Repetitive stepwise rotaxane formation toward programmable molecular arrays

Article abstract of DOI:10.1039/c3cc46859c

We present a novel strategy to synthesize multi-molecular arrays in a programmable way by stepwise elongation based on repetition of two-fold rotaxane formation and construction of threads. A cofacially triply stacked porphyrin array was obtained via the repetitive two-fold rotaxane formation.

Full text of DOI:10.1039/c3cc46859c

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Repetitive stepwise rotaxane formation toward
programmable molecular arrays†
Yasuyuki Yamada,ab Masa-aki Okadaa and Kentaro Tanaka*ac
Cite this: Chem. Commun., 2013,
49, 11053
Received 7th September 2013,
Accepted 4th October 2013
DOI: 10.1039/c3cc46859c
www.rsc.org/chemcomm
We present a novel strategy to synthesize multi-molecular arrays in rotaxane is applicable to pre- and post-metallation of both
a programmable way by stepwise elongation based on repetition porphyrin and phthalocyanine centers, the four-fold rotaxane
of two-fold rotaxane formation and construction of threads. framework is suitable for homo- and hetero-assemblage of
A cofacially triply stacked porphyrin array was obtained via the metal ions. In a homo-dinuclear Cu2+ complex of the four-fold
repetitive two-fold rotaxane formation.
rotaxane, the Cu2+–porphyrin and the Cu2+–phthalocyanine
showed a switching behavior in the spin–spin interaction
Programmable construction of a multiple molecular system is the through a change in the spatial distance between the two metal
best measure for bringing out the emergent functions evolved centers induced by protonation and deprotonation. Hence, the
from the intermolecular communication in the complex molecular rotaxane linkage is reasonably flexible to generate functionalized
assemblies. Especially, controlling sequential and spatial arrange- arrays composed of stacked porphyrins. In this communication, we
ments of building components in a molecular array is crucial for describe a repetitive stepwise two-fold rotaxane formation toward
developing novel functions of a designer nanoarchitecture. synthesis of sequence-controlled stacking arrays of multiple
Porphyrins, phthalocyanines, and their metal complexes have porphyrin components.
been recognized as the important building components of
The stepwise elongation of the stacked porphyrin array is
functionalized molecular systems due to their predictable and based on repetition of two steps (rotaxane formation and
robust frameworks, remarkable stabilities, ability of chemical construction of threads), which reminds us of a coupling–
modification, and photophysical and electrochemical properties.1–4 deprotection sequence of DNA or peptide syntheses, as outlined
One-dimensional stacked columnar arrays and cofacially stacked in Fig. 1. The first step is the two-fold rotaxane formation
dimers and oligomers of porphyrins are promising for the between a porphyrin having two threads (M1) and a porphyrin
above-mentioned functionalized systems.5–7 Sequential and having two peripheral rings (M2) to generate a stacked porphyrin
spatial control of the multiple porphyrin components in the dimer D1. The second step is the construction of two threads
arrays would permit us to generate precise functions and to (D2) to activate the extension end in preparation for the next
produce emergence in chemical properties.
Mechanically interlocked supramolecular linkages such as
catenanes and rotaxanes are flexible enough to provide switching
properties of the interactions among the components in
the assemblies.8–11 Recently we reported the formation of a
porphyrin–phthalocyanine heterodimer in which cofacially
stacked porphyrins and phthalocyanine were linked together
by four-fold rotaxane.12–14 Since the synthetic strategy for the
a Department of Chemistry, Graduate School of Science, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan. E-mail: kentaro@chem.nagoya-u.ac.jp;
Fax: +81-52-789-2940; Tel: +81-52-789-2940
b Research Center for Materials Science, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8602, Japan
c CREST, Japan Science and Technology Agency, Honcho 4-1-8, Kawaguchi-shi,
Saitama 332-0012, Japan
Fig. 1 Schematic representation of the repetitive stepwise synthesis of stacked
porphyrin arrays via two-fold rotaxane formation.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc46859c
c
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Scheme 1 Synthesis of a stacked porphyrin trimer 7 through two-fold rotaxane formation. Reagents and conditions: (i) 2 in CHCl3 : acetone = 8 : 1, (ii) P(OEt)3,
(iii) ion exchange (Clꢀ), 56% in three steps, (iv) TBAF, AcOH in THF, 84%, (v) Ts2O, pyridine in CH2Cl2, 75%, (vi) a-amino-a0-azide-p-xylene, (vii) AcOH, (viii) NaBARF,
7% in three steps, (ix) 2 in CHCl3 : acetone = 12 : 1, (x) P(OEt)3, (xi) ion exchange (Clꢀ), 62% in three steps.
rotaxane formation to give a trimer. Since repetition of these two terminal azide groups were introduced for the next two-fold
steps affords the stacked porphyrin array linked by the sequential rotaxane formation. Repetitive elongation reaction was carried
two-fold rotaxanes, this strategy could allow us to obtain pre- out in the second round of the two-fold rotaxane formation
programmed sequences of multiple porphyrins inside the array.
between 6 and 2 in a similar manner to the preparation of 3 to
Based on this strategy, we designed two porphyrin monomers, afford the stacked trimer 7, which was obtained in 64% yield
1 (BARFꢀ = tetrakis[(3,5-bistrifluoromethyl)phenyl]borate) as the from 6. During the course of the repetitive synthesis, the trimer 7
first terminal unit and 2 as the extension unit for the repetitive and the intermediate rotaxanes leading up to it were easily
synthesis (Scheme 1). The porphyrin 1 has two alkylammonium purified by standard silica gel column chromatography, despite
chains at the trans meso-positions, whereas the monomer 2 has their multivalent ionic character.
diagonally-disposed two 2,6-pyrido-24-crown-8 units whose
The repetitive elongation of the supramolecular array was
positions are spatially compatible with the alkylammonium clearly demonstrated by MALDI-TOF MS (Fig. S1 in the ESI†)
chains of the terminal porphyrin 1. When equimolar amounts and 1H-NMR spectroscopic studies. 1H-NMR spectra of the two-
(1.0 mM) of porphyrin monomers 1 and 2 were dissolved in a fold rotaxane dimer 3 (Fig. S2, ESI†) and the trimer 7 (Fig. S5,
mixture of CHCl3 and acetone (8 : 1 (v/v)), pseudorotaxane ESI†) were analyzed with the aid of 1H–1H-COSY and ROESY 2D
formation occurred immediately via threading of two alkyl- NMR spectroscopic experiments to reveal that they have
ammonium chains into respective crown ethers.15 Staudinger- C2-symmetric structures. Furthermore, the formation of the
phosphite reaction was subsequently employed to lock the two-fold rotaxane structure in 3 was supported by cross peaks
rotaxane linkages by conversion of the small terminal azide between the signals of the crown ethers derived from 2 and
groups into the phosphoramidate moieties which are large those of the alkylammonium chains derived from 1 in the
enough to prevent them from threading out,12–14,16 yielding ROESY spectrum (Fig. S4, ESI†) as in the case of the previously
the desired two-fold rotaxane dimer 3 in 87% yield. As the reported four-fold rotaxane.12 Significant upfield shifts of the
second step (construction of two threads), tert-butyldiphenylsilyl pyrrolic NH protons of the porphyrins in the dimer 3 (observed
(TBDPS) groups on the second unit of 3 were deprotected by at ꢀ3.16 and ꢀ3.74 ppm) and the trimer 7 (observed at ꢀ3.68,
treatment with tetrabutylammonium fluoride (TBAF) to alcohols ꢀ4.17, and ꢀ4.78 ppm) from that of the monomer 2 (observed
(4) which were tosylated to give 5. a-Amino-a0-azide-p-xylene at ꢀ2.00 ppm) are the result of shielded effects of the adjacent
moieties were nucleophilically introduced into 5, followed by porphyrins, which clearly indicate the formation of the stacked
protonation of the resulting secondary amines with AcOH; then structures (Fig. 2).
the counter ions were exchanged with BARFꢀ to yield a por-
The distinct stacking interactions among the porphyrins in
phyrin dimer 6 in which both secondary ammonium cations and the dimer 3 and the trimer 7 were also observed in UV-Vis
c
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In summary, we have developed a repetitive stepwise synthesis
of a cofacially-stacked porphyrin array via two-fold rotaxane
formation between a porphyrin bearing two crown ethers and a
porphyrin bearing two secondary ammonium ions. We demon-
strated the stepwise arrangement of three C2-symmetrically
stacked porphyrins inside a two-fold rotaxane array, whose
structure was confirmed by NMR and UV-Vis spectroscopies.
The most essential factor for development of emergence in
chemical properties is to organize intermolecular communica-
tion among multiple molecular components via molecular
assemblage based on accurate sequential and spatial arrange-
ments. Since our strategy would be applicable to arraying of
various kinds of metalloporphyrins and to formation of longer
sequences, it is a promising way to program intermolecular
communication in the assembly to develop molecular wires, spin
devices, and supramolecular catalysts which have both precise
steric structures and flexible linkages.
Fig. 2 Partial 1H-NMR spectra in CDCl3 of: (a) monomer 2; (b) dimer 3; (c) trimer 7.
This work was financially supported by Grants-in-Aid for
Scientific Research on Innovative Areas ‘‘Coordination Program-
ming’’ (area 2107, No. 21108012) to K. T. of the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
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absorption spectra (Fig. 3 and Fig. S8, ESI,† respectively). The
Soret bands of the monomer units S7 (a precursor of 1: the
detailed structure is shown in the ESI†) and 2 appeared at
442 nm and 421 nm, respectively. In the absorption spectrum
of the dimer 3, two Soret bands were also observed with
hyperchromicity and hypochromicity in the shorter band and
hypochromicity and bathochromicity in the longer band due to
the transition dipole interaction. Two Q-bands derived from
component 2 also red-shifted (600 nm to 608 nm and 691 nm
to 699 nm). These phenomena can be explained by molecular
exciton theory as the hetero H-dimer formation between the two
porphyrins.17 Although the band widths of these chromophores
were broadened by the transition dipole interactions in the
trimer 7 (Fig. S8, ESI†), the absorption spectrum indicated a
pronounced propensity of the stacked layer structure of the
trimer 7 (i.e. further red-shifts of the Q-bands reflect the efficient
interaction between the component 2 in the dimer and another
component 2 which was introduced as the third layer).
`
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 11053--11055 11055