J. Phys. Chem. 1982, 86, 2406-2409
2408
is no barrier in the entrance channel other than that arising from centrifugal energy, and that the F atom approaches the 0 atom along a Lennard-Jones potential curve. This gives
where rm is the critical distance between the F and the CFBOcenters of mass, ro is the equilibrium F-0 bond distance, and D Jis the F-O bond dissociation energy. We find that the critical distance between the two centers, and hence the effective reactive impact parameter, is 4.0 A from eq 16. This gives a gas-kinetic collision rate at 300 K which
is the upper limit for the rate of recombination of CF30 with F, equal to 1.7 X 10" L mol-'^-^. The value found for k3,which is 4-5 times smaller than this, suggests that a modest orientational requirement exists for the collision to be reactive, similar to (but less restrictive than) that observed in the recombination of CF3 radi~a1s.l~ Acknowledgment. This work was supported by the Air Force Office of Scientific Research, Grant No. AFOSR78-3725. We thank Dr. H. W. Galbraith for helpful discussions concerning the model for secondary IRMPD of CF30. (15) M. Rossi and D. M. Golden, Int. J.Chem. Kinet., 11,775 (1979).
Deazaflavln Photocatalyzed Methyl Viologen Reduction in Water. A Laser Flash-Photolysis Study Antone J. W. 0. Vlsser' and Janos H. Fendler' L7epartment of Chemistry, Texas AbM University, College Station, Texas 77843 (Received:November 18, 1981)
M 10-methyl-5-deazaisoalloxazine-3Photosensitized reduction of methyl viologen, MV2+, by 2 X propanesulfonicacid, dF,in the presence of 1 X 10-3-10 X M EDTA has been investigated in aqueous solutions by nanosecond laser flash photolysis. At substoichiometric concentrations of MV2+,the reactions ' d F 5 'dF* b 3dF*;3dF* + EDTA 2dF- EDTA+;2dF. MV2+ ldF + MV+. have been discerned; thus dF acts as an electron relay. At higher MV2+concentrations additional modes of dF catalyzed MV+. formation become available.
-
+
+
Introduction Photochemical solar energy conversion is an important and highly active area of re~earch.~-l~ Much attention has been focussed upon the photosensitized reduction (by an electron relay, R) of methyl viologen, MV2+,since the oxidized relay, R,,, can be regenerated by a suitable electron donor, D, and since reduced methyl viologen, MV+-,in the presence of a suitable catalyst is reoxidized to MV2+with concurrent water splitting
;z;x:::x;;:t
-
dF
photolysis mechanisms for photosensitized EDTA and MV2+,reductions have been elucidated.
(1)
Hydrogen is, therefore, photogenerated from water at the expense of a sacrificial donor (eq 1). Flavins, particularly proflavin, appear to be attractive relays because they have high quantum yields for MV2+ reduction.l3-l8 Deazaflavins have the additional advantage of being able to be regenerated from their photooxidized form by such a large variety of electron donors as amines and carboxylic and amino acids.lg They are also powerful photoreductants for many different redox enzymes and proteins.lg As part of our over& investigations of electron transfer processes we have examined a water-soluble 73unsubstituted 5-deazaflavin, l0-methyl-5-deazaisoalloxazine-3-propanesulfonicacid (dF) as a potential photocatalytic electron relay. Using nanosecond laser flash
* Address correspondence to this author at the Department of Chemistry, Clarkson College of Technology, Potsdam, NY 13676. 0022-3654/82/2086-2406$0 1.25/0
(1) Agricultural University, Wageningen, The Netherlands. (2) Kiwi, J.; Kalyanasundaram, K.; Gratzel, M. "VisibleLight Induced Cleavage of Water into Hydrogen and Oxygen in Colloidal Microheterogeneous Systems"; Springer-Verlag: Heidelberg, 1981. (3) Porter, G.; Archer, M. V. ISR Interdiscip.Sci. Reu. 1976, 1, 119. (4) Calvin, M. Acc. Chem. Res. 1978, 11, 4701. (5) Hautala, R. R.; King, R. B.; Kutal, C. "Solar Energy, Chemical Conversion and Storage";The Humana Press: Clifton, NY, 1979. (6) Bolton, J. R. 'Solar Power and Fuels"; Academic Press: New York, 1977. (7) Archer. M. D. In 'Photochemistrv. SDecialiste Periodical Rewrt": The Chemici Society: London, 1978; vol.-9, p 603; 1977; Vol. 8, 570; Vol. 7, p 567; 1976; Vol. 6, p 736. (8) Claesson, S.; Engstrom, M. "Solar Energy-Photochemical Conversion and Storage"; National Swedish Board for Energy Source Development: Stockholm, 1977. (9) Barber, J. "Photosynthesis in Relation to Model Systems"; Elsevier: New York, 1979. (10) Gerischer, H.; Katz, J. J. "Light Induced Charge Separation in Biology and Chemistry";Verlag Chemie: Berlin, 1979. (11) Kirch, M.; Lehn, J. M.; Sauvage, J. P. Helu. Chim. Acta 1979, 62, 1345. (12) Krasna, A. I. Photochem. Photobiol. 1979,29, 267. (13) Krasna, A. I. Photochem. Photobiol. 1980, 30, 75.
0 1982 American Chemical Society
Photocatalyzed Reduction of Methyl Vioiogen
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982 2407
Flgure 1. Absorption spectra of aqueous 2.0 X IO-' M dF in the ground state (-) and In the triplet state, 3dF*, 200 ns after laser excitation (0).Transient absorption spectra of *dF. (0) is recorded 2 j s after the laser pulse in an aqueous solution of 2.0 X lo-' M dF containing 1 .O X lo-' M EDTA.
Experimental Section l0-Methyl-5-deazaisoalloxazine-3-propanesulfonic acid (dF) was supplied by J. S. Santema, Agricultural University, Wageningen. The sulfonyl group at position 3 of the 5-deazaisoalloxazinering strongly enhances the water solubility without altering the spectral properties associated with deazaflavins. Concentrations were determined by taking € 3 9 2 ~= 1.2 X lo4M-' cm-l,l5 Methyl viologen (Aldrich) and other reagents were of the highest purity available. All solutions were made up in triply distilled water. Absorption spectra were taken on a Cary 118C spectrophotometer. Laser flash photolysis was carried out by using the third harmonic (353 nm) of a Quanta-Ray DCR Neodynium-Yag laser, delivering 5-6-ns pulses at 25-100 mJ/pulse. Excitation energy was varied by changing the amplifier setting. Samples, placed in a 1-cm quartz fluorescence cell, were continuously bubbled by argon. The 450-W Oriel xenon lamp analyzing beam, passed through the cell 90° to the laser beam, was focussed into the entrance slit of a 25.0-cm Jarrell-Ash monochromator. Light absorption was detected by a R-924 Hamamatsu PM tube operated at 800 V, connected to a Tektronix 7834 storage oscilloscope. A 7D12-M2 plug-in unit was used to monitor Io values (500 ns prior to the laser flash). Aqueous 20% (14) Krasna, A. I. In 'Biological Solar Energy Conversion"; A. San Pietro and A. Mitaui, Ed.; Academic Press: New York, 1977. (15) Duchstein, H. J.; Fenner, H.; Hemmerich, P.; Knappe, W. R. Eur. J. Biochem. 1979,95, 167. (16) Goldberg, M.; Pecht, I.; Kramer, H. E. A.; Traber, R.; Hemmerich, P. Biochim. Biophys. Acta 1981,673, 570. (17) Traber, R.; Vogelman, E.; Schreiner, S.; Werner, T.; Kramer, H. E. A. Photochem. Photobiol. 1981, 33, 41. (18) Kalyanasundaram, K.; Dung, D. J. Phys. Chem. 1980,84,2551. (19) Massey, V.; Hemmerich, P. Biochemistry 1978, 17, 9.
C
D
Figure 2. Oscillograms of transient species of photoexcited dF in the absence (a) and in the presence of 1.0 X lo-' M EDTA (b-d).
NaN02and Corion glass cuboff filters were used to protect the samples against photodegradation and to prevent stray light from entering the monochromator. Samples were changed frequently to avoid the buildup of photoproducts (typically after exposure to 10 shots; in some cases after 4 shots). Results and Discussion Laser Photolysis of dF. Figure 1shows the ground state (So - SI) and the transient absorptions spectra, obtained at 200 ns after the laser pulse, of dF in aqueous solutions. The triplet-triplet absorption spectrum is nearly identical with that previously obtained for 5-deazariboflavin and luminodeazaflavin."j Assuming an extinction coefficient = 3.6 x lo3 M-l cm-l,16 under the present exof perimental condition, a single laser pulse converts about 17% of the ground-state population into the triplet state; M triplets. Concentration depenLe., it forms 3.4 X dence of the triplet yield at a given laser intensity (-20 mJ/pulse) established the optimal ground-state concentration of dF to be 2.0 x lo4 M. This concentration was, therefore, used in all experiments. Figure 2A is a repre-
2400
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982
Visser and Fendler
TABLE I: Laser Flash Photolysis of d F in t h e Presence of EDTA4 semiquinone, 3dF. 103 x [EDTA],
f l m
t 1 m
105 x lZdF.I,
A = 0.016
M
PS
k , s-'
ps
M
k, M-' s-'
o
7.5 1.5 0.27
1.0 x 105 2.8 X 10' 2.3 x l o 6
42 21
2.0 2.0
3.2 X 10'' 3.8 x l o l o
1.0 10.0
5 x 10-6M M f V2'
Aqueous argon bubbled 2.0 x M dF. From transient absorbances a t 600 n m immediately following the laser pulse. From transient absorbances a t 510 nm 20 ps after the laser pulse. Rate constant calculated by assuming second-order kinetics taking into consideration the initial (extrapolated to t = 0) radical concentrations.
0'15
w
0.10
5x
A=
i o - 5 ~M V Z +
0.W6
I .f/
'
0
z
t% =
91s
2
A = 0.151
[r
Em
5 x 10-4M MV"
f a
-- - - - - -- - - - - - - - -
I
Flgure 4. Oscillograms of transient species obtained in the laser flash photolysis of aqueous 2.0 X lo4 M dF containing 1.0 X lo-' M EDTA and different amounts of MV2+ at 600 nm.
t% = 15ps
a
0.05
!tx = 0.00
I 0
33ps
I
I
1
1
25
50
75
100
1
LASER E N E R G Y (mJ)
Figure 3. Laser intensity dependence of 3dF' (700 nm. 0 )and 'dF. (510 nm, 0).
sentative oscillogram of the triplet transient absorption at 700 nm. The half-life is 7.5 ps (see also Table I). Both the initial triplet yield and the half-lives of the transient absorptions depend on the intensity of the laser beam (Figure 3). It is preferred to list half-lives rather than lifetimes, since the decay profiles do not follow clear-cut first- or second-order kinetics. As expected, the triplet state becomes longer lived if generated by a lower intensity laser pulse. In this case triplet-triplet annihilations become less important. No permanent ground-state photodestruction has taken place even after exposure to 30 laser shots. Equations 2-4 describe well, therefore, he photoprocesses following laser excitation of ground state dF. triplet formation 1dF ldF* 23dF* triplet-triplet annihilation 3dF*
-
+ 3dF*
'dF*
+ 'dF
(2)
triplet quenching a new species is formed with an absorption maximum of 520 nm (Figures 1and 2C). The rate of appearance of this species could not be followed due to overlapping absorption with the 3dF*. The transient absorption spectrum of this species (Figure 1)was obtained 2 ps after the laser pulse, where the triplet state of d F is fully depleted (Figure 2B). This spectrum is entirely consistent with d F semiquinone radical, 2dF.. Using an extinction coefficient of t520nm = 3.5 X lo3 M-' cm-' for deazariboflavin radical,16a simple calculation shows that about 17% of the ground-state population has been converted to 2dF. (i.e., 3.4 X M) by a single laser pulse. Furthermore, the intensity dependence of the radical formation parallels that of the triplet yield. Radical formation from the triplet is thus quantitative. The 2dF. semiquinone radical decays rather slowly (Figure 2D), mainly by a second-order process consistent with dismutation and dimerization. Exposure of an aqueous 2 X M dF solution containing 1.0 X M EDTA to continuous pulsing (-30 shots) and subsequently to air leads to some side reactions as indicated by the incomplete recovery of dF. These data are consistent with reaction 5-7. photoreduction 3dF* + EDTA
4
back reaction 2dF*+ EDTA+
2dF** EDTA+
-
'dF
+ EDTA
(5) (6)
dismutation and/or dimerization (3)
all other triplet deactivating processes 3dF* l(dF) (4) Laser Photolysis of d F in the Presence of EDTA. Addition of EDTA drastically decreased the triplet lifetime of dF. Figure 2B illustrates a typical triplet quenching. The data are collected in Table I. Concomitant with the
2dF. + 2dF.
products
(7)
Laser Photolysis of d F in the Presence of EDTA and M P . These experiments were carried out by using fixed M) and EDTA (0.01 M) concentrations of dF (2.0 X and at a constant laser intensity. Only the concentration of methyl viologen was varied. Figure 4 gives some representative traces. A t low concentrations of MV2+ the
The Journal of Physical Chemistty, Vol. 86, No. 13, 1982 2409
Photocatalyzed Reduction of Methyl Vioiogen
TABLE 11: Formation of Methyl Viologen Radicals at Different Times after the Laser Pulse' IMVz+l.Mb 5.0 X 10" 1.0x 10-5
5.0 x 10-5 1.0 x 10-4 5.0 x 10-4 1.0x 10-3
[MV+*],M; t = 500 ns
[MV+.],M;
2.4 X 4.2 X loe6 9.0 x 1.2 x 10-5 4.0 x 10-5 6.0 x 10-5
4.8 X 7.0 X 2.0 x 10-5 3.8 x 10-5 1.0 x 1.0 x 10-4
Solutions contain 2.0 x EDTA, adjusted to pH 7.0 by
A = 0.045
t = 50 us
M dF, 1.0 x
tK
-
MV"
5.9w
M
lo-' M sodium phosphate.
Concentration of MV2+prior to flashing.
electron transfer +
'dF
+ MV+.
600 nm
A = 0.040
rapid decay of 3dF* can be observed and the appearance of MV+- can be monitored at 600 nm. At higher MV2+ concentration the rate of MV+. buildup increases, as expected for a bimolecular reaction between 2dF.and MV2+. Table I1 lists the concentrations of MV+. formed at different times after the laser pulse. At the laser energy used (-80 mJ) each pulse creates 3 X M 2dF.. Data in Table I1 show that at substoichiometric concentrations of MV2+the conversion to MV+. is almost quantitative. At higher concentrations of MV2+,the conversion efficiency decreases, while the final concentration of MV+. exceeds that of 2dF.. The presence of a substantial amount of MV+-is seen even at 500 ns after the pulse (Table 11). Similar observations have been reported previously for photosensitized proflavin reactions.'* Apparently, MV+. can be formed via routes additional to 2dF. oxidation. In order to elucidate the nature of the side reaction 'dF was flashed in the presence of MV2+(i.e., in the absence of EDTA). Figure 5 shows typical traces. Addition of 5 X lob M MV+ leads to a decrease of 3dF*,at 510 nm. The half-life of triplet in the presence of 1 X M MV2+is estimated to be about 3 ps. Buildup of products with greater absorbances precluded more accurate determinations. No particular light absorption due to 2dF. could be seen, however. At 600 nm, formation and decay of MV+. is discernible (middle trace in Figure 5). At longer times (lower trace in Figure 5) a small amount (-40%) of the transient remains. These results indicate the formation of a kind of 3dF* MV2+ encounter complex. This complex is likely to dissociate to form MV+. and an unidentified cationic deazoflavin species and/or reform 'dF with concomitant MV+.. It should be realized that the maximum amount of MV+-formed in this side reaction is only 10% of that created in the presence of EDTA (Table 11). Reactions 2, 5,and 8 describe well dF photolysis in the presence of EDTA and substoichiometric MV2+. 2dF*+ MV2+
510nm
t%= 0 . 4 ~
(8)
f
A = 0.038
I
Figure 5. Oscillograms of transient species obtained in the laser flash photolysis of 2.0 X lo-' M dF in the absence (- in the top trace) and in the presence of 5.0 X 10" M MV2+ (+ in the top trace) and 1.0 X lo-' M MV2+ (middie and bottom traces).
At higher concentrations of MV2+,reactions 9 and 10 need to be additionally considered. collisional deactivation 3dF* + MV2+ 'dF MV2+ (9) electron transfer 3dF* MV2*-+ 'dF+ MV+. (10)
-
+
+
+
Conclusion 1O-Methyl-5-deazaisoalloxazine-3-propanesulfonic acid (dF) has been shown to be a useful electron relay for methyl viologen photoreduction if concentrations are well chosen. The overall reaction
can be fruitfully exploited in photochemical solar energy conversions. Acknowledgment. Support of this work by the Department of Energy (J.H.F.) and the Dutch Science Foundation (A.J.W.G.V.) is gratefully acknowledged. We thank Mr. David Danzeiser for his competent technical assistance.
