J. Phys. Chem. 1983, 87,328-331
328
dence to indicate the nature of these structures, except that they are of lower density than the bulk. The present measurements suggest that there is a positive coupling between these clusters and dissolved ethanol. The simplest intermetation is that the low-densitv structures in water and {he hydration shell of hydrophobic solutes are both in cagelike arrangements Of water structure to the water in crystalline clathrates.
Note Added in Proof. Halfpap and Sorensen28aand Oguni and Angel128bhave independently reached a very similar conclusion. Registry No. KC1,7447-40-7; EtOH, 64-17-5; H20, 7732-18-5. (28)(a) B. L. Halfpap and C. M. Sorensen, J. Chem. Phys., 77, 466 (1982); (b) M.Oguni and C. A. Angel], J. Phys. Chem., submitted for publication.
Interactions of Ruthenium( I I)Photosensitizers with Triton X-I 00 Krlsnagopal Mandal, B. L. Hauensteln, Jr., J. N. Demas,' The Department of Chemlstv, Unlversifj' of Vlrglnia, Charlonesvllle, Vlrglnia 2290 1
and B. A. DeGraff The Department of Chemistry, James Madlson Unlverslfy, Herrlsonburg, Vlrglnia 22807 (Received June 15, 1982; In Final Form: September 17, 1982)
Using excited-state lifetime measurements as a probe, we have studied the interactions between a series of ruthenium(I1)photosensitizers and Triton X-100 surfactant. The complexes are weakly bound to the surfactant assemblies,and the presence of the photosensitizers influences the assembly process. There is a good correlation between the strength of the binding and the nature of the ligands on the metal complex.
The use of surfactant assemblies in the study of photochemical processes has become widespread in recent years.' Most research has concentrated on using ionic micelles to bind photosensitizers and to produce a separation of photoproducts. We were interested in a surfactant system where electrostatic interactions alone would not be responsible for the binding so that we could examine more closely the interactions between the surfactants and the photosensitizers. Also, we wished to have a system which would exhibit more varied binding regions than the charged surfactants provided. This feature would provide more flexibility in the regions for binding the photosensitizers, the quenchers, and the reaction products. We report here the results of our study of the interactions between several tris(a-diimine)ruthenium(II)complexes and the neutral surfactant Triton X-100 (octylphenoxypolyethoxyethanol; C8H1,CGH4(0CH2CHz),0H, x = 9,lO). The neutral Triton should provide at least three distinct regions: the relatively polar potentially coordinating polyether region, the hydrocarbon region, and the potentially n-bonding aromatic region. We have found that the use of lifetimes of photosensitizers as a function of surfactant concentration is a powerful probe of photosensitizer surfactant interactions. In our earlier studies using cationic and anionic surfactants, we found the binding of the complexes to the micelles to be very tight with little change in the luminescence properties above the cmc. With Triton X-100, however, the binding between the micelle and the probe molecule is relatively weak, and the presence of the metal complex influences the nature of the surfactant assembly. Further, we can correlate the structure of the ligands with the strength and nature of the interactions between the com(1) For recent reviews see (a) Gratzel, M. Acc. Chem. Res. 1981, 14, 376. (b) Turro, N.J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980,19,675. (c) Yekta,A.; Aikawa, M.; Turro, N. J. Chem. Phys. Lett. 1979, 63,543.(d) Kalyanasundaram, K. Chem. SOC.Rev. 1978,4, 453. ( e ) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133.
TABLE I: Properties of Ruthenium( 11) Photosensitizers with Triton X-100"
RU(~PY)~I~+ Ru( phen),]l+ Ru( Clphen),lz Ru(Mephen),] ,+ Ru( 4,7-Me2phen),] R~(5,6-Me,phen),]~+ R~(Me,phen),]~+ R~(Ph,phen)(phen),]~+ Ru(Ph,phen),lz+ Ru( (SO,Ph,),phen)(phen),] Ru((S0,Ph ) hen),]" Ru(Me2bpy)3217 , +
+
0.64 0.95 5.25 5.70 10.2 33.3 56 1 5.05 9.26 7.26
0.585 0.923 0.97 1.39 1.72 1.97 1.81 3.18 3.41 3.38 3.75 0.350
1.58 2.18 2.85 2.92 2.16 4.69 5.32 3.79 5.34 0.401
1.49 1.61 1.55 0.250 0.830 0.315 0.070 0.00 0.00 0.00
a Experimental fits were typically made over the Triton X-100 ranges of 0-7 or 0-25 mM.
plexes and the surfactant assemblies.
Experimental Section The ligands, and our abbreviations, are as follows: 2,2'-bipyridine (bpy) 4,4'-dimethyl-2,2'-bipyridine (Me2bpy), 1,lO-phenanthroline (phen), 5-chloro-1 , l O phenanthroline (Clphen), 5-methyl-1,lO-phenanthroline (Mephen), 4,7-dimethyl-l,lO-phenanthroline(4,7Me2phen), 5,6-dimethyl-l,lO-phenanthroline(5,6Me2phen), 3,4,7,8-tetramethyl-l,lO-phenanthroline (Me4phen),4,7-diphenyl-l,lO-phenanthroline (Ph2phen), and disulfonated 4,7-diphenyl-l,lO-phenanthroline ((S03Ph)zphen). AU ligands were from G. Frederick Smith Chemical Co. and were used without further purification. [Ru(Phzphen)],C12was prepared by the method of Watts and Crosby2and purified by chromatography on alumina. The remaining tris homochelated complexes were prepared (2)Watts, R. J.; Crosby, G. A. J. Am. Chem. SOC.1971, 93,3184.
0022-365418312087-0328$01.50/00 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No.
Interactions of Ru(I1) with Triton X-100
2750
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Flgure 1. Luminescence lifetimes of [R~(4,7-Me,phen),]~+(A) and [Ru((S03Ph),phen),lC (B) vs. [TrRon X-1001. *'s represent the experimental points, and the soiM lines show the best fR with model 111.
as reported elsewhere.3a The mixed ligand [Ru(phed2L] complexes were prepared by reacting L with Ru(phed2(CZO,). The complexes studied are summarized in Table I. Triton X-100 was used as received from Sigma Chemical Co. Lifetime measurements were made with a pulsed nitrogen laser lifetime system described el~ewhere.~All solutions were deaerated with nitrogen in a deoxygenation cell described e l s e ~ h e r e . ~Sample ~ temperatures were controlled at 25.0 f 0.5 "C with an off-on ont troller.^^ Absorption spectra were obtained on a Cary 14 spectrophotometer. Emission spectra were recorded on an SLM 8000 spectrofluorimeter. All data fitting to the different models was carried out by a nonlinear least-squares simplex method. Simplex optimization fits were carried out on a Hewlett-Packard HP-85A microcomputer with a Basic version of D a n i e l ~ ' ~ Fortran program.
Results and Discussion Before carrying out a detailed study of the photokinetics of these systems, we surveyed the lifetime of each complex in both a 0 and an -6-7 mM solution of Triton X-100. We found that the addition of Triton X-100 generally increased the excited state lifetime of the complex. [Ru( b ~ y ) ~and ] ~ +[Ru(phen),12+ were notable exceptions, however, and showed no change in lifetime on going from 0 to 6 mM Triton. For most complexes, the lifetime (3) (a) Hauenstein, B. L., Jr.; Mandal, Krishnagopal; DeGraff, B. A.; Demas, J. N. Submitted for publication. (b) Turley, T. J. M.S. Thesis, University of Virginia, 1980. (4) (a) Buell, S.L.; Demas, J. N. Rev. Sci. Instrum. 1982,53,1298. (b) Buell, S. L.; Demas, J. N. Anal. Chem. 1982,54, 1214. (5) Daniels, R. W. "An Introduction to Numerical Methods and Optimization Techniques"; North Holland: New York, 1978; Chapter 8.
[ T R I T O N X-1003
(mM)
Flgure 2. Lifetime of several other ruthenium(1I)photosenslizers vs. [Triton X-loo]. Best fits were calculated from the simplex method with model 111: A, [Ru(Mephen),]*+; B, [R~((SO,Ph),phen)(phen)~];C , [Ru(5,6-Me,phen),12+; D, R~(Ph,phen),]+~.
changes were substantial and sometimes approached 50%. In contrast, we found no appreciable perturbation of the absorption or emission spectra by Triton X-100 either below or above the cmc. These results led us to carry out more extensive studies to clarify the effects of Triton X-100 on the excited-state behavior of the ruthenium(I1) complexes. A series of lifetime vs. surfactant concentration titrations were carried out on the complexes exhibiting a measurable lifetime change. The two different types of behavior observed are illustrated in Figure 1for [Ru(4,7-Me2phen),12+ and [R~((SO,Ph)~phen))~]". Other examples are shown in Figure 2. In all cases, regardless of the Triton concentration, the decay curves appeared exponential over at least 2 mean lifetimes. We also verified that [R~(phen)~]*+ showed no lifetime changes over the 0-25 mM Triton concentration range. A t concentrations well above the reported cmc of 0.32 mM,6 the rising portions of all of the curves are similar and appear to describe a weak binding. Differences arise at surfactant concentrations which are near or below the cmc. With the 4,7-Mezphen complex, and complexes exhibiting similar behavior, the lifetimes are essentially constant below the cmc and begin to rise rapidly at Triton concentrations near the cmc. In contrast, the complexes containing (S0,Ph)zphen or Me2bpyexhibit an immediate and rapid increase in lifetime even at Triton concentrations well below the cmc. In the latter cases, even as the Triton concentration passes through the cmc there is no evidence (6)(a) Helenius, A.; Simons, K. Biochim. Biophys. Acta 1975,415,29. (b) Kushner, L. M.; Hubbard, W. D. J. Phys. Chem. 1954,58,1163. (c) Law, K. Y. Photochem. Photobiol. 1981,33, 799.
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The Journal of Physical Chemistry, Vol. 87,No. 2, 1983
of any inflection or break in the curves. We assumed that the changes in lifetime were caused by partitioning of the complexes between free and surfactant bound forms and that the different forms exhibited different lifetimes. Since the lifetimes increased with increasing surfactant concentrations, it appeared that the bound forms had longer lifetimes than the free forms. As the decay curves appeared to be exponential, we had originally assumed that there was a rapid exchange between the bound and unbound forms of the complex. Subsequent calculations showed that a rapid exchange was not necessary to give our data. We envisioned several plausible models involving interactions between a donor molecule (D) and a surfactant monomer (SI, or between D and a surfactant assembly or micelle (MI. Model I is described by eq 1-5. We assume DhY-*D-D+hv DS -% *DS DM hY- *DM
-
+
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+ hv
DM
D+S+DS D+M+DM
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that D or *D can interact with S or M to form association species DS and *DS or DM and *DM. Our lifetime measurements only monitor species containing *D, but we assume that the ground- and excited-state equilibrium constants are very similar. The three forms of the donor molecule are characterized by lifetimes 7D, T D S , and 7DM. The observed lifetime T is then given by = f D T D + fDS7DS + f D M 7 D M
(6)
where the f s represent the fractions of each species present in solution. In addition, micelle formation is described by the cmc and the aggregation number. The reported values are 0.32 mM and 140,6 but because Triton X-100 is a mixture it is possible that our sample had different values. In all modeling we used an aggregation number of 140. Model I uses four fitting parameters T D S , T D M , K D S , and KDM.7 D is the lifetime of free D in pure water and is the lifetime in the absence of Triton. We assumed a cmc of 0.32 mM. Model I1 is a simpler version of model I. In model I1 we assume that there is no interaction between the donor and the free surfactant monomer S. Thus, only eq 1, 3, and 5 are employed. Again, the cmc was fixed at 0.32 mM. Model I1 requires two variables T D M and K D M . Model I11 employs the kinetic scheme of model I1 but the cmc is varied to give the best fit. Because the cmc in this model is affected by the presence of the sensitizer, we refer to it as an induced cmc which we abbreviate icmc. Model I is capable of fitting all of the experimental data. Our data can be fit with T ~ S ’ Swhich are not greatly different from the corresponding T ~ ’ s . Although we believe that DS species are probably present, there is so little information concerning the DS complex in the pre-cmc region that any derived KDS’sare virtually meaningless. Our results show that if DS species are present then they have low association constants or lifetimes very close to those of free D. Since we have no conclusive evidence for formation of DS species, we adopt a simpler model that fits all of the observed data (vide infra). With model 11, reasonable fits are observed for some of the complexes which exhibit the pre-cmc plateau such as the 4,7-Me2phen complex. Model I1 fails for all of the complexes which do not have the plateau, however, since
Model I1 requires that the lifetime remain constant until micelles begin to form at the cmc. For example, when using model 11, attempts to fit the data for [Ru((S03Ph)2phen)3]4-fail dramatically. Further, even for those complexes exhibiting a plateau the concentration ranges of the plateaus differ, suggesting that the cmc varies with the complex; this is inconsistent with Model 11. For example, compare the data of parts A and C of Figure 2 for the Mephen and 5,6-Me2phencomplexes, respectively. We discard model I1 as being inadequate. Model 111, with a variable cmc, fits all of the experimental data and yields a physically realistic set of parameters for the different complexes. We, therefore, restrict the remainder of our discussion to this model. A threeparameter simplex fit which used model I11 yielded the KDM, TDM, and the icmc values of Table I. For the systems exhibiting a plateau region, 7 D was estimated by averaging the first few lifetimes on the plateau. Figures 1 and 2 show that this model yields excellent fits to the experimental data. The best calculated fits are shown by the solid lines. Residuals plots are random and the errors fall well within experimental uncertainties. The failure of much of our data to reach a plateau at the higher Triton concentrations left open the possibility that we might be seeing changes in micelle structure with increasing surfactant concentration. For the 4,7-Me2phen complex we carried out a titration to 100 mM Triton. The previous titration curve was obtained in the lower concentration range and the curve rose to a plateau at 15 mM Triton. The lifetime then remained constant up to -100 M Triton. The lifetime in the plateau region was 2.73 p s which agrees well with the value derived from the simplex fit to the data of Figure 1A. Titrations carried out with several other complexes to Triton concentrations above 25 mM verify this result. We conclude that our model applies even to high Triton concentrations and that, at least to the photosensitizers, there are no discernible changes in micelle structure above the icmc. We return now to a consideration of the assumption of a rapid exchange between bound and unbound forms of *D. For an excited-state equilibrium to be established, the rate of escape of *D from the micelles must be at least 5 times faster than the excited-state decay rate. Using this estimate of the escape rate in eq 5 and from the KDM’s of Table I, we can estimate the forward bimolecular rate constants of eq 5. For the different complexes, these rates are 3 X 1O’O-90 X 1O’O M-’ s-’ which are physically unreasonable. We, therefore, must discard a rapid equilibrium in the excited state as the explanation of the apparent exponentiality of the decay curves. With our instrument we can only acquire good decay curves spanning two to three mean lifetimes. Digital simulations showed that even with a TDM that is 1.6 times T D we would see no detectable nonexponentiality in the decay curves even if there is no exchange. Further, digital simulations showed that collecting and reducing such decay curves gave the same lifetimes within
