Fluorophoric [2]rotaxanes: Post-synthetic functionalization, conformational fluxionality and metal ion chelation

Article abstract of DOI:10.1039/d0nj00353k

Herein, we report the synthesis of an amino-ether fluorophoric macrocyclic wheel (MC)-based multi-functional [2]rotaxane. This rotaxane was further functionalised into three different analogues, which were denoted as ROTa-c, upon appending three different

Full text of DOI:10.1039/d0nj00353k

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Fluorophoric [2]Rotaxanes: Post-Synthetic Functionalization,
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Conformational Fluxionality and Metal Ion Chelation
Saikat Santraa,b, Pradyut Ghosha*
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a
School of Chemical Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032,
India. Email. Id.: icpg@iacs.res.in
b Department of Chemistry, Taki Government College, Taki, North 24 Parganas, West Bengal, India, Pin Code
No. 743429.
Abstract
Herein, we report an amino-ether fluorophoric macrocyclic wheel (MC) based multi-functional
[2]rotaxane. The said rotaxane is further functionalised to three different analogues as ROTa-c upon
appending three different amide groups [-NC(O)R, where R = -CH3, 4-fluorophenyl and tertiary butyl
respectively]. Such post-synthetically analogues are characterised via mass spectrometry, 1D/2D NMR
and optical spectroscopy. Steric effects exerted by the tertiary amide groups on the rotamer induced
dynamic motions among ROTa-c are investigated via variable temperature NMR spectroscopy.
Although the tri-acetylated ROTa and tri-aryl substituted ROTb show rotamer induced conformational/
co-conformational diversities, one conformer is predominant in ROTc under identical experimental
conditions due to the restricted rotation of bulky tertiary butyl groups. Interestingly, intramolecular
exciplex formation between the wheel and the axle is exhibited by ROTa-c, which is established
through detail emission studies in solution state. The exciplex emission generated by rotaxanes is
monitored to predict the preferential coordination sites of metal ions (alkali and transition) in the above
interlocked systems. Finally, chelating property of fluorophoric rotaxanes is investigated with different
transition metal ions by absorption as well as emission spectroscopy studies.
Introduction
Mechanically Interlocked Molecules (MIM‟s) are the species in which more than one sub-component are
generally entwined and held together without any direct covalent interactions but the sub components are
unable to pass through one another. One of such MIM‟s is rotaxane which consist of a macrocyclic ring
wrapped around a linear rod shaped axle.1-4 Till date, significant progresses have been made for the
development of MIM‟s using different templating agents like cations,5 anions1,6, aromatic stacking
interactions,7 H-Bonds,8 halogen bonds,9 chalcogen bonds10 a etc.
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Now a day, the exploration of
MIM‟s including rotaxanes are increased dramatically for variety of applications such as sensing,1
recognition ,2,3 catalyst4, development of molecular machines etc.
11-48
One of such application is the

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metal ion binding studies where the chelation occurs through endotopic manner inDOroI:t1a0x.10a3n9e/Ds0.NJS0u03c5h3K
endohedral binding is predictable but the coordination may change the conformation and significantly
change their properties.48 (a)
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On the other side, tertiary amide bond is popular to generate a set of rotamers via rotation around N-
C(O) bond.49 Based on this concept, Clayden and others have synthesized smart non-interlocked
molecules as molecular gears upon introducing tertiary amide in the molecular frame work.28,50-53
However, the effect of such N-C(O) bond rotation of the tertiary amides embedded with the MIM‟s are
studied rarely.54, 55 Additionally, chemists are attracted to combine the electronic, chelating properties of
metal ions56-60 with the photo-physical properties of MIMs in multiple ways.61-66 Such property is useful
to sense or recognize a guest and to promote/ regulate molecular motions.63, 64 Intramolecular exciplex
formation between the non-covalently attached molecules are one kind of examples of aforesaid
phenomena. “Rotaxane exciplexes” are investigated by Swager et al. and their fundamental studies of
supramolecular photochemistry are relevant to develop (i) new compounds with various
photochemical/photophysical properties and (ii) chemical sensors.48 (b) In our recently published reports,
we have demonstrated that tri-acetylated ring components of multi-functional [2]rotaxanes show
rotamers induced dynamic property at room temperature, which can be controlled by the addition and
removal of alkali metal ions.67, 68 To reveal the cause of such controllable dynamic properties and effect
of steric crowding at N-C(O) bond, herein, we report heteroditopic fluorophoric macrocyclic wheel
based various tertiary amides appended [2]rotaxanes to explore (i) structure-property relationship in
terms of rotamers induced conformational/ co-conformational variation, (ii) mechanistic details of the
alkali metal ion triggered conformational/co-conformational changes, and (iii) transition metal ion
binding properties of interlocked species by absorption and emission spectroscopic studies.
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Results and Discussion
Designing aspects and strategies: Designing aspects and strategies: The targeted fluorophoric
[2]rotaxanes and their non-interlocked components are depicted in Chart 1. The component of the
rotaxanes integrates a naphthyl unit, which is strategically positioned opposite to the tris-amide pocket.
In this respect, our previous result demonstrate that rotaxane show various conformations/ co-
conformations upon tri acetylation.67 The existence of different conformation/ co-conformation is due to
the rotation around N-C(O) bonds in tertiary amides which are appended in the interlocked system
(Chart 1). To investigate the steric effect on such conformations/ co-conformations, we have
synthesized tertiary amide group functionalized new fluorophoric rotaxanes upon appending aryl rings
and tertiary butyl groups along with acetyl group to rotaxanes composed of napthyl integrated
macrocyclic wheel (Scheme 1 and 2).

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Chart 1. Chemical diagram of tertiary amide appended [2]rotaxanes ROTa-c, TROT and the non-
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interlocked components: MC,MCa-c, AXL, STP and DMBL with labelling (wherever necessary)

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Thus such modifications would be useful to establish the structure-property relationships of rotamers
induced dynamic property in multi-functional [2]rotaxanes and to confirm the actual binding site of
metal ions during metal ion assisted conformational locking via detailed absorption and emission
spectroscopic studies.
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Synthesis of Interlocked Species: The new fluorophoric macrocycle (MC) and their derivatives (MCa
c) are synthesised using high dilution method as described in the experimental section (Scheme 1).
[A]
[B]
Scheme 1 [A] Synthetic route of macrocycle MC (Yield: 70 %). [B] Synthetic route of the functionalised
macrocycles MCa-c.

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Scheme 2. Synthetic route of Ni II templated [2]pseudorotaxane, (PROT) (Yield : 76%); synthetic route
of ROT´ [Yield: 39%] where R = -C(O)OC(CH3)3; de-protected ROT´ and synthetic route of
functionalized [2]rotaxanes, ROTa-c within 79% to 82 % yield from [TROT].Chemical diagram of
[MROT] +2 .Chemical diagram of [TROT] and ROTa-c, are depicted in Chart 1. [Boc2O = Di-tertiary-
butyl dicarbonate].
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Characterization spectra of MC, MCa-c and their precursors are depicted in Figure 1SD-OI2: 130S.10i3n9/DE0SNJI0†0.35I3tK
is to be noted that absorption spectra of MCa-c show peaks at the region of 212 - 300 nm whereas upon
excitations of MCa-c at 285 nm, result in luminescence spectrum at ∼340 nm in 9:1 (v/v) CH3CN/
CHCl3 solvent mixture (Figure 23S in ESI†). Then the fluorophoric wheel, MC is employed to
synthesise rotaxanes via passive metal ion templation method. The reaction of MC with NiII and azide
terminated axle, AXL forms MC based [2]pseudorotaxane, PROT (Scheme 2, Chart 1 and Figure 24S-
28S in ESI†). PROT and alkyne-terminated stopper, STP are used as precursors to generate metallated
[2]rotaxane, MROT via click reaction (Chart 1, Scheme 2, Figure 29S in ESI†).
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From MROT, three functionalised [2]rotaxanes (ROTa-c) are synthesized as post synthetic
functionalization ROT´ and isolated in good yields in the range of 79-82% (Scheme 2). Successful
formation of tris tertiary amide functionalised rotaxanes are evidenced by mass spectrometry, 1H-NMR,
13C NMR, COSY, NOESY, ROESY, DOSY spectroscopy, MALDI mass spectrometry and optical
spectroscopic studies. Detailed synthesis, characterization and electronic spectral data are described in
experimental section. Characterization spectra are given in ESI† section (Figure 30S – 71S in ESI†).
Comparative NMR spectra of the post-synthetically functionalised rotaxane and their non-
interlocked subcomponents: The conjoint shielding effect among aromatic π systems due to the donor–
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acceptor interactions is clearly evident upon interlocking, from the comparative H-NMR spectrum of
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interlocked species and those of its non-interlocked counterparts. A comparison of the H NMR
spectrum of rotaxane (ROTa-c) with those of its non-interlocked components, MCa-c and axle (DMBL)
in DMSO-d6 reveals significant upfield shifts in the signals for the two lateral aromatic macrocycle
protons Ar-He and Ar-Hf and bipyridine protons Ar-H1, 2, 3 upon interpenetration (Figure 1).

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Figure 1.[A] Chemical diagram of functionalised Rotaxanes ROTa-c with labels. [B] Comparative 1H-
NMR spectra of (a) MCa, (b) ROTa, (c) Centre piece DMBL; [C] Comparative 1H-NMR spectra of (a)
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DMBL, (b) ROTb, and (c) MCb; [D] Comparative H-NMR spectra of (a) DMBL, (b) ROTc, and (c)
MCc. All the studies are carried out in DMSO-d6 solvent at 298 K ( 300 MHz ). The circle marked
proton (δ = 3.46 ppm) indicate the solvent. Labelling corresponds to the proton signals which are
mentioned in the chemical diagram of Chart 1.
It is mentioned in the introduction section that tertiary amides are capable to generate rotational isomers
due to rotation around N-C(O) bonds.28,49 Since the post-synthetically functionalised rotaxanes ROTa-c
composed of various tertiary amide moieties, it is expected that they will show existence of different
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rotational isomers in solution and indeed the H-NMR spectra of most of the rotaxane (ROTa-b) show
the peak multiplicities due to the existence of various rotational isomers. Comparative analyses of the
1H-NMR spectra of the functionalized rotaxanes show that with respect to the tri-acetyl substituted
rotaxane (ROTa), the peak multiplicities in aryl substituted rotaxane (ROTb) is notably decreased
(Figure 2). These observations clearly indicate that due to the restricted rotation of aryl ring compared
to the acetyl group appended rotaxanes, numbers of conformers and co-conformers are less in ROTb as

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compared to ROTa under identical conditions in both CDCl3 and in DMSO-d6 solvent.On the other side,
1H-NMR spectrum of ROTc shows the absence of peak multiplicity/ broadening that indicates the
preference of a single conformer/ co-conformer over other under identical experimental conditions. We
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have also carried out low temperature H-NMR of ROTb and ROTc in CDCl3 which show that our
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assumptions are quite precise, as the preferential existence of one conformer of ROTc is noticed upto
243 K (Figure 63S - 64S in ESI†). As the size of tertiary butyl group is larger, the three bulky group in
ROTc may not allow to rotate freely the tertiary –NC(O) bond in ROTc. But such rotations are
comparatively facile in ROTa due to the small size of acetamido group and as a consequence of that
multiple conformers/ co-conformers exist in ROTa and comparatively less numbers of rotational
isomers are exist in ROTb and in ROTc.
Figure 2. Comparative 1H-NMR spectra of ROTa (Top), ROTb (Middle), and ROTc (Bottom) at 298 K
(300 MHz) in CDCl3 (left) and in DMSO-d6 (right). The blue circle in the spectrum of ROTb in right

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indicates the peak (δ = 5.76 ppm) corresponds to dichloromethane solvent impurity. DTOhI:e10l.1a0b39e/lDs0NoJf00t3h5e3K
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protons are depicted in the chemical diagram of the [2]rotaxanes (Top).
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Rotamer induced conformational fluxionality studies via variable temperature 1H-NMR:
VT-1H-NMR spectroscopy are carried out in cases of all the substituted rotaxanes to imply the transition
from slow state to the time average state (Figure 60S, 61S and 62S for ROTa and ROTc respectively in
ESI†, Figure 3 for ROTb).
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ROTa: Multiplicity in case of ROTa resembles our previously published report.67, 68 1H-NMR spectral
evidences from Figure 60S in ESI† shows that aromatic protons of the bipyridine part of the centre
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piece, HAr , HAr and HAr which resonates at δ = 8.62 to 8.74 and 8.15 to 8.30 ppm as multiplets, start
to accumulate with increasing temperature and finally coalesce at 370 K. With increasing temperature,
amide -NH4 protons show gradual upfield shift due to the clevage of intermolecular H-bond with the
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polar solvent (DMSO-d6). At 370 K, -NH4 merged with the proton signals appeared for HAr and HAr
with proper integral value. Proton signals associated with the wheel component of ROTa shows peaks
multiplicity at δ = 6.25 to 6.73 ppm and δ = 5.70 to 6.09 ppm for HArf and HAre respectively, they started
to accumulate with increasing temperature and finally coalesce into two different set of peaks at 6.03
and 6.48 ppm with proper integration ratio at high temperurature. Methyl protons of the tertiary
acetamido groups which split in the ranges of δ = 1.95-2.25 ppm at room temperarute, show two broad
peaks with expected integral values at high temperature. Except Hk, Hk‟ and H8 protons, ROTa consist
seven other aliphatic protons, which includes five from wheel (Hc, Hd, Hg, Hh, Hi) and two from centre
piece (H5, H6) that resonate between δ = 3.25 to 4.50 ppm as multiplets at room temperature. But upon
heating, the multiplicity of all those seven proton signals convert to seven distinguishable peaks with
proper integration ratio in the range of δ = 3.42 to 4.57 ppm at 393 K (Figure 60S and 61S in ESI†).
ROTb: Figure 3a clearly shows that at higher temperature, the multiplets associated with the particular
peaks of ROTb are converged and resonate within a narrow range compared to the room temperature
NMR spectrum. Since the aryl ring contains 4-fluoro substituent, this kind of exchange is also
monitored via proton decoupled 19F-NMR spectroscopy. ROTb shows multiple signals at the range of δ
= -112.0 to -114.0 ppm for the fluorine at 298 K. At 353 K, the multiple signals of 19F-NMR is reduced
to only two which can ascribed to one middle and two for terminal aryl rings (Figure 4).There are only
one set of peaks (e.g. HAr1,2,3,7, -NH4) in ROTb which reveals that either there is only one conformer or
the exchange is rapid. Actually, the tertiary amide groups are appended in the wheel component and the
effect of rotamer induced conformational/ co-conformational variation should be more prominent in the
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wheel component rather than the DMBL part of the rotaxane. Thus, it is observed that the HAr proton

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show broad and multiple peaks at room temperature [Figure 2 and 3a]. Ar-Hl,m are the parts of the
wheel sub-component and thus the rotamer induced fluxionality should be prominent in the mentioned
proton. As the aromatic protons Ar-H9,10,11,a,a„,b,l,m are resonating within the same ppm range, it is hard
to isolate them separately. Being a part of ring component, Ar-Hl, Hm [Chart 1] should show broad or
multiple peak at room temperature. Indeed, Ar-Hl, Ar-Hm show the conformational variation at room
temperature.
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Figure 3.(a) Variable temperature H-NMR spectra of ROTb in DMSO-d6 at 300 MHz. The triangle
indicates the upfield shifting of the amide proton peak -NH4 upon increasing temperature. At 370 K, the
amide proton peak –NH4 merges with the aromatic proton Ar-H2, (b) Chemical diagram of ROTb.
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The F-NMR spectrum shows the real scenario. As fluorine is attached with the aromatic rings of the
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wheel component, the F signals show the multiplicity at room temperature (298 K) and upon heating
at 353 K (high temperature) spectrum shows only two peaks (2:1 ratio) attributed to middle and
terminal aryl rings (Figure 4). This supports fast inter-conversion on the NMR timescale at higher
temperature.

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Figure 4. Proton decoupled, temperature dependant 19F-NMR spectra of ROTb in DMSO-d6 at 500
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Figure 5. Variable temperature 1H-NMR data of ROTc in DMSO-d6 (300 MHz). The sDtaOrI: 1m0.1a0k3e9/dD0pNrJo00t3o5n3K
is corresponds to the amide –NH4 proton which show the upfield shift upon raising temperature
ROTc: Due to restricted rotation of the tris-tertiary butyl group appended rotaxane ROTc, the numbers
of conformers and co-conformers are less compared to the ROTa and ROTb. However, we have
undertaken the variable temperature 1H-NMR experiment of ROTC in DMSO-d6 solvent system and the
data analysis reveals that there is practically no significant changes are observed at higher temperature
NMR data except the amide peak of the centre piece component (-NH4) which is resonate around δ =
8.75 ppm at 298 K and shifted to upfield with raising temperature (Figure 5). Low temperature NMR
data also support our conclusion (Figure 63S and 64S in ESI†).
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Photophysical properties of rotaxanes: Absorption spectra of substituted wheels (MCa-c) show peak at
~ 285 nm whereas, ROTa-c show peak at the zone of 285-305 nm (Figure 6a for MCc and 23S in ESI†
for MCa and MCb, Figure 6b for ROTc). On the other side, photoluminescence spectra of macrocycles
MCa-c show peak at the region ~ 340 nm (excitation at 285 nm, Figure 6c for MCc and 23S in ESI† for
MCa and MCb). Interestingly, bipyridyl-based axle threaded wheel i.e. rotaxanes ROTa-c show
altogether different photoluminescence properties than their non-interlocked counterpart MCa-c (Figure
6d, Figures 23S, 59S in ESI†) when excited at 285 nm. An additional new broad band at around ~470
nm is observed along with a characteristic peak of naphthyl moiety at around ~340 nm. However, there
is no significant difference in the absorption spectra of the interlocked species when compared with
their non-interlocked components (Figure 66S-70S in ESI†). Thus, these results reveal that the
generation of such broad spectrum is an excited state phenomenon. To ascertain the nature of the broad
band (intra vs. inter molecular charge transfer), we have carried out the solvent and concentration
dependant photoluminescence studies of ROTa-c. Experimental evidences reveal that the broad band is
highly solvatochromic in nature. In fact, the broad band shifts from 491 nm in toluene to 527 nm in
tetrahydrofuran, 470 nm in CH3CN and the peak vanishes in CH3OH. When a polar protic solvent like
CH3OH is added to the CH3CN solution of tris-tertiary amide functionalized rotaxanes (Figure 7a, 7b,
and 72S-74S in ESI†) then also the broad peak gradually diminishes, and vanishes completely after the
addition of ~10% CH3OH. Further, the quenching of PL-intensity shows a linear relationship with
concentration (Figure 7c, 75S-77S in ESI†). These set of experiments suggest that the new broad band
must be arising due to an intramolecular exciplex formation and not from the intermolecular
aggregation within the experimental conditions. The exciplex is expected to be formed between the
electron-rich naphthyl group present in the wheel component and the electron-deficient bipyridine unit
of the thread (Figure 7e). Even 1:1 mixtures of functionalized macrocyclic ring and the centre piece do
not show the formation of such exciplex (Figure 71S in ESI†). Thus, it can be inferred that an electron-

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rich group in the wheel and an electron deficient functionality in axle are in close DpOrIo: 1x0i.1m03i9t/yD0aNnJ0d035is3K
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responsible for executing intra-molecular interaction in the excited state.
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Figure 6. Absorption spectra of (a) MCc (Concentration: 1.1 X 10-5 M) and (b) ROTc (Concentration:
1 X 10-5M) spectrum; Emission spectra of (c) MCc (Concentration = 3 X 10-5 M) and (d) ROTc
(Concentration: 2.9 X 10-5M). In all the cases analytes are excited at 285 nm in 9:1 (v/v) CH3CN/CHCl3
binary solvent mixture.
In this context, Swager et al. have reported “rotaxane exciplex” where intramolecular donor-acceptor
interaction exists between "bipyridine ring containing centre piece" and "phenyl acetylene group" of the
wheel counterpart.48 (b) Further, the higher temperature stability of the exciplex is checked by recording
the PL over a wide temperature range, i.e., from 283 K (10º C) to 348 K (75º C) in CH3CN (Figure 7d
and Figure 78S and 79S in ESI†).

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Figure 7. (a) Solvatochromic change of the exciplex peak; (b) quenching of the exciplex peak intensity
upon gradual addition of polar protic solvent CH3OH to the solution of ROTc in 9:1 (v/v) CH3CN/
CHCl3. (c) Relative intensity vs. concentration plot upon monitoring the exciplex peak indicating the
broad peak is appeared due to intramolecular donor-acceptor interaction; (d) Thermal stability of
exciplex monitored through variable temperature PL studies (Spectra are recorded in 283 K, 288 K,
293 K, 298 K, 303 K, 318 K, 328 K, 348 K temperature). Concentration was taken around 2.9 X 10-5M
for all the cases and particularly for (b), (c) and (d), 9:1 (v/v) CH3CN/ CHCl3 binary solvent system was
chosen. In every cases the solutions were excited at ex =285 nm. [In these diagrams all spectra are for

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ROTc. Same experimental data of ROTa and ROTb are presented in the ESI† section, 71S-79S in
ESI†],(e) Chemical diagram of ROTa-c with labelling showing plausible rotation around N-C(O) bond
and donor-acceptor interaction in the excited state which results the exciplex formation.
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These variable temperatures PL experiment suggest that even at 353 K (80oC), the wheel and axle
components are present in close vicinity in ROTa-c which is further supported by the high temperature
1H-NMR experiments. The aromatic protons of the bipyridine moiety (H1, H2 and H3) and the rings (He
and Hf) which are resonating at upfield region upon interpenetration, remain almost unperturbed at
higher temperature (Figures 60S, 62S in ESI†). These set of experiments confirm that the wheel resides
on the bipyridine part of the axle even at higher temperature. Next, the appearance of such exciplex
assists to investigate the metal ion complexation property in those multi-functional rotaxanes which are
discussed in the later section.
1
Effect of Na+/ Cu2+/ Ni2+on photophysical properties of rotaxanes: H-NMR studies reveal regulation
of the rotamers induced kinetic activity by alkali metal ions and crown ethers in case of ROTa (Figure
80S in ESI†). Experimental output reveals that the accumulation of peak multiplicities in the presence
of Na+. Conversely, sequestering the alkali metal ion by suitable crown ether regenerates the peak
multiplicities. Details of the peak accumulation in presence of alkali metal ions are shown in the
supporting information section (Figure 80S in ESI†). This result resembles our previous reports which
indicate the coordination of alkali metal ions through the MIM has a control towards conformational/
co-conformational fixation. [In brief, we are interested to demonstrate the effect of Na+ on rotamer
induced dynamic behaviour (originated due to tertiary amide bonds rotation), exhibit by ROTa via 1H-
NMR studies. Addition of equivalent amount Na+ salt solution in CD3CN in ROTa in 9:1 (v/v)
CDCl3:CD3CN solvent mixture executes a typical accumulation of most multiplets of ROTa.
Comparative 1H-NMR analysis of ROTa and (ROTa + Na)+ clearly indicate that the peaks corresponds
to the lateral aromatic rings of the wheel components i.e. He and Hf also congregate to a less number of
peaks compared to ROTa. Na+ assists to halt/retard the dynamic property upon coordinating with the
metal chelating sites present in the rotaxane. When equivalent amount of 18-crown-6 is added as an
input to the mixture of ROTa and Na+, regeneration of the multiple peaks are observed in the 1H-NMR
spectrum. This can be easily explained upon considering the phenomenon that crown ether removes Na+
from the system and as a consequence of that the multiple conformations/ co-conformations again
started to exhibit.]
At this juncture, it would be appropriate to study the effect of alkali metal ion on the photoluminescence
property of fluorophoric rotaxanes which might provide relevant information on the conformational and

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co-conformational fixation of this category of rotaxanes. In presence of Na+, thereDiOsI: p10r.1a0c3t9i/cDa0NllJy003n5o3K
perturbation in the absorption and emission spectra (Figure 8a and 8b). Thus it indicates that bipyridine
unit is also not involved towards coordination with such metal ions. If Na+ binds at the oxy-ether pocket
of the ring and/ or the bipyridine unit then one would expect an obvious change in the exciplex emission
intensity due to chelation. This experiment tells that the Na+ is not binding at the oxy-ether pocket
(Figure 8c). Then, the photophysical study of ROTa in the presence of transition metal ions such as CuII
/ NiII as a guest is carried out.70
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The UV/Vis study reveals the involvement of bipyridine part of the DMBL component (Figure 8d). As
the bipyridine unit is responsible for the exciplex formation, it is expected that upon suitable metal ions
coordination with the bipyridine unit, the exciplex formation might be obstructed. In fact, the PL
intensity of the exciplex is decreased to a significant extent in presence of CuII and NiII which also
confirms the involvement of bipyridine unit towards metal ion chelation (Figure 8e). Upon chelation,
particularly, between CuII / NiII with bipyridine unit, the donor-acceptor interaction between
naphthalene and bipyridine in the excited state is interrupted and as a result of that fluorescence
quenching of the exciplex emission is observed at ~470 nm. One may assume the binding mode of the
transition metal ions as depicted in Figure 8f where additional coordination may provide by the tris
tertiary amide moiety.
Thus along with the sp2 nitrogen atoms of bipyridine moiety, other coordination sites are tertiary amide
N-atoms and this assumption is based on the report by Bannwarth et al.72 These set of experiments
reveals that the probable site of the alkali metal ion coordination could be the O-atoms of tertiary amide
carbonyls in exotopic mode (Figure 8c), not within the cavity of the wheel, whereas, transition metal
ions are binding in the endotopic mode. In case of Na+, binding through the amide carbonyl might cause
the particular preferential orientation of the acetamido group and as a result, in presence of Na+,
conformational fixation might take place. It is important to mention that our recent report showed that
even in the absence of oxy-ether group, tris-acetamido containing amide based macrocyclic wheel
derived rotaxane (ROTa´, chemical diagram is shown in Figure 81S in ESI†) can also show
conformational fixation upon addition of investigated alkali metal ions.68

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Figure 8. (a) Unperturbed absorption spectra of ROTa (1.56 X 10-5 M) in the presence of equimolar
concentration of Na+; (b) Unperturbed photoluminescence spectra of ROTa (1.56 X 10-5 M) in the
presence of equimolar concentration of Na+, excited at ex =285 nm. (c) Exotopic binding mode of
alkali metal ion through amide carbonyl; (d) perturbed absorption spectra of ROTa (1.56 X 10-5 M) in
the presence of equimolar amount of CuII; (e) photoluminescence spectra of ROTa (1.56 X 10-5 M) in
the presence in the presence of equimolar amount of CuII, excited at ex =285 nm; (f) Endotopic binding
mode of transition metal ion. In all the cases 9:1 (v/v) CH3CN/ CHCl3 solvent is used.
On the other hand, report by Heinrich et al. states that the presence of proximal positive charge causes
the conformational fixation of tertiary amides.73
Detailed solution state transition meal ion binding with the rotaxanes: In the previous section, we
have observed that ROTa binds with the transition metal ions like CuII / NiII.Thus, here we have
undertaken detailed solution state studies on the metal ion coordination of MIM‟s using absorption and
emission spectroscopy. In a typical experiment, ROTa (~10-5 M in 9:1 CH3CN/CHCl3, v/v) is used as
host whereas, ~10-4 M solutions of various metal ions (NiII, CoII,CuII, FeIII, MnII, CrII) are used as guests
in CH3CN. Upon addition of guest to the host, the red shift of the absorption bands corresponding to n-
π* transition of the bipyridine unit are observed with a concomitant decrease of the parent absorption
peak intensity at 302 nm, particularly, in cases of CuII and NiII. Interestingly, a gradual increase in the
intensity of red shifted band at around 320-325 nm is observed in case of ROTa, up to the addition of
1.0 equivalent of corresponding metal salts (Figure 9a and 9d). Other investigated metal ions such as
FeII, MnII, CrIII, NaI, KI, LiI are unable to perturb the absorption spectrum (Figure 8, 83S in ESI†). A

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clear isosbestic point is maintained during the addition of 1.0 equiv of CuII and NiII. Job plot and
equivalent plot analysis of these absorption spectroscopic titration data shows an inflection point at 0.5,
supporting a 1:1 host-guest stoichiometric binding (Figure 9b and 9e and Table 1).
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Figure 9. Absortion spectroscopic titration profile of ROTa (1.0 X 10-5 M) with (a) CuII (1.2 X 10-4 M)
and (d) NiII (1.1 X 10-4 M). Job plot [Figure (b) for CuII and (e) for NiII] and Equivalent plot [Figure (c)
for CuII and (f) for NiII] indicates the 1:1 stoichiometric binding. Spectra are recorded in 9:1 CH3CN /
DMF (v/v) solvent mixture.
Binding constants are calculated by using non-linear curve fit analysis which shows that association
constants are 6.76 X 106 M-1, and 4.32 X 106 M-1 for CuII, and NiII as guests respectively (Table 1 and
Figure 86S in ESI†). Titration experiments have also been carried out for ROTb-c as host and above
metal ions (CuII, NiII, CoII, FeII, MnII, CrIII) as guests in a similar way as performed for ROTa. However,
here we do not observe any desirable stoichiometric binary complexation (Figures 84S-85S in ESI†).
Thus UV-Vis study confers that the electron donating effect as well as steric factor play important roles
towards the coordination with the transition metal ions by the tertiary amide group appended rotaxanes.
Likewise, upon addition of transition metal ions to the solution of ROTa, the exciplex peak gets
quenched (Figure 10(a) and Figures 87S in ESI†) in fluorescence spectra. No enhancement/ quenching
of monomer emission intensities at 340 nm is observed in the presence of any metal ions (Figures 87S
in ESI†). In fact, the PL intensity of the exciplex is decreased to a greater extent in case of CuII and NiII
whereas, upon addition of higher equivalent of other transition metal ions such as CrIII, MnII, FeII, CoII
to the solution of the respective rotaxanes (ROTa-c) quenching of exciplexes have been observed

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(Figures 87S-89S in ESI†). These observations exemplify that facile complexation with stoichiometric
ratios is obtained only in case of CuII and NiII which reflects the trends of Irving-William series.
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Figure 10. (a) Comparative photoluminescence spectra of ROTa (3.0 X 10-5 M) with equivalent amount
of (i) MnII, (ii) CrIII, (iii) FeII, (iv) Na+, (v) CoII, (vi) CuII and (vii) NiII in 9:1 CH3CN/ CHCl3 binary
solvent mixture. (Excited at ex =285 nm);(b) PL titration profile ROTa (3.0 X 10-5 M) with CuII in
CH3CN. (b) Job plot, and (c) equivalent plot for ROTa (3.0 X 10-5M) with CuII (2.89 X 10-4 M) in
CH3CN at 25 °C (Excited at ex =285 nm.)
Table 1. Stoichiometry and association constant of metal ion complexation using ROTa host
Guest
Inflection point Host:
Guest Association Constant (M-1)
from Job plot
Stoichiometry
CuII
NiII
~ 0.5
~ 0.5
1:1
1:1
6.76 X 106
4.32X 106
However in cases of both ROTb and ROTc, quenching of the exciplex peak intensity (Figures 88S, 89S
in ESI†) are observed as obtained in case of ROTa. This could be due to the interaction of bipyridine
unit with the metal ions which eventually disrupts the exciplex interaction as shown in Figure 8. Thus,
the involvement of bipyridine unit towards metal ion chelation and inhibition of the excited state donor-
acceptor interactions occur fully in a synergistic way. Our previously published results show that the

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macrocycle, metal ion and axle form 1:1:1 ternary complexes via pseudorotaxane formations.60, 62 In
this present study, potential redox quenching of luminescence may also occur but it is evident from our
previous studies that the CuII and NiII act as a potential candidate to act as guest and such binding is
examined upon monitoring the band of bipyridine via absorption spectroscopy. In this case also, upon
monitoring the bipyridine reveals the 1:1 binary complexation between functionalised rotaxane (ROTa)
and metal ion, particularly with NiII and CuII. One might assume that only bipyridine is involved in
binding along with the solvent molecules or the tris amide pockets are also involved towards
coordination. Whatever might be the case, absorption spectroscopic studies indicate that the bipyridine
unit is involved towards metal ion coordination and thus hamper the exciplex formation.
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Conclusions
A family of tertiary amide group appended multifunctional rotaxanes having complementary wheel and
axle components displays the excited state donor-acceptor interaction. Structure property studies are
exemplified upon varying bulkiness of the group that is attached to tertiary amide centers. Readable
dynamic properties are imposed due to the N-C(O) bond rotatiin which can be regulated by increasing
the bulkiness of the appended groups. On the other hand, conformational fixation of tri-acetylated
rotaxane can be achieved by Na+ as an input. The exciplex formation in rotaxane is also utilized for the
mechanistic investigation of Na+ assisted conformational/ co-conformational fixation. Photophysical
studies show the coordination of alkali metal ion through exotopic binding fashion of the tri-acetylated
rotaxane whereas; transition metal ions prefer endotopic coordination. Metal ion coordination behaviors
are also dependent on the substitution effects. CuII and NiII are found to be the choice of metal ions for
the coordination of tri-acetylated rotaxane, ROTa. Attachment of tertiary amide group on MIM‟s and
their dynamic controls are helpful towards the future development of more functionalized molecular
machines and the exciplex forming nature can be utilized towards sensing, recognition and overall
monitoring of the molecular machinery track by photophysical output.
Experimental Section and General Details:
All the reagents were obtained from commercial suppliers such as Sigma Aldrich, Merck, Alfa Aesar,
and Spectrochem, India and are used as received without further purification. 2,2′-bipyridine 5,5′-diacid,
it‟s di-acid chloride derivative and the bis-azide terminated axle AXL, were synthesized using
previously reported procedure.60,75 Peak assignments in the H NMR spectra were confirmed by using
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COSY, NOESY, ROESY NMR spectra of the compounds. In every cases of PL experiment, we have
excited at 285 nm which states that irradiation at 285 nm, the maximum of the wavelength in the PL
spectrum should be originally visualised at around 342 nm on the basis of the naphthalene ring.

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Materials: All reactions were carried out in a dry argon gas atmosphere, and workupDpOrI:o1c0.e10d3u9/rDe0sNJw00e3r5e3K
carried out under ambient conditions. Acetonitrile, dichloromethane was refluxed over CaH2 and was
collected prior to use. THF was dried by using Na-metal. Metal salts having perchlorate counter anions,
diethylene triamine, 4-fluoro benzoyl chloride, 5,5' dimethyl 2,2'-bipyridyl, were purchased from
Aldrich and were used as received. Ethanol, 2,3-dihydroxy benzene, 4-hydroxy benzaldehyde, 1,2-
dibromoethane, potassium carbonate, sodium borohydride, pivolyl chloride, thionyl chloride, 2-bromo
ethylamine hydrobromide, boc-anhydride, trifluoro acetic acid were purchased from Spectrochem Pvt.
Ltd., India. Potassium dichromate, potassium bicarbonate, sodium azide were procured from Merck.
Methods. HRMS analysis was performed on a QToF−Micro YA 263 mass spectrometer in positive ESI
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mode. H, C, DOSY, COSY, NOESY, ROESY, HSQC -NMR experiments were carried out on FT-
NMR Bruker DPX 300/ 400/ 500 MHz NMR spectrometer. Variable temperature 1H-NMR experiments
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were carried out on a 300 MHz Bruker DPX NMR spectrometer. Chemical shifts for H, C, and F-
NMR were reported in parts per million (ppm), calibrated to the residual solvent peak set. Absorption
spectra were recorded in a Perkin Elmer Lambda 900 UV/ vis/ NIR spectrophotometer (with a quartz
cuvette of path length 1 cm), and emission spectra were recorded in a Fluoro Max-3 spectrophotometer,
from Horiba Jobin Yvon. Elemental analysis was performed on PerkinElmer 2500 series II elemental
analyzer, PerkinElmer, USA. Caution! Perchlorate salts of the both alkali and transition metal ions and
organic azide are explosive under certain conditions, particularly when these salts/ compounds are used
in large amount and certain amount of heat energy was provided. All precautions should be taken during
the course of handling such chemicals.
Synthetic Procedures:
(a) Synthesis of compound I:
In a 250 mL R.B. excess 1, 2-dibromoethane (2.15 mL, 25.0 mmol, 5.0 eqv.) was added to the solution
of 4-hydroxybenzaldehyde (0.610 g, 5mmol, 1.0 eqv.) in the presence of K2CO3 (2.1 g, 15.0 mmol, 3.0
eqv.). The reaction mixture was then refluxed for 24h under inert condition having CH3CN as solvent.
Then the crude reaction mixture was dried under vacuum and extracted with CHCl3 and water. The
resultant organic layer was repeatedly washed with saturated brine solution. The organic layer was dried
over vacuum. The CHCl3 part was evaporated, and the yellow oily product was purified by column
chromatography by using 94:6 hexane/ ethyl acetate and using silica gel (60-120 mesh) as a stationary
phase to obtain the white crystalline product (930.5 mg, Yield 82%). The purity of the compound was
checked by NMR and ESI-MS which matches well with previously reported data.661H NMR (400 MHz,
CDCl3): δ (ppm) 4.45 (s, 4H, -CH2), 7.05 (d, 2H, J = 8 Hz, Ar-H), 7.85 (d, 2H, J = 8 Hz, Ar-H), 9.91 (s,

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1H, Ar-CHO). 13C NMR (75 MHz, CDCl3): δ (ppm) 28.7, 67.9, 114.8, 130.4, 131.9, 130.3, 162.9,
190.6.
(b) Synthesis of compound II:
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2,3-dihydroxy naphthalene (0.16 g, 1.0 mmol. 1.0 eqv.) was treated with compound I (0.48 g, 2.1
mmol. 2.1 eqv.) in the presence of anhydrous K2CO3 (0.40 g, 3.0 mmol, 3.0 eqv.) having CH3CN as a
solvent. The reaction mixture was refluxed for 24h under an inert atmosphere at 80°C. After the
completion of the reaction, the resulting solution was poured into ice cold solution and a white
precipitated was formed which was collected by filtration. Yield 0.38 g, 85%. M.P. 115º C, ESI-MS
(+ve): m/Z calculated for C28H24O6 = 456.1573, Found 456.1632. 1H NMR (400 MHz, CDCl3): δ (ppm)
4.46-4.51 (m, 8H, -CH2), 7.04 (d, 4H, J = 8.4 Hz, Ar-H), 7.24 (s, 2H, Ar-H), 7.35-7.38 (m, 2H, Ar-H),
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7.68-7.70 (m, 2H, Ar-H), 7.80 (d, 4H, J = 8.8 Hz, Ar-H), 9.86 (s, 2H, Ar-CHO). C NMR (75 MHz,
CDCl3): δ (ppm) 67.0, 67.7 (4C, -CHc+d), 109.7 (2C, Ar-C-H) 115.2 (4C, Ar-C), 124.8 (2C, Ar-C),
126.6 (2C, Ar-C), 129.7 (2C, Ar-C quaternary), 130.5 (2C, Ar-C quaternary), 132.1 (4C, Ar-C-H),
148.9 (2C, Ar-C-quaternary), 163.8 (2C, Ar-C-quaternary), 190.8 (C=O). Analytical Calculated for
C28H24O6: C: 73.67; H: 5.30; O: 21.03. Found: C: 73.52; H: 5.42, O: 21.40.
(c) Synthesis of MC:
In a 1 L two neck round bottomed flask, fitted with two pressure equalizer, one contains compound II
(0.456 g, 1.0 mmol, 1.0 eqv.) in 1:1 50 mL CH2Cl2/CH3OH solvent and another with diethylenetriamine
(0.1 mL, 1.0 mmol, 1.0 eqv.) dissolved in 50 mL of CH3OH. Then both of them were added drop wise
to the R.B. containing a large amount of 1:1 CH2Cl2/CH3OH binary solvent mixture. The addition was
continued with the specific time interval for 2 days. After complete addition of both the two reagents, a
macrocyclic compound was formed via imine bond formation. Then the resultant reaction mixture was
allowed to stir at room temperature for 24h, and the two imine bonds were reduced by using NaBH4
(150 mg, 4.0 mmol, 4.0 eqv.). After stirring it for additional 6h, the reaction mixture was evaporated,
and the resultant solid was dissolved in CHCl3 and washed thoroughly with water followed by brine
solution. Then the separated organic phase was dried over anhydrous Na2SO4. A white solid compound
was obtained as the product. Yield 370 mg, 70%. M.P. 170º C, ESI-MS (+ve): m/Z [(M+1)+] calculated
for C32H38N3O4: 528.2862, Found: 528.2858; FTIR in KBr disc (ν/cm−1 ): 3429, 2921, 2852, 1606,
1510, 1249, 1174, 1116, 1060, 750; 1H-NMR (300 MHz, CDCl3) δ (ppm) = 2.83 (b, 8H, -CH2), 3.70 (s,
4H, Ar-CH2), 4.43 (m, 8H, -CH2), 6.84 (d, 4H, J = 9.0 Hz, Ar-H), 7.16 (d, 4H, J = 9.0 Hz, Ar-H), 7.25
(m, 2H, Ar-H), 7.37 (m, 2H, Ar-H), 7.68 (m, 2H, Ar-H). 13C-NMR (75.47 MHz, CDCl3) δ (ppm) = 48.2
(2C, aliphatic-CH2), 48.6 (2C, aliphatic –CH2), 53.3 (2C, Ar-CH2, aliphatic), 67.4 (2C, aliphatic, -
OCH2CH2O-), 68.5 (2C, aliphatic -OCH2CH2O-), 110.1 (2C, Ar-C) 115.1 (4C, Ar-C), 124.6 (2C, Ar-C),

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126.6 (2C, Ar-C), 129.5 (4C, Ar-C), 129.8 (2C, Ar-C, quaternary), 133.0 (2C, Ar-C, qDuOaIt: e10r.n10a3r9y/D)0,N1J40093.553K
(2C, Ar-C, quaternary), 158.2 (2C, Ar-C, quaternary). Anal. calculated for C32H37N3O4: C, 72.84; H,
7.07; N, 7.96. Found: C, 72.48; H, 6.95; N, 8.09.
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(d) Synthesis of acetylated macrocycle, MCa:
MC (104 mg, 2.0 mmol, 2 eqv.) was taken in the 50 mL R.B., and dry CH2Cl2 was added. Then triethyl
amine (1.4 mL, 10.0 mmol) was added as base to the reaction mixture. After stirring for 10 -15 minutes
at room temperature, 7.0 equivalent distilled acetic anhydride (0.662 mL, 0.70 mmol) was added in the
reaction mixture and was allowed to stir at room temperature for another 8 hour. Then the reaction
mixture was poured into a 100 mL separating funnel and washed with distilled water and brine solution
for 3-4 times respectively. The organic solvent was evaporated under reduced pressure to obtain tri-
acetylated MC, MCa as a white powder. Yield 118 mg (91%). M.P. 198º C, MALDI-MS: m/Z
calculated for C38H44N3O7Na [M+Na]+, 676.7537, found 676.786.FTIR in KBr disc (ν/cm−1 ): 3433,
2923, 2854, 1745, 1633, 1510, 1448, 1413, 1367, 1290, 1253, 1178, 1112, 1066, 946, 811, 746, 671;1H-
k
NMR in CDCl3 (300 MHz, 298K) (mixture of rotamers), δ (ppm) 2.05-2.23 ( m, 9H, -C(O)CH3 , 3.10-
3.31 (m, 8H, -CH2h+i ), 4.36-4.43 (m, 12H, -CH2c+d+g), 6.8-7.24 (m, 10H, Ar-H e+f+b), 7.32 (b, 2H, Ar-H
a / a' ), 7.69 (b, 2H, Ar-H a' / a ). 1H-NMR in DMSO-d6 (300 MHz, 298K) (mixture of rotamers), δ (ppm) )
1.96-2.12 ( m, 9H, -C (O) CH3 k ), 3.10-3.31 (m, 8H, -CH2h+i ), 4.37-4.47 (b, 12H, -CH2c+d+g), 6.97-7.13
(m, 8H, Ar-H e+f), 7.32-7.36 (m, 2H, Ar-H a'/a) 7.40 (s, 2H, Ar-H ), 7.40 (m, 2H, Ar-H a/a'). C-NMR
(75.47 MHz, CDCl3) δ (ppm) = 21.5 (9C, aliphatic –CH3), 29.5-31.8 (4C, aliphatic, -NHCH2CH2NH-),
44.1-46.2 (2C, aliphatic, Ar-CH2), 67.5-68.4 (4C, aliphatic, -OCH2CH2O-) 108.6-109.2 (Ar-C), 115.5
(Ar-C), 124.9 (Ar-C), 126.8 (Ar-C), 128.2-131.4 (Ar-C), 149.1 (Ar-C), 158.5-158.8 (Ar-C), 161.2 (Ar-
C), 170.4 (Ar-C). Anal. Calculated for C38H43N3O7: C, 69.81; H, 6.63; N, 6.43; found: C, 69.72; H,
6.45; N, 6.55.
b
13
(e) Synthesis of triaryl ring substituted, MCb:
MC (104 mg, 2 mmol, 1.0 eqv.) was dissolved in dry CH2Cl2 in a 50 mL R.B. flask and 10.0 equivalent
triethyl amine (1.4 mL, 10.0 mmol., 5.0 eqv.) was added in the reaction mixture. Then the reaction
mixture was allowed to stir at room temperature for 10 min. After that, 4-fluorobenzoyl chloride (0.95
mL, 8.0 mmol, 4.0 eqv.) was dissolved in dry CH2Cl2 and taken in a 25 mL pressure equalizing
dropping funnel and attached with the R.B. flusk. The solution of acid chloride was then added
dropwise to the solution of MC in R.B. flusk at 0oC under inert atmosphere. After the complete addition
the reaction mixture was allowed to stir at room temperature for 12 h. Then the reaction mixture was

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poured into a 100 mL separating funnel and washed with plenty of water, saturated sodium bicarbonate
solution and brine solution for 2-3 times respectively. Then the organic layer was dried over anhydrous
Na2SO4. The organic phase was evaporated under reduced pressure to obtain aryl ring appended
functionalized wheel, MCb as a white powder. Yield 152 mg (79%). M.P. above 200º C, ESI-MS (+ve
mode): m/Z calcd. for C53H46F3N3NaO7 [M+Na]+, 916.3186, found 916.3190.FTIR in KBr disc
(ν/cm−1 ): 3434, 3068, 2923, 2854, 1633, 1606, 1546, 1510, 1454, 1419, 1299, 1247, 1176, 1163, 1120,
1099, 1062, 945, 848, 821, 757, 671, 599, 584;1H-NMR in CDCl3 (300 MHz, 298K) (mixture of
rotamers), δ (ppm) 3.44 ( b, 8H, -CH2 h + i ), 4.50 (b, 12H, -CH2 c + d + g ), 7.09-7.43 (b, 22H m + l + e + f +b + a
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/a'), 7.74-7.77 (m, Ar-H, 2H a' / a). H-NMR in DMSO-d6 (300 MHz, 298K) (mixture of rotamers), δ
1
(ppm) 3.26 (b, 8H, -CH2 h + i), 4.26-4.58 (b, 12H, -CH2c + d + g , 7.02-7.41 (m, 12H, Ar-H e + f + l + m + a'/a
)
7.76 (s, 2H, Ar-H a/ a'). C-NMR in CDCl3 (300 MHz, 298K) (mixture of rotamers), δ (ppm) = 28.8,
29.8, 43.1, 46.9, 53.2, 67.1, 68.1, 109.0, 115.4-116.0, 124.7, 126.7, 128.7, 129.2, 131.8, 149.1, 159.0,
161.8, 165.1, 171.4. Anal. Calculated for C53H46F3N3O7: C: 71.21; H: 5.19; N: 4.70; Found: C: 71.56,
H: 5.45, N: 4.59.
13
(f) Synthesis of rotaxane, MCc:
MC (104 mg, 2 mmol, 1.0 eqv.) was dissolved in dry CH2Cl2 in a 50 mL R.B. flask and 10.0 equivalent
triethyl amine (1.4 mL, 10.0 mmol., 5.0 eqv.) was added in the solution. Then the reaction mixture was
allowed to stir at room temperature for 5 min. After that, trimethylacetyl chloride/ pivaloyl
chloride(0.99 mL, 8.0 mmol, 4.0 eqv.) was dissolved in dry CH2Cl2 and taken in a pressure equalizing
dropping funnel. The solution of acid chloride was then added dropwise to the MC present in R.B. flusk
at 0oC under inert atmosphere. After the complete addition, the reaction mixture was allowed to
vigorous stirring at room temperature for 10 h. Then the reaction mixture was poured into a 100 mL
separating funnel and washed with water, saturated solution of sodium bicarbonate and brine for 2-3
times, respectively. Then the organic layer was collected and dried over anhydrous Na2SO4. The organic
phase was evaporated to obtain functionalized wheel, MCc as a white powder. Yield 140 mg (89%).
M.P. above 200º C, ESI-MS (+ve mode): m/Z calcd. for C47H61N3O7Na [M+Na]+, 802.4407, found
802.2400. FTIR in KBr disc (ν/cm−1 ): 3431, 2923, 2854, 1745, 1635, 1510, 1473, 1442, 1251, 1157,
1
372; H-NMR in CDCl3 (300 MHz, 298K) (mixture of rotamers), δ (ppm) 1.24 -1.35, (m, 9H,
C(O)(CH3)3), 3.24 (b, 8H, -CH2h + i), 4.37- 4.38 (m, 8H, -CH2c + d ), 4.67 (s, 4H, Ar-CH2 ), 6.95-7.21
(m, 10H, Ar-H e + f + b ), 7.34-7.36 (m, 2H, Ar-H a' / a), 7.67-7.70 (m, 2H, Ar-H a / a'). 1H-NMR in DMSO-
g
d6 (300 MHz, 298K) (mixture of rotamers), δ (ppm) 1.30 (m, 9H, C(O)(CH3)3), 4.08, 4.36-4.43 (b, 8H, -
CH2h + i),, 4.62 ((b, 4H, Ar-CH2 )), 7.01 (b, 8H, Ar-H e + f ), 7.36 (b, 2H, Ar-H a / a'), 7.41 (s, 2H, Ar-H b),
7.77 (b, 2H, Ar-H a' / a). C-NMR (CDCl3) δ (ppm) = 28.5 (9C, aliphatic, -CH3), 29.7 (3C, aliphatic,
g
13
quaternary), 39.2 (4C, aliphatic, -NHCH2CH2NH-), 45.9-46.4 (2C, aliphatic, Ar-CH2), 67.7-68.2 (4C,

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aliphatic, -OCH2CH2O-), 109.3 (Ar-C), 115.7 (Ar-C), 124.5 (Ar-C), 126.5 (Ar-C), 128.3 (Ar-C, two
signals merged here), 129.6 (Ar-C), 149.2 (Ar-C), 158.7 (Ar-C), 178.3 (3C, C=O). Anal. calcuated for
C47H61N3O7: C, 72.37; H, 7.88; N, 5.39; found C, 72.65; H, 7.95; N, 5.56.
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(g) Synthesis of bis-azido bis-amido 5,5′- bipyridyl based axle, AXL: The axle was made by following
our literature procedure, and the characterization data were matched with the literature report.69
(h) Synthesis of the stopper, STP: STP was prepared by our published synthetic protocol.61
(i) Synthesis of [2]pseudorotaxane, PROT:
A solution of Ni(ClO4)2, 6H2O (73 mg, 0.2 mmol) in CH3OH (5 mL) was added to a solution of MC
(114.2 mg, 0.2 mmol) in CH3OH/CH2Cl2 (1:1, 5 mL) at room temperature. After stirring for it for 0.5 h,
the solid di-azide axle AXL (76 mg, 0.2 mmol) was added to the reaction mixture and few drops of
CH3CN was added in the reaction mixture. During the course of the reaction, the suspension became
clear. Then the resultant mixture was stirred for another 4h, and the solvent was evaporated to dryness.
The greenish yellow solid was repeatedly washed with CH2Cl2 and dried in vacuum to obtain pure
product PROT, yields: 185 mg (76%).M.P. above 200º C. The complex is characterized by absorption
spectroscopy and ESI-MS spectrometry. ESI-MS (+ve): m/Z calculated for C48H53ClN13NiO10, 1064.30,
found 1064.27. Anal. Calculated for C48H53Cl2N13NiO14:C, 49.46; H, 4.58; N, 15.62, found: C, 49.92;
H, 4.22, N, 15.88. FTIR in KBr disc (ν/cm−1 ): 3371, 3353, 2102, 1660, 1608, 1510, 1446, 1251, 1114,
1076, 946, 852, 756, 626.
(j) Synthesis of [2]rotaxane, ROT′:
Azide terminated PROT (0.24 g, 0.2 mmol, 1.0 eqv.) was dissolved in 1:1 de-gassed CH2Cl2/CH3CN
binary solvent mixture inside the glove box. The alkyne-terminated stopper unit, STP (0.143 g, 0.44
mmol, and 2.2eqv.) was dissolved in CH2Cl2 and added to the stirring solution of PROT. Then Na2CO3
(0.010 g ) was added as a solid, followed by the addition of tetrakis (acetonitrile) copper(I)
hexafluorophosphate, Cu(CH3CN)4PF6 solution in CH3CN (75 mg, 0.2 mmol, 2.0 eqv.) to the reaction
mixture. The reaction was stirred for 12h inside the glove box at room temperature. Then the resulting
mixture was taken off from the glove box and evaporated to remove CH2Cl2/CH3CN. A saturated
solution of Na2EDTA (10 mL) in water was added to the resultant crude and stirred for 48h to remove
the metal ions. The solution was suspended in CHCl3 in a separating funnel and washed with plenty of
water followed by saturated brine solution. Then the organic layer was evaporated under vacuum to
obtain a yellowish solid. The resulting solid was used as a precursor to synthesize boc protected
rotaxane.

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The crude reaction mixture was taken in a 50 mL R.B. and dissolved in dry CH2Cl2. Then triethyl amine
(0.139 mL, 1.0 mmol, 5.0 eqv.) was added in it. After stirring for 10-15 min, excess di-tertiary butyl
dicarbonate (0.229 mL, 1.0mmol, 5.0 eqv.) was added in the reaction mixture. Then the mixture is
allowed to stir at room temperature for 4h under an inert atmosphere. After that, the solvent was
evaporated and purified by preparative TLC by using 1-3% CH3OH/CH2Cl2 as eluent and silica gel GF-
254 as a stationary phase to afford 162 mg (43%) of the targeted metal free boc-protected [2]rotaxane
ROT′, along with dumbbell-shaped component DMBL. Yield: 72 mg (35%). These processes were
repeated to synthesize large amount of ROT′ for next steps. ESI-MS (+ve): m/z calcd. for
C109H113N13O16Na [M+Na]+, 1884.13, found 1884.46. 1H-NMR (CDCl3, 500 MHz): δ (ppm) 1.43, 3.44,
3.82, 3.93, 3.98, 4.33, 5.24, 6.05-6.14, 6.74, 7.13-7.44, 7.65, 8.13-8.32, 8.41, 8.72.
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The main aim was to remove the DMBL and wheel component from the interlocked components. This
1
compound was used for the next steps without high degree of purification. H-NMR spectra and ESI-
MS spectra were recorded and the data were given in the SI section that helps to predict the formation
of interlocked components.
(k) Synthesis of acetylated rotaxane, ROTa:
[2]rotaxane, ROT′ (186 mg, 0.1 mmol, 1.0 eqv.) was dissolved in dry CH2Cl2 in a 50 mL R.B. flask
and 10.0 equivalent of trifluoroacetic acid (0.076 mL, 1.0 mmol, 10.0 eqv.) is added in the reaction
mixture. Then the resultant solution was allowed to stir at room temperature for 12h. The solvent along
with the excess acid was evaporated to obtain a sticky mass which mainly contains protonated rotaxanes
with trifluoro acetate counter anions. The crude product (TROT) was used in the next step without
further isolation and purification.
To the sticky mass of TROT which was obtained in the 50 mL R.B., dry CH2Cl2 was added. Then
excess triethyl amine (0.70 mL, 5.0 eqv. with respect to the trifluoro acetic acid) was added to the
reaction mixture. After stirring for 10 minutes at room temperature, 3.5 equivalent distilled acetic
anhydride (0.035 mL, 0.35 mmol, 3.5 eqv.) was added in the reaction mixture and was allowed to stir at
room temperature for another 6h. Then the reaction mixture was poured into a 100 mL separating funnel
and washed with plenty of water and brine solution for 2-3 times respectively. The organic phase was
evaporated under reduced pressure to obtain tri-acetylated [2]rotaxane, ROTa as a white powder. Yield
142 mg (84%). M.P. above 200º C. ESI-MS (+ve mode): m/Z calculated for C100H96N13O13 [M+H]+,
1687.7284, found 1687.7278. . FTIR in KBr disc (ν/cm−1 ): 3407, 2923, 2852, 1728, 1658, 1635, 1596,
1
1548, 1446, 1384, 1296, 1238, 1172, 1114, 1053, 950, 746. H-NMR in CDCl3 (300 MHz, 298K)
h + i + 5 + d
(mixture of rotamers), δ (ppm) = 1.88-2.2 (m, 9H, -CH3 k ), 3.4-3.9 (m, 16H, CH2
), 4.3 – 4.5
8
( m, 12H, -CH26 + g + c ), 5.22 – 5.37 (m, 4H, -CH2 ), 6.00 - 6.17 ( m, 4H, Ar-H e), 6.73 -6.80 (m, 4H, Ar-
H f), 7.16-7.24 (m, 34H, Ar-H 9 + 10 + 11 + a / a' + b ), 7.38-7.41 (m, 2H, Ar-H 7), 7.41 – 7.51 (m, 2H, -NH 4 ),

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7.71 (b, Ar-H, Ar-H a' / a), 8.23-8.30 (m, 4H, Ar-H 2 + 3), 8.65-8.76 (m, 2H, Ar-H 1) . 1H-NMR in DMSO-
d6 (500 MHz, 298K) (mixture of rotamers), δ (ppm) 1.88-2.25 (m, 9H, -CH3 k ), 3.44-3.71 (m, 12H,
h + i + 5
d
8
CH2
5.83-5.98 ( m, 4H, Ar-H e), 6.30-6.51 (m, 4H, Ar-H f), 6.92-7.52 (m, 36H, Ar-H 9 + 10 + 11 + a + a' + b ), 7.98
), 3.89-3.94 ( b, 4H, -CH2 ), 4.28-4.52 (m, 12H, -CH2 6 + g + c), 5.16-5.21 (m, 4H, -CH2 ),
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2 + 3
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(m, 2H, Ar-H ), 8.18-8.28 (m, 2H, - Ar-H
), 8.67-9.07 (b, 4H, Ar-H and –NH4). 13C-NMR
(CDCl3, 300 MHz, 298 K) δ (ppm) = 21.5 (3C, -aliphatic, -CH3), 46.2- 47.7 (4C, aliphatic, -
NHCH2CH2NH-, peak corresponds to wheel component), 49.5 (2C, aliphatic, peak corresponds to the
carbon where H6, Hg or Hc is attached), 58.8 - 59.2 (2C, aliphatic, peak corresponds to axle component
where H8 is attached), 65.3 (2C, aliphatic, aliphatic carbon in which any one of Hd, H5 is attached ),
67.7- 68.0 (2C, any one of the aliphatic carbon where either H6, Hg, or Hc is attached), 108.0 (Ar-C,
corresponds to the carbon where any one of Ha, Ha‟, Hb is attached), 113.1-113.8 (4C, Ar-C,
corresponds to the carbon where He is attached), 120.8 (Ar-C), 124.2-124.6 (Ar-C), 125.1-130.1 (Ar-C),
135.8-136.1(Ar-C), 142.3-142.7 (Ar-C), 147.9-148.6 (Ar-C), 155.8-156.9 (Ar-C), 165.6-165.7 (Ar-C),
178.1 (C=O). Anal. Calculated for C100H95N13O13:C, 71.20; H, 5.68; N, 10.79, found: C, 71.42; H, 5.58,
N, 10.88.
(l) Synthesis of rotaxane, ROTb:
[2]rotaxane, ROT′ (186 mg, 0.1 mmol, 1.0 eqv.) was dissolved in dry CH2Cl2 in a 50 mL R.B. flask and
4.0 equivalent of trifluoro acetic acid (0.076 mL, 10.0 eqv.) is added in the reaction mixture. Then the
reaction mixture was allowed to stir at room temperature for 12h. The solvent along with the acids was
evaporated to obtain a sticky mass which mainly contains protonated rotaxanes with trifluoro acetate
counter anions. The crude product was used in the next step without further isolation and purification.
To the sticky mass in the 50 mL R.B. dry CH2Cl2 was added. Then triethyl amine (0.70 mL, 5.0 eqv.
with respect to the trifluoro acetic acid) was added as a base. After stirring for 10 min at room
temperature, 3.5 equivalent 4-fluorobenzoyl chloride (0.05 mL, 0.4 mmol, 0.4 eqv) was added in the
reaction mixture and was allowed to stir at room temperature for another 6h. Then the reaction mixture
was poured into a 100 mL separating funnel and washed with plenty of water, saturated sodium
bicarbonate solution and brine solution for 2-3 times respectively. The organic phase was evaporated
under reduced pressure to obtain tris aryl ring appended functionalized rotaxane, ROTb as a white
powder. Yield 152 mg (79%) M.P. above 200º C. ESI-MS (+ve mode): m/z calcd. for
C115H98N13F3O13Na [M+Na]+, 1949.7290, found 1949.7250. FTIR in KBr disc (ν/cm−1 ): 3423, 3062,
2323, 2852, 1731, 1647, 1618, 1595, 1531, 1512, 1465, 1434, 1348, 1294, 1220, 1178, 1049, 1006, 854,
759, 702.1H-NMR in CDCl3 (500 MHz, 298K) (mixture of rotamers), δ (ppm) = 3.30 (b, 8H, -CH2h + i ),
3.65 – 4.02 (m, -CH2c + d + g + 5 +6), 5.23 (m, 4H, -CH2 ), 6.07-6.22 (s, 4H, Ar-H ), 6.51-6.80 (m, 4H,
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Ar-H f ), 7.05-7.47 (m, 50H, Ar-H 9 + 10 + 11 + a + a' + b + 7 + l + m), 7.74 (b, 2H, Ar-H3), 8.10-8.24 (m, 2H, -
NH4), 8.37 (b, 2H, Ar-H 2), 8.85 (s, 2H, Ar-H ). 1H-NMR in DMSO-d6 (300 MHz, 298K) (mixture of
rotamers), δ (ppm) 3.31 (b, 8H, -CH2h + i ), 3.52-3.77 (b, 8H, -CH2c + d), 4.42 (b, 8H, - CH2 6 + c), 4.62 (d,
1
9
g
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4H, -CH2 ), 5.16 (s, 4H, -CH2 ), 6.22 (s, 4H, Ar-H e ), 6.60 (b, 4H, Ar-H f ), 7.07-7.48 (m, 36H, Ar-H 9
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+ 10 + 11 + a / a' + b ), 8.05 (s, 2H, -Ar-H ), 8.28 (b, 2H, Ar-H 3), 8.36 (b, 2H, Ar-H ), 8.75 (b, 2H, -NH4),
8.91 (s, 2H, Ar-H ). 13C-NMR (CDCl3, 300 MHz, 298 K) δ (ppm) = 28.6 (4C, aliphatic, -
1
NHCH2CH2NH-), 50.3 (2C, aliphatic), 53.5 (2C, aliphatic), 55.4 (2C, aliphatic), 67.3 (2C, aliphatic),
78.3 (2C, aliphatic), 78.5, 79.7 (2C, aliphatic), 79.7 [total 9 types of aliphatic carbon], 108.7, 115.9,
121.1, 127.6, 128.4, 130.4, 136.9, 142.9, 149.1, 157.0, 165.4, 172.7, 177.5 (3C, C=O). ). Anal.
Calculated for C119H110F3N13O13:C, 71.92; H, 5.58; N, 9.17, found: C, 71.47; H, 5.78, N, 8.99.
(m) Synthesis of acetylated rotaxane, ROTc:
A similar procedure was followed to obtain crude protonated MC based rotaxane with trifluoro acetate
counter anions as applied for ROTb synthesis. To the sticky mass obtained in 50 mL R.B. was dissolved
in dry CH2Cl2 and triethyl amine (0.70 mL, 5.0 eqv. with respect to the trifluoro acetic acid) was added
as a base. After stirring it for 10 min at room temperature, 3.5 equivalent distilled pivaloyl chloride
(0.05 mL, 0.4 mmol, 0.4 equivalent) was added in the reaction mixture and was allowed to stir at room
temperature for additional 5h. Then the reaction mixture was poured into a 100 mL separating funnel
and water was added, then the biphasic layer was shaken well for 2-3 times. After that, a saturated
solution of sodium bicarbonate and brine solution was used for washing for 2-3 times respectively. The
organic phase was evaporated under reduced pressure to obtain three tertiary butyl group appended
functionalized rotaxane, ROTc as a white powder. Yield 265 mg (81%). M.P. above 200º C. ESI-MS
(+ve mode): m/z calcd. for C109H114N13O13[M+H]+, 1836.8590, found 1836.8563. H-NMR in CDCl3
1
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h
(500 MHz, 298K) (mixture of rotamers), δ (ppm) 1.27-1.40 (m, 9H, -C(O)(CH3 )3), 1.75 (b, 8H, -CH2
+ i ), 3.46 -3.64 (m, 8H, -CH2c + d), 3.83 (m, 4H, -CH2 6 ), 4.36 (m, 4H, -CH2 ), 4.68 (b, 4H, -CH2 ), 5.24
(s, 4H, -CH2 8 ), 6.08 (s, 4H, Ar-H e ), 6.72 (b, 4H, Ar-H f ), 7.17-7.47 (m, 36H, Ar-H 9 + 10 + 11 + a + a' + b +
7), 7.68 (b, 2H, Ar-H3), 8.22 (b, 2H, -NH4), 8.33 (d, 2H, Ar-H 2, J = 9 Hz), 8.83 (s, 2H, Ar-H 1).1H-NMR
5
g
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in DMSO-d6 (300 MHz, 298K) (mixture of rotamers), δ (ppm) ) 1.23 (m, 9H, -C(O)(CH3 )3), 3.47 (b,
8H, -CH2h + i ), 3.71 -3.79 (m, 8H, -CH2c + d), 4.32 (d, 4H, -CH2 6 + c), 4.56 (d, 4H, -CH2 g ), 5.13 (s, 4H, -
CH2 ), 6.08 (s, 4H, Ar-H e ), 6.62 (b, 4H, Ar-H f ), 7.05-7.44 (m, 36H, Ar-H 9 + 10 + 11 + a / a' + b ), 7.76 (b,
2H, Ar-H a ‟/ a), 8.02 (s, 2H, -Ar-H 7), 8.21 (b, 2H, Ar-H 3), 8.32 (d, 2H, Ar-H 2, J = 9 Hz), 8.71 (b, 2H, -
NH4), 8.88 (s, 2H, Ar-H 1). ). 13C-NMR (DMSO-d6, 300 MHz, 298 K) δ (ppm) = 31.3-31.7, 45.9, 59.1,
67.4, 67.7, 68.1, 108.7, 115.8, 121.1, 124.7, 125.7, 126.9, 127.5, 128.3, 128.4, 129.6, 130.1, 130.6,
134.0, 141.8, 143.1, 146.4, 149.1, 152.6, 157.0, 158.5, 162.2, 165.5, 166.1, 173.0, 177.5, 182.6. Anal.
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Page 29 of 34
New Journal of Chemistry
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View Article Online
Calculated for C113H125N13O13:C, 72.45; H, 6.73; N, 9.72, found: C, 72.64; H, 6.66, DNO,I: 910..61083.9/DF0TNIJR003i5n3K
KBr disc (ν/cm−1 ): 3413, 3058, 2921, 2852, 1730, 1662, 1625, 1614, 1598, 1510, 1483, 1411, 1367,
1294, 1242, 1176, 1114, 1083, 952, 842, 750, 702, 638.
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The rotation of the tertiary amide bonds (C=O)-NR2 is expected to be in the range of the room
temperature NMR time scale and has been observed at NMR experiments. The existence of such
rotations generates the fluxional nature and forms conformers/ co-conformers of the interlocked
molecules. Most importantly, not all of these conformers/ co-conformers are identical with respect to
energy. In the NMR spectrum, along with the major signals, the very tiny signals are correspond to the
protons which are actually representing the less populated isomers. Some of the signals of one
conformers/ co-conformers may superimpose with each other/ or with other NMR-signals to some
extent and thus, the evaluation is very difficult. At high temperature when all the amide rotations are
fast, the signals for each conformers/ co-conformers become averaged. As the macrocycle is
symmetrical and one of the amide groups (the middle one) is on the symmetry plane, we would
therefore expect two sets of signals, one is for the two terminals -R groups, and the other one is for the
middle R group. These facts are observed in case of ROTa (Figure 61S, SI at around 2.0-2.2 ppm
region) and ROTb (Figure 5C, 19F-NMR spectrum in the main text).
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However, we are pretty successful to evaluate the H-NMR signals of the interlocked compounds
(ROTa-c) by variable temperature 1H-NMR experiments, ROESY, NOESY and COSY spectra analysis.
We have tried to carried out high temperature 13C-NMR spectrum but our attempts was unsuccessful
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due to the (i) low solubility of the compound in DMSO-d6 (as large amount is required to record C-
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NMR) and (ii) requirement of higher number of scan at high temperature compared to the H-NMR
spectrum. Thus, we are unable to assign properly the 13C-NMR spectra.
Associated Contents: Spectral Data such as NMR, Electronic spectra are provided in Supporting
Information (ESI†). Saikat Santra: ORCID: https://orcid.org/0000-0002-4198-3192
Acknowledgements: PG is thankful to the Science and Engineering Research Board (SERB), New
Delhi (project SR/S1/IC-39/2012) for financial support. S. Santra acknowledges the Council of
Scientific & Industrial Research (CSIR) and IACS for fellowship. S.S. is thankful to Dhrubaa Halder
for PL data recording.
References:
(1)
(2)
M. J. Langton and P. D. Beer, Acc. Chem. Res. 2014, 47, 1935.
J. Y. C. Lim, I. Marques, V. Felix and P. D. Beer, Angew. Chem. Int. Ed. 2017, 220.