The effect of heteroatom conformation on optoelectronic properties of cyclopentadithiophene derivatives

Article abstract of DOI:10.1039/c3ob41648h

Cyclopentadithiophene (CPDT) derivatives with different heteroatom conformations have been synthesized. The optical, electrochemical and charge transport properties of these molecules are reported. The CPDT-anti-ketone not only exhibits the lowest optical and electronic bandgaps, but also exhibits reasonable hole mobility, 3 × 10^-3 cm² (V s) ^-1. Changing the carbonyl conformation to the syn position or incorporating the imine functionality results in a blue-shift in the lower energy band of the absorption spectrum indicative of the increased bandgaps. the Partner Organisations 2014.

Full text of DOI:10.1039/c3ob41648h

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The effect of heteroatom conformation on
optoelectronic properties of
cyclopentadithiophene derivatives†
Cite this: Org. Biomol. Chem., 2014,
12, 2474
Sompit Wanwong, Ambata Poe, Ganapathy Balaji and S. Thayumanavan*
Cyclopentadithiophene (CPDT) derivatives with different heteroatom conformations have been syn-
thesized. The optical, electrochemical and charge transport properties of these molecules are reported.
The CPDT-anti-ketone not only exhibits the lowest optical and electronic bandgaps, but also exhibits
reasonable hole mobility, 3 × 10−3 cm2 (V s)−1. Changing the carbonyl conformation to the syn position or
incorporating the imine functionality results in a blue-shift in the lower energy band of the absorption
spectrum indicative of the increased bandgaps.
Received 12th August 2013,
Accepted 13th February 2014
DOI: 10.1039/c3ob41648h
www.rsc.org/obc
Cyclopentadithiophene (CPDT) derivatives have attracted
research interest owing to their potential to serve as a building
block for the construction of organic semiconductors.1 The
fused ring system and planar geometry lead to enhanced con-
jugation, lower bandgap and an increase in intermolecular
interaction.1a,2 The CPDT unit has two locations where substi-
tuents can be incorporated: (a) the α-positions of thiophenes
and (b) the bridgehead position.3 Varying the substituents
at either of these positions provides the opportunity to tune
the electronic properties of the system. For example, it has
been shown that incorporation of electron-withdrawing func-
tionalities, such as carbonyl and dicyanomethylene, at the
bridgehead position of CPDT renders the system more electron
deficient, and often results in reduced bandgap.3c,4 In addition
to exploring CPDT based molecules for photovoltaics through
optical tuning, these molecules also have potential as semi-
conductors in thin film field effect transistors.1a,5
Although many CPDT-based molecules have low electro-
chemical bandgaps, the optical absorption in the longer wave-
length region (red edge) is typically low. This may be related to
the meta-linkage of the ketone or dicyanomethylene group
with the thiophene unit, in which the pi-conjugation may not
extend to the heteroatom of the bridgehead.3c,4a,6 We hypo-
thesized that the para-conjugated isomer, where sulfur atoms
and the bridgehead are located on the same side, may provide
a pathway to increased red-edge absorption. The reason for
this expectation stems from the possibility that the keto-group
Chart 1 Model of anti-CPDT and syn-CPDT.
in the para-isomer affords the opportunity to provide a keto-
cyanine type conjugation.7 Therefore, we targeted the anti
(meta-linkage) and syn (para-linkage) isomer of CPDT com-
pounds (Chart 1) to understand the effect of heteroatom con-
formation on the optical and electrical properties.
In our work here, ketone and imine will serve as the bridge-
head functionalities. Incorporation of the imine group will
provide extended conjugation at the bridgehead along with
the n-octyl solubilizing group. The bridgehead and the sulfur
atoms of anti-CPDT molecules are located on the opposite
side, while the bridgehead and the sulfur atoms of syn-CPDT
molecules are located on the same side. The structures of the
CPDT derivatives are shown in Fig. 1 and the synthetic routes
of the target molecules are described in Schemes 1 and 2.
The anti-CPDT derivatives were synthesized according to
the procedures shown in Scheme 1.8 Treatment of bithiophene
using bromine/acetic acid yielded the tetrabromo-biothio-
phene derivative 5 in 87% yield. The bromine moieties at the
α-position of 5 were first protected as trimethylsilyl groups.
This was achieved through bromine–lithium exchange with
only two equivalents of n-BuLi, which selectively converts the
α-bromo functionalities to the corresponding lithio derivative.
The dilithio intermediate was treated with trimethylsilyl
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts
01003, USA. E-mail: thai@chem.umass.edu
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, cyclic voltammograms and thermal analyses. See DOI: 10.1039/
c3ob41648h
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Fig. 1 Chemical structure of CPDT derivatives 1–4.
Scheme 1 Synthesis of CPDT-a-ketone and CPDT-a-imine.
chloride to obtain 6 in 79% yield. Compound 6 was then N-bromosuccinimide (NBS). Compound 8 was then coupled
treated with two more equivalents of n-BuLi to generate with 2-hexyl-5-trimethylstannyl thiophene 9 using Pd(II)-cata-
the corresponding β-dilithio intermediate, which was treated lyzed Stille coupling reactions in microwave to obtain targeted
with N,N-dimethylcarbamoyl chloride to obtain the ketone 7 CPDT-a-ketone molecule 1 in 47% yield. Condensation of 1
in only 12% yield. The TMS groups were then converted with 4-(octyloxy)aniline gave CPDT-a-imine molecule 2 in 63%
to bromo moieties to afford the dibromide derivative 8 using yield.
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Scheme 2 Synthesis of CPDT-s-ketone and CPDT-s-imine.
The syn-CPDT derivatives were synthesized starting from
lithiation of 3-bromothiophene and subsequent treatment
with 3-bromothiophene-2-carboxaldehyde to obtain 10 in 28%
yield. The alcohol 10 was then oxidized by PCC to obtain 11 in
53% yield. Microwave assisted Ullman coupling reaction of 11
using Cu powder as a catalyst provided syn-CPDT 12 in good
yield. The bromination of 12 using NBS afforded 13 in 65%
yield. Compound 13 was coupled with 2-hexyl-5-trimethylstannyl
thiophene 9 using Pd(II) catalyzed Stille coupling conditions via
microwave irradiation to obtain 3 in 57% yield. Condensation
of 3 with 4-(octyloxy)aniline gave compound 4 in 27% yield.
To determine the effect of the position and functionality of
the bridgehead on the optical absorption and the optical
energy gaps (Eg,opt), linear absorption spectra of the CPDT
derivatives were measured in CH2Cl2 solution (Fig. 2). The
optical bandgap is determined from the onset of the long-
wavelength absorption edge (Eg,opt = 1240/λonset).
Fig. 2 UV-vis spectra of CPDT derivatives in CH2Cl2 solution.
All molecules exhibited a relatively broad low energy absorp-
tion band and several higher energy bands. CPDT-a-ketone
exhibits absorption bands that are most red-shifted; the low
energy band of this molecule had the onset around 780 nm results in a significant blue shift (Δλmax = 33 nm), as discerned
with the λmax of this band centering around 592 nm. Changing from comparing CPDT-a-ketone with CPDT-s-ketone. The low
the carbonyl bridgehead from the anti- to the syn-position energy absorption band of the latter molecule centers around
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Table 1 Summary of optical absorption maxima, absorption coeffi-
Table 2 Summary of electrochemical data for CPDT derivatives
cients and the optical bandgaps of CPDT derivatives
Compounds
HOMO (eV)
LUMO (eV)
Eg, elec (eV)
εmax
λonset Eg,opt
(nm) (eV)
(×104 M−1 cm−1
)
CPDT-a-ketone
CPDT-s-ketone
CPDT-a-imine
CPDT-s-imine
−5.08
−5.27
−5.22
−5.23
−3.55
−3.46
−3.44
−3.43
1.53
1.81
1.78
1.80
Compounds
λmax (nm)
CPDT-a-ketone 234, 281, 320,
4.1, 4.9, 4.0,
4.7, 0.7
780
1.59
368, 592
CPDT-s-ketone 234, 286, 330, 559 4.8, 4.1, 4.2, 0.7
699
711
700
1.77
1.74
1.77
The cyclic voltammograms were recorded in anhydrous CH2Cl2 with
0.1 M terabutylammonium hexafluorophosphate (TBAPF6) at a scan
rate of 50 mV s−1. The electrochemical potentials were estimated from
the onset of the oxidation and reduction peaks. All voltammograms
were normalized to E1/2 (Fc/Fc+). The HOMO and LUMO energy levels
CPDT-a-imine
CPDT-s-imine
235, 289, 379, 547 6.7, 8.2, 8.5, 1.3
238, 332, 559 0.4, 1, 0.2
were calculated using the following relationship: HOMO = −(Eox,onset
4.8) eV and LUMO = −(Ered,onset + 4.8) eV.
+
599 nm with the onset at 699 nm. The change in absorption
wavelength maxima can be attributed to the less conjugated
quinoid form of CPDT-s-ketone, as compared to CPDT-a-
ketone. Nonetheless, the fact that the interruption of the
CPDT conjugation results in only a 33 nm blue shift in the
absorption also suggests that the keto-cyanine type contri-
bution to the optical properties exists. It is noteworthy however
that this positional variation had no effect on the absorption
coefficients of the low energy bands (Table 1).
Next, we investigated the effect of the imine substitution
upon the optical properties of the molecules. Incorporation of
the imine group, especially an aromatic imine, offers another
opportunity to evaluate the possibility of the keto-cyanine con-
tribution to the optical properties of the molecule. Here, the
charge distribution possibility in the aromatic ring of the
aniline moiety could provide an enhanced contribution from
the cyanine type structure. Upon analyzing the absorption
spectra of these molecules, we found that the bridgehead in
the anti-isomer, CPDT-a-imine, results in a large blue-shift
(Δλmax = 45 nm) with a higher absorption coefficient compared
to CPDT-a-ketone. Interestingly however, in the case of the syn-
isomer, the imine substitution had very little effect on the
absorption spectrum of the molecule, compared to the corres-
ponding ketone (Table 1). This supports the possibility that
the cyanine-type contribution allows the syn-isomers to exhibit
low energy absorption bands.
Next, the electrochemical properties of the compounds
were analyzed by cyclic voltammetry. The estimated HOMO
and LUMO energy levels are summarized in Table 2. The elec-
trochemical bandgaps of all the CPDT molecules were found
to be similar to their optical bandgaps (Table 1). Indeed,
CPDT-a-ketone exhibited the lowest bandgap of 1.53 eV and
the effect of incorporating imines was also consistent with the
photophysical measurements shown above.
Finally, we were interested in investigating the charge
carrier mobility of these CPDT derivatives using bottom-
contact field-effect transistors.1b,9 An example of the output
characteristics of CPDT and a summary of the mobilities of all
the semiconductors are given in Fig. 3 and Table 3, respect-
ively. All molecules were tested for both hole and electron con-
ductivity and the thin film was prepared by spin-casting the
molecules on the substrate. All CPDT molecules exhibited only
hole mobility. The mobility of CPDT-a-ketone was found to be
0.003 cm2 (V s)−1. Changing the position of the carbonyl group
from the anti to the syn-position results in a dramatically
Fig. 3 Output characteristics of CPDT-a-ketone at W/L = 500.
Table 3 Charge mobility of CPDT derivatives
µh [cm2 (V s)−1], as
cast
µh [cm2 (V s)−1],
annealed at 80 °C
Compounds
CPDT-a-ketone
CPDT-s-ketone
CPDT-a-imine
CPDT-s-imine
(3.0 0.8) × 10−3
(6.9 3.0) × 10−6
(1.4 0.9) × 10−5
(2.2 0.6) × 10−9
(8.2 0.7) × 10−3
(6.4 2.3) × 10−4
(1.3 0.9) × 10−5
No mobility
decreased mobility of 6.9 × 10−6 cm2 (V s)−1 as observed for
CPDT-s-ketone. We do not fully understand the reason for
such a large change in mobility, but these are usually attribu-
ted to the differences in molecular packing.10 The mobilities
of CPDT-a-imine and CPDT-s-imine were found to be 1.4 ×
10−5 cm2 (V s)−1 and 2.2 × 10−9 cm2 (V s)−1, respectively.
Overall, the syn-isomers show significantly lower mobilities
compared to the corresponding anti-isomers. In addition,
replacing the ketone group with the imine group lowered the
mobility. Thus, the syn-imine exhibited the lowest mobility,
since it has the unfavorable conformation and functionality.
The significant decrease in mobility in imines may be attribu-
ted to the insulation from the long alkyl chain of the imine
units that prevents efficient intermolecular packing in the
solid-state. Moreover, the effect of annealing was studied
for all compounds. CPDT-s-ketone exhibited the highest
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enhancement in mobility from 6.9 × 10−6 cm2 (V s)−1 to 6.4 ×
10−4 cm2 (V s)−1. However, this mobility was still lower than
that of the corresponding anti-isomer, CPDT-a-ketone. Also,
the thermal annealing effect is not significant for other CPDT
derivatives. Our attempts to grow single crystals of these semi-
conductor molecules were not successful. When analyzed by
differential scanning calorimetry (DSC), CPDT-a-ketone is the
only molecule that clearly exhibited a crystalline phase (see
ESI†).
In summary, four CPDT cores were designed and syn-
thesized to study the effect of heteroatom conformation on
optoelectronic properties of CPDT derivatives. We found that
CPDT-a-ketone exhibits the lowest optical and electrical
bandgaps and the highest hole mobility, 0.003 cm2 (V s)−1. On
the other hand, changing the carbonyl conformation to the
syn-position or replacing it with the imine group results in no
significant change of the absorption coefficient in the lower
energy region. However, these changes do result in increased
optical and electrochemical bandgaps, as well as reduced
charge carrier mobilities.
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