COMMUNICATIONS TO THE EDITOR
2413
No fluorescence has been observed from e ~ r . using a diffusion coefficient4J for the p-dioxane-oxygen system of 5 X loR5cm2 sec-I and an oxygen concenm-dioxane or tetrahydrofuran, whereas a weak fluoresM at 25", we calculate an effective tration of 3 X cence has been detected from tetrahydropyran and encounter radius for quenching of at least 13 8. This oxepane. A systematic study of the fluorescence from distance is obtained from the steady-state form of the these and other saturated cyclic ethers will be presented solution to the diffusion equation (see eq 15 of ref 6). elsewhere. Since this value is considerably larger than the sum of It is to be emphasized that all fluorescence quantum the molecular radii of p-dioxane and oxygen, it is yields and spectra reported in this communication postulated that the excited species being quenched is were determined a t 25". On cooling p-dioxane, a an aggregate. This hypothesis is supported not only marked increase in cpf has been noted accompanied by the decrease in fluorescence yield with increasing = by a slight red shift (e.g., a t 12",cpf = 0.040 and A, dilution with i s ~ o c t a n ebut , ~ also by the concomitant 2500 8). However, no fluorescence has been detected change observed in the radiative rate constant (see from solid p-dioxane (at -78"). A study of these Table I). temperature effects is currently in pr0gre~s.l~ (13) This work was supported by the U. S. Atomic Energy Commission, Document No. COO-913-34.
Acknowledgment. The authors wish to thank Mr. William Rothman for his assistance in obtaining some of the experimental data. DEPARTMENT OF CHEMISTRY UNIVERSITY OF MINNESOTA MINNEAPOLIS, MINNESOTA55455
Table I : Fluorescence Lifetimes and R a t e Constants for p-Dioxane under Various Experimental Conditions
FUMIO HIRAYAMA Neat" CRAIGW. LAWSON SANFORD LIPSKY Neatb
RECEIVED M.4RCH 6, 1970
System
Neata Neat' 10Minisooctane" 8 M in isooctane"
" Deaerated.
Fluorescence of p-Dioxane. Lifetime and Oxygen Quenching1
Xir: Using the time-correlated single photon technique,za,b we have been successful in obtaining the fluorescence decay of the saturated cyclic ether pdioxane excited at 1850 A. The instrument incorporated a gated (20 kHz) nanosecond flashlamp fitted with a Suprasil window and filled with 38 em of deuterium. Radiation of this energy was isolated by a Bausch and Lomb high-intensity grating monochromator, and the entire optical system was purged with dry nitrogen. The lamp decay was less than 1 nsec (l/e); lamp tailing was unimportant. Excited at this wavelength, neat p-dioxane has a fluorescence quantum yield of 0.029 at 25°.a This note reports the lifetimes of pdioxane systems in condensed media. All decay curves were deconvoluted with respect to the exciting light and were observed to follow a single exponential. The results are listed in Table I. Concerning oxygen quenching at 25" , Hirayama, Lawson, and Lipskya obtain a diminution in the fluorescence yield of 0.69 relative t o deaerated p-dioxane. Incorporating this value with the unquenched lifetime of 2.15 nsec, and
kF x 10-7,
Temp,
7,
OC
nseo
9fC
25 25 12 12
2.15
0.029
1.70
0.020
3.2 2.3 1.75 1.5
25 25
' Oxygen saturated.
see-1
kNR
x
10-8, seo-1
1.35
4.5
1.040 0.025
1.25
3.0
0.020
1.14 1.07
5.6 6.6
0.016 See ref 3.
As a check, a decay curve was synthetically generated from the time-dependent fluorescence intensity function, including the transient termJ6using appropriate values for the encounter radius, diffusion coefficients, oxygen concentration, and the unquenched lifetime. This plot showed only minute nonexponentiality at very short times (
