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Chemiluminescence from alkyl hyponitrites in viscous liquid media
21
J. Photochem. Photobtol. A: Chem., 83 (1994) 21-27
Chemiluminescence G. David
from alkyl hyponitrites in viscous liquid media
Mendenhall+
Deportment of Chemistry, Michigan Technological University, Houghton, MI 49931 (USA)
Lyda
Matisova-Rychla
Polymer Institute, Slovak Academy (Received
February
of Sciences, Dubmvskn cesta, 84236 Bmtislava (Slovak Republic)
18, 1993; accepted February 1, 1994)
Abstract We have studied the effect of dissolved polymers and mineral oil on the chemijuminescence from tert-butylbenzene solutions of trans-di-2-propyl hyponitrite (IPHN), *an.+di-1-phenylethyl hyponitrite (PEHN) and 3,3,4,4_tetramethyll,Z-dioxetane (TMD) in the presence of a fluorescent species (Z-tert-butyl-9,10-dibromoanthracene). It was found that the addition of polymers (poly(a-methylstyrene) and others) to solutions of the hyponitrites or TMD decreased their chemiluminescence. In contrast, addition of mineral oil decreased the luminescence from TMD but increased the luminescence from the hyponitrites. The decrease in luminescence was ascribed to the expected reduced rates of energy transfer to the fluorescent species in competition with decay of the carbonyl tripIets. The increase in luminescence on addition of mineral oil was too large to be accounted for by larger cage effects in the case of PEHN. The increase was tentatively ascribed to higher excited state formation efficiencies from alkoxyl radical dismutation in more viscous media.
1. Introduction A relatively large number of investigations have been reported on the use of chemiluminescence to monitor the oxidizability and other properties of polymers [l-3]. In addition, studies have been performed to determine the nature of the chemiluminescent reactions leading to excited states in fluid solutions of small molecules in well-defined systems [4-6]. In this paper, we present results intended to bridge the gap between these approaches; the rate and relative yields of chemiluminescence from trans-di-2-propyl hyponitrite (IPHN) and Wuns-di-1-phenylethyl hyponitrite (PEHN) were measured in pure solvents and in solvents made more viscous with dissolved polymers or mineral oil.
2. Experimental
details
Alkyl hyponitrites, 3,3,4,4-tetramethyl-1,2-dioxetane (TMD) and 2-tert-butyl-9,10-dibromoanthracene (tDBA) were available from earlier studies [5-7] or were synthesized in the same way. Solvents were of reagent grade. Poly(cu-methyl+Author to whom correspondence should be addressed.
molecular weight styrene) (number-average (M,) =i 11600; weight-average molecular weight (M,.,) = 372) was obtained from Scientific Polymer Products, Inc., Mineral oil (Fischer) was purified by stirring overnight with sulfuric acid, followed by washing with aqueous sodium ethylenediaminetetraacetate (EDTA) and distilled water, and passage through a column of alumina. Chemiluminescence was measured by photon-counting techniques in the apparatus described previously [4]. Computerized analysis of the data obtained in the isothermal mode was carried out by fitting to first-order kinetics by adjusting the background counting rate to optimize the F statistic. The total (relative) photon emission from each sample was calculated with corrections for both background and the unmeasured “tail” of the luminescence decay [8]. Analyses of decomposed samples of PEHN were carried out on a Microsorb MV Cl8 column (5 pm) with methanol as eluent and detection at 280 nm. The solutions of PEHN containing excess phenolic inhibitor were degassed in glass ampoules on a vacuum line, sealed and decomposed for at least ten half-lives in a water bath. A small correction was made for the acetophenone present * initially in the solutions of PEti.
lOlO-6030/94/$07.00 B 1994 Elsevier Science S.A. All rights resewed SSDI 1010-6030(94)03798-Y
22
G.D. Mendenhdl, L. Matisovo-Rychla I Chemiluminescence from aikyl hyponirrites
Viscosity measurements performed using Ubbelohde viscometer, according to and coworkers with a capillary, or Brookfield model viscometer. 3.
P
o-
,_/’
? ;i 34
'0
3.1. Decomposition of PEHN This hyponitrite was chosen for study because it shows a relatively high yield of triplet acetophenone from the cage reaction of the l-phenylethoxyl radicals resulting from thermal deeomposition [4] (PhCI-IMe-ON=),
/ ,/
0
3 u
//:’
, c..’
P-
-
n
/_---_
--
__--m---
0
[NZ + 2PhCHMeO’],,,
-
0
(1)
2
I
9
4
IRONNORl,mM
PhCOMe
+3[PhCOMe]
Acetophenone triplets in solution usually undergo quenching in preference to light emission, but the chemiluminescence from PEHN can be increased substantially using a fluorescent species, e.g. tDBA 3[PhCOMe]
+ tDBA -
c[tDBA]*
-
‘[tDBA]*} +- PhCOMe ‘[CDBA]” -
tDBA +hv
(3)
[Inhibitor]”
CL,,X
10-r
0.5
_
0.5 0.9 2.0 2.0 3.0 4.0
Yes Yes _ -
10.2 8.989 25.11 125.2 108.8 201.8 250.0
(mM)
‘Irganox 1076, 1.0 mM. bSum of photons counted,
400
corrected
i04Xk,,, (s-l)
tiR (min)
GCL) x 10-6s
6.43 6.37 7.3 1.9 1.8 8.4 8.8
17.9 18.3 16.4 15.0 14.8 13.6 12.9
0.274 0.235 0.559 3.15 2.27 5.08 6.18
for background
and tail.
of solutions of PEHN and tDBA in tert-butylbenzene were fitted to a first-order kinetic expression (r=0.993-0.999), but the rate constants so derived increase slightly with increasing starting concentration (Table 1). This result has previously been ascribed to induced decomposition [7]. If we consider the fate of the cage-escaped alkoxyl radicals associated with eqn. (1) above, we have the possible reactions
POD
9
TABLE 1. Rate constants and light emission (chemiiuminescence (CL)) from PEHN in tert-butylbenzene in the presence of 1.0 mM tDBA at 55.1 ‘C [PEHN],,
(2)
The initial chemiluminescence intensities and total light emission from solutions of hyponitrites containing tDBA increase in an approximately linear fashion as a function of hyponitrite concentration (Figs. 1 and 2). The luminescence decays
6
Fig. 2. Total light emission from PEHN (I) and IPHN (0) as a function of their initial concentrations in tert-butylbenzene solutions (cf Tables 1 and 2).
160
2 ‘ii
2 100
5
0
PhCHMeO’
SO
-+
PhCHMeO’+
PhCHO
Fig. 1. Maximum emission intensity from PEHN (M) and IPHN (0) as a function of their initial concentrations in tert-butylbenzene solutions (cf- Tables 1 and 2).
PhCHMeO’+AH
+ s‘ (S = solvent)
(5)
+ A‘ (AH = inhibitor)
(6)
-
PhCHMeOH PhCHMeO’
(4)
SH PhCHMeOH
7 5
+ Me’
+ 0, --+
PhCOMe
f HO0
(7)
23
G.D. Mendenhall, L. Matisova-Rychla I Chemiluminexence from alkyl hypnittites
Although the rate constants are not known for these reactions under our conditions, we may estimate the value ofk, as one-half the rate constant of p-scission of the cumyloxyl radical at this temperature in cumene [lo], or 2.2X106 s-l. The rate constant of reaction (5) with SH = tertbutylbenzene may be estimated from the reaction of the cumyloxyl radical towards the primary methyls in dicumyl peroxide (5.4X lo3 M-’ s-l per H atom at 37 ‘C [ll]), so that in neat tert-butylbenzene (4.5 M, 9H) the pseudo-first-order rate constant is estimated to be about 3 x lo5 s-l. If the activation energy of the reaction is 4 kcal mol-‘, the rate constant at 55 “C will be about 4x105 M-l s-r. The rate constant of reaction (6) with octadecyl3,5-di-tert-butyl-4-hydroxyhydrocinnamate (Irganox 1076) may be equated, to a first approximation, to that for the reaction of tert-butoxyl radicals with 2,6-di-tert-butyl-4-methylphenol [lo], or lo* M-r s-l. With an inhibitor concentration of 1 mM the initial pseudo-first-order rate constant in our system will be about 1tY s-‘. The rate constant for reaction (7) can be estimated from that of similar alkoxyl radicals in solution [12] as 1 X10* M-l s-l. If the concentration of oxygen in air-saturated tert-butylbenzene has a typical value of 1 mM, the pseudo-firstorder rate constant for quenching will be lo5 s-l. From these estimates, we assume that most of the cage-escaped 1-phenyl-1-ethoxy radicals in tertbutylbenzene will undergo p-scission (reaction (4)). The fate of the methyl radical is not known, but we assume it will react with oxygen to give methylperoxyl, or react with the aromatic solvent leading, after further reaction, to HO0 (also derived from reaction (7)) and/or SS-00’. These species will disappear predominantly by termination reactions with other radicals or by reactions with the inhibitor. In the absence of inhibitors and in the presence of dissolved oxygen, we infer that the induced decomposition of PEHN in tert-butylbenzene may be (R=H or alkyl) ROO’ + PEHN ROOH
+ PhCOMe
+ N2 + PhCHMeO’
(8)
d[PEHN]/&
If ROO’
- 2k,[R00’]2-k~[ROO-][PEHN] d[PhMeCHO’]/dt
Assuming ing these
-&[ROo-][PEHN]
is the methylperoxyl
radical,
19) then
(10)
= 2&[PEHN]
-k.,[PhMeCHO’]
+k,[ROO’J[PEHN]
(11)
the steady state approximation equations, we obtain
and add-
ekJPEHN]
=k,[ROOJ’
Substitution d [PEHN]/dr
into eqn.
(9) gives
= - k,[PEHN] - ks[ekl /kJ”,5[PEHN] l5
(12)
The rate constants and half-lives in Table 1 were obtained by fitting the experimental data to a simple exponential, corresponding to the equation d[PEHN]/dr= Substituting
-k,,,[PEHN]
(13)
eqn. (13) into eqn. (12), we obtain
koti = k, + ks[ek,/kJ”.5[PEHN]o.5 A plot of kobsvs. [PEHN]g5 (r=O.996) and gives intercept
(14)
from Table
=k, =klextrap=5.24X
slope=k,[ek,/k,]0~5=5.74X
lo-“
1 is linear
s-’
10e3 s-’
(15) (3
After eliminating kl from the above expressions, we calculate k8[e/k,]0.5=0.25. We obtained an experimental value of e= 0.72 in this study, comparable with e=0.70-0.71 in previous work [4]. The value of k, may be estimated from reported values of the termination rate constants for primary alkylperoxyl radicals [lo], e.g. Z&,=4X 107M~1s-’ for the methylperoxyl radical. Insertion of these values into eqn. (16) leads to an estimate of k,=2000 M-l s-l. This value appears to be unreasonably high, since the presumably related reaction shown below has a reported rate constant of only 32 M-r s-r at 70 “C [lo] PhCHMeOO’
-
Defining k, as the first-order rate of decomposition of PEHN, and e as the fraction of l-phenyl-lethoxyl pairs that escape the cage, we have
= -kJPEHN]
d[ROO’]l&=k,[PhMeCHO’]
+ PhCHMeOH
--+
PhCHMeOOH
+ PhC’MeOH
We infer that a different factor entirely is responsible for the apparent increase in rate constant with increasing concentration. This inference is consistent with the observation that the experimental rate constant for PEHN does not change when an inhibitor is present (Table 1). In addition, when tDBA is excited at 370 nm in a 1 mM solution of PEHN, the fluorescence intensity de-
G.D. Mendenhall, L. Malisova-Rychia I Chemiluminescence
24
creases by 19% after the hyponitrite has been decomposed by heating the solution for 140 min at 55 “C. The absorption bands of the fluorescent species in the visible region remain unchanged before and after heating. Apparently, a decomposition product in the solution decreases the luminescence yield as the reaction proceeds. The value of k,“t’ap for PEHN is very close to the value of 5.29 x 10m4 s-l calculated at 55.1 “C from the activation parameters of decomposition in tert-butylbenzene [4]. Plots of the maximum chemiluminescence emission from PEHN (Fig. 1) and the total emission (Fig. 2) are reasonably linear as a function of concentration, as expected. 3.2. Decomposition of IPHN Somewhat more limited experiments were carried out with IPHN in tert-butylbenzene, and the results are given in Table 2. Since this hyponitrite is kinetically more stable than PEHN in addition to being inherently less luminescent, the studies were carried out at a higher temperature in order to increase the precision of the measurements. As observed for PEHN, the rate constants derived from the isothermal decay of the chemiluminescence from IPHN increase somewhat with increasing IPHN concentration. A plot of kDbs21s. [IPHN],‘” for the three points is linear (r = 1.0000) and gives klext’;lP=5.88~ 10e4 s-l at 70.1 “C. This value is in excellent agreement with the value of 5.84 X 10s4 s-l obtained by extrapolating a published value of k, = 3.6 X 10m4 s-l in isooctane at 66.1 “C to 70.1 “C, assuming an activation energy of 28 kcal mol-’ as for the di-tert-butyl derivative [5]. The value of k, extrapis also close to that derived from the chemiluminescence of solutions containing inhibitor (Table 2).
TABLE 2. Rate constants and light emission (chemiluminescence (CL)) from IPHN in tert-butylbenzene in the presence. of 1.0 mM tDBA at 70.1 “C
[IPHNI” Inhibitor” (mM) 1.04 5.2 0.5 2.5 5.0
Yes
Yes _
CL, x 10-3
lo4XkC.l., (s-l)
t1n (min)
(XCL) x lo+
3.98 36.0 1.26 8.61 26.6
5.22 6.20 6.52 7.3 1.9
22.0 18.6 17.1 15.8 14.5
0.11 0.91 0.0354 0.271 0.67
pl.O mM tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane (Irganox 1010). %um of photons counted, corrected for background and tail.
from a&i hyponihites
3.3. influence of dissolved polymers on chemiluminescence ftom PENN and IPHN The decomposition of hyponitrites in solutions of tert-butylbenzene containing polymers was followed by chemiluminescence emission in the same way as in the pure solvent. The results from PEHN solutions containing poly(a-methylstyrene) are given in Table 3. The emission decreases monotonically with poly(cu-methylstyrene) concentration, showing a tenfold reduction for 11% polymer. The rate constant decreases by about 20% with a linear dependence on polymer concentration. The results for IPHN with the same polymer under slightly different conditions are very similar (Table 4), except that the rate constants show a smaller variation with polymer concentration. The same features, i.e. rate constants essentially invariant and a decrease in total light emission, are also observed with 3% or 11% poly(styrene) and 1.8% or 5.5% poly(n-butylmethacrylate), in each case at 70 “C with IPHN in solutions containing tDBA. 3.4. Triplet yields in polymer-containing solutions We measured the excited state yields by comparing the total light emitted on decomposition of a solution of an alkyl hyponitrite (PEHN or IPHN) with that from a matched solution of TMD TABLE 3. Effect of poly(cu-methylstyrene) on chemiluminescence (CL) from 1.8 mM PEHN in tert-butylbenzene at 55.0 “C cgntaining 0.91 mM tDBA Polymer*
CL,X
0 3.72 7.09 11.02
102.0 36.0 18.2 9.44
10-S
104xk, (s_‘)h
t1n (tin)
(CCL) x 10-S
7.86 7.28 6.80 6.50
14.7 15.8 16.8 17.0
2.53 0.896 0.472 0.233
‘Weight per cent of poly(ru-methylstyrene). bFrom chemiluminescence decay (r>0.992
in all cases).
TABLE 4. Effect of poly(cY-methylstyrene) on chemilurninescence (CL) from 3.08 mM IPHN in tert-butylbenzene containing 1.5 mM tDBA at 70.2 “C Polymer”
cL__x
0 3.41 6.78 10.1
28.8 14.4 6.08 4.75
10-3
104xk, (s-‘)a
r1n (min)
(,uzL) x lo-”
6.2 5.69 5.5 5.88
18.4 20.0 20.9 18.5
0.767 0.21 0.372 0.130
aWeight per cent of poly(wmethylstyrene). bFrom chemiluminescence decay (r>O.992
in all cases).
G.D. Mender&all, 15. Matisova-Rychla I Chemiluminescence from alkyd hyponitktes
(the latter was assumed to give 30% triplet acetone on decomposition). The results for IPHN and poly(a-methylstyrene) are given in Table 5. For each luminescent precursor, the ratio of the total emission with and without polymer is similar (0.24 for TMD and 0.20 for IPHN). We might have expected a larger ratio for IPHN because of an increased cage effect in the more viscous solution. Assuming the yield of triplets from TMD to be 0.3, and independent of solutes, we calculate S, = 3.7 X 10M4 for solutions of IPHN without polymer, and ST= 3.1 x 1O-4 in the presence of 6% polymer. For comparison, Quinga and Mendenhall [4] reported a triplet yield of 2.24 x low3 from IPHN in tert-butylbenzene. A similar pair of experiments was carried out with PEHN and TMD with solutions containing up to 20% poly(a-methylstyrene), and the results are given in Table 6. The calculations depend on a similar efficiency of energy transfer for triplet TABLE 5. Chemiluminescence (CL) from IPHN and TMD as a function of paly(wmethylstyrene) concentration in tert-butylbenzene containing 2.0 mM tDBA at 70.0 “C Luminescer
Polymer”
5.0x10-’ M 0 TMD 6.07 Z.OXIO-~ M 0 IPHN 6.07
CI+_,,X 10e3
104Xkl (s-‘)~
(CCL) x10-6
811
1.3
11.3
97.5
0.51
2.71
26.4
8.7
0.558
8,16
0.111
5.17
sWeight per cent of poly(L-u-methylstyrene).Each solution also contained initially 1.0 mM 2,6-di-tert-buty-4-methylphenol. bFrom chemiluminescence decay (r> 0.992 in all cases).
2.5
acetone and triplet acetophenone to tDBA, which is probably the case in tert-butylbenzene within error limits [13]. The yield of triplets from PEHN increases about twofold as the polymer concentration is increased to 20%. For comparison, Quinga and Mendenhall[4] reported a triplet yield of (3.25 *0.3)x 10e3 from PEHN in tert-butylbenzene, calculated with the same assumptions as described here. 3.5. Tripret y*elds from PEHN and IPHN in mixtures of mineral oil and teti-butylbenzene Since the polymers displayed limited solubilities in tert-butylbenzene, and the solutions were inconvenient to work with, we carried out some measurements in mixtures containing varying amounts of purified mineral oil. The results for IPHN and PEHN are given in Tables 7 and 8. Plots of total luminescence emission or CL, VS. mineral oil concentration are linear for TMD and IPHN over the ranges given in Tables 7 and 8. The corresponding plot for PEHN is linear except for the experiment in 67% mineral oil, where the point is far below the line extrapolated from the other points. For both PEHN and IPHN, the addition of mineral oil increases the measured amount of light based on starting hyponitrite, whereas the addition of polymers causes a decrease. In order to determine whether this increase is simply due to the increased amount of cage recombination in the more viscous solvent, the yields of acetophenone were determined as a function of mineral oil concentration. The results and yields of triplet acetophenone per cage-reacted phenylethoxyl radical are given in Table 9.
TABLE 6. Chemiluminescence (CL) from PEHN and TMD as a function of poly(a-methylstyrene) concentration in tert-butylbenrene at 55.0 “C Luminescer 5.0x10-'M TMD
Polymer” 0 5.09 10.1 20.1
2.0~10-~ M PEHN
0 5.09 10.1 20.1
CL,,X 10-3
l@Xk, (s-‘)b
(xcL)xlo-6
5$X ld
143
1.4
16.7
[3tw
25.4 11.1 5.23 166 56.1 29.6 17.4
0.98 0.93 0.96
4.26 1.89 0.821
9.5
3.11
1.4
9.09 8.7 8.5
1.09 0.599 0.367
1.9 2.4 3.2
‘Weight per cent of poIy(cu-methylstyrene). Each solution also contained initially 1.0 mM 2,~di-tert-butyl-4-methylphenol mM tDBA. bFrom chemihuninescence decay. CYield of triplet acetophenone per hyponitrite.
and 2.0
26
G.D. Mendenholl, L. Mottiova-Rychla I Chemilwninescence
TABLE 7. Chemilurninescence at 70.0 “C
(CL) from IPHN and TMD as a function
LtlrllineUX
oil”
1.9x lo-’ TMD
Mineral M
1.5~ 10-r M IPHN
CL=X
35.3 32.9 19.8
0
0.019
7.75
0.433
0.374
16.6 33.8 67.1
0.023 0.0275 0.0362
8.30 8.50 8.60
0.548 0.648 0.869
0.589 0.747 1.67
initially 4.89 mM 2,6-di-tert-butyl-4-methylphenol
and TMD as a function
CL,
x 10-X
‘Per cent by weight. bCentipoise. Yield of acetophenone ds;- = ST/(1 - e).
of concentration
llYXk,
(s-1)”
and 20.4 mM tDBA.
of mineral
(XL)
oil in tert-butylbenzene
x 10-S
137.0
1.22
18.9
16.6 33.8 67.1
118.0 106.0 62.7
1.21 1.22 1.26
16.5 14.6 8.37
0
26.1
8.4
0.544
1.74
16.6 33.8 67.1
30.2 37.1 33.8
8.85 8.93 9.06
0.615 0.717 0.714
2.27 2.97 5.16
initially 4.89 mM 2,6-di-tert-butyl-4-methylphenol
PhCOCH~
(%)’
28 38 39 44
e
lbXS,~~d
0.72 0.62 0.41 0.56
6.2 6.0 7.6 11.7
at
s; x l@
0
TABLE 9. Cage effects, viscosities and fully corrected yields of triplet acetophenone from PEHN in mixtures of mineral oil and tert-butylbenzene at 55.1 “C
0.688 0.945 1.50 4.43
L3W
7.50 1.66 7.80
“Weight per cent. Each solution also contained bFrom cbemiluminescence decay. rYield of triplet acetophenone per hyponitrite.
0
srcx l@
1.54 1.46 0.862
oil’
16.6 33.8 67.1
10-h
16.6 33.8 67.1
Mineral
oil” Viscosi@’
(XL)X 43.9
Luminescer
Mineral
(s-l)b
oil in tert-butylbensene
7.40
(CL) from PEHN
M
lO’Xk,
of mineral
1.92
TABLE 8. Chemiluminescence 55.2 “C
3.2~ lo+ PEHN
of the concentration
0
‘Weight per cent. Each solution also contained ‘From chemiluminescence decay. Yield of triplet acetone per hyponitrite.
6.45x10+ MTMD
1o-6
from olkyi hyponihies
[3001
and 20.4 mM tDBA.
from IPHN (Table 5), which indicate a decrease in the triplet yield with increasing polymer concentration even without correction for the cage effect. In Fig. 3, the corrected values of S, VS. the bulk viscosity of the medium are plotted. The data are probably insufficient to sustain a belief in a linear relationship. 4. Discussion
per hyponitrite
molecule.
From the data in Table 9, it can be observed that, after correction for the increased yields of acetophenone due to the cage effect, there is an inherent increase in the yield of excited acetophenone in the more viscous medium by about a factor of two. This result contrasts with the results
The addition of polymers or mineral oil to solutions of chemiluminescent alkyl hyponitrites should have several possible effects on the light emission yields: (1) an increase due to the increased lifetime of the excited state, as a result of the reduction in the rate of diffusional quenching by impurities or oxygen; (2) a decrease due to the reaction of excited state products with, or quench-
G.D. Mendenhall, L. Mntiova-Ryehla
/ Chemiluminescence from al!@ hyponiMes
27
The differences between the triplet yields obtained for PEHN and IPHN in this work and those reported previously are a little disconcerting. For PEHN, the differences are in line with estimated errors of 50% for the triplet yields of TMD [14]. In several of our experiments, we did not eliminate dissolved oxygen from the solution, which may contribute to chemiluminescence via luminescent peroxyl-peroxyl reactions; however, this contribution relative to that from alkoxyl radical self-reactions has been found to be small [5].
5e 0
Acknowledgments 1
Viscosity, Cp Fig. 3. Corrected yield of excited carbonyls from PEHN as a function of the viscosity of mineral oil-tert-butylbenzene mixtures (cfi Table 9).
ing by, the added (viscous) component; (3) a decrease due to the decreased fraction of energy transfer from the excited state carbonyl products to the fluorescent species; (4) an increase due to increased self-reaction of the initially formed alkoxyl pairs in the solvent cage, leading to more excited state carbonyl products. From the present experiments, we infer that, for dilute polymer solutions, the third factor is dominant, The exact contributions could be determined by time-resolved studies. A fifth factor, a change in the inherent yield of excited states from alkoxyl radical dismutation (hydrogen-atom transfer) with a change inviscosity, is of interest because the chemiluminescence from solid polymers represents a limiting case of high viscosity. Our experiments give a mixed answer to this question. The data in Tables 5 and 6 indicate small and opposite effects of polymers on S,. The effects on mineral oil (Table 9) are more dramatic and consistent with an inherent increase in S,. The conclusion must be tempered by the presumably small, but unknown, changes in the relative excitation efficiencies of tDBA by excited acetone vs. acetophenone, or changes in their relative lifetimes, with changes in composition of the medium. Moreover, we assume that the yield of triplets from TMD is insensitive to the medium.
Partial support for this work by the National Academy of Sciences (L.M.R.) and Himont USA, Inc. (G.D.M.) is gratefully acknowledged. We thank Mr. E. Lathova and Mr. Y. Geng for carrying out the viscosity measurements, and Dr. J.P. Riehl and Ms. C. Maupin for performing some fluorescence measurements. References 1 G.D. Mendenhall, 2 3 4 5 6 7 8 9 10
11 12 13 14
Anger. Chem. Int. Ed. En& 29 (1990) 362. L. Matisova-Rychla, 2s. Fodor, J. Rychly and M. Iring, Polym. Degrad. St&l., 3 (1980-1981) 371. EC. Ashby, J. Polym. Sci., 50 (1961) 99. E.M.Y. Quinga and G.D. Mendenhall, 1 Am. Chem. Sot., I08 (1986) 474. S.-H. Lee and G.D. Mendenhall, .I Am. Ciwm. Sot., 110 (1988) 4318. Q.J. Niu and G.D. Mendenhall,J Am. Chem. Sot., 114 (1992) 165. CA. Ogle, SW. Martin, M.P. Dziobak, M.W. Urban and G.D. Mendenhall, J. org. Chem., 48 (1983) 3728. G.D. Mendenhall, J.C. Tauschek and C.A. Ogle, I&. 3. Chem. finer., 13 (1981) 1217. G. Langhammer, R. Berger and H. Seide, Ploste fiutsch., II (1944) 472. J.A. Howard and J.C. Scaiano, in H. Fischer (ed.), Landolt-Bornstein’s Numerkal Data and Functional Relationships in Science and Technofogv New Series, Vol. 13d, Springer, Berlin, 1984. D.V. Avila, C.E. Brown, K.U. Ingold and J. Lusztyk, J. Am. Chem. SIX., 115 (1993) 446. J. Lusztyk, personal communication, 1993. G.D. Mendenhall, X.C. Sheng and T. Wilson, J. Am. Chem. Sot., 113 (1991) 8976. T. Wilson, D.E. Golan, MS. Harris and A.L. Baumstark, J. Am. Chem. Sot., 98 (1976) 1086.
J. Photochem. Photobtol. A: Chem., 83 (1994) 21-27
Chemiluminescence G. David
from alkyl hyponitrites in viscous liquid media
Mendenhall+
Deportment of Chemistry, Michigan Technological University, Houghton, MI 49931 (USA)
Lyda
Matisova-Rychla
Polymer Institute, Slovak Academy (Received
February
of Sciences, Dubmvskn cesta, 84236 Bmtislava (Slovak Republic)
18, 1993; accepted February 1, 1994)
Abstract We have studied the effect of dissolved polymers and mineral oil on the chemijuminescence from tert-butylbenzene solutions of trans-di-2-propyl hyponitrite (IPHN), *an.+di-1-phenylethyl hyponitrite (PEHN) and 3,3,4,4_tetramethyll,Z-dioxetane (TMD) in the presence of a fluorescent species (Z-tert-butyl-9,10-dibromoanthracene). It was found that the addition of polymers (poly(a-methylstyrene) and others) to solutions of the hyponitrites or TMD decreased their chemiluminescence. In contrast, addition of mineral oil decreased the luminescence from TMD but increased the luminescence from the hyponitrites. The decrease in luminescence was ascribed to the expected reduced rates of energy transfer to the fluorescent species in competition with decay of the carbonyl tripIets. The increase in luminescence on addition of mineral oil was too large to be accounted for by larger cage effects in the case of PEHN. The increase was tentatively ascribed to higher excited state formation efficiencies from alkoxyl radical dismutation in more viscous media.
1. Introduction A relatively large number of investigations have been reported on the use of chemiluminescence to monitor the oxidizability and other properties of polymers [l-3]. In addition, studies have been performed to determine the nature of the chemiluminescent reactions leading to excited states in fluid solutions of small molecules in well-defined systems [4-6]. In this paper, we present results intended to bridge the gap between these approaches; the rate and relative yields of chemiluminescence from trans-di-2-propyl hyponitrite (IPHN) and Wuns-di-1-phenylethyl hyponitrite (PEHN) were measured in pure solvents and in solvents made more viscous with dissolved polymers or mineral oil.
2. Experimental
details
Alkyl hyponitrites, 3,3,4,4-tetramethyl-1,2-dioxetane (TMD) and 2-tert-butyl-9,10-dibromoanthracene (tDBA) were available from earlier studies [5-7] or were synthesized in the same way. Solvents were of reagent grade. Poly(cu-methyl+Author to whom correspondence should be addressed.
molecular weight styrene) (number-average (M,) =i 11600; weight-average molecular weight (M,.,) = 372) was obtained from Scientific Polymer Products, Inc., Mineral oil (Fischer) was purified by stirring overnight with sulfuric acid, followed by washing with aqueous sodium ethylenediaminetetraacetate (EDTA) and distilled water, and passage through a column of alumina. Chemiluminescence was measured by photon-counting techniques in the apparatus described previously [4]. Computerized analysis of the data obtained in the isothermal mode was carried out by fitting to first-order kinetics by adjusting the background counting rate to optimize the F statistic. The total (relative) photon emission from each sample was calculated with corrections for both background and the unmeasured “tail” of the luminescence decay [8]. Analyses of decomposed samples of PEHN were carried out on a Microsorb MV Cl8 column (5 pm) with methanol as eluent and detection at 280 nm. The solutions of PEHN containing excess phenolic inhibitor were degassed in glass ampoules on a vacuum line, sealed and decomposed for at least ten half-lives in a water bath. A small correction was made for the acetophenone present * initially in the solutions of PEti.
lOlO-6030/94/$07.00 B 1994 Elsevier Science S.A. All rights resewed SSDI 1010-6030(94)03798-Y
22
G.D. Mendenhdl, L. Matisovo-Rychla I Chemiluminescence from aikyl hyponirrites
Viscosity measurements performed using Ubbelohde viscometer, according to and coworkers with a capillary, or Brookfield model viscometer. 3.
P
o-
,_/’
? ;i 34
'0
3.1. Decomposition of PEHN This hyponitrite was chosen for study because it shows a relatively high yield of triplet acetophenone from the cage reaction of the l-phenylethoxyl radicals resulting from thermal deeomposition [4] (PhCI-IMe-ON=),
/ ,/
0
3 u
//:’
, c..’
P-
-
n
/_---_
--
__--m---
0
[NZ + 2PhCHMeO’],,,
-
0
(1)
2
I
9
4
IRONNORl,mM
PhCOMe
+3[PhCOMe]
Acetophenone triplets in solution usually undergo quenching in preference to light emission, but the chemiluminescence from PEHN can be increased substantially using a fluorescent species, e.g. tDBA 3[PhCOMe]
+ tDBA -
c[tDBA]*
-
‘[tDBA]*} +- PhCOMe ‘[CDBA]” -
tDBA +hv
(3)
[Inhibitor]”
CL,,X
10-r
0.5
_
0.5 0.9 2.0 2.0 3.0 4.0
Yes Yes _ -
10.2 8.989 25.11 125.2 108.8 201.8 250.0
(mM)
‘Irganox 1076, 1.0 mM. bSum of photons counted,
400
corrected
i04Xk,,, (s-l)
tiR (min)
GCL) x 10-6s
6.43 6.37 7.3 1.9 1.8 8.4 8.8
17.9 18.3 16.4 15.0 14.8 13.6 12.9
0.274 0.235 0.559 3.15 2.27 5.08 6.18
for background
and tail.
of solutions of PEHN and tDBA in tert-butylbenzene were fitted to a first-order kinetic expression (r=0.993-0.999), but the rate constants so derived increase slightly with increasing starting concentration (Table 1). This result has previously been ascribed to induced decomposition [7]. If we consider the fate of the cage-escaped alkoxyl radicals associated with eqn. (1) above, we have the possible reactions
POD
9
TABLE 1. Rate constants and light emission (chemiiuminescence (CL)) from PEHN in tert-butylbenzene in the presence of 1.0 mM tDBA at 55.1 ‘C [PEHN],,
(2)
The initial chemiluminescence intensities and total light emission from solutions of hyponitrites containing tDBA increase in an approximately linear fashion as a function of hyponitrite concentration (Figs. 1 and 2). The luminescence decays
6
Fig. 2. Total light emission from PEHN (I) and IPHN (0) as a function of their initial concentrations in tert-butylbenzene solutions (cf Tables 1 and 2).
160
2 ‘ii
2 100
5
0
PhCHMeO’
SO
-+
PhCHMeO’+
PhCHO
Fig. 1. Maximum emission intensity from PEHN (M) and IPHN (0) as a function of their initial concentrations in tert-butylbenzene solutions (cf- Tables 1 and 2).
PhCHMeO’+AH
+ s‘ (S = solvent)
(5)
+ A‘ (AH = inhibitor)
(6)
-
PhCHMeOH PhCHMeO’
(4)
SH PhCHMeOH
7 5
+ Me’
+ 0, --+
PhCOMe
f HO0
(7)
23
G.D. Mendenhall, L. Matisova-Rychla I Chemiluminexence from alkyl hypnittites
Although the rate constants are not known for these reactions under our conditions, we may estimate the value ofk, as one-half the rate constant of p-scission of the cumyloxyl radical at this temperature in cumene [lo], or 2.2X106 s-l. The rate constant of reaction (5) with SH = tertbutylbenzene may be estimated from the reaction of the cumyloxyl radical towards the primary methyls in dicumyl peroxide (5.4X lo3 M-’ s-l per H atom at 37 ‘C [ll]), so that in neat tert-butylbenzene (4.5 M, 9H) the pseudo-first-order rate constant is estimated to be about 3 x lo5 s-l. If the activation energy of the reaction is 4 kcal mol-‘, the rate constant at 55 “C will be about 4x105 M-l s-r. The rate constant of reaction (6) with octadecyl3,5-di-tert-butyl-4-hydroxyhydrocinnamate (Irganox 1076) may be equated, to a first approximation, to that for the reaction of tert-butoxyl radicals with 2,6-di-tert-butyl-4-methylphenol [lo], or lo* M-r s-l. With an inhibitor concentration of 1 mM the initial pseudo-first-order rate constant in our system will be about 1tY s-‘. The rate constant for reaction (7) can be estimated from that of similar alkoxyl radicals in solution [12] as 1 X10* M-l s-l. If the concentration of oxygen in air-saturated tert-butylbenzene has a typical value of 1 mM, the pseudo-firstorder rate constant for quenching will be lo5 s-l. From these estimates, we assume that most of the cage-escaped 1-phenyl-1-ethoxy radicals in tertbutylbenzene will undergo p-scission (reaction (4)). The fate of the methyl radical is not known, but we assume it will react with oxygen to give methylperoxyl, or react with the aromatic solvent leading, after further reaction, to HO0 (also derived from reaction (7)) and/or SS-00’. These species will disappear predominantly by termination reactions with other radicals or by reactions with the inhibitor. In the absence of inhibitors and in the presence of dissolved oxygen, we infer that the induced decomposition of PEHN in tert-butylbenzene may be (R=H or alkyl) ROO’ + PEHN ROOH
+ PhCOMe
+ N2 + PhCHMeO’
(8)
d[PEHN]/&
If ROO’
- 2k,[R00’]2-k~[ROO-][PEHN] d[PhMeCHO’]/dt
Assuming ing these
-&[ROo-][PEHN]
is the methylperoxyl
radical,
19) then
(10)
= 2&[PEHN]
-k.,[PhMeCHO’]
+k,[ROO’J[PEHN]
(11)
the steady state approximation equations, we obtain
and add-
ekJPEHN]
=k,[ROOJ’
Substitution d [PEHN]/dr
into eqn.
(9) gives
= - k,[PEHN] - ks[ekl /kJ”,5[PEHN] l5
(12)
The rate constants and half-lives in Table 1 were obtained by fitting the experimental data to a simple exponential, corresponding to the equation d[PEHN]/dr= Substituting
-k,,,[PEHN]
(13)
eqn. (13) into eqn. (12), we obtain
koti = k, + ks[ek,/kJ”.5[PEHN]o.5 A plot of kobsvs. [PEHN]g5 (r=O.996) and gives intercept
(14)
from Table
=k, =klextrap=5.24X
slope=k,[ek,/k,]0~5=5.74X
lo-“
1 is linear
s-’
10e3 s-’
(15) (3
After eliminating kl from the above expressions, we calculate k8[e/k,]0.5=0.25. We obtained an experimental value of e= 0.72 in this study, comparable with e=0.70-0.71 in previous work [4]. The value of k, may be estimated from reported values of the termination rate constants for primary alkylperoxyl radicals [lo], e.g. Z&,=4X 107M~1s-’ for the methylperoxyl radical. Insertion of these values into eqn. (16) leads to an estimate of k,=2000 M-l s-l. This value appears to be unreasonably high, since the presumably related reaction shown below has a reported rate constant of only 32 M-r s-r at 70 “C [lo] PhCHMeOO’
-
Defining k, as the first-order rate of decomposition of PEHN, and e as the fraction of l-phenyl-lethoxyl pairs that escape the cage, we have
= -kJPEHN]
d[ROO’]l&=k,[PhMeCHO’]
+ PhCHMeOH
--+
PhCHMeOOH
+ PhC’MeOH
We infer that a different factor entirely is responsible for the apparent increase in rate constant with increasing concentration. This inference is consistent with the observation that the experimental rate constant for PEHN does not change when an inhibitor is present (Table 1). In addition, when tDBA is excited at 370 nm in a 1 mM solution of PEHN, the fluorescence intensity de-
G.D. Mendenhall, L. Malisova-Rychia I Chemiluminescence
24
creases by 19% after the hyponitrite has been decomposed by heating the solution for 140 min at 55 “C. The absorption bands of the fluorescent species in the visible region remain unchanged before and after heating. Apparently, a decomposition product in the solution decreases the luminescence yield as the reaction proceeds. The value of k,“t’ap for PEHN is very close to the value of 5.29 x 10m4 s-l calculated at 55.1 “C from the activation parameters of decomposition in tert-butylbenzene [4]. Plots of the maximum chemiluminescence emission from PEHN (Fig. 1) and the total emission (Fig. 2) are reasonably linear as a function of concentration, as expected. 3.2. Decomposition of IPHN Somewhat more limited experiments were carried out with IPHN in tert-butylbenzene, and the results are given in Table 2. Since this hyponitrite is kinetically more stable than PEHN in addition to being inherently less luminescent, the studies were carried out at a higher temperature in order to increase the precision of the measurements. As observed for PEHN, the rate constants derived from the isothermal decay of the chemiluminescence from IPHN increase somewhat with increasing IPHN concentration. A plot of kDbs21s. [IPHN],‘” for the three points is linear (r = 1.0000) and gives klext’;lP=5.88~ 10e4 s-l at 70.1 “C. This value is in excellent agreement with the value of 5.84 X 10s4 s-l obtained by extrapolating a published value of k, = 3.6 X 10m4 s-l in isooctane at 66.1 “C to 70.1 “C, assuming an activation energy of 28 kcal mol-’ as for the di-tert-butyl derivative [5]. The value of k, extrapis also close to that derived from the chemiluminescence of solutions containing inhibitor (Table 2).
TABLE 2. Rate constants and light emission (chemiluminescence (CL)) from IPHN in tert-butylbenzene in the presence. of 1.0 mM tDBA at 70.1 “C
[IPHNI” Inhibitor” (mM) 1.04 5.2 0.5 2.5 5.0
Yes
Yes _
CL, x 10-3
lo4XkC.l., (s-l)
t1n (min)
(XCL) x lo+
3.98 36.0 1.26 8.61 26.6
5.22 6.20 6.52 7.3 1.9
22.0 18.6 17.1 15.8 14.5
0.11 0.91 0.0354 0.271 0.67
pl.O mM tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane (Irganox 1010). %um of photons counted, corrected for background and tail.
from a&i hyponihites
3.3. influence of dissolved polymers on chemiluminescence ftom PENN and IPHN The decomposition of hyponitrites in solutions of tert-butylbenzene containing polymers was followed by chemiluminescence emission in the same way as in the pure solvent. The results from PEHN solutions containing poly(a-methylstyrene) are given in Table 3. The emission decreases monotonically with poly(cu-methylstyrene) concentration, showing a tenfold reduction for 11% polymer. The rate constant decreases by about 20% with a linear dependence on polymer concentration. The results for IPHN with the same polymer under slightly different conditions are very similar (Table 4), except that the rate constants show a smaller variation with polymer concentration. The same features, i.e. rate constants essentially invariant and a decrease in total light emission, are also observed with 3% or 11% poly(styrene) and 1.8% or 5.5% poly(n-butylmethacrylate), in each case at 70 “C with IPHN in solutions containing tDBA. 3.4. Triplet yields in polymer-containing solutions We measured the excited state yields by comparing the total light emitted on decomposition of a solution of an alkyl hyponitrite (PEHN or IPHN) with that from a matched solution of TMD TABLE 3. Effect of poly(cu-methylstyrene) on chemiluminescence (CL) from 1.8 mM PEHN in tert-butylbenzene at 55.0 “C cgntaining 0.91 mM tDBA Polymer*
CL,X
0 3.72 7.09 11.02
102.0 36.0 18.2 9.44
10-S
104xk, (s_‘)h
t1n (tin)
(CCL) x 10-S
7.86 7.28 6.80 6.50
14.7 15.8 16.8 17.0
2.53 0.896 0.472 0.233
‘Weight per cent of poly(ru-methylstyrene). bFrom chemiluminescence decay (r>0.992
in all cases).
TABLE 4. Effect of poly(cY-methylstyrene) on chemilurninescence (CL) from 3.08 mM IPHN in tert-butylbenzene containing 1.5 mM tDBA at 70.2 “C Polymer”
cL__x
0 3.41 6.78 10.1
28.8 14.4 6.08 4.75
10-3
104xk, (s-‘)a
r1n (min)
(,uzL) x lo-”
6.2 5.69 5.5 5.88
18.4 20.0 20.9 18.5
0.767 0.21 0.372 0.130
aWeight per cent of poly(wmethylstyrene). bFrom chemiluminescence decay (r>O.992
in all cases).
G.D. Mender&all, 15. Matisova-Rychla I Chemiluminescence from alkyd hyponitktes
(the latter was assumed to give 30% triplet acetone on decomposition). The results for IPHN and poly(a-methylstyrene) are given in Table 5. For each luminescent precursor, the ratio of the total emission with and without polymer is similar (0.24 for TMD and 0.20 for IPHN). We might have expected a larger ratio for IPHN because of an increased cage effect in the more viscous solution. Assuming the yield of triplets from TMD to be 0.3, and independent of solutes, we calculate S, = 3.7 X 10M4 for solutions of IPHN without polymer, and ST= 3.1 x 1O-4 in the presence of 6% polymer. For comparison, Quinga and Mendenhall [4] reported a triplet yield of 2.24 x low3 from IPHN in tert-butylbenzene. A similar pair of experiments was carried out with PEHN and TMD with solutions containing up to 20% poly(a-methylstyrene), and the results are given in Table 6. The calculations depend on a similar efficiency of energy transfer for triplet TABLE 5. Chemiluminescence (CL) from IPHN and TMD as a function of paly(wmethylstyrene) concentration in tert-butylbenzene containing 2.0 mM tDBA at 70.0 “C Luminescer
Polymer”
5.0x10-’ M 0 TMD 6.07 Z.OXIO-~ M 0 IPHN 6.07
CI+_,,X 10e3
104Xkl (s-‘)~
(CCL) x10-6
811
1.3
11.3
97.5
0.51
2.71
26.4
8.7
0.558
8,16
0.111
5.17
sWeight per cent of poly(L-u-methylstyrene).Each solution also contained initially 1.0 mM 2,6-di-tert-buty-4-methylphenol. bFrom chemiluminescence decay (r> 0.992 in all cases).
2.5
acetone and triplet acetophenone to tDBA, which is probably the case in tert-butylbenzene within error limits [13]. The yield of triplets from PEHN increases about twofold as the polymer concentration is increased to 20%. For comparison, Quinga and Mendenhall[4] reported a triplet yield of (3.25 *0.3)x 10e3 from PEHN in tert-butylbenzene, calculated with the same assumptions as described here. 3.5. Tripret y*elds from PEHN and IPHN in mixtures of mineral oil and teti-butylbenzene Since the polymers displayed limited solubilities in tert-butylbenzene, and the solutions were inconvenient to work with, we carried out some measurements in mixtures containing varying amounts of purified mineral oil. The results for IPHN and PEHN are given in Tables 7 and 8. Plots of total luminescence emission or CL, VS. mineral oil concentration are linear for TMD and IPHN over the ranges given in Tables 7 and 8. The corresponding plot for PEHN is linear except for the experiment in 67% mineral oil, where the point is far below the line extrapolated from the other points. For both PEHN and IPHN, the addition of mineral oil increases the measured amount of light based on starting hyponitrite, whereas the addition of polymers causes a decrease. In order to determine whether this increase is simply due to the increased amount of cage recombination in the more viscous solvent, the yields of acetophenone were determined as a function of mineral oil concentration. The results and yields of triplet acetophenone per cage-reacted phenylethoxyl radical are given in Table 9.
TABLE 6. Chemiluminescence (CL) from PEHN and TMD as a function of poly(a-methylstyrene) concentration in tert-butylbenrene at 55.0 “C Luminescer 5.0x10-'M TMD
Polymer” 0 5.09 10.1 20.1
2.0~10-~ M PEHN
0 5.09 10.1 20.1
CL,,X 10-3
l@Xk, (s-‘)b
(xcL)xlo-6
5$X ld
143
1.4
16.7
[3tw
25.4 11.1 5.23 166 56.1 29.6 17.4
0.98 0.93 0.96
4.26 1.89 0.821
9.5
3.11
1.4
9.09 8.7 8.5
1.09 0.599 0.367
1.9 2.4 3.2
‘Weight per cent of poIy(cu-methylstyrene). Each solution also contained initially 1.0 mM 2,~di-tert-butyl-4-methylphenol mM tDBA. bFrom chemihuninescence decay. CYield of triplet acetophenone per hyponitrite.
and 2.0
26
G.D. Mendenholl, L. Mottiova-Rychla I Chemilwninescence
TABLE 7. Chemilurninescence at 70.0 “C
(CL) from IPHN and TMD as a function
LtlrllineUX
oil”
1.9x lo-’ TMD
Mineral M
1.5~ 10-r M IPHN
CL=X
35.3 32.9 19.8
0
0.019
7.75
0.433
0.374
16.6 33.8 67.1
0.023 0.0275 0.0362
8.30 8.50 8.60
0.548 0.648 0.869
0.589 0.747 1.67
initially 4.89 mM 2,6-di-tert-butyl-4-methylphenol
and TMD as a function
CL,
x 10-X
‘Per cent by weight. bCentipoise. Yield of acetophenone ds;- = ST/(1 - e).
of concentration
llYXk,
(s-1)”
and 20.4 mM tDBA.
of mineral
(XL)
oil in tert-butylbenzene
x 10-S
137.0
1.22
18.9
16.6 33.8 67.1
118.0 106.0 62.7
1.21 1.22 1.26
16.5 14.6 8.37
0
26.1
8.4
0.544
1.74
16.6 33.8 67.1
30.2 37.1 33.8
8.85 8.93 9.06
0.615 0.717 0.714
2.27 2.97 5.16
initially 4.89 mM 2,6-di-tert-butyl-4-methylphenol
PhCOCH~
(%)’
28 38 39 44
e
lbXS,~~d
0.72 0.62 0.41 0.56
6.2 6.0 7.6 11.7
at
s; x l@
0
TABLE 9. Cage effects, viscosities and fully corrected yields of triplet acetophenone from PEHN in mixtures of mineral oil and tert-butylbenzene at 55.1 “C
0.688 0.945 1.50 4.43
L3W
7.50 1.66 7.80
“Weight per cent. Each solution also contained bFrom cbemiluminescence decay. rYield of triplet acetophenone per hyponitrite.
0
srcx l@
1.54 1.46 0.862
oil’
16.6 33.8 67.1
10-h
16.6 33.8 67.1
Mineral
oil” Viscosi@’
(XL)X 43.9
Luminescer
Mineral
(s-l)b
oil in tert-butylbensene
7.40
(CL) from PEHN
M
lO’Xk,
of mineral
1.92
TABLE 8. Chemiluminescence 55.2 “C
3.2~ lo+ PEHN
of the concentration
0
‘Weight per cent. Each solution also contained ‘From chemiluminescence decay. Yield of triplet acetone per hyponitrite.
6.45x10+ MTMD
1o-6
from olkyi hyponihies
[3001
and 20.4 mM tDBA.
from IPHN (Table 5), which indicate a decrease in the triplet yield with increasing polymer concentration even without correction for the cage effect. In Fig. 3, the corrected values of S, VS. the bulk viscosity of the medium are plotted. The data are probably insufficient to sustain a belief in a linear relationship. 4. Discussion
per hyponitrite
molecule.
From the data in Table 9, it can be observed that, after correction for the increased yields of acetophenone due to the cage effect, there is an inherent increase in the yield of excited acetophenone in the more viscous medium by about a factor of two. This result contrasts with the results
The addition of polymers or mineral oil to solutions of chemiluminescent alkyl hyponitrites should have several possible effects on the light emission yields: (1) an increase due to the increased lifetime of the excited state, as a result of the reduction in the rate of diffusional quenching by impurities or oxygen; (2) a decrease due to the reaction of excited state products with, or quench-
G.D. Mendenhall, L. Mntiova-Ryehla
/ Chemiluminescence from al!@ hyponiMes
27
The differences between the triplet yields obtained for PEHN and IPHN in this work and those reported previously are a little disconcerting. For PEHN, the differences are in line with estimated errors of 50% for the triplet yields of TMD [14]. In several of our experiments, we did not eliminate dissolved oxygen from the solution, which may contribute to chemiluminescence via luminescent peroxyl-peroxyl reactions; however, this contribution relative to that from alkoxyl radical self-reactions has been found to be small [5].
5e 0
Acknowledgments 1
Viscosity, Cp Fig. 3. Corrected yield of excited carbonyls from PEHN as a function of the viscosity of mineral oil-tert-butylbenzene mixtures (cfi Table 9).
ing by, the added (viscous) component; (3) a decrease due to the decreased fraction of energy transfer from the excited state carbonyl products to the fluorescent species; (4) an increase due to increased self-reaction of the initially formed alkoxyl pairs in the solvent cage, leading to more excited state carbonyl products. From the present experiments, we infer that, for dilute polymer solutions, the third factor is dominant, The exact contributions could be determined by time-resolved studies. A fifth factor, a change in the inherent yield of excited states from alkoxyl radical dismutation (hydrogen-atom transfer) with a change inviscosity, is of interest because the chemiluminescence from solid polymers represents a limiting case of high viscosity. Our experiments give a mixed answer to this question. The data in Tables 5 and 6 indicate small and opposite effects of polymers on S,. The effects on mineral oil (Table 9) are more dramatic and consistent with an inherent increase in S,. The conclusion must be tempered by the presumably small, but unknown, changes in the relative excitation efficiencies of tDBA by excited acetone vs. acetophenone, or changes in their relative lifetimes, with changes in composition of the medium. Moreover, we assume that the yield of triplets from TMD is insensitive to the medium.
Partial support for this work by the National Academy of Sciences (L.M.R.) and Himont USA, Inc. (G.D.M.) is gratefully acknowledged. We thank Mr. E. Lathova and Mr. Y. Geng for carrying out the viscosity measurements, and Dr. J.P. Riehl and Ms. C. Maupin for performing some fluorescence measurements. References 1 G.D. Mendenhall, 2 3 4 5 6 7 8 9 10
11 12 13 14
Anger. Chem. Int. Ed. En& 29 (1990) 362. L. Matisova-Rychla, 2s. Fodor, J. Rychly and M. Iring, Polym. Degrad. St&l., 3 (1980-1981) 371. EC. Ashby, J. Polym. Sci., 50 (1961) 99. E.M.Y. Quinga and G.D. Mendenhall, 1 Am. Chem. Sot., I08 (1986) 474. S.-H. Lee and G.D. Mendenhall, .I Am. Ciwm. Sot., 110 (1988) 4318. Q.J. Niu and G.D. Mendenhall,J Am. Chem. Sot., 114 (1992) 165. CA. Ogle, SW. Martin, M.P. Dziobak, M.W. Urban and G.D. Mendenhall, J. org. Chem., 48 (1983) 3728. G.D. Mendenhall, J.C. Tauschek and C.A. Ogle, I&. 3. Chem. finer., 13 (1981) 1217. G. Langhammer, R. Berger and H. Seide, Ploste fiutsch., II (1944) 472. J.A. Howard and J.C. Scaiano, in H. Fischer (ed.), Landolt-Bornstein’s Numerkal Data and Functional Relationships in Science and Technofogv New Series, Vol. 13d, Springer, Berlin, 1984. D.V. Avila, C.E. Brown, K.U. Ingold and J. Lusztyk, J. Am. Chem. SIX., 115 (1993) 446. J. Lusztyk, personal communication, 1993. G.D. Mendenhall, X.C. Sheng and T. Wilson, J. Am. Chem. Sot., 113 (1991) 8976. T. Wilson, D.E. Golan, MS. Harris and A.L. Baumstark, J. Am. Chem. Sot., 98 (1976) 1086.