Why are pi and pi* (anti-pi) transitions on conjugated double bonds such good absorbers of visible light?
Pi and pi* transitions in conjugated double bonds are excellent absorbers of visible light due to their electronic structure.
To understand why, we need to consider the concept of molecular orbitals and the nature of the bonding in conjugated systems.
In a conjugated molecule, such as a molecule with alternating single and double bonds, the p orbitals of the atoms involved in the double bonds align to form a delocalized pi (π) system. These delocalized pi electrons are able to move freely across the entire conjugated system.
Now, let's talk about the two types of pi transitions involved in conjugated systems:
1. Pi (π) to Pi* (π*) transition: In this transition, an electron in a pi bonding orbital (π) gets excited to a higher energy pi anti-bonding orbital (π*). This transition typically requires the absorption of visible light photons. The energy difference between the π and π* orbitals corresponds to the energy of visible light. The absorbed energy promotes the electron from the lower energy π orbital to the higher energy π* orbital.
2. Pi (π) to Sigma (σ) transition: In this transition, an electron occupying a pi bonding orbital (π) gets excited to a sigma bonding orbital (σ). However, this transition requires higher energy (ultraviolet or higher) photons because the energy difference between the π and σ orbitals is usually larger.
Now, coming back to your question: pi to pi* transitions are good absorbers of visible light because the energy difference between the π and π* orbitals corresponds well to the energy range of visible light. As a result, when visible light strikes a molecule with conjugated double bonds, it can be absorbed by exciting electrons from the lower energy π orbital to the higher energy π* orbital. The absorbed light energy is then often re-emitted as fluorescence or converted into thermal energy, leading to its strong absorption properties in the visible range.
So, the electronic structure of conjugated double bonds, specifically the presence of delocalized pi electrons and the energy difference between the π and π* orbitals, explains why pi to pi* transitions are excellent absorbers of visible light.