Singlet oxygen photo-oxygenation in water/pluronic F-127 hydrogels: Increased reaction efficiency coupled with a switch in regioselectivity

Article abstract of DOI:10.1002/chem.201202296

Pluronic F-127 hydrogels are highly efficient microenvironments for photochemical reactions, as demonstrated for singlet oxygen reactions of monoalkenes. Nonpolar substrates are localized in the nanosized polymer compartment, which can be visualized by neutron scattering. The efficiency of ¹O₂ reactions is strongly increased for tiglate derivatives and the regioselectivity of the ene reaction of trisubstituted alkenes is completely switched in comparison with solution phase and inverted in comparison with intrazeolite photo-oxygenations.

Full text of DOI:10.1002/chem.201202296

FULL PAPER
DOI: 10.1002/chem.201202296
Singlet Oxygen Photo-Oxygenation in Water/Pluronic F-127 Hydrogels:
Increased Reaction Efficiency Coupled with a Switch in Regioselectivity
Axel G. Griesbeck,*[a] Johannes Uhlig,[a] Thomas Sottmann,*[b]
Lhoussaine Belkoura,[b] and Reinhard Strey[b]
Abstract: Pluronic F-127 hydrogels are highly efficient microenvironments for
photochemical reactions, as demonstrated for singlet oxygen reactions of monoal-
kenes. Nonpolar substrates are localized in the nanosized polymer compartment,
which can be visualized by neutron scattering. The efficiency of O2 reactions is
strongly increased for tiglate derivatives and the regioselectivity of the ene reac-
tion of trisubstituted alkenes is completely switched in comparison with solution
phase and inverted in comparison with intrazeolite photo-oxygenations.
Keywords: alkenes · gels · neutron
diffraction · photo-oxidation · sin-
glet oxygen
1
Introduction
zeolites. These interactions shield trisubstituted alkenes so
that secondary hydroperoxides
3
are preferentially
Singlet oxygen reactions (Type II photo-oxygenation) are
highly sustainable processes that use triplet sensitizing dyes
and oxygen as reagent. They can be performed in virtually
all solvents, under solid-state conditions, or in the gas
phase.[1,2] A multitude of oxygenated products are available
from the photo-oxygenation of alkenes, alkynes, polyenes,
and heteroatom substrates.[3] The singlet oxygen ene reac-
tion, which transforms alkenes into allylic hydroperoxides, is
the most appealing and flexible process and produces useful
precursors to allylic and epoxy alcohols.[4,5] Dyes with excel-
lent singlet oxygen quantum yields are available for all reac-
tion conditions, the only limitation for solvents is substrate
solubility.[6] Concerning selectivity features,[7] the ene reac-
tion was long considered to be insensitive to external influ-
ences, due to the peculiar mechanism of the reaction (per-
formed.[15–17]
Under isotropic solvent conditions, these substrates give
1:1 regioisomeric mixtures of tertiary and secondary hydro-
peroxides 2 and 3 (Scheme 1). The directing cation–p inter-
Scheme 1. Tertiary (2) and secondary (3) hydroperoxides from the photo-
oxygenation of trisubstituted alkenes (1).
action operates only under highly confined conditions (in
zeolite supercages) and breaks down when less ordered mi-
croenvironments are applied, for example, in Nafion
beads.[18] Conceptually, when switching from these highly
electrostatic conditions to neutral conditions, one has to
compare the results obtained using nonpolar solvents to
those from nonpolar confined conditions, for example, with
monodisperse polystyrene nanobeads that are highly suita-
ble for porphyrin-sensitized photo-oxygenation, albeit no
changes in selectivity pattern are observed.[19,20] Singlet
oxygen photo-oxygenations are fast in chlorinated solvents
such as carbon tetrachloride, chloroform, or dichlorome-
ACHTUNGTRENNUNG
epoxide-like intermediate[8] or two-stage, no-intermedi-
ate[9,10]) and the size and inertness of the reagent singlet
oxygen towards complexation.[11,12] This opinion changed
when the first reports on intrazeolite photo-oxygenation ap-
peared.[13,14] As is now clearly established, the regioselectivi-
ty of the singlet oxygen ene reaction is influenced by specific
cation–alkene interactions when performed in the interior of
[a] Prof. Dr. A. G. Griesbeck, Dr. J. Uhlig
Organic Chemistry, Greinstr. 4
50939 Kçln (Germany)
À
À
thane due to the absence or limited amount of C H or O
1
H bonds that can physically quench O2.[21] Water-based sin-
Fax : (+49)221-470-5059
glet oxygen reactions have not been the focus of synthetic
applications primarily because of the low solubility of most
organic substrates and the reduced lifetime of singlet
oxygen (3 ms) in aqueous media. Pluronic hydrogels have
long been used as synthetic nontoxic materials for tissue ap-
plications.[22] Among a broad series of block copolymers,
Pluronic F-127 reversibly forms hydrogels at room tempera-
ture with 20–90% polymer content, and are converted into
E-mail: griesbeck@uni-koeln.de
[b] Dr. T. Sottmann, Dr. L. Belkoura, Prof. Dr. R. Strey
Physical Chemistry
Luxemburgerstr. 116
50939 Kçln (Germany)
Fax : (+49)221-470-5104
E-mail: thomas.sottmann@uni-koeln.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202296.
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the liquid phase either by heating or by cooling. This makes
these hydrogels highly suitable for the transport and release
of nonpolar drugs.[23–25] Mixtures of the block copolymer
Pluronic F-127 with water forms nanoheterogenous net-
works with unpolar domains that dissolve significant
amounts of organic substrate.[26] In this paper, we describe
for the first time the use of Pluronic hydrogels as a nano-
sized polymer network for a photochemical reaction, that is,
the photo-oxygenation of alkenes.
Scheme 2. Photo-oxygenation of tiglic acid (4a) and ethyl tiglate (4b).
tion of 10–20% block polymer Pluronic F-127. These micro-
heterogenous gels can dissolve organic compounds on a mil-
limolar scale. Citronellol 6, as an example of an electronrich
alkene, shows full conversion in water/methanol mixtures as
well as Pluronic solutions after 48 h irradiation (Scheme 3
and Figure 2).
Results and Discussion
Transmission electron microscopy: The structure of a 10%
aqueous Pluronic F-127 solution is visualized by transmis-
sion electron microscopy (TEM); Figure 1 shows the micro-
Scheme 3. Photo-oxygenation of citronellol (6).
Figure 1. Freeze fracture direct imaging (left) of a 10% aqueous Pluronic
F-127 solution. Note, that the nanoheterogeneous network become better
visible by subsequent freeze drying [FFDI(FD)], removing water. How-
ever, the structure is widened by this procedure (right).
Figure 2. Photo-oxygenation of citronellol (6) in protic environment and
in Pluronic F-127 hydrogel (20% w/w).
graphs obtained by using freeze fracture direct imaging
(FFDI) and FFDI with subsequent freeze drying
[FFDI(FD)].[27] Both micrographs show that the amphiphilic
Pluronic molecules self-assemble into a locally cylindrical,
network-like structures (Figure 1). Presumably the polyeth-
The electron-deficient substrates tiglic acid 4a and its de-
rivatives (Scheme 2), however, react slowly with singlet
oxygen, necessitating either the use of solvents that enable
high singlet oxygen lifetimes such as carbon tetrachloride, or
very long irradiation periods.[28] Figure 3 shows that photo-
oxygenations of 4a and 4b proceed significantly faster in
Pluronic solutions than in protic solutions. The “boosting
effect” becomes especially noticeable after 48 h irradiation.
Ethyl tiglate 4b photo-oxygenation showed a fivefold higher
conversion after 48 h irradiation; tiglic acid 4a showed 45%
conversion whereas it did not yield any products after 48 h
in the protic environment water/methanol.
ACHTUNGTRENNUNGylACHTUNGTRENNUNGene oxide (PEO) units are folded around the more hydro-
phobic polypropylene oxide (PPO) cores. Comparing the
micrographs, it becomes clear that the subsequent freeze
drying, which leads to a widening of the structure, carves
out the network-like shape.
Photo-oxygenation efficiency: Whereas the use of Pluronic
hydrogels as tensides, emulsifiers, and drug retardants has
been intensively studied, to the best of our knowledge, there
are no reports on the use of these systems as microenviron-
ments for organic reactions. We herein prove this general
applicability and show differences in reaction rates and re-
gioselectivities of singlet oxygen ene reactions. First, the sin-
glet oxygen ene reactions of the rather inactive tiglic acid
4a, its ethyl ester 4b (Scheme 2), and the highly reactive
substrate citronellol 6 were conducted in Pluronic hydrogels
and compared with protic solvents.
Regioselectivity: Ramamurthy et al. reported remarkable
changes in regioselectivity in favor of secondary hydroper-
oxides when conducting the ene reaction in zeolite super-
cages.[15–17] The cationic interactions of Na+ and Li+ with
the p system of the olefin are responsible for the specific
suppression of tertiary hydroperoxide formation.[29–31] For
comparison with the nonionic microdomains of Pluronic net-
1
Although most organic substrates are insoluble in water,
the nonpolar alkenes used in this study dissolve after addi-
works, we analyzed the regioselectivities of the O2 ene reac-
tion with the trisubstituted alkenes 9–12 (Table 1). Solution
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Photo-oxygenation in Hydrogels
FULL PAPER
tify the areas of substrate accumulation, because medium
polarity influences singlet oxygen lifetimes and confinement
can alter the hydrogen abstraction selectivity. Having deter-
mined the shape of the nanostructure formed in 10% aque-
ous Pluronic F-127 solution by TEM, we used small-angle
neutron scattering (SANS) as a complementary method to
determine the relevant length of the cylindrical struc-
tures.[32–35] The influence of substrate addition on the diame-
ter of the cylindrical structures (which can be taken as a
consequence of substrate accumulation within the inner
PPO-part of the cylinder) was studied by comparing the
scattering of the pure hydrogel (Sample A) with a sample
that was loaded with 1% (w/w) of the nonpolar alkene 3-
methyl-1-phenylbut-2-ene (11; Sample B). The samples were
prepared in almost pure bulk contrast, that is, by replacing
H2O with D2O. Figure 4 shows the absolute scattering inten-
Figure 3. Photo-oxygenation rates of tiglic acid (4a) and ethyl tiglate
(4b) in protic environment and in Pluronic F-127 hydrogel (20%w/w).
Table 1. Generation of tertiary vs. secondary hydroperoxides in chloro-
form (TPP sensitized), Pluronic F-127 hydrogel (20% w/w, HTPP sensi-
tized).
Substrate
Conditions
Regioisomeric ratio[a]
CHCl3/TPP
Pluronic F-127 (20%)
58:42
61:39
CHCl3/TPP
Pluronic F-127 (20%)
53:47
68:32
CHCl3/TPP
37:67
82:18
15:85
Pluronic F-127 (20%)
CHCl3/NaY[b]
CHCl3/TPP
Pluronic F-127 (20%)
55:45
72:28
Figure 4. SANS experiments with 10% (w/w) Pluronic hydrogels in D2O
*
without (Sample A; ) and with (Sample B; &) 1% (w/w) 3-methyl-1-
phenylbut-2-ene (11). Note that the addition of 11 causes a shift of both
the sequence of minimum and maximum as well as the structure peak to
smaller q values, which indicates an increasing cylindrical radius and a
swelling of the whole structure, respectively.
[a] Ratio of tertiary/secondary hydroperoxides was determined by NMR
spectroscopic analysis. [b] NaY zeolite, thionin sensitized, see Robbins
and Ramamurthy.[16]
photo-oxygenations were performed in chloroform and pro-
duced tertiary and secondary hydroperoxides with low re-
gioselectivities; for 11 a slight preference for the secondary
hydroperoxide was observed. This substrate serves also for
comparison with intrazeolite reactions.[16]
In contrast to the results obtained from reactions per-
formed in solution, the hydrogel-based photo-oxygenations
exhibit moderate to strong preference for tertiary hydroper-
oxide formation. This effect was most pronounced for the
aromatic substrates 11 and 12, whereas the less bulky sub-
strates 9 and 10 showed little to moderate change in regiose-
lectivity.
sity I [cmÀ1] as a function of the absolute value of the scat-
tering wave vector q=(4p/l)sinACHTUNGTRNEUNG
(q/2) [ꢁÀ1]. Both scattering
curves have qualitatively the same shape. Starting from low
values of q, the scattering intensity increases strongly and
runs through a pronounced maximum at intermediate q. A
further increase of q leads to a strong decrease of the scat-
tering intensity. Having traversed a shoulder (which stems
from the second order peak of the structure factor), this
steep decay ends in a minimum followed by a weak maxi-
mum. For even higher values of q, incoherent background
scattering is almost reached.
By comparing the scattering curves, it becomes clear that
the addition of 11 causes a shift of both the sequence of
minimum and maximum as well as the structure peak to
smaller q values. In a first approach we used the decoupling
approximation[36] and described the scattering curve as the
Small-angle neutron scattering: To understand the mecha-
nistic details of the efficiency and regioselectivity effects in
hydrogel photo-oxygenation reactions, it was crucial to iden-
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A. G. Griesbeck, T. Sottmann et al.
product of form and structure factors. Thereby, the form
factor of a rod-like polymeric micelle[37,38] given by its length
L and cross-sectional radius R (consisting of a PPO-core
and a PEO-shell) is used on the basis of the FFDI results.
To model the repulsive interactions of different cylinder
parts that occur due to the high fractions of Pluronic F-127,
we applied the well-known Percus–Yevick model for hard
unpolar substrates on a millimolar scale. The photostable
block polymer forms cylinder-shaped hydrophobic PPO net-
works that swell upon addition of alkenes by about 30% in
volume. Alkenes with low reactivity towards 1O2 exhibit
higher reaction rates in hydrogels than in pure protic envi-
ronments. This efficiency gain results from higher singlet
oxygen lifetimes in the hydrophobic microdomains. Addi-
tionally, hydrogel-based singlet oxygen ene reactions of tri-
substituted alkenes lead to inverted regioselectivities in
comparison with intrazeolite photo-oxygenation reactions.
The constrained conditions of zeolites lead to an excess of
secondary hydroperoxide through cation–p interactions,
whereas the corresponding tertiary hydroperoxides are pre-
ferred under the nonionic conditions of Pluronic F-127 hy-
drogels due to specific solvation in the hydrogel microenvi-
sphere solutions mainly determined by the effective interac-
[39]
tion distance dHS
.
As can be seen, the obtained fits de-
scribe most parts of the experimental scattering curve
almost quantitatively. The rather poor description of the
shoulder at intermediate q can be ascribed to the fact that a
hard sphere structure factor is used to model the scattering
of a cylindrical network-like structure. An improved descrip-
tion is in progress.
Using the aforementioned model, we obtained the follow-
ing parameters. The cross-sectional radius R of Samples A
and B were determined to be 3.5 and 5.1 nm, respectively.
Thus, the radius of the cylinders in block polymer solutions
increases due to the addition of the alkene substrate by
1.6 nm, indicating incorporation of 11 in the cylindrical net-
work-like structure (shown schematically in Figure5).
ACHTUNGTRENNrUNG onments.
Experimental Section
Transmission electron microscopy (TEM)
Cryo-TEM: The vitrified specimens were investigated by using a trans-
mission electron microscope LEO 912 (Zeiss, Oberkochen, Germany).
The instrument was operated at 120 kV acceleration voltage and “zero
loss” conditions to enhance image contrast, which is normally affected by
inelastically scattered electrons. A 2048ꢂ2048 slow-scan CCD camera
(sharp eye, TRS, Moorenweis, Germany) was used to acquire the elec-
tron micrographs. The remote control of the CCD camera and the trans-
mission electron microscope was assisted by the iTEM software version
5.1 (Olympus Soft Imaging Solutions GmbH, Mꢃnster, Germany). Low-
dose conditions were also used for imaging the vitrified specimens. With
an exposure time of 1 s, the total dose for individual images varied be-
tween 0.16 (10 000ꢂ) and 6.4 eꢁÀ2 (25 000ꢂ) because of the very low sta-
bility of the specimen in the electron beam.
FFDI and FFDI(FD): The basic technique of the freeze fracture direct
imaging (FFDI) has been described in detail by Gupte et al.[26] The mi-
crographs obtained from the FFDI technique revealed a weakly observa-
ble network-like structure of the amphiphilic Pluronic molecules in the
vitrified ice. A controlled freeze drying (FD) of the specimen inside the
electron microscope must then be performed to remove the main part of
the vitrified water and to allow carving out the network. For this purpose
the specimen, still mounted on the cryo-specimen holder (CT3500,
Oxford Instruments, Oxford, UK), was slowly warmed by passing an
electrical current through the heating coils of the specimen holder. In
steps of 5 K and a heating rate of 0.01 KsÀ1 the specimen was warmed
from À168 to À1008C and maintained for 15 min at this temperature.
The heating current was then turned off, allowing the specimen to cool
back to À1688C. Inside the microscope, the tip of the cryo-holder that
holds the specimen is surrounded by an anti-contaminator metal block
cooled with liquid nitrogen, reaching a temperature of À1758C.
Figure 5. Substrate accumulation in the less polar PP region of the colum-
nar hydrogel structure with change in cylindrical structures.
The distance between the cylinders can be estimated by
the effective interaction distance dHS, which increases from
dHS =25.8 to dHS =35.0 nm. These results indicate a strong
influence of 3-methyl-1-phenylbut-2-ene (11) on the net-
work structure, increasing the cylindrical radius as well as
swelling the whole structure and thus increasing distances
SANS measurements were carried out at the KWS 1 spectrometer at the
Jꢃlich Center for Neutron Science (JCNS) that is hosted at the For-
schungsneutronenquelle Heinz Maier-Leibnitz (FRM II) in Garching,
Germany. All samples were measured at a wavelength of l=4.5 ꢁ at
three detector distances of 2, 8, and 20 m. The collimation distance was
chosen to be 8 m for a detector distance of 2 and 8 m, respectively, and
20 m for the long detector distance. Thus, the scattering vector q=(4p/
between cylinders dHS
.
l)sinACHTNUGRTNEUNG
(q/2) ranged from 0.005 to 0.33 ꢁÀ1, where q is the scattering angle.
Conclusion
The samples were filled at room temperature in Hellma Quartz cells with
an optical path length of 1 mm and equilibrated to the desired tempera-
ture (T=258C) in a home-built cell holder before each measurement.
Thereby, the temperature was controlled with an accuracy of Æ0.1 K. To
obtain a one-dimensional scattering spectrum, the two-dimensional raw
Aqueous Pluronic F-127 hydrogels provide novel microen-
vironments with unique structural properties for Type II
photo-oxygenation that allow solubilization and recovery of
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Photo-oxygenation in Hydrogels
FULL PAPER
data were radially averaged and normalized to an absolute scale by using
the incoherent scattering of Plexiglas of 1.5 mm path length as reference.
[9] D. A. Singleton, C. Hang, M. J. Szymanski, M. P. Meyer, A. G.
Leach, K. T. Kuwata, J. S. Chen, A. Greer, C. S. Foote, K. N. Houk,
J. Am. Chem. Soc. 2003, 125, 1319–1328.
Photo-oxygenation reactions in Pluronic F-127 hydrogels
[10] D. H. Ess, S. E. Wheeler, R. G. Iafe, L. Xu, N. Celebi-Olcum, K. N.
Houk, Angew. Chem. 2008, 120, 7704–7713; Angew. Chem. Int. Ed.
2008, 47, 7592–7601.
[11] E. L. Clennan, Tetrahedron 2000, 56, 9151–9179.
[12] M. N. Alberti, M. Orfanopoulos, Synlett 2010, 999–1026.
[13] J. Shailaja, J. Sivaguru, R. J. Robbins, V. Ramamurthy, R. B. Sunoj,
J. Chandrasekhar, Tetrahedron 2000, 56, 6927–6943.
[14] L. S. Kaanumalle, J. Shailaja, R. J. Robbins, V. Ramamurthy, J. Pho-
tochem. Photobiol. A 2002, 153, 55–65.
[15] X. Li, V. Ramamurthy, J. Am. Chem. Soc. 1996, 118, 10666–10667.
[16] R. J. Robbins, V. Ramamurthy, Chem. Commun. 1997, 1071–1072.
[17] M. Stratakis, C. Raptis, N. Sofikiti, C. Tsangarakis, G. Kosmas, I.-P.
Zaravinos, D. Kalaitzakis, D. Stavroulakis, C. Baskakis, A. Stathou-
lopoulou, Tetrahedron 2006, 62, 10623–10632.
[18] S. Arumugam, J. Photochem. Photobiol. A 2008, 199, 242–249.
[19] A. G. Griesbeck, T. T. El-Idreesy, A. Bartoschek, Adv. Synth. Catal.
2004, 346, 245–251.
General procedure for singlet oxygen reactions in hydrogel carriers (GP1):
Pluronic F-127 hydrogels were made by dissolving the solid block poly-
mer in distilled water at 8–108C overnight. An aqueous solution of Plur-
onic F-127 (10%, 10 mL) was poured into a Petri dish and stirred with
olefinic substrate (1 mmol) and photosensitizer (7ꢂ10À6 mmol). For com-
plete homogenization, the covered Petri dish was left in the cold for
30 min, then stirred again. The covered Petri dish was irradiated with a
70 W sodium lamp at ambient temperature. For isolation of the product,
the hydrogel was poured into diethyl ether and the separated aqueous
phase was extracted twice with ether. The combined organic phases were
dried with magnesium sulfate and the solvent was removed by rotary
evaporation.
General procedure for singlet oxygen reactions in protic solution (GP2):
Photosensitizer (7ꢂ10À6 mmol) was added to a 0.02m alkene solution in
methanol/water (50 mL, 2:1) and the solution was added to a three-
necked irradiation tube equipped with magnetic stirrer and condenser.
During irradiation with a 70 W sodium lamp, air was bubbled through
the solution and the temperature was kept below 208C. The progress of
the reactions was monitored by TLC. After irradiation, diethyl ether
(100 mL) was added and the phases were separated. The aqueous phase
was extracted with diethyl ether (2ꢂ100 mL), the combined organic
phases were dried with magnesium sulfate, and the solvent was removed
by rotary evaporation. The products were analyzed by 1H and 13C NMR
spectroscopy.
[20] A. G. Griesbeck, T. T. El-Idreesy, A. Bartoschek, Pure Appl. Chem.
2005, 77, 1059–1074.
[21] C. Schweitzer, R. Schmidt, Chem. Rev. 2003, 103, 1685–1757.
[22] I. R. Schmolka, J. Biomed. Mater. Res. 1972, 6, 571–582.
[23] S.-J. Huang, S. L. Sun, T.-H. Feng, K.-H. Sung, W.-L. Lui, L.-F.
Wang, Eur. J. Pharm. Sci. 2009, 38, 64–73.
[24] M. E. Morales, B. Clarꢄs, M. Lꢅpez-Viota, M. A. Ruiz, Drug Deliv-
ery 2009, 16, 1–10.
[25] J. C. Gilbert, J. Hadgraft, A. Bye, L. G. Brookes, Int. J. Pharm. 1986,
32, 223–228.
[26] A. Gupte, R. Nagarajan, A. Kilara, Biotechnol. Prog. 1991, 7, 348–
354.
Acknowledgements
[27] L. Belkoura, C. Stubenrauch, R. Strey, Langmuir 2004, 20, 4391–
4399.
[28] W. Adam, A. G. Griesbeck, Synthesis 1986, 1050–1052.
[29] L. M. Stephenson, M. J. Grdina, M. Orfanopoulos, Acc. Chem. Res.
1980, 13, 419–425.
We gratefully acknowledge the support of the local contact H. Frieling-
haus and the Jꢃlich Center for Neutron Science (JCNS) hosted at the
Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II) in Garching,
Germany. We furthermore thank T. Foster for his valuable input during
the evaluation of the SANS-data.
[30] M. Stratakis, R. Nencka, C. Rabalakos, O. Krebs, W. Adam, J. Org.
Chem. 2002, 67, 8758–8763.
[31] M. Stratakis, G. Kosmas, Tetrahedron Lett. 2001, 42, 6007–6009.
[32] G. Allen, J. S. Higgins, Rep. Prog. Phys. 1973, 36, 1073–1133.
[33] M. Mihailescu, M. Monkenbusch, J. Allgaier, H. Frielinghaus, D.
Richter, T. Sottmann, B. Jakobs, Phys. Rev. E 2002, 66, 041504.
[34] M. Teubner, R. Strey, J. Chem. Phys. 1987, 87, 3195–3200.
[35] S. H. Chen, Ann. Rev. Phys. Chem. 1986, 37, 351–399.
[36] M. Kotlarchyk, S. H. Chen, J. Chem. Phys. 1983, 79, 2461–2469.
[37] Y.-Y. Won, H. T. Davis, F. S. Bates, M. Agamalian, G. D. Wignall, J.
Phys. Chem. B 2000, 104, 7134–7143.
[1] M. N. Alberti, M. Orfanopoulos in CRC Handbook of Organic Pho-
tochemistry and Photobiology, 3rd ed. (Eds.: A. G. Griesbeck, F.
Ghetti, M. Olgemoeller), CRC Press, Taylor and Francis, 2012, 1,
765–787.
[2] M. R. Iesce, F. Cermola in CRC Handbook of Organic Photochemis-
try and Photobiology, 3rd ed. (Eds.: A. G. Griesbeck, F. Ghetti, M.
Olgemoeller), CRC Press, Taylor and Francis, 2012, 1, 727–764.
[3] A. A. Frimer, Singlet O2, CRC Press, Boca Raton, FL, 1985.
[4] W. Adam, A. G. Griesbeck, E. Staab, Angew. Chem. 1986, 98, 279–
280; Angew. Chem. Int. Ed. Engl. 1986, 25, 269–270.
[5] W. Adam, M. Braun, A. G. Griesbeck, V. Lucchini, E. Staab, B.
Will, J. Am. Chem. Soc. 1989, 111, 203–212.
[38] T. Foster, T. Sottmann, R. Schweins, R. Strey, J. Chem. Phys. 2008,
128, 064902.
[39] D. J. Kinning, E. L. Thomas, Macromolecules 1984, 17, 1712–1718.
[6] F. Wilkinson, W. P. Helman, A. B. Ross, J. Phys. Chem. Ref. Data
1995, 24, 663–1021.
[7] M. N. Alberti, M. Orfanopoulos, Tetrahedron 2006, 62, 10660–
10675.
Received: June 28, 2012
Revised: August 25, 2012
[8] M. N. Alberti, M. Orfanopoulos, Chem. Eur. J. 2010, 16, 9414–9421.
Published online: October 18, 2012
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