Low band gap thiophene-perylene diimide systems with tunable charge transport properties

Article abstract of DOI:10.1021/ol1023486

Perylenediimide-pentathiophene systems with varied architecture of thiophene units were synthesized. The photophysical, electrochemical, and charge transport behavior of the synthesized compounds were studied. Both molecules showed a low band gap of ～1.4 eV. Surprisingly, the molecule with pentathiophene attached via β-position to the PDI unit upon annealing showed a predominant hole mobility of 1 × 10^-4 cm² V^-1 s^-1 whereas the compound with branched pentathiophene attached via β-position showed an electron mobility of 9.8 × 10 ^-7 cm² V^-1 s^-1. This suggests that charge transport properties can be tuned by simply varying the architecture of pentathiophene units.

Full text of DOI:10.1021/ol1023486

ORGANIC
LETTERS
2011
Vol. 13, No. 1
18-21
Low Band Gap Thiophene-Perylene
Diimide Systems with Tunable Charge
Transport Properties
Ganapathy Balaji,†,‡ Tejaswini S. Kale,‡ Ashok Keerthi,† Andrea M. Della Pelle,‡
S. Thayumanavan,*,‡ and Suresh Valiyaveettil*,†
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Singapore 117543, Singapore, and Department of Chemistry, UniVersity of
Massachusetts, Amherst, Massachusetts 01003, United States
thai@chem.umass.edu; chmsV@nus.edu.sg
Received September 29, 2010
ABSTRACT
Perylenediimide-pentathiophene systems with varied architecture of thiophene units were synthesized. The photophysical, electrochemical,
and charge transport behavior of the synthesized compounds were studied. Both molecules showed a low band gap of ∼1.4 eV. Surprisingly,
the molecule with pentathiophene attached via ꢀ-position to the PDI unit upon annealing showed a predominant hole mobility of 1 × 10-4 cm2
V-1 s-1 whereas the compound with branched pentathiophene attached via ꢀ-position showed an electron mobility of 9.8 × 10-7 cm2 V-1 s-1
.
This suggests that charge transport properties can be tuned by simply varying the architecture of pentathiophene units.
Donor-acceptor conjugated molecules have gained tremen-
dous scientific interest due to their unique optoelectronic
properties and have been successfully employed in applica-
tions such as organic field effect transistors, light-emitting
diodes, and photovoltaic devices.1 Perylene diimide (PDI)
belongs to the class of n-type semiconductors which exhibits
high molar extinction coefficients and high electron mobil-
ity.2 Significant research efforts have been focused on the
modification of PDI structures to improve their optoelectronic
and charge transport properties.3 The bay substitution of PDI
is an easy approach to fine-tune the properties of PDI by
modifying its frontier orbital energy levels.3 Oligothiophenes
are a well-known class of donor moieties with excellent
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Quante, H.; Geerts, Y.; Mu¨llen, K. Chem. Mater. 1997, 9, 495. (c) Thelakkat,
M.; Po¨sch, P.; Schmidt, H.-W. Macromolecules 2001, 34, 7441.
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J. Am. Chem. Soc. 2010, 132, 3375. (b) Scholz, M.; Schmidt, R.; Krause,
S.; Scho¨ll, A.; Reinert, F.; Wu¨rthner, F. Appl. Phys. A: Mater. Sci. Process.
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† National University of Singapore.
‡ University of Massachusetts.
(1) (a) Roncali, J. In Handbook of Conducting Polymers, 2nd ed.;
Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker:
NewYork, 1998; Chapter 12. (b) Garnier, F.; Hajlaoui, R.; Yassar, A.;
Srivastava, P. Science 1994, 265, 1685.
10.1021/ol1023486  2011 American Chemical Society
Published on Web 11/30/2010

charge transport properties and have been used as the active
material in organic electronic devices.4 Understandably, when
considering donor-acceptor systems, oligothiophenes at-
tached to PDI molecules have been the focus of many
studies.5 Among the oligothiophenes, pentathiophene absorbs
in the spectral region where the PDI absorption is minimum
and the fluorescence of pentathiophene overlaps well with
the absorption of PDI, which makes the PDI-pentathiophene
pair a favorable system for effective light harvesting and a
photoinduced electron transfer process.6 In our previous
work, we studied the effect of substituents on the electron
transport properties of bay substituted PDI derivatives.7 More
recently, the color tuning in PDI-terthiophene systems with
varied architecture of thiophene units has been reported in
the literature.8 In our work here, unlike the conventional
D-A-D systems, we have designed and synthesized
D-A-D systems 1 and 2 with restricted conjugation. Hence,
they are expected to interact Via an energy transfer process
(pentathiophenes interact well with PDI units)6 which does
not require conjugation. More specifically, we were interested
in investigating which of these molecules exhibit low band
gap features, while allowing for tuning the charge transport
properties.
Syntheses of target compounds 1 and 2 are depicted in
Schemes 1 and 2, respectively. N,N′-Bis(2-ethylhexyl)-1,7-
Scheme 1. Synthesis of Compound 1
The structures of the targeted molecules are shown in
Figure 1. In molecule 1, the linear pentathiophene units are
dibromoperylenediimide (4) was synthesized from 3,4,9,10-
perylenetetracarboxylic acid anhydride following a procedure
reported earlier.7 The dibromo compound 4 was reacted with
3-thiopheneboronic acid by a Suzuki coupling reaction to
get compound 5 in 87% yield. The compound 5 was
effectively brominated under Br2/CHCl3 conditions to obtain
the tetrabrominated product 6. This product upon Suzuki
coupling with 5′-hexyl-2,2′-bithiophene-5-boronic acid pi-
nacol ester yielded the target compound 1 in 78% yield
(Scheme 1).
Compound 2 was obtained from 4 following a similar
procedure. In this scheme compound 4 was coupled under
Suzuki conditions with 2-thiopheneboronic acid to afford
compound 7,5a followed by tetrabromination to obtain
compound 8. Compound 8, upon a final Suzuki coupling with
5′-hexyl-2,2′-bithiophene-5-boronic acid pinacol ester, yielded
the target compound 2 with an overall yield of ∼45%
(Scheme 2). The synthesized molecules are highly soluble
in common organic solvents such as dichloromethane,
chloroform, and THF.
The absorption spectra of 1 and 2 in solution (chloroform)
are shown in Figure 2. Both 1 and 2 showed two absorption
maxima with a weak low energy absorption band around
650 nm. The absorption spectrum of R-linked 2 is blue-
shifted compared to the ꢀ-linked compound 1. This may be
due to the branched nature of the thiophene segment, which
prevents the effective conjugation of the pentathiophene unit.9
The peak around 500 nm can be assigned to the PDI-based
transition. The high energy peak at 426 nm for 1 and 340
nm for 2 can be assigned to the thiophene centered transition
(terthiophene λmax ) 354 nm, pentathiophene λmax ) 428
nm),10 and the weak low energy absorption to the thiophene-
PDI charge transfer band.11 While PDI itself exhibits
Figure 1. Chemical structure of PDI-pentathiophene compounds
1 and 2.
attached to the bay position of the PDI moiety Via the central
thiophene unit through a ꢀ linkage, leading to a cross
conjugated D-A-D system. Whereas in molecule 2 the
donor unit (pentathiophene) is cross conjugated and attached
to PDI through a central thiophene through an R linkage.
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VCH: Weinheim, Germany, 1999. (b) Mishra, A.; Ma, C.-Q.; Ba¨uerle, P.
Chem. ReV 2009, 109, 1141.
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Mater. 2005, 17, 2208. (b) Huang, J.; Fu, H.; Wu, Y.; Chen, S.; Shen, F.;
Zhao, X.; Liu, Y.; Yao, J. J. Phys. Chem. C 2008, 112, 2689.
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A 2008, 112, 10193.
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Kazlauskas, K.; Jursenas, S.; Gruodis, A.; Simokaitiene, J.; Grazulevicius,
J. V.; Valiyaveettil, S. J. Phys. Chem. B 2010, 114, 1782.
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Mu¨llen, K. J. Phys. Chem. B 2010, 114, 14343.
Org. Lett., Vol. 13, No. 1, 2011
19

Scheme 2
.
Synthesis of Compound 2
Table 1. Optoelectronic Properties of Compounds 1 and 2
e
λa
Eb
Eb
HOMOc LUMOd Eg
max
oxid
red
compd (nm)
(V)
(V)
(eV) (eV) (eV)
1
2
426, 534 0.64, 0.51 -1.08, -1.38 -5.18
340, 508 0.74, 0.61 -1.01, -1.20 -5.29
-3.80 1.38
-3.93 1.36
a Recorded in chloroform. b Versus Ferrocene in 0.1 M Bu4NPF6 in
dichloromethane, platinum disk as working electrode with a scan rate of
100 mV s-1 c Calculated using the relationship EHOMO ) -(Eoxionset + 4.8).
.
d CalculatedusingtherelationshipELUMO )-(Eredonset -4.8).e Electrochemical.
ities.12,13 This is supported by the calculated frontier
molecular orbitals by density functional theory (Figure 3).
significant fluorescence (ΦF ) 0.95),5b neither 1 nor 2
exhibits any fluorescence. This is attributed to the possible
photoinduced intramolecular electron transfer between the
oligothiophene moiety and the perylene core.6
The electrochemical properties were investigated by cyclic
voltammetry. Both compounds 1 and 2 showed ambipolar
redox behavior (Figure S2). During the positive potential
sweep, 1 and 2 showed two reversible oxidation peaks
corresponding to the oxidation of thiophene units.12 In the
negative potential regime, two characteristic reversible
reduction waves of the PDI moiety were observed. The
HOMO and LUMO energy levels were determined from
oxidation and reduction onset potentials, respectively, and
their values are summarized in Table 1.
Interestingly, 1 and 2 possess similar energy gaps (Eg)
of 1.38 and 1.36 eV respectively, even as the positions of
HOMO and LUMO levels were different. In compound
1, the HOMO energy level resembles that of pen-
tathiophene while the LUMO level resembles that of PDI
due to the effective decoupling of these two functional-
Figure 3. Optimized geometric structure of compounds 1 and 2
and their frontier orbitals were calculated by density functional
theory (DFT) at the BLYP/3-21G level.
On the other hand, the HOMO and LUMO energy levels
are both lowered in the case of 2. We resorted to DFT
calculations to rationalize this experimental observation. The
DFT calculations showed a subtle distribution of HOMO
lobes on the PDI units and LUMO lobes on the thiophene
units, which is not present in compound 1 (Figure 3). We
speculate that, in the absence of an alternate explanation,
this leakage in electron density stabilizes the frontier mo-
lecular orbitals in 2.
(9) Xia, C.; Fan, X.; Locklin, J.; Advincula, R. C.; Gies, A.; Nonides,
W. J. Am. Chem. Soc. 2004, 126, 8735.
(10) (a) Oliva, M. M.; Pappenfus, T. M.; Helby, J. H.; Schwaderer,
K. M.; Johnson, J. C.; McGee, K. A.; Filho, D. A. S. F.; Bredas, J. -L.;
Casado, J.; Navarrete, J. T. L. Chem.sEur. J. 2010, 16, 6866. (b) Becker,
R. S.; de Melo, J. S.; Mac¸anita, A. L.; Elisei, F. J. Phys. Chem. 1996, 100,
18683.
(11) Zhan, X.; Tan, Z.; Zhou, E.; Li, Y.; Misra, R.; Grant, A.; Domercq,
B.; Zhang, X.-H.; An, Z.; Zhang, X.; Barlow, S.; Kippelen, B.; Marder,
S. R. J. Mater. Chem 2009, 19, 5794.
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J.; Neher, D.; Scherf, U.; Farrell, T. J. Am. Chem. Soc. 2006, 128, 3914.
(13) Li, C.; Scho¨neboom, J.; Liu, Z.; Pschirer, N. G.; Erk, P.; Herrmann,
A.; Mu¨llen, K. Chem.sEur. J. 2009, 15, 878.
Figure 2. Absorption spectra of PDI-pentathiophene compounds
1, 2, and PDI in chlorofom at room temperature.
20
Org. Lett., Vol. 13, No. 1, 2011

Next, we were interested in analyzing the charge transport
characteristics of these molecules. Bottom contact field effect
transistors were fabricated by spin coating the compounds
from 1 wt % solution in chlorobenzene on prepatterned
substrates. Thermal annealing was carried out under an inert
atmosphere at 80 °C for 1 h. The channel width and channel
length of the transistors were 10 mm and 5 µm, respectively.
Output characteristics of these transistors are as shown in
with an electron mobility of 2.23 × 10-7 cm2 V-1 s-1 and
hole mobility of 6.85 × 10-6 cm2 V-1 s-1. The compound 2
showed only electron transport behavior which is expected
for the PDI-based systems, with an electron mobility of 6.21
× 10-7 cm2 V-1 s-1.
The PDI-thiophene-based systems generally show elec-
tron transporting behavior.14 The predominantly hole trans-
porting feature of 1 is unusual for PDI-based systems. In
compound 1 the PDI unit is buried between the pen-
tathiophene chains which are not in plane with the PDI unit
(Figures 3 and S4). As a result of this, the interaction of
PDI units with adjacent molecules is likely to be restricted
and the mobility is dominated by thiophene units. Upon
annealing, the hole mobility of 1 increases to 1.04 × 10-4
cm2 V-1 s-1 and the electron mobility decreases to 9.13 ×
10-8 cm2 V-1 s-1, which is attributed to the better ordering
of pentathiophene units terminated with hexyl chains result-
ing in a more effective isolation of PDI units. Annealing
has marginal effect on the transport behavior of 2.
The low electron mobility of 2, compared to other reported
systems,11 can be attributed to the branched nature of the
side chain, which is expected to affect the packing of the
molecules.
In summary, we have synthesized highly stable and soluble
PDI-thiophene systems with varied thiophene architecture.
Such variations provide the ability to tune the charge
transport properties, while retaining the advantage of lower-
ing the band gap. Photovoltaic studies of these molecules
are currently underway in our laboratory.
Acknowledgment. The authors would like to thank the
Agency for Science, Technology and Research (A*STAR),
National University of Singapore for financial support. Both
G.B. and A.K. acknowledge the graduate scholarship from
the National Unviersity of Singapore. Partial support from
the U.S. Department of Energy’s Energy Frontier Research
Center at UMass is acknowledged.
Figure 4. Output characteristics of bottom contact field effect
transistors (a) 1 and (b) 2 at W/L ) 2000.
Figure 4. The charge transport properties are summarized
in Table 2. In as cast films, 1 showed ambipolar behavior
Supporting Information Available: Experimental pro-
cedures, cyclic voltammograms, AFM images of all com-
pounds, and BLYP calculated structures of compound 1. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Table 2. Mobility Data of Compound 1 and 2
mobility (cm2 V-1 s-1
)
compd
charge
beforeannealing
afterannealing
OL1023486
1
hole
electron
electron
6.85 × 10-6
2.23 × 10-7
6.21 × 10-7
1.04 × 10-4
9.13 × 10-8
9.83 × 10-7
(14) Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.;
Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2007, 129,
7246.
2
Org. Lett., Vol. 13, No. 1, 2011
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