A molecular plug - socket connector

Article abstract of DOI:10.1021/ja067739e

A monocationic plug-socket connector that is composed, at the molecular level, of three components, (1) a secondary dialkylammonium center (CH ₂NH₂⁺CH₂), which can play the role of a plug toward dibenzo[24]crown-8 (DB24C8), (2) a rigid and conducting biphenyl spacer, and (3) 1,4-benzo-1,5-naphtho[36]crown-10 (BN36C10), capable of playing the role of a socket toward a 4,4′-bipyridinium dicationic plug, was synthesized and displays the ability to act as a plug-socket connector. The fluorescent signal changes associated with the 1,5-dioxynaphthalene unit of its BN36C10 portion were monitored to investigate the association of this plug-socket connector with the complementary socket and plug compounds. The results indicate that (1) the CH₂NH₂⁺CH ₂ part of the molecular connector can thread DB24C8 in a trivial manner and (2) the BN36C10 ring of the connector can be threaded by a 1,1′-dioctyl-4,4′-bipyridinium ion only after the CH ₂NH₂⁺CH₂ site is occupied by a DB24C8 ring. The two connections of the three-component assembly are shown to be controlled reversibly by acid/base and red/ox external inputs, respectively. The results obtained represent a key step for the design and construction of a self-assembling supramolecular system in which the molecular electron source can be connected to the molecular electron drain by a molecular elongation cable.

Full text of DOI:10.1021/ja067739e

Published on Web 03/28/2007
A Molecular Plug-Socket Connector
Guillaume Rogez,†,# Bele´n Ferrer Ribera,†,£ Alberto Credi,*,† Roberto Ballardini,‡
Maria Teresa Gandolfi,† Vincenzo Balzani,*.† Yi Liu,§,¶ Brian H. Northrop,§ and
J. Fraser Stoddart*,§
Contribution from the Dipartimento di Chimica “Giacomo Ciamician”, UniVersita` di Bologna,
Via Selmi 2, I-40126, and Istituto ISOF-CNR, Via Gobetti 101, I-40129, Bologna, Italy, and the
California NanoSystems Institute and Department of Chemistry and Biochemistry, UniVersity of
CaliforniasLos Angeles, 405 Hilgard AVenue, Los Angeles, California 90095-1569
Received October 30, 2006; E-mail: stoddart@chem.ucla.edu; vincenzo.balzani@unibo.it; alberto.credi@unibo.it
Abstract: A monocationic plug-socket connector that is composed, at the molecular level, of three
components, (1) a secondary dialkylammonium center (CH2NH2+CH2), which can play the role of a plug
toward dibenzo[24]crown-8 (DB24C8), (2) a rigid and conducting biphenyl spacer, and (3) 1,4-benzo-1,5-
naphtho[36]crown-10 (BN36C10), capable of playing the role of a socket toward a 4,4′-bipyridinium dicationic
plug, was synthesized and displays the ability to act as a plug-socket connector. The fluorescent signal
changes associated with the 1,5-dioxynaphthalene unit of its BN36C10 portion were monitored to investigate
the association of this plug-socket connector with the complementary socket and plug compounds. The
results indicate that (1) the CH2NH2+CH2 part of the molecular connector can thread DB24C8 in a trivial
manner and (2) the BN36C10 ring of the connector can be threaded by a 1,1′-dioctyl-4,4′-bipyridinium ion
only after the CH2NH2+CH2 site is occupied by a DB24C8 ring. The two connections of the three-component
assembly are shown to be controlled reversibly by acid/base and red/ox external inputs, respectively. The
results obtained represent a key step for the design and construction of a self-assembling supramolecular
system in which the molecular electron source can be connected to the molecular electron drain by a
molecular elongation cable.
Introduction
machines. The challenge for the chemist in this field resides in
the programming7,11,13,15 of the devices and machines, that is,
in the design of components, which carry within their structures
all of the pieces of information necessary for the performance
of certain required functions. In the course of designing and
Molecular self-assembly,1 a process which is central and key
to nature’s forms and functions, provides2-18 a mechanism for
the construction of artificial molecular-level devices and
† Universita` di Bologna.
‡ Istituto ISOF-CNR.
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J. P. Angew. Chem., Int. Ed. 2000, 39, 3284. (d) Collin, J. P.; Dietrich-
Buchecker, C.; Gavina, P.; Jime´nez-Molero, M. C.; Sauvage, J. P. Acc.
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§ University of CaliforniasLos Angeles.
# Present address: Institut de Physique et de Chimie des Materiaux de
Strasbourg, UMR 7504, 23 rue du Loess, B.P. 43, 67034 Strasbourg Cedex
2, France.
£ Present address: Departamento de Qu´ımica, Universidad Polite´cnica
de Valencia, Camino de Vera s/n, 46022 Valencia, Spain.
¶ Present address: The Molecular Foundry, Lawrence Berkeley National
Laboratory, Berkeley, California 94720, USA.
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9
10.1021/ja067739e CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 4633-4642
4633

A R T I C L E S
Rogez et al.
Scheme 1. The Graphic Representation of a Molecular Connector
Results and Discussion
Design of the Molecular Connector. It has long been
known25 that crown ethers with the 18-crown-6 constitution form
face-to-face complexes with primary alkylammonium ions
(RNH3+) as a result of the stabilizing influence, primarily of
three [N+-H‚‚‚O] hydrogen bonds. More recently, it has been
demonstrated26,27 that, when the crown ether constitution is large
enough, such as, a 24C8 ring minimally and ideally, suitably
chosen secondary dialkylammonium (R2NH2+) ions can thread
through the ring to give [2]pseudorotaxanes,26a often used as
precursor 1:1 complexes in the syntheses of rotaxanes26b and
catenanes.28 Upon deprotonation of the CH2NH2+CH2 center,
the two strong [N+-H‚‚‚O] hydrogen bonds that give stability
to the 1:1 complex are broken, and dethreading of these
[2]pseudorotaxanes takes place. This assembly/disassembly
feature has been exploited previously21 in the construction of a
molecular plug-socket system for energy transfer.
Another well-known29 recognition motif is manifest in the
spontaneous threading of π-electron-rich crown ethers like
BN36C10 with π-electron-deficient bipyridinium dications to
form stable 1:1 complexes. This kind of supramolecular
assistance has also been harnessed30 in the synthesis of catenanes
and rotaxanes. Furthermore, it is known that such [2]pseudoro-
taxanes can be easily dethreaded by reduction of the electron-
constructing19 a supramolecular system in which a molecular
electron source can be connected (Scheme 1) to a molecular
electron drain by a molecular plug-socket connector, we have
synthesized and characterized the compound 1-H+ shown in
Chart 1. This compound is made up of three parts, (1) a
secondary dialkylammonium (CH2NH2+CH2) center, which can
play the role of a plug,20,21 (2) a rigid and conducting22 biphenyl
spacer, and (3) a 1,4-benzo-1,5-naphtho[36]crown-10 (BN36C10,
2) ring that is able to play the role of a socket.19,23 The connector
1-H+ is the most important component of a recently reported
self-assembling supramolecular system which mimics24 an
electrical extension cable at the nanoscale. We have investigated,
by absorption and fluorescence measurements, the connection
of the two ends of this wire-type molecule with socket (DB24C8,
3) and plug (1,1′-dioctyl-4,4′-bipyridinium, 42+) compounds.
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4634 J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007

A Molecular Plug−Socket Connector
A R T I C L E S
Chart 1
Scheme 2. Synthesis of the Molecular Connector 1-H‚PF6
accepting unit and then rethreaded by its reoxidation, thereby
constituting one of the simplest examples of red/ox-controlled
molecular machines.11,15,19,31-36
By taking advantage of the complexation/decomplexation
properties of these [2]pseudorotaxanes that can be subjected to
acid/base control on the one hand and red/ox control on the
other, as well as experience accumulated in previous investiga-
tions,37 including an attempt to govern the photoinduced electron
transfer in a triad that can be assembled/disassembled by
chemical inputs,19 we have designed 1-H+, a new type of
connector molecule containing a CH2NH2+CH2 center that
can play the role of a plug and a BN36C10-type unit that can
play the role of a socket. The next challenge was to make
1-H+.
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Synthesis of the Molecular Plug-Socket Connector. The
synthesis of 1-H‚PF6 is outlined in Scheme 2. Condensation of
4-bromobenzaldehyde (5) and benzylamine (6), followed by
reduction of the generated imine and subsequent N-protection
afforded the bromide 7. Pd(0)-catalyzed coupling between 7
and the boronic acid 8 generated the aldehyde 9. Reduction of
9, followed by bromination of the resulting alcohol 10, afforded
the bromide 11. The crown ether 14 was prepared from
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Ashton, P. R.; Balzani, V.; Credi, A.; Mattersteig, G.; Owen, M. A.;
Montalti, M.; Spencer, N.; Stoddart, J. F.; Venturi, M. Chem.sEur. J. 1997,
3, 1992. (c) Credi, A.; Montalti, M.; Balzani, V.; Langford, S. J.; Raymo,
F.; Stoddart, J. F. New J. Chem. 1998, 22, 1061. (d) Balzani, V.; Ceroni,
P.; Credi, A.; Go´mez-Lo´pez, M.; Hamers, C.; Stoddart, J. F.; Wolf, R.
New J. Chem. 2001, 25, 25.
9
J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007 4635

A R T I C L E S
Rogez et al.
Table 1. Luminescence Data (Air-Equilibrated Solutions, CH2Cl2,
298 K)
a
compound
Iem
τ (ns)
2
1
100
95
23
7.0
6.6
1-H+
1.3 (78%)b
5.6 (22%)b
1.9 (75%)b
7.0 (25%)b
7.0c
3⊃1-H+
30
2⊃42+
25c
13d
3⊃1-H+⊃42+
7.0d
a λexc ) 326 nm; λem ) 345 nm. b From the ratios of the pre-exponen-
tial factors of the decay curves. c Emission attributed to the fraction
of noncomplexed crown 2 after addition of 6 equiv of 42+ d Emission attri-
.
buted to the fraction of noncomplexed 1-H+ after addition of 6 equiv of
42+
.
Figure 1. Absorption and (inset) fluorescence spectra of 1-H+ (full line),
1 (dotted line), and 2 (dashed line). Experimental conditions: air-equilibrated
3.0 × 10-5 mol L-1 solutions in CH2Cl2 at 298 K; excitation wavelength,
326 nm.
reduction of the macrocyclization product between dibromide
1238 and the aldehyde 13. The etherification between 11 and
14 afforded 15, which was deprotected39 under mild conditions
employing a phenol/TMSCI/CH2Cl2 solution. Subsequent pro-
tonation with HCl and counterion exchange afforded the title
compound 1-H‚PF6.
Absorption and Fluorescence Properties of the Molecular
Plug-Socket Connector. Figure 1 shows the absorption and
(inset) luminescence spectra of 1-H+, 1 (obtained by depro-
tonation of 1-H+ with 1 equiv of tributylamine), and of the
BN36C10 reference compound 2. Above 300 nm, the absorption
spectra of the three compounds are quite similar. Comparison
with the absorption spectrum of 4-phenylbenzylamine shows
that the strong absorption band with λmax ) 260 nm of 1 and
1-H+ can be attributed to their wire-type moiety. The emission
spectrum of 1 is almost identical to that of model compound 2,
assigned to the fluorescence emission of the 1,5-dioxynaphtha-
lene unit.40 The spectrum of 1-H+ is much weaker and, contrary
to what happens for 1 and 2, exhibits two distinct emission
decays (Table 1). The absorption and emission properties of 2
are unaffected by the addition of 1 equiv of trifluoromethane-
sulfonic (triflic) acid or tributylamine (TBA). Upon addition of
1 equiv of triflic acid, the absorption and emission properties
of 1 become identical to those of 1-H+ and are reconverted into
the properties of 1 upon successive addition of 1 equiv of
tributylamine. Therefore, 1 and 1-H+ can be reversibly inter-
converted by acid/base inputs.
Figure 2. (a) Two perspectives of the global energy minimum conformer
of 1-H+, which is the threaded self-complex, as determined by molecular
force field calculations. (b) Two perspectives of the side-on self-complex,
which is a local-energy-minimum conformer of 1-H+ and has been
determined to be 1.1 kcal mol-1 less stable than the threaded self-complex.
All hydrogens except those involved in hydrogen bonding have been
removed for clarity.
complexation between the secondary dialkylammonium center
and the BN36C10-type moiety. Inspection of CPK space-filling
molecular models shows (Figure 2), indeed, that both threaded
and side-on self-complexation can occur. It seems likely that
the electronic interaction between the BN36C10 and 4-phenyl-
benzylammonium units of 1-H+ is more favored for a threaded
compared with a side-on structure. If this is the case, the
emission lifetime data (Table 1) show that about 80% of the
complexation occurs by threading.
Modeling of Molecular Plug-Socket Connector Conform-
ers. In order to gain a more thorough understanding of the
different low-energy conformers that may be adopted by 1-H+,
we have undertaken a theoretical investigation of the mono-
cationic plug-socket connector using molecular force field
computations. Computations were performed using the
AMBER* force field41 and GB/SA solvent model42 for CHCl3
in the program43 Maestro v3.0.038. A 1000-step conformational
search was used to determine the lowest energy conformer of
The decrease in the emission intensity of 1 upon protonation
shows that, in 1-H+, the fluorescence of the BN36C10-type unit
is quenched by the 4-phenylbenzylammonium moiety, most
likely because of an electron-transfer process from the BN36C10
singlet excited state to the 4-phenylbenzylammonium moiety.
The double exponential emission decay suggests that 1-H+ is
present as two different conformations originating from self-
(38) Menzer, S.; White, A. J. P.; Williams, D. J.; Belohradsky, M.; Hamers,
C.; Raymo, F. M.; Shipway, A. N.; Stoddart, J. F. Macromolecules 1998,
31, 295.
(39) Kaiser, E.; Picart, F.; Kubiak, T.; Tam, J. P.; Merrifield, R. B. J. Org.
Chem. 1993, 58, 5167.
(40) Ashton, P. R.; Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.;
Menzer, S.; Pe´rez-Garc´ıa, L.; Prodi, L.; Stoddart, J. F.; Venturi, M.; White,
A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1995, 117, 11171.
9
4636 J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007

A Molecular Plug−Socket Connector
A R T I C L E S
Figure 3. (a) Change in the fluorescence intensity of 1-H+ (2.0 × 10-5 mol L-1) upon addition of DB24C8 (3). (b) Change in the fluorescence intensity
of a solution containing 1-H+ (2.0 × 10-5 mol L-1) and 3 (1.5 equiv) upon addition of TBA. The corresponding processes are schematized at the top of each
diagram. Experimental conditions: air-equilibrated CH2Cl2 solution at 298 K; excitation wavelength, 326 nm; emission wavelength, 345 nm.
1-H+, which was found (Figure 2a) to be the threaded self-
complex. The slightly higher energy side-on self-complex
(Figure 2b) was also located. A 500 ps molecular dynamics
simulation at an elevated temperature of 400 K was then used
to equilibrate both the threaded and side-on self-complexes. Full
energy minimization of 500 randomly sampled conformers
generated during each molecular dynamics simulation gave the
global minimum threaded structure and local minimum side-
on self-complex (Figure 2). The threaded self-complex, which
is able to form two bifurcated hydrogen bonds with the
macrocyclic polyether, is found to be 1.1 kcal mol-1 more stable
than the side-on self-complex, which is only able to form two
hydrogen bonds with the macrocycle. This energetic difference
corresponds to a relative ratio of 84:16 in favor of the threaded
conformer. This theoretically predicted value is in good agree-
ment with experimental emission lifetime data and supports the
conclusions that (1) there are two distinct low-energy conforma-
tions available to 1-H+ and that (2) the threaded self-complex
is the more stable of the two.
Two-Component Supramolecular System 3⊃1-H+. Titra-
tion of 1-H+ (2.0 × 10-5 mol L-1) with the dibenzocrown
ether 3 caused an increase in the fluorescence of the BN36C10
unit of 1-H+. A plateau is reached (Figure 3a) after addi-
tion of 1 equiv of 3, indicating that the dibenzocrown ether 3
interacts with 1-H+ to give a remarkably stable (K > 105 L
mol-1) 1:1 adduct.26c This is an expected result since 1-H+
contains a dialkylammonium ion center that is known to
thread dibenzo[24]crown-8.20,26 Therefore, 3 prevents self-
complexation of 1-H+, giving rise to a supramolecular entity
3⊃1-H+. Under such conditions, the fluorescence of the
BN36C10 unit of 1-H+ shows two distinct decay times, differ-
ent from those exhibited by 1-H+ alone (Table 1). This
result suggests that 3⊃1-H+ is present in two distinct
conformations.
(41) Weiner, S. J.; Kollman, P. A.; Case, D. A.; Singh, V. C.; Ghio, C.; Alagona,
G.; Profeta, S., Jr.; Weinre, P. J. Am. Chem. Soc. 1986, 106, 765.
(42) Still, W. C.; Tempczyk, A.; Hawler, R. C.; Hendrickson, T. J. Am. Chem.
Soc. 1990, 112, 6127.
(43) Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.;
Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem.
1990, 11, 440. Molecular modeling calculations were executed with the
Maestro 3.0.038 package; Schro¨dinger, Inc.: Portland, OR.
9
J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007 4637

A R T I C L E S
Rogez et al.
Figure 4. Changes observed in the emission spectrum at 345 nm (b) and in the absorption spectrum at 500 nm (O) of a solution containing the BN36C10
model compound 2 (3.0 × 10-5 mol L-1) upon titration with 1,1′-dioctyl-4,4′-bipyridinium ion (42+). The corresponding process is schematized at the top
of the diagram. Experimental conditions: air-equilibrated CH2Cl2 solution at 298 K; excitation wavelength, 326 nm. Solubility reasons prevented the addition
of more than 6 equiv of compound 42+
.
Titration of the 3⊃1-H+ supramolecular entity with tributyl-
amine causes an increase in the intensity of the BN36C10
fluorescence (Figure 3b) that, upon addition of a stoichiometric
amount of base, reaches a value equal to that previously found
for the unprotonated species 1. Formation of 1 is confirmed by
the disappearance of the double emission decay and the
recovering of the monoexponential decay with τ ) 6.6 ns.
Successive addition of a stoichiometric amount of triflic acid
leads back to the emission intensity and decay times of the
3⊃1-H+ species. These results show that the adduct between
3 and 1-H+ can be reversibly assembled/disassembled by
acid/base inputs.
Two-Component Supramolecular System 2⊃42+. Violo-
gen-type molecules (i.e, 1,1′-dimethyl-4,4′-bipyridinium ions)
are known to thread aromatic crown ethers by virtue of
π-electron donor-acceptor and hydrogen-bonding interac-
tions.19,23 Formation of such pseudorotaxane structures causes
the quenching of the fluorescence of the crown ether and the
appearance of a broad and weak charge-transfer absorption band
in the visible region.26d,40
excited-state lifetime of 2 (τ ) 7 ns) and the low concentration
of 42+ (e2.0 × 10-4 mol L-1). These results indicate the
formation of an adduct between the electron-donating crown
ether 2 and the electron-accepting 42+ species, as already known
to occur upon threading bipyridinium-type units into crown
ethers incorporating benzo and naphtho substituents.19,23,40,44
Fitting of the titration curves (Figure 4) yielded a value of 21 000
( 1000 L mol-1 for the association constant and a molar
absorption coefficient of ca. 1000 L mol-1 cm-1 for the
maximum of the charge-transfer band, in good agreement with
previously reported values.40,45 Addition of Zn or Ag powder
to deaerated solutions of the adduct 2⊃42+ caused a complete
revival of the fluorescence of 2 and the appearance of the strong
absorption bands characteristic of the one-electron reduced form
of the bipyridinium unit.46 When dioxygen was allowed to enter
the solution, the changes caused in the absorption spectrum by
the addition of the metal were reversed. These results show that
42+ threads 2 and that the threading/dethreading process can
be redox controlled.
Two-Component Supramolecular System 1⊃42+. Upon
titration of 1-H+ (5.0 × 10-5 mol L-1) with 42+, no quenching
of the fluorescence of the BN36C10-type moiety was observed
In preliminary experiments, we found that titration of the
BN36C10 reference compound 2 (3.0 × 10-5 mol L-1) with
1,1′-dioctyl-4,4′-bipyridinium (42+) causes the quenching of the
fluorescence intensity of 2, accompanied by the appearance of
an absorption band with a maximum at 500 nm (Figure 4). It
should be noted that dynamic quenching of the fluorescent
excited state of 2 by 42+ can be excluded because of the short
(44) Ballardini, R.; Balzani, V.; Gandolfi, M. T.; Gillard, R. E.; Stoddart, J. F.;
Tabellini, E. Chem.sEur. J. 1998, 4, 449.
(45) Asakawa, M.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Beloradsky, M.;
Gandolfi, M. T.; Kocian, O.; Prodi, L.; Raymo, M. F.; Stoddart, J. F.;
Venturi, M. J. Am. Chem. Soc. 1997, 119, 302.
(46) Venturi, M.; Mulazzani, Q. G.; Hoffman, M. Z. Radiat. Phys. Chem. 1984,
23, 229.
9
4638 J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007

A Molecular Plug−Socket Connector
A R T I C L E S
Figure 5. Changes in the fluorescence intensity of a 3.0 × 10-5 mol L-1 solution of 1-H+ (O) and 1 (b) upon titration with 42+. The corresponding
processes are schematized at the top of the diagram. Experimental conditions: air-equilibrated CH2Cl2 solution at 298 K; excitation wavelength, 326 nm;
emission wavelength, 345 nm.
(Figure 5), showing that 42+ does not thread the crown-ether
moiety of 1-H+. The most likely explanation is that 1-H+ is
self-complexed (vide supra), and therefore, its macrocyclic
cavity is not available for intermolecular complexation. In fact,
if titration is performed in the presence of 1 equiv of tributyl-
amine (that causes deprotonation of 1-H+ and the consequent
disruption of the self-complexed species), the fluorescence
intensity of the BN36C10-type moiety decreases upon increasing
the 42+ concentration (Figure 5), and an absorption band with
maximum around 500 nm appears. These results indicate26d,40
that 42+ does thread the crown-ether moiety of the unprotonated
species 1. The association constant is substantially the same as
that for 2⊃42+ (vide supra). Successive addition of triflic acid
caused opposite spectral changes, showing that back self-
complexed structure of 1-H+ can be obtained not only by
deprotonation but also by surrounding the ammonium moiety
of 1-H+ with the dibenzocrown ether 3. Titration with 42+ of a
solution of 1-H+ containing 1.5 equiv of 3 caused47 the
quenching of the fluorescence intensity of the BN36C10-
type unit and the increase of absorbance at 500 nm (Figure 6).
These results show that, when the CH2NH2+CH2 center
in 1-H+ is surrounded by 3, 42+ threads the crown-ether
moiety of 1-H+. The association constant is estimated to be on
the order of 104 L mol-1 from the fitting of the titration plots.
Addition of Zn or Ag powder to the deaerated solution at the
end of the titration caused a revival of the BN36C10-type unit
fluorescence intensity and the appearance of the strong
absorption bands of the monoreduced bipyridinium unit. In
conclusion, the crown unit of 1-H+ can play the role of a socket
toward 42+ when the ammonium function is plugged in the
crown ether 3.
complexation of 1-H+ kicks out 42+
.
Three-Component Supramolecular System 3⊃1-H+⊃42+
.
Titration of 1-H+ with 42+ does not cause (Figure 5) any change
in the emission spectrum, indicating that threading of the crown-
ether moiety of 1-H+ by 42+ does not take place because 1-H+
is self-complexed. We have seen that disassembling of the self-
(47) Titration experiments showed that the association between 3 and 42+ is
negligible under the conditions employed.
9
J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007 4639

A R T I C L E S
Rogez et al.
Figure 6. Changes observed in the emission spectrum at 345 nm (b) and in the absorption spectrum at 500 nm (O) of a 3.0 × 10-5 mol L-1 solution of
1-H+ in the presence of 1.5 equiv of DB24C8 (3) upon titration with 42+. The corresponding process is schematized at the top of the diagram. Experimental
conditions: air-equilibrated CH2Cl2 solution at 298 K; excitation wavelength, 326 nm.
Conclusions
Experimental Section
We have designed, synthesized, and characterized a com-
pound (1-H+) that exhibits a fluorescence signal associated with
the dioxynaphthalene unit of its crown-ether moiety. Fluores-
cence studies performed in CH2Cl2 solution show that 1-H+ is
present as two different conformers, originating from self-
complexation between the CH2NH2+CH2 center and the crown-
ether moiety. Upon addition of a stoichiometric amount of
tributylamine, 1-H+ is converted into its unprotonated form 1;
subsequent addition of triflic acid causes reversible conversion
to the initial 1-H+ species. The secondary CH2NH2+CH2 center
in 1-H+ threads the DB24C8 crown ether 3 (Figure 2) and is
under conditions such that the crown-ether moiety of 1-H+ can
be threaded by 1,1′-dioctyl-4,4′-bipyridinium ion 42+ (Figure
5). Therefore, 1-H+, 2, and 42+ can self-assemble, giving rise
to the triad 3⊃1-H+⊃42+. Both of the self-assembling processes
are reversible and can be controlled by external inputs; as-
sembling/disassembling between 1-H+ and 3 is acid/base
controlled, and that between 1-H+ and 42+ is redox controlled.
In the 3⊃1-H+⊃42+ triad, the central component 1-H+ performs
as a connector between a socket-type component (crown ether
3) and a plug-type component (1,1′-dioctyl-4,4′-bipyridinium
ion). The results obtained in this work represent a fundamental
premise for the construction24 of more complex self-assembling
supramolecular systems in which a molecular electron source
can be connected with a molecular electron drain by a molecular
elongation cable.
Materials and General Procedures. Reagents were pur-
chased from Aldrich. BN36C10 (2) was available from previous
work. DB24C8 (3) and 4-phenylbenzylamine were obtained
from Aldrich. The 1,1′-dioctyl-4,4′-bipyridium hexafluorophos-
phate salt 4‚2PF6 was a gift from Professor A. Arduni
(University of Parma). The compound 1238 was prepared
according to literature procedures. Solvents were purified
according to literature procedures.48 Thin-layer chromatography
(TLC) was carried out using aluminum sheets, precoated with
silica gel 60F (Merck 5554). The plates were inspected by UV
light prior to development with iodine vapor. Melting points
were determined on an Electrothermal 9200 apparatus and are
uncorrected. Proton and carbon nuclear magnetic resonance
spectra (1H NMR and 13C NMR) were recorded on a Bruker
Avance500, Avance 600, or ARX500, using the deuterated
solvent as the lock and the residual protonated solvent as the
internal standard. All chemical shifts are quoted using the δ
scale, and all coupling constants (J) are expressed in Hertz (Hz).
Electrospray mass spectra (ESI-MS) were measured on a VG
ProSpec triple-focusing mass spectrometer.
7: 4-Bromobenzaldehyde (5) (2.50 g, 23.4 mmol) and
benzylamine (6) (4.32 g, 23.4 mmol) were heated together in
refluxing PhMe (100 mL) for 30 min. The reaction mixture was
(48) Perrin, D. D.; Armarego, W. F. L. Purification of Laboratory Chemicals;
Pergamon: Oxford, U.K., 1989.
9
4640 J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007

A Molecular Plug−Socket Connector
A R T I C L E S
cooled to room temperature, and the solvent was evaporated
off in vacuo. The residue was redissolved in MeOH (150 mL).
NaBH4 (1.7 g, 46.8 mmol) was added, and the mixture was
heated under reflux for 1 h. The reaction mixture was partitioned
in 2N HCl/CH2Cl2. The aqueous layer was extracted with CH2-
Cl2 (2 × 50 mL). The combined organic layer was dried
(MgSO4) and filtered, and the filtrate was evaporated to dryness.
The obtained oil was stirred together with Boc2O (5.53 g, 25.4
mmol) and DMAP (0.12 g, 0.1 mmol) in CH2Cl2 (100 mL) for
1 h at room temperature. The solution was evaporated, and the
residue was subjected to column chromatography (SiO2-EtOAc/
hexanes 1:10) to give compound 7 (4.70 g, 93%) as a colorless
liquid, which slowly solidified upon storage. Mp: 64-66 °C.
1H NMR (CDCl3, 500 MHz, 298 K): δ 7.59 (d, J ) 8.3 Hz,
2H), 7.56 (d, J ) 8.3 Hz, 2H), 7.45 (d, J ) 7.7 Hz, 2H), 7.36-
7.22 (m, 7H), 4.74 (s, 2H), 4.46 (br s, 2H), 4.38 (br s, 2H),
1.92 (s, 1H), 1.52 (s, 9H). 13C NMR (CDCl3, 125 MHz, 298
K): δ 156.0, 140.1, 139.9, 139.7, 137.9 (br), 137.0 (br), 128.5,
128.3 (br), 127.9 (br), 127.8 (br), 127.4, 127.1 (br), 127.1, 80.1,
64.9, 49.3 (br), 48.9 (br), 48.6 (br), 28.4. HRMS (ESI) (m/z):
[M - Boc]+ calcd for C19H22BrNO2, 274.0231; found, 274.0439.
10: A mixture of bromide 7 (2.00 g, 5.30 mmol), boronic
acid 8 (0.88 g, 5.85 mmol), Pd(PPh3)4 (0.31 g, 27 µmol), and
K2CO3 (1.47 g, 10.7 mmol) in degassed EtOH/H2O/THF (8/6/
50 mL) was stirred under reflux for 2 h. The resulting solution
was cooled down, evaporated, and partitioned in H2O/CH2Cl2.
The organic layer was dried (MgSO4) and filtered, and the
filtrate was evaporated to get a residue, which was subjected to
column chromatography (SiO2-EtOAc/hexanes 1:10) to give
9 (1.40 g, 67%) as a colorless oil. The oil was dissolved in
MeOH (30 mL). NaBH4 (0.19 g, 5.03 mmol) was added, and
the solution was heated under reflux for 2 h. The reaction
mixture was evaporated and subjected to column chromatog-
raphy (SiO2-EtOAC/hexanes 1:3) to give 10 as a colorless oil
(1.35 g, 96%), which slowly solidified upon vacuum. Mp: 91-
92 °C. 1H NMR (CDCl3, 500 MHz, 298 K): δ 7.59 (d, J ) 8.3
Hz, 2H), 7.56 (d, J ) 8.3 Hz, 2H), 7.45 (d, J ) 7.7 Hz, 2H),
7.36-7.22 (m, 7H), 4.74 (s, 2H), 4.46 (br s, 2H), 4.38 (br s,
2H), 1.92 (s, 1H), 1.52 (s, 9H). 13C NMR (CDCl3, 125 MHz,
298 K): δ 156.0, 140.1, 139.9, 139.7, 137.9 (br), 137.0 (br),
128.5, 128.3 (br), 127.9 (br), 127.8 (br), 127.4, 127.1 (br), 127.1,
80.1, 64.9, 49.3 (br), 48.9 (br), 48.6 (br), 28.4. HRMS (ESI)
(m/z): [M + Na]+ calcd for C26H29NO3, 426.2040; found,
426.2064.
a yellow oil (1.4 g). This yellow oil was dissolved in MeOH
(25 mL). NaBH4 (0.13 g, 3.50 mmol) was added, and the
mixture was heated under reflux for 2 h. After cooling down to
room temperature, the solvent was evaporated, and the residue
was extracted with CH2Cl2. After filtration, the filtrate was
concentrated to give 14 (1.40 g, 25% for two steps) as a white
solid. Mp: 52-54 °C. 1H NMR (CDCl3, 500 MHz, 298 K): δ
7.88 (dd, J ) 8.5, 4.5 Hz, 2H), 7.31 (q, J ) 8.2, 2H), 6.75 (d,
J ) 8.1 Hz, 2H), 6.62 (d, J ) 7.5 Hz, 1H), 6.59 (dd, J ) 8.5,
2.9 Hz, 1H), 6.45 (d, J ) 8.5 Hz, 1H), 4.47 (d, J ) 5.0 Hz,
2H), 4.24-4.21 (m, 4H), 3.99 (t, J ) 4.8 Hz, 2H), 3.97 (t, J )
4.8 Hz, 2H), 3.85-3.65 (m, 24H). 13C NMR (CDCl3, 125 MHz,
298 K): δ 154.2, 154.1, 152.9, 150.7, 131.3, 126.6, 126.5, 125.1,
125.0, 115.0, 114.5, 114.4, 114.1, 113.5, 105.6, 70.9, 70.7, 70.7,
70.7, 70.6, 70.6, 70.3, 69.6, 69.5, 68.6, 67.9, 67.7, 61.4. HRMS
(ESI) (m/z): [M + Na]+ calcd for C33H44O11, 639.2761,; found,
639.2776.
15: A solution of 14 (0.17 g, 0.28 mmol) in THF (2 mL)
was added into a suspension of NaH (20 mg, 0.73 mmol) in
THF (5 mL). The mixture was stirred under reflux for 0.5 h.
After cooled down, a solution of 11 (0.13 g, 0.28 mmol) in
THF (2 mL) was added dropwise, and the mixture was heated
under reflux overnight. The reaction mixture was filtered, and
the filtrate was evaporated to give a residue, which was subjected
to column chromatography (SiO2-EtOAC/hexanes 3:1) to give
15 (0.15 g, 54%) as a colorless oil. 1H NMR (CDCl3, 500 MHz,
298 K): δ 7.87 (dd, J ) 8.5, 4.5 Hz, 2H), 7.56 (dd, J ) 8.5,
2.6 Hz, 4H), 7.40 (d, J ) 8.1 Hz, 2H), 7.34 (d, J ) 7.5 Hz,
2H), 7.30-7.23 (m, 5H), 6.96 (d, J ) 2.9 Hz, 2H), 6.71 (d, J
) 8.1 Hz, 2H), 6.69 (d, J ) 7.5 Hz, 1H), 6.65 (dd, J ) 8.5, 2.9
Hz, 1H), 6.46 (d, J ) 8.5 Hz, 1H), 4.55 (s, 2H), 4.53 (s, 2H),
4.48 (br s, 2H), 4.39 (br s, 2H), 4.21-4.19 (m, 4H), 3.99 (t, J
) 4.8 Hz, 2H), 3.96 (t, J ) 4.8 Hz, 2H), 3.90 (t, J ) 4.8 Hz,
2H), 3.84 (t, J ) 4.8 Hz, 2H), 3.81-3.66 (m, 20H), 1.53 (s,
9H). 13C NMR (CDCl3, 125 MHz, 298 K): δ 155.9, 154.3,
154.2, 152.9, 150.4, 139.9, 137.7, 128.4, 128.2, 128.0, 127.1,
127.1, 126.8, 126.7, 126.6, 125.0, 125.0, 115.2, 114.5, 114.5,
113.7, 112.8, 105.5, 80.0, 72.0, 72.9, 70.8, 70.8, 70.8, 70.7, 70.6,
69.7, 69.7, 69.6, 68.4, 67.9, 67.9, 67.9, 66.9, 28.4. HRMS (ESI)
(m/z): [M + Na]+ calcd for C59H71NO13, 1024.4780; found,
1024.4818.
1-H‚PF6: A solution (10 mL) of phenol/trimethylsilyl chloride
(1:3 M) in CH2Cl2 was added to 15 (60 mg, 60 µmol), and the
mixture was stirred at room temperature for 5 min. The solution
was washed with aqueous NaOH solution (1 M, 3 × 10 mL),
and the organic layer was evaporated to dryness. The residue
was redissolved in Me2CO (5 mL), and HCl (1 M, 10 mL) was
added, followed by the addition of saturated NH4PF6 solution.
The mixture was extracted with CH2Cl2 (2 × 10 mL). The
combined organic layer was evaporated, and the residue was
subjected to preparative TLC (SiO2-CH2Cl2/MeOH 10:1) to
give 1-H‚PF6 (13 mg, 20%; difficulty in extracting the ionic
product off the TLC plate likely accounts for the relatively low
11: PBr3 (0.15 g, 0.55 mmol) was added dropwise into the
solution of 10 (0.20 g, 0.50 mmol) in Et2O (5 mL), and the
mixture was stirred at room temperature for 1 h. The resulting
suspension was filtered, and the filtrate was evaporated to
dryness. The residue was partitioned in H2O/CH2Cl2. The
organic layer was dried (MgSO4) and filtered, and the filtrate
was evaporated to give a sticky oil (0.13 g, 56%), which was
used directly for the next step without further purification.
14: Cesium carbonate (17.2 g, 89 mmol) was stirred in dry
DMF (180 mL). After the temperature was raised up to 100
°C, a solution of the dibromide 12 (5.70 g, 8.92 mmol) and the
aldehyde 13 (1.23 g, 8.92 mmol) in dry DMF (180 mL) was
added over 72 h. After cooling down to room temperature, the
reaction mixture was filtered, and the filtrate was concentrated
under reduced pressure. Purification of the resulting brown oil
by column chromatography (SiO2-EtOAC/hexanes 3:1) gave
1
yield). H NMR (CD3COCD3, 600 MHz, 298 K): δ 7.83 (dd,
J ) 9.1, 3.6 Hz, 2H), 7.64 (d, J ) 8.2 Hz, 2H), 7.62 (d, J )
8.2 Hz, 2H), 7.53 (d, J ) 8.0 Hz, 2H), 7.47 (d, J ) 7.5 Hz,
2H), 7.43 (d, J ) 8.0 Hz, 2H), 7.37 (t, J ) 7.5 Hz, 2H), 7.30
(t, J ) 8.2 Hz, 2H), 7.28 (t, J ) 7.8 Hz, 2H), 6.96 (s, 1H), 6.80
(d, J ) 7.8 Hz, 2H), 6.62 (s, 2H), 4.58 (s, 2H), 4.53 (s, 2H),
4.21 (m, 4H), 3.99-3.95 (m, 8H), 3.93 (t, J ) 5.0 Hz, 2H),
9
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A R T I C L E S
Rogez et al.
3.90 (t, J ) 5.0 Hz, 2H), 3.77-3.63 (m, 20H). HRMS (ESI)
(m/z): [M - PF6]+ calcd for C54H64F6NO11P, 902.4474; found,
902.4474.
interactions. An initial Monte Carlo multiple minimization
(MCMM) conformational search involving the generation and
minimization of 1000 starting conformations by application of
random variations to the internal coordinates was used to locate
the global energy minimum of 1-H+. The maximum number of
iterations for each energy minimization in the search was set
to 2000, and the energetic window for saving structures was
1000 kJ/mol. Both threaded and side-on self-complexes were
found, with the threaded conformer being the more stable. The
500 ps molecular dynamics simulations (1.5 fs time step) at a
temperature of 400 K were then used to relax both conformers.
The SHAKE algorithm was employed on all bonds to hydro-
gens, and 500 randomly selected structures generated from each
molecular dynamics simulation were energy minimized to full
convergence at 298K using the PRCG algorithm,43 resulting in
fully relaxed conformations of the threaded as well as side-on
self-complexes. The modeling procedure did not take into
account the influence of the PF6- counterion, as its position in
solution will be random, and subtle variations in counterion
position will average out and not affect the relative energetic
difference between the threaded and side-on self-complexes.
Absorption and Fluorescence Spectroscopy. The absorption
and fluorescence experiments have been performed at 298 K
in air-equilibrated CH2Cl2 (Merck Uvasol), unless otherwise
stated. The investigations were carried out in dilute solutions
(2.0 × 10-5-2.0 × 10-4 mol L-1) in order to avoid intermo-
lecular complexation among 1-H+ species (vide infra) and
dynamic quenching processes. The emission intensity values,
when necessary, were corrected to take into account inner filter
effects, geometrical factors (for solutions of different absor-
bances at the excitation wavelength), and reabsorption of the
emitted light.49 UV-vis absorption spectra were recorded with
a Perkin-Elmer λ40 spectrophotometer. Luminescence spectra
were obtained with a Perkin-Elmer LS-50 spectrofluorimeter
equipped with a Hamamatsu R928 phototube. Excitation was
performed at 326 nm, which is an isosbestic point of the
chromophoric groups involved (vide infra). Luminescence
lifetimes were measured by the time-correlated single-photon
counting technique with Edinburgh Instruments TCSPC equip-
ment. The exciting light was produced by a gas arc lamp (Model
nF900, filled with D2) that delivered pulses of about 1 ns
(fwmh). The light emitted was filtered by using a cutoff filter.
The detector was a cooled Hamamatsu R928 photomultiplier.
The experimental error on molar absorption coefficients,
fluorescence intensities, and fluorescence lifetimes is estimated
to be (5%.
Acknowledgment. Financial support in Italy from the EU
(STREP “Biomach” NMP2-CT-2003-505487), MIUR (PRIN
“Supramolecular Devices” and FIRB RBNE019H9K), and
Universita` di Bologna (Funds for Selected Research Topics)
and in USA by three NSF Grants DGE-0114443, CHE-9974928,
and CHE-0092036 is gratefully acknowledged. B.F. was sup-
ported by a Marie Curie individual fellowship (HPMF-CT-2002-
01916). B.H.N. was supported by a National Science Foundation
fellowship (IGERT (MCTP) - DGE-0114443).
Computational Procedure. Molecular force field calcula-
tions were performed using the program Maestro v3.0.038.43
The AMBER* force field41 and GB/SA solvent model42 for
CHCl3 were used along with an extended cutoff for nonbonded
Supporting Information Available: Complete ref 37a. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(49) (a) Credi, A.; Prodi, L. Spectrochim. Acta, Part A 1998, 54, 159. (b)
Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed; CRC-Taylor and Francis: Boca Raton, FL, 2006.
JA067739E
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4642 J. AM. CHEM. SOC. VOL. 129, NO. 15, 2007