Bis(azobenzene)-based photoswitchable, prochiral, Cα- tetrasubstituted α-amino acids for nanomaterials applications

Article abstract of DOI:10.1002/chem.201102609

Light-driven chirality: Sequential light-driven isomerization of prochiral, bis(azobenzene)-containing amino acids results in the formation of chiral entities that have been characterized by different techniques. Metal nanoparticles conjugated with these amino acids retain the photoswitching properties and show conformation-dependent magnetic susceptibility that can be reversibly controlled by irradiation (see figure).

Full text of DOI:10.1002/chem.201102609

DOI: 10.1002/chem.201102609
Bis(azobenzene)-Based Photoswitchable, Prochiral, Ca-Tetrasubstituted
a-Amino Acids for Nanomaterials Applications
Paola Fatꢀs,[a] Edoardo Longo,[b] Federico Rastrelli,[b] Marco Crisma,[c]
Claudio Toniolo,[b] Ana I. Jimꢁnez,[a] Carlos Cativiela,[a] and Alessandro Moretto*[b]
Great interest is currently devoted to molecular or supra-
molecular entities that have access to two or more forms,
the interconversion of which can be triggered by an external
stimulus.[1] Several switching systems have been reported
that are based on photochromic behavior, resulting in opti-
cal control of chirality, fluorescence, intersystem crossing,
electrochemically and photochemically induced changes in
liquid crystals, thin films, and membranes.[2] The design of
molecular compounds that exhibit photoinduced magnetiza-
tion and magnetic transitions is one of the main challenges
in the field of materials science because of their potential
application to optical memory and switching devices.[3]
Hence, the construction of new types of optically switchable
magnetic compounds that exhibit both large magnetization
changes and ferromagnetic order even at room temperature
is nowadays an issue of great interest.[3a,c,4] Particularly intri-
guing are photoactive molecules formed on the surface of
gold, silver, and platinum nanoparticles. Owing to the high
surface-to-volume ratio, the concentration of photoactive
compounds compared to the number of metal atoms allows
for standard characterization techniques, such as UV/Vis or
FTIR absorption spectroscopy, to be employed to detect
photochromic switching.[3b] Azobenzene ranks among the
first photochromic switches used and is still the subject of
extensive investigation.[5] Its cis- and trans-isomers (here-
after termed c and t, respectively) have a different spatial ar-
rangement of the aromatic moieties, and consequently show
significantly diverse physical and chemical properties, in-
cluding dipole moments.
We are currently expanding this field by developing a
novel family of Ca-tetrasubstituted a-amino acids, character-
ized by two azobenzene moieties covalently linked to the a-
carbon atom through a methylene group, to be exploited as
photoresponsive building blocks in different systems. These
compounds can be viewed as the result of introducing a phe-
nylazo group at either the para or meta position of the two
aromatic side chains in dibenzylglycine (Dbg). These sym-
metrically substituted, bis(azobenzene)-containing amino
acids, bis[p-(phenylazo)benzyl]glycine (pazoDbg) and bis[m-
(phenylazo)benzyl]glycine (mazoDbg), were prepared as the
ethyl esters, H-pazoDbg-OEt (1), and H-mazoDbg-OEt (2),
as illustrated for the former compound in Figure 1I (for syn-
thetic details, see the Supporting Information).
[a] P. Fatꢀs, Dr. A. I. Jimꢁnez, Prof. C. Cativiela
Department of Organic Chemistry, ISQCH
University of Zaragoza-CSIC
Figure 1. (I) Synthesis of 1. Reagents and conditions: (i) AcOH, nitroso-
benzene, RT, 24 h, 80%; (ii) THF, LiAlH4, RT, 24 h, 93%; (iii) triphenyl-
phosphine, N-bromosuccinimide, THF, RT, 12 h, 91%; (iv) ethyl isocya-
noacetate, tetrabutylammonium bisulfate, K2CO3, 508C, 24 h, 96%; (v)
HCl, EtOH, RT, 2 h, 100%. (II) X-ray diffraction structure of the “alba-
50009 Zaragoza (Spain)
[b] E. Longo, Dr. F. Rastrelli, Prof. C. Toniolo, Dr. A. Moretto
Department of Chemistry
University of Padova
via Marzolo 1, 35131 Padova (Italy)
Fax : (+39)049-827-5829
ꢀ
tross-like” compound 1. The intramolecular (amino) N H···O=C (ester)
H-bond is indicated by a dashed line.
E-mail: alessandro.moretto.1@unipd.it
[c] Dr. M. Crisma
The key step (iv) was the dialkylation of ethyl isocyanoa-
cetate with the corresponding p- or m-substituted bromide,
which proceeded with excellent yield under phase-transfer
conditions. The azobenzene-derived bromides were obtained
Institute of Biomolecular Chemistry, Padova Unit, CNR
via Marzolo 1, 35131 Padova (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102609.
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by condensation of p- or m-aminobenzoic acid with nitroso-
benzene in acetic acid and further elaboration of the carbox-
ylic acid group. The slow evaporation of a CH2Cl2/MeOH
solution of 1 furnished single crystals suitable for X-ray dif-
fraction analysis (Figure 1 II). The backbone of the a-amino
ester adopts an extended conformation [N1-C1A-C1-OT
(yT) torsion angle: ꢀ179.8(3)8], which allows for the pres-
ence of an intramolecular H-bond (C5 form) between the
N1-H1B nitrogen and the O1 carbonyl oxygen. The occur-
rence of the fully-extended conformation has been crystallo-
graphically documented for other Ca,a-symmetrically disub-
stituted glycines, including Dbg, in simple derivatives and
peptides.[6] The c1 torsion angles about the C1A-C1B1 (pro-
S side chain) and C1A-C1B2 (pro-R side chain) bonds are
for synthetic details, see the Supporting Information). This
derivatization was performed to subsequently exploit these
compounds as capping layers for metal nanoparticles. The
photoswitching capability of 3 and 4 was characterized by
UV/Vis spectroscopy, NMR, and reverse-phase HPLC. Both
compounds underwent multiple, reversible isomerizations
(compound 3, Figure 2; compound 4, see the Supporting In-
formation) in a variety of solvents upon irradiation with Vis
light (450 nm, c!t) or UV light (350 nm, t!c).
Specifically, 0.1 mm solutions of 3 and 4 in MeOH (as well
as in several different solvents; data not shown) were irradi-
ated separately at two different wavelengths, 350 and
450 nm, for multiple cycles. Under UV irradiation at
350 nm, the intensity of the strong absorption maximum at
330 nm, typical for the t form of the azobenzene moieties,
rapidly decreased in both samples while the weak absorp-
tion band at 435 nm concomitantly increased (Figure 2 II).
The opposite phenomenon occurred when the solutions
were subsequently illuminated with Vis light at 450 nm.
However, when 3 and 4 were dispersed in a paraffin
medium, the c$t photoswitch process was blocked. No sig-
nificant difference was observed between the para- and
meta-substituted compounds. Interestingly, when the photoi-
somerization process of 3 and 4 was monitored by HPLC up
to three isomeric species were detected, as shown in Fig-
ure 2III (inset) for 3. The peak eluting between the c and
t isomers was attributed to an intermediate state in the t!c
and c!t interconversion pathways, in which only one of the
in the g+ACHTUNGTRENNUNG
spectively. The two azobenzene moieties are in the t confor-
mation and are essentially flat, the angle between normals
to the aromatic rings in each azobenzene group being 10.38
and 2.68 for the pro-S and the pro-R chain, respectively. In-
terestingly, the distance between the terminal carbon atoms
of the two side chains is as long as 20.29 ꢃ and the overall
shape of the molecule is reminiscent of an albatross.
The isomerization properties induced by light on these bi-
s(azobenzene)-containing amino acids were investigated.
First, 1 and 2 were transformed into the free acids bearing
an S-(trityl)-3-mercaptopropionyl group at the amino func-
tion, Trt-S-(CH2)2-CO-pazoDbg-OH (3) (Figure 2I) and Trt-
S-(CH2)2-CO-mazoDbg-OH (4), respectively (Trt=trityl;
Figure 2. (I) Mechanism of light-driven isomerization for 3 (same mechanism, not shown for 4). (II) UV/Vis absorption spectra of the multiple, reversible
c$t isomerization of 3 in a 0.1 mm solution in MeOH. (III) DQF COSY sub-spectrum of 3 in CDCl3 run after 2 min irradiation at 350 nm (ratio of com-
pounds as in the HPLC profile; III inset), shows the concomitant presence of three isomeric species.
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A. Moretto et al.
two azobenzene moieties has isomerized. As a consequence
of this limited photoswitch, the Ca-atom becomes chiral and
therefore a mixture of two enantiomers (Figure 2I) is gener-
ated. The central peak in the chromatogram should then
correspond to a racemic mixture of the enantiomeric species
formed by isomerization of a single azobenzene unit,
namely the t/c and c/t forms. To confirm our hypothesis of
“mono” isomerization, double-quantum filtered (DQF) CO-
SY NMR experiments were carried out on 3 for the all-t
form, its all-c isomer, and a ꢁ1:1:1 mixture of the all-t, t/c+
c/t (racemate), and all-c isomers generated by 2 min irradia-
tion with UV light. In the latter case, the NMR spectrum
(Figure 2III) provides clear evidence for the occurrence of
three independent species in a photostationary equilibrium
(the geminal -CbH2- protons in the amino acid side chains
become diastereotopic).[7] Similar results were obtained for
4 (see the Supporting Information).
This phenomenon was further, albeit indirectly, demon-
strated by investigating the photoisomerization of the chiral
dipeptide containing the pazoDbg residue coupled with l-
leucine methyl ester, H-pazoDbg-l-Leu-OMe (5). In this
case, owing to the presence of l-Leu, the intermediate spe-
cies generated upon isomerization of a single azobenzene
unit are diastereomers, H-(R)-pazoDbg-l-Leu-OMe and H-
(S)-pazoDbg-l-Leu-OMe (Figure 3), and should therefore
be distinguishable in the HPLC and NMR experiments. A
solution of compound 5 in MeOH in the all-t conformation
was prepared and the isomerization process under irradia-
tion with UV light was followed by HPLC (Figure 3 II).
Beside the two peaks corresponding to the all-t and all-c
forms, two additional peaks of similar intensity were detect-
ed and assigned to the diastereomeric dipeptides exhibiting
each azobenzene group of the pazoDbg residue in a differ-
ent isomeric state, that is, the t/c and c/t species in Figure 3I.
This “mono” isomerization step was also analyzed by
DQF COSY experiments on a ꢁ 1:1:1:1 mixture of all four
isomers (Figure 3II). This study demonstrated that the
remote l-Leu stereocenter affects all of the diastereotopic
-CbH2- protons, which resulted in a further splitting of the
DQF COSY cross-peak patterns as compared to those in
Figure 2III. Indeed, these experiments provide evidence for
the presence of four independent species in a photostation-
ary equilibrium.
The behavior of different metal nanoparticles conjugated
with 3 was next investigated. Au, Ag and Pt nanoparticles
(Figure 4I) were prepared by chemical reduction (with
NaBH4) of HAuCl4, AgNO3, and H2PtCl6, respectively, in a
CHCl3/methanol/water mixture in the presence of 3.[8] Ac-
cording to our TEM analysis (Figure 4II) spherical Ag, Au,
and Pt nanoparticles (1.5–4 nm diameter) were obtained.
Formation of the 3-conjugated nanoparticles was confirmed
by the UV/Vis absorption spectra, in which weak (due to
the small nanoparticle size and the large absorption coeffi-
cient of the azobenzene moieties) metal-dependent plas-
monic bands were observed.
Figure 3. (I) Mechanism of light-driven isomerization for 5. (II) Left: DQF COSY sub-spectrum of 5 in CDCl3 run after 3 min irradiation at 350 nm high-
lights the concomitant presence of four isomeric species. Right: HLPC profile.
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Photoswitchable a-Amino Acids for Nanomaterials Applications
COMMUNICATION
Figure 4. Light-driven isomerization of the 3-conjugated metal nanoparticles. (I) Representations of AuNp-3 (left), AgNp-3 (center) and PtNp-3 (right).
(II) Representative TEM micrographs. (III) UV/Vis absorption spectra of the c$t isomerization of Ag, Au, Pt nanoparticles capped with 3 in MeOH.
Colors of the curves match those of the corresponding nanoparticle-conjugates in part (I). (IV) UV/Vis absorption spectra for the c$t isomerization of
1
AuNp-3 in a paraffin medium (solid state). (V) H NMR spectra of AuNp-3 run after different times of irradiation: (a) 0 min, all-t; (b) 6 min irradiation
1
at 350 nm; (c) 10 min irradiation at 350 nm, all-c; and (d) 20 min irradiation at 450 nm. (VI) H NMR spectra of PtNp-3 run after different times of irra-
diation: (a) 0 min, all-t; (b) 6 min irradiation at 350 nm; (c) 10 min irradiation at 350 nm, all-c; and (d) 20 min irradiation at 450 nm.
The photoswitch behavior of the conjugates was initially
investigated by UV/Vis spectroscopy in solution (Figure 4
III), which allowed us to follow the reversible isomerization
process. Then, the Au nanoparticles (AuNp-3) were exam-
ined in the solid state (dispersed in a paraffin medium;
Figure 4 IV), where the photoinduced molecular switch was
clearly seen over several cycles of irradiation. For the
AuNp-3 and Pt nanoparticles (PtNp-3) the c$t isomeriza-
tion was also studied by NMR (Figure 4V and VI) and
HPLC. In the NMR experiments, part of the complex enve-
lope of broad signals, corresponding to the aromatic protons
of both metallic nanoparticles, showed a significant upfield
shift during the irradiation process at 350 nm. The irradia-
tion was stopped after 10 min, when the NMR spectra did
not change anymore. Then, the NMR tube was irradiated
with Vis light for 20 min. At this point, the new NMR sig-
nals were almost superimposable to those in the initial spec-
tra recorded before UV irradiation. Monitoring the isomeri-
zation process by HPLC (see Supporting Information), it
was found that the peak corresponding to the all-t form of
AuNp-3 (and PtNp-3 as well) changed its retention time
under UV irradiation.
As recently reported by Einaga and co-workers,[3a,c] azo-
benzene-passivated gold nanoparticles show a “controlled”
ferromagnetism.[9] They demonstrated that this property can
be selectively modulated by alternating photoillumination
with UV and Vis light (that is, by exploiting c$t isomeriza-
tion) in the solid state. Based on this result, we investigated
the magnetic susceptibility c of AuNp-3 by solution-state
NMR at room temperature.[10] The method is based on the
fact that the resonance frequency for a given nucleus de-
pends, among other factors, upon the volume susceptibility
of the medium. We used tert-butanol as the inert indicator
compound and CD3OH as the solvent, and focused our at-
tention on AuNp-3. This conjugate was subjected to sequen-
tial irradiation at 350 nm (5 time points), 450 nm (3 time
points) and again 350 nm (3 time points).
The resulting chemical shifts for the tert-butanol CH3 pro-
tons (inner and outer NMR tubes) are reported in Figure 5.
The modulation of c as a consequence of irradiation is pro-
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A. Moretto et al.
Acknowledgements
Financial support from the Ministerio
de Ciencia Innovaciꢄn (grant
e
CTQ2010-17436, FPU fellowship to P.
F.), the University of Padova (“Proget-
to Strategico” HELIOS 2008, prot.
STPD08RCX), and the “Scientific
Equipment for Research” Initiative
(Oxford Gemini diffractometer) is
gratefully acknowledged. M. C. thanks
Prof. A. Dolmella for his help with X-
ray diffraction data collection and
processing.
Keywords: amino
acids
magnetic
nanomaterials
·
azobenzene
properties
prochirality
·
·
·
Figure 5. Modulation of the magnetic susceptibility of AuNp-3. Outer compartment (tube A): 4% tert-butanol
in CD3OH. Inner compartment (tube B): 4% tert-butanol and 2.5 mgmLꢀ1 AuNp-3 in CD3OH (the resonance
signals of CD3OH are not shown). Spectrum 1, all-t conformation for AuNp-3. From spectrum 2 to spectrum
5, four different periods of irradiation with UV light. Spectra 6 and 7 were recorded after two different times
of irradiation with Vis light. Spectra 8 and 9 were recorded after two different times of irradiation with UV
light.
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Received: August 22, 2011
Published online: September 28, 2011
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