J . Phys. Chem. 1986, 90, 1828-1830
1828
Luminescence Lifetimes of Ruthenium(I1) Polypyridyls in H20 and D20 at High Pressures M. L. Fetterolf and H. W. Offen* Department of Chemistry, University of California, Santa Barbara, California 931 06 (Received: November 15, 1985)
The pressure dependence of the luminescence lifetimes of Ru(bpy),CI, and R ~ ( p h e n ) ~ C(bpy l ~ = bipyridine; phen = phenanthroline) in H20and D 2 0 is reported in the 0.1-300-MPa range at several temperatures (2-70 "C). The observed activation volumes for electronic relaxation are analyzed in terms of a small, negative AVJ (-2 mL/mol) for the CT GS nonradiative process and a large, positive AV3+(10 mL/mol) for the CT LF decay channel. The observed magnitudes for the two intrinsic activation volumes are consistent with the changes expected from the calculated metal-nitrogen bonds of the complex in the ligand field (LF), charge transfer (CT), and ground (GS) electronic states. Superimposed on these principal trends is a small deuterium isotope effect in AV2t and a small ligand effect in AV,'.
-
-+
Introduction Recently, we reported the P-T dependence of the luminescence of Ruthenium(I1) and Osmium(I1) polypyridyls in acetonitrile.' One significant observation to emerge from those experiments is that the pressure effects on the lifetime T are strongly dependent on the temperature of the solution, with the largest changes occurring at the higher temperatures.' The temperature dependence of the electronic relaxation rate above 273 K is commonly analyzed within the framework of two excited states in which the population of the upper LF (ligand field) level is governed by the temperature.'-6 The appropriate equation is r-' = k ,
+ k2 + A, exp(-AEt/RT)
-
where k , and k 2 are the radiative and nonradiative C T GS (charge transfer to ground state) transition rates, respectively, and k 3 = A3 exp(-AEt/RT) describes the C T LF activation process. The corresponding expression for the activation volume of the observed decay rate
-
AV,-it = 8,AVrt + 82AV2+ + 83AVjt
(2) Here AV,-it is obtained from the observed In T-' vs. P plots and the intrinsic activation volumes AV,+ are associated with each energy dissipation pathway. The contribution of each decay channel to the observed activation volume is weighted by the respective quantum efficiencies ei= kiT-'. The application of eq 1 and 2 to Ru(I1) complexes in CH3CN predicts a low- and high-temperature limit for the observed AVr-lt.l The purpose of the present work is to test the underlying assumptions and to show that this analysis can be generalized and include protic solvents. This study includes Ru(I1) complexes with the ligands bipyridine (bpy) and phenanthroline (phen) and the solvents H 2 0 and D 2 0 . A significant solvent isotope effect has been identified for these polypyridine c o r n p I e x e ~ . ~ ~ The ~ ~ ~luminescence ~~-" decay rates also depend on the molecular structure of the x-ligand; Le., r is considerably longer for the tris(phenanthro1ine) than the tris(bipyridine) complex under identical conditions of solvent, temperature, and c o ~ n t e r i o n . ~Since * ' ~ the pressure dependence of both the solvent isotope and ligand effects is unknown, this work H. W. J. Phys. Chem. 1985, 89, 3320. (2) Van Houten, J.; Watts, R. J. J . Am. Chem. SOC.1976, 98, 4853. (3) Allsopp, S. R.; Cox, A,; Kemp, T. J.; Reed, W. J. J. Chern. SOC. Faraday Trans. I 1978, 74, 1275. (4) Watts, R. J. J . Chem. Educ. 1983, 60, 834. (5) Caspar, J. V.; Meyer, T. J . J . Am. Chem. SOC.1983, 105, 5583. (6) Cherry, W. R.; Henderson, L. J., Jr. Inorg. Chem. 1984, 23, 983. (7) Weber, W.; van Eldik, R.; Kelm, H.; DiBenedetto, J.; Ducommun, Y . : Offen, H.; Ford, P. C . Inorg. Chem. 1983, 22, 623. (8) Kirk, A. D.; Namaslvayam, C.; Porter, G. B.; Rampl-Scandola, M . A.; Simmons, A. J . Phys. Chem. 1983, 87, 3108. (9) Van Houten, J.; Watts, R. J. J. Am. Chem. SOC.1975, 97, 3843. ( I O ) Sriram, R.; Hoffman, M. 2. Chem. Phys. Lett. 1982, 85, 572. ( 1 1 ) Hauenstein, B. L., Jr.; Dressick, W. J.; Buell, S . L.; Demas, J. N.; DeGraff, B . A . J . A m . Chem. SOC.1983, 105. 4251. ( 1 ) Fetterolf, M. L.; Offen,
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TABLE I: The 1-atm Lifetimes of Ru(I1) Complexes at Different Temperatures T O , ns solvent 2-3.5 O C 25 OC 40 OC 60 O C 70 O C
H2O D2O
1250
H,O D2O
2030 2590
750
RU(bPY),CI2 600 430 980 660
290 340
200 190
210 150
120 95
Ru(phen),Cl, 960 1080
490
500
TABLE II: Pressure Dependence of the Lifetimes of Ru(I1) ComDlexes at 25.0 OC" 50
150
solvent
MPa
100 MPa
200 MPa
250 MPa
300 MPa
H2O D20 CH3CNb
0.94 0.97 1.08
Ru(b~~),C12 0.95 0.91 0.90 1.04 1.02 1.02 1.18 1.25 1.30
0.90 1.06
0.95 1.00 1.39
H20
1.08 1.19
1.47 1.71
1.60 1.88 2.15
MPa
Ru(phen),CI2 D2O CH3CNb
1.23 1.29 1.41
1.33 1.45
"Expressed relative to the 1-atm lifetime
1.43 1.55 1.71 TO.
bFrom ref 1.
will also provide information on these topics.
Experimental Methods The high-pressure optical cell, temperature controller, sample handling procedures, and lifetime instrumentation have been described previously.' The experimental ranges are 2 IT I70 "C and 0.1 IP I300 MPa. Luminescence is excited with a N2 laser and the intensity decays are traced with a fast oscilloscope and then analyzed with a linear least-squares program. The sources of the Ru(I1) complexes have been given.' The chemicals D 2 0 (Aldrich, Gold Label, 99.9 atom %, stored under nitrogen), urea (Aldrich, Gold Label, 99%), and LiCl (Mallinckrodt) were used as received. Water was purified to give a resistance greater than IO MQ cm. Results Listed in Table I are the 1-atm lifetimes 7" for excited Ru(I1) complexes in H 2 0 and D 2 0 at the several temperatures employed in this study. The TO values at 25 "C for both solvents are within 5% of other reported values, except for Ru(phen),Cl, in H 2 0which is within 10%.2-4The activation energies obtained from In [r-' - .,-'I vs. T 1 plots (eq I ) , using literature values3 for 7;' = k , + k2, are somewhat higher than those previously reported3 due to our limited temperature range. The solvent isotope effect,
0 1986 American Chemical Society
Luminescence Lifetimes of Ruthenium(I1) Polypyridyls
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 1829
TABLE 111: Observed Activation Volumes of the Reciprocal Lifetimes of Ru(I1) Complexes AV,-it, mL/mol solvent 2 OC 3.5 OC 25 "C 40 OC 60 OC 70 OC ~
10
0
~~
H2O D2O
R~(~PY),C&
-1.5
-1.0 0.9
0.0
2.5 3.7
4.2 5.9
0
1.5 8.9
-
Ru(phen),CI2 H2O D20
5.6 6.4
2.0 2.9
0
8.3 10.0
8.0 9.4
E
10.6 11.7
0
+
2
+
+
0
c
TABLE I V Ru(bpy)3C12 Lifetimes and Activation Volumes in Aaueous Solutions" 15 OC 60 OC TO AV.-it io A V.-i H2O 9.6 M LiCl 5.0 M urea
-1.0 -1 .O -0.4
650 620 700
290 330 250
c
I
xc
> Q
4.2 3.1 6.6
"The 1-atm lifetimes T O are expressed in nanoseconds; the activation volumes are given in milliliters per mole.
expressed as T ~ ( D ~ O ) / T ~ ( H ,isOgreater ), at 2 than 70 OC: 1.67 vs. 0.95 for Ru(bpy),Cl, and 1.28 vs. 0.79 for Ru(phen),Clz, respectively. The pressure dependence of the lifetime at 25.0 OC is given in Table I1 relative to 1-atm values for HzO, DzO,and CH3CN. The bpy complex undergoes small changes in 7 with increasing pressure, in sharp contrast to the pressure response of 7 for the phen complex which increases by 60% and 90%, respectively. The T / T ' ratios are larger in acetonitrile for both complexes when compared to their values in the protic solvents. The activation volumes for the observed reciprocal lifetimes, listed in Table 111, are obtained from a least-squares quadratic fitting of In 7-l = a bP cPz and equal to -bRTIJZwith an estimated error of fl.O mL/mol. The observed lack in the pressure response of T for the bpy complex in H,O at or near room temperature concurs with earlier report^.'^,'^ As shown in Figure 1, the lifetimes are much more sensitive to pressure at other temperatures, yielding a monotonic increase in AVT-ltvalues with rising temperatures. The positive values, observed at the upper end of the temperature range, indicate a slowing of the deactivation process with increasing pressure. The AV,-it values of both complexes in D 2 0 are larger than in H 2 0throughout the observed temperature range. In comparing ligands, the activation volumes are larger for Ru(phen)$l, regardless of the solvent. Some experiments were designed to note the effect of additives on the observed trends in water. Table IV compares both T O and AV,-lt values of Ru(bpy),Cl, in 9.6 M LiCl and 5.0 M urea with those of H 2 0 . The additives have only a small effect on the luminescence at 15 "C, but clear differences are observed at higher temperatures. Dissolved LiCl has a moderating effect at 60 OC: T O is longer3 and AV,-lt is less. Urea has just the opposite effect on these two quantities.
+ +
Discussion The experimental measurements depicted in Table I11 and Figure 1 can be analyzed in terms of eq 3. This equation derives AV,-it = ( 1 - e3)AVJ + 83AV3t (3) from eq 2 if the radiative contribution is negligible as assumed here because 8, of these complexes in water is sma11.2~5~'5 With this approximation, eq 4 predicts a low- and high-temperature
e, = 1 - e, = limit, governed by 8,
-
-
exp(-AE+/RT)s-' (4) 1 and €I3 1, respectively. In other
(12) Isaacs, N . S. Liquid Phase High Pressure Chemistry; Wiley: ChiChester, U. K., 1981; p 183. (13) Kirk, A. D.; Porter, G. B. J. Phys. Chem. 1980,84, 2998. (14) Lee, S. H.; Waltz, W. L.; Demmer, D. R.; Walters, R. T. Inorg. Chem. 1985, 24, 1531. (15) Nakamaru, K. Bull. Chem. SOC.Jpn. 1982, 55, 1639.
T ("C) Figure 1. The temperature dependence of the activation volume for luminescence decay AV,-it of the two Ru(I1) complexes in H 2 0 (+) and D20 (0).
-
TABLE V Intrinsic Activation Volumes (mL/mol) for the CT and CT LF Processes
-
GS
Ru(bPY),CIz H20 D20 CHiCN
-2.2 -1.0 -1.7
H2O D2O
-2.2 -1.1
9.1 9.1 12.5
10.2 10.6 10.8
Ru(phen),CI2 11.2
10.4 11.5
12.0
"The estimated errors are 10.2 mL/mol. bThe estimated errors are 1 1 . 0 mL/mol for Ru(bpy),C12 and 1 0 . 4 mL/mol for Ru(phen),C12.
words, the thermal population of excited states is a major factor in the magnitude of the experimental AV,-lt. If the intrinsic activation volumes for the two decay channels are assumed Tindependent, the results for AV,-it (Table 111) can be combined with the 1-atm data of Alsopp et aL3 for 0, (eq 4) to derive the best computer fit of eq 3 for AV; and AV3t consistent with the P-T measurements. The calculated values are shown in Table V. The results for R ~ ( p h e n ) ~ C inlCH3CN ~ could not be analyzed by this method because 8, is unavailable. An alternate method for obtaining AVJ based on eq 1 and used previously' is less reliable because it depends on the logarithm of the difference between two similar quantities. Nevertheless, the AVJ calculations using eq 1 at 70 "C, where errors are the smallest for our temperature range, are comparable to those based on eq 3, as shown in Table V. The inescapable conclusion from the above analysis is that the pressure dependence of T is characterized by a small and negative contribution to the activation volume of the C T GS relaxation process and by a large and positive contribution for the thermally activated C T L F process in both polar and hydroxylic solvents. This self-consistent treatment of activation volumes can be related to the molecular size of each electronic state. An average bond displacement of Ar = -0.015 8, for the C T GS transition of related complexes16translates to AV, = -1.9 mL/mol assuming r = 4.0 A and spherical contraction. Similarly, a 0.10-A increase
-
-+
-
(16) Caspar, J. V.; Westmoreland, T. D.; Allen, G . H.; Bradley, P. G.; Meyer, T. J.; Woodruff, W. H . J . Am. Chem. SOC.1984, 106, 3492.
J . Phys. Chem. 1986, 90, 1830-1834
1830
-
i n the metal-nitrogen bond in three directions would be accomin the pressure response of T , thus implicating AT‘,+. In the case L F process. The panied by AV, = 10 mL/mol for the C T of concentrated electrolyte solutions, molecular dynamic simu0.10-A geometrical factor is of the correct magnitude based on lations of ion hydration at 1 and 10000 bar suggest significant ’~ Franck-Condon analysis of L F states in related ~ o m p 1 e x e s . l ~ ~ ’ ~distortion of the H-bond network with increasing p r e s s ~ r e , in agreement with the loss in water structure observed in the difHence the sign and magnitude of A.V,’ for nonradiative electronic relaxation are explained by intrinsic volume changes of the two fraction patterns of LiCl solutions.20 If this decrease in water structure is the cause of the reduced pressure response of the k , states coupled in the transition. The data analysis reveals three additional effects in the pressure decay channel, then the opposite k,(P) results in urea solutions dependence of T . First, a solvent isotope effect in AV; is evident (Table IV) can be interpreted to suggest “structure-making’’ from Table 111. The influence of deuterium substitution on the abilities with pressure. This prediction has not been tested at high CT GS relaxation has been cited as evidence for the admixture pressures and further work is needed to mesh the interesting of charge-transfer-to-solvent states with effective vibronic coupling aspects of hydrogen-bonding and urea-water interactions2’ with between solvent molecules and the c ~ m p l e x . ~ ~The ~ .smaller ~ ~ ~ ’ ~ ~ its ’ ~ influence on excited-state solute processes. The inclusion of the temperature variable has been essential pressure increase of k2 in D 2 0 relative to H 2 0 and CH3CN (Table 111) is then attributed to poorer coupling with the OD environment in the interpretation of the high-pressure data on luminescent in the cybotactic cleft at high pressures. The presence of an isotope lifetimes of Ru(I1) complexes. Whenever thermally accessible effect on k2 and not k3 is entirely consistent with the weak coupling states are suspected to play a role in electronic relaxation, it will limit for the CT GS radiationless t r a n ~ i t i o n .Second, ~ the be imperative to include T in pressure studies of excited-state results in Table 111 demonstrate a small ligand effect in k , ( P ) , behavior. In summary, this project has demonstrated opposite with a greater pressure sensitivity indicated for phen ligands in pressure effects on the two nonradiative decay channels of ruthenium( 11) polypyridine complexes: a small and negative AT‘; H 2 0 and D20. The difference in AT‘,+ (Table V) can be assigned and a large positive AV,’. to the increased size and rigidity of phen vs. bpy ligands. Apparently, the ligand effect is only observable in the strongly coupled Acknowledgment. We acknowledge the donors of the Petrok3 decay channel. leum Research Fund, administered by the American Chemical Finally, significant differences in A V , - I are ~ observed with water Society, for partial support of this research. We are most grateful additives at a temperature where the thermally activated k3 process to Professor R. J. Watts for access to his lifetime instrumentation. is the dominant decay channel (Table IV). Electrolytes and urea in high concentrations evidently modify the solute-solvent inRegistry No. Ru(bpy),Clz, 14323-06-9;Ru(phen),CI,, 23570-43-6; HIO, 7732-18-5;DZO, 7789-20-0;CHlCN, 75-05-8. teractions in such a manner as to result in opposing effects on the net CT LF relaxation rate. The small effects noted in T” (Table IV) suggest that 0, is not responsible for the significant difference --+
-+
-+
(17) Hipps, K. W.; Merrell, G. A,; Crosby, G.A. J . Phys. Chem. 1976, 80, 2232. (18) Wilson, R. B.; Solomon, E. I . J . Am. chem. SOC.1980, 102, 4085.
(19) Jancso, G.; Heinzinger, K.; Kadnai, T. Chem. Phys. Lett. 1984, 110, 196. (20) Narten, A. H.; Vaslow, F.; Levy, H . A. J . Chem. Phys. 1973, 58, 5017. (21) Kuharski, R. A,: Rossky, P. J. J . A m . Chem. SOC.1984, 106, 5786.
SURFACE SCIENCE, CLUSTERS, MICELLES, AND INTERFACES Cooling of Water/Oil Mlcroemulsions: The Cupric Probe Peter Bruggeller Institut Fur Anorganische und Analytische Chemie. Universitat Innsbruck, A-6020 Innsbruck, Austria (Received: April 15, 1985; I n Final Form: November 7 , 1985) Cupric chloride has been used as an EPR spin probe to test the possibility of ice formation in contrast to a vitrification behavior of the aqueous part of water/oil (w/o) microemulsions when they are cooled. The investigated w/o microemulsions contained cationic, anionic, and nonionic surfactants and the influence of a short-chain alcohol used as cosurfactant is examined. Furthermore, a surfactantless system is studied. In all cases vitrification of the water content is readily obtained at comparatively low cooling rates: ice formation is in one case observable when a w/o microemulsion is cooled very slowly. The others are capable of being cooled at any rate without any observable ice formation. The g,,splittings of the cupric EPR signals are very sensitive to environmental effects in the solid state, giving information about the participation of alcohols and/or surfactants in the coordination sphere of the cupric ion and concentration gradients within a more concentrated cupric chloride/ice phase. Introduction The appearance of microemulsions is possibly the last marked sign of amphiphile association into large aggregates before essentially complete breakdown of supramolecular organization. However, the subdivision on a microscopic level into hydrophilic 0022-3654/86/2090-1830$01.50/0
and hydrophobic domains alters the self-diffusion of the molecules 1-2 orders of magnitude.] In normal micelles NMR data clearly show that the chain termini within the micelles are “wet” on a ( I ) Fabre, H . et ai. J . Phys. Chem. 1981, 85, 3493.
0 1986 American Chemical Society
