Steric hindrance-enforced distortion as a general strategy for the design of fluorescence "turn-on" cyanide probes

Article abstract of DOI:10.1039/c3cc46008h

For the rational design of fluorescence "turn-on" cyanide probes, a general strategy is developed by introducing a dicyanovinyl group at the sterically demanding position of a large π framework. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc46008h

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Steric hindrance-enforced distortion as a general
strategy for the design of fluorescence ‘‘turn-on’’
Cite this: DOI: 10.1039/c3cc46008h
cyanide probes†
Received 6th August 2013,
Bin Chen,a Yubin Ding,a Xin Li,b Weihong Zhu,a Jonathan P. Hill,c Katsuhiko Arigac
and Yongshu Xie*a
Accepted 4th September 2013
DOI: 10.1039/c3cc46008h
www.rsc.org/chemcomm
For the rational design of fluorescence ‘‘turn-on’’ cyanide probes, a
general strategy is developed by introducing a dicyanovinyl group
at the sterically demanding position of a large p framework.
Anion recognition chemistry has attracted intensive scrutiny in
recent years.1–3 Among various anions, cyanide poses continuing
environmental contamination problems due to its high toxicity
and widespread applications in industrial processes.4–6 Thus,
effective cyanide probes are highly desired. As a versatile probe
technique, elicitation of fluorescence has the advantages of
simplicity, high sensitivity, and low cost. However, probes that
rely on fluorescence quenching suffer from inherent drawbacks
including low signal-to-noise ratio and non-specific quenching so
that ‘‘turn-on’’ type fluorescence probes are preferred,7,8 and they
have been designed utilizing various binding moieties based on
Fig. 1 (a) Chemical structures of probes C1–C5. (b) Photographs of C1–C5
(40 mM) in CH2Cl2 showing the fluorescence changes upon addition of 3 equiv.
of CNÀ under a portable UV lamp.
various channels such as proton transfer, exciplex, intramolecular plane thus yielding a twisted intramolecular charge transfer
charge transfer (ICT), and metal coordination.9–13 In this respect, (TICT) state and quenching of its fluorescence.16 Furthermore,
the dicyanovinyl (DCV) unit has been widely employed due to the introduction of an additional electron donor into the p-framework
possible nucleophilic addition of CNÀ to the double bond, which may further attenuate fluorescence due to the possible TICT state
results in either enhancement or quenching of fluorescence, associated with the donor.17,18 Accordingly, C1, C2 and C3 were
although the predictability of the effect remains elusive.11a,14 synthesized by introducing DCV to a 9-anthryl moiety, with an
Hence, development of a general strategy towards rational design electron-donating group attached to the 10-position (Fig. 1).
of fluorescence ‘‘turn-on’’ CNÀ probes would be advantageous.
As expected, these molecules are non-planar, exhibiting weak
A good starting point for the rational development of fluorescence fluorescence which decreases in intensity with the increasing
‘‘turn-on’’ probes is the use of nearly non-fluorescent hosts15 donating ability of the donor. Interestingly, the addition of CNÀ
constructed from potential fluorophores. We envisioned that if induced fluorescence enhancement by up to 242-fold. High
DCV is introduced into a sterically demanding framework, such as selectivity and sensitivity to CNÀ can be realized with the
the 9-anthryl moiety, it would be forced to twist out of the anthryl advantages of low background fluorescence, tunable emission
wavelengths (Fig. 1), and excellent ‘‘turn-on’’ behaviour in both
CH2Cl2 and aqueous systems. To the best of our knowledge, this
is the first demonstration of introduction of DCV into a sterically
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science and Technology, Shanghai, P. R. China.
demanding framework opening a promising strategy for the
E-mail: yshxie@ecust.edu.cn; Fax: +86 21-6425-2758; Tel: +86 21-6425-0772
b Department of Theoretical Chemistry and Biology, School of Biotechnology,
rational design of fluorescence ‘‘turn-on’’ CNÀ probes.
KTH Royal Institute of Technology, Stockholm, Sweden
C1–C3 were readily synthesized by reactions between the respec-
tive 9-formyl anthracene derivatives and malononitrile (Scheme S1†),
and fully characterized using 1H-NMR, 13C-NMR, and HRMS
(Fig. S1–S3†). To our satisfaction, they exhibit rather weak fluores-
cence in CH2Cl2 with quantum yields of 0.49%, 0.42%, and 0.065%,
respectively (Table S1†). Thus, we continued to estimate their probe
c WPI-Center for Materials Nanoarchitectonics, National Institute for Materials
Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, Japan
† Electronic supplementary information (ESI) available: Full experimental and
crystallographic data and Fig. S1–S30 and Tables S1–S3. CCDC 923724 and
923725. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc46008h
c
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Fig. 2 (a) Fluorescence changes upon addition of CNÀ to C3 (20 mM, lex
=
Fig. 3 X-ray crystal structures of (a) C2 and (b) C3 showing the twisting
between the anthryl framework and DCV. Optimized planar structure of C4.
(c) Top view and (d) side view.
396 nm) in CH2Cl2. (b) Fluorescence changes with varying viscosity in a mixture of
CH2Cl2–polyetheramine D2000 for C3 (20 mM, lex = 401 nm).
behaviour for CNÀ. For example, in the case of C3 only CNÀ induced
a drastic colour change and an approximately 242-fold fluorescence
enhancement amongst the various anions added to its CH2Cl2
solution (Fig. 2a and Fig. S4a†). Concomitant UV-vis spectral
measurements revealed a clear isosbestic point at 396 nm
(Fig. S4b†). For C1 and C2, addition of CNÀ caused similar UV-vis
spectral changes, with prompt fluorescence enhancements
(Fig. S5–S8†) of approximately 29- and 54-fold, respectively.
For practical applications, it is essential that probes are available
for operation in aqueous systems. Hence, we investigated the probe
behaviour of C1–C3 in a mixed solvent of THF–H2O (4:1 v/v)
(Fig. S9–S11†). Interestingly, similar fluorescence ‘‘turn-on’’
behaviour was observed for C1–C3 in the aqueous systems upon
addition of CNÀ. In addition, the presence of competing anions
did not interfere significantly with CNÀ probing behavior, which
is indicative of the excellent selectivity of these probes. The
detection limit is also a key parameter based on the sensitivity of
anion probes with fluorescence titration measurements yielding
the detection limits19 for C1, C2 and C3 for CNÀ of 2.46, 1.29, and
1.14 mM (Fig. S12†), respectively. The values for C2 and C3 lie well
below the safe CNÀ levels in drinking water set by the World Health
Organization (WHO).20 In addition, the sensing systems can
detect CNÀ in the solvents containing high proportions of water
(Fig. S13†), showing good photostability (Fig. S14†), and they work
well over a pH range of 9–14 (Fig. S15†). These results demonstrate
that these probes are a promising prototype of highly selective and
sensitive fluorescence ‘‘turn-on’’ CNÀ probes.
As expected, the DCV and anthryl units have a small interplanar
angle of 0.61 in the optimized conformation (Fig. 3c and d), adopting
a nearly planar conformation. This conjugated planar conformation
likely suppresses non-radiative energy loss expected due to torsional
rotation. Thus, in contrast to C1–C3, C4 exhibits very strong
fluorescence with a high quantum yield of 64.1%. Moreover, the
fluorescence of C4 was drastically quenched upon addition of
CNÀ (Fig. S20†). The rather strong fluorescence of C4 further
supports the postulation that twisting and rotation of the
9-anthryl and DCV units due to steric crowding is responsible
for the weak fluorescence observed for C1–C3. Additionally, in
more viscous solvents, intramolecular rotation can be suppressed,
and the fluorescence will be recovered. For instance, fluorescences
of C1–C3 were significantly enhanced with increasing solvent
viscosity in a mixed solvent of CH2Cl2 and polyetheramine
D200021 (Fig. 2b, S21–S23†). In contrast, C4 exhibited a totally
different behaviour in that in a CH2Cl2 solution a broad intense
excimer-based emission peak22 was observed at 529 nm, probably
arising from its highly planar structure. With increasing viscosity,
the emission intensity of the 529 nm peak decreased, and peaks
due to monomerically dispersed C4 emerged at 419 and 401 nm
(Fig. S24b†), which is in good agreement with reported systems.23
To demonstrate the general applicability of our above-mentioned
strategy, we introduced DCV into another typical p-conjugated
framework, that of porphyrin, at its sterically demanding meso-
position to afford C5 (Scheme S1 and Fig. S25†). As expected, C5
exhibits weak fluorescence similar to that of C1–C3, with a quantum
yield of 0.53% in CH2Cl2. Its fluorescence intensity was also
significantly enhanced upon addition of CNÀ (Fig. S26†) or with
increasing solvent viscosity (Fig. S27†). These results indicate that
our design strategy is likely to have general applicability. It is
noteworthy that the emission wavelengths of the probe–cyanide
adducts can be modulated from blue to red (Fig. 1) by variation of
the framework in combination with the introduction of different
electron donors. As a matter of fact, the emission in the NIR
wavelength range makes such probes suitable for applications in
living systems.24
In order to understand the interaction of CNÀ with C1–C3, their
addition products were isolated and characterized (Fig. S16–S18†).
In the 1H-NMR spectrum of C3–CN, the vinylic proton signal of C3
at 8.97 ppm disappeared and two new peaks emerged at 6.08 and
5.00 ppm (Fig. S3 and S18†). These results are in agreement with a
nucleophilic addition mechanism.
As previously mentioned, the very weak fluorescence of C1–C3
may be ascribed to energy loss caused by intramolecular rotation
between the DCV and anthryl units in the nonplanar molecular
structures. As clearly evidenced from X-ray single crystal structures
(Fig. 3a and b), the dihedral angles between the DCV and anthryl
units in the molecules of C2 and C3 are 65.61 and 59.51, respectively.
These large dihedral values clearly demonstrate twisting of the
molecules to nonplanar structures, which facilitates intramolecular
rotation and non-radiative relaxation of the excited state, resulting
in the very weak fluorescence observed for C1–C3.
To gain further insights into the weak fluorescence of the
probes, time-dependent density functional theory (TDDFT)
calculations were carried out. In the lowest singlet excited state
(LUMO) of C1 and C2, the anthryl unit is nearly perpendicular
to DCV (Fig. S28 and Table S2†), and the HOMO and LUMO are
localized on these two units, respectively. For C3, the HOMO
is localized on the diphenylamino group, and the LUMO is
localized over the anthryl and DCV units. These results are
To further elucidate the effect of steric hindrance, the control
compound C4 was prepared by introducing DCV to the less sterically
demanding 2-position of anthracene (Scheme S1 and Fig. S19†).
c
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indicative of TICT states,15a,16–18 associated with DCV in C1 and C2,
and with the diphenylamino moiety in C3, leading to the weak and
decreasing fluorescence of C1–C3. Furthermore, their fluorescence
decay curves were observed to be biexponential (Fig. S29†), with the
slow decay component corresponding to the TICT states.25 Upon
addition of CNÀ, the curves became monoexponential (Fig. S29†),
concurrent with the disappearance of the TICT deactivation
pathway, resulting in a decrease in the intensity of the charge-
transfer peak and an increase in the intensity of the locally
excited peak (Fig. S4b, S5a and c, S9a–S11a†) in their absorption
spectra,26 which correspond to the electronic transitions
between HOMO À 1 and LUMO (Fig. S28†). Thus, fluorescence
recovery was observed. In contrast, C4 is nearly planar in both
the ground state and the lowest singlet excited state. Addition of
CNÀ can disrupt the large planar conjugation system, resulting
in a distinct decrease in the excimer-originated fluorescence
(Fig. S20†). Compared with C1–C4, C5 has a relatively larger and
more flexible porphyrin framework, which was also distorted by
the steric hindrance associated with DCV (Fig. S30†), so that weak
fluorescence could be observed. Addition of CNÀ disrupted the
DCV group, leading to recovery of the planarity of the porphyrin
macrocycle (Fig. S30†) resulting in drastic fluorescence enhance-
ment. Meanwhile, the initially biexponential fluorescence decay
curve for the distorted structure of C5 becomes monoexponential
for the planar structure of the cyanide adduct27 (Fig. S29 and
Table S3†). In addition, a sharper and blue-shifted Soret band was
observed upon reaction with CNÀ (Fig. S26a†), indicating that the
porphyrin aromaticity is increased with the improved molecular
planarity although the conjugation size was reduced after disrup-
tion of the DCV group.28
In conclusion, a general strategy for the rational design of
fluorescence ‘‘turn-on’’ cyanide probes has been developed by
introducing a dicyanovinyl unit at a sterically demanding
position of a large p framework. Such probes show very weak
background fluorescence due to the distorted nonplanar molecular
structure and intramolecular rotation with a potential TICT state.
Addition of CNÀ can induce fluorescence enhancement up to a
maximum of 242 fold for C3. By applying our design strategy for
substituted anthracene probes to the porphyrin ring system,
we have demonstrated the potential general applicability for
the development of highly sensitive and selective fluorescence
‘‘turn-on’’ cyanide probes applicable in aqueous systems with low
background fluorescence and tunable emission wavelengths.
Thanks to Prof. He Tian for valuable discussions. This work
was supported by NSFC/China (21072060, 91227201), NCET-11-
0638, SRFDP (20100074110015), the Oriental Scholarship, the
Fundamental Research Funds for the Central Universities
(WK1013002), and World Premier International Research Center
Initiative (WPI Initiative) on Materials Nanoarchitectonics.
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