Solid-state photopolymerization of a shape-persistent macrocycle with two 1,8-diazaanthracene units in a single crystal

Article abstract of DOI:10.1021/ja3038905

A macrocyclic monomer with two opposing 1,8-diazaanthracene units is polymerized in a single crystal by a photochemically induced [4 + 4] cycloaddition reaction between neighboring monomers in which the anthracene units are stacked face-to-face at the critical Schmidt distance. The severe structural changes associated with this are minimized by the monomer design, wherein the linkers between the two opposing photoreactive 1,8-diazaanthracene units are connected to the 4 and 5 positions of the latter, whose spatial positioning is changed the least during dimerization. This helps to keep the monomer's overall shape basically unchanged during the polymerization. The resulting new rigid-rod polymer is soluble in its protonated form, and after counterion exchange with a surfactant, it can be depolymerized back into monomer upon relatively mild thermal treatment (120 °C) in an organic solvent.

Full text of DOI:10.1021/ja3038905

Article
pubs.acs.org/JACS
Solid-State Photopolymerization of a Shape-Persistent Macrocycle
with Two 1,8-Diazaanthracene Units in a Single Crystal
Ming Li, A. Dieter Schluter,* and Junji Sakamoto
̈
Department of Materials, Laboratory of Polymer Chemistry, ETH Zurich, Wolfgang-Pauli Strasse 10, CH-8093 Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: A macrocyclic monomer with two opposing 1,8-diazaanthracene units is
polymerized in a single crystal by a photochemically induced [4 + 4] cycloaddition reaction
between neighboring monomers in which the anthracene units are stacked face-to-face at the
critical Schmidt distance. The severe structural changes associated with this are minimized by
the monomer design, wherein the linkers between the two opposing photoreactive 1,8-
diazaanthracene units are connected to the 4 and 5 positions of the latter, whose spatial
positioning is changed the least during dimerization. This helps to keep the monomer’s
overall shape basically unchanged during the polymerization. The resulting new rigid-rod
polymer is soluble in its protonated form, and after counterion exchange with a surfactant, it
can be depolymerized back into monomer upon relatively mild thermal treatment (120 °C) in
an organic solvent.
numerous cases, like the one mentioned above,5 where such
units are offset or not parallel at all.9 1,8-Diazaanthracenes are
considered to be particularly attractive photoactive units in this
context. They should exhibit an even stronger tendency for ftf
stacking in the single crystal than the parent anthracene because
of energetically favorable dipole moment compensation, and it
has been reported that they can be quantitatively converted into
the photodimer.10 Furthermore, if ftf stacking were not
observed, protonation of the nitrogen atoms would be a
potential handle to enforce it through cation−π interactions, as
shown for azaanthracene ftf dimers.11,12
INTRODUCTION
■
Since the pioneering work by Schmidt on photochemically
induced [2 + 2] cycloadditions1 and Wegner on diacetylene
polymerizations2 in single crystals, reactions in the crystalline
solid state have turned into an important tool for the creation
of complex structures.3 Particularly noteworthy are those
reactions in which the single crystals do not suffer
fragmentation, but rather, single-crystal-to-single-crystal trans-
formations are achieved. We recently applied photochemically
induced polymerization with tail irradiation4 to a shape-
persistent C3v-symmetric macrocycle with three 1,8-dialkyny-
lanthrylene units. Because of the layered packing of this
compound in the single crystal and the proper relative
positioning of the alkynyl and anthrylene units of neighboring
monomers, it was possible to induce a laterally proceeding
polymerization reaction based on [4 + 2] cycloaddition that
resulted in the synthesis of a two-dimensional polymer.5
Presently we are exploring the general applicability of this
concept and are interested to see whether similar but
bifunctional monomers can also be employed in such reactions.
They would result in novel linear polymers of the rigid-rod
type. Another and perhaps even more important aspect is
whether changing the photoreactive part would allow the
connection chemistry to be altered from the observed [4 + 2]5
to [4 + 4] cycloaddition6 between adjacent anthracene units of
different monomers. A connection pattern based on [4 + 4]
cycloaddition dimerization across the 9 and 10 positions of
anthracene derivatives would be rather interesting because it
would allow for depolymerization of once-formed polymers
under, for example, relatively mild thermal conditions,6c which
is in principle a matter of relevance to nanolithography.7 There
is a tendency of anthracene units to pack in face-to-face (ftf)
arrangement.8 However, it should also be noted that there are
RESULTS AND DISCUSSION
■
Here we describe the synthesis of macrocyclic monomer 1
containing two opposing 1,8-diazaanthracene units (Figure 1)
and its formation of single crystals in which the monomers are
lined up with the photosensitive units ftf-stacked. We further
report on photopolymerization within the single crystals, the
product of which is a novel linear polymer resulting from
anthracene dimerization.13 The polymer was analyzed by gel-
permeation chromatography (GPC) and tapping mode atomic
force microscopy (AFM) after deposition on a highly oriented
pyrolytic graphite (HOPG) substrate. Finally, evidence is
provided that the polymer can be depolymerized to the
monomer by mild thermal treatment.
The synthesis of monomer 114 started from known
compounds 215 and 416 which were synthesized on 5 and 2−
3 g scales, respectively (Scheme 1).17 Compound 2 was
trimethylsilylacetylenated using conventional Sonogashira
conditions to give 3a, the smooth desilylation of which
Received: April 23, 2012
© XXXX American Chemical Society
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were grown by first dissolving the compound in hot
tetrachloroethane (TCE), then adding tetrahydrofuran (THF)
under mixing until a TCE/THF ratio of 1:1 was reached (final
concentration 40 mg in 20 mL), and letting the solution cool to
room temperature in a vial closed with a plastic cap. In repeated
experiments, the time at which the first (yellow) crystals
formed ranged from a few hours up to a few days. Typical
crystal sizes were ∼100 μm. Figure 2a,b shows a false-color
Figure 1. Chemical structure of monomer 1 in two different
representations.
Scheme 1. Synthesis Sequence Leading to Macrocyclic
Monomer 1
Figure 2. (a) False-color OM micrograph of single crystals of
monomer 1 with cubic morphology obtained from TCE/THF. (b)
Structure of 1 in the crystal (ORTEP plot, 50% probability, H atoms
not shown). (c) 90°-turned views of the packing of monomer 1: width
(w) = 3.5 nm; height (h) = 1.2 nm. (d) Intra- and intermolecular
distances between the 9 and 10 positions of 1,8-diazaanthracenes,
which are ∼7.1 Å (7.034 Å and 7.220 Å) and ∼3.7 Å (3.653 Å and
3.623 Å), respectively. The latter are attractive for photochemical
reactions. The crystals contained both TCE and THF molecules,
which have been omitted for clarity. (e) False-color OM micrograph of
irradiated crystals of 1.
afforded building block 3b. The other starting material, 4, was
converted into dibromide 5, which served as coupling partner
for dialkyne 3b. Combining 3b and 5 gave the prolonged
dibromide 6, which in a final step was cyclized with another
equivalent of 3b to furnish monomer 1. This cyclization was
done at concentrations of 0.1 mM without special precautions
to suppress linear oligomerization. Compounds 3a, 3b, and 5
were purified by silica gel column chromatography, compound
2 by recrystallization, compounds 4 and 6 by filtration and
washing, and finally, monomer 1 by recycling-GPC (r-GPC).
After this procedure, 1 was obtained on a 40−70 mg scale
(overall yield: 12−21%) starting from 100−200 mg of 6 and
45−90 mg of 3b. The cyclization was repeated five times with
very similar results. For synthesis details and analytical data, see
the Supporting Information (SI) and Figures S1−S8.
optical microscopy (OM) image of a crystal of monomer 1 with
a cubic morphology obtained from TCE/THF and the
corresponding ORTEP plot of the structure. Crystallizations
were also performed in pure TCE and TCE/toluene mixtures
to give single crystals with rod and prism morphologies (Figure
S9). Crystals from these morphologies are also photoreactive
but suffer mechanical decomposition within a couple of days.18
Monomer 1 crystallizes in zigzag-type rows with rhombically
distorted macrocycles arranged alternating upside down. Figure
2c shows two perspectives of the packing within such a row.
The 1,8-diazaanthracene units are in fact ftf-stacked, and
important distances between the 9 and 10 positions are given in
Monomer 1 was characterized by NMR and UV/vis
absorption spectroscopy, high-resolution mass spectrometry,
1
and single-crystal X-ray diffraction. Both the H and 13C NMR
spectra showed the expected (small) set of signals in
accordance with the compound’s symmetry. Single crystals
B
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Article
the figure caption. While the intramolecular distances between
the diazaanthracene units are far too large for any reaction to
occur, the intermolecular ones are ∼3.7 Å and thus well within
the photochemically relevant range proposed by Schmidt.1
Before the polymerization of monomer 1 in the single crystal
is described, it is important to briefly consider the spatial
changes expected to take place when covalent bonds are formed
between the 9 and 10 positions of two ftf-stacked 1,8-
diazaanthracenes from adjacent monomer units. Severe
structural changes in going from the monomer to the resulting
repeat unit must be avoided in order to suppress phase changes
in the single crystal during polymerization. Such changes could
exert internal mechanical stress on the crystals that not only
would render polymerization impossible but also could lead to
macroscopic crystal disintegration. Figure 3 compares the
intensity at λ = 370−480 nm, but the signal now tailed all the
way up to λ = 650 nm (Figure S10).
Irradiation was monitored by H NMR spectroscopy. Every
1
couple of days, a small portion of crystals was dissolved in
deuterated trifluoroacetic acid (d-TFA) to give a homogeneous
solution, and the spectra were recorded (Figure S11). The
monomer signals steadily decreased in intensity until finally not
much of them was left; instead, broad, unstructured signals
grew into the spectra, indicating polymer formation and
reasonable conversion (the conversion was not determined
quantitatively). The chemical shifts of what was tentatively
assigned as polymer P were compared with those of model
compound 7, which has the same diazaanthracene dimer core
as expected for P (Figure 4; for the synthesis and X-ray
Figure 3. Structural changes of important positions (1 and 8, 2 and 7,
and 9) associated with dimerization of ftf-stacked anthracene as a
guideline for monomer structure design. The corresponding exact
distances in model dimer 7 are available (see below).
Figure 4. 13C NMR spectra of (top) model compound 7 and
(bottom) polymer P in d-TFA, with signal assignments. Residual
solvent signals are marked (#, TCE; *, THF). The signal marked with
“§” could not be assigned with certainty but may be due to carbon
atom e.
relevant inter- and intramolecular distances between positions
9, positions 1 and 8, and positions 2 and 7 for the parent,
anthracene, before and after dimerization. While positions 9
and positions 2 and 7 change considerably, positions 1 and 8
remain basically unchanged with respect to both relevant
distances. It is for this reason that the polymerizable 1,8-
diazaanthracene units of monomer 1 were integrated into the
structure at their 4 and 5 positions, which are equivalent to the
1 and 8 positions of anthracene.19 This was expected to result
in the least structural impact and thus the highest molar masses.
The UV/vis spectrum in reflection for monomer 1 in the
crystal (after it was ground into a fine powder) showed intense
practically unstructured bands of the diazaanthracenes up to λ =
450 nm (Figure S10). For irradiation, the crystals were air-dried
and deposited in an airtight vial filled with nitrogen. Irradiation
was performed with a 470 nm light-emitting diode and thus
into the tail of the solid-state UV/vis spectrum of the
monomer. During the entire irradiation time (27 days), the
vial was repeatedly turned to ensure that the crystals would
absorb light from all sides. While the initially yellow crystals
were translucent and had luster, they increasingly turned
brownish and nontranslucent. This is considered a sign of
mechanical disintegration, but the disintegration was not
pronounced enough to result in macroscopic crystal
disassembly. The reason for the color change to brown is not
yet understood, particularly in view of the fact that oxygen was
excluded. The UV/vis spectrum of the irradiated crystals
remained structureless and showed a significant reduction of
structure of 7, see the SI and Figures S12−S14). Particularly
characteristic was the bridgehead proton c of 7, which absorbed
at 6.3 ppm; polymer P shows a broad signal at this shift.20 The
molecular structure of the polymer was finally established by
comparison of the 13C NMR spectra of d-TFA solutions of 7
and P (Figure 4). As expected, the chemical shifts of the
bridgehead carbon atoms b and c as well as the methyl group a
at bridgehead atom b were virtually the same for the two
species. The same was true for the acetylenic and aromatic
carbon atoms d and f, respectively. The UV/vis spectra of
monomer 1 and polymer P in TFA are shown in Figure S15.
While polymer P easily dissolved under the acidic conditions
in TFA, the solubility in common organic solvents even at
elevated temperatures was negligible. It is therefore assumed
that TFA protonates P at an unknown number of its six
nitrogen atoms (which are of two different kinds) per repeating
unit (RU), converting the polymer into a polyelectrolyte. For
GPC molar mass determination it was necessary to render P
soluble in solvents such as chloroform or N,N-dimethylforma-
mide. This was achieved by exchanging the trifluoroacetate ion
with 4-dodecylbenzenesulfonate (DBSA), affording PDBSA
1.7
with ∼1.7 DBSA counterions per RU.21 By GPC against
polystyrene standards using chloroform as the eluent, the
C
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Article
(4 h), and 1 kDa (6 h). The signal at 1 kDa eluted at the
monomer retention time, as confirmed by coinjection. The H
NMR spectra of the depolymerized material were complicated
by the substantial amounts of surfactant present, but
particularly at low field, signals due to partially protonated
monomer were visible (Figures S16 and S17). Thus, polymer P
can be completely depolymerized back to the monomer.
number- and weight-averaged molar masses of PDBSA were
1.7
1
determined to be Mn = 44 kDa and Mw = 100 kDa, respectively
(polydispersity index = 2.3), corresponding to average degrees
of polymerization Pn = 20 and Pw = 47 (considering two TFA
and two DBSA counterions per RU). With the assumption that
the RU length is ∼1 nm, the typical contour length of P should
be in the range of a few tens of nanometers.
For AFM visualization, P was drop-cast from TFA solution
onto HOPG (Figure 5), resulting in many smaller (few tens of
CONCLUSIONS
■
In summary, we have shown that the 1,8-diazaanthracene units
of 1 in a single crystal prefer to be ftf-stacked in an antiparallel
fashion. This not only enabled the first photochemically
initiated polymerization of an anthracene-based monomer in
a single crystal via [4 + 4] cycloaddition but also a facile
depolymerization by retro-[4 + 4] cycloaddition of the novel
rigid-rod polymer obtained. We consider this a remarkable
result given the complexity of monomer 1. The unavoidable
structural reorganizations associated with 1,8-diazaanthracene
dimerization can effectively be minimized by proper monomer
design, in which the linkers between the dimerizing units are
connected to the latter at the 4 and 5 positions. The CC
bonds to these positions serve as hinges during dimerization
and keep the monomer’s overall shape basically unchanged.
Depolymerization of polymer P proved that the polymerization
went through a [4 + 4] cycloaddition, as proposed.
Furthermore, it is an attractive property wherever a polymeric
material needs to be cleanly removed (e.g., by laser treatment)
in such a way that the chemistry at the cutting point is defined.
Finally, we consider this a promising starting point for a new
2D polymer synthesis based on trifunctional monomers related
to 1.
Figure 5. Tapping-mode AFM height image of polymer P drop-cast
from a TFA solution onto freshly cleaved HOPG after air drying for 2
h followed by vacuum drying at 20 °C for 12 h.
nm) and larger (few hundreds of nm) ordered domains with
internally parallel features. The small domains are tentatively
assigned to nematically ordered single chains and the large
domains to the same order of end-on aggregated chains. The
domains are turned against each other by ∼60°, which suggests
epitaxial orientation on the graphite surface.22,23 The apparent
heights are happ = ∼0.8 nm, and the average distance between
the polymer chains is d ≈ 4.3 nm. For comparison, the crystal
structure of monomer 1 (Figure 2c) gives an approximate
height and width of each monomer row as h = 1.2 nm and w =
3.5 nm, respectively. We expect h = 1.2 nm to be an
overestimation. While during polymerization the monomer
zigzag conformation will most likely be transferred into the
same for P in the crystal, this conformation will not withstand
the attractive forces exerted on P when adsorbed onto a
substrate; h(Padsorbed) will rather assume a value of ∼0.8 nm
typical for anthracene dimers. Thus, this match looks perfect,
but it should be kept in mind that tapping-mode AFM heights
are apparent and considerable deviations from real heights have
been reported.24 The difference in the object width suggested
by the AFM periodicity, w = 4.3 nm, and the width of P in the
crystal, w = 3.5 nm (as derived from the width of a monomer
row), is not yet understood. The AFM image was taken from a
P without DBSA counterions, and it is unlikely that the TFA
ions alone account for a difference of 0.8 nm. Whether
graphite-induced epitaxial effects also play a role here requires
further investigation.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis, purification and characterization of compounds; UV/
vis spectra of monomer and polymer in TFA solution and in
1
the solid state; different crystal morphologies of 1; H NMR
spectroscopic monitoring of polymerization; and retro reaction
of P
. This material is available free of charge via the
DBSA1.7
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
ads@mat.ethz.ch
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support of this work by ETHZ (Grant ETH-26 10-2) and the
Swiss National Science Foundation is gratefully acknowledged.
We thank Dr. W. B. Schweizer and M. Solar for competent help
with X-ray structure analysis, M. Sc. A. Grotzky for her support
in UV/vis spectroscopical matters, and Dr. D. Kumar as well as
Dr. T. Schweizer (both at ETHZ) for their kind introductions
to AFM and GPC, respectively. Dr. K. Feldman and Prof. P.
Smith (both at ETHZ) are thanked for access to their OM
equipment.
Finally, thermally induced depolymerization was studied. A
solution of PDBSA in tetrachloroethane-d2 was placed in an
1.7
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■
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