the effect of c-cyclodextrin addition in the self-assembly ......the effect of c-cyclodextrin...

The Effect of c-Cyclodextrin Addition in theSelf-Assembly Behavior of Pyrene LabeledPoly(acrylic) Acid with Different Chain Sizes

TELMA COSTA, J. S. SEIXAS DE MELO

Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal

Received 4 April 2007; accepted 8 October 2007DOI: 10.1002/pola.22480Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The interaction between poly(acrylic acid) polymers (PAA) of low- (2000 g/mol) and high- (450,000 g/mol) molecular weight (Mw) hydrophobically modified withpyrene (PAAMePy) and b- and c-cyclodextrins (b-CD, c-CD) was investigated with flu-orescent techniques. The interaction with b-CD promotes little variation in the spec-tral and photophysical behavior of the polymer, whereas significant changes areobserved upon addition of c-CD. The degree of inclusion (between the pyrene groupsof the polymer and the cyclodextrins) is followed through the observation of thechanges in the absorption, excitation (collected in the monomer and excimer emissionregions) and emission (IE/IM ratio) spectra and from time-resolved data. Within thestudied range of c-CD concentration, the fluorescence decays of the long chain (highMw) PAAMePy polymers were found tri-exponential in the monomer and excimeremission regions in agreement with previous studies. In the case of the low Mw PAA-MePy polymers, tri-exponential decays were observed at the monomer and excimeremission wavelengths. However, when a c-CD concentration of 0.01 and 0.03 M isreached for, respectively, the low- and high-labeled pyrene short chain (low Mw) poly-mers, the fluorescence decays in the excimer region become biexponential (two exci-mers) with no rising component, thus showing that all pyrene groups are encapsu-lated (and preassociated) into the c-CD cavity. In the case of the high Mw polymers,the addition of c-CD has been found to change the level of polymer interaction frompure intramolecular (water in the absence of cyclodextrin) to a coexistence of intra-with intermolecular interactions. VVC 2008 Wiley Periodicals, Inc. J Polym Sci Part A: PolymChem 46: 1402–1415, 2008

Keywords: cyclodextrins; excimer; fluorescence; kinetics (polym.); photophysics;poly(acrylic acid); polymer photophysics; pyrene

INTRODUCTION

Hydrophobically modified polymers (HMP) arewater-soluble polymers modified with hydropho-bic moieties, which can be randomly distributed

or located in precise locations (usually in theterminal positions) of the polymer chain.Because of their capacity to form hydrophobicmicrodomains by inter- and intramolecularinteraction, HMP are very important in severalindustrial applications, for example, surfacemodification, rheology control, and dispersionstabilization.1,2 Because of its unique character-istics, long singlet lifetimes, excimer formation,vibronic structure dependence with polarity,2–4

Correspondence to: J. S. Seixas de Melo (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 1402–1415 (2008)VVC 2008 Wiley Periodicals, Inc.

1402

pyrene has a widespread use as hydrophobicmoieties in the colloidal domain. One of themost useful relations is the excimer-to-monomerratio (IE/IM), which can be directly related withmovements of the polymer where the probe isfound tagged to, and can be directly relatedwith the conformation of the polymer chain insolution. Additionally, the vibronically resolvedstructure of the monomer emission (i.e., the ra-tio between the first and third vibronic band ofmonomer emission, I1/I3 ratio) exhibits high sen-sitivity to changes in the polarity of the localenvironment felt by the chromophore and hasbeen extensively used as a polarity scale.5–13

Previous works showed that the intra-7,14–16

and intermolecular17–22 interactions induced bythe hydrophobes attached to the polymer chainare very dependent on the pH, solvent, tempera-ture, and presence of amphiphilic molecules,23–25 such as surfactants, block copolymers,26 andcyclodextrins (CDs).22 Addition of CDs to hydro-phobically modified polymers solutions have alsoshown to dramatically change the intramolecu-lar association behavior of the polymers.27

CDs have been widely used in pharmaceutical

science, catalyses, pesticides, foods, cosmetics, and

other areas.27–32 They are macrocyclic oligomers of

a-D-glucose and shaped like truncated cones withthe primary hydroxyl rim of the cavity openinghaving somewhat reduced diameter when com-pared with the hydroxy rim. The exterior of theCD is fairly polar and its interior non polar.31,33,34

This characteristic makes them of particular in-terest because of their ability to form host:guestcomplexes with different guest species.35,36 Thedriving forces involved in the inclusion complexa-tion of CD should include electrostatic, van derWaals, and hydrophobic interactions, togetherwith hydrogen bonding and charge-transfer inter-actions.34 The three most common CDs are: a-CD(6 glucose units, 4.5 Å), b-CD (7 glucose units, 7.8Å), and c-CD (8 glucose units, 9.5 Å).27,31,34

In aqueous solution, it is known that CDsform inclusion complexes with hydrophobic mol-ecules, such as pyrene and naphthalene, andamphiphilic molecules, such as surfactants37

and hydrophobically modified polymers.22,38,39

Inclusion guest:CD complexes of 1:1, 1:2, 2:1,and 2:2 have been reported. Several studieswere performed involving pyrene and c-CD. Twopyrene units are able to go inside the CD cavityforming a 2:1 (Py:c-CD) inclusion complex withthe appearance of the structureless long-wave-length excimer emission band.27,28,35,40,41

EXPERIMENTAL

Synthesis and Materials

The synthesis procedure of hydrophobic modifi-cation of the four poly(acrylic acid) polymerswith 1-pyrenylmethylaminehydrochloride is de-scribed in ref. 42. The poly(acrylic acid) poly-mers (PAA) with nominal weight Mn ¼ 2000 and450,000 g/mol (Aldrich), 1-pyrenylmethylaminehydrochloride (Aldrich), and all reagents andsolvents used were obtained from commercialsources and used as received. The pyrene (Py)labeling content was determined by UV spec-troscopy by comparison with the parent com-pound 1-pyrenylmethylamine (e ¼ 37,070 M�1cm�1 in methanol). The nomination of the poly-mers was based on the molecular weight of thePAA polymer and on the Py content of the poly-mers. The polymers investigated here are de-noted as PAAMePy(2)52, PAAMePy(450)53, PAA-MePy(2)77, and PAAMePy(450)87, where 52, 53,77, or 87 corresponds to number of PAA mono-mer units per Py chromophore, respectively, and2 and 450 stands for the 2000 and 450,000 g/molpolymers, respectively (Scheme 1).

Beta (Fluka) and gamma cyclodextrins(Aldrich) were used as received.

Scheme 1. Relative compositions (PAA versus pyrene unimer units) of polymersused and their corresponding acronyms.

SELF-ASSEMBLY BEHAVIOR OF PYRENE-LABELED PAA 1403

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Water used for the polymer solutions wastwice distilled and passed through a Millipore ap-paratus. The measured pH values were obtainedwith a Crison micropH 2000 and adjustments ofthe hydrogen ion concentration of the solutionswere made with dilute HCl solutions. The chro-mophore concentration of the aqueous PAAMePysolutions ranged from 10�5 to 10�6 M, which iswell below the critical value for coil overlap, c*.43

Absorption and Fluorescence Measurements

Absorption and fluorescence spectra wererecorded on Shimadzu UV-2100 and Jobin-YvonSPEX Fluorog 3-22 spectrometers, respectively.All the fluorescence spectra were corrected forthe wavelength response of the system. Fluores-cence decays were measured using a home-builtTCSPC apparatus as described elsewhere,44

except that as excitation source, a Horiba-JI-IBHNanoLED, kexc ¼ 339 nm, was used. The fluores-cence decays were analysed using the modulatingfunctions method of Striker.45 Temperature con-trol was achieved using a home-built systembased on cooled nitrogen and electric heating.

RESULTS

PAAMePy(2) and PAAMePy(450) Polymers inAqueous Solution

Previous studies on the PAAMePy(2) and PAA-MePy(450) polymers in aqueous solution showedthat their behavior is dependent on the pH, sol-vent, degree of labeling and size of the PAAchain.18,21,42,46 The absorption and fluorescencespectra (excitation and emission) displays thecharacteristic pyrene bands, found to be pH de-pendent.42 In the case of the fluorescence spec-tra, in addition to the vibronically resolvedmonomer (with maxima �374 nm) band, thestructureless excimer band (with maxima cen-tered at �480 nm) is also visible.

With the long chain polymers [PAAMePy(450)], the excimer-to-monomer (IE/IM) ratiodecreases in the alkaline region, which wasshown to be a consequence of the progressiveionization of the carboxylic (COOH) groups inthese, and related, PAA polymers labeled withfluorescent probes.17,21,24,25,42,43,47 This happensbecause the electrostatic repulsion between thecarboxylate (COO�) groups expands the polymerchain, affecting the distance between adjacentpyrene groups with less excimer (dynamic andstatic) being formed at higher pH values. In the

case of the short chain PAAMePy polymers anopposite trend in the variation of the IE/IM withpH was observed42,48 and explained on the basisof the adoption of a ‘‘micelle-like’’ conformationby the PAAMePy(2) polymers creating an‘‘hydrophobic’’ core where the pyrene groupscould be located.42 Similar formation of core-shell micelles was also observed with ionic poly-urethanes labeled with pyrene.49

Addition of c-Cyclodextrin to Aqueous Solutionsof PAAMePy(2) and PAAMePy(450)

Absorption Spectra

Figure 1(A) shows the variation of the absorp-tion spectra for PAAMePy(2)77 and PAA-MePy(450)87 polymers (top and bottom panels,respectively) with c-cyclodextrin (concentrationof c-CD ranging from � 0.005 to � 0.08 M), atpH ¼ 3.5. From the observation of Figure 1(A),where the PAA polymer concentration is con-stant, it can be seen that the gradual addition ofc-CD leads to significant changes in the absorp-tion bands. For the four studied polymers, thePA parameter (defined as the peak-to-valley ra-tio)17 decreases with the addition of c-CD [seeinsets in Fig. 1(B)]. The differences observed inthe S0 ? S2 transition indicate contributionsfrom the absorption of ground-state dimers(GSD; a band that is buried underneath themore intense pyrene monomer band).17 In addi-tion to the changes in the relative optical den-sity values, the long chain PAAMePy(450) poly-mers also display a blue shift from 345.6 to344.2 nm on going from low to high c-CD con-centrations. However, in the case of the shortchain polymers this shift is not as pronouncedas with the long chain polymers.

Fluorescence (Excitation and Emission) Spectra

Additional information concerning the level ofGSD comes by comparison between the excita-tion fluorescence spectra collected at the mono-mer and excimer emission wavelengths.2,17 Thedifferences consist of a more or less pronounceddisplacement of the wavelength maximum of thetwo spectra (Dk2); differences in the peak-to-val-ley ratio relative to the (0,1) transition observedin the monomer (PM) and excimer (PE) excita-tion spectra, respectively; and also from a signif-icant broadening of the S0 ? S1 absorptionband, leading to a more pronounced absorptionin the red part of the spectra.2,17,18,50,51 When

1404 COSTA AND SEIXAS DE MELO


GSDs are absent, i.e., when the diffusive en-counter between two pyrene molecules (one inthe ground-state with one in the excited state)is the only viable route for excimer formation (apurely dynamic process) then the two excitationspectra must superimpose.2,17,42 However, ascan be seen from Figure 1(B) (for low and highc-CD concentration), this is not what happenswith the excitation spectra of the PAAMePy pol-ymers. Moreover the differences between thespectra collected at kem ¼ 375 nm and kem ¼ 520nm become clearly more pronounced with theaddition of c�CD; these differences are mirroredby the increase in the PM � PE values [seeinsets in Fig. 1(B)].

The addition of b-CD (0.0005–0.008 M) to thePAAMePy polymers solution promotes nochanges on the absorption spectra and IE/IM ra-tio (data not shown). This clearly establishesthat in this case, as expected, the b-CD onlyallows the inclusion of a single pyrene unit (for-mation of a 1:1 inclusion complex) inducingminor changes in the photophysical properties ofthe PAAMePy polymers.

On the other hand, the addition of c-CD clearlyleads to changes in the emission spectra of thePAAMePy polymers mirrored by the gradual in-crement of the excimer emission (Fig. 2, left pan-

els). The variation of the IE/IM ratio with c-CD isalso presented in Figure 2 (middle hand panels),where it can be observed that this ratio increaseswith the concentration of c-CD. With the PAA-MePy(2) polymers the IE/IM ratio is constant upto a certain c-CD concentration. However, thisbehavior is not observed with the long-size chainPAAMePy(450) polymers. Indeed, with the PAA-MePy(450) polymers, the IE/IM ratio graduallyincreases, never reaching a plateau. It is worthnoting that the increase in the excimer emission,which occurs with the short and long PAA chainpolymers, is also accompanied by a � 6–7 nmblue-shift of the emission wavelength maximum.Moreover, from the dependence of the IE/IM ratiowith the excitation wavelength (kexc), qualitativeinformation on the presence (and level) of GSDcan also be obtained. In fact, from Figure 2(middle hand panels) it can be seen that the IE/IMratio increases both with the c-CD concentrationand with the kexc, showing an increment ofground-state association.

Inter- and intrapolymeric interactions are apriori possible to occur with the PAA polymersinvestigated. However, in water at the low con-centrations used,

that these began to play a determinant role (10g/L).24 Nonetheless, the absence of intermolecu-lar interactions was verified for these polymersin water.42 With the addition of c-CD new inter-actions could now be present, including poly-mer–polymer intermolecular interactions. Tocheck this hypothesis, a study on the depend-ence of the IE/IM ratio with the polymer concen-tration, at fixed c-CD concentration, was per-formed (Fig. 3). In the case of the PAAMePy(2)polymers, to a constant c-CD concentration (0.03M), the increase in the polymer concentrationleads to nonsignificant changes in the IE/IM ra-tio, a clear indication of the absence of (new)interpolymer interactions. On the other hand, inthe case of the PAAMePy(450) polymers, andcontrary to the behavior found in water,17,42 theincrease of polymer concentration leads to anaugment of the IE/IM ratio. In the case of themore (pyrene) labeled polymer, PAAMePy(450)53, this behavior can be observed for all c-CDconcentrations (lowest c-CD concentration is 0.5mM). The dependence of the IE/IM ratio with thepolymer concentration gains gradual importance

with the incremental addition of c-CD, as can beseen from the inset in Figure 3(B). For the low-labeled polymer, PAAMePy(450)87, the depend-ence of the IE/IM ratio versus polymer concentra-tion was only detected at c-CD concentrationshigher than � 2.5 mM of c-CD. Interpolymericinteractions promoted by the presence of c-CDwere also observed with the analogue PAA-MePy(150)55 polymer; a PAA polymer with anintermediate length chain (Mw ¼ 150,000 g/mol)between the two PAA polymers investigatedhere.22 From these results, it seems fair to con-clude that the presence of inter- (in addition tothe existent intra-) molecular interactions, pro-moted by the presence of c-CD, depend on thehydrophobicity, that is to say on the molecularweight and degree of labeling, of the polymer.

Time-Resolved Fluorescence

Figure 4(A) shows the fluorescence decays (fittedwith an individual analysis procedure42), for theshort chain PAAMePy(2)77 polymer with [c-CD]¼ 0.006 M, at pH ¼ 3.5. The fluorescence decays

Figure 2. Normalized emission fluorescence spectra (kexc ¼ 335 nm) of the (A) PAA-MePy(2)77 and (B) PAAMePy (450)87 polymers with different concentrations of c-CD, at pH ¼ 3.5 (left panels). Variation of the IE/IM (middle panels) and I1/I3 (rightpanels) ratios, obtained with three different excitation wavelengths as a function ofc-cyclodextrin concentration at pH ¼ 3.5.



collected at the monomer (375 nm) and excimer(520 nm) emission wavelengths were fitted withsums of three exponentials although providingfour distinct decay times, which were attributedto the emission of free monomers (longest decaytime, s0), monomers that are able to form exci-mer (shortest decay time, s1), and two conforma-tional different excimers (with the middle decaytimes, s2 and s3).

22,42 As a consequence, the ki-netic scheme introduced in refs. 22,42 andbriefly presented in the ‘‘Discussion’’ section (seebelow), for these polymers in aqueous solutionsis still valid in the presence of c-CD, with theimportant exception of the short chain polymersat high concentrations of c-CD. Indeed, for thePAAMePy(2) polymers, and for c-CD valueshigher than 0.01 M [PAAMePy(2)77] and 0.03 M[PAAMePy(2)52], the fluorescence decaysbecome monoexponential at the monomer emis-sion wavelength and biexponential at the exci-mer emission wavelength [see Fig. 4(B)].

DISCUSSION

Absorption and Steady-State Fluorescence Data

As it was previously described for the analogousPAAMePy(150) polymers (PAA with Mw ¼ 150kg mol�1), the presence of c-CD promotes signifi-cant changes in the absorption spectra of thesepolymers, with an inversion of the vibronicmodes relative to the S2 / S0 transition [Fig.1(A)],22 giving support for the inclusion of two

pyrene units in the c-CD cavity.27 In Figure 1(B)(insets) where the PA parameter is plotted as afunction of c-CD it can be seen that the PA valuefor the short chain polymers varies between 2.1and 1.5, while for the long chain polymers itvaries between 1.8 and 1.0. The observed grad-ual decrease of PA with the addition of c-CD isindicative of a gradual increment of preassoci-ated pyrene contribution. It is worth noting thatPA values close to 3.0 give support for the ab-sence of ground-state association whereas lowervalues are indicative of preassociated pyrenedimers.2,17

The excitation spectra provide additionalqualitative information on the presence of GSD.The spectra collected at the monomer and exci-mer emissions are very different over the wholerange of c-CD concentrations [see Fig. 1(B)]. Infact, the increase in the difference between thetwo spectra (increase of PM � PE value) agreeswell with the results obtained from the absorp-tion spectra (PA parameter). This means that athigh c-CD concentrations the contribution ofGSDs increases, which is also mirrored by theincrease of the excimer emission band [Fig.1(B)]. Moreover in Figure 2 (middle hand pan-els), it is shown that the IE/IM ratio increaseswith the excitation wavelength since GSD pre-dominantly absorb at longer wavelengths.17 Asmentioned in the ‘‘Introduction’’ section, the c-CD cavity can accommodate two pyrene (Py)units in a dimeric form.35 The observed increasein the ground-state association is due to theirpreferred location inside the CD cavity.

With the PAAMePy(2) polymers, the IE/IM ra-tio increases up to [c-CD] ¼ 0.01 and 0.03 M for,respectively, the low [Fig. 2(A), middle panels]and high-labeled polymers (data not shown),reaching a plateau thereon; this indicates thatall available Py groups are now found inside ac-CD cavity forming 1:2 or 1:1 c-CD:Py com-plexes. Consequently, the addition of cyclodextrinclearly favors Py-Py interactions. In contrast, inthe case of the long chain PAAMePy(450) poly-mers this plateau is never reached, and dynamicexcimer formation is observed. Since the numberof c-CD is also higher than the number of pyreneunits, the results suggest with the long chainPAAMePy(450) polymers the conformationacquired precludes the complete encapsulation ofall pyrene groups. Moreover and since with thePAAMePy(450) polymers intermolecular pyrene–pyrene interactions are induced by the presenceof c-CD it is likely that new conformations

Figure 3. Fluorescence spectra, normalized at themonomer emission maxima, for the (A) PAA-MePy(2)52 and (B) PAAMePy(450)53 polymers at var-ious concentrations, at pH ¼ 3.5, T ¼ 293 K. Shownas insets are the different IE/IM ratio values versusthe (pyrene) O.D. at 335 nm in the presence of a con-stant c-CD concentration.



adopted by the polymer will prevent the totalencapsulation of all pyrene groups. It is worthnoting that other order inclusion complexes,namely 2:1 or 2:2 c-CD:Py, are not viable since inthe case of the PAAMePy polymers one side ofthe pyrene chromophore is bound to the PAAchain (Scheme 1).

Another important parameter providing rele-vant information on the system is the I1/I3 ratio(ratio of the fluorescence intensity of the first[S0(v ¼ 0) / S1(v ¼ 0)] and the third [S0(v ¼ 1)/ S1(v ¼ 0)] vibronic bands) which reflects thepolarity of the local medium surrounding py-rene.5,11 For the PAAMePy(2) polymers, the I1/I3ratio shows a slight decrease with the c-CD con-centration, i.e., from 1.78 to 1.76 [Fig. 2(A), leftpanels] and from 1.82 to 1.77 (data not shown)for the low and highly labeled polymers, respec-tively, i.e., small variations are observed in thecase of the PAAMePy(2) polymers. It is worthremember that in aqueous solution the PAA-MePy(2) polymers adopt a micelle-like conforma-tion, protecting the pyrene groups from the con-tact with water.42 The addition of c-CD leads tothe encapsulation of the pyrene groups (present

both as free monomers or as dimers), leading toa change in this micelle-like structure.42 Adetailed conductivity study of the PAAMePy poly-mers:c-CD interaction, at different pH values,is currently under investigation and will be thesubject of a forthcoming publication. However,preliminary conductivity measurements (datanot shown) clearly show a change on the molarconductivity of the PAAMePy(2) polymers uponaddition of c-CD, i.e., the molar conductivityincreases with the formation of the Py:c-CDcomplex. This means that the polyacrylate chainbecomes more effective as charge carrier, whichconstitutes a further evidence of the change inthe structure adopted by these polymers inaqueous solution,52 in line with the proposed mi-celle-like structure.42

It is noteworthy that the dimensions of thePy groups (10.4 Å long) relative to the c-CD cav-ity (7 Å long) show that the Py group cannot betotally inside the c-CD cavity,28,35 remainingpartially in contact with water; as a conse-quence, the environment probed by pyrene israther polar even at high CD concentration. Forthe long size chain polymers the I1/I3 ratio pro-

Figure 4. Individual analysis of the fluorescence decays at the monomer (kem¼ 375 nm) and excimer (kem ¼ 520 nm) emission wavelengths, for the PAAMePy(2)77polymer in the presence of (A) [c-CD] ¼ 0.006 M and (B) 0.04 M of c-CD, at pH ¼ 3.5and kexc ¼ 339 nm.



gressively decreases from 1.77 to 1.62 and from1.75 to 1.58 for the low [PAAMePy(450)87] andhigh [PAAMePy(450)53] polymers, respectively.This shows that pyrene is sensing different po-larity microenvironments [see Fig. 2(B) left pan-els]. In aqueous solutions the PAAMePy(450)polymers display I1/I3 ratio values at pH ¼ 3.5equal to � 1.78 [PAAMePy(450)87] and �1.75[PAAMePy(450)53].42 These values are close tothose obtained at low c-CD concentrations show-ing that most of the Py groups are still in con-tact with water. At high c-CD concentrations theI1/I3 ratio approaches values around 1.62 and1.58, for PAAMePy(450)87 and PAAMePy(450)53, respectively, indicating that pyrene isexperiencing a less polar environment, since thislast value is close to the I1/I3 value found for py-rene in glycerol (I1/I3 ¼ 1.60).6 These pronouncedchanges in the environment probed by the Pygroups in PAAMePy(450) when compared withthe PAAMePy(2) polymers reveal the differentconformational nature adopted by the polymerchains resulting from interaction with c-CD.

From the Benesi-Hildebrand equation, the I1/I3 ratio has been used

28,53,54 as a way to deter-mine the stability constant53,54 of molecularcomplexes (involving CDs) and of the involvedstoichiometry.28,54 In this study the presence of

excimer emission tail beneath the monomer bandmay have implications in the relative vibronicI1/I3 ratio. As a consequence, the Benesi-Hilde-brand equations (eqs 1 and 2) have been used interms of the IE/IM ratio versus [CD]. For the 1:1Py:c-CD complex, the equation is given by:28,54

1

R� R0 ¼1

K1ðR� R0Þ½CD�0þ 1

R�R0 ð1Þ

where R and R0 are the IE/IM ratio in aqueoussolution in the presence of CD and K1 is theequilibrium constant for the 1:1 complex. Forthe 2:1 Py:c-CD complex, and assuming that[1:2] > [1:1], the equation is now given by:28

1

R� R0 ¼1

K2ðR� R0Þ½CD�20þ 1

R�R0 ð2Þ

where K2 is the equilibrium constant for the 2:1complex. The Benesi-Hildebrand plots for the1:1 and 2:1 Py:c-CD complexes are presented inFigure 5 (left and right panels, respectively) forthe PAAMePy(2)77 and PAAMePy(450)53 poly-mers. From Figure 5 it can be observed that, forthe two polymers, no linear trend is observedfrom the 1/(R � R0) versus 1/[c-CD] plot. How-ever, when 1/(R � R0)2 versus 1/[c-CD]2 is plot-

Figure 5. Benesi-Hildebrand plots for the (A) PAAMePy(2)77 and (B) PAAMePy(450)53polymers as a function of the 1/[c-CD] (right panels) and 1/[c-CD]2 (left panels).



ted a clear linear relationship is obtained, indic-ative of 2:1 Py:c-CD complexes. Nevertheless,from time-resolved fluorescence data coexistenceof 2:1 with 1:1 complexes is detected since anincrease of the decay time value associated withthe free monomers with the incremental additionof c-CD is observed (see Fig. 6 and Time-ResolvedFluorescence Data—‘‘Discussion’’ section).

As mentioned in the ‘‘Results’’ section, theaddition of b-CD to the PAAMePy polymersinduces minor changes in their photophysicalproperties. This behavior was previouslyobserved in similar PAAMePy polymers withmolecular weight of 150,000 g mol�1.22 Differentresults were obtained by Haldar et al.27 wherefor a pyrene-end-capped poly(ethyleneoxide)polymer and by addition of b-CD a decrease ofthe IE/IM ratio was found; a logical consequenceof the encapsulation of a single pyrene with con-sequent decrease in excimer formation. The ran-dom nature of the pyrene labeled polymersinvestigated here is responsible for a more rich(complex) kinetics with a clear departure fromthe kinetic formalism involving two species.With the PAAMePy polymers, free and MAGREmonomers coexist with two conformationally dif-ferent excimers, that is a 4-species system.42

Indeed, it is reasonable to consider that b-CDpreferentially forms 1:1 inclusion complexeswith the free monomers; monomers which eitherin a globule or expanded conformation are moreexposed and available to interact with the CDthan the MAGRE monomers are, since these arefound in a more hydrophobic environment andconsequently less available to interact with theCDs. As a consequently and in view that the b-CD essentially encapsulates the free monomers,there are no significant changes in the observedphotophysical parameters. In terms of thesteady-state data, the formation of inclusioncomplexes with b-CD is confirmed by the slightdecrease in the I1/I3 ratio; for the short chainpolymers the I1/I3 ratio decreases from 1.83 (inwater) to 1.78 (at high concentrations of CD),whereas for the long chain polymers it variesbetween 1.78–1.67 and 1.75–1.70 (low and high-labeled polymers respectively). By comparingthe variation of the I1/I3 ratio for the PAA-MePy(2) polymers in the presence of b-CD andc-CD, an identical variation is observed, due tothe fact that most of the labeled PAAMePy(2)polymers are single-labeled,42 i.e., form 1:1Py:CD inclusion complexes. Application of eqs 1and 2 to the PAAMePy polymers in the presence

of b-CD revealed to be inconclusive, since in thiscase no significant changes in the photophysicalproperties of the polymers were observed.

Time-Resolved Fluorescence Data

Figure 6 shows the dependence of the decaytimes and pre-exponential factors with the c-CDconcentration for the PAAMePy(450)87 andPAAMePy(2)52 polymers. The increase in c-CD

Figure 6. Fluorescence decay times (si) and preex-ponential factors at 520 nm (a2j) as a function of c-CDconcentration for (A) PAAMePy(450)87 and (B) PAA-MePy(2)52 polymers.



concentration leads, for the two polymers, to anincrease of the decay time values, a situationmore evident for the longest decay time, s0,associated with the isolated chromophores. Theraise on the decay time values with c-CD isrelated to the formation of 1:1 and 2:1 Py:CDinclusion complexes. The gradual inclusion of Pygroups into the CD cavity avoids the exposureto oxygen (an efficient fluorescence quencher),giving rise to higher fluorescence lifetime val-ues. This is particularly perceptible for thedecay times of the isolated chromophores (s0).Indeed, the presence of this decay time even athigh concentrations of CD confirms the forma-tion 1:1 Py:CD inclusion complexes.

The decay times s2 and s3 are associated tothe existence of two conformational differentexcimers. The existence of two excimers in py-rene-based systems has been observed witholigomers55–58 and polymers22,42 where thegrafting of the pyrene units is made at the posi-tion 1 of the probe. Asymmetric and symmetricsandwich-like structure are associated, respec-tively, to the shorter (s2) and longer decay time(s3) excimers.

42,57

It is also worth comment on the variation ofthe preexponential factors for the PAA-MePy(450) polymers in Figure 6(A). The a10 pre-exponential factor, associated with the fractionof isolated chromophores,17,42,43,47,48,59 decreaseswith the addition of c-CD. The presence of c-CDleads to an increase of the hydrophobic sites insolution (preferential location for the pyrenegroups). Since the c-CD cavity can accommodatetwo pyrene groups, the gradual encapsulation ofthese into the same c-CD cavity leads to moreformation of GSDs and less free monomers. It isonce more worth noting that because of theirlocation in a low-labeled region of the polymerchain, pyrene groups that are not able to formexcimer—the free monomers—are still present,leading to the formation of 1:1 c-CD:Py inclusioncomplexes. Moreover, with the addition of c-CD,the preexponential factor a21 becomes less nega-tive and the sum of the preexponential factorsat the excimer emission wavelength, a2j, gradu-ally approaches 1. However, even at high c-CDconcentration, a small negative preexponentialvalue is still observed. With the previouslyinvestigated PAAMePy(150)55 polymer,22 with aPAA chain length three times smaller than PAA-MePy(450)53 and identical degree of Py labeling,a negative pre-exponential was also observed.This suggests that this polymer behaves in many

ways similarly to the long chain PAAMePy(450)polymers and that, in both cases, some pyrenegroups are not involved in the inclusion complexformation and are still able to form excimerthrough a dynamic route.22 This behavior is op-posite to that found with the short chain polymer(as will be discussed later) where no rise timewas observed.

The addition of c-CD leads to a decrease andto an increase of, respectively, the preexponen-tial factors a22 and a23 (associated with the con-tributions of the excimers with asymmetric andsymmetric sandwich-like geometry, respec-tively). This supports the preferential encapsula-tion of the symmetric sandwich-like pyrene exci-mer. By using induced circular dichroism (ICD)Kano et al.40,60 and Kobayashi et al.61 havestudied the interaction of pyrene, and of 1,3-di(2-pyrenyl)propane, with c-CD. Based on thechange of the sign in the ICD spectrum it wasconcluded that the dimer formed inside the c-CD cavity had an asymmetric configuration, inapparent contrast with our results. However,the study with pyrene or pyrene-like (small)molecules clearly contrasts with pyrene-labeledpolymers, which introduces an additional degreeof complexity to the system.

With the PAAMePy(2) polymers, the decaytimes and preexponential factors change with c-CD follow the same tendency as observed withthe long chain PAAMePy(450) polymers (see Fig.6). However, the most significant finding regard-ing the interaction between the PAAMePy(2)polymers and c-CD is the fact that up to 0.01and 0.03 M, the fluorescence decays becomemono and biexponential at, respectively, themonomer and excimer emission wavelengths. Atthe monomer emission wavelength the decaytime is associated to the emission of free mono-mers whereas at the excimer emission wave-length the two conformational different excimersare now the (only) emissive species. It is impor-tant to stress that the decay time associatedwith MAGRE monomers (s1) has now vanishedand consequently the rise-time at the excimeremission is also absent at high c-CD concentra-tions. This gives support to the fact that the twoexcimers are preformed in the ground-state. Atlow c-CD concentrations, the presence of a nega-tive preexponential at the excimer emissionwavelength indicates that the pyrene groups areall not encapsulated by the c-CD. With the grad-ual addition of c-CD the a21 preexponential fac-tor becomes less negative (decrease of dynamic



excimer contribution) and above 0.01 or 0.03 M[PAAMePy(2)77 or PAAMePy(2)52, respectively]the sum of the preexponential factors at 520 nmbecomes equal to 1 which eventually shows thatall pyrene groups are now involved in the forma-tion of inclusion complexes with the c-CD.

Energetics

The dependence of the IE/IM ratio with tempera-ture can be presented by the so-called Stevens-Ban plots,43,62 from which the values for the acti-vation energy formation (slope of the low-temper-ature limit regime, LTL), Ea, and the enthalpy(slope of the high-temperature limit regimeHTL), DH, of the excimer are obtained.43 How-ever, as can be seen in Figure 7, the LTL is ill-defined for PAAMePy(2)52, at pH ¼ 3.5, since thetransition temperature between the LTL andHTL regimes (T*)4,43 is found at 28–30 8C. Conse-quently, what comes out from these plots isresumed to the enthalpy of excimer formation,with values of �23 and �13.7 kJ mol in purewater and in the presence of 0.03 M c-CD, respec-tively. Although the interpretation of these plotsis not as straightforward as it is in a Stevens-Banplot associated with a generic Birks’ kinetics,4 asignificant decrease in the binding energies ofthe excimers is observed upon addition of c-CD.In water, a predominance of the twisted-like con-

formation (more stable) over the parallel sand-wich-like (less stable) conformation (a22 > a23) isobserved.42 For [c-CD] ¼ 0.03 M the a22 and a23preexponential factors are equal, showing thatthe less stable excimer gains weight over themore stable excimer. This is most likely becauseof restrictions imposed by the polymer chain andby the presence of c-CD. From the above, it canbe concluded that the decrease in the absolutevalue of DH is related with the increase in thefraction of the less stable excimer.

Kinetic Scheme in the Presence of c-CD

As mentioned, the kinetic scheme presentedelsewhere22,42 and depicted in Scheme 2 is stillvalid in the presence c-CD. This occurs with thePAAMePy(450) (over all range of c-CD concen-trations) and PAAMePy(2) polymers (at concen-trations below 0.02 and 0.04 M). However, thedata obtained at (only) two emission wave-lengths is not sufficient to solve this kineticscheme and to determine the association, disso-ciation, and decay rate constants (kai, kdi, kEi,respectively, where i ¼ 1,2) and the fractions ofthe ground-state species present in solution (a,b, and c).42 This study is currently under pro-gress using a procedure and methodology analo-gous to the one used in ref. 63.

Nevertheless and for the short size chain pol-ymers at c-CD concentrations higher than 0.01and 0.03 M -for the low- and high-labeled poly-mers, respectively- the kinetic behaviour is sim-plified and can be described by Scheme 3, whereb (¼a10), a (¼a22), and c (¼a23) are the fractionsof light that excite the free monomers (MA), andthe excimers E1 and E2, respectively.

According to Scheme 3, the time-dependentprofile at the monomer and excimer emissionwavelengths can be described by eqs 3 and 4,respectively.

IMðtÞ ¼ I375nmðtÞ ¼ a10e�k0t ð3Þ

IEðtÞ ¼ I520nmðtÞ ¼ a22e�k2t þ a23e�k3t ð4ÞFigure 7. Plots of ln(IE/IM) versus 1/T obtained forthe PAAMePy(2)52 system with 0 M (n) and 0.03 M(*) of c-CD, at pH ¼ 3.5.

Scheme 2. Kinetic scheme for the PAAMePy poly-mers in water.



where, ki ¼ 1si :Since in Scheme 3, the species are kinetically

uncoupled the decay rate constants are given by:

ki ¼ 1si ð5Þ

The obtained rate constants are presented inTable 1. Up to c-CD concentration of 0.01 M[PAAMePy(2)77] or 0.03 M [PAAMePy(2)52] therate constants can be considered to be constantand independent of the c-CD concentrations.The rate constant (k0) for the free monomers isfound with values that can be found for pyrenesubstituted (in position 1) compounds.55,64,65 Therate constant of the more stable excimer (kE1)has a similar value to those reported for thePAAMePy(2) polymers in organic solvents(dioxane and methanol)22 and also with otherpyrene-labeled polymers.17,66,67 For the PAA-MePy(2) polymers, the rate constants seem to beindependent of the pyrene degree of labeling.This was also observed in organic solvents.42 Itis also interesting to compare the valuesobtained for these two excimers in c-CD cavitieswith analogous studies made with differenthosting systems. In fact, a recent study on theincorporation of pyrene onto the cavities of dif-ferent calix[n]arene (where n gives the diameterof the inner cavity) have showed that with n ¼

6 the dimer displays a lifetime of 65 ns and withn ¼ 8 the value is 130 ns.68 This suggests thatalso in these types of cavities different type ofdimers/excimers could be present and that themore stable excimer (twisted sandwich with alifetime of ca. 60 ns)42,57 is preferentially accom-modated by the lower diameter calixarenewhereas for the less stable excimer (parallelsandwich conformation with a longer lifetime130–150 ns42,57) a calixarene with a higher cav-ity diameter is preferred. The analogy with thestudied system seems evident, with the c-CDcavity accommodating preferentially the lessstable excimer.

CONCLUSIONS

The effect of the addition of c-CD to poly(acrylicacid) polymers with long [PAAMePy(450)] andshort [PAAMePy(2)] chain-size randomly labeledwith pyrene, was investigated with fluorescencetechniques. The addition of c-CD showed to pro-mote intermolecular interactions with the longchain polymers, while absence of interactionwas observed with the short chain polymers.With the two polymers (low and high Mw) theaddition of CD leads to the formation of 1:1 and1:2 c-CD:Py inclusion complexes. The degree ofdynamic versus static excimer formation alsochanges with the concentration of c-CD, withground-state association gradually overcomingexcimer formation from a dynamic mechanism.Nevertheless, with the long chain PAA-MePy(450) polymers, dynamic excimer forma-tion is still present, whereas with the shortchain polymers it vanishes at high c-CD concen-trations. The existence of two excimers with dif-

Table 1. Rate Constants for Excimer Formation with Symmetric (kE1) and Asymmetric (kE2) Configurationsand Free Monomers, k0, for the PAAMePy(2)77 and PAAMePy(2)52 Polymers at Different c-CD Concentrations,at pH ¼ 3.5 and T ¼ 293 K.

Polymer c�CD (M) s0 (ns) k0 (ns�1) sE1 (ns) kE1 (ns�1) sE2 (ns) kE2 (ns�1)

PAAMePy(2)77 0.02 194 0.005 60 0.017 110 0.0090.03 198 0.005 60 0.017 115 0.0090.04 200 0.005 57 0.018 121 0.0080.06 207 0.005 60 0.017 121 0.008

PAAMePy(2)52 0.04 195 0.005 51 0.020 110 0.0090.05 193 0.005 55 0.018 111 0.0090.07 200 0.005 57 0.018 109 0.0090.08 199 0.005 57 0.018 112 0.009

Scheme 3. Kinetic scheme in the presence of c-CD;see text for details.



ferent geometries was also observed from time-resolved fluorescence: twisted sandwich-like ge-ometry (longer lived) and parallel sandwich-likegeometry. The inclusion of two pyrene groups asa dimer into the CD cavity seems to favor thelatter geometry.

The coexistence of the intra- with intermolec-ular polymeric interactions (induced by the pres-ence of c-CD) is found only with the high-molec-ular weight polymers, whereas the low-mole-cular weight polymers only intrapolymericinteractions are present.

Financial support from the Portuguese Science Foun-dation (FCT) through FEDER and POCI is acknowl-edged. T. Costa acknowledges FCT for a Ph. D. grant(SFRH/BD/17852/2004). The authors thank A. Fran-cisco, Dr. S. Gago, and Prof. I. Gonçalves for theirhelp in the synthesis of the labelled polymers.

REFERENCES AND NOTES

1. Jönsson, B.; Lindman, B.; Holmberg, K.; Kron-berg, B. Surfactants and Polymers in Aqueous So-lution; Wiley: Chichester, 1998.

2. Winnik, F. M. Chem Rev 1993, 93, 587–614.3. Gilbert, A.; Baggott, J. Essentials of Molecular

Photochemistry; Blackwell Scientific Publications:Oxford, 1991.

4. Birks, J. B. Photophysics of Aromatic Molecules;Wiley: London, 1970.

5. Karpovich, D. S.; Blanchard, G. J. J Phys Chem1995, 99, 3951–3958.

6. Dong, D. C.; Winnik, M. A. Can J Chem Rev CanChim 1984, 62, 2560–2565.

7. Miguel, M. D.; Burrows, H. D.; Formosinho, S. J.;Lindman, B. J Mol Struct 2001, 563, 89–98.

8. Winnik, F. M.; Winnik, M. A.; Ringsdorf, H.; Ven-zmer, J. J Phys Chem 1991, 95, 2583–2587.

9. Street, K. W.; Acree, W. E. Analyst 1986, 111,1197–1201.

10. Castanheira, E. M. S.; Martinho, J. M. G.;Duracher, D.; Charreyre, M. T.; Elaissari, A.;Pichot, C. Langmuir 1999, 15, 6712–6717.

11. Valeur, B. In Molecular Luminescence Spectros-copy. Methods and Applications; Schulman, S. G.,Ed.; Wiley-Interscience: New York, 1993; Part 3,Chapter 2, pp 25–84.

12. Kalyanasundaram, K.; Thomas, J. K. J PhysChem 1977, 81, 2176–2180.

13. Kalyanasundaram, K.; Thomas, J. K. J Am ChemSoc 1977, 99, 2039–2044.

14. Turro, N. J.; Pierola, I. F. J Phys Chem 1983, 87,2420–2423.

15. Turro, N. J.; Pierola, I. F. Macromolecules 1983,16, 906–910.

16. Miguel, M. D. Adv Colloid Interface Sci 2001, 89,1–23.

17. Seixas de Melo, J.; Costa, T.; Miguel, M. G.; Lind-man, B.; Schillén, K. J Phys Chem B 2003, 107,12605–12621.

18. Turro, N. J.; Arora, K. S. Polymer 1986, 27, 783–796.

19. Arora, K. S.; Turro, N. J. J Polym Sci Part B:Polym Phys 1987, 25, 243–262.

20. Arora, K.; Turro, N. J. J Polym Sci Part A: PolymChem 1987, 25, 259–269.

21. Anghel, D. F.; Alderson, V.; Winnik, F. M.; Mizu-saki, M.; Morishima, Y. Polymer 1998, 39, 3035–3044.

22. Seixas de Melo, J.; Costa, T.; Oliveira, N.;Schillén, K. Polym Int 2007, 56, 882–899.

23. Relogio, P.; Martinho, J. M. G.; Farinha, J. P. S.Macromolecules 2005, 38, 10799–10811.

24. Schillén, K.; Anghel, D. F.; Miguel, M. D.; Lind-man, B. Langmuir 2000, 16, 10528–10539.

25. Anghel, D. F.; Toca-Herrera, J. L.; Winnik, F. M.;Rettig, W.; von Klitzing, R. Langmuir 2002, 18,5600–5606.

26. Costa, T.; Schillén, K.; Miguel, M. D. G.; Lind-man, B.; Seixas de Melo, J. In Coloides e Inter-fases; Salicio, M. M. V.; Moreno, M. D. M., Eds.;Ediciones Universidad Salamanca: Salamanca,2005; pp 313–319.

27. Haldar, B.; Mallick, A.; Purkayastha, P.; Burrows,H. D.; Chattopadhyay, N. Indian J Chem Sect A:Inorg Bioinorg Phys Theor Anal Chem 2004, 43,2265–2273.

28. Delapena, A. M.; Ndou, T.; Zung, J. B.; Warner, I.M. J Phys Chem 1991, 95, 3330–3334.

29. Xu, W. Y.; Demas, J. N.; Degraff, B. A.; Whaley,M. J Phys Chem 1993, 97, 6546–6554.

30. Dyck, A. S. M.; Kisiel, U.; Bohne, C. J PhysChem B 2003, 107, 11652–11659.

31. Connors, K. A. Chem Rev 1997, 97, 1325–1357.32. Szejtli, J. Chem Rev 1998, 98, 1743–1753.33. Zung, J. B.; Delapena, A. M.; Ndou, T. T.; Warner,

I. M. J Phys Chem 1991, 95, 6701–6706.34. Liu, L.; Guo, Q. X. J Inclusion Phenom Macrocycl

Chem 2002, 42, 1–14.35. Yorozu, T.; Hoshino, M.; Imamura, M. J Phys

Chem 1982, 86, 4426–4429.36. Cox, G. S.; Turro, N. J.; Yang, N. C. C.; Chen, M.

J. J Am Chem Soc 1984, 106, 422–424.37. Park, J. W.; Song, H. J. J Phys Chem 1989, 93,

6454–6458.38. Haldar, B.; Mallick, A.; Chattopadhyay, N. J Mol

Liq 2004, 115, 113–120.39. Duan, Q.; Miura, Y.; Narumi, A.; Shen, X.; Sato,

S.; Satoh, T.; Kakuchi, T. J Polym Sci Part A:Polym Chem 2006, 44, 1117–1124.

40. Kano, K.; Matsumoto, H.; Yoshimura, Y.; Hashi-moto, S. J Am Chem Soc 1988, 110, 204–209.

41. Werner, T. C.; Colwell, K.; Agbaria, R. A.; Warner,I. M. Appl Spectrosc 1996, 50, 511–516.



42. Seixas de Melo, J.; Francisco, A.; Costa, T.; Maça-nita, A.; Gago, S.; Gonçalves, I. S. Phys ChemChem Phys 2007, 9, 1370–1385.

43. Costa, T.; Miguel, M. G.; Lindman, B.; Schillén,K.; Seixas de Melo, J. S. J Phys Chem B 2005,109, 11478–11492.

44. Seixas de Melo, J.; Fernandes, P. F. J Mol Struct2001, 565, 69–78.

45. Striker, G.; Subramaniam, V.; Seidel, C. A. M.;Volkmer, A. J Phys Chem B 1999, 103, 8612.

46. Chu, D. Y.; Thomas, J. K. J Am Chem Soc 1986,108, 6270–6276.

47. Costa, T.; Miguel, M. G.; Lindman, B.; Schillén,K.; Lima, J. C.; Seixas de Melo, J. J Phys ChemB 2005, 109, 3243–3251.

48. Seixas de Melo, J.; Francisco, A.; Costa, T.; Gago,S.; Gonçalves, I. S. In Coloides e Interfases; Sali-cio, M. M. V.; Moreno, M. D. M., Eds.; EdicionesUniversidad Salamanca: Salamanca, 2005; pp291–298.

49. Buruiana, E. C.; Buruiana, T.; Strat, G.; Strat, M.J Polym Sci Part A: Polym Chem 2005, 43, 3945–3956.

50. Winnik, M. A.; Bystryak, S. M.; Liu, Z. Q.; Siddi-qui, J. Macromolecules 1998, 31, 6855–6864.

51. Ezzell, S. A.; Hoyle, C. E.; Creed, D.; McCormick,C. L. Macromolecules 1992, 25, 1887–1895.

52. Nilsson, M.; Cabaleiro-Lago, C.; Valente, A. J.M.; Soderman, O. Langmuir 2006, 22, 8663–8669.

53. Almgren, M.; Grieser, F.; Thomas, J. K. J AmChem Soc 1979, 101, 279–291.

54. Indirapriyadharshini, V. K.; Karunanithi, P.;Ramamurthy, P. Langmuir 2001, 17, 4056–4060.

55. Zachariasse, K. A.; Kuhnle, W.; Leinhos, U.; Rey-nders, P.; Striker, G. J Phys Chem 1991, 95,5476–5488.

56. Zachariasse, K. A.; Duveneck, G.; Busse, R. J AmChem Soc 1984, 106, 1045–1051.

57. Zachariasse, K. A.; Duveneck, G.; Kuhnle, W.Chem Phys Lett 1985, 113, 337–343.

58. Zachariasse, K. A.; Duveneck, G.; Kuhnle, W.;Reynders, P.; Striker, G. Chem Phys Lett 1987,133, 390–398.

59. Maçanita, A. L.; Horta, A.; Pierola, I. F. Macromo-lecules 1994, 27, 3797–3803.

60. Kano, K.; Matsumoto, H.; Hashimoto, S.; Sisido,M.; Imanishi, Y. J Am Chem Soc 1985, 107, 6117–6118.

61. Kobayashi, N.; Saito, R.; Hino, H.; Hino, Y.;Ueno, A.; Osa, T. J Chem Soc Perkin Trans 21983, 1031–1035.

62. Stevens, B.; Ban, M. I. Trans Faraday Soc 1964,60, 1515–1523.

63. Seixas de Melo, J.; Maçanita, A. L. Chem PhysLett 1993, 204, 556–562.

64. Duveneck, G. Georg-August-Universität, Göttingen:Göttingen, 1986.

65. Zachariasse, K. A.; Striker, G. Chem Phys Lett1988, 145, 251–254.

66. Kanagalingam, S.; Ngan, C. F.; Duhamel, J. Mac-romolecules 2002, 35, 8560–8570.

67. Prazeres, T. J. V.; Beingessner, R.; Duhamel, J.;Olesen, K.; Shay, G.; Bassett, D. R. Macromole-cules 2001, 34, 7876–7884.

68. Branco, T. J. F.; Ferreira, L. F. V.; do Rego, A. M.B.; Oliveira, A. S.; Da Silva, J. P. Photochem Pho-tobiol Sci 2006, 5, 1068–1077.



the effect of c-cyclodextrin addition in the self-assembly ......the effect of c-cyclodextrin...

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