Merocyanine-type dyes from barbituric acid derivatives

Article abstract of DOI:10.1016/S1386-1425(00)00461-3

The preparation and the solvatochromic behavior of two dyes, obtained by condensation of N,N′-dimethylbarbituric acid with dimethylaminobenzaldehyde and with 4,4′-bis(N,N-dimethylamino)benzophenone (Michler's ketone) are described. The latter dye is rathe

Full text of DOI:10.1016/S1386-1425(00)00461-3

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Spectrochimica Acta Part A 57 (2001) 1183–1190
www.elsevier.nl/locate/saa
Merocyanine-type dyes from barbituric acid derivatives
Marcos Caroli Rezende a,*, Paola Campodonico a, Elsa Abuin a,
Jean Kossanyi b
a Facultad de Qu´ımica y Biolog´ıa, Uni6ersidad de Santiago de Chile, Casilla 40, Correo 33 Santiago, Chile
b CNRS, 2-8 rue Henri Dunant, 94320 Thiais, France
Received 29 June 2000; accepted 14 November 2000
Abstract
The preparation and the solvatochromic behavior of two dyes, obtained by condensation of N,N%-dimethylbarbi-
turic acid with dimethylaminobenzaldehyde and with 4,4%-bis(N,N-dimethylamino)benzophenone (Michler’s ketone)
are described. The latter dye is rather sensitive to the polarity of the medium, and in particular, to the hydrogen-
bond-donor ability of protic solvents. The solvatochromism of both compounds is discussed in terms of the y* and
ET(30) solvent polarity scales and their differences in behavior interpreted with the aid of semiempirical calculations.
© 2001 Elsevier Science B.V. All rights reserved.
Keywords: Solvatochromic merocyanine dyes; Barbituric acid derivatives; Semiempirical calculations
widely studied photochromic spirooxazines [8–
13].
1. Introduction
Barbituric acid derivatives are versatile reagents,
capable of condensing with a wide range of ke-
tones. A particular example of this classical con-
densation is the use of g-pyrones as electrophilic
components, a reaction described nearly 40 years
ago by Eiden [14].
In the present communication, we employed this
reaction to obtain new dyes, which might exhibit
useful solvatochromic properties. Following Ei-
den’s general procedure, we prepared compounds
1 and 2, by condensation of N,N-dimethylbarbi-
turic acid with 4-dimethylaminobenzaldehyde and
4,4%-bis(dimethylamino)benzophenone (Michler’s
ketone), respectively, and investigated their spec-
troscopic behavior in various solvents (Scheme 1).
The pyrimidinetrione system of barbituric acid
may be envisaged as a potentially useful fragment
in the design of new cyanine dyes. Conjugation of
the carbonyl groups of this ring system with a
nitrogen or oxygen atom of another fragment by
means of a polymethine chain should lead to new
dyes with interesting spectroscopic properties.
Conspicuous examples of such compounds are
solvatochromic a- and g-vinylogous pyridones [1–
5] indoaniline dyes [6–8] and the open form of the
* Corresponding author.
E-mail address: mcaroli@lauca.usach.cl (M.C. Rezende).
1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00461-3

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1184 M.C. Rezende et al. / Spectrochimica Acta Part A 57 (2001) 1183–1190
(1.49 g, 0.01 mol) in acetic acid (5 ml) and acetic
anhydride (5 ml) was gently refluxed for 2 h. After
cooling, the precipitated product was filtered and
recrystallized in acetic acid to give 2.1 g (73% yield)
of 1, m.p. 224–226°C. Analysis, C, 63.01, H, 5.75%
C15H17N3O3 requires C, 62.72, H, 5.92%. IR (KBr)
1660, 1610, 1500, 1440, 1410, 1360, 1190, 1160 and
1080 cm−1. NMR (CDCl3) l 3.16 (s, 6 H,
N(CH3)2), 3.40 ( s, 6 H, CON–CH3), 6.70 ( d, 2 H,
J=9 Hz, Ar–H ortho to NMe2), 8.41 ( d, 2 H,
J=9 Hz, Ar–H meta to NMe2), 8.44 ( s, 1 H,
CHꢀC).
Scheme 1.
The comparison of their solvatochromic behav-
ior in 20 different solvents revealed some interesting
differences between these systems, which are dis-
cussed and rationalized in the present communica-
tion with the aid of semiempirical calculations.
2.2. 1,3-Dimethyl-5-[4,4%-bis(dimethylamino-
phenyl)methylene]-2,4,6(1H,3H)-pyrimidine-trione
(2)
2. Experimental
N,N%-dimethylbarbituric acid (1.56 g, 0.01 mol)
and 4,4%-bis(N,N-dimethylamino)benzophenone
(1.34 g, 0.005 mol) were gently refluxed in a mixture
of acetic acid (5 ml) and acetic anhydride (5 ml) for
8 h. The solution was rotary evaporated and the
residue redissolved in dichloromethane (70 ml) and
washed several times with a 1 M Na2CO3 solution
and finally with water. The dried ( anhydrous
MgSO4) organic phase was evaporated and the
solid residue purified by flash chromatography
(silica gel G-60, dichloroethane as eluent)
to give 0.81 g (40% yield) of the product, recrystal-
lized in acetone-petroleum ether, m.p. 227–230°C
(subl.). Analysis, C, 67.64, H, 6.29%. C23H26N4O3
requires C, 67.98, H, 6.40% IR (KBr) 1700, 1630,
1590,1400, 1360, 1330,1170 and 1080 cm−1. NMR
(CDCl3) l 3.13 (s, 12 H, N(CH3)2), 3.33 (s, 6 H,
CON-CH3), 6.65 (d, 4 H, J=9 Hz, Ar–H ortho to
NMe2), 7.25 (d, 4 H, J=9 Hz, Ar–H meta to
NMe2).
Melting points were recorded on an Electrother-
mal capillary melting point apparatus and were not
corrected. IR spectra were obtained with a Perkin–
Elmer 750B spectrometer, NMR spectra with a
Bruker AMX 300 instrument, utilizing tetramethyl-
silane as internal reference. UV–visible spectra
were recorded on a Hewlett–Packard Kayak XA
diode-array spectrophotometer.
Semiempirical calculations were performed with
the Gaussian 98w package [15]. All structures were
optimized fully with the AM1 hamiltonian, follow-
ing an eigenvector-following routine. In each case,
the first five singlet transition energies were then
calculated by single-point calculations employing
the ZINDO option and configuration interactions
involving monoelectronic transitions from the ten
highest occupied to the ten lowest unoccupied
molecular orbitals.
N,N%-Dimethylbarbituric acid, N,N-dimethyl-
aminobenzaldehyde and 4,4%-bis(N,N-dimethyl-
amino)benzophenone were purchased from
Aldrich. All employed solvents were analytically
pure.
3. Results and discussion
2.1. 1,3-Dimethyl-5-(4-dimethylaminophenyl)-
methylene-2,4,6(1H,3H)-pyrimidinetrione (1)
The umax values of the solvent-dependent absorp-
tion bands of dyes 1 and 2 in various solvents are
listed in Table 1.
A mixture of N,N%-dimethylbarbituric acid (1.56
g, 0.01 mol) and N,N-dimethylaminobenzaldehyde
Inspection of the table reveals some consistent
trends in the absorption spectra of these com-

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M.C. Rezende et al. / Spectrochimica Acta Part A 57 (2001) 1183–1190
1185
Table 1
umax Values of compounds 1 and 2 in various solvents
Solvent
umax Values, nm
Compound 1 Compound 2
n-Hexane
–
446
452
453
–
332
–
474
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Diethyl ether
Ethyl acetate
Toluene
Tetrahydrofuran
Glycerol triacetate
Acetone
342
343
346
345
345
350
354
354
356
354
356
359
500
501
504
510
510
512
518
519
526
526a
526a
526a
–
459
458
462
–
468
–
Acetonitrile
Dichloromethane
1,2-Dichloroethane
Dimethyl sulfoxide
Cyclohexanol
n-Butanol
–
Ethanol
Methanol
462
462
–
–
–
482
489
–
–
359 (522)b
(554)b
564
573
584
586
591
570
1,4-Butanediol
1,2-Ethanediol
1,2,3-Propanetriol
2,2,2-Trifluoroethanol
Water (pH 7)
Water (pH 1)
364
365
372
372
374
376
524
522
520
508
512
–
Fig. 1. UV-visible spectra of dye 1 (c$1.5×10−5 mol dm−3
)
in trifluoroethanol (a), dichloromethane (b) and diethyl ether
(c).
a Broad maximum.
b Overlap of the two bands makes these values less precise.
pounds. Dye 1 shows one major solvent-depen-
dent absorption band in the visible (around 440–
480 nm), with a bathochromic shift of 43 nm,
when going from diethyl ether to water. Dye 2
presents at least two bands, one in the 330–380
nm region, shifting bathochromically with the in-
creasing polarity of the medium. The other, in the
470–590 nm region, shows a more complex be-
havior; it is considerably broadened in protic sol-
vents and, in stronger hydrogen-bond-donor
(HBD) solvents (1,2,3-propanetriol, 1,2-ethane-
diol, trifluoroethanol, water), it splits into two
maxima, both of them being solvent-sensitive.
These maxima show contrasting behavior in
protic solvents. The longest-wavelength band un-
dergoes bathochromic and hyperchromic shifts in
strong HBD solvents, whereas the second band
exhibits the opposite behavior, shifting hypso-
and hypo-chromically with the increasing HBD
strength of the medium. These trends are illus-
trated with the spectra of Figs. 1 and 2.
Fig. 2. UV–visible spectra of dye 2 (c$2.0×10−5 mol
dm−3) in tetrahydrofuran (a), dimethylsulfoxide (b) and water
(c).

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1186
M.C. Rezende et al. / Spectrochimica Acta Part A 57 (2001) 1183–1190
Fig. 3. Variation of the longest-wavelength wavenumber value w¯ of dye 2 in non-protic solvents with the dipolarity-polarizability
value y* of the medium (y* values were taken from [17].
w−w0=c(w¯ −w¯0)
n2−1
The appearance of a second charge-transfer
band in the spectra of 2 in protic media reflects
different dye-solvent interactions in non-protic
and protic solvents. In the former set of solvents,
the solvatochromic shift is governed by the dipo-
larity-polarizability, while in the latter, the hydro-
gen-bond-donor strength of the medium is the
main factor. As a consequence, a good correlation
is obtained, in non-protic solvents, between the
absorption wavenumber values w¯ of dye 2 and the
dipolarity-polarizability values y* of the medium,
measured with Effenberger’s probe (Fig. 3) [16–
18]. This scale correlates well with other dipolar-
ity-polarizability scales [19,20]. In addition, unlike
other y* scales, which require more than one
probe [21,22], it is based on measurements with a
single, highly sensitive dye, the 5-dimethylamino-
5%-nitro-2,2%-bithienyl. As for dyes 1 and 2, its
solvatochromic behavior is caused by charge-
transfer from a dimethylamino donor group, dif-
fering for our dyes in the nature of the acceptor
fragment.
m −1 n2−1
ꢀ n ꢀ
n
r
=A
+B
−
(1)
2n2+1
mr+2 n2+2
that relates the frequency of transition w of the
dye in a given solvent with its refractive index (n)
and its static dielectric constant (mr ). The constant
w0 is the frequency of transition in the vacuum,
obtained, together with A and B, by linear regres-
sion of the data for the nine non-protic solvents,
and c is the velocity of light.
The calculated values of w0, A and B were
25 769, −25 298 and −2493 cm−1, respectively,
with a correlation coefficient of 0.991 for the
regression analysis. Fig. 4 compares the calculated
with the experimental w values for dye 2.
The interpretation of the spectral behavior of
both dyes in non-protic solvents is straightfor-
ward. The observed positive solvatochromism is
an indication of an excited state more polar than
the ground state. The absorptions at relatively
longer wavelength values of dye 2 are due to the
increasing conjugation of a second dimethyl-
aminophenyl donor group in the molecule. For-
mally, both dyes can be regarded as vinylogous
The values w of 2 in nine non-protic solvents
were fitted to the McRae equation [23,24]