3084
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
Bors, Saran, and Michel
Pulse-Radiolytic Investigations of Catechols and Catecholamines. 4. 3,4-Dihydroxybenzaldehyde and 3,4- Dihydrox y acetophenone Wolf Bors," Manfred Saran, and Chrlsta Mlchel Abteilung fur Strahlenbiologie,Institut fur Bioiogie, Gesellschaft fur Strahlen- und Umweltforschung (GSF), 8042 Neuherberg bei Munchen, West Germany (Received December 15, 1978; Revlsed Manuscript Received June 12, 1979) Publication costs assisted by Gesellschaft fur Strahlen- und Umweltforschung (GSF)
The model catechol compounds for adrenalone, 3,4-dihydroxybenzaldehydeand 3,4-dihydroxyacetophenone, were investigated by pulse radiolysis combined with kinetic spectroscopy. The rate constants with the oxidizing primary radicals in neutral aqueous solutions, hydroxyl radicals (4.8 X 109-5.9 X lo9 M-l s-l) and superoxide anions (1.4 X lo7-2.9 X lo7 M-' s-l), differ by no more than a factor of about 2 from those of adrenalone itself. The mechanism for the oxidation by .OHradicals or 02was identical for all three substances. The reduction rates with eaq-were determined both by kinetic analysis of the decay of e,, and by competition experiments. The fact that the model compounds exclusively form the ketyl radical upon reduction, whereas adrenalone also reveals side-chain cleavage, was not reflected in the respective transient spectra. The second-order decay rates of the three ketyl radicals in alkaline solution all exceed lo9 M-'s-l , with a fivefold acceleration in near neutral solutions.
Introduction The pulse-radiolytic investigation of adrenalone (see preceding paper, ref 1)was intended to shed light on the possible tautomerism with intermediates of the adrenaline oxidation sequence.2 Difficulties in the interpretation of the transient spectra of adrenalone required parallel pulse-radiolytic studies of the catechol model compounds 3,4-dihydroxybenzaldehyde(DHB) and 3,4-dihydroxyacetophenone (DHAP). The latter substance was furthermore a possible product of the reduction of adrenalone by hydrated electrons, in analogy to the reduction of protonated adrenaline.3 Experimental Section Materials. DHB (Fluka) was recrystallized from benzene. DHAP was synthesized from w-chloro-3',4'-dihydroxyacetophenone (Merck) according to the procedure of Stephen and Weizmann4 (mp 114 "C,with an absorption spectrum5 agreeing with the literature). We assume, however, that the absorption spectra for the hydroxylated, respectively methoxylated acetophenones as reported by Senoh et al.5 have been mixed up. Methods. Spectroscopy, high-pressure liquid chromatography, and pulse radiolysis were performed as outlined in the preceding paper.' Results Spectrophotometric titration resulted in pK, values of 6.5 for DHB and 7.8 for DHAP. As adrenalone gave a pK, of 6.8, the high value for DHAP is surprising and cannot be explained solely by the dissociation equilibrium proposed by Senoh et as this should be similar for each of the substances. The precautions concerning pH-dependent spectral shifts during or after the pulse-radiolytic investigations were generally observed. Several experiments, however, were started at pH values where a considerable buildup of the 350-nm absorption could be accounted for by a minor shift to higher pH (e.g., Figure 4c). Reduction. The transient spectra after reduction by hydrated electrons in neutral solutions (Figure 1)reveal, aside from the apparent regeneration of the 350-nm absorption, a strongly absorbing species at 275-280 nm. Shape and position of absorption bands closely agree for all three compounds in neutral solutions. In alkaline so0022-3654/79/2083-3084$01 .OO/O
lutions differences are mainly observed in the region 300-350 nm due to the dissimilar reconstitution of the original absorption. Furthermore, pH-dependent shifts occur for the far-UV absorption bands (230 to 245-250 nm and 275-280 to 290-295 nm). A compilation of the individual absorption maxima observed after reduction of DHB and DHAP by e, - is given in Table I, together with the respective spectra! and kinetic data. Second-order decay rates were obtained from the kinetic evaluation of the 300-nm absorption present in uncorrected spectra and are similar to the values of adrena1one.l Oxidation, The spectra after oxidation by .OH radicals are depicted in Figures 2 and 3 and appear after diffusion-controlled attacks of -OH radicals at a rate of N 5 X lo9 M-l s-l (see Table 111). We can differentiate between three (pH 9.5) and four (pH 7.0) dominant peaks, while additional shoulders appear at intermediate times (see also Table 11): (i) The peak at 235-250 nm with the wavelength shift is mainly due to pH-dependent changes of the correction spectra. (ii) A peak appears around 275-280 nm, but extends, in some cases, to 260 and 285 nm. Both short-wavelength peaks reach their absorption maxima several tens of microseconds after the pulse, suggesting that they belong to the same species and represent a secondary organic radical. (iii) An initial peak appears around 340 nm, which is more pronounced in neutral as opposed to alkaline solutions. This dependence on pH is even more clearly revealed if one looks at the kinetic behavior of this absorption peak; while in neutral solutions it is followed by a decrease in absorption before regeneration of the original 350-nm peak is observed, in alkaline solutions this initial 350-nm absorption is less stable and is immediately superseded by a refurbished absorption a t 350 nm. (iv) A very pronounced absorption is observed around 450 nm. This broad peak is not observed in the exclusive presence of 02-, similar to adrenalone, and is therefore generated by -OH radicals alone. While the transient spectra after oxidation by .OH radicals are quite similar to the case of adrenalone, there remains the oxidation by 02-. As shown in Figure 4a, the only species observed initially (5 ps after the pulse) is 02itself, the molar absorbtivity being close to the theoretical value of 2350 M-l cm-le6The peak at 350 nm is most likely due to the regenerated original absorption. Quantitative formation of 02is nevertheless surprising in view of the 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979 3085
Pulse Radiolysis of Catechol Compounds
TABLE I: Spectral and Kinetic Data of Reduced Transient Species Formed by Attack of Hydrated Electrons reduced species 2nd-order I1 IIIU I decay rate, solute DH h. n m E. M-' cm-' h. nm E. M - I cm-' h. nm E. M-' cm-' 2h X M - ' s-' DH B DHAP ADON
1.5 9.5 7.5 9.5c 1.5d 9.5
230 250 230 250 230 245
8 100 6 000 8 800 1400 7 300 1 0 900
215b 1 2 300 290 9 300 280b 9 IO0 290 1 0 600 215b 1 2 300 (295 shoulder)
1 2 100 2 100 9 100 4 500 1 5 800
350 355 325 360 340
Additional shoulder a t 310-315 nm (adrenalone a t 295 nm). Reconstituted original absorption peak. points a t 253, 324, and 410 nm. Isosbestic points a t 241, 261, 322, and 420 nm.
5.5 x 109 1.0x 109 3.6 x 109 1.1 x 109 5.7 x 109 1.9 x 1 0 9 Isosbestic
50t
c 50
X Inml
\
hinml
Flgure 1. Initial transient spectra after reduction by hydrated electrons (5 ps after the pulse; all solutions contain 0.1 M tert-butyl alcohol and are saturated with nitrogen). (a) 3,4-Dihydroxybenzaldehyde: (solid line) pH 7.25,concentration of solute was 0.1 mM; corrected with G = 2.8,initial radical concentration at a dose of 3.21 krdlpulse was 9.31 p M (dashed line) pH 9.6,concentration of DHB was 0.11mM; corrected with G = 3.4,values combined from two experiments with D = 0.82 krdlpulse, c = 2.88pM and D = 3.21 krdlpulse, c = 11.3 pM. (b) 3,4-Dihydroxyacetophenone: (solid line) pH 7.3;concentration of DHAP was 0.11 mM; corrected with G = 2.8,c = 8.02pM at a dose of 2.77 krdlpulse; (dashed line) pH 9.5, concentration of DHAP was 0.1 mM; corrected with G = 3.4, c = 14.0 pM at a dose of 4.0 krdlpulse.
fact that both DHB and DHAP react with 02-with rate constants of 1.4 X lo7 and 2.9 X lo7 M-l s-l , r espectively (see Table 111). As was the case with adrenalone, both compounds show almost identical isosbestic points at 241, 256, and 315 nm (Figure 4b). This again demonstrates that oxidation by 02-is a completely reversible process and common to all three compounds. Figure 4c depicts the effect of phosphate buffer (pH 7.1) on the occurrence of the 350-nm absorption. The higher absorption a t this wavelength in unbuffered solutions is evidence that it is partially due to small pH increases during the pulse (0.1-0.2 pH units). In the presence of phosphate buffer the reaction with 02-is evidently slowed, and until 60 pus after the pulse we observe exclusively 0'(data not shown). This gradual buildup of the o-semiquinone (285 nni in the uncorrected spectrum), which reaches its maximum after 730 ps, can be explained by reversible complexation of the catechol moiety with phos-
hate.^
4
L
\\
/
i
1
Flgure 2. Correction of transient spectra; initial transients (5 ps) after oxidation of DHB by .OH radicals in neutral (pH 6.0 (a)) and alkaline solutions (pH 9.5 (b)). All solutions are saturated with N,O: (solid line) corrected spectra, assuming C, = 5.6; initial concentration of radicals at a dose of 3.98 krd/pulse was 23.0 pM (a) and that at a dose of 2.76 krdlpulse was 16.0 pM (b); (dashed line) uncorrected spectra at the respective dose rate; (dotted line) correction spectra; depicts absorption of solute depleted by the calculated concentration.
The two individual species absorbing at 270 and 450 nm decay by second-order processes with rate constants (based on a 50% yield from the .OH adduct, see Discussion) of e2 X lo' M-l d. There is no effect of oxygen as compared to nitrous oxide on the decay rate, whereas in alkaline solutions the decay is about 6 times slower. Compiling the spectral parameters after oxidizing pulse radiolysis, we find, as a joint feature of the oxidation by 0'- and .OH radicals, transient species absorbing a t 240-250 and at 270-280 nm (Table 11). Assuming the same mechanism derived for the oxidation of catechol compounds such as adrenaline' or adrenalone,l we based the molar absorbtivity values of each of the semiquinones on a 1:l stoichiometry of the water elimination from the initial .OH adduct. This is expressed in the table as the effective radical concentration (erc).
3086
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
100-
..
63
, .
. P 0
., :..,:
., ;
-
.
I
Bors, Saran, and Michel
:
.
:..
200
280
360
440
520
A [nml
600
280
200
440 A[nml
360
520
600
Figure 3. Time-dependent transient spectra after oxidation of DHB and DHAP by .OH radicals (all solutions are saturated with N,O, yielding .OH radlcals with a Gvalue of 5.6): (a) DHB: 0.11 mM, pH 6.0, concentration was 23.0 pM at a dose of 3.98 krdlpulse, (solid line) 5 psec; (dashed line) 48 ps; (dotted line) 4.4 ms. (b) DHB: 0.1 mM, pH 9.5, concentration was 16.0 pM, D = 2.76 krd/pulse, (solid line) 5 ps; (dashed line) 100 1s;(dotted line) 44 ms. (c) DHAP: 0.1 mM, pH 7.0, concentration was 20.8 pM, D = 3.59 krdlpulse, (solid line) 5 ps; (dashed line) 60 ~ s (dotted ; line) 8.9 ms. (d) DHAP: 0.12 mM, pH 9.5, concentration was 25.1 pM, D = 4.33 krd/pulse, (solid line) 5 ps; (dashed line) 100 /AS; (dotted line) 44 ms.
TABLE 11: Spectral Data of Oxidized Transient Species of 3,4-Dihydroxybenzaldehyde and 3,4-Dihydroxyacetophenone oxidized mecies
solute
generating radical
DHB
.OH
DHAP
pH
6.0 9.5 .OH t 0 2 - 6.0 026.0' 7.5d 8.7 .OH 7.0 9.5 . O H + 02- 7.2f 0 2 7.lP9h 7.1Rz6
h,
nm
235 240 235 235 230 250 230 240 230 235
Ia
IP
e , M-'
e, M-'
cm-'
G
9950 8700 10 600 1 2 400 11 1 0 0 8 800 9500 11 300 8 850 11 400 nd
5.6 5.6 6.4 6.4 6.4 6.4 5.6 5.6 6.4 6.4
I11
cm-'
erc, G
280 270 285 280 275
1 2 100 12400 7 100 1 2 900 1 0 300
2.8 2.8 4.8 6.4 6.4
270 260 270 275 275
1 0 600 7 550 7 600 11 100 10 100
2.8 2.8 4.8 6.4 6.4
h,
nm
e , M-'
IV
M-' erc,
E ,
cm-'
erc, G h , nm
cm-'
G
340 340
6550 5350
5.6 5.6
440 460 420
5950 6400 5900
2.8 2.8 1.4
330e 345e 345
4500
5.6
430 440 430
7050 8200 7500
2.8 2.8 1.4
h,
nm
erc is the effective radical concentration (yield expressed as G a Maximal absorption at intermediate observation times. value) based o n 1 : l stoichiometry of 0 -and p-semiquinone formation from the initial .OH adduct, respectively exclusive generation of the o-semiquinone by 02-attack. Initial uncorrected transient represents mainly Oz- (see Figure 4a), E = Regenerated original absorption. Isosbestic points (exclusive initial time) at 241, 256, and 315 nm. 2800 M-'em-'. Isosbestic As in footnote c, E = 2350 M" cm-'. f Isosbestic points (exclusive initial time) at 252,,310, and 382 nm. Solution contains 0.15 M phosphate buffer;no isosbestic points are points (until 210 p s ) a t 239, 258, and 312 nm. observed. TABLE 111: Rate Constants with Primary Radicals in Neutral Aqueous Solutions rate constant, M-'s - ' radical DHB DHAP ADON -
.OH Oza
1.2 x 10" (1.16 -t 0.10) x 10" (4.84 + 0.24) x l o 9 (1.40 + 0 . 0 3 ) x l o 7
3.25 X (1.07 + (5.92 + (2.94 +
10'' 0.08) x 10" 0.66) x l o 9 0.22) x l o 7
2.7 X 10" (2.70 + 0.10) x lo1' (1.03 + 0.14) x 1O'O (2.34 + 0.31) x 10'
competitor
ref a
1.05 x 10'' (4.0 + 0.29) x 10" (2.14 + 0.05) x 10'
(NO;) (DCIP) (DCIP)
8 1 1
Direct observation of the decay of eaq' a t 6 7 0 n m (for interpretation see text).
Table I11 gives a compilation of the rate constants with the primary radicals. The reaction rates with ea;, determined both by competition with NO3- (h = 1.05 X 1O'O M-l s-l, ref 8) and by direct observation of the decay of eaq-at 670 nm in solutions where .OH radicals were scavenged by ethanol, exceed the theoretical limit for diffusion-con-
trolled reactions, as was the case for adrenalone,l where we already offered some possible explanations. The rate constants with .OHradicals were determined competitively by the bleaching of 2,6-dichlorophenolindophenol(DCIP, k = 4.0 X 1O1O M-ls-l, ref 1) at 600 nm to avoid interferences with the absorbing species of the catechol com-
Pulse Radiolysis of Catechol Compounds
The Journal of Physical Chemistty, Vol. 83, No. 24, 1979 3087
L " ".-. " ' " ' 1
transients of the three substances, we find a better correlation in neutral solution as compared to alkaline pH values. In neutral solutions the species absorbing at 275-280 nm, and possibly at 230 nm, is identical for all three carbonyl compounds and is proposed to represent the ketyl radical. The close correlation of the molar absorbtivities at 275 nm (12 300 M-' cm-' for DHB and adrenalone, 9700 M-l cm-' for DHAP) is taken as additional proof for the structural identity. An absorption at 300 nm, as reported for benzaldehydelO and acetophenone,11J2is observed only for uncorrected spectra. Absorption peaks above 300 nm are considered to be due to partial regeneration of the major absorption at 350 nm. Turning to the initial transient spectra after reduction in alkaline solutions, we find a greater dissimilarity, which can best be explained by different reconstitution rates and respectively product formation for the individual compounds. Basically we observe peaks or shoulders for all three substances a t 245-250 nm, at 290-300 nm, and between 325 and 350 nm. The former two peaks may be identical with those in neutral solutions except for a 2025-nm shift to longer wavelengths. As it is unlikely that the undissociated ketyl radical would manifest such a shift, we assume that the main reason for the shift is a pH-dependent change of the correction spectra. The latter peaks exhibit the greatest variance as they most prominently reflect the reconstitution of the original absorption and respectively formation of DHAP in the case of adrenalone. The bimolecular decay rates of the ketyl radicals closely correspond to each other (1.0 X 109-1.9 X lo9 M-l s-l in alkaline solutions);the major contribution of these spectral differences at 300-350 nm has therefore to result from variations in product formation. An absorption a t 450 nm after reduction by ea; was observed only in neutral solutions of DHAP with 10 mM tert-butyl alcohol added (data not shown). As this peak was absent when the solution contained 100 mM tert-butyl alcohol (Figure lb), it is most likely due to insufficient scavenging of .OHradicals. We do not consider it to represent a ketyl radical cation, as was previously reported for acetophenone and benzaldehyde with pK values of 10.0 and 9.25, respectively.12 As we did not observe a dissociation in the pH region 6.5-10.5, evidently the pK values of our catechol-derived ketyl radicals are even higher. The pK values of the parent compounds (adrenalone, 6.8; DHB, 6.5; DHAP, 7.8) seem to have only a minor influence on the reduced transient spectra. A final curiosity is the fact that the difference in the decay mechanism after electron attack a t adrenalone, forming DHAP only in alkaline solutions (see ref l),is not reflected in a change of the transient spectrum vis-a-vis of DHAP itself. Normally one would presume that the exclusive ability of adrenalone to form a hydrogen bond between the reduced side-chain carbonyl and the amine group facilitates differential attack of ea; as was suggested for protonated adrenaline,3 which should be reflected in different transient spectra. Oxidation by .OH Radicals. Contrary to the transient spectra obtained after reduction of the carbonyl compounds, there is no ambiguity for the transient spectra obtained after oxidation by both .OH radicals and OF, even though the wavelength maxima are somewhat different. Formation of both o- and p-semiquinones (at 270-280 and around 450 nm) from the initial .OH adduct is evident. Under the assumption of a 1:l stoichiometry for the formation of both semiquinones, which is difficult to prove, we arrived at the molar absorbtivities given in Table 11. Concomitantly this assumption allowed an estimation of
280
240
240 -
320
280
400
320
360
400
G.--
-
.
h lnml
I
.
0
360
h [nrnl
1001
&
.
I
.
/
'/
-
"
~ 9 -"280
_-
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'
320
a
I
360
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400
h[nrnl
Figure 4. Transient spectra alter oxidation by 01-(oxygenatedsolutions containing 10-100 mM sodium formate, Go,-= 6.4). (a) Uncorrected initial transient spectra, the peak at 245-250 nm is identical with 02itself: (solid line) 0.1 mM DHAP, 0.1 M sodium formate, pH 7.1 (unbuffered solution), D = 4.0 krd/pulse, c = 26.5 pM; (dashed line) 79.0 pM DHAP, 50 mM sodium formate, pH 7.1 (phosphate buffer, 0.15 M), D = 4.34 krd/pulse, c = 28.7 pM; (dotted line) 0.11 mM DHB, 10 mM sodium formate, pH 6.0 (unbuffered solution), D = 3.98 krd/pulse, c = 26.4 pM. (b) Time-dependent transient spectra of DHB (0.1 mM) at pH 7.5, 10 mM sodium formate Corrections were made with G = 6.4 and a dose of 3.66 krd/pulse, the concentration was 24.2 p M ~ Isosbestic points at 241, 256, and 315 nm: (-) 22-470 ps, (---) 2 1 oxidized DHAP ms, (-.-) 9.8 ms, (---) 44 ms. (c) Shift of pH in 02solutions, final transient spectra at 8.9 ms after the pulse, buffered (0.15 M phosphate buffer, formate 50 mu) vs. unbuffered solutions (formate 0.1 M) at pH 7.1: (-) unbuffered solution, corrected with D = 4.0 krd/pulse and G = 6.4 gave c = 26.5 pM; (---) buffered solution, corrected with D = 4.39 krd/pulse and G = 6.4: c = 28.7 pM; (--) unbuffered solution, uncorrected; (-e.-) buffered solution, uncorrected.
pounds. No direct observation is possible due to spectral and kinetic overlaps. The competitor for 02-was also DCIP, the redetermined rate constant1 closely agreeing with a previously reported value.g All rate constants are in the same range as for adrenalone, the values of which are included for comparison.
Discussion Basically the discussion can be limited to the demonstration of the analogy of the data of the model compounds with those of adrenalone, presented in ref 1. It is apparent that the greatest differences exist for the reduction by ea;, in which case the side chain adjacent to the carbonyl function seems to play a major role. Oxidation of the catechol moiety, on the other hand, involves only the aromatic ring system and is therefore identical for the three compounds. Reduction by Hydrated Electrons. Difficulties in interpreting the transient spectra after reduction of adrenalone, incidently, were the main reason for the investigation of the model compounds. Thus, comparing the initial
3088
The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
Gaines
*QH+ 02‘ 3 QH2 + 02
the second-order decay 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 a t 340-345 nm. The species disappears completely before the original absorption a t 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 a t 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 ruthenium(I1)-bipyridyl complexes by methyl viologen (l,lf-dimethyl-4,4’-bipyridinium chloride) in aqueous salt solutions. For the neutral complex (Ru(bpy)z(CN)z)o, no salt effect is observed and the quenching rate constant is near the diffusion-controlled limit. Another neutral complex, (R~(bpy)~(bpy(COO-)~))~, exhibits a small negative salt effect, perhaps due to its highly dipolar structure. The three positively charged complexes ~ (, b p y ) ~ ( b p y ( C H ~ and ) ~ ) )(Ru(bpy)z(bpy(COOH)2))2+, ~+, are all quenched with similar studied, ( R u ( b p ~ ) ~ () R ~+ 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
