radiolytically-induced one-electron reduction of methyl viologen in aqueous solution: stability of...

Radlat Phw Chem Vol 23 No 1 2, pp 229 236, 1984 0146 5724/84 $ 3 0 0 + 00 Printed in Great Britain Pergamon Press Ltd

RADIOLYTICALLY-INDUCED ONE-ELECTRON REDUCTION OF METHYL

VIOLOGEN IN AQUEOUS SOLUTION

STABILITY OF T H E R A D I C A L C A T I O N IN A C I D I C A N D H I G H L Y A L K A L I N E M E D I A ~1

MARGHERITA VENTURI IstItuto dl Sclenze Chlmlche, Facoltfi dl Farmacia, UniversitY, di Bologna, 40126 Bologna, Italy

and

QUINTO G. MULAZZANIt Istltuto dl Fotochxmlca e Radlazlonl D'Alta Energla, Conslgho Nazlonale delle Rlcerche, 40126 Bologna,

Italy

and

MORTON Z. HOFFMAN Department of Chemistry, Boston University, Boston, MA 02215, U S A

(Received 22 January 1983, accepted 21 March 1983)

Abstract--Pulse and continuous radiolyses have been used to investigate the stability of the reduced methyl viologen radical cation (MV + ) in acidic and highly alkaline aqueous solution The reaction of the methyl vlologen dlcation (MV 2+) with (CH3)2COH and (CH02CO- radicals generates MV + rapidly (k = 2 9 _+ 0 2 x 10 ° and 6 7 _ 0 3 x 109 M -~ s 1, respectively) The absorption spectrum of MV + is the same at pH 1, natural pH, and pH 13 suggesting that MV + is not involved in acid-base equlhbrla in that pH range Between pH 0 and 2, MV + disappears via second-order kinetics with kob s an inverse function of [MV 2+] and pH The decay of MV + occurs via H +-assisted dlsproportlonatlon and yields ultimately a hydrogenated species with /]'max 220, 255nm (Ema x 8 2 X 103, 4 1 X 103M lcm-~, respectively) At pH 13, MV ÷ is infinitely stable in the absence of 02, further reduction of MV ÷ yields the moderately stable MV ° species with 2m~ x 368nm(em~x36× 104M-Lcm ~) and a shoulder at 370 nm(E3 0 × 104 M - ~cm- i) Reaction of MV ° with O 2 yields MV 2+ quantitatively via two one-electron oxidation steps Acidification of MV ° In the absence of 02 yields the same air-insensitive hydrogenation product as is obtained from the disproportionatlon of MV ÷ in acidic solution The relationship of these observations to the use of MV 2 + as an electron relay species in photochemical solar energy conversion schemes is examined.

I N T R O D U C T I O N

THE PHOTOCHEMICAL sys tem based on me thy l viologen (1 ,1 ' -d lme thy l -4 ,4 ' -b lpy r ld lmum ion, M V 2 +) ac t ing as an e lec t ron relay and Ru(bpy) 2 + (bpy = 2 , 2 ' - b l p y n d m e ) ac t ing as a pho tosens l t l ze r has

been the m o s t extensively inves t iga ted a m o n g those p r o p o s e d for ef fec tmg the p h o t o g e n e r a t l o n o f H2 f rom water /2 16) In this system, M V 2+ reacts wi th

*Ru(bpy)~ +, the e lectronical ly exci ted s ta te o f the pho tosens l t l ze r , giving Ru(bpy)~ + and the me thy l v lo logen radical ca t ion , M V .+ .

T h r o u g h the l n t e r m e & a c y o f a sui table r edox catalyst , the la t ter species reduces wa te r to H2 regen- e ra t ing M W +, whereas Ru(bpy)~ ~ ts r educed to Ru(bpy)~ + by a sacrificial e lec t ron d o n o r , such as E D T A ( e t h y l e n e d m m m e t e t r a a c e t a t e ) o r T E O A ( tn-

tVlslting Scholar, Boston University, Fall 1981

H 3 C - + ~ ÷-c H 3

M V2+ (and o t h e r contTlbUtlng ~csonance forms)

M y .+

e thano lamlne ) T h e durabi l i ty o f these sys tems appears to be hml t ed by the occur rence o f a paras i t ic process leading to the h y d r o g e n a t i o n o f M V 2 + (6 8 ii 17)

In the course o f our inves t iga t ion "8) o f the Pt-

929

230 M V F N T U R I el al

ca ta lyzed f o r m a t i o n o f H 2 f r o m rad lo ly t l ca l ly gene ra -

ted M V ÷, we n o t e d t ha t in acidic s o l u t i o n a n d in the

a b s e n c e o f Pt the rad ica l cat~on d i s a p p e a r e d w~thout

g e n e r a t i n g h y d r o g e n , S mce the p r o d u c t o f th is reac-

t ,on is m o s t hke ly h y d r o g e n a t e d m e t h y l w o l o g e n , it

a p p e a r e d essent ia l to m v e s t t g a t e this p roce s s in deta i l

in o rde r to e s t a b h s h the e x t e n t to w h i c h it c o u l d

c o n t r i b u t e to the i n a c t i v a t i o n o f the p h o t o c h e m i c a l

s~s tem

In th is w o r k we h a v e u s e d the t e c h m q u e s o f pu l se

a n d c o n t i n u o u s rad lo lys t s m o rde r to g e n e r a t e M V

in a q u e o u s s o l u t m n a n d to i nves t i ga t e its s t a b l h t y as

a f u n c U o n o f p H T h e g e n e r a t m n o f M V * by pu l se

radsolys ls ha s been p r e v i o u s l y r e p o r t e d (m-24~

E X P E R I M E N T A L

Matenah Methyl vlologen dichloride (Aldrich) was recrystallyzed

from water by addition of acetone, collected on a slntered filter and dried over CaC12 in a vacuum desslcator. Aqueous solutions of MV 2 ~ showed 2m.~255 nm(%~x2 0 x 104 M Jcm t) in agreement w~th a recent determination ~25~ The punficatlon of 2-propanol, acetone, and H20 has been described before ~26~ The pH of the solutions was adjusted with H2SO 4 or NaOH (Merck, Suprapur) Solutions were deaerated by bubbling with argon or degassed by s tandard vacuum line technique

(2) OH/H + (CH3)2CHOH ~ H20/H 2 + (CHj)2COH k2= 1 3 × 109/50~ 107M ts I(Ref 28,29)

Hydrated electrons are ettic~ently scavenged by H ' (reaction 3) or by acetone (reaction 4~

(3) c,q 4 tt ~ ~|1

/~ 22 ~< 10 mM Is J(Ref 30)

H , (4) (CH02CO + eaq-- *(CHd.,(2OH

/ ,~= -59× 10"M ~s i (Ref 30~

Thus, in deaerated acidic solution containing 2-propanol, the (CH02COH radical is the main species that is reactive toward MV 2 * In neutral and alkaline solutions containing 2-propanol and acetone reactions (2) and (4! are opera- twe In alkaline solution, howe~er, the (CH02COH ra&cal undergoes deprotonatlon gwmg the (CHd2CO radical rcacnon 5)

(5) (( ' I{0,( 'OH,-~-(CtlJ~(O ~- tt

p / k = 122 (Ref ~1

R E S U L T S A N D D I S C U S S I O N

Pulse radlolysts W h e n s o l u t l o n s o f 2 5 x 10 ~ M M V :~ c o n t a i n i n g

0 1 M 2 - p r o p a n o l at p H 1. or 0 1 M each o f

Pro( edltreA Continuous radlolyses were carried out at room tem-

perature on 10-25 ml samples of degassed solution con- tamed m pyrex or silica cylindrical vessels (total volume = 100 ml) fitted with spectrosll optical cells on a side arm Absorption spectra were recorded with a Perkln-Elmer 555 spectrophotometer The absorbed irradiation dose from the two ~Co },-cell sources (Atomic Energy of Canada, Ltd ) used in the course of the work was determined by the Frlcke chemical dosimeter assuming G(Fe TM) = 15 5, where G(x) represents the number of molecules of species x formed or destroyed per 100eV of energy absorbed by the solutions, and had values of 7 50 x 10meV L t mln i ( 1 2 0 k r a d m m ~)and 1 4 4 x 1 0 2 1 e V L - l m i n 1 ( 2 3 0 k r a d mln ~). respectively Pulse radlolyses with opncal absorp- non detection were performed using the 12 MeV Linear Accelerator of the F R A E Institute, C N R , Bologna ~27) The irradiations were performed at room temperature on samples contained In a spectrosfl cell of 2 cm optical path length The exposure of the solutmns to unnecessary UV light was avoided or minimized by means of cut-off filters and a shutter The radlaUon dose per pulse of electrons was monitored with a physical dosimeter cahbrated with a 0 1 M KSCN aqueous solution saturated with 02 using G( _.2 15 x 104 at 500nm

Generation o/reducmg radwals The radlolysls of aqueous solution generates e.q, OH

radicals, H atoms, and the molecular products H 2 and H202 according to reaction (1), where the

(1) H20 ~ e.q(2 8), OH(2 8), H(0 6),H2(0 45), H202(0.8 )

numbers m parentheses represent the G-values of the species In the presence of 2-propanol H a toms and OH radicals generate (CH02(~OH radicals via reaction (2)

0 4

O3

O2

o/ r

350 450 5 5 0 650 nm

FIG 1 Average of the observed spectra obtained from the pulse radlolysls of deaerated aqueous solutions of MV 2" (2× 10 4M) c o n t a l n l n g 0 1 M 2 - p r o p a n o l a t p H l o r 0 1 M each of 2-propanol and acetone at natural pH and pH 13 Opncal path length = 2cm. d o s e / p u l s e - l krad At each

wavelength the qandard devtanon wa~ -< I0"

Ra&olytlcally-lnduced one-electron reduction of methyl vlologen m aqueous solution

2-propanol and acetone at natural pH or pH 13, are pulse irradiated with the same dose, the same spectrum after ~ 8#s is obtained irrespective of the condmons employed The spectrum obtained ~s un- questionably that of MV-+ for which ~-m~ 606nm(Em~137x 104M icm t) and )'max 396nm (Emax4.21 X 104M-~cm -~) have been recently rede- termmed. (2s) In Fig. 1 is shown the spectrum of MV -+ as the average of the three spectra obtained at pH 1, natural pH, and pH 13. At every wavelength the standard dewatlon was ~< 10~ From the absorption detected at 605 nm and from the knowledge of the dose delivered to the sample, G(MV -+) = 6.2 ___ 0 3 was calculated assuming E1 37 × 10~M ~cm m at this wavelength. F rom the pseudo first-order rate of formation of MV +, which was proport ional to [MV2÷], a value of k 6 = 2 9 + 0 . 2 x 109M-Is -~ was obtained at pH 1 and natural pH for the reduction of MV ~ ÷ by (CH3)2(~OH ra&cal (reaction 6).

(6) MV 2÷ + ( C H s ) 2 C O H ~ M V ~. + (CH3)2CO + H +.

For the same process, Melsel et al. I22) have reported k 6 = 3 5 _ + 0 . 2 × 109M-ls -~ . For the reduction of MV z+ by the (CH3)2CO radical (reaction 7)

(7) MV 2 + + (CH3)2CO ~ M V + + (CH3)2CO

we have obtained k 7 = 6.7 + 0 3 × 109 M ~s- 1 at pH 13.

Solar et al (241 have recently reported that two &fferent species are formed when H atoms react with MV z + at pH 1. They have suggested that one of these species is the protonated form of the radical catloni" (HMV 2 + ) for which they determmed p K a = 2.9 _ 0.1. According to these authors, the protonated form of the methyl wologen ra&cal cation absorbs in the same region in which the unprotonated form absorbs, but the intensity of the absorption is considerably reduced by the protonat~on.

Inasmuch as we observe that the spectrum of the methyl vlologen radical cahon is the same at pH 1, natural pH, and pH 13, we must conclude, m contrast w~th Solar et al., (24) that MV + does not appear to change its protonat ion state between pH 1 and pH 13 Because of this disagreement on the pro- tonatabfl~ty of MV ÷ in strongly acl&c solution, we have briefly reinvestigated the reaction of MV z ÷ and H atoms at pH 1. A deaerated solution of MV 2+ containing 0 1 M (CHs)sCOH at pH 1 was pulse

~ln this paper, a methyl vlologen symbol preceeded by an H represents protonaUon at a nng-mtrogen, the symbol followed by an H represents hydrogenation at a nng- carbon

231

irradiated with a dose -~ 0 5 krad per pulse. Under these conditions, e~ ~s converted into H according to reaction (3) and (CHs)3COH reacts with OH ra&cals according to reaction (8) Inasumch as H atoms,

(8) (CHs)sCOH + OH--÷'CH2C(CHa)2OH + H20

ks=2 .1 x 10SM i s i (Ref 28)

which are formed under these conditions with G = 3 3, react very slowly with (CH3)3COH (k = 105M ~s--~), (29) they are available to interact with MV 2÷. The spectrum generated under these conditions, taken -~ 100/~sec after the pulse, is, without a doubt, that of M V t . We never observed the formation of the band m the 470 nm region which Solar et al. (24) have observed under condmons similar to those we have employed. They attributed the band at 470nm, and an other band at 310nm, to the formation of an H-adduct to one rang-carbon of M W ÷. We wish to note here that a band at 470 nm is observed (20. 32) when MV 2 + reacts ( k = 1 8 x 10 s M -~s 1)(20/with OH ra&cals Taking t 1 37 x 104 M )cm- t at 605 nm for MV +, we calcu- late that, at pH 1 and in the presence of (CH3)sCOH, the yield for the formation of MV .+ corresponds to G(MV+. )= 1.8. The less-than-stolchlometnc yield obtained under these conditions can be attributed to incomplete scavenging of H atoms by M W + and to the occurrence of a process that removes MV -+. Th~s latter process is, presumably, a reaction of MV .~ w~th the 'CH2C(CH3)2OH radical; this radical is not ex- pected, due to its low reducing ability, to reduce MV 2+ to M V . + In conclusion, the system containing MV 2 + and (CH3)3COH does not exhibit clean behav- 1our. Therefore, the use of H atoms as a reducing agent toward M W + m the presence of ter t -

butylalcohol, does not appear to be a particularly good way to study the behavlour of MV + m acl&c solution

Between pH 1 and pH 13 the methyl wologen radical cation is stable on the longest t~me frame of the pulse radiolysis instrumentation (seconds) How- ever, at pH 0 (0.99 M H2504) , the absorption of MV + is seen to &sappear wa well-defined second- order kinetics. However, the second-order rate con- stant, kobs is an inverse function of [MV2÷]. A plot of l/kobs vs the mmal concentration of MV 2+ (Fig 2) yields a strmght line with intercept and slope described by equation (9)

(9) 1/kob s = 7.6 × 10 -6 + 0.12[MV2+], M s

C o n t m u o u s radtolysts at pH 0-2 The continuous radlolysls of deaerated aqueous

solutions of 1.0 x 1 0 - 4 M MV 2+ containing 0.1 M

2~2 M V~NTURI et a/

% 4

0 2 4 6 [MV 2.] xlO 4 M

FIG 2 Reciprocal of the observed second-order rate con- stant for the disappearance of MV * at pH 0 as a function of the lnmal concentration of MV 2 + Solutions were argon- purged and contained 0 1 M 2-propanol Each point represents the average of the values obtained using doses of 0 5,

1 5, and 3 krad/pulse

2-propanol at pH 0 causes a decrease of the absorption at 255 nm due to MV 2 + and the appearance of a new band in the 220nm region MV ~ is not detected after the irradiation consistent with the pulse ra&olys]s observation that at pH 0, MV + disappears m a few seconds As is shown in Fig 3, the absorbance at 220 nm increases and that at 255 nm decreases hnearly with the lrradmUon dose Two lsosbestlc points are observed at 210 and 228nm Further ]rradmt]on of the same sample produces plateau values for the absorbance at both 220 and 255 nm From the change in absorbance at 255 nm resulting from the exhausUve ]rradmUon of the sample, which produces the spectrum shown in the reset of Fig 3, a value of G ( - MV 2+) Is obtained that is approxi- mately one-half the value of G(MV +) determ]ned under the same condmons by pulse radmlysls Thus, the disappearance of the radical at pH 0 is very likely due to a d]sproport ]onatmn reactmn with MV 2t being one of the products o f the reactmn The spectral changes occurring during the ]rradmt]on of the system are very similar to those observed ~u when MV 2~ is hydrogenated to MVH~* (l-methyl-4- ( l ' -methyl-p]perldln-4'-yl)pyr]dm]um cation) We note, however, that under our condit ions the spectrum of the product is fully developed when the

04 E

0 28 290,

02

01

0 20 40 60 Dose k tad F~(, 3 Dependence of the absorbance of" a solution ol MV ~'- (1 x 10 4 M) containing 0 1 M 2-propanol at pH 0 as a function of the continuous irradiation dose C), / -255nm, 0 , 2 - 2 2 0 n m OpUcal path length=2mm lnset spectrum of solution after irradiation with 3 × 102~ cVL ~ (48 2 krad) Evaluation of c-values was made as-

suming complete conversion of MV 2* into product

H H

H 3 C - - + N ~ ~ ~ H~N CH3

MVH~

sample has recewed an lrradlaUon dose of -~ 35 krad corresponding to the generation of shghtly more than two equivalents of the reducing radical, (CH3)2~OH, per equivalent o f MV 2 + lnmally present Thus, under our cond]t]ons, the stolchlometry of the reaction appears to be that gwen by reaction (10) where MVH22+ Js a hydrogenated product

( 1 0 ) M V 2 ~ + 2 ( C H 3 ) 2 Q ' O H - + M V H 2 2 ~ + 2(CH02CO

We can rule out the posslblhty that the product of reaction (10) is further hydrogenated by radlolytlcally-generated H 2 In fact, the same value of G(H2) = 4.2 _+ 0.1 is obtained from the lrra&atlon of deaerated solut]ons of 2-propanol (0 1 M) at pH 1 In the absence and In the presence of 5 x 10 4 M MV 2+

Radmlytlcally-lnduced one-electron reductmn of methyl wologen m aqueous solutmn 233

No attempts were made to isolate and characterize the hydrogenated product formed under our condl- nons. If the product is indeed MVH22÷, this latter species absorbs similarly to MVH5 +. Some support for this hypothesis comes from the fact that further irradiation of the solution absorbmg at 220 and 255 nm causes a decrease of the spectrum without changing its form apprecmbly This seems to indicate that the spectra of denvatwes of MV 2÷, hydrogenated to dxfferent extents, are similar to each other. Assuming complete conversmn of MV 2+ into MVHf +, the values for the molar extinction coefficients of the latter species are those given in the inset of Fig 3, specifically, E2218.5 x 10SM-lcm -1 and E2554.0 x 103 M lcm- 1.

At pH 1 (0.18M H2SO4, 0.33M NaESO4) , the radmlync behavlour of this system is the same as that observed at pH 0, the only exception being that MV ÷ is detected after the irradiation As observed at pH 0 on the Ume frame of seconds, the radical catmn at pH 1 disappears vm second-order kinetics with the reaction going to completion within several minutes. As before, kob s is an reverse function of [MV2+]. Under these condltmns, the relatmnshlp between 1/kob s and the initial concentration of MV 2+ is gwen by equa- tmn (11).

(11) 1/kob s = 1 0 x 10 -4 + 8.7[MV2+], M s .

At pH 2 (2.2 x 10 -2 M H2SO4, 0 33 M Na2SO4), the second-order disappearance of MV .+ was slower yet with kobs agam a function of the mlnal concen- tratmn of MW ÷ (equation 12)

(12) 1/kobs = 2.4 x 10 -3 + 200[MV2+], Ms.

It was reported (33) some years ago that the MV +. radical cation is rapidly destroyed by protic acids xn aqueous solunon This was attributed to the occurrence of equdlbrium (13) followed by the protonatlon of MV ° (reactmn 14).

(13) 2MV. + ~ M V 2 + + M V °

(14) MV ° + H + - ,HMV +

H3C- ~ N -CH 3

M V °

HMV +

(15) 1/kobs= (1/2ki3) + ( k 13[MV2+]/2k13k,4[H+l)

According to equation (1 5), the value of the intercept of the plot 1/kobs vs [MW ÷] is equal to 1/2k13 and should be independent of pH. Furthermore, the value of the slope should increase hnearly as the concentration of H ÷ is decreased However, the experimental data summarized in Table 1 show that the intercept is a funcnon of [H + ] and the dependence of the slope on 1/[H +] is of an order greater than one. Upon modification of the mechamsm to introduce a coupled protonatmn-dlsproportionatlon initial step (equihbnum 16) followed by formation of the final product (MVH22+), reacnon (17), equation (18) as obtained.

(16) 2MV +- + H + ~ M V 2+ + M V H + ( o r HMV +)

(17) MVH +(or HMV+) + H+--, HMVH 2+

(18) 1/kobs = (1/2k16[H +]) + (k 16[MV 2+]

/2klrk ,7[H + ]2)

That equation (18) correlates well with the experimental data IS shown by the logarithmic plots of Fig. 4. The values of the rate constants derived from thin treatment suffer from a lack of premslon but do offer insight into their orders of magmtude Thus, 2 k t 6 = 9 ( _ 4 ) x 104M-t s 1 and k 16/k17 m

7( + 3) x 103. In fact, these values appear to decrease with decreasing [H ÷ ]

Although some of the details of the mechamsm by which MV .+ disappears in acidic solution have still

Eqm librlum (1 3) strongly f a v o u r s (34) MV + but reaction (14) provides the driving force for the disappearance of MV-+ by rapidly removing MV °. Application of the steady state approximanon to MV ° and describing the disappearance of MV t as - d[MV +]/dr =

kob~[MV +- ]2 results in equation (1 5). However, equation (1 5) does not represent the experimental data we have obtained for the decay of MV + at pH 0-2.

TABLE l INTERCEPT AND SLOPE OF PLOTS OF 1/kob s vs [MV 2+ ] FOR THE SECOND- ORDER DECAY OF M V + AS A FUNCTION OF

pH

pH intercept, Ms slope, s

0 7.6 × I0 6 0 12 1 1 . 0 × 10 4 87 2 2.4 x 10 -3 200

234 M VENTURI et al

- 2

- 3 w L

- 4

- 5

- 6

f i i i

o ~ pH

3

2 /S 6 ' 4

2pH

FIG 4 Plot of log intercept and log slope as a funcUon of pH from the data in Table 1

not been fully clarified, this invest igat ion has confirmed the mechanis t ic hypothesis previously made concerning the d l sp ropor t lona t lon of MV + in acidic m e d m m (3>381 In addi t ion, we have shown that H + is int imately revolved in the process, possibly via the p ro tona t lon of one of the two nng-n l t rogens of M V + At the present ume, however, there is no spectral evidence for react ion (19) in solutions as acidic as pH 0.

klnetlcally unfavorable for MV +. becomes facile for M V + and H M V 2+

(20) MV ~ + H M V 2+ ~-MV ~ ~ + H M V +

H M V ~ , which can be viewed initially as the conjugate acid of M V ~', could react fur ther with H +, as indicated by reaction (17), to yield a species ( H M V H 2~) tha t has undergone p ro tona t lon of the ca rbon adjacent to the qua te rnary mtrogen, the result is a hydrogenated product . Depro tona t lon of the qua te rnary ni t rogen would leave M V H + as the final p roduct This latter species has been proposed before (3~) as the product or iginat ing from the disappearance of MV~ in acidic solut ion

H M V H 2+

H + >

H H

H3C C H 3

M V H ÷

Replacing react ion (16) with reactions (19) and (2(I) in the mechan ism proposed above yields a kmetlcally Indls t ingmshable relat ionship of k,,b, to [MV 2 +] and [H ~].

(21) 1/kob s = (1/Klgk2o[H) ])

(19) M V + + H + ~ H M V 2 + + (k 20[MV 2 ~ ]/K, gk2oklv[H * ]2)

It IS entirely possible tha t H M V 2+ is a very s t rong acid ( p K , < 0 ) due to the powerful electron- wi thdrawing propert ies of the adjacent qulnold- type bond ing and the unpai red electron in the posi t ion para to the qua te rnary ni t rogen Of course, the actual s t ructure of H M W + is a composi te of many con- t rabutmg forms The p ro tona t lon of M V + will render its conjugate acid a weaker reducing agent but a s t ronger ox~dmng agent than ~s M V + As a result, d l spropor t lona t lon , which is the rmodynamica l ly and

. zk / H M V 2+

Contmuous radtolysts at pH 13 The con t inuous Irradiat ion of a deaerated aqueous

solution of 1 0 × 10 4M M V 2+ conta in ing 0 1 M 2-propanol at pH 13 causes the d isappearance of the absorp t ion of M W + centered at 2 5 5 n m and the appearance of the characteris t ic absorp t ion of MV + at 395 and 605 nm. Clean lsosbestlc points are observed at 225 and 291 nm The increase of the absorbance at 605 nm is not hnear with increasing irra- dlatxon dose, l e. G ( M V + ) decreases with increasing i r radia t ion dose As will be shown below, this non- l lneanty of the absorbance at 605 nm as a funct ion of the i r radia t ion dose, is mainly due to fur ther reduc- t ion of M V + to MV" The format ion of the dlmer (MV~)2, which absorbs less than the monomer , ~91 does not seem to be an impor t an t process under our

~E _v

uO

Radlolyt|cally-mduced one-electron reductmn of methyl vlologen in aqueous solution 235

condi t ions F rom a zero-dose extrapolat ion, a value of G ( M V + ) = 6 . 0 is obtained. F r o m the max imum value reached by the absorbance at 6 0 5 n m , we estimate tha t ~-90~o of M V -+ is obta ined f rom the reduct ion of M W + under these condi t ions The spectrum of M V + obta ined from the exhaustive radlolysls of a solution of M V 2 + conta in ing 0 1 M 2-propanol at pH 13 is shown in Fig. 5, curve I The spectrum has been corrected for the presence of abou t 10~o MV:' (see below)

Fur ther i r radia t ion of the same sample causes the d isappearance of the visible absorp t ion centered at 605 nm At the same time, the absorp t ion at 395 nm decreases and a new band at 386 nm with a shoulder at 3 7 0 n m is formed Dur ing this time, clean ISO- sbestlc points at 278, 318, 391, 400 and 465 nm are observed and the color of the solution turns f rom blue to yellow The spectrum of the yellow solution is shown m Fig. 5, curve II, the ext inct ion coefficients have been calculated assuming the complete conversion of MV 2 ~ into this species The decrease of the absorbance at 605 nm dur ing the convers ion of the blue species into the yellow species is linear with the i r radiat ion dose and corresponds to G( - M V + ) = 4 7. The i r radia t ion of the yellow solu- t ion results in a progressive decrease of its absorp t ion spectrum.

At pH 13, the yellow c o m p o u n d is moderate ly stable (tL,.2 -~ 40 hr) in air-free solution The admission

r ~

i

/ , / /

3~o 400 500 600 700 h n rn

FIG 5 Spectra of products originated from the irradiation of a deaerated aqueous solution of MV 2+ (1 × 10 4M) containing 0 1 M 2-propanol at pH 13 (--, Curve I), spectrum of MV ÷ obtained after irradiation w~th 1 27 × 1021 eVL i (20 3 krad) Concentration of MV + was evaluated from the absorbance at 605rim assuming E 1.37 × 104 M - Icm i The observed spectrum ofMV ÷ was corrected for the presence of -~ 10~o M V ( - - - , Curve II), spectrum of MV ~' obtained after irradiation of the previously irradiated solutmn with an addmonal 1 34 × 1021 eVL i (21 5 krad) The e-values for MV ° were evaluated

assuming complete conversion of MV 2 + mto MV °

of air into the solution causes, lmtlally, the appearance of the blue coloura t lon characteris t ic of M V .+ and, subsequently, its rapid d isappearance The spec- t rum of the resulting colorless solut ion shows tha t the react ion of the yellow species with oxygen regenerates M W + in a lmost quant i ta t ive yield These observat ions show that , at pH 13,. the reduct ion of M W + to M V -+ by e,q and (CH3)2CO is followed by the reduction of M V + to the yellow MV ° This latter species has been previously obta ined (4°) by dlthlOnlte reduction of M V 2+ in aqueous solution In cy- clohexane solution, M V '~ shows 2mdx 400 nm and a shoulder at 376 nm (401 In ethanol , the spectrum of M V °, obta ined by a thin-layer spectroelectrochemtcal technique, (25) is similar to the spect rum we observe m aqueous solution at pH 13 Tha t M W can be quantitatively oxidized to M V 2+ has been previously reported (40) We can note here tha t the reaction of MV ° with O2 seems to proceed according to react ion (22) The react ion of M V + with O2 has been previously investigated (19 20,41) and is likewise electron transfer in nature

(22) MV~' + O2--*MV + + 0 2

When a solut ion of radiolytical ly-generated MV '> at pH 13 is made acidic (final pH -~ 1) under oxygen- free con&tions , the yellow coloura t lon of the solution, due to the presence of M V ' , is seen to d isappear rapidly. The spectrum of the result ing colorless solu- t ion is no t affected by the exposure to air and is identical to the spectrum shown in the inset of Fig 3, obta ined by i r radia t ion of an acidic solut ion of M W +. The spectrum of yellow M V :> is no t reformed when the acidic solut ion IS made alkaline in the absence of oxygen indicat ing tha t the colorless species and M V ° are not involved in an acid-base equl- h b n u m . These observat ions are consis tent with the ass ignment tha t the species absorb ing at 220 and 2 5 5 n m is a hydrogenated derivative of MV 2+, presumably M V H +

C O N C L U S I O N S

In alkaline solution, M W + can be radlolytlcally reduced to M V + and, subsequent ly to M V °. M V + IS infinitely stable, in the absence of oxygen, in neutral and alkaline solut ion whereas MV ~' is moderate ly stable only in strongly alkaline solution. In acidic solution, M V + is unstable due to a dls- p ropor t iona t lon react ion assisted by H + followed by the irreversible p ro tona t lon of M V ~' to a hydrogenated species, presumably MVH22+ or M V H +, which absorbs at 220 and 255 nm No direct spectral evidence has been obta ined for M V + being involved in an acid-base equi l ibr ium in acidic solut ion (pH 0)

236 M VFNTURI et al

In the p h o t o c h e m i c a l sys tem con t a in ing Ru(bpy)] ~ , M V 2+, E D T A , and col lo idal Pt at

p H ~ 5, h y d r o g e n a t i o n o f M V 2 ~ . via the lnter- med lacy o f M V +, leads to the de t e r i o r a t i on o f the system.~6 ~t) As we have seen here, in the absence o f col lo idal Pt the r edox-ca t a lyzed h y d r o g e n a t i o n o f M V 2 + is rap id in s t r ong acidic so lu t ion w h d e M V

is modera t lve ly s table at p H 5 In the presence o f Pt, the ra te o f h y d r o g e n a t i o n o f M V + is very fast ~2~ 42 44)

F r o m these o b s e r v a t i o n s it can be c o n c l u d e d tha t in the p h o t o c h e m i c a l sys tem the paras i t ic hyd ro - gena t lon o f M V 2 + occurs only w h e n M V + in terac ts

wi th the ca ta lys t and c o m p e t e s wi th the h y d r o g e n - genera t ing p a t h w a y

Acknowledgement Stimulating discussions with Dr Gabrl- ella Poggl and Professor Guilford Jones are gratefully acknowledged

R E F E R E N C E S 1 Research supported in part by Conslgho Nazlonale

delle Rlcerche and in part by the Dlwslon of Chemical Soences, U S Department of Energy

2 A MORADPOUR, E AMOUYAL, P KELLER and H KA- GAN, Nout, J Chlm 1978, 2, 547 549

~, K KALYANASUNDARAM, J KIWI and M GRATZEL, Helv Chtm Acta 1978, 61, 2720-2730

4 M GOHN and N Gt'TOrF Z Naturyor,sch 1979, 34a, 1135 1139

5 I OKURA and N K1M-THUAN, J Mol Catal . 1979.5. 311 314

6 P KELLER, A MORADPOUR, E AMOUYAL and H B KAGAN, Nouv J Chlm 1980, 4. 377-384

7 A I KRASNA, Photochem Photobtol 1980, 31, 75 82 8 P KELLER, and A MORADPOUR, J Am Chem Soc .

1980, 102, 7193 7196 9 A HARRIMAN and A MILLS, J Chem Soc , Faraday

Trans 2 1981, 77, 2111 2124 10 D MILLER and G MCLENDON, Inorg ('hem 1981, 20,

950-953 11 O JOHANSEN, A LAUNIKONIS, J W LODER, A W - H

MAU, W H F SASSE, J D SWIFT and D WELLS, Aust J Chem 1981, 34, 981-991

12 A J FRANK a n d K L STEVENSON, J Chem S o c , C h e m Commun 1981, 593 594

13 M MAESTRI and D SANDRINI, Nour J Chtm 1981, 5, 637-641

14 E AMOUYAL, D GRAND, A MORADPOUR and P KEL- LER, Nout' J Chtrn 1982, 6, 241 244

15 A critical description of hydrogen generating systems not based on MV 2+ is gqven in Ref 16

16 N SUTIN and C CREUTZ, Pure Appl Chem 1980, 52, 2717-2738 and references therein

17 O JOHANSEN, A LAUNIKONIS, J W LODER, A W - H MAU, W H F SASSE, J D SWIFT and D WELLS, Aust J Chem 1981, 34, 2347-2354

18 (a) Q G MULAZZAN1, M VENTURI, M Z. HOFFMAN, 4th lnt Con[ on Photochem Confer and Storage o/

Solar Energy, Jerusalem, Israel, 8 13 August, Book of Abstracts, p 181, 1982, (b) M VFNTURI, Q G MULAZZA'ql and M Z HOFFMAN J Phy,~ Chem Accepted for pubhcatlon

19 J A FARRINGTON, M EaER~. E J LAND and K FLETCHER, Btochlrn Btophys Acta 1973, 314, 372-381

20 L K PATTERSON, L D SMALL JR and J C SCAIANO. Radtat Res 1977, 72, 218 225

21 J A FARRINGTON, M EBERT and E J LAND. J Chem Soc . Farada) Tran~ 1 1978. 74, 665 675

22 D MFISEL, W A MULA( and M S MATHESON J Phvs Chem 1981, 85, 179-187

23 M T NENADOVI(', O I MI('I(" and R R ADZI(", J Chem Sot , Faraday Trans l 1982, 78, 1065-1069

24 S SOLAR, W SOLAR, N GETOFF, J HOLCMAN and K SEHFSTED, J Chem Sm , Faradal" Tran~ 1 1982. 78, 2467-2477

25 T WATANABE and K HONDA, J Phys Chem 1982, 86, 2617-2619

26 Q G MULAZZANI, S EMMI, P G FUOCHI, M VENTURI, M Z HOtI~MAN and M G S1MI('. J Phvs Chem 1979, 83, 1582 1590

27 A HUTTON, G ROEIt and A MARrELLI, Quad Dell" Area Ru Dell' Emtha-Romagna 1974, 5, 67-74

28 FARHATAZlZ and A B Ross, Natl Stand Ref Data ~er. Natl Bur Stand No 59, 1977

29 M ANBAR, FARHATAZlZ and A B Ross, Natl Stand Ref Data Ser , Natl Bur Stand No 51, 1975

30 M ANBAR, M BAMBENEK and A B Ross, Natl Stand Ref Data Ser , Natl Bur Stand No 43, 1973

t l K -D ASMUS, A HENGLEIN, A WIGGER and G BECK, Ber Bunsenges Phys Chem 1966, 70, 756-758

32 M VENTURI and Q G MULAZZANI, Unpublished observations

~,3 A J BARD, A LEDWITH, and H J SHINE. Adt' Phys Org Chem 1976, 13, 155-278 and references thereto

34 Values of K~<10 ~. 2 6 x 10 ~, 2 5 x 10 7 and 5 × l0 s have been reported for equlhbrlbum 13 (Refs 35 38 respectively)

35 E STFCKHAN and T KUWANA. Ber Bunsenges Phys Chem 1974, 78, 253 259

36 J MOHAMMAD, R IQBAL, A Y KHAN, M BHATTI, K ZAHIR and R JAHAN, J Phvs Chem 1981, 85, 28 l 6-2820

37 1 V SHELEPIN and O A. USHAKOV, Russ J Phys Chem 1975, 49, 1021 1024

38 S Hf_INIG and H BARNFTH, Top Curr Chem 1980, 92, 1 44

39 E M KOSOWER, Free Radttal, s m Bzology, Vo[ II (Edited by W A Pryor) p l and references therein Academic Press, New York 1976

40 J G CAREY, J F CAIRNS and J E COLCHESTER, J Chem Soc . Chem Commun 1969, 1280-1281

41 C L BIRD a n d A T KUHN, Chem Soc Rev 1981, 10. 49-82 and references therein

42 M GRATZEL, Ber Bunsenges Phy~ Chem 1980, 84, 981-991 and references therein

43 P A BRUGGER, P CUENDET and M GRATZEL, J Am Chem Soc 1981, 103, 2923-2927

44 D S MILLER and G MCLENDON, fl Am Chem Soc 1981, 103, 6791-6796

radiolytically-induced one-electron reduction of methyl viologen in aqueous solution: stability of...

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