Molecular engineering of the internal charge transfer in thiophene-triphenylamine hybrid π-conjugated systems

Article abstract of DOI:10.1021/jo701390y

(Chemical Equation Presented) Introduction of electroaccepting groups at the periphery of triphenylamine-based derivatives leads to an internal charge-transfer band. Syntheses and spectroscopic, electrochemical, and theoretical studies of various derivati

Full text of DOI:10.1021/jo701390y

Molecular Engineering of the Internal Charge Transfer in
Thiophene-Triphenylamine Hybrid π-Conjugated Systems
Philippe Leriche,* Pierre Fre`re, Antonio Cravino, Olivier Ale´veˆque, and Jean Roncali
Groupe Syste`mes Conjugue´s Line´aires, CIMMA, UMR CNRS 6200, UniVersite´ d’Angers,
2 Bd LaVoisier, F-49045 Angers, France
philippe.leriche@uniV-angers.fr
ReceiVed June 27, 2007
Introduction of electroaccepting groups at the periphery of triphenylamine-based derivatives leads to an
internal charge-transfer band. Syntheses and spectroscopic, electrochemical, and theoretical studies of
various derivatives which differ by the strength and the number of electroacceptor groups are presented.
These various results show that the ICT band and the acceptor/donor abilities of derivatives can be finely
tuned.
Introduction
acenes6 or thiophene-based oligomers or polymers.4,5,7-9 How-
ever, due to their one-dimensionality, these materials present
anisotropic optical and charge transport properties7-12 which
pose specific problems for device fabrication. Thus, whereas a
vertical orientation of the conjugated chains on the substrate
improves the mobility of charge carriers in organic field-effect
transistors,10,11 such an orientation is detrimental for solar cells
as it strongly reduces the absorption cross-section of the incident
light as well as charge transport across the cell.7-9
Organic solar cells are subject to a considerable current
interest motivated by the perspective of achieving large area,
low-cost, lightweight, and flexible power sources.1-7 Based on
the intensive research effort invested in the optimization of
multilayered and bulk heterojunctions solar cells during the past
decade, power conversion efficiencies in the 4-5% range have
been reached.4,5
Many materials used as donors in organic heterojunction solar
cells are derived from linear π-conjugated systems such as
Organic glasses derived from triphenylamine (TPA)-based
compounds have been widely used as hole-transporting or
electroluminescent material in OLEDs.13 On the other hand, the
amorphous character of these materials offers the possibility to
develop organic semiconductors with isotropic optical and
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10.1021/jo701390y CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/03/2007
8332
J. Org. Chem. 2007, 72, 8332-8336

Internal Charge Transfer in Thiophene-Triphenylamine Systems
CHART 1
charge-transport properties. In the past few years, several groups
have developed hybrid systems obtained by fixation of conju-
gated chains on a TPA core as active materials for the fabrication
of OFETs or solar cells.14 However, the absorption maximum
of these materials in the 400-450 nm region is clearly
insufficient to ensure an efficient harvesting of solar light. A
possible solution to this problem consists in the covalent grafting
of electron-acceptor groups on the hybrid TPA-conjugated
system in order to increase the absorption in the visible region.15
In this context, we have shown that the creation of an
intramolecular charge transfer (ICT) by introduction of electron
acceptors such as indan-1,3-dione or dicyanovinyl groups at the
periphery of the hybrid TPA-thiophene systems leads to a
considerable improvement of the performances of the resulting
bilayer or bulk heterojunction solar cells thanks to the simul-
taneous increase of the open-circuit voltage and extension of
the spectral response.16
acceptor groups on the electronic properties of the system have
been analyzed on compounds 1-4, whereas the effect of the
number of tricyanovinyl acceptor groups was analyzed on
compounds 4-6 from both experimental and theoretical view-
points.
Results and Discussion
Synthesis. Trialdehyde 7 was synthesized according to the
already reported method.16a Compounds 1-3 have been syn-
thesized by condensation of trialdehyde 7 with indandione,
thiobarbituric acid, and malonodinitrile, respectively. The reac-
tion carried out at room temperature in chloroform and in
presence of triethylamine leads to derivatives 2 and 3 in good
yields (Scheme 1). The compounds were precipitated by addition
of methanol to the reaction mixture. The synthesis of 1 was
much more complicated because of the polycondensation
reactions of indandione,17 and it was only after several purifica-
tions by chromatography that compound 1 could finally be
isolated in ca. 1% yield.
We report here a further analysis of the structure-electronic
property relationships in a new series of TPA-based compounds
with ICT (1-6, Chart 1). The effects of different types of
Compounds 4-6 substituted by one, two, and three tricy-
anovinyl groups were synthesized following two routes.18 The
first involves the reaction of 8 with an excess of tetracyanoet-
hylene at 60 °C in DMF. Successive formation of 6, 5, and 4
(14) (a) Saragi, T. P. I.; Fuhrmann-Lieker, T.; Salbeck, J. Synth. Met.
2005, 148, 267. (b) Sonntag, M.; Kreger, K.; Hanft, D.; Strohriegl, P.;
Setayesh, S.; de Leeuw, D. Chem. Mater. 2005, 17, 3031. (c) Cravino, A.;
Roquet, S.; Ale´veˆque, O.; Leriche, P.; Fre`re, P.; Roncali, J. Chem. Mater.
2006, 18, 2584.
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Funct. Mater. 2004, 14, 83. (b) Sun, X.; Liu, Y.; Xu, X.; Yang, C.; Yu, G.;
Chen, S.; Zhao, Z.; Qiu, W.; Li, Y.; Zhu, D. J. Phys. Chem. B. 2005, 109,
10786. (c) Cremer, J.; Ba¨uerle, P. J. Mater. Chem. 2006, 16, 874.
(16) (a) Roquet, S.; Cravino, A.; Leriche, P.; Ale´veˆque, O.; Fre`re, P.;
Roncali, J. J. Am. Chem. Soc. 2006, 128, 3459. (b) Cravino, A.; Roquet,
S.; Ale´veˆque, O.; Leriche, P.; Roncali, J. AdV. Mater. 2006, 18, 3033.
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(b) Jonescu, M. Ber. Dtsh. Chem. Ges. 1927, 60, 1228.
J. Org. Chem, Vol. 72, No. 22, 2007 8333

Leriche et al.
SCHEME 1. Synthetic Procedures for Synthesis of
Derivatives 1-6
FIGURE 1. UV-vis absorption spectra of compounds 4 (bold dashed),
5 (solid line), and 6 (dashed) in methylene chloride.
TABLE 1. UV-vis Absorption Data (in CH2Cl2) and Cyclic
Voltammetric Data (in 0.10 M Bu4NPF6/CH2Cl2, Scan Rate 100 mV
s-1, Ref SCE) for Compounds 1-6
λ π-π* λ ICT Epa (V)
Epr (V)
∆Eopb (eV) ∆Eelc (eV)
was followed by TLC. The best results were obtained after 1
week of reaction. Compounds 6 and 5 were isolated in 50 and
29% yield, respectively, while compound 4 was only observed
in trace amounts. Increasing temperature, reaction time, and
TCNE quantities only led to degradation. Compound 4 was
finally obtained in 85% yield by treatment of compound 3
substituted by three dicyanovinyl groups, with successively
NaCN and Pb(OAc)4 in acidic conditions. All new compounds
were isolated as glassy solids and satisfactorily characterized
by NMR and mass spectrometry.
1
2
3
4
5
6
396
420
368
417
422
426
528
557
538
613
632
633
0.91
1.05
1.06
1.19
1.00
0.88
-1.11a
-0.95a
-1.23a
-0.56
-0.63
-0.64
2.35
2.23
2.30
2.02
1.96
1.96
2.02
2.00
2.29
1.75
1.63
1.52
a Irreversible process. b Optical gap based on λICT
≈ Epa - Epr.
.
c Electrochemical gap
presence of tetrabutylammonium hexafluorophosphate 0.10 M
as supporting electrolyte. The cylic voltammogram of all
compounds shows a reversible oxidation process indicative of
stable cation radical. As shown in Table 1, the potential of the
anodic peak (Epa) of the trisubstituted compounds increases with
the strength of the acceptor groups and shifts from 0.91 V for
indandione to 1.19 V for the strongest tricyanovinyl acceptor.
Comparison of the data for compounds 4-6 shows that the
increase of the number of tricyanovinyl acceptor groups from
1 to 3 leads to a 0.31 V positive shift of Epa. As for oxidation,
all compounds can be reduced at a potential which depends on
the strength and on the number of acceptor groups. Whereas
compounds 1-3 undergo irreversible reduction, the process is
fully reversible for compounds 4-6 (Figure 2). As shown in
Table 1, the potential of the corresponding cathodic peak (Epr)
shifts toward more positive values from -0.64 to -0.56 V when
the number of acceptor groups increases. The deconvoluted CV
of compounds 4-6 (Figure 2) show that the intensity of the
reduction wave relatively to that of the oxidation one increases
with the number of acceptor groups. Assuming a monoelectronic
oxidation process, integration of the reduction wave shows that
the reduction process involves one, two, and three electrons for
compounds 6, 5, and 4 respectively.
UV-vis Spectroscopy. The UV-vis spectrum of all com-
pounds presents a first absorption band in the 370-430 nm
region followed by a second transition at longer wavelengths.
Based on the spectrum of reference compounds devoid of
acceptor group,14c the first absorption band can be assigned to
a π-π* transition and the second band to an intramolecular
charge-transfer (ICT) transition between the TPA-thiophene
donor block and the peripheral acceptors groups. The UV-vis
data for the fully substituted compounds 1-4 show that the
maximum of the ICT band increases from 528 nm for compound
1 substituted by indanedione acceptors to 613 nm for compound
4 bearing the tricyanovinyl groups. These results show that, as
expected, the absorption maximum of the ICT band reflects the
relative strength of the acceptor groups. Comparison of the data
for compounds 6, 5, and 4 shows that increasing the number of
tricyanovinyl acceptor groups produces a decrease of the
intensity of the bands in the 300-450 nm region together an
increase of the relative intensity of the ICT transition around
600 nm (Figure 1). As appears in Figure 1, these changes are
accompanied with a hypsochromic shift of the ICT band. This
later result probably reflects changes in the dipole moment of
the molecules, but this question clearly requires further work.
Electrochemical Experiments. Compounds 1-6 have been
analyzed by cyclic voltammetry in methylene chloride in the
Theoretical Calculations. The electronic properties of
compounds 4-6 have been investigated by means of theoretical
calculations. Ab initio quantum calculations have been per-
formed with the Gaussian 03 package of programs at a hybrid
density functional theory (DFT) level. Calculations were done
using the restricted B3LYP/6-31G(d)* functions. HOMO and
(18) McKusick, B. C.; Heckert, R. E.; Cairns, T. L.; Coffman, D. D.;
Mower, H. F. J. Am. Chem. Soc. 1958, 80, 2806. (b) Lambert, C.; Gashner,
W.; Schma¨lklin, E.; Meerholz, K.; Bra¨uchle, B. J. Chem. Soc., Perkin Trans.
1999, 2, 577.
8334 J. Org. Chem., Vol. 72, No. 22, 2007

Internal Charge Transfer in Thiophene-Triphenylamine Systems
with electrochemical results. For compounds 5 and 4, the
localization of the LUMO at the terminal acceptor groups leads
to a 2- and 3-fold degenerated LUMO, respectively (Figure 3).
This indication of independent terminal acceptor groups agrees
well with the two- and three-electron reduction processes
indicated by electrochemical results for compounds 5 and 4. In
contrast, the persistence of a relatively delocalized HOMO for
all compounds is consistent with the observed one-electron
oxidation process. Calculations also show that the HOMO-
LUMO gap ∆E increases with the number of electro-acceptor
groups on the molecule, in agreement with optical and electro-
chemical results.
Conclusions
Star-shaped conjugated systems consisting of a hybrid TPA-
thiophene core substituted by electron-acceptor groups have been
synthesized. Optical and electrochemical data and theoretical
calculations provide a coherent picture showing that the
electronic properties of this class of compounds can be
controlled over a wide range by varying the nature and the
number of acceptor groups. Comparison of the characteristics
of compounds containing indandione, barbiturate, and dicyano-
and tricyanovinyl acceptor groups shows that the latter leads to
the systems with the smallest HOMO LUMO gap and the lowest
LUMO level.
The analysis of the effect of the number of tricyanovinyl
groups shows that increasing the number of acceptor groups
leads to a decrease of the HOMO and LUMO levels and to an
increase of the energy gap. On the other hand these three
compounds present a reduction potential higher than that of
fullerene C60. While these results show that these compounds
cannot be used as donor in bulk heterojunction solar cells, they
suggest that they could eventually behave as n type or as
ambipolar organic semiconductors. Work aiming at the confir-
mation of this hypothesis is now underway and will be reported
in future publications.
Experimental Section
Synthesis of N,N,N-Tris[4-[5-(1,3-dioxoindan-2-ylidenemethyl)-
2-thienyl]phenyl]amine (1). To a solution of tris[4-(5-formyl-2-
thienyl)phenyl]amine 7 (0.2 g 0.35 mmol) in 10 mL of CH2Cl2
were added 3.3 equiv of indandione and one drop of triethylamine.
After 24 h of stirring, 3 equiv of indandione were added, and the
mixture was stirred again for 24 h. Addition of petroleum ether
led to the precipitation of a crude product which was dissolved in
CH2Cl2. The organic phase was washed with 0.10 M aqueous
sodium hydroxide and water and dried over magnesium sulfate.
After solvent removal, the mixture was chromatographed twice on
a silica gel column using CH2Cl2 as eluent and once on a preparative
plate using the same eluent. Pure 6 was obtained in ca. 1% yield
as a purple glassy solid. 1H NMR (CDCl3) attributed by 2D
COSY: 7.98 (m, 12H, 4, 5, 6 and 6′), 7.79 (m, 6H, 7 and 7′), 7.75
FIGURE 2. Deconvoluted CV of compounds 6, 5, and 4 (from top to
bottom) in CH2Cl2, TBAPF6 0.1 M 100mV/S vs SCE.
LUMO levels of neutral molecules 4-6 are shown in Figure 3
(larger pictures of HOMO-LUMO levels are available in ESI).
As generally observed for TPA derivatives, the three N-C
bonds are coplanar which is characteristic of enamines and
indicates a certain degree of conjugation between the arms.
Steric hindrance between the phenyl rings grafted on the central
nitrogen atom generates the typical propeller shape of the
molecule. The HOMO is mainly located on the central node
with a significant contribution of the unsubstituted branches in
5 and 6. The LUMO is localized on the peripheral electron-
withdrawing groups and on the adjacent thiophene rings.
Distributions of HOMO and LUMO levels are separated in all
compounds indicating that the HOMO-LUMO transition can
be consider as a charge-transfer transition. As expected, the
HOMO and LUMO levels decrease when increasing the number
of electron-withdrawing groups on the molecule, in agreement
3
3
(d, 6H, 2, J ) 8.5 Hz), 7.45 (d, 3H, 3, J ) 4 Hz), 7.23 (d, 6H,
3
3, J ) 8.5 Hz). 13C NMR (CDCl3): 190.4, 189.8, 156.8, 147.5,
143.8, 142.0, 140.5, 136.3, 136.2, 135.0, 134.8, 128.7, 127.8, 124.7,
124.3, 123.7, 123.0, 122.8. MS (MALDITOF): C60H33NO6S3 M+
960.0. HRMS: M+• (calcd) (959.1470) 959.1495.
Synthesis of Compounds 2 and 3. To a solution of tris[4-(5-
formyl-2-thienyl)phenyl]amine 7 (0.2 g, 0.35 mmol) in 10 mL of
CH2Cl2 were added 3.3 equiv of malonodinitrile or thiobarbiburic
acid and one drop of triethylamine. The condensation was followed
by TLC: mono-, di-, and trifunctionnalized compounds appeared
successively. After 4 h (for 3) to 10 h (for 2) of reaction, addition
of petroleum ether allowed the precipitation of a crude product.
J. Org. Chem, Vol. 72, No. 22, 2007 8335

Leriche et al.
FIGURE 3. Calculated HOMO and LUMO levels for compounds 4-6.
Compound 3 was isolated pure in 90% yield as a red solid. In the
case of 2, the crude product was dissolved in CH2Cl2 and washed
with an aqueous solution of sodium hydroxide 0.1 M and then three
times with water after the organic phase was dried on magnesium
sulfate. After evaporation of the solvent, the pure derivative 2 was
isolated in 85% yield as a purple glassy solid.
N,N,N-Tris[4-[5-(2-thioxodihydropyrimidin-4,5-oxo-5-yliden-
emethyl)-2-thienyl]phenyl]amine (2). 1H NMR (CDCl3): 8.67 (s,
3H), 7.89 (d, 3H, 3J ) 4.5 Hz), 7.78 (d, 6H, 3J ) 8.5 Hz), 7.51 (d,
3H, 3J ) 4 Hz), 7.21 (d, 6H, 3J ) 8.5 Hz), 4.61 (m, 12H), 1.34 (m,
18H). 13C NMR (CDCl3): due to very poor solubility, several
signals were not observed (see the Supporting Information). MS
(MALDITOF): C57H51N7O6S6 M+ 1121.0. HRMS: M+• (calcd)
(1121.2225) 1121.2213.
week at 60 °C under nitrogen atmosphere followed by addition of
100 mL of methylene chloride. The organic phase was washed 10
times with water and dried over magnesium sulfate. After evapora-
tion of the solvent, the residue was chromatographed on silica gel
using methylenechloride/petroleum ether 1/1 (compound 1) and then
methylenechloride/EP 3/1 (compound 2) as eluent. Compounds 1
and 2 were isolated in 50 and 29 yields, respectively, as green glassy
solids. Note that compound 3 was observed in trace amounts.
N-[4-(5-Tricyanoethenyl-2-thienyl)phenyl]-N,N-bis[4-(2-thie-
1
3
nyl)phenyl]amine (6). H NMR (CDCl3): 8.03 (d, 1H, J ) 4.5
3
3
Hz), 7.59 (d, 2H, J ) 9 Hz), 7.58 (d, 4H, J ) 8.5 Hz), 7.44 (d,
1H), 7.29 (m, 4H, 3J ) 4.5 Hz), 7.17 (d, 2H, 3J ) 9 Hz), 7.11 (d,
2H, 3J ) 8.5 Hz), 7.09 (dd, 2H, 3J ) 5, 3.5 Hz). 13C NMR
(CDCl3): 160.0, 150.3, 145.1, 143.5, 141.8, 131.9, 131.7, 131.1,
128.2, 128.1, 127.2, 125.9, 124.9, 124.2, 123.0, 121.7, 112.9, 112.8,
112.5, 80.1. MS (MALDITOF): C35H20N4S3 MS M+ 592.1.
HRMS: M+• (calcd) (592.0850) 592.0833.
N,N,N-Tris[4-[5-(2,2-dicyanoethen-1-yl)-2-thienyl]phenyl]-
amine (3). Spectroscopic data are identical to those already
published.16
Synthesis of N,N,N-Tris[4-(5-tricyanoethenyl-2-thienyl)phe-
nyl]amine (4). To a solution of compound 3 (60 mg, 0.083 mmol)
dissolved at 0 °C under nitrogen atmosphere in 5 mL of dry DMF
was added 5 equiv of NaCN dissolved in 2 mL of 1:1 water-
DMF. The mixture turned brown immediately. After 15 min of
stirring at 0 °C, two drops of acetic acid and one drop of
concentrated hydrochloric acid were added followed by portionwise
addition of 3 equiv of Pb(OAc)4, and the solution turned blue. The
mixture was stirred for 2 h at 0 °C, and 100 mL of water was
added. The suspension was filtered on a Hyflo supercell. The solid
collected on the top of the column was dissolved in methylene
chloride and the solution dried over magnesium sulfate. After
evaporation of the solvent, the residue was chromatographed on
silica gel (eluent 3:1 CH2Cl2/PE) to give 65 mg (85%) of a blue
N,N-Bis[4-(5-tricyanoethenyl-2-thienyl)phenyl]-N-[4-(2-thie-
1
3
nyl)phenyl]amine (5). H NMR (CDCl3): 8.06 (d, 2H, J ) 4.5
Hz), 7.65 (d, 4H, 3J ) 8.5 Hz), 7.62 (d, 2H, 3J ) 8.5 Hz), 7.48 (d,
2H, 3J ) 4.5 Hz), 7.32 (m, 2H), 7.21 (d, 4H, 3J ) 9 Hz), 7.17 (d,
2H, 3J ) 8.5 Hz), 7.11 (dd, 1H, 3J ) 5, 3.5 Hz). 13C NMR
(CDCl3): 159.0, 149.1, 144.4, 143.1, 141.6, 132.3, 132.2, 132.0,
128.4, 128.3, 127.4, 126.6, 126.1, 125.3, 124.8, 123.8, 123.4, 112.8,
112.5, 112.2, 81.4. MS (MALDITOF): C40H19N7S3 MS M+• 693.0.
HRMS: (calcd) (693.0864) 693.0854.
Acknowledgment. We thank the SCAS of Angers for
analytical experiments, in particular, Dr. S. Fournier for HRMS.
1
3
coruscant glassy solid. H NMR (CDCl3): 8.08 (d, 3H, J ) 4.5
3
3
Hz), 7.70 (d, 6H, J ) 9 Hz), 7.51 (d, 3H, J ) 4.5 Hz), 7.23 (d,
6H, J ) 9 Hz). 13C NMR is not available due to the extremely
3
Supporting Information Available: 1H NMR spectra of
compounds 1, 2, 4-6; HOMO-LUMO figures and calculated bond
length for compounds 4-6. This material is available free of charge
via the Internet at http://pubs.acs.org.
low solubility. MS (MALDITOF): C45H18N10S3, 794.0. HRMS:
M+• (calcd) (794.0878) 794.0854.
Synthesis of Compounds 5 and 6. To a solution of tris[4-(2-
thienyl)phenyl]amine 8 (100 mg, 0.2mmol) was added 4 equiv of
TCNE dissolved in 5 mL of DMF. The mixture was stirred for 1
JO701390Y
8336 J. Org. Chem., Vol. 72, No. 22, 2007