UV-Visible Spectroscopy of Conjugated Systems
Substances appear to be of a particular color because its molecular constituents are constantly emitting the light with a frequency corresponding to that color. Dyes have very strong colors and have been used as coloring agents for clothes, food etc. Extensively conjugated organic molecules have been used as dyes from ubiquity and have their characteristic color corresponding the the transition between electronic levels. Typically, the electronic transitions require very high energy and hence fall in far UV region. However, in conjugated organic molecules, due to the extensive delocalization of the pi-electrons in the molecule, the energies (and frequencies) of these transition are lowered to near-UV and for some molecules to visible region. Such organic molecules (pigments) have color associated with this electronic transition. Shown below are some organic pigments and their structure.
Given a chemical structure, using Electronic Structure Theory, the energy levels and hence the energies of transition between them and hence the color of the molecule can be calculated. However, such a Quantum Mechanical calculation is computationally expensive. On the other hand, the aim is to typically extract some structural information from the spectra of a molecule of unknown structure. So a full Quantum Mechanical treatment is often not possible or at best uninformative. The typical strategy used is to keep in mind some structural feature and use the data from the spectra as input into an appropriate ''model'' for which the energy levels and hence the spectra can be approximated to the experimental spectra, and obtain all other extractable structural parameters.
For conjugated organic molecules, particle in a box model, which is one of the elementary model in quantum mechanics particle in a box model is very suitable because: (a) the conjugated pi-electrons are completely delocalised over the whole conjugated chain (b) there is a clear separation of the energy levels corresponding to the conjugated pi-system and rest of the molecule (ex: sigma bonds) (c) since the transitions corresponding to the conjugated pi-cloud are expected to be of the lowest energy transitions possible in these molecule.