Merocyanine Dyes Containing Imide Functional Groups: Synthesis and Studies on Hydrogen Bonding to Melamine Receptors

Article abstract of DOI:10.1021/jo0351670

The condensation of the CH acidic heterocycles 4-alkyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile (5a and b) and barbituric acid (15) with electron-rich thiophene aldehydes and benzaldehyde derivatives affords the respective monomethine dyes 10-1

Full text of DOI:10.1021/jo0351670

Mer ocya n in e Dyes Con ta in in g Im id e F u n ction a l Gr ou p s:
Syn th esis a n d Stu d ies on Hyd r ogen Bon d in g to Mela m in e
Recep tor s
Frank Wu¨rthner* and Sheng Yao
Institut fu¨r Organische Chemie, Universita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
wuerthner@chemie.uni-wuerzburg.de
Received August 8, 2003
The condensation of the CH acidic heterocycles 4-alkyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-
carbonitrile (5a and b) and barbituric acid (15) with electron-rich thiophene aldehydes and
benzaldehyde derivatives affords the respective monomethine dyes 10-13 and 17-19. The
formylation of 5a ,b and 15 with N,N′-diphenylformamidine or dibutylformamide in acetic anhydride
and further reaction with 4-picolinium salts 9a ,b provide the dimethine dyes 14 and 20a ,b. Triple
hydrogen bonding of the imide groups of merocyanine dyes 10-14 has been investigated by NMR
titration experiments with melamine 21. Despite rather pronounced variations of the charge-transfer
properties within the given series of dyes, minor changes of their binding constants have been
observed. These results could be rationalized by semiempirical calculations that reveal small changes
in the charge density at the oxygen functionalities involved in hydrogen bonding upon variation of
the electron-donating carbocyclic or heterocyclic groups at the terminal double bond. Although the
binding constants for triple hydrogen bonding between imides and melamines are rather weak in
chloroform, they proved to be strong enough to facilitate dissolution of some of these dyes in aliphatic
solvents by coordination to amphiphilic melamines and dipolar aggregation. UV-vis spectral
changes observed in methylcyclohexane vs chloroform suggest the formation of colloidal assemblies
through noncovalent polymerization.
In tr od u ction
bonds formed between imides and melamines are too
weak (binding constant of about 100 M-1) to effect
efficient association under dilute conditions in common
organic solvents (e.g., chloroform), and additional non-
covalent interactions, cooperativity, or weakly polar
aliphatic solvents are required to trigger self-assembly.5
On the other hand, recent work by Rotello et al.7 and
Smith et al.8 has demonstrated that the binding con-
stants between imides and diacyl diaminopyridines (dia-
cyl diaminopyridines exhibit similar binding constants
as melamines for the coordination to imides)9 increase
considerably upon electrochemical reduction of the imide
functionality to the anionic state (Scheme 1). Through
this electrochemical triggering, binding constants as high
as >10 000 M-1 could be achieved. Such strong nonco-
valent bonding should be enough for self-assembly of the
related bis receptors (e.g., bisimides and ditopic mela-
mines) to afford extended oligomeric dye assemblies
under reasonable experimental conditions, i.e., in the
Imide functional groups are important structural
constituents in pigment dyestuffs (Chart 1) because these
functional groups form hydrogen-bonded networks, which
contribute to the high lattice energies required to achieve
the desired insolubility of pigment particles.1
Recently, the dissolution of dyes with imide functional
groups has been realized in supramolecular assemblies
by means of triple hydrogen bonding to complementary
melamines and well-ordered self-assembled monolayers,
double rosettes, and mesoscopic and liquid-crystalline
materials have been obtained.2-6 However, these studies
have also provided evidence that the triple hydrogen
* Author to whom correspondence may be addressed. Tel: +49-(0)-
931-8885340. Fax: +49-(0)931-8884756.
(1) Herbst, W.; Hunger, K. Industrial Organic Pigments: Produc-
tion, Properties, Applications; Wiley-VCH: Weinheim, 1997.
(2) (a) Ariga, K.; Kunitake, T. Acc. Chem. Res. 1998, 31, 371-378.
(b) Bohanon, T. M.; Denzinger, S.; Fink, R.; Paulus, W.; Ringsdorf, H.;
Weck, M. Angew. Chem. 1995, 107, 102-104; Angew. Chem., Int. Ed.
Engl. 1995, 34, 58-60.
(7) Breinlinger, E.; Niemz, A.; Rotello, V. M. J . Am. Chem. Soc. 1995,
117, 5379-5380.
(8) Ge, Y.; Lilienthal, R.; Smith, D. K. J . Am. Chem. Soc. 1996, 118,
3976-3977.
(9) (a) Hamilton, A. D.; Pant, N.; Muehldorf, A. V. Pure Appl. Chem.
1988, 60, 533. (b) Simanek, E. E.; Li, X.; Choi, I. S.; Whitesides, G. M.
In Comprehensive Supramolecular Chemistry; Atwood, J . L., Davies,
J . E. D., MacNicol, D. D., Vo¨gtle, F., Eds.; Pergamon: Oxford, 1996;
Vol. 9, pp 595-621. (c) Brienne, M.-J .; Gabard, J .; Lehn, J .-M.; Stibor,
I. J . Chem. Soc., Chem. Commun. 1989, 1868-1870. (d) Beijer, F. H.;
Sijbesma, R. P.; Vekemans, J . A. J . M.; Meijer, E. W.; Kooijman, H.;
Spek, A. L. J . Org. Chem. 1996, 61, 6371-6380.
(3) Prins, L. J .; Thalacker, C.; Wu¨rthner, F.; Timmerman, P.;
Reinhoudt, D. N. Proc. Natl. Acad. Sci. U. S. A. 2001, 98, 10042-10045.
(4) (a) Yang, W.; Chai, X.; Chi, L.; Liu, X.; Cao, Y.; Lu, R.; J iang, Y.;
Tang, X.; Fuchs, H.; Li, T. Chem.-Eur. J . 1999, 5, 1144-1149. (b) Cao,
Y. W.; Chai, X. D.; Li, T. J .; Smith, J .; Li, D. Chem. Commun. 1999,
1605-1606.
(5) (a) Wu¨rthner, F.; Thalacker, C.; Sautter, A. Adv. Mater. 1999,
11, 754-758. (b) Wu¨rthner, F.; Thalacker, C.; Sautter, A.; Scha¨rtl, W.;
Ibach, W.; Hollricher, O. Chem.-Eur. J . 2000, 6, 3871-3885.
(6) Wu¨rthner, F.; Yao, S.; Heise, B.; Tschierske, C. Chem. Commun.
2001, 2260-2261.
10.1021/jo0351670 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/14/2003
J . Org. Chem. 2003, 68, 8943-8949
8943

Wu¨rthner and Yao
CHART 1. Rep r esen ta tive Exa m p les of P igm en t Dyestu ffs Th a t Con ta in Hyd r ogen -Bon d ed Netw or k s Du e
to th e Im id e F u n ction a l Gr ou p s: Isoin d olin e P igm en ts 1a (P igm en t Yellow 139) a n d 1b (P igm en t Red 260)
a n d P er ylen e P igm en t 2 (P igm en t Violet 29)
SCHEME 1. Effect of th e Electr och em ica l Red u ction of Im id e F u n ction a lities on Bin d in g Con sta n ts for
Hyd r ogen Bon d in g to Dia cyl Dia m in op yr id in es Accor d in g to Ref 8
SCHEME 2. Reson a n ce Str u ctu r es of
Mer ocya n in e Dyes
of such dyes is highly dependent on the strength of the
electron-donor substituent.11,12 In the case of weak donors
such as 4-alkoxyphenyl or 4-dialkylaminophenyl, poly-
ene-type dyes are formed with small ground-state dipole
moments and positive solvatochromism, whereas for
strong electron donors such as 2-dialkylaminothiophenes
or dihydropyridinemethylidene, polymethine-type dyes
are formed with large dipole moments and negative
solvatochromism. In the simple valence bond description
(Scheme 2), the dipolar and the optical properties are
explained in terms of charge transfer from the donor
carbo- or heterocycle to the acceptor heterocycle suggest-
ing an increase of the electron density at the imide
carbonyl groups (resonance structures D-F ) similar to
the aforementioned electrochemical reduction. Accord-
ingly, by means of strong electron-donor groups in
merocyanine dyes, an increase of their binding strength
to melamines seems possible for the dyes with R ) H
(Scheme 2).
millimolar regime.5b The rationalization for the observed
enhancement of hydrogen-bonding propensity of imides
upon electrochemical reduction was provided by theoreti-
cal studies that assign a major electrostatic contribution
to the strength of the hydrogen bond.10
By employment of this strategy, self-assembly of mero-
cyanine dyes and melamine receptors may be achieved
(11) (a) Wu¨rthner, F.; Wortmann, R.; Matschiner, R.; Lukaszuk, K.;
Meerholz, K.; DeNardin, Y.; Bittner, R.; Bra¨uchle, C.; Sens, R. Angew.
Chem. 1997, 109, 2933-2936; Angew. Chem., Int. Ed. Engl. 1997, 36,
2765-2768. (b) Wortmann, R.; Wu¨rthner, F.; Sautter, A.; Lukaszuk,
K.; Matschiner, R.; Meerholz, K. Proc. SPIE-Int. Soc. Opt. Eng. 1998,
3471, 41-49.
During the past years, we have studied merocyanine
dyes of the type shown in Scheme 2 (D ) electron donor
carbo- or heterocycle, R ) alkyl) for photorefractive
applications and showed that the zwitterionic character
(12) (a) Wu¨rthner, F.; Yao, S. Angew. Chem. 2000, 112, 2054-2057;
Angew. Chem., Int. Ed. Engl. 2000, 39, 1978-1981. (b) Wu¨rthner, F.;
Yao, S.; Debaerdemaeker, T.; Wortmann, R. J . Am. Chem. Soc. 2002,
124, 9431-9447.
(10) (a) Morukuma, K. Acc. Chem. Res. 1977, 10, 294-300. (b)
Guerra, C. F.; Bickelhaupt, F. M.; Snijders, J . G.; Baerends, E. Chem.-
Eur. J . 1999, 5, 3581-3594.
8944 J . Org. Chem., Vol. 68, No. 23, 2003

Merocyanine Dyes Containing Imide Functional Groups
SCHEME 3. Syn th esis of Mer ocya n in e Dyes 10-14a
a
(a) Ac2O, 90 °C, 1.5-2 h, yield 50% of 10, 48% of 11, 76% of 12a , and 74% of 12b; (b) Ac2O, 90 °C, 1 h, yield 88%; (c), (d) Ac2O, room
temperature, 0.5 h and 90 °C, 5 min, 9a , KOAc, 90 °C, 2 h, yield 39% of 14, (DPFA ) N,N′-diphenylformamidine).
in dilute solutions, and the packing of these dyes in the
solid state could be controlled. Here we report our results
on the exploration of this strategy, which include the
synthesis of properly designed merocyanine dyes with
free imide sites (R ) H) and the evaluation of their
coordination properties to complementary melamine
receptors through triple hydrogen bonding.
A second series of dyes containing barbituric acid
acceptor heterocycles was synthesized with the intention
to construct polymeric assemblies through coordination
of ditopic melamine receptors at both imide groups of the
heterocycle. Because of the high CH acidity of barbituric
acid (15), the same routes were applied as for pyridones
5a ,b to synthesize the dyes 17-20 in good yields (Scheme
4).
Bin d in g Stu d ies. To assess the effect of electron-
donor carbo- and heterocycles on the binding strength of
triple hydrogen bonding of merocyanine dyes to mela-
mines, constant host 1H NMR titration studies15 were
performed in CDCl3. In these experiments the concentra-
tions of the imide dyes were kept constant, while increas-
ing concentrations of the highly soluble complementary
melamine 215 were added (Scheme 5, Figure 1). Upon
coordination of melamine to imide dyes, the imide proton
is deshielded, leading to a strong downfield shift of the
signal. This resonance shift was used to calculate the
binding constant and the Gibbs binding energy through
nonlinear regression analysis, and the data are presented
in Table 1.
As shown in Table 1, all binding constants that could
be measured with high accuracy are about 120 M-1 for
the well-soluble dyes with R1 ) nC9H19 (dyes 11, 12a ,
and 14). Larger values have been measured for the dye
12b with R1 ) Me and for 10. However, both of these
values are prone to much larger errors because of either
low solubility (dye 12b) or base-catalyzed decomposition
(dye 10 upon addition of basic melamine 21). For dye 13,
this base-mediated decomposition is so fast that the
addition of the first drops of melamine solution already
caused pronounced destruction of 13. Despite the fact
that merocyanine dyes 11, 12a , and 14 exhibit signifi-
cantly different polarities (as expressed based on the
contribution of their zwitterionic resonance structures),
their binding constants do not differ accordingly. These
results lead to the conclusion that the hydrogen bonding
Resu lts a n d Discu ssion
Syn th esis. To explore the influence of electron-donat-
ing groups on the strength of hydrogen binding, a series
of appropriately functionalized merocyanine dyes are
required. Thus, the dyes 10-13 were synthesized through
Knoevenagel condensations of the highly CH-acidic pyr-
idones 5a ,b with thiophen aldehydes 7a -c and benzal-
dehyde 8. The dimethine dye 14 was obtained by formyl-
ation of pyridone 5a with N,N′-diphenylformamidine
(DPFA) followed by the reaction with pyridinium salt 9a
according to an earlier reported procedure.12b,13 In the
synthesis of these merocyanine dyes, we have encoun-
tered two problems. The first one is related to the low
chemical stability of the polyene-type merocyanine dyes.14
Thus, the derivative corresponding to 13 (Scheme 3) with
a 4-butoxyphenyl electron donor group could not be
isolated in a pure form and the next more polar dye 14
was so sensitive against bases that its binding to melamine
was accompanied by decomposition (probably by retro-
Knoevenagel reaction, see ref 2b). The second problem
is related to the solubility of merocyanine dyes that
decreases significantly with the increasing dipole moment
for the present series of dyes. For this reason, solubilizing
alkyl groups had to be introduced for the R1 substituent
of the pyridone acceptor heterocycle (5) and a dihydro-
pyridine donor with two isopropyl groups has been cho-
sen to obtain a reasonably soluble most polar dye 14.
(13) (a) Wu¨rthner, F. Synthesis 1999, 2103-2113. (b) Wu¨rthner, F.;
Yao, S.; Schilling, J .; Wortmann, R.; Redi-Abshiro, M.; Mecher, E.;
Gallego-Gomez, F.; Meerholz, K. J . Am. Chem. Soc. 2001, 123, 2810-
2814.
(14) Wu¨rthner, F.; Yao, S.; Wortmann, R. J . Information Recording
2000, 25, 69-86.
(15) (a) Connors, K. A. Binding Constants; Wiley & Sons: New York,
1987. (b) Wilcox, C. S. In Frontiers of Supramolecular Chemistry and
Photochemistry; Schneider, H. J ., Du¨rr, H., Eds.; VCH: Weinheim,
1991; pp 123-143.
J . Org. Chem, Vol. 68, No. 23, 2003 8945

Wu¨rthner and Yao
SCHEME 4. Syn th esis of Mer ocya n in e Dyes 17-20a
a
(a) Ac2O, 90 °C, 0.5-2 h, yield 67% of 18, 39% of 17, 93% of 19; (b) Ac2O, 120 °C, 2 h; (c) Ac2O, 120 °C, 2 h, yield 66% of 20a , 29% of
20b (DBF ) dibutylformamide).
SCHEME 5. F or m a tion of Tr ip le Hyd r ogen -Bon d ed 1:1 Com p lexes betw een Dyes 10-14 a n d Mela m in e 21
(R1 ) CH3 or C9H19, Eth ex ) 2-eth ylh exyl)
TABLE 1. Bin d in g Con sta n ts of Com p lexes of Dye
10-14 w ith Mela m in e 21 in CDCl3 a t 300 K
dipole
moment
(D)a
zwitterionic
character
(%)b
binding
constant
∆G°
dye
(M-1
)
(kJ mol-1
)
13
10
11
12a
12b
14
10.8
6.6
8.4
14.1
14.1
17.1
28
31
36
45
45
60
c
c
304 ( 21d
117 ( 8
-14.3d
-11.9
-12.0
-12.7
-12.0
123 ( 19
170 ( 40
123 ( 16
a
Determined for related dyes with n-butyl substituent at the
imide nitrogen and R1 ) Me (ref 11). Calculated based on a two-
b
state model from dipole moments in the ground and the excited
state according to ref 11. c Binding constant could not be deter-
mined because of immediate decomposition of the dye upon
addition of melamine. Because of decomposition during the
measurements, these values are susceptible to larger errors than
those indicated based on the statistical analysis.
F IGURE 1. Constant host 1H NMR titration of dye 14 (host)
with melamine 21 (guest) in CDCl3 at 300 K and the nonlinear
regression curve (s) based on a 1:1 model.15 The concentration
of the dye 14 was kept constant at 5.0 mM.
d
strength of merocyanine dyes does not increase with
increasing charge transfer from the donor to the acceptor
heterocycle.
tational analysis seems to be very reliable because the
experimental dipole moment data from Table 1 and the
X-ray crystallographic structural data of 12b13b are
reproduced well. With regard to the mesomeric formulas
in Scheme 2, we have to conclude that structures A, B,
and C, with negative charges on the polymethine chain,
are most important, whereas D, E, and F, with negative
charges on the carbonyl oxygens, are of minor impor-
tance. The negligible effect of the increased charge
transfer on the hydrogen-bonding strength is in good
accordance with this interpretation. On the other hand,
further calculations for radical anions derived from
molecule 12 have shown that here substantial negative
charge accumulates at the imide oxygen atoms, where
the SOMO exhibits high coefficients. The same behavior
was observed for naphthaline imide 3. Therefore, the
different impact of electrochemical reduction and charge
To rationalize this observation, AM1 calculations were
performed. Indeed, these theoretical studies revealed that
the charge densities at the imide oxygen atoms did not
notably change upon increasing the electron-donor
strength from thiophene to dialkylaminothiophene (Fig-
ure 2). Instead, the calculation showed that the negative
charge is highly delocalized over the whole acceptor
heterocycle with the strongest variability at the C5 and
C3 centers of the six-membered acceptor heterocycle.
Interestingly, these two carbon centers may be regarded
as an extension of the polymethine chain. By contrast,
the three outer heteroatomic centers (two oxygen atoms
and one nitrogen atom of the cyano group) experience a
much smaller increase of negative charge. This compu-
8946 J . Org. Chem., Vol. 68, No. 23, 2003

Merocyanine Dyes Containing Imide Functional Groups
F IGURE 3. UV-vis spetra of dye 14 in the presence of a half
F IGURE 2. Total charge distributions for methine dyes 10
(numbers on top) and 12 (numbers on bottom) with all alkyl
groups replaced by methyl groups according to AM1 calcula-
tions. The arrow marks the electron donor (hydrogen or
dimethylamino), and the circles mark the magnitude of the
increase of negative charges at the most influenced atoms.
equivalent of melamine 22 in chloroform (dotted line, λmax
535 nm) and in methylcyclohexane (solid line, λmax
506 nm).
)
)
transfer from electron-donor groups upon hydrogen-
bonding strength is well explained by these calculations.
Self-Assem bly. For the second series of dyes 17-20,
no binding studies could be accomplished owing to their
insufficient solubility in solvents where hydrogen bonds
exhibit reasonable strength (chloroform or solvents of
lower polarity). Notably, even the addition of stoichio-
metric amounts of melamine receptor 21 or 22 with
complementary hydrogen-bonding patterns did not afford
an increase of the solubility of these dyes in methylcy-
clohexane in contrast to the situation for bisimide dyes
reported in our earlier work.5 This low solubility is quite
likely related to the planarity of these dyes and the
presence of two imide units in one planar dye molecule
that can be used to form hydrogen-bonded networks
similar to those found in the pigments 1 and 2.
F IGURE 4. Reduced viscosity (in L/g) of a 2:1 mixture of dye
14 and melamine 22 vs concentration in methylcyclohexane
at 30 °C.
To address the last point, it seems quite reasonable
that upon coordination of melamine 22 by triple hydrogen
bonding, the two alkyl groups of the melamine support
the dissolution of 14. But how has the solvation of the
dipolar π-system been facilitated? This might be ac-
complished by dimerization of 14 to dimer aggregates as
shown in our earlier work.16 Such dimers have a vanish-
ing dipole moment which make them surprisingly soluble
in weakly polar solvents. As a characteristic feature,
these dimers exhibit strongly blue-shifted absorption
bands (H aggregate) as observed in Figure 3. Accordingly,
the dissolution of dyes 14 upon addition of melamine 22
in methylcyclohexane can be explained in terms of
formation of a supramolecular polymer (Scheme 6). In
contrast to an earlier reported17 supramolecular polymer
formed by dipolar aggregation of covalently coupled dyes
similar to 14, the present hydrogen-bonded assembly
does not lead to a well-ordered aggregate by further
hierarchical growth. Therefore, the hypsochromic shift
observed for the absorption band of 14 remains quite
modest as expected for dimer aggregates and no viscosity
increase could be observed by capillary viscosimetry
(Figure 4). The latter observation is in accordance with
the formation of a rather weakly bound colloid-type
assembly in contrast to covalently bonded polymeric
chains.
Therefore, another approach to extended self-as-
sembled hydrogen-bonded merocyanine dye networks
was employed, which is based on the combination of
hydrogen-bond formation of ditopic melamines with the
imide unit of dye 14 and the high dimerization constant
of this dye in aliphatic solvents. The latter effect is due
to the strong electrostatic interaction of this highly
dipolar dye leading to antiparallel dimers with vanishing
dipole moment.12,16
Figure 3 shows the absorption spectra of a 2:1 mixture
of dye 14 with the ditopic melamine 22 in chloroform and
in methylcyclohexane. The spectrum recorded in chloro-
form is identical with that of pure 14 (with λmax ) 535
nm), as melamine 22 and has no absorbance at wave-
length λ > 300 nm. On the other hand, the spectrum in
methylcyclohexane is remarkable for two reasons: First,
the absorption band of dye 14 is strongly blue shifted
(with λmax ) 506 nm), an effect that cannot be related to
the formation of hydrogen bonds. Second, the highly polar
imide dye 14 is not at all soluble in aliphatic solvents
such as methylcyclohexane but only in solvents of high
polarity that are able to solvate the dipolar π system
(µ ) 17 D!) and the hydrogen-bonding imide receptor.
(16) Wortmann, R.; Ro¨sch, U.; Redi-Abshiro, M.; Wu¨rthner, F.
Angew. Chem. 2003, 115, 2126-2129; Angew. Chem., Int. Ed. 2003,
42, 2080-2083.
(17) Wu¨rthner, F.; Yao, S.; Beginn, U. Angew. Chem. 2003, 115,
3368-3371; Angew. Chem., Int. Ed. 2003, 42, 3347-3350.
J . Org. Chem, Vol. 68, No. 23, 2003 8947

Wu¨rthner and Yao
SCHEME 6. F or m a tion of a Su p r a m olecu la r P olym er fr om Dye 14 a n d Mela m in e 22 th r ou gh Dim er iza tion
of Dip ola r Dye 14 a n d Tr ip le Hyd r ogen Bon d in g to Mela m in e 22
elemental analysis. 4-Methyl-2,6-dioxo-1,2,5,6-tetrahydro-pyr-
idine-3-carbonitrile (5b) was obtained from commercial sources,
and 4-nonyl-2,6-dioxo-1,2,5,6-tetrahydro-pyridine-3-carboni-
trile (5a ) was prepared according to ref 12b. AM1 calculations
have been performed with HyperChem 6.03 for Windows from
Hypercube, Inc. By comparison of the calculated bond length
and dipole moment data with experimental ones from crystal
structures and dipole moment measurements (refs 11-13), we
consider these calculations as reliable. Solution viscosities were
measured at 30 °C with capillary viscometers using an
automated viscosity measuring unit to obtain reproducible run
times. The effective capillary diameters were 0.47, 0.63, and
1.13 mm. The setup was mounted in a thermostat controlled
by circulating a 1:1 mixture of water-ethyleneglycol. The
measurements were run from concentrated to diluted solu-
tions. Dilution was achieved by using an automated device.
Con clu sion
This work has shown for merocyanine dyes with imide-
containing acceptor units that the Gibbs energy of imide-
melamine triple hydrogen bonding is not influenced by
the variation of the electron-donor moiety of the dye.
Despite the very diverse charge-transfer character (from
polyene- to betaine-type) these dyes are hydrogen bonded
to melamine 21 with identical binding constants of K )
120 M-1 in CDCl3. It was demonstrated that the triple
hydrogen-bonding coordination to ditopic melamine 22
is useful to dissolve the highly dipolar dye 14 even in
the little polar solvent methylcyclohexane. On the basis
of the optical properties of the resulting solutions, we
conclude that colloidal assemblies have formed by su-
pramolecular polymerization through hydrogen bonding
to melamines and dipolar aggregation between the dyes.
In contrast to the pyridone-derived dyes 10-14, the
barbituric acid-based dyes 17-20 retained their pigment-
type behavior even in the presence of complementary
melamines. Therefore, the dissolution of these dipolar
molecules from a strongly hydrogen-bonded crystalline
network necessitates a more refined complementary
binding partner. For this purpose, more elaborated
receptors such as the one introduced by Hamilton18 seem
promising, and such receptors are currently under in-
vestigation.
5-(Th iop h en e-2-ylm eth ylen e)-4-n on yl-2,6-d ioxo-1,2,5,6-
tetr a h yd r op yr id in e-3-ca r bon itr ile (10). Thiophene alde-
hyde 7a (0.28 g, 2.5 mmol) and 5a (0.66 g, 2.25 mmol, 90%)
were heated in acetic anhydride (2.5 mL) at 90 °C for 2 h. The
mixture was allowed to cool to room temperature with stirring.
The slurry formed was filtered by suction, washed with
2-propanol, and dried in vacuo at 60 °C to give 0.40 g (50%) of
10: mp 202-203 °C; 1H NMR (500 MHz, CDCl3) δ 8.65 (s,
1H, NH), 8.11 (m, 1H, Hthiophene), 8.08 (s, 1H, Hmethine), 7.84 (m,
1H, Hthiophene), 7.34 (m, 1H, Hthiophene), 3.02 (t, J ) 8.0 Hz, 2H,
CH2), 1.71 (m, 2H, CH2), 1.53 (m, 2H, CH2), 1.37 (m, 2H, CH2),
1.29 (m, 8H, CH2), 0.88 (t, J ) 7 Hz, 3H, CH3). Anal. Calcd for
C
21H26N2O3S (356.49): C, 67.39; H, 6.79; N, 7.86. Found: C,
67.08; H, 6.71; N, 8.07.
5-(5-Meth oxyth iop h en e-2-ylm eth ylen e)-4-n on yl-2,6-d i-
oxo-1,2,5,6-tetr a h yd r op yr id in e-3-ca r bon itr ile (11). Meth-
oxythiophene aldehyde 7b19 (0.51 g, 3.6 mmol) and 5b (0.85,
3.24 mmol) in acetic anhydride (3 mL) were heated at 90 °C
for 2 h. The mixture was allowed to cool to room temperature
Exp er im en ta l Section
The characterization of all new compounds was accom-
plished by 1H NMR spectroscopy, UV-vis spectroscopy, and
(18) Hamilton, A. D.; van Engen, D. J . Am. Chem. Soc. 1991, 113,
7640-7645.
(19) Sice´, J . J . Am. Chem. Soc. 1953, 75, 3697-3700.
8948 J . Org. Chem., Vol. 68, No. 23, 2003

Merocyanine Dyes Containing Imide Functional Groups
with stirring. The slurry formed was filtered by suction,
washed with 2-propanol, and dried in vacuo at 60 °C to give
0.60 g (48%) of 11: mp 175-176 °C; 1H NMR (200 MHz,
CDCl3) δ 8.98 (s, 1H, NH), 7.67 (s, 1H, Hmethine), 7.61 (d, J )
4.43 Hz, 1H, Hthiophene), 6.57 (d, J ) 4.4 Hz, 1H, Hthiophene), 4.15
(s, 3H, OCH3), 2.88 (t, J ) 7.4 Hz, 2H, CH2), 1.62 (m, 2H, CH2),
1.44 (m, 2H, CH2), 1.21 (m, 10H, CH2), 0.81 (t, J ) 5.9 Hz,
3H, CH3). Anal. Calcd for C21H26N2O3S (386.52): C, 65.26; H,
6.78; N, 7.25. Found: C, 65.19; H, 6.98; N, 7.35.
Anal. Calcd for C40H63N3O2 (617.97): C, 77.75; H, 10.28; N,
6.80. Found: C, 77.36; H, 9.98; N, 6.78.
Gen er a l P r oced u r e for th e P r ep a r a tion of 5-(4-Su b-
stitu ted -a r ylid en e)p yr im id in e-2,4,6-tr ion es 17-19. (Het-
ero)aromatic aldehyde (2 mmol), 2 mmol of barbituric acid, and
2 mL of Ac2O were mixed and heated at 90 °C for 2 h. After
the solution was cooled, the precipitated dye was filtered off,
washed with 2-propanol and hexane, and dried.
5-(4-Bu toxylben zylid en e)p yr im id in e-2,4,6-tr ion e (17).
Recrystallization from ethyl acetate (yield 39%): mp 239-242
°C; 1H NMR (200 MHz, [D6]-DMSO) δ 11.17 (s, 1H, NH), 11.07
(s, 1H, NH), 8.28 (d, J ) 9.0 Hz, 2H, Ar-H), 7.83 (s, 1H,
5-(5-Dibu tyla m in oth iop h en e-2-ylm eth ylen e)-4-n on yl-
2,6-d ioxo-1,2,5,6-tetr a h yd r op yr id in e-3-ca r bon itr ile (12a ).
Dibutylaminothiophene aldehyde 7c13b (0.43 g, 1.8 mmol) and
5a (0.52 g, 1.8 mmol, 90%) in acetic anhydride (2 mL) were
heated at 90 °C for 2 h. After the solution cooled, a deep-violet
oil was obtained that was purified by column chromatography
with CH2Cl2/MeOH ) 98:2. The solvent was concentrated to
5 mL, and the precipitated dye was filtered by suction and
dried in vacuo at 50 °C to give 0.66 g (76%) of 12: mp 156-
H
methine), 6.87 (d, J ) 8.7 Hz, 2H, Ar-H), 4.00 (t, J ) 6.3 Hz,
2H, OCH2), 1.71 (m, 2H, CH2), 1.42 (m, 2H, CH2), 0.90 (t, J )
7.3 Hz, 3H, CH3). Anal. Calcd for C15H16N2O4 (288.31): C,
62.49; H, 5.59; N, 9.72. Found: C, 62.05; H, 5.65; N, 9.72.
5-(4-Dib u t yla m in ob en zylid en e)p yr im id in e-2,4,6-t r i-
on e (18). Recrystallization from ethyl acetate (yield 67%): mp
1
158 °C; H NMR (200 MHz, CDCl3) δ 8.81 (s, 1H, NH), 7.53
1
214-217 °C; H NMR (200 MHz, [D6]-DMSO) δ 11.04 (s, 1H,
(s+d, 2H, Hthiophene and Hmethine), 6.41 (d, J ) 5.41 Hz, 1H,
NH), 10.91 (s, 1H, NH), 8.42 (d, J ) 8.9 Hz, 2H, Ar-H), 8.14
(s, 1H, Hmethine), 6.78 (d, J ) 9.1 Hz, 2H, Ar-H), 3.46 (m, 4H,
NH2), 1.57 (m, 4H, CH2), 1.34 (m, 4H, CH2), 0.94 (m, 6H, CH3).
Anal. Calcd for C19H25N3O3 (343.43): C, 66.45; H, 7.34; N,
12.24. Found: C, 66.33; H, 7.46; N, 12.19.
H
thiophene), 3.56 (t, J ) 7.6 Hz, 4H, NCH2), 2.86 (t, J ) 7.9 Hz,
2H, CH2), 1.81-1.63 (m, 6H, CH2), 1.52-1.27 (m, 16H, CH2),
0.99 (t, J ) 7.4 Hz, 6H, CH3), 0.88 (t, J ) 6.7 Hz, 3H, CH3).
Anal. Calcd for C28H41N3O2S (483.72): C, 69.53; H, 8.54; N,
8.69. Found: C, 69.28; H, 8.36; N, 8.61.
5-(5-Dibu tylam in oth ioph en e-2-ylm eth ylen e)pyr im idin e-
2,4,6-tr ion e (19). Recrystallization from ethyl acetate afforded
19 (yield 93%): mp 289-290 °C; 1H NMR (200 MHz, [D6]-
DMSO) δ 10.61 (s, 1H, NH), 10.53 (s, 1H, NH), 8.06 (s, 1H,
5-(5-Dibu tyla m in oth iop h en e-2-ylm eth ylen e)-4-m eth yl-
2,6-d ioxo-1,2,5,6-tetr a h yd r op yr id in e-3-ca r bon itr ile (12b).
Dibutylaminothiophene aldehyde 7c13b (0.75 g, 5.0 mmol) and
5b (1.26 g, 5.0 mmol) in acetic anhydride (5 mL) were heated
at 90 °C for 1.5 h. The mixture was allowed to cool to room
temperature with stirring. The slurry formed was filtered by
suction, washed with Ac2O and 2-propanol, and recrystallized
from acetic acid to give 1.38 g (74%) of 1d : mp 270-272 °C;
1H NMR (200 MHz, [D6]-DMSO) δ 10.88 (s, 1H, NH), 7.99 (d,
J ) 5.4 Hz, 1H, Hthiophene), 7.83 (s, 1H, Hmethine), 6.82 (d, J )
4.9 Hz, 1H, Hthiophene), 3.57 (t, J ) 7.6 Hz, 4H, NCH2), 2.43 (s,
3H, CH3), 1.65 (m, 4H, CH2), 1.33 (m, 4H, CH2), 0.92 (t, J )
7.4 Hz, 6H, CH3). Anal. Calcd for C20H25N3O2S (371.51): C,
64.66; H, 6.78; N, 11.31. Found: C, 64.78; H, 6.67; N, 11.18.
5-(4-Dibu tyla m in oben zylid en e)-4-m eth yl-2,6-d ioxo-1,-
2,5,6-tetr a h yd r op yr id in e-3-ca r bon itr ile (13). Dibutylami-
nobenzaldehyde 8 (1.16 g, 5.0 mmol) and 5b (0.75 g, 5.0 mmol)
were heated in acetic anhydride (5 mL) at 90 °C for 1 h. The
mixture was allowed to cool to room temperature with stirring.
The slurry formed was filtered by suction, washed with Ac2O
and 2-propanol, and recrystallized from acetic acid to give 1.6
H
methine), 7.89 (d, J ) 5.0 Hz, 1H, Hthiophene), 6.57 (d, J ) 5.0
Hz, 1H, Hthiophene), 3.51 (t, J ) 7.3 Hz, 4H, NCH2), 1.59 (m,
4H, CH2), 1.31 (m, 4H, CH2), 0.91 (t, J ) 7.3 Hz, 6H, CH3).
Anal. Calcd for C17H23N3O3S (349.46): C, 58.43; H, 6.63; N,
12.02. Found: C, 58.20; H, 6.49; N, 11.84.
Gen er a l P r oced u r e for P r ep a r a tion of 5-[(P yr id in e-
4-yliden e)eth yliden e]pyr im idin e-2,4,6-tr ion es (20a,b). Bar-
bituric acid 15 (1.5 mmol) and N,N-dibutylformamide (1.5
mmol) in Ac2O (3 mL) were heated at 120 °C for 2 h. To the
clear solution obtained was added pyridinium salt12b 9a or 9b
(1.5 mmol) and potassium acetate (0.15 g, 1.5 mmol), and the
mixture was heated at 120 °C for another 2 h. The resulting
solution was evaporated in vacuo and the isolated solid was
purified by column chromatography using silica gel and CH2-
Cl2/CH3OH ) 95:5 as eluent.
5-[2-(1-Dod ecyl-2,6-d iisop r op yl-p yr id in e-4-ylid en e)-
eth ylid en e]p yr im id in e-2,4,6-tr ion e (20a ). Recrystallization
from CH2Cl2/hexane afforded 20a (yield 66%): mp 223-224
°C; 1H NMR (400 MHz, CDCl3) δ 8.21 (d, J ) 15.2 Hz, 1H,
1
g (88%) of 13: mp 195 °C; H NMR (200 MHz, CDCl3) δ 8.19
(br, 2H, Ar-H), 8.08 (s, 1H, NH), 7.58 (s, 1H, Hmethine), 6.66
(d, J ) 9.4 Hz, 2H, Ar-H), 3.44 (t, J ) 7.6 Hz, 4H, NCH2),
2.59 (s, 3H, CH3), 1.65 (m, 4H, CH2), 1.38 (m, 4H, CH2), 0.99
(t, J ) 7.1 Hz, 6H, CH3). Anal. Calcd for C22H27N3O2 (365.48):
C, 72.30; H, 7.45; N, 11.50. Found: C, 72.17; H, 7.35; N, 11.49.
4-Non yl-2,6-d ioxo-5-[2-(1-d od ecyl-2,6-d iisop r op yl-1-h y-
d r op yr id in -4-ylid en e)eth ylid en e]-1,2,5,6-tetr a h yd r op yr -
id in e-3-ca r bon itr ile (14). Pyridone 5a (0.70 g, 2.7 mmol) and
N,N′-diphenylformamidine (0.58 g, 3.0 mmol) in Ac2O (3 mL)
were stirred at room temperature for 30 min until the mixture
solidified. To complete the reaction to 6, the mixture was
heated at 90 °C for another 5 min. After the solution cooled to
room temperature, pyridinium perchlorate salt 9a 11b (1.78 g,
3.0 mmol, 75%) and potassium acetate (0.3 g, 3.0 mmol) were
added and the mixture was heated at 90 °C for 2 h. The
resulting solution was concentrated and purified by column
chromatography using silica gel and CH2Cl2/CH3OH ) 96:4
as eluent. Recrystallization from 2-propanol/hexane afforded
0.60 g (39%) of 14: mp 165-167 °C; 1H NMR (400 MHz,
CDCl3) δ 7.95 (s, 1H, NH), 7.77 (d, J ) 14.9 Hz, 1H, Hmethine),
7.61 (d, J ) 14.9 Hz, 1H, Hmethine), 7.24 (s, 2H, pyridine), 4.16
(t, J ) 8.3 Hz, 2H, NCH2), 3.21 (m, 2H, CH), 2.81 (t, J ) 8.0
Hz, 2H, CH2), 1.78 (m, 2H, CH2), 1.65 (m, 2H, CH2), 1.44 (m,
16H, CH2 and CH3), 1.27 (m, 26H, CH2), 0.87 (m, 6H, CH3).
H
methine), 7.50 (s, 2H, NH), 7.47 (d, J ) 15.4 Hz, 1H, Hmethine),
7.23 (s, 2H, Hpyridine), 4.04 (t, J ) 8.3 Hz, 2H, NCH2), 3.11 (m,
2H, CH), 1.72 (m, 2H, CH2), 1.40 (m, 18H, CH2), 1.27 (m, 15H,
CH2 + CH3), 0.88 (m, 3H, CH3). Anal. Calcd for C29H45N3O3‚
H2O (501.72): C, 69.42; H, 9.44; N, 8.38. Found: C, 69.42; H,
9.32; N, 8.37.
5-[2-(1-Dod ecyl-p yr id in e-4-ylid en e)eth ylid en e]p yr im i-
d in e-2,4,6-tr ion e (20b). Recrystallization from CH2Cl2/hex-
ane afforded 20b (yield 29%): mp 264-265 °C; 1H NMR (400
MHz, [D6]-DMSO) δ 9.88 (s, 2H, NH), 8.17 (d, J ) 7.1 Hz, 2H,
H
pyridine), 8.00 (d, J ) 15.2 Hz, 1H, Hmethine), 7.42 (d, J ) 6.8
Hz, 2H, Hpyridine), 7.29 (d, J ) 14.9 Hz, 1H, Hmethine), 4.15 (t,
J ) 7.2 Hz, 2H, NCH2), 2.51 (s, 3H, CH3), 1.77 (m, 2H, CH2),
1.23 (m, 18H, CH2), 0.85 (t, J ) 6.7 Hz, 3H, CH3). Anal. Calcd
for C23H33N3O3‚H2O (417.56): C, 66.15; H, 8.46; N, 10.06.
Found: C, 66.07; H, 8.15; N, 9.95.
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support by the Deutsche Forschungsgemein-
schaft (Wu 317/1-3). We thank Prof. B. Engels for
helpful discussions.
J O0351670
J . Org. Chem, Vol. 68, No. 23, 2003 8949