21 14
KENNETH D. LEGGAND DAVID M. HERCULES
the doses of 150 to 200 rads used in the present experiments a concentration of 6 5 X lo-’ M C1 would be formed if all OH were converted into C1 atoms. M dye and 10-1 M C1- the lifetime At 2 X of C1 atoms would be very short due to its reactions with C1- and dye, leaving little chance for formation of C1*- via reaction of C1 atoms with cas-. Therefore energy transfer from C1”- to the dye cannot be involved in the present emission enhancement. I n fact irradiated NaCl crystals on dissolution probably give rise to the same species as obtained on irradiating NaCl solutions. Therefore the emissions obtained on dissolving irradiated NaCl in dye solutions are probably formed via a similar mechanism to the one now proposed. (5) E$ect of CNS-. From the experimental results it could be deduced that, like iodide, the corresponding reactions of CNS-dye products with eSq- would explain the enhanced emissions. The dotted lines of Figure 5b thus calculated using the appropriate rate constants given in Table I lead to the optimum relative values Fz,Fs,and Flo given in Table 11. ( 6 ) Conclusions. The enhancement of the emission from irradiated acriflavin solutions by addition of various halides or pseudo halides seems in all cases to be associated with the formation of dye products, via attack of halide intermediates X and-Xz--, which then react with eaq- in chemiluminescent reactions.
Neither the emission nor the absorption measurements carried out led to a definite decision as to whether the semioxidized dye and/or the OH, X, and Xzadducts of the dye participate in the emission-forming step. The relative reactivities of the various X and Xz- intermediates toward acriflavin follow the order I CNS > OH > ClzBrz- > (CNS)2- >> Iz-. This is similar to the order of oxidizing power: OH > Clz- > Brz- >> Iz- found by Langmuir and HayonZ3 from the measured reactivities of these species with various solutes, mainly alcohols. Advances in this system are possible by measurements of the absolute yields of emissions and by further absorption studies using both pulse radiolysis and flash photolysis in order to identify the spectrum of semioxidized acriflavin.
-
-
Acknowledgments. The authors wish to thank the late Professor K. Sommermeyer for stimulating this work, Dr. L. G. Lajtha and Dr. -11. Ebert for encouragement and support, and Dr. J. P. Keene and nh. B. W. Hodgson for constant supervision of the accelerator and pulse radiolysis apparatus. Financial support to W. P. was given by the Deutsche Forschungsgemeinschaft . (23) M . E. Langrnuir and E. Hayon, J . P h y s . Chem., 71, 3808 (1967).
Quenching of Lucigenin Fluorescence
by Kenneth D. Leggla Department of Chemistry and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts
and David M. Herculeslb Department of Chemistry, Uninersity of Georgia, Athens, Georgia
90601
(Received December 9, 1969)
A study is reported for the quenching of lucigenin (dimethylbis(acridinium) nitrate) fluorescence by various anions and amines. A linear relationship is found between quenching efficiency and ionization potential. Efficient quenchers such as chloride, cyanide, sulfite, thiocyanate, and sulfide ions have diffusion-controlled rate constants. The quenching of lucigenin fluorescence has been shown to proceed by formation of a transient charge-transfer complex. In the absence of quenchers, lucigenin undergoes a photochemical reaction which probably occurs from the lowest excited singlet state.
Introduction Weber2 has reported the quenching of lucigenin (dimethylbis(acridinium) nitrate) fluorescence by chloride, bromide, iodide, and thiocyanate ions. He noted that the quenching by iodide and bromide appeared to be due to the “heavy-atom” effectJ3while T h e Journal of Physical Chemistry
quenching by chloride and thiocyanate appeared to be related to their oxidation potentials. Leonhardt (1) (a) NIH Predoctoral Fellow, 1965-1968. (b) Address all correspondence to this author. (2) K. Weber, z. Phys, Chem., B50, 100 (1941). (3) J. G , Calvert and J. W, pitts, Jr,, C~photochemistry,”John Wiley and Sons, Inc., New York, N. Y., 1967.
QUENCHING O.F LUCIGENIN FLUORESCENCE
2115
and Weller4 observed charge-transfer quenching of perylene fluorescence by electron donors, notably amines. They confirmed what others5 surmised, namely, that quenching proceeds through a chargetransfer intermediate F*
+Q
-+
(F-Q ) - + F + Q +
In the case of perylene quenching by amines, they were able to observe the monoanion radical of perylene in polar solvents;. They were also able to show a relationship between the ionization potential of the amine and its quenching efficiency. These observations were in accord with the mechanism
P*
+ Q +(Pa-&-+)
(I’ * -Q * +) +P * -solvQ (Pa-&*+) +P
’ +solv
+Q
(1)
(3)
Experimental Section Chemicals. Lucigenin (dimethylbis(acridinium) nitrate) was obtained from Columbia Organic Chemicals and was recrystallized twice from 1: 1 methanol-ethanol. All other organic and inorganic chemicals were reagent grade and were used without purification. Solvents. Absolute ethanol (U. S.Industrial Chemicals Co.), dimethyl sulfoxide (DMSO), dimethylformamide (IDJIF) (Matheson Coleman and Bell, spectroscopic grade), and acetonitrile (AN) (Eastman, spectroscopic grade) were used as obtained. Apparatus. Absorption spectra were obtained on a Cary Model 14 spectrophotometer. Fluorescence studies used a Turner JIodel 210 absolute spectrofluorometer. Fluorescence lifetimes were measured using a TRW Model 31A nanosecond fluorometry system.’ The flash photolysis system was constructed from components manufactured by Xenon Corp. (Medford, Mass.) and has been described in detail by Bailey, et aL8 Results and Discussion The effect of a series of anions on the fluorescence intensity of lucigenin is shown in Table I. Iodide and bromide were excluded from this study because of the possibility of quenching by the “heavy-atom” effect. The rate constants for quenching, k,, were calculated from the Stern-Volmer relationship
Io -
= 1
+ kq7(Q)
Table I : Effect of Anions on Lucigenin Fluorescence in Water
(2)
They were not able to observe the charge-transfer complex (P-&+) due to its short lifetime (
