J.C.S. Faraday I, 1980, 76, 43-38Effect of Solvents on Dipole Moment of MerocyanineBY ZBIGNIEW PAWELKA AND LUCJAN SOBCZYK”Institute of Chemistry, University of Wroc€aw, 50-383 Wroclaw, PolandReceived 11 th December, 1978The energies of long-wave electron transitions and dipole moments of 3-ethyl-2-thio-5(3’-methyl-thiazolidyl-2’-ethylidene) oxasazolidine-2,4-diene(I) were determined in a number of solvents ofdifferent activity. Variations in both quantities correlated with one another and may be describedby the relative contribution of the resonance structures.In the generally accepted theory explaining strong solvatochromism of mero-cyanine dyes it is assumed that the electronic ground and excited states of a moleculeare described by two resonance structures : non-polar (quinoid) and polar(benzenoid)where D and A are the electron donor (basic) and electron acceptor (acidic) parts ofthe dye molecule, respectively.An increase in solvent activity (this term covers its ability to interact both non-specifically and specifically with the dissolved molecule) leads to a continuous shiftin the electron density distribution, making the dye structure closer to its polarresonance form.Indirect evidence of the shift induced by the environment througha conjugated n-electron system from the donor to acceptor end of the dye moleculeis the existence not only of strong solvatochromism (both positive and negative) butalso of a strong shift in the stretching vibration band vco of the carbonyl group in theelectron acceptor part of the molecule l o p and of the n.m.r.signal of the methineprotons.7* l2 Also, for weakly polar merocyanines in non-active solvents there is anapproximately lineax relationship between Amax and the root of the number of con-jugated double bonds (which is characteristic of the polyene structure), whereas inactive solvents there is a linear relationship between Amax and n (a structure similarto the polymethine system).** l3 Direct evidence of such changes in the electronstructure should be an increase in the dipole moment of the molecule. Quantum-mechanical calculations of solvation changes in the n-electron density distributionperformed recently for model dyes indicate that such an effect should be large enoughto be However, the above results have not been experimentally verifieduntil now, probably because of the low solubilities of most dyes.We have been studying the solvent effect on the polarity of rnerocyanine withthe structural formula (I)0 0D- $.CH=CH -f. ,CH=A D If: CH-CH+ .CH-AHZC-S0 NZC C=CH-CH=C-I I- \ /NICIC444 DIPOLE MOMENT OF MEROCYANINEThis dye, being sufficiently soluble in aromatic hydrocarbons, oxygen solvents andCHC13 for dielectric measurements, exhibits a strong positive solvatochromism. Anincrease in environment activity should then lead not only to changes in the groundstate energy but also to a measurable distortion in its electron density distribution.EXPERIMENTALThe absorption spectra of the dye in solvents under investigation were measured on aSpecord Carl Zeiss-Jena spectrophotometer.The reading error vmax did not exceed250 crn-l. Permittivities were determined by the superheterodyne beat method at 200 MHzin apparatus described earlier.14 The measuring circuit enabled us to measure the capaci-tance with relative error AC/C < 1 x Densities were measured by the pycnometricmethod and were accurate to 40.1 kg m-3 and the refractive index for the sodium D linewith an AbM refractometer was accurate to +O.OOO 02. All measurements were made at298.16+0.05 K. Within the concentration range 1 x < x2 < 1 x linear depen-dences of E, d and n2 against molar fraction of the dissolved dye were obtained. The slopesof the straight lines E = el(l+ax2), d = dl(l+px2) and n2 = nf(l+Yx2) and their root-mean-square errors were determined numerically by the method of least squares.Themolar orientation polarization at infinite dilution (P2,) was determined from the valuesof a, p and y, assuming two different models of the local electric field. In the case of non-polar solvents, Hedestrand’s formula l5 based upon the Lorentz field model was applied :where subscripts 1 and 2 denote the solvent and dye, respectively, and M is the molecularweight.For polar solvents the Onsager field model l6 is required for calculations. Accordingto this model, the orientation polarization is expressed by the formula :( E - n2)(2& + n2)E(n2 +2)2 . P =Assuming that the polarizations are additive, the expression for the limiting value of theorientation polarization for a solute (P$%) was obtained by differentiating eqn (2) withrespect to x2 :Neglecting the dependence of solvent polarization against concentration, the final equationhas the form :In the light of the studies by Cumper and Langley l7 this assumption seems to be justifiedonly in the case when the Onsager procedure is applied.The solvents applied were purifiedand dried by standard methods.18 The measurements in chloroform and in trichloro-ethylene were performed with freshly purified solvents.RESULTS AND DISCUSSIONThe energy values of the lowest energy electron transition (ET(,)) determined forthe dye in a number of solvents with various properties are summarized in table 1and fig. 1. The dye is characterized by a strong positive solvatochromism whosemagnitude is close to that shown by a number of other merocyanine dyes.19-23 Thedye transfer from n-heptane to rn-cresol is accompanied by the highest variation iZ .PAWELKA AND L. SOBCZYK 45the n + n* transition energy of 29.4 kJ mol-l in the series of solvents studied. Fig. 1shows a correlation of the ET(I) values in various solvents with the correspondinglong-wave n -+ n* transition energies (denoted by Brooker et aL21 as xR) in weaklypolar merocyanine characterized by an unusually high positive solvatochromism.According to Brooker et aL21 the xR parameter is a sensitive indicator of most solventeffects which are essential for these dyes. The relationship between ETCI) and xRis described by the linear equation ET(I) = 158.31 3-0.532~~ with a high correlationcoefficient of 0.975 (without the point for m-cresol). Acceptable correlations are alsosatisfied (except for cresols) by the transition energies of a number of other com-pounds where a charge displacement takes place due to a conjugation.19-23 Com-parison of the band shift in aromatic solvents with the shifts in aliphatic solvents ofsimilar permittivities and dipole moments shows that the former have much higheractivities. The reasons for such behaviour should be attributed to the high polar-izability of their n-electron systems and hence to the considerable energy of polar-ization interaction.Highest energy shifts occur in solvents with proton donornature, such as phenol (244.1 W mol-l, 323 K), rn-cresol (242.1 kJ mol-l), aniline(248.1 W mol-l), acetic acid (253.9 kJ mol-l) and trifluoroacetic acid (249.5 kJmol-l).For this group of solvents the specific interaction through the hydrogenbond to the active centre of the acidic dye part [for (I) it is the carbonyl group] isxR/kJ moklFIG. 1 ,-Correlation between transition energies (in kJ mol-l)ET(I) and for various solvents.1, n-Heptane (271.5) ; 2, cyclohexane (270.2) ; 3, triethylamine (268.6) ; 4, carbon tetrachloride(266.9) ; 5, n-butyl ether (266.1) ; 6,p-dioxan ; 7, ethyl ether (267.9) ; 8,p-xylene (263.6) ; 9, n-butylacetate ; 10, mesitylene (265.8) ; 11, o-xylene (262.1) ; 12, toluene ; 13, benzene ; 14, tetrahydrofuran(262.1) ; 15, acetonitrile (260.6) ; 16, acetone (260.9) ; 17, chlorobenzene (259.4) ; 18, dichloro-methane (257.1) ; 19, 2,6-lutidine (259.1) ; 20, bromobenzene ; 21, n-butyl alcohol (258.1) ; 22,cyclohexanone (256.1) ; 23, chloroform ; 24, nitromethane (258.3) ; 25, ethanol (255.6) ; 26, pyridine(253.5) ; 27, benzonitrile (255.0) ; 28, methanol (255.3) ; 29, nitrobenzene (254.1) ; 30, aniline(248.1) ; 31, m-cresol(242.1)46 DIPOLE MOMENT OF MEROCYANINEpredominant.The energy of that interaction depends strongly on the electron donor(basic) properties of oxygen exhibited by this part of the molecule. Since theseproperties depend on the electron structure and thus vary from one dye to another,the shifts for cresols are excluded from a general correlation. For the Brooker dyethe deviation for rn-cresol (xR = 140.6 kJ mol-l) is 9.2 kJ mol-l.The magnitudeof the effect related to the formation of the hydrogen bond m y be evaluated in agiven case by comparing the effect of proton donor solvents and their methyl deriva-tives (under conditions where their macroscopic properties are similar). Energydifferences between the electron transitions in phenol and anisol = 258.5 kJmol-l) as well as aniline and N,N-dimethylanilnie = 256.8 kJ mol-l) are 14.4and 8.7 kJ mol-l, respectively.The charge distribution in the n-electron system of merocyanine depends, aboveall, on differences in the electron donor-acceptor properties of the conjugated mole-cular ends. This obvious statement is supported by the results of measurements ofthe dipole moments in a number of merocyanine dyes performed by Kushner andSmyth 24 and Syrkin and The latter authors found that a change in thelong-wave band position in a group of structurally similar dyes is correlated withchanges in the dipole moment.The effect of the environment on the dipole moment of merocyanine has beenuntil now proven only by semi-empirical quantum-mechanical calculations by theSCF m.0.TABLE 1 .-TRANSITION ENERGIES AND DIPOLE MOMENTS OF MEROCYANINE(I) IN VARIOUSSOLVENTSbenzene 262.1 28.1 1 f0.78 1.318&0.071 1.292k0.022 99.4k1.9 900.1 127.6 22.14k0.33toluene 262.6 23.86k0.42 1.421 f0.143 1.259h0.095 97.4k8.2 926.1 k23.6 22.44&0.27bromobenzene 258.6 16.06k0.32 -0.255+0.040 1.565k0.082 1 1 1.0k4.6 976.6S52.2 23.04-fO.60trichloroethylene 261.1 22.16&0.57 0.413+0.035 1.470&0.110 91.5k4.6 872.9rt49.4 21.81 h O .6 0dioxan 263.1 24.8110.76 0.518&0.040 1.128h0.034 91.6k2.0 793.8131.0 20.7810.40butyl acetate 264.3 8.23&0.21 0.799&0.091 1.101 4Z0.117 103.018.7 758.8-164.9 20.31 zt0.87(896.21 30.5)* (22.08 S0.37)*(902.4k 13.2)* (22.14&0.17)*chloroform 256.4 23.75zk0.71 -0.100&0.028 1.965h0.188 110.1 rt6.3 1171.8k93.1 25.25rt1.00(766.2&27.2)* (20.41 &0.37)** Calculated according to eqn (1).Our experimental results presented in table 1 indicate that a change in the dipolemoment really takes place and exceeds the experimental error. Note here somepurely methodological problems of dipole moment determination in merocyanines.First, it is striking to find good agreement between the dipole moments in non-polarsolvents determined by the Hedestrand and Onsager formulae.CHCl, and bromo-benzene exhibit a considerable increase in the molar refraction determined from therefractive index for the sodium D line and calculated by extrapolation of the Hede-strand formula. This increase results mainly from dispersion of the refractive indexalthough D&ne and Nolte 26 suggest that the n-polarizability of merocyanines showsa solvent dependence. It should be emphasized that dispersive variations of themolar refraction are comprised within the experimental error of overall polarizationand have no significant influence on the solvent dependence of the dipole momentfound above. In spite of the fact that theoretical calculations concern the n-electrondensity distribution and our values are the overall dipole moments, taking into accounZ.PAWEEKA AND L . SOBCZYK 47that solvent variations comprise mainly the n-electron system it is possible to findqualitative agreement between the results achieved by two different methods. Aquantitative compaxison of our results with those estimated theoretically is difficultmainly because the group of solvents we used in measuring the dipole momentscorresponds to a fairly nurow interval in the activity changes. Therefore, the solventinduced increase found in the dipole moment is not too high. It appeared impossibleto extend the solvents applied to the least active aliphatic hydrocarbons because of23-0I5.s 22-21 -20 -191 . 256 258 260 262 264&(I)/kJ mol-lFIG. 2.-Correlation between dipole moments and transition energies for merocyanine(1) in varioussolvents.limited solubilities of merocyanines. However, from a qualitative point of view,the theoretical predictions find a satisfactory confirmation in our results. Theactivity parameter ET(I) of the solvent may be roughly related to the hamiltoniandescribing the interaction between polarizable dye molecule and surrounding layerof solvent molec~les.~ The interaction term has the form H’ = A4LM, where c)LMis the potential created by the oriented solvent molecules and ;1 varies from zeroto unity depending on the activity of the solvent. In not too broad limits of 2 andETCI) values, a nearly linear relationship between the dipole moment and the ET(I)value should be expected. Within the solvent under investigation the relationshipbetween p and ETcI) presented in fig.2 is in fact linear. If this relationship were satisfiedover the entire activity range of solvents then the dye molecule transfer from n-heptaneto the most active rn-cresol would correspond to an increase in the dipole momentof x 16 x C m. This value is close to that obtained theoretically for penta-methinemerocyanine which is similar to (I) with respect to its electron structure.Dielectric studies confirm an insignificant effect of the dielectric permittivity, foundearlier by the spectroscopic method, as an electron structure disturbing factor. Thelargest difference in the dipole moments appears between n-butyl acetate and chloro-form (Ap = 5 x C m), Le., in media of similar permittivities.In the latter48 DIPOLE MOMENT OF MEROCYANINEwhich exhibits proton donor properties, specific interactions through H-bond forrna-tion ‘C=Od-. . . H-C- / might occur inducing an additional charge displacement./ \It seems that the solvent effect on the transition energy and polarity of merocyanineis similar in magnitude and direction to that found for hydrogen bonded complexesbetween p-nitrophenol and triethylamine investigated previ~usly.~’ The similarbehaviour results from a similar electron density distribution in both systems and theso lut e-solven t interact ion mechanism .The work was performed under a project supported by the Polish Academy ofSciences.Th.Forster, 2. Elektrochem., 1939, 45, 548.L. G. S. Brooker and R. H. Spraque, J. Amer. Chem. SOC., 1941, 63,3214.L. G. S. Brooker, G. H. Keyes, R. H. Spraque, R. H. Van Dyke, E. Van Lare, G. Van Zandt,F. L. White, H. W. J. Cressman and S. G. Dent, J. Amer. Chem. SOC., 1951, 73, 5332.A. I. Kiprianov and E. S. Timoshenko, Zhur. obshchei Khiin., 1947, 17, 1468.J. R. Platt, J. Chem. Phys., 1956,25, 80.E. G. McRae, Spectrochim. Acta, 1958, 12, 192.H. G. Benson and J. N. 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