J Fluoresc (2014) 24:197–202
201
1
(27.1 mg, 34 % yield). H NMR (400 MHz, DMSO) δ ppm
Metal-sensing Application
8.03 (6H, d, J= 7.9 Hz), 7.67 (6H, d, J= 7.9 Hz), 7.55 (6H, d,
J= 8.0 Hz), 7.27 (3H, s), 7.24 (3H, s), 7.14 (6H, d, J= 8.0 Hz),
4.69−4.54 (6H, m), 4.17 (12H, t, J= 6.5 Hz,), 3.69 (12H, t,
J= 5.44 Hz), 1.94 (12H, m). 13C NMR (100 MHz, DMSO) δ
ppm 166.6, 153.2, 146.3, 132.7, 130.4, 129.5, 126.7, 124.0,
117.3, 116.6, 113.9, 112.3, 94.9, 93.8, 88.7, 85.8, 65.9, 57.2,
32.1. MS(MALDI-TOF) Calcd for C93H87NO12: 1422.52
Found: 1423.58.
For a preliminary screening of applications as fluorescence
sensors, we investigated the fluorogenic responses of 1 and 2
in the presence of metal ions such as Fe3+, Fe2+, Co2+, Ni2+,
Cu2+, Cd2+, Hg2+, Mn2+, Ba2+, Pd2+, Pb2+, Zn2+, Ca2+ and
Al3+. It was found that 1 exhibit a selective fluorescence
quenching by Fe3+ ion without showing significant signal
changes by Fe2+ or other metal ions (Fig. 3). This selectivity
renders an application of the fluorophore as a sensor for trace
analysis and diagnosis of iron-related diseases. Although,
Lohani and Lee [15] reported the effect of absorbance of Fe3+
as an interference effect of the excitation of fluorophore with an
excitation wavelength in the UV range, this may not be the case
for fluorophore 1 and 2 as both compounds exhibit similar
absorption properties but only 1 showed a selectivity toward
Fe3+. We proposed that the quenching mechanism should in-
volve a formation of complex between Fe3+ ion and the car-
boxylate groups. This complexation may lead to an aggregation
of fluorophore, which would become much less water-soluble
than the non-aggregate fluorophore for the case of 1.
Results and Discussion
Synthesis of Terphenyleneethynylene Fluorophores 1–2
Our target water-soluble terphenylene diethynylene fluorophores
were synthesized by mean of Sonogashira coupling. Prior to that
key step, the molecular branches were synthesized by O-alkyl-
ation of hydroquinone (Scheme 1). The di-n-butoxy benzene
(3a) was successfully iodinated by a reaction with ICl [14], but
the same reaction with 3b gave rise to an inseparable mixture of
the mono- and diiodo compounds. We then decided to acetylate
the hydroxyl group and found that 3c smoothly underwent the
diiodination upon treatment with I2 and KIO3. A Sonogashira
coupling of 4a and 4c with methyl 4-ethynylbenzoate gave rise
to 5a and 5c in moderate yields. Fluorophore 1 and 2 by were
then assembled via a series of Sonogashira couplings. First, the
tris(4-iodophenyl)amine [9] was coupled with trimethylsilyl
acetylene, followed by TMS deprotection. A sequential reaction
with the dendritic arms (5a or 5c) followed by base-hydrolysis
led to fluorophore (1 or 2) in fair yields.
Conclusion
Two new terphenylene diethynylene fluorophores containing
negatively charged peripheral groups were successfully syn-
thesized. Both compounds displayed high fluorescent quan-
tum yields in aqueous media, which was rationalized by
steric-induced deaggregation. The greater quantum efficiency
of 2 may result from the hydrophilic –OH groups facilitating
the solubility. Fluorophore 1 exhibited a selective fluorescent
quenching by Fe3+. The improvements on sensitivity as well
as the exploration of further applications for these
fluorophores are currently undertaken.
Photophysical Properties of Fluorophores 1–2
The photophysical properties of 1 and 2 in 50 mM phosphate
buffer (pH 8.0) were investigated and the results are summa-
rized in Table 1. Both fluorophores have a maximum absorp-
tion wavelength around 396–398 nm with an equal molar
extinction coefficient, which indicated that the two com-
pounds possessed a similar conjugated system. For the fluo-
rescent spectra, it was found that the maximum emission
wavelength of 2 appeared at a longer wavelength (508 nm)
compared to that of 1 (490 nm). It seems likely that the more
hydrophobic fluorophore 1 is less stabilized in aqueous media
upon excitation, while the excited 2 can be well stabilized and
a prolonged geometrical relaxation can cause a wider Stoke’s
shift. The quantum efficiency of 2 (0.33) was also consider-
ably higher than that of 1 (0.18). These results indicate that
hydrophobicity in 1 could lead to fluorescence self-
quenching. In comparison with our previously reported 3C-,
these new fluorophores possess more extended conjugation,
which results in the absorption and emission maxima at longer
wavelengths (Fig. 2).
Acknowledgements This work is financially supported by the Thailand
Research Fund (RTA5280002), the 90th Anniversary of Chulalongkorn
University Fund, and the Thai Government Stimulus Package 2
(TKK2555), under the Project for Establishment of Comprehensive Center
for Innovative Food, Health Products and Agriculture. This work is part of
the Higher Education Research Promotion and National Research Univer-
sity Project of Thailand, Office of the Higher Education Commission
(AM1006A-56) and the Ratchadaphiseksomphot Endowment Fund of
Chulalongkorn University (RES560530126-AM).
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