Polymeric Photosensitizers, 5. Synthesis and Photochemical Properties of Poly[(N-isopropylacrylamide)-co-(vinylbenzyl chloride)] Containing Covalently Bound Rose Bengal Chromophores

Macromol. Chem. Phys. 2001, 202, 1679–1688 1679

Polymeric Photosensitizers, 5a

Synthesis and Photochemical Properties of Poly[(N-isopropylacrylamide)-co-(vinylbenzyl chloride)]Containing Covalently Bound Rose BengalChromophores

Maria Nowakowska,* Mariusz Kepczynski, Monika Dabrowska

Faculty of Chemistry, Jagiellonian University, 30-060 Kraków, Ingardena 3, PolandE-mail: [email protected]

IntroductionThe anchoring of photosensitizers like Rose Bengal to aninsoluble support provides a splendid way to separatethem from reagents after the completion of a reaction.There are several ways of preparing supported photosen-sitizers:[1, 2]

– adsorption of the sensitizer onto a support,[3, 4]

– formation of an ionic bond between the sensitizer andan appropriate support, e.g. an ion-exchange resin,[5–7]

– incorporation of the sensitizer into a polymeric film,[8]

– formation of a covalent bond between the sensitizerand an appropriate polymer support.[9–11]

The last approach leads to the formation of the moststable systems.

A variety of photosensitizers based on polymeric sup-ports insoluble in common solvents have been developedover the years. They are used as heterogeneous photosen-sitizers. Although they are quite convenient for practicalapplications, they usually display relatively low photo-chemical activities. Gassmann[12] has found that the max-imum efficiency for insoluble sensitizers of colloidal size

Full Paper: The new polymeric photosensitizer poly[(N-isopropylacrylamide)-co-(vinylbenzyl chloride/Rose Ben-gal)] for singlet oxygen production was synthesized andstudied. The polymer is soluble in polar solvents, such asmethanol and water. It displays a lower critical solutiontemperature in aqueous solutions at 31.7 l 0.38C. Thequantum yield of singlet oxygen formation by the RoseBengal-polymer bound chromophore in methanol solutionis close to that determined for free Rose Bengal. In anaqueous solution of the polymer, the efficiency of singletoxygen formation is considerably lower. This phenom-enon results mainly from the aggregation of the Rose Ben-gal chromophores.

Macromol. Chem. Phys. 2001, 202, No. 9 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/0906–1679$17.50+.50/0

a Part 4: M. Nowakowska, Sz. Zapotoczny, A. Karewicz, Poly-mer, in press.

Electronic absorption spectra in the visible region of PNI-PAM-VBC/RB in methanol (solid line) and in aqueous solu-tion (dashed line).

1680 M. Nowakowska, M. Kepczynski, M. Dabrowska

is approximately 50% of the standard sensitizer in homo-geneous solution. This is due to the fact that the particlesof an insoluble support cause significant dispersion of theincident light. On the other hand, the polymeric photosen-sitizers which form homogeneous systems with the sol-vent used in the experiment are characterized by efficien-cies comparable to those found for the molecules of anunattached sensitizer.[13, 14] Unfortunately, it is usuallyvery difficult to separate the homogeneous polymericphotosensitizers from the reaction mixture when the reac-tion is completed.

It seems that some of these problems can be avoidedby the application of stimuli-responsive polymeric mate-rials as supports. Such stimuli-responsive polymeric sys-tems have been intensively studied in recent years. Theexternal stimuli may be temperature, pH, ionic strengthor a combination of them. Since the photosensitizershould be working in a wide range of pH and ionicstrength values, the stimulus that is the most suitable forthe changes is temperature. A well-known example of apolymer responding to external stimuli such as tempera-ture is poly(N-isopropylacrylamide) (PNIPAM). In aque-ous solutions, PNIPAM displays a lower critical solutiontemperature (LCST) at about 318C.

Recently, Bergbreiter et al.[15, 16] have reported applica-tions of PNIPAM as a soluble polymer support for thepreparation of transition metal catalysts.

The aim of the present work was to synthesize andstudy the properties of the thermoresponsive polymericphotosensitizer with Rose Bengal chromophores. A copo-lymer of N-isopropylacrylamide and vinylbenzyl chloride(PNIPAM-VBC) was used as a support.

Experimental Part

Materials

Commercial N-isopropylacrylamide (NIPAM, Aldrich, Mil-waukee, USA, 97%) was purified by recrystallization from amixture of n-hexane and toluene (volume ratio 1 :1). Vinyl-benzyl chloride (VBC, Fluka, Buchs, Switzerland, 70:30mixture of meta and para isomers) was purified by distilla-tion under reduced pressure. 2,29-Azoisobutyronitrile(AIBN) was recrystallized from ethanol at 408C under anitrogen atmosphere and dried under vacuum at room tem-perature. Rose Bengal (4,5,6,7-tetrachloro-29,49,59,79-tertraio-dofluorescein, RB, Aldrich, Milwaukee, USA, certifiedgrade) and sodium dodecyl sulfate (SDS, Aldrich, Milwau-kee, USA, 98%) were used as received. 1,3-Diphenylisoben-zofuran (DPBF, Aldrich, Milwaukee, USA) was purified byrecrystallizing three times from benzene in the dark. Anthra-cene-2-sulfonic acid sodium salt (ASN) was prepared byreduction of anthraquinone-2-sulfonic acid sodium salt(Aldrich, Milwaukee, USA) with zinc dust in the presence ofaqueous ammonia. K[Cr(NH3)2(SCN)4] was prepared fromcommercial Reinecke's salt and purified according to themethod of Wagner.[17]

Copolymer Synthesis

Poly[(N-isopropylacrylamide)-co-(vinylbenzyl chloride)](PNIPAM-VBC) was prepared by free-radical polymeriza-tion initiated with AIBN. A solution of NIPAM (5.87 g,51.9 mmol), VBC (0.35 cm3, 2.5 mmol) and DMSO (50 cm3)was purged with argon for 20 min. A solution of AIBN(60 mg, 0.36 mmol in 10 cm3) was then introduced and afterpurging with argon for 10 more min, the mixture was heatedin sealed ampoules at 60 8C for 24 h. The mixture was trans-ferred to dialysis tubes (cellulose tubing, cut-off 12000–14000 g N mol–1), exhaustively dialyzed against deionizedwater and freeze-dried. About 3.5 g of the copolymer (yield56%) was obtained. The composition of the copolymer wasdetermined from the NMR spectra and elemental analysis(C: 58.16%, H: 9.8%, N: 10.94%). It contained 95.6 mol-%of NIPAM and 4.4 mol-% of VBC.

Immobilization of Rose Bengal

RB was covalently attached to the polymer chain using themethod developed by Merrifield.[18] PNIPAM-VBC wasreacted with RB in DMF solution. A solution of the polymer(2.5 g), RB (1.85 g, 1.82 mmol) and solvent (50 cm3) wasstirred for 16 h at 808C. The mixture was purged with nitro-gen. After cooling to ambient temperature, the resultingpolymer was transferred to dialysis tubes and dialyzedagainst deionized water to remove unattached dye. The poly-mer was then recovered by freeze-drying. In order to avoiddegradation of the polymer, the mixtures were protectedagainst light during all these procedures. The RB content inthe resulting product was determined in methanol solutionby UV-vis absorption spectroscopy. The polymer shows thetypical ester carbonyl absorption at a frequency of 1740 cm–1

in the IR spectrum.[2]

Apparatus

UV-vis spectra were recorded using a Hewlett-Packard8452A diode-array spectrophotometer. Temperature-con-trolled experiments were performed by means of a Hewlett-Packard 89090A Peltier temperature control accessory. Thesample fluid was magnetically stirred and the temperaturewas measured with an external temperature sensor immersedin the sample fluid. The temperature of the fluid was variedfrom 10 to 458C at a constant rate (0.18C N min–1). Fluores-cence emission spectra were measured using a SLM-AMINCO 8100 Instruments spectrometer at room tempera-ture. Emission spectra were corrected for the wavelengthdependence of the detector response by using an internal cor-rection function provided by the manufacturer. IR spectra ofthe samples were recorded on a Bruker IFS 48 FT spectro-photometer. NMR spectra were measured on a Bruker AMX500 MHz spectrometer. Viscosity measurements were per-formed in THF solution at 278C with an Ubbelohde visco-meter for dilution sequences from Schott. Gel permeationchromatography (GPC) analyses were carried out using aWaters system equipped with a Waters 2487 dual-wave-length absorbance detector and a Waters 2410 refractiveindex detector. The separation was performed with PL-aqua-gel-OH 30, 40, and 50 columns using deionized water as the

Polymeric Photosensitizers, 5 1681

eluent. Elemental analysis was performed using an EA 1108CHNS-O elemental analyzer.

LCST Determination

Lower critical solution temperatures (LCSTs) for the poly-mer samples were determined by estimation of the cloudpoints from nephelometric measurements. The temperatureof the water-jacketed cell holder was controlled by a water-circulating bath and varied from 20 to 408C at a rate ofabout 0.28C N min–1. The exact temperature of the samplefluid was measured with a thermocouple. The intensity ofscattered light was plotted against the temperature of thesolution. The cloud point was determined graphically.

Irradiation of the Samples

The samples were irradiated using a medium-pressure mer-cury lamp (ASH 400) and an interference filter (kmax =556 nm) or a glass filter with a cut-off at k A 535 nm.

Results and Discussion

Polymer Characterization

In order to prepare a polymer support with reactivegroups that could be used as attachment points for RoseBengal chromophores, N-isopropylacrylamide (NIPAM)was copolymerized with vinylbenzyl chloride in dimethylsulfoxide (see Scheme 1). The resulting polymer,poly[(N-isopropylacrylamide)-co-(vinylbenzyl chloride)](PNIPAM-VBC), contained 95.6 mol-% of NIPAM and4.4 mol-% of VBC. The dye was covalently attached tothe polymer chain via a nucleophilic displacement pro-cess. The reaction was carried out in the dark in N,N-dimethylformamide (DMF). The content of RB chromo-phores in the resulting photosensitizer, poly[(N-isoprop-ylacrylamide)-co-(vinylbenzyl chloride)] with Rose Ben-gal chromophores covalently attached to the polymerchain (PNIPAM-VBC/RB), was estimated to be26 wt.-%, which indicates that 91% of the VBC poly-meric units were attached to RB. Lamberts and Neckershave demonstrated that the reaction between RB and ben-

zyl chloride (which can be used as a model for the reac-tive groups in our polymer) occurs only with the partici-pation of the carboxylic group (Merrifield reaction). Thephenolate centers are not active in this reaction.[19]

The polymer was soluble in water, methanol, acetone,and THF.

Viscosity-average molecular weights (M—

v) for PNIPAM-VBC and for PNIPAM-VBC/RB were determined from themeasurements of the viscosities of the copolymer solutionsin THF. The relationship between the limiting viscositynumber ([g]) and M

—v for the PNIPAM homopolymer was

established by Fujishige[20] ([g] = 9.59 N 10–3 M—

v0.65). Assum-

ing that this expression is also valid for our copolymer, theviscosity-average molecular weights were estimated. Theresults are summarized in Table 1.

The purity of the photosensitizer was very importantfor our further experiments. GPC analysis with UV detec-tion carried out for the PNIPAM-VBC/RB has shown thatthe polymer did not contain any low-molecular weightUV-absorbing impurities, especially unattached mole-cules of RB. Figure 1 shows the GPC profiles of PNI-PAM-VBC/RB detected at 200 nm and 562 nm. Bothprofiles have the maximum at the same retention time,which confirms that the RB species are covalently boundto the polymer chains.

Scheme 1. Synthesis of the polymeric photosensitizer PNI-PAM-VBC/RB.

Table 1. Physical properties of the copolymers.

Polymer ½g�aÞ

cm3 N gÿ1

M—

vb) LCST

�C

PNIPAM-VBC 19.61 124000 28.0 l 0.3PNIPAM-VBC/RB

34.07 290000 31.7 l 0.3

a) Copolymer solutions in THF.b) From relation [g] = (9.59 N 10–3) M

—v0.65, see ref.[20]

Figure 1. GPC traces for PNIPAM-VBC/RB monitored at twodifferent wavelengths (200 nm and 562 nm).


Effect of Temperature on the Solubility of PNIPAM-VBC/RB in Water

PNIPAM-VBC and PNIPAM-VBC/RB were soluble inwater at or below room temperature. Optically clear aque-ous solutions of these polymers can be prepared up to aconcentration of approximately 8–10 g N dm–3. Asexpected, the aqueous solutions became turbid duringheating above a certain temperature, indicating the occur-rence of an LCST. Phase diagrams for the PNIPAM-VBC-water and PNIPAM-VBC/RB-water systems areshown in Figure 2. (The insert in Figure 2 shows, as anexample, the dependence of the intensity of the scatteredlight on the temperature of the PNIPAM-VBC/RB solu-tion at a concentration of 3.9 g N dm–3. Such dependencieswere used for the determination of the cloud points). TheLCST value for PNIPAM-VBC/RB was determined to be31.7 l 0.38C for a polymer concentration higher than4 g N dm–3. It increased to 35.0 l 0.58C for solutions atlower concentrations. The homopolymer of NIPAM inaqueous solutions exhibits an LCST at 31–328C.[21, 22]

Physicochemical phenomena occurring in an aqueoussolution of PNIPAM at the LCST have been the subjectof studies carried out using a variety of experimentaltechniques. It has been demonstrated that the phaseseparation process of PNIPAM occurs in two steps.[23–25]

In the first step, an individual polymer chain collapsesfrom an extended coil into a globule. In the second step,an aggregation of the individual globules occurs leadingto a macroscopic phase separation. The extent to whichthis latter process is noticeable depends upon the concen-tration of polymer in the system.[26] This explains the

increase of the LCST with decreasing polymer concentra-tion.

The LCST of PNIPAM-VBC was found to be28.0 l 0.38C for aqueous solutions at a concentrationequal to or higher than 4 g N dm–3. This value is lowerthan the LCST determined for the polymer with theattached dye, PNIPAM-VBC/RB (see Figure 2). The dif-ference was about 3.78C. This observation is consistentwith the results of earlier experiments carried out by Tay-lor et al.[27] showing that the LCST value for a polymer inan aqueous solution decreases with an increase of itshydrophobicity. The attachment of RB molecules to thepolymer chain results in the replacement of hydrophobicgroups (chloromethylphenyl groups) by the hydrophilicRB groups. Thus, due to the increase in the ratio ofhydrophilic and hydrophobic groups within the macromo-lecule, the shift of the LCST to a higher temperaturecould be anticipated.

Absorption and Emission Spectra

Figure 3 shows the absorption spectra in the visible spec-tral region for PNIPAM-VBC/RB in methanol and inaqueous solution. The absorption bands occurring in thatregion were assigned to the RB chromophores attached tothe polymer chain. Both spectra exhibit one band with awell-defined maximum and a shoulder at a shorter wave-length. The positions of the maxima of absorption, themaxima of the emission spectra and the values of the ratioof the absorbance at the maximum (A1) to the absorbanceat the shoulder (A2) (A1/A2) for RB chromophoresattached to the polymeric chain and for free Rose Bengalmolecules are summarized in Table 2.

Figure 2. Phase diagrams for the PNIPAM-VBC-water (0) andPNIPAM-VBC/RB-water (f) systems. Insert: Dependence of theintensity of the scattered light on the temperature of the solutionfor PNIPAM-VBC/RB at a concentration of c = 3.9 g N dm–3.

Figure 3. Electronic absorption spectra in the visible region ofPNIPAM-VBC/RB in methanol (solid line) and in aqueous solu-tion (dashed line).


It is well-known that the photophysical properties ofRose Bengal are strongly dependent on the environment,especially on the hydrogen-donating power of the med-ium.[28, 29] In protic solvents, the dye molecules are boundto the solvent by their oxygen atoms. The positions of theabsorption maxima are influenced by the strength of thehydrogen bonds between dye and solvent. A shift of theabsorption spectra to shorter wavelengths (blue shift)with an increase in solvent hydrogen-bonding power isobserved for the Rose Bengal molecule (see Table 2).

The shape of the absorption bands and the value of theA1/A2 ratio provide information on the aggregationbetween dye molecules. The process of RB-RB aggrega-tion has been extensively studied.[30] The formation of RBdimers or higher aggregates affects the absorption spec-tra, causing a decrease in the A1/A2 ratio and a broadeningof the absorption bands.

It is observed that the positions of the absorption andemission maxima measured for PNIPAM-VBC/RB inmethanol are essentially the same as those for Rose Ben-gal in the same solvent. This indicates that the attachmentof the dye to the polymer chain does not disturb the for-mation of hydrogen bonds between RB moieties and thesolvent. The value of the A1/A2 ratio is only slightly lowerthan that measured for Rose Bengal in methanol and theshapes of the absorption bands are also very similar. Thisimplies that, in a methanol solution of PNIPAM-VBC/RBat room temperature, the aggregation of RB chromo-phores attached to the polymer chains is negligible. Theviscosity, osmometry and light-scattering measure-ments[31, 32] have suggested that the PNIPAM macromole-cule behaves as a flexible coil and adopts an open confor-mation in organic solvents. Therefore, the RB moietiesare kept far apart from each other and do not form aggre-gates.

The situation is quite different when PNIPAM-VBC/RB is dissolved in water. The data in Table 2 show thatthe absorption maximum of the polymeric photosensitizeris red-shifted compared to that recorded for the free dyemolecules in aqueous solutions. The shift is relativelylarge and amounts to 13 nm. Interestingly, the maximumof the absorption band for the RB chromophores in anaqueous solution of PNIPAM-VBC/RB was at the samewavelength as that for Rose Bengal-endcapped polysty-

rene dissolved in such aprotic solvents as methylenechloride, toluene or cyclohexane.[33] This suggests that theRB chromophores in PNIPAM-VBC/RB experience amicroenvironment quite different from that of pure water.Such a behavior of PNIPAM-VBC/RB in aqueous solu-tions can be explained on the assumption that the poly-meric support forms hydrophobic microdomains in whichthe RB chromophores are trapped. The formation ofhydrophobic cavities was proposed by Chee et al.[26] forthe copolymer of styrene and NIPAM. Based on studiesof the fluorescence decay of pyrene dispersed in the poly-mer solution, they concluded that intramolecular aggrega-tion occurs between styrene moieties and leads to theappearance of hydrophobic cavities. A similar picture ofPNIPAM with attached hydrophobic groups has emergedfrom the studies carried out by Winnik[31] with pyrene-labeled PNIPAM. At ambient temperature, the polymerin aqueous solution exists in the form of single-moleculemicelles in which the pyrene groups form a hydrophobiccore surrounded by the less hydrophobic polymeric back-bone, which accommodates the surrounding water mole-cules through hydrogen bonding.

In the case of PNIPAM-VBC/RB, the phenyl groups ofthe VBC monomer can form such a hydrophobic pseudo-phase. Due to their hydrophobic character these groupstend to avoid contacts with water. Because the chromo-phores of the dye are attached directly to the phenylgroups, they are most likely trapped within the polymericpseudophase. The polymeric hydrophobic pseudophasehas a limited volume. Thus, the distances between theimmobilized dye moieties are small and the probability ofRB-RB aggregation increases. Both the low value of A1/A2 (see Table 2) and the broadening of the absorption andemission bands for PNIPAM-VBC/RB in aqueous solu-tions confirm the formation of the RB aggregates in thissystem.

There is an interesting dependence of the A1/A2 ratiofor the aqueous solution of PNIPAM-VBC/RB (c =53 mg N dm–3) on the temperature (Figure 4). It wasobserved that an increase of the solution temperature inquite a wide range (15–408C) leads to an increase of theA1/A2 value, indicating that the RB aggregates originallypresent in the solution are destroyed while the conforma-tion of the polymer chain changes.

Table 2. Absorption and emission characteristics for PNIPAM-VBC/RB and Rose Bengal.

Sensitizer Solvent Absorption maximum A1/A2 Emission maximumnmk1

nmk2

nm

RB watermethanol

549557

515518

3.053.24

570580

PNIPAM-VBC/RB

watermethanol

562556

532520

1.482.95

569581


Determination of the Quantum Yield of Singlet OxygenFormation

Because of the different behavior of PNIPAM-VBC/RBin aqueous and methanol solutions, one can expect differ-ent values of the quantum yields of singlet oxygen forma-tion by the RB chromophores (U1O2

) in these two sol-vents. 1,3-Diphenylisobenzofuran (DPBF) and anthra-cene-2-sulfonic acid (ASN) were used as singlet oxygenacceptors in methanol and water, respectively.

Value of U1O2in Methanol

It is known that the interaction between molecules ofDPBF and singlet oxygen results mainly in the peroxida-tion of that compound to 1,2-dibenzoylbenzene.[34, 35] Thereaction can be easily followed by the measurement ofthe UV-vis absorption spectra of DPBF.

The rate of photooxidation of DPBF is given by Equa-tion (1).[14]

ÿ d½DPBF�dt

¼ IabsU1O2

½DPBF�½DPBF� þ b

ð1Þ

where Iabs is the mean intensity of the light absorbed bythe sample and b is the reactivity index.

Integration of Equation (1) leads to the expression:

½DPBF�0 ÿ ½DPBF�t þ b loge

½DPBF�0½DPBF�t

¼ IabsU1O2t ð2Þ

where [DPBF]0 and [DPBF]t is the concentration ofDPBF before and after irradiation, respectively, for a

given period of time t and b is the reactivity index, b =7.3 N 10–5 mol N dm–3, for the oxidation of DPBF inmethanol.[36]

An interference filter was used in order to obtainmonochromatic light. The mean intensity of the lightabsorbed by the samples was calculated according toEquation (3):

Iabs ¼ I546

Zk

TðkÞFðkÞð1ÿ 10ÿANVRBðkÞÞdk ð3Þ

where T(k) is the transmittance of the filter shown in Fig-ure 5, ANVRB(k) is the absorbance of PNIPAM-VBC/RBand F(k) is the spectral distribution of the light emittedby the lamp given as I(k) = I546 F(k) (see Figure 5). Inorder to determine the intensity of the light emitted by thelamp, a potassium reineckate actinometer[17] was used. Anaqueous solution of (K[Cr(NH3)2(SCN)4]) was placed inthe reaction cell and exposed to radiation while bubblingwith argon. The rate of reaction in the actinometer solu-tion, Vr, was measured and can be expressed as follows:

Vr ¼ I546

Zk

bRðkÞTðkÞFðkÞð1ÿ 10ÿARðkÞÞdk ð4Þ

where bR(k) is the quantum yield of the reaction andAR(k) is the absorbance of the actinometer. It was foundthat, under these experimental conditions, the mean inten-sity of the light absorbed by the PNIPAM-VBC/RB solu-tion was (7.97 l 0.15) N 10–7 einstein N dm–3 N s–1.

Figure 6 shows the dependence of the left side of Equa-tion (2) on time. Based on the experimental data, thequantum yield of singlet oxygen formation by PNIPAM-VBC/RB in methanol solution was determined to be0.55 l 0.05.

Figure 4. Dependence of the A1/A2 ratio determined for RoseBengal chromophores on the temperature in an aqueous solutionof PNIPAM-VBC/RB.

Figure 5. Spectral characteristics of the studied system inmethanol; ANVRB(k) – absorption band of RB chromophoresattached to PNIPAM-VBC polymer (c = 21.5 mg N m–3), AR(k) –absorption band of actinometer (c = 1.1 N 10–2 mol N dm–3), F(k)– spectral distribution of the light emitted by the lamp, T(k) –transmittance of the interference filter.


Value of U1O2in Water

The spectral characteristics of this system are shown inFigure 7. The cut-off filter was used during irradiation.The quantum yield of singlet oxygen formation in aque-ous solution was determined according to the proceduredescribed in the literature.[13] The procedure is based onthe relative actinometry method developed by Schaap etal.[10] This method requires the determination of the ratesof photooxidation of a singlet oxygen acceptor sensitizedby Rose Bengal (VRB) and by RB chromophores attachedto the polymer chain (VNVRB). Thus, these two systemswere irradiated under identical conditions. Both of themcontained a singlet oxygen acceptor, ASN (c =4.2 N 10–4 mol N dm–3), and a photosensitizer. In the firstsystem, Rose Bengal (c = 2.3 N 10–6 mol N dm–3) was usedas a photosensitizer, while in the other one, the polymericphotosensitizer PNIPAM-VBC/RB (c = 26.4 mg N dm–3)was applied. The systems were irradiated and the kineticsof the photooxidation of ASN was followed by the meas-urement of the fluorescence spectra.

The rate of acceptor oxidation can be expressed as fol-lows:

V ¼ IabsU1O2

kr½ASN�kd þ kr½ASN� ð5Þ

For the same concentration of acceptor and on theassumption that the presence of the polymer chain has noeffect on the reaction between ASN and singlet oxygen,the ratio of the oxidation rates with different sensitizerscan be expressed by Equation (6).

VNVRB

VRB¼

IabsðNVRBÞU1O2ðNVRBÞ

IabsðRBÞU1O2ðRBÞ

ð6Þ

The value of U1O2(RB) for Rose Bengal in aqueous

solution is equal to 0.75.[37] Iabs (NVRB) and Iabs (RB)were calculated using Equation (3). Using the experimen-tal data (VNVRB = 8.533 N 10–8 mol N dm–3 N s–1,VRB = 1.137 N 10–7 mol N dm–3 N s–1, Iabs(NVRB)/Iabs(RB)= 2.348), the quantum yield of singlet oxygen formationby PNIPAM-VBC/RB in aqueous solution was found tobe equal to 0.24 l 0.04.

The values of U1O2found for PNIPAM-VBC/RB in

aqueous solution and in methanol are lower than thevalues for Rose Bengal (free molecules) in these solvents.The quantum yields of 1O2 formation for the various poly-meric photosensitizers containing Rose Bengal as aphotoactive group are summarized in Table 3.

Singlet oxygen is produced in the system by energytransfer from the triplet state of the sensitizer (Sens) tothe ground state of the oxygen molecule:[39]

3Sens* + O2 e Sens + 1O2 (7)

Thus, U1O2is directly related to the quantum yield of

intersystem crossing (Uisc) of the sensitizer:

U1O2¼ Uiscuet ð8Þ

where uet is the efficiency factor of the energy transfer.The reduction of the U1O2

values for PNIPAM-VBC/RB can be explained by taking into account the followingfactors:

(i) effect of the environment;[28] the quantum yield ofintersystem crossing for RB is strongly dependent on the

Figure 6. Dependence of the kinetic parameters (left side ofEquation (2)) characterizing the photosensitized oxidation ofDPBF in methanol by PNIPAM-VBC/RB on the irradiationtime.

Figure 7. Spectral characteristics of the studied system inwater; ANVRB(k) – absorption band of PNIPAM-VBC/RB(c = 26.4 mg N dm–3), ANVRB,SDS(k) – absorption band of PNI-PAM-VBC/RB (c = 26.4 mg N dm–3) in SDS solution(c = 1 N 10–2 mol N dm–3), ARB(k) – absorption band of Rose Ben-gal (c = 2.3 N 10– 6 mol N dm–3), F(k) – spectral distribution ofthe light emitted by the lamp, T(k) – transmittance of the cut-offfilter.


ability of the environment to form hydrogen bonds and itis higher for more polar protic solvents, (ii) aggregationprocess;[41] the aggregation of the RB chromophoresinduces the self-quenching of the RB molecules in excitedtriplet states, (iii) effect of viscosity;[2] the introduction ofa polymeric support into the system results in an increaseof viscosity, which in turn influences the diffusion of oxy-gen to the Rose Bengal sites in the polymer solution.

In the case of the methanol solution of PNIPAM-VBC/RB, the reduction of the U1O2

values is caused only by theviscosity effect. In the aqueous solution, however, all thefactors contribute to the decrease in quantum yield ofsinglet oxygen formation.

In order to determine the influence of the aggregationof RB chromophores, we have carried out experiments inwhich an anionic surfactant, sodium dodecyl sulfate(SDS), was added to the aqueous PNIPAM-VBC/RBsolution. It is known that ionic surfactants such as SDSinteract with the polymers. In the polymer-surfactantsolution, except for the normal micelles formed by thesurfactant molecules at the critical micelle concentration(cmc), the appearance of mixed micelles at a surfactantconcentration lower than cmc was observed.[42] Thiseffect is especially evident in the case of water-solublepolymers with hydrophobic side groups. It was demon-strated that the mixed micelles can sequester the hydro-phobic side groups and isolate them from each other.

We have observed that the addition of SDS to an aque-ous solution of PNIPAM-VBC/RB (c = 53 mg N dm–3)influences the absorption spectra of the RB chromophores(see Figure 8). An increase in the concentration of SDSresults only initially in an increase of absorption at themaximum as well as in a decrease of the absorption at theshoulder (compare spectra (a) and (b) in Figure 8). Whenthe SDS concentration reaches the cmc region (about7 N 10–3 mol N dm–3), a small shift of the maximum from562 nm to 560 nm was also observed. Further increase inthe SDS concentration leads to a larger blue shift (to

558 nm at an SDS concentration of 9 N 10–3 mol N dm–3)and to a considerable increase of the intensity of absorp-tion at the maximum (spectrum (c) in Figure 8). Thechanges occurring in the system can be better demon-strated by monitoring the A1/A2 ratio. The dependence ofthe A1/A2 ratio as a function of surfactant concentration ispresented in Figure 9. It can be observed that the additionof SDS to the aqueous solution of PNIPAM-VBC/RBresults in an increase of this ratio. This indicates that theRB-RB dimers or higher aggregates are destroyed, mostlikely by the solubilization of the hydrophobic polymericpendant groups. A sharp increase occurs at a surfactantconcentration of approximately 8 N 10–3 mol N dm–3,which almost equals the cmc of SDS (8.2 N 10–3

mol N dm–3 [43]).

Table 3. Comparison of the literature values of the quantum yields of singlet oxygen formation by different polymeric photosensiti-zers.

Polymeric support System U1O2ref.

Chloromethylated styrene-divinylbenzene copolymer Heterogeneous – beads suspendedin CH2Cl2

0.43 [10]

Vinylbenzyl chloride-monomethacrylate ester of ethyleneglycol copolymer cross-linked by bismethacrylate esterof ethylene glycol

Heterogeneous – beads suspendedin methanol

0.48 [38]

Styrene-vinylbenzyl chloride copolymer Heterogeneous – polymer film 0.05 [39]Heterogeneous – powder dispersedin methanol

0.41a) [39]

Homogeneous – solution in CH2Cl2 0.38a) [2]Poly(sodium styrenesulfonate-styrene-vinylbenzyl chloride) Microheterogeneous system in methanol 0.75a) [13]Sodium styrenesulfonate-vinylbenzyl chloride copolymer Homogeneous – solution in methanol 0.73 [14]

a) The highest value for a series of polymers with different numbers of RB units attached to the chain.

Figure 8. Electronic absorption spectra of PNIPAM-VBC/RB(c = 53 mg N dm– 3) in the presence of sodium dodecyl sulfate:(a) without SDS; (b) 8 N 10–3 mol N dm– 3; (c) 1.2 N 10–2

mol N dm–3.


The separation of the RB chromophores resulted in anincrease of the quantum yield of singlet oxygen forma-tion. The value of U1O2

in the aqueous solution of PNI-PAM-VBC/RB containing SDS (c = 1 N 10–2 mol N dm–3)was determined using the experimental data (VNVRB =9.226 N 10–7 mol N dm–3 N s–1, VRB = 1.137 N 10–7

mol N dm–3 N s–1, Iabs(NVRB)/Iabs(RB) = 2.367) to be0.26 l 0.04.

The increase in the value of quantum yield of singletoxygen formation is, however, quite disappointing. Onecould expect a considerably higher value of U1O2

from thecomparison of the absorption spectra of the free RBmolecule and RB polymeric chromophores in the aqueousPNIPAM-VBC/RB-SDS solution. This observation canbe explained by taking into account that the isolated RBchromophores in the aqueous polymer-surfactant solutionare trapped in the interior of the micelle, which is charac-terized by a relatively low polarity.

ConclusionsA new type of polymeric photosensitizer soluble in polarsolvents, such as methanol and water, was prepared. ThePNIPAM-VBC/RB copolymer is an efficient generator ofsinglet oxygen in methanol solution. Its efficiency is alsoreasonable in aqueous solution. The phase diagram forPNIPAM-VBC/RB in aqueous solutions indicates that,while the polymer readily dissolves in water at room tem-perature, it can be easily precipitated at elevated tempera-tures. The process is reversible. This can be of impor-tance for a possible practical application of the polymer.The photosensitizer can be easily removed from the reac-tion mixture by increasing the temperature of the solution

to about 358C, which is not expected to be difficult froma technological point of view.

Received: May 10, 2000Revised: October 20, 2000

[1] M. Julliard, C. Legris, M. Chanon, J. Photochem. Photo-biol. A 1991, 61, 137.

[2] J. Paczkowski, D. C. Neckers, Macromolecules 1985, 18,1245.

[3] R. Nilsson, D. R. Kearns, Photochem. Photobiol. 1974, 19,181.

[4] H. Kautsky, Trans. Faraday Soc. 1939, 35, 3795.[5] S. L. Buell, J. N. Demas, J. Phys. Chem. 1983, 87, 4675.[6] J. R. Williams, G. Orton, L. R. Unger, Tetrahedron Lett.

1973, 46, 4603.[7] H. Takeshita, T. Hatsui, J. Org. Chem. 1978, 43, 3080.[8] R. A. Kenley, N. A. Kirshen, T. Mill, Macromolecules

1980, 13, 808.[9] E. C. Blossey, D. C. Neckers, A. L. Thayer, A.P. Schaap,

J. Am. Chem. Soc. 1973, 95, 5820.[10] A. P. Schaap, A. L. Thayer, E. C. Blossey, D. C. Neckers,

J. Am. Chem. Soc. 1975, 97, 3741.[11] S. Tamagaki, C. E. Liesner, D. C. Neckers, J. Org. Chem.

1980, 45, 1573.[12] E. Gassmann, PhD thesis, no. 519, Ecole Polytechnique

Federale de Lausanne, 1984.[13] M. Nowakowska, E. Sustar, J. E. Guillet, J. Photochem.

Photobiol. A 1994, 80, 369.[14] M. Nowakowska, M. Kepczynski, K. Szczubialka, Macro-

mol. Chem. Phys. 1995, 196, 2073.[15] [15a] D. E. Bergbreiter, Polym. Prepr. (Am. Chem. Soc.,

Div. Polym. Chem.) 1999, 40(2), 244; [15b] D. E. Bergbrei-ter, N. Koshti, J. D. Frels, Polym. Prepr. (Am. Chem. Soc.,Div. Polym. Chem.) 1999, 40(2), 246.

[16] D. E. Bergbreiter, B. L. Case, Y-S. Liu, J. W. Caraway,Macromolecules 1998, 31, 6053.

[17] E. E. Wagner, A. W. Adamson, J. Am. Chem. Soc. 1966,88, 394.

[18] R. B. Merrifield, J. Am. Chem. Soc. 1963, 85, 2149.[19] J. J. M. Lamberts, D. C. Neckers, J. Am. Chem. Soc. 1983,

105, 7465.[20] S. Fujishige, Polym. J. (Tokyo) 1987, 19, 297.[21] M. Heskins, J. E. Guillet, J. Macromol. Sci., Chem. 1968,

A2(8), 1441.[22] H. G. Schild, Prog. Polym. Sci. 1992, 17, 163.[23] C. K. Chee, S. Rimmer, I. Soutar, L. Swanson, Polymer

1997, 38, 483.[24] I. Yamamoto, K. Iwasaki, S. Hirotsu, J. Phys. Soc. Jpn.

1989, 58, 210.[25] F. M. Winnik, Polymer 1990, 31, 2125.[26] C. K. Chee, S. Rimmer, I. Soutar, L. Swanson, Polym.

Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1999, 40(2),291.

[27] L.D. Taylor, L.D. Cerankowski, J. Polym. Sci., Polym.Chem. Ed. 1975, 13, 2551.

[28] [28a] G. R. Fleming, A. W. E. Knight, J. M. Morris, G. W.Robinson, J. Am. Chem. Soc. 1977, 99, 4306; [28b] M. M.Martin, Chem. Phys. Lett. 1975, 35, 105; [28c] M. A. J.Rodgers, Chem. Phys. Lett. 1981, 78, 509; [28d] P. Bilski,R. Dabestani, C. F. Chignell, J. Phys. Chem. 1991, 95,5784; [28 e] L. E. Cramer, K. G. Spears, J. Am. Chem. Soc.1978, 100, 221.

[29] P. V. Kamat, M. A. Fox, J. Phys. Chem. 1984, 88, 2297.

Figure 9. Changes in the A1/A2 ratio in an aqueous solution ofPNIPAM-VBC/RB as a function of SDS concentration.


[30] [30a] K. K. Rohati, G. S. Singhal, J. Phys. Chem. 1966,70, 1695; [30b] I. L. Arbeloa, J. Chem. Soc., FaradayTrans. 2 1981, 77, 1725; [30 c] I. L. Arbeloa, J. Chem.Soc., Faraday Trans. 2 1981, 77, 1735; [30d] K. Kemnitz,K. Yoshihara, J. Phys. Chem. 1991, 95, 6095; [30e] K. K.Rohatgi, A. K. Mukhopadhyay, Photochem. Photobiol.1971, 14, 551; [30f] O. Valdes-Aguilera, D. C. Neckers,Acc. Chem. Res. 1989, 22, 171.

[31] F. M. Winnik, Macromolecules 1990, 23, 233.[32] H. Ringsdorf, J. Venzmer, F. M. Winnik, Macromolecules

1991, 24, 1678.[33] S. N. Gupta, S. M. Linden, A. Wrzyszczynski, D.C. Neck-

ers, Macromolecules 1988, 21, 51.[34] B. Stevens, J. A. Ors, M. L. Pinsky, Chem. Phys. Lett.

1974, 27, 157.[35] F. Wilkinson, J. G. Brummer, J. Phys. Chem. Ref. Data

1981, 10, 809.

[36] R. Young, K. Wehrly, R. L. Martin, J. Am. Chem. Soc.1971, 93, 5774.

[37] E. Gamdin, Y. Lion, A. Van de Vorst, Photochem. Photo-biol. 1983, 37, 271.

[38] A. P. Schaap, A. L. Thayer, K. A. Zaklika, P. C. Valenti, J.Am. Chem. Soc. 1979, 101, 4016.

[39] J. Paczkowski, D. C. Neckers, Macromolecules 1985, 18,2412.

[40] [40a] A. M. Braun, E. Oliveros, Pure Appl. Chem. 1990,62, 1467; [40 b] P. Murasecco-Suardi, E. Gassmann, A. M.Braun, E. Oliveros, Helv. Chim. Acta 1987, 70, 1760.

[41] [41a] J.J. Lamberts, D.R. Schumacher, D.C. Neckers, J.Am. Chem. Soc. 1984, 106, 5789; [41b] J. J. Lamberts, D.C. Neckers, Tetrahedron 1985, 41, 2183.

[42] See for example the review: E. D. Goddard, Colloids Surf.1986, 19, 225 and references therein.

[43] S. S. Berr, J. Phys. Chem. 1987, 91, 4760.

Documents

Polymeric Photosensitizers, 5. Synthesis and Photochemical Properties of Poly[(N-isopropylacrylamide)-co-(vinylbenzyl chloride)] Containing Covalently Bound Rose Bengal Chromophores