J. Phys. Chem. 1980, 84, 1853-1856
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Optical Detection of ESR Spectra of Short-Lived Ion-Radical Pairs Produced in Solution by Ionizinlg Radiationt Yu.
N. Molin,* 0. A. Anislmov, V. M. Grigoryants, V. K. Molchanov, and K. M. Sallkhov
Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, USSR (Received September 27, 1979)
A new and very sensitive technique to detect ESR spectra of short-lived radical pairs in solution is proposed on the basis of optical.detection of magnetic resonance (ODMR). An ODMR spectrometer has been constructed for detecting ESR spectra of ion-radical pairs created by ionizing radiation. The singlet ESR signal from (naphthalene)-/ (naphthalene)' pairs has been observed in a naphthalene solution in squalane at an average concentration of 20 radical pairs per sample with a signal-to-noise ratio equal to 10/1. The hyperfine structure has been resolved in the ESR spectrum of (diphenyl)-/(diphenyl)+ pairs at a low diphenyl concentration in squalane and spectrum narrowing due to charge-transfer reaction has been observed at a high diphenyl concen tration.
Introduction The subject of my report is a new and very sensitive technique of optical detection of magnetic resonance (ODMR) spectra of reacting radical pairs (RP) in solution as well as some preliminary results obtained with this technique. The method is based on detection of changes in the luminescence intensity of RP reaction products caused by ahlsorption of resonance microwave radiation by the radical pairs in an external magnetic field. The idea of the method is similar to that proposed by Frankevich et al. in Mocxowl and discussed in some detail in Wolfs lecture at that conference.2 The main difference is, however, that we investigated solutions instead of molecular crystals and used ionizing radiation to create RPs instead of UV irradlation. The extremely high sensitivity and the possibility of resolving the hyperfine structure of ESR spectra are achieved in our case. The Principle of the Method Let us now discuss the principle of the method in more detail considering, as an example, the recombination of aromatic ionrradical pairs produced in a nonpolar solvent by ionizing radiation. These systems were studied recently by Brocklehurst and Canadian researchers3 who observed the external magnetic field effects on aromatic ion-radical pair recombination (for details on magnetic field effects in free-radical recombination see ref 4). At a sufficiently high concentration (20.01 M) of aromatic molecules M in nonpolar solvent S the main reactions resulting in formation and recombination of aromatic ion radicals are as follows:
s S+
-- s++
e-
e-+M+MM S M+
+
+
+
(1) (2)
(3)
M-+M'-MM*+M (4) The last reaction is geminate recombination because the initial distance between two ion radicals is about 100 A and Coulombic attraction prevents the separation of this pair in a nonpolar solvent. An excited molecule can arise either in the singlet state and thus emit light or in the triplet state which is noiiluminescent, depending on the multiplicity of the M+-M- RP at the moment of recombination. The initial M+--M- pair is a singlet-born one because the unpaired electron spins were paired in the parent solvent molecule. It is known from the magnetic field This paper is dedicated to Sam Weissman.
0022-3654/80/20841853$01.OO/O
effect studies3 that prior to recombination the multiplicity of this pair can be changed from singlet to triplet due to the hyperfine interaction (hfi) mechanism. The hfi mechanism mixes the S and Tolevels in a high external magnetic field, as it is shown by the energy level diagram in Figure 1. The T+ and T- sublevels are off resonance with the S level because of the Zeeman splitting and hence cannot be populated due to the hfi mechanism. The time average depopulation of the singlet state can amount to 50% due to this S-To mixing in RPs containing many magnetic nuclei. If resonance ESR transitions are induced by microwave irradiation, all the triplet sublevels can then be populated. Hence, the depopulation of the singlet level can achieve 75% in the extreme case of equal average population of all the coupled levels. Therefore, the probability of singlet recombination and hence the luminescence intensity will decrease substantially under microwave irradiation which makes it possible to detect the ESR signal optically. A strong ESR signal can be obtained if the orientation of the radical electron spins changes substantially during the pair lifetime 7Rp, that is, if (7H1)-', the precession period of a spin in a resonance microwave magnetic field HI, is comparable with 7Rp. A typical RP lifetime in the system discussed is of the order of 10-7-10-8 s. It means that a microwave field of 1-10 G is required. Measurable signals can be obtained, however, with a smaller HIdue to a very high sensitivity of the ODMR technique.
Experimental Section The experimentalsetup is shown schematically in Figure 2. The solution studied (0.5-1 mL) was placed into a quartz tube. Fast positrons emitted by a radioactive 22Na source (3 &i) or fast electrons (Y3r, 15 pCi) were used as ionizing radiation. The source was inserted into the sample tube. The sample tube, together with the end of the quartz lightguide, was placed into the region of the maximum microwave magnetic field of a strip cavity with the resonance frequency 1704.609 MHz or of the cavity of a Xband Varian E-3 ESR spectrometer. The microwave oscillator was either a powerful 1700-MHz generator with the maximum output power up to 60 W, or a X-band klystron of Varian E-3 spectrometer with the power not exceeding 0.3 W. The light emitted by the sample was transmitted via the 560-mm long lightguide with a diameter of 8 mm to a photomultiplier (Feu-36) coated with a permalloy shield. The fluorescence intensity was measured by a single photon counting technique. The program block connected to a magnetic field sweep unit and to a 0 1980 American Chemical Society
1854
The Journal of Physical Chemistry, Vol. 84, No. 14, 1980
Molin et al.
bl
a)
T+
1 ’ T+ I ESR
1~
ESR
T‘u.TFlgure 1. Energy level diagram and S-T transitions in a radical pair at a high external magnetic field with (a) H, = 0, (b) H , # 0; hfi is S-To mixing due to hyperfine interactions; ESR is resonance microwave transitions.
20
G
L - - - L ) H g
609
G
Flgure 3. Optically detected ESR spectra of the ion-radical pair (naphthalene)-/(naphthalene)+for 1.15 X low2M naphthalene solution in squalane at room temperature at various microwave fields H,.
Figure 2. Block diagram of the experimental setup.
Nokia multichannel analyzer (LP4048) allowed us to sweep the field repeatedly, the information being stored in the analyzer memory and extracted to the display, recorder, or type printer. The solutions of naphthalene and diphenyl in squalane were degassed by argon bubbling. Naphthalene and diphenyl were “chemiclly pure” grade. Squalane, usually used for chromatography, was preliminarily distilled in vacuo.
Results and Discussion The ESR spectra of the C8Hlo--C8Hlo+pair detected optically in squalane at room temperature and various microwave powers are shown in Figure 3. Squalane was used as a solvent because of its high viscosity which provides a sufficiently long (some 100 ns) lifetime for radical pairs. The fluorescence intensities of the spectra are in fact dependent on the external field. The magnetic field was swept in the range of 550-650 G with a 1-G step. The photon counting time was 1.5 s per step. The field was swept five times and the data were stored in digital form. The overall time of spectrum recording was 12.5 min. The total number of counted photons was 3 X lo5 per each value of the magnetic field. The microwave power fed in the cavity was 1.3 (Figure 3a), 3.8 (Figure 3b) and 7.6 W (Figure 3c) which corresponds to the field amplitude estimates of 1.8, 3.2, and 4.5 G, respectively. No signal was observed a t 60 W. The ESR signal is seen from Figure 3 to be a single line, 14-G wide at the half-height. The line width obviously results from the unresolved hyperfine structure of negative and positive naphthalene ions, as the total hyperfine splitting (some 27 G for negative ions5)is comparable with this width. The center of the line lies at 609 G in the magnetic field which corresponds to the g value of a free electron at 1704.6-MHz operating frequency. This is the value expected for aromatic ions whose g values are close to those of free electrons. The signal intensity is seen to decrease both with increasing and decreasing microwave power from its opti-
T I M E . ns
Figure 4. The time dependence of triplet population of a singlet-born RP with 10 G difference in ESR line positions for various HI values.
mum value. This behavior can be easily explained qualitatively. At a low H1the intensity drops since the rate of spin flips become insufficiently fast. When H1considerably exceeds the hyperfine splitting, all the spins precess about H1(in rotating frame) with nearly the same frequency. The signal intensity drops again as this synchronous precession does not destroy the initial spin correlation of the radical pair. This qualitative picture can be illustrated by the results of numerical calculations. Figure 4 shows the S-T evolution of a simple RP containing two radicals whose ESR lines are singlets shifted by 10 G from one another due to the g value difference. At zero HIthis pair will oscillate between S and Tolevels with the frequency equal to the difference in the resonance frequencies of the two radicals. In this case the average population of the singlet state is one-half. With H1comparable to the splitting, the S-T evolution becomes more complicated but it can be readily seen that the average triplet population is now higher that it was at HI= 0. In fact, this is due to populating the T+ and T- sublevels along with the Tostate. At last, at a high H I the triplet population rate is again reduced.
Optical Detection of ESR Spectra
The Journal of Physical Chemistry, Vol. 84, No. 14, 1980 1855
7 ~~
b
L
~
1
th - 1 . 8 G
NAPHTHALENE
N A P H THAL € N E
Flgure 5. The time dependence of triplet population of a singlet-born naphthalene ion-radical pair for different values of H,.
The S-T evolution of a real RP with a number of RP subensembles having different splitting between the ESR resonance llines is more complex and involves the superposition of evolutions of various RP subensembles. The results of numerical calculations for the naphthalene system are shown in Figure 5. The hyperfine coupling constants for negative and positive ions, as well as their g values, are assumed to be equal. The microwave frequency coriresponds to the center of the spectrum. Again, one can see that the microwave field influences the S-T evolution, the average triplet population being maximum a t HI equal to about 2 G. Let us eskimate the average concentration of naphthalene RPs in the sample to compare the sensitivity of this technique t o that of standard ESR spectroscopy. Taking into account that the activity of the source used (22Na) corresponds to lo5 decays s-l and that a positron energy equals 0.5 MeV, one can evaluate the ionization rate in the sample to be 2 X BO9 s-l. The efficiency of charge trapping does not exceed 10% in low2M anthracene solutions in cyclohexane.6 In our case the efficiency of charge trapping seems to be close to this value too. Thus, the generation rate of aromatic pairs is about 2 X lo8 s-l, their lifetime in squalane being s . ~Hence, the average concentration of the radical pairs is about 20 per sample, with the signal-to-noise ratio being 10/ 1. The values can be compared with the sensitivity of modern ESR spectrometers equal to loll spins per sample at a line width 1 G. Figure 6 illustrates the possibility of resolving the hyperfine structure with the ODMR technique. This figure shows the central part of the ESR spectrum of (diphenyl)-/ (diphenyl)+RP and its first derivative taken in the X-band region. The RPs were produced in a 5 X M solution of diphenyl in squalane at room temperature with a wSr radioactive source. The estimated average RP concentration in this case was equal to 100 per sample, the
p.
@-@
1
4
d
-H
l
2
1
0.005M
4
1
2
8
4
I
5.9 G
Flgure 6. Optically detected ESR spectrum of (diphenyl)-/(diphenyl)+ pair and Its first derivative for 0.005 M solution of diphenyl in squalane at room temperature.
accumulation time being 1 h. Figure 6 also depicts a schematic representation of the hyperfine structure of a negative diphenyl ion; only the ortho- and para-proton splitting are taken into account, with slight meta-proton splitting being neglected (A, = 2.73
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J. Phys. Chem. 1980, 84, 1856-1863
Flgure 7. Optically detected ESR spectra of the pair (diphenyl)-/(diphenyl)' for various diphenyl concentrations in squalane at room temperature.
G, A , = 0.43 G, A, = 5.46 G'). The observed splitting (2.66 f 0.1 G) is seen to correspond well to that expected for a negative ion. As to a positive ion, its spectrum must be nearly identical with that of the negative ions8 Therefore, the spectrum observed is probably a superposition of negative and positive ion spectra. In order to obtain a good resolution in the ODMP spectra of radical pairs, two main contributions to the line width should be diminished. The first is the broadening due to a short RP lifetime. At a lifetime of 100 ns this broadening equals about 1 G. This contribution can be diminished by using either more viscous liquids or special techniques which permit one to select signals only from long-lived pairs. The other appreciable contribution to the line width is the broadening by the microwave field HI. This broadening can be diminished by using a low H1 at a sufficiently high signal-to-noise ratio. The two factors mentioned do not permit resolving any structure in the naphthalene system where the total evaluated broadening exceeds the distances between neighboring lines. In the case of diphenyl, the line split-
tings are higher than those for naphthalene ions and the microwave field Hl used is lower (0.5 G) which makes it possible to resolve the hyperfine structure. Figure 7 shows the ESR spectra taken at three different concentrations of diphenyl in solution. The spectrum narrowing is seen to take place at a high diphenyl concentration which is obviously due to the charge transfer between negative (and positive) ions and neutral diphenyl molecules. A rough evaluation made with these data shows that each collision results in charge transfer between an ion and a molecule. It should be mentioned in conclusion that the technique described can also be used to observe the ESR spectra of short-lived RPs produced photochemically or in thermal reactions provided the recombination product is electronically excited and can emit light. Because of its extremely high sensitivity, this technique can open up quite new possibilities in studying the nature, reactions, and molecular dynamics of short-lived RP intermediates in various chemical processes.
References and Notes (1) E. L. Frankevich, A. I. Pristupa, and V. I . Lesin, Chem. Pbys. Lett., 47, 304 (1977). (2) H. C. Wolf, Abstracts of papers presented at the International Symposium on Magnetic Resonance In Chemistry, Bolosy aml Physics, June 24-28, 1979, p 16. (3) R. S. Dixon, F. P. Sargent, V. J. Lopata, E. M. Gardy, and B. Brocklehurst, Can. J . Chem., 55, 2093 (1977). (4) R. 2 . Sagdeev, K. M. Sallkhov, and Yu. N. Molin, Usp. Kbim., 46, 569 (1977) (Engl. transl., Russ. Chem. Rev., 46, 297 (1977)). (5) F. Gerson, B. Weidmann, and E. Heilbronner, Heiv. Chim. Acta, 47, 1951 (1964). (6) J. K. Thomas, K. Johnson, T. Kllpper, and R. Lowers, J. Chem. Phys., 48, 1608 (1968). (7) A. Carrington and J. dos Santos-Velga, Mol. Pbys., 5, 21 (1962). (8) F. Gerson, "High ResolutionESR Spectroscopy", Wlley, New York, 1970.
An Investigation of the Photoelectron Emission from Solutions Containing Solvated Electrons, and the Physical Nature of the Solvated Electront A. M. Brodsky Institute of Elecfrochemlstry, Academy of Sciences, Moscow V-71, USSR (Received July 16, 1979)
This communication deals with theoretical hypotheses rather than with experimental analyses. This is mainly because more complete experimental results are not available at this time which would permit detailed quantitative treatment. The fundamental works of Delahay et al. lead us to believe that there will be advances in this area. These investigations have essentially stimulated the development of modern concepts about the physical nature of solvated electrons. 1. Introduction Photoelectron emission from solutions of solvated electrons obtained by dissolution of alkali metals in ammonia has long been known.lr2 A systematic quantitative study of this phenomenon did, however, become possible only after Delahay et al. developed the modern techniques to study photoemission from solutions in the early seventie~.~-'lDelahayll used a three-stage model of photoemission to describe the experimental results. In this model the photocurrent I from solutions is represented in the This paper was presented a t Colloque Weyl V. For a complete listing of the papers given a t this conference see the May 15, 1980 issue of T h e Journal of Physical Chemistry.
0022-3654/80/2084-1856$01 .OO/O
form of a product of three factors: I P1D where P is the probability of photoionization of solvated electrons per unit of time in a unit volume. C is the effective thickness of the layer which contributes to photoemission (it is determined by the laws of motion of delocalized electrons in a medium toward the surface), and D is the coefficient of transmission of electrons through the interface. In ref 11 P was estimated by using the formula for the photoionization cross section of hydrogen-like atoms, C was found by solving the random walk problem, and D was taken to be a constant independent of the frequency of light w. This choice of expressions for N
0 1980 American Chemical Society
