Photoswitchable metal coordinating tweezers operated by light-harvesting dendrimers

Article abstract of DOI:10.1021/ja307522f

A dendrimer bearing two cyclam units linked by an azobenzene moiety, and luminescent naphthalene units at the periphery performs three different functions (light-harvesting, photoisomerization and coordination of metal ions) which can cooperate or interfere depending on the nature of the metal ion. It is thus an example of light controlled molecular tweezers in which Zn(II) coordination allows 100% efficient photosensitization of azobenzene switching, while Cu(II) shuts down azobenzene isomerization.

Full text of DOI:10.1021/ja307522f

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Communication
Photoswitchable metal coordinating tweezers
operated by light-harvesting dendrimers
Enrico Marchi, Massimo Baroncini, Giacomo Bergamini, Jeroen Van Heyst, Fritz Vögtle, and Paola Ceroni
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja307522f • Publication Date (Web): 05 Sep 2012
Downloaded from http://pubs.acs.org on September 7, 2012
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Photoswitchable metal coordinating tweezers operated by
light-harvesting dendrimers
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Enrico Marchi,a Massimo Baroncini, a Giacomo Bergamini,a Jeroen Van Heyst,b Fritz Vögtle b and Paola
Ceroni a,c*
aDepartment of Chemistry “G. Ciamician”, via Selmi 2, 40126 Bologna – Italy, Eꢀmail: paola.ceroni@unibo.it
bKekuléꢀInstitut für Organische Chemie und Biochemie, Universität Bonn, GerhardꢀDomagk Strasse 1, Dꢀ53121 Bonn,
Germany
cInteruniversity Center for the Chemical Conversion of Solar Energy (SolarChem), Bologna unit, via Selmi 2, 40126 Bologna, Italy
each other. Therefore, this is an example of photocontrolled metal
coordinating tweezers in which the coordinated metal ion is not
only a spectator but it can switch ON/OFF the lightꢀharvesting
and photoisomerization functions.
ABSTRACT: A dendrimer bearing two cyclam units linked by
an azobenzene moiety, and luminescent naphthalene units at the
periphery performs three different functions (lightꢀharvesting,
photoisomerization and coordination of metal ions) which can
cooperate or interfere depending on the nature of the metal ion. It
is thus an example of light controlled molecular tweezers in which
Zn(II) coordination allows 100% efficient photosensitization of
azobenzene switching, while Cu(II) shuts down azobenzene isomꢀ
erization.
Artificial supramolecular structures containing multiple units
are highly attractive to perform complex functions not exhibited
by the single components. The value of these systems relies not
only on the total number of components, but also on the diversity
of the moieties and on the functions resulting from their mutual
interactions.1 To achieve these goals, a proper organization of the
different functional units is vital. Dendrimers,2,3 a class of multiꢀ
branched molecules that can – by design – exhibit a high degree
of order and complexity, are ideal scaffolds to organize multiple
units in a nanoobject. Dendrimers containing photochromic4,5 or
luminescent units6 or performing as ligands of metal ions7,8 have
been extensively investigated in the last decade. Herein we report
on two dendrimers (Scheme 1) containing all the three above
mentioned functions. They bear two coordinating units (1,4,8,11ꢀ
tetraazacyclotetradecane, hereafter called cyclam), linked by a
photoswitching azobenzene moiety, and functionalized at the peꢀ
riphery with six or 12 lightꢀharvesting (naphthalene) chromoꢀ
phores in G0(t-Azo) and G1(t-Azo), respectively.
Scheme 1. Synthesis and structures of the investigated comꢀ
pounds, evidencing the three different functional units.
Azobenzene has been chosen because it undergoes an efficient
and fully reversible photoisomerization reaction from the trans to
the cis form.9 For this reason, it has been extensively used to conꢀ
struct photoswitchable devices.10 Cyclam is one of the most exꢀ
tensively investigated ligands in coordination chemistry,11 able to
strongly bind a wide variety of metal ions.
Synthesis. The synthesis of G0(tꢀAzo) and G1(tꢀAzo) (Scheme
1) was achieved starting from a common intermediate: C(tꢀAzo)
that was prepared by monoꢀalkylation of two equivalents of a boꢀ
ron triꢀprotected cyclam12 with one equivalent of bis(4ꢀ
bromomethylꢀphenyl)diazene. C(tꢀAzo) was reacted with 2ꢀ
(bromomethyl)ꢀnaphthalene in DMF for two days to afford G0(tꢀ
Azo) in moderate yield. For the synthesis of G1(tꢀAzo) an excess
of 3,5ꢀbis(2’ꢀnaphtalenylmethyloxy)benzylbromide was reacted
with C(tꢀAzo) in DMF for four days. Both compounds G0(tꢀAzo)
and G1(tꢀAzo) were fully characterized by 1H and 13C NMR, ESIꢀ
MS, and UVꢀvisible absorption spectroscopy.
To the best of our knowledge, the present compounds represent
the first example of dendrimers containing photochromic (azoꢀ
benzene), luminescent (naphthalene) and metal coordinating units
(cyclam) and thus performing three different functions: lightꢀ
harvesting, metal coordination and photoswitching. Because of
their proximity, the various functional groups of the dendrimers
interact: the azobenzene unit enables to control the distance beꢀ
tween the two cyclam moieties upon light stimulation. This leads
to different coordination properties for the cis and trans isomers.
Metal coordination and photosensitized isomerization can control
Photophysical properties. The absorption spectra of G0(tꢀAzo)
and G1(tꢀAzo) in CH3CN/CH2Cl2 1:1 (v/v) (Figure 1) are similar
to those obtained by the sum of the component spectra. They
show the characteristic bands of both naphthalene at 275 nm and
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azobenzene at 336 (ππ* transition) and 450 nm (nπ* transition).
The lowest energy excited state is localized on the azobenzene
moiety.
In order to elucidate the interaction between naphthalene and
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azobenzene in the dendritic structures, photoisomerization of azoꢀ
benzene has been investigated upon selective excitation of the
naphthalene at 275 nm, where more than 95% of the light is abꢀ
sorbed by the naphthalene for G1(tꢀAzo) and G1(cꢀAzo). The reꢀ
sults show that azobenzene isomerisation takes place, demonstratꢀ
ing that energy transfer from the naphthalene to the trans isomer
of the azobenzene core occurs with an efficiency (ηET, Table 1) of
20% and 40% for G0(tꢀAzo) and G1(tꢀAzo), respectively.
Table 1. Most relevant photochemical data of the investigated
dendrimers and their Zn(II) complexes as well as model comꢀ
pound Azo in CH3CN/CH2Cl2 1:1 (v/v) solution at 298 K.
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λirr =275 nm
λirr =365 nm
Φt→c
ηET
Φt→c
% t
G0ꢀAzo
0.05
0.2
1.0
ꢀ
0.15
0.07
0.03
0.09
0.06
0.06
0.15
5
[Zn2(G0-Azo)]4+
[Cu2(G0-Azo)]4+
G1ꢀAzo
0.08
10
51
7
< 0.01
0.04
0.4
0.6
ꢀ
[Zn2(G1ꢀAzo)]4+
[Cu2(G1-Azo)]4+
Azo
0.04
13
36
5
< 0.01
0.13
Figure 1. Absorption spectra of G0(tꢀAzo) (full line), G1(tꢀAzo)
(dashed line), tꢀAzo (dotted line) and naphthalene (dashedꢀdotted
line) in CH3CN/CH2Cl2 1:1 (v/v). Inset shows emission spectra of
G0(tꢀAzo) (full line), G1(tꢀAzo) (dashed line) in CH3CN/CH2Cl2
1:1 (v/v). λex = 275 nm. T = 298 K.
ꢀ
Coordination properties. Upon titration of a 1.8 x 10ꢀ5 M soluꢀ
tion of G0(tꢀAzo) in CH3CN/CH2Cl2 1:1 (v/v) solution with
Zn(CF3SO3)2, the absorption spectrum shows a blue shift and an
increase in intensity of the ππ* band of azobenzene together with
a decrease of absorbance at 260 nm with isosbestic points at 268
and 342 nm (Figure 2). The plot of normalized absorption changes
at 304 nm versus the amount of Zn(II) (inset of Figure 2) is quite
linear and exhibits a plateau at ca. 2.3 eqs. of Zn(II) per denꢀ
drimer, demonstrating that up to two metal ions can be coordinatꢀ
ed, i.e. one per cyclam unit. By global fitting of the absorption
changes, an estimate of the association constants can be obtained:
K1 = 7 × 107 Mꢀ1, K2 = 5 × 106 Mꢀ1 for 1:1 and 2:1 metꢀ
al/dendrimer stoichiometry. In the 1:1 complex only one of the
two cyclam units is linked to Zn(II) because the transꢀazobenzene
unit keeps the two cyclam units quite far apart (Scheme 2a, left).15
By excitation at 275 nm where most of the light is absorbed by
the naphthalene chromophores, G0(tꢀAzo) and G1(tꢀAzo) exhibit
a very weak luminescence (Figure 1, inset). The former shows
two bands at 335 and 470 nm, and the latter exhibits a maximum
at 335 nm together with a shoulder at 400 nm. In analogy with the
behavior previously reported for the two dendrimers containing
only a cyclam core,13 we can assign the different emission bands
to naphthyl localized excited states (λmax = 335 nm), naphthyl exꢀ
cimers (λmax ca 390 nm), and naphthylꢀamine exciplexes (λmax
=
470 nm) as a result of the interaction of an excited naphthalene
with an amine group of the cyclam unit. The emission spectrum of
G1(tꢀAzo) does not show the band at 470 nm: this result is likely
due to the fact that energy transfer to the azobenzene core (see
below) is faster than exciplex formation, while in G0(t-Azo) the
close proximity of naphthalene and cyclam nitrogens enable exciꢀ
plex formation in competition with energy transfer to the azobenꢀ
zene core. The emission quantum yields of G0(tꢀAzo) and G1(tꢀ
Azo) (0.6 x 10ꢀ3 and 1.1 x 10ꢀ3, Table S1) are much lower than
those previously observed for cyclamꢀcored dendrimers (3 and 5 x
10ꢀ3 for generation 0 and 1, respectively).14 This result suggests a
quenching by the azobenzene unit (see below).
Titration with Zn(CF3SO3)2 has been performed also on the soꢀ
lution obtained at the photostationary state upon irradiation at 365
nm, where the ratio of cis and trans isomers is 95:5. The absorbꢀ
ance change at 304 nm reaches a plateau at ca. 1.4 eqs. of Zn(II)
ions per dendrimer, instead of 2.3 eqs. observed in the case of the
trans isomer (inset of Figure 2). These results can be rationalized
on the basis of a structural rearrangement which brings the two
cyclam units much closer, so that one Zn(II) ion can be coordinatꢀ
ed by both of them (Scheme 2a, right).16 The coordination of the
second Zn(II) ion is highly disfavored by the electrostatic repulꢀ
sion due to the close proximity of two 2+ ions. By fitting the abꢀ
sorption changes and taking into account the small percentage of
trans isomer present at the photostationary state, the association
constant is estimated as K1 = 1 × 108 Mꢀ1. This value is slightly
higher than that found for the trans isomer, evidencing a positive
effect of the second cyclam unit on the stability constant.17
Photochemical properties. Upon irradiation at 365 nm, where
only azobenzene is absorbing light, the absorption spectra of
G0(tꢀAzo) and G1(tꢀAzo) (Figure S1) show a decrease of the ππ*
band at 336 nm and an increase of the nπ* band of azobenzene.
These spectral changes are characteristic of trans → cis isomeriꢀ
zation. The cisꢀazobenzene species can be converted back to the
trans isomer by irradiation at 436 nm in the nπ* band.
Upon irradiation at 365 nm, the values of the photochemical
quantum yield Φt→c and the percentage of trans at the photostaꢀ
tionary state of the model compound tꢀAzo (Scheme 1) are identiꢀ
cal to those obtained for G0(tꢀAzo), within the experimental erꢀ
rors (Table 1). Small differences are observed for G1(tꢀAzo): a
lower value of Φt → c upon irradiation at 365 nm, and a somewhat
larger fraction of trans in the photostationary state reached by irꢀ
radiation both at 365 (Table 1) is indicative of a higher stability of
the trans isomer compared to the cis one in G1(tꢀAzo) with reꢀ
spect to the model compound.
Qualitatively similar results are obtained in the case of G1(tꢀ
Azo) upon titration with Zn(CF3SO3)2: K1 = 7 × 107 Mꢀ1, K2 = 1 ×
105 Mꢀ1. Upon titration with Zn(CF3SO3)2 of the photostationary
state of G1(cꢀAzo) obtained at λirr = 365 nm, the plot of absorbꢀ
ance at 304 nm is very similar to that reported for G0(cꢀAzo).
This results demonstrate the formation of a complex with 1:1
metal to dendrimer stoichiometry with K1= 1 × 108 Mꢀ1.
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expected, the cyclam moiety is able to coordinate both Zn(II) and
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Cu(II) ions, but profound differences are observed in the photoꢀ
physical and photochemical properties. Indeed, no sensitized
isomerization is observed in the Cu(II) complexes of G0(tꢀAzo)
and G1(tꢀAzo) upon excitation of the naphthalene units and a
lower isomerization quantum yield upon direct excitation of the
azobenzene core (Table 1, Scheme 2b). These results are conꢀ
sistent with the quenching of the naphthalene luminescence and,
to some extent, the azobenzene isomerization by energy/electron
transfer to a coordinated Cu(II) ion.
9
In conclusion, two dendrimers bearing two cyclam units as coꢀ
ordinating sites for metal ions, a photoisomerizable azobenzene
core, and naphthalene lightꢀharvesting units at the periphery have
been synthesized. The functions performed by the three compoꢀ
nents cooperate or interfere depending on the nature of the metal
ion: Zn(II) coordination allows 100% sensitization of azobenzene
by the lightꢀharvesting antenna, whereas Cu(II) prevents this efꢀ
fect. Zn(II) ions can be released in solution upon photochemical
stimulation. For the first generation dendrimer up to 12 naphthaꢀ
lene units can sensitize the azobenzene isomerization with larger
energy transfer efficiency compared to G0(tꢀAzo). Compared to
the pioneering work by Shinkai et al.,19 the presently investigated
systems couple the photocontrolled tweezering function, the light
harvesting antenna performed by the dendrons and the possibility
to switch ON/OFF the sensitization of azobenzene depending on
the nature of the coordinated metal ion (Scheme 2). The cyclam
moieties cannot discriminate between Zn(II) and Cu(II) ions, but
the resulting complexes can be differentiated on the basis of their
photochemical behavior. A Cu(II)ꢀlike behavior is expected for all
metal ions exhibiting lowꢀlying excited states or easy to oxidize
and reduce that can quench naphthalene by energy or electron
transfer.
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Figure 2. Absorption spectra of a 1.8 x 10ꢀ5 M solution of G0(tꢀ
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Azo) in CH3CN/CH2Cl2 1:1 (v/v) during the titration with
Zn(CF3SO3)2: 0 eq. (thick solid line), 3 eq. (thick dashed line).
Inset shows the normalized absorption changes at 304 nm for
G0(tꢀAzo) (open circles) and G0(cꢀAzo) (solid circles).
Photochemical experiments on the metal complexes of both
dendrimers have been performed upon addition of an excess of
Zn(II) ions (6 eqs. per dendrimer). The presence of two metal ions
per dendrimer in the trans isomer disfavor the trans → cis photoꢀ
reaction, as demonstrated by the lower value of Φt → c upon irradiꢀ
ation at 365 nm and higher molar fraction of trans isomer at the
photostationary state upon irradiation at 365.
Regarding the isomerization sensitized by naphthalene excitaꢀ
Acknowledgments. We gratefully acknowledge MAE (DGPSP Italyꢀ
China) MIUR (PRIN 20085ZXFEE, FIRB RBAP11C58Y) for financial
support and Prof. Vincenzo Balzani for useful discussions.
tion, a strong increase of the efficiency of energy transfer (ηET
)
from naphthalene to azobenzene is observed (Table 1, Scheme 2a)
with unitary values of ηET for metal complexes of G0(tꢀAzo). This
result is consistent with the fact that coordination of Zn(II) to the
cyclam prevents exciplex formation.
Supporting Information Available: Experimental procedures for the
synthesis and characterization, photochemical isomerization and Cu(II)
titration. This information is available free of charge via the internet at
http://pubs.acs.org.
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Scheme 2. Schematic representation of the functions perꢀ
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N atoms are coordinated to a Zn(II) at the same time.
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Journal of the American Chemical Society
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ACS Paragon Plus Environment