Structural control of the electronic properties of photodynamic azobenzene-derivatized π-conjugated oligothiophenes

Article abstract of DOI:10.1021/jp060254s

Quaterthiophenes containing a median 3,3′-dimethoxybithiophene (4MTZ) or bis(3,4-ethylenedioxythiophene) (4ETZ) block and an azobenzene group attached at two internal β-positions of the end thiophene rings have been synthesized. The analysis of the crystal structure of the two compounds by X-ray diffraction shows that, for 4MTZ, the quaterthiophene chain adopts a syn - anti - syn conformation similar to the parent system based on unsubstituted quaterthiophene. In contrast, 4ETZ presents an all-anti conformation stabilized by noncovalent intramolecular interactions between oxygen atoms and thiophenic sulfur atoms. The effect of the photoisomerization of the azobenzene group on the electronic properties of the attached π-conjugated quaterthiophene chain has been analyzed by UV-vis spectroscopy and cyclic voltammetry. The results obtained for 4MTZ suggest very limited geometrical changes due to the restricted rotation around the central interannular single bond caused by steric interactions between the methoxy groups. In contrast, upon trans to cis photoisomerization of the attached azobenzene group of 4ETZ, the quaterthiophene chain undergoes a reversible conformational switch to a final state with a lower HOMO level and a larger HOMO-LUMO gap than the initial state.

Full text of DOI:10.1021/jp060254s

3488
J. Phys. Chem. A 2006, 110, 3488-3494
Structural Control of the Electronic Properties of Photodynamic Azobenzene-Derivatized
π-Conjugated Oligothiophenes
Bruno Jousselme,† Philippe Blanchard,† Magali Allain,‡ Eric Levillain,† Maryle`ne Dias,† and
Jean Roncali†*
Groupe Syste`mes Conjugue´s Line´aires, CIMMA, UMR CNRS 6200, UniVersite´ d'Angers, 2 Bd LaVoisier, 49045
Angers, France, and CIMMA, UMR CNRS 620, UniVersite´ d'Angers, 2 Bd LaVoisier, 49045 Angers, France
ReceiVed: January 13, 2006
Quaterthiophenes containing a median 3,3′-dimethoxybithiophene (4MTZ) or bis(3,4-ethylenedioxythiophene)
(4ETZ) block and an azobenzene group attached at two internal â-positions of the end thiophene rings have
been synthesized. The analysis of the crystal structure of the two compounds by X-ray diffraction shows that,
for 4MTZ, the quaterthiophene chain adopts a syn-anti-syn conformation similar to the parent system based
on unsubstituted quaterthiophene. In contrast, 4ETZ presents an all-anti conformation stabilized by noncovalent
intramolecular interactions between oxygen atoms and thiophenic sulfur atoms. The effect of the photoi-
somerization of the azobenzene group on the electronic properties of the attached π-conjugated quaterthiophene
chain has been analyzed by UV-vis spectroscopy and cyclic voltammetry. The results obtained for 4MTZ
suggest very limited geometrical changes due to the restricted rotation around the central interannular single
bond caused by steric interactions between the methoxy groups. In contrast, upon trans to cis photoisomerization
of the attached azobenzene group of 4ETZ, the quaterthiophene chain undergoes a reversible conformational
switch to a final state with a lower HOMO level and a larger HOMO-LUMO gap than the initial state.
Introduction
while more recently, Kinbara, Aida and co-workers have
synthesized a photoresponsive molecular scissor based on a
combination of azobenzene and ferrocene units.10
Motion generation at the molecular level is a focus of
considerable current interest from the perspective of developing
nanoscale molecular actuators, machines, and motors.1-7 During
the past decade, various prototypes of dynamic molecular
architectures such gears2, shuttles,3 ratchets,4 motors,5 and
muscles6 capable of converting thermal, chemical, or photo-
chemical energy into molecular motion and, hence, mechanical
energy, have been reported. Initial prototypes of molecular
machines generating a repeatable and directionally controlled
motion were essentially powered by means of changes in the
oxidation state of a mobile part of the molecular architecture.
Thus, Stoddart and co-workers have developed molecular
shuttles in which modification of the oxidation state of a redox-
active system embedded in a rotaxane induces its translocation
along a linear axis.3a,7 Sauvage and co-workers have synthesized
molecular machines and muscles powered by chemically or
electrochemically induced changes of the redox state and, hence,
coordination sphere of metal complexes in rotaxanes containing
tetra- and pentacoordination sites.3b,c,6
Although the volume change associated with the redox
process of conjugated polymers has been used to develop bulk
electrochemical actuators,11 attempts to utilize π-conjugated
systems in dynamic molecular systems remain scarce. A first
step in this direction was reported by Marsella et al., who
investigated poly[cyclooctatetrathiophene] and tetra[2,3-thie-
nylene] as a basic unit for molecular actuation.12 Recently, we
have proposed a different concept in which stimulation of a
driving group covalently attached at two fixed points of an
oligothiophene chain generates reversible geometrical and
dimensional changes in the π-conjugated system.13 A specific
interest of this approach is that the oligothiophene chain
simultaneously constitutes the target of the generated motion
and an almost ideal optical and redox probe, allowing the real-
time monitoring of the chemo- or photomechanically produced
molecular motion by simple electrochemical and optical tech-
niques.
This concept was initially demonstrated for crown-annelated
oligothiophenes in which molecular motion was produced by
metal cation complexation.13a More recently, we have described
photochemically driven systems in which an azobenzene group
attached at two defined points of quarterthiophene (4TZ) and
sexithiophene (6TZ) chains served as a photoactuating governor
(Chart 1).13b,c The analysis of the photodynamic behavior of
these systems by 1H NMR, UV-vis spectroscopy, cyclic
voltammetry, and theoretical calculations has shown that trans
to cis photoisomerization of the azobenzene generates a con-
formational switch in the oligothiophene chain that results in
an increase of the HOMO level of the π-conjugated system.13b,c
We now report the synthesis and photodynamic behavior of
two new systems in which the median bithiophenic part of 4TZ
Among the various possible power sources, photons present
the advantages of direct and rapid access to the target active
site and absence of chemical reagent. Shinkai and co-workers
have synthesized a first generation of photoresponsive dynamic
molecular systems where photoisomerization of azobenzene was
used to modulate the affinity of macrocyclic crown ethers toward
metal cations.8 Photochemically powered molecular machines
undergoing circular motion have been developed by Ferringa
et al.5b,c Gaub et al. have investigated a polymeric system
capable of producing a single-molecule optomechanical cycle,9
* Corresponding author. E-mail: jean.roncali@univ-angers.fr.
† Groupe Syste`mes Conjugue´s Line´aires.
‡ CIMMA.
10.1021/jp060254s CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/18/2006

Azobenzene-Derivatized π-Conjugated Oligothiophenes
J. Phys. Chem. A, Vol. 110, No. 10, 2006 3489
SCHEME 1: Synthesis of 4MTZ and 4ETZa
CHART 1
has been replaced by 3,3′-dimethoxybithiophene (4MTZ) and
bis(3,4-ethylenedioxythiophene) (EDOT) (4ETZ). The crystal-
lographic structure of these compounds has been analyzed by
X-ray diffraction, and the photomechanically induced modifica-
tions of the electronic properties of the quaterthiophene (4T)
conjugated system have been investigated by UV-vis spec-
troscopy and cyclic voltammetry with reference to the model
compounds 4MT and 4ET.
a (i) 1. n-BuLi, THF, -78 °C. 2. Bu3SnCl, THF -78 °C. (ii) 3,
Pd(PPh3)4, Toluene ∆. (iii) 1. CsOH‚H2O, MeOH, DMF, 20 °C 20
min. 2. n-hexylbromide 20 °C 15 h. (iv) 1. CsOH‚H2O, MeOH, DMF,
20 °C 20 min. 2. 5, 20 °C high dilution.
Results and Discussion
Synthesis. The synthesis of the target compounds is depicted
in Scheme 1. Treatment of 3-methoxythiophene and EDOT with
n-butyllithium, followed by oxidative coupling with copper
chloride, gave 2,2′-bi(3-methoxythiophene) 1a and 2,2′-bi(3,4-
ethylenedioxythiophene) 1b14 in 34 and 66% yields, respec-
tively. Dilithiation of these two compounds and reaction with
tributyltin chloride afforded the bis(tributylstannyl) derivatives
2a and 2b. These compounds were immediately subjected to a
double Stille coupling reaction with 2-bromo-3-(2-cyanoethyl-
sulfanyl)thiophene 315 by using Pd(PPh3)4 as a catalyst to give
quaterthiophenes 4a and 4b with two protected thiolate groups
in 49 and 46% yields, respectively. Thiolate deprotection by
cesium hydroxide, followed by reaction with n-bromohexane,
gave the dihexylsulfanyl-substituted quaterthiophenes 4MT and
4ET in 53 and 60% yields, respectively. The two target
compounds 4MTZ and 4ETZ have been prepared in one pot
by deprotection of the thiolate groups of 4a and 4b by cesium
hydroxyde and ring closure by reaction with bis-p-bromom-
ethylazobenzene 516 under high dilution conditions, according
to the already-reported procedure.13
Figure 1. Crystallographic structure of 4MTZ.
non bonded S‚‚‚O distances S3-O1 ) 2.858(2) Å and S2-O2
) 2.830(2) Å are significantly shorter than the sum of the van
der Waals radii of sulfur and oxygen (3.35 Å). These results
show that the planar anti conformation of the central dimethoxy-
bithiophene core is stabilized by noncovalent intramolecular
interactions. The 12.385(1) Å distance found between the two
sulfur atoms serving as anchoring points for the azobenzene
group (S5-S6) agrees well with the 12.1 Å distance obtained
by geometrical optimization of 3,3′′′-dimethylsulfanyl quaterthio-
phene.13a
The crystal structure of 4ETZ contains two independent
molecules that are diastereoisomers (Figure 2). In fact, the last
step in the assembling of the molecule leads to two pairs of
enantiomers. As shown in Scheme 2, formation of enantiomers
is a consequence of the direction of the nucleophilic attack of
the dithiolate either above or below the azobenzene core. Then,
the spatial arrangement of the free lone pairs of the nitrogen
atoms results in the formation of diastereoisomers.
Crystallographic Analysis. Figure 1 shows the crystal-
lographic structure of a single crystal of 4MTZ. The 4T and
azobenzene chains lie in two quasiparallel planes, and the 4T
chain adopts an syn-anti-syn (SAS) conformation similarly
to the parent system based on a nonsubstituted 4T chain
(4TZ).13c Examination of crystallographic data shows that the

3490 J. Phys. Chem. A, Vol. 110, No. 10, 2006
Jousselme et al.
Figure 2. Crystallographic structure of the two stereoisomers of 4ETZ.
Figure 3. Molecular packing of the two stereoisomers of 4ETZ in
the crystal. Molecule 1 in green and molecule 2 in pink.
Although it is likely that the same process can occur with
the other azobenzene-derivatized oligothiophenes, such as 4TZ,
6TZ, and 4MTZ, until now, evidence for the formation of
stereoisomers is lacking. In fact, such isomers cannot by
distinguished by 1H NMR, while only one isomer was observed
in the crystallographic structure.
The molecular packing of 4ETZ shows that columns of
stereoisomers 1 and 2 alternate along the c axis (Figure 3). In
sharp contrast with 4MTZ and the other azobenzene-derivatized
oligothiophenes that present a syn-anti-syn conformation
(SAS),13b,c 4ETZ adopts a fully anti conformation (AAA)
(Figure 2). As for 4MTZ, the nonbonded S‚‚‚O distances, S1-
O1 ) 2.78(4) Å, S2-O3 ) 2.94(2) Å, S3-O2 ) 2.89(2) Å,
S4-O4 ) 2.81(5) Å, are much shorter than the sum of the van
der Waals radii of sulfur and oxygen (3.35 Å).
4ETZ is self-rigidified by intramolecular noncovalent interac-
tions between the oxygen atoms of EDOT and the sulfur atom
of the adjacent thiophene ring.17 A noticeable effect of this self-
rigidification of the 4T chain in a planar AAA conformation is
that the distance between the sulfur atoms of the two sulfide
groups serving as fixation sites for the azobenzene group
decreases from 12.38 Å for 4MTZ to ca. 10.90 Å for 4ETZ.
Again, this value agrees well the distance calculated for the
optimized AAA conformation of 3,3′′′-dimethylsulfanyl qua-
terthiophene (10.80 Å).13a
As a consequence of this shorter S‚‚‚S distance, the azoben-
zene group in 4ETZ is submitted to a stronger constraint than
in 4TZ and 4MTZ. Whereas, for these latter molecules, part
of the constraint is released through a forced rotation of
thiophene units, leading to a SAS conformation, for 4ETZ, the
These results show that, similarly to many conjugated systems
containing the EDOT unit, the planar AAA conformation of
SCHEME 2: Formation of Stereoisomers during the Synthesis of Azobenzene-Oligothiophene Cyclophanes

Azobenzene-Derivatized π-Conjugated Oligothiophenes
J. Phys. Chem. A, Vol. 110, No. 10, 2006 3491
Figure 4. Side view of 4MTZ (top) and 4ETZ (bottom) (hydrogen
atoms omitted for clarity).
TABLE 1: UV-Vis Spectroscopic Absorption Maxima (in
nm) for Compounds 4MTZ and 4ETZ and Their Respective
Reference Compounds 4MT and 4ET in Methylene Chloride
compd
4T chain
azobenzene
335, 439
DMeZ
4MT
436
4MTZ
4MTZa
4ET
4ETZ
4ETZa
4ETZa,b
450
446
411, 433, 460
421, 446, 474
416, 432, 460
410
353, 368
354, 369
350, 364
345, 368
320, 330
a After 5 h irradiation with monochromatic light at 360 nm. b Isolated
by HPLC.
Figure 5. Photomechanically induced changes in the UV-vis spectrum
of 4MTZ (2.0 × 10-5 M in CH2Cl2) (top) and 4ETZ (1.5 × 10-5
M
whole constraint is supported by the azobenzene group, which
results in a bending of its geometry (Figure 4).
in CH2Cl2) (bottom). Blue: initial state; red: after 5 h monochromatic
irradiation at 340 nm.
Optical and Photodynamic Properties. To evidence the
effects of the forced interactions between the azobenzene and
4T conjugated systems when assembled into a cyclophane
structure, Table 1 lists the UV-vis absorption data for 4MTZ
and 4ETZ and for the model compounds of their constitutive
parts, namely 4MT, 4ET, and p-dimethylazobenzene (DMeZ).
The UV-vis spectrum of 4MT (see Supporting Information)
exhibits a broad absorption band with a maximum at 436 nm.
In contrast, the spectrum of 4ET presents a well-resolved
vibronic fine structure with three maxima at 411, 433, and 460
nm. As for many EDOT-containing systems, these spectral
features typical for rigid conjugated systems result from the
already-discussed noncovalent intramolecular S‚‚‚O interac-
tions.17 The spectrum of DMeZ shows a main absorption band
at 335 nm, associated with a π-π* transition, and a weak band
at 439 nm, corresponding to a n-π* transition. Comparison of
these data to those of the 4T and azobenzene parts in compounds
4MTZ and 4ETZ reveals several noticeable differences. For
4MTZ, the λmax of the 4T chain shifts bathochromically from
436 to 450 nm, while the absorption band of the azobenzene
group undergoes a red-shift and splits into two subcomponents
at 353 and 368 nm. Similar effects are observed for 4ETZ for
which the three maxima of the 4T chain shift bathochromically
by 10-14 nm. These modifications of the optical features of
both constitutive parts of 4MTZ and 4ETZ can be attributed
to through-space interactions between the 4T and azobenzene
conjugated systems when covalently assembled into a cofacial
structure. As already observed for 4TZ and 6TZ, the presence
of the attached azobenzene group results in a strong quenching
of the photoluminescence of the 4T chain.13
showed that, as expected, irradiation of a 10-4 M solution of
p-dimethylazobenzene DMeZ leads to the bleaching of the π-π
transition with a blue-shift of λmax to 305 nm and a slight
intensification of the n-π* transition at 439 nm. As it appears
in Figure 5, irradiation of 4MTZ under the same conditions
produces a small decrease of the intensity of the azobenzene
absorption band, while that of the quaterthiophene chain
undergoes a small blue-shift and a slight decrease in intensity.
The small magnitude of these spectral changes suggests that,
in 4MTZ, the 4T chain undergoes only limited geometrical
modifications. In fact, the analysis of the dynamic process by
1H NMR according to the already-described procedure13b,c shows
that, after 5 h of irradiation, photoconversion is limited to ca.
10%, while 47% of conversion was reached for 4TZ under the
same conditions.13b,c
The initial spectrum of 4ETZ shows a first band with two
subcomponents at 350 and 364 nm, corresponding to the
absorption of the azobenzene group, and a second broad band
with two maxima at 446 and 474 nm and a shoulder at 421
nm, corresponding to the absorption of the 4T chain. Mono-
chromatic irradiation of the azobenzene group leads to a decrease
of the intensity of the azobenzene band with an enhancement
of the resolution of the spectrum. Concurrently, the intensity
of the two absorption bands of the 4T chain at 446 and 472 nm
also decreases, while the intensity of the 420 nm band increases.
These changes in the spectrum of the 4T chain occur around
two isosbestic points at 395 and 427 nm. The spectra in Figure
5 correspond to the mixture of the trans and cis forms of 4ETZ,
representing the quasistationary state reached after 5 h of
1
The photodynamic behavior of 4MTZ and 4ETZ has been
analyzed by recording their UV-vis spectrum under continuous
monochromatic irradiation at 360 nm. A control experiment
irradiation. As shown by H NMR data, this state corresponds
to 25% of photoconversion. To gain more information on the
spectral features of the pure final photomechanically produced

3492 J. Phys. Chem. A, Vol. 110, No. 10, 2006
Jousselme et al.
TABLE 2: Cyclic Voltammetric Data Recorded in 0.10 M
Bu4NPF6/CH2Cl2, Scan Rate 100 mV s-1, Ref Ag‚AgCl
compd
Epa1 (V)
Epa2 (V)
4MT
0.60
0.60
0.60
0.60
0.60
0.70
0.91
1.19
1.19
0.94
1.23
0.99
4MTZ
4MTZa
4ET
4ETZ
4ETZa
a After 5 h monochromatic irradiation at 360 nm
azobenzene and the 4T systems are brought nearer into a
cyclophane-like structure. Finally, it is worth noting that,
similarly to the previously described 4TZ and 6TZ, the
conversion process is fully reversible and the initial spectrum
is recovered after irradiation with 480 nm light.
Figure 6. UV-Vis absorption spectra of the two forms of 4ETZ
separated by HPLC in CH2Cl2. Dotted line: initial spectrum; solid
line: spectrum of the photogenerated twisted form.
To summarize, these results show that, in contrast to the
parent systems based on unsubstituted oligothiophenes 4TZ and
6TZ for which the photoinduced conformational switch results
in a decrease of the HOMO-LUMO gap of the oligothiophene
chain, the photomechanical conversion of 4ETZ leads to a final
state in which the oligothiophene presents a larger HOMO-
LUMO gap.
form of 4ETZ, the initial and photoconverted forms of 4ETZ
have been separated by HPLC. As shown in Figure 6, the
spectrum corresponding to the nonconverted molecule is, as
expected, identical to that of the initial spectrum in Figure 5.
On the other hand, the spectrum of the final state shows an
absorption band around 330 nm, corresponding to the azoben-
zene group, and a broad structureless absorption band with a
maximum at 410 nm, corresponding to the final geometry of
the 4T chain. Comparison of the initial and final spectrum shows
that the λmax of both the azobenzene and 4T systems has
undergone a considerable hypsochromic shift (ca. 30 nm for
the 4T chain).
Electrochemical Behavior. The electrochemical properties
of 4MTZ and 4ETZ have been analyzed by cyclic voltammetry
before and after photoirradiation. Table 2 lists the CV data of
the two molecules, together with data for the model compounds
4MT and 4ET. The CV of 4MT and 4ET (see Supporting
Information) shows two reversible one-electron oxidation
processes, with anodic peak potentials Epa1 and Epa2 around
0.60 and 0.91-0.94 V, respectively, corresponding to the
successive generation of the cation radical and dication of the
4T chain. Comparison of the CV data for 4MTZ and 4ETZ to
those of 4MT and 4ET shows that, while Epa1 is not affected
by the fixation of the azobenzene group, Epa2 shifts toward
positive potentials by 0.28 and 0.29 V for 4MTZ and 4ETZ,
respectively.
The contrasting photodynamic behaviors of 4MTZ and 4TEZ
must be discussed in relation with the chemical structure of the
4T chain and its implication for the geometry of the initial and
final states. As already discussed, in every case, the final state
of the 4T chain must adopt an ASA conformation in order to
accommodate the decrease of the distance between the anchoring
sulfur atoms produced by the photoisomerization of azobenzene.
For both molecules, conformational transition toward the final
ASA geometry implies rotation of one of the two inner
thiophene rings relative to the other (in fact, owing to the initial
SAS geometry of 4MTZ, the transition should also involve
rotation of an outer thiophene ring). On this basis, the different
photodynamic behaviors of 4MTZ and 4ETZ can be related to
differences in the possibilities of rotation around the central
single bond. For 4MTZ, such a rotation is restricted by steric
interactions between the methoxy groups. In contrast, for 4ETZ,
incorporation of the two facing alkoxy groups in their respective
ethylenedioxy bridges reduces steric hindrance, thus giving a
larger degree of freedom around the median single bond. Such
effects of reduction of steric hindrance by bridging have been
previously demonstrated in the case of 3,4-dimethylthiophene.18
Although this reduced steric hindrance opens larger possibili-
ties of rotation around the median single bond, a fully coplanar
syn conformation of the two median EDOT rings cannot be
reached due to steric interactions between the ethylenedioxy
groups, and the persistence of a torsion angle between the two
median thiophene rings in the final state can explain the blue-
shift of the λmax of the 4T chain after photoconversion of 4ETZ.
Finally, it must be underlined that, in addition to these purely
geometrical effects, the two EDOT units confere stronger donor
properties on the hybrid 4T chain. Consequently, the decrease
of the through-space interactions with the azobenzene group
consecutive to the trans to cis isomerization of the latter can be
expected to also contribute to a blue-shift of the band maxima.
This effect is, in fact, the reverse of that observed when the
A possible cause for this phenomenon could involve the
development of repulsive Coulombic interactions between the
electron-deficient azobenzene group and the cation radical of
the 4T chain. The fact that such effects were not observed for
4TZ and 6TZ suggests that, for 4MTZ and 4ETZ, the strong
electron donor methoxy and ethylenedioxy groups contribute
to localize the positive charge in the middle of the molecule
where it can interact with the azobenzene group, which is not
the case when the charge is expected to localize on the terminal
thiophenes as in 4TZ.19
After photoirradiation, the CV of 4MTZ remains practically
unchanged, confirming in agreement with optical data, that the
4T chain does not undergo significant conformational change.
In contrast, the CV of 4ETZ shows a positive shift of Epa1 from
0.63 to 0.70 V and the emergence of a new reversible redox
system with an anodic peak potential at 0.99 V (Figure 7).
In the case of 4TZ, the CV recorded after photoirradiation
exhibited a new redox system at less positive potential due to
an increase of the HOMO level of the 4T chain. The opposite
behavior observed here for 4ETZ can be attributed to the
already-discussed steric hindrance to coplanarity of the two inner
thiophene rings in the final ASA conformation (Scheme 3). On
the other hand, the negative shift of Epa2 can be attributed to
two causes. The first involves a decrease of the aleady-discussed
through-space interactions between the 4T cation radical and
the azobenzene group due to the increased distance between
the two systems after photoisomerization.

Azobenzene-Derivatized π-Conjugated Oligothiophenes
J. Phys. Chem. A, Vol. 110, No. 10, 2006 3493
annelated oligothiophenes on which the ASA conformation of
the cation radical was imposed by metal cation complexation.13a
Conclusion
Oligothiophene-based photoactuators with an azobenzene
group attached at the internal â-position of the end thiophene
rings and containing a dimethoxybithiophene and a bi-EDOT
median block have been synthesized. X-ray diffraction studies
have shown that, whereas for 4MTZ, the quaterthiophene chain
presents a SAS conformation similar to the parent system based
on unsubstituted quaterthiophene, 4TEZ adopts a fully anti
conformation stabilized by noncovalent intramolecular sulfur-
oxygen interactions.
The photomechanically induced modifications of the elec-
tronic properties of the quaterthiophene chain have been
investigated by UV-vis spectroscopy and cyclic voltammetry.
The results have shown that photoirradiation of the attached
azobenzene group of 4MTZ produces only limited conforma-
tional changes due to steric hindrance in the 3,3′-dimethoxy-
bithiophene median core. On the other hand, replacement of
3,3′-dimethoxybithiophene by bi-EDOT brings more freedom
to the system, thus allowing reversible geometrical changes of
larger magnitude. However, steric interactions between the
EDOT rings in the final state lead to a larger HOMO-LUMO
gap and a lower HOMO energy level than that of the initial
one. These results thus nicely complete the previously reported
ones, and they show that the manipulation of the chemical
constitution of the π-conjugated system allows development of
reversible photodynamic molecular architectures in which the
electronic properties of the photomechanically generated final
state can be controlled by design in order to reach at will a
smaller or a larger HOMO-LUMO gap. Thus, besides provid-
ing additional tools for the development of dynamic molecular
devices such as actuators or machines, these results open
interesting perspectives in the field of organic (opto)electronic
devices with the possibility of developing stimuli-responsive
organic semiconductors based on functional π-conjugated
systems.
Figure 7. Cyclic voltammograms recorded in 0.10 M Bu4NPF6/CH2-
Cl2, scan rate 100 mV s-1. Top 4MTZ, bottom 4ETZ ca. 10-4 M.
Dotted line: initial CV; solid line: after 5 h irradiation with 360 nm
monochromatic light.
The second is related to the specificity of the oxidation
process of π-conjugated systems with a nondegenerate ground
state. In such systems, formation of the cation radical is
accompanied by a transition from an aromatic to a quinoid
structure.20 Consequently, when the system is oxidized into the
cation radical state, it becomes locked into a defined conforma-
tion. Thus, once the conformation of the 4T chain of 4ETZ
has been photomechanically converted from an AAA to an ASA
one (at the eventual cost of some deformation of some thiophene
cycles21), further oxidation to the dication state is expected to
be easier than for that of a cation radical still in the initial AAA
conformation (Scheme 3). In fact, such a negative shift of the
second oxidation potential has been already observed for crown-
Supporting Information Available: Experimental proce-
dure for the synthesis of all compounds. UV-vis spectra and
SCHEME 3

3494 J. Phys. Chem. A, Vol. 110, No. 10, 2006
Jousselme et al.
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cyclic voltammograms of 4MT and 4ET. Crystallographic data,
tables of bond distances and angles, positional parameters, and
general displacement parameters for 4MTZ and 4ETZ. This
information is available free of charge via the Intertnet at http://
pubs.acs.org.
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