Flash Photolysis Study of Phenosafranine-Efr'I'A Systemnopr.niscair.res.in/bitstream/123456789/48931/1/IJCA 23A(8) 623-62… · The volume of photolyte solution (C= 1.0x 10-5 M) used

Indian Journal of ChemistryVol. 23A, August 1984, pp. 623-629

Flash Photolysis Study of Phenosafranine-Efr'I'A System

K K ROHATGI-MUKHERJEE* & MANASHI BAGCHIPhysical Chemistry Laboratories, Jadavpur University, Calcutta 700032

Received 18 November 1983; revised and accepted 26 March 1984

The photogalvanic system phenosafranine (PSFH +=DH +)(C = I x 10-5 M) in the presence of EDT A (0.1 M) has beensubjected to flash photolysis studies to identify the photoelectroactive species. The spectra of the transient species produced by200 J flash is recorded at a delay time of 0.4 ms between 300 nm and 700nm for solutions of pH 4.55, 6.2 and 7.00, in the absenceand presence of the electron donor EDTA. At low pH values in the absence of EDTA, )'m ax at 640 nm is assigned to semidyeradical anion of PSF i.e. -DH! generated by D-D mechanism

3DH~+ +3DH~+ """DH! +'DH~+where 3DH~ + results from the shift in the protolytic equilibrium in the triplet state: JDH ++ H +....•3DH~+. System exhibitssecond order decay constant, k2 =2.6 X 10"M -1 S -1. At higher pH values )'m ax is blue shifted to 620 nm and first order decaykinetics is observed: k 1 = 4.4 X 102; r 1;2 = 1.5 ms. In presence of EDT A at low pH, J.rna, appears at 620 nm and decay constantobeys first order kinetics. But at higher pH, Am •• shifts to 640 nm and absorbance increases withpH reaching a maximum atpH6.2 and decreasing again at pH 7, simulating the pH dependence of photo voltage (VoJ generation in a PSF+EDTAphoto galvanic cell. The appearance of maximum at pH 6.2 in both the cases is due to pK values of EDT A, dissociated speciesHyJ - and v- - only being good electron donors. The species responsible for photoelectroeffect is identified as the semi-radicalion, DH!.

The dye phenosafranine (3,7-diamino-S-phenyl-phenazinium chloride, PSF) has been observed 1 toproduce high photo potential in a photogalvanic (PG)cell with platinum electrodes in both, light and darkcompartments, in the presence of reducing agents suchas ethylenediaminetetraaceticacid (EDTA). Since theobservation of PG etTectby Eisenberg & Silverman 2, inrecent times Kaneko & Yamada.' have demonstratedhigh photopotential (- 844 mV) for Safranine-O(tolyISafranine)-EDTA system. Photochemistry ofdyes, an essentiai step in understanding the mechanismof PG effect, is very sensitive to substituents asdemonstrated by Bonneau et al.4- 6 for thionine, azureblue and methylene blue. Therefore, a study of thetriplet state photochemistry of PSF and its interactionwith EDTA was taken up using flash photolysistechnique although the transient photochemistry ofSafranine-O has been reported by Baumgartner et al,',

The cationic dyes are known to have different acid-base properties in the triplet state as compared to theground, as well as the singlet states. For thiazine dyesFischer", has observed three protonated species 3D,3DH + and 3DH ~+. These conjugate forms showditTerent reactivity towards reducing agents. For thephenazine dye Safranine-O also, stages of protonationobserved by Baumgartner et al,' with pKs at 7.S and9.2 respectively are

-H+3DH~+ ~. 3DH+

+H+

where 3DH~ +, 3DH + and 3D are respectively doublyprotonated, singly protonated and basic triplets of thedye, expressed in general notations. Besides these threetriplets, two radical ions are also reported 7:

-H+DH-

Materials and MethodsPhenosafranine (George T. Gurr Ltd, London) was

crystallised from water. EDTA (disodium saltdihydrate) was of AR (BDH) grade. The pH wasadjusted to 4.S5, 6.2 and 7.0 by adding HCI or NaOHas required. No butTerwas used since it was found togive reduced value for photovoItage and reducedprobability for photo bleaching. In fact Bonneau et al.5

have observed definite complex formation withphosphate ions of the butTer.

The Applied Photo physics Flash Kinetics Spectro-photometer model K-2, consisted of two xenon flashlamps of 200 J energy each with a flash duration of 0.1ms. The dimensions of the tubular photolysis cell were:internal diameter, 1.6 ern; and optical path length. 10.3em (internal dimensions). The volume of photolytesolution (C = 1.0 x 10- 5 M) used was 23 ml. Thesolutions were deoxygenated by bubbling purified N2-

gas through a syringe introduced through a rubberseptum. The spectra were obtained by flash kineticspectrophotometric technique. After each flash, themaximum growth point of the signal trace on thestorage oscilloscope, at any given wavelength,

623

INDIAN J. CHEM .• VOL. 23A. AUGUST 1984

produced by transmitted monitoring beam, was noted.This signal was proportional to the changes intransmittance (~T) of the sample at the givenwavelength and hence was proportional to theabsorption by the transient species produced by theflash energy (200 J). Prior to measurement, the base linewas adjusted to 100% transmission and the zero wasadjusted with the shutter of the monitoring beamclosed. Hence the changes in transmittance (~T) interms of changes in optical density was given by Eq. (1)

~T ~I~OD= -log-= -log- ... (1)To 10

where ~I is the change in the transmitted intensity. Thetime base was in millisecond. Since the duration of theflash is 0.1 ms, ~OD could not be observed at a shortertime period.

ResultsThe dye PSF absorbs maximally at 520 nm in dilute

aqueous solution with a broad absorption band lyingbetween 400 nm and 600 nm. At concentrations greaterthan C = 2 x 20 -5 M it tends to aggregate andspectrum shifts to 503 nm establishing a definitemonomereedimer equilibrium. In ethanol and glycerolthe maximum shifts to the red, A.max= 535 nm. Thespectrum is insensitive to pH in the range 3 and 10,indicating that in the ground state, only one species ispresent throughout this pH range. This species is thesingly protonated dye, the site of protonation being theamino nitrogen. The dye is weakly fluorescent and thefluorescence spectrum is the mirror image of theabsorption band. The fluorescence spectrum is alsoinvariant in the pH range 3 and 10. The EDTAquenches the fluorescence of PSF very weakly. In allprobability EDT A deactivates the singlet excited dyeto the ground state as observed by Bonneau? for thedye oxonine.

In the present study the transient absorption spectraof the dye phenosafranine have been obtained at threepH values: 4.5, 6.2 and 7.00, adjusted by addingappropriate acid or alkali to maintain conditionssimilar to that obtained in the photogalvanic set up.Buffers were avoided because a detailed study of theeffect of buffer salts has not been made as yet for PSF+EDT A system.

The triplet state is generated by flash photolysis ofdilute solution (C = 1 x 10 -5 M) of the dye atappropriate pH. The singlet excited state cannot beobserved as the time period of the flash is too large tocompete with the fast decay of the singlet. It is assumedthat triplet dye is formed with unit efficiency (cp = 1).The transient spectra do not appear unless thesolutions are completely deoxygenated. This estab-lishes that the spectra are due to the triplet forms of the

624

dye or its reaction products. The transient spectrabetween the wavelengths 300 nm and 700 nm arepresented in Fig. 1. The sensitivity of thephotomultiplier-detector system does not permitobservations beyond 700 nm. The spectra are recordedat 0.4 ms delay time after the flash at each wavelength.

In region between 400 nm and 590 nm lies theabsorption due to ground state dye. The negativevalues obviously are due to disappearance of theground state species by light absorption and one wouldhave expected strong negative differential absorptionat 520 nm, the absorption maximum of the dye in theground state. But the nature of the curve suggests thatthe triplet species also have strong absorption in thisregion. The negative peak at 460 nm is smaller at pH4.55 and increases at pH 7.00. It is difficult to identifyat this stage, the species responsible for absorbancechange in this region. Baumgartner et al.' have notconsidered this region of the spectrum at all, neitherthey have mentioned such observations. To avoidcomplications for the time being, awaiting furtherdetailed studies, we also will not discuss the absorptioncharacteristics in this wavelength region.

Interesting changes in the shapes and intensities ofthe transient absorption spectra are observed between590 nm and 700 nm when the pH of the solutions arevaried. At pH 4.55 the only transient species expectedto be present is the doubly protonated triplet dye3DH~+ epSFH~"'). One can calculate approximatenumber of photons emitted from two flash lamps of200J intensity per flash assuming that nearly 15% ofthe radiation emitted from a blackbody at a givencolour temperature are within 100 nm of wavelengthcentred at 500 nm. The calculations show that thenumber of photons available are three orders of

0.12,.----------------;:-;:;-;;:----,

0.11

0.'0

-0.0

Fig. I-Differential absorption spectra of transient produced byflash photolysis of dye PSF (C= 1.0x 10-, M) at different pH.

[(A}pH. 4.55; (B)pH. 6.25; and ro pH. 7.00]

ROHATGI-MUKHERJEE & BAGCHI: FLASH PHOTOLYSIS OF PHENOSAFRANINE-EOTA SYsTEM

magnitude greater than the number of moleculespresent in the given volume of solution ofconcentration I x 10 -5 M under study. Therefore onecan assume that all the ground state molecules arepromoted to the singlet state and they populate thetriplet state with unit efficiency. This may bepresumptuous but we start with this assumption. Theevolution of the doubly protonated triplet species canbe represented as shown in Scheme I.

-+ IDH+*-+ 3DH+

pK3DH+ + H+ ¢ 3DH~+

Scheme I

For Saf-O system, Baumgartner et al. 7 have calculateda pK = 7.5 for this acid-base equilibrium. Suchcalculations could not be made in the case of PSFbecause the nature of the spectrum at higher pH showsconsiderable variation in shape and intensity. Thespectra have been noted at 0.4 ms delay whereasBaumgartner et al. 7 had observed the spectra for Saf-Oat 50 J1.S delay. -The absorption maximum is fairlybroad and is identified at 640 nm. This peak apparentlyoverlaps the ground state absorption. A second peakappearing at 380 nm is also attributed to the samespecies as suggested for Saf-O. At higher pH values of6.25 and 7.00, the maximum shifts to 620 nm with thechange in the shape of the absorption band. Theexperiments repeated after 4 months with the sameflash lamp are recorded as the second line [repeat (R)experiments] in each spectrum.

In Fig. 2 are presented difference spectra in thepresence of EDTA, (C=O.I M). There is a sharp

0-08

0'07

0'06

~ 0-05inzwo~ 0-04..u

0-

o 0'03

0'02

0'01

7L1J

WaveLengthlnm J

Fig. 2-0ifTerential absorption spectra of transient produced byflash photolysis of dye PSF (C = 1.0 x 10 -s M) + EOTA (C = 0.1 M)

[(A) pH, 4.55; (B) pH, 5.51; (C) pH, 6.25; and (0) pH, 7.00]

decrease in ~OD at pH 4.55 with concomitant changein the shape of the differential absorption band. Themaximum shifts to 610 nm from 640 nm. There isconsiderable positive absorbance in the region ofground state absorption. Surprisingly the spectrumresembles that atpH 6.25 in the absence ofEDTA. Athigher pH values and in the presence of EDT A, themaxima again occur at 640 nm with concomitantincrease in ~OD. The intensity reaches a maximumvalue at pH 6.25 and decreases without any change inthe band shape at pH 7.00. In many ways these spectraresemble the spectrum of the dye at pH 4.55 in theabsence of EDT A suggesting that the two may be dueto the same species of the dye.

Decay kinetics of the transient speciesThe kinetics of decay of the transient species have

been studied at 600 nm and pH 4.55. For pH 6.25 and7.00, decay curves were monitored at 630 nm. Since aphotographic arrangement for recording the oscillog-ram has not been developed as yet, the decay curveshave been reconstructed from the recorded ~T as afunction of time for the given wavelength. The curvesare presented in Fig. 3a at the three pH values. Thedecay curves in the presence of EDT A are presented inFig. 3b. The plots of ~OD as a function of timeaccording to the first order as well as the second orderkinetic expression in the absence and presence of

(aJ

(a)'\ (b)

lJ. pH 4·55

D ... pH 6·25

O..• pH 7-00

195

200,L....---'_......L._.....I...._--'------.J

(b)

Fig. 3-Decay curves for transient species produced at different pH[(a) PSF + water, (b) PSF+EOTA]

625

INDIAN J. CHEM., VOL. 23A, AUGUST 1984

EDT A at pH 4.55, 6.25 and 7.00 respectively arepresented in Figs 4-6.

At pH 4.55 the plot is linear for second order decaykinetics in the absence of EDT A but not for the firstorder. The second order decay law for like moleculesonly, is expressed by Eq. (2)

I I---=k1tC Co

or ~ [!__l J= klt, ed C Co ed

I Ior, 00 = 00

0+ k2t/ed ... (2)

where 8 is the molar extinction coefficient of thetransient species and d is the length of the absorptioncell (d= 10.3 em), From the plot one obtains the slopeand the intercept extrapolated to zero time.

kSlope = 8;= 10.7 X 103 S-I

IIntercept = llCod = 4

Assuming 100% transformation to 3DH~ + onlight absorption, and Co = 1.0 x 10 -5 M

I4 x I x 10 5 M x 10.3 em

=2.4 x 103 M -1 cm "!

k2 =1O.7xI03s-IX8Xd=2.6 x 108 M-1 s :'

I'1/2 =k2 Co

=0.38 x 10 -3 s

=380 JJS

(a)210 x; 600nm }

pH~4S'j (0) &(0)10

DYE+WATER

IIME,mSf'(;

Fig. 4-Dccay kinetics at pH 4.55 of (a) PSF + water and (b) PSF+ EDTA. according to first order and second order rate expressions

626

Since spectrum is recorded at 0.4 ms delay time whichis nearly equal to the half-life of the species, theconcentration of initial triplet must be nearly half thatat zero time. Therefore, C after 0.4 ms delay ~ 4.9x 10-6 M.

The experiment was repeated after 4 months withfreshly prepared solution. 3DH~ + species formed inrepeat experiment after 4 months, i.e. CO<R) isobserved to decrease by nearly seven-fold (Fig. 4a),although the slope is the same. A likely explanation isthat the intensity of the flash lamp has decreased due toconstant use, so that there are not enough photons toexcite all the molecules to higher energy state. A

)..=61OfUnpH"i-2S

DYE.WATER

o

(0)

~ __ ......I'··I- 2·0

34567&91(..

,-,--r-,-,----,----,----'--"'--r-:oo.a(b)

1 120

I 100

~=USnmpH61':>OYE+EDTA

01234'5678910

TIJoIIE, ms ec

Fig. 5-Decay kinetics at pH 6.25 of (a) PSF + water and (b) PSF+EDTA. according to first order and second order rate expressions

200 ),= 6XlnmpH=1·00DVE+W4.TER

,·0

'10

160

,1.0

'10

-16100

80

60

~ 1~W 2~

•••••••0.e-2 ""'Q.4:----.'0'6----""o.e,.--,',''O,.....,.,'.1,.....,.,'.•,..-,',l°,--,~,.....,,.....,--r--r---.--.".O

(b).2

,·a ..Q

'.6 .!1-3 ~

0'00 2·0 j

Ie 80 2·2 J-060 1·4

1.0 A 1·6

20. 1~),0.

0. , 2 1 4 5 6 7 8

TlME.mMC

Fig. 6-Decay kinetics at pH 7.00 of (a) PSF +water and (b) PSF+ EDTA, according to first order and second order rate expressions

ROHATGI-MUKHERJEE & BAGCHI: FLASH PHOTOLYSIS OF PHENOSAFRANINE-EDTA SYSTEM

fraction of molecules therefore will remain in theground state even at zero time.

At pH 6.25 the decay curve at 630 nm is morecomplex (Fig. Sa). It may be suggested that a first orderdecay is taken over by a second order decay after 4 ms,i.e. three half-lives. The first order rate constants fortwo sets, the second one repeated after 4 months,are

Set (I) kl =4.4 X 102; 't1/2 = 1.5 msSet (R) kl =3.9 X 102

; 't 1/2 = 1.8 ms

The slopes of the second order plots as well as theintercepts vary slightly for the two sets;(1)slope = 10X 103 S -1; (I) intercept =4

(R) slope = 13 x 103 S-1; (R) intercept =20From (I) we have; (I) k2 = 1.2 X 108 M -1 S-1

At pH 7 the decay at 630 nm is very fast and firstorder for more than two half-lives (Fig. 6a). This pH isvery near the pK value for protolytic equilibriumbetween 3DH~ + and 3DH ". From the slope of theplot, unimolecular decay constant k 1=2.7 X 103 S -1

with a half-life 't1/2 of 0.36 ms is calculated. The decaycurve does not show complete recovery of absorbance(Fig. 3) indicating the presence of a very long livedspecies or a permanent change on flashing.

DiscussionAlthough the events immediately following the light

absorption cannot be monitored efficiently in thismodel of the flash unit, some interesting correlationscould be established from the absorption spectra of thetransient as a function of pH in the absence andpresence of the electron donor, EDT A. The decaykinetics are helpful in establishing the mechanism. Allrelevant data are presented in the Table I.

From a rough calculations of absorbances atdifferent pH values in the absence of EDT A, theprotolytic equilibrium: 3DH ~+~3DH +, works out tohave a pK ~ 7. Therefore the only species likely to bepresent at pH 4.55 is the doubly protonated dye in the

triplet state, i.e. 3DH2 + 2 if all the dye molecules areelectronically excited by the flash energy. The observedsecond order decay at this pH in the absence of anyreducing agent can be explained by two mechanisms:(i) quenching by T-T mechanism, i.e. interactionbetween two triplets; and (ii) quenching by T-Smechanism i.e. quenching by ground state (S) dye,giving the rate expression (3) for quenching

d[f]-~=kl[T] +k2[T] [T] +k3 [f] [S] ... (3)

where [T]= concentration of triplet dye species3DH~". This type of expression has been suggested 1 0

for chlorophyll triplets. But for PSF, quenching byground state triplets does not seem to be importantfrom following considerations. The slope I (I = initialset; R = repeat set) of the second order plot (Fig. 4a) iswell reproduced on repetition after 4 months (freshsolution but old flash lamp). Since the intercepts aredifferent, the concentration of the excited speciesinitially generated must have decreased because of thedecrease in the number of photons available from theold lamp. The percentage decrease can be evaluated bycomparing the ratios (slope/intercept) for two sets orcomparing the two intercepts. Both calculations showa reduction by a factor or 0.14 in the apparent initialconcentration of the triplet dye, suggesting that theground state is not completely emptied. In spite of thepresence of a large concentration of the dye in theground state the decay rate remains the same. Thissuggests that ground state dye molecules are noteffective quenchers of the triplet dye. One of thereasons could be that T-T quenching is a spin-allowedprocess whereas T-S quenching is spin-restrictedaccording to Wigner's spin conservation rule,

The appearance of a first order term in the repeatexperiment (k=4.6xI02s; 't1/2=1.5 ms) where

Table I-Rate and Spectroscopic Parameters from Flash StudypH 4.55 6.25 7.0

(,4,=600 nm) (,4,=630 nm) (A. =630 nm)

k2 (I) 2.6 X 108 M -I S -I (I) 1.2 X 108 M -I S-I

£.00 (I) 2.4 X 103 M -I em -I

(R) 3.2 x 103 M -I em -I

kl (I) None (I) 4.4 x 101 s -I (I) 2 X 103 S-1

(R) 4.6 x 102 s -I (R) 3.9 x 102 5 -I

I't'=- (1)2.2x 10-3 5

kl't'1/2 (1st order) 1.5 ms

't'1/1 = l/k2Co 370/.lS(2nd order)

(I) 2.3 x 10 -:3 5

(I) 1.5 ms(R) 1.7 msI ms

(I) 0.5 x 10-3 s

0.35 ms

627

INDIAN J. CHEM., VOL. 23A, AUGUST 1984

eDH!+] is much reduced may be due to the directdecay to the ground state

3DH~+ -+DH+ +H+

which becomes competitive at low concentrations ofdye triplets. The rate law can than be expressed by Eq.(4)

d [3DH22+]

kl eDH~+]dt

+k2 [3DH~+]2 '" (4)

with the involvement of the steps shown in Scheme 2.hvDH + -+ IDH+ -+

3DH+ + H+ -+

3DH~+

JDH~ + + 3DH~ +

-+ DH+ +H+

-+ 'DH~++'DH;oxidised reducedScheme 2

The last step is important. The triplet-tripletmechanism in dyes is known 11,12 to involve electrontransfer between each other producing the two radicalions, reduced semidye and oxidised dye radical. Wesuggest that the spectrum recorded at 0.4 ms delayperiod at pH 4.55 is due to semidye radical ion -DH;.This is substantiated by the discussion in the sequel.

The spectrum recorded at pH 6.25 is primarily due toabsorption by 3DH~ + as a shift in absorptionmaximum to 620 nm is observed. Due to shift in theproto lytic equilibrium towards singly protonatedspecies, [3DH~ +] will be reduced. Both first order andsecond order decay are observed. For the first orderdecay kl =4.4 x 102 s -1 and '1/2 = 1.5 ms. The valuesare the same as observed at pH 4.55 and must be due tothe first order decay of 3DH~ +. The second orderdecay rate also agrees within an order of magnitude.But due to decrease in [3DH~ +J, radical ion formationprobability is reduced and spectrum is preponderantlydue to 3DH~+. At pH 7, only first order decay isobserved but the rate constant is nearly five-timeslarger(k1 =2 x 103;, 1/2= 350 jls). There is an apparentdiscrepancy from k 1 values observed under lower pHconditions.

The recorded spectrum at pH 7 has nearly the sameshape as that at pH 6.25 but the absorbance is lowered.From Baumgartner et al's experience 7 it is expectedthat 3DH~ + and the semidye radical ion will absorb inapproximately the same region of spectrum whereasthe spectrum of 3DH;' i';shifted to longer wavelength.Interestingly, the spectra obtained at these pH valuesin the absence of EDTA (Fig. 1) resemble closely thespectrum recorded at 0.4 Ins delay when the solution isflashed in the presence of EDTA (C=O,1 M) at pH4.55, although with reduced absorbance (Fig. 2). This

628

is surprising because one expects electron transferquenching and formation of radical ion in the presenceof EDT A. One maybe tempted to assign this spectrumto the semidye radical ions generated by the step:3DH~+ +e-+"DH!. In that case, the spectra at higherpH in the absence of EDT A should also be assigned tothe radical ion.

But the observations are that as the pH of thesolution is increased at constant [EDT A], the natureof the spectra again change, maximum shifts to longerwavelength (Amax= 640 nm) with concomitant increasein absorbance. The absorbance increases to amaximum value at pH 6.25 and starts decreasing withfurther increase in pH. This behaviour is reminiscent ofvariation of photo voltage (Voc) with pH when thephotoredox reaction is carried out in a photogalvaniccell and is a strong indication that the speciesresponsible for absorption in this region is theelectroactive species. A very good correlation isobtained when dOD values at each pH are plottedalong Y -coordinate and Voc at the corresponding pHalong X-coordinate (Fig. 7), thus establishing thedirect proportionality between the two properties.Various data are recorded in Table 2. None of thetriplet species give maximum at pH 6.25 (Table 2).Therefore the spectra at higher pH are not due to anytriplet species and must be due to the semidyeradicalion which is the electroactive species.

It has been well documented in the literature 'P'!"

that the tetrabasic acid EDTA has pK values of 2.0,2.69,6.16 and 10.26. Bonneau et al:" have shown thatthe triply ionised and fully ionised species Hy3 - andy4 - only have free lone pair electrons on N-atomavailable for donation to suitable acceptors. ThereforeEDTA will act as an efficient donor at high pH only.The species available as electron acceptors at low pH ispredominantly 3DH~ ". At low pH the concentrationof 3DH~ + is high but the concentration of the electrondonor Hy3 - is low. As the pH increases, [3DH~+]decreases but [Hy3 -] increases. The electron transferproduces a radical ion semidye. The semidye

0·\

0." •o~O.Ob

oQ

0.04

1~''"_--+'-;:--',;,' ,,--.!';;;-----;;':' ....JI.i:5 6&0 700 720 7~C

0·02

Fig. 7-Plot of photovoltage V"" versus ODmu of transient speciesproduced on flashing (PSF + EDT A) -vstem at din-trent pH values

[(A) pH. 4.55; (B) pH. 5.SL (C) pH, 6.15; ami (D} pH, 7.00]

ROHATGI-MUKHERJEE & BAGCHI: FLASH PHOTOLYSIS OF PHENOSAFRANINE-EDTA SYSTEM

Table 2-0bserved Values of ODm •• at 640 nm in FlashStudy and V0< in Photogalvanic Study for a Solution of PSF(C = I x 10 -5 M) and EDTA (C = 0.1 M) as a Function of pH[3DH~+] and [3DH+] are concentrations of corresponding tripletspecies as calculated assuming a pK = 7 for the protolytic

equilibrium: 3DH~+ ¢3DH+ +H+

pH

ODma• at 640 nmv; in mV at 30°C'[3DH~+] in M[3DH+] in M

(a) Ref. I.

4.55

0.0365675

9.96 x 10-6

0.003 X 10-6

7.0

0.058710

5 x 10-65 X 10-6

5.5

0.070712

6.25

0.088735

8.4 x 10-61.5 x 10-6

concentration will increase with initial increase inpH,reach a maximum and decrease again for furtherincrease in pH since the concentration of the acceptormolecule, 3DH~+ will gradually decrease. This alsoindicates that 3DH ~+ is more easily reduced than thebasic triplet 3DH + in agreement with the observationof other workers 7.

From these observations one is forced to concludethat the spectra with absorption maximum at 640 nmat higher pH values in the presence of EDT A are due toprotonated semidye 'DH i .If that is so then the spectraat pH 4.55 in the absence of EDTA (A.max =640 nm) arealso due to protonate semidye, produced by T-Tinteraction which is an important mode of decay of3DH~+ of PSF. The spectra showing absorptionmaximum at 620 nm observed at high pH values in theabsence of EDT A and at low pH values in the presenceof EDTA must be due to 3DH~+. At low pH in thepresence of EDT A appearance of 620 nm maximum isevidently due to low concentration of donor ionsHy3 -. The direct dependence oflight intensity and VDC

with unit slope as reported earlier 1, indicates thatphotovoltage generation is a one-photon process. Thisfurther confirms the aotive participation of thesemidye radical ion, °DH i (PSFH i) in the electrodeprocess.

AcknowledgementThe financial assistance from Indian National

Science Academy and Department of Atomic Energy(DAE) to one of us (MB) is gratefully acknowledged.The facilities provided by University GrantsCommission is also acknowledged. We thank Drs S CBera and B B Bhowmik for fruitful discussions and GDas Mahapatro for assistance with the flash photolysisunit.

ReferencesI Rohatgi-Mukherjee K K, Bagchi Manashi & Bhowmik B B,

Electrochim Acta, 28 (1983) 293.2 Eisenberg M & Silverman H P, Electrochim Acta, 5 (1961) I.3 Kaneko M & Yamada A, J phys Chem, 81 (1977) 1213.4 Bonneau R, Joussot Dubien J & Faure J, Photochem Photobiol,

17 (1973) 313.5 Bonneau R, Forrier De Violet P & Joussot Dubien J, Photochem

Photobiol, 19 (1974) 129.6 Bonneau R & Pereyre J, Photochem Photobiol, 21 (1975) 173.7 Baumgartner C E, Richtol H H & Alkenes D A, Photochem

Photobiol, 34 (1981) 17.8 Fischer H, Z phys Chem, N F, 43 (1964) 177.9 Bonneau R, Photochern Photobiol, 25 (1977) 129.

10 Brown R, Harriman A & Porter G, reported by M A West inCreation and detection of the excited State, edited by W RWare, Ch. 5, P. 284.

II Heelis P F, Parson B J, Philips GO & McKellar J F, PhotochemPhotobiol, 30 (1979) 343.

12 Hemmerich P, Knappe W R, Kramer H E A & Traber R, Eur JBiochemistry, 104 (1980) 511.

13 Schwartzenbach G & Ackermann H, Helv chim Acta, 30 (1947)1798.

14 Chapman D, Lloyd D R & Prince R H, Proc chem Soc, (1963)336: J chem Soc (\ 963) 3645-.

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