New metal-coordination-inhibited charge transfer emission for terfluorenes: Highly sensitive and selective detection for Hg2+ with ratiometric "turn-on" fluorescence response

Article abstract of DOI:10.1039/c0cc02957b

Two terfluorenes TFOH and TFN exhibit high sensitivity and selectivity for the detection of Hg²⁺ with ratiometric fluorescence response. The sensory mechanism is attributed to a new metal-coordination-inhibited spiroconjugation-like charge tran

Full text of DOI:10.1039/c0cc02957b

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New metal-coordination-inhibited charge transfer emission for
terfluorenes: highly sensitive and selective detection for Hg2+ with
ratiometric ‘‘turn-on’’ fluorescence responsew
Linna Zhu, Jingui Qin and Chuluo Yang*
Received 1st August 2010, Accepted 5th October 2010
DOI: 10.1039/c0cc02957b
Two terfluorenes TFOH and TFN exhibit high sensitivity
and selectivity for the detection of Hg2+ with ratiometric
fluorescence response. The sensory mechanism is attributed to
a new metal-coordination-inhibited spiroconjugation-like charge
transfer emission.
To this end, we chose the typical terfluorene TFOH with
4-(bis(2-hydroxyethyl)amino)phenyl pendent group to exploit
its application as a fluorescent chemosensor for metal ions. To
study the binding position between the terfluorene derivatives
and metal ions, we also designed and synthesized a new
terfluorene derivative, namely TFN, with two 4-(diethylamino)-
phenyl side groups (Scheme 1). Their fluorescence responses
toward different metal ions are investigated. It turns out that
both terfluorenes are able to work as ratiometric fluorescent
chemosensors for Hg2+ exclusively, irrespective of the structure
of receptor units.
The synthesis of TFOH has been described elsewhere,10 and
that of TFN is presented in the ESI. TFN exhibits similar
photophysical behaviors to TFOH, with green emission in
THF solution and significant bathochromic shift with the
increase of solvent polarity (see Fig. S1 in ESI).
The significance of monitoring environmental contamination
is arousing more and more attention nowadays. Mercury, as a
poisonous heavy metal, could easily accumulate in organisms,
and cause irreversible damage to the central nervous and
endocrine systems.1 Highly sensitive and selective Hg2+
sensors are in high demand for mercury pollution monitoring.2
Most of the reported fluorescent chemosensors for Hg2+ are
turn-off detection because of the intrinsic property of Hg2+
to quench fluorophore emission.3 Designing fluorescent
chemosensors for Hg2+ with turn-on response is a considerable
challenge. In addition, ratiometric fluorescence is an alternative
approach for the detection of Hg2+ which can offer advantages
in eliminating the background interferences, and consequently be
of significance in biological contexts. Although only a few
examples of ratiometric Hg2+ detectors have been reported,
this method is gaining attention in the heavy-metal sensing
community.4 As far as the receptor unit of a Hg2+ fluorescent
chemosensor is concerned, a couple of heteroatoms (N, O, S,
et al.) are usually involved in order to afford high association
constants. To date the detection of Hg2+ ions with a single
heteroatom has rarely been reported due to the poor binding
ability and the lack of selectivity.5
We initially investigated their fluorescence emission changes
toward different metal ions. The metal ion detection procedures
were carried out in THF–MeOH mixtures, with TFOH/TFN
at a concentration of 5 mM in THF, and all metal ions
dissolved in MeOH at a concentration of 5 mM. Both
terfluorenes show distinct low energy emission around
500 nm, and quite weak high energy emission around 410 nm.
Of the metal ions investigated, both terfluorenes show unexpected
high selectivity toward Hg2+. Intuitively, their emission colors
change significantly from green to bright blue upon addition
of Hg2+, while the other metal ions have almost negligible
effect on the fluorescence change, except that Cu2+ only
Over the past few years, polyfluorenes as both chemosensors
and biosensors have been widely explored.6 Nevertheless,
oligofluorenes have rarely been applied in the detection area,
despite their unique electronic and photonic properties.6a,7 We
have recently reported a series of novel terfluorenes exhibiting
dual fluorescence emissions, which can be assigned to a p–p*
transition from the fluorene chain and a new intramolecular
through-space charge transfer emission (spiroconjugation-like
charge transfer) from the pendent aminophenyl group to the
fluorene chain. Furthermore, the dual emissions of terfluorene
derivatives could be facilely tuned by the electronic nature of
pendent aminophenyl groups.8 This may provide a good
model to develop ratiometric fluorescent chemosensors.9
Department of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan 430072,
P. R. China. E-mail: clyang@whu.edu.cn
w Electronic supplementary information (ESI) available: The synthesis
and characterization, UV-vis spectra, Job’s plots and non-linear
fitting. See DOI: 10.1039/c0cc02957b
Scheme 1 Chemical structures of TFOH and TFN.
Chem. Commun., 2010, 46, 8755–8757 8755
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TFOH and 294 fold for TFN. According to a non-linear
fitting equation,11 the binding constants are determined to
be 3.3 ꢀ 104 Mꢁ1 for TFOH–Hg2+, and 4.3 ꢀ 105 Mꢁ1 for
TFN–Hg2+. It is noteworthy that the simple receptor unit,
especially only N atoms for TFN, exhibits high sensitivity and
selectivity toward Hg2+
.
We also tested the interference of other metal ions toward
Hg2+ detection (Fig. S4 in ESI). The intensities of blue
emission only slightly increase when the examined metal ions
(Fe3+, Co2+, Zn2+, Ag+, K+, Pb2+, Cu2+, Mg2+, and Li+)
are added, however a prominent increase of the blue emission
is observed when Hg2+ is added to the above solution. The
result indicates that the two terfluorenes are able to detect
Hg2+ with the coexistence of other usual transition metal ions.
The recognition process of Hg2+ is reversible, as the
subsequent addition of an excess of EDTA aqueous solution
can fully recover the original green emission of TFOH (Fig. 3).
The same is true for TFN (Fig. S5 in ESI).
Fig. 1 (a) Fluorescence responses of TFOH (5 mM in THF) toward
different metal ions (5 mM in MeOH, each metal ion was of 50 equiv
of TFOH). (b) Fluorescence intensity ratio of 400 nm versus 494 nm
upon addition of 25 equiv different metal ions.
In view of the chemical structure of TFOH and TFN, as
well as their exceptional emission behaviours, we may deduce
that the coordination of Hg2+ with the N,N0-di(2-hydroxy-
ethyl)amino or N,N0-diethylamino groups weakens the electron-
donating ability of nitrogen atoms, thus the spiroconjugation-like
charge transfer process leading to green emission can be
suppressed. To further understand the complexation modes,
causes a small enhancement of the blue emission (Fig. 1 for
TFOH and Fig. S3 for TFN).
Fluorescence titration experiments of the two terfluorenes
toward Hg2+ are shown in Fig. 2. For both TFOH and TFN,
the intensities of green emission bands gradually decrease,
while those of the blue emission bands strikingly increase as
the concentration of Hg2+ ions grows. A comparison of the
1
we investigated the H NMR spectra of TFOH and TFN in
the absence and presence of Hg2+ (Fig. 4). After addition of
Hg2+ into TFOH in CDCl3 : CD3CN (v : v/1 : 3) mixture, the
resonances of the eight aromatic protons at 7.13 and 6.61 ppm
assigned to the pendent phenyl exhibit significant downfield
shifts, where they are merged with the protons of fluorene.
Evidently, the coordination of nitrogen with Hg2+ lowers the
electron density of the benzene rings, which causes a deshielding
effect. In addition, the resonances of the protons between
3.6 and 3.4 ppm assigned to two methene on the N,N0-
di(2-hydroxyethyl)amino groups become broadened. Similar
I
400/I494 ratio before and after the addition of Hg2+ gives an
approximate 205 fold ratiometric fluorescence change for
1
changes of H NMR are also observed for TFN before and
after the addition of Hg2+ (Fig. 4), indicating that a sole
nitrogen atom on the pendent groups can also bind Hg2+ ions.
Fig. 3 Fluorescence reversibility of TFOH (5 mM in THF) on
detection of Hg2+ (5 mM in MeOH). Inset shows the visual fluorescence
change of TFOH after the addition of Hg2+, and sequential addition
of EDTA aqueous solution, respectively. The photos were taken
under a handheld UV (365 nm) lamp as soon as the substances were
added.
Fig. 2 Fluorescence spectra of TFOH and TFN (5 mM in THF) in the
presence of Hg2+. lex = 350 nm. Inset shows the visual fluorescence
change of TFOH after the addition of 25 equiv Hg2+
.
c
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of trifluorenes in order to detect metal ions in aqueous systems
and biological contexts is under way.
We are grateful for financial support from the National
Natural Science Foundation of China (No. 20874077) and the
National Basic Research Program of China (973 Program-
2009CB623602) and the Fundamental Research Funds for the
Central Universities of China.
Notes and references
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In conclusion, we have successfully utilized the specific
spiroconjugation-like charge transfer to design new ratio-
metric fluorescent chemosensors for Hg2+ with high sensitivity
and selectivity. The monitoring event could facilely be
observed under UV light irradiation, as the emission color
changes intuitively from weak green to bright blue. We note
that the detection of Hg2+ ions is realized by only a single
nitrogen atom as receptor unit. Most previous fluorescence
sensing mechanisms for metal ions include photoinduced
electron transfer (PET), intramolecular charge transfer
(ICT), fluorescence resonance energy transfer (FRET), and
the heavy atom quenching effect. In this work, the metal-
coordination-inhibited spiroconjugation-like charge transfer
emission presents a new methodology for designing fluorescence
sensors. On the other hand, the sensory mechanism also
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Chem. Commun., 2010, 46, 8755–8757 8757