Energy transfer studies with fluorosiloxanes

Pergamon

0014-3057(94)00165-O

ENERGY

TRANSFER

STUDIES

CHRISTIAN

PHAM-VAN-CANG,‘.’

WITH

MITCHELL

and SYLVIE

Eur. Pol~m. J. Vol. 31, No. 3, pp. 227-231, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0014-3057195 $9.50 + 0.00

FLUOROSILOXANES

A. WINNIK,**

ROSITA

DORIG03

BOILEAU’*

‘Electriciti de France, Direction des Etudes et Recherches, 1 avenue du General de Gaulle, 92141 Clamart Cedex France, *Department of Chemistry, University of Toronto and Erindale College, 80 St George Street, Toronto, Canada M5S 1Al and 3College de France, 11 PI. Marcelin-Berthelot, 75231 Paris Cedex 05. France (Received 30 March 1994; accepted 10 May 1994)

Abstract-Four fluorinated siloxane polymers labeled with fluorescent dyes were prepared by hydrosilylation of a mixture of ally1 ethers with poly(methyl hydrogen siloxane). These comprised n-octyl ally1 ether, lH,lH,2H,-.2Hperfluorooctyle ally1 ether, and either 9-phenanthrylmethyl ally1 ether or 9-anthrylmethyl ether. In addition a non-fluorinated polymer was prepared with the octyl ally1 ether plus the phenanthryl ether. These yielded five polymers, mutually immiscible liquids labeled with chromophores appropriate for energy transfer studies. These experiments were carried out in 1,1,2-trichloro-1,2,2-trifluoroethane solution. Fluorescence decay profiles for the phenanthrene were all exponential, with lifetimes of 25 nsec for the ally1 ether and 30 nsec for the polymer bound chromophore. Quenching was diffusion controlled, as expected; and the polymer-polymer reaction had a rate 5-fold slower than that between the phenanthryl and anthryl ally1 ethers, independent of the fluorocarbon content of the polymer.

INTRODUCTION

Fluorinated polysiloxanes are of interest for a variety of reasons. For example, they represent a class of hydrocarbon-impermeable elastomers with significant potential aviation and aerospace applications (e.g. fuel lines). Because of their exceptionally high permeability to oxygen and low uptake of lipids, they, offer intriguing possibilities for biomedical applications (e.g. interoccular lens replacements). This paper examines the interactions in solution of a series of fluorinated and nonfluorinated polysiloxanes having a comb-like structure. These materials are prepared by the platinum-catalyzed addition of various ally1 ethers to the Si-H bonds of a single polymethylhydrosiloxane we,Si(OSiH(Me)),OSiMe,, n N 351 to give polymers with general structure given in Scheme 1. These polymers, containing 100, 75, 50, 25, and 0% fluorinated side arms, comprise mutually immiscible, highly fluid liquids. Our synthesis admits the introduction of fluorescent dyes (phenanthrene, Phe; anthracene, An) into the chain structure and permits us in principle to carry out a variety of experiments based upon energy transfer measurements on these materials. Our initial objective was to use energy transfer [1] to study interface structure in blends of these various copolymers. For this reason, alternating members of the series were substituted with donor (Phe) and acceptor (An) dyes. These experiments failed for the curious reason that these polymers were liquids of such high fluidity, low surface tension, and, of course, very different densities, that preparation of suitable samples for study eluded us. Samples always gave a single sharp interfalce, and the measured energy transfer varied with the position where the optics were *To whom all correspondence

should be addressed.

focused. As a consequence, we turned our attention to a technically simpler problem, that of polymer coil interaction in solution: if these polymers are so incompatible in bulk, how would this affect coil-coil interactions in solution as probed by energy transfer measurements? The donor group Phe was attached to polymers containing 0, 50, and 100 mol.% fluorinated side groups; and the acceptor An, to polymers containing 25 and 75 mol.% fluorinated arms. Each polymer contained 3 mol.% dye, corresponding to one fluorescent dye, on average, per chain. EXPERIMENTAL Polymer synthesis and characterization Reagents. Ally1 chloride (Janssen Chimca) was redistilled before use whereas I-octanol (Prolabo). lH.lH.2H.2Hperfluorooctan-l-01 (Atochem) and 9-anthracene-methanol (Aldrich) were used as received. 9-phenanthrene carboxaldehyde (Aldrich) was reduced to the corresponding alcohol by reaction with NaBH, according to the described procedure (m.p. 153-154°C) [2]. Tetrabutylammonium hydrogen sulfate (Merck) was used as the phase transfer catalyst without further purification. Commercial polymethylhydrogensiloxane, PMHS, (Merck) and trimethylvinylsilane (Petrarch Systems), were used as received. The molecular weight of PMHS, M. = 2200 (DP, = 35) was determined by ‘H-NMR and by vapor pressure osmometry. A platinumcyclovinylmethylsiloxane complex (Petrarch Systems) was used as the hydrosilylation catalyst. Ally/ ether synthesis. Ally1 ethers of I-octanol and of the perfluorooctanol C,F,,CH,CH, OH, were prepared by reaction with a large excess of ally1 chloride under PTC conditions, as previously described [3]. A similar procedure was used for 9-phenanthrenemethanol and 9-anthracenemethanol. The alcohol (20mmol) was added to aqueous NaOH (22.5 ml) plus freshly redistilled allylchloride (30 ml). Tetrabutylammonium hydrogen sulfate (0.74 mmol) was then added to the mixture, which was stirred mechanically under nitrogen at 40°C for 6 hr. The organic layer was extracted with 20ml of methylene chloride, washed with

227

Christian Pham-Van-Cang et al.

228

CH3 I

(CH3)sSi0 -(S&O),

-Si

@X3)3

!

I

3or4 Pt complex

1or2 Pt complex I

I

5

Pt complex

(CHs)$iO -(Si-O), I ('X)3

;

I CH2

-(Sia),

-(Si-@b

-(Si-O),

I

I

I (CfM3

&J-W3

b

b

I W2h

-Si

CH3b

CHz

!H

2

I

I

Cd417

Si CH3)3

FI = 9-anthryl [An] or 9-phenanthryl [Phe] Scheme 1 water until a neutral pH was obtained, and then dried over sodium sulfate. After filtration, the solvent and the excess of ally1 chloride were removed by evaporation. The remaining yellow oil was diluted with a mixture of water and ethanol, and white crystals (m.p. = 4OOC)were recovered with 84% yield in the case of the ally1 ether of 9-phenanthrenemethanol: C%, found: 86.88, talc.: 87.10, H%, found: 6.56, talc.: 6.45; O%, found: 6.58, talc.: 6.45. Yellow needles were obtained after crystallization in ethanol and in petroleum ether, in the case of the ally1 ether of 9-anthracenemethanol (m.p. = 72°C. C%, found! 87.36, talc.: 87.10: H%. found: 6.51. talc.: 6.45: 0% found: 6.13. talc.: 6.45. In both cases, the iH and 13CLNMRspectra are consistent with the expected structures.

polystyrene calibration curve in a mixed solvent of Freon 113/CHCl, (2/l v/v) at 25°C on a Waters apparatus equipped for refractive index detection. Fluorescence measurements

Fluorescence spectra were obtained on a SPEX Fluorolog 112 spectrometer. Decay profiles were obtained by the single photon timing technique [S]. All decays reported here were exponential. Samples were prepared in Freon I13 (1,1,2trichlorotrifluoroethane) and placed into 4mm id. quartz ESR tubes fitted via an O-ring seal to a vacuum line. After five successive freeze-pumpthaw cycles, they were sealed with a torch under a vacuum of better than 1 x low5 torr. RESULTS AND DISCUSSION

Polymer synthesis and characterization

Polymers were prepared by chemical modification of PMHS by hydrosilylation. In a typical experiment, ally1 ethers (18.6mmol) were mixed with the catalyst (1.6 x 10-3mmo1) in carefully dried toluene (IOml) for 10 min under nitrogen, then the polymer PMHS (15.8 mq of SiH) was added and the solution was stirred at 60°C. The reaction was followed by monitoring the decrease of the SiH i.r. band at 2160cm-‘. The polymer was recovered after evaporation of the solvent and precipitation into dry methanol. It was then dried under high vacuum. Polymers were examined by i.r. spectroscopy with a Perkin-Elmer 577 apparatus. The ‘H-and 13C-NMR spectra were recorded at 200 and 50.3 MHz, respectively, in CDCl, or in mixtures of CDCl,/CFCl, depending on the fluorine content of the polymers. The molecular weight of PMHS was measured by vapor pressure osmometty-in toluene at 45°C with 1,2-dinhenyl-l.Zethanedione (benail) as a standard using a Knauer-apparatus. Molecuiar weights of the modified polysiloxanes were determined by GPC using a

Synthesis

All polymers described here were prepared from a single sample of polymethyldrogensiloxane, PMHS, of IV, = 2200 (LIP, = 39, M,/M, - 2. This polymer was hydrosilylated, in the presence of a platinum zero complex as a catalyst, with a combination of the ally1 ether derivatives whose structures are shown below.

229

Energy transfer studies Table 1. Polymer composition (mol.%) Samule

Ad

Phed

a

b

’

Ph.0 An-25 Ph.50 An-75 Ph-100

0 3 0 3 0

3 0 3 0 3

0 25 49 75 97

91 72 49 22 0

tr tr tr tr tr

“C,F,,CH,CH,O(CH,), groups, see Scheme I. %Z,H,,O(CH,), groups. ‘Me,SiCh,CH, groups, trace amounts. dAnCH,O(CH:.), or Phe CH,O(CH,), groups.

We wish to point out that the synthesis of ally1 ethers 3 and 4 was very much improved by the use of phase transfer catalysis (PTC), compared to the method previously described [5]. In order to avoid possible complications due to the steric bulk of the fluorescent chromophores, this species was added first to the reaction mixture. The reaction was followed by thin layer chromatography. Once its attachment to the polymer was clearly evident, the fluoroallyl ether 1 and/or the octylallyl ether 2 were added and allowed to react until the band in the infra red spectrum due to the Si-H bond was no longer detectable. At this point, a small amount of trimethyl vinylsilane 5 was added. The last step was essential to prevent reticulation of polymers, leading to insoluble material. The role of the vinylsilane is to react with any vestigial Si-H bonds remaining on the polymer. This reaction is shown in Scheme 1. The polymers were characterized by a combination of ‘H- and 13C-MMR to determine the number of hydrocarbon branches, and by U.V. absorption spectroscopy to determine the chromophore content. The composition of the various polymer samples is indicated in Table 1. Gel permeation chromatography of these polymers in a mixed solvent of Freon 113/chloroform (2/l vol) indicated a similar broad polydispersity for all polymer samples. The Phe-labeled polymers were free of detectable (< 1 mol.%) low molecular weight phenanthrene derivatives, but the An samples contained traces of a low molecular weight antracene derivative, possible 4, which persisted after two reprecipitations of the polymer. For the An-75, the amount was significant, nearly 5 mol.%, and half that amount for the An-25 polymer sample. These quantities will have some effect on the fluorescence quenching experiments described below.

0

WAVELENGTH

(nm)

Fig. 1. Excitation and emission spectra of 9-allyloxymethylphenanthrene (3) and of the polymer Ph-0.

ENERGY

TRANSFER

DYNAMICS

Our objective in this work is to compare the energy transfer kinetics in three distinct types of processes: molecule-small molsmall molecule, small ecule-polymer, and polmyer-polymer. For the first case we examine the interaction of the allyloxy derivatives 3 and 4. For the second case we can examine the situation where the donor is polymer-bound and the acceptor free to diffuse, as well as that where the donor is attached to the polymer, and the acceptor is a small molecule. We have five possible combinations with which to study energy transfer between polymerbound chromophores. Energy transfer kinetics (61 can be described in terms of the simple reaction scheme: Phe + An *

9

Phe* + An k,

Phe + An* --, Phe + An.

The phenomenological rate coefficient for quenching, k, , describes the diffusion-controlled energy transfer

Spectroscopy

In Fig. 1 we present the excitation and fluroescence spectra of 9-allyloxymethylphenanthrene 3 and of one of the Phe-labeled polymers, Ph-0. In Fig. 2 we present the corresponding spectra of 9-allyloxymethylanthracene and the An-labeled An-75. These spectra are unexceptional for 9-alkyl Phe and 9-alkyl An derivatives. All Phe fluorescence decay profiles measured for samples in solution were exponential. This is expected when diffusion dominates in the energy transfer process. The one curious feature of these molecu1e.s is that the allyloxymethylphenanthrene has an unquenched lifetime (r,,) of 25 nsec and those of the Phe-labeled polymer were 30 nsec instead of the 45 nsec values always found for esters of 9-phenanthrenem’ethanol.

WAVELENGTH

(n ml

Fig. 2. Excitation and emission spectra of 9-allyloxymethylantracene (4) and of the polymer An-75

Christian Pham-Van-Cang et al.

230

in the An-75 sample. Since Allyl-An has a larger diffusion coefficient than the polymer, its effect as a quencher is magnified, i.e. 1 - =i+k,[A]+k;[A’]. T

Fig. 3. Stern-Volmer plots of the fluorescence quenching data for (0) 3+4; (+) Ph-100 +4; (m) 3+An-25; (0) Ph-100 + An-25.

process. Its magnitude is related to the diffusion coefficient of the reactants through the timeindependent Smoluchowski expression: k, = kdiff = 4nNx D, R,

(1)

where NA’is Avogadro’s number per mmol; and D, , the mutual coefficient, is commonly set equal to the sum of diffusion coefficients of the reactants. The term Rf describes the reaction radius. For energy transfer by the dipole coupling (Fiirster) mechanism, R, is an effective distance smaller than R,, the characteristic radius for energy transfer between non-diffusing reactants. R, represents the distance at which the energy transfer rate equals the unquenched decay rate of the donor, the for the Phe/An pair examined here, is equal to 22 A [7]. When diffusion-controlled quenching occurs predominantly by nonradiative energy transfer, Giisele [8] showed that R, can be approximated to: R, = 0.676(a/D,)“4

(2)

where CIcharacterizes the efficiency of the resonance energy transfer GI =

(R$/q.

(3)

Equations (1) and (2) form a system with two unknowns, which can be solved for each couple with known values of R, and zO, once values of k, have been determined. To obtain the rate constants of k,, fluorescence lifetimes (r) were fit to the Stern-Volmer equation [9] 1 - =;+k,[A]

r

(4)

where [A] is the molar concentration An groups. Plots of the data are shown in Fig. 3. For 2 + 3 we obtain a value of 1.1 x 10’OM-‘sec-’ in Freon 113. For reactions in which one partner is a polymer, k, drops by 60% to 4.7 x lo9 M-’ set-’ for Allyl-Phe + An-25 and to 4.2 x lo9 for Phe-100 + Allyl-An. For the polymer-polymer reactions, k, drops by an additional factor of cu 5 to 0.9 x lo9 M-’ see-‘, and is essentially independent of the fluorocarbon content of the polymers. Two features of the data require comment. The two polymer-An samples contain trace amounts of AllylAn as impurities, with a larger amount (ca 5 mol.%)

The effect of this impurity can be seen in larger values of k, in experiments involving An-75 The second feature is that the polymers themselves have a significant molecular weight polydispersity, and for the copolymers, a finite composition heterogeneity. One should properly express the quenching term as Zk,+[Ai] where the magnitude of kqi will depend upon the D,i characterizing a given pair of reactants. In the discussion that follows, it is important to keep in mind that the k, and D, values represent ensemble averages over the chain length and composition distributions in the samples. For the two ally1 derivatives, we find R, = 12.5 A and D, = 1.1 x 10m5cm* set-‘. Since D, represents the sum of the diffusion coefficients of the Phe and An derivatives, which ought to be similar for the two species, we obtain the very reasonable result that We have two D,, = DAn = 5 x 10e6 cm* see-‘. examples where we follow the reaction of a small molecule with a polymer-bound chromophore. For Ph-lOO+ the allylanthracene 3, we find R, = 15.5 A and D, = 3.4 x 106cm2 set-‘, and for the allylphenanthrene 4 + An-25 we find RI= 16 A and D, = 3.9 x lo6 cm* set-‘. These pairs of values are very similar, and indicate that the reaction is insensitive to whether the donor or acceptor chromophore is polymer-bound. It is interesting that the D, values determined here are smaller than D,, or D,, . One normally considers the reaction between a polymer-bound group and a mobile reactant to have a D, value half that of the reaction between two small molecules because the contribution of the polymer diffusion is small. The other possibility is that the comb structure of the polymer imposes a steric constraint to the energy transfer reaction, so that only a fraction of the diffusive encounters are effective. The Gdsele model described above does not take this factor into account when separating k, into its R, and D, components. For the polymer-polymer interactions, all pairs have similar rate constants and, as a consequence, (25 A) and D, values of R, similar (5.3 x lo-‘cm* set-‘). For us, the most significant feature of this result is that neither k, nor D, depends significantly on the fluorocarbon content of the polymers. We find this surprising, in spite of the low degree of polymerization of the polymers, because the neat liquids seem to be so completely immiscible. This suggests that, whatever the degree of coil adjacency or interpenetration necessary to effect energy transfer, it is not particularly inhibited by the composition of the polymer. Acknowledgements-The authors thank NSERC Canada and the Ontario Centre for Materials Research for their financial support, and NATO for a travel grant that made this collaboration possible.

Energy transfer studies REFERENCES

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