2411
COMMUNICATIONS TO THE EDITOR
t
q J.o
0
2,o
X
1
I
1.4
2.0
-
2.6
LOG ELECTRODE SPACING
Figure 7 . Calculated apparent specific resistivity vs. electrode spacing. See text for details.
graphs 2, 3, and 4. For fixed applied voltage the curve shape at small spacings is a result of the enhancement of current through the L-3 dependence of injected cur-
rent, and at large spacing as a result of the disappearance of injection. The apparent double layer thickness, 6, increases with increasing voltage owing also to the V2/L3 dependence of the injected current component. This means that for f close to unity, the ratio of injected current to bulk current is V / L 2and an increase in applied voltage requires an increase in spacing in order to keep the ratio, hence apparent resistivity, constant, e.g., at 50% of its large spacing (bulk) value. The graph also indicates the proper dependence of the effect on the bulk resistivity. For lower values of bulk resistivity, then a t fixed applied voltage a closer spacing is required to produce an injected current which matches bulk current and thereby reduces apparent resistivity to 50% of its large spacing value. This behavior is found experimentally, as was also anticipated by the Gavis theory on the basis that 6 decreases as ionic concentration increases. In conclusion, the systems studied do indeed show an increase in apparent conductivity with decrease in electrode spacing, but not as described by the Gavis theory. Rather it is due to the augmenting effect of ion injection from the anode.
C O M M U N I C A T I O N S TO T H E E D I T O R
Fluorescence of p-Dioxane
Xir: Fluorescence has recently been reported from a wide variety of saturated hydrocarbons implying the existence of a t least one excited electronic state in these systems which is either bound or a t least sufficiently stable to permit the development of an observable emission.lB2 I n this communication we report a fluorescence from the saturated cyclic ether, p-dioxane. However, as will be demonstrated in what follows, its emission characteristics differ importantly from those of the saturated hydrocarbons. For excitation of p-dioxane as neai liquid (at 25") in its first absorption system a t 1849 A, a structureless fluorescence is observed with A,, = 2470 A and quantum yield +f = 0.029.3j4 Upon dilution with isooctane,6 the emission yield is strongly reduced and the emission spectrum blue-shifted, as shown in Figure 1. It will be noted that both +f and,,,A continue to decrease strongly even in solutions which are already predominantly isooctane. An effect of the solvent to
simply perturb the emitting species appears inadequate to explain this behavior. No fluorescence (+f < 10-6) has been observed from the vapor (at 25 Torr) when excited at 1849 A. Since this excitation is only 1340 cm-l above what has been assigned as the first 0-0 (1) F. Hirayama and 8. Lipsky, J . Chem. Phys., 51, 3616 (1969). (2) F. Hirayama, W. Rothman, and S. Lipsky, Chem. Phya. Lett., in press. (3) ,Matheson Coleman and Bell pdioxane (Spectroquality) was purified immediately before use by refluxing for 24 hr over sodium and then distilling under nitrogen atmosphere. The experimental technique for fluorescence measurements has been previously described (see ref l l ) . All quantum yields have been determined ultimately relative to a fluorescence quantum yield of 1.0 for 2537 & . excitation of 9,lO-diphenylanthraceFe (2 X 10-8 in cyclohexane; degassed). Comparison of 1849-A and 2537-A excitation was achieved through the use of oxygenated pxylene whose internal conversion efficiency has been determined to be unity (ref 11). (4) During the course of this work, a similar fluorescence has been reported following high-energy pulsed electron irradiation of pdioxane liquid. [J. H. Baxendale, D. Beaumond, and M . A. J. Rodgers, Chem. Phys. Lett., 4, 3 (1969)l. (5) Isooctane (Matheson Coleman and Bell, Spectroquality) was chosen as diluent since, unlike most other saturated hydrocarbons, i t is negligibly fluorescent (see ref 2).
The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
COMMUNICATIONS TO THE EDITOR
2412 WAVELENGTH,^ 3200
3000
28W
2600
24W
WAVELENGTH, 2200
2000
1
1
6030 5000 4500 4030
3500
a
3000
2500
200c
t
1
WAVENUMBER x IO.',C~"
Figure 1. Fluorescence spectra (photons/cm-') of p-dioxane in isooctane (deoxygenated) corrected for spectral response of the analyzing system. 1, neat p-dioxane; 2, 10 M ; 3, 8 M ;
4, 6 M ; 5 , 4 M ; 6, 3 M; 7, 2 M ; 8, 1 M ; 9, 0.5 M ; 10, 0.2 M .
transition a t 1896 A in the vapor p h a ~ e , it~ 'is~ concluded that "mon~meric'~ dioxane does not fluoresce. The liquid fluorescence is therefore postulated to arise from some form of excited aggregate whose composition varies with dilution. Further evidence for an aggregate emission is obtained from calculation of an unusually large encounter radius for oxygen quenching. For neat dioxane, a t an oxygen concentration of 3 X M, the fluorescence intensity is reduced by a factor of 0.69 without change in spectrum. From this, Halpern and Ware,8 using a measured lifetime of 2.15 nsec and appropriate diffusion coefficients, have calculatead an encounter radius for quenching of a t least 13 A. Additionally, they observed a monotonic decrease in radiative lifetime on dilution with isooctane, consistent with a change in nature of the emitting species. If the aggregate responsible for emission is formed subsequent to light absorption, increasing the viscosity of the diluent solvent should reduce qh. This was indeed observed in a 1 M solution of p-dioxane in hexad e ~ a n e . ~The intensity was reduced by about half as compared t o that in isooctane while the viscosity increased by a factor of ea. 4.'" Additionally, we have studied the absorption spectrum of p-dioxane from 0.002 M to 0.32 M in perfluorinated hexane (which has adequate transmission t o 1670 A), An absorption spectrum of neat dioxane was also determined using a thin film of the liquid compressed between Suprasil windows. All samples exhibited the same structureloess spectrum with maximum absorptivity a t ea. 1720 A. Also for the fluorocarbon solutions the optical density was found t o be linear over the concentration range studied. Thus the liquid absorption spectrum gives no evidence for extensive ground-state aggregation. Nevertheless, the liquid and vapor spectra are markedly different. The vapor spectrum exhibits a pronounced vibrational structure on the long waveThe Journal of Physical Chemistry, Vol. 74, No. 11, 1970
WAVENUMBER x IO-',C~''
Figure 2. Fluorescence spectra (photons/cm-') of p-dioxane with added water. 1, neat p-dioxane; 2 ; 0.056 M (0.1 vol 70) HzO in p-dioxane; 3, 0.278 d4 (0.5%); 4, 0.556 M (170); 5,
2.78 M ( 5 % ) ; 6 , 5.56 iM (10%); 7, 11.1 M (20%); 8, 27.8 M (50%). The spectra are corrected for spectral response of the analyzing system.
length side of the first absorption system which $3 superimposed on a continuum peaking at 1810 A, ca. 2890 cm-l red-shifted from the liquid. Since the liquid spectrum remains invariant to dilution even with the very weakly perturbing fluorocarbon solvent,'l these differences between vapor and liquid spectra suggest a profound modification in the relative nuclear configurations of ground and excited states on condensation. The fluorescence of p-dioxane is found to be extremely sensitive to the presence of water as shown in Figure 2. Traces of mater noticeably reduce c$f and red-shift the fluorescence. Further addition of water continues to strongly red-shift the spectrum.12 The origin of these effects is still unknown, although the participation of a fluorescent dioxane-water complex is indicated. From dilute solutions of p-dioxane and water in isooctane, we have found, in preliminary experiments, a very broad fluorescence which appears t o consist of a superposition of two emission spectraone possibly from water-dioxane complex and the other from uncomplexed dioxane. (6) L. W. Pickett, N. J. Hoeflich, and T . C. Liu, J . Amer. Chem. Soc., 73, 4865 (1951).
(7) G. J. Hernandez and A. B. F. Duncan, J . Chem. Phys., 36, 1504 (1962). (8) A. Halpern and W. R. Ware, J . Phys. Chem., 74, 2413 (1970). (9) Although hexadecane is slightly fluorescent (see ref l ) , it absorbs a negligible fraction of the exciting light in the 1 M solution of dioxane. (10) The viscosity of the mixture was estimated by assuming each component's contribution is proportional t o its mole fraction. (11) C. W. Lawson, F. Hirayama, and 5 . Lipsky, J . Chem. Phys., 51, 1590 (1969). (12) Similar effects have been noted on addition of methanol.
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 (
