Water-binding solid scintillators: Synthesis, emission properties, and tests in 3H and 14C counting

Article abstract of DOI:10.1002/1521-3765(20000804)6:15<2809::AID-CHEM2809>3.0.CO;2-M

Spectral and time-resolved fluorescence properties as well as relative fluorescence quantum yields of carbodiimide derivatives of 2,5-diphenyloxazole (PPO) (prepared by H₂S elimination from the corresponding thioureas), of some intermediates in the preparation, and of several other PPO derivatives were investigated in solution and in the solid state to test their suitability as solid scintillators. The carbodiimides reacted slowly with water under acidic conditions to yield ureas. These systems were compared with solid mixtures of other PPO derivatives with sodium sulfate as a drying agent, as chemically water-binding solid scintillators in ³H and ¹⁴C counting. Both the chemically and the absorptive water-binding scintillators proved capable of counting ³H and ¹⁴C decay, and open a way to the counting of aqueous samples by solid scintillators without a drying step.

Full text of DOI:10.1002/1521-3765(20000804)6:15<2809::AID-CHEM2809>3.0.CO;2-M

FULL PAPER
Water-Binding Solid Scintillators: Synthesis, Emission Properties,
3
and Tests in H and 14C Counting
Hans-Joachim Meyer and Thomas Wolff*[a]
Abstract: Spectral and time-resolved
fluorescence properties as well as rela-
tive fluorescence quantum yields of
carbodiimide derivatives of 2,5-diphe-
nyloxazole (PPO)(prepared by H 2S
elimination from the corresponding thi-
oureas), of some intermediates in the
preparation, and of several other PPO
derivatives were investigated in solution
and in the solid state to test their
suitability as solid scintillators. The car-
mixtures of other PPO derivatives with
sodium sulfate as a drying agent, as
chemically water-binding solid scintilla-
bodiimides reacted slowly with water
under acidic conditions to yield ureas.
These systems were compared with solid
3
tors in H and 14C counting. Both the
chemically and the absorptive water-
binding scintillators proved capable of
Keywords: analytical methods ´ car-
bodiimides ´ fluorescence spectro-
scopy ´ heterocycles ´ scintillation
counting
3
counting H and 14C decay, and open a
way to the counting of aqueous samples
by solid scintillators without a drying
step.
solutions (cocktails)are available commercially. Although
this technique has been developed to give an excellent
performance, disposal of the weakly radioactive organic
liquid waste is problematic.
There is therefore growing interest in solid, disposable
scintillators that can be used for measuring aqueous samples
stemming from biological, pharmaceutical, and medical
research or diagnostics. Suitable scintillating solids can be
found,[5, 15] but before their application to aqueous samples a
drying procedure is necessary. Besides being time-consuming,
this drying step implies the risk of 3H emission since there may
be some tritium in the solvent water, arising from hydrogen
exchange with labeled solutes.
Here we describe attempts to find solid scintillators for use
without a drying step in aqueous samples. The scintillators (or
mixtures containing the scintillator), therefore, have to bind
the water of aqueous samples. This is achieved either by the
addition of a drying chemical or by synthesizing scintillators
bearing groups capable of binding the water chemically; the
carbodiimide group, for example, forms urea with water.
Introduction
Conversion of the energy of b particles (emitted upon
radioactive decay of unstable isotopes)into a corresponding
number of fluorescence quanta is known as scintillation.
Through collisions the particles promote nearby atoms or
molecules to electronically excited states capable of emitting
fluorescence. The number of excited states generated depends
on the energy of the b particles, that is, on the decaying
isotope. Since it was first described, the scintillation phenom-
enon has been exploited for detection, counting, and identi-
fication of radioactive radiation.[1] Inorganic materials such as
thallium-doped sodium iodide,[2±4] cerium-doped yttrium
silicate,[5, 6, 7] solid and liquid noble gases,[8] and organic
scintillators such as 2,5-bis(2-benzoxazolyl)phenol in poly-
styrene,[9] 2,5-diphenyloxazole (PPO), and 1,4-bis(2-methyl-
styryl)benzene (bis-MSB) in paraffin,[10, 11] or PPO and 1,4-bis-
2-(5-phenyloxazolyl)benzene (POPOP) in organic sol-
vents[12±14] are commonly used as scintillators. The latter
systems usually contain surfactants in order to form micro-
emulsions when water has to be taken up from biological and
medical samples containing radioactively labeled substances
in aqueous solution.
Results
The liquid scintillation counting method is superior for the
detection of weak b radiation from 3H (tritium)or 14C decay.
Liquid scintillation counters and many suitable scintillator
Choice of scintillator chromophore: Some principles of
traditional liquid scintillation counting hold for solid scintil-
lators. The fluorescence quantum yield should be as high as
possible; reabsorption of fluorescence in the system should be
low; the fluorescence maximum of the scintillator should be
not too far from 420 nm, to meet the maximum sensitivity of
common detectors (photomultipliers). Emission properties
[a] Prof. Dr. T. Wolff, Dr. H.-J. Meyer
Institut für Physikalische Chemie und Elektrochemie der Technischen
Universität Dresden
01062 Dresden (Germany)
Fax : ‡(49)351-463-3391
E-mail: thomas.wolff@chemie.tu-dresden.de
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T. Wolff and H.-J. Meyer et al.
FULL PAPER
often differ for the same fluo-
rophore in solution and in the
solid state. In most cases the
emission spectrum of the solid
is red-shifted relative to solu-
tions (in nonpolar solvents)be-
cause of interactions of the
closely arranged fluorescent
molecules.[16, 17] We therefore
tested five fluorescent substan-
ces (known to be used in liquid
scintillation counting)under
our conditions: 2,5-diphenylox-
azole (PPO), 1,4-bis(5-phenyl-
Scheme 1. Synthesis of 4'-substituted 2,5-diphenyloxazoles 4.
oxazol-2-yl)benzene (POPOP), trans-stilbene, 1,4-bis(2-meth-
ylstyryl)benzene (bis-MSB), and 9,10-dimethylanthracene.
These compounds in ternary mixtures with sodium sulfate
and sodium dodecyl sulfate (to adsorb water and to allow
homogeneous wetting, respectively; see Experimental Sec-
tion)were treated with [ 3H]water and scintillation was
counted. The counting efficiencies decreased in the order
PPO (7.6%), bis-MSB (5.6%), POPOP (4.2%), trans-stilbene
(1.1%), 9,10-dimethylanthracene (0.24%). This result
prompted us to use PPO as
chloride, the first two steps yielded ammonium salts 2 via
Â
the hexamethylene tetramine adduct 1 (Delepine reac-
tion).[20] In the presence of a base (pyridine[21] or aqueous
sodium carbonate[22]) 2 reacted with 4-substituted benzoic
acid chlorides to form amides 3. Dehydration thereafter
led to R-substituted PPOs: R ˆ F (4a) , Cl (4b) , Br (4c) , CH
3
(4d).
Radical substitution at the methyl group of 4d with N-
bromosuccinimide[23] led to 5. Compound 6, obtained in a two-
the basic chromophore in this
study. Another advantage of
PPO for our purpose was the
comparative simplicity of the
synthesis of carbodiimide-sub-
stituted derivatives exhibiting
the desired fluorescence prop-
erties.
Syntheses: Generally PPO de-
rivatives were synthesized in
four steps (Scheme 1)accord-
ing to Robinson and Lister[18]
and Gabriel.[19] From phenacyl
Scheme 2. Synthesis of aminomethyl-PPO 7.
Â
step Delepine reaction, formed the primary amine 7 upon
acidic alcoholysis (Scheme 2). Yields were moderate in 5 and
high in 6 and 7.
Abstract in German: Carbodiimid-Derivate des 2,5-Diphenyl-
oxazols (PPO) wurden aus den korrespondierenden Thio-
harnstoffen durch H2S-Eliminierung hergestellt. Fluoreszenz-
spektren und -lebensdauern sowie relative Fluoreszenzquan-
tenausbeuten dieser und einiger verwandter PPO-Verbindun-
gen wurden sowohl im gelösten als auch im festen Zustand
untersucht, um die Eignung der Substanzen als Feststoff-
szintillatoren zu prüfen. Unter sauren Bedingungen reagier-
ten die Carbodiimide mit Wasser allmählich zu Harn-
stoffen. Diese Systeme wurden als chemisch wasserbindende
Szintillatoren für die Zählung von 3H- and 14C-Zerfällen
getestet und verglichen mit festen Mischungen aus PPO-
Derivaten und Natriumsulfat als Trockenmittel. Sowohl die
chemisch wasserbindenden als auch die adsorptiv wasser-
The addition of isocyanates R' NCO[24] and isothiocyanates
À
[25]
À
R' NCS led to ureas 8 and thioureas 9 (Scheme 3): R' ˆ
C2H5 (8a, 9a), iPr (8b, 9b) , Ph (8c, 9c). Yields were excellent
for thioureas 9 and good for ureas 8. By elimination of H2S
employing HgO[26] the corresponding carbodiimides 10 were
obtained in moderate (10a)to fair ( 10b, 10c)yields.
Elimination of H2O from the ureas 8 by various meth-
ods[24, 27, 28] failed.
The hydration of the carbodiimides 10 in the solid state to
yield the ureas 8 was essential to this study, so we followed the
progress of urea formation in the scintillation test samples,
that is, in powdered carbodiimide (900 ± 1100 mg, sufficient
for stoichiometric reaction with water)and sodium dodecyl
sulfate (10%, w/w)after the addition of 50 mL of the aqueous
solution of the labeled probes (containing 0.05 molLÀ1
trifluoroacetic acid). Water consumption after one day was
35% in 10a, 27% in 10b, and 19% in 10c; after seven days it
3
bindenden Szintillatoren konnten H- und 14C-Zerfälle zählen
und eröffnen eine Möglichkeit, wässrige Proben mit Feststoff-
szintillatoren zu vermessen, ohne dass ein Trocknungsschritt
nötig ist.
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Water-Binding Solid Scintillators
2809±2817
constant kF), internal conver-
sion (kIC), and intersystem
crossing (kISC)are the only
processes deactivating the elec-
tronically excited fluorescent
state, according to Equa-
tion (2).
kF
FF ˆ
(2)
kF ‡ kIC ‡ kISC
Stokes shifts (the energy loss
between absorption and emis-
sion spectra)were calculated as
the difference between the
wavenumber of the 0 ± 0 tran-
sition (n00)and that of the
center of gravity of the emission
spectrum curve (nCG).[29] The
Scheme 3. Synthesis of PPO substituted by ureas, thioureas, and carbodiimides.
was 51%, 37%, and 24%, respectively. In [D6]DMSO
solution the reaction of the carbodiimides with aqueous
trifluoroacetic acid was complete within a few hours.
intersection of normalized absorption and emission spectra
was taken as n00. Large Stokes shifts were desirable to
suppress possible reabsorption of emitted fluorescence
quanta.
Fluorescence properties: Table 1 shows emission properties of
argon-saturated solutions of the synthesized compounds in
cyclohexane or methanol. Fluorescence quantum yields are
given relative to an absolute value FF ˆ 1 for PPO in
cyclohexane.[29] Natural lifetimes t0 were calculated from
measured lifetimes and fluorescence quantum yields accord-
ing to Equation (1), which is valid when fluorescence (rate
Table 1 reveals low quantum yields in solution for bromi-
nated compounds (4c, 5)and for the thioureas, especially the
phenyl derivative 9c, but the carbodiimides and their hydra-
tion products, the ureas 10 (except 10c), were highly
fluorescent and thus appeared suitable for this study.
In the solid state, however, the quantum yields of carbo-
diimides and ureas were lower (Table 2). Comparing the
spectral features le,max and lCG in Tables 1 and 2 the
tm
t0 ˆ
(1)
FF
Table 1. Emission properties of synthesized compounds.[a]
Table 2. Fluorescence quantum yield (relative to PPO) FF, lifetime t,
wavelength of the emission maximum le,max, and wavelength of the center
of gravity of the emission spectrum lCG. All for powdered solids at 258C.
Substance
FF
t
t0
Dn
[cmÀ1
]
la,max
[nm][b]
le,max lCG
Substance
FF
t [ns]
le,max [nm]
lCG [nm]
[ns]
[ns]
[nm] [nm]
PPO(4,R ˆ H)1
2.5
392
400
cyclohexane
[c]
4a
4b
4c
4d
5
0.76
0.63
0.18
0.68
0.05
0.35
0.20
0.44
0.24
0.36
0.43
0.31
0.18
0.17
0.13
0.16
2.3
2.3
1.3
1.7
2.2
1.9
2.2
2.2
2.3
1.7
2.1
1.5
2.2
2.5
2.2
1.8
398
396
393
387
460
398
438
426
407
412
414
438
441
401
408
393
410
412
407
402
395
407
437
426
419
400
426
438
433
410
415
411
PPO(4,R ˆ H)1
1.7
1.7 2860[c] 303(4.53)354
370
371
376
374
372
364
372
372
372
372
4a
4b
4c
4d
5
0.97 1.8
0.98 2.1
0.33 1.0
0.98 1.5
0.06 1.5
0.96 1.8
0.92 1.9
0.92 1.9
0.24 1.3
1.8 3000
2.2 2760
3.1 2490
1.5 2920
24.8 1540
2.0 2730
2.1 2700
2.0 2780
5.4 2590
methanol
2.5 2980
2.3 3060
2.2 2830
1.9 2940
2.3 2840
8.2 2420
8.0 2350
16.7 1730
302(4.49)355
307(4.52)360
309(4.54)361
304(4.48)355
314(4.51)359
305(4.50)358
306(4.47)359
306(4.51)359
307(4.49)357
6
7
7
8a
8b
8c
9a
9b
9c
10a
10b
10c
10a
10b
10c
PPO
6
0.79 2.0
0.64 1.5
0.64 1.4
0.69 1.3
0.62 1.4
0.17 1.4
0.15 1.2
0.09 1.5
302(4.48)362
308(4.47)372
305(4.51)362
304(4.50)362
305(4.50)363
305(4.52)351
306(4.52)355
306(4.55)350
372
379
374
375
375
367
364
356
8a
8b
8c
9a
9b
9c
[a] Fluorescence quantum yield FF (relative to PPO in cyclohexane[29]),
expected red shift in solids relative to solutions is evident.
The lCG values were in the high photomultiplier sensitivity
range, as desired. Powdered mixtures (as used in the
scintillation experiments described below)of the substances
investigated, Na2SO4, and sodium dodecyl sulfate showed
only slight deviations from the fluorescence properties of
lifetime t, natural lifetime t0, Stokes shift Dn, wavelength of the absorption
[b]
maximum la,max
, wavelength of the emission maximum le,max, and
wavelength of the center of gravity of the emission spectrum lCG. All
values for degassed solutions at c ˆ 50 mmoldmÀ3 in two solvents at 258C.
[b] Values in parentheses: log (e [molÀ1dm3cmÀ1]). [c] Ref.[29]: t ˆ 1.5 ns;
n ˆ 2900 cmÀ1
.
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T. Wolff and H.-J. Meyer et al.
FULL PAPER
Table 4. Scintillation counting efficiencies S (counts per minute/decays per
minute)for various solid powders measured 5 min after treatment of the
mixtures with aqueous solutions of labeled samples. The powdered
mixtures contain 10% sodium dodecyl sulfate (by weight), and 10%
sodium sulfate (if applicable).
the pure substances (compare Table 3 with Table 2), indicat-
ing that these additives did not quench fluorescence signifi-
cantly.
S(3H)
Mixtures with Na2SO4
S(14C)
S(3H)
Mixtures without Na2SO4
S(14C)
Table 3. Relative fluorescence quantum yields FF, lifetimes t, wavelength
of the emission maximum le,max , and wavelength of the center of gravity of
the emission spectrum lCG. All for powdered solids mixed with Na2SO4 and
sodium dodecyl sulfate (weight ratio 8:1:1)at 25 8C.
Substance
PPO(4,R ˆ H)0.076
0.251
0.151
4a
4b
4c
4d
5
0.023
Substance
FF
t [ns]
le,max [nm]
lCG [nm]
0.014
0.0032
0.014
0.0002
0.0064
0.0050
0.049
0.060
0.045
0.026
0.020
0.0007
0.14
0.02
0.04
0.005
0.15
0.24
0.63
0.66
0.44
0.52
0.49
0.02
PPO(4,R ˆ H)0.95
2.4
393
399
4a
4b
4c
4d
5
0.76
0.59
0.20
0.77
0.06
0.36
0.20
0.42
0.28
0.36
0.42
0.25
0.20
0.16
0.13
0.18
2.8
2.3
1.4
1.8
1.9
2.0
2.3
2.8
2.1
1.8
2.5
1.6
2.2
2.6
2.4
1.4
397
395
395
389
409
407
402
402
396
405
436
425
413
410
429
429
433
413
411
412
6
7
8a
8b
8c
9a
9b
9c
10a
10b
10c
[a]
±
6
7
395
437
431
399
402
424
442
439
401
392
398
8a
8b
8c
9a
9b
9c
10a
10b
10c
0.015
0.138
0.068
0.096
0.0013
0.0049
[a] Signal to noise ratio too low for the determination of lmax
.
Table 5. Scintillation counting efficiencies S (counts per minute/decays per
minute)for various solid powders measured at various times after treatment of
the mixtures with aqueous solutions of labeled samples. The mixtures contain
sodium dodecyl sulfate and sodium sulfate, each at 10% by weight.
Scintillation: Samples of 3H-labeled water and 14C-labeled
sodium benzoate (14C in the carboxyl group)in aqueous
solution were used to test scintillation in two series: i)in
powdered mixtures of the fluorescent substance (scintillator),
sodium sulfate, and sodium dodecyl sulfate; and ii)in
powdered mixtures of the synthesized carbodiimides (0.9 ±
1 g)and sodium dodecyl sulfate. In i), a weight ratio of 10%
sodium sulfate and 10% sodium dodecyl sulfate in 400 mg
samples was chosen. Scintillator contents exceeding 80% did
not increase the counting efficiency. In ii), the drying agent
sodium sulfate was omitted. Sodium dodecyl sulfate, a
surfactant, was added in both cases to allow wetting of the
organic material. The samples were treated with 50 mL of the
aqueous probes.
Substance
S(3H) S(3H) S(3H) S(3H)
at at At at
S(3H)
at
S(3H) S(3H)
at
at
5 min 39 min 73 min 107 min 141 min 24 h
7 d
PPO(4,R ˆ H)0.076 0.075
0.073 0.068 0.067
4a
0.023 0.021
0.014 0.014
0.019 0.018 0.017
0.014 0.013
4b
4c
4d
5
6
7
0.013
0.0032 0.0019 0.0017 0.0016
0.014 0.014 0.013 0.012
0.0002 0.0003 0.0003 0.0003
0.0064 0.0061 0.0061 0.0059
0.0050 0.0056 0.0057 0.0059
0.0014
0.012
0.0002
0.0059
0.0059
0.063
0.064
0.046
8a
8b
8c
9a
9b
9c
10a[a]
10b[a]
10c[a]
0.049 0.060
0.060 0.062
0.045 0.047
0.026 0.025
0.020 0.022
0.0007 0.0008 0.0008 0.0009
0.015 0.014 0.013 0.012
0.0013 0.0011 0.0011 0.0011
0.0049 0.0055 0.0059 0.0061
0.060
0.063
0.046
0.023
0.021
0.061
0.066
0.046
0.021
0.020
0.020
0.020
Intermediate products of the syntheses were included in the
investigation, for comparison. From Table 4, counting effi-
ciencies S of compounds with low fluorescence quantum
yields were low, as expected. Counting was much more
0.0009
0.010
0.0013
0.0059
0.012 0.0050
0.0014 0.0012
0.0062 0.0060
3
efficient for 14C than for H, reflecting the difference in b-
particle energy for tritium decay (b-emission extending to
18.6 keV)and for
[a] Without Na2SO4.
14C decay (b-emission extending to
156.5 keV).[1] The maximum counting efficiency was found
in the ureas 8 (in mixtures with sodium sulfate and sodium
dodecyl sulfate); these were the products of the reaction of
the corresponding carbodiimides 10 with water. Since carbo-
diimides 10 (in mixtures without sodium sulfate)counted less
efficiently, a time dependence of the counting efficiency was
to be expected.
Table 6. Scintillation counting efficiencies S (counts per minute/decays per
minute)for various solid powders measured at various times after
treatment of the mixtures with aqueous solutions of labeled samples. The
mixtures contain 10% sodium dodecyl sulfate (by weight).
Substance S(14C) S(14C) S(14C) S(14C)
at at at at
S(14C)
at
S (14C) S(14C)
at
at
Tables 5 and 6 show the temporal development of counting
5 min 39 min 73 min 107 min 141 min 24 h
7 d
3
efficiencies. In H counting, small changes with time were
generally observed. In 14C counting of 10c, however, the
counting efficiency increased by a factor of 1.7 within one
week.
10a
10b
10c
0.138 0.133
0.068 0.066
0.096 0.102
0.128
0.064
0.107
0.125
0.063
0.110
0.122
0.062
0.113
0.112
0.050
0.144
0.103
0.035
0.169
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Water-Binding Solid Scintillators
2809±2817
The influence of the spectral distributions of the fluores-
cence emission on the counting efficiency S may be assessed
by plotting S/FF, where FF is the corresponding fluorescence
quantum yield, as a function of the centers of gravity of the
Discussion
Synthesis: The PPO derivatives 4a ± 4d were prepared
according to literature procedures[31, 18, 21] with a few modifi-
cations: the intermediate 1 was obtained in better yields when
3
emission spectra, lCG, as shown in Figure 1 for H counting.
[31]
prepared in dichloromethane (85%)than in chloroform
Compounds with lCG around 420 nm are superior by far, in
accordance with the maximum sensitivity of the photomulti-
pliers used in the scintillation counter, which operates in this
wavelength range. Similar features for 14C counting are shown
in Figure 2.
(60%). Dichloromethane was also less toxic. In the prepara-
tion of 3a ± 3d toluene was used as a solvent and aqueous
sodium carbonate was used as a base[22] instead of pyridine.[21]
In the biphasic system the concentration of base was kept low
so that the concentration of free amines was also low, leading
to fewer by-products and slightly higher yields. Use of
POCl3 rather than sulfuric acid[22] in the dehydration of 3
[21]
to form 4 was found to be advantageous, since the workup was
facilitated. The yield of 5 prepared from 4d has been reported
to be 98% [23], but we obtained only 55 ± 60% in several
attempts.
Compounds 8a ± 10c have not been described previously.
The synthesis chosen led to high (sometimes excellent)yields
of the ureas 8 and the thioureas 9. Unfortunately the yields of
carbodiimides 10 were only moderate, probably due to the
presence of four phases in the reaction mixture (toluene,
water, solid 9, and solid HgO). The possible hydration of 10
during this reaction, however, was less likely to decrease the
yields, since it was found to proceed slowly (see above). In
contrast, the complete failure to prepare carbodiimides by
dehydration of ureas may have been due to fast rehydration of
carbodiimides under the reaction conditions.
3
Figure 1. Reduced H counting efficiences as a function of the centers of
gravity of the emission spectra (cf. Table 3).
Fluorescence: The fact that the quantum yields of brominated
compounds fall short of those of the other compounds can be
attributed mainly to the well-known heavy-atom effect, which
accelerates the spin-forbidden intersystem crossing process[33]
(kISC)to yield the excited triplet state at the expense of
fluorescence emission (kF). Additionally in these cases, the
strikingly high natural lifetimes t0 calculated for 5 and 9c
(Tables 1 and 2)may indicate that the condition of Equa-
tion (2)is not met. There may be two different reasons:
i)photochemical reactions may be competing with fluores-
cence, internal conversion, and intersystem crossing; or ii)
because of the comparatively small Stokes shift, considerable
reabsorption may be a partial cause of the low quantum yields
measured in 5 and 9c.
Figure 2. Reduced 14C counting efficiences as a function of the centers of
gravity of the emission spectra (cf. Table 3).
In the absence of effects specific to the scintillator, S(14C)/
S(3H)ratios as a function of lCG should reflect the ratio of b-
particle energy, E(14C)/E(3H), according to Equation (3), as
Scintillation: The counting efficiencies (Table 4)are generally
low compared with common liquid scintillation counting
results, largely due to the measuring geometry of the Packard
Tricarb 2550TR/LL counter, in which the detectors (photo-
multipliers)are positioned beside the sample. Modern
technical developments have led to scintillation counters
capable of multiple measurements simultaneously. For exam-
ple, the Top Count Reader (Packard Instrument BV, Gronin-
gen, The Netherlands)has an array of photomultipliers placed
on top of the samples in microtiter plates. This might provide a
more suitable geometry for counting with solid scintillators
such as those studied here, since most fluorescence quanta are
emitted upwards. Thus, improvements in absolute counting
efficiencies rest on optimizing the technical equipment.
Sꢀ14C† Eꢀ14C†
ˆ
ꢁ 8.6
(3)
3
3
Sꢀ H†
Eꢀ H†
was found for 4a ± 4c, 8a ± 8c, and 10a (Figure 3). Values
above and below 8.6, therefore, indicate a higher sensitivity
for 14C counting (7, 9a ± 9c, 10b, 10c)and for 3H counting
(PPO, 4d), respectively.
These observations can be explained by taking the molec-
ular interactions of the scintillators and [14C]benzoic acid into
account: values exceeding 8.6 indicate attraction, most
effective in polar or polarizable compounds (consequently
Chem. Eur. J. 2000, 6, No. 15
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0947-6539/00/0615-2813 $ 17.50+.50/0
2813

T. Wolff and H.-J. Meyer et al.
FULL PAPER
vary with the fluorescence quantum yields and with the
spectral distribution of the emission in relation to the
photomultiplier sensitivity of the scintillation counter. Mix-
tures of PPO derivatives containing a drying chemical
(sodium sulfate)count better than chemically water-binding
PPO derivatives bearing carbodiimide substituents, which are
accessible through elimination of H2S from the respective
thioureas.
Experimental Section
Syntheses: Chemicals were taken from the stocks of the Chemistry
Department, Technische Universität Dresden; purchased from Sigma ±
Aldrich, Acros, Merck, Baker, or Lancaster; or provided by Packard
Instrument.
Figure 3. Ratios of 14C and 3H counting efficiencies as a function of the
centers of gravity of the emission spectra (cf. Table 3).
1-(2-Oxo-2-phenylethyl)-3,5,7-triaza-1-azoniatricyclo[3.3.1.13,7]decane
chloride (1): At room temperature hexamethylene tetramine (226.9 g,
1.618 mol)was dissolved in dichloromethane (3.3 L.) Phenacyl chloride
(250.0 g, 1.617 mol)was added while the solution was stirred. After stirring
for 1 h a white precipitate was formed. The suspension was stirred for
another hour. The precipitate was filtered off, washed twice with dichloro-
methane (200 mL)and dried in vacuo for 6 h. Yield 403.7 g (1.370 mol,
84.7%); m.p. 145 ± 147 8C (decomp, lit. m.p. 1458C[31]).
exhibiting lCG at higher wavelengths), while values below 8.6
may reveal repulsion of scintillator and benzoic acid. Thus the
observed specificities of the scintillators are probably not due
to distinctions in the electronic excitation by collisions with
the b particles.
Temporal changes in counting efficiencies S were expected
from two effects: i)the aqueous sample being sucked into the
powder of the scintillation mixture; and ii)the reaction of
carbodiimides 10 with the water of the aqueous samples
yielding ureas 8 which in powdered samples count more
efficiently than the carbodiimides (Table 4), in accordance
with the differences in quantum yields (Table 3). Effect i) can
be expected to decrease S, since fluorescence quanta emitted
from the interior of the powder are less likely to be detected,
and therefore it may explain the data in Table 5. In contrast,
effect ii)should increase S values. This, however, is found in
the 14C counting of 10c only, although the hydration reaction
is faster for 10a and 10b.
In principle it is disadvantageous that we cannot fully
exploit the good scintillation properties of the ureas 8 since
the hydration reaction is slow compared with the scintillation
experiment. However, a fast reaction with water would raise
problems in storing the carbodiimides under ambient humid-
ity. Moreover, in fast hydration reactions warming of the
sample may lead to undesired evaporation of water. As the
reaction of the carbodiimides with water in [D6]DMSO
solution proceeds quantitatively within a couple of hours,
we may expect complete consumption of water in the solid
samples after sufficient time.
1-(2-Oxo-2-phenylethyl)ammonium chloride (2): Upon addition of con-
centrated HCl (37%, 400 mL)to 1 (403.0 g, 1.367 mol)suspended in
ethanol (96%, 3.3 L), 1 dissolved completely, with a slight warming. The
solution was stirred for three days at room temperature. The white
precipitate that had formed was filtered and washed twice with ethanol
(100 mL). The organic phases were combined and the solvent was
evaporated. The residue was crystallized from distilled H2O (50 mL)and
washed four times with acetone. The colorless crystals were dried in vacuo
for 4 h. Yield 199.4 g (1.162 mol, 85.0%); m.p. 194 ± 195 8C (decomp; lit.
m.p. 186 ± 1878C[31]).
4-Fluoro-N-[1-(2-oxo-2-phenylethyl)]benzamide (3a): 4-Fluorobenzoyl
chloride (12.1 mL, 100 mmol)was dissolved in dry pyridine (130 mL). At
room temperature 2 (17.2 g, 100 mmol)was added in portions to the stirred
solution within 15 min. The suspension was stirred and refluxed for 2 h. The
red solution was allowed to cool for 15 min, then distilled H2O (130 mL)
was added. A light yellow precipitate was formed. After the suspension had
been kept at 58C overnight the precipitate was filtered off. The crude
product was washed three times with distilled H2O (50 mL), recrystallized
from ethanol (96%, 250 mL)and dried in vacuo. Colorless needles; yield
18.8 g (73.1 mmol, 73.1%); m.p. 135 8C (lit. m.p. 134 ± 1358C[21]).
4-Chloro-N-[1-(2-oxo-2-phenylethyl)]benzamide (3b): This was prepared
analogously to 3a, from 4-chlorobenzoyl chloride (97%, 13.1 mL,
100 mmol)and 2 (17.2 g, 100 mmol). After crystallization from ethanol
(96%, 350 mL)and drying in vacuo, colorless needles were obtained. Yield
21.5 g (78.5 mmol 78.5%); m.p. 149 8C (lit. m.p. 148 ± 148.58C[21]).
4-Bromo-N-[1-(2-oxo-2-phenylethyl)]benzamide (3c): Preparation of 3c,
from 4-bromobenzoyl chloride (25.0 g, 114 mmol)and 2 (19.7 g, 115 mmol)
in dry pyridine (130 mL), was analogous to that of 3a. The crude product
was recrystallized from ethanol (96%, 600 mL)and dried in vacuo to yield
colorless needles. Yield 28.1 g (88.2 mmol; 77.4%): m.p. 167 ± 1688C (lit.:
m.p. 164 ± 1658C[21]).
Like the carbodiimides, the hexamethylene tetramine
derivative 6 is capable of reacting with water and thus may
be employed as a chemically water-binding solid scintillator.
We did not pursue this possibility since volatile products are
formed upon hydrolysis of hexamethylene tetramine so that
the emission of tritium with NH3 and H2CO is likely, contrary
to the aim of this study.
4-Methyl-N-[1-(2-oxo-2-phenylethyl)]benzamide (3d): To a stirred solu-
tion of 2 (85.8 g, 0.500 mol)in distilled H 2O (200 mL)at room temperature,
toluene (200 mL)and 1-methyl-4-chlorobenzene (98%, 1 ˆ 1.169 gmolÀ1
,
67.5 mL, 0.500 mol)were added. Under further vigorous stirring, aqueous
Na2CO3 (10% w/w, 600 mL)was added dropwise within 2 h. A light yellow
precipitate formed. After this suspension had been stirred for 2 h the
precipitate was separated, washed with distilled H2O until neutrality and
recrystallized from ethanol (96%, 300 mL). Colorless needles; yield 103.8 g
(0.410 mol, 82.0%); m.p. 125 ± 126 8C (lit. m.p. 1258C[18, 32]).
Conclusion
2-(4-Fluorophenyl)-5-phenyloxazole (4a): After 3a (15.4 g, 59.9 mmol)and
POCl3 (98%, 1 ˆ 1.68 g mLÀ1, 30 mL, 320 mmol)had been stirred and
thoroughly refluxed for 3 h, the hot solution was poured onto ice (1 kg).
Scintillation counting of aqueous samples using solid scintil-
lators without a drying step is possible. Counting efficiencies
2814
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0947-6539/00/0615-2814 $ 17.50+.50/0
Chem. Eur. J. 2000, 6, No. 15

Water-Binding Solid Scintillators
2809±2817
The suspension formed thereby was vigorously stirred for 30 min. A
precipitate formed which was filtered off, dissolved in dichloromethane
(500 mL)and washed with aqueous NaHCO 3 until neutrality. The organic
phase was dried over Na2SO4 and filtered. After evaporation of the solvent
the crude product was purified by column chromatography (twice)on
96%). Long light yellow needles; yield 18.4 g (58.6 mmol, 58.6%); m.p.
129 ± 1308C (lit. m.p. 1308C[25]); 1H NMR (500.13 MHz, [D1]CHCl3): d ˆ
3
8.07 (d, J(H,H) ˆ 8.4 Hz, 2H, C9 H, C9' H), 7.71 (d, 3J(H,H) ˆ 7.1 Hz,
À
À
3
À
À
À
À
2H, C3 H, C3' H), 7.50 (2H, d, J(H,H) ˆ 8.3 Hz, C10 H, C10' H), 7.42 ±
3
À
À
À
À
7.45 (m, 3H, C6 H, C2 H, C2' H), 7.34 (t, J(H,H) ˆ 7.5 Hz, 1H, C1 H),
4.52 (s, 2H, C12 H); 13C NMR (125.77 MHz, [D1]CHCl3): d ˆ 160.52 (C7),
À
Al2O3 (neutral, activity I)with CHCl
eluent (Rf ˆ 0.57 ± 0.61)and
3
recrystallization from ethanol (100 mL)(Merck, UVASOL). Yield 11.1 g
151.44 (C5), 139.85 (C11), 129.52 (C10, C10'), 128.93 (C2, C2'), 128.55 (C1),
127.81 (C4), 126.64 (C9, C9'), 127.31 (C8), 124.21 (C3, C3'), 123.49
(C6), 32.72 (C12); UV/Vis (cyclohexane): la,max (e) ˆ 314 nm
(46.4 mmol, 77.5%); m.p. 83 8C (lit. m.p. 81 ± 828C[21]); 1H NMR
(500.13 MHz, [D1]CHCl3): d ˆ 8.09 (dd, 3J(H,H) ˆ 8.8 Hz, 2H, 4J(H,F) ˆ
3
5.5 Hz, C9 H, C9' H), 7.70 (d, J(H,H) ˆ 7.4 Hz, 2H, C3 H, C3' H), 7.44
(32400 molÀ1 dm3 cmÀ1) ; C H12ONBr (314.2): calcd: C 61.17, H 3.85, N
À
À
À
À
16
3
À
À
À
(t, J(H,H) ˆ 7.8 Hz, 2H, C2 H, C2' H), 7.42 (s, 1H, C6 H), 7.34 (t,
4.46, Br 25.43; found: C 61.34, H 3.86, N 4.58, Br 25.51.
3J(H,H) ˆ 7.5 Hz, 1 H, C1 H), 7.16 (t, 3J(H,F) ˆ 8.7 Hz, 2H, C10 H,
À
À
1-[4-(5-Phenyloxazol-2-yl)benzyl]-3,5,7-triaza-1-azoniatricyclo[3.3.1.13,7]-
decane bromide (6): Hexamethylene tetramine (30.0 g, 214 mmol), 5
(62.8 g, 200 mmol), and dichloromethane (1 L) were mixed in a flask (2 L)
fitted with a reflux condenser. The suspension was stirred for 6 h at room
temperature and filtered. The solid residue was washed three times with
dichloromethane (100 mL)and dried for 6 h in vacuo to yield a colorless
powder. This salt was purified by extraction with dichloromethane (Merck,
UVASOL)in a Soxhlet extractor for 12 h. Yield 83.2 g (183 mmol, 91.5%);
C10' H); 13C NMR (125.77 MHz, [D1]CHCl3): d ˆ 165.01 (d, 1J(C,F) ˆ
À
251.5 Hz, C11), 160.31 (C7), 151.31 (C5), 128.93 (C2, C2'), 128.47 (C1),
3
128.35 (d, J(C,F) ˆ 8.6 Hz, C9, C9'), 127.89 (C4), 124.15 (C3, C3'), 123.82
(d, 4J(C,F) ˆ 3.5 Hz, C8), 123.40 (C6), 116.00 (d, 2J(C,F) ˆ 22.6 Hz, C10,
C10'); UV/Vis (cyclohexane): la,max (e) ˆ 302 nm (30800 molÀ1 dm3 cmÀ1).
2-(4-Chlorophenyl)-5-phenyloxazole (4b): This was prepared analogously
to 4a, from 3b (16.4 g, 59.9 mmol)and POCl (98%, 1 ˆ 1.68 gmLÀ1
,
3
30 mL, 320 mmol). The crude product was purified by column chromatog-
m.p. 185 ± 1878C (decomp); 1H NMR (500.13 MHz, [D6]DMSO): d ˆ 8.21
raphy (twice)on Al O3 (neutral, activity I)with CHCl 3 eluent (Rf ˆ 0.52 ±
2
3
À
À
À
(d, J(H,H) ˆ 8.1 Hz, 2H, C9 H, C9' H), 7.92 (s, 1H, C6 H), 7.87 (d,
0.56)and recrystallization from ethanol (130 mL)(Merck, UVASOL.)
3J(H,H) ˆ 7.7 Hz, 2 H, C3 H, C3' H), 7.71 (d, 3J(H,H) ˆ 8.1 Hz, 2H,
À
À
Yield 11.7 g (45.8 mmol, 76.5%); m.p. 116 8C (lit. m.p. 115 ± 1168C[21]);
3
À
À
À
À
C10 H, C10' H), 7.52 (t, J(H,H) ˆ 7.7 Hz, 2 H, C2 H, C2' H), 7.41 (t,
1H NMR (500.13 MHz, [D1]CHCl3): d ˆ 8.03 (d, 3J(H,H) ˆ 8.6 Hz, 2H,
3
À
À
À
À
J(H,H) ˆ 7.4 Hz, 1 H, C1 H), 5.19 (s, 6H, C13 H, C13' H, C13'' H), 4.60
3
À
À
À
À
C9 H, C9' H), 7.70 (d, J(H,H) ˆ 7.4 Hz, 2H, C3 H, C3' H), 7.42 ± 7.45 (m,
(d, 2J(H,H) ˆ 12.5 Hz, 3H, C14 He, C14' He, C14'' He), 4.46 (d,
À
À
À
3
À
À
À
À
À
5H, C10 H, C10' H, C6 H, C2 H, C2' H), 7.34 (t, J(H,H) ˆ 7.5 Hz, 1H,
2J(H,H) ˆ 12.5 Hz, 3H, C14 Ha, C14' Ha, C14'' Ha), 4.25 (s, 2H,
À
À
À
C1 H); 13C NMR (125.77 MHz, [D1]CHCl3): d ˆ 160.17 (C7), 151.49 (C5),
À
C12 H); 13C NMR (125.77 MHz, [D6]DMSO): d ˆ 159.57 (C7), 151.19
À
136.35 (C11), 129.11 (C10, C10'), 128.93 (C2, C2'), 128.57 (C1), 127.78 (C4),
127.48 (C9, C9'), 125.91 (C8), 124.20 (C3, C3'), 123.52 (C6); UV/Vis
(cyclohexane): la,max (e) ˆ 307 nm (33200 molÀ1 dm3 cmÀ1).
(C5), 133.23 (C10, C10'), 129.11 (C2, C2'), 128.77 (C1), 128.13 (C11), 127.93
(C4), 127.19 (C8), 126.41 (C9, C9'), 124.46 (C6), 124.09 (C3, C3'), 77.67
(C13, C13', C13''), 69.76 (C14, C14', C14''), 58.52 (C12); UV/Vis (meth-
2-(4-Bromophenyl)-5-phenyloxazole (4c): This was prepared in analogy to
anol): la,max (e) ˆ 308 nm (29800 molÀ1 dm3 cmÀ1) ; C H24N5OBr (454.4):
22
4a, from 3c (22.3 g, 70.1 mmol)and POCl (98%, 1 ˆ 1.68 g/ mL, 40 mL,
3
calcd: C 58.16, H 5.32, N 15.41, Br 17.59; found: C 57.93, H 5.41, N 15.22, Br
17.55.
430 mmol). The crude product was purified by column chromatography
(twice)on Al 2O3 (neutral, activity I)with CHCl eluent (Rf ˆ 0.42 ± 0.49)
3
4-(5-Phenyloxazol-2-yl)benzylamine (7): To
a mixture of 6 (22.7 g,
and recrystallization from 210 mL ethanol (Merck, UVASOL). Yield 14.1 g
(47.0 mmol, 67.0%): m.p. 116 ± 1178C (lit. m.p. 115 ± 1168C[21]); 1H NMR
50.0 mmol)and ethanol (96%, 350 mL)in a flask (500 mL)fitted with a
reflux condenser, conc. H2SO4 (10 mL)was added with stirring. The
mixture was refluxed thoroughly for 4 h. Ethanol was evaporated and the
residue was suspended in distilled H2O (100 mL). The suspension was
stirred for 1 h at room temperature, filtered, and the liquid phase was mixed
with toluene (600 mL)and aqueous NaOH (10% w/w, 200 mL.) The
aqueous phase was separated, extracted twice with toluene (200 mL), and
the combined organic phases were dried over KOH (30 g). The solvent was
evaporated from the dried solution and the light yellow residue was
recrystallized from cyclohexane (200 mL). Yield 11.0 g (43.9 mmol,
(500.13 MHz, [D1]CHCl3): d ˆ 7.94 (d, 3J(H,H) ˆ 8.5 Hz, 2H, C9 H,
À
3
C9' H), 7.69 (d, J(H,H) ˆ 7.3 Hz, 2H, C3 H, C3'-H), 7.59 (d, 3J(H,H) ˆ
À
À
3
À
À
À
À
8.7 Hz, 2H, C10 H, C10' H), 7.43 (t, J(H,H) ˆ 7.6 Hz, 2H, C2 H, C2' H),
3
7.42 (s, 1H, C6 H), 7.34 (t, J(H,H) ˆ 7.4 Hz, 1H, C1 H); 13C NMR
(125.77 MHz, [D1]CHCl3): d ˆ 160.17 (C7), 151.48 (C5), 132.01 (C10, C10'),
128.90 (C2, C2'), 128.55 (C1), 127.73 (C4), 127.62 (C9, C9'), 126.29 (C8),
124.69 (C11), 124.17 (C3, C3'), 123.52 (C6); UV/Vis (cyclohexane): la,max
(e) ˆ 309 nm (34400 molÀ1 dm3 cmÀ1).
À
À
5-Phenyl-2-p-tolyloxazole (4d): After 3d (47.1 g, 200 mmol)and POCl
3
87.9%); m.p. 109 ± 110 8C; 1H NMR (500.13 MHz, [D1]CHCl3): d ˆ 8.05
(98%, 1 ˆ 1.68 g mLÀ1, 90 mL, 970 mmol)had been stirred and refluxed
thoroughly for 3 h, the hot solution was poured onto ice (1 kg). The
suspension formed thereby was stirred vigorously for 30 min. A precipitate
formed which was filtered off, dissolved in dichloromethane (500 mL)and
washed with aqueous NaHCO3 until neutrality. The organic phase was
dried over Na2SO4 and filtered. After evaporation of the solvent the crude
product was purified by column chromatography (twice)on Al 2O3 (neutral,
3
(d, J(H,H) ˆ 8.3 Hz, 2H, C9 H, C9' H), 7.70 (d, 3J(H,H) ˆ 7.3 Hz, 2H,
À
À
À
À
À
À
À
À
À
C3 H, C3' H), 7.40 ± 7.43 (m, 5H, C6 H, C2 H, C2' H, C10 H, C10' H),
3
À
À
7.32 (t, J(H,H) ˆ 7.3 Hz, 1H, C1 H), 3.92 (s, 2H, C12 H), 1.47 (s, 2H,
C12 NH); 13C NMR (125.77 MHz, [D1]CHCl3): d ˆ 161.07 (C7), 151.05
À
(C5), 145.63 (C11), 128.85 (C2, C2'), 128.32 (C1), 127.98 (C4), 127.39 (C10,
C10'), 126.43 (C9, C9'), 125.95 (C8), 124.10 (C3, C3'), 123.35 (C6), 46.18
(C12); UV/Vis (cyclohexane): la,max (e) ˆ 305 nm (31400 molÀ1 dm3 cmÀ1);
C16H14N2O (250.3): calcd C 76.78, H 5.64, N 11.19; found C 76.55, H 5.64, N
11.20.
activity I)with CHCl
Remnants of the solvent were removed in vacuo and the product was
(Merck, UVASOL)eluent
(
Rf ˆ 0.55 ± 0.57).
3
recrystallized from n-hexane (200 mL)(UVASOL.) Colorless needles;
yield 39.3 g (167 mmol, 83.5%); m.p. 75 ± 76 8C (lit. m.p. 758C,[28] 778C;[18]
)
N-Ethyl-[4-(5-phenyloxazol-2-yl)benzyl]urea (8a): Ethyl isocyanate (98%,
1 ˆ 0.91 gmLÀ1, 2.0 mL, 25 mmol)was added dropwise to a stirred solution
of 7 (5.0 g, 20 mmol)in THF (dried, 100 mL)in a flask (250 mL). When
stirring was continued at room temperature for 16 h, a suspension was
formed. The solvent was evaporated and the colorless crude product was
recrystallized from 1,4-dioxane (320 mL). Yield 5.6 g (17 mmol, 87%); m.p.
1H NMR (500.13 MHz, [D1]CHCl3): d ˆ 7.99 (d, 3J(H,H) ˆ 8.1 Hz, 2H,
3
À
À
À
À
C9 H, C9' H), 7.71 (d, J(H,H) ˆ 7.9 Hz, 2H, C3 H, C3' H), 7.42 ± 7.45 (m,
3
À
À
À
À
3H, C6 H, C2 H, C2' H), 7.33 (t, J(H,H) ˆ 7.5 Hz, 1H, C1 H), 7.28 (d,
3
J(H,H) ˆ 7.9 Hz, 2H, C10 H, C10' H), 2.41 (s, 3H, C12 H); 13C NMR
(125.77 MHz, [D1]CHCl3): ˆ 161.41 (C7), 150.91 (C5), 140.60 (C11), 129.49
(C10, C10'), 128.87 (C2, C2'), 128.26 (C1), 128.09 (C4), 126.21 (C9, C9'),
124.75 (C8), 124.09 (C3, C3'), 123.32 (C6), 21.51 (C12); UV/Vis (cyclo-
hexane): la,max (e) ˆ 304 nm (29900 molÀ1 dm3 cmÀ1).
À
À
À
223 ± 2248C; 1H NMR (500.13 MHz, [D6]DMSO): d ˆ 8.04 (d, 3J(H,H) ˆ
3
À
À
À
À
8.2 Hz, 2H, C9 H, C9' H), 7.84 (d, J(H,H) ˆ 7.3 Hz, 2H, C3 H, C3' H),
3
À
À
À
7.83 (s, 1H, C6 H), 7.51 (t, J(H,H) ˆ 7.8 Hz, 2H, C2 H, C2' H), 7.41 (m,
3
À
À
À
2H, C10 H, C10' H), 7.39 (m, 1H, C1 H), 6.41 (t, J(H,H) ˆ 6.0 Hz, 1H,
4-(5-Phenyloxazol-2-yl)benzyl bromide (5): N-Bromosuccinimide (18.2 g,
100 mmol)and azobisisobutyronitrile (0.5 g)were added to 4d (23.5 g,
100 mmol)dissolved in dry CCl 4 (100 mL)in a flask (500 mL)fitted with a
reflux condenser. The suspension was stirred for 6 h under vigorous reflux,
the hot suspension was filtered, the residue was extracted three times with
hot CCl4 (10 mL), the CCl4 phases were combined, and the solvent was
evaporated. The crude product was recrystallized from ethanol (130 mL,
C12 NH), 5.96 (t, J(H,H) ˆ 5.6 Hz, 1H, C14 NH), 4.28 (d, 3J(H,H) ˆ
3
À
À
3
À
À
6.0 Hz, 2H, C12 H), 3.04 (m, 2H, C14 H), 1.01 (t, J(H,H) ˆ 7.2 Hz, 3H,
C15 H); 13C NMR (125.77 MHz, [D6]DMSO): d ˆ 160.31 (C7), 157.99
À
(C13), 150.56 (C5), 143.99 (C11), 129.12 (C2, C2'), 128.56 (C1), 127.61 (C10,
C10'), 127.45 (C4), 125.93 (C9, C9'), 125.12 (C8), 124.15 (C6), 124.00 (C3,
C3'), 42.59 (C12), 34.21 (C14), 15.70 (C15); UV/Vis (methanol): la,max (e) ˆ
Chem. Eur. J. 2000, 6, No. 15
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0947-6539/00/0615-2815 $ 17.50+.50/0
2815

T. Wolff and H.-J. Meyer et al.
FULL PAPER
305 nm (32600 molÀ1 dm3 cmÀ1) ; C H19N3O2 (321.4): calcd: 71.01, H 5,.6, N
13.08; found: C 71.19, H 6.12, N 13.20.
light yellow needles. Yield 33.2 g (94.4 mmol, 94.4%); m.p. 188 ± 189 8C;
19
1H NMR (500.13 MHz, [D6]DMSO): d ˆ 8.05 (d, 3J(H,H) ˆ 8.3 Hz, 2H,
3
À
À
À
À
C9 H, C9' H), 7.84 (d, J(H,H) ˆ 7.4 Hz, 2H, C3 H, C3' H), 7.83 (s, 1H,
N-Isopropyl-[4-(5-phenyloxazol-2-yl)benzyl]urea (8b): In a flask (500 mL)
fitted with a dropping funnel, 7 (9.0 g, 36 mmol)was dissolved in THF
(dried, 250 mL). Isopropyl isocyanate (98%, 1 ˆ 0.866 gmLÀ1, 3.7 mL,
37 mmol)in THF (dried, 50 mL)was added dropwise. After 15 min a
colorless precipitate was observed. This suspension was stirred for 16 h at
room temperature. The solvent was evaporated and the crude product was
crystallized from 1,4-dioxane (270 mL). The colorless needles obtained
were washed twice with methanol (UV quality, 30 mL)and dried in vacuo.
3
À
À
À
C6 H), 7.75 (b, 1H, C12 NH), 7.51 (t, J(H,H) ˆ 7.8 Hz, 2H, C2 H,
3
À
À
À
C2' H), 7.45 (d, J(H,H) ˆ 8.2 Hz, 2H, C10 H, C10' H), 7.44 (b, 1H,
3
À
À
À
C14 NH), 7.39 (t, J(H,H) ˆ 7.5 Hz, 1H, C1 H), 4.75 (b, 2H, C12 H), 4.27
3
(b, 1H, C14 H), 1.13 (d, J(H,H) ˆ 6.5 Hz, 6H, C15 H, C15' H); 13C NMR
(125.77 MHz, [D6]DMSO): d ˆ 181.56 (C13), 160.24 (C7), 150.60 (C5),
142.45 (C11), 129.11 (C2, C2'), 128.57 (C1), 127.88 (C10, C10'), 127.42 (C4),
125.94 (C9, C9'), 125.33 (C8), 124.16 (C6), 124.00 (C3, C3'), 46.49 (C12),
45.09 (C14). 22.30 (C15, C15'); UV/Vis (methanol): la,max (e) ˆ 306 nm
À
À
À
Yield 9.9 g (30 mmol, 82%); m.p. 228 ± 229 8C; 1H NMR (500.13 MHz,
3
(32800 molÀ1 dm3 cmÀ1) ; C H21N3OS (351.7): calcd: C 68.35, H 6.02, N
20
À
À
[D6]DMSO): d ˆ 8.04 (d, J(H,H) ˆ 8.3 Hz, 2H, C9 H, C9' H), 7.84 (d,
3J(H,H) ˆ 7.3 Hz, 2 H, C3 H, C3' H), 7.82 (s, 1H, C6 H), 7.51 (t,
11.96, S 9.12; found: C 68.60, H 6.04, N 12.02, S 8.99.
À
À
À
3
À
À
À
À
J(H,H) ˆ 7.8 Hz, 2 H, C2 H, C2' H), 7.41 (m, 2H, C10 H, C10' H), 7.39
N-Phenyl-[4-(5-phenyloxazol-2-yl)benzyl]thiourea (9c): Phenyl isothio-
cyanate (95%, 1 ˆ 1.13 gmLÀ1, 15.1 mL, 120 mmol)was added dropwise,
with stirring, to a solution of 7 (27.5 g, 110 mmol)in toluene (370 mL)in a
two-necked flask (1 L)fitted with a dropping funnel. After 40 min a light
yellow precipitate formed. The suspension was stirred at room temper-
ature. The solid was filtered off, washed twice with toluene (50 mL), and
purified by Soxhlet extraction with methanol (UVASOL). Yield 38.3 g
(99.4 mmol, 90.3%); m.p. 202 8C (dec.); 1H NMR (500.13 MHz,
(m, 1H, C1 H), 6.28 (t, 3J(H,H) ˆ 6.0 Hz, 1H, C12 NH), 5.83 (d,
À
À
3
3
À
À
J(H,H) ˆ 7.7 Hz, 1H, C14 NH), 4.28 (d, J(H,H) ˆ 6.0 Hz, 2H, C12 H),
3
À
À
À
3.70 (m, 1H, C14 H), 1.05 (d, J(H,H) ˆ 6.4 Hz, 6H, C15 H, C15' H);
13C NMR (125.77 MHz, [D6]DMSO): d ˆ 160.28 (C7), 157.41 (C13), 150.54
(C5), 144.05 (C11), 129.08 (C2, C2'), 128.52 (C1), 127.64 (C10, C10'), 127.43
(C4), 125.91 (C9, C9'), 125.12 (C8), 124.11 (C6), 123.97 (C3, C3'), 42,67
(C12), 41.01 (C14), 23.21 (C15, C15'); UV/Vis (methanol): la,max (e) ˆ
304 nm (31400 molÀ1 dm3 cmÀ1) ; C H21N3O2 (335.4): calcd: C 71.62, H
20
À
À
[D6]DMSO): d ˆ 9.74 (b, 1H, C12 NH), 8.29 (b, 1H, C14 NH), 8.07 (d,
6.31, N 12.53; found: C 71.45, H 6.26, N 12.42.
3
J(H,H) ˆ 8.2 Hz, 2H, C9 H, C9' H), 7.85 (2H, d, 3J(H,H) ˆ 7.5 Hz,
À
À
N-Phenyl-[4-(5-phenyloxazol-2-yl)benzyl]urea (8c): Phenyl isocyanate
(98%, 1 ˆ 1.096 gmLÀ1, 5.7 mL, 51 mmol)in THF (dried, 200 mL)was
added dropwise with stirring to 7 (12.5 g, 49.9 mmol)dissolved in THF
(dried, 200 mL)in a two-necked flask (1 L)fitted with a dropping funnel. A
colorless precipitate was observed. This suspension was stirred for 16 h at
room temperature. The solvent was evaporated and the colorless product
crystallized from 1,4-dioxane (1500 mL). Yield 14.8 g (40.2 mmol, 80.3%);
m.p. 235 ± 408C; 1H NMR (125.77 MHz, [D6]DMSO): d ˆ 8.63 (s, 1H,
À
À
À
À
À
C3 H, C3' H), 7.83 (s, 1H, C6 H), 7.52 (m, 2H, C10 H, C10' H), 7.48
3
À
À
À
À
(m, 2H, C2 H, C2' H), 7.45 (d, J(H,H) ˆ 7.8 Hz, 2H, C15 H, C15' H),
3
3
À
À
7.39 (t, J(H,H) ˆ 7.3 Hz, 1H, C1 H), 7.34 (t, J(H,H) ˆ 7.8 Hz, 2H, C16 H,
3
3
À
À
C16' H), 7.13 (t, J(H,H) ˆ 7.3 Hz, 1H, C17 H), 4.85 (d, J(H,H) ˆ 5.2 Hz,
2H, C12 H); 13C NMR (125.78 MHz, [D6]DMSO): d ˆ 180.94 (C13),
À
160.23 (C7), 150.59 (C5), 142.00 (C11), 139.05 (C14), 129.09 (C2, C2'),
128.68 (C16, C16'), 128.56 (C1), 128.01 (C10, C10'), 127.42 (C4), 125.93 (C9,
C9'), 125.36 (C8). 124.40 (C17), 124.17 (C6), 123.98 (C3, C3'), 123.44
(C15, C15'), 46.93 (C12); UV/Vis (methanol): la,max (e) ˆ 306 nm
C14 NH), 8.06 (d, 3J(H,H) ˆ 8.2 Hz, 2H, C9 H, C9' H), 7.84 (d,
À
À
À
3
À
À
À
J(H,H) ˆ 7.5 Hz, 2 H, C3 H, C3' H), 7.83 (s, 1H, C6 H), 7.51 (m, 2H,
(35500 molÀ1 dm3 cmÀ1) ; C H19N3OS (385.5): calcd: C 71.66, H 4.97, N
23
C2 H, C2' H), 7.48 (m, 2H, C10 H, C10' H), 7.42 (d, 3J(H,H) ˆ 7.8 Hz,
À
À
À
À
10.90, S 8.32; found: C 71.65, H 5.14, N 10.80, S 8.36.
3
À
À
À
2H, C15 H, C15' H), 7.39 (t, J(H,H) ˆ 7.4 Hz, 1H, C1 H), 7.23 (t,
3
J(H,H) ˆ 7.9 Hz, 2H, C16 H, C16' H), 6.90 (t, 3J(H,H) ˆ 7.3 Hz, 1H,
À
À
N-Ethyl-[4-(5-phenyloxazol-2-yl)benzyl]carbodiimide (10a): An aqueous
3
C17 H), 6.71 (1H, t, J(H,H) ˆ 6.0 Hz, C12 NH), 4.39 (d, 3J(H,H) ˆ
suspension of HgO (freshly prepared, 8.66 g, 40.0 mmol in 20 mL H2O[26]
)
À
À
6.0 Hz, 2H, C12 H); 13C NMR (125.77 MHz, [D6]DMSO): d ˆ 160.23
À
was added to 9a (13.5 g, 40.0 mmol)and toluene (400 mL)in a flask (1 L).
(C7), 155.24 (C13), 150.56 (C5), 143.29 (C11), 140.37 (C14), 129.07 (C2,
C2'), 128.61 (C16, C16'), 128.53 (C1), 127.72 (C10, C10'), 127.41 (C4), 125.98
(C9, C9'), 125.27 (C8), 124.13 (C6), 123.96 (C3, C3'), 121.11 (C17), 117.72
(C15, C15'), 42.52 (C12); UV/Vis (methanol): la,max (e) ˆ 305 nm
After thorough shaking for 20 min the suspension became dark green. The
solid was separated and washed twice with toluene (200 mL). The organic
phases were combined and dried over Na2SO4. After filtration the solvent
was distilled off. The residue formed very small light yellow crystals from
methylcyclohexane (UV quality, 100 mL)at À408C. Yield 7.1 g (23 mmol,
(31700 molÀ1 dm3 cmÀ1) ; C H19N3O2 (369.4): calcd: C 74.78, H 5.18, N
23
58%); m.p. 41 8C; 1H NMR (500.13 MHz, [D6]DMSO): d ˆ 8.08 (d,
11.37; found: C 74.86, H 5.19, N 11.33.
3
J(H,H) ˆ 8.2 Hz, 2H, C9 H, C9' H), 7.84 (d, 3J(H,H) ˆ 8.1 Hz, 2H,
À
À
N-Ethyl-[4-(5-phenyloxazol-2-yl)benzyl]thiourea (9a): Ethyl isothiocya-
nate (97%, 1 ˆ 0.997 gmLÀ1, 10.8 mL, 120 mmol)was added dropwise,
within 15 min, to a stirred solution of 7 (27.5 g, 110 mmol)and toluene
(370 mL)in a two-necked flask (1 L)fitted with a dropping funnel. A light
yellow precipitate formed. The suspension was stirred for 16 h at room
temperature; then the solid was filtered off, washed twice with toluene
(50 mL), and the crude product was recrystallized from ethanol (96%,
350 mL). Light yellow crystals; yield 35.4 g (105 mmol, 95.4%); m.p. 165 ±
3
À
À
À
À
C3 H, C3' H), 7.83 (s, 1H, C6 H), 7.51 (d, J(H,H) ˆ 7.9 Hz, 2H, C10 H,
C10' H), 7.50 (t, J(H,H) ˆ 7.7 Hz, 2H, C2 H, C2' H), 7.39 (t, 3J(H,H) ˆ
3
À
À
À
7.3 Hz, 1H, C1 H), 4.45 (s, 2H, C12 H), 3.16 (q, 3J(H,H) ˆ 7.2 Hz, 2H,
À
À
C14 H), 1.07 (t, J(H,H) ˆ 7.1 Hz, 3H, C15 H); 13C NMR (125.78 MHz,
[D6]DMSO): d ˆ 160.05 (C7), 150.69 (C5), 141.35 (C11), 140.33 (C13),
129.05 (C2, C2'), 128,55 (C1), 128.19 (C10, C10'), 127.37 (C4), 126.09 (C9,
C9'), 125.78 (C8), 124.17 (C6), 123.99 (C3, C3'), 49.08 (C12), 40.59
(C14), 16.50 (C15); UV/Vis (cyclohexane): la,max (e) ˆ 306 nm
3
À
À
1668C; 1H NMR (500.13 MHz, [D6]DMSO): d ˆ 8.05 (d, 3J(H,H) ˆ 8.2 Hz,
(29300 molÀ1 dm3 cmÀ1) ; CH17N3O (303.4): calcd: C 75.23, H 5.56, N
3
À
À
À
19
2H, C9 H, C9' H), 7.91 (b, 1H, C12 NH), 7.84 (d, J(H,H) ˆ 8.3 Hz, 2H,
13.85; found: C 75.47, H 5.70, N 13.81.
À
À
À
À
C3 H, C3' H), 7.83 (s, 1H, C6 H), 7.58 (b, 1H, C14 NH), 7.50 (t,
3J(H,H) ˆ 7.7 Hz, 2 H, C2 H, C2' H), 7.46 (d, 3J(H,H) ˆ 8.2 Hz, 2H,
À
À
N-Isopropyl-[4-(5-phenyloxazol-2-yl)benzyl]carbodiimide (10b): The
preparation, from 9b (14.1 g, 40.1 mmol), was analogous to that of 10a.
Compound 10b was obtained as light yellow crystals from methylcyclo-
hexane (UV quality, 150 mL)at À408C. Yield 8.9 g (28 mmol, 70%); m.p.
3
À
À
À
C10 H, C10' H), 7.39 (t, J(H,H) ˆ 7.5 Hz, 1H, C1 H), 4.75 (b, 2H,
3
À
À
À
C12 H), 3.40 (b, 2H, C14 H), 1.09 (t, J(H,H) ˆ 7.2 Hz, 3H, C15 H);
13C NMR (125.77 MHz, [D6]DMSO): d ˆ 182.23 (C13), 160.25 (C7), 150.59
(C5), 142.46 (C11), 129.09 (C2, C2'), 128.54 (C1), 127.84 (C10, C10'), 127.43
(C4), 125.91 (C9, C9'), 125.32 (C8), 124.14 (C6), 123.98 (C3, C3'), 46.55
(C12), 38.43 (C14), 13.43 (C15); UV/Vis (methanol): la,max (e) ˆ 305 nm
1
3
598C; H NMR (500.13 MHz, [D6]DMSO): d ˆ 8.08 (d, J(H,H) ˆ 8.2 Hz,
3
À
À
À
À
2H, C9 H, C9' H), 7.83 (d, J(H,H) ˆ 8.2 Hz, 2H, C3 H, C3' H), 7.83 (s,
3
À
À
À
1H, C6 H), 7.50 (d, J(H,H) ˆ 8.2 Hz, 2H, C10 H, C10' H), 7.49 (t,
(32900 molÀ1 dm3 cmÀ1) ; C H19N3OS (337.4): calcd: C 67.63, H 5.68, N
3J(H,H) ˆ 8.2 Hz, 2H, C2 H, C2' H), 7.38 (t, 3J(H,H) ˆ 7.4 Hz, 1H,
19
À
À
12.45, S 9.50; found: C 67.85, H 5.75, N 12.51, S 9.44.
3
À
À
À
C1 H), 4.43 (s, 2H, C12 H), 3.48 (sp, J(H,H) ˆ 6.4 Hz, 1H, C14 H),
1.05 (d, 3J(H,H) ˆ 6.4 Hz, 6H, C15 H); 13C NMR (125.78 MHz,
À
N-Isopropyl-[4-(5-phenyloxazol-2-yl)benzyl]thiourea (9b): To a well-stir-
red solution of 7 (25.0 g, 100 mmol)in toluene (350 mL)in a two-necked
flask (1 L)fitted with a dropping funnel, isopropyl isothiocyanate (97%,
11.5 g, 110 mmol)was added dropwise within 15 min. Upon further stirring
a light yellow precipitate formed after 30 min. After an additional 16 h of
stirring the solid was filtered off and washed twice with toluene (50 mL).
The product was recrystallized from ethanol (96%, 650 mL)to yield tiny
[D6]DMSO): d ˆ 160.03 (C7), 150.69 (C5), 141.31 (C11), 140.15 (C13),
129.04 (C2, C2'), 128.73 (C1), 128.36 (C10, C10'), 127.37 (C4), 126.12 (C9,
C9'), 125.82 (C8), 124.08 (C6), 123.99 (C3, C3'), 49.22 (C12), 48.31 (C14),
24.30 (C15, C15'); UV/Vis (cyclohexane): la,max (e) ˆ 306 nm
(32000 molÀ1 dm3 cmÀ1) ; CH19N3O (317.4): calcd: C 75.69, H 6.03, N
20
13.24; found: C 75.75, H 6.03, N 13.09.
2816
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0947-6539/00/0615-2816 $ 17.50+.50/0
Chem. Eur. J. 2000, 6, No. 15

Water-Binding Solid Scintillators
2809±2817
N-Phenyl-[4-(5-phenyloxazol-2-yl)benzyl]carbodiimide (10c): The prepa-
ration, from 9c (15.4 g, 40 mmol), was analogous to that of 10a. Compound
10c was obtained as light yellow crystals from methylcyclohexane (UV
quality, 150 mL)at 5 8C. Yield 7.5 g (21 mmol, 53%); m.p. 77.5 ± 78 8C;
Acknowledgements
This work was financed mainly by Packard Instrument BV, Groningen, The
Netherlands, who also provided the scintillation counter. Further financial
support from the Fond der Chemischen Industrie and from the Deutsche
Forschungsgemeinschaft (Graduiertenkolleg ªStruktur-Eigenschafts-Be-
ziehungen bei Heterocyclenº)is gratefully acknowledged.
1H NMR (500.13 MHz, [D6]DMSO): d ˆ 8.11 (d, 3J(H,H) ˆ 8.0 Hz, 2H,
3
À
À
À
À
C9 H, C9' H), 7.85 (d, J(H,H) ˆ 5.3 Hz, 2H, C3 H, C3' H), 7.84 (s, 1H,
3
3
À
À
À
C6 H), 7.60 (d, J(H,H) ˆ 8.0 Hz, 2H, C10 H, C10' H), 7.50 (t, J(H,H) ˆ
3
À
À
À
7.6 Hz, 2H, C2 H, C2' H), 7.39 (t, J(H,H) ˆ 7.3 Hz, 1H, C1 H), 7.32 (t,
J(H,H) ˆ 7.7 Hz, 2H, C16 H, C16' H), 7.13 (t, 3J(H,H) ˆ 7.3 Hz, 1 H,
3
À
À
3
À
À
À
C17 H), 7.06 (d, J(H,H) ˆ 7.7 Hz, 2H, C15 H, C15' H), 4.77 (s, 2H,
[1] Basic aspects of scintillation counting may be studied inJ. B. Birks, The
Theory and Practice of Scintillation Counting, Pergamon Press,
London, 1964.
[2] S. Buhl, H. Leutz, H. Muuss, Z. Phys. 1958, 152, 272.
[3] B. Grimeland, Phys. Rev. 1952, 86, 937.
[4] J. A. McIntyre, R. Hofstädter, Phys. Rev. 1950, 78, 617.
[5] S. W. Wunderly, Appl. Radiat. Isot. 1989, 40, 569.
[6] S. W. Wunderly, International Conference on Advances in Liquid
Scintillation Spectrometry 1992 (Eds.: J. E. Noakes, F. Schönhofer,
H. A. Polach), Radiocarbon, Tucson, 1993, p. 217.
[7] C. G. Potter, International Conference on Advances in Liquid Scintil-
lation Spectrometry 1992 (Eds.: J. E. Noakes, F. Schönhofer, H. A.
Polach), Radiocarbon, Tucson, 1993, p. 313.
C12 H); 13C NMR (125.78 MHz, [D6]DMSO): d ˆ 159.96 (C7), 150.76
À
(C5), 140.57 (C11), 139.58 (C14), 136.56 (C13), 129.59 (C16, C16'), 129.09
(C2, C2'), 128.61 (C1), 128.26 (C10, C10'), 127.36 (C4), 126.28 (C9, C9'),
126.06 (C8), 125.02 (C17), 124.22 (C6), 124.03 (C3, C3'), 123.41
(C15, C15'), 49.10 (C12); UV/Vis (cyclohexane): la,max (e) ˆ 307 nm
(31100 molÀ1 dm3 cmÀ1) ; C H17N3O (351.4): calcd: C 78.61, H 4.88, N
23
11.96; found: C 78.41, H 4.85, N 11.91.
Hydration of solid carbodiimides: In a polyethylene scintillation vial
(5 mL), a mortared solid mixture of a water-binding scintillator (10a ± 10c)
(2.8 mmol)and sodium dodecyl sulfate (10% w/w)was treated with 50 mL
of water containing trifluoroacetic acid (2 molLÀ1). After either one or
seven days, each mixture was homogenized by thorough mortaring.
1H NMR spectra were recorded for solutions of the mixtures (10 mg)in
[D6]DMSO. The amounts of ureas formed were determined by comparing
the integrals of the signals for C9 or C12 in 10a ± 10c and 8a ± 10c. For
comparison, the hydration was followed in solutions of 10a ± 10c in
[D6]DMSO for two days.
[8] E. Aprile, A. Bolotnikov, D. Chen, F. Xu, V. Peskov, Nucl. Instr.
Methods Phys. Res. A 1994, 353, 55.
[9] J. M. Kauffman, G. S. Bajwa, P. T. Litak, SCIFI 93, Workshop on
Scintillating Fiber Detectors (Eds.: A. D. Bross, R. C. Ruchti, M. R.
Wayne), World Scientific, Singapore, 1995, p. 353.
[10] M. Takiue, H. Fujii, T. Aburai, M. Yanokura, Appl. Radiat. Isot. 1995,
46, 191.
Fluorescence: Spectra were recorded at room temperature on a Shimadzu
(model RF-5000)spectral fluorimeter or on homemade apparatus descri-
bed previously.[34] Spectra from solid samples were taken from the surface
at 308 to the exciting light. Integrated fluorescence spectra obtained under
identical absorption conditions were taken as measures for relative
quantum yields, using PPO (scintillation grade; Packard Instrument)in
cyclohexane or solid powdered PPO, respectively, as the reference.
Measured values were reproducible within 3% in solutions and 5% in
solids.
[11] H. Fujii, M. Takiue, Appl. Radiat. Isot. 1989, 40, 495.
[12] J. G. Carter, L. G. Christophorou, US Patent 3444089 (May 13, 1969).
[13] L. I. Wiebe, C. Ediss, Liquid Scintillation: Science and Technology
(Eds.: A. A. Noujaim, C. Ediss, L. I. Wiebe), Academic Press, New
York, 1976, p. 93.
[14] D. N. Abrams, S. A. McQuarrie, C.Ediss, L. Wiebe, Liquid Scintilla-
tion: Science and Technology (Eds.: A. A. Noujaim, C. Ediss, L. I.
Wiebe), Academic Press, New York, 1976, p. 167.
[15] L. F. Costa, D. C. Harrington, R. S. Miller, US Patent 4692266
(September 8, 1987).
[16] M. Kasha, H. R. Rawls, M. A. El-Bayoumi, Pure Appl. Chem. 1965,
11, 371.
[17] M. Pope, C. E. Swenberg, Electronic Processes in Organic Crystals,
Oxford University Press, New York, 1982.
[18] J. Lister, R. Robinson, J. Chem. Soc. 1912, 101, 1297.
[19] S. Gabriel, Chem. Ber. 1910, 43, 1283.
Lifetimes were measured at room temperature in apparatus described
elsewhere,[35, 36] modified in that i)a nitrogen laser from ILEE (model NN-
100, pulse width 0.5 ns)was used as excitation source, ii)a photodiode from
Soliton (Model UPD-500UP, rise time < 500 ps)was employed as the
detector, and iii)signals were recorded on a Hewlett-Packard digital
oscilloscope (model 54615B). Measured lifetimes were reproducible with
deviations of Æ0.2 ns in solutions and Æ0.4 ns in solids.
Solutions for fluorescence experiments were deaerated by bubbling with
argon. Solid samples were prepared under argon.
[20] M. Hartmann, M. Räthe, Z. Chem. 1979, 19, 373.
[21] F. N. Hayes, B. S. Rogers, D. G. Ott, J. Am. Chem. Soc. 1955, 77, 1850.
[22] A. O. Doroshenko, Mol. Eng. 1994, 3, 353.
[23] E. M. Vernigor, V. K. Shalaev, E. A. Lukꢁyanets, Chem. Heterocycl.
Compd. (Engl. Transl.) 1981, 17, 328.
[24] H. Kamogawa, M. Nanasawa, S. Uehara, K. Osawa, Bull. Chem. Soc.
Jpn. 1979, 52, 533.
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[29] I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic
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Organic Molecules, VCH, New York, 1995.
Analyses: NMR spectra were recorded on a Bruker model DRX500
1
spectrometer at 500.13 MHz for H and at 124.77 MHz for 13C resonance.
2D NMR was employed to exclude equivocal assignments. Elemental
analyses were performed by the Institute of Organic Chemistry of the
Technical University of Dresden. Melting points are uncorrected.
Scintillation counting: A scintillation counter (model Tricarb 2550TR/LL)
and various scintillation cocktails were provided by Packard Instrument.
Counting efficiencies S (countsminÀ1/decaysminÀ1)were determined with
3
reference to H2O and to aqueous sodium [14C]benzoate solution contain-
ing trifluoroacetic acid (2 molLÀ1)in Ultima Gold cocktails (Packard
Instrument).
Compounds used as solid scintillators (PPO, 4a-4d, 5, 7, 8a ± 8c, 9a ± 9c)
were mixed with sodium dodecyl sulfate (a surfactant mediating wetting of
the organic solids)and sodium sulfate (a drying agent)in a ratio of 8:1:1.
Portions (400 mg)of these mixtures were mortared thoroughly and
transferred to a glass scintillation vial. The aqueous solution containing
the labeled probe (3H2O or sodium [14C]benzoate)(50 gL À1)was dropped
onto the powder. Scintillation counting was started 5 min later.
[34] T. Wolff, S. Weber, G. von Bünau, J. Photochem. Photobiol., A: Chem.
1990, 52, 157.
Each of the compounds 10a ± 10c (0.9 ± 1 g; the stoichiometric amount for
50 mL of water)used as chemically reactive scintillators was mixed with
sodium dodecyl sulfate (as a surfactant mediating wetting of the organic
solids; the drying agent was omitted)and treated as described above.
Counting efficiencies were reproducible within 5%.
[35] T. Wolff, K. Pfanner, C. Springob, J. Photochem. Photobiol., A: Chem.
1993, 74, 247.
[36] C. Springob, T. Wolff, J. Photochem. Photobiol., A: Chem. 1996, 101, 75.
Received: October 27, 1999 [F2106]
Chem. Eur. J. 2000, 6, No. 15
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