In Ultraviolet–Visible (UV–Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this process, the electrons in a molecule transit from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), moving to a higher energy state. This transition and the types of orbitals involved are central to understanding how molecules interact with UV–Vis light. The energy differences between the HOMO and the LUMO are referred to as the bandgap, which typically ranges from 125 to 650 kJ/mol.

A group of atoms in a molecule responsible for absorbing radiation is called a chromophore. When a chromophore undergoes structural changes, both the energy and intensity of the molecule's absorption change. Molecules with different chemical structures have band gaps and absorb radiation at different wavelengths, producing different absorption spectra.

In molecular orbital theory, the energy levels of orbitals influence the types of electronic transitions that can occur. Typically, the lowest-energy occupied molecular orbitals are the σ orbitals, corresponding to σ bonds. Next in energy are the π orbitals, followed by nonbonding (n) orbitals that contain an unshared pair of electrons. The highest energy levels are found in unoccupied or antibonding orbitals (π* and σ*). Certain restrictions, known as selection rules, must be considered during electron transitions. One important selection rule states that transitions involving a change in an electron's spin quantum number are not allowed. Such transitions are called "forbidden"transitions. A common example is a transition from the n to π* orbitals, which are often seen in molecules containing lone pairs.