On the route to mimic natural movements: Synthesis and photophysical properties of a molecular arachnoid

Article abstract of DOI:10.1039/c0cc03045g

The synthesis, photoswitchability and NIR emitting properties of a novel π-extended pyrene derivative, peripherally decorated with four azobenzenyl-ethynyl legs, are reported.

Full text of DOI:10.1039/c0cc03045g

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On the route to mimic natural movements: synthesis and photophysical
properties of a molecular arachnoidwz
Joceline Zeitouny,a Abdelhalim Belbakra,b Anna Llanes-Pallas,a Andrea Barbieri,b
Nicola Armaroli*b and Davide Bonifazi*ac
Received 4th August 2010, Accepted 9th September 2010
DOI: 10.1039/c0cc03045g
The synthesis, photoswitchability and NIR emitting properties of
a novel p-extended pyrene derivative, peripherally decorated
with four azobenzenyl-ethynyl legs, are reported.
luminescence properties, herein we describe the synthesis and
the photophysical studies in solution of a novel p-conjugated
compound in which a 1,3,6,8-tetrasubstituted pyrene derivative
(molecule 1, Fig. 1) has been peripherally connected to four
azobenzenyl moieties (the legs) through ethynyl linkers. Our
goal is to construct a system that may show molecular motion
of its azobenzenyl legs upon light stimulation. The syntheses of
the azobenzenyl legs and of the tetrasubstituted pyrene
conjugate 1 are outlined in Scheme 1 (see also ESIw).
4,40-diiodoazobenzene 4 was prepared by oxidative coupling
reaction of 4-iodo-aniline in the presence of KMNO4 and
CuSO4ꢀ5H2O in CH2Cl2. In parallel, 4-aminophenylethynyl 3
was synthesized by reacting trimethylsilylacetylene (TMSA)
with 4-iodo-N,N-dihexylaniline 2 under Sonogashira-type11
cross-coupling conditions ([Pd(PPh3)4], CuI and iPr2NH),
followed by cleavage of the TMS protecting groups upon
addition of a 2 M aq. solution of KOH. Subsequent
Sonogashira-type reaction between an equimolar mixture of
aminophenylacetylene 3 and diiodoazobenzene 4 afforded
unsymmetrical azobenzene 6 as an orange solid. Doubly-
substituted azobenzene 5 was also obtained as side product.
One of the most intriguing aspects in contemporary supra-
molecular chemistry is the engineering of architectures that
bridge the gap between molecules and applications, taking
advantage of molecular motion to perform programmed
functions in devices or materials.1 Proposed strategies often
find inspiration from components of natural (e.g. physiological
and biochemical processes of living systems, such as muscle
contraction/extension)2 or manmade (e.g. cogwheels and
shuttles)3 machineries from the macroscopic world. In this
respect, one of the first examples is the nanoscale elevator
reported by Stoddart, Balzani, Credi and co-workers,4 where
movement occurs via pH stimulus and develops an exceptional
force of up to 200 pN. Landmark results have been also
reported by Feringa and co-workers, who showed that macro-
scopic movements of a liquid crystal, induced by unidirectional
changes in the molecular chirality, reversibly rotate clockwise
or anticlockwise microscale objects placed on top of its
surface.5 Among others, light-induced switchable azobenzenes6
are good candidates for triggering movements at the molecular
scale, as they can reversibly switch from the thermodynamically
stable trans configuration to the cis one, by irradiation with
UV or VIS light.7 The back cis - trans reaction can be
accomplished either by light or heat. The versatility of
azobenzenes has been demonstrated in several supramolecular
structures8 and at the interfaces9 such as in self-assembled
monolayers (SAMs). Recently, the group of Tour has
employed the azobenzene group as the skeleton of a molecule
equipped with fullerene appendages, that, upon light irradiation,
should exhibit a worm-like movement if deposited on a surface.10
As part of our research program aiming at the preparation of
multicomponent architectures exhibiting, at the same time,
light switching properties, robust chromophoric character and
a
`
Dipartimento di Scienze Farmaceutiche, Universita di Trieste,
Piazzale Europa 1, 34127 Trieste, Italy
Istituto per la Sintesi Organica e la Fotoreattivita del CNR,
b
`
Via Gobetti 101, 40129 Bologna, Italy.
E-mail: nicola.armaroli@isof.cnr.it; Fax: +39 051 6399844
c Department of Chemistry, University of Namur (FUNDP),
Rue de Bruxelles 61, B-5000, Namur, Belgium.
E-mail: davide.bonifazi@fundp.ac.be; Fax: +32 81 725433
w Electronic supplementary information (ESI) available: Details for
the experimental procedures, computational studies and photo-
physical characterization. See DOI: 10.1039/c0cc03045g
z This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
Fig. 1 Schematic representation of the full switching of the molecular
arachnoid 1 from the 1-tttt to the 1-cccc configuration.
c
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Scheme 1 Reagents and conditions: i, TMSA, [Pd(PPh3)4], CuI, iPr2NH, 50 1C, 20 h (82%); ii, 1 M aq. KOH, MeOH/CH2Cl2, 2 h (98%); iii, 4,
[Pd(PPh3)4], CuI, Et3N/THF, rt, 20 h; iv, TMSA, [PdCl2(PPh3)2], CuI, iPr2NH/THF, rt, 20 h; v, TBAF/THF, rt, 20 h; vi, 8, [PdCl2(PPh3)2], CuI,
PPh3, iPr2NH/THF, rt, 20 h; vii, TMSA, [PdCl2(PPh3)2], CuI, PPh3, iPr2NH/THF, 80 1C, 12 h; viii, 1 M aq. KOH, MeOH/CH2Cl2, 12 h; ix, 2,
[PdCl2(PPh3)2], CuI, Et3N, 45 1C, 12 h. Right bottom: electronic absorption spectra of molecules tttt-1, t-8, 11 and tt-12 in toluene solution at
298 K.
Addition of the terminal ethynyl units (molecule 8) was
performed in 79% of yield by Pd-catalysed cross-coupling
reaction between 6 and TMSA followed by a cleavage of the
protecting group using TBAF. At last, cross-coupling reaction
between 1,3,6,8-tetrabromopyrene 912 at rt with five equivalents
of azobenzene 8 afforded tetra(4,40-azobenzenyl)-pyrene 1
(12%) and diazobenzene derivative 12 as darkish red and
orange solids, respectively. Separation of compounds 1 and
12 was achieved by repeated column chromatography. In
order to study the electronic effects of the azobenzenyl legs
on the pyrene emitting properties, tetrasubstituted pyrene
derivative 11 bearing four non-switchable aminophenylethynyl
legs has been also synthesized starting from tetra(ethynyl)pyrene
10 and 4-iodo-N,N-dihexylaniline 2 (see ESIw). All the structural
Fig. 2 (a) Time evolution of the absorption spectra of a 4.0 ꢁ 10ꢂ6 M
1
assignments were supported by H-NMR and UV-VIS-NIR
solution of tttt-1 in toluene under 460 nm irradiation, until the
spectroscopies (see ESIw) and mass spectrometry. In particular,
photostationary state is reached. (b) Back reaction observed over time
the molecular mass of each compound was unambiguously
under 313 nm irradiation. The initial spectrum of tttt-1 is fully
established by HR-MALDI-TOF-MS (matrix: DCTB), that
recovered showing complete reversibility.
displayed for 1 the molecular ion as prominent peak at m/z
2151.2788 (1, M+, C152H158N12+, calc. 2151.2732).
dark, the initial spectrum is recovered after 2 hours. The same
result is obtained upon light excitation at 313 nm but in a
much shorter time, i.e. about 90 seconds (Fig. 2b). Taken
together, the above observations suggest the occurrence of a
reversible trans–cis interconversion. Photoisomerization
experiments have been carried out also with the linear derivative
t-8. Due to the lack of the strongly-absorbing pyrenyl unit, this
molecule showed more pronounced spectral changes upon
illumination (Fig. S1, ESIw). The intense absorption features
of 1 and 8, which are attributable to the pyrene core and/or the
extended p-delocalization, do not allow us to individuate the
UV irradiation of azobenzene derivatives 1, 8 and 12
afforded the all-trans isomers in toluene solutions, their
electronic absorption spectra are depicted in Scheme 1 (bottom
right) along with that of tetrasubstituted pyrene reference 11.
The spectral onset is progressively red-shifted from 8 to 12 to 1
with enhanced p-delocalization. Upon irradiation of tttt-1 at
470 nm (see exp. section in ESIw), spectral changes are
observed, which are stopped after about 2 min. An isosbestic
point is detected at 372 nm, suggesting the occurrence of a
clean photoreaction (Fig. 2a). By keeping the solution in the
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452 Chem. Commun., 2011, 47, 451–453
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respectively). In both cases, the central spacer allows extensive
electronic delocalization between the 8-type branches, generating
a low-lying NIR emitting level. This shows that both bisethynyl
and pyrene p-spacers have a similar effect on the electronic
delocalization between the azobenzene units.
In summary, we have prepared an arachnoid-like molecular
system made of a chromophoric pyrene core and azobenzenyl-
ethynyl photoswitchable legs, which exhibits NIR emission, a
quite uncommon feature for organic switchable chromophores.
We are now carrying out more detailed photophysical
investigations to exploit the photoswitchable character of this
complex system also on surfaces and to rationalize and tune its
intriguing luminescence properties.
This work was supported by EU (MC-RTN ‘‘PRAIRIES’’
MRTN-CT-2006-035810 and MC-ITN FINELUMEN
PITN-GA-2008-215399), the CNR (PM.P04.010, MACOL),
the FRS-FNRS (2.4.625.08.F & F.4.505.10.F), the ‘‘Loterie
Nationale’’, the ‘TINTIN’ ARC project (09/14-023), the
University of Namur and the University of Trieste.
Fig. 3 Emission spectra of tttt-1, t-8, 11 and tt-12 (lexc = 330 nm) in
toluene at 298 K. Inset: absorption (colored lines) vs. excitation
spectra (black lines) of tttt-1 and tt-12. The emission spectrum of 11
is recorded with a conventional UV-Vis photodetector, the others with
a NIR sensitive one. Luminescence spectra are corrected for the
detector responses.
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moieties around 450 nm, thus hampering the determination of
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