1820
J. Phys. Chem. 1981, 85, 1820-1823
and IBare the ionization potentials of K+ and benzene while JK and JB are the respective correction factors suggested by Pitzerm and O b is the angle between the vector from the ion to the center of the bond and the vector in
the direction of the bond. No experimental information is available for the electron repulsion energy EER. This term was equated to the STO-3G calculated energy between argon and benzene.
Fluorescence Enhancement of Benzene Derivatives by Forming Inclusion Complexes with P-Cyclodextrin in Aqueous Solutions Miklo Hoshlno," Masashl Imamura, The Institute of Physical and Chemical Research, Wako-shi, Saitama 351, Japan
Klyoshl Ikehara, and Yoshlmasa Hama Science and Engineering Laboratory, Waseda University, Shinjuku-ku, Tokyo 160, Japan (Received: June 18, 1980; In Final Form: February 4, 198 1)
Benzene derivatives (benzene, phenol, ethoxybenzene, aniline, N-methylaniline, N,N-dimethylaniline, and NJV-diethylaniline)form inclusion complexes with P-cyclodextrin in aqueous solutions. The equilibrium constants of these inclusion complexes were obtained by absorption and/or fluorescence spectrophotometric methods. Fluorescence enhancement was observed upon inclusion for all benzene derivatives studied. Of the compounds, aniline and N-methylaniline showed marked effects of inclusion. These results are discussed on the basis of environmental changes around fluorescent molecules upon inclusion.
Introduction Cyclodextrins form inclusion complexes with organic solutes in aqueous solutions. The binding force between cyclodextrins and organic solutes has been assumed to be hydrogen bonding, van der Waals force, or hydrophobic interaction.'P2 Since absorption spectra of inclusion complexes differ from those of organic solutes dissolved in water, the absorption spectrophotometric method has been used for obtaining equilibrium constant^.^ A series of thermodynamic and kinetic studies on the formation of inclusion complexes showed that a molecule can be included when the size of the molecule is smaller than that of the cavity inside a cyclodextrin m ~ l e c u l e . ~ The first observation of fluorescence enhancement upon inclusion was reported for the aqueous P-cyclodextrin solutions of l-anilin0-8-naphthalenesulfonate.~ Recently, Kondo et al. studied fluorescence enhancement of 6-(pto1uidino)naphthalenesulfonate for the purpose of detecting ring opening of cyclodextrins catalyzed by Takaa m y l a ~ e . ~No other compounds were reported to show fluorescence enhancement in aqueous cyclodextrin solutions. In the present study, the spectra and the quantum yields of fluorescence were measured for benzene, phenol, ethoxybenzene, aniline, N-methylaniline, N,N-dimethylaniline, and N,N-diethylaniline in aqueous P-cyclodextrin solutions in order to elucidate the behavior of fluorescent molecules in their excited states upon inclusion. Experimental Section Reagent-grade benzene, ethanol, and phenol were used without further purification. Water, ethoxybenzene, aniline, N-methylaniline, N,N-dimethylaniline, and N,Ndiethylaniline were purified by distillation. P-Naphthol (1)Hall, E.S.;Ache, J. J. Phys. Chem. 1979,83,1805. (2)Hoffman, J. L.; Bock, R. H. Biochemistry 1970,9,3542. (3)Cramer, F.;Saenger, W.; Spatz, H. J.Am. Chem. SOC.1967,89,14. (4)Kondo, H.; Nakatani, H.; Hiromi, K. J. Biochemistry 1976,79,393. 0022-3654/81/2085-1820$01.25/0
was sublimed under reduced pressure before use. Reagent-grade P-cyclodextrin (P-CD) was purified by recrystallizing it twice from aqueous solutions. No fluorescent impurities were detected in aqueous solutions of the purified P-CD. Quinine sulfate was used as supplied. The purity of chemicals used was ascertained from absorption and fluorescence spectra. Absorption and fluorescence spectra were recorded on a Hitach 200-20 spectrophotometer and a MPF-4 spectrofluorimeter, respectively. All spectra were measured at 28 f 2 OC. Quantum fluorescence spectra were measured by calibrating the output of the spectrofluorimeter using the corrected fluorescence spectra of aniline and phenol in ethanol solutions, and quinine sulfate and @-naphtholin both aqueous and ethanol An acid aqueous solution of quinine sulfate was used as a standard for determination of fluorescence quantum yields. Solutions were not degassed because fluorescence intensities observed for deaerated aqueous solutions of benzene derivatives were decreased only by less than 2% after aeration.
Results Absorption Spectra. Figure 1 shows the absorption spectra of N-methylaniline (NMA) in aqueous solutions containing various amounts of P-CD. The absorption band around 283 nm shifts toward long wavelengths with increasing concentration of P-CD. The isosbestic points appearing at 270 and 280 nm indicate an equilibrium: NMA + P-CD F? NMA-P-CD The equilibrium constant, K (= [NMA.P-CD]/( [NMAI(5) Berlman, I. B. "Handbook of Fluorescence Spectra of Aromatic Molecules"; Academic Press: New York, 1965. (6) Lippert, E.; Nagele, W.; Seibald-Blankenstein, I.; Staiger, U.; Voss, W. Z. Anal. Chem. 1959,170, 1. (7) Parker, C. A.; Rees, W. T. Analyst (London) 1961,87, 83.
0 1981 American Chemical Society
Fluorescence Enhancement of Benzene Derivatives
The Journal of Physical Chemistry, Vol. 85, No. 13, 1981 1821
WAVELENGTH (nm) 300 250 1
"
.
450
I
0.6
400
WAVELENGTH ( n m ) 350 300
I
0.5 0.4
0.3
OD 0.2
10.1
25 30 WAVENUMBER ( k K 30
40
35 WAVENUMBER( k K
Figure 1. Absorption spectra of aqueous N-methylaniline solutions
(3.24X lo4 M)at various concentratlons of PCD: (A) without PCD; (B) 4.47 X lo-' M OCD; (C)1.72 X lo-' M PCD; (D) 2.64 X lo-'
M P-CD; (E) infinite concentration of p-CD.
35
Figure 3. Fluorescence spectra of aqueous N-methylaniline solutions (3.24X lo4 M) at various concentrations of PCD: (A) without PCD; (B) 1.19 x 10-3M P-CD; (c)2.91 x 10-3M PCD; (D) 4.48 x 10-3 M PCD; (E)9.59 x 10-3 M OCD; (F) 1.73 x io-* M ~ C D .
of the second one. The isosbestic points were observed at 278 and 274 nm with increasing concentration of P-CD. The first band shows a marked red shift with increasing concentration of p-CD, and the peak wavelength reaches 288 nm. The equilibrium constant was found to be (2.3 f 0.1) X lo2 M-' at 28 "C. The absorption spectra of N,N-diethylaniline (DEA) were measured at pH 10 to suppress protonation: DEA H2O F! DEAH+ + OH-
+
\ '
'
'
'
' -
The first absorption band of DEA in the aqueous solution is located around 290 nm as a shoulder of the second one. On addition of 0-CD, the peak of the first band appears at 305 nm with the isosbestic points at 288 and 278 nm. The equilibrium constant is (9.6 f 0.5) X lo2 M-' at 28 "C. The oscillator strength of DEA increases by forming the inclusion complex. Owing to small changes in absorption spectra, the equilibrium constants were not determined by the absorption spectrophotometric method for aniline, benzene, phenol, and ethoxybenzene in aqueous p-CD solutions. Spectra and Quantum Yields of Fluorescence in Aqueous p-CD Solutions. Figure 3 shows fluorescence spectra of N-methylaniline in aqueous solutions containing p-CD at various concentrations. The spectra were obtained by irradiating the solutions with 280-nm light at the isosbestic point. The fluorescence intensity is markedly enhanced, and the peak wavelength shifts toward shorter wavelengths with increasing concentration of p-CD. The Stokes loss obviously decreases when N-methylaniline forms the inclusion complex. The fluorescence quantum yield, a, of a fluorescent solute in an aqueous @-CDsolution is given by eq 2, where
+ zb)-'(*aza + @Jb)
= (1,
(2) Z stands for the number of photons absorbed per unit volume and time, and subscripts a and b refer to a solute and ita inclusion complex, respectively. The ratio za/zb and the equilibrium constant K are represented by eq 3 and 4, where t is the molar extinction coefficient at an exciting z a / z b = caCa/(tbCb) (3)
K = cb/(cacfl)
(4)
wavelength and C stands for the concentration; the sub-
1822
Hoshino et el.
The Journal of Physical Chemistry, Vol. 85, No. 13, 1981
TABLE I : Fluorescence Yields of Benzene Derivatives ( @ ( H 2 0 )and ) the Inclusion Complexes (@ (Inc)) in Aqueous Solutions solute @w2Ola @ (1nc)O
ethoxybenzene
phenol
benzene 0.0058'
0.14
0.0071
0.24
'
aniline
NMAC
DMA~
DEAe
0.025 0.10
0.029 0.17
0.048 0.13
0.036 0.076
0.18 0.22
a Experimental errors are ~ 5 . 0 8 % . Values from ref 8 . Diethylaniline.
N-Methylaniline.
N,N-Dimethylaniline. e N,N-
TABLE 11: Fluorescence Yields of Benzene Derivatives in Ethanol Solutions (@ (EtOH))a solute
benzene
0.003'
@(EtOH)b a
Abbreviations are the same as in Table I.
phenol
ethoxy benzene
aniline
0.20
0.23
0.085
' Experimental errors are +5.0%.
NMA
DMA
DEA
0.096
0.067
0.056
Value from ref 10.
50
40
Ask 30 20 1
I
10
20
30
4.0 51! $X162(M)
60
7.0
8.0 9 0
Figure 4. Plots of against reciprocal concentration of p-CD: (A) aniline; (B) N-methylaniline; (C) N,Ndimethylaniline; (D) N,N-
diethylaniline.
5.0
-2 $XI0
script /3 refers to 0-CD. Substituting eq 3 and 4 into eq 2, we obtain
+
9 = (1 + yKc@)-'(@a @b-fKC@)
where y =
10
tb/ta.
(5)
Equation 5 is transformed to
A 9 R - l = (@b,/@a - 1)-'(1
+ l/rKC@)
(6)
where AaR= (@/aa - 1). Figure 4 shows the plots of A 9 R - I vs. Cg' for aniline, N-methylaniline, N,N-dimethylaniline, and N,N-diethylaniline. Changes in refractive index, viscosity, and dielectric constant of the solution would be expected with increasing concentration of 0-CD. Good linear relation' C,, however, indicate that 9,and ships between A ~ R - vs. 9 b are scarcely affected by the addition of /3-CD. Figure 5 shows similar plots for benzene, phenol, and ethoxybenzene. From the slopes and intercepts of the lines, the equilibrium constants and the fluorescence enhancement ratios, +b/+a, are obtained. For example, N-methylaniline = 6.0 at 28 "C. The yields K = 52.6 f 2.6 M-' and former value is in good agreement with that obtained with the absorption spectrophotometric method. The latter indicates that the fluorescence quantum yield of the inclusion complex is 6 times as large as that of N-methylaniline itself in an aqueous solution. In Table I are listed the fluorescence quantum yields of the benzene derivatives and the inclusion complexes in aqueous solutions. Apparently, the molecules having an NH group exhibit marked fluorescence enhancement upon inclusion. Fluorescence Q u a n t u m Yields of Benzene Derivatives in Ethanol Solutions. The fluorescence studies of benzene revealed that radiationless processes are accelerated in
- 1 6.0 (MI
Figure 5. Plots of against reciprocal concentration of 6-CD: (A) phenol; (B) benzene; (C) ethoxybenzene.
water more efficiently than in organic solvents.8 In fact, the fluorescence quantum yields of benzene are 0.0058 and 0.033 in aqueous and ethanol solutions, respectively. Benzene derivatives, therefore, are considered to have lower fluorescence quantum yields in water than in ethanol. In Table I1 are listed the fluorescence quantum yields of benzene derivatives in ethanol solutions. From Tables I and 11, we can conclude that the fluorescence quantum yields in aqueous solutions are similar to those in ethanol solutions for phenol, ethoxybenzene, Nfl-dimethylaniline, and N,N-diethylaniline. Aniline and N-methylaniline, however, have lower fluorescence quantum yields in water than in ethanol as in the case of benzene. These results indicate that water molecules act as an effective quencher for the excited benzene derivatives having an NH group.
Discussion Equilibrium Constants. In Table I11 are summarized the equilibrium constants obtained by absorption and fluorescence spectrophotometric methods. For benzene, phenol, ethoxybenzene, and aniline, the equilibrium constants were determined solely by the fluorescence spectrophotometric method. The equilibrium constant increases in the order N,Ndiethylaniline, ethoxybenzene, N,N-dimethylaniline, benzene, N-methylaniline, aniline, and phenol. These results suggest that the hydrophobic interaction is important for the formation of the inclusion complexes. (8) Lauria, M.; Ofran, M.; Stein, G.
J. Phys. Chem. 1.974, 78,1904.
Fluorescence Enhancement of Benzene Derivatives
The Journal of Physical Chemistry, Vol. 85, No. 13, 1981
TABLE 111: Equilibrium Constants Obtained with Absorption and Fluorescence Spectrophotometric Methods" benzene phenol ethoxybenzene aniline NMA DMA DEA
47.6 * 2.4 230 i. 10 9601: 50
TABLE V: Relative Radiative Rate Constants k,,(Inc)/k,,(EtOH) for Benzene Derivativesa
1 9 6 f 10 40 i 2.0 2 8 6 t 15 50 f 3.0 52.6 ?- 2.6 217 f 10 8621: 40
benzene phenol ethoxy benzene aniline NMA DMA DEA
a Abbreviations are the same as in Table I. Values are obtained with absorption spectrophotometric method. Values are obtained with fluorescence spectrophotometric method.
TABLE IV: Relative Radiative Rate Constants ( a )and knr(H,O)/k,dEtOH) for Benzene Derivatives"
0.97b 0.83 f 0.84 f 0.74 f 0.74 f 0.82 f 0.65 f
benzene phenol ethoxybenzene aniline NMA DMA DEA
0.12 0.14 0.14 0.15 0.12 0.12
5.8 * 0.4 1 . 4 ?: 0.3 1.25 t 0 . 2 2.7 1: 0.8 2.7 f 0.8 1.3 f 0.3 1.1 * 0 . 3
a Abbreviations are the same as in Table I. from ref 8.
Value
= kr/(kr
+ knr)
(7)
k , and k,, are the rate constants for the radiative and radiationless processes of an excited state. From the molar extinction coefficient, t ( u ) , and the fluorescence intensity, I(v), at a wavenumber of u, the value of k , is computed by9 k , = 2.88
X
10-9n2(u~3)a;'Sc(u)/udu
(8)
where n is the refractive index of a solution and (uf3)a[1 =
I(u) du/$r31(u) du
(9)
In order to elucidate the difference in the quantum yields between aqueous and ethanol solutions, the rate constants of the radiationless processes were compared with the two solutions. From eq 7 , the relative rate constant, k,(H20)/k,(EtOH), is given by eq 10, where (H,O) knr(H2O)/kn,(EtOH)= CY(@(H~O)-')/(@(E~OH)-' - 1) (10) and (EtOH) stand for aqueous and ethanol solutions, respectively, and cy = k,(H,O)/k,(EtOH). The value of a is calculated from k , simulated with eq 8 for aqueous and ethanol solutions. In Table IV are listed cy and K,,(H20)/k,,(EtOH) for seven benzene derivatives studied. The former value is smaller than unity, indicating k,(EtOH) > k,(H20) for the benzene derivatives. The latter value is 1.1-1.4 for phenol, ethoxybenzene, N,N-dimethylaniline, and N,N-diethylaniline, whereas it is 2.7-5.6 for benzene, aniline, and N-methylaniline. Presumably, particular radiationless (9) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814.
0.1 0.2 0.2 0.15 0.2 0.2 0.2
( 7 ) and
4.7 t 1.0 0.7 t 0.2 0.95 * 0.3 0.7 f 0.3 0.7 i 0.4 0 . 5 * 0.15 0.72 * 0.3
Abbreviations are the same as in Table I.
processes are accelerated by water molecules for the latter three compounds. Fluorescence Enhancement Due to Inclusion. Seven benzene derivatives showed fluorescence enhancement by forming the inclusion complexes with P-CD in aqueous solutions. Except for benzene, the fluorescence quantum yields of the benzene derivatives are in the order W n c ) 1 @(EtOH)> @(H20) (11) where (Inc) stands for an inclusion complex, and those of benzene are $(etOH) > @(Inc)2 @(H20).These results are discussed on the basis of k,, of each compound. Similarly to eq 10, the relative rate constant, knr(Inc)/knr(EtOH), is expressed as
= y(@(Inc)-l - l)/(@(EtOH)-' - 1) (12) where y = k,(Inc)/k,(EtOH). In Table V are listed y and k,,(Inc)/k,,(EtOH). The former value is approximately unity except for aniline. The latter value is smaller than unity, indicating k,(Inc) < k,(EtOH). Since k,(Inc) < k,(EtOH) N k,(H20) for phenol, ethoxybenzene, N,N-dimethylaniline, and N,Ndiethylaniline, radiationless processes occurring in solutions are considered to be suppressed by forming the inclusion complexes. Inclusion probably gives rise to the decrease in intramolecular rotational freedom of these molecules by fixing them inside the cavity of 0-CD, resulting in the decrease in k,,. A particular radiationless process due to water is suggested for aniline and N-methylaniline based on comparison between k,(H20) and k,(EtOH). Since k,(Inc) < k,,(EtOH) < k,,(H20) for these two molecules, the decrease in k, due to inclusion may result from elimination of water molecules surrounding the fluorescent molecules and the decrease in their rotational freedom. Fluorescence enhancement of benzene due to inclusion is interpreted in terms of elimination of water molecules. Benzene included inside the cavity of 0-CD, however, is deduced to interact with water molecules because of knr(H20) 2 k,,(Inc) > k,,(EtOH). k,,(Inc)/k,,(EtOH)
Fluorescence Quantum Yields in Aqueous and Ethanol Solutions. The fluorescence quantum yields of benzene derivatives are larger in ethanol solutions than in aqueous solutions. The quantum yield, @, is given by eq 7 , where @
a
0.97 f 1.06 * 0.98 f 0.70 f 1.01 t 1.03 t 0.94 *
1823
Conclusion Fluorescence enhancement of benzene derivatives due to inclusion is ascribed to the increase in k,, the decrease in their rotational freedom, and elimination of water molecules surrounding fluorescent molecules in aqueous solutions. The excited states of the benzene derivatives having an NH group are efficiently quenched by water molecules. Fluorescence enhancement observed for these derivatives is principally due to elimination of water molecules by inclusion. Acknowledgment. We thank Professor Haruo Shizuka, Gumma University, for his valuable discussions.
