J . Am. Chem. SOC.1985, 107, 4655-4662
4655
57-3; 5-, 96897-63-1; 6(PF6)4, 96897-21-1; 65+, 96897-31-3; 63c, 96897-39-1; 62+,96897-45-9; 6+, 96897-50-6; 6 (neutral), 96897-58-4; 6-, 96897-64-2; 7(PF6)4, 96897-23-3; 75+,96897-32-4; 73+,96897-40-4; 7,+, 96897-46-0;7+, 96897-51-7; 7 (neutral), 96897-59-5;7-, 96897-66-4; 8(PF6)4, 96897-25-5;S5+, 96897-33-5;8’+, 96897-41-5;S2+,96897-47-1; 8+, 96897-52-8;8 (neutral), 96897-60-8;8-, 96897-65-3;8”, 96897-70-0; 8’-, 96897-72-2;9(PF6),, 96897-27-7; g5+, 96897-34-6; g3+,96897-42-6; 9,+, 96897-48-2;9+, 96897-53-9; 9 (neutral), 96913-26-7;9-, 96897-67-5; [Ru(Me,bpy),13+, 96897-35-7; [Ru(Me,bpy),]+, 96897-54-0; [Ru(Md,bpy),], 96897-61-9; [Ru(Me,bpy)J, 96897-68-6; [Ru[(COOEt)2bpy],(Me,bpy)] 3 + , 96897-36-8; [Ru [ (COOEt),bpy] (Me2bpy)](PF6),,96897-29-9; [R~[(COOEt),bpy]~(Me~bpy)]+, 9689755-1; [R~[(COOEt)~bpy]~(Me~bpy)], 96897-62-0; [Ru[(C0OEt),bpyl2(Me2bpy)]-, 96897-69-7; [ R ~ [ ( C 0 O E t ) ~ b p y ] ~ 96897-73-3; (Me2bpy)12-.96897-7 1-I ; [R~[(COOEt),bpy],(Me~bpy)]~-, Ru(bpy),CI,, 15746-57-3;Ru(Me2S0),CI2, 11070-19-2;Me2bpy, 113435-6; Me2-2DQ2+,96913-23-4; Me2-2DQ+, 96897-09-5; Me2-2DQ, 96913-24-5; Me2-3DQ2+,96897-10-8; Me2-3DQ+, 96897-1 1-9; Me23DQ, 96897-12-0; 1,2-dibromoethane, 106-93-4; 1,3-dibromopropane, 109-64-8.
Finally, while the electron-transfer products have lifetimes of
1-OC
A
1.0
-
0.5 I-
8.8
1
488
8m t InrErb)
I288
./”
1088
10
5
InCIntm.lty>
15
[CD] ,/1 0-3M
B
Figure 2. a/(l - a)vs. concentrationof @CD. a values were determined from Cuz+ quenching (0) and T1+ quenching ( 0 )of Py fluorescence. Table I. Bimolecular Quenching Rate Constants (M-’ s-l) in @CD
Systems
auencher
H,O 0 2 1.1 X 1O’O C H ~ N O ~8.1 x 109 TI+ 6.3 x 109 C2Pd+Br- 7.9 X lo9 cu2+ I ” C I n t . r u 1 ty>
C
4.5 X lo9
medium @CD + O*CD NaLS“ 1.2 X lo9
5.6 x
io8
2 x 107 7 X IO7 1.7 X 10’
6 X lo8 2.5 x 107 4 0 7
5 x 107 6.5 X lo8
@CD + CTAB* 6 X lo8
1.7 x 107 4 0 7
3.5 x 107 2.2 X 10’ 6.8 X 10’
MV+ 7.8 x 109 5 x 109 4.0 X lo9 C , ~ C ~ V 7.6 ~ + x 109 ClnPd+ 3.6 X lo9 “[PCD] = 13.7 mM; [NaLS] = 15 mM. b[/3CD] = 13.7 mM; [CTAB] = 10 mM.
a / ( l - a ) vs. [CD]. The association constant, K, = (44 & 6) M-I, was determined from the slope of the plot using eq 3 and 4. The concentration of Py did not affect the value of K, in the range between 5 X IO-’ and 4 X IO” M. The K, value determined in the present study was one-fourth that determined by NakajimaSsa The discrepancy may result from the different method used. A marked protective effect of P C D was observed for the Py quenching of fluorescence by oxygen, nitromethane, TI+, and Cu2+ as shown in Table I. The quenching in Table I may be divided into two groups, those that require contact of excited Py and quencher and those that do not. The latter category includes Cu2+ I . . . . . . . . . and MV2+. Quenching by MV2+ is electron transfer and is only 8.8 488 (IBB 1209 1088 t InrCnb) slightly affected by P C D , as the electron may be transferred over the distance larger than the collisional distance of Py and MV2+. Figure 1. Fluorescence decay curves of Py in P-CD systems: (A) 5 X A similar mechanism applies to Cu2+ although the P-CD does 10-7M Py in N,-saturated (a) and 02-saturated (b) 8.CD solution ([fl-CD] = 13.7 mM); (B) 5 X lO-’M Py in 13.7 mM 8.CD solution reduce the efficiency of reaction to a larger extent than MV2+. containing 0 (a), 5 (b), and 10 mM (c) of T1+;(C) 5 X lo-’ M Py in 13.7 Reaction of C H 3 N 0 2and C2Pd+Br- with Py are also electron mM @CD solution in the presence of 20 mM of CIoN(CH3)3*Br-(a), transfer in nature but are markedly affected by P-CD. This CloPd+Br-(b), and C8Pd+Br-(c). indicates that a close approach of the reactants is required for reaction. Thallium(1) ions cause intersystem crossing of excited in the Py-PCD associated systems, obeying the Stern-Volmer Py and require close contact of the two reactants. This mechanism relation: is clearly reflected in the dramatic reduction in reaction efficiency by P C D . Oxygen also causes intersystem crossing of excited Py, but the relatively small effects of the host molecule on the reaction The values of a, Klo, kl,, kz0, and kZqwere determined by a efficiency indicate that close contact is not required for this recomputer fitting of the experimental decay curves, typically shown action. in Figure IB, in terms of eq 2 at different concentrations of (2) Fluorescence Decay of Pyrene in B.CD Solution in the quencher, [Q]. A constant value of a was obtained at a fixed Presence of Amphiphilic Molecules. A marked change was obconcentration of PoCD irrespective of the various concentration served both for the absorption and the emission spectra of Py in of quenchers. Value of k l o = 2.6 X lo6 s-l and k20 = 4.8 X IO6 p-CD solution on addition of surfactant molecules to the system. s-l were obtained in the absence of quencher. The k20value agrees In particular, an intensity enhancement of the Py emission well with the reciprocal of the Py fluorescence lifetime obtained spectrum was clearly observed in the presence of a series of independently in water (4.4 X lo6 sd). Figure 2 shows a plot of C,S04-Na+ and C,NH3+Cl- (or C,N(CH3)3+Br-). No change
4658 J . Am. Chem. SOC.,Vol. 107, No. 16, 1985
Hashimoto and Thomas
111/1 I
Table 11. Association Constants and Reciprocal Lifetimes of Py in
LO1
1
Py-CD-Surfactant Systems
A
IO
__
no
-If
'
20
-I
30 40 C~urfactant]/ 10-3 M
100
Ill / I
B
IO
K., M-' a
surfactant (concn, mM)
.--
20
40
30
l100
~ ~ u r f a c t a n tI ~O/ - 3 ~ C
IO
20
30
[CPCII/ I O - 3 ~
Figure 3. (A) III/I ratio of Py fluorescence as a function of concentrations of C4S04-Nat ( 0 )and C6NH3+C1-(0). [PCD] = 13.7 mM; [Py] = 5 X lo-' M. (B) III/I ratio as a function of concentrations of NaLS (0),CTAB (p),and CPCl (0). [PCD] = 13.7 mM; [Py] = 5 X lo-' M. (C) Relative yield of Py fluorescence against the concentration of CPCI. [ P C D ] = 13.7 m M ;[Py] = 5 X IO-' M.
44
l/T",
C4S0;Nat (20) C6S0cNat (20) C8S0;Nat (20) CIoSOpNat (15) CI2SO4-Na+(15)
3000 1400 660 280 130
4.4 2.2 2.1 2.1 2.1 2.2
C4NH3+C1-(40) C6NH3'C1- (20) C8NHtC1- (20) C I ,,N(CH,) 3+Br- (20) C12N(CH3)3+Br-(15) C16N(CH3)3tBr-(10) Errors: f 15%.
86 1300 790 440 170 330
2.5 2.1 2.2 2.2 2.2 2.2
s-'
x 106 x io6 x 106 x 106
x 106 x 106 X lo6
x x x x
106 106 106 106 x 106
phenomenon was ascribed to the formation of micelles which remove Py from the P C D because of their larger affinity of the micelles for hydrophobic molecules. In the presence of C,Pd+X-, a similar increase and leveling off in III/I ratio were also observed as typically shown for Cl6Pd+C1-or CPCl in Figure 3B; however, a relative fluorescenceyield was significantly reduced at 12-14mM CPCl (cmc = 9 X 104M) as seen in Figure 3C. This is also an indication of micellar formation since it was observed that Py fluorescence was statically quenched in micellar CPCl solutions, and, therefore, fluorescence is considered to originate only from Py complexed with P C D in the present system containing CPC1. Both the decrease in the III/I ratio of P C D solutions in the presence of increasing amount of NaLS or CTAB and the decrease in Py fluorescence yield at higher concentrations of CPCl are indicative of a saturation of interaction between P C D and surfactant molecules. The saturation took place at a surfactant concentration of 10-13 mM (taking the cmc into account) which is a little below the concentration of P C D used (13.7 mM). This surfactant-CD interaction has previously been recognized by Okubo et a1.I2as an increased "apparent cmc" in micellar solutions of NaLS and CTAB by the addition of CD molecules, which was studied using conductivity measurements. They assumed intuitively that a 1:l surfactant-CD complex is formed. More strikingly, in the presence of surfactant molecules (ca. above 3-10 mM), a single exponential decay of Py fluorescence (a > 0.96 in eq 2) with a decay constant of 2.1-2.2 X lo6 s-l was 0 b ~ e r v e d . l ~A typical example is shown in Figure 1C for the system containing 20 mM CloN(CH3)3+Br-.This was rationalized by the increased association of Py and P C D in the presence of excess surfactant molecules, S, as shown in the following scheme: Py
+
CD-S
e
Py-CD-S
(7)
in the spectroscopic properties of Py can be observed when surfactant systems without P C D are examined. This intensity enhancement phenomenon is due to the change in microenvironment experienced by Py," and a convenient parameter, III/I ratio (which stands for the ratio of first and third vibronic bands of Py fluorescence), has been established as a measure of the polarity of the medium.llb Figures 3A and 3B depict the III/I ratio of 4, as a function of concentration of surfactants in several systems in 13.7 mM p-CD solution containing 5 X lo-' M Py. In general, a n increase was observed in the III/I ratio for the increasing concentrations of P C D , indicating that Py experiences a more hydrophobic environment, and finally a plateau was reached above a concentration of 2-8 mM, the exact value more or less depending on the surfactant. The plateau values, 1.4-1.7 in the presence of surfactants, is almost identical with the III/I ratio observed in hydrocarbon solution. For surfactants with relatively small cmc, a sharp decrease in III/I ratio was observed at higher concentrations. For example, the III/I ratio abruptly decreased at 17-20 mM CI2SO4-Na+,or N a L S (cmc = 8 X M), and also at 12-14 mM Cl,N(CH3)3+Br-or CTAB (cmc = 9 X lo4 M). This
The value of the association constant of surfactant and CD, K, in eq 7 is not known except in a few systems; Okubo et al.I2 have determined K,(P.CD-NaLS) = 356 M-' and K,(PCD-CTAB) = 2240 M-l, for example. However, the saturation of the III/I ratio may indicate that most Py is in the form of Py-CD-S at a sufficient high concentration of surfactant. Therefore, the association constant, K,, in the presence of excess surfactant as also determined by means of the analysis of Py fluorescence decay curves described in section 1 (see Table I1 for the values of K , ) . It is apparent from Table I1 that a larger value of K , is obtained in the presence of surfactants which have a shorter hydrocarbon chain. However, no interaction was observed between P-CD and C2 surfactants (C2NH3+C1-and C2PdtBr-), which may not be sufficiently hydr~phobic.'~The effect of surfactant in the present system can be understood by its complexaton with P C D , thus excluding water molecules originally occupying the cavity of @CD
(1 1) (a) Nakajima, A. Bull. Chem. SOC.Jpn. 1971,44, 3272-3277. (b) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chem. SOC. 1977, 99, 2039-2044.
(12) Okubo, T.; Kitano, H.; Ise, N. J . Phys. Chem. 1976,80, 2661-2664. (13) Complete single exponential decay of Py fluorescence was not achieved in the presence of NaLS or CTAB. (14) Only a slight interaction was observed for C4NHItCI- with &CD.
CD
!k+
S
Pyrene- and Naphthalene-Cyclodextrin Complexes
J . Am. Chem. SOC.,Vol. 107, No. 16, 1985 4659
Log k, , s-' A
9-
8-
7-
5
10
15
I
3.0
CN
I/T,
3.4 3.6 10-3 ~ - 1
Figure 5. Arrhenius plots of first-order quenching constants of C,, C,, C8 pyridinium surfactants. [PCD] = 13.7 mM; [Py] = 5 X IO'' M; [C,Pd+Br-] = 30 mM.
B
Table 111. Thermodynamic Parameters of Py Fluorescence Quenching in Py-Cd-C,PdBr Systems (21 "C) C6 C, C* E,, kcal/mol 1.8 I .9 2.7 AG', kcal/mol 6.3 7.1 8.0 AS*, cal/(mol-K) -17 -1 9 -20
a,..
'..
'._'.
.,
_ I I
20
30
b-a , A Figure 4. (A) Log k, vs. the number of carbon chain of pyridinium surfactants. [P-CD] = 13.7 mM; [Py] = 5 X lo-' M; [C,Pd+X] = 20 mM. (B) Relation between calculated k , and ( b - a ) , leading to a more hydrophobic environment. Accordingly, Py interacts more strongly with the cavity in the presence of surfactants. Shorter surfactant chains may provide more room for Py to interact in the cavity of CD-S complexes. The equilibrium constant, K,, of Py and P-CD decreases with temperature both in the absence and in the presence of surfactant. A linear relation was observed for the plot of In K, vs. l / T in the absence and in the presence of 20 mM C6S0,-Na+. From the plot, we obtained AH = -6.8 kcal/mol and A S = -15.5 cal/mol-K in the absence of surfactant, and AH = -12.0 kcal/mol and AS = -26.5 cal/mol.K in the presence of C6S04-Na+,respectively. Py fluorescence quenching by hydrophilic quenchers was observed in the presence of surfactants in p C D system. In Table I, the observed Stern-Volmer quenching rate constants in the presence of 15mM N a L S and lOmM CTAB were listed. AIthough larger rate constants are obtained in the NaLS-CD system for positively charged quenchers, the magnitudes of constants are
not so different from those in surfactant-free @CD system. It was also noticed that the quenching constant was significantly reduced for the system where complex formation is necessary for the quenching mechanism, namely, TI+ quenching; however, this was not the case for the Cu2+and MV2+quenching systems which undergo electron transfer. (3) Pyrene Fluorescence Decay in the Presence of Pyridinium Surfactants. A single exponential decay was also observed for Py fluorescence in the presence of C,,Pd+Br- or C,Pd+CI- (n > 5 ) . Representative decay curves are shown in Figure IC. However, a reduced lifetime, which is constant above certain concentration of surfactants, was observed. This suggests that a three-component complex, Py-CD-S, is exclusively formed a t excess concentration of pyridinium surfactants, and that an "intramolecular-like" quenching of Py fluorescence takes place within this complex. The length of the hydrocarbon chain of pyridinium surfactants as found to significantly affect the rate constant of this quenching reaction. The first-order quenching rate constants of pyridinium surfactants were plotted against the number of carbons in the chain of the surfactant in Figure 4A. A temperature study was carried out for the quenching by C6, C7, and C8 pyridinium bromides over the range from 0 to 75 O C . Arrhenius plots of the quenching constants are given in Figure 5. The thermodynamic parameters obtained are listed in Table 111, and it can be seen that one of the characteristics of these systems is the large negative values of LIS*. The "diffusion-controlled reaction model in a limited space" proposed by Tachiya15 was used to explain the chain-length dependent quenching constant of pyridinium surfactants. In this model, reactants A and B are confined in a sphere of radius b. The first-order quenching constant k , is given by:
k , = Da: (15) Tachiya, M. C'hem. Phy.r. L e f t . 1980, 69. 605-607
(8)
4660 J. Am. Chem. Soc.. Vol. 107, No. 16, 1985
1.0
'
Hashimoto and Thomas Table IV. Yields of Py' Fluorescence Quenching with CPCl in Different Media medium @(PY+). medium a(py+ja CH$N 0.63 0.1 M NaLS
