Volume 58, Number 7, 2004 APPLIED SPECTROSCOPY 8230003-7028 / 04 / 5807-0823$2.00 / 0q 2004 Society for Applied Spectroscopy

Thermodynamics of Complexation of Dimethyl Esters ofTere-, Iso-, and Phthalic Acids with a- and b-Cyclodextrins

ANTONIO DI MARINO and FRANCISCO MENDICUTI*Departamento de Quımica Fısica, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain

Thermodynamics of inclusion of a- and b-cyclodextrins (CDs) withthree diester precursors of polyesters, dimethyl terephthalate(DMT), dimethyl isophthalate (DMI), and dimethyl phthalate(DMP) were studied with different fluorescence spectroscopy tech-niques. The stoichiometry, association constants, and other ther-modynamics parameters were investigated from the fluorescence in-tensity decrease that takes place upon CD addition and temperaturechanges. Apart from DMP, which did not form a complex withaCD, all other systems showed 1:1 stoichiometry complexes withdifferent stabilities. The complexes formed with the DMT guest arethe most stable, whereas the stability constant for DMP : bCD is thelowest. Binding constants are also larger for complexes formed withbCD than for those formed with aCD. Fluorescence polarization,quenching, and lifetime measurements provide information aboutthe structure of the complexes formed. This geometry explains thevalues of enthalpy and entropy changes during complexation. Vander Waals interactions seem to be involved in the formation of thesecomplexes.

Index Headings: Cyclodextrins; Inclusion complex; Fluorescence;Dimethyl terephthalate; Dimethyl isophthalate; Dimethyl phthalate.

INTRODUCTION

Cyclodextrins (CDs) are torus-shaped macroringsmade up of a-glucopyranose units connected by a-1,4links and are well known for their ability to form inclu-sion complexes with low and high molecular weight mol-ecules.1–4 Fluorescence techniques can be used for study-ing complex association when the guest molecule con-tains fluorophores. The stoichiometry, the formation con-stants, and other thermodynamic parameters can beobtained by taking advantage of any change in the fluo-rescence emission intensity,5–11 in the excimer forma-tion,12–15 in the relative intensity of some bands of theemission spectra,16–24 in the fluorescence depolarization,25

or in the decay of fluorescence upon complexation.6,23,26,27

Values of enthalpy and entropy changes upon complex-ation provide information about the driving forces ofcomplexation and the possible arrangement of the com-plexes formed.

Thermodynamic studies on the inclusion of several es-ters containing naphthalene chromophores, such as 2-methyl-naphthalenecarboxylate (MN),20,21 dimethyl 2,6-naphthalenedicarboxylate (DMN),22 and diethyl 2,6-na-phthalenedicarboxylate (DEN)23 with three natural unsub-stituted a-, b-, and gCDs and 2-hydroxypropil-a-, -b-,and -gCDs (HPCDs)24 were carried out by our group.Emission spectra for all systems always showed twobands whose ratio of intensities R was very sensitive tomedium polarity. The binding constants (K), stoichiom-

Received 2 October 2003; accepted 1 March 2004.* Author to whom correspondence should be sent. E-mail: francisco.

mendicuti@uah.es.

etries, and enthalpy and entropy changes upon complex-ation were obtained from the variation of R with the CDconcentration and temperature. MN forms 1:1 complexeswith any of the six CDs. Binding constants for 1:1 com-plexes with unsubstituted CDs are KMN:bCD . KMN:aCD .KMN:gCD. Constants for the complexes of MN with HPCDsfollow a similar trend but they are slightly larger thanthose for complexes formed with their unsubstitutedcounterparts. DMN forms complexes of 1:2 and 1:1 stoi-chiometries with a- and bCDs, respectively. DEN, how-ever, prefers to form 1:2 complexes with both CDs. Thebinding constants for 1:2 complexes are obviously largerthan those for 1:1 complexes. They are also larger forguest : bCD2 than for guest : aCD2 complexes. Quantita-tive differences in binding constants and DH0 and DS0 for1:1 complexes were explained as being due to the dif-ferent cavity sizes, where van der Waals host : guest in-teractions dominate. In addition to this host–guest inter-action, the intermolecular hydrogen bonds between bothCDs also contributed to the stabilization of 1:2 complex-es. The hydrophobic effect seems to play only a minorrole in all these complexations.

In this work we have used several fluorescence tech-niques for studying the thermodynamics of inclusion ofcertain well-known diesters that are precursors in themanufacture of polyesters, dimethyl terephthalate (DMT),dimethyl isophthalate (DMI), and dimethyl phthalate(DMP) in the presence of a- and bCDs. The fluorescenceand conformations of such model compounds of poly-esters were widely studied by us earlier.28–31 The variationof the fluorescence emission intensity upon changing theCD concentration and temperature makes it possible toobtain complex stoichiometries, binding constants, andother thermodynamic parameters that accompany thecomplex formation. Fluorescence anisotropy, quenching,and lifetime measurements were also performed to obtaininformation about the structure of the complexes formed.Job plots32 were built to confirm the guest–host complexstoichiometries.

EXPERIMENTAL

Materials. Aldrich’s dimethyl terephthalate (DMT)was recrystallized from water, and dimethyl isophthalate(DMI) and dimethyl phthalate (DMP) were used withoutfurther purification. a- and bCDs were also from Aldrich.The Karl–Fisher analysis revealed a water content of8.5% and 12.5% (by weight) for a- and bCD, respec-tively. Milli-Q water was used for preparing solutions.CD aqueous solutions with guests were obtained byweight from filtered (1.0 mm diameter size teflon Milli-pore filters) aqueous solutions (OD at lexc ø 0.3–0.4) ofDMT, DMI, and DMP, respectively. Solutions were mea-

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sured after 48 hours of stirring. a- and bCD concentrationranged from 0 to about 1.0–1.8 3 1022 M while DMT,DMI, and DMP concentrations were held constant in allexperiments (;1024 M). A KBr (Aldrich 991%) aqueoussolution was used for quenching measurements.

Instrumentation. Steady-state fluorescence measure-ments were recorded on an SLM 8100 Aminco Spectro-fluorometer equipped with a Xenon lamp, a double (sin-gle) concave grating monochromator at the excitation(emission) path, and Glan–Thompson prism polarizers inboth paths. Except for the fluorescence polarization mea-surements, magic angle conditions were used. The pho-tomultiplier was cooled by a Peltier system. The excita-tion and emission slit widths were set at 8 nm. Fluores-cence decay measurements were performed on a time-correlated single-photon-counting FL900 EdinburghInstruments Spectrometer equipped with concave singlegrating monochromators at the excitation and emissionpaths with slits to 18 nm. The thyratron-gated lamp(nF900) was filled with hydrogen. No polarizers wereused. The signal was detected by a red-sensitive Peltiersystem cooled photomultiplier. The data acquisition wascarried out by using 1024–2048 channels of a multichan-nel analyzer with a time window width of 100 ns and thestop condition was 10 000 counts in the maximum inten-sity channel. Instrumental response functions were reg-ularly achieved by measuring the scattering of a Ludoxsolution and the quality of the fit was judged by the re-duced x2 criterium. Decay profiles were fitted to multiex-ponential trail functions by the iterative reconvolutionmethod.33

Most of the fluorescence measurements of different so-lutions were performed in the range of temperatures from5 to 45 8C at 10 8C intervals (Huber, ministat with Pt100temperature probe).

RESULTS AND DISCUSSION

Fluorescence Spectra. Figure 1 depicts emission spec-tra for DMT, DMI, and DMP water solutions in the ab-sence and in the presence of CD at 25 8C upon excitationwavelengths of 289, 277, and 289 nm, respectively. Thefluorescence emission spectra of DMT solutions in theabsence of CD and at different a- and bCD concentra-tions show a main band with a maximum around 330nm. This band is slightly extended to the red, especiallyat high aCD concentrations. The spectra of DMI and itsCD aqueous solutions revealed a main band centered at305 nm as well, but also showed a little shoulder (;360nm) at high CD concentrations. However, these spectrafor both DMI : aCD and DMI : bCD systems obtained atdifferent concentrations also depict an isoemissive pointat approximately 345 and 335 nm, respectively. In thecase of DMP and its solutions with a- and bCDs, thespectra show two bands centered at ;350 nm and ;380nm. The addition of CD does not affect the ratio of theintensities of the two bands. The main feature of all theseexperiments is the monotonic decreases in the fluores-cence intensity observed with CD concentration for mostof the systems (Figs. 1a–1d and 1f). This fact denotesthat CD hosts act as quenchers of guest fluorescence, i.e.,the fluorescence quantum yield of the guest when it iscomplexed is apparently smaller than when it is free.

However, as Fig. 1e illustrates for the DMP : aCD system,despite the wide range of aCD concentrations used (from0 to ;51 mM, more than three times the concentrationreached for other systems), no significant change was ob-served in the intensity of the emission spectrum. Thespectra obtained at other temperatures showed similarcharacteristics.

Binding Constants. For a simple one-to-one stoichi-ometry guest : host (G : CD) complexation, the equilibri-um can be written as:

G 1 CD s G : CD (1)

and the equilibrium constant as:

[G : CD]K 5 (2)

[G][CD]

where [G], [CD], and [G : CD] are the concentrations ofthe free guest, the free cyclodextrin, and the complex,respectively.

We can express the fluorescence intensity, I0, in theabsence of CD and at any CD concentration, I, as:34

I 5 k[G] and (3)0 0

I 5 k{[G] (1 2 f )e 1 [G] f d} (4)0 B 0 B

where [G]0 is the initial concentration of the guest, k isa constant depending on instrumental conditions like de-tector and monochromator response, geometry of the de-tection, etc., f B 5 [G : CD]/[G]0 is the complexed guestfraction, and e and d are the factors for the quantum yieldchange of the free and complexed guest, respectively, inthe presence of CD, relative to the quantum yield of theisolated free guest (in the absence of CD). Both the e andd parameters take into account the variation in quantumyields of the isolated free and complexed forms due tothe interactions with the new components of the systems.But the d factor also comprises the variation in quantumyield that takes place when the free guest complexes.Therefore, assuming that [G : CD] K [CD]0, where [CD]0

indicates the initial CD concentration, the followingequation can be derived:

I (1 2 e) 1 K(1 2 d)[CD]0 02 1 5 (5)I e 1 Kd[CD]0

If there is no quantum efficiency change for the freefluorophore, e 5 1 and Eq. 5 can be reduced to the fol-lowing equation:

I K(1 2 d)[CD]0 02 1 5 (6)I 1 1 Kd[CD]0

which can be rewritten in the linear form as:

I d 1 15 1 (7)

I 2 I 1 2 d K(1 2 d) [CD]0 0

In addition, if a non-fluorescent inclusion complex isformed, d 5 0, then Eq. 6 becomes linear.

Figure 2 depicts, as an example, (I0/I) 2 1 vs. [CD]0

plots for DMT : aCD, DMI : bCD, and DMP : bCD sys-tems at different temperatures ranging from 5 to 45 8C at10 8C intervals. The results for DMT : aCD and DMI :bCD, but also for DMT : bCD and DMI : aCD, which arenot shown, can reasonably be fitted to Eq. 6 by using a

APPLIED SPECTROSCOPY 825

FIG. 1. Uncorrected emission spectra of DMT, DMI, and DMP aerated aqueous solutions and different CD concentrations at 25 8C upon lexc 5289, 277, and 289 nm, respectively. (a) DMT with [aCD] 5 0, 0.63, 1.37, 2.92, 3.98, 5.24, 6.92, 8.28, 10.77, and 16.26 mM; (b) DMT with [bCD]5 0, 0.77, 1.71, 2.97, 4.18, 5.51, 7.84, 8.80, 11.03, and 13.05 mM; (c) DMI with [aCD] 5 0, 0.19, 2.21, 3.78, 6.24, 8.52, 10.41, 12.80, 15.01, and17.40 mM; (d) DMI with [bCD] 5 0, 0.50, 1.42, 2.30, 3.49, 4.99, 6.22, 7.74, 8.76, 10.32, and 11.92 mM; (e) DMP with [aCD] 5 0, 2.82, 5.75,8.69, 16.18, 21.56, 28.97, 37.55, 43.38, and 50.94 mM; and (f ) DMP with [bCD] 5 0, 0.46, 1.16, 2.04, 3.38, 4.78, 6.12, 7.35, 8.50, 9.77, and11.74 mM.

nonlinear regression program.35 The depicted dotted linesare the results of these adjustments. The representationsfor DMP : bCD, however, are reasonably linear. Thesedata infer that the quantum yields for free DMT, DMI,and DMP guests in the presence of CD are not affectedby interaction with medium components (e 5 1), and alsothat the DMP : bCD complex could be a non-fluorescentcomplex. However, when association constants and the dparameter are low, at the usual [CD] concentrations used,the whole Kd[CD]0 term in the denominator of Eq. 6 canbe neglected, thus also giving a linear relationship.

Figure 3, as an example, shows that the results forDMT : aCD and DMI : bCD systems (and for DMT : bCDand DMI : aCD, which are not depicted) can also be prop-erly fitted to the linear plots derived from Eq. 7.

Values of the binding constant K obtained by linearand nonlinear fits of the experimental data are collectedin Table I. The largest constants were obtained for thecomplexes of DMT with both CDs, whereas the lowestones were found for the DMP : bCD system. As with MN,DMN, and DEN guests studied, the complexes formedwith bCD are also more stable than those formed withaCD.20–24 The fluorescence intensity for the DMP : aCDsystem does not change sufficiently upon CD addition toconfirm the formation of any complex. Either the mea-sured property (the fluorescence intensity) is not sensitiveto the formation of this complex, or the binding constantof the complex is too small to form even at the largeaCD concentration used. Kitano et al.36 have reported theformation of DMP : aCD in aqueous medium, but a new

826 Volume 58, Number 7, 2004

FIG. 2. Variation of (I0/I) 2 1 with [CD] for (top) DMT : aCD, (cen-ter) DMI : bCD, and (bottom) DMP : bCD at different temperatures: 58C (▫), 15 8C (C), 25 8C (n), 35 8C (,), and 45 8C (#). Nonlinear fitswere obtained by adjusting experimental data to Eq. 6. Linear fits forDMP : bCD were obtained assuming that the Kd[CD]0 term in the de-nominator of Eq. 6 is negligible.

FIG. 3. Linear fits of I/(I0—I) with [CD] (top) DMT : aCD and (bot-tom) DMI : bCD at 5 8C (▫), 15 8C (C), 25 8C (n), 35 8C (,), and 458C (#). Linear fits were obtained by adjusting experimental data toEq. 7.

component was placed in this medium. Association con-stants were determined spectrophotometrically by usingmethyl orange as a probe. Semiempirical calculations,carried out by us,30,31 of the conformational energy ofDMT, DMI, and DMP as a function of the torsional an-gles that define the orientation of the ester groups relativeto the aromatic ring, reveal that the esters and benzenering are in the same plane for DMT and DMI. However,for DMP the situation is rather different. Due to the in-teraction between oxygens of the ester groups, the aro-matic ring and the ester group are rather distant from theplanarity. This should make the DMP significantly bulk-ier than DMT and DMI, thus hindering the DMP : aCDcomplex formation.

The location of ester substituents in the benzene ringobviously influences the complex formation when com-paring the formation constants of complexes of both CDswith different guests. At each temperature and for eachparticular CD, DMT forms more stable complexes thanDMI. DMP does not form complexes with aCD and thestability constant is very low for the DMP : bCD com-plex.

At 25 8C the average of the binding constants obtainedfrom both nonlinear and linear fits for DMT, DMI, andDMP with bCD are approximately 888, 300, and 69 M21,respectively. These values are larger than the 378 and 157M21 obtained for DMT and DMI with aCD. The twoformer values indicate that at 25 8C when [bCD] 5 12mM, ;91% of DMT, ;78% of DMI, and ;45% of DMPare complexed with bCD. However, ;82% of DMT and;65% of DMI are complexed with aCD at the same CDconcentration and temperature. Values of the d parameterare in the 0.10–0.39, 0.05–0.16, 0.12–016, and 0.31–0.66range for DMT : aCD, DMI : aCD, DMT : bCD, andDMI : bCD, respectively. Larger values for the DMI :bCD system indicate less change of quantum yield of thecomplexed form relative to the free isolated one.

Stoichiometry. One of the most reliable methods forconfirming stoichiometries is the continuous variationmethod (Job plot).32,37,38 In this study fluorescence inten-sity (I), measured as area under the spectrum, was usedas the physical parameter directly related to complex con-centration. Figure 4 depicts the Job plot of DI 3 [G],which is proportional to the complex concentration vs. q,where DI 5 I0 2 I is the difference of fluorescence in-tensities in the absence, I0, and in the presence of CD, I,and q is defined as:

[CD]q 5 (9)

[G] 1 [CD]

APPLIED SPECTROSCOPY 827

TABLE I. Binding constants, K and d parameters, as well as theirabsolute errors for DMT and DMI complexes with a- and bCDsand DMP with bCD at different temperatures, determined fromemission fluorescence measurements by using nonlinear regressionfits derived from Eq. 6 and linear regression fits from Eq. 7 (inparentheses). For the DMP:CD system, K values were obtained as-suming d 5 0.

DMT:aCD

T (8C) K (M21) d

515253545

957 6 41 (1019 6 29)639 6 27 (566 6 13)387 6 19 (368 6 10)289 6 11 (312 6 8)318 6 42 (308 6 17)

0.0991 6 0.004 (0.107 6 0.006)0.119 6 0.005 (0.098 6 0.008)0.192 6 0.009 (0.179 6 0.011)0.214 6 0.008 (0.234 6 0.010)0.386 6 0.022 (0.379 6 0.018)

DMT:bCD

T (8C) K (M21) d

515253545

1156 6 59 (1209 6 32)949 6 50 (902 6 27)923 6 30 (852 6 16)832 6 58 (768 6 24)738 6 48 (688 6 16)

0.117 6 0.005 (0.122 6 0.004)0.104 6 0.006 (0.098 6 0.007)0.133 6 0.004 (0.121 6 0.004)0.163 6 0.008 (0.153 6 0.007)0.155 6 0.008 (0.145 6 0.006)

DMI:aCD

T (8C) K (M21) d

515253545

206 6 23 (226 6 23)156 6 31 (164 6 27)140 6 25 (174 6 32)131 6 40 (178 6 55)103 6 5 (112 6 33)

0.052 6 0.028 (0.074 6 0.029)0.065 6 0.060 (0.078 6 0.055)0.159 6 0.054 (0.215 6 0.054)0.151 6 0.102 (0.238 6 0.089)0.120 6 0.102 (0.121 6 0.048)

DMI:bCD

T (8C) K (M21) d

515253545

271 6 18 (245 6 6)302 6 39 (223 6 13)343 6 36 (258 6 16)353 6 32 (343 6 26)488 6 113 (353 6 43)

0.312 6 0.014 (0.286 6 0.006)0.375 6 0.025 (0.302 6 0.008)0.441 6 0.017 (0.383 6 0.011)0.572 6 0.011 (0.569 6 0.010)0.661 6 0.020 (0.628 6 0.018)

DMP:bCD

T (8C) K (M21)

515253545

93 6 1881 6 1668 6 1756 6 1730 6 30

FIG. 4. Job’s plot for guest : host systems at 5 8C. Symbols are: DMTcomplexes with aCD (●) and bCD (n), DMI complexes with aCD (,)and bCD (#1), and DMP : bCD (n).

FIG. 5. (Top) Variation of r with aCD concentration for DMT : aCDsolutions at 5 8C (▫), 15 8C (C), 25 8C (n), 35 8C (,), and 45 8C (#).(Bottom) Variation of r of guests with (left) [aCD] and (right) [bCD]for all systems studied at 5 8C. Guests are: DMT (C), DMI (▫), andDMP (n).

where [CD] and [G] are the CD and G concentration foreach particular measured sample, respectively, and thesum [CD] 1 [G] was kept constant for all samples.

All continuous variation plots (Fig. 4) clearly showthat q ø 0.5. This value corresponds to a 1:1 stoichi-ometry, corroborating the analysis of the previous sec-tion.

Anisotropy Measurements. Fluorescence anisotro-pies, r, were obtained at the wavelength of the maximumof emission for each sample, 330 and 304 nm for DMTand DMI solutions with both CDs and 350 nm (the firstpeak of the emission) for DMP solutions. The same ex-citation wavelengths of the emission spectra were used.The anisotropy, measured by using L-Format, was de-fined as:39

I 2 GIVV VHr 5 (10)I 1 2IVV VH

where Ixy is the emission intensity when the excitationpolarizer is in position x (V 5 vertical and H 5 hori-zontal), the emission polarizer is in position y, and G

(5IHV/IHH) corrects for any depolarization produced bythe optical system.

The top of Fig. 5 shows, as an example, the variationof r with aCD concentration for DMT : aCD solutions atdifferent temperatures ranging from 5 8C to 45 8C at 108C intervals. r increases with [aCD] from the value forDMT in the absence of CD (r0) due to the larger amountof complexed DMT, which has a smaller rotational dif-fusion rate than the free guest. But r also decreases withtemperature, which may be due to two reasons: first, theincrease in the rotational diffusion rate of the guest, com-plexed and uncomplexed, and second, the decrease in theamount of the complexed form (if DH , 0, as it usually

828 Volume 58, Number 7, 2004

TABLE II. Fluorescence anisotropy values at [CD] 5 0 (r0) andextrapolated at [CD] → ` (r`) obtained from the data of r depictedin the bottom of Fig. 5 measured at 5 8C. Stern–Volmer constantsKSV for the quenching with KBr of free guests and in the presenceof CDs at 5 8C. [CD] for the quenching measurements used werethose for which 50% of the guests are complexed.

System r` (r0) KSV (M21)

DMTDMT:aCDDMT:bCDDMIDMI:aCDDMI:bCDDMPDMP:aCDDMP:bCD

(0.003)0.0860.057

(0.030)0.0620.047

(0.019)0.0270.072

11.3 6 0.56.7 6 0.37.7 6 0.36.6 6 0.33.7 6 0.21.0 6 0.25.3 6 0.34.2 6 0.31.2 6 0.2

FIG. 6. Stern–Volmer plots at 5 8C using KBr as quencher of (toppanel ) free DMT, (middle panel ) DMI, and (bottom panel ) DMPaqueous solutions (▫), and in the presence of aCD (C) and bCD (n).The concentration of CD used was that for which approximately 50%of the guests are complexed. For DMP, [aCD] 5 45.7 mM was used.

is). Other analyzed systems also revealed a similar be-havior.24 However, as the bottom of Fig. 5 depicts, theincrease of r with [CD] relative to r0 is considerablysmaller when using DMI and DMP as guests. The vari-ation of r with temperature for DMI and DMP systemsis also smaller and in the same order as the experimentalerror. r for DMP : aCD systems hardly shows any changewith [aCD]. Values at [CD] → `, r` can give an esti-mation of r for each guest : CD complex. Such valuesobtained at 5 8C are included in Table II. If both guestand CD rotate as a whole in the complex, r` should beproportional to the size of the complex. These resultsindicate that this assumption is not totally valid, as r`

decreases from aCD to bCD complexes. Therefore, eithera motion of the guest inside the CD cavity, which shouldobviously increase with the cavity size, takes place and/or the location of the guest is not the same for the dif-ferent complexes. If a part of the guest is outside the aCDcavity, due to steric hindrance, the size of the complexmay significantly increase. The small difference of r`

with r0 for the DMP : aCD system indicates that DMPdoes not form a complex with aCD.

Quenching Measurements. Figure 6 depicts thequenching data that were expressed as the ratio of thefluorescence intensity in the absence of KBr quencher,I[Q]50 and in the presence of it, I[Q], plotted versus thequencher concentration [Q] according to the knownStern–Volmer equation.40 The experiments were per-formed on aqueous solutions of free guests and of guestsin the presence CDs at CD concentrations for which ap-proximately 50% of the guests were complexed. ForDMP, which apparently does not form complexes withaCD, a relatively large CD concentration (45.7 mM) wasused. With the exception of the quenching for the DMTguest in the absence of CD, which shows a slight devi-ation from linearity, all representations are perfectly lin-ear in the range of the quencher concentrations used inour measurements. Values of the Stern–Volmer constants,KSV, are collected in Table II. Results clearly show thateven though each free guest is quenched in a differentamount, quenching for the free guests in the absence ofCD is always more effective than for the complexed ones.The quenching efficiency is related to the accessibility ofthe quencher to the fluorophore guest and it should giveinformation about its location in the complex. These re-sults may imply that DMT has a similar and quite largeaccessibility to the quencher with both CDs, suggesting

that a portion of the guest may be exposed to the quench-er in both complexes. However, the DMI guest is moreeffectively buried inside the bCD cavity than when com-plexed with aCD, in which case it probably penetratespartially into the cavity. Something similar occurs withthe DMP : bCD complex. DMP, which does not seem tocomplex with aCD, shows a Stern–Volmer quenchingconstant (KSV) similar to that of free DMP.

Lifetime Measurements. Time-resolved fluorescencemeasurements for DMT, DMI, and DMP and their aque-ous CD solutions were performed at the emission wave-lengths of 330, 305, and 350 nm, respectively, by usingthe same excitation wavelengths as in the previous ex-periments.

The fluorescence intensity decays of DMT, DMI, andDMP in the absence of CD were monoexponentials, andthey were very similar to the decay profile of the H2

lamp. The lifetime measurements of the free guest are atthe sensitivity limits of the apparatus. Nevertheless, theaverage of several measurements was used to estimatelifetimes of around 1.0–1.3 ns, 0.4–0.7 ns, and 0.5–0.7ns for free DMT, DMI, and DMP guests, respectively, inwater at 5 8C. Several years ago30 we obtained similarvalues in several organic solvents. As Fig. 7 shows, thedecays of the guests in the presence of CD, which canreasonably be fitted to the sum of two exponential decays,are slower. Of the two components, the fast one (0.4–1.3ns) can be attributed to the free guests. The other com-ponent, in the 8–16 ns range, is ascribed to the emissionfrom the complexed guests. In all cases, to a greater ora lesser degree, the proportion of the longest componentincreases with [CD]. This proves the formation of theguest–host complex with a remarkable increase in thelifetimes of the guests when complexed. However, due to

APPLIED SPECTROSCOPY 829

FIG. 7. Intensity decay profiles for the hydrogen lamp, for a DMIaqueous solution in the absence of CD and in the presence of bCD at5 8C upon lem 5 305 nm.

TABLE III. Values of enthalpy (DH0) and entropy (DS0) variationsfor DMT:aCD, DMT:bCD, DMI:aCD, DMI:bCD, and DMP:bCDcomplexation processes.

System DH0 (kJmol21) DS0 (JK21 mol21)

DMT:aCDDMT:bCDDMI:aCDDMI:bCDDMP:bCD

222.0 6 4.528.5 6 1.1

212.3 6 1.619.1 6 1.5

212.2 6 1.3

223.4 6 15.1128.1 6 4.310.3 6 5.3

178.2 6 5.326.0 6 4.3

FIG. 8. Van’t Hoff linear plots for the formation of DMT complexeswith aCD (n) and bCD (●), DMI complexes with aCD (n) and bCD(,), and DMP : bCD (the value of K at 45 8C was neglected due to thelarge uncertainty) (#).

the large uncertainties in the lifetime values, these datacannot be used quantitatively to obtain association con-stants as was done previously.23 In contrast, DMP in thepresence of aCD hardly reveals any variation of decayprofiles with respect to DMP in the absence of CD.

As we mentioned earlier, the formation of a non-fluo-rescent DMP : bCD complex is not probable on the basisof the observed changes of lifetimes, fluorescence an-isotropy, and quenching upon bCD addition. Then linear(I0/I) 2 1 plots derived from Eq. 6 are a consequence ofthe low association constants and d values.

Thermodynamic Parameters. DH0 and DS0 values,which are collected in Table III, were obtained from theaverage of the binding constants obtained from the non-linear and linear fits at each temperature by using theclassical van’t Hoff plots depicted in Fig. 8. As it illus-trates, assuming the uncertainties of binding constants forsome of the systems, especially at 45 8C, the represen-tations can reasonably be fitted linearly.

The following remarks could be useful for understand-ing the thermodynamics of complexation in a simple way.The G : CD complex formation is usually due to a com-bination of several effects, which mainly involve hydro-phobic, van der Waals (VDW), and/or hydrogen bonding(HB) interactions. Nevertheless, the steric effects alwaysplay a dominant role over these driving forces.

Attractive interactions, like van der Waals and/or hy-drogen bonding, are characterized by DH0 , 0. Van derWaals forces strongly depend on the distance and theydecrease as the CD cavity size relative to the guest mol-ecule increases. Theoretical simulation of the complexa-tion with CDs of MN, DMN, and DEN esters containingnaphthalene groups instead of benzene groups20–24 alwaysreveals that most of their stabilization comes from vander Waals interactions. Hydrophobic forces tend to re-duce the hydrophobic surface of the nonpolar guestsavailable to the polar solvent by burying it inside thenonpolar CD cavity. They are usually associated withsmall positive values of DH0 and the complexation pro-cess is obviously governed entropically. Negative valuesof the standard molar heat capacity DCp

0 are also char-acteristic. The polarity of the inner cavities relative to theguest should influence the amount of these interactions.Hydrophobic interactions seem to play a minor role inMN, DMN, and DEN complexation.20–24

The DS0 signs during association are usually the bal-ance of two opposite effects, the change in the rotationaland translational degrees of freedom of the system andthe variation in the solvating shells of the guest or thoseincluded inside the host during complexation (this con-tribution is part of the hydrophobic effect). The first ef-fect tends to decrease entropy and the second one to in-crease it. For a guest that is strongly bonded (VDW and/or HB interactions) to the host and cannot penetrate to-tally inside the cavity due to steric hindrance, a largenegative DS0 is expected. The process should be governedenthalpically. However, if a guest penetrates totally insidea relatively wide cavity where the guest motion is onlymoderately hindered, complexation should be accompa-nied by a large positive DS0. The process is then usuallygoverned entropically.

As mentioned previously DMP does not complex withaCD due to steric hindrance. In general, the enthalpyvariation of DMT, DMI, and DMP upon complexationfollows a trend that is similar to other studied MN, DMN,and DEN esters. As Table III shows, the enthalpy changesare less favorable or more unfavorable to complexationwhen the cavity size increases from a- to bCD. Theseresults confirm an important implication of van der Waals(and perhaps HB) interactions in these processes. Thesigns of entropy changes upon association also matchthose obtained for MN, DMN and DEN.20–24 When aguest complexes with aCD, DS0 # 0; however, when itcomplexes with bCD, DS0 . 0. The cavity size shouldtherefore also play an important role in the sign of entro-py change. The exception is the DMP : bCD complex,with a slightly negative DS0. This may indicate that thelocation of the guest may be different due to the steric

830 Volume 58, Number 7, 2004

hindrance of ester groups. In fact, anisotropy values showthat a portion of the guest may be outside the cavity,whereas the quenching data could indicate the opposite.DH0 . 0 and DS0 . 0 for the DMI : bCD complexationcould also suggest an important influence of the hydro-phobic effect. As mentioned previously, the sign of bothcontributions is a classical characteristic of the hydro-phobic effect but also a DCp

0 negative (here DCp0 5 0).

However, if this effect were important for the DMI com-plexation, it should be more important for the DMI : aCDcomplex than for the DMI : bCD complex, as the aCDcavity is less polar, which is not the case. Molecular mod-eling, which usually provides information on the geom-etry of the complex, would be useful here. These calcu-lations should be part of a future work.

CONCLUSION

In this work, thermodynamics parameters that describethe association of a- and bCDs with three well-knownmodel compounds of polyesters, DMT, DMI, and DMPin aqueous media, have been determined. With the ex-ception of DMP with aCD, which does not form the com-plex, probably due to steric hindrance, all other systemsgive thermodynamically stable 1:1 stoichiometry com-plexes. Association constants of each particular guestwith CDs increase from a- to bCD. DH0 values, however,are less favorable or more unfavorable to the complexa-tion when going from a- to bCD. This makes the entropicterm responsible for the larger stabilization of DMT andDMI complexes with bCD. The analysis also suggeststhat van der Waals interactions are mostly involved inthese inclusion processes.

ACKNOWLEDGMENTS

This research was supported by MCYT (BQU2001/1158). We alsoexpress our thanks to M. L. Heijnen for assistance with the preparationof the manuscript.

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