Photodecomposition of organic peroxides containing coumarin chromophore: Spectroscopic studies

Article abstract of DOI:10.1021/jp044554q

The photodecomposition of coumarin-3-t-Bu peroxyester (1) and coumarin-3-carbonyl-m-chlorobenzoylperoxide (2) has been studied using nanosecond and femtosecond spectroscopy to elucidate the nature of transient species involved. Excitation of the coumarin chromophore leads to its singlet excited-state decaying with the rate 9 × 10⁹ s^-1 that results from a composite of emission, intersystem crossing, thermal relaxation, and -O-O- bond homolysis. Dissociation of the weak oxygen-oxygen bond proceeds from both triplet and singlet excited states. The nature of this combination of states is predissociative rather than dissociative as demonstrated by the relatively slow rates of oxygen-oxygen bond rupture. The decomposition of 1 and 2 leads to the formation of coumarin-3-carbonyloxyl radical (R1). The later was observed spectroscopically on the nanosecond time scale using both time-resolved FTIR and UV-vis transient techniques. R1 is consumed in two competitive processes: unimolecular decarboxylation and bi-molecular hydrogen atom transfer. The rates of these reactions are 4.3 × 10⁵ s^-1 and 1 × 10⁶ M ^-1 s^-1 respectively. The transition state geometries and energies of decarboxylation of R1 have been determined using DFT calculations and are compared with values for the benzoyloxyl radical. The decarboxylation of R1 proceeds via a transition state in which the carboxyl group is almost perpendicular (dihedral angle 114°) to the plane of the coumarin chromophore. The transition state of the benzoyloxyl radical, in contrast, is flat (0°). The varied transition state energies of the radicals (13.6 kcal/mol for coumarin carboxyl radical vs 8 kcal/mol for benzoyloxyl radical) correlate with different decarboxylation rates of these two species.

Full text of DOI:10.1021/jp044554q

J. Phys. Chem. A 2005, 109, 2793-2800
2793
Photodecomposition of Organic Peroxides Containing Coumarin Chromophore:
Spectroscopic Studies
Dmitry E. Polyansky and Douglas C. Neckers*
Center for Photochemical Sciences,1 Bowling Green State UniVersity, Bowling Green, Ohio 43403
ReceiVed: NoVember 30, 2004; In Final Form: January 26, 2005
The photodecomposition of coumarin-3-t-Bu peroxyester (1) and coumarin-3-carbonyl-m-chlorobenzoylperoxide
(2) has been studied using nanosecond and femtosecond spectroscopy to elucidate the nature of transient
species involved. Excitation of the coumarin chromophore leads to its singlet excited-state decaying with the
rate 9 × 109 s-1 that results from a composite of emission, intersystem crossing, thermal relaxation, and
-O-O- bond homolysis. Dissociation of the weak oxygen-oxygen bond proceeds from both triplet and
singlet excited states. The nature of this combination of states is predissociative rather than dissociative as
demonstrated by the relatively slow rates of oxygen-oxygen bond rupture. The decomposition of 1 and 2
leads to the formation of coumarin-3-carbonyloxyl radical (R1). The later was observed spectroscopically on
the nanosecond time scale using both time-resolved FTIR and UV-vis transient techniques. R1 is consumed
in two competitive processes: unimolecular decarboxylation and bi-molecular hydrogen atom transfer. The
rates of these reactions are 4.3 × 105 s-1 and 1 × 106 M-1 s-1 respectively. The transition state geometries
and energies of decarboxylation of R1 have been determined using DFT calculations and are compared with
values for the benzoyloxyl radical. The decarboxylation of R1 proceeds via a transition state in which the
carboxyl group is almost perpendicular (dihedral angle 114°) to the plane of the coumarin chromophore. The
transition state of the benzoyloxyl radical, in contrast, is flat (0°). The varied transition state energies of the
radicals (13.6 kcal/mol for coumarin carboxyl radical vs 8 kcal/mol for benzoyloxyl radical) correlate with
different decarboxylation rates of these two species.
Introduction
by time-resolved infrared spectroscopy. To the best of our
knowledge, this is the first direct observation of an aroyloxyl
Organic peroxides are widely used to initiate free radical
reactions including polymerization.2-4 The lack of efficient
UV-vis absorption of conventional thermal peroxides has led
to extensive studies of compounds containing a light absorber
and peroxide functionality. Among these are peroxyesters
containing benzophenone,2 naphthalene,5 anthracene,6 fluo-
renone,7 fluorene,8 and other chromophores. Despite structural
differences, the photodecomposition of peroxyesters follows a
common pathway. Excitation leads to rapid cleavage of the
oxygen-oxygen bond and formation of the corresponding
aroyloxyl radical.8 Decarboxylation rates of the latter depend
mostly on structure and can vary from several picoseconds8 to
tens of microseconds.9 There are two predominant theories of
peroxide photodecomposition. According to the first, decom-
position is a concerted process involving simultaneous cleavage
of the O-O bond and the Cipso-Ccarbonyl bond leading to the
instantaneous evolution of carbon dioxide.10 However, recent
spectroscopic studies have shown that photodecomposition can
also be stepwise in which decarboxylation follows the O-O
bond cleavage. In some instances, vibrationally excited states
of the aroyloxyl radicals are formed resulting in ultrafast
decarboxylation kinetics and preventing one from clearly
distinguishing the two mechanisms.11
radical by TRIR.12 Nanosecond and femtosecond spectroscopic
studies as well as product analysis have been carried out to
elucidate the nature of transient species involved in the
decomposition mechanism.
Experimental Section
1
General. H and NMR spectra were recorded on Bruker
Avance 300 MHz system, and 13C NMR spectra were obtained
with Varian Unity+ 400 MHz instrument. GC-MS and DIP
mass spectra were measured on Shimadzu GC/MS-QP5050A
spectrometer. HPLC measurements were performed on Hitachi
LC-7000 series instrument equipped with Alltech Nucleosil
C18 column. UV-vis spectra were measured with Shimadzu
UV-2401PC spectrophotometer. Steady-state photoluminescence
spectra were recorded on Jobin-Yvon Fluorolog-3 FL-11
spectrofluorimeter (450 W Xe lamp, R928 Hamamatsu single
photon counting PMT detector). Photodecomposition quantum
yields were obtained by direct irradiation of samples with
OmNichrome Series 74XA HeCd laser (325 nm) interrupted
with Uniblitz SD-10 shutter drive timer. IR-ATR spectra of solid
samples were measured on Thermo Nicolet IR200 ATR
equipped spectrometer.
In the current work, we have designed new peroxide systems
containing an efficient light absorbing coumarin chromophore.
The presence of the carbonyl group in the coumarin moiety
allows direct observation of the coumarin-3-carbonyloxyl radical
Materials. Coumarin-3-carboxylic acid was purchased from
Aldrich (99%) and used as received. Coumarin-3-t-Bu-perox-
yester and coumarin-3-carbonyl chloride were prepared from
coumarin-3-carboxylic acid, purified and characterized as de-
scribed in the Supporting Information. All spectroscopic samples
were thoroughly dried, and the purity was verified by HPLC.
* To whom correspondence should be addressed. E-mail: neckers@
photo.bgsu.edu.
10.1021/jp044554q CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/04/2005

2794 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Polyansky and Neckers
TABLE 1: Photodecomposition Products of 3-coumarin-t-Bu
SCHEME 1: Structures of Coumarin-3-t-Bu-peroxyester
(1), Coumarin-3-carbonyl-m-chlorobenzoyl Peroxide (2),
Coumarin-3-carboxylic Acid (3),
Coumarin-3-carbonyloxyl Radical (R1), and Benzoyloxyl
Radical (R4)
Peroxyester in Different Solvents
concentration,
solvent
mM
photoproducts a
CCl4
CCl4
1.0
10.0
3-chlorocoumarin (100%)
3-chlorocoumarin (52%), 3-coumarin
carboxylic acid (35%), coumarin (13%)
3-coumarin carboxylic acid (31%),
coumarin (69%)
3-coumarin carboxylic acid (31%),
coumarin (69%)
CH3CN
CD3CN
1.0
1.0
a HPLC yields are shown in parentheses.
effects. For transition states, one imaginary vibrational frequency
was observed pertinent to the decarboxylation reaction coordi-
nate. Finally, the potential energy along the reaction path in
the neighborhood of the transition state was calculated requesting
IRQ keyword in Gaussian 98. The maximum of the potential
energy was observed at the transition state decreasing along
the reaction coordinate.
All solvents either for reactions or spectroscopic measurements
were dried via distillation over an appropriate drying agent
according to standard procedures.13
The chemical structures of compounds and intermediates
discussed below are shown in Scheme 1.
Results
Nanosecond Infrared Time-Resolved Experiments. Time-
resolved FTIR setup has been described elsewhere.14 Briefly,
the third harmonic of a Spectra Physics YAG:Nd3+ laser (354.7
nm) was used as an excitation source in all experiments. The
laser was operated in pulsed mode with a repetition rate of 10
Hz, and the energy was varied from 1.5 to 5 mJ per pulse. The
sample solutions were pumped through a 1 mm thick CaF2 flow
cell with a flow rate up to 150 mL/min. All spectra were re-
corded in a 1500-2800 cm-1 spectral window with a resolution
of 8 cm-1 every 20 ns. The raw data were processed and
visualized with custom written LabView based software.
Nanosecond UV-Vis Time-Resolved Experiments. The
detailed description of the time-resolved UV-vis spectrometer
is available elsewhere.15 Briefly, the third harmonic of a Con-
tinuum YAG:Nd3+ laser (354.7 nm) was used as an excitation
source in all experiments. The laser was operated with a
repetition rate of 5 Hz, and the energy was kept between 0.5
and 3 mJ per pulse. Solutions were pumped through a quartz
flow cell with three polished windows with flow rate up to 150
mL/min. Transient UV-vis spectra were acquired every 10 nm
in the 330-820 nm spectral interval.
Femtosecond UV-Vis Time-Resolved Experiments. A
detailed description of the home-built ultrafast UV-vis setup
is available elsewhere.16 Briefly, the frequency converted (340
nm) output from a Spectra-Physics Hurricane femtosecond laser
was used as an excitation source. The probe pulses were
generated in CaF2 crystal and overlapped with the pump inside
the sample flow cell. The solution was degassed with Ar to
avoid the formation of long living intermediates (over 1 ms)
and pumped through a 2 mm thick CaF2 flow cell with a flow
rate up to 150 mL/min. The absorption of the sample at
excitation wavelength was 0.8-1.0 (2 mm cell) and was
constantly checked to ensure the decomposition of the starting
material less than 10%.
Steady-State Studies. Irradiation of coumarin-3-t-Bu-per-
oxyester (1) in non-chlorinated solvents (e.g., acetonitrile) at
350 nm yields coumarin, t-Bu alcohol, acetone, and coumarin-
3-carboxylic acid as the major photochemical products as
observed by NMR and IR spectroscopy. In chlorinated solvents
(chloroform, dichloromethane, and carbon tetrachloride) forma-
tion of 3-chlorocoumarin and a complete or partial disappearance
of coumarin are the main differences. Product distribution does
not depend on substitution of hydrogen-containing solvents with
their deuterated analogues, which indicates no isotopic effect
on the hydrogen atom transfer from solvent to the coumarin
carbonyloxyl radical (Table 1). Remarkably, even in carbon
tetrachloride at concentrations equal to or higher than 10 mM,
a substantial amount of coumarin-3-carboxylic acid was ob-
served, whereas at concentrations about 1 mM, 3-chlorocou-
marin was the only photochemical product detected (Table 1).
Irradiation of a 1 mM solution of coumarin-3-carbonyl-m-
chlorobenzoylperoxide (2) in carbon tetrachloride yielded
3-chlorocoumarin as the only photochemical product. Due to
the limited solubility of 2 in CCl4 (1mM maximum), concentra-
tion dependence studies could not be performed for this
compound. The irradiation of 2 in acetonitrile gives photoprod-
ucts similar to those formed from 1.
The quantum yields of photodecomposition of 1 (Ø ) 0.67)
and 2 (Ø ) 0.70) were measured in acetonitrile by irradiating
optically concentrated (OD > 10) solutions in a 1 cm quartz
cell using the defocused output from a HeCd laser (325 nm).
Decomposition was followed by HPLC using biphenyl as the
internal standard.
Steady-state photoluminescence spectra of coumarin-3-car-
boxylic acid (3) were measured in degassed acetonitrile solutions
in the spectral range from 360 to 600 nm upon excitation at
350 nm. The emission maximum was observed at 425 nm, and
its intensity was slightly decreased upon purging with oxygen.
The fluorescence quantum yield for 3 (Φfl ) 0.03 ( 0.002)
was measured in acetonitrile using 9-bromoanthracene as a
standard.20 We were not able to obtain accurate Φfl values for
1 due to its decomposition under the experimental conditions.
However, an estimated fluorescence quantum yield of 1 in
acetonitrile is about 2.5 times lower than that of 3.
DFT Calculations. DFT calculations were done using the
Gaussian 98 package.17 The optimized structures and frequencies
have been visualized with Molekel software.18 The geometries
were optimized using the B3LYP level of theory with the
6-31+G(d,p) basis set. IR frequencies were calculated on
optimized structures using the same basis set. All frequencies
reported were scaled up by a factor of 0.96, which is traditionally
used for this size of the basis set in DFT calculations.19 The
transition state geometries were calculated with UB3LYP/
6-31+G(d,p), and energies were corrected to zero-point vibration
Nanosecond Time-Resolved FTIR and UV-Vis Studies.
Transient FTIR spectra of 1 taken between 700 ns and 2.5 µs
after the laser flash are shown in Figure 1. The formation of

Peroxides Containing Coumarin Chromophore
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2795
Figure 2. FTIR spectra of 1 (dashed line) and 2 (solid line) taken 1
µs after the laser pulse.
Figure 1. FTIR difference spectra of 1 in CCl4 acquired between 0.7
µs and 2.5 µs after the laser pulse. The inset shows an overlay of the
1740 cm-1 transient decay (solid line) and inverted 2335 cm-1
(triangles) growth.
vacuum. Calculations show excellent agreement with experiment
data and follow the trend of decreasing the vibrational frequency
of the carbonyl group with increasing solvent polarity.
the negative absorbance signals at 1765 cm-1 and 1795 cm-1
corresponding to the ground-state decay and positive transient
corresponding to transient absorption at 1740 cm-1 were
observed “instantaneously” (within the response time of the
instrument) after the laser pulse. Bleaching of the ground state
absorption had partially recovered after the first 5 µs and then
remained unchanged until the end of the acquisition time ca.
500 µs.
vibrational frequency, cm-1
solvent system
1744
1740
1727
1720
vacuum (calculated)
CCl4
CHCl3
CH3CN
Steady-state studies show that at higher concentrations
intermolecular hydrogen atom transfer to R1 from the tert-butyl
groups of the starting material takes place. In that one might
expect decarboxylation to compete with the hydrogen atom
transfer, we synthesized the coumarin peroxyester 2 which has
no labile hydrogen atoms to obtain an accurate value of the
decarboxylation rate of R1.
The photochemistry of 2 is essentially the same as that of 1.
The transient infrared spectrum of 2 contains a band at 1740
cm-1 that was previously assigned to R1 (Figure 2).
An interesting feature of the transient behavior of 2 compared
to 1 is the biexponential growth of the signal corresponding to
the formation of carbon dioxide (Figure 3). The first fast com-
ponent of this kinetic trace with the lifetime 160 ( 20 ns corre-
sponds to the decarboxylation of the 3-chlorobenzoyloxyl radi-
cal. The decarboxylation rate of this radical as reported by Ingold
et al.22 using time-resolved UV-vis spectroscopy was 5.5 ×
106 s-1 (≈ 180 ns), which shows good agreement with our data.
The lifetime of the second component was 2.8 ( 0.4 µs, whereas
the lifetime of the decay of the 1740 cm-1 transient was 3.3 (
0.3 µs. Apparently, the second, longer component of carbon
dioxide formation is due to the decarboxylation of R1.
We have obtained the transient UV-vis spectra from pho-
tolysis of 1 and 2 in argon-saturated carbon tetrachloride (Figure
4). The transient UV-vis spectra of aroyloxyl radicals have
characteristic broad absorption in the visible and infrared regions
of the spectrum.5,9,21,22 Both spectra show two spectral fea-
tures: one band around 400 nm and another broad band with a
maximum at ≈800 nm. Evidently, both spectra originate from
the same transient species as can be seen from the similarity in
their shape and positions of maxima. The transients observed
in our UV-vis experiments are like those assigned in previous
studies to aroyloxyl radicals.
The decomposition of peroxides at room temperature is an
irreversible process and the recovery of some ground state
bleaching in our case is due to the formation of photostationary
products, which in the case of carbon tetrachloride as a solvent
was mostly 3-chlorocoumarin (1757 cm-1): the product of
chlorine atom transfer from the solvent to the coumarin carbon-
centered radical. We did not observe any transient signals which
might possibly be associated with a coumarin carbon-centered
radical. The DFT calculations show that the frequency of the
carbonyl stretching band for these species should be around 1765
cm-1. The absence of any transient evidence for this radical is
probably due to overlapping absorptions of the starting material,
the 3-coumarin carbon centered radical, and 3-chlorocoumarin.
The transient at 1740 cm-1 decayed concomitantly with the
rise of the characteristic band at 2335 cm-1 (Figure 1, inset).
This transient was formed within the first 5 µs, and its amplitude
remained unchanged until the end of the acquisition. The strong
absorption at 2335 cm-1 was observed in the steady state IR
spectra of UV-irradiated peroxide solutions. The degassing of
irradiated solutions with argon for 10 min totally eliminated
the absorption at 2335 cm-1. Thus, the 2335 cm-1 band can be
unambiguously assigned to the absorption of carbon dioxide.
The kinetic data show that the 1740 cm-1 transient is a direct
precursor of carbon dioxide. This is the first strong argument
for the assignment of this transient to the coumarin-3-carbo-
nyloxyl radical (R1). Saturation of the peroxide solutions with
oxygen did not decrease the lifetime of the 1740 cm-1 band;
however, the addition of 20 mM of acrylonitrile totally
eliminated the observed transient. The sensitivity of the observed
species to acrylonitrile, a known radical scavenger, indicates
their radical nature, but the low reactivity toward oxygen points
to the oxygen centered radical.21,22
The table below summarizes vibrational frequency values of
R1 measured in different solvents and its calculated value in a
Convincing evidence for assignment of the broad band in
the visible region of the spectrum to the R1 radical would come

2796 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Polyansky and Neckers
Figure 5. Stern-Volmer plot of the quenching of coumarin-3-
carbonylxyl radical by di-t-Bu-peroxide.
Figure 3. Growth of 2335 cm-1 transient after the irradiation of 2
with a 355 nm laser pulse. The dots represent the experimental data
points, and the solid line is the biexponential fit (160 ns and 2.8 µs).
spectral information in a single experiment. The first approach
appeared to be faster and required less sample.
Based on these steady state and transient spectroscopy studies,
there are two major channels leading to the disappearance of
R1: intramolecular decarboxylation and intermolecular hydro-
gen atom transfer. Since the concentration of the products at
the early stages of irradiation is negligible compared to 1, we
assume that the source of the hydrogen atoms in the bi-molecular
reaction should be the tert-butyl groups of the starting material.
This suggested the use of di-tert-butylperoxide as a quencher
in kinetic experiments assuming that the chemical environment
of hydrogen atoms in the tert-butyl group is close to the
hydrogen-carbon bond energy of the starting peroxide. Di-tert-
butylperoxide cannot participate in primary photochemical
reactions due to the lack of absorption at 355 nm, and it plays
the role solely of hydrogen atom donor. Due to their insignificant
absorption in this spectral region, the possible products formed
from this quencher should also not interfere with the signal at
770 nm.23 A Stern-Volmer plot of the quenching of R1 with
di-tert-butylperoxide is shown in Figure 5. The value of
bimolecular rate determined from the slope was (1.01(0.02)-
106 M-1s-1 and the rate of decarboxylation reaction found as
Figure 4. Normalized transient UV-vis spectra of 1 (circles) and 2
(squares) measured 200 ns after laser pulse.
the intercept was (4.28(0.04)105 s-1
.
from direct comparison of the lifetime of the radical measured
in time-resolved IR experiments with lifetimes obtained using
transient UV-vis spectroscopy. The absorption at 770 nm was
chosen to monitor the kinetics of the decarboxylation process.
The kinetic studies indicate a strong dependence of the observed
lifetimes in both IR and UV-vis experiments upon excitation
energy. At laser energies higher than 1.5 mJ/pulse, the observed
lifetimes decreased with an increase in the laser energy, and at
energies over 8 mJ/pulse, the kinetic traces reveal multicom-
ponent character. We were only able to match lifetimes obtained
in the TRIR experiment (2.6 ( 0.3 µs) and TR UV-vis
measurements (2.3 ( 0.1 µs) at energies lower than 1.5
mJ/pulse. The similar lifetimes of the 1740 cm-1 transient and
the decay rate of the broad VIS-NIR absorption allow the
assignment of these two transient spectra obtained by different
spectroscopic techniques to the same species, namely, the
coumarin-3-carbonyloxyl radical.
Ultrafast Measurements. UV-vis transient spectra of 1 and
3 in acetonitrile were obtained with picosecond time resolution.
Excitation of the coumarin chromophore with a 340 nm laser
pulse leads to the “instantaneous” (within the pulse width)
formation of a broad absorption between 450 and 700 nm with
the maximum around 550 nm (Figure 6) observed for 1 as well
as for 3. The similar spectral features observed for each
compound indicates that the transient at 550 nm originates from
an excited state rather from the chemically produced transient
species since the model compound 3 does not possess substantial
reactivity under experimental conditions.
For 3 the rate of the decay of the 550 nm transient was 5 ×
109 s-1 which is a composite resulting from emission, inter-
system crossing, and thermal relaxation (Table 2). This decay
is accompanied by the growth of an absorption around 375 nm
with a rate24 3.3 × 109 s-1 (Figure 7). The same band was
observed decaying with a rate of ∼105 s-1 as measured using
nanosecond transient UV-vis experiment (Figure 8). The
sensitivity of this transient to oxygen and the similarity of its
spectroscopic features to those reported previously25 allow us
to assign this absorption to the triplet excited state of the
coumarin-3-carboxylic acid. We assign the 550 nm transient to
UV-vis transient spectroscopy was used to study the
decomposition kinetics of R1. The dispersive technique used
to acquire spectral information in the transient UV-vis setup
allowed us to use a single wavelength to monitor individual
kinetic traces as opposed to FTIR setup that collects complete

Peroxides Containing Coumarin Chromophore
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2797
Figure 6. Normalized transient UV-vis spectra of 1 (circles) and 3
(solid) measured 10 ps after the 340 nm femtosecond laser pulse.
Figure 8. Normalized transient UV-vis spectra of 3 taken 10 ps (solid)
and 2 µs (dot) after the laser pulse.
TABLE 3: Rates and Efficiencies of Photophysical and
Photochemical Pathways Measured for 1 and 3
kFL
,
kISC
,
kNR
,
kO-O,
compound s-1 x10-9 s-1 x10-9 s-1 x10-9 s-1 x10-9 ΦISC ΦO-O
S
1
3
0.15
0.15
3.3
3.3
1.55
1.55
4.0
0.37 0.44
0.37
elongated from 1.473 to 1.958 Å. The barrier height (∆ET) of
the decarboxylation reaction of R4 calculated as a difference
between ZPE corrected energies of the R4 and corresponding
transition state (TS4) was 8 kcal/mol. The geometry and ∆ET
obtained for R4 agrees well with previously reported values.26
In contrast to R4, the transition state geometry of R1 is not
planar but has an angle between the plane of coumarin
chromophore and carboxyl group about 66° and the C-CO2
distance 1.974 Å (Figure 9). The ∆ET of the decarboxylation
reaction of R1 calculated as a difference between ZPE corrected
energies of the R1 and corresponding transition state (TS1) was
13.6 kcal/mol. The graph of potential energy surface along
reaction coordinate corresponding to decarboxylation of R1
(Figures S1 and S2) and computational details can be found in
Supporting Information.
Figure 7. Transient UV-vis spectral evolution of 3 after the 340 fs
laser pulse.
TABLE 2: Decay Rates of Transient Species Observed for 1
and 3, Fluorescence Quantum Yield of 3, and
Photodecomposition Quantum Yield of 1
550 nm
decay,
s-1
375 nm
growth,
s-1
375 nm
decay,
s-1
ΦO-O ΦO-O
Discussion
compound
ΦFL
(Ar)
(O2)
1
3
9 × 109
0.67
0.56
Compounds containing the coumarin chromophore are well-
known laser dyes and possess high fluorescence quantum yields.
However, the photophysical properties of coumarin systems
greatly depend on their electronic structure.27,28,29 As opposed
to 7-substituted coumarins, coumarin-3-carboxylic acid and
corresponding esters have low fluorescence quantum yields due
to radiationless deactivation of the singlet excited state as
determined by steady-state experiments.28,29 Our ultrafast mea-
surements show that intersystem crossing and vibrational
relaxation are the dominant routes of singlet excited-state decay.
Nevertheless, the introduction of the carboxyl group into the
coumarin chromophore decreases the intersystem crossing rate
and increases the fluorescence quantum yield compared to
coumarin itself.29 The triplet excited state formed has an
appreciably long lifetime (∼10 µs) and decays predominantly
via vibrational relaxation and bimolecular quenching processes.
The comparison of the intersystem crossing rate and the rate
of oxygen-oxygen bond cleavage indicates that the peroxide
decomposition proceeds mostly from the singlet excited state
of the coumarin chromophore. However, the photodecomposi-
5 × 109 3.3 × 109
∼105
0.03
the singlet exited state absorption of the coumarin chromophore
based on the fact that this transient is the direct precursor of
the 375 nm absorption assigned previously to the T1-Tn
transition.
The presence of the peroxide functionality decreases the
lifetime of the 550 nm transient about two times which
corresponds to the rate 9 × 109 s-1. The difference in rates
between 1 and 3 could be used to calculate the rate (kO-O) and
quantum yield (ΦO-OS) of peroxide bond scission from the
singlet excited state (Table 3).
Ab Initio Calculations of Decarboxylation Thermodynam-
ics. A direct comparison of reactivity of benzoyloxyl radical
(R4) and coumarin-3-carbonyloxyl radical (R1) has been
performed by calculating the transition state energies of the
decarboxylation reaction (UB3LYP/6-31+G**). The ground-
state geometries of R1 and R4 are planar, with carboxyl groups
laying in the plane of the benzene or coumarin ring (Figure 9).
The transition state of R4 is also planar with the C-CO2 bond

2798 J. Phys. Chem. A, Vol. 109, No. 12, 2005
Polyansky and Neckers
Figure 9. Calculated (UB3LYP/6-31+G**) geometries of R1 (bottom) and R4 (top) and their transition states (TS1 and TS4) of decarboxylation
process. The dihedral angles between plane of the chromophore and CO2 group are shown next to each structure.
photodecomposition process. Consequently, the cleavage of the
O-O bond proceeds from the singlet as well as from the triplet
excited states with contributions of 66% and 34% respectively.
The sensitivity of the peroxyester photodecomposition quantum
yield to oxygen (Table 2) provides additional supporting
evidence for participation of the triplet state in the photo-
chemical reaction. As has been shown previously in studies of
the photosensitized decomposition of hydrogen peroxide, a
series of singlet and triplet excited states are adiabatically
correlated with ground states of hydroxyl radicals, and both
singlet and triplet sensitized decomposition of peroxide is
possible.34
Unfortunately, direct observation of the triplet state kinetics
in 1 was not possible due to instrumental limitations;35 however,
Figure 10. Schematic representation of photochemical reaction from
predissociative excited state. Legend: a, ground state; b, electronically
excited bound state; c, dissociative state; ∆E, barrier of crossing from
b to c.
taking into account the slow rates (∼105 s-1) of triplet
disappearance obtained for 3 and the lack of spectroscopic
evidence for the presence of the triplet state in 1 on a nanosecond
time scale, we assume that all of the triplet state population is
consumed by chemical reaction. This suggests that the quantum
yield of peroxide photodecomposition from the triplet excited
state equals the quantum efficiency of the triplet state production.
However, the sum of photodecomposition quantum yields from
triplet and singlet states is higher than the quantum yield
measured directly which we believe is due to overestimation
of the intersystem crossing rate constant and therefore the
quantum efficiency of the triplet state formation.
tion quantum yield (ΦSO-O) derived from the rates obtained
from kinetic measurements was about 30% lower than the
photodecomposition quantum yield measured directly (see Table
2 and Table 3) which points to the substantial contribution of
the triplet state into overall peroxyester decomposition. The
triplet excited state formed possesses sufficient energy30-32 to
provide efficient cleavage of the oxygen-oxygen peroxide
bond33 therefore providing another precursor for the peroxide
Figure 11. General photodecomposition scheme of 1. Species in parentheses are suggested (calculated) structures. The measured rates of chemical
and photophysical processes are indicated above corresponding arrows.

Peroxides Containing Coumarin Chromophore
J. Phys. Chem. A, Vol. 109, No. 12, 2005 2799
The magnitude of the rate of O-O dissociation provides
insight into the nature of the potential energy surface. It is known
that dissociative surfaces are barrierless and the promotion of a
molecule directly into a dissociative state leads to a very rapid
(≈ hundreds of femtoseconds) dissociation.36-39 In the case of
1, the rate of the O-O bond rupture is 250 ps, which is about
103 times longer than for dissociative states. Even though we
were not able to measure triplet state decay directly, the
sensitivity of the photodecomposition quantum yield to oxygen
indicates that at least a part of the triplet state is quenched in a
bimolecular process. Assuming triplet quenching to be diffusion
limited, the rate of O-O bond cleavage should be on the order
of nanoseconds which also argues against the repulsive nature
of the triplet state. Ruling out the dissociative nature of the
excited states, we propose peroxide decomposition proceeds
according to an electronic predissociation process.38,40 In this
case, molecules are promoted into bound excited states that do
not themselves dissociate but can cross into dissociative surfaces.
There is a certain barrier for this chemical process that depends
on coupling between two states, and this barrier determines the
rate of the dissociation (Figure 10).
The monoexponential character of the decay of R1 and the
formation of CO2 indicates that decarboxylation of R1 proceeds
from vibrationally excited ground state only. This is in contrast
to the situation reported with a series of aroyloxyl radicals,
which have been shown to undergo decarboxylation from both
vibronically excited and ground states.11,26 We believe that in
our case the excess of vibrational energy dissipates before
radicals are formed due to slower O-O bond scission.
The decarboxylation rate of R1 was about an order of
magnitude lower than the rate of decarboxylation of either the
benzoyloxyl radical or the 2-naphthyloxyl radical. DFT calcula-
tions of the transition states agree with this experimental
observation. The decarboxylation barrier for radical R1 is 5.6
kcal/mol higher than for R4, implying slower decarboxylation
rates of R1 compared to R4. We attribute this to the stabilization
of R1 by an electron-reach coumarin aromatic system and to
the geometrical specifics of the transition state. As proposed
by Ingold et al.22 stabilization of various aroyloxyl radicals via
substitution on the aromatic ring is due to conjugative electron
delocalization or hyperconjugation between the substituent and
the radical site.
Supporting Information Available: Synthesis and charac-
terization of compounds 1 and 2. The Cartesian coordinates of
optimized geometries of R1 and R4 and corresponding transition
states together with their energies and vibrational frequencies.
This material is available free of charge via the Internet at
http://pubs.acs.org.
References and Notes
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Conclusions
The decomposition of coumarin perester proceeds from both
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state is predissociative rather than dissociative as demonstrated
by the relatively slow rates of oxygen-oxygen bond rupture.
The decomposition of 1 and 2 is clearly a stepwise process since
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reactions are 4.3 × 105 s-1 and 1 × 106 M-1 s-1 respectively.
The decarboxylation of R1 proceeds from vibronically relaxed
ground state as can be seen from relatively slow monoexpo-
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Acknowledgment. Authors thank Dr. Evgeny Danilov for
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