Chemiluminescence during thermolysis of the (Ph3COOCPh3)n-Ph3C. peroxide containing captured triphenylmethyl radical

Article abstract of DOI:10.1023/A:1014090303274

Chemiluminescence (CL) in the thermolysis of (Ph₃COOCPh₃)_n-Ph₃C^. containing the triphenylmethyl radical captured during the synthesis of Gomberg's peroxide was found. Two CL emitters were identified: the triplet state of benzophenone (³Ph₂CO*) and Ph₃C^.*. Ph₃C^.* is formed due to the energy transfer from the excited ³Ph₂CO* generated in the disproportion of thermolysis intermediates, Ph₃CO^. radicals. This Ph₃C^.* luminescence is the first example of CL activation by an organic radical. Chemiluminescence during the thermolysis of Ph₃COOCPh₃ containing no Ph₃C^. is resulted from the emission of one emitter, ³Ph₂CO*. The solid-phase CL was found during the oxidation of Ph₃C^. with dioxygen after the destruction of the crystalline lattice as a result of the thermolysis of the (Ph₃COOCPh₃)_n-Ph₃C^. peroxide.

Full text of DOI:10.1023/A:1014090303274

1194
Russian Chemical Bulletin, International Edition, Vol. 50, No. 7, pp. 1194—1197, July, 2001
•
Chemiluminescence during thermolysis of the (Ph3COOCPh3)n—Ph3C
peroxide containing captured triphenylmethyl radical-
R. G. Bulgakov,à« S. P. Kuleshov,à L. I. Sharapova,à R. À. Sadykov,à and S. L. Khursanb
aInstitute of Petrochemistry and Catalysis, Bashkortostan Republic Academy
of Sciences and Ufa Scientific Center of the Russian Academy of Sciences,
141 prosp. Oktyabrya, 450075 Ufa, Russian Federation.
Fax: +7 (347 2) 31 2750. E-mail: ink@anrb.ru
bBashkortostan State University,
32 ul. Frunze, 450074 Ufa, Russian Federation.
Fax: +7 (347 2) 23 6778
•
Chemiluminesñence (CL) in the thermolysis of (Ph3COOCPh3)n—Ph3C containing the
triphenylmethyl radical captured during the synthesis of Gomberg´s peroxide was found. Two
•
CL emitters were identified: the triplet state of benzophenone (3Ph2CO*) and Ph3C *.
•
3
Ph3C * is formed due to the energy transfer from the excited Ph2CO* generated in the
•
•
disproportion of thermolysis intermediates, Ph3CO radicals. This Ph3C * luminescence is
the first example of CL activation by an organic radical. Chemiluminescence during the
thermolysis of Ph3COOCPh3 containing no Ph3C is resulted from the emission of one
emitter, 3Ph2CO*. The solid-phase CL was found during the oxidation of Ph3C with
•
•
dioxygen after the destruction of the crystalline lattice as a result of the thermolysis of the
•
(Ph3COOCPh3)n—Ph3C peroxide.
Key words: chemiluminescence, photoluminescence, triphenylmethyl radical, di(triphenyl-
methyl) peroxide, thermolysis of peroxides, activation of chemiluminescence.
•
argon. A solution containing Ph3C and its dimer was sepa-
rated from Zn by centrifuging, after which the reactor with the
solution was connected to a burette with Î2. Oxidation was
Thermolysis of several organic peroxides is accompa-
nied by chemiluminescence (CL), whose emitters are
ketones in a triplet state.1 In the presence of activators,
organic molecules or luminescent metal complexes,2 CL
is enhanced due to energy transfer from the primarily
excited emitters to activators or via the mechanism of
chemically initiated electron-exchange luminescence.2—4
In this work, we studied the thermolysis of the peroxide
•
carried out until Î2 absorption ceased and Ph3C and the
dimer were completely consumed. The precipitate of 1 was
washed with anhydrous toluene. Compounds 1 and 2 were
characterized by elemental analysis data, PL and ESR spectra,
•
and melting temperatures. The amount of the Ph3C radical in
sample 1 was determined by the ESR method relatively to the
standard (the Mn2+ sample produced at the VNIIFTRI). The
thermolysis of 1 and 2 (0.048 mmol) was carried out in a quartz
CL cell in an argon atmosphere with the temperature control
by a thermocouple. The activation energy (Åà) of the thermoly-
sis of 1 was determined from the stepped plot of the CL
intensity vs. temperature.9 The thermolysis products, Ph2CO
and Ph3COPh, were identified by the phosphorescence (PS)
and IR spectra and mass spectrometry.
PL and ESR spectra were recorded on an Aminco-Bowman
fluorimeter and an SE/X-2542 Radiopan spectrometer.
CL spectra were measured on a setup described previously.6
IR spectra (KBr) were measured on a Specord IR-75 spectro-
photometer, and mass spectra were recorded an MI-1201V
mass spectrometer.
•
(Ph3COOCPh3)n—Ph3C (1) containing the captured
•
Ph3C radical and showed for the first time that the
•
organic Ph3C radical can act as the CL activator.
Experimental
A sample of Ph3CCl (purity grade) was three times recrys-
tallized from hexane. Toluene and hexane (reagent grade) were
dried by standard procedures.5 Argon was purified from Í2Î
and O2 according to a previously described procedure.6 Perox-
ides were synthesized by the oxidation with O2 of a solution of
•
Ph3C obtained by the known7 reaction of Ph3CCl with zinc
•
powder. The Ph3C radical was identified in a solution before
its oxidation by the ESR spectra and photoluminescence (PL).
•
Ph3COOCPh3 (2) containing no Ph3C was synthesized by a
Results and Discussion
continuous O2 feed to a solution of Ph3CCl (0.18 mmol) in
toluene (298 Ê) followed by the addition of Zn (1.56 mmol).
Precipitate 2 that formed was recrystallized similarly to a
procedure described previously.8 To synthesize peroxide 1
•
The PL spectrum of the obtained solution of Ph3C
(λmax/nm: 525, 550) coincides with the PL spectra of
•
containing Ph3C , we performed the reaction of Ph3CCl
•
solutions of Ph3C in toluene and THF recorded for the
(0.18 mmol) in toluene (298 Ê) with Zn (1.56 mmol) under
first time10,11 at 300 Ê. When these Ph3C solutions are
•
- Dedicated to the 100th anniversary of the discovery by
M. Gomberg of the first free radical.
frozen to 77 or 90 Ê, a short-wave shift in the PL
spectra (λmax/nm: 518, 545) is observed,11—14 as well as
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1138—1141, July, 2001.
1066-5285/01/5007-1194 $25.00 ©ꢀ2001 Plenum Publishing Corporation

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Russ.Chem.Bull., Int.Ed., Vol. 50, No. 7, July, 2001
1195
Emission of Ph3C * in thermolysis of Ph3COOCPh3
I (rel. units)
a
160
120
3
80
40
0.5 mT
4, 5, 6
2
b
1
520
540
560
λ/nm
•
•
Fig. 2. PL spectra of the Ph3C radical: 1, solution of Ph3C
obtained by the reaction of Ph3CCl (0.18 mmol) with Zn
(1.56 mmol) in toluene (300 Ê); 2 and 3, suspension of
peroxides in a solution immediately after precipitation; 4, 5,
and 6, solid samples of 1 after the action of toluene, saturated
Í2Î (1 h), and air (1 h) and boiling in toluene (1 h). Tempera-
ture of PL detection: 298 (1, 2, 4, 5, 6) and 77 K (3).
2 mT
•
Fig. 1. ESR spectra of the Ph3C radical (298 Ê): a, solution in
toluene (10 mol L–1); and b, solid sample of 1.
–4
•
in the PL spectra of the Ph3C radical prepared by the
suspension in toluene at 353 K) with O2 does not result
photolysis15 or radiolysis16 of solutions of Ph3CCl or
Ph3CH vitrified at 77 K.
•
in CL usually observed10 for the autooxidation of Ph3C
•
in a solution, which confirms the stabilization of Ph3C
•
The ESR spectrum of a solution of Ph3C in toluene
in the peroxide matrix. It is most likely that the high
•
(Fig. 1, a) is a well resolved multiplet with the HFC
constants ao-H(6 H) = 0.255 mT, ap-H(3 H) = 0.278 mT,
and am-H(6 H) = 0.111 mT, which agree with published
values.17 Radical 1 has characteristics typical8 of
Gomberg´s peroxide, m.p. 458—459 Ê. Found (%):
Ñ, 87.7; Í, 6.20; O, 6.10. C38H30O2. Calculated (%):
Ñ, 88.0; Í, 6.17; O, 5.83. IR, ν/cm–1: 975 and 758
(ΗÎ). However, unlike compound 2, sample 1 at
298 Ê exhibits an intense green PL and an ESR signal,
stability of Ph3C is related to lower diffusion coeffi-
cients of dioxygen and water in the crystalline sample of
•
1 compared to those in solutions of Ph3C .
•
The stability of Ph3C could be a consequence of
the adsorption of these radicals on the solid peroxide
surface. To check this possibility, we prepared sepa-
•
rately a solution of Ph3C and solid sample 2. Then
these components were thoroughly mixed and stored for
2 h under argon, after which the solid phase was sepa-
rated by centrifuging. It turned out that the reaction
product exhibits neither PL nor ESR signal. Thus, the
•
which is characteristic of Ph3C . The ESR spectrum
(Fig. 1, b) represents a singlet line with ∆H1/2 ≈ 1.1 mT
and g = 2.0027. The estimations of the number of
radicals captured by the crystalline lattice show that 1 g
•
stabilization of Ph3C occurs, most likely, during the
formation of the peroxide crystalline lattice. To explain
of sample 1 contains 4.8•1017 Ph3C or one radical per
•
•
the effect of Ph3C stabilization, we propose the follow-
•
ing hypothesis. As known,7 Ph3C in a solution exists in
1000 peroxide molecules. The PL spectra of compound
1 (λmax/nm: 518, 545) measured at 77 and 298 Ê are
identical (Fig. 2) and coincide with the PL spectra of
solutions of Ph3C frozen at 77 K in which the radical
exists in the crystalline matrices.
an equilibrium with molecular dimer.
•
Ph3C
•
2 Ph3C
CPh2
(1)
H
•
The Ph3C radical captured by the peroxide is more
Among main7 reactions of peroxide formation by
radical autooxidation (reactions (2) and (3)), reaction (3)
is of the most interest because the components of com-
pound 1 simultaneously form precisely in this reaction.
stable than that in a solution. For example, when O2 is
fed to a solution containing Ph3C and its dimer,
•
5—10 min after (depending on the concentration of
•
Ph3C and dimer) the PL and ESR signal from the
radical disappear completely. At the same time, the PL
and ESR spectra remain unchanged upon prolonged
(>1.5 years) storage of compound 1 in air or in toluene
saturated with Í2Î (see Fig. 2). Heating of 1 in boiling
toluene (1 h) under Ar does not either result in a
•
•
Ph3CÎÎ + Ph3C
Ph3COOCPh3
(2)
Ph3C
H
•
Ph3COO
CPh2
+
•
(3)
decrease in the PL brightness and escape of Ph3C to a
solution (see Fig. 2). The contact of compound 1 (or its
•
Ph3COOCPh3
+
Ph3C

1196
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 7, July, 2001
Bulgakov et al.
IFL (rel. units)
ICL (rel. units)
Taking into account the capability of peroxide radi-
cals of forming π-complexes with aromatic com-
pounds,18,19 we can suggest that reaction (3) includes
stages of the formation and decomposition of this
π-complex.
8
80
1
2
6
4
60
40
20
Ph3C
•
Ph3COO
+
CPh2
H
OOCPh
•
•
•
4
Ph3C
H
5
(4)
2
CPh2
3
•
Ph3COOCPh3 + Ph3C + (Ph3COOCPh3)n–1 (5)
430
480
530
λ/nm
•
Fig. 3. Luminescence spectra during thermolysis of peroxides:
chemiluminescence of samples 2 (1), 1 (2), PL of Ph2CO at
λexc = 313 nm (3); FL before (4) and after thermolysis of
compound 1 (5) at λexc = 340 nm. Temperatures of detection:
473 (1, 2), 77 (3, 5), and 298 K (4).
(Ph3COOCPh3)n—Ph3C
(6)
Most these complexes are decomposed in reaction
(5) to form 2 and Ph3C , which remains in a solution
and then participates in the chain radical autooxidation
of the dimer. However, a minor portion of the
π-complexes is involved into the crystallization of the
peroxide (reaction (6)) to afford compound 1.
•
during the thermolysis of sample 2 (I(m.p.) = 5.1•106
–1
•
photon s
mL–1) containing no Ph3C is somewhat
lower than the CL intensity during the thermolysis of 1,
and the CL spectrum (503 Ê) is presented by the
emission of only one emitter, 3Ph2CO* (see Fig. 3).
The activation energy (Åà) of the thermodecom-
On heating sample 1 in an Ar atmosphere to the
melting point (459 Ê), CL appeared (I(m.p.) = 5.4•106
–1
photon s
mL–1). After thermolysis at temperatures
higher than m.p. and a 80% decrease in the CL intensity
compound 1 was cooled (393 Ê) to the complete lumi-
nescence decay. Then an O2 flow was fed to the cell
filled with the solidified sample, and a new CL appeared
due to the autooxidation of Ph3C . Therefore, the
thermal decomposition of 1 results in a release of the
position of compound 1 determined from the tempera-
–1
ture plot of the CL intensity (Fig. 4) is 27±2 kcal mol
.
It is known22 that the O—O bond strength (ÅΗÎ) for
different symmetric dialkyl peroxides is almost the same
and equals 38 kcal mol–1. A decrease in Åà for
Ph3COOCPh3 can be resulted from the destabilization
of the peroxide due to the steric repulsion of the bulky
phenyl substituents. The thermochemical estimation of
the ÅΗΠvalue for 2 gives 30 kcal mol–1, which agrees
with the experimental Åà value. In calculations,
•
•
captured Ph3C and its oxidation with O2, which is
accompanied by CL. Note that this luminescence is the
first example of CL detection during the oxidation of an
organic radical in the solid phase.
Among the thermolysis products we identified ben-
zophenone by its characteristic20 PL (Fig. 3) and absorp-
tion band in the IR spectra at 1665 cm–1 and PhOCPh3
by the mass spectrum (EI, 70 eV), m/z (Irel (%)):
336 [M]+ (33), 259 [M – Ph]+ (100), 182 [M – 2 Ph]+
(14), 166 [M – 2 Ph – O]+ (21). The same products
were previously21 found in the study of the dark ther-
molysis of sample 2.
–1
•
∆Hf° = 70 kcal mol for the Ph3CO radical was taken
from published data,23 and ∆Hf° = 110 kcal mol for
–1
Ph3COOCPh3 was calculated by the method of additiv-
ity of thermochemical increments.24
•
ÅO—Î = 2∆Hf°(Ph3CO ) – ∆Hf°(Ph3COOCPh3) =
–1
=2•70 – 110 = 30 (kcal mol
)
Since the luminescence intensity is weak, the CL
spectrum measured during the thermolysis of 1 (503 Ê)
using a set of boundary light filters is arranged in the
region of 400—600 nm (see Fig. 3). The short-wave
region of the CL spectrum, the PL spectrum (77 K) of
the thermolyzed sample 1, and the PL spectrum (77 Ê)
of benzophenone are arranged in the same region of
400—500 nm, i.e., the triplet state of benzophenone
3Ph2CO* is a CL emitter during the thermolysis
of 1. The long-wave component of the CL spectrum
(500—600 nm), which makes a great contribution to the
emission, lies in the PL region of the radical and is
lnI
6
5
–1
–3
2.10
2.15
T
•10
Fig 4. Plot of the logarithm of the CL intensity (lnI) vs.
temperature during thermolysis of compound 1.
•
resulted from the emission of Ph3C *. The CL intensity

•
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 7, July, 2001
1197
Emission of Ph3C * in thermolysis of Ph3COOCPh3
Based on the experimental results and published data
on CL during the thermolysis of other peroxides,1,25 we
can propose the mechanism of CL appearance during
the thermolysis of 1 (Scheme 1).
References
1. R. F. Vasil´ev, Kinet. Katal., 1999, 40, 192 [Kinet. Catal.,
1999, 40 (Engl. Transl.)].
2. K. D. Gundermann and F. McCapra, Chemiluminescence in
Organic Chemistry, Springer-Verlag, Berlin—Heidelberg—
New York—London—Paris—Tokyo, 1987, 23, 209 pp.
3. G. B. Schuster, Acc. Chem. Res., 1979, 12, 366.
4. G. L. Sharipov, V. P. Kazakov, and G. A. Tolstikov,
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cow, 1990, 288 pp. (in Russian).
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don, 1955, 290 pp.
Scheme 1
•
Ph3COOCPh3)n—Ph3C
[(Ph3COOCPh3)n–1
+
•
•
•
+ (Ph3CO ... OCPh3)...Ph3C ]
•
[(Ph3COOCPh3)n–1, PhOCPh3, Ph2C=O*, Ph3C ]
•
(Ph3COOCPh3)n–1, PhOCPh3, Ph2C=O, Ph3C
+
+ hν (400–500 nm)
6. R. G. Bulgakov, G. Ya. Maistrenko, B. A. Tishin, G. A.
Tolstikov, and V. P. Kazakov, Dokl. Akad. Nauk SSSR,
1989, 304, 1166 [Dokl. Chem., 1989 (Engl. Transl.)].
7. W. A. Waters, The Chemistry of Free Radicals, Fellow of
Balliol College, Oxford, 1946, 320 pp.
•
[(Ph3COCPh3)n–1, PhOCPh3, Ph2C=O, Ph3C *]
•
(Ph3COOCPh3)n–1, PhOCPh3, Ph2C=O, Ph3C
+
8. J. Tanaka, J. Org. Chem., 1961, 26, 4203.
+ hν (500–600 nm)
9. R. F. Vasil´ev, O. N. Karpukhin, and V. Ya. Shlyapintokh,
Dokl. Akad. Nauk SSSR, 1959, 125, 100 [Dokl. Chem., 1959
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11. R. G. Bulgakov, V. P. Kazakov, and G. A. Tolstikov,
J. Organomet. Chem., 1990, 387, 11.
It is known25 that the disproportionation of the tertiary
oxyl radicals R3CO in solutions occurs according to
•
the reaction
•
•
R3CO + R3CO
R3COR + R2CO,
(7)
and, as shown,25 excited ketones can also be formed. In
the peroxide melt, due to a lower mobility than that in a
solution, the probability of this reaction is likely higher
12. G. M. Lewis, D. Lipkin, and T. T. Magel, J. Am. Chem.
Soc., 1994, 66, 1579.
13. V. A. Smirnov and V. G. Plotnikov, Usp. Khim., 1986, 10,
1633 [Russ. Chem. Rev., 1986, 10 (Engl. Trasnl.)].
14. M. Ya. Mel´nikov and V. Ya. Smirnov, Fotokhimiya
organichenskikh radikalov [Photochemistry of Organic Radi-
cals], Izd-vo MGU, 1994, 335 (in Russian).
15. D. N. Shigorin, Yu. I. Kozlov, Optika i Spektroskopiya,
1961, 10, 600 [Opt. Spectr., 1961, 10 (Engl. Transl.)].
16. T. Izumida, Y. Tanabe, T. Ichikawa, and H. Yoshida, Bull.
Chem. Soc. Jpn., 1979, 52, 23514.
17. V. A. Rodionov and E. G. Rozantsev, Dolgozhivushchie
Radikaly [Long-lived Radicals], Nauka, Moscow, 1972,
198 pp. (in Russian).
18. V. G. Koloerova, A. N. Nikolaevskii, R. V. Kucher, and
T. A. Batrak, Dokl. Akad. Nauk SSSR, 1978, 242, 641
[Dokl. Chem., 1978 (Engl. Transl.)].
19. S. A. Khursan and R. L. Safiullin, Khim. Fiz., 1991, 10,
1625 [Russ. J. Chem. Phys., 1991, 10 (Engl. Transl.)].
20. J. A. Barltrop and D. Coyle, Excited States in Organic
Chemistry, J. Wiley and Sons, London—New York—
Sydney—Toronto, 1975, 418 pp.
21. J. Schmidlin and P. Hodgson, Chem. Ber., 1910, 43, 1152.
22. S. A. Khursan and V. V. Shereshovets, Kinet. Katal., 1999,
40, 167 [Kinet. Catal., 1999, 40 (Engl. Transl.)].
23. Yu. D. Orlov and Yu. A. Lebedev, Zh. Fiz. Khim., 1991, 65,
289 [Russ. J. Phys. Chem., 1991, 65 (Engl. Transl.)].
24. S. W. Benson, Thermochemical Kinetics, J. Wiley and Sons,
New York—London, 1976.
•
than that of the reaction of R3CO with other partici-
pants of the process. The calculation of the thermal
•
effect (∆H°) of the disproportional of the Ph3CO radi-
cals (see Scheme 1) using previously published data23,26
gives the value
•
∆H° = ∆Hf°(Ph3COPh) + ∆Hf°(Ph2CO) – 2∆Hf°(Ph3CO ) =
= 52.1 + 12 – 2•70 = –75.9 (kcal mol–1).
This value agrees well with the results in Ref. 1,
according to which the thermal effects of R3CO dis-
proportionation for the phenyl and alkyl derivatives,
including cyclic derivatives, differ slightly and range
within 75—82 kcal mol–1. Thus, the energy reserve of
•
•
the disproportionation of the Ph3CO radicals is suffi-
cient for the population of the triplet level of benzophe-
none (72 kcal mol–1). The radiative deactivation of
3Ph2CO* results in phosphorescence in the region of
400—500 nm (see Scheme 1). It is most likely that
Ph3C is excited due to the energy transfer to Ph3C .
The energy transfer to an acceptor decreases the lumi-
nescence intensity of a donor2 and, as can be seen in
Fig. 3, the intensity of the benzophenone component of
the CL spectrum during the thermolysis of 1 is lower
than that for 2.
•
•
25. R. F. Vasil´ev, Usp. Khim., 1970, 39, 1130 [Russ. Chem.
Rev., 1970, 39 (Engl. Transl.)].
26. NIST Standard Reference Database 19A, Version 2.02,
Gaithersburg, National Institute of Standard and Technol-
Chemiluminescence during the thermolysis of 1 is
the first example of the activation of chemical reaction
luminescence due to the energy transfer from an excited
organic molecule to a free radical.
ogy, 1994.
Received November 9, 2000;
in revised form March 12, 2001