Brooker's merocyanine (1-methyl-4- oxocyclohexadienylidene)ethylidene1,4-dihydropyridine, MOED) is an

organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ Chemistry * Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...

dye Juan de Guillebon, better known by his stage name DyE, is a French musician. He is known for the music video of the single "Fantasy Fantasy is a genre of speculative fiction that involves supernatural or Magic (supernatural), magical ele ...

belonging to the class of

merocyanine Merocyanines are a class of polymethine dyes which are clearly defined by set structural properties. Merocyanines belong to the group of dyes referred to as functional dyes, where their applications are not only determined by their colour, but ...

s. MOED is notable for its

solvatochromic In chemistry, solvatochromism is the phenomenon observed when the colour of a solution is different when the solute is dissolved in different solvents. The solvatochromic effect is the way the spectrum of a substance (the solute) varies when th ...

properties, meaning it changes color depending on the

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

in which it is dissolved. As shown in the

structural formula The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are connected to one another. The chemical bonding within the molecule is al ...

, MOED can be depicted using two

resonance structures In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

: neutral and

zwitterion In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively and negatively charged functional groups. : (1,2- dipolar compounds, such as ylides, are sometimes excluded from ...

ic. Research indicates that the zwitterionic structure is the major contributor to resonance hybrid when the compound exists in

polar Polar may refer to: Geography * Geographical pole, either of the two points on Earth where its axis of rotation intersects its surface ** Polar climate, the climate common in polar regions ** Polar regions of Earth, locations within the polar circ ...

solvents such as water, and the neutral form when it exists in nonpolar solvents such as

chloroform Chloroform, or trichloromethane (often abbreviated as TCM), is an organochloride with the formula and a common solvent. It is a volatile, colorless, sweet-smelling, dense liquid produced on a large scale as a precursor to refrigerants and po ...

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

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

s of the light absorbed will be, this is referred to as a

hypsochromic shift In spectroscopy, hypsochromic shift () is a change of spectral band position in the absorption, reflectance, transmittance, or emission spectrum of a molecule to a shorter wavelength (higher frequency). Because the blue color in the visible spect ...

. 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 Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...

, a less polar solvent. The effect stems in part from the stabilization of the

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state ...

of the merocyanine molecule in polar solvents, which increases the energy gap between the ground state and

excited state In quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...

s, 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.

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 In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

indicators. Further uses of MOED includes the production of certain photosensitive materials. Research into merocyanine dyes is ongoing.Valerii Z. Shirinian and Alexey A. Shimkin: "Merocyanines: Synthesis and Application", in ''Topics in Heterocyclic Chemistry'', Springer, 2008

Synthesis

Brooker's merocyanine can be prepared beginning with the

methylation Methylation, in the chemistry, chemical sciences, is the addition of a methyl group on a substrate (chemistry), substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replac ...

4-methylpyridine 4-Methylpyridine is the organic compound with the formula CH3C5H4N. It is one of the three isomers of methylpyridine. This pungent liquid is a building block for the synthesis of other heterocyclic compounds. Its conjugate acid, the 4-methylpyri ...

to produce 1,4-dimethylpyridinium iodide. Base catalyzed reaction with 4-hydroxybenzaldehyde and subsequent intramolecular dehydration provides Brooker's merocyanine.

Notes

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 • *Christian Reichardt: "Solvatochromic Dyes as Solvent Polarity Indicators", ''Chem. Rev.'', 1994, 94 (8), 2319-2358 • *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 • *Brooker, Keyes, et al.: "Studies in the Cyanine Dye Series. XI. The Merocyanines", ''J. Am. Chem. Soc.'', 1951, 73 (11), 5326-5332 • *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 • *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).

External links

*{{Commons category-inline Dyes Pyridinium compounds