Carbazol-N-yl and diphenylamino end-capped triphenylamine-based molecular glasses: Synthesis, thermal, and optical properties

Article abstract of DOI:10.1016/j.tetlet.2013.05.152

Novel low-molecular-weight amorphous organic glasses based on triphenylamine core end-capped by either di(4-methoxyphenyl)amino or 3,6-dimethoxycarbazol-N-yl groups have been synthesized and characterized. The molecules exhibit high glass-transition tempe

Full text of DOI:10.1016/j.tetlet.2013.05.152

Tetrahedron Letters 54 (2013) 4277–4280
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Carbazol-N-yl and diphenylamino end-capped triphenylamine-based
molecular glasses: synthesis, thermal, and optical properties
⇑
Thanh-Tuan Bui, Layla Beouch, Xavier Sallenave, Fabrice Goubard
Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI), Université de Cergy-Pontoise, 5 mail Gay Lussac, 95031 Neuville-sur-Oise, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel low-molecular-weight amorphous organic glasses based on triphenylamine core end-capped by
either di(4-methoxyphenyl)amino or 3,6-dimethoxycarbazol-N-yl groups have been synthesized and
characterized. The molecules exhibit high glass-transition temperatures (74–151 °C), excellent thermal
stability, and are soluble in common organic solvents. Moreover, most of them absorb mainly in UV
domain. These star-shaped triphenylamine derivatives are promising hole transporting materials for
solid-state dye-sensitized solar cell applications.
Received 29 April 2013
Revised 27 May 2013
Accepted 31 May 2013
Available online 10 June 2013
Keywords:
Molecular glasses
Ó 2013 Elsevier Ltd. All rights reserved.
Triphenylamine
Arylamine
Hole transporting materials
Solid-state dye-sensitized solar cells
Introduction
as hole conductor in solid-state DSSCs.9 In the present work, we
focus on the design and the synthesis of new p-type amorphous or-
ganic semiconductors based on TPA core with short p-conjugated
Nowadays, dye-sensitized solar cells (DSSCs)1 have been becom-
ing one of the most promising technologies in the field of renewable
energies thanks to their low weight, to the possibility of making
large flexible surfaces, and also to their photovoltaic efficiency
which are found to be more and more significant.2 One of the man-
ufacturing challenges for DSSCs is the need for a robust sealing pro-
cess that would prevent the liquid electrolyte in the cells from
leakage and evaporation. Moreover, the electrolyte, the iodide/trii-
odide redox couple, is quite corrosive to the metallic electrodes.
To overcome these problems, one of the promising solutions is
replacing the liquid electrolytes with low-molecular-weight
pathways in each branch. Structurally, each branch of these new
symmetrical star-shaped molecules is end-capped by either
di(4-methoxyphenyl)amino or 3,6-dimethoxycarbazol-N-yl. These
electron rich end-capping groups will help to modulate electro-
chemical properties of targeted compounds. Methoxy groups will
enhance pore filling properties of these molecules by (i) rendering
them more soluble in organic solvents and (ii) making them more
hydrophilic and increasing dye-sensitized TiO2/hole transporter
affinity. Additionally, the presence of methoxy groups in para posi-
tions of phenyl rings of the diphenylamino moiety should favor
charge transport properties.10 The structures of these compounds
are given in Figure 1.
amorphous organic
p-conjugated semiconductors to make
solid-state DSSCs.3 Despite their tendency to crystallization, small
molecules can form stable amorphous materials above ambient
temperature if their structures are meticulously designed4 and
become attractive hole conductors for solid-state DSSCs.5 Among
them, triphenylamine (TPA) derivatives having pseudo-three-
dimensional conjugated architecture constitute an attractive class
of material thanks to their interesting properties, including good
charge transport, good thermal, and morphological stability.6,7 Re-
cently, we have reported a series of star-shaped molecules derived
from thieno[3,2-b]thiophene unit and TPA core.8 Experimental and
theoretical investigations have shown that these visible-light-
harvesting molecular glasses have suitable characteristics for use
Results and discussion
The synthetic routes toward different targeted molecules are
outlined in Figure 2. All compounds were synthesized from
commercially available tris(4-bromophenyl)amine (TPA-Br).
Compounds 1–5 were obtained from their respective tribrominat-
ed TPA-based cores with either di(4-methoxyphenyl)amine or 3,
6-dimethoxycarbazole11 via threefold Pd-catalyzed Buchwald–
Hartwig amination.12 1 and 2 were synthesized from TPA-Br while
3 and 4 were obtained from tris(4-(5-bromothiophen-2-yl)pheny-
l)amine 7-Br.8 1 and 3 were isolated in high yields (95% and 78%,
respectively) after purification. However, 2 and 4 were obtained
in moderate yields (47% and 25%, respectively). These low yields
⇑
Corresponding author. Tel.: +33 1 34 25 70 22.
E-mail address: fabrice.goubard@u-cergy.fr (F. Goubard).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.152

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T.-T. Bui et al. / Tetrahedron Letters 54 (2013) 4277–4280
Figure 1. Molecular structures of compounds 1–6.
Figure 2. Synthetic routes toward compounds 1–6.

T.-T. Bui et al. / Tetrahedron Letters 54 (2013) 4277–4280
4279
Table 1
inserted. Simultaneously, the molar extinction coefficients were
doubled in accordance with the increasing of conjugation lengths.
The high
Thermal and optical properties of compounds 1–6
e
values of these compounds (70,000–94,000 L molÀ1 cm
d
e
Compound MW
g/mol
kmax
nm
konset
nm
e
(kmax
)
Egoptc Tg
Td
L molÀ1 cmÀ1 eV
°C
°C
À1) can be reasoned as a combined effect of high molecular weights
and of the presence of multi conjugated branches in each molecule.
Thermal behaviors of six synthesized compounds were then
investigated by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) under argon atmosphere. The temper-
atures corresponding to 5% weight loss (Td) and the glass transition
temperatures (Tg) were summarized in Table 1. Synthesized com-
pounds showed significant thermal stability with Td values in the
range of 420–440 °C in most cases. The Td of 5 is 313 °C which is
significantly lower than that of 3 (428 °C). This can be attributed
to the presence of a methyl moiety on the thiophene ring in the
1
2
3
4
5
6
927.09
921.05
354a 400
338a 396
41000
37000
85000
70000
55000
94000
3.10
—
437
439
428
3.13
2.76
2.88
2.85
2.86
—
1173.47 406b 449
1167.42 382b 430
1215.54 382b 435
1377.76 392b 434
99
134 426
74 313
151 424
a
b
c
In 1,2-dichlorobenzene.
In THF.
Egopt = hc/konset
.
d
e
determined from DSC analyses at heating rate: 20 °C/min.
determined from TGA analyses at heating rate: 20 °C/min.
a-position of phenyl-thiophene bond. Methyl moieties cause steric
hindrance and thus decrease thermal stability of corresponding
compound. However, the thermal stability of all compounds is
good enough for the applications in optoelectronic devices. Glass
forming properties of 1–6 were then investigated by DSC. In our
experiments, no glass transition was detected in the range of
0–200 °C for compounds 1 and 2. Other compounds (3–6) exhibit
glass transition temperatures (Tg) ranging from 74 to 151 °C and
their DSC curves were shown in Figure S2 in the Supplementary
data. Compound 3 exhibits the glass transition at 99 °C. The
introduction of a methyl moiety on the thiophene significantly
decreases its Tg (74 °C). Structurally, 3 differs from 4 by their end-
capping moieties. The replacement of di(4-methoxyphenyl)amino
in 3 by more rigid 3,6-dimethoxycarbazol-N-yl in 4 significantly in-
creases the corresponding Tg, (134 °C vs 99 °C). When 3-methylt-
are consequence of incomplete multiple cross-coupling reactions
leading to crude products containing complex mixtures of mono-
and di-coupling derivatives. In case of 5, the desired product was
obtained from tris(4-(5-bromo-2-methylthiophen-2-yl)phenyl)
amine 8-Br with di(4-methoxyphenyl)amine in good yield (61%).
A stille coupling between 2-tributylstannylthiophene and TPA-Br
gave 7 (75% yield) which was then brominated leading to tris
(4-(5-bromothiophen-2-yl)phenyl)amine 7-Br7 in good yield
(90%). The treatment of tris(4-(4,4,5,5-tetramethyl-[1,3,2]dioxa-
borolane)phenyl)amine13 with 2-bromo-3-methylthiophene14
under Suzuki cross-coupling conditions afforded tris(4-(3-methyl-
thiophen-2-yl)phenyl)amine 8 in high yield (94%). 8 was then bro-
minated before being converted into targeted compound 5.
Compound 6 was constructed in a different way starting from
2-bromo-3-methylthieno[3,2-b]thiophene which was prepared as
described for its analogues.15 The copper-catalyzed Ullmann
N-arylation conditions16 were then used to link 2-bromo-3-
hienothiophene was used as p-conjugated bridge (compound 6)
in place of thiophene (in 4), the rigidity increased leading to higher
Tg (151 °C vs 134 °C of 4, Table 1,). The compounds 3–6 were
obtained as amorphous materials as confirmed by DSC.
methylthieno[3,2-b]thiophene
and
3,6-dimethoxycarbazole
yielding 9 (73% yield), which was subsequently treated with NBS
providing 9-Br. The final molecule 6 was then achieved through
a threefold Suzuki coupling between 9-Br and tris(4-(4,4,5,5-tetra-
methyl-[1,3,2]dioxaborolane)phenyl)amine and isolated in moder-
ate yield (49%). Except 1 and 2, all other compounds have good
solubility in common organic solvents, such as dichloromethane,
chloroform, tetrahydrofuran, (di)chlorobenzene, and toluene.
Optical properties of synthesized compounds (1–6) were exam-
ined in dilute solutions. UV–vis absorption spectra were given in
the Supplementary data and related data are summarized in Table
1. Compounds 1 and 2 have the shortest kmax of absorption (354
and 338 nm, respectively) in this group due to limited conjugation
Conclusions
In summary, synthesis, thermal, and optical properties of novel
di(4-methoxyphenyl)amino and 3,6-dimethoxycarbazol-N-yl end-
capped triphenylamine-based molecular glasses were investigated
in detail. The synthetic routes allowed the preparation of highly sol-
uble compounds using short multistep syntheses starting from
tris(4-bromophenyl)amine in relatively good yields. Thanks to their
high glass transitiontemperatures (Tg varies from 74 to 151 °C), such
low-molar-weight compounds are able to stay in amorphous state
allowing the realization of a thin film with good contact with the
nanostructured inorganic layer. Moreover, most of them absorb
mainly in UV domain. These properties make them being promising
candidates for solid-state dye-sensitized solar cell applications.
in the molecular structures. The insertion of
TPA core and end-capping groups increases the delocalization of
electrons in the targeted molecules. As a consequence, the struc-
p-bridges between
p
ture of these new compounds involves a bathochromic shift in
comparison with 1 and 2. For instance, the insertion of a thiophene
Acknowledgement
unit as p-bridge in 3 and 4 causes a shift of 52 nm (vs 1) and 32 nm
This work was financed by the French National Research
Agency (ANR) as part of FMOCSOLE (Blanc SIMI 9 2010) project.
(vs 2), respectively, (Table 1). Adding a methyl moiety on the
thiophene in the adjacent position of phenyl-thiophene bond in
compound 5 causes higher deviation from planarity in comparison
with 3 resulting in a hypsochromic shift of 24 nm. Replacement of
Supplementary data
the thiophene in 4 by more conjugated p-bridge, thienothiophene
Supplementary data (full experimental detail, DSC curves and
UV–vis spectra of compounds 1–6) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2013.05.152.
in 6, causes a slightly bathochromic shift (10 nm vs 4). Obviously,
on the same TPA-based core, diphenylamino end-capping group
causes bathochromic shift in comparison with its analogue
carbazol-N-yl one (1 vs 2:
Dkmax = 16 nm; 3 vs 4: Dkmax = 24 nm).
The corresponding optical band gaps, which were determined
from the edge of the absorption bands, are summarized in Table
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