Electronic properties of hyperbranched compounds derived by pyrrole

Article abstract of DOI:10.1016/j.molstruc.2014.06.031

The electronic structure of hyperbranched polymers obtained from para-diaminodiphenyldiacetylenes as AB₂ type monomers by one-step polymerization is studied using DFT calculations. Five generations are modelled with optimization at each step. As the molecule grows it shows a helical shape conserving the C2 symmetry with wing like fragments covering the backbone. There is accidental degeneracy of the HOMO with the HOMO-1 and of the HOMO-2 with the HOMO-3. The LUMO is always located at the backbone and the HOMO set at the dianyline-pyrrole fragment. The HOMO-LUMO gap decreases as the molecule grows. Conductivity measurements as well as microscopy studies of para-hyperbranched polymer were carried out. Conductivity measurements as a function of temperature show semiconductor behavior. Optical microscopy reveals a macroscopic crystalline structure of this hyperbranched polymer.

Full text of DOI:10.1016/j.molstruc.2014.06.031

Journal of Molecular Structure 1074 (2014) 534–541
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Electronic properties of hyperbranched compounds derived by pyrrole
b
c
c
a
Lioudmila Fomina a, , Jorge Godínez Sánchez , J.A. Olivares , F.L.S. Cuppo , L. Enrique Sansores ,
⇑
Roberto Salcedo a
a Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito exterior s/n, Ciudad Universitaria Coyoacán 04510, México, DF, Mexico
b Escuela Nacional Preparatoria No 3, Justo Sierra, Universidad Nacional Autónoma de México, Av. Ing. Eduardo Molina No 1577, Gustavo A. Madero, 07400 México, DF, Mexico
c Centro de Investigación en Polímeros, Marcos Achar Lobaton No. 2 Tepexpan, 55885 México, Mexico
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
Hyperbranched molecules containing
pyrrole units were synthetized.
These hyperbranched molecules
show large electronic delocalization.
Theoretical results show that para-
hyperbranched polymer has an
uncommon symmetry.
ꢀ
ꢀ
The conductivity measurements
indicated an energy band gap of 0.2
eV.
Microscopy studies identify two
phases where self-assembling is
important.
a r t i c l e i n f o
a b s t r a c t
Article history:
The electronic structure of hyperbranched polymers obtained from para-diaminodiphenyldiacetylenes as
AB2 type monomers by one-step polymerization is studied using DFT calculations. Five generations are
modelled with optimization at each step. As the molecule grows it shows a helical shape conserving
the C2 symmetry with wing like fragments covering the backbone. There is accidental degeneracy of
the HOMO with the HOMO-1 and of the HOMO-2 with the HOMO-3. The LUMO is always located at
the backbone and the HOMO set at the dianyline-pyrrole fragment. The HOMO–LUMO gap decreases
as the molecule grows. Conductivity measurements as well as microscopy studies of para-hyperbranched
polymer were carried out. Conductivity measurements as a function of temperature show semiconductor
behavior. Optical microscopy reveals a macroscopic crystalline structure of this hyperbranched polymer.
Ó 2014 Elsevier B.V. All rights reserved.
Received 18 March 2014
Received in revised form 7 June 2014
Accepted 9 June 2014
Available online 19 June 2014
Keywords:
Hyperbranched polymers
Electronic properties
Diacetylenes
Semiconducting polymers
Morphology
Introduction
and the third group is constituted by the hyperbranched polymers
[4,5]. There are several species that belong to the third group [6].
The dendrimeric polymers can be classified into three large
groups [1]: the first one refers specifically to the basic dendrimers
i.e. molecules which grow in branches [2] and they are prepared
in steps; the second group is formed by the so called dendro-injert-
ed polymers [3] which have been described as the intermediated
species between the dendrimers and the hyperbranched polymers;
They are very interesting substances with highly branched struc-
tures, three dimensional architecture, globular shape, terminal
functional groups and particular electronic behavior [5]; they can
be prepared by polymerization in one or several steps synthesis [6].
The diacetylene derivatives shown in this communication
belong to this group. Recently, hyperbranched molecules contain-
ing pyrrole units were obtained by Godínez Sánchez et al. [6] from
ortho-, meta- and para-diaminodiphenyldiacetylenes as AB2 type
monomers by one-step polymerization. The new compounds can
⇑
Corresponding author. Tel.: +52 5556224727.
E-mail address: lioudmilafomina@gmail.com (L. Fomina).
http://dx.doi.org/10.1016/j.molstruc.2014.06.031
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

L. Fomina et al. / Journal of Molecular Structure 1074 (2014) 534–541
535
take orientations ortho, meta and para. All of them show a large
electronic delocalization, which makes them important targets
for high conductivity. Particularly, we will focus on the para ana-
logue, which has a curious geometry that can give it a large way
for electronic transport. This work is part of a research in which
our group has published some interesting results [6–8].
In this paper we report the structural characterization of
para-hyperbranched polymer and a theoretical study to under-
stand the nature of its properties. The molecule of the para-
hyperbranched compound is shown in Fig. 1 and its synthesis
was reported in [6]. It has been previously characterized by means
of infrared and NMR spectroscopy [9].
allowed to cool to room temperature. The mixture was diluted
with excess of acidified water. The precipitate was collected by
filtration and dried in vacuum. Method B. A mixture of compound
3 (0.5 g, 2.15 mmol), copper (I) chloride (0.05 g, 0.5 mmol) in diox-
ane (10 mL) was refluxed under nitrogen for 24 h at 70 °C in an oil
bath and allowed to cool to room temperature. The solution was
diluted with excess of acidified water. The precipitate was
collected by filtration and dried in vacuum.
Electrical conductivity measurements
The para-hyperbranched compound is partially soluble in
acetone; however, it was not possible to produce a continuous
homogenous film. The measurement of its conductivity was car-
ried out as a powder sample in a specially designed cell. This cell
consists of a transparent polystyrene tube, PSC, and two cylindrical
steel electrodes with a diameter, D = 3.19 mm. The powder sample
was pressed between the steel electrodes and the final samples
Experimental part
Synthesis [6]
Synthesis of the monomer compound
thickness, L = 1.46 mm was measured with
a Vernier scale
(Fig. 2). Then the cell was placed inside a microscope hot stage
adapted to heat uniformly the samples region. Conductivity mea-
surements were carried out as a function of the temperature using
and LCR meter. The measurements were carried out at 20 Hz fre-
quency. The initial temperature was 10 °C and then the sample
was heated at a ratio of 3.5 °C/min until a temperature of 50 °C
was reached and cooled again at a ratio of 1 °C/min.
(1) 4-(N-Boc-amino)phenylacetylene. To a solution of di-tert-
butyl dicarbonate (BOC2O) (27.56 g, 126.28 mmol) in
42 mL THF, was added aminophenylacetylene (4.85 g,
41.39 mmol). Solution was stirred and refluxed for three
hours. The solvent was removed in vacuum and the product
was purified by column chromatography using hexane–
ethyl acetate 10:1–5:1 as eluent.
(2) 4,40-Di(N-Boc-amino)diphenyldiacetylene. To a solution of
compound 1 (2.00 g, 9.2 mmol) in 20 mL isopropanol was
added (0.025 g, 0.252 mmol) of copper chloride and 0.3 mL
of N,N,N0,N0-tetramethylethylenediamine (TMEDA), the mix-
ture was stirring under oxygen atmosphere for 3 h, and
resulting solution was added to acidified water. The product
was separated by filtration, dried in vacuum and purified by
recrystallization from hexane.
Microscopy analysis
The para-hyperbranched sample was observed by optical
microscopy and scanning electron microscopy. Since the sample
is partially soluble in acetone, two different samples were pre-
pared. The first sample was a mixture of powder with deionized
water (0.1% wt) and dispersed by 15 min in an ultrasonic bath.
The second sample was prepared by mixing the powder sample
in acetone to study the dissolved part of the para-hyperbranched
polymer. Drops of these two samples were poured on clean glass
slides and observed directly by optical microscopy. Different illu-
mination techniques were used: bright field, polarize light and
dark field to enhance the sample contrast with the glass substrate.
The sample for scanning electron microscopy (SEM) was pre-
pared by pouring a drop from the deionized water mixture. After
water evaporates, the sample was coated with a thin layer of gold
for its observation under the SEM.
(3) 4, 40-Diaminodiphenyldiacetylene. To a suspension of 1.25 g
(2.89 mmol) of compound 2 in 110 mL of MeOH was added
125 mL of concentrated HCl, and the mixture was stirred
for 42 h at room temperature. The supernate was pipetted
off, and the solid was stirred in 100 mL of acetone overnight,
after which it was filtered off, washed four times with 15 mL
of acetone, and pumped dry.
Synthesis of para-hyperbranched compound
Method A. A mixture of compound 3 (0.1 g, 0.43 mmol), copper
(I) chloride (0.05 g, 0.5 mmol) in 10 mL dimethylformamide was
refluxed under nitrogen for 24–48 h at 110 °C in an oil bath and
Computational details
All structures have been optimized at B3PW91/3-21G using the
Gaussian09 code [10]. For all geometries, the starting point was a
non-symmetric structure. For the third and fourth generation,
before the last optimization the structure was very near C2 symme-
try, so we force to this symmetry and optimized again. The final
structure with lower energy is C2.
Fig. 2. Schematic cell for impedance measurements. Stainless Steel Cylindrical
Electrodes (SSE), Transparent Polystyrene Tube (PSC), Semi Cylindrical Copper
Cover (CC). The electrode radio is D and the sample thickness is L.
Fig. 1. Para-hyperbranched polymer.

536
L. Fomina et al. / Journal of Molecular Structure 1074 (2014) 534–541
Results
Synthesis
Monomer synthesis
The synthetic route to the monomer 3 is shown in Figs. 3–5.
This monomer was prepared from para-aminophenylacetylene, in
three steps.
Terminal amino groups have been converted into N-Boc-amino
groups by treating aminophenylacetylene with di-tert-butyl dicar-
bonate (BOC2O) in THF (Fig. 3) in order to protect amino groups for
the next reaction of oxidative coupling (Fig. 4).
Diyne compound 2 was obtained by a modified procedure of
Hay’s oxidative coupling using copper (I) chloride as a catalyst
[8] (Fig. 4).
The protective groups were removed by treating compound 2
by concentrated HCl giving the resulting monomer 3 (Fig. 5) con-
taining diacetylenic and terminate amino groups in 64–68% yield.
Fig. 7. Conductivity versus temperature for two cycles. Red Curve corresponds to
the first cycle. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
Fig. 3. Synthesis of 4-(N-Boc-amino)phenylacetylene (1).
Fig. 4. Synthesis of 4,40-di(N-Boc-amino)diphenyldiacetylene (2).
Fig. 5. Synthesis of 4,40-diaminodiphenyldiacetylenes (3).
Fig. 6. Synthesis of para-hyperbranched molecule (4) from 4,40-diaminodiphenyldiacetylene (3).

L. Fomina et al. / Journal of Molecular Structure 1074 (2014) 534–541
537
Fig. 8. Several morphologies founded by optical microscopy. Pictures from (a–f) were taken using bright field illumination. Picture (g) dark field illumination of (f). Picture (b)
was taken between cross polarizers.
Fig. 9. Dark field microscopy of sample dissolved in acetone. In the figure it can be appreciated dendrimers as well as regular geometrical shapes.

538
L. Fomina et al. / Journal of Molecular Structure 1074 (2014) 534–541
r
r0
Ea
2kT
Polymerization
Ln
¼ ꢁ
;
ð1Þ
Para-diaminodiphenyldiacetylene (3) as AB2 type monomer
was polymerized by two methods [8]. Method A – in DMF under
nitrogen at 110 °C and method B – in dioxane under nitrogen at
70 °C using copper (I) chloride as catalyst in both cases (Fig. 6).
Diacetylenic fragments and terminal amino groups reacted to yield
pyrrole units. These reactions are carried out by one step
polymerization.
where k is the Boltzmann constant, T the temperature in Kelvin and
r0 the ideal conductivity for a monocrystalline structure [11]. The
adjustment for each cycle gives the same activation energy,
Ea = 0.200 +/ꢁ .003 eV, while r0 (S cmꢁ1) increases from 39.64
S cmꢁ1 in the first cycle to 52.45 S cmꢁ1 in the second cycle.
The para-hyperbranched polymer was obtained from para-
diaminodiphenyldiacetylene as AB2 type monomer by one-step
polymerization. Due to the method of synthesis of this compound
we cannot to define a generation of this hyperbranched polymer.
The prepared polymer should be considered as a mixture in which
there are molecules of very different sizes, therefore, the general
macroscopic properties as conductivity and morphology corre-
spond to a response of all formed chains.
Microscopy analysis
Microscopy studies of para-hyperbranched polymer were car-
ried out too. In Fig. 8(a)–(f) bright field illumination was used.
For Fig. 8(b) the sample was placed between crossed polarizers
for probing anisotropy domains. In Fig. 8(d) oblique illumination
was used to enhance samples edges. Fig. 8(g) corresponds to the
same sample of Fig. 8(f) using the dark field technique to enhance
contrast at the edges.
Electrical conductivity measurements
Four different structures are relevant: amorphous flat structure
(flakes) as in Fig. 8(a), tubular or cylindrical, Fig. 8(b), and ellipsoi-
dal structures, Fig. 8(c) and (d). A crystalline structure with very
sharp edges is seen in Fig. 8(f) and (g). The flake-like structures,
Fig. 8(a), strongly absorb light and they appear always in yellowish
tones. Other structures are transparent, neither presenting optical
absorption nor refractive index contrast with the glass substrate. It
is noticeable in Fig. 8(d)–(f) the presence of regular geometrical
shapes. In Fig. 8(d) a solid cylinder appears while in Fig. 8(f) a per-
fect hexagonal cylindrical shape was found.
In Fig. 7, we show the two-cycle measurement of the conductiv-
ity,
r as a function of the temperature. The sample presents a
hysteresis behavior that can be related to polymer degradation
with temperature. As a consequence, the conductivity values were
reduced by 13% between the first and the second cycle. As can be
noticed in Fig. 7, the samples conductivity increases with temper-
ature in the range between T = 283 K and T = 315 K. This is a typical
behavior of a semiconductor material. After T = 315 K the conduc-
tivity decreases following an ohmic behavior [11]. The activation
The highly geometrical shapes suggest that the soluble part of
the sample has a crystalline structure with an optical anisotropy.
To observe the formation of the crystal structure a drop of the
energy, Ea, can be obtained by fitting
range, with the relation:
r data in the semiconducting
Fig. 10. Polarization microscopy. Optical anisotropy of the sample can be appreciated. Sample between cross polarizes and (a and c) and regular illumination, (b and d).

L. Fomina et al. / Journal of Molecular Structure 1074 (2014) 534–541
539
sample dissolved in acetone was placed on a clean glass slide and
then we left the acetone to evaporate freely. Micrographs of this
are shown in Figs. 9–11. In Fig. 9(a) a dendrimer structure is shown
using dark field technique and its corresponding oblique illumina-
tion 9(b) to enhance the dendrimers volume. In Fig. 9(c) a dendri-
mer structure appears as well as some regular triangular shapes. In
Fig. 11. Behavior of regular geometrical shape under different illuminations: (a–c) illuminated with polarized light. (d) Dark field illumination. (e–h) After a drop of acetone
was pour on sample.
Fig. 12. SEM microscopy. It can be observed large tubes (a) rhomboids (b) and triangles (c).
Fig. 13. Optimized geometries of the generations 0–4 of the para-hyperbranched polymer.

540
L. Fomina et al. / Journal of Molecular Structure 1074 (2014) 534–541
Fig. 14. Spatial representation of the LUMO, HOMO set and HOMO-1 set for generation 4.
Fig. 9(d) perfect isosceles triangles are displayed. Two regions of
highly concentrated polymer were found and shown in Fig. 9(e)
and (f), in which several regular parallelograms are displayed.
Some of the regular geometric shapes show birefringence when
they are illuminated between crossed polarizers. This is an indica-
tive of an optical uniaxial media. In Fig. 10 an isolated parallelo-
gram and a rod-like structure with a square base are shown
between cross polarizers, Fig. 10(a) and (c), and without analyzer
in Fig. 10(b) and (d).
In Fig. 11 a curious four-domain structure is shown. The struc-
ture changes its color as the polarization of the incident light is
rotated, Fig. 11(a)–(c). In Fig. 11(d) the sample is displayed under
dark field technique. A drop of acetone was pour onto this structure,
Fig. 11(e), its absorption by the polymer structure, Fig. 11(f) and (g)
and the crystallization process as solvent evaporates, Fig. 11(h).
Fig. 12 shows photographs of the sample illuminated by SEM.
Some of the previous structures observed in the optical microscope
also appear. In Fig. 12(a) rod shape structure is observed as long as
Computational results
Fig. 13 shows the sequence followed for the construction of the
molecule. We start with a central backbone fragment of dianyline-
pyrrole. In step 0 a dianyline-pyrrole fragment is added to the cen-
tral backbone. From here on, two dianyline-pyrrole fragments are
added in each step substituting two NH2, always selecting the
external NH2 and keeping the symmetry. Generation 1, 2, 3 and 4
are shown in the same figure. As the molecule grows it shows a
helical shape conserving the C2 symmetry for generation 2, 3 and
4. In all cases the steric hindrance was avoided.
The optimized molecule has a central backbone with a rod
shape, while the fragments are like wings covering the backbone.
As the molecule grows it takes a helical shape. It is expected that
the molecules grows in this same way for higher generations. This
arrangement resembles the shape of the crystals seen in the
microphotographs.
The main feature of this system is focused in the distribution of
the frontier molecular orbitals. At generation 0 the LUMO is located
at the central backbone with main contributions coming from the p
orbitals of the carbons. It has b symmetry and remains the same in
the next generations (see Fig. 14). The HOMO is located at the
dianyline-pyrrole fragment with irreducible representation a. It is
important to note that the next three occupied orbitals are very
near in energy but at this step they are not degenerated yet. In fact,
the HOMO-1 degenerates with the HOMO from generation 1 to 4.
Furthermore, HOMO-2 and HOMO-3 degenerate from generation
2 to 4. These are accidental degenerations since the point group
C2 does not have double irreducible representation. The symmetry
25 lm and a thickness of 1.24 lm. In Fig. 12(b) some geometrical
shapes similar to Fig. 8(g) are displayed. And in Fig. 12(c) an isos-
celes triangle is shown.
Table 1
Energy and symmetry of the frontier orbitals, HOMO–LUMO gap, total energy and
dipole.
Generation
LUMO
0
1
2
3
4
E (eV)
Sym
ꢁ1.380
b
ꢁ1.523
b
ꢁ1.641
b
ꢁ1.657
b
ꢁ1.783
b
HOMO
E (eV)
Sym
ꢁ4.432
ꢁ4.340
ꢁ4.243
ꢁ4.270
ꢁ4.195
a
b
a
a
a
HOMO-1
HOMO-2
HOMO-3
E (eV)
Sym
E (eV)
Sym
E (eV)
Sym
ꢁ5.064
b
ꢁ4.341
a
ꢁ4.243
b
ꢁ4.298
b
ꢁ4.195
b
ꢁ5.222
ꢁ5.239
ꢁ4.838
ꢁ4.533
ꢁ4.624
b
b
b
a
a
ꢁ5.973
b
ꢁ5.256
a
ꢁ4.838
a
ꢁ4.533
b
ꢁ4.625
b
Gap (eV)
Total E (keV)
Dipole (Debye)
3.052
ꢁ39.3
6.7
2.817
ꢁ78.6
4.2
2.602
ꢁ117.9
14.1
2.612
ꢁ157.2
19.8
2.411
ꢁ196.5
21.0
Fig. 16. Hyperbranched geometry adjusted by a diamond-like shape with two
Fig. 15. Energy scheme LUMO, HOMO set and HOMO-1 set.
symmetrical axes.

L. Fomina et al. / Journal of Molecular Structure 1074 (2014) 534–541
541
structures. It is expected that the antiparallel configuration is the
more favorable as shown in Fig. 17.
Conclusions
Hyperbranched structures containing pyrrole units were
obtained from diaminodiphenyldiacetylenes as AB2 type mono-
mers by one-step polymerization.
The para-hyperbranched polymer shows an uncommon sym-
metry that gives rise to an electronic structure with 8 electrons
available to contribute to the high conductivity. It is also important
the location of the LUMO at the backbone and of the HOMO sets at
the external dianyline-pyrrole fragments. These two features are
expected to be conserved, as the molecule grows further and
explain the high conductivity observed experimentally.
Conductivity measurements indicated an energy band gap of
0.2 eV, which is 10 times smaller to the theoretical value since
the later only considers 4 generations (it is to be expected that
the synthetized compounds have hundreds of generations) and
does not include the crystalline field found in a solid.
Microscopy studies identify two phases. The first is the insolu-
ble one which gives origin to flake type structures. The second is
acetone soluble phase, which generated crystalline structure
manifesting in optic anisotropy and rhomboids and triangles den-
drimeric structures. Self-assembled process is proposed by using a
diamond like structure, however, more work should be done to
study this point.
Fig. 17. Possible self-assembled macrostructures using as a building block the
diamond shape of Fig. 16.
of the LUMO is always b and that of the HOMO is a except for
generation 1 that is b. The orbital symmetries do not change from
generation 3 to 4 and it is to be expected that will not change for
further generations. The energy values and symmetries of the
orbitals are given in Table 1. It is important to mention that the
total energy is always a multiple of generation 0 total energy.
Fig. 15 shows a scheme of the energy levels in generation 4. The
important feature is that the molecule has 8 electrons available to
participate in the conducting process since the energy difference
between the LUMO and the HOMO and HOMO-1 sets is very small
(LUMO–HOMO energy gap 2.41 eV and HOMO, HOMO-1 gap
0.43 eV). This phenomenon is the qualitative explanation of the
experimentally measured high conductivity of the samples.
The position of the orbitals is also important since the LUMO is
always at the centre of the molecule while the HOMO set is always
at the external dianyline-pyrrole fragments. The HOMO-1 set is
always located at the fragment of the previous generation. In this
way there is an electron flow from the outside to the backbone.
The dipole moment increases with the generation (except for
generation 1) but is reaching saturation. It is always aligned with
the central bone.
Acknowledgments
We thank the Computing and Information Technology Division
of the UNAM for the computer resources.
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