Multi-responsive copolymers: Using thermo, light- and redox stimuli as three independent inputs towards polymeric information processing

Article abstract of DOI:10.1039/c1cc12652k

We report on triple responsive polymers, exhibiting a distinct and reversible lower critical solution temperature in water that can be altered by light and redox stimuli, and we suggest their evaluation for molecular information processing.

Full text of DOI:10.1039/c1cc12652k

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COMMUNICATION
Multi-responsive copolymers: using thermo-, light- and redox stimuli as
three independent inputs towards polymeric information processingw
Philipp Schattling,a Florian D. Jochuma and Patrick Theato*abcd
Received 5th May 2011, Accepted 31st May 2011
DOI: 10.1039/c1cc12652k
We report on triple responsive polymers, exhibiting a distinct
and reversible lower critical solution temperature in water that
can be altered by light and redox stimuli, and we suggest their
evaluation for molecular information processing.
by the RAFT polymerization technique.11 PPFPA was then
converted with N-(2-aminoethyl)-4-(2-phenyldiazenyl) benzamide
(azobenzene) and 4-amino-2,2,6,6-tetramethyl-1-oxyl-piperidine
(amino-TEMPO) followed by the reaction with an excess
amount of isopropylamine (Scheme 1) yielding statistical
copolymers based on thermo responsive poly(N-isopropyl-
acrylamide) (PNIPAM), containing light responsive azobenzene
moieties and redox sensitive TEMPO side groups. Azobenzene
is known for its wavelength dependent photo-induced cis–trans
isomerization. The reversible conformation change from trans to
cis is accompanied by an increase of local polarity due to a change
in the dipole moment of the chromophore.12 TEMPO is known to
undergo reversible electron transfer reactions and used in energy
storage applications.13 Recently, PNIPAM copolymers with
TEMPO moieties have been shown to act as double responsive
materials.14
The translation of external stimuli, e.g. light, pH or temperature,
into a physical response of a polymer has gained tremendous
attention recently due to the broad range of applications of these
materials ranging from drug delivery and tissue engineering to
coatings and microelectromechanical systems.1 Molecular logic
gates have broadened the perspective of responsive polymers.
However, so far mostly small molecules were investigated for
logical computations.2,3 Only a small number of examples take
advantage of polymers.2–5 For example when envisioning
multi-input logic gates, functional polymer synthesis with an
unprecedented precision and versatility is required, which was
not available until very recently. Hence, polymeric systems
promise a superior role due to their versatility in the type of
in- and output signals, which was only demonstrated in just a
small number of examples until now.5–7 Further, their application
as molecular memory devices has been discussed.8–10 Although
these systems currently suffer from the accumulation of side
products during data storage and a short retention time in the
range of minutes, they still have a great potential as a ‘‘bottom-up’’
strategy for designing molecular information processing devices.10
In this communication, we report on the synthesis and
characterization of triple responsive polymers, exhibiting a
distinct lower critical solution temperature (LCST) in water.
We focused on thermo-, light- and redox responsive copolymers
and their reversible change in LCST after applying an additional
external stimulus. These responsive systems were investigated in
the context of polymer information processing devices.
Following this strategy, two different triple responsive
copolymers PI and PII were synthesized, varying in their
amounts of azobenzene and TEMPO. The polymers
were purified by dialysis against water and one subsequent
precipitation step into diethylether. ESR spectroscopy was
used to confirm the existence of the paramagnetic TEMPO
functionality. The contents of azobenzene and TEMPO in PI
and PII were evaluated by 1H-NMR. However, due to the
paramagnetic character of the TEMPO moiety, this could only
be achieved after treatment of a sample with ascorbic acid to
reduce the TEMPO radical. PI contained 1.6 mol% of azobenzene
and 3.5 mol% of amino-TEMPO, while PII contained 2.3 mol%
of azobenzene and 5.7 mol% of amino-TEMPO.
The LCST behavior by means of cloud points of the triple
responsive polymers was investigated using turbidity measure-
ments. A concentrated solution (5 mg mLÀ1) of PI and PII in
Triple responsive statistical copolymers were synthesized
using a post-polymerization modification strategy utilizing
well-defined poly(pentafluorophenyl acrylate) (PPFPA), prepared
a Institute of Organic Chemistry, University of Mainz, Mainz, Germany
b World Class University Program of C2E2, School of Chemical and
Biological Engineering, Seoul National University, Seoul, Korea
c Department of Materials Science and Engineering,
University of Sheffield, Sheffield, UK
¨
d Current address: Universitat Hamburg, Institute for Technical and
Macromolecular Chemistry, Hamburg, Germany.
E-mail: theato@chemie.uni-hamburg.de; Fax: +49 (0)40 42838 6008
w Electronic supplementary information (ESI) available: Experimental
section, turbidity measurements, circuits. See DOI: 10.1039/c1cc12652k
Scheme 1 Synthesis of triple responsive PNIPAM based copolymers
containing azobenzene and TEMPO moieties.
c
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Table 1 LCST of PI and PII depending on three stimuli: temperature,
light and redox (temperature shift with respect to initial LCST)
However, it is known that the addition of salts might have a
significant influence on the LCST.16 The influence of added
salts was investigated on a PNIPAM model system (ESIw).
Being aware of this strong influence, the polymer solutions of
PI and PII were purified accordingly after treatment with
ascorbic acid or red prussiate. Thus, the results in Table 1 are
without the influence of additional salts, i.e. after purification.
To address the stimuli responsive moieties independently, a
second experiment was conducted by stimulation of the light-
responsive chromophore moieties first and then reduction of
the TEMPO moieties. Interestingly the impact of the light-
responsive moiety is increased in comparison to the previous
situation, with an increase of LCST by 0.5 1C for PI after
irradiation with UV light (365 nm). We believe that the effect
of the LCST on the azobenzene moieties depends on the
composition of the polymer, as shown previously.12,17 The
isomerization of the azobenzene moieties in the presence of the
less hydrophilic TEMPO led to a more pronounced impact in
comparison with the more hydrophilic hydroxylamine. The
subsequent reduction of TEMPO led to a further increase of
the LCST by 0.5 1C. Further irradiation with UV light raised
the LCST again by 0.3 1C, suggesting that the cis–trans
relaxation of azobenzene already happened along with the
oxidation process (see ESIw). With respect to the initial LCST
the temperature was shifted by 1.3 1C, which was similar to the
previous situation.
Stimuli
LCSTPI/1C (DLCST) LCSTPII/1C (DLCST)
None
Ascorbic acid
Irradiation 365 nm
Irradiation >400 nm 16.1 (+1.2)
Red prussiate 15.0 (+0.1)
14.9 (Æ0)
13.9 (Æ0)
16.1 (+1.2)
16.4 (+1.5)
17.7 (+3.8)
18.0 (+4.1)
17.7 (+3.8)
14.0 (+0.1)
0.1 M NaSO4 was heated in a cuvette and the transmittance was
measured as a function of temperature using a UV/Vis spectro-
meter. Although pure PNIPAM shows a sharp LCST,
introduction of further moieties required the addition of sodium
sulfate as a cosmotropic salt to obtain sharp LCSTs (see
ESIw).14 Generally, introduction of the TEMPO and azobenzene
groups in PI and PII resulted in more hydrophobic polymer
chains and thus decreased the LCST (Table 1). Adding
ascorbic acid to the polymer solution reduced the TEMPO
moieties,15 which resulted in an increase of the LCST. The
conversion from the oxyl- into the more hydrophilic hydroxyl-
amine group resulted in an improved hydrophilic interaction
with the solvent, likely also due to hydrogen bonding. The
amount of TEMPO functionalities within the polymer demon-
strated the impact of this stimulus: the higher the amount of
TEMPO moieties, the higher the change in LCST. For PI
(3.5 mol% TEMPO) the increase of the LCST was determined
as 1.2 1C. In contrast, the LCST of PII (5.7 mol% TEMPO)
increased by about 3.8 1C. Noteworthily, a stepwise reduction of
the TEMPO moiety allowed a successive increase of LCST.14
When PII was stepwise reduced by the addition of ascorbic acid
the LCST raised by 1.3 1C in a first reduction step and then by
0.6 1C in a second reduction step. This dependence of the
temperature shift on the amount of TEMPO moieties reduced
offers the possibility to fine-tune the LCST.
Translating this information into the context of information
processing, several combinations are possible with one system.
In general a logic gate is the physical expression of the Boolean
function and uses one or more inputs to produce a single
output.18 The stimuli responsive functionalities can be addressed
independently and therefore, in a triple responsive polymer,
three different inputs are available: temperature (input A: IA),
light (input B: IB) and the redox-sensibility (input C: IC). In
this context IA = 0 is correlated to keeping the polymer
solution below LCST, while IA = 1 is equal to heating the
solution to a temperature 0.4 1C higher than LCST. Accordingly
IB = 0 means the sample has not been irradiated and IB = 1
corresponds to the irradiated polymer solution and thus
isomerization into cis-azobenzene. IC = 0 indicates a sample
that has not been treated with ascorbic acid and IC = 1 is
equivalent to the hydroxylamine. The output signal (Out) can
either be Out = 0 for insoluble or Out = 1, which means the
polymer is soluble. The direct addressing of just two responsive
groups like temperature and light or temperature and redox-
sensibility corresponds to ‘‘A implies B’’ logic or ‘‘A implies C’’,
respectively, which was already described in the literature.6 The
combination of three stimuli leads to a new, more complex
logic circuit: if temperature and light stimuli are stimulated
first an ‘‘A implies B’’ output is generated. If the resulting
output OutA/B is subsequently combined with the redox input
as an ‘‘OR’’ logic gate, it results overall in the statement
‘‘always true, if not only A’’. The circuit fulfilled associativity,
i.e. it does not matter in which sequence the stimuli are applied
(see Boolean logic truth tables in Fig. 1).
Subsequent irradiation with UV light (365 nm) resulted in an
additional increase of the LCST by 0.3 1C, due to the isomer-
ization of the azobenzene moiety. The amount of the chromo-
phore in both polymers PI and PII was almost identical.
Consequently, the impact of the irradiation was identical for
both polymers as the LCST increased in both cases by 0.3 1C.
The photoisomerization could be reversed by irradiation
with light with l > 400 nm.12 Irradiation resulted in the trans
isomer of azobenzene, which lowered the local polarity and
thus decreased LCST again. The experiment confirmed that
the former increase of 0.3 1C (cis state) was fully reversible
after re-isomerization with visible light. Notably, this also
demonstrated the accuracy of the stimuli responses of the
polymers (Table 1, Fig. S4, ESIw).
Oxidation of the hydroxylamine moiety back to the radical
was achieved by addition of red prussiate (potassium ferri-
cyanide), which led to a significant decrease of the LCST,
demonstrating that the redox stimulus could be completely
reversed as the initial LCST was reobtained. A second redox
cycle with the same polymer proved the full reversibility of
the redox stimulus (see ESIw). The successive replication is
in accordance with the work of Nishide and Oyaizu and
confirms that decomposition of TEMPO groups or irreversible
side reactions can be excluded.13
A logic gate yields a logic output response as a result of an
input signal, but it does not allow storing the information. In
contrast, a flip-flop element offers the possibility of storing
information for sequential logic operations.9 The bistable
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responses of the TEMPO- and azobenzene moieties, two bits
can be stored successively. If both memory elements exist in
the logic state 1 then this is taken into account and can be
observed through the additional LCST at 16.4 1C. The truth
table of the flip-flop system is shown in the ESI.w Noteworthily,
the two states of the TEMPO moiety are stable over months,
while the relaxation of the cis isomer of the azobenzene to the
trans-conformation occurs with a half-life of 11 days in
the dark,12 i.e. the stored data are lost after this period.
The volatile character of molecular memories is a common
problem, however, the present polymer has a 1000 fold
increased half-life compared to previous molecular memories.8
In summary, post-polymerization modification provides an
easy access to multi-responsive polymers, which respond
independently to temperature-, light- and redox-sensitive
stimulations. These systems exhibited a distinct and an
adjustable LCST switching window. It was shown that this
property could be tailored during the synthesis by adjusting
the corresponding moiety, and additionally fine-tuned directly
in the application by the degree of conversion of the TEMPO
into the hydroxylamine functionality. The stability of every
state facilitates the implementation of different polymer
information processing systems. The realization of several
information processing techniques within only one system is
exceptionally fascinating. The unique application as a polymer
memory device, in the sense of Set/Reset flip-flop, for two bits
in one polymer is remarkable. It gives reason to except more
complex computing processes in the near future based on
polymeric materials, which exhibit multifaceted properties.
This research was supported by the DFG (IRTG 1404) and
by the WCU program through the NRF of Korea funded
by the MEST (R31-10013). The group of K. Heinze is
acknowledged for support in ESR measurements.
Fig. 1 Truth table for a logic system with 3 inputs. First, the stimuli
temperature (IA) and light (IB) give a logical output through an ‘‘A
implies B’’ logic, which is combined with the redox sensitive input (IC).
states of the TEMPO- and the azobenzene moieties suggest
the potential use as a Set/Reset (SR) flip-flop device. The
paramagnetic state and the reversible conversion into the
hydroxylamine as well as the cis–trans isomerization of
azobenzene offer the possibilities of saving two bits with only
one polymer. The saved information is translated into the fully
reversible change of the LCST.
The reduced TEMPO moiety will be considered as the memory
state output yielding logic Q1 = 1, while the paramagnetic state
will be defined as logic state Q1 = 0. The Set input S1 = 1
represents the addition of ascorbic acid and the Reset input
R1 = 1 means the addition of red prussiate. The operation of the
logic element is shown in Fig. 2. When the system is in logic state
Q1 = 0 or in logic state Q1 = 1 and the inputs S1 = 1 and
R1 = 0 are applied to the system, the system is set to state 1,
which is achieved by addition of ascorbic acid. But when the
system exists in the logic state 0 or in the logic state 1 and is
subjected to the S1 = 0 and R1 = 1, the system is reset to the
logic state 0, i.e. red prussiate is added.
The azobenzene moiety offers an additional SR flip-flop.
Similar to the previous situation the trans-isomer corresponds
to the logic state Q2 = 0, while the cis-state is considered as
logic Q2 = 1. If UV light with 365 nm is applied to the system,
existing in the state Q2 = 0, the memory element is set to logic
Q2 = 1, this corresponds to the S2 = 1, R2 = 0 setting and
results in the cis-configuration of the system. The Q2 = 1 state
can be deleted by the input configuration S2 = 0, R2 = 1 and
is realized by irradiation with light >400 nm. The exact state
of the flip-flop is read by temperature and determined by the
reversible change of the LCST. Because of the orthogonal
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Chem. Commun., 2011, 47, 8859–8861 8861