1833
J. Phys. Chem. lS82, 86,1833-1838
overtones, and combinations. We have (assume without loss of generality that
2 2 = av.W,-‘.av
do =
=
6) qt N
-9V.t2
Pt -9v.t N
(A7a) (A7b)
where ?V is the gradient of the upper potential. From the equations of motion for P, and Z,we have P, = i o 0 - V’ft (A84 z = 1 + iwot (A8b) where oo is the diagonal matrix of frequencies of the ground-state normal coordinates* Equations A*8 and A’2 yield, for the short-time expansion of A,
At N f/2[iwo + (wo2 - V”)t]
(-49)
Also Yt
Yo
- Et
(A131
cvk2/wOk k
consistent with eq 16. For the intensities of the fundamentals, we expand to order u k for each mode k. Using eq A12, A5, and 14, we easily obtain la1ok(w1)12
=
vk2 4&l( 2w0ka
‘I
”)
(A14)
For the overtones and combinations, we recall that there are two sources of intensity: (1)frequency changes (e.g., Wok2 z V k k ) , of order t’ and (2) nonzero slopes ( v k # 0). We treat them separately, because together they are cumbersome and not illuminating. First (case l),assume v k = 0 for the Kth mode. Examining the coefficients of u k 2 in the expansion of eq A12, and with eq 14 and A5, we have eq 23 for the overtone.
(A10)
The integral of interest, eq A6, becomes
+ i ( w 0 - V”)t]-G+ [2(00)1/2.0 - ibVt].+ - iEt - 0.0 (All)
Jmdt ealtImdq‘ -m exp(-y&[2wo
=
(
7P
)’2&mdt eiSItexp[-a2t2/2 det wo + 00) f/294w02- V”)&t - i8.9V.t - iEt] (A12)
where 9 = oo-1/2-0 and
Likewise, examining the coefficients of u k u k t , we derive easily eq 24, assuming v k = v k ! = 0. Second (case 2), if w o k 2 = v k k in some mode k but v k # 0, then we can expand the exponent of eq A12 so as to obtain a u k 2 (coefficient proportional to ( v k ) 2 ) , leading to Ia20k,2=
vk4 &2( 8W0k2U6
y)
(A16)
(see eq 25).
Fluorescence Quenching of Pyrene and Naphthalene in Aqueous Cyclodextrin Solutions. Evidence of Three-Component Complex Formation KoJIKano,’ Itsuro TakenoshRa, and Telkhlro Ogawa Depertnk?nt of Mobcular Scisnce and Technobgy, eaduate School of Englneedng Sciences, Kyushu Unlvers& Fukuoka 812, Japan (Receld: November 10, 1981)
The effects of a-,p-, and y-cyclodextrins on the fluorescencequenching of pyrene and naphthalene by aliphatic and aromatic amines have been studied in water and aqueous dimethyl sulfoxide solutions. When the smaller aliphatic amines, dimethylamine, diethylamine, trimethylamine, and triethylamine, were used as quenchers, 8- and y-cyclodextrinsaccelerated the fluorescence quenching of both fluorophores. The Stern-Volmer plots for the fluorescence intensities and the fluorescence lifetimes clearly indicated that static quenching occurred in these systems because of the formation of the three-component complexes of fluorophore, quencher, and cyclodextrin. On the other hand, cyclodextrins, especially &cyclodextrin, inhibited the fluorescence quenching of pyrene by N,N-dimethylaniline,suggesting formation of a tight 1:l complex of the quencher with 0-cyclodextrin.
Introduction Cyclodextrins, to be denoted here as CD, can include various kinds of molecules in their cyclic cavities and catalyze reactions such as hydrolyses of esters and decarboxylation of carboxylate anions.’ The catalytic effect of CD for ester hydrolysis can be interpreted in terms of the favorable geometric orientation of the substrate in the CD cavity for the attack of the hydroxide or hydroxyl anion (1) For a review, cf. M. L. Bender and M. Komiyama,“Cyclodextrin Chemistry”, Springer-Verlarg,New York, 1977. 0022-3654/82/2086-1833$01.25/0
groups of CD. the hydrophobic property of the CD cavity promotes the decarboxylation of the carboxylate anions. These typical reactions catalyzed by CD require the formation of a 1:l complex of the guest molecule with CD prior to reaction. Little is known about the possibility that two different kinds of guest molecules are simultaneously included in the same cavity of CD, leading to acceleration of a bimolecular reaction between the guest molecules. In a preliminary communication, we reported that the fluorescence quenching of pyrene by diethylamine in water is significantly accelerated by P-cyclodextrin (cyclohepta0 1982 American Chemical Society
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The Journal of Physlcal Chemlstty, Vol. 86, No. 10, 1982
amylose, #?-CD).2 This is the first case, to the best of our knowledge, that two different kinds of molecules are included in the same cavity of CD to catalyze a bimolecular reaction. More recently, Dideout and Breslow have found that a typical bimolecular reaction, the Diels-Alder reaction, is catalyzed by /3-CD.3 Kobashi et al. have reported recently the static quenching of 1-pyrenesulfonate with aniline in the 0-CD cavityq4 These effects of CD on bimolecular reactions may be interpreted in terms of the simultaneous inclusion of two different kinds of molecules in the @-CDcyclic c a ~ i t y . ~ , ~ The internal diameters of the rings of a-cyclodextrin (cyclohexaamylose, a-CD), 0-CD, and y-cyclodextrin (cyclooctaamylose, y-CD) are 4.5,7.0, and 8.5 A, respectively, and the depths of the cavities are about 6.7-7.0 A.1 The stability of an inclusion complex of an organic molecule with CD depends on the size, the hydrophobicity, the polarity, and the hydrogen-bonding ability of the guest molecule. Especially the relative size of the guest molecule and the CD cavity is a fundamental factor for forming a stable inclusion complex. It can be assumed very easily that, if a guest molecule is too large or too small relative to the size of the CD cavity, the guest molecule does not bind to CD. This is true, but several cases have been shown where large guest molecules bind to a CD host molecule having a smaller cavity size. For example, adamantane-1-carboxylicacid binds to ,8-CD.5 Since the size of adamantanel-carboxylic acid is too large to be included completely in the cavity of 0-CD, the guest molecule may bind on the face of the ~ a v i t y .Interestingly, ~ the study of the optical rotation suggests the formation of the 2:l complex of adamantane-1-carboxylic acid with P-CD:
*o-0 +
=
60 ,*-\
u COO'
On the other hand, when a guest molecule is too small to fit to CD, two guest molecules may be included simultaneously in the same CD cavity.68 The present study deals with the catalytic effects of CD on the fluorescence quenching of pyrene and naphthalene by aliphatic and aromatic amines. The purpose of this work is to determine the fundamental aspects of the CDcatalyzed bimolecular reactions by studying fluorescence quenching as a model reaction.
Experimental Section a- and 0-CD (Wako) were recrystallized from 60% aqueous propanol and water, respectively, followed by vacuum drying at ca. 60 "C. Elemental analysis showed that a-CD was anhydrous while P-CD was dihydrate. y C D (Nakarai) was used without purification. N,N-Dimethylaniline was purified by distillation. Reagent grades of dimethylamine (aqueous solution), diethylamine, trimethylamine (aqueous solution), and triethylamine were K. Kano, I. Takanoshita, and T. Ogawa, Chem. Lett., 1036 (1980).
D.C.Rideout and R. B d o w , J. Am. Chem. SOC.,102,7816(1980). H. Kobaehi, M. Takahaei, Y.Muramatau, and T. Morita, Bull. Chem. SOC.Jpn., 64, 2815 (1981). (5)R.Breslow, M. F. Czamiecki, J. Emert, and H. Hamaguchi, J. Am. Chem. SOC.,102,762 (19%). (6)k Ueno, K. Takahasi, and T. Osa, J.Chem.SOC.Chem. Commun., 921 (1980). (7)A. Ueno, K.Takahasi, Y.Hino, and T. Osa, J. Chem. SOC.Chem. Commun., 194 (1981). (8)N. Kobayaehi, A. Ueno, and T. Osa, J. Chem. SOC.Chem. Commun., 340 (1981).
Kano et al.
TABLE I: Binding Constants of Aliphatic and Aromatic Amines with Cyclodextrins in Water a t 25 "C ~~
guest n-propylaminea n-butylaminea diethylamine" trimethylamine" N,N-dimethylanilineb N, N-dimethylanilineb N, N-dimeth ylaniline
~
CD
4 4 4 4 Q
4 Y
K. M-7 914
lot
42 t 51 t 103 t 272 1 64t
5 23 7 30 22 12
a The detailed method for determining K of the aliphatic amines was shown in ref 12. "he concentrations of o-CD and sodium 4-(4-hydroxy-1-naphthylazo)-1-naphthalenesulfonate were 1 X and 4 x M, respectively. The K value of the azo compound with 4-CD was determined to be 2519 M-I. The changes in the absorbances of the azo compound upon addition of aliphatic amines were followed a t 525 nm. N,N-Dimethylaniline (1 X M) was excited a t 260 nm, and the changes in the fluorescence M ) were folintensities upon addition of CD (0-1 x lowed a t 360 nm.
commercially obtained (Nakarai) and used without purification. The concentrations of dimethylamine and trimethylamine were determined by pH titration. Pyrene (Nakarai) was carefully purified by means of silica gel column chromatography with cyclohexane as an eluent. Naphthalene (Ishizu) was used after determination of its purity by measuring its melting point and absorption spectrum. Sodium 1-naphthalenesulfonate (Wako) was recrystallized from aqueous methanol. Water was distilled and deionized by being passed through a column of ionexchange resin. Sodium 1-pyrenesulfonate was prepared and purified by Hideyuki Goto in this laboratory. The fluorescence emission and excitation spectra were measured on a Hitachi 650-10s spectrofluorometer whose cell compartment was thermostated. The absorption spectra were taken on a Jasco UVIDEC 505 spectrophotometer. The fluorescence decay curves of pyrene were obtained by using a nitrogen laser (Morectron UV-12,lO-ns pulse width) as described in a previous paper.g The fluorescence lifetime of naphthalene was kindly determined by Professor Kensuke Shima of Miyazaki University using a Hitachi MPF-4 time-resolved fluorometer. All experiments were carried out under aerobic conditions at 25 "C unless otherwise noted. The fluorescence measurements were performed within 2 h after samples were prepared. Results Binding Constants of Quenchers with Cyclodextrins. The binding constants of NJV-dimethylanilinewith p- and y-CD were determined by Benesi-Hildebrand plots for fluorescence intensities: AF' = ((~[guestl&[CD]~)-~ + (a[guestIo)-l (1) where AF is the change of fluorescence intensity upon addition of CD, [guest],, and [CD], are the initial concentrations of guest and CD molecules, respectively, and (Y is the proportionality constant (aF= a[complex]). Equation 1can be applied for a 1:l complex of guest with host. The linear relationship between hF1and [CD],-l gave K (Table I). Although this method was applied to determine the K values for the pyrene- and naphthalene-CD systems, the data could not be fitted with eq 1. Further detailed (9)K. Kano,H. Kawazumi, T. Ogawa, and J. Sunamoto, J. Phys. Chem., 85, 2204 (1981). (10)H. A. Benesi and J. H. Hildebrand, J. Am. Chem. SOC.,71,2703 (1949). (11)H. Kondo, H. Nakatani, and K. Hiromi, J. Biochem., 79, 393 (1976).
The Journal of phvsical Chemistry, Vol. 86, No. 10, 1982 1895
Fluorescence Quenching of Pyrene and Naphthalene
TABLE 11: Fluorescence Lifetimes of Pyrene, 1-Pyrenesulfonate, and Naphthalene in Aerobic Water in the Absence and the Presence of Cyclodextrins at 25 "Ca cyclofluorophore dextrin 7 , ns pyrene pyrene pyrene pyrene sodium 1-pyrenesulfonate sodium 1-pyrenesulfonate sodium 1-pyrenesulfonate sodium 1-pyrenesulfonate naphthalene naphthalene
01
B 7 01
B 7
P
154 161 226 21 3 65 63 72 82 37 48
q2ot
I
-
? 15
I
d
"ip 0
2
4
6
8
CTrimethyominell$ M
The concentrations of pyrene, sodium l-pyrenesulfonand 1 X lo-' ate, and naphthalene were 5 X lo-', 1 X M,respectively. The concentrations of cyclodextrins were 1 X lo-* M.
Flgure 1. Plots of l o l l(0, 0 )and T ~ / T(A)YS. [trimethylamine] for the p ~ e ~ t r b n e h y l a m l nsystem e in the absence (0)and the presence (0, A) of @-CDat 25 O C . The concentrations of pyrene and OCD
experiments are needed to determine the K values for these systems. The K values for nonfluorescent aliphatic amines, diethylamine and trimethylamine, were determined by a spectrophotometric method which applies an inhibition effect of guest molecule on the binding of sodium 444hydroxy-1-naphthy1azo)-1-naphthalenesulfonateto CD.12 Poor reproducibility of this method may be due to a small change in optical density of the azo dye upon addition of the aliphatic amines. The K values for various sytems are summarized in Table I. Fluorescence Lifetimes of Pyrene and Naphthalene. Judging from the CPK molecular model, the cavities of a- and 8-CD are too small to fit the pyrene molecule. The absorption and fluorescence spectroscopic data seem to be insufficient to conclude that the pyrene molecule is include in the CD cavity. Then the fluorescence lifetimes (7)of pyrene were determined in air-saturated water in the absence and the presence of CD to confirm the interaction between pyrene and CD. The results are shown in Table 11. The fluorescence lifetime of pyrene in the absence of CD was 154 ns. Since the T value of pyrene in nitrogenflashed water was ca. 400 ns and no excimer emission was observed under the present conditions ([pyrene] I 1 X lo4 M), the decrease of fluorescence lifetime in air-saturated water should be ascribed to the quenching by oxygen dissolved in water. No effect of a-CD on the fluorescence lifetime of pyrene was observed, suggesting no or weak interaction between pyrene and a-CD. By the way, a significant increase in the T values was observed when @and 7-CD were added to the solution, suggesting that 0and 7-CD prohibit the collisional quenching of excited pyrene by oxygen due to the inclusion complex formation. Virtually the same effect of CD on the fluorescencelifetime of sodium 1-pyrenesulfonate was observed, but it was not so remarkable as in the case of pyrene. This may be due to the weaker interaction of 1-pyrenesulfonatewith @- and yCD. The naphthalene excited state was also protected from the oxygen quenching by forming the naphthalene@-CDcomplex. Fluorescence Quenching by Aliphatic Amines. In homogeneous fluid solution, fluorescence intensity (I)and fluorescence lifetime (7)are correlated with quencher concentration ([Q]) by the following Stern-Volmer equation:
where Ksv and k, are the Stern-Volmer and quenching rate constants, respectively, and the subscript 0 denotes the absence of quencher. Table I11 summarizes the results of the fluorescence quenching of pyrene, naphthalene, and their sulfonate derivatives by various aliphatic amines at 25 "C. The Ksv and k, values were determined from the steady-state fluorescence measurements. As a typical example, the plot of Io/I vs. [Q]for the pyrene-trimethylamine-0-CD system is shown in Figure 1. As Figure 1shows, the fluorescence quenching of pyrene by trimethylamine was markedly accelerated by @-CD. In the presence of 8-CD, the I o / l values increased linearly with increasing quencher concentrations and leveled off at higher quencher concentrations. The Ksv and k, values were determined from a linear relationship at lower quencher concentrations. The quenching phenomena for the other systems were virtually the same as for the pyrene-trimethylamine-@-CD system. The results shown in Table I11 revealed that (1)the effects of CD on the fluorescence quenching are more remarkable for the hydrophobic fluorophores (pyrene and naphthalene) than for the hydrophilic ones (l-pyrenesulfonate and 1-naphthalenesulfonate),(2) @-CDis the most effective catalyst for accelerating the fluorescence quenching of pyrene and naphthalene except for the pyrene-triethylamine system, (3) a-CD slightly inhibits the fluorescence quenching of pyrene and does not affect the fluorsecence quenching of naphthalene, and (4) the fluorescence quenching by smaller aliphatic amines (dimethylamine and trimethylamine) is more sensitive toward addition of @-CDthan that by larger ones (diethylamine and triethylamine). Thus, the steady-state fluorescence measurements clearly showed the catalytic effect of CD for the fluorescence quenching of pyrene and naphthalene by the aliphatic amines. The fluorescence lifetimes of pyrene in aqueous CD solutions, however, were scarcely affected upon addition of quencher. The plot of T ~ / Tvs. [Q] for the pyrene-trimethylamine-@-CD system is also shown in Figure 1 in order to compare with the plot of Io/I vs. [Q]. The Stern-Volmer constant obtained from the plot of To/' vs. [Q] (Ksv7= 8.3 M-l) was much smaller than that from the plot of Io/I vs. [Q] (Ksvz= 3167 M-l), clearly indicating that static quenching predominates in the pyrene-trimethylamine-8-CD system.13J4 In other words, both fluorophore and quencher molecules are simultaneously included in the same cavity of p-CD and the fluorophore
Io/I =
TO/T
=1
+ Ksv[Q] = 1 + ~ , T ~ [ Q ] (2)
(12)Y. Matsui and K. Mochida, Bull. Chem. SOC.Jpn., 62, 2808 (1979).
were 5 X lo-' and 1 X lo-* M, respectively.
(13)W. R. Ware and J. S. Novros, J. Phys. Chem., 70,3246 (1966). (14)T.L. Nemzek and W. R. Ware, J. Chem. Phys., 62,477(1975).
1838
The Journal of Physical Chemistty, Vol. 86, No. 10, 1982
Kano et al.
TABLE 111: Effects of Cyclodextrins on Fluorescence Quenching of Pyrene, Naphthalene, and Their Sulfonate Derivatives by Aliphatic Amines in Water at 25 'C" fluorophore pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene pyrene naphthalene naphthalene naphthalene naphthalene naphthalene naphthalene naphthalene naphthalene sodium 1-pyrenesulfonate sodium 1-pyrenesulfonate sodium 1-pyrenesulfonate sodium 1-pyrenesulfonate sodium 1-naphthalenesulfonate sodium 1-naphthalenesulfonate sodium 1-naphthalenesulfonate
quencher dimethylamine dime thylamine dimethylamine dimethy lamine diethylamine diethylamine diethylamine diethylamine trimethylamine trimethylamine trimethylamine trimethylamine triethylamine triethylamine triethylamine triethylamine diethylamine diethylamine diethylamine diethylamine trimethylamine trimethylamine trimethylamine trimethylamine diethylamine diethylamine diethylamine diethylamine diethylamine diethylamine diethylamine
CD
Ksv,M-' 2.3 2.2 165 11 4.6 2.3 83 41 6.7 4.7 3167 29 29 24 382 54 8 17 25 102 36 5.6 6.1 195 30 23 19 33 29 27 45 32
(I
a 7 cy
4 7 cy
a 7 cy
B
Y cy
B 7 (I
B
Y cy
5
Y cy
B
10-*k M-1 sqi
k CD/ k>Zo
0.15 0.14 7.30 0.50 0.30 0.14 3.7 1.9 0.44 0.29 140 1.3 1.9 1.5 16.9 25.7 4.5
1 0.93 49 3.3 1 0.47 12 6.3 1 0.66 318 3.0 1 0.79 8.9 17 1
21.0
4.7
1.5
1
40.6
27
3.5 3.0 4.6 3.7
1 0.86 1.3 1.2
a Pyrene ( 5 X l o - ' M), naphthalene (1 X l o - ' M), sodium 1-pyrenesulfonate (1 X M), and sodium l-naphthalenesulfonate (1x 10'' M) were excited in water in the absence and the presence of CD (1 X l o - ' M) at 337, 280, 348, and 287 nm, respectively. The changes in the fluorescence intensities upon addition of quenchers were followed a t 394 nm for pyrene, 323 nm for naphthalene, 376 nm for sodium 1-pyrenesulfonate, and 334 nm for sodium 1-naphthalenesulfonate.
is quenched instantaneously by the neighboring quencher after excitation. Only decay of the fluorescence from pyrene molecules which do not form the three-component complex of pyrene, trimethylamine, and 8-CD may be observed by means of nanosecond time-resolved fluorometry. Virtually the Same results were obtained in the case of the pyrene-diethylamine-8-CD system (Ksv' = 4.4 M-' and Ksv*= 83 M-l). Although the effect of CD on the fluorescence quenching by primary aliphatic amines (n-propylamine and n-butylamine) were studied, the quenching ability of these amines was so poor that reproducible results could not be obtained. It can be said, however, that only a-CD shows an appreciable catalytic effect on the fluorescence quenching of pyrene and naphthalene: a-CD slightly accelerated the fluorescence quenching. Fluorescence Quenching by Aromatic Amine. NJVDimethylaniline has been well-know as a quencher which deactivates the fluorescent state of pyrene at a diffusioncontrolled rate.I5 Since the solubility of the inclusion complex of NJV-dimethylaniline with CD in water was quite low, the experiments were carried out in 50% aqueous dimethyl sulfoxide (Me,SO). The quenching parameters are summarized in Table IV together with the fluorescence lifetimes of pyrene in the aerobic aqueous MezSO solutions in the absence and the presence of CD. In contrast to the cases of the aliphatic amines, not only a-CD but also 8- and y C D inhibited the fluorescence (15) For example, P. Froehlich and E. L. Wehry, 'Modern Fluorescence Spectroscopy",Vol. 2, E. L. Wehry, Ed.,Plenum Press, New York, 1976, Chapter 8.
TABLE IV: Effects of Cyclodextrins o n Fluorescence Quenching of Pyrene by N,N-Dimethylaniline in 50% Aqueous Me,SO at 25 "C"
Q
D 7
193 208 294 230
489 439 66 219
25 21 2.2 9.5
1 0.84 0.09 0.38
a Pyrene ( 5 X lo-' M) in 50% aqueous Me,SO in the absence and the presence of CD (1 X lo-' M) was excited a t 340 nm, and the changes in the fluorescence intensities a t 394 nm were followed upon addition of N,N-dimethylaniline. Fluorescence lifetime in the absence of quencher .
quenching of pyrene. The inhibitory effects for the quenching of pyrene fluorscence increased in the order of a-CD < 7-CD < 8-CD. This order is compatible with that of the binding constants of NJV-dimethylaniline with CD (see Table I). Discussion The steady-state fluorescence measurements revealed that the fluorescencequenching of pyrene and naphthalene by relatively small aliphatic amines is significantly accelerated by 8- and 7-CD while that by a larger aromatic amine, NJV-dimethylaniline, is markedly depressed. In a series of aliphatic amines, the smaller amines such as dimethylamine and trimethylamine quench the excited fluorophore in aqueous CD solutions more efficiently than the large ones such as diethylamine and triethylamine. The following discussion is focused on the relationship
The Journal of Physical Chemistry, Vol. 86, No. 70, 1982 1837
Fluorescence Quenching of Pyrene and Naphthalene
between the quenching efficiency and the relative sizes of fluorophore and quencher to CD cavity. Judging from the CPK molecular model, the pyrene molecule is too large to fit into the cavities of a-and 8-CD. The fluorescence spectrum, as well as the fluorescence lifetime of pyrene, however, indicates the interaction between pyrene and CD. A dilute aqueous pyrene solution (5 X M) shows only monomer emission bands with maxima at 373,377,384,393, and ca. 410 nm. It is known that the fluorescence intensities at 373 and 384 nm are very sensitive to the solvent polarity: the ratio of the fluorescence intensity a t 384 nm to that at 373 nm (13&1/1373) increases with decreasing solvent polarity.16 The 1384/13,3 value in water was 0.57, while those in 1X M aqueous CD solutions were 0.58 for a-CD, 0.95 for p-CD, and 0.92 for yCD. These data clearly indicate that pyrene molecules in the aqueous a-CD solution are located at the aqueous bulk phase while those in the aqueous 8- and y C D solutions are translocated from the water phase to the hydrophobic cavities of CD. The same result has been reported for the pyrene-P-CD system by Edwards and Thomas.” The fluorescence lifetimes of pyrene and naphthalene in water with and without CD also suggested the interaction between these fluorophores and 8- and 7-CD (see Table I1 and the Experimental Section). A broad emission band from pyrene excimer (A,- = 475 nm) appeared upon addition of 7-CD (1 X 10-3-1 X M), Iexcimer/Imonomer being 0.086 a t [pyrene] = 5 X M M.’* The molecular model sugand [r-CD] = 1 X gests that two pyrene molecules can be included simultaneously in the same cavity of yCD. No excimer emission, however, could be observed in both cases of a-and 8-CD according to the same procedure. On the basis of these data, it can be concluded that (I) a-CD scarcely or weakly interacts with pyrene, (2) pyrene molecule is not included completely in the P-CD cavity but bind shallowly to p-CD to form a pyrene-capped CD complex, and (3) pyrene molecule is included completely in the cavity of yCD.
U
0-C 0
pyrene
pyrene-capped 0-C 0
It has been found that naphthalene forms 1:l and 2:2 complexes with p-CD at lower and higher concentrations of naphthalene, respectively; the binding constants are 685 M-’ for the 1:l complex and 4000 M-l for the 2:2 c0mp1ex.l~ The excitation of the 2:2 complex gives the naphthalene excimer emi~si0n.l~Recently, the 2:l complex of a-naphthylacetate with y C D has been suggested by measuring the excimer emission from the fluorophore. Under the present conditions, no excimer emission from naphthalene was observed. The naphthalene molecule may be included completely in both 6- and y C D cavities. The Stern-Volmer equation shown in eq 1could not be applied for the fluorescence quenching of pyrene by ali~
(16) K. Kalyanasundaran and J. K. Thomas, J. Am. Chem. SOC., 99, 2039 (1977). (17) H.Edwards and J. K. Thomas, Carbohydr. Res., 65,173 (1978). (18) K. Kano, I. Takenoshita, and T. Ogawa, to be submitted for publication. (19) S. Hamai, “Abstracts of Papers”,Symposium on Photochemistry, Mie, Oct 1980, IIIA-208.
phatic amines in water containing 0-CD. The inconsistency between Ksv’ and Ksv‘ indicates that static quenching predominantes in these systems. The static quenching can be interpreted in terms of the formation of three-component complexes of pyrene, aliphatic amine, and 0-CD. Since pyrene binds shallowly to p-CD, the pyrene-capped CD may still have a space in its cavity wherein a small quencher molecule is included:
pyrene-capped 0-CD
wencher
three-component complex
The pyrene chromophore of the three-component complex should be quenched instantaneously after excitation by the neighboring quencher. As shown in Figure 1 , 5 X 10” M of trimethylamine quenches ca. 90% of excited pyrene M of P-CD. Under molecules in the presence of 1 X the same conditions, no appreciable fluorescence quenching takes place in water in the absence of P-CD. Then, if the quencher molecules are distributed among 0-CD and pyrene-capped 8-CD in the same probability, only 50% of the pyrene molecules can associate with trimethylamine within the same cavity under the above conditions. Since trimethylamine molecules are also distributed to the bulk aqueous phase because of their high solubility in water, the association probability should be less than 50%. This simple calculation suggests that trimethylamine molecules are predominantly included in the cavity of pyrenecapped P-CD rather than the cavity of p-CD itself. The enhanced binding to a capped CD has been reported by Tabushi et al., who suggested that a favorable binding to the capped CD is ascribed to the decrease of the area of the hydrophobic surface of the guest molecule which is exposed in water.20 This concept is adequate to explain the present results. The effect of 8-CD on the fluorescence quenching of naphthalene by the aliphatic amines was not pronounced as compared with the case of pyrene. Since the naphthalene molecule can be included completely in the cavity of ,f3-CDas suggested by the molecular model, the threecomponent complex of naphthalene, aliphatic amine, and &CD may be difficult to form as compared with the case of pyrene. The fluorescence quenching of pyrene by secondary and tertiary aliphatic amines was always inhibited by a-CD. This should be ascribed to the unfavorable size of the a-CD cavity to form three-component complexes. Presumably, the 1:l complex formation between a-CD and aliphatic amine may reduce the effective quencher concentration in the aqueous bulk phase. Although the data are not presented here because of their poor reproducibility, the fluorescence quenching of pyrene and naphthalene by the primary aliphatic amines such as n-propylamine and nbutylamine was slightly accelerated by a-CD. This may mean that a considerably weak complex between fluorophore, primary amine, and a-CD forms to promote the static quenching. Since both pyrene and naphthalene molecules cannot be included in the a-CD cavity, these fluorophores may weakly bind to the external surface of wCD. (20) I. Tabushi, K. Shimokawa, N. Shimizu, H.Shirakata, and K. Fujita, J. Am. Chem. SOC., 98, 7855 (1976).
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The Journal of phvsical Chemistry, Vol. 86, No. 70, 1982
The effect of y C D on the fluorescence quenching was intermediate among the CDs used. The K values of diethylamine and trimethylamine with 7-CD were too small to determine accurately. The cavity of y C D may be too large to fit these amines. As mentioned above, the cavity of 7-CD is so large that pyrene and/or naphthalene can be included completely within the cavity. The molecular model indicates that the cavities of the 1:l complexes of naphthalene and pyrene with y C D are unfavorable for including an additional quencher molecule to form the three-component complexes. Therefore, the hydrophilic aliphatic atnines such as dimethylamine, diethylamine, and trimethylamine may be located predominantly in the aqueous bulk phase. Only in the pyrenetriethylamine system, was y C D most effective among the CDs for the fluorescence quenching. The hydrophobic property of triethylamine may promote the formation of the threecomponent complex. Since pyrene and triethylamine cannot be located simultaneously at the inside of the y C D cavity, either pyrene or triethylamine may bind shallowly to Y-CD. The effecta of CD on the fluorescence quenching greatly depend on the structure of the quencher molecule. As Table I11 shows, 0-CD enhanced the fluorescence quenching of pyrene by the smaller aliphatic amines such as dimethylamine (k m/k = 48.7)and trimethylamine (318) much more eficienhy than that by the larger ones such as diethylamine (12.3)and triethylamine (8.9). For understanding the effect of the structure of the quencher molecule, we compared the results of the fluorescence quenching of pyrene by diethylamine with those by trimethylamine. There are three conformational isomers of diethylamine:
In a homogeneous solution, most of the diethylamine molecules should be in the trans-trans conformation, which is thermodynamically stable. The molecular model suggests that the trans-gauche and eclipsed conformers can rotate in the cavity of 0-CD while the rotational motion of the trans-trans conformer is sterically restricted. The direction of the lone-pair electrons of the trans-trans
Kano et al.
conformer in the j3-CD cavity seems to be unfavorable for interacting with the pyrene molecule which binds to the face of the j3-CD cavity.
trans-trans
t rans-gauche
eclipsed
The trans-gauche and eclipsed conformers, which are less stable than the trans-trans conformer, can interact with the excited pyrene bound to j3-CD. On the other hand, trimethylamine does not have any conformer and the molecular model suggests that it can rotate freely in the cavity of j3-CD. Trimethylamine included in the 8-CD cavity, therefore, may quench the excited pyrene more efficiently than diethylamine. The same discussion applies to fluorescence quenching by dimethylamine and triethylamine in j3-CD solutions. In contrast with the fluorescence quenching by aliphatic amines, the quenching of the fluorescent state of pyrene by N,N-dimethylaniline was markedly inhibited by j3-CD (see Table IV). The binding constant shown in Table I indicates the tight complex formation between N,N-dimethylaniline and b-CD.2l The molecular model suggests that the N,N-dimethylaniline molecule occupies the whole cavity of 8-CD. Such a tight 1:l complex does not have a space for incorporating the pyrene molecule and is regarded as the deactivated quencher. Although the molecular model suggests that the formation of a three-component complex of pyrene, N,N-dimethylaniline, and y C D is possible, the fluorescence quenching of pyrene was depressed by y C D . The bulky dimethylamino group may inhibit the interaction between the lone-pair electrons of N,N-dimethylaniline and excited pyrene in the r-CD cavity. Fluorescence quenching is one model reaction to study a new type of CD-catalyzed bimolecular reaction. The present study reveals that the bimolecular reaction can be catalyzed under appropriate conditions by including two different kinds of substrates in the same cavity of CD. Further studies will expand the utilization of CD as a catalyst for various types of bimolecular reactions. (21) Hoahino et al. have also determined K of N,N-dimethylaniline with fl-CD to be 217 M-',which is in good agreement with our result: M. Hoehino, M. Imamura, K. Ikehara, and Y. Hama, J. Phys. Chem., 86, 1820 (1981).
