Photoinduced electron transfer in a triad that can be assembled/disassembled by two different external inputs. Toward molecular-level electrical extension cables

Article abstract of DOI:10.1021/ja025813x

We have designed, synthesized, and investigated a self-assembling supramolecular system which mimics, at a molecular level, the function performed by a macroscopic electrical extension cable. The system is made up of three components, 1²⁺, 2-H<

Full text of DOI:10.1021/ja025813x

Published on Web 10/04/2002
Photoinduced Electron Transfer in a Triad That Can Be
Assembled/Disassembled by Two Different External Inputs.
Toward Molecular-Level Electrical Extension Cables
Roberto Ballardini,† Vincenzo Balzani,*,‡ Miguel Clemente-Leo´n,‡,§ Alberto Credi,‡
Maria Teresa Gandolfi,*,‡ Ele´na Ishow,‡,| Julie Perkins, J. Fraser Stoddart,*,
Hsian-Rong Tseng, and Sabine Wenger
Contribution from Istituto FRAE-CNR, Via Gobetti 101, 40129 Bologna, Italy, Dipartimento di
Chimica “G. Ciamician”, UniVersita` degli Studi di Bologna, Via Selmi 2, 40126 Bologna, Italy,
and Department of Chemistry and Biochemistry, UniVersity of California Los Angeles,
405 Hilgard AVenue, Los Angeles, California 90095
Received February 5, 2002
Abstract: We have designed, synthesized, and investigated a self-assembling supramolecular system which
mimics, at a molecular level, the function performed by a macroscopic electrical extension cable. The system
is made up of three components, 12+, 2-H3+, and 3. Component 12+ consists of two moieties: a [Ru(bpy)3]2+
unit, which plays the role of an electron donor under light excitation, and a DB24C8 crown ether, which
fulfills the function of a socket. The wire-type component 2-H3+ is also composed of two moieties, a
secondary dialkylammonium-ion center and a bipyridinium unit, which thread into the DB24C8 crown-
ether socket of 12+ and the 1/5DN38C10 crown-ether socket 3, respectively. The photochemical,
photophysical, and electrochemical properties of the three separated components, of the 12+ ⊃ 2-H3+ and
2-H3+ ⊂ 3 dyads, and of the 12+ ⊃ 2-H3+ ⊂ 3 triad have been investigated in CH2Cl2 solution containing
2% MeCN. Reversible connection/disconnection of the two plug/socket systems can be controlled
independently by acid/base and redox stimulation. The behavior of the various different dyads and triad
has been monitored by light absorption and emission spectroscopies, as well as by electrochemical
techniques. In the fully connected 12+ ⊃ 2-H3+ ⊂ 3 triad, light excitation of the [Ru(bpy)3]2+ unit of component
1
2+ is followed by electron transfer (k ) 2.8 × 108 s-1) to the bipyridinium unit of component 2-H3+, which
is plugged into component 3. Possible schemes to obtain improved molecular-level electrical extension
cables are discussed.
Introduction
matter that have distinct structures, shapes, and properties. The
chemical, bottom-up approach to nanotechnology implies pass-
Miniaturization of the components for the construction of
useful devices is currently pursued by the large-downward (top-
down) approach. This approach, however, which leads solid-
state physicists and electronic engineers to manipulate progres-
sively smaller pieces of matter, has intrinsic limitations.1 An
alternative approach to the construction of nanoscale-sized
components and devices is the small-upward (bottom-up)
approach. Chemists, by the nature of their discipline, are in an
ideal position to develop bottom-up strategies because they are
able to manipulate molecules, that is, the smallest entities of
ing from molecules to supramolecular species and is driven by
the idea that the macroscopic concept of device can be extended
to the molecular level.2,3 A molecular-leVel deVice can be
defined as an assembly of a discrete number of molecules
designed to achieve a specific function: each molecular com-
ponent performs a simple act, while the entire supramolecular
structure performs a more complex function, which results from
the cooperation of the various molecular components.
Stimulated by this vision, we and others4,5 have constructed
and investigated a number of supramolecular systems specifi-
cally designed to play the role of molecular-level devices, that
is, aggregates able to perform a particular function upon
application of an external energy input. In this framework,
construction of chemical computers (i.e., of computers based
on molecular-level components) has been anticipated.6 Apart
* To whom correspondence should be addressed. E-mail: vbalzani@
ciam.unibo.it (V.B.).
† Istituto FRAE-CNR.
‡ Universita` di Bologna.
§ Present address: Instituto de Ciencia Molecular, Universitat de Va-
le`ncia, Dr Moliner 50, 46100 Burjassot, Spain.
| Present address: Department of Chemistry, ENS Cachan, 61 Avenue
du President Wilson, 94235 Cachan Cedex, France.
University of California, Los Angeles.
(2) (a) Balzani, V.; Moggi, L.; Scandola, F. In Supramolecular Photochemistry;
Balzani, V., Ed.; Reidel: Dordrecht, The Netherlands, 1987; pp l-28. (b)
Balzani, V.; Credi, A.; Venturi, M. In Stimulating Concepts in Chemistry;
Vo¨gtle, F., Stoddart, J. F., Shibasaki, M., Eds.; Wiley-VCH: Weinheim,
2000; pp 255-266.
(1) (a) Special issue on nanoscale materials, guest editor J. R. Heath: Acc.
Chem. Res. 1999, 32, 387-454. (b) Special issue on nanostructures, guest
editors E. A. Chandross and R. D. Miller: Chem. ReV. 1999, 99, 1641-
1990. (c) Schultz, M. Nature 1999, 399, 729-730. (d) Dagani, R. Chem.
Eng. News 2000, 78 (January 3), 22-26.
(3) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, 1995.
9
12786
J. AM. CHEM. SOC. 2002, 124, 12786-12795
10.1021/ja025813x CCC: $22.00 © 2002 American Chemical Society

Photoinduced Electron Transfer
A R T I C L E S
Figure 1. (a) Schematic representation of an extension. (b) The three components of the investigated self-assembling system.
from such a futuristic application, the design and realization of
a molecular-leVel electronic set (i.e., a set of molecular-level
systems capable of performing functions that mimic those
executed by the components of macroscopic electronic devices)
is of great scientific interest because it introduces new concepts
into the field of chemistry and stimulates the ingenuity of
research workers.
this paper, we report the results of a detailed investigation on
photoinduced electron transfer in a supramolecular triad that
can be assembled/disassembled in a controlled manner by two
different external inputs, namely, (i) acid/base and (ii) redox
stimuli. The investigated system mimics, at the molecular level,
the function performed by a macroscopic electrical extension
cable.
In the past few years, many systems that could prove useful
for information processing at the molecular level7 (e.g., wires,8
antennas,9 switches,10,11 rectifiers,12 plug/socket devices,13 memo-
ries,10,14 logic gates10,15) have been constructed and studied. In
Design
A macroscopic electrical extension cable is a wire with two
connecting ends used to transfer electrons from a power supply
(source) to a remote load (drain). A supramolecular system
capable of mimicking the function performed by a macroscopic
extension must consist of three molecular components (A, B,
(4) For recent reviews on molecular-level devices, see: (a) Balzani, V.; Go´mez-
Lo´pez, M.; Stoddart, J. F. Acc. Chem. Res. 1998, 31, 405-414. (b) Sauvage,
J.-P. Acc. Chem. Res. 1998, 31, 611-619. (c) Boulas, P. L.; Gomez-Kaifer,
M.; Echegoyen, L. Angew. Chem., Int. Ed. 1998, 37, 216-247. (d) Niemz,
A.; Rotello, V. M. Acc. Chem. Res. 1999, 32, 44-52. (e) Kaifer, A. E.
Acc. Chem. Res. 1999, 32, 62-71. (f) Leigh, D. A.; Murphy, A. Chem.
Ind. (London) 1999, 178-183. (g) Fabbrizzi, L.; Licchelli, M.; Pallavicini,
P. Acc. Chem. Res. 1999, 32, 846-853. (h) Piotrowiak, P. Chem. Soc. ReV.
1999, 28, 143-150. (i) Blanco, M.-J.; Jime´nez, M. C.; Chambron, J.-C.;
Heitz, V.; Linke, M.; Sauvage, J.-P. Chem. Soc. ReV. 1999, 28, 293-305.
(j) Tour, J. M. In Stimulating Concepts in Chemistry; Vo¨gtle, F., Stoddart,
J. F., Shibasaki, M., Eds.; Wiley-VCH: Weinheim, 2000; pp 237-254.
(k) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem.,
Int. Ed. 2000, 39, 3358-3391. (l) Special issue on molecular machines,
guest editor J. F. Stoddart: Acc. Chem. Res. 2001, 35, 409-522. (m) Issue
devoted to molecular machines and motors, guest editor J.-P. Sauvage:
Struct. Bonding (Berlin) 2001, 99, 1-281.
(5) For some recent examples of molecular-level devices, see: (a) Steinberg-
Yfrach, G.; Rigaud, J. L.; Durantini, E. N.; Moore, A. L.; Gust, D.; Moore,
T. A. Nature 1998, 392, 479-482. (b) Ashton, P. R.; Ballardini, R.; Balzani,
V.; Baxter, I.; Credi, A.; Fyfe, M. C. T.; Gandolfi, M. T.; Go´mez-Lo´pez,
M.; Mart´ınez-D´ıaz, M. V.; Piersanti, A.; Spencer, N.; Stoddart, J. F.;
Venturi, M.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1998, 120,
11932-11942. (c) Kelly, T. R.; De Silva, H.; Silva, R. A. Nature 1999,
401, 150-152. (d) Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.;
Harada, N.; Feringa, B. L. Nature 1999, 401, 152-155. (e) Ashton, P. R.;
Ballardini, R.; Balzani, V.; Credi, A.; Dress, K. R.; Ishow, E.; Kleverlaan,
C. J.; Kocian, O.; Preece, J. A.; Spencer, N.; Stoddart, J. F.; Venturi, M.;
Wenger, S. Chem.sEur. J. 2000, 6, 3558-3574. (f) Brouwer, A. M.;
Frochot, C.; Gatti, F. G.; Leigh, D. A.; Mottier, L.; Paolucci, F.; Roffia,
S.; Wurpel, G. W. H. Science 2001, 291, 2124-2128. (g) Willner, I.; Pardo-
Yissar, V.; Katz, E.; Ranjit, K. T. J. Electroanal. Chem. 2001, 497, 172-
177. (h) Goodman, A.; Breinlinger, E.; Ober, M.; Rotello, V. M. J. Am.
Chem. Soc. 2001, 123, 6213-6214.
(8) (a) Harriman, A.; Ziessel, R. Coord. Chem. ReV. 1998, 171, 331-339. (b)
Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature
1998, 396, 60-63. (c) Anderson, H. L. Chem. Commun. 1999, 2323-
2330. (d) Barigelletti, F.; Flamigni, L. Chem. Soc. ReV. 2000, 29, 1-12.
(e) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804.
(9) (a) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi,
M. Acc. Chem. Res. 1998, 31, 26-35. (b) Kuciauskas, D.; Liddell, P. A.;
Lin, S.; Johnson, T. E.; Weghorn, S. J.; Lindsey, J. S.; Moore, A. L.; Moore,
T. A.; Gust, D. J. Am. Chem. Soc. 1999, 121, 8604-8614. (c) Plevoets,
M.; Vo¨gtle, F.; De Cola, L.; Balzani, V. New J. Chem. 1999, 23, 63-69.
(d) Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701-1710.
(10) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim, 2001.
(11) (a) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.;
Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000,
289, 1172-1175. (b) Collier, C. P.; Jeppesen, J. O.; Luo, Y.; Perkins, J.;
Wong, E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc., submitted.
(12) (a) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950-957. (b) Xu, T.;
Peterson, I. R.; Lakshmikantham, M. V.; Metzger, R. M. Angew. Chem.,
Int. Ed. 2001, 40, 1749-1752. (c) Brady, C. A.; Sambles, J. R. In Electron
Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001;
Vol. 5, Part I, Chapter 4.
(13) Ishow, E.; Credi, A.; Balzani, V.; Spadola, F.; Mandolini, L. Chem.sEur.
J. 1999, 5, 984-989.
(14) (a) Special issue on photochromism: memories and switches, guest editor
M. Irie: Chem. ReV. 2000, 100, 1683-1890. (b) Luo, Y.; Collier, C. P.;
Nielsen, K. A.; Jeppesen, J. O.; DeIonno, E.; Perkins, J.; Pease, A. R.;
Stoddart, J. F.; Heath, J. R. Science, submitted.
(15) (a) de Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P. Nature 1993, 364,
42-44. (b) Credi, A.; Balzani, V.; Langford, S. J.; Stoddart, J. F. J. Am.
Chem. Soc. 1997, 119, 2679-2681. (c) Pina, F.; Maestri, M.; Balzani, V.
Chem. Commun. 1999, 107-114. (d) Collier, C. P.; Wong, E. W.;
Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams,
R. S.; Heath, J. R. Science 1999, 285, 391-394. (e) Wong, E. W.; Collier,
C. P.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. J. Am.
Chem. Soc. 2000, 122, 5831-5840. (f) de Silva, A. P.; Fox, D. B.; Huxley,
A. J. M.; Moody, T. S. Coord. Chem. ReV. 2000, 205, 41-57. (g) de Silva,
A. P.; McClenaghan, N. D. J. Am. Chem. Soc. 2000, 122, 3965-3966. (h)
Raymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2001, 123, 4651-4652.
(6) (a) Rouvray, D. Chem. Br. 1998, 34, 26-29. (b) Ball, P. Nature 2000,
406, 118-120. (c) Dagani, R. Chem. Eng. News 2000, 78 (October 16),
27-32. (d) Pease A. R.; Stoddart, J. F. Struct. Bonding (Berlin) 2001, 99,
189-236.
(7) (a) Molecular Electronics; Jortner, J., Ratner, M. A., Eds.; Blackwell:
Oxford, 1997. (b) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, 2001; Vol. 5, Part 1. (c) Shipway, A. N.; Katz, E.;
Willner, I. Struct. Bonding (Berlin) 2001, 99, 237-281.
9
J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002 12787

A R T I C L E S
Ballardini et al.
Scheme 1
and C in Figure 1a) programmed to operate as an electron source
(A), a wire (B), and an electron drain (C), in turn. The three
components must also be programmed so that one end of the
wire can be plugged in/out of the source and the other end can
be plugged in/out of the drain. Furthermore, each plug in/out
function has to be reversible, controllable by an external input,
and monitorable. It is, of course, very difficult to design, and
even more so to prepare, molecular components that will self-
assemble spontaneously into a supramolecular system capable
of satisfying all the described requirements. We shall demon-
strate, however, that, by using the molecular components 12+
,
2-H3+, and 3, shown in Figure 1b, we have succeeded in meeting
most of the essential requirements and that we are now in the
position to design more satisfactory, second-generation molec-
ular-level electrical extension cables. Compound 12+, which
fulfills the role of an electron-source component, is composed
of two moieties: (i) a [Ru(bpy)3]2+ unit, which is known16 to
be a strong electron donor in the excited state, and (ii) a dibenzo-
[24]crown-8 (hereafter indicated by DB24C8), which is capable
of playing the role of a socket.17 Compound 2-H3+, which fulfills
the role of a connecting wire-type component, contains two
moieties, a secondary dialkylammonium center, which is
known17c,d,f to thread as a plug into the DB24C8 socket, and a
bipyridinium moiety which can insert itself in a plug-like
manner18 into compound 3, namely 1,5-dinaphtho[38]crown-
10 (hereafter indicated by 1/5DN38C10), which fulfills the role
of an electron drain (socket) component. Because the wire-type
component 23+ is short and relatively rigid, the problem of
folding, which is often encountered in pseudorotaxane chemistry,
is not expected to be important. In this system, (i) the two plug/
socket functions can be controlled reVersibly and independently
by external acid/base and redox stimuli, respectively, and (ii)
the occurrence of the connections can be monitored by changes
in the absorption spectra, emission spectra, and electrochemical
potentials. In the fully connected system, light excitation of the
[Ru(bpy)3]2+ unit of compound 12+ is followed by electron
transfer to the bipyridinium unit of compound 2-H3+, which is
plugged into compound 3. The electron-transfer process can be
monitored by changes in the emission spectra, and its rate can
be determined from lifetime measurements.
Scheme 2
Results and Discussion
Syntheses. 1/5DN38C10 (3) was synthesized according to a
literature procedure.18a The syntheses of 12+ and 2-H3+ are
summarized in Schemes 1 and 2.
Synthesis of Component 12+. The synthesis of 12+ is outlined
in Scheme 1. 2-Acetylpyridine (6) was transferred into (2-
pyridacyl)pyridium iodide (7‚I) in 69% yield by its treatment
with I2 in C5H5N. The synthesis of 5-methyl-2,2′-bipyridine (8)
was accomplished almost quantitatively by pyridine ring an-
nulation of (2-pyridacyl)pyridium iodide (7‚I) in formamide
solution in the presence of methacrolein and ammonium acetate.
The bipyridine derivative 8 underwent AIBN-catalyzed bromi-
nation with NBS in CCl4, giving 5-bromomethyl-2,2′-bipyridine
(9) in 48% yield. Alkylation (NaH/PhMe) of the hydroxy-
methylated dibenzo[24]crown-8 derivative19 10 with 9 afforded
(40%) the bipyridine-tagged crown ether 11. The ruthenium
complex 12+ was obtained as its bis(hexafluorophosphate) salt
in 43% overall yield, after treating cis-bis(2,2′-bipyridine)-
dichloro-ruthenium(II)20 with the ligand 10 in aqueous EtOH,
followed by counterion exchange (NH4PF6/H2O/Me2CO).
(16) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277.
(17) (a) Kolchinski, A. G.; Busch, D. H.; Alcock, N. W. J. Chem. Soc., Chem.
Commun. 1995, 1289-1291. (b) Ashton, P. R.; Chrystal, E. J. T.; Glink,
P. T.; Menzer, S.; Spencer, N.; Schiavo, C.; Stoddart, J. F.; Tasker, P. A.;
White, A. J. P.; Williams, D. J. Chem.sEur. J. 1996, 2, 709-728. (c)
Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154-
1196. (d) Ashton, P. R.; Glink, P. T.; Mart´ınez-D´ıaz, M. V.; Stoddart, J.
F.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1996, 35,
1930-1933. (e) Montalti, M.; Ballardini, R.; Prodi, L.; Balzani, V. Chem.
Commun. 1996, 2011-2012. (f) Cartrill, S. J.; Pease, A. R.; Stoddart, J. F.
J. Chem. Soc., Dalton Trans. 2000, 3715-3735.
(18) (a) Ashton, P. R.; Chrystal, E. J. T.; Mathias, J. P.; Parry, K. P.; Slawin,
A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Tetrahedron Lett.
1987, 28, 6367-6370. (b) Credi, A.; Montalti, M.; Balzani, V.; Langford,
S. J.; Raymo, F. M.; Stoddart, J. F. New J. Chem. 1998, 22, 1061-1065.
(19) Diederich, F.; Echegoyen, L.; Go´mez-Lo´pez, M.; Kessinger, R.; Stoddart,
J. F. J. Chem. Soc., Perkin Trans. 2 1999, 1577-1586.
(20) Lay, P.; Sargeson, A. M.; Taube, H. Inorg. Synth. 1986, 24, 292-293.
9
12788 J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002

Photoinduced Electron Transfer
A R T I C L E S
Figure 3. Schematic representation of the threading of the NH2+ center in
component 2-H3+ into the cavity of the DB24C8 moiety of component 12+
.
important for selective connection/disconnection in our supra-
molecular system.
Component 2-H3+ shows an absorption spectrum (Figure 2)
dominated by the strong band for the bipyridinium unit in the
near UV spectral region,23 and no emission. In electrochemical
experiments, compound 2-H3+ shows (Table 1) the two char-
acteristic23 reversible, one-electron reduction peaks for the
bipyridinium unit.
Component 3 shows (Figure 2) absorption bands in the UV
region, a strong fluorescence band with λmax ) 346 nm24 (τ )
8 ns) and two irreversible, one-electron oxidation peaks which
can be assigned (Table 1)24b to its dioxynaphthalene ring
systems.
Figure 2. Absorption and (inset) emission spectra of components 12+
(s), 2-H3+ (s ‚ s), and 3 (s s) (CH2Cl2/MeCN, 98:2, v/v, room
temperature; λexc ) 450 and 295 nm for 12+ and 3, respectively).
Table 1. Electrochemical Dataa
cmpd
Eoxb (V)
E
b (V)
red
12+
2-H3+
3
+1.31c,d +1.41d +1.58d
-1.25
-0.24 -0.67
+1.11d
+1.25d
12+ ⊃ 2-H3+ +1.29c,d +1.41d +1.54d -0.24 -0.67 -1.27
Two-Component Supramolecular System 12+ ⊃ 2-H3+. It
is well-known17 that secondary dialkylammonium centers can
thread DB24C8 by virtue of strong N+-H‚‚‚O and C-H‚‚‚O
hydrogen-bonding interactions. In principle, although the bi-
pyridinium units can penetrate a DB24C8 ring partially, the
interaction (stabilized by charge transfer and weak C-H‚‚‚O
hydrogen bonding) is orders of magnitude weaker.5b,25 It can
therefore be expected that, on mixing components 12+ and
2-H3+, a supramolecular entity 12+ ⊃ 2-H3+ is obtained,26 with
2-H3+ ⊂ 3
+1.15d
+1.24d
-0.31 -0.67
a Argon purged CH2Cl2/MeCN 90:10 (v/v) solution, room temperature.
b Halfwave potential values in V vs SCE; reversible and monoelectronic
processes, unless otherwise indicated. c Shoulder. d Not fully reversible
process; potential value estimated from DPV peaks.
Synthesis of Component 2-H3+. The synthesis of 2-H3+ is
outlined in Scheme 2. The known compound,21 N-benzyl-4-
pyridium-4′-pyridine (12‚PF6), was treated with 5 equiv of 4,4′-
bis(chloromethyl)-1,1′-biphenyl in MeCN to afford, after coun-
terion exchange (NH4PF6/H2O/Me2CO), a reactive benzyl
chloride which was not isolated, but rather treated directly with
PhCH2NH2, to give the trication 2-H3+ as its tris(hexafluoro-
phosphate) salt following counterion exchange (NH4PF6/H2O).
The overall yield for the four reactions was 30%.
Properties of Components 12+, 2-H3+, and 3. All the
experiments have been performed at room temperature in
CH2Cl2 solution containing a fraction of MeCN (2% and 10%
for the photophysical and electrochemical experiments, respec-
tively) because trication 2-H3+ is insoluble in neat CH2Cl2.
Component 12+ (Figure 1b) is composed of a DB24C8 unit
appended to a [Ru(bpy)3]2+ moiety. The absorption spectrum
of this component (Figure 2) is dominated by the bands of the
[Ru(bpy)3]2+ moiety, which extend into the visible region, and
the emission spectrum shows (Figure 2, inset) only the well-
known16 [Ru(bpy)3]2+ band with λmax ) 608 nm (τ ) 420 ns).
Clearly, the emission of the DB24C8 ring is quenched by the
appended [Ru(bpy)3]2+ moiety. The redox behavior of 12+
(Table 1) is that expected from the behavior of the two separated
moieties.16,22 In particular, it does not show any redox process
in the potential range 0/-1 V versus SCE, a feature which is
the NH2 center of the wire-type component 2-H3+ threaded
+
(Figure 3) into the cavity of DB24C8. In this threaded
superstructure, one might expect27,28 that the luminescent excited
(23) (a) Summers, L. A. The Bipyridinium Herbicides; Academic Press: London,
1980. (b) Monk, P. M. S. The Viologens; Wiley: Chichester, U.K., 1998.
(24) (a) Gandolfi, M. T.; Zappi, T.; Ballardini, R.; Prodi, L.; Balzani, V.;
Stoddart, J. F.; Mathias, J. P.; Spencer, N. Gazz. Chim. Ital. 1991, 121,
521-525. (b) Ashton, P. R.; Baldoni, V.; Balzani, V.; Claessens, C. G.;
Credi, A.; Hoffmann, H. D. A.; Raymo, F. M.; Stoddart, J. F.; Venturi,
M.; White, A. J. P.; Williams, D. J. Eur. J. Org. Chem. 2000, 1121-1130.
(25) Ashton, P. R.; Ballardini, R.; Balzani, V.; Fyfe, M. C. T.; Gandolfi, M. T.;
Mart´ınez-D´ıaz, M. V.; Morosini, M.; Schiavo, C.; Shibata, K.; Stoddart, J.
F.; White, A. J. P.; Williams, D. J. Chem.sEur. J. 1998, 4, 2332-2351.
(26) For recent examples of supramolecular species containing [Ru(bpy)3]2+ and
bipyridinium moieties, see ref 5e and the following: (a) David, E.; Born,
R.; Kaganer, E.; Joselevich, E.; Du¨rr, H.; Willner, I. J. Am. Chem. Soc.
1997, 119, 7778-7790. (b) Ashton, P. R.; Balzani, V.; Credi, A.; Kocian,
O.; Pasini, D.; Prodi, L.; Spencer, N.; Stoddart, J. F.; Tolley, M. S.; Venturi,
M.; White, A. J. P.; Williams, D. J. Chem.sEur. J. 1998, 4, 590-607. (c)
Ashton, P. R.; Balzani, V.; Kocian, O.; Prodi, L.; Spencer, N.; Stoddart, J.
F. J. Am. Chem. Soc. 1998, 120, 11190-11191. (d) Benniston, A. C.;
Mackie, P. R.; Harriman, A. Angew. Chem., Int. Ed. 1998, 37, 354-356.
(e) Ashton, P. R.; Ballardini, R.; Balzani, V.; Constable, E. C.; Credi, A.;
Kocian, O.; Langford, S. J.; Preece, J. A.; Prodi, L.; Schofield, E. R.;
Spencer, N.; Stoddart, J. F.; Wenger, S. Chem.sEur. J. 1998, 4, 2413-
2422. (f) Hu, Y.-Z.; Bossmann, S. H.; van Loyen, D.; Schwarz, O.; Du¨rr,
H. Chem.sEur. J. 1999, 5, 1267-1277. (g) Hu, Y.-Z.; Takashima, H.;
Tsukiji, S.; Shinkai, S.; Nagamune, T.; Oishi, S.; Hamachi, I. Chem.s
Eur. J. 2000, 6, 1907-1916.
(27) Quenching of the luminescence of [Ru(bpy)3]2+ by bipyridinium species
has been extensively investigated. See, for example: Hoffman, M. Z.;
Bolletta, F.; Moggi, L.; Hug, G. L. J. Phys. Chem. Ref. Data, 1989, 18,
219-543.
(21) Barton, M. T.; Rowley, N. M.; Ashton, P. R.; Jones, C. J.; Spencer, N.;
Tolley, M. S.; Yellowlees, L. J. New J. Chem. 2000, 24, 555-560.
(22) For the electrochemistry of DB24C8 in MeCN, see: Ashton, P. R.;
Ballardini, R.; Balzani, V.; Baxter, I.; Credi, A.; Fyfe, M. C. T.; Gadolfi,
M. T.; Go´mez-Lo´pez, M.; Mart´ınez-D´ıaz, M. V.; Piersanti, A.; Spencer,
N.; Stoddart, J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Am.
Chem. Soc. 1998, 120, 11932-11942.
(28) For pioneering work on electron transfer involving covalently linked
[Ru(bpy)3]2+ and bipyridinium moieties, see: (a) Yonemoto, E. H.; Riley,
R. L.; Kim, Y. I.; Atherton, S. J.; Schmehl, R. H.; Mallouk, T. E. J. Am.
Chem. Soc. 1992, 114, 8081-8087. (b) Yonemoto, E. H.; Saupe, G. B.;
Schmehl, R. H.; Hubig, S. M.; Riley, R. L.; Iverson, B. L.; Mallouk, T. E.
J. Am. Chem. Soc. 1994, 116, 4786-4795. (c) Kelly, L. A.; Rodgers, M.
A. J. J. Phys. Chem. 1995, 99, 13132-13140.
9
J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002 12789

A R T I C L E S
Ballardini et al.
counterions. Such ion pairs between 2-H3+ and PF6 cannot
-
associate with 12+ on account of steric reasons and because of
the decrease in the hydrogen-bonding acceptor properties of the
+
NH2 centers. The role played by the presence of ion pairs in
preventing the inclusion of ammonium ions into neutral recep-
tors in low polarity solvents has been emphasized recently.30
A
confirmation of the role played by ion pairing on preventing
the association between 12+ and 2-H3+ was obtained by
experiments carried out in the presence of Cl- ions, which are
known31 to exhibit a strong tendency to form ion pairs. For
example, addition of 2 equiv of Bu4NCl to a solution containing
1.0 × 10-4 M 12+ and 2-H3+ caused a 2-fold increase of the
luminescence intensity in the [Ru(bpy)3]2+ moiety and a
corresponding decrease in the relative weight of the short
lifetime. From the initial part of the titration curve (insert in
Figure 4), where the formation of the adduct predominates with
respect to ion pairing, a value of about 105 M-1 was obtained
for the stability constant of 12+ ⊃ 2-H3+. This value is in
agreement17c,32 with the stability constants obtained for pseudo-
rotaxanes formed between DB24C8 and dibenzylammonium
derivatives in CH2Cl2. The restoration of the original lumines-
cence intensity and the disappearance of the short lifetime upon
addition of tributylamine is caused by the deprotonation of the
Figure 4. Quenching of the [Ru(bpy)3]2+ luminescence intensity (λexc
)
450 nm) upon titration of a 5.0 × 10-5 M solution of 12+ with 2-H3+
(CH2Cl2/MeCN, 98:2, v/v, room temperature). Inset a shows the titration
based on the luminescence intensity at 608 nm; inset b shows the
biexponential decay observed for an equimolar solution of 12+ and 2-H3+
For more details, see the text.
.
state of the [Ru(bpy)3]2+ moiety of 12+ is quenched by electron
transfer to the bipyridinium unit of 2-H3+. Upon titration of a
5.0 × 10-5 M solution of 12+ with 2-H3+, a decrease in intensity
of the [Ru(bpy)3]2+ luminescence is indeed observed (Figure
4) until a plateau is reached. Before addition of 2-H3+, the
luminescent signal exhibits a monoexponential decay with τ )
420 ns. On addition of 2-H3+, a double exponential decay is
observed (Figure 4, inset b) with lifetimes of about 3.5 and 400
ns. The two lifetimes are maintained throughout the titration,
with an increase in the relative weight of the short lifetime on
increasing concentration of 2-H3+, until the plateau is reached.
In the plateau region, the two lifetimes are still present. Addition
of tributylamine restores the original luminescence intensity and
causes the disappearance of the short lifetime.
NH2 center in 2-H3+ and the consequent dethreading of 12+
+
+
⊃ 2-H3+ (Figure 5). In conclusion, the NH2 center in
component 2-H3+ threads the DB24C8 ring in component 12+
(with a complexation efficiency around 60% for a 5.0 × 10-5
M equimolar solution of the components), the threading/
dethreading process is reversible and acid/base controllable, and
in the threaded structure, a photoinduced electron-transfer
process takes place from the [Ru(bpy)3]2+ moiety of component
12+ to the bipyridinium unit of 2-H3+
.
Two-Component Supramolecular System 2-H3+ ⊂ 3.
Contrary to what happens for DB24C8, 1/5DN38C10 has a
The results can be interpreted as follows. The decrease of
the [Ru(bpy)3]2+ luminescence intensity of component 12+ on
addition of 2-H3+, and the presence of only one short lifetime
that remains constant throughout the titration, show that the
quenching process is static in nature and takes place inside a
well-defined adduct.29 Most likely, such an adduct has a
pseudorotaxane-type superstructure (Figure 3) in keeping with
those observed in very similar systems.17,25 In the adduct, the
luminescent excited state of the [Ru(bpy)3]2+ moiety of com-
ponent 12+ is quenched27,28 by electron transfer to the bipyri-
dinium unit of 2-H3+ (Figure 5). The rate constant of the
electron-transfer quenching process, obtained from the equation
k ) 1/τ - 1/τ°, where τ and τ° are the short and long
(unquenched) emission lifetimes, respectively, was found to be
2.8 × 108 s-1. The lack of complete quenching of the
[Ru(bpy)3]2+ luminescence on increasing the amount of added
2-H3+ (Figure 4, inset a) shows that only a fraction of the 12+
species is engaged in adduct formation with 2-H3+. This result
can be attributed to the fact that addition of 2-H3+ implies the
simultaneous addition of 3 equiv of PF6- counterions. Therefore,
on increasing the amount of added 2-H3+, the concentration of
+
larger affinity for the bipyridinium unit than it has for the NH2
center in component 2-H3+. The reason for this different
behavior is twofold: (i) 1/5DN38C10 is much larger than
DB24C8 and lacks a contiguous ring of appropriately spaced
oxygen atoms; hence, its interaction with NH2+ center in 2-H3+
is disfavored,25 and (ii) the more extended aromatic ring system
and the stronger electron-donor power of the 1,5-dioxynaph-
thalene units favor18,33 the formation of π-π stacking and CT
interactions with the electron-acceptor bipyridinium unit of
2-H3+. Formation of pseudorotaxanes by self-threading of a
bipyridinium unit into 1/5DN38C10 has been demonstrated
previously.18a,25 It is, therefore, to be expected that, on mixing
components 2-H3+ and 3, a supramolecular 2-H3+ ⊂ 3 entity is
obtained in which the bipyridinium unit of the wire-type
component 2-H3+ is threaded (Figure 6) into the cavity of
1/5DN38C10.
Upon titration of a 1.0 × 10-4 M solution of 2-H3+ with 3,
a new absorption band with λmax ) 527 nm develops (Figure
(30) Bo¨hmer, V.; Dalla Cort, A.; Mandolini, L. J. Org. Chem. 2001, 66, 1900-
1902.
-
(31) The ability of Cl- ions to form ion pairs with ammonium ions in nonpolar
solvents has been exploited to drive the dethreading of a pseudorotaxane
formed between DB24C8 and an anthracenylammonium ion. See: Montalti,
M.; Prodi, L. Chem. Commun. 1998, 1461-1462.
PF6 counterions becomes large, and considering the low
polarity of the solvent used, the NH2+ center of 2-H3+ becomes
-
more and more engaged in tight ion pairs with the PF6
(32) Ashton, P. R.; Ballardini, R.; Balzani, V.; Go´mez-Lo´pez, M.; Lawrence,
S. E.; Mart´ınez-D´ıaz, M. V.; Montalti, M.; Piersanti, A.; Prodi, L.; Stoddart,
J. F.; Williams, D. J. J. Am. Chem. Soc. 1997, 119, 10641-10651.
(33) Balzani, V.; Ceroni, P.; Credi, A.; Go´mez-Lo´pez, M.; Hamers, C.; Stoddart,
J. F.; Wolf, R. New J. Chem. 2001, 25, 25-31.
(29) Dynamic quenching becomes appreciable only in the experiments with
higher concentrations of 2-H3+, as shown by the observed small decrease
of the long lifetime during titration.
9
12790 J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002

Photoinduced Electron Transfer
A R T I C L E S
Figure 5. Acid/base-controlled connection/disconnection of the 12+ and 2-H3+/22+ components. In the connected structure, the luminescence of the [Ru(bpy)3]2+
moiety of component 12+ is quenched by electron transfer to the bipyridinium unit of 2-H3+
.
in the threaded species.18a,25,34 The constant fluorescence lifetime
observed throughout the titration experiment (τ ) 8 ns) can be
assigned to uncomplexed 1/5DN38C10 molecules. From the
fitting of the absorption titration curve (Figure 7, inset), a
stability constant of (3 ( 1) × 104 M-1 can be obtained for the
2-H3+ ⊂ 3 adduct, with an ꢀmax value for the CT band of 450
M-1 cm-1. Since the interaction stabilizing the 2-H3+ ⊂ 3
connection is mainly CT in nature, it was anticipated that the
threading/dethreading process could be controlled by redox
stimulation.35 We have found that addition of an excess of Zn
powder to a deaerated solution containing 3.5 × 10-5 M 2-H3+
and 3 causes a recovery of the 1/5DN38C10 fluorescence
intensity and the appearance (Figure 8) of the strong absorption
bands of monoreduced bipyridinium units in the near UV-vis
spectral region. The absorbance values23 indicate a quantitative
one-electron reduction of the bipyridinium unit of 2-H3+. Since
a reduced bipyridinium unit, if connected to component 3,
would have quenched its fluorescence,36 we must conclude that
one-electron reduction of the bipyridinium units causes (Figure
9) dethreading of the 2-H3+ ⊂ 3 pseudorotaxane. Upon
subsequent addition of an excess of the oxidant NOBF4, the
absorption bands of the monoreduced bipyridinium units dis-
appear, and the fluorescence intensity of component 3 is again
Figure 6. Schematic representation of the threading of the bipyridinium
unit of component 2-H3+ into the cavity of 1/5DN38C10 (component 3).
(34) 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, 11171-11197.
(35) Ashton, P. R.; Ballardini, R.; Balzani, V.; Boyd, S. E.; Credi, A.; Gandolfi,
M. T.; Go´mez-Lo´pez, M.; Iqbal, S.; Philp, D.; Preece, J. A.; Prodi, L.;
Ricketts, H. G.; Stoddart, J. F.; Tolley, M. S.; Venturi, M.; White, A. J. P.;
Williams, D. J. Chem.sEur. J. 1997, 3, 152-170.
Figure 7. Charge-transfer absorption band exhibited by the 2-H3+ ⊃ 3
adduct. The inset shows the increase of the absorbance at 527 nm observed
upon titration of a 1.0 × 10-4 M solution of 2-H3+ with component 3
(CH2Cl2/MeCN, 98:2, v/v, room temperature).
(36) Analysis of the photophysical and redox properties of 2-H2+ and 3 (E°°(3)
∼ 3.8 eV, E°°(2-H2+) < 2 eV, E(2-H3+/2+) ) -0.31 V, E(30/-) ) -2.8 V)
reveals that both electron- and energy-transfer quenchings of the fluorescent
excited state of 3 by 2-H2+ are thermodynamically allowed processes.
7), while quenching of the intensity of the fluorescence band
of 1/5DN38C10 (λmax ) 346 nm in Figure 2) is observed. These
results can be assigned straightforwardly to the CT interaction
9
J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002 12791

A R T I C L E S
Ballardini et al.
as a model compound for the electron-acceptor unit of 2-H3+
)
to component 3 gives the same results. The shift of the first
oxidation peak is in agreement with the occurrence of CT
interactions between the dioxynaphthalene units of 3 and the
bipyridinium unit of the thread 2-H3+. The lack of displacement
of the second oxidation peak compared to that of free 3 suggests
that, after the first one-electron oxidation, dethreading occurs
and hence the second oxidation takes place on the free crown
ether. Therefore, dethreading can also be obtained by one-
electron oxidation. However, because of their irreversible nature,
the redox processes involving 3 are less suitable than the redox
processes associated with 2-H3+ to switch on/off the CT
interaction and so to control the connection between components
2-H3+ and 3. In conclusion, the bipyridinium moiety of
component 2-H3+ threads through component 3 with a com-
plexation efficiency of around 45% for a 5.0 × 10-5 M equi-
molar solution of the components. The dethreading/rethreading
process is reversible and can be easily controlled (Figure 9) by
Figure 8. Absorption bands of a solution containing 3.5 × 10-5 M of
2-H3+ and 3 before (s s) and after (s) the addition of an excess of Zn
powder. Successive addition of an excess of NOBF4 causes the complete
recovery of the original spectrum (CH2Cl2/MeCN, 98:2, v/v, room tem-
perature).
reduction/oxidation of the bipyridinium moiety of 2-H3+
.
Three-Component Supramolecular System 12+ ⊃2-H3+
⊂
3. Following the verification, in separated experiments, that the
12+ ⊃ 2-H3+ and 2-H3+ ⊂ 3 connections are robust and can be
acid/base (Figure 5) and redox (Figure 9) controlled, respec-
tively, we have examined the behavior of solutions containing
all the three components. From the results we obtained with
the 12+ ⊃ 2-H3+ and 2-H3+ ⊂ 3 two-component systems, it
was calculated that an equimolar mixture of the three compo-
nents at 5.0 × 10-5 M concentration should contain 27% of
the 12+ ⊃ 2-H3+ ⊂ 3 triad, assuming that the two connections
do not interfere with each other.
Addition of a 5.0 × 10-5 M solution of 2-H3+ to an equimolar
solution of component 12+ caused (Figure 4) a decrease of the
luminescence intensity of the [Ru(bpy)3]2+ unit to 40% of its
initial value. This result indicates that, in such a solution, 60%
of the 12+ and 2-H3+ species is threaded to give 12+ ⊃ 2-H3+
and 40% of each of the two components 12+ and 2-H3+ is free.
After successive addition of 1 equiv of component 3, we found
that (i) the [Ru(bpy)3]2+ luminescence intensity was unaffected,
(ii) the fluorescence intensity of component 3 was partially
quenched, and (iii) the CT band at 527 nm, characteristic of
the interaction between the bipyridinium unit of 2-H3+ and 3,
was observed. The result noted under point (i) demonstrates
that the connection between 12+ and 2-H3+ is not affected by
3. Because a quantitative evaluation of the intensity of the
fluorescence band of 3 was difficult, because of inner filter
effects37 caused by overlapping (Figure 2) between the absorp-
tion spectrum of 12+ with both the absorption and fluorescence
spectra of 3, we have used the intensity at 527 nm of the CT
Figure 9. Connection of 2-H3+ and 3 is controlled by reduction/oxidation
(chemical or electrochemical) of the bipyridinium unit of component 2-H3+
.
The luminescence of crown ether 3 is quenched by the presence of the CT
excited state. For more details, see the text.
quenched to the same extent as before addition of Zn, showing
that oxidation of the reduced bipyridinium units is followed by
a rethreading process of 2-H3+ into 3.
A more elegant means to disrupt/restore the CT interaction,
and therefore to control the connection between 2-H3+ and 3,
is by employing electrochemistry.35 The trication 2-H3+ under-
goes (Table 1) two reversible one-electron reduction processes
at -0.24 and -0.67 V while the crown ether 3 undergoes two
irreversible one-electron oxidation processes at +1.11 and +1.25
V. After addition of an excess of 3 to a solution of 2-H3+, in
order to ensure complete complexation of the bipyridinium units,
the first reduction process of 2-H3+ moves from -0.24 to -0.31
V, as expected for bipyridinium units engaged in CT interactions
with dioxynaphthalene units,34 whereas the second reduction
process is not affected. This behavior, which has been observed
previously for other pseudorotaxanes of the same family,35
establishes (Figure 9) that the threaded bipyridinium units
dethread after one-electron reduction. After addition of an excess
of 2-H3+ to a 2.5 × 10-4 M solution of 3, its first oxidation
peak is shifted toward a more positive potential (+1.15 V),
whereas the second one is practically unaffected (+1.24 V).
Addition of an excess of 1,1′-dibenzyl-4,4′-bipyridinium (taken
absorption band to estimate the fractions of 2-H3+ and 12+
⊃
2-H3+ threaded with 3. Titration of a solution containing 5.0 ×
10-5 M 12+ and 2-H3+ with 3 gave spectroscopic changes
identical to those obtained from the titration of 2-H3+ alone
with 3. Moreover, the intensity of the CT band of the 2-H3+
⊂
3 adduct did not change upon addition of 12+. These results
show that the connection between 2-H3+ and 3 is not affected
by the presence of 12+. From the absorbance at 527 nm, we
estimated that 45% of the crown ether is engaged in CT
interactions; that is, 3 is threaded by either 2-H3+ or 12+
2-H3+. Since under the experimental conditions used, the 12+
⊃
(37) Credi, A.; Prodi, L. Spectrochim. Acta, Part A 1998, 59, 154-170.
9
12792 J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002

Photoinduced Electron Transfer
A R T I C L E S
Figure 10. Photoinduced electron transfer in the 12+ ⊃ 2-H3+ ⊂ 3 self-assembled system and control of the two plug/socket functions by acid/base or redox
stimulation.
⊃ 2-H3+ species is 60% and the free 2-H3+ species is 40%, it
12+ ⊃ 2-H3+ and 2-H3+ ⊂ 3 connections can be controlled
reVersibly and independently by acid/base and redox external
follows that 27% of the 3 species is engaged with the 12+
⊃
2-H3+ two-component system to give the 12+ ⊃ 2-H3+ ⊂ 3 triad
(Figure 10). The invariance of the [Ru(bpy)3]2+ luminescence
intensity on addition of 3 to a solution already containing 12+
and 2-H3+ (point i) not only shows that the 12+ ⊃ 2-H3+
connection is not affected by 3 but also that quenching of the
[Ru(bpy)3]2+ excited state occurs both in the 12+ ⊃ 2-H3+ and
12+ ⊃ 2-H3+ ⊂ 3 adducts with the same rate constant (2.8 ×
108 s-1).
inputs, respectively (Figure 10). In the fully connected 12+
⊃
2-H3+ ⊂ 3 system, light excitation of the [Ru(bpy)3]2+ unit of
component 12+ is followed by electron transfer to the bipyri-
dinium unit of component 2-H3+, which is plugged into
component 3. Therefore, the triad mimics, at the molecular level,
the function played by a macroscopic electrical extension cable.
It should be noted that the 12+ ⊃ 2-H3+ ⊂ 3 supramolecular
triad (Figure 10) is in fact a pentad,38 because it is made of
five distinct molecular-level subunits: [Ru(bpy)3]2+, a DB24C8
crown ether, a secondary dialkylammonium center, a bipyri-
dinium unit, and a 1/5DN38C10 crown ether. Its operation is
goVerned by three different types of stimuli (light, acid/base,
redox), and its behaVior can be monitored by changes in one
electrochemical, two light-emission, and two light-absorption
signals.
On addition of tributylamine to the solution containing
the three components, the luminescence intensity of free
[Ru(bpy)3]2+ is recovered, as expected because of the dis-
engagement of component 12+ from both 2-H3+ and 2-H3+
3.
⊂
Since one-electron reduction of the bipyridinium moiety of
component 2-H3+ leads to its dethreading from 3 (vide supra),
it can be expected that photoinduced electron transfer from 12+
to 2-H3+ in the 12+ ⊃ 2-H3+ ⊂ 3 triad (Figure 10) leads to the
disconnection of the crown-ether socket 3 from the rest of the
system. Such a photoinduced disassembly would indeed com-
promise the performance of the supramolecular system as a
molecular-level extension device, unless the transferred electron
is rapidly removed from the reduced bipyridinium unit.
The results reported in this paper constitute a further
demonstration that looking at supramolecular chemistry from
the viewpoint of functions, keeping in mind the devices of
macroscopic world, is a very interesting exercise for a chemist.
This kind of research leads, indeed, to a better understanding
of the principles that govern intermolecular interactions as well
as of the effects caused by external energy inputs on supra-
molecular systems.
Conclusions
We have investigated the photochemical, photophysical, and
electrochemical properties of the three molecular components
(38) For recent reviews on multicomponent systems, see: (a) Gust, D.; Moore,
T. A.; Moore, A. L. In Electron Transfer in Chemistry; Balzani, V., Ed.;
Wiley-VCH: Weinheim, 2001; Vol. 3, Part II, Chapter 2. (b) Scandola,
F.; Chiorboli, C.; Indelli, M. T.; Rampi, M. A. In Electron Transfer in
Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. 3, Part
II, Chapter 3. (c) Chang, C. J.; Brown, J. D. K.; Chang, M. C. J.; Baker,
E. A., Nocera, D. G. In Electron Transfer in Chemistry; Balzani, V., Ed.;
Wiley-VCH: Weinheim, 2001; Vol. 3, Part II, Chapter 4.
1
2+, 2-H3+, and 3 (Figure 1b), their self-assembled 12+ ⊃ 2-H3+
and 2-H3+ ⊂ 3 two-component systems, and the 12+ ⊃ 2-H3+
⊂ 3 three-component system. We have shown that, on the basis
of the properties incorporated into the three components, the
9
J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002 12793

A R T I C L E S
Ballardini et al.
and washed with pyridine (75 mL). The black solid was then added to
a boiling suspension of carbon (1 spatula) in EtOH (2 L). Following
hot filtration, (2-pyridacyl)pyridium iodide (7‚I) (36.03 g, 69%) was
1
obtained as yellow-green solid. H NMR (300 MHz, CD3SOCD3) δ
6.52 (s, 2H), 7.82-7.86 (m, 1H), 8.06-8.09 (m, 1H), 8.12-8.17 (m,
1H), 8.27-8.31 (m, 2H), 8.71-8.77 (m, 1H), 8.87-8.89 (m, 1H), 9.02-
9.04 (m, 2H); 13C NMR (75 MHz, CD3SOCD3) δ 66.7, 122.1, 127.8,
129.2, 138.2, 146.3, 146.4, 149.6, 150.5, 191.5.
5-Methyl-2,2′-bipyridine (8). (2-Pyridacyl)pyridium iodide (7‚I)
(32.6 g, 0.10 mmol), methacrolein (7.88 g, 0.11 mmol), and ammonium
acetate (17.35 g, 0.23 mmol) were dissolved in formamide (312 mL).
The reaction mixture was stirred at 75 °C for 6 h. After addition of
H2O, the reaction mixture was extracted with Et2O. The combined
organic solution was washed with brine and dried (MgSO4). The solvent
was removed in vacuo, and the residue was subjected to column
chromatography (SiO2; CH2Cl2/MeOH, 20:1) to yield 5-methyl-2,2′-
1
bipyridine (8) (16.8 g, 99%) as an orange oil. H NMR (300 MHz,
CDCl3) δ 2.36 (s, 3H), 7.23-7.27 (m, 1H), 7.58-7.61 (m, 1H), 7.74-
7.80 (m, 1H), 8.25 (d, J ) 8.1 Hz, 1H), 8.31-8.35 (m, 1H), 8.48 (d,
J ) 1.8 Hz, 1H), 8.63, 8.65 (m, 1H); 13C NMR (75 MHz, CDCl3) δ
18.3, 120.6, 120.8, 123.4, 133.4, 136.9, 137.5, 149.1, 149.6, 153.6,
156.3.
Figure 11. Improved molecular-level model of a macroscopic extension
obtained by exchanging the bipyridinium moiety of component 2-H3+ with
a suitable appended crown ether capable of binding a bipyridinium dication.
5-Bromomethyl-2,2′-bipyridine (9). 5-Methyl-2,2′-bipyridine (8)
(2.80 g, 16.4 mmol), N-bromosuccinimide (2.93 g, 16.4 mmol), and
AIBN (673 mg, 4.1 mmol) were dissolved in CCl4 (250 mL). The
reaction mixture was stirred at 45 °C for 24 h. The solvent was
evaporated, and the residue was purified by column chromatography
(SiO2; CHCl3/Me2CO, 30:20) to yield 5-bromomethyl-2,2′-bipyridine
It should be noted that the photoinduced electron-transfer
process in the 12+ ⊃ 2-H3+ ⊂ 3 system does not take place
from the 12+ to the 3 component of the supramolecular system,
but rather from the “external” molecular moiety of 12+ to the
bipyridinium unit of component 2-H3+ that is threaded into 3.
To improve upon this system in supramolecular terms, the
bipyridinium moiety of component 2-H3+ should be exchanged
(Figure 11) with a suitable appended crown ether capable of
binding a bipyridium dication. Such a 12+ ⊃ 4-H+ ⊃ 52+ system
would more closely resemble a macroscopic extension cable,
where the wire component has a plug-and-socket function
instead of two plugs as in the system examined. In the case of
the system shown in Figure 11, light excitation would transfer
an electron from the first to the fifth unit of the pentad.
This design and other designs for constructing more intricate
molecular-level electrical extension cables are currently under
investigation in our laboratories.
1
(9) (987 mg, 48%) as a yellow solid. H NMR (300 MHz, CDCl3) δ
4.51 (s, 2H), 7.27-7.32 (m, 1H), 7.77-7.85 (m, 2H), 8.37 (d, J ) 8.1
Hz, 2H), 8.65-8.67 (m, 2H); 13C NMR (75 MHz, CDCl3) δ 29.7, 121.0,
121.2, 124.0, 133.6, 137.0, 137.6, 149.2, 149.3, 155.5, 156.0.
Crown Ether Derivative 11. Sodium hydride (15.1 mg, 0.63 mmol),
hydroxymethyl-functionalized19 dibenzo[24]crown-8 10 (288 mg, 0.60
mmol), and 5-bromomethyl-2,2′-bipyridine (9) (153 mg, 0.62 mmol)
were dissolved in PhMe (60 mL). The reaction mixture was heated
under reflux for 15 h. The solvent was removed, and the residue was
dissolved in CH2Cl2 and washed with H2O and brine. After the removal
of solvent, the residue was purified by column chromatography (SiO2;
CH2Cl2/MeOH, 90:10) to obtain the bipyridine-containing crown ether
1
11 (156 mg, 40%) as a yellow solid. H NMR (300 MHz, CDCl3) δ
3.79 (s, 8H), 3.88 (t, J ) 4.2 Hz, 8H), 4.14 (t, J ) 4.2 Hz, 8H), 4.48
(s, 2H), 4.56 (s, 2H), 6.81-6.90 (m, 7H), 7.25-7.31 (m, 1H), 7.76-
7.82 (m, 2H), 8.36 (d, J ) 8.0 Hz, 2H), 8.61-8.66 (m, 2H); 13C NMR
(75 MHz, CDCl3) δ 69.1, 63.9, 69.4, 69.8, 71.2, 72.2, 113.6, 114.0,
120.8, 121.1, 121.4, 123.7, 130.9, 133.8, 136.4, 136.5, 136.9, 148.5,
148.7, 148.8, 148.9, 149.2, 155.6, 155.9. MS(FAB) m/z 669 [M + Na]+,
647 [M]+.
Component 1‚2PF6. A solution of cis-bis(2,2′-bipyridine)dichloro-
ruthenium(II) hydrate20 (96.9 mg, 0.2 mmol) in a mixture of EtOH/
H2O (13.5 mL, 3:1 v/v) was added to a refluxing solution of bipyridine-
containing crown ether 11 (129 mg, 0.2 mmol) in a mixture of EtOH/
H2O (13.5 mL, 3:1 v/v). The reaction mixture was heated under refluxed
for 24 h. The solvent was evaporated, and the residue was purified by
column chromatography (SiO2; MeOH/2 M NH4Claq/MeNO2, 7:2:1)
to afford component 1‚2PF6 (70 mg, 43%) after counterion exchange.
1H NMR (300 MHz, CDCl3) δ 3.67-3.73 (m, 12H), 3.75-3.83 (m,
6H), 4.02-4.08 (m, 4H), 4.11-4.14 (m, 2H), 4.33 (s, 2H), 4.40-4.42
(m, 2H), 6.67-6.88 (m, 5H), 7.26 (t, J ) 6.4 Hz, 1H), 7.98-8.07 (m,
6H), 8.33 (d, J ) 8.0 Hz, 1H), 8.42-8.49 (m, 6H); 13C NMR (75 MHz,
CDCl3) δ 14.1, 23.4, 25.6, 29.5, 29.8, 29.9, 30.0, 30.1, 30.2, 30.3, 30.4,
32.4, 35.5, 64.0, 123.1, 125.3, 126.85, 128.6, 138.8, 149.3, 152.6, 152.7,
157.8, 157.9, 158.0, 173.9. MS(FAB) m/z 1373.1 [M + Na]+, 1205.2
[M - PF6]+, 1030.3 [M - 2PF6]+.
Experimental Section
General Synthetic Methods. All chemicals were purchased from
Aldrich and were used as received. Solvents were dried according to
literature procedures.39 Analytical thin-layer chromatography (TLC) was
carried out on aluminum sheets coated with silica gel (Merck 5554).
Column chromatography was performed on silica gel 60 (Merck 40-
60 nm, 230-400 mesh). Melting points (mp’s) were determined on an
1
Electrothermal 9200 apparatus. H NMR spectra were recorded on a
Bruker AC-300 at 300 MHz. 13C NMR Spectra were recorded on a
Bruker AC-300 at 75.5 MHz using broad band decoupling at 25 °C.
The chemical shift values are expressed as δ values, and the coupling
constant values (J) are in hertz. The following abbreviations are used
for the signal multiplicities or characteristics: s, singlet; d, doublet; t,
triplet; m, multiplet. Fast atom bombardment mass spectrometry
(FABMS) was performed on a ZAB-SE mass spectrometer, equipped
with a krypton primary atom beam, and utilizing a m-nitrobenzyl alcohol
matrix.
Syntheses. (2-Pyridacyl)pyridium Iodide (7‚I). Iodine (40.7 g, 0.16
mol) in warm pyridine (140 mL) was added under nitrogen to a solution
of 2-acetylpyridine (60 mL). The reaction mixture was stirred at 80 °C
for 4 h. After cooling, the precipitated solid was collected by filtration
Component 2-H‚3PF6. A solution of N-benzyl-4-pyridium-4′-
(39) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals;
Pergamon: Oxford, 1989.
pyridine21 (12‚PF6) (350 mg, 0.89 mmol) and 4,4′-bis(chloromethyl)-
9
12794 J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002

Photoinduced Electron Transfer
A R T I C L E S
1,1′-biphenyl (1.12 g, 4.5 mmol) in MeCN (20 mL) was heated under
reflux for 16 h. After cooling, the precipitated solid was collected by
filtration and washed with CH2Cl2. After counterion exchange (NH4PF6/
H2O/Me2CO), the product and benzylamine (95 mg, 0.89 mmol) were
dissolved in MeCN (20 mL) and heated under reflux. The solvent was
removed, and the residue was purified by column chromatography
(SiO2, MeOH/2 M NH4Claq/MeNO2, 7:2:1) to afford component 2-H‚
3PF6 (258 mg, 30%) after counterion exchange (NH4PF6/H2O). 1H NMR
(500 MHz, CD3CN) δ 4.31 (s, 2H), 4.75 (s, 2H), 5.85 (s, 2H), 5.89 (s,
2H), 7.51-7.82 (m, 18H), 8.39 (d, J ) 6.8 Hz, 2H), 8.41 (d, J ) 6.8
Hz, 2H), 9.00 (d, J ) 6.8 Hz, 2H), 9.03 (d, J ) 6.8 Hz, 2H); 13C NMR
(125 MHz, CDCl3) δ 45.6, 51.3, 64.2, 64.6, 127.3, 127.4, 127.4, 127.9,
127.9, 128.9, 129.2, 129.3, 129.5, 129.8, 129.9, 130.0, 130.1, 130.8,
131.7, 132.4, 137.7, 139.5, 141.7, 145.4, 150.2, 150.3. MS(FAB) m/z
824.7 [M - PF6]+, 678.3 [M - H-2PF6]+.
Photophysical Experiments. All the measurements were performed
at room temperature in air-equilibrated CH2Cl2/MeCN (98:2, v/v)
solutions. PF6- ions were the counterions in the case of all the cationic
compounds. UV-vis absorption spectra were recorded with a Perkin-
Elmer λ6 spectrophotometer. Uncorrected luminescence spectra were
obtained with a Perkin-Elmer LS-50 spectrofluorimeter equipped with
a Hamamatsu R928 phototube. Luminescence lifetimes were measured
by time-correlated single-photon counting with Edinburgh Instruments
DS199 equipment. The exciting light (λ ) 320 nm) 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 (λ ) 565 nm). The detector was a cooled Hamamatsu R928
photomultiplier. The estimated experimental errors are 2 nm on band
maxima and (5% on the molar absorption coefficients, fluorescence
intensity, and fluorescence lifetime values.
Autolab 30 multipurpose instrument interfaced to a personal computer.
The working electrode was a glassy carbon electrode (0.08 cm2, Amel);
its surface was routinely polished with a 0.05 mm alumina-water slurry
on a felt surface, immediately prior to use. In all cases, the counter
electrode was a Pt spiral, separated from the bulk solution with a fine
glass frit, and an Ag wire was used as a quasireference electrode.
Ferrocene was present as an internal standard. The concentrations of
the compounds were of the order of 5 × 10-4 M; the experiments were
carried out in the presence of 5 × 10-2 M TBAPF6. Cyclic voltam-
mograms were obtained with sweep rates in the range 0.05-0.5 V s-1
;
DPV experiments were performed with a scan rate of 20 or 4 mV s-1
,
a pulse height of 75 or 10 mV, and a duration of 40 ms. The reversibility
of the observed processes was established by using the criteria of (i)
separation of 60 mV between cathodic and anodic peaks, (ii) the close-
to-unity ratio of the intensities of the cathodic and anodic currents,
and (iii) the constancy of the peak potential on changing sweep rate in
the cyclic voltammograms. The same half-wave potential values were
obtained from the DPV peaks and from an average of the cathodic and
anodic CV peaks, as expected for reversible processes. The potential
values are in volts versus SCE; the experimental error was estimated
to be (10 mV.
Acknowledgment. Financial support from the EU (HPRN-
CT-2000-00029), the University of Bologna (Funds for Selected
Research Topics), MURST (Supramolecular Devices Project),
CNR, and the University of California at Los Angeles is
gratefully acknowledged. We thank the Ministerio de Educacio´n
y Ciencia (Spain) and the European Community (Marie Curie
Fellowship, Contract HPMF-CT-2000-00437) for postdoctoral
fellowships to M.C.-L.
Electrochemical Experiments. Cyclic voltammetric (CV) and
differential pulse voltammetric (DPV) experiments were carried out in
argon-purged CH2Cl2/MeCN (90:10, v/v) at room temperature with an
JA025813X
9
J. AM. CHEM. SOC. VOL. 124, NO. 43, 2002 12795