Time-resolved spectroscopy of the excited singlet states of tirapazamine and desoxytirapazamine

Article abstract of DOI:10.1021/jp0457040

Laser flash photolysis (LFP, 400 nm excitation) of the anti-cancer drug tirapazamine (TPZ) in acetonitrile produces the singlet excited-state S ₁ with λ_max = 544 nm. The lifetime of this state is 130 ps, in good agreement with the reported fluorescence lifetime. The excited state is reduced to the corresponding radical anion by KSCN or KI. The spectrum of the radical anion is in good agreement with previously reported pulse radiolysis studies and time-dependent density functional theory (TD-DFT) calculations. LFP of desoxytirapazamine (dTPZ) also produces the first excited singlet state, S₁. The fluorescence quantum yield and lifetime (5.4 ns) of the dTPZ singlet excited state are both much greater than the corresponding values of TPZ. This is explained by DFT calculations that predict that cyclization of TPZ to form an oxaziridine is thermodynamically facile but that cyclization of dTPZ to form an oxadiaziridine is not. Thus, the S ₁ state of TPZ has a short lifetime and low fluorescence quantum yield due to ready cyclization whereas the cyclization of the S₁ state of dTPZ is unimportant and does not limit either the fluorescence quantum yield or the fluorescence lifetime. This conclusion is confirmed by studies of dTPZ', an isomer of dTPZ containing the C=N-O moiety which has a low quantum yield and short fluorescence lifetime similar to that of TPZ.

Full text of DOI:10.1021/jp0457040

J. Phys. Chem. A 2005, 109, 1491-1496
1491
ARTICLES
Time-Resolved Spectroscopy of the Excited Singlet States of Tirapazamine and
Desoxytirapazamine
Xiaofeng Shi,† James S. Poole,‡ Ijeoma Emenike,† Gotard Burdzinski,†,§ and
Matthew S. Platz*,†
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue, Columbus, Ohio 43210 and
Department of Chemistry, Ball State UniVersity, Muncie, Indiana 47306, and Quantum Electronics Laboratory,
Faculty of Physics, Adam Mickiewicz UniVersity, Umultowska 85, 61-614 Poznan, Poland
ReceiVed: September 21, 2004; In Final Form: December 1, 2004
Laser flash photolysis (LFP, 400 nm excitation) of the anti-cancer drug tirapazamine (TPZ) in acetonitrile
produces the singlet excited-state S1 with λmax ) 544 nm. The lifetime of this state is 130 ps, in good agreement
with the reported fluorescence lifetime. The excited state is reduced to the corresponding radical anion by
KSCN or KI. The spectrum of the radical anion is in good agreement with previously reported pulse radiolysis
studies and time-dependent density functional theory (TD-DFT) calculations. LFP of desoxytirapazamine
(dTPZ) also produces the first excited singlet state, S1. The fluorescence quantum yield and lifetime (5.4 ns)
of the dTPZ singlet excited state are both much greater than the corresponding values of TPZ. This is explained
by DFT calculations that predict that cyclization of TPZ to form an oxaziridine is thermodynamically facile
but that cyclization of dTPZ to form an oxadiaziridine is not. Thus, the S1 state of TPZ has a short lifetime
and low fluorescence quantum yield due to ready cyclization whereas the cyclization of the S1 state of dTPZ
is unimportant and does not limit either the fluorescence quantum yield or the fluorescence lifetime. This
conclusion is confirmed by studies of dTPZ′, an isomer of dTPZ containing the CdN-O moiety which has
a low quantum yield and short fluorescence lifetime similar to that of TPZ.
1. Introduction
SCHEME 1
Tirapazamine (TPZ) is a promising anti-cancer drug that is
currently in Phase-III clinical trials.1 It is generally believed
that TPZ is enzymatically reduced to the corresponding radical
anion (Scheme 1).2 Protonation of the TPZ radical anion
produces a neutral radical, TPZOH, which is believed to
fragment to form the inactive metabolite desoxytirapazamine
(dTPZ, 3-amino-1,2,4-benzotriazine-1-N-oxide) and hydroxyl
radical.3 The hydroxyl radical or the iminyl radical (IR),4 or a
tautomer, formed by reaction of hydroxyl radical with dTPZ is
likely responsible for abstracting a hydrogen atom from a DNA
sugar and subsequent strand nicking. The sensitivity of the TPZ
radical anion to oxygen accounts for the greater potency of the
drug under hypoxic (oxygen-deficient) conditions (Scheme 1).1
We are interested in initiating these reactions photochemically
to develop a source of hydroxyl radical using visible light.5 Here
we report our studies of the excited singlet states of TPZ,
desoxytirapazamine and its isomer dTPZ′, quinoxaline di-N-
oxide (QXNO), and phenazine di-N-oxides (PNNO).
2. Experimental Section
Picosecond time-resolved measurements were performed at
the Ohio Laboratory for Kinetic Spectrometry at Bowling Green
State University and the Center for Chemical and Biophysical
Dynamics of The Ohio State University. The setup used for
the ultrafast transient absorption spectrometric experiments at
BGSU has been detailed elsewhere.6 Briefly, the output of a
* To whom correspondence should be addressed.
† The Ohio State University.
‡ Ball State University.
§ Adam Mickiewicz University.
10.1021/jp0457040 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/05/2005

1492 J. Phys. Chem. A, Vol. 109, No. 8, 2005
Shi et al.
Ti:sapphire laser (Spectra-Physics, Hurricane) (fwhm ) 150 fs)
was coupled into a second-harmonic generator (CSK Super
Tripler) to obtain the 400 nm excitation wavelength. The energy
of the probe pulse was less than 1 µJ/cm2 at the sample. The
pump beam used was approximately 5 µJ/pulse with a spot size
of 1-2 mm diameter. After the sample cell both beams were
coupled into 200 µm fiber optic cables and input to a CCD
spectrograph (Ocean Optics, S2000-UV-vis) to obtain time-
resolved spectral information (425-800 nm). Five thousand
excitation pulses were averaged to obtain the transient spectrum
at a particular delay time. From the accumulated spectral data
kinetic traces at different wavelengths were assembled. The
sample flow cell had an optical path of 2 mm and was connected
to a solution reservoir and pump system. Sample solutions were
prepared with an absorbance of 0.2-1.0 at the excitation
wavelength and studied under an argon atmosphere. All
measurements were conducted at ambient temperature, 22 ( 2
°C. The ground-state absorption spectra of the compounds
studied in this work are given in the Supporting Information
(SI, Figure S1)
The Ohio State University Center for Chemical and Bio-
physical Dynamics consists of a short pulse oscillator (Coherent/
Positive Light, Mira) generating ∼30 fs pulses at ∼800 nm that
seeds a high-energy regenerative amplifier (Coherent/Positive
Light, Legend HE USP). The regenerative amplifier produces
∼2.5 mJ, ∼40 fs pulses at 1 kHz. The remaining fundamental
is used for harmonic and white light generation. The pump beam
is chopped with a frequency of 333 Hz. The probe beam is
derived from the small portion of the fundamental output (800
nm). It is passed through an optical delay line consisting of a
retroreflector mounted on a computer-controlled motorized
translation stage. The probe beam is then used to generate a
white light continuum in a 1- mm sapphire plate. This is
followed by interference filtering. Transmission signals were
detected by Si photodiodes and measured with a digital lock-in
amplifier (SRS 830). The samples were circulated in a flow
cell; the optical path length was 1 mm. To avoid polarization
effects, the angle between polarizations of the pump beam and
the probe beam was set to the magic angle by a λ/2 plate.
Nanosecond time-resolved measurements were performed at
The Ohio State University using the instrument described
previously.7 Samples in 1-cm quartz cuvettes were excited with
a Spectra Physics LAB-150-10 (∼5 ns) water-cooled Nd:YAG
laser at the third harmonic frequency (355 nm) with an energy
of ∼0.045 J/pulse.
Figure 1. Transient absorption spectrum produced upon excitation (400
nm) of TPZ in acetonitrile under an argon atmosphere (1) 8.3, (2) 53,
and (3) 218 ps after the laser pulse.
TPZ was dissolved in dioxane in the presence of 150 µL of
azo-tert-butyl and refluxed overnight. The reaction was moni-
tored by HPLC until all the staring material was consumed.
The reaction solution was then evaporated under reduced
pressure and eluted with ethyl acetate/hexane through a silica
column. dTPZ′ was the third main fraction (the first two
fractions were dTPZ and 3-amino-1,2,4-benzotriazine, respec-
tively) to yield 15 mg of final product (55% yield). The
spectroscopic properties (UV-vis, NMR, MS) of the product
were measured and were identical to literature reports.3d
Quinoxaline di-N-oxide and phenazine di-N-oxide were pre-
pared by the oxidation of parent quinoxaline and phenazine by
acetic acid and hydrogen peroxide according to the literature.10
DFT11 and TD-DFT12 calculations were performed using the
Gaussian 98 suite of programs13 at The Ohio Supercomputer
Center. Geometries were optimized at the unrestricted B3LYP/
6-31G* level of theory with single-point energies obtained at
the B3LYP/6-31+G**//B3LYP/6-31G* level of theory. Vibra-
tional frequency analyses at the B3LYP/6-31G* level were
utilized to verify that stationary points obtained corresponded
to energy minima. The zero-point vibrational energy correction,
scaled by a factor of 0.9804, was also obtained from the
frequency analysis. Solution structures and energies were
computed using the polarizable continuum model (PCM)14
provided by the Gaussian 98 suite of programs. The electronic
spectra were computed using time-dependent density function
theory of Gaussian 98 at the B3LYP/6-31G* level, and 10
allowed electronic transitions were calculated.
3. Results
Fluorescence quantum yield (ΦF) and fluorescence quenching
experiments were carried out using a Spex Fluorolog 1680
double spectrometer (JY Horiba Inc., Edison, NJ). Measure-
ments of the fluorescence quantum yield were performed as
described in the JobinYvon Ltd. website.8 Acridine orange (ΦF
) 0.46, λex ) 475 nm, integrated from 500 to 750 nm, in
ethanol) was used as the standard for TPZ and PNNO, and
harmane ((ΦF ) 0.81, λex ) 390 nm, integrated from 405 to
650 nm, in 0.1 M H2SO4) was used as the standard for dTPZ,
dTPZ′, and QXNO. A series of fluorescence intensities were
plotted versus the corresponding absorptions at the excitation
wavelengths to obtain the slopes (S) of the samples and
standards. The quantum yields of samples were calculated by
ΦX ) ΦST(SX/SST)(ηX/ηST)2, where η is the refractive index of
the solution.
Tirapazamine. The absorption spectra, emission spectra, and
fluorescence lifetime of TPZ have been studied as a function
of solvent.5 The interaction of ground and excited (S1) state
TPZ with amino acids and nucleosides has also been reported.5
Laser flash photolysis (LFP, 400 nm) of TPZ in acetonitrile
produces the transient absorption spectrum of Figure 1. Bleach-
ing of TPZ is evident below 500 nm. The negative band
observed between 600 and 700 nm is attributed to TPZ
stimulated fluorescence. The fluorescence has been previously
reported in the 530-650 nm region.5 The quantum yield of TPZ
fluorescence was measured to be 0.002 in acetonitrile. The
positive peak at 544 nm decays with the same time constant
(130 ps) as the stimulated fluorescence decay and the recovery
of the ground-state absorption below 500 nm. Thus, the peak
at 544 nm is attributed to the S1 state of tirapazamine. This
assignment is consistent with our previous report that the
fluorescence lifetime of TPZ in acetonitrile is 110 ps.5
The fluorescence lifetime of dTPZ was measured by TCSPC
at Bowling Green State University as described in the literature.9
TPZ and dTPZ were prepared according to the procedure of
Fuchs et al.2 dTPZ is a synthetic precursor to TPZ. dTPZ′ was
prepared by the deoxygenation of TPZ. A 30 mg amount of
The addition of the neutral electron donor DABCO increases
the pseudo-first-order rate constant of decay of the transient

Spectroscopy of Tirapazamine and Desoxytirapazamine
J. Phys. Chem. A, Vol. 109, No. 8, 2005 1493
Figure 3. Fluorescence spectra of TPZ (λex ) 432 nm, A ) 0.302),
dTPZ (λex ) 422 nm, A ) 0.301), dTPZ′ (λex ) 420 nm, A ) 0.310),
QXNO (λex ) 400 nm, A ) 0.305), and PNNO (λex ) 422 nm, A )
0.308) in acetonitrile at ambient temperature.
Figure 2. Transient spectrum produced upon excitation (355 nm) of
0.9 mM TPZ in water containing 4 M KSCN (1) 10 ns after the laser
pulse, (2) 5 µs after the laser pulse, and (3) calculated UV-vis of the
TPZ radical anion (B3LYP/6-31G*).
TABLE 1: Rates of Reaction for Electron Transfer between
Desoxytirapazamine and Various Substrates Determined
from Stern-Volmer Analysis of Fluorescence Quenching
absorption of TPZ and its fluorescence, as expected (see
Supporting Information, Figure S2). However, the transient
spectrum of the TPZ radical anion is not observed. Presumably,
reverse electron transfer between the TPZ radical anion and
DABCO radical cation is extremely rapid, leading to undetect-
ably short lifetimes and low concentrations of these species.
Nanosecond time-resolved LFP (355 nm) of 1.9 mM TPZ in
water containing either 4-5 M KSCN or 3-5 M KI produces
the transient spectrum shown in Figure 2. The bleaching
observed between 430 and 500 nm is due to the ground-state
absorption of TPZ in this region of the spectrum. The transient
absorptions at 385 and 540 nm decay with the same time
constant (1.1 µs), arguing that a common intermediate is
responsible for both spectral features. The carrier of the transient
absorption is assigned to the TPZ radical anion, and the spectrum
itself is consistent with the spectrum of this species obtained
by pulse radiolysis.15 The pulse radiolysis studies demonstrated
that the pKa of TPZ-OH, the conjugate acid of the radical anion,
is 5.6 ( 0.2.15 The transient spectrum of TPZ radical anion is
also consistent with TD-DFT calculations, which predict vertical
absorptions of the following wavelengths: 592 (f ) 0.0376),
493 (f ) 0.0276), and 411 nm (f ) 0.0329), see Figure 2 and
the Supporting Information. The lifetime of the radical anion
is greatly shortened in the presence of oxygen (Supporting
Information, Figure S3). This result is consistent with the
mechanism of action of TPZ and in particular its selectivity
toward hypoxic cells.
linear fit
quencher,
Q
solventa [Q] (M) kQτfb (M-1
)
R2 c
kq (M-1 s-1
)
AMPd
NaN3
W
W
W
W
W
W
0.0-0.10 22.8 ( 0.4 0.995 (4.22 ( 0.07) × 109
0.0-0.3 16.4 ( 0.3 0.994 (3.03 ( 0.05) × 109
0.0-0.17 20.5 ( 0.6 0.990 (3.80 ( 0.10) × 109
0.0-0.15 21.6 ( 0.5 0.993 (4.00 ( 0.09) × 109
0.0-0.09 21.8 ( 0.5 0.992 (4.04 ( 0.09) × 109
0.0-0.06 33.4 ( 1.0 0.987 (6.2 ( 0.2) × 109
KSCN
tyrosinee
GMPf
tryptophane
a W ) aqueous phosphate buffer, 0.025M, pH 6.9. b Errors quoted
are standard deviations on the line of best fit. c Goodness-of-fit
parameter. d Adenosine monophosphate disodium salt. e Amino acids
used as methyl ester hydrochlorides. f Guanosine monophosphate,
monosodium salt.
4-Desoxytirapazamine. The visible absorption band of dTPZ
has a maximum at 413 nm in water (log ꢀ ) 3.68), 404 nm in
acetonitrile (log ꢀ ) 3.63), and 402 nm in dichloromethane (log
ꢀ ) 3.70) (SI, Figure S1). The solvatochromic behavior of this
band is much less pronounced than that of the visible band of
tirapazamine and follows the opposite trend: the absorbance
maximum is lower in energy in polar, hydrogen-bonding
solvents.
Desoxytirapazamine fluoresces more intensely than does TPZ
under comparable experimental conditions (Figure 3). We
measured the fluorescence quantum yield of dTPZ as 0.12 in
acetonitrile.
The rates of electron transfer from various donors to singlet
excited state dTPZ were determined by fluorescence quenching
experiments, and the data obtained are summarized in Table 1.
The measured quenching rate coefficients have magnitudes
approaching the diffusion limit but are, in general, slightly
smaller than the corresponding values obtained with TPZ.5 There
is only marginal evidence to suggest that there is any significant
curvature in the Stern-Volmer behavior, and so it cannot be
determined whether dTPZ forms hydrogen-bonded complexes
Figure 4. Transient absorption spectrum produced upon LFP (400 nm)
of dTPZ in acetonitrile (1) 1.29 and (2) 1033 ps after the laser pulse.
with some quenchers in the same manner as TPZ.5 The larger
rate constants obtained with TPZ may reflect the greater
tendency of this drug to form complexes with quenchers.
Adenosine monophosphate is an exception to the rule. It can
be seen that unlike TPZ, dTPZ will undergo photochemical
electron transfer with adenosine monophosphate. This result may
be due to the fact that the photochemical energy available from
excitation of dTPZ (assuming that the S0,0 f S1,0 electronic
transition for dTPZ in water is approximately 450 nm or 2.8
eV) is greater than that available from excitation of TPZ (S0,0
f S1,0 approximately 510-520 nm or 2.4 eV).
Picosecond time-resolved LFP (400 nm) of dTPZ in aceto-
nitrile produces the transient spectrum of Figure 4. The carrier
of the transient absorption at 640 nm is assigned to the S1 state
of dTPZ. The lifetime of the transient absorption is much greater
than 200 ps, the limit of the spectrometer. This is consistent
with the fluorescence lifetime measurements. The fluorescence
lifetime of dTPZ, obtained by TCSPC technique, was deter-

1494 J. Phys. Chem. A, Vol. 109, No. 8, 2005
Shi et al.
Figure 5. Transient absorption spectrum produced upon LFP (355 nm)
of 0.9 mM dTPZ and saturated KSCN (∼0.15 M) in acetonitrile (1)
10 ns after the laser pulse, (2) 1 µs after the laser pulse, and (3)
calculated radical anion of the UV-vis spectrum of dTPZ (B3LYP/
6-31G*, gas phase).
mined to be 5.4 ns, which is about 50-fold longer than that of
TPZ.5
Nanosecond time-resolved LFP (355 nm) of dTPZ in aceto-
nitrile containing either 0.15 M KSCN or 0.08 M NaN3 produces
new transient absorptions which decay with microsecond
lifetimes (Figure 5). The transient absorption is rapidly quenched
by oxygen (Supporting Information, Figure S4). The carrier of
the transient spectrum is attributed to the radical anion of dTPZ
based on TD-DFT calculations and by analogy to the TPZ
studies and the oxygen dependence of the transient species.
1-Desoxytirapazamine. A second, minor metabolite of
tirapazamine is dTPZ′, an isomer of dTPZ.3
After photoexcitation of dTPZ′, we observe stimulated
fluorescence decay (Figure 6a) at 450 nm; the S1 lifetime is
1.6 ( 0.3 ps. The fluorescence quantum yield of dTPZ′ was
measured to be 0.0024 in acetonitrile, which is similar to that
of TPZ and much smaller than that of its isomer dTPZ.
Figure 6. Transient absorption kinetic traces after photoexcitation of
dTPZ′ (a), QXNO (b), and PNNO (c) in acetonitrile at 400 nm.
Related experiments were performed with quinoxaline di-N-
oxide (QXNO) and phenazine-di-N-oxide (PNNO), Figure 6b
and c, respectively.
often noted interesting differences between the photochemistry
of a CdN-O unit in aromatic N-oxides and the analogous Nd
N-O units in a comparable aromatic system.16-18
The excited S1 states of QXNO and PNNO have fluorescence
quantum yields of less than 0.001 and 0.011 in acetonitrile,
respectively, and lifetimes of 3.1 and 400 ps respectively.
The photochemistry of aromatic N-oxides is complex, and it
is unlikely that a single mechanism can explain all known
observations.16-18 Photolysis of nitrones produces oxaziridines,
which can be isolated as stable compounds.18
4. Discussion
Photolysis of aromatic N-oxides does not produce isolable
bicyclic heterocyclic species. The evidence suggests that ox-
aziridines are formed as reactive intermediates by cyclization
of the excited singlet state.18
Photolysis of TPZ and dTPZ produces the analogous excited
singlet states which can be detected by picosecond transient
absorption spectroscopy. The excited singlet states can be
reduced by anionic electron donors to form observable radical
anions, key species in the cascade of reactions involved in the
biological activity of TPZ.
A key finding is that the excited singlet state of dTPZ is about
50 times longer lived than that of TPZ. Previous workers have

Spectroscopy of Tirapazamine and Desoxytirapazamine
J. Phys. Chem. A, Vol. 109, No. 8, 2005 1495
TABLE 2: Summary of Computational and Photophysical Data
a Estimated by the UV-vis and fluorescence spectra. b Measured in acetonitrile. c Measured by picosecond transient absorption. d Measured by
fluorescence decay.9
A complex mixture of stable products is then formed
depending on the specific reaction conditions and the aromatic
N-oxides employed.
To better understand our data DFT calculations were per-
formed on the N-oxides and di-N-oxides of this study (Table
2). The cyclization of TPZ to form an oxaziridine is predicted
by DFT to be endothermic by 31.7 kcal/mol. The singlet (S1)
energy of TPZ is 54-55 kcal/mol.
Cyclization of dTPZ′ produces an oxaziridine, which exists
in a well-defined minimum.
The energy cost of cyclization of dTPZ′ (34.2 kcal/mol) is
well below the singlet energy (64 kcal/mol) of the N-oxide.
Thus, the fluorescence lifetime and quantum yield are lower
than its isomer dTPZ.
The cyclization of TPZ to form an oxadiaziridine is a much
more endothermic process, 68.0 kcal/mol.
The endothermicities of cyclization of quinoxaline-di-N-oxide
and phenazine-di-N-oxide are 30.1 and 42.3 kcal/mol, which
compare with their singlet energies of 69 and 57 kcal/mol,
respectively. The short singlet lifetimes are consistent with these
results.
Oxadiaziridine itself is not a known compound but has been
studied previously by computational methods.19 We used DFT
calculations to study the following isomeric reaction in which
both the oxaziridine and oxadiaziridine isomers are stable
minima.
Thus, the S1 state of TPZ should readily cyclize to from an
oxaziridine, and this will result in a low fluorescence quantum
yield and a short fluorescence lifetime.
We attempted the analogous calculation with dTPZ. However,
the oxadiaziridine is no longer predicted to be a stationary point.
Attempts to find a minimum for the oxadiaziridine always led
to ring opening.

1496 J. Phys. Chem. A, Vol. 109, No. 8, 2005
Shi et al.
DFT calculations predict that oxadiaziridines are 40 kcal/
mol less stable than isomeric oxaziridines. Thus, we conclude
that cyclization of the excited state of dTPZ (S1 ) 63-64 kcal/
mol) to an oxadiaziridine is endothermic by at least 7 kcal/mol
and as a result is a slow and unimportant process. Consequently,
the excited state of dTPZ does not enjoy a rapid deactivation
process and thus has a greater fluorescence quantum yield and
longer fluorescence lifetime than TPZ.
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Laser flash photolysis of tirapazamine (TPZ) and desoxy-
tirapazamine (dTPZ) produces their excited singlet states. The
S1 states have been observed by picosecond time-resolved
absorption spectroscopy. The lifetime of TPZ is much shorter
than that of dTPZ, and the quantum yield of TPZ fluorescence
is much smaller than that of dTPZ. DFT calculations indicate
that the S1 lifetime of TPZ is controlled by reversible cyclization
to an oxaziridine. The corresponding process in dTPZ forms a
higher energy oxadiaziridine. This latter process is endothermic
and does not influence the photophysics of the singlet state of
dTPZ. The S1 states of TPZ and dTPZ are reduced to radical
anions by KSCN, KI, and NaN3. Studies of the S1 studies of
4-desoxytirapazamine (dTPZ′), which can cyclize to form an
oxaziridine, are consistent with this rule.
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Acknowledgment. Support of this work by the National
Science Foundation and the Ohio Supercomputer Center is
gratefully acknowledged. The authors thank the Ohio Laboratory
of Kinetic Spectrometry at Bowling Green State University,
Professor Michael A. J. Rodgers, Dr. Alex Gusev, and Professor
Felix Castellano for their assistance of the ultrafast spectroscopic
and fluorescence lifetime studies. We also thank Professor Terry
Gustafson, Director of the OSU Center for Chemical and
Biophysical Dymanics (CCBD), for this assistance. The authors
also thank Professor Clemens Burda for the assistance of his
laboratory in obtaining preliminary data.
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Supporting Information Available: The absorption spectra
of the compounds studied, the influence of oxygen on the
lifetimes of transient species, and tables of energies, Cartesian
coordinates, and vibrational frequencies related to the calcula-
tions. This material is available free of charge via the Internet
at http://pubs.acs.org.