Metallo supramolecular assemblies of bis-squaraines by allosteric Ca 2+ ion binding

Article abstract of DOI:10.1021/ol070582q

Alkyl chain tethered bis-squaraines bind to Ca²⁺ ions through the participation of the negatively charged oxygen of the central cyclobutene moiety to form folded H-type aggregates. The initially formed Ca²⁺ complex is preorganized to

Full text of DOI:10.1021/ol070582q

ORGANIC
LETTERS
2007
Vol. 9, No. 10
1999-2002
Metallo Supramolecular Assemblies of
+
Bis-squaraines by Allosteric Ca2 Ion
Binding
Shigeyuki Yagi,*,† Yutaka Hyodo,† Masahiko Hirose,† Hiroyuki Nakazumi,*,†
Yoshiaki Sakurai,‡ and Ayyappanpillai Ajayaghosh§
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture
UniVersity, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan, Technology
Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan,
and Photosciences and Photonics Group, Chemical Sciences and Technology DiVision,
Regional Research Laboratory (CSIR), TriVandrum 695019, India
yagi@chem.osakafu-u.ac.jp
Received March 15, 2007
ABSTRACT
+
Alkyl chain tethered bis-squaraines bind to Ca2 ions through the participation of the negatively charged oxygen of the central cyclobutene
+
+
moiety to form folded H-type aggregates. The initially formed Ca2 complex is preorganized to facilitate cooperative allosteric binding of Ca2
,
resulting in the formation of extended supramolecular arrays. The electronic absorption, IR, and ESI-MS studies support the formation of
metallo supramolecular architectures of the folded H-type dimers of the bis-squaraines.
Squaraine dyes (squarylium dyes) are zwitterionic dyes
consisting of aromatic or heterocyclic π-electron systems at
both ends of a cyclobutenoate core that exhibit unique optical
and electrochemical properties.1 They have been receiving
much attention from fundamental and technological view-
points and have been extensively used in xerographic2 and
electroluminescent devices,3 organic solar cells,4 chemosen-
sors,5 and so on. Due to their planar structures, squaraine
dyes often form aggregates in solutions as well as in solid
states.6 Such aggregate formation significantly affects the
optical and electronic properties of the dyes due to excitonic
interaction among the constituent chromophores. Thus,
† Osaka Prefecture University.
(2) Law, K. Y. Chem. ReV. 1993, 93, 449 and references therein.
(3) (a) Mori, T.; Miyachi, K.; Kichimi, T.; Mizutani, T. Jpn. J. Appl.
Phys. 1994, 33, 6594. (b) Matsui, M.; Tanaka, S.; Funabiki, K.; Kitaguchi,
T. Bull. Chem. Soc. Jpn. 2006, 79, 170.
(4) Merritt, V. Y.; Hovel, H. Appl. Phys. Lett. 1976, 29, 414. (b) Otsuka,
A.; Funabiki, K.; Sugiyama, N.; Yoshida, T.; Minoura, H.; Matsui, M. Chem.
Lett. 2006, 35, 666.
‡ Technology Research Institute of Osaka Prefecture.
§ Regional Research Laboratory.
(1) (a) Yagi, S.; Nakazumi, H. In Functional Dyes; Kim, S.-H., Ed.;
Elsevier: Oxford, 2006; pp 215-255. (b) Ajayaghosh, A. Acc. Chem. Res.
2005, 38, 449. (c) Ajayaghosh, A. Chem. Soc. ReV. 2003, 32, 181. (d)
Fabian, J.; Nakazumi, H.; Matsuoka, M. Chem. ReV. 1992, 92, 1197.
10.1021/ol070582q CCC: $37.00
© 2007 American Chemical Society
Published on Web 04/20/2007

controlling the aggregate formation is one of the important
issues to optimize the performance of the materials based
on organic functional dyes. In general, two type of ag-
gregates, H- and J-types, are formed, where the dye
molecules are aligned in card-pack and slipped-stack man-
ners, respectively, resulting in different types of excitonic
interactions of the chromophores.7 The H-type aggregation,
in which the transition dipole moments of the chromophores
are arranged in a parallel mode, yields a hypsochromic shift
of the electronic absorption band. On the other hand, in the
J-type aggregates, the transition dipoles are arranged in a
head-to-tail manner, leading to a bathochromic shift.8
Recently, Ajayaghosh and co-workers have exploited exciton
interaction in a series of squaraine-tethered podands to
colorimetric sensing of Ca2+ and Mg2+ ions.9 In these cases,
the podand chains play a crucial role in the binding of the
cations. However, in an unprecedented way, we found
another type of metal-cation-induced H-aggregate formation
of squaraine dimers. Herein we report that polymethylene-
bridged squaraine dimers form metallo supramolecular
assemblies through extended allosteric chelation of the folded
H-aggregates in which the negatively charged oxygens in
the central cyclobutene ring of the squaraine skeleton play
a crucial role.10
Figure 1. Squaraine dimers 1 and 2 and monomer 3.
of 1a in CHCl3/CH3CN (3/1, v/v) in the presence of varying
concentrations of Ca(ClO4)2 are shown in Figure 2. As the
The polymethylene-bridged squaraine dimers 1a-c (Figure
1) with varying spacer length were prepared from 4-[4-(N,N-
dibutylamino)phenyl]-3-hydroxy-3-cyclobutene-1,2-dione and
the corresponding N,N′-dimethyl-N,N′-diphenyl-R,ω-alkanedi-
amines in 42-58% yields, using triethyl orthoformate as a
dehydrating reagent (see Supporting Information). The
synthesis of the dimer 2 was reported previously.11 The
dimers 1 and 2 exhibit their absorption maxima at 640-645
nm in CHCl3/CH3CN solutions, similar to that of the
squaraine monomer 3. On the other hand, the dimers exhibit
large hypsochromic shifts upon addition of increasing
amounts of Ca2+. The electronic absorption spectral changes
Figure 2. Electronic absorption spectral changes of 1a in CHCl3/
CH3CN (3/1, v/v) at 293 K upon addition of varying concentrations
of Ca(ClO4)2 ([Ca2+]/[1a] ) 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, 1.0). [1a] ) 4.0 × 10-6 M.
(5) (a) Ros-Lis, J. V.; Mart´ınes-Ma´n˜es, R.; Rurack, K.; Sanceno´n, F.;
Soto, J.; Spieles, M. Inorg. Chem. 2004, 43, 5183. (b) Nakazumi, H.; Col-
yer, C. L.; Kaihara, K.; Yagi, S.; Hyodo, Y. Chem. Lett. 2003, 32, 804.
(c) Welder, F.; Paul, B.; Nakazumi, H.; Yagi, S.; Colyer, C. L. J.
Chromatogr. B 2003, 793, 93. (d) Kukrer, B.; Akkaya, E. U. Tetrahedron
Lett. 1999, 40, 9125. (e) Oguz, U.; Akkaya, E. U. Tetrahedron Lett. 1998,
39, 5857.
(6) (a) Chen, H.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1996, 100,
5949. (b) Chen, H.; Herkstroeter, W. G.; Perlstein, J.; Law, K. Y.; Whitten,
D. G. J. Phys. Chem. 1994, 98, 5138. (c) Das, S.; Thanulingam, T. L.;
Thomas, K. G.; Kamat, P. V.; George, M. V. J. Phys. Chem. 1993, 97,
13620. (d) Buncel, E.; MacKerrow, A.; Kazmaier, P. M. J. Chem. Soc.,
Chem. Commun. 1992, 1242. (e) Law, K. Y. J. Phys. Chem. 1987, 91, 5184.
(f) Kim, S.; Furuki, M.; Pu, L. S.; Nakahara, H.; Fukuda, K. J. Chem. Soc.,
Chem. Commun. 1987, 1201.
(7) (a) Jelly, E. E. Nature 1936, 138, 1009. (b) Brooker, L. G. S.; White,
F. L.; Heseltine, D. W.; Keyes, G. H.; Dent, S. G.; VanLare, E. J. J. Photogr.
Sci. 1953, 1, 173.
(8) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721.
(9) (a) Ajayaghosh, A.; Arunkumar, E.; Daub, J. Angew. Chem., Int. Ed.
2002, 41, 1766. (b) Arunkumar, E.; Chithra, P.; Ajayaghosh, A. J. Am.
Chem. Soc. 2004, 126, 6590. (c) Arunkumar, E.; Ajayaghosh, A.; Daub, J.
J. Am. Chem. Soc. 2005, 127, 3156.
concentration of Ca2+ increased, the absorbance at 645 nm
decreased, and a new absorption band appeared at 574 nm,
accompanied by two isosbestic points at 614 and 677 nm.
These spectral changes were similar to those observed in
the cation-induced H-type foldamer formation previously
reported by Ajayaghosh et al.9 Taking into consideration that
Ca2+-induced spectral changes were not observed in the case
of the monomer 3, the hypsochromic shift indicates that the
complexation of 1a with Ca2+ led to formation of the
H-foldamer.
The plot of the absorbance at 574 nm of the H-aggregate
of 1a upon titration with Ca(ClO4)2 is shown in Figure 3.
The absorbance change reached a plateau when an equimolar
amount of Ca(ClO4)2 was added, indicating that the stoichi-
ometry of the dimer-Ca2+ complexation is 1:1. The Job’s
analysis also supported 1:1 stoichiometry. However, the
sigmoidal profile of the absorbance changes implied that the
(10) For supramolecular dye aggregates, see: (a) Ogawa, K.; Kobuke,
Y. J. Photochem. Photobiol. C 2006, 7, 1. (b) Wu¨rthner, F.; Chen, Z.; Dehm,
V.; Stepanenko, V. Chem. Commun. 2006, 1188. (c) Wu¨rthner, F. Chem.
Commun. 2004, 1564. (d) Tamiaki, H.; Miyatake, T.; Tanikaga, R.;
Holtzwarth, A. R.; Schaffner, K. Angew. Chem., Int. Ed. Engl. 1996, 35,
772.
(11) Hyodo, Y.; Nakazumi, H.; Yagi, S. Dyes Pigm. 2002, 54, 163.
2000
Org. Lett., Vol. 9, No. 10, 2007

of the absorbance of the H-aggregate to that of the cation-
free dimer varied with the alkyl and ferrocene linkages. As
the length of the alkyl linker is increased in 1a-c, the
formation of the H-foldameric aggregates is suppressed. One
might see that the entropic disadvantage upon the complex-
ation may reduce the stability of the H-foldamer. The
H-aggregate formation was remarkable on 2 compared to
that of 1. We previously reported that 2 showed the broader
absorption band in CHCl3 solution than the monomeric
squaraine dye, due to the intramolecular interaction between
two squaraine chromophores.11 Thus, the chromophores in
2 might be more preorganized by the ferrocene linkage,
preferably due to the complexation-induced formation of the
folded H-aggregate.
In order to confirm the complexation mode between the
dimers and Ca2+ ions, the IR spectroscopic study and
electrospray ionization mass (ESI-MS) measurements were
performed. The IR absorption bands assigned to squaraine
moieties of the metal-free dimers, including CdO stretching
at the cyclobutenoate core, were observed as strong peaks
at 1582-1616 cm-1,14 whereas these peaks coalesced into
one at ca. 1590 cm-1 upon complexation with Ca2+ (see
Supporting Information). This result indicates that the bound
Ca2+ should interact with the negatively charged oxygen
atoms of the cyclobutenoate core. Thus, one can see that
Figure 3. Absorbance changes at 574 nm of 1a titrated with Ca-
(ClO4)2; [1a] ) 4.0 × 10-6 M. The inset shows the Job’s plot for
the 1a-Ca2+ system; [1a] + [Ca2+] ) 4.0 × 10-6 M.
H-aggregate formation was caused not only by a simple 1:1
complexation between 1a and Ca2+ but also by their
cooperative supramolecular assembly. This cooperative as-
sembling behavior was also indicated by the Hill’s analysis,
where the Hill’s constant, n, and the logarithm of the appar-
ent association constant, log Kapp, were determined as 4.4
(SD ) 0.1) and 25.7 (SD ) 0.6), respectively.12,13
Table 1. Electronic Absorption Spectral Properties of the
Squaraine Dimers and Their Ca2+ Complexes in CHCl3/CH3CN
(3/1, v/v) at 298 Ka
metal-free
complex
λmax
(nm)
ꢀ
λmax
(nm)
d
compound
(×10-5 M cm-1
)
Acomplex/Afree
1a
1b
1c
2
645
641
640
640
644
4.8
4.7
4.3
4.2
2.6b
574
575
577
564
c
0.85
0.27
0.16
1.2
3
c
a 4.0 × 10-6 M. b 8.0 × 10-6 M. c No spectral change was observed.
d Acomplex and Afree are absorbances at λmax of H-aggregate (ca. 575 nm) and
metal-free dye (ca. 640 nm), respectively.
As summarized in Table 1, similar Ca2+-induced spectral
changes were observed in the other dimers, although the ratio
(12) The Hill’s analysis was carried out according to the equation: log-
[Y/(1 - Y)] ) n log[Ca2+] + log Kapp, where Y, n, [Ca2+], and Kapp represent
the fractional saturation (occupancy of the binding sites), the Hill’s constant,
the concentration of Ca2+, and the apparent association constant, respec-
tively. Monitoring the absorbance changes of 1a at 574 nm, Y was
determined by the equation of (A - A0)/(Amax - A0), where A0, A, and Amax
are the absorbance in the absence of Ca2+, the absorbance in the presence
Figure 4. A possible structure of the H-aggregate supramolecule
of squaraine dimer with an alkaline earth metal cation.
chelation of the two squaraine moieties in the dimer to a
Ca2+ affords the H-foldamer (Figure 4a). This folded “unit
of Ca2+, and the absorbance upon addition of an excess amount of Ca2+
respectively. See also ref 13.
,
(13) (a) Hill, A. V. J. Physiol. 1910, 40, 4. (b) Pfeil, A. Lehn, J.-M. J.
Chem. Soc., Chem. Commun. 1992, 838. (b) Takeuchi, M.; Imada, T.;
Shinkai, S. Angew. Chem, Int. Ed. 1998, 37, 3020.
(14) Yagi, S.; Hyodo, Y.; Matsumoto, S.; Takahashi, N.; Kono, H.;
Nakazumi, H. J. Chem. Soc., Perkin Trans. 1 2000, 599.
Org. Lett., Vol. 9, No. 10, 2007
2001

H-type aggregate” possesses another preorganized chelating
site to bind with one more Ca2+ ion, thereby resulting in the
formation of a bimetallic H-aggregate (Figure 4b). This will
further complex with another bis-squaraine dye, propagating
a metallo supramolecular polymeric structure. Thus, a one-
dimensional array of the H-type folded aggregates of the Ca2+
complex should be afforded (Figure 4c). This is supported
by the ESI-MS study of the dimers 1a-c. The ESI-MS
spectrum of 1a (4.0 × 10-4 M) in the presence of 3 molar
equiv of Ca(ClO4)2 in CHCl3/CH3CN (3/1, v/v) is shown in
Figure 5. Parent peaks assigned to the supramolecular
Table 2. Assignment of ESI-MS Peaks of Dye-Ca2+
Complexes in CHCl3/CH3CN (3/1, v/v)
m/z (relative intensity)
molecular
composition
1a
1b
1c
[M+Ca+ClO4]+
1029
(29%)
910
1071
(11%)
952
1113
(21%)
994
(17%)
2087
(3%)
1481
(31%)
1600
(9%)
[2M+Ca]2+
(18%)
1919
(20%)
1355
(55%)
1474
(7%)
(22%)
2003
(6%)
1418
(54%)
1537
(9%)
[2M+Ca+ClO4]+
[3M+Ca]2+
[3M+2Ca+2ClO4]2+
with other alkali and alkaline earth metal cations (using
perchlorates of Li+, Na+, K+, Mg2+, and Sr2+). Only small
electronic absorption spectral changes were observed for the
dimers 1a-c and 2 upon addition of Mg2+ and Sr2+, where
the absorbance ratios of the H-aggregates to the free dimers
ranged from 0.07 to 0.16. On the other hand, addition of
Li+, Na+, and K+ did not show any change in the absorption
spectrum. Thus, it was confirmed that Ca2+ is a good trigger
for the formation of the metallo supramolecular assemblies
of the folded H-aggregates.
Figure 5. ESI-MS spectrum of 1a with 3 molar equiv of Ca(ClO4)2
in CHCl3/CH3CN (3/1, v/v). [1a] ) 4.0 × 10-4 M.
In summary, we have demonstrated that the squaraine
dimers with flexible alkyl chain linkers form H-type su-
pramolecular aggregates by the extended complexation with
Ca2+ cations. Chelation of the negatively charged oxygens
in the central cyclobutenoate to Ca2+ ions is essential to the
formation of the H-foldamer. This preorganized unit foldamer
facilitates cooperative allosteric chelation to Ca2+ ions,
resulting in the unprecedented formation of a metallo
supramolecular one-dimensional array of squaraine H-
aggregates. The present results provide new insights to
explore the potential of squaraine dyes in the field of
supramolecular dye chemistry.
- +
complexes ([2M + Ca2+]2+, [2M + Ca2+ + ClO4 ] , [3M
- 2+
+ Ca2+]2+, and [3M + 2Ca2+ + 2ClO4 ] ) were observed
in addition to the monomeric 1:1 complex ([M + Ca2+
+
- +
ClO4 ] ), indicating that three unit H-type folded aggregates
were at least involved. Similar supramolecular complexes
were also observed in the other dimers, but not observed in
the monomer 3 (Table 2). Taking into account that the
complexation-induced absorbance changes of squaraine
dimers get saturated upon addition of 1 molar equiv of Ca2+
ion, the metallo supramolecular structures should involve the
one-dimensional array of the folded H-aggregates (Figure
4c). The cooperative behavior observed in the electronic
absorption spectroscopic titration is explained by the allos-
teric Ca2+ binding facilitated by the preorganized chelating
sites of the initially formed folded H-aggregates.
Supporting Information Available: Experimentals and
UV-vis spectral changes of the squaraine dimers upon
addition of metal cations. This material is available free of
charge via the Internet at http://pubs.acs.org.
The complexation-induced formation of the supramolecu-
lar assemblies of the folded H-aggregate was also examined
OL070582Q
2002
Org. Lett., Vol. 9, No. 10, 2007