J . Phys. Chem. 1985,89, 4065-4070
4065
quenching of the initial fluorescence yield. Such processes have to be much faster than the shortest half-life of 15 ps to be observed reliably by our experimental technique.
acknowledge comments of reviewer (C) which helped to improve the presentation and our results. We also thank Ms. Kajander for the purification of diethylaniline.
Acknowledgment. We thank the H M I for support of one of us (J.K.T.), and also the National Science Foundation. We also
Registry No. DE,91-66-7; toluene, 108-88-3;benzyl chloride, 10044-7; biphenyl, 92-52-4.
Effect of Conformation of Poly(methacry1ic acid) on the Photophysical and Photochemical Processes of Trls( 2,2'-bipyridine)ruthenium( I I)' Deh Ying Chu and J. K. Thomas* Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 (Received: January 17, 1985; I n Final Form: May 13, 1985)
The pH induced phase transition of poly(methacry1ic acid), PMA, and poly(acry1ic acid), PAA, over the pH range from 2 to 10 has been investigated by means of the fluorescent probe tris(2,2'-bipyridine)ruthenium(II), Ru(bpy),'+. At low pH, in the region of 2 , this probe does not bind to PMA as no ionic sites are available on the polymer and fluorescence is exactly that found in an aqueous solution. At high pH, pH 10, Ru(bpy)?+ is completely bound to the anionic stretched polymer; however, it is still in close contact with the aqueous phase and again the photophysics of the probe are identical with those observed in an aqueous solution. However, at intermediate pH the photophysics, for example, the lifetime and the fluorescence intensity of Ru(bpy)32f,show maxima at a pH of about 5 . The probe also exhibits a blue spectral shift at this particular pH compared to other pH. The photophysical experiments show that these properties are due to a binding of the R u ( b ~ y ) , ~ + into the coiled or swollen polymer of PMA at pH 5 . This binding is such that there is a restriction on the ligands of the organometallic probe so that unlike more mobile systems such as water or a stretched polymer complete relaxation of the excited state is not achieved. Hence, the lifetime and yield of the fluorescence increase accordingly and the spectrum shows a blue shift. The binding of more than one Ru(bpy),*+ to a polymer coil leads to an alternative site on the polymer which is closer to the aqueous phase and is more indicative of that particular environment. Quenching studies also follow this behavior over the full pH range and are interpreted according as to whether the probe is in the water phase, pH 2 , on a stretched polymer at pH 10, or in the interior of a polymer at pH 5 . At pH 5 R ~ ( b p y ) , ~is+shielded from attack by molecules from the aqueous phase. Other molecules which bind to the polymer such as cupric and chromic also quench the Ru(bpy),2+, and the kinetics follow a Poisson distribution. Hence, it is possible to calculate the concentration of polymers coils. The agreement was excellent with that measured by viscosity studies. This confirms the concept or picture of the kinetics that have been developed. Furthermore, the rate of quenching of Ru(bpy)32+by copper on the PMA coil is much more rapid than in anionic micellar systems. Here again, the kinetics show that the ruthenium and copper are in close proximity in the polymer coil rather than on the exterior surface as in the micellar system. These studies show the nature of the interaction of cationic species with a polyelectrolyte under different conformations or geometries of the polymer. PMA shows a very sharp expansion transition at pH greater than 4, and at a pH of 5 the polymer is partly swollen and penetration of water into the polymer occurs. This enables polar cationic species to penetrate into the polymer and bind at internal sites thereby affecting the structure of the polymer coil. This type of behavior is minimized in polymers that do not show sharp transitions, such as poly(acry1ic acid), PAA.
Introduction Over the past decade there has been an increased interest in the interaction or solubilization of guest molecules in colloidal systems such as micelles, microemulsions and in large biopolymers such as DNA.*+ Such systems often provide unique environments for the guest molecules and promote certain sought after features of chemical and photochemical reactions that cannot be found in homogeneous media. The use of polyelectrolytes has several interests, because these molecules are used often to stabilize large colloidal particles in aqueous solution and also because of their resemblance to biopolymer such as DNA. Earlier work with poly(methacry1ic acid), PMA, and poly(acry1ic acid), PAA, showed that these polymers can solubilize hydrophobic molecules such an arenes at low pH where the polymers are coiled.5a It (1) We thank the National Science Foundation for support of this work via Grant No. CHE-82-01226. (2) Fendler, J. "Membrane Mimetic Chemistry"; Academic Press: New York, 1983. (3) Thomas, J. K. 'The Chemistry of Excitation at Interfaces"; American Chemical Society: Washington, DC, 1984; ACS Monograph No. 181. (4) Geacintov, N. E.; Prusik, T.; Khosrofian, J. J . Am Chem. SOC.1976, 98, 6444.
is also possible to solubilize cationic species in these polymers at higher pH where the polymer is extensively ionized and here the solubilization is via electrostatic interactions. Similar studies have been carried out for the interaction of cations with other strong polyelectrolytes such as poly(viny1 sulfate): and for the interaction of cationic surfactants with polyelectrolytes thereby creating hydrophobic domains to solubilize the hydrophobic molecules.' This work shows that the polyelectrolytes introduce unique features into the photochemistry or chemistry of molecules adsorbed into them, features that can be interpreted with experience gained in micellar and other colloidal s y ~ t e m s . * ~ ~ ~ * Earlier workSbshowed that the conformation of PMA or PAA was extremely important in determining the solubilization ( 5 ) (a) Borone, G.; Crescenzi, V.;Quadrifoglio, F. J . Phys. Chem. 1967, 71, 2341. Treloar, F. E. Chem. Phys. Lett. 1980, 73, 234 and references therein. (b) Chen, T.; Thomas, J. K. J . Polymn. Sci., Part A-1 1979, 17, 1103. (6) Meisel, D.; Matheson, M. S.; Rabani, J. J. Am. Chem. SOC.1978, 100, 117. Meisel, D.; Rabani, J.; Meyerstein, D.; Matheson, M. S. J. Phys. Chem. 1978, 82, 985. (7) Abuin, E. B.; Scaiano, J. C. J . Am. Chem. SOC.1984, 106, 6274. Turro, N. J.; Baretz, B. H.; Kuo, P. L. Macromolecules 1984, 17, 1321. (8) Lachish, U.; Ottolenghi, M.; Rabani, J. J . Am. Chem. SOC.1977, 99,
8062.
0022-3654/85/2089-4065$01.50/0
0 1985 American Chemical Society
4066
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
characteristic of arenes in aqueous solutions of these polyelectrolytes. More recent work,g where aromatic molecules are COvalently bound to polymers, showed that again the conformation of the polymer plays an important role in determining the nature of the photochemistry of the arene in the polyelectrolyte system. We have also established'O that the probe molecule tris(2,2'-bipyridine)ruthenium(II), Ru(bpy),*+, is also an excellent monitor of the environment where it residues. The excited state of Ru(bpy)?+ is a charge-transfer metal to the ligand state. Following excitation and charge transfer to the ligands, the conformation of the ligands with respect to the metal ion is changed. In restricted environments, such as low-temperature rigid media, or in porous environment,I0 and quite unlike fluid media, the excited state cannot adjust to its fully relaxed form. This effect is reflected in the photophysics and can be used to monitor the nature of the environment of the excited state. Similar effects have been observed in clay systems" where it has been shown that the a-a* absorption spectrum is dramatically altered when the probe molecule is introduced between the layers of an expanding clay, whereas absorption on the clay surface gives rise to an absorption spectrum that is very similar to that of an aqueous environment. Hence, the probe molecule Ru(bpy),2+ is a useful monitor of the nature of its environment. In the present experiments we have used Ru(bpy)?+ to monitor the nature of the conformational transition of PMA over the pH range of 2-10, from the tight compact form at low pH into the larger linearly extended form at higher pH. The interaction of the probe with PMA is electrostatic in nature, and we can rely on earlier work to give us guidelines in the interpretation of the data and to anticipate events in such systems. Another feature investigated is whether the probe molecule can itself significantly affect the conformation of the polymer. The purpose of the work is twofold, to investigate the nature of the transition of the PMA polymer coil with increasing pH, and to note any unique effects that are introduced by charged molecules on polyelectrolytes.
Experimental Section Polymer Materials. Poly(methacry1ic acid), PMA, and poly(acrylic acid), PAA, used in this work were obtained from the Polyscience and Aldrich Chemicals Inc., respectively. Stable concentrated PMA solution was prepared by repeated suction filtering of the initial solution in order to separate undissolved particles. The molecular weight of PMA was measured by the viscosity method to be 1.1 X 104. The molecular weight of PAA was given as 2.5 X lo5 by the Aldrich Chemicals Inc. In solutions that were used the concentrations of polymer chains are of the order 10-5-10-4 M, while the exact numbers are given at the appropriate places. Various PMA samples used in comparison work were synthesized by y-irradiation or by using different initiators, such as hydrogen peroxide (Fisher), benzoyl peroxide (Fisher), AIBN (Aldrich), etc. The monomer methacrylic acid (Aldrich) was purified by vacuum distillation. Polymerization and purification of polymers were carried out as described previo~sly.~ Chemicnls. Tris(2,2'-bipyridine)ruthenium(II) chloride ( G . Fredrick Smith), Ru(bpy)32+,was purified by double recrystallization from deionized water. Cupric sulfate (Baker), chromic chloride (Fisher), methylviologen dichloride (MV2+)(Aldrich), nitrobenzene (Kodak), benzoquinone (Fisher), potassium ferricyanide (Baker), potassium persulfate (Baker), sodium dodecyl sulfate (BDH), Igepal CO-880 (Anspec), and urea (Baker) were used as received. Methanol was purchased from Mallinckrodt Co. as spectral AR reagent grade, sodium 1-pyrenesulfonate was prepared by neutralization of I-pyrenesulfonic acid with sodium hydroxide and recrystallization from ethanol. Equipment. A Lambda Physik 100 excimer laser (A = 337.1 nm; pulse width = 6 ns; 8 mJ/pulse) and a Candela SLL-66A (9) Chu, D. Y . ;Thomas, J. K. Macromolecules 1984, 17, 2142. (10) Wheeler, J.; Thomas, J. K. J . Phys. Chem. 1982,85, 4540. (11) DellaGuardia, R.; Thomas, J. K. J. Phys. Chem. 1983, 87, 990.
Chu and Thomas
w
A
I-
z -
W
-t
> U -I
w
LT
h WnbELENTH
(nm)
Figure 1. A. Luminescence spectra of Ru(bpy),zf i n PMA aqueous solution at the following pH's (a) 5 , (b) 4, (c) 6 , (d) 7 , (e) 2, (f) 9, (9) 11; [ R ~ ( b p y ) ~=~ 1.0 +] X M, [PMA] = 1.9 X lod M, X(excitation) = 450 nm. Note: absorption spectra of Ru(bpy),2+at 450 nm are not pH dependent. B. Relative intensity (0) and the wavelength of the maximum ( X ) of luminescence of Ru(bpy)32+in PMA aqueous solution as a function of pH
dye laser (A = 490 nm, pulse width = 120 ns; 0.1 J/pulse) were used to excite the luminescence probe Ru(bpy),2+. The excited states were monitored as in earlier work.I0 The quenching rate constants of excited R ~ ( b p y ) , ~by + added quenchers were measured directly by the increased rate of decay of the excited R ~ ( b p y ) on ~ ~addition + of quenclier molecules. In this technique, the quenching rate constant, k,, is related to ko and k by the following relationship k = ko k , [ Q ] , where [Q] is the concentration of quenchers and kt, and k are the first-order decay rate constants in the absence and in the presence of quenchers, respectively. The percentage of bound cation quenchers on PMA were obtained by the measurement of queriching rate constants of sodium 1-pyrenesulfonatein water (k,)and in PMA aqueous solution ( k p ) . The percentage of bound cation
+
It is assumed that the decreased quenching constant in PMA aqueous solution is due to the absorption of quencher cations on PMA chains and that there is no adsorption of I-pyrenesulfonate anions on the polymer chain. The percentage of bound cation quenchers on PMA in aqueous solution at pH 5 thus obtained were Cu2+90 f 5%, Cr3+60 f 5%, and MV2+45 f 5%. These values were in good agreement with those calculated from the Poisson quenching data (see Tables 111 and IV and Discussion section). Steady-state absorption spectra were obtained with a PerkinElmer 552 spectrophotometer. Luminescence spectra and fluorescence polarization studies were carried out with a Perkin-Elmer MPF-448 fluorescence spectrophotometer. All aqueous solutions were prepared with deionized water and all samples were deaerated by bubbling with nitrogen, unless stated to the contrary.
Results and Discussion Effect of p H on the Luminescence of Ru(bpy)32+in PMA. Steady-State Studies. P H markedly affects the luminescence spectra of R ~ ( b p y ) , ~in+ aqueous solutions of poly(methacry1ic acid). Figure 1 shows the luminescence spectra of R ~ ( b p y ) ~ ~ + in aqueous solutions of PMA at various pH. The relative intensity of the emission exhibits a marked enhancement on increasing the pH from 2 to 5, followed by a decrease to the original spectrum on increasing the pH above 5. It is noted that the luminescence intensity of Ru(bpy),2+ reaches a maximum at pH 5 . This figure also shows that the spectral wavelength maximum exhibits a
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4067
Phase Transition of Poly(methacry1ic acid) TABLE I:
01
Values’ of Ru(bpy),” in PMA Aqueous Solutions at pH 5 a
[Ru(bpy),’+] 105, M 4.0 2.0 1.o 0.5
X
11.0 X 10-’[PMA]
5.3 x 1 0 + [ PMA]
0.50
0.57 0.56 0.55 0.56
0.50 0.50 0.52
2.7 X 1 O-’ [ PMA]
I
0.70 0.67 0.64 0.60
1.3 X lO+[PMA]
I
0.80 0.78 0.68 0.64
ox
0.7 X [PMA]
I
0.95 0.88 0.75 0.68
[PMA]
I
1 1 1 1
CY value is calculated from the double-exponential fit equation (see text for detail): It = I , [CY exp(-k,r) + (1 - a ) exp(-k,r)] that at the right side of dark line, the average number of Ru(bpy),’+/PMA is larger than 1 and CY approaches 1 .
pronounced blue shift with a minimum (A = 595 nm) at pH 5. The excitation wavelength of these samples was 450 nm, i.e., into the charge-transfer metal to the ligand band. It is pertinent to note that luminescence spectra of Ru(bpy),2+ in aqueous PMA solution at pH 2 and 9 are identical with that in water, where the wavelength maximum is 614 nm. The luminescence lifetime of R ~ ( b p y ) ~vs.~ pH + shows a similar bell trend to that exhibited by the luminescence intensity, i.e., an increase on going from pH 2 to 5 and a decrease at larger pH beyond 5. At 77 K the emission at pH 5 also shows a marked blue shift compared to other pH. Therefore, aqueous solutions of PMA at pH 5 show unique effects on the wavelength maximum, relative intensity, and lifetime of luminescence of R ~ ( b p y ) , ~ + . Increasing the pH of poly(methacry1ic acid) solutions from pH 2 through 5 to 9 results in an increasing degree of ionization of the acid. At a pH of 2 the polymer is in an undisassociated condition and tight coils of PMA are formed. At pH 5 the polymer is partially neutralized, and the degree of ionization is about 24%. At pH larger than 9, the degree of ionization is large and greater than 97%, resulting in an uncoiled anionic polymer species which is strongly negatively charged. Up to a pH 5 the polymer ionizes and swells, whereas at higher pH, a long flexible molecule is formed. R ~ ( b p y ) , ~ +which , is cationic, strongly binds to the anionic sites on the polymer. However, at pH 5 the environment of the binding site is quite different from that at other pH, and to other clustered anionic systems such as anionic micelles. The increased fluorescence lifetime and the blue spectral shift are indicative of a restricted environment for Ru(bpy)?+, as observed in other studies. The presence of structure as a shoulder in the emission spectrum, the increased lifetime, and the blue spectral shift, indicate a restriction of the ligands around the metal, a situation which is usually observed in condensed systems at low temperature, or in porous colloidal particles such as porous silica.l0 A simple explanation of the observed events is as follows: anionic sites are formed on the polymer as it opens with increasing pH, leading to binding of R ~ ( b p y ) , ~to+ these sites; R ~ ( b p y ) , ~ on +, binding to these sites, causes a restriction which tends to pull the polymer chain around the probe molecule. At higher pH with an uncoiled highly charged polymer, this restriction by the probe molecule is not observed. At pH 2 no cationic binding is observed, and the R ~ ( b p y ) , ~is+in the aqueous phase. Other studies have shown that the conformational transition of PMA could be induced by addition of methanol12 or urea.], The induction of a phase transition in un-ionized PMA by urea or methanol had no effect on the photochemistry of added R ~ ( b p y ) ~ Hence, ~ + . the interaction between Ru(bpy)t+ and PMA at pH 5 is not one of simple solubilization, but rather electrostatic in nature. Various PMA samples synthesized by y-ray radiation or by using different initiators also show the same unique effect at pH 5. This indicates that the method of preparation and the nature of the end groups are not important, and that the conformational transition induced by pH is the important feature in the present data. Lifetime Studies. Figure 2A shows that the luminescence lifetimes of R ~ ( b p y ) , ~at+ pH 2 and 9 are equivalent and identical with those normally observed in simple aqueous solution, while (12) Anufrieva, E. V.; Birshtein, T. M. et al. J . Polym. Sci., Parr C, 1968, 16, 3519. (13) Jager, J.; Engberts, J. B. F. N. J . Am. Chem. SOC.1984, 106, 3 3 3 1 . Dubin, P.; S t r a w , U. P. J . Phgs. Chem. 1973, 77, 1427.
,
It is noted
T I M E , ns T I M E , ns Figure 2. A. In (Intensity) of luminescence of Ru(bpy)3Z*vs. time in PMA aqueous solution at the following pH’s which are monitored at 610 nm; (a) 5, (b) 4, (c) 6, (d) 7, (e) 2, (f) 9, ( 9 ) 11. Note: In I vs. time at pH 5 is not single-exponential decay. B. Double-exponential fit of the time-dependent luminescence decay of Ru(bpy),2+,in PMA aqueous solution at pH 5 , by the equation
I = I0[aexp(-k,t) + (1 - CY)exp(-k,t)] lo6 s-I, k2 = 6.5 X lo5 s-’, a = 0.5, [Ru(bpy)?+] = 2 X
k , = 1.7 X M, [PMA] = 0.94
X
lo4 M .
the decay curves are simple exponential. However, in PMA at pH 5 the luminescence lifetime is longer, and not simple exponential. Shown in Figure 2B is a double-exponential fit of the time-dependent luminescence decay of R ~ ( b p y ) , ~in + PMA aqueous solution at pH 5, which follows the form
I , = Zo[aexp(-k,t)
+ ( 1 - a ) exp(-k,t)]
(2)
where k, and k2 are two different exponential decays of excited R ~ ( b p y ) , ~ a+ ,is the fraction that decays with a rate constant k , , and I, and Io are the luminescence intensities at time t and zero, respectively. The decay lifetime in the first fast decay region is 588 ns and that in the second and slower region is 1.54 11s. The excellent fitting of experimental data and calculation is indicative of at least two binding sites for R ~ ( b p y ) , ~to+ the polymer, one where it is severely restricted and removed away from the water phase, and another where it is bound on the surface of the polymer coil and in close proximity to the water, which gives rise to behavior which is similar to the water. Other processes which would lead to nonsimple exponential decay kinetics for R ~ ( b p y ) , ~are + ground-state quenching and triplet-triplet annihi1ati0n.I~ However, these possibilities are ruled out on the basis of the experimental facts that there is no effect of laser intensity on the luminescence lifetime of R ~ ( b p y ) in ~ ~PMA + at pH 5. A triplettriplet annihilation process should have given an intensity dependence resulting in shorter lifetimes at higher laser intensities. The negative result rules out any interaction of two R ~ ( b p y ) , ~ + bound to the same polymer coil. The triplet-triplet quenching is more efficient than the triplet-ground-state quenching. It is pertinent to note that increasing the Ru(bpy)32+to polymer ratio increases the proportion of fast decay and also leads to a red shift in the luminescence spectrum. Both factors suggest that the second R ~ ( b p y ) , ~bound + to a PMA coil is in a more aqueous environment, Le., toward the polymer water interface. (14) Milosavljevic, B. H.; Thomas, J. K. J . Phys. Chem. 1983, 87, 616.
4068
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
Chu and Thomas
TABLE 11: Rate Constants (K' s - l ) for the Quenchingof Ru(bpy),'+ by Various Compounds PMA aqueous solution'
water k, 1.7 X l o 6
PH 2 k, 1.7 X l o 6
k, 1 1.7 X l o 6
K,' 6.5 X l o 5
3.0 x 109 7.0 x 109
2.9 x i o 9 7.3 x 109
6.1 x i o * 3.4 x i o 9
Q=C+ C. anionic quenchers
1.1 x
8.7 x 109
5.5 x
io9
K, Fe(CN), K*S,O* D. cationic quenchers
3.2 x 10'" 6.8 X l o 9
3.2 x l o i o 4.6 X l o 9
5 x 109 >i x 109 -1 x 109
quencher A. noneb B. neutral quenchers &NO.
cuso, CrC1, MVZ
+
' [ Ru(bpy), "1
1010
M, [ PMA] = 0.94 X
=2 X
-
PH 5
M.
pH 9
0.5
K, 1.7 X l o 6
k, 1.3 X l o 6
1.8 x i o 8 2.0 x i o *
0.5 0.5
1.8 3.6
x 109 x 109
2.9 x 109 5.0 x 109
2.1 x 109 1 x 109
1.0 x 109
0.5
5.6 x 109
9.0 x 109
cy
x 107
