Infrared absorbing croconaine dyes: Synthesis and metal ion binding properties

Article abstract of DOI:10.1021/jo702209a

(Graph Presented) Quinaldine-based croconaine dyes synthesized by the condensation reaction between croconic acid and the respective quinaldinium salts are described. These dyes exhibit absorption maximum in the infrared region (840-870 nm) with high molar extinction coefficients (1-5 × 10 ⁵ M^-1 cm^-1) and have very low fluorescence quantum yields. Upon binding to divalent metal ions, these dyes were found to form complexes with a 2:1 stoichiometry having high association constants of the order of 10¹¹-10¹⁴ M^-2, while the monovalent metal ions showed negligible affinity. The binding of the croconaine dye 3d with divalent metal ions especially Zn²⁺, Pb²⁺, and Cd ²⁺ led to significant chelation-enhanced fluorescence emission. The broadening of the aromatic signals, vinylic and N-methyl protons and the negligible changes at the aliphatic region of the dye 3d in the ¹H NMR spectrum in the presence of Zn²⁺, indicate that the binding occurs at the carbonyl groups of the croconyl ring. The shift in the croconyl carbonyl stretching frequency in the [3d-Zn²⁺] complex analyzed through FT-IR analysis further confirms the involvement of two electron-rich carbonyl groups of the croconyl moiety in the complexation. These results demonstrate that the binding of the divalent metal ions at the carbonyl oxygens of these infrared absorbing dyes can be favorably utilized for the development of potential sensors for the detection of metal ions and further can be exploited as sensitizers for photodynamic therapeutic applications.

Full text of DOI:10.1021/jo702209a

Infrared Absorbing Croconaine Dyes: Synthesis and Metal Ion
Binding Properties
Rekha R. Avirah, Kuthanapillil Jyothish, and Danaboyina Ramaiah*
Photosciences and Photonics, Chemical Sciences and Technology DiVision, National Institute for
Interdisciplinary Science and Technology (NIIST), CSIR, TriVandrum, India 695 019
d_ramaiah@rediffmail.com; rama@csrrltrd.ren.nic.in
ReceiVed October 11, 2007
Quinaldine-based croconaine dyes synthesized by the condensation reaction between croconic acid and
the respective quinaldinium salts are described. These dyes exhibit absorption maximum in the infrared
region (840-870 nm) with high molar extinction coefficients (1-5 × 105 M-1 cm-1) and have very low
fluorescence quantum yields. Upon binding to divalent metal ions, these dyes were found to form
complexes with a 2:1 stoichiometry having high association constants of the order of 1011-1014 M-2,
while the monovalent metal ions showed negligible affinity. The binding of the croconaine dye 3d with
divalent metal ions especially Zn2+, Pb2+, and Cd2+ led to significant chelation-enhanced fluorescence
emission. The broadening of the aromatic signals, vinylic and N-methyl protons and the negligible changes
1
at the aliphatic region of the dye 3d in the H NMR spectrum in the presence of Zn2+, indicate that the
binding occurs at the carbonyl groups of the croconyl ring. The shift in the croconyl carbonyl stretching
frequency in the [3d-Zn2+] complex analyzed through FT-IR analysis further confirms the involvement
of two electron-rich carbonyl groups of the croconyl moiety in the complexation. These results demonstrate
that the binding of the divalent metal ions at the carbonyl oxygens of these infrared absorbing dyes can
be favorably utilized for the development of potential sensors for the detection of metal ions and further
can be exploited as sensitizers for photodynamic therapeutic applications.
Introduction
NIR dyes due to their remarkable absorption and emission
properties.6 However, photobleaching and low chemical stability
of these dyes has necessitated the development of alternate dyes
with absorption in the NIR and infrared (IR) region. In this
regard, the electron donor-acceptor-donor concept7 has been
utilized for the synthesis of long-wavelength absorbing dyes
including squaraines and croconaines.8
Dyes that absorb or emit in the long-wavelength (>650 nm)
region of the optical spectrum have gained increasing attention
in the past few years,1 especially because of their potential
optoelectronic2 and biomedical3 applications. Several attempts
have been made toward developing compounds that absorb in
the near-infrared region. One commonly employed approach is
to increase the extent of π conjugation, which would eventually
lead to its convergence limit.4 Further, most of the near-infrared
(NIR) absorbing dyes require tedious synthetic methodologies.5
Of these, cyanine dyes have received considerable attention as
Recently, we have reported the synthesis of phloroglucinol-
based squaraine dyes9 for photodynamic therapeutic (PDT)
applications.10 However, the absorption maximum of these dyes
is in the short-wavelength region (600-650 nm) where most
of the biological tissues have significant absorption. Since the
10.1021/jo702209a CCC: $40.75 © 2008 American Chemical Society
Published on Web 12/11/2007
274
J. Org. Chem. 2008, 73, 274-279

Infrared Absorbing Croconaine Dyes
tissue penetration of light increases with increasing wavelength,
we were interested in the development of dyes having longer
wavelength absorption. This was achieved by replacing the
electron-donor phloroglucinol moiety with a quinaldine group.
These quinaldine-based squaraine dyes were prepared by the
condensation reaction between substituted quinaldinium salts
and squaric acid in 2:1 equiv.11 It was observed that quinal-
dinium salts with electron-donating substituents gave only the
semisquaraine as the isolable product due to the reduced
electrophilicity at the squaryl ring of the semisquaraine.
However, the quinaldinium salts with electron-withdrawing
substituents gave the corresponding squaraine dyes in quantita-
tive yields. These dyes exhibit long-wavelength absorption (λ
) 700-750 nm) and high molar extinction coefficients (ꢀ )
1-3 × 105 M-1 cm-1). We felt that if the squaryl moiety in
FIGURE 1. Normalized absorption spectra of the croconaine dyes
3a-d.
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FIGURE 2. Changes in the absorption spectra of the croconaine dye
3d (7.75 µM) in THF with the addition of Zn2+: [Zn2+] ) 0 (a), 0.94
(b), 1.88 (c), 2.82 (d), and 3.87 µM (e). The inset shows the
corresponding changes in the emission of 3d on complexation with
Zn2+. Excitation wavelength, 700 nm.
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FIGURE 3. Job’s plot for the complexation of the croconaine dye 3d
with Zn2+ in THF. The inset shows the Benesi-Hildebrand analysis
of the emission changes for the complexation between the dye 3d and
Zn2+
.
these dyes is replaced with its higher homologue croconic acid
having a stronger acceptor moiety, the absorption can be further
shifted to longer wavelengths12 and hence such longer wave-
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J. Org. Chem, Vol. 73, No. 1, 2008 275

Avirah et al.
SCHEME 1. Synthesis of Croconaine Dyes 3a-d
length absorbing dyes can have potential PDT applications.13
Further, as observed previously with the semisquaraines,14 these
dyes could also act as a bidentate ligand and form stable
complexes with metal ions. Due to the higher penetration and
minimal autofluorescence of infrared light by the tissues, these
dyes could function as novel metal ion sensors in biological
applications. Our results demonstrate that the appropriately
substituted quinaldine derivatives undergo efficient condensation
reaction with croconic acid yielding the dyes in quantitative
yields. These dyes exhibit absorption in the infrared region (λmax
) 800-900 nm) and were found to undergo selective com-
plexation with divalent metal ions, indicating thereby their
potential use as sensors for metal ions and sensitizers in PDT
applications.
after evaporation of the solvent was subjected to column
chromatography to yield the cholesterol-linked croconaine dye
3d in 75% yield.
The croconaine dyes 3a-d exhibited sharp and intense
absorption in the IR window with absorption maximum ranging
from 800 to 900 nm and have high molar extinction coefficients
in the range ꢀ ) 1-5 × 105 M-1 cm-1. Figure 1 shows the
absorption spectra of the various croconaine dyes. The unsub-
stituted croconaine dye 3a showed absorption maximum at 840
nm in DMF, while the halogenated dyes 3b and 3c exhibited
absorption maximum, which is ca. 20 nm red-shifted from the
parent unsubstituted derivative, at 860 and 865 nm, respectively
(Table S1, Supporting Information). The dyes 3a-c have low
solubility in common organic solvents. However, substitution
of the quinaldine ring with the cholesterol moiety resulted in
increased solubility of the dye 3d in THF and CHCl3. The dye
3d showed an absorption maximum at 871 nm in THF. All these
dyes were found to have negligible fluorescence quantum yields.
Results and Discussion
Synthesis and Photophysical Properties of the Croconaine
Dyes. With the objective of preparing the croconic acid-based
dyes, we synthesized the substituted quinaldines by modifying
the reported procedure.15 Since heavy atom substitution would
result in higher intersystem crossing efficiency as well as higher
triplet yields, we introduced heavy atoms like iodine in the
quinaldine ring. Besides, as observed previously for the
squaraine dyes, the incorporation of electronegative and electron-
withdrawing groups in the quinaldine ring would facilitate the
formation of croconaine dyes in quantitative yields. As shown
in Scheme 1, the reaction of the substituted quinaldines 1a-c
with methyl iodide at 100 °C in a sealed tube yielded the
corresponding quinaldinium salts 2a-c (Figures S1-S7, Sup-
porting Information). The condensation reaction between 2:1
equiv of the quinaldinium salts 2a-c and croconic acid in
ethanol at 80 °C with quinoline as catalyst gave the correspond-
ing croconaine dyes 3a-c in 80-90% yields. With a view to
improving the solubility and cellular permeability of the dyes,
cholesterol-anchored croconaine dye 3d was synthesized from
the corresponding quinaldinium salt 2d, following the same
synthetic strategy as in the previous cases. The reaction mixture
Metal Ion Binding Properties of the Croconaine Dyes.
With a view to investigate the ability of these dyes as bidentate
ligands and thereby their potential use as probes, we have carried
out their interactions with various mono- and divalent metal
ions. The derivative 3d was selected as a representative example
because of its higher solubility and stability. Figure 2 shows
the changes in the absorption spectrum of the dye 3d on addition
of Zn2+. With increasing concentration of Zn2+, we observed a
decrease in the absorption band at 871 nm, with the concomitant
formation of a new band at 788 nm. The corresponding changes
in the fluorescence spectra of the dye 3d with increasing addition
of the metal ion are shown in the inset. The dye 3d alone is
weakly fluorescent. With the addition of Zn2+ a significant
increase in the fluorescence intensity was observed with an
emission maximum at 814 nm. The progressive increase in the
fluorescence intensity reached saturation with the addition of
0.5 equiv of the metal ion and ca. 28-fold fluorescence
enhancement could be observed with Zn2+ (Φ ) 2.5 × 10-3).
The stoichiometry of the complex formed between the crocon-
aine dye 3d and the representative metal ion Zn2+ was found
to be 2:1 as evidenced from the Job’s plot (Figure 3). Benesi-
Hildebrand analysis of the fluorescence titration data of com-
plexation between the dye 3d and Zn2+ gave a binding constant
of 2.2 ( 0.3 × 1014 M-2, which is in good agreement with the
value obtained from the absorption changes.
(13) (a) Hilmey, D. G.; Abe, M.; Nelen, M. I.; Stilts, C. E.; Baker, G.
A.; Baker, S. N.; Bright, F. V.; Davies, S. R.; Gollnick, S. O.; Oseroff, A.
R.; Gibson, S. L.; Hilf, R.; Detty, M. R. J. Med. Chem. 2002, 45, 449-
461. (b) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W.
M.; O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10624-10625. (c)
Stefflova, K.; Li, H.; Chen, J.; Zheng, G. Bioconj. Chem. 2007, 18, 379-
388.
The interaction of the dye 3d with different alkali, alkaline
earth, transition, and heavy metal ions was also examined. Figure
4 shows the changes in the absorption spectra of the dye 3d
(14) Avirah, R. R.; Jyothish, K.; Ramaiah, D. Org. Lett. 2007, 9, 121-
124.
(15) Doebner, A.; von Miller, W. Ber. 1884, 17, 708.
276 J. Org. Chem., Vol. 73, No. 1, 2008

Infrared Absorbing Croconaine Dyes
FIGURE 4. Changes in the absorption spectra of the dye 3d (7.75
µM) in THF with the addition of Pb2+: [Pb2+] ) 0 (a), 0.94 (b), 1.88
(c), 2.82 (d), and 3.87 µM (e). The inset shows the corresponding
changes in fluorescence intensity. Excitation wavelength, 700 nm.
FIGURE 6. Relative fluorescence enhancement of the croconaine dye
3d (7.75 µM) upon interaction with different metal ions.
TABLE 1. Absorption and Fluorescence Maxima, Association
Constants, and Quantum Yields of the Various Metal Ion
Complexes of 3d in THFa
association
constant (Kass),c
M-2
λ
max, nm
b
complex
[3d-Zn2+
absorption emission
ΦF
]
788
735
804
805
750
735
755
814
816
818
815
818
812
820
2.5 × 10-3 1.9 ( 0.4 × 1014
(2.2 ( 0.3 × 1014)
[3d-Pb2+
]
2.2 × 10-3 1.0 ( 0.3 × 1013
(1.3 ( 0.5 × 1013)
[3d-Cd2+
]
1.4 × 10-3 5.2 ( 0.4 × 1012
(5.3 ( 0.3 × 1012)
[3d-Mg2+
[3d-Ba2+
[3d-Hg2+
[3d-Ca2+
]
5.5 × 10-4 6.0 ( 0.5 × 1012
(6.5 ( 0.4 × 1012)
]
4.7 × 10-4 2.7 ( 0.3 × 1011
(3.1 ( 0.2 × 1011)
]
4.2 × 10-4 5.1 ( 0.5 × 1011
(5.9 ( 0.3 × 1011)
3.2 × 10-4 3.1 ( 0.3 × 1011
(3.5 ( 0.4 × 1011)
FIGURE 5. Changes in the absorption spectra of the dye 3d (7.75
µM) in THF with the addition of Cd2+: [Cd2+]) 0 (a), 0.94 (b), 1.88
(c), 2.82 (d), and 3.87 µM (e). The inset shows the corresponding
changes in fluorescence intensity. Excitation wavelength, 700 nm.
]
a Average of more than 2 experiments and the error is ca. (5%.
b Fluorescence quantum yields were calculated by using the IR-125 as the
standard (Φ ) 0.13, in DMSO). c Association constants were calculated
based on fluorescence changes while the values obtained from absorption
changes are given in parentheses.
with the addition of Pb2+. With the addition of Pb2+ to the dye
solution in THF, we observed a decrease in the absorption band
of the dye at 871 nm, with the concomitant formation of a new
band at 735 nm. The corresponding change in the fluorescence
spectra of the dye on addition of Pb2+ is shown in the inset.
With increasing concentration of Pb2+, the initially weakly
fluorescent dye shows ca. 26-fold enhancement in fluorescence
intensity (Φ ) 2.2 × 10-3) with the emission maxima at 816
nm. Figure 5 shows the changes in the absorption spectra of
the dye 3d with increasing concentration of Cd2+. As observed
in the case of Pb2+, the addition of Cd2+ resulted in the decrease
of the absorption band of the dye at 871 nm. However, the new
band corresponding to the [3d-Cd2+] complex was observed at
804 nm. In the fluorescence spectra a ca. 14-fold fluorescence
enhancement (Φ ) 1.4 × 10-3) was observed with the emission
maxima at 818 nm (inset, Figure 5). On the basis of the
fluorescence data, we calculated the binding constants for Pb2+
and Cd2+ and these values are found to be 1.3 ( 0.5 × 1013
and 5.3 ( 0.3 × 1012 M-2, respectively.
chelation-enhanced fluorescence intensity. However, the relative
changes were found to vary with different metal ions (Figures
S8 and S9, Supporting Information). Similar, but lower effects
were observed in the absorption and emission spectra of the
dye 3d upon adding other divalent metal ions like Mg2+, Hg2+
,
Ba2+, and Ca2+ (Table 1). With the addition of a strong chelating
agent such as ethylenediaminetetraacetic acid (EDTA) to the
dye-metal ion complex, we observed the disappearance of the
new band formed during metal ion complexation as well as the
quenching of the fluorescence, indicating thereby the revers-
ibility of the complex formation between 3d and various divalent
metal ions.
Characterization of the Metal Ion Complexation. The
complexation between the metal ion and the dye 3d was further
confirmed through the 1H NMR titrations and the infrared (FT-
1
IR) spectral analysis of the [3d-Zn2+] complex. The H NMR
The relative changes in the fluorescence intensity of the
croconaine dye 3d upon the addition of different metal ions
are shown in Figure 6. The monovalent metal ions such as Li+,
Na+, and K+ caused negligible changes in absorption and
fluorescence emission. In contrast, the addition of other divalent
metal ions like Pb2+, Cd2+, Mg2+, Hg2+, Ca2+, and Ba2+ to the
dye solution in THF resulted in a decrease in absorption and
spectrum of the croconaine dye 3d in CDCl3 showed five
aromatic protons as multiplets in the region between δ 7 and
9.1, while the olefinic and N-methyl protons appeared as singlets
at δ 6.4 and 3.9 ppm, respectively (Figure 7). With the addition
of Zn2+, a broadening of the aromatic signals as well as the
vinylic and N-methyl protons could be observed. However, no
significant changes were observed in the aliphatic region,
J. Org. Chem, Vol. 73, No. 1, 2008 277

Avirah et al.
divalent metal ions through the electron-rich carbonyl group of
the croconyl ring in a 2:1 binding mode.
The binding of the metal ion at the carbonyl groups results
in the broadening of the peaks of the aromatic protons and
vinylic and N-methyl groups seen in the 1H NMR titration data.
Moreover, the binding of the positively charged metal ions at
the carbonyl group reduces the bond energy of the conjugated
CdO bond and reduces the carbonyl stretching frequency in
the [3d-Zn2+] complex.
The ground (S0) and excited state (S1) of the croconaine dyes
are intramolecular D-A-D charge transfer (ICT) states.16 The
S0-S1 electronic excitation in these systems involves a CT
process that is primarily confined to the central C5O3 cyclo-
pentane ring, with the major contribution from the oxygen
atoms. The intramolecular charge-transfer character (ICT) of
this transition, combined with an extended π conjugation, is
responsible for the sharp and intense absorption bands of these
dyes. The strong binding of the divalent metal ions at the
croconyl ring could be attributed to the presence of two electron-
rich carbonyl groups in the croconyl ring that can act as a
bidentate ligand for various divalent metal ions. The interaction
of the metal ions at the carbonyl group lowers the π conjugation
as well as the CT character, resulting in the formation of a
hypsochromically shifted absorption band for the dye-metal
complex. This interaction of the metal ion with the carbonyl
groups of the dye 3d is strongly supported by the clear shifts
in the stretching frequency of the croconyl carbonyl groups in
the [3d-M2+] complex. Further, the possibility of the metal ion
binding with a keto-enol tautomeric form can be ruled out,
since such an interaction of the metal ion with the keto-enol
form of the dye would have resulted in the loss of conjugation
and thereby a much lower wavelength absorption band than
observed for the [3d-M2+] complex. On the basis of this
evidence we propose that the metal ion binding is through the
dicarbonyl rather than through a keto-enolate tautomer. The
presence of electron-rich carbonyl oxygens allows only the
interaction with divalent metal ions and the sensitivity of the
FIGURE 7. 1H NMR spectra of the dye 3d in CDCl3 with increasing
concentration of Zn2+ in CD3CN. The mole ratio of [Zn2+] to [3d] is
0 (a), 0.25 (b), and 0.5 (c).
FIGURE 8. IR spectra of the croconaine dye 3d and the [3d-Zn2+
]
complex.
indicating that the cholesterol moiety has no significant interac-
tion with the metal ion. The IR spectrum of the dye showed
four carbonyl bands of the dye 3d (Figure 8). The bands at
1660, 1598, and 1560 cm-1 are characteristic of the carbonyl
groups of the croconyl moiety, while the band at 1755 cm-1 is
assigned to the carbonyl group attached to the cholesterol
moiety. However, in the IR spectrum of the [3d-Zn2+] complex,
the carbonyl band at 1660 cm-1 shifted to 1610 cm-1, while
the bands at 1598 and 1560 cm-1 merged to give a new band
at 1564 cm-1. This clearly indicates the involvement of two
carbonyl groups of the croconyl moiety of 3d in the complex-
ation with Zn2+. As expected, the band at 1755 cm-1 corre-
sponding to the carbonyl group attached to the cholesterol
moiety showed negligible changes. Figure 9 shows the mode
of binding of the divalent metal ions to the croconaine dye 3d.
As shown in the figure, the croconaine dye 3d binds to the
binding follows the order Zn2+ > Pb2+ . Cd2+ . Mg2+
≈
Hg2+ ≈ Ca2+ ≈ Ba2+
.
Conclusions
In conclusion, a new class of substituted quinoline-based
croconaine dyes have been synthesized in quantitative yields
by the condensation reaction between croconic acid and the
corresponding quinaldinium salts, and their interactions with
metal ions have been investigated. These dyes show sharp and
intense absorption in the IR region (λmax ) 800-900 nm) with
high extinction coefficients (ꢀmax ) 1-5 × 105 M-1 cm-1).
Due to the presence of a stronger acceptor moiety, these
FIGURE 9. Schematic representation of the complexation between divalent metal ion, M2+, and the croconaine dye 3d.
278 J. Org. Chem., Vol. 73, No. 1, 2008

Infrared Absorbing Croconaine Dyes
3a (80%, based on conversion): mp >300 °C; IR (KBr) νmax
(cm-1) 3035, 1604, 1558, 1454; 1H NMR (DMF, 300 MHz) δ 9.14
(1H, d, J ) 9.4 Hz), 7.9 (2H, d, J ) 9.4 Hz), 7.82 (2H, m), 7.53
(1H, m), 6.46 (1H, s), 4.14 (3H, s); FAB-MS m/z calcd for
C27H20N2O3 420.46, found 420.24. Anal. Calcd for C27H20N2O3:
C, 77.13; H, 4.79; N, 6.66. Found: C, 76.92; H, 4.65; N, 6.54.
3b (85%): mp >300 °C; IR (KBr) νmax (cm-1) 3055, 1654, 1608,
1543; FAB-MS m/z calcd for C27H18Br2N2O3 578.25, found 579.35.
Anal. Calcd for C27H18Br2N2O3: C, 56.08; H, 3.14; N, 4.84.
Found: C, 55.74; H, 2.97; N, 5.10.
croconaine dyes exhibited around 100 nm red-shifted absorption
compared to the corresponding squaraine dyes. The presence
of the two carbonyl groups facilitates complex formation of the
croconaine dyes with divalent metal ions. The lowered ICT
character and decreased conjugation in the ring resulted in the
formation of a new absorption band at the shorter wavelength
region for the dye-metal ion complex and the metal binding is
signaled through “turn on” fluorescence intensity and high
association constants (1014-1011 M-2). The broadening of the
1
3c (85%): mp >300 °C; IR (KBr) νmax (cm-1) 3055, 1653, 1606,
1546; FAB-MS m/z calcd for C27H18I2N2O3 672.25, found 672.31.
Anal. Calcd for C27H18I2N2O3: C, 48.04; H, 2.70; N, 4.17. Found:
C, 48.21; H, 2.90; N, 4.16.
aromatic, vinylic, and N-methyl peaks during the H NMR
titrations indicates binding of the metal ion at the carbonyl group
of the croconyl moiety. This was further substantiated by the
clear shifts in the carbonyl stretching frequency of the croconyl
ring upon metal ion binding. These features, together with the
high affinity and sensitivity of the dyes for Zn2+, Pb2+, and
Cd2+, could be exploited for the design of IR absorbing metal
ion sensors and sensitizers for PDT applications.
Procedure for the Synthesis of Dye 3d. A mixture of the
cholesterol-linked quinaldinium salt 2d (0.06 mmol), croconic acid
(0.03 mmol), and quinoline (0.5 mL) was refluxed in ethanol (6
mL) for 24 h. The solvent was distilled off under reduced pressure
to obtain a residue, which was then subjected to column chroma-
tography over silica gel. Elution of the column with a mixture (1:
9) of methanol and chloroform gave 75% of the croconaine dye
3d: mp 290-292 °C; IR (KBr) νmax (cm-1) 2947, 1755, 1660,
Experimental Section
Calculation of Quantum Yield of the [3d-M2+] Complexes.
The fluorescence quantum yields were determined by using optically
matched solutions with IR-125 as the standard (Φ DMSO ) 0.13).6b
The quantum yields of the [3d-M2+] complexes have been
determined by measuring the integrated fluorescence intensity after
the addition of 0.5 equiv of the M2+ ion to 1 equiv of the dye 3d
and substituting in the equation
1
1598, 1560; H NMR (CDCl3, 300 MHz) δ 8.99 (2H, d, J ) 9.4
Hz), 7.55-7.39 (8H, m), 6.39 (2H, s), 5.37 (2H, s), 4.55 (2H, s),
3.95 (6H, s), 2.44 (4H, s), 1.96-0.75 (82H, m); 13C NMR (CDCl3,
75 MHz) δ 196.6, 188.3, 186.8, 183.2, 154.1, 153.1, 140.4, 130.3,
128.2, 124.9, 123.2, 121.4, 118.2, 117.5, 114.3, 111.1, 108.4, 103.3,
81, 73.3, 72.1, 58.2, 57.7, 51.5, 43.8, 41, 37.7, 37.3, 33.4, 31.2,
30.9, 29.7, 29.5, 25.8, 25.4, 24.4, 24.2, 24.1, 22.6, 20.8, 20.3, 13;
FAB-MS m/z calcd for C83H108N2O9 1277.75, found 1277.69. Anal.
Calcd for C83H108N2O9: C, 78.02; H, 8.52; N, 2.19. Found: C,
77.85; H, 8.30; N, 2.13.
2
ΦsAsFunu
Φu )
AuFsns2
where Φu and Φs are the quantum yield of the unknown and
standard, Au and As are the absorbance at the excitation wavelength
of the unknown and standard, Fu and Fs are the integrated
fluorescence intensity of the unknown and standard, and nu and ns
are the refractive index of the solvent of the unknown and standard,
respectively.
General Procedure for the Synthesis of Croconaine Dyes. A
mixture of the corresponding quinaldinium salt (0.06 mmol),
croconic acid (0.03 mmol), and quinoline (0.5 mL) was refluxed
in ethanol (6 mL) for 24 h. The solvent was distilled off under
reduced pressure to obtain a residue, which was washed with
methanol and DMSO to give the corresponding croconaine dyes
3a-c.
Acknowledgment. We thank the Council of Scientific and
Industrial Research, Department of Science and Technology,
Government of India for the financial support of this work. This
is contribution No. NIIST-PPD-251 from the National Institute
for Interdisciplinary Science and Technology (CSIR), Trivan-
drum, India.
Supporting Information Available: Synthetic details, spec-
troscopic data, and photophysical properties of the croconaine dyes,
1H and 13C NMR spectra of the representative dyes, and Figures
S1-S10 showing the changes in NMR, absorption, and fluorescence
spectra of 3d in the presence of various metal ions. This material
is available free of charge via the Internet at http://pubs.acs.org.
(16) Bigelow, R. W.; Freund, H.-J. Chem. Phys. 1986, 107, 159-174.
JO702209A
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