J. Phys. Chem. 1995, 99, 12109-12114
12109
Inclusion Complexes and the Room-Temperature Phosphorescence of 6-Bromo-2-naphthol in Aerated Aqueous Solution of a-Cyclodextrin Sanyo Hamai Department of Chemistry, College of Education, Akita University, Tegata Gakuen-machi 1-1,Akita 010, Japan Received: April 11, 1995@
The inclusional complexation between a-cyclodextrin (a-CD) and 6-bromo-2-naphthol (BN) in aqueous solutions is investigated by means of electronic absorption, induced circular dichroism, emission, and 'H NMR spectroscopies. With stoichiometries of 1:l and 2:1, a - C D forms inclusion complexes with BN. The room-temperature phosphorescence of BN in aerated aqueous solution is observed from a 2:l a-CD-BN inclusion complex, whereas a 1:1 a-CD-BN inclusion complex does not phosphoresce at room temperature. No observation of the BN room-temperature phosphorescence in deaerated solutions without a-CD suggests that a-CD decelerates not only the oxygen-quenching rate but also the other deactivation rates of the triplet state of BN. Analyses of chemical shift differences in 'HNMR signals of BN indicate that the first a-CD molecule accommodates an end substituting a Br atom on a naphthalene ring in BN and then the second a - C D molecule encapsulates the other end substituting a hydroxyl group on the same naphthalene ring to form the 2:1 a-CD-BN inclusion complex.
Introduction Cyclodextrins (CDs) are cyclic oligosaccharides composed of six, seven, and eight D-glucopyranose units, which are named a-, /3-, and y-CD, respectively. Since these CDs have a unique molecular architecture that is shaped like a truncated cone with a hollow, hydrophobic cavity, organic guest molecules of an appropriate size can be accommodated into cavities of CDs to form inclusion complexes.'-* Due to the accommodation of a guest molecule into the CD cavity, a guest in the excited state is sterically protected from an attack of a quencher such as an iodide. Consequently, quenching rate constants for the guest fluorescence are reduced by the formation of inclusion complexes with C D S . ~For , ~ temary inclusion complexes that contain two different kinds of guest molecules or for a 2:2 /3-CDnaphthalene inclusion complex, the more prominent protection effects on the quenching are observed compared to 1:1 inclusion c o m p l e x e ~ . ~The . ~ additional guest within the CD cavity or an additional CD molecule further shields an excited fluorophore from quenchers. A heavy atom introduced into an organic molecule strongly induces intersystem crossing from the excited singlet state to the triplet state.5 At room temperature, however, it is very difficult to observe phosphorescence of aromatic hydrocarbons or their derivatives in homogeneous, even degassed solutions, because triplet states of phosphorophores in solutions are predominantly deactivated radiationlessly without emitting phosphore~cence.~~~ On the other hand, a halogenated guest incorporated into the CD cavity often exhibits room-temperature phosphorescence that is not usually detected in liquid media.*-I0 Although a CD molecule considerably protects the triplet state of a guest, which is located within the CD cavity, from dissolved oxygen, in the case of binary inclusion complexes of CD roomtemperature phosphorescence is not necessarily detected in airsaturated solutions. For ternary inclusion complexes that include both a halogenated guest (phosphorophore) and an additional guest having no heavy atom, room-temperaturephosphorescence has also been observed.1'.12When ternary inclusion complexes contain a nonhalogenated phosphorophore and a second guest @
Abstract published in Advance ACS Absfructs, July 15, 1995.
0022-3654/95/2099-12109$09.00/0
having a heavy atom, the extemal heavy atom effect exerted by a second guest effectively generates the triplet state of a phosphorophore, which is incorporated into the CD cavity. As a consequence, the room-temperature phosphorescence appears in solution^.'^-'^ Similarly, bromine-substituted CD efficiently produces the triplet state of an incorporated guest, resulting in the emission of the room-temperature phosphorescence.I6 It should be noted that, in the CD inclusion complexes exhibiting room-temperature phosphorescence, only one CD molecule is associated with a phosphorophore, except for a 2: 1 a-CD-6-bromo-2-naphthol (BN) inclusion c0mp1ex.I~ In addition, not a-CD but /3- and y-CDs (or their derivatives) are known to act as hosts of phosphorophores emitting roomtemperature phosphorescence in the CD cavity, with the exception of the a-CD-BN system. Previously, we have found room-temperature phosphorescence of an inclusion complex that is formed among two a-CD molecules and a single BN molecule (2:l a-CD-BN inclusion complex) in aerated aqueous ~ o l u t i o n . ' ~In this paper, in addition to the room-temperature phosphorescence of BN in the 2: 1 inclusion complex, we report on the complexation of a-CD and BN and the disposition of the two accommodating a-CD molecules relative to a BN molecule on the basis of induced circular dichroism (icd) and 'HNMR spectra of BN.
Experimental Section 6-Bromo-2-naphthol (BN) was purchased from Tokyo Kasei Kogyo, Ltd., and was recrystallized twice from benzene. a-Cyclodextrin (a-CD) purchased from Nacalai Tesque, Inc., was used as received. /3-CD obtained from Nacalai Tesque, Inc., was recrystallized twice from water. Aqueous solutions of BN were prepared according to the method previously described.l8 Concentrations of BN were about 9 x M throughout this work, except for measurements of an icd spectrum for a BN solution containing P C D . In this solution, M. the BN concentration was about 2 x Absorption spectra were obtained with a Shimadzu 260 spectrophotometer. Fluorescence and phosphorescence spectra were recorded on a Shimadzu RF-50 1 spectrofluorometer equipped with a cooled Hamamatsu R-943 photomultiplier. 0 1995 American Chemical Society
Hamai
12110 J. Phys. Chem., Vol. 99, No. 32, 1995
0.4
r
I
00
Wavelength 1 nm 300 350
'9'1
1 .
t:
0'
0
2
4
6
0
1
0
I
1
2
I l o 2 mol'' dm3 Figure 2. Plot of 1/(A - A") against 1/[a-CDlofor BN in aqueous solution. [a-CD],,"
Wavenumber I lo3 cm-' Figure 1. Absorption spectra of BN in aqueous solutions containing varying concentrationsof a-CD: (1) 0, ( 2 ) 1.0 x (3) 2.0 x (4) 5.0 x and ( 5 ) 1.0 x M.
These emission spectra were corrected for the spectral response of the spectrofluorometer. Because the fluorescence intensity of aqueous BN solutions containing a-CD was slightly increased during measurements, prolonged irradiation of samples was avoided as much as possible. However, this enhancement of the fluorescence intensity hardly affected the phosphorescence intensity. Phosphorescence and triplet lifetimes were measured, with a conventional flash photolysis apparatus, by monitoring decays of phosphorescence and triplet-triplet absorption intensities, respecti~e1y.I~ Induced circular dichroism (icd) spectra were taken with a JASCO 5-600 spectropolarimeter. 'H NMR spectra were recorded on a Varian VXR-500s spectrometer, operating at 500 MHz. About 500 transients were collected for each 'H NMR spectrum. Chemical shifts were expressed in ppm relative to an external standard of sodium 3-(trimethylsily1)- 1-propanesulfonate-d4 in D20. Spectroscopic measurements were made for aerated sample solutions at 25 f 0.1 'C, except for measurements of icd spectra and lifetimes, for which experiments were performed at room temperature.
Results and Discussion Absorption Spectra of 6-Bromo-2-naphthol. Figure 1 shows absorption spectra of 6-bromo-2-naphthol (BN) in aqueous solutions in the absence and presence of a-CD. Because a pKa value of BN has been found to be 9.2, BN in aqueous solution is in neutral form. As the a-CD concentration is increased, absorption maxima at 323 and 337 nm are redshifted with an enhancement of the absorption band intensity. In the low a-CD concentration range (below =5 x M), isosbestic points are observed at 287, 300, and 321 nm. Over all the a-CD concentration range examined, however, isosbestic points are lost, indicating that an inclusion complex other than a 1:l a-CD-BN inclusion complex (a-CD-BN) is also involved in the equilibria of the a-CD-BN system. As shown in Figure 2 , experimental data do not fit a usual double-reciprocal plot of 1/(A - Ao) against l/[a-CD]o, where A, Ao, and [a-CD]o are the absorbance of an aqueous BN solution with a-CD, that without a-CD, and the initial a-CD concentration, respectively. This provides additional evidence for the existence of an inclusion complex other than the 1:l a-CD-BN inclusion complex. Induced Circular Dichroism Spectra of BN in Solutions Containing a-CD or P-CD. To further confirm the existence of at least two kinds of inclusion complexes, we examined induced circular dichroism (icd) spectra of BN as a function of a-CD concentration (Figure 3a). Over the wavelength regions from 290 to 360 nm, icd spectra show positive signals. As the a-CD concentration is increased below about 3.0 x M,
35
.
a -235
Wavenumber I lo3 cm-1 33 31 29
27
1
33 31 29 Wavenumber 1 103 cm-1
27
Figure 3. (a) Induced circular dichroism spectra of BN in aqueous solution containing varying concentrations of a-CD: (1) 1.0 x (2), 3.0 x and (3) 1.0 x lo-* M. (b) Induced circular dichroism M) in aqueous solution containing p-CD spectrum of BN (2.4 x (1.0 x lo-? M).
the intensity of the icd spectrum in the wavelength range from 290 to 360 nm is monotonously enhanced, whereas above about 3.0 x M a-CD the intensity of the icd band from 290 to 330 nm is rather reduced compared to that for a BN solution containing 3.0 x M a-CD. This finding definitely supports our conclusion that there is more than one inclusion complex in an a-CD solution of BN. In the higher a-CD concentration range, the second inclusion complex significantly contributes to the apparent intensity of the icd spectrum, thereby resulting in the reduction of the band intensity in the 290-330-nm region. As in the case of 2-naphthol, the 290-nm and 330-nm bands of BN are assigned to the Lb and La transition, respectively. Whenever, inside the cavity, angles between the direction of an electronic transition moment and the molecular symmetry axis of CD are less than 54.44" and are greater than -54.44", positive signals are expected for icd spectra;20.2'signs of plus and minus mean a counterclockwise and a clockwise rotation from the symmetry axis of CD, respectively. For the Lb and La bands in 2-naphthol, the directions of the transition moments are rotated by -62.4' and -77.9", respectively, from the longitudinal axis (x-axis) of a naphthalene nucleus.22 In a-CD-BN, as mentioned below, a hydrophobic Br atom that is substituted on a naphthalene nucleus of BN is buried inside the a-CD cavity. This implies that an angle between the a-CD symmetry axis and the longitudinal axis of BN is ~ 3 0 'in a-CDBN. If the directions of the Lb and La transition moments of BN are the same as those of 2-naphthol, the angles between the directions of the Lb and La transitions and the a-CD symmetry axis in a-CD.BN are ca. -92.4 (87.6)' and ca. -107.9 (72.1)", respectively. In the case of such large angles, negative intensities in the icd spectrum are anticipated for a-CD-BN. However, this is not true for the observed icd spectra shown in Figure 3a. Consequently, the directions of the Lb and La transitions are perturbed by the Br substitution and are tilted
Inclusion Complexes of 6-Bromo-2-naphthol
J. Phys. Chem., Vol. 99, No. 32, 1995 12111
less than ca. 84.4' and are greater than ca. -24.4' from the longitudinal axis of BN. In contrast to the icd spectra for the a-CD-BN system, the icd spectrum for a BN solution containing p-CD (1.O x M) exhibits negative intensities in the wavelength region from 290 to 360 nm, as shown in Figure 3b. On the basis of absorbance changes in the BN absorption spectrum (not shown), it was found that /3-CD forms a 1:l inclusion complex with BN. Consequently, the icd spectrum in Figure 3b is due to the formation of the 1:l /3-CD-BN inclusion complex. The icd spectral intensity of the a-CD-BN system is opposite in sign to that of the /3-CD-BN system, evidently indicating that the inclusion positions of BN in the inclusion complexes of a-CD and /3-CD are different from each other. The icd spectrum for the /3-CD-BN inclusion complex is similar to that for a 1:l /3-CD-Znaphthol inclusion complex.23 An axial inclusion of 2-naphthol into the p-CD cavity has been concluded.23 Like 2-naphthol, BN is expected to be included with a naphthalene longitudinal axis being nearly parallel to the molecular axis of /3-CD. Consequently, the angle between the direction of the Lb (or La) transition and the longitudinal axis of BN is greater than 54.44' or is less than -54.44'. Combined with the deduction on the relative orientation of a-CD and BN in a-CDBN, the Lb and La transition moments are most likely to be directed with angles of 54.44-84.44" relative to the longitudinal axis of BN. This finding implies that the orientations of the Lb and La transition moments in BN are varied by the Br substitution because the negatively greater values (-62.4' and -77.9') have been estimated for the directions of the Lb and La transition moments in 2-naphthol. Evaluation of Equilibrium Constants for the Formation of Inclusion Complexes. Since the a-CD cavity is too small to accommodate a full part of BN, it is most likely that an additional a-CD molecule tends to associate with the other end of BN, whose opposite end is already accommodated into the a-CD cavity; a 2:l a-CD-BN inclusion complex is formed. Thus, we analyzed the a-CD concentration effect on the absorbance of BN under the assumption that in the a-CD-BN system there are two equilibria: a-CD a-CDBN
+ BN ==K , a - C D B N
(1)
+ a-CD =+K? (a-CD),*BN
(2)
where (a-CD)2*BN stands for the 2:l a-CD-BN inclusion complex, and KI and K2 are the equilibrium constant for the formation of a-CDBN and that of (a-CD)yBN, respectively. Under the conditions that the concentration of a-CD is considerably greater than that of BN, the absorbance of BN, A, is expressed as
A = [BN], (eo
+ E ~ K , [ ~ - C D+] , E,K,K,[~-CD]:)/ (1 + K,[a-CD], + K,K,[a-CD]:)
(3)
where [BN], is the initial concentration of BN, and EO,€ 1 , and €2 are the molar absorption coefficients of free BN, a-CDBN, and (a-CD)yBN, respectively. Figure 4 depicts the a-CD concentration dependence of the absorbance observed at 345 nm together with the best-fit curve that was calculated using a known EO (345 nm) value (500 M-' cm-I) and assumed values KI = 560 M-I, K2 = 530 M-I, E I (345 nm) = 930 M-' cm-I, and €2 (345 nm) = 2730 M-' cm-I. The best-fit curve for the absorbance change excellently fits the experimental data, indicating that the second inclusion complex is assigned to (aCD)yBN. A 2:l inclusion complex of a-CD and 2-naphthol
0.0
'
2
0
4
6
I 1 0 1 2
8
[a-CD]o I lom3mol dm-3
Figure 4. a-CD concentration dependence of the BN absorbance observed at 345 nm. Circles represent experimental points. The bestfit curve was calculated with K I = 560 M-', K2 = 530 M-I, el = 930 M-' cm-I, and €2 = 2730 M-I cm-I. Wavelength I nm 300
I '
350
600 700
400 450 500
1
'
30 25 20 Wavenumber 1103 cm-1
15
Figure 5. Corrected emission spectra of BN in aqueous solutions containing varying concentrations of a-CD: (1) 0, (2) 1.O x (3) 2.0 x (4)5.0 x and (5) 1.0 x M. A,, = 287 nm. Wavelength I nm 500
550
600
650 700
L
22
Wavenumber 1 103 cm-1
Figure 6. Comparison between the longer wavelength emission spectrum of a BN aqueous solution containing a-CD (1.0 x M) at 25 "C and the phosphorescence spectrum of BN in a 1:1 mixture of methanoUethano1 at 77 K.
has similarly been suggested.24 In the a-CD-2-naphthol system, KI and K2 have been estimated to be 250 f 50 and 35 f 5 M-', respectively. The values of KI and K2 for BN are about 2- and 15-fold greater than those for 2-naphthol, respectively. Phosphorescence Spectroscopic Studies of BN. Figure 5 illustrates corrected emission spectra of BN in aqueous solutions containing varying concentrations of a-CD. In the absence of a-CD, only the fluorescence of BN is ~bserved.'~Upon the addition of a-CD, the fluorescence intensity of BN is enhanced accompanied by an appearance of vibrational structures. At the same time, a new emission band appears in the wavelength range from 480 to 700 nm. Figure 6 shows both the new emission band at 25 'C and the phosphorescence spectrum of BN in methanovethano1 (1:l) at 77 K. In addition to a redshift of the former band, the former band is less structured
Hamai
12112 J. Phys. Chem., Vol. 99,No. 32, I995 -!
1.0
[-
-
i
ID 0
'
c 0
0.6
F 0.4 i
0 0.2 c
0
Y
0.0
0
2
4
6
8
1
0
1
2
[a-CD]o / 10" mol dm-3
Figure 7. a-CD concentration dependence of the room-temperature phosphorescence intensity observed at 535 nm (Zp) and the concentrations of free BN, a-CD.BN, and (a-CD)2.BN. The concentrations of these species were calculated using K I(560 M-I) and K2 (530 M-') determined in this study and were normalized as [BN]o = 1.0.
compared to the latter. Taking into account the temperature differencebetween the two spectra, these spectra are very similar to each other. Therefore, the new emission appearing in the wavelength range 480-700 nm can be assigned to the roomtemperature phosphorescence of BN. The triplet state of BN, which is induced by the internal heavy atom effect of Br and is guarded by bound a-CD, emits room-temperature phosphorescence. Under our experimental conditions, the intensity of the roomtemperature phosphorescence of BN is proportional to the concentration of a phosphorescent species. To identify which inclusion complex is the phosphorescent species, a comparison was made between the concentration of a-CDBN or (aCD)yBN and the room-temperature phosphorescence intensity. The concentrations of free BN, a-CDmBN, and (a-CD)z.BN are respectively given: [BN] = [BN]d( 1
+ K,[a-CD], + K,K2[a-CDl,2)
(4)
[a-CDBN] = K , [BN],[a-CD]d
(1
+ K,[a-CDI, + K,K,[a-CDl;)
(5)
[(a-CD),*BN] = K,K,[BN],[a-CD]~/
(1
+ K,[a-CD], + K,K2[a-CD],2)
(6)
Figure 7 displays the room-temperature phosphorescence intensity observed at 535 nm as a function of a-CD concentration, together with concentration curves simulated for free BN, a-CDBN, and (a-CD)yBN. These concentration curves have been calculated using the K Iand K2 values that have previously been evaluated from the a-CD concentration dependence of the absorbance. The excellent fit of the calculated (a-CD)yBN concentration curve to the observed intensity of the roomtemperature phosphorescence provides evidence that (aCD)z*BN is responsible for the room-temperature phosphorescence of BN. Phosphorescence and Triplet Lifetimes of BN. The triplet lifetime of BN in Nz-purged aqueous solution was evaluated to be 0.023 ms. When 1.0 x M a-CD was added to the deaerated aqueous solution of BN, the triplet lifetime was significantly lengthened to 0.29 ms. On the other hand, the phosphorescence lifetime of BN in an aerated aqueous solution containing a-CD (1.0 x M) was determined to be 0.16 ms, which was about half of the triplet lifetime in a deaerated solution. Since the phosphorescencelifetime should be the same as the triplet lifetime, the triplet state of a BN molecule being located within (a-CD)2*BN is quenched to some extent by oxygen in the aerated solution. In Nz-purged aqueous solutions
4
5 1
1
-
8.4
7
-
8
7
, I
1
l i 3
I
8.0
7.6
'
I
7.2
6 I PPm Figure 8. 'H NMR spectra of BN in D20 containing varying concentrations of a-CD. The a-CD concentrations (in M) are described at the left-hand sides of the corresponding spectra. Numbers near signal peaks at zero concentration of a-CD represent proton positions in BN.
without a-CD, BN was found to exhibit no room-temperature phosphorescence in spite of the absence of oxygen quenching. In addition to the blocking effect of a-CD on the oxygen quenching, therefore, the radiationless process and triplet-triplet quenching of the BN triplet state are considerably retarded by the formation of (a-CD)yBN. Although a single a-CD molecule is not protective enough to inhibit the deactivation of the BN triplet state within its cavity, two a-CD molecules in (a-CD)yBN, which seem to respectively include each end of a single BN molecule, significantly suppress the deactivation processes of triplet-state BN, resulting in the appearance of the room-temperature phosphorescence of BN. Because the inclusion of BN by two a-CD molecules reduces the degree of freedom for the BN movements to a great extent, the deactivation rate of the BN triplet state is most likely to be diminished. 'H NMR Spectroscopic Studies of BN. As a function of a-CD concentration, 'H NMR spectra of BN in D20 are shown in Figure 8. Proton assignments for BN in Figure 8 were made on the basis of an analysis of a COSY spectrum of BN in D20 (Figure 9). In the low a-CDconcentration region, addition of a-CD results in the downfield shifts of all the proton signals. M, In the a-CD concentration range exceeding about 3 x H-1, H-3, and H-4 signals of BN are successively shifted downfield, while H-5, H-7, and H-8 signals of BN are shifted upfield. At an a-CD concentration of 1.0 x M, observed chemical shift differences, Adobs, for H-1, H-3, and H-4 are 0.471, 0.240, and 0.252 ppm, respectively. On the other hand, hdobs for H-5, H-7, and H-8 are 0.040, 0.161, and 0.098 ppm at the same a-CD concentration, respectively, although Adobs for these protons at 3.0 x low3M a-CD are slightly greater than those at 1.O x M a-CD. For each BN proton, Adobs is represented as the sum of a contribution from the intrinsic chemical shift difference for a-CD-BN (Ad(a-CDBN)) and that
Inclusion Complexes of 6-Bromo-2-naphthol
J. Phys. Chem., Vol. 99, No. 32, I995 12113
7
i
3 811
'
7*2 j 7.4
7.0
0.2
1
96
I
I 0.1
8 8 7
0.0
k 1 0
2
4
6
[a-CD], /
1
8
1
0
1
2
mol dm-3
Figure 10. Plots of Adobs for H-1 and H-5 of BN against the a-CD 0.2
8.0 7.8
7.6
7.4
7.2
81 I PPm Figure 9. COSY spectrum of BN in D20. Numbers represent proton
concentration. Open and solid circles represent experimental data of Adobsfor H-1 and H-5, respectively. Also shown are the best-fit curves for Adobsof H-1 and H-5 calculated with Ad(a-CDBN) = 0.0934 ppm and Ad((a-CD)yBN) = 0.588 ppm for H-1 and Ad(a-CD.BN) = 0.256 ppm and Ad((a-CD)yBN) = 0.007 27 ppm for H-5, respectively.
positions in BN.
for (a-CD)yBN (Ad((a-CD)yBN)): Ad,,, = (Ad(a-CDBN)[a-CDBN]
Er
5
4
+
Ad((a-CD)2*BN)[(a-CD)2*BN])/[BN], (7) For Adobs of all the BN protons, simulations were made assuming values of Ad(a-CDBN) and Ad((a-CD)yBN). As representative examples, the best-fit curves for H-1 and H-5 along with experimental data are shown in Figure 10. The values of Ad(a-CDBN) and Ad((a-CD)*.BN) thus obtained are plotted against the proton position of BN (Figure 11). In the case of a-CNBN, the intrinsic Ad values for H-5, H-7, and H-8 are remarkably greater than those for H-1, H-3, and H-4. This finding indicates that, in a-CDBN, BN inserts its naphthalene nucleus into the a-CD cavity from an end having a Br atom. In (a-CD)yBN, conversely, the Ad values for H-1, H-3, and H-4 are much greater than those for H-5, H-7, and H-8, indicating that the second a-CD molecule accommodates a BN molecule, which is already incorporated in the other a-CD cavity, into its cavity from an end possessing a hydroxyl group. In a BN molecule, a Br substituent is the most hydrophobic portion. Consequently, as expected, the first a-CD molecule is preferentially bound to the BN end substituting a Br atom to form a 1:l a-CD-BN inclusion complex. Since the a-CD cavity is not wide enough to thoroughly include a BN molecule, the opposite end having a hydroxyl group extrudes from the first a-CD cavity into the bulk water environment. So, the second a-CD molecule encapsulates the same BN molecule that is located within the 1:1 inclusion complex from the extruded end substituting a hydroxyl group, leading to the formation of a 2: 1 a-CD-BN inclusion complex. Since the naphthalene-ring end possessing a hydroxyl group is more hydrophilic than that possessing a Br atom, it seems to be unfavorable that the second a-CD molecule associates with a BN molecule residing within the a-CD cavity. However, this is not the case for the a-CD-BN system; K2 (530 M-I) for the formation of (a-CD)yBN is nearly the same as K1 (560 M-I) for the formation of a-CD.BN. This result may imply that hydrogen bonding between the second a-CD molecule and a hydroxyl group of BN andor hydrogen bonding between two a-CD molecules facilitate the binding of the second a-CD molecule to a-CD-BN. Due t o the presence of a Br atom in BN, the relative position of a guest to the a-CD molecule in a 1:l inclusion complex is most likely to be different between a-CD-BN and a 1:l a-CD-2-naphthol inclusion complex. Consequently, the association between a-CDBN and the second
1
3 4 5 7 8 H-Position Figure 11. Plots of intrinsic Ad values estimated for a-CD*BN and (a-CD)2.BN against the proton position in BN.
a-CD molecule is sterically favored compared to that between the 1:1 a-CD-2-naphthol inclusion complex and the second a-CD molecule; K2 (530 M-I) for BN is 1 order of magnitude greater than KZ (35 f 5 M-'1 for 2-naphthol. As stated previously, K I for BN is about twice that for 2-naphthol, suggesting that the introduction of a hydrophobic Br atom urges the insertion of BN into the a-CD cavity from the Br end.
Conclusions In addition to a 1:l inclusion complex, a-CD forms a 2:l inclusion complex with BN in aqueous solutions. The 2:l a-CD-BN inclusion complex exhibits room-temperature phosphorescence of BN in aerated aqueous solution, whereas the 1:l a-CD-BN inclusion complex does not phosphoresce at room temperature. The room-temperature phosphorescence of BN has not been detected in deaerated solutions without a-CD. Consequently, the absence of oxygen quenching of the BN triplet state does not straightforwardly enable us to observe the room-temperature phosphorescence. Although the oxygenquenching rate is decelerated to a great degree by the formation of the 2: 1 a-CD-BN inclusion complex, the appearance of the room-temperaturephosphorescencedoes not seem to come from only the protection effect of a-CD on the oxygen quenching. Two a-CD molecules i n the 2:1 a-CD-BN inclusion complex effectively suppress the deactivation processes, of the triplet BN molecule bound into their cavities, which involve triplettriplet quenching. BN enters the cavity of the first a-CD molecule from an end substituting a Br atom on a naphthalene ring to form a 1:l a-CD-BN inclusion complex. Then, the second a-CD molecule binds to the opposite end substituting a hydroxyl group on the naphthalene ring, resulting in a 2: 1 a-CD-BN inclusion complex.
12114 J. Phys. Chem., Vol. 99, No. 32, 1995 Acknowledgment. I thank Professor Fujio Toda, Professor Akihiko Ueno, and Dr. Keita Hamasaki of Tokyo Institute of Technology for measurements of icd and 'HNMR spectra. I also thank Professor Koichi Kikuchi of Kitasato University for measurements of phosphorescence and triplet decays. References and Notes (1) Bender, M. L.; Komiyama, M. Cyclodextvin Chemistry; SpringerVerlag: Berlin, 1978. (2) Hamai, S.; Ikeda, T.; Nakamura, A.; Ikeda, H.; Ueno, A,; Toda, F. J. Am. Chem. SOC. 1992, 114, 6012. Hamai, S. J. Phys. Chem. 1989, 93, 2074. (3) Hamai, S. Bull. Chem. Soc. Jpn. 1982, 55, 2721. (4) Hamai, S. J . Phys. Chem. 1990, 94, 2595. ( 5 ) Hamai, S. Bull. Chem. SOC.Jpn. 1984, 57, 2700. (6) Tsai, S. C.; Robinson, G. W. J . Chem. Phys. 1968, 49, 3184. (7) Langelaar, J.; Rettschnick, R. P. H.; Hoijtink, G. J. J. Chem. Phys. 1971, 54, 1. (8) Turro, N. J.; Okubo, T.; Chung, C. J. Am. Chem. SOC. 1982, 104, 1789. (9) Turro, N. J.; Cox, G. S.; Li, X. Photochem. Phorobiol. 1983, 37, 149. (10) Bissell, R. A.; Prasanna de Silva, A. J . Chem. Soc., Chem. Commun. 1991, 1148. (11) TUEO,N. J.; Bolt, J. D.; Kuroda, Y.; Tabushi, I. Photochem. Photobiol. 1982, 35, 69.
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