system. The electrochemical and optical gaps, deduced from
the CV and UV-Vis spectra, strongly suggest differences of
magnitude in conductivity between the on (dialkoxybenzenes I)
and off (quinone bisketals II) states at low voltage. Although
the proposed off state II is electrochemically irreversible, it can
be changed again to the on state using some known chemical or
physical processes.18
´
We thank Junta de Andalucıa (Projects P06-FQM-01726 and
Fig. 3 Comparison between the UV-vis spectra of off structures 4a, 9a
and 10a (0.00001 M in CH2Cl2).
P09-FQM-04571), MICINN (Grants CTQ2008-4091,
CTQ2007-60494/BQU) and Comunidad de Madrid (Grant
SOLGEMAC-S2009/ENE-1617) for financial support. The
project CONSOLIDER INGENIO-2010 (CSD2006-00015)
provided X-ray structural facilities for this work. We
gratefully acknowledge the BM16 Spanish beamline at ESRF
(Grenoble, France) for access to synchrotron radiation.
DChL, NF and LAdC thank CSD2006-00015, Junta de
The phenyl alkyndiyl substituted dimethoxybenzene
derivative 2 showed a cyclic voltammogram where the first
reversible peak appeared at 0.91 V. We also observed two
irreversible peaks at 1.12 and 1.32 V which must be related also
with the on/off voltage. Similar anodic peaks (0.91 rev., 1.10
irrev. and 1.30 irrev.) were obtained in the cyclic
voltammogram of compound 4. The corresponding off states
2a and 4a showed a similar behavior to 1a and 3a. Both were
electrochemically inert from À2.2 to + 1.7 V vs. Fc/Fc+. The
electrochemical behaviour of compounds 5–10 possessing
substituents showed the same trend.11
´
Andalucıa and UGR, respectively, for research contracts.
Notes and references
1 C. Joachim and M. A. Ratner, Proc. Natl. Acad. Sci. U. S. A., 2005,
102, 8801–8808.
2 P. Faller, A. Pascal and A. W. Rutherford, Biochemistry, 2001, 40,
6431–6440.
In the standard theory of quantum transport through
molecules, the conductivity at low bias is related to the
HOMO–LUMO energy gap, being maximized with small
values of such gap. Consequently, the measure of the
HOMO–LUMO difference of energy could help to predict
the different ability of our systems to transfer the electrons
through the reduced and oxidized molecules I and II depicted
in Scheme 1. The HOMO–LUMO energy gap could be
deduced from the CV studies considering the difference
between the first oxidation and reduction potentials, or from
the UV-vis absorption wavelengths. These two values were
similar, showing an increase in the HOMO–LUMO gap of
about 1.2 eV for tri-p-phenylene-type structures (1a, 3a, 5a–8a)
and 0.85 eV from expanded alkyndiyl tri-p-phenylene-type
structures (2a, 4a, 9a and 10a) with respect to the
corresponding on structures (1–10).11 These electrochemical
and optical gap values and our previous theoretical
calculations17 support our hypothesis that structures 1–10
have conductivity values higher than 1a–10a. This fact is also
supported by the comparison between UV-vis spectra of off
structures 2a, 9a and 10a (Fig. 3). It can be observed that the
UV-vis spectrum of asymmetric 10a is, within the experimental
error, a simple addition of the UV-spectra of symmetric 2a and
9a. This fact strongly suggests that an electronic disconnection
between the two ends of the molecule is taking place owing to
the expected insulating character of the quinone bisketal moiety.
In conclusion, we have synthesized a series of dialkoxybenzene/
quinone bisketals, which mimic some properties of a macroscopic
fuse. The key synthetic processes used warrant a trouble-free
incorporation of these pairs in more elaborated structures for
future applications in molecular circuitry. The X-ray structures
of different pairs showed that no significant geometrical
changes occurred from the reduced to the oxidized forms,
which, to our knowledge, has no precedent in the literature.
CV studies also determined that quinone bisketals show
differences between oxidation and reduction peaks/waves
that at least exceed 4 V, evidencing the irreversibility of the
3 K. Szacilowski, Chem. Rev., 2008, 108, 3481–3548.
4 K. Seo, A. V. Konchenko, J. Lee, G. S. Bang and H. Lee, J. Am.
Chem. Soc., 2008, 130, 2553–2959, references cited therein.
5 For a 2D memory circuit: E. Green, J. W. Choi, A. Bouka,
Y. Bunimovich, E. Johnston-Halprin, E. DeIonno, Y. Luo,
B. A. Sheriff, K. Xu, Y. S. Shin, H.-R. Tseng, J. F. Stoddart and
J. R. Heath, Nature, 2007, 445, 414–417.
6 M. C. Carreno, J. M. Cuerva, M. Ribagorda and
A. M. Echavarren, Angew. Chem., Int. Ed., 1999, 38, 1449–1452.
7 J. S. Swenton, Acc. Chem. Res., 1983, 16, 74–81.
8 Under low bias we presume that the off-resonant electron transport
(ET) mechanism predominates and no charge is developed in the
molecule. At higher bias, near the on-resonant regimen by the
HOMO level, vibronic couplings and/or hole-based ET begins to be
important with the consequent development of charged species,
which is required for our proposed switching event.
9 Electronic Materials: The Oligomer Approach, ed. K. Mullen and
G. Wegner, Wiley-VCH, New York, 1998.
¨
10 R. L. McCreery, Chem. Mater., 2004, 16, 4477–4496.
11 See ESIw for details.
12 A. C. Whalley, M. L. Steigerwald, X. Guo and C. Nuckolls, J. Am.
Chem. Soc., 2007, 129, 12590–12591.
13 Additionally, three key intermediates in the oxidation process have
been studied theoretically, showing again essentially planar
structures, which suggest that this geometric feature is retained
during all the switching process, including transition states. See
ESIw.
14 Crystal structure of compound 2 has been recently reported:
R. Thomas, S. Varghese and G. U. Kulkarni, J. Mater. Chem.,
2009, 19, 4401–4406.
15 All E values were measured vs. E1/2 of the Fc+/Fc reference couple.
16 Noteworthy, the irreversibility of the process in the presence of a
nucleophilic oxygen is related with the formation of a stable
quinone bisketal and not with a usual oxidative decomposition.
17 Conductivity of structures I at low bias, the desired working
voltage, is at least two orders of magnitude higher than through
structures II: N. Fuentes, A. Parra, E. Oltra, J. M. Cuerva,
S. Rodrı
´
Villanueva, J. E. Carceller, E. Bunuel and D. Ca
guez-Bolı
´
var, F. M. Go
´
mez-Campos, J. A. Lo
rdenas,
´
pez-
´
Proceedings of the 13th International Workshop on Computational
Electronics, 2009, 238–241.
18 See for example: (a) B. L. Chenard, M. J. Manning, P. W. Raynolds
and J. S. Swenton, J. Org. Chem., 1980, 45, 378–384;
(b) M. P. Capparelli and J. S. Swenton, J. Org. Chem., 1987, 52,
5360–5364; (c) T. M. Swager, M. M. Rock and R. H. Grubbs, New
Polym. Mater., 1990, 2, 1–10.
c
1588 Chem. Commun., 2011, 47, 1586–1588
This journal is The Royal Society of Chemistry 2011