Full text of DOI:10.1016/S0040-4039(00)01424-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8163–8166
The reversible complexation of a tetrathiafulavene
functionalised self-assembled monolayer by
cyclobis(paraquat-p-phenylene)
Graeme Cooke,a,* Florence M. A. Duclairoir,a Vincent M. Rotellob and
J. Fraser Stoddartc
aDepartment of Chemistry, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK
bDepartment of Chemistry, University of Massachusetts at Amherst, Amherst, MA 01002, USA
cDepartment of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Avenue,
Los Angeles, CA 90095, USA
Received 10 July 2000; revised 16 August 2000; accepted 23 August 2000
Abstract
The first example of redox controlled host–guest complexation of a tetrathiafulvalene functionalised
self-assembled monolayer with the electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenyl-
ene) is reported. © 2000 Elsevier Science Ltd. All rights reserved.
The study of electro-active self-assembled monolayers (SAM’s) which have the propensity of
both controlling and signalling the specific molecular recognition of a guest is a burgeoning field
of study.1 In particular, the development of systems utilising the tetrathiafulvalene (TTF, 1) unit
are especially promising due to the ability of this system to exist2 in three stable oxidation states
2+
(TTF0, TTF+ and TTF ). However, all of the systems reported to date have involved the use
’
of crown ether functionalised derivatives and the study of their subsequent metal cation
recognition properties. Remarkably, the recognition properties of immobilised TTF units for
charged and neutral organic molecules have not been reported. In recent solution-phase studies,
it was shown that the TTF0 redox state has the ability to form charge-transfer complexes with
the electron deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene) 2.3 Furthermore,
upon electrochemical oxidation of the TTF unit to its electron deficient states, a rapid and
reversible decomplexation occurs. In the present work, we show for the first time using cyclic
voltammetry (CV) that the reversible host–guest complexation can also occur at a solid/liquid
interface of a SAM between an immobilised TTF disulfide 3 and the tetracationic cyclophane 2.
* Corresponding author. Fax: 0131 451 3180; e-mail: G.Cooke@hw.ac.uk
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01424-6

8164
TTF disulfide 3 was synthesised in high yield from 4-hydroxymethyl-TTF4 and thioctic acid
using DCC/DMAP-catalysed esterification procedures.5 The solution electrochemistry of this
system gave rise to two reversible oxidations at E1/21=+0.40 V and E1/22=+0.76 V corresponding
2+
6
to the formation of 3+ and 3 , respectively (Fig. 1(a)). Upon the addition of a 10-fold excess
’
of 2 to the CV cell, the oxidation giving rise to the 3+ species is immediately displaced by 19 mV
’
to a more positive potential, whilst the oxidation due to the formation of 3(++) is unaffected (Fig.
1(b)). Although this shift is somewhat smaller than that reported for [1·2], the shift is considerably
greater than shifts attributed to experimental error commonly associated with CV measurements
( 3 mV).3c The complexation behaviour is consistent with previously reported CV data for [1·2]
and suggests that [2·3] reversibly decomplexes when the 3(0) guest experiences its first electrochem-
ical oxidation.3c Solution-phase complexation between 2 and 3 was further confirmed by UV-vis
and 1H NMR spectroscopic studies.3 Mixing 2 and 3 in equimolar proportions in MeCN resulted
in an immediate formation of an emerald green-coloured solution as a result of the appearance
1
of a charge-transfer absorption band centred at 843 nm. The H NMR spectrum of the
green-coloured complex [2·3] shows small, but significant, changes in chemical shifts relative to
1
those of non-complexed 2 and 3 (Table 1). The changes in chemical shifts of H NMR signals
for the bipyridinium unit of [2·3] are remarkably similar to that obtained for [1·2]; however, the
changes in the chemical shift for the TTF protons of the former are less pronounced.3c
Figure 1. Cyclic voltammograms (MeCN, 0.1 M Bu4NClO4, 298 K, scan rate 100 mV s−1) of compound 3 (a) and in
the presence of 2 (b)

8165
Table 1
The H NMR chemical shift data (l and Dl) for 2, 3 and [2·3] in CD3CN at ambient temperatures
1
Compound or complex
a-Bipy-CH
b-Bipy-CH
C6H4
7.52
+NCH2
5.74
TTFa
TTFb
2
8.86
8.16
3
[2·3]
Dl
6.52
6.43
(−0.09)
6.47
6.37
(−0.10)
8.94
(+0.08)
7.92
(−0.24)
7.68
(+0.16)
5.73
(−0.01)
a Refers to the proton attached to the functionalised 1,3-dithiole ring.
b Refers to the protons on the non-functionalised 1,3-dithiole ring. a and b are with respect to N in the cyclophane.
With solution complexation between 2 and 3 unequivocally verified, we turned our attention
to complexation at a solid/liquid interface of a SAM. Mixed monolayers were fabricated by
immersing a gold wire (99.9999%, length 50 mm, diameter 0.25 mm) into an equimolar mixture
of 3 (5 mmol) and 1-butanethiol (5 mmol) in MeCN for 12 hours. After washing the electrode
with THF and MeCN and allowing it to dry in air, the SAM modified electrode was placed into
a solution of 0.1 M Bu4NClO4–MeCN and their CV’s were recorded (Fig. 2(a)). In accordance
with previously reported work using a thioctic acid anchoring unit, the resultant SAM’s were
remarkably stable to repeated electrochemical oxidation/reduction cycles.2d Indeed, repeated
electrochemical cycling of several days resulted in negligible change in peak currents. The CV’s
following the addition of an excess of 2 to the CV cell directly mirrored the solution-phase
studies, as the first oxidation process was immediately anodically shifted by 19 mV, whilst the
second oxidation process remained unaffected (Fig. 2(a)). The displacement of the first oxidation
process of immobilised 3 is consistent with donor–acceptor interactions occurring between this
Figure 2. Cyclic voltammograms (MeCN, 0.1 M Bu4NClO4, 298 K, scan rate 500 mV s−1) of SAM’s of 3 (a) and in
the presence of 2 (b)

8166
unit and 2. This result mirrors the solution-phase studies and shows that one-electron oxidation
causes very fast (compared with the scan rate used, 100–1000 mV s−1) reversible dethreading of
the complex, and successive one-electron reduction causes a very fast re-formation.7 The
remarkably similar changes in the electrochemical properties of SAM’s of 3, following complex-
ation with 2, clearly illustrates that electrochemically controlled host–guest complexation, which
we have confirmed in solution for 2 and 3, can also be observed in the solid/liquid interface of
a SAM.
In summary, we have shown that we can exploit the multi-stage redox properties of SAM’s
of 3 to produce electrochemically controlled host–guest complexes with the tetracationic
cyclophane 2. Further work is underway in our group to maximise the interaction between the
immobilised TTF unit and 2, and exploit the tuneability of TTF moieties oxidation potentials
to create addressable SAM’s.
Acknowledgements
G.C. gratefully acknowledges the EPSRC and the Royal Society of Chemistry for supporting
this work. We thank Dr. Gunter Mattersteig for providing a sample of 2.
References
1. Goldenberg, L. M.; Bryce, M. R.; Petty, M. C. J. Mater. Chem. 1999, 9, 1957.
2. (a) Hansen, T. K.; Jorgensen, T.; Stein, P. C.; Becher, J. J. Org. Chem. 1992, 57, 6403; (b) Yip, C.; Ward, M. D.
Langmuir 1994, 10, 549; (c) Moore, A. J.; Goldenberg, L. M.; Bryce, M. R.; Petty, M. C.; Monkman, A. P.;
Morenco, C.; Yarwood, J.; Joyce, M. J.; Port, S. N. Adv. Mater. 1998, 10, 395. (d) Liu, H.; Liu, S.; Echegoyen,
L. Chem. Commun. 1999, 1493. (e) Fibbioli, M.; Bandyopadhyay, K.; Liu, S.-G.; Echegoyen, L.; Enger, O.;
Diederich, F.; Buhlmann, P.; Pretsch, E. Chem. Commun. 2000, 339. (f) Liu, S.-G.; Liu, H.; Bandyopadhyay, K.;
Gao, Z.; Echegoyen, L. J. Org. Chem. 2000, 65, 3292.
3. (a) Philp, D.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams, D. J. Chem. Commun. 1991, 1584; (b)
Devonport, W.; Blower, M. A.; Bryce, M. R.; Goldenberg, L. M. J. Org. Chem. 1997, 62, 885. (c). Ashton, P. R;
Balzani, V.; Becher, J.; Credi, A.; Fyfe, M. C. T.; Mattersteig, G.; Menzer, S.; Nielsen, M. B.; Raymo, F. M.;
Stoddard, J. F.; Venturi, M.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 3951.
4. Garin, J.; Orduna, J.; Uruiel, S.; Moore, A. J.; Bryce, M. R.; Wegner, S.; Yufit, D. S.; Howard, J. A. K. Synthesis
1994, 489.
5. Selected data for 3: isolated as a yellow wax in 86% yield. lH (CD3CN/TMS) 6.51 (s, 1H), 6.47 (s, 2H), 4.83 (s,
2H), 3.56 (m, 1H), 3.16 (m, 2H), 2.42 (m, 1H), 2.35 (t, 2H), 1.94 (m, 1H), 1.63 (m, 4H), 1.46 (m, 2H); MS (EI)
m/z=422; exact mass calc. for C15H18S6O2: 421.9631. Found: 421.9627.
6. All electrochemical experiments were performed using a BAS-100W system. Electrolyte solutions were prepared
from recrystallised Bu4NClO4 using spectroscopic grade MeCN and purged with nitrogen prior to use. A three
electrode configuration was used with an Ag/AgCl reference electrode and a platinum wire as the counter
electrode.
7. Asakawa, M.; Ashton, P. R.; Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.; Montalti, M.; Spencer, N.;
Stoddart, J. F.; Venturi, M. Chem. Eur. J. 1997, 3, 1992.
.