Optically active dendrimers with a binaphthyl core and phenylene dendrons: Light harvesting and enantioselective fluorescent sensing

Article abstract of DOI:10.1021/jo001565g

Optically active dendrimers containing a 1,1′-binaphthyl core and cross-conjugated phenylene dendrons were synthesized and characterized. The chiral optical properties of these phenylene-based dendrimers are different from the previously reported phenyleneethynylene-based dendrimers probably because of the increased steric interaction between the adjacent phenylene units. UV and fluorescence spectroscopic studies demonstrate that the energy harvested by the periphery of the dendrimers can be efficiently transferred to the more conjugated core, generating much enhanced fluorescence signal at higher generation. The fluorescence of these dendrimers can be quenched both efficiently and enantioselectively by chiral amino alcohols. The energy migration and light-harvesting effects of the dendrimers make the higher generation dendrimer more sensitive to fluorescent quenchers than the lower ones. Thus, the dendritic structure provides a signal amplification mechanism. These materials are potentially useful in the enantioselective recognition of chiral organic molecules.

Full text of DOI:10.1021/jo001565g

2358
J . Org. Chem. 2001, 66, 2358-2367
Op tica lly Active Den d r im er s w ith a Bin a p h th yl Cor e a n d
P h en ylen e Den d r on s: Ligh t Ha r vestin g a n d En a n tioselective
F lu or escen t Sen sin g
Liu-Zhu Gong, Qiao-Sheng Hu, and Lin Pu*
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904-4319
lp6n@virginia.edu
Received November 3, 2000
Optically active dendrimers containing a 1,1′-binaphthyl core and cross-conjugated phenylene
dendrons were synthesized and characterized. The chiral optical properties of these phenylene-
based dendrimers are different from the previously reported phenyleneethynylene-based dendrimers
probably because of the increased steric interaction between the adjacent phenylene units. UV
and fluorescence spectroscopic studies demonstrate that the energy harvested by the periphery of
the dendrimers can be efficiently transferred to the more conjugated core, generating much enhanced
fluorescence signal at higher generation. The fluorescence of these dendrimers can be quenched
both efficiently and enantioselectively by chiral amino alcohols. The energy migration and light-
harvesting effects of the dendrimers make the higher generation dendrimer more sensitive to
fluorescent quenchers than the lower ones. Thus, the dendritic structure provides a signal
amplification mechanism. These materials are potentially useful in the enantioselective recognition
of chiral organic molecules.
In tr od u ction
antenna to the porphyrin core.5a Fre´chet and co-workers
also reported the use of dye-labeled dendrimers to carry
out energy migration.7
The discovery of light-harvesting phenomena in pho-
tosynthesis has stimulated a tremendous amount of
research.1 Various types of light-harvesting materials
have been designed and synthesized to mimic the pho-
tosynthetic process. Among these studies, the use of
dendrimers has received particular attention.2-7 For
example, a phenyleneethynylene-based dendrimer (1)
prepared by Moore and co-workers shows that its den-
dritic arms can act as antenna to absorb as well as
transfer light energy to the more conjugated center,
leading to greatly enhanced fluorescence signal.2 Fox and
co-workers have studied the light harvesting properties
of phenyl benzyl ether dendrimers.3d J iang and Aida have
found that porphyrin-centered dendrimers having a
continuous array of aryl ether peripheral framework
exhibit very efficient energy transfer from the dendritic
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(c) Mcdermott, G.; Prince, S. M.; Freer, A. A.; Hawthornwaite-Lawless,
A. M.; Rapiz, M. Z.; Cogdell, R. J .; Isaacs, N. W. Nature 1995, 374,
517. (d) Ku¨hlbrandt, W. Nature 1995, 374, 497.
In our laboratory, we are interested in using the light-
harvesting and energy-migration properties of chiral
dendrimers8 to develop efficient enantioselective fluores-
cent sensors. Chiral discrimination in luminescence has
been studied in the past two decades.9-11. These studies
involve a variety of luminescent materials including
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2000, 1701-1710.
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10.1021/jo001565g CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/03/2001

Dendrimers with a Binaphthyl Core and Phenylene Dendrons
J . Org. Chem., Vol. 66, No. 7, 2001 2359
Ch a r t 1
3-
inorganic complexes,9 organic molecules,10,11 and en-
zymes.11g,h Enantioselective responses have been observed
when chiral luminophores are treated with chiral quench-
ers or enhancers. In a few cases, the relationship between
the fluorescence properties of the luminophores and the
enantiomeric purity of the substrates have been estab-
lished.9b,10g,11a,b,h For example, Richardson and co-workers
studied the circular polarized luminescence of a racemic
complex Tb(dpa)3 in the presence of the Λ and ∆
3+ 9b
enantiomers of Ru(Phen)3
.
A linear relationship be-
tween the enantiomeric composition of Ru(Phen)33+ and
the steady-state emission dissymmetry factor of Tb(dpa)33-
was observed.
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Emgl. 1983, 22, 550.
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Chem. 1996, 68, 3775. (b) Grady, T.; J oyce, T.; Smyth, M. R.; Harris,
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L. Anal. Chem. 1999, 71, 1, 1958-1962.
1
Using H NMR and CD spectroscopy, Diederich and
co-workers have demonstrated that chiral dendrimers
such as 2 are able to conduct enantioselective recognition
of monosaccharides (Chart 1).12 Recently, we have pre-
pared the chiral cross-conjugated dendrimer (S)-3 and
have shown that it can be used as an enantioselective
fluorescent sensor for the recognition of chiral amino
alcohols.13 The fluorescence intensity change of this
dendrimer in the presence of amino alcohol quenchers
(12) (a) Smith, D. K.; Diederich, F. J . Chem. Soc., Chem. Commun.
1998, 2501-2502. (b) Smith, D. K.; Zingg, A.; Diederich, F. Helv. Chim.
Acta 1999, 82, 1225-1241.
(13) (a) Pugh, V.; Hu, Q.-S.; Pu, L. Angew. Chem., Int. Ed. 2000,
39, 3638-3641. (b) Hu, Q.-S.; Pugh, V.; Sabat, M.; Pu, L. J . Org. Chem.
1999, 64, 7528.

2360 J . Org. Chem., Vol. 66, No. 7, 2001
Gong et al.
Sch em e 1. Syn th esis of P h en ylen e Den d r on s
was found to be much larger than that of the small 1,1′-
bi-2-naphthol molecule. This is because the fluorescence
quenching occurs through the interaction of the quench-
ers with the binaphthyl core where the light energy
harvested by the dendrons is transferred. Thus, the
dendrimer is a much more sensitive fluorescent sensor
than the corresponding small molecule. Efficient and
enantioselective fluorescent sensors are potentially useful
in the real-time determination of the enantiomeric com-
position of chiral organic compounds. A few binaphthyl-
based chiral dendrimers containing flexible dendrons
have also been studied by other researchers.14,15 For
example, Diederich and co-workers found that a binaph-
thyl-based chiral dendrimer was capable of molecular
recognition.15c On the basis of our work on (S)-3, we have
further prepared optically active phenylene-based den-
drimers as enantioselective fluorescent sensors. Herein,
our synthesis of these materials and their use in the
fluorescent recognition of chiral amino alcohols are
reported.
F igu r e 1. Phenylene-based chiral dendrimers.
dibromo-5-trimethylsilylbenzene (5)18 gave 5-trimethyl-
silyl-1,3-diphenylbenzene (6). Reaction of 6 with BBr3
followed by hydrolysis produced 3,5-diphenylphenyl-
boronic acid (7). The Suzuki coupling of 7 with 5 gave 8,
which was then converted to 9.
The optically active (R)-4,4′,6,6′-tetrabromo-2,2′-di-
hexyloxy-1,1′-binaphthyl [(R)-11] was synthesized in two
steps from (R)-1,1′-bi-2-naphthol [(R)-BINOL] (Scheme
2). Alkylation of (R)-BINOL gave (R)-10 in over 95%
yield, which was then brominated at room temperature
to (R)-11 in over 80% yield.13
The Suzuki coupling of the boronic acids 4, 7, and 9
with (R)-11 in refluxing THF in the presence of 3 mol %
of Pd(PPh3)4 and 2 M K2CO3 (aq) gave compounds (R)-
12-(R)-14, respectively. Because their Rf values were
very close to that of PPh3, these compounds were difficult
to purify by flash column chromatography. They were
thus passed through a short plug of silica gel without
further purification for the next step. Treatment of the
crude (R)-12-(R)-14 with BBr3 followed by hydrolysis
gave the corresponding BINOL-based generation zero
dendrimer (R)-15, the generation one dendrimer (R)-16,
and the generation two dendrimer (R)-17. These com-
pounds were purified by flash chromatography on silica
gel. After elution with ethyl acetate and hexane, (R)-15-
(R)-17 were obtained in 80%, 71%, and 58% yields,
respectively. The lower yield of (R)-17 was due to its
lower solubility in the eluent.
Resu lts a n d Discu ssion
1. Syn th esis a n d Ch a r a cter iza tion of th e Str u c-
tu r a lly Rigid Ch ir a l Den d r im er s. Miller and co-
workers have used the aryl boronic acids such as 4, 7,
and 9 to synthesize meta-phenylene-based dendrimers.16
To synthesize phenylene-based rigid chiral dendrimers,
we have also prepared the aryl boronic acids by using a
slightly modified procedure of Miller’s (Scheme 1). The
Suzuki coupling17 of phenylboronic acid (4) with 1,3-
(14) (a) Peerlings, H. W. I.; Meijer, E. W. Eur. J . Org. Chem. 1998,
573, 3-577. (b) Rosini, C.; Superchi, S.; Peerlings, H. W. I.; Meijer, E.
W. Eur. J . Org. Chem. 2000, 61-71.
(15) (a) Chen, Y.-M.; Chen, C.-F.; Xi, F. Chirality 1998, 10, 661. (b)
Yamago, S.; Furukawa, M.; Azuma, A.; Yoshida, J .-i. Tetrhedron Lett.
1998, 39, 3783. (c) Bahr, A.; Felber, B.; Schneider, K.; Diederich, F.
Helv. Chim. Acta 2000, 83, 1346-1376.
(16) Miller, T. M.; Neenan, T. X.; Zayas, R.; Bair, H. E. J . Am. Chem.
Soc. 1992, 114, 1018-1025.
(17) (a) Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981,
11, 513. (b) Suzuki, A. Acc. Chem. Res. 1982, 15, 178.
(18) Benkeser, R. A.; Hickner, R. A.; Hoke, D. I.; Thomas, O. H. J .
Am. Chem. Soc. 1958, 80, 5289-5294.

Dendrimers with a Binaphthyl Core and Phenylene Dendrons
J . Org. Chem., Vol. 66, No. 7, 2001 2361
Ta ble 1. P h ysica l P r op er ties of th e Den d r im er s (R)-15-(R)-17
dendrimer
(R)-15 (G0)
(R)-16 (G1)
(R)-17 (G2)
mass spectral data
GPC Mn (PDI) (THF, polystyrene standard)
CI, 591 (M + H)+
570 (PDI ) 1.03)
EI, 1199 (M + H)+
1100 (PDI ) 1.03)
FAB, 2417 (M + H)+
2200 (PDI ) 1.03)
Ta ble 2. Op tica l Rota tion Da ta of th e Ch ir a l Den d r im er s
dendrimer
(R)-15 (G0)
590.7
-391
-66.3 (c ) 1.03)
(R)-16 (G1)
1199.5
-704
-51.6 (c ) 1.04)
(R)-17 (G2)
molecular weight
molar optical rotation (M)
specific optical rotation [R]D (THF)
2417.1
-754
-31.2 (c ) 1.04)
Ta ble 3. CD Sp ectr a l Da ta of th e Den d r im er s (R)-15-(R)-17
dendrimer
(R)-15 (G0)
(R)-16 (G1)
(R)-17 (G2)
CD [Θ] (λ, nm)
(CH2Cl2)
6.37 × 10-5 (254)
-4.75 × 10-5 (282)
2.72 × 10-5 (244)
-6.79 × 10-5 (272)
-3.64 × 10-5 (287)
Sch em e 2. Syn th esis of Op tica lly Active (R)-11
drimers to the higher generations increases the dihedral
angle of the 1,1′-binaphthyl core, causing significantly
increased molar optical rotation. However, we found that
in the case of the phenyleneethynylene-based dendrimers
represented by (S)-3 there was only a 30% increase in
molar optical rotation from generation zero to generation
two. This indicates that substitution at the 4,4′,6,6′-
positions of the binaphthyl core as in (S)-3 cannot
significantly change the 1,1′-binaphthyl dihedral angle
at least from generation zero to generation two.
Therefore, we propose that the observed large increase
in molar optical rotation from (R)-15 to (R)-17 might also
not be due to the change of the binaphthyl dihedral angle.
Instead, it might be contributed by the chiral conforma-
tions of the biaryl units in the dendrons induced by the
chiral binaphthyl core. The steric interaction between the
adjacent arylene units in the dendrons of (R)-15-(R)-17
is much larger than that in (S)-3. This might have
allowed the chiral effects of the binaphthyl core in (R)-
15-(R)-17 to propagate better along the dendritic
branches, producing the significantly increased molar
optical rotation.
Dendrimers (R)-15-(R)-17 are pale yellow solids. They
are soluble in common polar organic solvents such as
acetone, ethyl acetate, methylene chloride, chloroform,
and THF but are insoluble in nonpolar solvents such as
hexane and toluene. In the 1H NMR spectra of these
dendrimers, a signal at δ 8.18 (d, J ) 1.8 Hz), 8.49 (s),
or 8.59 (s) is attributed to the 5,5′-protons of (R)-15, (R)-
16, or (R)-17. This indicates a C2 symmetric structure of
these compounds in solution. The expected molecular ions
of (R)-15-(R)-17 were observed in their CI, EI, and FAB
mass spectra, which support their structures. Table 1
summarizes the mass spectral results and gel permeation
chromatography (GPC) data of these dendrimers. The
GPC results are consistent with the monodisperse nature
of these materials.
2. Op t ica l Sp ect r oscop ic St u d ies of t h e Ch ir a l
Den d r im er s. The optical rotation of the dendrimers was
studied. As shown in Table 2, the molar optical rotation
increases about two times as the molecular weight
increases about four times from the generation zero
dendrimer to the generation two dendrimer. Earlier,
Chen15a and Meijer14 have studied the effect of the
dendritic branches at the 2,2′-positions of the 1,1′-
binaphthyls on the optical rotation of the chiral binaph-
thyl-core-based dendrimers. They found that the growing
steric interaction between the dendritic branches at the
2,2′-positions from the lower generation binaphthyl den-
The circular dichroism (CD) spectra of the dendrimers
(R)-15-(R)-17 in methylene chloride solution were ob-
tained at 2.0 × 10-6 M (Figure 2). The molar ellipticities
([θ]) of the major CD signals are listed in Table 3. From
the generation zero dendrimer to the generation two
dendrimer, the major negative CD signal has shifted 15
nm to the red and the intensity is decreased to 54% of
the original value. This is also very different from the
phenyleneethynylene-based dendrimers, which show no
significant change among different generations. Both the
CD spectra and the optical rotations of (R)-15-(R)-17
suggest that the dendritic arms of these materials might
have made certain contributions to the chiral optical
effects probably due to the increased steric interaction
between the adjacent arylene units in the dendrons. The
observed red-shift in the CD signals indicates that there
is a small increase in conjugation from the lower genera-
tion dendrimers to the higher ones.
Figure 3 shows the UV-vis absorption spectra of (R)-
15-(R)-17 in CH2Cl2 at 2 × 10-6 M. The absorption
maximum and molar extinction coefficients of these
dendrimers as well as their dendron precursors 6 and 8
are summarized in Table 4. There are two major absorp-
tion bands in the UV spectra of the dendrimers. One

2362 J . Org. Chem., Vol. 66, No. 7, 2001
Gong et al.
F igu r e 2. CD spectra of the dendrimers (R)-15-(R)-17.
F igu r e 3. UV-vis absorption spectra of the dendrimers (R)-15-(R)-17 in methylene chloride (2.0 × 10-6 M).
strong band at 250-300 nm is due to both the phenylene
dendrons and the binaphthyl moieties. The absorption
in this area increases significantly as the dendritic
generation grows because of the increased phenylene
units. The molar extinction coefficient of the dendrimers
at 256 nm is increased over 5 times from the generation
zero (R)-15 to the generation two (R)-17 when the number
of phenylene units is increased 4 times. The second
absorption band at 300-370 nm is due to the conjugated
4,4′,6,6′-tetraphenyl-1,1′-bi-2-naphthol core. Its intensity
does not show significant change as the dendrimer
generation increases.
The dendrimers emit strong blue light under UV
irradiation. Figure 4 shows the fluorescence spectra of
the dendrimers in CH2Cl2 solution at 2.0 × 10-7 M. When
the dendrimer samples were excited at 256 nm where
the absorption is mostly due to the benzene units of the
dendrons, the emission wavelengths of these rigid den-
drimers are almost identical with a maximum at 395 nm
for (R)-15, 402 nm for (R)-16, and 405 nm for (R)-17,
respectively. At the same molar concentration, the gen-
eration two dendrimer (R)-17 emits about 2.5 times more
intense than the generation one (R)-16 which emits about
three times more intense than the generation zero (R)-
15. Thus, from the generation zero dendrimer to the
generation two, the fluorescence intensity has increased
about 7.5 times.
The fluorescence spectra of dendrons 6 and 8 are
obtained (Figure 5). Comparison of these spectra with
those of the dendrimers shows that the emissions at 340-
350 nm for the monodendrons have disappeared in the
dendrimers. Thus, the energy absorbed by the dendrons
has completely migrated to the more conjugated core,
leading to the greatly enhanced fluorescence intensity as
the dendrimer generation grows.
The excitation spectra of the dendrimers also support

Dendrimers with a Binaphthyl Core and Phenylene Dendrons
J . Org. Chem., Vol. 66, No. 7, 2001 2363
F igu r e 4. Fluorescence spectra of dendrimers (R)-15-(R)-17 in methylene chloride (2.0 × 10-7 M) (uncorrected).
F igu r e 5. Fluorescence spectra of the generation one dendron 6 and the generation two dendron 8 (uncorrected).
Ta ble 4. UV Absor p tion Ma xim u m s a n d Extin ction Coefficien ts of th e Den d r im er s a n d Den d r on s
dendrimer
dendron
(R)-15 (G0)
(R)-16 (G1)
(R)-17 (G2)
6
8
λmax (nm) (ꢀ × 10-4
)
351 (1.0)
310 (2.5)
270 (8.8)
256 (8.4)
351 (1.0)
310 (2.7)
256 (16.6)
356 (2.0)
310 (5.0)
256 (44.3)
252 (30.2)
256 (10.8)
the observed efficient energy migration (Figure 6). When
the emission was set at 405 nm, the excitation maximum
of the dendrimers at 256 nm increases dramatically from
generation zero to generations one and two. The fluores-
cence intensity of (R)-17 is also much stronger than that
of (R)-BINOL at the same concentration of 2.0 × 10-7
M. To achieve a similar fluorescence intensity, the
concentration of (R)-BINOL needs to be over 100 times
higher than that of the generation two dendrimer.
Therefore, the dendrimer is much more suitable as a
fluorescent sensor and only a very small amount of the
material is needed for the measurement.
3. E n a n t ioselect ive F lu or escen t R ecogn it ion of
Ch ir a l Am in o Alcoh ols. We have studied the fluores-
cence of the generation two dendrimer (R)-17 in the
presence of chiral amino alcohols such as 2-amino-1-
propanol (18).10,13 In methylene chloride solution, the
fluorescence of (R)-17 was not significantly quenched by
18. However, with the addition of hexane as a cosolvent,
the fluorescence quenching became much more efficient.

2364 J . Org. Chem., Vol. 66, No. 7, 2001
Gong et al.
-7
F igu r e 6. Excitation spectra of the dendrimers (R)-15-(R)-17 in methylene chloride (2.0 × 10
M).
F igu r e 7. Stern-Volmer plot of (R)-17 in the presence of (R)- and (S)-18.
This is probably due to better hydrogen bonding interac-
tion in a less polar solvent between the hydroxyl groups
of the dendrimer and the amino alcohol. The use of the
mixed solvent of hexane and methylene chloride (9:1 by
volume) allows the fluorescence of the dendrimer to be
quenched by the amino alcohols in the desired range of
20-80% for minimum operational errors. Both the den-
drimer and the amino alcohols are not soluble in pure
hexane.
where I0 is the fluorescence intensity in the absence of a
quencher and I the fluorescence intensity in the presence
of a quencher. [Q] is the quencher concentration. Ksv is
the Stern-Volmer constant, which measures the ef-
ficiency of quenching.
Figure 7 is the Stern-Volmer plot of the dendrimer in
the presence of (R)- and (S)-18. From these plots, it is
found that KsvR ) 243.5 M-1 and KsvS ) 216.0 M-1. Thus,
the fluorescence quenching of (R)-17 by the amino alcohol
R
S
S
is enantioselective with Ksv - Ksv ) 27.5 and KsvR/Ksv
We found that the fluorescence quenching of dendrimer
(R)-17 at 1.0 × 10-7 M by the optically pure (R)- and (S)-
18 in the concentration range studied obeys the Stern-
Volmer equation:20
) 1.13. The R enantiomer of the amino alcohol quenches
(19) For a recent study of the conformation of chiral dendrimers,
see: Recker, J .; Tomcik, D. J .; Parquette, J . R. J . Am. Chem. Soc. 2000,
122, 10298-10307.
(20) Lakowicz, J . R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer Academic/Plenum: New York, 1999.
I0/I ) 1 + Ksv[Q]

Dendrimers with a Binaphthyl Core and Phenylene Dendrons
J . Org. Chem., Vol. 66, No. 7, 2001 2365
Ta ble 5. Ster n -Volm er Con sta n ts of th e Den d r im er
(R)-17 in th e P r esen ce of th e Ch ir a l Am in o Alcoh ols
18-20
amino alcohol
(R)-18 (S)-18 (R)-19 (S)-19 (R)-20 (S)-20
Ksv (M-1
)
243.5
216.0 191.5
27.5
1.13
162.5 145.5
29.0
1.18
133.5
12.0
1.09
R
S
Ksv - Ksv
S
KsvR/Ksv
dendrimers than for the lower ones. The quenching of
the dendrimer fluorescence probably occurs when the
dendritic core hydroxyl groups interact with the amino
alcohol quencher. This allows the light harvested by the
dendrons to be quenched at the core where the energy is
funneled. Therefore, the higher the dendrimer genera-
tion, the more the quenched emission.
The fluorescence of (R)-17 (1.0 × 10-7 M) in the
presence of leucinol (19) (Figure 10) and 2-amino-3-
methyl-butanol (20) (Figure 11) were also studied under
the same conditions as above. The Stern-Volmer con-
stants of (R)-17 in the presence of the chiral amino
alcohols 18-20 are summarized in Table 5. These studies
F igu r e 8. Stern-Volmer plot of (R)-15 in the presence of (R)-
and (S)-18.
demonstrate that the fluorescence response of the den-
drimer to chiral amino alcohols is enantioselective. The
enantioselectivity of dendrimer (R)-17 is higher for the
amino alcohol 19 than for 18 and 20. The magnitude of
S
the KsvR/Ksv ratio of dendrimer (R)-17 for chiral amino
alcohols is similar to those of (S)-313a and BINOL,10e,13a
but smaller than that of a calix[4]arene-based fluorescent
sensor.11b The fluorescence of these dendrimers can be
quenched both efficiently and enantioselectively by chiral
amino alcohols. In all the cases, the (S)-amino alcohols
quench the fluorescence of the chiral dendendrimers (R)-
15 - (R)-17 more efficiently than the (R)-amino alcohols.
This indicates that the interaction of the (R)-dendrimers
with the (S)-amino alcohols is better than with the (R)-
amino alcohols.
F igu r e 9. Stern-Volmer plot of (R)-16 in the presence of (R)-
and (S)-18.
the fluorescence of the dendrimer more efficiently than
the S enantiomer.
Under the same conditions, the fluorescence quenching
of the generation zero dendrimer (R)-15 and the genera-
tion one dendrimer (R)-16 by (R)- and (S)-18 also followed
the Stern-Volmer equation (Figures 8 and 9). The
4. Con clu sion
In summary, optically active dendrimers containing a
1,1′-binaphthyl core and cross-conjugated phenylene den-
drons have been synthesized and characterized. The
chiral optical properties of these phenylene-based den-
drimers are different from our previously reported phen-
yleneethynylene-based dendrimers probably because of
the increased steric interaction between the adjacent
arylene units in the phenylene dendrons. These new
dendrimers show significantly increased molar optical
rotation as the dendrimer generation increases. UV and
fluorescence spectroscopic studies demonstrate that the
energy harvested by the periphery of the dendrimers is
efficiently transferred to the more conjugated core,
generating much enhanced fluorescence signal at higher
generation. The energy migration and light harvesting
observed Stern-Volmer constants are Ksv ) 75.5 M-1
R
and KsvS ) 63.5 M-1 for (R)-15 (KsvR/KsvS ) 1.19), and Ksv
R
S
S
) 99.9 M-1 and Ksv ) 88.4 M-1 for (R)-16 (KsvR/Ksv
)
1.13).
Thus, there is a significant increase in the Stern-
Volmer constant as the dendrimer generation increases.
This demonstrates that the fluorescence of the higher
generation dendrimer is more sensitive toward quenchers
than the lower ones. The increased nonpolar dendrons
for the higher generation dendrimers may enhance the
hydrogen bonding between their core hydroxyl groups
and the amino alcohols. The difference between the
fluorescence intensity of the sensor without and with the
quencher is also much larger for the higher generation

2366 J . Org. Chem., Vol. 66, No. 7, 2001
Gong et al.
F igu r e 10. Stern-Volmer plot of (R)-17 in the presence of (R)- and (S)-19.
using Perkin-Elmer 2400 Series II CHN S/O Analyzer. Gel
permeation chromatography (GPC) analysis utilized Waters
510 HPLC pump, Waters 410 Differential Refractometer and
Ultrastyragel Linear GPC columns. THF was used as the
eluent solvent for GPC and polystyrene standards were used.
UV-vis spectra were recorded on a Hewlett-Packard 8452A
diode array spectrophotometer. Fluorescence spectra were
recorded on a Perkin-Elmer LS-50B luminescence spectrom-
eter. Both the excitation and emission slits were set at 2.5 nm,
and the scan speed was set at 100 nm/min. Circular dichroism
spectra were recorded using a J ASCO J -710 spectropolarim-
eter. Optical rotations were measured on a J ASCO polarimeter
at λ ) 589 nm. CI and EI mass spectra were recorded on LCQ
FINNIGON. FAB mass spectroscopic analysis was performed
by the University of CaliforniasRiverside mass spectroscopy
facility. THF was dried with sodium/benzophenone. CH2Cl2
was dried with CaH2. Tetrakis(triphenylphosphine)palladium
was purchased from Strem and used directly. Other chemicals
were purchased from Aldrich and used directly.
P r ep a r a tion a n d Ch a r a cter iza tion of 3,5-Dibr om o-1-
(tr im eth ylsilyl)ben zen e, 5.16,18 Under nitrogen, to a solution
of 1,3,5-tribromobenzene (9.4 g, 30 mmol) in THF (80 mL) was
added n-BuLi (2.5 M in hexane, 12 mL, 30 mmol) at -78 °C.
The resulting mixture was stirred at this temperature for 30
min, to which trimethylsilyl chloride (7.6 mL, 60 mmol) was
added via a syringe. After another 30 min, the reaction mixture
was warmed to room temperature and stirred overnight. To
the resulting solution, saturated aqueous NH4Cl was added
to quench the excess n-BuLi. The aqueous layer was extracted
with EtOAc, and the combined organic layer was washed with
brine. After evaporation of the solvent, the residue was
distilled under reduced pressure to give 5 as needle crystals
in 79% yield (7.2 g, 23.7 mmol): mp 40-41 °C; 1H NMR
(CDCl3, 300 MHz) δ 7.64 (d, J ) 1.8 Hz, 1H), 7.51 (d, J ) 1.8
Hz, 2H), 0.27 (s, 9H); 13C NMR (CDCl3, 75 MHz) δ 146.01,
134.45, 123.16, -1.37.
P r ep a r a tion of 5-Tr im eth ylsilyl-1,3-d ip h en ylben zen e,
6. A 100 mL Schlenk flask charged with 3,5-dibromo-5-
(trimethylsilyl)benzene (5) (6.16 g, 20 mmol) and phenylboronic
acid (4) (5.36 g, 44 mmol) was brought into a drybox and was
combined with Pd(PPh3)4 (1.39 g, 1.20 mmol). To the resulting
mixture, aqueous K2CO3 (2 M, 25 mL, degassed with nitrogen)
and THF (50 mL) were syringed in under nitrogen. The
reaction mixture was degassed again by three freeze-pump-
thaw cycles and was then refluxed under nitrogen. After 24
F igu r e 11. Stern-Volmer plot of (R)-17 in the presence of
(R)- and (S)-20.
effects of the dendrimers make the higher generation
dendrimer more sensitive to fluorescent quenchers than
the lower ones. Thus, the dendritic structure provides a
signal amplification mechanism.21 These materials are
potentially useful in the enantioselective recognition of
chiral organic molecules.
Exp er im en ta l Section
Gen er a l Da ta . NMR spectra were recorded on a Varian-
300 MHz spectrometer. Elemental analysis was carried out
(21) Using energy migration as a fluorescence signal amplification
mechanism in linear conjugated polymers was studied by Swager:
Zhou, Q.; Swager, T. M. J . Am. Chem. Soc. 1995, 117, 7017.

Dendrimers with a Binaphthyl Core and Phenylene Dendrons
J . Org. Chem., Vol. 66, No. 7, 2001 2367
h, the reaction mixture was poured into a mixture of H2O and
EtOAc. The aqueous layer was extracted with EtOAc. The
combined organic layer was washed with brine and dried over
anhydrous Na2SO4. After removal of the solvent, the residue
was passed through a silica gel column using hexanes as the
eluent, and the resulting material was further recrystallized
from EtOH to yield 6 as colorless crystals in 80% yield (4.8 g,
16 mmol). The 1H and 13C NMR data of 6 matched those in
the literature.16
g, 4.4 mmol), 7 (1.205 g, 4.4 mmol), or 9 (2.543 g, 4.4 mmol),
and Pd(PPh3)4 (138 mg, 0.12 mmol) were added degassed THF
(20 mL) and aqueous K2CO3 (2 M, 12 mL). After the mixture
was degassed with three freeze-pump-thaw cycles, it was
refluxed for 48 h and then cooled to room temperature. The
resulting mixture was poured into a mixture of EtOAc and
H2O. The aqueous layer was extracted with EtOAc. The
combined organic solution was washed with brine and dried
over anhydrous Na2SO4. After concentration, the residue was
passed through a silica gel column to remove the inorganic
impurity. The crude mixture was dissolved in CH2Cl2 (20 mL),
and BBr3 (0.28 mL, 3 mmol) was added dropwise under
nitrogen at -78 °C. The reaction solution was then warmed
to room temperature for a period of 20 h and was quenched
with H2O. The aqueous layer was extracted with CH2Cl2. The
combined organic solution was washed with brine and dried
over anhydrous Na2SO4. After removal of the solvent, the
residue was purified with a silica gel column (eluent: EtOAc/
hexane ) 1:10 to 1:4) to give the dendrimers as pale solids.
P r ep a r a tion of 5-Tr im eth ylsilyl-1,3-bis(1′,3′-d ip h en yl-
p h en yl)ben zen e, 8. The same procedure as the preparation
of 6 was followed. The synthesis of 8 was carried out by using
5 (1.57 g, 5.1 mmol), 7 (3.1 g, 11.0 mmol), Pd(PPh3)4 (381 mg,
0.33 mmol) in THF (25 mL), and aqueous K2CO3 (2 M, 16 mL).
The reaction mixture was heated at reflux under nitrogen for
48 h. The crude product was recrystallized from EtOH to give
1
8 as white crystals in 90% yield (2.8 g, 4.6 mmol). The H and
13C NMR data of 8 matched those in the literature.16
P r ep a r a tion a n d Ch a r a cter iza tion of (R)-2,2′-Hexyl-
oxy-1,1′-bin a p h th yl, (R)-10. To a solution of (R)-BINOL (6.0
g, 21.0 mmol) and 1-bromohexane (12 mL, 105.0 mmol) in
acetonitrile (100 mL) was added K2CO3 (15 g, 105 mmol). The
resulting mixture was degassed with nitrogen and then
refluxed for 16 h. After the mixture was cooled to room
temperature, H2O was added and the mixture was extracted
with hexanes. The organic layer was washed with brine.
Evaporation of the solvent followed by flash chromatography
on silica gel with hexane as eluent afforded (R)-10 as a
colorless oil in 95% yield (9.1 g, 20 mmol): [R]D ) +60.5 (c )
(R)-15: yield 80%; mp 164-167 °C dec; [R]D ) -66.3 (c )
1.03, THF); 1H NMR (CDCl3, 300 MHz) δ 8.18 (d, J ) 1.8 Hz,
2H), 7.32-7.68 (m, 26H), 5.21 (s, 2H); 13C NMR (CDCl3, 75
MHz) δ 152.28, 144.22, 141.03, 139.82, 136.97, 133.16, 129.93,
128.80, 128.54, 128.24, 127.85, 127.26, 127.17, 127.10, 125.25,
124.81, 119.17, 110.30; MS (CI) m/e 591 (M + H+, 100); UV
λmax (CH2Cl2) nm 351, 310, 270, 256. Anal. Calcd for C44
30O2: C, 89.40; H, 5.12. Found: C, 89.27; H, 4.96.
(R)-16: yield 71%; mp 207-210 °C dec; [R]D ) -51.6 (c )
-
H
1.04, THF); 1H NMR (CDCl3, 300 MHz) δ 8.49 (s, 2H), 7.97 (d,
J ) 1.2 Hz, 6H), 7.76-7.78 (m, 16H), 7.57-7.64 (m, 12H),
7.34-7.51(m, 24H), 5.35 (s, 2H); 13C NMR (CDCl3, 75 Hz) δ
152.94, 144.58, 142.79, 142.71, 142.58, 141.40, 141.20, 137.77,
133.86, 129.36, 129.29, 128.72, 128.14, 128.07, 127.92, 127.78,
127.68, 126.21, 125.90, 125.75, 125.66, 119.85, 111.02; MS (EI)
m/e 1199 (M + H+); UV λmax (CH2Cl2) nm 351, 310, 256. Anal.
Calcd for C92H62O2: C, 92.12; H, 5.21. Found: C, 92.07; H,
5.58.
1
1.0, THF); H NMR (CDCl3, 300 MHz) δ 7.92 (d, J ) 9.0 Hz,
2H), 7.84 (d, J ) 8.1 Hz, 2H), 7.41 (d, J ) 9.0 Hz, 2H), 7.33-
7.17 (m, 6 H), 3.98-3.89 (m, 4H), 1.44-1.37 (m, 4H), 1.07-
0.91 (m, 12H), 0.77-0.72 (m, 6H); 13C NMR (CDCl3, 75 MHz)
δ 154.54, 134.23, 129.27, 128.97, 127.72, 125.95, 125.50,
123.34, 120.76, 115.89, 69.79, 31.30, 29.35, 25.25, 22.41, 13.88.
P r ep a r a tion a n d Ch a r a cter iza tion of (R)-4,4′,6,6′-Tet-
r a br om o-2,2′-h exyloxy-1,1′-bin a p h th yl, (R)-11. To a solu-
tion of (R)-10 (4.5 g, 10 mmol) in AcOH (100 mL) was added
Br2 (5.2 mL, 100 mmol) over 30 min at room temperature. The
reaction mixture was stirred for 6 h and monitored by 1H NMR
spectroscopy. Then, NaHSO3 was added to quench excess Br2.
After being extracted with EtOAc, the combined organic
solution was washed with brine. After removal of the solvent,
the residue was purified by column chromatography on silica
gel with hexanes as the eluent to give (R)-11 in 80% yield (6.1
g, 8 mmol) as a slightly yellow oil: [R]D ) + 50.0 (c ) 0.808,
(R)-17: yield 58%; mp 231-234 °C dec; [R]D ) -31.2 (c )
1.03, THF); 1H NMR (CDCl3, 300 MHz) δ 8.59 (s, 2H), 8.07 (s,
6H), 7.97-7.76 (m, 32H), 7.67-7.60 (m, 36H), 7.30-7.44 (m,
48H), 5.36 (s, 2H); 13C NMR (CDCl3, 75 MHz) δ 152.58, 144.06,
142.67, 142.50, 142.25, 141.78, 140.91, 140.84, 136.85, 133.56,
128.78, 128.38, 128.20, 127.50, 127.30, 125.91, 125.58, 125.34,
119.45, 110.64; MS (FAB) m/e 2417 (M + H+); UV λmax (CH2-
Cl2) nm 356, 310, 256. Anal. Calcd for C188H126O2: C, 93.33;
H, 5.25. Found: C, 93.21; H, 5.37.
1
THF); H NMR (CDCl3, 300 MHz) δ 8.39 (d, J ) 2.1 Hz, 2H),
7.70 (s, 2H), 7.30 (dd, J ) 2.1 Hz, 9.0 Hz, 2H), 6.97 (d, J ) 9.0
Hz, 2H), 3.89-3.96 (m, 4H), 1.38-1.43 (m, 4H), 0.72-1.08 (m,
18H); 13C NMR (CDCl3, 75 MHz) δ 154.56, 133.26, 130.72,
129.55, 129.02, 127.49, 122.55, 120.52, 119.40, 119.36, 69.99,
31.37, 29.26, 25.47, 22.63, 14.04. Anal. Calcd for C32H34Br4O2:
C, 49.90; H, 4.45. Found: C, 50.07; H, 4.39.
P r ep a r a tion a n d Ch a r a cter iza tion of th e Ch ir a l Den -
d r im er s. A typical procedure: Under nitrogen, to a mixture
of (R)-11 (0.764 g, 1 mmol), the boronic acid dendron 4 (0.536
Ack n ow led gm en t. The support of this work from
the National Institutes of Health (1R01GM58454) is
gratefully acknowledged. We also acknowledge the
partial support from the donors of the Petroleum
Research Fund, administered by the American Chemi-
cal Society.
J O001565G