Brooker's merocyanine
Chemical compound
From Wikipedia, the free encyclopedia
Brooker's merocyanine (1-methyl-4-[(oxocyclohexadienylidene)ethylidene]-1,4-dihydropyridine, MOED)[1] is an organic dye belonging to the class of merocyanines.
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| Names | |
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| IUPAC name
1-methyl-4-[(oxocyclohexadienylidene)ethylidene]-1,4-dihydropyridine | |
| Identifiers | |
3D model (JSmol) |
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| ChemSpider | |
| ECHA InfoCard | 100.255.640 |
PubChem CID |
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| UNII | |
CompTox Dashboard (EPA) |
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| 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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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.
| 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.
