Merocyanines solvatochromic dyes

In 1951, Brooker suggested for the first time that solvatochromic dyes could be used to obtain measures of solvent polarity. This author constructed the Xr scale on the basis of the solvatochromism of the merocyanine dye (5), the electronic transition of which gives rise to a charge-transfer fi-om flie amine nitrogen to a carboxamide group at the other end of tire molecule. Hence, the excited status is more dipolar than the ground state, and the resulting band is shifted bathochromically as solvent polarity increases. Xr values reflect the position of the maximum of the first band for the chromophore in kcal mol . 

The application of solvatochrome dyes for characterizing the strengths (polarities) of solvents was first proposed by Brooker [Br 51]. His solvent scale is based on the shifts in the absorption bands of the following two merocyanine dyes (I and II) ... 

Merocyanines are widely used as solvatochromic dyes. Fujita et al. demonstrated that the Brooker s dye analogue (Fig. 32, 24) shows a colour change based on protonation and deprotonation in a densely packed SAM (Fig. 32, 24) [196]. Surface plasmon spectroscopy and X-ray photoelectron spectroscopy indicate that the monolayers are closely packed. The layer thickness is consistent with the molecules having a tUt angle of 30° to the surface normal IRRAS spectroscopy shows that the chromophores are located in a polar local dielectric situation even in non-polar solvents. Nonetheless, the deprotonated, zwitterionic form of the dye shows a distinct negative solva-tochromism depending on the solvent polarity. 

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

The merocyanine dye mentioned above shows solvatochromism, which means that the absorption band maximum of the quinoid form (D form) is sensitive to solvent polarity [40,41]. In Fig. 3, the absorption maximum of the solvatochromic band for M-Mc (a low molecular weight merocyanine analog) is plotted against the dielectric constant of 1,4-dioxane/water mixtures [42]. With the relationship... 

S.-R Heum, M.-S. Hur, P. M. Kazmaier, and E. Buncel, Thermo- and photochromic dyes Indolino-benzospiropyrans. Part 1. UV-VIS spectroscopic studies of 1, .3,.3-spiro(2 //-1 -benzopyran-2,2 -indolines) and the open-chain merocyanine forms solvatochromism and medium effects on spiro ring formation, Can. J. Chem., 69, 1940-1947 (1991). 

A particularly interesting solvatochromic merocyanine dye is l-methyl-4-[(4-oxocyclohexadienylidene)ethylidene]-l,4-dihydropyridine also called Brooker s merocyanine [48]. First it exhibits a bathochromic and then a hypsochromic shift of the long-wavelength n n absorption band as the solvent polarity inereases [309] cf. also entry 14 in Table 6-1. 

Combining the idea of solvent-induced changes in molecular structure with the concept of a solvent continuum around the solvatochromic molecule, a micro-structural model of solvatochromism has been developed by Dahne et al., which reproduces, qualitatively correctly and quantitatively satisfactorily, the solvatochromic behavior of simple merocyanine dyes [95b], The results obtained with this model for 5-(dimethylamino)penta-2,4-dienal are in good agreement with the solvent-dependent experimental data such as transition energies, oscillator strengths, r-electron densities, and r-bond energies [95b] cf. also [326, 327],... 

The solvent dependence of the n n transition energies of two meropoly-methine dyes was used by Brooker et al. [77] to establish the solvent polarity parameters /r and Xb ( / Table 7-2). is based on the positively solvatochromic merocyanine dye no. 1 in Table 6-1 of Section 6.2.1 (red shift with increasing solvent polarity), while Xb represents the transition energies of the negatively solvatochromic merocyanine dye no. 13 in Table 6-1. 

In 1994, a review on the further development and improvement of the n scale was given by Laurence, Abboud et al. [227], They redetermined n values for a total of 229 solvents, this time using only two (instead of seven) solvatochromic nitroaromatics as indicator compounds, i.e. 4-nitroanisole and A,A-dimethylamino-4-nitroaniline, for good reasons see later and reference [227] for a more detailed discussion. A thermodynamic analysis of the n scale [and the t(30) scale] has been reported by Matyushov et al. [228]. Using six novel diaza merocyanine dyes of the type R-N=N-R (R = N-methylpyridinium-4-yl or A-methylbenzothiazolium-2-yl, and R = 2,6-disubstituted 4-phenolates or 2-naphtholate) instead of nitroaromatics as positively solvatochromic probe compounds, an analogous n azo scale was developed by Buncel et al., which correlates reasonable well with the n scale, but has some advantages for a detailed discussion, see references [333], Another n scale, based solely on naphthalene, anthracene, and y9-carotene, was constructed by Abe [338], n values are mixed solvent parameters, measuring the solvent dipolarity and polarizability. The differences in the various n scales are caused by the different mixture of dipolarity and polarizability measured by the respective indicator. The n scale of Abe is practically independent of the solvent dipolarity, whereas Kamlet-Taft s n and Buncel s n azo reflect different contributions of both solvent dipolarity and polarizability. 

The structure of the open-chain form was assigned on the basis of its negative solvatochromic behaviour, which is similar to that of other meropolymethines such as the pyridinium A-phenolate betaines [108]. The correlation shown in Fig. 7-4 allows one to calculate absorption maxima of the merocyanine dye in other solvents for which x(30) values are known. 

These results may be compared with those of recent SERS experiments performed by Schneider et al.,54 who postulated that SERS spectra ofnitro-BIPS derivatives in methanolic solutions originated from neutral open merocyanine species. The presence of open forms in solutions of 7, in the absence of any UV light, probably arises from solvatochromism, which is an efficient process in the opening of nitro-substituted spiro compounds in polar solvents.55 Moreover, as discussed above, the detection of photomerocyanines, even at trace levels, is favored with respect to that of the nonresonant closed form of the photochromes by a SERRS effect excited at 5l4.5nm. It should be noted that in the case of cyanine dyes, SERRS spectra, in Ag colloids, from 10 I7M solutions have been recently reported.56... 

A CS INDO scheme incorporating solvent polarity effects according to Klopman s model is applied to examine geometric modification and solvatochromism of merocyanine dyes exhibiting peculiar behaviours. Qualitatively speaking, the model reproduces quite well the most important effects. The advantages of the CS INDO-solvaton approach with respect to other semiempirical procedures including solvation are discussed. 

The proposed calculation procedure will be first tested by analysing in detail the effects of the solvent polarity on the structure and electronic spectra of the simple merocyanine Ml. Afterwards, the selected calculation procedure will be applied to the more complex dyes M2 and M3, characterized by equal length of the conjugated path connecting the donor and acceptor group, but exhibiting opposite solvatochromic effects. To be precise, the acyclic merocyanine M2 shows, like the simpler chromophore Ml, positive solvatochromism [25] (i.e. bathochromic shift of the first absorption band on increasing solvent polarity),... 

Baraldi, L Momicchioli, F. Ponterini. G. Vanossi. D. Solvent effects within the CS INDO method. Geometrical distortion and solvatochromism of merocyanine dyes. Chem. Phys. 1998, 238, 353-364. 

Solvatochromism of merocyanine dyes is accompanied also with the solvent dependence of their dipole moment [Pa 80], So dipole moment measurements have also contributed to the explanation of solvatochrom behaviour of these systems. In such systems the electronic ground and excited states of a molecule are described by two resonance structures non-polar (quinoid) and polar (benzenoid). 

P. Jacques,/. Phys. Chem., 90, 5535 (1986). On the Relative Contributions of Nonspecific and Specific Interactions to the Unusual Solvatochromism of a Typical Merocyanine Dye. 

Lu, L., Lachicotte, R.J., Penner, T.L., Perlstein, J., and Whitten, D.G., Exciton and charge-transfer interactions in nonconjugated merocyanine dye dimers novel solvatochromic behavior for tethered bichromophores and excimers, /. Am. Ghem. Soc., 121, 8146,1999. 

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