A water-soluble non-aggregating fluorescent octa-carboxylic acid derived from tetraphenylmethane: Synthesis and optical properties

Article abstract of DOI:10.1016/j.tetlet.2004.06.019

The synthesis and optical properties of an octa-substituted dendritic fluorescent compound having tetraphenylmethane as the core and bithiophene as the chromophoric arms, and its water-soluble octa-carboxylic acid derivative are reported.

Full text of DOI:10.1016/j.tetlet.2004.06.019

Tetrahedron
Letters
Tetrahedron Letters45 (2004) 6173–6177
A water-soluble non-aggregating fluorescent octa-carboxylic
acid derived from tetraphenylmethane: synthesis
and optical propertiesq
Xue-Ming Liu, Chaobin He* and Junchao Huang
Institute of Materials Research and Engineering, National University of Singapore, 3 Research Link, Singapore 117602, Singapore
Received 7 April 2004; revised 22 May 2004; accepted 3 June 2004
Available online 2 July 2004
Abstract—The synthesis and optical properties of an octa-substituted dendritic fluorescent compound having tetraphenylmethane as
the core and bithiophene as the chromophoric arms, and its water-soluble octa-carboxylic acid derivative are reported.
ꢀ 2004 Elsevier Ltd. All rights reserved.
During the last decade, branched and dendritic organic
materials based on several tetrahedral core compounds
such as tetraphenylmethane,1 tetraphenylsilane,2 and
adamantane3 have attracted increasing interest in
chemistry and materials science. They have shown many
unusual properties including optical, electronic and
thermal properties, and have been investigated as light-
emitting materials,1–4 electronically active materials,5
and molecular caltrops in scanning probe microscopy,6
etc. Such materials also represent an important class of
diagnostic and sensoring agents.7 It hasbeen well estab-
lished that good water solubility, minimal self-aggrega-
tion in aqueousmedia, and preferably, multiple
chromophoresare important propertiesfor a fluorescent
agent to be used for biomedical applications.8 Assuch,
water-soluble dendritic fluorescent materials based on
the above-mentioned coresare interesting candidatesfor
these applications. However, such dendritic materials
have not been used for clinical applications, mainly due
to the difficulties in their syntheses.9 We have previously
reported a novel class of water-soluble fluorescent
polymerscomprising hydrophobic tetraphenylmethane–
oligothiophene chromophoric coresand hydrophilic
poly(ethyleneglycol) arms.10 We found that the self-
aggregation of the polymersin aqueousmedia was
minimized efficiently due to their branched structures.10
In continuation of our studies on water-soluble non-
aggregating multi-chromophoric fluorescent probes, we
herein report the synthesis and optical properties of an
eight-armed tetrahedral fluorescent compound and its
water-soluble octa-carboxylic acid derivative. These
compoundshave dendrimer-like highly hindered
structures yet were much more easily synthesized. The
octa-carboxylic acid exhibited absorption and PL pH-
sensitivity and did not show obvious aggregation in
aqueousmedia.
The synthesis of the eight-armed tetraphenylmethane–
bithiophene adduct 2 and itsocta-carboxylic acid
ꢀ
derivative 3 isshown in Scheme 1.
Nitration of tetra-
phenylmethane with fuming nitric acid and subsequent
Pd/C-catalyzed hydrazine reduction gave C(p-
C6H4NH2)4.11 Bromination of C(p-C6H4NH2)4 with
neat Br2 gave the octa-brominated 1 in 75% yield.5a The
1H NMR spectrum of 1 in DMF-d7 exhibited a broad
peak at d 5.60 (NH2) and a singlet at d 7.85 (Ar-H).
Compound 1 underwent Suzuki coupling with 2,20-bi-
thiophene-5-boronic acid to give 2 in 20% yield. Al-
though 2 hasa highly hindered tsructure, unlike other
similar tetrahedral molecular glass materials,2 it only
Keywords: Tetrahedral material; Tetraphenylmethane; Oligothioph-
enes; Water-soluble fluorescent probe; Non-aggregating luminescent
material; pH-indicator.
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2004.06.019
1
exhibited a Tm at 210 ꢁC and a Tc at 185 ꢁC. The H
NMR spectrum of 2 in CDCl3 showed one doublet of
3
doublets(dd) igsnal at
d 7.02 (dd, 8H, JH–H
¼
3
0
JH–H ¼ 3:6 Hz), which istypical for the b-H of bithienyl
moieties. In addition, three well-separated doublets at d
7.06, 7.18 and 7.22 (bithienyl-H) and one singlet at d
* Corresponding author. Tel.: +65-68748145; fax: +65-68727528;
e-mail: cb-he@imre.a-star.edu.sg
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.06.019

6174
X.-M. Liu et al. / Tetrahedron Letters 45 (2004) 6173–6177
NH2
NH2
Br
Br
Br
Br
Br
Br
NH2
1 fuming HNO3, 0 oC
Br2
C
H2N
C
C
H2N
NH2
2 NH2NH2, Pd/C,
THF, refluxing
0 oC
Br
Br
NH2
NH2
1
[Pd(PPh3)4]
toluene/2M K2CO3
3:2 (V/V), refluxing
S
S
B(OH)2
S
CO2H
l
HO2C
S
S
C
3
S
H
CO2H
S
NH2
S
N
HO2C
S
S
S
S
S
S
S
S
1 n-BuLi (8.8 eq.)
-20 oC, THF
S
S
ClH3N
S
C
NH3Cl
H2N
C
NH2
S
2 CO2
3 H3O+
S
S
S
S
S
S
S
S
S
HO2C
CO2H
CO2H
S
S
NH2
S
S
HO2C
S
2
3
Scheme 1. Synthesis of the eight-armed fluorescent compounds 2 and 3.
7.08 (Ar-H) were observed (Fig. 1). The 13C NMR
spectrum of 2 showed 12 aromatic-C atoms. The
MALDI-TOF MS of 2 showed a molecular ion peak at
1695.1 (Mþ). These data prove unambiguously the
complete substitution of the bromo groups in 1 with
bithienyl moieties. Finally, deprotonation and sub-
sequent reaction of 2 with CO2 gave 3, quantitatively, as
a yellow solid. Compound 3 wassoluble in both organic
os lvents(THF and DMF) and dilute aqueousalkaline
solutions. The full substitution of the acidic a-H atoms
in the bithienyl moietiesof 2 by eight carboxyl groups
wasproved by 1H NMR and MALDI-TOF MS spec-
Figure 1. The 1H NMR spectrum of 2 in CDCl3.
1
troscopic studies. For example, the H NMR spectrum
of 3 in DMF-d7 exhibited three well resolved doublets at
d 7.45, 7.57 and 7.77, and one singlet at d 7.50, due to
protonsfrom the bithienyl moietiesand the aryl groups,
respectively (Fig. 2). The characteristic dd signal of the b
protonsof the bithienyl groupswasnot observed, indi-
cating complete carboxylation of the a-C positions of
the bithienyl groups. The MS spectrum of 3 showed a
molecular ion peak at 2052.4 (Mþ+6). The IR spectra of
1–3 exhibited strong absorptions at 3431–3439 (major)
and 3350–3358 cmꢀ1 (shoulders), due to the N–H
stretching of primary amino groups. In addition, the IR
spectrum of 3 also exhibited strong absorptions at 1657
and 1104 cmꢀ1, due to C@O and O–H stretches of the
carboxylic acid groups.
ꢀ
Physical data for compounds 1–3. Tetra(4-amino-3,5-dibromo)phen-
ylmethane (1). Mp 310 ꢁC. Anal. Calcd for C25H16Br8N4: C, 29.7; H,
1.6; N, 5.5. Found: C, 30.1; H, 1.4; N, 5.2. 1H NMR (DMF-d7): d
5.60 (br, 8H, suppressed by D2O, NH2), 7.85 (s, 8H, Ph-H). IR (KBr,
cmꢀ1): 3439, 3358, 1609, 1469, 738. MS (EI), m/z: 1011.7 (Mþ, 22%).
Tetra(4-amino-3,5-di(2,20-bithiophene-5-yl))phenylmethane (2). Mp
210 ꢁC. Anal. Calcd for C89H56N4S16: C, 63.1; H, 3.3; N, 3.3. Found:
C, 62.8; H, 3.2; N, 3.0. 1H NMR (CDCl3): d 4.61 (br, 8H, suppressed
3
3
0
by D2O, NH2), 7.02 (dd, 8H, JH–H ¼ JH–H ¼ 3:6 Hz, Hb of
3
bithienyl), 7.06 (d, 8H, JH–H ¼ 3:2 Hz, bithienyl-H), 7.08 (s, 8H,
3
Ph-H), 7.08–7.09 (m, 8H, bithienyl-H), 7.18 (d, 8H, JH–H ¼ 3:6 Hz,
bithienyl-H), 7.22 (d, 8H, JH–H ¼ 5:2 Hz, bithienyl-H). 13C NMR
3
(CDCl3): d 64.4, 123.7, 124.2, 124.6, 124.8, 125.0, 128.3, 129.0, 133.8,
136.3, 136.7, 137.5, 141.4. IR (KBr, cmꢀ1): 3439, 3351, 1609, 1465,
791, 699. MS (MALDI-TOF, matrix: 4-hydroxyazobenzene-2-car-
boxylic acid), m/z: 1695.1 (Mþ, 10%). Tetra[4-amino-3,5-di(50-
carboxyl-2,20-bithiophene-5-yl)]phenylmethane (3). Mp >300 ꢁC
(dec). Anal. Calcd for C97H56N4O16S16Æ4HCl: C, 52.8; H, 2.7; N,
2.6. Found: C, 52.4; H, 2.8; N, 2.4. 1H NMR (DMF-d7): d 7.45 (d,
3JH–H ¼ 3:6 Hz, 8H, bithienyl-H), 7.50 (s, 8H, Ph-H), 7.48–7.52 (m,
Figure 3 shows the UV and PL spectra of 2 in THF.
The UV spectrum of 2 exhibitstwo bandsat 248 nm ( e
6:8 ꢁ 104 Mꢀ1 cmꢀ1) and 396 nm (e 9:0 ꢁ 104 Mꢀ1 cmꢀ1
)
with red tailing to 450 nm. The long wavelength maxi-
mum of 2 is substantially red shifted relative to bithi-
ophene (302 nm), indicating the formation of an
extended p-delocalized system among the two bithienyl
groupsand the aryl ring. The excitation spectrum of 2,
however, isnot identical to itsUV spectrum; it exhibits
two peaks at 343 and 440 nm. The emission spectra of 2
3
8H, bithienyl-H), 7.57 (d, JH–H ¼ 3:6 Hz, 8H, bithienyl-H), 7.77 (d,
3
8H, JH–H ¼ 3:6 Hz, bithienyl-H). IR (KBr, cmꢀ1): 3432, 1657, 1506,
1447, 1300, 1104, 1019, 791. MS (MALDI-TOF, matrix: dithra-
nol+silver trifluoroacetate), 2052.4 (Mþ+6, 25%); GPC, Mw 2220, Mn
2210.

X.-M. Liu et al. / Tetrahedron Letters 45 (2004) 6173–6177
6175
Figure 4. The UV spectral changes of 3 (1 · 10ꢀ5 M) in different
aqueous pH buffer and THF solutions. From bottom to top: pH ¼ 1.0,
3.0, 4.0, 4.5, 5.0, 5.5, 6.0, 7.0, 9.0, 11.0, 13.0 and THF. Insert: The
absorbance ratio of k400 and k350 asa function of pH.
Figure 2. The 1H NMR spectrum of 3 in DMF-d7.
Figure 5. The PL spectral changes of 3 (1 · 10ꢀ5 M) in different
aqueous pH buffer and THF solutions. From a to m: pH ¼ 1.0, 3.0,
4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 13.0, 11.0, 9.0 and THF. Insert: The
intensity ratio of 500 and 450 nm as a function of solution pH. The
potassium salt of 3 wasused for UV and PL determinationsin THF.
Figure 3. The UV and PL spectra of 2 in THF (concentration at ca.
1 · 10ꢀ7 M): (a) the UV spectrum; (b) the excitation spectrum
(kem ¼ 478 nm); (c) and (d) the emission spectra with kex at 440 and
396 nm, respectively. The intensity of spectrum d is normalized relative
to that of c.
obtained by excitation at 440 and 396 nm, respectively,
show a similar spectral pattern with a similar emission
maximum at 478 nm and a shoulder peak at 460 nm.
However, the former emission is much stronger than the
latter (quantum yields: 11.2% vs 0.4%). It should be
noted that the PL spectra were recorded with a dilute
sample (abs. <0.05) to avoid absorption saturation. The
significant red shift of the excitation maximum relative
to absorption maximum indicates that the aromatic
ringsof 2 adopt a more nearly planar conformation,
thusa higher degree of conjugation in the excited state.
Such differencesare due to the highly hindered and
multi-chromophoric structure of 2, in which conforma-
tion and complex intramolecular electron delocaliza-
Table 1. The optical data of 3 in different pH buffer and THF solu-
tions
Solvent
pH
kmax (nm)
UPL (%)
Absorption
(e ꢁ 104,
Emission
(fwhm/nm)
M
ꢀ1 cmꢀ1
)
AqueouspH
buffer solu-
tions
1.0
348 (2.1)
347 (2.6)
347 (2.7)
347 (2.8)
347 (3.2)
350 (3.1)
388 (3.2)
393 (3.5)
397 (4.0)
397 (4.2)
398 (4.2)
398 (4.1)
428 (70)a
430 (75)a
444 (94)a
450 (95)a
480 (102)a
486 (100)a
504 (88)b
503 (87)b
504 (85)b
503 (85)b
503 (85)b
503 (86)b
5.7
4.2
3.5
3.1
2.2
2.9
5.0
4.1
5.4
6.1
5.9
5.3
3.0
4.0
4.5
5.0
5.5
6.0
6.5
7.0
12
tionsand p–p interactionsinfluence itsPL properties.
9.0
11.0
13.0
The absorption and emission spectra of 3 in different pH
buffer and THF solutions are shown in Figures 4 and 5,
respectively. A summary of the optical data of 3 isgiven
in Table 1. In pH 1.0–5.5 solutions, the UV spectra ex-
hibit long wavelength maxima around 348 nm, and
weaker shoulder bands around 400 nm. With an increase
in pH, the band around 400 nm becomestsronger and
finally it becomesthe major band at pH 6.0. The
absorbance ratio of 400 and 350 nm becomes constant at
pH 7.0–13.0 solutions (Fig. 4, insert). The evolution of
THF
––
400 (4.0)
485 (78)b
5.4
a Excited at 350 nm.
b Excited at 406.5 nm.
emission spectra of 3 with the increase of pH follows a
similar tendency to that of the UV spectra. In pH 1.0–
3.0 acidic solutions, 3 emitsindigo light with the

6176
X.-M. Liu et al. / Tetrahedron Letters 45 (2004) 6173–6177
emission maxima centered around 430 nm. With in-
creased pH, the emission maximum is red shifted grad-
ually and reaches504 nm with a shoulder band around
486 nm in the pH 6.0 solution. The PL intensity in-
creases significantly with an increase of pH from 5.5 to
9, whereas it decreases slightly when the pH is increased
further from 11 to 13. The spectral patterns and emis-
sion maxima are the same in pH 6.0–13.0 buffer solu-
tions. An analysis of I500=I450 versus pH shows a similar
trend to that of A400=A350 (Fig. 5, insert). In THF solu-
tion, 3 exhibits an emission maximum at 486 nm and a
shoulder band around 503 nm. It is important to note
that, however, the quantum yield and fwhm (full width
at half-maximum) valuesof 3 in THF and aqueous
buffer solutions are similar. This is in contrast to similar
poly(ethyleneglycol) linked four-armed water-soluble
tetrahedral fluorescent derivatives of tetraphenylme-
thane, which showed significant red shift and broaden-
ing in the emission spectrum, and a decrease in quantum
efficiency due to self-aggregation of the compounds in
aqueousmedia. 10 The similarities of the emission spectra
and quantum yieldsof 3 in aqueoussolution and THF
suggest that, as expected, self-aggregation of 3 in
aqueousoslution isnot obviousdue to itshighly hin-
dered structure.
Figure 6. Emission spectra of 3 in pH 1.0 (a and a0), pH 4.5 (b and b0),
and pH 7.0 (c and c0) with excitation at 350 nm (a, b, and c) and
406.5 nm (a0, b0, and c0), respectively. (*) From the excitation light.
The UV and PL pH-sensitivity of 3 isapparently due to
itscarboxyl and amino groups. Furthermore, it islikely
that the two distinct absorption and emission bands are
due to two species (Scheme 2), since the evolution of the
emission and absorption spectra versus pH exhibit
similar trends.13 To further confirm this, we examined
the PL spectra of 3 in pH solutions at excitation wave-
lengthsof 350 and 406.5 nm, respectively. The excitation
of the pH 4.5 solution of 3 at these wavelengths gave two
different emission spectra indicating that two different
species were present at this pH. Similar experiments
showed that the pH 1.0–3.0 solutions were not emissive
in nature when excited at 406.5 nm, whereasexcitation
of a pH 7.0 solution at the two wavelengths gave almost
identical emission spectra, indicating that the fully
protonated or fully deprotonated form ispreesnt pre-
dominately in these pH solutions, respectively (Fig. 6).
In pH 4.0–6.0 buffer solutions, the gradual change in
the UV and PL spectra of 3 indicate that complex
ionization processes þand multiple species are present.
Figure 7. The aqueousGPC tracesof 3 (1 · 10ꢀ5 M in buffer solutions).
From bottom to top: pH 1.0, 3.0, 5.0, 7.0, and 9.0, respectively. The
UV detector wasset at 400 nm.
Finally, we investigated the self-aggregation behavior of
3 in aqueousmedia by aqueousgel permeation chro-
matography (GPC), which hasbeen developed asa
useful method to determine molecular weights and
aggregation numbersof aggregatesof amphiphilic
14
polymersin aqueousmedia.
In view of the strong
absorption of 3 at long wavelengths, we chose a UV
detector setting of 400 nm to monitor the GPC elute.
Figure 7 shows aqueous GPC traces of 3 in variouspH
solutions. In all the pH solutions, only one species,
which isthe parent compound, wasdetected (retention
time: 38.0 0.1 min, Mw ꢂ1850 with polydispersity of
1.01). Therefore 3 wasnot prone to self-aggregation in
aqueousmedia probably due to itshighly hindered
structure and static repulsion from COOꢀ or NH3
þ
The CO2H and NH3 are electron-withdrawing groups
groups present in the outer sphere.
ꢀ
whereasCO
and NH2 are electron-donating groups,
2
which account for the significant difference in the
absorption and emission maxima of their respective
species.
In conclusion, we have developed a facile method for the
synthesis of an octa-carboxylic acid from an eight-
armed tetraphenylmethane–bithiophene adduct. The
-
+
-
-O2C
NH2
HO2C
CO2
CO2H
+
+
NH3
FL core
NH3
-O2C
-O2C
CO2
-O2C
-O2C
CO2H
NH3
FL core
NH3
NH3
FL core
NH3
-
-O2C
H2N
-O2C
CO2H
HO2C
+H3N
-
-
CO2
NH2
CO2
CO2
NH2
OH-
H+
OH-
H+
+
+
FL core
+H3N
-O2C
+
+H3N
-O2C
NH3
......
+
NH3
-
CO2H
CO2H
HO2C
HO2C
-
-
CO2
CO2
-
CO2
+
-O2C
+
+
-
-
-O2C
CO2
-O2C
CO2
NH2
CO2
pH 1-3, protonated form
indigo
pH 4-5.5, a mixture of
indigo-blue
pH 6-13, deprotonated form
blue-green
Scheme 2. The acid–base equilibrium of 3 in aqueousmedia.

X.-M. Liu et al. / Tetrahedron Letters 45 (2004) 6173–6177
6177
4. (a) Robinson, M. R.; Wang, S.; Bazan, G. C.; Cao, Y.
Adv. Mater. 2000, 12, 1701–1704; (b) Yeh, H.-C.; Chan,
L.-H.; Lee, R.-H.; Chen, C.-T. SPIE-Int. Soc. Opt. Eng.
2000, 4105, 348–349; (c) Zhao, H. D.; Tanjutco, C.;
Thayumanavan, S. Tetrahedron Lett. 2001, 42, 4421–4424;
(d) Langhals, H.; Ismael, R.; Yuruk, O. Tetrahedron 2000,
56, 5435–5441; (e) Langhals, H.; Wagner, C.; Ismael, R.
New J. Chem. 2001, 25, 1047–1049; (f) Zimmerman, T. J.;
compound is water-soluble and showed no obvious
self-aggregation in aqueous media. Compound 3 emits
indigo light in strongly acidic solution and emits blue-
green light in neutral and alkaline solutions, whereas its
UV and PL spectra are highly sensitive in pH 4.0–6.0
solutions. To our best knowledge, compounds 2 and 3
are the first examples of eight-armed tetrahedral lumi-
nescent compounds derived from tetraphenylme-
thane.1–4 It hasbeen well etsablihsed that an ‘ideal’
fluorescent probe should bear a large but distinct num-
ber of chromophoresaswell asa isngle biologically
active group.8;15 By using the eight reactive a-carbon
sites of bithienyl moieties in 2, it would be easy to
introduce a selective binding ligand to one arm and
several hydrophilic segments to other arms in a stepwise
way. Studies on the synthesis of such fluorescent probes
and detailed studies on the photophysical behavior of 3
are in progress.
€
Muller, T. J. J. Eur. J. Org. Chem. 2002, 2269–2279; (g)
Zimmerman, T. J.; Muller, T. J. J. Synthesis 2002, 1157–
€
1162; (h) Kramer, C. S.; Zimmerman, T. J.; Sailer, M.;
€
Muller, T. J. J. Synthesis 2002, 1163–1170; (i) Minard-
Basquin, C.; Weil, T.; Hohner, A.; Radler, J. O.; Mullen,
K. J. Am. Chem. Soc. 2003, 125, 5832–5838; (j) Tinnefeld,
P.; Weston, K. D.; Vosch, T.; Cotlet, M.; Weil, T.;
Hofkens, J.; Mullen, K.; De Schryver, F. C.; Sauer, M. J.
Am. Chem. Soc. 2002, 124, 14310–14311.
5. (a) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett.
2001, 3, 2887–2890; (b) Zimmermann, T. J.; Freundel, O.;
€
Gompper, R.; Muller, T. J. J. Eur. J. Org. Chem. 2000,
3305–3312.
6. (a) Li, Q.; Rukavishnikov, A. V.; Petukhov, P. A.;
Zaikova, T. O.; Keana, J. F. W. Org. Lett. 2002, 4,
3631–3634; (b) Mongin, O.; Gossauer, A. Tetrahedron
Lett. 1996, 37, 3825–3828; (c) Sengupta, S.; Sadhukhan, S.
K.; Muhuri, S. Synlett 2003, 2329–2332.
7. Stenger, D. A.; Gross, G. W.; Keefer, E. W.; Shaffer, K.
M.; Andreadis, J. D.; Ma, W.; Pancrazio, J. J. Trends
Biotechnol. 2001, 19, 304–309.
Acknowledgements
We are grateful to the Agency for Science, Technology
and Research (A*Star), Singapore for financial support.
8. (a) Toutchkine, A.; Kraynov, V.; Hahn, K. J. Am. Chem.
Soc. 2003, 125, 4132–4145; (b) Minard-Basquin, C.; Weil,
References and notes
€
T.; Hohner, A.; Radler, J. O.; Mullen, K. J. Am. Chem.
Soc. 2003, 125, 5832–5838.
€
9. Krause, W.; Hackmann-Schlichter, N.; Maier, F. K.;
€
10. (a) Liu, X.-M.; He, C.; Xu, J.-W. Tetrahedron Lett. 2004,
45, 1593–1597; (b) Liu, X.-M.; Xu, J.-M.; He, C. Tetra-
hedron Lett. 2004, 45, 1507–1510.
11. Thaimattam, R.; Xue, F.; Sarma, J. A. R. P.; Mak, T. C.
W.; Desiraju, G. R. J. Am. Chem. Soc. 2001, 123, 4432–
4445.
1. (a) Wilson, L. M.; Griffin, A. C. J. Mater. Chem. 1993, 3,
991–994; (b) Sengupta, S.; Sadhukhan, S. K. Tetrahedron
Lett. 1998, 39, 1237–1238; (c) Sengupta, S.; Sadhukhan, S.
K. Tetrahedron Lett. 1999, 40, 9157–9161; (d) Sengupta,
S.; Sadhukhan, S. K. Organometallics 2001, 20, 1889–
1891; (e) Sengupta, S.; Sadhukhan, S. K.; Muhuri, S.
Tetrahedron Lett. 2002, 43, 3521–3524; (f) Sengupta, S.;
Purkayastha, P. Org. Biomol. Chem. 2003, 1, 436–440.
2. (a) Chan, L.-H.; Lee, R.-H.; Hsieh, C.-F.; Yeh, H.-C.;
Chen, C.-T. J. Am. Chem. Soc. 2002, 124, 6469–6479; (b)
Yeh, H. C.; Lee, R. H.; Chan, L. H.; Lin, T. Y. J.; Chen,
C. T.; Balasubramaniam, E.; Tao, Y. T. Chem. Mater.
2001, 13, 2788–2796; (c) Chan, L.-H.; Yeh, H.-C.; Chen,
C.-T. Adv. Mater. 2001, 13, 1637; (d) Yeh, H.-C.; Lee,
R.-H.; Chan, L.-H.; Lin, T.-Y. J.; Chen, C.-T.; Balasubra-
maniam, E.; Tao, Y.-T. Adv. Mater. 2001, 13, 2788–2796;
(e) Fournier, J. H.; Maris, T.; Wuest, J. D.; Guo, W. Z.;
Galoppini, E. J. Am. Chem. Soc. 2003, 125, 1002–1006; (f)
Fournier, J. H.; Wang, X.; Wuest, J. D. Can. J. Chem.
2003, 81, 376–380.
Muller, R. Top. Curr. Chem. 2000, 210, 261–308.
ꢀ
12. (a) Adronov, A.; Frechet, M. J. Chem. Commun. 2000,
1701–1710; (b) Adronov, A.; Gilat, S. L.; Frechet, M. J.;
ꢀ
Ohta, K.; Neuwahl, F. V. R.; Fleming, G. R. J. Am.
Chem. Soc. 2000, 122, 1175–1185; (c) Weil, T.; Wiester, U.
M.; Herrmann, A.; Bauer, R.; Hofkens, J.; De Schryver,
€
F. C.; Mullen, K. J. Am. Chem. Soc. 2001, 123, 8101–8108;
(d) Maus, M.; Mitra, S.; Lor, M.; Hofkens, J.; Weil, T.;
€
Herrmann, A.; Mullen, K.; De Schryver, F. C. J. Phys.
Chem. A 2001, 105, 3961–3966; (e) Liu, D. J.; De Feyter,
S.; Cotlet, M.; Stefan, A.; Wiesler, U.-M.; Herrmann, A.;
€
Grebel-Koehler, D.; Qu, J. Q.; Mullen, K.; De Schryver,
F. C. Macromolecules 2003, 36, 5918–5925.
13. Valeur, B. Molecular Fluorescence, Principles and Appli-
cations; Wiley-VCH: Weinheim, Germany, 2001.
14. (a) Patrickios, C. S.; Forder, C.; Arms, S. P.; Billingham,
N. C. J. Polym. Sci. A: Polym. Chem. 1997, 35, 1181–1195;
(b) Schappacher, M.; Deffieux, A.; Putaux, J.-L.; Viville,
P.; Lazzaroni, R. Macromolecules 2003, 36, 5776–5783.
15. Langhals, H.; Speckbacher, M. Eur. J. Org. Chem. 2001,
2481–2485.
3. (a) Wang, S.; Oldham, W. J.; Hudack, R. A.; Bazan, G. C.
J. Am. Chem. Soc. 2000, 122, 5695–5709; (b) Zheng, S. Y.;
Shi, J. M.; Mateu, R. Chem. Mater. 2000, 12, 1814; (c)
Hong, S. B.; Raushel, F. M. Bioorg. Med. Chem. Lett.
1994, 4, 2705–2708; (d) Lee, Y. K.; Jeong, H. Y.; Kim, K.
M.; Kim, J. C.; Choi, H. Y.; Kwon, Y. D.; Choo, D. J.;
Jang, Y. R.; Yoo, K. H.; Jang, J.; Talaie, A. Curr. Appl.
Phys. 2002, 2, 241–244.