Photomechanical actuation and manipulation of the electronic properties of linear π-conjugated systems

Article abstract of DOI:10.1021/ja029754z

A photodynamic molecular architecture has been synthesized by covalent fixation of a photoisomerizable azobenzene group at two fixed points of a conformationally flexible π-conjugated quaterthiophene chain. The crystallographic structure shows that the tw

Full text of DOI:10.1021/ja029754z

Published on Web 02/13/2003
Photomechanical Actuation and Manipulation of the Electronic Properties of
Linear π-Conjugated Systems
†
,†
‡
‡
‡
Bruno Jousselme, Philippe Blanchard,* Nuria Gallego-Planas, Jacques Delaunay, Magali Allain,
§
,†
Pascal Richomme, Eric Levillain, and Jean Roncali*
Groupe Syst e` mes Conjugu e´ s Lin e´ aires, IMMO, UMR CNRS 6501, and SerVice Commun d’Analyses
Spectroscopiques, UniVersit e´ d’Angers, 2 Bd LaVoisier, 49045 Angers, France
Received December 16, 2002 ; E-mail: Jean.Roncali@univ-angers.fr
Dynamic molecular devices capable of converting thermal,
chemical, or photochemical energy into motion are a focus of
considerable current interest, and a wide range of systems producing
longitudinal or rotational motion have recently been reported.1
Although bulk electrochemical actuators based on volume changes
associated with the redox process of conjugated polymers have been
2
extensively investigated, attempts to synthesize molecular π-con-
jugated dynamic structures remain scarce. A first step in that
direction was recently reported by Marsella et al., who investigated
tetra[2,3-thienylene] as a basic unit for molecular actuation.3
We now report that the covalent fixation of a photostimulable
group on two fixed points of an oligothiophene chain allows one
to produce conformational changes and thus reversible modifications
of the electronic properties of the π-conjugated system.
As the photoactive driving group, we used the azobenzene
chromophore which can be reversibly switched between an extended
trans and a shorter cis configuration.4
Figure 1. Optimized geometries for compound 1. Top, 1-SAS-t; bottom,
1-ASA-c.
The target compound 1 was synthesized by reacting the depro-
5
tected thiolate groups of bis-cyanoethylsulfanyl quaterthiophene
with bis-p-bromomethylazobenzene.6
The structure of 1 was modelized by theoretical calculations
7
based on the density functional method. Becke’s three parameter
Figure 2. Crystallographic structure of 1 (hydrogens omitted).
gradient corrected functional (B3lyp) with a polarized 6-31G* basis
for all atoms was used to optimize the geometry and to compute
the electronic structure at the minima found. The geometries of
the two conformations of 1 and that of models of its two constitutive
parts, that is, 3,3′′′-dimethylsulfanyl-quaterthiophene (Me4T) and
para-dimethyl-azobenzene (MeAz), were analyzed independently.
Examination of the various minimal energy conformations of Me4T
shows that the distance (d) between the two sulfur atoms serving
as anchoring points for the azo group decreases from 12.1 Å for
the syn-anti-syn conformation (SAS) to 10.8 (AAA), 10.7 (SSA),
ASA conformational transition of the 4T chain which reduces d to
7.5 Å. Calculations show that the trans-azo-SAS form (1-SAS-t)
is more stable than its isomer containing cis-azobenzene (1-ASA-
c) by 52.6 kJ/mol, while the SAS to ASA transition raises the
HOMO level from -5.10 to -5.02 eV and decreases the HOMO-
LUMO gap from 3.24 to 3.03 eV.8
Figure 2 shows the X-ray structure of a single crystal of 1. The
4T and azo systems are quasi-planar and parallel, the 4T chain
presents the expected SAS conformation, and the 12.3 Å d value is
in excellent agreement with the theoretically predicted one (12.1 Å).
1
0.3 (AAS), and 7.5 Å (ASA). Comparison of these data with the
distances between the two methyl groups of trans and cis MeAz
12.1 and 8.6 Å) shows that the SAS and ASA conformations are
1
(
Figure 3 shows the aromatic region of the H NMR spectrum of
the best fit for the trans and cis azo group, respectively.
The optimized geometry of 1 shows that with the trans azo group,
the 4T chain adopts a SAS conformation with d ) 12.1 Å (Figure
compound 1 in CDCl before and after irradiation with 360 nm
monochromatic light. The initial spectrum exhibits two anoma-
lously shielded signals: a broadened singlet at 6.79 ppm assigned
3
9
1). Trans to cis isomerization of the azo group induces a SAS to
to the four benzenic protons at the ortho positions of the methylene
group of azobenzene (H
to the two hydrogens at the 4′ and 3′′ positions of the median
thiophenes (H ).
1
) and a doublet at 6.58 ppm corresponding
†
Groupe Syst e` mes Conjugu e´ s Lin e´ aires, IMMO.
‡
IMMO.
§
Service Commun d’Analyses Spectroscopiques.
d
2888
9
J. AM. CHEM. SOC. 2003, 125, 2888-2889
10.1021/ja029754z CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
1
Figure 3. Aromatic region of the H NMR spectrum of compound 1 (9 ×
-
4
1
0
M) in CDCl3. Before (top) and after 5 h of irradiation at 360 nm
-
4
Figure 4. Cyclic voltammogram of 1 (5 × 10 M in 0.1 M Bu4NPF6/
(bottom).
-
1
CH2Cl2; 100 mV s ). Before irradiation (dashed line) and after 2 h of
irradiation at 360 nm (solid line).
The unusual upfield chemical shifts of these hydrogens are due
to their location in the shielding cone of their respective facing
thiophenic or benzenic system. The spectrum recorded after
irradiation at 360 nm shows new signals associated with the new
parameters for 1 (PDF and CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
geometry of the molecule. The chemical shifts of H
1
and H
d
protons
References
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(
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(
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(
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(
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(
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0
The CV of 1 shows a reversible oxidation wave (E ) 0.94 V)
corresponding to the generation of the 4T cation radical (Figure 4).10
Irradiation at 360 nm produces a decrease of the intensity of this
wave with emergence of a new redox system at lower potential
0
(
E ) 0.78 V). These changes are fully reversible, and the initial
CV is restored by irradiation at 480 nm. These results show in
agreement with theoretical results that the photoinduced SAS to
ASA transition of the 4T chain significantly increases the HOMO
level, thus confirming that photomechanical control of the electronic
properties of the conjugated system has been achieved.
(8) This gap value corresponds to the HOMO - LUMO + 1 gap because
the LUMO is essentially localized on the azobenzene group.
(9) Irradiation was performed with a 150 W xenon lamp using band-pass
filters.
Supporting Information Available: Synthetic procedure and
characterization of compound 1, crystallographic data, tables of bond
distances and angles, positional parameters, and general displacement
(10) A control experiment on p-dimethylazobenzene shows that the azo group
is electrochemically inert up to +1.70 V versus Ag/AgCl.
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