Supramolecular polymerization triggered by molecular recognition between bisporphyrin and trinitrofluorenone

Article abstract of DOI:10.1002/anie.201107655

Networking: A bisporphyrin compound possessing an electron-deficient guest moiety assembles in a head-to-tail manner to form polymeric aggregates. Diffusion-ordered NMR spectroscopy and viscosity measurements of the aggregates confirmed the formation of s

Full text of DOI:10.1002/anie.201107655

Angewandte
Chemie
DOI: 10.1002/anie.201107655
Supramolecular Polymers
Supramolecular Polymerization Triggered by Molecular Recognition
between Bisporphyrin and Trinitrofluorenone**
Takeharu Haino,* Akihide Watanabe, Takehiro Hirao, and Toshiaki Ikeda
Self-assembled supramolecular polymers consist of molecular
components that are held together through noncovalent
interactions. The reversible noncovalent interactions can be
used to produce healable, stimuli-responsive, and switchable
supramolecular polymers. This new class of intelligent
polymer materials, with macroscopic properties that might
be turned on and off by external stimuli, has helped
supramolecular polymer chemistry to gain momentum
within the field of polymer science.[1] The design of well-
organized polymer architectures requires the integration of
certain supramolecular components that must be capable of
creating the strong noncovalent interactions necessary for
producing an appreciable degree of polymerization. Multiple
hydrogen-bonding,[2] hydrophobic,[3] cation–dipole,[4] CH/p,[5]
and aromatic electron donor–acceptor[6] interactions are
often employed in the synthesis of functional supramolecular
polymers.
acceptor-type host–guest motif was incorporated into the
heteroditopic monomer 1. The electron-deficient guest
moiety, 4,5,7-trinitrofluorenone-2-carboxylate (TNF), can
bind within the bisporphyrin cleft through a charge-transfer
interaction, and iterative head-to-tail host–guest complex-
ation should produce
a new supramolecular polymer
(Figure 1). Herein, we report the novel molecular recogni-
tion-directed supramolecular polymerization of monomer 1
in solution and solid state.
Supramolecular porphyrin polymers have recently
attracted attention because of their creative applications in
photoactive devices. Coordination-driven self-assembly is one
of the most useful approaches for building large and elaborate
porphyrin architectures.[7] However, self-assembly of por-
phyrins in organic media, driven by weak noncovalent forces,
such as van der Waals and CH/p interactions, is very limited,[8]
even though the porphyrin moiety possesses a flat and
electron-rich surface that creates the possibility of attractive
van der Waals, stacking, and charge-transfer interactions.
Recently, we have developed a bisporphyrin cleft connected
by a pyridine dicarboxamide linker that assembles to form a
unique complementary dimer in organic media.[9] The com-
petitive complexation of a flat, electron-deficient aromatic
guest into the bisporphyrin cleft leads to a p donor–acceptor-
type host–guest complex.[10] These supramolecular motifs
should be useful for the synthesis of supramolecular porphy-
rin polymers.[11] To investigate this strategy, a p donor–
Figure 1. Heteroditopic monomer 1, its analogue 2, and the supra-
molecular polymer.
The self-assembly of 1 was studied in solution using
fluorescence and UV/Vis absorption spectroscopies (see
Figure S1 in the Supporting Information). The fluorescence
spectrum of 1 in toluene was temperature dependent; strong
emission bands (lex = 501 nm) at 363 K were observed at 657
and 719 nm, which are characteristic of a porphyrin core, but
the emission bands gradually diminished upon cooling the
solution. When the temperature reached 263 K, 80% of the
emission was quenched. The TNF moiety is a good energy
acceptor. Therefore, this quenching can be rationalized by
accommodation of the TNF moiety of 1 into the bisporphyrin
cleft. The UV/Vis absorption spectrum of 1 was concentration
dependent at 298 K in toluene, and plotting the molar
extinction coefficients versus the concentrations gave hyper-
bolic curves. The isodesmic model was applied for the curve-
fitting analysis to obtain the association constant (KE =
42000 Æ 4000 LmolÀ1).[12] In chloroform, the association con-
stant could not be determined in the same way because of
competitive protonation of the porphyrin nitrogen. To
examine the solvent effect for p donor–acceptor-type host–
guest complexation in chloroform and toluene, the associa-
[*] Prof. Dr. T. Haino, A. Watanabe, T. Hirao, Dr. T. Ikeda
Department of Chemistry, Graduate School of Science
Hiroshima University, 1-3-1 Kagamiyama
Higashi-Hiroshima 739-8526 (Japan)
E-mail: haino@sci.hiroshima-u.ac.jp
[**] We are grateful to Prof. Dr. Shin-ichi Kihara at the department of
chemical engineering, graduate school of engineering, Hiroshima
University for helpful discussions about solution rheology. This
work is supported by the JSPS (Japan), the Yamada Science
Foundation, and the Electric Technology Research Foundation of
Chugoku through grant-in-aids for scientific research (B; grant
number 21350066) and the challenging exploratory research (grant
number 23655105).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107655.
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tion constants of 2 and 2,4,7-trinitrofluorenone were deter-
mined (Ka = 250000 Æ 7000 LmolÀ1 and 35000 Æ 1000 LmolÀ1
for chloroform and toluene), and the results clearly indicated
that the host–guest complexation was enhanced more in
chloroform than in toluene.
coefficient of 1 was strongly dependent on its concentration.
At a concentration of 2.34 mmolLÀ1, 1 had a diffusion
constant of 2.96(2) ꢀ 10À10 m2 sÀ1. The similarity to 2 indicates
that, at this concentration, 1 exists in its monomeric form.
Upon concentrating the solution, the diffusion coefficient of 1
decreased nonlinearly to 0.340(5) ꢀ 10À10 m2 sÀ1. The large
difference between the D values of 1 at high and low
concentrations indicates considerably different hydrodynamic
radii, implying that large polymeric aggregates were formed
through self-assembly at the higher concentrations. The
average degree of aggregation can be estimated by simplis-
tically assuming that all aggregates are hydrodynamically
spherical.[14] The average size of the supramolecular polymer
(DP) was calculated to be approximately 660 at a concen-
tration of 66.0 mmolLÀ1.
The head-to-tail complexation between the bisporphyrin
cleft and the TNF moiety was confirmed using 1H NMR
spectroscopy (Figure 2).
A large upfield shift (Dd =
À1.58 ppm) for the inner porphyrin NH of 1 was observed
Viscometry provides fruitful information concerning the
size and structure of supramolecular assemblies in solution.
The specific viscosities for solutions of 1 in chloroform and in
toluene were measured as a function of the concentration at
293 K using a micro-Ubbelohde viscometer (Figure 4). A
double-logarithmic plot of the specific viscosity (hsp) versus
Figure 2. 1H NMR spectra of a) monomer 1 (5.0ꢀ10À3 molLÀ1) and
b) analogue 2 (5.0ꢀ10À3 molLÀ1) at 296 K in [D1]chloroform. The
asterisks denote the aromatic protons of the TNF moiety.
when the acetyl group of 2 was displaced by the TNF moiety
(Figure 2). The aromatic TNF protons of 1 emerged in
unusually upfield regions (d = 4.56, 5.77, 6.36, and
7.05 ppm).[13] Both the porphyrin NH and the TNF moiety
obviously experienced a large shielding effect, placing the
TNF moiety within the porphyrin cleft. Significant intermo-
lecular nuclear Overhauser effects (NOEs) between the TNF
protons and the porphyrin NHs provided evidence for a close
contact, implying the formation of head-to-tail supramolec-
ular polymers (see Figure S11 in the Supporting Information).
Diffusion-ordered spectroscopy (DOSY) was used to
determine the hydrodynamic radii for molecular aggregates
formed in solution. The DOSY experiments for 1 and 2 were
performed in [D1]chloroform (Figure 3). Compound 2 did not
Figure 4. Specific viscosity (h) of 1 in chloroform (*) and toluene
(*) at 293 K. The values denote the slopes.
the concentration of 1 in chloroform or toluene yielded a
linear relationship below the critical polymerization concen-
tration (CPC). The change of viscosity in dependence of the
concentration is indicative of a transition from the dilute to
the semidilute concentration regime where overlap between
chains contributes to a viscous drag. The change was sharper
in chloroform than in toluene, but it occurred in approx-
imately the same concentration region for each solvent
(CPCs: 23.7 mmolLÀ1 and 24.1 mmolLÀ1 in chloroform and
in toluene).
In the dilute concentration regime, the viscosities are low,
and the plots have slopes of 0.95 and 1.15 in chloroform and
toluene, suggesting that the oligomeric assemblies are too
short to be entangled. Above the critical concentrations, the
plots curve upward, reaching a slope of 3.07 in the case of
chloroform. Sizable supramolecular polymers are clearly
formed, and their chains become entangled when the
polymers come into contact, which is commonly observed in
polymer solutions.
^
Figure 3. Diffusion coefficients D of 1 (*) and 2 ( ) at 298 K in
[D1]chloroform.
form any aggregates in solution; in fact, its diffusion
coefficient (Davg = 3.02(8) ꢀ 10À10 m2 sÀ1) was not noticeably
influenced by its concentration. In contrast, the diffusion
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Since the host–guest complexation of the bisporphyrin
and the TNF moieties is driven by noncovalent interactions,
the dissociation and recombination of the supramolecular
polymers occurs on an experimental time scale. The supra-
molecular relaxation regime should obey the mixed reputa-
tion-breakage regime.[15] When the noncovalent bond lifetime
tbreak is greater than trep, the relaxation time t is approximately
equal to trep. This leads to a scaling law exponent of 3.0–3.4,
which is quite consistent with the slope of 3.07 observed in
chloroform. In contrast, the slope of 1.92 in toluene is a little
smaller, most likely suggesting that shorter supramolecular
polymers are formed because of the weaker head-to-tail
connections, which leads to a large polydispersity index
(PDI).
a perpendicular alignment that enhances the oriented fibrillar
nanoassemblies. The development of supramolecular func-
tional materials is currently underway.
Received: October 31, 2011
Revised: December 6, 2011
Published online: January 3, 2012
Keywords: charge transfer · host–guest systems ·
.
molecular recognition · porphyrins · supramolecular chemistry
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