Communications
DOI: 10.1002/anie.200705313
Energy Conversion
The Direct Conversion of Light into Continuous Mechanical Energy by
Photoreversible Self-Assembly: A Prototype of a Light-Powered
Engine**
Stefano Masiero,* Stefano Lena, Silvia Pieraccini, and Gian Piero Spada
There is general agreement on the need for alternative
sources of energy owing to the continuous growth in the
consumption and the depletion of fossil fuel reserves.[1] In this
context the production of clean energy from renewable
sources is particularly attractive. Several forms of renewable
energy are available, and most of them originate more or less
directly from the sun.[2,3] Solar energy conversion systems fall
into three categories[4] according to their primary product:
solar electricity, solar fuels, and solar thermal systems. The
challenge is to improve these technologies or to try and devise
new ones with the goal of converting as much of this energy as
possible. Herein, we describe a new strategy for the contin-
uous production of clean energy in the form of mechanical
work from light. The method relies on the photoreversible
isomerization of an azobenzene (diphenyldiazene) derivative.
Several classes of compounds that can undergo photo-
reversible transformations are known in the literature,[5] and,
among these, azobenzene-based systems are possibly the most
widely studied.[6,7] Azobenzene can be photoisomerized
reversibly from the more stable E to the Z isomer. When
this happens, the molecule is modified in several ways (e.g.
size, shape, electronic properties, polarity), and this has been
exploited to control material properties.[7] One intriguing
aspect of this photoreversible isomerization is that the
chemical system depends on several factors besides temper-
ature, and in particular, it depends on the number of particles
which constitute it. Thus, if the photoisomerization can be
coupled to a change in system concentration, this could pave
the way to the conversion of the stored energy into an
exploitable form (e.g. mechanical work). If a continuous
conversion of energy is sought, this concentration change
must be reversible, like photoisomerization itself. In addition,
the variation of the number of particles does not necessarily
imply that matter enters or leaves the system: the system just
has to be fooled to believe it. A supramolecular approach is
best suited to implement these qualities into an azobenzene-
based molecular structure.
The first step was therefore to design an azobenzene
derivative with a built-in capability of undergoing an aggre-
gation change as a consequence of photoisomerization. The
idea which has been developed is that of an azobenzene
derivative able to form polymeric supramolecular aggre-
gates[8] when in the E form but unable to self-assemble when
in the Z form.
It is known that in (Z)-azobenzene the two phenyl rings
=
are roughly facing each other: the N N-C-C dihedral angle is
538 according to X-ray diffraction experiments.[9] Inspection
of models indicates that, upon the introduction of two
2’-carboxyphenyl residues (thin line in Figure 1) in the
4-positions of diphenyldiazene (thick line in Figure 1), the
distances and geometries might be conducive for the forma-
tion of two intramolecular hydrogen bonds between the two
carboxylic groups, as long as the Z isomer adopts the
conformation in which the two groups point towards each
other (Figure 1, structure B). In order to populate preferen-
Z isomer has an energy content higher (about 50 kJmolÀ1
)
than the E form, meaning that, during photoisomerization,
some energy is “harvested” from light and “stored” in the
Z isomer in the form of a potential. Usually this energy is
dissipated thermally during the back-isomerization to the
E form, and the thermal effects of this process are so weak
that they are useless for practical purposes. With the aim of
finding a way to collect this energy and transform it into a
useful form, one can reason that the energy content of a
[*] Prof. S. Masiero, Dr. S. Lena, Dr. S. Pieraccini, Prof. G. P. Spada
Alma Mater Studiorum-Università di Bologna
Dipartimento di Chimica Organica “A. Mangini”
Via San Giacomo 11, 40126 Bologna (Italy)
Fax: (+39)051-2099-5688
E-mail: stefano.masiero@unibo.it
[**] We thank Profs. G. Gottarelli andC. Zannoni (Bologna) for helpful
discussions and suggestions. Thanks are also due to Dr. M. A.
Cremonini andProf. G. Placucci for their kindhospitality at the
NMR facility, Campus di Scienze degli Alimenti (Forlì-Cesena).
Technical assistance by G. Gnudi (F.A.V.S. Scientific Glassworks,
Bologna) is gratefully acknowledged. Financial support was pro-
vided by MiUR (PRIN Projects) and the University of Bologna.
Supporting information for this article is available on the WWW
Figure 1.
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 3184 –3187