Chiral 1,2- and 1,3-diol-functionalized chromophores as Lego building blocks for coupled structures

Article abstract of DOI:10.1021/jo051097g

Chiral nitroanilines containing 1,2- or 1,3-diol functionalities have been synthesized by nucleophilic aromatic substitution of fluoronitroanilines with 1-aminopropane-2,3-diols and 2-aminopropane-1,3-diol in the melt. X-ray structure analyses confirm ret

Full text of DOI:10.1021/jo051097g

robustness are guaranteed. For this it is necessary for
the molecules on the periphery to be functionalized in
such a way that they can be chemically bonded to and
separated again from other functional molecules. We
chose solvatochromic organic chromophores with 1,2- and
1,3-diol functions. Bonding with boronic-acid-based sen-
sor molecules is of particular interest for developing
bifunctional and adjustable chromophores.7-13 Boronic-
acid-containing organic compounds are becoming increas-
ingly popular as macromolecular substrates in organic
synthesis and combinatorial chemistry.14,15
By reaction with functionalized boronic acids 1,2- and
1,3-diols can be converted into boronate esters.7-15 These
can easily be cleaved using base-catalyzed hydrolysis.
Besides this functionalization, this class of molecule has
other uses:
Chiral 1,2- and 1,3-Diol-Functionalized
Chromophores as Lego Building Blocks for
Coupled Structures
Stefan Spange,*,† Katja Hofmann,† Bernhard Walfort,‡
Tobias Ru¨ffer,‡ and Heinrich Lang‡
Department of Polymer Chemistry and Department of
Inorganic Chemistry, Institute of Chemistry, Chemnitz
University of Technology, 09111 Chemnitz, Germany
stefan.spange@chemie.tu-chemnitz.de
Received June 1, 2005
(i) The chromophoric compounds can be obtained as
(R)- or (S)-enantiomers or as racemic mixtures. This
allows for easy integration of chiral information into the
chromophore.
(ii) 1,2- and 1,3-diol functions can also be functionalized
in other ways, for example, through the introduction of
fatty acid chains. They can thus be used as solvatochro-
mic components for chiral lipid layers for localized
measurement of micropolarity.16,17
(iii) Solvatochromic chromophores give information on
the polarity of the surroundings of the molecule (sol-
vent, surface, micelles) through the measurement of the
UV/vis absorption maximum.18,19
We chose nitroaniline derivatives for the preliminary
investigations as these have been widely used in materi-
als science (NLO)20-22 and analysis (solvatochromism).23-31
There are a large number of substituted p-nitroaniline
Chiral nitroanilines containing 1,2- or 1,3-diol functionalities
have been synthesized by nucleophilic aromatic substitution
of fluoronitroanilines with 1-aminopropane-2,3-diols and
2-aminopropane-1,3-diol in the melt. X-ray structure analy-
ses confirm retention of the configuration of the chiral center.
The novel chromophores are suitable to link reversibly to
various substituted arylboronic acids which allows the
construction of new solvatochromic sensor molecules suitable
to response to solvent and anion coordination by fluoride.
The solvatochromism of the new compounds has been
studied using the Kamlet-Taft LSE relationship.
According to the definition supramolecular structures
are built up as a result of noncovalent interactions
between individual building blocks.1-3 The principle of
reversibility guarantees that the individual building
blocks are assembled according to a plan and can be
separated again as with Lego building blocks. In 1988,
Michl and Stoddart developed, independently of one
another, the idea of combining organic and/or inorganic
molecules using a building plan, as with Lego.4-6 How-
ever, there is still no successful synthetic concept of
assembling molecular building blocks to form an unsym-
metrical superstructure using a plan. An important
disadvantage of many compounds whose formation is
reversible is the thermal lability of the binding points,
which also determines the average lifetime of the func-
tional units.
(7) James, T. D.; Shinkai, S. Top. Curr. Chem. 2002, 218, 159-200.
(8) Shoji, E.; Freund, M. S. J. Am. Chem. Soc. 2002, 124, 12486-
12493.
(9) Takeuchi, M.; Taguchi, M.; Shinmori, H.; Shinkai, S. Bull. Chem.
Soc. Jpn. 1996, 69, 2613-2618.
(10) Wang, W.; Gao, Y.; Wang, B. Curr. Org. Chem. 2002, 6, 1285-
1317.
(11) Hartley, J. H.; James, T. D. Tetrahedron Lett. 1999, 40, 2597-
2600.
(12) Cooper, C. R.; James, T. D. J. Chem. Soc., Perkin. Trans. 1 2000,
963-969.
(13) Ward, C. J.; Patel, P.; Ashton, P. R.; James, T. D. Chem.
Commun. 2000, 229-230.
(14) Wulff, G. Angew. Chem. 1995, 107, 1958-1979; Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1812-1831.
(15) Gravel, M.; Hall, D. G. J. Org. Chem. 2002, 67, 3-15.
(16) Tsukamoto, I.; Ogli, K. J. Colloid Interface Sci. 1994, 168, 323-
326.
To construct bifunctional probes, we looked for com-
pounds with covalent binding points where both the
principle of reversibility and also a high degree of
(17) Tada, E. B.; El Seoud, A. O. Prog. Collect. Polym. Sci. 2002,
121, 101-109.
(18) Reichardt, C. Chem. Rev. 1994, 94, 2319-2358.
(19) Reichardt, C. Solvents and Solvents Effects in Organic Chem-
istry, 3rd ed., updated and enlarged ed.; Wiley-VCH: Weinheim, 2003.
(20) Kato, M.; Kiguchi, M.; Sugita, N.; Taniguchi, Y. J. Phys. Chem.
B 1997, 101, 8856-8859.
(21) Ledoux, I.; Zyss, J. In Novel Optical Materials & Applications;
Khoo, I., Simoni, F., Umeton, C., Eds.; John Wiley & Sons: New York,
1997; Chapter 1, pp 1-48.
(22) Gangopadhyay, P.; Venugopal-Rao, S.; Narayana-Rao, D.;
Radhakrishnan, T. P. J. Mater. Chem. 1999, 9, 1699-1707.
(23) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1976, 98, 377-
383.
(24) Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027-
6038.
(25) Kamlet, M. J.; Abboud, J.-L. M. M. H.; Abraham, M. H.; Taft,
R. W. J. Org. Chem. 1983, 48, 2877-2887.
† Department of Polymer Chemistry.
‡ Department of Inorganic Chemistry. To whom correspondence
pertaining to X-ray structure analyses should be addressed.
(1) Lehn, J.-M. Supramolecular Chemistry-Concepts and Perspec-
tives; VCH: Weinheim, 1995.
(2) Balzani, V. F.; Credi, F.; Raymo, M.; Stoddart, J. F. Angew.
Chem. 2000, 112, 3484-3530; Angew. Chem., Int. Ed. 2000, 39, 3348-
3391.
(3) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Angew. Chem.
2001, 113, 2446-2492; Angew. Chem., Int. Ed. 2001, 40, 2382-2426.
(4) Kaszynski, P.; Michl, J. J. Am Chem. Soc. 1988, 110, 5225-5226.
(5) Stoddart, J. F. Chem. Br. 1988, 24, 1203-1208.
(6) Michl, J.; Magnera, F. PNAS 2002, 99, 4788-4792.
10.1021/jo051097g CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/22/2005
8564
J. Org. Chem. 2005, 70, 8564-8567

TABLE 1. Synthesized 1,2- and 1,3-Diol-functionalized
Nitroanilines by Nucleophilic Aromatic Substitution in
the Melt According to Scheme 1
SCHEME 1. Nucleophilic Aromatic Substitution of
Aminopropandiols with Activated
Fluoronitrobenzenes, e.g., of Compound 1
R1
R2
R3
R4
1
2
NO2
H
NO2
NO2
H
NO2
NO2
H
H
CH3
CH3
CH3
H
rac-CH2(CHOH)CH2OH
rac-CH2(CHOH)CH2OH
rac-CH2(CHOH)CH2OH
(R)-CH2(CHOH)CH2OH
(R)-CH2(CHOH)CH2OH
(R)-CH2(CHOH)CH2OH
(S)-CH2(CHOH)CH2OH
(S)-CH2(CHOH)CH2OH
CH(CH2OH)2
derivatives which are used as solvent-sensitive indicators
for setting up polarity scales,23-26 as lipophilic indicators
in micelles, bilayers, and biological membranes,27-31 or
guests in cyclodextrine.32
NO2
NO2
H
3
4
5
NO2
NO2
H
NO2
H
NO2
NO2
H
6
H
The aim of the present work was to synthesize and
structurally characterize a range of new hydrophilic
nitroaniline derivatives and to investigate their solvato-
chromic properties in detail. In particular, we wanted to
show whether and how the free 1,2- and 1,3-diol functions
and those capped by reaction with various aromatic
boronic acids have an effect on the chromophoric π-elec-
tron system as a result of interactions with the surround-
ings of the molecules and what proportion of these are
dipole-dipole and/or hydrogen bond or acid-base inter-
actions. Also, the effect of substituents on the solvato-
chromic behavior of the boronic acid esters was investi-
gated. To separate the individual solvation effects we
used the simplified Kamlet-Taft equation25 (eq 1) from
which the coefficients of the individual interaction con-
tributions can be determined using multiple correlation
analysis.18,19
7
H
8
H
9
NO2
H
NO2
H
10
11
H
CH(CH2OH)2
H
CH(CH2OH)2
of choice was found to involve performing the reaction
in the melt with CaCO3 as the catalyst. Enantiomerically
pure and racemic nitroanilines with 1,2- and 1,3-diol
functions were thus obtained in high purity (Table 1).
Complete retention of the (R)-configuration during
nucleophilic substitution was established unambiguously
by X-ray structure analysis of (R)-N-[1-(2,3-dihydroxy-
propyl)]-2-nitroaniline (5) (Flack-x: 0.1(15)).
The crystal structure of 5 (Figure 1A) shows weak
intermolecular hydrogen bonding interactions between
O1-H1‚‚‚O1A and O2-H5‚‚‚O2A (O1‚‚‚O1A 263.6(2),
O2‚‚‚O2A 267.2(2) pm). The molecules are arranged in a
helical chain, whereby the aryl rings are on the exterior
and are parallel to one another. Overall there is a double-
helix structure (Figure 1B). Weak intramolecular hydro-
gen-bonding interactions between N1-H8‚‚‚O3 (N1‚‚‚O3
260.8 (2) pm) and intermolecular hydrogen-bonding
ν˜max ) ν˜max,0 + aR + bâ + sπ*
(1)
ν˜max is the longest wavelength UV/vis absorption
maximum of the compound measured in a particular
solvent, ν˜max,0 is that of a nonpolar reference solvent, R
is the hydrogen-bonding acidity, â describes the hydrogen-
bonding basicity, and π* describes the dipolarity/polar-
izability of the solvent. a, b, and s are the solvent-
independent correlation coefficients, which allow the
effects of a particular parameter on the solvatochromic
properties of the compounds to be determined.
According to the literature,29,32 the class of compound
to be investigated should be readily accessible by nucleo-
philic substitution of fluoronitroaromatics with 1-ami-
nopropane-2,3-diols and 2-aminopropane-1,3-diol (serinol)
(Scheme 1).
Carrying out the synthesis according to Scheme 1,
however, was not so trivial as many experiments per-
formed using literature procedures in solvents such as
DMSO, NMP, or DMF failed to allow clean isolation of
products, and consequently chiral compounds could not
be obtained enantiomerically pure. Finally, the method
(26) Laurence, C.; Nicolet, P.; Tawfik-Dalati, M. J. Phys. Chem.
1994, 98, 5807-5816.
(27) Helburn, R.; Ullah, N.; Mansour, G.; Maxka, J. J. Phys. Org.
Chem. 1997, 10, 42-48.
(28) Helburn, R.; Dijiba, Y.; Mansour, G.; Maxka, J. Langmuir 1998,
14, 7147-7154.
(29) Mansour, G.; Creedon, W.; Dorrestein, P. C.; Maxka, J. Mac-
Donald, J. C.; Helburn, R. J. Org. Chem. 2001, 66, 4050-4054.
(30) Ibata, T.; Isogami, Y.; Toyoda, J. Bull. Chem. Soc. Jpn. 1991,
64, 42.
(31) Novaki, L. P.; El Seoud, O. A. Phys. Chem. 1996, 100, 648-
655.
(32) Lo Meo, P.; D’Anna, F.; Gruttadauria, M.; Riela, S.; Noto, R.
Tetrahedron 2004, 60, 9099-9111.
FIGURE 1. (A) X-ray crystal structure of (R)-N-[1-(2,3-
dihydroxypropyl)]-2-nitroaniline (5), crystals from acetonic
solution, and (B) helical structure of 5.
J. Org. Chem, Vol. 70, No. 21, 2005 8565

TABLE 2. Values of the Solvent-Independent
Correlation Coefficients (a, b, and s of the Kamlet-Taft
Parameters r, â, and π*), Solute Property of a Reference
System ν˜max,0, Standard Deviation (sd), Correlation
Coeffiecent (r), and Number of Solvents (n) for the
Solvatochromism of Compounds 1, 4, 7, 9, and 12-17
(Scheme 2)
SCHEME 2. Studied Compounds Regarding Their
Solvatochromic Behavior
compd ν˜max,0
a
b
s
n
r
sd
1
27.8 -0.61
28.9 -0.66
-0.78 -2.70 26 0.96 0.24
-1.62 -2.64 26 0.96 0.26
-1.65 -2.57 25 0.96 0.26
-1.65 -2.69 25 0.95 0.27
-1.01 -2.94 25 0.98 0.21
-0.83 -2.71 27 0.96 0.26
-1.38 -3.19 24 0.97 0.22
-1.09 -2.96 26 0.97 0.24
-1.11 -3.02 27 0.96 0.30
4
7
9
28.9 -0.71
29.0 -0.59
28.2 -0.62
28.0 -0.88
28.5 -0.44
28.3 -0.89
28.3 -0.79
12
13
14
15
16
17
28.3 -3.8 × 10-4 -1.47 -2.99 24 0.96 0.24
The correlation coefficient r is greater than 0.95 for
LSE relationships, which indicates a high validity of the
multiparameter equations and allows significant conclu-
sions to be drawn. The arrangement of the nonpolar
solvents, such as benzene, toluene, tetrachloromethane,
triethylamine, diethyl ether, or p-xylene, in the LSE
relationship shows that no significant contribution by
dimers or associated compounds to the solvatochromism
could be observed. As expected, all compounds show
positive solvatochromism with regard to the π*-param-
eter, which correlates with the higher dipole moment of
the electronically excited state, and it was expected that
p-nitroaniline derivatives in particular could be used to
set up the π*-polarity scale.24-26 Protic solvents, which
can act as hydrogen bond donors (HBD) and electron-
pair acceptors (EPA), also interact with the oxygen atoms
of the nitro groups (a is negative). However, the effect of
the a coefficient is only very small and, in comparison to
s and b, is of little significance.
The highly positive solvatochromic effect of the â-pa-
rameter in the 1,2-diol-and N-methyl-substituted p-
nitroaniline derivative 1, caused by specific solvation of
the diol groups by hydrogen bonding to the solvent, is
new and noteworthy. We found a similar effect of the
â-parameter in investigations of the solvatochromism of
N-(2-hydroxyethyl)-substituted Michler’s ketone deriva-
tives.36,37 The stronger contribution of the HBA capacity
(â-term) on the solvatochromism for compounds 4, 7, and
9 was expected and can be readily explained through
NH-solvent interactions.38-41
interactions between the oxygen of the nitro group and
the hydrogen atoms on C1A and C9A of the neighboring
molecule (O4‚‚‚C1A 321.7(2), O3‚‚‚C9A 342.5(2) pm) lead
to further binding of the helices in the periphery.
Comparable X-ray structure analyses of 1,2-diol-func-
tionalized nitroanilines have not been described. A paper
by Ojala et al. reports the crystal structure of a sugar-
functionalized aniline whereby the 4-nitroaniline unit is
bonded to C1 of the sugar forming a â-N glycoside.33
To demonstrate the concept of reversible combination,
reactions of compound 1 with variously substituted
phenylboronic acids were investigated. The esterification
is an equilibrium reaction so the water formed was
removed by azeotropic distillation from the reaction
mixture.34 Thermogravimetric analysis of the boronic
esters obtained showed that the thermal stability was
considerably higher than that of the starting diols.
Decomposition of the esters begins at about 220 °C,
whereas the 1,2-diol-functionalized nitroanilines began
to decompose at ca. 150 °C.
Thus, treatment with arylboronic acids not only pro-
tects the 1,2-diol function but also increases the stability.
In further work it will be shown how, building on the
Lego principle, chiral information on chromophoric struc-
tures can be integrated into nanosystems.
For the solvatochromic investigations we chose the four
new nitroaniline derivatives 1, 4, 7, and 9 and the boronic
esters 12-17 synthesized from 1 (Scheme 2), and the
measurements were taken in 27 different solvents to
determine the coefficients a, b, and s in eq 1.
The solvatochromism of the boronic esters 12-17 also
surprisingly shows a clear effect of the basicity of the
(33) Ojala, C. R.; Ostmann, J. M.; Hansen, S. E.; Ojala, W. H.
Carbohydr. Res. 2001, 332, 415-417.
(34) Wulff, G.; Stellbrick, H. Recl. Trav. Chim. Pays-Bas 1990, 109,
216-221.
Because of a high polarity and HBD/HPA capacity of
the diol-functionalized chromophore, it is likely that
association will occur in nonpolar solvents, and this can
have an effect on the UV/vis absorption maximum. For
this reason, very low concentrations (c ∼ 10-5 M) of the
compounds were used for the UV/vis measurements. This
posed particular requirements on the purity of the
solvents. The UV/vis absorption maxima were then
correlated with the Kamlet-Taft solvent parameters.35
The results of the multiple square correlation analyses
are shown in Table 2.
(35) Marcus, Y. Chem. Soc. Rev. 1993, 22, 409-416.
(36) El-Sayed, M. M.; Mu¨ller, H.; Rheinwald, G.; Lang, H.; Spange,
S. J. Phys. Org. Chem. 2001, 14, 247-255.
(37) El-Sayed, M. M.; H. Mu¨ller, H.; Rheinwald, G.; Lang, H.;
Spange, S. Chem. Mater. 2003, 15, 746-754.
(38) Kamlet, M. J.; Jones, M. E.; Taft, R. W.; Abboud, J. J. Chem.
Soc., Perkin Trans 1979, 342-348.
(39) Wang, C.-K.; Wang, Y.-H.; Su, Y.; Luo, Y. J. Chem. Phys. 2003,
119, 4409-4412.
(40) Bosch, E.; Rived, F.; Rose´s, M. J. Chem. Soc., Perkin Trans. 2
1996, 2177-2184.
(41) Vokin, A. I.; Shulunova, A. M.; Krivoruchka, I. G.; Krylova, O,
V.; Lopyrev, V. A.; Turchaninov, V. K. Russ. J. Gen. Chem. 2002, 72,
449-455.
8566 J. Org. Chem., Vol. 70, No. 21, 2005

solvent on the positive solvatochromic shift, although
both OH groups are completely protected. The influence
of the â-term (basicity) of the solvent on the contribution
to the bathochromic shift of compounds 12-15 can be
explained in two ways. First, the boron atom can build
up a five-membered ring involving the amino nitrogen
atom of the aromatic ring. Then the external base
competes with the lone electron pair of the amino group.
Some sugar indicators are based on this mechanism.10,11
Since that intramolecular boron atom complex formation
takes place, a significant hypsochromic UV/vis shift of
the compounds 12-15 compared to compound 1 is
expected. However the difference of ∆ν˜ measured in
various solvents (Table 2) does not exceed 400 cm-1. In
dichloromethane or toluene the effect on the shift of the
UV/vis band is negligible.
Second, it is possible that the specific solvation of the
sp2-hybridized boron by electron pairs of bases increases
the electron-donating properties of the neighboring amino
group. This effect is particularly clear in the case of the
1,4-phenyldiboronic-acid-bridged bis(4-nitroaniline) de-
rivative. Apparently, the electron-donating effect of the
complexed boronic acid groups is increased by a second
boronic acid group, which has also been discussed briefly
by Hoeg-Jensen for diboronates.42 In the case of the
p-methoxy group (compound 13), complex formation at
the boron atom is obviously compensated for by the +M
effect (lower b value). In other cases, the effect of the
substituent on the phenylboronic acid on the b coefficient
is within experimental error. To support the hypothesis
of complex formation at boron, 1 equiv of strongly basic
fluoride in each case was added to compounds 12-17.
Fluoride should show this effect particularly clearly,
as many boron-based fluoride sensors are based on this
complex formation.43,44 The effect of fluoride ion on the
solvatochromic behavior of compounds 12-17 was de-
termined using tetra-n-butylammonium fluoride (n-
Bu4N+F-) in thf. The pure boronic ester 13 has an
absorption maximum at λmax ) 385 nm in thf. On addition
of n-Bu4N+F-, the whole UV/vis band clearly undergoes
FIGURE 2. (A) UV/vis absorption spectra of compound 13 in
tetrahydrofuran (THF) and after addition of the strong coor-
dinative base fluoride (tetra-n-butylammonium fluoride).
In the case of compound 17 which contains no N-H
groups, the measurable effect of the basicity of the solvent
will have on ν˜max is of the same order of magnitude as in
the use of NH-acidic substituents.
The results of this work show that the functionalization
of the amino nitrogen of p-nitroaniline derivatives with
polar groups which are not in direct n-π conjugation with
the aromatic system also affects the solvatochromism of
the pull-push system in a complex fashion.
Experimental Section
General Procedure for Synthesis of Diol-Functionalized
Nitroanilines. All diolfunctionalized nitroanilines were syn-
thesized by the same experimental procedure.
The aminopropandiol (5.5 × 10-3 mol) and the corresponding
fluoronitrobenzene were heated to 130 °C with calcium carbonate
in the melt for 8 h. After being cooled to room temperature, the
mixture was washed at first with water and then with diethyl
ether. After that, 40 mL of acetone was added, whereas the nitro-
aniline derivatives were solved and it was possible to remove
the calcium fluoride from the mixture by filtration. The solvent
was evaporated, and the corresponding 1,2- and 1,3-diolfunctional-
ized nitroanilines were obtained as solids and oils, respectively.
General Procedure for Synthesis of the Boronate Es-
ters. Equimolar amounts of compound 1 and the corresponding
aromatic boronic acid were suspended in 80 mL of toluene and
heated to 100 °C. The mixture were stirred for 1 h at this
temperature, whereby a clear solution were formed. The water
formed was removed by azeotrop distillation from the reaction
mixture. After the mixture was cooled to room temperature, the
yellow precipitate formed was filtered, washed with toluene, and
dried in air to afford the corresponding ester of the boronic acid.
a bathochromic shift to λmax ) 400 nm (∆ν˜ ) 970 cm-1
)
(Figure 2). For the other compounds, the following ∆ν
values were found: 12 (970 cm-1), 14 (1230 cm-1), 15 (960
cm-1), 16 (580 cm-1), and 17 (1160 cm-1). The compounds
14 and 17 have the highest b coefficients; consequently,
they also show the strongest UV/vis shift on addition of
fluoride. An excess of fluoride does not lead to any further
shift of the UV/vis band. The pure 1,2-diol derivative 1
shows a weaker bathochromic shift on addition of fluoride
(∆ν˜ ) 323 cm-1). We explain this effect by the fact that
fluoride ions interact with the hydroxy groups similar to
HBA solvents.
These results support the interpretation that complex-
ation of the boron atoms with bases strengthens the
electron-donating effect of the amino group and thus the
push-pull character of the aromatic system. When
ordered structures, such as polar crystals, are assembled
using these building blocks, this effect can be significant.
Acknowledgment. Financial support by the Deut-
sche Forschungsgemeinschaft, Bonn, and the Fonds der
Chemischen Industry, Frankfurt am Main, is gratefully
acknowledged.
Supporting Information Available: General experimen-
tal procedures, characterization data of compounds 1-17
thermogravimetric analysis of 1 and 12-17, all UV/vis data,
Uv/vis spectra of 1 on addition of fluoride and X-ray crystal-
lographic data of 5 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(42) Hoeg-Jensen, T. QSAR Comb. Sci. 2004, 23, 344-351.
(43) Kubo, Y.; Yamamoto, M.; Ikeda, M.; Takeuchi, M.; Shinkai, S.;
Yamaguchi, S.; Tamao, K. Angew. Chem. 2003, 115, 2082-2086;
Angew. Chem., Int. Ed. 2003, 42, 2036-2040.
(44) DiCesare, N.; Lakowicz, J. R. Anal. Biochem. 2002, 301, 111-
116.
JO051097G
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