Merocyanine solvatochromism

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). 

Other solvatochromic probes have been proposed. Mukerjee et al. used nitrox-ides for this purpose, finding that their transition energies correlate linearly with Z and t (30). Brooker et al. prepared a polar merocyanine that shows a blue shift... 

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... 

Azomethine ylides such as 412 react with triafulvenes again by analogy with cyclopropenones. (3 + 2) Cycloaddition of the 1,3-dipole to the CVC2 bond and subsequent loss of C02 produces l,4-dihydro-4-methylene-N-alkyl pyridines 559, which as merocyanines show marked solvatochromic and thermochromic effects260. ... 

Unlike the still-unknown 277-pyran which exists exclusively in the ring-opened form, 277-thiopyran is a well-characterized molecule. Nevertheless, the S-C(2) bond can be cleaved and this is the basis of the photochromic properties observed with spirobenzothiopyrans. Irradiation at 365 nm of the spiro[2/7-l-benzothiopyran-2,2 -indoline] 273 in both the solid state and in solution results in opening of the thiopyran ring and the formation of a colored metastable zwitterionic merocyanine (Equation 21). The open form exhibits solvatochromism, with Amax 588 nm in methanol and 673 nm in acetone. In solution, the thiopyran unit reforms rapidly when irradiation ceases, but continuous irradiation leads to the growth of crystals of the open form <2002JOC533>. 

The condensation of 2,3,3-trimethylinolenine with aryl aldehydes gave the corresponding 2-styrylindolenines. Condensation with 5-nitrosalicylaldehyde and its 3-bromo, 3-methoxy, and 3-nitro derivatives similarly gave the 2-(2-hydroxy-styryl)indolenines (46, 73, 68 and 91% yields, respectively), rather than the corresponding merocyanine or spiropyran forms. These indolenines were neither photochromic nor solvatochromic.53... 

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). 

S.-R. Keum and K.-W. Lee, Unusual solvatochromic behavior of the open-chain merocyanine forms of 5-chlorinated l,3,3-spiro(2/7-l-benzo-2,2 -indoline) derivatives, Bull. Korean Chem. Soc., 14, 16-18 (1993). 

Compound 12, incorporating two heterocyclic nuclei, is very polarizable and shows a large solvatochromic behavior.9 A polar solvent shifts the equilibrium toward the opened form as shown in Table 2.7. Nuclear magnetic resonance (NMR) experiments (400 MHz 1H) showed that the open forms of merocyanines are transoid toward the azomethine bridge. The delocalized electronic structure tends to become more quinoidal with decreasing polarity of the medium.9... 

The negative entropy changes observed in all solvents are a result of an ordering of solvent molecules in the environment of the zwitterionic form. Since polar solvents are per se more structured than apolar solvents, proportionally less negative entropy changes are obtained in more polar solvents such as ethanol. The rate of the spiropyran/ merocyanine interconversion is also solvent-dependent, as is the position of the visible absorption band of (27a), which, as is typical for a merocyanine, exhibits a pronounced negative solvatochromism (see Section 6.2) [99c, 99d]. 

That the electronic ground-state structure of a dipolar solute is indeed affected by solvent polarity has been independently shown by NMR [20, 50, 73, 75, 78], NMR [77], and IR measurements [20] of merocyanines. Some of these results observed with the positively solvatochromic 3-(dimethylamino)propenal are presented in Table 6-2. 

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),... 

This description disregards instantaneous effects due to solvent polarization induced by the change in charge density associated with the electronic transition [19]. In principle such effects may be appreciable [24], but we will shortly show (section 3) that the solvatochromic behaviours of merocyanines can be correctly predicted in terms of the only effects related to the dielectric constant of the solvent. 

On the basis of the results discussed in section 3.1, concerning solvation effects on structure and electronic spectrum of chromophore Ml, we decided to carry out a corresponding study for merocyanines M2 and M3 (Fig.5) constituted by TT-conjugated chromophores of the same length , yet characterized by opposite solvatochromic behaviours. 

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