EFFECTOF VARIOUS“DRYELECTRON” SCAVENGERS tion 4. The fate of the resulting PF has not been determined, since the ultraviolet spectrum of PF is not observed, and the earlier esr studies show no evidence for phosphorus atoms which would be formed by further photolysis. The P and/or PF fragments would remain close to the formed PF, molecule, and this perturbation may result in broad, undetectable spectra. P2F4
P2F4
-%
2PFz
PF3
+ PF
4059
ments using heated tubing inside the cell resulted in some aggregation of PFs and PzF~.These aggregates have broad intense infrared absorptions that effectively block the infrared spectral region of greatest interest and the experiments are of no value. Solan6has used a better furnace design and has evidence for having isolated PF and further evidence for having isolated PF2 from the thermal decomposition of PzF4.
(5)
(6)
The thermally decomposed samples passed through 5 cm of 7-mm glass tubing before expanding inside the cell. Only a small fraction of the formed radicals ever reached the cold window. The relative importance of ( 5 ) and (6) therefore could not be determined. Experi-
Acknowledgments. We wish to thank Mr. Harvey Schiller for the sample of PF21and Dr. Ralph Rudolph for several useful discussions. Dr. David Solan has generously provided us with a copy of his thesis and other information and criticisms which have been most helpful. This work was supported in part by the National Science Foundation.
Effect of Various “Dry Electron” Scavengers on the Radioluminescence
of Indole in Polar Solution by H.B. Steenl Norsk Hydro’s Institute for Cancer Research, Montebelb, Oslo 9, Norway
(Receiued M a y 19, 1070)
The electron scavengers HzOz, ClCHzCOOH, CClaCOOH, and NaNOs have been found to reduce significantly the yield of the X-ray-induced phosphorescence of indole in ethylene glycol-water glass a t 77’K when present in concentrations >0.1 M . The reactions involved obey homogeneous competition kinetics. Since the major fraction of this phosphorescence is supposedly due to the geminate recombination between positive indole ions and nonhydrated electrons (e-), this effect is attributed to the reaction between the electron scavengers and e-. No effect of HC1 was found. This is in agreement with the hypothesis of Hamill that H30aq+is unreactive toward e-. The relative scavenging constants determined by the present experiments agree with those obtained for what is believed to be e- by picosecond pulse radiolysis of aqueous solutions, while they are significantly different from the scavenging constants reported both for “mobile electrons” in strongly acidic glasses as well as for eaq- in water,
Considerable experimental evidence exists that radiation-induced electrons in frozen aqueous media are able to react with molecular entities while they are still in the mobile presolvation ~ t a t e . However, ~~~ the exact nature of these “mobile electrons” is still o b ~ c u r e . ~Recently Hunt, et U Z . , ~ , have ~ performed picosecond pulse radiolysis studies of aqueous solutions a t room temperature which seem to demonstrate that the precursor of the hydrated electron, ie., presumably the nonhydrated electron, reacts efficiently with a variety of hydrated electron scavengers except H,, +. These phenomena are now attracting renewed interest in consideration of a new model for the radiolysis of water proposed by Hami11.7*8 A major assumption of this model is that
electrons may react with molecular entities prior to hydration, that is while they are still moving with thermal energies as quasifree particles. According to this (1) Fellow of the Norwegian Cancer Society. (2) (a) P. N. Moorthy and J. J. Weiss, Advan. Chem. Ser., 50, 180 (1965); (b) L. Kevan, “Radiation Chemistry of Aqueous Systems,” G. Stein, Ed., Wiley, New York, N.Y.,1968,pp 21-71. (3) T. Sawai and W. H. Hamill, J . Phys. Chem., 73,2760(1969). (4) P. N. Moorthy, C. Gopinathan, and K. N. Rao, Radht. Ef., 2,176 (1970). (5) R. K.Wolff, M. J. Bronskill, and J. W. Hunt, submitted for publication in J . Phys. Chem. (6) J. W. Hunt, et al., private communication. (7) W.H. Hamill, J . Chem. Phya., 49,2446 (1968). (8) W. H.Hamill, J . Phys. Chem., 73,1341(1969). The Journal of Phgsical Chemhtry, Vol. 74, No. $8, 1070
H. B. STEEN
4060 hypothesis these "dry electrons" (e-) should in general react with molecules which are scavengers of hydrated electrons (eaq-) with the important exception of Haq+ which should be essentially unreactive toward e-. The rate constants of such scavengers toward e- are generally expected to be different from those valid for eaq-. It is of considerable interest to see if this can be experimentally verified and, if so, to obtain the relevant rate constants. I n the present communication we report experiments which seem to support the above hypothesis. Thus, we have studied the effect of various electron scavengers on the radioluminescence of indole in a polar solution a t 77°K. The basis for these experiments is our previous studies of the X-ray-induced luminescence of various aromatic molecules in similar solution^^-^^ from which it was concluded that approximately 90% of the phosphorescence observed during X-irradiation a t 77 "K results from the recombination between an ionized solute molecule and an electron, the radiative process being the 2'1 -+ 80transition of the neutral solute molecule. This recombination appears t o be a geminate process which takes place before solvation occurs. Hence, the effect of various additives on this X-ray-induced phosphorescence should provide a simple test of their efficiencies as scavengers of e-. It should be emphasized that our previous data seem to exclude that any substantial part of the X-ray-induced phosphorescence observed here results from solvent-solute energy transfer, e.g., from trapping of positive holes by solute molecules followed by neutralization. Thus, a reduction of the X-ray-induced phosphorescence caused by the competition for positive holes between the solute and the scavenger can be disregarded in the present case. Reactions between positive solute ions and scavenger molecules are also ruled out since neither one is able to diffuse in the solvent under the present conditions. The solve$ was a 1 :1 mixture of ethylene glycol and water which appears as a rigid and completely transparent glass a t 77°K. The concentration of indole was 2 X M . &he scavengers were HzOz, ClCH2COOH, CCl&OOH, NaNOa, and HC1which were mixed into the sample solutions within a few minutes before cooling to 77°K. The samples were irradiated in a vacuum with 50-keV X-rays a t a dose rate of approximately 50 rads/sec and the emission spectrum was recorded during irradiation. I n order to correct for the possible effects of the scavengers on the radiative properties of the solute, we measured also the relative quantum yields of the fluorescence and phosphorescence induced by 270-nm uv light. Thus, it was found that NaNOs, CClaCOOH, and H20amarkedly reduced these yields. The apparatus and experimental procedure have been described el~ewhere.~ The yields of fluorescence and phosphorescence were obtained by integration of the respective spectral bands. Taken separately these bands were identical for the two The Journal of Physical Chemktry, Vol. 74,No. db, 1970
types of irradiation and, apart from the reduction in intensity, they were not affected by the presence of any of the scavengers except NaNOa. For both types of irradiation NaNOa produced an additional fluorescence band which presumably reflects the presence of a chargetransfer complex involving the excited solute. The yield, YT, of X-ray-induced excitation of the phosphorescent triplet level is calculated fram
where YP is the yield of X-ray-induced phosphorescence and tpF and ~ J are P the quantum yields of the uv-induced fluorescence and phosphorescence, respectively. The superscript denotes the yields observed with no scavenger present. Equation 1is valid only provided the rate of intersystem crossing from the fluorescent singlet level to the phosphorescent triplet level is independent of the scavenger concentration. It was found that, with one exception, all the scavengers studied significantly reduced YT, hence indicating that reactions between scavengers and e- can compete with recombination. The exception is HCI which, in agreement with Hamill's hypothesis, gives no noticeable reduction of YT when present in a concentration of 2 M. The appropriate Stern-Volmer graphs are presented in Figure 1. It can be seen that the quenching of the X-ray-induced triplet excitation, that is, supposedly the
.i
4
0.2 0.4 0.6 0.8 1.0 SCAVENGER CONCENTRATION (MOL)
Figure 1. Stern-Volmer graphs of the yield of X-ray-induced excitation of the phosphorescent triplet level of indole in ethylene glycol-water glass containing various electron scavengers at 77°K. (9) H.B.Steen, Photochem. Photobwl., 6,805 (1967). (10) H.B.Steen, Radiat. Res., 38,260 (1969). (11) H.B.Steen, ibid., 41,268 (1970).
EFFECT OF VARIOUS “DRYELECTRON” SCAVENGERS Table Io Scavenger
C7,
M-1
H202 1 . 0 (1.7)b ClCHzCOOH 1.1 (1.5)‘ CClsCOOH 0.26 (0.5)“ NaNOa 0.31 (0.45)b >10 HC1
C‘d,re1 units
1.0 0.9 0.29 0.26
k’,d re1
kQ, M - 1
units
1.6 1.6 6.6 5.5 (0.2
1.6 0.85
. 1.1 1.4 3.0
a The scavenger concentration Ca? needed to reduce the yield of X-ray-induced triplet excitation to 37% and the corresponding quenching constants k ~ .The numbers in parentheses are those obtained by Hunt, et al., and C37’ are these values normalized so as to match the present value for HzOz. k’ are the relative rate constants for eaq-. ,i See ref 5. See ref 6. See ref 12.
quenching of e-, obeys homogeneous competition kinetics within the range of concentration studied. The quenching rate constants obtained from the slopes of these curves are given in Table I together with the scavenger concentrations needed to reduce YTto l / e . The latter values are compared to those found by Hunt, et al., for these scavengers in water a t room temperature. It can be seen that the quenching efficiencies are somewhat smaller under the latter conditions. This can be accounted for by the longer lifetime of the nonhydrated ions in the highly viscous solvent used in the present experiments. However, it appears that when compared on a relative scale the quenching efficiencies obtained
406 1
here are approximately the same as those found by Hunt, et al. This result strongly indicates that the numerical data obtained by these two types of experiments indeed refer to the same reactions, namely the scavenging of e-. From Table I it appears that the quenching rate constants obtained here are reciprocally quite different from those known for ea,- from pulse radiolysis measurements.12 Kevan2 reports that the relative rate constants which he obtained for the scavenging of mobile electrons by various electron scavengers in strongly acidic glasses a t low temperatures are fairly similar to those known for eras-. Hence, it seems indicated that the nonhydrated electrons observed by Hunt, et al., and in the present work are different both from the mobile electrons discussed by Kevan as well as from the hydrated electrons. I n view of the high “solute” concentrations characterizing living organisms, reactions involving “dry electrons” are likely to have particular importance in radiobiology. Acknowledgments. The author is deeply indebted to J. W. Hunt for a series of stimulating discussions which initiated the present work. Valuable discussions with M. Kongshaug are also appreciated. (12) M. Anbar, and P. Neta, Int. J. A p p l . Radiat. Iaotop., 18, 493 (1967).
The Journal of Physical Chemistry, Vol. 74, No. 33,1970
