444
A. J. Elliot and J. K. S. Wan
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
to be polarized along the C2 axis. A photoselection polarization rnea~urernent'~ was attempted in this work by studying the cyclohexadienyl radical in the polycrystalline benzene at 4.2 K and EPA glass at 77 K. At best, we can only conclude that all four bands are polarized in the plane. Finally, the results of fluorescence lifetime measurement are reported. The lifetime of the first excited state of the cyclohexadienyl radical is very short. Since the fluorescence decay is almost coincident with the laser pulse (fwhh = 8 ns), a computer program was written to deconvolute the time intensity profile of the emission by using an iterative pr0~edure.l~ The lifetime of the cyclohexadienyl radical, obtained in this manner is 2 f 1ns. We have also determined the lifetime of the deuterated analogue to be 6 f 1 ns. It is interesting to note that the decay rate of the excited CsH7. is three times faster than that of CsD7. due to nonradiative processes.
Acknowledgment. The author thanks Dr. Gordon Hug for helpful discussions.
References and Notes (1) S.Gordon, A. R. Van Dyken, and T. F. Doumani, J. Phys. Chem., 62, 20 (1958). (2) V. A. Tolkachev, Y. N. Molin, I. I. Tchkheidze, N. Y. Buben, and V. V. Voevodsky, Dokl. Akad. Nauk USSR, 141, 911 (1961). (3) H. Fischer, J . Chem. Phys., 37, 1094 (1962). (4) R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 38, 773 (1963). (5) M. C. Sauer, Jr., and B. Ward, J . Phys. Chem., 71, 3971 (1967). (6) B. D. Michael and E. J. Hart, J . Phys. Chem., 74, 2878 (1970). (7) T. Shida and W. H. Hamill, J . Am. Chem. Soc., 88, 3689 (1966). (8) T. Shida and I.Hanazaki, Bull. Chem. SOC.Jpn., 43, 646 (1970). (9) J. E. Jordan, D. W. Pratt, and D. E. Wood, J. Am. Chem. Soc., 96, 5588 (1974). (10) K. Nakagawa and N. Itoh, Chem. Pbys., 16, 461 (1976). (11) C. W. Jacobsen, G. H. Hong, and S. J. Sheng, to be submitted for publication. (12) G. Herzberg, "Infrared and Raman Spectra", Van Nostrand, Princeton, N.J., 1945, p 362. (13) A. C. Albrecht, J . Mol. Spectrosc., 6, 84 (1961). (14) A. E. McKinnon, A. G. Szabo, and D. R. Miller, J . Phys. Chem., 81, 1564 (1977). (London), (15) H. C. Longuet-Higgins and J. A. Pople, Proc. Phys. SOC. 68, 591 (1955). (16) M. J. S.Dewar and H. C. Longuet-Higgins,Proc. Pbys. SOC.(London), 67, 795 (1954).
A CIDEP Study of the Photoreduction of Quinones in the Presence of Phenols and 2-Propanol A. John Elllot and Jeffrey K.
S. Wan*
Department of Chemlstty, Queen's University, Kingston, Ontario, Canada K7L 3N6 (Received September 26, 1977) Publication costs assisted by the National Research Council of Canada
A method is described on how to evaluate the triplet quenching (chemical abstraction) rate constants from the CIDEP observed when quinones are photoreduced in the presence of a hydrogen donor. The technique, which utilizes an intermittent light source in conjunction with a 100-kHz ESR spectrometer, has been used to determine the relative triplet quenching rate constants for pentachlorophenol, 2-methylphenol, phenol, and 2-propanol with a number of p-quinones. In general 2-propanol was found to be about two orders of magnitude less effective a quencher than the phenols.
In roduc ion To establish the role of the photochemical triplet mechanism in CIDE(N)P, phenols and alcohols have been used extensively in this laboratory as hydrogen donors for the photoreduction of quinones.1-6 However, little is known about the relative efficiences of these two classes of compounds to chemically quench the quinone triplet state. Phenols were assumed to have quenching rate constants ( h ) about four to five orders of magnitude greater than dcohols. This assumption was based on a few, not totally related, studies: those where triplet duroquinone was quenched by durohydroquinone (a phydroxyphenol) and by a l c o h ~ l s ;and ~ - ~one where triplet biacetyl was quenched by phenol and 2-propan01.l~ Obviously laser flash photolysis would be the first experimental method chosen to determine the relative (or absolute) k,'s. However, while duroquinone is quite amenable to this t e ~ h n i q u e ,this ~ - ~is not the case for other less substituted p-benzoq~inones.~ Possible reasons for this are that their triplet lifetimes are too short and/or their triplet extinction coefficients may be less than that of duroquinone. Now that the mechanisms which give rise to the CIDEP observed when quinones are photoreduced are well understood,ll we can estimate the relative k,'s for a series of hydrogen donors by following the dependence of the initial 0022-365417812082-0444$0 1.OO/O
(triplet) polarization on the concentration of the hydrogen donors. Furthermore, if the triplet spin-lattice relaxation time (3T1)is known, we can estimate the absolute k,. Ideally these experiments should be carried out using a rapid response time ESR spectrometer ( N 1 p s ) in conjunction with a nanosecond laser hash light source.12 However, because of the large bandwidth required to obtain this response time, the signal-to-noise ratio is often poor which makes finding the signals and recording their time profile d i f f i ~ u l t .In~ this paper we will describe how, under favorable conditions, a conventional ESR spectrometer using 100-kHz modulation (time response of -500 p s ) with its greatly enhanced signal-to-noise characteristics can be employed in conjunction with a rotating sector chopped light source to give the same information as the rapid response arrangement. With this experimental technique we have determined the relative h,'s for 2-propanol, phenol, 2-methylphenol, and pentachlorophenol quenching the triplet state of a series of benzoquinones and naphthaquinones to produce semiquinone radicals. As the 3T1for the duroquinone triplet is known,13an estimate of the kq's has been made. Furthermore, we have also shown that if EA (radical pair) and initial (triplet mechanism) polarizations1' are present together in an ESR spectrum, then the initial polarization can be separated by adding the time profiles of peaks @ 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
CIDEP Study of the Photoreduction of Quinones
445
equally disposed around the spectral center. This is important in experiments where there is no central line in the ESR spectrum. We have also evaluated the intrinsic EA polarization for the durosemiquinone radical.
The Triplet Mechanism and Quenching Rate Constants The triplet mechanism of CIDEP in which the ESR spectrum is emissively polarized requires that the semiquinone radicals are produced via an electronically excited quinone molecule in a spin-polarized triplet state. This triplet state is polarized because the rates of intersystem crossing (ISC) from the photoexcited singlet [Q(Sl)] to the three triplet sublevels [Q(Ti)]are unequal. (In all quinones studied to-date, it is the T+l sublevel that has the higher population.) So, provided these polarized triplet molecules react with spin conservation to form radicals within the spin-lattice relaxation time of the triplet (3T1 to s), the ESR spectra of the semiquinone radicals [QH] will be emissively polarized. The polarization is measured in terms of an enhancement factor V
-
v = ( H - Ho)/Ho
(1)
where H is the height of the observed peak and Ho is the height of the peak if all the spin states were populated thermally. To measure Ho for polarization due to the triplet mechanism, the light is turned off and the peak height (H) will relax to its Boltzmann value (Ho)with the Tl of the radical provided no chemical decay occurs during that time. To see how the rate of hydrogen abstraction affects the magnitude of V, consider the following simplified scheme: ISC
hv
Q ( S o ) - Q ( S l ) + Q(T+,),QU.,), Q ( T o )
(2)
( 3T1 )-
Q(T+i),Q(T-i), Q(To)-
Qo(T+i),Qo(T-i),Qo(To) ( 3 )
kg
Q(Tj) + HD+ &I3 t r) QH, r) -+ products
(4) (5)
Reaction 2 describes the formation of the spin-polarized triplet, reaction 3 indicates that the polarized triplets are relaxing toward thermal equilibrium with a rate constant of (3T1)-1,and reaction 4 describes the quenching by a hydrogen donor HD of the triplet with a rate constant of k,. Reaction 5 involves the fate of the radicals which does not concern us a t the moment. It is the competition between the thermalization of the triplet state populations (reaction 3) and the chemical quenching of the triplet (reaction 4) which determines the enhancement factor of the polarization. This can be expressed as
V = Vokq[HD]/(3T;1 + k,[HD]) (6) where Vois the enhancement factor at infinite quencher concentration. This equation can be rearranged into a more convenient form:
1 / V = l / V o + 1/(Vo3Tlkq[HD]) (7) A plot of 1/ V against 1/ [HD] should yield a linear plot where the intercept a t 1 [HD] = 0 gives l / V o , and from the slope of the plot (Vo T1hq)-’,if 3T1is also known, the value of k, can be estimated. In general 3T1is not known.
$!
However, if different donors are used, their relative quenching rate constants can be estimated from the slopes.
Experimental Section The chemicals were supplied by Aldrich (benzoquinone, 2,6-dimethylbenzoquinone,duroquinone, and penta-
Flgure 1. The time profile of an ESR peak obtained when the light beam was chopped by a rotating sector. The downward spike in the trace is the “light-on” point and the upward spike is the “light-off” point. The second-order kinetic plot to deconvolute through the machine dead time is shown on the right-hand side (see text).
chlorophenol), Eastman (2,5-dimethylbenzoquinone, 1,4-naphthoquinone), K and K (2-methyl-1,4-naphthoquinone and 2-methylbenzoquinone), Baker (phenol), and Fisher (2-methylphenol). All chemicals were sublimed before use except duroquinone, 2-methylnaphthoquinone, and pentachlorophenol which were used as supplied. Unless otherwise stated, the solvent was 15 vol % toluene:2-propanol; the toluene was added to improve the solubility of some of the quinones. The concentration of quinone was 0.04-0.08 mol dm-3. All solutions were deoxygenated by bubbling with nitrogen gas. Static solutions were photolyzed in Spectrosil tubes directly in the ESR cavity with the sample thermostated at 0 “C. In flow experiments, a Spectrosil flat cell was used and the in situ photolyses of the solutions were carried out a t room temperature. To minimize the heating of the solution by the photolyzing light, the cavity was purged with cooled nitrogen gas. The ESR spectra were recorded on a Bruker ER-420 spectrometer with a Varian Mark I1 field dial and 9-in. magnet system. The spectrometer response time was lowered to -500 p s by broadening the bandwidth of the 100-kHz tuned amplifiers in the detection system so that the time profile of the signal could be followed more effectively as the light beam (from a 1 kW high-pressure mercury lamp) was chopped by a rotating sector. The light-to-dark ratio for the sector was 1:3, with a “light-on” period of 12 ms. This time was normally sufficient for the ESR signal to reach a “steady-state” height (see Figure 1). The rise and fall time (
