1H NMR spectral evidence for a specific host-guest complexation induced charge localization in squaraine dyes

Article abstract of DOI:10.1021/ol050612u

(Chemical Equation Presented) Experimental support is provided for the charge localization in squaraines, a class of fundamentally and technologically important organic dyes, by ¹H NMR analysis through a host-guest complexation approach. Specific binding of Ca²⁺ ions to the squaraine 2 with a podand sidearm resulted in a charge-localized structure 2a with dramatic shifts and resolution of the proton signals when compared to those of 2.

Full text of DOI:10.1021/ol050612u

ORGANIC
LETTERS
1H NMR Spectral Evidence for a Specific
Host Guest Complexation Induced
2005
Vol. 7, No. 15
3135-3138
−
Charge Localization in Squaraine Dyes
Ayyappanpillai Ajayaghosh* and Easwaran Arunkumar
Photosciences and Photonics Unit, Chemical Sciences DiVision, Regional Research
Laboratory (CSIR), TriVandrum-695019, India
aajayaghosh@rediffmail.com
Received March 21, 2005
ABSTRACT
Experimental support is provided for the charge localization in squaraines, a class of fundamentally and technologically important organic
+
dyes, by 1H NMR analysis through a host
−
guest complexation approach. Specific binding of Ca2 ions to the squaraine 2 with a podand
sidearm resulted in a charge-localized structure 2a with dramatic shifts and resolution of the proton signals when compared to those of 2.
Squaraine dyes are important from both fundamental and
technological viewpoints. They are extensively used in
imaging process,1 photovoltaics,2 nonlinear optics,3 sensor
design,4 photoconducting devices,5 photodynamic therapy,6
and the design of conjugated polymers.7 They show sharp
and intense absorption bands from visible to the near-IR
wavelengths, depending upon the structure, due to a donor-
acceptor-donor type of charge transfer and also due to the
extensive conjugation.8 Theoretical studies have proved that
squaraine dyes show significant bond delocalization.9 From
crystallographic studies Dirk et al. have reported that the
average C-C bond length in squaraines lies between single
and double bonds, suggesting that there is an extensive
delocalization of the electronic charge along the molecule,
resulting in resonance structures as shown in Scheme 1.10
Kazmaier et al. have demonstrated that a dipolar cyanine
structure and not a cyclobutene diylium structure best
(1) Law, K.-Y. Chem. ReV. 1993, 93, 449.
(2) Loutfy, R. O.; Hsiao, C. K.; Kazmaier, P. M. J. Photogr. Sci. 1983,
27, 5.
(3) (a) Chen, C.-T.; Marder, S. R.; Cheng, L.-T. J. Chem. Soc., Chem.
Commun. 1994, 259. (b) Andrews, J. H.; Khagdarov, J. D. V.; Skinger, K.
D.; Hull, D. L.; Chuang, K. C. Nonlinear Opt. 1995, 10, 227. (c) Ashwell,
G. J.; Jefferies, G.; Hamilton, D. G.; Lynch, D. E.; Roberts, M. P. S.; Bahra,
G. S.; Brown, C. R. Nature 1995, 375, 385.
(4) (a) Das, S.; Thomas, K. G.; Thomas, K. J.; Kamat, P. V.; George,
M. V. J. Phys. Chem. 1994, 98, 9291. (b) Thomas, K. G.; Thomas, K. J.;
Das, S.; George, M. V. Chem. Commun. 1997, 597. (c) Oguz, U.; Akkaya,
E. U. Tetrahedron Lett. 1998, 39, 5857. (d) Chenthamarakshan, C. R.;
Ajayaghosh, A. Tetrahedron Lett. 1998, 39, 1795. (e) Chenthamarakshan,
C. R.; Eldo, J.; Ajayaghosh, A. Macromolecules 1999, 32, 5846.
(5) (a) Tam, A. C. Appl. Phys. Lett. 1980, 37, 978. (b) Emmelius, M.;
Pawlowski, G.; Vollmann, H. W. Angew. Chem., Int. Ed. Engl. 1989, 28,
1445.
(7) (a) Ajayaghosh, A.; Eldo, J. Org. Lett. 2001, 3, 2595. (b) Eldo, J.;
Ajayaghosh, A. Chem. Mater. 2002, 14, 410. (c) Bu¨schel, M.; Ajayaghosh,
A.; Arunkumar, E.; Daub, J. Org. Lett. 2003, 5, 2975. (d) Ajayaghosh, A.
Chem. Soc. ReV. 2003, 32, 181.
(8) Law, K.-Y. J. Phys. Chem. 1987, 91, 5184.
(9) Bigelow, R. W.; Freund, H.-J. Chem. Phys. 1986, 107, 159.
(10) Dirk, C. W.; Herndon, W. C.; Cervantes-Lee, F.; Selnau, H.;
Martinez, S.; Kalamegham, P.; Tan. A.; Campos, G.; Velez, M.; Zyss, J.;
Ledoux, I.; Cheng, L.-T. J. Am. Chem. Soc. 1995, 117, 2214.
(6) (a) Ramaiah, D.; Eckert, I.; Arun, K. T.; Weidenfeller, L.; Epe, B.
Photochem. Photobiol. 2002, 76, 672. (b) Ramaiah, D.; Eckert, I.; Arun,
K. T.; Weidenfeller, L.; Epe, B. Photochem. Photobiol. 2004, 79, 99.
10.1021/ol050612u CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/28/2005

techniques, HRMS, and elemental analysis. The FT-IR and
NMR spectral data indicate considerable charge delocaliza-
tion in 2, which could be due to the resonance hybridized
canonical forms, 2 and 2′, as shown in Scheme 3. There are
Scheme 1
Scheme 3
represents the electronic configuration of these compounds.11
They have been able to support the dipolar canonical
structure with partial double bond character to the bond
connecting the four-membered ring and the six-membered
aromatic ring. These studies have established the zwitterionic
structure that is the preferred structure for squaraines. Herein,
we provide H NMR experimental evidence for the charge
localization, which is an indirect support for the zwitterionic
structure of squaraines.
1
four pairs of aromatic protons H1, H2, H3, and H4 and two
different types of NCH3 protons as indicated in Scheme 3.
The H NMR spectrum of 2 in CD3CN (Figure 1) shows
1
two peaks at δ 3.18 and 3.17 ppm for the -NCH3 (a) and
-NCH3 (b), respectively, corresponding to a total of nine
protons. The singlet at δ 3.27 ppm is assigned to -OCH3
protons. The multiplet observed at δ 3.69-3.72 ppm is
attributed to -NCH2 protons and the neighboring -OCH2
protons. The multiplet at δ 3.42-3.57 ppm is due to the
-OCH2 protons. Two quartet like peaks observed at δ 7 and
8.2 ppm, which are comprised of four doublets, correspond
to the four different pairs of aromatic protons H1, H2, H3,
On the basis of our recent findings on squaraine foldamers
as cation probes,12,13 we speculated that an unsymmetrical
squaraine dye with a podand sidearm upon binding of Ca2+
may localize the charge to the opposite nitrogen atom of the
dye. To verify this hypothesis we studied Ca2+-induced
1
changes in the H NMR spectrum of the squaraine dye 2,
which is prepared as shown in Scheme 2.
1
and H4. Each proton is characterized using H-1H COSY,
Scheme 2
NOESY, and ROESY analyses (Figures S1 and S2, Sup-
porting Information). 1H NMR ROESY and NOESY spectra
indicate that -NCH3 (a), and -NCH2 proton signals couple
1
with the H4 aromatic protons. The H-1H COSY experi-
ments reveal that the H1 protons couple with H2 protons,
indicating that they are neighboring pairs. Similarly H3 and
H4 protons couple each other since they are a neighboring
pair of aromatic protons.
Synthesis of 2 started with the preparation of N-(hexa-
(ethyleneoxy)methyl)-N-methylaniline 3 according to stan-
dard procedures starting from N-methyl aniline in 65% yield.
The squaryl derivatve 4 was obtained by the reaction between
N,N-dimethyl aniline and squaryl chloride.14 Reaction be-
tween 3 and 4 in 2-propanol under refluxing in the presence
of tributyl orthoformate yielded the squaraine dye 2 (32%,
Scheme 2), which is characterized using FT-IR, 1H, 13C, 1H-
1H COSY, NOESY, and ROESY NMR spectroscopic
(11) Kazmaier, P. M.; Hamer, G. K.; Burt, R. A. Can. J. Chem. 1990,
68, 530.
(12) Arunkumar, E.; Chithra, P.; Ajayaghosh, A. J. Am. Chem. Soc. 2004,
126, 6590.
(13) (a) Ajayaghosh, A.; Arunkumar, E.; Daub, J. Angew. Chem., Int.
Ed. 2002, 41, 1766. (b) Arunkumar E.; Ajayaghosh, A.; Daub, J. J. Am.
Chem. Soc. 2005, 127, 3156.
Figure 1. 1H NMR spectrum of 2 (8.5 mM) in CD3CN. (A)
Aliphatic region. (B) Aromatic region.
(14) Keil, D.; Hartmann, H. Dyes Pigm. 2001, 49, 161.
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Org. Lett., Vol. 7, No. 15, 2005

Figure 2. 1H NMR spectral changes of 2 (8.55 mM) in CD3CN
before and after the addition of Ca(ClO4)2. [2:Ca2+]: (a) [1:0], (b)
[1:0.58], (c) [1:1.41], and (d) [1:10].
In acetonitrile, 2 showed an absorption maximum at 630
nm and an emission maximum at 657 nm with a quantum
yield of 0.21. Addition of Ca(ClO4)2 to a solution of 2 in
acetonitrile could not induce any changes in the absorption
or emission properties, indicating that the structure or
electronic properties of the molecule are not affected.
Figure 3. 1H-1H NOESY NMR spectra of the dye 2 (8.55 mM)
after the addition of Ca(ClO4)2 (85.47 mM).
1
However, H NMR titration studies of 2 in the presence of
Ca(ClO4)2 showed significant shift in the proton signals
(Figure 2). During the initial additions, broadening and shift
of the NMR signals were observed.15 This could be due to
the formation of complexes of different stoichiometry and
due to the reversible exchange between the complexed and
the uncomplexed states. However, subsequent additions of
Ca(ClO4)2 clearly resolved the peaks due to the shift of the
equlibrium toward complexation between the dye and Ca-
(ClO4)2. The most prominent changes are noticed for the H3
and H4 aromatic protons and the -NCH3 (b) protons. The
-NCH3 protons that appeared together in the initial spectrum
of the dye significantly resolved into two peaks at δ 3.12
ppm [-NCH3 (a)] and δ 3.23 ppm [-NCH3 (b)] after the
addition of excess Ca(ClO4)2.
1A. Upon Ca2+ binding, the positive charge is localized on
the nitrogen atom opposite to the binding site. Consequently
-NCH3 (b) protons would be experiencing the least shielding,
which is clearly exhibited by a downfield shift when
compared to their original chemical shift value. On the other
hand, the -NCH3 (a) protons are shifted to the upfield relative
to their initial chemical shift value. The separation of the
H1, H4 and H2, H3 protons into four well-resolved doublets
are a clear indication of the magnetic anisotropy of these
protons due to the complexation-induced charge localization
of the dye 2. After complexation, protons on the aniline
moiety, where the polyether chain is attached, experience
more electron density than the uncomplexed state, as
indicated by the upfield shift of these protons. Similarly the
1
After the addition of 10 equiv of Ca(ClO4)2, H-1H
1
NOESY (Figure 3) and H-1H ROESY (Figure S3, Sup-
porting Information) spectra were recorded, which could
distinguish between the aromatic protons H1, H2, H3, and
H4 and the aliphatic protons, -NCH3 (a), -NCH3 (b), -OCH3,
and -NCH2. Variable temperature NMR studies after adding
Ca(ClO4)2 to 2 between 300 and 343 K showed a gradual
change in the resonance signals of -NCH3 protons, indicating
the decomplexation of Ca2+ from the dye 2 (Figure 4).
Binding of Ca2+ resulted in a downfield shift of the water
peak from 2.13 to 3.68 ppm. This could be due to the
coordination of the oxygen atom of the water molecule with
Ca2+, which weakens the O-H bond. Upon increasing the
temperature, the water peak shifted toward upfield along with
-NCH3 protons to their initial position, suggesting the
decomplexation of the metal ion at higher temperatures.
Since the positive charge of the dye 2 is equally distributed
to the -NCH3 (b) and -NCH3 (a) moieties, these protons
appeared together in the initial NMR spectrum as in Figure
Figure 4. Temperature-dependent 1H NMR spectra of 2 (8.55 mM)
in CD3CN (a) in the absence and in the presence of added Ca-
(ClO4)2 (85.47 mM) and measured at (b) 300, (c) 313, (d) 328,
and (e) 343 K.
(15) For similar observations, see: Kobiro, K.; Inoue, Y. J. Am. Chem.
Soc. 2003, 125, 421.
Org. Lett., Vol. 7, No. 15, 2005
3137

-OCH2 and -OCH3 protons showed significant downfield
shifts due to the decrease in electron density around the
oxygen atom upon coordination with the Ca2+. It must be
noted that the neutral -OCH3 protons experience more
downfield shift when compared to that of the partly positive
-NCH3 (a) protons. These observations are in support of the
involments of the -NCH3 (a) group and the podand chain to
the Ca2+ binding and the consequent charge localization as
indicated in Scheme 3. Interestingly, addition of alkali metal
salts such as NaClO4 and KClO4 did not show any change
as proof of the host-guest assisted charge localization within
the dye molecule, thereby indicating that the initial 1H NMR
spectrum of the dye is an average of the charge delocalized
resonance forms.
Acknowledgment. This work was supported by the
Department of Science and Technology and Council of
Scientific and Industrial Research, Government of India. This
is contribution number RRLT-PPD-206 ,which is dedicated
to Professor Jo¨rg Daub, University of Regensburg, Germany
on the occasion of his 65th birthday.
1
in the H NMR spectrum of 2, indicating that these metal
ions do not bind with the dye.
Supporting Information Available: Synthetic details of
In conclusion, through a rational approach, we provided
experimental evidence for the charge localization in an
unsymmetrical squaraine dye that specifically binds Ca2+ at
the attached podand arm, which is an indirect support for
the charge delocalized structure of squaraine dyes. 1H NMR
titration experiments of the dye in the presence of Ca(ClO4)2
showed remarkable resolution of the various proton signals
1
2. NOESY, ROESY, and H-1H COSY NMR spectra and
table showing change in the resonance signals of 2 with
increasing Ca2+ ion concentration and temperature. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL050612U
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Org. Lett., Vol. 7, No. 15, 2005