Two new fluorescent Zn2+ sensors exhibiting different sensing mode with subtle structural changes

Article abstract of DOI:10.1080/10610278.2016.1242730

Two novel receptors HL-1 and HL-2 without and with hydroxyl groups were designed and synthesised. Both receptors showed highly selective coordination towards Zn²⁺ and exhibited diverse sensing behaviour due to the structural variations. HL-1 showed monotonous ‘turn-on’ response towards Zn²⁺ while HL-2 displayed highly Zn²⁺ sensitive ‘turn on’ and ‘ratiometric’ properties. Detailed Job plot experiment, single crystal data, ¹H NMR, ESI-MS, UV–vis and density functional theory calculation studies were conducted to understand the binding modes of HL-1 and HL-2 with Zn²⁺. These results revealed the binding stoichiometric ratio between receptors and Zn²⁺ were 1:1 with low detection limits and high binding constants.

Full text of DOI:10.1080/10610278.2016.1242730

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
2+
Two new fluorescent Zn sensors exhibiting
different sensing mode with subtle structural
changes
Hassan Mehdi, Hongchang Pang, Weitao Gong, Ali Wajahat, Kalavathi
Dhinakaran Manivannan, Shaheen Shah, Jun-wei Ye & Guiling Ning
To cite this article: Hassan Mehdi, Hongchang Pang, Weitao Gong, Ali Wajahat, Kalavathi
Dhinakaran Manivannan, Shaheen Shah, Jun-wei Ye & Guiling Ning (2016): Two new fluorescent
2+
Zn sensors exhibiting different sensing mode with subtle structural changes, Supramolecular
Chemistry, DOI: 10.1080/10610278.2016.1242730
To link to this article: http://dx.doi.org/10.1080/10610278.2016.1242730
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Date: 26 October 2016, At: 02:50

Supramolecular chemiStry, 2016
ꢁꢁꢂ://dx.dꢃꢄ.ꢃꢅg/10.1080/10610278.2016.1242730
ꢀ
2+
Two new fluorescent Zn sensors exhibiting different sensing mode with subtle
structural changes
Hassan Mehdi, Hongchang Pang, Weitao Gong, Ali Wajahat, Kalavathi Dhinakaran Manivannan, Shaheen Shah,
Jun-wei Ye and Guiling Ning
Sꢁꢆꢁꢇ Kꢇꢈ lꢆbꢃꢅꢆꢁꢃꢅꢈ ꢃf Fꢄnꢇ cꢀꢇꢉꢄꢊꢆꢋs, Sꢊꢀꢃꢃꢋ ꢃf cꢀꢇꢉꢄꢊꢆꢋ engꢄnꢇꢇꢅꢄng, Dꢆꢋꢄꢆn unꢄvꢇꢅsꢄꢁꢈ ꢃf tꢇꢊꢀnꢃꢋꢃgꢈ, Dꢆꢋꢄꢆn, p.r. cꢀꢄnꢆ
ABSTRACT
ARTICLE HISTORY
Two novel receptors HL-1 and HL-2 without and with hydroxyl groups were designed and
synthesised. Both receptors showed highly selective coordination towards Zn2 and exhibited
diverse sensing behaviour due to the structural variations. HL-1 showed monotonous ‘turn-
rꢇꢊꢇꢄvꢇd 28 mꢆꢅꢊꢀ 2016
aꢊꢊꢇꢂꢁꢇd 25 Sꢇꢂꢁꢇꢉbꢇꢅ 2016
+
KEYWORDS
2
+
2+
on’ response towards Zn while HL-2 displayed highly Zn sensitive ‘turn on’ and ‘ratiometric’
Fꢋꢌꢃꢅꢇsꢊꢇnꢊꢇ; ꢊꢀꢇꢉꢃsꢇnsꢃꢅ;
ꢅꢇꢊꢃgnꢄꢁꢄꢃn; zꢄnꢊ;
ꢊꢃnfꢃꢅꢉꢆꢁꢄꢃn
1
properties. Detailed Job plot experiment, single crystal data, H NMR, ESI-MS, UV–vis and density
functional theory calculation studies were conducted to understand the binding modes of HL-1 and
2
+
2+
HL-2 with Zn . These results revealed the binding stoichiometric ratio between receptors and Zn
were 1:1 with low detection limits and high binding constants.
Two novel receptors HL-1 and HL-2 without and with hydroxyl groups were designed and
synthesised via TCR reaction. Both of them showed selective coordination to Zn2 ions but exhibited
incongruent sensing mode due to structural variations. HL-1 showed monotonous‘turn-on’whereas
+
2+
HL-2 displayed highly sensitive ‘turn on’and ‘ratiometric’properties towards Zn .
CONTACT Wꢇꢄꢁꢆꢃ Gꢃng
wꢁgꢃng@dꢋꢌꢁ.ꢇdꢌ.ꢊn; Gꢌꢄꢋꢄng Nꢄng
nꢄnggꢋ@dꢋꢌꢁ.ꢇdꢌ.ꢊn
tꢀꢇ sꢌꢂꢂꢋꢇꢉꢇnꢁꢆꢅꢈ ꢄnfꢃꢅꢉꢆꢁꢄꢃn fꢃꢅ ꢁꢀꢄs ꢆꢅꢁꢄꢊꢋꢇ ꢊꢆn bꢇ ꢆꢊꢊꢇssꢇd ꢀꢇꢅꢇ: ꢀꢁꢁꢂ://dx.dꢃꢄ.ꢃꢅg/10.1080/10610278.2016.1242730.
©
2016 infꢃꢅꢉꢆ uK lꢄꢉꢄꢁꢇd, ꢁꢅꢆdꢄng ꢆs tꢆꢈꢋꢃꢅ & Fꢅꢆnꢊꢄs Gꢅꢃꢌꢂ

2
H. MEHDI ET Aꢀ.
1
. Introduction
Design and synthesis of efficient fluorescent sensors for
metal ions, especially transition metal ions, is a vigorous
research area of supramolecular chemistry (1–3). During
2
+
past few years, selective fluorescent sensors for Zn have
attracted great attention, owing to the biological and envi-
ronmental significance (4–6). As a second most abundant
2
+
transition metal in human body, Zn is responsible for
blood sugar (7), epilepsy (8), ischemic stroke (9), abnor-
mal growth (10) and Alzheimer’s disease (11). Furthermore,
2
+
Zn also has an essential role in environmental pollution,
and agricultural disorders (12).
2
+
Although, a variety of Zn fluorescent sensors have
been designed with di-2-picolylamine (DPA) (13), acyclic,
cyclic polyamines, quinoline, bipyridine and Schiff-bases
(14–16). However, most of them suffered from the interfer-
ence of other heavy metal and transition metal ions such
2
+
2+
2+
2+
+
2+
as Fe , Co , Ni , Cu and Hg (17), particularly Cd is in
the same group of the periodic table and displays identi-
2
+
cal properties to Zn (18, 19). Accordingly, development
of new chelators with unique fluorescent selectivity and
2
+
sensitivity towards Zn is still a hot topic in the supramo-
lecular field.
Tandem claisen rearrangement (TCR) reaction devel-
oped by Hiratani, represents a useful strategy to construct
various building blocks for supramolecular chemistry (20)
including macrocycles (21), rotaxanes, catenanes (22) and
helicate-type complexes by the efficient introduction of
multiple hydroxyl groups into the structure (23–26).
By exploiting these building blocks, some admirable
fluorescent chemosensors have been established with
potential application towards anions and rare earth met-
als (27). Moreover, the building blocks before TCR and
after TCR show diverse structural conformations due to
the differences in structural connectivity and existence of
hydroxyl groups. So, based on this efficient phenomenon,
different supramolecular structures can be constructed
Figure 1. (cꢃꢋꢃꢌꢅ ꢃnꢋꢄnꢇ) Fꢋꢌꢃꢅꢇsꢊꢇnꢊꢇ ꢅꢇsꢂꢃnsꢇs ꢃf (ꢆ) HL-1, (b)
HL-2 ꢁꢃwꢆꢅds vꢆꢅꢄꢃꢌs ꢉꢇꢁꢆꢋ ꢄꢃns (10 μm, ch cN /h o, 8:2 v/v).
3
2
2+
ability towards Zn . Just as expected, ligand HL-1 showed
2
+
monotonous ‘turn-on’ response towards Zn , while in
2
+
contrast, receptor HL-2 displayed highly Zn sensitive
turn-on’ along with ‘ratiometric’ sensing properties. The
detailed sensing behaviours of receptors HL-1 and HL-
(
25, 28). However, no investigation on metal ion sensing
‘
was reported till now.
In present work, two novel receptors HL-1 and HL-2
were constructed and their selective fluorescent sensing
2
+
2
towards Zn have been investigated by fluorescence
2+
spectroscopy, UV–vis, ESI-MS experiment, X-ray crystal-
behaviours towards Zn ion were investigated. Both HL-1
and HL-2 consist of naphthalene fluorophores as signal-
ling part and pyridine moieties act as potential chelating
1
lography, partial H NMR titrations and density functional
theory (DFT) calculations.
2+
sites with Zn . Isobutenyl group was introduced with two
aims: (1) introducing multiple hydroxyl groups via TCR
reaction readily; (2) producing certain rigidity in structure
and reducing free molecular rotations via intramolecular
restrictions. The key structural difference in HL-2 and HL-
2
. Results and discussion
2
.1. Fluorescence spectroscopic studies of HL-1 and
HL-2 in the presence of various metal ions
1
is relief of two hydroxyl groups via TCR, which would
Fluorescence spectroscopic studies were recorded in
CH CN:H O (8:2) with 10 μM solutions of receptors and
metal ions. As shown in Figure 1(a), receptor HL-1 displays
induce clear variation in structural conformations. We
expected that this difference will show diverse sensing
3
2

SUPRAMOꢀECUꢀAR CHEMISTRY
3
weak fluorescence emission. Upon addition of equimolar
2
+
of Zn into receptor HL-1 showed significant enhance-
ment in fluorescence intensity at 405 nm using excita-
tion maxima at 353 nm, whereas no obvious fluorescent
response could be observed by addition of other metal
2
+
2+
3+
2+
3+
2+
2+
2+
ions such as Mg , Mn , Fe , Co , Cr , Cd , Cu , Pb ,
2
+
+
+
3+
Ni , Hg , Ag and Ce .
This observation clearly demonstrated that receptor
HL-1 showed highly selective responsiveness towards
2+
Zn among various other metal ions (Figure 1(a)). Unlikely,
HL-1, receptor HL without C=C double bond in structure
2
+
exhibited weak fluorescence enhancement with Zn
because of free intramolecular rotations (Figure S9). This
result revealed that the remarkable fluorescence emission
2
+
enhancement of receptor HL-1 with Zn ion is due to the
introduction of the rigid environment, which restricted to
free intramolecular rotation. Such rigidity in structure of
2
+
receptor provided opportunity to coordinate with Zn .
Interestingly, receptor HL-2 containing two hydroxyl
groups showed quite different sensing mode towards
2
+
Zn compared with receptor HL-1. Only receptor HL-2
displayed weak fluorescence emission at 506 nm. Upon
2
+
addition of Zn into HL-2 exhibited clear ratiometric flu-
orescence property via drastically increase in fluorescence
intensity along with red shift about 40 nm. A new peak
2
+
appeared at 546 nm after addition of 1 eq of Zn lead-
ing to a striking modulation in the colour of solution from
blue-green to yellow-green under the UV-lamp at 360 nm.
In contrast, for all competitive metal ions, receptor HL-2
exhibited common quenching effect (Figure 1(b)).
Figure 2. (cꢃꢋꢃꢌꢅ ꢃnꢋꢄnꢇ) (ꢆ) Fꢋꢌꢃꢅꢇsꢊꢇnꢊꢇ sꢂꢇꢊꢁꢅꢆ ꢃf HL-1 wꢄꢁꢀ
ꢄnꢊꢅꢇꢆsꢄng ꢁꢀꢇ ꢊꢃnꢊꢇnꢁꢅꢆꢁꢄꢃn ꢃf Zn . (b) Fꢋꢌꢃꢅꢇsꢊꢇnꢊꢇ sꢂꢇꢊꢁꢅꢆ ꢃf
HL-1-Zn ꢄn ꢁꢀꢇ ꢂꢅꢇsꢇnꢊꢇ ꢃf vꢆꢅꢄꢃꢌs ꢉꢇꢁꢆꢋ ꢄꢃns.
2
+
The fluorescent emission intensity of both receptors
HL-1 and HL-2 are remarkably increased with increasing
2
+
the concentrations of Zn until 1.0 eq, while the emission
intensity tends to be the same with further increasing the
Moreover, high binding constants and low detection
2
+
limits of HL-1 and HL-2 towards Zn were calculated as
2
+
7
−1
7
−1
−6
−6
concentrations of Zn . Furthermore, a good linear rela-
4
.7 × 10 M , 2.5 × 10 M , and 1.29 × 10 M, 1 × 10 M
2
tionship (R = 0.99) in both cases were obtained between
by fluorescence titration experiment respectively. In order
the maximum fluorescence intensity and the minimum
fluorescence intensity (Figure 2(a), Figure S6). In addition,
to investigate the counter ions effect, salts other than
Zn(ClO ) such as Zn(NO ) , ZnCl and Zn(SO ) were used
to quantify the binding ratio between both receptors HL-
2
+
1
and HL-2 with Zn , fluorescence Job plot experiments
were carried out. The results indicated that the stoichio-
2
+
metric complex ratio of HL-1 and HL-2 with Zn ion was
:1 (Figure S11).
4
2
3 2
2
4 2
which showed similar spectral changes (Figure S10).
To validate the high selectivity of HL-1 and HL-2 as a
turn on and ratiometric chemosensors for the detection
1
2
+
2.2. Absorption spectroscopic studies
of Zn , competitive experiments were carried out by the
2+
addition of 1.0 eq of Zn into the HL-1 and HL-2 solutions
UV–vis spectroscopic measurements were also conducted
in the presence of 1.0 eq of other metal ions. Receptors
in CH CN:H O (8:2) with 10 μM solutions. Receptor HL-1
3
2
2
+
HL-1 and HL-2 could easily detect Zn in the presence
of interfering metal ions. These results demonstrated that
HL-1 and HL-2 displayed excellent selectivity and sensitiv-
exhibited an absorption peak at 285 nm whereas HL-
showed two prominent absorption peaks at 295 and
60 nm which originating from high electronic transition
2
3
2
+
ity towards Zn from the interference of other heavy and
and long conjugation system (Figure 3(a)). Upon addition
2
+
2+
2+
2+
2+
transition metal ions such as Fe , Co , Ni , Cu , Hg and
of above-mentioned metal ions into the receptors HL-1
2
+
2
+
Cd (Figure 2(b) and Figure S8).
and HL-2, only Zn induced a noticeable change in the

4
H. MEHDI ET Aꢀ.
absorption spectra over other common metal ions (Figure
S7).
As for receptor HL-1, absorption peak recorded at
2+
2
85 nm, while after the addition of Zn exhibited a remark-
able enhancement in the absorption intensity. Upon grad-
2+
ual addition of Zn into HL-1, its absorbance intensity was
significantly enhanced with the absorption peak centred
at 285 nm. This process was indicating the formation of
2
+
a well-defined complex between HL-1 and Zn (Figure
2
+
S7c). Furthermore, upon incremental addition of Zn into
receptor HL-2, both bands showed clear bathochromic
shift from 295 to 305 and 360 to 370 nm along with red
shift about 10 nm (Figure 3(b)). Essentially, these absorp-
tion spectra also demonstrated that the receptors HL-1
and HL-2 have unique, diverse and outstanding selectivity
2
+
and sensitivity response towards Zn .
.3. Partial H NMR of HL-1 and HL-2 with Zn2+
1
2
To further investigate the differences in molecular struc-
1
ture of HL-1 and HL-2, we recorded H NMR spectra, which
showed the characteristic distribution in chemical shifts. In
1
previous studies, Hiratani et al. reported H NMR signal of
several compounds based on conformational changes in
isobutylene chain. These compounds were classified into
three groups such as (a) skew–skew (b) syn–skew (c) syn–
syn conformers with respect to conformation–chemical
shift relationship (29, 30).
Based on the conformation–chemical shift correla-
tion, we were able to follow the key structural differences
between two receptors. According to the theoretical calcu-
lation (DFT), receptor HL-1 (C=C–C–O) adopted syn–skew,
while HL-2 (C=C–C– (Neph) followed syn–syn conforma-
tion with respect to the isobutylene chain (Figure 4). In
this study, we focused on isobutylene protons such as
H-1 and H-2 which showed comparatively large changes
in chemical shift.
Figure 3. (cꢃꢋꢃꢌꢅ ꢃnꢋꢄnꢇ) (ꢆ) absꢃꢅꢂꢁꢄꢃn sꢂꢇꢊꢁꢅꢃsꢊꢃꢂꢈ ꢃf HL-1
ꢆnd HL-2 (10 μm) ꢄn ch cN:h o (8:2) ꢆnd (b) wꢄꢁꢀ ꢄnꢊꢅꢇꢉꢇnꢁꢆꢋ
ꢆddꢄꢁꢄꢃn ꢃf Zn ꢄnꢁꢃ ꢅꢇꢊꢇꢂꢁꢃꢅ HL-2.
3
2
2
+
In receptor HL-1, the chemical shift of protons H-1 and
H-2 were recorded at δ 5.70 and δ 5.04 ppm, while shifts of
HL-2 were observed at δ 4.41 and δ 4.03 ppm, respectively.
This result indicates that in receptor HL-2, protons H-1 and
H-2 undergo more shielded due to the presence of two –
OH groups and ring current effect. The upfield shifting of
H-1 and H-2 revealed that receptor HL-2 follows syn–syn
conformation which is fully consistent with its dihedral
angle of calculated structure (3°, 9°). While the chemical
shifts of receptor HL-1 adopted syn–skew conformation.
Interestingly, this result was also in agreement with the
conformations of the isobutenylene moiety, which was
observed via dihedral angle of calculated structure (9°,
Figure 4. pꢅꢃꢂꢃsꢇd ꢊꢃnfꢃꢅꢉꢆꢁꢄꢃnꢆꢋ sꢁꢅꢌꢊꢁꢌꢅꢇ ꢃf ꢅꢇꢊꢇꢂꢁꢃꢅs (ꢆ) HL-1
ꢆnd (b) HL-2.
To further elucidate the nature of coordination and con-
1
11°). This conformation–chemical shift correlation results
formational changes of HL-1 and HL-2 after binding with
2
+
1
suggested that these two receptors have different struc-
tural conformations.
Zn , we carried out partial H NMR titrations in CD CN.
3
Chemical shifts of H-1 and H-2 of both free receptors

SUPRAMOꢀECUꢀAR CHEMISTRY
5
1
2+
Figure 5. (cꢃꢋꢃꢌꢅ ꢃnꢋꢄnꢇ) pꢆꢅꢁꢄꢆꢋ h Nmr wꢄꢁꢀ ꢄnꢊꢅꢇꢆsꢄng ꢊꢃnꢊꢇnꢁꢅꢆꢁꢄꢃn ꢃf Zn ꢄn cD cN ꢆꢁ ꢅꢃꢃꢉ ꢁꢇꢉꢂꢇꢅꢆꢁꢌꢅꢇ (ꢆ) HL-1 ꢆnd (b) HL-2.
3
and their Zn-complexes are shown in Figure 5. For HL-1,
This result indicated that conformational structure of
HL-1-Zn adopted as syn–syn which deviated from syn–
skew conformation of free receptor HL-1. Whereas in HL-2
2
+
upon addition of Zn protons H-1 and H-2 were shifted
towards upfield from 5.70 to 5.55 ppm and 5.04–4.95 ppm,
respectively.
2
+
upon addition of Zn , H-1 and H-2 shifted quite opposite

6
H. MEHDI ET Aꢀ.
system, non-planer structures in solution. The turn-on and
ratiometric fluorescence properties of receptors HL-1 and
2
+
HL-2 after coordinating with Zn may be attributed to
different signalling mechanisms. As for HL-1, fluorescence
enhancement can be ascribed to the conformational
restriction by CHEF effect, while for HL-2, ratiometric flu-
orescence change can be attributed to the binary signal-
ling mechanism like ICT and conformational restriction via
CHEF effect.
As illustrated in single crystal structure (Figure 6), com-
2
+
plex HL-1-Zn followed by chelation of the Zn with the
N-atoms of pyridine groups. Consequently, the free rota-
tion of receptor gets restricted and the system becomes
more rigid and planar. Additionally, due chelation of Zn2
with N-atoms of pyridine moieties drastically increased
the electronic mobility through conjugations resulting
in remarkable enhancement in the fluorescence intensity
(31).
+
Figure 6. (cꢃꢋꢃꢌꢅ ꢃnꢋꢄnꢇ) X-ꢅꢆꢈ ꢊꢅꢈsꢁꢆꢋ sꢁꢅꢌꢊꢁꢌꢅꢇ ꢃf HL-1-Zn.
to each other, H-1 experienced downfield shift from 4.41 to
2
+
4.75 ppm and H-2 shifted towards upfield 4.03–3.89 ppm
Moreover, in the presence of Zn , emission colour
readily modulated from blue (HL-1-Zn) to yellow-green
(HL-2-Zn) under the UV-lamp at 360 nm, which is criti-
cally dependent on the molecular structure and higher
electron mobility in the π-conjugation system. Compared
with HL-1, receptor HL-2 has two additional –OH groups
and two newly formed C–C bonds which were introduced
via TCR reaction. These –OH groups do not get involved
in the binding processes but only acted as electron-do-
nating groups to regulate the electronic system, and
promote electrons from donor to acceptor via excitation.
Furthermore, according to the theoretically optimised
structure, as HL-1, receptor HL-2 also chelated with Zn2
by N-atoms of pyridine moieties, owing to changes in the
structural conformation of HL-2-Zn. These results sub-
sequently lead to binary sensing mechanism as ICT and
conformational restrictions via CHEF effect. The molecular
restrictions via CHEF effect in the combination with ICT
due to the non-equivalent chemical and electronic envi-
ronments. This result revealed that the conformation of
the complex HL-2-Zn adopted as skewed-trans (50°, 30°)
C=C–C–C (Neph) which diverged from the syn–syn con-
formation of the free receptor through the steric effect of
two –OH groups, anisotropic effect, γ-steric effect and ring
current effects. On the other hand, upon addition of Zn in
HL-1, all pyridine protons shifted towards downfield along
with a reduction in intensity, such as H-13, H-12, H-11 and
H-10 from δ 8.63, 8.15, 7.11, and 8.02 ppm to 8.67, 8.46,
2+
7
.50 and 8.36 ppm. Similarly, in HL-2 clear downfield shift-
+
ing were noted in pyridine moieties, while –OH protons
shifted towards upfield from 11.85 to 11.22. Hence, both
receptors showed minute changes in naphthalene plat-
form as compared to the other protons within the system.
1
Collectively, H-NMR titration suggested the N-atoms of
2
+
pyridine moieties are strongly coordinated with Zn .
2
+
process upon interacting with Zn perhaps results in the
enhancement of the fluorescence intensity along with a
red shift of ∼40 nm.
2
.4. ESI-MS experiment
The binding mode of HL-1 and HL-2 with Zn2+ were
also investigated by ESI-MS. Mass spectra of complexes
HL-1-Zn [C H N O + Zn] and HL-2-Zn [C H 8N O + Zn]
Further, above evidence of our proposed sensing
mechanism were verified by the theoretical calculation of
HL-1, HL-2, and complexes HL-1-Zn, HL-2-Zn. To identify
36
28
4
4
36
2
4
4
2
+
recorded at ‘321.9888’ and ‘643.1281 ’, respectively (Figure
S2 and Figure S4). So ESI-MS results, single crystal data, DFT
calculations were consistent with the results of emission
the optical response of receptors towards Zn and the
association between the structural changes of receptors
and complexes, we carried out DFT calculations with the
B3ꢀYP/6-31 + G (d, p). The optimised geometries along
with highest occupied molecular orbital (HOMO), lowest
unoccupied molecular orbital (ꢀUMO) of both receptors
1
spectroscopy, UV–vis, and partial H NMR titrations, which
revealed that the stoichiometric ratio was 1:1 in complexes
HL-1-Zn and HL-2-Zn.
(HL-1, HL-2) and complexes (HL-1-Zn, HL-2-Zn) were
calculated and presented in Figure 7. HOMO and ꢀUMO
energy diagrams showed that the energy gap for the
complexes HL-1-Zn and HL-2-Zn were reduced as com-
pared to the free receptors. This result revealed the fluo-
rescence enhancement due to strong coordination with
2
+
2
.5. Signalling mechanisms of Zn ion with HL-2
and HL-1
Initially, the weak fluorescence of receptors HL-1 and HL-2
may be due to the free molecular rotation, less conjugation

SUPRAMOꢀECUꢀAR CHEMISTRY
7
Figure 7. (cꢃꢋꢃꢌꢅ ꢃnꢋꢄnꢇ) enꢇꢅgꢈ dꢄꢆgꢅꢆꢉs ꢃf homo ꢆnd lumo ꢃꢅbꢄꢁꢆꢋs ꢃf HL-1 ꢆnd HL-1– Zn (ꢆ) HL-2 ꢆnd HL-2-Zn (ꢊ). oꢂꢁꢄꢉꢄsꢇd
sꢁꢅꢌꢊꢁꢌꢅꢇs ꢃf HL-1 ꢆnd HL1–Zn (b) HL-2 ꢆnd HL-2–Zn (d).
2
+
Zn . Besides that, the HOMO–ꢀUMO energy gap of the
complexes HL-1-Zn and HL-2-Zn were also decreased
successively from 3.626 to 3.021 eV. This result perhaps
led to the fluorescence enhancement along with red shift
3. Experimental
.1. Instruments and reagents
3
All required chemicals were obtained from commercial
suppliers unless otherwise specified. Solvents such as
dichloromethane, ethyl acetate, petroleum ether and
methanol were used without distillation. Anhydrous
DMF and THF were dried and distilled immediately
2
+
(
40 nm) upon chelation of the HL-2 with Zn ions in the
proposed fashion. These results indicated that receptors
2+
HL-1 and HL-2 are strongly coordinated with Zn formed
stable complexes with significant changes in geometry
and structural conformations. Furthermore in HL-1-Zn dis-
tances of N1–Zn (2.13Å) and N2–Zn (2.14 Å), while in com-
plex HL-2-Zn distance between N1–Zn (2.13Å) and N2–Zn
1
prior to use. H NMR spectra were recorded on Bruker
AVANCE-400 spectrometer. Mass spectra were obtained
on Agilent 6310 MS spectrometer and Q-TOF MS spec-
trometer. UV–vis spectra were recorded using HITACHI
U-4100 spectrophotometer. Fluorescence spectroscopic
measurements were conducted on JASCO FP-8500
spectrofluorimeter.
(
2.15 Å) were in the range of typical coordination bond
distances, besides that, clear variations were also recorded
in dihedral angles of free receptors and complexes. (Figure
S13). These results expected to play an essential role in
stabilising the molecules. Selected orbitals and their cor-
responding energies for both receptors and Zn-complexes
shown in Figure 7 might have been playing a vibrant role
in the optical spectral outcome.
For absorption and fluorescence measurements, recep-
tors were dissolved in CH CN to obtain 1 mM stock solu-
3
tions. These stock solutions were diluted about 10 μM with
CH CN/H O (8:2 v/v) solution.
3
2

8
H. MEHDI ET Aꢀ.
Scheme 1. Sꢈnꢁꢀꢇꢁꢄꢊ ꢅꢃꢌꢁꢇ ꢃf ꢅꢇꢊꢇꢂꢁꢃꢅs HL-1 ꢆnd HL-2.
Receptor HL-1 was synthesised according to the lit-
erature (28) and used as a starting material to obtain
receptor HL-2 via TCR reaction with two additional
hydroxyl groups and newly formed C–C bonds. As a
comparison, receptor HL without C=C bond was syn-
thesised (Scheme S1) in order to explore the effect of
coordination capability of receptors towards metal ions
due to molecular restrictions in free intramolecular rota-
tion. The synthetic routes to receptors HL-1 and HL-2
are shown in Scheme 1.
3.3. Preparation of HL-1-Zn
HL-1 (0.058 g, 0.1 mmol) was dissolved in CH Cl and was
2
2
added into the CH CN solution of ZnCl (0.013 g, 0.1 mmol).
3
2
After two weeks, transparent X-ray crystals of HL-1-Zn
were obtained by diffusion of diethyl ether into solution.
3.4. Synthesis of compound HL-2
Receptor HL-1 (0.580 g, 1 mmol) was dissolved in 5 ml of
NMP (N-methyl-2-pyradone) under the argon with constant
stirring at 160 °C for 50 min. After addition of H O into reac-
2
tion mixture, we obtained precipitations of HL-2, filtered and
3
.2. Synthesis of compound HL-1
washed three times with H O yielded 80% pure product.
2
+
1
Compound 2 prepared according to the literature (28) and
used as a starting material for receptor HL-1. As like in
literature, compound 2 (0.23 g, 0.5 mmol) was dissolved
into dried THF and added dropwise into 3-aminopyridine
ESI-MS m/z‘603.2021’(M +Na). H NMR (400 MHz, CD CN) δ
3
11.85 (s, 2H, –H,), 9.36 (s, 2H, –NH), 8.90 (s, 2H, Py-H12), 8.51
(s, 2H, Nep-H7), 8.45 (d, J = 8 Hz, 2H, Py-H11), 7.97 (d, J = 8 Hz,
2H, Py-H9), 7.85 (d, J = 4 Hz, 2H, Py-H6), 7. 56 (d, J = 8.0 Hz,
2H, Nep-H3), 7.53 (t, J = 8.0 Hz, 2H, Py-H10), 7.48–7.37 (m, 4H,
(
0.3 g, 1 mmol) containing triethylamine (0.2 g, 2 mmol)
13
with continuous stirring at 0 °C. Subsequently, reaction
mixture was stirred for 12 h at 25 °C. THF was evaporated
under reduced pressure. Purification was performed by
column chromatography through CH Cl /ethyl acetate
Nep-H5 & Nep-H4), 4.41 (s, 2H, H-1), 4.03 (s, 4H, H-2). C NMR
(100 MHz, DMSO-d6) δ169.50, 153.69, 146.31, 145.81, 143.39,
135.71, 134.84, 129.88, 129.82, 129.74, 129.82, 127.63, 126.74,
123.94, 119.26, 117.48, 110.21 ppm.
2
2
(
3:2) and obtained HL-1 with 74% yield. ESI-MS m/z
+
1
‘603.9048’ (M +Na). H NMR (400 MHz, CD CN) δ 9.69 (s,
3
3
.5. Synthesis of compound HL
2H, –NH2), 8.63 (s, 2H, Py-H13), 8.41(s, 2H, Nep-H8), 8.15
(
(
7
d, J = 4, 2H, Py-H12), 8.02 (d, J = 8 Hz, 2H, Py-H10), 7.91
d, J = 8 Hz, 2H, Nep-H7), 7.71(d, J = 8 Hz, 2H, Nep-H4),
.52(t, J = 8 Hz, 2H, Nep-H6), 7.44–7.42 (m, 4H, Nep- H5,
Synthetic procedure of HL is same as HL-1. ESI-MS m/z
1
‘569.2179 ’, H NMR (400 MHz, DMSO-d ) δ 10.62 (s, 2H, –
6
NH), 8.97 (s, 2H, Py-H12), 8.29 (s, 2H, Nep-H8), 8.22 (d, 2H,
J = 8.0 Hz Py-H11), 8.03 (d, J = 8.0 Hz, 2H, Py-H9), 7.97 (t,
J = 4 Hz, 2H, Py-H10), 7.84 (d, J = 8.1 Hz, 2H, Nep-H7), 7.63
(m, 4H, Nep-H5, Nep-H6), 7.63 (m, 2H, Nep-H4), 7.46 (s,
2H, Nep-H3), 4.40 (s, 4H, H-2), 2.38 (s, 2H, H-1). 13C NMR
H3), 7.12–7.08 (m, 2H, Py-H11), 5.74 (s, 2H, H-1), 5.08 (d,
1
3
4
1
1
1
H, H-2). C NMR (100 MHz, DMSO-d ) δ 165.35, 152.60,
6
44.42, 141.04, 139.62, 135.79, 134.69, 129.36, 128.22,
27.67, 127.39, 126.55, 126.34, 124.54, 123.62, 114.81,
07.63, 68.50 ppm.
(100 MHz, DMSO-d ) δ 164.80, 152.74, 143.78, 140.44,
6

SUPRAMOꢀECUꢀAR CHEMISTRY
9
1
1
29.42, 128.61, 127.90, 127.42, 127.10, 126.65, 126.10,
24.04, 123.35, 106.90, 64.79, 28.00 ppm.
Research Funds for the Central Universities [grant number DUT-
4ꢀK08].
1
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3
.6. Preparation of sample solution for analysis
(
(
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HL-2 were prepared in CH CN solvent. Wherein prepared
3
1
0 μM dilute solution in CH CN/H O (8:2 v/v) for sample
3 2
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. Conclusion
In conclusion, we synthesised and demonstrated two novel
receptors HL-1 and HL-2 exhibiting incongruent sensing
mode due to dissimilar conformational structures. Based
on conformational–chemical shift described the structural
variations in both receptors. Furthermore, differences
in molecular conformation in HL-1 and HL-2 were also
observed after coordination with Zn . Complex HL-1-Zn
changed towards syn–syn while HL-2-Zn adopted as
skewed-trans from syn–skew and syn–syn conformations
of free receptors, respectively.
(
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Acknowledgements
The authors are grateful for financial support from the Nation-
al Natural Science Foundation of China (NO.21206016) and the
Fundamental Research Funds for the Central Universities (DUT-
14ꢀK08).
Disclosure statement
No potential conflict of interest was reported by the authors.
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Funding
This work was supported by the National Natural Science Foun-
dation of China [grant number 21206016]; and the Fundamental
(31) Wu, J.; ꢀiu, W.; Ge, J.; Zhang, H.; Wang, P. Chem. Soc. Rev.
2011, 40, 3483–3495.