Superaromatic terpyridines based on corannulene responsive to metal ions

Article abstract of DOI:10.1039/c3dt52013g

Two superaromatic terpyridine ligands (1 and 2) incorporating a corannulene unit at the 4′-position are reported. The optical and metal sensing properties of both ligands were investigated by the naked eye, and UV-vis and fluorescence spectroscopy in this work. In 1, the corannulene motif is directly connected to the 4′-phenylterpyridine domain, while in 2, the corannulene motif and the 4′-phenylterpyridine domain are separated by an acetylene linker. Both 1 and 2 can work as chemosensors for metal ions and display different optical responses to various metal ions. It is shown that both ligands exhibit a colorimetric sensing ability for Fe²⁺ through an obvious color change from colorless to magenta, and this color change can be observed easily by the naked eye. The addition of Fe²⁺ also leads to significant changes in the absorption spectra of the ligands. A characteristic red shift in the emission spectra is observed in the presence of Zn ²⁺, which facilitates the discrimination of Zn²⁺ from other metal ions. In addition, density functional theory (DFT) and time-dependent-density functional theory (TD-DFT) calculations were performed and shown to be consistent with the observed experimental results. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3dt52013g

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Superaromatic terpyridines based on corannulene
responsive to metal ions†
Cite this: Dalton Trans., 2014, 43,
1753
Difeng Wu, Tao Shao, Jian Men, Xiaochuan Chen and Guowei Gao*
Two superaromatic terpyridine ligands (1 and 2) incorporating a corannulene unit at the 4’-position are
reported. The optical and metal sensing properties of both ligands were investigated by the naked eye,
and UV-vis and fluorescence spectroscopy in this work. In 1, the corannulene motif is directly connected
to the 4’-phenylterpyridine domain, while in 2, the corannulene motif and the 4’-phenylterpyridine
domain are separated by an acetylene linker. Both 1 and 2 can work as chemosensors for metal ions and
display different optical responses to various metal ions. It is shown that both ligands exhibit a colorimetric
sensing ability for Fe2+ through an obvious color change from colorless to magenta, and this color
change can be observed easily by the naked eye. The addition of Fe2+ also leads to significant changes in
the absorption spectra of the ligands. A characteristic red shift in the emission spectra is observed in the
presence of Zn2+, which facilitates the discrimination of Zn2+ from other metal ions. In addition, density
functional theory (DFT) and time-dependent-density functional theory (TD-DFT) calculations were per-
formed and shown to be consistent with the observed experimental results.
Received 24th July 2013,
Accepted 8th October 2013
DOI: 10.1039/c3dt52013g
www.rsc.org/dalton
been designed and synthesized for use as chemosensors.12–14
Polycyclic aromatic hydrocarbons (PAHs) are ideal candidates
Introduction
In the past decade, the development of chemosensors with to be used as substituents on terpyridine due to their unique
high selectivity and sensitivity has been a field of great interest electronic and optoelectronic properties. Hexa-peri-hexabenzo-
in medical, environmental and biological applications.1–3 coronene (HBC), the largest aromatic PAH, was coupled to a
Designing and synthesizing chemosensors that can detect terpyridine unit at the 4′-position and the resulting terpyri-
various metal ions has attracted huge attention from dines displayed intriguing photophysical properties.15
chemists.4,5 Fe2+ and Zn2+ play vital roles in many biological However, other PAHs have rarely been substituted onto terpyri-
processes, such as oxygen transport and metabolism, electron dine until now.
transfer and enzymatic reactions with the mitochondrial
Corannulene is composed of five benzene rings fused to a
respiratory chain. However, disruptions in the normally strict central five-membered cycle and has a bowl-shaped π-conju-
regulation of these ions are responsible for severe systemic gated system, adding one more dimension to the chemistry of
disorders, such as anemia, hemochromatosis, Alzheimer’s the usually planar PAHs.16,17 Due to their distinct properties,
disease and Parkinson’s disease.1,6,7 Thus, the development of corannulene and its derivatives can be considered as promis-
an efficient chemosensor for these cations is important for ing building blocks for the construction of functional
fundamental research and biological applications.
materials.18–24 For instance, corannulene derivatives have been
It is well known that terpyridine has a strong and directed shown to emit blue light and are of interest in the fabrication
metal coordination capacity.8,9 However, terpyridine itself has of light emitting diodes.24 Lithium can be stored/intercalated
not been considered for use as a chemosensor, possibly owing between corannulenes, yielding a promising anode material
to an expected lack of selectivity (given the diverse range of for batteries.21 Liquid crystalline phases that respond to an
metal ions it is able to bind) and perhaps also due to its low electric field have also been created using corannulenes.22
emission properties.10,11 Consequently, a number of functiona- Recently, corannulene connecting to triphenylamine was used
lized terpyridines (especially 4′-substituted terpyridines) have as a chemosensor for detecting TNT, which opened up a new
area in the applications of corannulene chemistry.18 However,
other corannulene derivatives have rarely been studied as
chemosensors, especially for metal ions. Herein, we reported
the synthesis and characterization of two superaromatic terpyri-
College of Chemistry, Sichuan University, Chengdu, China.
E-mail: gaoguowei@scu.edu.cn; Fax: +86 28 85412095; Tel: +86 28 85412095
†Electronic supplementary information (ESI) available. See DOI:
dine ligands (1 and 2) incorporating corannulene units at the
10.1039/c3dt52013g
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4′-position. The photophysical properties of the ligands and PdCl2(PPh3)4 and CuI as catalyst to afford ligand 1 in moder-
their potential applications as chemosensors for metal ions ated yield.
have been investigated.
In the case of 2, 4-bromobenzaldehyde was reacted with
ethylene glycol employing p-toluenesulfonic acid as catalyst to
give the ethylene ketal 5, which was converted into the aryl
lithium and then reacted with triisopropyl borate followed by
acid hydrolysis to afford 6. The Kröhnke condensation reaction
of 6 with 2-acetylpyridine afforded the important terpyridine
precursor 7, used to generate the target ligand 2 via Suzuki
coupling with bromocorannulene.
Results and discussion
Synthesis
By far, the most common approach for the construction of
4′-substituted terpyridine is based on the so-called Kröhnke-type
condensation reaction.25,26 In addition, cross-coupling reac-
tions such as the Sonogashira and Suzuki coupling reactions
are the most versatile tools for the construction of π-conju-
gated molecules containing two different units connected via
acetylene or phenylene linkers.27,28 Accordingly, the synthesis
of the ligands (1 and 2) incorporating corannulene units at the
The title compounds were initially characterized on the
1
basis of H, 13C, elemental and HRMS analysis, which firmly
established the structures. The detailed synthetic procedures
and spectroscopic data are available in the ESI.†
UV-vis absorption spectroscopy
4′-position is illustrated in Scheme 1. 4-Bromobenzaldehyde The UV-vis absorption studies of 1 and 2 were carried out in a
was employed as the starting material for both ligands. The THF–CH3CN (v : v = 1 : 19) solution at a moderate concen-
important precursor bromocorannulene was synthesized via tration (1.0 × 10−5 mol L−1). As shown in Fig. 1 and Table 1,
bromination of corannulene, which was prepared by the solu- compound 2 exhibited a strong absorption band at 282 nm
tion phase synthesis.29,30
(ε = 3.74 × 104 L mol−1 cm−1) and a shoulder band at 295 nm
For 1, in the presence of NaOH and concentrated NH4OH, (ε = 3.23 × 104 L mol−1 cm−1), which are assigned to the π–π*
4-bromobenzaldehyde was reacted with 2-acetylpyridine to transition within the compound. For compound 1, π–π* bands
afford 3. In order to generate the acetylene-terminated 4, 3 was at 284 nm (ε = 4.23 × 104 L mol−1 cm−1) were also observed,
reacted with 2-methyl-3-butyn-2-ol via the Sonogashira coup- but the corresponding shoulder band increased and was red-
ling reaction, followed by a base-promoted deprotection. 4 was shifted to 303 nm (ε = 4.53 × 104 L mol−1 cm−1). This could be
isolated in 72% yield following two-step column chromato- attributed to the fact that the introduction of acetylene linkage
graphy. The final transformation also employed Sonogashira extended the molecular conjugation and reduced the flexibility
coupling between 4 and bromocorannulene in the presence of of the single bond connection, resulting in a more delocalized
Scheme 1 Synthetic route to 1 and 2. Reagents and conditions: (i) NaOH, NH4OH, EtOH, reflux, 4 h, 60%; (ii) (a) 2-methyl-3-butyn-2-ol,
PdCl2(PPh3)2, PPh3, CuI, Et3N, THF, reflux, 12 h, (b) KOH, toluene, reflux, 1 h, 72% overall; (iii) PdCl2(PPh3)2, PPh3, CuI, Et3N, reflux, 24 h, 55%;
(iv) TsOH, toluene, reflux, 24 h, 86%; (v) (a) n-BuLi, B(OiPr)3, THF, −78 °C to r.t. overnight, (b) 3 N HCl, THF, r.t., 2 h, 71% overall; (vi) (a) NaOH, EtOH,
25 °C, 7 h, (b) NH4OH, 65 °C, 12 h, 72% overall; (vii) Pd(PPh3)4, Na2CO3, toluene, CH3OH, reflux, 24 h, 64%.
1754 | Dalton Trans., 2014, 43, 1753–1761
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THF–CH3CN (v : v = 1 : 19) by comparison with a solution of
corannulene (Φ = 0.07).31 As shown in Table 1, the quantum
yield of 1 was found to be approximately 0.19, which is higher
than that of 2 (ca. 0.16). This can be attributed to the presence
of the acetylene bridge which facilitates electron and energy
transfer, resulting in a more efficient internal conversion
process in 1.15
Cation-sensing studies
As terpyridine is a well-known metal chelating unit, we
Fig. 1 UV-vis absorption spectroscopy of 1 and 2 (10 μM) in THF– wondered if 1 and 2 could be used as a chemosensor for metal
CH3CN (v : v = 1 : 19) solution.
ions. The chemosensor behavior of 1 and 2 with a variety of
metal cations (Na+, K+, Mg2+, Ba2+, Al3+, Mn2+, Fe2+, Fe3+, Co2+,
Ni2+, Zn2+ and Cd2+) were investigated by visual examination,
UV-vis and fluorescence measurements.
Table 1 UV-vis absorption, emission and quantum yields for 1 and 2a
Entry
Absorption λmax/nm (log ε)
Emission Imaxb/nm
Φc
Colorimetric signaling. The sensing ability of the metallo-
receptors towards various cations was studied qualitatively by
visual examination of the cation-induced color and the fluo-
rescence changes of the receptors in THF solution. As exempli-
fied in Fig. 3, upon the addition of one equivalent of Fe2+ ions
to the solution of 1, the color in daylight (Fig. 3a) of the solu-
tions changes drastically from colorless to purple. In contrast,
the addition of other metal cations has little effect on the
color of the solution of 1. A similar phenomenon is also
observed for 2 (Fig. 3c). It is important to note that color
change is one of the most convenient visual detection
methods used in classical chemical analysis, as well as being
straight-forward and inexpensive. The results indicate that
both 1 and 2 can serve as sensitive “naked-eye” indicators for
Fe2+ ions.
1
2
284 (4.63), 303 (4.66), 364 (4.29)
282 (4.57), 295 (4.51)
416, 437
437
0.19
0.16
a Measurements recorded in THF–CH3CN (v : v
= 1 : 19) solution.
b Excitation wavelengths were fixed at 300 nm for 1 and 295 nm for 2.
c Fluorescence quantum yields were measured relative to corannulene,
Φ = 0.07.
π system. In addition, there is a new shoulder band at 364 nm
(ε = 1.97 × 104 L mol−1 cm−1), which is not observed in the
spectrum of 2.
Emission spectroscopy
Luminescence spectra were recorded for 1 and 2 in a THF–
CH3CN (v : v = 1 : 19) solution at a concentration of 1.0 × 10−5
mol L−1. For both compounds, their spectrum consists of a
characteristic broad band at ∼400–600 nm (shown in Fig. 2
and Table 1).
For 1, the most intense peak within this band occurs at
437 nm, but another peak at 416 nm also dominates the spec-
trum. In the case of 2, the most intense peak within the emis-
sion band is only observed at 437 nm, which is relatively less
intense as compared to the corresponding peaks in 1. The
fluorescence quantum yields of 1 and 2 were measured in
In addition, chelation experiments showed that the
response is irreversible. The addition of the well-known cation
chelator, ethylenediamine tetraacetic acid (EDTA), did not
recover a colorless solution of the ligands, indicating the
strong coordination ability of these ligands to Fe2+.
Fig. 3 Photographs of 1 and 2 in THF solution (200 μM) in the presence
of different metal cations (1.0 eq.): (a) daylight for 1; (b) UV light for 1;
(c) daylight for 2; (d) UV light for 2.
Fig. 2 Emission spectroscopy of 1 and 2 (10 μM) in a THF–CH3CN
(v : v = 1 : 19) solution (λex = 300 nm for 1 and 295 nm for 2).
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Under UV illumination (Fig. 3b and 3d), both 1 and 2 addition of other metal ions such as Na+, K+, Mg2+, Ba2+, Al3+,
exhibit cyan fluorescence in THF solution. The presence of Mn2+, Fe3+, Co2+, Ni2+, Zn2+ and Cd2+ cannot produce the sig-
Zn2+ slightly enhances the fluorescence of the solution. nificant changes in the UV-vis spectra observed for the solu-
However, the group VIIIB metal ions (Fe2+, Fe3+, Co2+ and Ni2+) tion of Fe2+ ions and the ligands. This indicates that both
and Mn2+ ions are found to efficiently quench the emission. terpyridine ligands have high selectivity for Fe2+ over other
The dramatic cation-specific response makes the receptors metal ions. Therefore, they both have potential as Fe2+ chemo-
especially effective colorimetric cation sensors.
sensors through UV-vis spectrometry.
Absorption signaling. The sensing of the receptors towards
To further determine the stoichiometry and binding ability
various cations in THF–CH3CN (v : v = 1 : 19) was studied of our chemosensors toward Fe2+, titration experiments were
through UV-vis spectroscopy. As shown in Fig. 4, the addition performed as shown in Fig. 5. Upon the addition of 0–1.5 eq.
of one equivalent of Na+, K+, Mg2+, Ba2+ and Al3+ ions to the of Fe2+ cations to 1, the absorption intensity of the π–π* bands
solution of 1 induced little change in the absorption spectra of at 303 nm gradually weakened while simultaneously, the
1 and 2. In contrast, when exposed to Mn2+, Co2+, Ni2+, Zn2+ absorption shoulder bands around 373 nm and the MLCT
and Cd2+ ions, the absorbance intensity at 303 nm decreased band at 574 nm increased progressively, resulting in an iso-
and simultaneously, the absorption shoulder bands around sbestic point at 322 nm. The variation of A574 nm/A303 nm with
373 nm increased. Interestingly, the addition of Fe2+ induced the molar fraction of Fe2+ (Fig. 5, top, inset) indicated that the
the appearance of a metal-to-ligand charge transfer (MLCT) binding of 1 with Fe2+ showed a 1 : 1 stoichiometry. In the case
band centered at 574 nm due to the Fe2+ ion coordination to of 2, upon the addition of Fe2+, two new bands centered at
1.32 In the case of 2, the addition of Na+, K+, Mg2+, Ba2+ and 571 nm and 322 nm appeared while the band at 295 nm dis-
Al3+ ions induced the increase or decrease of the absorbance appeared gradually. The ratio of A571 nm/A303
against the
nm
at 295 nm with no appearance of any additional bands. Other molar fraction of Fe2+ (Fig. 5, bottom, inset) also indicated
metal ions apart from Fe2+ not only decreased the absorbance that the binding of 2 with Fe2+ showed a 1 : 1 stoichiometry.
intensity at 295 nm, but also exhibited absorption shoulder
The binding stoichiometry of 1 and 2 with Fe2+ ions was
bands around 326 nm. In contrast, the addition of Fe2+ led to further estimated using the Benesi–Hildebrand equation,33
significant changes in the absorption spectra. An absorption
peak at 571 nm appeared upon the addition of Fe2+. The above
results reveal the unique absorbance featured by both ligands
which is induced by coordination with Fe2+, giving rise to the
solution color change from colorless to purple. Moreover, the
n
1=ðA ꢀ A0Þ ¼ 1=ðA ꢀ A Þ ꢁ ½1=K½Fe2þꢂ þ 1ꢂ
ð1Þ
1
Fig. 5 Changes in the UV-vis absorption spectra of 1 and 2 in a THF–
acetonitrile (v : v = 1 : 19) solution (10 μM) upon titration (0–15 μM) of
Fe2+. Inset: titration curve of 1 and 2 with Fe2+ (the absorbance ratio at
574 nm and 303 nm for 1 and at 571 nm and 295 nm for 2).
Fig. 4 Absorption spectra of 1 and 2 (10 μM) in THF–CH3CN (v : v =
1 : 19) induced by various metal cations (1.0 eq.).
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Table 2 The association constants of Fe2+, Zn2+ and Cd2+ to 1 and 2,
calculated using the Benesi–Hildebrand equation
1
2
Entry
λa (nm)
303
K (M−1
)
λa (nm)
295
K (M−1
)
Fe2+
Zn2+
Cd2+
4.73 × 104
8.92 × 103
3.89 × 103
1.42 × 105
1.50 × 104
5.16 × 103
a The most changed wavelength selected for the Benesi–Hildebrand
plot.
Fig. 6 Time course of the absorption response of 1 and 2 (10 μM) upon
the addition of 1.0 equiv. of Fe2+ in a THF–acetonitrile (v : v = 1 : 19) solution.
where A0 is the absorbance of free 1 or 2, A∞ is the absorbance
measured with excess amounts of Fe2+, A is the absorbance
measured with Fe2+, n represents the binding stoichiometry of
Fe2+ to 1 or 2, K is the association constant and [Fe2+] is the
concentration of Fe2+.
As shown in Fig. S4.1,† the plot of 1/(A − A0) against 1/[Fe2+]
shows a linear relationship (R = 0.9959), indicating that 1
associates with Fe2+ in a 1 : 1 stoichiometry. The association
constant, K, between 1 and Fe2+, was determined from the
ratio of the intercept to the slope to be 4.73 × 104 M−1. Similar
phenomena were observed in the association of 2 with Fe2+.
The binding constant was estimated as 1.42 × 105 M−1, which
was larger than the value for 1. This can be attributed to the
fact that the introduction of acetylene linkage enhances the
conjugation of the molecule and thus reduces the coordi-
nation ability of the N atom of terpyridine in 1.15,34
These observations were reinforced by the results of the
addition of Zn2+ or Cd2+ to the solution of our ligands
(Fig. S4.2, S4.3† and Table 2). The association constants of
Zn2+ and Cd2+ to 1 were calculated as 8.92 × 103 M−1 and 3.89 ×
103 M−1, which are smaller than the values of Zn2+ (1.50 ×
104 M−1) and Cd2+ (5.16 × 103 M−1) to 2. The results indicate
that the coordination ability of terpyridine in 2 is stronger
than that in 1. It is worthy of note that the conjugated size of
the substituent group at the 4′-position of terpyridine plays
a vital role in the coordination ability of the ligands.
Therefore, the introduction of various PAHs with different con-
jugated sizes could be used for regulating the binding ability
of terpyridine, which achieves the aim of enhancing the
selectivity and sensitivity of terpyridine sensors for specific
metal ions.
Fig. 7 Fluorescence emission spectra of 1 and 2 (10 μM) upon the
addition of various metal ions (1.0 equiv.) in THF–acetonitrile (v : v =
1 : 19) solution (λex = 300 nm for 1 and 295 nm for 2).
The detection limit is one of the most important para-
meters in cation sensing for practical purposes. Therefore, the
limitations of detection (LOD) of our ligands for Fe2+ were
obtained according to the equation: detection limit = 3σ/slope,
where σ is the standard deviation of blank measurements,
slope is the slope between intensity versus sample concen-
tration.35 The LOD for Fe2+ were measured as 1.1 × 10−7 M and
8.9 × 10−8 M for 1 and 2, respectively (Fig. S5.1 and S5.2†).
These data indicate that our ligands show high sensitivity
toward Fe2+.
function of time (Fig. 6), which indicate that rapid response
within a few seconds was observed for 1 and 2.
Fluorescence signalling. The fluorescence properties of 1
and 2 upon the addition of several metal ions were also investi-
gated by fluorescence spectroscopic measurements and titra-
tion studies. Fig. 7 shows the fluorescence response of 1 and 2
towards various metal ions in THF–acetonitrile (v : v = 1 : 19)
solution at λex = 300 nm for 1 and λex = 295 nm for 2. In the
case of 1, the addition of Na+, K+, Mg2+, Ba2+, Al3+ and Fe3+
induced a very minor decrease in the emission band. In
the presence of Fe2+, the fluorescence decreased obviously,
accompanied by the emission band red-shifting to 488 nm.
Additionally, the presence of Ni2+, Co2+ and Mn2+ quenched
Response time is also an important practical parameter for
chemosensors. After the addition of 1.0 equiv. of Fe2+ to the
solutions of the ligands, absorption changes were tracked as a
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the fluorescence almost completely. Interestingly, when Zn2+
ions were added to the solution of 1, the fluorescence
spectrum of the complex showed a significant bathochromic
shift. In the case of 2, Na+, K+, Mg2+, Ba2+ and Al3+ could not
induce any changes in the emission bands, while the fluo-
rescence was quenched to some extent in the presence of Fe3+
ions. Fe2+, Ni2+, Co2+ and Mn2+ were found to efficiently
quench the emission of 2. When exposed to Zn2+, ligand 2 also
showed a significant bathochromic emission shift.
The above results reveal the unique fluorescence features of
both ligands induced by the coordination with Zn2+, implying
that our ligands could be used as fluorescence sensors for
Zn2+. However, one of the most important aspects that should
be explored for a sensor is its ability to detect a specific cation
in the presence of competing ions. In this case, the selectivity
of both ligands for Zn2+ over other metal ions was revealed by
measuring the fluorescence spectra of both ligands with Zn2+
in the presence of other competitive cations. As shown in
Fig. S6,† 2.0 equiv. of competitive cations were added to the
complex of 1 or 2 with Zn2+ and the fluorescence response was
detected. All the cations except Co2+ exhibit only small or no
interference with the affinity of our ligands with Zn2+. In con-
trast, Co2+ ions can almost completely quench fluorescence
emission due to the strong affinity of our ligands and Co2+.
These results confirm that our ligands show a good specificity
toward Zn2+ and hence could be used to detect Zn2+ ion in a
complicated matrix.
Fig. 8 Fluorescence emission spectra of 1 and 2 (10 μM) in THF–aceto-
nitrile (v : v = 1 : 19) solution upon titration of Zn2+ (0–15 μM). Inset: titra-
tion curve of 1 and 2 with Zn2+ (intensity ratio at 495 nm and 437 nm for
1; ratio at 500 nm and 437 nm for 2).
To further determine the stoichiometry and detectability of
our chemosensors toward Zn2+, detailed investigations were
carried out and the results are displayed in Fig. 8. With the
addition of Zn2+ to 1 (Fig. 8, top), the intensity of the emission
band peaks at 416 nm and 437 nm decreases gradually, while
a new emission band emerges with a λmax located at 495 nm.
As a result, an isoemissive point at 459 nm can be found. The
intensity ratio at 495 nm and 437 nm increases with the incre-
mental addition of Zn2+ until [Zn2+]/[1] reaches 1.0. Even
higher concentrations of Zn2+ do not lead to any further
change. This indicates that the 1–Zn2+ complex is formed in
approximately 1 : 1 stoichiometry. The fluorimetric limitation
of detection (LOD) for 1 with Zn2+ is determined to be ∼1.6 ×
10−8 M (Fig. S5.3†).35 A similar situation is found in the
addition of Zn2+ to 2 (Fig. 8, bottom). The fluorimetric LOD for
2 with Zn2+ is determined to be ∼2.0 × 10−8 M (Fig. S5.4†). The
addition of increasing amounts of Zn2+ to 2 induces a red-shift
of 63 nm in the emission peak from 437 to 500 nm. The inten-
sity ratio at 500 and 437 nm (I500 nm/I437 nm) increases from
0.27 to 3.09 and the stoichiometry of the coordinating species
Fig. 9 Time course of the fluorescence response of 1 and 2 (10 μM)
upon the addition of 1 equiv. of Zn2+ in THF–acetonitrile (v : v = 1 : 19)
solution.
is monitored, is highly desirable in practical sensors, in order
to cancel out variations in intensity arising from background
changes in the analytical environment. In addition, the
response time of our ligands for Zn2+ was also studied. As
shown in Fig. 9, only 5 s was required for 1 to reach 90% of the
saturation fluorescence, and for 2, the fluorescence intensity
reached 90% of the saturation value within 20 s.
Cation sensing in aqueous CH3CN–THF solution. The
sensing of cations in aqueous media or water–organic mixed
solvents is important for practical applications.36,37 Therefore,
we studied the effect of the content of the aqueous 4-(2-
hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer
is also found to be 1 : 1 according to the variation of I500 nm
I437 nm with the molar fraction of Zn2+. In addition, Job’s plot
analysis also demonstrates 1 : 1 binding stoichiometry
/
a
between our ligands and Zn2+ (Fig. S7†). That the binding of
Zn2+ to ligands leads to red-shifts is not surprising, since the
positive charge on the metal ion will promote the intramolecu-
lar charge transfer (ICT) process from pendant corannulene to
terpyridine.10,11 It is worth noting that such ratiometric fluori-
metry, where the ratio of signals at two emission wavelengths
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(30 mM, pH = 7.4) in CH3CN–THF solution on the colorimetric central pyridine ring (ring 3) are twisted about 30°, which can
response of our ligands toward ferrous ion. As shown in be ascribed to the unfavourable interaction between the adja-
Fig. S8.1,† when the water content was increased up to a cent protons. Due to the introduction of acetylene linkage in 1,
20–30% volume ratio, the purple color of the solution was still the phenyl ring in corannulene (ring 1) is calculated to be
clearly visible, indicating that our ligands acted as effective coplanar with the connecting phenyl ring (ring 2). In contrast,
chemosensors toward the detection of Fe2+ in the aqueous ring 1 in 2 is twisted about 40° with respect to ring 2. This
CH3CN–THF solution. Therefore, HEPES buffer–CH3CN–THF indicates that the introduction of acetylene linkage has lead to
(4 : 15 : 1, v : v : v, pH 7.4) was selected as the water–organic improvement in the planarity of the ligands.
mixed solvent to study the chemosensor behavior of our
A schematic representation of the molecular orbitals for 1
ligands. In the case of 1, when 1.0 equiv. of various cations and 2 is reported in Fig. 10. In the case of 2, the electron
was added, only Fe2+ triggered a new peak at 574 nm in the clouds in the terpyridine unit are mainly concentrated on the
absorption spectra while the other cations induced no changes central pyridine ring and partly localized on the side pyridine
at this wavelength, indicating that ligand 1 can be established rings. However, in 1, the electron clouds are only localized on
as a highly sensitive colorimetric sensor for Fe2+ in water– the central pyridine ring. In other words, the electron clouds
organic mixed solvents (Fig. S8.2†). Changes in the lumine- of the terpyridine unit in 1 are found to be smaller and more
scence spectrum (λex = 300 nm) of 1 (10 μM) in a HEPES shifted towards the corannulene domain in comparison with
buffer–CH3CN–THF (4 : 15 : 1, v : v : v, pH 7.4) solution upon that of ligand 2. Therefore, the coordination ability of terpyri-
the addition of different cations showed that only Zn2+ could dine in 2 is stronger than that in 1, which is in line with the
induce a significant bathochromic shift of about 60 nm, imply- optical titration results. As shown in Fig. 11, the energy gap
ing that ligand 1 can act as a highly sensitive fluorescent Zn2+ (ΔE) between the highest occupied molecular orbital (HOMO)
probe in aqueous CH3CN–THF solution (Fig. S8.3†). However, and the lowest unoccupied molecular orbital (LUMO) for both
competition experiments showed that the sensing properties ligands was also obtained via chemical calculations. The ΔE
of 1 for Fe2+ and Zn2+ encountered interference by Co2+, which value of 1 is 0.43 eV lower than that of 2, which can be
was in line with the results observed in organic solvents. explained in that the maximum absorption wavelength of
Similar measurements for 2 also showed that ligand 2 has a ligand 1 is found to be bathochromic with respect to ligand 2.
good selectivity toward Fe2+ and Zn2+ in aqueous CH3CN–THF
TD-DFT calculations were also carried out at the B3LYP
solution (Fig. S8.2 and S8.3†). Accordingly, our ligands can be level of theory with a 6-31G* basis set. Calculated wavelengths
used as multichannel sensors for multiple target cations in and oscillator frequencies ( f ) are listed in Table 3. According
both organic solvents and water–organic mixed solvents.
to TD-DFT calculations for 1, three significant transitions are
predicted to be observed at 403.74 nm ( f = 1.0101), 383.81 nm
( f = 0.0022) and 366.46 nm ( f = 0.2513). In contrast, only two
significant transitions for 2 are obtained at 361.95 nm ( f =
0.0366) and 351.43 nm ( f = 0.0083). This is in line with the
absorption results that there is one more shoulder band in the
spectrum of ligand 1 than that of ligand 2. The calculated
wavelengths mismatch with the observed bands in the experi-
mental absorption spectra. This could be ascribed to the
solvent effect and intermolecular bowl-to-bowl interactions
which are not accounted for by the TD-DFT B3LYP level of
theory. Despite all this, the calculated maximum wavelength of
DFT studies
To gain insight into the geometry and optical properties of
ligands 1 and 2 we performed density functional theory (DFT)
and time-dependent-density functional theory (TD-DFT) calcu-
lations on the geometrical configuration and absorption spec-
trum of 1 and 2. We employed the B3LYP exchange-correlation
functional with a 6-31G* basis set, as implemented in the
Gaussian 09 suite of programs.38 The B3LYP/6-31G* geometry
optimized structures for 1 and 2 are presented in Fig. 10. In
both structures, the connecting phenyl ring (ring 2) and the
Fig. 10 B3LYP/6-31G* calculated geometries for 1 (top) and 2 (bottom).
Fig. 11 A schematic representation of the molecular orbitals for 1 and 2.
Dalton Trans., 2014, 43, 1753–1761 | 1759
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Table 3 Calculated TD-DFT wavelengths (nm) and oscillator strengths
( f ) for 1 and 2
Notes and references
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f
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403.74
383.81
366.46
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0.0366
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Conclusions
In summary, the first two terpyridine ligands based on cor-
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