TTF derivative of 2,5-aromatic disubstituted pyrrole, synthesis and electronic study

Article abstract of DOI:10.1016/j.molstruc.2015.12.010

TTF derivative of substituted pyrrole was obtained by means electrochemical synthesis, the resultant colored mix was characterized by Mass spectrometry, NMR and EPR studies, its intrinsic electronic behavior was measured by a four point probe method, besides theoretical calculations were carried out on the possible structures of the resultant molecular adduct. All the results show that there is a net transfer of an electron between both organic moieties in solution giving place to a semiconductor species.

Full text of DOI:10.1016/j.molstruc.2015.12.010

Journal of Molecular Structure 1108 (2016) 370e377
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
TTF derivative of 2,5-aromatic disubstituted pyrrole, synthesis and
electronic study
Lioudmila Fomina a, *, Chistopher Le oꢀ n , Monserrat Bizarro , Alejandro Baeza ,
a
a
b
c
a
a
Virginia G oꢀ mez-Vidales , L. Enrique Sansores , Roberto Salcedo
Instituto de Investigaci oꢀ nes en Materiales, Universidad Nacional Aut oꢀ noma de M ꢀe xico, Circuito Exterior s/n, Ciudad Universitaria, Coyoac aꢀ n 04510, M ꢀe xico
a
D.F., Mexico
b
Facultad de Química, Universidad Nacional Aut oꢀ noma de M ꢀe xico, Circuito Escolar s/n, Ciudad Universitaria, Coyoac aꢀ n 04510, M ꢀe xico D.F., Mexico
Instituto de Química, Circuito Exterior s/n, Ciudad Universitaria, Coyoac aꢀ n 04510, M ꢀe xico, D.F., Mexico
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 August 2015
Received in revised form
TTF derivative of substituted pyrrole was obtained by means electrochemical synthesis, the resultant
colored mix was characterized by Mass spectrometry, NMR and EPR studies, its intrinsic electronic
behavior was measured by a four point probe method, besides theoretical calculations were carried out
on the possible structures of the resultant molecular adduct. All the results show that there is a net
transfer of an electron between both organic moieties in solution giving place to a semiconductor
species.
4
December 2015
Accepted 4 December 2015
Available online 11 December 2015
©
2015 Elsevier B.V. All rights reserved.
Keywords:
TTF derivative
Substituted pyrrole
Synthesis
Electrochemical synthesis
Electronic study
1
. Introduction
the HOMO of TTF and the LUMO of the other species give place to an
electronic flux making a redox complex with semiconductor
behavior. One of the most studied pairs is that in which TTF in-
teracts with TCNQ [11] which has demonstrated a very good
organic semiconductor behavior; however, this is only one example
of a large quantity of combinations with interesting electronic
characteristics [12].
The study of electronic materials has been growing and opening
new important areas [1e3], the study of this kind of compounds
takes account mainly on the energy of the highest occupied mo-
lecular orbital (HOMO) and lowest unoccupied molecular orbital
(
LUMO) levels and the orbital interactions. These last interactions
are fundamental for the communication between two different pair
of molecules that can show interesting electronic interchanges [4].
The combination of TTF (tetrathiafulvalene) (Fig. 1) with several
organic molecules showing electron donor-acceptor and charge
transfer complexing abilities has given place to several interesting
chemical mixes with important semiconducting behavior [3e7].
TTF molecule is known since 1970 [8e10] and one of the groups
interested in its synthesis was the one to find the very important
redox behavior of this substance [10]. The authors [10] suggest for
the first time that the molecule can reach an aromatic situation on
the consecutive lack of two electrons (Fig. 2), therefore, this ion can
interact with an electron acceptor counterpart in such a way that
In other context, the synthesis, characterization and study of the
semiconductor behavior of 2,5-aromatic disubstituted pyrrole de-
rivatives have been an interest of our research group [13]. These
studies have yielded interesting results in which the importance of
the substituent of the aromatic ring joint to the nitrogen atom of
the pyrrole unit have been established as a promotion agent of
semiconductor behavior.
The goal of this work is to show the nature of the redox
complex arising from the interaction among pyrrole derivative and
TTF in order to get semiconductor species. The method of electro-
chemical synthesis, the characterization by several spectroscopic
techniques and the study of conductivity are included. A theoretical
study of the interaction between TTF and one of the derivatives (the
one with the eNO
very interesting pathway of electronic promotion and give a chance
2
substituent) is also shown, the results show a
*
Corresponding author. Tel.: þ52 5556224727.
E-mail address: lioudmilafomina@gmail.com (L. Fomina).
http://dx.doi.org/10.1016/j.molstruc.2015.12.010
022-2860/© 2015 Elsevier B.V. All rights reserved.
0

L. Fomina et al. / Journal of Molecular Structure 1108 (2016) 370e377
371
Fig. 1. Molecular structures of TTF (a) and 1-(4-nitrophenyl)-2,5-diphenyl-pyrrole (b)
studied in this work.
Fig. 5. Voltammetric behavior of compound 4 (TTF), (1 mM), v ¼ 10 mV/s, at platinum
disk in 0.1 M TBAP in acetonitrile. I: TTF ꢁ 1eꢁ / TTF ; II: TTF ꢁ 1e / TTF
þ
þ
ꢁ
2þ
.
Fig. 2. TTF redox transformation.
for other kind of studies which have started in our group and will be
reported in short time.
2
. Experimental part
2.1. Synthesis
The synthesis and characterization of 1, 4-diphenylbuta-1, 3-
diyne (compound 1); 1-(p-nitro-phenyl)-2, 5-triphenylpyrrole
Fig. 6. Voltammetric behavior of compound 2, (1 mM), v ¼ 10 mV/s, at platinum disk
(
(
compound 2) and 1-(p-carboxy-phenyl)-2,5-triphenylpyrrole
compound 3) have been recently described [13,14].
in 0.1 M TBAP in acetonitrile.
2
.2. Electrochemical synthesis
reference electrode. The silver reference potential according to
couple ferrocene/ferricinium was 0.4V. A great surface Au foil is
used as auxiliary electrode. Constant potential bulk electrolysis was
performed in a 50 mL electrolytic cell using a great surface plat-
Voltammetric analysis of fresh solutions of substrates, TTF and
-(4-nitrophenyl)-2,5-diphenylpyrrole, in order to determined the
1
electrochemical oxidation properties prior to bulk electrolysis, was
carried out.
2
inum plaque of about 20 cm with constant stirring using a fresh
solution containing 1 mM of compounds 1-(4-nitrophenyl)-2,5-
diphenylpyrrole and TTF in presence of TBAP 1 mM as well.
The cells were maintained in a pure nitrogen atmosphere at all
times. Voltammograms and chronoamperometric bulk electro-
Pure acetonitrile, AN, (AR grade) dried over molecular sieves
was used as solvent. The supporting electrolyte was tetrabuty-
lammonium perchlorate (TBAP, reagent grade), dried at 90
overnight before use. As working electrode a platinum disk
microelectrode of about 0.031 cm was used. A silver wire
ꢀ
C
2
synthesis were obtained with
analyzer (Radiometer-Tacussel).
a DEI-digital electrochemical
3
immersed in a solution of 0.01 mol/L in AgNO was used as quasi-
Fig. 3. Chemical reaction of diphenylacetylene with different amines used to obtain the pyrrole derivatives.
Fig. 4. Proposed reaction of formation of CTC 2e4.

372
L. Fomina et al. / Journal of Molecular Structure 1108 (2016) 370e377
3. Results
3.1. Synthesis
The diacetylenic molecule 1,4-diphenylbuta-1,3-diyne (1) (see
Fig. 3) was prepared following the Glaser method taking into ac-
count the modification of Hay [18,19] which considers the oxidative
coupling of terminal alkynes with cooper salts as catalysts and a
bidentate ligand, TMEDA, in the present particular case.
As shown in Fig. 3, 1-(p-nitro-phenyl)-2,5-triphenylpyrrole (2)
and 1-(p-carboxy-phenyl)-2,5-triphenylpyrrole (3) were synthe-
sized by the reaction of diphenyldiacetylene with different aro-
matic amines employing copper chloride (I) as catalyst, the
synthesis and characterization of these compounds have been
recently described [13,14]. These compounds were obtained by a
modification of a reported procedure [20].
Fig. 7. Voltammetric behavior of compounds 2 and 4 (TTF), (1 mM each), v ¼ 10 mV/s,
at platinum disk in 1 mM M TBAP in acetonitrile, before bulk electrolysis.
3.2. Electrochemical synthesis
The proposed electrochemical reaction is shown in Fig. 4.
The idea is to reach the two oxidative potentials of compound 4
(TTF) in order to share both resultant electrons with molecule 2. It is
known the large capability of compound 4 to form free radicals [8],
this characteristic made it a very popular generator of stable
conductor systems [3,12]. On other context, compound 2 can have
acceptable reduction potentials [13], therefore, the combination of
both species is promising to get interesting electronic interactions.
Therefore, the idea is to prepare a transfer charge complex be-
tween compounds 2 and 4, in a first step, a voltamperometric study
of both compounds was carried out in order to determinate the
nature and potentials for each participant in the reaction, after the
compounds were mixed and the electrosynthesis was carried out
by means the chronoamperometric technics. The cyclic voltam-
perograms of 4 and 2 compounds are shown in Figs. 5 and 6,
respectively; the potential scan was from ꢁ1000 to 1800 mV vs an
Ag/AgCl electrode and the solvent was anhydride acetonitrile.
Fig. 5 shows the voltammetric behavior, at 10 mV/s, of com-
pound 4, TTF (1 mM), on platinum in 0.1 M TBAP in AN. From this
figure it is observed that TTF presents two oxidation peaks and two
corresponding reduction peaks in the reverse scan, with a relative
Fig. 8. Chronoamperometric plot during bulk electrolysis of compounds 4 (TTF) and 2,
1
mM each, at platinum plate in 1 mM M TBAP in acetonitrile. The color solution
changes from orange to deep red.
c
a
quotient (Ip /Ip ) y 1. The latter behavior suggests two mono-
electronic quasi-reversible oxidation processes according to:
I= TTF ꢁ 1e /TTFþ Ep Ia ¼ 0:258 V
ꢁ
þ
ꢁ
2þ
a
II= TTF ꢁ 1e /TTF
Ep ¼ 0:629 V
II
a
c
A (Ep ꢁ E
p
) ¼ 0.173 V for both oxidationereduction processes
þ
shows that the cation TTF electrogenerated is stable enough in
2
þ
solution as well as the second cation observed, TTF
.
Fig. 6 shows the voltammetric behavior, at 10 mV/s, of com-
pound 2, 1 mM, on platinum in 0.1 M TBAP in AN. From this figure it
is observed that compound 2 presents one oxidation peak,
Fig. 9. Voltammetric behavior of compounds 2 and TTF (4) (1 mM each), v ¼ 10 mV/s,
at platinum disk in 1 mM M TBAP in acetonitrile, after 250 min bulk electrolysis.
a
c
Ep ¼ 1.04 V, and a corresponding reduction peak, Ep ¼ 0.90 V,
2.3. Computational details
c
a
with a relative quotient (Ip /Ip ) y 1. The latter behavior suggests a
mono-electronic quasi-reversible redox that shows that the
reduced form of compound 2 is stable in the oxidation range of TTF.
Fig. 7 shows the voltammetric behavior, at 10 mV/s, of the
mixture of compound 4 (TTF), and compound 2, both 1 mM, on
platinum in 1 mM TBAP in AN prior to bulk electrolysis. From this
All calculations were carried out using a pure DFT method for
energy evaluations, and it was applied Becke's gradient corrections
15] for exchange and Perdew-Wang's for correlation [16]. This
[
scheme gives place to the B3PW91 method which forms part of the
Gaussian09 code [17]. All calculations were performed using the 6-
figure it is observed that the voltammetric profile is strongly
31G** basis set. Frequency calculations were carried out at the same
a
modified since de first oxidation peak of TTF, Ep
0
I
, is displaced from
level of theory to confirm that the optimized structures were at a
minimum of the potential surfaces.
a
.258 mV to 0.786 V as well as for the second oxidation peak, EpII, is
displaced from 0.629 V to 1.24 V. The corresponding reduction
a
c
peaks are also modified since (Ep ꢁ E
p
) ¼ 0.173 V is increased to A

L. Fomina et al. / Journal of Molecular Structure 1108 (2016) 370e377
373
Fig. 10. Infrared spectrum of CTC 2e4.
a
c
(
Ep ꢁ E
p
) ¼ 0.400 V. The latter behavior strongly suggests chemical
3.3. Spectroscopic analysis
interactions between substrate 2 and TTF (4) previous to bulk
electrolysis.
The latter solution is submitted to bulk electrolysis on a great
platinum plate with strong stirring. Electrolysis current is
measured with respect to time imposition of ¼ 1.00 V with respect
to the silver quasi-reference electrode. Electrolysis is stopped after
50 min of potential imposition. The orange color of the initial
mixture of compounds 2 and TTF (4) changes to deep red at the end
of the bulk electrolysis procedure.
Fig. 8 shows the typical chronoamperometric plot during the
bulk electrolysis. The latter plot shows that exhaustive electrolysis
was achieved.
Fig. 9 shows the voltammetric behavior, at 10 mV/s, of the
mixture of TTF (4) and compound 2, both 1 mM, on platinum in
The IR spectrum of charge transfer complex of compounds 2 and
4 (TTF) is shown in Fig. 10. Unfortunately, the counterion found in
the environment (tetrabutylammonium perchlorate (TBAP)) is
present and its bands appear also in the analysis, however, the
corresponding IR spectrum of this substance (TBAP) (see Fig.11) can
be subtracted from the total and the important peaks at 2980, 2950,
2
ꢁ
1
1
1370 cm coming from the complex can be appreciated. The H
NMR of CTC 2e4 (see Figs. 12 and 13) shows the corresponding
peaks for compound 2, but there are no evidence of the protons of
the compound 4, this feature arises from the dicationic nature of
þþ
the TTF which makes so strong the magnetic moment of the ion,
13
however, the signal of compound 4 appears in the C spectrum
together with the corresponding signal of the compound 2 (see
Figs. 14 and 15).
1
mM TBAP in AN after 250 min of bulk electrolysis. From this figure
it is observed that the voltammetric profile is strongly modified
The EPR spectrum is shown in Fig. 16, whereas the hyperfine
structure of the same is shown in Fig. 17. This analysis gives better
evidences of the formation of the charge transfer complex because
is very clear the presence of the free radical with a g value of
2.0068, but even the hyperfine analysis shows the presence of the
other electron and the interaction of it with the one localized on the
charge transfer complex, the couples constant is 0.145 mT and the
corresponding g is 2.00882. The free electron of the complex is
a
since only de first oxidation peak of TTF is observed. The (Ep -
c
E
p
) ¼ 0.173 V of the original electrochemical oxidation of TTF is
a c
strongly increased until (Ep - E
Since the first oxidation of TTF is the electrochemical process
that leads the whole mechanistic reaction between compound 2
p
) ¼ 0.554 V.
and TTF (4), a electrochemical mechanism of the type E
posed where TTF is stabilized by the presence of compound 2
r
C
r
is pro-
þ
according to scheme:
2
localized on the aromatic ring which supports the eNO functional

374
L. Fomina et al. / Journal of Molecular Structure 1108 (2016) 370e377
where n is the number of neighbors with no cero spin and l is the
magnitude of an individual spin, substituting n ¼ 4 (i.e. the four
hydrogens of the mentioned aromatic ring) and l ¼ 1/2, the result
corresponds with the five lines in the hyperfine spectrum, these
results match with the corresponding theoretical simulation which
localize the LUMO on the same region of compound 2 and can
reproduce a very similar spectra. With the respect of the theoretical
simulation mentioned above, the results of this were compared
with the experimental spectrum as well as a simulation obtained
from the spectrophotometer software in which the experimental
data were supplied, the result of these comparisons is shown in
Table 1.
This analysis is the better evidence of the formation of the
charge transfer complex. However, the electronic spectra and other
theoretical calculation were carried out in order to assure the na-
ture of the resultant complex and the transfer of the electron
involved in this complex itself. This analysis is shown below in the
next section.
Fig. 11. Infrared spectrum of tetrabuthylammonium perchlorate (TBAP).
group of the pyrrolic counterpart because the number of lines is
obtained by means:
h
¼ 2 nl þ 1
Fig. 12. 1H NMR spectrum of CTC 2e4.
Fig. 13. Expansion of 1H NMR spectrum of CTC 2e4 from Fig. 12.

L. Fomina et al. / Journal of Molecular Structure 1108 (2016) 370e377
375
Fig. 14. 13C NMR spectrum of CTC 2e4.
Fig. 15. Expansion of 13C NMR spectrum of CTC 2e4.
3
.4. Electronic spectroscopy, conductivity and theoretical
band is localized at 294 nm (4.21 eV), this value is compared with
those obtained by means the theoretical calculations which cor-
responds to the energy gap between HOMO and LUMO which
yields 3.89 eV, the matching is very good even considering that the
spectrum was captured in chloroform solution and it is considered
calculations
The UVeVisible spectra of the compounds was achieved, it is
shown in Fig. 18. The important point is that the main absorption
Fig. 16. EPR spectrum of 2e4.

376
L. Fomina et al. / Journal of Molecular Structure 1108 (2016) 370e377
Fig. 17. Hyperfine structure of CTC 2e4.
Table 1
Comparison of parameters experimental EPR analysis and theoretical and experimental simulations.
Parameters measured/simulated
Experimental
Theoretical simulation
Experimental simulation
Number of equivalent atoms
Spín
4
0.5
4
0.5
4
0.5
Factor g
2.00882
0.145
0.089
335.747
2.0088
0.150
0.09
335.75
2.0088
0.142
0.09
Coupling constant (mT)
Peak broad (mT)
Central signal (mT)
335.748
moieties of the complex was also simulated, Fig. 19 show it, the
schemes b) and c) show the shape of the HOMO and LUMO
respectively making emphasis on the strong division of the mo-
lecular orbitals on each part of the complex, i.e. the HOMO
distributed on all the surface and the LUMO focused on the pyrrole
derivative moiety.
Table 3 shows the conductivity of both pyrrole derivatives 2 and
3, the measurements were carried out by means the four point
probe.
4. Conclusions
In this work, the synthesis and characterization of organic
compounds containing 2,5-disubstituted pyrroles were carried out.
These compounds were the 1-(4-nitrophenyl)-2,5-diphenylpyrrole
and 1-(4-carboxyphenyl)-2,5-diphenylpyrrole. There are previous
reports of their synthesis by our group.
Fig. 18. UVeVisible spectrum of CTC 2e4.
We also conducted the synthesis and characterization of CTC
formed by 1-(4-nitrophenyl)-2,5-diphenylpyrrole and tetrathia-
fulvalene (TTF) (which has no previous reports in literature) by
electrochemical method. The results of the characterization of the
CTC synthesized show that the compounds involved share an
electron from the TTF that migrates to the structure of 1-(4-
nitrophenyl)-2,5-diphenylpyrrole.
that the calculated molecule is placed in vacuum.
Table 2 shows the eigenvalues of the frontier molecular orbitals
of both studied derivatives of pyrrole, in both cases the values
correspond to the classification of semiconductors, in fact, the
present study contains conductivity measurements which will be
discussed below. In the same table is registered the corresponding
band gap value of the charge transfer complex, the value is 4.6 eV,
but this result even being classified as an insulator should be
handled with care because it is a salt susceptible to narrow its gap
in solution or by means a dopant agent. The interaction of both
It was also carried out the calculation of the difference in
HOMO-LUMO energy levels (band gap) of compounds 1-(4-
nitrophenyl)-2,5-diphenylpyrrole,
1-(4-carboxyphenyl)-2,5-
1-(4-nitrophenyl)-2,5-
diphenylpyrrole
and
CTC
Table 2
Energies of orbitals calculated by Gaussian.
Compound
HOMO (Ev)
LUMO (Ev)
g
E (Ev)
1
1
-(4-Nitrophenyl)-2,5-diphenylpyrrole (2)
-(4-Carboxyphenyl)-2,5-diphenylpyrrole (3)
5.578
5.363
9.93
2.558
1.621
5.33
3.02
3.741
4.6
CTC of 1-(4-nitrophenyl)-2,5-diphenylpyrrole (2)eTTF (4)

L. Fomina et al. / Journal of Molecular Structure 1108 (2016) 370e377
377
Fig. 19. a) Structure of CTC proposed, b) energetic level of LUMO, c) energetic level of HOMO.
Table 3
Conductivities of obtained compounds.
Compound
Conductivity
s (S/cm)
3.5 ꢂ 10ꢁ
7
1
1
1
-(4-Nitrophenyl)-2,5-diphenylpyrrole (2)
-(4-Carboxiphenyl)-2,5-diphenylpyrrole (3)
-(4-Nitrophenyl)-2,5-diphenylpyrrole e TTF (CTC 2e4)
ꢁ
7
3.94 ꢂ 10
1.07 ꢂ 10ꢁ
9
diphenylpyrroleeTTF. We found the three compounds to exhibit
band gaps that place them within the classification of materials
with potential semiconducting behavior, and this can be demon-
strated by analyzing their electrical properties.
We obtained a good approximation of the theoretical band gap
compared with the band gap obtained experimentally by UVeVi-
sible spectroscopy, which value may be considered among mate-
rials with semiconducting behavior.
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Acknowledgments
[
[
[
16] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244.
The authors wish to express their gratitude to Teresa V ꢀa zquez,
Oralia Jim eꢀ nez for technical help and to Joaquín Morales Rosales
and Alberto Lopez for computers support. Thanks are also due to
Gerardo Cedillo for the assistance in NMR analysis, to Miguel Angel
Canseco for IR analysis and to Damaris Cabrero Palomino for ther-
mal analysis.
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