Zwitterionic polymers—IV. Pyrene as a photophysical probe for coulombic interactions in poly(vinylsulphobetaines)

Eur. Polym. J. Vol. 26, No. 10, pp. 1065-1070,1990

0014-3057/90$3.00+ 0.00 Copyright ~ 1990PergamonPress plc

Printed in Great Britain.All rights reserved

ZWITTERIONIC POLYMERS--IV. PYRENE AS A PHOTOPHYSICAL PROBE FOR COULOMBIC INTERACTIONS IN POLY(VINYLSULPHOBETAINES) THOMAS A. WIELEMAand JAN B. F. N. ENGBERTS* Department of Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands (Received 23 October 1989)

A~tract--The fluorescence spectra and fluorescence quenching of a pyrene-labelled poly(vinylsulphobetaine) were measured in the presence of various salts. The extent of excimer fluorescencewas found to be only slightly affected by the solubility behaviour of these types of zwitterionic polymers. However, the fluorescence quenching experiments clearly demonstrate the interactions between anions and poly(vinylsulphobetaines) in the solubilization process of these polymers in aqueous salt solutions. Further support for these interactions is provided by the observation of a CT band for the poly(vinylsulphobetaine)derived from 4-vinylpyridine in an aqueous 0.05 M NaI solution. The fluorescence quenching experiments did not provide additional evidence for interactions between cations and poly(vinylsulphobetaines).

INTRODUCTION

In a recent publication [1] we discussed the influences of the natures of the anion and cation on the critical salt concentration (CSC) for dissolutions of zwitterionic polymers. In the anion series, we observed a Hofmeister series (CIO~- > I - > Br- > CI- > F - ) for the solubilization process. Among the cations, the differences were less profound and only changing an alkali metal cation for a tetraalkylammonium or a transition metal cation resulted in a significant change in the CSC. From these experiments we concluded that particularly the weakly hydrated ions exhibit strong interactions with zwitterionic polymers resulting in a lowering of the CSC. To obtain additional evidence for these conclusions, we have utilized pyrene, covalently bound to a poly(sulphobetaine), as a photophysical probe. Pyrene is a useful fluorescence probe molecule because of its well-resolved absorption and fluorescence spectrum. It exhibits a long fluorescence lifetime. The vibrational structure of the fluorescence spectrum is known to be sensitive to the local polarity of the medium [2]. In particular, the ratio of the intensities of the first and the third peaks (I~/I3) increases with increasing polarity of the solvent. This solvent dependence can be rationalized in terms of specific interactions between the excited singlet state of the pyrene and solvent molecules [2]. The dipolar character of the excited state of pyrene is also manifested in the ease with which pyrene forms exciplexes with a wide variety of solutes as well as the ease of excimer formation. An excimer is a complex between an excited and a ground-state molecule and in the case of pyrene it has a characteristic band in the emission spectrum ()'max= 480 rim) [3]. This excimer formation has been utilized to determine the aggregation number of micelles of anionic, cationic and *To whom all correspondence should be addressed. EPJ 26:10--B

1065

zwitterionic surfactants [4]. Pyrene, covalently bound to poly(acrylic acid), has been used as a probe for chain expansion of the polymer upon ionization of the carboxyl groups [5]. Drastic chain expansion results in a decrease in excimer fluorescence. The conformational transition of poly(methacrylic acid) from a compact coil to a random coil upon ionization of the carboxyl groups can also be investigated using free and bound pyrene molecules [6]. The excimer fluorescence of bound pyrene has also been employed to study polymer-surfactant interactions [7, 8]. A third possibility for using pyrene as a photophysical probe involves fluorescence quenching. Pyrene fluorescence is quenched by a number of anions, cations and neutral molecules. The anion-induced quenching proceeds via a charge-transfer complex produced by electron transfer from the anion (donor) to the excited singlet state of pyrene (acceptor) [9]. Electron transfer is excluded as a possible quenching mechanism for cationic quenchers (closed-shell heavy metal ions) but instead results from a rapid transformation of the singlet excited state into a triplet state via intersystem crossing [10]. The quenching of pyrene covalently bound to poly(acrylic acid) is influenced by the degree of chain expansion since the efficiency of quenching was greater at high pH in comparison with quenching at low pH [11]. In the present report excimer formation and fluorescence quenching were investigated for pyrene covalently bound to a poly(vinylsulphobetaine). EXPERIMENTAL PROCEDURES

Fluorescence measurements

Steady-state fluorescence spectra were recorded using an SLM-Amico (SPF-500c) spectrophotometer, equipped with a thermostated cell unit, at 25.0°. Fluorescence quenching experiments were performed in duplicate by measuring the fluorescence intensity at 377 nm at different quencher concentrations. Absorption spectra were recorded on a

1066

THOMASA. WIELEMAand JAN B. F. N. ENGBERTS

Perkin-Elmer Lambda-5 u.v./VIS spectrophotometer. GPC analyses were performed as described previously [12].

Synthesis l-Bromo-4-(l-pyrenyl)butane. To a suspension of 4-(1pyrenyl)butanoie acid (5.00 g; 17.4 mmol) in 40 ml of dry THF was added a suspension of LiA1H4 (0.80 g; 21.0 mmol) in 20 ml of dry THF at 10°. After stirring for three days at room temperature, the reaction mixture was poured into a 1 : 1 mixture of ice and 33% NaOH. The aqueous suspension was filtered over celite. The residue was washed with ether and the aqueous layer was extracted with ether. The combined ether layers were washed with water, dried and concentrated under reduced pressure. The crude alcohol (5.06 g) was dissolved in 70 ml of CHCI 3. To this solution was added pyridine (0.70g; 9.0mmol) and PBr 3 (4.80g; 8 mmol) at - 1 0 °. After standing at room temperature for 4 days, the reaction mixture was poured into water and extracted with ether. The ether layers were washed (NaHCO3), dried and concentrated in vacuo. The pure product was obtained after column chromatography (kieselgel 60, eluent: CHCI3) and crystallization from cyclohexane; yield 2.68 g (46%), m.p. 74.3-75.1 °. IH-NMR (CDCI3): 6 = 1.77-2.10 (m, 4H), `5 = 3.10-3.48 (m, 4H), `5 = 7.65-8.10 (m, 9H) ppm. Analysis: calc. for C20HI7Br (M = 337.26); C = 71.22, H=5.08, Br=23.70, found: C=71.I0, H=5.11, Br = 23.69.

N,N-Dimethyl- (2-methacryloxyethyl)- I- (4- (l-pyrenyl)butyl)ammonium bromide (M). A solution of l-bromo-4-(1pyrenyl)butane (l.00g; 3.0mmol) and N,N-dimethylaminoethylmethacrylic acid (0.50 g; 3.0 mmol) in 15 ml of acetone, containing a few crystals of NaI and m-dinitrobenzene, was kept in the dark at room temperature for 16 days. The crude product was isolated by filtration and crystallized from acetone/ethanol (9 : 1). The pure monomer M was obtained in a yield of 40%, (0.59 g), m.p. 187.6-188.3 °. IH-NMR (CD3OD); ,5 = 1.62-1.93 (m, 4H), `5 = 1.83 (s, 3H), `5 = 3.08 (s, 6H), `5 = 3.16-3.47 (m, 4H), 6 = 3.533.71 (m, 2H), `5 =4.36-4.58 (m, 2H), `5 = 5.56 (s, IH), 6 = 6.02 (s, 1H), `5 = 7.73-8.29 (m, 9H) ppm. Analysis: calc. for C2sH32NO2Br (M=494.47); C = 68.01, H=6.52, N=2.83, Br= 16.16; found: C=68.03, H =6.62, N =2.92, Br= 16.14.

Pyrene-labelled poly[dimethyl-(2-methacryloxyethyl)-(l(2-sulphoethyl)) ammonium betaine] (P). Into a polymerization tube was introduced N,N-dimethyl-(2-methacryloxyethyl)-(1-(2-sulphoethyl))ammonium betaine [13] (2.00g, 7.6mmol), M (37.0mg, 0.076mmol) and 4,4'-azobis-4cyanovaleric acid (2.1 mg, 0.1 mol%). After the addition of 10ml of double distilled water, a 0.70M solution of monomer was obtained. After degassing, the tube was sealed-off under N2 and placed at 60° for two days. Then a 1.0 M NaCI solution was added to the heterogeneous reaction mixture, giving a clear solution, which was dialysed against deionized water. The homogeneous contents of the dialysis tube were concentrated by lyophilization and vacuum-drying at 60°, providing the labelled polymer P in a yield of 94% (1.92 g). Analysis: calc. for Ci022Hi9.26Ni.01Os02C10.01 (M = 268.98), C = 45.63, H = 7.22, N = 5.26, S = I 1.92; found: C =45.16, H = 7.21, N = 5.20, S = 11.56. GPC measurements: h~, = 3.00 x 105, At,, = 1.01 × 106 and h,-],,.//ff,= 3.42. RESULTS AND DISCUSSION There are several routes for attaching a pyrene group onto a polymer backbone, including grafting, copolymerization and quenching of a living polymer [6]. In the first two procedures, the pyrene functionality will be randomly distributed along the polymer

chain. The third approach leads to a polymer with pyrene at the end of the polymer chain. We focused our attention on the synthesis of a pyrcne derivative containing a polymerizable group which can be used in copolymerization with a monomeric vinylsulphobetaine. To achieve a good random copolymerization, the structures of the two monomeric species must be very similar. We therefore choose dimethylaminoethyl methacrylate ( D M A E M ) , a tertiary monomeric amine, as starting material for both the vinylsulphobetaine and the synthesis of a pyrene derivative containing a polymerizable group. Aromatic amines cannot be used because aromatic quarternary amines are very effective quenchers of pyrene fluorescence [14]. The desired monomeric pyrene functionality was prepared from commercially available 4-(l-pyrenyl)butanoic acid, which was first reduced to the corresponding alcohol and subsequently transformed into the bromide in an overall yield of 46%. In the last step D M A E M was quarternized with the bromide, to obtain the monomeric fluorescence label M in 40% yield (see Scheme 1). The monomeric fluorescence label was copolymerized with the vinylsulphobetaine derived from D M A E M [13], to obtain the pyrene-labelled poly(vinylsulphobetaine) P in 94% yield (see Scheme l).

C1..!2=01C~:O I

(CH) 1®2 2 CH3-N-CH 3

[-- CH2- ?-];[-C 1"12-C=O ? -]q C-O I 0 0 [ I (CI~"" H.,)., (CI@ H2),," CH_-N--CH CH-N-CH. (CH) Br (CH)

so~ (M)

~

(P)

Scheme 1 The concentration of pyrene was fixed at 1% (mol/mol). This amount of label should be sufficient to observe intramolecular excimer formation in aqueous solution [5]. The actual amount of covalently bound pyrene in P (0.8% mol/mol) was calculated from the absorbance due to the (0, 0) transition (343 nm) of pyrene in the absorption spectra of M and P in water [5]. The monomeric fluorescence label M can be viewed as a surfactant because of the presence of a quarternary a m m o n i u m headgroup and a hydrophobic n-butylpyrene tail. At sufficiently high concentration, we therefore can expect M to aggregate, resulting in a drastic increase in excimer fluorescence. To determine the concentration at which the first excimer fluorescence occurs, we measured the fluorescence spectra at different concentrations of M. The data in Fig. 1 show that above ca 10 -4 mol/l of M a broad excimer fluorescence at 480 nm starts to appear. At 10-Smol/l, no excimer formation is observed and this concentration is used in all the subsequent fluorescence experiments. To obtain a concentration of 10-Smol/l of covalently bound

Zwitterionic polymers--IV

1067

100

m

.el

50

c

qu

0 ~

"J600 &SO EM wavetength (nm)

&O0

550

Fig. 1. Fluorescence spectra of M at different concentrations; (--), 1 x 10-~ mol/I; (---), 1 × 10-4 mol/l and ( . . . . . ), 2 x 10-4 mol/l. with concomitant increase in chain expansion with pyrene in P, a 10 -3 mol/l polymer solution in water increasing salt concentrations. To probe this chain was used. At these low concentrations, intermolecu- expansion, we have measured the ratio of monomer lar interactions between polymer molecules were to excimer fluorescence (I,,/I~) at various concenasumed to be insignifican( and the shape of the trations of NaCI. Surprisingly however, we find that fluorescence spectrum is expected to be determined by P is soluble in water. The fluorescence spectrum of P intramolecular interactions within the polymer coil. in water is depicted in Fig. 2. A plot of Im/I¢ vs [NaCI] Due to the polymerization procedure used in the shown in Fig. 3. preparation of P, there will be a random distribution is In water we observe very low excimer fluorof pyrene groups in the polymer chain, and therefore, escence (I~/I~= 11.9), indicating that a statistical the amount of excimer fluorescence will be dependent copolymerization has taken place in the synthesis on the conformation of the polymer coil. Poly(vinyl- of P. The salt effect on the Im/l¢ values and also sulphobetaines) are insoluble in water, as a result of the solubility behaviour of P are rather peculiar. strong intra- and intermolecular Coulombic inter- Figure 3 shows that lm/l¢ decreases upon the first actions [13]. In salt solutions, the polymers dissolve

100

m

m

L

o,

50

ca o u

ol o

i

J

I

4OO

450

I

500 EM wavelength (nrn)

Fig. 2. Fluorescence spectrum of P in water.

550

1068

THOMASA. V~/IELEMAand JAN B. F. N. ENGBERTS

2O |m Ie

15

10 ~

.¢

5

0

~ 50

J 100

J 150

[NaCt] (raM)

Fig. 3. Plot of the lm/1 e values of P at different concentrations of NaCI.

addition of salt. At 15 mM NaC1, the polymer partly precipitates. Unexpectedly, the value of lm/le is only slightly affected by this precipitation. Above 50 mM NaCI, the polymer dissolves again, but here again we observe hardly any influence on the excimer fluorescence. This unusual solubility behaviour has also been observed for another aliphatic poly(vinylsulphobetaine) [15]. Schultz et al. [15] observed a dramatic lowering of the hydrodynamic diameter, measured by quasi-elastic light scattering (QELS), of the poly(vinylsulphobetaine) derived from DMAEM and 1,3-propanesultone in aqueous solution upon the first addition of salt. This behaviour was rationalized by the fact that, even at low polymer concentration (3.5mM), small aggregates are formed which do not contribute to the overall viscosity. Further addition of salt led to an increase of the hydrodynamic diameter. In a publication of Liaw et al. [16], the CSC was measured for the poly(vinylsulphobetaine) derived from DMAEM and 1,3-propanesultone. They did not report solubilization of the polymer in water. The CSC for NaCI was found to be 71 mM. We submit that the solubilization of P in water may result from the formation of interassociations, which results in opening of the polymeric chain. In the presence of salt, these weak interassociations are broken up and then intramolecular Coulombic interactions dominate the solubility behaviour. Thus the polymer is insoluble at low salt concentrations and dissolves at higher salt concentrations. Alternatively, the solubilization of P in water may be explained by assuming very weak intramolecular Coulombic interactions for this type of aliphatic poly(vinylsulphobetaines). Therefore, the sulphonate and the quarternary ammonium group will be hydrated, resulting in a solubilization of the polymer in water. With the addition of NaCI, this hydration will be reduced and again intramolecular Coulombic interactions dominate the solubility behaviour. We observe that the addition of salt has only a small influence on the Im/le values. An explanation for this result could be that the pyrene units are too few to probe changes in the chain conformation. It is also possible that these changes in chain conformation are too small to be accurately probed by excimer fluorescence. The quenching of the pyrene fluorescence of M

and P both in water and in 0.50MNaCI was measured using the quenchers NaI, TINO, CuCI2, nitromethane and l-(2-sulpboethyl)pyridinium betaine (SPB). These quenchers were selected on the basis of their expected interactions with poly(vinylsulphobetaines). Thus, the I-, T! + and Cu ~+ ions were selected to probe the presence of specific interactions between anions or cations and poly(vinylsulphobetaines) in the solubilization process of these polymers in aqueous solution. Nitromethane, an uncharged water-soluble molecule, will only undergo dipolar interactions. Finally, the interactions between a low molecular weight sulphobetaine and the polymeric sulphobetaine P were also examined using fluorescence quenching. The quenching of pyrene fluorescence in aqueous solutions of M and P was found to fit the Stern-Voimer equation: = l +

--~

!

K,~ [Q]

Herein I0 and I are the intensities of the (0, 0) peak in the fluorescence spectrum in the absence and the presence of quencher respectively, [Q] is the concentration of the quencher and K , is the Stern-Volmer constant. Plots of Io/I against [NaI] for M and P both in water and in 0.50 M NaCI solution are depicted in Figs 4 and 5, respectively. For all quenchers, satisfactory Stern-Volmer plots were obtained at low quencher coacentrations. At higher quencher concentrations, deviations from linearity were observed. The K,v values are listed in Table I. In the case of the anionic quencher I-, enhanced quenching for the polymer-bound pyrene is observed. Upon addition of NaCI, the quenching is considerably lowered. This enhanced quenching for I- is a strong indication for a Coulombic interaction between poly(vinylsulphobetaines) and the iodide anion. It is anticipated that this interaction may contribute to the solubilization of poly(vinylsulpbobetaines) in aqueous solution. Upon the addition of 0.50 M NaCl, the specific Coulombic interaction with I- is partly overruled by interactions with the excess of CI-, resulting in a smaller K,v value. We did not observe an enhanced quenching by the cationic quenchers TI + and Cu 2+ . The K,v values of TINO3 for P are even smaller than those for M. This effect can 2.0 I0 I ~ 1.5

~

-

(P)

~ -

(I,I) 1,0 0

J 1.0

i 2.0

i 3,0 [Na[] (rnM)

Fig. 4. Stern-Volmer plot of P and M for NaI in water.

Zwitterionic polymers---IV 2.0 (P)

I0

o o o

I

o

1.5 ~

~

1,0

1,0

0

IM)

D

i 2.0

i [Nall(mM)

3,0

Fig. 5. Stern-Volmer plot of P and M for NaI in 0.50 M NaCI. be explained by assuming steric hindrance experienced by Tl + upon approaching the polymer backbone. With the addition of 0.50 M NaCl, the K,v value for TI + increases for both M and P, apparently because the electrostatic repulsion between the cationic quencher and the positively charged label is decreased upon addition of NaCl. For the neutral quencher nitromethane, we observe a decrease in Ksv values for P, which can also be rationalized in terms of steric hindrance by the polymer backbone. Comparison of the K~v values of nitromethane and PSB, shows that the sulphobetaine PBS can be considered as an overall neutral quencher, which undergoes no specific interactions with P. In the fluorescence experiments, we observed CouIombic interaction between I - and polymer P. To obtain additional support for these Coulombic inter-

1069

actions between I - and poly(vinylsulphobetaines), we recorded the absorption spectrum of a poly(vinylsulphobetaine) derived from 4-vinylpyridine [13] in the presence of NaI. Quarternary pyridinium iodides exhibit an intramolecular charge-transfer (CT) band, which can serve as an intrinsic polarity probe [17-19]. This CT band has also been utilized to study the micellization of N-alkylpyridinium iodide surfactants [20, 21]. The absorption spectra of poly[4-vinyl-l-(2-sulphoethyl)pyridinium betaine] (I) in 0.50 M NaCI and 0.05 M NaI solutions are depicted in Fig. 6. In 0.05 M NaI an additional broad absorption band is observed, partially hidden beneath the strong n - n * transition of the heterocyclic system. The position of the absorption maximum of this CT band (290 nm) was determined by means of the band-match method [22]. The observation of this CT band is fully consistent with Coulombic interactions between I - and the zwitterionic polymer, and is reminiscent of the CT band originating from counterion binding in micelles formed from N-alkylpyridinium iodide surfactants [20]. CONCLUSIONS In this paper we have reported fluorescence measurements using a pyrene-labelled poly(vinylsulphobetaine) P. The solubility properties of P in aqueous solution exhibit quite unusual behaviour; the polymer is soluble in water and upon addition of salt the polymer precipitates. However, at sufficiently high salt concentration, the polymer redissolves. This solubility behaviour may be rationalized in terms of a change from dominating inter- to intramolecular Coulombic interactions or by salting-out of the

Table 1. Stern-Volmerquenching constants for M and P using various quenchers in water and in 0.50M NaCI Solvent Nal TiNO3 CuCIz CH3NO2 SPB Water 142 255 0.50 M NaCI 73 365* P Water 1888 87 P 0.50M NaCI 232 140* *Precipitationof TICI occurs above [1"1+] = 2 raM.

150 282 61 212

M

M

492 459 359 285

495 353 148 106

0.5

o:

al 6

o

O.t,

0.3

0.2

(b) ~

0.1

0 250

i

l

i-

300

350

~00

,

wavelength (nrn)

Fig. 6. Absorption spectrum of I in aqueous solutions containing 0,05 M NaI (a) and 0.50 M NaCI (b).

1070

THOMASA, WIELEMAand JAN B. F. N. ENGBERTS

polymer. Unfortunately, the excimer fluorescence is only slightly affected by the addition o f salt. Fluorescence quenching experiments clearly demonstrate the interaction between anions and poly(vinylsulphobetaines) in the solubilization process of these polymers in aqueous salt solutions. This interaction between anions and poly(vinylsulphobetaines) is also supported by the observation of a C T band for the poly(vinyisulphobetaine) derived from 4-vinyipyridine in an aqueous 0.05 M N a I solution. The fluorescence quenching experiments did not provide additional evidence for interactions between cations and poly(vinylsulphobetaines). Acknowledgements--The investigations were supported by the Koninklijke Shell Laboratories, Amsterdam (Shell Research BV). We are indebted to Drs T. Graafland, P. Maarsen and F. Binsbergen for valuable discussions.

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