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(3) C. F. Giese and W. B. Maier, J . Chem. Phys., 39, 739 (1963). (4) R. F. Barrow, P. G. Dodsworth, A. R. Downie, E. A. N. S. Jeffries, A. C. P. Pugh, F. J. Smith, and J. M. Swinstead, Trans. Faraday SOC., 51, 1354 (1955). (5) R. C. Pierce and R. F. Porter, J. Phys. Chem., 78, 93 (1974). (6) J. K. Kim, L. P. Theard, and W. T. Huntress, Jr., Int. J . Mass Spectrom. Ion Phys., 15, 223 (1974). (7) D. L. Smith and J. H. Futreil, J . Phys. B , 8, 803 (1975). (8) A. Flaux, D. L. Smith, and J. H. Futreii, Int. J . Mass Spectrom. Ion Phys., 15, 9 (1974). (9) G. Herzberg, "Molecular Spectra and Molecular Structure, I. Spectra of Diatomic Molecules", 2nd ed,Van Nostrand, Princeton, N.J., 1955. (10) C. E. Moore, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 35, Vol. 2 (1971). (11) C. R. Blakley, M. L. Vestal, and J. Futreil, J. Chem. Phys., 66, 2392 (1977), and references therein. (12) W. A. Chupka and J. Berkowitz, J . Chem. Phys., 54, 4256 (1971).
Gafney et al. (13) F. P. Lossing and G. P. Semeluk, Can. J . Chem., 48, 955 (1970). (14) Calculations based on proton affinity of H2obtained from ref 11, and heats of formation from J. L. Franklin, J. G. Diliard, H. M. Rosenstock, J. T. Herron, K. Draxl, and F. H. Field, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 26 (1969). (15) D. L. Smith and J. H. Futreil, Chem. Phys. Lett., 40, 229 (1976). (16) M. E. Schwartz and L. J. Schaad, J. Chem. Phys., 47,5325 (1967). (17) I. G. Csizmadia, R. E. Kari, J. C. Poianyi, A. C. Roach, and M. A. Robb, J. Chem. Phys., 52, 6205 (1970). (18) M. Karpius in "Molecular Beams and Reaction Kinetics", Academic Press, New York, N.Y., 1970. (19) Calculations based on heats of formation (see ref 14) and ionization Dotential of Zn obtained from ref 10. (20) P. F. Fenneiiy, R. S. Hemsworth, H. I. Schiff, and D. K. Bohme, J. Chem. Phys., 59, 6405 (1973). (21) F. C. Fehsenfeid, W. Lindinger, and D. L. Aibritton, J. Chem. Phys., 63, 443 (1975).
Photoinduced Electron Transfer Reactions between Tris(2,2'-bipyridine)ssmium(II) and Various Acidopentaamminecobalt(111) Complexes Edward Flnkenberg, Peter Fisher, Sze-Ming Y. Huang, and Harry D. Gafney" Department of Chemistry, The City Unlversity of New York, Queens College, Flushing, New York 11367 (Received October 19, 1977) Publication costs assisted by the Research Corporation and the Research Foundatlon of The Clty Universlty of New York
The quenching of the luminescence of O ~ ( b p y )by ~ ~a+number of acidopentaamminecobalt(II1) complexes leads to O ~ ( b p y )and ~ ~ +Co2+in equimolar amounts. The consistancy of the reaction stoichiometry as well as the high limiting yields of Co2+found with this low energy donor is presented as evidence of an electron transfer quenching mechanism. Consistent with the proposed mechanism, the limiting yield of Co2+found for the reduction of Co(NH&C12+by *Os(bpy);+ is 1.09 f 0.18 whereas that found for the reduction of the same complex by *Ru(bpy)3Z+ is 0.52 f 0.21. The difference in the limiting yields is thought to arise from the difference in the reduction potentials of the M ( b p ~ )complexes. ~~+ The overall efficiency of the redox reaction then reflects the rates of the back reactions relative to the rates of spin conversion and/or ligand dissociation. These reactions, which convert the reduced cobalt moiety to a form not readily oxidizable by the M ( b p ~ )complex, ~ ~ + must be competitive with the back reaction in order for a net reaction to occur.
Introduction The mechanism by which various inorganic substrates quench the luminescence of tris(2,2'-bipyridine)ruthenium(II), Ru(bpy):+, is a topic of current interest. There is now general agreement that the luminescent state of the complex, designated as * R ~ ( b p y ) ~possesses ~+, both oxidizing and reducing properties and, depending on the substrate used, the quenching mechanism may involve electron transfer,l energy transfer,2 or possibly both. Although a low energy donor, 2.08 eV, * R ~ ( b p y ) ~is ' +a strong reductant with a reduction potential estimated to be -0.84 V in aqueous ~ o l u t i o n .With ~ many individual quenchers, however, the choice of a quenching mechanism cannot be made completely free of a m b i g ~ i t y In . ~ deciding how quenching occurs, it may be informative, on the other hand, to look a t the effect of various properties of these excited-state reductants on the reaction efficiency. From our studies of the quenching of *Ru(bpy):+ by various C O ( N H ~ ) ~ X complexes ~+ and the subsequent photochemical reaction^,^ we conclude, like Navon and Sutin,lc that quenching occurs by an electron transfer mechanism and that the variation in the quantum yield of Co(II), is not due to unfavorable thermodynamics. Since * R ~ ( b p y ) ~a~strong +, reductant, is converted on electron transfer to a strong oxidant, Ru(bpy),3+ (E" = 1.24 V in 1M acid),6the variation in the limiting yield of Co(I1) is thought to reflect the kinetics of the reactions subse-
quent to the electron transfer step. These reactions, which convert the reduced cobalt moiety to a form not readily oxidizable by Ru(bpy)$+, must be competitive with reverse electron transfer in order for a reaction to occur. Preliminary experiments in our laboratory indicated that * O ~ ( b p y ) also ~ ~ +possesses strong reducing properties.7 Recently, Lin and Sutin have estimated the reduction potential of the luminescent state of this complex to be -0.96 Vn3 There are, however, substantial differences between these excited-state reductants. In aqueous solution, the radiative lifetime of *Os(bpy)?+ a t 25 "C, 19.2 n ~is ,considerably ~ less than that of * R ~ ( b p y ) ~which ~+, under similar conditions is 600 f 20 ns.lc Also the reduction potential of O ~ ( b p y ) (E" ~ + = 0.88 V in 1 M acid) is less than that of R u ( b p ~ ) ~These ~ + . differences might then be exploited to yield some information on the quenching mechanism. If an energy transfer mechanism were operative, the lower energy and shorter lifetime of *Os(bpy)?+ suggests, a priori, that this donor would be less efficient than *Ru(bpy):+. In an electron transfer mechanism, on the other hand, the inverse would be expected. Larger limiting yields of electron transfer products are thermodynamically favored because of the lower reduction potential of the Os(bpy),3+ ~ o m p l e x . ~ In this paper, we report the results of a study of the photochemical reactions which occur when Os(bpy):+ is irradiated in the presence of various Co(NH3)6Xn+com-
0022-3654/76/2082-0526$01.00/00 1978 American Chemical Society
Mechanlsm of Luminescence Quenching
plexes. The quantum yields of reduction of the Co(II1) complexes, reaction stoichiometries, and quenching constants have been measured. Due to the shorter lifetime of * O s ( b p ~ ) , ~the + , quantum yields of Co(I1) at a given concentration of the Co(II1) complex are less than those found with Ru(bpy)?+. On extrapolation to infinite Co(II1) concentration, however, the limiting yields of Co(I1) are essentially unity. This result, which is thought to reflect the lower reduction potential of Os(bpy),3+, and the pattern of reactivity found with the Os(bpy)32+,are presented as evidence of an electron transfer mechanism.
Experimental Section Materials. The acidopentaamminecobalt(II1)complexes were prepared by published methods.8 The absorption spectra of the complexes, which were twice recrystallized as nitrate salts by the addition of H N 0 3 and/or NaN03, agreed with published spectraag The Os(bpy),Clz.3HZ0was prepared by literature methoddo and purified by chromatography on a 122 cm X 4 cm Cellex-P column (Bio-Rad control no. 13552). The reaction mixture was placed on the column, washed with distilled water, and eluted with 0.05 M HC1. On evaporating some of the eluants to dryness, however, a reddish material was found on the edges of the evaporating dish, To eliminate this impurity from all of the evaporated samples, the dark green Os(bpy),Cl,.SH,O was removed from the evaporating dish with care not to include the red material. The solid was dissolved in distilled water, returned to the reconditioned column, and again eluted with 0.05 M HC1. On evaporating these eluants, lustrous dark green crystals were obtained whose absorption spectrum was in excellent agreement with published spectral1 (at 483 nm, E is 1.31 X lo4 M-l cm-l ). O ~ ( b p y ) , ~was + prepared in situ by Clz or PbOz oxidation of known concentrations of Os(bpy):+. The other chemicals used were reagent grade. All solutions were prepared with water distilled in a Corning distillation apparatus and adjusted to an ionic strength of 0.5 M with 0.166 M Na2S04and 0.01375 M NaHS04 (see below). Emission Measurements. The phosphorescence measurements were made on a Perkin-Elmer Hitachi MPF-2A emission spectrophotometer equipped with a red sensitive Hamamatsu R818 photomultiplier. Due to the short lifetime of the luminescent state, relatively high concentrations of these Co(II1) complexes were required and standard equations were used to correct the observed emission intensities for trivial effects.lc To minimize these corrections, the samples were excited at 640 nm and the emission monitored at the uncorrected maxima of 720 nm. With many of these Co(II1) complexes it was not possible, due to limited solubilities, to obtain reliable quenching data beyond ca. 20% quenching. Within this limited range, however, the Stern-Volmer plots were found to be linear and in good agreement with the photochemical data. Photolysis Procedures. The photolysis apparatus used in these experiments has been previously described.12 Solutions at room temperature, 23-24 "C, containing M Os(bpy)?+ and varying amounts of the Co(II1) complex, were irradiated at 436 nm in 1-or 5-cm spectrophotometer cells. To obtain reliable and reproducible values of C $ C ~ ~ ) , we found it was necessary to vigorously stir the solution by nitrogen or helium bubbling during photolysis. The bubbler was centered in the photolysis cell and did intercept the light beam, but tests with Fe(Cz0,)33-showed that bubbling had no measurable effect on the measured intensity. The light intensity was measured by ferrioxalate actinometry1, before and after photolysis and an average value of the intensity was used to calculate the quantum yield. The variation in intensity was less than 5%.
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978 527
Flash photolysis experiments made use of previously described equipment." The samples were exposed to unfiltered 250-5 flashes. Analytical Procedures. Cobalt(I1) was analyzed by the thiocyanate procedure of Kitson.14 Based on the formation of a tetrahedral Co(I1)-SCN- complex in a 50% acetone-water mixture, this colorimetric procedure is the only analytical technique we are aware of with sufficient sensitivity to analyze the small amounts of Co(I1) formed in these experiments. Unfortunately, we have found the procedure fraught with difficulties. The ionic strength chosen for these experiments, 0.5 M (0.166 M NaZSO4and 0.01375 M NaHS04) was dictated by this analytical procedure. With higher concentrations of Na2S04or other electrolytes such as NaC1, NaC104, and NaOOCCH3, precipitation or cloudiness occurred on adding the acetone. In a number of tests with known amounts of CoClz added to the solution, the precipitates were centrifuged off and the absorbance of the centrifugate measured at 625 nm. The amount of Co(I1) calculated from the absorbance of the centrifugate, however, was found to be variable and generally less than the added amount. An additional difficulty with the procedure, at least for these systems, is that at the analyzing wavelength of 625 nm, the oxidation of Os(bpy),2+ causes a large spectral change. Experiments with Os(bpy),3+ showed that the complex could be quantitatively reduced the O~(bpy)~'+ by the excess amounts of I-, NO,, OH-, and SCN-. With the exception of SCN-, these reducing agents were not used because undesirable side reactions occurred during the Co(I1) analysis, or, with some of the Co(II1) complexes, precipitation occurred on adding the acetone. Since a large excess of SCN- is used in the Co(I1) analysis, this reagent was used to both reduce the Os(bpy)gS+to Os(bpy),2+ and complex the Co(I1). Tests showed that on adding the SCN- to known amounts of O ~ ( b p y ) ~and ~ + CoC12, a quantitative reduction of Os(bpy)gS+to Os(bpy)?+ occurred and, on dilution with acetone, yielded an extinction coefficient of 1.80 f 0.07 X lo3 M-l cm-l at 625 nm for the Co(I1)-SCN- c0mp1ex.l~ The procedure adopted for the analysis of the photolyte was to measure the decrease in absorbance at 650 and 625 nm following photolysis. From the change in absorbance at 650 nm, the concentration of Os(bpy),3+formed in the photochemical reaction was calculated. An aliquot of 6 M NH4SCN was added to the photolyte and the absorbance again measured at 625 nm to ensure a quantitative reduction of O~(bpy),~+. The sample was then diluted to volume with water and acetone and the absorbance of the Co(I1)-SCN- complex was measured at 625 nm relative to an unirradiated sample treated in an identical manner. Physical Measurements. Absorption spectra were recorded on either a Cary 14 or Techtron 635 spectrophotometer. The latter instrument, equipped with thermostated cell holders, was connected to a Haake FK2 constant temperature bath and was used to measure the thermal reduction of Ru(bpy),3+ or Os(bpy),3+. Infrared spectra were recorded on a Perkin-Elmer 635B spectrophotometer calibrated against polystyrene. A Beckman SS2 expandomatic pH meter, standardized with a Beckman pH 7 buffer, was used to make the pH measurements. Results Similar to a thermal oxidation of Os(bpy)Z+ to Os( b ~ y ) 3 by ~ + either Clz or Pb02, extensive photolysis of ~ ~ + M in CO(NH,)~X~+ solutions lo4 M in O ~ ( b p y )and causes a change from a dark green to a pale red color. Spectra recorded periodically during the photolysis show
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TABLE I: A Summary of Various Reaction Parameters for the Reduction of CO(NH,)~X"+Complexes by *Os(bpy)3a+
Complexa
Stoichiometry mol of Co(II)/ mol of Co(II)/ mol of R ~ ( b p y ) ~ ,mol + of O s ( b ~ y ) ~ ~ '@ c ~ ( I I ) " ~ ~
RH,O~+
1.09 f 0.16
-1
KsV,CM-' 4t 2 -2 7+2 4f 2 11 f 2 (11 f 2) 17 .I: 4 (14i 2)
1.03 f 0.05 RCO,' 1.03 t 0.27 -1 RN,2' 0.99 t 0.07 0.94 f 0.21 -1 RSO,+ 0.97 f 0.07 0.97 f 0.19 -1 1.03 f 0.06 RC12+ 0.92 i: 0.21 1.09 ?r 0.18 RBrz' 1.01 t 0.05 0.89 i: 0.27 1.11 f 0.21 R'C,O,+ 0.97 f 0.05 a R = Co(NH,),; R' = Co(NH,),. Extrapolated from Figure 2, estimated uncertainty (see text). Figure 2, value in parentheses from luminescence quenching experiments.
I
Ir)
-
'0 X 0
\ \
0
', \
I
400
I
I
500
I
I
\
I
\
'\
600
wavelength (nm)
Figure 1. The absorption spectra of Os(bpy);+ (-) and Os(bpy);+ (----)recorded in a 0.166 M NaS0, and 0.01375 M NaHSO, aqueous solution.
a progressive decline in absorbance throughout the 500680-nm region, which, from the spectra of Os(bpy),2+and O s ( b p ~ ) ~Figure ~ + , 1, suggests that an oxidation of the Os(I1) complex occurs during the photochemical reaction. Treating the photolyte with an excess of a reducing agent such as NOz-, I-, or SCN- quantitatively regenerates the original amount of Os(bpy),2+. Diluting the solution treated with SCN- with acetone also indicated the presence of Co(I1). Although these experiments gave a qualitative indication of the reduction of the Co(II1) complex, the quantitative measurement of the amount of reduction was made difficult by the spectral change at 625 nm due to the oxidation of O ~ ? b p y ) ~ ~ + . Unlike Ru(bps)d+ which slowly reverts to Ru(bpy),'+ on standing,'l O&bpy),3+ is much more stable. At 25 "C and an ionic strength of 0.5 M (0.166 M NaS04 and 0.01375 M NaHS04), we were unable to detect any increase in absorbance of a M O ~ ( b p y ) solution ~~+ over a 24-h period. Similarly, in a 2.65 X low3M Na00CCH3-1.25 X M HOOCCH3 buffer at 25 OC, no change in absorbance was detected over a 7-h period. On the other hand, Os(bpy),3+ is readily reduced by a number of anions. As mentioned above, however, a number of these anions caused difficulties in the Co(I1) analysis and NH4SCN was chosen to reduce the Os(II1) complex and complex the Co(I1). Adjusting the concentration of a lo4 M O~(bpy),~+ solution to 1.2 M in NH4SCN (the concentration of NH4SCN used in the Co(I1) analysis) reduced the Os(II1) complex within the time of mixing. Absorbance measurements at 625 and 650 nm indicated the reduction was
kh, M-'S-' X lo*
2.1 f 1.0 -1 3.7 f 1.2 2.1 t 1.0 5.7 f 1.0 8.9 i: 1.1
Obtained from
quantitative and was unaffected by these Co(II1) complexes. To calibrate the spectrophotometric analysis for M Os(bpy),3+, 5 X Co(II), solutions which contained M C O ( N H ~ ) ~ Xand ~ +varying , amounts of CoClz were made 1.2 M in NH4SCN. The increase in absorbance a t 650 nm indicated that the Os(II1) complex was quantitatively reduced. The solution was then diluted with water and acetone and the absorbance measured relative to a solution which contained the same concentrations of Co(NH3)5Xn+and Os(bpy)gS+. Using this procedure and concentrations of CoClz of 7.5 X loM5to M, an extinction coefficient of 1.80 f 0.07 X lo3 M-l cm-l was found for the Co(I1)-SCN- complex. This value is in good agreement with the reported value1*and this procedure was adopted for the analysis of the photolyte. Due to dilution errors, however, some uncertainty in the measurement of the Co(I1)-SCN- absorbance was introduced. A t lower concentrations of the Co(II1) complexes, the uncertainty is appreciable and we estimate the uncertainty in -&o(II) to be ca. 15%. The progressive decline in absorbance throughout the 500-680-nm region during photolysis and the eventual formation of a pale red solution indicates the formation of Os(bpy),3+. In one experiment, an aliquot of the photolyte was treated with 0.01 M NaNOz to reduce the Os(II1) complex and then with 0.01 M FeS04.(NH4)zS04 to determine if free bipyridine was present in the photolyte. Relative to an unphotolyzed solution, no increase in absorbance at 510 nm was detected indicating that &, I 5X This result and the regeneration of the origind amount of Os(bpy),'+ on adding NH4SCN indicates a one electron oxidation of the complex without disruption of the coordination sphere. The stoichiometries, summarized in Table I, indicate that the photochemical reaction is *Os(bpy)32++ Co(NH,),X2+ + O ~ ( b p y ) , ~++ Co(I1) + 5NH, -t X-
(1)
Within the experimental error, $co(II) was found to be independent of whether the solutions were bubbled with compressed air, Nz, or He. To obtain reproducible values of 4co(II), however, it was necessary to vigorously stir the solutions during photolysis. For this reason, the results reported here were obtained in solutions stirred by Nz or He bubbling. Due to the limited solubility of these Co(II1) complexes and the short lifetime of *O~(bpy):+,~ it was not possible to obtain quenching data beyond 20% quenching. Within this limited range, the more efficient quenchers, CO(N#3)5Br2+and C O ( N H ~ ) ~ C yielded ~ ~ + , linear plots with Ksy)sof 14 f 2 and 10 f 3MW1, respectively. With the other Co(II1) complexes, however, the data were scattered, but did indicate that the Ks,)s were less than 10 M-l. It was apparent that the photochemical data could be measured with higher precision and reproducibility. Thus, the
Mechanism of Luminescence Quenching
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978 529
solutions containing 5 X M of the bipyridine complexes were exposed to unfiltered 250-5 flashes. The R ~ ( b p y ) solutions ~~+ were analyzed at 452 nm and the at 480 nm. In distilled water, both O ~ ( b p y ) solutions ~~+ R ~ ( b p y ) ~and ~ + Os(bpy),2+ exhibit a slight transient bleaching. Consistent with the observations of Meisel and co-workers, we find that the recovery of the absorbance of both bipyridine complexes takes place in two steps: a relatively fast process which occurred within 70-80 ps and a slower process which occurred on the order of 1 ms or longer. Since the more rapid process has been discussed in the study mentioned above,16 we will limit this discussion to the results we have obtained on these slower processes. For times 2 100 ps, plots of In (A0 - A,) vs. time were linear and yielded a rate constant for the regeneration of the Ru(bpy)?+ absorbance of 9.2 f 0.3 X lo3 s-l while [CdNH,),X"']-' that for the regeneration of the Os(bpy)Qs+absorbance was 1.0 f 0.2 x IO3 s-l. Although O ~ ( b p y ) ~due ~ + to , its high Figure 2. The dependence of cc(rI)on the concentration of the various absorptivity through the visible region, absorbs a larger Co(II1) complexes. fraction of the flash energy, extrapolating the plots to t majority of the KBv)slisted in Table I were obtained from = 0 indicated that the amount of reaction induced in the vs. ~ / [ C O ( N H ~ ) ~ X "With + ] . the less efplots of 1/q5co(II) O ~ ( b p y ) sample ~ ~ + was approximately half of that which ficient quenchers C O ( N H ~ ) ~ S OCO(NH3)5C03t, ~+, Cowas induced in the Ru(bpy)QP+sample. At a pH of 3 (0.166 (NH3)5H203+,and CO(NH~)~N$+, least-squares analysis of M NaZSO4and 0.01375 M NaHSOJ, the rate of recovery the data in Figure 2 gave intercepts which were less than of the R ~ ( b p y ) absorbance ~~+ was slower, h = 1.89 f 0.23 one or negative. Unlike similar experiments with Rux lo2 s-l while flash photolysis of O ~ ( b p y )induced ~ ~ + only (bpy)?+ where the quantum yields are approximately 30 a very slight bleaching. Due to noise, however, it was not times larger, the principal cause of these unusual intercepts possible to quantitate this rate, but it appears to be on the appears to be due to the uncertainty associated with order of 0.1 s-l. The oscilloscope traces also showed that measuring $c0(II)a t lower Co(II1) concentrations. Rethe regeneration of the R ~ ( b p y ) , ~absorbance + was gardless of whether the mechanism is energy transfer or complete. The trace recording the change in the Oselectron transfer, it would be unlikely that the limiting (bpy)QP+sample, on the other hand, did not return to the yields are greater than one. Thus, the values listed in initial absorbance of the sample. Spectra recorded before Table I are taken to be one. The dependence of 1/C#Jc0(~~) and after the flash show approximately a 10% decline in on l/concentration of Co(NH3),C12+and C O ( N H ~ ) ~isB ~ ~ +absorbance throughout the visible region. The experiment not as steep and the quantum yields at lower Co(II1) was repeated using steady-state photolysis techniques (Aex concentrations could be measured with greater precision. 436 nm), but no change in the absorption spectrum was For these Co(II1) complexes, least-squares analysis gave detected. The spectral change induced by the flash persists limiting yields of unity (Table I). The bimolecular rate for a considerable period of time; the spectra of aliquots constants, h b , were calculated from the expression K,, = recorded periodically over 6 h gave no indication of rehbq, where T,, the radiative lifetime of *Os(bpy)QP+, is taken generating the original O~(bpy),~+. The spectral change to be 19.2 ns. induced by the flash is consistent with the formation of The pH dependent transient bleaching observed in Os(bpy)$+, but the exact nature of the reaction(s) which + to the previous flash photolysis studies of R ~ ( b p y ) , ~led cause this change are not presently known. Suffice it to suggestion that a bipyridine ligand partially dissociates say, at this point, that the flash photolytic behavior of from the Ru(I1) ion and this species can act as a reducthese bipyridine complexes is qualitatively similar. Extant.15 The agreement between the Stern-Volmer constant trapolating plots of In (A, - A,) vs. time to t = 0 and the obtained from luminescent quenching and that obtained amount of bleaching occurring during the flash also X ~ + ) , as well from plots of l/q5 vs. ~ / ( C O ( N H ~ ) ~ however, suggests that the efficiency of the photoinduced reaction as the low efficiency of formation of this intermediate, C#J of O ~ ( b p y ) is ~ ~less + than those of R~(bpy)~O+. in contrast to the relatively high limiting yields Discussion of Co(II), q5Co(II)lirnI0.3 (see below), leaves little question In a study of this type, the fundamental question to be that the partially dissociated species is not the principal reductant of these Co(II1) complexes. On the other hand, resolved is the mechanism by which quenching and senthe yields of Co(I1) found in these experiments with sitization occur. A number of experiments have estabOs(bpy)$+ are considerably smaller than those found with lished the reducing properties of the luminescent state of Ru(bpy)QP+and we were concerned that a partially disRu(bpy)32+.1 In a recent paper, Lin and Sutin have presented evidence that the luminescent state of Ossociated Os(I1) complex might play some role in the reduction of these Co(II1) complex. To explore the possi(bpy)Qs+also possess similar proper tie^.^ The calculated bility and to compare the behavior of these bipyridine oxidation potentials of these excited state reductants complexes, a number of flash photolysis experiments were indicate that they are strong reducing agents, yet some carried out. As these experiments were being completed, ambiguity remains in the choice of a quenching mechanism a comprehensive study of the flash photolysis and pulsed for systems such as these. The data gathered by Navon radiolysis of R ~ ( b p y ) ~was ~ +published,16 but it was and Sutin on the R U ( ~ ~ ~ ) , ~ + - C O ( Nphotochemical H~)~X~+ gratifying to see that our results were in general agreement reactions suggest an electron transfer mechanism.lc A with those of Meisel and co-workers. Having a different point of contention in the interpretation of the original objective, however, our experiments were carried out under experiments has been the stoichiometry of the photodifferent conditions and monitored only the bleaching of chemical reaction. Subsequent experiments in this labthe bipyridine complexes.- Room temperature aqueous oratory, which are summarized in Table I, have shown that
-
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The Journal of Physical Chemistry, Vol. 82, No. 5, 1978
Gafney et al.
the stoichiometry of the photochemical reaction is Ru(bpy),*++ Co(NH,),X"' Coz++ 5NH, t X"-9
hv
Ru(bpy),,+ t (2)
where X = Br-, C1-, Sot-,and C2042-. In this regard, the stoichiometry found with C O ( N H ~ ) ~ C ~ifOinterpreted ~+, in terms of an energy transfer mechanism, requires the oxalate radical to be an oxidant, yet such behavior differs from previous studies which have shown that it is a reductan t.9d The reaction stoichiometries found in these experiments with O ~ ( b p y ) ~Table ~ + , I, are also consistent with an electron transfer mechanism, but additional information germane to the choice of mechanism can be obtained from the limiting yields of Co(I1) found with Ru(bpy)?+ and those found with O ~ ( b p y ) ~ ~As+ .mentioned in the Introduction, there are significant differences in the properties of the luminescent excited states of these Os(I1) and Ru(I1) complexes. Since the intersystems crossing yields are unity in both c o m p l e x e ~ the , ~ ~ shorter lifetime and lower energy of *Os(bpy)$+ suggests that it would be a less efficient energy transfer reagent than the more energetic and longer lived *Ru(bpy)gP+. If an electron transfer mechanism were operative, however, the lower oxidation potential of Os(bpy),3+ (0,88 V in 1M acid) as compared to that of Ru(bpy),3+ (1.24 V in 1 M acid) would favor higher limiting yields of electron transfer product^.^ In our studies, we generally find larger limiting yields of Co(I1) with O ~ ( b p y ) , ~than + with R ~ ( b p y ) ~ ~For + . ex~ ample, under identical conditions of ionic strength (0.166 M NaS04 and 0.01375 M NaHS04),temperature, and pH, the limiting yield of Co(I1) found with Ru(bpy)?+ and Co(NH3)&lZ+is 0.52 A 0,21, whereas that found with Os(bpy)t+ and C O ( N H ~ ) ~ is C essentially P unity, 1.09 f 0.18. The uncertainty in the limiting yields is large, nonetheless the results do coincide with the behavior predicted by an electron transfer mechanism. In comparing the two possible mechanisms, we find it difficult to accept an energy transfer mechanism because of the consistency of the stoichiometry of these reactions and the low energy required for the presumed T T state of these Co(II1) complexes. With R ~ ( b p y ) , ~reduction +, of these Co(II1) complexes requires an energy of the presumed 3CT state of 1,64 pm-', while these experiments with Os(bpy)t+ further demand a ,CT state energy of 1.35 prn-l. Wet1*and others,lc have noted that these energies are unrealistically low and require exceptionally large 1CT-3CT splittings. It is difficult to conceive that both conditions, 3CT energies of 11.35 pm-l and radicals which quantitatively oxidize these bipyridine complexes, are simultaneously met by all of the Co(II1) complexes studied here. Thus, we interpret the data within the hypothesis of an electron transfer mechanism. Within this hypothesis then, the limiting yields of unity found in these experiments with Os(bpy):+ (Table I) indicate that all quenching encounters lead to electron transfer and that a quenching reaction sdch as catalytic deactivation of *Os(bpy)32+must be inefficient or nonexistent. Quite different results are obtained with Ru(bpy)32+and C O ( N H ~ ) ~where C ~ ~ +the limiting yield is 0.52 f 0.21. Since the limiting yield is less than one, the photochemical data do not rule out the possibility of catalytic deactivation of *Ru(bpy)?'. Demas and Addington have pointed out, however, that deactivation of *Ru(bpy)QP+by either a spin-orbit coupling or a paramagnetic mechanism is remote with these diamagnetic Co(II1) complexes.2b Navon and Sutin have also reported that the efficiency of electron transfer per quenching encounter between *Ru(bpy)?+ and Co-
(NH3),C12+to be 0.86 f 0.15 in 0.5 M H2S04.1C These results indicate that catalytic deactivation of *Ru(bpy)t+ must also be inefficient or nonexistent. The reduction potentials or *Ru(bpy),2+and *Os(bpy)t+ While are calculated to be -0.84 and -0.96 V, re~pectively.~ the standard potentials of these Co(II1) complexes are unknown, the half-wave potentials are more positive than that of Co(NH3)8+which has a standard reduction potential of 0.1 V.18 Thus it is generally agreed that these redox reactions proceed with a favorable driving force.lc In view of this favorable driving force and the above results, it seems reasonable to assume, in the subsequent analysis, that the efficiency of electron transfer per quenching encounter is essentially 100%. With this assumption in mind, the data gathered in this study suggest the general reaction sequence represented by 3-8, where
Ia@ic
M(bPY),a+
*M(bPY),a+
(3)
k
*M(bpy),a' -2M(bpy)3z++ heat
(4)
k
+ hv *M(bpy),l++ Co(NH,),X"+ 5 M(bpy),,+, *M(bpy),'+ -2 M(bpy),"
Co( NH,),Xtn-'
(6) k,
I-
(5)
1
!%
M(bPY)q2+t Co(NH,),X"+
(7)
M(bpy),'+ + Co2++ 5NH, t Xtn-,
(8 1
M represents ruthenium or osimium. I , denotes the absorbed light intensity, ca. einstein/L min, and dic denotes the intersystems crossing yield to form the luminescent charge-transfer state of the bipyridine complexes. The latter is assumed to have a value of unity in the following e q ~ a t i 0 n s . l ~Equation 6 represents the quenching reaction which, as mentioned above, is assumed to lead to electron transfer products with 100% efficiency. The redox products, M(bpy)$+ and C O ( N H , ) ~ X + ~are -~, enclosed within square brackets to indicate the formation of these products within a solvent cage. The brackets do not imply composition of structure, but simply imply that these reactive products exist in an effective reaction volume or solvent cage for some finite period of time. The remaining equations then represent back electron transfer to form the initial reagents, k7, and dissociation to products, k,. Steady-state analysis of the reaction sequence yields for emission quenching
& / I = 1 f 70k6[CO] (9) where T~ = l / ( k 4 + k5) and Toke = Ksv. The majority of the Ks;s listed in Table I, however, were obtained from plots of l/&,(II) vs. 1/[Co(NH3),Xn+] where the variables are related by the equation
With the more efficient quenchers Co(NH3)&12+ and CO(NH~)~B luminescence ~~+, quenching yields Ks,)s of 10 f 3 and 14 f 2 M-l, respectively. These values agree with the respective values of 11 f 2 and 17 f 2 M-l calculated ~ ~the + from the data in Figure 2 and establish * O ~ ( b p y ) as reductant of these Co(II1) complexes. A similar comparison could not be made with the remaining Co(II1) complexes, but the K,'s and limiting yields obtained from
Mechanism of Luminescence Quenching
Figure 2 indicate a short-lived reductant which is formed in high efficiency. Since the flash photolysis experiments indicate that the Os(I1) transient is long lived, ca. 10-100 ms, and formed in low efficiency, reduction by this transient species can be ruled out and the principal reaction is that given by eq 1. An unusual aspect of these reactions is that a strong reductant, * M ( b ~ y ) ~is~converted +, on electron transfer to a strong oxidant, M(bpy)$+. Thus, the oxidant must undergo an irreversible reduction in order for a net chemical reaction to occur. The results of these experiments with O ~ ( b p y ) ~summarized ~+, in Table I, indicate that the limiting yields of Co(I1) are unity. These limiting yields then imply that k8 > k,. In other words, those processes which convert the reduced cobalt moiety to a form not readily oxidizable by Os(bpy),3+, a doublet to quartet spin conversion within the reduced cobalt moiety and dissociation of the coordinated ligands, are more rapid than the back electron transfer reaction, eq 7. In the reduction of Co(NH3)&12+ by *Ru(bpy)?+, however, the limiting yield is less than one. As discussed above, if we assume that each quenching encounter leads to electron transfer, the diminished yield must reflect the reactions subsequent to the quenching reaction. The limiting yield of 0.52 f 0.21 then implies that k7/k8is 1.3 f 0.9 and that the back reaction is competitive with the spin conversion and/or ligand dissociation of the cobalt complex. The exact rate of these processes are not known for this cobalt complex. The analogy is not precise, but the rate of dissociation of the first two ammonia ligands from CO("~)~'+, k I lo6 s-l,19 and the spin relaxation I30 s,~Oleads us to suspect that the times of C~(terpy),~+, rates of these rather exothermic back reactions are competitive with the spin conversion and/or ligand dissociation of these reduced cobalt complexes. The difference in the limiting yields found with Ru( b ~ y ) and ~ ~ Os(bpy)Qa+ + is attributed to differences in the rate of the back reactions. Quantitating this difference in terms of Marcus theory, however, flounders in the absence of the self-exchange rates and the standard potentials for these cobalt complexes. Qualitatively, however, the data which are available on these bipyridine complexes suggest that the rate of the back reaction would be more rapid with Ru(bpy)$+ than with Os(bpy)$+. The self-exchange rate for the R ~ ( b p y ) ~ ~reaction + / ~ + is taken to be 2 X lo9 M-l s-1,1f,21 while an estimate based on the Fe(DMP):+l3+ reaction (DMP denotes 4,7-dimethyl-l,lO-phenanthroline) suggests that the O ~ ( b p y ) ~ ' +self-exchange /~+ rate is 3 X lo8 M-l Even if we assume that the intrinsic barriers to electron transfer are the same for Ru(bpy),3+ and Os(bpy)$+, a faster rate might be expected with Ru(bpy),3+ because of the additional 0.36 V driving force. This favorable driving force is a clear disadvantage in applying these Ru(I1) excited-state reductants to solar energy conversion. Perhaps the more highly absorbant Os(I1) complexes warrant closer attention since their disad-
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978 531
vantage, a short excited-state lifetime, could be rectified by working in the solid phase. Conclusion The quenching of the luminescence of Os(bpy),2+by a number of acidopentaamminecobalt(II1) complexes leads to Os(bpy),3+ and Co2+. The quenching reactions are interpreted in terms of an excited-state electron transfer reaction. Within this hypothesis, the larger limiting yield observed with Os(bpy)?+ suggests that the overall efficiency of the reaction is determined by the rate of the back reaction relative to the rate of the reactions which convert the reduced cobalt complex to a form not readily oxidizable by the M ( b p ~ ) complex. ~~+
Acknowledgment. Financial support of this research from the Research Corporation and The Research Foundation of The City University of New York is gratefully acknowledged. References and Notes (1) (a) H. D. Gafney and A. W. Adamson, J. Am. Chem. Soc., 94,8238 (1972);(b) J. N. Demas and A. W. Adamson, ibid., 95,5159 (1973); (c) G. Navon and N. Sutin, Inorg. Chem., 13,2159 (1974);(d) C. R. Bock, T. J. Meyer, and D. G. Whitten, J. Am. Chem. Soc., 96, 4710 (1974);97,2909 (1975);(e) C. Creutz and N. Sutin, ibid., 96, 6384 (1976);Inorg. Chem., 15,496 (1976); (f) R. C. Young, F. Richard Keene, and T. J. Meyer, J. Am. Chem. Soc., 99, 2468 (1977). (2) (a) J. N. Demas and A. W. Adamson, J. Am. Chem. Soc., 93,1800 (1971);(b) J. N. Demas and J. W. Addington, ibid., 98,5800 (1976); (c) M. Wrighton and J. Markham, J. Phys. Chem., 77,3042 (1973). (3) C. T. Lin and N. Sutin, J . Phys. Chem., 80,97 (1976). (4) P. Natarajan and J. F. Endicott, J. Fhys. Chem., 77,971,1823 (1973). (5) P. K. Lam and H. D. Gafney, unpublished observations. (6) F. E. Lytle and D. M. Hercules, Photochem. photobiol., 13,123 (1971). (7) E. Finkenburg and H. D. Gafney, "Excited State Electron-Transfer Reactions of Trls(2,2'-bipyridlne)osmium(II)",Proceedings of the East Coast Science Conference, April, 1975. (8) (a) H. H. Willard, H. Diehl, and H. Clark, Inorg. Syn., 1, 186 (1939); (b) H. S.Booth, ibid., 1, 187 (1939);(c) F. Basolo and K. Murmann, /bid., 4, 171 (1953);(d) M. Linhard and H. Flygave, 2.Anorg. Allg. Chem., 262, 328 (1950); (e) H. Siebert, ibid., 327, 63 (1964). (9) (a) C. K. Jorgensen, "Absorption Spectra and Chemical Bonding in Complexes", Addlson-Wesley, Reading, Mass., 1962; (b) V. Balzani et al., Inorg. Chim. Acfa Rev., 1, 7 (1967);(c) A. W. Adamson, Discuss. faraday Soc., 29, 163 (1960);(d) J. F. Endicoti and M. Z. Hoffman, J . Am. Chem. Soc., 90,4740 (1968). (10) (a) F. H. Burstall, F. P. Dwyer, and E. C. Gyarfas, J . Chem. Soc., 953 (1950);(b) C. F. Liu, N. C. Liu, and J. C. Bailar, Inorg. Chem.,
3, 1085 (1964). (11) C. Creutz and N. Sutin, Proc. Natl. Acad. Sci. U.S.A ., 72,2858 (1975). (12) M. Katz and H. D. Gafney, Inorg. Chem., in press. (13) (a) C. A. Parker, Proc. R . SOC.London, Ser. A , 220,104 (1953); (b) C. G. Hatchard and C. A. Parker, ibid., 235, 518 (1956). (14) R. E. Kitson, Anal. Chem., 22, 664 (1950). (15) P. Natarajan and J. F. Endicott, J. Am. Chem. Soc., 94,5909 (1972). (16) D. Meisel, M. S. Matheson, W. A. Mulac, and J. Rabini, J . Phys. Chem., 81, 1449 (1977). (17) J. N. Demas and G. A. Crosby, J. Am. Chem. Soc., 93,2841(1971). (18)W. M.Latimer, "Oxidation Potentials", Prentice-Hall. New York, N.Y., 1952,p 214. (19) M. Simic and J. Lilie, J . Am. Chem. Sac., 96, 291 (1974). (20) M. G. Simmons and L. J. Wilson, Inorg. Chem., 16, 126 (1977). (21) M. Chou, C. Creutz, and N. Sutin, J. Am. Chem. Soc., 99,5615 (1977). (22) A. Haim and N. Sutin, Inorg. Chem., 15,476 (1976).
