J . Phys. Chem. 1989, 93, 2074-2078
2074
Inclusion Compounds in the Systems of 0-Cyclodextrin-Alcohol-Pyrene in Aqueous Solution Sanyo Hamai Department of Physics, Miyazaki Medical College, Kiyotake, Miyazaki, 889- 16, Japan (Received: April 1I , 1988; In Final Form: September 9, 1988)
Pyrene (Py)forms a 1:l inclusion compound PyC with P-cyclodextrin (CDx). When 1-pentanol (Pe) is added to a Py aqueous solution containing CDx, a sharp absorption band with a maximum at 339 nm appears accompanied by isosbestic points at 328 and 337 nm. The 339-nm band is attributed to the formation of a ternary 1:l:l inclusion compound PyPeC. By use of a curve-fitting procedure for the observed intensities of the Py fluorescence,the equilibrium constant K3for the formation of PyPeC from PyC and Pe has been evaluated to be 40000 mol-] dm3. From a relationship among equilibrium constants, K4, which is an equilibrium constant for the formation of PyPeC from Py and PeC (1:l inclusion compound of Pe with CDx), has been determined to be 4800 mol-I dm3. For other primary alcohols from methanol to 1-octanol and for cyclic alcohols from cyclobutanol to cyclohexanol, absorption changes similar to that for Pe have been observed, indicating the formation of a 1:1: 1 inclusion compound corresponding to PyPeC for Pe. The value of K3 for the primary alcohols increases on going from methanol to 1-octanol. In contrast, K4 for the primary alcohols increases first, reaches a maximum value for 1-propanol, and then decreases with a further increase in the alkyl chain length of alcohol. In the case of the cyclic alcohols, both K3 and K4 have maximum values for cyclopentanol whose molecular length is very close to that of 1-propanol. For the 1:l:l inclusion compounds of the primary alcohols, a variation of the Py fluorescence lifetime parallels the variation of K4. For the cyclic alcohols, the fluorescence lifetimes of the 1:l:1 inclusion compounds are almost the same as that containing 1-propanol.
Introduction In order to prepare a sample of a pyrene (Py) aqueous solution, a very small amount of a concentrated Py alcoholic solution is often diluted with water because of a very low solubility of Py in water.’ Py in aqueous solution is known to form a 1:1 inclusion compound PyC with P-cyclodextrin ( C D X ) . ~ -However, ~ as seen in the systems of CDxaniline-Py6 and CDxaliphatic amine-Py,’ PyC can further include an aniline or an aliphatic amine molecule inside its CDx cavity, into which a relatively large Py molecule is already partially incorporated, to form a ternary 1:1:1 inclusion compound. In the course of our work on the association of inclusion compounds in the CDx-aniline+ system,6 we have noticed that a small amount of alcohol affects the absorption spectrum of a Py aqueous solution containing CDx. Thus, we have investigated the interaction between PyC and primary or cyclic alcohols in aqueous solutions. Experimental Section Py purchased from Tokyo Kasei was purified by column chromatography. CDx (Nakarai) was recrystallized from water three times.* Primary alcohols (Wako) were distilled under reduced pressure except for methanol, ethanol, and 1-propanol which were distilled under atmospheric pressure. Cyclic alcohols (Wako) were distilled under reduced pressure except for cyclobutanol (Tokyo Kasei) which was used as received. In order to examine the effects of alcohols on the inclusion process of Py, Py aqueous solutions that did not contain alcohol were prepared according to the method described previously.6 The concentration of Py in the samples was about (2-3) X lo-’ mol d ~ n - ~ . Throughout this work, aerated sample solutions were employed, and the measurements were made at 25 OC. Absorption spectra were run on a Shimadzu UV-260 spectrophotometer, and an accumulation of spectral data, which were acquired at a slow scan speed, was repeated 20 or 30 times. ( 1 ) Patonay, G.; Rollie, M. E.; Warner, I. M. Anal. Chem. 1985, 57, 569. (2) Edwards, H. E.; Thomas, J. K. Curbohydr. Res. 1978, 65, 173. (3) Nakajima, A. Spectrochim. Acta, Part A 1983, 39, 913. (4) Hashimoto, S.; Thomas, J. K. J . Am. Chem. SOC.1985, 107, 4655. (5) Kusumoto, Y . Chem. Phys. Lett. 1987, 136, 535. (6) Hamai, S. J . Phys. Chem. 1988, 92, 6140. (7) Kano, K.; Takenoshita, I.; Ogawa, T. J . Phys. Chem. 1982, 86, 1833. (8) Hamai, S.Bull. Chem. SOC.Jpn. 1982, 55, 2721.
0022-3654/89/2093-2074$01.50/0
Fluorescence spectra were recorded on a Shimadzu RF-501 spectrofluorometer equipped with a cooled Hamamatsu R-943 photomultiplier. Fluorescence spectra were corrected for the spectral response of the fluorometer. Fluorescence decay data were obtained with a time-correlated single-photon counting apparat~s.~ Results and Discussion I : I Inclusion Compound of Py with CDx. Figure 1 shows the absorption spectra of the Py S2band in aqueous solution containing varying amounts of CDx. The absorption maxima of Py are slightly red-shifted on adding CDx with isosbestic points at 320, 327, and 335 nm. The spectral changes shown in Figure 1 can be attributed to an equlibrium represented by
Py
+ CDx & PyC
(1)
where PyC is a 1 :1 inclusion compound of Py with CDx, and K , is an equilibrium constant for the formation of PyC. The above equilibrium has already been suggested by Edwards and Thomas,2 N a k a j i ~ n a ,Hashimoto ~ and tho ma^,^ and Kusumoto.s The fluorescence spectra of Py in the absence and presence of CDx are illustrated in Figure 2. When CDx is added to a Py aqueous solution, the intensity of the 373-nm band relative to that of the 393-nm band is reduced while the intensity of the 383-nm band is enhanced. These effects of CDx on the intensities of the three vibronic bands are essentially the same as those reported by N a k a j i ~ n a .From ~ studies on the fluorescence quenching of Py by TI+, Hashimoto and Thomas have revealed that the rate for association of Py with CDx or that for dissociation of Py from CDx is not fast enough to allow Py to associate with CDx or to dissociate from CDx during an excited lifetime of Py? Therefore, one can use the fluorescence method to evaluate K 1 in the ground state of Py. The following relationship holds for the fluorescence intensities excited at the wavelength of an isosbestic point8 and also holds for those excited at any wavelength when the absorbance of a sample is very low: l/(Zf-
IF) = l / a
+ l/aK,[CDx],,
(1)
where If and IF are the integrated fluorescence intensity of Py in the presence and absence of CDx, respectively, a is a constant, (9) Hamai, S. Bull. Chem. SOC.Jpn. 1987, 60, 3505.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2075
Inclusion Compounds in 0-Cyclodextrin-Alcohol-Pyrene A / nm
h/nm
0.01
1
300
350 I
I
A?
il
1 C
nni
"
34
32
30
u"/io3 cm-' Figure 1. Absorption spectra of Py in aqueous solution in the presence of CDx. Concentration of CDx: (1) 0, (2) 1.68 X lo-), (3) 4.2 X lo-), and (4) 8.4 X lo-' mol dm-).
32
30
28
E/ io3cm-' Figure 4. Absorption spectra of Py in aqueous solution containing CDx mol dm-)) in the presence of Pe. For comparison, the ab(8.4 X sorption spectrum of Py alone in aqueous solution is shown by (1). Concentration of Pe: (2) 0, (3) 2.78 X lo4, and (4) 9.27 X lo4 mol
dm-).
A/nm 400
350
34
28
450
500
A/nm 350
-m
400
450
500
h
-m
E
h
8
E
v
8
r
I
Y
c
I
28
26
24
22
20
u"/ 10~cm-l Figure 2. Normalized fluorescence spectra of Py in aqueous solution in the presence of CDx. Concentration of CDx: (1) 0, (2) 2.52 X lo-), and (3) 8.4 X lo-' mol dm-'. A,, = 339 nm. 0.04
I
I
I
I
28
26
24
22
20
C/103cm-' Figure 5. Normalized fluorescence spectra of Py in aqueous solution containing CDx (8.4 X lo-) mol dm-') in the presence of Pe. Concentration of Pe: (1) 0, (2) 2.78 X lo4, and (3) 9.27 X lo4 mol dm-). &, = 339 nm.
0
0.0 3
-
.r I
'
0.02
r I
-
Y
\
0.01 0
0
200
400
600
800
[CDxl0-l/ mol-' d m 3 Figure 3. Plot of 1/(1f - 1;) against 1/[CDxlo. A,, = 340 nm.
and [ lo stands for an initial concentration. Upon excitation at the isosbestic point (335 nm), the integrated fluorescence intensity of the Py solution varied only slightly in the presence of CDx. To follow a large intensity change and then evaluate a reliable K I value, we have chosen 340 nm as the excitation wavelength. Figure against 1/[CDxIo. From this plot, 3 displays a plot of l/(If K , was determined to be 7.6 mol-' dm3.10311 This K , value is 5.8-25 times smaller than those obtained by Hashimoto and Thomas (44 mol-' dm3),4 Kusumoto (128 mol-' dm3),5and Na-
It)
(10) The K 1value is estimated to be accurate to about 8%. (1 1) From the absorbance change, 20 & 12 mol-' dm' was obtained as a
K1value.
kajima (190 mol-' dm3),3all of which were evaluated from Py fluorescence data but by using different methods from ours. At present it is not clear why such a smaller Kl value than the reported ones has been obtained. 1:1:1 Inclusion Compound Composed of Py, CDx, and IPentanol. Addition of 1-pentanol (Pe) to a Py aqueous solution containing 8.4 X mol dm-3 CDx results in the change in the absorption spectra as shown in Figure 4. On addition of Pe, a new absorption band at 339 nm appears with isosbestic points at 328 and 337 nm. The 339-nm band is sharper and stronger than the 0-0 vibronic band (334 nm) of Py in aqueous solution or the corresponding 335-nm band of Py in the presence of CDx (see Figure 1). Such a drastic spectral change has never been observed in the absence of CDx. From this experimental result, we can ascribe the spectral change shown in Figure 4 to the formation of a ternary inclusion compound composed of Py, CDx, and Pe. The fluorescence spectra of a Py solution containing both CDx and Pe are given in Figure 5 . An enhancement of the intensity ratio of the 383- or 393-nm band to the 373-nm band, which is also observed upon the addition of CDx alone, becomes more prominent upon the further addition of Pe. In the low-concentration range of Pe, both the absorbance of Py at 339 nm and the fluorescence intensity of Py excited at 339 nm are proportional to the concentration of Pe added (Figure 6 ) . This finding indicates that the inclusion compound responsible for the 339-nm band includes only one Pe molecule. Although a continuous variation method has been usually employed to binary systems in order to decide a stoichiometry of a
2076
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
I
I
I
1
I
I
100 -
I
I
I
0-
To
-
/O
7 r I
I
40
+0
I
I
1
1
2
3
(Pe),/
4
0
mol dm-3
Figure 6. Plots of the fluorescence intensity (0)excited at 339 nm and the absorbance ( 0 )at 339 nm for a Py solution containing CDx (8.4 X lo-’ mol d d ) against [Pelo.
I , 0
I
I
02
I
I
0.4
I
I
06
I
0.8
1
I
I
4
6
a
(Pe), /
1 10
mol dm-3
Figure 8. Comparison of a simulation curve with the observed fluorescence intensities of Py solutions containing CDx (8.4 X lo-’ mol dm-’) and varying concentrations of Pe. The maximum data point is normalized to 100. X, = 339 nm. The simulation curve has been calculated by use of assumed parameters, A 5 107, B = 0.0507 mol dm-’, and C = 0.000685 mol dm-’.
K3, and K4 are equilibrium constants for the formation of PeC, PyPeC from PyC and Pe, and PyPeC from Py and PeC, respectively. For y-cyclodextrin of which cavity is larger than that of CDx, a 1:1:1 inclusion compound containing cyclohexanol and 1-naphthyloxyacetic acid14 and that containing 1-butanol and Pyi5 have also been suggested without stoichiometric investigations. Because of the extremely low concentration of Py, the light intensities absorbed by Py, PyC, and PyPeC are proportional to the absorbance of each species. Therefore, the total fluorescence intensity, Zf,is given by a sum of the fluorescence intensities of Py, PyC, and PyPeC: If= b~0(4l~i[PYl + 42dPYCI + 43c3[PyPeCI) (11) where b is an instrumental constant, Io is the excitation light intensity, 4 is the fluorescence quantum yield, c is the molar absorption coefficient at the excitation wavelength (339 nm), and subscripts 1, 2, and 3 refer to Py, PyC, and PyPeC, respectively. By introducing K,, K2, and K3, one can rewrite eq I1 as I, = A - B / ( C + [Pelo) (111)
1
‘$\
I
2
O’Ool
*\lo 10
[CDXJO [CDxI, + [Pel, Figure 7. Continuous variation plots of the fluorescence intensity (0) excited at 339 nm and the absorbance ( 0 )at 339 nm for Py solutions under the conditions of [CDxIo [Pelo = 3.0 X lo-) mol dm-’.
+
c o m p l e ~ , it~ can ~ . ~be~ applied to a ternary system such as the CDx-Pe-Py system when the concentration of a ternary complex is low.6 Under the conditions that a sum of the initial concenmol dm-3 ([CDxIo trations of CDx and Pe was fixed at 3 X [Pelo = 3 X mol dm-3) and that the F‘y concentration was kept constant, the continuous variation method was applied to our system. Figure I depicts the difference in the fluorescence intensity of Py excited at 339 nm and the difference in the absorbance of Py monitored at 339 nm. The two experimental curves go through maximum values at a molar fraction of 0.5, indicating a 1:l stoichiometry concerning CDx and Pe. The species responsible for the 339-nm band shown in Figure 4, of course, includes only one Py molecule. From these results, the absorption change in Figure 4 can be attributed to a 1:l:l inclusion compound PyPeC which is composed of one Py,one Pe, and one CDx molecule. The formation of PyPeC is represented by the following equilibria:
+
+C DS ~ PeC P ~ C + Pe 2PyPeC ~y + PeC & PyPeC Pe
(2) (3)
(4) where PeC is a 1 :1 inclusion compound of CDx with Pe, and K2, (12) Harada, A.; Fume, M.; Nozakura, S. Macromolecules 1977, I O , 616. (13) Tamaki, T. Chem. Lett. 1984, 53.
When the values of K1 and K2 are known, one can determine a K3 value if the value of C in eq I11 is estimated from the experimental data. Figure 8 illustrates the least-squares fit of eq I11 to the plot of Ifagainst [Pelo using the following values: A = 107, B = 0.0507 mol dm-3, and C = 0.000685 mol d ~ n - ~Substituting . our value of 7.6 mol-l dm3 for K 1 and a literature valuei6of 63.1 mol-’ dm3 for K2, we evaluate K3 to be 40 000 mol-l dm3. From a relationship of KiK3 = K2K4, K4 is determined to be 4800 mol-’ dm3. As stated already, the absorption maximum of the 1:l:l inclusion compound is located at 339 nm. On the other hand, the 0 4 band maxima of the Py S2band in cyclohexane, 1,Cdioxane, ethyl acetate, ethanol, methanol, and acetonitrile are at 335, 336, 334, 334, 334, and 334 nm, respectively. The maximum wavelength, 339 nm, of the 1:l:l inclusion compound is rather anomalous compared with those in organic solvents. Furthermore, the isosbestic points (320, 327, and 335 nm) appearing in the equilibrium of eq 1 do not coincide with those (328 and 337 nm) in the equilibrium of eq 3 (or 4), implying that some interaction is (14) Ueno, A.; Takahashi, K.; Hino, Y.; Osa, T. J. Chem. SOC.,Chem. Commun. 1981, 194. (15) Kano, K.; Takenoshita, I.; Ogawa, T. Chem. Lett. 1982, 321. (16) Matsui, Y.; Mochida, K. Bull. Chem. SOC.Jpn. 1979, 52, 2808.
Inclusion Compounds in P-Cyclodextrin-Alcohol-Pyrene
r--l
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2077 TABLE I: Equilibrium Constants for the Formation of Inclusion Compounds and Ratios of K3 to K,"
K,/moP dm'
/
~~b/moI-' K3/moP dm' dm3
methanol ethanol 1-propanol
7.6
1-butanol
7.6
1-pentanol
7.6
1-hexanol
7.6
219
1-heptanol 1-octanol
7.6 7.6
708 1480
cyclobutanol
7.6
cyclopentanol cyclohexanol
7.6
7.6 7.6
7.6
0.32 0.93 3.72 16.6
63.1
15.1
120 501
K4/mol-' dm3 KJK, 260 34 3000 400 2000 15000 6100 780
11
370 7 600
13000 40 000 60 000
4800
630
2100
270
100 000
1100
140
160000
820
110
21 000
10000 32000 5400
1400 4300
510000 350000
700
"The error limits for K, and K4 are estimated to be no larger than f15%. bData of Matsui and Mochida (ref 16). 0
I/ 0
, 0.2
,
, 0.4
,
, 0.6
,
, 0.8
1.0
Figure 9. Continuous variation plots of the uncorrected and the corrected fluorescence intensities excited at 339 nm for Py solutions under the mol d ~ n - ~Open . circles conditions of [CDxIo+ [ethanolIo= 1 X
represent the observed fluorescence intensities, and closed circles represent the fluorescence intensities corrected for a contribution from the fluorescence intensity of PyC. occurring between Py and Pe. These findings may suggest that a molecular complex is formed between Py and Pe inside the CDx cavity. On the basis of an infrared spectroscopic study, Lianos and Georghiou have revealed the existence of 1:1 stoichiometric molecular complexes of Py with alcohol^.'^ Nakajima has assigned the Py-alcohol complex to a hydrogen-bonded complex.'* 1:I :I Inclusion Compounds of Py with CDx and Alcohols Other Than Pe. All the primary alcohols from methanol to 1-octanol exhibit similar absorption changes to that shown in Figure 4, indicating the formation of a 1:1:1 inclusion compound analogous to that of Pe. For short-chain alcohols like ethanol, however, two or more alcohol molecules may be incorporated into the CDx cavity which already includes a Py molecule. To test the above possibility, a continuous variation method has been applied to a system of CDx-ethanol-Py (Figure 9). As in the case of Pe, the difference in the fluorescence intensity of Py reaches a maximum value at a molar fraction of 0.5, indicating a 1:l stoichiometry concerning CDx and ethanol. Unfortunately, such a continuous variation method could not be applied to a system involving methanol owing to the limited solubility of CDx. Nevertheless, the same 1:l stoichiometry is expected for the case of methanol from a spectral change similar to that shown in Figure 4 and from the fact that even a free CDx molecule, which does not accommodate a 4, molecule, associates with only one methanol molecule to form a 1:l inclusion compound.16 Therefore, the same procedure as that used for Pe was employed to evaluate K3 and K4 for all of these primary alcohols. Since absorption changes similar to that shown in Figure 4 were also observed for cyclic alcohols (cyclobutanol, cyclopentanol, and cyclohexanol), K3 and K4 for cyclic alcohols were determined by the same method. These K3 and K4 values for the primary and cyclic alcohols examined in this study are listed in Table I together with our K , value and K2 values obtained by Matsui and Mochida.16 Table I also tabulates the ratios of K3 to K2. The K2 value increases with increasing the carbon number of the primary or cyclic alcohols.16 Similarly, the K3 value for the primary alcohol monotonously increases on going from methanol to 1-octanol. The K 3 value is, however, always 1-3 orders of magnitude larger than the corre-
sponding K2 value, indicating that the alcohol molecule is bound to the CDx cavity more easily when a Py molecule is already incorporated into the cavity. On the other hand, K4for the primary alcohol increases at first as the alkyl chain in alcohol is lengthened, reaching a maximum value of 15 000 mol-' dm3 for 1-propanol. Then K4 decreases on going from 1-propanol to 1-octanol. This finding indicates that the best fit of a size of a Py molecule into a vacant space inside the CDx cavity, in which an alcohol molecule already binds, is accomplished for 1-propanol. An alcohol molecule bearing a short alkyl chain such as methanol or ethanol is nbt large enough to occupy the empty space of the CDx cavity binding a Py molecule, giving a smaller K4 value than that for 1-propanol. Since an alcohol molecule from 1-butanol to 1-octanol is longer than a 1-propanol molecule, a vacant zone of the CDx cavity including an alcohol molecule such as 1-butanol, etc., becomes narrow, and then it becomes difficult to further accommodate a Py molecule. Furthermore, a Py molecule may be slightly pushed out from the CDx cavity by a co-included alcohol molecule when Py enters the cavity accommodating an alcohol molecule. For the alcohols from 1-butanol to 1-octanol, therefore, such an unfitness appears to lead to a smaller K4 value than that of 1propanol. In cyclic alcohol series, K3 is 2-3 orders of magnitude larger than the corresponding K2 value, which increases with increasing the carbon number of alcohol. In contrast to the case of the primary alcohols, K3 for the cyclic alcohol has a maximum value for cyclopentanol. Similarly, the largest value of K4 has been obtained for cyclopentanol. With respect to the molecular length (diameter), cyclobutanol, cyclopentanol, and cyclohexanol resemble ethan01,'~1-propanol, and 1-butanol, respectively, although the cyclic alcohols are much bulkier than the primary alcohols similar in molecular length (diameter). Consequently, it is reasonable that cyclopentanol, which is closest in molecular length to 1-propanol, has the largest K4 among the cyclic alcohols examined. The K4 value of cyclopentanol, however, is larger than that of 1-propanol. This suggests that much bulkier cyclopentanol than 1-propanol can be more tightly buried in the CDx cavity which already accommodates a Py molecule. The ratio K 3 / K 2 represents the extent to which an alcohol molecule is incorporated into the CDx cavity which a Py molecule already enters. Unlike K3, K3/K2 reaches a maximum value at 1-propanol and then decreases with an increase in the carbon number of alcohol. This finding shows that 1-propanol is bound most strongly to the CDx cavity in which Py is already enclosed. This trend for K3f K , is identical with that for K4 of the primary alcohols. In the cyclic alcohols, K3/K2 as well as K4has the highest value for cyclopentanol. For the systems of CDx-sodium n-alkylsulfate-Py and CDxn-alkyltrimethylammonium bromide-Py, 1:1:1 inclusion compounds, which are composed of CDx, the above amphiphile, and Py, have been investigated by Hashimoto and tho ma^.^ Py as~~
s.
(17) Lianos, p.; Georghiou, Photochem. Photobiol. 1979, 29, 843. (18) Nakajima, A. Bull. Chem. SOC.J p n . 1983, 56, 929.
~
~~~~
~
~
~
~~~~
(19) Strictly speaking, the molecular length of cyclobutanol is intermediate between that of ethanol and I-propanol.
2078
The Journal of Physical Chemistry, Vol. 93, No. 5. 1989 400
350
v)
c 300
I
\
(.’
250
200 1
2
3
4
5
6
7
8
9
CARBON NUMBER OF ALCOHOL
Figure 10. Plot of the fluorescence lifetimes of the 1:l:l inclusion compounds composed of a Py, a CDx, and a primary alcohol molecule (0) or a cyclic alcohol molecule ( 0 )against the carbon number of alcohol.
sociates with 1:1 inclusion compounds formed between CDx and the amphiphiles. The association constants K,,, have been determined by means of the analyses of Py fluorescence decay curves. In the former systems, K,,,, is largest for n-butyl sulfate and becomes smaller when the carbon number in n-alkyl sulfate is larger or smaller than four. In the latter systems, the largest K,, has been obtained for n-hexyltrimethylammonium bromide. On the other hand, in the case of the CDx-alcohol-Py systems, K4, which corresponds to K,,,,, passes through a maximum value at three in the carbon number of the primary alcohol. At present, we cannot answer the question of why in these systems such differences occur in the carbon number at which the equilibrium constant reaches a maximum value. Fluorescence Lifetimes of 1:1:1 Inclusion Compounds. The fluorescence lifetime of Py in aqueous solution has been observed to be 122 ns, for which nearly the same values (126 f 3 and 127 ns) have been reported.20~21The lifetime of a Py aqueous solution containing 8.4 X mol dm-3 CDx was determined to be 192 xz2 A comparable value, 226 ns, has been reported for a Py (20) Geiger, M. W.; Turro, N. J. Photochem. Photobiol. 1975, 22, 273. (21) Kano, K.; Matsumoto, H.; Hashimoto, S.; Sisido, M.; Imanishi, Y . J . Am. Chem. SOC.1985, 107, 6117.
Hamai aqueous solution with mol dm-j CDx.’ For all the 1:l:l inclusion compounds, fluorescence lifetimes were measured, and these lifetimes are plotted as a function of the carbon number of alcohol in Figure 10. The lifetime for the primary alcohols increases with an increase in the carbon number of alcohol, reaches a maximum value at 1-propanol, and then decreases on going from 1-propanol to 1-octanol. Such a variation of the lifetimes for the 1:1:1 inclusion compounds containing the primary alcohols is analogous to the variation of K4 for the primary alcohols. Our conclusion that the 1:l:l inclusion compound of 1-propanol is most rigidly complexed in the primary alcohol series is further confirmed by the experimental result regarding the lifetimes of the 1:l:l inclusion compounds. The lifetimes of the 1:1:1 inclusion compounds are considerably long compared with that of Py or PyC, and the lifetimes of 300-350 ns are rather close to those in deaerated organic solvent^.^^-^^ The shorter lifetime of the 1:1:1 inclusion compound including methanol or ethanol may imply that a water molecule(s) is (are) incorporated into the CDx cavity which already accommodates both a Py and a methanol or ethanol molecule. For all the cyclic alcohols examined, nearly the same lifetimes as that for 1-propanol were obtained. This may arise from the fact that the molecular length (diameter) of the cyclic alcohols does not remarkably vary, unlike the primary alcohols, and then the molecular lengths of the cyclic alcohols are approximately identical with that of 1-propanol or 1-butanol, although the bulkiness of the cyclic alcohols is different from 1propanol or I-butanol.
Acknowledgment. The author thanks Professor Fumio Hirayama for his helpful discussion. Registry No. PyPeC, 118320-08-4; CDx-methanol-Py, 1 18320-04-0; CDx-ethanol-Py, 1 18320-05-1; CDx-1-propanol-Py, 118320-06-2; CDx-1-butanol-Py, 118320-07-3; CDx-1-hexanol-Py, 118320-09-5; CDx-1-heptanol-Py, 118320-10-8; CDx-1-octanol-Py, 118320-1 1-9; CDx-cyclobutanol-Py, 1 18320- 12-0; CDx-cyclopentanol-Py, 11832013- 1; CDx-cyclohexanol-Py, 1 18320- 14-2. (22) The excitation wavelength for lifetime measurements was 337 nm except for 1:l:l inclusion compounds for which 339 nm was selected. As shown in Figure 1, at 337 nm, at least about half of the absorbance is due to PyC, although most of Py molecules exist in the aqueous bulk phase. In addition, in our apparatus for lifetime measurement, the fluorescence decay of Py in a 8.4 X lo-’ mol dm-’ CDx solution could be followed over a range of only 2.5 times the observed lifetime. Under such circumstances, a fluorescence decay curve may be analyzed as a one-component decay. Therefore, a small K,value of 7.6 mol-’ dm’ does not seem to be inconsistent with the observation of the one-component decay. (23) Birks, J. B.; Lumb, M. D.; Munro, 1. H. Proc. R. SOC.London, A 1964, 280, 289. (24) Palmans, J. P.; Van der Auweraer, M.; Swinnen, A. M.; De Schryver, F. C . J . Am. Chem. SOC.1984, 106, 7721. (25) Sadovskii, N. A,; Shilling, R.-D.; Kuzmin, M. G. J. Photochem. 1985, 31, 247.
