NOTES
203
was retained in the organic phase. Also, in each instance, no more than 0.2% of p-nitrobenzyl iodide was extracted into the aqueous phase. Measurements of Radioactivity. The liquid samples were counted with a well-type scintillation detector to expected standard deviations of 1% or less.
Results and Discussion The reaction investigated was p-NO2CeH4CHd
+ ((>4H9)4NI*
=
p-K02C6H4CH21*
+ (CdH,)&”
where the asterisks indicate radioactive atoms. The experimental data were evaluated with the aid of the logarithmic form of the first-order isotopic exchange law.9 The reaction was found to be first order with respect to both the p-nitrobenzyl iodide and iodide ion. In solvent mixtures of dielectric constant as low as 12.53, the absence of any systematic variation of the observed specific rate constant, k , suggests that the salt is completely dissociated; for the solvent mixtures of dielectric constant of 10.00 and 8.12, the values of k increase with decrease in tetrabutylammonium iodide concentration, indicating the presence of kinetically inactive ion psirs.1° Values of the degree of dissociation, a, calculated from the association constants for tetrabutylammonium iodide, l1 are in accord with the foregoing observations. In all instances, a was calculated employing the limiting form ‘of the Debye-Huckel activity coefficient expression12which should be
valid a t the low ionic strengths of the reaction mixtures. A semilogarithmic plot of k / a us. the reciprocal of the dielectric constant is shown in Fiq. 1. Over the range of dielectric constant from 34.69 to 12.53, the plot is linear with a positive slope, in agreement with the theory of Laidler and Eyring.2 At lower values of the dielectric constant the specific reaction rate is less than would be predicted from this theory. This departure from theory may be attributed to the accumulation of nitrobenzene around the iodide ion, causing the microscopic dielectric constant to be higher than the bulk v a l ~ e ~ ~ , ~ ~ and/or the increase of the “effective size” of the iodide ion with decrease in dielectric constant by virtue of “selective solvation” with nitrobenzene.11 G. Friedlander and J. W. Kennedy, “Nuclear and Radiochemistry,” John Wiley and Sons, Inc., New York, N. Y., 1955, p. 315. (10) C. C. Evans and S. Sugden, J . Chem. SOC.,270 (1949). (11) E. Hirsch and R. 31. Fuoss, J . Am. Chem. Soc., 82, 1018 (1960). (12) C. B. Monk, “Electrolytic Dissociation,” Academic Press, Inc., New York, N. Y., 1961, p. 30. (13) L. R. Dawson, J. E. Berger, and H. C. Eckstrom, J . Phys. Chem., 65, 986 (1961). (9)
?-Excitation of the Singlet and Triplet
States of Naphthalene in Solution by B. Brocklehurst, G. Porter, and J. M. Yates
I
I
f
I
1
I
90
Department of Chemistry, The UniTersity, Shefield 10, Englanil (Received August 1, 1963)
80
70 60 I
50 I
m
d
40 c
a‘ I&
30
20
II
20
40
60
I 80
f . . 100
I
I
120
140
108/D.
Figure 1 . constant.
Dependence of specific reaction r a t e on dielectric
The luminescence produced by high energy irradiation of organic solutions has been much studied but the mechanisms of excitation have not yet been fully elucidated. In the case of a dilute solution of a compound with excited states lying lower than those of the solvent, energy transfer processes are commonly involved, Le., energy absorbed by the solvent is transferred to the solute which emits light. In the field of radiation chemistry, similar eff ects-sensitized reactions and “protection”-are observed, The transfer of energy is usually discussed in terms of transfer of electronic excitation, but it is also possible that positive charge is transferred, followed by the recombination of the solute ion with an electron or negative ion to produce luminescence or chemical change. In frozen organic solutions (glasses) which have been bombarded with high energy radiation, the existence of trapped electrons and negative ions has been demonVolume 68, Number 1
January, 196.4
NOTES
204
&rated,*but apparently positive ions have not been observed directly. In glassy solutions of easily ionizable compounds which have been irradiated with ultraviolet light, trapped positive ions and electrons have been 0bserved~2~ and it has been shown that luminescence is produced by their recombination; at 77°K. this is a slow process but it can be speeded up by warming (thermoluminescence) or irradiation with infrared light. One criterion of ion recombination should be that triplet and singlet states will be excited in the statistical weight ratio of 3 : 1, at least approximately, i.e., the phosphorescence/3uorescence ratio should be enhanced, the extent of this depending on the radiationless processes involved. Such an enhancement has been only observed in one case2 but in a n ~ t h e r surprisingly, ,~ phosphorescence was seen. Some qualitative observations of thermoluminescence of y-irradiated glassy solutions have been reported p r e v i o ~ s l y ,but ~ no measurements appear to have been published on the ratio, R, of phosphorescence to fluorescence directly excited by high energy radiation or in thermoluminescence following such irradiation. We report here some preliminary results obtained with naphthalene. Observations were made on a deoxygenated il/l solution in MP (isopentane-methylcyclohexane) at 77'K. For ultraviolet excitation, the mercury 2537 8. line was isolated by filters or a monochromator; ?-irradiation was carried out with a 500-c. cobalt source. After y-ray doses of 1000-10,000 rads, the sample became green in color. (The naphthalene negative ion is known to be green and the positive ion would be expected to have a similar epectrum.) On removing the sample from the liquid nitrogen it emitted a green glow as the temperature rose. Photographs of the emission spectra in each case showed naphthalene fluorescence and phosphorescence and only traces of emission from impurities in the solvent. Both fluorescence and phosphorescence were observed during ultraviolet irradiation, y-irradiation, and thermoluminescence experiments, but it was immediately apparent that, relative to ultraviolet excitation, phosphorescence was enhanced by ?-excitation and the enhancement was still greater in thermoluminescence. Quantitative measurements have been begun using a photomultiplier and filters (Chance glasses OX 9A and OY 18) to isolate the two emissions. The first approximate measurements confirm the enhancement of the phosphorescence. Absolute measurement of the ratio, R, has not yet been attempted, so we put R = 1 for ultraviolet excitation; then for ?-excitation we find R m 2 and for thermoluminescence R m 5. The behavior of ultraviolet-excited naphthalene is The .Journal of Physical Chemistry
not yet fully worked out. Gilmore, et aZ.,6 give the absolute value of R as 0.17, the absolute values of the fluorescence and phosphorescence yields being 0.52 and 0.09, respectively. We define here 9 s and +T, the quantum yields of singlet and triplet emission referred to the excitation of singlet and triplet states, respectively. Following Gilmore, et aZ.,5we assume that the excited singlet state can only decay by light emission or conversion to the triplet state; i.e., radiationless decay to the ground state can only occur from the triplet state. Then the normally defined phosphorescence yield (i.e., referred to singlet excitation) is given by (1 - +s)&. Gilmore, et al., calculate +T = 0.23 and combine this value with their measured phosphorescence lifetime, T , to calculate the true radiative lifetime, T ~ of , the triplet state as 11 sec. This, however, is not consistent with recent measurement~6~~ of the lifetime (7) of triplet CloD8as 16.9, 18.0, or 20.0 sec.; these suggests that the true radiative lifetime of both CloDBand ClaHs is a t least 20 sec. This figure together with the value of 2.1 sec. for CloHsmeasured under the same conditions7 gives $T 5 0.1. Because of the difficulty of making accurate absolute measurements of light intensities, we put ( b ~= 0.1 and combine this with the ratio, R = 0.17, of Gilmore, et al., to calculate = 0.37. Then in the case where direct excitation of triplets as well as singlets occurs (in the ratio z :1),R is given by
9T (1 - 9s 9s
--
+ ).
and we can write
R
-=I+Ro
X
1 - 9s
where $Eo refers to the case z = 0 (ultraviolet excitation). Then x = 3 gives R / R o = 5.8; our measured value is about 5 for thermoluminescence, which suggests that it results from recombination of trapped naphthalene ions. Our value of R/Ro = 2 for direct y-excitation corresponds to x = 3/h; if we assume that energy transfer from the solvent only excites singlet
(1) (2)
(3)
(4) (5)
(6)
(7)
P. S.Rao, J. R. Nash, J. P. Guarino, M .R. Ronayne, and U'. H. Hamill, J . Am. Chem. SOC.,84, 500, 4230 (1962). E. Dolan and A. C. Albrecht, J . Chem. Phys., 37, 1149 (1962). H. Linschitz, M . G. Berry, and D. Schweitzer, J . Am. Chem. Soc., 76, 5833 (1954). H. T. J. Chilton and G. Porter, J . Phys. Chem., 63, 904 (1959). E. H. Gilmore, G. E. Gibson, and D. S. McClure, J . Chem. Phys., 20, 829 (1952); 23, 399 (1955). C. A. Hutchison and B. 1%'.Mangum, ibid., 32, 1261 (1960); M . S. de Groot and J. H. van der Waals, Mol. Phys., 4, 189 (1961). B. Smaller, J . Chem. Phys., 37, 1578 (1962).
205
SOTES
states, then this process and charge transfer followed by recornbjriatiori are occurring 111 equal proportions. These calculations are, of course, very approximate and are open t o the further objection that we have assumed that the rate of singlet to triplet conversion is independent of the type of excitation; this is usually correct, but exceptions have been found in recent workS on chrysene and hexahelicene where excitation of the higher singlet btates leads to a larger phosphorescence/ fluorescence ratio. This effect may explain our results with direct y-excitation ; the amount of energy transferred from the solvent is not known, but it is probably much greater than that of the first excited singlet state of naphthalene. Support for this interpretation comes from recent work in this laboratoryQon the variation of luminescence with solute concentratJim in similar systems; the light intensity obeys a simple Stern-Volmer relation which suggests that only one transfer process, rather than two, is involved. It seems improbable that excitation and charge transfer would take place a t the same rate. On the other hand, it seems reasonable to ascribe the thermoluminescence to naphthalene ion recombmation since the enhancement of phosphorescence is much larger in this case; if energy is released in the solvent on warming and is transferred to the naphthalene, a different transfer process involving more energy must be invoked. The nature of the negative species is unknown a t present; previous on electron trapping has always involved much more polar solvents. One would not expect electrons to be trapped in aliphatic hydrocarbons, so we have to assume that, they are attached to polar impurities or to other naphthalene molecules, the latter possibility being supported by the green coloration of the irradiated glass. ~~
(8) M. F. O’Dwyer, M. A. El-Bayoumi, arid S. J. Strirkler, J . Chem. Phys., 36, 1395 (1982). (9) G. I’ortcr and J. M. Yates, unpublished work.
Radiation Synthesis of Iodoriiuni Compounds
by A . Alac1,achlan Radiation Physics Laborulotv, Engin~eriiylDeparlmmt, E I du Poiit de .?Ternoiirs and CompcznU, Wilminglon. Ddaware (Received August A, 19G3)
Diphenyliodonium triiodide (I) has been detected when iodoberizene is irradiated with y-rays.l,* Investigations a t this Laboratory concerning the radiation chemistry of aromatic compounds found that I is also
formed wlien iodobeiizeiie is irradiated with 2-6-Mev. electrons. Furthermore, pure iodobenzene is not necessary for excellent yields and, in fact, unsymmetrical iodonium compounds can be obtained when the proper mixture is used. T h k results prompted the brief mechanism study reported here. Maximum total yields of iodoniurn compound could be obtained with only 7%. iodoberizene in chlorobenzene or broniobenzene. 1)iphenyliodonium chloride and bromide are obtained, respectively, complexed with iodine. The iodine complexes were identified by comparison of infrared spectra and melting points with either authentic samples or with the Merature value^.^,^ Precipitation of the iodoriium salts occurs spontaneously during irradiation. Dissolution of the iodine iodonium complexes in acetone results in eventual crystallization of the iincomplexed iodoriium halides. Comparison of tlie melting points of the compounds with t,hose given by Beringer, et al.,5 served as further structure proof. The effect of iodobenzene concentration was examincd by irradiating 03% bromobenzenc-7% iodobenzene and 99.2% bromobenzene--O.8~,iodobenzene and pure iodobenzene with the same radiation dose. At t8hehigher iodobcnzene concentration in bromobenzene, 0.7 g. of product formed, while a t the lower only 0.2,i g. could be isolated. Irradiation of pure iodobenzene yielded 0.75 g. of diphenyliodonium iodide triiod ido. Free iodine was found to be unnecessary for efficient production of I, arid in fact was detrimental. Pure iodobenzene and a slurry, consisting by volume of 95% iodobenzene-5% aqueous sodium thiosulfate (lo%), were irradiated with the same t,otal dose. Stirring of the slurry was accomplished wit.h a magnetic stirring bar, such that. rapid dissolution of iodine occurred but no emulsion was formed. Irradiation of pure iodobenzene yielded 1.01 X 1W2 molc of diphcnyliodonium iodide diiodide along wit,h free iodirie. Thc iodobenzene scrubbotl with aqueous tJhiosulfete duriitg irradiation (thus maint,aining the free 1, conccntiatioii at. a low level) yielded 1.5 X 10 * rrivlar euuivalerit of iodonium compound ns t h c thiosulfntr salt. Thr bisdiphonyliodoriium thiosulfate was dissolvcd in hoiling water containing sodium bromide, which on coolirig yielded white crystals of diphenyliodonium bromide. Irradiat,ion of J. I ). Parrack, G. A. SWWI,ant1 I). Wright, .I. CIwn. Sor.. 911 (1982). P.1;. I>. Shaw, ibid., 443 (1951). 11.D. Forster arid J. 11. Svhnegpi, ibid.. 101, 382 (1912). cy. ITartmrl~~nand V. hIayer. B e r . , 27, 502 (1894). F. AI. Beringer. 11. I>rexler. 15. M.Giridler, and C. C. Lumpkin, J . Am. Chen. Sor 75, 2705 (1953).
.
Voltrmc GR. S i t m b r ~1 January. 1964
