Photoswitchable conductivity in a rigidly dendronized salt

Article abstract of DOI:10.1002/anie.201206010

Light switch: A dendronized salt exhibiting photoswitchable conductivity was designed and synthesized. The salt consists of tetrabutylammonium cations and large, rigidly dendronized borate anions, each bearing eight photoresponsive azobenzene moieties. The conductivity of solutions of this salt can be reversibly switched by irradiation, owing to light-induced changes in the overall size of the dendronized anion and the density of its polyphenylene shell. Copyright

Full text of DOI:10.1002/anie.201206010

Angewandte
Chemie
DOI: 10.1002/anie.201206010
Dendrimers
Photoswitchable Conductivity in a Rigidly Dendronized Salt
Thi-Thanh-Tam Nguyen, David Tꢀrp, Manfred Wagner, and Klaus Mꢀllen*
Advances in synthetic chemistry have enabled the synthesis of
responsive macromolecules[1] that can undergo reversible
structural changes (size, shape, configuration) in response to
an external stimulus, such as a change in temperature,[2] pH,[3]
electric field,[4] or irradiation with light.[5] The structural
changes in macromolecules are usually reflected in a macro-
scopic change in their physical properties (for example,
polarity, mobility, electrical conductivity, catalytic activi-
ty).[1n,5d,6] Light-stimulated systems[5a,7] are receiving increas-
ing attention because they can be remotely controlled and
rapidly changed in a clean and non-invasive manner.[6c,8] For
the design of light-responsive materials, azobenzene contin-
ues to attract considerable attention owing to the substantial
change in its geometry upon cis–trans isomerization. As
a result, azobenzene has led to a variety of photoresponsive
functional materials and devices, including smart polymer-
s,[1g,i,n] molecular machines,[1j] molecular switches,[1h] and
optical storage devices.[9]
Dendrimers,[10] a class of synthetic macromolecules with
well-defined chemical structures, offer the possibility to
precisely insert azobenzene units at well-defined sites.[11]
Depending upon the chemical design of the dendrimer
scaffold, dendritic structures can be rigid[12] or flexible.[10a,b,13]
In the case of flexible dendrimers, the configurational change
in the inserted azobenzene can only affect its direct molecular
environment. Within rigid dendrimers, local changes in
azobenzene configuration can translate throughout the
entire stiff scaffold and thus affect the macromolecule as
a whole. Polyphenylene dendrimers (PPDs),[12a,d] a unique
class of semi-rigid dendrimers, exhibit a significant change in
overall shape upon cis–trans photoisomerization if an azo-
benzene moiety is employed as their core.[14] Recently, we
succeeded in synthesizing a fluorescent polyphenylenene
dendritic host, PDI(Py2-azo2-G3)4, which bears eight azoben-
zene moieties within its scaffold (Figure 1a).[1m] The place-
ment of eight azobenzenes into the rigid scaffold of PDI(Py2-
azo2-G3)4 effectively amplified the impact of azobenzene
configuration on the overall size and shape of the dendrimer;
according to fluorescence correlation spectroscopy (FCS), the
hydrodynamic volume of PDI(Py2-azo2-G3)4 decreased by up
to 66% as a result of the light-induced trans-to-cis isomer-
ization of its azobenzene moieties. This change in overall size
and shape was utilized for a physical encapsulation of guest
molecules within the dendritic interior (retarded leakage) by
Figure 1. a) Structure of PDI(Py2-azo2-G3)4.[1m] b) General structure of
rigidly dendronized fluorinated borate anions.[15]
way of switching from an “open” to a much more compact
“closed” form of the dendritic host.[1m]
In a parallel approach, we took advantage of the rigidity
of PP scaffolds to reduce coordinative interactions between
ions for the ultimate purpose of producing very weakly
coordinating anions (WCAs) through rigid dendronization.
Using the concept of divergent dendritic growth, we were able
to generate rigid, bulky, and specifically fluorinated borate
anions with very large overall sizes in the range of several
nanometers (Figure 1b).[15] In these ions, the rigidity of the
polyphenylene shell prevents backfolding of the PP dendrons
towards the interior, and coordinative interactions between
ions can be effectively reduced. Another effect of polyphen-
ylene dendronization of borates is an increase in their
hydrophobicity, which is reflected in the largely increased
solubility of their salts, even in solvents of low polarity such as
[*] Dr. T.-T.-T. Nguyen,[+] Dr. D. Tꢀrp,[+] Dr. M. Wagner, Prof. Dr. K. Mꢀllen
Max-Planck-Institut fꢀr Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
E-mail: muellen@mpip-mainz.mpg.de
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206010.
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chloroform (dielectric constant e = ca. 4.8) or toluene (e =
ca. 2.4).
We then realized that both concepts could be combined to
generate salts with photo-switchable properties (Scheme 1). It
Scheme 1. Illustration of size and the density switching of a rigidly
dendronized anion by azobenzene photoisomerization.
is known that the conductivity (s) of a solution depends on
the degree of dissociation (a) of the electrolyte and the
mobility (m) of its charge carriers.[16] Upon switching the
configuration of several azobenzenes within the scaffold of
a rigidly dendronized anion from trans to cis, the overall size
of the anion should decrease, resulting in an increase of its
mobility and thus of its solution conductivity. We also
reported that[15] dendronized ions with denser shells are less
coordinating than those with more open dendritic shells. Thus,
the enhanced shielding effect of the dendritic shell in the cis
form of the salt (more densely packed polyphenylene
branches around a borate core) might also induce an increase
in the degree of dissociation between anion and cation, as
compared to the trans form. Both effects
always produces a large number of side-products that cannot
be completely removed, even after several rounds of chro-
matographic purification.
Diels–Alder cycloaddition of branching reagent 6 to the
fluorinated and ethynyl-functionalized tetraphenylborate 7[15]
afforded the first-generation dendronized salt
8
(Scheme 2).[15] To activate borate 8 for a second dendroniza-
tion step, the TiPS-protected ethynyl groups were depro-
tected with tetrabutylammonium fluoride in THF at room
(increased mobility and reduced coordina-
tive strength) should be reflected in an
increase of conductivity upon exposure of
the electrolyte to UV light.
For the synthesis of the desired rigidly
dendronized anion 1, suitably functionalized
AB2 building block 6 was required. Unlike
the tetracyclone used for the synthesis of
dendritic host PDI(Py2-azo2-G3)4,[1m] tetracy-
clone 6 does not contain any pyridine groups
and was synthesized either by Knoevenagel
condensation of azobenzene-ethynyl-TiPS
functionalized benzil 5[1m] (TiPS = triisopro-
pylsilyl) with commercially available 1,3-
diphenylpropan-2-one
in
ethanol
(Scheme 2, Route 2) or by Suzuki coupling
between dibromo-tetracyclone 2 and the
disubstituted azobenzene
4
(Scheme 2,
Route 1). The latter was synthesized from
asymmetrically substituted azobenzene 3[1m]
using an excess of bis(pinacolato)diboron in
the presence of Pd(dppf)Cl2 (dppf = 1,1’-
bis(diphenylphosphino)ferrocene)
and
Scheme 2. Synthesis of second generation dendronized borate 1 with eight photo-switch-
able azobenzene hinges within its polyphenylene scaffold. Reaction conditions: a) NaOH,
EtOH, 15 min, 80%; b) bis(pinacolato)diboron, Pd(dppf)Cl2, AcOK, dioxane, 808C, 24 h,
argon, 95%; c) Pd(PPh3)4, K2CO3, nBu4NBr, toluene/H2O, 808C, 24 h, 93%; d) TBA+OHꢀ,
tBuOH, 808C, 53%; e) o-xylene, 1558C, 40 h, 36%; f) TBAF, THF, 84%; g) o-xylene,
1558C, 12 h, 58%. AcO=acetate, dppf=1,1’-bis(diphenylphosphino)ferrocene, TBA=
tetrabutylammonium.
potassium carbonate. The selected sequence
of reactions (Route 1) furnishes pure tetra-
cyclone 6 in good yield because it avoids
performing the Knoevenagel reaction
directly on the expensive compound
(Route 2): the latter condensation reaction
5
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temperature. The resulting borate 9 was then converted into
the second generation PP-dendronized borate 1 by Diels–
Alder cycloaddition with commercially available tetracyclone
10 in o-xylene.
The reaction progress was monitored by means of
MALDI-TOF mass spectrometry. After completion of the
reaction, conventional column chromatography was sufficient
to isolate borate salt 1 as an orange powder with high purity,
as evidenced by both MALDI-TOF and NMR spectroscopy.
The MALDI-TOF mass spectra of each compound (8, 9, and
1) revealed a single intense signal corresponding to the
calculated mass of each dendrimer (Figure 2; see also the
Supporting Information, Figures S6 and S7).
Figure 3. Change in UV/Vis absorption spectra (absorption band of
azobenzene) of a solution of borate 1 in THF (1.25ꢁ10ꢀ6 m) after
exposure to light of either 365 nm (trans to cis) or 450 nm (cis to trans)
wavelength.
and shell density. These models (Figure 4) indicate a notice-
able change in anion size and polyphenylene density around
the borate anion upon switching azobenzenes from trans to
cis.
Unfortunately, independent methods for detecting the
size change of borate anion 1 before and after irradiation,
such as the use of dynamic light scattering (DLS), size
exclusion chromatography (SEC), or fluorescent correlation
spectroscopy (FCS), were precluded owing to the individual
limitations in these methods: 1) The size of dendritic salt
1 (ca. 4 nm in diameter) is below the lower threshold
detection by DLS and thus the difference between cis-1 and
trans-1 could not be distinguished.[18] 2) The electrolyte
character of such a salt causes an undesired interaction with
GPC gel, and consequently no product was recovered after
passing through a GPC column.[19] 3) Dendritic salt 1 does not
emit any fluorescence, and thus cannot be tracked by FCS, as
with PDI(Py2-azo2-G3)4.
Figure 2. MALDI-TOF mass spectrum for second generation PP-
dendronized borate 1. THF as solvent, dithranol as matrix, analyte/
matrix=1:250.
To test whether the change in configuration of azobenzene
moieties (trans to cis) is possible within dendronized borate
salt 1, UV/Vis absorption spectra of a THF solution of
1 before and after irradiation at 365 nm were recorded.
Figure 3 shows the absorption spectra of 1 in THF before
(solid black curve) and after (solid red curve) irradiation. The
measurement confirms that a configurational change in
azobenzene (AB) moieties within borate anion 1 is possible.
Compared to the non-irradiated solution, the absorption band
at 370 nm (p–p* transition of trans-AB) clearly decreases,
whereas the absorption band at 460 nm (n–p* transitions of
cis-AB)[17] increases slightly. Similar to the azobenzene–
dendritic host PDI(Py2-azo2-G3)4,[1m] the change in absorption
band intensities indicates a conversion of the all-trans
dendrimer 1 (8t) into species containing on average 3.04
trans and 4.96 cis AB moieties, at the photostationary state
(PSS; Figure S1). The quantum yields of the 1(8t)!1(3t5c)
and 1(3t5c)!1(8t) photoisomerization reaction were deter-
mined to be Ft-c = 0.22 ꢁ 0.04 and Fc-t = 6.70 ꢁ 0.25 (Figur-
es S2 and S3).
However, DOSY-NMR spectra of 1 before and after UV
irradiation at 365 nm were recorded in [D8]THF (Figure S8).
Figure 4. Effect of irradiation at different wavelengths (365 nm and
450 nm) on the molar conductivity (L) of a solution of 1 in THF
(5ꢁ10ꢀ4 molLꢀ1); left: all azobenzene moieties in the trans configura-
tion; right: all azobenzene moieties in the cis configuration.
Space-filling models of anion 1 were generated to
illustrate the effect of azobenzene switching on anion size
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Table 1: Results of concentration-dependent measurements of conductivities of borate salt 1 in THF
before and after irradiation at 365 nm.
Although DOSY-NMR spectros-
copy enables direct measuring of
molecular mobility, the method is
naturally limited to non-infinitely
diluted solutions. The mobility
values observed by means of
DOSY-NMR are therefore aver-
aged over associated and non-asso-
ciated ions. Intrinsic ion mobility
without ion association cannot be
directly obtained, and the observed
signals are broadened because of
the ion association effects.[20] Nev-
Entry c [molLꢀ1
]
Non-irradiated
Irradiated at 365 nm
s [mScmꢀ1
]
L [Scm2 molꢀ1
]
a
s [mScmꢀ1
]
L [Scm2 molꢀ1
]
a
1
2
3
4
5
6
1.56ꢁ10ꢀ5
0.56
1.05
1.97
3.61
6.49
35.84
33.60
31.52
28.88
25.96
22.50
1.014
0.951
0.892
0.817
0.735
0.65
1.24
2.36
4.40
8.00
41.60
39.68
37.76
35.20
32.00
28.00
1.006
0.960
0.913
0.851
0.774
0.677
3.13ꢁ10ꢀ5
6.25ꢁ10ꢀ5
1.25ꢁ10ꢀ4
2.50ꢁ10ꢀ4
5.00ꢁ10ꢀ4 11.25
0.637 14.00
a=dissociation, L=molar conductivity, s=conductivity.
ertheless, a general increase in average ion mobility could be
detected as a result of azobenzene switching from trans to cis
within borate 1. At a concentration of about 5 ꢀ 10ꢀ4 molLꢀ1,
the average mobility of the anion increased by about 17%
from 5.6 ꢀ 1010 skgꢀ1 to 6.6 ꢀ 1010 skgꢀ1, which corresponds to
a decrease in its hydrodynamic radius from 1.9 nm to 1.6 nm,
as calculated from the different diffusion values of compound
1 before and after irradiation at 298 K in [D8]THF.
Conductivity measurements were performed to monitor
the impact of configurational changes in the azobenzene
moieties within salt 1. The non-irradiated solution of 1 in dry
THF (0.5 mm) exhibited a molar conductivity (L) of (22.5 ꢁ
0.1) Scm2 molꢀ1 at room temperature. During the time period
after exposure of the solution to 365 nm UV light (to convert
the azobenzenes from trans to cis) until the system reached
the photostationary state, the L of the solution reached
a value of (28.0 ꢁ 0.1) Scm2 molꢀ1, which corresponds to an
increase in conductivity by 24%. The L could then be
reduced back to about 23 Scm2 molꢀ1 by irradiating the
solution with 450 nm blue light (cis to trans).[21] The observed
increase in solution conductivity upon switching azobenzene
from trans to cis can be attributed to two effects:
as a converges to 1 in both solutions, irrespective of their
irradiation history. According to the Ostwald law of dilution,
a limiting molar conductivity (Lo) at infinite dilution can be
determined by linear extrapolation of the inverse molar
conductivity 1/Lo to a concentration of c = 0 molLꢀ1. As
a approaches 1 at infinite dilution within each electrolyte,
a change in Lo owing to irradiation reflects an intrinsic
property change in the electrolyte. An increase in Lo can thus
be solely attributed to an increase in ion mobility. In the case
of borate 1, the limiting molar conductivity Lo at infinite
dilution increases by about 17% (from (35.34 ꢁ 0.70) to
(41.36 ꢁ 0.52) Scm2 molꢀ1) as a result of azobenzene switching
from trans to cis. This increase in limiting molar conductivity
should thus reflect a proportional increase in anion mobility.
The increase in the conductivity of salt 1 upon trans to cis
photoisomerization of azobenzene therefore mainly stems
from an increase in the mobility of the anion (a decrease in its
overall size). Nevertheless, the decrease in the coordination
strength between anion and cation (the enhanced shielding
effect of the dendritic anion shell) also plays a role, albeit
much less pronounced.
In summary, we have herein described the first example of
a borate salt in which the ion conductivity could be switched
by light. This salt is made of a rigidly dendronized anion that
contains several photo-switchable azobenzene moieties
within its scaffold and a tetrabutylammonium ion as the
counterion. The successful synthesis of this nanometer-sized
and specifically functionalized borate anion is a result of
combining several synthetic strategies recently developed
within our group. A photochemical study on the obtained
macromolecular salt 1 by UV/Vis spectroscopy confirmed
that the azobenzene moieties within the dendritic scaffold of
the anion could be switched from trans to cis and back. It was
further shown that this configurational switch had a marked
effect on the conductivity of electrolyte solutions of the
dendritic salt. This example demonstrates the great potential
of rigid PPD chemistry for the specific design of defined
macromolecules with tailored and even switchable material
properties. In the future, the impact of azobenzene config-
uration on electrolyte properties could be further enhanced
by utilizing bulkier and highly fluorinated building blocks for
dendronized ion end-capping.
1) The effect of azobenzene configuration on the poly-
phenylene density around the borate core: if the azobenzene
moieties in borate 1 are in a cis configuration, the negatively
charged core can be more efficiently screened, owing to
denser packing of the polyphenylene shell, as compared to
that of the trans configuration.[15] One therefore expects
higher degrees of dissociation (a) if the azobenzene moieties
within borate 1 predominantly adopt the cis configuration.
2) The effect of azobenzene configuration on anion size:
along with increased density, the overall size and extension of
borate anion 1 decreases if its azobenzene moieties are
switched from trans to cis. This size decrease should give rise
to an increase in ion mobility.
To obtain information about the individual contribution
of both the degree of dissociation and the mobility (m) to
overall conductivity, we measured the conductivity as a func-
tion of the concentration of electrolyte 1.
The conductivities of solutions of 1 varying in concen-
tration from 1.56 ꢀ 10ꢀ5 m to 5.00 ꢀ 10ꢀ4 m in dry THF were
measured before and after UV-irradiation. This series of
measurements (Table 1) reveals that the degree of dissocia-
tion increases with decreasing salt concentration. Then, the
difference in the degree of dissociation of irradiated and non-
irradiated solutions decreases with decreasing concentration
Received: July 26, 2012
Revised: September 21, 2012
Published online: November 27, 2012
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