’s gradient corrections [28] for exchange and Perdew-Wang’s parameterization
for correlation [29] were applied. This is the scheme of the B3PW91 method. The calculations were
performed using the 6-31G** basis set. In all cases, frequency calculations were carried out in order
to assess thermodynamic stability as well as the theoretical IR spectra, which were compared with
the experimental ones.
RESULTS AND DISCUSSION
The diphenyl-pyrrole based monomer syntheses are shown in Diagram 2. The diyne compound
precursor for that reaction was obtained through a modification of Hay´s oxidative coupling,
employing CuCl (I) as a catalyst [30].
Diagram 2. Reactions of diacetylene with amines.
The diacetylene thus obtained was then made to react with aromatic amines with electron-
withdrawing substituents, according to the Reisch-Shulte procedure under nitrogen, using CuCl (I)
at 153º C during 48 hours [19,24-26]. Polar solvents like DMF favor the formation of pyrroles. The
1
conversion of the diacetylenic group was studied by FTIR and NMR spectroscopy. The H spectra
of the monomer compounds showed singlets at 6.47-6.74 ppm. In the 13C NMR spectrum, signals at
13
73.92 and 81.55 ppm disappear after reaction. The signals at 107.20-111.70 ppm in the C spectra

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are attributed to 2,5-disubstituted pyrrole. The monomers derived from ortho-substituted amines
exhibit lower yields than those obtained from meta- and/or para-substituted amines, due to the steric
hindrance.
As mentioned earlier, all the monomers in this study correspond to the general formula shown in
Diagram 2. The differences in all of them arise from the nature and position of the substituents on
the terminal aromatic ring. It is well known that the substituents have a very important effect when
trying to obtain the desired properties. The substitution with electron withdrawing substituents
produces lower energetic differences between the HOMO-LUMO frontier orbitals, higher electron
affinities and higher ionization potentials. In contrast, electron donating substituents have an
opposite effect on HOMO and LUMO energy orbitals because of enlarged electron density in the
structure [31,32]. Studies of monomers with only one electron withdrawing substituent in the para
position have previously been reported [21], obtaining a narrow band gap between the frontier
orbitals. In this work, the study has been extended to improve the semiconducting behavior by
changing the nature of the substituent or introducing more substituents which are also electron
withdrawing and may reinforce the phenomenon. The geometry of the general compound leads to
an electronic flow which can run from the pyrrole to the aromatic ring or in the reverse direction,
depending on the nature of the substituents. This study focuses on the influence of this effect on the
global behavior of the resulting compounds. The results are presented in two separate sections, one
about the theoretical calculations and another with the experimental data. A comparison and the
corresponding discussion is made at the end.
a) Theoretical results
The theoretical calculations were carried out to explain the experimental data. The main features to
consider are the frontier molecular orbitals behavior and the theoretical spectroscopy. Considering
the data shown in Diagram 1, it is important to ascertain the semiconducting nature of the several
compounds reported there. The main feature to consider is the presence in all the important cases of
the –NO2 substituent; there are other important electron withdrawing groups, but it seems that the
presence of this particular group yields the best results, therefore the discussion focus on this
isolated case. The particular phenomenon in which the presence of the electron withdrawing groups
are related to the semiconducting behavior has been previously studied and discussed [21].
Nevertheless, in this work it is important to highlight that the electronic effect is larger when the
group is –NO2, therefore the first compounds in Table 1 were chosen in such a way that this group
be included, with several other electron withdrawing substituents for comparison.

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Figure 1 shows the HOMO-LUMO set for the molecules which bear this substituent. The case
shown there corresponds to a dinitro case; however, the behavior is exactly the same for all
molecules which have at least one nitro substituent. HOMO is localized on the pyrrole and its
lateral aromatic rings moiety, whereas LUMO is completely localized on the aromatic ring joined to
the nitrogen atom from the pyrrole and with a strong participation of the substituents. The HOMO
and LUMO values calculated for different monomers are shown in Table 1. If monomers are to be
used as organic semiconductors, it’s necessary to separately consider the values of HOMO and
LUMO energies. The p-type semiconductors, or hole conveyors, have low levels of HOMO energy,
allowing easy hole injection and high hole mobility. On the contrary, n-type semiconductors will be
those that have low LUMO energy levels and high electron mobility [33]. The HOMO values
shown in Table 1 are very similar for all the monomers, suggesting that they follow the same trend
regarding hole mobility. The lower value corresponds to monomer 1, which is an indication that this
monomer could be used as p-type semiconductor. On the other hand, the monomer 8, with the -F
group, seems to behave as n-type semiconductor. It is known that electron-attracting functional
groups, such as -F or -CF3, can promote electronic conduction [33]. This fits with the observation in
Table 1 that monomer 7 is the second one with the highest HOMO after monomer 8. Apparently,
the presence in the same molecule of two electron-attracting groups, as occurs in monomer 6, does
not favor electronic conduction, since it has two active sites from which electrons can flow.
Regarding LUMO, it can be observed in Table 1 that there are important differences among the
studied monomers. As in the HOMO case, monomer 1 is the one with the significantly lowest
LUMO value and would behave as n-type semiconductor, facilitating electron mobility. The
HOMO and LUMO energy values obtained for monomer 1, makes it a strong candidate for use in
the manufacture of devices, such as OLEDs, because, upon substituent change, it can be used as
both p-type semiconductor and n-type semiconductor.

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Figure 1. Frontier molecular orbitals of molecules which contain -NO2
Table 1. HOMO and LUMO values and comparison of experimental and theoretical band gap
results of the synthesized compounds.
Frontier Orbitals
(eV)
Band gap
(eV)
Com-
Structure
Name
pound
HOMO LUMO DFT Tauc Cody
1-(3,5-dinitrophenyl)-
N
1
2
-5.796
-5.714
-3.700
-3.374
2.096
2.340
2.1
2.2
2.1
2.1
2,5-diphenyl-1H-pyrrole
O2N
NO2
1-(4-nitro-3-
N
(trifluoromethyl)phenyl)-
2,5-diphenyl-1H-pyrrole
CF3
NO2
N
1-(2-nitro-4-
NO2
3
4
(trifluoromethyl)phenyl)-
2,5-diphenyl-1H-pyrrole
-5.660
-5.697
-3.238
-3.238
2.422
2.449
2.5
2.8
2.6
2.7
CF3
N
1-(2,4-dinitrophenyl)-
NO2
2,5-diphenyl-1H-pyrrole
NO2

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1-(4-nitro-2-
N
CF3
5
(trifluoromethyl)phenyl)-
-5.551
-2.802
2.749
2.7
2.9
2,5-diphenyl-1H-pyrrole
NO2
1-(4-fluoro-3-
N
6
7
(trifluoromethyl)phenyl)-
2,5-diphenyl-1H-pyrrole
-5.523
-5.442
-1.632
-1.442
3.891
4.0
3.3
3.3
3.3
3.3
CF3
F
2,5-diphenyl-1-(4-
(trifluoromethyl)phenyl)-
1H-pyrrole
N
CF3
N
1-(4-fluorophenyl)-2,5-
diphenyl-1H-pyrrole
8
-5.306
-0.761
4.545
2.7
2.4
F
Another important aspect of the semiconductor materials used in the manufacture of optoelectronic
devices is a band gap that, in monomers and polymers, generally varies between 1.5 y 4eV [9]. The
theoretical model suggests that the inductive effect is fundamental to get a short band gap, the
explanation being that the electronic flow caused by the aromatic cloud has a definitive direction
which is defined by the delocalized negative charge between both oxygen atoms of the –NO2 group.
Therefore, the HOMO (bearing to the majority aromatic system) which has the least electrons tends
to approach towards the empty function of the LUMO, shortening the gap. The trend is similar in
other compounds with electron-withdrawal effect, but is stronger in this case. Figure 2 shows the
shape and location of frontier molecular orbitals corresponding to the structures which do not
include –NO2 substituents. In these cases, the influence of the groups –CF3 and –F is generally
evident. The DFT band gap in these substances is relatively larger than in the –NO2 cases, the
explanation of this situation arising from the same MO analysis.

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Figure 2. HOMO-LUMO set of molecules without -NO2 substituent
The HOMO in these cases shows exactly the same behavior as in their –NO2 counterparts, but the
LUMO is predominantly localized on the aromatic ring joined to the nitrogen atom from the pyrrole
ring, and this situation differs from the first case, in which the majority of the LUMO functions is
on the –NO2 substituent. This suggests that the electronic flow is almost the same in all cases, i.e.
the electrons flow from the lateral aromatic rings of the pyrrole to the substituted ring, but the
strongest electronic flow is directed towards the –NO2 molecules. The DFT band gap shown in
Table 1 suggests that monomer 8 has an insulating behavior, with a band gap higher than 4 eV.
Nevertheless, the DFT band gap should first be compared to experimental values in order to clearly
define its electrical properties.
Comparison of the theoretical and experimental data validates the calculations, Figure 3a/b shows
the experimental and theoretical spectra for the monomers 1, 2, 7, 8. The main peaks for the specific
substituents are indicated, the comparison of all the cases shows that the structure of the new
molecules is correct, the more relevant matching arises when the C-N stretching peaks of all cases
are compared because they appear at the expected position (with slight variations since theoretical
model does not take into account the environmental effect). These bands are located between 1280
and 1390 cm-1. In the case of monomer 8 the peak has low oscillator strength and appears at 1287
and 1310 cm-1 for both the experiment and the calculations. It is important to note that this
particular signal gains importance when the –NO2 substituent is present, but this is not the case of
monomer 8. The case of monomer 7 is different because the corresponding peak of the C-N

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stretching does not appear because of the overlap with a strong band at 1332 cm-1 originated from
the stretching modes of the -CF3 substituent. Another important effect is a shift of the C-N
stretching band when the -NO2 substituent is present, due to the strong inductive effect of this
group. The cases of the monomers 1 and 2 which have the –NO2 group are easy to interpret, the
symmetric stretch band is strong and appears between 1301 and 1335 cm-1, it seems that the
difference in position arises from the fact that monomer 1 has two –NO2 groups. Monomer 2 has the
–NO2 group as well as –CF3, the last one appears at 1245 cm-1 in the experimental spectrum
whereas it is localized at 1288 cm-1 in the theoretical one, this same signal appears in the spectrum
of monomer 7 at 1332 and 1353 cm-1 respectively. Another important band to note is the C-F
stretching of the monomer 8, this band is important because the group C-F corresponds to the F
atom directly linked to the aromatic ring, this frequency appears at 1229 cm-1 for the experimental
and 1270 cm-1 for the theoretical spectra. Despite small differences arising from gas phase
calculations, all cases show good agreement with experiment.

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Figure 3. (a) Experimental infrared spectra and (b) theoretical infrared spectra for the compounds 1,
2, 7, 8.
Experimental results
Optical absorption measurements are widely used to characterize the electronic properties of
monomers through the determination of parameters, such as the band gap, describing the electronic
transitions [34]. The optical absorbance spectra of the monomers were recorded from 250 to 700 nm
and are shown in Figure 4a. The intermolecular band appearing in the high energy region between

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280 and 380 nm refers to electronic transitions inside the monomer, because there is an electronic
delocalization along the π-conjugated structure of the monomer. The positions of the absorbance
bands were weakly influenced by the structure of these monomers (see Diagram 2), including
factors such as the size of the monomer and, above all, the presence of acceptor and donor electrons
units. Since the main difference between monomers is the radical present in each molecule, the
important point to consider is the presence of spectrum peaks corresponding to the electronic
transitions derived from the new substituents on the bottom ring. In several cases, there are fine and
hyperfine structures arising from interactions between both substituents; these interactions lead to
expected, though complex, structures in the spectra [35]. In a different context, the known
electronic transitions are shown in Figure 4b; their presence helps to confirm the structure of the
synthesized substances. The presence of a nitro group (-NO2) and its position in the aromatic ring
seem to be the cause of the additional features of the spectrum. In monomer 4, where there are two
nitro groups in the ortho and para positions, the absorbance peaks are sharpest, followed by
monomer 5, in which there is a nitro group in the ortho position and a -CF3 substituent in the para
position. The nitro substituent –NO2 is a deactivating group, i.e., is an electron attractor group; it is
also a meta director. Due to this inductive effect, molecular stabilization is achieved by resonance.
Groups such as –CF3, are a special case, as they are deactivating groups and ortho-para directors
because of the same inductive effect. In these groups, two unpaired electrons coming from the
intrinsic negative charge are donated to the aromatic ring. The electrons, however, are attracted to
the same substituent due to the electronegativity of the group.
From the UV-vis spectra, it is noticed that monomer 1 has two nitro groups which are meta
directors, both groups in meta positions and attracting electronic density. This causes an inductive
effect directed towards the same nitro groups, leading to dipole formation. Therefore, electronic
transitions are represented in the UV-vis spectrum by a very wide absorption band. Monomer 2 has
a meta director, electro-attractor group (-NO2), and an ortho-para director, electro-attractor group (-
CF3). Since both groups are deactivating, they have the same effect that the groups in monomer 1;
electronic density is attracted by these groups, generating an inductive effect directed towards
themselves and favoring dipole formation. Electronic transitions in the UV-vis spectrum are
characterized by a slightly narrower absorption band than that in monomer 1, due to a substituent
change. Monomer 3 has a meta director, electro-attractor group (-NO2) and an ortho-para director,
electro-attractor group (-CF3), both of which are deactivating groups. In this case, as in the previous
ones, both substituents attract electronic density; however, because of the position of the –NO2
(ortho) substituent, dipole formation is weaker. The presence of this pair of substituent groups, and
particularly their relative positions (ortho and para), also allows for a wide absorption band in the

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UV-vis spectrum. Monomer 4 has two nitro substituent groups, both of them meta directors and
electro-attractors. Both groups tend to attract electronic density because of their positions (ortho and
para), although dipole formation is not as strong as in the previous case. Nevertheless, the presence
of a nitro group in the para position gives a higher definition to the absorption band of the UV-vis
spectrum. In monomer 5 both groups are deactivating and attractors of electronic density. In this
case, the nitro group is in para position and the –CF3 group is in ortho position, so that dipole
formation occurs in the same way as in the previous case, though not as intense as in monomers 1
and 2; the presence of the nitro group in the para position contributes to the additional features
found in the absorption band of the UV-vis spectrum, as occurs in monomer 4. In monomer 6, both
substituents are halogens, which are electro-attractors, so both attract electronic density that leads to
dipole formation, though not as strong as in monomer 1. The lack of the nitro group makes the
absorption band in the UV-vis spectrum narrower than in those cases where there is at least one
nitro substituent. For monomer 7, there is only a deactivating substituent (-CF3) in para position,
which attracts electronic density and promotes an inductive effect leading to dipole formation;
however, since there is only one substituent, the inductive effect is less noticeable than in the
previous cases. As was the case for monomer 6, there is no nitro group in monomer 7, so the
absorption band in the UV-vis spectrum is narrow. Finally, monomer 8 has only one electro-
attractor substituent (-F) in para position, which attracts electronic density; nevertheless, the effect
is even less noticeable that in the previous cases. This monomer does not have nitro groups or -CF3,
so the absorption band of the UV-vis spectrum is wide.

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Figure 4. (a) UV-vis spectra of monomers and (b) characteristic UV-Vis absorptions in monomers
The band gap is a fundamental parameter to establish the semiconducting character of these
materials. It refers to the energy required by charge carriers to jump between the HOMO and the
LUMO. Charge carrier mobility in monomers can be explained in terms of a hopping mechanism,
according to which, charge carriers jump between adjacent molecules [36, 37]. Unlike copolymers,
monomers have only one type of molecule and their hopping process can be described in terms of
an electronic exchange taking place through direct or indirect transitions. Direct transitions are
typical for semiconductors with ordered structure, while indirect transitions take place in
amorphous semiconductors [11-14]. The E-k diagrams of a semiconductor with direct band gap and
another with indirect band gap, can be observed in Figure 5a/b. The distinction concerns the relative
positions of the minimum of the conduction band and the maximum of the valence band. In a direct
band gap material, both positions occur at the same value of k (k=0 in Figure 5a). In an indirect
band gap material, the minimum of the conduction band does not occur at the same position as the
maximum of the valence band, but at another k value. In a direct band gap (see Figure 5a) the
electron wave vector does not change significantly during the photon absorption process. For the

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indirect band gap in Figure 5b, the electron wave vector must change significantly for a transition
from the valence band to the bottom of conduction band to occur. It is not possible to perform this
jump only by absorption of a photon: the transition must involve a phonon for momentum
conservation [38]. Thus, it is worth determining whether the synthetized monomers have a
crystalline structure permitting direct transitions. For this, DRX analysis of the monomers was
performed and the results are shown in Figure 6. The X-ray diffraction patterns of the monomers
present well defined diffraction peaks, indicating polycrystalline samples and the likelihood of
direct-type electronic transitions.
Figure 5. (a) Direct band gap and (b) indirect band gap

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Figure 6. DRX spectrum for monomers
The direct band gap was determined first from the Tauc model, which assumes that the variation in
the absorption coefficient (α) depends on the photon energy (hν) [11, 14]. The absorption
coefficient (α) is thus given by:
In the above expression, T is the transmittance and d is the film’s thickness. The photon energy is
obtained through:
Where λ is the wavelength, h is Planck’s constant and c is the speed of light. From these
expressions, a plot of the absorption coefficient vs photon energy can be made, the absorption
coefficient being (αhv)n, with n=2 for direct transitions. Figure 7 shows the resulting graph, in
which the linear zone is identified, a tangent line is drawn and the intersection point with the hv axis
is obtained. This point corresponds to the direct band gap. The numerical results are given in Table

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1 and show that monomers 6, 7 and 8 have a band gap considerably different from that estimated by
DFT calculation. It is worth noting that the Tauc band gap for monomer 8 has a value of 2.7 eV,
within the range established for organic semiconductors [33], though much smaller than the
theoretical value (4.5 eV). The difference may be due to a hopping-based charge transport process,
in which electrical charges are easily displaced between ordered monomers.
Figure 7. Tauc plots for the determination of the direct band gap
In order to (i) corroborate the band gap obtained through Tauc method and (ii) determine the
influence of thickness in the monomers’ band gap, another analysis based on the Cody model was
performed. Cody et al. [12, 14] hypothesize that the band gap associated with the monomers should
be determined by extrapolating the linear trend observed in the spectral dependence of (α/hν)2, over
a range of hν values (Figure 8). The abscissa axis intercept of this linear extrapolation corresponds
to the Cody band gap. The results obtained can be compared to the DFT calculations and Tauc band

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gap in Table 1. From these results it is observed that there are no significant differences between the
Tauc and Cody band gaps. Notice that both the Tauc and the Cody band gaps for monomers
containing -NO2 groups is similar to that calculated using DFT. In monomers with –CF3 and –F
groups, the smaller values of the experimental band gaps with respect to the theoretical band gaps
may be due, as stated earlier, to a polycrystalline structure facilitating charge transport via a
hopping process. Finally, it is observed that both Tauc and Cody band gaps are in the interval
stablished for organic semiconductors [9]. Thus, the synthetized monomers may be considered for
use in the production of optoelectronic devices. Monomers with –NO2 group should behave as p
semiconductors and those having –CF3 and –F substituents as n semiconductors.
Figure 8. Cody plot for the determination of the direct band gaps.
CONCLUSIONS
In this work, 8 new monomers containing pyrrole fragments with electron withdrawing groups have
been processed according to the Reisch-Shulte procedure to synthesize the corresponding diphenyl-
pyrrole derivatives. The HOMO energy values obtained for the synthetized monomers show

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relatively small variations, while the LUMO energy values change significantly. The differences
may be related to the presence of substituents of differing nature in the skeleton of the monomers,
which is a differentiating parameter in their charge-carrying properties. Based on the HOMO and
LUMO values, monomer 1 shows the most promising behavior for semiconductor applications.
Electronic transitions in the monomers depend on the type of radicals in the molecule and its
crystallinity as well as of the position of the HOMO and LUMO of their orbitals. The band gap
calculated by DFT for direct transitions is similar to that obtained by the Tauc and Cody methods
for those monomers presenting –NO2 in their structure. While for monomers with electron-
attracting functional groups, such as –F o –CF3, electronic transport is favored and the experimental
band gaps are considerably different from those obtained by DFT, although they are in an
acceptable range for use in the manufacture of optoelectronic devices, such as OLEDs.
Acknowledgments: The authors want to thank Adriana Tejeda, María Teresa Vázquez, Oralia
Jiménez, Alberto López, Miguel A. Canseco and Alejandro Pompa for technical help. Many thanks
are also due to DGAPA-UNAM for grants corresponding to the DGAPA PAPIIT IN203816 and
RN203816 projects. Olivia Monroy thanks CONACYT for financial support. M.E. Sánchez-
Vergara acknowledges financial support from Anahuac México University, project number
NNAIASEVM16070616
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HIGHLIGHTS



Pyrroles trisubstituted containing electron-withdrawing groups were synthesized.
Band gaps obtained by Tauc and Cody methods were compared with DFT calculations.
Inductive effect of electron-withdrawing groups is vital to get a short band gap.