Journal Name
ARTICLE
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a.
Instituto de Ciencia de Materiales de Aragón, Departamento de Química
Orgánica, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain.
b.
LASCAMM CR-INSTM, CNR-NANOTEC Lab LiCryL, Dipartimento di Fisica,
Universitá della Calabria, 87036 Rende, Italy.
c.
Instituto de Nanociencia de Aragón, Departamento de Química Orgánica,
Universidad de Zaragoza, 50009 Zaragoza, Spain.
‡
Present adress: Department of Chemistry, Massachusetts Institute of Technology,
MA 02139, Cambridge, United States.
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
High hole mobility and light-harvesting in discotic nematic
dendrimers prepared via ‘click’ chemistry
Alberto Concellón,
a,‡
Roberto Termine,
b
Attilio Golemme,
b
Pilar Romero,
a
Mercedes Marcos,
a
and
José Luis Serrano*
,a,b
We report a new family of liquid crystalline porphyrin-core dendrimers with coumarin units at the periphery of the
dendrimer. These compounds have been prepared by copper-catalyzed azide-alkyne “click” cycloaddition (CuAAC). The
mesomorphic properties have been investigated by polarized optical microscopy (POM), differential scanning calorimetry
(DSC) and X-ray diffraction (XRD). The peripheral coumarin units play a key role in the liquid crystal behavior, contributing
to the appearance of discotic nematic mesophases with hole mobility values among the highest values reported for
discotic liquid crystals (of the order of 1 cm
2
·V
-1
·s
-1
). It has also been demonstrated that excitation of the coumarin
moieties leads to energy transfer (antenna effect) to the luminescent porphyrin core. Therefore, this strategy, that
involves ‘click’ chemistry, has proven to be a powerful and elegant synthetic tool for the preparation of optoelectronic
materials based on complex dendritic architectures.
Introduction
In the last few years the research field of organic electronics –
i.e. the use of conjugated molecules as active components in
electronic devices– has expanded impressively and diversified
into organic light-emitting diodes (OLEDs), organic field-effect
transistors (OFETs) or organic photovoltaic devices (OPVs). The
ideal organic semiconductor must have closely-packed
molecules with few defects between molecules or domains
because charge transport depends on molecular order.
1-4
The
highest charge carrier mobility in organic systems has been
measured in single crystals of pentacene and rubrene (20
cm
2
V
-1
s
-1
),
5
but the preparation of single-crystalline thin films
is very tedious and not applicable industrially. To overcome
this limitation, liquid crystals (LCs) offer an interesting
approach as they self-organize into nanostructured phases
that provide properties similar to those of organic single
crystals, while the dynamics is vital for the processability and
the self-healing of structural defects.
6-14
In particular, columnar
LCs have attracted considerable attention because they
represent one-dimensional charge carrier pathways, whereby
the inner aromatic core is conducting and the outer alkyl
periphery has an insulating character. In such phases, charge
mobilities of the order of 1 cm
2
·V
-1
·s
-1
along the direction of
the columns have been described.
15-19
Despite this promising
properties, columnar mesophases still suffer from the
disadvantage that orientationally uniform domains that are
large enough to be used in most devices are often hard to
obtain. Nonetheless, very recently we reported a new family of
LC dendrimers with charge mobilities of the same order of
magnitude as those found in columnar phases, in spite of
exhibiting a discotic nematic (N
D
) phase.
20
The N
D
mesophase
makes possible to obtain large homogeneously oriented
domains with very low density of defects due to its lower
order degree in comparison to columnar mesophases.
On the other hand, LC dendrimers have been recognized as an
attractive tool for the preparation of polymeric materials for
optoelectronic applications.
21-24
They combine in the same
molecule the structural and synthetic versatility of dendrimers
with the anisotropic properties of the LC state. The topology of
these molecules bearing several external reactive groups
allows the introduction of different active moieties that self-
assemble into a LC arrangement with a supramolecular order
that facilitates charge transport.
25-29
Although the preparation
of dendrimers has been improved over the past decade, their
synthesis requires extremely efficient reactions. “Click”
chemistry provides efficient routes for the synthesis of
dendritic structures because it requires mild reaction
conditions, tolerates a wide range of functional groups, and
leads to high yields.
30-34
The aim of the work reported here is to exploit the ‘click’
chemistry as an effective and elegant approach for the
preparation of LC dendrimers with potential applications in
organic optoelectronic devices. Herein, we describe a new
approach which allows us to connect four azido-functionalized
dendrons with an alkyne functionalized porphyrin core. This
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strategy gives high yields and resulted in products that can be
easily purified by non-chromatographic procedures.
Following this synthetic approach, three porphyrin-core
dendrimers have been synthetized and their chemical
structure are shown in Figure 1. Two of them are the first- and
second-generation dendrimers bearing coumarin moieties
(ZnP-d
1
Cou and ZnP-d
2
Cou). The third porphyrin-core
dendrimer was prepared to evaluate the role of coumarin
peripheral units in these supermolecular systems. It is formed
by a first-generation dendron containing dodecyloxy alkyl
chains without coumarin moieties (ZnP-d
1
C12). In this article,
we report the synthesis, chemical and physical
characterization of these porphyrin-core dendrimers, with the
aim of assessing their possible use in organic electronics.
Figure 1. Chemical structure of the porphyrin-core dendrimers prepared by ‘click’ chemistry.
Results and discussion
Synthesis and structural characterization
The synthesis of the azido functionalized dendrons (N
3
-d
1
Cou,
N
3
-d
2
Cou, and N
3
-d
1
C12) and the alkyne functionalized
porphyrin core (ZnP-YNE) was described in the Supporting
Information.
A convergent synthetic approach was used for the preparation
of the porphyrin-core dendrimers (Scheme 1). In the final
synthetic step, the azido functionalized dendrons and the
alkyne functionalized porphyrin were coupled by copper-
catalyzed azide-alkyne cycloaddition (CuAAC) using THF/water
as solvent and CuSO
4
·5H
2
O/sodium ascorbate as the catalytic
system. An excess of the azide dendron (1.5 equivalents per
alkyne group) was employed to ensure the completeness of
the reaction and was removed by using a previously
synthesized alkyne-functionalized Wang resin. The efficiency of
the CuAAC coupling was determined by FTIR, NMR
spectroscopy (Figure 2a and Figures S1-S10), MALDI-TOF MS
(Figures S11-S13), and size exclusion chromatography (SEC). All
characterization methods indicated that the target porphyrin-
core dendrimers were monodisperse.
Scheme 1. Convergent synthetic approach used for the
preparation of the porphyrin-core dendrimers.
N
N
N
N
Zn
O
O
O
N
N
N
O
O
O
N
N
N
O
O
O
N
N
N
O
O
O
N
N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OO
OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OO
OO
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
N
N
N
Zn
O
R
O
R
O
O
R
O
O
N
N
N
O
R
O
R
O
O
R
O
O
N
N
N
O
R
O
R
O
O
R
O
O
N
N
N
O
R
O
R
O
O
R
O
O
N
N
N
O OO
CH
3
ZnP-d
1
Cou
:
ZnP-d
1
C12
:
ZnP-d
2
Cou
R =
R =
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4
N
N
N
N
Zn
N
N
N
O
4
N
N
N
N
Zn
N
N
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
ZnP-d
1
Cou
ZnP-d
2
Cou
O
O
O
O
O
4
N
N
N
N
Zn
N
N
N
O
ZnP-d
1
C12
N
N
N
N
O
O
O
O
Zn
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
N
3
O
O
O
O
O
N
3
N
3
-d
1
Cou
N
3
-d
1
C12
N
3
-d
2
Cou
CuSO
4
.H
2
O, sodium ascorbate
THF/H
2
O
CuSO
4
.H
2
O, sodium ascorbate
THF/H
2
O
CuSO
4
.H
2
O, sodium ascorbate
THF/H
2
O
ZnP-YNE
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Figure 2. (a)
1
H NMR spectra (400 MHz, CD
2
Cl
2
, 298K), (b) FTIR spectra, (c) MALDI mass spectrum, (d) SEC traces of ZnP-YNE
(red), ZnP-d
1
Cou (black) and N
3
-d
1
Cou (blue).
Evidence for the formation of the dendrimers was collected
from the
1
H NMR spectra (Figure 2a), where new peaks
corresponding to the formed triazol ring appeared at around
7.95 ppm (H
V
) and the ethynyl proton signal of the alkyne
functionalized porphyrin core disappeared upon CuAAC
coupling. Furthermore, there were changes in the
1
H shift of
the methylenic protons linked to the new formed triazol (H
U
and H
T
).
FTIR spectra also confirmed the coupling, the bands at around
3280 (≡C-H), 2120 (C≡C) and 2110 (N
3
) disappeared after the
CuAAC reaction (Figure 2b). Further confirmation of the
coupling was provided by SEC analysis and MALDI-TOF mass
spectroscopy. The MALDI-TOF mass spectra showed the
expected peak and no other signals corresponding to
dendrimers with a partial functionalization were detected
(Figure 2c). The SEC curves showed monomodal molar mass
distributions and residual precursors were not detected
(Figure 2d). As can be observed, CuAAC coupling of the
precursors blocks gave rise to a shift of the molar mass
distribution towards lower retention times that indicates the
porphyrin-core dendrimers’ formation.
Thermal stability and liquid crystalline properties
The thermal stability of all the compounds was studied by TGA.
All of the samples showed good thermal stability with the 2%
weight loss temperature (T
2%
) more than 150 ºC above the
clearing point (Table 1). Thermal transitions and mesomorphic
properties were studied by POM, DSC and XRD, and the results
are listed in Table 1. Three cycles were carried out in DSC
experiments and data were taken from the second cycle. In
some cases, temperatures were taken from POM observations
because transition peaks were not detected in DSC curves
(Figures S14-S17).
Table 1. Thermal Stability and Transition Temperatures.
T
2%
(°C)
[a]
Phase transitions
[b]
N
3
-d
1
Cou
213
g 20 Cr 61 I
N
3
-d
2
Cou
230
g 11 N 73
[c]
I
N
3
-d
1
C12
241
Cr 30 I
ZnP-d
1
Cou
344
g 24 N
D
112
[c]
I
ZnP-d
2
Cou
309
g 21 N
D
98
[c]
I
ZnP-d
1
C12
335
g 122 Cr 151
[c]
M 155
[c]
I
[a] Temperature at which 2% mass lost is detected in the
thermogravimetric curve. [b] DSC data of the second heating
process at a rate of 10 °C/min. Temperatures are read at the
maximum of the peak. g: glass, Cr: crystal, N: nematic
mesophase, N
D
: discotic nematic mesophase, M: unidentified
mesophase, I: isotropic liquid. [c] POM data.
X
U T
V
≡C─H
3
C
C’
7
2
P, E’
F, F’, H, H’
E
B, B’
L
J
K, M
N, N’
N
3
-d
1
Cou
ZnP-d
1
Cou
ZnP-YNE
N
N
N
N
23
7
Zn
O
O
O
O
OO
O
O
O
O
O O
O
O
E
L
J
N'
M'
B
C
F
H
K
M
N
P
P
O
N
N
N
T
U
V
X
4
7
3
2
X
C
E
P
F
H
B
T U
K, M
L
4000 3500 3000 2500 2000 1500 1000 500
Transmitance
Wavenumber (cm
-1
)
2000 3000 4000 5000 6000 7000
Intensity
m/z
12 14 16 18 20 22
Normalized signal
Elution time (min)
(a) (b)
(c)
(d)
C≡C
N
3
≡C─H
[M]
+
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Only the azido-functionalized second generation dendron (N
3
-
d
2
Cou) exhibited LC behavior. The other azido-functionalized
dendrons (N
3
-d
1
Cou and N
3
-d
1
C12) were not mesomorphic.
N
3
-d
2
Cou showed a birefringent texture by POM at room
temperature on applying mechanical stress (Figure 3a).
Figure 3. POM microphotographs for: (a) N
3
-d
2
Cou taken at
room temperature in the second cooling process, (b) ZnP-
d
1
Cou taken at room temperature in the second cooling, (c)
ZnP-d
2
Cou taken at room temperature in the second cooling
process, and (d) ZnP-d
1
C12 at 152 ºC in the second heating
process.
All the porphyrin-core dendrimers exhibited mesogenic
properties and the thermal behavior was different depending
on the compound. In the case of the porphyrin-core
dendrimers with coumarin functional units around the
porphyrin core (ZnP-d
1
Cou, and ZnP-d
2
Cou), enantiotropic LC
mesophases were observed. The DSC curves showed only a
glass transition freezing the mesomorphic order at room
temperature, while clearing temperatures were established
from POM observations. All the compounds had a high
tendency to homeotropic alignment and the mesophase was
observed by POM on applying mechanical stress to the
samples showing birefringent textures (Figure 3).
The assignment of the mesophase type was carried out by
XRD. The XRD pattern of the coumarin-containing second
generation dendron (N
3
-d
2
Cou) showed only diffuse scattering,
suggesting the absence of long-range positional order in the LC
phase (Figure S18). This kind of pattern corresponds to a
nematic mesophase which has only orientational order. This
nematic behavior can be explained by taking into account that
the flexibility of the ester bonds allows adopting a rod-shaped
geometry that locates the two dendrons in opposite
directions.
35
On the other hand, functionalization of the
porphyrin core with the coumarin-containing dendrons gives
rise to disk-like structures.
36-38
Thus, the XRD pattern recorded
for ZnP-d
1
Cou is characteristic of a discotic nematic (N
D
)
mesophase. It displayed one relatively strong reflection in the
low-angle region corresponding to the presence of a certain
degree of short-range columnar order within the nematic
phase, as has been previously described in other N
D
phases,
39-
41
and a broad halo in the high-angle region which corresponds
to the liquid-like order of the disordered alkyl chains (Figure
4a). The calculated d-spacing (26 Å) for the signal in the low-
angle region represents the average inter-disk distance and
approximates the diameter of the porphyrin core. The spacing
from the high-angle diffraction is 4.6 Å, which mainly
originates from the liquid-like correlations between the
conformationally disordered alkyl chains.
On the other hand, the XRD pattern of ZnP-d
2
Cou only showed
diffuse scattering in the low-angle region, and a broad diffuse
halo in the high-angle region (Figure 4b). This pattern also
confirms the presence of a N
D
mesophase.
42
It is noteworthy
the disappearance of the intense maximum in the low-angle
region in the XRD pattern of ZnP-d
2
Cou (Figure 4c). The
greater number of alkyl chains in ZnP-d
2
Cou could be
responsible for these differences of XRD, since they may
hinder the lateral disk-disk interactions between
metalloporphyrin units, and consequently, prevent the
formation of columnar alterations within the N
D
phase.
With ZnP-d
1
C12, the DSC curve showed an exothermic peak at
122 ºC (cold crystallization) and an endothermic peak
corresponding both to the crystal-to-mesophase transition and
the transition from mesophase to isotropic liquid. These two
transitions were observed by POM and detected as only one
endothermic peak in the DSC which corresponds to the sum of
the two transition peaks, and thus the nature of the
mesophase could not be identified by XRD. These results
confirm the key role of the coumarin terminal units in the
mesogenic behavior of these materials leading to the
apparition of the mesophase at room temperature and
increasing significantly the mesophase range. Both features
are fundamental parameters for their practical applications in
organic electronics.
Figure 4. (a) Room temperature XRD pattern of ZnP-d
1
Cou, (b) room temperature XRD pattern of ZnP-d
2
Cou, (c) 1D profiles of
the XRD patterns.
(c)
(b)
(d)
(a)
10
Intensity (a. u.)
q (nm
-1
)
ZnP-d
1
Cou
ZnP-d
2
Cou
(b)(a) (c)
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Table 2. Photophysical data in DCM solutions and in thin film.
λ
abs
(nm)
Coumarin
λ
abs
(nm)
Soret band
λ
abs
(nm)
Q-bands
λ
em
(nm)
Coumarin
λ
em
(nm)
Q (0‒1)
λ
em
(nm)
Q (0‒2)
Φ
F
[a]
Φ
ET
ZnP-YNE
DCM
‒
426
558, 599
-
600
644
0.09
‒
film
‒
426
560, 602
‒
606
651
‒
‒
N
3
-d
1
Cou
DCM
324
‒
‒
388
‒
‒
‒
‒
film
314
‒
‒
411
‒
‒
‒
‒
N
3
-d
2
Cou
DCM
323
‒
‒
388
‒
‒
‒
‒
film
314
‒
‒
411
‒
‒
‒
‒
ZnP-d
1
Cou
DCM
322
426
552, 595
383
603
643
0.12
16%
film
322
434
562, 603
‒
[b]
605
649
‒
‒
ZnP-d
2
Cou
DCM
321
427
552, 594
383
601
642
0.11
23%
film
322
436
563, 604
‒
[b]
605
648
‒
‒
ZnP-d
1
C12
DCM
‒
417
540
‒
600
644
0.14
‒
film
‒
424
544
‒
611
650
‒
‒
[a]
The Φ
F
values were calculated from DCM solutions with tetraphenylporphyrin (0.11 in benzene) as standard, excitation at 420
nm. [b] Not detectable.
Absorption and emission properties.
The UV-Vis absorption and fluorescence spectra of porphyrin-
core dendrimers and precursors were collected on dilute
solutions (10
-5
to 10
-7
M) in DCM, and in thin films prepared by
casting DCM solutions onto quartz substrates. Relevant data
are presented in Table 2.
The two coumarin-containing dendritic precursors, N
3
-d
1
Cou
and N
3
-d
2
Cou, showed an absorption band in DCM solution at
324 and 323 nm respectively, and an emission band at 388 nm.
Interestingly, in thin film the absorption bands showed a
hypochromic shift of about 10 nm, while the emission band
showed a significant bathochromic shift of 23 nm probably due
to the aggregation effect. The porphyrin precursor ZnP-YNE
showed in the UV-visible spectrum a Soret band at 426 nm,
and two Q bands at 558 and 599 nm. Excitation of the Soret
band resulted in the apparition of two emission bands at 606
(Q(0-1)) and 651 nm (Q(0-2)).
On the other hand, the UV-Vis absorption spectra of the three
porphyrin-core dendrimers in DCM solution are, within
experimental error, a combination of the spectra of the alkyne
functionalized porphyrin and the corresponding azido
functionalized dendron (Figure 5a). The absorption bands
corresponding to the coumarin units showed similar values
both in solution and in thin film, whereas the absorption bands
corresponding to the porphyrin core in thin film exhibited a
bathochromic shift of about 10 nm in relation to those in
solution, due to the aggregation effect.
The fluorescence emission spectra of the three porphyrin-core
dendrimers (λ
exc
= 425 nm) consisted of two bands with a Q(0-
1)-band at around 600 nm and a Q(0-2)-band at around 645
nm (Figure 5a). The fluorescence quantum yields were also
measured with tetraphenylporphyrin as standard (Φ
F
= 0.11 in
benzene), obtaining low quantum yields. The fluorescence
spectra were also recorded in thin films. Compared to the data
from DCM solutions, the emission Q(0-1) and Q(0-2) peaks are
red-shifted ca. 5-10 nm.
Figure 5. a) UV-Vis absorption spectra in DCM solution (black
line) and emission spectra (λ
exc
= 425 nm) in DCM solution
(blue line) and in thin film (red line) of ZnP-d
1
Cou. b) UV-Vis
absorption spectra in DCM solution (black line) and emission
spectra (λ
exc
= 320 nm) in DCM solution (blue line) and in thin
film (red line) of ZnP-d
1
Cou.
Nonetheless, when peripheral coumarin units of the
dendrimers were excited at 320 nm, significant differences in
200 300 400 500 600 700 800
Coumarin band
Soret band
Q(0-2)
Q(0-1)
Intensity
Absorbance
Wavelenght (nm)
200 300 400 500 600 700 800
Coumarin band
Q(0-2)
Intensity
Absorbance
Wavelenght (nm)
Soret band
Q(0-1)
(a)
(b)
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