Effect of the molecular size of analytes on polydicetylene chromism

Article abstract of DOI:10.1002/adfm.201000262

The pH chromism of polydiacetylenes (PDAs) is examined with respect to the molecular size and acidity of acid analytes, along with the alkyl spacer length of primary-amine-functionalized diacetylene (DA) lipids. pH turns out to be an important parameter to charge amine headgroups of PDA but a change in pH does not necessarily result in a PDA color change. The molecular size of acid analytes is identified as another factor that can produce a configurational change in PDA amine headgroups, followed by perturbation of the ene-yne conjurgated backbone.In addtion. the length of a alkyl spacer between the amine headgroup and the amide group of the diacetylene lipids is found to strongly affect thedegree of PDA chromatic transition. The longer alkyl spacer shows a smaller chromatic transition from blue to red phase. The alkyl spacer seems to provide a certain degree of freedom to the amine headgroup, thus decreasing the transfer of headgroup steric effects to the PDA backbone. These correlations found for PDA; chromism are applied, to, he development of a system that colori metrically detects diethyl phosphate (DEP), a degraded nerve agent simulant. PDA liposomes show a selective chromatic transition upon binding with DEP compared to other acid analytes.

Full text of DOI:10.1002/adfm.201000262

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Effect of the Molecular Size of Analytes on
Polydiacetylene Chromism
By Donghwan Seo and Jinsang Kim*
Salt formation or electrostatic interac-
tions have not been only employed in the
assembly of DA molecules, but PDA
chromism based on pH changes has also
been studied. More specifically, carboxylic
acid functionalized PDAs have been widely
investigated with regard to pH change. A
studyof10,12-pentacosadiynoicacid(PCDA)
monolayers suggests that the ionization of
the carboxylic acid group at high pH causes
a disordered configuration due to salt
formation with counterions. This change
in configuration induces the optical transi-
tion from blue phase to red phase.[24] When
the pH is lowered back towards the original
level, bathochromic shifts are observed in
absorption spectra of PDAs, where yellow
phase transitions to red phase[25] or red
phase to blue phase.[26–30] These chromatic
transitions are attributed to a conforma-
tional change that results in an increase in
the degree of conjugation of the PDA
backbone.[9] It seems to have been consis-
The pH chromism of polydiacetylenes (PDAs) is examined with respect to the
molecular size and acidity of acid analytes, along with the alkyl spacer length
of primary-amine-functionalized diacetylene (DA) lipids. pH turns out to be an
important parameter to charge amine headgroups of PDA but a change in pH
does not necessarily result in a PDA color change. The molecular size of acid
analytes is identified as another factor that can produce a configurational
change in PDA amine headgroups, followed by perturbation of the ene–yne
conjugated backbone. In addition, the length of a flexible alkyl spacer between
the amine headgroup and the amide group of the diacetylene lipids is found to
strongly affect the degree of PDA chromatic transition. The longer alkyl spacer
shows a smaller chromatic transition from blue to red phase. The alkyl spacer
seems to provide a certain degree of freedom to the amine headgroup, thus
decreasing the transfer of headgroup steric effects to the PDA backbone.
These correlations found for PDA chromism are applied to the development
of a system that colorimetrically detects diethyl phosphate (DEP), a degraded
nerve agent simulant. PDA liposomes show a selective chromatic transition
upon binding with DEP compared to other acid analytes.
tently shown that the replacement of hydrogens bonds with
electrostatic interactions may cause a significant change in the
configuration of DA or PDA headgroups, as well as a change in the
electronic state of PDA’s conjugated backbone. pH change, which
can be a driving force for electrostatic interaction, is one of stimuli
known to cause color change from blue phase to red phase, as well
as fluorescent emission in PDA. The intensity of this interaction is
controlled by the pH level and is believed to be a direct result of the
degree of the conformational change in the PDA conjugated
backbone. Accordingly, the acidityorbasicityrepresentedby pHlevel
has been solely correlated with these PDA chromatic transitions.
Furthermore, an interesting result was reported about basic-
headgroup-functionalized DA, which provides more detailed
information on the configuration of DA headgroups due to salt
formation.[31] The polymerization of these initially unpolymeriz-
able monomers functionalized with hydrazide was achieved by the
addition of HCl. This result was explained by a change in the
distance between adjacent headgroups. According to computa-
tional studies, hydrogen bonding between amine headgroups
1. Introduction
The molecular design of diacetylenes (DAs) has been of great
interest since the mechanism of topochemical polymerization was
uncovered.[1] To satisfy the requirements for their polymeriza-
tion,[2] the interactions between adjacent functional groups on DA
moleculeshavebeenscrutinized. Aromaticinteractions, hydrogen
bonding, and hydrophobic interactions were found to be effective
for the self-assembly and polymerization of DA molecules when
they are properly combined and tailored.[3–7] In addition, external
stimuli such as heat,[8] pH,[9] mechanical stress,[10] or ligand-
receptor binding[11] have been found to induce a polydiacetylene
(PDA) color change and various sensor applications have been
proposed based on these chromisms.[12–23]
[*] Prof. J. Kim
Departments of Materials Science
& Engineering, Chemical
Engineering, and Macromolecular Science & Engineering
University of Michigan
Ann Arbor, 48109 (USA)
˚
made the distance between DA units (3 A) too short for
polymerization. HCl salt formation on amine functional groups
E-mail: jinsang@umich.edu
˚
resulted in a larger distance between molecules (4.9 A), which
Dr. D. Seo
Department of Mechanical Engineering
University of Michigan
Ann Arbor, 48109 (USA)
seems to be more favorable for topochemical polymerization.
In this contribution, it is demonstrated that a certain pH level is
necessarytocreatesaltformationthroughanacid–basereactionon
the PDA headgroup. However, it is not the only criteria to induce
DOI: 10.1002/adfm.201000262
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the chromatic transition of PDA. Considering
the space occupied by the counterion, the
chromatic transition of PDA liposomes was
investigated in terms of the molecular weight
and the pH level of acid analytes. Molecular
weight was adopted in convenience to produce
appreciable data for the effect of molecular size.
It was found that a strong correlation exists
between the extent of PDA chromatic transition
and the molecular size of acid analytes. When
the acidity of analytes is strong enough to pull
counterions into the surface of the amine
headgroups, it is suggested that the molecular
size of acid analytes causes steric effects and
induces the chromatic transition of amine-
functionalized PDAs. The size-selectivity of
PDA chromism was applied to distinguish
diethyl phosphate (DEP) from general acids.
This could be used in the detection of nontoxic
degraded acids from the hydrolysis of toxic
organophosphorusagents(Fig. 1). In addition, it
was shown that flexible alkyl spacers affect this
chromism by decreasing the extent of PDA
chromatic transition as the length of the alkyl
spacers increases.
Figure 2. DA monomers of APCDAn (n ¼ 2 ꢀ 6) and HPCDA2.
betweenamineandamidegroupsofAPCDAnwasvariedfromtwo
to six methylene groups.
2. Results and Discussion
2.1. Effects of Molecular Size and pH on Polydiacetylene
Chromism
The primary-amine-functionalized DAs of N-(n-aminoalkyl)-
10,12-pentacosadiynamide (APCDAn, n ¼ 2 ꢀ 6) and the
hydroxy-functionalized DA of N-(2-hydroxyethyl)-10,12-pentaco-
sadiynamide (HPCDA2) were synthesized and their chemical
structures are shown in Figure 2. The length of the alkyl spacer
In order to study the effects of molecular size and pH of acid
analytes on PDA chromism, a series of alkanoic acids were chosen
Figure 1. Schematic illustration of PDA’s selective chromatic transition dependent on the molecular size of acid analytes.
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blue to red phase. Although raising the concentration of formic
acid to 1 and 10 mM increased the extent of the transition, these
transitions were still not considerable enough to distinguish the
color transition by the naked eye. The additions of other acids at
10 mM concentrationsinducedsuccessivelyincreasingtransitions.
A remarkable color transition from blue to red phase was observed
with a CR over 50% when butyric acid was added to the APCDA2
liposome solution. On the other hand, 0.1 and 1 mM concentra-
tions of those acids induced only slight transitions.
Individually, each alkanoic acid showed APCDA2 color
transitions that were dependent on acid concentration or pH
level. As the pH level of each acid was lowered, the CR values of the
APCDA2 liposome solution increased. To the contrary, the
transitions induced by the addition of 10 mM alkanoic acids
showed a strong dependency on molecular weight rather than pH
level. These data suggest that the molecular size effect is dominant
for pH chromism in PDA, where the strength of acidity is still a
necessary factor since it is the driving force for salt formation. At
lower acidity of ca. pH > 4, alkanoic acids did not induce any
noticeable chromatic transition, probably because their electro-
staticinteractionswiththeamineswerenotstrongenoughtocause
a significant change in the configuration of the APCDA2
headgroup. Note that the hydrophobicity of the acids increases
along with the molecular size. However, because the acids are
completely soluble in water and do not form any assembly the
hydrophobicity is not likely responsible for the trend of the PDA
chromism.
Figure 3. Acidity of alkanoic acids with respect to molecular weight.
and PDA liposomes were prepared with APCDA2. Alkanoic acid is
the simplest carboxylic acid (RꢁCOOH, R ¼ H or alkyl group)
having moderate acidity, as most carboxylic acids are generally
weak acids. The acidity of short-chain alkanoic acids tends to
decrease as thenumber of methylene groupincreases, as shown in
Figure 3. Formic acid (MW 46.03, pKa 3.75) shows the strongest
acidity among these acids and the acidity decreases in the order of
acetic acid (MW 60.05, pKa 4.76), propionic acid (MW 74.08, pKa
4.87), and butyric acid (MW 88.11, pKa 4.83).[32] This tendency
gradually lessens and the acidity of butyric acid is almost similar to
that of propionic acid. These acids were added to the APCDA2
liposome solution in such a way that the final concentration of the
liposome solution was 0.05 mM and that of the acids was either 0.1,
1, or 10 mM. The absorption spectra of APCDA2 liposome
solutions were monitored upon the addition of the acids. The
extent of the chromatic transition from blue to red phase was
quantified by colorimetric responses (CRs) as previously defined.[33]
As shown in Figure 4, the addition of 0.1 mM formic acid in
APCDA2 liposome solution hardly induced the transition from
2.2. Molecular-Size-Dependent Detection of DEP
Simple, low-cost, and reliable detection of nerve agents is a great
challengeowingtotheelusivereactivityoftoxicorganophosphorus
compounds.[34–38] Although great efforts have been made to
implement their detection,[39–44] conventional analytical methods
such as gas/liquid chromatography, mass spectroscopy, and NMR
spectroscopy are still considered as the most assuring techniques
for their accurate identification.[45,46] It is known that toxic
organophosphorus compounds are degraded to nontoxic phos-
phonic acids by hydrolysis [45–49] and the detection of these acids
has been recently interesting as an indirect alternative to infer any
use of their toxic parent compounds.[50–53] The
molecular size effect found for the chromism of
the APCDA2 liposome was applied to distin-
guish DEP from general strong acids frequently
used in the laboratory. DEP was obtained by the
hydrolysis of diethyl chlorophosphate (DCP),
one of nerve agent simulants, as shown in
Figure 5. Since HF and HCl among other acids
may come not only from the degradation of
those agents or simulants but also from other
routes, the distinction between these acids and
DEP provides an advanced selectivity in this
field.
Absorption and photoluminescence (PL)
spectra of APCDA2 liposomes were measured
followingeachadditionofacidandtheCRswere
calculated, as shown in Figure 6. The acidities of
0.1 mM solutions of these acids were not strong
Figure 4. APCDA2 liposome chromatic transitions induced by alkanoic acids. A) CRs on
additions of 0.1, 1, and 10 mM acids with respect to pH. B) Absorption spectra on additions
of acids (10 mM).
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the molecular weight of the acid analytes, as
observed in the study of alkanoic acids. The
additionofDEP, whichhasthelargestmolecular
weight among the acids and is the second
weakest acid to HF in this study, induced almost
a full transition to red phase (CR 90.0%, Fig. 6B)
with strong fluorescent emission (Fig. 6D). The
addition of HNO3 (MW 63.01, CR 9.0%), H3PO4
(MW 98.00, CR 12.5%), and H2SO4 (MW 98.08,
CR 21.7%) induced small transitions to the red
phase. The stronger acidity of H2SO4 may have
caused a larger chromatic transition than that of
H3PO4 and induced a slight color change toward
purple to the naked eye.
Figure 5. G-type nerve agents with DCP simulant and their hydrolytic degradation.
Intriguingly, diatomic acids induced unusual
chromatictransitionsinAPCDA2liposomes. As
enough to induce chromatic transitions comparable to 0.1 or 1 mM
solutions of alkanoic acids. The 1 mM solutions of these acids have
a narrow acidity range of pH 2.8 ꢀ 3.4, which is strong enough to
induce a transition. The molecular weights of the acids vary from
20.01 (HF) to 154.10 (DEP). The degree of the chromatic transition
of the APCDA2 liposomes again showed a strong correlation with
shown in Figure 6A, the addition of HF, HCl, and HBr induced
bathochromic shifts as well as considerable hyperchromic effects
in the absorption spectra of APCDA2 liposomes. Instead of
chromatic transitions from blue to red phase, the additions of HF,
HCl, and HBr red-shifted the absorption maxima of the APCDA2
liposomes by 11, 15, and 10 nm with intensity increases of 30%,
Figure 6. APCDA2 liposomes chromatic transitions induced by general acids and DEP (1 mM). A) Absorption spectra on additions of diatomic acids.
B) Absorption spectra on additions of multiatomic acids. C) CRs with respect to molecular weight and pH. D) PL spectra (excitation: 503 nm).
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38%, and 24%, respectively. The negative CR numbers of HF
(ꢁ8.8%), HCl (ꢁ13%), and HBr (ꢁ6.3%) added to APCDA2
liposomes came from a considerable increase of the blue phase.
The addition of HBr to APCDA2 liposomes also induced a slight
transition to the red phase, while HFand HCl did not develop any
red phase. Similar hyperchromic effects with bathochromic shifts
of absorption spectra have been previously reported as the result of
increased conjugation in the polymer backbone.[54–57] These
unusual results by diatomic acids indicate that intact APCDA2
liposome headgroups may not form the most favorable spatial
arrangement for the conjugation of PDA backbone. Since the
distance or angle between adjacent DAs usually changes during
topochemical polymerization,[58,59] whereas interactions between
headgroups are retained, these possibly discordant behaviors
between PDA backbone and headgroup may cause an unrelaxed
conformation in the PDA backbone.[60] In addition, it was
previously shown that the hydrogen bonds between amine
headgroups do not make an optimal configuration for DA
assembly and topochemical polymerization, while HCl salt on
amine headgroups was found to provide a polymerizable
configuration. Therefore, our data also imply that the salt
formation on amine headgroups by small-molecular-weight
diatomic acids replaces the hydrogen bonds by the electrostatic
interaction and rearranges the configuration of the amine
headgroups, which improves the conjugation in the PDA
backbone. Furthermore, the rearrangement of the charged
headgroup by HCl seems to increase the degree of conjugation
more than HF and HBr. This ‘‘salt annealing effect’’ by these
diatomic acids is indeed similar to the effect of heat annealing on
PDA. Thermally pre-annealed DA films show bathochromic shifts
with hyperchromic effects through polymerization compared to
untreated DA films.[61]
Figure 7. CRs of APCDAn and HPCDA2 liposomes on addition of general
acids and DEP.
selectivity, decreases as the length of alkyl spacer increases. The
liposome solutions with longer alkyl spacers have smaller
chromatic transitions from blue to red phase when the same
analytes are compared, as previously reported.[25] The CR of
APCDA2 liposomes induced by DEP is about 95.5%, whereas that
of APCDA6 by DEP is 34%. The control sample, the HPCDA2
liposome solution, did not have any noticeable chromatic
transition when any of the acids were added.
According to both studies, the length of alkyl spacer seems to
substantially influence the configuration of the self-assembled DA
as well as the conformation of PDA’s conjugated backbone. The
unfavorable effect of hydrogen bonding between amine head-
groups decreases after polymerization and the degree of PDA
chromatic transition also decreases as the length of alkyl spacer
increases. It is inferred that the alkyl spacer may provide flexibility
or a degree of freedom, thus, reducing the association between the
conditions of the outer headgroup and the inner hydrophobic part
or conjugated backbone.
2.3. Effect of Alkyl Spacer on PDA Chromism
APCDAn, aseriesof DAwithalkylspacerlength varied fromtwoto
six methylene groups, was examined by the addition of general
acids and DEP. After photopolymerization, it was interesting to
find that liposomes with longer alkyl spacers efficiently developed
the blue phase with shorter UV irradiation time (see Supporting
Information, Fig. S4). The full development of the blue phase from
APCDA2 liposomes took about 90 s of UV irradiation. The
irradiation time necessary for a full development of the blue phase
decreased to about 60 and 40 s for APCDA3 and APCDA4
liposomes, respectively. Only about 30 s was enough to achieve the
full blue phase of APCDA5 and APCDA6 liposomes. Prolonged
UV irradiation after formation of the full blue phase continued to
develop the vibronic peak of the blue phase but eventually the peak
from the red phase became dominant. As mentioned above, the
hydrazide-functionalized DA liposomes without any spacer
between the amine headgroup and amide group (APCDA0) is
notpolymerizable. Ourdataobtainedfromthisstudyshowthatthe
alkyl spacer promotes topochemical reaction.
3. Conclusions
In this study, the molecular size and acidity of analytes were
correlated with PDA chromism and the effects of different alkyl
spacer lengths were investigated. The degree of the chromatic
transition of PDA liposomes was shown to be strongly dependent
on the molecular size of acid analytes as well as their pH level.
Small diatomic acids proved that pH change doesnot solely induce
PDA chromatic transition from blue to red phase. Rather than
producing significant chromatic transitions, diatomic acids
produced bathochromic and hyperchromic shifts in the absor-
bance of APCDA2 liposomes as a result of the salt annealing effect.
APCDA2 liposomes showed that the degree of chromatic
transition is dependent on the molecular size of alkanoic acids.
Acid analytes were added to polymerized APCDAn liposome
solutions. As shown in Figure 7, it was found again that the sterics
created by salt formation at the PDA headgroup induce chromatic
transitions that depend on the molecular size of the acid analytes.
Furthermore, the degree of discrimination by molecular size, or
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phase of the DA liposome solution was monitored by
spectrophotometer after every UV irradiation.
a UV-Vis
Hence, we propose that the charge-induced configurational
change on the PDA headgroup may result from the combined
effects of molecular size and pH level. These findings were applied
to the development of PDA liposomes for the detection of DEP, a
hydrolyzed nerve agent simulant. DEP, thelargest molecule in this
study, induced almost full chromatic transition in APCDA2
liposomes. In addition, the variation in alkyl spacer length of DA
provided evidence that flexible alkyl spacers may reduce the degree
of association between the PDA headgroup and the conjugated
backbone.
Addition of Acid Analytes and pH Measurement: Each acid (2 mL, 2 mM)
was added to 2 mL of PDA liposome solution (0.1 mM) for final
concentrations of 1 and 0.05 mM. pH measurement was carried out using
a SA520 pH meter (Orion Research Inc.) after the calibration of the pH
meter by pH buffer solutions of red (pH 4), yellow (pH 7), and blue
(pH 10). Each acid (2 mM) was diluted to 1 mM by adding deionized water
and the pH was recorded. The chromatic transitions by the addition of
acids were observed by UV-Vis and PL measurements at 30 s, 1 min, and
10 min after the acid addition.
UV-Vis and PL Measurement: UV-Vis absorption spectra were obtained
on a Cary 50 UV-Vis spectrophotometer (Varian Inc.) and PL spectra were
recorded on a PTI QuantaMaster fluorometer (Photon Technology
International), using a quartz cuvette with a 1 cm optical path length.
4. Experimental
Materials and Syntheses: 10,12-pentacosadiynoic acid (PCDA) was
purchased from GFS Chemicals (Powell, OH). All other reagents and
solvents were purchased from Sigma-Aldrich and Acros Organics and used
as received. Deionized water was used for the synthesis of DAs, DEP, and
the preparation of liposomes and diluted solutions of acid analytes.
APCDAn (n ¼ 2 ꢀ 6) were synthesized based on the synthetic routes of
APCDA2 and HPCDA2 reported previously [62] and characterized by NMR
and mass spectroscopies.
APCDA3 (N-(3-aminopropyl)-10,12-pentacosadiynamide). 94.4% yield.
1H NMR (300 MHz, CDCl3, d): 6.26 (br, 1H), 3.36 (dd, J ¼ 6.2, 12.4, 2H),
2.80 (t, J ¼ 6.3, 2H), 2.23 (t, J ¼ 6.9, 4H), 2.15 (t, J ¼ 7.6, 2H), 1.68–1.15
(m), 0.87 (t, J ¼ 6.7, 3H); MS (ESI): [M þ H]þ calcd for C28H50N2O:
431.4001; found: 431 .4005.
APCDA4 (N-(4-aminobutyl)-10,12-pentacosadiynamide). 90.1% yield.
1H NMR (300 MHz, CDCl3, d): 5.74 (br, 1H), 3.21 (dd, J ¼ 6.7, 12.5, 2H),
2.73 (t, J ¼ 6.6, 2H), 2.19 (t, J ¼ 6.9, 4H), 2.17 (t, J ¼ 7.6, 2H), 1.66–1.15
(m), 0.87 (t, J ¼ 6.7, 3H); MS (ESI): [M þ H]þ calcd for C29H52N2O:
445.4158; found: 445.4169.
APCDA5 (N-(5-aminopentyl)-10,12-pentacosadiynamide). 67.6% yield.
1H NMR (300 MHz, CDCl3, d): 5.46 (br, 1H), 3.25 (dd, J ¼ 7.0, 13.0, 2H),
2.69 (t, J ¼ 6.7, 2H), 2.23 (t, J ¼ 6.9, 4H), 2.18–2.10 (t, J ¼ 7.7, 2H), 1.66–
1.16 (m), 0.88 (t, J ¼ 6.7, 3H); MS (ESI): [M þ H]þ calcd for C30H54N2O:
459.4314; found: 459.4315.
APCDA6 (N-(6-aminohexyl)-10,12-pentacosadiynamide). 88.8% yield.
1H NMR (300 MHz, CDCl3, d): 5.44 (br, 1H), 3.24 (dd, J ¼ 7.1, 13.0, 2H),
2.70 (t, J ¼ 6.8, 2H), 2.24 (t, J ¼ 7.0, 4H), 2.18–2.10 (m, 2H), 1.66–1.18 (m),
0.88 (t, J ¼ 6.7, 3H); MS (ESI): [M þ H]þ calcd for C31H56N2O: 473.4471;
found: 473.4489.
DEP. To a solution of 10.34 g (59.93 mmol) of DCP in 5 mL of water in an
ice bath (4 ꢀ 5 8C) was added dropwise a solution of 4.79 g (119.85 mmol)
of NaOH in 45 mL of water. The mixture solution was allowed to stir for 2 h
at room temperature and acidified with 10% HCl. The solvent was
evaporated in a vacuum. The residue was dissolved in chloroform, dried
over MgSO4, and the solvent was removed by rotary evaporator. The
resulting compound was purified by silica gel column chromatography
eluting with CHCl3/MeOH (8:1) to yield DEP as a colorless oil (7.33 g,
47.56 mmol, 79.3%). 1H NMR (300 MHz, CDCl3, d): 8.44 (s, 1H), 4.10 (dq,
J ¼ 14.2, 7.1, 4H), 1.34 (td, J ¼ 7.1, 1.0, 6H); 31P NMR (121 MHz, CDCl3, d):
0.95.
Preparation and Polymerization of Liposome Solution: Each DA dissolved
in chloroform was placed in a 50 mL Erlenmeyer flask and the solvent was
dried. Deionized water was added to make a 0.1 mM solution of each DA.
The mixture solution was heated in a water bath of 90ꢀ95 8C and probe-
sonicated for 20 min with a high-intensity ultrasonic processor (750 W–
20 kHz, Cole-Parmer). The resulting solution was filtered through a 0.8 mm
filter (Millex-AA, Millipore) and stored at 4 8C for 2 h followed by warming
up to room temperature for an additional 2 h prior to photopolymerization.
DA liposome solution (2 mL) was put in a quartz cuvette and polymerized
by 254 nm light from a hand-held UV lamp (UVGL-25, UVP). UV irradiation
was performed for 10 s repeatedly until the excitonic peak of the blue phase
did not develop noticeably by the naked eye. The development of the blue
Acknowledgements
We gratefully acknowledge financial support from the National Science
Foundation (DMR Career 0644864) and the American Chemical Society
(PRF 46485-G7). Supporting Information is available online from Wiley
InterScience or from the author.
Received: February 8, 2010
Published online: April 1, 2010
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