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The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
Gaines
*QH+ 02‘ 3 QH2 + 02
the second-orderdecay rates (-2 X lo’ M-ls-l ). In neutral solutions of both DHB and DHAP (Figure 3a,c), the “OH adduct can be easily recognized as an initial peak absorbing at 340-345 nm. The species disappears completely before the original absorption at 350 nm is reconstituted. In alkaline solutions and in the case of adrenalone the .OH adduct is so labile that it cannot be observed at all. Rather it is superimposed by the rapidly appearing 350-nm peak. Regeneration of this peak suggests dismutation of the semiquinones which is also inferred from the second-order decay of the absorption at 450 nm. Reversible Oxidation by 02-.This feature is also identical with the case of adrenalone and implies that it is a common mechanism for catechol compounds with a acarbonyl function in C4position. The almost quantitative presence of 02-(Figure 4a) before reacting with DHB or DHAP with rate constants of >lo7 M-l s-l was also observed with Tiron13 but not in the case of adrena1one.l
(2)
Stability and/or reactions of these semiquinones are dependent on the respective univalent redox potentials, while the side chains also determine final product formation, e.g., cyclization to indole compounds as in the case of adrenaline.2
Acknowledgment. Thanks are due to Mr.A. Kruse for operating the accelerator and to Ms. C. Fuchs for preparing dihydroxyacetophenone.
References and Notes Bors, W.; Saran, M.; Michel, C. J. fhys. Chem. 1979, 83, 2447. Bors, W.; Saran, M.; Michel, C.; Lengfelder, E.; Fuchs, C.; Spottl, R. Int. J. Radiaf. Siol. 1975, 28, 353-371. Gohn, M.; Getoff, N.; Bjergbakke, E. Int. J. Riidi8f. fhys. Chem. 1976, 8, 533-538. Stephen, H.; Weizmann, C. J. Chem. SOC.1914, 105, 1046-1057. Senoh, S.; Dab, J.; Axelrcd, J.; Witkop, B. J. Am. Chem. Soc. 1959, 8 1 , 6240-6245. Blelskl, 8. H. J. fhofochem. fhotoblol. 1978, 28, 645450. Chaix, P.; Morin, G.-A.; Jezequel, J. Blochlm. Siophys. Act8 1950, 5,472-488. Peled, E.; Crapskl, G. J. fhys. Chem. 1970, 74, 2903-2911. Greenstock, C. I. Ruddock, ; G. W. Inf. J R8dkd. fhys. Chem. 1976, 8, 367-369. Lllie, J.; Hengleln, A. Ber. Sunsenges. fhys. Chem. 196gZ 73, 170-176, Adams, G. E.; Michael, 8. D.; Willson, R. L. Adv. Chem. Ser. 1988, NO, 81, 289-308. Hayon, E.;Ibata, T.; Lichtin, N. N.; Simic, M. J. Phys. Chem. 1972, 76, 2072-2078. Bors, W.; Saran, M.; Michel, C. Blochlm. Siophys. Acta 1979, 582, 537-542. Bklski, E. H. J.; Gebldti, J. M. Adv. Radlat. Chem. 1970,2, 177-279.
Conclusions The hypothesis of Bielski and Gebicki14on the different behavior of aromatic diols and their respective semiquinones with .OH and/or Oc, as discussed extensively for the pulse-radiolytic oxidation of adrenaline,2can thus be extended by another group, in which both .OH and 0, oxidize the catechol quite rapidly to the semiquinone. In the exclusive presence of Or, however, no further oxidation to the quinone takes place, rather the semiquinone is quantitatively reduced by H20z and/or 02-. QH2 + 02- *QH + HO2(1) +
Coulombic Effects in the Quenching of Photoexcited Tris(2,2’-bipyridlne)ruthenlum( 11) and Related Complexes by Methyl Viologen George L. Gaines, Jr. General Nectric Corporate Research end Development, Schenectady, New York 1230 1 (Received July 2, 1979) Publication costs assisted by the General flectrlc Company
Both intensity and lifetime measurements have been used to study the quenching of luminescence of several chloride) in aqueous ruthenium(I1)-bipyridyl complexes by methyl viologen (l,lf-dimethyl-4,4’-bipyridinium salt solutions. For the neutral complex (Ru(bpy)z(CN)z)o, no salt effect is observed and the quenching rate exhibits constant is near the diffusion-controlled limit. Another neutral complex, (R~(bpy)~(bpy(COO-)~))~, a small negative salt effect, perhaps due to its highly dipolar structure. The three positively charged complexes ) ~ ) )(Ru(bpy)z(bpy(COOH)2))2+, ~+, are all quenched with similar studied, ( R u ( b p ~ ) ~()R~~+(,b p y ) ~ ( b p y ( C H ~ and rate constants and show similar large positive salt effects; lz, increases approximately sixfold when [NaCl] is increased from 0.03 to 1.5 M, and at the highest salt content is within a factor of 2 of the diffusion-controlled limit. While the results are qualitatively consistent with the conventional Bronsted-Debye treatment of ionic reaction rates, large specific ion effects are indicated by limited data with NaC10, as a neutral salt.
Introduction Electron transfer reactions involving the excited state of the tris(2,2’-bipyridine)ruthenium(II) cation, (Ru(bpy)3)2+,and related complexes are currently under intensive study, both for their intrinsic interest and because they may offer promise as models for solar energy conversion pr0cesses.l In particular, it has been found2 that methyl viologen (1,l’-dimethyl-4,4’-bipyridiniumchloride) reacts rapidly with [ R ~ ( b p y ) ~ ~via + ]electron * transfer. If an irreversibly oxidizable donor (such as cysteine, tri0022-3654/7912083-3088$0 1.OOlO
ethanolamine, or ethylenediaminetetraaceticacid) is present to react with the R ~ ( b p y ) , ~which + is formed, the reduced methyl viologen radical can accu~nulate,~ and, with the addition of catalysts such as PtO,, it has been shown that hydrogen can be liberated efficiently from watera4 This sequence of reactions is of special interest since it appears to simulate one-half of a photochemical water splitting procem6 While several s t u d i e ~ ”have ~ discussed the electron transfer quenching of ruthenium-bipyridyl complex ex@ 1979 American Chemical Society
The Journal of Physical Chmisfty, Vol. 83, No. 24, 1979
Luminescence Quenching of Ru(I1)-bpy Complexes
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TABLE I: Quenching of the Luminescence of RuII-Bipyridyl Complexes by Methyl Viologen in Aqueous Salt Solutions a t Constant Ionic Strength l--_l_.
--
Ru( bPY )3'+ 0.027 M NaCl 0.042 M NaCl 0.23 prl NaCl 0.52 M NaCl 1.0 M NaCl 1.5 M NaCl 0.20 M NaC10, W b w ),(bpy(CH,), 1'' 0.023 M NaCl 0.52 M NaCl Ru(bpy),(bpy(COOH),)2t (0.1 M HC1) 0.037 M NaCl 0.52 M NaCl Ru(bpy),(bpy(COO-),)O (neutral) 0.029 M NaCl 0.53 M NaCl W b P Y ),(CN)Zn 0.024 M NaCl 0.5 2 M NaCl
l l l _ . l _ l
0.62
0.40
0.41 0.41
290 350 710 940 1120
225
5.6
610 810
1.5
3.6 3.7
X
10'
x 109 2.0 x 109 x 109 x 109
0.44 0.38
1810 1560
1600 1410
0.33 0.34
230 780
2 20 690
6.7 X 2.0 x
0.26
0.23 0.22
105 155
140 220
6.1 X 10' 1.0x 109
0.52
0.40 0.39
1250 870
1010 740
2.5 1.9
x 109 x 109
0.26
0.19 0.20
1180 1130
1010 1060
5.3 5.3
x 109 x 109
0.60
0.49
cited states by viologens, no attention seems to have been paid to Coulombic effects in these reactions, although salt effects on other quenching reactions involving Ru(bpy)gP+ have been studied.1° Methyl viologen and Ru(bpy)QP+, both divalent cations, are expected to exhibit substantial repulsive interaction, and neutral salt effects on their reaction rates correspondingly should be appreciable. In connection with an on-going study of surfactant Ru(bpy), derivatives,11J2information was needed on such effects in the electron transfer reactions of the corresponding water-soluble complexes in homogeneous aqueous solutions. The results of an examination of the salt effect are presented here.
Experimental Section Ru(bpy),C12.6H20was obtained from the G. Frederick Smith Chemical Co. and methyl viologen from the Aldrich Chemical Co.; both gave clear aqueous solutions whose UV-visible absorption spectra were in agreement with those in the literature, and they were used without further purification. The preparation and characterization of the other complexes have been reported el~ewhere.ll-~~ Luminescence spectra and intensity measurements were made with an Aminco-Bowman spectrophotofluorometer equipped with an AH-4 mercury arc source and an RCA C31034 photomultiplier. The 436-nm Hg line was used for excitation. Luminescence lifetime measurements utilized a dye laser system described el~ewhere;'~ the excitation wavelength was 424 nm. In all cases, luminescence was measured at the maximum sensitivity for the system under study (wavelengths, uncorrected, 585-675 nm). Stock solutions of the ruthenium complexes (1.0 X 10-5-8.0 X M) were prepared in redistilled water or salt solutions. To aliquots of these, solid methyl viologen was added at the maximum concentration (0.4 X 10-2-2.4 X M) to be used, while an appropriate amount (= 3[MV2+]) of solid NaCl or NaC104 to give equal ionic strength was added to other aliquots. These were then mixed in various ratios for the quenching measurements. Neither instrument was equipped for temperature control, and all measurements were made at room temperature (23 f 2 "C). Solutions were not deaerated, except as noted. (Temperature variations may contribute substantially to the scatter of the results because of the effect of temperature on the lifetime and yield of lumine~cence.'~)
lo8 1 0 9
Results In all cases, luminescence decay was strictly logarithmic over at least two lifetimes, The added yuencher was transparent (OD < 0.005 at the highest added concentration) throughout the visible region, so that no corrections were needed for trivial absorption effects. The addition of neutral salt (NaC1or NaC10,) up to the highest concentrations reported here had no detectable effect on the absorption or emission spectra of any of the complexes studied. A small effect of salt concentration on luminescence lifetime was detect,able; this may be ascribed to the reduction of oxygen solubility at high salt content. Thus, the solubility of O2in 1.5 M NaCl solution is 61% of that in pure H20, which is 2.5 X lov4 M in equilibrium with air.16 Using these values together with the Stern-Volmer constant for oxygen quenching given by Demas et a1.17 and T~ = 0.60 ps for Ru(bpy),2+,one obtains values of T = 0.40 ps for pure water and 0.46 ps for 1.5 M NaC1, which may be compared with experimental values of 0.40 ps in 0.027 M NaCl and 0.44 ps in 1.5 M NaC1. It was also confirmed that nitrogen deaeration of these solutions gave the same lifetime, which agreed with literature values (cf. Table I). The observed lifetime for the neutral complex Ru( b p ~ ) ~ ( c Nand ) ~the , change due to aeration, agree with the results reported by Demas et The present results for the complex R~(bpy)~(bpy(COOH),), however, are not identical with those obtained by Giordano et a1;18 while the value of 0.39 ps for the deprotonated form agrees with the earlier results, I find a somewhat shorter lifetime in the protonated state. It is possible that the presence of small amounts of nonionizing impurities (e.g., Ru(bpy),2+) in the earlier preparation could account for this discrepancy. While there are no directly comparable literature data for R~(bpy)~(bpy(CH,)~),~+ it may be noted that Sutin et al. have reported r,, = 0.33 ps for Ru(bpy(CH,)2),zs.1 The rate constants for O2 quenching which can be estimated from the observed lifetimes in aerated and deaerated solutions are also similar to those obtained by Demas et al. for the other complexes. Stern-Volmer plots of typical intensity (0and lifetime ( 7 ) quenching data at constant ionic strength are shown in Figure 1; in all cases the plots are linear within the experimental uncertainty. The mean values of the %ern-Volmer constants evaluated from intensity (Ks,')
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The Journal of Physical Chernisfry, Vol. 83, No. 24, 1979
&/I
$r 5
4
3
2
I
0
Figure 1. Stern-Volmer plots for the quenching of luminescence by methyl viologen in intensity, I (X), or lifetime, r (0),of Ru(bpy):+ aqueous solution at various ionic strengths adjusted with NaCI. [Ru= 1.0 X M. (bpy),CI,]
and lifetime (Ksvr)measurements, and the quenching rate are constants, k,, calculated from k, = KsvT/~([Q]=O), collected in Table I.
Discussion As is apparent from Figure 1and Table I, there is always some difference between Ka: and Ka/ values, although all of the Stern-Volmer plots appear linear; in all cases but one, Ka: > Kavr. Such differences have usually been ascribed to contributions due to static (associational) quenching in the intensity measurement^.'^-^^ In the present case, where [quencher] >> [donor] and Keg,the association constant for donor-quencher pairs, appears to be
