J. Phys. Chem. 1985,89, 4081-4087 N,,,(Rho-O:-) = 1.2, we obtain N,(Rho-02-) = 2.5. Possible structures for the metal-support interface are discussed in ref 18. In summary, we have found experimental evidence for a coordination of interfacial rhodium atoms with oxygen ions of the support. The bond length is 2.68 8, with an estimated average coordination number of 2.5 atoms. Short oxide-like coordination distances have not been detected in fully reduced rhodium catalysts. The metal-support interaction is possibly through an ioninduced dipole interaction due to the registry of the metal particle with the support. The results presented in this work show that metal to support-oxygen bonds in supported metal catalysts can be detected when both the dispersion of the metal particles and the signal-to-noise ratio of the EXAFS data are high enough.
408 1
Acknowledgment. This work was done at SSRL (Stanford University), which is supported by the Department of Energy, The National Science Foundation, and the National Institutes of Health. We gratefully acknowledge the assistance of the SSRL staff and the assistance of Mr. E. F. Gleason and Dr. J. B. Michel in the preparation and characterization of the catalyst. This study was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for the Advancement of Pure Research (ZWO). D.C.K. and R.P. thank Z W O for supplying a travel grant (R71-34) and NATO for a research grant (1972), respectively. Registry No. Rh, 7440-16-6; 02,7782-44-7; A120,, 1344-28-1.
Kinetic Analysis of the Anomalous Behavior in the Fluorescence Quenching of Phenanthryl Groups Covalently Linked to Polyelectrolytes Yotaro Morishima,* Takaomi Kobayashi, and Shun-ichi Nozakura Department of Macromolecular Science, Faculty of Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: February 26, 1985)
The fluorescence quenching of amphiphilic polyanions containing phenanthryl chromophores (APh) by bis(2-hydroxyethyl) terephthalate (BHET) was investigated in aqueous solutions over a concentration range of the phenanthryl residue of 8 X 10-5-3 X M. The steady-state experimental data were analyzed in terms of a kinetic model based on a stepwise association for the incorporation of BHET, an amphiphilic quencher, into the hydrophobic microdomains of the amphiphilic polymer. The polyanion with a phenanthryl content of 41 mol % (APh-41) showed fluorescencequenching for which Stern-Volmer-type plots gave a downward curvature. The initial slope was extremely steep and was dependent on the residue concentration of the phenanthryl chromophore. The quenching process was reasonably approximated as entirely static in nature. The first-step association constant for the BHET incorporation was estimated to be K 1 = 1.1 X lo3 M-I, which is in fair agreement with the binding constant of K = 1.2 X lo3 M-' obtained by the equilibrium dialysis technique. The analysis also led to estimates of the average aggregation number of the phenanthryl chromophores as n = 2.6 for APh-41, through which energy migration could take place. The rate constant for the intradomain fluorescence quenching was estimated by computer curve fitting to be kq,]= 5.5 X IO7 s-I. Under conditions where the BHET concentration is high enough for all the microdomains to incorporate the quencher, the fluorescence decay curve showed apparently single exponential nature, yielding kq,]= 4.8 X lo7 s-l which is comparable to the computed value. In contrast, the amphiphilic copolymer with a phenanthryl content of 2 mol % showed entirely dynamic quenching because of the absence of hydrophobic association of the quencher with the polymer.
Introduction Polyelectrolytes chemically modified with polycyclic aromatic groups have been demonstrated to form microscopically phaseseparated structures in aqueous solutions because of their strong amphiphilic Within a polymer chain of these polyelectrolytes the attractive hydrophobic interaction between the hydrophobic groups competes with the repulsive electrostatic interaction between the segmental electric charges as conceptually illustrated in Figure 1. The consequence of the competition critically determines the conformation of the macromolecular chains. In case the hydrophobic interaction prevails over the segmental Coulombic repulsion, the hydrophobic groups would end up forming hydrophobic microdomains. In this situation the charged segments would surround the periphery of the microdomains, thus the amphiphilic polyelectrolytes may provide a (1) Y . Morishima, Y . Itoh, and S. Nozakura, Makromol. Chem., 182, 3135 (1981). (2) Y . Itoh, Y. Morishima, and S. Nozakura, J . Polym. Sci., Polym. Chem. Ed., 20, 467 (1982). ( 3 ) Y . Morishima, Y . Itoh, T. Hashimoto, and S. Nozakura, J . Polym. Sei., Polym. Chem. Ed., 20, 2007 (1982). (4) Y . Morishima, T. Tanaka, Y . Itoh, and S. Nozakura, Polym. J . , 14, 861 (1982).
0022-3654/85/2089-4081$01.50/0
"unimolecular" m i ~ e l l e . ~This . ~ is actually the case for polyelectrolytes with a higher content of aromatic segments as will be discussed later. On the other hand, if the content of the hydrophobic segments is not high enough, the segmental Coulombic repulsion allows the polymer chain to expand, as usually occurs in the common polyelectrolytes. In the expanded conformation a hydrophobic group would be isolated from another along an electrolyte main chain and completely exposed to an aqueous environment. The photochemical behavior of the polycyclic aromatic groups is considerably dependent on whether the polymer conformation is expanded or miceller like. In a recent article concerning the photophysics of poly(methacry1ic acid) containing pendant 9,10-diphenylanthracene,the emission properties and quenching rate constants of the photoactive group were shown to be strongly affected by the conformation of the polymer.' We have reported that the fluorescence quenching of the photoexcited phenanthryl groups in amphiphilic random copolymers of 9( 5 ) Y . Itoh, Y . Morishima, and S. Nozakura, Photochem. Photobiol., 39, 603 (1984). (6) Y. Morishima, Y. Itoh, S. Nozakura, T. Ohno, and S. Kato, Mucromolecules, 17, 2264 (1984). (7) J. A. Delaire, M. A. J. Rodgers, and S. E. Webber, J . Phys. Chem., 88, 6219 (1984).
0 1985 American Chemical Society
4082
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 Hydrophobic attract ion
repulsion
Figure 1. Competition between hydrophobic attraction and electrostatic repulsion in an amphiphilic polymer chain that would critically determine the chain conformation.
vinylphenanthrene and sodium 2-acrylamido-2-methylpropanesulfonate (APh, a numerical suffix represents the phenanthryl
Morishima et al. a methanol solution into an excess of ether and then dissolved in water. The polymer solution was dialyzed against pure water. After the solution was neutralized with an equimolar amount of NaOH, the polymer was finally freeze-dried. The related monomer model compound, sodium 9phenanthrylmethanesulfonate (AM) was prepared as reported el~ewhere.~ Bis(2-hydroxyethyl) terephthalate (BHET) was provided by Toyobo Co. in pure form and used without further purification. Equilibrium Dialysis. Experiments were performed according to the literature.’’ The cellulose bags filled with polymer solutions were immersed in an aqueous solution of BHET and allowed to stand at 30 “C for 3 days with occasional shaking to facilitate equilibration. The concentration of BHET was determined spectroscopically after the equilibrium was established. Measurements. The steady-state fluorescence spectra were recorded on 4 Shimadzu RF-502A spectrofluorometer at room temperature. Prior to measurement the sample solutions were deaerated by bubbling with nitrogen for 30 min. The transient fluorescence decay was measured using a small Blumlein-type pulsing N2laser ( 1-ns pulse width) as described previously.* The output from the photomultiplier tube was displayed on a Hewlett-Packard oscilloscope (type 485) and photographically recorded. The sample solution was evacuated and sealed in a 2-mm quartz tube on a vacuum line. Surface tension was measured by a modification of the conventional drop-weight method.”J2 The number of drops was counted at 30 O C by a stalagnometer held in a thermostat.12 N
APh- 2 ( X ~ 0 . 0 2 ) A P h - 1 9 ( X = 0.19) APh-41 ( X =0.41)
CH2$
COO CH C H20H AM
BHET
group content in the copolymer in mol %), by an amphiphilic quencher, bis(2-hydroxyethyl) terephthalate (BHET), occurs with the apparent rate constants strongly dependent on the content of the phenanthryl groups in the polymers2 The particular interest we have encountered lies in the fact that the apparent second-order rate constant for APh-58 showed a value of 3 X 1Olo M-’s-’ which was obviously higher than the diffusion limiting value. In the Stern-Volmer-type plot for this system, the relation showed downward deviation from the normal linear Stern-Volmer relation. Recently, we have proposed a specific kinetic model for such systems on the basis of a multistep equilibrium for the binding of the amphiphilic quenchers to the hydrophobic microdomains.* In this article we describe in further detail the kinetic feature of the fluorescence quenching of APh by BHET in aqueous systems. For the most part interest has been centered on the role of hydrophobic microdomains in the quenching process. Further, we present an extended quantitative treatment of the fluorescence quenching data in terms of the new kinetic model. Experimental Section
Materials. Synthesis and purification of the random copolymers of 9-vinylphenanthrene and sodium 2-acrylamido-2-methylpropanesulfonate (AMPS) were carried out as reported elsewhere.2 The contents of the phenanthrene residues were determined from the C/N and C/Sratios in the microanalysis. Three water-soluble copolymers with different phenanthrene content were prepared in the present study. Homopolymerization of AMPS in the acid form was performed in a methanol solution using 2,2’-azobis(isobutyronitrile) as initiator at 60 O C . The polymer was purified by precipitation from (8) Y. Morishima, Y.Itoh, and S . Nozakura, Chem. Phys. Lett., 91, 258 (1982).
Results and Discussion Kinetic Model. In a previous work we have reported a kinetic model for the fluorescence quenching of amphiphilic polyelectrolytes that is based on the stepwise association model.8 The following assumptions have been made in the model. All the hydrophobic chromophores in the polymer self-associatein aqueous systems to form hydrophobic microdomains with an average aggregation number of n. Amphiphilic quenchers are incorporated into the microdomain via hydrophobic interaction according to the multistep equilibrium: M,,i-l
+Q
e k
*
Mn,i, i = 0, 1, 2, ...
K, = k,/ik-, where M , , denotes the hydrophobic microdomain of average aggregation number n associated with i quencher molecules, Q the quencher in the bulk aqueous phase, k, the rate constant for tbe association of quencher, and ik-, the rate constant for the ith quencher escape, respectively. As we previously discussed,2 the segmental motion of the phenanthryl groups in APh is highly restricted when they are self-associated into hydrophobic microdomains. This is probably due largely to the stacking tendency of phenanthrene itself. Therefore, it is reasonable to assume that the excitation is delocalized throughout the microdomain as a result of intradomain energy m i g r a t i ~ nand ~ * ~that the intradomain quenching process is described by the first-order rate constant kq,l, independent of the number of quenchers incorporated.8 Thus, fluorophores that may be buried in the rigid microdomain and shielded from direct contact with quenchers are also susceptible to quenching. In this article the term “static quenching” hereafter means quenching that takes place with quenchers already associated hydrophobically with fluoropbores while they are in the ground state. This does not imply, therefore, the presence of a ground-state charge-transfer interaction that could lead to the (9) Y.Itoh, Y. Morishima, and S . Nozakura, Photochem. Photobiol., 39, 451 (1984). (10) P. Molyneux and H. P. Frank, J. Am. C h e y . Soc., 83,3169 (1961). (11) A. E. Alexander and J. B. Hayter in ”Technique of chemistry,
Physical Methods of Chemistry, Part V”,A. Weissberger and B. Bryant, Ed., Wiley-Interscience, New York, 1971, p 5’12. (12) K. Tsuji and T. Takagi, Protein, Nucleic Acid Enzyme, 19, 705 (1974).
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4083
Fluorescence Quenching of Phenanthryl Groups
TABLE I: Binding Constants ( K ) Estimated by Equilibrium Dialysis for the Hydrophobic Association of BHET with Amphiphilic Copolymers oolvmers fnhQ 8JCb Kl 1O2 M-' PAMPSC 0 6.25 (0.502)d 0.58 APh-2 APh-19 APh-4 1
0.02 0.19 0.41
1.09 (0.506)d 0.39 (0.500)d 0.14 (0.502)d
1.5
3.1 12
Mole fraction of 9-vinylphenanthrene in the copolymer. Reduced viscosity in pure water (dL/g) at 30 O C . cHomopolymer of AMPS. dConcentration (g/100 mL) at which viscosities were measured.
0
0.1
0.2 0.3 ( 1I
.,
0 0.2 0.4 0.6 0.8 [ Q l t r e e 1 I I O 5 M-'
1
Figure 2. Relationship between the reciprocal concentrations of bound (1 / [Qlb) and free BHET (1 / [Q]frcc) for the equilibrium dialysis at 30 OC: 0,PAMPS;
APh-2; 0 , APh-19; 0, APh-41.
most effective quenching.13 O n the basis of above assumptions the quenching process can be represented by a combination of dynamic and static contributions kq.zlQ1
+ 117
M*n,o
-
Mn,0
k , i + 117
M*,,(
Mn,i, i = 1, 2,
(3)
...
(4)
where kq,2 is the second-order rate constant for diffusional quenching, and 7 the natural lifetime of the excited state. The steady-state solution for these schemes leads to the mathematical expression8
350
400 450 500 Wavelength I nm
550
Figure 3. Normalized fluorescence spectra measured in aqueous solutions APH-19; - - - APh-41: at room temperature: -, AM; ---, APh-2; residue concentration of the phenanthryl group, 5.0 X lo4 M; excitation -.-a,
zo/z=
(1
+ 7 k q , & 4 + B[Qlo)/[A + B[Qlo + 7(Akq,1nkq,z[Qlo) e x ~ H n K 1 / 4 [ Q l o H (5)
+
where A = K1[M1 n and B = n7kq,*. The initial slope of the curve for eq 5 in the Zo/Zvs. [Ql0 coordinates, Ksv,o,is related to the total residual concentration of chromophores as follows:
- 1- K,,o
K1 47kq,2 + Kl) IM1
1
+
7kq,2+ K l
(6)
In the extreme case where the quenching process is entirely static, Le., K , >> 7kq,2, eq 6 is approximated as
(7) In another extreme case where the quenching occurs only dynamically, i.e., 7kq,2>> K I ,the simple Stem-Volmer relation K , = 7kq,2 holds. Binding of BHET. In order to probe into the hydrophobic binding of the quencher molecules by the amphiphilic polyelectrolytes, equilibrium dialysis was studied. Results were plotted according to the Klotz equationI4.l5 in Figure 2. The binding constants ( K ) calculated from the slope and intercept of the plots according to the literatureI4*l5are summarized in Table I. The value of K showed a strong dependence on the mole fraction of 94nylphenanthrene in the copolymer Cfph), i.e., the value of K increased rather gradually with an increase offph untilfph = 0.19, but showed a sharp increase betweenfph = 0.19 and 0.41. This is indicative of the presence of hydrophobic microdomains in APh-41 with which BHET is preferably associated. It is worth noting that electrolyte homopolymers (PAMPS) can also take up BHET to some extent, which may be explained in terms of the (13) T. L. Nemzek and W. R. Ware, J . Chem. Phys., 62, 477 (1975). (14) I. M. Klotz, F. M. Walker, and R. B. Pivan, J. Am. Chem. Soc., 68, 1486 (1946). (15) I. Klotz and D. Hunston, Biochemistry, 10, 3065 (1971).
wavelength, 332 nm.
hydrophobicity of the main chain of PAMPS. Table I also includes the reduced viscosities of the polymers measured in pure water. These values are not only related to molecular weight, but they also reflect the extent of intramolecular hydrophobic association of the phenanthryl groups on which the hydrodynamic volume of the polymers would strongly be dependent.' The value of the reduced viscosity of PAMPS in Table I is remarkably large. Steady-State Fluorescence Quenching. Figure 3 compares the fluorescence spectra of the amphiphilic copolymers with various fph and the related monomer model (AM) in aqueous solution. As the fph value increases, the spectral shape becomes broad with considerable tailing in the longer-wavelength region, which implies that the electronic interaction between the phenanthryl groups occurs increasingly with an increase of fph. Although, in general, hydrophobic interactions between the phenanthryl groups are expected to facilitate excimer formation,',' no clear excimer emission was observed even for APh-41 in the longer-wavelength region. In fact, phenanthrene is known as a compound that would not show an excimer emission in the solution state.I6 It has been speculated, however, that poly(9-vinylphenanthrene) may form an excimer with a fluorescence quantum yield that is so low that no clear excimer emission appears." The phenanthryl groups incorporated in the hydrophobic microdomain in the amphiphilic copolymer of the present study can be envisioned as being under highly favorable circumstances for excimer formation. Considering the fact that phenanthrene crystals show a broad excimer emission at 440 nm under a high pressure,I8 the emission tailing in the longer-wavelength region in Figure 3 may involve a contribution from excimer emission. It should be noted that the fluorescence (16) E. A. Chandross and H. T. Thomas, J . Am. Chem. Soc., 94, 2421 (1972). (17) A. Itaya, K. Okamoto, and S. Kusabayashi, Bull. Chem. SOC.Jpn., 50, 52 (1977). (18) P. F. Jones and M. Nicol, J . Chem. Phys., 48, 5440 (1968).
4084
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
Morishima et al. TABLE II: Parameters Obtained from the Stern-Volmer Analyses of Fluorescence Quenching of the Amphiphilic Copolymers and the Monomer Model bv BHET system" K,!/102 M-' rc/ns kOd/1O9M-' s-l AM 1.33 40 3.33
3 /
APh-2 APh-19 APh-41 I
43 38/ 3 4
1.40 3.71 26. l g (kq,app)
'Residue concentration of the phenanthryl group; 5.0 X M. Stern-Volmer constant. 'Natural lifetime of the fluorescence. Second-order rate constant for the fluorescence quenching. 'Calculated from the initial slope of the Stern-Volmer plots. fEstimated as a half-life since the fluorescence decay contained an initial fast component. 8 Apparent rate constant calculated from the Stern-Volmer equation.
4
-
0.60 1.41 8.88' (Kpy,o)
2
1
0
10
30
20
[BHETI
I
40
1.3
M
Figure 4. Stern-Volmer-type plots for the fluorescence quenching of phenanthryl group in the presence of BHET in aqueous solution; the residue concentration of the phenanthryl group is 5.0 X lo4 M: 0, APh-41; 0 , APh-19; W, APh-2; 0,AM.
1.2
-. I
intensity decreased with increasingfph as reported in some detail in our earlier paper,2 which may partly be a result of the excimer contribution. The phenanthryl groups in APh-2 are considered to exist in isolation from one another in pure water as a result of the prevailing electrostatic repulsion of the charged segments. Therefore, the difference in the fluorescence spectra between APh-2 and A M may simply be attributed to the difference in the microenvironments such as local electrostatic potential, local polarity, etc. There was a subtle difference in the absorption spectra of the amphiphilic copolymer (data are not shown), Le., the absorption bands due to phenanthryl groups tend to get broad with a noticeable tailing on the longer-wavelength side as fph increases, which may suggest the presence of an electronic interaction as a consequence of hydrophobic association of the chromophores. However, quantitative estimation of hypochromism at the 250-nm band of the phenanthryl groups was not possible because of a considerable overlap of the absorption associated with the AMPS residues in the copolymer. Upon addition of BHET, quenching of the fluorescence intensity occurred at the emission maximum as well as in the longerwavelength region, Some examples of the relations between the relative fluorescence intensity (Zo/l)and the quencher concentration are given in Figure 4. The fluorescence quenching of APh-2 and APh-19 obeyed the Stern-Volmer relation as in the case of the monomer model (AM). The remarkable feature to be noted is that with APh-41 quenching was extremely effective and the result showed a downward deviation from the simple Stern-Volmer equation. The apparent Stern-Volmer slope of 888 M-' calculated from the initial slope in Figure 4 yielded an apparent second-order rate constant of kqSapp = 2.61 X loEoM-' s-' (Table II), which significantly exceeds the diffusion limit. This is thought to be a consequence of the associative binding of quencher molecules by APh-41, On the other hand, the quenching of APh-2 is extremely ineffective as compared with that of APh-41. It should be pointed out that APh-2 showed even less effective quenching than AM. Similar phenomena have also been encountered in other systems, Le., when the quenching is totally dynamic in nature without any associative binding of the quenchers present, the quenching tends to occur at a faster rate with the related low-molecular weight model systems than with the polymeric system^.^*^*'^ As we discussed in previous article^,^^^^*^ this difference between small molecules and macromolecules can be
interpreted in terms of the hindering effect of the polymer segments on the diffusion of the quencher molecules through the polymer free volume.z't22 Figure 5 compares another aspect of the quenching behavior between APh-41 and AM. The quenching efficiency of APh-41 depends on the residual concentration of phenanthryl groups, i.e., the lower is the residual concentration, the more efficient is the quenching. Obviously, this is not the case for AM. No such concentration dependence was observed for APh-2 and APh-19 as compared in Figure 6, where the reciprocal of the apparent Stern-Volmer constants were plotted against the residue concentration of the phenanthryl group. The striking point to note is that only APh-41 yielded a sloping line apparently obeying eq 6 . In general, the fluorescence quenching of phenanthrene by BHET is by electron transfer from the singlet excited phenanthrene, the rate of which is diffusion limited.2 In fact, with A M the Stern-Volmer slope of 133 M-' observed in Figure 5, along with T = 40 ns calculated from Figure 9c gives k, = 3.33 X lo9 M - k ' which is close to the diffusion limit. Using this value and T = 34 11s (fluorescence half-life for APh-41 calculated from the decay data in Figure lOc), one finds that the term rkq,2.in eq 6 for APh-41 does not ex& 112 M-I, taking into consideratlon the fact that the diffusion-limiting rate constant for the reaction between a polymer and a small molecule is usually smaller than that for the reaction between small molecules.20-22 In fact, this
(19) Y . Morishima, T. Kitani, T. Kobayashi, Y . Saeki, and S. Nozakura, Photochem. Photobiol., in press.
(20) A. Behzadi, V. Borgwardt, A. Henglein, E. Schamberg, and W. Schnabel, Eer. Bunsenges. Phys. Chem., 74, 649 (1970).
0
1.1
1
[BHETI I
M
Figure 5. Stern-Volmer-type plots for the fluorescence quenching by BHET at the various concentrations of phenanthryl residue: 0 A 0, APh-41; 0 A m, AM; the residual concentration of phenanthrene: I , 8.0 x 10-5 M; 2, 8.0 x 10-4 M; 3, 2.0 x 10-3 M.
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4085
Fluorescence Quenching of Phenanthryl Groups
I:
-.
?
0
. Yu)
3.0-
78t 74p
n
LJ-
70t--10
14
10 I
0
1
10
5
15
20
Figure 6. Dependence of the reciprocal of K,,,o on the residue concentration of phenanthryl groups for the APh-2 (m), A M ( O ) , APh-19 ( O ) , and APh-41 (0)systems.
Figure 8. Simulation using eq 5 with the optimally fit values of kq,, = 5.5 X 10's-1 and kq,*= 1.8 X lo9 M-l s-' for the APh-41-BHET system: [Ph](residue) = 5.0 X M , T = 3.4 X lo-* s, K , = 1.1 X lo3 M-I, and n = 2.6 were used: plots in the figure represent experimental data. TABLE III: Parameters for the Fluorescence Quenching of the APh-41 System Obtained by Analysis Using the Kinetic Model parameters
K , , M-l
n k , i , s-I kq,*,M-'
s-I
1.1 x 1030 2.6" 5.5 x 1076 1.8 x 1096
"Calculated from the data in Figure 6 by using eq 7. 6Estimated by computer best fit using eq 5 (Figure 8).
h
\
Figure 7. Relationship between surface tension and the residual concentration of the phenanthryl group in aqueous solution: 0 , APh-19; 0, APh-41.
can be seen in the comparison of the APh-2 and AM systems as listed in Table 11. The value of k for APh-2 is less than half of that for AM. From the intercept of the plots for APh-41 in Figure 6, 7kq,2 K, = 1.1 X lo3 M-' was obtained, in which the contribution of the first term, Tkq,2,would be, therefore, less than 10%. Thus, quenching of the APh-41 system can approximately be regarded as all static in nature. Consequently, the plots for APh-41 in Figure 6 can be fit with eq 7 . The K1value thus estimated is in fair agreement with the binding constant obtained from dialysis experiments (Table I), although the latter showed a little larger value than the former. This discrepancy may have arisen from the fact that hydrophobic association that could be observed in the equilibrium dialysis would include any types of binding, and not be limited to the type of the association of the quencher to the microdomain through which the fluorescence is quenched. From the slope of the plots for APh-41 in Figure 6, n = 2.6 was obtained. This means that APh-41 forms a micro-
+
(21) J. F. Pratte and s. E. Webber, Macromolecules, 15, 417 (1982). (22) K. Horic, K. Tomomune, and I. Mita, Polym. J., 11, 539 (1979).
domain consisting of 2.6 phenanthryl groups on the average in the concentration range employed in the present study. It should be noted, however, that this number does not necessarily mean the actual aggregation number, but rather reflects the number over which energy migration could occur. Now a question arises as to whether this hydrophobic microdomain is formed intra- or intermolecularly. If the latter is the case, the aggregation number may increase as the polymer concentration increases, which is expected to reflect the surface-active properties. To probe this question the surface tension data for aqueous solutions of APh-41 were compared with those for APh-19 in Figure 7 . The results suggest that in the case of APh-41 the hydrophobic microdomains are formed intramolecularly when the residue concentration of the phenanthryl group is lower than about 2 mM a t which concentration an inflection point was clearly observed in the relationship between the surface tension and the phenanthryl residue concentration for APh-41. Above this concentration the amphiphilic polymer showed a strong tendency to form intermolecular aggregates. In contrast to APh-41, APh-19 showed only a slightly surface-active property, implying that in APh-19 self-association of the phenanthryl groups is suppressed by the segmental charge repulsion prevailing over hydrophobic interaction between the phenanthryl groups. However, from the fact that for APh-19 the Stern-Volmer constant and the BHET binding constant are both significantly larger than those for the APh-2 system, the fluorescence quenching for APh-19 may be considered as involving some static contribution. The experimental data obtained for the APh-41 system were compared with the computed curves based o n eq 5 in Figure 8. The solid line drawn through the data plots is the theoretical curve optimally fit by varying the two parameters k,, and kq,*. The s, Kl = 1.1 X lo3 M-l, and n = 2.6 were values of 7 = 3.4 X used for this computation. Fairly good agreement between experiment and theory could be observed. The parameters that resulted in a best fit for the computation are listed in Table 111. The value of k,,z = 1.8 X lo9 M-' s-' thus obtained seems to be a reasonable value as compared with the second-order quenching constant for APh-2 ( k , = 1.40 X lo9 M-' s-l a s shown in Table
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
4086
Morishima et al. decay. The initial rapid decay behavior implies complexity involved in the primary photophysical process in the polymer system, which awaits further study. Figure 1Ob demonstrates the nonsingle exponential nature of the fluorescence decay in the presence of a low concentration of BHET, whereas at a higher quencher concentration the decay apparently follows a single exponential process with a lifetime of r = 13 ns (Figure loa). According to the stepwise association model,8 the concentrations of the microdomain with and without the quencher incorporated in it are given by
0
40
80 0
40
80 0
40
(9)
80
Timelns Figure 9. Fluorescence decay of AM in the presence of various concentrations of BHET in aqueous solution monitored at 375 nm: (a) [BHET] = 18 mM; (b) [BHET] = 4 mM; (c) in the absence of BHET: [Phl(residue) = 1.5 mM.
respectively, where C represents the fraction of microdomain with no quencher associated and is related to the association constant as C = exp(-K,[Q]) as discussed in the preceeding paper.8 For simplicity, we consider here only the situation for which quenchers are completely incorporated in the microdomains and hence no collisional quenching process occurs. Actually, the value of K I for APh-41, as discussed above, seems to be large enough for this assumption to be reasonable. Accordingly, the fluorescence intensity at time t relative to that at t = 0 is given by
E[M*.II,/C!=O[M*n,rlr=o-
r=O
c c 3 LLL 40 $ 1
.-
U
0
40
80 0
80 0
40
80
Time/ ns Figure 10. Fluorescence decay of APh-41 in the presence of various concentrations of BHET in aqueous solution monitored at 375 nm: (a) [BHET] = 4 mM; (b) [BHET] = 0.7 mM; (c) in the absence of BHET: [Ph](residue) = 1.5 mM.
11). The value of kq,l = 5.5 X lo7 s-I obtained, however, appears to be quite small for the typical static quenching process in rigid systems and rather similar to those values observed for surfactant micellar system^.^^-^^ This may suggest that the chromophore in the microdomain and the associated quencher are not held in an optimum juxtaposition for the instantaneous quenching. Fluorescence Decay. For further information on the quenching process the fluorescence decay was also investigated. Figure 9 illustrates the decay of the A M system. The data show single exponential decay where the lifetime linearly decreases with increasing quencher concentration, indicating the quenching process is entirely dynamic. The value of k, = 3.45 X lo9 M-' s-I was obtained from these fluorescence decay experiments, which is in good agreement with that obtained from steady-state SternVolmer analysis (Table 11). Similarly, the APh-2 system showed single exponential decay both in the absence and presence of BHET. The Stem-Volmer plots about ro/r agreed well with those about I o / I , indicating that the quenching of the APh-2 system is also entirely dynamic. Figure 10 shows fluorescence decay profiles for the APh-41 systems. The decay profile of APh-41, in the absence of quencher (Figure lOc), is not as straightforward as those of AM and APh-2, Le., an initial rapid decay was observed in the time domain of 5 ns after the laser pulse, although a large portion of the total process showed apparently single exponential (23) P. P. Infelta, Chem. Phys. Lett., 61, 88 (1979). (24) S. S. Atik and L. A . Singer, Chem. Phys. Lett., 59, 519 (1978). (25) S. S. Stik and L. A. Singer, Chem. Phys. Lett., 66, 234 (1979). (26) S. S. Atik and J. K. Thomas, J . Am. Chem. Soc., 103, 3550 (1981).
where T is the fluorescence lifetime in the absence of the quencher, Le., r = 34 ns for APh-41. With the quencher concentration of [BHET] = 4 mM, the first exponential term is negligible because the value of Cis very small (C = 0.012 using K, = 1.1 X lo3 M-I). Thus, the decay process is approximated to be a single exponential decay to give a straight line in the semilogarithmic plots of eq 10. This is a reasonable explanation of the apparently single exponential decay nature presented in Figure loa. From the slope of this data k,,, = 4.8 X lo7 S-I was obtained (using r = 34 ns), which is in fair agreement with the value estimated by the computer curve fitting as mentioned earlier. In the case of [BHET] = 0.7 mM (Figure lob), the value of C = 0.46 indicates that there is about equal contributions of the first and second exponential terms in eq 10, which explains the nature of the decay curve illustrated in Figure lob. In conclusion, we have shown that the fluorescence quenching properties of the amphiphilic copolymers containing pendant phenanthryl groups are strongly dependent on the mole fraction The fluorescence of the phenanthryl group in the copolymers quenching of the copolymer with fph = 0.02 (APh-2) occurred via an entirely dynamic process in the presence of bis( 2-hydroxyethyl) terephthalate (BHET). The quenching was a little less effective than that of the low-molecular-weight model (AM) system. This was explained by a hindering effect of the polymer segments on the diffusion of the quenchers. In APh-2 the chromophores are considered to exist in isolation from one another due to the segmental electrostatic repulsion prevailing over the attractive hydrophobic interaction between the phenanthryl groups. The quenching of the copolymer with&, = 0.41 (APh-41) was essentially static in nature as a result of the formation of the hydrophobic microdomains by the chromophores that would incorporate BHET into the microdomains. The quenching was extremely effective and the Stern-Volmer-type plots gave a downward curvature. The steady-state experimental data were analyzed by a kinetic model based on a stepwise association for the incorporation of the quencher molecules. The binding constant for the association of BHET with the microdomain resulting from the kinetic analysis showed a fair agreement with the value estimated by equilibrium dialysis. intradomain energy migration was shown to play a role to the efficient quenching process. The first-order quenching rate constant was also evaluated by computer
up,,).
J . Phys. Chem. 1985, 89, 4087-4090
curve fitting. The copolymer withf, = 0.19 showed an apparently dynamic nature although some static contribution was noticeable. These results suggest that the nature of the amphiphilic copolymer would change rather critically betweenfph = 0.19 and 0.41. The
4087
data obtained in the equilibrium dialysis and surface tension experiments evidenced that the extent of the intramolecular hydrophobic association of the phenanthryl groups is significantly changed between fph = 0.19 and 0.41.
Matrix Isolation Infrared Spectra of Carbonyl Complexes of Iron( II) Tetraphenylporphyrin T. Kuroi,] H. Oshio, and K. Nakamoto* Todd Wehr Chemistry Building, Marquette University, Milwaukee, Wisconsin 53233 (Received: March 22, 1985)
The IR spectra of cocondensation products of Fe(TPP) with CO/Ar exhibit six v(C0) bands which are assigned to CO-H20 (2 148/2 100), monomeric CO (2 135/2088), Fe(TPP)(C0)2 (2036/ 1997), Fe(TPP) (CO)O2 (2OO4/ 1961) , Fe(TPP)(CO)( H20) (1988/1944), and Fe(TPP)(CO) (1974/1929). (The numbers in the parentheses are in cm-I and indicate v(I2CO) and v(I3CO), respectively.) Formation of Fe(TPP)(CO)(H20) and Fe(TPP)(C0)02 results from the presence of trace H 2 0 and 02, respectively, in the system. In fact, the bands due to these species become the strongest when Fe(TPP) is cocondensed with CO/H20/Ar and C 0 / 0 2 / A r , respectively, at -20 K and warmed to -40 K. Upon forming Fe(TPP)(C0)02, the v ( 0 2 ) of Fe(TPP)02 at 1195 cm-' is shifted to 1190 cm-I while the v(C0) of Fe(TPP)(CO) at 1974 cm-I is shifted to 2004 cm-I. These results indicate that ~7donation occurs from CO to O2in Fe(TPP)(CO)02. The antisymmetric and symmetric 6 ( F 4 - 0 ) bands of Fe(TPP)(C0)2 are observed at 583 (IR) and 569 cm-I (Raman), respectively, while the b(Fe-C-0) band of Fe(TPP)(C0)02 is at 552 cm-I (IR).
Introduction
Recently, a number of novel and transient complexes have been synthesized via matrix cocondensation reactions of metal atom vapors with ligands such as CO, NO, and 02,and their structures determined by using IR and Raman spectroscopy.2 We3+4and other workers5 have extended this technique to the reactions of metal halide vapors with similar ligands. In 1981, we6 carried out matrix cocondensation reactions of bulky molecules such as Co(TPP) (TPP: tetraphenylporphyrin) with CO, NO, and O2 diluted in inert gases (Ar or Kr) and obtained the IR and Raman spectra of Co(TPP)L and Co(TPP)LL' type complexes, where L and L' denote the axial ligands mentioned above. Since then, our work has been concentrated on IR and R R (resonance Raman) studies of dioxygen adducts of Co(1I) Schiff base complexes' and Fe(I1) porphyrinss which serve as model compounds of oxyhemoglobin and oxymyoglobin. In this work, we have directed our attention to C O adducts of Fe(TPP) because CO binding with hemes has not been studied as extensively as O2binding. To our knowledge, the only report on Fe(TPP)(CO) and Fe(TPP)(CO), is that of Wayland et a1.: who measured the IR spectra of these compounds in the solid state. Here, we report the IR spectra of novel mixed-ligand complexes such as Fe(TPP)(C0)02 and Fe(TPP)(CO)(H20) and discuss (1) On leave from the Chemical Institute of Oita University, Oita, Japan. (2) Moskovits, M.; Ozin, G. A. "Crywhemistry";Wiley: New York, 1976. (3) Tevault, D.; Nakamoto, K. Inorg. Chem. 1976, 15, 1282. (4) Tevault, D.; Strommen, D. P.; Nakamoto, K. J . Am. Chem. SOC.1977, 99, 2997. ( 5 ) Van Leirsburg, D. A,; DeKock, C. W. J . Phys. Chem. 1974, 78, 134; J . Am. Chem. SOC.1972, 94, 3235. (6) Kozuka, M.; Nakamoto, K.J . A m . Chem. SOC.1981, 103, 2162.
(7) For example, see: Nakamoto, K.; Nonaka, Y.;Ishiguro, T.; Urban, M. W.; Suzuki, M.; Kozuka, M.; Nishida, Y.;Kida, S. J . Am. Chem. SOC. 1982, 104, 3386. (8) For examle, see: Watanabe, T.; Ama, T.; Nakamoto, K. J . Phys.
Chem. 1984,88, 440. (9) Wayland, B. B.; Mehne, L.F.;Swartz, J. J. Am. Chem. Soc. 1978,100, 2379.
0022-3654/85/2089-4087$01.50/0
TABLE I: IR Spectra of CO Cocondensed with Fe(TPP) in Ar Matrices (cm-I)
designation' a b C
d e f
species
v(wo) v(i3c0)
CO-H20
monomeric CO Fe(TPP)(C0)2 Fe(TPP)(C0)02 Fe(TPP)(CO)(H20) Fe(TPP)(CO)
V('2CO)/ v(13c0)
2148 2135 2036
2100 1997
1.020
2004 1988 1974
1961' 1944' 1929
1.022
2088
1.023 1.023
1.023 1.023
"See Figure 1. bThese frequencies were obtained at -35 K where the spectrum shows distinct maxima (see Figure 3B). the nature of interactions between the two axial ligands in terms of vibrational frequencies. Experimental Section
Compounds. Fe(TPP)(pip), (pip: piperidine) was prepared by the literature method.IO The gases, Ar (99.9995%, Matheson), I6O2(99.5%, Airco), lSO2(95.91%, Monsanto Research), I2CO (99.995%, MG Scientific), and I3CO (90.5%, Merck), were used without further purification. Mixed gases of various ratios of C O and O2 were prepared in our vacuum system equipped with a Mcleod type gauge and diluted by adding argon. The accuracy of pressure reading was rt0.l torr a t pressures below 5 torr. Red-violet crystals of Fe(TPP)(C0)2 were obtained by saturating a toluene solution of Fe(TPP) with CO. Spectral Measurements. Since Fe(TPP) is highly air sensitive, a stable, "base-bound" complex, F e ( T P p ) ( ~ i p )was ~ , placed inside the Knudsen cell of our matrix isolation system, and heated in a vacuum of torr at -370 K for 4 h or longer until the vacuum gauge indicated complete dissociation of the base ligand from the complex. Heating was continued at -400 K for 2 h (10) Epstein, L. M.; Straub, D. K.; Maricondi, C . Inorg. Chem. 1967,6, 1720. (11) Dubost, H.; Abouaf-Marguin, A. Chem. Phys. Lett. 1972, 27, 269.
0 1985 American Chemical Society
