Thiophene-based conjugated polymers with photolabile solubilizing side chains

Article abstract of DOI:10.1021/ma502289n

This paper describes a series of thiophene-based conjugated polymers that become insoluble upon irradiation with ultraviolet light. Stille or Suzuki reactions of appropriately substituted 2,5-bromothiophene derivatives yielded terthiophene and polythiophe

Full text of DOI:10.1021/ma502289n

Article
pubs.acs.org/Macromolecules
Thiophene-Based Conjugated Polymers with Photolabile Solubilizing
Side Chains
Zachary C. Smith,† Dianne M. Meyer,‡ Marc G. Simon,§ Cristian Staii,§ Deepak Shukla,‡
and Samuel W. Thomas, III*,†
†Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States
‡Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York 14650, United States
§Department of Physics and Astronomy and Center for Nanoscopic Physics, Tufts University, Medford, Massachusetts 02155, United
States
S
* Supporting Information
ABSTRACT: This paper describes a series of thiophene-based
conjugated polymers that become insoluble upon irradiation with
ultraviolet light. Stille or Suzuki reactions of appropriately
substituted 2,5-bromothiophene derivatives yielded terthiophene
and polythiophene derivatives with either o-nitrobenzyl (ONB)
ester or ONB ether photolabile side chains with n-octyl
substituents. Light-induced cleavage of these ONB side chains
with ultraviolet light at 365 nm cleaves the octyl chains
responsible for solubilization of the polymers away from the
conjugated main chains, rendering them insoluble. Consistent
with the accepted mechanism of ONB photolysis, those structural modifications that would yield a more stable benzylic radical
methyl substitution on the benzylic position, replacement of the ester with an ether, or bothyielded more efficient photolyses
as determined by (i) quantum yields of photolysis of ONB-substituted terthiophenes, and (ii) the percentage of polymer that
persists in UV-irradiated thin films upon rinsing with chloroform. These polymers behave as negative-tone photoresists, enabling
both direct photopatterning of conjugated polymers, and fabrication of multilayer conjugated polymer films by irradiating with
UV light after each spin-casting step. Although hole-mobility values of these polymers in thin film transistors were only ∼10−5
cm2 V−1 s−1, photolysis and rinsing did not cause significant degradation in performance.
Several approaches exist for addressing the drawbacks of
solubilizing chains. Cross-linking of polymer films is a popular
choice for limiting solubility upon film deposition, although the
alkyl chains remain in the film.9 There have been a number of
examples of thermally cleavable solubilizing chains, focusing on
tertiary esters that eliminate olefins and produce insoluble
carboxylic acid-functionalized CPs.10,11 The resultant films can
have performance that is superior to the films prior to
thermolysis.10 More recently, acid catalysis for cleavage of
solubilizing chains at lower temperatures has been reported,
including of silane-substituted polythiophenes.12,13 Approaches
that do not require high temperatures are important for
eventual coating of CPs on inexpensive, flexible substrates such
as poly(ethylene terephthalate).14 Photochemical approaches to
changing the solubility of CPs have the advantage of
spatiotemporal control and the possibility of mild conditions.
Some approaches have included photo-oxidative cleavage of the
CP backbone,15 photocleavage of solubilizing xanthate side-
INTRODUCTION
■
This paper describes conjugated oligomers and polymers (CPs)
with solubilizing chains connected to main chains through
different photocleavable linkers. Conjugated polymers are
active components of a number of solid-state optoelectronic
technologies, such as sensors, transistors, and photovoltaics.1−3
Consistent among the vast majority of reported CPs are the
presence of long alkyl chains, which impart solubility to these
otherwise insoluble materials.4 Solubility of CPs is critical both
during their preparationprecipitation during polymerization
limits yield and molecular weight of polymersand for solvent-
based processing of materials into thin films, which is an
important advantage that organic semiconductors have over
their inorganic counterparts.5 Although state of the art
semiconducting polymers generally possess them, solubilizing
chains can present some drawbacks: (i) they restrict fabrication
of solution-deposited multilayer films to materials that are
orthogonally soluble,6 (ii) they occupy volume with optoelec-
tronically inactive groups,7 and (iii) they participate in CP
degradation processes, such as hydrogen atom abstractions.8 In
a comprehensive study relating chemical structure of CPs to
their photochemical stability, Krebs and co-workers recom-
mended limiting their presence to “as low as possible”.7
Received: November 12, 2014
Revised: January 23, 2015
© XXXX American Chemical Society
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chains,16 and photopatterning of a conjugated polymer using a
Chart 1. Rationale for Design of Photolabile Linkers:
Increasing Radical Stability upon Intramolecular Hydrogen
Atom Abstraction during Photocleavage
photobase generator.17
The application of photocleavable groups to the design of
light-responsive polymers and surfaces is a rapidly growing area
of research.18,19 Although a number of photocleavable linkers
exist, o-nitrobenzyl (ONB) groups are among the most popular,
due to their ease of synthesis and the range of materials and
experimental conditions in which they function.20,21 Pauly and
Theato have used ONB groups in the photopatterning of
conjugated poly(phenylacetylene)s.21 Our group has reported a
polythiophene derivative comprising n-octyloxy chains con-
nected to the conjugated poly(thiophene) backbone through
photolabile ONB esters that cleave upon irradiation at 365 nm
(Scheme 1).22 Upon irradiation, films of this polymer became
Scheme 1. o-Nitrobenzyl Ester Photocleavage of Previously
Reported Polymer (P1) Resulting in the Analogous
Carboxylic Acid Containing Polythiophene and the Nitroso
Aldehyde Byproduct
of 1 with NaBH4 yielded primary nitrobenzyl alcohol 2, while
reduction of 1 with CH3MgBr/TiCl4 gave the corresponding
secondary nitrobenzyl alcohol 4. Acylation of either 2 or 4 with
known compound 2,5-dibromothiophene-3-carboxylic acid24
gave the nitrobenzyl ester-functionalized dibromide-function-
alized monomers 3 and 5. Stille cross-coupling of 3 with either
2-(tributylstannyl)thiophene or 2,5-bis(trimethylstannyl)-
thiophene yielded terthiophene O1 and polythiophene P1,
while cross-coupling of 5 under similar reaction conditions
yielded terthiophene O2 and polythiophene P2.
The key intermediates to prepare analogous targets with
nitrobenzyl ether linkers were 2,5-dibromothiophenes 7 and 9
(Scheme 3). Halo-dehydroxylation reactions of benzyl alcohols
2 and 4 yielded the corresponding benzyl bromides with PBr3
(6) or PPh3/CBr4 (8). Alkylation of 2,5-dibromo-3-(4-
hydroxyphenyl)thiophene with either of these alkyl bromides
gave the nitrobenzyl ether-functionalized monomers 7 and 9.
Stille cross-coupling of 7 with the appropriate stannanes yielded
terthiophene O3 and polythiophene P3, while cross-coupling of
9 gave analogues O4 and P4, each of which has one methyl
group in the benzylic position.
Our synthesis of O4 by Suzuki coupling highlights an
advantage of the ether linkage over the more commonly used
esters, because the standard, strongly basic conditions for
Suzuki coupling that we used can saponify esters.25 In addition,
the use of Suzuki coupling reactions increases the number of
easily accessible polymers because there are more commercially
availableand less toxicdiboronic acids and esters than
distannanes. As an example of this extended synthetic space, we
synthesized the donor−acceptor polymers P5 and P6 using
monomers 7 and 9, respectively, with the commercially
available diboronic ester of benzothiadiazole (Scheme 4).
Optical Properties of Conjugated Oligomers and
Polymers. Tables 1 and 2 summarize the properties of
absorbance and fluorescence emission of the terthiophenes and
conjugated polymers studied here, while Figure 1 shows
absorbance, emission, and excitation spectra of terthiophene
O4. Absorbance spectra of the ONB-containing materials
showed bands due to both the ONB chromophore in the UV
region of the spectrum at approximately 285−320 nm, as well
as the conjugated thiophene-containing main chains: terthio-
insoluble in toluene without significant photobleaching of the
polythiophene backbone. The work reported herein summa-
rizes detailed studies of this approach: (i) the effect of the
chemical structure of the ONB group on the efficiency of
photocleavage of solubilizing chains from terthiophenes and
polythiophenes, (ii) the preparation of multilayer CP films
using only one deposition solvent, (iii) the photopatterning of
CP films, and (iv) hole mobility measurements in thin film
transistors before and after photocleavage of solubilizing chains.
EXPERIMENTAL DESIGN
■
For the purposes of efficiency in processing, reducing the time
required for photochemical cleavage of solubilizing chains is
important. In addition, because both the CP backbones and
photocleavage byproducts compete with ONB groups for
absorption of light, reducing the number of photons required to
render polymers insoluble also reduces photobleaching. We
therefore designed conjugated oligomers and CPs with the
photocleavable units shown in Chart 1. The key difference
between these ONB groups is the stability of the corresponding
benzylic radical photolysis intermediate. Our hypothesis was
that those linkers with increased substitution or increased
electron density at the benzylic position would cleave more
efficiently: Bochet and co-workers recently showed a positive
correlation between radical stabilization energy of ONB radicals
and their quantum yields of photocleavage.23
RESULTS AND DISCUSSION
■
Synthesis of Conjugated Oligomers and Polymers. We
prepared nitrobenzyl ester-linked terthiophenes and polythio-
phenes, which either lacked (O1 and P1) or contained (O2 and
P2) benzylic methyl groups, as summarized in Scheme 2. The
first steps in the synthesis of these targets was to define the
substitution pattern at the benzylic position while reducing the
aldehyde group of previously described compound 1; reduction
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Scheme 2. Synthesis of Terthiophenes and Polythiophenes
Containing n-Octyl Solubilizing Chains Bound through
ONB Ester Photocleavable Linkers That Either Contain
Either Zero (O1, P1) or One (O2, P2) Benzylic Methyl
Scheme 3. Synthesis of Terthiophenes and Polythiophenes
Containing n-Octyl Solubilizing Chains Bound through
ONB Ether Photocleavable Linkers That Either Contain
a
Zero (O3, P3) or One (O4, P4) Benzylic Methyl Group
a
Group
a
Key: (i) PBr3, 0 °C, 46%; (ii) K2CO3, DMF, 50 °C, 69%; (iii)
Pd(PPh3)4, DMF, 90 °C, 43%; (iv) Pd2(dba)3, tri-o-tolylphosphine,
toluene, 90 °C, 85%; (v) CBr4, PPh3, 52%; (vi) K2CO3, DMF, 50 °C,
84%; (vii) NaHCO3, Pd(PPh3)4, DMF/H2O, 80 °C, 82%; (viii)
Pd2(dba)3, tri-o-tolyl phosphine, toluene, 90 °C, 92%.
Scheme 4. Synthesis of Copolymers P5 and P6
a
Key: (i) NaBH4, MeOH, 70%; (ii) 2,5-dibromo-3-thiophenecarbox-
ylic acid, DCC, DMAP, CH2Cl2, reflux, 43%; (iii) 2-(tributylstannyl)-
thiophene, Pd(PPh3)4, DMF, 90 °C, 83%; (iv) Pd2(dba)3,trio-
tolylphosphine, 2,5-bis(trimethylstannyl) thiophene, toluene, 90 °C,
95%; (v) TiCl4, CH3MgBr, Et2O, −78 °C, 95%; (vi) 2,5-dibromo-3-
thiophene carboxylic acid, DCC, DMAP, CH2Cl2, reflux, 50%; (vii)
Pd(PPh3)4, 2-(tributylstannyl)thiophene, DMF, 90 °C, 54%; (viii)
Pd2(dba)3, tri-o-tolylphosphine, 2,5-bis(trimethylstannyl)thiophene,
toluene, 90 °C, 82%.
phene chromophores absorbed in the UV between 320 and 350
nm, while polythiophenes P1−P4 have absorbance maxima
between 510 and 530 nm, reflecting the extension of the
conjugated π-systems upon comparing trimers to polymers.
The absorbance and fluorescence spectra of the thiophene-co-
benzothiadiazole polymers (P5-P6) are significantly red-shifted
from those of the polythiophenes (P1−P4), which we attribute
to their donor−acceptor character. The fluorescence quantum
yields of those materials without ONB substituents have
quantum yields of fluorescence similar to reported terthio-
phenes26 and polythiophenes;27 differences in molecular weight
and regioregularity of polythiophenes also contribute to
differences in optical properites.
The nature of the interactions between chromophores is key
to the study of any multichromophoric material. To help
understand the interactions of the ONB and thiophene-based
chromophores, we compared them to analogous compounds
that did not contain ONB groups. Terthiophenes O5 and O6,
which do not contain ONB groups, had 3−5 fold higher
quantum yields of fluorescence than ONB-containing analogues
Chart 2. Chemical Structures of Oligomers O5 and O6
Table 1. Relevant Properties of Terthiophenes O1−O6
λmax(abs), nm ε, M−1 cm−1 λmax(em), nm
ΦF
Φrxn
O1
O2
O3
O4
O5
O6
320
321
332
342
351
350
15000
17000
21000
17000
15000
19000
469
438
438
434
476
436
0.009
0.006
0.005
0.004
0.03
0.011
0.054
0.12
0.18
−
−
0.02
C
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excitation of the terthiophene and not the ONB moiety. We
therefore conclude that energy transfer from the ONB groups
to the terthiophenes is not highly competitive with other
photophysical or photochemical pathways, including bond
cleavage, indicating the general applicability of using nitro-
benzyl-based photocleavable groups in multichromophoric
materials.
Table 2. Characterization Details of P1−P7
λmax(abs), nm
λmax(em), nm
ΦF
Mn, g/mol
PDI
P1
P2
P3
P4
P5
P6
P7
520
509
529
520
546
544
464
627
590
626
579
676
663
581
0.14
0.19
0.10
0.13
0.13
0.17
0.33
18400
7000
1.20
1.14
2.14
1.58
1.65
2.12
1.42
12800
14800
46400
10200
6050
ONB Photocleavage. As we have observed in previous
studies of conjugated small molecules bound to nitrobenzyl
groups,22,28 we found that fluorescence emission intensity from
the terthiophene of O4 in solution increased upon irradiation
with UV light, as shown in Figure 1. After irradiation at 365 nm
for 3 min in CH2Cl2, the emission intensity of the sample
increased 3-fold. This observation is consistent with photo-
cleavage of the fluorescence quenching ONB groups, as upon
their removal from the conjugated terthiophene backbone, the
concentrations of any photolysis byproducts in solution are too
small for diffusional quenching to compete with the
unimolecular relaxation of the terthiophene excited states.
One of our key objectives was to determine the relationship
between the structure of the photocleavable ONB linkers and
the efficiencies of their photocleavage reactions when bound to
otherwise identical conjugated terthiophene backbones. We
therefore monitored the progress of the photoreaction of each
1
terthiophene oligomer by H NMR spectroscopy as a function
of the number of photons absorbed from a 200 W Hg/Xe lamp
at 365 nm, the flux of which we determined using the
potassium ferrioxalate chemical actinometer. As the reactions
proceeded, new resonances corresponding to each terthiophene
photoproduct appeared with chemical shifts and multiplicities
that matched the resonances observed for the expected
terthiophene photoproducts, each of which we synthesized
and characterized independently. Integrations of the resonances
assigned to the starting material and product gave percent
conversion.
On the basis of our measurements of extinction coefficients
of the terthiophene oligomers and model compounds designed
to mimic the structures of the ONB pendants, the terthiophene
chromophores absorb 87−90% of the incident light at 365 nm.
Therefore, determination of quantum yields required account-
ing for the percentage of absorbed photons that the nitrobenzyl
chromophore absorbed. As samples suitable for NMR spec-
troscopy were optically dense (OD > 3) at 365 nm, ratios of the
extinction coefficients of the oligomeric chromophores and the
ONB chromophores were sufficient to correct for the
competitive absorbance due to the additive nature of the
terthiophene and ONB extinction coefficients (see Supporting
Information). These ratios, combined with the results of
chemical actinometry, yielded the rate of photon absorbance of
the photocleavable ONB pendants.
Table 1 summarizes the quantum yields of photocleavage of
the ONB groups of O1-O4. These results are consistent with
our expectations detailed in the Experimental Design section:
those structures that yield more stable benzylic radicals upon
hydrogen atom abstraction have higher quantum yields of
photocleavage than those that yield less stable benzylic radicals.
The most dramatic example of this trend is the ONB
photocleavage of compound O4, which has a methylated
nitrobenzyl ether side chain: its photolysis was approximately
18-times more efficient than that of compound O1, which has
an unmethylated nitrobenzyl ester.
Figure 1. Top: Absorbance (solid lines), and emission (dashed lines)
spectra for O4. Bottom: excitation spectra for O4. Black lines are the
spectra of O4 in dichloromethane before irradiation; blue lines are the
spectra of the same sample after 3 min of irradiation at 365 nm (40
mW/cm2). λex = 350 nm for emission spectra; λem = 450 nm for
excitation spectra.
O1−O4; this observation is indicative of photoinduced
electron transfer quenching of the fluorescence of the ONB-
containing fluorophores, as we have observed in previous
studies of ONB-functionalized conjugated materials, and as is
utilized in some photoactivatable dyes. Comparing ONB-
substituted polythiophenes P1−P4 to model polymer P7
showed a similar trend.
In contrast to the evidence that the ONB group perturbs the
excited state of the conjugated backbone by quenching
fluorescence, the conjugated thiophene backbones have no
more than a small effect on the excited states of ONB groups.
As shown in Figure 1, there is a peak in the same range of
absorbance of the nitrobenzyl group of O4 at 290 nm in the
excitation spectrum. Photons absorbed in this range, however,
contribute to only a small amount of terthiophene fluorescence,
as the intensity of this peak in the excitation spectrum is heavily
suppressed relative to its intensity in the absorbance spectrum.
In addition, control oligomer O6, which has the same
terthiophene chromophore, but lacks the ONB group, also
has a band at 290−300 nm, indicating that the small peak at
290 nm in the excitation spectrum of O4 is mostly due to direct
Photoinduced Film Insolubility. As most applications of
conjugated polymers require that they be in the solid state, it is
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intermediates. Figure 3 and Figure S3 shows comparisons that
highlight this trend. Results from the thinnest films we prepared
Figure 3. Film retention of P2 and P4 as a function of film thickness
under identical irradiation and washing conditions. Each film was
irradiated for 4 min and then washed with CHCl3 for 30 s. Film
retention was measured by monitoring the change in optical density in
peak absorbance.
(thickness between 3 and 5 nm) showed that the largest
difference of photoinduced insolubility between any two
polymers was between P4 and P1: the solubility of P4, which
has methylated ONB ether linkers, decreases significantly faster
than that of P1, which has unmethylated ONB ester linkers,
during UV irradiation (Figure S3). Thicker films showed an
even larger difference in photoinduced film retention efficiency,
as the comparison of films of P4 and P2 in Figure 3 shows:
after 4 min of irradiation followed by rinsing in CHCl3 3−5 fold
more of P4 is retained. Donor−acceptor polymers P5 and P6,
which have ONB ether linkers, also showed photoinduced
insolubility, with 90% retained of a 27 nm thick film of P6 after
irradiation at 365 nm for 20 min and rinsing with CHCl3
(Figure 4). We attribute the hypsochromic shift of the polymer
Figure 2. NMR spectra from the photolysis of (a) O1 and (b) O3 in
CDCl3. Faint vertical lines indicate resonances attributed solely to the
terthiophene photoproduct, the integrals of which we used to
determine conversion.
important to understand how structure of the photocleavable
groups affects the photoinduced resistance to dissolution of
polymer films. Photobleaching, which is often observed upon
exposing organic chromophores to light and air simultaneously,
is a primary concern in the design of our materials and the
execution of these experiments. Addressing this potential
problem is an important reason that we focused on
thiophene-based polymers, which are known to be relatively
resistant to photooxidation.29 Thin films of our polymers were
resistant to photobleaching upon irradiation with UV light
under ambient conditions: Figure S2 shows that 1−2 nm thick
films of P3 showed less than 7% bleaching of its main
absorbance band at 520 nm upon irradiation at 365 nm (40
mW/cm2, 10 min). All polymers described here displayed
similar robustness with respect to photobleaching, which we
observed during film retention experiments (vide infra).
Irradiating an identical film of P3 in an argon atmosphere
prevented even this small amount of photobleaching.
Having established the photostability of our materials, we
investigated how photocleavable linker structure impacted the
efficiency of rendering polythiophene films insoluble with UV
irradiation. To do so, we irradiated spun-cast thin films of
polymers P1−P4 on glass substrates at 365 nm, followed by
rinsing in a bath of CHCl3 for 30 s. Comparisons of UV/vis
absorbance spectra of films before and after irradiation yielded
the degree of photobleaching, while comparisons of spectra
before and after rinsing the irradiated films yielded the percent
retention of the polymer backbone on the substrate. In all cases,
unirradiated spun-cast films were freely soluble in CHCl3.
In agreement with the quantum yields of photocleavage we
measured in solution, as well as our expectations based on our
experimental design, the solubility of films of polymers with
more stable benzylic radical intermediates were more sensitive
to UV irradiation than polymers with less stable radical
Figure 4. Absorbance spectra of a film of P6 before irradiation (black
line), after 20 min of irradiation at 365 nm (orange line), and after
washing with chloroform for 30 s (blue dashed line).
upon rinsing to a decreased density of conjugated polymer in
the film combination of swelling with solvent and dissolution of
shorter conjugated polymer chains.
In contrast to the retention of material as determined by
UV/vis spectrophotometry, film thicknesses as determined by
AFM decreased significantly upon irradiation and rinsing. Spun
cast films of P4 that were initially 33 3.0 nm decreased to 23
1.5 nm after irradiation for 20 min. After rinsing of the
irradiated films, the film thickness decreased to 16 3.0 nm,
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even though absorbance decreased by only ∼10%. Prelimi-
narily, we attribute these changes to (i) annealing and closer
packing of the polymer chains during UV irradiation, and (ii)
removal of quantities of both small molecule photocleavage
products and shorter polymer chains that remained soluble
even after photolysis (vide infra). This explanation is consistent
with the consistent red-shifting of absorbance spectra of these
polymer films after irradiation but before rinsing (Figure 4).
Our experiments showed that even after prolonged
irradiation times (up to 40 min), less than 100% of the
polymer film remained after rinsing. UV/vis spectroscopy of
the CHCl3 rinsing solution for a film of polymer P6 that had
been irradiated at 365 nm for 20 min shows a peak at 325 nm
that is due, at least in part, to the o-nitrobenzyl side-chains, and
a broad peak with maximum at 565 nm corresponding to the
poly(thiophene-co-benzothiadiazole) backbone. This peak in
the rinsing solution, however, is shifted hypsochromically from
the as-synthesized P6 in CHCl3 by ∼16 nm (Figure 5). In
Figure 6. Optical microscopy images of a photomask (A and B) and
images of films of P4 photopatterned using the photomask (C and D).
group of moderately polar organic solvents. This shared
characteristic presents a challenge for using solution-based
processes to fabricate films with multiple layers, which organic
photovoltaics (OPVs) and organic light emitting devices
(OLEDs) often require. The capability to render a solution-
cast conjugated polymer film insoluble in a postdeposition
processing step, such as with photochemical cross-linking,30 or
photochemical acid generation leading to a change in polymer
solubility,31 can allow for the preparation of multilayer films
from solution while only minimally disturbing previously
deposited layers.
Figure 5. Left: The normalized absorbance spectra of P6 in CHCl3
(black) and the rinsing solution from an irradiated film of P6 (blue).
Right: The normalized GPC chromatogram for P6 (black) and its
corresponding rinsing solution (blue).
Figure 7 illustrates the capability of our approach to yield
multilayer films with all solution-based processing. We first
performed multiple spin-casting steps of a solution of P6 in
CHCl3 onto a glass substrate without any irradiation in
between deposition steps. The absorbance of the film did not
increase significantly upon spin-casting additional polymer after
deposition of the initial film, as the solvent used during each
spin-casting step dissolved the previously deposited film. We
irradiated the film with 365 nm light (34 mW/cm2) for 10 min
after the fourth deposition step, which resulted in a decrease of
intensity and bathochromic shift of the UV-absorbing band,
consistent with cleavage of the ONB groups, but no significant
change in the band in the visible region of the spectrum, which
indicated minimal photobleaching of the conjugated poly-
(thiophene-co-benzothiadiazole) backbone. A subsequent spin-
casting step of P6 under identical conditions as used in the first
four deposition steps increased the absorbance of the film.
Rinsing that film with CHCl3 confirmed that this increase in
thickness was due to light-induced insolubility of the irradiated
film, as the absorbance spectrum of the resultant, thinner film
closely resembled that observed after irradiation of the
previously deposited film. We could repeat this process
multiple times with either the same polymer (P6), which has
potential utility in the fabrication of thicker films with polymers
that have limited solubility, or switch between photoreactive
polymers that have different optoelectronic properties between
deposition/irradiation steps (alternating layers of P4 and P6).
Fabrication and Evaluation of Organic Thin Film
Transistor Devices. To probe carrier transport properties, p-
type thin film transistor devices incorporating polymers P1, P2,
P5, and P6 were made using the bottom-gate, top-contact
addition, GPC analysis of the rinsing solution shows low
molecular weight polymer chains and small molecule by-
products of photolysis, with less contribution from higher
molecular weight chains of the as-synthesized sample of P6
used to prepare the films. This evidence indicates that some
shorter polymer chains remain soluble, even upon ONB
photolysis, and their dissolution is a primary cause for the 10−
20% decrease in absorbance upon rinsing of irradiated films.
Photopatterning of Conjugated Polymer Films. The
patterning of electronically active materials, using photo-
lithography as a key step, is a cornerstone technology of the
electronics industry. Conjugated polymers that show light-
sensitive solubility open the possibility of semiconducting
organic materials that can form intricate patterns easily and
directly with minimal additional processing steps.15,16 Our
approach of photocleavage of solubilizing chains enables
conjugated polymers to be used as negative photoresists.
Figure 6 shows typical results of a photopatterning experiment
with polymer P4. Irradiation of a 23 nm thick of P4 at 365 nm
(34 mW/cm2) for 20 min through a chrome photomask
followed by rinsing with CHCl3 for 30 s yielded a pattern of
conjugated polymer: those areas of polymer film exposed to
light remained on the substrate, while those polymers in areas
shielded by the mask dissolved. Although imperfections such as
rounded corners indicate room for improvement of photo-
lithographic conditions, our nonoptimized patterning proce-
dure could resolve features on the order of 10 μm.
Multilayer Films Enabled By Photoinduced Insolubil-
ity. Most reported conjugated polymers are soluble in the same
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Figure 7. Multilayer deposition experiments: Left: Thin film multilayer experiment without irradiation until after spinning P6 onto the substrate four
times. Middle: Thin film multilayer of P6 made by sequentially spin coating and irradiating layers for the first three deposition steps. The fourth layer
was deposited and then washed away without irradiation, demonstrating the necessity of UV irradiation to retain the film on the substrate. Right:
Multilayer film made from alternating deposition of P4 (blue lines) and P6 (black lines). Each layer was spun on top of the previous layer after a 10
min irradiation at 365 nm. After the final layer was deposited, it was not irradiated and the layer was washed away with chloroform to give the final
spectrum (black dashed line).
geometry, in which heavily doped silicon (resistivity 0.005−
0.025 Ω-cm) substrate served as the gate electrode. The gate
dielectric was a thermally grown SiO2 layer with a thickness of
190 nm modified with hexamethyldisilazane (HMDS). P1, P5,
and P6 showed typical p-type field effect operation with
excellent output and transfer characteristics. Representative
output characteristics (drain current (IDS) versus source-drain
voltage (VDS) curves at different gate voltages (VGS)) and
transfer characteristics (ISD vs VGS plotted on a logarithmic scale
and (ISD)1/2 vs VGS, at VDS = 100 V) of the transistors based on
P1 at 25 °C are shown in Figure S5.
For a device with 150 μm channel length (L) and 1000 μm
channel width (W), a saturation regime hole mobility of 2.6
( 0.05) × 10−5 cm2 V−1 s−1 and Vth of 52 V and current on/off
ratio of 4.5 × 103 was extracted for VDS = −100 V. These
devices exhibit low contact resistance and as a consequence
saturation mobility does not decrease significantly with
decreasing channel length. The electrical performance of
polymers P5 and P6 were similar to P1 (Table S1). Polymer
P2 did not show any measurable hole mobility in this device
architecture, to which impurities in the polymer sample may be
a contributing factor. We also compared electrical performance
of our polymers with commercially available regioregular P3HT
(Aldrich), which showed higher carrier mobility (ca. 10−3 cm2
V−1 s−1) than our polymers. Preliminarily, we attribute these
differences in mobility in part to higher crystallinity of P3HT,
due to the lack of regioregularity and relatively low molecular
weights of polymers described in this paper.
CONCLUSION
■
This work demonstrates that photocleavable solubilizing side
chains can give conjugated polymers the properties of negative
photoresists, enabling photopatterning and the fabrication of
multilayer films using solution processing. Our observations
that photolysis of ONB groups need not cause significant
photobleaching or complete disruption of the charge transport
properties of these materials supports the potential applicability
of this approach. Photocleavable side chains generally have the
advantages that they neither require the addition of additional
compounds in formulations such as photoacid or photobase
generators, nor do they depend on degradation of the
conjugated backbone. The ONB photocleavable moiety has a
number of specific advantages in this context: (i) their
photolysis efficiencies are not affected significantly by potential
energy transfer deactivation pathways to the conjugated
material, (ii) they have tunable sensitivity to light through
controlling the stabilization of the intermediate benzylic
radicals, (iii) their functionalization and installation is
straightforward, and (iv) they do not require highly hydrophilic
environments to cleave efficiently. Primary impediments to the
eventual success of this approach include the presence of
potentially deleterious photolysis byproducts, and the creation
of voids within films. Questions this work has not addressed are
the potential role of minor extents of photoinduced cross-
linking of polymer chains in the thin films due to radical
intermediates of ONB photolysis, or the sharpness of the
interfaces between different polymers in multilayer films, but
we anticipate that the degree of interpenetration of polymers
may be tunable based on the extent of photolysis. Current work
in our laboratory is focused on expanding the classes of
monomers that incorporate photocleavable solubilizing chains,
as well as the development of conjugated polymers that behave
as positive-tone photoresists, which could overcome the
challenges described here.
Importantly, the electrical performance of P1 and P6 did not
degrade significantly after the polymers were exposed to light
and rinsed with chloroform (Figure S5, Table S1). For example,
OTFT devices made with P1 after exposure to light and rinsing
with chloroform did not show any significant change in
electrical performance or drop in carrier mobility. Thus, for
devices with W = 1000 μm and L = 100 μm, saturation mobility
μ = 1.6 ( 0.05) × 10−5 cm2 V−1 s−1 was only slightly lower
than for devices before irradiation and rinsing (Figure S5).
G
DOI: 10.1021/ma502289n
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
(21) Zhao, H.; Sterner, E. S.; Coughlin, E. B.; Theato, P.
Macromolecules 2012, 45, 1723−1736.
ASSOCIATED CONTENT
■
S
* Supporting Information
(22) Smith, Z. C.; Pawle, R. H.; Thomas, S. W. ACS Macro Lett.
2012, 1, 825−829.
General considerations, all experimental procedures, including
procedures for syntheses of new compounds, NMR spectra,
and additional absorbance and fluorescence spectra, transistor
and I−V characteristics, AFM images, and quantum yield
calcualtions. This material is available free of charge via the
Internet at http://pubs.acs.org.
(23) Solomek, T.; Mercier, S.; Bally, T.; Bochet, C. G. Photochem.
Photobiol. Sci. 2012, 11, 548−555.
(24) McTiernan, C. D.; Chahma, M. h. New J. Chem. 2010, 34,
1417−1423.
(25) Blettner, C. G.; Konig, W. A.; Stenzel, W.; Schotten, T. J. Org.
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Chem. 1999, 64, 3885−3890.
(26) DiCesare, N.; Belletete, M.; Leclerc, M.; Durocher, G. Synth.
Met. 1998, 98, 31−40.
AUTHOR INFORMATION
■
(27) Cook, S.; Furube, A.; Katoh, R. Energy Environ. Sci. 2008, 1,
294−299.
Corresponding Author
*(S.W.T.) Telephone: 617-627-3771. E-mail: sam.thomas@
tufts.edu.
(28) Pawle, R. H.; Eastman, V.; Thomas, S. W. J. Mater. Chem. 2011,
21, 14041−14047.
Notes
(29) Jørgensen, M.; Norrman, K.; Krebs, F. C. Sol. Eng. Mater. Sol.
Cells 2008, 92, 686−714.
The authors declare no competing financial interest.
(30) Lee, J.; Han, H.; Lee, J.; Yoon, S. C.; Lee, C. J. Mater. Chem. C
2014, 2, 1474−1481.
ACKNOWLEDGMENTS
■
(31) Bouffard, J.; Watanabe, M.; Takaba, H.; Itami, K. Macromolecules
2010, 43, 1425−1429.
Generous support for this work was provided by the National
Science Foundation (CAREER Award DMR-1151385 and
CBET-1067093).
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DOI: 10.1021/ma502289n
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