J . Phys. Chem. 1989, 93, 41 16-41 20
4116
of exciting the B state of acetylene as well as the A state. Thus, it is possible that the electronically excited intermediate proposed2' to explain the 193-nm photolysis is not vinylidene. Finally, it should be mentioned that much of the early motivation for this study was provided by the large discrepancy that existed between the value for k , obtained in this laboratory8 and that determined by Laufer and Lechleider.6 This discrepancy has been completely resolved by the present study. When CO(c = 1 0) was monitored under conditions similar to those used by Laufer and Lechleider (100 mTorr of CF3C2H;100 mTorr of 0,; and 50 Torr of He), a rise time almost identical with that observed
-
previously by Laufer and Lechleider was obtained. It is now clear that this slow rise time resulted from a complex coupling of vibrational relaxation and two different formation mechanisms and should not be interpreted in terms of the rate constant for 0, reaction. the C,H
+
Acknowledgment. This work was supported by the Department of Energy under Grant DE-FG05-85ER 13439 and The Robert A. Welch Foundation under Grant C-1084. Registry No. C,H, 2122-48-7; 02,7782-44-7; CO, 630-08-0; CF,C,H, 661-54-1; C2H2, 74-86-2; CO,, 124-38-9.
Mechanism of Quenching of Electronically Excited Ruthenium Complexes by Oxygen Cliff J. Timpson,+ Charles C. Carter,*and John Olmsted III*st Department of Chemistry and Biochemistry and Department of Physics, California State University, Fullerton, Fullerton, California 92634 (Received: October 17, 19881
Stern-Volmer quenching studies have been carried out on the electronic excited states of ruthenium complexes of bipyridine, bipyrimidine, and bipyrazine, using as quenchers oxygen, 9-anthracenecarboxylic acid (an energy-transfer quencher), and nitrobenzenes (charge-transfer quenchers). Luminescence lifetimes were determined as a function of quencher concentration, by using single-photon counting with nanosecond resolution. The rate constants for quenching are always significantly lower than those for diffusion-controlled reaction, indicating that the rate-limiting step is charge or energy transfer within the encounter complex, which have unimolecular rate constants in the vicinity of IO9 s-'. Oxygen quenching shows significant variations with both ligand and solvent that are best explained by a mechanism dominated by energy transfer, even though the charge-transfer path is energetically allowed for [Ru(bpy)3]2+*.
among both organic' and inorganiclo systems; and a theoretical Introduction framework has been developed within which to try to account for Oxygen quenching of electronic excited states plays an imsuch slower quenching rates." portant role in a large number of photochemical systems, ranging We have undertaken a study of the quenching of rutheniumfrom aromatic organic compounds through inorganic complexes tris(diimine) complexes in which the energies of the charge-transfer to biochemically important systems such as porphyrins. The and energy-transfer products relative to the excitation energy of ubiquitousness of oxygen is partially responsible for its importance the complex can be systematically varied. The basic idea is to in quenching processes, but equally significant is the fact that it "switch off" one or the other of the possible quenching processes can quench in at least three distinct ways: by catalytically inducing by making it significantly endothermic. intersystem crossing,' by energy transfer to give singlet oxygen,, Recent synthesis and characterization of tris(che1ate) complexes or by electron transfer to give s ~ p e r o x i d e . ~ of ruthenium containing diimine ligands other than bipyridineI2 I n the case of inorganic complexes, both the energy-transfer have made it possible to "tune" the electrochemical properties of and the electron-transfer mechanisms can generally operate, and these c o m p l e ~ e s . ' ~In particular, R u ( b p ~ ) ~(bpz ~ + = bipyrazine) the determination of the relative importance of each is complicated is much more difficult to oxidize than is Ru(bpy)?+. As a result, by the possibility that superoxide can react rapidly with an oxidized the charge-transfer pair R~(bpz),~+-O,-is higher in energy than complex, in a reverse electron-transfer process, to yield singlet ~ x y g e n . Thus ~ either quenching mechanism could give rise to the same final products, and previous investigators have not been ( I ) Turro, N. J.; Renner, C. A.; Waddell, W. H.; Katz, T. J. J . Am. Chem. able to determine if singlet oxygen arises in a single, energySOC.1976, 98, 4320. transfer step or in a coupled pair of electron-transfer steps. This (2) Kearns. D. R. Chem. Reu. 1971. 71. 395. (3) Srinivasan, V. S.; Pcdolski, D.; Westr'ick, N . J.; Neckers, D. C. J . Am. ambiguity pertains both to the porphyrins5 and to metal-trisSOC.1978, 100, 6513. (diimine) complexes.6 Recent work on the [ R ~ ( b p y ) ~ ] ~ + - O ~ - Chem. (4) Miller, S. S.; Zahir, K.; Haim, A. Inorg. Chem. 1985, 24, 3978. system (bpy = bipyridine) has shown that electron-transfer (5) Cox, G. S.; Whitten, D. G.; Giannotti, C. Chem. Phys. Lett. 1979, 67, 51 1 . quenching between these species when independently generated (6) Demas, J. N.; Harris, E. W.; McBride, R. P. J. Am. Chem. SOC.1977, does not lead to singlet oxygen; by inference, it was concluded 99, 3547. that oxygen quenching of [Ru(bpy)J2+* must be exclusively via (7) Mulazzani, Q. G.; Ciano, M.; D'Angelantonio, M.; Venturi, M.; the energy-transfer path.' Rodgers, M. A. J. J. A m . Chem. SOC.1988, 110, 2451. Another point of interest concerns the magnitude of the uni(8) Lamola, A. A. In Energy Transfer and Organic Photochemistry; Lamala, A. A,, Turro, N. J., Eds.; Interscience: New York, 1967; p 17. molecular rate constant for energy transfer once the encounter (9) Turro, N. J. Molecular Photochemistry; Benjamin/Cummings: Menlo pair has formed by diffusion: under what conditions it is suffiPark, CA, 1978; p 331. ciently rapid to render energy-transfer quenching as diffusion ( I O ) Balzani, V. S.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Laurence, G. controlled. Although such quenching is conventionally considered S. Coord. Chem. Rec. 1915, I S , 321. ( 1 I ) Balzani, V.; Bolletta, F.; Scandola, F. J . Am. Chem. SOC.1980, 102, always to be diffusion controlled when sufficiently exothermiq8 2152. there are numerous examples of lower quenching efficiencies (12) Allen, G . H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J. Am. Chem. SOC.1984, 106, 2613.
Department of Chemistry and Biochemistry Department of Physics.
0022-3654/89/2093-4116$01.50/0
(13) Rillema, D. P.;Allen, G.; Meyer, T. J . ; Conrad, D. Inorg. Chem. 1983, 22. 1617.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4117
Quenching of Electronically Excited Ru Complexes Ru(bpy)32’
+3
PNB’
Ru(bpz),’+ +3PNB* R ~ ( b p z ) , ~++P N B
Ru(bpz)$’ + 0,
TABLE I: Lifetimes and Quenching Parameters for [Ru(L)#+* Quenched by O2 solvent dimethyl sulfoxide
i
1
t
water
propylene carbonate
Ru(bpy),’+ + 0, (a)
R u ( b p z ) p + 0, (b)
Ru(bpy),,+ (C)
+ PNB
Ru(bpz),,+ + P N B
N-methylformamide
(d)
Figure 1. Schematic diagram of energetic relationships for excited complexquencher combinations involving Ru(bpy),2+, Ru(bpz)32+,02,and p-nitrobenzaldehyde.
1 -propanol acetone
the excitation energy of R ~ ( b p z ) , ~ +whereas *, the charge-transfer pair R ~ ( b p y ) , ~ + - O is ~ -somewhat lower in energy than the excitation energy of R ~ ( b p y ) , ~ + * . There is no convenient way to ”tune” the energy of the oxygen excited states, but there are organic charge-transfer quenchers that lack low-lying excited electronic states but have reduction potentials that are nearly identical with that of O2.I4 The O2 reduction potential is somewhat solvent dependent but is about -0.85 V (vs SCE) in a number of organic solvents.lS Thus, a quencher such as p-nitrobenzaldehyde, whose reduction potential is -0.86 V vs S C E in a~etonitrile,’~ has charge-transfer energetic properties that are like those of oxygen but is incapable of inducing energy-transfer quenching. The energetics for four different complex-quencher combinations are schematically displayed in Figure 1. For combination (a), Ru(bpy)32+-02, both energy-transfer and charge-transfer quenching are exothermic; for (b), Ru(bpz)?++, energy-transfer quenching is exothermic but charge-transfer is endothermic; for (c), R~(bpy),~+-p-nitrobenzaldehyde(PNB), charge transfer is exothermic but energy transfer is endothermic; for (d), Ru(bpz)?+-p-nitrobenzaldehyde, both paths are endothermic. We therefore expect to see no quenching in (d), charge-transfer quenching only in (c), energy-transfer quenching only in (b), and perhaps to elucidate the quenching mechanism in (a) by appropriate comparison of the quenching rate constants. In hopes of elucidating the roles of the two possible quenching pathways for oxygen interacting with transition-metal complexes, we have carried out studies of the quenching rates for these combinations as well as others of intermediate energetics. This paper presents the results of those quenching studies.
Experimental Section Materials. RuCl3-3H20(Alfa), Ru(bpy),CI2, 2,2’-bipyrimidine (bpm), 2,2’-bipyrazine, p-nitrobenzaldehyde (all from Aldrich), and p-dinitrobenzene (Eastman) were obtained from the indicated commercial suppliers and were used without further purification. Propylene carbonate was purified by fractional distillation under reduced pressure. The ruthenium complexes of bipyrimidine and bipyrazine were prepared and purified by the method described by Rillema et aL13 Each complex was converted to the PF, salt by addition of excess KPF6 solution to an aqueous solution of the chloride complex. Following chromatography, each compound showed a single spot upon thin-layer chromatography analysis and gave luminescence lifetimes that were equal to or longer than reported values.I3 Solutions. Solutions of the complexes were prepared to give optical densities of about 0.5 a t the wavelength of maximum absorbance. Organic quencher concentrations were established by mixing appropriate aliquots of gravimetrically prepared stock solutions of quencher with stock solutions of a complex and then (14)Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J . A m . Chem. Soc. 1979, 101, 4815. (15)Peover, M. E.; White, B. S. Electrochim. Acta 1966, 11, 1061.
ethanol methanol chloroform
ligand
io, ns
780 1250 567 72 845 930 140 920 810 115 1040 805 88 605 800 125 810 660 645 595 835 865
k,, 109/(M s) k J k , 3.26 0.163 0.74 0.037 3.9 0.150 0.049 1.2, 0.022 0.58 2.77 0.140 0.99 0.050 0.020 0.39 2.2, 0.103 0.81 0.037 0.39 0.016 2.01 0.101 0.93 0.0465 0.39 0.020 l.g5 0.048 0.49 0.013 0.19 0.0049 1.08 0.043 0.49 0.020 1.32 0.041 0.20 0.0062 0.68 0.022
ket,
109/s 2.1 0.41 2.4 0.70 0.30 1.7 0.54 0.21 1.3 0.44 0.185 1.2 0.52 0.22 1.0 0.27 0.10 0.58 0.265 0.73 0.105
0.35
diluting volumetrically. Oxygen concentrations in the solutions were varied by saturating with O2or N2gas prior to measurements, and solutions containing organic quenchers were purged of 0, by bubbling with N, for 5 min prior to measurements. The oxygen concentration in air-saturated propylene carbonate was taken from the literature16 (mole fraction 5.45 X lo4) and the concentration when under 1 atm of 0, was computed assuming adherence to Henry’s law. Lifetime Measurements. All lifetimes were determined by using a time-correlated single-photon-counting apparatus constructed locally by using conventional components. Excitation was by means of a free-running air discharge with 5 4 s pulse duration, filtered through a Corning 450-nm bandpass filter, and emission was viewed through a Corning 600 nm band-pass filter. Count rates were kept below 10% of the pulse rate to minimize lifetime skewing from double events. Data were collected by using a multichannel analyzer until the peak channel contained a t least 7000 counts. The data set was truncated a t early times to eliminate the need for convolution and was then fitted to a single-exponential decay by using a weighted least-squares computer program that varied the background count level until a best fit, as measured by the shape of the autocorrelation function,” was obtained. Calibration of the time base was verified by determining the lifetime of Ru(bpy),*+* in degassed aqueous solution and comparing to the standard value’s of 575 f 10 ns. Our apparatus and computer fitting routine gave 567 f 10 ns.
Results Oxygen Quenching in Various Solvents. Lifetimes of the [Ru(L),] 2+* complexes (L = bipyridine, bipyrimidine, bipyrazine) were measured in oxygen-free, air-saturated, and oxygen-saturated solutions in a set of solvents of varying characteristics. Results for the unquenched lifetimes and the experimental quenching constants are given in Table I. Also listed in Table I are values for the ratio of the quenching constant k, to the diffusion constant kd (see Discussion for a description of how kd was evaluated); this ratio measures the fraction of bimolecular encounters resulting (16)Solubility Data Series, Battino, R., Ed.; Pergamon Press: Oxford, 1981;Vol. 7, p 183. (17) Grinvald, A,; Steinberg, I. Z . Anal. Biochem. 1974, 59, 583. (18) (a) Mandal. K.; Hauenstein, B. L., Jr.; Demas. J. N.: DeGraff, B. A. J . Phys. Chem. 1983,87, 328. (b) Hauser, A,; Krausz, E. Chem. Phys. Lett. 1987, 138, 355. (c) PRA Pulse, Quarterly publication of PRA International Inc , Autumn, 1986.
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The Journal of Physical Chemistry, Vol. 93, No. 10, 1989
Timpson et al.
TABLE II: Quenching Parameters for [RII(L)#+* Quenched by Organic Quenchers"
quencher 9-anthracenecarboxylic acid p-dinitrobenzene p-nitrobenzaldehyde oxygen
ligand bPY bPm bPZ bPY bPm bPZ bPY
eV 1.21 1.71
-0.27 -0.21
k,, 109/(M s) 2.4 2.0
1.81
-0.30
1.8
>0.6 >0.6 >0.6 >0.6 >0.6 >0.6
1.7 0.1 0 1.35 0
-0.16 0.34
0.49
bPm
0.01 0.51
bPZ
0.66
bPY bPm bPZ
0 0.50 0.65
AEET,' eV
-1.10 -1.04 -1.13
0 2.8
k,/kd
0.89 0.74 0.67 0.63 0.037
0
0
0.50
0.82d 0
0 0
.o
0.140 0.050
0.39
0.020
1
k,,, 109/s 6.7 2.3 1.7 1.4 0.032d
0 1.7
0.54 0.021
Solvent is propylene carbonate. Energy change associated with charge-transfer quenching, estimated from reduction potentials of quenchers, oxidation potentials of complexes, and excitation energies of complexes. Positive values are endothermic. CEnergychange associated with triplettriplet quenching, estimated from triplet energy levels. Positive values are endothermic. dLower limit; k,, cannot be easily computed since the kinetic analysis is complicated by possible contributions from the reverse transfer process, IC-,. See Discussion. 12,
1 .o
2.0
10:l Ill '"d"
Stern-Volmer plots of O2quenching of RU(L)~I** complexes in propylene carbonate solvent. Ligands are bipyridine, bpy; bipyrimidine, bpm; bipyrazine, bpz. Figure 2.
in quenching. Stern-Volmer plots for O2quenching in propylene carbonate, which are typical of the plots obtained for all solvents, are shown in Figure 2. Quenching by Organic Species. To further elucidate the role of charge-transfer and energy-transfer quenching, quenching rates were measured for [Ru(L)J2+* quenched by an organic species capable only of energy-transfer quenching (9-anthracenecarboxylic acid) and by two organic species capable only of charge-transfer quenching (p-dinitrobenzene, Eo = -0.69 V, and p-nitrobenzaldehyde, E' = -0.86 V). For these studies, propylene carbonate was chosen as the solvent, since the reduction potentials for the ruthenium complexes have been measured in that solvent. These results are collected in Table 11, in which the k,/kd ratio is also listed along with the value for k,, (intracomplex unimolecular charge- or energy-transfer rate constant; see Discussion) derivable from these results. Discussion We have examined how the quenching of [Ru(L)J2'* complexes varies with conditions, looking both at oxygen quenching in a variety of solvents (Table I) and a t quenching by a variety of species in a common solvent (Table 11). The correlation of k,, with energetic factors is evident from the data collected in Table 11, which shows that when both energy transfer and charge transfer are substantially endothermic-[Ru(bpz)J2+* quenched by p nitrobenzaldehyde-quenching does not occur, whereas if either energy transfer-[ Ru( L)J2+* quenched by anthracenecarboxylic acid-or charge transfer-[Ru(bpy),I2+* quenched by p-dinitrobenzene-is exothermic, relatively efficient quenching takes place. When both pathways are exothermi~-[Ru(bpy),]~+* quenched by oxygen-we wish to know whether the mechanism of quenching is via one or both processes. To elucidate that question, we need to examine the oxygen-quenching results collected in Table I.
The main features shown by the oxygen quenching of the three ruthenium tris(diimine) complexes are the following: (1) The excited states of all three complexes are quenched by oxygen, even though the charge-transfer mechanism is substantially endothermic ( > O S V) for both the bipyrimidine and bipyrazine complexes. (2) In all solvents, the rate constant for oxygen quenching becomes larger as the charge-transfer energetics become more favorable. (3) For none of the complexes does the quenching rate constant approach the diffusion-controlled limit; in the most efficient case, [Ru(bpy)J*+* in dimethyl sulfoxide, k, = 0.16kd. (4) When corrected for the "trivial" variation of k, with solvent viscosity, there is a small but real dependence of the quenching constant on solvent, as evidenced by variations both in the fractional efficiency k,/kd and in the computed energy-transfer rate constant k,,. From the first feature, we can unequivocally conclude that the direct energy-transfer mechanism of quenching plays a major role. The higher rate constants for quenching of the bipyridine complex than for the bipyrazine complex might a t first glance appear to indicate that, when it is exothermic, the charge-transfer mechanism is contributing to the quenching. However, we note that the quenching rate for the bipyrimidine complex is higher than that for the bipyrazine complex, even though the charge-transfer path is still much too endothermic for the charge-transfer rate to be measurable. Thus, the energy-transfer path must somehow be mediated by the charge-transfer energetics, as will be discussed below. The low quenching efficiency for oxygen quenching of Ru complexes can be contrasted with the extremely efficient energy-transfer quenching by anthracenecarboxylic acid (quenching fraction of 0.89 for [ R ~ ( b p y ) ~ J ~see + * ;Table 11) and equally efficient energy-transfer quenching by oxygen of aromatic organic excited singlet states, which is found to be virtually diffusion On the other hand, oxygen quenching of aromatic organic triplet states consistently occurs with an efficiency of or smaller because of the spin-statistical factor resulting from the fact that the two spin vectors of the interacting triplets can align in nine different ways, only one of which has the net singlet character needed for quenching to occur.2 Before carrying out a detailed analysis of the quenching kinetics, it is necessary to assess whether oxygen quenching of ruthenium complexes behaves in a singlet-like or triplet-like manner, that is, whether to expect the maximum quenching rate to approach diffusion control or of diffusion control. The following reaction scheme pertains to this assessment:
(19) Berlman, I . B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic Press: New York, 1971. (20) Ware, W. R.J . Phys. Chem. 1962, 66, 455.
Quenching of Electronically Excited Ru Complexes 5[ R u L32+
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4119
*. ..0,l
The two (formally) triplet-state species diffuse together at the diffusional rate given by the second-order rate constant k d . Quintet, triplet, and singlet encounter complexes are formed with the appropriate statistical probabilities, and each dissociates back to unchanged reactants with unimolecular rate measured by k4. Spin evolution occurs within the encounter complexes, linking all three encounter spin states, with unimolecular rate constant k,,. Energy transfer within the encounter complex occurs only in the singlet spin state, with rate constant k,,; this transfer step is sufficiently exothermic (- 1 eV) to be completely irreversible. If k,, > k4, the spin states become quickly scrambled and the quenching rate constant can approach kd. The back-diffusion rate constant k4 can be evaluated by using the Fuoss-Eigen approach (see below), giving values of ca. 1OIo/s for the solvents used in these studies. While spin evolution rates have not been determined for complexes of this type, cage escape yields for the [ R U ( ~ ~ ~ ) ~ + . - M complex V * + ] (MV = methylviologen) allow computation of the rate of back-electron transfer, which for that complex is equal to the rate of spin evolution from triplet to singlet.21 Since spin evolution is dominated by spin-orbit and d-d coupling, both of which arise almost entirely from the Ru site in the complex, the spin evolution rate constant within the [ R U L ~ ~ + * . - O complex ~] should be similar. This allows us to estimate k,, = 16 X 1010/s,2'an order of magnitude greater than k-d. On the basis of this analysis, we conclude that all [RUL,~+*.-Q] encounter complexes are close to the spin-scrambled limit; thus the appropriate upper limit for the quenching rate constant is kd, and kinetic analysis can be carried out without differentiating among encounter spin states. In further support of this conclusion, we find that when the data are analyzed assuming either that spin evolution does not take place or that it is consistently intermediate in rate, the computed energy-transfer rate constants do not form an internally consistent set of values. We assume that bimolecular quenching of an excited complex C* by a quencher Q is describable by diffusion together of a pair, quenching within the encounter complex, and diffusion apart of the products. Following the analysis of Balzani," we consider only conditions under which the separated products decay sufficiently quickly that back-diffusion followed by back-reaction can be neglected:
Two cases will be of interest to us: (a) significantly exothermic quenching; (b) near-isoenergetic charge transfer. Energy-transfer quenching of all three Ru complexes by either oxygen or 9anthracenecarboxylic acid fall within case (a), as does chargetransfer quenching of [Ru(bpy),] 2+* by p-dinitrobenzene. Charge-transfer quenching of [ R ~ ( b p y ) ~ ] ~by+ *oxygen or pnitrobenzaldehyde falls within case (b). All other potential quenching processes that we have examined are sufficiently endothermic (>0.3 eV) to be inoperative. In case (a), k,,
