Oligoamylose-entwined porphyrin: Excited-state induced-fit for chirality induction

Article abstract of DOI:10.1039/c5cc08488a

An oligoamylose-strapped porphyrin displayed circularly polarized luminescence (CPL) in the S₁ state despite being silent in circular dichroism (CD) in the ground state, suggesting chirality induction in the photoexcited porphyrin moiety from t

Full text of DOI:10.1039/c5cc08488a

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Oligoamylose-entwined porphyrin: excited-state
induced-fit for chirality induction†
Cite this:DOI: 10.1039/c5cc08488a
Mitsuhiko Morisue,*a Takashi Yumura,b Risa Sawada,c Masanobu Naito,d
Yasuhisa Kurodaa and Yoshiki Chujoc
Received 13th October 2015,
Accepted 14th November 2015
DOI: 10.1039/c5cc08488a
www.rsc.org/chemcomm
An oligoamylose-strapped porphyrin displayed circularly polarized
In contrast to the conventional approaches, we investigated
luminescence (CPL) in the S1 state despite being silent in circular whether an induced-fit system8 could be adaptable to induce
dichroism (CD) in the ground state, suggesting chirality induction in chirality in an intrinsically achiral chromophore in the photo-
the photoexcited porphyrin moiety from the oligoamylose-strap in excited state in a chiral environment. In this context, a spiral
the photoexcited state.
channel of a chiral biological entity may be appropriate to
transfer chirality to the intrinsically achiral chromophore. This
Chromophores that show circularly polarized luminescence report describes a comprehensive proof-of-concept study to
(CPL) are an attractive class of optical materials.1 CPL originates tackle challenges in developing a new class of CPL materials.
from chiral dispersive interaction in the photoexcited state, as
A spiral channel can be created by length-mismatched chain
approximated by the quotient of the electronic dipole moment clamping between each terminal of a rigid rotatable axle, which
divided by the magnetic dipole moment.2 Therefore, CPL is an leads to the axle moiety self-encapsulating in the channel. For
alternative application of electromagnetic properties of light in example, clipping the side-groups of the oligomeric p-systems has
Maxwell’s equation. A photoexcited chiral chromophore can been explored to provide helical p-systems, which have been coined
9
¨
¨
generate CPL even when no chiroptical properties are observed as ‘‘gelander oligomers’’ by Vogtle and coworkers. Gladysz and
in the ground state.3 However, organic chromophores that exhibit co-workers also reported a similar strategy to form spiral structures
large differential luminescence of left- or right-handed CPL have entwining a stiff axle.10 Such a vine-twining analogy is also
been primarily limited to chiral chromophore aggregates.1a–c intriguing to tune the electronic properties by twisting the axle of
Otherwise, the structurally rigid p-framework including helicenes,4 the chromophore.11 Here we present a biological entity as a chiral
biaryls,5 and sophisticatedly designed molecules6 has been adaptable channel that includes an achiral chromophore.
indispensable to suppress racemisation both in the ground state
Amylose is a polysaccharide composed of a D-glucose backbone
and the excited state. In this context, exploration of non-aggregated connected via a(1 - 4)-glycoside linkages, adopting a 61 left-handed
and non-polymeric CPL-active chromophores is a significant helix (6 glucopyranose units in one turn).12 This is of particular
challenge.1d,7 Given that there is a trade-off between the electronic interest because the formation of the amylose–iodine inclusion
dipole moment and the magnetic dipole moment, a conforma- complex, which is known as a ‘‘starch blue reaction’’, is one of
tional change of a chromophore in the photoexcited state could the most representative examples of an induced-fit host–guest
be a potential strategy to impart chiral nature to the photophysical binding of a helical polymer.13 The interior channel of the folded
properties of the chromophore.
amylose encapsulates versatile lipophilic guest molecules, such
as chromophores and polymers.14,15 The most representative
examples are cyclic analogues of oligoamylose (cyclodextrins),
which have been key elements in host–guest chemistry,16 and
some of which can exhibit CPL.17
Herein, we design an oligoamylose-strapped porphyrin 1
(Scheme 1), with which an achiral porphyrin axle is self-folded
by a helically twisted polysaccharide. A flexible oligoamylose-
strap (per-O-methylated maltoheptaose) was tethered to the rigid
porphyrin, wherein an interaction between the oligoamylose and
porphyrin moiety occurred based on the fact that cyclodextrin
derivatives were able to form exceptionally stable inclusion
a Faculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology,
Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. E-mail: morisue@kit.ac.jp
b Faculty of Material Science and Engineering, Kyoto Institute of Technology,
Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
c Department of Polymer Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
d National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki, 305-0044,
Japan
† Electronic supplementary information (ESI) available: Synthetic procedures,
NMR spectra, time-resolved fluorescence spectra, and computational studies. See
DOI: 10.1039/c5cc08488a
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Self-folded conformations were rationally simulated by the
computational energy-minimized model study (B97D). In the folded
conformation of 1 (Fig. 1A), the distance between the 5-oxygen atom
of the glucose-(III) ring and the zinc atom is 2.3 Å, suggesting the
presence of a coordination bond. The presence of multiple CH–p
interactions between the glucose and porphyrin plane was also
predicted. The folded conformation gave rise to the structural strain
of the oligoamylose-strap, leading to a ‘‘chair’’-to-‘‘boat’’ conforma-
tional change of the glucose-(III) ring.
The folded conformation of 1 was verified by NMR spectroscopy
in toluene-d8. The aromatic resonances of the porphyrin ring were
heterogeneously multiplied due to the asymmetric environment of
the oligoamylose strap (Fig. 1A). Similarly, some of the methyl
protons of the oligoamylose-strap shifted upfield due to shielding
in the vicinity of the porphyrin moiety. The computational structure
was also supported by unambiguous nuclear Overhauser effect
(NOE) correlations between the protons of the oligoamylose-strap
and the aromatic resonances of porphyrin-b and meso-aryl groups
(Fig. S1, ESI†).25 Therefore, 1 formed a self-inclusion complex at
ambient temperature in toluene.
The addition of pyridine-d5 as a competitively coordinating
axial ligand (Ka = 2.6 Â 104 MÀ1) disturbed the glucose-to-zinc
axial coordination (Fig. 1B). Then, 1ÁPy showed no substantial NOE
correlation between the aromatic resonances of the porphyrin ring
and the oligoamylose strap, unlike the results observed in toluene-d8.
The glucose C1-protons (5.2–4.0 ppm) mostly shifted downfield
upon addition of pyridine-d5, suggesting that the protons of
Scheme 1 Chemical structure of oligoamylose-strapped porphyrin 1.
complexes with aqueous porphyrinoid.18,19 With the aim of creating
an induced-fit system through intramolecular host–guest inter-
actions, the rotatable ethynylene linkages may be effective in
inducing helicity around the porphyrin plane. Moreover, achiral
porphyrin macrocycle is a privileged substrate to induce chirality.20
At the same time, such a macrocyclic cavity composed of a
porphyrin–cyclodextrin hybrid is also intriguing as a novel structural
unit, the so-called ‘‘element-block’’,21 in host–guest systems.22
The synthetic route is outlined in Scheme 2. Single scission
of the glycoside linkage opened the ring of per-O-methylated
b-cyclodextrin,23 and both the 1(I)-hemiacetal and 4(VII)-hydroxy
terminal of the methylated maltoheptaose 2 was simultaneously
subjected to the Dudley’s benzyl-transfer protocol24 with 3, and 4 was
obtained in 30% yield. Then silylated acetylene was introduced
to each bromine-terminal of 4 by the Sonogashira–Hagihara
coupling reaction to give 5, which was then desilylated. The
linear oligoamylose 6 was re-cyclized with the corresponding
porphyrin. The oligoamylose-strapped porphyrin 7 was metallated
with zinc ion. The products were satisfactory to identification by
NMR and MALDI-TOF MS spectra, as described in the ESI.†25
Fig. 1 1H NMR spectra and computer-optimized structures of folded 1 in
toluene-d8 (A) and semifolded 1ÁPy in the presence of 5% of pyridine-d5
(B) at 298 K. Orange and green circles indicate the b-protons as indicated
in Scheme 1.
Scheme 2 Synthetic route of oligoamylose-strapped porphyrin 1.
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the oligoamylose strap were less shielded by the porphyrin ring was +4.5 Â 10À4 at 452 nm for folded 1 and +6.0 Â 10À4 at
(Fig. 1B). At the same time, the methyl protons at 3.0–2.5 ppm 454 nm for semifolded 1ÁPy. In contrast, the CD signal was only
shifted downfield. These results suggest that the folded conforma- faint at the Q band (S1 state approximately 650 nm; gabs
o
tion of 1 was disturbed in the presence of pyridine. Nevertheless, B10À5 for 1 and 1ÁPy). In these cases, the Qx transition is
the oligoamylose-strap retained helical conformation. Actually, the predominantly based on the TD-DFT calculation.25,32 Such diagonal
b-protons at 10.3–9.9 ppm indicated an increase in dissymmetrisa- orientations of the Qx transition dipole could account for the
tion along the x direction following the addition of pyridine. The contrasting chiroptical properties of the Q band in the CD spectra.
optimized structure suggested that the porphyrin moiety remained
Porphyrin 1 showed fluorescence with absolute quantum
in the self-included structure even after pyridine bound the axial yield (F) = 0.12 and 0.13 in the absence and presence of pyridine,
position. These experiments prove that there are intramolecular respectively (Table 1). Upon excitation of the Soret band (450 nm),
interactions between the oligoamylose-strap and the porphyrin fluorescence was obtained from the Q band through internal
moiety. It is known that dipole interactions play a crucial role in conversion. The time-resolved fluorescence spectroscopy exhibited
the host–guest interaction of the polar cavity of cyclodextrin26 in biexponential decay components regardless of the absence or
aqueous media18a,27 and in nonpolar solvents.28 Eventually, in the presence of pyridine, suggesting structural relaxation in the excited
presence of pyridine, 1 adopted the semifolded conformation.
state,25 i.e., the induced-fit conformational change in the photo-
The chiroptical properties of 1 in the ground state were excited state. Considering the fact that the photoexcited porphyrin
verified by circular dichroism (CD) (Fig. 2B). An intense positive plane is ruffled,33 the porphyrin plane could be reorganized into a
sign of induced CD (ICD) emerged at the Soret band (S2 state at helical fashion at the photoexcited state. Moreover, it was recently
452 nm) of the achiral porphyrin moiety. It is known that reported that the vibrational mode is coupled with the photoexcited
glucose alone is not sufficient to induce a CD signal on the Qy transition.30 Together with the above TD-DFT results, we
Soret band.29 Instead, assuming that 1 was amenable to the hypothesize that the electronic transition anisotropy is diagonally
ICD rule for an encapsulated molecule in a cyclodextrin cavity,30 reorganized from the Qx to Qy transition at the photoexcited state.
the positive CD signal suggested that the orientation of the Under these conditions, 1 could display CPL.
porphyrin moiety was laid along the axis of the helical channel
Indeed, the emission from the S1 state of 1 was circularly
of the oligoamylose-strap, where the Soret transition along the y polarized (Fig. 2B). The luminescence with an anisotropy factor,
direction is predominant in electronic transition based on the glum, of +2 Â 10À4 at 664 nm and +4 Â 10À4 at 674 nm was
time-dependent (TD)-DFT calculation.25,31 The helically twisted observed as folded 1 and semifolded 1ÁPy, respectively. These
conformation of the meso-aryl groups along the y direction may values maintained similar magnitudes of the gabs values at the
contribute to chiroptical properties to some extent. The absorp- Soret band. These results are in sharp contrast to the Q band
tion dissymmetry factor, gabs = De/e = 2(eleft À eright)/(eleft + eright), that is substantially silent in the CD spectra in the ground state.
From these results, we conclude that the conformational
change caused by photoexcitation induced chirality in the
intrinsically achiral S1 state of the porphyrin moiety of 1.
Moreover, the somewhat enhanced glum value resulting from
the addition of pyridine suggests that the more adaptable the
induced-fit system the more effective the induction of chirality.
In conclusion, we demonstrated chirality induction based
on the excited state induced-fit mechanism. A spiral channel
created by a flexible oligoamylose-strap definitely had a chiral
impact on the photoexcited conformational change of the
achiral porphyrin axle to exhibit explicit CPL, although the glum
magnitude of 1 still ranged at a moderate level for a non-
aggregated chromophore. Excited-state induced-fit mechanisms for
chirality induction have highlighted a new potential approach to
develop CPL-active chromophores. A further study of host–guest
Table 1 Photophysical properties of the S1 state of 1 upon excitation at
450 nm
max/nm Fa
t (a)b
glum
c
l
1
1ÁPy
a
659 (0.12)
666 (0.13)
0.51 (19%), 1.98 (81%)
0.44 (21%), 1.68 (79%)
+2 Â 10À4 (664 nm)
+4 Â 10À4 (674 nm)
Fig. 2 (A) Absorption spectra of 1 (1 Â 10À6 M) in the course of addition of
pyridine (up to 3750 equiv.; red to green) in toluene at 298 1C. The inset
shows normalized fluorescence spectra (lex = 450 nm). (B) CD spectra of
folded 1 (red) and semifolded 1ÁPy (green) in toluene. The inset shows CPL
spectra (lex = 450 nm) with fitted curves (thick line).
b
Emission maxima (lmax) and absolute quantum yield (F). Fluores-
cence lifetime (t) and the normalized amplitude (a) determined from
c
the fluorescence decay profiles. Luminescence anisotropy factor,
glum = DI/I = 2(Ileft À Iright)/(Ileft + Iright).
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This work was partly supported by a Grant-in-Aid for Scientific
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Innovative Area ‘‘Stimuli-responsive Chemical Species for the Crea-
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