A Cd(ii) and Zn(ii) selective naphthyl based [2]rotaxane acts as an exclusive Zn(ii) sensor upon further functionalization with pyrene

Article abstract of DOI:10.1039/d0dt03645e

A new multi-functional [2]rotaxane, ROTX, has been synthesized via a Cu(i) catalysed azide-alkyne cycloaddition reaction between Ni(ii) templated azide terminated pseudorotaxane composed of a naphthalene based heteroditopic wheel, NaphMC, and an alkyne terminated stopper. Subsequently, ROTX has been functionalized with pyrene moieties to develop a bifluorophoric [2]rotaxane, PYROTX, having naphthalene and pyrene moieties. Detailed characterization of these two rotaxanes is performed by utilizing several techniques such as ESI-MS, (1D and 2D) NMR, UV/Vis and PL studies. Comparative metal ion sensing studies of NaphMC (a fluorophoric cyclic receptor), ROTX ([2]rotaxane with a naphthyl fluorophore) and PYROTX ([2]rotaxane having naphthyl and pyrene fluorophores) have been performed to determine the effect of dimensionality/functionalization on the metal ion selectivity. Although NaphMC fails to discriminate between metal ions, ROTX serves as a selective sensor for Zn(ii) and Cd(ii). Importantly, PYROTX shows exclusive selectivity towards Zn(ii) over various transition, alkali and alkaline earth metal ions including Cd(ii).

Full text of DOI:10.1039/d0dt03645e

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DOI: 10.1039/D0DT03645E
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A Cd(II) and Zn(II) selective naphthyl based [2]rotaxane acts as an
exclusive Zn(II) sensor upon further functionalization with pyrene
Received 00th January 20xx,
Accepted 00th January 20xx
Somnath Bej, Mandira Nandi and Pradyut Ghosh*
DOI: 10.1039/x0xx00000x
A new multi-functional [2]rotaxane, ROTX has been synthesized via Cu(I) catalysed azide-alkyne cycloaddition reaction
between Ni(II) templated azide terminated pseudorotaxane composed of naphthalene based heteroditopic wheel,
NaphMC and an alkyne terminated stopper. Subsequently, ROTX has been functionalized with pyrene moieties to develop
a bifluorophoric [2]rotaxane, PYROTX having naphthalene as well as pyrene moieties. Detailed characterizations of these
two rotaxanes are performed by utilizing several techniques such as ESI-MS, (1D and 2D) NMR, UV/Vis and PL studies. A
comparative metal ion sensing studies of NaphMC (fluorophoric cyclic receptor), ROTX ([2]rotaxane with naphthyl
fluorophore) and PYROTX ([2]rotaxane having naphthyl and pyrene fluorophores) have been performed to find out the
effect of dimensionality/ functionalization towards the metal ion selectivity. Although the NaphMC fails to discriminate
any metal ion, the ROTX serves as selective sensor for Zn(II) and Cd(II). Importantly, PYROTX shows exclusive selectivity
towards Zn(II) over various transitions, alkali and alkaline earth metal ions including Cd(II).
generate a metal binding pocket that selectively binds Li+ over
Na+ and K+ through “switch on” fluorescence response.12 In
2018, Goldup and co-workers reported rotaxanes based
Introduction
Mechanically interlocked molecules have attracted much
attention due to their synthetic challenges as well as
numerous applications in the construction of various
molecular machineries,1 devices,2 switches,3 chemo/
fluorescent sensors4 and fluorescent imaging probes.5 In the
literature, various types of interlocked molecules are
constructed by several groups through the incorporation of
suitable functionalities either in the wheel or axle moiety by
employing several templation methods, such as, cation/ anion
coordination,6 hydrogen/ halogen bonding,7 - stacking,8
hydrophobic interaction etc.9 Rotaxanes having suitable
chelating groups to bind with the cations/ anions are good
candidates for selective sensing of a particular analyte
depending on the overall construction of the mechanically
interlocked system having specific cavity dimension. In this
direction, Beer et al., and others have extensively explored
both electrochemical and optical anion sensing using
interlocked architectures which are found to be highly
selective for anionic guests.10 On the other hand, cationic
guests bound interlocked molecules have also been explored
towards the development of fluorescent sensors, catalysts,
and molecular rotors.11 In 2004 Hiratani and co-workers have
reported [1]rotaxane, where mechanical bond is utilized to
fluorescent sensors for binding of metal ions.13
It is noteworthy to mention that the imbalance of Zn(II) may
generate numerous problems such as superficial skin disease,
diabetes, prostate cancer, alzheimer's disease, parkinson's
disease, age-related macular degeneration etc.14 Hence,
designing of fluorescent sensors for selective sensing of Zn(II)
is becoming a general area of interest to the scientific
community.15 In the literature, several metal ion sensors are
reported which are mainly based on acyclic, cyclic and non-
interlocked ligands.16 Herein, we demonstrate selective
sensing of Zn(II) over Cd(II) by modifying a fluorophoric
[2]rotaxane to a bifluorophoric [2]rotaxane upon further
functionalization.
Results and discussion
Designing aspects:
Our choice of wheel for the construction of a mechanically
interlocked fluorophoric rotaxane is a naphthalene based amino-
ether heteroditopic macrocycle, NaphMC having three secondary
amine functionalities. Multi-functionality in the newly developed
[2]rotaxane, ROTX having three-dimensional cavity can be
introduced by threading of 5,5’-dimethyl-2,2’-bipyridine derivatized
ligand into the fluorophoric wheel via Ni(II) templation as well as -
 stacking interaction. Subsequently, amine functionalities of the
wheel in ROTX can be utilized to attach pyrene moieties to achieve
rotaxane PYROTX having two types of fluorophoric moieties. Thus,
fluorophoric hosts of different dimensionalities, NaphMC, ROTX
School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A &
2B Raja S. C. Mullick Road, Kolkata 700032, India. E-mail: icpg@iacs.res.in
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0DT03645E
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and PYROTX having metal ion chelating pockets can be generated
to explore for the comparative metal ion sensing studies. Here it is
important to mention that the naphthalene or pyrene fluorophoric
units can be connected to the nitrogen centres through -CH2-
spacer, so that the photo-induced electron transfer (PET)
mechanism could be operative during such sensing studies.
Scheme 1. Synthetic route of [2]pseudorotaxane [NiPR(ClO4)2]. (where, L = azide
Synthesis of rotaxanes (ROTX and PYROTX):
In this aspect, a heteroditopic macrocycle, NaphMC consisting
of tris-amine, oxy-ether moieties and adjacent naphthalene
groups and its Ni(II) complex, NaphMC-Ni(II) are synthesized
following the reported procedures17 [Scheme 1S and scheme
3S, ESI†]. Further, the precursor azide terminated bidentate
ligand (L) is also synthesized in high yield starting from 5,5’-
dimethyl-2,2’-bipyridine unit following the reported synthetic
route18 [Scheme 2S, ESI†]. Subsequently, NaphMC-Ni(II)
complex and bidentate chelating ligand (L) are engaged to
achieve a new heteroleptic ternary complex upon threading of
an azide terminated bidentate chelating ligand (L) into the
NaphMC-Ni(II) complex to form Ni(II) templated
[2]pseudorotaxane, [NiPR(ClO4)2] [Scheme 1]. Detailed
synthetic procedure of [NiPR(ClO4)2] complex is described in
the experimental section. Characterization of the
pseudorotaxane [NiPR(ClO4)2] is performed by several usual
techniques such as ESI-MS, UV/Vis spectroscopy, IR and
elemental analysis. Initially, ESI-MS spectrum has shown the
appearance of a prominent molecular ion peak at m/z =
terminated bidentate chelating ligand and S = Solvent molecule) (Yield: 91 mg,
75%)
one solvent molecule as similar to the reported Ni(II) complex
having alike coordination environment.6c Herein, CH3OH molecule
might acts as coordinating solvent to attain hexa-coordination of
Ni(II) centre in the solution state. However, the elemental analysis
data supports the Ni(II) in [NiPR(ClO4)2] complex without the
solvent composition. This could be attributed to the solvent
evaporation from the complex during the ignition process. Hence,
the experimental result matches close to the value of the calculated
formula C52H55Cl2N13NiO14. Subsequently, attempt is made to
prepare fluorescent [2]rotaxane following the capping strategy by
using azide-alkyne cycloaddition reaction. In this regard, a suitable
newly designed alkyne terminated bulky stopper unit (STP) has
been synthesized by following scheme 4S [ESI†] and their
corresponding characteristic spectra are given in the Supporting
Information [Fig. 4S- Fig. 15S, ESI†]. Thus, the successful synthesis of
Ni(II) templated [2]rotaxane {NiROTX(ClO4)2} is achieved by the
reaction between azide terminated pseudorotaxane {NiPR(ClO4)2}
- +
1114.3239 which corresponds to [NiPR(ClO4)2 - ClO4 ]
component [Fig. 1S, ESI†]. As well as, the simulated isotopic
distribution pattern is merged well with the experimental
pattern that supports the formation of azide terminated
[2]pseudorotaxane [NiPR(ClO4)2]. The UV/Vis titration studies
[Fig. 2S, ESI†] are also performed to support the formation of and alkyne terminated stopper units (STP) via click reaction by
[NiPR(ClO4)2] complex in the solution state. During UV/Vis
titration experiment, it is found that the absorption maxima of
L at 298 nm gradually decreases upon addition of one
equivalent of NaphMC-Ni(II) with a concomitant increases of
absorption maxima at 319 nm due to the formation of
heteroleptic complex, [NiPR(ClO4)2]. Equivalence plot
evaluated from UV/Vis titration study indicates the presence
of 1:1 binding interaction between NaphMC-Ni(II) complex and
bidentate chelating ligand (L) [Fig. 2S, ESI†]. Thus, ESI-MS and
absorption studies validate the threading of L into the cavity of
NaphMC-Ni(II) complex, resulting the formation of
[2]pseudorotaxane. It is expected that Ni(II) ion in
[NiPR(ClO4)2] complex might prefer common hexa-
coordination through the engagement of tridentate
macrocycle and bidentate azide terminated ligand along with
addition of the catalytic amount of Cu(CH3CN)4PF6 salt. In-situ de-
metallation of the metal templated rotaxane {NiROTX(ClO4)2} by di
sodium salt of ethylene diamine tetra acetic acid (Na2EDTA) and
aqueous ammonia solution results into the formation of the metal
free rotaxane (ROTX) along with the dumbbell shaped bidentate
ligand with stopper unit (AXLE) [Scheme 2]. Furthermore, to induce
another fluorophore in ROTX, we have functionalized the tris amine
unit of ROTX with the pyrene moieties which attain pendant like
arrangement at the tris amine site. In the course of synthesis of
PYROTX, equivalent amount of ROTX is reacted with three
equivalents of pyrene bromide in the presence of N,N-
diisopropylethylamine (DIPEA) as base in CH2Cl2 solvent [Scheme 2].
These two rotaxanes (ROTX and PYROTX) are further characterized
by mass, NMR, absorption (UV/Vis), photo- luminescence (PL) and
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Scheme 2. Synthetic route of pyrene derived fluorophoric [2]rotaxane (PYROTX). (S = Solvent molecule) yields: (ROTX: 70 mg, 42%; PYROTX: 180 mg,78%).
infrared spectroscopic (IR) studies. Initially, the formation of non-interlocked NaphMC and AXLE units. Such observation
[NiROTX(ClO4)2] has been examined by the ESI-MS studies, in which demonstrates that in the interlocked rotaxane system aromatic unit
appearances of the peaks at m/z 1830.6373 and 865.8442 of bipyridine moiety is situated in between two naphthalene
- +
correspond to the single {NiROTX(ClO4)2 - ClO4 } and double moieties of the macrocycle and thus they are interacting with each
{NiROTX}2+ charged components respectively which designate the other through π-π stacking interaction. As well as, downfield
formation of metallated [2]rotaxane [NiROTX(ClO4)2] [Fig. 16S, shifting of amide H4 proton of the axle unit and Ha, Hb protons of
ESI†]. Subsequently, metal free interlocked [2]rotaxane (ROTX) is NaphMC occur due to the interlocked nature of rotaxane system.
confirmed by ESI-MS, (1D, 2D) NMR, UV/Vis and PL spectroscopic But such type of interactions are completely absent in 1:1 mixture
studies [Fig. 17S- Fig. 28S, ESI†]. Mass spectrometric analysis of the of NaphMC and AXLE unit, as no shifts of proton signals are
ROTX shows prominent peaks at m/z 1674.7616 and 837.8848 that observed there. Another evidence, such as DOSY NMR
attributes to the [ROTX + H]+ and [ROTX + 2H]2+ respectively and spectroscopic study of rotaxane in CDCl3 at 298 K shows that all of
also the simulated isotope distribution pattern matches perfectly the peaks correlated to the proton signals in the chemical shift
with the experimental data [Fig. 17S, ESI†].
dimensions, are in a same horizontal line [Fig. 20S, ESI†]. Thus,
Further, comparative 1H NMR studies have been carried out to [2]rotaxane acquires the single diffusion coefficient value of D = 4.5
confirm the formation of interlocked [2]rotaxane (ROTX) in CDCl3 at x 10-10 m2/s, which indicates that NaphMC and AXLE units are the
1
room temperature. A comparison of H NMR spectra of NaphMC, integrated parts of the same molecule. Moreover, we have carried
ROTX, azide terminated bidentate ligand with stopper unit (AXLE) out other 2D NMR studies (COSY and ROESY) to support the
and 1:1 mixture of NaphMC and AXLE unit shows a correlation of interlocked nature of ROTX system [Fig. 21S- Fig. 22S, ESI†]. The
respective chemical shift values which is clearly observed in Fig. 1. ROESY-NMR spectrum of ROTX shows the through space interaction
Here, the bipyridine protons of axle moiety H1, H2, H3 and between macrocyclic wheel proton (Hh) and AXLE proton (H1),
macrocyclic wheel (NaphMC) protons Hc, Hd, He, Hf, Hg, Hh, Hj have which designates that the bipyridine moiety of the axle unit is
shown upfield shifts in rotaxane in comparison to corresponding present in the core of macrocyclic wheel.
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upon addition of perchlorate salt of various metal ions (Ni2+, Cu2+,
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2+
3+
2+
2+
2+
+
+
2+
2+
3+
2+
2+
DOI: 10.1039/D0DT03645E
Mn , Cr , Co , Hg , Cd , Li , Na , Zn , Ca , Fe , Mg and Pb )
and PF6 salt of K+ in an excess amounts (10 equiv.) in THF via
-
emission spectroscopy [Fig. 35S, ESI†]. Apparently, the emission
maximum of NaphMC is appeared at max = 357 nm when it is
excited at 300 nm. However, the fluorescence intensity of NaphMC
does not show significant change towards any of the above-
mentioned metal ions. Hence, it is obvious that, there is no
selectivity towards metal ions in case of NaphMC. Subsequently,
the naphthyl based ROTX having three dimensional cavity
composed of tris-amine and bipyridine chelating units is examined
towards the metal sensing properties.
When RTOX is excited at 302 nm, it exhibits a broad emission band
with the emission maximum at 360 nm. However, upon addition of
various metal salts (10 equiv.) to the THF solution of ROTX, the
emission intensity is found to be slightly increased only in cases of
Zn2+ and Cd2+ [Fig. 2]. In order to inspect such changes in the
emission intensity of the ROTX towards Zn2+ and Cd2+, ESI-MS (+ve)
study is carried out. The analysis indicates the presence of
molecular ion peaks at m/z = 1836.6314 and 1886.6054 for uni-
positive species of Zn2+-bound and Cd2+-bound rotaxane
respectively [Fig. 36S and Fig. 38S, ESI†]. In both the complexes,
experimentally obtained isotopic distribution patterns are perfectly
matched with the theoretically obtained calculated patterns.
Hence, the above experiments confirm the formation of both Zn2+
and Cd2+ bound ROTX molecules. Such metal binding affinity of
ROTX could be due to the suitable co-ordination of those two ions
in the ROTX cavity of appropriate dimension through three
secondary amine and two bipyridine nitrogen centres. However,
ROTX is unable to differentiate between Zn(II) and Cd(II).
Fig. 1: Stacked 1H NMR spectra (500 MHz) in CDCl3 at 298 K of (a) NaphMC, (b)
Rotaxane, (c) AXLE (d) NaphMC: AXLE (1:1 mixture). The labels correspond to
those shown in the onset pictures of NaphMC and AXLE.
Thereafter, solution state UV/Vis absorption spectroscopic study of
ROTX is recorded in dry THF medium, in which peak maxima arrives
at 302 nm that is due to π→π* electronic transition of naphthalene
moieties [Fig. 23S, ESI†]. Emission spectrum of ROTX is also
recorded in dry THF, which exhibits λmax at 360 nm when it is
excited at its absorption maxima (302 nm) [Fig. 23S, ESI†]. Thus, all
the above mentioned experimental results confirm the interlocked
nature of multi-functional rotaxane (ROTX), which is consisting of
several functionalities such as tris-amine, oxy-ether, naphthalene,
amide, bipyridine etc.
After being confirmed the interlocked nature of ROTX system, the
ROTX molecule is further derivatized with fluorophoric pyrene
groups to obtain a bifluorophoric rotaxane (PYROTX) system, which
is further characterized by several methods such as mass
spectrometry, NMR spectroscopy, UV/Vis, IR and PL studies [Fig.
29S- Fig. 34S, ESI†]. Firstly, ESI-MS analysis shows prominent peaks
at m/z = 2316.9960 and 1158.9970 which are designated as
[PYROTX + H]+ and [PYROTX + 2H]2+ species respectively [Fig. 29S,
ESI†]. Experimentally obtained isotopic distribution pattern of the
molecular ion peak is matched well with the theoretically calculated
pattern that supports the formation of PYROTX molecule.
To fine tune the selectivity, we have further attached three pyrene
moieties at the amine centres of the wheel to introduce more steric
crowding near to the metal ion binding site. Post functionalization
of ROTX is done by attaching pyrene moieties with –CH2- spacer at
the tris amine site to synthesize PYROTX. Detailed synthetic
Moreover, variable temperatures 1H NMR (VT-1H NMR) spectra are
recorded to obtain the information about the stability of the
compound even at higher temperature. Accordingly, 1H NMR
spectra of PYROTX solution in DMSO-d6 in different temperatures
from 300 to 370 K (10 K interval) are presented in figure 33S (ESI†).
It is evident from the VT-1H NMR analysis that, PYROTX is stable
even at 370 K as the 1H NMR spectral patterns do not change
significantly at the higher temperature.
Sensing studies:
Metal ion sensing studies of NaphMC, ROTX and PYROTX
containing well-defined cavities and suitable chelating units are
carried out in details. In case of NaphMC, naphthyl group acts as
responsive unit whereas tris amine moiety could serve as metal
chelating site. The sensing studies are performed with NaphMC
Fig. 2: Emission spectra of ROTX (5x10-6 M) in the presence of 10 equivalents of
Ni2+, Cu2+, Mn2+, Cr3+, Co2+, Hg2+, Cd2+, Li+, Na+, K+, Ca2+, Fe3+, Mg2+, Zn2+ and Pb2+
ion in THF at 298 K, λ exc=302 nm.
4 | J. Name., 2012, 00, 1-3
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procedure of PYROTX is described in the experimental section.
Consequently, we have examined the absorption changes of
PYROTX in THF upon addition of various metal ions (Ni2+, Cu2+,
Mn2+, Cr3+, Co2+, Hg2+, Cd2+ Li+, Na+, Ca2+, Fe3+, Mg2+, Pb2+) in large
excess (10 equiv.) including one equiv. of Zn2+ having perchlorate
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DOI: 10.1039/D0DT03645E
-
counter ion except K+ which is used as PF6 salt [Fig. 3]. The
characteristic absorption peaks of PYROTX are observed at 315 nm,
330 nm and 346 nm as due to π→π* electronic transition of the
pyrene chromophore, which matched well with literature data.19 It
is observed that among all the ions, only Zn2+ shows a small
perturbation of the initial absorption band of PYROTX with the
increase in absorption intensity at 330 nm with the concomitant
decrease in absorption maxima at 346 nm. Thereafter, the emission
spectra of PYROTX molecule are recorded in THF at room
temperature upon excitation of two different fluorophores i.e.
naphthalene as well as pyrene moieties. During the excitation at
naphthalene fluorophore (max at 302 nm) the emission maxima of
PYROTX are observed at 355 nm, 380 nm and 398 nm due to both
naphthalene and pyrene units [Fig. 40S, ESI†]. Again, when PYROTX
Fig. 4: Emission spectra of PYROTX in the presence of 10 equivalents of Ni2+
equiv. of Zn2+ ion in THF at 298 K, λexc= 346 nm.
,
Cu2+, Mn2+, Cr3+, Co2+, Hg2+, Cd2+, Li+, Na+, K+, Ca2+, Fe3+, Mg2+ and Pb2+ and one
is excited at the pyrene (max at 346 nm) the emission maxima occur pyrene groups act as responsive unit [Fig. 4]. In fact, the binding
at 383 nm and 401 nm [Fig. 32S, ESI†]. The PL studies are carried ability of PYROTX with the metal ion instantly prevents the photo
out upon excitation at both the fluorophoric centres (naphthalene electron transfer process from tris-amine N atom to fluorophoric
as well as pyrene units) in presence of various metal ions.
units, which may result in the increment of fluorescence intensity of
Noticeably, during PL study upon excitation at the pyrene groups the pyrene moieties. Moreover, in order to evaluate the efficiency
of PYROTX towards this particular Zn2+ metal ion, detailed UV/Vis
and PL titration studies are also performed by utilizing PYROTX
solution as host and Zn2+ ion as guest molecules in THF. In case of
absorption titration studies, upon addition of equivalent amounts
of Zn2+ ion solution in THF into the host PYROTX solution, a clear
isosbestic point is observed. As well as the molar ratio plot obtained
from the absorption titration experiments, suggests the 1:1 binding
interaction between PYROTX and Zn2+ [Fig. 5]. Sequentially, the
emission titration experiments are also performed. It has been
observed that, the emission intensity of pyrene moiety at 382 nm
and 398 nm is gradually increased upon gradual addition of
equivalent amount of Zn2+ ion in the solution of PYROTX in THF [Fig.
6] upon excitation at 346 nm. In order to estimate the binding
(max at 346 nm) shows significant perturbation only in case of Zn2+,
whereas addition of other metal ions (Ni2+, Cu2+, Mn2+, Cr3+, Co2+,
Hg2+, Cd2+, Li+, Na+, K+, Ca2+, Fe3+, Mg2+ and Pb2+) even with excess
amounts (10 equiv.) do not cause any changes in the PYROTX [Fig.
4]. It is observed that, in presence of Zn2+, PYROTX experiences
relatively higher emission intensity when it is excited at the pyrene
fluorophore as compared to naphthalene [Fig. 41S, ESI†]. Thus, the
individual emission response of PYROTX shows the emission
intensity enhancement about ~5 fold at 382 nm and 398 nm which
ensures the effective sensing of Zn2+ by PYROTX where mainly
Fig. 3: Comparative UV/Vis spectra of PYROTX in the presence of one equiv. of
Zn2+ ion and ten equiv. of other metal ions such as Ni2+, Cu2+, Mn2+, Cr3+, Co2+
,
Hg2+, Cd2+ Li+, Na+, K+, Ca2+, Fe3+, Mg2+ and Pb2+ in THF at 298 K.
Fig. 5: UV/Vis titration profile of PYROTX (1x10−5 M) upon addition of Zn2+
(2x10−4 M) in THF, (B) molar ratio plot of UV/Vis titration data in THF at 298 K.
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selective detection of Zn2+ ion even in presence of large excess of
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DOI: 10.1039/D0DT03645E
other metal ions.
Furthermore, in order to justify the above spectroscopic results, we
have analysed the titrated solution with mass spectrometry study
[Fig. 43S, ESI†]. However, the ESI-MS spectrum shows a molecular
ion peak at m/z = 2478.8662 which indicates the mono positive
- +
molecular ion species of [PYROTX + Zn2+ + ClO4 ] . Thus, the
additional mass analysis is a clear evidence of the formation of Zn2+
bound PYROTX complex. Moreover, we have also carried out the
sensing studies of PYROTX molecule in more competitive solvent
mixture (5% water in THF medium) through the absorption and
emission spectrometric analysis. The experimental results obtained
from such studies are found to be almost similar with the results
acquired in case of only THF medium. Related plots are provided in
supporting information [Fig. 45S- Fig. 47S, ESI†]. Thus, it may be
concluded that Zn2+ occupies the cavity of PYROTX molecule via the
formation of penta coordinated Zn2+ complex by the participation of
corresponding tris tertiary amine centres and bipyridine moieties
[Fig. 8]. The selective binding of metal ion in PYROTX can be
explained as possibly due to the different sizes of the metal ions or
by sterically preferred coordination environment of metal ion
where pyrene groups also may provide important role for selective
sensing of Zn2+ ion.
Fig. 6: Emission titration profile of PYROTX (1x10−6 M) upon addition of Zn2+
(1x10−5 M) in THF at 298 K, λexc= 346 nm (inset picture: molar ratio plot).
stoichiometry, the molar ratio plot is drawn from the emission
titration data which also suggests the 1:1 (H: G) binding between
PYROTX and Zn2+. The association constant, calculated from
fluorescence titration experiment by using nonlinear curve fitting
method is found to be Ka= 6.22x107 M-1 [Fig. 42S, ESI†]. These
results can be attributed to the stoichiometric binding of PYROTX
with the Zn2+ ion through the tertiary amine group of macrocycle
and bipyridine moiety of the axle, which provides the suitable
coordination environment around the Zn2+ metal ion in PYROTX.
We have recorded the change in emission intensity of PYROTX upon
addition of various transition and alkali metal ions in excess
amounts including one equivalent of Zn2+ in THF [Fig. 7]. It is found
that addition of Zn2+ in presence of excess amount of other metal
salts in the PYROTX solution in THF shows similar kind of
enhancement of emission intensity of PYROTX like previously
obtained in the case of individual addition of Zn2+ ion in PYROTX
[Fig. 4]. Thus, PYROTX is found to be an effective sensor for the
Fig. 8: Proposed coordination sphere of the Zn2+ ion in PYROTX.
Conclusions
In summary, we have successfully developed fluorophoric
[2]rotaxanes, ROTX and PYROTX based on heteroditopic
macrocyclic wheel through azide alkyne click reaction and
investigated their properties via different spectroscopic techniques.
Pyrene derived [2]rotaxane (PYROTX) is found to be highly selective
towards Zn2+ ion over the various transition, alkali metal ions and
thus it acts as a good candidate as a Zn2+ ion sensing agent. We
envisage that these kind of interlocked molecular systems are
capable to show various utility in different direction towards the
development of new molecular machineries.
Fig. 7: Selectivity studies PYROTX with Zn2+ ion in the presence of competing
metal ions in THF. Red bars show the emission intensity of PYROTX in the
presence of other competitive metal ions and Zn2+ ion and blue bars show the
emission intensity of PYROTX (1x10-6 M) in the presence of all metal ions.
Experimental section
General details
6 | J. Name., 2012, 00, 1-3
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All commercially available reagents were used as received without point, the solvent was evaporated and the resultedViseowlAidrticrleesOidnluinee
DOI: 10.1039/D0DT03645E
further purification. Solvents were distilled following the was washed repeatedly with CHCl3 and diethylether. The ternary
appropriate procedures and were dried over molecular sieves. All complex, [NiPR(ClO4)2] was isolated as yellow coloured solid
perchlorate salts of metal (Zn2+, Ni2+, Cu2+, Mn2+, Cr3+, Co2+, Hg2+, product. Yield: 91 mg, 75%.
Cd2+, Li+, Na+, Ca2+, Fe3+, Mg2+, Pb2+) and hexa fluoro phosphate salt
of potassium (K+) used for spectroscopic studies were stored in
vacuum desiccator prior to use. 1D and 2D NMR spectra were
measured on a FT-NMR Bruker DPX 300/400/500 MHz NMR
spectrometer. The electronic spray ionization (ESI) mass spectra
were achieved on a QToF–Micro YA 263 Mass spectrometer in
positive ion mode. Fourier transform infrared (FT-IR) spectra were
Characterization of [NiPR(ClO4)2]: Anal. calcd for NiPR(ClO4)2,
C52H55Cl2N13NiO14 (MW = 1215.6712) C, 51.38; H, 4.56; N, 14.98;
found: C, 51.29; H, 4.52; N, 14.89. M.P: above 200 °C. HRMS (ESI-
MS) {NiPR(ClO4)2-ClO4− = C52H55ClN13NiO10}+: calcd, m/z =1114.3237;
−
found, m/z = 1114.3239; {[NiPR(ClO4)2-2ClO4 = C52H55N13NiO6}2+:
calcd, m/z = 507.6876; found, m/z = 507.6878. IR (KBr,  cm-1): 624,
obtained on a SHIMADZU FTIR-8400S IR spectrophotometer with 754, 929, 1095, 1230, 1251, 1280, 1452, 1471, 1504, 1546, 1583,
KBr pellets. The UV/Vis absorption spectra and fluorescence spectra 1660, 2102 (azide), 2925, 3064, 3271, 3382. Absorption spectra
were recorded using Perkin-Elmer Lambda 900 UV–Vis–NIR studies: λmax = 319 nm.
spectrometer (NIR = near-infrared) (with a quartz cuvette of path
(iii) Synthesis of L3:
length 1 cm) and FluoroMax-3 spectrophotometer, from Horiba
Jobin Yvon respectively. Elemental analysis was performed on a
Perkin-Elmer 2500 series II elemental analyzer, Perkin-Elmer, USA.
Compound L2 was prepared by following usual procedure. L2 (546
mg, 3 mmol) and K2CO3 (1.25g, 9 mmol) were taken in a 100 ml
round bottom flask in CH3CN. Benzyl bromide (857 µL, 7.2 mmol)
was added to it and refluxed under N2 for 24 h. Then, after cooling
the reaction mixture, solvent was removed under vaccum. The
residue was extracted with CHCl3 and water. By washing the organic
layer with brine solution, organic layer was dried over sodium
sulfate and evaporated in vacuo. Further, washing the solid residue
by hexane, the white coloured product was collected as pure form
in high yield (913 mg, 84%).
M.P: 76 °C. HRMS (ESI-MS) for {C23H22NaO4 = [L3 + Na]+} calcd, m/z =
385.1416; found, m/z =385.1414. IR (KBr,  cm-1): 698, 738, 767,
846, 966, 1035, 1054, 1108, 1164, 1230, 1298, 1348, 1379, 1446,
1593, 1708 (ester C = 0), 2898, 2987, 3031. 1H NMR (400 MHz,
CDCl3): δ (ppm) 1.39 (t, 3H, J = 7.2 Hz, - CH3), 4.36 (q, 2H, J = 7.2 Hz
- CH2), 5.08 (s , 4H, - CH2), 6.80 (t, 1H, J = 2.4 Hz, Ar-H), 7.30-7.44 (m,
12H, Ar-H). 13C NMR (125 MHz, CDCl3): δ (ppm) 14.5, 61.3, 70.5,
107.2, 108.6, 127.8, 128.3, 128.8, 132.6, 136.7, 160.0, 166.4.
Caution! Generally metal perchlorate salts are explosive in presence
of open flames, sparks or heat. Hence, the usage of perchlorate
salts should be maintained with precautions.
Calculation of Association Constant:
The association constant was calculated from emission titration
experiments by plotting the emission changes (ΔI) at a fixed λ value
against the guest concentration by using nonlinear fitting of the
curves. Following equation was used for 1: 1 (host: guest) binding
model20
2
퐼
1
1
2
∆퐼 =
∗
퐺0 + 퐻0 +
―
퐺0 + 퐻0 +
+ 4퐺0퐻0
(
)
(
)
(
)
{
}
2 ∗ 퐻
퐾
퐾
Where, I= emission intensity value upon each addition of the guest,
change in emission intensity ΔI = (I − I0), [H]0 = initial concentration
of the host, [G]0 = initial concentration of the guest and K is the
association constant.
(iv) Synthesis of L4:
LiAlH4 (45 mg, 1.2 mmol) was added to the solution of compound L3
(362 mg, 1 mmol) in dry THF. The resultant solution was stirred
under N2 for 12h at RT followed by reflux for 5h. Then, strong KOH
solution was added drop by drop to this reaction mixture until the
white precipitate appeared. By filtering the white precipitate,
filtrate was evaporated and extracted with CHCl3 and water. The
organic layer was completely evaporated to get pure off white solid
product. Yield: 208 mg, 65%.
M.P: 84 °C. HRMS (ESI-MS) for {C21H20NaO3 = [L4 + Na]+} calcd, m/z =
343.1310; found, m/z =343.1312. IR (KBr,  cm-1): 632, 696, 752,
833, 908, 1026, 1049, 1159, 1218, 1286, 1371, 1448, 1593, 2867,
2906, 3029, 3336 (O–H). 1H NMR (400 MHz, CDCl3): δ (ppm) 1.73 (s,
1H, - OH), 4.63 (s, 2H, -CH2), 5.04 (s, 4H, -CH2), 6.56 (t, 1H, J = 2.0 Hz,
Ar-H), 6.63 (d, 2H, J = 2.0 Hz, Ar-H), 7.31-7.44 (m, 10H, Ar-H). 13C
NMR (125 MHz, CDCl3): δ (ppm) 65.5, 70.3, 101.5, 106.0, 127.7,
128.1, 128.7, 137.0, 143.6, 160.4.
Experimental procedures:
(i) Synthesis of NaphMC, L and NaphMC-Ni(II) complex:
The heteroditopic macrocycle having naphthalene moieties
(NaphMC), the azide terminated 5,5’-dimethyl-2,2’-bipyridine
derivatized ligand (L) and NaphMC-Ni(II) complex were synthesized
by following the literature procedures.17,
18
All the characteristic
data of NaphMC, L and NaphMC-Ni(II) complex were exactly
matched with the reported data.
(ii) Synthesis of [2]pseudorotaxane [NiPR(ClO4)2]:
Ni(ClO4)2.6H2O (36.5 mg, 0.1 mmol) was dissolved in CH3OH and
added to a solution of NaphMC (58 mg, 0.1 mmol) in CHCl3. The
reaction mixture was stirred for 2 hours at room temperature (RT),
resulting into formation of NaphMC-Ni(II) complex. In situ, azide
terminated bidentate ligand as L (38 mg, 0.1 mmol) was added to
the above solution. The resultant reaction mixture was stirred for
another 3-4 hours at RT. Therefore, Ni(II) templated threaded
complex i.e. [2]pseudorotaxane {[NiPR(ClO4)2]} was formed. At that
(v) Synthesis of STP:
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J. Name., 2013, 00, 1-3 | 7
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Compound L4 (320 mg,1 mmol) was added dropwise using pressure Data of AXLE: M.P: above 200 °C. HRMS V(ieEwSIA-rMticSle)Onflioner
+
DOI: 10.1039/D0DT03645E
equalizer funnel at ice cold condition (0°C) to the NaH (48 mg, 2 {C64H60N10NaO8 = [AXLE + Na] } calcd, m/z = 1119.4493; found, m/z
=1119.4495. IR (KBr,  cm-1): 698, 738, 835, 1056, 1157, 1296, 1375,
1454, 1548, 1595, 1662 (amide C=O), 2864, 2920, 3062, 3299, 3384.
1H NMR (500 MHz, DMSO-d6): δ (ppm) 3.75 (d, 4H, - CH2), 4.42 (s,
4H, - CH2), 4.54-4.60 (m, 8H, - CH2), 5.05 (s, 8H, -CH2), 6.57 (s, 6H,
Ar-H), 7.30-7.42 (m, 20H, Ar-H), 8.15 (s, 2H, Ar-H), 8.26 (d, 2H, J =
8.0 Hz, Ar-H), 8.45 (d, 2H, J = 8.5 Hz, Ar-H), 8.93 (t, 2H, J = 5.0 Hz, -
NH), 9.04 (s, 2H, Ar-H). 13C NMR (125 MHz, DMSO-d6): δ (ppm)
48.2, 62.8, 69.2, 70.8, 100.8, 106.3, 120.4, 124.3, 127.6, 127.7,
128.3, 130.0, 136.2, 136.9, 140.7, 143.7, 148.3, 156.2, 159.4, 164.8.
mmol) solution in THF. Then, propargyl bromide (103 µL,1.2 mmol)
was dissolved in THF and added dropwise to the reaction mixture at
stirring condition at 0°C under N2 for 12h followed by reflux for 12h.
After cooling the reaction mixture, minimum amount of CH3OH was
added for deactivating the NaH. Solvent was removed under vacuo
and extracted with CHCl3 and water. The organic layer was
evaporated and purified via silica gel column chromatography using
2% ethylacetate-petether as eluent. White coloured solid was
isolated as pure form. Yield: 272 mg, 76%.
M.P: 56 °C. HRMS (ESI-MS) for {C24H22NaO3 = [STP + Na]+} calcd, m/z
= 381.1467; found, m/z =381.1466. IR (KBr,  cm-1): 628, 696, 748,
837, 908, 1031, 1087, 1161, 1218, 1292, 1377, 1448, 1595, 2111
(vii) Synthesis of pyrene derived [2]rotaxane (PYROTX):
ROTX (167 mg, 0.1 mmol) was dissolved in a CH2Cl2. N,N-
diisopropylethylamine (DIPEA) was added to it and stirred for 0.5 h
at RT. After addition of pyrene bromide (88 mg, 0.3 mmol), the
reaction mixture was stirred for 24 h at RT. Solvent was evaporated
and dried. The crude residue was dissolved in CHCl3. It was washed
with dil HCl (0.1 M) and brine solutions sequentially. Then, the
organic layer was dried over Na2SO4. The solid residue was collected
after evaporation of the organic layer. Furthermore, by washing the
1
(C≡C), 2867, 2916, 3031, 3259 (H–C≡). H NMR (400 MHz, CDCl3): δ
(ppm) 2.47 (d, 1H, - CH), 4.17 (d, 2H, - CH2), 4.57 (s, 2H, - CH2), 5.05
(s, 4H, -CH2), 6.58 (s, 1H, Ar-H), 6.64 (s, 2H, Ar-H), 7.31-7.44 (m,
10H, Ar-H). 13C NMR (125 MHz, CDCl3): δ (ppm) 57.2, 70.3, 71.6,
74.8, 79.8, 101.8, 107.1, 127.7, 128.1, 128.7, 137.0, 139.9, 160.3.
(vi) Synthesis of [2]rotaxane (ROTX):
NiPR(ClO4)2 (122 mg, 0.1 mmol) and STP (79 mg, 0.22 mmol) were solid residue with hexane, yellowish white coloured solid product
dissolved in a CH2Cl2-CH3CN (1: 1) binary solvent mixture. Then, the was isolated as pure form in 78% yield (180 mg).
solid Na2CO3 (5 mg, 0.05 mmol) was added to the reaction M.P: 152-156 °C. HRMS (ESI-MS) for {C151H130N13O12 = [PYROTX +
container followed by addition of Cu(CH3CN)4PF6 (56 mg, 0.015 H]+} calcd, m/z = 2316.9962; found, m/z =2316.9960; [PYROTX +
2H]2+ calcd, m/z = 1159.0020, found, m/z = 1158.9970. . IR (KBr, 
cm-1): 551, 698, 757, 846, 935, 1056, 1097, 1157, 1249, 1371, 1454,
1504, 1593, 1666 (amide C=O), 2854, 2923, 3035, 3398. 1H NMR
(300 MHz, DMSO-d6): δ (ppm) 2.73 (s, 8H, - CH2), 3.90 (s, 4H, -CH2),
4.06 (s, 4H, - CH2), 4.19 (s, 4H, - CH2), 4.36-4.68 (m, 22H, - CH2), 4.97
(s, 8H, - CH2), 5.75 (d, 2H, J = 7.5 Hz, Ar-H), 6.09 (s, 2H, Ar-H), 6.48-
6.55 (m, 6H, Ar-H), 7.00-7.09 (m, 6H, Ar-H), 7.24-7.42 (m, 22H, Ar-
H), 7.88-9.04 (m, 41 H, Ar-H & -2NH). 13C NMR (75 MHz, DMSO-d6):
δ (ppm) 41.2, 48.6, 56.9, 57.9, 62.8, 62.9, 64.9, 69.2, 70.3, 70.6,
70.8, 71.1, 77.4, 79.2, 100.8, 106.3, 111.8, 120.5, 123.4, 123.8,
124.1, 124.2, 124.4, 125.0, 125.3, 126.1, 126.7, 127.5, 127.6, 127.7,
128.3, 128.4, 129.9, 130.1, 130.3, 136.2, 136.9, 137.0, 140.6, 140.6,
143.7, 148.4, 151.9, 155.4, 156.3, 159.4, 165.4.
mmol) in the glove box. The reaction mixture was stirred for 12h at
RT. Solvent was completely evaporated. Saturated Na2EDTA
aqueous solution and aqueous ammonia solution were added to
the solid residue and stirred for 32-36 h. The crude product was
extracted with CHCl3 and water. The organic layer was evaporated
and dried. The crude mixture was purified by using neutral alumina
column chromatography eluting with 2% CH3OH/CH2Cl2, to obtain
off white coloured metal free [2]rotaxane (70 mg, 42%) along with
white coloured AXLE (38 mg, 35%).
Data of [2]rotaxane (ROTX): M.P: 182-185 °C. HRMS (ESI-MS) for
{C100H100N13O12 = [ROTX + H]+} calcd, m/z = 1674.7614; found, m/z
=1674.7616; [ROTX + 2H]2+ calcd, m/z = 837.8846, found, m/z =
837.8848. IR (KBr,  cm-1): 613, 648, 698, 740, 754, 833, 935, 1026,
1054, 1095, 1157, 1251, 1296, 1377, 1454, 1548, 1595, 1662 (amide
C=O), 1726, 2852, 2921, 2954, 3064, 3317, 3404, 3573. 1H NMR
(300 MHz, CDCl3): δ (ppm) 2.96 (s, 4H, - CH2), 3.05 (s, 4H, - CH2),
3.80 (s, 4H, -CH2), 3.99 (d, 4H, - CH2), 4.13 (s, 4H, - CH2), 4.45 (s, 4H, -
CH2), 4.52 (s, 4H, - CH2), 4.64-4.71 (m, 8H, - CH2), 4.94-4.99 (m, 8H, -
CH2), 5.79 (d, 2H, J = 7.8 Hz, Ar-H), 6.50-6.61 (m, 8H, Ar-H), 7.01-
7.07 (m, 8H, Ar-H), 7.31-7.36 (m, 20H, Ar-H), 7.55 (s, 2H, Ar-H), 7.76-
7.85 (m, 4H, Ar-H), 8.11 (d, 2H, J = 8.4 Hz, Ar-H), 8.41 (d, 2H, J = 8.1
Hz, Ar-H), 8.79 (s, 2H, -NH), 9.00 (s, 2H, Ar-H). 13C NMR (100 MHz,
CDCl3): δ (ppm) 40.0, 49.6, 62.0, 63.7, 63.8, 66.0, 70.1, 70.2, 72.6,
72.7, 101.5, 106.9, 107.0, 111.9, 121.0, 121.3, 122.3, 123.5, 123.8,
124.7, 125.3, 127.7, 128.1, 128.7, 129.6, 136.0, 136.9, 140.3, 145.3,
145.5, 148.2, 148.4, 153.2, 157.5, 160.2, 166.1.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
P.G. gratefully acknowledges the Science and Engineering
Research Board (SERB), New Delhi (Project SR/S1/IC-39/2012)
for financial support. S. Bej acknowledges the Council of
Scientific & Industrial Research (CSIR) and IACS for SRF. M.
Nandi acknowledges DST-Inspire and IACS for SRF.
8 | J. Name., 2012, 00, 1-3
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5. (a)J.-J. Lee, A. G. White, J. M. Baumes and B. D.VSiemw iAtrhtic,leCOhnelmine.
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Table of Contents
A Cd(II) and Zn(II) selective naphthyl based [2]rotaxane acts as an exclusive Zn(II)
sensor upon further functionalization with pyrene
Somnath Bej, Mandira Nandi and Pradyut Ghosh*
A new pyrene derived multi-functional [2]rotaxane is synthesized using click chemistry via Ni2+ templation and - stacking interaction
which is served as selectively responsive sensor for Zn2+ ion.
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