J. Phys. Chem. 1980, 84. 994-999
994
during the quenching. Thus, k3 is given by k3 = kisl + ki; (20) That is, k3 is comprised of fast intersystem crossing and internal conversion induced by inorganic anions and its value is close to that of %., In conclusion, it can be said that electron transfer (or charge transfer) is a key step in the anian-induced quenching of aromatic molecules. The quenching rate constants for EDA systems can be estimated by the present calculation based on the Polanyi rule,22although the Polanyi equation has a restriction in the range of AG. Acknowledgment, This work was supported by a Scientific Research Grant-in-Aid (No. 310905) from the Ministry of Education of Japan. The authors are grateful to the reviewers for their useful comments.
References and Notes
(11) For anthracene, biphenyl, fluorene, and pyrene, higher concentrations
of EtOH (50% EtOH aqueous solutions) were needed since they were insoluble in 20% EtOH aqueous solutions at 300 K. In the present paper, 1:l H20-EtOH mixtures were used In every case. (12) Ware, W. R.; Wan, D.; Holmes, J. D. J . Am. Chem. SOC. 1974, 96, 7853. (13) (a) Masuhara, H.; Hino, T.; Mataga, N. J . Phys. Chem. 1975, 79, 994. Hino, T.; Akarawa, H.; Masuhara, H.; Mataga, N. Ibid. 1978, 80, 33. (b) Masuhara, H., private communication. As for quenching of aromatic molecules by metal cations, ionlc and radical species which were expected are scarcely observed in laser experiments. 14) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8 , 259;Ber. Bunsenges. Phys. Chem. 1969, 73, 834. 15) For the reaction of D -tA D'A-, the Coulomb term (-e:/ta) should be added to the right side in eq 3 or 4,where t is the dielectric constant of the solvent used and a the encounter distance between the electron donor D and acceptor A. 16) Treinin, A.; Hayon, E. J. Am. Chem. SOC. 1976, 98, 3884. 17) Irie, M.; Yorozu, T.; Hayashi, K. J . Am. Chem. Sac. 1978, 100,
-
2236. 18) Kestner, N. R.; Logan, J.; Jortner, J. J. Phys. Chem. 1974, 78, 2148. 19) Jortner, J. J. Chem. Phys. 1976, 64, 4860,and references therein. 20) Marcus, R. A. J. Chem. Phys. 1956, 24,966;1965, 43,679;Annu.
Rev. Phys. Chem. 1964, 15, 155.
(1) Presented at the "Proceedings of VI1 IUPAC Symposium on Photochemistry at Leuven", July, 1978,p 303. (2) Jett, E.; West, W. Proc. R . SOC. London 1928, 727, 199. (3) Watkins, A. R. J . Phys. Chem. 1974, 78, 2555,and references therein.
(4) Forster, Th. "Fluoreszenz organischer Verbindungen"; Vandenhoeck and Ruprecht: Gottingen, 1951;Chapter 10. (5) Majumdar, D. K. Z. Phys. Chem. 1961, 217, 200. Beer, R.; Davis, K. M. C.; Hodgson, R. Chem. Commun. 1970, 840. Brooks, G. A. G.; Davis, K. M. C. J . Chem. SOC..Perkin Trans. 1972, 1 , 1649. Harriman, A.; Rockett, B. W. J . Chem. Soc., Perkin Trans. 1973,
2,1624. (6) Leonhardt, H.; Weller, A. Z. Phys. Chem. (Frankfurtam Main) 1961, 29,277. (7) Horrocks, A. R.; Medinger, T.; Wilkinson, F. Photochem. Photoblol. 1967, 6, 21. Kearvell, A.; Wilkinson, F. Mol. Cryst. 1968, 4 , 69. (8) Watkins, A. R. J. Phys. Chem. 1973, 77, 1207. (9) (a) Shizuka, H.; Matsui, K.; Okamura, T.; Tanaka, I. J. Phys. Chem. 1975, 79, 2731. Shizuka, H.; Matsui, K.; Hirata, Y.; Tanaka, I. Ibid. 1976, 80,2070;1977, 81, 2243. Shizuka, H.; Nakamura, M.; Morita, T. Ibid. 1979, 83, 2019. (b) Tsutsumi, K.; Shizuka, H. Chem. Phys. Lett. 1977, 52, 485;Z. Phys. Chem. (Frankfurt am Main) 1978, 711. 129. Shizuka. H.: Tsutsumi. K. J . Photochem. 1978. 9. 334. Shizuka, H.; Tsutsumi, K.; Takeuch, H.; Tanaka, I. Chem, h y s : Left. 1979, 62, 408. (10) Shizuka, H.; Saito, T.; Morita, T. Chem. Phys. Lett. 1978, 56, 519.
(21) Scandola, F.; Balzani, V. J . Am. Chem. SOC. 1979, 101, 1640. (22) Horiuchi, J.; Polanyi, M. Acta Physicochim. URSS, 1935, 2 , 505. Evans, M. G.; Polanyi, M. Trans. Faraday SOC. 1938, 34, 11. (23) For the Polanyi equation, eq 13 holds only in the range -0.493 eV 5 AGRW5 0.079 eV as described in the text. However, plots of ' k , calculated from the Polanyi equation fit the experimental values even beyond the range for AGRwas shown in Figure l b . (24)Indelli, M. T.; Scandola, F. J . Am. Chem. SOC. 1978, 100, 7733. (25) Efrima, S.;Bixon, M. Chem. Phys. Lett. 1974, 25, 34. (26) Eriksen, J.; Foote, C. S. J . Phys. Chem. 1978, 82, 2659. (27) Vogelmann, E.; Chreiner, S.; Rauscher, W.; Kramer, H. E. A. Z.Phys. Chem. (Frankfurt am Main) 1976, 101, 321. (28)(a) Wilkinson, F.; Schroeder, J. J . Chem. Soc., Faraday Trans. 2, 1979, 75, 441. (b) Schroeder, J.; Wilklnson, F. IbM. 1979, 75, 896. (29) Tamura, S.;Kikuchl, K.; Kokubun, H.; Usui, Y. Z. Phys. Chem. (Frankfurt am Main) 1978, 111, 7. (30) These values were estimated from the experimental values (anthracene, @F = 0.36; phenanthrene, aF= 0.23)with the assumption that abc-I-aF= 1. (31) Steiner, U.; Winter, G. Chem. Phys. Left. 1976, 55, 364. (32) Watkins, A. R. J. Phys. Chem. 1974, 78, 1885. A similar quenching mechanism Is given by Mazzucato and his group: Bortolus, P.; Bartocci, G.; Mazzucato, U. Ibid. 1975, 79, 21. (33) Kaptein, R. J . Am. Chem. SOC. 1972, 94, 6251. Lepley, A. R.; Closs, G. L., Ed. "Chemically Induced Magnetic Polarization", Wliey: New York, 1973.
Fluorescence Enhancement of Dibenzo-I8-crown-6 by Alkali Metal Cations' Haruo Shizuka,* Klyoshl Takada, and Toshifuml Morlta Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received November 7, 1979) Publication costs assisted by Gunma University
Dynamic behaviors of the excited state of the dibenzo-18-crown-6and alkali metal ion complexes in alcohol have been studied quantitatively. It is originally found that the fluorescence intensity of dibenzo-18-crown-6 (DC) is enhanced by alkali metal cations (M+). On the basis of kinetic analyses, it is shown that the internal quenching rate kTm due to the rotational diffusion rate krd decreases in the S1state of DC-M' complexes having a rigid structure and the decrease in kTIM leads to fluorescence enhancement. The maximum enhancement is observed in the DC-Kf complex which has the most rigid structure. The chelate strength of DC-M+ complexes related to the process represented by rate kTm has been discussed from the viewpoints of atomic number, ionic radius r of M+, and ring radius R of DC. For the heavy cations Rb+ and Cs', both intersystem crossing rate constants for SI TI and T1 So increase due to a heavy atom effect. Slight phosphorescence enhancement of DC by M+ (Li+,Na', and K+) is also observed in a MeOH-EtOH rigid matrix at 77 K.
-
-
Introduction Quenching experiments give US much information about bimolecular interactions between excited molecules and quenchers. It is well known that quenching processes involve electron transfer (or charge transfer), intersystem crossing due to a heavy atom effect, energy transfer, and chemical reactions. For metal cation quenchers, a number 0022-3654/80/2084-0994$01 .OO/O
of quenching studies of the singlet2-' and triplet"15 states of organic molecules by metal ions have been extensively reported. These studies are very important not only in Physical chemistry but also in biochemistry. Since the original work of Pedersen16 in 1967, there has been considerable recent interest in the chemical and physical properties of crown ethers.l' However, little at@ 1980 American Chemical Society
Fluorescence Enhancement of Dibenzo-18-crown-6
tention to the excited state of crown ethers has been given until recently. Sousa and Larson18 have reported the fluorescence quenching and enhancement of crown ether-naphthalene derivatives by alkali metal ions. The reason the fluorescence enhancement occurs in the complex between 1,8-naphtho-21-crown-6 and K+ (or Rb+)8a is unknown. In the course of our quenching studies of aromatic it was found that the fluorescence intensity of dibenzo-18-crown-6 in alcohol is enhanced by alkali metal cations.' The fluorescence (or lifetime) enhancement of dibenzo-18-crown-6 (DC) is caused by chelation of alkali metal cations (M+)and depends upon the atomic number of M+. In the present work, the relaxation processes of the excited singlet and triplet states of the DC-M+ complexes have been investigated quantitatively.
Experimental Section Dibenzo-18-crown-6 (a reagent grade product from Nihon Soda) was purified by repeated recrystallizations from ethanol. LiCl, NaC1, KCl, RbC1, and CsCl (GR grade products from Wako) were used without further purifications. Methanol (a SG product from Wako) was used as the solvent except for the experiments at 77 K. A mixed solvent of methanol-ethanol (1:4 in volume) was used a t M) 77 K. In usual cases, dibenzo-18-crown-6 (1.3 X plus alkali metal chlorides (MCl), 6.5 X lo4 M in methanol or a mixture of methanol-ethanol (1:4), were used. All samples were thoroughly degassed by freeze-pump-thaw cycles on a high vacuum line. Experimental apparatus and procedures were the same as reported previously.21B22Fluorescence quantum yields were determined by comparison with that of a quinine bisulfate-0.1 N H2S04 solution (@F = 0.54).23y24Phosphorescence quantum yields were evaluated from the relative intensities of total emission spectra and the corresponding fluorescence quantum yields a t 77 K. Fluorescence lifetimes were measured by a Hitachi nanosecond time-resolved fluorimeter (11-ns pulse width). The convolution method was applied to determine the fluorescence lifetimes by using a Facom 230-25 computer. A cryostat (Oxford DN704) for optical spectroscopy was employed a t temperatures 5250 K. Phosphorescence lifetimes were taken on a Hitachi MPF 2A fluorimeter with an oscilloscope (Iwatsu SS-6200). Results and Discussion ( 1 ) Absorption and Emission Spectra of the Complexes of Dibenzo-18-crown-6 (DC) and Alkali Metal Ions (M+). It is well known that DC-M+ (1:l) complexes are easily formed in Sample solutions of 1.3 X M DC plus 6.5 X M alkali metal chlorides in methanol or a mixture of methanol and ethanol (1:4) were used in the present work,26and the two-component equilibriaz5were completely shifted to the complexes under the experimental conditions. Absorption spectra of the DC-M+ complexes in methanol at 300 K are shown in Figure 1. A vibrational structure appears at the 274-nm band due to complex formation as has been shown by Pedersen.16b The vibrational spectra indicate that the chelating strength of the cations to DC is in the order K+ > Na+ > Rb+ > Cs+ > Li'. The molar extinction coefficients of the complexes a t absorption maxima were almost the same as that ( E = 5.2 X lo3 M-' cm-' a t 274 nm) of DC in methanol. Figure 2 shows the fluorescence, phosphorescence, and excitation spectra of the complexes. These spectra of the complexes were scarcely changed compared with those of DC (blank) [fluorescence maxima amRg= 3.28 X lo4 cm-l (DC and DC-Li+) and pmax = 3.33 X lo4 cm-l (DC-M+ except for
The Journal of Physical Chemistry, Vol. 84, No. 9, 1980 995 0.8 I
I
I
I
Wavelength Inm
Flgure 1. Absorption spectra of DC-M+ complexes ([DC] = 1.3 X M and [MCI] = 6.5 X M) in methanol at 300 K. 600
500
Wavelength In m 400 300
250
1
Wavenumber I 104cm-'
Flgure 2. Typical fluorescence, excitation, and phosphorescence solid line, in methanol at spectra of DC-M+ complexes (M+ =:)'K 300 K;dotted line, in a MeOH-EtOH rigid matrix at 77 K. See the text for details.
Li+) in methanol at 300 K, vmax= 3.41 X lo4 cm-l (DC and DC-M+) in a MeOH-EtOH rigid matrix a t 77 K; phosphorescence maxima vmax = 2.04 X lo4 cm-l (DC and DCM+) in a MeOH-EtOH rigid matrix at 77 K]. The excitation spectra were very close to those of the corresponding absorptions of the complexes, showing that the fluorescence originates from the lowest excited singlet state l(a,a*). There was no excimer or exciplex emission under these experimental conditions. (2) Fluorescence Quantum Yields (@FM) and Lifetimes ( T M ) of DC-M+ Complexes. Measurements of the fluorescence quantum yields @FM and lifetimes T~ a t various temperatures have been carried out. The results are summarized in Table I. It was found that the values of @FM and TM of DC in alcohol are appreciably enhanced by alkali metal cations. Of course, the counteranion (Cl-) effect on the values of @ F ~ and J TpJ was negligibly small under the experimental ~0ndition.l~ The @m(or T M ) value in a MeOH-EtOH mixture at 300 K was the same as that in methanol. Plots of @FM or 7 M vs. ionic radii r27of the alkali metal ions are interesting and are shown in Figures 3 and 4, respectively. In the present work, we adopted the Gourary-Adrian ionic radii scale P7instead of those of PaulingB and G o l d s ~ h m i d tsince , ~ ~ the Gourary-Adrian ionic radii scale is more consistent with our explanation of the QFM (or T M ) dependence on r than others.30 In comparison with the @FM value of DC (0.15 a t 300 K), the @FM values of DC-M+ complexes increase with increasing r value to give a maximum value of @FMmaX (0.24 a t 300 K) at rm = 1.49 A for the DC-K+ complex. After passing through the point r, the @FM values of the complexes DC-Rb+ and DC-Cs+ decrease with increasing the r values. Similar results are
996
Shizuka, Takada, and Morita
The Journal of Physical Chemistry, Vol. 84, No. 9, 1980 0.4 -
0.39 M 0.2
I
1 0;.
I
I
Blank L:
-11
k
N i
1.0
"
-rlA
F;!
ii 2 .o
1.5
Figure 3. Plots of @FM vs. r a t various temperatures. See the text for details.
6.05.0
-
4.0
-
4-220 K
3.0-
c280K
Z M ns
-
J77 e190 K ~ 2 5 K0
e300K
u.
t
T
t,
-
Blank L:
wFigure 4. Plots of
Na
1.o '
T M vs.
rti
?+ t , K Rb
7,
C,
1.5
2.0
I
r a t various temperatures.
obtained at various temperatures. The fluorescence enhancement is significant at higher temperatures (300 K). Plots of TM vs. r are very similar to those of @FM vs. r as shown in Figure 4. One can estimate the values of the radiative rate constants kFMfrom eq 1. These kFMvalues @FM = ~ F M T M
(1)
are also listed in Table I and do not change compared with that of DC (blank). That is, the fluorescence enhancement of DC by M" is not due to an increase in the k F M values. It is noteworthy that the rmvalue (1.49 A) of the Kf ion corresponds to that of the ring radius of DC ( R = 1.3-1.6 The chelate strength for the DC-M+ complexes increases with increasing the atomic number of M+ in the range r < R, judging from the vibrational structures at the 274-nm band in Figure 1. Recently, Rode and Gstrein31 have investigated 1:l complexes of biuret with alkali and alkaline earth metal ions by means of ab initio MO SCF calculations and have shown that there is a linear relation between the relative energy for chelation and the atomic number. For DC-M+ complexes, the chelate strength does not increase with increasing atomic number in the range r 1 R in accord with the geometrical restriction of the ring size of DC where it is impossible for M+ to enter into the DC ring. Thus, the chelate strength for the DC-Rb" or DC-Cs" complex having r greater than R may decrease with increasing atomic number. This corresponds to the order of chelate strength derived from the vibrational structures a t the 274-nm band as described above. Furthermore, a F M and T M values for DC-Rb+ and DC-Cs" complexes decrease because of the heavy atom effect due to heavy metal cations as will be discussed in the next section. Temperature effects on @FM and T M were markedly observed as shown in Table I and Figures 3 and 4. These values increase with decreasing temperature in all cases. The experimental results show that the radiationless rate
.E? a0
rl
2 . z
?
8
.3 'm
125
The Journal of Physical Chemistry, Vol. 84, No. 9, 198Q 997
Fluorescence Enhancement of Dibenzo- 18-crown-6
TABLE 11: Intersystem Crossing Yields ( @ T M ) and Intersystem Crossing Rate Constants ( k T M ) for S, + T,, Phosphorescence Quantum Yields ( @ p ~ and ) Lifetimes ( T T ) , and Rate Constants for Radiative ( k p ~ ) Intersystem , Crossing T, + So ( k G T ) , and Triplet Decay Processes ( k T ) in a MeOH-EtOH Rigid Matrix at 7 7 Ka
Li' Na+
a
0.64 0.60 K+ 0.59 Rb' 0.66 cs+ 0.72 Experimental errors within 20%.
constant kIMT for the excited singlet state of DC-M+ complexes (or DC) decreases with decreasing temperature. The temperature-dependent rate constant KIMT becomes negligibly small in a rigid MeOH-EtOH matrix at 77 K, as can be seen in Figures 3 and 4. (3)Phosphorescence Quantum Yields (apT) and Lifetimes (rT)of DC-M+ Complexes at 77 K . Measurements of amand 7T for DC-M+ complexes have been carried out in a MeOH-EtOH rigid matrix a t 77 K, and the results are listed in Table 11. Slight phosphorescence enhancement of DC by M+ (Li+, Na+, and K+) was observed in a MeOH-EtOH rigid matrix at 77 K. The order of the phosphorescence quantum yields @pT (K+ > Na+ > Li+ > blank E Rb+ > Cs') is the same as that of @FM at 77 K, although the triplet decay rate kT increases with increasing the atomic number of M+. Equation 2,32where @TM is the @FM @TM N 1 at 77 K (2) intersystem crossing yield for S1 T1,approximately holds in a rigid MeOH-EtOH matrix at 77 K, since there was no appreciable photochemical reaction of the DC-M+ complexes. The values of @TM can be calculated from eq 2 where the @FM values at 77 K are given in Table I. The values of @pTand T~ in Table I1 are relatively small in spite of the moderate values of @TM. The intersystem crossing rate constants kGT for T1 So, which can be obtained from eq 3, are relatively large compared with the radiative rate
+
0.67 0.42 0.28 0.22 0.075 0.055
0.08 0.14 0.24 0.46 0.72 0.40
3.7 4.1 6.0 3.5 1.6
1.1 1.0 1.0 1.2 1.5
1.4 2.2 3.3 4.1 12.6 17.8
1.5 2.4 3.5 4.6 13.3 18.2
SO Flgure 5. Schematic energy state diagram for the relaxation processes of DC-M+ complexes. See the text for details.
-
-
(3)
-
constants km for T1 So. Of course, the phosphorescent state TI is 3 ( ~ , ~ * )The . large values of kGT is probably due to the charge-transfer character in the TI state of the DC-M+ complexes, considering the broad and structureless phosphorescence spectrum and the relatively large red shift of the emission (see Figure 2). The intersystem crossing rate constants kTM for S1 T1at 77 K are also obtained from eq 4, where TMO denotes
-
(4) the fluorescence lifetimes a t 77 K (see Table I). Considerable increases both in the intersystem crossing rate constants kTM and kGT for the DC-Rb+ and DC-Cs+ complexes indicate a heavy atom effect due to heavy metal cations. As for the radiative rate constants kpT for T1 So, the values of kpT increase with increasing atomic number of the alkali metal ions. ( 4 ) Fluorescence Enhancement Mechanism. Let us consider dynamic processes in the excited state of DC-M+ complexes. The experimental results of DC-M+ complexes can be accounted for by diagram in Figure 5. The fluorescence lifetimes TM (at temperatures >77 K) and TMO (at 77 K) can be expressed by eq 5 and 6, respectively, ~ T M = @TM/TM'
-
TM
TMO
=
=
+~
(~FM
(kFM
+
G M ~ T M ) -=~ (kFM f ~ I M ) - '
+ ~ G M ' + ~TM')-'
=
+
(5)
( ~ F M k ~ ~ ' ) - l (6)
TZ-I
io4des
P-1
Flgure 6. Internal quenching rate constants kIMT via rotational diffusion as a function of Tg-' in methanol. See eq 7.
where kIM and kIMo denote internal quenching rate constants at >77 and 77 K, respectively, and k G M o ( T T - ~is )therefore ~ given by kIMT
= Q(T/q)+ p
(8)
where a and P are constants (see Table 111). The value of the rate constant for rotational diffusion in methanol, hrd, can be expressed by the Debye rotational correlation time T ~ : ~ ~ T~ (=&I)
= 4nao3q/(3~T)
(9)
998
The Journal of Physical Chemistry, Vol. 84, No. 9, 1980
Shizuka, Takada,
TABLE 111: Specific Values in t h e I n t e r n a l Quenching via R o t a t i o n a l D i f f u s i o n for DC-M' Complexes in Methanola a/
samples
l o 3 deg-' P-'
PI l o 7 s-l
blank
4.1 3.3 2.4 2.0 2.7 2.8
-- 1.0
Lit Na' K' Rb' CS' a
-1.5 - 2.3 -2.0 - 2.4 -1.7
24-crown-$ and alkali metal ion complexes were also examined. However, fluorescence enhancement was scarcely observed in their systems. One possible explanation for this is that it is very hard to form a planar or semiplanar structure in such complexes with a large ring size.37
[TIS I,/
l o 4 deg P-' 0.24 0.45 0.96
1.0 0.89 0.61
See t h e t e x t for details.
where a. is the radius of the rotating particle and Boltzmann constant. We thus obtain
K
the
krd = RT/(Vv)
(10)
+p
(11)
kIMT = dk,d
where a' = aVR-l. From the slopes and intercepts in Figure 6, the values of a and p in eq 8 can be obtained, and are listed in Table 111. The values of a are related to overall efficiencies for the internal quenching rate kIdr via the rotational diffusion rate krd. Very recently, similar analyses have been carried out for the intramolecular fluorescence quenching of phenylalkylamines.21 The experimental values of a! are in the order blank > Li+ > Cs+ > Rb+ > Na+ > K+, indicating that the order of internal quenching rate constants kIMTdue to dynamic motion (Brownian motion) of excited DC-M+ complexes is the same as that for a values. The order of a values is also in accord with the reverse order of the chelate strength of the DC-M+ complexes as described in sections 1 and 2. It is of interest that the larger slopes ( a ) become the smaller threshold values ( T V - as ~ )shown ~ in Figure 6 and Table 111. These findings show that a rigid structure of DC-M+ complexes is responsible for a decrease in the internal quenching rate constant kIMTand therefore 7 M values become large compared with that of DC (blank). As a result, fluorescence enhancement occurs in DC-M' complexes having a rigid structure. The fluorescence enhancement is therefore significant at higher temperatures and smaller viscosity. The internal quenching constant kIMTseems to be mainly comprised of the T7-l dependent internal conversion. It is known that the rate constant for internal conversion is markedly enhanced by a conformational change in the excited state.34 This was found to be the case. A conformational change between the bichromophores of DC in the S1 state may give rise to an exciton interaction or a charge-transfer interaction leading to the nonradiative decay SI So. Complex formation of DC with M+ prevents such a conformational change in the excited state. It is assumed that DC-M+ complexes, especially for M+ = Na+ and K+, have a planar or semiplanar structure both in ground and excited states when considering the experimental results as described above. A semiplanar structure for the DC and potassium p toluenesulfonate complex, analyzed from dipole moment data,35IR and X-ray diffraction supports this assumption. Sousa and Larson8 have reported that for the complex between 2,3-naphtho-20-crown-6 and K+ the fluorescence quantum yield decreases probably due to increased internal quenching. Therefore, decreased kIMT caused by complexation and related stiffening of crown ethers is probably not general, but it may hold in crown ethers having bichromophores such as DC. The fluorescence enhancement was observed in the DC-sodium p toluenesulfonate complex [ @pM= 0.21(k0.02) in methanol a t 300 K]. Fluorescence measurements of the dibenzo-
-
and Morita
Concluding Remarks Fluorescence enchancement of dibenzo-18-crown-6(DC) by alkali metal cations (M+)was especially found at higher temperatures (300 K). The reasons why the maximum values of @FM and T M are obtained in the DC-K+ system are as follows: (1)The structure of the DC-M+ complex is rigid. The internal quenching rate kIMT(probably internal conversion) due to rotational diffusion in the S1 state of DC-M+ complexes becomes appreciably small when compared with that of DC (blank), since DC-M+ complexes have a rigid structure by chelation. This phenomenon results in the fluorescence enhancement of DC-M+ complexes. The chelate strength in the complexes of M+ to DC increases with increasing atomic number for r < R and then decreases with increasing the atomic number for r 1 R; the order of the chelate strength is K+ > Na+ > Rb+ > Cs+ > Li+. (2) The heavy atom effect is present. For DC-Rb+ or DC-Cs+, the decrease in @FM or T M is due to the enhancement of intersystem crossing induced by the heavy metal cation. That is, the effects of M" on DC in the excited state involve two photophysical actions: a decrease in internal quenching kIMT(fluorescence enhancement) and an increase in intersystem crossing induced by heavy atom cations (fluorescence quenching). Slight phosphorescence enhancement of DC by M+ (Li+, Na', and K+) was also observed in a MeOH-EtOH rigid matrix at 77 K. Acknowledgment. This work was partially supported by a Scientific Research Grant for Solar Energy of the Institute of Physical and Chemical Research.
References and Notes Presented at the Symposium on Molecular Structure, Tokyo, Oct 9-12, 1979, p 226. Foss, R. P.; Cowan, D. 0.; Hammond, G. S. J. Pbys. Cbem. 1964, 68, 3747. Weber, G. Biochem. J. 1950, 47, 114. Rutter, W. J. Acta Cbem. Scand. 1958, 12, 436. Varnes, A. W.; Dodson, R. B.; Wehry, E. L. J. Am. Chem. Soc.1972, 94, 946. Kemlo, J. A.; Shepherd, T. M. Cbem. Pbys. Lett. 1977, 47, 158. Saito, T.; Yasoshima, S.; Masuhara, H.; Mataga, N. Cbem. Pbys. Lett. 1978, 59, 193. (a) Linschitz, H.; Sarkanen, K. J. Am. Cbem. SOC. 1958, 80,4826. (b) Linschitz. H.: Pekkarinen, L. Ibid. 1960, 82, 2411. (c) Steel, C.; Linschitz, H. J. Pbys. Cbem. lQ62, 66, 2577. Porter, G.; Wright, M. R. Discuss. Faraday SOC.1959, 27, 18. (a) Moore, W. M.; Hammond, G. S.; Foss, R. P. J. Cbem. Pbys. 1960, 32, 1594; J. Am. Cbem. Soc. 1961, 83, 2789. (b) Fry, A. J.; Lin, R. S. H.; Hammond, G. S. Ibid. 1966, 88, 4781. (c) Wasgestian, H. F.; Hammond, G. S. Tbeor. Cbim. Acta 1971, 20, 186. Binet, D. J.; Gokiberg, E. L.; Foster, L. S. J . Pbys. Chem. l W 8 , 72, 3017. Ohno, T.; Kato, S. Bull. Cbem. SOC. Jpn. 1969, 42, 3385. Wilkinson, F. Pure Appl. Cbem. 1975, 41, 661. Schroeder, J.; Wilkinson, F. J. Cbem. SOC.,Faraday Trans. 21979, 75, 896, and preceding papers listed thereln. Baizani, V.; Carassiti, V. "Photochemistry of Coordination Compounds"; Academic Press: New York, 1970. Marshall, E. J.; Pilling, M. J. J. Cbem. Soc., Faraday Trans. 21978, 74, 579. (a) Pedersen, C. J. J. Am. Chem. SOC.1967, 89, 2495. (b) Ibid. 1967, 89, 7017. (a) Pedersen, C. J.; Frendsorff, H. K. Angew. Cbem., Int. Ed. Engl. 1972, 1 I , 16. (b) Lehn, J. M. "Structure and Bonding"; Vol. 16; Springer-Verlag: Berlin, 1973; p 1. (c) Truter, M. R. I n ref 17b, p 71. (d) Christensen, J. J.; Eatough, D. J.; Izatt, R. M. Chem. Rev. 1974, 74, 351. (e) Cram, D. J.; Helgeson, R. C.; Sousa, L. R.; Timko,
J. Phys. Chem. 1980, 84, 999-1005
(18) (19) (20)
(21)
J. M.; Newcomb, M.; Moreau, P.; Dejong, F.; Gokel, G. W.; Hoffman, D. H.; Domeier, L. A.; Peacock, S. 0.; Madan, K.; Kapian, L. Pure Appl. Chem. 1975, 43, 327. (f) Hiraoka, M. “Crown Compounds: Their Characteristics and Applications” (in Japanese); Kohdansha Scientific: Tokyo, 1978. (a) Sousa, L. R.; Larson, J. M. J. Am. Chem. SOC. 1977, 99, 307. (b) Larson, J. M.; Sousa, L. R. J. Ibld. 1978, 100, 1943. Shizuka, H.; Saito, T.; Morita, T. Chem. Phys. Lett. 1978, 56,519. (a) Tsutsumi, K.; Shizuka, H. Chem. Phys. Lett. 1977, 52,485; (b) 2.Phys. Chem. (Frankfurtam Main) 1978, 7 7 I , 129. (c) Shizuka, H.; Tsutsumi, K. J . Photochem. 1978, 9 , 334. (d) Shizuka, H.; Tsutsumi, K.; Takeuchi, H.; Tanaka, I. Chem. Phys. Lett. 1979, 82, 408. Shizuka, H.; Nakamura, M.; Morita, T. J. Phys. Chem. 1979, 83, 2019. fa) Shizuka. H.: Matsui. K.: Okamura. T.: Tanaka. I. J . Phvs. Chem. 1975, 79,2731. (b) Shizuka, H,; Matsui, K.; Hirata, Y.;ianaka, I. Ibid. 1977, 87, 2243; 1976, 80, 2070. Meihuish, M. H. J. Phys. Chem. 1961, 65,299. Demas, J. N.; Crosby, G. A. J . Phys. Chem. 1971, 75,991 Frensdorff, H. K. J . Am. Chem. SOC.1971, 93, 600. Large equilibrium constants in DC-Ms systems are reported. For example, there was no change in the fluorescence intensity of the DC-Na’ s stem with variation In the NaCl concentration (1.3 X 10-4-1 x io- M). Gourary, B. S.;Adrian, F. J. Solid State Phys. 1960, 70, 127. The Gourary-Adrian ionic radii are 0.94 (Lis), 1.17 (Na’), 1.49 (K’), 1.63 (Rb’), and 1.86 A (Cs’).
. ..
(22), (23) (24) (25) (26)
Y
(27)
999
(28) Pauling, L. “The Nature of the Chemical Bond”, Corneli University Press: Ithaca, New York, 1948. (29) Goldshmidt, V. M. “Geochemishe Verteilungsgesetze der Eiemente”; Skrifter Norske Videnskaps-Adad.; Oslo, I.Mat.-Naturv. Ki., 1926. (30) For example, the ionic radii scale of Gourary and Adrian is fitted to the temperature dependence of the viscosity of concentrated aqueous electrolyte solutions more than the Pauiing ionic radii scale as reported by Goklsack and Franchetto. See the reference of Goldsack, D. E.; Franchetto, R. C. Can. J . Chem. 1978, 56, 1442. The GouraryAdrian ionic radii are much bigger than the ionic radii taken from X-ray work on crowns and cryptands as suggested by one of referees. (31) Rode, B. M.; Gstreid, K. H. J . Chem. Soc., Faraday Trans. 2 1978, 74, 889. (32) Birks, J. E. “Photophysics of Aromatic Molecules”; Wiley-Interscience: London, 1970. (33) Debye, P. ”Polar Molecules”, Dover Publications: New York, 1974. Bioembergen, N.; Purceli, E. M.; Pound, P. V. Phys. Rev. 1948, 73, 679. (34) (a) Oster, G.; Nishijima, Y. J . Am. Chem. SOC.1958, 78,1581. (b) Forster, Th.; Hoffmann, G. Z. Phys. Chem. (Frankfurt am Main) 1971, 75,63; (c) Magde, D.; Windsor, M. W. Chem. Phys. Lett, 1974, 24, 144; (d) Kordas, J.; El-Bayoumi, M. A. J . Am. Chem. SOC.1974, 96, 3034. (e) Shizuka, H.; Seki, I.; Morita, T.; Iizuka, T. Bull. Chem. SOC.Jpn. 1979, 52,2074. (35) Gruenwald, T. I. J . Am. Chem. SOC. 1974, 96, 2879. (36) Dale, J.; Kristiansen, P. 0. Acta Chem. Scand. 1972, 26, 1471. (37) Fenton, D. E.; Mercer, M.; Poonia, N. S.; Truter, M. R. J. Chem. Soc., Chem. Commun. 1972, 66.
Vibrational Spectra and Molecular Conformations of Potassium trans-2-Pentenoate and Potassium trans-2-Hexenoate. Conformation Change Due to Micelnzation Hirofumi Okabayashi” Department of Industrial Chemistty, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya 466, Japan
and Motoko Abe Shoh Women’s University, NakaJimadori,Fukiai-ku, Kobe 65 1, Japan (Received November 13, 1979)
The Raman spectra of potassium trans-2-hexenoate and potassium trans-Zpentenoate in the crystalline state and aqueous solution were measured. To aid vibrational assignments of the Raman bands of these molecules, we also measured the Raman spectra of trans-2-hexene and trans-2-pentene(in the solid and liquid states and made the calculations of normal mode vibrations for the possible rotational isomers of trans-Zhexene. The observed spectra in the region of 600-200 cm-l, where the skeletal deformation bands appear, show that the skew-trans and skew-gauche isomers of potassium trans-2-hexenoate are stable in aqueous solution. The intensities of the Raman bands of the skewgauche isomer relative to those of the skew-trans isomer increase with increasing concentration of the molecules in aqueous solution, and the intensity change is remarkable near the minimum concentration necessary for micelle formation. Such a concentration dependence of Raman intensity was also observed in the region of 1200-800 cm-’. Micelle formation affects the conformation of the trans-2-hexenoate ion, and the skew-gauche form of this ion is more stable in the micelle than in the monomolecular dispersion state. For potassium trans-2-pentenoate, cis and skew forms coexist in the crystalline state and aqueous solution, and no remarkable concentration dependence of the Raman intensity was observed.
Introduction In a conformation study of saturated hydrocarbon chains, the vibrational spectra have been used successf~lly.l-~ The concept of acoustical modes developed by Mizushima and Shimanouchi,l and Schaufele6i7has played an important role in the study of structural ordering of the saturated hydrocarbon parts of fatty acids and phospholipids.6-11 The Raman spectra of potassium n-alkyl carboxylate and sodium n-alkyl sulfate have recently been reported,12J3 and the conformations of these molecules in aqueous solutions have been discussed. From a comparison of the longitudinal accoustical mode in the crystalline state with that in aqueous solutions, it has been concluded that the 0022-3654/80/2084-0999$01 .OO/O
conformational randomness of the n-alkyl hydrocarbon skeleton increases with the number of carbon atoms.12 Moreover, by measuring the Raman intensities of such an accordionlike mode at various concentrations, Okabayashi et have shown that for potassium 11-alkyl carboxylates (carbon number n = 5,6) the all-trans form of the hydrocarbon chain predominates aftter micelle formation. These observations are relevant to our understanding of the structural ordering of the hydrocarbon part of lipid molecules in biomembranes and their model systems. The present work concerns the study of conformation changes due to the micelle formation of surfactants having unsaturated hydrocarbon chains. For detailed investigation below the critical micelle concentration (cmc), sur0 1980 American
Chemical Society
