BODIPY-based donor-acceptor π-conjugated alternating copolymers

Article abstract of DOI:10.1021/ma200839q

Four novel π-conjugated copolymers incorporating 4,4-difluoro-4-borata- 3a-azonia-4a-aza-s-indacene (BODIPY) core as the "donor" and quinoxaline (Qx), 2,1,3-benzothiadiazole (BzT), N,N′-di(2′-ethyl) hexyl-3,4,7,8-naphthalenetetracarboxylic diimide (NDI), and N,N′- di(2′-ethyl)hexyl-3,4,9,10-perylene tetracarboxylic diimide (PDI) as acceptors were designed and synthesized via Sonogashira polymerization. The polymers were characterized by ¹H NMR spectroscopy, gel permeation chromatography (GPC), UV-vis absorption spectroscopy, and cyclic voltammetry. Density functional theory (DFT) calculations were performed on polymer repeat units, and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were estimated from the optimized geometry using B3LYP functional and 6-311g(d,p) basis set. Copolymers with Qx and BzT possessed HOMO and LUMO energy levels comparable to those of BODIPY homopolymer, while PDI stabilized both HOMO and LUMO levels. Semiconductor behavior of these polymers was estimated in organic thin-film transistors (OTFT). While the homopolymer, Qx, and BzT-based copolymers showed only p-type semiconductor behavior, copolymers with PDI and NDI showed only n-type behavior.

Full text of DOI:10.1021/ma200839q

ARTICLE
pubs.acs.org/Macromolecules
CB Oo pD oI Pl yYm- Be ar ss ed DonorꢀAcceptor π-Conjugated Alternating
Bhooshan C. Popere, Andrea M. Della Pelle, and S. Thayumanavan*
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States
S Supporting Information
b
ABSTRACT: Four novel π-conjugated copolymers incorporating
4,4-difluoro-4-borata-3a-azonia-4a-aza-s-indacene (BODIPY) core
as the “donor” and quinoxaline (Qx), 2,1,3-benzothiadiazole
0 0
BzT), N,N -di(2 -ethyl)hexyl-3,4,7,8-naphthalenetetracarboxylic
(
0
0
diimide (NDI), and N,N -di(2 -ethyl)hexyl-3,4,9,10-perylene
tetracarboxylic diimide (PDI) as acceptors were designed and
synthesized via Sonogashira polymerization. The polymers
1
were characterized by H NMR spectroscopy, gel permeation
chromatography (GPC), UVꢀvis absorption spectroscopy,
and cyclic voltammetry. Density functional theory (DFT)
calculations were performed on polymer repeat units, and
the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were
estimated from the optimized geometry using B3LYP functional and 6-311g(d,p) basis set. Copolymers with Qx and BzT
possessed HOMO and LUMO energy levels comparable to those of BODIPY homopolymer, while PDI stabilized both HOMO
and LUMO levels. Semiconductor behavior of these polymers was estimated in organic thin-film transistors (OTFT). While the
homopolymer, Qx, and BzT-based copolymers showed only p-type semiconductor behavior, copolymers with PDI and NDI
showed only n-type behavior.
’
INTRODUCTION
,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacene dyes, more
solar cells, it is reasonable to assume that the corresponding
homopolymers involving the BODIPY core in the π-conjugated
backbone should also function as donor polymers in polymerꢀ
PCBM BHJ solar cells. Indeed, an ethynyl-bridged alternating
copolymer of BODIPY with thiophene has shown electron
donor characteristics in BHJ solar cells. The optical bandgap of
the resulting copolymer, however, was identical to that of the
4
commonly known as BODIPY dyes, have been long since
recognized for their excellent optical properties such as large
absorption coefficients, high fluorescence quantum yields,
1
and remarkable photostability. Additionally, straightforward
chemical synthesis and structural robustness have enabled fine-
tuning of optical properties of BODIPY dyes via systematic
structural variations. For instance, small-molecule BODIPY
derivatives with solution-state absorption maxima ranging from
7
homopolymer.
The electrochemical characteristics of several small-molecule
BODIPY derivatives show excellent reversibility during both
8
oxidation and reduction. To us, this implies that the BODIPY
5
00 to 700 nm have been reported and are commonly used in
2
core is capable of stabilizing an additional electron as well as a
hole, both in small molecules and in π-conjugated polymers.
Indeed, the cyclic voltammogram of our BODIPY-based homo-
polymer (vide infra) clearly indicates both reduction and oxida-
tion behavior in thin films. This provides us with a unique and
rare opportunity to systematically study the effect of different
electron-rich or electron-poor comonomers on the optical and
electrochemical properties of BODIPY-based conjugated alter-
nating copolymers, which is currently lacking in the literature.
Several acceptor units have been designed synthesized and
used in conjunction with various donor segments, effecting
biological imaging. Additionally, the optical properties, ease of
synthetic modification, and chemical and photostability make
BODIPY-based systems attractive candidates for light-harvesting
3
applications. Owing to large extinction coefficients, intense
absorption spectra that extend into the red region of the visible
spectrum, and decent hole mobility, small-molecule BODIPY
derivatives have been recently employed as p-type or donor
materials in conjunction with PCBM in bulk heterojunction
4
(
BHJ) solar cells.
π-Conjugated polymers based on the BODIPY core naturally
extend the absorption into the deep-red regions of the visible
spectrum and, in some cases, are excellent near-infrared emitters.
The relatively high extinction coefficients of these polymers
combined with their photostability make them attractive candi-
dates for optoelectronic applications, specifically polymeric
9
5
reduced bandgaps in the resulting π-conjugated polymers.
Although the end result of most DꢀA combinations has been
Received: April 11, 2011
6
photovoltaics. On the basis of the prior knowledge that small
Revised:
May 6, 2011
molecule BODIPY derivatives function as electron donors in BHJ
Published: May 19, 2011
r 2011 American Chemical Society
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Chart 1. Chemical Structures of the Synthesized BODIPY-Based Homopolymer and Copolymers
a reduced bandgap compared to the corresponding donor homo-
deeper insight into the peculiar characteristics of each of these
polymers, we have performed DFT calculations on the respective
model compounds that constitute the repeating units of the
corresponding polymers. Furthermore, semiconductor behavior
of these polymers was evaluated using thin-film field-effect
transistor measurements.
10
polymers, each combination has a unique set of properties. Among
1
1
the myriad of acceptor units studied so far, o-quinoid acceptors
12
12g,13
such as 2,1,3-benzothiadiazole, quinoxaline,
pyrazine,
thieno[3,4-b]-
1
0g,14
0
benzo[1,2-c,4,5-c ]bis[1,2,5]thiadiazole, and
11b,16
15
[1,2,5]thiadiazolo[3,4-g]quinoxaline
have garnered much
attention. Alternating copolymers made from aromatic donors
and these o-quinoid acceptor heterocycles are highly polarized
and possess small HOMOꢀLUMO gaps, leading to absorption
spectra extending into the near-infrared region.
’
RESULTS AND DISCUSSION
Synthesis and Characterization. To synthesize the targeted
0
Second, small-molecule and polymeric systems based on N,N -
polymers, we first synthesized the corresponding monomer
units. 2,6-Diethynyl BODIPY was synthesized according to
the reported procedure with small modifications wherever
necessary as outlined in Scheme 1. Briefly, condensation of
4-(2 -ethyl)hexyloxybenzaldehyde (1) with 2,4-dimethylpyr-
17
dialkyl-1,4,5,8-naphthalenetetracarboxylic diimide (NDI) and
0
18
N,N -dialkyl-3,4,9,10-perylenetetracarboxylic diimide (PDI)1
9
6
have been demonstrated to possess excellent electron mobility.
0
However, reports on alternating DꢀA type copolymers synthe-
sized using these rylene acceptors are rather scarce compared with
o-quinoid acceptors. In addition to possessing reduced optical
bandgaps, the copolymers based on π-conjugated PDI or NDI
units (substituted through the aromatic core) show excellent
role gave the corresponding dipyrromethane. This was then
oxidized by DDQ to generate dipyrromethene followed by
coordination with trifluoroborane dietherate to give the
BODIPY core (2) in 25% overall yield. Iodination of the
BODIPY core at the 2- and 6-position using N-iodosuccini-
mide yielded 2,6-diiodo BODIPY (3) in about 80% yield. This
was then subjected to Pd(II)- and CuI-catalyzed Sonogashira
cross-coupling reaction with trimethylsilyl (TMS) acetylene
to generate 2,6-bis(trimethylsilylethynyl) BODIPY (4). The
TMS groups were subsequently deprotected using tetrabuty-
lammonium fluoride at reduced temperatures to finally yield
2,6-diethynyl BODIPY (5). It is important to carry out this
reaction at reduced temperatures because the diacetylide
anions generated during this reaction are nucleophilic enough
to react with the boron center at room temperature and
20
electron transport characteristics. Consequently, these polymers
have been used as fullerene substitutes in all-polymer organic solar
cells with reduced optical bandgaps, broadened absorption across
the visible spectrum, and deep HOMO and LUMO energy
2
1
levels.
In light of the above discussion, we have designed, synthesized,
and characterized four BODIPYꢀacceptor alternating π-conju-
gated polymers (Chart 1). We have chosen two acceptor units
each from the o-quinoid and rylene type acceptors. Quinoxaline
(
Qx) and 2,1,3-benzothiadiazole (BzT) constitute the o-quinoid
0
acceptors, while N,N -di(2-ethyl)hexyl-1,4,5,8-naphthalenetetra-
carboxylic diimide and N,N -di(2-ethyl)hexyl-3,4,9,10-perylene-
0
22
thereby replace the fluorine atoms irreversibly.
tetracarboxylic diimide (PDI) constitute the rylene acceptors.
Steady-state UVꢀvis absorption spectroscopy and thin film
cyclic voltammetry were used to determine the optical and
electrochemical characteristics of these polymers. To gain a
Scheme 2 outlines the synthesis of the o-quinoid acceptor
units, namely 4,7-dibromo-2,1,3-benzothiadiazole and dibromo-
quinoxaline. 2,1,3-Benzothiadiazole (6) was subjected to bromi-
nation under acidic conditions according to a previously reported
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Scheme 1. Synthesis of BODIPY Monomer
Scheme 2. Syntheses of Acceptor Monomers
13l
24
procedure to yield the corresponding dibromo compound 7.
sulfuric acid and bromine. Reaction of the corresponding
Desulfurization of 4,7-dibromo-2,1,3-benzothiadiazole resulted
in the 2-amino-3,6-dibromoaniline intermediate that was then
condensed in situ with glyoxal to give dibromoquinoxaline (8) in
dibromo dianhydride (10) with 2-ethylhexylamine gave com-
pound 11 in moderate yields. Similarly, N,N -di(2 -ethyl)hexyl-
0
0
1,7-dibromo-3,4,9,10-perylenetetracarboxylic diimide (14) was
2
3
25
moderate yields.
synthesized according to a previously reported protocol.
0
N,N -Di(2-ethyl)hexyl-2,6-dibromo-1,4,5,8-naphthalene-
tetracarboxylic diimide (11) diimide was synthesized by first
brominating the corresponding anhydride (9) using fuming
Briefly, 3,4,9,10-perylenetetracarboxylic acid dianhydride (12)
was brominated under hot sulfuric acid conditions in the
presence of a catalytic amount of molecular iodine. This reaction
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Scheme 3. Syntheses of BODIPY-Based Polymers via Sonogashira Polymerization
yielded a product mixture primarily comprising of the desired
,7-dibromo isomer, along with, 1,6-dibromo and the 1,6,7-
Table 1. Molecular Weights of the BODIPY-Based Conju-
gated Polymers
1
tribromo PTCDA isomers that could not be separated at this
stage. Imidation with 2-ethylhexylamine in hot N-methylpyrro-
lidinone yielded the corresponding imides. Once again, the
product of this reaction contained a mixture of the 1,7-dibromo
and the 1,6-dibromo PDI isomers that were used as such. The
ratio of 1,7- and 1,6- isomers was found to be 80:20 as investigated
a
a
a
polymer
M
n
[g/mol]
M
w
[g/mol]
PD
p(BODIPY)
4200
2900
2300
4000
4100
7100
4300
3200
8200
9200
1.69
1.48
1.39
2.05
2.24
p(BODIPY-alt-Qx)
p(BODIPY-alt-BzT)
p(BODIPY-alt-NDI)
p(BODIPY-alt-PDI)
1
by H NMR spectroscopy (See Supporting Information).
All the polymers were synthesized via the Sonogashira con-
densationpolymerization reaction using 1:1 monomer feed ratios.
In a typical experiment, 2,6-diethynyl BODIPY was reacted with
the corresponding dihalide in presence of Pd(PPh ) and CuI
a
M , M , and PD of polymers were determined by GPC using
polystyrene standards with THF as eluent.
n
w
3
4
to yield the corresponding polymer. The polymerizations were
carried out in a degassed mixture of anhydrous diisopropylamine
and anhydrous toluene at 60 °C for 24 h. The crude polymers
were precipitated from methanol, filtered, and dried. In each case,
the crude polymer was then purified by Soxhlet extraction with
methanol, acetone, and chloroform. The chloroform fraction was
then concentrated under reduced pressure, reprecipitated from
methanol, filtered, and dried to yield a deep violet to deep blue
polymer. The resulting polymers were readily soluble in chloro-
form, THF, chlorobenzene, and o-dichlorobenzene.
Optical Properties. The UVꢀvis absorption spectra of the
polymers in dilute chloroform solutions and as thin films are
shown in Figure 1. Steady-state absorption spectra of p-
(BODIPY), p(BODIPY-alt-Qx), and p(BODIPY-alt-BzT) show
two distinct absorption bands: a high-energy band between 350
and 475 nm that is relatively independent of the nature of the
acceptor units and a low-energy band between 500 and 750 nm
6
that is characteristic of all BODIPY-based conjugated polymers.
The high intensity of this band reflects the large extinction
coefficient of the BODIPY core. The absorption maxima of
p(BODIPY), p(BODIPY-alt-Qx), and p(BODIPY-alt-BzT) in
chloroform solutions at room temperature are 656, 596, and
604 nm, respectively, while the corresponding absorption
onsets are at 690, 640, and 650 nm, respectively. It is interesting
to note that the absorption spectra of p(BODIPY-alt-Qx) and
p(BODIPY-alt-BzT) show a considerable blue shift relative to
that of p(BODIPY). This is contrary to other π-conjugated
1
The polymers were characterized with H NMR spectroscopy
and gel permeation chromatography (GPC). The proton signals
in the NMR spectra were broadened compared to the monomers
indicative of successful polymerization of the monomers. For
copolymers, integration of the proton signals in the aromatic
regionindicated 1:1 polymercomposition. Themolecularweights
of the polymers were estimated by GPC using THF as the mobile
phase and calibrated against polystyrene standards (Table 1).
1
3d,e,g
polymers that incorporate BzT and Qx as “acceptor” units.
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Figure 1. UVꢀvis absorption spectra of BODIPY-based polymers represented as extinction coefficients in chloroform solutions (left) and normalized
absorption in thin films annealed at 100 °C (right).
Table 2. Summary of Optical Properties of BODIPY-Based Conjugated Polymers
thin films
solution
as cast
annealed
opt a
elec b
polymer
p(BODIPY)
λ
max [nm]
λ
onset [nm]
λ
max [nm]
λ
onset [nm]
λ
max [nm]
λ
onset [nm]
E
g
[eV]
E
g
[eV]
656
596
604
634
656
690
640
650
715
765
670
595
634
618
656
735
712
720
732
770
678
593
598
660
660
745
750
740
762
788
1.66
1.86
p(BODIPY-alt-Qx)
p(BODIPY-alt-BzT)
p(BODIPY-alt-NDI)
1.65
1.67
1.63
1.57
1.75
1.86
1.79
p(BODIPY-alt-PDI)
opt
1.67
b
a
elec
Eg calculated from the intersection of the tangent drawn to the lowest energy absorption edge with the baseline in annealed thin films. E
=
g
elec
elec
LUMO ꢀ HOMO as determined from cyclic voltammetry (vide infra).
However, the corresponding absorption spectra in annealed thin
films are significantly broadened and red-shifted (∼50 nm)
compared to the solution-state spectra, presumably due to
planarization of the polymer backbone and enhanced interchain
interactions in solid state. Thin-film spectra of p(BODIPY) and
p(BODIPY-alt-BzT) show additional features, while that of
p(BODIPY-alt-Qx) remains featureless. Conservatively esti-
mated optical bandgaps from the low-energy absorption edges
for p(BODIPY), p(BODIPY-alt-Qx), and p(BODIPY-alt-BzT)
are 1.66, 1.65, and 1.67 eV, respectively.
The solution-state absorption spectrum of p(BODIPY-alt-
NDI) shows a global maximum at 634 nm while the correspond-
ing absorption onset is at 715 nm. In thin films, the absorption
maximum and the onset red shift to 660 and 762 nm, respec-
tively, implying planarization of the polymer backbone in the
solid state. The absorption spectrum of p(BODIPY-alt-PDI),
however, is markedly different from all the other polymers
discussed above in several ways. First, in addition to the
BODIPY-based transition at 656 nm, we see a distinct low-
energy band centered at 715 nm in solution that can be attributed
to the BODIPY-to-PDI intramolecular charge-transfer (ICT)
transition that tails off into the deep-red region of the spectrum.
Furthermore, contrary to most low bandgap conjugated poly-
mers, the intensity of this ICT band is comparable to the most
intense feature in the absorption spectrum. Second, we also see a
distinct band between 450 and 575 nm that can be attributed to
21b,26
the PDI-based transition.
Once again, owing to the large
extinction coefficient of the PDI core, the intensity of this feature
is comparable to that of the BODIPY-based transition. Thus,
p(BODIPY-alt-PDI) shows a uniform absorption spectrum that
spans over most of the visible spectrum. Third, we do not see any
bathochromic shift in the solid-state absorption spectrum. This
could be due to the lack of further planarization of the polymer
backbone. This has been observed with a few examples of
2
6
conjugated polymers employing PDI units in their backbone.
The optical bandgap of p(BODIPY-alt-PDI) estimated from the
absorption onset in the red region is 1.57 eV.
Electrochemical Properties. The redox properties of BODI-
PY-based polymers were evaluated using cyclic voltammetry.
Polymer films were drop-cast from chloroform solutions onto a
2 mm diameter Pt disk electrode. The cyclic voltammograms were
þ
recorded against Ag/Ag reference electrode in anhydrous acet-
onitrile with 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF ) as the supporting electrolyte at a scan rate of
6
50 mV/s. The electrochemical potentials were estimated from
the onset of the oxidation and reduction sweeps. All voltammograms
þ
were calibrated using the Fc/Fc redox couple. The redox
potentials thus obtained were converted to the corresponding
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energy levels assuming the absolute HOMO energy level of
ferrocene to be ꢀ4.8 eV (eq 1).
characteristic features of the respective rylene diimide in the
reduction scans of the cyclic voltammograms. p(BODIPY-alt-
NDI) shows two characteristic quasi-reversible reductions, the
2
7
28
HOMO ¼ ꢀðE
þ 4:8Þ eV
ox,onset
first of which corresponds to reduction of NDI and the peak
around ꢀ1.5 V corresponds to the reduction of the BODIPY
core. Similarly, p(BODIPY-alt-PDI) shows three quasi-reversible
reduction peaks, the first two of which can be assigned to PDI-
centered reduction while the last one is characteristic of BODIPY-
ð1Þ
LUMO ¼ ꢀðEred,onset þ 4:8Þ eV
The onset oxidation and reduction potentials along with the
estimated HOMO and LUMO energy levels are summarized in
Table 3 for comparison, while the representative scans of the
polymer thin films are shown in Figure 2. The voltammograms
are displaced along the y-axis for clarity. All polymers show
irreversible oxidation and quasi-reversible reduction behavior.
Evidently, the redox potentials for p(BODIPY-alt-Qx) and p-
29
based reduction as discussed above. Both NDI and PDI small
molecules typically show two distinct reversible reduction peaks.
However, in the case of p(BODIPY-alt-NDI), we were unable to
see these two reduction peaks. It is possible that the second
reduction of NDI core and the reduction of the BODIPY core
occur around the same potential. We did not see any additional
peaks even at a lower scan rate (10 mV/s). The HOMO energy levels
thus estimated for p(BODIPY-alt-NDI) and p(BODIPY-alt-
PDI) are ꢀ5.55 and ꢀ5.73 eV, respectively, while the corre-
sponding LUMO energy levels are ꢀ3.76 and ꢀ4.09 eV. It is
gratifying to note that introduction of the PDI units in the
polymer backbone causes significant stabilization of the
HOMO and LUMO energy levels with a concurrent reduction
(BODIPY-alt-BzT) copolymers bear strong resemblance to the
that of p(BODIPY). Furthermore, the reduction scans of all three
polymers show a distinct peak at ∼ꢀ1.5 V that is characteristic of
the BODIPY reduction. The HOMO energy levels of p-
(BODIPY), p(BODIPY-alt-Qx), and p(BODIPY-alt-BzT) are
ꢀ
5.33, ꢀ5.33, and ꢀ5.44 eV, respectively, while the correspond-
ing LUMO energy levels are ꢀ3.47, ꢀ3.58, and ꢀ3.58 eV,
respectively. The electrochemical data and the optical data
unanimously indicate that BzT and Qx as acceptor units do
not significantly perturb the HOMO and LUMO energy levels of
the corresponding copolymers compared with the energy levels
of p(BODIPY) (Figure 2).
2
6
in the optical and electrochemical bandgaps. Although intro-
ducing NDI unit in the polymer backbone does not effectively
reduce the HOMOꢀLUMO separation of p(BODIPY-alt-
NDI), the resulting molecular orbitals are relatively stabilized
compared with p(BODIPY), p(BODIPY-alt-Qx), and p-
(BODIPY-alt-BzT).
p(BODIPY-alt-NDI) and p(BODIPY-alt-PDI), on the other
hand, show markedly different redox chemistry compared
with the other polymers in the series. Both polymers show
Theoretical Calculations. In order to gain a deeper insight
into the optical and electrochemical properties of our polymers,
we performed density functional theory (DFT) anaylsis on
model compounds constituting the corresponding repeat units.
HOMO and LUMO energy levels were estimated from the
optimized geometry using B3LYP functional and 6-311g(d,p)
basis set. The surface plots for the model compounds are shown
in Figure 3 for comparison. In principle, the HOMO and LUMO
energy levels of DꢀA type conjugated polymers depend on the
relative placement of the HOMO and LUMO energy levels of the
respective donor and acceptor segments. Our calculations sug-
gest that the LUMO energy levels of Qx (ꢀ2.62 eV) and BzT
Table 3. Summary of Electrochemical Data for BODIPY-
Based Polymers
ox a
elec b
red a
elec b
E
HOMO
E
LUMO
polymer
p(BODIPY)
[V]
[eV]
[V]
[eV]
þ0.64
þ0.64
þ0.75
þ0.86
þ1.04
ꢀ5.33
ꢀ5.33
ꢀ5.44
ꢀ5.55
ꢀ5.73
ꢀ1.22
ꢀ1.11
ꢀ1.11
ꢀ0.93
ꢀ0.60
ꢀ3.47
ꢀ3.58
ꢀ3.58
ꢀ3.76
ꢀ4.09
p(BODIPY-alt-Qx)
p(BODIPY-alt-BzT)
p(BODIPY-alt-NDI)
p(BODIPY-alt-PDI)
a
ox
red
(
ꢀ2.97 eV) lie very close to and, in the case of Qx, slightly above
E
and E determined from the onset potentials of the oxidation and
b
elec
ox
reduction waves, respectively. HOMO
= ꢀ(E þ 4.8) eV and
the LUMO energy level of BODIPY (ꢀ2.90 eV). This implies
elec
red
LUMO = ꢀ(E = 4.8) eV.
that BODIPY has a comparable electron affinity with Qx and
Figure 2. Cyclic voltammograms of all the polymers cast as thin films from chloroform solutions on platinum disk electrode (left) and comparative
HOMO and LUMO energy levels thus estimated (right).
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BzT, which reflects in the almost identical optical bandgaps for all
the three polymers: p(BODIPY), p(BODIPY-alt-Qx), and p-
function resides almost entirely on the NDI core, resulting in
an ineffective orbital mixing. Thus, there seems to be very
poor intramolecular π-electron delocalization along the
polymer backbone in p(BODIPY-alt-NDI). This explains
why although having comparable electron affinity, PDI and
NDI have different effect on the energy levels of the resulting
copolymers.
Thus, as seen from optical and electrochemical measurements
and theoretical calculations, both PDI and NDI cause a net
stabilization of HOMO and LUMO energy levels. This is not
unusual for rylene-based acceptors, as they are known to exert
strong electron-withdrawing forces thereby lowering the LUMO
energy levels, making the resulting copolymers efficient n-type
semiconductors. We believe that the broad absorption spectrum,
large extinction coefficients in the entire visible region, and
reduced bandgap could make p(BODIPY-alt-PDI) and p-
(BODIPY-alt-NDI) potentially useful as electron transporting
materials.
(BODIPY-alt-BzT). Our DFT calculations indicate that in the
case of BODIPY-Qx and BODIPY-BzT models both HOMO
and LUMO wave functions are delocalized with significant
contribution to the LUMO from the BODIPY core (Figure 3).
This is also reflected in the cyclic voltammograms of the
polymers that predominantly show redox potentials correspond-
ing to the BODIPY core (Figure 3).
On the other hand, the calculated LUMO energy level of PDI
is ꢀ3.68 eV, and hence, it has a higher electron affinity compared
to BODIPY. Thus, in principle, PDI can act as an acceptor while
used in conjunction with BODIPY while designing conjugated
polymers with reduced optical bandgaps. This is evident from the
absorption and electrochemical features discussed above. The
HOMO wave function in the case of BODIPYꢀPDI model is
extensively delocalized along the backbone, while the LUMO
resides on the PDI unit. This feature is characteristic of most
3
0
donorꢀacceptor polymers reported in the literature.
Charge Carrier Mobility Measurements. We investigated
the ability of our BODIPY-based polymers to behave as both
p-type and n-type semiconductors by analyzing the charge carrier
mobility in thin films using the field-effect transistor configura-
tion. Bottom-contact field-effect transistors were fabricated by
spin-coating 10 mg/mL polymer solutions from chlorobenzene
onto octadecyltrimethoxysilane (OTS) treated, heavily doped
Although the calculated LUMO energy level of NDI (ꢀ3.69 eV)
is lower than that of BODIPY and almost comparable to that
of PDI, the absorption spectrum of p(BODIPY-alt-NDI) does
not show an ICT band as observed in p(BODIPY-alt-PDI). Our
calculations on BODIPYꢀNDI units show that there is signifi-
cant orbital partitioning between the two components. While
the HOMO is primarily BODIPY-centered, the LUMO wave
SiO /Si substrates. The substrates were annealed at 80 °C for 1 h
2
before analyzing them for charge carrier mobility. All the
measurements were performed in a glovebox under an argon
atmosphere, and the results are summarized in Table 4. The
charge carrier mobilities were calculated from the saturation
regime using the following equation:
I ¼ ðμC W=2LÞ½ðV ꢀ V Þꢁ
D
i
GS
t
Analogous with small-molecule and polymeric BODIPY deri-
vatives, p(BODIPY), p(BODIPY-alt-BzT), and p(BODIPY-alt-
Qx) showed p-type semiconductor behavior. The hole mobility
ꢀ
6
2
for p(BODIPY) was found to be ∼3 ꢂ 10 cm /(V s), which is
7
comparable to the previously reported value. The incorporation
of BzT and Qx in the polymer backbone decreased the hole
mobility by a few orders of magnitude. On the other hand,
p(BODIPY-alt-NDI) and p(BODIPY-alt-PDI) showed n-type
semiconductor behavior. The saturation regime electron mobility
ꢀ
6
2
for p(BODIPY-alt-NDI) was found to be ∼4 ꢂ 10 cm /(V s),
while that for p(BODIPY-alt-PDI) was an order of magnitude
ꢀ
5
2
higher (∼1.5 ꢂ 10 cm /(V s)) (Figure 4). Although the
mobilities are relatively modest compared with some other
2
0a,c
PDI-based n-type semiconducting polymers,
to the best of
our knowledge, p(BODIPY-alt-NDI) and p(BODIPY-alt-PDI)
are the only polymers based on the BODIPY core that show
any n-channel activity. This supports our hypothesis that
p(BODIPY-alt-PDI) and p(BODIPY-alt-NDI) could potentially
Figure 3. HOMO and LUMO surface plots for BODIPYꢀacceptor
model compounds.
Table 4. Summary of Field-Effect Mobilites for BODIPY-Based Conjugated Polymers Determined from Bottom-Contact Thin-
Film Transistors
2
2
polymer
μ
max [cm /(V s)]
μ
avg [cm /(V s)]
V
t
[V]
I
on/off
ꢀ
6
þ
ꢀ6
ꢀ7
þ
3
p(BODIPY)
2.9 ꢂ 10 (h )
2.3 ꢂ 10 ((4.8 ꢂ 10 , h )
ꢀ75
ꢀ53
ꢀ44
þ43
þ37
10
10
10
10
10
ꢀ
9
þ
ꢀ10
ꢀ8
ꢀ10
þ
2
2
3
4
p(BODIPY-alt-Qx)
p(BODIPY-alt-BzT)
p(BODIPY-alt-NDI)
p(BODIPY-alt-PDI)
1.6 ꢂ 10 (h )
1.7 ꢂ 10
((5.5 ꢂ 10 , h )
ꢀ
8
þ
ꢀ9
þ
3.1 ꢂ 10 (h )
2.2 ꢂ 10 ((9.3 ꢂ 10 , h )
ꢀ
6
ꢀ
ꢀ6
ꢀ7
ꢀ
3.9 ꢂ 10 (e )
3.7 ꢂ 10 ((2.3 ꢂ 10 , e )
ꢀ
5
ꢀ
ꢀ5
ꢀ7
ꢀ
1.4 ꢂ 10 (e )
1.3 ꢂ 10 ((6.4 ꢂ 10 , e )
773
4
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Macromolecules
ARTICLE
Figure 4. Output characteristics (left) and transfer characteristics (right) for p(BODIPY-alt-PDI)-based bottom-gate bottom-contact thin-film field-
effect transistors at W/L = 2000.
be useful as n-type semiconducting materials. The output
were estimated by size exclusion chromatography on a single injection
mode Polymer Laboratories PL-GPC 50 using THF as the mobile phase
and toluene as the internal reference. The number-average molecular
characteristics show saturation behavior at a channel length of
3
5
μm, an on/off ratio of about 10 , and a threshold voltage of
about 35 V.
n
weights (M ) were calibrated against PS standards, and the output was
received and analyzed using an RI detector. Electrochemical measure-
ments were performed on a BASi Epsilon potentiostat in anhydrous
acetonitrile.
’
CONCLUSIONS
In summary, we have successfully synthesized and characterized
Mobility Measurements. Thin-film transistors were fabricated
four alternating π-conjugated polymers based on the BODIPY
core utilizing o-quinoid acceptors like quinoxaline and benzothia-
diazole, and rylene acceptors like NDI and PDI. The optical and
electrochemical properties of the BODIPYꢀacceptor conjugated
polymers depend on the electron affinity of the corresponding
acceptor units. While units with comparable electron affinity to
BODIPY do not significantly perturb the energy levels of the
corresponding copolymers, stronger acceptor units like PDI tend
to bring out the relative electron donor characteristics of the
BODIPY core. These polymers can be considered low bandgap
polymers with low-lying HOMO energy level. UVꢀvis absorption
spectra ofthese polymers reflect the characteristicsof the BODIPY
core. Specifically, the absorption spectrum of p(BODIPY-alt-PDI)
spans most of the visible spectrum (350ꢀ800 nm) while retaining
the intense absorption characteristics of both BODIPY and PDI
units. Preliminary charge transport measurements indicate that
BODIPY-based conjugated polymers can be active as both p-type
and n-type semiconductors and that the switch between p-type
and n-type can be accomplished by a choice of suitable comono-
mers. Thus, we believe that p(BODIPY-alt-PDI) holds a strong
potential to function as an efficient n-type semiconducting poly-
mer, with stable energy levels, reduced optical bandgap, and
panchromatic absorption.
using heavily doped silicon substrates with 230 nm thermally grown
2
SiO (capacitance of 14.9 nF/cm ). Gold electrodes were prepatterned
2
onto the dioxide layer with channel lengths of 2.5, 5, 10, and 20 μm. Prior
to film deposition these substrates were cleaned with acetone and
2-propanol followed by oxygen plasma cleaning for 2 min. The cleaned
substrates were then exposed to OTS vapors for at least 12 h at room
temperature where the surface was modified by slow evaporation and
condensation of the silane molecules. The substrates were then washed
with anhydrous toluene and then heated at 80 °C for 1 h to remove the
unbound silanes. The substrates were then coated by spinning 10 mg/mL
polymer solutions in anhydrous chlorobenzene (1500 rpm for 45 s) and
annealedat80 °C for1 h under a nitrogen atmosphere. The channel width
was fixed at 10 mm for all devices. Field-effect mobility was estimated
using a three-electrodeAgilent 4156C precisionsemiconductorparameter
analyzer.
Materials. All reagents were used as received without further
purification, unless otherwise specified. All monomers and polymers were
synthesized according to reported procedures with a few modifications
wherever necessary. Air- and moisture-sensitive reactions were conducted
in oven-dried glassware using a standard Schlenk line or drybox techniques
under an inert atmosphere of dry nitrogen.
General Procedure for Polymerization via Sonogashira
Polycondensation Reaction. A 5 mL pear-shaped Schlenk flask was
charged with both monomers and a magnetic stirrer. The flask was then
taken inside a glovebox maintained under an inert atmosphere of
’
EXPERIMENTAL SECTION
3 4
nitrogen. The catalyst Pd(PPh ) and cocatalyst (CuI) were added
1
Instrumentation. H NMR spectra were recorded on a 400 MHz
inside the glovebox. A 1:1 mixture of anhydrous diisopropylamine and
anhydrous toluene was degassed using freezeꢀpumpꢀthaw cycles. This
degassed solvent mixture was then added to the monomers, and the
reaction temperature was raised to 60 °C. The polymerization was
continued for 24 h. The polymers were then precipitated in excess
methanol and isolated via gravity filtration. The crude polymers were
then purified via continuous extraction using a Sohxlet apparatus. The
monomers and the catalysts were removed by extraction with methanol.
Acetone extraction removed the low molecular weight oligomers, and a
Bruker NMR spectrometer using the residual proton resonance of the
solvent as the internal standard. Chemical shifts are reported in parts per
million (ppm). When peak multiplicities are given, the following abbre-
1
3
viations are used: s, singlet; d, doublet;t, triplet; m, multiplet; b, broad. C
NMR spectra were proton decoupled and recorded on a 100 MHz Bruker
spectrometer using the carbon signal of the deuterated solvent as the
internal standard. UVꢀvis absorption spectra were recorded on a Cary
100 scan UVꢀvis spectrophotometer. Molecular weights of the polymers
4
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dx.doi.org/10.1021/ma200839q |Macromolecules 2011, 44, 4767–4776

Macromolecules
ARTICLE
final chloroform extraction yielded the desired polymers. We found that
in all polymerizations there remained a residue that was partly soluble
in THF.
as well as output and transfer characteristic curves. This material
is available free of charge via the Internet at http://pubs.acs.org.
p(BODIPY). 2,6-DiiodoBODIPY (50 mg, 0.07 mmol) and 2,6-diethy-
nylBODIPY (36 mg, 0.07 mmol) were added to a 5 mL Schlenk flask.
Pd(PPh ) (6.0 mg, 5.2 μmol) and CuI (2.0 mg, 10.4 μmol) were added
’
AUTHOR INFORMATION
Corresponding Author
*
3
4
E-mail: thai@chem.umass.edu.
to the flask under nitrogen. A degassed mixture of anhydrous diisopro-
pylamine (1.5 mL) and anhydrous toluene (1.5 mL) was added to the
flask, and the polymerization was carried out according to the general
procedure. The chloroform fraction was concentrated under reduced
’
ACKNOWLEDGMENT
1
pressure to yield a deep violet colored polymer (48 mg). H NMR (400
We thank partial support from the U.S. Department of Energy
3
MHz, CDCl ): δ 7.13 (br, 2H), 7.03 (br, 2H), 3.89 (br, 2H), 2.63
through the EFRC at UMass Amherst (DE-SC0001087) and the
U.S. Army Research Office (54635CH) through the Green
Energy Center.
(
s, 6H), 1.73 (br, 1H), 1.59 (s, 6H), 1.53ꢀ1.32 (br, 8H), 0.97 (br, 6H).
p(BODIPY-alt-Qx). Dibromoquinoxaline (23.0 mg, 0.08 mmol) and
2,6-diethynylBODIPY (40.0 mg, 0.08 mg) were added to a 5 mL Schlenk
flask. Pd(PPh ) (6.0 mg, 5.2 μmol) and CuI (2.0 mg, 10.4 μmol) were
3
4
added to the flask under nitrogen. A degassed mixture of anhydrous
diisopropylamine (1.5 mL) and anhydrous toluene (1.5 mL) was added
to the flask, and the polymerization was carried out according to the
general procedure. The chloroform fraction was concentrated under
’
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’
ASSOCIATED CONTENT
Supporting Information. Text giving detailed synthetic
S
b
procedure, characterization, and figures showing NMR spectra
4
775
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Macromolecules
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