Water-soluble anionic fluorophores from truxene

Article abstract of DOI:10.1016/j.dyepig.2011.10.009

Two new water-soluble fluorophores are synthesized from truxene and ester-substituted aryl acetylenes. The truxene core is decorated by 2-(2′-methoxy)ethoxyethyl groups to enhance the hydrophilicity of these fluorophores as well as to prevent the aggregation by π-stacking in aqueous media. The conjugated structures are assembled by iodination of the truxene core and subsequent Sonogashira coupling with aryl acetylenes. Upon the hydrolysis of the ester groups, water-soluble fluorophores are obtained in good to excellent yield (30-71% for 3 steps). A photophysical investigation reveals that these compounds exhibit strong fluorescence signals (quantum yield 46-63%) with maximum emission wavelength around 390-400 nm in aqueous phosphate buffer. Preliminary screening on sensing application shows that their fluorescent signals can be selectively quenched by porphyrin-containing metalloproteins.

Full text of DOI:10.1016/j.dyepig.2011.10.009

Dyes and Pigments 93 (2012) 1428e1433
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Water-soluble anionic fluorophores from truxene
1
*
Nopporn Earmrattana , Mongkol Sukwattanasinitt, Paitoon Rashatasakhon
Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new water-soluble fluorophores are synthesized from truxene and ester-substituted aryl acetylenes.
The truxene core is decorated by 2-(20-methoxy)ethoxyethyl groups to enhance the hydrophilicity of
Received 8 April 2011
Received in revised form
6 September 2011
Accepted 9 October 2011
Available online 20 October 2011
these fluorophores as well as to prevent the aggregation by p-stacking in aqueous media. The conjugated
structures are assembled by iodination of the truxene core and subsequent Sonogashira coupling with
aryl acetylenes. Upon the hydrolysis of the ester groups, water-soluble fluorophores are obtained in good
to excellent yield (30e71% for 3 steps). A photophysical investigation reveals that these compounds
exhibit strong fluorescence signals (quantum yield 46e63%) with maximum emission wavelength
around 390e400 nm in aqueous phosphate buffer. Preliminary screening on sensing application shows
that their fluorescent signals can be selectively quenched by porphyrin-containing metalloproteins.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Truxene
Phenyleneethynylene
Fluorophore
Sensor
Metalloprotein
Quenching
1. Introduction
a result, fluorescent dendrimers have recently been of growing
interest in the development of chemical and biological sensors
Fluorescence signal transduction has been widely used as
a selective and sensitive method for the detection of chemical and
biological substances [1e3]. Conjugated polyelectrolytes have
usually served this purpose due to their tunable optoelectronic
properties and good water solubility. By a signal amplification
mechanism in which the exciton can migrate along the conjugated
backbone, polymeric fluorophores are more sensitive toward the
analytes than their small molecule analogs [4e9]. Linear poly(-
phenyleneethynylene) [8] and poly(fluorene) [9] are among the
major classes of compound used as fluorescence signal transducers.
They are usually prepared by Pd-catalyzed reactions such as Heck
[10], Suzuki [11], and Sonogashira coupling [12]. In most cases,
these polymers are polydispersed and their molecular weight
distribution or degree of polymerization depends variably on the
conditions in which they are produced. Furthermore, their optical
properties cannot be correlated with a high degree of certainty to
their three-dimensional structure due to their random coil
conformation [13]. In comparison with linear polymers, den-
drimers usually possess more predictable geometry as well as
a monodispersity achieved by stepwise synthetic routes [14]. As
[15e17].
During the course of our research program on design and
synthesis of dendritic polyelectrolytes, we have established the
synthesis and applications of a polyanionic dendrimer as a selective
fluorescence sensor for mercuric ion [18]. Most recently, we
demonstrated the use of variously charged dendritic poly-
electrolytes in sensor array along with a multivariate statistical
analysis for protein discrimination [19]. In the development of
p-
conjugated materials as fluorescent signal transducers via
quenching process, high quantum efficiencies of the fluorophores
are challenging and desirable, especially for application in aqueous
system. In the present work, we report the design and synthesis of
a new class of water-soluble fluorophores containing a truxene core
which showed high quantum efficiency and selective fluorescence
quenching by porphyrin-containing metalloproteins.
Truxene or 10,15-dihydro-5H-diindeno[1,2-a
;10,20-c]-fluorene
has been used as a photoactive moiety in materials for organic
light-emitting diodes and phosphorescent metal complexes
[20e24]. With its molecular rigidity and a highly emissive property,
we chose to incorporate a truxene unit into our target molecules 1
and 2 (Fig. 1) and demonstrate the first application of this fluo-
rescent building block as water-soluble sensing materials. However,
the structure of truxene is highly hydrophobic and relatively planar
which can lead to a poor water solubility and lower quantum
efficiency due to aggregation. We therefore opted to install six
* Corresponding author. Tel.: þ66 2 2187620; fax: þ66 2 2187598.
E-mail address: paitoon.r@chula.ac.th (P. Rashatasakhon).
1
Deceased April 4, 2011.
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.10.009

N. Earmrattana et al. / Dyes and Pigments 93 (2012) 1428e1433
1429
COOH
HOOC
COOH
OMe
O
OMe
O
MeO
MeO
O
O
OMe
OMe
OMe
O
O
HOOC
HOOC
O
O
HOOC
OMe
O
O
O
O
MeO
MeO
COOH
MeO
MeO
COOH
HOOC
1
2
Fig. 1. Structure of fluorophores 1 and 2.
hydrophilic chains of 2-(20-methoxy)ethoxyethyl at the methylene
positions of truxene. This structural modification should facilitate
for 30 min, then 1-iodo-2-(2-methoxyethoxy)ethane (0.53 g,
2.32 mmol) was added. After stirring for 24 h, the mixture was
poured into water and extracted with EtOAc. The volatile solvents
were removed under reduced pressure and the crude product was
purified by column chromatography on silica gel eluting with EtOAc
to yield 5 (182 mg, 66%) as white crystalline solid. 1H NMR (CDCl3,
the water solubility and prevent molecular aggregation by
stacking. Three phenylethynylene moieties were connected to the
truxene core to extend the -conjugated system and shift the
p-
p
emission wavelength into the visible region. The ionizable
carboxylic acid groups installed at the peripheries could promote
the solubility in water and serve as interaction sites for analytes.
400 MHz, ppm)
(m, 6H), 3.19e3.28 (m, 36H), 3.06 (m, 12H), 2.50e2.72 (m, 18H). 13C
NMR (CDCl3, 100 MHz, ppm) 151.5, 143.0, 138.1, 127.4, 127.3, 125.1,
d
8.34 (d, J ¼ 8.3, 3H), 7.58 (d, J ¼ 8.3 Hz, 3H), 7.41
d
2. Experimental
122.8, 71.5, 69.7, 67.1, 58.8, 51.2, 36.0.
2.1. Chemicals and instruments
2.2.3. 2, 7, 12-Triiodo-5, 50, 10, 100, 15, 150-hexa-2-(2-
methoxyethoxy)ethyltruxene (6)
The starting materials and reagents were purchased from
commercial sources and used without further purification. All 1H
and 13C NMR spectra were obtained on a Varian Mercury NMR
spectrometer, which operated at 400 MHz for 1H and 100 MHz for
13C nuclei (Varian Company, CA, USA). Mass spectra were recorded
on a Microflex MALDI-TOF mass spectrometer (Bruker Daltonics)
A solution of 5 (0.20 g, 0.20 mmol) in 20 ml of solvent mixture
(CH3COOH : H2SO4 : H2O : CCl4 ¼ 100 : 5 : 20 : 8) was heated to 40 ꢀC
with stirring. After adding KIO3 (0.05 g, 0.23 mmol) and I2 (0.08 g,
0.32 mmol), the mixture was heated to 80 ꢀC and stirred for 3 h. After
the mixture was cooled to room temperature, it was poured into water
and extracted with EtOAc. The volatile solvents were removed under
reduced pressure and the crude product was purified by column
chromatography on silica gel eluting with EtOAc to yield 6 (0.26 g, 93%)
using doubly recrystallized
and dithranol as a matrix.
a-cyano-4-hydroxy cinnamic acid (CCA)
as yellow solid. 1H NMR (CDCl3, 400 MHz, ppm)
3H), 7.85 (s, 3H), 7.68 (d, J ¼ 8.1, 3H), 3.01 (m, 18H), 2.63 (m,12H), 2.44
(m, 6H). 13C NMR (CDCl3, 400 MHz, ppm)
154.0, 143.5, 137.4, 137.3,
d
7.97 (d, J ¼ 8.1 Hz,
2.2. Synthesis
d
2.2.1. Truxene (4)
136.5, 132.4, 126.6, 93.5, 71.6, 69.9, 67.0, 58.9, 51.7, 35.8, 29.7.
A mixture of 3-phenylpropionic acid (3) (10.0 g, 66.7 mmol) and
polyphosphoric acid (50 g) was heated at 60e65 ꢀC for 60 min
under nitrogen atmosphere. Water (5 mL) was then added to the
reaction and temperature was raised to 160 ꢀC and maintained for
3 h. After the reaction was allowed to cool to room temperature, the
mixture was poured into ice/water and gray powder was filtered
and washed with water. The product was purified by column
chromatography on silica gel eluting with hexanes to yield 1 as
light-yellow solid (4.3 g, 57%). 1H NMR (CDCl3, 400 MHz, ppm)
2.2.4. Triester 7
To a mixture of 6 (180 mg, 0.14 mmol), PdCl2(PPh3)2 (8 mg,
0.01 mmol), CuI (6 mg, 0.02 mmol) in toluene 13 (mL) was added
DBU 1 (mL) and the mixture was stirred at room temperature for
24 h. The reaction mixture was diluted with EtOAc then filtered
through a pad of silica gel. The filtrate was washed with water and
the organic phase was dried over NaSO4, filtered, and concentrated
under reduced pressure. The residue was purified by flash chro-
matography on silica gel eluting with gradient solvents from ethyl
acetate/hexane (1/2) to pure EtOAc to afford 7 as a yellow amor-
d
7.97 (d, J ¼ 7.2 Hz, 3H), 7.70 (d, J ¼ 7.6 Hz, 3H), 7.51 (t, J ¼ 7.6 Hz,
3H), 7.40 (t, 3H, J ¼ 7.6 Hz, 3H) 4.29 (s, 6H).
phous solid (0.18 g, 93%). 1H NMR (CDCl3, 400 MHz, ppm)
d 8.34 (d,
2.2.2. 5, 50, 10, 100, 15, 150-hexa-2-(2-methoxyethoxy)ethyltruxene
(5)
To a stirred solution of 4 (0.10 g, 0.29 mmol) in DMF (10 ml) at
0 ꢀC under nitrogen, NaH (0.09 g, 2.32 mmol) was added. Then the
solution was allowed to warm to room temperature and stirred
J ¼ 8.2 Hz, 3H), 8.07 (d, J ¼ 8.4 Hz, 6H), 7.78 (s, 3H), 7.67 (d,
J ¼ 8.4 Hz, 6H), 7.61 (d, J ¼ 8.2, 3H), 3.94 (s, 9H), 3.09e3.28 (m, 48H),
2.58e2.76 (m, 18H). 13C NMR (CDCl3, 100 MHz, ppm)
d 166.5, 151.9,
144.5, 138.4, 137.7, 131.6, 131.0, 129.6, 128.5, 127.8, 126.2, 125.1, 121.8,
92.5, 90.0, 71.6, 69.9, 67.1, 58.9, 52.2, 51.6, 36.1.

1430
N. Earmrattana et al. / Dyes and Pigments 93 (2012) 1428e1433
2.2.5. Hexaester 8
d6, 100 MHz, ppm) d 166.6, 152.3, 144.4, 138.5, 137.1, 131.5, 131.4,
A mixture of 6 (100 mg, 0.075 mmol), PdCl2(PPh3)2 (3 mg,
0.004 mmol), CuI (2 mg, 0.008 mmol), in toluene 8 (mL) was added
DBU 0.5 (mL) and the mixture was stirred at room temperature for
24 h. The reaction mixture was diluted with EtOAc then filtered
through a pad of silica gel. The filtrate was washed with water and
the organic phase was dried over NaSO4, filtered, and concentrated
under reduced pressure. The residue was purified by flash chro-
matography on silica gel eluting with gradient solvents from ethyl
acetate/hexane (1/2) to pure EtOAc to afford 8 as a yellow amor-
130.9, 129.7, 126.6, 126.2, 124.9, 120.6, 92.4, 89.5, 70.8, 69.1, 66.4,
57.8, 51.5, 35.3. HRMS (EI) Calcd. for C84H90O18 (M þ K)þ 1425.5764,
found 1425.5757.
2.2.7. Hexaacid fluorophore (2)
A mixture of 8 hexaester (30 mg, 0.018 mmol) in THF (5 mL) and
methanol (5 mL) was added saturated KOH aqueous solution
(0.5 mL) and the mixture was stirred at room temperature. After
24 h the solution was evaporated and the residue dissolved in water
(30 mL). Ice was added to the aqueous layer which is acidified and
the mixture kept in refrigerator. The product was filtered to afford 2
as a yellow solid (21 mg, 75%). 1H NMR (DMSO-d6, 400 MHz, ppm)
phous solid (52 mg, 43%). 1H NMR (CDCl3, 400 MHz, ppm)
d 8.67 (d,
J ¼ 1.5 Hz, 6H), 8.46 (d, 1.5 Hz, 6H), 8.36 (d, 8.4 Hz, 3H), 7.80 (s, 3H),
7.62 (d, 8.4 Hz, 3H), 4.00 (s,18H), 3.09e3.28 (m, 48H), 2.58e2.76 (m,
18H). 13C NMR (CDCl3, 100 MHz, ppm)
d
165.6, 152.0, 144.6, 138.5,
d
8.46 (s, 3H), 8.42 (d, J ¼ 7.8 Hz, 3H), 8.31 (s, 6H), 8.14 (s, 3H), 7.77
137.7, 136.5, 131.1, 130.2, 126.3, 125.2, 124.3, 121.5, 91.4, 88.6, 71.6,
69.9, 67.1, 58.9, 52.5, 51.6, 36.1, 30.9.
(d, J ¼ 7.8, 3H), 2.92e3.12 (m, 48H), 2.59 (m, 18H). 13C NMR (DMSO-
d6, 100 MHz, ppm)
d 165.9, 152.3, 144.4, 138.5, 137.1, 135.4, 132.3,
130.9, 129.7, 126.4, 124.9, 123.3, 120.5, 91.3, 88.4, 70.8, 69.1, 66.4,
57.8, 51.5, 35.3. HRMS (EI) Calcd. for C87H90O24 (M þ K)þ 1557.5459,
found 1557.5489.
2.2.6. Triacid fluorophore (1)
A mixture of 7 (50 mg, 0.035 mmol) in THF (8 mL) and methanol
(8 mL) was added saturated KOH aqueous solution (0.5 mL) and the
mixture was stirred at room temperature. After 24 h the solution
was evaporated and the residue dissolved in water (30 mL). Ice was
added to the aqueous layer which is acidified and the mixture kept
in refrigerator. The product was filtered to afford 1 as a yellow solid
3. Results and discussion
3.1. Synthesis of anionic fluorophores 1e2
(40 mg, 82%). 1H NMR (CD3CN, 400 MHz, ppm)
d
8.46 (d, J ¼ 8.4 Hz,
Our synthesis began with a dehydration-trimerization of the
commerically available hydrocinnamic acid (3) using a procedure
reported by Yuan et al. [22] Scheme 1. Hexa-alkylation at three
3H), 8.04 (d, J ¼ 8.4 Hz, 6H), 7.94 (s, 3H), 7.71 (d, J ¼ 8.4 Hz, 9H),
3.01e3.22 (m, 48H), 2.72 (m, 12H), 2.60 (m, 6H). 13C NMR (DMSO-
R
R
R
PPA, 60oC, 1h
NaH, DMF
MeOCH2CH2OCH2CH2I
66%
COOH
Ph
R
then 160oC, 3 h
3
R
57%
R
4
5
; R = -(CH2CH2O)2CH3
Ar
I
R
R
Ar
H
R
R
R
R
KIO3, I2, AcOH,
CuI, Pd(PPh3)2Cl2,
R
R
Ar
I
H2SO4,H2O,CCl4
96%
DMF, DBU, PhMe
R
R
R
R
I
6; R = -(CH2CH2O)2CH3
Ar
Ar = p-MeOOC-C6H4 (93%)
7
; R = -(CH2CH2O)2CH3
8; R = -(CH2CH2O)2CH3
Ar = 3,5-(MeOOC)2-C6H3 (34%)
KOH, MeOH
THF, reflux
1 (75%) or 2 (83%)
Scheme 1. Synthesis of fluorophore 1 and 2.

N. Earmrattana et al. / Dyes and Pigments 93 (2012) 1428e1433
1431
Table 1
methylene units of truxene (4) was found very challenging. Various
Photophysical properties of 1 and 2.
combinations of bases, solvents, and electrophiles such as 2-(20-
methoxy)ethoxyethyl tosylate or mesylate have been tested, most
of which yielded mixtures of partially alkylated products. After
several variations, it was found that the use of NaH and 2-(20-
methoxy)ethoxyethyl iodide in DMF could cleanly afford the hexa-
alkylated truxene core 5 in 66% yield after a simple chromatog-
raphy on silica gel column. Iodination of 5 using KIO3 and I2 gave
rise to the 2,7,12-triiodo 6 as a single regiosiomer in excellent yield
of 96%. The Sonogashira coupling of 6 with aryl acetylene 7 or 8
followed by the hydrolysis of ester groups afforded ionizable tri-
and hexa-carboxylic acid (1 and 2), respectively.
Cmpd.
Absorption
Emission
lmax (nm)
3
(Mꢂ1 cmꢂ1
)
lmax (nm)
VF
a
Truxeneb
275
298
336
356
332
350
e
346
359
387
402
383
400
e
e
e
1
2
47700
44400
39300
40300
0.46
0.63
a
2-aminopyridine in 0.1 M H2SO4 (VF ¼ 0.60) was the reference.
b
dissolved in 9:1 DMSO:H2O.
3.4. Fluorescent responses by proteins
3.2. Photophysical properties of fluorophores 1e2
Several literature reports on fluorescent protein sensor arrays
[26e28] and the presence of anionic peripheral groups inspired us
to use 1 and 2 as fluorescent sensors for proteins. The solution of
The electronic absorption spectra of 1 and 2 were acquired from
1
m
M solutions in phosphate buffer pH 8. The absorption peak of
both compounds displayed typical vibrational progression
a
fluorophores (1
mM) was tested with 8 protein solutions
pattern of truxene (Fig. 2). Extensions of the conjugated system
caused longer absorption wavelengths in 1 and 2 compared to the
starting truxene. The slightly longer wavelength of absorption of 1
suggested that the substitution of eCOOH groups at the para-
(A280 ¼ 0.05) with a broad range of pI values and molecular
weights. The selected proteins included metalloproteins such as
myoglobin (Myo, pI ¼ 7.2, 17.0 kDa), cytochrome c (CytC, pI ¼ 10.7,
12.3 kDa) and carbonic anhydrase (CA, pI ¼ 5.9, 29.0 kDa) and non-
metalloproteins such as bovine serum albumin (BSA, pI ¼ 4.8,
66.3 kDa), histone (His, pI ¼ 10.8, 21.5 kDa), human serum albumin
(HSA, pI ¼ 5.2, 69.4 kDa), lysozyme (Lys, pI ¼ 11.0, 14.4 kDa), and
papain (Pap, pI ¼ 9.6, 23.0 kDa). The dependence of fluorescence
responses on the nature of proteins is illustrated in Fig. 4. It was
evident that both compounds exhibited selective fluorescence
quenching by metalloproteins containing porphyrin unit e CytC
and Myo. In order to evaluate the quenching efficiency of the two
proteins, SterneVolmer plots were constructed by varying the
protein concentration and monitoring the fluorescent signal of the
fluorophores. In the presence of CytC, the KSV of 2.53 ꢁ 105 Mꢂ1 and
2.01 ꢁ 105 Mꢂ1 were obtained for 1 and 2, respectively (Fig. 5a,b).
For the detection of Myo, the KSV was 1.59 ꢁ 105 Mꢂ1 for 1 and
1.47 ꢁ 105 Mꢂ1 for 2 (Fig. 5c,d). The fact that CytC and Myo
exhibited fluorescence quenching suggested that these responses
might cause by the electron transfer process between metal-
porphyrin moiety in the proteins and the fluorophore. This
hypothesis is currently under investigation.
positions could cause a slightly larger p-ststem. The emission peak
of 1 and 2 were at 402 and 400 nm in 10 mM phosphate buffer pH
8.0 with high quantum yields of 46 and 63%, respectively (Table 1).
The larger overlapping between the absorption and emission
spectral of 1 could cause the lower quantum yield, presumably due
to the re-absorption of the emitted photon by molecules at the
ground state [25].
3.3. Effect of pH on fluorescent intensities
The pH-dependent fluorescence studies of 1 and 2 were con-
ducted in phosphate buffer at pH range of 2e10 (Fig. 3). At pH
below 3, both 1 and 2 showed no fluorescence, which could be
attributed to their poor solubility as a result of low degree of
ionization of the carboxylic acid groups under highly acidic
condition. The fluorescent intensities reach their maximum
plateaus at pH 6. While 1 exhibited one-step fluorescent intensity
increase starting around pH 4, compound 2 however displayed
two-step increments taking off at pH 3 and 4, respectively. The two-
step increase of the fluorescent intensity corresponded well with
the two ionization processes of the dicarboxylic acid groups on
each branch of the structure of 2.
Fig. 3. pH dependence of the fluorescence intensity for 1 and 2 measured at room
temperature. Excitation wavelength is 356 nm for 1 and 350 nm for 2, observed
emission wavelength is 400 nm for both compounds.
Fig. 2. Normalized absorption and emission spectra of 1 and 2 (1 mM) in phosphate
buffer pH 8.

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N. Earmrattana et al. / Dyes and Pigments 93 (2012) 1428e1433
Fig. 4. (a) and (b) Fluorescence responses of 1; (c) and (d) fluorescence responses of 2 (1
Excitation wavelength is 356 nm for 1 and 350 nm for 2, observed emission wavelength is 400 nm for both compounds.
m
M) to various protein solution (A280 ¼ 0.05) in phosphate buffer saline (10 mM, pH 7.4).
Fig. 5. (a) and (b) SterneVolmer plots for fluorescence quenching of 1 and 2 by Cytochrome C, (c) and (d) SterneVolmer plots for fluorescence quenching of 1 and 2 by Myoglobin.

N. Earmrattana et al. / Dyes and Pigments 93 (2012) 1428e1433
1433
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4. Conclusion
In conclusion, we have accomplished the synthesis of two new
anionic fluorophores from truxene. To the best of our knowledge,
they are the first examples of water-soluble truxene derivatives in
the literatures. With six 2-(20-methoxy)ethoxyethyl groups deco-
rated at the methylene units and three or six carboxylic acid
peripheries, both compounds possessed excellent solubilities in
aqueous media. They also exhibited outstanding emission quantum
efficiencies typical for truxene derivatives which indicated a low
degree of aggregation. Both fluorophores showed a selective fluo-
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Acknowledgment
This work was financially supported by the Center of Petroleum,
Petrochemicals and Advanced Materials, National Nanotechnology
Center, and Faculty of Science, Chulalongkorn University (RES-
A1B1-5). The UV/Vis and fluorescence spectrometers were kindly
supported by Prof. Thawatchai Tuntulani.
Appendix. Supplementary material
[20] Ventura B, Barbieri A, Barigelletti F, Diring S, Ziessel R. Energy transfer
dynamic in multichromophoric arrays engineered from phorphorescent PtII/
RuII/OsII centers linked to a central truxene platform. In Org Chem 2010;49:
8333e46.
Supplementary data related to this article can be found online at
doi:10.1016/j.dyepig.2011.10.009.
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branched ladder-type poly(p-phenylene)s containing truxene units. Macro-
molecules 2010;43:731e8.
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