Final answer:
A more highly conjugated molecule will display a vibrant color because the degree of conjugation affects the light it absorbs and reflects, with highly conjugated molecules absorbing longer (lower energy) wavelengths, resulting in brighter colors observed.
Step-by-step explanation:
When observing conjugated compounds in the UV and visible regions of the electromagnetic spectrum, a more highly conjugated molecule will have a vibrant color. This is because the degree of conjugation affects the gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). A smaller HOMO-LUMO gap, as seen in highly conjugated systems, corresponds to the absorption of longer wavelengths (lower energy) within the visible range of the spectrum. As a result, more highly conjugated compounds often appear brighter in color since they absorb higher energy light and reflect or transmit light in the visible spectrum that gives them their distinct hue.
For instance, a compound absorbing ultraviolet light with a wavelength of 217 nm exhibits less conjugation compared to a complex that absorbs light in the visible range. An example of this can be seen in complexes such as [Cr(NH3)6]³+, which has strong-field ligands and absorbs higher energy photons corresponding to blue-violet light, giving it a yellow color. In contrast, [Cr(H₂O)]³+ absorbs lower-energy photons corresponding to the yellow-green portion of the spectrum, imparting a deep violet color.