Chromo-and fluorophoric water-soluble polymers and silica particles by nucleophilic substitution reaction of poly(vinyl amine)

Article abstract of DOI:10.3762/bjoc.6.79

Novel chromophoric and fluorescent carbonitrile-functionalized poly(vinyl amine) (PVAm) and PVAm/silica particles were synthesized by means of nucleophilic aromatic substitution of 8-oxo-8H-acenaphtho[1,2-b]pyrrol-9- carbonitrile (1) with PVAm in water. The water solubility of 1 has been mediated by 2,6-O-β-dimethylcyclodextrin or by pre-adsorption onto silica particles. Furthermore, 1 was converted with isopropylamine into the model compound 1-M. All new compounds were characterized by NMR, FTIR, UV-vis and fluorescence spectroscopy. The solvent-dependent UV-vis absorption and fluorescence emission band positions of the model compound and the carbonitrile-functionalized PVAm were studied and interpreted using the empirical Kamlet-Taft olvent parameters ?^* (dipolarity/polarizability), α (hydrogen-bond donating capacity) and β (hydrogen-accepting ability) in terms of the linear solvation energy relationship (LSER). The solvent-independent regression coefficients a, b and s were determined using multiple linear correlation analysis. It is shown, that the chains of the polymer have a significant influence on the solvatochromic behavior of 1-P. The structure of the carbonitrile 1-Si bound to polymer-modified silica particles was studied by means of X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) measurements. Fluorescent silica particles were obtained as shown by fluorescence spectroscopy with a diffuse reflectance technique.

Full text of DOI:10.3762/bjoc.6.79

Chromo- and fluorophoric water-soluble polymers
and silica particles by nucleophilic substitution
reaction of poly(vinyl amine)
Katja Hofmann1, Ingolf Kahle1, Frank Simon2 and Stefan Spange*1
Full Research Paper
Open Access
Address:
Department of Polymer Chemistry, Chemnitz University of
Technology, Straße der Nationen 62, Chemnitz 09111, Germany and
Leibniz Institute of Polymer Research Dresden, Hohe Straße 6,
Beilstein J. Org. Chem. 2010, 6, No. 79. doi:10.3762/bjoc.6.79
1
Received: 21 May 2010
Accepted: 07 July 2010
Published: 22 July 2010
2
Dresden 01069, Germany
Email:
Guest Editor: H. Ritter
Katja Hofmann - katja.hofmann@chemie.tu-chemnitz.de;
Ingolf Kahle - ingolf.kahle@chemie.tu-chemnitz.de; Frank Simon -
frsimon@ipfdd.de; Stefan Spange* -
© 2010 Hofmann et al; licensee Beilstein-Institut.
License and terms: see end of document.
stefan.spange@chemie.tu-chemnitz.de
*
Corresponding author
Keywords:
carbonitrile; cyclodextrin; fluorescence; hybridmaterials; poly(vinyl
amine)
Abstract
Novel chromophoric and fluorescent carbonitrile-functionalized poly(vinyl amine) (PVAm) and PVAm/silica particles were synthe-
sized by means of nucleophilic aromatic substitution of 8-oxo-8H-acenaphtho[1,2-b]pyrrol-9-carbonitrile (1) with PVAm in water.
The water solubility of 1 has been mediated by 2,6-O-β-dimethylcyclodextrin or by pre-adsorption onto silica particles. Further-
more, 1 was converted with isopropylamine into the model compound 1-M. All new compounds were characterized by NMR,
FTIR, UV–vis and fluorescence spectroscopy. The solvent-dependent UV–vis absorption and fluorescence emission band positions
of the model compound and the carbonitrile-functionalized PVAm were studied and interpreted using the empirical Kamlet–Taft
solvent parameters π* (dipolarity/polarizability), α (hydrogen-bond donating capacity) and β (hydrogen-accepting ability) in terms
of the linear solvation energy relationship (LSER). The solvent-independent regression coefficients a, b and s were determined
using multiple linear correlation analysis. It is shown, that the chains of the polymer have a significant influence on the solvato-
chromic behavior of 1-P. The structure of the carbonitrile 1-Si bound to polymer-modified silica particles was studied by means of
X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) measurements. Fluorescent silica particles were
obtained as shown by fluorescence spectroscopy with a diffuse reflectance technique.
Introduction
Fluorescent polymers are an important tool for the study of great interest in the synthesis, characterization and applications
molecular dynamics and also find use as sensor probes for bio- of fluorescent polymers [2-4]. In general, there are two
logically active compounds [1]. In recent years, there has been approaches for the synthesis of fluorescent polymers: First, the
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Beilstein J. Org. Chem. 2010, 6, No. 79.
polymerization of a monomer that contains a fluorescent chro- the introduction of alkyl or aryl groups, thus mediating solu-
mophore is possible. However, in some cases the new fluoro- bility in organic solvents [26].
phore-carrying polymer [1,5] is accompanied by a non-adequate
effort concerning its synthesis and purification. An alternative The use of cyclodextrins (CD) provides a further opportunity to
pathway is the chemical modification of commercially avail- solve these problems. Due to the formation of host–guest
able polymers containing reactive groups which can react with complexes with CD both reactants become completely soluble
fluorescent dyes [6]. Poly(vinyl amine)s (PVAm) are copoly- in water. According to Ritter et al., the 2,6-O-β-dimethylcyclo-
mers made by the hydrolysis of poly(vinyl formamide) (PVFA). dextrin (β-DMCD) derivative has been found to be suitable
Fully hydrolyzed PVFA contains ca. 5% of formamido groups because of its higher solubility in water compared to β-cyclo-
and 95% of primary amino groups. The water-soluble PVAm dextrin (800.5 g L−1 vs. 18.5 g L−1) [27-29].
copolymer has been widely applied for a number of purposes,
e.g., in catalysis [7], chelation [8], treatment of waste water [9], Chemical reactions between two incompatible reactants can also
paper making [10], recovery of oil [11] and as superabsorber be achieved under heterogeneous reaction conditions (the reac-
[
12]. Modified PVAm is expected to be useful in the prepara- tion is localized at the interphase of the two contacting phases).
tion of polymeric water-soluble dyes [13], in mimicking natural This approach was developed to carry out functionalization
enzymes [14] and as a polymeric surfactant [15]. Depending on reactions between the water-soluble PVAm and suitable chro-
the pH of the aqueous solution, the primary amino groups of mophores [30] or fluorophores. In these reactions, silica can be
PVAm can be partly protonated (−NH3+) or they can be present considered as ‘solubilizer’ because PVAm, as well as the reac-
as charge-neutral amines (−NH2). In this context, PVAm can be tants, are consecutively adsorbed on the silica particles. The
considered as a weak cationic polyelectrolyte.
functionalization reactions take place on the silica surface
simultaneously. The large specific surface area of the silica
The pH-variation is an excellent tool to control and adjust the particles employed (ca. 400 m2 g−1) guarantees the presence of
net-charge density along the polymer chain or in a layer made a high amount of the adsorbed polymer and modifiers [31].
from PVAm [16,17]. Full conversion of the amino groups into
their cationic form leads to a polymer with the highest known The electron-deficient heterocycle 8-oxo-8H-acenaphtho[1,2-
charge density along the polymer backbone. Such highly b]pyrrol-9-carbonitrile (1) is a type of novel fluorescent chro-
protonated polymers appear to be very interesting compounds mophore with long-wavelength absorption and fluorescence, the
for the prevention of bacteria adhesion on surfaces [18]. Non- excitation and emission wavelengths of which can reach 530
protonated primary amino groups in the PVAm polymer have a nm and 590 nm, respectively [36]. It is well known, that
high synthetic potential which can be used for versatile carbonitrile 1 is able to react with nitrogen-, oxygen-, or sulfur-
subsequent derivatization reactions [17,19-21].
containing nucleophiles by nucleophilic aromatic substitution
reactions (SNArH) [32-36]. The reaction of 1 with 3-thio-
In previous articles we reported the nucleophilic aromatic propionic acid has used to produce a fluorescent sensor for cys/
substitution of activated fluoroaromatic compounds with PVAm Hcy with a 75-fold fluorescence enhancement [34]. The SNArH
[
22-24]. Reactions of PVAm with derivatization agents seem at reaction of 1 with the thiolated RGD peptide cyclo(Arg-
first glance to be simple to carry out, but in fact they require a Gly_Asp-Phe-Lys(mpa))(c(RGDFK)-SH) leads to a fluorescent
considerable synthetic effort because the reactants must share a sensor for imaging tumor cells [37].
solvent or a homogeneous mixture of solvents. For many reac-
tions it is profitable to remove or to buffer low-molecular mass In this paper we report our current studies on the functionaliza-
products formed during the reaction between PVAm and the tion of PVAm with carbonitrile 1 to introduce a chromophore as
derivatization agent.
well as a fluorophore into the polymer chain. The synthesis was
achieved by two different approaches. The first method
High molecular mass products of PVAm copolymers (Mr > included the use of β-DMCD to render 1 compatible to PVAm
500 g mol−1) are soluble in water only, while fluoroaromatic dissolved in water. The SNArH reaction between the two reac-
1
compounds are sparingly soluble or even insoluble in water. tants was carried out in homogeneous phase. The second
The lack of solvents capable of dissolving all the reactants approach was to synthesize a hybrid material by the consecu-
complicates subsequent derivatization reactions or requires a tive adsorption of PVAm and 1 onto silica particles. The
high synthetic effort. Micelle techniques commonly used to adsorbed substances were reacted directly on the silica particle
carry out reactions between water-soluble and insoluble com- surface. The reaction products (yield on the surface of silica
pounds cannot be cleanly applied to modify polyelectrolytes particles) were analyzed using high-resolution X-ray photoelec-
[
25]. Alternatively, water-soluble polymers can be modified by tron spectroscopy (XPS). The XPS method is very surface-sen-
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Beilstein J. Org. Chem. 2010, 6, No. 79.
sitive and gives spectroscopic information for sample depths of The success of the SNArH reaction of 1 with PVAm (Scheme 2)
less than 8 nm.
mediated by β-DMCD in water could be established using
3C-{1H}-CP-MAS NMR spectroscopy.
1
Results and Discussion
Synthesis
Figure 1 compares the solid state 13C NMR spectra of pure
PVAm, fluorophore-functionalized PVAm 1-P and the model
compound 1-M. It can be seen that the signals observed for 1-M
Synthesis and characterization of the fluorescent
polymer
Carbonitrile 1 was synthesized by a very efficientl two-step and pure PVAm are also visible in the 13C-{1H}-CP-MAS
literature procedure [32,34]. As shown in Scheme 1, the starting NMR spectrum of 1-P. This is an excellent evidence for the
material acenaphthylene-1,2-dione undergoes a Knoevenagel functionalization of the PVAm fluorophore 1. The signal at
condensation with malononitrile to give mono-adduct 2.
δ = 169 ppm in the solid state 13C NMR spectra [(a) and (b),
Figure 1] are due to the residual formamido groups while the
Compound 2 was originally synthesized by Junek et al. [38]. signal at δ = 67 ppm is caused by the methine carbon of
However, no detailed NMR data were reported. In a second poly(vinyl alcohol) [39,40].
step, cyclization with anhydrous K2CO3 converted 2 to 1 in a
good yield. In contrast to the procedure described in [34], a ten- Further confirmation of the success of the SNARH reaction was
fold excess of K2CO3 did not give rise to the target compound obtained from the FTIR spectra. Pure PVAm copolymers show
1
. Only the use of a 10 % mol. equivalent of the base led to 1. characteristic IR absorption of N–H groups in the range of
Scheme 1: Synthesis of carbonitrile 1.
Scheme 2: Coupling of the β-DMCD-caged carbonitrile derivative 1-DMCD with PVAm to yield the fluorophore-functionalized PVAm 1-P and synthe-
sis of the model compound 1-M.
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Beilstein J. Org. Chem. 2010, 6, No. 79.
units occurs at ca. = 1665 cm−1 which appear to be over-
lapped by N–H deformation vibration bands. The presence of
the C≡N stretching band at = 2221 cm−1 confirms the exist-
ence of the organo-substituted fluorophore.
The glass transition temperature Tg is an important parameter
for polymers, which indicates the transition of the amorphous
phase of the polymer from its rubbery to its glassy state (and
vice versa). Pure PVAm shows a Tg of 103 °C. The glass tran-
sition temperature is a function of chain flexibility. Because of
the presence of hydrogen-bond donor and -acceptor centers
along the polymer backbone, PVAm is able to form hydrogen
bonds between the amino groups (and residual formamido
groups). The carbonitrile-functionalized PVAm 1-P shows a
lower Tg of 87 °C. Obviously, the introduction of the rigid
fluorophore leads to a separation of the individual polymer
chains, resulting in a decreased glass transition temperature.
Figure 1: Solid state 13C-{1H}-CP-MAS NMR spectra of pure PVAm
a), functionalized PVAm 1-P (b) and the model compound 1-M (c) [“*”
indicates the signals of the methine carbon of poly(vinyl alcohol) (PVA)
and “#” corresponds to non-hydrolized formamido groups still present
in the PVAm chains].
(
Synthesis and characterization of the hybrid
material
=
=
3450–3150 cm−1 due to the stretching vibration and at As can be seen from Scheme 3, carbonitrile 1 was adsorbed
1600 cm−1 due to the deformation vibration. Asymmetric from its dichloromethane solution onto carefully dried silica
as well as symmetric stretching modes of C–H groups particles (Kieselgel 60, Merck). The 1-loaded silica particles
= 2880–2950 cm−1) and deformation vibrations of the CH2 were transferred into an aqueous solution of PVAm. In order to
(
and CH group at = 1430 cm−1 as well as 1381 cm−1 were initiate the SNARH reaction between 1 and PVAm the solution
observed. Additionally, the IR band at = 1665 cm−1 corres- was heated and kept for 16 h at its boiling point. The silica
ponds to residual formamido groups. The IR spectrum of the particles were extracted twice, first with water to remove non-
model compound shows a sharp N–H stretch band at ca. adsorbed functionalized PVAm and then with acetone to
=
3334 cm−1, stretching modes of aromatic and aliphatic C–H remove unreacted 1. 1-Si was obtained as intensively colored
groups between 2879–3080 cm−1, C≡N stretching at 2213 cm−1 purple solid.
and aromatic C=C stretching vibrations at 1627 cm−1. The IR
spectrum of the carbonitrile-functionalized PVAm 1-P shows The 13C-{1H}-CP-MAS NMR spectrum of 1-Si shows the char-
the characteristic absorption bands of both the PVAm and acteristic signals of the fluorophore as well as those of the
carbonitrile moiety which confirms the functionalization. A aliphatic carbons of the polymer backbone. The FTIR spectrum
broad N–H stretching band of the NH2 groups of the polymer of 1-Si confirms the adsorption of the functionalized PVAm
and the NH group of the fluorophore at = 3550–3150 cm−1 onto the silica particle surface. As expected, a strong IR absorp-
was observed. Bands between = 2890–2990 cm−1 correspond tion band was present in the range of = 1200–1000 cm−1
to the stretching modes of the aliphatic CH groups of the corresponding to Si–O–Si stretching vibrations. Furthermore, a
polymer backbone. The IR absorption of the aromatic C=C broad N–H stretching vibration at = 3150–3500 cm−1 was
Scheme 3: Synthesis of 1-Si by nucleophilic aromatic substitution of 1 adsorbed onto silica particles.
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Beilstein J. Org. Chem. 2010, 6, No. 79.
observed. Additionally, the IR spectrum shows stretching vibra- atoms of the amino (and residual formamido) groups lead to the
tions at 2207 cm−1, 1651 cm−1 and 1372 cm−1 (νC≡N, νC=C and N 1s peak (Figure 2b). The corresponding C 1s high-resolution
δCH) which are evidence for the adsorption of carbonitrile-func- spectrum of the PVAm-loaded silica shows saturated hydrocar-
tionalized PVAm.
bons from the methylene units of the polymer backbone and
surface contaminations (component peak A). Component peak B
XPS spectra were taken from the unmodified silica, PVAm shows C–N bonds of primary amino groups of the polymer and
adsorbed onto silica and the fluorophore PVAm-modified silica the amine-side carbon atoms (O=CH–NH–C) of residual amido
particles 1-Si (Figure 2).
groups. Formamido groups (O=CH–NH–C) contribute to the
component peak C. The area of component peak C is ca. 4.9%
Figure 2a shows a typical XPS wide-scan spectrum of the bare of the C 1s area. As noted above, the amino groups of the
silica support which was unloaded. The spectrum contains the PVAm polymer can be protonated by hydronium ions. In
expected peaks from silicon (Si 2s and Si 2p) and oxygen (O 1s, Figure 2b the N 1s high-resolution spectrum of the PVAm/silica
O 2s and the O KLL Auger series). The small C 1s peak shows hybrid material shows such protonated amino groups
the presence of typical hydrocarbon surface contaminations. (component peak G), clearly separated from the non-protonated
PVAm, adsorbed onto the silica particle surface, strongly amino groups (component peak F). Nitrogen involved in amide
increases the amount of carbon on the silica surface. Nitrogen groups contributes to component peak F. The ratio of the two
Figure 2: Wide-scan X-ray photoelectron spectra (left), C 1s and N 1s high-resolution spectra (right) of bare silica particles (a), PVAm adsorbed onto
silica particles (b) and PVAm reacted with 1 on the surface of silica particles (c).
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Beilstein J. Org. Chem. 2010, 6, No. 79.
component peak areas [G]:[F] can be considered as the protona-
tion–deprotonation equilibrium of the amine-functionalized
hybrid surface.
The reaction of adsorbed PVAm with adsorbed 1 changes the
corresponding XPS spectra slightly (Figure 2c). In the wide-
scan spectrum the amounts of carbon and nitrogen increase: The
elemental ratio [N]:[O] of the PVAm loaded silica sample was
found to be [N]:[O] = 0.094, while the ratio after reaction of
PVAm with 1 was [N]:[O] = 0.152 (nitrogen can be considered
as label for the organic adsorption layer and oxygen the label
for the silicon support). The coupling of PVAm with the fluoro-
phore group 1 introduces an additional amount of carbonyl
groups (carbon atoms of the cyclic amide in structure 1-P) into
the organic adsorption layer. Hence, in the corresponding C 1s
spectrum (Figure 2c) the area of component peak C is increased
to 8.11%. Carbon atoms of the C≡N group, C–N bonds in struc-
ture 1 and C–NH–C links between 1 and the PVAm polymer
contribute to component B. The attachment of the fluorophore
Figure 3: Pore-size distribution of bare silica, PVAm/silica and 1-Si.
to the PVAm also changes the ability of the nitrogen atoms to the inner silica surface. The corresponding BET measurements
be protonated. Nitrogen atoms in structure 1 cannot be involved show a smaller value of specific surface area (Table 1). After
in the protonation–deprotonation equilibrium of the amino the adsorption of 1 or PVAm the fraction of the pore sizes
groups of the PVAm polymer. The N 1s spectrum in Figure 2c between 4 and 10 nm is apparently higher. However, the values
shows a significantly decreased area of component peak G given in Figure 3 are expressed in percent terms. The increased
which is due to the C–N+ species (its binding energy is BE ≈ values profit from the decreased value of the accessible narrow-
4
01.4 eV while the binding energy of the non-protonated size pores.
species is BE ≈ 399.5 eV).
Solvatochromic and fluorescence properties
Table 1 summarizes the results of the BET analysis. In addition, Interactions of solvatochromic dyes with pure solvents or
Figure 3 shows the pore-size distribution histograms of bare solvent mixtures are a combination of many effects [41-44]. We
silica, a typical PVAm/silica hybrid material and 1-Si.
wanted to investigate the influence of the PVAm on the chro-
mophoric and fluorophoric π-electron system of 1 resulting
The bare silica support used for the functionalization reaction from intermolecular interactions with the surroundings of the
showed a BET surface of 411 m2 g−1. The main average pore molecules, and which of these are dipole–dipole and/or
radius ranges between 2 and 4 nm. The adsorption of PVAm as hydrogen-bond interactions. To separate the effects of non-
well as 1-Si significantly reduces the specific surface area. specific van der Waals interactions including electrostatic
Small molecules such as the fluorophore 1 preferably fill the effects (dipolarity/polarizability) from specific interactions
narrow pores. Hence, the fraction of the narrow pore radii 2–4 (hydrogen bonding), we used the simplified Kamlet–Taft equa-
nm is considerably decreased. PVAm also reduces this. But the tion (Equation 1) [45,46].
macromolecules are also able to cover the porous silica surface
(1)
and prevent the access of the BET probe molecules (nitrogen) to
Table 1: Specific surface areas (according to BET, ABET) and fractions of the average pore-size radii range of the bare silica support, a PVAM/silica
hybrid material and 1-Si, compared with the relative carbon-content (Cfound/Ccalc·100%) determined from data of the elemental analysis.
sample
Fraction of the average pore radii (%)
Cfound/Ccalc
100%
ABET (m2 g−1)
2–4 nm
4–10 nm
bare silica
411
207
197
85
27
6
–
PVAm/silica
45
33
75.50
89.05
1-Si
0.3
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Beilstein J. Org. Chem. 2010, 6, No. 79.
According to Equation 1, the influence of the hydrogen-bond = 333 cm−1 and Δ (1-P) = 905 cm−1, respectively. In general,
donor capacity (HBD) [47], the hydrogen-bond acceptor capac- the carbonitrile-functionalized PVAm 1-P absorbs at shorter
ity (HBA) [48] and the dipolarity/polarizability [45,49] of a wavelengths as compared to the model compound 1-M, which
solvent can be expressed by α, β and π*, respectively.
at firstl indicates an influence of the polymer chains on the
corresponds to a standard process, referenced to a nonpolar solvatochromic behavior of the chromophoric unit.
medium. a, b and s represent solvent-independent regression
coefficients which reflect the relative influence of each of the In order to determine the relative contributions of the solvent
three parameters.
properties on
, the simplified form of the Kamlet–Taft
linear solvation energy relationship was used (Equation 1). The
UV–vis spectroscopy
qualitatively best regressions of 1-M and 1-P are shown in
The solvatochromism of 1-M and 1-P was investigated in a set Table 3.
of only ten solvents due to the low solubility of 1-P in organic
solvents. Table 2 shows the UV–vis absorption maxima of 1-M The correlation coefficients r are greater than 0.92 for LSERs,
and 1-P measured in solvents of different polarity and which indicates a high quality of the two multi-parameter equa-
hydrogen-bonding ability, and the Kamlet–Taft parameters used tions and allows significant conclusions to be drawn. When
for the multiple linear correlation analysis.
increasing the HBA strength of the solvent, a bathochromic
shift of the UV–vis absorption maxima of 1-M is observed,
For both compounds, the shortest UV–vis absorption maxima which is readily explained by interaction of the solvents with
were observed at λmax(1-M) = 568 nm and λmax(1-P) = 513 nm the NH function of the chromophore. The negative sign of the
in HFIP. 1-M shows the longest wavelength UV–vis absorption correlation coefficient s of 1-M indicates that the electronically
band at λmax = 580 nm in DMSO, whereas 1-P has the strongest excited state of these molecules becomes more strongly sol-
bathochromic shift in ethanol at λmax = 538 nm. These band vated and is consequently stabilized with increasing the
shifts correspond to a small solvatochromic range of Δ (1-M) solvent’s dipolarity/polarizability. This correlates with a higher
Table 2: UV–vis absorption maxima,
, of 1-M and 1-P investigated in ten solvents of different polarity and hydrogen-bond ability and the empir-
ical Kamlet–Taft parameter α, β and π* [41,43].
Solvent
Kamlet–Taft parameters
1-M
1-P
λmax
λmax
(nm)
α
β
π*
(nm)
(103 cm−1)
(103 cm−1)
DMFa
0
0.69
0.76
0.76
0.10
0.41
0.90
0.75
0.66
0
0.88
0.88
1.00
0.82
0.75
0.52
0.54
0.60
0.73
0.65
578
579
580
571
573
575
574
575
569
568
17.30
17.27
17.24
17.51
17.45
17.39
17.42
17.39
17.57
17.61
534
535
532
— g
531
536
538
535
521
513
18.73
18.69
18.79
— g
DMAAb
DMSOc
DCMd
0
0
0.13
0.19
0.84
0.86
0.98
1.51
1.96
acetonitrile
18.83
18.66
18.59
18.69
19.19
19.49
1-propanol
ethanol
methanol
TFEe
HFIPf
0
aN,N-dimethylformamide, bN,N-dimethylacetamide, cdimethyl sulfoxide, ddichloromethane, e2,2,2-trifluoroethanol, f1,1,1,3,3,3-hexafluoro-2-propanol,
gprobe is insoluble in this solvent.
Table 3: Solvent-independent correlation coefficients a, b and s of the Kamlet–Taft parameters α, β and π*; solute property of the reference system
,
correlation coefficient (r), standard deviation (sd), number of solvents (n) and significance (f) for the solvatochromism of 1-M and 1-P.
comp.
(103 cm−1)
a
b
s
r
sd
n
f
1
-M
-P
17.857
18.260
0
0
−0.311
−0.813
−0.386
0.995
0.922
0.014
0.122
10
9
< 0.001
1
0
4 × 10−4
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Beilstein J. Org. Chem. 2010, 6, No. 79.
dipole moment of the electronically excited state. In contrast to
-M, 1-P shows no significant influence of the π* term of the
solvent on , whereas the influence of the β term is greater
1
compared to that of 1-M. This effect reflects the impact of the
NH2 groups of the PVAm on the UV–vis absorption maxima of
the chromophore. Probably, the chains of the polymer are able
to insulate the chromophores from the solvent molecules.
Hence, the chromophores interact more strongly with the
polymer chains than they do with the solvents. The HBD ability
of the solvents shows no influence on the solvatochromic
behavior of 1-M and 1-P.
In addition, all compounds were measured as powders by means
of the diffuse reflectance technique. The UV–vis absorption
bands of the solids are non-symmetric and show several UV–vis
absorption maxima. The longest-wavelength UV–vis absorp-
tion maxima were observed at λmax (1-P, 1-Si) = 545 nm and
λmax (1-M) = 546 nm. Figure 4 shows the UV–vis absorption
and the emission diffuse reflectance spectra of 1-Si.
Figure 4: UV–vis absorption and emission diffuse reflectance spectra
of sample 1-Si.
Figure 5 shows an UV–vis absorption spectrum and a fluores-
cence emission spectrum of 1-M and 1-P measured in meth-
anol as well as a photograph of solutions of these compounds.
Fluorescence
Both the carbonitrile-functionalized PVAm 1-P and the model
compound 1-M show strong fluorescence. This can be Similar to the UV–vis measurements, 1-M shows the longest-
explained by two factors: Firstly, the fluorophore is a very rigid wavelength emission maximum in DMSO at λmax = 595 nm and
and planar molecule and secondly, the fluorophore contains 1-P in ethanol at λmax = 586 nm. The longest hypsochromic
both strongly electron-withdrawing and electron-donating shifts of the fluorescence emission maximum were observed at
groups along the axis of the π-conjugated system. Therefore, an λmax(1-M) = 577 nm in HFIP and λmax(1-P) = 561 nm in
efficient intramolecular charge transfer (ICT) system is possible DMMA. These band shifts correspond to a small solvato-
[
32]. The fluorescence emission maxima of 1-M and 1-P are chromic range of Δ (1-M) = 524 cm−1 and Δ (1-P) = 760
shown in Table 4.
cm−1, respectively. Furthermore, for 1-M only a small Stokes
Table 4: Fluorescence emission maxima
of 1-M and 1-P, measured in ten solvents of different polarity and hydrogen-bond ability and the
corresponding Stokes shifts.
Solvent
1-M
1-P
λmax,em
Stokes shift
(nm)
λmax,em
(nm)
Stokes shift
(nm)
(nm)
(103 cm−1)
(103 cm−1)
DMFa
593
592
595
584
586
586
587
588
582
577
16.86
16.89
16.81
17.06
17.06
17.06
17.04
17.01
17.18
17.33
15
13
15
13
13
11
13
13
13
9
562
561
564
— g
563
585
586
582
580
576
17.79
17.83
17.73
— g
28
26
32
—
DMAAb
DMSOc
DCMd
acetonitrile
17.76
17.09
17.06
17.18
17.24
17.36
32
49
48
61
59
63
1-propanol
ethanol
methanol
TFEe
HFIPf
aN,N-dimethylformamide, bN,N-dimethylacetamide, cdimethyl sulfoxide, ddichloromethane, e2,2,2-trifluoroethanol, f1,1,1,3,3,3-hexafluoro-2-propanol,
gprobe is insoluble in this solvent.
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Beilstein J. Org. Chem. 2010, 6, No. 79.
Figure 5: Normalized absorption and emission spectra of 1-M (a) and 1-P (b).
shift (9–15 nm) is observed, whereas 1-P shows a larger one this group resulting in a band shift to longer wavelengths
(
(
26–63 nm), which reaches a maximum in strong HBD solvents (b < 0). To confirm these conclusions drawn from the regres-
HFIP).
sion analysis with regard to the α term of the solvents, the emis-
sion spectra of 1-P were measured in aqueous solutions at four
Again, the solvent-dependent fluorescence emission maxima different pH values (Figure 6).
can be interpreted with regard to the dipolarity/polarizabilty and
the hydrogen-bond capacity of the solvents using the simplified In comparison to the UV–vis absorption measurements of the
Kamlet–Taft equation (Equation 1). The qualitatively best solids of 1-M, 1-P and the hybrid material 1-Si, the fluores-
regressions are shown in Table 5.
cence spectra recorded by diffuse reflection seem to be smaller
Figure 4). The model compound 1-M exhibits an emission
(
In both, absorption and fluorescence of 1-M, the polarity as maximum at λem = 753 nm. For the compounds 1-Si and 1-P a
well as the hydrogen-bond accepting ability of the solvents hypsochromic band shift to λem(1-Si) = 745 nm and λem(1-P) =
leads to a bathochromic band shift and both contribute to the 741 nm was observed. This behavior also shows the strong
stabilization of the excited state as well as to the interaction influence of the chains of the PVAm on the solvatochromic
with the NH group. However, this effect is slightly more behavior of the polymer.
pronounced on the fluorescence side, as shown by the slightly
higher b and s coefficients. In contrast to the results of the As depicted in Figure 6, in aqueous solution at pH = 11, 1-P
regression analysis obtained from the UV–vis absorption shows two emission bands at λem,1 = 515 nm and λem,2 = 579
spectra, the HBD ability of the solvents shows a significant nm. With decreasing pH value, the intensity of the first emis-
influence on the emission maxima of 1-P. With increasing HBD sion band decreases, whereas for the emission band at the
strength of the solvent a bathochromic shift of the emission longer wavelength a slightly bathochromic shift is observed.
maximum was observed. Additionally, solvents which can act This result confirms the conclusions drawn by the LSER of 1-P
as hydrogen-bond acceptors also interact with the NH function with regard to the HBD strength of the solvents. A possible
of 1-P, which leads to an enhancement of the push character of explanation is that at higher pH values the polymer chains are
Table 5: Solvent-independent correlation coefficients a, b and s of the Kamlet–Taft parameters α, β and π*; solute property of the reference system
,
correlation coefficient (r), standard deviation (sd), number of solvents (n) and significance (f) for the fluoro-solvatochromism of 1-M and 1-P.
comp.
(103 cm−1)
a
b
s
r
sd
n
f
1
-M
-P
17.638
17.465
0
−0.357
−0.772
−0.573
0.970
0.922
0.043
0.144
10
9
< 0.001
0.003
1
−0.576
0
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Beilstein J. Org. Chem. 2010, 6, No. 79.
Figure 6: Normalized emission spectra of 1-P in water at four different pH values and a sketch of the assumed chain conformations of PVAm.
coiled and the fluorophore is not accessible for interactions with Pre-adsorption of carbonitrile 1 on silica and subsequent
the surrounding molecules. At lower pH values the polymer has nucleophilic substitution of PVAm in water at 100 °C results in
a more straight conformation and interactions of the solvent fluorophore-functionalized PVAm/silica particles. The
with the fluorophore are possible. The explanation is in good PVAm–carbonitrile layer remains irreversibly bound to the
agreement with the known behavior of weak cationic polyelec- silica, as shown by extraction experiments. Hence, the synthe-
trolyte molecules. With lowering the pH of the surrounding sis of a fluorescent hybrid material was possible.
aqueous solution the degree of protonation increases. The
increased net charge along the polymer stretches and Experimental
strengthens the chain.
General details: The aqueous solution of poly(vinyl amine)
copolymer (Mn = 15,000 g mol−1, pH = 11) was kindly
provided by BASF SE (Ludwigshafen, Germany). 2,6-O-β-
Conclusion
In this paper we present the functionalization of PVAm with the dimethylcyclodextrin (β-DMCD) was supplied by Wacker
known fluorophore 1 in water, mediated by 2,6-O-β-dimethyl- Chemie (Burghausen, Germany).
cyclodextrin. The characterization of the new fluorescent
2-(2-Oxo-2H-acenaphthylene-1-ylidene)-
PVAm 1-P has been carried out by means of 13C-{1H}-CP-
MAS NMR, FTIR spectroscopy and DSC.
malononitrile (2)
Compound 2 was synthesized as described earlier [34].
The influence of the solvents on the solvatochromic behavior of Acenaphthene-1,2-dione (2.00 g, 11 mmol) and malononitrile
-M and 1-P can be quantitatively described by means of a LSE (0.73 g, 11 mmol) were dissolved in 40 mL of CH3CN and re-
1
relationship using the well-established empirical Kamlet–Taft fluxed for 3 h. After cooling, the orange precipitate was filtered
equation. The most dominant effect on the UV–vis absorption off and washed several times with CH3CN to yield 2 as an
and fluorescence is caused by the HBA strength as well as by orange crystalline solid (2.00 g, 8.3 mmol). Yield 80%, mp
the dipolarity/polarizabilty of the solvents. In particular it has 241–244 °C (lit. 243–245 °C); 1H NMR (250 MHz, CD2Cl2): δ
been shown that the polymer chains have a significant influ- (ppm) 8.61 (dd, J = 7.5 Hz, J = 0.6 Hz, 1H), 8.33–8.29 (m, 2H),
ence on the solvatochromic behavior of 1-P. With decreasing 8.21 (dd, J = 7.2 Hz, J = 0.8 Hz, 1H), 7.96–7.88 (m, 2H);
pH value in aqueous solutions a bathochromic band shift was 13C NMR (69 MHz, CD2Cl2): δ (ppm); 142.9, 132.6 (ArH),
observed.
132.3 (ArH), 130.5, 128.3, 112.7, 129.2 (ArH), 129.0 (ArH),
Page 10 of 13
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Beilstein J. Org. Chem. 2010, 6, No. 79.
1
1
24.5 (ArH), 123.6 (ArH); FTIR (KBr): (cm−1) 3093, 2225, Yield 47%, mp > 300 °C (lit. >300 °C [33]); 1H NMR (250
717, 1597, 1574; C15H6N2O (230.23) Anal. calcd. C, 78.26; MHz, CD2Cl2): δ (ppm) 8.41 (d, 1H, J = 7.7 Hz), 8.15 (t, J =
H, 2.63; N, 12.17; found C, 78.03; H, 2.66; N, 12.18.
7.6 Hz, J = 9.4 Hz, 2H), 7.69 (t, J = 7.6 Hz, J = 7.7 Hz, 1H),
.36 (d, J = 9.4 Hz, 1H), 4.22–4.31 (m, 1H) NHCH) 1.49–1.51
m, 6H, CH(CH3)2; 13C-{1H}-CP-MAS NMR (100 MHz, 12.5
kHz): δ (ppm) 185.3, 174.7, 171.8, 154.3, 137.9, 130.0, 1278.0,
-(2-Oxo-2H-acenaphthylene-1-ylidene)-malononitrile (2) (0.80 124.8, 123.5, 121.9, 118.7, 117.3, 115.5, 111.6, 107.3, 104.8,
7
8
-Oxo-8H-acenaphtho[1,2-b]pyrrol-9-carboni-
(
trile (1)
2
g, 3.5 mmol) and K2CO3 (48 mg, 0.35 mmol) in 5 mL of 48.2, 23.2, 22.2; FTIR (KBr):
(cm−1) 3334, 3049, 2982,
CH3CN were refluxed for 1 h. After cooling, the yellow-orange 2213, 1627, 1576, 1539; C18H13N3O (287.32) Anal. calcd. C,
precipitate was filtered off and washed several times with 75.25; H, 4.56; N, 14.63; found C, 74.85; H, 4.37; N, 14.49.
CH3CN to yield 1 as a yellow–orange solid (0.70 g, 3.1 mmol).
Fluorophoric, carbonitrile-functionalized
Yield 88%, mp 278–280 °C (lit. 275–277 °C [32]); 1H NMR
(
250 MHz, CD2Cl2): δ (ppm) 8.80 (dd, J = 7.4 Hz, J = 1.3 Hz, PVAm/silica particles (1-Si)
1
7
3
H), 8.51–8.41 (m, 3H), 7.99 (dd, J = 7.6 Hz, J = 0.5 Hz, 1H), For functionalization of silica with fluorophores and PVAm in
.89 (dd, J = 8.3 Hz, J = 1.0 Hz, 1H); FTIR (KBr): (cm−1) aqueous media, 1 (0.1 g, 0.4 mmol) was dissolved in 50 mL of
084, 2231, 1713, 1645, 1576, 1549; C15H6N2O (230.23) Anal. CH2Cl2, then suspended with silica (2.5 g, 41 mmol) and kept
calcd. C, 78.26; H, 2.63; N, 12.17; found C, 77.96; H, 2.61; N, overnight. The solvent was removed in a rotary evaporator at 40
1
1.79.
°C. Distilled water (60 mL) and 2.2 g of the aqueous solution of
PVAm (0.18 g of polymer, 4.1 mmol) were added to the freshly
prepared 1-loaded silica particles. The mixture was refluxed for
16 h. After cooling to room temperature, the PVAm-functional-
Fluorophoric, carbonitrile-functionalized
PVAm (1-P)
For the β-cyclodextrin complex formation, stoichiometric ized silica was filtered off and washed carefully with water. In
amounts of 1 (0.29 g, 1.28 mmol) and β-DMCD (1.68 g, 1.28 order to remove non-adsorbed functionalized PVAm and unre-
mmol) were dissolved in methanol and stirred overnight at acted 1 from the silica particles surface, two Soxhlet extraction
room temperature [29]. The light orange solid complex of cycles were carried out, the first one with water and the second
1
-DMCD was isolated by removing the methanol in vacuum with acetone. 1-Si was obtained as purple solid.
and was used without further purification.
1
3C-{1H}-CP-MAS NMR (100 MHz, 12.5 kHz): δ (ppm)
For functionalization of PVAm in aqueous media, the solid 181.2, 164.7, 151.4, 128.3, 44.6, 37.4; FTIR (KBr): (cm−1)
β-DMCD complexes (10 mol %) was dissolved in 50 mL of 3277, 2207, 1651, 1572, 1569; 1087, 1025; Anal. calcd. C,
distilled water and 6.10 g of the aqueous solution of PVAm 6.30; H, 0.82; N, 2.52; found C, 5.61; H, 1.24; N, 2.08.
containing 0.50 g of the polymer added. The mixture was re-
fluxed for 8 h. Then the fluorophore-functionalized PVAm was Acknowledgements
precipitated by addition to ice-cold acetone (refrigerator), Financial support from the Deutsche Forschungsgemeinschaft,
washed with acetone and dried in vacuum to yield an intensive Bonn and the delivery of chemicals from the BASF SE,
purple-colored solid.
Ludwigshafen, is gratefully acknowledged. The 2,6-O-β-
dimethylcyclodextrin was kindly supplied by Wacker Chemie.
1
3C-{1H}-CP-MAS NMR (100 MHz, 12.5 kHz): δ (ppm) The authors thank Dr. A. Seifert and R. Jaeschke for 13C-{1H}-
1
81.3, 168.6, 128.4, 71.5, 46.0, 31.6, 23.5; FTIR (KBr):
CP-MAS NMR measurements and the determination of the
(
cm−1) 3239, 2895, 2838, 2221, 1665, 1590, 1497, 1386; Anal. glass-transition temperatures by means of differential scanning
calcd (for 100% substitution). C, 55.73; H, 10.11; N, 31.20; calorimetry (DSC).
found C, 57.27; H, 7.45; N, 15.71.
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