Angewandte
Chemie
DOI: 10.1002/anie.200903301
Tautomeric Switches
Exploiting Tautomerism for Switching and Signaling**
Liudmil Antonov,* Vera Deneva, Svilen Simeonov, Vanya Kurteva, Daniela Nedeltcheva, and
Jakob Wirz
Herein, we demonstrate a conceptual idea for a tautomeric
switch based on implementation of a flexible piperidine unit
in 4-(phenyldiazenyl)naphthalen-1-ol. The results show that a
directed shift in the position of the tautomeric equilibrium
can be achieved through protonation/deprotonation in a
number of solvents. The developed molecular switch, in spite
of the simple host–guest system, has shown acceptable
complexation ability towards small alkali- and alkaline-
earth-metal ions and can be a promising basis for further
development of effective molecular sensors through imple-
mentation of azacrown ethers.
Scheme 1. Molecular switch based on molecular recognition (left) and
Organic molecular materials are increasingly recognized
as suitable molecular-level elements (such as switching,
signaling, and memory elements[1]) for molecular devices,
because the wide range of molecular characteristics can be
combined with the versatility of synthetic chemistry to alter
and optimize molecular structure in the direction of desired
properties. Virtually every molecule changes its behavior
when acted upon by external fields or other stimuli. True
molecular switches undergo reversible structural changes,
caused by a number of influences, which give a variety of
possibilities for control. Several classes of photoresponsive
molecular switches are already known; these operate through
processes such as bond formation and bond breaking, cis–
trans isomerization, and photoinduced electron transfer upon
complexation.[2] A conceptual scheme of a molecular switch
based on molecular recognition is shown in Scheme 1. The
host–guest system represents, for instance, a crown ether that
can bind ions or a cyclodextrin that can bind other small
molecules. It is bound to a signal converter. The complexation
behavior is monitored by the state of the signal converter, and
in turn its optical or electronic properties are determined by
the complexation state of the host–guest system.
conceptual idea for a tautomerism-based molecular switch (right).
The main requirement in the design of new molecular
switches is to provide fast and clean interconversion between
structurally different molecular states (on and off). Tauto-
merism could be a possibility, because change in the
tautomeric state can be accomplished by a fast proton
transfer reaction between two or more structures, each of
them with clear and different molecular properties.[3] There-
fore, our aim herein is to show how tautomerism can be
exploited for signal conversion. The conceptual idea of such a
device is presented in Scheme 1. In this structure, a change in
tautomeric state, labeled A and B, is linked to changes in the
complexation abilities of the host–guest system by modulating
the propensity of the system to hydrogen bond to the antenna.
At the same time, engagement of this antenna causes a change
in the tautomeric state. The sensitivity of the electronic
ground and excited states of the tautomeric forms to environ-
ment stimuli (light, pH value, temperature, solvent) and to
the presence of a variety of substituents or to hydrogen
bonding can be exploited in the design of flexible tools for
control.
Obviously, such a device should be based on a tautomeric
structure with easy proton exchange between the tautomers,
which means that they must coexist in solution. At the same
time, a main feature of systems of tautomers coexisting in
solution is that the overall optical response is a mixture of the
optical responses of the individual tautomers. Consequently,
in the design of tautomeric switches, conditions for obtaining
pure end tautomer in the corresponding off and on states must
be provided.
Herein we report the properties of two tautomeric
switches, namely 3 and 4 (Scheme 2), based on 4-(phenyl-
diazenyl)phenol (1) and 4-(phenyldiazenyl)naphthalen-1-ol
(2).
[*] Prof. Dr. L. Antonov, V. Deneva, S. Simeonov, Dr. V. Kurteva,
D. Nedeltcheva
Institute of Organic Chemistry with Centre of Phytochemistry
Bulgarian Academy of Sciences
Acad. G.Bonchev str., bl.9, 1113 Sofia (Bulgaria)
Fax: (+359)2-8700-225
E-mail: lantonov@orgchm.bas.bg
Prof. Dr. J. Wirz
Departement Chemie, University of Basel
Klingelbergstrasse 80, 4056 Basel (Switzerland)
[**] The work was supported by The Bulgarian Science Found (projects
TK-X-1716 and UNA-17/2005) and The Swiss National Science
Foundation (JRP IB7320-110961/1).
The parent compound 2 is the first dye that was shown to
tautomerize by Zincke and Bindewald in 1884.[4] It has been
the object of many spectral and theoretical studies[5] because
its tautomeric forms coexist in solution and the equilibrium
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2009, 48, 7875 –7878
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7875