Chemiluminescent Reactions after Pulse Radiolysis
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cent Reactions after Pulse Radiolysis of Aqueous Dye Solutions.
a
A. f'WtZ
lnsiiiul for Btophysik und Strahlenbtologte der Universtfdt Fretburg, 7800 Fretburg, Germany
and E. J. Land* FWerson Laboratories, Chrfstte Hosptfal and Holt Radium lnsf/tufe, Manchester M 2 0 SEX, England (Received October 30, 7972, Ftevsed Wanuscrtpf Received February 25, 7974)
The number of fluorescence quanta emitted per reaction [dye(oxidized by -0")-+ eaq-], denoted &h, has been estimated for several dyes. Two standards were used involving high energy irradiation: (A) Cerenkov light induced fluorescence of the dye itself, (B) p-terphenyl fluorescence in benzene. Both methods agreed within experimental error and gave mean values of &, for Acriflavin, Rhodamine B, Fluorescein (at pH lo), and anthranilic acid of 0.011,0.018,0.031, and 0.020, respectively.
Introduction Oxidized intermediates of various dyes, formed after pulse radiolysis of aqueous dye solutions by the action of OH radicals, have been found to yield singlet excitation of these dyes by subsequent reaction with hydrated electron~.~ Time - ~ dependencies of such chemiluminescences, which build up and decay over tens of microseconds after a single pulse, and the effect of dye concentration and of adding various radical scavengers to the solutions, have been described ear1ier.l Possible chemiluminescent processes were also discussed in these papers. The present article reports determinations of the absolute yields of such emissions. s3
Experimental Section Two different methods (A and B) were applied using 0.005-1-psec pulses of 8-14-MeV electrons from a Vickers accelerator and the pulse radiolysis apparatus of Keene.4 Solutions were irradiai ed at room temperature in a blackquartz cell with Supxasil end windows (0.7-cm electron path and 2.5-crri optical path) and emissions observed at right angles to the electron beam. Commercial dyes were used as b e f o d . 3 without further purification, the extinction coefficient obtained a t the absorption maximum was ~(452)3.8 x lo4 M - l cm-1 for Acriflavin (Fluka AG) at pW 7, e(554) 1 1 X lo5 M - l cm-l for Rhodamine B (Merck AG) a t p H 7, t(491) 8.3 X lo4 M - l cm-1 for Fluo) X lo3 M - l rescein (Merck AG) at, pH 10, and ~ ( 3 1 0 2.8 em- for anthranilic acid (Merck AG) at pH 7. Method '4. During the electron pulse the dye solutions show a strong prompt emission which is assumed to be due predominantly to excitation of the dye by Cerenkov Light. The number of dye molecules thus excited can be calculated from the theoretical yield of Cerenkov light and the geometry of its propagation (see, e.g., ref 5 ) . This fluorescence can be used as a reference for each particular dye for the determination sf the chemiluminescence yield.. Since these cbemilumi nescences actually involve the formation of the same singlet excited dye molecule^^,^ this method avoids any spectroscopic correction. According to Gerenktov light theory5 the number of photons ( N ) emitted per unit path within the spectral region X toX i- dA is given by
d2N/dx = 2xa sini 0(dX/X2) where a ( = 1/137) is the fine structure constant and 0 is the angle of light emission with respect to the electron path, which is connected to the refractive index n(X) and to p, the ratio of the electron velocity to that of light in vacuo, via the Cerenkov relation cos 0 = l/ni3. The Cerenkov relation in this case @ = 1) ensures total light reflection at the side wall of the cell. Since the cell had black front and end walls, with respect to the electron beam, no Cerenkov light was generated in the entrance window and no light reflection occurred at the end surface. The Cerenkov light produced at any point with distance x from the inner entrance surface of the electron beam will be absorbed only along the light path ( I - x)/cos 8, where 1 is the cell depth. Integrating the total absorption of light produced along the electron path 1 by the dye at concentration c we obtain
where G A is the number of fluorescence quanta produced by Cerenkov light per 100 eV absorbed in the solution. The literature values used for the fluorescence quantum efficiencies, &, which are independent of the excitation w a ~ e l e n g t h , are ~ ? ~given in Table I. (dE/dx) = 2.0 X lo6 eV/cm8 was used for the mean differential energy loss of the electrons. Dye concentrations of around 5 x loW5M were used €or the reference solutions, which ensured similar uniformity of excitation as occurs in the chemiluminescence situation. Calculated values of GA at a dye concentration of 4 x M , for example, were of the order of 5 x 10- 3 to 10- quanta per 100 eV for each dye. Direct excitation of the dye by electron impact can be neglected since it has been found to yield only about 2 X quanta/lQO eV a t 10- M dye from low-energy X-irradiation experiment^.^ Absolute chemiluminescence yields Gch, in quanta/100 eV, were obtained using the integrated areas under the emission intensity us. time curves for Cerenkov light induced fluorescence (10- to 10- ?I M dye) M and those for chemiluminescence (0.05 LO 1 X dye) a t a wavelength where no reabsorption takes place. A typical oscilloscope trace of such chemiluminescence is inserted in Figure 2. In the case of Cerenkov light induced The Journal of Physical Chemistry, Vol. 78,No. 13, 1974
W . A. Prutz and E. J. Land
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fluorescence the 1123. One reason for the lower quantum efficiencies in the chemical excitation of the dye may be that both singlet and triplet states are formed in reaction 2 . Such possible triplet yields are too low to be observed by absorption spectroscopy at these concentrations. Another more important reason may be that both D.+ and D O H adducts are probably formed3J4 in reaction 1 but only one of these leads to emission via reaction 2. Presumably the OH products which do not lead to luminescence are formed in large excess. The above effects may also explain why there is no apparent correlation between the changes in ,&I and @f in going from one dye to another.
Acknowledgments. This work was supported. by grants from the Cancer Research Campaign and the Medical Research Council. Funds were also given by the Deutsche Forschungsgemeinschaft . References and Notes W. Prutz and E. J. Land, Biophysik, 3, 349 (1967). (a) L. I. Grossweiner and A. F. Rodde, Jr., J. Phys. Chem., 72, 756 (1968); (b) L. I. Grossweiner, Radiaf. Res. Rev., 2, 345 (1970). W. A. Prutzand E. J. Land, J. Phys. Chem., 74, 2107 (1970). J. P. Keene, J. Sci. Instrum., 41, 493 (1964). J. V. Jelly. "Cerenkov Radiation and Its Application," Pergamon Press, London, 1958. W. R. Dawson and M. W. Windsor, J. Phys. Chem., 72, 3251 (1968). A. Schmillen and R. Legler, "Landolt-Bornstein, New Series I l/3," K. -H. Hellwege and A. M. Hellwege, Ed., Springer-Verlag, Berlin, 1967, p 270. H. Bichsel, "Radiation Dosimetry, 1," F. H. Attix and W. C. Roesch, Ed., Academic Press, New York, N. Y.. 1968, p 157. W. Prutzand K. Sommermeyer, Biophysik, 4,48 (1967). P. Skarstad, R. Ma, and S. Lipsky, Mol. Crysr., 4, 3 (1968). R. R. Hentz and L. M, Perkey, J. Phys. Chern., 74, 3047 (1970). 8 . M. Zarnigar and D. G. Whitten. J. Phys. Chem., 76, 198 (1972). C. Fuchs, F. Heisel, and R. Voltz, J. Phys. Chem,, 76, 3867 (1972). R. Cooper and J. K. Thomas, J. Chem. Phys., 48, 5097 (1968). S. Lipsky, W. P. Helman, and J. F. Merklin, "Luminescence of Organic and Inorganic Materials," H. P, Kallmann, Ed., Wiley, New York, N. Y . , 1962, p83. (a) W. P. Helman, Ph.D. Dissertation, University of Minnesota, 1964; (b) J. B. Birks, Proceedings of the University of New Mexico Conference on Organic Scintillation Detectors, G. Daub, F. M. Hayes, and E. Sullivan, Ed., U. S. Atomic Energy Commission Report No. TID-7612,1960. W. Stotz, Acta Phys, Polon., 26, 501 (1964). I. 8. Berlman, "Handbook of Fluorescence Spectra of Aromatic Molecules," Academic Press, New York, N. Y., 1971. W. H. Melhuish, J. Phys. Chem., 65, 229 (1961). G. Weber and F. W. J. Teale, Trans. Faraday Soc., 53, 646 (1957). E. J. Bowen and F. Wokes, "Fluorescence of Solutions," Longmans, Green and C o . , London, 1953, p 22. M. S. Matheson and L. M. Dorfman, "Pulse Radiolysis," MIT Press, Cambridge, Mass., 1969, p 64. The IBM-7040 computer of the Freiburg University Computer Center was used in these calculations. P. Cordier and L. I. Grossweiner, J. Phys. Chem., 72. 2018 (1968).
The Journal of Physical Chemistry, Vol. 78, No. 13, 1974
