In summary, amide-attached pyrrole-based p-conjugated anion
receptors showed solvent-dependent assembled modes such as
H-aggregates soluble in octane and supramolecular gels in
CH2Cl2 and 1,4-dioxane. The introduction of hydrogen-bonding
interaction sites to the p-conjugated planes enabled the
receptors to exhibit tunable stabilities, thereby resulting in the
dramatic changes observed in the gels and dispersed solutions in
the absence and presence of anions, respectively. Further
modifications of receptors could provide various charge-based
soft materials by combination with appropriate salts of anions.
These detailed investigations are currently underway.
Fig. 4 SEM images of xerogels of 2d from (a) CH2Cl2 (10 mg mLÀ1
and (b) 1,4-dioxane (10 mg mLÀ1).
)
hand, CH2Cl2, CHCl3 and 1,4-dioxane prefer p-planes over alkyl
chains (Fig. 3c(ii)). While these solvents cause less significant
spectral changes as discussed above, they provide highly
organized bundled structures based on columnar structures that
are suitable for gels.
This work was supported by PRESTO/JST (2007–2011),
Grants-in-Aid for Young Scientists (B) (No. 21750155) and
(A) (No. 23685032) from the MEXT and Ritsumeikan
R-GIRO project (2008–2013). We thank Prof. Atsuhiro
Osuka, Dr Naoki Aratani, Mr Taro Koide and Mr Tomohiro
Higashino, Kyoto University, for single-crystal X-ray analysis,
Dr Takashi Nakanishi, NIMS, for SEM measurements, Dr
Noboru Ohta, JASRI/SPring-8, for synchrotron radiation
XRD measurements (BL40B2 at SPring-8) and Prof. Hitoshi
Tamiaki, Ritsumeikan University, for various measurements.
The organized structures of xerogels of 2d prepared on
silicon substrates from CH2Cl2 and 1,4-dioxane were observed
using scanning electron microscopy (SEM) (Fig. 4). In
the SEM measurements, the CH2Cl2 xerogel showed the
formation of fibrous structure of widths of ca. 50–100 nm
(Fig 4a), whereas the xerogel obtained from 1,4-dioxane
exhibited similar wire-like structures with widths of 25–75 nm
(Fig. 4b). As already reported, 1d did not form such fibrous
structures,6a suggesting that the presence of amide units
improves the efficiency of the fabrication of stable fibres for
supramolecular gels. Further, the packing structure of the
CH2Cl2 xerogel was examined using synchrotron X-ray
diffraction (XRD); the XRD exhibited the diffraction peaks
corresponding to d = 6.96 (001), 3.51 (002), 2.34 (003), 1.39
(005) and 1.11 (006) nm. All the peaks were due to the lamellar
structure in the xerogel. The diffraction peak value of 6.96 nm
is nearly consistent with the AM1-optimized9 molecular length
of 6.61 nm, suggesting the formation of 1D aggregates.
Moreover, the 1,4-dioxane xerogel exhibited similar diffraction
peaks with a d value of 5.91 (001) nm, presumably due to the
interdigitation or tilting of the components. On the other
hand, the solid state of nongelated 3d exhibited diffraction
peaks with d values of 2.71 (100), 1.57 (110), 1.35 (200), 1.00
(210) and 0.90 (300) nm, all of which were derived from a
hexagonal columnar structure consisting of one molecule
(Z = 1) as a circle unit with a lattice constant a = 3.13 nm.
These results are consistent with the respective preferred
conformations, trans for 2d and cis for 3d, and the presence
or absence of hydrogen bonding of amide units.
Notes and references
1 G. A. Jeffrey and W. Saenger, Hydrogen Bonding in Biological
Structures, Springer, Berlin, 1991.
2 Examples of hydrogen-bond-based p–p stacking assemblies:
(a) V. Percec, C.-H. Ahn, T. K. Bera, G. Ungar and D. J. P.
Yeardley, Chem.–Eur. J., 1999, 5, 1070; (b) V. Percec, M. R. Imam,
T. K. Bera, V. S. K. Balagurusamy, M. Peterca and P. A. Heiney,
Angew. Chem., Int. Ed., 2005, 44, 4739; (c) Y. Sagara, S. Yamane,
T. Mutai, K. Araki and T. Kato, Adv. Funct. Mater., 2009,
19, 1869; (d) A. Das and S. Ghosh, Chem.–Eur. J., 2010, 16, 13622.
3 Selected books for further information on supramolecular gels:
(a) Low Molecular Mass Gelators, Topics in Current Chemistry,
ed. F. Fages, Springer-Verlag, Berlin, 2005; (b) T. Ishi-i and
S. Shinkai, Supramolecular Dye Chemistry, in Topics in Current
Chemistry, ed. F. Wurthner, Springer-Verlag, Berlin, 2005, p. 119;
¨
(c) Molecular Gels, ed. R. G. Weiss and P. Terech, Springer,
Dordrecht, 2006.
4 Selected books for anion binding: (a) Supramolecular Chemistry of
Anions, ed. A. Bianchi, K. Bowman-James and E. Garcıa-Espana,
´
Wiley-VCH, New York, 1997; (b) Anion Sensing, Topics in Current
Chemistry, ed. I. Stibor, Springer-Verlag, Berlin, 2005; (c)
P. A. J. L. Sessler, P. A. Gale and W.-S. Cho, Anion Receptor
Chemistry, RSC, Cambridge, 2006; (d) Anion Complexation in
Supramolecular Chemistry, Topics in Heterocyclic Chemistry,
ed. P. A. Gale and W. Dehaen, Springer-Verlag, Berlin, 2010,
vol. 24.
5 Reviews for anion-responsive supramolecular gels: (a) H. Maeda,
Chem.–Eur. J., 2008, 14, 11274; (b) G. O. Lloyd and J. W. Steed,
Nat. Chem., 2009, 1, 437; (c) M.-O. M. Piepenbrock, G. O. Lloyd,
N. Clarke and J. W. Steed, Chem. Rev., 2010, 110, 1960.
6 (a) H. Maeda, Y. Haketa and T. Nakanishi, J. Am. Chem. Soc.,
2007, 129, 13661; (b) H. Maeda, Y. Terashima, Y. Haketa,
A. Asano, Y. Honsho, S. Seki, M. Shimizu, H. Mukai and
K. Ohta, Chem. Commun., 2010, 46, 4559; (c) Y. Haketa,
S. Sasaki, N. Ohta, H. Masunaga, H. Ogawa, F. Araoka,
H. Takezoe and H. Maeda, Angew. Chem., Int. Ed., 2010,
49, 10079; (d) H. Maeda, K. Naritani, Y. Honsho and S. Seki,
J. Am. Chem. Soc., 2011, 133, 8896.
7 As an example: X.-Q. Li, X. Zhang, S. Ghosh and F. Wurthner,
¨
Chem.–Eur. J., 2008, 14, 8074.
8 F. Camerel, G. Ulrich and R. Ziessel, Org. Lett., 2004, 6, 4171.
9 M. J. Frisch, et al., , Gaussian 03, Revision C.01, Gaussian, Inc.,
Wallingford, CT, 2004.
10 (a) B. W. Laursen and F. C. Krebs, Angew. Chem., Int. Ed., 2000,
39, 3432; (b) B. W. Laursen and F. C. Krebs, Chem.–Eur. J., 2001,
7, 1773.
In a manner similar to an octane gel of 1d,6a the supra-
molecular gel of 2d from CH2Cl2 was found to be responsive
to anions.5 The addition of TBACl salt as a solid to the gel of
2d (10 mg mLÀ1) provided a solution state with lmax and lem
(excited at lmax) values of 529 and 577 nm, respectively.
Interestingly, instead of a bulky TBA cation, the introduction
of ClÀ as a planar 4,8,12-tripropyl-4,8,12-triazatriangulenium
(TATAC3) cation salt10 also afforded a solution state; this was
in sharp contrast to the gelated charge-by-charge assemblies
observed in 1dÁClÀÁ(TATAC3)+.6c The introduction of a
planar cation appears less efficient for 2d, presumably because
pyrrole inversion by anion binding prevents the effective
hydrogen bonding by amide moieties and the size of
(TATAC3 +
)
does not match that of 2dÁClÀ.
c
7622 Chem. Commun., 2011, 47, 7620–7622
This journal is The Royal Society of Chemistry 2011