J. Phys. Chem. 1989, 93, 6080-6088
6080
One-Electron Reduction of Tris(2,2'-bipyrimidIne)ruthenium(2+) Ion in Aqueous Solution. A Photochemical, Radiation Chemical, and Electrochemical Study Gilda Neshvad,la Morton Z. HOffman,**laQuinto G. Mulazzani,*JbMargherita Venturi,lbqc Mauro Ciano,Ib and Mila D'AngelantonioIb Department of Chemistry, Boston University, Boston, Massachusetts 0221 5, Istituto di Fotochimica e Radiazioni d'Alta Energia, Consiglio Nazionale delle Ricerche. 401 26 Bologna, Italy, and Dipartimento di Chimica "C. Ciamician", Universitd di Bologna, 401 26 Bologna, Italy (Received: January 13, 1989)
The reduction of R ~ ( b p m ) , ~in+ aqueous solution has been investigated by use of photochemical, radiation chemical, and electrochemical techniques. The luminescent excited state of the substrate, *Ru(bpm)32+,has a lifetime (T,,) of 0.081 ps and a standard reduction potential of 1.2 V; it is quenched by electron donors (D) such as ethylenediaminetetraacetic acid (EDTA), triethanolamine (TEOA), ascorbate ion, deprotonated cysteine, and reduced glutathione with values of k, that depend on the pH of the solution and the reducing ability of the quencher. The one-electron-reduced species, Ru(bpm)3+, is formed in the quenching reaction; it is also produced electrochemically and from the reaction of radiolytically generated COC- with R ~ ( b p m ) , ~(k+ = 6.7 X lo9 M-l s-l). Ru(bpm),+ is a good reducing agent (Eoxo= 0.73 V) and reduces MV2+ (methylviologen) to MV" (k = 1.0 X lo9 M-' s-I ). Ru(bpm),+ also undergoes protonation; its acidic form (pK, 6.3) is a milder reducing agent (E,' = 0.50 V) but is still capable of reducing MV2+(k = 1.0 X lo6 M-I s-l ). Both forms of Ru(bpm),+ are unstable with respect to long-term storage; it is likely they engage in disproportionation and/or reaction with the solvent. The continuous photolysis of a solution containing Ru(bpm)?+, MV2+,and a sacrificial reductive quencher (EDTA, TEOA) generates MV". The values of @(MV'+) correlate very well with the rate constants and efficiencies of the various steps of the mechanism according to the following expression: @(MV'+)= qqqqoc(qrrd+ qd'), where 7. is the efficiency of population of *R~(bpm),~+, qq is the efficiency of quenching of *R~(bpm),~+, qw is the efficiency of escape of the redox products into the bulk solution, and qrd and qrd' are the efficiencies of the reactions of Ru(bpm),+ and Drd*, the reducing radical from the irreversible transformation of the initially formed oxidized-quencher radical, respectively, with MV2+. Values of qcc of 0.64 and -0.7 for TEOA and EDTA, respectively, in alkaline solution have been obtained. For D = TEOA, evidence is cited indicating that the back electron transfer between the geminate pair (Ru(bpm),+ and the oxidized-TEOA radical) within the quenching solvent cage occurs in the "inverted Marcus" region. The absorption spectra of *Ru(bpm)32+and the acid-base forms of Ru(bpm),+ are also characterized.
-
Introduction One of the many complexes of Ru(I1) that has been considered for use as a photosensitizer in model systems for the conversion and storage of solar energy is R ~ ( b p m ) ~(bpm ~ + = 2,2'-bipyrimidine).2 Like its isomer, Ru(bpz)?+ (bpz = 2,2'-bipyrazine), the reduction potentials of the various ground and excited states of R ~ ( b p m ) , ~are + more positive than those of R ~ ( b p y ) , ~(bpy + = 2,2'-bipyridine),, making their luminescent excited states more amenable to reductive quenching than to oxidative quenching. Indeed, the reductive quenching of *Ru(bpz)?+ to Ru(bpz),+ by sacrificial electron donors, such as EDTA, TEOA (triethanolamine), and C2042-,the one-electron-oxidized forms of which undergo irreversible transformations, gives rise to high cage escape yields (qcc L 0.5) and, in the presence of methylviologen (N,N'-dimethyL4,4'-dipyridinium dication; MV2+), values of the quantum yield of formation of the methylviologen radical cation, @(MV'+), in excess of ~ n i t y . ~ , ~ Despite the fact that * R ~ ( b p m ) , ~and + * R ~ ( b p z ) , ~have + similar energies and reduction potentials (although the former is a somewhat weaker oxidant), they have strikingly different lifetimes (-0.1 and 1 ps, respectively) in fluid solution at ambient t e m p e r a t ~ r e . ~ .Nevertheless, ~ they follow a similar + pattern of reactivity, with the quenching of * R ~ ( b p m ) , ~by electron donors (D) proceeding via the formation of Ru(bpm),+ which, as a strong reductant, is capable of reducing MV2+.'
-
( I ) (a) Boston University. (b) Istituto FRAE-CNR. (c) Universita di Bologna. (2) Juris, A.; Barigelletti, F.; Campagna, S.; Balzani, V.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 8 5 . (3) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D.Inorg. Chem. 1983, 22, 1617. (4) Prasad, D.R.; Hessler, D.;Hoffman, M. Z.; Serpone, N. Chem. Phys. ( 5 ) Neshvad, G.; Hoffn
(6) Kawanishi, Y.; Kitan Chem. Res. (Jpn.) I'
0022-3654/89/2093-6080$01.50/0
Values of (P(MV'+) in aqueous solution have been reported,bE indicating that R ~ ( b p m ) , ~has + potential utility as a photosensitizer. In our recent examination of the reductive quenching of * R ~ ( b p z ) , ~and + the chemistry of Ru(bpz),+ in aqueous solution, using the techniques of photochemistry, radiation chemistry, and electrochemistry, we reported on the rate constants of quenching and the steps of the mechanism in the presence and absence of MV2+as a function of pH, the acid-base properties of Ru(bpz),+, and the electrochemical parameters of the system; values of (P(MV'+) were reconciled with the efficiencies of the quenching, cage escape, and secondary reduction reactions in the mechanism.4S,9-l 1 In this paper, a similar approach is taken; we present the rate and equilibrium constants for the various steps of the mechanism involving *Ru(bpm)32+,examine the kinetic and thermodynamic properties of Ru(bpm),+, and assess the utility of R ~ ( b p m ) , ~ + as a photosensitizer. Comparison is made, at all stages, with the results obtained previously with R ~ ( b p z ) ~ ~ + .
Experimental Section Materials. R ~ ( b p m ) , ~ as + , the PF6- salt, was prepared by the method of Rillema et al., The compound was stable under all the concentration and pH conditions used in this study. For the photochemical experiments, methylviologen dichloride (Aldrich) and TEOA-HCI (Aldrich) were recrystallized three times from ethanol and water/methanol, respectively, and were both dried by suction for 2 days; some laser flash photolysis experiments (7) Diirr, H.; Dorr, G.; Zengerle, K.; Mayer, E.; Curchod, J.-M.; Braun, A. M. Nouu. J. Chim. 1985, 9, 717.
(8) Kitamura, N.; Kawanishi, Y.; Tazuke, S. Chem. Leu. 1983, 1185. (9) Prasad, D. R.; Hoffman, M. Z. J. Am. Chem. SOC.1986, 108, 2568. (IO) Venturi, M.; Mulazzani, Q. G.; Ciano, M.; Hoffman, M. Z. Inorz. Chem. 1986, 25, 4493. ( 1 I ) Mulazzani, 0.G.; Venturi, M.; Hoffman, M. 2.Radiat. Phys. Chem. 1988, 32, 7 1.
0 1989 American Chemical Society
One-Electron Reduction of R ~ ( b p m ) ~ ~ + utilized TEOA (Merck). All other materials were Baker or Aldrich reagent grade and were used without further purification. For the radiation chemical and electrochemical experiments, methylviologen dichloride (Aldrich), NaHCO, (Merck), and N a 2 S 0 4 (Merck, Suprapur) were used as received. Solutions. Distilled water was further purified by passage through a Millipore purification train. The solutions were saturated with NzO, purged with Ar or N,, or degassed by standard vacuum line techniques as needed. The pH of the solutions was adjusted with H2SO4, HC1, or NaOH, or with phosphate, hydrogen phthalate, or bicarbonate buffers. In the continuous photolysis experiments, the ionic strength was adjusted to 1.0 M with NaZSO4. Apparatus. Luminescence quenching experiments were made by using a Perkin-Elmer MPF-2A spectrofluorometer set at 460 nm for excitation and 622 nm for emission. Excited-state-lifetime measurements and laser flash photolysis experiments at Boston University were made by using a Nd:YAG pulsed laser system with excitation at 355 nm; details of the apparatus have been described before.I2 Laser flash photolysis experiments were also performed at the Center for Fast Kinetics Research (CFKR), University of Texas, Austin; 12-11s pulses at 355 or 532 nm were provided by Nd:YAG Q-switched lasers under computer contr01.l~ Continuous photolyses were performed at 440 nm by using a Bausch & Lomb high-intensity monochromator in conjunction with a 100-W quartz halogen lamp and photon counter. Continuous radiolyses were carried out in a 'To-Gammacell (Atomic Energy of Canada, Ltd.) with a dose rate of 6.0 Gy min-l; pulse radiolyses with optical absorption detection were performed by using the 12-MeV linear accelerator of the FRAE Institute of CNR, Bologna, Italy.14 Procedures. Air-equilibrated solutions for luminescence quenching experiments, with at least four different concentrations of the quencher, were contained in 1-cm spectrofluorometer cuvettes. Continuous photolyses in the presence of MV2+ were performed at -22 OC on Ar-purged solutions contained in 1-cm cuvettes with septum-sealed and plastic film sealed long necks; the absorbance of the solution at 605 nm was monitored as a function of irradiation time. Solutions for laser flash photolysis at Boston University were contained in a 1 X 2 cm cuvette with a long neck that could be sealed, after saturation with Ar, air, or Oz, with a septum and plastic film; each data point was the average of at least 10 shots. Solutions for laser flash photolysis at CFKR were contained in a 1 X 0.5 cm cuvette, excited along the larger dimension and probed by the analyzing light along the smaller one; they were deaerated and continuously mixed with a stream of N2. Data were collected for at least 10 shots per point. Continuous radiolyses were carried out at room temperature on 10-20-mL samples contained in silica or Pyrex vessels that were provided with silica optical cells on a side arm. The absorbed radiation dose was determined with the Fricke chemical dosimeter by taking G(Fe3+) = 15.5, where C(X) = number of molecules of species X formed per 100 eV of energy absorbed by the solution. The pulse irradiations were performed at room temperature on samples contained in Spectrosil cells of 2-cm optical path length; the solutions were protected from the analyzing light by means of a shutter and appropriate cutoff filters. The radiation dose per pulse was monitored by means of a charge collector placed behind the irradiation cell and calibrated with a NzO-saturated solution containing 0.1 M HC02- and 0.5 mM MVZ+using Gc = 9.32 X lo4 at 602 nm.Is Cyclic voltammetry was performed on Ar-purged solutions containing 0.1 M Na2S04as a supporting electrolyte with an Amel 448 oscillographic polarograph with a glassy-carbon electrode (GCE) as the working electrode, a platinum wire as the counter (12) Malba, V.; Jones, G., 11; Poliakoff, E. Photochem. Phofobiol. 1985, 42, 45 1. (13) Foyt, D. C. Comput. Chem. 1981, 5 , 49. (14) Hutton, A,; Roffi, G.; Martelli, A. Quad. Area Ric. Emilia-Romagna 1974, 5, 67. (15) Mulazzani, Q. G.;D'Angelantonio, M.; Venturi, M.; Hoffman, M. Z.; Rodgers, M. A. J. J . Phys. Chem. 1986, 90, 5347.
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6081 1.0
2 0.8 '0 C
0.8
=-g 0.4 0
0)
a 0.2
0.0 2
3
4
5
6
7
0
9
PH Figure 1. I , (m) and r0 (0)relative to plateau values at pH 6-10 for *Ru(bpm)32+as a function of pH measured in air-saturated solutions.
electrode, and a standard calomel electrode (SCE) as the reference electrode; the pH of the solutions was adjusted to 3.0 with HzS04 or 13.0 with NaOH. Generation of Reducing Radicals. The radiolysis of NzOsaturated aqueous solutions containing HCO, generates strongly reducing COz'- radicals as the only reactive species; E o(CO,/ C02'- = -2.0 V).16 The relevant processes are reactions a-d; their rate constants have been given before:l0
H20
--
e,;(2.8),
'OH(2.8), H'(0.6), H,(0.45), H20,(0.8) (a)
with the numbers in parentheses representing the G values for the individual species. eaq-
+ N20
H20
Nz
eaq- + H+ H'/'OH
+ HC02--+
+ *OH + OH-
-
H'
H2/Hz0
(b) (c)
+ C02'-
( 4
Results Continuous and Time-Resolved Luminescence. The broad, but structured emission from *Ru(bpm)?+ in the 600-800-nm rangel' decays via first-order kinetics. The lifetime of the luminescence ~ l/ko) at in Ar-purged H 2 0 in the absence of quenchers ( T = natural pH (6.4) and ambient temperature was 0.081 ps; in air-saturated solution, 7,fr = 0.075 ps. We also found that 70 in Ar-purged CH3CN is 0.12 ps. Our value of T~ in HzO agrees very well with that of Akasheh et al. (0.083 and moderately well with that of Kitamura et al. (0.06 ps)? but poorly with that of Durr et al. (0.19 ps).' Other reported values are 0.067 ps (CH3CN),lE0.10 ps (CH30H),lEand 0.13 ps (propylene carbonate).3 The intensity of emission (Z,)and 70 are diminished in acidic solution (Figure 1) due to the successive protonation of the peripheral nitrogen atoms of the ligands. Rillema et al.3 estimated an apparent pKa* of 2.2 for the conjugate acid from the first protonation of * R ~ ( b p m ) from ~ ~ + the midpoint of the break in a similar "titration curve'' of 1,. In comparison, values of 2.0 (3.8 ~ a l c u l a t e d )and ' ~ 3 S 3 have been assigned for * R ~ ( b p z ) ~ ~ + . Values of the rate constants for the quenching of * R ~ ( b p m ) ~ ~ + (IC& calculated from linear Stern-Volmer plots (Zlo/Z, vs [D]), are given in Table I as a function of pH for EDTA, TEOA, (16) Breitenkamp, M.; Henglein, A.; Lilie, J. Ber. Bunsen-Ges. Phys. Chem. 1976,80, 973.
(17) Hunziker, M.; Ludi, A. J . Am. Chem. SOC.1977, 99, 7370. (18) Akasheh, T. S.; Beaumont, P. C.; Parsons, B. J.; Phillips, G. 0. J . Phys. Chem. 1986, 90, 5651. (19) Crutchley, R. J.; Kress,N.; Lever, A. B. P. J . Am. Chem. Soc. 1983, 105, 1170.
6082 The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
Neshvad et al.
TABLE I: Values of k , for the Quenching of *Ru(bpm)3z+ in Aqueous Solution"
quencher (concn range) EDTA (0.010-0.10 M)
TEOA (0.005-0.20 M) ascorbate ion (0.0002-0.08 M) cysteine (0.025-0.15 M)
k,, M-' s-IC 1.5 X lo7
7.8
pH 6.0 7.3 9.5 11.0 10.0
4.0, 11.3
10.0
2.8 x 109
6.4
3.0 x 107
7.1
1.0 x 1.8 x 5.8 X 8.6 X 1.3 x 1.5 x 1.8 x
PK,b 0.0, 1.5. 2.0,
2.7, 6.1, 10.2
1.9, 8.2, 10.3
7.4 8.1 8.5
9.1 9.4 10.0 10.5 reduced glutathione (0.002-0.015 M)
2.3, 3.6, 8.8, 9.7
6.3 x 107 8.0 x 107 9.9 x 107 2.3 x 107
2.3 x
11.1 11.4
2.5 x 2.5 x
7.5 9.1 10.0
3.2 x 1.2 x 2.3 x
11.5
2.7 x
108 108
320
IO8 lo8
109 109 109 109 109 109 107 109 109 109
1.2
480
520
560
600
: .J
0.Q-
-
1.5 -
-
5i 0.6 0.6-
Y)
n U
0 0 0.0
'E 1.0
440
Figure 3. Transient absorption spectrum obtained 5 ps after the 355-nm laser flash photolysis of Ar-purged solutions containing 64 pM Ru(bpm),2+and 0.01 M ascorbate ion at pH 7.8.
0.075 ps; T 22 OC. bpK, of the conjugate acid. cValues reproducible to within *5-10%.
5.
400
A, nm
" [ R ~ ( b p m ) ~=~64 + ]pM; air-saturated solutions; p = 1.0 (Na,SO,); 7oair =
360
-
50
100
150,
200
250
33 IO
300
1, sec
72
Figure 4. Absorbance at 605 nm as a function of irradiation time for the
x
continuous photolysis of Ar-purged solutions containing 64 pM Ru( b ~ m ) , ~and ' 20 mM MVZ+at pH 9.3. [TEOA]: A, 0.10 M; 0.20 M; W, 0.80 M.
w
+,
0.0 300
A. is the absorbance obtained from the extrapolation of the decay (kobs= 2.9 X lo7 s-I) back to the midpoint of the flash, and Af 350
450
400
500
550
X,nm
Figure 2. Absorption spectra of *Ru(bpm)?+ (0)and R ~ ( b p m ) (-) ~~+ in water at natural pH. Inset: absorbance change recorded 20 ns after the 355-nm laser pulse; average of I O shots.
cysteine, reduced glutathione, and ascorbate ion. N o quenching of the luminescence intensity was observed for solutions containing 0.33 M C2042-or 0.10 M MVZ+at pH 7.5. As expected, the reducing abilities of the quenchers diminish markedly upon their protonation, although EDTA and the thiols (RS-) still exhibit quenching into mildly acidic solution, and ascorbate ion would be expected to do so as well. Pulsed Laser Flash Photolysis. The absorbance changes at 20 ns after the laser flash (A,, 355 nm), corresponding to the bleaching of the ground state and the formation of *Ru(bpm)32+, are shown in the inset to Figure 2; the light incident on the sample was adjusted so as to minimize the occurrence of biphotonic processes. The absorption decayed completely via first-order kinetics with ko = 1.4 X lo7 s-l ( T ~= 0.071 ps). When monitored at 460 or 490 nm in the presence of 0.5 M TEOA at pH 10.2, the absorbance of the solution is decreased as a result of the laser pulse (A,,, 532 nm). Over the course of the next few microseconds, the absorbance increases via first-order kinetics and reaches a plateau at a level greater than that of the 460 nm, A f / A o= -0.34 0.03, where original solution. For A,
*
is the absorbance at the plateau. The quenching of *Ru(bpm)32+by ascorbate ion yields the transient absorption spectrum in Figure 3 which disappears over the course of a few hundred microseconds via second-order kinetics. From the slope of a plot of 1/Abs vs time, the value of k / A c at 490 nm is determined to be 2.8 X lo5 cm SKI. Continuous Photolysis. The Ru(bpz)32+ (50 pM)/MV2+/ EDTA (0.10 M) system at pH 8-9, for which @(MV'+) = 1.3 (Aexc 440 nm) independent of MV2+ (0.5-40 mM) in the absence of air: was used as the actinometric standard. The rate of MV" formation of the actinometer solution, determined at 605 nm with c605 = 1.37 X lo4 M-' cm-' ,2o was compared with that of the system of interest under conditions of constant light intensity at the same A,,; the intensity of absorbed light (I,) of the sample solution at 440 nm was calculated from Beer's Law. Plots of Abssos as a function of irradiation time were linear and generally passed through the origin. For those systems with low rates of MV" production, an induction period, representing the time necessary for the scavenging of adventitious O2 by MV", was observed. Typical plots are shown in Figure 4. Values of @(MV'+) are given in Table I1 for systems with sacrificial quenchers (D = EDTA or TEOA), which were the only ones studied. (20) Watanabe, T.; Honda, K. J . Phys. Chem. 1982, 86, 2617.
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6083
One-Electron Reduction of R ~ ( b p m ) ~ ' + TABLE 11: @(MV'') from the 440-nm Continuous Photolysis of Deaerated Aqueous Solutions Containing 64 pM Ru(bpn&*' IMV2'1, M DI, M PH +(MV'+)O D = EDTA 0.020 0.10 0.034 0.020
0.10 0.10 0.10 0.10
0.020 0.020 0.020 0.10 0.034 0.010 0.034 0.020
0.20 0.60 0.80 0.80 0.10 0.60 0.60 0.80
6.0 6.0 9.5 10.0
0.053 0.057 0.59 0.53
D = TEOA
-
"p
T
0.32 0.68 0.76 0.72 0.18 0.56 0.64 0.72
9.3 9.3 9.3 9.3 10.0 10.0 10.0 10.7
= 1.0 M (Na,SO,); values of +(MV") 22 "C.
are reproducible to *5%;
0.10
Figure 6. Observed first-order rate constants a s a function of [MV2+] obtained from the changes in absorbance a t 602 nm from the pulse radiolysis of N20-saturated solution containing 0. IO mM Ru(bpm)32' and 0.1 M H C 0 2 - a t pH 13.0. Optical path = 2.0 cm; dose per pulse = 2.0 Gy.
D
n
a 0.05
60
0.00 240
300
420
360
400
540
600
660
720
h,nm
Figure 5. Observed spectra obtained from the pulse radiolysis of a N20-saturated solution containing 70 pM Ru(bpm)?+ and 0.1 M HCO, a t pH 3.0 ( 0 )and pH 13.0 (0). Optical path = 2.0 cm; dose per pulse = 9.7 Gy. Inset: effect of pH on the absorbance a t 490 nm.
40
-
20
-
+
-
R ~ ( b p m ) ~++C 0 2
(e)
containing 0.1 M HCO2- and 25-70 pM complex at pH 3.0, 7.0, and 13.0; the radiation dose was varied between 5 and 20 Gy per pulse. The value of k, was (6.7 f 0.7) X lo9 M-* s-I, independent of pH; the rate constant for the reduction of Ru(bpz):+ by C02'is 1.3 X 1 O l o M-' s-l.Io As shown in Figure 5, the spectra obtained at pH 3.0 and pH 13.0 are different. From the change in absorbance at 490 nm as a function of pH, a "titration curve" (inset, Figure 5) is obtained, from which the pK, of the conjugate acid of Ru(bpm),+ is evaluated to be 6.3 f 0.3. Because of the localization of the electron on one of the ligands of the reduced species~'~22 the basic and acidic forms of R ~ ( b p m ) ~are + written as [Ru"(bpm),(bpm')]+ and [ R ~ * ~ ( b p m ) ~ ( b p m H 'respectively. )]~+, Henceforth, the reduced forms will be indicated generically as R ~ ( b p m ) ~ + , with the specific designations used as necessary for clarification. Unlike Ru(bpz),+, the basic form of which is stable indefinitely in the absence of O2 (although the acidic form undergoes slow disproportionation),'O both forms of R ~ ( b p m ) ~are + unstable. At pH 3.0, two decay processes were observed, the first of which gave a good first-order plot with k = (3.6 f 0.2) s-l; due to the instability of the analyzing light in the longer time frame, only the value of the first half-life (-2.5 s) of the second process could be obtained. At pH 7.0 (unbuffered; defined by the presence of 0.1 M HC02-), a decay on the millisecond time frame was ob(21) Tait, C. D.; Donohoe, R. J.: DeArmond, M. K.: Wertz, D. W. Inora. Chem. 1987, 26, 2154. (22) Gex, J.-N.; Brewer, W.; Bergmann, K.;Tait, C. D.; DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. J. Phys. Chem. 1987, 91, 4176.
,
/I
I
c (10
4
a
Pulse Radiolysis. The reduction of R ~ ( b p m ) ~by~ + C02'radicals (reaction e) was accomplished in N20-saturated solutions R ~ ( b p m ) ~ ~C02'+
I
-
Dol8
;! '
8
xl
[MV"]
0
e
, NM
'
so
6084
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
Neshvad et al.
''o
0.001 240
I 360
600
480
X,nm
Figure 8. Spectral changes (vs air) from the continuous radiolysis (dose rate = 6.0 Gy min-') of a N20-saturated solution containing 50 FM Ru(bpm)32tand 0.1 M HC0,-at pH 13.0 optical path = 1.0 cm. The spectra correspond to 0, 3.3, 6.6, 9.9, and 13.2 min of irradiation; the arrows indicate the direction of the changes. The dashed line represents the spectrum of the irradiated solution equilibrated with air; the lowest solid line shows the spectrum of H 2 0 vs air.
generated quantitatively (C = 6.5 h 0.5, independent of [MV2+]), via first-order kinetics (Figure 6) with a [MVzt]-dependent rate constant. At pH 3.0, the initial formation of MV" via reaction f was followed by a much slower process (reaction e), leading to the generation of more MV'+; the MV" formed under these conditions was, however, unstable, disappearing in 1 min. The first-order rate constant of the slower formation of MV" increased linearly with increasing [MVz+]with, however, a nonzero intercept (Figure 7 ) ; kobs= 6.3 (1.0 X 106)[MV2+]s-'. In addition, as the inset to Figure 7 indicates, the value of G(MV'+) increased nonlinearly with increasing [MV2+]. These two facts indicate that an equilibrium is established among the reactants and products of the reaction of Ru(bpm)3+with MV2+ in acidic solution. Continuous Radiolysis. The continuous irradiation (dose rate = 6.0 Gy min-I) of a N,O-saturated solution containing 0.1 M HCO, and 50 KM Ru(bpm)?+ at pH 13.0 produced an increase of absorbance across the entire spectral range examined (270-600 nm) (Figure 8). The exposure of the irradiated solution, which had received a dose corresponding to the generation of about 1 equivalent of reducing species per equivalent of complex initially present, to air resulted in spectral changes, the extent of which depended on the time elapsed after the irradiation and the presence of absence of NzO. Specifically, when the equilibration with air was performed immediately after the end of the irradiation period, the spectral changes occurred rapidly and indicated that the starting material was regenerated in good yield. When the N20-saturated solution was kept free of air, spectral changes indicating that the starting material was regenerated in good yields occurred on the time frame of 3-4 h. At that stage, the sample was analyzed for gaseous products; G(H2) = 0.9 i 0.1 and G(N2) = 5.2 f 0.2. An identical value for G(H2) was obtained from the irradiation of a solution that had been degassed rather than having been saturated with N20. This value of G(H2) is readily explained in terms of the yield of H2 generated in the primary radiolysis act (0.45) and the scavenging of radiation-generated H' atoms (0.6) by HCO;. The value of G(Nz) is in excess of about 2 units with respect to the yield of N, (3.1) that is obtained from the scavenging of eaQ-by N 2 0 , indicating that, under these conditions, -30%
-
+
A ,nm Figure 9. Spectral changes (vs air) from the continuous radiolysis (dose rate = 6.0 Gy min-I) of a N20-saturated solution containing 50 pM Ru(bpm),2+ and 0.1 M HCOT at pH 7.0; optical path = 1.0 cm. The spectra correspond to 0, 6.6, 13.2, 19.8, and 26.4 min of irradiation; the arrows indicate the direction of the changes. The dashed line represents the spectrum of the irradiated solution equilibrated with air; the lowest solid line shows the spectrum of H 2 0 vs air.
of Ru(bpm)3+, or its degradation products, are able to reduce N 2 0 to N2. When the irradiation was performed on a degassed solution, the spectral changes induced by the irradiation were virtually the same (for the same dose) as those obtained in the presence of N20. Under these conditions and with the sample kept free of air, slow (over many hours) postirradiation spectral changes were observed. When no further changes were detected, equilibration with air had no effect on the spectrum of the solution, which differed considerably from the spectrum obtained prior to the irradiation, particularly in the 350- and 500-nm regions. Continuous irradiation at pH 3.0 and 7.0 of N20-saturated solutions produced spectral changes that were very much alike, but different from those observed at pH 13.0. Three well-defined isosbestic points were observed at 334, 436, and 466 nm and were maintained for a total dose corresponding to the generation of up to about 2 equiv of reducing species. The spectral changes observed at pH 7.0 are shown in Figure 9. The exposure of the irradiated solutions to air caused a slow partial recovery, over the course of hours, of the spectrum of the starting materials. After the end of the period of continuous radiolysis, a degassed solution of MV2+ was added immediately to the irradiated solution under an atmosphere of He. At pH 3.0, the formation of MV'+ was not detected. At pH 13.0, MV" was seen to form rapidly with C(MV") = 4.8; a correction was applied in order to take into account the OH--induced reduction of MV2+.23 The amount of MV" formed in the blank experiments was less than 10% of that obtained by mixing MV2+and irradiated Ru(bpm)32+solutions. Cyclic Voltammetry. These measurements were performed on Ar-purged solutions containing 0.30 mM R ~ ( b p m ) , ~and + 0.1 M Na2S0, at pH 3.0 and pH 13.0. In alkaline solution, three cathodic peaks at -1.0, -1.18, and -1.32 V and two anodic peaks at -0.94 and -1.12 V (vs SCE) were observed. In Figure loa is shown the voltammogram obtained at pH 13.0, limited to the two reversible processes. At pH 3.0, as shown in Figure lob, a cathodic peak, which was followed by another process occurring close to the same potential, was obtained at --0.70 V vs SCE. Under (23) Novakovic, V.; Hoffman, M. 2. J . Am. Chem. Sac. 1987. 109,2341.
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6085
One-Electron Reduction of R ~ ( b p m ) , ~ +
solution, and tcewould be given by eq B.
b
a
-
* R ~ ( b p m ) , ~++ D [Ru(bpm),+--D,,']
-0.2
-0.6
-1.4 -0.2 E ,Volt
-1.0
-0.6
-0.4
-0.8
I
tce
Figure 10. Cyclic voltammograms of Ar-purged solutions containing 0.30 mM Ru(bpm)32+and 0.1 M Na2S04at (a) pH 13.0 and (b) pH 3.0 at a glassy carbon electrode with a saturated calomel reference electrode. Scan rate = 0.4 V s-l.
these conditions, an anodic peak at --0.67 V vs S C E was observed, indicating a partial reversibility of the system.
Discussion Based on what is known about R ~ ( b p z ) , ~ + the , ~behavior of R ~ ( b p m ) , ~as + a photosensitizer upon the reductive quenching of its excited state by an electron donor (D) can be described, in the first instance, by the general mechanism of reactions 1-10, where Do< is the radical produced as a result of the one-electron oxidation of the quencher and Drd*is the reducing radical derived from the transformation of DOx'.
+ + + + + hu
R~(bpm),~+ *R~(bpm),~+
(1)
* R ~ ( b p m ) , ~ + R ~ ( b p m ) , ~+ + huf
(2)
k0
k,
*R~(bpm),~+D
Dox*
Ru(bpm),+
+ Dox* R ~ ( b p m ) , ~++ D
Ru(bpm)3+
ke.
(3) (4)
ku
Dox*
Dred*
kra
Drd* + R ~ ( b p m ) , ~ + Ru(bpm),+ Ru(bpm)3+ Drd*
MV2+
MV2+
(5)
Ru(bpm),+ K,
H+
kd
+ products
protonation
+ R~(bpm),~+ + products
MV'+
kd!
MV"
(6) (7) (8)
(9)
k k
Dred*
products
(10)
The efficiency of reaction 3 with regard to the escape of the redox pair from the solvent cage, in which they were generated by the quenching interaction (reaction 3a), into bulk solution can be expressed as eq A, where kbt and k, are the rate constants for the back electron transfer between the geminate radical pair (reaction 3b) and the diffusion apart of those species, respectively (reaction 3c). * R ~ ( b p m ) , ~++ D [Ru(bpm),+.-D,,'] [Ru(bpm),+-D,,'] %e
-
[R~(bpm)~+.-D,,']
(3a)
+D
(3b)
k*
[Ru(bpm),+.-D,,']
Ru(bpm)?+
5 Ru(bpm),+ + Dox*
= kce/(kbt + kce)
(3c) (A)
For sacrificial donors (EDTA and TEOA), transformation reaction 5, in which EDTAd* and TEOArd* are formed, is rapid and irreversible and competes with reaction 4. We have suggestedS that this irreversible transformation may occur to some extent within the quenching solvent cage of reaction 3 in competition with back electron transfer between the geminate pair (reactions 3b and 3d). If so, reactions 4 and 5 would not occur in bulk
(3a)
R ~ ( b p m ) ~ ~D+
+
(3b)
[Ru(bpm),+-Drd*]
(3d)
k*
k,'
[Ru(bpm),'-D,,']
[Ru(bpm),+-.D,,']
kd
Ru(bpm),+
+ Dred*
= kt,'/(kbt -k kt;)
(3e) (B)
For thiols, which are semisacrificial donors, reaction 3 generates RS' radicals into bulk solution; in the presence of a sufficiently high concentration of quencher, reaction 4 does not compete with lo9 M-' s-' ) r eaction of RS' with RS- that the very rapid ( k establishes an equilibrium (reaction 5a) with reducing RSSR'radicals ( K lo3 M-1).24
-
-
RS'
+ RS-
s RSSR'-
(5a)
For ascorbate ion, a nonsacrificial donor because its oxidized radical does not convert into a nonoxidizing species, reaction 5 is not operative, and reaction 4 annihilates the redox products. Redox Properties of R ~ ( b p m ) , ~ +Because . the quenching reaction and the secondary reduction steps are electron transfer in nature, the reduction potentials of the ground- and excited-state complexes will control, in large measure, the rate constants of the reactions. From the average of the reversible cathodic and anodic potentials at pH 13.0, we obtain values of -0.97 and -1.16 V vs SCE for the first two reductions of R ~ ( b p m ) , ~in+ HzO, corresponding to the formation of Ru(bpm),+ and Ru(bpm),O, respectively; Rillema et al., reported -0.91 and -1.08 V vs SCE in CH,CN for the same processes measured by differential pulse polarography. From our data, we obtain a value of -0.73 V vs N H E for the first reduction potential of R ~ ( b p m ) , ~ +By . comparison, EO(Ru(bpz)?+/+) = -0.50 V vs NHE.'O Thus, it is harder to reduce R ~ ( b p m ) , ~than + R ~ ( b p z ) , ~ +correspondingly, ; Ru(bpm),' is a stronger reductant than is Ru(bpz),+. The cyclic voltammogram in acidic solution is only partially reversible, indicating the instability of [ R ~ " ( b p m ) ~ ( b p m H * ) ] ~ + during the scan sweep. It is possible to estimate a reduction potential of --0.5 V for the R ~ ( b p m ) , ~ + H+/[Rull(bpm)Z(bpmH')I2+ couple from the data. The corresponding potential for R ~ ( b p z ) , ~is+ -0.30.'0.11 Inasmuch as *Ru(bpm):+ lies 1.9 V above the ground state,3 the reduction potential of the excited state is 1.2 V; the corresponding value for R ~ ( b p z ) , ~is+ 1.6 V.5 Thus, *Ru(bpm)?+ is a weaker oxidant than is its tris(bpz) analogue by about 0.4 V. Properties of * R ~ ( b p m ) , ~The + . excitation of R u ( b ~ m ) , ~ + generates the luminescent metal-to-ligand charge-transfer (MLCT) state ( * R ~ ( b p m ) , ~ = ' *[R~~~'(bpm)~(bpm'-)]~+) via reaction 1; the efficiency of population of that state (7,) has been taken before to be 1.25 The difference in the values of T~ for * R ~ ( b p m ) , ~and + *Ru( b p ~ ) , ~has ' been speculated upon in great detail by Allen et al.25 By writing T~ in terms of eq C, where krd and k,, are the rate
+
-
-
-
-
l / r 0 = krd
+ k,, + k'exp[-AE'/kT]
(C)
constants for the radiative and nonradiative modes of decay of the excited state, respectively, k'is the rate constant of population of near-lying dd states, and AEf is the MLCT dd energy gap, they showed that krd and the temperature-dependent term were essentially the same for the tris(bpm) and tris(bpz) complexes, at least in propylene carbonate. They concluded that the difference in T~ resides almost exclusively in the values of k,,, which differ by a factor of 10. They attributed the significantly lowered
-
-
(24) Hoffman, M. 2.; Hayon, E. J . Phys. Chem. 1973,77, 990. ( 2 5 ) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J. Am. Chem. SOC.1984, 106, 2613.
6086
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
70 values for complexes possessing bpm ligands, which is demonstrated by the whole series of mixed bpy, bpz, and bpm com' plexes, to electronic structural differences between the ligands which govern the important vibrationally induced electronic coupling term. Perhaps important as well is the involvement of vibronic coupling to the solvent in enhancing the nonradiative decay of * R ~ ( b p m ) , ~ +It. is anticipated that dipolar interactions between the solvent and that complex in the ground or excited state will be greater than for the bpz analogue because of the adjacent positions of the peripheral nitrogen atoms. This feature could serve as the explanation of our observation that ro is shorter for *Ru(bpm),2+ in water than in organic solvents, the reverse of that observed for * R ~ ( b p z ) ~ ~ + . ~ ~ The differential absorption spectrum of * R ~ ( b p m ) , ~relative + to that of the ground state (inset, Figure 2) reflects the change in absorbance of the solution upon flash excitation; A(Abs) = ( e . - t2)Ic, where e* and t2 are the molar absorptivities of the excitedand ground-state complexes at the wavelength of observation, respectively, and c is the concentration of * R ~ ( b p m ) formed ~~+ and R ~ ( b p m ) , ~lost + in reaction 1 . Inasmuch as the value of c depends on the intensity of absorbed light, which is a function of the intensity of incident light and [ R ~ ( b p m ) ~ ~the + ]obtaining , of values of tt as a function of X becomes a problem in actinometry. By using R ~ ( b p y ) , ~(er+ = 7.0 X lo3 M-'cm-' at 450 nm)27and benzophenone in benzene (e. = 7.2 X lo3 M-l cm-' at 533 nm)28 as the actinometric standards, the corrected spectrum of *Ru(bpm)32+,based on the extrapolation of its decay back to the midpoint of the laser pulse, is calculated and shown in Figure 2; for comparison, the ground-state absorption spectrum of Ru(bpm)32+is also given. The spectrum of * R ~ ( b p m ) , ~ + which , is very similar to that reported in CH30H,I8shows an enhanced absorption at 360 nm which has been attributed to that of the bpm'- ligand. In the 370-540-nm region the excited state absorbs less than does the ground state, a situation similar to that for the bpz29,30and bpy3I analogues. Quenching ~ f * R u ( b p m ) ~From ~ + . the values of the excitedstate reduction potentials, it is understandable why C2042- is an ineffective quencher of * R ~ ( b p m ) , ~in + comparison to *Ru(bpz):+; the values of k , for *Ru(bpm):+ are, in general, as much as an order of magnitude lower, except for ascorbate ion and the completely deprotonated thiols which still quench at, or near, the diffusion-controlled limit. From our examination of the reductive quenching of * R ~ ( b p z ) ~ ~we + , estimated ' that the E,,' values for the one-electron oxidation of TEOA and EDTA in alkaline solution, and C20d2-,could be as negative as --1.5 V and --1.7 V, respectively, if one assumed that the rate constants for the self-exchange electron transfer of the oxidized radicals were very large. E,' for generic RS- species has been calculated to be -0.84 V,32and the corresponding value for ascorbate ion is -0.68 V,,, making those quenching reactions highly exoergic. The general dependence of k , on the energetics of the quenching reaction is consistent with the assumption that there is no appreciable intrinsic barrier to the adiabatic, or nearly so, excited-state electron transfer.34 From the values of k, and ko, and [D] in the solution, the efficiency of the quenching process can be easily calculated: qq = k,[D]/(k,[D] + ko). Because ko is larger and k , values of the
(26) Haga, M.-A.; Dodsworth, E. S.; Eryavec, G.;Seymour, P.; Lever, A. B. P. Inorg. Chem. 1985, 24, 1901. (27) Rougee, M.; Ebbesen, T.; Ghetti, F.; Bensasson, R. V . J . Phys. Chem. 1982, 86,4404. (28) Carmichael, I.; Hug,G. L. J. Phys. Chem. Ref. Data 1986, I S , 1 . (29) Kalyanasundaram, K. J . Phys. Chem. 1986, 90, 2285. (30) Barqawi, K. R.; Akasheh, T. S.; Beaumont, P. C.; Parsons, B. J.; Phillips, G. 0. J . Phys. Chem. 1988, 92, 291. (31) Bensasson, R.; Salet, C.; Balzani, V. J . Am. Chem. Sor. 1976, 98, 3722. (32) Surdhar, P. S.; Armstrong, D. A. J . Phys. Chem. 1986, 90, 5915. (33) Creutz, C. Inorg. Chem. 1981, 20, 4449. (34) Balzani, V.; Scandola, F. In Energy Resources through Phofochemisfry and Catalysis; Gratzel. M., Ed.; Academic: New York, 1983; pp 1-48.
Neshvad et al.
3
t
i
i 0
300
400
500
A,
600
700
nm
Figure 11. Spectra, obtained from the data of Figure 8, of acidic ( 0 ) and basic (0)forms of Ru(bpm),+, taking G(-Ru(bpm)32+) = 6.2.
sacrificial donors are smaller for *Ru(bpm)?+ compared to * R ~ ( b p z ) , ~ +it ,is clear that the values of q, for the former case will be considerably smaller for comparable [D]. Generation of Ru(bpm),+. The difference spectra in Figure 5, when coupled with the G value of the formation of C02'-, the radiation dose, the absorption spectrum of R ~ ( b p m ) , ~ + and , the assumption that reaction e is quantitative, leads to the spectra of the acidic and basic forms of Ru(bpm),+ (Figure 11). As in the case of the spectra of the corresponding forms of Ru(bpz),+," bands identified as Lo- intraligand (350 nm)and MLCT (380-500 nm) are seen; in general, the acidic form of the reduced species is more weakly absorbing than is the basic form. The spectrum of Ru(bpm),+ has the same absorption bands, albeit with t values 20% lower, as those reported for electrochemically generated Ru(bpm),+ in DMF.21 Immediately after the laser pulse in the flash photolysis experiments, the absorbance of the solution at 460 nm in the presence of a sacrificial donor, A,, is equal to (e* - e2)1c0,where the symbols have the same meaning as above, with co equaling the concentration of R ~ ( b p m ) , ~lost + and the concentration of * R ~ ( b p m ) , ~ + formed in the flash. After the quenching reaction, which is temporally separated from subsequent reactions, is complete, the absorbance of the solution, Af, is equal to ( e l - t2)Zcf,where e l is the molar absorptivity of Ru(bpm),+ and cf equals the concentration of Ru(bpm),+ formed and the concentration of Ru( b ~ m ) , ~ still ' missing; absorption by Dox*or Drd* would be negligible at that wavelength. The quantum yield of R ~ ( b p m ) ~ + formation, 4, is the ratio of the concentration of Ru(bpm),+ formed in the quenching process to the concentration of *Ru( b ~ m ) , ~generated ' in the flash; 4 = q / c o = Af(t*- t2)/Ao(el -
-
t2).
By taking t2, e., and t 1 as 7.5 X lo3, 3.8 X lo3, and 1.1 X lo4 M-I cm-I, respectively, at 460 nm from Figures 2 and 1 1, we obtain a value of 4 of 0.3 f 0.1 for a solution containing 0.5 M TEOA at pH 10.2; the same value of 4 was obtained at 490 nm, by taking e2 and t l as 2.3 X lo3 and 9.7 X lo3-' cm-', respectively. Inasmuch as 4 = qqqcc,a value of qcc of 0.6 f 0.2 is calculated from qq = 1 (ko/kObs) = 0.52. The relatively large uncertainty is due, at least in part, to the error in the extrapolation of the very rapid decay of * R ~ ( b p m ) , ~under + the quenching conditions. In comparison, qce for the quenching of the longer lived * R ~ ( b p z ) , ~by+ TEOA (0.05 M) at pH 12 was found to be 0.47 under flash photolytic conditions.' Properties ofRu(bpm),+. The transient absorption spectrum + ascorbate in Figure 3 from the quenching of * R ~ ( b p m ) , ~with ion (HA-)33 is clearly that of a superimposition of Ru(bpm),+ and the ascorbate free radical (A'-; A, 360 n ~ n ) ~and ' the loss of the ground-state substrate. From the values of k / A e (2.8 X lo' cm s-l) and At ( e l - t2 = 7.4 X lo3 M-l cm-' ) at 490 nm given above, we calculate k = 2.1 X lo9 M-' s-' for the diffusion-con-
+
(35) Bielski, B. H. J.; Comstock, D. A,; Bowen, R. A. J . Am. Chem. SOC. 1971, 93, 5624.
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6087
One-Electron Reduction of R ~ ( b p m ) ~ , + trolled reaction 11 at pH 10 and p = 1.0 M (Na2S04) between
+
R ~ ( b p m ) ~ +A'-
+H+
R ~ ( b p m ) , ~++ HA-
(1 1)
R u ( b ~ m ) ~and + A'-, good reducing (E,,' = 0.73 V) and oxidizing (Ercd' = 0.05 V)33agents, respectively. In comparison, the rate constant for the reaction of R ~ ( b p z ) ~(E,,' + = 0.50 V) with A'is 1.5 X lo9 M-' S - ~at p = 1.0 (Na2S04).5 The pKa of [R~~'(bpm),(bpmH')]~+ (6.3), representing the reverse of reaction 7, is about 0.8 unit lower than the corresponding value for [ R ~ ~ ~ ( b p z ) , ( b p z H ' )(7.1).1° ]~+ In comparison, the apparent pK,* values quoted by Rillema et aL3 for * R ~ ( b p m ) ~ , + and * R ~ ( b p z ) are ~ ~ +2.2 and 3.5, respectively. In both cases, the bpm systems are the stronger acids by comparable amounts. The difference in the pK, and pKa* values of -4 for the reduced and excited complexes that contain a reduced ligand coordinated to the Ru" and Ru"' centers, respectively, is easily rationalized on the basis of the charges of the acidic species (+2 and +3, respectively). In contrast, the first protonation of ground-state Ru(bpz)?+, and presumably ground-state Ru(bpm)?+, occurs in 3.5% H2S04,19indicating the importance of the extra electron in the ligand system in rendering the complex a stronger base in the excited and reduced state. As pointed out above, both the acidic and basic forms of Ru( b ~ m ) are ~ + unstable toward long-term storage in aqueous solution; in contrast, the basic form of R u ( b p ~ ) ~is+infinitely stable in the + pulse absence of 0,.The second-order decay of R ~ ( b p m ) ~under radiolysis conditions in the pH 5-7 region where both the acidic and basic forms are at equilibrium is attributed to disproportionation reaction 12; [Ru"(bpm),(bpm'-)]+
-
+ [ R ~ " ( b p m ) ~ ( b p m H * ) ]H+~ + Ru(bpm)?+ + Ru(bpm),(bpmH,),+
(12)
TABLE III: Literature Values of CP(MV'+) for the Reductive Quenching of *Ru(bpm)d+ in Aqueous Solution [MVz+], M [Dl pH @(MV'+) ref 0.05 M EDTA 5 0.035' 7 0.002 0.002 0.02
8 b
0.05 M TEOA 0.6 M TEOA
0.021' 0.85c
7 6, 8
"Based on C , ~ ~ ( M V ' +=) 1.1 X lo4 M-I cm-'. , A r-purged solutions. Not reported. c(MV'+) not reported; evacuated solution.
/ 1
o
! 0
.
, 2
,
,
10
4
l/[TEOA],
1
12
8
6
M-'
Figure 12. l/@(MV'+) vs l/[TEOA] for solutions at pH 10.0 containing 10-100 m M MV2+; = 1.0 M (Na2S04).
equal to the rate constant for the reaction in the forward direction ( k f )and the intercept represents k,[Ru(bpm),2+], values of k f (1.0 X lo6 M-' s-l ) a nd k , (6.3 X lo4 M-ls-I ) a re obtained. From these values, K13-- 16 is obtained. From the absorbance values
+
5 -H+
+
[ R ~ I ~ ( b p m ) ~ ( b p m H ' ) ] ~MV2+ + R ~ ( b p m ) ~ , + MV'+ it must be remembered that the basic form is a better reducing agent than is the acidic form and that the reverse situation will (13) exist for their powers as oxidizing agents. The reduced product at 602 nm (inset, Figure 7), and from a knowledge of the dose would most likely contain a dearomatized and hydrogenated and the e value of MV'+, and by correcting for the effect of the ligand, thereby destroying the ability of the complex to act as a competition between reactions e and f, a value of K13 = 9 f 3 photosensitizer. is obtained. By taking the average of these values (K13 = 13 f Unlike the acidic form of R u ( b p ~ ) ~which + , seems to decay via 3) and Eo(MV2+/MV'+) = -0.44 V,39 a value of E' for the disproportionation with itself, R ~ ( b p m ) at ~ +pH 3.0 under pulse H+/[R~~~(bpm),(bpmH')]~+ couple of -0.50 f Ru(bpm),,+ radiolysis conditions disappears via two first-order processes; at 0.01 V is obtained, a value that is consistent with the cyclic pH 13.0, the decay is via a single first-order process. Possible voltammetric data presented above. The instability of the MV'+ decay modes involve reaction with the solvent, possibly through (in the time frame of minutes) formed under these conditions at the intermediacy of a covalent hydrate species. The more rapid relatively low [MV2+]is presumably due to the intrinsic decay decay of R ~ ( b p m ) ~in+alkaline solution suggests an interaction of [R~"(bpm),(bpmH')]~+,which would shift the position of with OH-; in comparison, the decay of Ru(bpy),+ under the same equilibrium 13 to the left. conditions occurs with a rate constant of 0.2 s-1.36 The continuous MV'+ is also formed from the reaction of Drd* with MV2+ radiolysis results confirm the interaction of R ~ ( b p m ) ~or + , its according to reaction 9. For D = EDTA and TEOA in alkaline degradation products, with N 2 0 and show that the species ultisolution, k9 = 1.5 X lo9 and 2.7 X lo9 M-I s-I, respectively, at mately formed in the various reactions are reactive, at least in p = 1.0 M (Na2S04);519at pH 4.7, k9 = 5.9 X lo5 M-' s-I for part, with 0,; the interaction of N20with low-valent metal ions Reaction 9 has also been investigated by pulse radiolysis and complexes to generate N 2 have been observed p r e v i ~ u s l y . ~ ~ ~EDTA.9 ~~ for D = EDTA; rate constants of 2.8 X lo9, 7.6 X lo8, and 8.5 Generation o f M V ' + . From the data in Figure 6 , the secondX lo6 M-' s-' were obtained at pH 12.5, 8.3, and 4.7, respectively." order rate constant for the quantitative reaction of Ru(bpm),+ Quantum Yields. Values of (P(MV'+) for the Ru(bpm)32+/ with MV2+ ( k r d ;reaction 8) in alkaline solution is evaluated to for aqueous MV2+/D system as reported by previous be (1.0 f 0.1) X lo9 M-l s-I; under comparable conditions, the solutions, given in Table 111, compared quite well with those same reaction of Ru(bpz),+ has a rate constant of 1.3 X lo8 M-' obtained in this work (Table 11). s-l under the conditions used for pulse radiolysis1° and 4.5 X lo8 For sacrificial quenchers in alkaline solution in the presence M-' s-I under pulsed laser flash photolysis condition^.^ of MV2+,where reactions 4,6, 7, and 10 are not operative (and/or The same type of analysisIO*"that was applied to the reactions of R u ( b p ~ ) ~and + its conjugate acid with MV2+ and C r ( b ~ y ) , ~ + reactions 3a,b,d,e replace reactions 3-5), the general mechanism is simplified considerably. Under these conditions, (P(MV'+) = was used to evaluate the equilibrium constant for the reaction vqqvm(qd + vd'), where p , tq,and vce have been defined before, between MV2+and [Ru"(bpm),(bpmH')I2+. From the analytical and qrd and qreJ are the efficiencies of formation of MV" from expression for the plot in Figure 7, where the slope of the line is reactions 8 and 9, respectively. In the absence of any competitive mode of decay of Ru(bpm),+ and Drd', v r d and vrd' are unity, ( 3 6 ) Mulazzani, Q.G.;Emmi, S.;Fuochi, P. G.; Hoffman, M. Z . ; Venturi,
+
M. J. Am. Chem. SOC.1978, 100, 981. ( 3 7 ) Buxton, G. V.;Sellers, R. M. Natl. Stand. Ref: Data Ser. (US.Natl. Bur. Stand.) 1978, No. 6 2 . ( 3 8 ) Mulazzani, Q.G.; Emmi, S.; Hoffman, M. 2.;Venturi, M. J . Am. Chem. SOC.1981, 103, 3362.
( 3 9 ) Summers, L. A. The Bipyridinium Herbicides; Academic: London, 1980. (40) Mulazzani, Q.G.; Venturi, M.; Hoffman, M. Z . J. Phys. Chem. 1%S, 89, 122.
6088
J . Phys. Chem. 1989, 93, 6088-6094
and @(MV'+) is independent of [MV2+]. Thus, @(MV'+) = 2qqqce.This treatment predicts that a plot of I/@(MV'+)vs l/[D] be linear with an intercept equal to 1/2qce and intercept/slope = kq/ko. Figure 12 shows a double-reciprocal plot of the data in Table I 1 for D = TEOA; slope = 0.48, intercept = 0.78, correlation coefficient = 0.996. The value of qceis calculated to be 0.64, which is in very good agreement with the value of 0.6 i 0.2 from the flash photolysis experiments. The intercept/slope ratio is 1.6 M-I; from measured values of k (2.3 X lo7 M-' s-l ) a nd ko (1.2 X IO' s-I), the ratio is 1.9 M-7 . The excellence of the agreement demonstrates the efficacy of the continuous photolysis method for the determination of qce,the surety of the knowledge of the overall mechanism, and the validity of the t values of * R ~ ( b p m ) , ~ + and Ru(bpm),+ as shown in Figures 2 and 5 toward the calculation of qce from the flash photolysis experiments. In comparison, the value of vcefor the quenching of *Ru(bpz)?+ by TEOA (0.05 M) at pH 12 is 0.47; in the presence of MV*+, the maximum value of @(MV'+) obtained in alkaline solution as qq + 1 is Inasmuch as kbt/k,, (Or kbt/k,,') = (l/qce) - 1 according to eq A or B, if k,, (or k t i ) is the same for TEOA,,' in both systems, then kb:P' = 2kb:pm. From the values of E,,' for Ru(bpm),+ and Ru(bpz),+ (0.73 and 0.50 V, respectively), we see that E' for reaction 3b, the back electron transfer reaction between the geminate pair within the solvent cage, is 0.23 V more positive (exoergic) for [Ru(bpm),+-.TEOA,,'] than for [Ru(bpz),+.-TEOA,,']. Thus, the more exoergic reaction displays the smaller value of kbt,placing this process within the "inverted Marcus" region. Recently, Ohno et showed that, in the reductive quenching of the excited states of some Ru(I1)-polypyridine complexes by nonsacrificial neutral aromatic amines in 1:l C H 3 C N / H 2 0 , kbt as a function of AGO of the redox pair -1.7 follows a "bell-shaped" curve with a maximum at AGO eV, encompassing the "normal" and "inverted Marcus" regions. From the data for D = EDTA in alkaline solution in Table I1 where qq 0.4, we calculate qce to be -0.7. At pH 6.0 where qq = 0.1 1, the near constancy of @(MV'+) with [MV2+]suggests that Ru(bpm),+ (and its conjugate acid) are completely scavenged by MV2+ and that EDTArd' is scavenged to the highest extent possible in competition with its H+-promoted decay modes. However, the uncertainty of the value of qd' makes it impossible to calculate a definite value of qce. Inasmuch as qd' has boundary
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(41) Ohno, T.; Yoshimura, A,; Mataga, N. J . Phys. Chem. 1986, 90, 3295.
Dynamics of the Triplet &e
values of 0 and 1, voewould lie in the range 0.25-0.5. Regardless of its exact value, qcc is significantly lower in acidic media than in alkaline; in comparison, qce for the quenching of * R u ( b p ~ ) , ~ + by EDTA at pH 4.7 and in alkaline solution is 0.5 and 0.7, respectively. It is reasonable to assume that, at a given pH, k , (or kw') for EDTA,,,' is the same for both complexes. In that case, kbtbPm kb:PZ in alkaline solution and kb?Pm > kb>pr in acidic solution. Summary. The one-electron reduction of Ru(bpm):+ generates Ru(bpm),+, which is a stronger reducing agent (E,,' = 0.73 V) than is Ru(bpz),+ (E,,' = 0.50 V). The conjugate acid of Ru(bpm),+, [R~"(bpm)~(bpmH')]~+, is a stronger reducing agent (E,' = 0.50 V) and acid (pKa 6.3) than is the corresponding form of Ru(bpz),+ (E,,' = 0.30 V; pKa 7.1). As a result, both the acidic and basic forms of Ru(bpm),+ reduce MV2+ (1 .O X 1O6 and 1.O X lo9 M-I s-I, respectively); only the basic form of Ru(bpz),+ is capable of that reaction. The efficiency of formation of Ru(bpm),+ (qce)from the quenching of *Ru(bpm)32+by TEOA and EDTA in alkaline solution is at least as high as in the comparable quenching reactions of *Ru(bpz)z+; in the presence of MV2+,the quantum yield of MV" is given by a simple expression: @(MV'+) = 2 qqqce. Unfortunately, three properties of R ~ ( b p m ) , ~and + its conjoiners conspire to make it a less desirable photosensitizer than Ru(bpz)?+: (1) a significantly shorter excited-state lifetime (-0.1 vs 1 p s ) ; (2) a less positive excited-state-reduction potential (1.2 vs l .6 V), causing values of k,, and hence qq for comparable quencher concentrations, to be smaller; (3) the instability of both acid-base forms of Ru(bpm),+ in aqueous solution toward degradative processes, making them unamenable to long-term storage, unlike [Ru"(bp~)~(bpz')]+, which is very stable in alkaline solution.
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Acknowledgment. This research was supported in part by the Office of Basic Energy Sciences, Division of Chemical Sciences, U S . Department of Energy, in part by Consiglio Nazionale delle Ricerche of Italy (Progetto Strategic0 "Tecnologie e Metodologie Radiochimiche Avanzate"), and in part by Minister0 della Pubblica Istruzione of Italy (Quota 40%). The collaboration between Q.G.M. and M.Z.H. is part of the US.-Italy Cooperative Research Program. CFKR is supported jointly by the Biomedical Research Technology Program of the Division of Research Resources of N I H (RR 00886) and by The University of Texas, Austin. G.N. and M.Z.H. acknowledge the assistance of Jennifer Meyer (NSF-REU participant, summer 1987) in the preliminary lifetime and quenching measurements.
and the Reverse Proton Transfer of 3-Hydroxyfiavone
William E. Brewer, 4hannon L. Studer, Michael Standiford, and Pi-Tai Chou* Department of Chemislry, University of South Carolina, Columbia, South Carolina 29208 (Received: January 23: 1989) The dynamics of the d q y of the tautomer species and triplet state of 3-hydroxyflavonehave been studied in detail by transient absorption, two-step lasdr excitation, and steady-state photolysis experiments. The decay of the tautomer species is observed to be 6.9 X lo4 s-' in nfheptane at room temperature and is only slightly dependent on various chosen nonpolar solvents. This rate is more than $ orders of magnitude smaller than the rate of excited-state proton transfer, in agreement with the experimental results re orted by Itoh et al. The bimolecular quenching rate constant of the tautomer of 3-hydroxyflavone by oxygen is determine to be 3.2 X lo9 M-I s-l in n-heptane. Two proposed mechanisms that involve the tautomer ground state and triplet state for the reverse proton transfer are presented in order to interpret the experimental results. An observed T I T, absorption in the transient absorption spectrum at -395 nm is assigned as originating from the normal species with a 7.2-ps lifetime in n-heptane. The yield of the triplet states is determined to be 0.18 i 0.02 as a lower limit.
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I. Introduction In 1979, Sengupta and Kashal first proposed the mechanism of excited-state proton transfer to explain the dual fluorescence (1) Sengupta, P.; Kasha, M. Chem. Phys. Lett. 1979, 68, 382.
0022-3654/89/2093-6088$01.50/0
of 3-hydroxyflavone (3-HF). Since then, 3-HF has been used as a Prototype molecule to study the mechanism of excited-state intramolecular proton transfer. Most of the studies on 3-HF and its derivatives rely on steady-state electronic spectroscopy and nanosecond to picosecond time-resolved fluorescence measurements 0 1989 American Chemical Society
