A taco complex derived from a bis-crown ether capable of executing molecular logic operation through reversible complexation

Article abstract of DOI:10.1021/jo300622b

As learned from natural systems, self-assembly and self-sorting help in interconnecting different molecular logic gates and thus achieve high-level logic functions. In this context, demonstration of important logic operations using changes in optical resp

Full text of DOI:10.1021/jo300622b

Article
pubs.acs.org/joc
A Taco Complex Derived from a Bis-Crown Ether Capable of
Executing Molecular Logic Operation through Reversible
Complexation
Amal Kumar Mandal, Priyadip Das, Prasenjit Mahato, Suhash Acharya, and Amitava Das*
CSIR-Central Salt and Marine Chemicals Research Institute, Bhavnagar, 364002, Gujarat, India
S
* Supporting Information
ABSTRACT: As learned from natural systems, self-assembly and self-sorting help
in interconnecting different molecular logic gates and thus achieve high-level logic
functions. In this context, demonstration of important logic operations using
changes in optical responses due to the formation of molecular assemblies is even
more desirable for the construction of a molecular computer. Synthesis of an
appropriate divalent as well as a luminescent crown ether based host 1 and paraquat
derivatives, 2(PF6)2 and 3(PF6)2, as guests helped in demonstrating a reversible
[3](taco complex) (1·{2(PF6)2}2 or 1·{3(PF6)2}2) formation in nonpolar solvent.
Detailed 1H NMR studies revealed that two paraquat units were bound
cooperatively by the two crown units in 1. Because of preorganization, the flexible
host molecule 1 adopts a folded conformation, where each of two paraquat units
remain sandwiched between the two aromatic units of each folded crown ether
moiety in 1. Disassembly of the “taco” complex in the presence of KPF6 and
reassembly on subsequent addition of DB18C6 was initially demonstrated by 1H NMR spectral studies, which were subsequently
corroborated through luminescence spectral studies. Further, luminescence spectral responses as output signals with appropriate
and two independent molecular inputs could be correlated to demonstrate basic logic operation like OR and YES gates, while the
results of the three molecular inputs could be utilized to demonstrate important logic operation like an INHIBIT gate.
assemblies.1a,4 Formation of such assemblies and changes in
INTRODUCTION
■
conformations of associated host and/or guest component(s)
in most instances are primarily studied by 1H NMR
spectroscopy. Literature examples, wherein such processes are
probed by monitoring changes in optical responses are very
limited.5 Further, examples of such supramolecular assembly
wherein the reversible assembly and or disassembly process
could be demonstrated as a function of external stimulation in
the form of molecular input are rare.
Earlier, Stoddart and co-workers reported formation of
pseudorotaxanes from paraquats and bisarylene crown ethers
bearing 32−34 core atoms.6 They have also shown that both a
pseudorotaxane and an “exo” complex can form independently
under different crystallization conditions for 1,5-bis((3,5-di-tert-
butylbenzyl)ammonium)pentane bis(hexafluororophosphate)
and bis(p-phenylene)-34-crown-10.7 This suggests that both
complexes can and possibly do exist in solution. More recently,
Gibson et al. showed that complexation between bis(5-
hydroxymethyl-1,3-phenylene)-32-crown-10 and paraquats de-
rivatives led to an exo- or taco-complex formation,8 which was
influenced by the favorable π−π stacking interaction(s) owing
to the folded conformation of the flexible host molecule. They
also reported the influence of counteranions on Ka during
“taco” complexation of paraquat-based guest and crown ether
In supramolecular chemistry, the controlled and reversible
formation of molecular assemblies like rotaxanes/pseudorotax-
anes or of taco complex formation between tailor-made host
and guest component molecules along with changes in relative
conformations of these individual components on formation of
these assemblies are of enormous significance for the
construction of molecular machines or developing information
storage materials.1 In this regard, design of the appropriate
building block(s) having exciting properties for reversible
formation of a predicted supramolecular assembly with desired
conformation and property are crucial, as this allows switching
between different assembled and disassembled states under the
influence of various external stimuli. These designer assemblies
have potential for use as organized molecular-scale devices,
which are able to interpret, store, process, and dispatch
information similar to the sophisticated machines found in
natural systems. During the last three decades, recognition
motifs based on crown ethers as host and organic cations, e.g.,
secondary dialkylammonium salts,2 or paraquat derivatives,3 as
guest have played a significant role in demonstrating such
possibilities. Paraquat derivatives (N,N′-dialkyl-4,4′-bipyridi-
nium salts) are commonly being used as guest molecules in
achieving such assemblies because of their easy availability,
interesting physicochemical properties, and abilities to adopt
different conformations (threaded or taco complex formation)
based on the final conformation of the host component in these
Received: March 31, 2012
Published: July 26, 2012
© 2012 American Chemical Society
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Figure 1. Schematic representation of the methodology that was adopted for synthesis of 1 and 4, guest molecules (2(PF6)2 and 3(PF6)2), and the
model macrocyclic host 5, which were used in this study.
based host molecules.8b It has been argued that the favorable
entropy change that results from preorganization of the host
molecule while adopting the folded state during the complex-
ation processes influences the overall stability in such taco
complexes.9 Folding being an addressable step for the
formation of such “taco” complexes, proper choice and
introduction of substituents allows a smart control in improving
the stability of such taco-complex and thereby a host−guest
interactions.
the crown ether-based host fragments in 1 to form eventually
[3](taco complex). Because of preorganization of the flexible
host molecule during binding, 1 adopts a folded conformation,
wherein the paraquat-based guest component forms a
sandwiched type assembly. Reversible formation, disassembly
and reformation of the “taco” complex could be demonstrated
using external molecular stimulations in the form of KPF6 and
dibenzo-18-crown-6 (DB18C6) and through 1H NMR spectral
studies. Achieving three states, namely, assembled, disas-
sembled and reassembled by sequential addition of KPF6 and
DB18C6 could also be probed by monitoring distinct changes
in luminescence spectra. By adopting appropriate threshold
values and a logic convention, molecular input and output
signals could be used to encode binary information.
Monitoring the observed fluorescence response at different
wavelength, as output signals we obtained two basic gate OR
and YES by using two inputs and an INHIBIT gate by using
three inputs. To the best of our knowledge, demonstration of
such logic operations using a self-assembled “exo” complex is
extremely rare in the contemporary literature.
In the macroscopic world, logic devices are incorporated in
machines and devices for demonstrating desired logic functions.
The challenge for the miniaturization problem faced by silicon-
based electronics has actually led to the development of
molecular logic gates as an emerging research area.10 Since the
first unambiguous demonstration of Boolean logic operation
using a specified set of molecules and external stimulation in
the form of molecular inputs by de Silva et al.,11 numerous
molecules capable of basic logic functions, like AND,12 OR,13
NAND,14 INHIBIT,15 NOR,16 XOR,17 and XNOR,18 have
been reported. Molecular combinatorial logic circuits, such as a
digital adder,19 subtractor,20 comparator,21 and multiplexer,22
have also been realized. Although complex logic functions could
be achieved or demonstrated by using a single molecule,
realization of a true molecular computer is far from reality.
Interconnection between different logic operations is required
to create more complex logic circuits,10b,23 and this still remains
as a challenge for researchers active in this area. As learnt from
natural systems, self-assembly24 and self-sorting25 may work as
a glue to interconnect different molecular logic gates and thus
achieve higher-level logic functions. Such an approach may
open up the possibility of achieving higher logic operation(s)
through appropriately designed molecular assemblies and by
correlating different optical responses as a result of the
formation of such assembly.26
RESULTS AND DISCUSSION
■
Synthesis. Molecular structures and schematic representa-
tion of the bis-crown ether based host molecule 1, two guest
molecules as their hexafluorophosphate salt {2(PF6)2 and
3(PF6)2}, along with two reference compounds 4 and 5 are
shown in Figure 1. The 1,5-naphthalenediamine bridged
divalent host 1 was synthesized in a convergent manner after
four intermediate steps in a reasonable yield (80%). Two
paraquat derivatives, 2(PF6)2 and 3(PF6)2, were synthesized by
treating 4,4′-bipyridine with an excess of the appropriate alkyl
halide in acetonitrile, and their desired hexafluorophosphate
salts were isolated as insoluble solids by anion exchange in
aqueous media. The other model compound 5, along with
intermediates A and B, were synthesized following previously
reported procedures.5a
In this contribution, we report the design and synthesis of
divalent crown ether 1 having two photoactive naphthalene
units as signaling fragments and two dibenzo-[24]-crown-8
ether moieties as the receptor fragments for two different
Complexation of Divalent Host 1 with 2(PF6)2 and
3(PF6)2. The divalent host 1 in CDCl3:CD3CN (4:1, v/v)
solution was found to be light brown in color. However, this
solution turned dark brown when 2 mol equiv of 2(PF6)2 were
1
parquat derivatives in nonpolar solvent. Detailed H NMR
studies reveal that two parquat units bind cooperatively with
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added, and this change in color was due to the formation of a
charge-transfer complex between the electron-rich aromatic
rings of the host 1 and the electron-deficient bipyridinium rings
(Δδ = −0.16 ppm)) were observed, which revealed that these
aromatic rings were also under the shielding influences of the
bipyridinium rings. Shifts of the aromatic protons of the central
naphthalene unit were rather different.
1
of 2(PF6)2. A Job’s plot analysis (see Figure 4b) based on H
NMR data revealed that the stoichiometry for the host−guest
complex was 1:2. This was confirmed by electrospray ionization
mass spectrometry (ESI-MS) and MALDI-TOF mass data
(Supporting Information Figures S2 and S4). In both mass
spectral techniques, for a mixture of 1 and 2(PF6)2 in the molar
ratio of 1:2, the peak for [1·{2(PF6)2}2 − PF6]+ was observed at
m/z 1983.6. The change in solution color on addition of benzyl
derivative 3(PF6)2 to 1 was not so prominent compared to that
discussed above for 2(PF6)2 under identical experimental
condition; however, this gave the first evidence for the
formation of a complex between 1 and 3(PF6)2. A Job’s plot
A gradual but small downfield shift (Δδ = 0.04 ppm) for the
Ho proton was observed on inclusion complex formation. Two
other protons Hn and Hp as well as the imine proton (Hm)
initially moved downfield, and then their position remained
unchanged as the titration progressed. These observations were
consistent with the proposed sandwich type complex formation
(Figure 3). Possible conformation for the host−guest complex
1
analysis (see Figure 6b) based on H NMR data demonstrated
that the binding stoichiometry was 1:2 in CDCl3:CD3CN (4:1,
v/v) solution. This was confirmed by ESI-MS and MALDI-
TOF mass data (Supporting Information Figures S3 and S5).
The peak for [1·{3(PF6)2}2 − PF6]+ was observed at m/z
2287.1 recorded using both techniques for a 1:2 mixture of 1
and 3(PF6)2.
Let us first establish the complexation process for the
formation of a 1:2 complex between 1 and 2(PF6)2. The
complexation of 1 and 2(PF6)2 in CDCl3:CD3CN (4:1 v/v)
Figure 3. (a) Partial 2D-NOESY spectrum (500 MHz, 298 K)
recorded for 4.2 mM 1 with 8.4 mM 2(PF6)2 in CDCl3:CD3CN (4:1,
v/v). (b) Schematic representation of the mode of binding of 1 and
2(PF6)2 due to formation of a [3](taco complex).
1
solution was investigated in detail by H NMR spectroscopy
using different stoichiometric ratios by systematically varying
the concentration of the guest fragment. Partial 1H NMR
spectra of 1, 2(PF6)2, and a mixture of 1 and 2(PF6)2 are shown
in Figure 2. Only one set of peaks was observed in the 1H NMR
(Figure 3) suggests that central naphthalene ring and the imine
protons are not within the shielding influence of the aromatic
rings of the paraquat guest and formation of an inclusion
complex with cationic guest component accounts for their small
but definite downfield shifts. All the ethyleneoxy protons of the
crown ether were upfield shifted on complexation, and shifts
were most significant for Hd and Hi (Δδ = −0.21 ppm).
To gain more insight into the complexation process and the
relative orientation of the individual host and guest
components in the inclusion complex, we performed 2D-
NMR studies. The 2D-NOESY spectrum of 1 in presence of 2
mol equiv of 2(PF6)2 was recorded in CDCl3:CD3CN (4:1 v/v)
at room temperature and is shown in Figure 3a. Strong
correlations between the bipyridinium proton H3 and the
ethyleneoxy protons (Hd and Hi) of the crown unit indicates
that these two sets of protons come closer to the bipyridinium
units during binding. Two sets of cross peaks were also
observed for the interaction of H3 with Ha and Hb of the
terminal naphthalene unit as well as Hl of the phenyl unit of the
divalent host. These results suggest that the terminal
naphthalene unit and the phenyl unit are in a position after
binding, where they interact simultaneously with the bipyr-
idinium units. The cross peak was also observed for the
interaction of H2 of the bipyridinium unit and Ha of the
terminal naphthalene unit. A correlation between the Hm and
Hp proton in the 2D-NOESY spectrum also suggests that the
central aromatic units of the divalent host remains planar while
it forms the host−guest complex. The cross peaks in the 2D-
COSY spectrum of 1 in the presence of 2 mol equiv of 2(PF6)2
were recorded in CDCl3:CD3CN (4:1, v/v) at room temper-
ature, which also supported the formation of the host−guest
adduct (Supporting Information Figure S6).
1
Figure 2. Partial H NMR spectra (500 MHz, 298 K) recorded in
CDCl3:CD3CN (4:1, v/v) for (a) 4.2 mM 1, (b) 4.2 mM 1 with 4.2
mM 2(PF6)2, (c) 4.2 mM 1 with 8.4 mM 2(PF6)2, and (d) 8.4 mM
2(PF6)2.
spectrum of the mixed solution of 1 and 2(PF6)2, which implied
that the equilibrium kinetics were fast within the 1H NMR time
scale at the 500 MHz frequency. After complexation,
appreciable upfield shifts in signals corresponding to bipyr-
idinium protons H2 (Δδ = −0.34 ppm), H3 (Δδ = −1.40 ppm)
and the N-methyl protons H1 (Δδ = −0.12 ppm) were
observed, which suggested that these paraquat protons resided
under the shielding influence of the aromatic rings of the host
molecule 1 during binding. Chemical shifts for two sets of
aromatic protons of the divalent host were quite interesting.
Significant upfield shifts for aromatic protons (Ha, Hb and Hc)
of the terminal naphthalene unit and the protons of the phenyl
ring (Hj (Δδ = −0.33 ppm), Hk (Δδ = −0.14 ppm), and Hl
Previous literature reports on inclusion complex formation
between crown ether derivatives and paraquat units suggest two
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Figure 4. (a) Scatchard plot for the complexation of 1 ([1]0 = 4.2 × 10−3 M) with 2(PF6)2 in CDCl3:CD3CN (4:1, v/v) at 25 °C (y = −383.34x2 +
391.48x + 29.88, R2 = 0.99), where p is fraction of 1 units bound. Error bars in p: 0.01; error bars in p/2(PF6)2: 3.06. (b) Job’s plot that reveals a
1:2 binding stoichiometry between 1 and 2(PF6)2 in CDCl3:CD3CN (4:1, v/v) using changes in chemical shift (δ) data for Hd, while [1]0 +
[2(PF6)2]0 = 7.3 × 10−3 M.
possible modes of binding during complexation;6−9 either
formation of a pseudorotaxane type interwoven complex takes
place wherein the paraquat ion is threaded through the
macrocyclic host or formation of a “taco” complex wherein the
guest is sandwiched within the aromatic units of the folded
host. Mainly three possible binding interactions, e.g., hydrogen
bonding, face-to-face π-stacking, and N+···O ion dipole
interactions, are expected to contribute to the stability of the
adduct. The most prominent binding interactions are hydrogen
bonding between OCrown and acidic hydrogens of the paraquat
unit and π−π stacking interactions between the electron poor
pyridinium ring and aryl units of the crown ether-based host.
Gibson et al. reported a [3](taco complex) from a linear
bis(crown ether) as host and paraquat ion as guest; taco
apparent average association constant (Kav). To find out the
stoichiometric binding constants, the following equations were
used: K1 = [1·2(PF6)2]/{[1]·[2(PF6)2]} and K2
=
[1·{2(PF6)2}2]/{[1·2(PF6)2]·[2(PF6)2]}. The slope of the
first six data points for low p (Figure 4a) gave the value of
(2K2 − K1), while the slope of the last 10 data points for high p
(Figure 4a) gave the value of −2K2.31f Thus, the value for K1
was found to be 1.0 ( 4.4) × 102 M−1, while that for K2 was 1.6
( 0.4) × 102 M−1. The ratio K2/K1 = 1.6 is significantly higher
than the value of 0.25 that is expected for statistical
complexation31b,g and supports the cooperative nature of
binding. Formation of the 1:1 complex is expected to effectively
restrict conformational changes of crown moiety due to the
hydrogen bonding interactions between the paraquat protons
with crown ether oxygen atoms, and this conformational
restriction presumably facilitates complexation of the second
crown ether site and accounts for the apparent observed
cooperativity. As mentioned by Hayman,31b in case of positive
cooperativity it is misleading to say that the first ligand is bound
less strongly than the subsequent ligands. Rather, positive
cooperativity means that in the bis-complex both ligands are
bound more strongly than the first ligand is in the
monocomplex. In the present study, the apparent average
association constant was found to be Kav = (K1 + K2)/2 = 1.3
( 1.6) × 102 M−1, which is bigger than that reported for 2:1
complex formation between dibenzo-[24]-crown-8 and the
paraquat.28 But the apparent average association constant is
lower than the association constant value of 3.7 ( 0.4) × 102
M−1 reported for paraquat-based [3](taco complex) prepared
from a linear bis-crown ether-based host having bis(m-
phenylene)-32-crown-10 as binding motif.27 The average
binding constant, evaluated for the present study, is also
lower than the value reported by Chen et al., 4.0 × 103 M−1 for
a triptycene paraquat based [2](taco complex) having dibenzo-
[24]-crown-8 as the host.30b It may be mentioned here that we
have ignored the possibility of the ion pairing of paraquat ion
while computing the binding constant. The observed lower
value of the binding constant in the present study is
understandable, as the stability of “taco” complex depends on
different parameters like the flexibility of the crown unit and the
nature of substituent on the aromatic moiety.8,9 Other factors
like steric and conformational effects caused by the
introduction of a substituent on the crown ether moieties are
also expected to influence the overall binding constant.
1
complex formation was confirmed by H NMR spectra as well
as by single crystal X-ray structure.27 They also reported a
capsular structure that results from an opposing, head-to-head
orientation of the two DB24C8, wherein the benzene rings
π−π stack with the pyridinium units of the paraquat guest.28
However, a report on analogous complex formation by
Stoddart et al. with the diazapyrenium salt features a head-to-
tail structure of the two host species in which one pair of benzo
rings does not interact with the guest.29 A recent report by
Chen et al. reveals that a triptycene-based cylindrical macro-
tricyclic polyether containing dibenzo-24-crown-8 cavities form
“taco” complex with different functional paraquat derivatives.30
1
Comparison of the literature reports and results of our H
NMR experiments discussed above led us to conclude that a
[3]taco complex involving 1 and two molecules of 2(PF6)2
probably has the conformation shown in Figure 3b. The
flexibility of the crown ether in solution is expected to allow
rapid folding such that the phenyl rings and the terminal
naphthalene ring interact with pyridinium rings of the guest to
adopt a folded [3](taco complex) conformation.
In order to study the relationship between the two crown
ether binding sites during formation of the [3](taco complex),
1H NMR spectra were recorded for a series of CDCl3:CD3CN
(4:1, v/v) solutions of 1 (4.2 mM) with systematic variation of
1
[2(PF6)2]. On the basis of these H NMR data, the extent of
complexation, p (Supporting Information page 4), of the crown
ether units was determined and a Scatchard plot was made
(Figure 4a). The nonlinear nature of this plot with a maximum
demonstrated that positive cooperativity had driven the
complex formation processes.31
Analysis of the Scatchard plot enabled us to find out the
stoichiometric binding constants K1 and K2 and also the
Formation of the complex between 1 and 3(PF6)2 in
CDCl3:CD3CN (4:1, v/v) was also studied in detail using H
1
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NMR spectroscopic studies. Only one set of peaks was
observed in the 1H NMR spectrum, which revealed fast
exchange at 500 MHz.
−2K2. Thus, the value for K1 was found to be 0.5 ( 9.0) × 102
M−1, and K2 was 1.2 ( 5.8) × 102 M−1. Any possibility of ion
paring of this paraquat derivative was also ignored. Thus the
Partial 1H NMR spectra of 1, 3(PF6)2, and a mixture of 1 and
3(PF6)2, are shown in Figure 5. As observed in case of 2(PF6)2,
apparent average association constant was found to be Kav =
(K1 + K2)/2 = 0.8 ( 1.8) × 102 M−1, which is lower than that
we observed earlier for 2(PF6)2. Presumably, the presence of
benzyl units accounts for the lower formation constant for
1·3(PF6)2.
Photophysical Study. An overview of the steady state
spectroscopic data measured in air equilibrated CH2Cl2
solutions at room temparature is given in Table 1. The
Table 1. Spectroscopic Data Recorded for Compounds 1,
2(PF6)2, 3(PF6)2 and Respective Complexes 1·{2(PF6)2}2
and 1·{3(PF6)2}2 in CH2Cl2 Solution at Room Temperature
host/guest
ε × 10−3 (L mol−1 cm−1
)
λabs (nm) λemi (nm) Φ (%)
1
26.9
19.9
7.3
280
365
260
260
280
373
280
365
365
280
342, 420
1.5
2(PF6)2
342
287
342
17.6
0.5
3(PF6)2
10.9
46.0
21.4
35.4
20.3
11.1
9.2
1
Figure 5. Partial H NMR spectra (500 MHz, 298 K) recorded in
1·{2(PF6)2}2
5.9
CDCl3:CD3CN (4:1, v/v) for (a) 4.2 mM 1, (b) 4.2 mM 1 with 4.2
mM 3(PF6)2, (c) 4.2 mM 1 with 8.4 mM 3(PF6)2, and (d) 8.4 mM
3(PF6)2.
1·{3(PF6)2}2
342
1.0
4
5
374
320
all bipyridinium protons (Hv and Hvi) including the N-
methyelene (Hiv) and phenyl protons (Hi,ii,iii) were shifted
upfield on formation of the host−guest complex. Though the
extent of shifts observed was less compared to that of previous
one, a similar shift pattern was observed for the protons of
divalent host 1 after complexation. Cross peaks that were
observed in the 2D-NOESY and COSY spectrum also
confirmed the complex formation between 1 and 3(PF6)2 in
mixed solvent medium (Supporting Information Figure S7).
To study the relationship between the two crown ether
absorption spectrum of 1 mainly showed two band maxima at
280 nm and a broad band at 365 nm (Table 1), which could be
attributed to electronic transitions associated with two different
photoactive moieties, D1 and D2 (Scheme 1). To understand
Scheme 1. Schematic Drawing of 1 Indicating Two Different
Units D1 and D2
1
binding sites of 1 during the complexation with 3(PF6)2, H
NMR spectra were recorded for a series of solutions with a
fixed concentration for 1 and varying concentration of 3(PF6)2.
On the basis of these data, the extent of complexation, p, of the
crown ether units was determined and a Scatchard plot was
drawn (Figure 6a). The nonlinear nature of this plot with a
maximum confirmed positive cooperativity in the complex
formation process.31 The slope of the first six data points for
low p (Figure 6a) gave the value of 2K2 − K1, while the slope of
the last 13 data points for high p (Figure 6a) gave the value of
the nature of these electronic transitions better, two new
reference compounds (4 and 5) were also synthesized, and
their spectroscopic and physicochemical properties were
measured for comparison. The absorption band at 280 nm
for 5 was attributed to a naphthalene-based π−π* transition,
and for 4, the broad band at 365 nm was ascribed to an
intracomponent charge transfer (CT) band. Thus, a compar-
ison of absorption spectra of 1 with that of 4 and 5 suggests
that the broad band at 365 nm is due to the charge transfer
transition of D2 unit, while the other higher energy band at 280
nm is associated with the napthalene-based transition of D1
unit (Scheme 1). Absorption spectra for 1 were also recorded
in the presence of 2 equiv of 2(PF6)2 or 3(PF6)2. Interestingly
on addition of 2(PF6)2, a red shift of 8 nm for the charge
transfer band (λmax = 365 nm for 1) was observed, and the
broadness of the band was also enhanced. A similar trend in the
spectral change was also observed when spectra for 1 were
recorded in presence of 2 mol equiv of 3(PF6)2, although the
extent of change was less (Table 1) compared to spectra
recorded in the presence of comparable concentrations of
Figure 6. (a) Scatchard plot for the complexation of 1 ([1]0 = 4.2 ×
10−3 M) with 3(PF6)2 in CDCl3:CD3CN (4:1, v/v) at 25 °C (y =
−380.71x2 + 354.69x + 28.16, R2 = 0.99), where p is fraction of 1 units
bound. Error bars in p: 0.01; error bars in p/[3(PF6)2]: 2.63. (b)
Job’s plot that reveals a 1:2 binding stoichiometry between 1 and
3(PF6)2 in CDCl3:CD3CN (4:1, v/v) using changes in chemical shift
(δ) data for Hd, while [1]0 + [3(PF6)2]0 = 7.3 × 10−3 M.
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Scheme 2. Relative Energies for the HOMO and LUMO Levels for 1, 4 and 5, 1·{2(PF6)2}2 and 1·{3(PF6)2}2, Which Were
Evaluated from Respective Ground State Redox Potentials and E0−0 Values
2(PF6)2. However, in both cases, features are indicative of the
new charge transfer complex formation in the ground state in
presence of 2 equiv of 2(PF6)2 or 3(PF6)2.
HOMO − 1 based transitions, respectively. Further, to get a
better understanding about electronic and fluorescence spectral
results, we also calculated the energy levels of the two different
charge transfer complexes of 1 formed in presence of 2 equiv of
either 2(PF6)2 or 3(PF6)2. In the case of 1·{2(PF6)2}2 and
1·{3(PF6)2}2, the E0−0 value for the charge transfer transition
(HOMO − 1 to LUMO transition) was lower than the E0−0
value of 1. This decrease is more prominent for 1·{2(PF6)2}2
than that of 1·{3(PF6)2}2 (Scheme 2), which corroborates well
with the more significant spectral changes for 1·{2(PF6)2}2 than
that of 1·{3(PF6)2}2. Thus, the energy levels evaluated on the
basis of results of electrochemical experiments agree well with
the results of the absorption and emission spectral results.
Luminescence decay profiles for the excited state were
measured for 1 in presence and absence of 2(PF6)2 and 3(PF6)2
by time correlated single photon counting technique using 280
nm nano-LED as an excitation source (Table 2). Lumines-
The emission spectrum of 1 in CH2Cl2 solution was
recorded following excitation at 280 and 365 nm. On exciting
at 280 nm, 1 showed two different emission bands with maxima
at 342 and 420 nm, while one emission band with maxima at
420 nm was observed when λExt of 365 nm was used. The weak
emission of 1 (Φ = 0.0147) was attributed to the flexible nature
of the crown moiety and the presence of lower energy ICT
state.32 2(PF6)2 exhibited a strong emission band (λExt = 260
nm, Φ = 0.1762) with a maximum at 342 nm. The significant
decrease of fluorescence quantum yield (Φ = 0.0593) observed
for 1:2 mixture of 1 and 2(PF6)2 also supports the formation of
a CT complex. But the emission spectra of 1 in presence of 2
equiv of 3(PF6)2 was a little different, and an overall decrease in
emission quantum yield supports the CT complex formation
(Table 1).
To obtain a better understanding about the relative energies
of the frontier molecular orbitals, cyclic voltammetry (CV)
studies were carried out for measuring the oxidation/reduction
potential of the respective host−guest system. The results of
the cyclic voltametric studies are provided in Table 3, and the
respective energy levels associated with host and guest
components are shown in Scheme 2.
Two oxidation potentials (Eox) for 1, corresponding to the
highest occupied molecular orbital (HOMO) and (HOMO −
1) levels, were recorded. Two band gap energy (E0−0) values
for 1 were obtained from the intersection of the normalized
absorption and emission spectra and were assigned to D1
(HOMO to LUMO + 1 transition) and D2 (HOMO − 1 to
LUMO transition) based transitions, respectively (Supporting
Information page 15). Interestingly, the E0−0 value of the D1
unit was lower (0.12 eV) compared to the reference compound
5. On the other hand, the E0−0 value that was assigned for D2
unit almost remains unchanged (0.03 eV). This signifies a little
alteration of the charge transfer state of the compound 4 due to
the attachment of an additional moiety D1. Thus, these results
indicate the presence of an intramolecular charge transfer state
(ICT) in 1. The results also suggest that the absorption bands
of 1 at 280 and 365 nm could be ascribed to (HOMO to
LUMO + 1) and (HOMO − 1 to LUMO) transitions,
respectively. Similarly, the emission bands of 1 at 342 and 420
nm originated from LUMO + 1 to HOMO and LUMO to
Table 2. Fluorescence Life Time Data (τ in ns) Obtained by
Using 280 nm NanoLED As an Excitation Source and
Monitoring Wavelength (λMon), in Dichloromethane at
a
25°C
host/guest
λMon (nm)
τ (ns)
rel. (%)
χ2
1
340
425
340
340
340
340
340
0.55, 7.59
46, 54
1.11
0.97
1.06
1.23
1.17
1.08
1.15
4
0.57, 2.00
87, 13
5
11.00
100
1{2(PF6)2}2
1{3(PF6)2}2
2(PF6)2
3(PF6)2
0.40, 1.36, 9.46
0.05, 1.29, 9.60
0.56, 1.53
40, 44, 16
25, 12, 63
40, 60
0.41, 5.66
56, 44
a
χ2 is a numerical value that reflects the overall goodness of fit
(Supporting Information page 12).
cence, monitored at 340 nm for 1, showed a biexponential
decay with τ1 = 0.55 ns and τ2 = 7.39 ns. A comparison of these
with the emission decay lifetimes recorded for two model
compounds 4 and 5 (Table 2) suggests that the contribution of
the longer-lived component of the biexponential decay for 1
was due to decay of the naphthalene-based excited state in the
D1 moiety, while the shorter-lived component could be
ascribed to a ICT-based excited state in the D2 unit.
The observed decrease in lifetime of both components also
supports the alteration of the charge transfer-based excited
states in 1. Emission decay profiles for 1 were also recorded in
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the presence of 2 mol equiv of 2(PF6)2 and 3(PF6)2. In both
cases the decay profile showed a triexponential decay,
suggesting the presence of three components in the excited
state. The decrease of lifetime of the three components
compared to their corresponding individual compound
supports the presence of a lower energy charge transfer state
in 1·2(PF6)2 or 1·3(PF6)2, which favors the faster deactivation
according to the energy gap law.32 This also agrees well with
the electronic spectral data, which reveal that 1 forms charge
transfer complexes with 2(PF6)2 and 3(PF6)2 and the energy of
this CT transition is lower than that of 1.
to the model compound, 5. These observations are attributed
to the presence of charge transfer interactions between D1 and
D2 units in 1, which was also evident in the absorption spectra
and reported earlier for analogous crown ethers.34 Upon
complexation of 1 with 2(PF6)2 or 3(PF6)2, the oxidation
processes for D1 and D2 were found to respond differently.
Appreciable anodic shifts in potential were observed for the
oxidation process associated with D1, while cathodic shifts were
evident for the D2 fragment (Table 3), when compared with
respective values for 1. Interestingly, with this cathodic shift the
oxidation potential for the D2 fragment in 1·{2(PF6)2}2 or
1·{3(PF6)2}2 became comparable to that for the model
compound 4 (Table 3). This observation clearly indicates
that the D1 fragment in 1·{2(PF6)2}2 is involved in a new
charge transfer interaction with the bipyridinium unit, while
fragment D2 in 1·{2(PF6)2}2 or 1·{3(PF6)2}2 is no longer
under the influence of any charge transfer interaction.
Control of Reversible Complexation of 1 and 2(PF6)2.
Potasium ion is known to form a stable complex with dibenzo-
24-crown-8 (DB24C8), and this high stability (Ka = 7.6 × 103
M−1 in acetonitrile at 25 °C) has actually helped in using K+ as
an effective templating agent for the synthesis of 24-crown-8
derivatives.35 Literature reports also revaled that dibenzo-18-
crown-6 forms a much stronger complex with the K+ (Ka = 1.0
× 106 M−1 in CH2Cl2 + 2% acetonitrile at 25 °C).36 We have
used the difference in binding constants to demonstrate
reversible [3](taco complex) formation as well as self-
assembled phenomena for implementing logic operations
based on the fluorescence response.
Electrochemical Study. Redox potential values for all
compounds and the taco-complexes were recorded and are
summarized in Table 3. Paraquat (PQ2+) derivatives usually
Table 3. Electrochemical Data for the Molecular
Components Measured in Argon-Purged CH3CN:CH2Cl2
a
Solution (1:1) at Room Temperature
host/guest
oxidation (V vs SCE)
reduction (V vs SCE)
b
1
+1.24, +1.55
b
b
1{2(PF6)2}2
1{3(PF6)2}2
2(PF6)2
3(PF6)2
4
+1.10, +1.91
−0.91, −1.00
−0.71, −0.85
−0.39, −0.80
−0.28, −0.69
b
b
+1.03, +1.88
c
c
c
+1.02
c
5
+1.62
a
Et4NPF6 was used as supporting electrolyte, and glassy carbon was
b
used as working electrode. Not fully reversible process; potential
value was estimated from DPV peaks. Half-wave potential values are
c
On addition of 8.5 mM of KPF6 to a CDCl3:CD3CN (4:1, v/
expressed in V vs SCE for reversible and one-electron redox processes,
unless otherwise indicated.
v) solution of 4.2 mM 1 and 8.5 mM 2(PF6)2, an instantaneous
1
change in the H NMR spectrum was observed. Spectral shifts
characteristic for the taco complex disappeared, and the original
1H NMR spectra for 2(PF6)2 was restored (Figure 7c).
Significantly, a simultaneous visual change in solution color
from dark brown to pale brown was observed. On further
addition of 8.5 mM of KPF6, the chemical shifts of protons of
2(PF6)2 also remain the same (Figure 7d). Control experiments
revealed that on addition of KPF6, the chemical shifts of
protons on 2(PF6)2 remains unchanged. All these data indicate
undergo two consecutive one-electron reduction processes, and
both processes are chemically and electrochemically rever-
sible.33 The first reduction leads to the generation of a cation
radical species (PQ•+) (E1/2 = −0.39 V), and the second
reduction yields the fully neutral residue (PQ) (E1/2 = −0.80
V) species. However, the first and second reductions of the
3(PF6)2 occur at less negative potentials than 2(PF6)2. The
presence of the benzyl substituent is expected to contribute to
this observed difference. The diminished current observed for
these redox processes on complexation, compared to those for
individual components, confirm that the complexes have
smaller diffusion coefficients. Cathodic shifts in potentials
were observed for both these redox processes on complex
formation, which suggests that bipyridinium unit is engaged in a
charge transfer interaction with the electron donating host. As
expected, the observed cathodic shift for the first reduction
wave (520 mV) is larger than that of the second reduction wave
(200 mV), as the CT interaction with the host molecule
becomes weaker after the first reduction of the bipyridinium
unit.
Two oxidation processes were observed for the divalent host
1 at +1.24 V and +1.55 V. A comparison of these values with
those for two reference compounds 4 and 5 (Table 3) suggests
that the first oxidation process at +1.24 V is due to the redox
process involving the D2 unit, while the second one at +1.55 V
is associated with the D1 unit. Thus, for the divalent host
molecule 1, the oxidation process associated with the D2 units
takes place at a potential that is 220 mV more positive than that
of the corresponding reference compound 4, and the oxidation
process for D1 unit in 1 becomes easier by 70 mV as compared
1
Figure 7. Partial H NMR spectra (500 MHz, 298 K) recorded in
CDCl3:CD3CN (4:1, v/v) for (a) 4.2 mM 1; (b) 4.2 mM 1 with 8.5
mM 2(PF6)2; (c) 4.2 mM 1 with 8.5 mM 2(PF6)2 and 8.5 mM KPF6;
(d) 4.2 mM 1 with 8.5 mM 2(PF6)2 and 17 mM KPF6; (e) 4.2 mM 1
with 8.5 mM 2(PF6)2, 17 mM KPF6 and 17 mM DB18C6; (f) 8.5 mM
2(PF6)2.
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Scheme 3. Schematic Representation of the Formation of the [3](Taco Complex), Its Disassembly in Presence of K+, and
a
Reassembly on Subsequent Addition of DB18C6
a
The [3](taco complex) undergoes de-complexation in the presence of 2 equiv of K+, as K+ formed an even stronger complex with 1. However,
subsequent addition of DB18C6, which has an affinity toward K+ even higher than that of 1, allowed the regeneration of the complex 1·{2(PF6)2}2.
that complete dissociation of the taco complex, 1·{2(PF6)2}2,
takes place in the presence of 8.5 mM KPF6. However, upon
subsequent addition of 17 mM of DB18C6 to this resultant
solution, chemical shifts revert back to those for 1·{2(PF6)2}2
(Figure 7e), indicating the reformation of the taco complex.
Appropriate design of the host functionality and the choice of
the host and guest moieties also allowed us to probe the
complexation and decomplexation process for 1 and 2(PF6)2 in
the presence of other molecular inputs like KPF6 and DB18C6
by monitoring changes in fluorescence spectral patterns
(Supporting Information Figure S17).
luminescence spectra. Our main focus is to demonstrate the
logic operation by monitoring the optical response of 1 in
presence of different inputs. By adopting appropriate threshold
values and a logic convention, the physicochemical input and
output signals could be used to encode binary information.10
Monitoring the luminescence output signals at appropriate
wavelengths, two basic logic operations like OR and YES could
be demonstrated, while more complicated logic operations like
INHIBIT gate could be demonstrated using three ionic/
molecular inputs. Such behavior is rather uncommon when
compared with previous reports,26a,c,38 where mostly acid and
base were used as two inputs for assembly/disassembly
purposes. Here we have used three chemical input channels,
e.g., DB18C6 (In1), KPF6 (In2) and 2(PF6)2 (In3), and either
one of three optical output channels (using luminescence
intensity as the readout data) to describe OR, YES and
INHIBIT logic operation at 310 (Out1), 330 (Out2), and 360
nm (Out3), respectively. We have adopted a positive logic
convention (i.e., a signal below the threshold corresponds to
logic ⟨0⟩) for demonstrating these logic functions. The
threshold values selected for the three output channels are
indicated as the dashed lines in bar diagrams shown in Figures
8a,b and 9a. For inputs, the logic ⟨0⟩ is represented by no
addition of reactant, and the logic ⟨1⟩ is represented by the
addition of 2 equiv of reactant.
Earlier we have shown that a significant decrease of
fluorescence intensity of free 2(PF6)2 (Φ = 0.1762, Table 1)
was observed upon formation of the taco complex,
1·{2(PF6)2}2 (Φ = 0.0593, Table 1). On addition of 2 mol
equiv of KPF6, the fluorescence intensity at 342 nm was
enhanced (Φ = 0.0821), which confirmed the decomplexation
1
of 1·{2(PF6)2}2 as was also evident in the H NMR spectra.
The difference in fluorescence intensity of the solution having
KPF6 (Φ = 0.0821) with that of 2(PF6)2 (Φ = 0.1762) was
attributed to the quenching influence of the hexafluorophos-
phate ion37 or crown moiety present in the solution mixture. In
presence of added KPF6, decrease of the dissociation of 2(PF6)2
could have also contributed to this. Further, on addition of 2
equiv of DB18C6 to the solution containing (1·{2(PF6)2}2 + 2
mol equiv of KPF6), K+ binds preferentially to DB18C6 to form
K{DB18C6}+ and favors the regeneration of the taco complex
formation between 1 and 2(PF6)2, and this was evident by
further decrease in fluorescence intensity. Thus, the results of
the fluorescence studies agree well with those obtained from 1H
NMR studies, and all these results demonstrated that the
reversible complex formation between 1 and 2(PF6)2 can be
controlled in presence of molecular inputs like KPF6 and
DB18C6 (Scheme 3). Similar phenomena were also observed
for the complexation between 1 and 3(PF6)2 (Supporting
Information Figure S16).
The output of OR gate is normally switched on in presence
of either one or both inputs. To use 1 as a molecular OR gate,
we used two inputs In1 (DB18C6) and In3 (2(PF6)2), and the
change of luminescence intensity at 330 nm was monitored as
output (Out2). The truth table for the OR gate and the
corresponding bar diagram are shown in Figure 8a.
In the absence of any of these inputs, the fluorescence
intensity at 330 nm of 1 was relatively low (<2, output: ⟨0⟩),
whereas the fluorescence intensities were obviously enhanced
(>2, output: ⟨1⟩) in the presence of each one or both of these
two inputs. As a result, a two-input OR logic gate was obtained
according to the truth table shown in Figure 8a. It was also
possible to construct a YES gate with 1, by using another set of
inputs (In1 and In2) and monitoring the fluorescence intensity
at 310 nm as output (Out1). The general feature of YES gate is
to transform one input signal to output neglecting another
input signal.39 The truth table of 1 for YES gate and the
corresponding bar diagram are shown in Figure 8b. The
emission intensity at 310 nm for the solution of 1 in the
presence of both inputs (In1 and In2) was relatively high (>14,
output: ⟨1⟩).
Logic Operations through Control of Reversibility.
The results described above showed that the chemically driven
reversible “taco” complex formation between 1 and 2(PF6)2
could be achieved through sequential molecular inputs like
KPF6 and DB18C6. Three states of this process, namely,
assembly (that generated 1·{2(PF6)2}2 on addition of 2(PF6)2
to 1), disassembly (that regenerated individual component
2(PF6)2 and 1{KPF6}2) on addition of KPF6, and reassembly
(that regenerated 1·{2(PF6)2}) on addition of DB18C6, were
characterized by probing distinct and specific changes in
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table is presented in Figure 9a. When none of these inputs were
on, the gate was off (<4, output: ⟨0⟩). Switching on happened
only when one input combination (In2 + In3) was used and
turned the gate on (>4, output: ⟨1⟩). However, use of any other
input combination did not switch the gate on (<4, output: ⟨0⟩).
Thus, one input combination (In2 + In3), out of eight
possibilities could switch the gate on (Figure 9a), and thus, the
observed luminescence response of divalent host 1 monitored
at 360 nm met the requirements for an INHIBIT logic
operation.
CONCLUSION
■
In summary, we have demonstrated the complexation behavior
of newly synthesized divalent host 1 with two different paraquat
derivatives. Detailed H NMR study reveals that two paraquat
Figure 8. (a) Bar diagrams showing the experimental output values
(λex = 280 nm; Out2, λMon = 330 nm) for the OR logic states of the
system, obtained on the same solution by the sequential addition of
DB18C6 and 2(PF6)2 inputs. The dashed lines mark the threshold
values for the corresponding logic operations. (b) Bar diagrams
1
units bind cooperatively with this host, forming a [3](taco
complex). Because of preorganization of the flexible host
molecule during binding, the [3](taco complex) adops a folded
shape, wherein the two paraquat units are sandwiched between
the two aromatic units of the folded crown ether fragment of 1.
The steady and the excited state spectra along with the results
of electrochemical studies of the respective compounds and the
[3](taco complexes) suggest the presence of a charge transfer
state in the divalent host 1, while on complexation with
respective paraquat derivative, a new charge transfer complex is
formed. The energy levels calculated on the basis of
electrochemical data and E0−0 value for the respective molecule
also agree well with spectral responses for different CT
processes. We have also investigated the reversible and
controlled complexation processes involving 1 and 2(PF6)2/
3(PF6)2 in presence of independent molecular inputs like KPF6
and DB18C6. Preferential binding of K+ to crown ether
moieties of 1 leads to the dissociation of the [3](taco complex).
Further, on subsequent addition of DB18C6 to this resulting
solution, the higher affinity of DB18C6 toward K+ allows
reassembly of the [3](taco complex). These three processes,
namely, assembly, disassembly, and reassembly, were probed by
monitoring distinct changes in luminescence spectra associated
with each process. These observed fluorescence responses at
appropriate monitoring wavelengths could be used as a output
signals to demonstarte in principle two basic logic gates, OR
and YES, using two independent inputs (In1 and In3; In1 and
In2), while an INHIBIT gate could be demonstrated in
principle using three inputs. Results described in this report are
expected to contribute to the development of new concepts in
the field of nanoscience and to the cross-fertilization between
traditionally separated disciplines such as chemistry and
information science.41
showing the experimental output values (λex = 280 nm; Out1, λMon
=
310 nm) for the YES logic states of the system, obtained on the same
solution by the sequential addition of DB18C6 and KPF6 inputs.
Schematic representation of (c) an OR logic gate (d) a YES logic gate.
Figure 9. (a) Bar diagrams showing the experimental output values
(λex = 280 nm; Out3, λMon = 360 nm) for the INHIBIT logic states of
the system, obtained on the same solution by the sequential addition
of DB18C6, KPF6 and 2(PF6)2 inputs. The dashed lines mark the
threshold values for the corresponding logic operations. (b) Schematic
representation of an INHIBIT logic gate.
The relatively higher emission quantum yield for DB18C6
(In1), when coordinated to K+ of KPF6 (In2), contributes to
the interruption of the photo induced electron transfer (PET)
process associated with the crown ether moiety, whereas the
fluorescence intensity of 1 is relatively low (<14, output: ⟨0⟩)
in the presence and absence of In2. Factors like flexibility and
an ICT state contribute to their observed lower quantum yield.
Further, emission intensity at this monitoring wavelength in the
presence of the input In1 alone was higher than the threshold
value (>14, output: ⟨1⟩) due to the weak but sufficient emissive
nature of DB18C6. These observations all together complied
with the YES logic gate function and are presented in Figure 8b.
Until now, no example is available in the literature that
demonstrates the INHIBIT logic operation by utilizing the
control and reversible formation of the taco complex with three
different molecular inputs. The INHIBIT logic operation with
three inputs provides eight possibilities with noncommutative
behavior, where one of these inputs can immobilize the whole
system.40
EXPERIMENTAL SECTION
■
Reagents and Methods. 2,3-Dihydroxynaphthalene, 1,5-diami-
nonaphthalene, 2,2′,2″-chloroethoxyethoxyethanol and 3,4-dihydrox-
ybenzaldehyde were used as received. NH4PF6 was recrystallized from
ethanol before use. All solvents were of reagent grade, which were
further dried and distilled prior to use following standard procedures.
Two intermediates (A and B), which were used for synthesis of 1,
were prepared following a literature procedures.5a Model compound 5
was synthesized following one of our previously reported procedures.5a
Details about various instrumentation techniques that were used for
this study are provided in the Supporting Information section.
Methodologies adopted for calculation of stoichiometric binding
constant from 1H NMR titration study and the corresponding errors in
solution preparation and instrumental analysis are provided in the
Supporting Information.
By combining these three molecular inputs with eight
possible input combinations, we could succeed in achieving the
much more complex INHIBIT logic operation with divalent
host 1. To demonstrate this, we used three inputs (In1, In2 and
In3), and the luminescence intensity was monitored at 360 nm
as output (Out3). The bar diagram with corresponding truth
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Synthesis of A. A was synthesized following a literature procedure,
and the pure compound was isolated after purification as a sticky dark
brown semisolid.5a Analytical and different spectroscopic data agreed
well for the proposed molecular formula and the desired purity. Yield
Synthesis of 5. Model compound 5 was prepared and isolated as a
white powder (mp 90 °C) following a previously reported
procedure.5a Yield 5 (63.0%): H NMR (500 MHz, CDCl3, δ ppm)
1
7.65−7.63 (2H, m), 7.32−7.30 (2H, m), 7.09 (2H, s), 6.89−6.84 (4H,
m), 4.25 (4H, t, J = 4), 4.15 (4H, t, J = 4), 3.98 (4H, t, J = 4), 3.93
(4H, t, J = 4), 3.89−3.85 (8H, m); 13C NMR 150.9, 131.1, 128.0,
125.9, 123.2, 116.1, 109.7, 73.1, 71.9, 71.1. Elemental Analysis Calcd
for C28H34O8: C, 67.45; H, 6.87. Found: C, 67.32; H, 6.84. (ESI−MS)
Calcd for C28H34O8: 498.56. Found: 521.58 [M + Na]+.
1
(53.0%): H NMR (500 MHz, CD2Cl2, δ ppm) 7.68−7.66 (2H, m),
7.33−7.31 (2H, m), 7.15 (2H, s), 4.25 (4H, t, J = 4.5), 3.93−3.91 (4H,
m), 3.74−3.73 (4H, m), 3.66−3.65 (8H, m), 3.56−3.54 (4H, m).
Elemental Analysis Calcd for C22H32O8: C, 62.25; H, 7.60. Found: C,
62.48; H, 7.53. ESI−MS calcd for C22H32O8: 424.48. Found: 447.24
[M + Na]+.
ASSOCIATED CONTENT
Synthesis of B. Intermediate compound B was synthesized,
purified, and isolated as a sticky brown mass following a previously
reported procedure.5a Yield B (82.0%): 1H NMR (500 MHz, CD2Cl2,
δ ppm) 7.75 (4H, d, J = 8.5), 7.66−7.65 (2H, m), 7.30 (6H, d, J =
7.5), 7.14 (2H, s), 4.20 (4H, t, J = 4.5), 4.09 (4H, t, J = 4.5), 3.85 (4H,
t, J = 4.5), 3.65−3.62 (8H, m), 3.57−3.55 (4H, m), 2.36 (6H, s); 13C
NMR 148.9, 144.8, 132.9, 129.8, 127.8, 126.3, 124.2, 108.3, 70.7, 69.4,
68.6, 68.3, 29.6. Elemental Analysis Calcd. for C36H44O12S2: C, 59.00;
H, 6.05; S, 8.75. Found: C, 59.16; H, 6.00, S, 8.70. ESI−MS Calcd. for
C36H44O12S2: 732.86. Found: 755.14 [M + Na]+.
■
S
* Supporting Information
1
Details about general instrumentation techniques, H NMR,
13C NMR, and ESI-Mass spectra, 2D-COSY spectra for
1·{2(PF6)2}2 and 1·{3(PF6)2}2. Detailed spectroscopic results
(UV−vis, Fluorescence) and cyclic voltammograms. Details of
determination of binding constants, methods of error
calculation, methods of determining energy levels. This material
is available free of charge via the Internet at http://pubs.acs.org.
Synthesis of C. 3,4-Dihydroxybenzaldehyde (0.38 g, 2.8 mmol)
was dissolved in 40 mL of freshly dried DMF. To this solution K2CO3
powder (1.3 g, 9.6 mmol) was added. This mixture was allowed to stir
for 15 min, and compound B (ditosylate of oligoethylene glycol
derivative of naphthalene) (2.0 g, 2.7 mmol) dissolved in 50 mL of dry
DMF was added in a dropwise manner over 2 h at 60 °C. Then the
temperature was raised to 80 °C, and the mixture was allowed to stir
for 5 days. The solvent was removed under reduced pressure; the
residue was extracted three times with CHCl3 and water. The organic
layers were combined and dried over anhydrous sodium sulfate.
Removal of the solvent under a vacuum gave crude product, which was
purified on a silica gel column using CHCl3:CH3OH (98:2, v/v) to
yield pure C (0.90 g, 62%), as a brown solid (mp 80 °C −85 °C): 1H
NMR (200 MHz, CDCl3, δ ppm) 9.78 (1H, s), 7.64−7.63 (2H, bs);
7.38−7.35(2H, bs), 7.32−7.30 (2H, bs), 7.08 (2H, s), 6.91(1H, s),
4.25 (4H, bs), 4.22−4.20 (4H, broad m), 4.00−3.99 (4H, broad m),
3.97−3.93 (4H, broad m), 3.89−3.86 (8H, broad m); 13C NMR 190.9,
162.6, 148.9, 130.1, 129.2, 126.8, 126.2, 124.2, 111.8, 110.9, 107.8,
71.5, 69.5, 69.0. Elemental Analysis Calcd for C29H34O9: C, 66.15; H,
6.51. Found: C, 65.93; H, 6.48. ESI−MS Calcd for C29H34O9: 526.22.
AUTHOR INFORMATION
Corresponding Author
*E-mail: amitava@csmcri.org.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.D. thanks DST (India) and CSIR (India) for financial
support to this research. A.K.M., P.D., and P.M. acknowledge
CSIR for their research fellowship. S.A. acknowledges Indian
Academy of Sciences for supporting his stay at CSMCRI under
the Summer Fellowship Programme.
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+
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Synthesis of 1. Compound C (0.2 g, 0.4 mmol) was dissolved in
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1
°C). Yield (0.16 g, 80%): H NMR (500 MHz, CDCl3, δ ppm) 8.43
(2H, s), 8.19 (2H, d, J = 8.5); 7.70 (2H, s); 7.66 (4H, m); 7.47 (2H, t,
J = 7.5); 7.40 (2H, d, J = 8.5); 7.31 (4H, m); 7.12 (4H, s); 7.08 (2H, d,
J = 7); 6.97 (2H, d, J = 8.5); 4.30 (4H, t, J = 4); 4.27 (8H, t, J = 2);
4.24 (4H, t, J = 4); 4.00 (8H, t, J = 4); 3.97 (8H, s); 3.88 (16H, s); 13C
NMR 159.6, 149.2, 130.3, 129.9, 126.2, 125.7, 124.4, 121.5, 113.3,
112.6, 111.6, 106.0, 71.4, 69.7, 69.5. HRMS spectrum calculated for
C68H74N2O16: 1175.5117. Found: 1175.5026.
Synthesis of 4. 3,4-Dimethoxybenzaldehyde (0.48 g, 2.9 mmol)
was dissolved in 20 mL of dry methanol, and a solution (0.2 g, 1
mmol) of 1,5-diaminonaphthalene was added. The reaction mixture
was stirred for 2 days at room temperature. After 2 days a dark green
precipitate appeared, which was filtered through a G3 crucible and
washed 3−4 times with cold ethanol. Then the product was dried in
vacuum. Yield (0.45 g, 80.0%), as a dark color solid (mp 230−235
°C): 1H NMR (500 MHz, CDCl3, δ ppm) 8.47 (2H, s), 8.21 (2H, d, J
= 8.5); 7.77 (2H, bs); 7.49 (2H, t, J = 7.5); 7.41 (2H, dd, J = 2, 8);
7.09 (2H, d, J = 7.5); 6.98 (2H, d, J = 8.0); 4.03 (6H, s); 3.98 (6H, s).
Elemental Analysis Calcd for C28H26N2O4: C, 73.99; H, 5.77; N, 6.16;.
Found: C, 73.75; H, 5.74; N, 6.13. (ESI−MS) calcd for C28H26N2O4:
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