Chemical-stimuli-controllable circularly polarized luminescence from anion-responsive π-conjugated molecules

Article abstract of DOI:10.1021/ja203206g

Introduction of a BINOL-boron moiety to dipyrrolyldiketones as precursors of anion-responsive π-conjugated molecules results in the formation of a chiral environment in the form of anion-free receptors and anion-binding complexes. Conformation changes by

Full text of DOI:10.1021/ja203206g

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Chemical-Stimuli-Controllable Circularly Polarized Luminescence
from Anion-Responsive π-Conjugated Molecules
Hiromitsu Maeda,*,† Yuya Bando,† Konomi Shimomura,† Ippei Yamada,† Masanobu Naito,‡,§
Kazuyuki Nobusawa,‡ Hiroyuki Tsumatori,‡ and Tsuyoshi Kawai‡
†College of Pharmaceutical Sciences, Institute of Science and Engineering, Ritsumeikan University, Kusatsu 525-8577, Japan
‡Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Ikoma 630-0192, Japan
§PRESTO, Japan Science and Technology Agency (JST), Kawaguchi 332-0012, Japan
S Supporting Information
b
ABSTRACT: Introduction of a BINOLꢀboron moiety to
dipyrrolyldiketones as precursors of anion-responsive π-
conjugated molecules results in the formation of a chiral
environment in the form of anion-free receptors and anion-
binding complexes. Conformation changes by inversion
(flipping) of two pyrrole rings as a result of anion binding
can control the chiroptical properties of the anion receptors.
In particular, appropriate pyrrole β-substituents induce
distorted receptor π-planes and, as a result, give larger
circularly polarized luminescence (CPL), which can be
tuned by chemical stimuli (anions). This is the first example
of chemical-stimuli-responsive CPL properties.
reaking of symmetry in molecules provides chiral environ-
ments, which exhibit chiral electronic and electrooptical
B
Figure 1. (a) π-Conjugated acyclic anion receptors 1aꢀc, 2aꢀc, 3aꢀc,
and 4aꢀc. (b) Anion-binding mode of 1a.
properties. Among them, circularly polarized luminescence
(CPL),1,2 as observed in emissive chiral molecules2c and assem-
blies2g such as one-handed helicenes,2b,h lanthanide complexes
with chiral ligands,2d polymers,2e,f and liquid crystals of helicenes,2a
becomes an essential property for biological probing and display
technologies. Such CPL-active materials have a tremendous
amount of potential for highly sophisticated optical devices in
comparison with conventional circular dichroism (CD)-active
materials. However, there are few examples of the control
and switching of CPL properties by external stimuli. As emis-
sive π-conjugated molecules that are responsive to anions as
chemical stimuli,3 boron complexes of 1,3-dipyrrolyl-1,
3-propanediones (e.g., 1a and 2a in Figure 1a), which show
pyrrole inversion by anion binding (Figure 1b), afford various
derivatives by substitution at the pyrrole R- and β-positions and
the boron moiety of the central six-membered ring.4ꢀ6 In
particular, substitution of diol moieties on the core boron has
been found to provide various receptor molecules with useful
functionalities.6 As one of the strategies to achieve chiroptical
properties, the introduction of chiral diols would induce a chiral
conformation of the core receptor π-planes. In this communica-
tion, we report the synthesis and anion-responsive chiroptical
properties of diolꢀboron-substituted π-conjugated anion recep-
tors, especially supported by bulky pyrrole β-substituents. The
inversion of the pyrrole rings by anion binding enables the
induction of conformational changes of the receptor π-planes
followed by changes in the electronic and electrooptical properties.
According to literature procedures,6 β-phenyl-substituted and
β-dihydronaphtho-fused catecholboron complexes 3a and 4a
were prepared by treatment of the corresponding pyrroles7,8 with
malonyl chloride and the subsequent reaction with BCl3 and
catechol. As chiral substituents, we focused on 1,10-bi-2-naphthol
(BINOL) moieties. As in the case of 1a, 2a, 3a, and 4a, (R)-(þ)-1,
10-bi-2-naphtholboron compounds 1b, 2b, 3b, and 4b and
(R)-(þ)-3,30-diphenyl-1,10-bi-2-naphtholboron compounds 1c,
2c, 3c, and 4c were obtained via the precursory diketone
derivatives. The single-crystal X-ray structure of 2c showed that
the tilted dihedral angle between the two pyrrole rings is 21.61°
as a result of steric hindrance between the ethyl moieties at the
pyrrole β-positions, whereas that of 4a with the totally pyrrole-
inverted conformation is 22.37° (Figure 2).9 It is essential to
point out that 2c forms intramolecular πꢀπ stacking between a
pyrrole ring and a phenyl ring attached to the BINOL unit. In the
packing diagram, even though 2c shows no significant assembled
structures, 4a forms self-complementary dimers based on hydro-
gen bonding between NH and the catechol oxygen. Further-
more, the X-ray structure of 4a Clꢀ as the tetrabutylammonium
3
(TBA) salt showed a [1 þ 1]-type receptorꢀanion complex
wherein the dihedral angle between two pyrrole rings is 6.58°,
Received: April 15, 2011
Published: May 21, 2011
r
2011 American Chemical Society
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Table 1. Summary of UVꢀVis Absorption Maxima (λmax),
Fluorescence Emission Maxima (λem) and Emission Quan-
tum Yields (ΦF) Excited at These λmax, and Negative CD
Maxima (λCD) in CH2Cl2
λmax (nm)
λem (nm)
ΦF
λCD (nm)
1b
1c
2b
2c
3b
3c
4b
4c
432
437
452
456
470
473
513
518
438
441
467
472
505
499
555
554
0.005
0.005
0.006
0.009
0.005
0.004
0.28
432
433
451
452
467
470
517
500
Figure 2. Single-crystal X-ray structures (top and side views) of (a) 2c,
(b) 4a, and (c) 4a TBACl. Atom color code: brown, carbon; pink,
3
hydrogen; yellow, boron; blue, nitrogen; red, oxygen; yellow-green
(sphere), chlorine. Solvent molecules [hexane and CH2Cl2 for (a)
and (b), respectively] and the TBA cation [in (c)] have been omitted
for clarity. The solid-state structure of 4a shown in (b) is quite different
from that in solution (CDCl3), wherein the two pyrrole rings are not
inverted.
0.51
π-extended 4b and 4c bearing a BINOLꢀboron moiety exhibit
fairly strong emissions, presumably because of tuning of the
molecular orbitals (MOs) suitable for preventing intramolec-
ular electron transfer. Calculations at B3LYP/6-31þG(d,p)//
B3LYP/6-31G(d,p) level showed that the orders and energy
gaps of the MOs localized at the core dipyrrolyldiketone moieties
and the aryldiol moieties are correlated with the fairly high ΦF
values of 4b and 4c relative to other diolꢀboron derivatives.
Furthermore, Cotton effects in the CD spectra were observed for
the chiral molecules 1b, 1c, 2b, 2c, 3b, 3c, 4b, and 4c (Table 1;
see Figure 3a for 4c); these molecules exhibit negative Cotton
effects derived from core π-planes in CH2Cl2, suggesting that
enantiomerically distorted M-like conformations are induced by
the substituents on boron.
The conformations of π-conjugated anion receptors and their
electronic and electrooptical properties can be controlled by
anion binding. From the UVꢀvis absorption spectral changes
upon the addition of anions such as TBA salts in CH2Cl2
(Figure 3a), the binding constants (Ka) of, for example, 1c, 2c,
3c, and 4c for Clꢀ and CH3CO2ꢀ were estimated as 2500 and
86 000 Mꢀ1 for 1c, 95 and 4400 Mꢀ1 for 2c, 3700 and 55 000 Mꢀ1
for 3c, and 8200 and 51 000 Mꢀ1 for 4c, respectively, suggesting
efficient anion binding by all of these except for 2c.11 There
seemed to be no significant correlation between the Ka values and
the stabilities of the molecular conformations estimated by DFT
calculations.12 Some of the receptors showed anion-modulated
emission properties, as observed in the increasing ΦF values of
0.40 (4b) and 0.72 (4c) upon the addition of excess TBACl
(Figure 3b).13 The increased emission quantum yields were
correlated with the factors observed in the HOMO and LUMO
localized at the dipyrrolyldiketone moieties of the receptorꢀ
anion complexes (also see the Supporting Information). Upon
the addition of Clꢀ as a TBA salt, the CD signals of, for example,
1c, 2c, 3c, and 4c in CH2Cl2 (1 ꢁ 10ꢀ5 M) exhibited changes in
the intensities and λCD at 437, 452, 469, and 506 nm as negative
signals, respectively (Figure 3a). The CD anisotropy factors gabs
(defined as Δε/ε at the wavelengths of the first Cotton effects)
for 1c, 2c, 3c, and 4c were found to be 104gabs = 7.7, 6.8, 5.0, and
2.0, respectively, which were moderately changed to 7.1, 6.2, 4.2,
and 4.9, respectively.14 The gabs values were correlated with
multiple factors such as the distortion of the π-conjugated units
in the core receptor π-planes.
Figure 3. Spectral changes for 4c (1.0 ꢁ 10ꢀ5 M in CH2Cl2) in (a)
UVꢀvis absorption (bottom) and CD (top) and (b) fluorescence
(bottom) and CPL (top) excited at the isosbestic point of UVꢀvis
absorption spectrum, upon the addition of Clꢀ as the TBA salt (50 equiv
for UVꢀvis and 200 equiv for the other measurements as amounts
sufficient for almost complete complexation; 4c, black line; 4c Clꢀ, red
3
line), and corresponding solution photographs (insets).
which is smaller than that of anion-free receptor 4a. On the other
hand, conformational changes such as pyrrole inversion by anion
binding in solution were suggested by 1H NMR analysis, includ-
ing nuclear Overhauser effect spectroscopy (NOESY).
The UVꢀvis absorption maxima (λmax) and corresponding
fluorescence emission maxima (λem) and emission quantum
yields (ΦF) of 1b, 1c, 2b, 2c, 3b, 3c, 4b, and 4c in CH2Cl2
(see Figure 3 for the spectra of 4c) are summarized in Table 110
The λmax and λem of 4b and 4c are red-shifted relative to those
of the other receptors as a result of the introduction of fairly
coplanar π-units at the β-positions. Even though most diol-
substituted boron complexes show significantly weak fluorescence,
On the basis of the chemical-stimuli-responsive behaviors in
the steady state, induction of chirality of π-conjugated molecules
in the excited state by anion binding was examined by CPL
measurements.1,2 In fact, the anion-responsive CPL property was
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observed in 4c by complexation with Clꢀ and CH3CO2
Muller, G.; Riehl, J. P.; Venkataraman, D. J. Am. Chem. Soc. 2003,
125, 11808–11809. (c) Kawai, T.; Kawamura, K.; Tsumatori, H.;
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S. Chem. Lett. 2009, 38, 254–255. (f) Kawagoe, Y.; Fujiki, M.; Nakano, Y.
New J. Chem. 2010, 34, 637–647. (g) Tsumatori, H.; Nakashima, T.;
Kawai, T. Org. Lett. 2010, 12, 2362–2365. (h) Kaseyama, T.; Furumi, S.;
Zhang, X.; Tanaka, K.; Takeuchi, M. Angew. Chem., Int. Ed. 2011,
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(3) Selected books on anion binding: (a) Supramolecular Chemistry
of Anions; Bianchi, A., Bowman-James, K., García-Espa~na, E., Eds.;
Wiley-VCH: New York, 1997. (b) Fundamentals and Applications of
Anion Separations; Singh, R. P., Moyer, B. A., Eds.; Kluwer Academic/
Plenum Publishers: New York, 2004. (c) Anion Sensing; Stibor, I., Ed.;
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(d) Sessler, J. L.; Gale, P. A.; Cho, W.-S. Anion Receptor Chemistry; RSC
Publishing: Cambridge, U.K., 2006. (e) Recognition of Anions; Vilar, R.,
Ed.; Structure and Bonding, Vol. 129; Springer-Verlag: Berlin, 2008.
(4) (a) Maeda, H. In Handbook of Porphyrin Science; Kadish, K. M.,
Smith, K. M., Guilard, R., Eds.; World Scientific: Hackensack, NJ, 2010;
Vol. 8, Chapter 38. (b) Maeda, H. Top. Heterocycl. Chem. 2010,
24, 103–144.
(5) Selected reports: (a) Maeda, H.; Kusunose, Y. Chem.—Eur. J.
2005, 11, 5661–5666. (b) Maeda, H.; Kusunose, Y.; Mihashi, Y.;
Mizoguchi, T. J. Org. Chem. 2007, 72, 2612–2616. (c) Maeda, H.;
Haketa, Y.; Nakanishi, T. J. Am. Chem. Soc. 2007, 129, 13661–13674. (d)
Maeda, H.; Terasaki, M.; Haketa, Y.; Mihashi, Y.; Kusunose, Y. Org.
Biomol. Chem. 2008, 6, 433–436. (e) Maeda, H.; Haketa, Y. Org. Biomol.
Chem. 2008, 6, 3091–3095. (f) Maeda, H.; Mihashi, Y.; Haketa, Y. Org.
Lett. 2008, 10, 3179–3182. (g) Maeda, H.; Ito, Y.; Haketa, Y.; Eifuku, N.;
Lee, E.; Lee, M.; Hashishin, T.; Kaneko, K. Chem.—Eur. J. 2009,
15, 3706–3719. (h) Maeda, H.; Terashima, Y.; Haketa, Y.; Asano, A.;
Honsho, Y.; Seki, S.; Shimizu, M.; Mukai, H.; Ohta, K. Chem. Commun.
2010, 46, 4559–4561. (i) Maeda, H.; Bando, Y.; Haketa, Y.; Honsho, Y.;
Seki, S.; Nakajima, H.; Tohnai, N. Chem.—Eur. J. 2010, 16,
10994–11002. (j) Haketa, Y.; Sasaki, S.; Ohta, N.; Masunaga, H.; Ogawa,
H.; Mizuno, N.; Araoka, F.; Takezoe, H.; Maeda, H. Angew. Chem., Int.
Ed. 2010, 49, 10079–10083. (k) Haketa, Y.; Maeda, H. Chem.—Eur. J.
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ꢀ
(Figure 3b), both of which exhibited a CPL anisotropy factor
glum (defined as ΔI/I, where ΔI and I are the CPL and
fluorescence intensities, respectively) of 2 ꢁ 10ꢀ3, whereas 4c
showed negligibly small CPL. To the best of our knowledge, this
is the first example of chemical-stimuli-responsive CPL. The
(S)-(ꢀ) isomer of 4c showed the mirror images of these anion-
responsive CD and CPL spectral changes. The enhancement of
the glum value of 4c by anion binding is greater than that of gabs by
Clꢀ binding, suggesting that the anion-driven chiral induction,
which is mainly due to conformation changes of the π-conju-
gated system, is more pronounced in the excited state than in the
steady state. Such a distinct on/off switching of glum was not
observed for the other receptors 1b, 1c, 2b, 2c, 3b, 3c, and 4b,
some of which could not provide exact glum values because of the
smaller CPL intensities. In any case, our research suggests that
the anion triggers CPL and that CPL measurements can be used
for ion sensing.
In summary, we have demonstrated chemical-stimuli-respon-
sive chiroptical properties using anion-responsive π-conjugated
molecules. In regard to the structures of fluorescent molecules, it
is essential to develop π-conjugated molecules possessing fairly
planar but chirally distorted geometries, which are present in the
BINOLꢀboron complexes of β-substituted dipyrrolyldiketones.
At present, the combination of 3,30-diphenyl-1,10-bi-2-naphthol
and β-dihydronaphthopyrrole has been found to be the most
efficient for achieving anion-driven CPL enhancement. Thus, the
property reported in this communication seems to result from
the pyrrole modifications; a series of the related anion receptors
have been reported to date,4ꢀ6 and a myriad of unexplored
properties are likely to be revealed. Further investigations to
elucidate the fascinating properties of these pyrrole-based
π-conjugated molecules are currently underway.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures, anion-
b
binding properties, crystallographic data (CIF), and complete
ref 12b. This material is available free of charge via the Internet at
http://pubs.acs.org.
(6) (a) Maeda, H.; Fujii, Y.; Mihashi, Y. Chem. Commun. 2008,
4285–4287. (b) Maeda, H.; Takayama, M.; Kobayashi, K.; Shinmori, H.
Org. Biomol. Chem. 2010, 8, 4308–4315.
(7) Synthesis of β-diphenylpyrrole: (a) Fukuda, T.; Sudo, E.;
Shimokawa, K.; Iwao, M. Tetrahedron 2008, 64, 328–338. (b) Zonta,
C.; Fabris, F.; Lucchi, O. D. Org. Lett. 2005, 7, 1003–1006.
(8) Synthesis of β-dihydronaphthopyrrole: (a) Lash, T. D.; Denny,
C. P. Tetrahedron 1995, 51, 59–66. (b) Manley, J. M.; Roper, T. J.; Lash,
T. D. J. Org. Chem. 2005, 70, 874–891.
’ AUTHOR INFORMATION
Corresponding Author
maedahir@ph.ritsumei.ac.jp
(9) See the crystallographic data in the Supporting Information.
(10) (a) The corresponding BF2 complexes 1d, 2d, 3d, and 4d
exhibited high ΦF values of 0.96,5d 0.98,5b 0.83, and 0.80, respectively.
(b) Data for 1a, 1d, 2a, 2d, 3a, 3d, 4a, and 4d are summarized in the
Supporting Information.
(11) (a) Ka values for anions are summarized in the Supporting
Information. (b) Chiral receptors 2c, 3c, and 4c showed almost no
selectivity for recognizing asymmetric guest species such as L- and D-Phe
anions as TBA salts.
(12) (a) The relative energies between the stable and preorganized
pyrrole-inverted geometries of 2aꢀc and 4aꢀc are 5.32,6a 5.85, 6.06,
5.65, 6.50, and 4.83 kcal/mol, respectively, which are small compared to
the values 9.05,6a 9.32, 9.55, 9.89, 11.10, and 9.81 kcal/mol for 1aꢀc and
3aꢀc, respectively. (b) Frisch, M. J.; et al. Gaussian 03, revision C.01;
Gaussian, Inc.: Wallingford, CT, 2004.
(13) Anion-modulated emission properties were also observed in
the increases in ΦF values of 0.008 (1b), 0.008 (1c), 0.062 (2b), 0.047
(2c), 0.010 (3b), and 0.024 (3c) upon the addition of excess TBACl.
’ ACKNOWLEDGMENT
This work was supported by PRESTO/JST (2007ꢀ2011),
SENTAN/JST(2008ꢀ2011), Grants-in-Aid for Young Scientists (B)
(21750155) and (A) (23685032) from MEXT, the Green Photonics
Project at NAIST sponsored by MEXT, and the Ritsumeikan R-GIRO
Project (2008ꢀ2013). We thank Prof. Atsuhiro Osuka, Dr. Shohei
Saito, Mr. Eiji Tsurumaki, Mr. Taro Koide, and Mr. Tomohiro
Higashino (Kyoto University) for X-ray analysis and Prof. Hitoshi
Tamiaki (Ritsumeikan University) for various measurements.
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(14) Upon the addition of Clꢀ as a TBA salt, negative Cotton
effects were observed for 1b, 2b, 3b, and 4b in CH2Cl2 with λCD at
435, 452, 460, and 506 nm, respectively, along with changes
in 104gabs from 4.2, 3.9, 3.5, and 4.7 to 3.5, 4.2, 4.0, and 3.6,
respectively.
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