J . Phys. Chem. 1991, 95, 1924-1933
1924
EPR Spectroscopy of Electron Spin Polarized Biradicals in LlquM Solutions. Technique, Spectral Simulation, Scope, and Limitations Gerhard L.Closs*~~,* and Malcolm D.E.Forbest** Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, and Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: June 5, 1990)
Application of the time-resolved EPR technique to the study of unstable organic biradicals in liquid solution is described. Results for a homologous series of polymethylene chain biradicals, produced by laser flash photolysis of cyclic ketones, are presented as experimental examples. To account for the shape and decay of the spectra with time, two models for the EPR spectra are put forward. The first uses a perturbation theory approximation for the eigenvectors,eigenvalues,and populations of the states, and then applies kinetic decay rate constants for nuclear spin dependent chemical reaction and spin relaxation. The second method shows how symmetry factoring the nuclear basis allows the block diagonalizationof the spin Hamiltonian, so that the eigenvalue problem, even for 16 hyperfine interactions, can be solved exactly. A comparison of the two models shows that the perturbation treatment is adequate for most cases. The shape and time dependence of the EPR spectra for both acyl-alkyl and bis(a1kyl) polymethylene biradicals are simulated and good agreement with experiment is obtained. The electronic spinspin coupling interaction,J, and the end-to-end contact rate of the chains, k,,are extracted from the simulations for each chain length. The lifetimes are 1-2 pus for the bis(alky1) biradicals and about 0.5 ps for the acyl-alkyl species.
Introduction The chemistry of reactive biradicals has been under active investigation in recent years.' The foundation for this activity has been the development of techniques making it possible to study highly unstable reaction intermediates by physical methods. Essentially, two strategies have been used: one renders the reactive molecule unreactive by matrix isolation,2 and the other relies on speeding up the experiment so it can be performed during the short lifetime of reacting biradicals in liquid solution.' Clearly, if one is interested in reactivity measurements, the latter is the preferred approach. Optical measurements have been made using either strategy, but EPR spectroscopy so far has been limited to matrix isolation. This is in spite of the fact that useful EPR measurements on stable biradicals have routinely been made for a long timee4 If one contemplates the EPR detection of unstable biradicals under normal reaction conditions in the liquid phase, it is easy to foresee a number of difficulties which, on the surface, might make the experiment extremely difficult or even impossible to carry out. The most important is the inherent instability of most biradicals which, in contrast to radicals, have a unimolecular reaction channel to diamagnetic products. The resulting short lifetime may make it impossible to get a high enough concentration for EPR detection even with modern, high-sensitivity spectrometers. Next, one might worry about the spin relaxation times which, because of the electron dipole-dipole interaction, might be too short to obtain high-resolution spectra. Finally, in flexible biradicals, the variation of the magnetic parameters with conformational changes might lead to excessive line broadening. It is the purpose of this paper to show that it is possible to overcome these problems by the choice of the appropriate experimental method and biradical precursor. This paper will first deal with the experimental methodology and the choice of a suitable chemica1,schemeto generate biradicals, then develop the theoretical background necessary for the spectral simulations and extraction of magnetic and kinetic parameters, and finally examine the scope and limitations of the technique. A separate paper, to be published shortly, will deal with the application of this method to the study of the magnetic and dynamic properties of a homologous series of polymethylene biradicals5 Experimental Technique To estimate the magnitude of the sensitivity problem, let a solution in a typical EPR flat cell of volume 0.1 mL absorb a light 'The University of Chicago. Argonne National Laboratory. I Present address: Chemistry Department, CB No. 3290, University of North Carolina, Chapel Hill, N C 27599.
*
beam with a wavelength of 300 nm and power of 1 W. With unit quantum yield for formation and a typical decay rate constant for a triplet biradical of (2-10) X lo6 s-I, the steady-state concentration of the biradical will be at best lo-* M. This is about 1-2 orders of magnitude too low to detect depending on the width of the signal. Increasing the light intensity by the necessary factor is not practical because of excessive heating. It appears therefore that a conventional steady-state EPR experiment is not very promising. A more advantageous route is to generate the biradicals with short laser pulses, creating initial high concentrations and detecting the short-lived signals in a time-resolved fashion. Several variations of this type of experiment have been described in the literature? all working without field modulation, which would limit the time response to many microseconds. The one chosen for the experiments described in this paper is a direct detection method using a boxcar integrator as a sample-and-hold device. The advantage of this over other time-resolved experiments is that it involves only minor modifications to a commercial spectrometer. The experiment and its timing sequence are summarized schematically in Figure 1. The laser, in these experiments a Lambda Physik EMG 103-MSC excimer laser working at 308 nm (XeCl), initiates the sequence by providing a sync-out pulse 1 before the laser flash. This pulse triggers a two-channel (PAR Model 162) boxcar signal averager. Between the sync-out pulse and the laser flash, the boxcar samples the voltage from the microwave bridge preamplifier and holds it in channel B. The sampling aperture is typically set at 100 ns. Next, the laser fires and initiates the chemistry producing the biradicals in the cavity of an X-band EPR spectrometer (Varian E-9). At a prechosen delay T from the laser pulse, the boxcar samples the preamplifier output voltage again and holds it in channel A. The sequence ( I ) (a) Borden, W. T. Diradicals; Borden, W. T., Ed.; Wiley: New York, 1982. (b) Platz, M. Kinetics and Spectroscopy of Carbenes and Biradicals; Plenum: New York, 1990. (2) (a) Buchwalter, S.L.; Closs, G. L. J . Am. Chem. Soc. 1975,97,3857. (b) Jain, R.; Snyder, G.J.; Dougherty, D. A. J . Am. Chem. Soc. 1984,106, 7294. (c) Jain, R.; Sponsler, M. 8.; Coms, F. D.; Dougherty, D. A. J . Am. Chem. Soc. 1988,110, 1356. (3) Closs, G.L.; Forbes, M. D. E. J . Am. Chem. Soc. 1987, 109, 6585. (4) Krinitskaya, L. A.; Buchachenko, A. L.; Rosantsev, E. G. Zh. Org. Khim. 1966, 2, 1301. (5) Closs, G. L.; Forbes, M. D. E.;Piotrowiak, P. To be published. (6) (a) Norris, J. R.; Trifunac, A. D.; Thurnauer, M. C. Chem. Phys. k t t . 1978, 57,471. (b) Kam, E.; Craw, M. C.; Depew, M. C.; Wan, J. K. S.J . Magn. Reson. 1986, 67, 556. (c) Trifunac, A. D.; Lawler, R. G.; Bartels, D. M.; Thurnauer, M. C. Prog. React. Kinet. 1986, 14, 43. (d) Basu, S.; McLauchlan, K. A.; Scaly, G. R. J . Phys. E 1984, 16, 767. (e) Prisner, T.; Dobbert, 0.;Dinse, K. P.; van Willigen, H. J . Am. Chem. Soc. 1988, 110, 1622. (0 Angerhofer, A.; Toporowicz, M.; Bowman, M. K.; Norris, J. R.; Levanon, H. J. Phys. Chem. 1988, 92, 7164.
0022-3654/91/2095- 1924S02.50/0 , 0 1991 American Chemical Society I
,
~
1
The Journal of Physical Chemistry, Vol. 95, No. 5, 1991 1925
EPR Spectra of Biradicals in Solutions
SCHEME I
A
I BOXCAR
B
I i I
BRIM M'cRow*vE
I. SAMPLE
2
1
nc out
-4
1 pa
0 '0
variable delay
4
Figure 1. (A) Block diagram of the time-resolved EPR apparatus. (B) Timing sequence for the experiment.
is repeated until the averaging produces a satisfactory signal-to noise ratio. The voltage difference between channels B and A of the boxcar integrator is fed into an analog-to-digital converter of a computer (IBM PC-AT) and stored. The repetition rate of the experiment is 60-100 Hz. To obtain an EPR spectrum at a given T after the laser pulse, the magnetic field is swept across the spectrum. In a typical experiment the magnetic field sweep of 200 G is divided into 1000 data points and it takes approximately 4 min to scan this range. With a repetition rate of 100 Hz this amounts to signal averaging 24 times per spectrum. In principle there is a variation of this experiment in which the magnetic field is held constant and 7 is swept. This should yield a time profile of the intensity of the spectrum. In the work described here, this mode has not been used because it has been found that the bridge output voltage is a function of the delay time T even in the absence of a signal from the sample. When kinetic information was desired, complete EPR spectra at various delay times were recorded and then compared. The key to the success of this experiment is the two-channel configuration. By subtracting the baseline voltage sampled before the firing of the laser from the signal voltage sampled after the biradicals are generated, noise of a frequency less than the inverse of the sampling interval is rejected. The time interval between the two sampling points is usually in the order of 1 ps or less, resulting in rejection of noise with frequencies below 1 MHz. This is an order of magnitude better than can be obtained with the standard field modulation technique (100 kHz). Another advantage of the pulsed detection is the fact that the detection is turned off when the biradical concentration has decayed to zero. From that point on to the next laser pulse only noise would be added to the signal already collected. However, probably the most impottant factor in making this type of experiment possible is the strong electron spin polarization created in the biradicals. As will be shown below, this can increase the signal intensity by 3 orders of magnitude. The time resolution of the experiment is determined by the width of the sampling gates and the general response time of the preamplifier. Gate widths of 50-250 ns were used for spectra shown here. Gate apertures of less than 50 ns led to significant signal deterioration. Preamplifier response times of the Varian E-line microwave bridges are in the vicinity of 300-400 ns. By using other preamplifiers it is possible to improve the response
4
rJ 3
time to cavity quality factor limit. The feasibility of the experiment was tested first on a series of polymethylene chain biradicals generated from cyclic ketones by laser flash photolysis. Scheme I summarizes the chemistry underlying these experiments. The cyclic ketones are excited by the laser pulse (15 ns fwhm) to generate the n,r* state of the ketone.' Intersystem crossing to the lowest triplet state occurs within the laser pulse.8 Norrish Type I cleavage from the triplet state produces the acyl-alkyl biradical 1. A subsequent thermoactivated loss of carbon monoxide yields the bis(alky1) biradical 2. Although not necessary for formation of the acyl-alkyl biradicals, loss of carbon monoxide is facilitated by substituting with methyl gioupsg at the a and a' positions of the ketones. In this case the formation of the bis(a1kyl) biradicals occurs on a time scale of several hundred nanoseconds instead of microseconds. T h e methyl groups also accelerate the Norrish Type I cleavage. This is important for larger ring ketones where intramolecular hydrogen atom abstraction can compete with ring cleavage if the latter is (7) Vassilev, R. F. Prog. Reocr. Kiner. 1967, 4, 305. (8) Turro, N . J. Modern Molecular Phofochemislry; Benjamin/Cummings: Menlo Park, CA, 1978; p 290. (9) For the preparation of a-methylated cyclic ketones, two standard methods were used, depending on the desired substitution pattern. To prepare the tri- or tetramethyl ketone, the following procedure was repeated three or four times: the enolate of the parent ketone was formed by refluxing a dry THF solution of the ketone and 1.1 q u i v of sodium amide until evolution of ammonia ceased. Methyl iodide was then added at room temperature at such a rate so as to maintain steady reflux. The a,#-dimethyl product could also be generated in this fashion. If the symmetric a,a'-dimethyl product was desired, then lithium diisopropyl amide (LDA) was used to generate the sterically favored enolate at -78 OC. Addition of the iodide at
