ring cleavage occurs in the parent ion, ring closure occurs in one of the products, thercby lowering the total energy required for the reaction. The processes and stucture proposed are those which best fit the thermochemical data. It has bcen shown that some of the
ions from C6HaSD issue from the six-rnernbcred ring structure where the hydrogen and deuterium arc not equivalent, but othcr ions issue from another structure, such as the seven-membercd ring, where the hydrogcns and deuteriumareequivalent.
Quenching of the Scintillation Process in Plastics by Organometallics’
by Stanley R. Sandler and K. C. Tsou Central Research Laboratory, Borden Chemical Company, Philadelphia, Pennsylvania, and Harrison Department of Surgical Research, School of Medicine, University of I’nnsylvania, Philadelphia. Pennsglvania (Received August 18. 1963)
Organometallics of the group 11, IVA, and VA metals were found to be quenchers for the scintillation process in polyvinyltoluene plastics, Copolymerizing vinyltoluene and p triphenyltiristyrene produced a slight increase in the degrcc of quenching above triphenyl-4ethylphenyltin. In addition, copolymerizing p-triphenylleadstyrenc with vinyltoluerie also produced slightly morc quenching than triphenyl-4-ethylpheriyllead physically dissolved in the plastic scintillator. A modified Stcrn-Volmcr relationship was used to correlate t,he scintillation quenching of the organometallics and the atomic numbcr. Tho mechanism of encrgy transfcr arid qut:nching arc discussed in relation to the results found in this Investigation.
ing in the polystyrene scintillator containing 1,1,4,4tetraphenylhutadienc. afore recently, Haisle3 evaluated diphenylmercury and Hyman4 evaluated tripheriylbismuth, tetraphenyllead, and various lead methacrylates in polyvinyltoluane scintillators and found the organometallics to be quenchers of the light output. In aromatic solutions, organometallic compounds were found not to act as scintillators6 but as quenchers of the light output.“-R In the present investigation, the group IVA and VA organometallics were evaluated in polyvinyltoluene plastics toward &irradiation of l’a234. The scintillaThe Journal of Phvsical Chemistrv
This work was supported by the 1:. 8. Atomic I’hergy Commission under Contract No. A T (30-1)-1931. (2) L. I’iohat, P. Pesteil, and J. Clement. J . C h i m . Phus., 50, 26 (1)
(1953).
(3) (4)
L. J. Baisle, J. Chem. Fhye., 27, 801 (1957), M. Hymen and J. J. Ryan, I R E , Trans. Nucl. Sci., 5, 87. (1958).
(5)
H. Gilman, E. A. Weipert, and F. N. Hayes, J . Oro. Chcm., 23, 860 (1958).
F. 11. Brown. M . I:urst, and €1. Kallmann in “Organic Scintillation Detectors.” TID 7G12, U. S. .4tomic Energy Commission 1961, I.’.3T. (7) V. N. Kerr. F. N. lIayes, and D. O t t , Intern. .I. A p p l . Radiation Isotopes. 1 , 284 ( 1 957). (8) J. L. Kropp and M . Burton, J . Chem. Phys., 37, 1752 (1962). (6)
QUENCHING I N PLASTICS BY ORGANOMETALLICS
30 1
--
Experimentall Vinyltoluene monomer was obtained in 99.9% purity from the Dow Chemical Co. and distilled through a 3-ft. Nester-Faust spinning-band column. The di-, tri-, and tetraarylmetallics were obtained from commercial sources and repeatedly recrystallized. The syntheses of the new compounds are described below. Preparation of Triphenyl-4-ethylphenyllead. To an ether solution of 4-ethylphenylmagnesium bromide (0.27 M )was added 102.3 g. (0.215 mole) of triphenyllead chloride in ether. The solution was refluxed for 3 4 hr. and then hydrolyzed with aqueous ammonium Anal. chloride to yield 63.6 g. (54.6%), m.p. 75-77'. Calcd. for C2&4Pb: C, 57.50; 13, 4.42; Pb, 38.10. Found: C, 56.98; H,3.89; Pb,37.2!8. Preparation of Triphenyl-4-ethylphenyltin. To 0.27 mole of 4-ethylphenylmagnesium bromide was added an equimolar amount of triphenyltin chloride in ether. The reaction was worked up as in the preceding preparation to yield 53.7 g. (55%), n1.p. 83-84'. Anal. Calcd. for C26H24Sn:C, 68.70; 13, 5.27; Sn, 26.00. Found: C,68.06; H, 5.07; Sn, 26.19. Preparation of p-Triphenylleadstyrene and p-Trzphenyltznstyrene. The following procedure for monomer synthesis represents an improvement over that reported in thie l i t e r a t ~ r e . ~ ~ ~ ~ p-Chlorostyrene was treated with the theoretical amount of magnesium in 50 ml. of tetrahydrofuran. A few millilit>ersof methyl iodide was used to initiahe the formation of the' Grignard reagent. The pchlorostyrene was added in 100 ml. of tetrahydrofuran (THF) a t such a rate that the reaction mixture refluxed. Healing was necessary throughout the reaction. After all the p-chlorostyrene had been added the solution was refluxed for 15-20 min. and the solution turned a clear dark green color. Triphenyllead chloride or triphenyltin chloride was dissolved in 100 ml. of THF and added dropwise. After the latter was added, the solution was kept at 50-66' for 1-1.5 hr. and then cooled with ice. The reaction mixture was hydrolyzed with aqueous ammonium chloride and the organic layer separated. The water layer was extracted twice with 100 ml. of ether. The combined organic layer was attached to an aspirator and the T H F and ether were removed. Then petroleum ether (60-30') was added to the residue to extract the product. The product was recrystallized from petroleum ether or isopropyl alcohol to yield either 77% p-triphenylleadstyrene, m.p. 112--113' (lit.9 m.p. 107-log'), alX: : : 258 mp ( E 28,'700), or 46% p-triphenyltinstyrene, m.13. 104-105' (lit.l0 m.p. 105,5-108'), 258 n-1l.c ( e 23,000).
-
An excess of magnesium in this preparation tends to give lower yields, possibly due to the formation of hexaphenylditin or hexaphenyldilead and their cleavage products. l i The polyvinyltoluene scintillators were prepared as previously described12 and evaluated toward p-irradiation of Paza4,2.3 Mev., 0.01 mc. The fluorescence spectra were obtained using an Aminco-Bowman spectrophotofluorometer equipped with a xenon light source, and the ultraviolet spectra were obtained with a Beckman DK-U spectrophotometer.
Results In an effort to understand the scintillation process of organometallic scintillators a series of molecules containing different heavy atoms was evaluated. For this study aryl metals such as (C6H5)nMwere chosen so that n = 2 for the group IIB series, 4 for group ITJA, and 3 for group VA. The organometallics were dissolved in vinyltoluene containing 3% p-terphenyl and 0.05% POPOP (1,4-bis(2,5-phenyloxazoyl)benzene) and polymerized at 100-110' under vacuum for 7 days. A series of various concentrations (0-15%) was used in most cases. The scintillation plastics were machined to 0.5 X 0.813 in. and evaluated relative to an anthracene crystal of the same size with a Pa234P, 2.3 Mev. source (0.01 mc.). A plot of the dif-
- G e LO
;a
10
ATOMIC
20
$0
10
do
d
NUMBER
-4
Figure 1. The difference in relative pulse height of organometallic loaded scintillators and unloaded scintillators ( R.P.H.IIR.P.H.) is plotted against the atomic number for the group IVA and group VA metals.
(9) H. G. Pars, W. A. Graham, E. R. Atkinson, and C. C. Morgan, Chem. Ind. (London), 693 (1960). (10) J. R. Leebrick and H. E. Ramsden, J . Org. Chem., 2 3 , 935 (1958). (11) C. Tamborski and E. J. Salaski, J . Am. Chem. Soc., 8 3 , 373 (1961). (12)
R. K. Swank and W. L. Buck, Phys. Rev.,91, 928 (1953).
Volume 68, Number 2
February, 2864
11.1
.".O
R .l>.H. -
T 30-
i n d eo-
-
05
roi
LO
UNITS OF
1.5 0-1
20
2.5
I
I
3.0
3S
MOLAL CONCENTRATION
Figure 2 . I~.P.H.O/R.I'.H.- 1 us. concentration of triphenylstibine [Q] X lo-' r n .
ference of tlir relative pulsc height of scintillators m i taining 0.14 m concentration of the organometallic compared to an unloaded scintillator us. the atomic riurribcr gave a straight line for the group IVA and another linc for the group L'A metals ab shown in I'ig. 1. It is interesting tllat the gro,lp y-1 are yuenchers than group IVA with t,ctraphenylnietharic and t,etraphcnylgcrmaiiiuni showing no quenching. The study of thc effect on the quenching process by incorporation of lead or titi organomctallics in the polymcr backbone was rnadc possible by copolymerizing p-tripheriyltinstyrenc or p-triphcnyllradstyrenc ith vinyltoluene. 'l'he results are shown in Table I.
- 1 =
/C,,[Q]
where R.P.H., is the rclative pulse height of a standard scintillator containing no organonictallic evaluated toward ?'a2348 ; 1Z.P.H. is thc relative pulse height of an organornctallic loaded scintillator. 'fhe dctcrmniation of relative pulse height is that rtconimcndcd by Swank aiid Buck, l 2 using anthraceiie cbrystal of the same size as standard. IC, is the quenching constant and [&I is the concentration of the organomctallic in molcs. .I plot of 11.1).H.o,It.I'.tI. - 1 us. [Q] for triphenylstibinc is sho\in i r i Fig. 2 as a rcpreseutative plot to indicate the method for a11 thc materials studied. The quenching coiistarits are tabulated in Tablc I. The ,llt,raviolct arl(ifluoresct,rirc spectral of the fluor arid orgallonlet,a~~~cs are sholvn in Table 11.
Table 11 : Sprrtral Propertics of Fluors and Quenc4iers Fluorescenccc
I . l l l d \ ll>l?t"
Coin pound
al)sorpti(in ransx
Amax,
mrr
Pniission rangc
A,,, rng
gTerphenvl Polyviny Itolrirnr
210-310
279 30,XOO 322
310-445
3.50
290 345
310-423
350
Y ~ rent" S l'o1.y- p-triphcny Itinsty rcrioh 1riphrnyl-4-et hylphcnyltin r. 1riptienyl-4-tIt hylphcny llcad Tetraphcny llead 'l'etraphrnylt in l't.t,r;iphen).IEcrmaniuiri Tctr~pheiiylsilic*ciii 'I'rI~nphcii.yInicthnna Triphcny IursiIio 'I'riphmjlnt ibinr Triphrnylhismuth 1)iphcny lriiercurj
'L.iO-3X5
322
3 0 0 600
370
250-400
330
225-285
259
1 .7"0
350-525
3x5
200 -300 257 238 2~10-%)0 260
2,110 2, I30
I,oly-p-trlphenylleatl-
r ,
200-2x0
252 243--300 266 230-200
230 300
256
200-290 21x 200--300 255 200-315 280 200 .280 255
290 -530 273 dOO 1,386 280-300 1 ,5 i . i 310 425 1 ,3!)0 395 $90 1,910 290-J'2*; 13,540 290 17,i 13,870 4 , x40
I ,370
290-475 3 1 0 43; avo 430
:%io 360 34.; 330 348
335 3 17 3.15
3,;o 348
" The u1t ritvlolet spcctrn were determined in chloroforrii iiqiiig the I3eckman I ) K U spcctrophotomcter * Spwtral propcrtirs of the solid polyrncr ot)tained using thc Aminco Bownan specstrophotofluoromrter. < T h c fluorescenre spwtra were ohtained in chlorofornn quenrhing c:onst:mt for (C,H:),Hg apparently \\'as lower at low c:oncent,rrrtioris (0-0.15 V I , ) and ~ v a ec a l ~ ~ ~ l i i to t c dhe 0 . 5 5 .
The .loumnl of Physicfll Cheminlry
~
i ~ ~ ~ ~ The rcsults in Fig. 1 indicate t,hat the relat'ivc pulse
~
IXphcnylmercury is also included and falls on the line for the group IVA metallics since the atomic number of mercury (80) is close to that of lead (82). b’rom tho results in Fig. 1 the group VA triaryl organometallics appear t,o bc more effective queiichers than the group IVA tetraaryl orgaiiometallics. The relationship dtxrived from Fig. 1 (R.P.H.0 - R.P.H. = Ic’Z) can be combined with the Stern-Volmer relatioriship (cq. 1) to give 1
l”
where k’, %, and l~.l’.II.are the proportionality constant, atomic number, and relative pulse height at 0.14 m organometallics, respectively. The other quantities are the same as described earlier. It is seen from the derived relationship that the quenching constant should increase with an increase in Z and more so with a decrease in II,I’.H. Since k’ can be evaluated for the group IVA (k’ = 0.384) and group VA (k’ = 0.50)
kq
Rot
1
I
I
Figure 3. The quenching constant k , for the organomctallics a~ shown in Table I tis k’Z/R.F‘.H. for the group VA metals.
r~
I
I
QP
0.4
1
I
I
I
I
0.5
c I
0.3
kq O0.1. 1 I 0
0.6
I
I
1
0.8
1.0
1.2
organometallics from Fig. 1 it can be uscd to prepare a plot of /c’Z/R.P.H. 1)s. IC,, as shown in Fig. 3 and 4. In most cases knowing the value of k’ and the I1.P.H. a t 0.14 m concentration is enough to determine the kq of the organometallic from Fig. 3 arid 4. The obscrvcd quenching of the scintillation process by the above organometallics may be due to a decrease in the fluorescence by an internal conversion mechanism as has been suggested for oxygen.l The scintillation mechanism incorporating a queneher can be described for convenience as
+ + +
(1) S hvl ----f s* S* S -e- S S* (2) S* 1; + S F* (3) (4a) (4b) (54 (5b) (5c) (6) (7)
’*
I;*
+ +
Act ivatiori Transfer of excitation Transfer to fluor
+ Q --+F ++*‘Q* t Transfer to quencher
+
----f
Q* +Q
S* --+ S E’* --+-p’ F* --+-I? S* +S
Internal conversion
+ hvz
+ hv,
Emission from the fluor Emission from the solvent in the absence of a fluor
The evidence presented in this investigat,ion suggests that steps 4a, 4b, and 5a are involved in quenching by organometallics in plastics. In order for energy transfer to occur from an excited donor to an acceptor, as in step 3, the fluorescence spectrum of the donor must overlap the ahsorption spectrum of the acceptor.’5 It is scen from an examination of Table I1 that the spectral absorption properties of tripheriylbisrriuth are similar to that of p terphenyl. However, sincc t h e former is a poor fluorescence emitter, as t h e concentrations of triphenylbismuth is increased above p-terphenyl t h e former will capture most of the excitation of tho solvent and rcemit very little. This masking effect by an inefficient fluor leads to quenching of t h e sciiitillatiori process by steps 4b arid sa. On the other harid, the organometallics may have an absorption spectrum overlapping the solvent as does poly-p-triphenylleadstyrerie arid polyp-triphenyltiristyrerie so that there exists a competition for the excitation energy o f t he solvent polyvinyltoluene. If, for example, poly-p-tnphenylleadstyrene re-emits very little of its energy to p-terphenyl then quenching of the scintillation process will occur. If energy transfer occurred along the polymer backbone then introducing a quencher into the polymer by
1.4
E:. J. Howen rind I I . A. Williarns, Trans. Faraday Soc., 35, 765 (1939). (15) T. Forster. I’luoresaenz Organischer Verbindungen,” VandenIioeck a n d Ituprecht, Giittingen. Germany. 1951. (14)
Figure 4. The quenching constant k , for the organonictal1ic.s a~ shown in Table I va. k’Zl1i.P.H. for the group I\.A nictnls.
( ‘
Volume 68. .Vumher 2
February, 1964
H. KIKI AND GILBERTJ, MAINS
304
copolymerization may show a reduction in efficiency compared to physically dissolving the quencher. A slight reduction was observed since p-triphenyltinstyrene shows a small reduction in efficiency compared to triphenyl-4-ethylphenyltin as seen in Table I. In addition, p-triphenylleadstyrene shows slightly more quenching than triphenyl-4-ethylphenyllead. Since energy transfer probably takes place by a a-bond overlap between the excited solvent and fluor, the quencher
The
3P,
competes for this orbital overlap whether physically dissolved or as part of a polymer molecule. This is likely the reason that Kerr, Hayes, and Ott7 have found that the aromatic quenchers have a greater detrimental effect on the scintillation process than similarly substituted aliphatic compounds. Acknowledgment. We are grateful to Dr. B. D. Halpern for his interest and helpful suggestions in this work.
Mercury-Photosensitized Decomposition of Monosilane
by H. Niki and Gilbert J. Mains D e p a r t m e n t of Chemistry, Carnegie Institute of Technology, Pittsburgh 13, Pennsylvania (Received A u g u s t 21, 1968)
The yP1Hg-sensitized decomposition of SiH4 and a 1: 1 SiH4-SiD4 mixture was studied at 1 cm. pressure and 25’. Quantum yields of hydrogen and disilane were estimated to be 1.8 and 0.6, respectively, a t low conversion, but are subject to considerable uncertainty. Polymeric silicon hydride was deposited on the walls as the reaction proceeded toward a photostationary state. The uniformity of the polymeric film is taken as evidence that the “zebra” effect is not important in his system. The failure of a large concentration of ethylene to reduce the yield or isotopic distribution of hydrogens from the decomposition of the SiH4-SiDl mixture indicates that the reaction between atomic hydrogen and monosilane is very rapid. Monosilane may find future use as an efficient hydrogen atom scavenger.
Introduction Silicon hydrides are the closest structural analogs of paraffin hydrocarbons. While the decomposition of small paraffin hydrocarbons has been studied by direct photolysis,’i2 mercury sensitized photolysis, 3-6 direct radiolysis,6 and mercury-sensitized radiolysis,6 no comparable investigations have been reported for the silanes. Because the silanes are thermodynamically unstable relative to the elements at room temperature, and because olefinic compounds, a serious complication in interpreting hydrocarbon systems, are absent among the silicon hydrides, the photochemistry of silanes is of considerable interest. This laboratory has initiated a systematic study of the photochemistry and radiation chemistry of silanes. The data given here represent the first part of this study. T h e Journal of Physical Chemistry
Emelhus and Stewart? initially reported the mercuryphotosensitized decomposition of monosilaiie into hydrogen and a brown film of polymeric silicon hydride. The empirical formula of the solid hydride varied from SiHo.31 to SiHo,48depending upon the extent of conversion. According to these authors, the polymeric film was opaque to ultraviolet light and its deposition J. R. McNesby and H. Okabe, J . Chem. P h y s . , 34, 668 (1961). B. Mahan and R. Mandal, ibid., 37, 207 (1962). R. Back and Van der Auwera, Can. J . Chem., 40, 2339 (1962). L. Dorfman, E. Spittler, P. Jordan, and M. Snuer, J . P h y s . Chem., in press. ( 5 ) G. J. Mains and A. S. Newton, ibid., 6 5 , 212 (1961). (6) P. Ausloos and S. G. Lias, J . Chem. P h y s . , 38, 2207 (1963). (7) H. J. Ernel6us and K. Stewart, Trans. Faraday SOC.,32, 1577 (1) (2) (3) (4)
(1936).
