There are two common approaches when determining the LUMO (Lowest Unoccupied Molecular Orbital) energy level for a Donor-Acceptor (D-A) system. Let's explore both methods:

1. Absorption Edge Method:

○ In this method, you analyze the absorption spectrum of the D-A system.

○ The LUMO energy corresponds to the energy at the absorption onset (i.e., where the absorption curve starts to rise).

○ This approach provides information about the electronic transitions involving the LUMO.

○ However, it assumes that the absorption onset directly corresponds to the LUMO energy, which may not always be accurate due to vibrational effects and other factors.

2. Cyclic Voltammetry (CV) Method on the Reduction Side:

○ CV is an electrochemical technique that measures a molecule's oxidation and reduction potentials.

○ For the LUMO energy determination, focus on the reduction potential (E_red).

○ The LUMO energy level is related to the energy difference between the reduced form (LUMO) and the neutral form (HOMO).

○ By measuring E_red, you can estimate the LUMO energy.

○ This method considers the redox behavior of the D-A system, which can provide more accurate results.

In summary, both methods have their merits. The absorption edge method is straightforward but may have limitations, while the CV method accounts for redox behavior. Researchers often use a combination of experimental techniques to obtain a more comprehensive understanding of the LUMO energy in D-A systems.

Source(s)

4.13C: Hückel MO Theory - Chemistry LibreTexts

30.1: Molecular Orbitals of Conjugated Pi Systems

HOMO and LUMO Conjugated System - Chemistry Stack Exchange