Brooker's merocyanine

Brooker's merocyanine
Identifiers
CAS number 23302-83-2
PubChem 258436
ChemSpider 226778
Jmol-3D images Image 1
Properties
Molecular formula C14H13NO
Molar mass 211.26 g/mol
Appearance Red crystals
Melting point

220 °C (decomposes)

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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Brooker's merocyanine (1-methyl-4-[(oxocyclohexadienylidene)ethylidene]-1,4-dihydropyridine, 'MOED')[1] is an organic dye belonging to the class of merocyanines.

MOED is notable for its solvatochromatic properties, meaning it changes color depending on the solvent it is dissolved in. Because of this interesting effect and the fact that it is relatively simple to prepare, its synthesis is suitable as an exercise in an undergraduate organic chemistry laboratory course.[2]

As shown in the structural formula, MOED can exist in two resonance forms: A neutral molecule and a zwitterion. Research indicates that the zwitterion form is most representative when the compound exists in polar solvents such as water, and the neutral form when it exists in nonpolar solvents such as chloroform. [3]

Contents

Solvatochromatic effects

When MOED is dissolved in various liquids, its color will vary, depending on the solvent and its polarity. In general, the more polar the solvent, the shorter the wavelengths of the light absorbed will be. When light of a certain color (wavelength) is absorbed, the solution will appear in the complementary color of the one absorbed. Therefore, in water, a highly polar solvent, MOED appears yellow (corresponding to absorbed blue light of wavelengths 435-480 nm), but is purple or blue (corresponding to absorbed green to yellow light of wavelengths 560-595 nm) in acetone, a less polar solvent. The effect stems in part from the stabilization of the ground state of the merocyanine molecule in polar solvents, which increases the energy gap between the ground state and excited states, which corresponds to shorter wavelengths (increased energy) of the absorbed light.

Uses

Because of its solvatochromatic properties MOED, and solvatochromatic dyes in general, are useful as solvent polarity indicators, and for creating solutions that absorb light at a specific frequency. Additional potential areas of use include pH sensors and transition metal cation indicators. Research into merocyanine dyes is ongoing.[4]

Synthesis

Brooker's merocyanine can be prepared beginning with the methylation of 4-methylpyridine to produce 1,4-dimethylpyridium iodide. Reaction with 4-hydroxybenzaldehyde and subsequent treatment with aqueous base provides Brooker's merocyanine.

Notes

  1. ^ L.G.S. Brooker, G.H. Keyes, R.H. Sprague, R.H. VanDyke, E. VanLare, G. VanZandt, and F.L. White: "Studies in the Cyanine Dye Series. XI. The Merocyanines", Journal of the American Chemical Society, Volume 74. Number 11., November 1951, p. 5350 link
  2. ^ M. J. Minch and S. Sadia Shah: "A Merocyanin Dye Preparation for the Introductory Organic Laboratory", Journal of Chemical Education, Volume 54. Number 11, November 1977, p. 709
  3. ^ "Fundamental Studies on Brooker’s Merocyanine", Morley et al., J. Am. Chem. Soc., 1997, 119 (42), 10192-10202 • DOI: 10.1021/ja971477m
  4. ^ Valerii Z. Shirinian and Alexey A. Shimkin: "Merocyanines: Synthesis and Application", in Topics in Heterocyclic Chemistry, Springer, 2008

References