Synthesis and characterization of pentafluorophenyl-substituted perylenebis(dicarboximides)

Article abstract of DOI:10.1002/hlca.200900218

A series of 1,7-bis(perfluorophenyl)substituted perylenebis(dicarboximides) were synthesized by Cu-mediated Ullmann coupling of bromopentafluorobenzene with 1,7-dibromoperylenebis(dicarboximides) (Scheme). The photophysical and electrochemical properties

Full text of DOI:10.1002/hlca.200900218

Helvetica Chimica Acta – Vol. 92 (2009)
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Synthesis and Characterization of Pentafluorophenyl-Substituted
Perylenebis(dicarboximides)
by Tobias Schnitzler, Chen Li, and Klaus Mꢀllen*
Max-Planck-Institut fꢀr Polymerforschung, Ackermannweg 10, D-55128 Mainz
(phone: þ 49-6131379151; fax: þ 49-6131379100; e-mail: muellen@mpip-mainz.mpg.de)
Dedicated to Professor Jean-Claude Bꢀnzli on the occasion of his 65th birthday
A series of 1,7-bis(perfluorophenyl)substituted perylenebis(dicarboximides) were synthesized by
Cu-mediated Ullmann coupling of bromopentafluorobenzene with 1,7-dibromoperylenebis(dicarbox-
imides) (Scheme). The photophysical and electrochemical properties of these dyes were investigated
with regard to their potential use as building blocks in organic photovoltaics (OPV) or organic field-
effect transistors (OFET).
Introduction. – In recent years, molecular semiconductors for electronics have
approached the same level of importance as the traditionally used inorganic materials.
There are some key advantages of the former approach as small molecules or
conjugated polymers are inherently inexpensive and easily processible [1 – 4]. These
features offer the potential of achieving the long-term goal of developing organic
electronics that are inexpensive, flexible, and suitable for large-area applications, such
as organic field-effect transistors (OFET) [1], organic photovoltaics (OPV) [5], and
organic light-emitting diodes (OLED) [6 – 8].
In the field of organic semiconductors, p-type (hole-conducting) materials have
been investigated extensively, and some have fulfilled many of the needs of industrial
applications due to their high charge-carrier mobilities and easily scalable, high-speed
printing processibility [9]. However, n-type (electron conducting) materials remain less
investigated. In general, crucial issues in the development of n-type semiconductors
include mobilities, air-stability, and solubility. In addition to these aspects, a functional
n-type material should also exhibit high electron affinity to provide efficient electron
injection to the interface of common metal electrodes.
Perylenebis(dicarboximides) (¼ perylene diimides ¼ PDIs) have been intensively
investigated since the beginning of the last century. Their remarkable chemical stability
and the variety of possible chemical functionalization, as well as their outstanding
optical and electronic properties make PDIs outstanding and versatile candidates for
applications in organic electronic devices, biochemistry, sensors, and single-molecule
spectroscopy [10 – 12]. With two imide groups, PDIs present themselves as promising
organic n-type materials [13]. Furthermore, the introduction of strong electron-
withdrawing groups into the PDI scaffold has been developed as a strategy to improve
the performance of PDI derivatives in n-channel FETs. Meanwhile, core-cyanated [14],
core-fluorinated [15][16] and core-perfluoroalkylated [17] PDIs have been synthesized
ꢁ 2009 Verlag Helvetica Chimica Acta AG, Zꢀrich

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and exhibit excellent stability in air. Furthermore, it has been demonstrated that
electron-withdrawing groups at the imide groups afford promising substituents for n-
type arylene-based OFET materials (e.g., naphthalenes [18] and perylenes [19]).
Fluoroalkyl and fluorophenyl-functionalized imide groups are particularly noteworthy
because of their strong influence on the LUMO level and the solid-state packing of PDI
molecules [16][20].
The synthesis of core-phenyl-substituted PDI was up to now only accomplished by
Pd-catalyzed Suzuki coupling of phenylboronic acids with 1,7-dibromo-PDIs [21][22].
The disadvantage of this route lies in the use of potentially difficult to synthesize
molecules (i.e., boronic acids or esters) and expensive Pd catalysts.
Herein, we report the first synthesis of core-phenyl-substituted PDI by the Cu-
catalyzed Ullmann reaction. With the use of simple aryl halides and Cu bronze,
Ullmann coupling offers a promising alternative to ꢂclassicalꢃ Suzuki coupling towards
core-phenyl-substituted PDIs. By employing bromopentafluorobenzene as a coupling
reagent, we were able to synthesize PDIs with two strong electron-withdrawing groups
in the 1,7-positions of the perylene. The PDIs 3a – 3c were synthesized in high yields.
The chemical structures of the new perylene compounds were established by 1H-, 13C-,
and 19F-NMR, IR and mass spectroscopy. Their optical properties were investigated by
UV/VIS and fluorescence spectroscopy. The electrochemical properties of the PDIs
were characterized by cyclic voltammetry.
Results and Discussion. – Synthesis. The 1,7-dibromo-PDIs have already been
established as very useful intermediates for the synthesis of a broad range of 1,7-
substituted PDIs [21][23 – 25]. The imidisation of perylene-3,4,9,10-tetracarboxylic
acid dianhydride (PDA; 1) with octan-1-amine or 2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentade-
cafluorooctan-1-amine to form the bis-imide 2a and 2b, respectively, followed the
published standard conditions and was performed in AcOH in good yields (75 and
78%, resp.) [20][26] (Scheme). Due to the very low nucleophilicity of pentafluoroani-
line, the reaction to form 2c required 1-methylpyrrolidin-2-one (NMP) as a solvent and
Scheme. Synthesis of 3a – 3c

Helvetica Chimica Acta – Vol. 92 (2009)
2527
a large excess of pentafluoroaniline (yield 22%) [27]. The purifications of 2a – 2c were
carried out by column chromatography (silica gel, CH2Cl2).
The subsequent conversion into the 1,7-bis(pentafluorophenyl)perylenebis(dicar-
boximides) 3a – 3c was performed under Ullmann conditions [28] with Cu-bronze as
catalyst. The Cu-bronze was activated, as others have noted that the activation of the
Cu-bronze is crucial for the yield in Ullmann couplings [29][30]. The PDIs 2a – 2c were
suspended together with an excess of bromopentafluorobenzene (10 equiv.) and
activated Cu-bronze in dry DMF (Scheme). After flushing with Ar for 15 min, the
reaction mixtures were heated overnight at 1408 in a sealed microwave tube. In this
way, the PDIs 3a – 3c were obtained in good yields (60 – 68%) as red-orange solids.
Optical Properties. Fig. 1 shows the UV/VIS absorption spectra of PDIs 3a – 3c in
CHCl3. The bathochromic shifts of the absorbance of the pentafluorophenyl-
substituted PDIs 3a – 3c are, compared to the parent PDIs 2a – 2c, remarkably small
(2 nm, for details see Table 1). This can be explained by the steric restrictions of the
bulky pentafluorophenyl substituents. These 1,7-substituents are forced into an
orthogonal orientation with respect to the perylene core, minimizing the p-conjugative
interaction (as can be seen in the molecular model in Fig. 2; structure produced with
PM3 by Hyperchem 6) [22].
Fig. 1. UV/VIS Spectra of 3a – 3c in CHCl3
Table 1. Photophysical Properies of PDIs 2a – 2c and 3a – 3c in CHCl3
lmax (abs.) [nm]
e [kmꢀ1 cmꢀ1
]
lmax (fl.) [nm]
Ff [%]
2a
2b
2c
3a
3b
3c
527
527
530
529
529
532
47.8
46.1
49.2
46.8
43.8
34.6
554
552
554
555
556
559
0.77
0.80
0.88
0.93
0.92
0.95

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Fig. 2. 3D-Model of 1,7-bis(pentafluorophenyl)-substituted PDI
The absorption maxima of the dyes 3a – 3c are quite similar as the effect of the imide
substituent R on the absorption wavelength is minor. A significant spectral broadening
of the absorption is observed when comparing the spectra of unsubstituted PDIs with
3a – 3c. This phenomenon can be explained by loss of the vibronic fine structure. This is
caused by the strong twist of the perylene cores due to the space-demanding
pentafluorophenyl substituents.
The PDIs 3a – 3c are also highly fluorescent. Their emission maximum lies in the
range of 555 – 559 nm with remarkably high fluorescence quantum yields from 0.92 –
0.95 (for details see Table 1). The fluorescence of 3a is, in comparison with the parent
compound 2a, only slightly red-shifted (1 nm), whereas the bathochromic shift is
stronger in case of the PDIs 3b and 3c (4 and 5 nm). This might be explained by the
additional electron-withdrawing effect of the fluorinated imide substituents.
Electrochemical Properties. Cyclic voltammetry (CV) was performed to investigate
the energy levels of the frontier orbitals. In CV, the dyes 3a – 3c showed two reversible
reduction waves, a typical pattern for PDIs [31]. Due to the limited potential range of
the solvent (CH2Cl2), no oxidation wave was observed. The band gaps (Eg) were
estimated from the onset of the absorption in the UV/VIS spectra (for results, see
Table 2). The energy levels for the LUMO and HOMO of 1,7-unsubstituted PDIs lie
near ꢀ 3.8 and ꢀ 5.8 eV, respectively. As our results show, the introduction of the 1,7-
bis(pentafluorophenyl) substituents causes a significant decrease of the HOMO and
LUMO levels (see Table 2). This is highly important for the use in OFETs. It should be
noted that the imide substituents have also a profound influence on the energy levels.
The LUMO level is also lowered by 0.10 eV for the N-(pentafluorophenyl)-substituted
PDI 3c in comparison with the N-alkyl-substituted PDI 3a. The extremely low LUMO
levels of 3a – 3c (ꢀ4.21 to ꢀ 4.33) compared with other PDIs render these dyes
promising candidates for use in OPV and OFETs. The low LUMO levels improve the
charge driving force in OPV and also stabilize the FET device.

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Table 2. Half-Wave Reduction Potentials E (vs. Fc/Fcþ) of 3a – 3c Measured in CH2Cl2
E (PDIꢀ/PDI2ꢀ) [V]
E (PDI/PDIꢀ ) [V]
Eg [eV]
LUMO [eV]
HOMO [eV]
3a
3b
3c
ꢀ 0.77
ꢀ 0.83
ꢀ 0.66
ꢀ 0.57
ꢀ 0.59
ꢀ 0.47
2.23
2.23
2.20
ꢀ 4.23
ꢀ 4.21
ꢀ 4.33
ꢀ 6.40
ꢀ 6.44
ꢀ 6.25
Conclusion. – We established a convenient synthetic route for the synthesis of 1,7-
bis(pentafluorophenyl)-substituted PDIs by Ullmann coupling. The PDIs 3a – 3c were
obtained in high yields. Due to their significant lower LUMO level, as compared to
unsubstituted PDI, these chromophores are promising n-type molecular semiconduc-
tors for use in either organic photovoltaic or organic field-effect transistors. OPV and
OFET measurements are currently in progress.
Financial support of this work by the BASF AG and Deutsche Forschungsgemeinschaft (DFG)
priority program (SPP 1355) Elementary Processes of Organic Photovoltaics is gratefully acknowledged.
We also thank Ashlan Musante for her help in preparing this manuscript.
Experimental Part
General. The 1,7-dibromoperylene-3,4,9,10-tetracarboxylic acid 3,4 :9,10-dianhydride was provided
by BASF AG. All other starting materials were purchased from Aldrich, Acros, ABCR, or Lancaster, and
used as received. Cyclic voltammetry (CV) data were obtained with a standard commercial electro-
chemical analyzer in a three-electrode single cell. CV Measurements were carried out in dry CH2Cl2
under Ar with Bu4N(PF6) as a supporting electrolyte. The experimental setup was calibrated with
ferrocene. Potentials were obtained from the half-wave reduction potentials. UV/VIS Spectra: Perkin-
Elmer-Lambda-900 spectrophotometer; at r.t.; lmax (log e) in nm. Fluorescence spectra: T&D TIDAS
diode-array spectrometer. IR Spectra: Nicolet-730-FT-IR spectrometer with a Endurance diamant ATR
unit; ˜n in cmꢀ1. 1H-, 13C- and 19F-NMR Spectra: Bruker-Avance-300 and Bruker-Avance-700 in CD2Cl2,
with the residual H-atom resonance or the C-atom signal of the solvent as the internal standard; d in ppm,
J in Hz. FD-MS: VG Instruments ZAB 2-SE-FPD; in m/z (rel. %).
1,7-Dibromoperylene-3,4 :9,10-bis(dicarboximides) (¼ 5,12-Dibromoanthra[2,1,9-def :6,5,10-d’e’f’]-
diisoquinoline-1,3,8,10(2H,9H)-tetrones) 2a – 2c, were synthesized according to standard literature
procedures.
Ullmann Coupling of 1,7-Dibromoperylene-3,4 :9,10-bis(dicarboximides) with Bromopentafluoro-
benzene: General Procedure. The 1,7-dibromo-PDI (100 mg), bromopentafluorobenzene (10 equiv.),
and activated Cu-bronze (150 mg) were suspended in dry DMF (0.7 ml). The mixture was flushed with
Ar and stirred for 16 h at 1408 in a sealed tube. After cooling at r.t., H2O (15 ml) was added, and the
reaction mixture was filtered and washed thoroughly with H2O (50 ml). The filter cake was dried and
washed out with CH2Cl2 (100 ml) to extract the crude product. The org. phase was filtered, and the
solvent was evaporated in vacuo. The crude product was purified by column chromatography (silica gel,
CH2Cl2/petroleum ether 1:1). Compound 3c was additionally purified by prep. TLC (CH2Cl2/petroleum
ether 1:1).
N,N’-Bis(octyl)-1,7-bis(2,3,4,5,6-pentafluorophenyl)perylene-3,4 :9,10-bis(dicarboximide) (¼2,9-Di-
octyl-5,12-bis(2,3,4,5,6-pentafluorophenyl)anthra[2,1,9-def :6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H,9H)-
tetrone; 3a). Yield 65%. Orange-red solid. M.p. 2058. IR: 2955, 2925, 2856, 1703, 1663, 1601, 1590, 1522,
1496, 1462, 1434, 1412, 1401, 1350, 1331, 1246, 1089, 990, 813. 1H-NMR (300 MHz, CD2Cl2): 8.55 (s, 2 H);
8.34 (d, J ¼ 8.1, 2 H); 7.83 (d, J ¼ 8.1, 2 H); 4.25 – 4.09 (m, 4 H); 1.79 – 1.64 (m, 4 H); 1.49 – 1.21 (m, 24 H);
0.87 (t, J ¼ 6.6, 6 H). 13C-NMR (75 MHz, CD2Cl2): 163.09; 162.98; 135.41; 134.95; 134.45; 131.16; 128.92;
128.41; 127.61; 124.24; 123.77; 123.73; 41.04; 32.21; 29.70; 29.60; 28.39; 27.48; 23.03; 14.24. 19F-NMR

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Helvetica Chimica Acta – Vol. 92 (2009)
(471 MHz, CD2Cl2): ꢀ 142.16 (d, J ¼ 16.1); ꢀ 153.69 (t, J ¼ 20.5); ꢀ 160.67 (t, J ¼ 17.9). FD-MS: 943.4
(100, [M ꢀ 3]þ, C52H37F10N2Oþ4 ; calc. 943.3).
N,N’-Bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-1,7-bis(2,3,4,5,6-pentafluorophenyl)pery-
lene-3,4 :9,10-bis(dicarboximide) (¼2,9-Bis(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-5,12-
bis(2,3,4,5,6-pentafluorophenyl)anthra[2,1,9-def :6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H,9H)-tetrone;
3b). Yield 68%. Orange-red solid. M.p. 2368. IR: 2920, 2850, 1730, 1718, 1682, 1524, 1499, 1463, 1418,
1351, 1335, 1240, 1207, 1148, 1087, 1022, 991, 815. 1H-NMR (300 MHz, CD2Cl2): 8.69 (s, 2 H); 8.51 (d, J ¼
8.1, 2 H); 7.99 (d, J ¼ 8.1, 2 H); 5.06 (t, J ¼ 15.7, 4 H). 13C-NMR (75 MHz, CD2Cl2): 162.87; 162.80; 136.25;
135.66; 135.13; 132.08; 129.15; 128.68; 128.02; 124.79; 123.01 (2 C); 30.08. 19F-NMR (471 MHz, CD2Cl2):
ꢀ 81.09 (d, J ¼ 9.1); ꢀ 115.20 (s); ꢀ 121.77 (s); ꢀ 122.15 (s); ꢀ 122.86 (s); ꢀ 123.64 (s); ꢀ 126.29 (s); ꢀ
140.82 (d, J ¼ 16.3); ꢀ 151.93 (t, J ¼ 20.5); ꢀ 159.18 (t, J ¼ 17.9). FD-MS: 1485.7 (75, Mþ, C52H10F40N2O4þ ;
calc. 1486.0), 1466.0 (100, [M ꢀ F]þ), 1446.3 (65, [M ꢀ 2F]þ), 1427.4 (10, [M ꢀ 3F]þ).
N,N’-1,7-Tetrakis(2,3,4,5,6-pentafluorophenyl)perylene-3,4 :9,10-bis(dicarboximide) (¼2,5,9,12-Tet-
rakis(2,3,4,5,6-pentafluorophenyl)anthra[2,1,9-def :6,5,10-d’e’f’]diisoquinoline-1,3,8,10(2H,9H)-tetrone;
3c). Yield 60%. Orange-red solid. M.p. > 3008. IR: 2930, 1724, 1688, 1516, 1496, 1398, 1337, 1310, 1243,
1188, 1122, 1105, 1022, 989, 811, 801, 723, 698. 1H-NMR (300 MHz, CD2Cl2): 8.70 (s, 2 H); 8.52 (d, J ¼ 8.1,
2 H); 8.03 (d, J ¼ 8.1, 2 H). 13C-NMR (75 MHz, CD2Cl2): 161.14; 161.09; 136.08; 135.61; 135.07; 131.93;
129.20; 128.51; 127.82; 124.66; 122.63 (2 C). 19F-NMR (471 MHz, CD2Cl2): ꢀ 141.88 (d, J ¼ 15.9); ꢀ
144.79 (d, J ¼ 18.0); ꢀ 152.77 (t, J ¼ 20.5); ꢀ 153.57 (t, J ¼ 21.0); ꢀ 160.14 (t, J ¼ 17.8); ꢀ 163.30 (t, J ¼
19.2). FD-MS: 1054.4 (100, Mþ, C48H6F20N2O4þ ; calc. 1054.5).
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Received June 8, 2009