Laser Flash Photolysis of tert-Butyl Aroylperbenzoates: Kinetics of the Singlet and Triplet States and the Aroylphenyl Radicals

Article abstract of DOI:10.1021/ja030231g

tert-Butyl aroylperbenzoates (1-4) were studied by laser flash photolysis (LFP). LFP (380 nm, pulse width ～350 fs) of 2 and 3 allowed direct observation of their singlet states, which showed broad absorption (λ_max ～ 625 nm; τ ～ 20 and ～7.9 ps, respectively). The triplet state of each (λ_max ～ 530-560 nm) rapidly dissociates by O-O cleavage as indicated by the short triplet lifetimes (e.g., triplet lifetime of 3 ～0.74 ns). The ～550 nm absorption obtained from the 355 nm LFP (pulse width ～7 ns) of 1, 2, and 4 has been assigned to the corresponding aroylphenyl radicals. Two representative radicals (4-benzoylphenyl 5 and 3-(4′-methylbenzoyl)phenyl 6) investigated in detail showed solvent-dependent lifetimes. Absolute bimolecular rate constants of reactions of these radicals with various quenchers including double-bond-containing monomers have been observed to range from 7.56 × 10⁷ to 1.68 × 10⁹ M^-1 s^-1 in CCl₄ at room temperature. A possible structure of the aroylphenyl radicals and the transition responsible for the 550 nm absorption are discussed.

Full text of DOI:10.1021/ja030231g

Published on Web 01/27/2004
Laser Flash Photolysis of tert-Butyl Aroylperbenzoates:
Kinetics of the Singlet and Triplet States and the Aroylphenyl
Radicals1
Bipin K. Shah and Douglas C. Neckers*
Contribution from the Center for Photochemical Sciences, Bowling Green State UniVersity,
Bowling Green, Ohio 43403
Received April 15, 2003; E-mail: neckers@photo.bgsu.edu
Abstract: tert-Butyl aroylperbenzoates (1-4) were studied by laser flash photolysis (LFP). LFP (380 nm,
pulse width ∼350 fs) of 2 and 3 allowed direct observation of their singlet states, which showed broad
absorption (λmax ∼ 625 nm; τ ∼ 20 and ∼7.9 ps, respectively). The triplet state of each (λmax ∼ 530-560
nm) rapidly dissociates by O-O cleavage as indicated by the short triplet lifetimes (e.g., triplet lifetime of
3 ∼0.74 ns). The ∼550 nm absorption obtained from the 355 nm LFP (pulse width ∼7 ns) of 1, 2, and 4
has been assigned to the corresponding aroylphenyl radicals. Two representative radicals (4-benzoylphenyl
5 and 3-(4′-methylbenzoyl)phenyl 6) investigated in detail showed solvent-dependent lifetimes. Absolute
bimolecular rate constants of reactions of these radicals with various quenchers including double-bond-
containing monomers have been observed to range from 7.56 × 107 to 1.68 × 109 M-1 s-1 in CCl4 at room
temperature. A possible structure of the aroylphenyl radicals and the transition responsible for the 550 nm
absorption are discussed.
nm (n f π* transition).4,5 These radicals have been found to
Introduction
initiate polymerization of double-bond-containing monomers4,6
Diaroyl peroxides and peroxyesters are important thermal
initiators for the formation of vinyl polymers in a number of
commodity applications. For instance, most commercial poly-
styrene is made from styrene monomer with benzoyl peroxide
as the initiator in a thermal reaction. Peroxide initiators also
stimulate the free radical reactions that provide relatively high
molecular weight polymers from many other vinyl monomers
including butadiene and acrylonitrile.2 Photoinitiators for vinyl
polymerization, in contrast, almost always differ from their
thermal counterparts. Bond homolysis initiation of monomer
to polymer conversions by photochemical processes most often
involve aromatic ketones that initiate chains either via radicals
produced from a Norrish type I cleavage or by bimolecular
electron-transfer processes that also produce chain-initiating
radicals.3 Save for those reported here, there are virtually no
commercially employed photoinitiators that are also peroxides,
either diaroyl or dialkyl, or peresters. Thus, the work below
was initially undertaken to develop methodology to produce
radical intermediates directly by the photodecomposition of
selected peresters.
and to cause photo-cross-linking and photografting.7,8 Rapid
intramolecular triplet energy dissipation accounts for the efficient
dissociation of the perester bond and the attendant high quantum
yields of cleavage. Laser flash photolysis (LFP) of tert-butyl
(4-benzoyl)perbenzoate [1] revealed a relatively short-lived
triplet state (∼0.87 ns).9 The following steps are, in principle,
involved in the photodissociation of BPs:
ArCOPhCOOOCMe3 + hν f 1[BP]*
1[BP]* f 3[BP]* f ArCOPhCOO• + •OCMe3
ArCOPhCOO• f ArCOPh• + CO2
Reports dealing with the kinetics of the singlet and/or triplet
states of organic peresters are limited.9,10 Furthermore, though
the rate of intersystem crossing (ISC) of benzophenone is often
quoted (∼3.33 × 1010 s-1 measured in aromatic solvents such
(4) Thijs, L.; Gupta, S. N.; Neckers, D. C. J. Org. Chem. 1979, 44, 4123.
(5) Shah, B. K.; Neckers, D. C. J. Org. Chem. 2002, 67, 6117.
(6) Allen, N. S.; Hardy, S. J.; Jacobine, A. F.; Glaser, D. M.; Yang, B.; Wolf,
D.; Catalina, F.; Navaratnam, S.; Parsons, B. J. J. Appl. Polym. Sci. 1991,
42, 1169.
(7) (a) Gupta, S. N.; Thijs, L.; Neckers, D. C. J. Polym. Sci., Polym. Chem.
Ed. 1981, 19, 855. (b) Gupta, I.; Gupta, S. N.; Neckers, D. C. J. Polym.
Sci., Polym. Chem. Ed. 1982, 20, 147.
tert-Butyl aroylperbenzoates (BP) efficiently produce aroyl-
benzoyloxyl and aroylphenyl radicals when irradiated at ∼355
* To whom correspondence should be addressed: Telephone (419) 372-
2034; fax (419) 372-0366.
(1) Contribution 481 from the Center for Photochemical Sciences.
(2) Kirchgessner, R. J.; Kamath, V. R.; Sheppard, C. S.; Stromberg, S. E. Mol.
Plastics 1984, 61, 66.
(3) (a) Roffey, C. G. Photopolymerization of Surface Coatings; John Wiley &
Sons: New York, 1982; p 67. (b) Lai, Y.; Quinn, E. T. Photopolymerization
Fundamentals and Applications; Scranton, A. B., Bowman, C. N., Peiffer,
R. W., Eds.; ACS Symposium Series 673; American Chemical Society:
Washington, DC, 1997; p 35.
(8) (a) Neckers, D. U.S. Patent 4,754,659, 1988. (b) Takeshi, K.; Mamoru, S.
Eur. Patent 126541 A1, 1984. (c) Neckers, D. C. U.S. Patent 4,498,963,
1983.
(9) Morlino, E. A.; Bohorquez, M. D.; Neckers, D. C.; Rodgers, M. A. J. J.
Am. Chem. Soc. 1991, 113, 3599.
(10) Krongauz, V. V., Trifunac, A. D., Eds. Processes in PhotoreactiVe
Polymers; Chapman & Hill: New York, 1995; p 72.
9
1830
J. AM. CHEM. SOC. 2004, 126, 1830-1835
10.1021/ja030231g CCC: $27.50 © 2004 American Chemical Society

Laser Flash Photolysis of tert-Butyl Aroylperbenzoates
A R T I C L E S
Chart 1
demonstrating the virtue of attaching a chromophore to an
otherwise difficult to observe transient species.4,20 We also report
here the kinetics of decay processes of the 4-benzoylphenyl and
3-(4′-methylbenzoyl)phenyl radicals obtained by the 355 nm
LFP of 1 and 4.
Experimental Section
Chemicals and solvents required were obtained from commercial
suppliers. Compounds 1-3 were synthesized as previously reported,4,5
and 4 was synthesized from methyl 3-(4′-methylbenzoyl)benzoate that
had been itself synthesized by a modified literature method21 (see
Supporting Information). Solvents used for LFP experiments were either
spectrometric-grade or distilled over benzophenone/sodium as required.
The laser system and apparatus for the ultrafast transient absorption
spectroscopy experiments has been described elsewhere.22 It included
a Ti:sapphire seed laser pumped with a diode-pumped 5 W cw Nd:
YAG laser. The excitation pulse (380 nm) was the second harmonic
of the tuned fundamental frequency (760 nm). The energy of the pump
beam was arranged to be ∼150 µJ/pulse. Typically, 500 excitation
pulses were averaged to obtain the transient spectrum at a particular
delay time. In-house LabVIEW (National Instruments) software allowed
automatic spectral acquisition over a series of delay line settings. Kinetic
traces at appropriate wavelengths were assembled from the accumulated
spectral data. The instrument rise time was ∼350 fs. Sample solutions
were prepared to have an absorbance of 0.7-0.8 at the excitation
wavelength in the 2 mm flow cell and were used without deaeration.
Nanosecond flash photolysis studies were performed on a kinetic
spectrometric detection system previously described.23 The excitation
pulse (355 nm, 12-19 mJ/pulse) was the third harmonic of a
Q-switched Nd:YAG laser. The excitation pulse width was ∼7 ns.
Transients produced were followed temporally and spectrally by a
computer-controlled kinetic spectrophotometer. The sample solutions
showing an absorbance in the range of 0.15-0.33 at the excitation
wavelength in 1-cm2 quartz cuvettes were used and degassed continu-
ously with argon during experiments. Fresh samples were used for
obtaining each kinetic trace. All measurements have been made at room
temperature.
as benzene)11 and multitudes of data are available on the triplet
state of benzophenones,11,12 there are limited reports on the
spectroscopic observation of the singlet state of benzophenones.
We recently carried out LFP studies of 1, tert-butyl 4-(4′-
methylbenzoyl)perbenzoate [2], tert-butyl 4-(4′-bromomethyl-
benzoyl)perbenzoate [3], and tert-butyl 3-(4′-methylbenzoyl)-
perbenzoate [4] (Chart 1). Transient spectroscopy provided
insights into the kinetics of their photophysical and photochemi-
cal processes. LFP at 380 nm (pulse width ∼350 fs) of 3 allowed
direct observation of its singlet state. We report here the kinetics
of decay of the singlet and triplet states of 2 and 3.
Photoinduced peroxy bond fission and the decarboxylation
of the aroyloxyl radical so obtained have been studied in detail
and different rates of peroxy bond fission have been obtained
for various perester systems.13 Detailed spectroscopic studies
of photochemically generated aroyloxyl radicals and their
absolute rate constants have also been reported.14 However, the
same cannot be said of phenyl radicals. A detailed study of the
relative reactivities of the thermally produced phenyl radical
toward a number of substrates has been reported earlier.15
Because of their weak UV-visible signals in solution at ambient
temperature, indirect approaches have been applied to obtain
absolute rates of reactions of the phenyl radical in solution.16
Their UV-vis,17 IR,18 and Raman19 spectra have, however, been
reported and interpreted in the gas phase and in an argon matrix
at low temperatures.
Results and Discussion
Absorption and Kinetics of Singlet and Triplet States. LFP
of 2 in benzene at room temperature (380 nm excitation, pulse
width ∼350 fs) resulted in a broad absorption having a
maximum at ∼560 nm (Figure 1). The triplet states of
benzophenones have been known to show absorption at ∼530
nm.12a,c A similar 560 nm transient absorption obtained from 1
has been assigned to its triplet state.9 Thus, the 560 nm transient
obtained from 2 can be assigned to its triplet state, the rise time
of which was found to be ∼20 ps.
This work first shows that aroyl substitution conveys a strong
and easily detectable mid-UV absorption to phenyl radicals,
(11) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
Marcel Dekker: New York, 1993; p 16.
(12) (a) Godfrey, T. S.; Hilpern, J. W.; Porter, G. Chem. Phys. Lett. 1967, 1,
490. (b) Rentzepis, P. M. Science 1970, 169, 239. (c) Hochstrasser, R. M.;
Lutz, H.; Scott, G. W. Chem. Phys. Lett. 1974, 24, 162. (d) McGarry, P.
F.; Doubleday, C. E., Jr.; Wu, C.; Staab, H. A.; Turro, N. J. J. Photochem.
Photobiol. A 1994, 77, 109. (e) Bhasikuttan, A. C.; Singh, A. K.; Palit, D.
K.; Sapre, A. V.; Mittal, J. P. J. Phys. Chem. A 1998, 102, 3470.
(13) (a) Falvey, D. E.; Schuster, G. B. J. Am. Chem. Soc. 1986, 108, 7419. (b)
Allen, N. S.; Hardy, S. J.; Jacobine, A. F.; Glaser, D. M.; Navaratnam, S.;
Parsons, B. J. J. Photochem. Photobiol. A 1990, 50, 389. (c) Aschenbru¨cker,
J.; Buback, M.; Ernsting, N. P.; Schroeder, J.; Steegmu¨ller. U. J. Phys.
Chem. B 1998, 102, 5552 and references therein.
In fact, time-resolved transient absorption spectra of 2 indicate
that, in the early time (∼2 ps), the broad absorption has a
maximum at ∼625 nm that later shifts to ∼560 nm. This
suggests that there may be two species involved, though it is
not that clear in the case of 2. An attempt to extract kinetic
(14) (a) Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J. Am. Chem. Soc. 1988,
110, 2877. (b) Chateauneuf, J.; Lusztyk, J.; Ingold, K. U. J. Am. Chem.
Soc. 1988, 110, 2886.
(20) (a) Abu-Abdoun, I. I.; Thijs, L. B.; Neckers, D. C. Macromolecules 1984,
17, 282. (b) Gottschalk, P.; Neckers, D. C. J. Org. Chem. 1985, 50, 3498.
(c) Valdes-Aguilera, O.; Rajadurai, S.; Linden, S. M.; Zakrzewski, A.;
Neckers, D. C. Tetrahedron Lett. 1988, 29, 5109. (d) Linden, S. M.;
Williams, B. L.; Zakrewski, A.; Neckers, D. C. J. Org. Chem. 1989, 54,
131. (e) Neckers, D. C.; Tsuchiya, M.; Raghuveer, K. S.; Valdes-Aguilera,
O. M. Tetrahedron Lett. 1990, 31, 5143.
(21) (a) Murakami, Y.; Hara, H.; Okada, T.; Hashizume, H.; Kii, M.; Ishihara,
Y.; Ishikawa, M.; Shimamura, M.; Mihara, S.; Kato, G.; Hanasaki, K.;
Hagishita, S.; Fujimoto, M. J. Med. Chem. 1999, 42, 2621. (b) Smith, M.
E. J. Am. Chem. Soc. 1922, 43, 1921.
(15) Bridger, R. F.; Russell, G. A. J. Am. Chem. Soc. 1963, 85, 3754.
(16) Scaiano, J. C.; Stewart, L. C. J. Am. Chem. Soc. 1983, 105, 3609.
(17) (a) Dick, B.; Engert, J. M. Appl. Phys. B 1996, 63, 531. (b) Wallington, T.
J.; Egsgaard, H.; Nielsen, O. J.; Platz, J.; Sehested, J.; Stein, T. Chem.
Phys. Lett. 1998, 290, 363. (c) Radziszewski, J. G. Chem. Phys. Lett. 1999,
301, 565. (d) Tonokura, K.; Norikane, Y.; Koshi, M.; Nakano, Y.;
Nakamichi, S. Goto, M.; Hashimoto, S.; Kawasaki, M.; Andersen, M. P.
S.; Hurley, M. D.; Wallington, T. J. J. Phys. Chem. A 2002, 106, 5908.
(18) Friderichsen, A. V.; Radziszewski, J. G.; Nimlos, M. R.; Winter, P. R.;
Dayton, D. C.; David, D. E.; Ellison, G. B. J. Am. Chem. Soc. 2001, 123,
1977.
(22) Nikolaitchik, A. V.; Korth, O.; Rodgers, M. A. J. J. Phys. Chem. A 1999,
103, 7587.
(23) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98, 3822.
(19) Lapinski, A.; Spanget-Larsen, J.; Langgard, M.; Waluk, J.; Radziszewski,
J. G. J. Phys. Chem. A 2001, 105, 10520.
9
J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004 1831

A R T I C L E S
Shah and Neckers
Figure 1. Transient absorption spectra obtained from the 380 nm laser
flash photolysis of 2 in benzene, recorded (blue) 2 ps, (red) 50 ps, and
(green) 300 ps after the laser pulse. Inset: kinetic trace (triplet growth)
monitored at 570 nm.
Figure 3. Kinetic traces showing growth and decay of the singlet and triplet
states of 3. (a) Decay of the singlet state monitored at 675 nm. (b) Growth
of the triplet state monitored at 525 nm. (c) Decay of the triplet state
monitored at 475 nm.
heavy atom-enhanced ISC (S1 f T1) in 3 that contains a
bromine atom.26
Figure 2. Transient absorption spectra obtained from the 380 nm laser
flash photolysis of 2 in benzene, recorded (blue) 2 ps, (red) 20 ps, and
(green) 1200 ps after the laser pulse.
The steady-state photolysis of the BPs has revealed that the
corresponding aroylbenzoic acid and benzophenones are always
the major products in a H-donating solvent like methanol or
cyclohexane.4,5 When benzene was used as the solvent, adduct
products such as ArCOPh-Ph and ArCOPh-COOPh were also
observed in addition.5 Biphenyl, phenyl tert-butyl ether, acetone,
and tert-butyl alcohol were observed as minor products.5 In the
case of CCl4 as the solvent, the major product, however, was
found to be the corresponding chlorobenzophenones. The decay
of the triplet state of BPs, thus, represents dissociation of the
perester bond and the resulting radicals give rise to different
photoproducts. Both concerted (simultaneous scission of the
O-O and R-C bonds) and stepwise (scission of the O-O bond
followed by scission of the R-C bond) mechanisms have been
reported and debated for photodissociation of peresters.6,13c,14b,27
Thus, the triplet state of BPs can dissociate either to the
aroylbenzoyloxyl radical or to a combination of the aroylben-
zoyloxyl and aroylphenyl radicals.
data at ∼625 nm failed because of the weak signal. However,
under similar experimental conditions, LFP of 3 resulted in two
clearly distinguishable transients having absorption maxima at
∼530 and ∼625 nm (Figure 2). The 530 nm transient can be
unequivocally assigned to the triplet state of 3 and its lifetime
was found to be ∼0.74 ns.
The 625 nm transient from 3 appears instantly after the laser
pulse excitation and decays rapidly. Since the singlet state of
benzophenone has been suggested to have absorption at ∼580
nm, at a higher wavelength than that at which its triplet state
absorbs,24 we assign the 625 nm absorption to the singlet state
of 3.25 Conclusive evidence for this assignment comes from
kinetic analysis. Decay of the 625 nm transient and the rise
time of the triplet state of 3 were found to be the same (∼7 ps)
within the error range (Figure 3). Thus, it is clear that the 625
nm transient is the precursor of the 530 nm transient (the triplet
state). The rate of ISC increases from 4.93 × 1010 s-1 to 1.27
× 1011 s-1 going from 2 to 3. This can be attributed to the
In both 2 and 3, the absorption at ∼530-560 nm does not
go to the baseline over the data accumulation time (1.4 ns).
This likely indicates the triplet state produces a species that also
absorbs at these wavelengths. In our case we cannot distinguish
whether the residual absorption at ∼560 nm is due to the
(24) (a) Greene, B. I.; Hochstrasser, R. M.; Weisman, R. B. J. Phys. Chem.
1979, 70, 1247. (b) Miyasaka, H.; Morita, K.; Kamada, K.; Mataga, N.
Bull. Chem. Soc. Jpn. 1990, 63, 131. (c) Tamai, N.; Asahi, T.; Masuhara,
H. Chem. Phys. Lett. 1992, 198, 413.
(25) After the submission of this paper, it came to our attention that similar
absorption of the singlet state of benzophenone in water has been observed
by Ramseier et al. (J. Phys. Chem. A 2003, 107, 3305).
(26) Basu, G.; Kunasik, M.; Anglos, D.; Secor, B.; Kuki, A. J. Am. Chem. Soc.
1990, 112, 9410.
(27) (a) Bevington, J. C.; Lewis, T. D. Trans. Faraday Soc. 1958, 54, 1340. (b)
Koenig, T.; Hoobler, J. A. Tetrahedron Lett. 1972, 1803.
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1832 J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004

Laser Flash Photolysis of tert-Butyl Aroylperbenzoates
A R T I C L E S
p-iodobenzophenone was photolyzed in benzene, the well-
established absorption of the iodine-benzene complex16 having
a maximum at ∼470 nm was obtained in addition to the 550
nm absorption; (d) the 550 nm transients were quenched by a
H-atom-donating quencher like 1,4-cyclohexadiene and an
analysis of the reaction mixture revealed formation of an
increased amount of products formed by hydrogen abstraction
(corresponding benzophenones); and (e) the corresponding
chlorobenzophenones were the major photoproducts in all cases,
indicating chlorine abstraction by the aroylphenyl radicals from
the solvent.
Aroylbenzoyloxyl, tert-butoxyl, and trichloromethyl radicals
are also formed in the reaction, though none contributes to the
550 nm absorption. Aroylbenzoyloxyl radicals are not the
absorbing species because the 550 nm absorption is also
observed from p-iodobenzophenone, a compound that cannot
produce the benzoylbenzoyloxyl radical.31 The tert-butoxyl
radical absorption at 320 nm is neglible.32 Similar absorptions
were obtained during LFP of 2 in either benzene or CCl4, ruling
out contribution by any solvent-derived radicals such as the
ArCOC6H4C6H6• adduct radical5,33 or the trichloromethyl radical.
Similar spectra were also obtained from 4 in both CCl4 and
acetonitrile. There were no differences in the spectra recorded
on either flowing or static samples, indicating that there is no
involvement of any product of a secondary photoreaction.
Figure 4. (a) Transient absorption spectra obtained from laser flash
photolysis of 1 (1.18 × 10-3 M) in CCl4, recorded (b) 20 ns and (O) 2 µs
after the laser pulse. Inset: kinetic trace monitored at 550 nm. (b) Transient
absorption spectra obtained by laser flash photolysis of p-iodobenzophenone
(1.15 × 10-3 M) in CCl4, recorded (b) 20 ns and (O) 2 µs after the laser
pulse. (Excitation at 355 nm.)
The triplet states of benzophenones have also long been
known to abstract available H-atoms, forming ketyl radicals that
show absorption at ∼535 nm.34 However, since the lifetimes
of the triplet state of these BPs are in the picosecond regime,
formation of a ketyl-type radical seems unlikely. The solvent
(CCl4) also cannot furnish such an H-atom. In fact, intramo-
lecular triplet energy transfer causing scission of the perester
bond is much faster than are the bimolecular rates of H-
abstraction by the triplet states of benzophenones.35 Conse-
quently, no ketyl radical-derived product was observed when
BPs were irradiated at 350 nm in either benzene or methanol.5
The estimated lifetime of the triplet state of p-iodobenzophenone
is ∼0.2 ns, and no ketyl radical-derived product was observed
during its photolysis either, even when an H-donor solvent like
cyclopentane was used.36,37 This along with the picosecond
lifetime of the triplet state rules out the 550 nm transient being
a ketyl-type radical.
aroylbenzoyloxyl or the aroylphenyl radical. Nevertheless, after
∼7 ns of laser excitation (355 nm), we observe transient
absorption having a maximum at ∼550 nm that is convincingly
due to the aroylphenyl radicals (vide infra). Thus, if these BPs
undergo stepwise decomposition, decarboxylation seems to take
place between ∼1 and ∼7 ns.28
Characterization and Kinetics of Aroylphenyl Radicals.
Two absorption bands having maxima at 320 and 550 nm were
obtained from each of 1, 2, and 4 when they were laser flash
photolyzed at room temperature in CCl4 with 355 nm excitation
(pulse width ∼7 ns). The 550 nm transient showed a single-
exponential decay in each case and has been assigned to the
corresponding aroylphenyl radical on the basis of the following
observations: (a) similar 550 nm transients were obtained during
LFP of 1 and p-iodobenzophenone in CCl4 (Figure 4);29 (b) the
550 nm transients were chemically quenched by oxygen,
indicating they were carbon-centered radicals;30 (c) when
(28) The aroyloxyl radicals are known to show absorption in the 600-800 nm
range and their reported lifetimes are 0.2-0.5 µs (ref 14). However, we
observed no absorption in that range during our experiments, and attempts
to observe the aroylbenzoyloxyl radicals were unsuccessful. Ingold and
co-workers (ref 14b) had studied a single aroylbenzoyl peroxide and, as
was the case with the system we report in this paper, saw no aroylben-
zoyloxyl radical absorption. One of the reviewers has pointed out that two-
photon absorption could be responsible for extremely fast decarboxylation
of the aroylbenzoyloxyl radicals, and we believe this could be the case.
The aroyloxyl radicals are known to decarboxylate faster, leading to the
formation of the aryl radicals when they absorb at either 308 or 700 nm
(ref 14a). It has also been shown that the benzoyloxyl radical that has been
stabilized by preparation at low temperatures (67 K) can be photodecar-
boxylated with light of 550-1300 nm (Karch, N. J.; Koh, E. T.; Whitsel,
B. L.; McBride, J. M. J. Am. Chem. Soc. 1975, 97, 6729).
(29) In optically matched experiments, the lifetimes of the 550 nm transient
obtained from both 1 and p-iodobenzophenone were found to be same
within experimental error. The initial OD obtained in the case of
p-iodobenzophenone was, however, much smaller than in the case of 1.
The quenching rate constant of the benzoylphenyl radical with p-
iodobenzophenone was found to be 1.7 × 109 M-1 s-1 (the error involved
in this measurement is large due to the low OD at 550 nm).
(31) The only common radical that can be obtained from both 1 and p-
iodobenzophenone is the 4-benzoylphenyl radical. The similar ∼560 nm
absorption observed previously during nanosecond LFP of 1 was tentatively
assigned to the 4-benzoylbenzoyloxyl radical on the basis of the resemblance
of its lifetime to that of the benzoyloxyl radical (ref 14). However, the
lifetimes of the aroylphenyl radicals have also been found to be in a similar
range (∼0.4 µs) and depend on the solvent and concentration of the
precursor BPs. The evidence presented in the text clearly suggests that the
550 nm absorption is due to the aroylphenyl radicals.
(32) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J. C.; Woynar, H. J. Am. Chem.
Soc. 1981, 103, 3231.
(33) Griller, D.; Marriott, P. R.; Nonhebel, D. C.; Perkins, M. J.; Wong, P. C.
J. Am. Chem. Soc. 1981, 103, 7761.
(34) (a) Hammond, G. S.; Baker, W. P.; Moore, W. M. J. Am. Chem. Soc. 1961,
83, 2795. (b) Bell, J. A.; Linschitz, H. J. Am. Chem. Soc. 1963, 85, 528.
(c) Topp, M. R. Chem. Phys. Lett. 1975, 32, 144.
(35) The possibility that a bimolecular rate constant of H-abstraction by the
triplet state of the BPs in the concentration range as low as 8 × 10-4 mol/L
could be 109 M-1 s-1 or faster seems to be very unlikely. The rates of
reactions of the triplet state of benzophenone with benzene and toluene
have been found to be 0.8 × 104 and 2 × 104 M-1 s-1, respectively (ref
34a).
(30) Since the triplet states are also quenched by oxygen, the 550 nm absorption
might be mistakenly assigned to the triplet state of BPs, as was done by
Allen et al. (ref 6), who assigned the 550 nm absorption obtained from the
nanosecond LFP of 1 in acetonitrile to its triplet state.
(36) Wagner, P. J.; Waite, C. I. J. Phys. Chem. 1995, 99, 7388.
(37) We have recently measured the triplet lifetime of p-iodobenzophenone and
it was found to be 98.46 ( 2.16 ps in benzene (detailed results will be
published elsewhere).
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J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004 1833

A R T I C L E S
Shah and Neckers
Table 1. Lifetimes of 4-Benzoylphenyl (5) and
3-(4′-Methylbenzoyl)phenyl (6) Radicals in Different Solvents
Table 2. Bimolecular Rate Constants of Reactions of
4-Benzoylphenyl (5) and 3-(4′-Methylbenzoyl)phenyl (6) Radicalsa
solvent
5a (τ, µs)
6b (τ, µs)
kq × 108 (M-1 s-1
)
C6H6
CCl4
CH3CN
0.66 ( 0.03
0.41 ( 0.01
0.21 ( 0.01
0.68 ( 0.04
0.62 ( 0.02
0.29 ( 0.01
quencher
BP
1,4-CHD
toluene
MMAb
MMA
LAC
5
6
16.8 ((1.40)
15.8 ((1.10)
5.59 ((0.46)
0.76 ((0.03)
3.14 ((0.19)
3.57 ((0.12)
8.67 ((0.29)
9.43 ((0.69)
8.75 ((0.12)
4.14 ((0.16)
1.07 ((0.05)
2.86 ((0.11)
2.07 ((0.18)
9.59 ((0.47)
a Concentration of the precursor BP (1): 1.18 ×10-3 M. b Concentration
of the precursor BP (4): 1.41 ×10-3 M.
Scheme 1
NVP
a Solvent, CCl4; concentration of the radical sources, 7.82 × 10-4 to
2.41 × 10-3 M; abbreviations, 1,4-CHD ) 1,4-cyclohexadiene, MMA )
methyl methacrylate, LAC ) lauryl acrylate, NVP ) 1-vinyl-2-pyrrolidi-
none. b The commercially available sample of MMA containing monomethyl
ether hydroquinone as the radical inhibitor was used; in all other cases of
monomers, radical inhibitor removed samples were used.
Scheme 2
When experiments were carried out at ∼2 and ∼6 mJ/pulse,
similar spectra were obtained from a 1.20 × 10-3 M solution
of 1 in CCl4. The initial ODs at 550 nm were observed to be
lower than those obtained from the experiment when ∼12 mJ/
pulse energy was employed. The lifetime monitored at 550 nm,
however, was essentially the same (0.42 µs). A similar 550 nm
transient was obtained from LFP of tert-butyl 4-(4′-bromoben-
zoyl)perbenzoate in CCl4. Since each of the BPs studied, as
well as p-iodobenzophenone, gave the 550 nm transient, this
transient seems generic to this class of radicals. Transients that
form during the photodissociation of BPs are presented in
Scheme 1.
the quencher and are given in Table 2. The radicals were
quenched by precursor BPs with comparatively high rate
constants. Electron spin resonance studies have shown that
substituted phenyl radicals with para and meta carboxyl groups
have high reactivity.38 Although it was difficult to distinguish
whether the quenching was due to oxidation by the peroxyester
bond or H-abstraction, it is suggested that quenching by the
precursor BP primarily occurs by H-abstraction. When benzoyl
peroxide was used as quencher, it showed no measurable
quenching in the concentration range of 10-2-10-4 M. The
addition of these radicals to the aromatic rings of BP can also
be one possible mode of the quenching. However, products from
such an adduct were not observed during product analysis.4,5,36
Lifetimes of the 4-benzoylphenyl (5) and 3-(4′-methylben-
zoyl)phenyl (6) radicals were recorded in different solvents
(Table 1) and reflect the reactivity of these radicals toward the
solvents employed. The lifetime of 5 was observed to be higher
in benzene (0.66 µs) than in CCl4 (0.41 µs) and it is consistent
Although 6 is a little longer lived than 5, there are not many
differences in their reactivities as reflected by their similar kq
of reactions with all the studied quenchers. Both radicals showed
higher reactivity toward 1-vinyl-2-pyrrolidinone than toward
acrylate monomers. It is also noted that the kq did not change
significantly in the presence of monomethyl ether hydroquinone
(MMEH, the commercially used radical inhibitor).
with the higher rate of chlorine abstraction (7.8 × 106 M-1 s-1
)
by the phenyl radical from CCl4 than the rate of its quenching
(4.5 × 105 M-1 s-1) by benzene.16 However, the rate of
quenching of the phenyl radical by acetonitrile (1.05 × 105 M-1
s-1) has been found to be nearly comparable with that by
benzene,16 while we observed lower lifetimes of both 5 (0.21
µs) and 6 (0.29 µs) in acetonitrile.
Possible Structure and Transitions of the Aroylphenyl
Radicals. The odd electron is localized in the σ orbital in the
Solvent polarity may influence the stability of these radicals.
So we tried to deduce their lifetimes in cyclohexane and
methanol. However, in such H-donating solvents, the kinetics
became complex at 550 nm, and other unidentified transients
having very long lifetimes were observed. Given the nonpolar
nature of their ground states (vide infra), it is clear that polarity
and H-donating incapability of a solvent is critical in observing
such radicals.
ground (doublet) state (2A1) of the phenyl radical.17c,38,39
A
similar σ radical structure can be assumed for aroylphenyl
radicals; so one might expect a contribution from the resonance
structures shown in Scheme 2 for the 4-aroylphenyl radicals.
However, this does not seem to be the case because such
resonance is not possible for the 3-aroylphenyl radical. Since
both of these radicals show the 550 nm absorption, they should
have similar structures in their ground states.
Absolute bimolecular rate constants of quenching (kq) of the
two representative radicals (5 and 6) by various additives were
obtained by monitoring the absorption at 550 nm. The kq values
were calculated by plotting 1/lifetime against concentration of
(38) Zemel, H.; Fessenden, R. W. J. Phys. Chem. 1975, 79, 1419.
(39) (a) Porter G.; Ward, B. Proc. R. Soc. Ser. A 1965, 287, 457. (b) Johnson,
R. P. J. Org. Chem. 1984, 49, 4857. (c) Kim, G.; Mebel, A. M.; Lin, S. H.
Chem. Phys. Lett. 2002, 361, 421.
9
1834 J. AM. CHEM. SOC. VOL. 126, NO. 6, 2004

Laser Flash Photolysis of tert-Butyl Aroylperbenzoates
A R T I C L E S
The lowest energy optical transition of phenyl radicals has
been assigned as 2A1 f 2B1 (σ radical f π radical).39a The 2B1
state is the next low-lying electronic state in which one of the
π electrons has moved to the unpaired σ orbital, creating a
radical cation center in the aromatic ring. The existence of the
σ ground state and low-lying π radical state of the phenyl radical
has been confirmed by several ab initio calculations.39b,c
Low-temperature matrix studies have shown phenyl radical
bands at 534, 288, and 235 nm,40 and recently, the low-energy
absorption at ∼530-560 nm, have been confirmed to be short-
lived species (τ < ∼1 ns in benzene). This is due to rapid
intramolecular triplet energy dissipation to cause efficient
scission of the perester bond forming reactive aroylbenzoyloxyl
and aroylphenyl radicals. The latter absorbs in the mid-UV-
visible region (∼550 nm) and shows solvent-dependent life-
times. Both the 4-benzoylphenyl and 3-(4′-methylbenzoyl)-
phenyl radicals obtained from the 355 nm LFP of 1 and 4,
respectively, have been found to rapidly react with various
substrates including double-bond-containing monomers, the
bimolecular rate constants for which have been observed to be
7.56 × 107 to 1.68 × 109 M-1 s-1 in CCl4 at room temperature.
band (∼511 nm) has been assigned to the 2A1
f
2B1
2
2
transition.17c It is possible that the A1 f B1 transition is
responsible for the 550 nm absorption of the aroylphenyl
radicals. The contribution of the electron density on the carbonyl
group may be shifting absorption to higher wavelengths.
Substituents such as carboxylic acid or aldehyde groups have
long been known to shift and usually intensify the UV
absorption of benzene.41 Investigation by up-to-date methods
of ab initio calculations might shed some theoretical light on
electronic excitation energies of the aroylphenyl radicals.
The symmetrically allowed 2A1
f
2B1 transition seems to
account for the 550 nm absorption of aroylphenyl radicals.
Acknowledgment. We acknowledge helpful discussion with
Professor M. A. J. Rodgers, Dr. G. S. Hammond, and Professor
T. H. Kinstle. Thanks are also due to Dr. E. Danilov and Mr.
T. Gunaratne, who helped with the laser flash photolysis
experiments. B.K.S. thanks the McMaster Endowment for
financial support. We thank the National Science Foundation
Division of Material Research (DMR 9803006) for financial
support of this work. This work is dedicated to Professor Binne
Zwanenburg on the occassion of his 70th birthday.
Conclusion
LFP (380 nm, 350 fs) allowed observation of the singlet and
triplet states of BPs. The singlet states of BPs (2, 3) showed
broad absorption having a maximum at ∼625 nm and the rates
of ISC were observed to be 4.93 × 1010 s-1 and 1.27 × 1011
s-1 in 2 and 3, respectively. The triplet states, which show
Supporting Information Available: Synthetic procedure and
41 figures showing NMR spectra of 4, various transient spectra,
lifetimes, and quenching rate constant graphs and fittings. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(40) (a) Miller, J. H.; Andrews, L.; Lund, P. A.; Schatz, P. N. J. Chem. Phys.
1980, 73, 4932. (b) Hatton, W. G.; Hacker, N. P.; Kasia, P. H. J. Chem.
Soc., Chem. Commun. 1990, 227.
(41) Jaffe´, H. H.; Orchin, M. Theory and Applications of UltraViolet Spectros-
copy; John Wiley & Sons: New York, 1962; p 257.
JA030231G
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