Time-resolved electron spin resonance with electron spin polarization (CIDEP) as a sensitive probe of degenerate electron exchange reactions

Volume 105, number 4

TIME-RF%OLVED

ELECTRON

AS A SENSlTIVE

Chemirtry

Rcccived

SPIN RESONANCE

PROBE OF DEGENERATE

S BASU, K.A. McLAUCHLAN Physical

13 March 1984

CHEMICAL PHYSICS LETTERS

Loboratory.

and A.J.D.

Sourh Parks Road,

WITH ELECTRON

ELECTRON

SPIN POLARIZATION

EXCHANGE

(CIDEP)

REACTIONS

RJTCHIE Oxford,

UK

31 October 1983

It is shown that the o~currente of electron-exchange processes produces a dislinctive effect in the time-resolved electron spin resonance spectra of spin-polarized radicals observed in the continuous presence of a micro\vave field. The effects observed are consistent with a theory based upon the Bloch equations.

1. Introduction

usually neghgble in carbon-centred radicats but in its presence lines of different degeneracy have differing

Degenerate electron exchange processes of the type k,

effective relaxation times This has been shown for both T, and spin--spin (T-J relaxation processes in both

form an important nificance

class of reactions

in photosynthesis

of particular

and biology.

However

sigthe

identity of the reactants and products poses unusual difficulties to their study. Here a new and particularly sensitive method for their detection and quantitative study is reported, based upon therr effect on the time evolution of the electron spin resonance spectra ofspin-polarized radicals. Profound effects are observed whether spin polarizatmn arises via the triplet mechanism (TM) or the radical pair mechanism (RPM) of chemic~ly-induced dynamic electron pola~zatioR {CLDEP). Here the particular case of TM polarization is considered. When a radrcal IS created by flash photolysis in a spin-polarized state the lines in its electron spun resonance spectrum exmbrt time-dependent intensitres as the radical attains thermal equihbrium with its surroundings. For a TM-polarized radical all the lines have the same polarization, chosen here to be emissive, and the rate of this process depends mainly on the electron spin-lattice relaxation time (Tr) and on the magnitude of the microwave field (wt) applied ~ou~out. In the absence of an electron exchange process any hyperfine dependence of relaxation is 0 009-2614/84/S (North-Holland

03.00 0 Elsevier Science Publishers B.V. Physrcs Publishing Division)

steady-state

[l]

and transient

studies

[2]

but

accurate me~urcment of T, and Tz remains difficult and small changes in them may go unremarked. However if the time dependence of the entire spectrum is monitored the differentral relaxation rates cause the various lines to invert in phase at different times after their creation: all lines of the same degeneracy behave similarly, with the most intense lutes chan~ng phase first. Electron exchange is consequently recognised very directly in the form of phase information of a characteristic nature and not, as is usual, by its often small effect on rate constants and linewidths. Furthermore experiments at a single concentration of substrate only are required_ The physical origin of the phenomenon follows from consideration of a spectrum of well-resolved lines m which at any one tune only one transition is excited. The over-population of the upper energy level in the polarization process is aged in time by relaxation and also by stimulated transitions under the influence of the applied microwave field. At the same time, the electron hops from the radical ion to the neutral molecule in a random manner which populates the nuclear states of the new radical statistically. Since the total degeneracy of the nuclear states perforce exceeds the degeneracy of a single line, the mol447

Volume 105. number

CHEMlCAL

4

PHY SlCS LETTERS

ecules constitute a large reservoir from which the smaller reservoir associated with the particular energy level involved

is replenished

at a rate which depends

upon

iadeg eneracy _ By uarytig the magnitude of the observingmicrowave field a competition between the loss and gain of electrons from or to this level can be established and the effective relaxation time can be “tuned” cess. The phasised: spinecho

to respond to the rate of the exchange proseminal role of the microwave field is emthe phenomenon would not be evident in experiments.

Tr

23 March 19E4

on both the microwave

field strength

and the

degeneracy_ Exchange effects are usually detected in maguetic resonance by their influence on the linewidths of transitionqi~e~ upon 2-2 for a kWXItzian hue, and the effects reported here can be detected in this way. However the known exchange rate constant in

the case, for example, of the duroquinone anion implies a maximum variation in linewidth proportional to cDm* - Dmin)lD of only about 3% The effect of exchange on the tune evolution of the entire spectrum is more dramatic, as is shown below.

2. Theory

3. Experimental

The theory of electron exchange effects on the time evolution of lines in the ESR spectra of polarized radicals has been given elsewhere [2,3] although rts implications to the appearance of full spectra have not

The method, based upon a modified Bruker ER 200 spectrometer and a Lambda Physik excimer laser operating at 308 run, has been described recently [5] _

been discussed. It is based upon the Bloch equations modified to account for the electron exchange process,

using McConnell’s general approach

[4]. The basic assumption, mentioned above, is that the microwave field which excites one lure has a negligible effect on any other, I e_ that (km [A])-1 is less than the smalest hyperfine coupling constant. In the theoretical spectra shown below the further approximation has been made that radical termination reactrons are neghgibly slow compared with electron-transfer ones, a condition easily satisfied at low radical concentrations_ In these simulations it was necessary to compute the time-dependent intensities and lineshapes of transitions of different degeneracy individualhy before adding them together with the correct normalisation to reproduce the complete spectrum. The details of these calculations will be published elsewhere although it is noted here that some unusual lineshapes are reproduced within experimental error. Extension of the published analysis gives expres-

sions for the effective spin-spin laxation times for a chosen line l/T%

= l/T2

+ [W

-D,)/Dl

and spin-lattice

re-

?“I

= l/Tr + LJ: (l/T%

kFT [Al

- l/z-r)-1

,

where D is the total degeneracy and Dj that of line “i”. These equations show the predicted dependence of 448

The theoretical spectra were integration of the theoretical tervals. with small integration gration can be nusleadmg in expenmental tion_

obtained by numerical curves over the same mwindows analytical inte-

a comparison made with results obtamed by numerical integra-

Materials were used as supplied with the solutions de-oxygenated by passage of nitrogen before flowing through the irradiation region. Initial radical concentrations were limited to lop5 M by attenuatron of the laser flash.

4. Results In fig. 1 is shown the tune evolution of the spectrum of the duroquinone anion (LIQ’) as it undergoes degenerate electron exchange with the parent molecule (DQ). Each spectrum was obtained by integration for 1 us centred at a specific time following the flash which created the radical, and is shown with its in-

and l/G

Chosen parts of the decay curves obtained at different values of the magnetic field were integrated in time.

tensity normal&d so that the decay of the signal with tune is apparent only in the progressive worsening of the signal-to-noise ratio. As predicted above the most degenerate line changes phase first and is followed by the next most degenerate pair, which show similar behavrour. These are followed in turn by the other pairs of lines in order of decreasing degeneracy until, at a

CHEMICAL

Volume 1 OS number 4

PHYSICS LETTERS

23 March 1984

I /

3

Frg 1. The time dependence of the emissive spectrum of the duroquinone anion produced by flash photolysls of a 0.08 31 soluiron of duroqumone in a propan-2-l. trrethylamine 4- 1 (VI V) mixture. The mrcrowave field strength was 2.5 rad MHz and all spectra are normalised to the same height.

time beyond those shown, the entire spectrum has relax from its initial emissive state to absorption. This behaviour 1s symptomatic of the occurrence of the exchange process, as is the distinctive and unusual hneshape. That this phenomenon is capable of detailed theoretical analysis is illustrated in fig. 2 in which the spectra correspondtig to just three times after the flash are simulated, with the integration period corresponding to that used experimentally. The calculations depend

upon several input parameters besides these times and for fig. 2 the following values were taken: Imtial polarization ratio = -100 times the Boltzmann population difference, T, = 6.6 m, T2 = O-67 JJS, wl = 2-5 rad MHz and (km [DQ])-’ = 1.5 w-l _All the major features of the observations are reproduced and it is apparent that the exchange rate constant can be extracted if the values of the other parameters are known accurately. At this stage this is not the case and the calculations given here must be considered illustrative only. However they can all be obtained by careful observation of the time dependencies and lineshapes of spin-polanzed spectra obtained in the absence of exchange. or even, with great care, with it. This timeconsuming work is in progress. Similar exchange effects have been observed in the spectra of a wide range of radical anions, includmg I, II and Ill formed from pyromellitic acrd, pyromellitic dianbydride and phtbahc anhydride.

Fig. 2. Theoretical spectra calculated from the amended Bloch equations at (i) 10.5 ~5. (ii) 14.5 JIS and (iii) 17.5 ps after the Ilash. AU the specLra have the same set of parameters Listed in the text, which satisfactorily account for changes in the spectra with time

Volume

105, number 4

CHFMICAL

PHYSICS

LETTERS

23 March 1984

its difference from other lines of other degeneracies, is particularly evident. The Lime dependence of the spectra of spin-polarized radicals has been shown to provtde a very direct indication of the occurrence of electron-exchange processes. It has been shown further, for the first time, that the theory of the effect of these processes on electron spin resonance spectra accounts fully for the behaviour observed. This verification represents a considerable advance on the much less demanding fitting to simple decay curves or individual lines [2,3,6] reported previously. A notable feature of the experiments is that a method has been provided for recogmsing electron exchange processes outside the fast exchange region in which lines are shifted, with much greater sensitivity than is usually associated with lineshape measurements A quantitative study of all the chemical systems mentioned is at hand.

Acbowledgement Fig. 3. The time dependence of the emissive spectrum of the radical trianion (I) from pyromellitic acid, with the spectra shown at the rruddle of their integration periods. The microwave field strenght was 1.0 rad MHz. It is seen that lines of the same degeneracy invert phase at the zame rates. The solution was 0.1 hl of pyromelhtlc amine 4: 1 (vr v) mixture.

acid in a propan-2+l,

SB and AJDR thank the S.E.R C. for maintainance awards.

triethyl-

References

[ 1) An example is shown in fig 3 for the radical trianion (1) in whch lines of degeneracy, 1,2 and 4 are present as a result of dissimilar coupling to two pairs of equivalent protons. In this case exchange is with the dlanion, which 1s the stable form of the parent molecule in the presence of triethylamine, and integration was over 1 .O p for each spectrum_ The parallel behavrour of lines of the same degeneracy, and

450

C.P. Cheng and S 1. Weissman, J. Phys. Chem. 80 (1976) 872.

121 PJ. Hore and K-A. McLauchlan, 533. [3] [4] [5]

P-J. Hore and K-A. McLauchlan. Mol. Phys 42 (1981) 1009. H.M. McConnell, J. Chem. Phys. 25 (1956) 709 S. Basu. K-A. hlclauchlan (1986)

[6]

hlol. Phys. 42 (1981)

and G.R. Scaly. J. Phys. E 16

767_

PJ. Hore.KA. M&m&Ian, S. Frydkjaer and L.T. Muus. Chem. Phys. Letters 77 (1981) 127.