Structure of Merocyanines

Crystals of all MCs investigated (MC1-MC6) have been isolated from ethanol by recrystallization under UV radiation by Gal bershtam,1 Kholmanskii, and the author. 

the study of the MC crystals shows that the structure of open forms of SPs can be described by resonance forms A (which makes the greatest contribu- 

In MCI andMC2, /j-N02 substituents in the phenolate ring displace the equilibrium toward the quinoid form B and hence stabilize the unstable zwitterionic form. It was suggested31 that introduction of an additional N02 group into the position ortho to the 0(1) atom of this moiety should increase the stability of the open MC form. X-ray diffraction analysis of such a merocyanine, MC3, showed that this compound crystallizes in two polymorphic forms—a monoclinic form and rhombic form. 

The trans-transoid isomer, which constitutes 50% of the crystal, is solvated by a water molecule [0(1 )- - Oho distance = 2.44 A], which also occupies 50% of independent positions. The hydrogen bond formed in the solvate is a short one. 

Crystals of the rhombic form of MC4 also contain equal amounts of two isomers—the trans-cisoid and the trans-transoid isomer. The pattern of distribution of the bond lengths in MC4 is in line with that of MC3 within the limit of experimental error. 

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A monolayer of the pyridine-substituted alkyl merocyanine (12) was prepared in the 1970s (67), and a noncentrosymmetric multilayer structure of merocyanine amphiphiles was later prepared (68) using derivatives, but introducing long-chain amines as the counter layer in an ABABAB system (69,70). 

Fig. I Chemical structure of merocyanine (MC) dye used in this work.

Fig. 2 Examples of the structures of a few fast-response electric filed sensitive dyes N-(4-sulpho-butyl)-4-(4-(4-(dipentylamino)phenyl)butadienyl)pyridinium inner salt (RH421, a styrylpyridinium dye), ANNINE 5 (an annellated hemicyanine dye), merocyanine 540, and N-[(4 -dimethylamino)-3-hydroxy-6-flavonyl mcthyl-N,N-trimcthyl ammonium (F4N1, a 3-hydroxychromone dye)...

Figure 3. General structure of Hiinig s violene-cyanine hybrid. One or two of the moieties X=C-Y represent cyanines, oxonols, or merocyanines.

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

An X-ray structure of the photochromic spirobenzothiopyran 46 shows an elongated bond from sulfur to the spiro carbon atom while the bond from S to the aromatic ring is shortened. Irradiation at 365 nm cleaves the former bond and generates the zwitterionic blue-green merocyanine with an s-trans, s-trans conformation in the solid state (Equation 3). In solution, NMR studies indicate an s-trans, s-cis conformation <2002JOC533>. 

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... 

Figure 8 Molecular structures of some interesting materials (a) copper phthal-ocyanine (CuPc) (b) perylene tetracarboxylic derivative (PV) (c) hydroxysquarylium (d) merocyanine dye.

However, it is a truism in materials science that the pathway from vision to reality or from an idea to a marketable product is hardly ever as straightforward as it may seem. The field of organic materials for non-linear optics is no exception. Many problems are encountered when it comes to the translation of a molecular property into a bulk property. It has transpired that some of these problems are not easy to solve with classical NLO-phores that mostly belong chemically to the class of merocyanine dyes. New design strategies for organic molecules and their respective bulk structures, crystals or oriented composite materials like polymers, are needed. Only a more fundamental understanding of these issues will allow rational optimization of molecular and bulk properties. 

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. 

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. 

The bicyclic compounds obtained from unsymmetrical pyrimidine-thiones (104,107, and 110) can be represented by either structures 105 or 106 (50BRP634951), 108 or 109 (83M13), and 111 or 112 (75FRP22337I, 75GEP2317109 76JAP74126828), respectively (Scheme 24). No characterization of the cyclized products was attempted although cyclized product obtained from 104 was used in the preparation of merocyanine dyes (50BRP634952). 

The Structure of Permanent Merocyanines—Models of the Open Forms of Indolinospiropyrans... 

E. Miler-Strenger and R. Guglielmetti. Structure of 6- 2-(3-ethyl-2,3-dihydro-l,3-benzothiazol-2-ylidene)-ethylidene -2-methoxy-4-nitro-2,4-cyclohexadienone A model compound for a photo-merocyanine, Acta Crystallogr., Sect. B 38, 1980-1984 (1982). 

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