The organic molecules shown here are derivatives of benzene in which six-membered rings are 'fused' at the edges of the hexagons.
(e) Benzene, naphthalene, and anthracene are colorless, but tetracene is orange. What does this imply about the relative HOMO–LUMO energy gaps in these molecules? See the 'Chemistry Put to Work' box on orbitals and energy.

Key Concepts

HOMO-LUMO Gap

The HOMO-LUMO gap refers to the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in a molecule. This gap is crucial in determining a molecule's electronic properties, including its color. A smaller gap typically allows for electronic transitions that can absorb visible light, leading to color, while a larger gap results in colorless compounds.

Aromatic Compounds

Aromatic compounds, such as benzene and its derivatives, are characterized by their stable ring structures and delocalized π-electrons. This delocalization contributes to their unique electronic properties, including stability and specific absorption characteristics. The presence of fused rings, as seen in naphthalene and phenanthrene, can influence the HOMO-LUMO gap and, consequently, the color of the compound.

Color and Electronic Transitions

The color of a compound is often a result of electronic transitions between energy levels when light is absorbed. If a molecule has a small HOMO-LUMO gap, it can absorb visible light, leading to the appearance of color. In contrast, compounds with larger gaps, like benzene and naphthalene, do not absorb visible light and thus appear colorless, while tetracene's orange color suggests a smaller gap allowing for such transitions.