Photoresponsive supramolecular self-assembly of monofunctionalized pillar[5]arene based on stiff stilbene

Article abstract of DOI:10.1039/c4cc02760d

We report a photoresponsive monofunctionalized pillar[5]arene based on stiff stilbene. The Z isomer, Z-1, tends to form self-complexing [1]pseudorotaxanes and [c2] daisy chains, whereas the E analog, E-1, forms supramolecular polymers. the Partner Organis

Full text of DOI:10.1039/c4cc02760d

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Photoresponsive supramolecular self-assembly of
monofunctionalized pillar[5]arene based on stiff
stilbene†
Cite this: Chem. Commun., 2014,
50, 7001
Received 14th April 2014,
Accepted 11th May 2014
Yuan Wang,‡ab Jiang-Fei Xu,‡ab Yu-Zhe Chen,a Li-Ya Niu,a Li-Zhu Wu,a
Chen-Ho Tunga and Qing-Zheng Yang*a
DOI: 10.1039/c4cc02760d
www.rsc.org/chemcomm
We report a photoresponsive monofunctionalized pillar[5]arene based
on stiff stilbene. The Z isomer, Z-1, tends to form self-complexing
[1]pseudorotaxanes and [c2] daisy chains, whereas the E analog, E-1,
forms supramolecular polymers.
Pillar[n]arenes (n = 5–10), composed of hydroquinone units
linked by methylene bridges at para positions, are rigid and
easily soluble in organic solvents as well as conveniently
functionalizable with various substituents at the hydroquinone
units.1 Both cations and neutral molecules can be incorporated
into their cavities.2 These attributes make pillararenes attrac-
tive macrocyclic hosts in supramolecular chemistry.3 Mono-
functionalized pillararenes have been shown to have very
interesting self-assembly behaviour, including the formation
of [1]rotaxanes (or [1]pseudorotaxanes),4 [c2] daisy chains5 and
supramolecular polymers.6 The control of the self-assembly
behaviour of monofunctionalized pillararenes by external
stimuli is promising for use in smart functional materials,
but it remains rarely explored.7 Among diverse external stimuli
(chemical, electrostatic and electromagnetic), light is unique in
allowing a remote control of assembly behaviour with poten-
Scheme 1 Chemical structures of Z-1, E-1, E-2.
Stiff stilbene as a novel chromophore in photoresponsive
tially excellent temporo-spatial resolution and without adding supramolecular systems possesses advantages over other chromo-
any chemical agents to the system. phores such as azobenzene. High stability of its Z isomer (with a
Here, we report a photoresponsive monofunctionalized half-life of B109 years at 300 K) and relatively high quantum
pillar[5]arene based on stiff stilbene (1,1-biindane, Scheme 1). yields of photoisomerization of Z and E isomers as well as easy
The configuration of Z-1 is favourable for forming self- peripheral substitution make it a good candidate as a component
complexing [1]pseudorotaxanes or [c2] daisy chains, while E-1 of photoresponsive supramolecular systems.8 Here, we mono-
forms supramolecular polymers. Furthermore, the degree of functionalized pillar[5]arene with stiff stilbene to control the
polymerization of E-1 can be controlled by pH, by protonation self-assembly behaviour by light.
of the imidazole guest.
The 1H NMR spectra of Z-1 at 5 mM in CDCl3 (Fig. S7, ESI†)
contained two sets of resonances for alkyl protons H2–H6
(Scheme 1). We assigned the major set of the resonances that
were substantially shifted upfield to the alkyl ‘‘tail’’ of Z-1 threaded
through the cavity of pillar[5]arene.9 The strong correlations
between these resonances and the bridging methylene protons
Ha of pillar[5]arene (Scheme 1) in a 2-D ROE spectrum (Fig. 1)
further confirmed that the alkyl part was included into the
cavity of pillar[5]arene.
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: qzyang@mail.ipc.ac.cn; Fax: +86-10-62554670
b University of the Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available: Experimental details,
synthesis of compounds, 1H NMR, COSY, DOSY, and other materials. See DOI:
10.1039/c4cc02760d
‡ These authors contributed equally to this work.
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viscosity increased with concentration to the power of 1.45,
suggesting the formation of supramolecular polymers.6a,10,11
The DOSY experiments were consistent with the results of the
viscosity studies (Fig. 2b): when the concentrations of E-1
increased from 20 to 150 mM, the measured weight-average
diffusion coefficients decreased from 9.83 Â 10À10 to 2.14 Â
10À10 m2
s
À1. These results indicate that the E-1 has the
tendency to form linear supramolecular polymers at concentra-
tions above B33 mM, which can be ascribed to the linear and
stiff structure of E-1.
Fig. 1 Particle ROE spectra of Z-1 at 150 mM in a CDCl3 solution.
Owing to the relatively low binding constant of imidazole to
pillar[5]arene in the absence of preorganization (B230 MÀ1),
the degree of polymerization of E-1 was low, as suggested by the
relatively low viscosity of E-1 solutions (Fig. 2a). Because pro-
tonation of imidazole to imidazolium increases the association
constant by B50-fold (to B104 MÀ1) we studied polymerization
of E-2 (Scheme 3).
Scheme 2 The illustrations of the supramolecular assemblies of Z-1.
Adding trifluoroacetic acid to a solution of E-1 at o38 mM
had a negligible effect on solution viscosity. At concentrations
438 mM, protonation of imidazole was associated both with
an increase in the solution viscosity and the dependence of the
viscosity on concentration (i.e., increase in the slope of the
doubly-logarithmic plot from 1.45 to 1.84, Fig. 2). The totality
of the data suggests that protonation of E-1 increases its degree
of polymerization by increasing the affinity of imidazole for
pillar[5]arene.
We studied the aggregate size of Z-1 at 5–150 mM in CDCl3
by two-dimensional diffusion-ordered 1H NMR spectroscopy
(DOSY).10 At 5 mM one set of DOSY signals was observed (Fig. S9,
ESI†), which in conjunction with the presence of two sets of
1
signals in H NMR spectra (above) suggests that at 5 mM Z-1
forms self-complexing [1]pseudorotaxane. Two sets of signals
were found at 20–100 mM, with one set having the diffusion
constant close to that at 5 mM. These data suggest an equilibrium
between [1]pseudorotaxane (higher diffusion coefficient) and
[c2] daisy chains (lower diffusion coefficient, Scheme 2). Finally, at
150 mM, the high-diffusion-coefficient signals disappear, suggest-
ing that the equilibrium is dominated by [c2] daisy chains. We
suggest that the bent conformation of Z-1 sterically suppresses
polymerization. This conclusion was confirmed by viscometry,
which is a classic method to estimate weight distributions of
self-assembled systems. We measured the specific viscosities of
Z-1 solutions in CHCl3 at 291 K (Fig. 2a). We observed a slope
of 1.02 in the double logarithmic plots of specific viscosity versus
concentration at 5–200 mM, consistent with the negligible poly-
merization of Z-1 at these concentrations.
We also obtained E-2 nearly quantitatively by irradiating of
protonated Z-1 which was determined by 1H NMR spectroscopy
(Fig. S6, ESI†). In contrast to our observation of photoiso-
merization of Z-1/protonated Z-1 pseudorotaxanes to E-1/E-2,
a related pseudorotaxane based on the Z isomer of a crowded
aromatic chromophore and protonated amine/crown-ether host–
guest complex (5, Scheme 4) was reported to be stable towards
photoinduced dethreading upon irradiation at the wavelength
where the free Z isomer photoisomerizes efficiently to the E
+
analog.12 In other words, the R2NH2 /crown ether host–guest
complex acted as a ‘‘lock’’ on photoisomerization of the Z
chromophore. Understanding the difference in the response
of Z-1 and Z-5 to irradiation may yield insights into which
of the two chromophores is the more efficient photoactuating
moiety, i.e., one that can operate against larger loads.13
Because photoisomerization of CQC bonds is much faster
(4 ps)8f than host–gust decomplexation (typically ms or longer),9b
the two processes needed for photoinduced dethreading of a
Irradiating of Z-1 at 387 nm resulted in the formation of E-1
with 97% yield which was determined by 1H NMR spectroscopy
(Fig. S5, ESI†). Viscosity of chloroform solutions of E-1 increased
linearly with concentration up to B33 mM, consistent with the
presence of only low-molecular weight species. Above 33 mM
Fig. 2 (a) Specific viscosity of Z-1, E-1 and E-2 in CHCl3 solutions versus the
concentration (291 K). Values on the curves indicate the slope. (b) Concen-
tration dependence of diffusion coefficient D (from 1H NMR spectroscopy
600 MHz, CDCl3, 298 K) of E-1, E-2.
Scheme 3 The illustrations of the supramolecular assemblies of E-1, E-2.
7002 | Chem. Commun., 2014, 50, 7001--7003
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This work was financially supported by the 973 Program
(2013CB933800, 2013CB834505), National Natural Science Foun-
dation of China (21222210, 21072202, 91027041), and the Chinese
Academy of Sciences (100 Talents Program).
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Scheme 4 The structure of Z-5 reported in reference and the illustra-
tions of the photoisomerization processes of Z-1.
pseudorotaxane (photoisomerization of the CQC bond and host–
guest decomplexation, Scheme 4) must occur sequentially. Two
limiting mechanisms are plausible. In a small-molecule analogy
of the power-stroke mechanism,8d,14 absorption of a photon by the
Z chromophore of the pseudorotaxane quickly generates a highly
strained transoid form of the CQC bond. It then slowly (ms) relaxes,
by thermally activated decomplexation (yielding the E isomer) or
thermal isomerization of the highly strained CQC bond (yielding
the reactant). Compressive loads on E-olefins are known to increase
the quantum yield of E - Z photoisomerization and the rate of
thermal E - Z isomerization.8b If the power-stroke mechanism
dominates photodethreading of Z-1 and Z-5, the difference in
photochemical behaviour of Z-1 and Z-5 suggests that either the
quantum yield of E - Z photoisomerization of stiff stilbene is less
sensitive to load than that of the olefin in Z-5, or the larger barrier of
E - Z isomerization of stiff stilbene prevents it from unproductive
relaxation back to Z-1 before undergoing thermal relaxation.
In the alternative (Brownian ratchet),15 photoisomerization occurs
only in the non-pseudorotaxane fraction of Z-1 or Z-5 (i.e., the fraction
+
in which imidazole and pillar[5]arene of Z-1 or R2NH2 /crown ether
of Z-5 are uncomplexed). This thermally populated minor fraction is
trapped by photoisomerization. The dominance of this mechanism
would be suggested if the self-associated constants of Z-1 were
considerably lower than Z-5. We are currently conducting experi-
mental and computational studies to probe these possibilities.
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alized pillar[5]arene based on stiff stilbene. The two isomers of the
molecule, Z-1/E-1, exhibited different self-assembly behaviour. Z-1
forms self-complexing [1]pseudorotaxanes and [c2] daisy chains,
depending on concentration. E-1 tends to form linear polymers,
whose degree of polymerization is sensitive to solution pH. The
complexation of pillar[5]arene/imidazole does not prevent photo-
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