PyC60-naphthacrown system: A new supramolecular recognition element

Article abstract of DOI:10.1016/j.saa.2014.09.020

(Graph Presented) Fulleropyrrolidine (PyC₆₀) and a binaphthyl bridged crown ether macrocyclic receptor (1) undergoes spontaneous self-assembly phenomenon in solution to generate a new supramolecular recognition element having binding constant value of ～5.83 × 10⁴ dm³ mol^-1. Lifetime measurement reveals a static quenching mechanism behind the deactivation of photoexcited state of 1 in presence of PyC₆₀. The results obtained from this work would definitely reinforce bridged cyclic crown ether as one of the most suitable fragments for the molecular recognition of various macrocyclic receptor(s) in near future.

Full text of DOI:10.1016/j.saa.2014.09.020

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Short Communication
PyC60-naphthacrown system: A new supramolecular recognition
element
Debabrata Pal a, Kshama Kundu b, Sandip K. Nayak b, Sumanta Bhattacharya a,
⇑
a Department of Chemistry, The University of Burdwan, Golapbag, Burdwan 713 104, India
b Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
PyC60-naphthacrown (1) non-
covalent interaction is established in
solution.
2.0
1.8
1.6
1.4
1.2
1.0
Magnitude of binding constant of
PyC60-1 system is estimated to be
58,300 dm3 molÀ1
.
Photo-excited decay of 1 in presence
of PyC60 takes place through static
quenching.
Solvent reorganization energies
indicate energy transfer upon photo-
excitation.
0.8
2x104 3x104 4x104 5x104 6x104 7x104 8x104 9x104
1/[PyC60], dm3 mol-1
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 May 2014
Received in revised form 23 August 2014
Accepted 11 September 2014
Available online xxxx
Fulleropyrrolidine (PyC60) and a binaphthyl bridged crown ether macrocyclic receptor (1) undergoes
spontaneous self-assembly phenomenon in solution to generate a new supramolecular recognition
element having binding constant value of ꢁ5.83 Â 104 dm3 molÀ1. Lifetime measurement reveals a static
quenching mechanism behind the deactivation of photoexcited state of 1 in presence of PyC60. The results
obtained from this work would definitely reinforce bridged cyclic crown ether as one of the most suitable
fragments for the molecular recognition of various macrocyclic receptor(s) in near future.
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
PyC60
Naphthacrown
Self-assembly
Binding constant
MEP
Introduction
phthalocyanine (Pc) [10–14] occupy the major part of the literature
in terms of their unique binding motif as well as appealing photo-
The search for proper macrocyclic receptors with complimen-
tary sites for efficient binding of fullerenes is gaining current impe-
tus [1–4]. An interesting aspect of the chemistry of fullerenes and
various macrocyclic receptors is that they are spontaneously
attracted to each other, as a result of ground state complexation.
Among all the receptors reported so far, porphyrin [5–9] and
physical properties. An Ir(III) metalloporphyrin demonstrates the
highest value of binding constant (K), i.e., K = 1.25 Â 108 dm3
molÀ1, with C60 measured in 1,2-dichlorobenzene [15]. It is already
reported that both C60 and C70 form ground state molecular com-
plexes with naphthalene [16] and crown ether [17,18] as a result
of formation of electron donor–acceptor (DA) or charge transfer
(CT) complex. However, the estimated K values for the fullerene–
naphthalene and fullerene–crown ether complexes are not so sig-
nificant in comparison to what fullerene exhibits with other host
⇑
Corresponding author. Fax: +91 3422634200.
E-mail address: sum_9974@rediffmail.com (S. Bhattacharya).
http://dx.doi.org/10.1016/j.saa.2014.09.020
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: D. Pal et al., PyC60-naphthacrown system: A new supramolecular recognition element, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.020

2
D. Pal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
molecule as mentioned above. For example, the K values for the
60-naphthalene and C70-naphthalene complexes are determined
to be only 0.4 [16] and 0.2 dm3 molÀ1, [16] respectively. Similarly,
the values for the electron donor–acceptor complexes of
60-benzo-15-crown-5 and C70-benzo-15-crown-5 complexes are
Synthesis of 1a
C
A
mixture of 2-naphthol (1.44 g, 10 mmol), benzaldehyde
K
(0.584 g, 5.5 mmol) and ( )-camphor-10-sulfonic acid (CSA,
0.232 g, 1.0 mmol) was stirred in dry acetonitrile at ambient tem-
perature (25 °C). After 16 h of reaction, TLC indicated complete con-
sumption of both the starting materials. The synthesized material
was filtered followed by washing with water and dried in air. The
crude solid was recrystallized with hexane–ethyl acetate solvent
mixture to afford pure 1a (1.41 g, 75%), a pink colored solid; mp
C
estimated to be 641 and 116 dm3 molÀ1, respectively [17,18]. As
a result of this, the design of macrocyclic receptor molecule having
naphthalene based crown ether moiety (1, Scheme 1) would be of
potential interest to examine its binding affinity and photophysical
properties in presence of fullerene derivative, e.g., C60 pyrrolidine
tris-acid ethyl ester (PyC60) (Scheme 2). The design of the macrocy-
clic receptor 1 conserves the basic feature of the propeller shaped
crown receptor but includes binaphthyl bridged alkyl linker. We
envisaged that this sort of study is expected to provide information
about the formation of supramolecular architecture which spans
from the nanoscopic to the macroscopic level across multiple
length scales, and affect the intermolecular binding strength
between 1 and fullerene because of different wave-function mixing
in the non-covalently linked DA pair.
195–196 °C; IR (CHCl3):
t
3471, 3423, 3019, 1618, 1597, 1513,
1491, 1468, 1390, 1253, 1046, 877 cmÀ1
;
1H NMR (200 MHz,
CD3COCD3): d 7.13–7.38 (m, 12H, ArCHAr, ArH), 7.74–7.84 (m, 4H,
ArH), 8.12 (d, J = 8.3 Hz, 2H, ArH); 13CNMR (50 MHz, CD3COCD3):
d 43.0, 120.1, 120.4, 123.6, 123.7, 126.8, 127.5, 128.8, 129.1,
129.7, 130.0, 130.5, 135.2, 143.3, 154.0; MS, m/z (%) = 376 (M, 25),
375 (95), 353 (8), 349 (11), 339 (16), 337 (10), 325 (10), 321
(100), 311 (11), 309 (16), 293 (22), 283 (9), 265 (16), 231 (58),
143 (39); HRMS: m/z calc. for C27H20O2Na (M + Na): 399.1361;
found: 399.1365 (see Supporting Information).
Materials and methods
Synthesis of 1
Synthesis of macrocyclic naphthacrown 1 starts with the acid
catalyzed (CSA) condensation of 2-naphthol with benzaldehyde
to afford bis-naphthol 1a in good yield. The compound 1 was finally
achieved by the alkylation of 1a with pentaethyleneglycol ditosy-
late. No dimeric product was isolated. Detailed synthetic procedure
is delineated below.
A mixture of bis-(2-naphthol) 1a (0.790 g, 2.1 mmol), pentaeth-
yleneglycol ditosylate (1.26 g, 2.3 mmol, 1.1 equiv) and Cs2CO3
(1.73 g, 5.3 mmol, 2.5 equiv) was refluxed in dry acetonitrile
(30 ml) for 8 h. TLC experiment revealed absence of both 1a and
the ditosylate. The solvent was then removed under vacuo, and
allowed to cool in room temperature; the reaction is then
quenched with 1 N HCl and extracted with ethyl acetate. The
organic layer was washed with water, brine and dried (Na2SO4)
in a sequential manner. Removal of solvent afforded a thick mass
which was purified by silica gel column chromatography (using
CHCl3 as eluant) to afford pure 2 (0.948 g, 78%), a light yellow col-
ored solid having mp 155–156 °C; IR (CHCl3):
1622, 1598, 1511, 1492, 1451, 1295, 1259, 1243, 1215, 1176, 928,
806, 697 cmÀ1 1H NMR (200 MHz, CDCl3): d 2.88–2.98 (m, 2H,
t 3058, 3016, 2874,
;
OCH2CH2O), 3.10–3.21 (m, 2H, OCH2CH2O), 3.31–3.35 (m, 4H,
2 Â OCH2CH2O), 3.45–3.48 (m, 8H, 4 Â OCH2CH2O), 3.69–3.71 (m,
2H, OCH2CH2O), 3.77–3.83 (m, 2H, OCH2CH2O), 7.03–7.14 (m, 6H,
ArCHAr, ArH), 7.23–7.30 (m, 6H, ArH), 7.73–7.81 (m, 6H, ArH);
13CNMR (50 MHz, CDCl3): d 43.9, 68.4, 68.9, 70.3, 116.1, 122.7,
124.0, 124.8, 125.2, 125.8, 127.5, 128.1, 128.2, 128.7, 129.4,
133.4, 144.9, 155.2; MS, m/z (%) = 578 (M, 100), 577 (43), 259
Scheme 1.
Fig. 1. Steady state UV–vis titration experiment of 1 (at a fixed concentration of
8.65 Â 10À6 mol dmÀ3) in presence of variable concentration of PyC60 (6.7 Â 10À6 to
4.5 Â 10À5 mol dmÀ3
) at 298 K; the inset of figure shows UV–vis BH plot.
kobs = 430 nm. The bottom black and green lines represent UV–vis spectrum of
uncomplexed 1 and PyC60, respectively, recorded in toluene. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)
Scheme 2.
Please cite this article in press as: D. Pal et al., PyC60-naphthacrown system: A new supramolecular recognition element, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.020

D. Pal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
3
Job type: Geometry optimization.
2.0
1.8
1.6
1.4
1.2
1.0
0.8
Method: RHF; Basis set: STO-3G; Number of shells: 301; Number of basis functions: 667
Multiplicity: 1
SCF model:
A restricted Hartree-Fock SCF calculation has been performed using Pulay DIIS + Geometric
Direct Minimization.
Optimization:
Step Energy
Max Grad.
0.045343
0.030271
0.023523
0.021064
0.019731
0.019111
0.019078
0.019143
0.019369
0.019444
0.019421
0.019205
0.018650
0.017739
0.016843
0.016036
0.015174
0.014232
0.012530
0.009517
0.006163
0.004459
0.003714
0.003201
0.002817
0.002894
0.002879
0.003118
0.002927
0.002929
0.002829
0.002674
0.002599
0.002679
0.002685
0.002819
0.002738
0.002523
Max Dist.
0.049827
0.045699
0.063321
0.062370
0.065238
0.067744
0.066051
0.065474
0.070331
0.062673
0.117577
0.050016
0.170625
0.097666
0.157535
0.079918
0.087361
0.046911
0.044133
0.057205
0.032116
0.050861
0.035299
0.054163
0.031358
0.048523
0.036344
0.034825
0.041490
0.037754
0.040323
0.062859
0.045278
0.047905
0.033373
0.024353
0.031189
0.036640
2x104 3x104 4x104 5x104 6x104 7x104 8x104 9x104
1
-5050.201969
1/[PyC60], dm3.mol-1
2
-5050.241354
-5050.256934
-5050.261119
-5050.263332
-5050.264862
-5050.266131
-5050.267141
-5050.267920
-5050.268520
-5050.269030
-5050.269444
-5050.269805
-5050.270138
-5050.270517
-5050.270980
-5050.271403
-5050.271818
-5050.272225
-5050.272594
-5050.272867
-5050.273065
-5050.273245
-5050.273397
-5050.273525
-5050.273652
-5050.273789
-5050.273919
-5050.274049
-5050.274165
-5050.274278
-5050.274405
-5050.274533
-5050.274642
-5050.274740
-5050.274812
-5050.274878
-5050.274952
3
4
5
Fig. 2. Steady state fluorescence spectral variation of 1 (8.65 Â 10À7 mol dmÀ3) in
presence of PyC60 (2.45 Â 10À6 to 3.45 Â 10À5 mol dmÀ3) recorded in toluene at
298 K; the inset of figure shows modified fluorescence BH plot. kex = 337 nm;
kem = 365 nm.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
Table 1
Binding constant (K) of PyC60-1 system done by steady state UV–vis and fluorescence
investigations in toluene along with the average value of K. Temp. 298 K.
System
K, dm3 molÀ1
UV–vis
Kav, dm3 molÀ1
Fluorescence
54,660
PyC60-1
61,950
ꢁ58,300
Fig. 3. Time-resolved fluorescence decay profile of (a) 1 (5.45 Â 10À6 mol dmÀ3) in
presence of (b) PyC60 (6.25 Â 10À5 mol dmÀ3
kex = 337 nm; kdecay = 365 nm.
) recorded in toluene at 298 K;
(35), 171 (31), 169 (25); HRMS: m/z calcd. for C37H39O6 (M + H):
579.2747; found: 579.2731; anal. calc. for C37H38O6: C, 76.79; H,
6.62%. Found: C, 76.96; H, 6.51% (see Supporting Information).
The fullerene derivative, e.g., PyC60 (shown in Scheme 2) is
obtained from Aldrich, USA and used after checking the purity of
the compound. The purity of the material has been checked in
accordance with the work done by Isacs and Diederich [19]. The
motivation behind selecting the PyC60 molecule as an electron
acceptor comes from the work of Sessler et al. in which they have
employed fulleropyrrolidine bearing a guanosine moiety as a rec-
ognition motif for the construction of C60-Pc dyad system [20].
HPLC grade toluene (Merck, Germany) is used as solvent to favor
non-covalent interaction between PyC60 and 1 and, at the same
time, to ensure good solubility and photo-stability of the samples.
Scheme 3.
UV–vis spectral measurements are performed on a Shimadzu
UV-2450 model spectrophotometer using quartz cell with 1 cm
optical path length. Emission spectra have been recorded with a
Please cite this article in press as: D. Pal et al., PyC60-naphthacrown system: A new supramolecular recognition element, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.020

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D. Pal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
39
40
41
42
43
-5050.275027
-5050.275096
-5050.275152
-5050.275193
-5050.275228
0.002211
0.002290
0.002766
0.002952
0.003114
0.050540
0.042741
0.051204
0.049638
0.087615
PyC60 is selectively excited by 532 nm light from a Nd:YAG laser
(6 ns fwhm) with 7 mJ power. For the transient absorption spectra
in the visible region, a photomultiplier tube has been used as a
detector for the continuous Xe-monitor light (150 W). Theoretical
calculations are performed using Gaussian ‘03 and SPARTAN’06
V1.1.0 (USA) Windows version software.
Results and discussions
44
45
-5050.275265
-5050.275303
0.003251
0.003308
0.110031
0.134577
Binding studies by UV–vis absorption and steady state fluorescence
techniques
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
-5050.275341
-5050.275380
-5050.275418
-5050.275452
-5050.275484
-5050.275512
-5050.275534
-5050.275552
-5050.275568
-5050.275583
-5050.275597
-5050.275608
-5050.275610
-5050.275614
-5050.275619
-5050.275622
-5050.275625
-5050.275628
-5050.275631
-5050.275634
-5050.275637
0.003492
0.003489
0.003399
0.003121
0.002680
0.002260
0.001834
0.001499
0.001088
0.001011
0.000792
0.000603
0.000510
0.000488
0.000394
0.000480
0.000371
0.000350
0.000326
0.000361
0.000389
0.126616
0.098317
0.096945
0.098085
0.112027
0.110388
0.117449
0.101228
0.090044
0.048023
0.027774
0.107712
0.033548
0.039305
0.030301
0.025238
0.019542
0.038229
0.047463
0.027815
0.035002
It is observed that both PyC60 and 1 attract each other sponta-
neously in toluene and the binding constant (K) of PyC60-1 system
is determined to be 61,950 dm3 molÀ1 as evidenced from UV–vis
investigations (Fig. 1). The K value of PyC60-1 system gets further
support from steady state fluorescence measurements (Fig. 2); K
of PyC60-1 system (see Table 1) is evaluated employing modified
Benesi–Hildebrand equation in both UV–vis and fluorescence
experiments [21]. An excellent straight line plot is obtained with
correlation factor of 0.99 (inset of Figs. 1 and 2). The average value
of K, viz., Kav for the PyC60-1 system is estimated to be 58,300 dm3
molÀ1. Noteworthy is that, PyC60-1 system is reported to exhibit
ꢁ91.0 times higher magnitude of K (as reported in Table 1) com-
pared to C60-benzo-15-crown-5 system [17].
Time-resolved fluorescence study
Apart from steady state fluorescence measurements, we have
performed detailed nanosecond time-resolved fluorescence
experiment for PyC60-1 supramolecule. The experiment has been
67
68
-5050.275640
-5050.275643
0.000469
0.000493
0.020973
0.025605
69
70
71
72
73
74
75
76
-5050.275646
-5050.275649
-5050.275651
-5050.275653
-5050.275655
-5050.275656
-5050.275657
-5050.275658
0.000506
0.000483
0.000442
0.000378
0.000343
0.000298
0.000294
0.000282
0.034947
0.027593
0.020003
0.010493
0.003711
0.006220
0.005214
0.005810
Reason for exit: Successful completion
Quantum Calculation CPU Time : 14:37:50.50
Quantum Calculation Wall Time: 45:29:01.37
Scheme 3 (continued)
steady state Hitachi F-7000 model spectrofluorimeter. Fluores-
cence decay curves are measured with a HORIBA Jobin Yvon single
photon counting set up employing nanoled as excitation source.
Fig. 4. Ab initio optimized geometric structure of PyC60-1 system done in vacuo.
Please cite this article in press as: D. Pal et al., PyC60-naphthacrown system: A new supramolecular recognition element, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.020

D. Pal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
5
carried out at a fixed concentration of 1. The time-resolved fluores-
orbital (LUMO). DFT calculations reveal that, in the ground state,
the HOMO is exclusively located on 1, while the LUMO is precisely
positioned in fullerene entity of the complex (Fig. 5); this observa-
tion evokes that 1 and PyC60 act as donor and acceptor, respec-
tively. The absence of HOMO on fullerene and LUMO on 1 also
suggests absence of charge transfer (CT) interaction between 1
and fullerene, which is consistent with the fact that no CT bands
are observed for PyC60-1 supramolecule in the UV–vis experiment
as mentioned earlier. Moreover, in all the cases studied, the posi-
tion of the LUMO + 1 to LUMO + 6 states are exclusively found on
the fullerene unit (see Supporting Information). If we look into
detail regarding feature of HOMO, it is observed although HOMO
– 1 is precisely centered on naphthacrown unit, HOMO – 2, HOMO
– 3 and HOMO – 4 are positioned on fullerene (see Supporting
Information). Molecular electrostatic potential (MEP) maps have
been generated for PyC60 (see Supporting Information), 1 (see
Supporting Information) and PyC60-1 (Fig. 6) system in vacuo.
Interestingly, in the supramolecular complex, the original deep
reddish-green color of the separated PyC60 has been changed to
pale reddish yellow color, and deep blue color of 1 is changed to
bluish green, indicating strong propensity of electrostatic interac-
tion between these two chromophores upon photo-excitation.
cence measurement shows mono-exponential decay for uncom-
plexed 1; life time value of the singlet excited state (
measured to be 6.0 ns Fig. 3(a). In presence PyC60, however, no sig-
nificant amount of quenching in the
s value of 1 (5.95 ns) has been
s
s) of 1 is
s
observed and decay follows mono-exponential pathway Fig. 3(b).
Lifetime measurement, therefore, suggests a static quenching
mechanism behind the deactivation of photoexcited state of 1 in
presence of PyC60
.
Theoretical calculations
In our present study, we have done exclusive density functional
theoretical (DFT) calculations (single point) with the help of
restricted B3LYP method using 6-31G(D) basis set in vacuo. Before
energy calculations, we have optimized the geometry of PyC60-1
complex by ab initio calculations using STO 3G basis set. Detailed
optimization steps are given as Scheme 3. The DFT calculations
confirm very good fitting of the PyC60 molecule with the plane of
1. The heat of formation value of the PyC60-1 complexation process
is enumerated to be 4.35 kcal molÀ1. Single projection geometric
structure of the PyC60-1 system, optimized by ab initio calculation
in vacuo, is depicted in Fig. 4. The most significant observation in
Fig. 4 is that during complexation, the electron deficient part of
unsubstituted C60, is pointed towards the cavity of 1 keeping the
electron rich functionalized part (i.e., pyrrolidine unit) in opposite
space of 1. This particular binding motif of PyC60 towards 1 envis-
age that electrostatic interactions originating from the electron
density at the surface of the PyC60 and 1 are supposed to play vital
role in our present work. The existence of the electrostatic interac-
tions between fullerene and 1 have been evidenced by the results
obtained on frontier molecular orbitals, viz., highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
The DA distance (rDA), viz., 3.090 Å (H40 of 1, Bond191 of PyC60
)
estimated by DFT calculations also supports strong association
between PyC60 and 1 during complexation. Utilizing the value of
rDA, the solvent reorganization energy (R) [22] of the PyC60-1 sys-
tem is estimated to be À0.815 eV. R of PyC60-1 system exhibits
higher value compared to fullerene–diporphyrin system [23] and
very much comparable with PyC60-Pc systems reported in recent
past. The magnitude of R, however, does not indicate possibility
of electron transfer mechanism behind photoactive decay of 1 in
presence of PyC60 and rather indicative of photoinduced energy
transfer process [24,4,25].
Fig. 5. (a) HOMO and (b) LUMO of PyC60-1 system done in vacuo along with schematic diagram of orbital energies at various electronic states.
Please cite this article in press as: D. Pal et al., PyC60-naphthacrown system: A new supramolecular recognition element, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.020

6
D. Pal et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.09.020.
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In summary, we have described a binaphthyl bridged crown
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Acknowledgment
Grant-In-Aid through UGC (CAS), New Delhi is greatly
acknowledged.
Please cite this article in press as: D. Pal et al., PyC60-naphthacrown system: A new supramolecular recognition element, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2014), http://dx.doi.org/10.1016/j.saa.2014.09.020