J. Phys. Chem. 1996, 100, 769-774
769
Facile Dimerization of Viologen Radical Cations Covalently Bonded to β-Cyclodextrin and Suppression of the Dimerization by β-Cyclodextrin and Amphiphiles Joon Woo Park,* Nak Hee Choi, and Jung Ha Kim Department of Chemistry, Ewha Womans UniVersity, Seoul 120-750, Korea ReceiVed: July 20, 1995; In Final Form: October 6, 1995X
Dimerization of radical cations of mono-6-(1-alkyl-4,4′-bipyridino)-β-cyclodextrins (β-CD-CnV•+, n ) 6, 7, 8) formed via Ru(bpy)32+-sensitized photochemical reduction of β-CD-CnV2+ was studied spectroscopically, and the results were compared with those of methylalkyl viologens. Spectral characteristics of monomer and dimer of viologen radical cations were derived. The dimerization constants have been determined to be 4.0 × 104 M-1 for β-CD-C6V•+, 8.9 × 105 M-1 for β-CD-C7V•+, and 6.8 × 106 M-1 for β-CD-C8V•+. These values are 2-3 orders of magnitude greater than those for the corresponding methylalkyl viologen radical cations. The dimer formation is driven by a large enthalpy decrease and a moderate entropy increase. The dimerization is suppressed upon addition of β-CD or amphiphilic molecules that are included into β-CD. These results indicate that the dimers are stabilized by inclusion of the alkyl chain of the 1-alkyl-4,4′-bipyridino moiety of the β-CD-CnV•+ molecule into the β-CD cavity of the counter molecule. Association constants of β-CD-CnV•+ with β-CD and amphiphilic molecules have been determined from the dependence of dimerization constants on the concentrations of β-CD or amphiphiles. The results suggested that the terminal methyl and ethyl groups of the alkyl chains of β-CD-C7V•+ and β-CD-C8V•+, respectively, are included in the β-CD cavities of the same molecules, and this intramolecular inclusion affects the association of β-CD-CnV•+ with β-CD and amphiphiles.
Introduction V2+)
have received Viologens (1,1′-dialkyl-4,4′-bipyridinium, much attention as electron mediators in various types of photochemical, electrochemical, and chemical applications extending their uses in solar energy conversion systems,1-5 electrochromic displays,6,7 herbicides,8 and electron transfer catalyst for reductive transformation of organic compounds.9,10 The efficacy of viologens in these applications has been mostly related with the facile one-electron reduction of V2+ to radical cation V•+. V•+ is known to dimerize in solution.5-7,11-32 The dimer formation affects the electrochromic properties of viologens6,7,28 and further reactions where V•+ participates.5,15,16 The dimers have been characterized by UV-vis,5,7,12,14,21-27,31,32 IR,17 Raman,26,28-30 and ESR13 spectroscopies. The reported monomer-dimer equilibrium constant (KD) for dimethyl viologen radical cation (C1C1V•+) in aqueous solutions around 25 °C is in the range of 380-840 M-1.11,14,20,21,23 The ambiguity of the UV-vis spectroscopic parameters of the viologen dimer might be responsible for the wide range of the reported KD values. The dimerization of viologen radical cations is greatly enhanced by the presence of low concentrations of anionic surfactants18,19 or polyelectrolytes19,20 or by Nafion films,21-23 and the phenomena have been attributed to the concentrating effect of the anionic microparticles or films for the cationic species. The radical cations of unsymmetric viologens with long hydrocarbon chains24-26 or symmetric viologens with relatively long hydrocarbon chains27-30 form much more stable dimers than C1C1V•+. However, the exact KD values for the viologens have not been reported, presumably due to the complications arising from adsorption of the viologens on electrode surface or glassware.15,33 Radicals of bisviologens linked by flexible organic bridges form dimers of remarkable stability by intramolecular process.30-32 X
Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0769$12.00/0
Cyclodextrins (CDs) are cyclic oligosaccharides that possess hydrophobic cavities capable of forming inclusion complexes with a variety of organic molecules in aqueous solution.34-36 Several investigators have reported the results of studies on the interaction of viologens with CDs and the effect of CDs on the dimerization of viologen radicals.5,7,16,24,25,27,28 Though the exact behavior depends upon the length of the alkyl chains of the viologen, it is generally recognized that R- and β-CDs prevent the dimer formation,5,7,16,25,27,28 while γ-CD facilitates the dimerization.7 The covalent attachment a viologen to CDs provides supramolecules which exhibit quite different physicochemical properties from the simple mixtures of viologens and CDs by intraand intermolecular interactions between viologen and CD moieties.37,38 In this paper, we report facile dimerization of viologen radical cations, β-CD-CnV•+, which are covalently bonded to β-CD. Also, it is demonstrated that the addition of β-CD or amphiphilic molecules which form inclusion complexes with β-CD prevents the dimerization. From the dependence of the apparent dimerization constants of β-CD-CnV•+ on the concentration of β-CD or amphiphiles, the dimerization constants of β-CD-CnV•+ and the association constants of β-CD or amphiphiles with β-CD-CnV•+ are determined. In addition, UV-vis spectroscopic features of viologen radical dimer and thermodynamic parameters of the dimer formation are presented. Experimental Section Materials. β-CD was purchased from Aldrich, and the concentration of β-CD solutions was calculated from optical rotation data using [R]25D ) 162.5°.39 1-Heptyl-4,4′-bipyridinium bromide was obtained from Aldrich. Other 1-alkyl-4,4′bipyridinium halides (alkyl ) methyl, hexyl, octyl) were prepared by reacting 4,4′-bipyridine (Aldrich) with the corresponding alkyl halide (Aldrich) according to a literature procedure.37 Mono-6-(1-alkyl-4,4′-bipyridino)-β-CD (β-CDCnV2+) was synthesized by reacting C-6-mono-tosylated β-CD © 1996 American Chemical Society
770 J. Phys. Chem., Vol. 100, No. 2, 1996 (0.5 g, 0.39 mmol)40 with 1-alkyl-4,4′-bipyrdinium halide (2 mmol) in 10 mL of DMF at 95 °C for 40 h under a nitrogen atmosphere.41 After precipitation with acetone, the product was purified by ion-exchange chromatography on a Sephadex CM25 column using a linear gradient of NaCl (0.0-1.0 M) containing 0.02 M phosphate buffer (pH 7.0). The fractions showing optical rotation (measured with a JASCO DIP-140 polarimeter) and a UV absorption band near 260 nm were combined, concentrated, desalted by gel chromatography on a Sephadex G-25 column with 0.01 M HCl as eluent, ultrafiltrated by using Spectrum type C cellulose membrane (MW cutoff 500), and finally freeze-dried. The β-CD-CnV2+ samples were assayed by UV absorption data using 262 ) 21000 M-1 cm-1.42 All compounds gave appropriate 1H NMR spectra37 and elemental analysis data. Molar optical rotation, [φ]25D, was 140 deg M-1 dm-1. 1-Methyl-1′-alkyl viologen dichlorides (C1CnV2+ 2Cl-: n ) 7, 8) were prepared by reacting 1-alkyl-4,4′bipyridinium bromide with methyl iodide, followed by anion exchange by stirring the viologen solution in the presence of AgCl. Other chemicals were obtained from Aldrich and used without further purification. Water was deionized and distilled in glass. Ru(bpy)32+-Sensitized Photochemical Reduction of Viologens. The photosensitized reduction of viologens was performed in a gas-purging cuvette of light pathlength 1.0 cm containing pH 10 aqueous 0.1 M NaCl solutions which include 5.0 × 10-5 M Ru(bpy)32+ as a photosensitizer, 5.0 × 10-5 M β-CD-CnV2+ (n ) 6, 7, 8), and 0.10 M ethylenediaminetetraacetate (EDTA) as a sacrificial electron donor. Photoreduction of C1CnV2+ and β-CD-C1V2+ were carried out in a similar manner as above using a cuvette of light pathlength 0.20 cm, and with 2.5 × 10-4 M Ru(bpy)32+ and 2.5 × 10-4 M viologen solutions. The desired amount of β-CD or amphiphiles was added to the solutions. The solution in the cuvette was purged with solvent-saturated argon gas for at least 30 min and then irradiated with a 200 W tungsten lamp of which light was filtered through water. Absorption spectra were recorded with a Hewlett-Packard HP-8452 diode array spectrophotometer or GBC UV-vis 920 spectrophotometer at various irradiation times. The solution prior to irradiation was used as a blank. Spectral Decomposition. The monomer and dimer spectrum of viologen radical cations were deduced from the spectra of C1C8V•+ and β-CD-C8V•+, respectively. The dimer component in a spectrum of monomer-dimer mixture was obtained by subtracting the monomer spectrum (multiplied a factor) from the mixture spectrum: the multiplication factor was adjusted until the resulting spectrum is indistinguishable from the dimer spectrum in shape in the wavelength region of 300-800 nm (for details, see text). Results and Discussion We will first show the UV-vis absorption spectral behavior of the photosensitized reduction products of the viologens covalently-bonded to β-CD (β-CD-CnV2+) and present the spectral characteristics of the viologen radical cation and its dimer. Then, equilibrium constants for the dimerization of β-CD-CnV•+ and thermodynamic parameters for the dimerization will be presented. Finally, we will describe the effects of β-CD and amphiphiles on the dimerization and present the binding constants of the additives with β-CD-CnV•+. UV-Vis Absorption Spectral Characteristics of Viologen Radical Cation and Its Dimer. Irradiation of deaerated aqueous solutions of viologens in the presence of Ru(bpy)32+
Park et al.
Figure 1. Absorption spectra of the one-electron reduction products of C1C8V2+ (spectrum A) and β-CD-C8V2+ (spectrum B) obtained by irradiation of the aqueous solutions containing 5.0 × 10-5 M viologen, 5.0 × 10-5 M Ru(bpy)32+, and 0.1 M EDTA at pH 10. The solution prior to irradiation is used as a blank. The spectra are those taken after quantitative conversion of the viologens to radical cations.
and EDTA at pH 10 reduces the viologen dication (Figure 1).9a-c,f,38 The spectra of the reduction products of 5.0 × 10-5 M dimethyl vilogen (C1C1V2+), methyloctyl viologen (C1C8V2+), and β-CD-C1V2+ were virtually that of viologen radical cation monomer. On the other hand, the spectra of the reduction products of β-CD-C7V2+ and β-CD-C8V2+ show all the absorption characteristics of the dimer of viologen radical cations25-27 and only hints of those corresponding to the monomer. The spectrum grows continuously upon irradiation and reaches maximum. Prolonged irradiation resulted in diminution of the absorption band above 450 nm and an increase in absorbance near 390 nm. The latter behavior is attributed to the formation of neutral viologens (V0) by further reduction of the viologen radical cations or their dimer.38 Since the equilibrium constant of the disproportionation reaction of viologen radical cations (2V•+ ) V0 + V2+) is ∼10-7,9b virtually quantitative conversion of V2+ to V•+ is required to observe appreciable amount of V0.38 Thus it is sound to assume that V2+ is reduced to V•+ quantitatively, when the absorbance reaches maximum. Figure 1 shows the absorption spectra of the reduction products of 5.0 × 10-5 M C1C8V2+ (spectrum A) and β-CDC8V2+ (spectrum B) taken after quantitative one-electron reduction. Though spectra A and B resemble very closely the spectra of the monomer and dimer of viologen radical cation, respectively, there is some contribution of the dimer in spectrum A and monomer in spectrum B. Contribution of the dimer in the spectrum of the reduction product is smaller as the total concentration of the reduction product is lower. We assumed that the spectrum of the reduction product of C1C8V2+ taken at ca. 10% conversion, i.e., the total concentration of the radical cation of ca. 5 × 10-6 M, is the spectrum of monomer.43 The monomer spectrum of 5.0 × 10-5 M C1C8V•+ was deduced by expanding the spectrum taken at ca. 10% conversion until the absorbance at 548 nm, an isosbestic point of the monomerdimer equilibrium (eq 1), coincides with the absorbance at 548 nm of spectrum A. The absorbances of the expanded spectrum at other isosbestic points correspond well with those in Figure 1. Subtraction of 1% of the monomer spectrum from spectrum B resulted in disappearance of the shoulder near 396 nm. The resulting spectrum can be regarded as the spectrum of dimer of β-CD-C8V•+, where the dimer concentration is 2.48 × 10-5 M. The UV-vis spectral characteristics of the monomeric viologen radical cation and its dimer as well as the isosbestic points of
Dimerization of Viologen Radical Cations Bonded to β-CD
J. Phys. Chem., Vol. 100, No. 2, 1996 771
TABLE 1: UV-Vis Absorption Spectral Characteristics of Viologen Radical Cation and Its Dimer in Aqueous Solutiona,b
monomer dimer
λmax/nm
× 10-4/ (M-1 cm-1)
λmax/nm
× 10-4/ (M-1 cm-1)
396 362
4.2 5.0
602 518
1.37 2.10
a Isosbestic points (absorption coefficient per viologen unit) for the monomer-dimer equilibrium are 373 (16 300), 430 (960), 548 (8920), and 754 nm (1720 M-1 cm-1). b Estimated uncertainty of values is 5% (see text).
the monomer-dimer equilibrium are summarized in Table 1. KD
2 V•+ {\} (V•+)2
(1)
Two major error sources could be envisaged for the absorption coefficients reported here: one is incomplete or further reduction, and the other is concentration of viologen. We estimate that the magnitude of these uncertainties are 1-2%. Thus, we could say that the values of absorption coefficients in Table 1 are accurate within 5%. It was shown that the spectrum of viologen radical cation monomer depends little on the nature of the solvent.42,44 The absorption coefficients of C1C8V•+ monomer (Table 1) are in excellent agreement with those reported for MV•+ determined from a spectroelectrochemical study42 or from nonaqueous solutions of isolated MV•+PF6 crystals.44 This indicates that the max values of viologen radical cation monomers are insensitive to the variation of the alkyl substituents on the bipyridine ring. A number of groups have reported the UV-vis spectral characteristics of dimers of viologen radical cation. From the results of computer decomposition, Stargardt and Hawkridge14 reported absorption coefficients of 89 700 M-1 cm-1 at λmax ) 367 nm and 32 400 M-1 cm-1 at λmax ) 537 nm for the dimer of dimethyl viologen radical cations, (C1C1V•+)2. These values are about twice the corresponding values obtained in this work. On the other hand, Neta and co-workers presented 23 000 M-1 cm-1 at λmax ) 365 nm and 9600 M-1 cm-1 at λmax ) 545 nm for the same dimer, claiming less than 20% uncertainty.32 These values are ca. 25-45% lower than the results of this work. Another report on the absorption coefficients is for (C1C1V•+)2 and the dimer of the one-electron reduction product of 1-methyl1′-(3-sulfonatopropyl)-4,4′-bipyridinium (MPVS+) in Nafion films from a spectroelectrochemical study by Johanson et al.23 The maximum absorption coefficients are estimated to be 46 000 M-1 cm-1 at λmax ) 360 nm and 20 000 M-1 cm-1 at λmax ) 520 nm from a figure presented in the report: considering 15% uncertainty in the ensurance of completion of the reduction,23 our results agree with the report. Dimerization Equilibrium Constants of β-CD-CnV•+ and Thermodynamic Parameters for the Dimerization. The absorption spectra of the reduction products of β-CD-C6V2+ and β-CD-C7V2+ were taken at various irradiation times. The spectra were typical of those of the mixture of the viologen radical cation monomer and dimer, and the contribution of the dimer to a spectrum was greater as more viologen dication was reduced. Each spectrum of the mixture was decomposed to monomer and dimer spectra by the linear combination of the respective spectrum, as exemplified in Figure 2. The concentrations of the monomer and dimer in the mixture were determined by using the spectral parameters given in Table 1, and then KD value was calculated. Good agreement among KD values calculated at different irradiation times, ie., different total
Figure 2. Separation of an absorption spectrum (a) of the one-electron reduction product of β-CD-C6V2+ into the dimer spectrum (b) and monomer spectrum (c).
TABLE 2: Dimerization Equilibrium Constants (KD) of Viologen Radical Cations and Binding Constants (KC) of β-CD-CnV•+ with β-CD or n-Octyl Sulfate at 25.0 °Ca KC/M-1 viologen radical cation
KD/M-1
β-CD
n-octyl sulfate
C1C1V•+ C1C7V•+ C1C8V•+ β-CD-C1V•+ β-CD-C6V•+ β-CD-C7V•+ β-CD-C8V•+
500 770 850 100 4.0 × 104 8.9 × 105 6.8 × 106
320 350 380
3000 1500 950
a The uncertainty of K and K values obtained in this work is D C estimated to be (15%.
concentration of the reduced viologen was found. The KD values were calculated at seven different irradiation times and they were averaged. The value at 25.0 °C was (4.0 ( 0.3) × 104 M-1. The KD values of β-CD-C1V•+, C1C1V•+, C1C7V•+, and C1C8V•+ were determined by the same method, using a 0.20 cm pathlength cell and 2.5 × 10-4 M viologen solutions rather than the usual 1.0 cm cell and 5.0 × 10-5 M viologen solutions. The results are given in Table 2. The KD value of C1C1V•+, 500 ( 75 M-1, is in the agreement with previously reported values, 380-840 M-1.1,11,14,20,21,23 We also attempted to determine KD values of β-CD-C7V•+ and β-CD-C8V•+ from the spectra of reduction products of the respective viologens. However, the fractions of monomer in the mixtures were too small to give reliable results. For these viologens, the KD values were obtained from the dependence of dimerization behavior of the viologens on the concentrations of β-CD or n-octyl sulfate (Vide infra). The results are summarized in Table 2. Table 2 shows that the KD values of β-CD-CnV•+ (n ) 6, 7, 8) are larger than the corresponding values of C1CnV•+ by 2-4 orders of magnitude. This is quite a contrast to the observation of dissociation of viologen radical cation dimers upon addition of β-CD.5,7,16,25,27,28 Inclusion of the alkyl chain of the viologen moiety of a β-CD-CnV•+ molecule into the β-CD cavity of the other β-CD-CnV•+ molecule (Scheme 1) might be responsible for the remarkable stability of the dimers of β-CD-CnV•+. This argument is supported by the fact that the KD value of β-CDC1V•+ is considerably less than that of C1C1V•+.
772 J. Phys. Chem., Vol. 100, No. 2, 1996
Park et al.
SCHEME 1. Schematic Representation for the Monomer-Dimer Equilibria of β-CD-C8V•+ in the Presence of β-CD or n-Octyl Sulfate. Similar Schemes Can Be Drawn for β-CD-C6V•+ and β-CD-C7V•+ and in the Presence of Other amphiphilic Molecules Which Form Inclusion Complexes with β-CD
Figure 3. Dependence of the dimerization constant (KD) of β-CDC6V•+ on temperature.
We also measured KD values of β-CD-C6V•+ as a function of temperature. The plot of the logarithm of KD values against the reciprocal of temperature showed good linearity (Figure 3), from which the standard free energy (∆G°), enthalpy (∆H°), and entropy (∆S°) of the dimerization of β-CD-C6V•+ are estimated to be -26.3 kJ/mol, -23.4 kJ/mol, and 9.73 J/K/ mol, respectively at 25 °C. This implies that the dimer formation is driven by a large favorable enthalpy change but only a small favorable entropy change.45 Perturbation of Dimerization of β-CD-CnV•+ by the Addition of β-CD and Amphiphiles. Figure 4 shows absorption spectra of β-CD-C7V•+ taken in the presence of various concentrations of β-CD. Disruption of the dimerization of β-CD-C7V•+ by β-CD is clearly seen. Similar effects of β-CD were observed with other viologens bound to β-CD. The presence of amphiphiles which form inclusion complexes with β-CD also inhibited the dimerization of β-CD-CnV•+. These are attributed to the complexation of β-CD or the amphiphiles with β-CD-CnV•+. The dimerization of β-CD-CnV•+ in the presence of β-CD or an amphiphile, e.g. n-octyl sulfate, is proposed as Scheme 1. If we assume that the complexes do not form dimer, two competing equilibria (2) and (3) should be considered KD
2 β-CD-CnV•+ {\} (β-CD-CnV•+)2 KC
β-CD-CnV•+ + X {\} β-CD-CnV•+-X
(2) (3)
where X denotes β-CD or amphiphiles and KC is the equilibrium constant for the complexation reaction between β-CD-CnV•+ and X.
Figure 4. Effect of added β-CD on the spectrum of β-CD-C7V•+. The total concentration of β-CD-C7V•+ is 4.69 × 10-5 M.
The apparent dimerization constant (KD′) in the presence of X is defined as eq 4 and is related to KD, KC, and [X] by eq 5.
KD′ )
[(β-CD-CnV•+)2] ([β-CD-CnV•+] + [β-CD-CnV•+-X])2 KD′ )
KD (1 + [X]KC)2
(4)
(5)
Equation 5 gives the following linear equation.
(KD′)-1/2 ) (KD)-1/2 + (KD)-1/2[X]KC
(6)
Since we used large excess of X compared to β-CD-CnV2+, we can substitute [X] by the total concentration of β-CD or amphiphiles. We have taken absorption spectra of β-CD-CnV•+ in the presence of various concentrations of β-CD or amphiphiles at various irradiation times. The spectra were decomposed into dimer and monomer spectra, and KD′ values were calculated from the absorbances of the separated spectra using parameters in Table 1. The KD′ values were determined at more than seven different irradiation times for a given concentration of β-CD or amphiphiles, and they were averaged: the standard deviation of the averaged values was usually less than 10%. Figure 5 shows the plots of the experimental data obtained from β-CD and n-octyl sulfate (C8OS) according to eq 6. When the
Dimerization of Viologen Radical Cations Bonded to β-CD
J. Phys. Chem., Vol. 100, No. 2, 1996 773 TABLE 3: Binding Constants (KC) of Amphiphiles with β-CD-C7V•+ and β-CD at 25. °C KC/M-1 amphiphiles
β-CD-C7
n-C7H15SO3n-C8H17SO3n-C8H17OSO3n-C8H17NH3+ a
Figure 5. Plots of (KD′)-1/2 against the concentration of β-CD (b, 2, 9) and n-octyl sulfate (O, 4, 0) for β-CD-C6V•+ (O, b), β-CD-C7V•+ (4, 2), and β-CD-C8V•+ (0, 9).
concentration scales of C8OS are expanded by 9.48, 4.16, and 2.48 times for β-CD-C6V•+, β-CD-C7V•+, and β-CD-C8V•+, respectively, the experimental data obtained with C8OS were on the same line with those with β-CD for each β-CD-CnV•+. The combined data gave good linearity with coefficients of correlation better than 0.998. The KD and KC values were calculated from the plots, and the results are included in Table 2: the KD value of β-CD-C6V•+ obtained from the plots agrees with that obtained from spectra in Figure 2 which were taken in the absence of β-CD (see previous section). Data in Table 2 show several interesting features. One is that dependence of the KD value on the alkyl chain length of viologen moieties is much greater than the usual ca. 3-fold increase in association constant of alkyl derivatives with β-CD per increment of a methylene group.35,36 This can be attributed to cooperative inclusion of each alkyl chain of two interacting monomers into the cavity of the β-CD moiety of the countermolecule to form a dimer. Another is the small dependence of the binding constant of β-CD-CnV•+ with β-CD on the alkyl chain length: the binding constant is 320 M-1 for β-CD-C6V•+ and increases to 380 M-1 when the alkyl chain is octyl. These values are slightly larger than the corresponding value for n-hexylamine, 240 M-1, but much smaller than that for n-heptylamine (KC ) 650 M-1) or n-octylamine (KC ) 1690 M-1) with native β-CD.36 This implies that the effective alkyl chain of β-CD-CnV•+ molecules, where n ) 6, 7, and 8, for the inclusion complexation with β-CD is much the same as the hexyl chain. This could be due to the intramolecular inclusion of the terminal methyl group of the heptyl chain of β-CD-C7V•+ and ethyl group of the octyl chain of β-CD-C8V•+ in the β-CD cavity of the same molecule (see Scheme 1 for β-CD-C8V•+): the favorable free energy change from intermolecular inclusion of the terminal groups into β-CD is roughly compensated by the energy required to release the groups from the intramolecular inclusion. CPK space-filling models indicated that the intramolecular inclusion is geometrically feasible. The above arguments are also supported by the dependence of the binding constants of β-CD-CnV•+ with C8OS on the alkyl chain length. In a previous report, we showed that the binding constants of C8OS with native β-CD is 2560 M-1.35 Considering the electrostatic effect on the association, the KC value of 3000 M-1 for the β-CD-C6V•+/C8OS system corresponds well with the result. The observation of smaller a KC value as the alkyl chain of β-CDCnV•+ is longer can be explained in terms of the aforementioned intramolecular partial inclusion of the alkyl chain into the cavity of the β-CD moiety: the maximum number35 of carbon atoms
320 840 1500 330
V•+
β-CD 430a 1180a 2560a 750b
Reference 35. b Reference 36.
of an aliphatic hydrocarbon chain that can be accommodated inside the cavity of a β-CD is eight, and the intermolecular association of C8OS with β-CD-CnV•+ competes with the intramolecular association of the host molecules. The effects of other amphiphilic molecules on the dimerization of β-CD-CnV•+ are also investigated by the same method. The association constants of various amphiphiles with β-CDC7V•+ were determined and compared with those of the amphiphiles with native β-CD in Table 3. The binding constants of anionic guests to β-CD-C7V•+ are 60-70% of the corresponding values obtained with native β-CD. However, the binding of a cationic guest, n-C8H17NH3+ with β-CD-C7V•+ is only about 44% of that with native β-CD. This implies that electrostatic effect also contributes to the binding. Conclusions Radical cations produced by Ru(bpy)32+-sensitized photochemical reduction of 1-alkyl-1′-β-CD-viologens (β-CD-CnV2+), where the alkyl groups (Cn) are hexyl, heptyl, or octyl, form dimers of remarkable stability. The molar absorptivities of the dimer at absorption maxima are 21 000 M-1 cm-1 at 518 nm and 50 000 M-1 cm-1 at 362 nm. The equilibrium constants for the dimerization are 2-3 orders of magnitude greater than those of the corresponding 1-alkyl-1′-methyl viologens. The dimer is stabilized by mutual inclusion of the alkyl chain of the viologen moiety into the cavity of the β-CD moiety of the counter molecule, in addition to the π-π interaction between the bipyridine rings. The dimer formation is favored by a large enthalpy decrease and moderate entropy increase. Addition of β-CD or amphiphiles such as alkyl sulfates suppresses the dimerization. This is due to the inclusion of the alkyl chain of β-CD-CnV•+ into the cavity of added β-CD or inclusion of the alkyl chains of the amphilphile into the β-CD cavity of β-CDCnV•+, respectively. From the dependence of apparent dimerization constants on the concentration of β-CD or amphiphiles, the association constants of β-CD with β-CD-CnV•+ or amphiphiles with β-CD-CnV•+ are determined. The alkyl chains of β-CD-CnV•+, when n > 6, interact with a β-CD moiety of the same molecule intramolecularly. This results in smaller association constants of β-CD-CnV•+ with added β-CD or amphiphiles than those of the corresponding alkylamines or the amphiphiles with native β-CD, respectively. The suppression of dimerization of β-CD-CnV•+ by amphiphiles can be used for studies of association of amphiphiles with β-CD-CnV•+. Also the sensitivity of the dimer formation on the concentrations of added β-CD or amphiphiles and the large difference of spectra between dimer and monomer of the viologen radical cations provide us a convenient means for controlling the color of electrochromic systems based on β-CD-CnV•+ by addition of β-CD or amphiphiles, and for spectroscopic analysis of the additives. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (91-0300-03 and 94-050105-01-3) and by the Ministry of Education of the Republic of
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