Benzil-tethered precipitons for controlling solubility: A round-trip energy-transfer mechanism in the isomerization of extended stilbene analogues

Article abstract of DOI:10.1021/ja068211c

We are investigating photoresponsive molecules called " precipitons" that undergo a solubility change co-incident with isomerization. Isomerization can be induced by light or by catalytic reagents. Previous work demonstrated that covalent attachment of a

Full text of DOI:10.1021/ja068211c

Published on Web 03/10/2007
Benzil-Tethered Precipitons for Controlling Solubility: A
Round-Trip Energy-Transfer Mechanism in the Isomerization
of Extended Stilbene Analogues
Mark R. Ams and Craig S. Wilcox*
Contribution from the Department of Chemistry and The Combinatorial Chemistry Center,
UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260
Received November 16, 2006; E-mail: daylite@pitt.edu
Abstract: We are investigating photoresponsive molecules called “precipitons” that undergo a solubility
change co-incident with isomerization. Isomerization can be induced by light or by catalytic reagents.
Previous work demonstrated that covalent attachment of a metal complex, Ru(II)(bpy)3, greatly accelerates
photoisomerization and influences the photostationary state. In this paper, we describe precipitons (1,2-
biphenylethenes; analogous to stilbenes) that are activated by a covalently attached organic sensitizer
(benzil). We find that isomerization of these stilbene analogues is little effected by the presence of benzil
in solution but that the intramolecular benzil effect is to increase the rate of isomerization and to significantly
change the photostationary state. What is most interesting about these observations is that the precipiton
is the primary chromophore in this bichromophoric system (precipiton absorbance is many times greater
than benzil absorbance in the 300-400 nm range), yet the neighboring benzil has a significant effect on
the rate and the photostationary state. The effect of unattached benzil on the rate was small, about a 24%
increase in rate as compared with 4-6-fold changes for an attached benzil. We speculate that the
isomerization process occurs by a “round-trip” energy-transfer mechanism. Initial excitation of the precipiton
chromophore initiates a sequence that includes (1) formation of the precipiton singlet state, (2) singlet
excitation transfer from the precipiton unit to the benzil, (3) benzil-centered intersystem crossing to the
localized benzil triplet state, (4) triplet energy transfer from the benzil moiety back to the precipiton, and (5)
isomerization.
Introduction
We are interested in crystallization and dissolution and have
been investigating stilbene-like molecules that, upon isomer-
ization, exhibit large changes in solubility or phase affinity.
Molecules with this property can be used as labels or tags for
starting materials or reagents and can be used for removal of
unwanted reaction byproducts or metal ions.1 We call such tags
precipitons because isomerization can cause precipitation.
Derivatives of 1 undergo photoisomerization, and the isomers
have large differences in solubility (Figure 1). Isomer 1Z is
readily soluble in most organic solvents, while the trans form,
1E, is very insoluble. The solubilities of such isomers (measured
as saturated concentrations) can differ by factors greater than
2000.1a The melting points of the trans isomers are greater than
300 °C, while the corresponding cis isomers melt at less than
200 °C. This suggests that the difference in solubility may be
caused by enhanced intermolecular contacts (and the corre-
Figure 1. Molecules and equilibria of interest in this study.
(1) (a) Bosanac, T.; Yang, J.; Wilcox, C. S. Angew. Chem., Int. Ed. 2001, 40,
1875. (b) Bosanac, T.; Wilcox, C. S. Chem. Commun. 2001, 1618. (c)
Bosanac, T.; Wilcox, C. S. Tetrahedron Lett. 2001, 42, 4309. (d) Bosanac,
T.; Wilcox, C. S. J. Am. Chem. Soc. 2002, 124, 4194. (e) Honigfort, M.;
Brittain, W. J.; Bosanac, T.; Wilcox, C. S. Macromolecules 2002, 35, 4849.
(f) Honigfort, M.; Shingtza, L.; Rademacher, J.; Malaba, D.; Bosanac, T.;
Wilcox, C. S.; Brittain, W. J. ACS Symposium Series 854; American
Chemical Society: Washington, DC, 2003; p 250. (g) Bosanac, T.; Wilcox,
C. S. Org. Lett. 2004, 6, 2321.
sponding increased enthalpy of fusion) in the solid form of the
trans isomer compared to that of the cis isomer.
Stilbene isomerization has been studied in great detail for
many years.2 In recent work, we investigated the effect of a
covalently attached photosensitizer, Ru(II)(bpy)3, on stilbene-
9
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J. AM. CHEM. SOC. 2007, 129, 3966-3972
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Benzil-Tethered Precipitons for Controlling Solubility
A R T I C L E S
Scheme 1 . Synthesis of Benzil Precipiton Esters
Table 1. Spectroscopic Data in THF at 298 K
absorption
emission
λ
max/nma
1
compound
λ
max/nm (
ꢀ
/M-1 cm-
)
2Z
313 (22 610)
344 (63 267)
387, 409, 432,
465
384, 409, 433,
2E
464
413
benzil
3Z
249 (39 103), 255 (43 193),
261 (35 851), 282 (8 303)
249 (12 394), 255 (13 902),
261 (11 862), 319 (25 931)
249 (11 761), 255 (12 771),
261 (99 979), 341 (67 480)
249 (59 937), 255 (60 602),
261 (53 967), 306 (23 272)
249 (97 959), 255 (109 682),
261 (85 832), 340 (49 315)
386, 408, 431
3E
386, 408, 431,
464
385, 407, 428,
461
385, 407, 432
4Z
4E
a λexc ) 320 nm.
coupling 4-(bromomethyl)benzil to precipiton alcohol 1Z by
ether formation using halide abstractor AgO3SCF3 resulted in
either decomposition (in CH3NO2 solvent) or no reaction (in
CH3CN solvent).
based precipiton isomerization. In the presence of light (>400
nm), the isomerization rate is increased more than 200-fold by
the intramolecular Ru(II)(bpy)3.3 In this paper, we report our
first studies of precipiton isomerization activated by a covalently
attached organic sensitizer. The preparation, photophysical
properties, and photoisomerization kinetics of precipitons that
are covalently linked and unlinked to a benzil4 unit are described.
The photoisomerization kinetics and photostationary state ratios
of the benzil-linked precipitons 3Z/E and 4Z/E at a low
concentration (10 µM) were compared to those of unlinked
examples 2Z/E. Compounds 3Z/E and 4Z/E differ in regard to
the position of the benzil sensitizer relative to the precipiton
(para vs meta). The differences in photoisomerization kinetics
and photostationary state ratios will be discussed, and a
mechanism for the isomerization will be proposed.
We, therefore, pursued coupling based on an ester linkage.
Treatment of 4-(bromomethyl)benzil with aqueous AgNO3 in
THF afforded the benzyl alcohol 6 and nitrate ester 7. Purifica-
tion by column chromatography on silica gel eluted with 1%
EtOAc in CH2Cl2 gave alcohol 6 in 56% yield.
For the preparation of conjugate 3Z (Scheme 1), esterification
of alcohol 6 with the para-substituted phenylboronate ester 8
afforded a 94% yield of the benzil-tagged boronic ester 9. Suzuki
coupling between 9 and 1-[2-(4-bromophenyl)]biphenyl ethene
10 afforded the benzil-modified 1,2-bis(biphenyl)ethene ana-
logue 3Z in 80% yield. Sensitized photoisomerization of 3Z
using erythrosin B in THF afforded trans isomer 3E in 80%
yield.
Synthesis of the meta-linked isomers 4Z and 4E followed a
similar pathway. Esterification of alcohol 6 with the meta-
substituted phenylboronate ester 11 afforded a 59% yield of
the benzil-tagged boronic ester 12 (not shown). Suzuki coupling
between 12 and 1-[2-(4-bromophenyl)]biphenyl ethene 10
afforded the benzil-modified 1,2-bis(biphenyl)ethene analogue
4Z in 90% yield. Sensitized photoisomerization of 4Z using
erythrosin B in THF afforded trans isomer 4E in 92% yield.
Absorption and Emission Spectroscopy. The photophysical
data for the compounds are summarized in Table 1. Figure 2
shows the absorption spectra for each compound in neat THF
at room temperature. Absorption in the region of 321-377 nm
is attributed to the π f π* transition localized on the ethylene
functionality for each precipiton.3 As expected, the ethylene
absorption in this region is more intense for the trans isomers
compared to that for the cis isomers. The most intense
absorptions are seen for the para-substituted precipiton 3E and
the untethered precipiton 2E. The meta-substituted precipiton
4E is the weakest absorbing trans isomer. This sequence of
absorbance intensity is repeated for the cis isomers.
Results and Discussion
Synthesis. The preparation of 1,2-bis(biphenyl)ethene alcohol
1Z,1d 1,2-bis(biphenyl)ethene TBS ether 2Z/E,1g,3 and precipiton
precursor 101a have previously been reported from our labora-
tory. 4-(Bromomethyl)benzil (5) is commercially available.
Our original plan was to join the benzil and precipiton
moieties through an ether linkage. However, 4-(bromomethyl)-
benzil was not stable under base treatment, and attempts at
(2) (a) Hammond, G. S.; Saltiel, J.; Lamola, A. A.; Turro, N. J.; Bradshaw, J.
S.; Cowan, D. D.; Counsell, R. C.; Vogt, V.; Dalton, C. J. J. Am. Chem.
Soc. 1964, 86, 3197. (b) Herkstroeter, W. G.; Hammond, G. S. J. Am. Chem.
Soc. 1966, 88, 4769. (c) Hammond, G. S.; Saltiel, J. J. Am. Chem. Soc.
1963, 85, 2516. (d) Saltiel, J.; Charlton, J. L.; Mueller, W. B. J. Am. Chem.
Soc. 1979, 101, 1347. (e) Saltiel, J.; Marchand, G. R.; Kirkor-Kaminska,
E.; Smothers, W. K.; Mueller, W. B.; Charlton, J. L. J. Am. Chem. Soc.
1984, 106, 3144. (f) Saltiel, J. S.; Ganapathy, S.; Werking, C. J. Phys.
Chem. 1987, 91, 2755. (g) Waldeck, P. H. Chem. ReV. 1991, 91, 415. (h)
Saltiel, J.; Waller, A. S.; Sears, D. F. J. Photochem. Photobiol., A 1992,
65, 29. (i) Saltiel, J.; Waller, A. S.; Sears, D. F. J. Am. Chem. Soc. 1993,
115, 2453.
(3) Ams, M. R.; Wilcox, C. S. J. Am. Chem. Soc. 2006, 128, 250.
(4) (a) Herkstroeter, W. G.; Lamola, A. A.; Hammond, G. S. J. Am. Chem.
Soc. 1964, 86, 4537. (b) Subhas, C. B.; Mukherjee, R.; Chowdhury, M. J.
Chem. Phys. 1969, 51, 754. (c) Wrighton, M. S.; Pdungsap, L.; Morse, D.
L. J. Phys. Chem. 1975, 79, 66. (d) Flamigni, L.; Barigelletti, F.; Dellonte,
S.; Orlandi, G. J. Photochem. 1983, 21, 237. (e) Darmanyan, A. P.; Foote,
C. S.; Jardon, P. J. Phys. Chem. 1995, 99, 11854. (f) Tokunaga, Y.;
Akasaka, K.; Hisada, K.; Shimomura, Y.; Kakuchi, S. Chem. Commun.
2003, 17, 2250. (g) Lopes, S.; Gomez-Zavaglia, A.; Lapinski, L.; Chatto-
padhyay, N.; Fausto, R. J. Phys. Chem. A 2004, 108, 8256.
Benzil exhibits only very weak absorption in the UV range
above 300 nm and has insignificant absorption above 340 nm.
In our studies of the Ru(II)(bpy)3 intramolecularly sensitized
isomerization of these stilbenes, we used light of wavelengths
greater than 400 nm. Our interest here, for practical reasons,
was in the behavior of these molecules under the influence of
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A R T I C L E S
Ams and Wilcox
Figure 4. Reaction progress upon irradiation of 10 µM THF solutions of
2Z (×) and 2E (2) in the presence of 1 equiv of benzil.
Table 2. Isomerization Rate Constants, Half-Lives, and
Photostationary State Ratios for the Photoisomerization Process of
2Z, 2E, 3Z, 3E, 4Z, and 4E upon Irradiation at 10 µM
1
reaction
k (s-
)
×
104
t1 2 (sec)
/
Z/E ratio
2Z f 2Ea
2E f 2Za
2Z f 2Eb
2E f 2Zb
3Z f 3E
3E f 3Z
4Z f 4E
4E f 4Z
1.7
2.1
9.5
6.7
4077
3300
730
19/81
22/78
90/10
76/24
Figure 2. Room-temperature absorption spectra for degassed 10 µM THF
solutions of 2Z, 2E, 3Z, 3E, 4Z, 4E, and benzil.
1034
a Reaction observed only for the first half-life. All others were observed
for the first five half-lives. b Reaction carried out in the presence of benzil
(10 µM).
The same quenching effect is observed for 3Z compared to the
untethered 2Z. The luminescence of 4Z is interesting in that it
is identical with that of 2Z. The origin of this effect is not clear
but is probably not due to the movement of the substituent to
the meta position for 4Z. It has been observed that methyl
substitution in different positions on the phenyl rings in trans-
stilbene has an insignificant effect on absorption and fluores-
cence spectra.5
Photoisomerization Kinetics. To test whether isomerization
of the tethered precipiton is influenced by an intramolecular
interaction with the nearby benzil chromophore, quantitative rate
studies were undertaken. Separate degassed solutions of pure
2Z and 2E (10 µM, THF) were irradiated with a 25 W
incandescent lamp in the presence of 10 µM benzil sensitizer,
and changes in absorbance were monitored at 345 nm. The
concentration of 2Z and 2E present is plotted as a function of
irradiation time in Figure 4. After 7 h of irradiation, solutions
containing each pure isomer came to a final Z/E photostationary
state, the average ratio of which was determined to be 22/78.
The plots analyzed according to a reversible first-order rate curve
and the sum of the forward and reverse rate constants for each
reaction were averaged and determined to be k1 + k-1 ) 2.1 ×
10-4 s-1 (Table 2). There are small deviations from first-order
behavior here and with 4E/Z (Figure 5b) but not in 3Z/E (Figure
5a). The extent of imperfection correlates with the extent of
singlet participation in the process (see discussion below) and
is probably caused by minor side reactions from the singlet
excited states.
Figure 3. Room-temperature emission spectra for degassed 10 µM THF
solutions of 2Z, 2E, 3Z, 3E, 4Z, 4E, and benzil.
inexpensive visible light sources. Therefore, our isomerization
studies were conducted with irradiation from an incandescent
source without filtering. For the lamp we used, the power output
in the 300-320 nm wavelength range is less than 1% of the
power output in the 320-400 nm range. We note, before further
discussion, that the absorbance of the precipiton moiety is far
larger than that of the benzil moiety in the region of highest
output from this light source.
Figure 3 illustrates the fluorescence of the investigated
compounds upon excitation at 320 nm. Viewing 2E as the
control, these results indicate that the observed precipiton
luminescence is quenched when the precipiton is covalently
linked to the benzil moiety. The luminescence of 2E is two
times more intense than the luminescence of 3E and seven times
more intense than that of 4E. The emission positions and
vibronic progressions for each precipiton remain unchanged.
Separate solutions of each isomer 3Z/E and 4Z/E were
irradiated under the same conditions as those used in the
(5) Samsonova, L. G.; Kopylova, T. N.; Svetlichnaya, N. N.; Andrienko, O.
S. High Energy Chem. 2002, 36, 276.
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Benzil-Tethered Precipitons for Controlling Solubility
A R T I C L E S
Figure 6. Fluorescence of 10 µM 2Z and 2E compared to a solution of
2Z/E + benzil at the same concentration in THF; λexc ) 320 nm.
In contrast, when the benzil moiety is covalently attached
and thus restricted in distance and orientation relative to the
precipiton, the final photostationary states for the isomerizations
are significantly shifted in favor of the cis isomers. This effect
is most pronounced for the para-substituted benzil-tethered
isomers 3Z/E. Here, the final equilibrium consisted of 90%
Z-isomer, whereas the intermolecular benzil effect led to only
20% Z-isomer. The half-lives for isomerization also decreased
significantly for the benzil-tethered precipitons. The first half-
life for para-substituted 3Z/E was completed 4.5 times faster
than that for 2Z/E + benzil, and meta-substituted 4Z/E was
completed 3 times faster than that for 2Z/E + benzil.
“Round-Trip” Precipiton-Benzil Energy Transfer. An
interesting question is posed by these data. How does an attached
chromophore (the benzil chromophore), which does not directly
absorb light energy, have such a large effect on the photosta-
tionary state and also affect the rates of isomerization? We
propose that photoisomerization for the precipitons begins with
the singlet excited state of the precipiton. The absorbance of
this chromophore is dominant within the range of available
wavelengths. It is known that isomerization of stilbene upon
direct photoexcitation proceeds via a singlet excited state and
not a triplet excited state (unless a substituent that promotes
intersystem crossing is present).7 We expect, therefore, that
isomerization of 2Z/E (Table 2) through direct irradiation also
proceeds via a singlet pathway. There was no effect on rate of
isomerization of 2Z/E when an equal concentration of benzil
sensitizer was added. This is good evidence against any
intermolecular triplet sensitization in the isomerizations of 3Z/E
and 4Z/E.
Figure 5. (a) Reaction progress upon irradiation of 10 µM THF solutions
of 3Z (×) and 3E (2). (b) Reaction progress upon irradiation of 10 µM
THF solutions of 4Z (×) and 4E (2).
isomerization of 2Z/E. For all isomers, the photostationary state
was reached within 70 min. The average ratio of the photosta-
tionary state was determined to be 90/10 for 3Z/E and 76/24
for 4Z/E (Table 2). The concentration of E and Z precipiton
present is plotted as a function of irradiation time and fits well
to a reversible first-order rate curve (Figure 5). The sum of the
forward and reverse rate constants for each reaction were
averaged and determined to be k1 + k-1 ) 9.5 × 10-4 s-1 for
the para-tethered benzil isomers 3Z/E and k1 + k-1 ) 6.7 ×
10-4 s-1 for the meta-tethered benzil isomers 4Z/E.
A control experiment was performed to evaluate the inter-
molecular effect of benzil (Table 2). Separate solutions of 10
µM 2Z and 2E were prepared in degassed THF and irradiated
using the same experimental conditions as those described
above. After 7.5 h of irradiation, solutions containing each pure
isomer came to a final Z/E photostationary state, the average
ratio of which was determined to be 19/81. The sum of the
forward and reverse rate constants for each reaction were
averaged and determined to be k1 + k-1 ) 1.7 × 104 s-1
.
The effect of covalent attachment of benzil to the precipiton
can be observed in the resulting isomerization kinetic data and
final photostationary state ratios (Table 2). The first half-life
for the direct irradiation of 2Z/E (no benzil) was weakly effected
(1.2 times longer) upon addition of benzil. We conclude that,
under these experimental conditions, singlet excitation of the
precipiton via direct irradiation plays a major role in the
isomerization process6 and that benzil contributes only to a
minor extent as an intermolecular agent at 10 µM. This is not
unexpected upon consideration of the low-light absorption of
benzil compared to that of the precipiton in this wavelength
range. The final photostationary state ratios were changed only
very slightly (toward the cis isomer) by the presence of benzil.
Does benzil, at these concentrations, effectively quench the
singlet excited state of the precipiton? To test this possibility,
fluorescence spectra of 2Z/E were compared to those of 2Z/E
+ benzil (Figure 6). The fluorescence intensity from the
precipitons was essentially unchanged when benzil was present,
signifying that intermolecular quenching is not occurring at these
concentrations (10 µM).
(7) Schuster, D. I.; Nuber, B.; Vail, S. A.; MacMahon, S.; Lin, C.; Wilson, S.
(6) Bortolus, P.; Monti, S. J. Phys. Chem. 1979, 83, 648.
R.; Khong, A. Photochem. Photobiol. Sci. 2003, 2, 315.
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Isomerization kinetics are significantly faster for the benzil-
tethered precipitons 3Z/E and 4Z/E. Why? As shown in Figure
3, the normal stilbene-derived fluorescence caused by singlet
excitation for 2E and 2Z is considerably quenched for 3Z/E
and 4E. We propose that this singlet-singlet energy transfer
to the benzil moiety is then followed by efficient intersystem
crossing to the triplet state of benzil. Benzil has a very low
singlet emission quantum yield (∼0) and a very high intersystem
crossing quantum yield (0.9).8 Our proposed pathway is
consistent with studies in the Rothe labs, wherein they describe
benzil acting as an effective “singlet-triplet converter” for
polyfluorene sensitization.8 Bergamini et al. recently reported
a similar forward and backward energy transfer in a dendrimer
with peripheral naphthalene units and a benzophenone core.9
Ketonic steroids have also been shown to serve as singlet-
triplet switches for intramolecular olefin sensitization, although
not through a round-trip pathway.10
Following singlet energy transfer to benzil and intersystem
crossing, back-transfer of excitation energy to the precipiton
will yield the precipiton triplet excited state. The isomerization
rate enhancement for the tethered precipitons over their inter-
molecularly sensitized unbound analogues is consistent with this
triplet energy transfer back to the precipiton, which allows the
precipiton to enter upon the triplet energy surface and partake
of the usual triplet isomerization pathway.2a Benzil fluorescence
overlaps only to a minor extent with precipiton absorption.
Therefore, FRET is not likely to contribute significantly to the
operative mechanism, and a Dexter-type pathway is more
likely.11
Figure 7. Energy diagram illustrating the “round-trip” precipiton/benzil
energy-transfer pathway. PE0-benzil0 and PZ0-benzil0 represent the ground
states of the precipiton-benzil bichromophore. Changes in superscripts
identify changes in localized electronic excited states (1 ) singlet, 3 )
triplet). T3 represents the minimum-energy conformer of the precipiton
triplet.
excitation, lead to the formation of the benzil triplet state. In
the case of 4Z/E, only one isomer (4E) is quenched by the
tethered benzil. The failure of 4Z to carry singlet energy to the
benzil chromophore results in a reduced rate of precipiton triplet-
state formation, and the photostationary state is more influenced
by the direct singlet pathway.
A summary of the proposed precipiton-benzil round-trip
energy transfer is presented in Figure 7. The approximate
energies of the relevant excited levels are based on stilbene and
benzil photophysical data. Radiationless transitions are repre-
sented by wavy arrows. Our proposal is that excitation of the
precipiton affords a singlet excited state localized on the
precipiton and that this excitation energy is transferred to the
benzil. (The evidence for this lies in the fluorescence quenching
observed in the benzil-tethered precipiton isomers.) Rapid ISC
affords the benzil-localized triplet, and back-transfer to the
precipiton results in the precipiton triplet state and allows
efficient isomerization.
Conclusions
Our goal was to identify a precipiton derivative that would
efficiently isomerize the insoluble trans form of our precipitons
to the soluble cis form. Benzil-modified precipiton 4E/Z meets
this need. The molecule will be applied in further investigations
of the mechanism of solubilization-isomerization and solubility-
based photoswitchable materials.
The round-trip energy-transfer pathway that we discuss here
is speculative. The experiments we carried out do not establish
this as the only acceptable explanation of the observed results.
More sophisticated time-dependent studies will be required to
support or refute this proposal. The basic scheme for round-
trip energy transfer was proposed by Hammond in 1965 to
explain enhanced naphthalene phosphorescence in a benzophe-
none-naphthalene bichromophoric system.12 In that case, in the
Rothe work with benzil,8 and in the recent work by Bergamini
et al. with benzophenone,9 the round trip concluded with
enhanced triplet-state emission from a readily excited chro-
mophore that was inefficient in intersystem crossing. In those
studies as in ours, the enhancement occurred due to the effect
of a nearby partner. In those studies, the evidence for the round
trip was based on total emission measurements. To our
knowledge, the work described here provides the first example
of a round trip leading to a chemical event; here, it is the
chemical event that provides the evidence of the round trip. It
may interest the reader to note that the benzil effect is
geometrically selective. The round-trip mechanism opens access
The observed photostationary state difference among 2Z/E,
3Z/E, and 4Z/E can best be explained as arising from a
competition between direct singlet-state isomerization (in which
benzil plays no part) and the round-trip pathway. Introduction
of benzil into the solution of 2Z/E induces a small change in
rate and a small change in the photostationary state toward the
cis isomer. The propinquity of the benzil chromophore in 3Z/E
and 4Z/E increases the efficiency of singlet energy transfer to
the benzil chromophore, and through back-transfer, increases
the effect of the triplet energy pathway of isomerization. The
round-trip pathway competes most effectively with the direct
pathway in the case of 3Z/E, where both isomers, upon
(8) Rothe, C.; Palsson, L. O.; Monkman, A. P. Chem. Phys. 2002, 285, 95.
(9) Bergamini, G.; Ceroni, P.; Maestri, M.; Balzani, V.; Lee, S. K.; Vogtle, F.
Photochem. Photobiol. Sci. 2004, 3, 898.
(10) Agyin, J. K.; Timberlake, L. D.; Morrison, H. J. Am. Chem. Soc. 1997,
119, 7945.
(11) (a) Fo¨rster, Th. H. Discuss. Faraday Soc. 1959, 27, 7. (b) Dexter, D. L. J.
Chem. Phys. 1953, 21, 836. (c) Mondal, J. A.; Ramakrishna, G.; Singh, A.
K.; Ghosh, H. N.; Mariappan, M.; Maiya, B. G.; Mukherjee, T.; Palit, D.
K. J. Phys. Chem. A 2004, 108, 7843.
(12) Lamola, A. A.; Leermakers, P. A.; Byers, G. W.; Hammond, G. S. J. Am.
Chem. Soc. 1965, 87, 2322.
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Benzil-Tethered Precipitons for Controlling Solubility
A R T I C L E S
solution of 4-(hydroxymethyl)benzil 6 (1.02 g, 4.26 mmol) and meta-
substituted carboxylic acid pinacol ester 11 (1.10 g, 4.21 mmol) in
anhydrous CH2Cl2 (43 mL), at 0 °C, were added 4-(dimethylamino)-
pyridine (DMAP, 103 mg, 0.843 mmol) and dicyclohexylcarbodiimide
(DCC, 957 mg, 4.64 mmol). The solution was stirred for 10 min at 0
°C and then warmed to room temperature and stirred for 2.5 h. The
white precipitate that formed was removed by filtration, and the volatile
components of the filtrate were removed in vacuo to give a yellow oil
that was purified by flash chromatography (SiO2, 1% EtOAc in CH2-
Cl2) to afford 12 as a yellow oil (1.21 g, 59%): Rf 0.61 (1% EtOAc in
CH2Cl2); IR (thin film) 3057, 2978, 2931, 1740, 1674, 1608, 1430,
to the precipiton triplet state but does so to different extents for
the Z and E isomers and for the meta- and para-linked
chromophores. This is surely because the geometry of the
molecules and their conformational microstates influence the
efficiency of intramolecular singlet energy transfer and also
affect the back-transfer of triplet excitation energy. Which effect
is most important, if either, is unknown. We believe it may be
productive to contemplate potential uses of this stereoselectivity
in energy transfer for any investigator interested in new
separation strategies and new chemoselective processes.
1
1360, 1323, 1213, 1144, 1099, 1000, 709 cm-1; H NMR (300 MHz,
Experimental Section
CDCl3) δ 7.95 (app t, J ) 14, 6 Hz, 4H), 7.74 (m, 2H), 7.64 (app t, J
) 14, 7, 1 Hz, 1H), 7.50 (app t, J ) 15, 8 Hz, 2H), 7.43-7.31 (m,
4H), 5.19 (s, 2H), 3.71 (s, 2H), 1.34 (s, 12H); 13C NMR (75 MHz,
CH2Cl2) δ 194.2, 193.8, 170.9, 143.1, 135.1, 134.8, 133.6, 132.8, 132.5,
132.0, 130.0, 129.8, 128.9, 128.0, 127.8, 83.4, 65.3, 41.0, 24.8; HRMS
(ES) m/e calcd for C29H29BO6Na, 507.1955; found, 507.197.
Introduction. The compound 4-(bromomethyl)benzil (5) and the
boronic acid pinacol esters 8 and 11 were commercially available and
were used as received. Preparation and purification of the tethered
precipitons 3Z/E and 4Z/E were carried out in a darkened laboratory
under red-light illumination.
[4′-(2-Biphenyl-4-yl-vinyl)biphenyl-4-yl]acetic Acid 4-(2-Oxo-2-
phenylacetyl)benzyl Ester (3Z). A solution of 1-[2-(4-bromophenyl)]-
biphenyl ethene 10 (555 mg, 1.66 mmol), pinacol boronic ester 9 (884
mg, 1.82 mmol), and Pd(PPh3)4 (82.5 mg, 0.071 mmol) in THF (10.3
mL) was degassed via three freeze-pump-thaw cycles and placed
under nitrogen. Sodium carbonate (527 mg, 4.97 mmol), which had
been dissolved in a minimum amount of water in a separate flask and
degassed similarly, was transferred to the THF solution via cannula.
The resulting mixture was heated at reflux for 20 h. After the mixture
was cooled, volatile components were removed in vacuo, and the crude
residue was combined with water (100 mL) and extracted with CH2-
Cl2 (3 × 100 mL). The CH2Cl2 extracts were combined, washed with
brine (300 mL), dried with MgSO4, and filtered. Volatile components
of the filtrate were removed in vacuo, and the residue was purified by
flash chromatography (SiO2, 7:3 CH2Cl2/Hex) to afford 3Z as a
light-yellow wax (81.3 mg, 80%): Rf 0.39 (7:3 CH2Cl2/Hex); mp 82-
84 °C; IR (thin film) 3028, 1738, 1671, 1608, 1596, 1496, 1486, 1449,
1416, 1213, 1173, 1146, 1006, 883 cm-1; 1H NMR (300 MHz, CDCl3)
δ 7.98-7.94 (m, 4H), 7.71-7.30 (m, 22H), 6.66 (s, 2H), 5.22 (s, 2H),
3.74 (s, 2H); 13C NMR (75 MHz, CH2Cl2) δ 194.2, 193.9, 171.0, 143.1,
140.6, 139.9, 139.7, 139.3, 136.4, 136.3, 134.9, 133.0, 132.7, 132.6,
130.1, 130.0, 129.9, 129.7, 129.4, 129.3, 129.0, 128.7, 128.0, 127.4,
127.3, 127.1, 127.0, 126.9, 126.85, 126.75; HRMS (EI) m/e calcd for
C43H32O4, 612.230060; found, 612.232163.
4-(Hydroxymethyl)benzil (6) and 4-(Hydroxymethyl)benzil Ni-
trate (7). To a solution of 4-(bromomethyl)benzil (5, 100 mg, 0.33
mmol) in 33 mL of THF was added a solution of AgNO3 (653 mg,
0.38 mmol) in 11 mL of water. The resulting mixture was heated at
reflux for 3.5 h. After the mixture was cooled, precipitated AgBr was
removed by filtration and rinsed with acetone. Volatile components of
the filtrate were removed in vacuo, and the resulting mixture was
extracted with CH2Cl2 (3 × 50 mL). The CH2Cl2 extracts were
combined, washed with brine (150 mL), dried with MgSO4, and filtered.
Volatile components of the filtrate were removed in vacuo, and
purification of the residue by flash chromatography (SiO2, 1% EtOAc
in CH2Cl2) afforded, first, 4-(hydroxymethyl)benzil nitrate 7 as a yellow
oil (42.8 mg): Rf 0.79 (1% EtOAc in CH2Cl2); IR (thin film) 2924,
1672, 1626, 1607, 1596, 1580, 1450, 1279, 1211, 1173 cm-1; 1H NMR
(300 MHz, CDCl3) δ 8.04-7.97 (m, 4H), 7.69 (dddd, J ) 8, 8, 1, 1
Hz, 1H), 7.54 (app d, J ) 6 Hz, 3H), 7.51 (m, 1H), 5.50 (s, 1H); 13C
NMR (75 MHz, CH2Cl2) δ 194.0, 193.6, 139.4, 135.0, 133.6, 132.8,
130.3, 129.9, 129.1, 128.8, 73.3; HRMS (EI) m/e calcd for C15H11-
NO3, 285.063723; found; 285.063638. Further elution (SiO2, 1% EtOAc
in CH2Cl2) afforded alcohol 6 as a light-yellow wax (25.9 mg, 56%):
Rf 0.21 (2% EtOAc in CH2Cl2); IR (thin film) 3419, 3062, 2923, 1674,
1
1605, 1579 cm-1; H NMR (300 MHz, CDCl3) δ 7.98 (app d, J ) 8
Hz, 4H), 7.68 (dddd, J ) 8, 8, 2, 2 Hz, 1H), 7.53 (m, 4H), 4.82 (s,
2H); 13C NMR (75 MHz, CH2Cl2) δ 194.6, 194.2, 148.4, 134.9, 133.0,
132.2, 130.3, 129.9, 129.0, 126.9, 64.4; HRMS (EI) m/e calcd for
C15H12O3, 240.0786; found, 240.0776.
[4′-(2-Biphenyl-4-yl-vinyl)biphenyl-4-yl]acetic Acid 4-(2-Oxo-2-
phenylacetyl)benzyl Ester (3E). A solution of precipiton 3Z (100 mg,
0.163 mmol) and erythrosine B (6.8 mg, 0.008 mmol) in THF (1.63
mL) was degassed via three freeze-pump-thaw cycles and then placed
under nitrogen. The resulting mixture was stirred and irradiated with a
250 W incandescent lamp for 20 h. The resulting precipitate 3E (70.8
mg, 71%) was isolated by filtering the reaction mixture through filter
paper and washing it with THF until all traces of the red dye were
removed: mp >240 °C; IR (KBr) 3030, 1729, 1665, 1608, 1496, 1450,
1409, 1374, 1243, 1162, 1003, 971 cm-1; 1H NMR (300 MHz, CDCl3)
δ 7.99-7.96 (m, 4H), 7.68-7.34 (m, 22H), 7.21 (s, 2H), 5.24 (s, 2H),
3.76 (s, 2H); 13C NMR, too insoluble to obtain; HRMS (EI) m/e calcd
for C43H32O4, 612.230060; found, 612.232850.
[4-(2-Oxo-2-phenylacetyl)phenyl]acetic Acid 4-(4,4,5,5-Tetra-
methyl-[1,3,2]dioxaborolan-2-yl)benzyl Ester (9). To stirred a solution
of 4-(hydroxymethyl)benzil 6 (968 mg, 4.03 mmol) and para-substituted
carboxylic acid pinacol ester 8 (1.05 g, 3.99 mmol) in anhydrous CH2-
Cl2 (40 mL), at 0 °C, were added 4-(dimethylamino)pyridine (DMAP,
97.4 mg, 0.798 mmol) and dicyclohexylcarbodiimide (DCC, 905 mg,
4.39 mmol). The solution was stirred for 10 min at 0 °C and then
warmed to room temperature and stirred for 3 h. The white precipitate
that formed was removed by filtration, and the volatile components of
the filtrate were removed in vacuo to give a yellow oil that was purified
by flash chromatography (SiO2, 1% EtOAc in CH2Cl2) to afford 9 as
a yellow oil (1.81 g, 94%): Rf 0.55 (CH2Cl2); IR (thin film) 3399,
2978, 1736, 1678, 1608, 1450, 1360, 1322, 1274, 1213, 1174, 1143
cm-1; 1H NMR (300 MHz, CDCl3) δ 7.99-7.94 (m, 4H), 7.78 (d, J )
8 Hz, 2H), 7.68 (dddd, J ) 8, 8, 1, 1 Hz, 1H), 7.55-7.50 (m, 2H),
7.42 (d, J ) 9 Hz, 2H), 7.30 (d, J ) 8 Hz, 2H), 5.19 (s, 2H), 3.72 (s,
2H), 1.35 (s, 12H); 13C NMR (75 MHz, CH2Cl2) δ 194.2, 193.9, 170.8,
143.1, 136.6, 135.1, 134.8, 132.9, 132.6, 130.1, 129.9, 129.0, 128.6,
127.9, 83.8, 65.5, 41.5, 24.8; HRMS (ES) m/e calcd for C29H29BO6Na,
507.1955; found, 507.1947.
[4′-(2-Biphenyl-4-yl-vinyl)biphenyl-3-yl]acetic Acid 4-(2-Oxo-2-
phenylacetyl)benzyl Ester (4Z). A solution of 1-[2-(4-bromophenyl)]-
biphenyl ethene 10 (372 mg, 1.11 mmol), pinacol boronic ester 12 (592
mg, 1.22 mmol), and Pd(PPh3)4 (55.4 mg, 0.048 mmol) in THF (6.9
mL) was degassed via three freeze-pump-thaw cycles and then placed
under nitrogen. Sodium carbonate (353 mg, 3.33 mmol), previously
dissolved in a minimum amount of water in a separate flask and
degassed similarly, was transferred to the THF solution via cannula.
The resulting mixture was heated at reflux for 23 h. After the mixture
was cooled, volatile components were removed in vacuo, and the crude
residue was combined with water (100 mL) and extracted with CH2-
[4-(2-Oxo-2-phenylacetyl)phenyl]acetic Acid 3-(4,4,5,5-Tetra-
methyl-[1,3,2]dioxaborolan-2-yl)benzyl Ester (12). To stirred a
9
J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007 3971

A R T I C L E S
Ams and Wilcox
Cl2 (3 × 100 mL). The CH2Cl2 extracts were combined, washed with
brine (300 mL), dried with MgSO4, and filtered. Volatile components
of the filtrate were removed in vacuo, and the residue was purified by
flash chromatography (SiO2, 8:2 Hex/CH2Cl2) to afford 4Z as a light-
yellow wax (608 mg, 90%): Rf 0.16 (5:5 Hex/CH2Cl2); mp 44-46
°C; IR (thin film) 3056, 3029, 1739, 1672, 1607, 1579, 1485, 1449,
1417, 1241, 1212, 1174, 1147, 883 cm-1; 1H NMR (300 MHz, CDCl3)
δ 8.03 (app t, 4H), 7.68-7.28 (m, 22H), 6.73 (s, 2H), 5.25 (s, 2H),
3.81 (s, 2H); 13C NMR (75 MHz, CH2Cl2) δ 194.0, 193.7, 170.7, 142.9,
140.8, 140.3, 139.6, 139.1, 136.3, 136.0, 134.7, 133.9, 132.7, 132.4,
129.9, 129.7, 129.6, 129.2, 128.9, 128.8, 128.5, 128.0, 127.8, 127.6,
127.1, 126.8, 126.6, 125.6, 65.3, 40.1; HRMS (ES) m/e calcd for
C43H32O4Na, 635.2218; found, 635.2198.
too insoluble to obtain; HRMS (EI) m/e calcd for C43H32O4, 612.230060;
found, 612.230060.
Photochemical Experiments. All of the photochemical measure-
ments were performed in degassed THF. UV-vis absorption spectra
were recorded with a PerkinElmer Lambda 9 UV-vis spectrophotom-
eter. Photoisomerization experiments were monitored by recording the
UV-vis absorption of 2Z/E, 3Z/E, and 4Z/E at 345 nm exclusively.
All experiments were performed in the dark or under red light due to
the high sensitivity of the compounds to normal ambient lighting.
Emission spectra were recorded with a 1680/1681 Spex Fluorolog
spectrometer. The excitation light used for all of the photoisomerization
experiments was produced by a 25 W incandescent lamp positioned 5
cm from the degassed cuvette which contained the sample.
[4′-(2-Biphenyl-4-yl-vinyl)biphenyl-3-yl]acetic Acid 4-(2-Oxo-2-
phenylacetyl)benzyl Ester (4E). A solution of precipiton 4Z (66.8 mg,
0.109 mmol) and erythrosine B (4.6 mg, 0.006 mmol) in THF (1.09
mL) was degassed via three freeze-pump-thaw cycles and then placed
under nitrogen. The resulting mixture was stirred and irradiated with a
250 W incandescent lamp equipped with a λ e 400 nm cutoff filter
for 4 h. The resulting homogeneous solution was then purified by flash
chromatography (SiO2, 8:2 Hex/CH2Cl2) to afford 4E as a light-yellow
solid (61.4 mg, 92%): mp 181-183 °C; IR (KBr) 3434, 3030, 1736,
1670, 1607, 1596, 1579, 1486, 1449, 1408, 1375, 1213, 1174, 1146,
Acknowledgment. Financial support from the National
Institute of General Medical Sciences is gratefully acknowl-
edged. The authors would also like to recognize the help of
Prof. David Waldeck and Prof. Stephane Petoud.
Supporting Information Available: Experimental details, 1H
and 13C NMR spectra, and complete characterization data for
all compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
1004, 970 cm-1; H NMR (300 MHz, CDCl3) δ 7.99-7.95 (m, 4H),
7.69-7.30 (m, 22H), 7.21 (s, 2H), 5.24 (s, 2H), 3.79 (s, 2H); 13C NMR,
JA068211C
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3972 J. AM. CHEM. SOC. VOL. 129, NO. 13, 2007