Competitive influence of carboxylic groups in ionic complex formation of 4-hydroxybenzylidene alkanones with polyamidines

Article abstract of DOI:10.1021/ma049466i

Interactions of phenolic and carboxylic group containing chromophores (hydroxyarylidene alkanones) with polyamidines were investigated by UV/vis and ¹H NMR spectroscopy. Because of the strong basicity of the polyamidines, deprotonation of the c

Full text of DOI:10.1021/ma049466i

Macromolecules 2004, 37, 8389-8393
8389
Competitive Influence of Carboxylic Groups in Ionic Complex Formation
of 4-Hydroxybenzylidene Alkanones with Polyamidines
Ma r in a M. Du d k in a ,† An d r ey V. Ten k ovtsev,† Ha r tm u t Kom ber ,‡
Lia n e Ha1u ssler ,‡ a n d F r a n k Bo1h m e*,‡
Institute of Polymer Research Dresden e.V. Hohe Strasse 6, 01069 Dresden, Germany,
and Institute of Macromolecular Compounds, Russian Academy of Sciences, 199004,
Bolshoy pr. 31, St. Petersburg, Russia
Received March 18, 2004; Revised Manuscript Received J uly 16, 2004
ABSTRACT: Interactions of phenolic and carboxylic group containing chromophores (hydroxyarylidene
alkanones) with polyamidines were investigated by UV/vis and 1H NMR spectroscopy. Because of the
strong basicity of the polyamidines, deprotonation of the chromophores was observed. Deprotonation of
the phenolic group is accompanied by a change of the electronic structure of the chromophore resulting
in a strong bathochromic shift of the longest wavelength absorption band. Increased interactions are
observed after introduction of a carboxylic group into the chromophore. It could be shown that
deprotonation of the carboxylic group is favored so that the deprotonation of the phenolic group only
starts after a major part of the carboxylic groups is deprotonated. In highly diluted solutions, the
interaction of the carboxylic group with the polyamidine also increase the extent of deprotonation of the
phenolic groups. A physical network formation is assumed that also increases the glass transition
temperatures of the mixtures strongly.
Sch em e 1
In tr od u ction
Complexes of low molecular weight compounds with
polymers have been the object of intense research. On
the basis of electrostatic attraction between polyelec-
trolytes and oppositely charged surfactants, it was
possible to obtain systems with self-organization effects
resulting in the formation of mesomorphic and su-
pramolecular structures.1-6 Other examples comprise
structure formation of complexes between dendritic
ionomers and polyelectrolytes.7,8
Nowadays, special attention is paid on optical proper-
ties of such polymeric systems.9-11 Thunemann and
Ruppelt9,10 described supramolecular structures based
on carboxylate group containing polymers and cationic
low molecular weight compounds that showed electro
luminescence. Behnke and Tieke11 pointed out that
coloring agents could be bonded ionically to amino group
containing polymers.
Recently, we reported that aliphatic polyamidines are
able to interact with proton donating compounds result-
ing in deprotonation of the later. It was shown that in
ethanolic solution poly(1,10-decamethyleneacetamidine)
(P DA) forms ionic complexes with different 4-hydroxy-
benzylidene alcanones.12-15 Due to deprotonation, the
chromophores exhibit a bathochromic shift of their
longest wavelength absorption band by more that 100
nm. Additionally, third-order nonlinear optical activity
was evidenced on solution cast films. The reason for the
changes in optical properties is the much stronger linear
π-conjugation in the deprotonated chromophore mol-
ecule and a high portion of the quinoid form (see Scheme
1).
formation accompanied by an increase of the glass
transition temperature about 60 K.12,13 It could be
shown that the deprotonation of the second OH group
of 1b did not influence the optical behavior of the
mixture significantly.12 In other words, although the
second hydroxyl group favors network formation, it is
not required with respect to optical properties. This
raised the idea to exchange one hydroxyl group with
other functional groups possessing more or less pro-
nounced acidity. With the aim to promote network
formation, we replaced one hydroxyl group by one
carboxylic group. It is known that the interactions of
carboxylic groups with polyamidines are strong, result-
ing in almost complete deprotonation of the carboxylic
groups.16
Since the chromophore possesses two binding sites
(usually two hydroxyl groups, 1b), the noncovalent
interaction with the polymer leads to ionic network
In the present article, the competitive influence of the
carboxylic group in complex formation of 4-hydroxyben-
zylidene alkanones with poly(1,9-nonamethylene aceta-
midine) (P NA) or poly(1,8-octamethylene acetamidine)
* Corresponding author. E-mail: Boehme@ipfdd.de.
† Russian Academy of Sciences
‡ Institute of Polymer Research Dresden, e.V.
10.1021/ma049466i CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/28/2004

8390 Dudkina et al.
Macromolecules, Vol. 37, No. 22, 2004
(P OA) is discussed. For this, interactions of the three
different chromophores 1a -c (see Scheme 1) with P NA
in solution and with P OA in the solid were investigated.
Mixtures of low molecular weight chromophores with
polymers are regarded as a promising alternative for
polymers with covalently bonded chromophores. They
are easier to prepare and possess better processability,
and their properties can be tailored by changing the
composition. The properties of the mixtures described
here strongly depend on the interactions between the
components. A better understanding of these interac-
tions and their influence on optical, thermal, and
mechanical properties may open new possibilities for the
development of optical and pH sensitive materials for
information storage and optical switching.
On the other hand, the alterations of the optical
behavior of the chromophores in the presence of proton
donor groups might be a very sensitive probe for
interactions in the interface of polymer blends or
composite materials. For this the chromophore has to
be bonded covalently to one of the components. In the
case of 1c, this can be done with the carboxylic group
side without to lose the optical sensitivity of the
molecule. This will be subject of future investigations.
F igu r e 1. UV/vis spectra of 1c in ethanol in the presence of
different amounts of P NA (concentration of 1c, 4.34 × 10-5
mol/L; molar amidine-chromophore ratio (a) 0, (b) 1.19, (c) 2.05,
(d) 3.41, and (e) 13.64).
and left overnight. The mixture was diluted with 30 mL of
water, and acidified with diluted acetic acid. Yellow crystals
precipitated. The product 1c was filtered off and recrystallized
from an ethanol-water (1:2 v/v) mixture. Yield: about 3 g
(45%). Data for 1c: Tm ) 260-262 °C; 1H NMR (CD3OD) δ
(ppm) ) 8.07 (d, H1), 7.81 (d, H2), 7.79 (d, H6), 7.78 (d, H3),
7.60 (d, H7) 7.37 (d, H4), 7.08 (d, H5), 6.85 (d, H8).
Exp er im en ta l Section
Ma t er ia ls. p-Hydroxybenzaldehyde, p-carboxybenzalde-
hyde, and acetone were purchased from Aldrich and used
without further purification.
Mea su r em en ts. UV measurements were recorded on a
Cary 100 Varian spectrophotometer on ethanol solutions (l )
10 mm).
P olya ceta m id in es. The phenol catalyzed preparation of
polyacetamidines by conversion of aliphatic diamines with
triethyl orthoacetate was carried out as described earlier.17
1-(4-Hyd r oxyp h en yl)-5-(4-m et h oxyp h en yl)-p en t a -1,4-
d ien -3-on e (1a ) a n d 1,5-Bis(4-h yd r oxyp h en yl)-p en ta -1,4-
d ien -3-on e (1b). Preparation of compounds 1a and 1b was
described elsewhere.12
1H NMR spectra were recorded on a Bruker DRX 500 NMR
spectrometer operating at 500.13 MHz for 1H. CD3OH was
used as solvent and tetramethylsilane (TMS) as internal
standard. In the titration experiments a P NA solution in
CD3OH (0.08 and 0.008 mol repeating unit/L, respectively) was
added in 25 µL portions to 1 mL of chromophore solution (0.01
and 0.001 M of 1c in CD3OH, respectively) directly placed in
a NMR tube.
4-[5-(4-H yd r oxy-p h en yl)-3-oxo-p en t a -1,4-d ien yl]-b en -
zoic a cid (1c). 1c was prepared as follows:
DSC measurements were performed on a Perkin-Elmer DSC
7 at a heating rate of 20 K/min in the temperature range -30
to +180 °C.
Resu lts a n d Discu ssion
As shown earlier, aliphatic polyamidines are basic
enough to be able to deprotonate chromophores of type
1. Upon deprotonation strong changes in their absorp-
tion behavior are observed. As an example, UV/vis
spectra of 1c in the presence of different amounts of
P NA are shown in Figure 1. Two absorption bands are
seen, the intensities of which alter in opposite directions.
The absorption band at 370 nm belongs to the structure
with a protonated phenolic group, whereas the band of
the deprotonated structure appears at 460 nm. The
deprotonation of the carboxylic group has no influence
on the UV/vis spectrum of 1c. The spectral behaviors
of compounds 1a and 1b are very similar.12
The influence of the degree of deprotonation on the
absorption behavior allows one to record the titration
curves of 1a -c by UV/vis spectroscopy. The absorbance
of the maximum at 460 nm serves as a measure for the
degree of deprotonation. In Figure 2, the respective
titration curves of 1b with NaOH and P NA are shown.
As expected, the deprotonation proceeds at much lower
base concentrations when the stronger base NaOH was
used as a titrator. In both cases, a distinct excess of base
is required to achieve complete deprotonation. This is
not typical for strong bases, such as NaOH, but can be
explained by the high dilution of the titrant (concentra-
tion of 1b ) 3.58 × 10-5 mol/L). In such highly diluted
A solution of 2.65 g (66 mmol) of NaOH in 10 mL of water
was added to a solution of 4.00 g (33 mmol) of p-hydroxyben-
zaldehyde in 15 mL of acetone. A precipitate was formed. The
mixture was heated to 50 °C for 15 min (precipitate dissolves)
and then cooled and left overnight. The mixture was diluted
with water (about 80 mL) until the precipitate dissolved
completely. After acidification with diluted acetic acid, a yellow
oil was formed, which turned into a crystalline product within
5 min. The intermediate 4-(4-hydroxyphenyl)-but-3-en-2-one
was filtered off and recrystallized from an ethanol-water (1:2
v/v) mixture. Yield: about 4 g (75%); Tm ) 108-110 °C.
A 3.87 g (26 mmol, 10% excess) sample of p-carboxybenzal-
dehyde was dissolved in a solution of 2.81 g (70 mmol) of NaOH
in 15 mL of water. This solution was added to a solution of
3.80 g (23 mmol) of 4-(4-hydroxyphenyl)-but-3-en-2-one in 15
mL of ethanol. A precipitate was formed. The mixture was
heated to 50 °C for 15 min (precipitate dissolves), then cooled

Macromolecules, Vol. 37, No. 22, 2004
Competitive Influence of Carboxylic Groups 8391
F igu r e 2. UV/vis titration curves of 1b (absorbance measured
in the maxima at about 470 nm in dependence on NaOH and
P NA added; concentration of 1b ) 3.58 × 10-5 mol/L).
F igu r e 4. 1H NMR spectra of 1c in CD3OD in the presence
of P NA: (a) pure sample; (b) equimolar sample; (c) sample
with distinct excess of amidine groups.
The UV/vis spectroscopy gives only information about
the degree of deprotonation of the phenolic group of 1c.
Conclusions as to the deprotonation state of the car-
boxylic group can only be drawn indirectly since its
deprotonation does not influence the UV/vis spectrum.
Simultaneous information about both groups can be
1
obtained by NMR spectroscopy. In Figure 4, H NMR
spectra of 1c in the presence of different amounts of
P NA are shown. It can be seen that the position of the
signals of the aromatic group protons H1, H2, H7, and
H8 strongly depend on the amount of base in the system.
With increasing degree of deprotonation a distinct high-
field shift of these signals is observed. Comparing the
three spectra shown in Figure 4, it is obvious that the
behavior of the H1 and H2 signals differs from that of
H7 and H8. The proton signals at the carboxylic group
side (H1 and H2) change their position already at
relatively low amidine concentrations, whereas a dis-
tinct shift of the proton signals at the phenolic side is
only seen at higher amounts of P NA added. This reflects
the different deprotonation behavior of the carboxylic
and the phenolic groups.
F igu r e 3. UV/vis titration curves of 1c (absorbance measured
in the maxima at about 460 nm in dependence on NaOH and
P NA added; concentration of 1c ) 4.34 × 10-5 mol/L).
systems the competing interactions with the solvent
(ethanol) reduces the activity of the base strongly.
The titration curves of the carboxylic group containing
chromophore 1c are shown in Figure 3. In contrast with
1b, the deprotonation of the phenolic group of 1c with
NaOH is retarded. At a NaOH/chromophore ratio of
about 1.5 (mol/mol) the deprotonation of the phenolic
group of 1c is negligible whereas in the case of 1b
deprotonation already starts at a ratio distinctly below
1. This can be explained by the presence of carboxylic
groups which are stronger acidic than phenolic groups.
If NaOH is added to the solution, the base is first
consumed by the neutralization of the carboxylic groups
with no influence on the UV/vis absorption. Significant
deprotonation of the phenolic groups only starts after
the majority of carboxylic groups is neutralized.
Provided the chemical shifts of the completely proto-
nated (δR)0) and the completely deprotonated chro-
mophore (δR)1) are known, it is possible to calculate the
degree of deprotonation R for both groups from the
actual chemical shifts:
[A-]
[A-] + [AH]
δR - δR)0
δR)1 - δR)0
R )
)
[A-] and [AH] are the molar concentrations of the
particular deprotonated and protonated group, respec-
tively. Here, δR)0 was determined from a saturated
solution of the chromophore and δR)1 from a solution
with a 5-fold molar excess of the base.
Because of the low sensitivity of NMR spectroscopy,
the measurements had to be carried out at higher
concentrations than the UV/vis measurements. Figure
5 shows the calculated R values of 1c in dependence on
the chromophore/amidine ratio at a chromophore con-
The shape of the curve obtained for the titration of
1c with P NA was completely unexpected. In contrast
with 1b, deprotonation of the phenolic groups starts at
much lower base concentrations (amidine) and occurs
even easier compared with the NaOH titration curve.
It is reasonable to assume that this behavior is caused
by the competing interaction of the phenolic and the
carboxylic groups with the polymer. An explanation for
this behavior will be given later.

8392 Dudkina et al.
Macromolecules, Vol. 37, No. 22, 2004
F igu r e 5. 1H NMR titration curves of 1c, with P NA as
titrator (concentration of 1c, 1 × 10-2 mol/L).
F igu r e 6. Glass transition temperatures of P MA/chro-
mophore mixtures in dependence on their composition: (a) 1a ;
(b) 1b; (c) 1c.
centration of 1 × 10-2 mol/L. The curves clearly show
that the deprotonation of the carboxylic groups starts
first and that of the less acidic phenolic groups subse-
quently. In the pure chromophore solution without base,
about 15% of the carboxylic groups are already depro-
tonated by self-dissociation. Complete deprotonation of
the whole chromophore is reached at about equimolar
ratio (chromophore/amidine ) 2).
are completely soluble in the polymer, they contribute
to a molecular reinforcement of the matrix polymer,
resulting in reduced mobility of its chain segments and
eventually in increased Tg. The differences between the
three compounds can be explained by the nature of their
functional groups.
Additional measurements showed that at higher
dilution (1 × 10-3 mol/L) the self-dissociation of the
carboxylic groups is more pronounced (R approximately
75%) and that deprotonation of the phenolic group starts
already at distinctly lower base concentration (chromo-
phore/amidine approximately 0.4). Furthermore, com-
plete deprotonation (COOH and OH) was only observed
if a distinct excess of base (approximately 100%) was
added to the solution. These observations are in good
accordance with the results obtained by UV/vis spec-
troscopy.
As mentioned above, UV/vis titration of the carboxylic
group containing chromophore 1c proved that the
phenolic group is easier deprotonated by P NA than by
NaOH. This can be explained by the polymeric nature
of P NA. As approximated from the NMR results, one
has to assume that the carboxylic group of 1c is
completely dissociated at the concentration commonly
used for UV/vis measurements. As soon as P NA or
NaOH is added to the solution of 1c, the free protons
are neutralized. In the case of P NA, proton absorption
results in a positively charged polymer chain that is
assumed to form immediately ionic complexes with the
anions of 1c by electrostatic attraction. In the vicinity
of the polymer, the local concentration of amidine groups
is much higher than average resulting in an earlier
deprotonation of the phenolic group of 1c. In other
words, the carboxylic group of 1c favors the approach
of the chromophore to the polymer and subsequently
also the deprotonation of the phenolic group.
Generally, the influence of the chromophores to the
glass transition behavior is very strong. From our earlier
investigations, it has been known that the deprotonation
of phenol group containing chromophores by poly-
amidines is much higher in the solid state than in
solution, but not complete.13 Deprotonation of carboxylic
groups, however, proceeds quantitatively.16 In the case
of 1a , it is assumed that in the bulk mixture with P OA
the degree of deprotonation of the single phenolic group
is high at low chromophore concentration, but it dimin-
ishes with increasing chromophore content. That means,
that at high chromophore concentrations near the
equimolar ratio (chromophore/amidine ) 1.0) some
chromophore molecules are not demobilized by com-
plexation. This explains the strong increase in Tg at low
chromophore contents and the leveling off at equimolar
ratio (Figure 6a).
An additional contribution to enhancement of Tg can
be expected by cross-linking. Chromophore 1b with two
phenolic groups is assumed to form a physical network
with P OA by ionic complex formation. Because of this,
mixtures of 1b and P OA possess distinctly higher Tg
values (Figure 6b) than mixtures of 1a . For 1b, the
equimolar ratio is already reached at a chromophore/
amidine ratio of 0.5. At this ratio, maximum network
formation is assumed. Further increase in Tg at higher
chromophore/amidine ratios is attributed to the different
contributions of decreasing network formation and
increasing reinforcement by the rodlike chromophores.
The strongest influence on Tg is seen for chromophore
1c (Figure 6c). The addition of 1c to P OA results in an
increase of Tg from 7 °C for pure P OA to 146 °C at
equimolar ratio (chromophore/amidine ) 0.5). At higher
chromophore contents a decrease of Tg can be observed.
It is reasonable that this is caused by the presence of
the carboxylic group.
The interactions of the chromophores with the poly-
mer also influence the thermal behavior of the mixtures
in bulk. Mixtures of 1a , 1b , and 1c with poly(octa-
methyleneacetamidine) (P OA) were prepared by casting
from ethanolic solutions. After complete evaporation of
the solvent, deeply red films were obtained, the thermal
properties of which were investigated by DSC. DSC
scans of all mixtures showed pronounced glass transi-
tions which were strongly dependent on their composi-
tion. The results are demonstrated in Figure 6. In each
case a strong increase in Tg upon addition of chro-
mophore is observed. Since the rodlike chromophores
One can assume that the interactions of the carboxylic
groups with the amidine groups are almost quantitative
over the whole composition range investigated. That
means that at equimolar ratio about 50% of the amidine
groups are occupied by carboxylic groups and a certain
part of the remaining amidine groups by hydroxyl

Macromolecules, Vol. 37, No. 22, 2004
Competitive Influence of Carboxylic Groups 8393
groups. This reflects a high degree of physical cross-
linking. With increasing amounts of chromophore, the
bonded hydroxyl groups are more and more replaced by
carboxylic groups because of their higher acidity. Even-
tually, a completely un-cross-linked associated side
chain polymer with free hydroxyl groups is formed at a
chromophore/amidine ratio of 1. In the case of chro-
mophore 1b, a certain amount of cross-links beside
completely unassociated molecules has to be assumed
at this ratio since both phenolic groups are equal. This
explains the higher Tg of the mixture with 1b at high
chromophore concentrations compared to the mixture
with 1c which is assumed not to be cross-linked.
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Con clu sion s
(9) Thunemann, A. F.; Ruppelt, D. Langmuir 2001, 17, 5098-
It could be shown that the interactions of polyamidine
chromophore mixtures both in solution and in bulk are
strongly influenced by the incorporation of a carboxylic
group into the chromophore molecule. Because of the
pronounced acidity of the carboxylic group, the chro-
mophore is stronger bonded to the polymer that also
results in an improvement of the thermal stability of
the mixture. The change in UV/vis absorption caused
by deprotonation of the phenolic group is not effected
by the interactions of the carboxylic group. In solution,
these interactions even contribute to strengthen this
optical effect. The incorporation of the carboxylic group
also opens the possibility to bond the chromophore
covalently to a polymer. This will be subject of future
investigations.
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Ack n ow led gm en t . We thank the Sa¨chsische
Staatsministerium fu¨r Wissenschaft und Kunst for
financial support.
MA049466I