UV-Vis Spectroscopy of Dyes

When light reaches a substance, a portion is absorbed by it, while the rest is either reflected or transmitted through it. The color of the substance, as we perceive it, depends on which wavelengths it is most likely to reflect. For example, a piece of fabric that we see as blue contains a dye that strongly reflects blue light and strongly absorbs orange and red light.

Dyes are typically conjugated compounds, meaning that they have alternating double and single bonds. Electrons can move freely within the conjugated system. Differently colored dyes must vary in the wavelengths of light that they absorb. When we look at a few examples, we see that the absorbed wavelength increases with the amount of conjugation.

So, how is wavelength related to the degree of conjugation? Let's consider molecular energy levels. We can think of delocalized electrons as occupying molecular orbitals, or MOs. A molecule absorbs light with the exact energy needed to excite an electron to a higher energy molecular orbital. The most likely transition is from the highest occupied molecular orbital, called the HOMO, to the lowest unoccupied molecular orbital, or LUMO. So, we expect that the most absorbed wavel