734
J. Phys. Chem. 1996, 100, 734-743
Alcohol Effect on Equilibrium Constants and Dissociation Dynamics of Xanthone-Cyclodextrin Complexes Yuan Liao and Cornelia Bohne* Department of Chemistry, UniVersity of Victoria, P.O. Box 3055, Victoria, B.C., Canada V8W 3P6 ReceiVed: June 20, 1995; In Final Form: August 31, 1995X
The effect of alcohol addition on the ground state complexation of xanthone with cyclodextrins (CDs) and on the dissociation rate constants of triplet xanthone from these complexes was studied by fluorescence and laser flash photolysis experiments. In the case of β- and Hp-β-CD, the addition of alcohol led to the formation of weaker ternary complexes when compared to the xanthone CD binary complexes. In contrast, for γ-CD a slight increase of the complexation strength was observed for the ternary complexes. Addition of alcohols decreased the dissociation rate constant of triplet xanthone from β- and γ-CD by at least a factor of 5. The fact that the dissociation processes was slowed down for both CDs suggests that the effect of ternary complexation agents on the dynamics of complexation was not related to the strength of the ternary complexes formed.
Introduction Chemical reactivity can be drastically altered when processes are carried out in microheterogeneous environments, such as host-guest complexes. Cyclodextrins (CDs) and several of their derivatives are host molecules that have been employed as complexation agents,1-3 chiral discriminating compounds in chromatography,4-7and enzymatic mimetic systems.8-10 CDs are cyclic oligosaccharides containing 6(R), 7(β), and 8(γ) D-glucose units, and guest molecules can be included in their relatively hydrophobic cavities. The equilibrium constant, a thermodynamic parameter, yields information on how strong the complex is, whereas kinetic parameters can be related to the lifetime of a particular complex. There is a wealth of information in the literature about how guest structure, addition of cosolvents, and temperature affect equilibrium constants for CD complex formation. Conversely, the dynamics of complexation (i.e., association and dissociation rate constants) has not been extensively described, probably reflecting the necessity for time-resolved experiments. However, to employ host-guest complexes for the purpose of influencing chemical processes, it is important to learn how to fine-tune the dynamics of complexation as well as how to change the strength of hostguest associations. Temperature-jump experiments have been employed to study the complexation dynamics of nitrophenols and azo derivatives with R-CD.11 The association and dissociation rate constants varied over 7-8 orders of magnitude, showing that the complexation dynamics was very dependent on the guest’s structure. Ultrasonic relaxation studies for the complexation of anions with CD have shown that although the equilibrium constants were small, the association and dissociation dynamics was fast ((4-7) × 107 M-1 s-1 and (0.3-2) × 107 s-1, respectively).12 Photophysical probe molecules have been employed to study the complexation dynamics with CDs. The advantage of photophysics is the accessibility to excitation pulses with short duration which enables the kinetics of fast processes to be determined. Quenching experiments were the most common methodology employed to investigate the complexation dynamics.13-16 Quencher molecules were chosen so that quenching of excited states occurs primarily in the aqueous phase.13,17 X
Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0734$12.00/0
Addition of coincluding molecules, such as alcohols or alkyl sulfates, has been shown to dramatically alter the equilibrium constants between CDs and guest molecules, such as pyrene,15,18-26naphthalene derivatives,15,16,22,27 acenaphthene,28 acridine,29 and azulene.30 Two of these studies15,16 employed quenching of excited states to determine the effect of coincluded molecules on the complexation dynamics. The exit rate constant of pyrene from β-CD decreased from 5 × 104 to 4 × 103 s-1 in the presence of 1-butyl sulfate,15 whereas naphthalene exit from glucosyl-β-CD was only moderately dependent on the coincluded alcohol.16 The relocation of an excited state guest from the CD cavity was for the first time directly measured for triplet xanthone complexed to R-, β-, and γ-CD.31 Xanthone is a suitable probe molecule for studies in the microsecond time domain since the lowest triplet state is fairly unreactive, and the maximum for the triplet-triplet absorption spectrum depends on the solvent polarity.32 In addition, triplet ketones with π,π* configuration have a higher dipole moment in the excited state when compared to their ground state.33 In the case of xanthone-CD complexes, excitation led to relocation from the CD cavity to the aqueous phase. The triplet-triplet absorption maxima for xanthone in water and when complexed to CD were different. Thus, the kinetics for relocation could be measured directly.31 This process was much faster than the decay of triplet xanthone in water, and both rate constants could be independently quantified. We recently34 measured the equilibrium constant for triplet state xanthone with β-CD (48 M-1), hydroxypropyl-β-CD (Hp-βCD, 20 M-1), and γ-CD ( 650 nm). Samples for fluorescence measurements were contained in 10 × 10 mm2 quartz cells, whereas laser flash photolysis experiments were performed in 7 x 7 mm2 cells constructed from Suprasil tubing. UV-vis spectra were recorded on a Varian Cary 5 or Cary 1 at room temperature. Fluorescence spectra were acquired with a Perkin-Elmer MPF 66 fluorimeter. The samples were kept at 20.0 ( 0.5 °C (Haake F3 circulating bath). Emission and excitation slits were set close to 5 nm, and xanthone was excited at 320 nm. Construction of the laser flash photolysis apparatus was based on the design of similar equipment.32 The two excitation sources available are a Lumonics Excimer laser Model EX510 operated with a Xe/HCl gas mixture (308 nm, e40 mJ/ pulse) or a Spectra Physics YAG laser Model GCR-12 (266 nm, e40 mJ/pulse; 355 nm, e70 mJ/pulse; 532 nm, e150 mJ/ pulse). The laser pulse energies are typically attenuated to less than 20 mJ/pulse. The YAG laser beam is directly aligned onto the sample holder whereas the excimer laser beam is concentrated, but not focused, by the use of spherical lenses. A 90° angle arrangement between the excitation source (laser) and analyzing beam is employed. The analyzing beam consists of a pulsed 150 W xenon lamp (Oriel housing Model 66057, PTI power supply Model LPS-220). The pulser is custom-made, and upon triggering, the output of the Xe lamp is increased significantly for 4 ms in a wavelength-dependent manner. Two detection systems are available. Light intensities at fixed wavelengths are detected using a photomultiplier (PMT, Hamamatsu R446, five dynodes)/monochromator (CVI Digikrom 240)) system. The high voltage for the PMT tube is set by a custom built programmable power supply interfaced to
J. Phys. Chem., Vol. 100, No. 2, 1996 735 the computer. Signals from the PMT are fed into a base line compensation circuit that incorporates a sample and hold amplifier with digital memory based on a published circuit.35 This unit offsets the background intensity of the Xe lamp. On receiving a trigger pulse, it holds constant the value of the background intensity (V0) and provides a dc output proportional to its magnitude. The transient signal which remains can then be measured with high precision using a Tektronix TDS 520 digital oscilloscope (50 Ω input impedance for transmission experiments) or a Tektronix SCD 1000 digitizer. The SCD 1000 is used for accurate measurements of lifetimes below 100 ns. Spectrally resolved data are recorded with an intensified dualdiode array system from Princeton Instruments (DIDA 700/RG, detector controller ST116, high-voltage gating pulse generator PG200 and ISA spectrometer HR-320). Timing for laser pulsing, lamp pulsing, sample and hold on the back-off unit, and acquisition on the oscilloscope, digitizer, or DIDA controller are set by a custom-built pulse generator (millisecond delays) driving a Stanford Research System delay generator Model DG535 (nanosecond delays). The timing is set so that the “flat” portion of the 4 ms lamp pulse is used, which in our system occurs about 2.2 ms after the start of the lamp pulse. The lasers fire at a repetition rate of 1 Hz, and data collection occurs at 0.3 Hz. The latter is determined by the time it takes to recharge the capacitors of the Xe lamp pulser. The system is fully integrated to a Macintosh IIci computer. The oscilloscope, digitizer, monochromator, DIDA controller, and Stanford delay generator are interfaced to a GPIB bus (National Instruments GPIB NI-488.2 board). All settings on these units are controlled directly by the computer program. Digital outputs on an I/O data acquisition board (National Instruments Lab-NB) are employed to set logic levels (and, nand, and or gates) to configure different experiments. Besides triggering the lasers, oscilloscopes, DIDA controller, and backoff unit, these logic settings are also employed to open or close shutters and to set filter wheels. Depending on the experimental conditions, the transient absorption signal may have to be corrected for fluorescence from the sample or for slope in the base line. The latter correction is always necessary when data are collected for a total collection time equal or longer than 20 µs. A correction shot is performed after each signal shot; i.e., for fluorescence correction the lamp shutter remains closed, whereas for base line correction the laser shutter remains closed. The correction is then subtracted from the signal shot after transfer of the data to the computer. Two filter wheels with cutoff filters at different wavelength (no filter, 320, 375, 435, and 590 nm) are set between the Xe lamp and the monochromator: one before and the second after the sample holder. The computer program sets both wheels to the first cutoff filter with wavelength smaller than the monochromator setting. The filter wheel in between the Xe lamp and sample is employed to avoid irradiation at short wavelength that could lead to degradation of the sample. The second filter avoids the detection of overtones of emission that occur at half the wavelength being monitored. A computer-controlled filter wheel containing neutral density filters (no filter, 63%, 40%, 25%, and 10% transmission) is employed to decrease the intensity of the excimer laser. The intensity of the YAG laser is set by adjusting the high voltage for the flash lamp or by employing solution filters. One analog output of the Lab-NB board is employed to set the voltage on the PMT power supply, whereas two inputs are employed to measure the temperature at the sample holder (Omegameter Model DP2000) and the held V0 signal from the backoff unit. All experiments are performed at room temperature (20 ( 2 °C).
736 J. Phys. Chem., Vol. 100, No. 2, 1996
Liao and Bohne
The computer program to control the experiment was written using Labview 3.1.1 (National Instruments). The program subtracts correction shots, transforms the data collected (Vt) on the oscilloscope into absorbance values (∆A ) -log(1 corrected Vt/V0)), averages sets of measurements, and saves the data into disk files. Absorbance values are equal to the negative logarithm of the ratio of light intensities being detected on the PMT in the absence and presence of a chromophore. This relationship assumes a linear response of the signal measured at the PMT with light intensity irradiating the detector. The response of PMTs is not linear. For this reason only small changes of light intensities (∆A < 0.2) are measured, and we incorporated an algorithm into the program to acquire the data at a constant target V0 value (default value of 250 mV). At wavelengths where the Xe lamp has a high light intensity output, the high voltage on the PMT is initially kept at 800 V, and the slit on the monochromator is adjusted according to an experimentally determined calibration curve. Toward both ends of the spectrum the maximum slit is employed (2000 µm), and the voltage is increased to a value determined by calibration. During data acquisition the high voltage on the PMT (V) is continuously adjusted when the experimental V0 value is off by more than 10% from the target V0 (Vnew ) [(V0,target/V0,exp)(Vold)6]1/6; the relationship of V0 ∝ V6 was determined by calibration). If the maximum voltage is employed and the target V0 is not reached, the program will increase the monochromator slit. We measured the triplet-triplet absorption of anthracene in ethanol (sharp absorption maximum at 420 nm) at different slit widths (4002000 µm), and no broadening was observed when increasing the slit width. A program capable of several different fitting routines for different kinetic mechanisms based on the Levenberg-Marquardt algorithm was employed to evaluate the data. This algorithm was written in C as a code interface node for Labview (A. D. Kirk, Department of Chemistry, University of Victoria). Alternatively, data were fitted employing the general fitting procedure of the Kaleidagraph software (Synergy software v. 3.0).
Figure 1. Change of the fluorescence intensity of xanthone with addition of Hp-β-CD in the absence of (O) and presence of ()) 0.16 M 1-BuOH. The solid lines correspond to the fit of the experimental data to eq 6, and the recovered Kapp are 2010 and 584 M-1 in the absence and presence of alcohol. The inset shows the fluorescence emission spectra (λex ) 320 nm) measured at different Hp-β-CD concentrations (A, 0; B, 0.3; C, 0.6; D, 0.9; E, 1.5; F, 3.0; G, 6.0; H, 12.0 mM) in the presence of 0.16 M 1-BuOH.
and only alcohol, respectively. An overall equilibrium can be written if we assume that the fluorescence quantum yields for xanthone bound to CD in the absence and presence of alcohol were the same. K
Xan + CD + ROH y\z (Xan)bound
where (Xan)bound corresponds to the sum of X-CD and X-CD-ROH concentrations. At constant alcohol concentrations, which are higher than the CD or xanthone concentrations, the apparent equilibrium constant (Kapp) is given by
K[ROH] ) Kapp )
Ground-State Complexation of Xanthone to CDs in the Presence of Alcohols. The fluorescence quantum yield of xanthone is dependent on the polarity of the solvent, a decrease being observed for solvents with lower polarities. Addition of increasing concentrations of CDs to an aqueous solution of xanthone leads to a decrease in the fluorescence intensity as a consequence of xanthone inclusion into the less polar CD cavity. This property was previously employed31 to determine the equilibrium constants of xanthone with R-, β-, or γ-CD from the nonlinear fit of the change in fluorescence intensity with increasing CD concentration.36,37 The individual equilibria that have to be considered to describe the complexation of xanthone in the presence of alcohols are shown in eqs 1-3. K1
Xan + CD y\z X-CD K2
Xan + CD-ROH y\z X-CD-ROH K3
CD + ROH y\z CD-ROH
(1) (2) (3)
where X-CD, X-CD-ROH, and CD-ROH correspond to the cyclodextrin complexes with xanthone, xanthone and alcohol,
[Xan]bound [Xan][CD]0
(5)
This equation is similar to that in the absence of alcohol, and Kapp38 can be obtained from the same modified BenesiHildebrand treatment as previously employed,31
∆I )
Results
(4)
Kapp∆R [Xan]0[CD]0 1 + Kapp[CD]0
(6)
where ∆I is the change in fluorescence intensity, ∆R includes the difference in the fluorescence quantum yields for bound and free xanthone, and subscripts “0” denote total concentrations. The variation of Kapp with alcohol concentration can be related to the equilibria described in eqs 1-3 by38
Kapp )
K1 + K2K3[ROH] 1 + K3[ROH]
(7)
The xanthone fluorescence intensity decreased with increasing CD concentrations (inset, Figure 1). Nonlinear fits of the experimental data to eq 6 (Figure 1, solid line) were obtained with the general fitting procedure of the Kaleidagraph software, and errors correspond to those recovered from the fitting procedure. The decrease of the fluorescence intensities at equal CD concentrations was smaller in the presence of alcohols, suggesting a weaker interaction of xanthone with CD. There were two possible reasons for this observation. Exclusion of xanthone from the CD cavity, due to occupation of the internal space by alcohol molecules, or ternary complex formation between CD, alcohols, and xanthone. In the latter case, the smaller change of fluorescence intensity in the presence of alcohol suggested a smaller equilibrium constant for the ternary
Xanthone-Cyclodextrin Complexes
J. Phys. Chem., Vol. 100, No. 2, 1996 737 TABLE 2: Apparent Equilibrium Constants (Kapp, M-1) for the Complex between Xanthone and γ-CD in the Presence of 0.05 M Alcohol
Figure 2. Dependence of the apparent equilibrium constants (Kapp) between ground state xanthone and CDs with alcohol concentration: (0) xanthone/Hp-β-CD with 2-BuOH; (b): xanthone/β-CD with 2-BuOH; ()) xanthone/β-CD with c-PeOH. Error bars correspond to those obtained from the fitting of the experimental data to eq 6. The solid lines correspond to the fit of the experimental data to eq 7 when K1 and K3 were fixed (see text).
TABLE 1: Equilibrium Constants (K2, M-1) for Ternary Complexes between Xanthone and β- or Hp-β-CD in the Presence of Alcohols and Literature Values for the CD-Alcohol Equilibrium Constants (K3, M-1)40,41 β-CD
Hp-β-CD
alcohol
K2
K3
K2
K3
1-BuOH 2-BuOH t-BuOH 1-PeOH c-PeOH c-HeOH
4.4 (1.7 92 ( 10 70 ( 18 56 ( 19 20 ( 4 17 ( 6
17 15 48 63 121 501
49 ( 34 45 ( 25 45 ( 33 44 ( 6 40 ( 49
16 12 25 59 a
a The value for K recovered when this parameters was floated was 3 very different from that in the literature. K2 was obtained when K3 was floated.
complex when compared to the xanthone CD equilibrium (K2 < K1). Weaker ternary complex formation had been previously observed for acridine inclusion within β-CD in the presence of alcohols.29 To resolve this issue, Kapp values were measured at different alcohol concentrations,39 and the data were fitted to eq 7 (Figure 2). Two different fitting procedures were employed. The K1 values were determined in the absence of alcohols and were fixed at 1160 and 1800 M-1 for β- and Hpβ-CD, respectively. The Kapp vs [ROH] data were fitted to eq 7 by leaving K2 and K3 as floating parameters. All recovered K3 values were close to the values published in the literature for the association of alcohols with β-CD.40 For this reason, the data was refitted by also fixing the K3 values to those from the literature. The values recovered for K2 are shown in Table 1. The degree of substitution of Hp-β-CD is very dependent on the source of this CD. The degree of substitution of our sample is close to that employed by Tee et al.41 in their measurements of association constants between Hp-β-CD and alcohols. When only K1 was fixed, the recovered K3 values were close to those in the literature for the alcohols shown in Table 1, but c-PeOH and c-HeOH, and the K2 values were determined when K1 and K3 were fixed. For c-PeOH, only K1 was fixed. In the case of c-HeOH, the Kapp value dropped significantly (37 M-1) at the lowest alcohol concentration employed (0.05 M), and no fit of the experimental data to eq 7 was possible. In the presence of all alcohols, a weaker complexation was observed for xanthone with β- or Hp-β-CD. We realize that the errors associated with the recovered K2 values are large. However, addition of alcohols greatly influenced the exit rate constant of triplet xanthone from β-CD (vide infra), being consistent with ternary complex formation. More importantly, for alcohols which have a K2 value close to
alcohol
Kapp/102
alcohol
Kapp/102
no alcohol 1-BuOH 2-BuOH
2.2 ( 0.3 2.8 ( 0.2 3.0 ( 0.4
t-BuOH 1-PeOH c-PeOH c-HeOH
2.2 ( 0.3 3.7 ( 0.3 3.1 ( 0.6 3.3 ( 0.3
zero (1-butanol and β-CD), no change was observed for the dissociation rate constant of triplet xanthone from the CD cavity (vide infra). In the case of γ-CD, the values for Kapp were measured at alcohol concentrations between 0.05 and 0.2 M.39 The average equilibrium constants between xanthone and γ-CD in the absence of alcohols was 222 ( 31 M-1 (12 experiments). Table 2 shows the averaged Kapp values for γ-CD and xanthone in the presence of alcohols (0.05 M). The Kapp values were constant within experimental errors when the alcohol concentration was increased further. Exit Rate Constant of Triplet Xanthone from Ternary CD/Alcohol Complexes. Triplet xanthone has a higher dipole moment than its ground state, and for this reason the equilibrium constant for this excited state with CDs was much smaller than for the ground state.34 When CD complexed xanthone was excited to its triplet state, a nonequilibrium situation was established, and the system relaxed to a new equilibrium for triplet xanthone and CD. Since the triplet lifetime of xanthone in water was much longer than the relocation process from the CD cavity, both decays were separable, and the corresponding rate constants could be determined.31 The relaxation to the new equilibrium was followed directly since the absorption of xanthone in CD and in water was different. The association and dissociation rate constants and equilibrium constants for the triplet state were obtained from the variation of the observed relaxation rate constant with CD concentration.34 In the initial study on the relocation of xanthone from CD cavities,31 the direct spectroscopic methodology was compared to quenching studies of triplet xanthone by cupric ions. It is important to discuss these results in some detail since in this work the quenching methodology was also employed. Cu2+ resides primarily in the aqueous phase, and in the presence of CD, a curved quenching plot (kobs vs [Cu2+]) was observed, suggesting that at high quencher concentrations the exit of triplet xanthone from CD was rate limiting. Similar rate constants were observed both for direct spectroscopic measurements and in quenching studies.31 However, a more detailed analysis of the relocation kinetics established that this similarity was fortuitous, since the observed rate constants measured in spectroscopic studies were dependent on the CD concentration.34 At high Cu2+ concentrations, the relaxation model predicts that the observed rate constants are independent of the CD concentration and represent the exit rate constants. This picture was probably correct for β- and Hp-β-CD, for which the same rate constants were obtained within the experimental error from the direct method and in quenching studies.31,42 However, for γ-CD, a higher rate constant is observed for the quenching studies, suggesting that Cu2+ has some access to the xanthone within the CD cavity.31,42 For this reason we refer to rate constants obtained in quenching studies as apparent rate constants. Ternary complex formation for xanthone, alcohols, and γ-CD was somewhat stronger than the complexation of xanthone in the absence of alcohols. At high γ-CD concentrations (30 mM) in the presence of 0.05-0.2 M alcohols most of the xanthone (>87%) was complexed to CD as either a ternary or a binary complex. In the presence of alcohol, there are three first order
738 J. Phys. Chem., Vol. 100, No. 2, 1996
Liao and Bohne TABLE 3: Dissociation Rate Constants of Triplet Xanthone from the Ternary Complex with γ-CD (30 mM) and Alcohols (0.10-0.11 M) and Preexponential Factors Recovered from the Fit of the Triplet Xanthone Decay at 620 nm (See Text)a alcohol 1-BuOH 2-BuOH t-BuOH 1-PeOH c-PeOH c-HeOH
k2-/105, s-1 k- /k2-b 3.6 ( 0.2 6.1 ( 0.8 4.2 ( 1.1 2.9 ( 0.6 5.9 ( 0.7 6.4 ( 0.6
21 ( 2 12 ( 2 17 ( 5 25 ( 5 12 ( 2 11 + 1
A1
A2
A3
0.16 ( 0.01 0.12 ( 0.01 0.17 ( 0.01 0.21 ( 0.02 0.16 ( 0.04 0.13 ( 0.04
0.34 ( 0.02 0.29 + 0.01 0.15 + 0.07 0.31 ( 0.06 0.17 ( 0.10 0.23 + 0.06
0.50 ( 0.02 0.56 ( 0.03 0.68 ( 0.08 0.49 ( 0.09 0.68 ( 0.06 0.64 ( 0.01
a
Values and errors correspond to averages from two independent experiments. b The values for k-/k2- represent the ratio between the dissociation rate constant for the binary (k-, 7.3 × 106 s-1) and ternary (k2-) complexes.
Figure 3. (A) Decay of triplet xanthone in the presence of c-PeOH (0.11 M) and γ-CD (30 mM) monitored at 620 nm. The decay was fitted to the sum of three exponentials. The values of kw and k- were fixed at 4.79 × 104 and 7.3 × 106 s-1, respectively. The inset of (A) shows the triplet xanthone decay in presence of c-PeOH and γ-CD at a long time base. The recovered value for k2- was (6.4 ( 0.4) × 105 s-1. ∆∆A corresponds to the residuals between the experimental and calculated data for the fit of the data to the sum of three (B) and to the sum of two exponentials (C).
kinetic processes to be considered. k-
XAN*-CD 98 3XAN* + CD
3
k2-
XAN*-CD-ROH 98 3XAN* + CD-ROH
3
3
kw
XAN* 98 XAN
(8) (9) (10)
where k- corresponds to the exit of triplet xanthone from the CD cavity in the absence of alcohols, k2- corresponds to the dissociation from the CD cavity in the presence of alcohols, and kw is the decay rate constant for triplet xanthone in water. This mechanism assumes that reassociation of triplet xanthone was negligible. This assumption was reasonable in the case of γ-CD where only a very small dependence was observed for the relaxation kinetics on CD concentration.34 The value for k- determined earlier was (7.3 ( 0.5) × 106 s-1.34 The lifetime of xanthone in water was longer than the relocation processes (kw , k- and k2-) and was very dependent on residual amounts of oxygen in solution. For this reason, kw was determined for each sample from the decay at a longer time base (inset, Figure 3). The observed relaxation kinetics (Figure 3) was adequately fitted to the sum of three exponentials corresponding to the three process shown in eqs 8-10.43
∆A ) A1e-k-t + A2e-k2-t + A3e-kwt
(11)
Figure 3 also shows the residuals obtained when the experimental data were fitted to the sum of two exponentials with the value for kw fixed. The comparison of both residuals shows that the experimental data could not be fitted to the sum of two exponentials, suggesting that more than two triplet xanthone species were present in solution. The values for kand kw were fixed, and the recovered k2- values are shown in Table 3. For all alcohols studied the dissociation rate constant
for triplet xanthone in the ternary complex was smaller than that observed for the binary complex. The preexponential factors are indirectly related to the concentrations of complexed (A1 and A2) and free (A3) xanthone.43 At 620 nm the molar absorptivity for complexed xanthone was higher than for aqueous xanthone, and all preexponential factors were positive. The actual concentrations could not be calculated since the absolute values for the molar absorptivities for triplet xanthone in water and when bound to the CD cavity were not known, and a blue shift was observed for the ground state absorption of xanthone when complexed to CD. The consequence of this latter observation was a higher excitation efficiency at 355 nm for free xanthone when compared to complexed xanthone. This difference in the excitation efficiencies had no effect on the first-order rate constants, but decreased the values for the preexponential factors which correspond to complexed xanthone. Nevertheless, the fact that the values for A2 (Xan-CD-ROH) were of the same order of magnitude as A1 (Xan-CD) indicated that an appreciable amount of xanthone was involved in ternary complexation. An alternate method to measure the exit dynamics from the CD cavity is to employ a triplet state quencher (Cu2+) that mainly resides in the aqueous phase and cannot efficiently quench the complexed excited state. The kinetic scheme is the same as represented by eqs 8-10, but the lifetime of triplet xanthone in water is very short and is determined by the concentration of Cu2+. At 0.8 M Cu2+ this decay rate constant was 4.0 × 107 s-1. The exit rate constants from the binary and ternary complex will be expressed as k-(app) and k2-(app), respectively, to identify that they were determined in quenching experiments which is an indirect methodology. The xanthone triplet decay in the presence of 0.8 M Cu2+ and γ-CD was measured at 600 nm, the isosbestic point between xanthone complexed to CD and free in water.31 In the presence of 20 mM γ-CD but the absence of alcohol, the decay trace for triplet xanthone was fitted to the sum of two exponentials since some xanthone was free in solution prior to excitation. One rate constant was fixed to the value obtained for the decay of triplet xanthone in water in the presence of 0.8 M Cu2+ (4.0 × 107 s-1). The second rate constant, recovered from the fit of the experimental data (1.0 × 107 s-1), corresponds to an apparent exit rate constant (k-(app)). This rate constant was higher than measured with the direct method, probably due to the ability of the quencher to intercept triplet states before they exited completely from the CD cavity.31 In the presence of alcohols, the decay curve was not adequately fitted to the sum of two exponentials (Figure 4). A third exponential term, which we assigned to the dissociation of xanthone from ternary complexes with γ-CD, was added to the fitting procedure. The rate constants corresponding to the decay of xanthone in water (kw
Xanthone-Cyclodextrin Complexes
J. Phys. Chem., Vol. 100, No. 2, 1996 739
Figure 4. (A) Decay trace of triplet xanthone in presence of 20 mM γ-CD, 0.055 M c-PeOH, and 0.8 M Cu2+ measured at 600 nm. The experimental data were fitted to the sum of three exponentials by fixing the values for kw ) 4.0 × 107 s-1 and k-(app) ) 1.0 × 107 s-1. The recovered value for k2-(app) was (6.8 ( 2.0) × 105 s-1. ∆∆A corresponds to the residuals for the fit of the experimental data to the sum of three (B) and the sum of two exponentials (C).
TABLE 4: Apparent Dissociation Rate Constants of Triplet Xanthone from the Ternary Complex with γ-CD (20 mM) and Alcohols (0.10-0.11 M) and Preexponential Factors Recovered from the Fit of the Triplet Xanthone Decay at 600 nm in the Presence of 0.8 M Cu2+ (See Text)a alcohol
k-/ k2-(app)/ 105, s-1 k2-(app)b
1-BuOH 2-BuOH t-BuOH c-PeOH c-HeOH
4.8 ( 1.4 4.8 ( 1.0 2.5 ( 0.2 4.4 ( 2.2 5.3 ( 2.4
15 ( 4 15 ( 3 29 ( 3 17 ( 8 14 ( 6
A1
A2
A3
0.31 ( 0.07 0.43 ( 0.01 0.33 ( 0.16 0.36 ( 0.16 0.55 ( 0.13
0.10 ( 0.04 0.13 ( 0.01 0.12 ( 0.02 0.10 ( 0.03 0.08 ( 0.01
0.60 ( 0.11 0.44 ( 0.01 0.56 ( 0.18 0.54 ( 0.16 0.38 ( 0.13
a Values and errors correspond to averages from two independent experiments for 1-BuOH, 2-BuOH, and t-BuOH and three experiments for c-PeOH and c-HeOH. b The values for k-/k2-(app) represent the ratio between the dissociation rate constant for the binary (k-, 7.3 × 106 s-1) and ternary (k2-(app)) complexes.
) 4.0 × 107 s-1) and xanthone complexed to γ-CD (k-(app) ) 1.0 × 107 s-1) were fixed, and the rate constants for the exit from the ternary complex were recovered from the fit of the experimental data to eq 11 (Table 4). No experiments could be performed in the presence of 1-PeOH, since the solution became cloudy with the addition of 0.8 M Cu2+. The values obtained in the direct spectroscopic measurements and in the quenching studies were the same within experimental errors (compare Tables 3 and 4). For this reason we compare the k2-(app) values to the dissociation rate constant from the binary complex measured with the direct spectroscopic method (k-). The fact that k2- and k2-(app) were the same suggested that in the presence of alcohols the quencher had no access to the triplet state within the γ-CD cavity. This result is important, since for β- and Hp-β-CD we were not able to apply the direct method, and all the data were obtained from quenching studies (vide infra). Table 4 also shows the preexponential factors for the fits of the experimental data to the sum of three exponentials. Since we measured the decay of the triplet state at the isosbestic point, the values for the molar absorptivities were equal for all xanthone species. However, the shift of the ground state spectrum to shorter wavelength for complexed xanthone led to a higher excitation efficiency for xanthone in water when
Figure 5. (A) Decay trace of triplet xanthone in presence of 10 mM β-CD, 0.055 M c-PeOH, and 0.8 M Cu2+ measured at 600 nm. The experimental data were fitted to the sum of three exponentials by fixing the values for kw ) 4.0 × 107 s-1 and k-(app) ) 9.5 × 106 s-1. The recovered value for k2-(app) was (1.7 ( 0.1) × 106 s-1. ∆∆A corresponds to the residuals for the fit of the experimental data to the sum of three (B) and the sum of two exponentials (C).
compared to complexed xanthone. This in turn led to higher values of A3. It is worth noting that we did not expect the same preexponential values for the decays measured at 620 nm in the absence of quencher and 600 nm in the presence of Cu2+, since the preexponential factors include molar absorptivities and rate constant values43 which are different for these two measurements. Ternary complexes between xanthone, alcohols, and β- or Hp-β-CD had smaller equilibrium constants than the binary complexes. The lower equilibrium constants and the much lower solubility of β-CD in aqueous solution, when compared to γ-CD, made it impossible to work under conditions when all xanthone was complexed. For this reason, there were always three xanthone species present, i.e., free xanthone in solution and xanthone complexed to CD in binary or ternary complexes. The presence of an appreciable amount of xanthone in water precluded the use of the direct methodology, since the changes for the triplet-triplet absorption spectra were very small. The exit of triplet xanthone from the β-CDs was measured by monitoring the triplet decay in the presence of 0.8 M Cu2+. The same methodology described above for γ-CD was employed. The apparent rate constants for the exit of xanthone from the binary complexes determined in the absence of alcohols, but in the presence of 0.8 M Cu2+, were 9.5 × 106 and 6.6 × 106 s-1 for β-CD and Hp-β-CD, respectively. In the presence of β-CD and most alcohols used, the decay was only satisfactorily fitted to the sum of three exponentials (Figure 5). The recovered rate constants for the dissociation of triplet xanthone from ternary complexes were always smaller than the exit from the binary complex (Table 5). In the case of 1-butanol, where in the fluorescence experiments we determined that no ternary complex was formed, the rate constants recovered for exit from the CD cavity (k-(app) and k2-(app)) from the fit to the sum of three exponentials were the same. Thus, no slow component was observed. This result clearly indicates that for alcohols, other than 1-butanol, the formation of a ternary complex, as determined in the fluorescence studies, was related to the smaller dissociation rate constant for triplet xanthone from CD/alcohols.
740 J. Phys. Chem., Vol. 100, No. 2, 1996
Liao and Bohne
TABLE 5: Apparent Dissociation Rate Constants of Triplet Xanthone from the Ternary Complex with β-CD (10 mM) and Alcohols (0.10-0.11 M) and Preexponential Factors Recovered from the Fit of the Triplet Xanthone Decay at 600 nm in the Presence of 0.8 M Cu2+ (See Text)a alcohol
k2-(app)/ k-(app)/ 105, s-1 k2-(app)b
1-BuOH 2-BuOH t-BuOH 1-PeOH c-PeOH c-HeOH
96 ( 5 7.5 ( 2.7 16 ( 1 4.6 ( 2.5 17 ( 1 10 ( 3
1.0 ( 0.1 13 ( 5 5.9 ( 0.7 21 ( 11 5.6 ( 0.3 10 ( 3
A1
A2
A3
0.49 ( 0.11 0.55 ( 0.11 0.53 ( 0.05 0.45 ( 0.01 0.50 ( 0.05 0.43 ( 0.05
0.04 ( 0.02 0.19 ( 0.02 0.06 ( 0.01 0.27 ( 0.06 0.08 ( 0.01
0.52 ( 0.11 0.41 ( 0.09 0.28 ( 0.03 0.49 ( 0.01 0.23 ( 0.01 0.50 ( 0.04
a Values and errors correspond to averages from two independent experiments for 1-BuOH, 2-BuOH, 1-PeOH, and c-HeOH and three experiments for t-BuOH and c-PeOH. b The values for k-(app)/k2-(app) represent the ratio between the apparent dissociation rate constant for the binary (k-,(app), 9.5 × 106 s-1) and ternary (k2-(app)) complexes.
Preliminary experiments were also performed at different alcohol concentrations. For example, the exit rate constant of triplet xanthone from the ternary complex with β-CD and t-BuOH increased from (1.2 ( 0.1) × 106 to (2.4 ( 0.2) × 106 s-1 when the alcohol concentration was increased from 0.05 to 0.21 M, suggesting that changes in the composition of the bulk solvent might be important on the exit dynamics. However, this trend was not observed for all alcohols. For Hp-β-CD (12 mM) the triplet decay was monitored in the presence of 0.05-0.1 M 1-BuOH, 2-BuOH, t-BuOH, 1-PeOH, c-PeOH, or c-HeOH. In all cases, the third rate constant determined in the fitting procedure was the same within the experimental error as that for the exit of xanthone from the binary complex with Hp-β-CD. The studies of the ground-state complexation indicated that ternary complexes were formed with Hp-β-CD, but the triplet decay results suggested that alcohols did not influence the exit rate constant of triplet xanthone from ternary complexes. Discussion Coinclusion of ternary complexation agents can lead to significant changes in the thermodynamics and dynamics of the host-guest interactions involving CDs.15,16,18-27,29,30 Depending on the nature of the guest molecules, host-guest complexation with cyclodextrins can be either enthalpically or entropically driven,40,44 and an enthalpy-entropy compensation effect has been proposed.44 Ternary complexation agents, such as alcohols and alkyl sulfates, were suggested to replace water molecules for the solvation of the empty CD cavity and/or CD-guest complex. If the solvation of the empty cavity by these molecules is strong, an inclusion complex is formed, and complexation of other guest molecules will not occur. In this case, displacement of guest molecules into the aqueous phase will be observed with increasing concentrations of “ternary complexation agents”. However, ternary complexes (CD/guest/ alcohol) can be formed when the solvation of the CD with a ternary complexation agent does not lead to the formation of strong inclusion complexes but is thermodynamically favorable when compared to the solvation by water molecules. Changes of the complexation efficiency in ternary complexation has been explained with a space-filling model.15,16,24,25,29,30 The role of the ternary complexation agent was to fill up “void space” within the CD cavity that was not occupied by the guest molecule. In the absence of these molecules, this space would contain water, a thermodynamically unfavorable situation. In several studies specific stoichiometries involving alcohol and CD molecules were proposed.16,19,24,26-28,30 Other effects of cosolvents have
been proposed, especially when high concentrations were employed. For example, preferential solvation of the entrances of β-CD complexes with 1-naphthylethylamines by DMSO in a 6:4 DMSO:water mixture was proposed to explain the photophysics of the guest molecule.45 In addition, the decrease of the equilibrium constant of guest molecules with CDs in the presence of cosolvents, such as urea, guanidinium chloride, and ethanol, has been attributed to the antihydrophobic nature of these molecules.46,47 Xanthone is an excellent probe molecule to study dynamic processes in microheterogeneous systems31,34,48,49 and has been employed to compare the dynamics of ground and excited state complexation with CDs.34 The primary objective of this work was to establish the effect of alcohol addition on the dissociation dynamics of triplet xanthone. In order to study this dynamic process, we had to ensure that xanthone-CD complexes which contained alcohol molecules were formed. The strength of ternary complexes between alcohols, xanthone, and CDs showed a dependence on the CD cavity size. The equilibrium constant for binary complex formation was higher for β- and Hp-β-CD than for γ-CD. For the β-CDs, a decrease was observed in the strength of ternary complex formation. This suggested that the cavity was not large enough to accommodate xanthone and the hydrophobic portion of the alcohols. Ternary complex formation with β-CD was very sensitive to the structure of the alcohols as shown by the different K2 values for the butanol isomers (Table 1). It was quite surprising that no ternary complex was formed for 1-BuOH, whereas complexes with comparable strength were formed for 2-BuOH and t-BuOH. For larger alcohols, such as c-PeOH and c-HeOH, a decrease was observed for the equilibrium constant for the ternary complexes when compared to the complexes with 2-BuOH or t-BuOH. This indicated that the bulkiness of the hydrophobic portion of the ternary complexation agent was determinant for complex formation and that steric crowding occurred within the cavity. The value of Kapp, from which the equilibrium constants for the ternary complexes were determined, incorporated a term for the alcohol concentration. This was a valid approach since the alcohol concentration in all cases was much higher than the CD concentration. From this treatment, we cannot establish whether only one or more alcohol molecules were involved in the complexation. In the case of Hp-β-CD, no clear trend was observed for the equilibrium constant for ternary complex formation with alcohol structure. This may in part reflect the large experimental errors obtained. However, a smaller sensitivity to the alcohol structure could also be related to the extended hydrophobic cavity in this substituted CD, for which addition of alcohol would not increase the hydrophobicity of the microenvironment around the guest molecule. This might also be the reason why no change was observed for the dissociation rate constant of triplet xanthone. The addition of all alcohols, but t-BuOH, in the presence of γ-CD led to a slight increase of Kapp. The formation of ternary complexes was confirmed in the studies related to the dynamics of triplet xanthone relocation. The equilibrium constants for the ternary complexes (K2) could not be determined, since Kapp did not vary significantly with alcohol concentration, and the values for K1 and K2 were too similar for a fit of the experimental data to eq 7 to adequately differentiate between these equilibrium constants. These results indicated that for γ-CD, which has a larger cavity, alcohol molecules were more easily accommodated with the xanthone molecule inside the CD cavity than for the smaller β-CD cavity. Thus, in the presence of alcohols, stronger complexes were formed in absolute terms with γ-CD than with the β-CDs, reverting the pattern observed
Xanthone-Cyclodextrin Complexes for the binary complexes where the equilibrium constants are higher for the β-CDs. In addition to steric effects, we should also consider the role of hydrophobic interactions,50,51 specifically the interaction of xanthone with alcohols in homogeneous solution. Ethanol (1020%) in water was shown to decrease the equilibrium constants of host molecules with β-CD, and this effect was attributed to an increased dispersion of the host in water which was assisted by the interaction with the hydrophobic surface of the alcohol (antihydrophobic effect).46 Although we employed much lower alcohol concentrations (1% v/v), we should discuss the importance of this interaction. The increased solubilization of xanthone in the homogeneous phase could play a role for complexes with β- and Hp-β-CD. If this interaction was the dominant factor leading to the decrease of Kapp, we would not expect to have observed a decrease of the dissociation rate constant for triplet xanthone, since the properties of the bulk solvent did not change appreciably. In addition, if small changes in the constitution of the bulk solvent would be important, we should have observed changes for the dissociation rate constants for all alcohols. No effect was observed for 1-butanol/β-CD. Thus, the decrease of the dissociation rate constant suggests that alcohol molecules have to participate in the CD complex. In the case of γ-CD, the value for Kapp increased at low alcohol concentrations, suggesting that the antihydrophobic effect of alcohols was not significant. However, the fact that the value for Kapp was constant with a further increase in the alcohol concentration indicated that xanthone might be better solubilized in the homogeneous solution as the mole fraction of the cosolvent increased. Formation of ternary complexes increases the protection of the guest from collisions with molecules residing primarily in the aqueous phase. This phenomenon was previously established in quenching experiments of excited state guest molecules, where the addition of alcohols or alkyl sulfates significantly decreased the quenching efficiency of the complexed excited state by quenchers from the aqueous phase.15,16,22 Quenching experiments were also employed to determine the effect of alcohols16 or alkyl sulfates15 on the complexation dynamics with CDs and guest molecules. For example, it was determined that the dissociation rate constant of pyrene from β-CD decreased from 5 × 104 to 4 × 103 s-1 in the presence of 1-butyl sulfate,15 whereas 1-bromonaphthalene exit from glucosyl-β-CD was moderately dependent on the coincluded alcohol; e.g., the dissociation rate constants were 2.0, 7.9, and 15 s-1 in the presence of 1-BuOH, c-HeOH, and 2-BuOH, respectively.16 The dynamics of xanthone complexation with CDs can be followed directly by measuring the absorption change due to the triplet state relocation.31,34 The advantage of the direct observation is that no mechanistic assumptions have to be made. In contrast, for the quenching methodology a quenching mechanism has to be assumed. In most quenching studies, it was assumed that quencher molecules have no access to the excited state guest molecules within the CD cavities. Studies with xanthone and CDs31 have shown that this assumption does not always hold. For example, in the case of γ-CD, the exit rate constant obtained from the quenching studies was significantly higher than that determined by the spectroscopic method, suggesting that the quencher molecule can intercept the excited state before it completely dissociates from the CD cavity. In the present study, we were able to employ, for γ-CD, the spectroscopic method to investigate how ternary complex formation affects the exit rate constant of complexed triplet xanthone. At high γ-CD concentrations, most xanthone was complexed as either a binary or a ternary complex. Thus, the
J. Phys. Chem., Vol. 100, No. 2, 1996 741 processes contributing to the triplet xanthone decay were deactivation of the triplet in water (slow process) and the exit of triplet xanthone from the binary and ternary complexes. If no ternary complex was formed, the triplet decay should have been adequately fitted to the sum of two exponentials. The poor fit to the sum of two exponentials (Figure 3) suggests that a third xanthone species existed, which was assigned to xanthone complexed with CD/alcohols. The exit rate constant from the ternary complex was much smaller than the dissociation from the binary complex. The relative decrease of the exit rate constant was not very dependent on the structure of the alcohol. The dissociation rate constants were also determined by employing the quenching methodology. The errors for the recovered rate constants are larger since the rate constants for the three processes, decay of triplet in water in the presence of Cu2+ and exit of triplet xanthone from the CD complexes, have similar values due to the shortening of the xanthone triplet lifetime in water. However, it is very significant that the rate constants for the exit of triplet xanthone from the ternary complex are the same within the experimental error when determined by the direct spectroscopic measurements or with the quenching methodology. This result is in contradiction to what was observed for the binary complex, suggesting that the alcohols protected the guest molecule from the aqueous environment. In the case of β-CD, we could not measure the dissociation rate constant for triplet xanthone from the ternary complex, since the equilibrium constant for this complex was small, and the low solubility of this CD precluded the use of conditions where all xanthone in the ground state was complexed. The exit rate constant was determined in quenching experiments. We believe that the values recovered correspond to true dissociation rate constants, since the experiments with γ-CD showed no differences for the values obtained from quenching experiments and by direct spectroscopic measurements. The dissociation rate constants from the ternary complexes with β-CD were smaller than those observed for the binary complex, and they showed a slightly higher dependence on the alcohol structure than observed for γ-CD. There was no correlation between the relative decrease of the dissociation rate constant and the strength for ternary complex formation. The addition of alcohol had an opposite effect on the ground state xanthone complex formation with β- and γ-CDs. This can be attributed to the different cavity sizes. The smaller β-CD cavity has less space to accommodate xanthone and the alcohol, resulting in less favorable xanthone complexation. However, for the larger γ-CD cavity there was enough space to incorporate the alcohol molecules. In contrast, the effects of alcohol addition on the dissociation rate constants of triplet xanthone were similar for CDs with different cavity sizes. In the case of β-CD, where a weaker ternary complex was formed, xanthone could have been partially displaced from the interior of the CD cavity. In this case, we would expect the dissociation rate constants in the presence of alcohols to be very different for the complexation with β- and γ-CD since in the former case xanthone would be “closer” to the aqueous phase. Indeed, a faster dissociation rate constant of triplet xanthone from the ternary complex than measured for the binary complex with β-CD would be expected, if xanthone had been partially displaced. The fact that the magnitude of the decrease of the dissociation rate constant was independent of the cavity size suggested that in both cases xanthone was protected by the alcohols from the aqueous phase. Aromatic ketones which have a lowest triplet state with π,π* configuration have higher dipole moments in their triplet excited
742 J. Phys. Chem., Vol. 100, No. 2, 1996 states when compared to the ground state.33 The relocation of triplet xanthone from the relatively hydrophobic CD cavity to the aqueous phase was attributed to the higher dipole moment of the excited triplet state.31 One possible effect of the alcohols was to create a more hydrophobic environment for the guest molecule within the CD cavity. This has been observed for the complexation of pyrene with CDs where the alcohols decreased the I/III vibronic ratio of the emission spectrum, suggesting a substantial decrease of the polarity being sensed by pyrene.19,23-25 By creating a more hydrophobic environment, the polarity difference within the CD cavity and the water phase would be increased. If exit of triplet xanthone was driven by the higher dipole moment, we would expect the dissociation rate constant from the ternary complex to be higher than for the binary complex. This is contrary to our experimental evidence, and the effect of alcohol has to be of a different nature. An alternate explanation is that the relocation was not driven by the higher dipole moment of the excited state, but by its higher basicity, and the better hydrogen donor ability of bulk water would be the driving force for relocation. A recent linear free energy relationship analysis of the incorporation of nonionic solutes into aqueous micelles suggested that dipolarity of the solute was not relevant and that the magnitude of the partitioning coefficient was dependent on the molar volume and the basicity of the solute.52 To explain the alcohol effect by this mechanism, we have to assume that the alcohol molecules within the CD cavity would hydrogen bond better to the carbonyl group of xanthone than the solvating water molecules or the hydroxyl groups at the CD entrances. This hydrogen bonding would decrease the basicity of the excited state, and the driving force for exit into the aqueous phase would be smaller. However, it is difficult to explain why alcohol molecules would be much more efficient than the hydroxyl groups of the CDs in hydrogen bonding to xanthone. At least in the case of γ-CD, where the cavity is big, there should be enough mobility of the CD frame to attain optimum conformation for hydrogen bonding. In addition, the xanthone molecules in the binary complex with γ-CD should be surrounded by more water molecules than for β-CD. Thus, if the difference in the basicity was the driving force for relocation, we would expect a smaller effect of alcohol addition on the dissociation rate constant for γ-CD than β-CD, contrary to what was observed. The effect of alcohols on the dissociation rate constants of triplet xanthone from the ternary complexes was not dependent on the CD cavity size and the structure of the alcohols. If only one alcohol molecule was involved in ternary complexation, we would expect the structure of the alcohol to have influenced the magnitude of the xanthone dissociation rate constants. Our result are indicative that more than one alcohol molecule was involved in the ternary complex. A small amount of alcohol molecules could coinclude in the CD cavity, but other molecules could be solvating preferentially the entrances of the cavity. Thus, the alcohols would create a barrier between the CD cavity and the homogeneous aqueous phase. This barrier would shield the triplet state from the aqueous phase, leading to the extra protection for the quenching by Cu2+ observed for complexes with γ-CD. In addition, in the presence of an alcohol “layer”, the triplet xanthone would have to explore a larger volume before sensing the more polar environment of the water phase and would need to shed some of the solvating alcohol molecules during the dissociation process. All these effects are expected to lead to a decrease of the dissociation rate constant. We are fully aware that the suggestion of preferential solvation is speculative at this point, and we are designing experiments in
Liao and Bohne which product studies from excited states formed in the CD cavity should indicate if such solvation occurs. In summary, the addition of alcohols decreased the equilibrium constants between ground-state xanthone and β-CD but increased slightly the association with γ-CD. In contrast, alcohols decreased the dissociation rate constant for triplet xanthone for both CDs, indicating that the effect on the dynamics does not depend on the strength of the ternary complex. These studies show that addition of coincluding molecules can be employed to fine-tune the dynamics of hostguest complexation, and this understanding could have important implications for general applications of CDs, such as drug delivery or separation technology. Acknowledgment. This research was supported by the Natural Sciences and Engineering Research Council of Canada. The University of Victoria is also gratefully acknowledged for providing setup funds. The authors thank L. Netter for developing and maintaining the data acquisition and analysis software for the laser flash photolysis system, F. H. Quina for providing a preprint to their work, and G. A. Reed at American Maize for the gift of the CD samples. Supporting Information Available: Derivation of eqs 6, 7, and 11 and tables containing the Kapp values for different CDs and alcohol concentrations (15 pages). Ordering information is given on any current masthead page. References and Notes (1) Saeger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344-362. (2) Szejtli, J. Cyclodextrins and Their Inclusion Complexes; Akade´miai Kiado´: Budapest, 1982. (3) Szejtli, J. Cyclodextrin Technology; 1st ed.; Kluwer Academic Publishers: Dordrecht, 1988. (4) Armstrong, D. W.; Ward, T. J.; Armstrong, R. D.; Beeley, T. E. Science 1986, 232, 1132-1135. (5) Steveson, D.; Wilson, I. D. Chiral Separations; Plenum Press: New York, 1988. (6) Allenmark, S. Chromatographic Enantioseparation: Methods and Applications, 2nd ed.; Ellis Horwood Limited: New York, 1991. (7) Li, S.; Purdy, W. C. Chem. ReV. 1992, 92, 1457-1470. (8) Tabushi, I. Acc. Chem. Res. 1982, 15, 66-72. (9) Tee, O. S. AdV. Phys. Org. Chem. 1994, 29, 1-85. (10) Breslow, R. Acc. Chem. Res. 1995, 28, 146-153. (11) Cramer, F.; Saenger, W.; Spatz, H. C. J. Am. Chem. Soc. 1967, 89, 14-20. (12) Rohrbach, R. P.; Rodriguez, L. J.; Eyring, E. M.; Wojcik, J. F. J. Phys. Chem. 1977, 81, 944-948. (13) Turro, N. J.; Bolt, J. D.; Kuroda, Y.; Tabushi, I. Photochem. Photobiol. 1982, 35, 69-72. (14) Turro, N. J.; Okubo, T.; Chung, C.-J. J. Am. Chem. Soc. 1982, 104, 1789-1794. (15) Hashimoto, S.; Thomas, J. K. J. Am. Chem. Soc. 1985, 107, 46554662. (16) Ponce, A.; Wong, P. A.; Way, J. J.; Nocera, D. G. J. Phys. Chem. 1993, 97, 11137-11142. (17) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279-291. (18) Matsui, Y.; Mochida, K. Bull. Chem. Soc. Jpn. 1979, 52, 28082814. (19) Kano, K.; Takenoshita, I.; Ogawa, T. Chem. Lett. 1982, 321-324. (20) Nelson, G.; Patonay, G.; Warner, I. M. Anal. Chem. 1988, 60, 274279. (21) Nelson, G.; Patonay, G.; Warner, I. M. J. Inclusion Phenom. 1988, 6, 277-289. (22) Nelson, G.; Warner, I. M. J. Phys. Chem. 1990, 94, 576-581. (23) Mun˜oz de la Pen˜a, A.; Ndou, T. T.; Zung, J. B.; Greene, K. L.; Live, D. H.; Warner, I. M. J. Am. Chem. Soc. 1991, 113, 1572-1577. (24) Zung, J. B.; Mun˜oz de la Pen˜a, A.; Ndou, T. T.; Warner, I. M. J. Phys. Chem. 1991, 95, 6701-6706. (25) Schuette, J. M.; Will, A. Y.; Abaria, R. A.; Warner, I. M. Appl. Spectrosc. 1994, 48, 581-586. (26) Hamai, S. J. Phys. Chem. 1989, 93, 2074-2078. (27) Hamai, S. J. Phys. Chem. 1990, 94, 2595-2600. (28) Hamai, S. J. Am. Chem. Soc. 1989, 111, 3954-3957.
Xanthone-Cyclodextrin Complexes (29) Schuette, J. M.; Ndou, T. T.; Mun˜oz de la Pen˜a, A.; Mukundan, Jr., S.; Warner, I. M. J. Am. Chem. Soc. 1993, 115, 292-298. (30) Hamai, S.; Ikeda, T.; Nakamura, A.; Ikeda, H.; Ueno, A.; Toda, F. J. Am. Chem. Soc. 1992, 114, 6012-6016. (31) Barra, M.; Bohne, C.; Scaiano, J. C. J. Am. Chem. Soc. 1990, 112, 8075-8079. (32) Scaiano, J. C. J. Am. Chem. Soc. 1980, 102, 7747-7753. (33) Fessenden, R. W.; Carton, P. M.; Shimamori, H.; Scaiano, J. C. J. Phys. Chem. 1982, 86, 3803-3811. (34) Liao, Y.; Frank, J.; Holzwarth, J. F.; Bohne, C. J. Chem. Soc., Chem. Commun. 1995, 199-200. (35) Janata, E. ReV. Sci. Instrum. 1986, 57, 273-275. (36) The linearized form of this treatment corresponds to the BenesiHildebrand method.37 Nonlinear fits were employed since they provide equal weighting of the experimental data. (37) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 27032707. (38) A complete derivation of eqs 6 and 7 is given in the supporting information. The assumptions are equal fluorescence quantum yields for xanthone bound to CD in the absence and presence of alcohol and [alcohol] . [CD] . [Xan]. (39) Tables containing the Kapp values at different alcohol concentrations are listed in the supporting information. (40) Matsui, Y.; Nishioka, T.; Fujita, T. Top. Curr. Chem. 1985, 128, 61-89.
J. Phys. Chem., Vol. 100, No. 2, 1996 743 (41) Tee, O. S.; Gadosy, T. A.; Giorgi, J. r. B. J. Chem. Soc., Perkin Trans. 2 1993, 1699-1700. (42) Liao, Y.; Bohne, C. To be published. (43) A complete derivation of eq 11 is given in the supporting information, which also states the equations for the preexponential factors. (44) Inoue, Y.; Hakushi, T.; Liu, Y.; Tong, L.-H.; Shen, B.-J.; Jin, D.S. J. Am. Chem. Soc. 1993, 115, 475-481. (45) Tran, C. D.; Fendler, J. H. J. Phys. Chem. 1984, 88, 2167-2173. (46) Breslow, R.; Halfon, S. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 6916-6918. (47) We thank one of the referees for bringing this point to our attention. (48) Barra, M.; Bohne, C.; Scaiano, J. C. Photochem. Photobiol. 1991, 1-5. (49) Abuin, E. B.; Scaiano, J. C. J. Am. Chem. Soc. 1984, 106, 62746283. (50) Tanford, C. The Hydrophobic Effect, 2nd ed.; John Wiley and Sons: New York, 1980. (51) Blokzijl, W.; Engberts, J. B. F. N. Angew. Chem., Int. Ed. Engl. 1993, 32, 1545-1579. (52) Quina, F. H.; Alonso, E. O.; Farah, J. P. S. J. Phys. Chem. 1995, 99, 11708-11714.
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