Brooker’s Merocyanine and Solvatochromism: How a chemical changes colour depending on its solvent

Picture Credit: NileRed (CC BY-SA 4.0)

Dissolved in each member of this assortment of vials seen to the right is Brooker’s Merocyanine (or MOED). This prismatic array of solutions arises from very different interactions seen in each vessel between solvent (the liquid) and MOED (the solute). The fascinating property that allows the colour changes is explored in NileRed’s  magnificent YouTube  video (called “Making Brooker’s Merocyanine”). Brooker’s Merocyanine exhibits solvatochromism (the ability for a chemical to change colour depending on what it is dissolved in).

MOED exists simultaneously as somewhere between two resonance forms.  The positively and negatively charged ‘zwitterion’ (a chemical with both positively and negatively charged groups) is very polar and a Hydrogen bond (H-bond) acceptor; it appears yellow in solution, such as in AcOH, a weak acid which is more commonly known as vinegar (good for chips).  Whereas the resonance form is neutral and will interact less strongly with polar solvents whilst also being a poorer hydrogen bond acceptor; it appears blue in solution, such as in DCM.

We see that MOED dissolved in Acetone and DMSO solutions both appear as shades of violet; both solvents are very polar liquids and will strongly interact with the zwitterion form to solvate, thus stabilising it. Hence, the zwitterion’s representation in the resonance structure is increased, which is then seen in the solutions as their colours being less blue.

Next, when MOED is mixed with i-PrOH, EtOH and MeOH solvents, the solutions become increasingly less blue and more yellow; going from left to right. These three liquids are alcohols and as a university student you might well be familiar with EtOH, ethanol. Alcohols are able to H-bond to the zwitterion’s negatively charged (anionic) Oxygen via their -OH group. This interaction will further stabilise the zwitterion beyond what the polar solvents seen right of i-PrOH are able to do, and thus increase the zwitterion’s representation in the resonance structure.  

So, we see lighter solutions on the left. Moreover, the further left one looks, the more yellow the solutions become. This is due to the liquids becoming more acidic on the left too which means the ability of the solvent to H-bond increases. We then reach H2O, water, which is somewhat similar to an alcohol (though less hangover inducing), and more acidic still. Hence, we see an even yellower solution.

Finally AcOH. The acidity of acetic acid means the solvent will fully donate its acidic Hydrogen to the anionic Oxygen of MOED.  This traps the MOED as a new form which has very similar properties to the zwitterion as described before. There is no neutral form of MOED present at all here and so no blue colour is seen, therefore, the AcOH solution is entirely yellow.

We have now finished exploring the exciting world of solvatochromism in Brooker’s Merocyanine, and how solvent liquids interact with their dissolved solutes. I find this example particularly refreshing because in Chemistry, a lot of practical labs end up being just writing down what colours you see when you mix some chemicals together in a test tube and then rationalising colour changes by a chemical reaction… To be able to explain this phenomenon just by considering the major form of MOED in solution is brilliant to me.

If you want to find out more, check out NileRed’s YouTube video on the synthesis of MOED for the amateur Chemist and further discussion of solvatochromism.