Chasing Rainbows: The Colorful World of Solvatochromism

By Jacob A. Brooks

So What is Solvatochromism Anyways?

Solvatochromism is the change in absorption property of a chromophore in a variety of solvents, with differing polarity of said solvents. Understanding this now may lead you to the following questions: What is a chromophore, and what is absorption?

A chromophore is the part of a given molecule that is responsible for the color that it is perceived as. Therefore, if you are looking at an object that is perceived to be red, the chromophore is the part of its molecules reflecting the red light entering your eye.

Fig 1: Interactions of Silica-Based Composites

Now, what is absorption. Absorption, in terms defined for its use in chemistry, is when light waves hit a molecule and are “absorbed” by the molecule. When this occurs and in the terms of solvatochromism, the light waves hit the chromophore and some of them are absorbed. The chromophore can then enter an excited state. For use in biochemistry, the now excited state can cause conformational changes in the molecule when it is hit by light. Solvatochromism is used then to estimate when these molecules change from their ground state to the excited state previously mentioned.

Fig 1 shows some potential types of interaction of these molecules, that of which I won’t get into in this blog post, but it does show that shifts in absorption and emission spectrum as a possibility of the interactions.

Fig. 2: Red Shifts and Blue Shifts



What is a “Blue Shift” and “Red Shift”

These shifts are just in reference to if the emission spectrum moves to longer wavelength or to a shorter wavelength of where the initial measurement fell. Below in Fig. 2 this is clearly labeled. Therefore, if there is a shift to a shorter wavelength of the spectrum it is a blue shift, and if it shifts to a longer wavelength, it is a red shift.

What Causes Red and Blue Shifts?

Fig. 3: Energy Level of Ground State and Excited State for Shifts

Red shifts and blue shifts are caused by the change in energy level of the molecule, or in others words the change from a ground state to an excited state like I mentioned earlier. This is caused by the type of solvent used to dissolve our chosen compound. The solvent can either have stabilizing effects or destabilizing effects on the molecule dissolved in the solution. Therefore, the solvent can either bring the molecule closer to its ground state, or further away from that ground state. If the gap between the molecules ground state and excited state is smaller, then a red shift will occur. This happens because a lower energy level is required to excite the molecule and occurs at longer wavelengths of light. If the gap between the molecule’s ground state and excited state is larger, then a blue shift will occur. This is because the larger gap requires high energy, or shorter wavelengths of light, to excite the molecule. Below in Fig. 3 is an image depicting the energy levels required to excite the molecule changing to cause a specific type of shift. In the image a hypsochromic shift is another name for a blue shift, and a bathochromic shift is another name for a red shift.

Why is Solvatochromic Studies “Chasing Rainbows”

Fig. 4: Color Change Among Solvents

This is due to the color of the solution changing when using different solvents. For example, this is like if you were to dissolve something in water and your solution being clear compared to dissolving it in ethanol and now the solution is purple. That is just a theoretical example, but you get the idea. This is due to the interaction between your solute (what is being dissolved) and your solvent (what is doing the dissolving). The observed differences in color changes because of the dielectric constant and the hydrogen bonding capacities of the solute vary based off the solvent they are in. Below in Fig. 4 is an example of the differences in color when using one type of solute in a bunch of different solvents. You’ll see that when the solute is dissolved in Chloroform the solution is a greenish color. However, when the solute is dissolved in Ethanol the solution is a purple color.

Solvatochromic species have been used as probes in a variety of environmental settings. This includes, but is not limited to, polymers, micelles, zeolites, inorganic glasses, and a variety of surfaces. This is because solvatochromic species can easily allow for the polarity of these environments to be determined, and quantified. Solvatochromic species are also used to study polarity of macrosystems, and the conformational binding of proteins or other molecules.

Another use for solvatochromic species is to probe the chemical interfaces of molecules, such as microplastics. This is so that we can characterize their behavior in various solvents, but that is a topic for another blog post!