HOMO and LUMO

HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. The difference of the energies of the HOMO and LUMO, termed the band gap can sometimes serve as a measure of the excitability of the molecule: the smaller the energy, the more easily it will be excited.

Those who are already familiar with inorganic semiconductors see that the HOMO level is to organic semiconductors what the valence band is to inorganic semiconductors. The same analogy exists between the LUMO level and the conduction band. The energy difference between the HOMO and LUMO level is regarded as band gap energy.

When the molecule forms a dimer or an aggregate, the proximity of the orbitals of the different molecules induce a splitting of the HOMO and LUMO energy levels. This splitting produces vibrational sublevels which each have their own energy, slightly different from one another. There are as many vibrational sublevels as there are molecules that interact together. When there are enough molecules influencing each other (e.g. in an aggregate), there are so many sublevels that we no longer perceive their discrete nature: they form a continuum. We no longer consider energy levels, but energy bands.

See also

• Diels-Alder reaction
• Electron configuration
• Koopmans' theorem
• Ligand
• Molecular orbital
• Organic semiconductor
• Polymer

Bibliography

• Martin Pope et Charles E. Swenberg, Electronic Processes in Organic Crystals and Polymers, 2nd ed., Oxford Science Publications, Oxford University Press, New York, 1999