Brooker's merocyanine

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

Brooker's merocyanine
Names
IUPAC name
1-methyl-4-[(oxocyclohexadienylidene)ethylidene]-1,4-dihydropyridine
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.255.640
Properties
C14H13NO
Molar mass 211.26 g/mol
Appearance Red crystals
Melting point 220 °C (428 °F; 493 K) (decomposes)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references
Brooker's merocyanine in different solutions

MOED is notable for its solvatochromic properties, meaning it changes color depending on the solvent in which it is dissolved.

As shown in the structural formula, MOED can be depicted using two resonance structures: neutral and zwitterionic. Research indicates that the zwitterionic structure is the major contributor to resonance hybrid when the compound exists in polar solvents such as water, and the neutral form when it exists in nonpolar solvents such as chloroform.[2]

Solvatochromic effects

When MOED is dissolved in various liquids, its colour 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, this is referred to as a hypsochromic shift. When light of a certain colour (wavelength) is absorbed, the solution will appear in the complementary colour 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. Similarly, protic and aprotic solvents also affect MOED in solution differently. Solvents that are hydrogen donors (i.e. water, acids), will affect the visible absorption spectra by engaging in hydrogen bonding/ or donating the hydrogen outright, making the molecule favor the zwitterionic resonance form; an example of this may be seen in the picture where acetic acid, though less polar than water, was able to produce a more yellow solution.

Colors of MOED Solutions in Various Solvents[3]
Solvent Color λ(max, nm) Relative solvent polarity[4]
Water Yellow 442 1
Methanol Red-orange 509 0.762
Ethanol Red 510 0.654
2-Propanol Violet 545 0.546
DMSO Blue-violet 572 0.444
Acetone Blue-violet 577 0.355
Pyridine Blue 603 0.302
Chloroform Blue 618[5] 0.259

Uses

Because of its solvatochromic properties MOED, and solvatochromic 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. Further uses of MOED includes the production of certain photosensitive materials. Research into merocyanine dyes is ongoing.[6]

Synthesis

Brooker's merocyanine can be prepared beginning with the methylation of 4-methylpyridine to produce 1,4-dimethylpyridinium iodide. Base catalyzed reaction with 4-hydroxybenzaldehyde and subsequent intramolecular dehydration provides Brooker's merocyanine.

Synthesis of Brooker's merocyanine from 4-methylpyridine, methyl iodide, and 4-hydroxybenzaldehyde. Step 2 is catalyzed by weak base.
MOED crystals after one recrystallisation in water

Notes
  1. Brooker, L.G.S.; Keyes, G.H.; Sprague, R.H.; VanDyke, R.H.; VanLare, E.; VanZandt, G.; White, F.L. (November 1951). "Studies in the cyanine dye series. XI. The Merocyanines". Journal of the American Chemical Society. 74 (11): 5326–5332. doi:10.1021/ja01155a095. link
  2. "Fundamental Studies on Brooker’s Merocyanine", Morley et al., J. Am. Chem. Soc., 1997, 119 (42), 10192-10202 • doi:10.1021/ja971477m
  3. Minch, M.J. (1977). "Merocyanin dye preparation for the introductory organic laboratory". J. Chem. Educ. 54 (11): 709. Bibcode:1977JChEd..54..709M. doi:10.1021/ed054p709 – via ACS Publications.
  4. Reichardt, Christian (2003). Solvents and Solvent Effects in Organic Chemistry. Wiley-VCH Publishers.
  5. Wang, Yuheng (2018). "A Short Spectroscopic Studies on MOED".
  6. Valerii Z. Shirinian and Alexey A. Shimkin: "Merocyanines: Synthesis and Application", in Topics in Heterocyclic Chemistry, Springer, 2008
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References
  • M J Minch and S Sadiq Shah: "Spectroscopic studies of hydrophobic association. Merocyanine dyes in cationic and anionic micelles". Journal of Organic Chemistry, 44:3252, 1979.
  • Amaresh Mishra, et al.: "Cyanines during the 1990s: A Review", Chemical Reviews, 2000, 100 (6), 1973-2012 • doi:10.1021/cr990402t
  • Christian Reichardt: "Solvatochromic Dyes as Solvent Polarity Indicators", Chem. Rev., 1994, 94 (8), 2319-2358 • doi:10.1021/cr00032a005
  • S. J. Davidson2 and W. P. Jencks: "The Effect of Concentrated Salt Solutions on a Merocyanine Dye, a Vinylogous Amide", Journal of the American Chemical Society,1969, 91 (2), 225-234 • doi:10.1021/ja01030a001
  • Brooker, Keyes, et al.: "Studies in the Cyanine Dye Series. XI. The Merocyanines", J. Am. Chem. Soc., 1951, 73 (11), 5326-5332 • doi:10.1021/ja01155a095
  • Brooker, Keyes, et al.: "Color and Constitution. XI.1 Anhydronium Bases of p-Hydroxystyryl Dyes as Solvent Polarity Indicators", J. Am. Chem. Soc., 1951, 73 (11), 5350-5356 • doi:10.1021/ja01155a097
  • Mohamed K. Awad and Shakir T. Abdel-Halim: "Mechanism of Water Attacking on Brooker’s Merocyanine Dye and Its Effect on the Molecular and Electronic Structures: Theoretical Study", Bull. Chem. Soc. Jpn. Vol. 79, No. 6, 838–844 (2006)
  • H.S. Freeman and S.A. McIntosh, "Some Interesting Substituent Effects in Merocyanine Dyes", Educ. in Chem., 27(3) 79(1990).
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