Extended triphenylamine conjugated systems derivatized by perfluorophenyl groups

Article abstract of DOI:10.1016/j.tetlet.2011.09.129

Two triphenylamine derivatives bearing terminal perfluorophenyl groups have been synthesized. Their HOMO, LUMO levels and electronic band gap have been evaluated by spectroscopic and electrochemical measurements and rationalized with theoretical calculati

Full text of DOI:10.1016/j.tetlet.2011.09.129

Tetrahedron Letters 52 (2011) 6573–6577
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Extended triphenylamine conjugated systems derivatized
by perfluorophenyl groups
⇑
Emilie Ripaud, Charlotte Mallet, Magali Allain, Philippe Leriche , Pierre Frère, Jean Roncali
LUNAM Université, Université d’Angers, MOLTECH-Anjou UMR CNRS 6200, Groupe Systèmes Conjugués Linéaires, 2 boulevard Lavoisier, 49045 Angers, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2011
Revised 26 September 2011
Accepted 28 September 2011
Available online 5 October 2011
Two triphenylamine derivatives bearing terminal perfluorophenyl groups have been synthesized. Their
HOMO, LUMO levels and electronic band gap have been evaluated by spectroscopic and electrochemical
measurements and rationalized with theoretical calculations. X-ray structure analysis of crystals allowed
the observation of multiple intermolecular interactions due to the presence of the perfluorophenyl pen-
dant groups. The multiplication of these interactions explains the differences between calculated (in gas
phase) and observed (in solid states) structures.
Keywords:
Triphenylamine
Ó 2011 Elsevier Ltd. All rights reserved.
Perfluorinated compounds
Conjugated systems
X-ray
Intermolecular interactions
Organic semi-conducting materials based on triphenylamine1
(TPA) are widely investigated as hole-transporting materials for
OLEDs or as donor materials for organic solar cells. Indeed, TPA
based materials combine processability, amorphous structure and
isotropic optical and electronic properties.2 As recently shown,
increasing the crystallinity of these materials can lead to improved
hole-mobility.3 In this regard, recent studies have shown that the
grafting of moieties promoting intermolecular interactions such
as dithiolethione4 or benzothiadiazole5 at the periphery of TPA,
can lead to enhanced intermolecular interactions.
Perfluorophenyl groups (PFP) are known to promote intermo-
lecular interactions via PFP–PFP, PFP–aryl aromatic interactions,
or C–HÁ Á ÁF contacts.6 In this work we report on the synthesis of
two TPA systems 1–2 functionalized at their periphery with PFP
groups (Scheme 1) and show the importance of these interactions
on the supra-molecular organization in crystals.
Compounds 1 and 2 were obtained by Wittig reaction between
((perfluorophenyl)methylene)triphenylphosphorane7 5 and trisal-
dehydes 38 and 49 in 35 and 25% yield.10 1H NMR spectra of com-
pounds 1 and 2 (ESI, Figs. S1 and S2) show clearly J-coupling
constants for the vinyl protons in the range of 16–17 Hz, thus indi-
cating the formation of all trans-configurations.
The optical and electrochemical properties of compounds 1 and
2 have been analyzed by UV–vis absorption and fluorescence emis-
sion spectroscopies and cyclic voltammetry. The results are col-
lected in Table 1.
The CV of compound 1 presents two reversible one-electron
oxidation processes with anodic peak potential Epa1 and Epa2 at
0.85 and 1.29 V respectively leading to stable radical cation and
dication (Fig. 1). Compound 2 also oxidizes in two steps at more
positive potentials of 1.03 and 1.68 V, the second process being
irreversible. The lower, more reversible and less separated oxida-
tion potentials for compound 1 (Epa2 À Epa1 = 0.65 V for 2, and
0.44 V for 1) reflect the longer conjugated system which stabilizes
the oxidized states and favors the access to dicationic state by
decreasing the intramolecular coulombic repulsions between the
positive charges.
The UV–vis absorption spectra of the two compounds in CH2Cl2
exhibit two absorption maxima at 342 and 431 nm for 1 and 292
and 405 nm for 2 (Table 1, Fig. 2). As expected compound 1 which
presents the longest conjugated system, absorbs at longer wave-
lengths and shows energy transitions with higher molar absorption
coefficients (k00, e , e
(1) = 115000 L molÀ1 cmÀ1 (2) = 81000 L molÀ1
cmÀ1).
Solvatochromic studies were performed on these compounds
(ESI, Fig. 3) and showed that their absorption maxima are nearly
independent on the nature of the solvent that shows a low degree
of charge transfer in the absorption bands. This result contrasted
with the others obtained with TPA systems derivatized with stron-
ger acceptors11 shows that the perfluorophenyl group is probably
not electron-withdrawing enough to generate a significant polarity
in these molecules.
The fluorescence emission spectra of 1 and 2 in CH2Cl2 (Table 1,
Fig. 2) present emission maxima at 494 and 451 nm, respectively,
and high emission quantum yields of 65 and 73%. Compound 1
⇑
Corresponding author.
E-mail address: philippe.leriche@univ-angers.fr (P. Leriche).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.129

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E. Ripaud et al. / Tetrahedron Letters 52 (2011) 6573–6577
Scheme 1. Syntheses of compounds 1–2.
Table 1
Electrochemical and spectroscopic data for compounds 1 and 2
Epa1, Epa2a (V)
kmax (nm)
kem (nm)
U
(%)
b
c
Compd
1
2
0.85, 1.29
1.03, 1.68
342, 431
292, 405
494
451
65
73
a
b
c
0.10 M TBAHP/CH2Cl2, scan rate 100 mV sÀ1, ref. SCE.
10À5 M in CH2Cl2.
In CH2Cl2 against perylene in cyclohexane as standard.
Figure 1. CV of compounds 1 (solid line) and 2 (dotted line) in 0.10 M TBAHP/
CH2Cl2, scan rate 100 mV sÀ1).
presents a larger Stokes shift (0.36 eV) than 2 (0.31 eV) due to larger
conformational changes between fundamental and excited states
related to the higher flexibility of its conjugated skeleton.
Figure 2. UV–vis absorption (line) and fluorescence emission (dotted line) spectra
of compounds 1 (top) and 2 (bottom) in CH2Cl2.

E. Ripaud et al. / Tetrahedron Letters 52 (2011) 6573–6577
6575
Table 2
Theoretical data for compounds 1-2
HOMOa (eV) LUMOa (eV)
À5.03
Compd
D
E Calcda (eV)
D
Eb (eV)
1
2
À2.22
À2.08
2.81
3.09
2.88
3.06
À5.17
a
DFT-calculated occupied (HOMO) and unoccupied (LUMO) molecular orbital
energies, band gap calculated for 1–2.
b
Estimated from UV–vis data in solution.
suggests enhanced intermolecular interactions in the solid state. In
contrast, only compound 2 still retains some fluorescence emission
(not quantified here) in the solid state.
The electronic properties of compounds 1 and 2 were evaluated
by ab initio theoretical calculation using Gaussian 09 (DFT-B3LYP/
6-31g (d, p)).12 The optimized geometries of the two molecules are
shown in Figure 3. Both compounds present a characteristic pro-
peller shape typical of TPA derivatives. For compound 2, each
arm is perfectly planar but the benzene rings adjacent to the cen-
tral nitrogen are not coplanar due to steric hindrance. In contrast,
for compound 1, periplanar interactions between the protons of
phenyl and adjacent thiophene rings explain the twist angle ob-
served between the two aromatic rings.
Table 2 lists the calculated HOMO, LUMO (degenerated due to
the C3 geometry imposed for calculation) levels as well as calcu-
lated and experimental values of the energy gap (DE) for com-
pounds 1–2. Compound 1 which presents the most extended
conjugated system shows lower LUMO and higher HOMO levels
and consequently a smaller
agreement with the electrochemical and spectroscopic data and
confirm that 1 presents a lower oxidation potential and E than
DE. Theoretical calculations are in good
D
2. The electronic levels and distributions of HOMO, LUMO, and
higher and lower levels are presented in ESI (Figs. S4 and S5).
While the HOMO level is mainly located on the central node with
negligible coefficients on the perfluorinated adjacent cycles, the
degenerated LUMO are dissymmetrical and localized on adjacent
arms from the central node to the lateral functionality.
Although TPA is generally amorphous materials, crystals of
compound 2 obtained by slow evaporation from a dichlorometh-
ane/hexane solution could be isolated and analyzed by X-ray dif-
fraction.13 The molecule crystallizes in the centrosymmetric
triclinic p1 space group. The structure of the molecule is shown
in Figure 4. As expected, the three C–N bonds are coplanar and
the three phenyl rings grafted on the nitrogen atom present the
Figure 3. Optimized geometries of compounds 1 (top) and 2 (bottom).
The optical properties of the compounds were also studied in the
solid-state on spun-cast or vacuum deposited films. In both cases,
the UV–vis absorption spectrum shows a bathochromic shift of kmax
of 25 and 21 nm for 1 and 2 respectively compared to solution what
ꢀ
Figure 4. Crystallographic structure of compound 2.

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E. Ripaud et al. / Tetrahedron Letters 52 (2011) 6573–6577
Figure 5. Stacking mode of molecules along the a axis.
Figure 6. C–HÁ Á ÁF contacts (red dotted lines) in one sheet of molecules 2 viewed along the a axis.
typical propeller shape of TPA derivatives.14 All vinylic bonds are in
trans-configuration that is in agreement with NMR data.
In contrast to theoretical optimization, the conjugated system
of compound 2 is slightly bent and the two aromatic rings of each

E. Ripaud et al. / Tetrahedron Letters 52 (2011) 6573–6577
6577
Gallego-Planas, N.; Frère, P.; Roncali, J. New. J. Chem. 2009, 33, 801; (c) Ripaud,
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2011, 9, 1034.
arm present torsion angles of 31.2°, 38.6°, and 39.4°, respectively.
These twists which are not due to intramolecular steric hindrance
are imposed by intermolecular interactions as shown by the
numerous short intermolecular distances observed in the crystal.
The packing arrangement of the molecules in the crystal is
characterized by layers of molecules stacking along the a axis
separated by short distances (3.37 Å, Fig. 5, ESI Fig. S6). In these
stacks, the 3 PFP rings of each molecule are engaged in PFP–PFP
interactions with two other molecules. Numerous short distances
ranging from 3.19(9) and 3.39(5) Å shown in blue on Figure 5 are
observed between fluorine and carbon atoms of the two super-
posed PFP rings.
Moreover, in layers, the molecules are imbricated within each
other and present many C–HÁ Á ÁF contacts. Figure 6 shows a set of
molecules in a sheet, the red dotted lines symbolise the interac-
tions between hydrogen and fluorine atoms. The distances dH–F
represented here range between 2.58 and 2.89 Å.
Finally, it may be noted that the structure is built from C–HÁ Á ÁF
interactions in sheets and from inter-PFP ring interactions in stacks
thus allowing a strong three dimensional arrangement. These mul-
tiple interactions probably explain the deviation from planarity of
the lateral arms for compound 2 in the crystal.
In conclusion we have synthesized two new TPA systems with
peripheral PFP groups. The presence of these electron withdrawing
groups leads to high oxidation potentials. On the other hand, the
results of spectroscopic and X-ray diffraction analyses are consis-
tent with the existence of multiple intermolecular interactions in
the solid state. The reminiscence of fluorescence associated with
strong intermolecular interaction in the solid states may allow
the incorporation of such derivatives as active layer in OLEDs and
in organic solar cells.
5. Wang, J.-L.; He, Z.; Wu, H.; Cui, H.; Li, Y.; Gong, Q.; Cao, Y.; Pei, J. Chem. Asian J.
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10. Compound
1.
To
1.53 mmol
of
((perfluorophenyl)methylene)
triphenylphosphorane
6
dissolved in 20 mL of THF at À78 °C, 200 mg
(0.35 mmol) of trisaldehyde 5 dissolved in 30 mL of THF are dropped. The
mixture is stirred 1 h at À78 °C then 12 h at room temperature. After
evaporation of THF, the residue is dissolved in methylenechloride, washed
with water and dried on magnesium sulfate. After evaporation of solvent,
compound is purified threw chromatography on silica gel using toluene as
eluent. 130 mg (35%) of yellow powder are isolated. 1H NMR (C6D6): 7.47 (d,
2H, 3J = 8.5 Hz); 7.40 (d, 1H, 3J = 16.5 Hz); 7.09 (d, 2H, 3J = 8.5 Hz); 6.92 (d, 1H,
3J = 3.5 Hz); 6.73 (d, 1H, 3J = 16.5 Hz); 6.72 (d, 1H, 3J = 3.5 Hz). 13C NMR (C6D6)
147.1; 145.1; 141.2; 132.5; 132.4; 130.2; 129.5; 127.3; 124.9; 123.7; 111.5; 19
F
NMR À163.8 (td, 2F); À157.7 (t, 1F); À144.0 (dd, 2F); MP (DSC): 213 °C; HRMS
(Calcd/Found, [M+.], g molÀ1): 1067.0826/1067.0828. Compound 2. Analogous
procedure than for 1 starting from 0.18 mmol of 4 after chromatography on
silica gel (hexane/toluene 7:3), 87 mg (25%) of a yellow solid are isolated. 1H
NRM (C6D6): 7.31 (d, 1H, 3J = 16.5 Hz); 7.20 (d, 2H, 3J = 9 Hz); 7.08 (d, 2H,
3J = 9 Hz); 6.75 (d, 1H, 3J = 16.5 Hz); 13C NMR (C6D6): 147.8; 132.2; 128.5;
128.1; 127.8; 124.7; 111.7; MP (DSC): 212 °C; HRMS (Calcd/Found, [M+],
g molÀ1): 821.1194/821.1192. (in both cases, carbons bearing fluorine which
are known to produce large signals and low intensity were not observed).
11. (a) Leriche, P.; Frère, P.; Cravino, A.; Alevêque, O.; Roncali, J. J. Org. Chem. 2007,
72, 8332; (b) Ripaud, E.; Yoann, O.; Leriche, P.; Cornil, J.; Roncali, J. J. Phys. Chem.
B 2011, 115, 9379.
Acknowledgments
Authors thank the PIAM for analytical measurements and the
French Minister of research for E.R. and C.M. granting.
12. Gaussian 09, Revision A.02, M. J. Frisch, et al. Gaussian, Inc., Wallingford CT,
2009.
Supplementary data
13. X-ray single-crystal diffraction data for 2 were collected at 293 K on a BRUKER
KappaCCD diffractometer, equipped with a graphite monochromator utilizing
MoK
a radiation (k = 0.71073 Å). The structure was solved by direct methods
Supplementary data NMR spectra of compound 1–2; solvato-
chromic studies for compounds 1–2, HOMO, LUMO and higher and
lower levels for compounds 1–2 associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2011.09.129.
using SIR92 (Altomare et al.,1993) and refined on F2 by full matrix least-
squares techniques using SHELXL97 (G.M. Sheldrick, 1998). All non-H atoms
were refined anisotropically and absorption was corrected by SADABS program
(Sheldrick, Bruker, 2000). The
H atoms were included in the calculation
without refinement. Crystallographic data for 2:
C42H18F15N, M = 821.57,
yellow prism, 0.27 Â 0.22 Â 0.04 mm3, triclinic, space group p1,
ꢀ
a = 11.836(1) Å, b = 12.166(3) Å, c = 13.015(2) Å,
a = 95.22(1)°, b = 98.84(1)°,
References and notes
c
l
= 109.04(1)°,V = 1730.4(5) Å3,
(MoK F(000) = 824, hmin = 2.20°, hmax = 26.04°, 33738
Z = 2,
q
calc = 1.577 g/cm3,
a
) = 0.149 mmÀ1
,
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and wR2 = 0.1365 using all data, GOF = 1.103, À0.241 <
< 0.184 e ÅÀ3
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r
D
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