Solvatochromism – One solute, multiple colors, endless possibilities

Introduction:

Most chemicals impart almost the same colour irrespective of the solvent they are dissolved in. However, there are some exceptional compounds that impart ‘solvent dependent colour’ when dissolved fully. Such compounds are said to be Solvatochromic (No prizes for guessing!)

Solvatochromism is the phenomenon observed when the colour due to a solute, differs depending on the solvent used^[1]. Some common solvatochromic dyes are shown below:

In this article, let us explore this super-cool phenomenon of Solvatochromism and one of the popular solvatochromic dyes, Brooker’s Merocyanine (or) 1-methyl-4-[(oxocyclohexadienylidene)ethylidene]-1,4-dihydropyridine (MOED).

Solvatochromism in action

Here is a set of vials containing about 10 mg of MOED in each.

Image source: NileRed Youtube

Let’s see the magic after adding the solvents to their respective vials and dissolving the solute completely…

Image source: NileRed Youtube

The pleasant spectrum of colours produced by this compound in various solvents certainly motivates us to know more about how this phenomenon works. We shall use MOED to understand it better.

Some history…

It was known for quite some time that UV/vis/near-IR absorption spectra of chemical compounds may be influenced by the surrounding medium, and that solvents can bring about a change in the position, intensity, and shape of absorption bands.

This phenomenon was later termed as solvatochromism by Arthur Hantzsch. Some scientists recommended to replace solvatochromism by the term “perichromism” (peri=around, in Greek) in order to stress that spectroscopic probe molecules cannot only measure the polarity of liquid environments, but also that of solids and surfaces. ^[2] Nevertheless, the name “solvatochromism” stuck around… 

So much for the history, let us now get into the fun chemistry.

How does Solvatochromism work?

The solvatochromic effect is studied as the way the spectrum of a solute differs when it is dissolved in solvents of different polarity. With various solvents, there is a different effect on the electronic ground state and excited state of the solute. As a result, the size of the energy gap between them changes as the solvent changes. This is reflected in the spectrum of the solute as differences in the position, intensity, and shape of the spectroscopic bands. When the spectroscopic band occurs in the ‘visible’ part of the spectrum, solvatochromism is observed as a change of colour.

So, we can think of solvatochromism to be caused by differential solvation of the ground and first excited state of the light-absorbing molecule. If, with increasing solvent polarity, the ground-state molecule is better stabilized by solvation than the molecule in the excited state, negative solvatochromism will result (and a corresponding hypsochromic shift), and vice versa. 

This is exactly what we see happening in our example compound MOED. When the light of a certain colour is absorbed, the solution will appear in the complementary colour of the one absorbed. Therefore, a highly polar solvent like water, MOED appears yellow (corresponding to absorbed blue light of wavelengths 435-480 nm), whereas, acetone, a less polar solvent would show us purple or blue (corresponding to absorbed green to the yellow light of wavelengths 560-595 nm). And as MOED is sensitive to small changes in polarity, we see a band of vibrant colours.

But look at the above result… Something is odd, isn’t it? You might be thinking if the above were true, then why is acetic acid, being much less polar than water, also yellow? 

Things are not as binary as they seem. This is where the solvent specific effects like proticity kick in, and change the energy difference between the two states. Solvents that are hydrogen donors, affect the visible absorption spectra by engaging in hydrogen bonding or donating the hydrogen outright, making the molecule favour the zwitterionic resonance form. Hence, we see MOED in acetic acid as yellow, even though acetic acid is less polar than water.

What compounds exhibit solvatochromism?

Although it is difficult to arrive at a generalisation due to specific solute-solvent interactions, thanks to various studies, we can say that dyes with a large change in their permanent dipole moment upon excitation exhibit strong solvatochromism^[2]. These are usually compounds that form the class of polymethine dyes, commonly called merocyanines. 

Do these compounds have real life uses?

Yes, they do. In fact, they have a wide range of applications. Solvatochromic dyes because of their unique property, are promising candidates for use as polarity and pH sensors. They can also be used to indicate the presence of transition metal cations. 

Further, merocyanine dyes have found use in silver halide photography, optical data storage, and photodynamic therapy, and research is ongoing in these areas.

References

1) Marini, A., Munoz-Losa, A., Biancardi, A., and Mennucci, B. What is solvatochromism? The Journal of Physical Chemistry B 114, 51 (2010), 17128-17135. 

2) Reichardt, C. Solvatochromic dyes as solvent polarity indicators. Chemical Reviews 94, 8 (1994), 2319-2358.

3) Shirinian, V., and Shimkin, A. Merocyanines: Synthesis and Application, vol. 14. 05 2008, pp. 75-105.

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