2028
J . Phys. Chem. 1988, 92, 2028-2032
Spectroscopic Investlgatlons of the Interactions between PolypyrMyl-Ruthenium Complexes and Anionic Polyelectrolytes Gert L. Duveneck, Challa V. Kumar, Nicholas J. T w o , * and Jacqueline K. Barton* Department of Chemistry, Columbia University, New York, New York 10027 (Received: June 8, 1987; In Final Form: October 23, 1987)
The interactions between three ruthenium(I1) complexes [Ru(bpy),CI2, Ru(phen),C12, and Ru(DIP),C12] and two anionic polyelectrolytes [sodium poly(acrylate), NaPA, and sodium poly(styrenesulfonate), NaPSS] in aqueous solution have been investigated by steady-state and time-resolved luminescence methods. The interactions are of electrostatic and of hydrophobic nature. Strong interactions, resulting in binding of the metal complex to the polyanion (NaPSS),’ are indicated by marked enhancementsof luminescence intensities and lifetimes, with the enhancementsparalleling the increase in hydrophobic interactions. The luminescence decays are nonexponential for the stronger interacting systems [Ru(DIP),C12 with NaPA and NaPSS and R ~ ( p h e n ) ~ with C l ~ NaPSS]. The interaction of the complexes with the plyanions leads to protection of the metal complexes against quenching processes by both negatively charged quenchers (ferrocyanide) and neutral quenchers (molecular oxygen). The quenching by ferrocyanide is found to be static in nature, and the protection against quenching is explained in terms of electrostatic interaction and by the induced formation of hydrophobic domains.
Introduction Ruthenium complexes display interesting photophysical and photochemical properties which make them attractive probes of interfacial systems and macromolecules: (a) These complexes possess strong absorption bands in the visible region (for example, tris(2,2’-bipyridine)ruthenium(II) chloride, Ru(bpy),C12, c ca. 14000 at 450 nm2), which are assigned to spin-allowed metal to ligand charge-transfer (MLCT) transitions? (b) These complexes possess strong emission which arises from the lowest excited triplet MLCT state4 at about 600 nm and is rather long-lived, with reported lifetimes between 600 and 700 ns for R~(bpy):+.2~-~ (c) These complexes possess low-lying d states: which are thermally accessible from the MLCT state, so that the emission lifetimes are strongly influenced by changes of temperature or of the microenvironment. The photophysics and photochemistry of these luminescent metal complexes have been investigated in solvents of different polarity and polarizability,6,’ micelles,* and polyelectrolyte~.~For example, it has been shown that the rate of photoinduced redox reactions of the ruthenium complexes in aqueous solution can be strongly enhanced in the presence of anionic micelles or anionic polyelectrolytes?-‘ mainly due to the increase in effective metal complex concentration in a restricted environment and to electrostatic interactions. The role of hydrophobic interactions in determining the photophysics and photochemistry of ruthenium complexes is also of considerable interest because of the relationship of such factors in determining the binding of metal complexes to biological molecules.1° Re(1) (a) Kurimura, Y.; Yokota, H.; Shigehara, K.; Tsuchida, E. Bull. Chem. SOC.Jpn. 1982,55, 55. (b) Rabani, J.; Sassoon, R. E. J. Phorochem. 1985, 29, 7. (2) Demas, J. N.; Adamson, A. W. J . Am. Chem. SOC.1973, 95, 5159. (3) Demas, J. N.; Adamson, A. W. J. Am. Chem. SOC.1971, 93, 1800. (4) (a) Klassen, J. N.; Crosby, G. A. J. Chem. Phys. 1968,48, 1853. (b) Demas, J. N.; Crosby, G. A. J. Mol. Spectrosc. 1968, 26, 72. (c) Lytle, F. E.; Hercules, D. M. J. Am. Chem. SOC.1969, 91, 253. (5) (a) Lin, C.-T.; BBttcher, W.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. SOC.1976, 98, 6536. (b) Creutz, C.; Chou, M.; Netzel, L.; Okumara, M.; Sutin, N. J . Am. Chem. SOC.1980, 102, 1309. (6) Caspar, J. V.; Meyer, T. J. J. Am. Chem. SOC.1983, 105, 5383. (7) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. Am. Chem. SOC.1982, 104, 630. (8) (a) Turro, N. J.; Yekta, A. J . Am. Chem. SOC.1978,100, 5951. (b) Meisel, D.; Matheson, M. S.; Rabani, J. J . Am. Chem. SOC.1978, ZOO, 117. (c) Lee, P. L.;Meisel, D. J. Am. Chem. SOC.1980,102, 5477. (d) Rodgers, M. A. J.; Baxendale, J. H. Chem. Phys. Letr. 1981,81, 347. (e) Hauenstein, B. L., Jr.; Dressick, W. J.; Buell, S. L.; Demas, J. N.; De Graff, B. A. J. Am. Chem. SOC.1983, 105,4251. (9) (a) Meisel, D.; Matheson, M. S. J. Am. Chem. SOC.1977, 99, 6577. (b) Meyerstein, D.; Rabani, J.; Matheson, M. S.; Meisel, D. J. Phys. Chem. 1978,82, 1879. (c) Meisel, D.; Rabani, J.; Meyerstein, D.; Matheson, M. S . J. Phys. Chem. 1978,82,985. (d) Sassoon, R. E.; Rabani, J. J . Phys. Chem. 1980, 84, 1319. (e) Kelder, S.; Rabani, J. J . Phys. Chem. 1981. 85, 1637.
0022-3654/88/2092-2028$01.50/0
cently, octahedral metal complexes, existing in different stereoisomeric forms, have been the center of particular interest in biochemistry, since these molecules display enantiomeric selectivity, revealing different binding modes (intercalative, hydrophobic, electrostatic) upon binding to chiral biopolymers such as DNA.Io The influence of specific properties of the biopolymers, such as differing hydrophobicity, on the binding strength and dynamics of the metal complexes to the biopolymers can be examined with polyelectrolytes as model systems. In the present work, the influence of hydrophobicity, of both the ligands and the polyelectrolyte, on the luminescence properties of metal complexes is investigated. The photophysics of three polypyridyl-ruthenium complexes (Figure l ) , tris(2,2’-bipyridine)ruthenium(II) chloride, [Ru(bpy),]C12, tris( 1,lOphenanthroline)ruthenium(II) chloride, [ R ~ ( p h e n ) ~ ] Cand l~, tris(4,7-diphenyl-l,1O-phenanthroline)ruthenium(II) chloride, [Ru(DIP),]C12, upon binding to polyelectrolytes, sodium poly(acrylate), NaPA, and sodium poly(styrenesulfonate), NaPSS, are examined. Experimental Section Chemicals. The ruthenium complexes, Ru(bpy)?+ (Aldrich),lk R ~ ( p h e n ) ~and ~ + ,Ru(DIP),~+,were prepared as described elsewhere.SaJOfPotassium ferrocyanide, K.,Fe(CN),, was purchased from Aldrich (Gold Label) and used without further purification. The polymer samples NaPA (mol wt 250000) and NaPSS (mol wt 70000) were extensively dialyzed with deionized distilled water. Luminescence Titratiom. All measurements were carried out in distilled deionized water a t 20 OC. In luminescence titration measurements, small aliquots of a concentrated (20 IJIM monomer units) polymer stock solution (pH adjusted to ca. 8.5 by addition of dilute sodium hydroxide) were added to the aqueous solution of the ruthenium complex (ca.10 pM). In quenching experiments, small amounts of an aqueous solution of bFe(CN), ( 5 mM) were added to a mixture of the ruthenium complex and the polymer (the concentration ratio of ruthenium complex to monomer units of polymer being 1:30). The solutions were thermally equilibrated
-
-
(10) (a) Barton, J. K. Science 1986, 233,727. (b) Barton, J. K. J. Biomol. Struct. Dyn. 1983,1, 621. (c) Barton, J. K.; Basile, L. A,; Danishefsky, A,; Alexandrescu, A. Proc. Narl. Acad. Sci. W.S.A.1984.81, 1961. (d) Barton, J. K.; Danishefsky, A.; Goldberg, J. M. J. Am. Chem. SOC.1984, 106, 2172. (e) Kumar, C. V.; Barton, J. K.; Turro, N. J. J. Am. Chem. SOC.1985, 107, 5518. (f) Barton, J. K.; Goldberg, J. M.; Kumar, C. V.; Turro, N. J. J . Am. Chem. SOC.1986, 108, 2081. (g) Barton, J. K.; Kumar, C. V.; Turro, N. J. J . Am. Chem. SOC.1986, 108, 6391. (h) Fleisher, M.; Waterman, K. C.; Turro, N. J.; Barton, J. K. Inorg. Chem. 1986, 25, 3449. (i) Barton, J. K.; Paranawithana, S. R. Biochemistry 1986, 25, 2205. (k) Turro, N. J. Polym. Prepr. (Am. Chem. Soc., Diu. Polym. Chem.) 1986, 27, 318.
0 1988 American Chemical Society
Polypyridyl-Ruthenium Complexes
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 2029
D , D RulDIPl:+
A , A Rulbpyl,Z+
SOSeNae NaPA
NaPSS
Figure 1. Ruthenium complexes tris(2,2'-bipyridine)ruthenium(II), Ru(bpy)32+,tris( 1,lo-phenanthroline)ruthenium(II), R~(phen),~+, and tris(4,7-diphenyl-1,IO-phenanthroline)ruthenium(II), RU(DIP),~+and
polyelectrolytes sodium poly(acrylate),NaPA, and sodium poly(styrenesulfonate), NaPSS. for at least 15 min. The steady-state luminescence measurements were carried out on a Perkin-Elmer LS-5 spectrometer, by excitation at the isosbestic points (462 nm with R ~ ( b p y ) , ~ + 464 , nm with R ~ ( p h e n ) , ~ and + , 483 nm with RU(DIP),~+'OC+) of the ruthenium complex-polyelectrolyte systems. Emission intensities were monitored at 610 nm with R ~ ( b p y ) , ~and + R ~ ( p h e n ) , ~and + a t 630 nm with RU(DIP),~+.The observed values of emission intensity were corrected for changes in metal complex concentration upon addition of the polymer stock solution or of the quencher solution. Luminescence Decay Measurements. Emission lifetime measurements were performed using a PRA single-photon-counting unit and a TN 1710 MCA, interfaced with a PDP-11/03 computer. The samples [either pure solutions of the ruthenium complexes, 10 rM, or mixtures of the ruthenium complexes with polyelectrolytes (ratio 1:30), in the presence and absence of ferrocyanide, 0.1 mM] were excited in the intense metal to ligand charge-transfer band, between 400 and 500 nm,lOd using cutoff filters. Emission traces were collected at 610 nm, with R ~ ( b p y ) , ~ + and R ~ ( p h e n ) , ~ and + , at 630 nm, with RU(DIP),~+.The decays were analyzed, using PRA deconvolution software.
Results and Discussion Steady-State Luminescence. The luminescence intensity of the ruthenium complexes in aqueous solution is strongly enhanced (up to a factor of 3) by the addition of NaPSS. The enhancement of emission intensity increases (Figure 2) with of the metal complex. Consequently, the strongest increase was observed with RU(DIP),~+and the weakest with R ~ ( b p y ) , ~ + . However, by the addition of NaPA to aerated solutions of the metal complexes, only a weak enhancement of luminescence intensity (less than 20%) was observed, with minor differences between the three ruthenium complexes (Figure 2). Since the linear charge densities of the two polymers, NaPA and NaPSS, are essentially the same, these results suggest that the differences in interaction between the metal complexes and the polyelectrolytes are mainly determined by the influence of hydrophobicity. The strong enhancement of luminescence intensity (between a factor of 2 and a factor of 3) by NaPSS reveals its great affinity' for the ruthenium complexes, an affinity that arises from the strong hydrophobic interactions between the polymer backbone and the ligands of the metal complexes." NaPA, which (1 1) (a) Turro, N. J.; Okubo, T. J . Am. Ckem. SOC.1982,104,2985. (b) Morawetz, H. Acc. Chem. Res. 1970, 3, 354. (c) Ise, N. Adu. Pofym. Sci. 1971, 7, 536.
I
NaPA
I
IO0
[ Polyion ] / [ R u ] 2 + Figure 2. Changes in luminescence intensity of aqueous solutions of
Ru(bpy)32t and R~(phen),~+ (both recorded at 610 nm) and of Ru(DIP)32+(measured at 630 nm) by the addition of sodium poly(styrenesulfonate), NaPSS, and of sodium poly(acryIate), NaPA. is more hydrophilic, has a less pronounced influence on the emission from the metal complexes. The enhanced emission intensities are accompanied by lengthened excited-state lifetimes (vide infra). The conclusion of binding of the ruthenium complexes to NaPSS is supported by the results of quenching experiments, using potassium ferrocyanide, K,Fe(CN),, as the quencher. l 2 The excited-state ruthenium complexes, in the presence of anionic polyelectrolytes, are expected to be protected against quenching by ferrocyanide due to electrostatic repulsion between the highly negatively charged quencher molecule and the polyanion. In complete agreement with these expectations, with NaPSS as the polyelectrolyte, almost no quenching was observed (Figure 3a). In the case of R ~ ( b p y ) , ~and + R~(phen),~+ the , changes in luminescence intensity were not significant (smaller than 5% in the presence of a quencher concentration up to 1 mM). Unexpectedly, with RU(DIP),~+,the addition of ferrocyanide even led to a considerable increase (ofabout 30%) in the measured luminescence intensity. Again the changes of emission intensity are paralleled by similar changes of the luminescence decay times (vide infra). In the presence of NaPA, the addition of ferrocyanide caused a decrease of emission intensity in the case of all ruthenium complexes investigated (Figure 3b), revealing the weaker interaction between NaPA1*and the metal complexes, compared to NaPSS. The quenching was the most efficient with R ~ ( p h e n ) ~ ~ + and the least with Ru(DIP),~+. The quenching is, however, still strongly reduced, compared to that of the free metal complexes in aqueous solution in the absence of polyanions. For example, the luminescence intensity of R ~ ( p h e n ) , ~ +in, the presence of NaPA, is reduced by a factor of 4 at 1 mM quencher concentration. At this ferrocyanide concentration, more than 90% of the luminescence intensity of free Ru(phen):+ in aqueous solution, in the absence of a polyelectrolyte, is quenched.loe The SternVolmer plots (Figure 3b) are nonlinear, suggesting that the quenching mechanism cannot be explained by a simple diffusion model but that a more complex mechanism is operating under these conditions. These conclusions are supported by the results (12) Ferrocyanide is an efficient quencher of the luminescence from the free ruthenium complexesik with rate constants (in the order of 3 X loioM-I s-I), which have been explained in terms of the Debye equation" for diffusion rates of oppositely charged, ionic species.
2030
Duveneck et al.
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
TABLE I: Luminescence Decay Parameters (in p s ) of Mixtures of Polypyridyl-Ruthenium Complexes with Sodium Poly(acrylate), NaPA, and Sodium Poly(styrenesulfonate), NaPSS, in the Presence of Different Amounts of Oxygen (Nitrogen-Saturated,Aerated, and Oxygen-Saturated Aqueous Solutions; 20 "C); Influence of Ferrocyanide, [Fe(CN)6P, as a Negatively Charged Quencher [Fe(CN)6I4- = 0 mM, with [O,]"of [Fe(CN),I4- = 0.1 mM, with [O,]" of complex Ru(bp~),~+ pure , R ~ ( b p y ) , ~++ N a P A Ru(bpy),2+ NaPSS
+
R ~ ( p h e n ) , ~ pure +, Ru(phen),*+ NaPA R ~ ( p h e n ) , ~++ NaPSS
+
0.28 mM
1.35 mM
0.64 0.60 0.85
0 mM
0.42 0.45 0.68
0.18 0.22 0.44
T
1.19
T
1.02
TIC
0.77 (24) 1.69 (76)
0.53 0.60 0.55 (19) 1.56 (81)
3.38 0.60 2.24 0.94 4.02
0.94 0.52 2.38 1.99 4.52
Tb T T
72
RU(DIP)~,+,pure Ru(DIP),~++ N a P A
+
T
Ti 72
Ru(DIP),~+ NaPSS
71
72
(15) (85) (2) (98)
(12) (88) (30) (70)
0 mM
0.28 mM
1.35 mM
0.60 0.86
0.45 0.69
0.22 0.44
0.18 0.23 0.35 (27) 1.05 (73)
0.96 0.85 (29) 2.03 (71)
0.57 0.62 (24) 1.93 (76)
0.23 0.32 (24) 0.99 (76)
0.24 0.24 (9) 1.17 (91) 1.20 (53) 4.31 (47)
0.59 2.42 1.79 5.14
0.46 2.21 2.15 5.44
(11)
0.26 (13) 1.18 (87) 1.28 (51) 4.92 (49)
(17)
(83) (11) (89)
(89)
(35) (65)
a Molar oxygen concentrations were determined by using Henry's law.I6 bSingle-exponential decay, characterized by one decay time T . Decay A , exp(-t/r,). In parentheses is the contribution of the two-exponential terms, I, = parameters from double-exponential fit: f(t) = A , exp(-t/rl) A t ~ , / ( A I+~ A2r2) I (i = 1, 2), to the decay curve.
+
of time-resolved luminescence measurements. Luminescence Decays. In order to obtain more insight into the underlying quenching mechanisms, the time dependence of the luminescence from the various ruthenium complex-polyelectrolyte systems, with and without quencher, was studied by the technique of time-correlated single-photon counting. Experiments were performed in the presence and in the absence of oxygen, in order to separate the influence of different quenchers on the emission from the metal complexes interacting with polyions. The luminescence lifetimes of the three ruthenium complexes in aqueous solution are affected in a similar manner by the presence of oxygen, with a quenching rate constantI4 k, = 3 X 109 M-1 s-1 (Table I). However, principal differences in the luminescence decay behavior of the three metal complexes show up by the addition of polyelectrolytes, revealing the different extent of interaction. The decays of Ru(bpy):+ remain single exponential in the presence of both polyelectrolytes. In the case of Ru(phen),*+, decay curves are single exponential in the presence of NaPA but double exponential in the presence of NaPSS (Figure 4). Luminescence decay curves of R u ( D I P ) ~ ~by + , the addition of either NaPA or NaPSS, could only be fitted to a sum of two exponentials (Table I). This general behavior is not affected by the amount of dissolved oxygen or by the addition of ferrocyanide as a negatively charged quencher. The results with the three ruthenium complexes will now be discussed in detail.
Interaction with Ru(bpy)3z+ For R ~ ( b p y ) , ~the + rate constant of oxygen quenching is reduced, from 2.9 X lo9 to 2.2 X lo9 M-' s-l in the presence of NaPA and 0.8 X lo9 M-' s-l upon binding to NaPSS, respectively (Figure 5). In the absence of oxygen, the luminescence decay times of the free complex and of a mixture of R ~ ( b p y ) , ~and + NaPA are almost the same, whereas the emission lifetime of Ru(bpy)?+, bound to NaPSS, is clearly longer. With increasing amount of oxygen, the emission lifetimes in the presence of either polymer become longer than observed for the free metal complex. From these data, it can be concluded that the moderate increase of luminescence lifetime of aerated and of oxygen-saturated solutions of Ru(bpy)?+ in the presence of NaPA, compared to that of the puTe compound, is mainly caused by the reduced mobility of the metal complex upon interaction with the polymer. The remarkable increase of emission lifetime in the presence of NaPSS, ~~~
(1 3) Debye, P. Trans. Electrochem. SOC.1942, 82, 265. (14) Quenching rate constants k, were calculated by using the following equation derived from Stern-Volmer kinetics l / =~ k o w = ko
l/kOM
+ kq[Q]
with 7 = being the observed luminescence decay time, 70 = l/ko the decay time in absence of the quencher, and [Q] the quencher concentration.
I
O
'
I
'
I
'
I
~
No PSS 1.2 -
+
Ru Ibpyl,Z+
A
Ru Iphenll2+
-
8 R u IDIPI,Z+
lo/l
[ Fe lCNI6l4-
[IO-'
M
]
4
3 lo/l
2
1
C
2
4 [FeICN16I4-
6
8
1 0 1 2
[ 10-4M]
Figure 3. Steady-state quenching of luminescence intensities of polypyridyl-ruthenium complexes by potassium ferrocyanide, in the presence of two different polyelectrolytes ([R~]~'/[polyion]= 1:30): (a, top) sodium poly(styrenesulfonate), NaPSS; (b, bottom) sodium poly(acrylate), NaPA.
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 2031
Polypyridyl-Ruthenium Complexes
VI
i3
e
CHlSQ
7.85
- 1.121
o
-7.85
"I cVI
z z n W
I-
n W N
1
a
T,
5
= 352 ns Il = 23% 1050ns 12 = 77%
r 2=
e 1 .o
TIME Ips1
20 30 T I M E (psl
40
Figure 4. Luminescence decay curves of Ru(phen)32+in aqueous, oxygen-saturated solutions (1.35 m M 02), at 20 "C, in the presence of two different polyelectrolytes ([R~]~'/[polyion] = 1:30): (a, left) single-exponential decay in the presence of NaPA; (b, right) double-exponential decay in the presence A2 exp(-t/~*). The contributions I, = A z i t / ( A l i l ,4272) ( i = 1,2) are displayed. The insert shows the autocorrelation of NaPSS: At) = A, exp(-f/7,) function, C(I), displaying the correlation between the first and the second half of the fit.
+
+
8
amount of oxygen is much snialler than in case of the single decay time of Ru(phen),'+-NaPA solutions. Treating the two decay constants as caused by two independent, noninteracting species would lead to rate constants for oxygen quenching of 1.O X lo9 M-' s-l, calculated from i1 and 0.2 X lo9 M-' s-l, calculated from i2, respectively.
I
PURE
-
-
e
WITH NoPA
A
WITH N O E S
00
2 02 [lO-'M]
Figure 5. Oxygen quenching of Ru(bpy)32+ luminescence in aqueous solution (20 "C); reduction of the quenching rate (slope) by the presence of polymers ([R~]~+/[polyion] = 1:30).
even in the absence of oxygen, and the simultaneous reduction of the quenching rate of oxygen by more than a factor of 3 indicate, in accordance with the results of photostationary measurements, a strong interaction between the metal complex and this polymer, resulting in a decrease of the rate constants for the nonradiative deactivation of the excited state.
Interaction with Ru(phen)?+ Interaction with NaPA has a similar effect on the luminescence decay times of Ru(phen),*+ as that described for R ~ ( b p y ) , ~ + . Again, the observation of a longer luminescence decay time of the aerated solution of the metal complex in the presence of NaPA, compared to that of an aqueous solution of pure Ru(phen),'+, is due to a reduced constant for oxygen quenching (2.4 X lo9 M-' s-l). In the absence of oxygen, the observed decay time of a mixture of Ru(phen)32+and NaPA was even shorter than in the I). case of pure R ~ ( p h e n ) , ~(Table + In the presence of NaPSS, however, the decays become double exponential. The dependence of both decay constants on the
Interaction with RU(DIP)~*+ In the presence of polyanions, Ru(DIP);+ shows a luminescence decay behavioi, which is again different from the other two ruthenium complexes. In the presence of NaPSS or NaPA, luminescence decay curves of R u ( D I P ) ~ ~are + nonexponential but could be fitted to a sum of two exponentials. With NaPA, in the absence of oxygen, both decay times are shorter than the luminescence lifetime of pure Ru(DIP)~'+, similar to the results obtained with the other two ruthenium complexes. Again, the decay constants, determined from the measurements in the presence of NaPA, show a stronger dependence on the amount of oxygen than the data calculated for RU(DIP),~+, in the presence of NaPSS (Table I). Although a direct determination of quenching rates by means of Stern-Volmer kinetics, applied to noninteracting excited-state species, is not possible in the case of R U ( D I P ) , ~ + ,these ' ~ results support the assumption that the interaction of Ru(DIP),'+ with NaPSS is considerably stronger than with NaPA. Influence of Ferrocyanide In general, the addition of ferrocyanide has no significant influence on the emission decays of the ruthenium complexpolyelectrolyte systems (Table I). However, in the case of Ru(DIP),Z+, in the presence of NaPSS, a slight but systematic increase in both decay times, by the addition of the quencher, was observed. This surprising result is in complete agreement with the results from the steady-state luminescence experiments, (1 5 ) In the case of noninteractingspecies the luminescence decay times are identical with the excited-state lifetimes, and the ratio of the preexponential factors, A I / A 2 obtained , from fitting the data to a double exponential represents the concentration ratio of these species at the moment of excitation. These initial concentrations will not be affected by dynamic quenching processes. Thus, the dependence of this ratio on the amount of oxygen (a slight decrease in the presence of NaPA but a strong increase in the presence of NaPSS, with increasing oxygen content) indicates that for this particular system an interaction between at least two kinetically different species has to be considered, resulting in a more complicated kinetic scheme. (16) For example, see: Gucker, F. T.;Seifert, R. L. Physical Chemistry; Norton: New York, 1966; pp 459-461.
J. Phys. Chem. 1988, 92, 2032-2036
2032
R~L:’ (L
PSS
(
RuL
‘‘
+ pss)
(camplexed)
bpy, phen, D I P )
Figure 6. Scheme of induced formation of hydrophobic domains by interaction of polypyridyl-ruthenium complexes with poly(styrenesulfonate), PSS, in the presence of a negatively charged quencher, Q-.
where an enhancement of emission intensity of R u ( D I P ) ~ ~bound +, to NaPSS, was observed upon addition of the quencher. This unexpected luminescence behavior could be explained if NaPSS is coiled around the metal complex, forming hydrophobic domains which are difficultly accessible to the quencher, as shown schematically in Figure 6 . The approach of a highly negatively charged quencher molecule to the polyion could increase this hydrophobic effect, resulting in an enhanced interaction between the polymer and the bound metal complex. The lack of any noticeable influence of ferrocyanide on the decays of the ruthenium complexes in the presence of NaPA, together with the observation of an efficient quenching of the luminescence intensity of these systems, leads to the conclusion that this quenching process is static. It thus appears that quenching occurs by ferrocyanide molecules which are close to the metal complexes at the moment of light excitation, whereas the diffusion of the quencher molecules toward the metal centers is inhibited by repulsion due to the negative polyion.
Conclusions It has been shown that the interactions between polypyridylruthenium complexes and polyelectrolytes depend not only on electrostatic factors but also on the hydrophobicity of both the ruthenium complex and the polyelectrolyte. Interaction of the metal complexes with the polymers results in an increase of luminescence intensity and decay times. In case of strong interactions, double-exponential decay behavior was observed, which may be explained by different microenvironments, such as coiled and extended form of the polymer, which are experienced by the probe molecule. The interaction with the anionic polymers leads to protection of the metal complex against a negatively charged quencher, such as ferrocyanide, and to a reduced quenching rate by a neutral molecule, such as oxygen. Quenching by ferrocyanide, in case of interaction of the ruthenium complexes with NaPA, was found to be static. The protection against dynamic quenching is probably caused by repulsion of a free, negatively charged quencher molecule from the polyanion. As an explanation for the absence of any quenching by ferrocyanide upon binding of the ruthenium complexes to NaPSS, especially the surprising enhancement of luminescence intensity with R u ( D I P ) ~ ~the + , formation of hydrophobic domains by the coiling of the polymer chain (so that the metal complex is no longer accessible to the quencher) is proposed. Acknowledgment. We thank the National Science Foundation, the National Institutes of Health, and the Army Office of Research for their generous support of this research. Registry No. [Ru(bpy),]CI2, 14323-06-9; [Ru(phen),]Cl,, 2357043-6; [Ru(DIP),]CI,, 36309-88-3; NaPA, 9003-04-7; NaPSS, 9080-79-9; [Fe(CN),I4-, 13408-63-4; 02,7782-44-7.
Solvent Interactions at Crystal Surfaces: The Kinetic Story of a-Resorcinol Roger J. Davey,* Chemicals and Polymers Group, Imperial Chemical Industries plc, Runcorn, Cheshire, UK
Bogdan Milisavljevic, and John R. Bourne Technisch Chemisches Laboratorium, Eidgennosische Technische Hochschule, Zurich, Switzerland (Received: June 29, 1987; In Final Form: October 14, 1987)
New kinetic data are reported for the growth of {Ol1) and {OiT)faces of the polar crystal a-resorcinol from aqueous solution. Careful confrontation with crystal growth theory suggests that both surfaces should grow by spiral, dislocation mechanisms. The existence of dead zones in the measured kinetics is in conflict with this prediction and is explained on the basis of solvent adsorption at crystal surfaces. The data suggest that a water molecule is bound more strongly to a growth site on the (011) surfaces than it is on the (Oii]surfaces. This difference in binding energy is estimated to be about 1 kcal mol-’, in good agreement with previous calculations.
1. Introduction a-Resorcinol belongs to the space group Pna2, and grows from aqueous solution as ( 110) prisms bounded by the (011) and (OTi) faces. It was Wells in 1949, who first pointed out that because of the noncentric nature of the resorcinol structure, the c axis was polar and hence one end of the crystal must be “hydroxyl-rich” and the other “benzene-rich”. In addition, the existence of inclusions in only one of the c-axis growth sectors suggested that there was a significant difference in crystal growth rate between the (011)and (OTi}faces. Wells was unable to assign the absolute direction of growth along the c axis but surmised that strong adsorption of water on the hydroxyl-rich end would impede its (1) Wells, A. F. Discuss. Faraday SOC.1949, 5 , 197.
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growth and that the fast-growing end was in fact benzene-rich. More recently, firstly M i l i s a ~ l j e v i cand ~ ~ ~then Shimon et aL4s5 have returned to this problem and concluded that Well’s assignment was in fact incorrect and that the hydroxyl-rich end is actually the faster growing. This conclusion has been arrived at (2) Miliivljevic, B. C. Ph.D. Thesis, Swiss Federal Institute of Technology (E.T.H.), Zurich, Switzerland, 1982, Dissertation N6898. (3) Davey, R.J.; Milisavljevic, B. C.; Bourne, J. R. Paper presented at a Discussion Conference ‘Crystallisation Processes in Condensed Phases”; Faraday Division of the Royal Society of Chemistry; Girton College, Cambridge, England, July 1983. (4) Shimon, L.J. W.; Wireko, F. C.; Wolf, J.; Wiessbuch, I.; Addadi, L.; Berkovitch-Yellin, 2.;Lahav, M.; Leiserowitz, L. Mol. Cryst. Liq. Cryst. 1986, 137, 67. ( 5 ) Wireko, F. C.; Shimon, L. J. W.; Frolow, F.; Berkovitch-Yellin, 2.; Lahav, M.; Leiserowitz, L. J . Phys. Chem. 1987, 91, 472.
0 1988 American Chemical Society
