Time-resolved spectroscopy of sulfur- and carboxy-substituted N-alkylphthalimides

Article abstract of DOI:10.1002/1521-3765(20010401)7:7<1530::AID-CHEM1530>3.0.CO;2-L

The photophysical and photochemical properties of N-phthaloylmethionine (1), S-methyl-N-phthaloylcysteine methyl ester (2) and N-phthaloyltranexamic acid (3) were studied by time-resolved UV/Vis spectroscopy, using laser pulses at 248 or 308 nm. The quant

Full text of DOI:10.1002/1521-3765(20010401)7:7<1530::AID-CHEM1530>3.0.CO;2-L

FULL PAPER
Time-Resolved Spectroscopy of Sulfur- and Carboxy-Substituted
N-Alkylphthalimides
Helmut Görner,*[a] Axel G. Griesbeck,[b] Thomas Heinrich,[b]
Wolfgang Kramer,[b] and Michael Oelgemöller[b]
Abstract: The photophysical and pho-
tochemical properties of N-phthaloyl-
methionine (1), S-methyl-N-phthaloyl-
cysteine methyl ester (2) and N-phtha-
loyltranexamic acid (3) were studied by
time-resolved UV/Vis spectroscopy, us-
ing laser pulses at 248 or 308 nm. The
quantum yield of fluorescence is low
than for 3 or N-methyltrimellitimide (5')
at ambient temperatures. The quantum
yield (FD) of singlet molecular oxygen
O2(1Dg) formation is substantial for 3
and 5' in several air- or oxygen-saturated
solvents at room temperature, but small
for 2 and 1. The quantum yield of
decomposition is substantial (0.2 ± 0.5)
for 3 and small (<0.05) for 2 and 1. It is
postulated that photoinduced charge
separation in the spectroscopically un-
the cyclization products of 1 and 2. In
aqueous solution, this also applies for 3,
whereas in organic solvents cyclization
involves mainly the lower lying 3p,p*
state. Triplet acetone, acetophenone and
xanthone are quenched by 1 ± 3 in ace-
tonitrile; the rate constant is close to the
diffusion-controlled limit, but smaller
for benzophenone. While the energy
transfer from the triplet ketone occurs
for 3, a major contribution of electron
transfer to the N-phthalimide derivative
is suggested for 1 and 2, where the
radical anion of benzophenone or
4-carboxybenzophenone is observed in
alkaline aqueous solution.
2
(Ff < 10 ) for 1 ± 3 in fluid and glassy
media, whereas that of phosphorescence
is large (0.3 ± 0.5) in ethanol at 1968C.
The triplet properties were examined in
several solvents, at room temperature
and below. The spectra and decay ki-
netics are similar, but the population of
the p,p* triplet state, as measured by T±
T absorption, is much lower for 1 and 2
3
detectable n,p* state may account for
Keywords: electron transfer ´ pho-
tochemistry
´
photocyclization
´
phthalimide ´ quantum yields
formation of a photoisomer as the main product upon direct
excitation occurs through an upper excited n,p triplet state.[11]
Charge separation prior to cyclization was assumed to occur
either upon direct irradiation of 1 ± 3 in acetonitrile or after
energy transfer from triplet acetone or benzophenone, but
different distributions of products were obtained for 1 and 2 in
the absence and in the presence of high energy sensitizers.[1, 2]
The role of energy and/or electron transfer has been discussed
for various systems.[26±31] Examples for electron transfer
substrates include sulfur-containing amino acids, in which, in
alkaline aqueous solution, transfer occurs from the sulfur
atom in methionine or cysteine derivatives to the triplet state
of 4-carboxybenzophenone (CB).[30, 31] A further example for
energy transfer versus electron transfer is the combination of
ketones and DNA bases.[32]
Introduction
The photochemical behaviour of N-phthaloylmethionine (1),
S-methyl-N-phthaloylcysteine methyl ester (2) and a phthal-
imidocarboxylic acid containing a trans-cyclohexane-1,4-diyl
spacer group between the nitrogen and the carboxyl group (3)
has been intensively investigated.[1±11] Phthalimides have been
the subject of various intramolecular photoprocess studies
and some photophysical properties of N-substituted phthali-
mides have been reported.[12±25] N-Alkylphthalimides exhibit
fluorescence with a quantum yield of 0.01 or less and a
lifetime of 0.2 ± 4 ns in solution at room temperature.[5, 18, 25] In
aprotic media, fluorescence is absent for N-methylphthal-
imide (5) and N-propylphthalimide (5'').[20, 21] For N-phthal-
oylvaline methyl ester (4) we have proposed that the
In this study, the photophysics and photochemistry of three
N-phthaloyl aliphatic amino acid derivatives (1 ± 3) and, for
comparison, N-methyltrimellitimide (5') was studied by time-
resolved spectroscopy. The features of the observable triplet
states of 1 ± 3 in polar solvents are discussed and the quantum
yield of triplet formation (FT) is compared with the quantum
yields of decomposition (Fd) and formation of singlet
molecular oxygen (FD). Measurements of the quenching by
[a] Dr. H. Görner
Max-Planck-Institut für Strahlenchemie
45413 Mülheim an der Ruhr (Germany)
[b] Prof. Dr. A. G. Griesbeck, Dipl.-Chem. T. Heinrich, Dr. W. Kramer,
Dr. M. Oelgemöller
Institut für Organische Chemie der Universität zu Köln
Greinstrasse 4, 50939 Köln (Germany)
1530
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0707-1530 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 7

1530±1538
Table 1. Quantum yield Fd of decomposition of phthalimide derivatives.[a]
1
2
3
3'
4[b]
5[b]
5'
[c]
CH3CN
±
< 0.005 0.2
0.06
0.08
0.001 < 0.01
EtOH < 0.04 < 0.04 0.4 (0.05)[d] 0.1 (0.09) 0.16 (0.14) < 0.02 < 0.01
[c]
H2O
< 0.01 0.3 (0.2)
±
0.12 (0.1)
0.01
0.003
[a] In argon-saturated solution upon irradiation at 290 nm. [b] Taken from
ref. [11]. [c] Solubility too low. [d] Values in parentheses refer to oxygen-
saturated solution.
1 ± 3 of the triplet states of acetone, acetophenone, xanthone,
CB and benzophenone were carried out in aqueous solution,
acetonitrile or mixtures thereof.
Results
Photolysis: Phthalimides have two absorption maxima,
around 220 and 295 nm. Upon irradiation (290 nm) of 3 in
argon-saturated ethanol, the 290 nm absorption disappears
and a new maximum appears at <250 nm (Figure 1, part a).
Scheme 1. Photocyclization of 2 by intramolecular charge separation
between the thiomethyl and phthalimide moieties, yielding 2a as the
major product.
of 3 in the presence of 1 M water yielded 3a as the major
cyclization product (68%) and 3b (12%), the product of
decarboxylation without cyclization (Scheme 2).[3, 7] The re-
sults were similar when CB was used as sensitizer.[33]
Emission: The literature values for 5 in acetonitrile are lf ˆ
390 nm, Ff ˆ 8  10 4 and tf ˆ 0.19 ns, while the rate constants
1
1 [25]
are kf ˆ 4  106 s and kisc ˆ 4  109 s .
The fluorescence
Figure 1. Absorption spectra of 3, a) in argon-saturated ethanol prior to
(full line) and after irradiation at 290 nm (broken and dashed lines: 1 and
2 min) and b) in aqueous solution at pH 7 (1 ± 11: 2 s interval).
properties are sensitive to solvent polarity and, in protic
solvents, to hydrogen bonding.[21, 25] In ethanol or acetonitrile
solutions at room temperature, the N-alkylphthalimides
exhibit a weak fluorescence with quantum yields of <1 Â
3
Similar changes were obtained for 3 in water (Figure 1, part b)
and acetonitrile, and for 3' and 4,[11] whereas the degree of
conversion is much smaller for 1, 2 and the other compounds
under examination. The quantum yield of decomposition
ranges from Fd < 0.001 for 5 or 5' to Fd ˆ 0.4 for 3 in ethanol
(Table 1). In contrast to 4[11] and 3', in which the Fd values are
not significantly changed in the presence of oxygen, Fd of 3
depends on the oxygen concentration. The value of Fd is
drastically reduced in oxygen-saturated ethanol, but the effect
of oxygen is smaller in aqueous solution (Table 1).
Practically no photoproducts were found for 1 and 2.[1, 2] For
2 in deoxygenated acetonitrile, Fd is < 0.005, but upon
acetone-sensitized excitation the quantum yield is 0.12, of
which a value of 0.08[2] is accounted for by the major
photoproduct 2a (Scheme 1). Acetone-sensitized photolysis
10 for 1 ± 5. In aqueous solution, in which Ff is low for 1, 2
(or 2') and 5', but larger than in ethanol, the emission maxima
are around 430 nm (Figure 2 and Table 2).
Phosphorescence lifetimes (tp) of approximately 1 s at
1
1968C and triplet energies of ET ˆ 286 ± 297 kJmol are
known for several phthalimides.[18±20] The phosphorescence
spectra of 1 ± 5 and 5' in ethanol at 1968C are very similar
(Figure 2). They have a maximum at approximately 450 nm
1
and an onset at ꢀ405 nm, corresponding to ET ˆ 293 kJmol .
The phosphorescence excitation spectrum generally coincides
with the absorption spectrum (at 248C). The decay follows a
first-order law for 1 ± 5 below 1508C and tp ˆ 0.7 ± 0.9 s at
1968C. The quantum yield is Fp ˆ 0.3 ± 0.7 (Table 2), where-
as fluorescence is practically absent (on the overall lumines-
cence intensity scale).
Chem. Eur. J. 2001, 7, No. 7
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0707-1531 $ 17.50+.50/0
1531

H. Görner et al.
FULL PAPER
no triplet could be detected for
2 or 2' in acetonitrile upon 248
or 308 nm laser excitation, in
contrast to the case of 2 in
ethanol (Figure 3b) or water.
The transient absorbance at
lmax (DAmax) in acetonitrile, us-
ing optically matched condi-
tions and lexc ˆ 308 nm, increas-
es from <0.04 to 0.3 and 1.0
(relative values) for 2 (or 1) 3
and 5, respectively. They are
converted into quantum yields
of population of the observable
Scheme 2. Photoreactions of 3 yielding 3a and 3b as the major and a minor product, respectively.
triplet (FT) based on FT ˆ 0.8 for 5 in acetonitrile. Similar
values were obtained using lexc ˆ 248 nm (Table 3). The DAmax
Table 3. Properties of the triplet state of phthalimide derivatives.[a]
[b]
Com-
pound
Solvent lmax
tT
kox
FT
1
[nm]
[ms]
[109 m 1 s
]
1
2
CH3CN
EtOH
H2O
CH3CN < 330
EtOH
H2O
310
3
3
3
1.5
1.5
1.2
0.05 (<0.05)[c]
315 [320][d]
310
0.15 (<0.05)
0.05 (<0.05)
< 0.03 (<0.05)
0.03 (<0.05)
0.08 (0.05)
325 [340][e]
325
2
3
1.6
1.2
2'
CH3CN < 330
3
< 0.05
3
dioxane
CH2Cl2
CH3CN
EtOH
330
320
330
340 [335][e]
340
330
350
360
350
3
(3)
5
5
8
0.8
(1.1)
1.5
1
1.5
1.2
0.2
(0.4)
0.6 (0.4)
0.5 (0.4)
0.3 (0.2)
0.5
Figure 2. Absorption, fluorescence and phosphorescence spectra, in
ethanol, in water, both at 248C and in ethanol at 1968C (left, middle
and right, respectively), of a) 5, b) 1 and c) 3; lexc ˆ 290 nm.
H2O
3'
5'
CH3CN
dioxane
CH2Cl2
CH3CN
EtOH
5
5
(1.2) (0.3)
5
8
(0.8)
0.8
0.4
0.5
0.3
Table 2. Fluorescence and phosphorescence maxima, quantum yields and
phosphorescence lifetime of phthalimide derivatives.[a]
350
[a] In argon-saturated solution at room temperature, lexc ˆ 248 nm. [b] Us-
Compound Solvent lf
[nm]
Ff (Â103) lp
Fp tp
ing FT ˆ 0.8 for 5 in acetonitrile. [c] Values in parentheses refer to lexc
ˆ
[nm]
[s]
308 nm. [d] Values in brackets refer to 1658C. [e] A second (faster)
component appears below 1308C.
1
2
H2O
EtOH
H2O
< 420
< 420
< 420
< 420
< 420
430
< 1
< 0.1
0.2
445
0.3 0.85 (ꢂ0.3)[b]
EtOH
EtOH
H2O
< 0.1
< 0.1
4
450
452
0.3 0.85 (ꢂ0.3)
2'
3
0.4 0.86
EtOH
EtOH
H2O
EtOH
EtOH
H2O
< 420
< 400
415
< 405
< 400
422
< 0.2
< 0.2
2
< 0.5
< 0.5
0.5
450
450
0.3 0.75 (0.6)
0.5 0.85
4[c]
5[c]
5'
450
455
0.6 0.75 (0.3)
0.6 0.73
5''[c]
EtOH
< 400
< 0.5
453
0.7
[a] In air-saturated solution at 24 and
1968C for fluorescence and
phosphorescence, respectively. [b] Values in parentheses: triplet lifetime
(absorption) at 1708C. [c] Values for 4, 5 and 5'' were taken from ref. [11].
Transients observed upon direct excitation: The triplet
properties of 3, 4 and 5 are similar, and the assignment of
the observed main transient to the lowest triplet state is based
on the similarities with 5 (Table 3). The T± T absorption
spectra of 3 in acetonitrile (Figure 3c) and other solvents show
a lmax ꢁ 330 nm; that of 1 (Figure 3a) is rather weak. Virtually
Figure 3. Transient absorption spectra in argon-saturated solution of a) 1
in ethanol, b) 2 in ethanol and c) 3 in acetonitrile (in the absence of
*
*
additives) at 30 ns ( ) and 5 ms ( ) after the 248 nm pulse, A248 ꢀ2; insets:
decay kinetics at 330 nm.
1532
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0707-1532 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 7

Time-Resolved Spectroscopy
1530±1538
Table 4. Quantum yield of singlet molecular oxygen, FD.[a]
values of 1 and 2 are much smaller than those of 3, the last of
which is fairly independent of solvent polarity. The triplet
lifetimes of 1 ± 3 in several solvents are in the 1 ± 10 ms range.
1
2'
3
4[b]
5'
toluene
CH3CN
EtOH
D2O
0.4
(0.32)
0.4
(0.38)
0.5
(0.38)
0.2
0.3
(0.18)[c]
0.9
The decay is quenched by oxygen with rate constants of kox
ˆ
1
(1 ± 2) Â 109 M 1 s . For 3 in water an additional minor
transient was observed, with the two lexc ˆ 248 or 308 nm.
The T± T absorption of 5 in ethanol is observable in the
temperature range down to 1968C; the triplet lifetime
under argon changes by five orders of magnitude (Figure 4).
The DA330 value increases only slightly on going from 25 to
1968C, using 308 or 248 nm laser pulses. This could be due
to changes in the ground state at lexc and indicates that FT at
room temperature is close to its maximum value, which should
be almost unity (assuming that internal conversion does not
take place). For 1 ± 5 in the 150 to 1968C range, virtually
the same lifetime was observed both for phosphorescence and
for T± T absorption (Table 2), demonstrating that the triplet
states, observed by emission and absorption, are identical. The
T± T absorption spectrum and triplet lifetime of 1 ± 3 at
intermediate temperatures under the same conditions are also
similar to those of 5, whereas the yield (measured by the DA330
value) is significantly smaller and increases on decreasing the
temperature (Figure 4). It is noteworthy that for 2 a second
< 0.04
0.04
0.15
(0.10)
0.2
(0.20)
0.1
(0.65)
< 0.04
< 0.05
[d]
< 0.05
±
(0.06)
[a] In oxygen-saturated solution (unless otherwise indicated), lexc
308 nm. [b] Taken from ref.[11] [c] Values in parentheses: in air-saturated
solution. [d] Solubility too low.
ˆ
the triplet state are intercepted by oxygen. Generally, the
fraction of the triplet state intercepted by oxygen is larger
(typically >98%), on the basis of the triplet lifetimes of
<70 ns and 3 ± 10 ms in oxygen-saturated and argon-saturated
solutions, respectively.
Photoreactions upon ketone-sensitized excitation: In addition
to energy transfer from the triplet state of the ketone (3K*) to
the acceptor molecule (Q), electron transfer to 3K* may also
occur; that is, the two are, in principle, competing processes
(Scheme 3).[26±31] When T± T annihilation can be ignored, the
decay of the triplet state is influenced by self-quenching and
quenching by oxygen (Scheme 3, reaction 3). In the presence
of an electron donor, the two major subsequent transients are
.
the ketyl radical ( KH)Ðdiphenylhydroxymethyl radical in
the case of CB or benzophenoneÐand the radical anion
.
(K ). Electron transfer (Scheme 3, reactions 4,5) or hydro-
Figure 4. Effect of temperature on FrTel (right scale) and the reciprocal
triplet lifetime (left scale) of 5 (circles), 1 (triangles), 2 (diamonds) and 3
(squares) in argon-saturated ethanol; open and full symbols refer to
phosphorescence and absorption, respectively; lexc ˆ 248 nm.
Scheme 3. Primary ketone-sensitized reactions pathways of N-phthal-
imides (Q).
decay component appears below 1308C; its spectrum does
not differ markedly from that of the major one, but the
lifetime is about two orders of magnitude shorter. This species
could be tentatively assigned to a second, higher-lying excited
triplet state.
gen-atom abstraction (Scheme 3, reactions 4,6), both coupled
by equilibrium (7) (Scheme 3), energy transfer (Scheme 3,
reactions 4,8) or physical quenching of the collision complex
may also occur.
Formation of singlet molecular oxygen: The quantum yield
FD in several air- or oxygen-saturated solvents at room
temperature is 0.1 ± 0.25 for 3, up to 0.7 for 5 and 5', but
virtually zero for 1 and 2' (Table 4). The largest change on
going from air-saturated to oxygen-saturated solution was
found for 5', for which the FD values in dichloromethane are
0.18 and 0.44, respectively. This is in agreement with the kox
and tT values (Table 3), since only 80% of the molecules in
The pKa of equilibrium (7) for benzophenone or acetophe-
none in aqueous solution is ꢀ9 ± 10 and for CB, pKa ˆ 8.3.[30, 34]
.
Acetone or xanthone, due to their low e values for KH and
.
K
, are not appropriate sensitizers for the detection of
hydrogen-atom and/or electron transfer reactions in the
presence of N-phthalimides.[11] These processes can be
observed with acetophenone or benzophenone and even
better with CB.
Chem. Eur. J. 2001, 7, No. 7
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0707-1533 $ 17.50+.50/0
1533

H. Görner et al.
FULL PAPER
Table 6. Rate constant for quenching of ketone triplet states by phthal-
Transients observed with benzophenone and the carboxy
derivative: Excitation of benzophenone in argon-saturated
acetonitrile (or in a mixture with water) in the presence of 1, 2
imides, k4 [109 m 1 s 1].[a]
1
2'
3
4[b]
5[b]
3
or 3, using 248 or 308 nm laser pulses, produces K* (with
benzophenone
CB[c]
xanthone
acetophenone
acetone[d]
3
4
6
6
2
4
4
1
2
6
0.9
2
2
6
7
lmax ˆ 325 and 525 nm) as an initial species. For 3, a second
4
9
transient with lmax ˆ 330 nm appears after a few ms (Figure 5,
10
5(9)[e]
9(9)
[a] In acetonitrile (unless otherwise indicated), lexc ˆ 248 nm. [b] Taken
from ref. [11]. [c] In water. [d] Build-up kinetics for the phthalimide triplet.
[e] Values in parentheses refer to lexc ˆ 308 nm.
Similar results were registered with CB, in which the three
.
.
transients (3K*, KH and K ) have maxima in the 335 ±
360 nm range and, in addition, at lmax ˆ 550, 570 and
660 nm, respectively. The respective molar absorption coef-
ficients are e550 ˆ 5.2  103, e570 ˆ 5.2  103 and e660 ˆ 6.0 Â
1 [28±31]
103 M 1 cm .
Excitation of CB in argon-saturated aque-
.
ous solution in the presence of 1 yields KH (Figure 6, part
Figure 5. Transient absorption spectra of benzophenone in argon-saturat-
ed acetonitrile/water 4:1 in the presence of a) 3 (0.5 mm, pH 7), b) 1
*
*
(0.1 mm, pH 7) and c) 1 (pH 10) at 30 ns ( ) and 3 ms ( ) after the 248 nm
pulse; insets: kinetics at 340 nm (left) and 530 nm (right).
part a); we assign this to the lowest triplet state of 3, reactions
.
(4) and (8) (see Scheme 3). As secondary transients KH
.
(lmax ˆ 550 nm) and K (lmax ˆ 640 nm) could be detected for
1 in acetonitrile-water mixtures at pH 4 ± 6 (Figure 5, part
b) and pH 10 ± 11 (Figure 5, part c), respectively (Table 5).
Virtually no signal appears for 2' after triplet quenching; that
3
is, decay of K* occurs through physical quenching of the
collision complex. The rate constant of decay increases
linearly with the quencher concentration, and the rate
constants k4 are obtained from the slope (Table 6).
Figure 6. Transient absorption spectra of CB in argon-saturated aqueous
solution in the presence of a) 1 (0.5 mm) at pH 7, b) 1 at pH 9.5, and c) 2'
&
*
*
(0.5 mm, pH 9.5) at 30 ns ( ), 10 ms ( ) and 100 ms ( ) after the 248 nm
pulse; insets: kinetics at 360 nm (left) and a) 550 nm, or b) and c) 650 nm
(right).
Table 5. Transients obtained with phthalimide derivatives in the presence
of ketones.[a]
[c]
Ketone
Phthalimide Conditions[b] lmax [nm] t1/2[d] [ms] Species
.
benzophenone none
I
I
II'
I
II'
III
III'
III
III'
I
III'
I
I
320, 520
550
660
330
330
570
660
660
570
330
440
330
330
330
6
> 3
> 10
4
3K*[e]
a) and K (Figure 6, part b) as secondary transients at pH < 7
.
1
1
3
5
KH
.
and 8.5 ± 10, respectively. K could also be detected for 2'
.
K
(Figure 6, part c and Table 5), but not for 3. Electron transfer
from triplet CB or benzophenone to 1 and 2', possibly in
addition to energy transfer, is also indicated by the larger rate
constant of quenching with respect to the cases of 3 or 5
(Table 6).
33*
5
35*
.
CB
1
1
2'
2'
3
1
3
3
3
> 3
> 10
> 10
> 3
4
> 5
4
< 10
< 10
KH
K
KH
.
.
.
K
xanthone
acetophenone
33*
.
K
Transients observed with xanthone, acetophenone and ace-
33*
33*
33*
3
tone: Excitation of xanthone produces K* (lmax ˆ 300 and
acetone
620 nm) in the absence and presence of Q (Table 5). The
secondary transient in the presence of 5 with lmax ˆ 330 nm
(not shown) is assigned to the triplet state of 5, reactions (4)
plus (8). A similar result was observed for 3, but virtually no
remaining signal (after a few ms) appears for 1 or 2 (not
shown). For the system of 3 or 5 and xanthone, in contrast to
III
[a] In argon-saturated solution, using [phthalimide] ꢂ 1 mm and A(lexc) ˆ
1 ± 3 for ketones; lexc ˆ 248 nm. [b] I: in acetonitrile, II': CH3CN/H2O 1:1 at
pH > 9, III: water at pH ꢁ 7, III': pH > 9. [c] No remaining acceptor triplet
3
was observable for 1 or 2. [d] Half-life for the cases of radicals. [e]The K*
triplet is the precursor in all cases.
1534
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0707-1534 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 7

Time-Resolved Spectroscopy
1530±1538
the cases with benzophenone and acetophenone, the observed
Discussion
signal at 330 nm increases with time; that is, the e330 value of
3
3Q* is larger than that of K*. The rate constant of triplet
Photoprocesses with N-alkylphthalimides: Generally, low Ff
values were observed for compounds 1 ± 5'' (Table 2). The
fluorescing singlet state of N-substituted phthalimides is the
1p,p* state, which is believed to be above the 1n,p*
state,[18, 19, 25] but could be below it.[11] The observed lowest
triplet of 3 is probably a 3p,p* state. Whether the higher lying
3n,p* state is below the 1p,p* state is controversial even for 5
or 5'', and this question cannot be conclusively answered in
the cases of other N-substituted phthalimides, such as 1 ± 3.
The quantum yield of intersystem crossing of 5 in acetonitrile
is FT ˆ 0.7 ± 0.8.[11, 25] For 3 and 5' FT is slightly smaller, but it is
much smaller for 1, 2 and 2' (Table 3).
decay (at 620 nm) increases linearly with [Q], the rate
constant is k4 ˆ (4 ± 6)  109 m 1 s 1 (Table 6).
The triplet state of acetophenone has lmax < 330 nm;
addition of 3 (not shown) leads to a longer lived transient
with lmax ꢀ 330 nm in argon-saturated acetonitrile. The rate
constant of decay of triplet acetophenone increases with the
concentration of 3, but this effect is partly obscured, due to
comparable overlapping absorption of the secondary transi-
.
ents; e.g., K or 3Q*. Similar results were obtained in
aqueous solution for 1 (pH 4 ± 5) and 2 or 3 (pH 7 ± 8). On
.
increasing the pH to 11,
K
(lmax ˆ 440 nm, e ꢀ
4000 m 1 cm )[27] was observed for 1.
Energy transfer to the phthalimide type acceptor (reactions
(4) and (8)) occurs from the triplet state of high energy
sensitizers, such as acetone, acetophenone or xanthone, with
1
Excitation of acetone in argon-saturated acetonitrile, using
248 or 308 nm laser pulses, produces the triplet state (lmax
<
1
330 nm with small DA). The same T± T absorption spectrum
appears in aqueous solution at pH 7; with lexc ˆ 248 nm at
high intensities the hydrated electron (e aq) also appears in
low yield. Addition of 5' to acetone in aqueous solution leads
to a new transient with lmax ˆ 330 nm (Figure 7, part a). The
value of DA330, owing to the small e value of 3K* with respect
to that of 35'*, markedly increases, reaches a maximum
(DA330max) and then decays within a few microseconds. For 3, a
ET ˆ 309, 305 or 300 kJmol , respectively. The quenching rate
constants in acetonitrile at room temperature are close to the
diffusion-controlled limit (Table 6). For benzophenone, with
1
ET ˆ 288 kJmol , this rate constant is significantly smaller, in
1
agreement with the above estimate of ET ˆ 293 kJmol for
the lowest triplet state of N-substituted phthalimides.
Direct and sensitized photoreactions with 3: In the case of
direct excitation of 3 in several solvents at room temperature,
the lowest triplet state is rather efficiently populated. This is
confirmed by the FT and FD values in Table 3 and Table 4. A
singlet pathway for cyclization of 3 under direct excitation
conditions is unlikely in view of the large Fd value and the
short singlet lifetimes for either 3 or those phthalimides (5, 5'
and 5'') that are chemically unreactive. Intramolecular
electron transfer should cause charge separation and CO2
elimination, leading mainly to cyclization, to which the
observed chromophore loss of 3 is ascribed. Triplet quenching
by oxygen should strongly reduce the effect of intramolecular
electron transfer. Therefore, one would expect that the Fd
values should be much lower in the presence of oxygen. In
fact, the results in ethanol are in agreement with this lowest
triplet pathway (Table 1).
build-up (increase of DA330 with time, see Figure 7, part c) with
max
smaller DA330
DA330
than for 5' was observed. The increase of
max
versus [Q] is more pronounced with acetone than
xanthone. The build-up rate constant increases linearly with
[Q]; the slope equals k4. For 1 (Figure 7, part b) or 2, however,
In aqueous solution, however, Fd is only 30% smaller
under conditions of saturation with oxygen than with argon.
Therefore, we propose that the observed 3p,p* state is mainly
deactivated by intersystem crossing and that photodecompo-
sition as a measure of photocyclization is a fast competing
3
reaction, involving the spectroscopically undetectable n,p*
state. For 3 in aqueous solution, this level should be equal to
or only slightly above the 3p,p* state, but significantly lower in
organic solvents, in ethanol particularly. An analogous argu-
ment that the 3p,p* state is not reactive with respect to
cyclization (the Fd values are not affected by oxygen) has
already been used for the isomerization in the case of 4 in the
absence and in the presence of water.[11] The hypothesis of the
Figure 7. Transient absorption spectra of acetone in argon-saturated
aqueous solution (1:9) in the presence of a) 5' (0.1 mm, pH 7), b) 1 (0.1 mm,
*
pH 4) and c) 3 (0.2 mm, pH 7) at 30 ns ( ) after the 308 nm pulse and at 0.5
*
*
(
) and 2 ms ( ) for 5', 1 and 3, respectively; insets: grow-in and decay
kinetics at 330 nm.
3
3p,p* state as the observed triplet and the n,p* state as a
there was triplet quenching but virtually no new transient
could be detected either in acetonitrile or in aqueous solution
(on increasing the pH to 11). The lack of an observable triplet
state of 1 and 2 after quenching of high-energy sensitizers
indicates efficient higher excited state reactivity.
spectroscopically undetectable triplet has been suggested for
other aromatic compounds containing a nitrogen atom, such
as acridines, although the energy gap here is larger.[35]
For the methyl ester 3', cyclization is possible but decar-
boxylation is not. In fact, Fd is significantly smaller than for 3
Chem. Eur. J. 2001, 7, No. 7
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0707-1535 $ 17.50+.50/0
1535

H. Görner et al.
FULL PAPER
(Table 1). The triplet properties are reminiscent of those of N-
alkylphthalimides, in which virtually no reactive 3n,p* state is
involved.[11, 18, 19]
The radiation chemistry of methionine is well known.[36]
The OH radical-induced decarboxylation involves a sulfur
radical cation with lmax ˆ 400 nm and a lifetime of ꢀ 0.2 ms and
a longer lived dimeric sulfur radical cation with lmax ˆ 480 nm.
However, under the applied conditions, no transient attribut-
able to a sulfur-centred radical cation was found for 1 or 2.
Instead, the observed weak transient is most probably the
lowest p,p triplet state. The proposed major cyclization of 1
and 2 after excitation occurs by means of intramolecular
charge separation between the thiomethyl and phthalimide
moieties (P' S Me ), as illustrated in Scheme 4 and
Scheme 1, respectively. Photoinduced charge separation has
also been proposed for N-(2-methyl-2-propenyl)phthalimides,
to account for the cyclization products.[2±16] The further
reaction steps of 1 and 2 have already been discussed.[1, 2]
Efficient triplet population is also the case for ketone-
sensitized excitation (reactions (4) and (8)), since the triplet of
3 was observed under all conditions investigated (Figure 5,
part a and Figure 7, part c). Quenching by electron transfer is
.
.
unlikely, since neither KH or K could be detected for the 3/
CB or 3/benzophenone systems, in contrast to the cases of 1 or
2'. To account for the rather effective acetone-sensitized
photolysis of 3,[3] we postulate population of the 3n,p* state in
.
.
‡
3
aqueous solution and of the p,p* state in the absence of
water. Intramolecular electron transfer should cause charge
separation, CO2 elimination, cyclization and eventually pro-
tonation to complete the formation of 3a (Scheme 2).
Sensitized photoreactions with 1 and 2: Reaction (4) of the
ketone triplet state with the phthalimide is the major process,
since the k4 values of 1 and 2 are similar to those of 5 (Table 6).
After quenching of 3K*, however, practically no triplet could
be generated for 1 (Figure 5, parts b and c, Figure 6, parts a
and b) and 2' (Figure 6, part c); that is, the energy transfer
reaction (8) is too fast for observation or does not take place
Photoreactions of 1 and 2 upon direct excitation: On the basis
of flash photolysis studies of N-substituted phthal-
imides,[14, 19, 23] it is conceivable to assign the transient absorp-
tion spectrum of 1 or 2 (Figure 3, part a and b) at room
temperature to the lowest triplet state. The phosphorescence
results from the phthalimides in ethanol at 1968C (Figure 2
and Table 2) support this. The shorter triplet lifetime of 1 ± 3
with respect to rather unreactive N-alkylated phthalimides[19]
could be due to subsequent reactions. However, the similar
lifetimes for the N-phthalimides examined (Table 3), or at
least those within the same order of magnitude, rule this out.
Singlet states have been assumed to contribute to the
reactivity of N-substituted phthalimides upon direct excita-
tion.[1, 2] The fluorescence lifetime of phthalimide esters in
several solvents at room temperature are approximately
0.2 ns.[5, 18] Because of the similar Ff (Table 2) and tf values
of reactive 1, 2 and 2' and unreactive 5 and 5', a triplet route
upon direct excitation is more likely than a singlet route. Why,
however, is almost no triplet state detectable for 1 and 2 at
room temperature (Table 3)? The increase in DA330 on
decreasing the temperature (Figure 4) indicates a fast reaction
competing with population of the observed triplet state, and
that this reaction is suppressed at lower temperatures. A
possible explanation for the fast competing reaction in the
cases of 1 and 2 (but not 4)[11] is the population of an upper
excited triplet state, followed by intramolecular electron
transfer and then back-transfer.
3
at all. The low amount of observed p,p* state should not be
due to physical quenching without chemical reactivity, since
the quantum yield of formation of photoproduct 2a is 0.08
upon acetone-sensitized excitation.[2, 9] One possibility might
be
a
too rapid charge separation in the 3n,p* state
.
.
‡
(3*P' S Me ! P' S Me ) and another that quenching of
3K* generates, to some degree, the 3n,p* state in which charge
separation should occur. The most likely reason is intermo-
lecular electron transfer (reaction (5) or reactions (6) plus
(7)). In fact, for benzophenone or CB, formation of the radical
and the radical anion was observed for 1 and gradually for 2'
in the presence of water at pH < 7 and > 9, respectively
(Figure 5, part bÐFigure 6, part c). This could be interpreted
by the respective pathways b) and c) in Scheme 5.
Intermolecular electron transfer: The rate constant for
quenching of the benzophenone or CB triplet state by 1 is
significantly larger than that in the case of 3 (Table 6). This is
in agreement with the redox properties. The measured
oxidation potential (in acetonitrile vs. ferrocene) is Eox
ˆ
Scheme 4. Photocyclization of 1 through intramolecular charge separation between the thiomethyl and phthalimide moieties, yielding 1a and 1b as products.
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0947-6539/01/0707-1536 $ 17.50+.50/0 Chem. Eur. J. 2001, 7, No. 7
1536

Time-Resolved Spectroscopy
1530±1538
Table 7. Quantum yields of phthalimide derivatives.[a]
IET[c] Type[d]
[b]
Compound Ff
FT
Fp
FD
Fd
1
2
< 0.001 0.05 0.3
< 0.001 0.05 0.3
< 0.05
< 0.02 yes
< 0.01 yes
yes
C
C
C
A/B
B
2'
3
3'
4[e]
5[e]
< 0.001
< 0.002 0.5
0.5
0.4
0.3
< 0.05
0.13
0.3
no
< 0.02 no
0.16 no
< 0.01 no
< 0.002 0.2
< 0.005 0.5
0.5
0.6
0.3
0.6
B
D
[a] From Tables 1 ± 4 at 248C using averages of values in acetonitrile and
water. [b] At 1968C. [c] Intermolecular electron transfer upon benzo-
phenone- or CB-sensitized excitation. [d] See text. [e] Values taken from
ref. [11].
Scheme 5. Ketone-sensitized reactions pathways a ± c accounting for
cyclization of the thiomethyl and phthalimide (P') moieties.
Conclusion
The excited singlet states of all N-phthalimides investigated
are too short-lived and decay preferentially by intersystem
crossing to the corresponding triplet states and only margin-
ally (Ff < 0.01) by fluorescence. The observable triplet state is
of p,p* type and shows strong phosphorescence (Fp ˆ 0.3 ±
0.5). The photoreactive N-phthalimides are converted into
products with remarkable differences in their quantum yields
of chromophore decomposition: Fd ˆ 0.2 ± 0.4 for 3 and
< 0.05 for 1 and 2. For 3 in ethanol and water, Fd is too
large for the involvement of the singlet pathway. The quantum
yields for formation of phthalimide triplet states and singlet
oxygen are substantial (FT ˆ 0.2 ± 0.5, FD ˆ 0.1 ± 0.2) for 3 and
4, but small for 1 and 2. A new triplet pathway and
intramolecular electron transfer are postulated for the photo-
cyclization. The involvement of the spectroscopically unde-
tectable higher lying 3n,p* state prior to intramolecular
electron transfer accounts for the difference between Fd
and FT in 4 (photoconversion into an isomer) and is suggested
for the photocyclization (in low yield) of 1 and 2. In the case of
3, photodecarboxylation and product formation involve the
3p,p* and 3n,p* states in organic solvents and aqueous
solution, respectively. Thus, the chemical behaviour of excited
3 and 4 points to a situation in which the classical Kasha rule
(in its photochemical version) is circumvented.
1.28, 1.47 and > 1.9 V for 1, 2' and 3, respectively.[37] The free
energy change for electron transfer is given by the Rehm ±
Weller relationship, where the term Ec takes account of ion
pairing (<0.06 V in acetonitrile or mixtures with water).[38]
*
DG ˆ Eox Ered Ec
(1)
Using the energy of the T1 state of benzophenone (3.0 V) and
the reduction potential ( 1.83 V), it follows that the reduc-
*
tion potential in the triplet state is Ered ˆ 1.2 V (versus SCE;
note that Eox versus SCE is approximately 0.3 V more positive
than versus ferrocene). Thus, with DG ˆ 0.2, ꢀ 0 and > 0.4 V
(versus SCE), for 1, 2' and 3, respectively, electron transfer
from the phthalimide to triplet benzophenone is feasible for 1
and 2, but not for 3. The results presented in Table 6 reveal
that the choice of ketone sensitizers examined does not seem
to be critical concerning the reaction step (4). The possibility,
.
.
for a given phthalimide, of observing KH or K (Table 5) is a
consequence of the specific sensitizer properties: essentially
the e values. For time-resolved measurements, benzophenone
or CB are favoured, whereas for product composition analysis
acetone may be better.
Under conditions of sensitized excitation (acetone, aceto-
phenone, xanthone and, less efficiently, benzophenone)
product formation is enhanced for 1 or 2, in contrast to 4.
The evidence for intermolecular electron transfer from 1 or 2
to triplet benzophenone or the carboxy derivative is based on
spectroscopic and kinetic characterization of the radical anion
of the ketone and its conjugated acid. Thus, the triplet state of
the ketone acts as an electron acceptor.
Pathways of photocyclization: Four cases can be distinguished
mechanistically, and results supporting the different types of
photochemical reactivity are summarized in Table 7. Virtually
no photochemical reactivity at all (low Fd) is expected for 5, 5'
or 5''. This case (D) is characterized by efficient triplet
population (large FT and FD values). The other extreme, case
A, of high photochemical reactivity (large FT, FD and Fd
values), through the 3p,p* state accounts for 3 in organic
solvents. A second triplet route (case B), of cyclization
Experimental Section
3
through the higher lying n,p* state for 4 in the absence and
±
presence of water and for 3 in aqueous solution, has large Fd
and somewhat reduced FT and FD values. In case C,
intermolecular electron transfer from 1 and 2 to triplet
benzophenone or CB was observed. Inefficient triplet pop-
ulation (small FT and FD values) and low Fd (as a measure of
cyclization) upon direct excitation of 1 or 2 can be explained
by intramolecular back electron transfer as a major effect.
Compounds 1 ± 3 were as used previously.[1 3] The molar absorption
coefficient of 2 is e296 ˆ 1.7  103 and e219 ˆ 4.7  104 M 1 cm
.
1 [2] N-Methyl-
phthalimide (5, EGA), the sensitizers and most solvents (acetone and
acetonitrile: Merck, Uvasol) were used as commercially available; ethanol
was purified by distillation and water was deionized by a Millipore
(Milli Q) system. Where indicated, the pH values refer to ordinary aqueous
solution; otherwise they were adjusted with NaOH. N-Alkylphthalimides
in alkaline solution are known to hydrolyze;[21] that is, they are thermally
Chem. Eur. J. 2001, 7, No. 7
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0707-1537 $ 17.50+.50/0
1537

H. Görner et al.
FULL PAPER
unstable. The stability in acetonitrile-water mixtures (1:1) at pH 10.5 was
found to increase in the order 5, 2 and 1; the last two could be used within a
few minutes after establishing the pH, to record transient absorption
spectra. Compound 2' was used rather than 2 in some cases, due to its better
solubility.
[14] U. C. Yoon, J. W. Kim, J. Y. Ryu, S. J. Cho, S. W. Oh, P. S. Mariano, J.
Photochem. Photobiol. A: Chem. 1997, 106, 145.
[15] Z. Su, P. S. Mariano, D. E. Falvay, U. C. Yoon, S. W. Oh, J. Am. Chem.
Soc. 1998, 120, 10676.
[16] T. C. Barros, G. R. Molinari, P. Berci Filho, V. G. Toscano, M. J. Politi,
J. Photochem. Photobiol. A: Chem. 1993, 76, 55.
Absorption and emission spectra were recorded on spectrophotometers
(Perkin ± Elmer 554 and Hewlett Packard 8453) and spectrofluorimeters
(Perkin ± Elmer LS5 and Spex-Fluorolog), respectively. Note that the
fluorescence excitation spectrum does not generally coincide with the
absorption spectrum, owing to a very small Ff value and superposition of
fluorescence from trace impurities. Phosphorescence of O2(1Dg) at
1269 nm[39±41] was detected after the pulse, using a cooled Ge detector
(North Coast, EO 817FP), a silicon filter, an interference filter and an
amplifier (Comlinear, CLC-103) as described elsewhere.[11] The lifetimes
range from 10 ± 14 ms in ethanol to 60 ± 90 ms in dichloromethane. The
signal, extrapolated to the end of the 20 ns pulse (ID), was found to show a
linear dependence on the absorbed energy and on the laser intensity; the
slope of the latter plot is denoted as qD. The quantum FD in toluene was
obtained from qD values using optically matched solutions (A308 ˆ 0.8) and
acridine as reference FDref ˆ 0.71.[40] For the values in other oxygen-
saturated solvents a correction has to be applied, using the rate constant
kr for radiative deactivation of O2(1Dg) relative to that in benzene (kor †. The
kr/kor ratios taken from the literature range from 0.12 in D2O to 0.96 in
toluene.[41] The 248 or 308 nm output from one of two excimer lasers
(Lambda Physik EMG 200 and EMG 210 MSC; energy less than 100 mJ,
pulse width 20 ns) was used for excitation. The laser flash photolysis
apparatus and the detection system have been described elsewhere.[11, 39]
[17] T. C. Barros, S. Brochsztain, V. G. Toscano, P. Berci Filho, M. J. Politi,
J. Photochem. Photobiol. A: Chem. 1997, 111, 97.
[18] J. D. Coyle, G. L. Newport, A. Harriman, J. Chem. Soc. Perkin Trans. 2
1978, 133.
[19] J. D. Coyle, A. Harriman, G. L. Newport, J. Chem. Soc. Perkin Trans. 2
1979, 799.
[20] H. Hayashi, S. Nagakura, Y. Kubo, K. Maruyama, Chem. Phys. Lett.
1980, 72, 291.
[21] P. Berci Filho, V. G. Toscano, M. J. Politi, J. Photochem. Photobiol. A:
Chem. 1988, 43, 51.
[22] F. C. L. Almeida, V. G. Toscano, O. dos Santos, M. J. Politi, M. G.
Neumann, P. Berci Fo, J. Photochem. Photobiol. A: Chem. 1991, 58,
289.
[23] E. C. Hoyle, E. T. Anzures, P. Subramanian, R. Nagarajan, D. Creed,
Macromolecules 1992, 25, 6651.
[24] D. Creed, E. C. Hoyle, J. W. Jordan, C. A. Panday, R. Nagarajan, S.
Pankasem, A. M. Peeler, P. Subramanian, Macromol. Symp. 1997, 116,
1.
[25] V. Wintgens, P. Valet, J. Kossanyi, L. Biczok, A. Demeter, T. Berces, J.
Chem. Soc. Faraday Trans. 1994, 90, 411.
[26] I. Carmichael, G. L. Hug, J. Phys. Chem. Ref. Data 1986, 15, 1.
[27] a) E. Hayon, T. Ibata, N. N. Lichtin, M. Simic, J. Phys. Chem. 1972, 76,
2072; b) G. E. Adams, R. L. Willson, J. Chem. Soc. Faraday Trans. 1
1973, 69, 719.
.
For selective production of KH, 2-propanol was used; the radical anion
was generated by electron transfer from 1,4-diazabicyclo[2.2.2]octane or in
the presence of 2-propanol in water at pH 10 ± 11.[11]
[28] a) S. Baral-Tosh, S. K. Chattopadhyay, P. K. Das, J. Phys. Chem. 1984,
88, 1404; b) J. K. Hurley, H. Linschitz, A. Treinin, J. Phys. Chem. 1988,
92, 5151; c) K. Okada, M. Yamaji, H. Shikura, J. Chem. Soc. Faraday
Trans. 1998, 94, 861.
[29] K. Bobrowski, B. Marciniak, G. L. Hug, J. Photochem. Photobiol. A:
Chem. 1994, 81, 159.
[30] a) K. Bobrowski, B. Marciniak, G. L. Hug, J. Am. Chem. Soc. 1992,
114, 10279; b) K. Bobrowski, G. L. Hug, B. Marciniak, H. Kozubek, J.
Phys. Chem. 1994, 98, 537.
[31] a) B. Marciniak, G. L. Hug, K. Bobrowski, H. Kozubek, J. Phys. Chem.
1995, 99, 13560; b) G. L. Hug, B. Marciniak, K. Bobrowski, J.
Photochem. Photobiol. A: Chem. 1996, 95, 81.
For photodecomposition, lirr of 290 nm from a 1000 W Xe/Hg lamp
combined with a monochromator or 254 nm from a low pressure Hg lamp
was used. The concentration was adjusted such that the absorbance (in a
1 cm cell) was 1 ± 3 at lirr. The quantum yield Fd was obtained by using the
decrease of absorbance at 290 nm (or the increase in absorbance at the
initial minimum ꢀ 250 nm). Actinometry was performed with Aber-
chrome 540 for lirr ˆ 290 nm.[42] The measurements (unless specified
otherwise) were carried out after purging with argon prior to and during
irradiation at 24 Æ 28C.
[32] a) I. G. Gut, P. D. Wood, R. W. Redmond, J. Am. Chem. Soc. 1996,
118, 2366; b) P. D. Wood, R. W. Redmond, J. Am. Chem. Soc. 1996,
118, 4256.
Acknowledgements
We thank Professor K. Schaffner for his support, Dr. E. Bothe for
electrochemical measurements and Mr. L. J. Currell for technical assis-
tance.
[33] A. G. Griesbeck, private communication.
[34] E. Hayon, M. Simic, Acc. Chem. Res. 1974, 7, 114.
[35] a) D. G. Whitten, Y. J. Lee, J. Am. Chem. Soc. 1971, 93, 961; b) H.
Görner, P. Nikolov, R. Radinov, J. Photochem. Photobiol. A: Chem.
1995, 85, 23, and references therein.
[36] a) R. S. Glass, Top. Curr. Chem. 1999, 205, 1; b) K. Bobrowski, C.
Schöneich, J. Holcman, K.-D. Asmus, J. Chem. Soc. Perkin Trans. 2
1991, 353; c) C. Schöneich, K. Bobrowski, J. Holcman, K.-D. Asmus,
Oxidative Damage and Repair, Pergamon, Oxford, 1991, p. 380;
d) B. l. Miller, K. Kuczera, C. Schöneich, J. Am. Chem. Soc. 1998, 120,
3345.
[1] A. G. Griesbeck, H. Mauder, I. Müller, E.-M. Peters, K. Peters, H. G.
von Schnering, Tetrahedron Lett. 1993, 34, 453.
[2] A. G. Griesbeck, J. Hirt, K. Peters, E.-M. Peters, H. G. von Schnering,
Chem. Eur. J. 1996, 2, 1388.
[3] A. G. Griesbeck, A. Henz, W. Kramer, J. Lex, F. Nerowski, M.
Oelgemöller, K. Peters, E.-M. Peters, Helv. Chim. Acta 1997, 80, 912.
[4] A. G. Griesbeck, H. Mauder, Angew. Chem. Int. Ed. Engl. 1992, 31,
73; Angew. Chem. 1992, 104, 97.
[37] E. Bothe, private communication.
[38] a) D. Rehm, A. Weller, Isr. J. Chem. 1970, 8, 259; b) D. Rehm, A.
Weller, Ber. Bunsenges. Phys. Chem. 1969, 73, 834.
[39] H. Görner, Chem. Phys. Lett. 1998, 282, 381.
[5] A. G. Griesbeck, A. Henz, J. Hirt, V. Ptatschek, T. Engel, D. Löffler,
F. W. Schneider, Tetrahedron 1994, 50, 701.
[6] A. G. Griesbeck, Liebigs Ann. 1996, 1951.
[40] a) F. Wilkinson, W. P. Helman, A. B. Ross, J. Phys. Chem. Ref. Data
1993, 22, 113; b) R. W. Redmond, S. E. Braslavsky, Chem. Phys. Lett.
1988, 148, 523.
[41] a) R. D. Scurlock, S. Nonell, S. E. Braslavsky, P. R. Ogilby, J. Phys.
Chem. 1995, 99, 3521; b) R. D. Scurlock, P. R. Ogilby, J. Phys. Chem.
1987, 91, 4599; c) R. Schmidt, E. Afshari, J. Phys. Chem. 1990, 94, 4377.
[42] H. G. Heller, J. R. Langan, J. Chem. Soc. Perkin Trans. 2 1981, 341.
[7] A. G. Griesbeck, Chimica 1998, 52, 272.
[8] A. G. Griesbeck, EPA Newsletter 1998, 62, 3.
[9] A. G. Griesbeck, J. Hirt, W. Kramer, P. Dallakian, Tetrahedron 1998,
54, 3169.
[10] A. G. Griesbeck, W. Kramer, M. Oelgemöller, Synlett. 1999, 7, 1169.
[11] A. G. Griesbeck, H. Görner, J. Photochem. Photobiol. A: Chem. 1999,
129, 111.
[12] Y.Sato, H. Nakai, T. Mizoguchi, M. Kawanishi, Y. Hatanaka, Y.
Kanaoka, Chem. Pharm. Bull. 1982, 30, 1263.
[13] a) K. Okada, K. Okamoto, M. Oda, J. Am. Chem. Soc. 1988, 110, 8736;
b) K. Okada, K. Okamoto, N. Morita, K. Okubo, M. Oda, J. Am.
Chem. Soc. 1991, 113, 9402.
Received: June 26, 2000
Revised version: November 3, 2000 [F2567]
1538
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0707-1538 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 7