Metal ion clip: Fine-tuning aromatic stacking interactions in the multistep formation of carbazole-bridged zinc(ii) complexes

Article abstract of DOI:10.1039/c5cc03281d

A carbazole-based triple bridging ligand (LH) consisting of two imidazole moieties at 3,6 positions with a diketone unit at the carbazole nitrogen forms carbazole-bridged zinc(ii) complexes with structures of [(L^-)₄(Zn²⁺)<

Full text of DOI:10.1039/c5cc03281d

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Metal ion clip: fine-tuning aromatic stacking
interactions in the multistep formation of
Cite this:DOI: 10.1039/c5cc03281d
carbazole-bridged zinc(II) complexes†
Yuki Imai,a Tsuyoshi Kawai*a and Junpei Yuasa*ab
Received 21st April 2015,
Accepted 11th May 2015
DOI: 10.1039/c5cc03281d
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A carbazole-based triple bridging ligand (LH) consisting of two imid- ‘‘loose’’ and ‘‘tight’’ dimer contacts between the aromatic rings
azole moieties at 3,6 positions with a diketone unit at the carbazole for the first time. In this method, hereafter referred to as metal
nitrogen forms carbazole-bridged zinc(II) complexes with structures of ion clip, three distinct potential binding sites for metal ions were
[(LÀ)4(Zn2+)n] (n = 2–6), where the strength of aromatic stacking introduced into a carbazole ring (LH),9 two imidazole moieties at
interactions between the carbazole rings increases with an increase 3,6 positions and a diketone unit at the carbazole nitrogen, the
in the number of Zn2+ ions bridged by the imidazole moieties.
latter of which is the stronger binding site. Diketone ligands often
give L2M type complexes through deprotonation of the a-proton
The strength of aromatic stacking interactions is an important when they interact with divalent cations such as zinc(II) (Zn2+).10 In
determinant of the chemical and physical properties of aromatic such preorganization [(LÀ)2(Zn2+)m], the two aromatic surfaces are
compounds as well as chemical and biological recognition.1 adjacent to each other, resulting in four distinct potential binding
Stacking interactions for typical aromatic–aromatic contacts sites for metal ions (Scheme 1). In this communication, we demon-
are weak,2 and normally afford only stacked (on) and unstacked strate how this approach can modulate aromatic stacking inter-
(off) states in solutions. The strength of attractive interactions actions, enabling ‘‘loose’’ and ‘‘tight’’ dimer contacts.
between aromatic units can be improved if additional inter-
Complex formation of LH with Zn(OTf)2 (OTf = OSO2CF3)
actions are involved,3 such as metal–ligand interactions.4–7 Hence, was examined using UV-vis titration experiments at two different
self-assembly via metal–ligand complexation could be a unique initial LH concentrations: [LH]0 = 2.0 Â 10À5 M and 2.0 Â 10À4 M
approach to modulate stacking interactions. Previously, metal– in methanol. In the case of low initial concentration of [LH]0 =
ligand directed assembly approaches containing distinct binding 2.0 Â 10À5 M (Fig. 1a), LH shows a biphasic UV-vis spectral
sites have mainly focused on the cooperative binding of metal ions change in response to the molar ratio of [Zn2+]/[LH]0 = 0–0.5 (red
for development of a nonlinear response to achieve ‘‘on/off’’ lines) and 0.5–3.1 (blue lines). The absorption band at the peak
switching under specific threshold conditions,4 or for development wavelength (lmax = 311 nm) decreases with a clear isosbestic
of excimer based turn-on fluorescence sensors.5 Alternatively, we point at l = 355 nm at [Zn2+]/[LH]0 = 0–0.5 (Fig. 1a, red lines),
have recently reported metal-assisted p-association (on) and dis- while the peak shoulder at around l = 340 nm gradually increases
sociation (off) of aromatic rings consisting of imidazole moieties, with the increasing ratio of [Zn2+]/[LH]0 = 0.5–3.1 (Fig. 1a, blue
in which multistep complex formation regulated by the metal ion lines). Such biphasic UV-vis spectral changes are ascribed to
concentration can produce only two distinct complexes with (on) stepwise complex formation8,11 between LH and Zn2+, a first
and without (off) stacked aromatic units.8
Zn2+ ion binds to the diketone unit through deprotonation of
We report herein the fine-tuning of aromatic stacking inter- the a-proton (Scheme 1a), after which the second Zn2+ ion is
actions regulated by the metal ion concentration, enabling coordinated to the imidazole nitrogen above [Zn2+]/[LH]0 = 0.5
(Scheme 1b). Negative control experiments were conducted
with a reference compound (LH’) to confirm that the imidazole
moieties serve as the distinct binding sites for the second Zn2+
ion. The reference compound (LH’) has no imidazole moiety
but has the diketone unit at the carbazole nitrogen (Scheme 1).
LH’ shows a monotonous UV-vis spectral change during the
titration of Zn2+ with clear isosbestic points (see ESI,† S2),
suggesting formation of a single complex species upon binding
of Zn2+ to the diketone unit. The negative result obtained with
a Graduate School of Materials Science, Nara Institute of Science and Technology,
8916-5 Takayama, Ikoma, Nara 630-0192, Japan. E-mail: yuasaj@ms.naist.jp,
tkawai@ms.naist.jp; Fax: +81 743-72-6179
b PRESTO, Japan Science and Technology Agency, Kawaguchi, 332-0012, Japan
† Electronic supplementary information (ESI) available: Experimental section,
titration of LH’ with Zn2+, excitation spectra, NMR titration of LH with Zn2+
,
titration of LH with triflic acid, calculated isotopic distributions, temperature
dependence of emission spectra, and monomer–dimer simulation. See DOI:
10.1039/c5cc03281d
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Fig. 1 UV-vis absorption spectra of LH (a) (2.0 Â 10À5 M) and (b) (2.0 Â 10À4 M)
(1 mm path-length) in the presence of Zn2+ in methanol at 298 K. Emission
spectra of LH (c) (2.0 Â 10À5 M) and (d) (2.0 Â 10À4 M) in the presence of Zn2+ in
methanol at 298 K. Excitation wavelength: (c) l = 330 nm and (d) 340 nm. Insets:
plots of (a) and (b) absorbance at 311 nm, (c) relative emission intensity (I/I0)
at 388 nm, and (d) emission intensity at 388 nm (I388: red circles) and 458 nm
(I458: blue circles) vs. [Zn2+]/[L]0.
Scheme 1 (a)–(e) Multistep complex formation between LH and Zn2+
.
(f)–(j) Stacked–unstacked equilibrium. (k)–(n) Sequential binding of Zn2+ to
the stacked complexes. Structures of H+(LÀ)2Zn2+ and (LÀ)2(Zn2+)2.
with the increasing ratio of [Zn2+]/[LH]0 along with the develop-
ment of an absorption tail observed at wavelengths longer than
LH’ is clear evidence that the imidazole moieties in LH act as the 360 nm. This spectral patterning resembles that arising from
distinct binding site for the second Zn2+ ion. Then, the binding exciton coupling between adjacent carbazole chromophores in a
stoichiometry of LH with Zn2+ was estimated from the titration plot. face-to-face position.12
The titration curve shows two breaks at [Zn2+]/[LH]0 = 0.5 and 1.0
Multistep complex formation of LH with Zn2+ was also investi-
(inset of Fig. 1a). The binding stoichiometry of [Zn2+]/[LH]0 = 0.5 and gated by fluorescence titration under the two different conditions
1.0 should correspond to (LÀ)2(Zn2+) and (LÀ)2(Zn2+)2, respectively.
([LH]0 = 2.0 Â 10À5 and 2.0 Â 10À4 M). In the case of low initial
In the case of high initial concentration of [LH]0 = 2.0 Â 10À4 M, concentration ([LH]0 = 2.0 Â 10À5 M, Fig. 1c), the emission peak at
LH shows three-step spectral changes during the titration of Zn2+ 388 nm due to carbazole monomer fluorescence decreases mono-
(Fig. 1b), where the titration curve also reveals two breaks at around tonously with the increasing ratio of [Zn2+]/[LH]0 (0–2.9 eq.), where a
[Zn2+]/[LH]0 = 0.5 and 1.0 (Fig. 1b inset). Upon addition of more two-step saturation curve is also present in the titration plot at [Zn2+]/
than 1.3 equivalents of Zn2+ (Fig. 1b green lines), the absorption [LH]0 = 0.5 and 1.0 (inset of Fig. 1c). Conversely, when the initial
band at the peak wavelength (lmax = 311 nm) gradually decreases concentration of LH is increased 10-fold ([LH]0 = 2.0 Â 10À4 M),
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the carbazole fluorescence peak at 388 nm decreases with con-
comitant increase of a broad structureless emission band at around
458 nm in the concentration range of [Zn2+]/[LH]0 = 0–0.33, where a
clear iso-emission point is observed at l = 431 nm (Fig. 1d red
lines).13,14 The broad emission band at around 458 nm is char-
acteristic of the excimer formed by face-to-face stacking of carb-
azole rings.12 In the concentration range of [Zn2+]/[LH]0 = 0.37–1.67,
the emission intensity decreases over the whole wavelength region
with the increasing ratio of [Zn2+]/[LH]0 (Fig. 1d blue lines). At
higher Zn2+concentrations ([Zn2+]/[LH]0 = 2–47), the monomer
carbazole fluorescence at shorter wavelength is diminished almost
completely, and the carbazole excimer emission at longer wave-
length is enhanced with a clear iso-emission point at l = 413 nm
(Fig. 1d green lines).14 The titration plots are shown in the inset
of Fig. 1d, where there are also two breaks at [Zn2+]/[LH]0 = 0.5 and
1.0. This is a clear indication of 2 : 1 (or 4 : 2) and 2 : 2 (or 4 : 4)
binding stoichiometry even at the high initial concentration of LH
(2.0 Â 10À4 M). The remarkable difference in spectral features
resulting from the difference in the initial concentration of LH
(2.0 Â 10À5 and 2.0 Â 10À4 M) clearly suggests the existence of a
stacked–unstacked equilibrium of the preorganized complexes
[(LÀ)2(Zn2+)m] (Scheme 1f–j).15
Fig. 2 The structure of (LÀ)4(Zn2+)6(CH3OH)8 optimized at B3LYP/
LANL2DZ: (a) top view and (b) side view.
Fig. 3 Positive-ion ESI MS of a solution of LH (1.3 Â 10À3 M) in methanol in
the presence of Zn(OSO2CF3)2 (1.3 Â 10À3 M). The inset shows isotopically
resolved signals at m/z = 1345.2 and the calculated isotopic distributions for
{[Zn4(L)4](OSO2CF3)2}2+ combined with {[Zn2(L)2](OSO2CF3)}+ (1 : 1 ratio). m/z =
1283.28: {[Zn(L)2H2](OSO2CF3)}+, m/z = 1495.16: {[Zn2(L)2H](OSO2CF3)2}+,
m/z = 1707.03: {[Zn3(L)2](OSO2CF3)3}+, m/z = 2092.27: {[Zn3(L)3](OSO2CF3)2}+,
and m/z = 2454.10: {[Zn4(L)3](OSO2CF3)4}+ (see ESI,† S6).
Scheme 1 explains the stacked–unstacked equilibrium. Upon
addition of less than 0.5 equivalents of Zn2+ (Scheme 1a),
the first Zn2+ ion is coordinated to form (LÀ)2(Zn2+) through
deprotonation of LH. This process may be accompanied by
partial protonation of the imidazole nitrogen atoms. The proto-
The stacked complex, (LÀ)4(Zn2+)4, was successfully identi-
nation constant (K) was estimated by UV-vis titration of LH with fied by the positive-ion ESI mass of a methanol solution of LH
triflic acid (see ESI,† S5). The determined-protonation constant (1.3 Â 10À3 M) containing 1 equivalent of Zn(OSO2CF3)2 (Fig. 3).
K = 9.4 Â 104 MÀ1 enables us to estimate that ca. 50% of the The mass signal at m/z = 1345.2 (red circles) corresponds to
imidazole nitrogen undergoes protonation in the methanol {[Zn4(L)4](OSO2CF3)2}2+, which is largely overlapped with that of
solution of [LH]0 = 2.0 Â 10À4 M containing 0.5 equivalents of {[Zn2(L)2](OSO2CF3)}+ (1 : 1 ratio). Characteristic distribution of
Zn2+ (1.0 Â 10À4 M). The stacked–unstacked equilibrium between isotopomers in those signals agrees closely with their calcu-
(H+)2(LÀ)4(Zn2+)2 and H+(LÀ)2(Zn2+) (Scheme 1f) gives both the lated isotopic distributions (inset of Fig. 3). The fragment mass
monomer carbazole fluorescence and the carbazole excimer in signals were also found at m/z = 1707.03 (L2M3), 2092.27 (L3M3),
the concentration range of [Zn2+]/[LH]0 = 0–0.33 (Fig. 1d red lines). and 2454.10 (L3M4). The unstacked complexes were detected at
The protons bound to the imidazole nitrogen should be gradually m/z = 1283.28 (L2M2) with a weak signal at m/z = 1495.16 (L2M).
replaced by Zn2+ (Scheme 1k and l) during the concentration
In light of these results, we tried to estimate the strength of
range of [Zn2+]/[LH]0 = 0.5–1.0, since the titration curve showed aromatic stacking interactions by temperature dependence of
two breaks at [Zn2+]/[LH]0 = 0.5 and 1.0 (inset of Fig. 1b and d, emission spectra of LH (2.0 Â 10À4 M) in the presence of 0.5,
vide supra). For this reason, no distinct spectral change was 0.9, and 50 equivalents of Zn2+ (Fig. 4a–c, respectively), which
observed in this concentration region (Fig. 1d blue lines, vide supra). correspond to DG00, DG20, and DG40, respectively (Scheme 1f, h,
At higher Zn2+concentrations, the excess Zn2+ ion should interact and j, respectively). In the presence of 0.5 and 0.9 equivalents of
with the free imidazole nitrogen to give (LÀ)4(Zn2+)6 (Scheme 1n), Zn2+ (Fig. 4a and b, respectively), the excimer emission intensity
which stabilizes the stacked state and shifts the stacked–unstacked normalized by the monomer fluorescence intensity (I464/I390
)
equilibrium to the stacked complex. Thus the carbazole excimer was decreases with increasing temperature (Fig. 4d and e), indicating
enhanced in the concentration region of [Zn2+]/[LH]0 = 2–47 (Fig. 1d that the stacked complexes [(LÀ)4(Zn2+)n] are thermodynamically
green lines, vide supra). The structure of (LÀ)4(Zn2+)6 was optimized favored as compared to the unstacked complexes [(LÀ)2(Zn2+)n].
at B3LYP/LANL2DZ, where the vacant sites of Zn2+ were occupied by In contrast, LH with 50 equivalents of Zn2+ shows no apparent
methanol molecules to satisfy the tetrahedral geometry around Zn2+ emission spectral shape change at 203–333 K, where no monomer
(Fig. 2). The model structure suggests that the Zn2+ ions bridged by fluorescence peak is detected over the whole temperature range
the imidazole moieties bring the two carbazole rings in a face-to-face (Fig. 4c). Although the strength of aromatic stacking interactions
position (Fig. 2), which is consistent with the above assumption. The in the multistep equilibrium could be difficult to determine
ethylene spacer allows the rotational flexibility around imidazole accurately, simple monomer–dimer simulation suggests that such
groups. This appears to play an important role in the face-to-face insensitive temperature dependence observed at 50 equivalents of
arrangement of the carbazole rings.
Zn2+ (Fig. 4f) can be obtained with ÀDG40 larger than 10 kcal molÀ1
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Scientific Research (C) (No. 26410094) and Scientific Research
(A) (No. 25248019) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Notes and references
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Fig. 4 Normalized emission spectra of LH (2.0 Â 10À4 M) in the presence
of Zn2+ (a) (1.0 Â 10À4 M), (b) (1.8 Â 10À4 M), and (c) (1.0 Â 10À2 M) in
methanol at 203–333 K. Excitation wavelength: l = 340 nm. Plots of
relative emission intensity (d), (e) (I464/I390), and (f) (I464/I443) vs. T.
(g) Estimated DG00, DG20, and DG40. The spectra are normalized at (a) and
(b) 390 nm and at (c) 443 nm (for non-normalized spectra, see ESI,† S7).
(tight dimer contact, see ESI,† S8). On the other hand, the
temperature dependent change in the intensity ratio of the
monomer fluorescence and the excimer emission observed at
0.5 and 0.9 equivalents of Zn2+ (Fig. 4d and e, respectively) is
likely obtainable with ÀDG00 and ÀDG20 = 3–7 kcal molÀ1 (loose
dimer contact, see ESI,† S8). The LH sample containing
0.9 equivalents of Zn2+shows a higher intensity ratio of excimer
emission at 203 K (Fig. 4a) as compared to that containing
0.5 equivalents of Zn2+ (Fig. 4b), indicating that ÀDG02 is slightly
9 Carbazole derivatives have been extensively utilized as building
blocks for metal–ligand directed self-assembly, see: J.-R. Li and
H.-C. Zhou, Nat. Chem., 2010, 2, 893.
10 C. Nie, Q. Zhang, H. Ding, B. Huang, X. Wang, X. Zhao, S. Li,
H. Zhou, J. Wu and Y. Tian, Dalton Trans., 2014, 43, 599.
larger than ÀDG00. The estimated ÀDG0 values (Fig. 4g) are con- 11 (a) J. Yuasa, A. Mitsui and T. Kawai, Chem. Commun., 2011, 47, 5807;
(b) N. Inukai, T. Kawai and J. Yuasa, Chem. Commun., 2011, 47, 9128;
siderably higher than typical aromatic–aromatic interactions
(c) N. Inukai, T. Kawai and J. Yuasa, Chem. – Eur. J., 2013, 19, 5938.
(ca. 2.4 kcal molÀ1),2 suggesting that the metal–ligand interactions
12 Ohkita et al. extensively studied excimer formation in rigid carb-
between Zn2+ and the imidazole moieties mainly contribute to the
stability of the stacking arrangements. In contrast, the stabilization
energy of the carbazole excimer should depend on the distance
azolophane, see: H. Ohkita, S. Ito, M. Yamamoto, Y. Tohda and
K. Tani, J. Phys. Chem. A, 2002, 106, 2140.
13 Excitation spectra recorded at 392 and 450 nm are almost identical
to the absorption spectra (see ESI† S3).
between the carbazole rings in the face-to-face position. In such a 14 No appreciable change in excimer emission maxima was observed
case, the coordination arrangements around Zn2+ largely contribute
during the titration (Fig. 1c and d), indicating no significant change
in the stabilization energy of the carbazole excimer. The term ‘‘loose
and tight dimer contacts’’ used in this study denotes the thermo-
to the stabilization energy of the carbazole excimer.14
In conclusion, we have demonstrated the efficiency of the metal-
ion clip method in fine-tuning aromatic stacking interactions.
Sequential binding of Zn2+ to the imidazole moieties bridges
the two preorganized complexes [(LÀ)2(Zn2+)m] and strengthens
the stacking interaction between the carbazole rings in stages.
This approach then becomes of interest in finding a way to
control aromatic stacking interactions, enabling ‘‘loose’’ and
‘‘tight’’ dimer contacts between the aromatic rings.
dynamic stability of the stacking arrangements (mainly due to the
metal–ligand interactions).
15 LH (2.0 Â 10À2 M) shows line broadening in NMR spectrum upon
addition of 0–0.2 equivalents of Zn2+in CD3OD, where the aromatic
protons of the carbazole ring (C1–H and C4–H) show upfield shifts
(see ESI† S4). The upfield shifts clearly suggest the shielding effects
of the carbazole rings, a clear indication of the stacked carbazole
units. In addition, the diffusion coefficient of LH (D = 8.8 Æ 0.4 Â
10À10 m2 sÀ1) decreases significantly (D = 4.3 Æ 0.8 Â 10À10 m2 sÀ1
)
in the presence of Zn2+, indicating that the size of the stacked
complex is larger than the unstacked complex. Precipitation was
observed at the higher Zn2+ concentrations under these conditions
due to low solubility of the stacked complexes in CD3OD.
This work was partly supported by JST-PRESTO ‘‘Molecular
technology and creation of new functions’’, a Grant-in-Aid for
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