Brown Carbon and Color

Yesterday I was tasked with doing a pH study of the absorption of another brown carbon compound called 4-Nitrocatechol. As I took it from low pH to high pH, I noticed it had some stronger color properties than other compounds we have worked with. Check it out in the video below:

4-Nitrocatechol is a brown carbon compound that absorbs light in the visible region, but its absorption (and therefore its color!) changes drastically with pH. Why does this happen? It has a lot to do with the structure of the molecule and why molecules absorb light in the first place. 

Here’s a picture of 4-Nitrocatechol’s molecular structure:

Taken from http://www.chemspider.com

Taken from http://www.chemspider.com

Notice the double bonds in the ring and the positive and negative charges on the oxygen (O) and the nitrogen (N). The negative charge indicates unbonded electrons. Right now the negative charges on the upper Oxygen have nowhere more favorable to go. They are essentially stuck where they are. This is what 4-Nitrocatechol looks like in a low pH (acidic) environment, when Hhydrogen ions are in abundant supply.

But when a solution is basic and the pH is higher, it means that there now a lot of OH–  ions in solution. The hydrogens attached to the bottom oxygens will now want to leave to join an OH: H+ + OH –> H2O. This means that our 4-Nitrocatechol molecule will now look something like this:

Romero, R.; Salgado, P. R.; Soto, C.; Contreras, D.; Melin, V. Frontiers in Chemistry 2018, 6.

Romero, R.; Salgado, P. R.; Soto, C.; Contreras, D.; Melin, V. Frontiers in Chemistry 2018, 6.

One or both of the two oxygens have now lost their hydrogen and become negatively charged. This opens up a lot of opportunity for resonance in the molecule. Resonance is when negative charges can be distributed in different ways across the molecule. It’s common in compounds with negative charges and double bonds.

Resonance is a big factor in visible light absorption. When a molecule absorbs light it can excite electronic transitions. The more conjugated a molecule is (the more double bonds and resonance potential it has), the smaller the energy gap between the ground state and the excited state. Smaller energy gaps mean larger wavelengths, and as wavelengths get larger through the UV range to the visible, the more likely it is that a visible color will be observed. The point is that more resonance means greater absorption in the visible range, and this compound has a lot more resonance in basic conditions. That’s why we observe the color change when we add strong bases like NaOH.

You can see this color change happening in absorption measurements. In my experiments, we use an instrument called a UV-Vis to measure the change in absorption in our samples over time. The UV-Vis works by shining light in the UV and visible wavelength range through the sample into a detector. The detector measures how much light is blocked by the sample at each wavelength. This gives us a measure of absorption at every wavelength from 200-600 nanometers (nm). Here’s a graph showing the changes in absorption of 4-Nitrocatechol at different pHs:

Click on the graph to get a better look

Click on the graph to get a better look!

At low pH, we see some absorption at ~350nm, which is just outside of the visible range. It makes sense that our compound is clear at low pH. At high pH, we see a big absorption peak with a high intensity at ~450 nm. At this wavelength our compound is absorbing blue light, which means it reflects orange light back at us. This is why we see the yellow to orange to red transition as we add a base. As the pH rises, the molecule gets more resonance, and our absorption wavelength changes. This is similar to how pH indicators work!

I love the chemistry of color and it really goes all the way down to the most basic physical chemistry principles. When we start talking about the kinds of electronic and energetic transitions that are possible things get complicated very quickly. I’m still learning about the relationship between structure and color and it’s really fun to take opportunities to learn when I stumble upon demonstrations of these concepts in lab work. 4-Nitrophenol has similar (but less extreme) color trends, but we usually run our experiments at low pH to mimic atmospheric conditions so I don’t often get to play around with cool colors. Thanks for reading and I hope it was fun. Please feel free to leave any thoughts questions comments down below!



  1. This is really cool…love the video!