3864
J. Phys. Chem. 1981, 85,3864-3868
Photoinduced Electron Ejection from Methylene Blue in Water and Acetonitrile Prashant V. Kamat and Norman N. Lichtln" Department of Chemistry, Boston University, Boston, Massachusetts 02215 (Received: March IO, 1981;In Final Form: June 22, 1981)
Excitation of all of the methylene blue dissolved in alkaline water or neutral CH&N by a 694.3-nm laser flash results in prompt production (C0.5 11s) of triplet methylene blue, MB+(Tl),and semioxidized methylene blue, MB2+.. Production and decay of these transients were characterized by means of laser flash photolysis-kinetic spectrometry. The prompt production of MB2+.is ascribed to photoinduced electron ejection from excited methylene blue. The decay of MBYTl), a much slower process, was not accompanied by production of MB2+. under conditions of complete excitation. Dependences of prompt yields of MB2+.and MB+(TI)on flash energy were identical. Increasing the acidity of the medium decreased both the prompt yield of MB2+.and its lifetime but had no effect on fluorescence spectrum or yield. The data are most consistent with electron photoejection from MB+(T2).Laser flash excitation of only part of the methylene blue dissolved in alkaline or neutral CH3CN results in formation of MB2+.by both photoinduced ejection of an electron and slower ground-state quenching of MB'(T1).
Introduction We have recently shown that quenching of triplet methylene blue in acidic medium by the ground state of the dye, MB+(So),involves net electron transfer to give MB2+.and MBH+..' Net electron transfer in ground-state quenching of triplet methylene blue was first proposed by Kato et to account for results obtained when methylene blue was flash-excited in neutral and alkaline aqueous media. They identified a long-lived transient absorbing at 520 nm as the semioxidized ion radical of methylene blue (MB2+.). In some of their experiments, essentially all of the dye was excited. Ground-state quenching cannot be a major process when almost all of a solute is excited and hence cannot account for the production of MB2+. under the latter conditions. Danziger et ala3also observed a transient absorption at 520 nm generated by flash photolysis of MB' in unbuffered aqueous solution and assigned it to a charge-transfer state resulting from an internal electron transfer in a dimeric triplet. Nilsson et a1.4 were unable to draw any definite conclusion from their flash-photolytic data concerning the nature of the transient absorbing at 520 nm other than that it is not a triplet state of the dye. We report here the results of a laser flash photolysiskinetic spectrophotometric study of methylene blue in aqueous and acetonitrile media in alkaline, neutral, and weakly acidic solution which provides evidence for electron ejection from the excited dye. Experimental Section Apparatus. A Holobeam Series 630 laser system with a Q-switched ruby laser with a nominal pulse width of 19 ns at 694.3 nm was used. A flash energy of 1.0 J per flash (equivalent to 5.8 X lo4 einstein per flash) was sufficient to excite M methylene blue completely in the 1-cm3 volume illuminated by the beam. Other technical details have been reported earliere5 Fluorescence measurements were carried out with a Perkin-Elmer MPF 44A spectroN
(1)Kamat, P. V.; Lichtin, N. N. J. Phys. Chem. 1981, 85, 814-8. (2) Kato, S.;Morita, M.; Koizumi, M. Bull. Chern. SOC. Jpn. 1964,37, 117-24. (3) Danziger, R.M.;Bar-Eli, K. H.; Weiss, K. J. Phys. Chem. 1967,71, 2633-40. (4)Nilsson, R.;Merkel, P. B.; Kearns, D. R. Photochem. Photobiol. 1972,16,109-16. ( 5 ) Ohno, T.; Osif, T. L.; Lichtin, N. N. Photochem. Photobiol. 1979, 30, 541-6. 0022-365418112085-3864$01.25/0
fluorimeter. pH was measured with an Orion Research Model 601A meter. Materials. Methylene blue was Fluka puriss grade. Purification was carried out by column chromatography on neutral A1203to give MB'C1--2H20, mol w t 376.l Solutions were prepared in laboratory distilled water which had been further purified by passage through a Millipore deionizer and filter or in Burdick and Jackson UV-grade acetonitrile. N 2 0 was supplied by Matheson. All other chemicals were Fisher Certified ACS grade. Test solutions were deaerated by bubbling for 15-20 min with deoxygenated purified-grade Union Carbide N2which had first been bubbled through the same solvent used in the test solution. The nitrogen was deoxygenated by bubbling through chromous perchlorate solution which was stored over zinc amalgam. Unless otherwise stated, the components of pH 8.2 buffer solution were 0.0125 M sodium borate and 0.019 M HCl. Measurements. Pseudo-first-order decay of protonated triplet methlene blue, MBH2+(T1),was monitored5 at 710 nm and unprotonated methylene blue, MB+(T1), at 840 nm (in water) or 850 nm (in CH3CN). Semioxidized methylene blue (MB2+.)was monitored' a t 520 nm as a prompt transient, 1K S after the laser flash. Semireduced methylene blue was monitored6 a t 880 or 420 nm, 100 ps after the laser flash. The temperature of measurement was 23 k 1 "C. Results and Discussion Evidence for Electron Photoejection from Fully Excited Methylene Blue in Aqueous Medium. The absorption M spectrum of transients formed during excitation of MB' in aqueous borate buffer at pH 8.2 is illustrated in Figure 1. The absorption spectrum recorded 1ps after the flash exhibits maxima a t 420, 520, and 830 nm, and a shoulder around 750 nm. The absorption spectrum recorded 100 ps after the flash retains an absorption maximum at 520 nm dong with a broad maximum around 880 nm. The absorption spectrum recorded 111s after the flash closely matches the reported absorption spectrum of unprotonated triplet methylene blue, MB+(T,).5 The pK, of protonated triplet methylene blue, MBH2+(T1),in water is reported to be 7.2.5 The absorption band observed at 880 nm, recorded 100 ps after the flash, matches the ab~
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(6)Kamat, P. V.; Lichtin, N. N. Photochern. Photobiol. 1981. 33, 109-13.
0 1981 American Chemical Society
The Journal of Physical Chemistty, Vol. 85, No. 25, 198 1 3885
Photoinduced Electron Ejection from Methylene Blue
o
.
4
t
TABLE I: Influence of Composition and Concentration of Buffer and Concentration of Excited Dye o n Prompt Yield of MBa+. in Water a t p H 8.2'
i
buffer
concn. M
sodium borate sodium borate sodium borate potassium dihydrogen phosphate ",OH
0.0625 0.125 0.250 0.05
lo6 x
1
1
360 400
I
I
I
,I
I
500
1
I
700
800
I
900
10-4 x
[MB+(T,)],b M
h,j,c
5.0 7.5
2.70 2.78 2.81 2.85
10.0 15.0
A,nm
Flgure 1. Absorption of transients obtained by laser flash photolysis of M MB+ in water at pH 8.2 (borate buffer): (0)1 and (0)100 ps alter the flash.
sorption characteristics of monoprotonated semireduced methylene blues6MBH+., the pKa of which is reported to be 9.0 in aqueous solution.' The absorption band observed a t 520 nm 100 ps after the flash matches the absorption spectrum previously assigned to semioxidized methylene blue ion radical (MB2+.).' The first half-life of the 520-nm transient is about 300 ps. The formation of MB2+.is apparently prompt. Absorbance a t 520 nm is recorded immediately (i.e., less than 0.5 ps) after the flash, and no further increase in absorption at 520 nm is seen during the decay of triplet methylene blue (half-life 25 ps). This rules out the possibility that either triplet-triplet annihilation or interaction of MEV(T1)with MB+(So)is a major mechanism of generating MB2+..The proposal of Danziger et al.3 that the species absorbing at 520 nm is dimeric triplet produced by excitation of the ground-state dimer of methylene blue is also largely ruled out under the present conditions. It can readily be calculated from the reported value of the dimerization constant8 of aqueous MB+, 2.5 X lo3 M-l, that, as [MBt] increases from 5 X lo4 to 15 X lo4 M, the proportion of the dye which is dimerized increases from 1.2% to 3.5%. As shown in Table I, the fraction of the dye converted promptly to species absorbing at 520 nm (taken as MB2+.)remains constant over this concentration range while essentially all of the dye is excited. The constancy of the fraction of MB+ converted to MB2+.with threefold variation in [MB+(T,)] also confirms the conclusion that neither interaction of MB+(T+)with MB+(So)nor triplet-triplet annihilation is significantly involved. The prompt (Le.,
