Electron transfer in the quenching of protonated triplet methylene blue

Electron transfer in the quenching of protonated triplet methylene blue by ground-state molecules of the dye. Prashant V. Kamat, and Norman N. Lichtin...
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J. Phys. Chem. 1981, 85,814-818

Electron Transfer in the Quenching of Protonated Triplet Methylene Blue by Ground-State Molecules of the Dye Prashant V. Kamat and Norman N. Lichtln" Department of Chemistry, Boston University, Boston, Massachusetts 022 15 (Received: September 29, 1980; In Final Form: December 18, 1980)

A Q-switched pulsed ruby laser emitting at 694.3 nm was used in an investigation by means of flash photolysis-kineticspectrophotometry of the mechanism of quenching of the monoprotonated lowest triplet state of methylene blue, 3MBH2+,by the ground state of the dye, MB+. Quenching in 0.01 N acid is accompanied by electron transfer to give the half-oxidized and half-reduced ion radicals, MB2+.and MBH+.. The absorption spectrum of MB2+.has been characterized in several media from 360 to 600 nm. The rate constant for quenching, k,, varies with solvent, ionic strength, and nature of anions with values around 1 X lo8 M-' s-l in water, aqueous CH3CN,and aqueous EtOH. The efficiency of net electron transfer in quenching, F1(= ket/k,), varies with solvent but is independent of the ionic strength or the nature of the anions. Fl varies inversely with polarity of the solvent from 0.055 in water to 0.48 in 90% (vol/vol) aqueous CH3CN. On the basis of analogy to the behavior of a number of other quenchers and the observed linear variation of the function In [(l/Fl)- 11 with Kosower's polarity parameter 2, it is suggested that reversible electron transfer is the only significant mechanism of quenching of 3MBH2+by MB+(S,J. Both MBH+. and MB2+-decay by second-order processes in solvents containing 75% (vol/vol) or less of organic component but the specific rates are different for the two species in most media. It is suggested that in the latter media both cross reaction of MBH+.with MB2+.and bimolecular reaction of two molecules of the same radical occur. This study shows that ground-state quenching can significantlyreduce the sunlight engineering efficiency of photogalvanic conversion in systems incorporating relatively concentrated dyes.

Introduction Albery and Foulds have recently concluded that optimization of the efficiency of photogalvanic transduction of incident light into electricity requires concentrations of light-absorbing species at least as high as 0.1 M.l However, use of such high concentrations of dyes or other solutes which absorb light intensely increases the possibility of wastage of absorbed quantum energy via a variety of quenching processes. Thus, we earlier reported that absorption of light by ground-state thionine dimer resulted in little, if any, transduction to electricity in a totally illuminated, thin-layer acidic iron-thionine cell with water as solvent and with a 1.2 mM stoichiometric concentration of thionine.2 We also reported small increases in the first-order specific rate of decay of the protonated triplet methylene blue, 3MBH2+,in both water and 50% (vol/vol) aqueous CH3CN when the stoichiometric concentration of the ground-state dye was increased from 12 pM to 1.0 mM and speculatively assigned these increases to quenching by ground-state MB+.3 Other workers have suggested that the observed decrease in the lifetime of methylene blue triplet with increasing dye concentration in neutral water solution is due to quenching by the ground-state dimera4 Ground-state quenching does not necessarily result in a net loss of charge carriers and attendant wastage of light energy. The possibility exists that such quenching may proceed with net electron transfer and that relatively long-lived charge carriers may be formed in the quenching process or in subsequent reactions. Net electron transfer in the ground-state quenching of methylene blue triplet has, in fact, been ~ b s e r v e d . We ~ report here the results

Experimental Section Apparatus. A Holobeam Series 630 laser system with a Q-switched ruby laser capable of providing up to 3.6 J per flash at 694.3 nm with a nominal pulse width of 19 ns was used. Details of the laser and monitoring apparatus have been reported.' Flash energies were 0.2-1.0 J in the present work, sufficient to excite of the order of 10% of the dye in a given test solution. Stable absorption spectra were recorded with a Cary 118 spectrophotometer. Materials. Small amounts of azure B present in our sample of Fluka puriss methylene blue chloride interfered with the accurate measurement of the spectra and the kinetics of radicals because of the relatively high concentrations of dye used in the present work. Adequate purification was not achieved by extraction procedure^.^.^ Satisfactory results were obtained by column chromatography on Fisher Certified ACS grade neutral alumina of Brockman Activity 1,using as solvent 7:3 ethanol-benzene containing 0.4 mL of glacial acetic acid per 100 mL.'O

(1) Albery, W. J.; Foulds, A. W. J. Photochem. 1979,10, 41-57. (2) Hall, D. E.; Clark, W. D. K.; Eckert, J. A.; Lichtin, N. N.; Wildes, P. D. Am. Ceram. SOC.Bull. 1977,56, 409-411. (3) Wildes, P. D.; Lichtin, N. N.; Hoffman, M. Z.; Andrews, L.; Linschitz, H.Photochem. Photobiol. 1977,25,21-25. (4) Nilsson, R.; Merkel, P. B.; Kearns, D. R. Photochem. Photobiol. 1972,16, 109-116.

(5) Kato, S.;Morita, M.; Koizumi, M. Bull. Chem. SOC. Jpn. 1964,37, 117-124. (6) Ohno, T.;Lichtin, N. N. J. Am. Chem. SOC.1980,102,4636-4643. (7) Ohno, T.;Osif, T. L.; Lichtin, N. N. Photochem. Photobiol. 1979, 30, 541-546. (8) Hall, F. H.; Marple, L. W. Anal. Chem. 1975, 47, 912-914. (9) Bergmann, K.; OKonski, C. T. J.Phys. Chem. 1963,67,2169-2177. (10) Nerenberg, C.;Fischer, R. Stain Technol. 1963,38,75-84.

0022-3654/81/2085-0814$01.25/0

of a laser flash-photolytic, kinetic-spectrophotometric study of the kinetics and mechanism of quenching of 3MBH2+by MB+ in water, aqueous CH3CN, and aqueous

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EtOH. The data are interpreted in the light of our recent investigation of the quenching of 3MBH2+by a number of coordination complexes of Fe(II).6

@ 1981 American Chemical Society

The Journal of Physical Chemlstty, Vol. 85, No. 7, 1981 815

Methylene Blue Triplet Quenching

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Figure 1. Absorption spectra of radicals produced by ground-state quenching of %BH2+ in 50% (vol/vol) aqueous AN, 0.01 M in HCI, 0.39 M in KCI, 200 pM in MB': W, MB2+. MBH'.; 0 , MBH'.; A MB2+..

+

Spectrally pure fractions were crystallized by concentration of eluate under vacuum. Crystals were dried to constant weight in a vacuum desiccator at room temperature and analyzed for C, H, N, S, and C1 by Atlantic Microlabs, Inc., Atlanta, GA. Anal. Calcd for MB+C1-.2H20: mol w t 356; C, 53.93; H, 6.18; N, 11.79; S, 8.99; C1,9.97. Found C, 53.45 f 0.12; H, 6.04 f 0.17; N, 11.73; S, 8.95; C1, 9.84, where uncertainties are mean deviations of triplicate analyses. Hydrochloric, sulfuric, and acetic acids, KC1, and Na2SO4were Fisher Certified ACS grade. Solvents included U.S.I. reagent grade ethanol, Burdick and Jackson UV grade acetonitrile, Fisher Certified ACS grade thiophene-free benzene, and laboratory distilled water which had been further purified by passage through a Millipore deionizer and filter. Nitrogen gas was prepurified grade supplied by Union Carbide and deoxygenated by bubbling through chromous perchlorate solution which was stored over zinc amalgam. Test solutions were deaerated by purging for 15-20 min with deoxygenated N2. Measurements. Pseudo-first-order decay of protonated triplet methylene blue was monitored at 370 and 710 nm.' Initial concentrations of 3MBH2+were calculated with the aid of molar absorbancy indices (which had been determined under the conditions of the experiments) by extrapolating pseudo-first-order decay data to the time of the flash. Semireduced methylene blue (mostly MBH+.) was monitored at 880 nm, taking as 24000 M-' cm-' in water, 38000 in aqueous CH3CN,11 and 30000 in aqueous EtOH.6 Semioxidized methylene blue (MB2+.) was monitored at 520 nm,5 by using values of €520 which are given below.

Data Absorption Spectrum of MB2+.. With solutions initially containing 50-250 pM MB+ and 0.01 M acid, flashing at 694.3 nm produced a prompt spectrum characteristic of 3MBH2+.7 This spectrum decayed by a rapid pseudofirst-order process, the rate constant of which, kd,varied linearly with [MB+Io,to give a relatively long-lived spectrum which decayed via a process which was second order in transient. The long-lived spectrum (corrected for reduced absorption by MB+) is characterized by intense bands with A,, at 520 (shoulder at -470 nm) and 880 nm (shoulder at -800 nm) and an even more intense band, located between the latter two bands, the overlap of which with the intense absorption of MB+ (A,=. = 665 nm) precluded its full characterization. Absorption from 700 (11) Kamat, P.V.;Lichtin, N. N. Photochem. Photobiol. 1981, 33, 109-114.

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Figure 2. Dependence of pseudo-first-order specific rate of decay of %BH*, kd,on stoichiometric concentrationof dye, [MB'] , Inaqueous CH&N mixtures .01 M In HCI, with p = 0.01 M unless otherwlse designated.

to 900 nm coincided exactly in a given medium, as illustrated in Figure 1,with the known spectrum of the semireduced methylene blue ion radical in that medium.'l The band at 520 nm is similar to the one ascribed to the half-oxidized methylene blue ion radical, MB2+.,generated by ground-state quen~hing.~ This transient absorption band is also generated in the quenching of MB+(S1) by Fem(H20),3+in acidic solutions of high ionic strength12and in the quenching of 3MBH2+or 3MB+by benzoquinone, Com(phen)2+or Com(tpy)2+in the pH range 0.5-10.'3 It is accordingly concluded that the only net chemical transformation associated to a significant degree with the observed quenching of 3MBH2+in the presence of large excesses of MB+(So)is electron transfer to yield equal amounts of MBH+. (in 0.01 M acid") and MB2+.. Since absorption by MB2+.in the vicinity of 880 nm is apparently negligible, the absorption spectrum of MB2+. could be calculated from the differential spectrum obtained at constant flash intensity by means of eq 1where (AOD)x exMBa+*

[(AoD)A]/l[MB2+.]o+ 2cXMB+ - EAMBHC* (1)

is the measured maximum change in absorbance at a given wavelength relative to the absorbance of the initial solution, 1 is the optical path length, [MB2+-Io is th,e initial, i.e., maximum, concentration of MB2+., and exMB and exMBH+* are, respectively, molar absorbancy indices at the given wavelength of MB+(So)and (MBH+.).'' Values of [MB2+.lO were taken as equal to those of [MBH+.Iocalculated from maximum absorbances at 880 nm. The assumption inherent in eq 1that dimerization of MB+ is negligible under the conditions of measurement is taken up in the Discussion section. The absorption spectrum of MB2+9in a 0.01 M solution of HCl in 50% (vol/vol) aqueous CH3CN with 0.4 M ionic strength made up with KC1 is shown in Figure 1along with the spectrum of MBH+. (plus a small amount of MBH?'.) in the same medium (obtained by reductive quenching of 3MBH2+with diphenylamine") and the sum of the spectra of MB2+.and MBH+. calculated from eq 1 b transferring eAMBH+' to its left side. From = 58500 M-' cm-' in 50% (vol/vol) aqueous Figure 1, ' solCH3CN. Similarly obtained values of t ~ ~ 2int other vents are 60000 M-' cm-' in water and 42 500 M-' cm-' in 50% (vol/vol) aqueous ethanol. Kinetics of Quenching and of Electron Transfer. Second-order rate constants for quenching of 3MBH2+by MB+(So)were calculated with eq 2 from the linear dekd = K O + k,[MB+]o (2) (12) Ohno, T.;Lichtin, N. N., J.Phys. Chem. 1980,84, 3485. (13) Ohno,T.;Lichtin, N. N. to be submitted to J. Phys. Chem.

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The Journal of Physical Chemistry, Vol. 85, No. 7, 1981

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TABLE 11: MBH'., h,,,, solvent composition,b vol % 0 50, AN 50, AN 50, AN

Second-Order Rate Constants for Decay of and MB*+.,k,,,, at 23 i: 1 "Ca

50, AN

SO,*-/

anion

c1-

C1C1-

SO,*-/

p, M

0.01 0.01 0.4 0.01

HSO,' 0 01

0 00 [MB%/b

109MS

Figure 3. Determination of k , from [MBH+.],,/[3MBH2+] = k, [MB+],/kd, in aqueous CH3CN and aqueous EtOH mixtures 0.01 M in HCI, with p = 0.01 M.

TABLE I: Quenching Constants, k,, and Rate Constants for Electron Transfer het, in the Quenching of 3MBHZ+by MB+ solvent composi1 0 - 7 1 2 ~ , 10-7ket, Z,e tion,c M-I M-I Fld kcal/ vol % anion p , M s-l s-l ( k e t / k q ) mol 0 c10.01 11.3 0.63 0.056 94.6 50,AN Cl0.01 7.2 1.65 0.23 87.5 50, AN 0.4 C114 3.5 0.25 87.5 50, AN SO,*-/ 0.01 6.6 1.6 0.24 87.5 HS0,50, AN SO,*-/ 0.2 11.2 2.8 0.25 87.5

HS0,75, AN 90, AN 50, EtOH 75, EtOH 90, EtOH

C10.01 3.2 0.39 82.0 8.3 Cl0.01 9.3 4.5 0.48 80.0 C10.01 8.2 1.17 0.14 89.5 C10.01 2.0 8.3 0.24 84.0 0.01 10.4 Cl4.3 0.41 81.0 a T = 23 f 1 "C. ProAll solutions 0.01 M in acid. portion of organic component. Other component of sole See ref 19 for vent water; AN = CH,CN. d F , = k,Jk,. source of 2 values.

pendence upon [MB+],, of observed pseudo-first-order rates of decay of the triplet which is illustrated in Figure 2. Second-orderrate constants for net electron transfer in the quenching process, ket, were calculated with eq 3 from

[MBH+.l,,/[3MBH2+lmax = ket[MB+I~/kd (3) maximum concentrations of MBH'. and 3MBH2+.6Typical sets of data used in this determination are illustrated in Figure 3. Values of k,, ket, and F1= ket/k, in a number of different media are assembled in Table I. It is apparent that k, and k,. vary significantly with the solvent, the nature of the anions, and the ionic strength but that F,, the efficiency of net electron transfer in quenching, varies significantly only with the solvent, increasing with the proportion of organic cosolvent. Under all the conditions investigated, less than half the quenching events resulted in net electron transfer. Decay of MB2+.and MBH'.. Both 1/(OD)520and 1/ (OD)880varied linearly with time in all the media investigated except in solvents containing 90% (vol/vol) of the organic component. In the latter cases the kinetic order was intermediate between first and second. Rate constants for all the conditions under which decay of absorbance at 520 and 880 nm was clearly second order, calculated with the aid of values of tgf2+' and egEH+'given above, are assembled in Table 11.

10M-1 9ks-l 520,c

1 0 - ~ 880,c k M-'s-l

3.5 f 0.5 3.3 i: 0.4 5.7 i: 0.6 3.6 i: 0.2

2.2 * 2.2 e 3.1 f 1.9 *

0.3 0.2 0.2 0.2

4.3 i: 0.3 0.2 2.8 f 0.1 HS0,75, AN C10.01 2.8 i: 0.2 3.0 f 0 . 3 50, EtOH C10.01 1.31 i: 0.11 1.09 f 0.07 75, EtOH C1' 0.01 0.69 i: 0.02 1.24 f 0.06 a All solutions 0.01 M in acid. Proportion of organic component. Other component of solvent water; AN = CH,CN. Uncertainties are standard deviations. I

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A Aqueous

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Flgure 4. Dependence of RTln [(l/F,) - 11 for groundstate quenching of 3MBH2+upon Kosower's solvent polarity parameter, Z: H20, A; 0 aqueous EtOH; 0, aqueous CH3CN.

Discussion Role of Dimerization of MB+. Dimerization of MB+(So) was determined spectrometrically to be negligible in solvents containing 50% (vol/vol) or more of organic component. Association is significant in neat water and from the dimerization constant at 25 "C with p I 0.001 M, 2.5 X lo3 M-l,14 it can be calculated that, in a solution with [MB+Istoieh= 200 pM, -83% of the dye is present as monomer. The values of k,, Ket, and F1in neat HzO which are given in Table I are formal values calculated without taking dimerization into account. The similarity of the value of k in neat water to its values in aqueous-organic solvents rdes out the possibility that dimer is a much more effective quencher than monomer. The small value of Fl in water and the fit of the point for neat water to the linear dependence of In [(l/Fl) - 11 upon Kosower's 2 (Figure 4) rule out the possibility that F1is unusually high in the quenching of 3MBH2+by ground-state dimer. Thus, the values of k,, ket, and F1for neat water given in Table I are reasonably good approximations to the correct values for monomeric MB+. Implications of Ground-State Quenching f o r Photogalvanic Conversion. The relatively low values of Fl for ground-state quenching of 3MBH2+presented in Table I imply that competition by this process can significantly reduce the efficiency of generation of charge carriers by (14) Zadorozhnaya, E. M.; Nabivanets, B. I.; Maslei, N. N. Zh.Anal. Khim. 1974, 29, 993-997.

The Journal of Physical Chemistry, Vol. 85,No. 7, 1981 817

Methylene Blue Triplet Quenching

a quencher such as Fe11(H20)62+.Use of concentrations of dye as high as 0.1 M advocated by Albery and Fouldsl would lead to such competition with Fen(H2O):+ because k, for quenching by MB+(So)is equal to or greater than k for quenching by Fe11(H20)t+.3 Comparison under identical conditions is available only in 50% (vol/vol) aqueous CH3CNwith p = 0.2 M and S042-/HS04-as anion where k, for quenching of MB+(So)and Fe"(HzO),2+ are, respectively, 11 X lo7 and 7 X lo7 M-l s-l. Mechanism of the Quenching Process. We have presented substantial evidence that the only significant mechanism of quenching of 3MBH2+by a number of stable coordination complexes of Fe(I1) proceeds via reversible electron transfer in the encounter complex of excited dye and quencher.6 The same conclusion has been drawn with respect to the quenching of 3MBH2+or 3MB+by several organic quenchers.13 The most important type of evidence presented to support these conclusions is the observations that, for these cases, F1 F2 = 1where Fl = ket/k, is the efficiency of net electron transfer in quenching events, F2 = k r & / k D is the efficiency of net reverse electron transfer in encounters with a diffusion-controlled rate constant, k D , between the products of forward electron transer, and k,,, is the specific rate of net reverse electron transfer. In contrast, quenching of 3MBH2+by four coordination complexes of Co(I1) is characterized by (Fl+ F2)