J. Phomchem. Photobid. A: Chem., 80 (1994) 369-376 369

Studies of the antenna effect in polymer molecules 25. Photozymes with rose bengal chromophores

Maria Nowakowska+, Eddy Sustar and James E. Guillet Deparhnent of Chemimy, Univemiry of Toronto, Toronto, Ont., M5S fA1 (Canada)

Abstract

Several copolymers of poly(sodium styrenesulfonate-styrene-vinylbenzylchloride) and poly(sodium styrenesul- fonate-2-vinylnaphthalene-vinylbenzykhloride) containing various amounts of rose bengal chromophores attached to the polymer chain were synthesized. The copolymers are soluble in water and can solubilize large hydrophobic compounds. They are efficient generators of singlet oxygen, and act as photosensitizers in the oxidation of singlet oxygen acceptors which are dissolved in the water phase and solubilized in the hydrophobic polymeric microdomains.

1. Introduction

It has been shown in previous papers in this series that the novel antenna polyelectrolytes, re- ferred to as “photozymes”, behave as efficient photocatalysts [l-103. In aqueous solutions these polymers adopt a compact conformation, resulting in the formation of hydrophobic microdomains, which are capable of solubilizing sparingly water- soluble organic compounds. The antenna chro- mophores, covalently attached to the polymer chain, absorb light and the excitation energy is transferred to substrate molecules solubilized within the polymer coil.

Ideally, solar light can be used to drive the photochemical reactions. In order to increase the probability of utilization of solar energy, the overlap between the absorption spectrum of the antenna units and the spectrum of solar light should be maximized.

This paper reports studies on the synthesis and properties of photozymes with rose bengal (RB) chromophores. The most important features of the RB chromophores are their ability to absorb visible light and to use this energy for the efficient formation of singlet oxygen.

2. Experimental details

2.1. Mater-ids Commercial 2-vinylnaphthalene (VN, Aldrich)

was purified by absorption chromatography, using

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cyclohexane as eluent and alumina as adsorbent. Commercial sodium styrenesulfonate (SSS, Mon- omer-Polymer & Dajac) was purified by three

recrystallizations from a 9 : 1 (v/v) mixture of meth-

anol and water, followed by two recrystallizations from pure methanol (Caledon, spectrograde). Sty-

rene (S, Aldrich) and vinylbenzylchloride (VBCh,

Monomer-Polymer & Dajac) were washed three times with 2% NaOH and four times with distilled water. The monomers were dried over anhydrous CaC12 and were distilled under reduced pressure before use.

Rose bengal (RB, Aldrich, certified grade) and rose bengal, bis(triethylammonium) salt (RBA, Aldrich, dye content 90%) were used as received. Benzoyl peroxide was purified by three recrystal- lizations from methanol. Dimethyl sulfoxide (DMSO, Caledon, ACS reagent grade), methanol (Caledon, spectrograde), 1-butanol (Caledon, re- agent grade) and tetrahydrofuran (THF, Caledon, glass distilled) were used without further purifi- cation. Distilled water was passed through a deion- izer and subsequently filtered through a trace organic removal cartridge (Norganic, Millipore).

1,3-Diphenylisobenzofuran (DPBF, Aldrich) was purified by recrystallization three times from ben- zene in the dark. 1,2_Dibenzoylbenzene (DB, Ald- rich) and perylene (P, Aldrich} were used as received. Anthracene-2-sulfonic acid, sodium salt (ASN) was prepared by reduction of anthraqui- none-2-sulfonic acid, sodium salt (Aldrich) with Zn dust in the presence of N&OH.

370 M. Nowakowska et al. / Photqmes with rose bengai chromophores

2.2. Polymer synthesis Poly(sodium styrenesulfonatestyrene-vinyl-

benzylchloride) (PSSS-S-VBCh) and poly(sodium styrenesulfonate-2-vinylnaphthalene-vinylbenzyl- chloride) (PSSS-VN-VBCh) were prepared by free-radical polymerization initiated with benzoyl peroxide (see Table 1 for details). The polymer- izations were carried out in degassed DMSO (three freeze-thaw cycles under high vacuum) in sealed ampules at 60 “C. The polymers were precipitated with an excess of 1-butanol, filtered, washed with ether and dried. The terpolymers were then dis- solved in water, exhaustively dialyzed (Fisher, cel- lulose tubing, cut-off 12000-14000 g mot-‘) against deionized water and freeze dried. Eiemental anal- ysis was used to determine the composition of the terpolymers.

RB was attached to the polymer chain using the method developed by Merrified [ll] and Blos- sey et al. [12]. It has been shown that the dye can be covalently bound to vinylbenzylchloride polymeric units. The reaction involves a nucleo- philic displacement with the C-2’ carboxyl of RB as the nucleophile

P-CHJJ + RCO,-Na’ -

I? P-CH,OC-R+NaCl (I)

Polymers containing different amounts of im- mobilized RB were obtained by reacting the same amount of the polymer with different quantities of the dye (see Table 2) added to the aqueous polymer solutions_ The solutions were stirred and

TABLE 1. Feed compositions and yield after purification for PSSS-SVBCh and PSSS-VWVBCh copolymers

Polymer SSS s VN VBCh Initiator Yield

(9) (9) (g) (9) (mg) (%)

PSSSS-VBCh 15.00 4-55 - 2.29 90 56 PSSS-VWVBCh 15.00 - 6.78 2.29 90 48

TABLE 2. Amount of rose bengal added to 1 g of polymer (mt”) and amount attached to polymer chain (m::)

Polymer

PSSS-S-VBCh-RBI PSSS-SVBCh-RB2 PSSSS-VBCh-RB3 PSSS-VWVBCh-RBl PSSSVWVBCh-RB2 PSSS-VN-VBCh-RB3

RB wl m:B (g) (9)

0.1 0.0093 0.3 0.0165 0.6 0.0282 0.1 0.0073 0.3 o.LMOo 0.6 0.0486

reacted at 80 “C for 20 h, then cooled to ambient temperature, transferred to dialysis tubes and ex- haustively dialyzed against deionized water to re- move unattached RB. The resulting polymers were then recovered by freeze drying. In order to avoid degradation of the polymers, the mixtures were protected against light during all these procedures. The RB content in the polymers was determined by UV-visible absorption spectroscopy.

2.3. Apparatus The UV-visible absorption spectra of the sam-

ples were measured using a Hewlett-Packard 8451A diode-array spectrophotometer.

Steady state fluorescence spectra were recorded at room temperature using an SLM Instruments fluorescence spectrometer.

Quantitative and qualitative analyses of the sys- tems studied were carried out using a Hewlett- Packard 5890 gas chromatograph equipped with a flame ionization detector (FID) and a DB-1 capillary column (polyrnethylsiloxane, 30 m X 0.25 mm, 0.25 pm film thickness).

2.4. Procedures 2.4.1. Solubilization of probes Solubilization of DPBF and P in aqueous poty-

mer solutions was achieved by slowly injecting microliter quantities of the probe (1 X 10m3 M) dissolved in THF into milliliter quantities of poly- mer solution. The mixture was shaken for 5 min and equilibrated in the dark for 2-4 h. THF was removed before the measurements by bubbling with argon for 15 min.

2.4.2. Irradiation of the samples Irradiation was carried out using a High Effi-

ciency High Uniformity Illumination System (Sciencetech Inc.) equipped with a 1000 W mer- cury/xenon lamp and an IR filter. A dichroic mirror (266320 nm) and a 313 nm interference filter were used to obtain monochromatic light at 313 nm. The incident light intensity was determined using a ferrioxalate actinometer, I0 = 3.3 X 10e8 einstein dmp3 s-‘. An aluminum mirror, neutral density filter (T= 48%) and monochromator were applied to obtain radiation at A=546 nm. The incident light intensity at this wavelength was determined using the potassium reineckate acti- nometer [13], I0 = 9 X 1O-6 einstein dmp3 s-l. Dur- ing irradiation, the solutions were stirred using a magnetic stirring bar and bubbled with oxygen.

3. Results and discussion

3.1. Polymer characterization Polymer compositions were determined by el-

emental analysis and the contents of RB were calculated from the UV-visible absorption spectra of the aqueous polymer solutions. These data are summarized in Table 3. The PSSS-S-VBCh poly- mers contained 59.6 mol.% of SSS, 37.5 mol.% of S and 2.9 mol.% of VBCh. The PSSS-VN-VBCh polymers contained 56.9 mol.% of SSS, 39.9 mol.% of VN and 3.2 mol.% of VBCh. Although all polymers obtained are soluble in water, PSSS-S-VBCh displays higher solubility than PSSS-VN-VBCh. This is due to the higher content of the hydrophilic SSS units and also reflects the lower hydrophobicity of S monomer in comparison with VN. All the polymers are soluble in methanol up to 0.2 g dme3.

The content of RB changes from zero to about 0.5 mol.% in PSSS-S-VBCh polymers and from zero to about 1 mol.% in PSSS-VN-VBCh systems. The analysis of the data presented in Tables 2 and 3 indicates that the efficiency of immobilization of RB is quite low (no more than 13% of the dye added was immobilized). The total concentration of VBCh binding sites and the initial concentration of RB used were much higher than those needed in the reaction, and so they could not be considered to be the factors limiting the efficiency of RI3 binding. This shows that large fractions of VBCh units present in the polymer are not accessible to RB molecules. This may be caused by steric effects since the RB molecule is very large. In an earlier study, a decrease in the efficiency of immobilization of RB with an increase in the dye concentration added to polymeric beads was also attributed to steric effects [14]. In addition, PSSS-VN-VBCh and PSSS-S-VBCh chains adopt a compact con- formation in aqueous solution (see below), and the VBCh polymeric units may be trapped within

TABLE 3. Polymer composition and content of rose bengal

the hydrophobic domains where the access to negatively charged ionized RB molecules may be very difficult.

The absorption and fluorescence bands in the visible spectral region for all the polymers can be assigned to the RB chromophores attached to the polymer chain. All absorption spectra (Fig. 1) display one band with a well-defined maximum (around 550 nm) and a shoulder at a shorter wavelength (A=512 nm). Differences in the in- tensities of the respective absorption and emission bands can be correlated with the polymer com- position. The positions of the absorption and emis- sion maxima and the values of the ratio of the absorption of RB at the maximum to the absorption at the shoulder are given in Table 4.

In methanol solutions the shape and position of the absorption and emission bands are almost the same for all the polymers. The main absorption band has a maximum at 554 nm, and an emission band with a maximum at 576f3 nm is also ob- served. The absorption and emission bands for RB attached to the polymeric chain are slightly blue shifted from those recorded for RB in meth- anol solution. There is also a change in the ratio of the absorption at the peak maximum of RB to the absorption at the shoulder, but there is no defined dependence on the loading of the polymers with the dye. These phenomena indicate that there are some interactions between the chromophores themselves, or between the chromophores and the polymer chain. Interactions between the chro- mophores may lead to aggregation. The aggregation of xanthene dyes has been shown to result in red shifts in the absorption spectrum and an increase in the absorption at the shoulder of the peak (AJ 115, 161. Although the A,/& ratio is generally lower for the polymers than for the free RB molecule (see Table 4), a blue shift, not red, was observed in the spectra. This suggests that, due to the presence of the polymer chains, the RI3

Polymer sss S VN VBCh (mol.%) (mol.%) (mol.%) (mol.%)

Rose bengal

(mol.%) (wt.%)

PSSS-S-VBCh 59.6 PSSS-S-VBCh-RBl 59.6 PSSS-ZX’BCh-RB2 59.6 PSSS-S-VBCh-RB3 59-6 PSSS-VN-VBCh 56.9 PSSS-VN-VBCh-RBl 56.9 PSSS-VN-VBCh-RB2 56.9 PSSS-VN-VBCh-RB3 56.9

37.5 2.90 37.5 2.73 37.5 _ 2.60 37.5 2.39

39.9 3.20 39.9 3.05

_ 39.9 2.40 39.9 2.23

0.17 1.03 0.30 1.83 0.51 3.31

_

0.15 0.81 0.80 4.44 0.97 5.40

Wavelength Cm-n)

Fig. 1. Electronic absorption spectra of aqueous solutions of PSSS-S-VBCh-RB3 (-) and PSSS-VN-VBCb-RB3 (-- -) in the visible region.

chromophores experience a microenvironment dif- ferent from that of pure methanol.

The photophysical properties of xanthene dyes are strongly dependent on the environment, es- pecially on the hydrogen-donating power of the medium [17-251. This “environmental” effect is even more pronounced when the polymers are dissolved in water, as can be seen by the consid- erable difference between the spectral character- istics of the PSSS-S-VBCh-RB and PSSS-VN- VBCh-RB copolymers. The shapes and positions of the absorption and emission maxima for all styrene-containing polymers (PSSS-S-VBCh- RB) are very similar and almost the same as those of RB. The values of A,/!, are also very similar. This indicates that the RB chromophore experi- ences an environment of similar polarity. The spectra for PSSS-VN-VBCh-RB polymers are

broader and blue shifted. Figures 1 and 2 show the absorption and emission bands in the visible region for the PSSS-VN-VRCh-RB and PSSS-S-VBCh-RB polymers with the highest con- tent of RB. (The spectra were normalized to the same absorption or intensity at the maxima of the bands.) The spectra of RB (free molecule) in an aqueous solution of PSSS-VN-VBCh were mea- sured in order to separate the possible mixed- solvent effect, resulting from the presence of the hydrophobic polymer in water, from the confor- mational effect. It was shown that the addition of PSSS-VN-VBCh to an aqueous RR solution does not influence the spectroscopic properties of the dye. However, solubilization of RBA (an RB de- rivative soluble in non-polar media) in an aqueous solution of PSSS-VN-VBCh gives a typical RB spectrum with a slightly red-shifted absorption band (maximum at 548 nm).

Taking the above experimental data into account, it can be concluded that the differences in the absorption and emission spectra of RB chromo- phores attached to the polymer chain result from differences in the microenvironment experienced by the chromophores, not from interactions be- tween the chromophores themselves. The fact that the more pronounced differences are observed for PSSS-VN-VBCh-RB in aqueous solution strongly supports these suggestions. The broadening and blue shift of the absorption and fluorescence bands and the decrease in the Al/A, ratio are indicative of the occurrence of hydrogen-bond formation between the dye and the polymer chain [17].

The lack of spectral changes when RB was simply added to the aqueous polymer solution shows that, in this system, the hydrophobic in- teractions between the dye and the hydrophobic

TABLE 4. Spectral characteristics of rose bengai chromophores

Polymer Methanol Water

PSSS-SVBCh-RBl PSSS-S-VBCb-RB2 PSSS-S-VBCh-RB3 PSSS-Vh’-VBCh-RBl PSSS-VN-VBCh-RBZ PSSS-VN-VBCh-RB3 RB PSSS-VN-VBCh + RBb PSSS-VN-VBCh-RBA’

554 576 2.76 554 578 2.92 554 579 3.04 554 576 2.68 552 574 2.59 554 577 2.55 55s 580 3.20

546 568 546 568 546 568 540 545 540 563 532 559 546 570 546 570 548 565

2.63 2.62 2.49 2.11 1.63 1.n 3.04

“Ratio of the absorbance at the maximum of the RB peak to the absorbance at the absorption shoulder. “RB dissolved in aqueous solution of PSSS-VN-VBCb. ‘RBA solubilized in aqueous solution of PSSS-VN-VBCh.

hf. Now&owska et ai. I Photqmes wifh rose bmgal chmwwphores 373

Wavelength (nm)

Fig. 2. Steady state fluorescence spectra of PSSS-S-VBCb-RB3 (-) and PSSS-Vl-VBCh-RB3 (---) in aqueous solution (A,=480 ml).

PSSS-VN-VBCh-RB copolymers than for PSSS-S-VBCh-RB photozymes because of the higher hydrophobicity of the polymeric microdo- mains in the former. The solubiliiing ability of DPBF is generally higher for the photozymes with a higher content of RB. This can be explained by considering that the VBCh units that are not substituted with RB may be solubilized in the hydrophobic pockets of the photozymes. This prob- ably diminishes the volume of the microdomains accessible for DPBF. However, VBCh units sub- stituted with RB are repulsed by the electrostatic interactions with the chain after ionization of the dye. The solubilization of P occurs in a slightly different way, possibly due to the considerably higher hydrophobicity, molecular volume and mo- lecular geometry of this compound.

TABLE 5. Dishibution coefficients of P and DPBF between polymer pseudophase and aqueous phase

3.3. Determination of the quantum yie,!ds of singlef osen formation

Polymer KmF X 10s &Xl@ (*5%)” (*5%)b

PSSS-S-VBCh-RI31 0.42 0.95 PSSSS-VBCh-RB2 0.74 1.94 PSSSS-VBCh-RB3 0.92 2.10 PSSS-W-VBCh-RBl 1.72 9.67 PSSS-VN-VBCh-RB2 1.37 9.24 PSSS-VN-VBCh-RB3 3.08 6.03

?~&,~=6.7SxlO-~ g kg-‘. bw;9=3.03xlO-’ g kg-‘.

polymeric pseudophase are not strong enough to overcome the electrostatic interactions between the polymer chain (polyanion) and the ionized negatively charged RB. Thus the RB units bound to the polymer chain are not expected to be solubilized in the interior of the hydrophobic po- lymeric pseudophase but directed to the aqueous phase.

The quantum yields of singlet oxygen formation by the RB chromophores bound to PSSS-S- VBCh-RB and PSSS-VN-Vl3Ch-RB were deter- mined in methanol solution using the relative actinometry method [27]. The method requires determination of the rates of photo-oxidation of a singlet oxygen acceptor sensitized by RB (VRB) and by RB chromophores attached to the polymer chain (&_,,). The reactions must be carried out at an acceptor concentration high enough to ensure that the reaction is zero order with respect to the acceptor. Under these conditions, the quantum yield of singlet oxygen formation by the RB chro- mophores attached to the polymer chain can be calculated using

WO&-_RB = 4J(‘O&B(~~-RF3lv,B) (3)

where +(‘O,),_- and +(‘O,), are the quantum yields of singlet oxygen formation by the RB attached to the polymer chain and free RIB mol- ecules respectively.

3.2. Solubilization of probes in aqueous solutions of ptiotozynes

In aqueous solutions, photozymes solubilize large molecules of sparingly water-soluble compounds such as DPBF and P. The distribution coefficient Q of DPBF and P between the polymeric pseu- dophase and aqueous phase can be defined as the ratio of the weight fractions of the probe in the respective phases [26]

The distribution coefficients are listed in Table 5. The distribution coefficients are higher for

The quantum yields of singlet oxygen formation were determined for all the polymers studied at the same total concentration of the RB chro- mophores (cRB = 2.3x10-’ M). DPBF, a known efficient singlet oxygen acceptor, was used. The interaction between molecules of this compound and singlet oxygen results mainly in the peroxi- dation of DPBF. No measurable physical quenching of singlet oxygen by DPBF has been observed; thus the photobleaching of the compound is a sensitive and quantitative counter for singlet oxygen [28]. The initial concentrations of DPBF in all experiments were adjusted values (c,~, = 1.1 X

low4 M). At this concentration the photoperox-

374 M. Nowakowskn er al. / Pbmymes with rose bengal chmmophores

id&ion can be fitted by a zero-order kinetic equa- tion as shown in Fig. 3. The quantum yields of singlet oxygen formation so determined are given in Table 6. It can be seen that RB chromophores attached to the polymer chain maintain their high efficiency of formation of singlet oxygen; the quan- tum yields of singlet oxygen formation are com- parable with that for the free molecule of RB (C#J = 0.76 [29]). This indicates that the interactions between the RB chromophores and the polymer chain do not affect the efficiency of singlet oxygen generation when the incident radiation is directly absorbed by the RB chromophores. The quantum yields generally increase with RB loading. Although the dependence of C#J on RB concentration is quite weak, it is important to observe that the highest value was obtained for the polymer with the highest loading of RB. This shows that there is no self-

Fig. 3. (A) UV-visible absorption spectra of DPBF during ox- idation photosensitized by PSSS-WJ-VBCh-RB3 in methanol solution. Irradiation times: 0 min {-); 5 min (---)_ (B) Consumption of DPBF during irradiation of PSSS-VN- VBCh-RB3 and DPBF in methanol at A=546 nm.

TABLE 6. Quantum yields of singlet oxygen formation by the polymers and quantum efficiencies of photo-oxidation of DPBF and ASN during direct excitation of rose bengal chromophores (A..,-546 nm)

Polymer &W” y&F x 103b ?%NC

PSSS-S-VBCh-RBl 0.71 5.0 0.23 k%SS-S-VBCh-RB2 0.73 5.2 0.24 PSSS-S-VBCh-RB3 0.75 5.4 0.25 PSSS-VN-VFJCh-RI31 0.68 4.9 0.19 PSSS-VWVBCh-RB2 0.77 5.5 0.21 PSSS-W.-VBCh-RB3 0.85 6.2 0.31 RB 0.76d - Cl.15

quantum yield of singlet oxygen formation by polymers in methanol. bQuantum efficiency of oxidation of DPBF solubilized in aqueous polymeric solutions, c~‘aF = 3 x 10m6 M, cRB = 2 x lo-’ M. Quantum efficiency of oxidation of ASN dissolved in aqueous polymeric solutions, ceN = 4 X 10m4 M, cRB = 2 X 10Y5 M. ‘%ken from ref. 28.

quenching in the photozymes with attached RB chromophores and that the RB chromophores are accessible to oxygen.

The quantum yields of singlet oxygen formation by the photozymes containing RB are considerably higher than those obtained for RB-functionalized poly(styrene-co-vinylbenzylchloride), where the maximal value of 4 was found to be 0.38 [30], and similar to those observed for polymeric beads with RB attached [14]. Thus the photozymes rep- resent a new type of efficient polymeric generator of singlet oxygen.

4. Singlet oxygen reactions in aqueous polymer solutions

4.1. Reaction initiated by direct excitation of RE chromophores

The reactions between singlet oxygen generated by photozymes containing REJ in aqueous solution and two different types of acceptor have been studied. The first type was the water-soluble ac- ceptor ASN. The second was DPBF, which is only slightly water soluble, but can be efficiently sol- ubilized by the photozymes (see Table 5). Both of these compounds are easily oxidized by singlet oxygen. The reaction rate with ASN in water is 3 X lo8 M-l s-’ [31], while for DPBF in methanol it is 6X 10’ M-’ s-’ [32].

Irradiation of photozymes containing solubilized DPBF or dissolved ASN with light at A = 546 nm (which is absorbed only by RB) results in the photosensitized oxidation of these compounds (see Figs. 4 and 5). The quantum efficiencies of photo- oxidation are given in Table 6. It should be noted that the oxidation of DPBF requires fast transport

I I I , I I I I I.2 -

$ c o.a- x b I.3

2

450 500 !i!m 600 650 700 Wavelength (nm)

Fig. 4. UV-visible absorption spectra of PSSS-VKVEtCh-RB2 in aqueous solution containing solubilizing DPBF before (-) and after (---) irradiation with light at L-546 nm for 5 min (,+,=O.S g I-‘, c,D~~~=~.~X~O-~ M).

M. Nowakowska el al. I Phofozymes with rose bengal chromophores 37s

A ‘-

t 2 0.75 - c

$ 5 :: 0.50 - E 2 GZ

P 0.25 - .- -L

B 0 * I

360 400 440 480 Wavelength Cnm)

Fig. 5. Steady state fluorescence spectra of ASN dissolved in an aqueous solution of PSSS-VN-VBCh-RB2 before (--) and after (- --) irradiation with light at A = 546 nm for 30 min.

TABLE 7. Quantum efficiencies of photo-oxidation of DPBF during excitation with light absorbed by naphthalene polymeric chromophores (h =313 m-n)

System

PSSS-VN-VBChb PSSS-VI+VBCh + RB,’ PSSS-VWVBCh f RBAd PSSS-VN-VBCh-RBZ

7&F x lff

1.84 2.10 0.38 3.50

a~,DPBF=2.1~10-5 M, c,,=2x11Y5 M. %+= 0.5 g 1-l.

‘RB dissolved in aqueous polymer solution. dRBA solubilized in aqueous polymer solution.

of singlet oxygen from the aqueous phase, where it is most probably formed, to the hydrophobic polymeric microdomains where DPBF is solubi- lized.

4.2, Reaction initiated with light absorbed by naphthalene chromophores

In order to determine the importance of the site at which singlet oxygen is formed (aqueous phase or polymeric microdomains), and to establish whether energy migration in the polymer molecules influences the oxidizing abilities of RB chromo- phores attached to the polymer chain, the photo- oxidation of DPBF solubilized in the interior of the PSSS-VN-VBCh photozyme with light ab- sorbed by naphthalene chromophores (A = 313 nm) was studied (see Table 7). In the case of PSSS-VN-VBCh {no RB attached), singlet oxygen is formed by oxygen quenching of the electronically excited naphthalene chromophores, while in the other systems it is generated mostly by the deac- tivation of excited Rl3.

The highest efficiency of oxidation of the probe solubilized in the hydrophobic polymeric pseu- dophase is observed for the PSSS-VN-VBCh-RI32 polymer, where the RB chromophores (attached to the chain) are excited via energy transfer from the polymeric naphthalene chromophores (see Fig. 6) and the singlet oxygen is probably formed in the aqueous phase. This enhancement in the ef- ficiency of DPBF oxidation, when the reaction is sensitized by polymeric RB chromophores, suggests the participation of energy migration in the process. The oxidation of DPBF is also efficiently pho- tosensitized by the system in which free RB is dissolved in an aqueous polymer solution. The efficiency of sensitization by REA solubilized in the hydrophobic polymeric microdomains was found to be considerably lower. This may result from the lower quantum yield of singlet oxygen formation; RBA displays a generally lower #I value than that found for RB [33] and, in addition, self- quenching may be expected due to the high local concentration of the dye in the polymeric micro- phase. The above experimental data indicate that the efficiency of DPBF photo-oxidation is depen- dent on the efficiency of singlet oxygen formation rather than on the site at which singlet oxygen is formed. This observation is in agreement with literature data, indicating that singlet oxygen easily penetrates the interior of micelles and that both the natural decay rate of singlet oxygen and its bimolecular rate constant for reaction with the acceptor (DPBF) are insensitive to the site at which ‘0, is produced [34, 351. The present study suggests that this may also be valid in photozymes. This is important because it shows that it may be useful to locate the chromophores outside the restricted volume of the polymeric pseudophase,

Wavelength (run)

Fig. 6. (A) Steady state fluorescence spectra of PSSS-S-VBCh (-) and PSSS-S-VEKh-RB2 (---) in aqueous solution (A,,=254 nm, cpo, =O.l g I-‘). (B) The RB fluorescence band recorded for PSSSS-RBZ in aqueous solution (A,.=254 nm, ~~~-0.1 g I-‘) at higher sensitivity.

376 M Nowakowska et al. / Photoqmes with rose bengal chromophores

which can eliminate or at least diminish the in- teractions leading to the loss of energy via self- quenching or excimer formation.

5. Conclusions

Two types of photozymes with various amounts of RB chromophores have been prepared. It was demonstrated that RB chromophores covalent& attached to the polymer chain efficiently generate singlet oxygen. Singlet oxygen generated in the aqueous phase efficiently oxidizes molecules of organic compounds in both the aqueous phase and polymeric pseudophase. Due to the spectral char- acteristics of the RB chromophores, photozymes containing RB can initiate photochemical reactions when exposed to visible light. Thus the polymers can be used to drive singlet oxygen reactions using solar light.

Acknowledgments

The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. M.N. thanks the Polish State Committee for Scientific Research for sup- port in the form of a research grant (no. 2 0665 91.01).

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