merocyanine-type dyes from barbituric acid derivatives

Spectrochimica Acta Part A 57 (2001) 1183–1190

Merocyanine-type dyes from barbituric acid derivatives

Marcos Caroli Rezende a,*, Paola Campodonico a, Elsa Abuin a,Jean Kossanyi b

a Facultad de Quımica y Biologıa, Uni�ersidad de Santiago de Chile, Casilla 40, Correo 33 Santiago, Chileb 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 �* andET(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

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1. Introduction

The pyrimidinetrione system of barbituric acidmay be envisaged as a potentially useful fragmentin the design of new cyanine dyes. Conjugation ofthe carbonyl groups of this ring system with anitrogen or oxygen atom of another fragment bymeans of a polymethine chain should lead to newdyes with interesting spectroscopic properties.Conspicuous examples of such compounds aresolvatochromic �- and �-vinylogous pyridones [1–5] indoaniline dyes [6–8] and the open form of the

widely studied photochromic spirooxazines [8–13].

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 �-pyrones as electrophiliccomponents, a reaction described nearly 40 yearsago by Eiden [14].

In the present communication, we employed thisreaction to obtain new dyes, which might exhibituseful solvatochromic properties. Following Ei-den’s general procedure, we prepared compounds1 and 2, by condensation of N,N-dimethylbarbi-turic acid with 4-dimethylaminobenzaldehyde and4,4�-bis(dimethylamino)benzophenone (Michler’sketone), respectively, and investigated their spec-troscopic behavior in various solvents (Scheme 1).

* Corresponding author.E-mail address: [email protected] (M.C. Rezende).

1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S1 386 -1425 (00 )00461 -3

M.C. Rezende et al. / Spectrochimica Acta Part A 57 (2001) 1183–11901184

Scheme 1.

(1.49 g, 0.01 mol) in acetic acid (5 ml) and aceticanhydride (5 ml) was gently refluxed for 2 h. Aftercooling, the precipitated product was filtered andrecrystallized 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 and1080 cm−1. NMR (CDCl3) � 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,CH�C).

2.2. 1,3-Dimethyl-5-[4,4 �-bis(dimethylamino-phenyl)methylene]-2,4,6(1H,3H)-pyrimidine-trione(2)

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 mixtureof acetic acid (5 ml) and acetic anhydride (5 ml) for8 h. The solution was rotary evaporated and theresidue redissolved in dichloromethane (70 ml) andwashed several times with a 1 M Na2CO3 solutionand finally with water. The dried ( anhydrousMgSO4) organic phase was evaporated and thesolid 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) � 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 toNMe2), 7.25 (d, 4 H, J=9 Hz, Ar–H meta toNMe2).

3. Results and discussion

The �max values of the solvent-dependent absorp-tion bands of dyes 1 and 2 in various solvents arelisted in Table 1.

Inspection of the table reveals some consistenttrends in the absorption spectra of these com-

The comparison of their solvatochromic behav-ior in 20 different solvents revealed some interestingdifferences between these systems, which are dis-cussed and rationalized in the present communica-tion with the aid of semiempirical calculations.

2. Experimental

Melting points were recorded on an Electrother-mal capillary melting point apparatus and were notcorrected. IR spectra were obtained with a Perkin–Elmer 750B spectrometer, NMR spectra with aBruker AMX 300 instrument, utilizing tetramethyl-silane as internal reference. UV–visible spectrawere recorded on a Hewlett–Packard Kayak XAdiode-array spectrophotometer.

Semiempirical calculations were performed withthe Gaussian 98w package [15]. All structures wereoptimized fully with the AM1 hamiltonian, follow-ing an eigenvector-following routine. In each case,the first five singlet transition energies were thencalculated by single-point calculations employingthe ZINDO option and configuration interactionsinvolving monoelectronic transitions from the tenhighest occupied to the ten lowest unoccupiedmolecular orbitals.

N,N �-Dimethylbarbituric acid, N,N-dimethyl-aminobenzaldehyde and 4,4�-bis(N,N-dimethyl-amino)benzophenone were purchased fromAldrich. All employed solvents were analyticallypure.

2.1. 1,3-Dimethyl-5-(4-dimethylaminophenyl)-methylene-2,4,6(1H,3H)-pyrimidinetrione (1)

A mixture of N,N �-dimethylbarbituric acid (1.56g, 0.01 mol) and N,N-dimethylaminobenzaldehyde

M.C. Rezende et al. / Spectrochimica Acta Part A 57 (2001) 1183–1190 1185

Table 1�max Values of compounds 1 and 2 in various solvents

Solvent �max Values, nm

Compound 2Compound 1

–n-Hexane 332 474 –– –446 –Diethyl ether

342 500Ethyl acetate –452343 501453 –Toluene

–Tetrahydrofuran 346 504 –345 510– –Glycerol triacetate345 510Acetone –459350 512458 –Acetonitrile354 518Dichloromethane –462354 519– –1,2-Dichloroethane356 526Dimethyl sulfoxide –468354 526a– –Cyclohexanol

n-Butanol – 356 526a –359 526a462 –Ethanol359 (522)bMethanol (554)b462364 524– 5641,4-Butanediol365 5221,2-Ethanediol 573–372 520– 5841,2,3-Propanetriol372 5082,2,2-Trifluoroethanol 586482374 512489 591Water (pH 7)376Water (pH 1) –– 570

a Broad maximum.b Overlap of the two bands makes these values less precise.

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).

Fig. 2. UV–visible spectra of dye 2 (c�2.0×10−5 moldm−3) in tetrahydrofuran (a), dimethylsulfoxide (b) and water(c).

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 2presents at least two bands, one in the 330–380nm region, shifting bathochromically with the in-creasing polarity of the medium. The other, in the470–590 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 twomaxima, both of them being solvent-sensitive.These maxima show contrasting behavior inprotic solvents. The longest-wavelength band un-dergoes bathochromic and hyperchromic shifts instrong HBD solvents, whereas the second bandexhibits the opposite behavior, shifting hypso-and hypo-chromically with the increasing HBDstrength of the medium. These trends are illus-trated with the spectra of Figs. 1 and 2.


Fig. 3. Variation of the longest-wavelength wavenumber value � of dye 2 in non-protic solvents with the dipolarity-polarizabilityvalue �* of the medium (�* values were taken from [17].

The appearance of a second charge-transferband in the spectra of 2 in protic media reflectsdifferent dye-solvent interactions in non-proticand 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 themain factor. As a consequence, a good correlationis obtained, in non-protic solvents, between theabsorption wavenumber values � of dye 2 and thedipolarity-polarizability values �* 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, unlikeother �* scales, which require more than oneprobe [21,22], it is based on measurements with asingle, highly sensitive dye, the 5-dimethylamino-5�-nitro-2,2�-bithienyl. As for dyes 1 and 2, itssolvatochromic behavior is caused by charge-transfer from a dimethylamino donor group, dif-fering for our dyes in the nature of the acceptorfragment.

The values � of 2 in nine non-protic solventswere fitted to the McRae equation [23,24]

�−�0=c(�− �0)

=A� n2−1

2n2+1n

+B��r−1

�r+2−

n2−1n2+2

n(1)

that relates the frequency of transition � of thedye in a given solvent with its refractive index (n)and its static dielectric constant (�r ). The constant�0 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 �0, A and B were25 769, −25 298 and −2493 cm−1, respectively,with a correlation coefficient of 0.991 for theregression analysis. Fig. 4 compares the calculatedwith the experimental � values for dye 2.

The interpretation of the spectral behavior ofboth dyes in non-protic solvents is straightfor-ward. The observed positive solvatochromism isan indication of an excited state more polar thanthe ground state. The absorptions at relativelylonger wavelength values of dye 2 are due to theincreasing conjugation of a second dimethyl-aminophenyl donor group in the molecule. For-mally, both dyes can be regarded as vinylogous


Fig. 4. Comparison of the experimental and the calculated wavenumber values of dye 2 in non-protic solvents, obtained from datafitted to Eq. (1).

amides, and the electronic transitions explained interms of structures (I) and (II) ( shown for dye 1),where the contribution of (I) is greater in theground state, whereas (IIa/b) are the major con-tributors to the lowest excited state (Scheme 2).

A different spectral behavior is observed inprotic solvents, dominated by hydrogen-bondinginteractions of the dyes with the medium. Theseinteractions help stabilizing the polar excitedstate, and shift bathochromically the charge-trans-fer band of both dyes in the visible with the

increasing HBD strength of the solvent. However,in contrast with dye 1, this band is broadenedfurther for dye 2 and finally splits into two max-ima in very polar media.

The transition from the behavior in non-proticsolvents, dominated by the dye dipolarity-polariz-ability, to that in protic media, determined mainlyby hydrogen-bonding interactions with the sol-vent, is illustrated in Fig. 5, where spectralchanges are recorded in various acetonitrile–wa-ter mixtures. The predominance of hydrogen-bonding interactions in affecting thesolvatochromism of 2 in the latter media is sug-gested by a reasonable correlation observed be-tween the wavenumber values of itslongest-wavelength maxima and the ET(30) valuesof the solvent (graph not shown). This scale,based on the solvatochromic behavior of a pyri-diniophenoxide dye, is sensitive to the hydrogen-bonding interactions between a protic solvent andthe phenoxide oxygen atom of the probe. A simi-lar situation exists in the present case, with theScheme 2.


Fig. 5. UV–visible spectra of dye 2 (c�1.9×10−5 moldm−3) in pure acetonitrile (a) and in 70% (b), 55% (c), 45%(d) and 35% (e) solutions of acetonitrile–water mixtures.

Scheme 3.

2a, and 2b. These theoretical results may be com-pared with the observed trends in the spectra ofdyes 1 and 2 in protic solvents.

In the gas phase, or in non polar solvents, dye1 exhibits only one charge-transfer band. By con-trast, dye 2 exhibits two bands, in agreement withthe calculations.

In the gas phase, the calculations predict similartransition energies for the longest-wavelengthband of both dyes, with dye 2 absorbing atslightly longer �max value (368 nm) than dye 1(365 nm). This trend is observed experimentally,although the difference in �max values in non polarsolvents is larger (ca. 30 nm). In agreement withcalculations, this difference increases with proto-nation by the solvent, from ca. 50 nm in ethylacetate, it increases to 104 nm in 2,2,2-trifl-uoroethanol. Calculations show that this variationreflects the fact that, compared to dye 1, dye 2 ismore sensitive to the environment, and particu-larly to protic solvents. A bathochromic shift ofthe longest-wavelength band of only 17 nm ispredicted theoretically for dye 1, when structure1a is compared with 1. In the case of dye 2, thesame shift amounts to 132 nm.

Calculations also explain the appearance of athird band in the spectra of 2 in protic solvents.Hydrogen-bonding with the solvent simply shiftsbathochromically the solvatochromic band of dye

three basic oxygen atoms present in the barbituricacceptor moiety of 1 and 2.

In order to investigate the effect of hydrogen-bonding interactions of protic solvents with thedyes, we performed calculations with variouslyprotonated forms of compounds 1 and 2, employ-ing configuration interactions and the semiempiri-cal ZINDO/S program, especially useful forestimating electronic transition energies (Scheme3).

We first optimized all structures with the AM1method. The barbituric ring was found to becoplanar with the dimethylaminophenyl ring indye 1, but not in dye 2, where a dihedral angle of57° was obtained between the donor and acceptorrings. Protonation of structure 1, as in 1a, led toloss of coplanarity, with a resulting dihedral anglebetween donor and acceptor rings of 49°. Thecalculated charges on all heavy atoms indicatedthat the preferential sites of protonation in bothdyes 1 and 2 were the oxygen atoms of thebarbituric moiety, and not the nitrogen atoms ofthe dimethylamino groups. Among the former,the oxygen atom at C-2 was more basic than theones at C-4 and C-6.

Table 2 summarizes the results obtained withthe ZINDO/S calculations for structures 1, 1a, 2,


Table 2Calculated �max values of variously protonated forms of dyes 1 and 2

Transition energya, eV MO’s involvedbStructure Oscillator strength f�max Value, nm a

1 365 3.398 55�56 (0.686) 0.993.249 53�56 (−0.226)382 1.031a

55�56 (0.649)3.3722 78�79 (0.653)368 0.323.831 77�79 (0.589)324 0.162.480 78�79 (−0.261)2a 1.20500

78�80 (0.641)458 2.706 78�79 (0.612) 0.17

78�80 (0.256)78�81 (−0.221)

3.701335 74�79 (0.598) 0.2777�80 (−0.294)

2.348 78�81 (0.681) 1.442b 528

a From the five lowest singlet transitions with f�0.15.b Main contributing monoelectronic transitions, with corresponding coefficients in parentheses.

1 (see structures 1�1a). In the case of dye 2,protonation of the oxygen atoms (structure 2a)leads to a splitting of the longest-wavelength bandinto two absorption bands, both shiftedbathochromically in comparison with the spec-trum of structure 2. These two bands coalesce toa single absorption when dye 2 is fully protonatedat its three oxygen atoms (structure 2b), in agree-ment with the spectrum of this dye found in waterat pH 1.

In conclusion, the solvatochromic behavior ofthe new, readily prepared, barbituric acid deriva-tives 1 and 2 reveals a transition from solute–sol-vent interactions in non-protic solvents,dominated by the dipolarity-polarizability proper-ties of the dye, to hydrogen-bonding interactionswith the solvent in protic media. The latter inter-actions are responsible for the bathochromicshifts of the longest-wavelength absorptions ofboth dyes. Of the two compounds, dye 2 exhibitsa greater sensitivity to this effect. In addition,hydrogen-bonding with the solvent causes a split-ting of the longest-wavelength band in the case ofcompound 2 but not of compound 1. These ef-fects could be rationalized with the aid of semiem-pirical calculations employing configurationinteractions and the ZINDO/S program, appliedto variously protonated forms of both dyes.

Acknowledgements

This work was supported by USACH-DICYTand by Fondecyt, project 1980211. We are alsograteful to Dr Gloria Cardenas for allowing usaccess to the Gaussian 98w program.

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merocyanine-type dyes from barbituric acid derivatives

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